RESUMEN
A new spectroscopic cell suitable for the analysis of heterogeneous catalysts by fluorescence EXAFS (extended X-ray absorption fine structure), transmission EXAFS and X-ray diffraction during in situ treatments and during catalysis is described. Both gas-phase and liquid-phase reactions can be investigated combined with on-line product analysis performed either by mass spectrometry or infrared spectroscopy. The set-up allows measurements from liquid-nitrogen temperature to 973 K. The catalysts are loaded preferentially as powders, but also as self-supporting wafers. The reaction cell was tested in various case studies demonstrating its flexibility and its wide applicability from model studies at liquid-nitrogen temperature to operando studies under realistic reaction conditions. Examples include structural studies during (i) the reduction of alumina-supported noble metal particles prepared by flame-spray pyrolysis and analysis of alloying in bimetallic noble metal particles (0.1%Pt-0.1%Pd/Al(2)O(3), 0.1%Pt-0.1%Ru/Al(2)O(3), 0.1%Pt-0.1%Rh/Al(2)O(3), 0.1%Au-0.1%Pd/Al(2)O(3)), (ii) reactivation of aged 0.8%Pt-16%BaO-CeO(2) NO(x) storage-reduction catalysts including the NO(x) storage/reduction cycle, and (iii) alcohol oxidation over gold catalysts (0.6%Au-20%CuO-CeO(2)).
RESUMEN
Tremendous changes of the structure of Rh particles occurred during partial methane oxidation to hydrogen and carbon monoxide over a 2.5 wt % Rh/Al(2)O(3) catalyst upon ignition of the catalytic reaction. Furthermore, near the ignition temperature a variation in the Rh-valence state along the catalyst bed was observed. By combining hard X-ray absorption spectroscopy (X-ray absorption near edge structure, XANES) with a charged coupled device (CCD) camera and using a suitable spectroscopic cell with gas supply and on-line mass spectrometry, we demonstrate that 2D-mapping of the Rh-oxidation state in a catalyst bed can be achieved during the catalytic reaction. For this purpose, X-ray images were recorded with the CCD camera at each energy around the Rh K-edge with and without the spectroscopic cell. This resulted effectively in the transmitted and incident intensity at each energy and at each pixel of the spectroscopic cell. Reconstruction of the full Rh K-edge XANES spectra at each pixel revealed the local distribution of oxidized and metallic Rh-species in the catalyst bed. Along the catalyst bed, structural changes were found with a steep gradient within less than 100 microm. Furthermore, a characteristic cone toward the inlet of the spectroscopic cell was observed.
RESUMEN
Peroxynitrous acid was reduced by cathodic linear sweep voltammetry at a gold electrode and by iodide at pH 3.2 and 5.6. The cathodic reduction wave was identified by measuring its decay in time, which was the same as observed by optical spectroscopy. The iodide oxidation was followed by optical measurement of the triiodide formation. Both reductions show one-electron stoichiometry, with the product n(alpha)alpha = 0.23 +/- 0.04 from the electrochemical experiments, in which alpha is the transfer coefficient and n(alpha) the number of electrons transferred, and an diiodine yield of ca. 0.5 equiv per equivalent of peroxynitrous acid. The voltammetric reduction was irreversible up to scan rates of 80 V s(-1). Both reductions were pH independent in the range studied. The voltammetric reduction is most likely an irreversible elemental reaction followed by a chemical decay that cannot be observed directly. Because of the pH independence, we conclude that both reductions have a common short-lived intermediate, namely [HOONO]*-. We estimate the electrode potential of the likely ONOOH/ONOOH*- couple to be larger than 1 V. The commonly used electrode potential E degrees (ONOOH, H+/NO2*, H2O) does not describe the chemistry of peroxynitrous acid.