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Reactivity of Surface Species in Heterogeneous Catalysts Probed by In Situ X-ray Absorption Techniques

X-ray absorption spectroscopy (XAS) has been applied to numerous problems in soil science, mineralogy, and geochemistry. X-ray absorption spectroscopy was developed in the early 1970s (Sayers et al., 1971) and is widely used at synchrotron radiation facilities. Regardless of the complexity of the sample, the XAS signal comes from all of the atoms of a single element as selected by the X-ray energy. High-quality XAS spectra can be collected on heterogeneous mixtures of gases, liquids, and/or solids with little or no sample pretreatments, making it ideally suited for soils and many other systems. The structural information obtained from XAS is useful for identifying the chemical speciation of an element, including mineral, noncrystalline solid, or adsorbed phases. With the addition of X-ray focusing, samples can be interrogated on length scales comparable to or smaller than some level of natural heterogeneity, thus making it possible to study differences in the atomic environment of an element within or between individual particles and grain boundaries. The acronym XAS covers both X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies. X-ray absorption near edge structure can be used to determine the valence state and coordination geometry, while EXAFS can be used to determine the local molecular structure of a particular element within a sample. Micrometer-length scale X-ray measurements are designated m-EXAFS and m-XANES. Another variation to the XAS technique utilizes the natural linear polarization of synchrotron X-rays. The use of polarized X-rays allows the atomic environment of the absorber to be probed in the polarization direction and is particularly suited for minerals with layer-type structures such as phyllosilicates and manganese oxides (Manceau et al., 1999). The nature of a complex sample is best revealed by the application of several different experimental techniques, with each individual measurement providing both unique and complementary information. The most common technique for characterizing abundant soil minerals is X-ray diffraction (XRD), which relies on long range ordering of atomic planes to probe crystalline structure at a length scale of approximately 50 Å or more. X-ray absorption spectroscopy probes the immediate environment of the selected element, within about 6 Å, and its theory and interpretation does not rely on any assumption of symmetry or periodicity. While both XRD and XAS can be used to determine distances between atoms, the information is derived from two very different X-ray interactions with the sample. For most systems the application of XRD and XAS is complementary.

Analysis of Soils and Minerals Using X-ray Absorption Spectroscopy

The effect of composition, microstructure, and defect chemistry on sensing performance of gas sensors based on CuO-doped SnO2 is investigated using sol–gel derived nano-sized powders (about 20 nm). The particle size of copper oxide doped tin oxide is varied by annealing at different temperatures and a significant grain growth is observed at temperatures above 1000°C due to the liquid phase sintering effect of copper oxide. The reduction of particle size to nanometers, or to the dimension comparable to the thickness of charge depletion layer, leads to a dramatic improvement in sensitivity and speed of response. It appears that the substitution of Sn by Cu in the cassiterite structure increases the concentration of oxygen vacancies and decreases the concentration of free electrons. In particular, the existence of cuprous ions (Cu+), due to partial reduction of Cu2+ during sintering, plays an important role in enhancing the sensor response to nitric oxide (NO) and CO2.

Effect of particle size and dopant on properties of SnO2-based gas sensors

Study of CeO2 XPS spectra by factor analysis : reduction of CeO2

Maleic anhydride, a precursor to polyester resins, is made by oxidation of n -butane over vanadium phosphate catalysts. This system is of general interest because it is the only heterogeneously catalyzed, alkane-selective oxidation reaction in commercial use. Time-resolved in situ x-ray absorption spectroscopy shows that when either α I -VOPO 4 /SiO 2 or (VO) 2 P 2 O 7 /SiO 2 catalysts are exposed to n -butane, the rate of maleic anhydride formation is proportional to the rate of decay of V 5+ species in the catalyst. Thus V 5+ species are kinetically significant for the production of maleic anhydride and not just for the production of by-products. The results also suggest that V 5+ species may play a role in the initial hydrogen abstraction from n -butane, the rate-determining step in the reaction sequence. V 4+ sites appear to be responsible for by-product formation.

https://science.sciencemag.org/content/sci/275/5297/191.full.pdf

The Kinetic Significance of V5 in n-Butane Oxidation Catalyzed by Vanadium Phosphates

In this paper we describe the interpretation of X-ray absorption near-edge structure (XANES) data using factor analysis to follow the temperature-programmed reduction (TPR) of CuPd/KL-zeolite catalysts. With this tool it is possible to identify and quantify the chemical compounds in which a metal is present during a reduction treatment. When the method is applied to the Cu K-edge XANES-TPR spectra of two bimetallic catalysts containing (1% Pd + 0.5% Cu), and (1% Pd + 0.25% Cu), it is shown that Cu is initially present in both as two oxidic species, a Cu-Pd mixed oxide and a Cu-exchanged in L-zeolite. Both catalysts show a similar behavior under reduction yielding only one zero valent phase, a Cu-Pd alloy. 25 refs., 6 figs., 3 tabs.

XANES-TPR Study of Cu-Pd Bimetallic Catalysts: Application of Factor Analysis

Carbon monoxide (CO) and nitric oxide (NO) are among the major components of exhaust gases produced from vehicle engines and industrial boilers. Apart from noble metals (Rh, Pt, Pd, ...), which are very active for the simultaneous NO reduction and CO oxidation, copper based catalysts have been widely studied as less expensive alternatives for this reaction. Here we use `in situ` X-ray absorption near edge structure (XANES) to study the state of copper species during the NO + CO reaction. The correlation of copper surface species present on the catalyst with its catalytic performance allows us to deduce that a redox-type mechanism (Cu{sup 2+}/Cu{sup +}) is working under reaction conditions and that the NO adsorption and/or dissociation is the slow step of the reaction. 22 refs., 2 figs., 1 tab.

C. Marquez Alvarez

Papers

XANES-TPR Study of Cu-Pd Bimetallic Catalysts: Application of Factor Analysis

New Insights on the Mechanism of the NO Reduction with CO over Alumina-Supported Copper Catalysts