It has been known for centuries that 'particle size' can very strongly affect chemical reactions.

At 'normal' scales, it's generally the case that the smaller the size of reactant particles, the faster the reaction progresses. This is often simply the result of increased surface area.

In recent years however, the possibility of creating nano-scale particles has shown that they often have radically different reactive properties. (see example Gold Catalysisplugin-autotooltip__plain plugin-autotooltip_bigGold Catalysis

"Catalysis by gold has rapidly become a hot topic in chemistry, with a new discovery being made almost every week. Gold is equally effective as a heterogeneous or a homogeneous catalyst and in this Review we attempt to marry these two facets to demonstrate this new found and general efficacy of gold. The latest discoveries are placed within a historical context, but the main thrust is to highlight the new catalytic possibilities that gold‐catalyzed reactions currently offer the …)

The development of nanomaterials has made its mark in nanocatalytic applications and ensuing plethora of nanotechnological advances. The size‐dependent chemistry of nanomaterials and consequent controlled and designed synthesis of smart materials with desired end application has provided us a number of new products which are already making impact on quality of human life. The fundamental nano paradigm shift would influence future research advances in field of nanoscience chemistry, ranging from drug delivery to exquisite designs of novel catalysts that drive innovations in chemical synthesis and transformations. "

Source : Nano Science Nanochemistry and Nanocatalysis Science: Research advances and future perspectives Journal of Materials Nano Science 6 (1), 1‐6

However, a lack of understanding of the exact atomic processes involved means that it's very hard - or sometimes impossible - to accurately predict how chemicals will react with each other when the particle sizes are at nanoscale.