802
J. Phys. Chem. 1991,95, 802-808
Structure and Catalytic Activity of Alumina-Supported Pt-Co BimetalWc Catalysts. 2. Chemisorption and Cataiytlc Reactions Uszlo Guczi,* TamQsHoffer, ZoltQnZsoldos, Institute of Isotopes of the Hungarian Academy of Sciences, P.O. Box 77, H-1525 Budapest, Hungary
Souad Zyade, Gilbert Maire, and Fmncois Garin Laboratoire de Catalyse et Chimie des Surfaces. U.A. 423 du CNRS, Universite Louis Pasteur, 4, rue BIaise Pascal, 67070, Strasbourg, France (Received: January 30, 1990; In Final Form: June 25, 1990)
A series of Ptl-xC~x/A1203 bimetallic catalysts have been characterized by temperature-programmed reduction (TPR), chemisorption of hydrogen and CO, deuterium exchange using both methanol and methane, and activity for the CO/H2 reaction. A Pt-assisted reduction mechanism over the entire range of composition was established by the TPR studies as well as by the chemisorption results. An enhanced metallic dispersion for the Pt-rich catalyst and formation of bimetallic particles on the Co-rich side was also indicated. In the CO hydrogenation over the Pt-rich catalysts the predominant products are methanol and dimethyl ether whereas on the Co-rich samples hydrocarbons and higher alcohols are produced. The mechanisms of deuterium exchange with methane and methanol are significantly different, the former being catalyzed solely by metallic sites while the latter utilizes both oxide and metallic sites for stepwise and multiple exchange, respectively. On the basis of the XPS data (preceding article) as well as the chemisorption results reported here, a surface model is introduced for interpretation of the catalytic results.
Introduction The increasing production of methanol has initiated considerable effort in finding new routes for expanding its utilization. Research has been done on the use of methanol in phenol alkylation' and in the low-temperature route for methanol formation from carbon monoxide using methyl formate in the homogeneous phase,2 and methanol can also be a starting compound for production of specialty chemicals such as methyl a ~ e t a t e . ~ In all these processes methanol must be activated; thus, information about its interaction with catalysts becomes of primary interest. Furthermore, this interaction could attract great attention also in fundamental catalysis, because in methanol there are three types of bonds, C-H,C-O, and 0-H, which have different reactivities over various catalysts. The selectivity measured here reflects the contributions of the different reaction pathways involving various bond ruptures in methanol, and is therefore, characteristic of the catalyst system. In the syntheses of methanol and higher alcohols from syngas, transition noble metals were also found to be active components. First Pd4 and then Pt were established as good catalystss7 for the production of oxygenates in the medium-pressure range. Most recently, modified iridium catalysts showed activity in this rea c t i ~ nand ~ * this ~ tendency was generalized for most noble metals (1) (a) Namba, S.;Yashima, T.; Itaba, Y.; Hara, N. In Catalysis by Zeolites; Imelik, B., et al., Eds.; Elsevier: Amsterdam, 1980; p 105. (b) Tanabe, K.; Nishizaki, T. Proceedings of the 6th International Congress on Catalysis; Bond, G.C.; Wells, P. B.,Tompkins. F. C., Eds.; The Chemical Society: London, 1977; Vol. 2, p 863. (2) Forster, D.; Singleton, T. C. J . Mol. Catal. 1982, 17, 299. (3) (a) Moser, W. R.; Cnossen, J. E. Presented at the 1 Ith North American Meeting of the Catalysis Society, Dearborn, May 7-11, 1989. (b) Roeper, M.; Loevenich, H. In Catalysis in CI Chemistry; Keim, W. Ed.; Reidel: Dordrecht. The Netherlands, 1983; p 105. (c) Fakley, M. E.; Head, R. A. Appl. Catal. 1983, 5. 3. (4) Poutsma, M. L.; Elek, L. F.; Ibarbia, P. A.; Risch, A. P.; Rabo, J. A. J . Catal. 1978, 52, 157. (5) Naccache, C. Discuss. Faraday Soc. 1986. (6) Jaeger, N. I.; Jourdan, A,; Schulz-Ekloff, G.; Svensson. A.; Wildebotr, G . Chem. Express 1986, I , 697. (7) Guczi, L.iStefler, G.; Matusek, K.; Bogyay, 1.; Engels, S.;Lausch, H.; Schuster, L.; Wilde, M. Appl. Catal. 1988, 37, 345. (8) Niemantsverdriet, J. W.; van der Kraan, A. M. SurJ Interface Anal. 1986, 9, 221. (9) Niemantsverdriet, J. W.; Louwers, S.P. A.; van Grondelle, J.; van der Kraan, A. M.; Kampers, F. W. H.; Koenigsberger, D. C. Proceedings of the 9th International Congress on Catalysis; Phillips, M. J., Ternan, M., Eds.; The Chemical Institute of Canada: Ottawa, 1988; Vol. 2, p 674.
0022-3654191 12095-0802$02.50/0
in group VI11 transition metals. One of the best additives was iron,'*'5 the effect of which could be further amplified by using a zeolite framework.I6l7 Zeolite cages were presumed to confine the particle size by which a further enhancement in the selectivity toward oxygenates could be achieved. Co, Mo, Mn, and Ru supported on AI3O3could also be good additives because they are stabilized, at least partly, in low valence oxidation states due to the enhanced metal-support Here the partial electron deficient environment is ensured which promotes associative CO chemisorption. Iridium behaves somewhat anomalously as indicated by the lack of alcohols among the products of the CO/H2 reaction on the alumina-supported Ir-Co catalyst under atmospheric pressure.22 Although here the conditions for alcohol formation were provided, i.e., the presence of bimetallic particles and some amounts of Co in low valence no oxygenates were produced. In contrast, alcohol was formed on Ir-Fe catalysts at higher pressure, but only after a 10-20-h induction period! On the other hand, cobalt has already been added to Ru to enhance the rate for ethylene hyd r o f o r m y l a t i ~ n . ~The ~ Pt-Co/SiO, system was also found to (IO) Sachtler, W. M. H.; Ichikawa, M. J . Phys. Chem. 1986, 90,4752. (1 1) Fukuoka, A.; Kimura, T.; Ichikawa, M. J . Chem. Soc., Chem. Commun. 1988, 428. (12) Lazar, K.; Nimz, M.; Lietz, G.; Volter, J.; Gucd, L. Hyperf. Inreracr. 1988, 41, 657. (13) Lietz, G.; Nimz, M.; Volter, J.; Lazar, K.; Guczi, L. Appl. Coral. 1988,45,7 1. (14) Fuhoka, A.; Ichikawa, M.; Hriljac, J. A,; Shriver, D. F. Inorg. Chem. 1987, 26, 3643. (15) Delgass, W. N. Presented at the 1 lth North American Meeting of the Catalysis Society, Dearborn, May 7-1 1, 1989. (16) Ichikawa. M.; Fukuoka, A.; Kimura, T. Proceedings of the 9th International Congress on Catalysis; Phillip, M. J., Ternan. M., Eds.; The Chemical Institute of Canada: Ottawa, 1988; Vol. 2. p 569. (1 7) Jaeger, N. I.; Schulz-Ekloff, G.;Svensson, A. Proceedings of the 7rh International Zeolite Conference; Kodansha: Tokyo, 1986; p 923. (18) Tri, T. M.; Candy, J.-P.; Gallezot, P.;Massardier, J.; Primet, M.; Vedrine, J. C.; Imelik, B. J. Catal. 1983, 79, 396. (19) Tri, T. M.; Massardier, J.; Gallezot, P.; Imelik, B.J. Catal. 1984,85, 244. (20) Inoue, M.; Miyake, T.; Tagekami. Y.; h i , T. Appl. Catal. 1987,29, 285. (21) Inoue, M.; Kurusu, A.; Wakamatsu, H.; Nakajima, K.; Inui, T. Appl. Caral. 1987, 29, 361. (22) Guczi, L.; Matusek, K.; Bogyay, I.; Garin, F.; Esteban Pugs, P.; Girard, P.; Maire, G. CI. Mol. Chem. 1986, I , 355. (23) Esteban Puges, P.; Garin, F.; Weisang, P.; Bernhardt, P.; Maire, G.; Schay, Z.; Guczi, L. J. Carol. 1988, 114, 153.
0 1991 American Chemical Society
Alumina-Supported Pt-Co Catalysts produce methanol9 and here earlier studies have also indicated formation of bimetallic particles.25 These bimetallic catalysts may also catalyze the decomposition of methanol. It is well-known that a part of the methanol easily decomposes on interaction with Fe( Pd( 11 l)?' Ni( 11 1),28 and even with ZnO(0001) and ZnO films.29 Earlier, isotope exchange between methanol and deuterium gases was applied to study the interaction of methanol with Pt/A1203.at different metal dispersion^^^ and it was established that this interaction is significantly enhanced on highly dispersed platinum particles. In part 1 of this series the surface composition of the Pt,-xCox/A1203catalysts has been established by XPS." Here the bimetallic samples will be further characterized by TPR and hydrogen and C O chemisorption. Activity and selectivity of the CO/H2 reaction will also be investigated on the PtCo/A1203 catalysts for oxygenate formation. To clarify the mechanism of methanol transforamtion, hydrogen-deuterium exchange in methanol and methane will be studied.
Experimental Section Catalyst Preparation. H2PtC16and C O ( N O ~were ) ~ used with total metal loading of 10 wt % to impregnate 7-A1203(Woelm) by the incipient wetness method described earlier." In addition, samples of the Pto,sCoo,2composition were also prepared by successive impregnation. In sample A, H2PtC16was deposited first and then reduced at 720 K for 24 h, followed by the impregnation with Co(N03)2 and reduction again at 720 K for 24 h. In sample C the sequence was reversed. Two different treatments were applied before and between the reactions. In treatment I the samples were treated in O2at 570 K for 1 h and then reduced in hydrogen a t 570 K for 1 h. In treatment 11 after the calcination the samples were reduced at 720 K for 24 h. Catalyst Characterization. Reduction of the catalysts was characterized by means of temperature-programmed reduction (TPR) in a flow apparatus. Calcination of the samples was carried out at 573 K in oxygen for 1 h; then TPR was performed using 1% H2 in argon (flow rate 40 cm3 m i d ) with 5 K m i d ramp rate in the temperature range between 298 and 723 K. At 723 K the sample was purged with helium for 1 h and then it was cooled down to 373 K. At this stage hydrogen was chemisorbed at 298 K. Co was also chemisorbed at room temperature either after hydrogen desorption (in flowing Ar up to 723 K) or in a hydrogen atmosphere. Catalytic Reaction. The CO/H2 reaction after treatment I1 was performed in a tubular reactor (10 cm3) containing 0.3 cm3 of calcined catalyst placed between two plugs of quartz wool. Temperature was controlled with a precision of 2 K. The premixed feed gas (CO/H2 = 1:2) was purified by passing it through activated carbon and then prereduced manganese acetate impregnated on molecular sieve and finally through silica gel. The effluent flow was controlled by a valve usually set at 15 cm3 min-' and periodically measured by wet test meter. Effluent gases were anlyzed by using a Packard-437 gas chromatograph. Separation was camed out using a Chromosorb 101 column (100-200 mesh) which was connected to an exit of the reactor via a six-way sampling valve. Selectivity and activity values were defined in terms of carbon balance. Total rate, ro, was calculated from CO consumption and it was expressed in mol s-I (g of cat.)-' units. (24) Fukuoh, A.; Matsuzah, H.; Hidai, M.; Ichikawa, M. Chem. Leu. 1987,941. (25) Zyade, S.; Garin, F.; Maire, G. Nouv. J. Chim. 1987, 11, 429. (26) Benzinger. J. B.; Madix, R. J. J. Caral. 1980, 65, 36. (27) Kok, 0 . A.; Noordermeer, A.; Nieuwenhuys, B. E. Surf. Sci. 1983, 135, 65. (28) Gates, S.M.; Russel, J. N., Jr.; Yates, J. T., Jr. J . Coral. 1985, 92, 2s. (29) Hirschwald, W.; Hofmann, D. Proceedings of the 5rh Inrernarinal Symposium on Heferogeneous Caralysrs; Bulgarian Academy of Sciences Sofia, 1983; Part 2, p 295. (30) Guczi, L.; Hoffer, T.; Shestov, A. A,; Tetenyi, P. Chem. Eng. Commun. 1989, 83, 15. (31) Zsoldos, 2.; Hoffer, T.; Guczi, L. J. Phys. Chem., preceding article in this issue.
The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 803
I-
7 7
c o l o o O2 300c
w
Pt-Co 33:67 02 300C
Pt-Co
50:50
02 300C
Pt-Co 80:20/Co/ 02 300C Pt 100 O2 300C
I
1 350
'
I
~
450 550 650 Temperature/K
I
'
750
I
'
I
850
Figure 1. TPR profiles after 1 h oxidation in flowing O2 at 573 K.
When selectivity was calculated, dimethyl ether was included in the oxygenates but not in methanol. Hydrogen-Deuterium Exchange. Isotope exchange between methanol and deuterium gases was conducted after treatment I. The rate of the exchange was measured by mass spectrometer using a capillary leak to continuously introduce the sample to a AEI MS 20 type mass spectrometer. Experimental details are given in a previous paper.30 Corrections were made for the naturally occurring I3C, D concentrations and for the fragmentation. Initial rates of the formation of the products containing different numbers of deuterium atoms (k,,where i = 1...4) as well as the rate of the disappearance of the "light methanol" molecule (kh) were calculated from the initial slope of the corresponding curves and expressed in mol s-I (g of cat.)-' units. The ratio of the rates of stepwise to multiple exchange32was evaluated by the equation Ri/R2 ki/(k2 + k3 + (where stepwise and multiple exchange is defined by the number of hydrogen atoms exchanged (one or several, respectively) a t one sojourn of methanol on the surface). For the sake of better understanding the exchange mechanism in the methyl group of methanol, methane-deuterium exchange was also investigated according to the method of Kemball.32 Here the exchange was characterized by the initial rate, k, which is defined as the number of D atoms entering 100 molecules of methane per unit time at the beginning of the reaction. Dimethyl ether formation was characterized by K D M E which is the initial slope of the XDME vs time curves. (XDM, is the amount of ether formed per initial amount of methanol per gram of catalyst.)
Results In Figure 1 the H2 TPR data are presented. On pure Pt/A1203 the main peak appears at 560 K along with a small shoulder at around 420 K. The peak, which is characteristic of platinum reduction, also appears for Pt,,.8Coo.2, Pto.5Coo.5,and pt,,.3$00.67 compositions. On the last sample another doublet at 460 and 410 K is also observed the intensity of which increases toward higher cobalt content (Pto.I5co035). For the latter sample further two peaks can also be observed at 660 and 550 K. On pure Co/A1203 (32) Kemball, C. Adu. Caral. 1959, 11, 223.
Guczi et al.
804 The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 TABLE I: HITPR and Cbemlrorptlon of H, and CO on 10 wt 96 Ptl-,Co,/A1200s after C.kimtioa h 1 bar of O2 at 570 K' TPRCo) chemisorptn, . . nH2.
XC,, at 5%
0 20
so 67 85 100
20 (A) 20 (C)
amol
nnJnco9
pmol (g of cat.)-' (8 of cat.)-' 0 0.67 (0.0) 80 0.34 (0.3) 135 0.69 (0.5) 475 0.81 (0.7) 866 1175 0.70 (0.0)
X I 0 4 mol (g of cat.)-', of
HI 13.5 1 1.1
2.7 2.8 0.9 0 3.3 47.2
CO 21.9 19.6 19.6 27.7 18.2 5.2 2.2 88.3
CO(a)l H2(a) I .62 1.77 7.26 9.89 20.22 0.67 1.87
80
-
60
-
PR
v;
;
.-.-
al
Y
-
>
(r
'3 400
-alal
v,
20
"The figures in parentheses represent the ratio of reduced (metallic) cobalt as was found by XPS after a treatment by 1 bar of H2 at 570 K for 1 h (see part 1).
-
0
two broad peaks are measured at 520 and 670 K. At room temperature on the Pto.33Coo.a7 and Ptp.15Coo.85 samples, a small amount of hydrogen is taken up immediately after calcination (=6 X lod mol (g of cat.)-' which might be due to reduction to a small extent. The amount of hydrogen uptake measured by TPR and the results of the hydrogen and CO chemisorptions are presented in Table I. For the bimetallic samples the amount of hydrogen used for Co reduction presented in column 2 is calculated by the following method. First, from the H2 consumption measured for pure platinum, the amount of H2 used for the reduction of platinum in the corresponding bimetallic samples was calculated. This value was subtracted from the total amount of H2 measured by TPR and presented in column 2. These H2 TPR(Co) values were divided by the moles of cobalt in the corresponding sample. Thus, the fractions of the reduced cobalt were estimated (column 3). The main characteristics of the hydrogen consumption during the TPR measurements is a monotonic increase in the hydrogen uptake with increasing cobalt content. A similar trend was observed for the extent of cobalt reduction in the bimetallic catalysts with X,, = 0.5, 0.67,and 0.85 atomic fraction. The amounts of the chemisorbed hydrogen measured on the catalysts after TPR are presented also in Table I (column 4). Here two parts can be distinguished. On the samples of the pure Pt and F'b.aC%.2prepared by coimpregnation as well as by sequential impregnation (C) hydrogen chemisorption is large compared to sample (A) and the other coimpregnated catalysts. This characteristic can also be confirmed by the ratios of the adsorbed CO to H2 listed in Table I (column 6),the values being about 2 for sample C, for pure Pt/A1203,and for the coimpregnated pto.8C%.2 whereas much higher for all other catalysts. The C O + H2 reaction carried out at 505 K after treatment I1 indicates that the reaction rate is very high on the pure Co/ Al20?and on cobalt-rich samples and decreases with rising atomic fraction of platinum as shown by the rate of reaction presented in Figure 2. On the cobalt-rich samples olefins and C2+.hydrocarbons are the main products whereas oxygenate selectivity drastically drops. On the other hand, on the Pt-rich samples methanol and DME are the predominant products the amounts of which decrease with rising cobalt content. Among higher alcohols only ethanol is formed when the atomic fraction of Co is equal to or above 0.5, that is, when metallic cobalt or bimetallic particles arc also present. At higher temperatures the reaction selectivity is shifted toward hydrocarbons. In order to investigate further the transformation of the methanol formed in the CO/H2 reaction, the reactivity of the various bonds in C H 3 0 H must be established. Isotope exchange between methanol and deuterium appears to be a convenient way to study this iateraction. The exchange is indicative of either the interaction between the alcoholic hydrogen and the catalyst which results in the formation of methanol-dl, or that between the methyl group and deuterium leading to methanol-d, as well as methan~l-d~-~.
20
0
80
60
40
IO0
c o IB Figure 2. Product selectivities and rate of CO/H2 reaction (in terms of carbon balance) vs bulk concentration of Co, T = 505 K. Symbols: (*) log r,, total reaction rate, log (mol/s g of catalyst); Selectivities: (A) for C1+ hydrocarbons; (m) for olefins; ( 0 )for oxygenates including D M E ( 6 ) for ethanol (in the total oxygenates).
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40
60
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c o IB Figure 3. Rate of methanoldeuterium exchange vs bulk Co concentration: rate of consumption of methanol-do (kdoin mol s-' (g of cat.)-'. T = 413 K.
From previous studies30 it is known that the O H group in methanol is more reactive in exchange reaction over Pt/AI2O3 than the CH3 group, so the monodeuterated product consists mostly of CH30D. In Figure 3 the rate of exchange is characterized by In k, (where k, is the initial rate of consumption of light methanol) plotted vs cobalt content. Here the rate drastically increases on the sample containing a small amount of cobalt, and after a maximum the rate decreases up to the composition of Pto.sCoo.s. From this composition on a second maximum is observed, whereas pure cobalt shows negligible activity in deuterium-methanol exchange. The RI/R2ratios for methanol exchange are plotted in Figure 4. On pure Pt/A1203 methanol-dl is predominant but with increasing Co content R1/R2decreases, Le., C-H bond dissociation is facilitated. At higher Co content, when bimetallic particles are likely methanol-d, is the main product again. However, the calculated ratios at high cobalt content are rather uncertain because the overall rate is very small. Dimethyl ether is also formed during the exchange and a maximum is observed as the cobalt content increases as shown in Figure 5 . On the samples processed by treatment I1 there is no change in the general trends of the results obtained for methanol exchange after treatment I.
.
The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 805
Alumina-Supported Pt-Co Catalysts
70
60
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-
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Figure 6. Initial rates of methane-deuterium exchange vs bulk concentration of Co,T = 533 K. (H) rate of consumption of methanedo (kh in mol s-' (g of cat.)-l; (A) k,. Interconnected points refer to the coimpregnated samples, individual points at 20% c o marked with (A) and (C) refer to the successively impregnated samples of A and C, respectively.
7 I
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% Figure 5. Rate of dimethyl ether formation during methanol-ieuterium exchange. XDME is the amount of ether per initial amount of methanol for 1 g of catalyst. ~ D M Eis the rate of XDMEchange, T = 473 K.
Although it is most likely that methanol-dl consists of CH30D, one cannot exclude the presence of CH2DOH. We must, therefore, obtain separate information about the reactivity of C-H bonds in methanol. Methanedeuterium exchange was, therefore, measured on the Pt-Co samples because thereby the exchange in CH3 group of methanol can be modeled. After treatment I the rates measured at 533 K were determined either by the consumption of methanedo or by the formation of methane-d, to methane-d4. The initial rates characterized by In k, and In k, are plotted vs composition and presented in Figure 6. Here, as in methanol, the rates pass through a maximum and a considerably high value is measured for pure cobalt. For both curves the points determined for samples A at Pb.8C00.2composition significantly deviate from the interconnected data points of the coimpregnated samples. However, sample C and the coimpregnated Pt0,8C00.2behave in similar manner. In Figure 7 the R I / R 2ratio is plotted. It is characteristic that with increasing cobalt content the ratio of stepwise to multiple exchange continuously decreases; that is, the catalyst becomes more and more cobalt-like where the multiple exchange is predominant.
Discussion To interpret the catalytic data, the composition and the structure of the surface of the Pt-Co bimetallic catalysts must be understood. Based upon the results obtained here by the surface
\
\
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Guczi et al.
806 The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 to Co0.33*34It is further supported by the results of XPS indicating the presence of COOafter reduction at 570 K.)’ The second peak at 670 K can be attributed to the reduction of cobalt monoxide. However, here the ratio of TPR peaks corresponding to c3.04 to COO and COO to Coosteps is not 1/3 as expected from the l~terature.’~The deviation is attributed to the redispersion of Co2+ions spreading over the alumina, increasing the amount of the Co surface phase (CSP). The CSP can be reduced only at temperatures above 850 K which were attained in TPR but not during the XPS measurements; hence the discrepancy between the reports of reducibility of this by these techniques is anticipated. Now, we have to consider the Pt-Co bimetallic samples with high amounts of cobalt. The doublet at 460 K and at around 410 K can be ascribed to the two-step reduction of Co304to COO and further to zerovalent Co.” It must be a Pt-assisted process because it proceeds at 540 K, but only in the presence of platinum. It has to be noted that a part of the cobalt is not associated with platinum as indicated by the TPR peak at 670 K. The question now arises as to why this shift in TPR toward lower temperature appears only at these two compositions and is nonexistent at higher platinum content. The possible explanation originates from the XPS.)’ Accordingly, at these two compositions the dispersion of platinum is high enough to form a large interface with the cobalt oxide. The minute amounts of zerovalent platinum being present at the beginning of TPR measurements, thus, easily spread out on the support and the hydrogen atoms generated on Pt sites facilitate the hydrogenation of cobalt oxide species. It can be concluded from the hydrogen uptake in TPR that the amount of the Pt-Co interface responsible for the platinumpromoted reduction of cobalt oxide increases with rising cobalt content. The enhancement of this interface can be attributed solely to the increase of the surface platinum dispersion as was clearly demonstrated by the XPS results, because the size of cobalt oxide particles must increase with rising cobalt content. Thus, we may assume that in this range of composition larger cobalt oxide particles are associated with small platinum particles. The cobalt surface phase indicated by XPS cannot be. measured by the TPR since it is not reduced below 850 K3) which temperature has not been ever reached in our experiments. The quantitative data shown in columns 2 and 3 in Table I are characteristic of the reducibility of the cobalt in the different samples. The drop in reducibility of the pure cobalt sample as well as its monotonic increase with decreasing platinum content (with the exception of Pt,)&00,2 sample) clearly demonstrate that platinum, being present on the catalyst surface, promotes reduction of the ionic cobalt. The contradiction between the present results obtained for Co/A1203and those measured by XPS can be attributed to the fact that here the reduction was conducted up to higher temperature (800 K vs 570 K in XPS). In addition, TPR results also account for the H2 consumption originating from the reduction of Co3+ formed on calcination whereas metallic cobalt cannot be detected by XPS. The large fraction of cobalt reduced in the P t o . & ) 2 sample may be ascribed to the error in the hydrogen uptake calculation. The estimated hydrogen consumption for cobalt is negligible compared to that used for platinum, and thus a small mistake in the latter amount affects the accuracy to a large extent. However, the extent of reduction in the middle range of Pt-Co samples fully agrees with those measured by XPS because here the difference in reduction conditions affects the reducibility to a smaller extent. The ease of reduction of nonnoble metals in the presence of platinum has also been demonstrated by Niemantsverdriet et a1.9 This finding can be easy to rationalize as P o l a n ~ k y has ) ~ shown that there is no reduction of cobalt even on silica and it is known that interaction on this support is weaker than that on alumina. The results of H2 and C O chemisorption measurements could be considered as a third significant experimental finding. It is (33) Arnoldy, P.; Moulijn, J. A. J. Curul. 1985, 93, 38. (34) Sexton. B. A.; Hughes,A. E.; Turney, T. W. J. Curd. 1986,97,390. (35) Polansky, C. A. Ph.D. Thesis, Texas A t M University, College Station, TX, 1988.
SCHEME I: Transformation of Pt and Co Phwes over AI& Reduction calcined
“2
-
during
reduced
Part A . npt > y o
alumina
metallic platinun
cobalt curface phase
cobalt oxide
ionic platinum
metallic cobalt
0 Pto- Coo
interface or bimetallic PtCo
apparent from the data displayed in Table I that the drastic drop in the H2 chemisorption between the samples containing 0.2 and 0.5 atomic fraction of cobalt indicates a substantial change in the metal surface area. It is further supported by the ratios of the chemisorbed CO to H2 which can be seen in the last column of Table I. It we assume that H2 and CO are chemisorbed on the same metallic sites in the 1:l H,:M, or the CO,:M, ratios, the (CO/H2), should amount to a value of 2. It is, indeed, the case for pure platinum, for sample C, and for the coimpregnated Pt&00.2, whereas all other catalysts show much higher values. From these results it can be concluded that the platinum-like surface below X, = 0.5 loses its character at composition of XQ > 0.5.This change may be correlated to the formation of Pt-Co bimetallic particles via the decoration of platinum particles by metallic cobalt. This assumption is further supported by the formation of bimetallic Pt$o and PtCo species as was found by EXAFS and XRD after reduction of pt0.sC~.4,25Similar ob servation was found also on Co-Ir samples.*) Based on the results of the various surface characterization techniques, a model that describes the surface composition and morphology can be suggested (Scheme I). Here two types of surface transformations are shown separately in the upper (part A) and the lower part (part B) of the scheme. After calcination in the bimetallic catalysts with compositions of nR > nco (where nR and are the moles of platinum and cobalt, respectively) cobalt is located only in the surface phase in monolayer thickness, while platinum of ionic state is found in large particles (part A). After reduction in H2the Co surface phase (CSP) cannot be reduced under the mild conditions applied (570 K) but ionic platinum particles are converted into Pto and a part of the cobalt surface phase becomes covered by zerovalent platinum. The metallic platinum is thereby dispersed onto the cobalt surface. phase (CSP) and the DR is, therefore, higher than that on alumina as proven by XPS and by chemisorption of H2 and CO for the P~.&o,,~ sample. On the other hand, the very low H2 chemisorption measured on sample A may be due to the decoration of the platinum particles with cobalt oxide as here Co(NOd2was the second component in s u h u e n t impregnation.
The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 807
Alumina-Supported Pt-Co Catalysts In the case of the bimetallic catalysts with nR < kocompositions (part B) calcination results in the formation of CSP, platinum oxide particles, and Co304phase. On subsequent reduction by H2 both platinum and CSP are transformed into their metallic particles. This latter transformation is a platinum-assisted process which points to the existence of intimate contact between metallic platinum and cobalt particles. It is, therefore, assumed that on these samples the highly dispersed platinum is partly covered by metallic cobalt and a PtCo bimetallic interphase is formed. The presence of bimetallic particles was also detected by EXAFS as well as by XRD on the &6Coo.4 samples.25 The particle sizes measured by CO chemisorption and TEM are also comparable.z For a composition of P4,.&0,3.~the average particle size measured by TEM is 14.5 nm, whereas for pt0,5C00,5sample the same value determined by chemisorption is about 1 1.2 nm. The surface model is in full agreement with the rate and selectivity values obtained for the CO H2 reactions. The overall reaction rate increases with increasing cobalt content because the intrinsic activity of Co is higher than that of platinum. Platinum dispersion displayed by XPS and the H2 TPR data indicates an increasing metal fraction on the bimetallic catalysts. The decrease in chemisorption capacity on the pure cobalt can be explained by the lack of platinum-activated hydrogen chemisorption. Thus, cobalt stays mainly in an oxide form which does not chemisorb hydrogen. It is, however, a reasonable assumption that reduction takes place to some extent under reaction conditions thereby ensuring the promotion of active sites for C O dissociation and hydrocarbon formation. Olefin and C2+ selectivities increase with Co concentration, indicating the enhancement in chain-growing probability of the adsorbed C , units generated by the dissociative chemisorption of CO on the cobalt sites. On Pt/Alz03 and on the platinum-rich catalysts, oxygenates (methanol, dimethyl ether) are formed with a shallow maximum in methanol selectivity at the composition of Pto,&oo.ls where there is a maximum in Pt dispersion. However, the overall oxygenate selectivity keeps decreasing as the fraction of cobalt increases. This is an additional evidence for the formation of bimetallic particles. Paralleling the drop in methanol selectivity, the selectivity to ethanol increases in the range of pto.5co0.5 to pt0.&00.85. In this range of composition zerovalent cobalt is present in a larger extent and thus Co sites are generated for C O dissociation. The CH, units are available either for chain growth or CO insertion. The associatively chemisorbed C O at the Pt/AIz03 or Pt/ Co0,/AIz03 interface provides activated C O molecules to be inserted into the CH,-metal bond, as shown in the following scheme:
+
CH,
C=O /
CO
-C
v
/”.
A0-R C o O ~ A ~ O-C , C=O
-C
CH3CH20H
‘C&
A possible second role of platinum is for the activation of hyrogen chemisorption to control the Co oxidation state because a partially reduced metal is also advantageous for the oxygenate f~rmation.’~*’’ The balance between the two pathways is partly controlled by the ratio of the two metals and by the hydrogen activation on the metal site. The fact that platinum is modified by increasing its dispersion or/and by modifying the underlaying alumina support with CSP is further reflected in the methanol deuterium exchange revealing a rate maximum at pt0,8C~,z composition. The exchange is a more sensitive test for this platinum-like behaviour than that indicated by the selectivity of CO + H2 reaction because the rate of exchange is increased to large extent. Since the deuterium-methanol exchange is also initiated by methoxide formation of Al+ sites,’O we may assume that the CSP-modified alumina support sites are (36) Takeuchi, K.; Matsuzaki, T.; Arakawa, H.; Hanoaka, T.; Sugi, Y. Appl. Catal. 1989, 48. 149. (37) Derule, H.;Blanchard, M.; Canesson, P. Appl Coral. 1989.50, L1.
responsible for the 1 order of magnitude increase in the rate of exchange. The multiple exchange takes place through the involvement of metallic sites methyl group of the methoxide species already bound to the Lewis acid sites,30and we can establish that the higher the dispersion, the more multipleexchanged methanol should be formed. This again points out that in this composition region the main effect of the cobalt is the increase in the dispersion of the platinum. The more dispersed the platinum, the larger interphase is available for the methyl-platinum interaction resulting in the increase of multiple exchange. This is, indeed, shown in the large drop of R I / R 2ratio in this region. This surface model can explain the several orders of magnitude deviation in the rates of methane exchange. Platinum is an excellent catalyst for hydrocarbon e ~ c h a n g e ; thus, ) ~ here only the metallic sites play a role in the methandeuterium exchange, and conseqeuntly, the more dispersed the platinum, the higher the rate of exchange is. The stepwise exchange of methane is predominant both on pure Pt/A1203 and on Pto&oo,z sample as indicated by the change of R l / R 2 . An additional effect involving the interaction between platinum and the underlying ionic cobalt surface may be present but it requires further studies. Sample C, in which Pt was deposited on top of the already present cobalt surface phase, shows a behavior very similar to the corresponding coimpregnated sample while sample A in which the cobalt was deposited after Pt impregnation resembles the cobalt-rich samples (see Figures 6 and 7). This result supports the assumption based on XPS studies3’ that platinum (at least in part) covers the cobalt surface phase of the coimpregnated Pto,8Coo,zsample. As the amount of Coo gradually increases with the simultaneous formation of an intermetallic phase or bimetallic particles, nonalloyed Pt particles are available to lesser extent for the exchange reaction. The subsequent decrease in the activity may be explained by the formation of PtCo bimetallic particles having an intrinsic activity lower than that of pure supported Pt or Co. In the paper published by Dees and P ~ n e on c ~silica-supported ~ Pt-Co samples no extreme points in the rate and selectivity were found by only monotonous changes. It is likely due to the more severe condition of hydrogen reduction that they employed causing the weaker interaction between metals and the support. However, with increasing cobalt concentration the cobalt surface phase (CSP) uniformly covers the support and Pt-Co particles are likely formed. For methanol this results partly in the change of the interphase to where methoxide species are connected and partly in changing the metal phase composition, so the R l / R 2ratio rises again with a decreasing trend in the rate of methanol exchange. Since more and more oxide phase is also generated on the surface the depletion in surface deuterium may partly be responsible for the decrease of rate of both methanol and methanol in the middle range of composition. The change from platinumlike to cobalt-like surface is further indicated by the increasing importance of multiple exchange as indicated by decreasing R l / R z ratio for methane exchange. The increase in R I / R zfor methanol exchange at the end of the curve may be due to (a) the drastic decrease in the rate of exchange, which is characteristic of cobalt, causing large experimental errors in the calculated ratio, or (b) the increasing amount of cobalt oxide chemisorbing methanol in the way that alumina does, Le., via methoxide groups which can leave the surface as monodeuterated product but which do not seem to participate in multiple exchange while in this adsorbed state. The drop in the total rate (especially for pure cobalt) may be d6e to the lack of platinum being the active sites for the dissociation of hydrogen/deuterium molecules. A similar trend was found in dimethyl ether formation and for methanol exchange which indicates similarities in both processes. For both reactions the first step is the formation of the methoxide species which can react with chemisorbed deuterium atoms or with themselves to form dimethyl ether. As the nature of the support is changed, the rate of formation decreases as shown in Figures (38) Dees, M. J.; Ponec, V. J . Caral. 1989, 119, 376.
808
J . Phys. Chem. 1991,95,808-813
2 and 5. Obviously, high platinum dispersion favors DME formation as in the case of Pt/A1203 catalysts.30 Finally, the fact that alcohols on cobalt-rich catalysts are not formed is not due to the enhanced rate of decomposition of methanol formed from C O H2. At this composition the interaction of methanol with the surface is weak as shown by deuterium exchange, so the main reason of the absence of oxygenates is a lack of C O insertion being devoted to the presence of Pt sites.
+
Conclusions PT-and Cerich catalysts are active for alcohol and hydrocarbon formation, respectively. On bimetallic particles ethanol is also formed by the insertion of the adlineated C O molecules created at the perimeter of the Pt/A1203/CSP interface to the C , units formed on the Co site. Within the range of compositions used in this study, the high activity in both methanol and methane exchange on the platinum-rich catalysts compared to that of pure platinum is due to
the influence of finely dispersed ionic cobalt surface phase partly covered by platinum. The low activity on the cobalt-rich samples compared to that of the monometallic cobalt catalyst is attributed to the formation of Pt-Co bimetallic particles. Both C-H and 0-H bond cleavage takes place on the highly dispersed nonalloyed platinum. Splitting of the first and second C-H bonds in methanol and methane, respectively, occurs by different mechanism. While methanol activation is entirely attributed to platinummodififed support interphase, the C-H bonds in methane could be split by both metals as indicated by the rate of deuterium exchange on pure cobalt but in the latter case the multiple exchange predominates shown by low R l / R 2ratio.
Acknowledgment. We are indebted to Mrs. G. Stefler and Mr. I. Bogyay for the help in the experimental part. The fund granted by the National Scientific Research Foundation (OTKA No 1309) are greatly acknowledged.
NMR Longitudinal Relaxation Study of the Fluidity at Octadecylsllka Surfaces: Acetonitrile as a Probe of Surface Viscosity Eric H.Ellison and David B. Manhall* Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322-0300 (Received: March 16, 1990; In Final Form: July 10, 1990)
Deuterium and nitrogen-14 longitudinal (spin-lattice) relaxation measurements are used to assess the surface fluidity under conditions typical of reversed-phase high-performance liquid chromatography. A model of two phases in rapid exchange and values of the observed 2Hand I4N spin-lattice relaxation time ( T I )in suspensions of microporous silica gel (bare and C-18 modified) and acetonitrile-d, (CD3CN) are used to assess the rotational motion of surface-associated CD3CN. With use of bulk theories of relaxation and predicted values of the longitudinal relaxation rate of surface-bound nuclei ( R I , = 1 /TIS), the reorientational correlation time, r,, of surface-associated CD3CN is predicted and used to determine a surface microviscosity. The predicted value for octadecylsilica (ODS)surface viscosity of 4.6 cP, 13 times the value for neat acetonitrile, is shown to be representative of intermolecular interactions occurring in the hydrocarbon fraction of the heterogeneous ODS surface. Comparisons are made between the values For ODS surface viscosity given here and those in previous investigations.
Introduction Measurements of the surface fluidity of alkyl-modified microporous silica gel yield information for describing the separation mechanism in reversed-phase high-performance liquid chromatography (RP-HPLC). To better understand the relative roles of surface versus bulk diffusion in the overall transport of species through the stationary phase and the chromatographic column, the surface fluidity must be more closely examined. Previous studies of the fluidity at alkylsilica surfaces have been based on determinations of the time dependence of pyrene excimer formation and decay on solvated octadecylsilica (ODS) surfaces.’V2 The ability of pyrene monomers to form an excimer was evidence for a fluid or fluidlike surface environment, and partitioning was proposed, in contrast to 2-D physisorption, for the uptake of solute molecules by the surface phase.’ In other studies of ODS surface fluidity, the mobility of immobilized octadecyl chains was examined under chromatographic conditions by using I3C NMR.3-S ( I ) &gar, R. G.; Thomas, J. C.; Callis, J. B. Anal. Chem. 1984.56, 1080. (2) Stahlberg, J.; Almgren, M.; Alsins, J. Anal. Chem. 1988, 60, 2487. (3) Gilpin, R. K.; Gangoda, M. E. And. Chem. 1984,56, 1470. (4) Gilpin, R. K.; Gangoda, M. E. J . Magn. Reson. 1988, 64, 408. ( 5 ) Bayer, E.; Paulus, A.: Peters, B.; Laupp, G.: Reinen. J.; Albert, K. J . Chromatogr. 1986, 364, 25.
0022-3654/9 1/2095-0808$02.50/0
Changes in the mobility of terminal methyl groups (determined by using I3C relaxation measurements) were correlated with changes in solvent viscosity and polarity as well as changes in immobilized ligand density.’A The overall mobility of the C-18 chain was also shown to be influenced by whether or not endcapping (trimethyl silylation) was employed as a second step in the derivatization process. In this report, an alternative method is proposed for examining the surface fluidity of solvated ODs. In a previous study of the 2H longitudinal (spin-lattice) relaxation time ( T I )of D20in association with solvated ODs, we showed a minimum in T I to occur at 5% D 2 0 in the mobile phase, indicating a unique association between water and the surface phase.6 The 2H T I in suspensions was also shown to be influenced by changes in surface character, in spite of the presence of interface-induced field inhomogeneity effects. This result was expected since T I values, in contrast to T2values, are relatively insensitive to magnetic field inhomogeneity effects.’ Here we report observations of the 2H and I4N TI in suspensions of solid particles and CD3CN. Both 2H and I4N are quadrupolar (I > nuclei and therefore relax predominantly through the (6) Marshall, D. B.; McKenna, W. P. Anal. Chem. 19%4,56, 2090. (7) Glasel, J. A.; Lee, K. H. J . Am. Chem. Soc. 1974, 96, 970.
0 199 1 American Chemical Society
