Preparation And Catalytic Properties Of New Catalysts For Hydrodesulfurization Of Carbonyl Sulfide

As one of the main source of energy in China, it is necessary to develop "clean coal" techniques for its utilization so as to adapt to the sustainable economic development and environment protection of our country. While some kinds of sulfur-containing compounds are inevitably existed in the transforming process of coal, therefore, the removal of organic sulfur (carbonyl sulfur, COS) in coal-based gases becomes a key technology. However, there are lots of defects for the traditional catalysts for the hydrodesulfurization (HDS) of COS, generally the oxides or sulfides of Ni, Co and Mo supported on γ-Al2O3, such as a high reaction temperature (>280℃), the presulfiding treatment and the strong interaction between γ-Al2O3support and the active constituent, which severely limit their performance and application. Therefore, the developing of new support materials and/or the modifying of active components for HDS catalysts are very important.Graphene, as a new type of carbon material, has a series of excellent properties, for example two-dimensional plane structure, a large specific surface area, good biocompatibility as well as high adsorptive capacity property. So that graphene has great promises for potential applications in technological fields such as sensors, hydrogen storage materials, electrode materials, catalysis and so on. As for the current situation of HDS catalysts, it is very important to develop new methods for the preparation of noble metal or transition metal supported on graphene sheets, and the discussions about the HDS activities of these catalysts are also necessary not only for basic research but also for practical utilization. Thus, the target of this paper has been focused on the research of graphene-based catalysts, including the preparation of noble metal and transition metal supported on graphene sheets and the evaluation of their HDS activities for COS conversion. A series of novel results have been obtained and are described as follows.Firstly, a facile method by using dielectric barrier discharge (DBD) plasma technology has been developed to synthesize graphene sheets by the reduction of graphite oxides (GO) precursor, with the advantage of high efficiency, no pollution and low energy consumption. Characterizations using XRD, FT-IR, Raman spectra, N2adsorption and HRTEM techniques reveal that the GO has been exploited successfully and single or little layers graphene sheets (GS) have been obtained.Then, palladium (Pd) hydrochloric and GO were chosen as the precursor, a new route which is the impregnation with the assistance of ultrasonic combined with DBD plasma to prepare metallic Pd supported on GS (PL-Pd/GS) catalyst has been established. The optimal technological condition for the preparation of PL-Pd/GS with a high dispersity has been determined by investigating the temperature and pH values in impregnation process, and other plasma discharge parameters. The as-prepared PL-Pd/GS catalyst shows high catalytic efficiency and good stability for the COS hydrogenation in the low temperature range (200-250℃), compared to Pd/GS prepared by traditional hydrogen reduction at high temperature or by ethylene glycol reduction in liquid phase, as well as the classical Pd/activated carbon catalyst. The characterizations of all these catalysts reveal that the improved performance of PL-Pd/GS for COS conversion should be attributed to very small size (～2nm) and uniform dispersion of metallic Pd nanoparticles on GS support, which is derived from the simultaneous reduction of Pd (Ⅱ) and GO by low temperature treatment of DBD plasma.While considering the little amount reserve of noble metals and their high price, and the advantage of transition metal (Ni, Co) catalysts for hydrogenation reaction, DBD plasma was exploited to synthesize single nickel (or cobalt)/GS composite catalysts and Ni-Co/GS bimetal composite catalysts. Some characterizations have been also applied to confirm that well-dispersed metal nanoparticles with the same particle size supported on graphene sheets. The examinations of these catalysts used for COS conversion show that under the same conditions, the HDS activity of single Ni/GS is better than Co/GS catalyst, and the bimetal Ni-Co/GS exhibits the highest activity, especially under the low reaction temperature (140-240℃). The reason should be that the synergistic effect between nickel and cobalt can help the mutually promotion. Furthermore, the method by the assistance of low-temperature plasma discharge reduction has been confirmed again an effective technique to prepare metal catalysts with high dispersion supported on graphene sheets, and it is also demonstrated that the graphene-based new catalysts with supported noble metal and transition metal have great potential application in the HDS of COS.In addition, the reports about the facet catalytic activity of inorganic nanocrystals are increasing in recent years. Based on the previous work of ours, Co3O4with different morphologies (nanorod, nanopolyhedron) modified by the second constituent (NiO) was prepared through ethylene glycol route in liquid phase, and the HDS activities for COS over these unsupported catalysts were investigated. Compared to other NiO-Co3O4complexes prepared by impregnation or deposition method, the sulfided NiO-Co3O4catalysts obtained by one step of co-precipitation shows the excellent HDS activity and is also better than pure Co3O4nanorods. Moreover, under the same amount of NiO (5%wt), the HDS activity of NiO-Co3O4nanorods is superior to NiO-Co3O4nanopolyhedron especially in the low temperature range of140-200℃, which is mainly caused by the different exposure facets of Co3O4nanocrystals associated with the synergistic effect between NiO and Co3O4revealed by the BET and TPR analysis.