Steam reforming of methanol over copper-containing catalysts: influence of support material on microkinetics

Steam reforming of methanol (SRM) was investigated over copper-containing catalysts supported on four different oxides and mixed oxides: Cu/ZnO/Al2O3, Cu/ZrO2/CeO2, Cu/SiO2 and Cu/Cr2O3/Fe2O3. After observing slight differences in the way of catalyst aging and experimental exclusion of mass transport limitation effects, a detailed kinetic study was carried out at 493 K. The dependence of the reaction rate on the molar ratio of methanol and water was determined as well as the influence of addition of inert nitrogen and the main reaction products hydrogen and carbon dioxide to the reactant mixture. Although there were remarkable differences in the catalytic activity of the samples, the main mechanistic steps reflected in the rate law appeared to be similar for all catalysts. The reaction rate is mainly determined by the methanol partial pressure, whereas water is not involved in the rate determining step, except over Cu/Cr2O3/Fe2O3, where several differences in the chemistry were observed. Hydrogen and carbon dioxide were found to inhibit the reaction. These results were confirmed by a DRIFTS study at 493 K using an equimolar reactant mixture and an excess of 4:1 of water and methanol, respectively. The same surface species could be identified on each catalyst but neither kinetic modelling nor the DRIFTS spectra could give a clear answer if the reaction pathway occurs via a dioxomethylene or a methyl formate species as intermediate. Similar activation energies of SRM confirm the assumption, that the surface chemistry of SRM over copper-based systems is independent of the catalyst support material

In situ surface coverage analysis of RuO₂-catalysed HCl oxidation reveals the entropic origin of compensation in heterogeneous catalysis

In heterogeneous catalysis, rates with Arrhenius-like temperature dependence are ubiquitous. Compensation phenomena, which arise from the linear correlation between the apparent activation energy and the logarithm of the apparent pre-exponential factor, are also common. Here, we study the origin of compensation and find a similar dependence on the rate-limiting surface coverage term for each Arrhenius parameter. This result is derived from an experimental determination of the surface coverage of oxygen and chlorine species using temporal analysis of products and prompt gamma activation analysis during HCl oxidation to Cl2 on a RuO2 catalyst. It is also substantiated by theory. We find that compensation phenomena appear when the effect on the apparent activation energy caused by changes in surface coverage is balanced out by the entropic configuration contributions of the surface. This result sets a new paradigm in understanding the interplay of compensation effects with the kinetics of heterogeneously catalysed processes

The impact of nitrogen mobility on the activity of zirconium oxynitride catalysts for ammonia decomposition

A zirconium oxynitride catalyst was used for the decomposition of ammonia to hydrogen and nitrogen. The onset of catalytic activity at 550 °C coincided with the onset of nitrogen ion mobility in the material and a phase change from the initial β′ phase ( Zr7O11N2) to the nitrogen-rich β″ ZrON phase ( Zr7O9,5N3). No hydrazine formation during an extended time on stream was detectable. Moreover, the onset of activity was also correlated to a rapid change in the electronic structure of the surface accompanying formation of the more active β″ ZrON phase. The results presented here show for the first time a direct correlation among the onset of ion conductivity as a bulk property, a modified electronic structure of the surface, and the catalytic performance of a heterogeneous catalyst

Nitrides as ammonia synthesis catalysts and as potential nitrogen transfer reagents

In this article, an overview of the application of selected metal nitrides as ammonia synthesis catalysts is presented. The potential development of some systems into nitrogen transfer reagents is also described