New active site for CO hydrogenation formed on promoted Rh catalyst by spillover hydrogen

T. Inui et al. (Editors), N e w Aspects of Spillover Effect in Catalysis 0 1993 Elsevier Science Publishers B.V. All rights reserved.

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New active site for CO hydrogenation formed on promoted Rh catalyst by spillover hydrogen H. Kato," T.M o ~M. , ~Nakajima? Y.Mori," T. Hatton," and Y.Murakamia EDepartment of Applied Chemistry, School of Engineering, Nagoya University. Furo-cho, Chikusa-ku, Nagoya 464, Japan Qovernment Industrial Research Institute, Nagoya, Hirate-cho, Kita-ku, Nagoya 462, Japan

Abstract By adding Ti02 and Cr2O3 to a RNSi02 catalyst, Their effect on CO hydrogenation was studied with pulse surface reaction rate analysis (PSRA) and steady-state reaction under pressure. Chemisorption measurement indicated that these oxides covered a part of the Rh metal surface. TPR or 02-uptake measurement suggested that the oxides on the Rh metal particles were reduced partially by spillover hydrogen. PSRA revealed that the oxides added increased the rate constant of the step of C-0 bond dissociation. As a result, turnover frequency (TOF) was also increased. This promotion effect was explained in terms of the newly formed active site composed of surface Rh atom(s) and a partially reduced cation of the oxide on the Rh metal particles. On such an active site, C-0 bond is dissociated in the way that the oxygen side is bound to the partially reduced cation. High oxophilicity of the partially reduced cation must be the cause for the promotion in CO hydrogenation. 1. INTRODUCTION Among metals active for CO hydrogenation, Rh exhibits an interesting feature in the sense that C2-oxygenates can be produced selectively. The addition of an appropriate promoter to a Rh catalyst is reported to result in activity enhancement and also in selectivity control [l-31. The activity enhancement is explained in terms of an active site consisting of surface Rh atom(s) and a promoter element: with the aid of the oxophilic property of the promoter element, C-0 bond is readily dissociated [2,4]. So that the validity of this explanation is confirmed, the promotion effect should be examined on the basis of the rate of C-0 bond dissociation. It is also desired that the new active site responsible for enhanced activity is characterized further in a more detailed way. We developed the method of pulse surface reaction rate analysis (PSRA) for the purpose of measuring the rates of individual steps involved in CO hydrogenation, especially of the steps of C-0 bond dissociation and of the hydrogenation of the resultant surface carbon species [5]. This paper describes the promotion effect of the oxide on CO hydrogenation, especially on C-0 bond dissociation, on the basis of the rate of this step by combining the characterization of promoted Rh/Si@ by temperature-programmedreduction (TPR) and @-uptake measurement. 2. EXPERIMENTAL

Supported Rh catalyst was prepared by impregnating Si02 (Fuji Davison 57) in an aqueous solution of RhC13 followed by drying at 393 K overnight and subsequent reduction at 673 K

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for 2 h. Promoted Rh catalysts were prepared by impregnating the Rh/SiOz in an aqueous solution of ( N H ~ ) ~ T ~ O ( C ~ O ~or ).H NHqVO3, ZO followed by the same treatment as that for the Rh/SiO2. The amounts of CO adsorbed and 02-uptake on the catalyst were measured at 298 K with a conventional flow type pulse adsorption apparatus by repeatedly pulsing a small amount of CO or 0 2 into the He carrier gas. Before measurement, the catalyst was reduced in flowing H2 at 623 K for 0.5 h. The TPR experiment was performed by using an apparatus for temperature-programmed desorption. After being pretreated in flowing 9at 673 K for 2 h, the catalyst was heated in the flowing TPR gas (6.15 96 H2 in Ar) from 298 to 1123 K at the rate of elevating temperature of 10 b i n - ’ . The amount of H2 consumed by reduction was determined from a TPR peak area. The rate constant of C-0 bond dissociation was determined by using a PSRA apparatus from the dynamics of CHq produced by the hydrogenation of CO adsorbed on the catalyst. The details were described elsewhere [5]. Using a continuous flow reaction apparatus, steady-state CO hydrogenation (HdCO= 2) was carried out at 453 K under the pressure of 2 MPa [6].

3. RESULTS AND DISCUSSION 3.1. Characterization of promoted Rh/SiOz catalysts For the purpose of characterizing the active site on the promoted Rh/Si02. CO adsorption, TPR, and 02-uptake measurements were carried out. Table 1 summarizes the result of CO adsorption. Both the oxides added considerably suppressed CO adsorption. Since these oxides do not adsorb CO, covering a part of the Rh metal surface with the oxide is the cause for the suppression in CO adsorption Therefore, the coverage of the Rh metal surface with the oxide can be calculated from the decrease in the amount of adsorbed CO and Table 1 also summarizes the result. For the both oxides, the coverage increased with their amount added. Figure 1 displays a typical example of TPR spectra. A sharp single peak appeared at ca. 315 K for the unpromoted Rh/SiOz and a broad one at 700 K for 1 wt% Cr203/Si02. The amount of H2 consumed was 0.732 (mmo1)g-l for the former reduction and 0.274 (mmo1)g-1 for the latter reduction. The former value was much larger than the amount of CO adsorbed, due to the reduction of the bulk Rh203. For Cr2Oypromoted Rh/SiO2, three TPR peaks were observed. Table 1 Amount of CO adsorbed, coverage of the Rh metal surface with the oxide, and turnover frequency for different lots of Rh/SiO2 catalysts Promoter content atomic ratio Ti/Rh = 0 0.05 0.1 0.2 0.3 O/Rh = 0 0.05 0.1 0.2

CO adsorbed @mol)g-1 139 105 96 85 72 143 117 100 70

Coverage

Turnover frequency

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25 31 39 48

82.4 131 149 226 198

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Figure 1. TPR spectra for C~q-prornotedand unpromoted Rh/Si@ catalysts.

From the comparison with the unpromoted RNSiOz and the Cr203/Si02, the peaks at 33 1 K and 685 K were assigned to the reductions of the oxidized Rh metal and the isolated Cr2O3. respectively. The peak at 440 K was assignable to the reduction of Cr2O3 located on the Rh metal particles or at their peripheral portion. As a result of the additional reduction, the amount of H2 consumed, i.e., 1.50 (mmo1)g-1. was larger than the total amount for the unpromoted Rh/SiO2 and the Cr203/Si02. TPR of Ti&-promoted Rh/SiOz, on the other hand, gave no evidence for the panial reduction of the promoter oxide. In order to confirm the reduction of the promoter oxide, the amount of 02-uptake was compared with that of CO adsorbed. The ratio between these two amounts was close to 0.5 for unpromoted Rh/SiOz, as expected from the stoichiomeaiesof CO/Rh(s)= 1 and Oz/Rh(s)= 1/2 for CO adsorption and @-uptake, respectively, where Rh(s) means surface Rh atoms. For CrzO3-promoted Rh/SiO2, the ratio was larger than 0.5, as expected from TPR. TiO2promoted Rh/SiOz also gave larger ratio than 0.5, i.e., 0.64 for the atomic ratio of Ti/Rh = 0.3. This is the evidence for the partial reduction of Ti&. In conclusion, the promoter oxide located on the Rh metal particles or at their peripheral portion is partially reduced by the Hz-pretreatment for reaction. Spillover hydrogen from the Rh metal surface should play a key role in such easier reduction.

3.2. Promotion of C - 0 bond dissociation and overall CO hydrogenation on the new active site formed by spillover hydrogen The rate constant for C-0 bond dissociation was plotted in Fig. 2 as a function of the coverage. In Fig. 2. the rate constant was normalized by that for the unpromoted Rh/Si@. because the catalyst was prepared from different lots of Rh/Si@. As shown, a promotion effect on the dissociation was observed for the both oxides. It should be noted in Fig. 2 that the slope of the line corresponds to the promotion with the exclusion of the effect of the amount of the oxide added. The effect was obviously larger for Ti& than for Cr@3. TOF of overall CO hydrogenation is also listed in Table 1. It was obvious that both Ti&

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Coverage I % Figure 2. Relative rate constant of C-0 bond dissociation for promoted Rh/SiO2.

and (32% increased TOF.as a result of the promotion of C-0 bond dissociation. In CO hydrogenation, C-0bond is considered to be dissociated in the way that the oxygen side was bound to the site adjacent to the site on which CO is adsorbed. If the cation of the partially reduced promoter oxide on the Rh metal particles has oxophilicity higher than the surface Rh atom, the site consisting of a combination of surface Rh atom(@and such a cation should be more active than the site of surface Rh atoms. The promotion effect of Ti02 and Cr2O3 should be the case. The difference in the promotion effect between these two oxides must result from their oxophilicity different from each other. The heat of formation of oxide may be a measure for oxophilicity: the oxide with the larger value of heat formation is recognized to have higher oxophilicity. The heat of formation is 471 and 375 kJ(mol.atom-O)-l for Ti@ and Cr2O3. respectively, the difference of which explains their promotion effect different from each other. This supports the idea that the newly formed active site is responsible for activity enhancement. 4. REFERENCES 1 M.M. Basin, W.J. Bertley, P.C. Ellgen, and T.P. Wilson. J. Catal., 54 (1978) 120. 2 M.Ichikawa. T. Fukushima, and K. Shikakura. Proc.8th Int. Congr. Catal., 2 (1984) 69. 3 H. Arakawa, T. Hanaoka, K. Takeuchi, T. Matsuzaki. and Y. Sugi, Proc. 9th Int. Congr. Catal., 2 (1988) 602. 4 M. Ichikawa, A.J. Lang, D.F.Shriver, and W.M.H. Sachtler, J. Am. Chem. SOC.,107 (1985) 7216. 5 T. Mori, A. Miyamoto, H. Niizuma, N. Takahashi, T. Hattori, and Y.Murakami, J. Phys. Chem.. 90 (1986) 109. 6 Y.Mori, T. Mori, T. Hattori, and Y.Murakami, Appl. Catal., 66 (1990) 59.