Towards a Molecular Level Understanding of CO and H$_{2}$ Adsorption and Dissociation on Cobalt Nanoparticles

The development of sustainable energy technologies, including the production of synthetic fuels, is of global importance. Fischer-Tropsch synthesis (FTS) has recently gained increased attention as it involves the formation of hydrocarbons via the catalytic conversion of syngas (CO and H$_{2})$, which can be derived from renewable sources. FTS is often performed using Co-based catalysts that are greatly affected by the adsorption state of reactants, as well as nanoparticle shape and size. Here we have used low-temperature scanning tunneling microscopy (LT-STM) to study the interaction of syngas with well-defined Co nanoparticles grown onto Cu {\{}111{\}}, an inert metal for FTS. Hydrogen adsorbs dissociatively on the Co surfaces, resulting in three unique coverage-dependent phases. We demonstrate that these phases can resolve crystal packing ambiguities of the underlying Co nanoparticles, a question that has been debated in the literature. Simultaneous exposure of the Co to H$_{2}$ and CO results in segregated islands of the adsorbates on the nanoparticle surface at 80 K, and we propose that atomic H blocks CO adsorption, causing the build-up of CO at the nanoparticle step edges. With increasing CO coverage, a two-dimensional phase compression of H by CO is observed, providing \textit{the first direct} visualization of this phenomenon in a catalytically relevant system. Our data suggest that FTS reactivity may be dominated by the interface length between the adsorbates and be subject to unforeseen kinetic restraints as a function of particle size.