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Post-steaming treatment of Mo/HZSM-5 catalysts results in more molybdenum species migrating into and residing in the HZSM-5 zeolite channels. This is confirmed by XRF and XPS measurements. 1H MAS NMR and 29Si MAS NMR also demonstrate that the number of free Brönsted acid sites decreases in the Mo/HZSM-5 catalysts that underwent post-steaming treatment, compared to untreated Mo/HZSM-5 catalysts. As a result, the deactivation rate constant (kd) on the Mo/HZSM-5 catalyst after post-steaming treatment for 0.5h is much smaller, and the catalyst therefore shows remarkable stability in the probe reaction of methane dehydro-aromatization. The results suggest that a more beneficial bi-functional balance between active Mo species for methane activation and acid sites for the following aromatization is developed over those Mo/HZSM-5 catalysts that have experienced post-steaming treatment for 0.5h, in comparison with the untreated Mo/HZSM-5 catalysts. [ABSTRACT FROM AUTHOR]

Hydrogen production from the steam reforming reactions of ethanol and glycerol has been studied over ceria-supported Ir, Co and Ni catalysts with respect to the nature of the active metals and the reaction pathways. For ethanol steam reforming, ethanol dehydrogenation to acetaldehyde and ethanol decomposition to methane and carbon monoxide were the primary reactions at low temperatures, depending on the active metals. At higher temperatures where all the ethanol and the intermediate compounds, like acetaldehyde and acetone, were completely converted into hydrogen, carbon oxides and methane, steam reforming of methane and water gas shift became the major reactions. The Ir/CeO2 catalyst was significantly more active and selective toward hydrogen production, and the superior catalytic performance was interpreted in terms of the intimated contact between Ir particles and ceria based on the ceria-mediated redox process. Additionally, hydrogen production from steam reforming of glycerol was also examined over these ceria-supported metal catalysts, and the Ir/CeO2 catalyst again showed quite promising catalytic performance with hydrogen selectivity of more than 85% and 100% glycerol conversion at . [Copyright &y& Elsevier]

Abstract: The catalytic behavior of Ni/CeO2 catalysts was investigated for methane decomposition at 773K. Ni/CeO2 catalysts prepared by impregnation and deposition–precipitation methods showed much higher hydrogen formation rates than that of Ni/CeO2 catalyst prepared by coprecipitation method, which exhibited relatively higher CO formation rate. CH4-TPSR measurements confirmed that CO formation is inevitable during methane decomposition, since the lattice oxygen of ceria could react with the deposited carbons. TEM observations further indicated that the morphologies as well as the reactivities of the deposited carbons are strongly dependent on the interaction degrees between Ni and ceria, which in turn explains the differences in the catalytic activity of the Ni/CeO2 catalysts for methane decomposition. The Ni/CeO2 catalyst prepared by coprecipitation method exhibited rather strong metal–support interaction probably through the formation of Ni–O–Ce solid solution, resulting in much lower hydrogen formation rate and relatively higher CO production. [Copyright &y& Elsevier]

Abstract: MnO x –CeO2 mixed oxides prepared by sol–gel method, coprecipitation method and modified coprecipitation method were investigated for the complete oxidation of formaldehyde. Structure analysis by H2-TPR and XPS revealed that there were more Mn4+ species and richer lattice oxygen on the surface of the catalyst prepared by the modified coprecipitation method than those of the catalysts prepared by sol–gel and coprecipitation methods, resulting in much higher catalytic activity toward complete oxidation of formaldehyde. The effect of calcination temperature on the structural features and catalytic behavior of the MnO x –CeO2 mixed oxides prepared by the modified coprecipitation was further examined, and the catalyst calcined at 773K showed 100% formaldehyde conversion at a temperature as low as 373K. For the samples calcined below 773K, no any diffraction peak corresponding to manganese oxides could be detected by XRD measurement due to the formation of MnO x –CeO2 solid solution. While the diffraction peaks corresponding to MnO2 phase in the samples calcined above 773K were clearly observed, indicating the occurrence of phase segregation between MnO2 and CeO2. Accordingly, it was supposed that the strong interaction between MnO x and CeO2, which depends on the preparation route and the calcination temperature, played a crucial role in determining the catalytic activity toward the complete oxidation of formaldehyde. [Copyright &y& Elsevier]

Abstract: The effect of pre-reduction with H2 at a low temperature, i.e. 623K, on the structure of the Mo/HZSM-5 catalyst prepared by impregnation has been studied and characterized by means of 1H MAS NMR and EPR techniques. It has been revealed that there are three kinds of Mo species on the Mo/HZSM-5 catalyst, namely, the Mo6c 6+ oxide species with distorted octahedral coordination, the Mo5c 6+ oxide species with square pyramidal coordination, and the Mo6+ oxide species associated with the Brönsted acid sites. The first two Mo oxide species exist mainly on the external surface of the HZSM-5 zeolite, and can be readily reduced to form the β-Mo2C species, while the last one resides primarily in the channels, and is difficult to be fully reduced to form Mo carbide species. The Mo oxide species associated with the Brönsted acid sites can be partially reduced to the MoO x C y species during the induction period, which play a crucial role in the methane dehydro-aromatization (MDA) reaction. The MoO x C y species with face centered cubic (fcc) structure are more active and more selective towards benzene, yet more stable than those Mo carbide species with hexagonally close packed (hcp) structure. It is likely that pre-reduction with H2 at 623K can enhance the topotactic transformation of the Mo specie from the hcp structure into the fcc structure, thus can improve greatly the catalytic activity and selectivity towards mono-aromatics, as well as the stability of the Mo/HZSM-5 catalyst. [Copyright &y& Elsevier]

Abstract: The catalytic performances of methane dehydroaromatization (MDA) under non-oxidative conditions over 6wt.% Mo/HZSM-5 catalysts calcined for different durations of time at 773K have been investigated in combination with ex situ 1H MAS NMR characterization. Prolongation of the calcination time at 773K is in favor of the diffusion of the Mo species on the external surface and the migration of Mo species into the channels, resulting in a further decrease in the number of Brönsted acid sites, while causing only a slight change in the Mo contents of the bulk and in the framework structure of the HZSM-5 zeolite. The MoO x species associated and non-associated with the Brönsted acid sites can be estimated quantitatively based on the 1H MAS NMR measurements as well as on the assumption of a stoichiometry ratio of 1:1 between the Mo species and the Brönsted acid sites. Calcining the 6wt.% Mo/HZSM-5 catalyst at 773K for 18h can cause the MoO x species to associate with the Brönsted acid sites, while a 6wt.% Mo/SiO2 sample can be taken as a catalyst in which all MoO x species are non-associated with the Brönsted acid sites. The TOF data at different times on stream on the 6wt.% Mo/HZSM-5 catalyst calcined at 773K for 18h and on the 6wt.% Mo/SiO2 catalyst reveal that the MoC x species formed from MoO x associated with the Brönsted acid sites are more active and stable than those formed from MoO x non-associated with the Brönsted acid sites. An analysis of the TPO profiles recorded on the used 6wt.% Mo/HZSM-5 catalysts calcined for different durations of time combined with the TGA measurements also reveals that the more of the MoC x species formed from MoO x species associated with the Brönsted acid sites, the lower the amount of coke that will be deposited on it. The decrease of the coke amount is mainly due to a decrease in the coke burnt-off at high temperature. [Copyright &y& Elsevier]

TEOS as the exotic silicon source was used to modify HZSM-5 zeolite by chemical vapor deposition (CVD) in a vapor phase flow system. Mo/HZSM-5 catalysts prepared by using silicon-modified zeolite showed higher activity and stability than those prepared by using non-modified HZSM-5 zeolite in methane dehydro-aromatization (MDA) under non-oxidative conditions. The effect of silanation on the acid sites and the interaction between the Mo species and the acid sites have been studied by using FT-IR, NH3-TPD and 1H MAS NMR. The exotic silicon species not only selectively covered and eliminated almost all of the Brönsted acid sites on the external surface of HZSM-5 zeolite, but also affected the internal Brönsted acid sites, which are close to the channel openings. This results in an obvious decrease in the number of Brönsted acid sites per unit cell. In the MDA reaction, the Brönsted acid sites provided active sites not only for the polymerization and aromatization of the active intermedia, but also for the formation of carbonaceous deposits, which is a crucial factor leading to the deactivation of Mo/HZSM-5 catalyst. Only a small fraction of Brönsted acid sites, ca. 0.6 per unit cell, was required to accomplish the aromatization, and the superfluous free Brönsted acid sites would cause severe carbonaceous deposits under non-oxidative condition at a temperature as high as 973 K. [Copyright &y& Elsevier]

The production of ethylbenzene from the alkylation of dilute ethylene in fcc off-gases with benzene has been commercialized in China over a newly developed catalyst composed of ZSM-5/ZSM-11 co-crystallized zeolite. The duration of an operation cycle of the commercial catalyst could be as long as 180 days. The conversion of ethylene could attain higher than 95%, while the amount of coke deposited on the catalyst was only about 10 wt.%. Thermogravimetry (TG) was used to study the coking behavior of the catalyst during the alkylation of fcc off-gas with benzene to ethylbenzene. Based on effects of reaction time, reaction temperature, reactants and products on coking during the alkylation process, it is found that the coking rate during the alkylation procedure follows the order: ethylbenzene>ethylene>propylene>benzene for single component, and benzene–ethylene>benzene–propylene for bi-components under the same reaction condition. Furthermore, the coking kinetic equations for benzene–ethylene, benzene–propylene and ethylbenzene were established. [Copyright &y& Elsevier]