Engineered core@shell nanomaterials are widely studied for applications ranging from sensing over drug delivery to sorption and catalysis. For catalysis, many investigations have highlighted the remarkable thermal stability of this class of catalysts. However, beyond stability, core@shell materials with porous shells enable rational catalyst design for reaction selectivity: Core@shell nanocatalysts can be considered "nano-reactors" with membrane walls which allow selective access to the embedded active sites and hence tuning of reaction selectivity. We demonstrate the principle of core@shell nanomaterials as tuneable catalysts using Ni@SiO2 as a model catalyst. By tailoring the SiO2 shell thickness via careful control of synthesis parameters, we demonstrate preferential diffusion of reactants through this shell during catalytic oxidation of CH4 as model reaction. We then demonstrate application of this diffusion control to preferential conversion of H2 in H2-CH4 mixtures using preoxidized NiO@SiO2. Our results suggest that core@shell nanocatalysts can be rationally engineered and utilized as tuneable catalysts to tailor reaction selectivity in a rather straightforward, predictive way.