2500
Anal. Chem. 1985, 57,2500-2503
Quantitative Raman Spectrometric Determination of Molybdenum Trioxide and Tungsten Trioxide in Supported Catalysts John P. Baltrus,' Leo E. Makovsky? John M. Stencel? and David M. Hercules*'
Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, and United States Department of Energy, Pittsburgh Energy Technology Center, Pittsburgh, Pennsylvania 15236 A method employing Raman spectrometry is presented for accurate and easy quantltatlon of crystalline phases commonly found in commercial hydrodesulfurizatlon Catalysts. The method Is based on the use of an Internal standard and provides an alternative method for quantitative measurement of crystalline phases uslng Raman spectrometry. The method Is accurate to within &6% (relative) for MOO, and WO, concentrations greater than 0.1 %. The method should be especially valuable for species having large Raman scattering cross sections such as NIMoO,, Ai,MoO,, Cog04,and NIWO,. It is shown that estimation of relative concentrations of species present on a Catalyst Is valuable for modeling a number of different Catalyst systems.
Raman spectrometry is useful for the investigation of promoted and unpromoted Mo03/A1203(1-5) and W03/A1203 catalysts (6-8). In addition to bands assigned to the metaloxygen vibrations of the metal oxides, other bands are often present in Raman spectra. These additional bands may be due to molybdate and tungstate compound formation involving the support or promoter and the active metal, as well as amorphous phases such as Mo and W interaction species (4, 5, 8). Quantitative estimation of these species is not possible using conventional analytical methods. A recent study by Chan et al. (91,reporting the relative Raman cross sections of various tungsten oxides, has demonstrated the feasibility of a quantitative approach for determination of molybdenum and tungsten species commonly found on hydrodesulfurization (HDS) catalysts. Comparison of relative Raman peak intensities from different species on a catalyst surface can lead to an erroneous estimate of the relative amounts of such species, if their relative Raman-scattering cross sections are not known. This was pointed out clearly for W/A120, catalysts (9). Estimating the relative amounts of crystalline Moo3 formed on molybdenum-containing HDS catalysts is another prime example. Several studies (8,10-12) could benefit from a more accurate estimation regarding speciation of Mo or W on the catalyst. Quantitative measurement of the fraction of Mo as MOO, or W as WO, would give an accurate estimate of the relative amounts of these species formed on the catalyst surface and allow subsequent development of better catalyst models. The purpose of the present paper is to report an analytical method that was used to quantitate the amount of crystalline Moo3 and W 0 3 formed on molybdenum or tungsten-containing catalysts, supported on alumina. The importance of the technique will be illustrated by applying it to a series of Mo/A1203catalysts previously characterized (12) along with a new series of Ni/Mo/A1203 catalysts.
EXPERIMENTAL SECTION Catalyst Preparation. Ni/Mo/Alz03 catalysts, containing from 1% to 21% Ni, were prepared by wet impregnation of University of Pittsburgh. Pittsburgh Energy Technology Center.
Harshaw y-AlzO, (190 M2/g) with Ni(N03)z.6Hz0(Fisher) followed by calcination of 400 OC for 5 h. Ammonium hepta(Fisher) was impregnated, molybdate [ (NH4)6M07024.4H20] subsequently followed by calcination at 400 "C for an additional 5 h. the Mo loadings on all catalysts were determined by atomic absorption to be 11.5 h 0.4%. The percentage of Ni in each catalyst, also determined by atomic absorption, is designated by the last two digits in the catalyst code. For example, the code MNA421 means that the Ni/Mo/A1203catalyst (MNA indicates that Ni was impregnated before Mo) was calcined at 400 "C (indicated by the first digit) and contains 21% Ni. The actual Ni concentrations for the catalysts are within 0.5% absolute of the nominal value. The preparation of the Mo/Alz03and W/ A1203 catalysts has been described previously (8, 12). For Mo/Alz03catalysts the Mo loadings are reported as percentages of MOO, (conventional procedure in catalysis) although the Mo is present as a variety of species; all Mo/A1203catalysis were calcined at 500 "C. Catalysts 1and 3 were calcined for 16 h and catalyst 2 for 1 h. Catalysts 1 and 2 contain 20% MOO, and catalyst 3 contains 30% MOO,. Instrumentation. The Raman instrument used for the present work has been described elsewhere (8). Approximately 45 mW of power (measured at the sample) were employed. The spectral slit width was 4 cm-l, and reported peak positions are accurate to f2 cm-'. A Datamate computer was added to allow computer control of the monochromator, as well as data acquisition and manipulation. Powdered catalysts were pressed onto a KBr backing which was used as a support for the pressed wafers. The samples were rotated off-axis to avoid sample decomposition. Sample Preparation. KNOB was chosen as an internal standard to be added to each catalyst because it does not interfere with peaks from other species commonly found in the catalysts. Calibration samples of MOO, or WO, and KN03 only, weighed to five decimal places, were prepared by taking approximately 1-1.5 g mixtures containing various weight percentages of MOO, or W03 and KNO, and grinding them for 7 min in an agate mortar. Analysis of the mixtures by scanning electron microscopy showed that the range of particle sizes for MOO, and WO, is about 1-5 Fm and that they are distributed evenly among aggregates of KNO,. A portion of the mixture was then pressed into a KBrbacked, 13-mm pellet for subsequent Raman analysis. The procedure outlined above was used for Mo/Al2O3and Ni/Mo/ A1203 catalysts mixed with KNOB. A known amount of KNOB was added to each catalyst to give a mixture containing a weight percentage of approximately 7% KNOB. In order to check the validity of the method, a Ni/Mo/A1203 catalyst (11.5% Mo) that showed no peaks attributable to MOO, was mixed with different known weights of MOO, and KNOB. Increasing amounts of MOO, were used for three different mixtures. A check of the reliability of this method for WO, determinations was made by spiking a 11.9% W/A1203catalyst, which showed no bands due to W03, with increasing amounts of WO, and KNO,. Quantitation of Moo3 and W 0 3 is based on measurement of relative peak areas and peak heights of the Raman peaks for each species, along with the KNO, standard. Peak areas were measured from a linear base line drawn between lower limits, reflecting the difference in broadness of the peaks, approximately 75 cm-' to each side of the peak maximum for MOO, and WO, and 20 cm-l to each side of the peak maximum for KNO,. Peak heights were measured from the peak maximum to a point on the base line directly below the maximum.
0003-2700/85/0357-2500$01.50/00 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 13, NOVEMBER 1985
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Table I. Determination of MOOSfor Ni/Mo/Al,O, Catalysts Mixed with Moo3 amt of Moo3, g samplen
from areab
actual
1 2 3
0.0038 f 0.0002 0.0184 & 0.0004 0.0456 f 0.0023
0.0039 0.0179 0.0431
Measurements were made on three separate wafers made from the same mixture. bMeasurements are for the 820-cm-l peak of Moo3 and 1050-cm-' peak of KNOB.
*
t v) z W If
Table 11. Determination of WO, for W/A1203Catalysts Mixed with W03 amt of W03, g samplen
from areab
actual
1 2 3
0.0054 f 0.0006 0.0144 f 0.0007 0.0397 f 0.0028
0.0053 0.0143 0.0393
a Measurements were made on three separate wafers made from the same mixture. *Measurements are for the 808-cm-' peak of WOs and 105O-cm-' peak of KNOR.
600
850
I100
A WAVENUMBER, cm-'
Figure 1. Raman spectra of a series of Ni/Mo/AI,O, catalysts containing 11.5% Mo and increasing Ni loadings, mixed with approximately 7 % KNO,.
We also determined the Raman scattering cross sections, relative to Moo3,for the Mo "interaction" species responsible for a band at 950 cm-l in Mo/A1203and Ni/Mo/A1203 catalysts (4, 5). The term "interaction species" refers to two-dimensional islands of Mo in strong interaction with the support. This was accomplished by using mixtures of Moo3 and two catalysts prepared under different conditions and having different molybdenum loadings (9.5% and 11.7% Mo). These catalysts did not show the presence of MOO, in their Raman spectra, only the Mo interaction species. A similar approach was used to determine the relative Raman scattering cross section for the tungsten interaction species, giving the 960-cm-' band in W/A1203catalysts. The 11.9% W/A12O3 catalysts did not give any evidence for the presence of W03.
RESULTS Calibration Mixtures. Calibration curves for mixtures containing MooBwere constructed by using the relative intensities of the 820-cm-' Moos peak due to a bridging Mo0-Mo stretching vibration (3, 12) and the 1050-cm-' KN03 peak due to the NO< symmetric stretching band. Intensities of the 820-cm-' and 105O-cm-' peaks were measured by using both peak heights and peak areas. Differences between the two methods were determined and compared. A calibration curve of the ratio of the MooBpeak area to the KNOBpeak area vs. the weight ratio of MooBto KNOB in the mixtures produced excellent linearity (coefficient of linear determination = 0.999) over the range of interest. The slope of the curve is 60.03 with an intercept of 0.085. The relative standard deviation for the peak area ratio at a given weight ratio is *7.8%. Additional sample grinding did not change the peak area ratios. Calibration curves for the mixtures are linear from 0 to 0.429 g of Mo03/g of KNOBusing peak areas. Limitations on linearity are observed because of concentration effects which lead to intense scattering from Moo3 compared to KNOB. Calibration curves were also established to determine the amount of W03 in W/AlpOB catalysts. The most intense band of W 0 3 at 808 cm-l, due to a W=O stretching vibration, was measured along with the KN03 band at 1050 cm-'. The range
of W03 and KNOBweight ratios used in this study can easily be obtained by adding appropriate amounts of KNOBto each catalyst. A calibration curve for the ratio of WO, peak area to KNOBpeak area vs. the weight ratio of WO, to KNO, in the mixtures showed excellent linearity (coefficient of linear determination = 0.996). The slope of the curve is 72.38 with an intercept of -0.078. The relative standard deviation for the peak area ratio a t a given weight ratio is k6.0%. Accuracy of the Method. The procedure used to check the accuracy of the internal standard method for determination of MOO, concentrations produced spectra that were similar to those for Mo/A1203 catalysts containing MOO,. Raman spectra of the catalysts containing MooBand the KNOBinternal standard are shown in Figure 1. The band a t 950 cm-' is due to the Mo interaction species; bands at 820 cm-I and 996 cm-' are due to MOO,. The 1050-cm-' band is due to KNOBand a band at 878 cm-' is due to a CaMoOI impurity present in some samples. The amounts of Moo3, determined from the calibration curves, were compared with the amount of Moos added. These data are found in Table I. The known amounts of W03 in catalyst mixtures used to determine the accuracy of the technique for WO, quantitation and those determined from the calibration curves are reported in Table 11. Spectra of W/A1203catalysts containing W03 and the internal standard are shown in Figure 2. W03 has intense bands at 716 cm-' and 808 cm-'. The tungsten interaction species has a band at approximately 960 cm-l. The KNOBband is a t 1050 cm-l. Quantitation in Catalysts. The amounts of MOO, present in the Ni/Mo/A1203catalysts were determined by the internal standard method. Raman spectra of typical catalysts are shown in Figure 1. Except for catalyst MNA411, sufficient KNOBwas added to each catalyst to give similar peak heights for the bands due to MOO, and KNO,. The percentage of Mo present as Moos in each Ni/Mo/A1203 catalyst is reported in Table 111. The same type of quantitative determinations, using a series of Mo/A1203 catalysts, gave the spectra shown in Figure 3. The spectra are identical with those reported earlier (12) except for the addition of the KNOBpeak. These catalysts show a larger range of MOO, contents. The importance of the preparation variables will be discussed later. The percentages of Mo as MOO, for the Mo/A1203 catalysts are reported in Table 111.
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 13, NOVEMBER 1985
I
E. > t In
>
cIn W z
5 I-
c
z
f
I
C.
I
C.
3 )
A
825 WAVENUMBER, ern-'
A
111
Figure 2. Raman spectra of a 11.9% W/AI,O, catalyst mixed with various amounts of WO, and KNO,: (A) 0.0393 g of WO,; (B) 0.0143 g of WO,; (C) 0.0053 g of WO,. ~
Table 111. Mooo Contents of Ni/Mo/A1203and Mo/Al,03 Catalysts catalyst
% Mo as MOO,"
catalyst ( b )
% Mo as MOO,'
MNA421 MNA411 MNA408 MNA401
0.41 0.04 0.67
l(16 h/20%) 2 (1 h/20%) 3 (16 h/30%)
0.7 3.1 7.2
0.96
'Determined from peak area ratio Mo03/KN03. Calcination time at 500 "C/Mo loading expressed as % MooB.
Relative Cross Sections. Raman cross sections of the Mo interaction species relative to MOO, and the tungsten interaction species relative to W03 were calculated. The MOO, or WO, peak area intensities were normalized to the weight of MOO, or WO, present in the mixture. The intensities of the peaks due to the Mo or W interaction species were normalized to the amount of each species assuming their chemical formulas are MOO, or WO,, respectively, since no accurate chemical formula can be assigned to either interaction species. This provided a common basis for calculation of relative cross sections. This procedure produced a relative cross section of MOO, to that of the interaction species of 1 5 1 for the 9.5% Mo/A1203catalyst and 1 8 1 for the 11.5% Mo/A1203catalyst; the average was 17:l. There is no trend in relative cross sections with increasing Mo loading. The difference in values represents the error associated with the measurements. The relative cross section of WO, to that of the tungsten interaction species was determined to be 137:l. The smaller values for Moo3 relative cross sections are due to the much greater broadness of the molybdenum interaction species peak compared to the tungsten interaction species peak. DISCUSSION The ability to construct linear calibration curves for Mo03/KN03 and W03/KN03 peak intensity ratio vs. the weight ratios of Moo3 or WO, and KNO, indicates that KN03 can be used as an internal standard for detkrmination of MOO, and WO, in HDS catalysts. A large range of amounts of Moo3 and W03 can be quantified by dilution of catalyst samples with an appropriate amount of KNOBto give a weight ratio
875 WAVENUMBER, cm-1
II!
Flgure 3. Raman spectra of Mo/Ai,O, catalysts calcined at 500 O C mixed with KNO,: (A) catalyst 1, 20 wt % MoO,/caicined 16 h; (B) catalyst 2, 20 wt % MoO,/calcined 1 h; (C) catalyst 3, 30 wt % MoO,/calcined 16 h.
in the range covered by the calibration curves. The validity of the method and accuracy of the calibration curves are illustrated by application of the technique to catalyst samples containing a large range of known amounts of MOO, and WO,, as reported in Tables I and 11. The application of the internal standard technique for determination of a known amount of MOO, added to a Ni/Mo/A1203 catalyst (Table I) showed that the use of Mo03/KN03 peak area ratios is accurate for determining very small amounts of MOO,. The results reported in Table I1 show that the technique is quantitatively reliable for determining known amounts of W03 in W/A1203catalysts. The results from the peak area method are in excellent agreement with the actual values (Table 11);relative standard deviation is not greater than &lo%. Similar results were obtained when MOO, (WO,) and KNO, peak heights were used instead of peak areas provided that KN03 accounted for at least 25% (by weight) of the sample to be measured. Use of smaller amounts of KNOBis accompanied by a change in KNO, peak shape. In quantitative Raman studies of catalysts, one must also be concerned with particle size differences between bulk compounds and species found on a catalyst surface. Differences in peak shape and relative peak intensities caused by interaction of MOO, or W03 with the catalyst support and with other species present on the catalyst surface may also lead to problems. It has been demonstrated that band intensities, including relative band intensities from the same species, may change as a function of catalyst loading (13). It was observed for our samples that the intensity of the 996-cm-' band relative to the 820-cm-l band of Moo3, reported in Table IV, did not change between different calibration mixtures or catalyst mixtures. This constancy indicates a similarity between bulk crystalline Moo3 and the MOO, surface species. The ability to quantify amounts of Moo3 and WO, formed on supported catalysts permits a more accurate estimate of the extent of formation of these compounds than comparison of their peak intensities with intensities of other species present in the catalyst sample. This has also been reported recently for the case of WO, in W/A1203 catalysts (9). A typical example is determination of the extent of Moo3 formation for sets of Ni/Mo/A1203 and Mo/A1203 catalysts
ANALYTICAL CHEMISTRY, VOL.
Table IV. Raman Intensity Ratio of the 996-cm-' to SZO-cm-' Bands of MOO, for MOO, Containing Samples sample
bulk MOOS calibration m i x t u r e 0.00761 g of Mo03/g of
zW6,(l cm-'
h p ,Icm-'
97 19
200 39
0.485 0.481
60
123
49 10 30
101 b 62
0.488 0.485
z996/z8Z0
KN03
MNA4Ol MNA408 MNA411 MNA421 a A r b i t r a r y u n i t s of intensity. measure veak intensity.
* Signallnoise
0.484
is t o o small t o
prepared in this laboratory. A catalyst may show a large MOO, peak, as in catalyst MNA401 shown in Figure 1; however, the percentage of Mo as Moo3 is quite small (less than 1%).The other catalysts which contain increasing amounts of nickel, MNA408 through MNA421, have even smaller amounts of MOO,; as low as 0.04% Mo as MOO, for catalyst MNA411 (determined after scale expansion). The results for the Mo/A1203catalysts also show that while a Raman spectrum may be dominated by peaks due to Moo3, the amount of MOO, may be only a small fraction of the total Mo content. This is especially evident for the 30% MoO3/A1,03 catalyst. For the Mo/A1203 series of catalysts, the results from the quantitative Raman study, reported in Table 111,confirm that the conditions favorable for the formation of a maximum amount of MOO, are shorter calcination time and higher Mo loading (12). Most Mo-containing catalysts supported on A1203also show a peak around 950 cm-l due to the Mo interaction species. Depending on preparation conditions, this species is usually observed for a wide range of Mo loadings. Figure 3A shows the Raman spectrum for a Mo/A1203catalyst having a large Mo loading. Although Moo3 and the interaction species are the only species detected on the catalyst, and the percentage of Mo as MooBwas shown to be very small, the intensities of the main peaks for the species are similar. This prompted determination of the relative Raman cross sections of MOO, and the Mo interaction species and to determine if the cross sections are affected by catalyst composition or preparation conditions. Since both species in the mixtures, MOO, and the interaction species, were sampled at the same time, all sampling conditions were the same for both species. The value of the relative cross sections of MOO, to the Mo interaction species are similar for the mixtures of MOO, and the 9.5% or 11.5% Mo/A1203catalysts, within the standard deviation of results expected from the technique. It is important to note that the relative cross sections are not significantly affected by a difference in Mo loading or the presence of Ni in one of the catalysts. Determination of the relative Raman cross sections of W 0 3 and the W interaction species is also important in order to demonstrate the magnitude of difference between their values. Results reported by Chan et al. (9) show that the relative b a n cross section of WO, to the tungsten interaction species is 160:l. Our method for determination of relative cross sections is different from the above because all species are present in the same sample for our method. The value reported by Chan et al. compares favorably to the value of 137:l reported here, showing that the methods are complementary. The magnitude of difference in the cross sections of MOO, or WO, and the respective Mo and W interaction species indicates that the interaction species are poor Raman scatterers compared to Moo3 and W03. This has important implications for characterization of HDS catalysts, since a small change in Moo3 concentration or even a small MOO,
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NOVEMBER 1985
2503
concentration in a catalyst will easily be detected. Conversely, small amounts of interaction species or small changes in the amounts of interaction species will not be detected by inspection of changes in peak intensities and then will also be more difficult to measure. The same has been shown to be true for WO, and the interaction species in W/A1203catalysts (9). Also, as indicated earlier, similar peak intensities for MOO, or WO, and the respective Mo or W interaction species do not imply that Mo or W are equally distributed between the two species. The advantages of the internal standard method for quantitating the amount of MOO, and WO, is illustrated in its application to Mo- and W-containing supported catalysts. In addition to allowing quantitative measurement of phases that cannot be quantitated by other analytical methodology, using an internal standard allows one to minimize problems associated with maintaining accurate excitation power, laser focus, and sampling volume from sample to sample. All of the above parameters would require close attention if an internal standard were not used, as illustrated in a study of relative Raman cross sections involving measurements on separate, pure samples (9). In that study, the excitation power had to be accurately measured from sample to sample so that peak intensities could be properly normalized. For our method, an initial calibration curve can be used indefinitely. Our study of relative Raman cross sections involved species that were present in the same sample, thereby eliminating some possible sources of error. Minimization of error is extended by use of an internal standard which provides a common reference from sample to sample to allow routine analysis of "compoundlike" species found in catalysts. This study has shown two important results. The first is that, as was shown for the case of WO, in W/A1203catalysts (9),the appearance of large MOO, peaks in the Raman spectra of supported molybdenum-containing catalysts does not necessarily imply that the compound accounts for a large fraction of the species present in the catalyst. Second, by using an internal standard in a procedure similar to that given above, one can obtain an accurate estimate of the amount of MOO, or W 0 3 present in a catalyst. This method should also be readily adaptable to the reliable quantitation of other Raman-active species commonly found in supported catalysts. Registry No. MOO,, 1313-27-5; WO,, 1314-35-8; KN03, 1151-19-1; W, 7440-33-1; Mo, 7439-98-7; Ni, 7440-02-0.
LITERATURE CITED Jeziorowskl, H.; Knozinger, H.J. Phys. Chem. 1979, 83, 1166-1173. Dufresne, P.; Payen, E.; Grimblot, J.; Bonelle, J. P. J. Phys. Chem. 1981, 85,2344-2351. Brown, F. R.; Makovsky, L. E. Appl. Spectrosc. 1977, 37, 44-46. Brown, F. R.; Makovsky, L. E.; Rhee, K. H. J. Catal. 1977, 50,
162- 171.
Jezlorowski, H.; Knozinger, H., J. Phys. Chem. 1978, 82,
2002-2005. Thomas, R.; Kerkhof, F. P. J. M.; Moulijn, J. A.; Mederna, J.; de Beer, V. H. J. J. Catal. 1980, 67, 559-561. Iannibello, A.; Villa, P. L.; Marengo, S . Gazz. Chim. Ita/. 1978, 709,
521-528. Salvatl, L.; Makovsky, L. E.; Stencel, J. M.; Brown, F. R.; Hercules, D. M. J. Phys. Chem. -1981, 85,3700-3707. Chan, S. S.;Wachs, I.E.; Murrell, L. L. J. Catal. 1984, 90, 150-155. Schrader, G. L.; Cheng, C. P. J. Catal. 1984, 85,488-498. Cheng, C. P.; Schrader, G. L. J. Cafal. 1979, 60, 276-294. Zingg, D. S.;Makovsky, L. E.;Tlscher, R. E.;Brown, F. R.; Hercules, D. M. J . Phvs. Chem. 1980. 8 4 . 2898-2906. Jeziorowski; H.; Knozinger,' H.; ' Grange, P.; Gajardo, P. J . Phys, Chem. 1080, 84, 1825-1829.
RECEIVED for review April 22,1985. Accepted June 27,1985. Reference in this report to any specific commercial product, process, or service is to facilitate understanding and does not necessarily imply its endorsement or favoring by the United States Department of Energy.
