A combined theoretical/experimental study highlighting the formation of carbides on Ru nanoparticles during CO hydrogenation

Formation of stable carbides during CO bond dissociation on small ruthenium nanoparticles (RuNPs) is demonstrated, both by means of DFT calculations and by solid state 13C NMR techniques. Theoretical calculations of chemical shifts in several model clusters are employed in order to secure experimental spectroscopic assignations for surface ruthenium carbides. Mechanistic DFT investigations, carried out on a realistic Ru55 nanoparticle model (∼1 nm) in terms of size, structure and surface composition, reveal that ruthenium carbides are obtained during CO hydrogenation. Calculations also indicate that carbide formation via hydrogen-assisted hydroxymethylidyne (COH) pathways is exothermic and occurs at reasonable kinetic cost on standard sites of the RuNPs, such as 4-fold ones on flat terraces, and not only in steps as previously suggested. Another novel outcome of the DFT mechanistic study consists of the possible formation of μ6 ruthenium carbides in the tip-B5 site, similar examples being known only for molecular ruthenium clusters. Moreover, based on DFT energies, the possible rearrangement of the surface metal atoms around the same tip-site results in a μ-Ru atom coordinated to the remaining RuNP moiety, reminiscent of a pseudo-octahedral metal center on the NP surface.The authors thank the HPCs CALcul en MIdi-Pyrénées (CALMIP-OLYMPE, grant P1415) and GENCI (grant 2018-A0050810168) for generous allocations of computer time. The computational resources provided by the high-performance computational facility of the Babeş-Bolyai University (MADECIP, POSCCE, COD SMIS 48801/1862) are also acknowledged. I.-T. M. acknowledges financial support from the grant of the Romanian CNCS–UEFISCDI, project number PN-III-P4-ID-PCE-2016-0351, within PNCDI III. I.-T. M. also acknowledges the financial support received through the project: Entrepreneurship for innovation through doctoral and postdoctoral research, POCU/360/6/13/123886 co-financed by the European Social Fund, through the Operational Program for Human Capital 2014–2020. We additionally thank ERC Advanced Grant (MONACAT 2015-694159) for financial supportPeer reviewe