RESUMO
Monometallic and bimetallic Cu:Ni catalysts with different Cu:Ni molar ratios (3:1, 2:1, 1:1, 1:2, 1:3) were synthesized by wetness impregnation on activated carbon and characterized by TPR (temperature programmed reduction), XRD (X-ray diffraction) and XPS (X-ray photoelectron spectroscopy). The synthesized catalysts were evaluated in the gas phase production of diethyl carbonate from ethanol and carbon dioxide. The largest catalytic activity was obtained over the bimetallic catalyst with a Cu:Ni molar ratio of 3:1. Its improved activity was attributed to the formation of a Cu-Ni alloy on the surface of the catalyst, evidenced by XPS and in agreement with a previous assignment based on Vegard law and TPR analysis. During the reaction rate experiments, it observed the presence of a maximum of the reaction rate as a function of temperature, a tendency also reported for other carbon dioxide-alcohol reactions. It showed that the reaction rate-temperature data can be adjusted with a reversible rate equation. The initial rate as a function of reactant partial pressure data was satisfactorily adjusted using the forward power law rate equation and it was found that the reaction rate is first order in CO2 and second order in ethanol.
RESUMO
Hydrodeoxygenation (HDO) kinetics of glycerol into 1,2-propanediol (1,2-PDO) in the liquid phase is studied on Cu-Pd/TiO2 catalysts. At a stirring speed higher than 480 rpm and an average diameter of the catalyst particles smaller than 89.5 µm, no mass transfer resistance artifacts are observed. The increasing temperature and H2 concentration promote the glycerol conversion and the selectivity to 1,2-PDO and disfavor the selectivity to acetol. Based on the experimental data, empirical kinetic pseudo-homogeneous expressions are proposed for glycerol disappearance, 1,2-PDO formation, and acetol formation in the catalytic system. Dependence of the disappearance rate of glycerol is closer to 1 with respect to glycerol and nonmeaningful with respect to H2. The formation rate of 1,2-PDO is not highly dependent on the initial concentration of glycerol or H2, and the formation rate of acetol is directly dependent on glycerol and inversely dependent on H2, since it accelerates acetol conversion to 1,2-PDO. The activation energies for glycerol disappearance (77.8 kJ/mol), 1,2-PDO formation (51.2 kJ/mol), and acetol formation (84.6 kJ/mol) evidenced the selective formation of 1,2-PDO in this catalytic system.
RESUMO
We demonstrated recently that CuPd/TiO2-Na bimetallic catalysts synthesized by sequential wet impregnation are active, selective, and stable for the hydrodeoxygenation (HDO) of glycerol into propylene glycol at low H2 pressure. The present study reports on the nature and distribution of Cu and Pd surface species in CuPd/TiO2-Na bimetallic catalysts using different scanning transmission electron microscopy techniques that supply cluster-specific alloying details. In particular, we used atomic-resolution Z-contrast imaging, X-ray energy-dispersive spectroscopy, and electron energy-loss spectroscopy. We also include X-ray photoelectron spectroscopy results. Our analysis shows that the metallic nanoparticles adopt mainly five different structures according to how the Cu and Pd atoms coordinate among themselves: a homogeneous CuPd alloy structure (45-61%), a Cu shell/CuPd core (15-23%), a smaller number of particles formed by Cu on the surface and Pd in the nucleus (10-17%), and there are also nanoparticles formed only by Pd (4-7%) or by Cu (8-13%). We determined that there is a inhomogeneous distribution of Cu and Pd in the bimetallic nanoparticles, with Cu being predominant on the surface (between 76 and 90% of the total area analyzed for each particle). Most bimetallic nanoparticles have sizes below 6 nm, the Pd monometallic nanoparticles are in the 2-4 nm range, whereas the monometallic Cu nanoparticles are larger than 8 nm. Bimetallic nanoparticles with sizes smaller than 6-7 nm are fundamentally made up of Cu0-Pd0 and Cu1+-Pd0. The nanoparticles with sizes greater than 7 nm consist of Cu2+ and Cu2+-Pd2+. Our obtained results also help describe reports about the activation of HDO by Pd-Cu in the absence of H2, an effect apparently not observed with other bimetallic systems.