Humidity And Plasma Treatment Effects On Reaction Performance Of Gold-based Catalysts And Its Mechanism Investigation

Gold nanoparticles have promising applications in environmental protection, fuel cells, petrochemical and other fields. Catalytic oxidation removel of CO in air over gold nanoparticles at room temperature is of importance in environmental application. In this reaction, humidity is a key factor to affect the catalytic performance of gold nanoparticles. Therefore, in this thesis, humidity effect on the performance of gold nanoparticles in CO oxidation was first carried out, and its mechanism was proposed. Then O2 plasma treatment effects on the reaction performance of gold nanoparticles and its mechanism were investigated. In addition, selective hydrogenation of acetylene in excess ethylene is a very important reaction in petrochemical field. At last, H2 plasma treatment effects on performance of gold-based catalysts in selective hydrogenation of acetylene were investigated. The main results are as follows:1. Au/CeO2 and Au/TiO2 were prepared by deposition-precipitation with urea (DPU), and the humidity effects on induction period, initial activity and deactivation rate of Au/CeO2 and Au/TiO2 in CO oxidation were investigated. The catalysts before and after CO oxidation were characterized by XPS, TEM/HRTEM, FTIR and H2-TPR. Induction and deactivation periods were observed over Au/CeO2. Compared with dry stream, the induction period was significantly reduced by 0.58% of H2O addition, while it was slightly shortened by further increasing water content. The maximum initial activity of Au/CeO2 was achieved at 0.74% H2O. In-situ DRIFTS measurements suggested that humidity contributed to the formation and consumption of reaction intermediates [COOH]S and hence enhanced the initial activity. Deactivation of Au/CeO2 was observed under both dry and wet streams. In dry stream, only slow deactivation was observed. However, in wet stream, the catalyst showed a rapid deactivation after the induction period and then underwent a slow deactivation stage. Induction period of Au/CeO2 arised from unreduced cationic Au, and a mechanism was proposed to illustrate the existence of induction period under both dry and wet streams. For Au/TiO2, no significant induction period was observed whether in dry or wet stream. For all tested humidities, the initial activity of Au/TiO2 decreased linearly with increasing water content.2. The treatment of Au/TiO2 catalyst by atmospheric-pressure dielectric barrier discharge (DBD) O2 plasma for CO oxidation was conducted. To elucidate the plasma treatment effects, TEM/HRTEM, XPS, FTIR, UV-vis spectroscopy and sub-ambient CO pulse chemisorption techniques were used for catalyst characterization. The activity of the Au/TiO2 sample, which was prepared by DPU, was not improved after O2 plasma treatment, due to the blockage of active sites by the [NOy]s species formed in the O2 plasma treatment, while further thermal treatment in inert gas resulted in better activity than the caclined one. However, for Au/TiO2 prepared by deposition-precipitation with Na2CO3, one-step treatment of O2 plasma (without thermal treatment) showed higher activity than the caclined one, which was attributed to the fact that smaller Au particle size, higher Au dispersion and more low coordinated Au0 and [OH]S species could be achieved by O2 plasma treatment.3. Au/TiO2 catalyst, which was prepared by deposition-precipitation with NH3, was further modified by small amount of Pd (Au/Pd atomic ratio= 14) by impregnation method to obtain Pd/Au/TiO2 for the sective hydrogenation of acetylene. Low-pressure radio frequency H2 plasma was applied to treat the Pd/Au/TiO2. XPS, in-situ FTIR spectra of CO adsorption, H2 pulse chemisorption and TGA/DTG were used for catalyst characterization. Compared with the conventional thermal reduction, the acetylene conversion over plasma-treated catalyst was improved significantly while the ethylene selectivity showed the opposite trend. With additional thermal reduction on the plasma-treated catalyst, the acetylene conversion decreased and the ethylene selectivity increased with increasing reduction temperature. The high acetylene conversion of the plasma-treated catalyst was ascribed to its large amount of surface Pd sites for acetylene adsorption, while its poor ethylene selectivity was due to the formation of contiguous Pd ensembles, which caused the over hydrogenation of acetylene.