Font Size: a A A

The Low-temperature Combustion Reaction Of Methane Over Pd-supported Zeolite Catalysts

Posted on:2004-10-16Degree:DoctorType:Dissertation
Country:ChinaCandidate:C K ShiFull Text:PDF
GTID:1101360122966912Subject:Physical chemistry
Abstract/Summary:PDF Full Text Request
Since methane (CH4, main constituent of natural gas) is the most difficult to oxidize among all the hydrocarbons and has a much larger greenhouse effect than carbon dioxide, low-temperature complete combustion of methane is a challenging research subject in terms of energy generation and methane emission control. Pd-related catalysts are considered as the most active ones for CH4 combustion, and palladium metal is richer and more inexpensive than other noble metals. Thus, Pd-supported catalysts for CH4 combustion are got an especial attention. In the practical applications, the catalyst musi ignite reaction below 300℃, oxidize nearly 100% of CH4 below 400℃, and possess a long catalytic lifetime. Therefore, some Pd-supported oxide catalysts, such as Pd/Al2O3, Pd/SnO2, Pd/ZrO2 and Pd/36NiO-Al2O3 etc., were not efficient enough for CH4 combustion at low temperatures. The Pd-supported zeolite catalysts showed excellent low-temperature activity and conversion efficiency for CH4 combustion. However, these catalysts, which were typically prepared by the ion-exchange method, included generally high Pd-loadings and still showed poor thermal and hydrothermal stabilities. In addition, there is a general agreement that the activity of Pd catalysts was greatly affected by the kinds of supports and additives. And active sites on Pd catalysts were differently described in literatures. So it is very important that Pd catalysts are researched for low-temperature combustion of CH4 from the point of application and theory study.In this thesis, Pd/HZSM-5 catalysts without and with additives of oxides were prepared by the impregnation method and investigated for the first time as catalysts for low-temperature methane combustion. In point of application and theory study, we optimized firstly the desired catalysts by researching into the catalytic activities and thermal/hydrothermal stabilities of various catalysts for CH4 combustion. Then, the various characterization techniques such as X-raydiffraction (XRD) for the crystallization of PdO on catalysts, CO adsorption for Pd dispersion of catalyst surface, ammonia temperature-programmed desorption (NH3-TPD) for acid-basic properties of catalyst surface, oxygen temperature-programmed desorption (O2-TPD) for oxygen desorption, methane temperature-programmed reduction (CH4-TPR) for methane reduction of PdO, in/ex-situ Raman spectra for the states and redox behavior of PdO were used. Finally, reaction mechanism of methane combustion was speculated according to the activity data and characterization results of catalysts. The primary content of the research is as follows:1. Optimization of catalysts for methane combustion at low-temperaturePd/HZSM-5, Pd/NaY, Pd/Mordenite, Pd/Al2O3 and Pd/SiO2 were investigated as catalysts for low-temperature methane combustion. Pd/HZSM-5 was found to have the best activity among these catalysts. The T10%, T50% and T100% (the temperatures corresponding to 10, 50 and 100% methane conversions) of Pd/HZSM-5 were 32, 96 and 70℃ lower than those of Pd/Al2O3, respectively. It was confirmed that the HZSM-5 support showed the best enhancing effect on the activity of supported PdO species than other supports. Moreover, Pd/HZSM-5 catalysts with different Si/Al2 did not show the significant discrepancy of activity.The activity results of catalysts with 0.5~4wt% Pd-loadings found that the low Pd content showed better promoting effect on activity of Pd/HZSM-5 than high Pd content.Pd/HZSM-5 (Si/Al2 = 50 or 165) catalysts without and with additives of oxides of Zr, Ce, Zn, Cu, La, Ba, Fe, Mn, Ca, Mg, Li, Y, Al, Sm, Nd and Tb were prepared by the impregnation method and were investigated for low-temperature CH4 combustion. Addition of Zr and Ce were found to have asignificant promoting effect on the activity of Pd/HZSM-5. The T10%, T50% and T100% of Pd-Zr/HZSM-5 and Pd-Ce/HZSM-5 were found to be ca. 15~29, 6~ 18 and 35~60 ℃ lower than those of Pd/HZSM-5, respectively. Addition of Zn was found no appreciable change to activity of Pd/HZ...
Keywords/Search Tags:Low-temperature Catalytic Combustion of CH4, The Modified Pd/HZSM-5 Catalyst, Thermal and Hydrothermal Stability
PDF Full Text Request
Related items