Search

Abstract: Comparative studies using high temperature in situ Raman spectroscopy, TPSR, pulsed reactions of CH4, H2-TPR and O2-TPD techniques were carried out on Rh/Al2O3 calcined at 600 and 900°C to elucidate the effect of calcination temperatures on the nature of the Rh species in the catalysts and its relationship with the catalytic performance of Rh/Al2O3 for partial oxidation of methane (POM) to synthesis gas. The results indicate that significant difference in the POM performance of Rh/Al2O3 catalysts calcined at 600 and 900°C can be attributed to the formation of rhodium species of different nature after the samples were calcined in the air at above temperatures. Calcining a Rh/Al2O3 sample in the air at 900°C caused not only the formation of more than 81% of rhodium species irreducible by H2 at the temperature below 600°C, but also affected the redox property of the RhOx species reducible at the temperature below 250°C. Compared to the rhodium species in the Rh/Al2O3 calcined at 600°C, those in the catalyst calcined at 900°C possesses stronger Rh–O bonds and higher oxygen affinity. These factors would affect the concentration of oxygen species on the two catalysts under the POM reaction conditions and consequently affect the primary products of the reaction. The results of TPSR and in situ Raman characterizations on the POM reaction over the catalysts indicate that, Rh/Al2O3 in fully oxidized form are mainly responsible for complete oxidation CH4 to CO2 and H2O. The catalysts abruptly changed its oxidation state at the temperature when POM reaction starts. The results of high temperature Raman characterizations also suggest that a rhodium species in which Rh may substitute for the Al site of Al2O3 was formed in the Rh/Al2O3 sample calcined in air at 900°C. This rhodium species is difficult to be reduced to Rh metal by H2 at temperature below 600°C. [Copyright &y& Elsevier]

Abstract: Catalytic performance for partial oxidation of methane (POM) to synthesis gas was studied over the Rh/Al2O3 catalysts with Rh loadings between 0.1 and 3wt%. It was found that the ignition temperature of POM reaction increased with the decreasing of the Rh loadings in the catalysts. For the POM reaction over the catalysts with high (≥1wt%) Rh loadings, steady-state reactivity was observed. For the reaction over the catalysts with low (≤0.25wt%) Rh loadings, however, oscillations in CH4 and reaction products (CO, H2, and CO2) were observed. Comparative studies using H2-TPR, O2-TPD and high temperature in situ Raman spectroscopy techniques were carried out in order to elucidate the relation between the redox property of the Rh species in the Rh/Al2O3 with different Rh loadings and the performance of the catalysts for the reaction. Three kinds of oxidized rhodium species, i.e. the rhodium oxide species insignificantly affected by the support (RhO x ), that intimately interacting with the Al2O3 surface (Rh i O x ) and the Rh(AlO2) y species formed by diffusion of rhodium oxides in to sublayers of Al2O3 [C.P. Hwang, C.T. Yeh, Q.M. Zhu, Catal. Today, 51 (1999) 93.], were identified by H2-TPR and O2-TPD experiments. Among them, the first two species can be easily reduced by H2 at temperature below 350°C, while the last one can only be reduced by H2 at temperature above 500°C. The ignition temperatures of POM reaction over the catalysts are closely related to the temperature at which most of the RhO x and Rh i O x species can be reduced by CH4 in the reaction mixture. Compared to the Rh/Al2O3 with high Rh loadings, the catalysts with low Rh loadings contain more Rh i O x species which possess stronger Rhrength and are more difficult to be reduced than RhO x by the reaction mixture. Higher temperature is therefore required to ignite the POM reaction over the catalysts with lower Rh loadings. The oscillation during the POM reaction over the Rh/Al2O3 with low Rh loadings can be related to the behaviour of Rh(AlO2) y species in the catalyst switching cyclically from the oxidized state to the reduced state during the reaction. [Copyright &y& Elsevier]

Abstract: Pulse reaction method and in situ IR spectroscopy were used to characterize the active oxygen species for oxidative coupling of methane (OCM) over SrF2/Nd2O3 catalyst. It was found that OCM activity of the catalyst was very low in the absence of gas phase oxygen, which indicated that lattice oxygen species contributed little to the yield of C2 hydrocarbons. IR band of superoxide species (O2 −) was detected on the O2-preadsorbed SrF2/Nd2O3. The substitution of 18O2 isotope for 16O2 caused the IR band of O2 − at 1128cm−1 to shift to lower wavenumbers (1094 and 1062cm−1), consistent with the assignment of the spectra to the O2 − species. A good correlation between the rate of disappearance of surface O2 − and the rate of formation of gas phase C2H4 was observed upon interaction of CH4 with O2-preadsorbed catalyst at 700°C. The O2 − species was also observed on the catalyst under working condition. These results suggest that O2 − species is the active oxygen species for OCM reaction on SrF2/Nd2O3 catalyst. [Copyright &y& Elsevier]

Highlights: [•] Mesoporous SiO2 with incorporated V and Te is tested for propane oxidation. [•] Acrolein formation rate positively correlates with content of highly dispersed TeO x . [•] Highly dispersed VO4 in close proximity to TeO x constitutes the active site. [•] The active site is capable of catalyzing direct oxidation of propane to acrolein. [Copyright &y& Elsevier]

In situ microprobe Raman and XRD techniques were used to follow the oxidation state of Rh/Al2O3 and Ru/Al2O3 catalysts during the catalytic ignition process of the partial oxidation of methane (POM) to synthesis gas. It was found that the catalyst was in the fully oxidized state before ignition of the POM reaction, and abruptly changed its oxidation state at the temperature at which the POM reaction started. After the POM reaction was ignited, the amount of Rh2O3 or RuO2 in the catalyst at the entrance of the catalyst bed was below the detection level of Raman spectroscopy. Due to the greater Mrength of Rud with Rhce:inf loc="post">2O3 demonstrated a greater tendency to oxidize than Rh/Al2O3 under the POM conditions. This factor affects the oxygen coverage on the two catalysts under reaction conditions and consequently affects the pathways of synthesis gas formation. [Copyright &y& Elsevier]