Electronic State of Ruthenium Deposited onto Oxide Supports: An XPS Study Taking into Account the Final State Effects
Applied Surface Science
||Differential charging, Electronic state, Oxide supports, Ruthenium, XPS
Larichev Yurii V.
Moroz Boris L.
Bukhtiyarov Valerii I.
G.K. Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, 5 Avenue Akademika Lavrentieva, 630090 Novosibirsk, Russia
Novosibirsk State University, 2, Pirogova Street, 630090 Novosibirsk, Russia
The electronic state of ruthenium in the supported Ru/EOx (EOx = MgO, Al2O3 or SiO2) catalysts prepared by with the use of Ru(OH)Cl3 or Ru(acac)3 (acac = acetylacetonate) and reduced with H2 at 723 K is characterized by X-ray photoelectron spectroscopy (XPS) in the Ru 3d, Cl 2p and O 1s regions. The influence of the final state effects (the differential charging and variation of the relaxation energy) on the binding energy (BE) of Ru 3d5/2 core level measured for supported Ru nanoparticles is estimated by comparison of the Fermi levels and the modified Auger parameters determined for the Ru/EOx samples with the corresponding characteristics of the bulk Ru metal. It is found that the negative shift of the Ru 3d5/2 peak which is observed in the spectrum of ruthenium deposited onto MgO (BE = 279.5–279.7 eV) with respect to that of Ru black (BE = 280.2 eV) or ruthenium supported on γ-Al2O3 and SiO2 (BE = 280.4 eV) is caused not by the transfer of electron density from basic sites of MgO, as considered earlier, but by the differential charging of the supported Ru particles compared with the support surface. Correction for the differential charging value reveals that the initial state energies of ruthenium in the Ru/EOx systems are almost identical (BE = 280.5 ± 0.1 eV) irrespectively of acid–base properties of the support, the mean size of supported Ru crystallites (within the range of 2–10 nm) and the surface Cl content. The results obtained suggest that the difference in ammonia synthesis activity between the Ru catalysts supported on MgO and on the acidic supports is accounted for by not different electronic state of ruthenium on the surface of these oxides but by some other reasons.