2016
J. Phys. Chem. 1989, 93, 2016-2018
surfaces to the top-layer rhenium atoms. It was also noted that the HDS activity of the rhenium surfaces did not correlate with surface openness but instead could be related to the coordination numbers of the top layer rhenium atoms. In Figure 10, the HDS activity of the rhenium single-crystal catalysts is plotted as a function of the coordination number of the top layer rhenium atoms (see Figure 2). The “volcano” shaped curve shown in Figure 10 is common throughout heterogeneous catalysis and can be understood by relating the catalytic activity to the chemisorption energy of species involved in the r e a ~ t i o n . ~If ’ we assume that the active sites for thiophene HDS on the rhenium surfaces are composed of the top-layer rhenium atoms, the trend in activity can be explained. On the left side of the curve, a C6 surface atom is the most coordinately unsaturated compared to the other toplayer surface atoms and would be expected to bind species involved in the reaction the most strongly. Although the coverage of reactant and intermediate species will be high, the species may be so stable that they do not decompose into products, therefore causing the Re( 1121) surface to have a relatively low activity. In addition, this surface is the most likely to experience product inhibition due to strong binding of product molecules on the active sites. At the other extreme, a C9 surface atom is the most coordinately saturated and will therefore be expected to interact the weakest with species involved in the reaction. As a result, the coverage of reactant and intermediate species will be lower and the surface will be less able to “activate” thiophene molecules. Therefore the turnover frequency of the Re(0001) surface would also be expected to be low. In addition, the close-packed structure of this surface may sterically hinder adsorption of reactant species. The most active surfaces, the Re( lOi0) and Re( 1020) surfaces, are those that have top layer atoms with coordination numbers (C,, C,) intermediary between the two extremes described above. (31) Gasser, R. P. H. Introduction to Chemisorption and Catalysis by Metals; Clarendon Press: Oxford, 1985; pp 185-190.
Unfortunately, it is not possible to directly determine the chemisorption energies of thiophene on the different surfaces as thiophene decomposes to carbon, sulfur, and hydrogen instead of desorbing molecularly. However, desorption energies have been determined for sulfur on the Re(0001) and Re(lOi0) surfaces. At low coverages, sulfur desorbs from Re(0001) at 1600 K (Ed = 98 kcal/mol) and from Re(lOT0) at 1700 K (Ed = 104 kcal/mol).12 Sulfur is believed to be adsorbed on 3-fold sites formed between three C9 atoms on the Re(0001) surface and on 3-fold sites formed between two top layer CB atoms and one second-layer C,, atom on the Re( lOi0) surface. Clearly, sulfur chemisorbs more strongly on Re( lOiO), which may indicate that thiophene and other species in the reaction pathway also bind more strongly to this surface.
5. Conclusion We have shown that rhenium single crystals and polycrystalline foils are active catalysts for the hydrodesulfurization of thiophene. Both adsorbed sulfur and carbon block active HDS sites on the rhenium surfaces, indicating that the active catalyst surfaces are free of strongly bound deposits of sulfur and/or carbon. Thiophene HDS is a structure-sensitive reaction over rhenium, in contrast to earlier work over molybdenum for which the reaction was found to be structure insensitive. The rhenium single crystals studied were found to be 1-6 times more active than low Miller index planes of molybdenum ((loo), (1 lo), (1 11)) which may be due to their ability to remain free of irreversibly bound deposits of sulfur and carbon while HDS over the molybdenum catalysts occurs on a strongly bound overlayer composed primarily of adsorbed carbon. Acknowledgment. This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U S . Department of Energy under Contract No. DE-AC03-76SF00098. Registry No. Thiophene, 110-02-1;rhenium, 7440-15-5.
Effects of Alkali-Metal Promoters (Potassium and Cesium) on a Mo/y-Al,O, Catalyst Chi-Lin O’Young Texaco. Inc.. P.O.Box 509, Beacon, New York 12508 (Received: May 6, 1988; In Final Form: August 1 , 1988)
Effects of alkali-metal promoters (Alk = K and Cs) on the surface structure of a Mo/y-A1203 catalyst were studied by TGA, NO chemisorption,and FT-IR techniques. The results clearly reveal that two levels of Alk-Mo interaction can be distinguished, depending on the atomic ratio, Alk/Mo. At low atomic ratios, Alk/Mo < 1, the Alk-Mo interaction is not strong enough to suppress the reducibility of molybdenum. The NO uptake decreases mainly due to the site blocking effect of alkali-metal promoters. However, the interaction is sufficient to perturb the electronic structure of molybdenum and to cause significant changes in the IR spectra of adsorbed NO species, such as the NO band position, bandwidth, and bond angle of the Mo(NO)~ dimer. The NO band intensities decrease more rapidly than the NO uptake, indicating that there is a significant decrease in the extinction coefficient of NO bands upon the addition of a small amount of alkali-metal promoters. Consequently, the IR band intensity might be no longer a reliable measure of the quantitative adsorption when the position and shape of bands are changed. At high atomic ratios, Alk/Mo > 1, the Alk-Mo interaction increases. Both the reducibility and NO uptake are suppressed significantly. The NO band intensities almost vanish. The interaction reaches a maximum after Alk/Mo > 2, and the reducibility is suppressed to zero. It is postulated that K2Mo04 or Cs2MoO4might form on the surface.
Introduction Alkali-metal ions, mainly potassium and cesium, are used as promoters in many catalysts to improve the selectivity and activity. For example, potassium is used in Fischer-Tropsch iron catalysts to increse the olefin selectivity and the chain growth probability.’.*
Similar effects have been observed for the alcohol synthesis from synga$s4 the alcohol selectivity and the chain growth probability are increased with alkali-metal promoters. Alkali-metal promoters also increase the activity of ammonia synthesis over transitionmetal catalysts5 and the activity of the water gas shift reaction
( 1 ) Storch, H. H.; Golumbic, N.; Anderson, R. B. The Fischer-Tropsch and Related Synthesis; Wiley: New York, 1951. (2) Dry, M. E. In Catalysis, Science, and Technology; Anderson, J. R., Boudart, M., Eds.) Springer-Verlag: New York, 1981; Vol. 1, pp 159-256.
(3) Natta, G.; Colombo, U.;Pasquon, I. Catalysis; Emmett, Rheinhold: New York, 1957; Vol. 5, p 141. (4) Smith, K. J.; Anderson, R. B. J . Catal. 1984, 85, 428. (5) Aika, K.; Hori, H.; Ozaki, A. J . Catal. 1971, 27, 424.
0022-3654/89/2093-20 16$01.50/0 0 1989 American Chemical Society
P.H., Ed.;
Effects of Alkali-Metal Promoters on Moly-AlO,
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989 2017
TABLE I: Reducibility, NO Chemisorption, and IR Spectral Parameters of Alk/Mo/y-AI20, Catalysts' NO uptake, IR int v(sym). cm-' Alk wt,b % Alk/Mo' redd cm3 at STP/g of cat. NO/Mo relat wavenumber fwhm' K K
K K
cs cs cs cs
cs cs
K2M004 CS~MOO~
0
0.00
1 3 6 10 1 3 6 10 15 20
0.36 0.93 1.76 2.88 0.07 0.24 0.50 0.84 1.48 2.18 2.00 2.00
56.90 53.30 52.00 19.30
0.00 53.40 53.20 53.80 53.50 32.91
0.00
7.872 5.828 4.252 1.SO2 1.337 6.383 5.052 4.402 4.1 14 3.258 1.939
0.37 0.31 0.23 0.08 0.08 0.31 0.25 0.23 0.22 0.19 0.12
100.0 19.0 7.8 1.5 1.1 63.4 25.4 7.9 5.2
v(asym), cm-I wavenumber fwhm
O/deg
1814 1804 1786
38 41 62
1708 1694 1657
66 86 226
106.2 112.2 115.0
1811 1802 1785 1771
39 45 46 58
1701 1687 1657 1608
66 84 93 200
108.4 110.6 114.7 122.4
0.00 0.00
'All catalysts were reduced at 500 OC for 3 h. bNominal weight percent of alkali-metal ions. cBased on elemental analyses. dReducibility measured bv TGA after 3-h reduction. cfwhm = full width at half-maximum and is given in reciprocal centimeters. 'Calculated by the equation,
over molybdena catalysts.6 The effects of alkali-metal promoters on catalysts have been studied by different groups.61o In general, it is believed that the alkali-metal ions can donate electrons (+I effect) to the transition metals to enhance their back 7r bonding capability. But the exact nature and role of alkali-metal promoters is still not well understood. Molybdena catalysts have been used in many important industrial reactions. These catalysts can also produce either hydrocarbons or mixed alcohols from syngas depending on the type of promoter and the reactor In the previous report,14we studied the surface structure of Moly-A1203catalysts by using different techniques. To extend the study, we have investigated the effects of alkali-metal promoters on the surface structure of a 9 wt % Mo/y-A1203 catalyst by using thermogravimetric analysis (TGA), N O chemisorption, and Fourier transform infrared (FT-IR) techniques. The results clearly reveal that two levels of Alk-Mo interaction can be differentiated depending on the atomic ratio, Alk/Mo. Experimental Section A 9% Mo/ y-A1203catalyst was prepared by an aqueous incipient wetness impregnation t e ~ h n i q u e . ' ~Ammonium heptamolybdate, (NH4)6M07024-4H20, was used as the molybdenum salt. Norton 6375 y-A1203with a surface area of 216 m2/g and a pore volume of 1.1 cm3/g was used as the support. After impregnation, the catalyst was dried at 120 OC for 16 h and calcined at 500 OC for 3 h under a stream of air (60 cm3/min). This catalyst has approximately a monolayer coverage of molybdenum on the support. Aqueous solutions of K2C03 and Cs2C03with the desired concentrations were subsequently added on the catalyst to form Alk/Mo/y-A1203 catalysts. The catalysts were then dried and calcined at 500 OC. Final alkali-metal and molybdenum compositions of the catalysts were measured. X-ray diffraction (XRD) of the catalysts was studied to see the formation of any bulk molybdenum phases. The reducibility of molybdenum in Alk/Mo/y-Al,O, catalysts was measured by TGA techniques. A 100% reducibility means all of the Mo6+03is reduced to Mo4+02and dehydrated. Detailed procedures of TGA, NO chemisorption, and FT-IR studies were described previo~sly.'~ (6) Kantschewa, M.; Delannay, F.; Jeziorowski, H.; Delgado, E.; Eder, S.;
Ertl, G.; Knozinger, H. J . Catal. 1984, 87, 482. (7) Dry, M. E.; Shingles, T.; Boshoff, L. J. J . Catul. 1972, 25, 99.
(8) Gonzalez, R. D.; Miura, H. J . Catal. 1982, 77, 338. (9) King, D. L.; Peri, J. B. J . Cutal. 1983, 79, 164. (10) Rankin, J. L.; Bartholomew, C. H. J . Curul. 1986, 100, 533. ( 1 1) Murchison, C. B. Proceedings, 4th International Conference on the Chemistry and Uses of Molybdenum; Climax Molybdenum Co., Ann Arbor, MI. 1982. II 197. (12) C o k h a , B. E.; Bartholomew, G. L.; Bartholomew C. H. J . Cutal. 1984, 89, 536. (13) Tatsumi, T.; Muramatsu, A.; Tominaga, H. Appl. Cutal. 1987, 34, 77. (14) O'Young, C.-L.;Yang, C.-H.; DeCanio, S. J.; Patel, M. S.; Storm, D. A. J . C a r d 1988, 113, 307.
Results and Discussion Table I shows the results of reducibility, NO chemisorption, and IR spectral parameters. For the potassium promoter, the reducibility of molybdenum is almost unchanged at low loadings, K I 3 wt %. At high loadings, K 2 6 wt %, the reducibility decreases rapidly and reaches zero at 10 wt %. The N O uptake and NO/Mo ratio decrease slowly before K I 3 wt % and then decrease rapidly at high loadings. For the cesium promoter, the reducibility is unchanged at low loadings, Cs I10 wt %. It decreases rapidly at high loadings, Cs 2 15 wt %, and reaches zero at 20 wt %. The NO uptake decreases slowly before Cs I 10 wt % and then decreases rapidly at high loadings. On a weight basis, potassium is more effective in suppressing the reducibility and N O chemisorption. But on an atomic ratio basis, Alk/Mo, both potassium and cesium have about the same capability for suppressing the reducibility and NO uptake. The results suggest that there are two levels of Alk-Mo interaction, depending on the atomic ratio. At low atomic ratios, Alk/Mo < 1, the interaction is not strong enough to suppress the reducibility. It is believed that the decrease in both NO uptake and NO/Mo ratio at low atomic ratios is mainly due to the site blocking effect of alkalimetal promoters. At high atomic ratios, Alk/Mo > 1, the interaction increases and the reducibility, NO uptake, and NO/Mo ratio are suppressed significantly. The interaction reaches a maximum after Alk/Mo > 2, and the reducibility is suppressed to almost zero. One possible explanation for the suppression of the reducibility is due to the formation of an alkali-metal molybdate species. When Kantschewa et aL6 studied the effect of potassium on a NiMo/y-A1203 catalyst, they observed that at K/Mo = 2.5, the original octahedral coordination of molybdenum was transformed to a tetrahedral coordination and the reducibility of molybdenum was strongly decreased. The formation of a stoichiometric compound, such as K2M004, at high atomic ratios was proposed. K2Mo04and Cs2MoO4both have a tetrahedral Moo4" structure and can be prepared by fusion of the alkali-metal oxide or carbonate with the appropriate quantity of Moo3. The reducibility of K2Mo04and Cs2Mo04was found to be zero at 500 OC (see Table I). However, XRD of the high Alk/Mo samples does not show any detectable K2M004or Cs2Mo04phase, perhaps due to the well-dispersed nature of the molybdenum phases. It is quite probable that well-dispersed species of K2M004or Cs2M004might form on the surface at high ratios, Alk/Mo > 2, although there is no direct evidence. Alternatively, the suppression of the reduction of transitionmetal ions with the addition of alkali-metal promoters6J0 and the site blocking effect (geometric effect) of alkali-metal p r o m o t e r ~ ' ~ J ~ have been reported in the literature. Our results are consistent with the literature data. Moreover, we are able to distinguish two (15) Stoop, F.; Toolenaar, F. J. C. M.; Ponec, V. J. J . Catal. 1982, 73, 50. (16) McClory, M. M.; Gonzalez, R. D. J. Catal. 1984, 89, 392.
2018 The Journal of Physical Chemistry, Vol. 93, No. 5, 1989
O’Young
Alk/Mo 1, the Alk-Mo interaction is sufficient to perturb the electronic structure of Mo and to cause significant changes in the IR spectra of adsorbed N O species, even though the interaction G is not strong enough to suppress the reducibility. Besides the NO bands, the spectra also show bands in the region between ca. 1324 and 1233 cm-I. These band intensities increase with the alkali-metal loading, especially for those catalysts with potassium promoter. King and Peri have observed similar bands on K/ Fely-AlzO3 catalysts.’ They assigned these bands to the formation of nitrite ions or NO, species from the reaction between N O and surface oxides. At low atomic ratios, Alk/Mo < 1, the relative N O band intensity decreases significantly, while the N O uptake and the W V NO/Mo ratio decrease more slowly. These results suggest that c z m there is a significant decrease in the extinction coefficient of NO U 4 0 bands upon the addition of a small amount of alkali-metal prom a moters. Redey et aL20have observed that a small fraction of N O uptake on a 500 OC reduced Mo/y-A1203 catalyst at room temC perature was adsorbed on the free exposed support, and also another small fraction was decomposd to N 2 0 and N2. The contribution of nitrite or NO, species to the N O uptake is negligible. This can be observed by comparing the N O uptake and IR intensity of the K/Mo/y-A1203 catalysts with 6 wt % K (Figure 1D). The nitrite or NO, band intensities are significant, but the N O uptake and the NO/Mo ratio are quite low, indicating that the contribution of nitrite or NO, species to the N O uptake is negligible at low atomic ratios. With all of these factors taken into consideration, the real NO uptake and NO/Mo ratio on the 2250 ZiOO 1950 le00 1650 1500 1350 1200 ldS0 WRVENUMBERS reduced molybdenum portion of the catalysts might be lower than those shown in Table I. Nevertheless, one can still confidently Figure 1. IR spectra of NO adsorbed on 500 “C reduced K/Mo/y-A1203 conclude that the drop of N O band intensities is not due to the and Cs/Mo/y-A1203 catalysts. All spectra were taken after the catalysts were exposed to 30-Torr NO and evacuated at room temperature: (A) decrease of N O uptake but due to the decrease of the extinction 0% Alk, (B) 1% K, (C) 3% K, (D) 6% K, (E) 1% Cs, (F) 3% Cs, (G) coefficients of the N O bands. If this is the case, then these results 6% Cs, (H)10% Cs. suggest that the IR band intensity is no longer a reliable measure of the quantitative adsorption when the position and shape of the levels of interaction depending on the atomic ratio. bands are changed. Figure 1 shows the IR spectra of N O adsorbed on 500 “ C In conclusion, all of these results clearly suggest that there are reduced K/Mo/y-A1203 and Cs/Mo/y-A1203 catalysts, and Table two levels of Alk-Mo interaction, depending on the atomic ratio. I shows the spectral parameters. The IR spectra of N O adsorbed At low atomic ratios, Alk/Mo C 1, the Alk-Mo interaction is not on the reduced Moly-A1203catalyst show two strong peaks at strong enough to suppress the reducibility. The N O uptake deca. 1814 and 1708 cm-I. These peaks were assigned to the symcreases mainly due to the site blocking effect of alkali-metal metric and asymmetric stretches of Mo(NO),, r e s p e c t i ~ e l y . ~ ~ J ~ promoters. However, the interaction is sufficient to perturb the As shown in the figure and table, the intensities of the N O bands electronic structure of molybdenum and to cause significant decrease rapidly at low atomic ratios, Alk/Mo C 1, and almost changes in the IR spectra of adsorbed NO species, such as the vanish at high atomic ratios, Alk/Mo > 1. At low atomic ratios, N O band position, bandwidth, and bond angle of the Mo(NO), the band positions shift to lower frequencies; the asymmetric dimer. The N O band intensities decrease more rapidly than the stretch has larger red shifts, approximately 2 times that of the N O uptake, suggesting that there is a significant decrease in the symmetric stretch. The red shifts of N O bands are due to the extinction coefficient of N O bands upon the addition of a small electron donation ( + I effect) from alkali-metal ions, which has amount of alkali-metal promoters. Consequently, the IK band been reported by other researchers.*~’ The asymmetric stretch intensity might be no longer a reliable measure of the quantitative has a broader bandwidth (fwhm, full width at half maximum) adsorption when the position and shape of the bands are changed. than the symmetric stretch, and both bandwidths, especially the At high atomic ratios, Alk/Mo > 2, the Alk-Mo interaction asymmetric stretch, increase significantly with the atomic ratio. increases, and the reducibility, N O uptake, and NO/Mo ratio are The bond angle of the M o ( N O ) ~dimer also increases with the suppressed significantly. The NO band intensities almost vanish. atomic ratio. The increase of bandwidths suggests that there is The interaction reaches a maximum after Alk/Mo > 2, and the a wider adsorption site distribution upon the addition of alkalireducibility is suppressed to zero. It is postulated that K2Mo04 metal promoters. It is possible that the Alk-Mo interaction causes or Cs2Mo04might form on the surface. more heterogeneous molybdenum phases.” The increase in the bond angle is probably due to a decrease of adsorbate-adsorbate Acknowledgment. I thank Texaco, Inc., for permission to interaction. All these results suggest that at low atomic ratios, publish this article as well as C.-H. Yang, R. J. Barresi, J. E. Broas, and M. A. Behrens for their experimental assistance. Registry No. Mo, 7439-98-7; K, 7440-09-7; Cs, 7440-46-2; NO, (17) Millman, W. S.; Hall, W. K. J . Phys. Chem. 1979, 83, 427. 1233
L
(18) Peri, J. B. J . Phys. Chem. 1982, 86, 1615. (19) Yao, H. C.; Rothschild, W. G. Proceedings, 4th International Conference on the Chemisrry and Uses of Molybdenum; Climax Molybdenum Co., Ann Arbor, MI, 1982, p 31.
10102-43-9. (20) Redey, A.; Goldwasser, J.; Hall, W. K. J . Coral. 1988, 113, 82.
