Recent advances in catalysis instrumentations include synchrotronbased facilities where time-resolved X-ray scattering and absorption techniques are combined in the same in situ or operando experiment to study catalysts at work. To evaluate the advances and limitations of this method, we performed a series of experiments at the new XAFS/XRD instrument in the National Synchrotron Light Source. Nearly simultaneous X-ray diffraction (XRD) and X-ray absorption finestructure (XAFS) measurements of structure and kinetics of several catalysts under reducing or oxidizing conditions have been performed and carefully analyzed. For CuFe 2 O 4 under reducing conditions, the combined use of the two techniques allowed us to obtain accurate data on kinetics of nucleation and growth of metallic Cu. For the inverse catalyst CuO/CeO 2 that underwent isothermal reduction (with CO) and oxidation (with O 2 ), the XAFS data measured in the same experiment with XRD revealed strongly disordered Cu species that went undetected by diffraction. These and other examples emphasize the unique sensitivity of these two complementary methods to follow catalytic processes in the broad ranges of length and time scales.
Undoped and Nd3+-doped TiO2 nanoparticles were synthesized by chemical vapor deposition in order to tailor the band gap of TiO2. The doping reduced the band gap. The band gap was measured by ultraviolet-visible light absorption experiments and by near-edge x-ray absorption fine structure. The maximum band gap reduction was 0.55 eV for 1.5 at. % Nd-doped TiO2 nanoparticles. Density functional theory calculations using the generalized gradient approximation with the linearized augmented plane wave method were used to interpret the band gap narrowing. The band gap narrowing was primarily attributed to the substitutional Nd3+ ions which introduced electron states into the band gap of TiO2 to form the new lowest unoccupied molecular orbital.
The decomposition and dehydrogenation of cyclohexene (c-C 6 H 10 ) are used as probe reactions to compare the surface reactivities of clean and carbide-modified W(111). The reaction mechanisms have been studied using temperature-programmed desorption (TPD), Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), high-resolution electron energy loss spectroscopy (HREELS), and near-edge X-ray absorption fine structure (NEXAFS). On the clean W(111) surface, cyclohexene molecules decompose to produce hydrogen, atomic carbon and cyclohexane. In contrast, on the carbide-modified W(111) surface, cyclohexene undergoes primarily dehydrogenation to form benzene and hydrogen. The selectivity to the production of gas-phase benzene on C/W(111) is similar to that observed on the Pt(111) surface. † Part of the special issue "John T. Yates, Jr. Festschrift".
We have determined the activation energies of sodium diffusion from the soda-lime glass substrate through the Mo back-contact layer, as well as through copper indium gallium diselenide (CIGS) deposited on the Mo back-contact layer of CIGS thin-film solar cells. The activation energies were determined by X-ray photoelectron spectroscopy (XPS) to measure surface sodium concentrations before and after thermally induced diffusion. The activation energies were found to be similar for the diffusion of Na through the Mo/glass and CIGS/Mo/glass thin films, approximately 8Á6 and 9Á6 kcal/mol, respectively. Furthermore, the sodium diffusion was found to occur by annealing in an environment of 1Á0 Â 10 À5 Torr of air, oxygen, or water vapor, but not in vacuum of less than 1 Â 10 À8 Torr. In addition, the diffusion of Na was found to occur faster in the presence of oxygen than in water under identical annealing conditions.
The rate and selectivity of chemical reactions on transition-metal surfaces can be controlled by using different bimetallic combinations. The interaction of bimetallic components leads to a change in the electronic properties of the surface, which in turn produces a change in chemical reactivity. In the current paper, we illustrate the correlation of the electronic properties of bimetallic surfaces with the reaction pathways of C2 hydrocarbons. Density functional theory (DFT) was used to study the binding of hydrogen, ethylene, acetylene, ethyl, and vinyl on monometallic and bimetallic transition-metal surfaces. The binding energies of these species were found to correlate with the d-band centers of these surfaces. The binding energies for hydrogen atoms on bimetallic surfaces were lower than for those on the corresponding parent metal surfaces. This trend was consistent for ethylene and acetylene binding. Comparative studies between acetylene and ethylene revealed that acetylene was more strongly bonded to the monometallic and the bimetallic surfaces than was ethylene. Bond order conservation (BOC) theory was used to calculate the activation barriers for ethyl dehydrogenation to ethylene and vinyl dehydrogenation to acetylene. The activation barriers for these reactions were correlated with the surface d-band center of the substrates.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.