Interplay between pore size and nanoparticle spatial distribution: consequences for the stability of CuZn/SiO2 methanol synthesis catalysts

Abstract

Particle growth is a major deactivation mechanism for supported metal catalysts. This study reveals that the impact of pore size on catalyst stability is very sensitive to the nanoscale metal distribution. A set of ex-nitrate CuZn/SiO2 catalysts was synthesised using SiO2-gel supports (pore size 5-23 nm). The catalyst compositions were adjusted to attain series of catalysts with either constant pore volumetric (1.6 Cu nm-3) or surface (2.0 Cu nm-2) overall metal loading. The procedures of thermal decomposition of the metal nitrate precursors were adjusted to achieve <10 nm Cu particles displaying markedly different nanospatial distributions, i.e either gathered in high-metaldensity domains with small interparticle spacings or evenly distributed over the support with maximum interparticle spacings. Under industrially relevant methanol synthesis conditions, a strong increase of the deactivation rate with the support pore size is observed for catalysts with high-density domains of Cu particles. For these samples the local, nanoscale Cu surface loading is determined by pore size rather than by the overall metal content, as ascertained by HAADF-STEM/EDX. Conversely, Cu nanoparticles evenly spaced on the surface of the SiO2 carrier show improved stability, being the deactivation rate chiefly independent of the support pore size. The differences in catalyst stability are ascribed to the dominance of different particle growth mechanisms. Our study highlights the significance of local, nanoscale properties for rationalizing the relevance of structural parameters such as pore size for catalyst stability.