Chemical Control of Noble Metal Catalysis by Main Group Elements
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INTRODUCTION Main group elements have often been used as additives for the modification of catalysis of noble metals. Heavy 13-16 main group elements such as Se and Sn show unique synergistic catalyses when they are present on the surface of noble metal particles. For example, Se° adsorbed on Rh particles promoted CO insertion reaction[l,2] and Sn° on Pt and Rh particles enhanced the NO decomposition reaction[3-5]. Rh-Ge,-Sn, and -Pb were also active for the selective ester and citral hydrogenation to alcohol[6,7]. In these catalysts, the main group elements in low valence states have been demonstrated to have direct bondings with noble metal atoms to modify the active metal sites electronically and geometrically as well as to provide new active sites owing to their oxophillic properties[l, 4, 5]. On the other hand, oxides of heavy 13-16 main group elements have accepted interests as one component of mixed oxides catalysts such as MoBi, [8] FeSb, VSb, [9-12], SnSb [13, 14] oxides which are used for selective oxidation. In these catalysts, the main group elements work as oxygen donor sites, hydrogen extraction sites and adsorbed sites of allyl species[8, 9, 13, 141. The reaction sites may be coupled with each other through the diffusion of adsorbed species and reaction intermediates. From these points of view, it is possible to prepare active surfaces composed of noble metal 99
Mat. Res. Soc. Symp. Proc. Vol. 497 ©1998 Materials Research Society
particles and main group metal oxides for selective oxidation and hydrogenation, where the reducibility of heavy 13-16 main group metal oxides may regulate the catalysis of noble metal particles. In this paper we will report our recent investigation of synergistic modifications of Rh and Pt catalyses by one-atomic layer GeO 2 and bulk SbOx respectively[15-22]. EXPERIMENTAL
The one-atomic layer GeO 2 on Si0 2 (aerosil 300 pretreated at 473 K for 1 h) was prepared by the reaction of surface OH groups of Si0 2 with Ge(OMe)4116]. The obtained sample was calcined at 693 K for 1 h under 20.0 kPa of oxygen in a closed
circulating system. The loading of Ge was 7.4 wt% which was a maximum loading by one cycle deposition of Ge(OMe) 4 . Then the one-atomic layer GeO 2 on SiO 2 was impregnated with a Rh 6(CO)1 s chloroform solution under Ar atmosphere[19]. Rh loading was 2.0 wt%. The sample thus obtained was reduced at given temperatures for 2 h under 13.3 kPa of hydrogen. The Sb01 support was prepared by hydrolysis of SbCI5 with aqueous ammonia solution, followed by calcination at 773 K. Pt was supported on the SbOx by an impregnation method using an acetone solution of Pt(acac) 2 , followed by drying at 323 K and calcination at 773 K. Pt loading was 0.5 wt%[17, 22].
Reaction1 CO hydrogenation was carried out at PCO-=PHi= 13.3 kPa and 523 K in a closed 4 circulating system. NO reduction by CO was carried out at P~j=Pco= kPa and 423 K in a closed circulating system. The 0.3 g Rh/one-atomic layer GeO 2 /Si0 2 was used for both reactions. The ethylacetate hydrogenation reaction on t
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