Ind. Eng. Chem. Res. 1988,27, 1391-1396
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Resistivity to Sulfur Poisoning of Nickel-Alumina Catalysts Ienwhei C h e n * and Dar-Woei Shiue Department of Chemical Engineering, Tatung Institute of Technology, Taipei, Taiwan, R.O.C.
Resistivity t o sulfur poisoning has been investigated over nickel-alumina catalysts with promoters such as Mo, W, Li, K , Mg, Ca, and La, respectively. Studies were carried out by performing hydrodesulfurization (HDS) of thiophene with a fixed bed reactor at 673 K and atmospheric pressure. After the hydrodesulfurization of thiophene, sulfur content and quality of the used catalysts were analyzed by a n elemental analyzer and X-ray diffractometer, respectively. The nickels of Ni-Mo, Ni-W, and Ni-La catalysts are found to be poisoned by sulfur; however, the nickels of Ni-Li, Ni-K, Ni-Mg, and Ni-Ca catalysts are found to be still kept well from t h e sulfur poisoning. The poisoning of catalyst, which reduces activity of the catalyst, is caused by the impurity presented in the feed stream. One of the important impurities in coal-derived liquids and petroleum from the point of view of catalytic processing of distillate fractions is sulfur compounds. Various amounts of organic sulfur compounds vary from as little as 0.05 wt % to as high as 5 wt % sulfur in all crude oils. Hydrogen sulfide is also often present, and sometimes there are traces of elemental sulfur. Many of the organic sulfur compounds can be desulfurized by treating them with catalyst, but many still leave little organic sulfur compounds after the hydrodesulrization process. The catalyst can be poisoned by sulfur compounds a t concentrations in the range around 1 ppm (Satterfield, 1980). Nickel catalysts find widespread industrial applications in the hydrogenation of unsaturated organics, steam reforming, and methanation. Nickel catalysts are very sensitive to sulfur compounds. The nickel is poisoned by sulfur to become inert Ni,S2 (Kerry et al., 1980). A number of surface studies (Delescluse and Masson, 1980; Erley and Wagner, 1978; Sargent et al., 1980) have provided useful information regarding the structure of the sulfur adsorbed on various faces of nickel. Surface chemisorbed species can exist under equilibrium conditions. For the poisoning of Ni/Si02 catalysts with H2S, Ng and Martin (1979) also suggested that H,S adsorption involves four surface nickel atoms per adsorbed H2S a t room temperature. A number of studies (McCarty and Wise, 1980; Fowler and Bartholomew, 1979) provided evidence showing surface nickelsulfur bonds to be substantially more stable than bulk nickel-sulfur bonds. Because the strong adsorption of sulfur on metals prevents or modifies the further adsorption of reactant molecules, its presence on a catalyst surface usually causes substantial or complete loss of activity in many important reactions, particularly in hydrogenation reactions. The effects of sulfur poisoning on activity and selectivity properties of Ni, Co, Fe, and Ru catalysts in methanation were well investigated by Dalla Betta et al. (1975), Bartholomew and co-workers (Fowler and Bartholomew, 1979; Gardner and Bartholomew, 1981). One area of interest, being common to all catalysts, has been the use and performance of promoters in improving catalytic activity, selectivity, and catalyst life. The reason of adding promoters may be grouped into two general postulates: (1)the promoter increases the number of active catalyst sites, or (2) it increases the activity of the active catalyst sites. In fact, both approaches could be applicable simultaneously in the catalyst treatment (Laine et al., 1985). * T o whom all correspondence should be addressed.
Methanation and Fisher-Tropsch studies on supported nickel catalysts with alkali-metal promoters as well as alkaline-earth promoters have been discussed (Gonzalez and Muira, 1982; Chai and John, 1985). Alkali-metal promoters apparently improve the resistance of FischerTropsch catalysts to poisoning. For example, Anderson et al. (1965) found that alumina-promoted iron evidenced somewhat great resistance to the sulfur poisoning. It is interesting to increase the steady-state activity and the life of nickel catalysts for the steam reforming reactions if feedstocks still have little organic sulfur compounds after hydrodesulrization process. The objective of this study is to develop the nickel-alumina catalyst, giving excellent protection from the poisoning of sulfur compounds contaminated in the catalytic reaction system of the steam reforming. The thiophene hydrogenolysis can be used for the simulation of the resistivity to sulfur poisoning of the nickel-alumina catalysts. The tasks of the present study are to investigate characterizations of the Ni-Mo, Ni-W, Ni-Li, Ni-K, Ni-Mg, Ni-Ca, and Ni-La catalysts and to understand the role of the promoters in resistivity to sulfur poisoning. Experimental Section Materials. The y-alumina (Merck) was coimpregnated with aqueous solutions of nickel nitrate (Hayashi) and aqueous solutions of promoters according to a composition reported as 15 wt % Ni and desired wt % promoter, respectively. The solutions were dried in air a t 333 K followed by calcination a t 973 K for 5 h and the subsequent reduction in flowing hydrogen a t 673 K for 5 h. Salts of metal promoters were lithium nitrate (Wako), calcium nitrate (Wako), potassium nitrate (Wako), lanthanum nitrate (Fluka), ammonium (meta) tungstate (Fluka), magnesium nitrate (Wako), and ammonium molybdate (Wako). The prepared catalysts were found to have 15 wt % Ni, 2-10 wt % promoter, and 75-83 wt % 7-A1203. Apparatus a n d Procedure. A diagram of the apparatus used for continuous flow experiments is given in Figure 1. The hydrogen (hl) for prereduction is regulated by the pressure regulator (a) and the needle valve (b) and is measured by the manometer (c) and flow meter (d). For prereduction, hydrogen passes a bypass (f) and the fixed bed reactor (r). Hydrogen (h2) for hydrodesulfurization is passed through vessels (e) filled with thiophene and carries the vapor of the thiophene to a fixed bed reactor (I). The product gas is led to a three-direction switch (t) and a loop in the sample valve. The fixed bed reactor is loaded with 1.0 g of catalyst, and it is electrically heated. The catalyst sample is reduced by flowing hydrogen for 5 h at 673 K before the experiment was carried out. Thiophene hydrogenolyses are conducted at 673 K and a t atmospheric pressure. Analysis of reactor effluent gas was 0 1988 American
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1392 Ind. Eng. Chem. Res., Vol. 27, No. 8, 1988
Figure 1. Test unit of promoted nickel catalysts for resistivity to sulfur poisoning.
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Figure 3. Activity behavior (673 K) of Ni-W catalyst with various concentrations of W. (0) 0 wt 90W, (A)2 wt 70W, (0)4 wt % W, (0) 6 wt 70W, ( X ) 8 wt % W, (0)10 wt % W.
3 pb 1
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Figure 2. Activity behavior (673 K) of Ni-Mo catalyst with various concentrations of Mo. (0) 0 w t % Mo, (A)2 wt % Mo, (0) 4 wt % Mo, (0) 6 wt % Mo, ( X ) 8 wt % Mo, ( 0 ) 10 wt 70 Mo.
performed by the on-line gas chromatograph set (HP 5890A) equipped with thermal conductivity detector. The separation column is packed with 20% Carbowax 20 M treated with 2% KOH on 60/80-mesh Chromosorb W. The catalytic activities are reported as the conversion of thiophene at a flow rate of 80 mL (H, + C,H4S) per minute. After hydrodesulfurization of thiophene, the sulfur content and quality of the used catalysts were analyzed by using the elemental analyzer (Tacussel coulomax 78) and X-ray diffractometer (Rigaku D/MAX-A), respectively. Fe K a radiation lines were applied in the X-ray diffractometer.
Results and Discussion Ni-Mo and Ni-W Catalysts. The influences of the presence of Mo and W promoters on the catalytic activity behavior of hydrodesulfurization of the nickel catalysts are shown in Figure 2 and Figure 3, respectively. It is observed that the presence of Mo and W promoters remarkably affects the activity behavior of the nickel catalyst: the activities of catalyst samples with different concentrations of promoter are significantly higher than that of a catalyst without using promoter within the running time of 24 h. The maximum activities appear within 4 h of the running time for those catalyst samples having several different concentrations of promoter. Occurrence of the maximum value of activity with respect to the running time is a result of two opposing effects: (1)the fraction of catalyst reduced to the metal increases with running time; and the promoters of Mo and W are gradually sulfidated to MoSz and WS2, respectively. It was found that the MoSz and WS, enhance the activity of hydrodesulfurziation greatly. Thus, the activity of catalysts containing Mo or W exhibits a continuous increment up to a maximum value during the initial running time. (2) However, at the mean time, the degree of sulfur poisoning on nickel is increased, and the
Concentrotion of Promoter 1 w t % 1
Figure 4. Effect of concentration of promoter on steady-state activity and sulfur content. (0)Ni-Mo, (A)Ni-W, (m) Ni-Mo, (A) Ni-W, (EI) estimated sulfur content from formation of MoS2, (A) estimated sulfur content from formation of WS2.
dispersion of Ni, Mo, and W can be decreased due to the sulfidation of Ni, Mo, and W as the running time increases. Consequently, the catalytic activity of hydrodesulfurziation declines in the latter stage, and it approaches steady state when the sulfidation of Ni, Mo, and W reaches the equilibrium state. Figure 4 shows the effect of Mo and W concentrations on the steady-state activity of hydrodesulfurization and sulfur content of the used catalyst, respectively. It is observed that a maximum steady-state activity occurs at 4 wt 9'0 Mo. This optimum composition of the Ni-Mo catalyst can be explained in two ways. First, from the surface model shown in Figure 5, active MoSz develops during hydrodesulfurization in a two-dimensional direction to form a plane of the monolayer within the region of Mo content up to 4 wt % ,and subsequently three-dimensional MoSz crystals (multiply layer) develop with increasing concentrations of Mo above 4 wt %. It is said that the active sites for hydrodesulfurization of MoSz probably lie on the basal plane only; the edge plane is quite inert (Yasuaki et al., 1980). The surface area of the basal plane is relatively bigger than that of the edge plane as the
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Figure 5. Surface model of developing of MoSz and WS2 on alumina. (A) Ni-Mo catalyst, (B) Ni-W catalyst.
two-dimensional MoS, layer develops. This results in increasing steady-state activity of hydrodesulfurization. However, the edge plane can be significantly developed by the further addition of Mo. As a result of increasing the Mo content above 4 w t %, the exposed surface area of the basal plane decreases as the three-dimensional MoSz layer is formed. This results in decreasing the steady-state activity. Second, the higher the concentration of Mo, the more likely the formation of AI,(MoO~)~ may occur during the calcination. Since A ~ , ( M o O ~is)an ~ inert compound and it is also difficult to reduced, it is of no help in catalytic activity. From Figure 4 it can be seen that the steady-state activity of Ni-W increases as the concentration of W increases within 10 wt % in this experiment. It is possible to make an explanation that the active WS2 develops in a two-dimensional direction to form the monolayer plane within this region; i.e., the surface area of the WS2 layer increases with increasing concentrations of W up to 10 wt % . These results indicate that the dispersion of W is much better than that of Mo on the surface of the alumina support. From Figure 4, the sulfur content of the used catalysts is increased as the concentration of Mo or W is increased, respectively. The sulfur content and its increasing rate of the used Ni-Mo catalysts are more than those of used Ni-W a t each concentration of promoter. By comparison of these sulfur contents with those obtained from stoichiometry assuming Mo and W to be sulfided and formed MoS, and WS2, respectively, their trends are in accord with each other. In other words, the formation of MoS, and WS2 is very highly dependent on and proportional to the concentrations of Mo and W in the catalysts. Besides, the amounts of sulfur content of the used Ni-Mo and Ni-W catalysts are much more than those obtained from stoichiometry assuming Mo and W to be sulfided and formed MoS2 and WS,, respectively. These results indicate that the major sulfur content of used catalysts is contributed to by the formation of nickel sulfide. On the other hand, the nickel is poisoned during the hydrodesulfurization. The X-ray diffraction spectra of used Ni-Mo and Ni-W catalysts are shown in Figure 6. By comparison of them to that of fresh Ni-Mo catalyst, the peak of Ni(lll), the most intensive peak of nickel crystallite, does not appear in both used Ni-Mo and Ni-W catalysts according to the "Powder Diffraction File". These results also prove that the nickel of used Ni-Mo and Ni-W catalysts is poisoned during hydrodesulfurization. It is recognized that the
28 Figure 6. X-ray diffraction spectra of (A) Ni-Mo sample pretreated with calcination, (B) Ni-Mo sample pretreated with calcination followed by reduction, and (C) Ni-Mo and (D) Ni-W samples pretreated with calcination and reduction followed by hydrodesulfurization of thiophene.
Running Time ( hours)
Figure 7. Activity behavior (673 K) of Ni-Li catalyst with various concentrations of Li. (0) 0 wt % Li, (A)2 wt '70Li, (0) 4 w t % Li, (0)6 wt % Li, ( X ) 8 wt '3 Li, (V) 10 wt % Li. S
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Figure 8. Activity behavior (673 K) of Ni-K catalyst with various concentrations of K. (0) 0 wt '3 K, (A)2 wt % K, (0) 4 w t 5% K, ( 0 ) 6 wt 70 K, ( X ) 8 wt % K, (V) 10 wt % K.
addition of promoters Mo and W into the nickel catalysts is just to enhance the activity of hydrodesulfurization, whereas it bears no function of resistivity to sulfur poisoning. Ni-Alkali Metal and Ni-Alkaline Earth Metal Catalysts. Figures 7-10 show the results of the activities of nickel catalysts not containing promoter and containing
1394 Ind. Eng. Chem. Res., Vol. 27, No. 8, 1988 I
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Figure 9. Activity behavior (673 K) of Ni-Mg catalyst with various 0 wt 70 Mg, (A) 2 wt YO Mg, (0) 4 wt % concentrations of Mg. (0) Mg, (0) 6 wt % Mg, (x) 8 wt % Mg, (V)10 wt % Mg.
Concentration of Promoter [ w t */, )
Running Time i h o u r s ) Figure 10. Activity behavior (673 K) of Ni-Ca catalyst with various 0 wt % Ca, (A) 2 wt % Ca, (0)4 wt % concentrations of Ca. (0) Ca, (0)6 wt 70Ca, (X) 8 wt % Ca, (V) 10 wt % Ca.
the respective different promoters Li, K, Mg, and Ca, during the hydrodesulfurization of thiophene. It is observed that the activities of all these catalysts with or without promoters decrease as the running time increases. The activities of catalysts with promoter are lower than that of the unpromoted nickel catalyst a t each running time. The higher the concentration of promoter, the more obvious the retardation of activity will be. Among these four kinds of promoters, Li inhibits the activity of hydrodesulfurization most remarkably. The steady-state activities of the hydrodesulfurization of Ni-Li, Ni-K, Ni-Mg, and Ni-Ca catalysts, as well as sulfur contents of these used catalysts, are shown in Figure 11as a function of promoter and its concentrations. It may be seen that the steady-state activities of Ni-alkali metal and Ni-alkaline earth metal catalysts, as well as sulfur contents of these used catalysts, decrease with increasing concentration of promoters. At most, the steady-state activities of these promoted catalysts for hydrodesulfurization of thiophene would drop to zero. This result suggests that the chance of contact with sulfur commpound or adsorbing ability of sulfur compound on the nickel surface is decreased for the promoted nickel catalysts as the concentration of the promoter increases. The sulfur contents of these used catalysts are inverse proportional to the amount estimated by assuming all promoters to be sulfided during the hydrodesulfurization of thiophene. These results also imply that the higher the concentration of promoter, the higher the resistivity to sulfur poisoning will be. The resistivity to sulfur poisoning of Ni-alkali metal is greater than that of Ni-alkiline earth metal. The tendency of resistivity to sulfur poisoning for the promoters is found to be in the order of Li > K > Ca > Mg at each concentration of promoter. The catalytic properties of nickel catalysts were modified by adding different promoters of Li, K, Mg, and Ca (Chai
Figure 11. Effect of concentration of promoter on steady-state Ni-K, (0) Ni-Mg, (0) activity and sulfur content. (A) Ni-Li, (0) Ni-Ca, (A)Ni-Li, (m) Ni-K, ( 0 )Ni-Mg, (V)Ni-Ca, (A)estimated sulfur content from formation of Li2S, (a) estimated sulfur content estimated sulfur content from formation from formation of K2S, (0) of MgS, ( 0 )estimated sulfur content from formation of Cas.
and John, 1985). It has been suggested that a reconstruction of both the electronic state and the geometry of the surface layer of catalysts, as well as a change in the surface free energy of the various crystal planes, may result from atom interactions. The same suggestions are proposed in the studies on the Pt catalysts (McCarroll, 1975). The addition of alkali metal and alkaline earth metal, which are easy to release electrons, causes the electronic transfer to nickel crystallite and weakens the formation of nickel sulfide during hydrodesulfurization of thiophene. Therefore, nickel catalysts promoted by alkali and alkaline earth metals are not easily poisoned by sulfur compounds, and the sulfur content in the used Ni-Li, Ni-K, Ni-Mg, and Ni-Ca becomes less than that of unpromoted nickel catalyst. Owing to the difficulty of adsorption of sulfur compounds on the nickel surface, the steady-state activity of this promoted nickel catalyst is accordingly lower than that of unpromoted nickel catalyst. The higher the concentration of the promoter, the more difficult the adsorption of sulfur compounds on the nickel surface and the formation of nickel sulfide would be because the change in the electronic structure of nickel becomes more significant. This results in the steady-state activity of the promoted catalyst and the sulfur content of this used catalyst being decreased with an increase the concentration of the promoter. The higher the standard electrodic potential of the promoter, the easier the release of electrons from the promter is, and the degree of modification on the electronic structure of nickel will be increased with increasing the ability of electron releasing. So the absorbing ability of the sulfur compound on nickel is inversive to the order of standard electrodic potential. It is reasonable to show that the tendency of the steady-state activity of hydrodesulfurization and sulfur content for the catalysts is in the order of Ni-Mg > Ni-Ca > Ni-K > Ni-Li at each concentration of promoter because the tendency of the standard electrodic potential for the promoters is Li > K > Ca > Mg. It is also found that the degree of poisoning
Ind. Eng. Chem. Res., Vol. 27, No. 8, 1988 1395
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28 Figure 14. X-ray diffraction spectra of (E) 15 wt % Ni sample pretreated with calcination followed by reduction and (J) 15 wt % Ni-6 wt % La sample pretreated with calcination and reduction followed by hydrodesulfurization of thiophene.
up to 24 h. The more the concentration of La, the higher the catalytic activity of Ni-La catalyst will be. After 24 h of running, the steady-state activity of Ni-La catalysts 65 60 55 50 is close to that of unpromoted Ni catalyst. The particle 29 sizes of nickel of Ni-La catalysts with different concenFigure 12. X-ray diffraction spectra of (E) 15 wt % Ni sample trations of La are determined by examination of their pretreated with calcination followed by reduction, (F)15 wt % Ni-8 X-ray diffraction spectra based on the assumption of wt % Li, (G) 15 wt % Ni-8 wt % K, (H) 15 wt % Ni-8 wt % Mg, spherical nickel crystallite. The result reveals that the (I) 15 wt % Ni-8 wt % Ca. (F),(GI,(HI, and (I) samples were particle sizes of nickel are 113, 91, 73, and 63 A, with pretreated with calcination and reduction followed by hydrocorresponding Ni-La catalysts having the respectively 0, desulfurization of thiophene. 2, 6, and 10 w t % La. Thus, the possible reason for the relationship between activity and concentration of La is that both dispersion and surface area exposed to sulfur compound of nickel in the Ni-La catalyst are greater as compared with that of nickel in unpromoted nickel catalyst, and they increase as the concentration of La increases. However, La does not cause the atom interaction of Ni and La in the Ni-La catalysts nor the change of the electronic state and the geometry of surface layer of nickel crystallite. Therefore, the steady-state activity and resistivity to sulfur poisoning of Ni-La catalyst are not remarkably modified. Sulfur content of the used Ni-La catalyst with 6% La U fJ is 4.05%, which is a little more than that of the used OO 2 4 6 18 24 Running Time ( hours) unpromoted nickel catalyst. By comparing the X-ray diffraction spectra of the used Ni-La catalyst with that Figure 13. Activity behavior (673 K) of Ni-La catalyst with various of the fresh Ni catalyst shown in Figure 14, the peak of 0 wt % La, (A)2 wt % La, (0) 6 wt % La, concentrations of La. (0) Ni(ll1) is not found to appear after hydrodesulfurization (0) 10 wt % La. of thiophene. This result reveals that nickel is poisoned by the sulfur compound during the hydrodesulfurization by sulfur compound of nickel catalyst promoted with Li of thiophene. It is conclusively seen that the addition of is less than that of the other promoted nickel catalysts. La enhances neither the catalytic activity nor the resistivity In other words, the nickel promoted with Li provides the to sulfur poisoning during the hydrodesulfurization of greatest resistance to sulfur poisoning among these four thiophene. kinds of catalyst for hydrodesulfurization of thiophene. By comparing the X-ray diffraction spectra of the used Conclusions Ni-Li, Ni-K, Ni-Ca, and Ni-Mg catalysts with that of the fresh unpromoted nickel catalyst shown in Figure 12, the The hydrodesulfurization of thiophene is used to inpeak of Ni(ll1) is found still to exist after hydrovestigate the resistivity to sulfur poisoning of the nickeldesulfurization of thiophene. This result suggests that the alumina catalyst by using a plug flow reactor of fixed nickel Li, K, Ca, and Mg promoters function to protect the nickel catalysts bed. To the nickel catalysts are added individfrom being poisoned by sulfur compound during hydroually Mo, W, Li, K, Ca, Mg, or La as the promoter. Their desulfurization of thiophene. Therefore, the nickel catalyst catalytic activities are determined by using a gas chropromoted by Li, K, Ca, or Mg is considered to be applied matograph which is on-line. The sulfur contents of these as the steam reforming catalyst, giving excellent protection used catalysts are analyzed by an elemental analyzer. The from the poisoning of sulfur compounds contaminated in sulfur poisoning of nickel is also comfirmed by an X-ray the catalytic reaction system. diffractometer. All additives used as the promoter are Ni-La Catalyst. The effect of running time on the reviewed and classified into three sorts by their characactivity of Ni-La catalysts for the hydrodesulfurization of terizations. The promoter of both Mo and W bears no function to thiophene a t different La contents ranging from 2 to 10 resist the sulfur poisoning during hydrodesulfurization of wt % is illustrated in Figure 13. As can be seen, all the thiophene. However, they can enhance the catalytic accatalytic activities of the Ni-La catalysts are higher than tivity of hydrodesulfurization of nickel catalysts. that of unpromoted nickel catalyst within the running time
Ind. Eng. Chem. Res. 1988,27, 1396-1407
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Those promoters such as Li, K, Ca, and Mg can enhance the resistivity to sulfur poisoning of nickel catalysts significantly during hydrodesulfurization of thiophene due to the poor adsorption of sulfur compound on nickel in the presence of these additives. The tendency of resistivity to sulfur poisoning for the promoters is found to be in the order Li > K > Ca > Mg with respect to resistivity because the higher ability of electron releasing would cause greater modification of the electronic structure of the nickel catalysts. However, they can reduce the nickel catalytic activity of hydrodesulfurization of thiophene. La is no help in both resistivity to sulfur poisoning and the steady-state activity of nickel catalysts in hydrodesulfurization of thiophene.
Acknowledgment The authors thank the Tatung Company and the Chinese National Council for financial aid. Registry No. Ni, 7440-02-0; S, 7704-34-9; Mo, 7439-98-7; W, 7440-33-7; La, 7439-91-0; Li, 7439-93-2; K, 7440-09-7; Mg, 743995-4; Ca, 7440-70-2; thiophene, 110-02-1.
Literature Cited Anderson, R. B.; Karn, F. S.; Schultz, J. F. J . Catal. 1965, 4, 65. Chai, G. Y.; John, L. F. J . Catal. 1985, 93, 152. Dalla Betta, R. A.; Piken, A. G.; Shelef, M. J . Catal. 1975, 40, 173. Delescluse, P.; Masson, A. Surf. Sci. 1980, 100, 423. Fowler, R. W.; Bartholomew, C. H. Znd. Eng. Chem. Prod. Res. Deu. 1979, 18, 339. Gardner, D. C.; Bartholomew, C. H. Znd. Eng. Chem. Fundam. 1981, 20, 229. Gonzalez, R. D.; Miura, H. J . Catal. 1982, 77, 338. Kerry, C. P.; John, V. S.; Nicholus, T. J . Catal. 1980, 66, 82. Laine, J.; Brito, J.; Gallardo, J.; Severino, F. J . Catal. 1985,91, 64. McCarroll, J. J. Surf. Sci. 1975, 53, 297. McCarty, J. G.; Wise, H. J . Chem. Phys. 1980, 72, 6332. Ng, C. F.; Martin, G. A. J . Catal. 1979, 54, 384. Sargent, G . A.; Freeman, G. B.; Chao, J. L. Surf. Sci. 1980,100,342. Satterfield, C. N. Heterogeneous Catalysis in Practice; McGraw-Hill: New York, 1980. Yasuaki Okamoto; Hiroyuki Tomioka; Toshinobu Imanaka; Shiichiro Teranishi J . Catal. 1980, 66, 93. Received for review May 26, 1987 Revised manuscript received December 24, 1987 Accepted February 10, 1988
Steady-State and Transient Studies of Carbon Monoxide Oxidation on Alumina-Supported Rhodium via Transmission Infrared Spectroscopy Michael R. Prairie,+Byong K. Cho, Se H. Oh, and Edward J. Shinouskis Physical Chemistry Department, General Motors Research Laboratories, Warren, Michigan 48090
James E. Bailey* Department of Chemical Engineering, California Institute of Technology, Pasadena, California 91125
Transmission infrared (TIR) spectroscopy was used t o examine the behavior of adsorbed CO on a Rh/A1203 catalyst during transient CO chemisorption experiments and during steady-state, step-response, and forced-cycling oxidation experiments a t 900 Torr. At 300 "C, the catalyst initially supported primarily a dicarbonyl CO species but after use exhibited spectra characteristic of a surface mostly covered by linearly bound CO. A model t h a t describes transient, diffusion-influenced CO adsorption and desorption for the supported catalyst is presented. It suggests that the CO desorption energy depends linearly on coverage and t h a t the magnitude of this dependence is a function of temperature. Observed rate dependence on bulk CO concentration for O2effluent levels of 0.5% and 0.25 % is interpreted considering the effects of internal and external mass transport a t 300 "C. Step-response and forced-cycling oxidation experiments across stoichiometric conditions exhibit oxygen and CO storage effects characteristic of CO oxidation catalysts. Data indicating autonomous oscillation of CO coverage and C 0 2 production are also presented. Many researchers have used infrared (IR) spectroscopy to study CO chemisorption on supported rhodium (e.g., Cavanagh and Yates (1981), Primet (1978), Rice et al. (1981), Yates et al. (1979)). The predominant features in the IR spectrum for this system are a doublet at about 2100 and 2030 cm-l corresponding to a dicarbonyl species (Rh(CO),) and a singlet near 2065 cm-l attributed to linearly bound (terminal) CO (RhCO). Bridge-bonded CO (Rh,CO) exhibits a broad peak near 1900 cm-I. Although the chemical nature of the adsorbed CO species and the effects of experimental conditions (e.g., temperature, gas composition, dispersion, loading, support material, preireatment, Rh oxidation state, etc.) on these species have been studied extensively, reports describing the application
* To whom
correspondence should be addressed, Present address: Institut de genie chimique, Ecole Polytechnique FBdBral de Lausanne, CH-1015 Lausanne, Switzerland.
088S-5885/88/2627-1396$01.50/0
of IR spectroscopy to study steady-state and dynamic behavior of adsorbed CO during catalytic reaction with oxygen are less common. Direct IR measurement of surface concentration (coverage) during forced and autonomous transients is a powerful method for elucidating elementary adsorption and desorption processes and is also extremely valuable for studying more complicated multispecies catalytic reactions. While gas-phase composition measurements are usually adequate for characterizing the steady-state kinetics of a catalytic reaction, those types of measurements are sometimes neither sensitive enough nor robust enough to obtain an accurate description of surface phenomena that may dictate transient behavior. Such descriptions of surface phenomena are essential for modeling of dynamic catalytic reactors. Also, since the molar capacity of a catalyst is usually very small with respect to the overall reactor capacity, gas-phase transients can be difficult to 0 1988 American Chemical Society
