关键词: 分子筛, 介孔, Pd, 甲烷, 催化燃烧

The environmental problems caused by the release of unburned methane from stationary and mobile combustion processes is becoming more serious in recent years. The catalytic combustion of methane has been regarded to be superior to the conventional combustion, and consequently efficient catalysts play important roles during the process. In this study, mesoporous Beta zeolites have been prepared in the presence of a kind of polymer polydiallyldimethylammonium chloride (PDADMAC) as the mesoscale template and been used for the catalytic combustion of methane after supporting Pd. The samples have been intensively characterized by a series of techniques including X-ray diffraction (XRD), N₂ adsorption, scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), temperature program reduction (H₂-TPR) etc. It was found that mesoporous volume of the samples could be controlled by altering the amount of mesoporous template (PDADMAC) and Pd nanoparticles supported on mesoporous Beta zeolites were smaller and more reducible than those supported on the conventional Beta zeolites. Furthermore, Pd(II) species were assigned to be the crucial active sites based on the results of XPS together with H₂-TPR, and the ratio of the Pd(II) determined the catalytic performance. The relatively strong interaction between Pd and the hydroxyl group (caused by the formation of mesopores) of mesoporous Beta zeolites benefit the high ratio of Pd(II) species. As a result, mesoporous Beta zeolites supported Pd showed better catalytic performance in the catalytic combustion of methane than the conventional Beta zeolites supported Pd. As a typical example, Pd/meso-Beta-H2 sample gave T₉₀ (90% conversion temperature) at only 342 ℃, which was much lower than that of Pd/Beta without mesopores (384 ℃) at the same condition. Additionally, catalytic life test exhibited that Pd/meso-Beta-H2 sample was stable and the conversion remained even after 50 h at high temperatures. Kinetics analysis revealed that the apparent activation energies for mesoporous Beta zeolites supported Pd (86~118 kJ/mol) were much lower than that of the conventional Beta zeolites supported Pd (140 kJ/mol). The excellent catalytic performance of mesoporous Beta zeolites supported Pd would be potentially important for the catalytic combustion of methane in the future.

Key words: zeolite, mesopores, Pd, methane, catalytic combustion