J. Phys. Chem. 1993,97, 470-477
470
An Infrared Spectroscopy and Temperature-Programmed Desorption Study of CO on MoO3/Al203 Catalysts: Quantitation of the Molybdena Overlayer Anthony L. Dipz and Mark E. Bussell' Department of Chemistry, M.S.- 9058, Western Washington University, Bellingham. Washington 98225 Received: August 3, 1992; In Final Form: October 20, 1992
The adsorption of CO on pure y-Al203 and a series of MoOs/A1203 catalysts with different Mo loadings has been investigated using infrared (IR) spectroscopy and temperature-programmed desorption (TPD).Carbon monoxide adsorbs selectively on AI3+ sites on the uncovered, free alumina portion of the surface; these sites are created by dehydroxylation of the alumina. As the Mo loading is increased, the CO adsorption capacity of the catalysts decreases in a linear fashion as the MoO3 overlayer covers an increasing fraction of the alumina support. Overlayer growth saturates at a loading of (42 i 3) X 1013 Mo atoms/cm2 and corresponds to less than a complete monolayer as 14 i 5% of the alumina support remains uncovered. Extrapolation of the overlayer growth curve to "monolayer" coverage allows calculation of a MoO3 cross-sectional area of -22 A2, in good agreement with values predicted by theoretical models.
htroduciion The chemistry of metal oxide surfaces is a subject of considerable technological interest given their extensive use in catalytic processes as high surface area supports, as catalytic agents themselves, and as precursors to other catalytic species.' Additionally, metal oxide surfaces are gaining attention as models for investigating environmentally important phenomena such as the interaction of contaminant species with mineral constituents in soils.2 Alumina-supported molybdenum trioxide (MoO3/A1@3) is no exception in its technological importance; it is a precursor of molybdenum-based hydrotreating catalysts for which the demand is enormous. Numerous studies can be found in the literature in which Mo03/AlzOa catalysts have been characterized by a variety of techniques, and these have been reviewed in detail el~ewhere.~*~ These investigations have resulted in a good understanding (described briefly below) of the structure of the MoO3 species on the alumina support and how loading affects the structure of the overlayer. Missing from the literature, however, is a consensus on what the absolute coverage of the molybdena overlayer is as a function of loading as well as what loading corresponds to saturation coverage of MOO:, on the alumina support. Alumina-supported MoO3 catalysts are generally prepared by pore volume impregnation of the y-alumina (yA1203)support with aqueous solutions of ammonium heptamolybdate ((NH4)6Ms024). Following drying, the catalyst precursors are calcined at temperatures between 750 and 950 K to produce the A1203 catalysts. As summarized by Okamoto et al., different Mo species adsorb on the alumina support depending upon the impregnation condition^.^ At low metal loadings (< 10 X 1 O I 3 Mo atoms/cm2), molybdenum adsorption occurs predominantly in the form of monomeric M004*- species via the following reaction:
-
H Mo0d2-
+
I
H
0
0
I Ar
I Ar
I
-
OQO
o/ I Ar
\o + 20HI Ar
These tetrahedral molybdenum species, when sulfided, exhibit low hydrodesulfurization activitiea,6 and as a result, molybdenumbased HDS catalysts are prepared with metal loadings in excess of 10 X lOI3 Mo atoms/cm2. Above this loading, the principal molybdenum species found to adsorb onto the alumina surface To whom correspondence should be addrmed.
0022-3654/58/2097-0470$04.00/0
is the polymeric M s 0 ~ ion. 4 ~ This octahedral molybdenum species also reacts with OH groups on the surface of y A l 2 0 3 although the stoichiometry of this reaction would be expected to be lower than the 20H-/Mo predicted for the monomeric Mo species. The reason for this difference is that the polymeric molybdate ion has a multilayer structure, and therefore, some of the Mo03 units probably do not interact directly with surface hydroxyl groups. The strongest evidence for the elimination of hydroxyl groups from the alumina surface comes from infrared (IR) and nuclear magnetic resonance (NMR) spectroscopic investigations of the oxide catalysts. Infrared spectroscopy has proven to be an invaluable tool for studying hydroxyl groups on metal oxide surfaces. In the case of yA1203, five different OH stretching frequencies have been identified at 3798,3732,3680,3585, and 3500 cm-' for an alumina sample outgassed in vacuum.7.* The different OH stretching frequencies can be traced to differences in the coordination (e.&, tetrahedral vs octahedral) and number of aluminum atoms to which the OH groups are bound and to the extent of hydrogen bonding with nearat-neighbor OH groups! Hall and co-workers9as well as otherdohave used IRspectroscopy to show that the intensity in the OH stretch region decreasca with increasing molybdenum loading, indicating that replacement of surface OH groups by tetrahedral and octahedral molybdate species does indeed occur. Interestingly, it was found that OH groups with high stretching frequencies were removed preferentially and that, even in the limit of high loading, some OH intensity remained for the catalysts. This latter discovery was found to be independent of the preparation method, even when catalysts were prepared by the equilibrium adsorption method in which excess solution is used, OH intensity was observed for the catalysts with high weight 1oading.Il The equilibriumadsorption method eliminates the possibility of nonuniform loading due to preparation, and such a loading problem can therefore be ruled outasthecausefortheobservedOH intensity at highMoloadings. This result, along with proton NMR data which suggested that areas of unperturbed Al-OH groups existed at all Mo loadings,I2 led Hall and co-workers to conclude that a complete monolayer of MoO3 dom not occur on the alumina surface, but rather polymeric molybdate clusters separated by patches of free a1umina.I' Site selective adsorption has proven to be a useful technique for measuring the coverage of one oxide component at the surface of mixed oxide systems.I3 In the case of the MoO,/Al203 system, COz chemisorption has been employed to characterize the growth
Ca 1993 American Chemical Society
CO on MoO,/A1203 Catalysts of the molybdena o ~ e r l a y e r . ~Hall J ~ and co-workers had shown previously that C02 adsorbs selectively on alumina at room temperat~re.’~Okamoto et al. used IR spectroscopy of C02 adsorbed at room temperature to characterize the growth of the MoO3 overlayer on y - a l ~ m i n a .Analysis ~ of the IR data was qualitative (Beer’s law was not utilized) and complicated by the fact that C02 reacts with the alumina support to produce bicarbonate and carbonate species. Nevertheless, a smooth decrease in the IR intensity of the bicarbonate stretch (1237 cm-1) with molybdenum loading was observed with complete elimination of the bicarbonate stretch, corresponding to no free alumina surface remaining, at a loading of 26 X 1013Mo atoms/ cm2. This ‘monolayer” coverage is well below the theoretical monolayer coverage of (5040) X l O I 3 Mo atoms/cm2 l6*I7 and the experimentally determined ‘monolayer” loadings of (40-57) X 10” Mo atoms/cm2 based upon X-ray photoelectron (XPS) and laser Raman ( L E )spectroscopicmeasurements.18-20 These results suggest that the intensity of the bicarbonate stretching frequency produced by C02 adsorption on MoO3/A203 catalysts is not a good measure of the MoO3 coverage. It should also be noted that since the XPS and LRS results discussed above provide only a relative calibration of the coverage, no information is given concerning what fraction of the alumina surface is covered when monolayer growth of the MoO3 overlayer stops. O’Young et al. used site selective C02 chemisorption to determine the percentage of free alumina surface as a function of loading for MoO3/Al203 catalysts. Utilizing both volumetric chemisorption and infrared measurements, a similar curve to that of Okamoto et al. showing a decrease in chemisorbed CO2 (percent free alumina) as a function of Mo loading was determined.“ They associated a ‘monolayer” of MoO3 on the alumina support with a Mo loading of 28 X 1013Mo atoms/cm2, also well below the theoretical monolayer coverage. An additional troublesome result of the two C02 chemisorption studies is that both studies suggest no free alumina surface is present at the ‘monolayer” loading which contradicts infrared data which show significantOH intensityat thesesameloading~.~~~J~ As discussed above, OH intensity is associated with OH groups bound to free alumina sites. Finally, it should be noted that differences in the method of catalyst preparation and in the specific surface area of the y-Al203 support are not likely to be responsible for the discrepancies between ‘monolayer” loadings determined by the different analytical techniques. The LRS study found the structure of the MoO3 overlayer to be independent of the preparation method while the XPS results of Dufresne et al. indicate the ‘monolayer” loading of Mo to be independent of the specific surface area of the s ~ p p o r t . ~ ~ ~ ~ ~ In order to address the discrepancies between the CO2 chemisorption studies and the theoretical monolayer predictions as well as the XPS and hydroxyl region IR results discussed above, we have undertaken an investigation of the MoO3/Al203 system using the site selective adsorption properties of carbon monoxide. Ballinger and Yates have recently used IR spectroscopy to show that an empirical correlation exists between the quantity of CO adsorbed on yA1203 ( T = 180 K, PCO = 5.0 Torr) and the number of OH groups removed from the alumina surface by high-temperature dehydroxylation.8 As we will show, CO titration of MoO,/Al203 catalystsdehydroxylated in ultrahigh vacuum permits a quantitative determination of the coverage of the molybdena overlayer. ExperimenW Section Catalyst Preparatioa and Mouating. Md3/Al203 catalysts were prepared by impregnation of y-Al203 (Engelhard AL-3945) with aqueous solutionsof ammonium heptamolybdate (J. T. Baker Co.). The y’Al2O3 had a BET surface area of 255 m2/g and a pore volume of 0.60 mL/g. The alumina support (1/12-in. extrusions) was ground to a fine powder prior to use. Following impregnation, the catalysts were dried for 24 h at 393 K and then
The Journal of Physical Chemistry, Vol. 97, No. 2, 1993 471 POWER
r
LN2
LE
Figure 1. Design drawing of IR-TPDsample holder (scale 1:2).
calcined for 3 h in air at a temperature of 773 K. BET surface areas were measured for each calcined catalyst using a Micrometria Flowsorb I1 2300 system. Catalyst samples (- 15.0 mg) used for IR-TPD studies were pressed at 10 OOO psi into a tantalum metal mesh (50 X 50 mesh size, 0.003-in. wire diameter). In a recent study, Yates and c e workers have shown that alumina samples prepared similarly exhibit no apparent diffusion limitations when compared to a sample prepared by a more complicated spraying technique.21 The area of the pressed samples was 0.90 cm2, and sample thicknesses were normalized using their masses. The temperature of the sample was monitored by means of a chromel-alumel thermocouple spot-welded to the tantalum mesh. U l ~ V l c u u m / H i g h - ~ u System. re The research d e scribed was carried out in a bakable, stainless steel ultrahighvacuum (UHV) chamber equipped with a high-pressure cell that can be isolated from the vacuum system. The UHV chamber is pumped by a 110 L/s ion pump, and after changing samples, a base pressure of 2.0 X 1 Torr can easily be achieved. System base pressures are limited by sample outgassing. Pressures in the UHV chamber are measured by means of an ion gauge while pressures in the high-pressure cell are measured using a capacitance manometer. The high-pressure cell consists of a 2’/4-in. cube cross outfitted on two parallel faces with flange-mounted CaF2 windows. The cell is connected to the UHV system via an angle-in-linevalve (1 ‘/2-in. tube) for high gas conductance. The sample holder is mounted to the cell via the top face of the cube cross, perpendicular to the CaF2 windows, permitting infrared experiments to be conducted in the transmission mode. The sample holder, shown in Figure 1, consists of a 23/4-in. conflat flange outfitted with feedthroughs for resistive heating, temperaturemeasurement, and liquid nitrogen cooling via in. stainless steel tubing. Samples supported on the tantalum metal mesh are clamped to the copper-beryllium sample supports via stainless steel screws and can be cooled to 140 K and heated to 1200 K. This method for clamping the sample to the sample holder is essentially the same as that described previously by Yates and c+workers.22 Samplecooling is achievedvia gravity feed of liquid nitrogen to a copper reservoir separated from the heating arms of the sample holder by 1/16-in. sapphire spacers which are electrically isolating but thermally conducting. Sample heating is accomplished using a home-built temperature controller which allows linear sample heating at rates of 0.1-10 K/s. Attached to the remaining face of the high-pressure cell via ’/4-in. tubing is a welded, stainless steel gas handling manifold through which gases are leaked into the system. Infrared spectroscopic mea-
472 The Journal of Physical Chemistry, Vol. 97, No. 2, 1993
surements are accomplished using a Mattson RS-1 FTIR spectrometer which has a water-cooled source and a narrowband MCT detector and is interfaced to a personal computer for dataacquisitionand treatment. For TPD experiments,the UHV system is outfitted with a Leybold-Inficon Quadrex 200 quadrupole mass spectrometer which is interfaced to a personal computer for acquisition of up to six ma- simultaneously.The mass spectrometer was calibrated for TPD measurements by simulating a CO TPD spectrum with a known amount of gas measuredvolumetricaly in the high-pressurecell. This calibration was readily reproducible and is believed to be accurate within 10%. IR/TPD ExpCriments. Following mounting of a catalyst sample in the vacuum system, it was outgassed for 16 h at a temperature of 475 K. After the sample was cooled to 140 K, an IR spectrum of the sample was recorded. In all cases, IR spectral acquisition consisted of 32 scans of the region 4OOOlo00 cm-I at a resolution of 4 cm-I and took less than 15 s to acquire. For IR spectra recorded in vacuum, the subtracted background was of a blank tantalum mesh mounted in the sample holder. Following acquisition of the predose IR spectrum, the high-pressure cell was isolated from the UHV system and pressurized to an equilibrium pressure of 5.0 Torr of CO. The CO (Matheson Gas Products, 99.99% minimum purity) was passed through a /s-in. stainleas steel coil submerged in a pentane slush prior to dosing in order to remove metal carbonyl impurities. Following acquisition of an IR spectrum at a CO pressure of 5.0 Torr (using a similar background as above but with PCO= 5.0 Torr),the carbon monoxide was evacuated using a mechanical pump prior to opening the valve linking the high-pressure cell to the UHV system. When the system pressure had lowered to 6.0 X 10-* Torr (-5 min), 1R and TPD spectra were acquired. The sample was then heated to the next annealing temperature for 30 min and then cooled, and the procedure for acquiring IR and TPD spectra was repeated. Temperature-programmeddesorption experiments were carried out over a temperature range of 140475 K using a heating rate of 1 K/s while acquiring data for masses 2 (H2). 18 (HzO), 28 (CO), and 44 (COz) at a sampling frequency of 5 points/K. All IR and TPD spectra are reproduced without any further background or smoothing treatment. Integrated IR absorbances were calculated in the VOH region from 3827 to 3200 cm-I and in the vco region from 2250 to 2150 cm-I. TPD peak areas were calculated between points on either side of the peak for which the mass spectrometer signal was at its base line value.
Diaz and Bussell I
I
3886
T
I
p
4
0 3493
I
37\
-
Results
Effect of Mo Lording on the IR Spectra of MOO3/AbO3 Catalysts. Infrared spectra were acquired for pure y-AlzO3 and MoOp/Al20p catalysts with different Mo loadings following outgassing at a temperature of 600 K for 30 min. The most noticeable changes in the IR spectra of the different samples were observed in the OH bending and stretching regions. The intensity of the OH bending mode ( 1555an-’) decreased rapidly as the Mo loading was increased and could not be distinguished above the background for loadings greater than 15 X 1013Mo atoms/cmZ. Shown in Figure 2 is the OH stretch region for pure y-AlzOp and MoOp/AlzOp catalysts with different Mo loadings. These spectra are very similar to those published previously by Hall and CO-workersand Comac et al.9J0 As the loading of Mo is increased, the integrated intensity in the OH stretch region decreases and the different VOH frequencies easily resolved for pure y-Al2Op become ill-defined. For loadings greater than 31 X 10’) Mo atoms/cm2, the OH intensity is present in the form of a single very broad peak. As noted by Hall and CO-workers, the high-frequencys t r e i n g vibrations (particularly 3769 cm-l) are preferentially reduced in intensity with respect to the lower VOH frequencies and significant OH intensity remains wen at high Mo 10ading.s.~As will be discussed shortly, CO adsorption
/
(e)
0
I
I
I
3800
3600
3400
3200
Wavenumber (cm-1) 2. Infrared spectra of the UOH region of (a) pure y A l 2 0 3 and MoOp/Al203 catalysts with Mo loadingsof (b) 15 X 10”. (c) 20 X loL3, (d) 31 X lo”, and (e) 45 X 10” Mo atome/cm2 of catalyst.
on the dehydroxylated catalysts is sensitive to the replacement of these high stretching frequency OH groups at low Mo loadings. Finally, if it is assumed that the integrated extinction cafficient in the OH stretching region (%H) is constant, then the decrease in integrated absorbance in the OH stretching region is proportional to the concentration of OH groups on the catalyst. Integration of the spectra in Figure 2 over the region from 3827 to 3200 cm-I shows a decrease in integrated absorbance from pure y-AlzO3 to the highest weight loading sample in Figure 2 (45 X 10’) Mo atoms/cm2) of 80%. As revealed shortly, this loading is beyond the saturation coverage of MoOp on y-Al203. Debydroqktim Of r-AIfl3 d M003/AI203 Catilpk Reproduced in Figure 3 are infrared spectra of the OH stretching region for y-AhO3 after annealing at temperatures of 475,600, 800, 1O00, and 1200 K in vacuum. Sample annealing was for a period of 30 min in all cases but 475 K, for which the anneal timewas 16 h. Thespectrawereaq~atasampletemperature of 140 K and are similar to those of Ballinger and Yates.s The only significantdifferencesapparent in the two sets of spectra are in the peak position of the lowest VOH frequency (3405 vs 3500 cm-l) and of the highest VOH frequency (3769 vs 3798 cm-l). The position of the low-frequency stretch is sensitive to the amount of hydrogen bonding between neighboring OH groups; the fact that this stretch appears at a lower position in our spectrum suggests a higher OH concentrationand therefore more hydrogen bonding. This difference between the two spectra may due to slightlydifferent heating characteristiaof the samplesor perhaps because Ballinger and Yates outgassed their sample at 475 K for 48 h versus 16 h in our case. An explanation for the different positions of the highest VOH frequency is not obvious; it should be noted, however, that in our 1O00 K spectrum a shoulder is apparent at 3798 cm-l, the position of the high-frequency peak observed by Ballinger and Yates? The MoOp/AlzOp catalysts heated to the different dehydroxylation temperatures exhibited IR spectra in the VOHregion similar to pure y-Ah03 with the exception that the well-resolved UOH absorbances were not observed for the catalysts with high Mo
-
The Journal of Physical Chemistry, Vol. 97, No. 2, 1993 413
CO on MoO3/Al203 Catalysts
2201
2223
I
'
1200 K
475
1 I DO
I
O.1SA
I
I
I
3800
3600
3400
3:
IO
Wavenumber (cm-1) Figure 3. Infrared eptctra of the UOH region for pure y-Ah03 dehydroxylated in vacuum at the indicated temperatures.
loadings. In each case, complete dehydroxylationof the catalyst was accomplished by annealing at temperatures I 1 2 0 0 K. Infr8redSpectroscopy of Adsorbed CO 011 y'AI203. Following dehydroxylation of pure y-Al2O3 at temperatures of 475, 600, 800,1000, and 1200 K,CO adsorption studies were performed. Upon cooling from the dehydroxylation temperature, a sample was exposed to an equilibrium pressure of 5.0 Torr of CO. After reevacuation to UHV pressures, infrared and TPD spectra were recorded. In Figure 4 are shown IR spectra in the CO stretch region for a pure y-AlzO3 sample at a temperature of 140 K for the different annealing temperatures. These spectra are similar to those O ~ S C Nby~ Ballinger ~ and Yates under high-pressure conditions of 5.0 Torr of CO and a sample temperature of 180 K.8 The CO IR spectrum following dehydroxylation at 475 K exhibits a single peak located at 2197 cm-l. This UCO frequency is blue-shifted with respect to gas-phase CO (uco = 2143 cm-l) and has been assigned by others to CO weakly chemisorbed on dehydroxylated Al3+ sites on the y-Al& surface.8.23 In their higher pressure IR spectrum of CO adsorbed on y'Al203 dehydroxylated at 475 K,Ballinger and Yates observed a second uco absorbance at 2154 cm-1, which has been assigned to CO hydrogen bonded to hydroxyl groups on the alumina surface?' This second vcoabsorbancewas also observed in our high-pressure spectra (PCO = 5.0 Torr), albeit with much greater intensity because of our lower substrate temperature of 140 K. When the CO was evacuated from the cell and the sample reexposed to the UHV environment, this low-frequency absorbance disappeared from the IR spectrum, confirming that this species is very weakly bound to the surface. Reference to Figure 4 shows that as the dehydroxylation temperature is increased, the 2197-C"' v a absorbancegradually shifts to higher frequency and a second peak at an even higher frequency grows in for dehydroxylation temperatures of 800 K and above. Using the curvefitting routine in the Mattson FTIR software, peak positions of 2201 and 2223 cm-1 were determined for thew two vco psalu for the surface dehydroxylated at 1200 K. Kn6dngcr and c+workcrs o h e d similar IR spectra in the CO stretch region (vco = 2195 and 2226 cm-1) and assigned the
K
-21
2197
4
2300
2250
2200
2150
2100
Wavenumber (cm-1) Figwe 4. Infrared spectra of the v c o region for pure y - A I 2 0 3 in UHV following dehydroxylation at the indicated temperature and doring with an equilibrium pressure of 5.0 Torr of CO at 140 K.
former to CO bound on octahedral A l 3 + sites and the latter to CO bound on tetrahedral A13+ sites.25 Following acquisition of the IR spectra, temperatureprogrammed desorptionspectra were acquired for the y-AlzO3 sample dosed with CO. Shown in Figure 5 are the CO TPD spectra from y-Al2O3 dehydroxylated at 475,600,800,1000, and 1200 K.In addition to CO, TPD spectra were acquired for masse3 2 (Hz), 18 (H20), and 44 (COZ), but in each caw the signal was 1-2 orders of magnitude less than that for CO, indicating that Hz, HzO, and C02 were most likelyjust minor surface contaminants. Two important observations can be made based upon the CO TPD spectra shown in Figure 5. As the dehydroxylation temperature is increased, the amount of CO that deeorbs from the surface increases and the temperature at which the rate of maximum CO desorption occurs also increases from 170 K following dehydroxylation at 475 K to -185 K following dehydroxylation at 1200 K. The observed increase in quantity of adsorbed CO with dehydroxylation temperature reflects the fact that theconcentrationof A13+sites and thereforeof adsorbed CO increases, which is in agreement with the CO IR data. Quantitative analysis of the shift of the CO TPD temperature maxima is complicated for porous catalysts due to the likeliood that adsorption equilibrium of CO occurs during the TPD e x p e r i m ~ n t . ~Herz ~ * ~and ~ co-workers carried out a TPD study of CO on Pt/Al2O, in vacuum and found that adsorption equilibrium is approached under conditions similar to Calculations modeling the CO TPD experiment showed that if CO readsorption equilibrium was accounted for, then broadened desorptionpeaksandshiftsofpeakmaxima t o w e r temperature8 were predicted. The CO TPD spectra shown in Figure 5 in fact exhibit peak broadening and a shift to increasing p a k "a as thedehydroxylationtemperature (and, therefore,CO coverage) is increased. This suggests that adrorption equilibrium is being approached as the concentration of CO on the alumina surface incream. Due to this complication, TPD is ured in this work mlely to quantify the amount of CO adsorbed on the diffrrent samples.
-
Diaz and Bussell
The Journal of Physical Chemistry, Vol. 97, No. 2, 1993 I
I
n
I
I
400.0 -
u
: 300.0 -
e
10
P
z0
200.0
-
100.0
-
.e 4
m
ko 4
e
I
0.0 -
I
.
1
.
I
'
'
'
I
"
'
CO Molecules/cmz (x 1 0 - 1 3 ) Figure 7. Correlation of the integrated absorbance in the VOH region of pure y-Al203 with the integrated CO TPD peak areas for the different anneal temperatures.
1 1
I
I
I
I
200
300
400
500
Temperature ( K ) Figure 5. Temperature-programmed desorption spectra for CO on pure yAl2O3 following dehydroxylation at the indicated temperature and dosing with an equilibrium pressure of 5.0 Torr of CO at 140 K. The TPD spectra were acquired in UHV.
t
0.0Ob'
0.0
h
'
2.0
'
I
4.0
'
I
6.0
"
8.0
"
10.0
o
"
12.0
'
I
14.0
i
"
16.0
CO Molecules/cmz Sample Disk (x 1 0 - 1 7 ) , Flgure 6. Beer's law plot for CO on pure y-Al2O3 using the IR and TPD data from Figures 4 and 5, respectively. The CO TPD data are plotted using units of CO molecules per cross-sectional area of the sample disk. The 1200 K data point (open circle) was not included in the linear regression analysis for reasons described in the Results section.
Corrchtion of IR Ud TPD Dnta for y-AI&. Figure 6 is a Beer's law plot for CO on y-Al2O3 using the integrated CO IR intensity (A) from Figure 4 versus the TPD peak areas from Figure 5. In order to facilitate calculation of an extinction coefficient for CO (Lco), the CO TPD data in Figure 6 are plotted using units of CO molecules per cross-sectional area of the sample disk.28 Linear regression of the first four points yields an integrated extinction coefficient for CO of k~ (4.8 f 0.4) X cm/molecule. The data point from the 1200K anneal (open circle) was not included in the linear regression analysis because theCO IR spectrumfor this annealcontains considerableintensity in the vco absorbance at 2223 cm-I which, given the deviation
-
of this point from the straight-line fit, appears to have a different extinction coefficient than the 2197-cm-I vco absorbance. The extinction coefficient determined from Figure 6 is similar to the value published by Ewing and co-workers of ZCO = 5.2 X lo-'* cm/molecule for CO adsorbed on NaCl and slightly larger than the gas-phase value of ZCO = 3.9 X cm/m~lecule.~~ Given that CO is very weakly bound to cations for both yAl2O3 and NaCl surfaces, it is not surprising that the extinction coefficient for CO is little perturbed from its gas-phase value. In addition to the correlation between the CO IR and TPD data, a second correlation, shown in Figure 7, exists between the integrated OH IR intensity and the CO TPD peak areas. This latter correlation is similar to that of Ballinger and Yates between the integrated CO IR and OH intensities, except that we use CO TPD area instead.* Although not shown here, a linear correlation exists between integrated CO IR and OH intensitia for our data; we simply choose to use the CO TPD area instead because it is more readily quantified. Depending on the crystallographicface of y-Al2O3 exposed, the number of OH groups on the surface is (93-145) X 10" OH/cm2.'0 Using the CO TPD peak area of 5.4 X 10" CO molecules/cm2 for the completely dehydroxylated y-A1203sample (1200 K anneal), an approximate ratio of CO molecules to OH group removed of 1:20 can be calculated. If it is assumed that one A13+site is created per two OH group removed, then the CO coverageon the completely dehydroxylated surface in UHV is -0.1 CO/Al3+ at 140 K. CO Adsorption on M003/AljOJ Catalysts. Parallel IR spectroscopy and TPD experiments to those described above for pure y-A1203were also carried out for Mo03/A1203 catalysts with different Mo loadings. In general, the CO IR and TPD spectra were similar to those in Figures 4 and 5, and the correlations between integrated CO and OH infrared intensities with CO TPD areas were equally as good as those shown in Figures 6 and 7. However, important differences were observed in the CO IR and TPD spectra as the Mo loading was increased. Reproduced in Figure 8 is a wries of CO IR spectra for pure y-Al2O3 and Mo03/A1203 catalysts with different Mo loadings after dehydroxylation at 1200 K. As the Mo loading is increased to 20 X lOl3 Mo atoms/cm2, the intensity of the high-frequency uco absorbance decreases to the background level. As discuaPedabove, this CO stretching frequency becomes apparent in the IR spectrum only after high-temperature dehydroxylation ( T,nnar> 800 K) and is associated with OH group with high VOH frequencies as these require the highest annealing temperatures to remove. As will be discussed later, this preferential suppression of the highfrequency vco species correlates nicely with the preferential removal of the high-frequency UOH groups as the Mo loading is
The Journal of Physical Chemistry, Vol. 97, No. 2.1993 475
CO on MoO3/A1203 Catalysts
5.0
-
4.0
-
3.0
-
’3
2.0
-
8
1.0
-
2 I
2
3 X
Y
>
9 u
0.0
‘
400
,
I
I
600
600
1000
J
I
1200
1400
Annealing Temperature ( K )
Flpllre9. Plot of the integratedCO TPDpeak a m versus dehydroxylation temperature for (a) pure y-AlzO3 and M&3/4203 catalpta with Mo
loadings of (b) 15 X lo”, (c) 20 X 10” Mo atom/cm2.
(d) 31 X
0
1200K
0
1OOOK 800K
A
2300
2250
2200
2150
and (e) 42 X
2100
Wavenumber (cm-1) Firpn 8. Infrared spectra of the YCO region for (a) pure y - 4 2 0 3 and (c) 15 X M&3/4203catalystS with Moloadingsof (b) 7.3 X and (d) 20 X 10” Mo atom/cm2 in UHV following dehydroxylation at 1200 K and dosing with an equilibrium pressure of 5.0 Torr of CO at 140 K.
0
0 I
increased. It should be pointed out that no new features are seen in the CO IR spectrum and that the position of the 2197-cm-l v w stretch shifts little with Mo loading. The absence of new features in the CO IR spectra suggests that there is negligible adsorption of CO on MoO3 under the conditions employed. Temperature-programmed desorption spectra for CO on the MoO3/Al203 catalysts were essentially the same as those for pure y-Al2O3; the temperature at which the maximum rate of desorptionoccurred wasin therange 170-185 K. This provides further evidence that under UHV conditions CO adsorbs only on A13+sites at the surface of MoO,/A1203 catalysts at 140 K.The only observed effect of Mo loading was dramatic suppression of the CO adsorption capacity of MoOJA1203 catalysts. Figure 9 is a plot of the CO TPD peak areas for samples with different Mo loadings at the different dehydroxylation temperatures. It is clear that the quantity of CO adsorbed on the catalysts decreases as the loading of molybdenumisi n c r d . Additionally, it should be pointed out that as the Mo loading is increased, the fractional increase in adsorbed CO for the higher dehydroxylation temperatures (e.g., T.-l > 800 K) decreases significantly. This reflects the fact that the higher frequency, &ore strongly bound OH groups are preferentially replaced as Mo loading is increased, and as a result, lower dehydroxylationtemperatures are sufficient to completely dehydroxylate the surface. If the data from 800, 1O00, and 1200 K anneals in Figure 9 along with data from additional samples with different weight loadings are replotted as single temperature slices as shown in Figure 10, a clear break in the decrease in CO adsorption capacity is apparent at a Mo loading of (42 3) X l O I 3 Mo atoms/cm2. Above this loading, no further decrease in the CO adsorption capacity is observed and, therefore, no additional alumina surface is being covered by
-
0 !
1.0
,
1
~
I
*
10.0 20.0 30.0
I
,
l
40.0 50.0
,
1
I .
1~
.
60.0 70.0 t
0
M o atoms/cm2 (x 10-13) Figure 10. Plot of the integrated CO TPD peak arcas versus Mo loading for dehydroxylation temperatures of 800, 1o00, and 1200 K.
MoO3. Thus,asaturationcoverageof (42f 3) X 1013Moat-/ cm2can be assigned for the MoO3 overlayer on y-Al2O3. Above this loading, only three-dimensional growth of bulklike MoO3 crystallites occurs on the alumina support as confiied by X-ray diffractionanalysisof the catalysts.31 Thisvalue for the saturation coverage is in good agreement with the XPS results of Dufrcsne et al. and Hercules and c+workers which indicate that monolayer growth stops at Mo loadings of 40 X lOI3 and 44 X loL3Mo atoms/cm2, respactively.18J9 Our results are not in agreement with the site selective C02 chemisorptionstudies discusssd in the Introduction, and this disagreement will be addressed later. A s m n d important observation which can be made based upon the data plotted in Figure 10 is that a complete monolayer (e+, 100% coverage of the y-Al203 support) is not achieved for the M&3/M203 catalysts. If the CO adsorption capacity in the flat region beyond the saturation coverage is divided by the CO capacity for completely dehydroxylated y-Al2O0,sample, a value of 14 & 5% free alumina surface at the saturation coverage of MoO3 can be calculated. DircuMioo
The major results of this study can be summarized as follows: (1) site selective adsorption of CO can be used to quantify the
Diaz and Bussell
476 The Journal of Physical Chemistry, Vol. 97, No. 2, 1993
free alumina surface of calcined MoO,/A1203 catalysts, (2) the saturation coverage of MoO3 on y-A1203is (42 f 3) X lo1)Mo atoms/cm2, and (3) at the saturation coverage, 14 5% of the ?-AI203 surface remains free of MoO3. These results, while in general agreement with the consensus understanding of the structure of MoOa/A1203 catalysts, provide the first calibration of the absolute coverage of MoO3 on y-Al203. The difficulty in characterizing mixed metal oxide systems such as MoO3/Al203 stems in large part from the lackof selective chemisorption probes which adsorb on the oxide surfaces at easily accessible temperatures and do not undergo irreversiblereaction with the oxide components. As discussed earlier, Ballinger and Y a m have recently establiihed that an empirical correlation exists between the quantity of CO adsorbed on y-MzO3 and the number of OH groups on the surface as determined by IR spectroscopy.6 As hydroxyl groups are removed from the alumina surface by heating in vacuum, the quantity of CO adsorbed on the surface increases. For each pair of OH groups removed by dehydroxylation, one AP+ site is created at the surface and CO weakly chqimrbs in these sites. Our results have confirmed the cotrelation established by Ballinger and Yates and, through the use of CO TPD,shown that the amount of CO adsorbed on the alumina surface can be easily quantified. Since the reaction of MoO3 with 7-A1203involves removal of OH groups from sites on the alumina surface to which the MoO3 is bound, it follows that the remaining OH groups must be in bare alumina patches and a chemisorption probe which is sensitive to the number of OH groups which remain can be used to quantify the surface coverage. The research described in this study suggests that in cases such as MoO,/A1203 in which one of the two constituent oxides can be dehydroxylated,reversible chemisorption of CO on cation sites (e+, A13+)can be used to determine the coverage of the respective oxide species. Our results, while in agreement with XPS results previously published by other^,^^-^^ differ significantly from site selective chemisorption studies using C02.5J4 Both Okamoto et al. and O'Young et al. have utilized room temperature chemisorption of C02 on MoOs/A120p catalysts calcined at 773 K and then outgassed in vacuum at this same temperat~re.~J~ In the case of Okamoto et al., the relative amount of adsorbed COz was measured using the IR intensity of the bicarbonate C-CLH bending mode at 1237 cm-I and was determined as a function of Mo loading.S OYoung et al. utilized both the integrated IR intensity of the symmetric CO stretch at 1485 cm-l of this same bicarbonate species and volumetric C02 uptake measurements for their calibration of free alumina surface as a function of Mo loading. The two studies are in good agreement with complete suppression of C02 adsorption occurring at loadings of 26 X 1013 and 28 X loL3Moatoms/cm2, respectively. On the basis of these "monolayer" loadings, one can calculate cross-sectional areas for the individual MoO3 speciesof 38 and 36 A2. Given the nonlinear dependence of the suppression of C02 adsorption as a function of Mo loading, even larger cross-sectional areas are predicted at lower coverages. At a coverage of 6.8 X 1013Mo atoms/cm2, for which 50% suppression is observed by OYoung et al., a MoO3 cross-sectional area of 73.5 A2 is predicted. Using an idealized model for the surface of y-Al2O3, Sonnemansand Mars calculated cross-sectional areas for tetrahedral and octahedral MoO3of 15.626.6 and 22.0-26.6 A?, respectively, depending upon the crystal face of y-A1203to which the MoO3 species are bound.l6 Using a different model for the structure of the MoO,/Al203 surface, Schuit and Gates computed a surface area of 18 A2 per MoO3 g r o ~ p . ' ~Clearly, , ~ ~ the "monolayer" coverages determined by c02 chemis0rption predict MoO3 structures with unrealistically large cross-sectional areas. In contrast to the C02 chemisorption results, our calibration of MoO3 coverage on y-Al203 is in good agreement with both theoretical predictions and the existing body of experimental evidence. Replotted in Figure 11is the CO TPD data from Figure 10 for the completely dehydroxylated surfaces (1200 K anneal)
\. 8oL \.
100
'
10
QOl
*
i
70
60
e,
50
h
cr,
K
40
30 20 10
0
0.0
10.0
20.0 30.0 40.0
50.0
M o atoms/cmz ( x
60.0 70.0 80.0
IO-13)
Figure 11. Plot of the free alumina surface versus Mo loading luring the CO TPD peak areas from the 1200 K anneal in Figure 10. The vertical axis has been scaled by expressing the CO TPD peak areas of the MOO,/ A1203 catalysts as a percentage of the pure y-Al203 CO TPD peak area.
where the vertical axis now represents the percentage of free alumina on the catalyst surface. Only the 1200 K anneal data are appropriate for this plot as it reflects the true adsorption capacity of the catalysts. At lower anneal temperatures, some hydroxyl groups remain on the free alumina patches and CO chemisorption does not occur on these sites in UHV. The downward sloping line through the data is a linear regression fit of the data excluding the points in the flat region (>45 X 1013 Mo atoms/cm2). If this line is extended to the horizontal axis, corresponding to complete "monolayer" growth, a coverage of (46 4) X loL3Mo atoms/cm2 is calculated for the MOOS monolayer. This experimehtal value is close to the theoretically predicted monolayer coverages of (50-60) X 10" Mo atoms/ cm2 16.17 discussed earlier and corresponds to a cross-sectional area of -22 A2/Mo03 species. Recent LRS investigation of MoO,/Al203 catalysts by Williams et al. indicates that under dehydrating conditions (e.g., UHV) polymericmolybdatespecies spread to form octahedrally coordinated MoO3 units on the surface of ~ - A l 2 0 3 .Thus, ~ under the conditions of our study, one would expect the MoO3 overlayer to consist of octahedral Mo species, and our experimentally predicted cross-sectional area of -22 A2 agreeswell with thevalue of 22.0-26.6 A2per MoO3 unit predicted for octahedral MoO3 by the theoretical model of Sonnemansand Mars.16 The data plotted in Figure 11, in addition to allowing determination of an experimental ymonolaycr" coverage, show that the MoOt coverage on y-Al203 saturates prior to formation of a full monolayer. This saturation coverage occurs at a loading of (42 f 3) X 10') Mo atoms/cm2 and corresponds to -8696 of the alumina surface being covered with MoO3. As dimwed earlier, on the basis of IR and NMR spectroscopic results, Hall and co-workers proposed that full monolayer merage was not achieved but rather that free alumina patches existed even at high loadings. To our knowledge, the results presented here provide the fiqt absolute determination of the amount of free alumina surface at %hesaturation coverage. Interestingly, the fact that 14 5% free alumina remains at high loadings carelates approximately with the decrease in integrated IR OH intensity to -20% of the value of pure y'Al203 for catalysts with loagreater than that corresponding to the saturation coverage. It should be reiterated that the C02 chemisorption studies ruggest a complete monolayer of MoO3 is formed at high loadings which is at odds with the IR data in the OH region. There are at least two possible explanationsfor the differences between our coverage calibration determined using eelective chemisorption of CO and those in which C02 was utilized as the
*
CO on MoO3/Al203 Catalysts chemisorption probe. The first is that some free alumina sites on the MoO3/Al203 catalysts (e+, between MoO3 islands) are simply inaccessible to the bulkier C02 molecule while the smaller CO molecule can adsorb in thest sites. If this is the case, there would be a periphery surrounding MoO3 islands on y-Al203 on which CO2 chemisorption is not possible. This would lead to a lower than expected C02 adsorption capacity and, therefore, a predicted MoO3 cross-sectional area that is too large to be structurally realistic. A second explanation for the apparently low C02 adsorption capacity of MoO,/Al203 catalysts can be formulated based on the surface chemistry of C02 on y-Al2O3. The adsorption of C02 on y-AlzO3 has been studied with IR spectroscopy by numerous investigat~rs.~*~~~~~*~~ Under high-pressure conditions ( P aP 10 Torr), three modes of adsorption have been proposed for ambient temperatures: a physisorbed species, a carbonate speciesweakly bound to one or more A13+sites, and an irreversibly bound bicarbonate species produced by reaction of C02 with OH g r o ~ p s . ~Under J ~ the vacuum conditions employed in the C02 chemisorption studies of MoO3/Al203 catalysts, only the bicarbonate species is present on the free alumina surface. Interestingly, the reaction to produce the bicarbonate species has been found to be site selectivein that the IR intensity of only the three high-frequency VOH absorbances (3769,3732, and 3686 cm-l in our spectrum) are reduced while a new peak grows in at 3616 cm-'which has been assigned to the OH stretch of the bicarbonate species? Lower frequency YOH absorbances are unaffected by adsorption of CO2. Parkym has proposcd that the bicarbonate species is formed via reaction of C02 adsorbed in an A13+ site with an adjacent OH group.37 This is consistent with the fact that the highest VOH frequency absorbances have been assigned to isolated OH groups (i.e., those next to A13+sites).30 Focusing on the MoO,/Al203 system, the selectivitywith which C02 reacts with OH groups to form bicarbonatespecies is important because MoO3 also reacts selectively with these same OH groups as discussed earlier. Therefore, it is possible that the C02 chemisorption capacity is reduced disproportionately with respect to the surface coverage of MoO3 as CO2 chemisorption sites on y-Al203 are eliminated preferentially by the MoO3. Thus, as the Mo loading is increased, the concentration of C02 adsorption sites on the free alumina surface would be reduced with respect to pure y-Al203, giving anomalouslylow COZ adsorptioncapacities for the free alumina patches on Mo03/Al203 catalysts. One final point which needs to be addressed is the fact that the CO adsorption studies described here provide additional evidence to the IR studies of Hall and co-workers which showed that MoO3 reacts preferentially with high stretching frequency OH groups on the y-Al2O3 ~urface.~ The effect of Mo loading on the IR spectrum of y-Al2O3 and MoO3/Al203 catalysts in the YCO region shows that, by a Mo loading of 20 X lOI3Mo atoms/ cm2, the higher frequency CO stretch (2223 cm-') has been eliminated. This is approximately the same loading at which Okamotoet al. observed eliminationof the intensity of the highest frequency VOHabsorbance in their semiquantitative investigation of the effect of Mo loading on the OH stretch region of MoO3/ A 1 2 0 3 catalysts.5 As noted earlier, Zaki and Knbzinger assigned the higher frequency CO stretch (2226 cm-l in their work) to CO chemisorbedon a tetrahedral A13+site.25 It is tempting, therefore, to suggest that, at low Mo loadings where tetrahedral MoO3 speciap predominate, adsorption of Mo occursvia oxygen linkages with tetrahedrally coordinated aluminum atoms on the surface Of r-Al203.
conclolsion The selective adsorption of CO has been used to quantify the coverage of MoO3 on y-Al2O3 as determined by IR spectroscopy and TPD. Monolayer growth of MoO3 occurs on the alumina surface up to a Mo loading of (42 3) X 1013 Mo atoms/cm2, above which three-dimensionalgrowth of MoO3 particles occurs.
The Journal of Physical Chemistry, Vol. 97, NO. 2, 1993 411
At the saturation coverage, 14 f 5% of the alumina support remains uncovered, which is in good agreement with IR studies of the hydroxyl region. Low-temperature CO adsorption should provide a useful probe for determining the location of cobalt atoms in calcined Co-Mo/Al~03 catalysts and may have applicability for probing the surface chemistry of other mixed metal oxide systems.
Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the ACS, for partial support of this research. This research was also supported by an award from the Research Corporation. The authors also acknowledge Don J. Stone and John S.Harold for preparation of the MoO3/ A 1 2 0 3 catalysts as well as P r o f m r Charles T. Campbell for helpful discussions.
Referencea md Notes (1) Satterfield, C. N. Heterogeneous Catalysis in Practice; McGrawHill Book Co.: New York, 1980. (2) Karichoff, S. W. J . Hydraul. Eng. 1984, 110, 1984. (3) Hall, W. K. In ChemistryandPhysicsofSo1idSu~mesIIY;Vonrclow. R., Howe, R., Eds.; Springer-Verlag: Berlin, 1986; p 73. (4) Knbzinger, H. In Proceedings, Ninth International Congress on Catalysis,Calgary, 1988; Phillip, M. J., Ternan, M., Eds.;Chemical IMtitUtC of Canada: Ottawa, 1988; pp 20-53. (5) Okamoto, Y.; Imanaka, T. J . Phys. Chem. 1988, 92,7102. (6) de Beer, V. H. J.; van der Aalst, M.J. M.;Machiels, C. J.; Schuit, G. C. A. J. Catal. 1976,43, 78. (7) Peri, J. B.; Hannan, R. B. J. Phys. Chem. 1960,64, 1526. (8) Ballinger, T. H.; Yates, J. T., Jr. Lungmuir 1991, 7, 3041. (9) Segawa, K. I.; Hall, W. K. J. Catal. 1982, 76, 133. (10) Comac, M.;Janin,A.; Lavalley, J. C. InfraredPhys. 1984,24,143. (11) Hall, W. K. In Proceedings of the Climax Fourth Internotional Barry,H. F., Mitchell, Con/mnnceontheChemistryandUsesofMo~~emtm; P. C. H., Eds.; Climax Molybdenum Co.: Ann Arbor, MI, 1982; p 224. (12) Cirillo, A. C.; Dollish, F. R.; Hall, W. K. J. Catal. 1980, 62, 379. (13) Biaglow, A. I.;Gorte, R. J.; Srinivasan, S.;Datye, A. K. Catal. Lett. 1992, 13, 313. (14) O'Young, C. L.; Yang, C. H.; DeCanio, S. J.; Patel, M. S.;Storm, D. A. J . Catal. 1988,113, 307. (15) Segawa, K. I.; Hall, W. K. J . Catal. 1982, 77, 221. (16) SOnneman~,J.; Mars, P. J. Catal. 1973, 31,209. (17) Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. Chemistry ofcatalytlc Processes; McGraw-Hill: New York, 1979; p 413. (18) Dufresne, P.; Payen, E.; Grimblot, J.; Bonnelle, J. P. J. Phys. Chem. 1981,85,2344. (19) Zingg,D.S.,Makovsky,L.E.;Tischer,R.E.;Brown,F. R.;Hcrcules. D. M.J. Phys. Chem. 1980,84, 2898. (20) Williams,C.C.;Ekerdt,J.G.;Jehng,J.M.;Hardcartle,F.D.;Waclu, I. E. J. Phys. Chem. 1991,95,8791. (21) Ballinger, T. H.; Wong, J. C. S.; Yates, J. T., Jr. Lungmuir 1992, 8. 1676. (22) Basu, P.; Ballinger, T. H.; Yates, J. T., Jr. R w . Scl. Instrum. 1988, 59, 1321. (23) Della Gatta, G.; Fubini, B.; Ghiotti, G.; Morterra,C. J. Catal. 1976, 43,90. (24) Zaki, M.I.; Knbzinger, H. Mater. Chem. Phys. 1987,17,201. (25) Zaki, M.I.; Knbzinger, H. Spectrochfm.Acta 1987,43A, 1455. (26) Falconer, J. L.; Schwarz, J. A. Catal. Reu.-Scl. Eng. I=, 25,141. (27) Hen, R. K.; Kiela, J. B.; Marin, S. P. J. Catal. 1982, 73, 66. (28) For a discussion of the procedure used to calculate h:w: Bell. A. T. In Vibratlonal Spectroscopy of Molecules on Surfmess; Yata, J. T.. Jr., Madey, T. E., Eds.;Plenum Press: New York, 1987; pp 105-134. (29) Richardson, H. H.; Baumann, C.; Ewing, G. E. Surf. Scf. 1987,185, IS. (30) Knbzinger, H.; Ratnasamy, P. Catal. Rw.-Sci. Eng. 1978,17, 31. (31) Unpublished results. (32) Schuit. G. C. A,; Gates, E. C. AIChE J . 1973, 19,417. (33) Pen, J. B. J. Phys. Chem. 1975, 79, 1582. (34) Pen, J. B. J. Phys. Chem. 1966, 70, 3168. (35) Morterra, C.; Zecchina.A.;Coluccia, S.;Chiorino, A. J . Chem.Scc., Faraday Trans. I 1977, 73, 1544. (36) Morterra, C.; Coluccia, S.;Garrone, E.; Ghiotti, G. J. Chem. Soc., Faraday Trans. I 1979, 75,289. (37) Parkyns, N. D. J . Chem. Soc. A 1969,410.
