Chemical Quantification of Mo-S, W-Si and Ti-V by Energy Dispersive X-Ray Spectroscopy

Elemental chemical identification of a specimen and its quantification is fundamental to obtain information in the characterization of the materials (Angeles et al., 2000; CortesJacome et al., 2005). Energy dispersive X-ray spectroscopy (EDXS) is the technique that allows obtaining information concerning the elemental chemical composition using the EDX spectrometer. Generally is attached to a scanning electron microscope (SEM) (Goldstein & Newbury, 2003) and/or in a transmission electron microscope (Williams & Barry-Carter, 1996). The technique is very versatile because the spectrometer gives results in few minutes. The instrument is compact, stable, robust and easy to use and its results can be quickly interpreted. The analysis is based in the detection of the characteristic X-rays produced by the electron beam-specimen interaction. The information can be collected in very specific local points or on the whole sample. So, both electron microscopy and EDXS, give valuable information about the morphology and chemical composition of the sample. In order to give an accurate interpretation of the data collected by the instrument is important to know the fundaments of the technique. The characteristic X-rays are produced by the atoms of the sample in a process called inner-shell ionization (Jenkins & De Dries, 1967). This process is carried out when an electron of inner-shell is removed by an electron of the beam generating a vacancy in the shell. At this moment the atom remain ionized during 10-14 second and then an electron of outer-shell fills the vacancy of the inner-shell. During this transition a photon is emitted with a characteristic energy of the chemical element and its shell ionized. The emitted photons are named by the shell-ionized type as K, L, M lines.... and ┙, ┚, ┛... by the outer-shell corresponding to the electron that filled the inner-shell-ionized. For atoms with high atomic numbers, is important to note that some transitions are forbidden. Permissible transitions can be followed by the quantum selection rules and the notation can be followed by Manne Siegbahn and/or, IUPAC rules (Herglof & Birks, 1978). During the beam-sample interaction, another X-ray source is produced and it is known as Bremsstrahlung radiation or continuum X-rays which are generated for the deceleration of the electron beam in the Coulombic field of the specimen atoms. When the electrons are braked, they emit photons with any energy value giving rise to a continuous electromagnetic spectrum appearing in the EDX spectrum as

The instrument is compact, stable, robust and easy to use and its results can be quickly interpreted.The analysis is based in the detection of the characteristic X-rays produced by the electron beam-specimen interaction.The information can be collected in very specific local points or on the whole sample.So, both electron microscopy and EDXS, give valuable information about the morphology and chemical composition of the sample.In order to give an accurate interpretation of the data collected by the instrument is important to know the fundaments of the technique.The characteristic X-rays are produced by the atoms of the sample in a process called inner-shell ionization (Jenkins & De Dries, 1967).This process is carried out when an electron of inner-shell is removed by an electron of the beam generating a vacancy in the shell.At this moment the atom remain ionized during 10 -14 second and then an electron of outer-shell fills the vacancy of the inner-shell.During this transition a photon is emitted with a characteristic energy of the chemical element and its shell ionized.The emitted photons are named by the shell-ionized type as K, L, M lines.... and , , ... by the outer-shell corresponding to the electron that filled the inner-shell-ionized.For atoms with high atomic numbers, is important to note that some transitions are forbidden.Permissible transitions can be followed by the quantum selection rules and the notation can be followed by Manne Siegbahn and/or, IUPAC rules (Herglof & Birks, 1978).During the beam-sample interaction, another X-ray source is produced and it is known as Bremsstrahlung radiation or continuum X-rays which are generated for the deceleration of the electron beam in the Coulombic field of the specimen atoms.When the electrons are braked, they emit photons with any energy value giving rise to a continuous electromagnetic spectrum appearing in the EDX spectrum as obtain overall elemental chemical composition representative of the sample by EDXS.Specifically of elements with overlapping characteristic X-ray lines.In order to carry out this study the Mo-S, W-Si and Ti-V systems were choice because they are widely used in the catalysis field and their accurate chemical composition are required.Additionally, their more intense characteristic X-ray lines used to calculate the composition are overlapped.In order to know if the instrument is able to solve this problematic giving reliable composition results, samples of known Mo, S, W and V composition were prepared.

Experimental section
Samples of Al 2 O 3 , MoO 3 and S-MoO 3 , V 2 O 5 -TiO 2 and SiO 2 -WO 3 were chemically analyzed in an EDX spectroscope EDAX which is attached to the environmental scanning electron microscope PHILIPS XL30.The EDAX instrument has a detector type UTW-Sapphire, an energy resolution of 0.129 keV, tilt of 0.0 and take-off of 35.90.As the microscope is designed to operate at different conditions and to obtain useful information of the samples, a review of the variables of time constant, acceleration voltage and acquisition time in order to obtain more accurate overall quantification results was performed.In this study, the dead time, the magnification and the work distance were kept constant.The DT was adjusted with the spot size and it kept around of 30%.The deconvolutions of the experimental results were followed using the Phoenix software version 3.3 included with the instrument.The software uses the socalled segment Kramer´s fit method to model the background which include the collection efficiency of the detector, the processing efficiency of the detector and the absorption of X-ray within the specimen.This method is useful when the spectrum contains overlapping peaks.The chemical composition was calculated using the ZAF correction method.In order to establish the operation conditions of the instrument, samples of Al 2 O 3 and MoO 3 were used.The experimental results were compared with their theoretical compositions and a statistical study of the exactitude and precision of the instrument was determined (Ellison, et al., 1995).Subsequently, under these conditions samples containing S-MoO 3 , V 2 O 5 -TiO 2 and SiO 2 -WO 3 were analyzed and their average compositions were reported.Al 2 O 3 , MoO 3 , S, V 2 O 5 , TiO 2 , SiO 2 and WO 3 analytic grade were used to prepare the samples.Samples with 1, 5, 10 and 15 wt% of Mo were prepared for the MoO 3 -Al 2 O 3 system.Same S concentrations in the S-MoO 3 system were prepared.For the V 2 O 5 -TiO 2 system, the samples were prepared at 0.5, 1, 3, 5 wt% of V while the SiO 2 -WO 3 system the tungsten concentration in the samples were 20, 40, 50 and 60 wt% of W. All samples were mechanically mixed in an agate mortar.V 2 O 5 -TiO 2 and SiO 2 -WO 3 systems were mixed and annealed at 500 °C.In order to calculate the chemical composition of the elements, more intense characteristic energy lines were used.Five measurements in different region were performed in each sample.The concentration values reported of each component for each sample was the average value.A monolayer of sample was deposited and dispersed on carbon tape to have a flat surface.

Review of the operation conditions of the instrument
The best operation conditions in the instrument to calculate the more accurate elemental chemical composition were established using Al 2 O 3 and MoO 3 samples.These samples were selected because they are components of the hydrodesulfuration catalysts.Al 2 O 3 and MoO 3 are very stable under environmental conditions of temperature, humidity and pressure.Their chemical composition remains constant with the time. www.intechopen.com

X-Ray Spectroscopy 122
The effect of the Tc, AV and AT variables on the chemical composition was studied using MoO 3 .Figure 1 shows the typical EDX spectra of the MoO 3 displaying their characteristic energy lines located at 0.523 keV for OK ,2.015,2.293,2.394 keV,for MoLl,MoL 1 and MoL 1 and 17.376,17.481,19.609keV for MoK 2, MoK 1 and MoK 1.All lines were displayed in five Gaussian peaks.The more intense peaks correspond to the OK and MoL 1 lines.The software calculates the composition using the energy lines corresponding to the shell ionized, for instance the OK and MoL lines for MoO 3 .The first calculus was made with the EDX spectrum obtained to a Tc of 35.0s, 25kV AV, 100s of lecture, around 30% of DT, 11 mm of WD and 100X of magnification using the most intense peaks, OK and MoL lines.The results obtained were 32.30 wt% O and 67.70 wt% Mo, composition close to the theoretical composition (33.35 wt% O and 66.65 wt% Mo).This result differs of the results obtained with MoK line.The chemical composition obtained considering MoK line was 23.40 wt% O and 76.60 wt% Mo, composition very far of the theoretical composition.
Here is important to note the importance of consider the more intense characteristic energy lines to obtain more accurate composition results in samples containing an element with two line series as K and L or M and L. Knowing that with the MoL line the calculus of chemical composition gives a better approximation to the theoretical composition of MoO 3 , the study of the effect of Tc in the chemical composition can be performed.Tc variable is the time allowed for the pulse processor to evaluate the magnitude time of pulse.For longer Tc, the ability of the system to assign an energy to the incoming pulse is better, but a few counts can be processed in a given analysis time.Therefore, longer Tc will give better spectrum resolution but the count rate will be lower.Shortest Tc will allow to process more counts per second but with a large error in the assignment of a specific energy to the pulse, and so the energy resolution will be poor.This can be observed in the Gaussian curves of MoO 3 EDX spectra displayed in Figure 1.A careful analysis of Figure 1 indicates that the resolution between the MoL 1 and MoL 1 lines were better resolved at 100.0s than at 10.0s of Tc.In general, at shorter Tc larger intensity, wider broadening and minor resolution, while at higher Tc, minor intensity, minor broadening and better resolution is obtained.Nevertheless, the peak intensity and its broadening are important in the calculus of the composition and the resolution is also important to discern the characteristic energy of the chemical elements and as it can be seen in the Gaussian curves of Figure 1, the intensity, broadening and resolution changes for each Tc measured.Its effect in the chemical composition is displayed in table 1.A relative error was calculated between the theoretical and experimental composition data to analyze the results.The relative error given in the last column is lesser than 2.5% and it does not show some tendency.This means that the Tc has little effect in the chemical composition.The larger and smaller error was obtained at Tc=50.0s and Tc=100.0s,respectively.Therefore, a Tc optimum must be choice without affect both, intensity and resolution, too much.In the instrument used in this work the Tc value suggested by the provider was 35.0s that combine resolution and intensity giving a relative error of 1.6%.With a Tc=35.0s and 100s of acquisition time, the effect of accelerating voltage on molybdenum oxide composition was studied.The DT was kept around of 30% manipulating the spot size.Figure 2 displays the EDX spectra of the OK and MoL characteristic energy lines.The Figure 2 shows a decreasing in the peak intensity with the increase of the accelerating voltage, see inset in Figure 2.However, the peak width fits with the others Gaussian peaks.This overlapping of curves was more evident for MoL line than OK line.The decreasing of the intensity produced by increase of the accelerating voltage without affect the broadening in the Gaussian curves suggests that the accelerating voltage has strong effect in the determination of the composition.This was revealed in the calculus of the composition, see  The acquisition time is directly related with the intensity and it is expected to affect the calculus of the composition.Therefore, it is important to know how AT affects the MoO 3 composition and determine if there is an optimum acquisition time for more precise experimental results.Measurements carried out at Tc 35s and 25kV and different acquisition time are displayed in the EDX spectra of Figure 3.The Gaussian peaks of the OK and MoL lines show an increase in the intensity and broadening with an increase on the acquisition time.This behavior suggests that the acquisition time has little effect in the chemical composition as it was revealed in the calculus displayed in table 3. A relative error of 1.7% was obtained at 100 and 200s of lecture while at 300 and 400s the error obtained was 1.4%.Therefore, the time acquisition only minimizes the integration error of the experimental results, and it does not affect the chemical composition.An acquisition time of 100s is adequate for performance the chemical analysis.Knowing the values of the variables as Tc, accelerating voltage and acquisition time, the instrumental error was estimated performing measurements in a same region for five times.
The measurements were performed at 35.0s of Tc, 25kV of accelerating voltage 100s of acquisition time, around of 30% of DT, 100X of magnification and 11mm of WD.The results obtained are displayed in EDX spectra of Figure 4.As expected, the Gaussian peaks of the OK and MoL energy lines overlap with each measurement in the EDX spectra; however, the variations in the intensity of the peaks without affecting their broadening could affect the results of the chemical composition as was observed previously in the voltage study.These variations are related with the ability of the instrument to reproduce a measurement.The composition results and the relative errors calculated for every measurement is illustrated in table 4. As can be seen, the composition in every measurement is different indicating the instrumental error in the determination of the chemical composition.The variations are small without show any tendency.Under the measurement conditions used, the average error for the O was 1.9% while for the Mo it was 0.9%.From these results, it is possible to calculate the accuracy and precision of the instrument.Therefore, the instrument has an accuracy around 0.6wt% and a precision around 0.2wt%.Measurement

MoO 3 -Al 2 O 3 system
With the operation conditions established on the instrument and knowing the exactitude and precision grade, a study on the determination of molybdenum in alumina was carried out.The purpose was to evaluate the response capability of the instrument to quantify molybdenum at different concentration in the alumina matrix.Theoretically, the instrument can detect concentrations around of 0.1wt% under special operation conditions.In this study, the samples analyzed were prepared with nominal molybdenum concentration of 1, 5, 10 and 15wt%.The results obtained are displayed in the EDX spectra of Figure 5.The more intense Gaussian peaks corresponding to the OK, AlK and MoL characteristic energy lines are observed.Figure 5 shows the EDX spectra of MoO 3 /Al 2 O 3 system with low and high Mo concentration.As expected, at low Mo concentration the MoL Gaussian peak is very small, while at high Mo concentration the MoL Gaussian peak is more intense indicating the qualitative increase of Mo.Additionally, in the Gaussian peaks can be observed the calculated curve, black line, overlapping the experimental curve, red line.
Observing the difference between both curves, a very well fit can be determined.Latter indicates that all necessary considerations were made for a good interpretation of the experimental results.This analysis is very important when in the samples are present elements with very close characteristic energy lines that cannot be resolved by the www.intechopen.comX-Ray Spectroscopy 128 instrument and/or when the concentration of one of the components is very low that cannot be detected its corresponding characteristic energy lines.Generally, the next energy line occurs at high energy value; for molybdenum the next energy line is the MoK and it is not detected at low concentration.Then, a careful analysis of calculated and experimental curves helps to resolve the presence of elements with characteristic energy lines very close and consequently their concentration can be calculated.The following systems analyzed in this work correspond to some examples to resolve the problematic presented when elements with similar energy lines are containing in a specimen.As can seen in the Figure 5, both EDX spectra the experimental and calculated curves follow the same trajectory indicating that chemical elements identified in the sample were correct.The average chemical composition calculated for each sample is displayed in table 7.Each molybdenum value obtained was into measurement error.Therefore, the results obtained operating the instrument under the conditions previously mentioned were enough reliable.
Exp. data Calc.data

S-MO 3 system
According with the previous results, measurements into the reliability range marked by the instrumental error can be obtained.Now, the capabilities of the instrument to resolve the chemical composition of two elements that have characteristic energy values very close were analyzed.The first example was the determination of S in S-MoO 3 samples.Their more intense characteristic energy lines are very close and when both components are present in the sample only one experimental Gaussian peak is generated overlapping the presence of both elements.The case was studied using an S-MoO 3 system where sulfur concentration was varied from 1 to 15 wt%.Experimentally, one Gaussian peak was detected in the EDX spectrum.Figure 6 shows experimental Gaussian peak around of SK and MoL lines in the sample with the lower sulfur concentration.This experimental curve can be explained if we analyzed the values of the characteristic energies of both elements.The characteristic energies of the sulfur are SK and SK with values of 2.307 and 2.464 keV while for molybdenum are MoLI, MoL and MoL with values of 2.015, 2.293 and 2.394 keV, respectively.The energy difference between more intense lines, SK and MoL , is 0.014 keV and the energy resolution of the instrument is 0.129 keV, which is larger than the separation between both energies.Therefore, only one experimental Gaussian peak is generated, see Figure 6.A careful analysis the Gaussian peaks evidence the presence of Mo by the presence of MoL line that it is separated 0.101 keV from MoL line and 0.087 keV from SK line.However, the SK line is contained in it.To evidence the presence of sulfur component the overlapping of the calculated Gaussian peak with the experimental was made considering first individual elements and after both of them.The results obtained are displayed in Figure 6 for the sample with 1wt% S. Figure 6a show the calculated Gaussian curve considering only SK line, the fit with the experimental Gaussian curve was not good, see Figure 6a.Considering only the MoL line the calculated and experimental Gaussian curves are very close, Figure 6b, however, in the maximum of the calculated peak does not match with the maximum of the experimental one.The calculated curve is shift to the left of the experimental curve.In Figure 6c, both SK and MoL lines were considered in the calculus and the result obtained is lightly better than that obtained with individual MoL line in Figure 6b.As both results of Figure 6b and c almost are likeness, the presence of S in the sample can be doubtful.Then, the deconvolution and composition results must be reviewed.An integration error is given by the Phoenix software after chemical composition calculation.This error must be below 10% and for the analysis of sulfur in this sample was around 2.5% indicating good fit and confirming the presence of sulfur in the sample.

Conclusion
The optimization of the operation conditions of the instrument to obtain more reliable elemental chemical composition for alumina and molybdenum oxide was reviewed.For the environmental scanning electron microscope PHILIPS XL30 the better operation conditions were a constant time of 35s, 25kV of accelerating voltage and 100s of acquisition time keeping constant the dead time, the magnification, and the work distance.Under these operation conditions the exactitude and precision of the instrument to calculate the O, Al and Mo compositions for the alumina and molybdenum oxide was 0.6wt% and 0.2wt%, respectively.The more intense characteristic energy lines must be used to calculate the composition.The instrument is able to resolve very well the chemical composition in samples having elements with their more intense characteristic energy lines overlapped and they cannot be resolved by the limit of energy resolution of the instrument.Additionally, the preparation of the sample to be analyzed by SEM plays an important role for the determination of the chemical composition, then; a study of powder sample preparation is suggested.The results reported in this work correspond only to the samples analyzed with the instrument used.

Fig. 1 .
Fig. 1.EDX spectra showing the O and Mo experimental Gaussian peaks at different Tc.The spectra were obtained under operation conditions of DT around 30%, 25kV and 100s of acquisition time.

Fig. 2 .
Fig. 2. EDX spectra showing the O and Mo experimental Gaussian peaks at different accelerating voltage.The spectra were obtained under operation conditions of Tc=35s, DT around 30% and 100s of acquisition time.

Fig. 3 .
Fig. 3. EDX spectra showing the O and Mo experimental Gaussian peaks at different acquisition time.The spectra were obtained under operation conditions of Tc=35s, DT around 30% and 25kV.

Fig. 4 .
Fig. 4. EDX spectra showing the O and Mo experimental Gaussian peaks obtained in a same region under operation conditions of Tc=35s, 25kV, DT around 30% and 100s of acquisition time.

Fig. 5 .
Fig. 5. EDX spectra showing the peak intensities corresponding to OK, AlK and MoL lines a)l.0 Wt% Mo and b)15Wt% Mo

Fig. 6 .Fig. 7 .
Fig. 6.EDX spectra of the sample with 1 wt% S showing the experimental and calculated Gaussian curves around of the SK and MoL lines.Calculated Gaussian curve considering a) SK line b)MoL line and c) SK and MoL lines.

Fig
Fig. 12. EDX spectra of the sample with 5 wt% V showing the experimental and calculated Gaussian curves around of the TiK and VK lines.Calculated Gaussian curve considering a) TiK line and c) VK and TiK lines

Table 1 .
Chemical composition of MoO 3 obtained with the OK and MoL lines at different time constant.

table 2 .
At low accelerating voltage high error was obtained and it decreased with the increase of accelerating voltage obtaining the lowest error at 25 kV.Therefore, in order to obtain more accurate overall chemical composition is very important acquire experimental data at 25kV in the instrument used in this work for this sample.

Table 2 .
Chemical composition of MoO 3 obtained with OK and MoL lines at different accelerating voltage.

Table 3 .
Chemical composition of MoO 3 obtained with OK and MoL lines at different acquisition time.

Table 4 .
Chemical composition of MoO 3 obtained with OK and MoL lines in the same analysis region.Once the precision and exactitude of the instrument are known, the homogeneity of the O, Al and Mo composition in the Al 2 O 3 and MoO 3 samples was evaluated.As the samples analyzed were analytical grade is expected a homogenous chemical composition and then the measurement error will be equal or minor to the instrumental error.To carry out the study, ten measurements in different regions of the Al 2 O 3 sample deposited on the carbon tape were obtained using the same operation conditions of the instrument to determine the instrumental error.The results obtained are displayed in table 5. Characteristic energy corresponding to OK and AlK lines located at 0.523 and 1.486 keV were used to calculate the composition.Chemical composition expected is 47.07 wt% O and 52.93 wt% Al.This theoretical chemical composition was used in the statistical analysis.The average experimental composition obtained was around 46.46 wt% O and 53.54 wt% Al.The average calculated error was around of 1.3% for O and 1.2% for Al.The precision calculated for this sample was around of 0.2 wt% and the exactitude was around 0.6 wt%.The results are similar to the obtained in the study of the instrumental error.This similitude is indicating the high homogeneous distribution of the O and Al atoms in the alumina sample.Then, the measurement error is attributed to the instrument.

Table 5 .
Chemical composition of Al 2 O 3 obtained with OK and AlK lines in different regions of the sample.Experimental results corresponding to the molybdenum oxide is displayed in table6.OK and MoL characteristic lines were used to calculate the chemical composition.In this sample, the average experimental composition obtained was 32.86 wt% O and 67.14 wt% Mo.The average measurement error calculated was 1.3% for O and 1.2% for Mo with an accuracy and precision of 0.5wt% and 0.3wt%, respectively.The exactitude value is into range calculated for the instrument but not for the precision.Precision is the degree of dispersion of the data, and then the result is indicating a high dispersion of the values of oxygen and molybdenum composition in the sample compared with the dispersion produced by the instrument.This dispersion could be related with the homogeneity grade in the chemical composition of the sample.

Table 6 .
Chemical composition of MoO 3 obtained with OK and MoL lines in different regions of the sample.

Table 7 .
Average chemical composition of the O, Al and Mo obtained of Al 2 O 3 and MoO 3 samples at different molybdenum compositions.

Table 8 .
Average chemical composition of the O, Mo and S obtained of S and MoO 3 samples at different sulphur composition.

Table 10 .
Average chemical composition of the O, Ti and V obtained from samples of V 2 O 5 and TiO 2 at different vanadium composition.