The Investigation Of Methane Reforming With Carbon Dioxide Over Rh-based Catalysts

The investigation of methane reforming with carbon dioxide over Rh-based catalystsMethane is the main chemical of natural gas and coal bed gas, and carbon dioxide is the final oxide of the carbon containing compounds and a greenhouse gas. Reforming is an attractive way of utilizing methane. Among reforming reactions, the dry reforming of methane with carbon dioxide attracts much attention as the reaction converting methane and carbon dioxide to valuable oxygenated chemicals through synthesis gas.The dry reforming reaction of methane with carbon dioxide is strongly exothermal in thermodynamics and performs at high temperature (?800 K). The main obstacle is the sintering of the active species at high reaction temperature and coking, which leads to the loss of the catalytic activity of the catalysts. Thus, it is necessary to understand the catalytic reaction mechanism, which is beneficial for the catalyst design. This dissertation is aimed to understand the effect factors of the catalyst property such as metal dispersion and support role on the catalytic activity, and the disclosure of the reaction intermediates and the mechanism over the Rh-based catalysts. Rh/H-Beta with high dispersion shows high activity and coke resistance in the dry reforming reaction.1H MAS NMR under controlled atmosphere was employed in the detection of the adsorbed species (CHx*, CHxO*, x=1-3) with high sensitivity, clear signal discrimination and no Knight shift effect from rhodium surface. The reaction intermediates of CH3O*, CH2O* and HCOOH* were detected in the dry reforming reaction over Rh/H-Beta. Also, the mesoporous materials of MCF and SBA-15 with high thermal stability and large surface area were used as the support of rhodium based catalysts with different rhodium dispersion. The results show that high rhodium dispersion, hydrogen spillover between metal and support of Rh/SBA-15 and Rh/MCF could influence the dry reforming reaction.The main results are summarized as follows:1. The investigation of methane dry reforming with carbon dioxide over Rh/H-BetaThe effect of Rh dispersion on reforming of CH4 with CO2 over H-Beta supported Rh catalysts has been investigated. The CH4 and CO2 conversion over the catalysts increase with increasing Rh loading from 0.5 to 4.0 wt% in the reaction temperature range of 823-1123 K. The high TOF of CH4 over 0.5 and 1.0 wt% Rh/H-Beta was observed, which may be attributed to highly dispersed rhodium species. The catalysts before and after the reaction were characterized by XRD, TEM, and TG-DTA and the results indicate the catalysts with Rh loading of 0.5 and 1.0 wt% exhibit high resistance to coke deposition. Under different gas atmosphere, we confirm that the coke is originated from methane dissociation and can be substantially oxidized by active oxygen species dissociated from the adsorbed carbon dioxide on the catalyst with high dispersion of Rh species.2. The reaction intermediates study in the dry reforming of CH4 with CO2 over Rh/H-BetaThe reaction intermediates in the dry reforming of CH4 with CO2 over 2.0 wt% Rh/H-Beta were investigated using 13C and 1H MAS NMR under controlled atmosphere. No intermediates information but the carbon exchange and the dry reforming reaction were found by 13C MAS NMR due to the Knight shift effect from rhodium surface, while H-containing adsorbed species such as CH3O*, CH2O*, HCOOH* were detected by 1H MAS NMR. Based on the results obtained, the dry reforming mechanism was proposed. The decomposition of CH2O* species may be considered as the rate determined step of the dry reforming of CH4 with CO2, whereas the formation of HCOOH* species may be attributed to reverse water gas shift reaction.3. The investigation of methane dry reforming with carbon dioxide over Rh/MCF and Rh/SBA-15On Rh/MCF and Rh/SBA-15 there is metal-support interaction in the behavior of hydrogen spillover. When rhodium dispersion increases above 50%, the strength of the hydrogen spillover greatly enhances. The catalytic test shows that high TOF of methane over the catalysts with high rhodium dispersion is directly influenced by the strength of the hydrogen spillover. For the catalysts of Rh/MCF and Rh/SBA-15 with low rhodium dispersion (< 30%), no hydrogen spillover can be observed and the catalytic activity is relatively low and shows little difference, suggesting the activity of the catalysts with low rhodium dispersion has less influence from the support.4. Mechanistic studies of CH4 reforming with CO2 over Rh/SBA-15 by controlled atmosphere Solid State 13C and 1H MAS NMRThe mechanism of methane reforming with carbon dioxide over the efficient catalyst of 1.0 wt% Rh/SBA-15 was investigated by the 13C and 1H MAS NMR under controlled atmosphere condition. Under mild conditions (573-773 K), the carbon exchange reaction between methane and carbon dioxide was observed. From 773 K, the adsorbed species of CH3O* could be detected. In the high temperature range of 873-1023 K, the dry reforming occurs with the appearance of CH2O* signal besides the strengthening CH3O* signal, indicating that formation of CH3O*, decomposition of CH3O* to CH2O*, and decomposition CH2O* species to CO and H2 are crucial steps in methane reforming with carbon dioxide.