Reactants-induced dynamic responses of the surface of heterogeneous catalysts monitored by microcalorimetry beyond adsorption

摘要

For a detailed understanding of complex reaction networks we need thermodynamic data of high accuracy and information about the nature of the catalyst surface. Therefore, we quantitatively study the adsorption, activation, and reaction phenomena close to the reaction parameters. Furthermore, we simulate reactants-induced responses of the surface via adsorption/desorption cycles in order to stepwise create an active surface, and thus can get new insights into the dynamic behaviour of the surface. Since perhaps only a minor fraction of all surface atoms form active sites, the quantification requires a very sensitive analytical method. We focus on microcalorimetry beyond adsorption. [1-3] This is a direct method to determine the number, strengths and energy distribution of adsorption sites, as well as adsorption constants and the active surface area. To facilitate the correlation of microcalorimetric results with the catalytic performance, a molecule similar to the reactant is used. Tads, is chosen lower than Treact. to try to separate the adsorption and activation process from the catalytic reactions. In this work, we present i) CO chemisorption cycles on a Ni/MgAl oxide catalyst for dry reforming of methane (DRM), [3] ii) propane and ethane adsorption/desorption cycles on a MoV oxide catalyst for oxidative dehydrogenation of alkanes and iii) CO chemisorption on Ir based catalysts for oxygen evolution reaction (OER). [2] In the first example, the influence of structural and compositional properties of Ni/MgAl catalysts on the catalytic performance in dry reforming of methane (DRM) has been studied.[3] In general, the most active 50wt.-%Ni/MgAl catalyst (XCH4,10h=73%) is characterized by bigger Ni particles (>20nm) due to the high Ni content. The particles are mainly metallic and partially covered by an overgrowth (NiA1204). Reactant-induced changes of the surface via adsorption/desorption CO cycles are revealed by a drastic change in the differential heat profiles due to restructuring of metallic Ni sitee. This behavior evidences a highly dynamic surface. In contrast, the less active 5wt.-%Ni/MgAl oxide catalyst (XCH4,10h=50%) shows no dynamic features. We can conclude that an active Ni-based catalyst needs metallic Ni sites and has dynamic capabilities to restore deactivated metallic Ni sites. The less active catalyst has a comparably high nickel oxide content, which probably prevents the dynamics. The second example presents MoV oxide as an active catalyst in the oxidative dehydrogenation of alkanes. In-situ photoelectron spectroscopy has shown that with time on stream the surface of the catalyst is progressively enriched in V5+, which leads to a decrease in selectivity.[4] This surface modification was simulate via adsorption/desorption cycles of propane measured by microcalorintry. At very low coverages of propane the differential heat increases over the cycles. The newly generated sites might be related to the segregation of V5+. The very strong interaction of propane with these sites explains the decrease in selectivity caused by V-segregation. Microcalorimetry close to the reaction conditions suggests that segregation occurs already at low temperature and is possible in absence of water. However, traces of water might be formed during the experiment, because, analyses of the heat profiles prove that the alkane is already activated at room temperature on MoV oxide.