Preparation And Catalytic Performance Of Supported Pd-Based Catalysts For Low-temperature CO Oxidation

Low-temperature CO oxidation is an important research subject because of its wide applications in the fields of industry, military, environment protection and citizen's daily life. In addition, in academic fields, CO oxidation reaction has been regarded as a "model reaction" to study the relationship between catalysts structure and their performances and surface catalytic reaction mechanisms.Among catalysts for low-temperature CO oxidation, most high active catalysts show low stabilities, and catalysts with high stabilities usually show low activities. Therefore, it is a challenge to prepare a catalyst with high activity and high stability at low temperature. In order to solve this problem, supported Pd-Fe-Ox and Pd-Cu-Clx catalysts are respectively prepared by co-precipitation-deposition method and coordination-impregnation method in this dissertation. (ⅰ) The activities and stabilities of catalysts prepared are investigated under different reaction conditions, and their structures, physical and chemical properties are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), hydrogen temperature-programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS). (ⅱ) By means of kinetic study and in situ diffuse reflectance infrared fourier transform spectroscopy (DRIFTS), the adsorption of the reaction gases (CO, O2 and H2O), the mechanism of CO catalytic oxidation reaction over Pd-Cu-Clx/Al2O3 and the deactivation mechanism under different reaction conditions are systematically studied. (ⅲ) Based on the above results, a novel preparation method—two-step coordination impregnation method was developed. Using this method, a high active and high stable Pd-Cu-Clx/Al2O3 catalyst has been obtained, which is appropriate for the purification requirement of tunnel exhaust gases. The main results are as follows.1. Preparation of supported Pd-Fe-Ox catalysts and its catalytic performanceSBA-15, CeO2 nano-particle and CeO2 nano-rod supported Pd-Fe-Ox catalysts are prepared by deposition-precipitation method. When using CeO2 nano-rod as the support, the Pd-Fe-Ox catalyst shows the highest activity and its complete CO conversion temperature is 10℃, and 100% CO conversion can be maintained for 10 h at 50℃, but after reaction for 10 h at 25℃the CO conversion decreases from 100% to～80%. The results show that, the particle size of CeO2 nano-rod is smaller and high-index crystal planes dominate its surface, which is benefit for the high dispersion of Pd and Fe active species and the formation of oxygen vacancies on its surface.2. A synthesis of high-efficiency Pd-Cu-Clx/Al2O3 catalyst and its catalytic performancePd-Cu-Clx/Al2O3 catalysts (PC) are prepared by a NH3 coordination-impregnation (CI) method and wet impregnation (WI) method, respectively. Compared with that of PC-WI catalysts, the activities of PC-CI catalysts are much higher. For the oxidation of 1500 ppm CO in～0.1% moisture, over the PC-CI catalyst prepared using water as a dilution, its T100 is 0℃. When using isopropanol as a dilution to prepare the PC-CI catalyst, its T100 decreases to-10℃; for the oxidation of 400 ppm CO,T100 further decreases to -30℃. At 0℃, over PC-CI catalyst the specific reaction rate is 0.18 molCO/gPd/h. The studies of the physical and chemical properties show that, the high activities of PC-CI catalysts are attributed to three reasons. (ⅰ) The active Cu2Cl(OH)3 phase highly disperses on the surface of PC-CI catalysts, resulting in the acceleration of the rate of Cu+ re-oxidation and the strong interaction between Cu and Pd species. (ⅱ) On the surface of PC-CI, the Cu phase is more easily reduced, indicating that Pd0 is more readily re-oxidized to high-valent Pd species by Cu2+. (ⅲ) There are much more amounts of active Pd and Cu species on the surface of PC-CI catalysts.3. The catalytic reaction mechanism of low temperature CO oxidation over Pd-Cu-Clx/Al2O3 catalysts prepared by NH3 coordination-impregnation methodThe catalytic reaction mechanism of low temperature CO oxidation over Pd-Cu-Clx/Al2O3 catalysts was investigated by the kinetic study and in-situ DRIFTS technique. The results show that, highly dispersed Cu2Cl(OH)3 and Pd+ species are the active sites of PC-CI catalysts, in which Pd+ species is the active sites for CO oxidation and more active than the Pd2+ active sites of PC-WI catalyts, and the rate-determining step is the re-oxidation of Pd0 to Pd+ by Cu2+ on the PC-CI catalyst instead of the re-oxidation of Cu+ to Cu2+ by O2 on the PC-WI catalyst. The presence of H2O can improve the re-oxidation rate of Cu+ to Cu2+, and H2O can also provide active oxygen species for the oxidation of CO to CO2.4. The stability and deactivation mechanism of low temperature CO oxidation over Pd-Cu-Clx/Al2O3 catalysts prepared by NH3 coordination-impregnation methodThe reaction stabilities of Pd-Cu-Clx/Al2O3 for low temperature CO oxidation were studied. Under～0.1% moisture, CO conversion over PC-CI catalyst can maintain 100% for 30 h at 0℃, but it deactivated reversibly at 25℃in the～0.6% moisture and irreversibly deactivated at 25℃in the～3.1% moisture or at 0℃in～0.6% moisture. The reversible deactivation is resulted from physical capillary condensation in the small pores of catalysts. The irreversible deactivation is due to a breakage of close-knit structure of Pd-Cu-Cl to lead an aggregation or transformation of the active copper species, and the formation of carbonate species on the catalyst surface, which results in that the inactive sites of Pd0 species over the surface of Pd-Cu-Clx/Al2O3 catalyst is hard to be re-oxidized back to active sites of Pd+ by copper species in the high moisture reaction condition.5. High active and stable Pd-Cu-CIx/Al2O3 catalysts prepared by two-step coordination-impregnation method and its catalytic preformanceA novel preparation method, two-step coordination-impregnation (TCI) method has been developed to prepare the Pd-Cu-Clx/Al2O3 (PC-TCI) catalysts. The results show that, the activity and stability of PC-TCI are improved obviously, such as, the specifc reaction rate of CO oxidation over PC-TCI catalyst reaches 1.24 molCO gPd-1 h-1 at 30℃, which is nearly treble over the PC-CI catalyst. In the presence of 0.6% moisture,100% CO conversion over PC-TCI can be maintained for more than 100 h at 25℃, and its activity is hardly changed during 10 h at 0℃in～0.6% moisture and at 25℃in～3.1% moisture.The studies show that the catalytic reaction mechanism of CO oxidation over PC-TCI is the same as that over PC-CI catalysts. Compared with the PC-CI catalyst, there are much more amount of active Pd+ and Cu2+ species on the surface of PC-TCI, which may be due to the stronger charge-transfer interaction between them, resulting in higher activities and stabilities of PC-TCI for low temperature CO oxidation in the presence of moisture.Using the monolithic Pd-Cu-Clx catalysts prepared by TCI method,50 ppm CO can be completely oxidized to CO2 at room temperature (0～25℃) for stimulated tunnel atmosphere and its activity is hardly varied after reaction for 100 h. The preparation process of the two-step coordination-impregnation method is simple without liquid waste. Furthermore before Pd-Cu-Clx catalyst will be used, no pretreatment is required. Therefore, TCI method is fit for the preparation of monolithic catalysts.