Chemical state

About: Chemical state is a research topic. Over the lifetime, 2378 publications have been published within this topic receiving 78183 citations.

Investigation of the physico-chemical state and aggregation mechanism of surface Cr species on a Phillips CrOx/SiO2 catalyst by XPS and EPMA

Abstract: X-ray photoelectron spectroscopy (XPS) and electron probe microanalysis (EPMA) were jointly applied to achieve some basic understandings of the physico-chemical state and aggregation mechanism of surface Cr species on an industrial Phillips CrO x /SiO 2 catalyst calcined in dry air at 800°C for 20 h with 0.4 Cr/nm 2 . The XPS results showed the coexistence of surface-stabilized hexavalent chromate species (70.4% expressed as Cr(VI)O x , surf ) and trivalent chromium oxide (29.6%) on the catalyst. The latter derived from calcination-induced reduction of the Cr(VI)O x , surf species is mostly chemically-bonded to the silica surface (expressed as Cr(III)O x , surf ). The EPMA map and line curves of the Cr distribution state on the catalyst revealed a small amount of the trivalent chromium oxide existed as a few aggregates in sizes of 200–300 nm on the surface of each particle, which were supposed to be crystallized aggregates of Cr 2 O 3 . Consequently, the calcination-induced reduction of Cr(VI)O x , surf to Cr(III)O x , surf species and the formation of Cr 2 O 3 microcrystals on the Phillips catalyst with relatively low Cr loading were specifically confirmed. The variation of distribution and oxidation states of surface Cr species for the catalyst after being further calcined at 800°C for 2 h in the presence of moisture had also been studied in terms of the role of moisture and effect of atmosphere (pure air or N 2 ). It was found that the purposely introduced moisture induced the transformation of all the Cr(III)O x , surf and one-seventh of Cr(VI)O x , surf species into aggregates of Cr 2 O 3 at high temperature, whereas oxidizing and inert atmospheres made no obvious difference. Finally, the formation mechanism of aggregates of Cr 2 O 3 induced by moisture through cleavage of Cr(III)O x , surf species during the calcination had been speculated considering of the indispensable evolution of traces of moisture from the simultaneous dehydroxylation of residual hydroxyl groups on silica surface. The advantages of the combination of XPS and EPMA techniques for the basic investigation of Phillips catalysts were substantiated by the results obtained in this study.

Nitrogen dioxide sensing using tungsten oxide microspheres with hierarchical nanorod-assembled architectures by a complexing surfactant-mediated hydrothermal route

Abstract: WO3 microspheres with hierarchical nanorod-assembled architectures were successfully synthesized by a complexing surfactant-mediated hydrothermal method in the presence of K2SO4 and H2C2O4 with a molar ratio of 1 : 1. Microstructural characterization by means of X-ray diffraction, scanning electron microscopy and transmission electron microscopy showed that WO3 microspheres with diameters ranging from 3 to 5 μm were assembled by 90 nm diameter nanorods and had a single crystal hexagonal structure. The analysis results of the elemental composition and chemical state demonstrated that the obtained WO3 microspheres were nearly stoichiometric. Based on the experimental results, a possible growth mechanism consisting of nucleation, Ostwald ripening, and self-assembly of WO3 crystals was proposed. Gas sensing properties demonstrated that WO3 microspheres exhibited not only a high response and excellent reversibility to NO2, but also a good linear relationship between the response and NO2 concentration in the range of 1 to 10 ppm. The response and recovery times significantly decreased as the operating temperature increased gradually. The highest response of 790 to 20 ppm NO2 was obtained at a relatively low operating temperature of 100 °C, which revealed that WO3 microspheres were very promising for fabricating low-consumption chemical gas sensors. The electron depletion theory was used for explaining the gas sensing mechanism by the chemical adsorption and reaction of NO2 gas molecules on the surface of WO3 microspheres.

XPS Study of Interface and Ligand Effects in Supported Cu2O and CuO Nanometric Particles

Abstract: This paper reports an analysis of the changes in the photoemission parameters of copper in small particles of copper oxides deposited on silicon dioxide. This study is of relevance for investigations in the fields of heterogeneous catalysis and coordination chemistry. Copper oxides (Cu2O and CuO) have been deposited on the surface of a flat SiO2 substrate by evaporation of copper and subsequent oxidization of the deposited particles. XPS has been used to analyze the chemical and coordination state of copper. Large variations in the Cu 2p(3/2) binding energy (BE) and Auger parameter (alpha') have been found as a function of the type and amount of deposited copper oxide. The differences in BE calculated from the values of the lowest amount of deposited material and those of the bulk compounds were -0.4 eV (Cu2O) and -1.9 eV (CuO), while those in alpha' amounted to 2.9 (Cu2O) and 1.6 eV (CuO). The observed changes have been described in terms of the chemical state vector (CSV) concept in a Wagner plot and rationalized by considering the characteristics of bonding and electronic interactions that occur at a given oxide/oxide interface. These interactions have been modeled by means of quantum mechanical calculations with cluster models simulating the Cu-O-Si bonding at the interface. The effect of the polarization of the surrounding media around the copper cations has been also estimated for both the dispersed clusters supported on the SiO2 substrate and for the copper oxide materials in bulk form. A change in the values of alpha' and BE of copper (ie., delta alpha' = 1.1 eV, deltaBE = 0.1 eV) upon adsorption on the Cu+ species of Cu2O moieties dispersed on SiO2 of a phenyl-acetylene molecule illustrates the use of XPS to study the formation of cation-ligand complexes in heterogeneous systems. A detailed description of the bonding interactions of these coordinated Cu+ species in terms of initial and final state effects of the photoemission process has been also carried out by means of quantum mechanical calculations and cluster models.