Lanthana-promoted RhSiO2: II. Studies of CO hydrogenation

doi:10.1016/0021-9517(88)90091-7

Journal of Catalysis

Volume 111, Issue 2, June 1988, Pages 325-335

https://doi.org/10.1016/0021-9517(88)90091-7 Get rights and content

Abstract

A study of the influence of lanthana promotion on the hydrogenation of CO over $Rh SiO_{2}$ has been conducted. Lanthana-promoted $Rh SiO_{2}$ exhibits higher turnover frequencies for the synthesis of CH₄, C₂C₄ hydrocarbons, CH₃OH, and C₂ oxygenates than unpromoted $Rh SiO_{2}$ . The turnover frequency for each product goes through a maximum with increasing lanthana addition. Lanthana promotion is also found to increase the selectivity for the formation of CH₃OH, C₂ oxygenates, and C₂C₄ hydrocarbons and to decrease the selectivity for CH₄. The selectivity for C₂ oxygenates is optimized at low levels of lanthana addition. In situ infrared observations show that lanthana promotion blocks the chemisorption of CO onto surface Rh sites. Of particular interest is the observation of a band at 1725 cm⁻¹. This feature is assigned to CO adsorbed on a Rh site immediately adjacent to an LaO_x island. The low frequency of this vibration is attributed to a weakening of the CO bond caused by the interaction of lanthana cations with the oxygen end of the adsorbed CO. It is postulated that the interaction of the lanthana with the CO is responsible for the enhanced rates of CO dissociation and the formation CH₄ and C₂₊ products. Infrared spectroscopy also provides evidence for acyl, formate, and acetate groups on the surface of lanthana-promoted $Rh SiO_{2}$ . The possible role of these species in the synthesis of products is discussed.

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This band can be attributed to CO species bonded at the metal–support interface in such a way that the molecule interacts with the support via either the carbon atom (species I) or the oxygen atom (species II) [41]: Results of several studies suggest that weakening of the C–O bond caused by the interaction of the metal oxide additive with the oxygen end of adsorbed CO (species II) is responsible for the observed bands at frequencies around 1725 cm−1 [41–43]. Summarizing, results of Fig. 1 show that doping with CaO results in an increase of the relative intensity of the LF-bands and the thermal stability of the corresponding linear and bridged CO species adsorbed at the metal–support interface.
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