Thermodynamic and kinetic aspects of surface acidity

Our research in the general area of acid catalysis involves the characterization of solid acidity and the corresponding assessment of catalytic performance of acidic materials. Acid characterization studies are required to provide essential information about the type of acid site (i.e., Lewis versus Bronsted), the strength of the sites, and the mobility of molecules adsorbed on the acid sites. An accurate measure of acid strength is given by the heat of adsorption of a basic probe molecule on the acid site. A thermodynamic representation of the mobility of adsorbed species on these sites is given by the entropy of adsorption. Important techniques used in these acid site characterization studies include microcalorimetry, thermogravimetric measurements, temperature programmed desorption, infrared spectroscopy and solid state nuclear magnetic resonance. The combination of these acid site characterization studies with reaction kinetics measurements of selected catalytic processes allows the elucidation of possible relationships between surface thermodynamic and kinetic properties of acidic sites. Such relationships are important milestones in formulating effective strategies for the effective utilization of solid acid catalysts. Current work in this direction involves methylamine syntheses over various zeolites, and the basic probe molecules employed include ammonia, methanol, water and mono-, di- and tri-methylamines. 31 refs., 18 figs., 1 tab

MÖSSBAUER SPECTRA OF STOICHIOMETRIC AND NONSTOICHIOMETRIC Fe3O4 MICROCRYSTALS

On a trouvé dans cette étude que des sauts d'électrons ont lieu dans des microcristaux superparamagnétiques de Fe3O4. Ceci est en contradiction avec des études précédentes qui ont probablement été faites sur des échantillons non stœchiométriques. On montre que même une exposition à l'air à température ambiante suffit pour créer des défauts de stœchiométrie. On peut conclure de la variation des spectres Mössbauer en fonction du temps que la diffusion à travers une couche d'oxyde du type γ-Fe2O3 est l'étape lente du processus.In this study electron hopping is found to occur in superparamagnetic Fe3O4 microcrystals. This is contrary to previous studies in which the samples were probably nonstoichiometric. It is shown that even room temperature exposure to air can result in nonstoichiometry. From the time change of the Mössbauer spectra, it can be implied that the diffusion through a γ-Fe2O3 like oxide layer is the slow step of the process

Oxidation of furfural in aqueous H2O2 catalysed by titanium silicalite: Deactivation processes and role of extraframework Ti oxides

Titanium silicalites (TS-1) with different Ti/Si atomic ratio (0.01–0.08) have been prepared, characterised by different techniques (X-Ray Diffraction, Transmission Electron Microscopy and Raman, Diffuse Reflection Infrared Fourier Transform and X-Ray Photoelectron spectroscopies), and their catalytic properties were investigated for the liquid phase oxidation of furfural with aqueous HO. For samples prepared with an initial Ti/Si > 0.01, extraframework Ti oxide is present and become more important as Ti loading increases. Two types of extraframework Ti have been identified: (i) anatase Ti oxide located at the external surface of zeolite crystals and (ii) Ti oxide nanodomains occluded within the channels and cavities of the zeolite. The latter can block the access of reactants to the active framework Ti(IV) sites for samples prepared with an initial Ti/Si > 0.04 and their removal results in an improvement in the selective oxidation of furfural to valuable products. Fouling by deposition of heavy by-products and Ti leaching were identified as two main causes of catalyst deactivation. The former can be reverted by removal of the deposits through calcination under air at 773 K. Leaching affects both the extraframework Ti and the active framework Ti species. During the first runs of reutilization, the Ti leaching is severe, affecting mainly to extraframework Ti oxide. Initially this leaching may be positive because its removal leads to the elimination of species blocking the access of reactants to the active Ti species; but in the longer term, the continuous leaching of active framework Ti irreversibly deactivates the catalyst.Financial support from the Spanish Ministry of Economy and Competitiveness is gratefully acknowledged (project CTQ2012-38204-C03-01). M.L.G also thanks the Ministery of Education, Culture and Sports for a “Salvador de Madariaga” grant (PRX14/00234) and CSIC for financial support from i-link project (i-link1048). The authors acknowledge the use of facilities and instrumentation supported by the University of Wisconsin Materials Research Science and Engineering Center (grant DMR-1121288).Peer Reviewe