Jacinto Sá

A von Hamos x-ray spectrometer based on a segmented-type diffraction crystal for single-shot x-ray emission spectroscopy and time-resolved resonant inelastic x-ray scattering studies

We report on the design and performance of a wavelength-dispersive type spectrometer based on the von Hamos geometry. The spectrometer is equipped with a segmented-type crystal for x-ray diffraction and provides an energy resolution in the order of 0.25 eV and 1 eV over an energy range of 8000 eV-9600 eV. The use of a segmented crystal results in a simple and straightforward crystal preparation that allows to preserve the spectrometer resolution and spectrometer efficiency. Application of the spectrometer for time-resolved resonant inelastic x-ray scattering and single-shot x-ray emission spectroscopy is demonstrated.

SpaciMS: spatial and temporal operando resolution of reactions within catalytic monoliths

Monolithic catalysts are widely used as structured catalysts, especially in the abatement of pollutants. Probing what happens inside these monoliths during operation is, therefore, vital for modelling and prediction of the catalyst behavior. SpaciMS is a spatially resolved capillary-inlet mass spectroscopy system allowing for the generation of spatially resolved maps of the reactions within monoliths. In this study SpaciMS results combined with 3D CFD modelling demonstrate that SpaciMS is a highly sensitive and minimally invasive technique that can provide reaction maps as well as catalytic temporal behavior. Herein we illustrate this by examining kinetic oscillations during a CO oxidation reaction over a Pt/Rh on alumina catalyst supported on a cordierite monolith. These oscillations were only observed within the monolith by SpaciMS between 30 and 90% CO conversion. Equivalent experiments performed in a plug-flow reactor using this catalyst in a crushed form over a similar range of reaction conditions did not display any oscillations demonstrating the importance of intra monolith analysis. This work demonstrates that the SpaciMS offers an accurate and comprehensive picture of structured catalysts under operation.

High energy resolution off-resonant spectroscopy at sub-second time resolution: (Pt(acac)2) decomposition.

We report on the decomposition of platinum acetylacetonate (Pt(acac)(2)) in hydrogen induced by flash heating. The changes in the local Pt structure were followed by high energy resolution off-resonant spectroscopy uniquely performed with sub-second time resolution. The decomposition consists of a two-step reduction process of the Pt(II) species.

Increased dispersion of supported gold during methanol carbonylation conditions

The active site in supported gold catalysts for the carbonylation of methanol has been identified as dimers/trimers of gold which are formed from large gold particles >10 nm in diameter. Methyl iodide was found to be critical for this dispersion process and to maintain the catalyst in the active form. This study also shows that it may be possible to redisperse gold catalysts, in general, after reaction.

Scientific opportunities for heterogeneous catalysis research at the SuperXAS and SNBL beam lines.

In this short review, we describe the complementary experimental capabilities for catalysis research at two beam lines available to the Swiss community, SuperXAS at SLS (Swiss Light Source, Villigen) and SNBL (Swiss Norwegian Beam lines) at ESRF (European Synchrotron Radiation Facility, Grenoble). Over the years, these two facilities have been developed to provide powerful techniques for structural studies under in situ and operando conditions. These techniques, X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and X-ray emission spectroscopy (XES) in combination with Raman or infrared spectroscopy provide new avenues for structure-performance studies of catalysts. Several exemplary studies are used to demonstrate the capability of these facilities.

Particle size and support effects in hydrogenation over supported gold catalysts

Particle size and support effects were determined for the hydrogenation of nitrobenzene over gold supported on alumina and titania by kinetic experiments, TEM and in situ high energy resolution fluorescence detected X-ray absorption spectroscopy (HERFD XAS). Especially when supported on alumina, the catalytic activity correlated well with the fraction of particles smaller than 2 nm. These particles are able to split hydrogen on undercoordinated gold atoms, hence the strong particle size effect. In addition, on titania the increased length of the metal–support interface for smaller particles leads to enhanced activity because of the beneficial effect of the metal–support interface on the splitting of hydrogen. Reporting the average particle size is not always relevant in describing catalytic performance.

Hydrogenation of Nitrobenzene Over Au/MeOx Catalysts—A Matter of the Support

The heterogeneous hydrogenation of substituted nitrobenzenes is a reaction of great interest, because aniline and its derivates are valuable substances in the chemical industry for the production of polymers, pharmaceuticals, herbicides, and dyes. The state-of-the-art catalysts are mostly active metals, such as Pt, Pd, Ni, Cu, and Ir, which are supported on various materials, such as activated C, CaCO3, and SiO2, depending on their application. To achieve high selectivity to substituted anilines in the presence of other reducible groups and to prevent arylhydroxylamine accumulation in the reaction mixture, state-of-the-art catalysts are often modified with environmentally harmful additives, such as Pb and V promoters and Fe salts. Since the discovery that Au, when present as nanoparticles in the range of 1–3 nm, catalyzes CO oxidation, more and more reactions have been shown to be catalyzed by Au, among them the hydrogenation of nitrobenzene. Hydrogenation of nitroaromatics containing additional unsaturated groups over unmodified Au/TiO2 and Au/Fe2O3 shows a high selectivity to the nitro group. Thus, Au/MeOx (Me corresponds to a metal) catalysts have been presented as a “green” alternative in reactions where a high selectivity under moderate reaction conditions is required. Haber proposed a reaction scheme (Scheme 1) for the electrochemical hydrogenation of nitrobenzene and its derivates in 1898; however, there is an ongoing debate about the reaction mechanism over heterogeneous catalysts. Haber proposed two main reaction routes, namely the “direct” (left hand side) and the “condensation” route (right hand side). In the direct route, nitrobenzene is reduced to nitrosobenzene, then to phenylhydroxylamine, and finally to aniline (Steps I–III). A variation of the direct route is the “no-nitroso route” (Step IV), in which nitrobenzene directly reacts to phenylhydroxylamine and then to aniline. The condensation route occurs when the two intermediates nitrosobenzene and phenylhydroxylamine condensate to form azoxybenzene (Step VI). This species is then hydrogenated to aniline in consecutive steps via the intermediates azobenzene and hydrazobenzene (Steps VII–IX). Another possible step in the transformation of nitrobenzene to aniline is the decomposition of phenylhydroxylamine into nitrosobenzene and aniline (Step V). Aniline is produced by the disproportion of phenylhydroxylamine. The nitrosobenzene generated by the disproportion reenters the catalytic cycle and is subsequently transformed into phenylhydroxylamine. These findings are based on measurements of nitrobenzene hydrogenations over Ir/C poisoned by Hg. Azoxybenzene is the first intermediate that is formed in the condensation route, which is observed when the reactions are performed in the presence of a base. Azoxybenzene can also be detected at slow reaction rates, for example, over Pd/SiO2 in methanol at 25 8C. Recently, the selective catalytic hydrogenation of functionalized nitroarenes has been reviewed. The authors describe precisely the tailoring of selective catalysts by using organic and inorganic modifiers and their application for different catalytic problems. Also, the effect of solvent, particle size, and support are discussed. The discussion on the influence of the support focuses on selectivity, activity, and stabilization of the metal nanoparticles. Other reports detail the effect of the composition of the reaction mixture, the noble metal, and the Scheme 1. Possible reaction pathways for the hydrogenation of aromatic nitro compounds to the corresponding anilines. NB: nitrobenzene, NSB: nitrosobenzene, PHA: phenylhydroxylamine, AN: aniline, AOB: azoxybenzene, AB: azobenzene, HAB: hydrazobenzene. Adapted from Ref. [6] .

Influence of Methyl Halide Treatment on Gold Nanoparticles Supported on Activated Carbon

Catalyzing concept: Methyl halides have been used to disperse large (around 20 nm) gold nanoparticles supported on carbon into dispersed gold atoms/dimers at low temperature and atmospheric pressure (see picture). The process occurs through the progressive removal of gold–halogen entities from the metal nanoparticles and a gradual decrease in the size of the gold nanoparticles on the minute timescale.

Subsecond and in Situ Chemical Speciation of Pt/Al2O3 during Oxidation Reduction Cycles Monitored by High-Energy Resolution Off-Resonant X-ray Spectroscopy

We report an in situ time-resolved high-energy resolution off-resonant spectroscopy study with subsecond resolution providing insight into the oxidation and reduction steps of a Pt catalyst during CO oxidation. The study shows that the slow oxidation step is composed of two characteristic stages, namely, dissociative adsorption of oxygen followed by partial oxidation of Pt subsurface. By comparing the experimental spectra with theoretical calculations, we found that the intermediate chemisorbed O on Pt is adsorbed on atop position, which suggests surface poisoning by CO or surface reconstruction.

Characterization of silica-supported dodecatungstic heteropolyacids as a function of their dehydroxylation temperature

Dodecatungsto-silicic H(4)SiW(12)O(40) and -phosphoric acids H(3)PW(12)O(40) were deposited on silica by a classical impregnation technique. The resulting materials were studied by in situ Raman and infrared spectroscopy, XPS and by solid-state (1)H MAS NMR as a function of their dehydroxylation temperature. The data show that in the case of H(3)PW(12)O(40) three silanol groups are protonated while in the case of H(4)SiW(12)O(40) at least one acidic proton remains. Upon heating this proton reacts leading to a disordered structure and a broadening of the W-O Raman bands.