Sebastián E. Collins

Stability of formate species on β-Ga2O3

Gallia (gallium oxide) has been proved to enhance the performance of metal catalysts in a variety of catalytic reactions involving methanol, CO and H(2). The presence of formate species as key intermediates in some of these reactions has been reported, although their role is still a matter of debate. In this work, a combined theoretical and experimental approach has been carried out in order to characterize the formation of such formate species over the gallium oxide surface. Infrared spectroscopy experiments of CO adsorption over H(2) (or D(2)) pretreated beta-Ga(2)O(3) revealed the formation of several formate species. The beta-Ga(2)O(3) (100) surface was modelled by means of periodic DFT calculations. The stability of said species and their vibrational mode assignments are discussed together with the formate interconversion barriers. A possible mechanism is proposed based on the experimental and theoretical results: first CO inserts into surface (monocoordinate) hydroxyl groups leading to monocoordinate formate; this species might evolve to the thermodynamically most stable dicoordinate formate, or might transfer hydrogen to the surface oxidizing to CO(2) creating an oxygen vacancy and a hydride group. The barrier for the first step, CO insertion, is calculated to be significantly higher than that of the monocoordinate formate conversion steps. Monocoordinate formates are thus short-lived intermediates playing a key role in the CO oxidation reaction, while bidentate formates are mainly spectators.

Fully Reversible Metal Deactivation Effects in Gold/Ceria–Zirconia Catalysts: Role of the Redox State of the Support

Gold nanoparticles supported on reducible oxides are highly interesting catalytic materials. In particular, they are known to exhibit exceptional activity for CO oxidation, lowtemperature water–gas shift (LT-WGS), and selective oxidation of CO in the presence of a large excess of hydrogen (PROX). 20–24] Despite the extraordinary research effort devoted to these catalysts, there are still some key questions about the nanostructural constitution and chemical properties of the gold sites involved in the above reactions that require further clarification. The relationship between the redox state of the support and the chemical properties of gold nanoparticles is one of these major open questions. 5] Its understanding, however, is critically important to fully interpret catalysis by Au/reducible oxide systems, particularly in the case of processes like LTWGS, 19] PROX, and hydrogenation reactions, which typically occur under net reducing conditions. To gain further information on this issue, we investigated CO adsorption on an Au/Ce0.62Zr0.38O2 (Au/CZ) sample subjected to different redox pretreatments. In our experimental approach, FTIR spectroscopic and volumetric adsorption techniques were combined with studies on ultimate oxygen storage capacity (OSC), metal dispersion, as determined by high-resolution TEM (HRTEM) and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), and X-ray photoelectron spectroscopy (XPS). By this approach, the influence of the redox state of the support on the CO adsorption capability of Au nanoparticles could be established on a quantitative basis. In contrast with earlier proposals suggesting that gold catalysts do not exhibit a strong metal/support interaction (SMSI) effect, the results presented and discussed herein indicate that the behavior of our Au/CZ shows close resemblances with those of a number of noble metal/reducible oxide systems which are acknowledged to show this effect. Our experimental approach also allowed us to show that the absorption coefficient of the n[CO(Au)] band depends on the redox state of the support. The implications of this finding for the use of integrated absorption data as a quantitative tool for characterizing the changes of CO adsorption capability occurring in gold nanoparticles supported on reducible oxides is discussed. Figure 1 shows the n(CO) FTIR spectra recorded on the same sample disk successively submitted to a) oxidizing treatment at 523 K (Au/CZ-O523); b) pretreatment (a) followed by reduction at 473 K (Au/CZ-O523-R473); and

Infrared and Raman Investigation of Supported Phosphotungstic Wells- Dawson Heteropolyacid

Fil: Bonivardi, Adrian Lionel. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico - Conicet- Santa Fe. Instituto de Desarrollo Tecnologico Para la Industria Quimica (i); Argentina

Molecular recognition of an acyl–enzyme intermediate on the lipase B from Candida antarctica

This investigation provides evidence of the acyl enzyme species involved in the interaction of R/S-ketoprofen with the lipase B from Candida antarctica. The interaction between the profen and the enzyme was studied by in situ time-resolved ATR-FTIR under both static and transient conditions. Particularly, modulation excitation spectroscopy (MES) with phase sensitive detection (PSD) allowed us to univocally distinguish the signals belonging to the interaction between ketoprofen and the enzyme from the strong background signals. These experimental tools coupled with theoretical DFT analysis allowed us to propose various species derived from the interaction of ketoprofen with serine through H bonding (without reaction) and the acyl enzyme species (ester bond formation) which are the intermediates in the biocatalytic assisted esterification and hydrolysis using lipases.

Resolution of intermediate surface species by combining modulated infrared spectroscopy and chemometrics

In this work, aiming at exploiting the essential particularities of modulation excitation spectroscopy (MES) coupled to phase sensitive detection (PSD) and chemometrics, a data analysis procedure was implemented to analyze phase-resolved infrared (IR) data. The fundamental principle of the proceedings is the application of successive multivariate curve resolution-alternating least squares (MCR-ALS) resolutions to MES-PSD IR data. The applicability of the strategy was evaluated in several cases of simulated data considering the effect of spectral band overlapping and presence of noise. Outcomes related to data-processing are depicted in detail. As a proof of concept, the data resolution approach was validated by resolving an experimental real system related to the adsorption-desorption dynamic of oxalic acid on titanium dioxide by in situ IR spectroscopy in attenuated total reflection (ATR) mode. After data resolution, different oxalate species were assigned to each spectral band and information about the kinetics in terms of phase lag was obtained. In the light of the obtained results, this approach is rather appealing in other research fields, very helpful for the non-chemometric community and foresees manifold applications, specially, in catalytic system investigations.