MECHANISM OF BRANCHED CARBON-CHAIN FORMATION FROM CO AND H2 OVER OXIDE CATALYSTS .1. ADSORBED SPECIES ON ZRO2 AND CEO2 DURING CO HYDROGENATION

The adsorbed species on ZrO2 and CeO2 during CO hydrogenation forming branched carbon chains, especially isobutene, (over ZrO2), have been investigated by chemical trapping, in situ IR, and solid-state NMR methods. These indicated that methoxide and formate were present as the surface species on ZrO2 . CO hydrogenation on ZrO2 with pre-adsorbed methoxide or formate showed that the pre-adsorption of methoxide promotes the formation of the higher hydrocarbons containing mainly isobutene but that formate retards the reaction. Chemical trapping experiments on CeO2 after CO hydrogenation at 523 and 673 K led to the formation of methane together with methanol, indicating the presence of methyl, mu-methylene, or carbene as a surface species. Eta2-Formaldehyde was suggested to be the precursor of the methyl, mu-methylene, or carbene species, the latter of which in turn gives C2 species by insertion of CO. Following the products of CO hydrogenation and chemical trapping on CeO2 at 523 K with time indicated that an aldol-condensation-type reaction leads to C3 and branched-chain C4 products from C2 oxyhydrocarbon.