The role of surface structure and defects in the oxidative coupling of methane (OCM) was studied over magnesium oxide as a model catalyst. Pure, nano-structured MgO catalysts with varying primary particle size, shape and specific surface area were prepared by sol-gel synthesis, oxidation of metallic magnesium, and hydrothermal post treatments. The initial activity of MgO in the OCM reaction is clearly structure-sensitive. Kinetic studies reveal the occurrence of two parallel reaction mechanisms and a change in the contribution of these pathways to the overall performance of the catalysts with time on stream. The initial performance of freshly calcined MgO is governed by a surface-mediated coupling mechanism involving direct electron transfer between methane and oxygen. The two molecules are weakly adsorbed at structural defects (steps) on the surface of MgO. The proposed mechanism is consistent with high methane conversion, a correlation between methane and oxygen consumption rates, and high C2H4 selectivity after short times on stream. The water formed in the OCM reaction causes sintering of the MgO particles and loss of active sites by degradation of structural defects, which is reflected in decreasing activity of MgO with time on stream. At the same time, gas-phase chemistry becomes more important, which includes the formation of ethane by coupling of methyl radicals formed at the surface and the partial oxidation of C2H6. The mechanistic concepts proposed in this work (Part I) will be substantiated in Part II by spectroscopic characterization of the catalysts (Schwach et al. 111). (C) 2015 Elsevier Inc. All rights reserved.
The subjects of the research were barium-promoted ruthenium catalysts for ammonia synthesis supported on graphitized carbon. The purpose of this work was to study in detail the process of an active Ba-Ru/C catalyst formation. Another goal was to characterize the active state of the Ba promoter, that is the state corresponding to ammonia synthesis conditions. In situ XRD and TPR-MS techniques were applied to monitor the changes in the Ba-Ru/C specimens when heating in hydrogen (or H_2 + N_2) and H_2 + Ar mixtures, respectively. The post-activation state of the catalyst was characterized chemically via interaction of the reduced samples with water vapour at 50 °C and also via interaction with oxygen at 0 °C. The above mentioned experiments were supplemented with those of ammonia synthesis. It was shown that ruthenium facilitates decomposition of the promoter's precursor (Ba(NO_3)_2) deposited onto the surface of Ru/C catalysts when heating the specimens in a hydrogen-containing stream. The Ba(NO_3)_2/C reference materials, which do not contain ruthenium, are stable in a flowing H_2 + Ar mixture up to about 400 °C, whereas the Ba(NO_3)_2 decomposition starts at 100-120 °C in the Ba(NO_3)_2-Ru/C systems (XRD, TPR-MS). The decomposition of Ba(NO_3)_2 in hydrogen leads to barium oxide (BaO) and metallic barium. Under steady-state conditions BaO is the only Ba-containing phase detected by the X-ray diffraction technique. Characterization of the post-activation catalysts showed that barium is partially reduced during the aforementioned operations and that these catalysts react with oxygen and water vapour. Based on the comparison of the O_2 consumption and H_2 evolution data one may deduce that the active form of the promoter is a mixture: Ba~0 + BaO. It can be stated that the temperature and content of the promoter (C_(Ba)) have a significant influence on NH_3 formation. The shape of reaction rate vs. barium content function is assumed to be an outcome of the promoter distribution on the active carbon surface and ruthenium surface. The trend of the integral reaction rate clearly reflects that of the Ru coverage by the barium-containing species, which is controlled by the heats of adsorption on ruthenium and carbon, respectively.