Mechanism of branched carbon-chain formation from CO and H2 over oxide catalysts. Part 1.—Adsorbed species on ZrO2 and CeO2 during CO hydrogenation

Abstract

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, µ-methylene, or carbene as a surface species. η² Formaldehyde was suggested to be the precursor of the methyl, µ-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.