J. Phys. Chem. 1993,97, 9028-9033
9028
Characterization of Sulfided Mo/A1203 Catalysts by Temperature-Programmed Reduction and Low-Temperature Fourier Transform Infrared Spectroscopy of Adsorbed Carbon Monoxide B. Miiller,t A. D. van Langeveld,* J. A. Moulijn,* and H. Knozinger'J Institut fur Physikalische Chemie, Uniuersitiit Miinchen, Sophienstrasse 1I , 80333 Miinchen, Germany, and Faculty of Chemical Technology and Materials Science, Deljt University of Technology, Julianalaan 136, 2682 BL Deljt, The Netherlands Received: March 30, 1993
Sulfided Mo/A1203 catalysts were studied by temperature-programmed reduction (TPRS) and transmission infrared spectroscopy of adsorbed CO at low temperatures. Reductive elimination of nonstoichiometric and stoichiometric sulfur from edge and corner sites creates coordinatively unsaturated MoZ+sites. C-0 stretching bands at 2100 cm-l and at ca. 2060-2070 cm-l were attributed to CO adsorption complexes on edge and corner sites, respectively. Evidence of morphological changes of MoS2 slabs during H2 treatment at 770 K is provided. IR spectroscopy of adsorbed CO at low temperature provides unique possibilities for the characterization of sulfided catalysts.
thus, be inferred from TPRS results that, in contrast to the stoichiometricsulfur, excess and edge/corner sulfurs have limited One of the main processes in the conversion of crude oil into stability under industrial hydrotreating conditions. This is various products is mild hydrotreating over sulfided molybdenumfavorable in catalysis, and, as a consequence, excess sulfur may or tungsten-based catalyst^.^-^ Undesired heteroatoms, namely, play a role in the hydrodesulfurization mechanism of crude oil S,N, and 0, are removed in this process. The primary goals of molecules. Consequently, the nature of the binding sites for this sulfur and nitrogen removal were and still are the protection of reactive form of sulfur is of major interest. Clearly, the controlled downstream reforming catalysts from sulfur-poisoning and generation and removal of excess and edge/corner sulfurs cracking catalysts from deactivation by adsorption of nitrogencombined with infrared spectroscopyof suitable probe molecules containingmolecules, respectively. Environmentallegislation can is expected to provide detailed information on the nature of cus be expected in the near future to be the driving force for even sites on sulfided catalysts. deeper hydrodesulfurization (HDS) so as to desulfurize the more Nitric oxide has been used extensively as a probe molecule for stable sulfur-containing molecules. A detailed understanding of the characterization of sulfided catalysts.1G1s However, this the reaction mechanism and the possible nature of active sites molecule is not entirely unreactive and may lead to corrosive may play a key role in future developments of new catalysts and chemisorption.1618 Therefore, carbon monoxide has been applied processes. as an alternative. Bachelier et al.19 first demonstrated that CO A convenient technique to characterize sulfided catalysts is adsorption at 273 K correlated linearly with HDS activity for temperature-programmed reduction of the sulfide (TPRS).* With thiophene. Peril0 reported the appearance of a carbonyl infrared this technique, four types of sulfur species have been identified band at 2100 cm-1 when CO was adsorbed on a sulfided Mo/ on sulfided, AlzOs-supported molybdenum catalysts, namely,*v9 A1203catalyst at room temperature. This band was attributed (i) adsorbed HzS; (ii) adsorbed SH groups; (iii) stoichiometric to CO coordinated to Mob+ sites. An analogous band position sulfur; and (iv) excess sulfur, being present in excess of the (2105 cm-I) was later confirmed by Bachelier et a1.20 These stoichiometricsulfide. The existence of this so-called excess sulfur authors also observed a shoulder at 2070 cm-l which they was inferred from the appearance of a characteristic HIS evolution tentatively attributed to CO coordinated to a Mo center in still in TPRS spectra. This peak might also include contributions lower oxidation state. Since CO typically adsorbs onlyvery weakly from edge and corner sulfurs on MoS2 platelets which are expected on cationic sites having low d-electron density, saturation of to be reactive. The amount and thermodynamic stability of the coordination sites usually requires higher pressures. Delgado et stoichiometricsulfur correspondto that of the bulk sulfideM O S ~ , ~ a1.Z1 and Zaki et have therefore studied sulfided Mo-based while the excess sulfur is highly reactive toward hydrogen at catalysts by infrared spectroscopy of CO adsorbed at 77 K. relatively low temperatures. The hydrogenation of the latter can Carbonyl bands were observed in the 2100-21 20-cm-1 range and be observed in almost all TPRS patterns, depending on the details were attributed to Mo2+C0adsorption complexes, the MoZ+sites of the sulfiding procedure. Quite remarkably, the so-called excess presumably being located along the edges of MoSz platelets. sulfur is relatively stable in the absence of hydrogen (e.g. during Angulo et al.23 and Qin et al.24 also reported carbonyl bands near purging with inert gas at 673 K), suggesting that it may concern 21 10 cm-1 on sulfided catalysts, and Maugd and LavelleyZs were sulfur chemisorbed on coordinatively unsaturated (cus) sites on able to determine the integrated molar extinction coefficient for the microcrystalline MoS2 phase.9 This conclusion is also this band. These authors also reported on the thermal stability supported by the correlation between the dispersion of the MoSz of the correspondingadsorption complex relative to CO adsorbed and the maximum amount of this reactive form of sulfur which on the A1203support. can be generated on a given catalyst. A systematic study by CO adsorption of the influence on the Hydrotreating reactions are typically performed at pressures nature of adsorption and potential active sites of parameters such between 1 and 200 bar in the temperature region of 600-670 K, as molybdenum loading, sulfiding conditions, and Hz-treatment the reaction atmosphere essentially consisting of Hz.1 It can, of AlzO3-supported sulfided Mo catalysts is still lacking to the best of our knowledge. The present study therefore aims at filling To whom correspondence should be addressed. this gap by correlating results from low-temperature infrared t Universitit Miinchen. spectroscopy of adsorbed CO with those of TPRS experiments. Delft University of Technologie. 1. Introduction
0022-3654/93/2097-9028$04.00/0
0 1993 American Chemical Society
Characterization of Sulfided Mo/A1203 Catalysts
The Journal of Physical Chemistry, Vol. 97, No. 35, 1993 9029
TABLE I: Composition of Catalysts catalyst code
Mo content,
Mo(0.5) Mo( 1.O)
0.5
atoms/nm2
Mo(2.2)
1.o 2.2
Mo content, W t % MOO3 2.4 4.9 10.8
I
I '
0
7
As
2. Experimental Section 2.1. Catalyst Preparation. High-purity yA1203(Ketjen CK 300, specific surface area 213 m2/g, pore volume 0.51 cm3/g) was used as a support. The particle size fraction of 63-160 Mm was impregnated with aqueous solutionsof (NH4)6Mo70~-6H20 (BDH, spectroscopy grade) by the incipient wetness technique. Subsequently the material was dried isothermally at 353 K for 57.6 X l o 3 s. It was then subjected to thermal treatment by increasing the temperature at a rate of 3.3 X 1 t 2K/s up to 383 K, at which temperature the material was held isothermally for 3.6 X lo3 s. Final calcination was performed by heating the material at a rate of 8.3 X le2 K/s up to 823 K and holding this temperature for 7.2 X lo3 s. The material having the highest Mo loading could only be prepared by a second impregnation after standard drying because of solubilitylimitations in the molybdate solution. The catalyst precursors thus obtained are denoted Mo(x) where x represents the Mo content expressed as Mo atoms per nm2 of the support. Table I summarizes x-values and the corresponding loadings expressed as the weight percent of Moo3 for the three materials under consideration. 2.2. Temperature-ProgrammedReduction of the SuVide. The TPRS technique has been described in some detail by Scheffer et al.z6 Concentrations of H2S and H2 were measured by a UV spectrometer at 195 nm and a thermal conductivity detector (TCD), respectively. The oxidic materials were sulfided in an H2 flow containing 15% HzS, the total flow rate being 13.7 mol/s. The sulfided catalysts were then reduced in a mixture of H2 and Ar (67% H2) at a total flow rate of 9.1 mol/s. The amount of catalyst used was about 50 mg in all cases. A typical TPRS experimentconsistedof the following individual steps: (1) purging the catalysts with Ar to remove 0 2 prior to the sulfidation; (2) sulfiding for 1.8 X lo3sat 293 K; (3) sulfiding at heating rate of 0.167 K/s up to 673 K; (4) isothermal sulfiding at 673 K for 3.6 X lo3 s; (5a) purging in Ar at 673 K for 3.6 X lo3 s to remove adsorbed H2S or (5b) cooling to 293 K in the sulfiding mixture in about 6 X lo3 s and (5c) purging the sulfided catalyst with H2/Ar at 293 K for 7.2 X lo3 s; (6) temperatureprogrammed reduction of the sulfide at a heating rate of 0.167 K/s. 2.3. Infrared Spectroscopy. For transmission infrared spectroscopy, the oxidic catalyst precursors were ground and pressed (200 bar) into self-supporting wafers having a weight of approximately 10-1 5 mg/cm2. The low-temperature infrared transmission cell has been described previou~ly.~~ The cell was connected to a reactor in which in situ sulfidation could be performed. Standard sulfidation conditions of the sample wafers consistedinsteps 1-4and 5boftheTPRSexperiments,thedetailed conditions being reproduced as closely as possible, although the Hz/H2S mixture contained only 10% H2S. Freshly sulfided catalysts were obtained by purging with dry N2 at 293 K for 7.2 X lo3 s followed by evacuation (2 X 10-5 mbar) at 293 K for 1.8 X 103s. The reduction in 100%flowing Hz followed the conditions prescribed in step 6 of the TPRS experiment; however, the reduction temperature was kept constant at selected values for 300 s prior to evacuation at 2 X 1t5mbar and cooling to liquid N2 temperature. Low-temperature carbonyl spectra of adsorbed CO were then recorded prior to continuing the reduction process at the next higher temperature. The transmission spectra of adsorbed CO could be taken at temperatures as low as 77 K under controlled pressure conditions.
250
350
450
550
650
750
temperature, K Figure 1. TPRS spectra of sulfided Mo/Al2O3 at different loadings: (a) Mo(0.5); (b) Mo(l.0); (c) Mo(2.2). Samples were purged in Ar 673 prior to temperature-programmed reduction.
They were recorded at a spectral resolution of 6.8 cm-* on a Perkin-Elmer 580 B spectrometercoupledto a Perkin-Elmer series 3600data station. The relatively low resolution had to be tolerated since the sample wafers had rather low transmission of typically 15%.
3. Results and Discussion 3.1. Temperature-Programmed Reduction of the Sulfided Catalysts. The results of the TPRS experiments with the three catalysts after standard sulfidation (steps 1-5a as described in the Experimental Section) are shown in Figure 1, where the H2S evolution as measured by the UV extinction at 195 nm is plotted against the temperature. The small peak seen at 340 Kin the reduction trace of catalyst Mo(0.5) having the smallest Mo loading, is not accompanied by a simultaneous consumption of H2. This suggests that some H2S was still adsorbed on the catalyst surface, presumably largely on the alumina support, and is simply desorbed rather than being produced in a reduction step. At increasing temperatures, H2S is evolved between 400 and 500 K with simultaneous consumption of H2, indicating the hydrogenation of sulfur species. The relatively sharp peaks appearing in the temperature range between about 410 and 480 K are attributed to the hydrogenation of the so-called excess and edge/corner s u l f ~ r s . ~The . ~ temperatures of maximum hydrogenation rate for the three catalysts are indicated in Figure 1. As the reactive form of sulfur was suggested to be chemisorbed on cus Mor+ sites on the MoS2 phase,9 its removal must liberate these sites and provide coordinative unsaturation. HIS evolution continues at temperatures beyond the reduction peak at 410-480 K (see Figure l), although at temperatures below approximately 770 K the H2S production is not accompanied by H2 consumption. This suggests that the H2S formation stems from condensation of neighboring S H groups on the MoS2 platelets (similar to the known dehydroxylation processes of oxides viacondensation of OH groups) in addition to small contributions from reductive sulfur elimination. Desorption of molecularly adsorbed H2S may also contribute to the H2S evolution. These processes must lead to the creation of cus Mo sites which would most likely comprise pentacoordinated Mo4+centers, as can be expected from the structural characteristics of MoS2 with Mo4+ being located in a prismatic environment of sulfur ligands. Some experimental evidence for the existence of such cus Mo4+centers has been obtained by perturbated angular correlationspectroscopy (PAC) .28 When the reduction behavior of the three catalysts is compared, it can be noted in Figure 1 that the temperature of maximum rate
Muller et al.
9030 The Journal of Physical Chemistry, Vol. 97, No. 35, 1993
8 0
f a 8
t
.:
2300
. ,
,
2200
.-
,
2100
k. P1
2000
2300
1900
of hydrogenation of the excess and edge/corner sulfurs shifts to lower temperatures when the Mo loading increases. These results are almost identical to those reported earlier by Scheffer et a1.z6 Since the reductive elimination of sulfur particularly in the temperature range 400-500 K most probably involves several forms of sulfur such as nonstoichiometric sulfur, stoichiometric sulfur located at edges and corners, and perhaps disulfide species and SH groups, we prefer to speak about reactive sulfur in the present context. 3.2. InfraredSpectraof Adsorbed Carbon Monoxide. Infrared spectra of C O were taken a t equilibrium adsorption pressures up to 75 mbar in the temperature range between 77 K and room temperature. CO adsorption on all threecatalystswas performed after sulfidation prior to any reductive treatment and after reduction in flowing Hz at critical selected temperatures as suggested by the TPRS results. These temperatures were 353, 453, and 773 K. As can be seen in Figure 1 and as was discussed above, thermal treatment a t 353 K only removes adsorbed HIS without any detectable reduction; hence reactive sulfur must still be present after this treatment. Reductive elimination of the corresponding sulfur species occurred in the temperature range 370-550 K. The corresponding cus MoX+ must therefore be accessible for CO adsorption after reduction at 453 and/or 523 K. Finally, additional cus MoX+(and A P ) sites are expected to be present after reduction a t 773 K due to condensation of SH groups. For comparison purposes, a pure y-A1203sample was sulfided according to the mentioned standard conditions. The carbonyl IR spectra of CO adsorbed at 77 K and 75 mbar are shown in Figure 2. Only an extremely weak band is seen near 21 50 cm-1 on the freshly sulfided alumina, while bands a t 2154 and 2184 cm-l grow in after HZtreatments a t 423 and 773 K, respectively. The first of these bands can be attributed to CO molecules being H-bonded to surface hydroxyl groups of the y - A l ~ O 3 . ~As~ the -~~ intensity of the 21 54-cm-l band does not significantly increase when the reduction temperature is brought from 353 to 773 K, alumina OH groups are obviously largely freed from adsorbed HzS at the lower temperature. In contrast, the intensity of the band at 2184 cm-l does increase after reduction a t 773 K, indicating adsorption sites which bind H,S more tightly. These are cus A13+sites, as indicated by the carbonyl stretching frequency of the coordinated C0.30-33This result suggests that cus Al3+ sites may also be created on the molybdenum containing catalysts (at least at loadings significantly below monolayer coverage) when the reduction is carried out at 773 K. CO Adsorption on Mo(2.2). The CO adsorption will first be discussed for the catalyst Mo(2.2) which contains 10.8 wt % Moo3 corresponding to approximately 70% of the theoretical monolayer capacity of the y-Alz03support used. Figure 3 shows
2200
2100
wavenumbers,
cm-’ Figure 2. Carbonyl IR spectra of adsorbed CO (75 mbar) on y-AlzO3 freshly sulfided (a) and after H2 reduction at (b) 423 K and (c) 773 K. Spectra recorded at 77 K. wavenumbers,
2000
1900
cm-’
8 0
f
a
8
2300
2200
2100
wavenumbers,
1900
2000
cm-‘
I 0,
C
K I
a C
b
-a 2300
2200
2100
2000
1900
cm-‘ Figure 3. Carbonyl IR spectra of CO adsorbed on Mo(2.2) at 77 K and pressures of 5 (a), 25 (b), 50 (c), and 75 mbar (d): (A) catalyst freshly sulfided; (B) catalyst H2 reduced at 453 K, (C) catalyst H2 reduced at wavenumbers,
773 K.
the development of the carbonyl spectra as the C O pressure is increased at 77 K. The first band that appears and saturates already a t low pressure on the freshly sulfided catalyst (Figure 3A) is located at 2105 cm-I. Bands at approximately this position had been observed earlier and attributed to Moa+ CO1OJO or Mo2+ C021,22complexes. Hence, some cus sites with Mor+ in low oxidation state, x presumably being 2, must be exposed even after mere evacuation. The only additional band that continuously grows in intensity as the pressure is increased is the band at 21 54 cm-’ of H-bonded CO. There may be an indication of physically adsorbed CO by a weak absorption near 21 30 cm-1. The carbonyl spectra are essentially identical when the catalyst is exposed to flowing Hz at 353 K (see Figure 4) since no reduction did occur a t this low temperature as indicated by the TPRS experiment (vide supra).
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Characterization of Sulfided Mo/A1203 Catalysts
The Journal of Physical Chemistry, Vol. 97, No. 35, 1993 9031 appear. The only explanation for the disappearance of the shoulder after high-temperature reduction-if it was to assign to Moo CO species-is an excessive aggregation to very large metal particles exposing an extremely small metal surface area. This interpretation appears to be rather unlikely, the more so as Wambeke et al.35 have shown that reduction to metallic Moo occurs at reduction temperatures greater than 973 K. Also, the present TPRS results did not provide any evidence for Moo formation at temperatures below 800 K. We therefore prefer an alternative though tentative explanation. Kasztelan et al.36 and Wambeke et al.35 have developed a geometrical model for highly dispersed sulfided Mo/A1203 catalysts according to which the morphology of the supported MoS2 can, e.g., be described as hexagonal platelets. These expose (lOi0) and (1010) edge planes in addition to the (0001) basal planes. Different types of sulfur and molybdenum ions having different local environments along the edges and corners of the slabs can be distinguished.37.38 Upon reduction, the lowcoordinated S2- and SH- ions at edge and corner positions are removed and cus MV+ in low oxidation states will be exposed.35 Wambeke et have also suggested that the most likely formal oxidation state of the cus Mo sites is x = 2. These Mo2+ sites will occur in a variety of different degrees of unsaturation (depending on the reduction temperature) and different coordination environments with regard to the local environment of the remaining sulfur neighbors. It is known31933that not only the oxidation state but also the coordination number of the central ion determines the carbonyl stretching frequency of coordinated CO ligands in a purely electrostaticchemisorption model. In addition, r-back-donation may become efficient as the coordinative unsaturation increases, when the coordination site is a transition metal ion in a low oxidationstatesuchas,e.g., Md+(d4). Comer siteson thereduced MoS2 slabs will provide the highest unsaturation. We therefore propose that the shoulder appearing in the carbonyl spectra at 2060 cm-l after mild reduction at 453 K is due to CO molecules coordinated to highly cus Mo2+ sites at corner positions. The principal band at 2105 cm-I is then to be attributed to CO molecules bonded to less unsaturated Mo2+sites, probably being located along edges. When the reduction temperature is increased to 773 K, it is feasible that restructuring of the slabs and possible slab growth occurs. This would lead to a decrease of the number of corner sites relative to edge sites and might lead to an erosion of the 2060-cm-1 shoulder in the carbonyl spectra. We therefore infer that both the principal carbonyl band at 2105/2110 cm-l and the shoulder at 2060 cm-l are indicative of Mo2+ CO complexes with the Mo2+sites being distinct with regard to their coordinative unsaturation. The lower frequency is supposed to be due to enhanced r-back-donation becoming possible by improved orbital overlap in the more open coordination sphere of the more highly cus corner sites. It has been mentioned above that the absorbances at peak maximum of the principal Mo2+ CO band at 2105/2110 cm-l were essentially the same after reduction at 453 and 773 K (see Figure 4) under saturation conditions at 77 K. Although the positions of the peak maxima of this band do not vary much for the different treatments, the thermal stabilityof the corresponding Mo2+ CO complexes seems to increase slightly with reduction temperature. This is demonstrated by the series of spectra shown in Figure 5 which were recorded at 75-mbar equilibrium CO pressure and at approximately 210 K. Clearly, only very weak absorptions remain under these conditions for the freshly sulfided catalyst and after H2 treatment at 353 K (spectra a and b in Figure 5). Also, the absorbance at peakmaximum after reduction at 453 K (spectrum c in Figure 5) is now only about 75% that measured after reduction at 773 K (spectrum d of Figure 5). This effect may be due to the formation of cus Mo2+ with somewhat
-
2300
2200
2100
ZOO0
1900
wavenumbers, cm-‘ Figure 4. Carbonyl IR spectra of CO (75 mbar) adsorbed on Mo(2.2) at 77 K after (a) sulfidation, (b) Hz reduction at 353 K, (c) Hz reduction at 453 K, and (d) Hz reduction at 773 K.
A much stronger band at 2105 cm-1 (note the different absorbance scales in Figure 3A and 3B) develops when Mo(2.2) was reduced at 453 K and the reactive sulfur had been eliminated indicating that the density of Mo2+sites had been increased by this treatment. Most interestingly, a pronounced shoulder also occurs at 2060 cm-l. This absorption already saturates at 5 mbar CO at 77 K, suggesting a rather stable adsorption complex, this also being supported by its relative thermal stability as the temperature is increased at constant pressure. The possible nature of this species will be discussed below. Relatively weak bands of Al3+ + CO (2190 cm-I) and of H-bonded CO (2153 cm-l) are also seen as expected. These latter bands (slightlymore pronounced) are also observed after reduction at 773 K (Figure 3C) with the most prominent band of CO coordinated to cus MoX+ now being located at 21 10 cm-1. Pressure and temperature dependences of the relative intensities of these bands clearly show that the Mor+ CO complex is the most stable. In contrast to the lower reduction temperature of 453 K, the shoulder below 2100 cm-l does not appear after reduction at 773 K although the band at 21 10 cm-l has a significant tailing toward lower wavenumbers. The absorbances at peak maximum of the bands at 2105 and 21 10 cm-1, respectively, of the samples being reduced at 453 and 773 K are very similar. This correlates nicely with TPRS results, indicating that the sites which bind reactive sulfur are already liberated at 453 K and no or only minor reduction occurs at higher temperatures up to 773 K. For better comparison, the final spectra obtained at 75 mbar CO and 77 K after the various treatments are shown in Figure 4. The formation of a carbonyl species that absorbs at 2060 cm-l exclusively on the catalyst that was reduced at 453 K is intriguing. Bachelier et al.20 have first reported a shoulder at 2070 cm-l on the low-frequency flank of the 2105-cm-l band of sulfided Mo/ A1203 catalysts after evacuation. These authors attributed this shoulder to CO molecules coordinated to Mo sites “in an even more reduced state” than the Moa+ sites giving rise to the 2105-cm-1 band. When oxidic molybdena catalysts were reduced in H2 at elevated temperatures, bands between 2025 and 2080 cm-1 were reported by Peril0 and at 2000 and 2070 cm-l by DeCanio and Storm,34 which were assigned to CO chemisorbed on zero-valent Moo sites. The shoulder at 2060 cm-I observed in the present study of sulfidedcatalysts after reduction at 453 Kmight also be attributed to Mor+ CO complexes with x < 2 if the band at 2105 cm-1 is due to Mo2+ CO complexes. Hence, the presence of Moo sites might be envisaged. However, the shoulder is not seen after reduction at 773 K although the higher temperature would be expected to lead to deeper reduction. The intensity of the 2060-cm-1 shoulder should therefore increase rather than dis+
--
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9032
IT
g 5 I
0.1
na
I1
8
6
e y = 166q
2300
2200
2100
ZOO0
1900
cm-' Figure 5. Carbonyl IR spectra of CO (75 mbar) adsorbed on Mo(2.2) at 210 K after (a) sulfidation, (b) Hz reduction at 353 K, (c) Hz reduction at 453 K, and (d) H2 reduction at 773 K. wavenumbers,
0
s e
d
n
C
2300
2200 2100 ZOO0 wavenumbers, cm-'
1900
Figure 6. Difference of carbonyl spectra of CO adsorbed on Mo(2.2) at 75 mbar and 25 mbar (77 K) (a) after sulfidation, (b) after Hz reduction at 353 K, (c) after H2 reduction at 453 K, and (d) after Hz reduction at 773 K.
different average local environments at the two reduction temperatures which may influence the bond energy of the coordination bond without detectably altering the carbonyl frequency. The TPRS experiments (vide supra) had suggested that cus Mod+ sites may also be created by condensation of neighboring SH- groups. The spectra shown in Figures 3-5 do not provide any immediateevidenceofcarbonyl bands that might be attributed to Mo4+ CO complexes. Unfortunately, the characteristic carbonyl stretching frequency of these complexes is expected in the frequency range 2180-2190 cm-1,21,22 very close to that of A13+ CO complexes. The spectra of Figure 3C show that the bandat21 lOcm-lsaturatesat25mbarat77K(equaIabsorbances at peak maximum at 25 and 75 mbar of CO). The same is obvious for the band at 2190 cm-1 which would be attributed to the Al3+ CO complex. However, the depth of the valley between this band and the band of H-bonded CO at 2155 cm-l appears to be reduced, and an asymmetry at the low frequency flank of the band at 2190 cm-1 seems to develop as the pressure is increased. Difference spectra (spectrum d minus spectrum b of Figure 3C) have therefore been inspected, and an example is shown in Figure 6. Neither the band at 2110 cm-1 nor that at 2190 cm-1 are detectable. Only the band of the H-bonded CO at 21 55 cm-1 and anew bandat 2180cm-lshowpositiveabsorbanceinthedifference spectrum. The latter band may well provide evidence for the existence of cus Mo4+ sites which are occupied by CO as the pressure increases. Much smaller site densities of Mo4+ were found for the catalyst reduced at 453 K, and no evidence for their
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Miiller et al.
The Journal of Physical Chemistry. Vol. 97, No. 35, 1993
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4
2300
2200
2100
ZOO0
1900
cm-' Figure 7. Carbonyl IR spectra of CO (75 mbar) adsorbed on Mo( 1 .O) at 77 K after (a) sulfidation, (b) Hz reduction at 353 K, (c) Hz reduction at 453 K, and (d) Hz reduction at 773 K. wavenumbers,
2300
2200
2100
ZOO0
1900
wavenumbers, cm-'
Figure 8. Carbonyl IR spectra of CO (75 mbar) adsorbed on Mo(0.5) at 77 K after (a) sulfidation, (b) Hz reduction at 353 K, (c) H2 reduction at 453 K, and (d) Hz reduction at 773 K.
existence at lower reduction temperature or on the evacuated catalyst could be obtained. Effect of Mo Loading. When the Mo loading was reduced (catalysts Mo( 1.O) and Mo(0.5)), the general appearance of the carbonyl IR spectra of CO adsorbed at 77 K is very similar to those recorded for Mo(2.2) under identical conditions. Series of spectra for the four standard pretreatments of catalysts Mo( 1.O) and Mo(O.5) are shown in Figures 7 and 8, respectively. Due to the low loadings, namely, 4.9 and 2.4 wt % Moo3corresponding to 30 and 15%of the theoretical monolayer capacity, respectively, the absorbances of the characteristic bands of AP+ CO and of H-bonded CO are relatively high. Furthermore bands appear at 2103 and 2100 cm-l, respectively, on Mo(1.0) and Mo(0.5) suggestive of the presence of Mo2+ CO species. Interestingly, spectrum c in Figure 7 of the Mo( 1.O)catalyst after reduction at 453 K also shows a shoulder, though weak, at 2060 cm-1, which disappears upon reduction at 773 K. Simultaneously, the absorbance of the band at 2105 cm-l dramatically decreases. This observation is consistent with the assumption of restructuring and slab growth at the higher reduction temperature. Further support for morphological changes of the dispersed MoS2 under high-temperature Hz treatment is provided by the fact that prolonged thermal treatment for up to 10 h further decreases the absorbance of this band. Mo4+sites appear to exist in addition to the majority MoZ+ sites also on Mo( 1.O) after reduction at 773 K, and to a lesser extent after reduction at 453 K, as evidenced by difference spectra. As shown in Figure 8, catalyst Mo(0.5) behaves very similarly. The relative abundance of Al3+ CO and H-bonded CO species
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Characterization of Sulfided Mo/A1203 Catalysts
The Journal of Physical Chemistry, VoL 97, No. 35, 1993 9033
increases and that of Mo2+ CO species decreases further as compared to Mo( 1.0) as a result of the lower molybdenum loading. It is interesting to note, however, that a shoulder at 2060 cm-l is not detected after reduction a t 453 K (spectrum c of Figure 8), probably due to the low abundance of highly cus Mo2+corner sites which might be related to different slab morphologies a t these low loadings. The decreasing absorbance at 2100 cm-l is still evidence for restructuring and possible slab growth at 773 K (spectrum d in Figure 8).
Acknowledgment. The work done in Delft was supported by the Commision of European Communities, under Contract no. JOUF 0049.C. The authors gratefully acknowledge Mr. H. Muller for the TPR measurements and Dr. Xu Xiadeng for the preparation of the catalysts. The work done in Munich was financially supported by the Deutsche Forschungsgemeinschaft (Sonderforschungsbereich SFB 338), the Bundesminister fur Forschung und Technologie, and the Fonds der Chemischen Industrie.
4. Conclusions
References and Notes
Sulfided Mo/A1203 catalysts contain reactive sulfur species which may consist of nonstoichiometric chemisorbed (excess) sulfur but also of S2-, SH-, and perhaps Sz2- species being located at edges and corners of MoS2 slabs. This reactive sulfur can be removed by H2 treatment a t temperatures between 400 and 500 K. Cus M@+sitesin reduced oxidation state become thus exposed. In addition, condensation of SH-groups occurs in the temperature regime 500-770 K so that cus M o ~ +sites also become exposed. Carbon monoxide adsorption a t low temperature provides the unique possibility to detect and characterize these M e + sites with respect to their (formal) oxidation state and coordination unsaturation. The sites being created by reductive elimination of the reactive sulfur species lead to the formation of CO adsorption complexes that are characterized by a C O stretching band a t ca. 2100 cm-*. This band is attributed to a MoZ+ CO adsorption complex which should be lcoated at edge positions of the MoS2slabs. A shoulder appearing at ca. 2060-2070 cm-l after mild reduction (453 K) is attributed to a second MoZ+ CO complex, the Mo2+ being located a t corners. It has been shown that CO adsorbs on ionic oxides via electrostatic interaction^.^^,^^ This type of interaction leads to frequency shifts to higher values relative to the gas-phase frequency (2143 cm-l). The characteristic C-O stretching frequencies observed for C O coordinated to Mo sites in the present and previous studies were below the gas-phase frequency. This must be due to contributions of n-back-donation from Mo d-orbitals to the antibonding orbitals of C O which requires Mo centers in low valence state. n-Back-donation becomes possible here because of the highly covalent Mo-S bond character. The necessary orbital overlap for efficient n-back-donation requires a sufficiently close approach of the C O probe molecule at the coordination site, this being possible at cus sites along the edges and even closer at corners of the slabs. On the basis of these arguments, it is inferred that the two observed sites are to be attributed to cus Mo2+ edge and corner sites. Since the corner sites disappear a t higher reduction temperatures (773 K), it has been concluded that morphological modifications and perhaps slab growth occur a t these higher temperatures, thus leading to a decreasing number of corner relative to edge sites. Very similar phenomena are observed for catalysts containing 70 and 30% of a Moo3 monolayer in the oxide precursor material. When the loading was reduced to only 15% of a monolayer of MOOJ,the corner sites were no longer detected. This may be due to the low density of sites, perhaps below the limits of detectability of the method, or else to morphological differences of the MoS2 slabs relative to those formed at higher loadings. It may be speculated that sites of the type characterized in this work could be involved in HDS catalysis on unpromoted Mobased catalysts. As discussed above, the Mo2+ sites are produced by reductive elimination of the reactive sulfur. Sulfur being produced during catalytic HDS might well be bonded to these sites. The hydrogen present under reaction conditions at temperatures similar to the reduction temperatures used in the present study would then remove the sulfur from the sites and make them accessible for the reactants again.
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