On the Structure Sensitivity of CO2 Hydrogenation over Cu/ZrO2: Insights into the Role of the Support and the Active Sites

Abstract

The well-known structure sensitivity of CO₂ hydrogenation to methanol has shown to be an impactful topic for the performance of the catalyst and yet remains unaddressed for Cu nanoparticles supported on ZrO₂, a material that has shown to be involved in the active site and the reaction mechanism of methanol formation. Herein, Cu/ZrO₂ catalysts were studied to unravel the underlying structure–activity relationships by combining surface and bulk characterization techniques, kinetic measurements, operando-DRIFTS and DFT calculations. Contrary to Cu over inert supports, the results showed different trends and two distinct kinetic regimes. For Cu nanoparticles larger than 2 nm, they are in accordance with previously reported results, this is, a change in the number of active sites, without affecting the nature of them. Conversely, it is demonstrated that the active sites are markedly different over the regime of nanoparticles smaller than 2 nm, accessed for ultralow Cu contents of 0.1 wt %, as evidenced from the systematic change of kinetic parameters and from operando-DRIFTS. The distinct active sites were identified as isolated Cu species (i.e., single atoms and Cu incorporated into the lattice of ZrO₂) and highly stable Cu clusters, both of which would allocate the formation of products for low metal contents. The results are certainly related to the interaction between Cu and ZrO₂ and unequivocally disclose the relationship between activity regimes and the nature of active sites as a function of the Cu particle size. Furthermore, they demonstrate that distinct active sites can be accessed just by varying the metal content on active reducible supports. As such, the findings are of particular relevance for the fundamental understanding of the interaction between Cu and ZrO₂ and its interdependence with the size of the Cu nanoparticles, as well as for the rational design of catalysts for CO₂ hydrogenation to methanol.