Spectroscopic assignment of intermediate products of radiolysis of n-hexane on the aluminum surface.
\r\n\tThe diversity of the issues will be covered from algorithms, mathematical models, and software engineering by design methodologies and technical or practical solutions. This book intends to provide the reader with a comprehensive overview of the current state-of-the-art, cases studies, hardware and software solutions, analytics and data science in dependability engineering.
",isbn:"978-1-78923-908-9",printIsbn:"978-1-78923-907-2",pdfIsbn:null,doi:null,price:0,priceEur:0,priceUsd:0,slug:null,numberOfPages:0,isOpenForSubmission:!1,hash:"9342a056651f34acc565b467a71e1e27",bookSignature:"Prof. Fausto Pedro García Márquez, Dr. Isaac Segovia, Dr. Tamás Bányai and Dr. Péter Tamás",publishedDate:null,coverURL:"https://cdn.intechopen.com/books/images_new/8453.jpg",keywords:"Waste, Manufacturing, Lean Manufacturing, Strategy, Planning, Resources, Organization, Financial, Cost, Profict, Efficence, Transport, Inventory, Movement, Algorithms, Mathematics Methods, Heuristic, Alarms, Scada, Condition Monitoring, Software",numberOfDownloads:1299,numberOfWosCitations:0,numberOfCrossrefCitations:0,numberOfDimensionsCitations:0,numberOfTotalCitations:0,isAvailableForWebshopOrdering:!0,dateEndFirstStepPublish:"September 26th 2018",dateEndSecondStepPublish:"October 17th 2018",dateEndThirdStepPublish:"December 16th 2018",dateEndFourthStepPublish:"March 6th 2019",dateEndFifthStepPublish:"May 5th 2019",remainingDaysToSecondStep:"a year",secondStepPassed:!0,currentStepOfPublishingProcess:5,editedByType:null,kuFlag:!1,editors:[{id:"22844",title:"Prof.",name:"Fausto Pedro",middleName:null,surname:"García Márquez",slug:"fausto-pedro-garcia-marquez",fullName:"Fausto Pedro García Márquez",profilePictureURL:"https://mts.intechopen.com/storage/users/22844/images/system/22844.jpeg",biography:"Dr. Fausto Pedro García Márquez is a full professor at the Universidad de Castilla-La Mancha (UCLM), Spain (accredited as a full professor since 2013), an Honorary Senior Research Fellow at Birmingham University, UK, and a lecturer at the Postgraduate European Institute. Dr. Garcia Marquez also worked as a Senior Manager in Accenture from 2013 to 2014. He obtained his European Ph.D. with maximum distinction. He has been awarded several prizes including the Advancement Prize for Management Science and Engineering Management Nominated Prize (2018), the First International Business Ideas Competition 2017 Award (2017); Runner (2015), Advancement (2013) and Silver (2012) by the International Society of Management Science and Engineering Management (ICMSEM); and the Best Paper Award in the international journal of Renewable Energy (Impact Factor 3.5; 2015). He has published more than 150 papers (65% ISI, 30% JCR and 92% international), some recognized as 'Renewable Energy” (as 'Best Paper 2014”); 'ICMSEM” (as 'excellent”), 'Int. J. of Automation and Computing” and 'IMechE Part F: J. of Rail and Rapid Transit” (most downloaded), etc. He is the author and editor of 25 books (Elsevier, Springer, Pearson, Mc-Graw Hill, IntechOpen, IGI, Marcombo, AlfaOmega), and holds 5 patents. Dr. Garcia Marquez is the editor of 5 international journals and has acted as a committee member of more than 40 international conferences. He has been a principal investigator on 4 European projects, 5 National projects, and more than 150 projects for various universities and companies. His main interests are Maintenance Management, Renewable Energy, Transport, Advanced Analytics, Data Science.",institutionString:"University of Castile-La Mancha",position:null,outsideEditionCount:0,totalCites:0,totalAuthoredChapters:"7",totalChapterViews:"0",totalEditedBooks:"6",institution:{name:"University of Castile-La Mancha",institutionURL:null,country:{name:"Spain"}}}],coeditorOne:{id:"266521",title:"Dr.",name:"Isaac",middleName:null,surname:"Segovia",slug:"isaac-segovia",fullName:"Isaac Segovia",profilePictureURL:"//cdnintech.com/web/frontend/www/assets/author.svg",biography:"Isaac Segovia is an expert in Lean manufacturing, production, operations research, Renewable Energies. He is currently involved as researcher in several industrial collaborative research projects related to the renewable energy sectors which are financially supported by the European Commission and the Spanish Goberment. 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He has 25 years of teaching and research experience in logistics and supply chain management, with special emphasis in heuristic optimization of large scale systems.\nHe is currently a member of the Committee of Logistics and the Committee of Engineering and Computer Sciences of the Hungarian Academy of Sciences and is also editorial board member of international journals. Tamás has been a member and manager of more than 50 R&D projects. He has published over 150 research papers, book chapters and conference proceedings. Away from academia, his other interests include playing the piano and taking photographs.",institutionString:"University of Miskolc",position:null,outsideEditionCount:0,totalCites:0,totalAuthoredChapters:"0",totalChapterViews:"0",totalEditedBooks:"0",institution:{name:"University of Miskolc",institutionURL:null,country:{name:"Hungary"}}},coeditorThree:{id:"288310",title:"Dr.",name:"Péter",middleName:null,surname:"Tamás",slug:"peter-tamas",fullName:"Péter Tamás",profilePictureURL:"https://mts.intechopen.com/storage/users/288310/images/system/288310.jpeg",biography:"Péter Tamás graduated in 2006 from the Faculty of Mechanical Engineering and Informatics at the University of Miskolc, Hungary with an MSc. in mechanical engineering. He completed his Ph.D. work in the field of material handling systems and logistics information technology in 2012. He has worked as an assistant professor since 2011, an associate professor since 2017 and vice dean from 2017. As a lecturer on numerous logistics subjects in the fields of process improvement at the BSc, MSc and Ph.D. levels, he has professional responsibility in more lean educational programs. Dr. Tamás’ research area is focused on simulation modeling, lean process improvement, and planning of the logistics systems. 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From chapter submission and review, to approval and revision, copyediting and design, until final publication, I work closely with authors and editors to ensure a simple and easy publishing process. I maintain constant and effective communication with authors, editors and reviewers, which allows for a level of personal support that enables contributors to fully commit and concentrate on the chapters they are writing, editing, or reviewing. I assist authors in the preparation of their full chapter submissions and track important deadlines and ensure they are met. I help to coordinate internal processes such as linguistic review, and monitor the technical aspects of the process. As an ASM I am also involved in the acquisition of editors. 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Obtaining a package of experimental data and information on the study of RHP using complex physical-chemical methods, including spectroscopic methods, also opens up new opportunities for solving a number of problems on the surface of metals and oxides. The activated metal surface accelerates the radiation-chemical decomposition of paraffins and increases the efficiency of hydrogen production in the radiolysis of hydrocarbons. At the same time, surface adsorption of paraffins significantly influences the course of the radiation-heterogeneous decomposition process and, therefore, the yield of radiolysis products. In the literature, there are a number of experimental and theoretical studies devoted to the study of the interaction and activation of paraffins on the surface of metals [1, 2, 3, 4].
However, until now, the problems associated with the adsorption of hydrocarbons on the radiation-modified surface of metals have not been studied sufficiently [5, 6, 7, 8]. In fact, there are no data on the radiation-chemical decomposition of hydrocarbons on the surface of metals and the radiation hydrogenation of these surfaces under the influence of gamma radiation. This chapter presents the results of work on IR spectroscopic studies of radiation-stimulated heterogeneous processes of adsorption, radiolysis, and hydrogenation on the surface of metals in contact with hydrocarbons [5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17].
Metal plates of aluminum (Al) and beryllium (Be) reactor materials having a smooth polished surface with a high reflection coefficient (R = 0.88 + 0.05) in the middle IR region (λ = 2.2–25 μm) were used and investigated [9, 10]. As the hydrocarbon adsorbate, the authors selected unsaturated pairs of n-hexane (n-C6H14), the purification and adsorption of which are described in detail in [10, 11]. The formation of gaseous decomposition products—molecular hydrogen and hydrocarbons—was monitored chromatographically and spectroscopically. IR absorption spectra of gaseous hydrocarbons were obtained in a gas cuvette with an optical path length of ~1 m. Radiation-chemical yields of these products have been determined [12]. The heterosystem Al (Be)/ads.n-hexane was irradiated with γ-quanta on an isotope 60Со source with dose rates dФγ/dt = 0.80 and 1.03 Gy/s at room temperature. The absorbed dose was Фγ = 0.5–120 kGy.
Molecular vibrational spectroscopy of substances in various aggregate states has long and firmly established itself as a method for studying their structure, properties, and all kinds of transformations in external interactions. Similar information on the adsorbed state of molecular systems would be of great value. However, it is not possible to mechanically transfer to it the whole arsenal of experimental, theoretical, and calculated methods of vibrational spectroscopy. In the experimental plan, the main difficulty is that a useful optical signal turns out to be small and, in the end, one must deal with a very poor signal-to-noise ratio, since a small number of molecules (at the level of one monomolecular layer) participate in the absorption, emission, or scattering events.
In the IR range, for measurement of the vibrational spectra of thin films on metals with a nanosized scale, the most promising are methods in which the probe electric field has a maximum intensity on a smooth surface of the metal or very close to it. This condition is satisfied when the grazing angle effect is applied to the metal in the IR reflection-absorption spectroscopy (IRRAS) method [18]. The advantage of this method is the wide range of wavelengths available for investigation. One of the most important conditions for successful application of methods of vibrational spectroscopy is a wide range of the spectrum, usually 200–4000 cm−1—the area of the “fingerprint” of the overwhelming number of molecular systems, where most of the vibrational frequencies fall. In this respect, the IRRAS method seems to be the most wide-range and practical than the method of surface electromagnetic wave spectroscopy (SEW), which has high sensitivity. Therefore, in obtaining the spectra of the reflection of oxides in the region of lattice vibrations, in studying radiation-heterogeneous adsorption processes, in the transformation of water and hydrocarbon molecules, and in studying the processes of oxidation and hydrogenation of aluminum and beryllium metals in contact with water and n-hexane. The choice of this method is due to the fact that it has already proved itself as a noncontact nondestructive optical probing method in the study of adsorbed molecules and oxide films on metal surfaces. The IRRAS method allows one to simultaneously obtain complete information about some chemical stages of radiation-heterogeneous transformations in metal-water, metal-hydrocarbon systems, the formation of adsorption—and catalytically active surface functional groups, and also to trace the formation of oxide and hydride films during oxidation and hydrogenation metal.
According to the theory of IRRAS [18], the efficiency of the interaction of infrared radiation with a smooth metal surface is achieved with incidence coal close to a sliding (i.e., close to 90°) and with p-polarization. This method is sensitive to the components of the dipole moments of vibrational transitions in thin films perpendicular to the metal surface.
The IR reflection spectra at the drop of linearly polarized radiation on the sample at an angle close to the sliding angle φ = 88° were measured at room temperature by means of a special reflecting device (Shimadzu, Japan), adapted to Specord 75 IR spectrophotometer, as well as Harrick prefixes. The optical scheme of the prefix of reflection with the coal of incidence of rays near 88° collected on the base of the attachment “Harrick” is shown in Figure 1. Mirror 1 sent a beam of radiation to the surface of sample 2 (for the axial beam φ0 = 88°±0.5°). Narrow mirrors 3 and 4 of a special shape, as well as screen 7, provided passage through the prefix to only those rays for which the angle of incidence on the sample lay in the interval φ0 – 1°< φ < φ0 + 1°. The screens in front of mirror 2, which were not shown in the figure, made it possible to illuminate a part of the sample surface occupied by the film and located between two horizontal dashed lines.
Optical scheme of the attachment of reflection: 1,4,6 - flat mirrors, 5 - concave mirror, 7 - screen, φ - angle of incidence of radiation on the sample. The inset shows the sample used.
After reflection from the rotary mirror 6, the radiation was directed to the detector (receiver). At φ = 88°, the rays reflected from the entire illuminated surface of the sample were captured. When using a reflection attachment, polarizers in the form of Al gratings were placed in both channels of the spectrophotometer on plates of KRS-5 with a polarization degree of 99%, which transmitted radiation polarized parallel to the plane of incidence of radiation on the sample. Another hand-held ZnS polarizer was also used. The IR reflection spectra at the drop of linearly polarized radiation on the sample at an angle close to the sliding (φ = 88°) were measured in the region of wavenumbers ν = 3400–650 cm−1 at room temperature. The optical densities of the absorption bands were determined from D = −lg(R/Ro). According to this formula, the optical densities D and D0 of the bands of stretching vibrations С▬Н and Ме▬Н of adsorbed n-hexane (D0—optical density in the initial sample, D—in the processed samples) were calculated and their ratios D/Do were determined [18].
In this section, we present and discuss the results of experimental studies devoted to the spectroscopic study of radiation-stimulated adsorption of hydrocarbons, in particular n-hexane, on the surface of reactor materials-metal plates of aluminum and beryllium. To this end, the authors used the IR reflection-absorption spectroscopy method and developed a special vacuum optical cell that takes into account the specifics of conducting radiation studies. Let’s consider some important aspects of this IR research, which in our opinion are the most interesting.
IR absorption spectra of n-hexane in the stretching vibration region of СН adsorbed on the dehydroxylated aluminum surface at room temperature are presented in Figure 2a (curve 1) [8, 10]. As can be seen from the figure, the adsorption of n-hexane on the aluminum surface is accompanied by the formation of a number of absorption bands (a.b.): narrow ones at νmax = 2950 and 2920 cm−1, weak at 2900 and 2880 cm−1 and intense wide at 2680 cm−1. The narrow bands are close in position to the bands characteristic of ν(CH) in the spectra of n-C6H14 in the gas phase [19], which allows them to be attributed to physically adsorbed n-hexane. An asymmetric broad band with a maximum at 2680 cm−1 is attributed to the vibration of one of the C▬H bonds in the n-C6H14 molecule perturbed by the surface centers of aluminum. The unusually low frequency of oscillations, as well as the relatively high intensity and width, indicates a strong perturbation of n-hexane molecules upon adsorption. The complex to which this a.b. object belongs is not stable and is destroyed by evacuation at room temperature. This allows us to classify this band as n-hexane adsorbed in molecular form.
IR absorption spectra of n-hexane adsorbed on the surface of aluminum (a) and beryllium (b): 1-initial and 2-γ-irradiated at Фγ = 10 kGy. T = 300 K, P = 20 Pa.
The formation of a molecular complex was theoretically proved in the framework of an ab initio quantum chemical calculation of the profile of the potential energy of dissociative methane adsorption on the Ni surface and was experimentally established when studying its adsorption on metallic surfaces (Fe, Ni, Pt) [20]. Weak a.b. with maxima at 2900 and 2880 cm−1 indicate a slight dissociative adsorption of n-hexane on the Al surface, which is associated with even lower concentrations of such forms than for H-bound complexes, especially at a relatively low interaction temperature.
When studying the adsorption of n-hexane on preliminarily γ-irradiated aluminum samples, we found that starting from certain values of the irradiation dose (Φγ = 10 kGy) at room temperature, strong dissociative chemisorption is observed (Figure 2a, curve 2). It occurs as a result of the interaction of n-hexane with surface-active centers formed under the action of γ-quanta in aluminum. This is evidenced by the increase and redistribution of the intensities of a.b. at 2900 and 2880 cm−1, as well as the appearance in the IR spectrum of a new a.b. with a maximum at 2840 cm−1. The observed absorption bands apparently belong to the stretching vibrations of the CH bonds of the fragments CH3, C2H5, etc. bound by aluminum (aluminum alkyls) [6, 7, 8, 10].
The dissociative adsorption is also confirmed by the appearance in the region of 2000–1700 cm−1 of bands with frequencies of 1920, 1830, and 1760 cm−1 of Al▬H bonds in the IR spectrum, which are related to surface aluminum hydrides [3, 4]. The formation of hydrides in the interaction of n-hexane with aluminum is consistent with the hydride mechanism of interaction of metals with hydrocarbons [4, 6, 7, 8].
In order to reveal the spectrokinetic regularities of radiation-stimulated adsorption of n-hexane on the aluminum surface, the kinetic adsorption curves, that is, the dependence of the changes in the relative optical densities of the D/D0 bands of the molecularly and dissociatively adsorbed n-hexane forms on the absorbed dose of irradiation, were studied. The spectra are shown in Figure 3a. As can be seen from Figure 3a (curve 1), the kinetic feature of radiation-stimulated chemisorption consists of a certain initial induction period at Фγ ≤ 5 kGy, related to the healing of biographical defects, the linear region at 2 ≤ Фγ ≤ 25 kGy, caused by the generation of adsorption active centers and the adsorption of additional n-C6H14 molecules at these centers, as well as from the stationary saturation region occurring at Фγ > 25 kGy.
(a) Dependences of the relative optical densities of the bands molecularly (1) and dissociative (2) adsorbed n-hexane from the absorbed γ-irradiation dose in the Al/ads.n-hexane heterosystem: νmax = 2680 (1) and 2880 cm−1 ( 2). (b) Dependences of the optical densities of the absorption bands of aluminum surface hydrides on the absorbed dose of γ-irradiation: νmax = 1920 (1b), 1830 (2b) and 1760 cm−1 (3b).
Apparently, under the action of γ-radiation in aluminum, new active surface states are generated, whose density increases with increasing γ-radiation dose in aluminum, and the probability of their interaction with adsorbed n-hexane molecules increases, which causes their dissociation. At the same time, the kinetic curve of molecularly adsorbed n-hexane is characterized by two regions: in the region 5 ≤ Фγ ≤ 10 kGy, formation is observed, and at Фγ > 10 kGy, the molecular H-complex decays (Figure 3a, curve 2). The presence of activated dissociative chemisorption in the region 5 ≤ Фγ ≤ 25 kGy is also confirmed by the course of the kinetic curve obtained for surface aluminum hydrides (Figure 3b).
Thus, in the region of absorbed dose 5 ≤ Фγ ≤ 25 kGy, activated dissociative chemisorption is detected, which is explained both by an increase in the number of centers of activated adsorption due to surface-excited states of aluminum with increasing γ-radiation dose and by decomposition of H-bound complexes as a result of energy transfer excitation to n-hexane molecules. Activated adsorption of n-hexane was also observed on the nickel surface [1, 20], and according to [1, 21], dissociative adsorption of methane on metals is usually activated. Similar results were also obtained in the study of the radiation-stimulated adsorption of n-hexane on the beryllium surface. The observed narrow bands refer to physically adsorbed n-hexane. The asymmetric broad band with a maximum at 2640 cm−1 is attributed to the vibration of one of the C▬H bonds in the n-C6H14 molecule perturbed by the surface centers of beryllium. The unusually low frequency of oscillations, and also the relatively high intensity and width (ν1/2 = 50 cm−1) indicate a strong perturbation of n-hexane molecules during adsorption. This band refers to n-hexane adsorbed in the molecular form. Weak a.b. with peaks at 2860 and 2840 cm−1 indicate a slight dissociative adsorption of n-hexane on the Be surface. When studying the adsorption of n-hexane on preirradiated beryllium samples, it was established that starting from certain values of the irradiation dose (Φγ = 10 kGy), strong dissociative chemisorption is observed at room temperature (Figure 2b, curve 2). It occurs as a result of the interaction of n-hexane with surface-active centers formed under the action of γ-quanta in beryllium. This is evidenced by the appearance in the IR spectrum of new a.b. with maxima at 2860 and 2820 cm−1. The observed absorption bands seem to refer to stretching vibrations of the C▬H bonds of the fragments CH3, C2H5, etc., associated with beryllium (beryllium alkyls) [14, 15]. The dissociative adsorption is also confirmed by the appearance in the IR spectrum in the 2000–1700 cm−1 region of bands with frequencies of 1790 and 1740 cm−1, which relate to the Be▬H bonds of surface beryllium hydrides [14, 15]. The kinetic curves of radiation-stimulated adsorption of n-hexane on the beryllium surface have also been studied in [14] (Figure 4). It was found that the kinetic feature of radiation-stimulated chemisorption consists of a certain initial induction period at Фγ ≤ 10 kGy, a linear region at 10 ≤ Фγ ≤ 25 kGy, and also from a stationary saturation region at Ф > 25 kGy. Apparently, the generation of new active surface states under the action of γ-radiation in beryllium, the density of which increases with increasing dose of γ-radiation in beryllium, and the probability of their interaction with adsorbed n-hexane molecules, which causes their dissociation. At the same time, the kinetic curve of molecularly adsorbed n-hexane is characterized by two regions. It is seen that the kinetic feature of radiation-stimulated adsorption in the 5 ≤ Фγ ≤ 10 kGy region is observed, and at Фγ > 10 kGy, the molecular H-complex decays (Figure 4a, curve 2). The presence of activated dissociative chemisorption in the region 5 ≤ Фγ ≤ 25 kGy is also confirmed by the kinetic curve obtained for surface beryllium hydrides (Figure 4b).
Dependence on the absorbed dose of gamma irradiation in the Be-adsorbed n-hexane system of the relative optical densities of the bands molecularly (1) and dissociatively (2) adsorbed n-hexane (a) and optical densities of absorption bands of surface beryllium hydrides (b): νmax = 2640 (1a), 2880 (2a), 1790 (1b), and 1740 cm−1 (2b).
Thus, in the region of absorbed dose 5 ≤ Фγ ≤ 10 kGy, activated dissociative chemisorption is detected, which is explained both by an increase in the number of centers of activated adsorption due to surface excited beryllium states with increasing dose of γ-radiation and by decomposition of H-bound complexes as a result of transmission excitation energy to n-hexane molecules.
In this section, some aspects of IR spectroscopic studies of the radiation-chemical transformation of n-hexane on the surface of aluminum and beryllium at room temperature are made to determine the role of surface intermediate-active particles and their products in the dynamics of changes in the radiolysis process.
The IR spectra of the reflection of the Al/ads.n-hexane heterosystem before (curve 1) and after the action of γ-quanta (curves 2–4) at room temperature in the frequency range ν = 3500–650 cm−1 are shown in Figure 5. Changes in the spectra were observed in both the region of stretching (Figure 5a) and deformation vibrations of C▬H (Figure 5b). It is seen that in the unirradiated heterosystem after the adsorption of n-hexane on the aluminum surface in the stretching vibration region of CH there is an appearance of absorption bands (a.b.) indicating the occurrence of three forms of adsorption: physical adsorption (narrow bands at νmax = 2950 and 2920 cm−1), the molecular form of adsorption (intense broad band at 2680 cm−1), and insignificant dissociative chemisorption (weak bands at 2900 and 2880 cm−1) (Figure 5a, curve 1). The flow of three forms of adsorption is also confirmed by the formation of a number of bands in the region of deformation vibrations of C▬H with maxima at νmax = 1540, 1480, 1460, 1400, and 1360 cm−1 (Figure 5b, curve 1).
IR absorption spectra of the Al/ads.n-hexane system: 1—initial, 2–4—after γ-radiolysis at Фγ = 10 (2), 20 (3), and 30 kGy (4).
Irradiation of the Al/ads.n-hexane heterosystem by γ-quanta at Φγ = 10 kGy leads to the formation of additional a.b. in the spectra. (Figure 5a and b, curves 2), which indicates the radiation-chemical decomposition of n-hexane on the Al surface. Radiolysis of n-hexane in the Al/ads.n-hexane system in the stretching vibration region of C▬H (νCH) is accompanied by the disappearance of the a.b. at 2950 and 2920 cm−1, a decrease in the intensity of the broad band at 2680 cm−1 and its narrowing and the appearance of a series of narrow bands at 2960, 2940, 2900, 2880, 2840, and 2800 cm−1, and also comparatively broad at 2520 and 2440 cm−1 bands indicating the formation of surface aluminum alkyls and molecular complexes (Figure 5a, curve 2). The observed new narrow a.b. at 2980–2780 cm−1 are associated with the formation of adsorbed C1–C5 hydrocarbons, and relatively broad bands at 2500–2400 cm−1 are possibly due to heavier than n-hexane hydrocarbons [19, 22, 23, 24, 25, 26, 27]. The formation of aluminum alkyls is also confirmed by the presence in the spectrum of a.b. in the regions of planar and out-of-plane deformation vibrations of δCH relatively narrow with maxima at −1470, 1450, 1430, 1420, 1400, 1380, and wide at 855 and 720 cm−1 (Figure 5b, curve 2). A weak broad band at 720 cm−1 refers to the pendular vibrations of the CH2 group, which are not connected with the skeleton vibrations and is characteristic of long paraffin chains of the type ▬(CH2)n▬ (n ≥ 4) [25, 26, 27].
As a result of the decomposition of n-hexane in the Al/n-hexane system, various olefins, mainly trans-ethylene, propylene, butylene-1, hexene-1, and trans-hexene-3, are formed similarly to homogeneous radiolysis. This is evidenced by a.b. with maxima at 1650, 1620, 1600, 1570, and 1530 cm−1, characterizing the stretching vibrations νc〓c in the π-complexes of these adsorbed olefins with the Al3+ cation predominantly by the octahedral coordination of Al atoms [26, 27, 28, 29, 30] (Figure 5b, curve 2). The appearance of a number of a.b. in the regions of stretching νCH(ν ~ 3140–3075 cm−1), planar (ν ~ 1350–1200 cm−1), and out-of-plane (ν ~ 1100–950 cm−1) strain δCH vibrations of CH containing 〓CH2 and 〓CH▬ groups is one of the main criteria for proving the formation of olefins on the surface of π-complexes [26, 27]. In this case, a.b. with peaks of 1320 and 1280 cm−1 are attributed to the plane deformation vibrations ν1 and ν2 of two different symmetry classes, and a.b. with maxima of 980 cm−1—the out-of-plane deformation vibration of 〓CH disubstituted ethylene (trans) [10, 26, 27]. The formation of π-complexes of olefins is also confirmed by the presence of a.b. with maxima at 3300, 3240, and 3200 cm−1 in the overtone coupling region νc〓c (Figure 5a and b, curve 2) [26, 27].
With an increase in the γ-radiation dose Фγ up to 20 kGy, the spectra are transformed: the intensities of some a.b. associated with adsorbed hydrocarbons and olefins decrease, indicating that they are partially decomposed (Figure 5a and b, curve 3). Partial decomposition of olefins is accompanied by their dissociative adsorption and the formation of π-allyl complexes on the aluminum surface [26]. A further increase in the value of Фγ to 30 kGy leads to a complete decomposition of hydrocarbons and olefins (curve 4). In addition, an interesting fact was discovered, consisting in an increase in the intensities of a.b. associated with the formation of surface hydrides. At irradiation doses up to 10 kGy, bands with frequencies of 1920, 1830 and 1760 cm−1 appear in the IR spectrum in the region 2000–1700 cm−1, which refer to Al▬H [2, 3, 4]. With an increase in the dose of γ-irradiation up to 20 kGy, the intensities of these a.b. increase, which indicates the accumulation of hydrogen in the form of its hydrides (Figure 5b, curves 2, 3). A further increase in the value of Фγ to 40 kGy is accompanied by the formation of aluminum hydride Al▬H3 stable at room temperature (a.b. with a maximum at 1830 cm−1) (Figure 5b, curve 4) [10]. As follows from IR spectroscopy, the radiolysis of n-hexane in the γ-irradiated Al-n-hexane heterosystem is accompanied by the formation of intermediate decomposition products—surface aluminum alkyls, π-complexes of olefins and aluminum hydrides (Table 1).
Sample | Surface aluminum alkyls (ν сm−1) | Surface π-complexes of olefins (ν, сm−1) | Surface hydrides (ν, сm−1) |
---|---|---|---|
Al/ads.n-hexane | 2960, 2940, 2900, 2880, 2840, and 2800 (region of stretching vibrations C▬H), 2520 and 2440 Narrow bands 1470, 1450, 1430, 1420, 1380 (region of planar and nonplanar deformation of the vibrations δCH), broad 855 and 720 | 1650, 1620, 1600, 1570, and 1530 (C〓C bond) 3300, 3240, and 3200 (overtone coupling region C〓C) ν ~ 3140–3075 (the stretching vibration region νCH) ν ~ 1350–1200 and ν ~ 1100–950 (the region of planar and out-of-plane deformation δCH vibrations of C▬H containing 〓CH2 and 〓CH▬ groups) | 1920, 1830 (stable form AlH3), 1760 |
Spectroscopic assignment of intermediate products of radiolysis of n-hexane on the aluminum surface.
The main end products of decomposition are gaseous hydrocarbons and molecular hydrogen, the formation of which was monitored by spectroscopic and chromatographic methods. IR absorption spectra of gaseous hydrocarbon products of n-hexane radiolysis in the region of out-of-plane deformation vibrations of C▬H are shown in Figure 6a. It can be seen that for small values of the absorbed dose of γ-irradiation (Φγ = 10 kGy) in the Al/n-hexane system, a.b. with maxima at 830, 800, 785, 770, 750, and 720 cm−1 (Figure 6a, curve 1), whose location and half-width allows them to be assigned to C1–C5 hydrocarbons in the gas phase [19]. With an increase in the absorbed dose of Φγ to 30 kGy, the intensities of these bands are redistributed and increased (curves 2–3), and a further increase in Φγ to 40 kGy does not lead to appreciable changes in the spectrum. Comparison of dose changes in the absorption spectra of intermediate and final hydrocarbon products of the decomposition of n-hexane in the γ-irradiated Al/ads.n-hexane system, which occur as a function of the absorbed dose of γ-irradiation, shows that they have an antibatic character. According to the chromatographic analysis, the total radiation-chemical yield of hydrocarbons ΣG(C1–C5) is 0.36 molecule/100 eV. The kinetic regularity of the accumulation of molecular hydrogen in the radiolysis of n-hexane in the Al/ads.n-C6H14 system was studied in [10]. Based on the kinetic curve of H2 accumulation, the rate of the radiative formation of H2 is determined and the radiation-chemical yield of molecular hydrogen Gads (H2) in the Al/ads.n-hexane heterosystem is calculated, taking into account the total absorbed radiation dose of gamma quanta, which is Gads (H2) = 29.6 molecule/100 eV. It should be noted that the yield of hydrogen for a homogeneous phase (in the absence of aluminum) is G (H2) = 5.2 molecule/100 eV [29, 30, 31].
IR spectra of the absorption of gaseous hydrocarbon products of n-hexane radiolysis on the surface of Al (a) and Be (b) in the region of out-of-plane deformation vibrations of CH at doses of gamma irradiation Фγ = 10 (1), 20 (2), and 30 kGy (3).
A comparison of Gads (H2) and G (H2) under identical conditions indicates the radiation-catalytic activity of aluminum during the decomposition of n-hexane. Similar results were obtained with the radiation-chemical transformation of n-hexane on the surface of beryllium at room temperature. Radiolysis of n-hexane in the presence of beryllium is accompanied by the formation of intermediate decomposition products‑surface beryllium hydrides, beryllium alkyls, and π-olefin complexes (Table 2).
Sample | Surface beryllium alkyls (ν, сm−1) | Surface π-complexes of olefins (ν, сm−1) | Surface hydrides (ν, сm−1) |
---|---|---|---|
Ве/ads.n-hexane | 2960, 2930, 2900, 2880, 2840, 2820 (region of stretching vibrations С▬Н), 2520 and 2420 narrow bands 1468, 1450, 1430, 1415, 1400, 1370 (region of planar, out-of-plane deformation vibrations δСН), broad bands 855 and 720 сm−1 | 1660, 1610, 1580, 1565, 1550, 1530 (C〓C bond) 3300, 3840, 3200 and 3140 (overtone region C〓C) ν(3140–3050) (region stretching vibrations νСН) ν(2140–3050) ν(1060–950) (region of planar, out-of-plane deformation vibrations of С▬Н containing 〓СН2 and 〓СН groups) | 1790, 1740 (stable form ВеН2) |
Spectroscopic assignment of intermediate products of radiolysis of n-hexane on the surface of beryllium.
The variety of products of n-hexane radiolysis is a consequence of the formation of highly reactive radicals upon irradiation of hydrocarbons. Thus, the formation of higher hydrocarbons in the radiolysis of n-C6H14 is due to the appearance of a number of alkyl radicals that combine with each other to form C7H16, or n-hexane or C7H16, and give products of more complex composition. The main end products of decomposition are gaseous hydrocarbons and molecular hydrogen. IR absorption spectra of gaseous hydrocarbon products of n-hexane radiolysis on the beryllium surface in the region of extraplane deformation vibrations of C▬H are presented in Figure 6b.
According to the chromatographic analysis, the total radiation-chemical yield of hydrocarbons ΣG(C1–C5) is 0.28 molecule/100 eV. The kinetic regularity of the accumulation of molecular hydrogen in the radiolysis of n-hexane in the system Be/ads.n-C6H14 [14, 15] has been studied. Based on the kinetic curve of H2 accumulation, the rate of the radiative formation of H2 is determined and its activation energy is calculated, which is Ea ≈ 4.5 kC/mol. According to the calculation, the radiation-chemical yield of molecular hydrogen is Gads (H2) in the heterosystem Be/ads.n-C6H14, taking into account the total absorbed radiation dose of γ-quanta, is Gads(H2) = 24.8 molecule/100 eV. It should be noted that the yield of hydrogen for a homogeneous phase (in the absence of beryllium) is G(H2) = 5.2 molecule/100 eV. A comparison of Gads(H2) and G(H2) under identical conditions indicates the radiation-catalytic activity of beryllium and aluminum during the decomposition of n-hexane.
A comparative analysis of the studies shows that gamma irradiation of aluminum (beryllium)/ads.n-hexane heterosystems with γ-quanta in the region of absorbed dose of 5 < Φγ ≤ 50 kGy leads to a radiation-chemical decomposition of n-C6H14. In contrast to the homogeneous phase, the radiolysis of n-hexane in the presence of aluminum and beryllium is accompanied by the formation of intermediate products of decomposition of surface aluminum and beryllium hydrides, aluminum (beryllium) alkyls, and π-complexes of olefins. The authors revealed the limits of doses of complete radiolysis of n-hexane, below which its partial decomposition occurs, and at higher levels, a stationary saturation region sets in. The kinetics of the accumulation of molecular hydrogen has been studied and its radiation-chemical yields, which constitute the are determined Gads(H2) = 24.8 (in the presence of Be) and 29.6 molecule/100 eV (in the presence of Al), respectively.
In the heterogeneous radiolysis of n-hexane in contact with a metal, γ-quanta are exposed to both n-hexane and metal [28, 29, 30]. Since metals are a core of positive ions immersed in an electron gas, excitation and ionization produced by radiation in them create only defects that arise during elastic collisions [28, 29]. Such defective centers are the radiation-induced active states S* (ions, localized charges, etc.), whose density on the surface of metals with thin oxide films is much larger compared to the surface of metals (Al, Be) with continuous oxide layers. The interaction of the active surface states of S* and the release of secondary electrons from the metal under the action of γ-radiation
The study of radiation-stimulated hydrogenation of the surface of metals in contact with hydrocarbons is of interest from the point of view of the development of radiation-heterogeneous catalysis processes [29, 30]. The identification of the mechanisms and kinetic regularities of the formation of hydride layers at the initial stage of the process is necessary to solve the problems of radiation embrittlement of metals and alloys, as well as the production of molecular hydrogen by its accumulation in the form of hydrides [23, 24, 29, 30].
In this section, the features of the radiation-stimulated hydrogenation of the surface of aluminum and beryllium contacting with n-hexane are exposed under the action of gamma irradiation at room temperature.
Radiation-chemical conversion of n-hexane in the Al-n-hexane heterosystem takes place at an absorbed dose of Фγ > 0.5 kGy and is accompanied by the formation of aluminum hydrides, which is confirmed by the appearance and IR reflection spectra in the bands ν = 2000–1500 cm−1 of the absorption bands 1920, 1830, and 1760 cm−1, related to the Al▬H bonds (Figure 7a, curve 1). The formation of hydrides in the interaction of n-hexane with aluminum occurs via the hydride mechanism of interaction of metals with hydrocarbons [3, 4] according to which in the first stage of the reaction after orientation of n-hexane molecules on the aluminum surface and mutual polarization of molecular and atomic layers at the aluminum/ads.n-hexane interface, a new metal-hydrogen bond is formed, and a new chemical compound, the hydride, appears on the surface of the metal. The formation of various types of hydrides is associated with different coordination numbers of a coordinative unsaturated aluminum atom [2, 3, 4]. Figure 6a shows the change in the bands of Al▬H stretching vibrations as a function of the contact time τ of aluminum with n-hexane under radiation exposure (T = 300 K, dФγ/dt = 0.80 Gy/s). Increasing the contact time from 10 min to 40 h (absorbed dose of 0.5 and 120 kGy, respectively) leads to the transformation of spectra accompanied by the redistribution of the intensities of these bands and their fusion and the formation of aluminum hydride in a more stable form of Al-H3 (ν = 1830 cm−1) at room temperature (curves 2–4). According to [2, 3, 4], the frequencies of stretching vibrations of Al▬H3 are in the region of 1850–1770 cm−1, while the intensity increases by almost an order of magnitude and the half-width ν1/2- by 3.6 times (from 50 to 180 cm−1).
The change of the bands of stretching vibrations of Al▬H and Be▬H on the dependence of contact time τ of aluminum (a) and beryllium (b) with n-hexane under radiation action τ: 10 min (1), 5 h (2), 20 h (3), and 40 h (4). (T = 300 K, dФγ/dt = 0.80Gy/s).
A similar picture is also observed when the bands of stretching vibrations of Be▬H are varied as a function of the contact time τ of beryllium with n-hexane under radiation action (T = 300 K, dФγ/dt = 0.80 Gy s). Increase in the contact time from 10 min to 40 h is accompanied by the transformation of hydride bands and the formation of beryllium hydride in the stable form of Be▬H2 (ν = 1740 cm−1) (Figure 7b, curves 2–4). In this case, the intensity increases almost by ~4, and the half-width ν1/2- by ~2.2 times. The observed increase in ν1/2 for the vibration bands of Al▬H and Be▬H in hydride layers is due to the inhomogeneous broadening caused by the effect of γ-irradiation.
It is known that irradiation leads to the nucleation and growth of defects such as dislocation loops and pores and stimulates diffusion processes, initiating the effect of segregation of impurities and/or the appearance of new phases, the formation of which in conventional thermal conditions is impossible. Thus, it can be argued that in the general case, irradiation initiates the creation of microstructural inhomogeneities in the metal [32], which possibly lead to broadening of the Al▬H and Be▬H vibration bands of the hydride layers obtained by radiation hydrogenation of Al and Be in contact with n-hexane.
It should be noted that the probability of formation of MeC precipitates on the surface of Al and Be plates contacting with n-hexane in the region of the absorbed dose Фγ ≤ 120 kGy is small. Therefore, in the IR reflection spectra, it was not possible to detect the absorption bands of the skeleton vibration of Al (Be)▬C bonds [33]. With the purpose of experimental confirmation of the role of hydrogen nanoclusters (accumulations) during defect formation on the metal surface during their interaction by substances under the action of gamma radiation, a microscopic study of the aluminum surface after radiative hydrogenation was carried out. Figure 8a and b shows 3D AFM images of surfaces of aluminum plates up to (a) and after hydrogenation at an absorbed dose of Фγ = 120 kGy. The surface of the original Al plate with a thin natural oxide film (d = 3.6 nm) is characterized by a high degree of density defect (a). Hydrogenation of aluminum leads to the formation of hydride phase islands on its surface and the formation of a continuous hydride layer with a thickness d ~ 450 nm as a result of the introduction of H+ ions and their migration into the volume either from a defective surface or from internal traps along the grain boundaries. In the 3D images of hydrogenated aluminum, the areas indicating carbon nanotube-like formations are clearly distinguished [7, 21].
3D images of the surface of the initial (a) and hydrogenated (b) aluminum samples.
The yields of hydrogen accumulated in the form of aluminum and beryllium hydrides have also been determined. To this end, the kinetics of hydrogen desorption by aluminum and beryllium at a temperature of T = 423 K was studied by the method of [6, 7]. The yields of H2 are found to be 0.12 molecule/100 eV for aluminum and 0.07 molecule/100 eV for beryllium. The experimental results obtained once again testify to the high ability of aluminum to accumulate hydrogen in the form of its hydride. It should be noted that the development of systems related to storage and generation of hydrogen for autonomous power plants based on oxygen-hydrogen fuel cells in efficiency (27 g components per 1 g of hydrogen) is inferior to aluminum only to systems with lithium hydride and lithium metal. This shows that one of the promising methods is the radiation-chemical reduction of hydrogen from water by metallic aluminum.
IR spectroscopic studies revealed that hydrogen was partially accumulated in the form of Al (Be) hydride [6, 7, 16] upon radiolysis of n-hexane on the surface of aluminum and beryllium metals. The dynamics of formation of the hydride layer shows that the process of radiation-stimulated hydrogenation at room temperature on the surface of these metals, contacting with n-hexane, with γ-irradiation in the absorbed dose range of 0.5–120 kGy has a multistage nature. IR spectroscopic data are in good agreement with the results of electrophysical measurements [11, 12]. In works on electrophysical measurements, it was shown that the transition from the first stage to the latter is accompanied by a decrease in the electrical conductivity of aluminum and beryllium by several times and by an order of magnitude increase in the thickness of the resulting hydride layer. The growth of the electrical resistivity of metals at the last stage is explained by the formation of a subsurface layer of dissolved hydrogen that dissipates the conduction electrons and the formation of defects in the structure of metals.
As noted above, the problems associated with the occurrence of radiation processes in a heterogeneous metal-hydrocarbon system have not been studied sufficiently; there are also no experimental data on the effect of the state of the metal surface on the course of radiation-heterogeneous processes of paraffin decomposition. Most metals are usually characterized by the presence of thin protective oxide films on their surface, the passivation of which is disturbed under the condition of increased radiation. This leads to a change in the surface state of metals and significantly affects the course of radiation-heterogeneous processes of paraffin decomposition [5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17]. In the nanostructured surface of metals, the mechanisms and rates of radiation-chemical transformations change‑local charges and their distribution, ionization energies and electron affinity, conformations and reactivity; new “forced” reactions appear; and many other anomalies are found [34, 35]. The process of radiolysis of n-hexane on the surface of previously radiation-oxidized aluminum plates containing thin oxide films of various thickness to reveal the role of Al nanostructured surface in the dynamics of the decomposition process and its influence on the rate of formation and on the yield of final products of radiolysis is considered in [9, 11]. The kinetics of the accumulation of molecular hydrogen during the decomposition of n-hexane on the surface of Al in a relationship with the growth of oxide films was studied.
Nanostructuring of the Al surface was created by preliminary oxidation of aluminum plates in contact with water under the action of γ-radiation at room temperature by the method of [17]. This involved the modification of the metal surface and the formation of a nanostructured oxide coating with an unusual property. 3D AFM-images of these surfaces are shown in Figure 9. Radiation-oxidized plates of Al contained oxide films with a thickness of 8–600 nm on the surface [5].
3D images of radiation-oxidized surfaces of aluminum with thickness d = 8 (a) and 600 nm (b).
The kinetics of the accumulation of molecular hydrogen in the radiolysis of n-hexane in radiation-oxidized Al/ads.n-hexane systems at room temperature has been studied. Based on the kinetic curves of H2 accumulation shown in Figure 10, the rates of formation and radiation-chemical yields of molecular hydrogen are determined. The radiation-chemical yields of H2 are calculated in two ways [31].
Kinetics of accumulation of molecular hydrogen on radiation-oxidized aluminum surfaces containing oxide films with a thickness of 8 (1), 40 (2), 80 (3), 200 (4), 400 (5), and 600 nm (6).
For comparison with the homogeneous phase and the characteristics of the radiation-catalytic activity, the values of the radiation-chemical yield of H2-Gads (H2) are calculated, taking into account the energy absorbed by the substance subjected to radiolysis (n-hexane) and the entire system of Gtotal(H2). Comparison of the values of Gads(H2) = 21.4–127.2 molecule/100 eV in the heterogeneous radiolysis of n-C6H14 with a value of G(H2) = 5.2 molecule/100 eV for the homogeneous phase (in the absence of aluminum) under identical conditions of our experiments indicates radiation-catalytic activity of aluminum during the decomposition of n-hexane. The energy efficiency of converting the energy of ionizing radiation to the energy of molecular hydrogen is characterized with the aid of Gtotal(H2), and therefore, in the future, their values are used in the discussion.
The dependence of the rate of formation of molecular hydrogen W(H2) in the radiolysis of n-C6H14 in radiation-oxidized Al/ads.n-hexane systems on the thickness of oxide films on the aluminum surface is shown in Figure 11. As can be seen from Figure 11, this dependence has an exponential character. The decrease in the thickness (d) of oxide films by ~2 orders of magnitude (from 600 to 8 nm) is accompanied by an increase in the rate of formation of molecular hydrogen W(H2) by ~7 times(from 1.1 to 7.6∙1015 g−1 s−1). In this case, the value of the yield of molecular hydrogen Gtotal(H2) increases from 4.3 to 8.2 molecule/100 eV. An abrupt increase in the value of the rate of formation of H2 is observed in the region of small thicknesses (d = 8–80 nm), where the surface of aluminum shows comparatively high catalytic activity. A further increase in the thickness of oxide films from 80 to 600 nm leads to a monotonic decrease in the rate of formation of molecular hydrogen.
Dependence of the rate of formation of molecular hydrogen upon the radiolysis of n-hexane in systems of radiation-oxidized aluminum/ads.n-hexane from the thickness of oxide films on the Al surface.
The effect of the topography of the powder surface on the rate of its oxidation and on the yield of molecular hydrogen was also found by the authors of [36, 37] in the oxidation of compact and porous beryllium and beryllium powder by steam. It was found that the interaction of compact and porous beryllium with water vapor under certain conditions of experiments and oxidation states, the growth of the rate constant acquires the character of a jump and the yield of molecular hydrogen increases by an order of magnitude. The results obtained are explained with the dynamic instability of the oxide layer structure on the surface of compact and porous beryllium and the change in the relief surface area of the “recess-protrusion” type [36]. A direct evidence of the role of the nanostructured surface of Al containing oxide films of various thicknesses in the process of n-hexane decomposition in the heterogeneous system of radiation-oxidized Al/ads.n-hexane is the AFM data. Figure 9 shows the AFM images of surfaces of radiation-oxidized aluminum containing oxide films of different thicknesses (a, b). Comparison of the images shows that the surface of Al containing thin oxide films (d = 8 nm) is characterized by its high defectiveness (Figure 9a).
It is known that at room temperature, the interaction of oxygen with aluminum produces clusters with a small number of atoms [38, 39] and there are more than 10 complexes on the surface of Al, differing not only in localization sites but also in structure and electronic structure. As a rule, such complexes coexist even at sufficiently high degrees of filling of the surface and interact with each other. Complexes with the greatest reactivity play the role of active centers in heterogeneous catalysis [35]. The presence of such complexes on the surface of Al in the initial stages of its oxidation is confirmed by the registration of multiquantum vibrational transitions of oxygen clusters using scanning tunneling microscopy and topographic images [35], as well as the results of RTL studies [15, 17]. Depending on the size and shape of the oxygen nanoclusters, the specific surface energy of aluminum also changes [38]. The presence of a hydroxyl cover also contributes to an increase in the adsorption of n-hexane on the oxidized aluminum surface, since OH groups are active adsorption centers [39]. Further oxidation of aluminum leads to the formation of oxide islands on its surface and the formation of a continuous intrinsic oxide layer (d ~ 80–600 nm) as a result of introduction of oxygen atoms and migration of oxygen hole centers to the volume either from a defective surface or from internal traps along grain boundaries (Figure 9b). Depending on the degree of oxidation of the aluminum surface, its adsorption capacity with respect to n-hexane will vary.
The surface of Al containing thin oxide films in comparison with the surface with continuous oxide layers is characterized by a greater adsorption capacity. These processes are most effective if the thicknesses of oxide films are commensurable with the values of the mean free path of charge carriers (electrons and holes) in the metal and oxide, and the total energy transfer absorbed by the adsorbent (AI–AI2O3) to surface adsorbed n-hexane molecules is determined, which causes its decomposition by the recombination mechanism [27, 29].
This chapter presents the results of IR spectroscopic studies (IR reflection-absorption spectroscopy) of radiation-stimulated heterogeneous processes of adsorption, radiation-chemical decomposition (radiolysis) of hydrocarbons on the metal surface, and the radiation hydrogenation of these surfaces under the action of gamma radiation. The role of intermediate surface-active decomposition products in the process of heterogeneous radiolysis of hydrocarbons is discussed, as well as the influence of the surface relief of the metal in the dynamics of the change in the decomposition process on the rate of formation and the yield of final products of radiolysis. The chapter deals with the spectrokinetic regularities, their features, and the mechanisms of radiation-stimulated adsorption and radiolysis of hydrocarbons in heterogeneous metal-hydrocarbon systems, in particular in heterogeneous Al (Be)-n-hexane systems first time. It is shown that n-hexane absorption in Al (Be) surface happens by the molecular and dissociative mechanisms. It has been found that the decomposition of excited n-hexane molecules upon their radiolysis on the surface of aluminum and beryllium metals is accompanied by the formation of active intermediate decomposition products that can interact with the surface-active states of metals and form their hydrides, alkyls, and π-complexes of olefins. The final decomposition products are hydrocarbons and molecular hydrogen (H2). It is shown that during the radiolysis of n-hexane on the surface of Al and Be metals, hydrogens are partially accumulated in the form of their hydrides.
I am grateful to Melikova S.Z., PhD, for the rendered help at registration of work.
Aortic valve stenosis (AS) is the most common valvular heart disease in developed countries. When symptomatic, AS is known to have significant morbidity and mortality. While the prevalence of AS is expected to rise with the aging population, there is no pharmacological treatment option to prevent its progression at this time [1, 2]. Aortic valve replacement (AVR) is the only treatment demonstrated to improve survival and symptoms [3, 4]. Therefore, in the management of patients with AS, it is essential to accurately diagnose the disease severity and determine the proper timing of surgical referral. According to the ACC/AHA guidelines, AVR is class I indication for patients with symptomatic severe AS with high transaortic mean gradient (MG) ≥ 40 mmHg and left ventricular (LV) ejection fraction (LVEF) < 50% and/or who are undergoing another surgery [5]. Over the past decade, challenges due to discrepancies with grading AS severity and the necessity of integrating the valve gradient with flow patterns were recognized when a significant subset of patients were found to have small AVAs suggestive of severe AS with lower gradients despite preserved LVEF [6]. As a result, under the umbrella of severe AS (based on AVA < 1.0 cm2), a new hemodynamic classification of AS was proposed which can be categorized into six subgroups based on LV flow state [normal flow (NF) vs. low flow (LF)] and pressure gradient [very high gradient (VHG) vs. high gradient (HG) vs. low gradient (LG)]. These six flow-gradient patterns (NF/VHG, NF/HG, LF/HG, LF/LG with reduced LVEF, LF/LG with preserved LVEF, and normal NF/LG) have shown to represent distinct pathophysiologic types of severe AS with different clinical outcomes (see Table 1).
\nHemodynamic classification of severe aortic stenosis (AVA < 1.0 cm2).
AS is a progressive valvular heart disease with gradual valvular narrowing resulting in LV outflow tract (LVOT) obstruction over time. Degenerative calcific AS is the most common type of this disease process and predominantly affects the elderly. With this condition, there is a long latent period during which the patient is asymptomatic although there is progression of obstructive physiology at the aortic valve and LV pressure overload. Survival in asymptomatic patients undergoing conservative management with watchful waiting is not statistically different from age- and gender-matched controls [7]. However, once symptoms of angina, syncope, or heart failure develop, there is a very rapid decline. Patients with AS who develop angina have a 5-year survival, syncope 3-year survival, and heart failure, the most ominous of all, 2-year survival (see Figure 1) [8, 9]. Thus, when symptoms are corroborated by established echocardiographic criteria for severe AS, some form of intervention is required because these individuals only have a 3-year survival of about 25%. In severe asymptomatic AS, the rate of symptom onset is higher when significant calcification of the aortic valve is present and in older patients [7]. Other factors demonstrated to predict symptom onset and surgical outcome include brain natriuretic peptide (BNP) [10]. While the risk of sudden death is a major concern in patients with asymptomatic AS undergoing conservative management, numerous studies have shown that the risk is very low, <1% per year [7, 11, 12].
\nNatural history of aortic stenosis. A long, latent, asymptomatic period is present followed by a very rapid decline in survival with the onset of symptoms of angina, syncope, and/or heart failure in severe AS patients [8].
Over the years, there has been marked decrease in the operative risk of AS. Furthermore, while prior studies have shown rather benign prognosis of asymptomatic severe AS patients, suggesting that delay in surgery can be safe until the development of symptoms, there is controversy as to the optimal timing of AVR and whether elective or early intervention during the asymptomatic stage might be better long term. At present, the surgical mortality for AVR is <2% for severe AS in patients with New York Heart Association (NYHA) functional class I or II heart failure, whereas this risk is significantly higher with class III or IV [13]. Thus, even though the patient may be asymptomatic, AS severity can progress and cause LV dysfunction during the conservative management period and significantly increase the surgical risk [14]. Furthermore, there is concern regarding the development of significant LV myocardial hypertrophy and irreversible myocardial fibrosis due to pressure overload which may result in persistent postoperative diastolic dysfunction and heart failure, even if AVR is successful [15, 16]. However, a general recommendation cannot be made at this time due to insufficient evidence to justify the benefit of AVR in asymptomatic patients to outweigh the risks of surgery and complications related to prosthesis long-term. However, those patients who may benefit from early surgical intervention should be identified through risk stratification [17]. Over the past decade, transcatheter aortic valve replacement (TAVR) has emerged as an alternative treatment strategy for symptomatic severe AS patients who are not suitable or prohibitive for surgical AVR (SAVR) [18, 19] or at high risk for surgery [20, 21]. This technology then expanded to benefit patients with intermediate operative risk, where TAVR using a self-expanding prosthesis was noninferior to SAVR at 24 months follow-up [22]. More recently, TAVR using a balloon-expandable SAPIEN 3 system in low-risk patients was shown to be superior to SAVR based on a composite of death, stroke, and rehospitalization at 1-year follow-up, despite excellent surgical results [23]. Long-term follow-up studies are underway to help determine the true therapeutic impact of TAVR vs. SAVR.
\nAS severity quantitation is based on the degree of LVOT obstruction caused by progressive narrowing of the aortic valve orifice. Echocardiography with Doppler evaluation is the main modality for diagnosing AS. Traditionally, hemodynamic severity of AS has been described based on peak aortic jet velocity (Vmax), MG, and AVA. According to the 2014 ACC/AHA guidelines, severe AS is defined as Vmax ≥ 4.0 m/s, MG ≥ 40 mmHg, and AVA < 1.0 cm2 [24]. The rate of hemodynamic progression in AS is highly variable. The average rate of progression was reported as increase in Vmax by 0.3 m/s/year and MG by 7 mmHg/year and decrease in AVA by 0.1 cm2/year [11]. Studies have shown that the strongest predictors of outcomes in AS were severity of the aortic valve obstruction. During a follow-up period of 2 years, progression of symptoms requiring AVR was about 80% for patients with Vmax > 4.0 m/s vs. 35% with Vmax of 3.0–4.0 m/s and 15% for patients with Vmax < 3.0 m/s. MG and AVA, other parameters of stenosis severity, were also strong predictors of patient outcomes [25].
\nEchocardiography is the current standard modality for evaluating AS severity. However, challenges due to inconsistencies between measurements of the MG and the calculated AVA in patients with normal systolic function were noted (see Figure 2). This finding was attributed primarily to differences in stroke volume and flow across the aortic valve. While it seems possible that discrepancies can occur when the cardiac output is low from reduced LVEF, inconsistent measurements in patients with preserved LVEF were observed. Another potential explanation for the discrepancies was that effective valve area derived by Doppler echocardiography is often smaller than the anatomic valve area measured during cardiac catheterization or by planimetry or at autopsy. So while the initial guidelines for determining AS severity were based on invasive measurements (reflecting the anatomic valve area), echocardiographic Doppler measurements are currently used to make clinical decisions for AS patients still based on the original anatomic valve area criteria. Thus, based on AVA, it is possible that more patients may be categorized as having severe AS relative to the peak flow velocity and MG. Therefore, some authors have suggested that AVA cutoff value for severe AS be changed to 0.8 cm2 [26].
\nComparison of AVA vs. MG in AS patients with preserved LVEF. The predicted values from the Gorlin equation and the fitted curve of the study cohort are shown. The quadrants depict severe AS cutoff points based on the guidelines, and the percentages represent patients per quadrant. Thirty percent of the severe AS patients were diagnosed based on AVA, but not by MG [26].
There are other potential etiologies of discrepant AVA and MG measurements in the setting of preserved LVEF which also need to be taken into consideration. First, technical errors need to be excluded. For example, LVOT diameter measurement may be inaccurate, and/or LVOT velocity time integral may be underestimated due to misplacement of the pulsed wave Doppler sample in the LVOT, leading to the underestimation of the stroke volume and the AVA. Second, patients with small body habitus and small LV dimensions could have lower stroke volume and lower transaortic gradient. Therefore, additional diagnostic studies such as dobutamine stress echocardiography (DSE), calcium scoring using multi-detector computed tomography (MDCT), and/or BNP may be necessary to corroborate AS severity and guide management strategy.
\nIn patients with AVA < 1 cm2, there are six flow-gradient patterns: NF/VHG, NF/HG, LF/HG, LF/LG with reduced LVEF, LF/LG with preserved LVEF, and NF/LG. VHG is defined as MG ≥ 60 mmHg, and HG is defined as MG ≥ 40 mmHg; stroke volume index (SVI) of normal flow is ≥35 ml/m2. Low flow is defined as SVI < 35 ml/m2. Low gradient is defined as MG < 40 mmHg. LF/LG AS with reduced LVEF is present when the gradient is low, the flow is low, and the LVEF is abnormal (<50%). LF/LG AS with preserved LVEF is present when the gradient is low and the flow is low but the LVEF is normal (>50%) (see Table 1).
\nSevere VHG AS (Vmax ≥ 5.0 m/s) has significantly worse prognosis than severe HG AS (Vmax ≥ 4.0 m/s) [3], so we acknowledge VHG AS as a separate entity from HG AS. However, most studies assessing AS severity using the new classification system combined NF/VHG and NF/HG as one entity under the subgroup of NF/HG. Thus, we will characterize these two groups together and highlight some of the relevant findings for VHG AS.
\nNF/VHG AS pattern is defined as AVA < 1.0 cm2, MG ≥ 60 mmHg, Vmax ≥ 5.0 m/s, and LVEF ≥ 50% with SVI ≥ 35 ml/m2. NF/HG AS is defined as MG ≥ 40 mmHg and Vmax ≥ 4 m/s with the same criteria for AVA, LVEF, and SVI as NF/VHG. Patients with these two flow-gradient patterns are the most prevalent (up to 70%) of all the AS groups. These patients tend to have more severe valvular stenosis suggesting more prolonged exposure to the progressive disease process. Compared with the NF/LG group, there is preservation of LV longitudinal function. However, these patients have higher BNP level and lower cardiac-event free survival [27].
\nWhen evaluating AS severity, Vmax is an important parameter which closely correlates with outcome. One study assessing the outcome of asymptomatic patients with very severe AS found that the higher the velocity, the lower the event-free survival with most patients experiencing some event within 3 years (see Figure 3). Patients with Vmax ≥ 5 m/s were symptomatic at presentation. Furthermore, asymptomatic patients with Vmax ≥ 5.5 m/s were highly likely to develop rapid onset of symptoms [3]. A landmark study evaluating the rate of hemodynamic progression and predictors of outcome in asymptomatic AS patients demonstrated that when Vmax exceeds 4 m/s, virtually all patients become symptomatic in 5 years. The velocity traditionally reflects the chronicity of the degenerative process. Vmax between 3 and 4 m/s were also found to be not benign, and only 20% of patients remained asymptomatic over 5 years. Only when Vmax was <3 m/s, there was an 85% chance that the patient will remain asymptomatic for 5 years [11] (see Figure 4).
\nEvent-free survival with very severe AS. Kaplan-Meier estimates demonstrate that maximum aortic jet velocity closely correlates with outcome, with higher the velocity, the lower the event-free survival [3].
Effect of Vmax on outcomes in asymptomatic AS. Cox regression analysis demonstrating event-free survival in asymptomatic AS patients categorized by initial peak aortic jet velocity [11].
MG is another well-recognized parameter for defining AS severity. One study assessed the prognostic impact of MG on all-cause mortality in severe AS with preserved LVEF. They found that MG > 60 mmHg at baseline was associated with greater risk of all-cause mortality than lower values, thereby justifying a separate hemodynamic classification. The higher MG also reflected the chronicity of the disease process [28] (see Figure 5).
\nImpact of MG on outcomes in severe AS. Kaplan-Meier estimates of survival based on MG [28].
AVA < 1.0 cm2 also correlated with poor outcome compared to moderate or mild categories. More severe AVAs carried worse prognosis, and like Vmax and MG, they reflected disease chronicity. While the rate of progression is highly variable, the often quoted number is 0.1 cm2/year [29] (see Figure 6). However, when Vmax was high or very high (4–6 m/s), there was no significant difference in the outcome based on the calculated AVA [3].
\n(A) Adjusted event-free survival based on AVA. (B) Cumulative hazard of death based on AVA [29].
According to the current ACC/AHA guidelines, symptomatic NF/HG and NF/VHG severe AS patients have a class I indication for AVR. When asymptomatic, these AS subgroups are recommended to undergo further risk stratification.
\nThis pattern of AS is defined as AVA < 1.0 cm2, MG ≥ 40 mmHg, and LVEF ≥ 50% with SVI < 35 ml/m2. The prevalence of this AS subtype is much less (8%) [30]. These patients have LV remodeling with reduced longitudinal function despite preserved LVEF. As a consequence, LV output is reduced with resultant lower than expected MG. LF/HG AS patients have shown to have high BNP, and their prognosis is similar or worse than those with NL/HG AS. When symptomatic, these patients have better survival with AVR [27, 31].
\nThree types of low-gradient severe AS have been described based on the LVEF and the flow state. LF/LG AS with reduced LVEF (<50%) is present when there is LV systolic dysfunction with reduced stroke volume in the setting of severe AS which results in decreased transvalvular velocity/gradient. If the LVEF is normal (≥50%), the stroke volume index (SVI) helps determine the presence of LF/LG AS with preserved LVEF (if the SVI is low, <35 ml/m2) or NF/LG AS (if the SVI is normal, ≥35% ml/m2) [32] (see Table 2).
\nSubclassification of low gradient AS.
This AS subtype, also known as “classical” LF/LG AS, is defined as AVA < 1.0 cm2, MG < 40 mmHg, SVI < 35 ml/m2, and LVEF < 50%. LF/LG AS with reduced LVEF accounts for about 5–10% of the AS population [33, 34] and has the worst outcome among all the AS categories [30, 33, 34]. The low flow state is usually associated with LV systolic dysfunction either from pressure overload due to the underlying severe AS or cardiomyopathy of another etiology.
\nIn order to differentiate true-severe AS from pseudo-severe AS, low-dose DSE is the initial recommended study to determine whether there is normal flow reserve (an increase in stroke volume of >20%) or diminished flow reserve (see Figure 7). Patients with normal flow reserve may have true-severe AS (MG ≥ 40 mmHg with AVA < 1.0 cm2 at any stage of DSE) which requires AVR or pseudo-severe AS (MG < 40 mmHg with AVA > 1.0 cm2) where medical therapy is recommended [32, 35]. In patients where the increase in stroke volume with DSE is <20% but >15% and MG is <40 mmHg, the definitive diagnosis of AS severity may remain questionable. In this case, the projected AVA calculation using normal flow rate may be beneficial where a value <1.0 cm2 is suggestive of true-severe AS [36] (see (Eq. (1)). However, if the stroke volume increase is <15%, further evaluation beyond DSE is often required, and calcium quantification of the aortic valve using MDCT is helpful in confirming the AS severity. The cutoff values for true-severe AS is >1200 AU in women and >2000 AU in men [37, 38].
\nAlgorithm for diagnosing LF/LG AS with reduced LVEF. AS, aortic stenosis; AVA, aortic valve area; MG, mean gradient; LVEF, left ventricular ejection fraction; SV, stroke volume; AV, aortic valve; MDCT, multi-detector computed tomography.
Valvuloarterial impedance (Zva) is an index to evaluate global LV hemodynamic load using Doppler echocardiography (see Eq. (2)). Zva > 5 has been shown to predict adverse outcomes in patients with AS and LV dysfunction. Since AS is a disease of the elderly, in addition to valvular stenosis, vascular stiffness due to various factors including age and hypertension may be present. As a result, the LV may be subject to a double afterload, known as global LV afterload or Zva. In general, higher Zva is associated with worse outcome. However, since Zva is a flow-dependent parameter, this index may be less reliable in low flow states since small changes in stroke volume can produce large changes in Zva [39].
\nTwo-dimensional and three-dimensional transesophageal echocardiography may also be beneficial for confirming AS severity via direct visualization of the aortic valve anatomy and physiology.
\nIn general, LF/LG AS has the worst prognosis compared to the other categories in part because the severity of AS is often under-recognized and surgical treatment is delayed. Patients with LF/LG AS with reduced LVEF have higher adverse event rates and mortality than LF/LG AS with preserved LVEF. The operative risk is also high in this AS subgroup. However, AVR has shown to have significant survival benefit compared to patients undergoing conservative management [40]. Furthermore, TAVR in LF/LG AS with reduced LVEF has demonstrated to have significant survival benefit compared with standard medical therapy in patients who are not suitable for surgery and similar outcomes compared with SAVR for patients at high surgical risk [41]. According to the ACC/AHA guidelines, true-severe LF/LG AS with reduced LVEF has a class IIa indication for AVR [42] (see Table 3).
\nRecommendations for aortic valve replacement in LF/LG AS.
LF/LG AS with preserved LVEF, also described as “paradoxical” LF/LG AS, is defined as AVA < 1.0 cm2, AVA indexed < 0.6 cm2/m2, MG < 40 mmHg, SVI < 35 ml/m2, and LVEF ≥ 50%. This AS pattern has generated much controversy among investigators. Studies have reported that low flow state is present in about 30% of AS patients with normal LVEF [31, 43, 44, 45, 46]. This AS subgroup accounts for about 15–35% of the symptomatic and 5–10% of the asymptomatic AS patients [30]. The classic characteristics described with this AS subtype are small LV cavity size with marked concentric hypertrophy, myocardial fibrosis, restrictive diastolic physiology, reduced LV longitudinal systolic function, and increased global LV afterload resulting in reduced SVI and worse outcome [6, 31, 47]. Other factors associated with this pattern include women, older age, systemic and/or pulmonary hypertension, atrial fibrillation, mitral regurgitation, and right ventricular dysfunction [27, 46]. Some studies have shown that these patients have one of the worst prognoses as the disease severity is often under-recognized and surgery is delayed. This pattern has shown to have better outcomes than LF/LG AS with reduced LVEF but worse outcomes than moderate AS, HG AS, and NF/LG AS [31, 41, 48]. The likelihood of remaining alive in 3 years without AVR has been reported about five fold lower than normal flow state [43].
\nWhen evaluating patients with this AS entity, it is essential to first exclude potential technical errors which may affect the gradient, stroke volume, and AVA measurements. Next, an integrated approach assessing the different criteria to support severe AS needs to be evaluated. These parameters include clinical characteristics such as physical examination suggestive of severe AS, patient symptoms, and the presence of hypertension. Potential etiologies of low flow state need to be considered. Qualitative imaging analyses such as the presence of left ventricular hypertrophy and LV strain measurements should also be assessed. Once LF/LG AS with preserved LVEF status is confirmed, quantitation of aortic valve calcification using MDCT may be helpful in differentiating true-severe vs. pseudo-severe AS [35, 49] (see Figure 8). One small study showed that low-dose DSE may be useful in confirming the diagnosis with this entity [50].
\nAlgorithm for diagnosing LF/LG AS with preserved LVEF. LF, low flow; LG, low gradient; AS, aortic stenosis; AVA, aortic valve area; MG, mean gradient; LVEF, left ventricular ejection fraction; SVI, stroke volume index; AV, aortic valve; MDCT, multi-detector computed tomography.
According to the ACC/AHA guidelines, LF/LG AS with preserved LVEF has a class IIa indication for AVR, if clinical, anatomic, and hemodynamic data support that the patient’s symptom is from the obstructive pathophysiology of the aortic valve [42] (see Table 3). One randomized trial data showed significant survival benefit after TAVR compared to standard medical treatment or similar clinical outcomes vs. SAVR [41]. In patients with greater degree of LV myocardial fibrosis, more advanced stage of diastolic dysfunction and low SVI demonstrated worse outcomes after TAVR [51, 52].
\nIn contrast to the findings described above, some other investigators have shown differing results for this AS entity. In one prospective study with a large number of patients with asymptomatic AS, there was no difference between the moderate stenosis and the low-gradient “severe” AS groups in terms of valve-associated events, major cardiovascular events, or cardiac death, even when the groups were subcategorized into low flow and normal flow states [53]. Another large study demonstrated that patients with LF/LG AS with preserved LVEF had better spontaneous survival than the patients with HG severe AS, and the results are unaffected by flow states. Furthermore, the patients with LF/LG AS with preserved LVEF progressed to develop HG AS over time, and in all patients who showed a reduction in transvalvular gradients over time, this decrease was associated with reduction in LVEF [54]. Another study showed that patients with severe LF/LG AS with preserved LVEF had similar outcomes as patients with mild to moderate AS, and there was no significant benefit of AVR in this group [55]. However, a comparison of two studies by Hachicha et al. [31] and Jander et al. [53] showed that there were some differences between the study group findings which may, at least in part, have contributed to the differing outcomes. Some investigators have proposed for reducing the AVA cutoff value for severe AS closer to ≤0.8 cm2 to avoid overestimation of AS severity [56].
\nThis AS pattern is defined as AVA < 1.0 cm2, AVA indexed < 0.6 cm2, MG < 40 mmHg, and LVEF ≥ 50% with SVI ≥ 35 ml/m2. NF/LG AS has shown to be present in about one third of AS patients [30], and some studies have suggested that this AS pattern may be due to marked reduction in transaortic gradient from systemic hypertension and decreased aortic compliance [57, 58]. Patients with NL/LG AS are reported to have less severe disease than the other AS categories with lower BNP and preserved LV longitudinal function [35]. In terms of diagnosis, technical measurement errors need to be excluded, and aortic valve calcium scoring using MDCT may be beneficial to further determine the AS severity [38]. According to the 2017 European Association of Cardiovascular Imaging and the American Society of Echocardiography Recommendations, however, this entity is considered to be due to measurement errors or the consequence of inconsistent cutoff values for transaortic velocity/gradient and AVA [35]. Some studies have supported this thought as patients in the NF/LG AS subgroup demonstrated the same outcome as patients with moderate AS [59].
\nThere are no particular recommendations for this subgroup in the current guidelines, and AVR should only be considered in symptomatic patients with confirmed severe AS. One study showed survival benefit in these patients [43], while another study showed no difference in survival in patients who underwent early AVR compared to conservative management [60].
\nProjected AVA calculation
\nProjected AVA at a normal flow rate (250 ml/s) <1.0 cm2 suggests severe AS.
\nAVArest, aortic valve area at rest; DSE, dobutamine stress echocardiography; AVApeak, aortic valve area at peak; Qrest, stroke volume at rest; Qpeak, stroke volume at peak DSE.
\nValvuloarterial impedance calculation
\nThe different hemodynamic categories of severe AS have shown to have varying clinical outcomes. Low flow state has exhibited the worst prognosis due to intrinsic myocardial dysfunction and/or under-recognition of the disease severity resulting in inappropriate delay in AVR. Low-gradient AS with low flow state is of particular challenge for clinical decision-making, especially when differentiating true-severe AS (where AVR may be beneficial) vs. pseudo-severe AS (where conservative medical management is appropriate). In LF/LG AS with reduced LVEF, DSE is beneficial for the confirmation of AS severity and risk stratification. In the setting of partial or no flow reserve, projected AVA and/or calcium scoring with MDCT may be useful to guide management. LF/LG AS with preserved LVEF is an entity where the natural history and the pathophysiology are not well understood. There has been much controversy and differing schools of thought around this AS subgroup. Numerous studies have shown that LF/LG AS with preserved LVEF is associated with poor prognosis, and therefore, careful evaluation and identification of these patients are necessary to ensure proper management. Calcium quantification using MDCT has shown to be the preferred technique for confirming AS severity with this subgroup. However, other investigators have reported that this AS entity represents moderate AS with no significant difference in outcomes between the groups. These discrepant findings may be resolved based on more randomized studies with large cohorts and with the application of more advanced diagnostic imaging techniques capable of overcoming the limitations of the currently available technology to better assess AS severity. In symptomatic high-gradient severe AS, regardless of the flow state, AVR is the only treatment option that has demonstrated to improve symptoms and survival. In asymptomatic high-gradient severe AS, regardless of the flow state, the current guidelines recommend watchful waiting and conservative management, although controversy exists about the optimal timing of intervention.
\nOver the years, the operative risk for SAVR for severe AS has significantly decreased, and TAVR has emerged as a promising alternative treatment for these patients with different operative risk profiles—high, intermediate, and more recently low risk. Recent data have supported that TAVR is superior or noninferior to SAVR in the treatment of severe AS and long-term follow-up assessment will better validate the true comparison between the two approaches and determine the optimal treatment strategy. As the TAVR technology continues to advance, the next generations of bioprostheses will be introduced which may further improve outcomes. Therefore, it is vital to accurately diagnose AS severity and identify those individuals who may benefit from AVR in a timely manner to optimize patient care and clinical outcomes.
\nNone.
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