Selective C-C coupling catalyzed by solid catalysts is pertinent to the manufacture of fuel and chemical products from renewable or available carbon sources such as biomass and C1 compounds including methane and CO2. A major pathway that accomplishes C-C coupling between organic oxygenates is aldol addition. Through a combination of ultra-high vacuum (UHV) experiments on synthetic CeO2(111) thin films, including temperature programmed reaction (TPR) and reflection absorption infrared spectroscopy, and density functional theory based theoretical modeling, we have identified common as well as distinct features in the formation of C-C coupled products from acetaldehyde and acetic acid, i.e. crotonaldehyde and ketone, respectively. Combined experimental and theoretical evidence suggests that C-C coupling is mediated by enolate intermediates, and that for both molecules enolization is facilitated by oxygen vacancies. In particular, we have devised a special TPR procedure to significantly enhance the formation of crotonaldehyde on partially reduced CeO2(111) surfaces under UHV conditions, which is reported for the time in the literature. This procedure sheds light on a hitherto unrecognized aspect in the C-C coupling of acetaldehyde, the dynamic formation of adsorbate-stabilized oxygen vacancy dimers as active sites for C-C coupling.

Mechanistic insights for C-C coupling of simple carbonyl compounds on CeO2(111)

2019 Spring National Meeting

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