Spectral and spatial anisotropy of the oxide growth onRu(0001)

Abstract

Scanning photoelectron spectromicroscopy has been used to study the onset and the initial stages of oxidation of Ru(0001) at three oxidation temperatures, 625, 675, and 775 K, and oxygen exposures of about 105 Langmuir. The lateral heterogeneity developed during oxide nucleation and growth and the local composition of the coexisting phases have been determined using as fingerprints the O 1s and Ru 3d spectra, thus combining chemical mapping with spectroscopy from selected features from the maps. The onset of oxide formation is characterized by the appearance of randomly distributed small islands (⩾0.5 μm) identified as germinal patches exhibiting some spectral features of bulk RuO2. The following anisotropic growth of the RuO2 phase and in particular the shape of the oxide islands shows a strong dependence on the oxidation temperature. The spectroscopic information obtained for the areas surrounding the oxide islands reveals an intermediate oxygen state characterized by distinct O 1s and Ru 3d features different from both the chemisorbed and the oxide form. It is assigned to an intermediate state acting as precursor of the oxide. The experimental data are discussed in the framework of the oxidation pathway and of core level fingerprints for the various oxygen–Ru phases suggested in recent theoretical models.

Catalytically active states of Ru(0001) catalyst in CO oxidation reaction

“…Although the energy position of the Ru x O y component is practically identical with the Ru(I)-2O ad one, the Ru 3d spectra are very different, because of the strong attenuation of Ru bulk component due to screening by the 'surface oxide' film. The RuO 2 (110) phase has two components corresponding to the cus-Ru and six-fold coordinated Ru atoms and a broad satellite at ≈ 283 eV [4,13,27]. The O 1s spectra from the adsorption and 'surface oxide' states appear at 530.0 eV, the latter being only a bit broader, indicative of multiple bonding configurations [13].…”

“…The RuO 2 (110) phase has two components corresponding to the cus-Ru and six-fold coordinated Ru atoms and a broad satellite at ≈ 283 eV [4,13,27]. The O 1s spectra from the adsorption and 'surface oxide' states appear at 530.0 eV, the latter being only a bit broader, indicative of multiple bonding configurations [13]. The O 1s spectrum of the RuO 2 (110) phase has two components, reflecting the emission from the bridge-O atoms at the RuO 2 surface (528.7 eV) and bulk RuO 2 oxygen (529.5 eV) [4,13].…”

“…The recent XPS microscopy and TDS studies demonstrated that starting from an atomically clean Ru(0001) surface, the formation of a stoichiometric RuO 2 phase is kinetically hindered at temperatures lower than 500 K and readily occurs at higher temperatures [2,13,14,21]. The main reason for such temperature dependence of the Ru oxidation state is that the incorporation up to 3 ML of oxygen at T < 500 K is limited to the top 2-3 Ru layers.…”

“…1 shows schematically the 'oxidation' states of the Ru(0001) that have attained experimental proof [2,4,13,14,21] In the following we will call them: adsorption phase, referring the state when oxygen in present only on the surface with maximum coverage 1 ML (A), 'surface oxide' with ~ 1-3 ML oxygen incorporated within the top few Ru layers (B and C) and rutile RuO 2 phase (D). An important finding of XPS microscopy is that 'surface oxide' and RuO 2 phases coexist in a wide T-pressure range, even when formed in a pure O 2 ambient [13,21] Undoubtedly under reaction conditions, the CO will drive the oxidation state away from the equilibrium achieved in O 2 ambient, which implies that the T-p space of coexistence of the two phases may be expanded.…”

“…The inspired UHV surface science and theoretical studies of CO oxidation reaction on a model RuO 2 (110) single crystal surface reached the consensus that the elementary reaction steps involve CO and O adsorption on the cus-Ru, followed by reaction between cus-CO and cus-or bridge O atoms [5][6][7][8][9]. However, the RuO 2 (110) surface is an idealised case of an active Ru catalyst, which has been prompted by the parallel studies focused on the oxidation mechanism of the Ru(0001) surface and the stability of the Ru oxidation states at different partial pressures and temperatures [10][11][12][13][14][15][16]. After the completion of the O-(1x1) adsorption phase [17] , the suggested oxidation pathway of the Ru(0001) surface involves as an important intermediate step the incorporation of O atoms between the first and second Ru layers and the formation of an O ad -Ru-O sub trilayer hosting 2 ML of oxygen [18,19] 1 ML equals the number of Ru atoms on the (0001) surface.…”