X-ray spectroscopic fingerprints of reactive oxygen sites at the MoO3(010) surface

Abstract

The identification of oxygen sites at metal oxide surfaces and the characterization of their properties is of great importance for an understanding of the catalytic activity of such materials and, thus, for a rational design of efficient and selective catalysts. In the case of the clean MoO 3 (010) surface we show that an unambiguous discrimination of the different reactive oxygen sites can be obtained by angle-resolved Near-Edge X-Ray Absorption Fine Structure (NEXAFS) combined with Density-Functional-Theory (DFT) based spectrum analyses for different photon polarization directions. In particular, we are able to unequivocally discriminate the characteristic spectral signatures of singly coordinated molybdenyl oxygen covering the topmost molybdenum layers from those of other oxygen centers that have very similar local environment and only differ by their spatial orientation in the crystal. Theoretical predictions are also successfully used to identify and interpret characteristic features in the NEXAFS spectra that arise from specific vacancy sites present at oxygen deficient surfaces.

Learning from the past: Are catalyst design principles transferrable between hydrodesulfurization and deoxygenation?

“…In other words, after the removal of an oxygen atom (terminal or asymmetric), the residual terminal or asymmetric oxygen atom attached to the central molybdenum atom rearranges to compensate for the vacancy and the final geometry obtained in both cases is similar. This was also reported in earlier experimental and theoretical studies on MoO 3 …”

Efficient Visible‐Light‐Driven CO2 Reduction Mediated by Defect‐Engineered BiOBr Atomic Layers

“…Importantly,t he visible-light absorption reached the highest level after UV irradiation for 480 minutes,w hich indicated that the oxygen defect concentration reached am aximum at this time.I ns upport of this finding, electron paramagnetic resonance (EPR) spectra ( Figure 3B;F igure S5) revealed that 480 minutes of UV-light irradiation produce the strongest EPR signal at g = 2.004, which was identified as the electrons trapped on oxygen vacancies [11] and agreed fairly well with the UV/Vis absorption analysis in Figure 3A.T hus,t he BiOBr atomic layers after 480 minutes of irradiation with UV light were selected as the oxygen-deficient BiOBr atomic layers.In contrast, the pristine BiOBr atomic layers and bulk counterpart did not show any obvious EPR signal, suggesting there were almost no oxygen vacancies.M oreover,a s illustrated by high-resolution O1SX -ray photoelectron spectroscopy (XPS;F igure 3C;F igure S6), after UV irradiation of the pristine BiOBr atomic layers for 480 minutes,a ne xtra peak centered at 531.4 eV was observed, which was attributed to the oxygen atoms in the vicinity of an oxygen vacancy [11,15] Given the extent of the peak area at 531.4 eV,itwas concluded that 480 minutes of irradiation with UV light resulted in the largest amount of oxygen vacancies,w hile the other two samples presented av ery low amount of oxygen vacancies,i na greement with the EPR results in Figure 3B.F urthermore, as imilar finding was deduced from the O K-edge X-ray absorption near-edge structure (XANES) spectra ( Figure 3D), in which the extra peak centered at 529.1 eV was assigned to oxygen vacancies resulting from 480 minutes of irradiation with UV light. [16,17] Thus,a ll the aforementioned results clearly demonstrate that 480 minutes of irradiation with UV light leads to abundant oxygen vacancies in the BiOBr atomic layers,w hile the pristine BiOBr atomic layers and bulk counterparts contain almost no oxygen vacancies. As ac onsequence of the abundant oxygen vacancies,t he oxygen-deficient BiOBr atomic layers offered additional light harvesting in the visible-light range,w hile pristine BiOBr atomic layers and bulk counterparts only responded to UV light ( Figure 3A).…”

The impact of steam on the electronic structure of the selective propane oxidation catalyst MoVTeNb oxide (orthorhombic M1 phase)

“…In this context features VII (p* resonance) and VIII (s* resonance) can be assigned to transitions into orbitals with the antibonding Mo4d-O2p character indicating covalent bonds between molybdenum and oxygen atoms, 41 while feature IX is attributed to transitions into antibonding orbitals with Mo5s-O2p character. 41 In a pure ionic oxide, the oxygen 2p orbitals are completely filled and no resonance occurs. Thus the oxygen K-edge dominated by contributions from molybdenumoxygen bonds indicates that molybdenum to a large extent is covalently bonded in M1.…”