Tuning adsorption energies and reaction pathways by alloying: PdZn versus Pd for CO2 reduction to methanol
Résumé
The reduction of CO2 greenhouse gas emissions and the availability of a reliable, and secure carbon-neutral energy supply are vital in order to slow down the rate of climate change while replacing fossil sources [1]. Anthropogenic CO2 conversion to methanol can help to face this double challenge. Methanol is a safe, easily transportable and fastly biodegradable liquid fuel which can be used with today’s vehicle technology at minimal incremental costs. Catalytic CO2 hydrogenation to methanol (CO2 + H2 → H2O + H3COH), with hydrogen produced from water electrolysis, has proven to be a viable process at the pilot plant scale in several countries. Because CO2 is a very stable molecule, highly efficient catalysts are required to reduce it to methanol. For now, the Cu-ZnO-Al2O3 catalyst, initially developed for CO hydrogenation to methanol, is used in combination with harsh pressure conditions (> 50 bar). In this context, the design of more efficient catalysts for decreasing the pressure as well as the temperature is an active field of research.
Several Pd-based compounds are recognized as attractive catalysts for CO2 reduction to methanol. Compared to Cu-based catalysts, they can be chemically stable, resistant to sintering, and present superior activity toward methanol production at lower temperatures. They can be tuned by alloying different elements to design catalysts with greater activity, selectivity, and stability than single metals. By comparing the Pd(111) and PdZn(111) model catalysts for CO2 hydrogenation to methanol, we show that intermetallic alloying is a possible strategy to control the reaction pathway from the tuning of adsorbate binding energies. In comparison to Pd, the strong electron donor character of PdZn weakens the adsorption of carbon-bound species and strengthens the binding of oxygen-bound species. As a consequence, the first step of CO2 hydrogenation more likely leads to the formate intermediate on PdZn, while the carboxyl intermediate is preferentially formed on Pd. This results in the opening of a pathway from carbon dioxide to methanol on PdZn similar to that previously proposed on Cu.
These findings rationalize the superiority of PdZn over Pd for CO2 conversion into methanol, and suggest guidance for designing more efficient catalysts by promoting the proper reaction intermediates [1].
[1] Olah, G. A. Beyond oil and gas: the methanol economy, Angew. Chem. Int. Ed. 44 (2005) 2636-2639.
[2] Palo, D. R. et al., Methanol steam reforming for hydrogen production, Chemical reviews 107 (2007) 3992-4021.
[3] Trimm, D. L. and Önsan, Z. I. Onboard fuel conversion for hydrogen-fuel-cell-driven vehicles, Catalysis Reviews 43 (2001) 31-84.
[4] Brix, F. et al, Tuning adsorption energies and reaction pathways by alloying : PdZn versus Pd for CO2 reduction to methanol, J. Phys. Chem. Lett. 11 (2020) 7672-7678.