Design of nanostructured catalysts for H2 production and CO2 hydrogenation.

In a sustainable energy economy, hydrogen will become very important as it is considered one of the key energy carriers in terms of energy content, as fuel for transportation and intermediate in the conversion of renewable energy sources. The aim of this work was the design of novel catalysts for hydrogen production from both methane and methanol- ethanol/water streams. Special interest was also focused on the CO2 hydrogenation reaction, as a potential chemical route to its valorization to mitigate its greenhouse effect. Active catalysts, resistant to the sinterization under severe reaction conditions, were developed through a simple and low cost synthetic route, which was based on the encapsulation of pre-formed Rh nanoparticles into porous oxides. This procedure reduces the mobility of the metal particles at high temperature. The embedded catalysts, tested for the methane partial oxidation, presented higher thermal stability with respect to a reference catalyst obtained by conventional incipient wetness impregnation. In order to obtain a better understanding of the interaction between Rh nanoparticles and the alumina support in the embedded catalysts, an X-Ray Photoelectron Spectroscopy (XPS) study on three model embedded Rh systems was performed. The extension of the embedding approach to Ni/Cu-based systems was also handled. In addition, Ni(x%)Cu(y%)/Al2O3 catalysts with different Ni and Cu contents were synthesized using the conventional impregnation method. All the samples were tested towards the partial oxidation of methane and the steam reforming of methanol and ethanol. Neither copper nor nickel alone supported on alumina appeared as suitable catalysts for ethanol steam-reforming at low temperatures (T < 500 °C). The activity of the bimetallic systems, during the first run-up experiment, is not very different from the monometallic Ni system. Furthermore, the Ni:Cu ratio does not seem to affect the product distribution. Notably, the bimetallic systems show promising catalytic activity in the methanol steam reforming. Finally, the formation of metal alloys between Cu and Ni, after high temperature reduction, leads to a strongly reduction of coke deposition under methane partial oxidation conditions, increasing the life-time of the catalyst. On Ni(x%)Cu(y%)/Al2O3 samples, CO2 hydrogenation was also investigated. All catalysts did not show the ability to activate the CO2 molecule, as well as the corresponding unsupported systems. No CO was observed if hydrogen was not introduced into the stream. These results are in good agreement with the data obtained on Ni single crystal (Ni(110)), under UHV conditions. In this case, stable hydrogenation intermediates/products were observed during the reaction by means of X-Ray Photoelectron Spectroscopy (XPS), Temperature Programmed Desorption (TPD) experiments and High Resolution Electron Energy Loss Spectroscopy (HREELS) in the -180/230 °C temperature range. The evolution of the surface species and concentrations as a function of the annealing temperature were examined. This work was supported by parallel DFT calculations, in order to model both experimentally and theoretically the CO2 hydrogenation reaction.