J. Phys. Chem. 1992, 96, 362-366
362
Selective Chemisorption of Carbon Monoxide and Hydrogen over Supported Molybdenum Carbide Catalysts Jae Sung Lee,*'+Kyung Hee Lee,+ and Jeong Yong Leet Department of Chemical Engineering, Pohang Institute of Science and Technology, and Research Institute of Industrial Science and Technology, P.O. Box 125, Pohang, Korea, and Department of Material Science, Korea Advanced Institute of Science and Technology, Daeduk Science Town, Daejon, Korea (Received: May 7, 1991)
In order to establish procedures to determine the number of exposed metal sites and the particle size of molybdenum carbide catalysts by selective chemisorption of CO and H2, alumina supported molybdenum and its carbides were studied by CO and H2chemisorptionat different temperatures, TPD, XRD, and TEM. Carbon monoxide appeared to chemisorb molecularly on molybdenum carbide at 196 K, yet substantial amounts of chemisorbed CO desorbed upon heating to room temperature, Hydrogen chemisorption was an activated process and required elevated temperatures. Hence, CO chemisorption at 196 K and H2 chemisorption as the sample is cooled from 620 K to room temperature provided the number of exposed metal sites and the particle sizes, which are consistent with each other and with TEM results as well.
Introduction Molybdenum carbide has received attention as a potential substitute for noble metals, especially Ru, as a catalytic material. In the past decade, significant progress has been made in the preparation of supported and unsupported molybdenum carbide catalysts with desired purity and high specific surface a r e a ~ . l - ~ Excellent catalytic performance has been reported for hydrocarbon synthesis from CO and H2,6-10hydrogenation of b e n ~ e n e , ~ . ' ~ hydrogenolysis of alkanes,sJ I alcohol synthesis,I2 and hydrotreatingI3J4over the molybdenum carbide catalysts prepared by the new procedures. When new catalytic materials are evaluated, it is essential to compare their intrinsic activities with those for established materials. For example, turnover rates defined as reaction rates per exposed catalytic site are most frequently compared. For metal catalysts, the catalytic sites are conveniently defined as exposed metal atoms and the number of exposed metal atoms is usually titrated by selective chemisorption of CO or H2 at a low temperature. For this purpose, the stoichiometry of chemisorption between exposed metal atoms and the employed probe molecule should be known and a temperature of chemisorption is chosen such that the employed stoichiometry is satisfied without significant error. This titration method also allows the calculation of an average particle size provided that the shapes of the catalyst particles are known and that there is no contaminant blocking the surface sites. The adsorption of carbon monoxide on molybdenum has been investigated quite intensively for many years.lS On an Mo(100) single crystal plane, CO initially adsorbed molecularly at a low temperature was found to completely dissociate upon heating to 270 K.I6-l8 Also it is well known that hydrogen chemisorbs dissociatively on molybdenum over a wide range of temperatures. When a monolayer of elemental carbon (one carbon atom per surface molybdenum atom) was deposited on the Mo(100) surface, the dissociation of CO and chemisorption of H2 were completely suppre~sed.'~Hence, CO chemisorbed molecularly on the carburized Mo( 100) surface at 200 K. The presence of carbon on the Mo( 110) surface was also reported to decrease the activation energy and preexponential factor of the dissociation reaction of initially molecularly adsorbed C0.20 In previous works on unsupported molybdenum carbide catalysts,2,3the number of exposed metal sites was estimated by CO chemisorption at room temperature assuming the molecular adsorption and the stoichiometry of one CO molecule per exposed Mo atom. Adsorption of H2 at room temperature on molybdenum carbide proceeded too slowly to be practically used as a titration *To whom correspondence should be addressed.
'Pohang Institute of Science and Technology.
Korea Advanced Institute of Science and Technology.
method. The CO titration method for molybdenum carbide has been adopted without further ~crutiny.'~ This work was initiated to investigate if this procedure is adequate to determine the number of exposed metal sites and an average particle size of the molybdenum carbide catalysts. Alumina-supported molybdenum and molybdenum carbides with high metal dispersions are studied for which this type of titration method is more useful than other techniques such as X-ray diffraction (XRD). In addition to the chemisorption of CO and H2, temperatureprogrammeddesorption (TPD),XRD, and transmission electron microscopy (TEM)were employed.
Experimental Section To obtain better Mo dispersion, the procedures to prepare alumina-supported molybdenum and molybdenum carbide samples described elsewhereSwere slightly modified. A yalumina (Alfa, 160 m2 g-I) was impregnated with a solution of ammonium heptamolybdate (Alfa), dried at 270 K for 10 h, and calcined in air at 770 K for 15 h to obtain Mo03/A1203. This extended calcination period was needed to spread out Moo3 evenly onto the alumina surface and thus to obtain a well-dispersed M0O3/Al2o3showing no XRD peaks due to bulk MOO,. It is well established that at Mo03 loadings corresponding to less than a monolayer, Moo3 spreads over the alumina surface forming islands of less than two Mo atoms thick.2' This is the case with (1) Leclerq, L.; Imura, K.; Barbee, T.; Boudart, M. In Preparation of Catalysis Ik Delmon, B., Grange, P., Jacobs, P. A., Poncelet, G., Eds.; Elsevier: Amsterdam, 1978; p 627. (2) Lee, J. S.; Oyama, S. T.; Boudart, M. J . C a u l . 1987, 106, 125. (3) Lee, J. S.; Volpe, L.; Ribeiro, F. H.; Boudart, M. J . Catal. 1988, 112, 44.
(4) Volpe, L.; Boudart, M. J . Solid State Chem. 1985, 59, 348. (5) Lee, J. S.; Yeom, M. H.; Park, K. Y.;Nam, I.; Chung, J. S.; Kim, Y. G.; Moon, S. H. J. Catal. 1991, 128, 126. (6) Saito, M.;Anderson. R. B. J . Catal. 1980, 63, 438. (7) Kojima, I.; Miyazaki, E. J . Catal. 1984,89, 168. (8) Ranhotra, G. S.;Bell, A. T.; Reimer, J. A. J . Catal. 1987, 108, 40. (9) Logan, M.; Gellman, A.; Somorjai, G. A. J . Catal. 1985, 94, 60. (10) Lee, J. S.; Yeom, M. H.;Lee. D.4. J. Mol. Catal. 1990, 62, L45. (11) Lee, J. S.; Locatelli, S.; Oyama, S. T.; Boudart, M. J. Catal. 1990, 125, 157. (12) Woo,H. C.; Park, K. Y.; Kim, Y. G.; Nam, I.; Chung, J. S.; Lee., J. S. Appl. Catal. 1991, 75, 267. (13) Schlatter, J. C.; Oyama, S. T.; Metcalfe, J. E., 111; Lambert, J. M., Jr. Ind. Eng. Chem. Res. 1988, 27, 1648. (14) Lee, J. S.; Boudart, M. Appl. Catal. 1985, 19, 207. (15) Ford, R. R. Adu. Catal. 1970, 21, 5 1 and references therein. (16) Semancik, S.;Estrup, P. J. J . Vac. Sci. Technol. 1980, 17, 233. (17) Felter, T. E.; Estrup, P. J. Surf.Sci. 1976, 54, 179. (18) KO, E. I.; Madix, R. J. Surf.Sci. 1980, 100, L505. (19) KO, E. I.; Madix, R. J. Surf.Sei. 1981. 109, 221. (20) Erickson, J. W.;Estrup, P. J. Surf.Sci. 1986, 167, 519.
0022-365419212096-362503.00/0 0 1992 American Chemical Society
Chemisorption over Supported Molybdenum Carbide
The Journal of Physical Chemistry, Vol. 96, No. I, 1992 363
lo
P/ KPa
20
30
40
P/KPa
Figure 1. Adsorption (top) and back-adsorption (bottom) isotherms for N-Mo2C/A1203of CO at 196 K (a) and of H2 at 620 K-RT (b).
our experiments where the Moo3 loading of 5 wt % as Mo was employed. The oxide was transformed to carbide by three methods: (i) reduction in H2 at 1220 K for 2 h followed by carburization with a 20% CH4/H2mixture at 950 K for 2 h; (ii) direct reduction/carburization in the 20% CH4/H2mixture in a temperature-programmed reaction (TPR) mode between 670 and 950 K with a heating rate of 30 K h-I; (iii) nitriding in NH3 to form M02N/Al203 followed by carburization in the 20% CH4/H2, both in the same TPR mode as before. These supported molybdenum carbides are denoted as M-Mo2C/A1203,D-Mo2C/ A1203,and N-Mo2C/A1203,respectively. The metallic intermediate during the preparation of M-Mo2C/A1203 is called Mo/A1203. Sample preparation was performed in a typical atmospheric pressure flow system made of Pyrex. Each preparation employed 100 mg of Mo03/A1203loaded on a coarse quartz fritted disk in a cell which was designed to be used for preparation, adsorption, and TPD/TPR. A flow of reducing gases was maintained at 50 pmol s-l by mass flow controllers. Methane (Matheson, 99.97%) and H2 (Airco, 99.99%) were passed through a series of liquid nitrogen, 4A molecular sieve, and MnO/Si02 t r a p for purification before entering the cell. Ammonia (Matheson, anhydrous) was used as received. An electric furnace coupled to a controller and a thermocouple contacting the sample through a thermowell regulated the sample temperature. To measure the chemisorption of CO and Hz, the cell was isolated using Teflon stopcocks in the cell and transported to a volumetric adsorption system (Micromeritics Accusorb 2 100E). Samples were treated in Hz flow at 620 K for 1 h and evacuated at 5 X 1W2 Pa for 0.5 h before chemisorption measurements. For CO chemisorption, the conventional adsorption and back-adsorption method was employed at 196 K or room temperature (RT), and the difference of the two isotherms extrapolated to zero pressure was taken as the amount of chemisorbed CO. The 196 K was acheived by a dry iceacetone bath. For H2 chemisorption, employed was a procedure often used for samples whose H2 chemisorption was an activated p r o ~ e s s . ~After ~ , ~ ~evacuation at 620 K, H2 was allowed to adsorb on the sample as it is cooled from 620 K to RT over a period of 1-2 h at an equilibrium adsorption pressure of ca. 40 kPa. The initial isotherms were then measured at decreasing pressures. Back-adsorption isotherms were then measured in the usual manner at increasing pressures. Again, (21) Hall, W. K. In Proceedings of the 4th International Conference on the Chemistry and Uses of Molybdenum; Barry, H. F., Mitchel, P. C. H., Eds.; Climax Molybdenum Co.: Ann Arbor, 1978; p 224. (22) Amelse, J. A.; Schwartz, L. H.; Butt, J. B. J . Catal. 1981, 72, 95. (23) Mcmahon, K. C.; Suib, S. L.; Johnson, B. G.; Bartholomew, C. H., Jr. J . Catal. 1987, 106, 47.
the difference between these two isotherms extrapolated to zero pressure was taken as chemisorbed H1. For TPD, CO was adsorbed on a sample by flowing a 30% CO/He mixture at 196 K for 1 h. After flushing with He at this temperature for 2 h, the sample was heated at a rate of 0.17 K s-I in flowing He and desorbed gases were continuously monitored by a quadrupole mass spectrometer (VG, Micromass). The XRD and TEM experiments were performed on samples passivated before they were removed from the preparation cell by flowing 1% O2 in He at RT for 3 h. A Rigaku Dmax-B diffractometer with Cu Ka radiation was used for powder XRD measurements. For TEM, the passivated samples were dispersed ultrasonically in ethanol and then deposited on a TEM grid that had been coated with a holey carbon film. A Jeol JEM-2OOEX microscope was operated at 200 kV.
ReSults The prepared supported molybdenum carbide samples showed only broad XRD peaks which were difficult to distinguish from the XRD pattern of alumina itself. This is an indication that small particles of molybdenum and molybdenum carbides were formed on alumina. Furthermore, the y-alumina also has XRD peaks near Bragg angles of 35-40° where major peaks of MozC and Mo would appear, and thus the latter, if they were present, would have been masked by broad peaks due to alumina. In Figure 1, isotherms of CO at 196 K and of H2 on N-MozC are shown as typical examples. Very similar isotherms were obtained for other samples. The CO adsorption showed two nearly parallel isotherms for adsorption and back-adsorption and thus render the usual procedure of extrapolating to zero pressure applicable to obtain the irreversibly chemsorbed CO. The amount of weakly adsorbed CO obtained during back-adsorption corresponded to 35-40% of total for carbides and ca. 50% for Mol A1203. Hydrogen adsorption also showed two parallel lines, However, the back-adsorption isotherms, when extrapolated to zero pressure, passed through the origin. Namely, there is no reversible adsorption for H2 and no need to take the back-adsorption isotherms. Results for chemisorption processes are summarized in Table I. In general, 25-35% more CO was adsorbed at 196 K than at RT. These values were corrected for adsorption for bare alumina. This correction was needed only for the CO adsorption at 196 K where alumina itself adsorbed 5 pmol g-I of irreversible CO. No irreversible adsorption was observed at RT. The treatment between CO-adsorption experiments for the same sample was found to be important to obtain reproducible results. Much less CO adsorbed in the second adsorption experiment although the sample was treated in H2 at 620 K for 2 h after the first run. After a CO chemisorption, the sample was subject to
364 The Journal of Physical Chemistry, Vol. 96, No. 1, 1992 TABLE I:
Selective Chemisorption of CO and
sample M0/A1203 M-M02C/A1203 D-M02C/A1203 N-M02C/A1203
H2(620-RT)/ @molg-' 54 40 56 65
Lee et al.
H1on Alumha-Supported Molybdenum and Molybdenum Carbide Catalysts __ CO(196 K)/ CO(RT)/ CO(196 K)/ CO(RT)/ % ' metalc bmol g-' j " l g-' HJ620-RT)" CO(196 K)* exposed 73 103 131 151
52 76 93 98
1.35 2.57 1.34 2.32
0.7 1 0.74 0.7 1 0.65
20.0 19.8 25.1 29.0
D,d/ nm 10.8 10.9 8.6 7.4
"The ratio of CO uptake at 196 K to H2 uptake at 620 K-RT. *The ratio of CO uptake at RT to that at 196 K. 'On the basis of CO uptake at 196 K assuming 1:l stoichiometry for CO and exposed Mo metal for molybdenum carbides. For Mo/A1203, CO uptake at RT was used assuming one CO molecule per two exposed Mo atoms. dAssuming spherical particles and a site density of lOI5 cm-2. The same titration data and stoichiometry of CO chemisorption as in c were employed.
H2 TPR. Two methane peaks around 600 and 900 K were detected. Hence, chemisorption deposits carbon which is not removed by the H2treatment at 620 K. Reproducible results were obtained only when the conditions employed in the original preparation were used after every CO adsorption experiment, Le., H2 treatment at 1220 K for Mo/A1203and 20% CH4/H2treatment at 950 K for Mo2C/A1203.The Mo/A1203chemisorbed much less CO than its corresponding carbide, M-Mo2C/A1203. The amounts of H2 chemisorption obtained while cooling the samples in H2 from 620 K to RT (denoted as 620-RT in Table I) were less than for CO chemisorption at 196 K by factors of 2.3-2.6 for molybdenum carbides. Contrary to the case of CO, Mo/A1203chemisorbed more H2 than M-Mo2C/A1203. Table I also contains the data of percent metal exposed based on CO uptake at 196 K and assuming molecular adsorption and, hence, 1:l stoichiometrybetween CO and exposed Mo metal for molybdenum carbides (vide infra). For Mo/Al2O3,CO uptake at RT was used assuming dissociative adsorption and a stoichiometry of one CO per two exposed Mo atoms. Average particle sizes D, were also estimated using these stoichiometries of CO chemisorption. For spherical particles, D,is given by 6/pSg, where p is the solid density and S, denotes the specific surface area of Mo or M q C particles excluding the area of alumina support. The S could be calculated from CO uptake and assuming site density of 1015 cm-2. Figure 2 shows CO TPD spectra taken for Mo/A1203 and M-Mo2C/A1203.The effluent of the cell was analyzed for CO, C02,and O2by a quadrupole mass spectrometer. Desorption of CO started much below RT. For M-Mo2C/A1203,CO was the only desorbing species while C 0 2 was observed for Mo/A1203at high temperatures. This different desorption behavior of Mo/ A1203and its carbide is noteworthy. Because of the oxygen and carbon atom balance, one C atom must be retained on the surface per C 0 2 molecule desorbed. This could be a route, at least for M0/A1203,for leaving carbon atoms on the surface after a CO adsorption experiment as mentioned earlier. No oxygen was detected. Figure 3 shows bright field TEM images of M-Mo2C/Al2O3 and N-Mo2C/A1203.Fairly uniform particles with diameters of 5-10 nm are seen. It is evident that particles in M-Mo2C/A1203 are considerably larger than those in N-Mo2C/A1203. Discussion As mentioned, adsorption of CO and Hzhas been studied over an Mo(100) single crystal surface and its carburized surface Mo( lOo)-~?~.On Mo( loo), CO is completely dissociated above 270 K,16 and t h e resulting carbon and oxygen atoms reside on interstitial +fold sites. Additional CO can also adsorb molecularly on the vacant on-top sites. Hydrogen dissociates on Mo(100) even at 200 K. Upon carburization, carbon atoms fill the interstitial sites up to one monolayer (one carbon atom per surface Mo atom). Thus on Mo( loo)
