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Abstract

Oxide-supported copper nanoparticles attract interest as solid catalysts for the selective hydrogenation of CO₂ and its organic derivatives carbonates to methanol. Although, reaction rates and selectivity are known to depend strongly on the nature of the oxide support/promoter at the metal periphery, the achievement of a quantitative description of such metal/oxide promotion effects, which is an essential step towards a more rational catalyst design, remains a major challenge. Therefore, in this thesis, the promotion effects exerted by the oxide support are investigated systematically. Particularly, the relative electron-withdrawing capacity (Lewis acidity) of coordinatively unsaturated metal surface sites (cus) on the surface of the oxide support is used as an experimentally accessible and rather generally valid descriptor for catalytic performance.
First, a series of model catalysts is prepared via the dispersion of Cu nanoparticles on a mesoporous γ-Al₂O₃ carrier, which had been previously overlaid with different transition metal or lanthanide oxides, e.g. SmO_x, YO_x, ScO_x, ZrO_x, and TaO_x, spanning in a broad range of Lewis acidity. The Lewis acidity of cus on the surface of the oxide support materials is quantified on the basis of the intramolecular charge transfer (IMCT) of alizarin (1,2-dihydroxy-9,10-anthracenedione) as a probe molecule. The nanospatial distribution and dispersion of Cu on the series of model catalysts is complementarily assessed by means of STEM-EDX and XPS.
In the reaction of methanol synthesis by CO₂ hydrogenation, the apparent activation energy (E_a) for methanol formation is found to down-scale linearly with the surface Lewis acidity of the oxide support, making this single physicochemical parameter a suitable reactivity descriptor in the whole study space. In correspondence with this performance trend, in situ Fourier-Transform infrared (FTIR) spectroscopy reveals that both the ionic character as well as the relative reactivity of bidentate formate species, developed on the catalyst surface under reaction conditions, vary systematically with the surface Lewis acidity of the oxide support. These findings support the involvement of oxide-adsorbed bidentate formate species as reaction intermediates, and point to the relative electron-accepting character of the Lewis cus on the oxide surface as the factor determining the stability of these intermediates and the overall energy barrier for the reaction.
In the hydrogenation of propylene carbonate, the initial reaction rate as well as the apparent activation energy show linear correlations with the surface Lewis acidity of the oxide support. In this case, oxide supports exposing cus centers of increasingly higher Lewis acidity result in higher apparent activation energies and lower reaction rates. The selectivity to methanol shows a volcano dependence with a maximum at intermediate oxide Lewis acid-base character. Higher selectivities to carbon oxide byproducts (CO and CO₂) are observed for both markedly Lewis acidic and basic oxide supports, which promote undesired decarbonylation and decarboxylation reaction pathways, respectively. Different modes of activation of the carbonate substrate on cus Lewis centers of the oxide supports are invoked to explain the systematic evolution of the catalytic performance with the surface Lewis acidity of the oxide at the periphery of the supported Cu nanoparticles.
The results and analyses presented in this thesis contribute towards a unifying and quantitative description of support effects in the hydrogenation of CO₂ and organic carbonates to methanol on oxide-supported copper nanoparticles, and provide a blueprint for a predictive description of metal-oxide promotion effects, which are ubiquitous in heterogeneous catalysis.