Alloying Effect in Low Loaded Rh Catalysts Supported on High Surface Area Alumina on Their Activity in CH4 and NO Decomposition

In the alloys of transition metals (TM) and main group ones (MGM) the valence electrons from MGM can fill the d band of the TMs entirely and slightly increase the number of electrons in their s band (Azaroff, 1960; Kittel, 2005). Such Rh atoms enriched in electrons, present in Al-Rh alloys, were found to be very active electron donors (Pietraszek et al., 2007). It was revealed that they transfer the electrons to the antibonding Л orbital of adsorbed nitric oxide molecules causing NO decomposition to dinitrogen and dioxygen (Pietraszek et al., 2007).


The activity of Rh sites in Al-Rh alloys nanocrystallites in the reaction based on the electron transfer to the adsorbed molecules
In the alloys of transition metals (TM) and main group ones (MGM) the valence electrons from MGM can fill the d band of the TMs entirely and slightly increase the number of electrons in their s band (Azaroff, 1960;Kittel, 2005). Such Rh atoms enriched in electrons, present in Al-Rh alloys, were found to be very active electron donors (Pietraszek et al., 2007). It was revealed that they transfer the electrons to the antibonding Л orbital of adsorbed nitric oxide molecules causing NO decomposition to dinitrogen and dioxygen (Pietraszek et al., 2007).
Due to the great difference in the Al and Rh electro-negativity, the alloys are sometimes considered as nonstoichiometric chemical compounds. Because of the large range of nonstoichiometry those compounds should rather be estimated as intermediate phases (IP) (Azaroff, 1960).
It is well known that in bimetallic alloy crystallites (Gasser, 1985) the metal with the lower melting point segregates in their surface layers. The melting point of aluminum is much lower than that of rhodium (Lide, 2004(Lide, -2005. The enrichment in aluminum of the surface of Since a C-H bond can be broken on Rh sites at relatively low temperatures (Wang et al., 1996), the endothermic methane dissociation to C atoms and dihydrogen, followed by carbon oxidation, is most frequently considered as a plausible mechanism of the partial methane oxidation to the synthesis gas over the Rh/Al 2 O 3 catalysts (Enger et al., 2008;Hofstad et al., 1998;Mallens et al., 1997;Nakagawa et al., 1999;Wang et al., 1996). At PMO temperatures hydrogen practically does not adsorb on the Rh surface and thus the possibility of its interaction with surface oxygen or OH groups is rather low ( Rieck & Bell, 1985).
The high price of rhodium limits the use of the high loaded Rh/Al 2 O 3 catalysts. Therefore it is of interest to investigate the possibility of the use of the low-loaded ones in PMO. In such catalysts Al-Rh alloys can be formed, besides clusters of metallic Rh, as a result of the strong metal-support interaction (SIMS) (Pietraszek et al., 2007;Zimowska et al., 2006).

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Alloying Effect in Low Loaded Rh Catalysts Supported on High Surface Area Alumina on Their Activity in CH 4 and NO Decomposition

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As it was discussed elsewhere (Zimowska et al., 2006), calcination of the precursors of the low loaded Rh/δAl 2 O 3 catalysts (with the specific surface area ca 287m 2 /g) causes Rh incorporation into the near-to-surface layers of the alumina nanocrystallites resulting in a Rh x+ /Al 2 O 3 solid solution (Rh x+ /Al 2 O 3 s.s.) formation. The reduction of such a s.s. leads to Al-Rh alloys.

The NO direct decomposition in the oxygen presence over alumina supported precious metal catalysts
Nitrogen oxides NO x in the flue gases from fuel combustion contain more then 90% of NO. It is formed from air in the endothermic N 2 oxidation at temperatures above 1273K. NO is a thermodynamically unstable molecule with a high, positive formation enthalpy ∆Hº f (298) = 90,2 kJ/mol. Therefore it could decompose to N 2 and O 2 at low temperatures (between 293 and 973K ) (Chuang & Tan, 1997;Garin, 2001;Tonetto et al., 2003).
The NO decomposition seems to be the best way of NO removal from the flue gases of stationary sources of emission from a practical, environmental as well as an economical point of view. This process does not require any reducer that allows the avoidance of secondary pollutants like oxygenated hydrocarbons, CO, CO 2 , NH 3 or even cyanate and isocyanate (Chuang & Tan, 1997;Parvulescu et al., 1998;Tonetto et al., 2003). However, in the case of the direct NO decomposition N 2 O can be formed aside from N 2 in one of the two possible paths of the process (Parvulescu et al., 1998): Oxide supported Pt, Rh and Pd high-loaded catalysts are known to be the most active in the direct NO decomposition among the metal catalysts supported on metal oxides (Almusaiteer et al., 2000;Garin, 2001;Gorte et al., 1981;Ishii et al., 2002;Papp & Sabde, 2005;Pietraszek et al., 2007;Rahkamaa & Salmi, 1999;Root et al., 1983;Sugisawa et al., 2001;X. Wang et al., 2004). However, the NO dissociation proceeds on Rh active species in rhodium clusters with a satisfactory rate only above 623K and it is greatly suppressed by the oxygen presence. The reactive atomic species at such temperatures favor NO oxidation. On the other hand, the direct NO decomposition on the Rh active sites in Al-Rh alloy nanocrystallites (Pietraszek et al., 2007) formed on the low loaded alumina supported catalysts may proceed with a relatively high rate at 473K, below the temperature of the oxygen dissociation. The investigation of the alloying effect on the NO direct decomposition in the presence of oxygen and methane on a 0.18 wt.% Rh/δAl 2 O 3 catalyst with rhodium present mostly in the Al-Rh nanocrystallites should give relevant information about the direct NO decomposition in off gasses formed during methane combustion.

Experimental
Two 0.18 wt.% Rh/δAl 2 O 3 catalysts were obtained by one-or three-step Rh 3+ deposition on δ-alumina support. It was expected that such a procedure would produce catalysts with Rh sites of different coordination.

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The structure of the Rh sites was determined by the FTIR spectroscopy of CO adsorbed and by XPS.
The one-step (1-S) and three-step (3-S) Rh depositions were performed from a RhCl 3 . 2H 2 O aqueous solution of the same concentration.
The details of the Rh deposition as well as those of oxidising and reducing thermal treatments were described elsewhere (Pietraszek et al., 2007;Zimowska et al. 2006 In the third step of the 3-S synthesis the 0.12 wt.% Rh/δAl 2 O 3 catalyst is impregnated by the same RhCl 3 . 2H 2 O aqueous solution. The next drying, calcinations and reduction yields the 0.18 wt. % Rh/ δAl 2 O 3 catalyst.
The 1-S synthesis of the 0.18 wt.% Rh/δAl 2 O 3 catalyst consists of: i/ the impregnation of the δAl 2 O 3 support with a triple portion of the RhCl 3 . 2H 2 O aqueous solution, ii/ the drying of a wet precursor, iii/ the calcination of a dry precursor and the reduction of a calcined precursor.
The FTIR spectroscopy of CO adsorbed and XPS measurements were performed for the fresh one-step ( F-1-S) and three-step (F-3-S) catalysts as well as for the one-step and threestep catalysts subjected to the interaction with six methane pulses (U-1-S) and (U-3-S).
CO was adsorbed on the surface of the catalysts at room temperature and the spectra were taken also at room temperature after outgasing at 373K using a Nicolet 5700 Nexus spectrometer at a resolution of 4 cm -1 .
The XPS measurements for F-1-S, F-3-S, U-1-S, and U-3-S catalysts were carried out using a scanning photoelectron spectrometer PHI 5000 VersaProbe (Physical Electronics USA/ULVAC Japan) with monochromatic Al Kα radiation (1486.6eV). The signal was collected from 200 x200μm area using an x-ray beam focused to 100μm diameter with 25 W power. For the charge shift compensation all the measurements were made with neutralization by low energy electrons and argon ions. The survey spectra were obtained with a pass energy of 117.4eV in 0.4eV increments. The detailed spectra of the Rh3d, and the Al2p regions were measured with a pass energy of 23.5eV in 0.1eV increments. The binding energy scale (BE) was calibrated based on the Al2p in a Al 2 O 3 support signal (BE = 74.3eV). Signal components were fitted using a non-linear least squares fitting Casa XPS software. The quantitative analysis was done after a Shirley-type background subtraction using the Scofield sensitivity factors.

Alloying Effect in Low Loaded Rh Catalysts Supported on High Surface Area Alumina on Their Activity in CH 4 and NO Decomposition 357
The investigation of the interaction of the catalysts with methane pulses was carried out at 873K in an acid-resistant steel tubular reactor with a 6mm internal diameter and 30cm length, under the pressure of 280 kPa and at GHSV =9000h -1 . The 700 mm 3 methane pulses were introduced into the Ar stream flowing through the reactor, the Carboxen 1000 packed column and the TCD detector of a Hewlett-Packard 5890 chromatograph. The time between consecutive methane pulses was ca. 32min.
The direct NO decomposition in methane and the oxygen presence was investigated under the steady-state conditions in 473-673K temperature range. The reaction mixture of 150ppm NO 1500ppm CH 4 , 7%O 2 and Ar as a balance passed over the catalyst placed on quartz-wool in a quartz reactor with GHSV = 20 000h -1 . Before the steady-state experiment the catalyst was heated in the reaction mixture from the r.t. to 773K and consecutive measurements were performed stepwise starting from 673K to 473K. A FID detector was used to follow the total concentration of hydrocarbons and the N 2 formation was checked by a micro-GC.

The physicochemical characterisation of 0.18wt.%Rh/Al 2 O 3 catalysts by FTIR spectroscopy of CO adsorbed and XPS
Fig. 1. presents the FTIR spectra of CO adsorbed on the F-3-S 0.18wt.%Rh/Al 2 O 3 catalyst (a) and on the F-3-S 0.18 wt.% Rh/Al 2 O 3 one (b). In agreement with the data extensively reported in the literature (Finocchio et al., 2007 ;Hadjiivanov &Vayssilov, 2002;Kraus et al., 1989;Lavalley et al., 1990;Paul et al., 1999;Yates et al., 1979) two main bands at 2087 and 2013 cm -1 , present in the spectra of both the catalysts, can be assigned to symmetric and asymmetric stretching vibrations of gem-bicarbonyls adsorbed on the Rh atomically dispersed Fig. 1. DRIFT spectra CO adsorbed on the surface of the 0.18 wt.% Rh /δAl 2 O 3 catalysts obtained by: a/three-step rhodium deposition, b/ one-step rhodium deposition on the alumina surface. However, the exact position of the atomically dispersed Rh in the structure of catalyst has yet to be determined (Finocchio et al., 2007 ;Hadjiivanov &Vayssilov, 2002;Kraus et al., 1989;Lavalley et al., 1990;Paul et al., 1999;Yates et al., 1979). According to above cited papers the peak at 2058cm -1 , clearly seen in the spectrum of the F-1-S catalyst, results from C-O stretching vibrations in CO linearly adsorbed on the Rh clusters. Fig. 2. DRIFT spectra of CO adsorbed on the surface of the 0.18 wt.% Rh /δAl 2 O 3 catalysts: a/ obtained by a three-step rhodium deposition and subjected to interaction with six methane pulses, b/ obtained by a one-step rhodium deposition and subjected to interaction with six methane pulses.
Thus one may conclude that on the surface of the F-1-S catalysts the atomically dispersed Rh coexists with the Rh clusters. However, on the surface of the F-3-S catalyst atomically dispersed species are hardly seen. Fig. 2 presents the FTIR spectra of CO adsorbed on the three-step (a) and one-step (b) 0.18wt.% Rh/Al 2 O 3 catalysts subjected to interaction with 6 methane pulses at 873K.
The distinct decrease of the ratio of Rh in clusters to Rh in alloy nanocrystallites as a result of the catalysts interaction with methane pulses is observed. The increase of the width of the gem-bicarbonyl peaks reveals that during the pulse experiment atomically dispersed Rh with a coordination slightly different than in the fresh catalysts is formed.
The comparison of XPS results for the fresh and used catalysts gives further insight in the nature of atomically dispersed rhodium species and in the mechanism of their creation as well as their interaction with the clustered metallic rhodium species.
In Table 1 the BEs of peaks obtained by the deconvolution of the Al2p 3/2 and Rh3d 5/2 bands as well as the percentage of the particular Al and Rh species ((Me x /ΣRh x +ΣAl x ) . 100) in F-1-S, U-1-S, F-3-S and U-3-S catalysts are presented.  Table 1. The XPS results for the fresh one-step (F-1-S) and the three-step (F-3-S) 0.18 wt.% Rh/ δ Al 2 O 3 catalysts and for those catalysts subjected to the interaction with six methane pulses (U-1-S and U-3-S).
The Al2p 3/2 bands in the spectra of the particular catalysts were deconvoluted into two peaks. The major peaks were ascribed to Al in alumina -BE=74.3eV (Wagner et al., 2007).
The BE of the less intensive Al2p 3/2 peaks as well as those of the Rh 3d 5/2 peaks were determined with respect to this one. The less intensive Al2p 3/2 peaks with exceptionally high BE (75.4-75.5eV) were ascribed to Al in the Al-Rh alloy. The transfer of the valence Al electrons mainly to Rh unoccupied d-orbitals (Azaroff, 1960;Kittel, 2005) in the alloy should cause the increase of the Al2p 3/2 BE and decrease of Rh3d 5/2 one with respect to those in separate metals (Wagner, 2007).
The Rh3d 5/2 bands in the XPS spectra of the F-1-S and F-3-S catalysts were deconvoluted into the peaks of cationic species with BE equal to 310.0 and 310.2eV and of Al-Rh alloy ones with BE equal to 307.4 and 307.8 eV, a little lower than that of metallic Rh (Wagner, 2007).
The presence of only one 3d 5/2 peak of the Rh species with BE =307.4 eV in the spectrum of the F-1-S catalyst reveals an electronic interaction between the Al-Rh intermediate phase nanocrystallites and the Rh clusters. It shows that the Rh clusters occur in the immediate proximity of the Al-Rh nanocrystallites. Such an electronic interaction was earlier observed by Thiam et al. (Thiam et al., 2004) between aluminum foil and a rhodium overlayer.
A higher percentage of the metallic Rh species in the F-3-S catalyst (60 %) than in the F-1-S one (ca 40%) could easily be explained by the differences in the syntheses. The one-step Rh deposition is thought to have occurred preferably on the smallest nanocrystallites with the highest surface/bulk ratio, possibly by an Rh 3+ cation exchange with protons of the accessible acidic OH groups on alumina. Thus, the Rh deposition calculated per volume unit of alumina crystallites is higher the smaller their dimensions are. Rhodium clusters are formed from an excess of the aqueous Rh 3+ ions solution present in pores.
On the other hand, the three-step Rh deposition occurs mostly by exchange of the Rh cations with the acidic OH group protons on the surface of the smallest δ alumina crystallites, which is renovated in each synthesis step in the course of the calcinations. It results in the exceptional enrichment of these crystallites in Rh. An incorporation of rhodium into δ alumina during the thermal treatments causes the formation of the Rh x+ /Al 2 O 3 solid solution. The reduction of the s.s. nanocrystallites, resulting in inter-metallic Al-Rh alloy formation, is easier the higher the rhodium concentration is.
It is obvious that the enrichment of Rh atoms in electrons is the greater the higher their dispersion is in the aluminium matrix. Earlier this problem in particular was discussed regarding the Cu-Ni alloys (Kittel, 2005).
The interaction of the F-1-S catalyst with six CH 4 pulses causes a complete disappearance of the cationic Rh species and a great decrease in the content of the metallic Rh species.
Simultaneously the Rh species with 3d 5/2 BE equal to 305.0 eV and the Al species with an exceptionally high 2p 3/2 BE (75.4eV) do appear.
The disappearance of the cationic species reveals the formation of the new nanocystallites of the Al-Rh alloy by the reduction of the Rh x+ /Al 2 O 3 s.s. of low Rh content.
The major decrease of the content of Rh species with 3d 5/2 BE close to 307 eV and the appearance of the species with 3d 5/2 BE = 305 eV and the Al ones with 2p 3/2 BE = 75.4 eV could be the result of the Al surface segregation in the alloy nanocrystallites formed at 773K. However, Rh species with 3d 5/2 BE = 305 eV and the Al ones with 2p 3/2 BE = 75.4 eV could also be formed as a result of the reduction of the Rh x+ /Al 2 O 3 s.s. with a low Rh content.
The heating of the F-3-S catalyst and its interaction with 6 methane pulses causes a great decrease in the total contents of the Rh species in the surface nanolayers, the disappearance of the species with 3d 5/2 BE = 307.8 eV, the appearance of the ones with 3d 5/2 BE = 307.2 eV and an increase of the content Al species with 2p 3/2 BE = 75.4 eV.
The major decrease in the content of the cationic and metallic Rh species, the the decrease in the 3d 5/2 BE of the Rh metallic species as well as the increase of the content of the Al ones with 2p 3/2 BE = 75.4 eV reveal the reduction of the Rh/Al 2 O 3 s.s. areas with a small Rh content and Al segregation in the Al-Rh alloy nanocrystallites formed at 773K. The great extent of the surface Al segregation can be caused by great difference in Al and Rh melting points and the proximity of the used temperature and the temperature of Al melting (Lide, 2004(Lide, -2005
The CH 4 conversion in the first pulse over the 1-S catalyst is equal to 33%, the selectivity to H 2 -68%, the selectivity to CO -61% and to CO 2 -10%. In the following methane pulses C CH4 continuously decreases down to 23% in the sixth pulse. The S H2 increases up to 78 % in the sixth pulse, S CO decreases, almost linearly, down to 22% in the sixth pulse and the S CO2 drops to 0% in the fourth pulse.
The simultaneous decrease of the C CH4 , the S CO and the S CO2 and the increase of S H2 in the consecutive methane pulses can be explain by the decreasing concentration of the atomic species of oxygen adsorbed on the surface Rh species in Al-Rh alloy nanocrystallites and in Rh clusters present on the surface of the 1-S catalyst.
The initial S H2 increase accompanied by a decrease in S CO2 reveals that further formation of alloy nanocrystallites with a low concentration of Rh sites have a greater ability to transfer electrons to methane molecules. The 2.8 fold decrease in the S CO in the sixth methane pulse with respect to that in the first methane pulse with a simultaneous increase of the selectivity to hydrogen, suggesting deeper methane dissociation, clearly shows a carbon deposition on the catalyst surface as a result of the deficit of oxygen species adsorbed.
The C CH4 in the first methane pulse is equal to 42%, the S H2 -83%, the S CO -72% and the S CO2 -11%. In the next methane pulses the C CH4 decreases down to 22% in the sixth pulse. The S H2 increases up to 98% in the third pulse and slowly decreases to 94% in the sixth pulse. The S CO decreases almost linearly down to 22% in the sixth pulse and S CO2 drops to 0% in the fourth pulse.
The simultaneous decrease in the S CO2 and increase of the S H2 in the consecutive methane pulses up to the forth one could be ascribed to the formation of the new Al-Rh alloy nanocrystallites of a low content of Rh sites with a great ability to transfer the electrons to the methane molecules adsorbed.
There is a 50% decrease in the C CH4 and approximately a 70% decrease in the S CO in the sixth pulse in comparison with the first one, not accompanied by the decrease in the S H2 which reveals carbon deposition on the Al-Rh alloy nanocrystallites.
The formation of the new alloy nanocrystallites by the reduction of the Rh/Al 2 O 3 s.s. of the relatively low rhodium concentration causes the creation of a very active Rh species.
The stronger S CO decrease on the 3-S catalyst (3.3 fold) of a higher activity than on the 1-S one (2.8 fold) of lower activity reveals that carbon is preferably deposited on the Al-Rh nanocrystallites.
The higher conversion of the first methane pulse over the F-3-S catalyst than over the F-1-S reveals that the Rh active species in the Al-Rh alloy nanocrystallites are more active in the methane dissociation than those in the Rh clusters ( Figs.1 and 2). The decrease of the activity of both the catalysts in the consecutive pulses corresponds to the decrease of the total content of the Rh species accessible for methane adsorption.
The slower the decrease of the methane conversion over the 1-S catalyst than that over the 3-S one may be assigned to a slower carbon deposition on the Rh clusters than on the Al-Rh nanocrystallites.
On the other hand, the higher selectivity to hydrogen over the 3-S catalyst than over the 1-S one clearly shows a beneficial influence of the Al proximity on the Rh sites ability to transfer electrons to antibonding orbitals of methane. Fig. 5 presents CH 4 conversion and selectivity to H 2 , CO and CO 2 in 41 consecutive CH 4 pulses over the 3-S 0.18wt.% Rh/δAl 2 O 3 catalyst (873K, 280kPa, 9000 h -1 ). The methane pulses were introduced into the reactor mostly in time intervals close to 30min. Less regular time intervals (30-37min.) were applied between the 11 th and 17 th pulses as well as the 25 th and 33 rd . The C CH4 and S CO decrease and S H2 increases in consecutive pulses up to the pulse 11. Irregular intervals between the 11 -17 pulses and the 25 -33 ones cause fluctuations in the C CH4 , the S CO and the S H2 that can be ascribed to the changes in the amount of the carbon deposit oxidized by the oxygen from the gas phase. The 75 min. time interval between 34 th and 35 th pulses resulted in the C CH4 increase of 11% and of the S CO of 110%. The increase of the time between the 40 and 41 pulses to 110 min. results in the increase of the C CH4 of 25% and of the S CO of 230%. However, the selectivity to hydrogen does not change distinctly in any case. Thus, one can conclude that a long term interaction of the surface of the Al-Rh nanocrystallites, containing the carbon deposit, with the gas phase containing traces of oxygen causes both a carbon deposit removal by oxidation and oxygen adsorption. The oxidation of the carbon deposit causes the recovery of the Rh sites active in the methane dissociation that results in the increase of the C CH4 . The surface oxygen species cause oxidation of the carbon formed due to methane dissociation, that leads to the S CO increase. However, their population is not high enough to have an effect on the S H2 . Thus, to perform the PMO with the high methane conversion and the high selectivity to CO and H 2 the proper CH 4 :O 2 ratio is needed. Fig. 5. CH 4 conversion (•) and selectivity to H 2 (■), CO (♦) and CO 2 (▲) in 41 consecutive CH 4 pulses over the 3-S 0.18wt.% Rh/δAl 2 O 3 catalyst (873K, 280kPa, 9000 h -1 ).

Direct NO decomposition over the three-step (3-S) 0.18 wt.% Rh/δAl 2 O 3 catalyst
NO conversion (C NO ), selectivity to N 2 (S N2 ) calculated from the results of the steady-state experiments over the 3-S 0.18wt.%Rh/Al 2 O 3 catalysts, are presented in Figure 6.
The steady-state experiment was performed with the use of the reaction gas containing 150ppm NO, 1500ppm CH 4 , 7%O 2 and Ar as a balance. The highest NO conversion, increasing with a temperature increase, from 38 to 47%, is observed at a temperature range 473K -573K. The highest but decreasing from 100% to 85% selectivity to N 2 is achieved in the same temperature. A simultaneous sudden drop of the S N2 and a deep decrease in C NO are observed at 623K.
The direct NO decomposition over the high-loaded rhodium catalysts supported on alumina was earlier observed only at temperatures higher than 623K (Garin, 2001).
Thus, the low-temperature NO decomposition on the low loaded 0.18wt.%Rh/δAl 2 O 3 catalyst confirms exceptional activity of the Rh sites in Al-Rh alloy.
A simultaneous increase of the C NO and the decrease of the S N2 with the temperature increase in the 473K -573K range could be explained by the competitive dissociative O 2 adsorption on the Rh active sites, increasing with temperature. The atomic oxygen species block the Rh sites active in the direct NO decomposition and facilitate the NO oxidation.
The simultaneous sudden drop of the S N2 and a great decrease in the C NO at 623K shows that blocking the Rh sites-active in the NO dissociation becomes much stronger. Such blocking may originate in a methane dissociation on the same sites. In Fig. 7, the CH 4 conversion (C CH4 ) and selectivity to CO 2 (S CO2 ), measured during the thermo-programmed catalyst heating at the rate of 3K/min in the reaction mixture containing 150ppm NO, 1500ppm CH 4 , 7%O 2 and Ar as a balance, are presented.

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Alloying Effect in Low Loaded Rh Catalysts Supported on High Surface Area Alumina on Their Activity in CH 4 and NO Decomposition

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The temperature of the sudden drop of the S N2 and C NO (Fig. 6) corresponds to the initial temperature of the total methane oxidation to CO 2 (Fig. 7). This confirms competitive methane adsorption on the Rh active sites leading to the formation of the carbon species undergoing oxidation to CO 2 . . CH 4 conversion (•) and selectivity to CO 2 (▲) over three-step 0.18 wt.% Rh/δAl 2 O 3 catalyst (150ppmNO +1500ppm CH 4 + 7%O 2 /He , 20000h -1 , data from TPSR experiment)

Conclusions
The DRIFT spectroscopy of the adsorbed CO and XPS were used to show the formation of the Al-Rh alloy nanocrystallites on the surface of the 0.18 wt % Rh/δAl 2 O 3 catalysts obtained by the one-step or three-step impregnation of the high surface area alumina support with the RhCl 3 aqueous solution (1-S and 3-S catalysts).
The effect of Al-Rh alloying on the catalyst activity in the methane dissociation and the direct NO decomposition was investigated.
The activity and selectivity to dihydrogen and to carbon monooxide of both the catalysts were determined at 873K. The results showed unambiguously that alloying distinctly enhances the Rh site activity in the electron donation to antibonding orbitals of methane molecules adsorbed. The observed enhancement of the Rh site activity was ascribed to the Al valence electrons transfer to the Rh 4d shell and slightly to the Rh 5s shell. The direct NO decomposition in the presence of oxygen and methane was investigated in the temperature range of 673-473K over a 3-S catalyst. The NO conversion with 100% selectivity to dinitrogen on the Rh active sites was revealed at an unexpectedly low temperature (473K). Simultaneous increase of the activity and decrease in the selectivity to dinitrogen with a temperature increase up to 573K clearly revealed a competitive dissociative oxygen adsorption on the same active sites. However, the sudden decrease at 623K of the catalyst activity in the NO decomposition and the selectivity to dinitrogen, accompanied by an initial methane oxidation to CO 2 , undoubtedly showed competitive methane dissociation.