Metal-Specific Reactivity in Single-Atom Catalysts: CO Oxidation on 4d and 5d Transition Metals Atomically Dispersed on MgO

[EN] Understanding and tuning the catalytic properties of metals atomically dispersed on oxides are major stepping-stones toward a rational development of single-atom catalysts (SACs). Beyond individual showcase studies, the design and synthesis of structurally regular series of SACs opens the door to systematic experimental investigations of performance as a function of metal identity. Herein, a series of single-atom catalysts based on various 4d (Ru, Rh, Pd) and Sd (Ir, Pt) transition metals has been synthesized on a common MgO carrier. Complementary experimental (X-ray absorption spectroscopy) and theoretical (Density Functional Theory) studies reveal that, regardless of the metal identity, metal cations occupy preferably octahedral coordination MgO lattice positions under step-edges, hence highly confined by the oxide support. Upon exposure to O-2-lean CO oxidation conditions, FTIR spectroscopy indicates the partial deconfinement of the monatomic metal centers driven by CO at precatalysis temperatures, followed by the development of surface carbonate species under steady-state conditions. These findings are supported by DFT calculations, which show the driving force and final structure for the surface metal protrusion to be metal-dependent, but point to an equivalent octahedral-coordinated M4+ carbonate species as the resting state in all cases. Experimentally, apparent reaction activation energies in the range of 96 +/- 19 kJ/mol are determined, with Pt leading to the lowest energy barrier. The results indicate that, for monatomic sites in SACs, differences in CO oxidation reactivity enforceable via metal selection are of lower magnitude than those evidenced previously through the mechanistic involvement of adjacent redox centers on the oxide carrier, suggesting that tuning of the oxide surface chemistry is as relevant as the selection of the supported metal.XAS experiments were performed at B18 beamline, Diamond Light Source, United Kingdom (proposals Nr. SP17377 and SP19072) and BL22 beamline, ALBA Light Source, Spain (experiment 2019023278). Beamline scientists D. Gianolio (Diamond) and L. Simonelli and C. Marini (ALBA) are acknowledged for their assistance with the beamline setup during XAS experiments. The authors are grateful to M. Garcia, E. Andres, M. E. Martinez, and I. Lopez (ITQ) for assistance during the XAS experiments. J. Ternieden (MPIKOFO) is acknowledged for the performance of XRD experiments. J.M. Salas (ITQ) is acknowledged for his experimental contribution to the CO-FTIR studies, and M.D. Soriano and A. Munoz for the recording of XP spectra. P.N.P. and F.S. acknowledge support by the state of BademWurttemberg through bwHPC (bwUnicluster and JUSTUS, RV bw17D01) and support from the Helmholtz Association is also gratefully acknowledged. This research received funding from the Alexander von Humboldt Foundation (postdoctoral grant to B.B.S.), the Max Planck Society and the Fonds der Chemischen Industry (FCI, Germany). The authors are grateful to Prof. Ferdi Schuth for the provision of lab facilities and support throughout the project. Funding from the Spanish Ministry of Science, Innovation and Universities (projects SEV 2016-0683 and RTI2018-096399-A-100) is also acknowledged.Sarma, BB.; Plessow, PN.; Agostini, G.; Concepción Heydorn, P.; Pfänder, N.; Kang, L.; Wang, FR.... (2020). 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Sustainable land-use alternatives in tropical rainforests? Evidence from natural and social sciences.

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