Unravelling structure sensitivity in CO2 hydrogenation over nickel

Abstract

T he reduction of CO 2 emissions into the Earth's atmosphere is gaining legislative importance in view of its impact on the climate [1][2][3][4][5] . Reduction of the harmful effect of these emissions through reclamation of CO 2 is made attractive because CO 2 can be a zero-or even negative-cost carbon feedstock 6,7 . The conversion of renewably produced hydrogen and CO 2 into methane, or synthetic natural gas, over Ni is a solution that combines the potential to reduce CO 2 emissions with a direct answer to the temporal mismatch in renewable electricity production capacity and demand [8][9][10][11][12][13][14][15][16][17] . Chemical energy storage in the form of hydrogen production by electrolysis is a relatively mature technology; however, the required costly infrastructure, and inefficiencies in distribution and storage deem it inconvenient for large-scale application in the near future. Point-source CO 2 hydrogenation to methane represents an alternative approach with higher energy density. Furthermore, methane is more easily liquefied and can be stored safely in large quantities through infrastructures that already exist 18,19 . Power-to-gas (in this case methane) is thus actively considered as being capable of balancing electric grid stability, which will allow us to increase the renewable energy supply 20 .The search for fossil fuel alternatives, and application of a process such as that described above can arguably be achieved only with the help of advances in catalysis and the closely related field of nanomaterials. Continuous efforts in both fields have allowed us to make increasingly smaller and catalytically more active (metal) particles. However, it is already known that making progressively smaller supported catalyst particles does not necessarily linearly correspond to higher catalytic activity [21][22][23] . This phenomenon, where not all atoms in a supported metal catalyst have the same activity, is called structure sensitivity and is often attributed to the distinctly different chemistries on different lattice planes for π -bond activation in CO 2 , or σ -bond activation in, for example H 2 dissociation and C-H propagation 21,24 . The availability of stepped (less coordinated) versus terrace (more coordinated) sites on the surface of supported catalyst nanoparticles obviously changes with particle size, and atomic geometries become particularly interesting below 2 nm where, for example, π -bond activation is believed to not be able to occur 21 . While particle-size effects have been extensively studied for CO hydrogenation over Co 23,25 , the understanding of such structure sensitivity effects for these critical smaller metal particle sizes is lacking as sub-2-nm particles prove difficult to synthesize for first-row transition metals (Co, Fe and Ni). However, a particle-size effect for CO 2 hydrogenation is much less well established 26 .Here, we used a unique set of SiO 2 -supported Ni nanoparticles with diameters ranging from 1 to 7 nm in size, and show not only the existence of a distinct pa...