Quantitative relationship between support porosity and the stability of pore-confined metal nanoparticles studied on CuZnO/SiO2 methanol synthesis catalysts

Abstract

Metal nanoparticle growth represents a major deactivation mechanism of supported catalysts and other functional nanomaterials, particularly those based on low melting-point metals. Here we investigate the impact of the support porous structure on the stability of CuZnO/SiO2 model methanol synthesis catalysts. A series of silica materials with ordered cagelike (SBA-16 mesostructure) and disordered (SiO2-gel) porosities and varying pore sizes were employed as catalyst supports. Nitric oxide moderated nitrate decomposition enabled the synthesis of catalytically active Cu nanoparticles (3-5 nm) exclusively inside the silica pores with short interparticle spacings. Under relevant reactive conditions, confinement of the Cu particles in cagelike silica pores notably enhances catalyst stability by limiting Cu particle growth as compared to catalysts deposited in SiO2-gel host materials with also 3D and highly interconnected though unconstrained porosity. For both pore morphologies, we find a direct relationship between catalyst stability and support porosity, provided the narrowest characteristic pore dimension is employed as a porosity descriptor. For cagelike porosities this corresponds to the size of the entrances to the nanocages. Our results point to nanoparticle diffusion and coalescence as a relevant growth mechanism under reactive conditions and underscore the significance of the narrowest pore constrictions to mitigate growth and improve catalyst stability. This finding contributes to the establishment of general and quantitative structure-stability relationships which are essential for the design of catalysts and related functional nanostructures with long lifetimes under operation conditions.