CuO/ZrO₂ Water-gas Shift Reaction:Preparation And Structure-performance Relationship

The water-gas shift (WGS) reaction (CO+H2O(?)H2+CO2), which can not only clean up CO but also generate H2, plays a critical role in supplying H2 fuel for proton exchange membrane fuel cell (PEMFC). The traditional WGS catalysts are not suitable for the specific application in PEMFC due to their inevitable drawbacks. Hence, it becomes urgent and of grate importance to explore new and more promising catalysts for WGS reaction.In this dissertation, catalytic performance of the CuO/ZrO2 catalyst was promoted through different approaches. The highly efficient CuO/ZrO2 catalysts, which are suitable for the specific application in hydrogen-rich reformed gases under low stream/gas ratio, were successfully prepared. The structure and physical/chemical properties of CuO/ZrO^ catalysts were characterized by ICP-OES, XRD, N2-physisoption, N2O-titration, Raman, UV-Vis-DRS, SEM, (HR)TEM, H2-TPR, CO-TPR, H2O-TPD, XPS techniques. Herein, the relationship between the structure and catalytic performance of CuO/ZrO; catalysts was fully discussed. The nature of the strong interaction between CuO and ZrO2 was clarified. Moreover, the nature of active site and reaction mechanism on CuO/ZrO2 catalysts for the WGS reaction were also identified. The results are as follows:(1) Several common preparation methods were chosen to prepare CuO/ZrO2 catalysts. It is found that the one prepared by deposition-precipitation method possesses the novel mesocrystal structure, smallest ZrO2 crystallite size, largest pore volume, highest Cu dispersion and largest Cu metal area, exhibits excellent mono-dispersed property, thus showing the strongest interaction between CuO and ZrO? and best catalytic performance.(2) A series of CuO/ZrO2 catalysts with different Cu loading (4.1,6.1 and 8.4 wt%) were prepared by deposition-precipitation method. The catalytic evolution results show that the sample which has a Cu loading of 6.1 wt% showes the highest catalytic activity. The characterization results for parent and (NH4)2CO3-leached catalysts disclose that three types of supported CuO species are present on the as-prepared CuO/ZrO2 catalysts:(a) highly dispersed CuO clusters; (P) strongly bound Cu-[O]-Zr species, possibly associated with the surface oxygen vacancy of ZrO2; (y) crystalline CuO. The reaction rate correlates well with the amount of Cu-[O]-Zr species, moreover, CO-TPR results show that only the reduction of Cu-[O]-Zr species can activate the surface WGS reaction (i.e. the surface hydroxyl groups to react with CO to produce H2 and CO2), revealing the highly dispersed metallic Cu strongly interacted with the surface oxygen vacancy of ZrO2 (i.e. Cu-[]-Zr species) is the catalytically active copper species. The presence of Cu-[O]-Zr species significantly promotes the reactivity of surface hydroxyl groups, as is believed to be responsible for the high catalytic activity of CuO/ZrO2 catalysts for WGS reaction. Based on the role of Cu-[]-Zr species, surface hydroxyl groups and surface oxygen vacancy on ZrO2, a hybrid mechanism, named "surface formate associative mechanism with ZrO2 redox regeneration" was proposed.(3) The influence of calcination temperature of the catalyst (300,400 and 500℃) and ZrO2 (120,250,350 and 450℃) on the structure and catalytic activity of CuO/ZrO2 catalysts was investigated in detail. The characterization results show that increasing catalyst calcination temperature from 300℃ to 400℃ leads to the redispersion of crystalline CuO on ZrO2, which is mainly transformed into catalytically active Cu-[O]-Zr species, thus enhancing the catalytic activity of the catalyst calcined at 400℃. However, with a further increase of calcination temperature to 500℃, partial Cu2+ from Cu-[O]-Zr species incorporated into the ZO2 lattice, thus degrading the catalytic activity of the catalyst calcined at 500℃. Increasing the calcination temperature of ZrO2 brings about two completely opposite effects:on one hand, high temperature calcinations pretreatment eliminates the surface hydroxyl on ZrO2, which helps in the formation of the strong interaction between CuO and ZrO2, thus improving the reducibility of Cu-[O]-Zr species; on the other hand, high temperature calcination pretreatment results in the growth of ZrO2 crystallite, which is disadvantage to the dispersion of CuO and formation of Cu-[O]-Zr species. Thus, the CuO/ZrO2 sample supported on ZrO2 calcined at 250℃ gives the best catalytic activity.(4) The effect of the doping method and doping amount of Al additive on the structure and performance of the CuO/ZrO2 WGS catalysts was studied in detail. The results show that the introduction of Al promoter (2%) both during the hydrothermal synthesis of ZrO2 (denoted as HHP) and the deposition-precipitation of CuO (denoted as DP) can significantly improves the catalytic performance of the CuO/ZrO2 catalysts. The HHP introduction method is more attractive than the DP method, and the Al-doped CuO/ZrO2 sample prepared by the HHP method shows higher catalytic activity than the commercial Cu-Zn-Al catalyst. However, further improving the amount of Al dopant to 5% leads to a decrease in the improvement of catalytic activity, as compared to the pure CuO/ZrO2 sample. Based on the above results, the effect of the Y additive on the structure and performance of the CuO/ZrO2 WGS catalysts was also studied in detail. Results indicate that CuO/ZrO2 catalyst doped with 2% of Y (i.e., CZY2 sample) shows the highest catalytic activity for the WGS reaction. Furthermore, the promoting mechanism of Al and Y additives in WGS reaction on CuO/ZrO2 catalyst was fully discussed on the basis of the characterization results.It can be established that the nature of the interaction between CuO and ZrO2 is the presence of Cu-[O]-Zr species, which is formed by incorporating an 0 atom from CuO into an oxygen vacancy on the ZrO2 surface. Thus, the amount of Cu-[O]-Zr species and the intensity of Cu-O-Zr bond determine the intensity of the interaction between CuO and ZrO2. Most important, the Cu-[O]-Zr species is the catalytically active copper species and the reducibility of surface hydroxyl (i.e. the ability of surface hydroxyl that participates in the WGS reaction) is closely related to that of Cu-[O]-Zr species. Therefore, the amount and the reducibility of Cu-[O]-Zr species are the two most important factors that determine the catalytic performance of CuO/ZrO2 catalysts.In addition, this dissertation presents a facile method for the synthesis of ZrO2 mesocrystal by a hydrothermal homogeneous precipitation procedure. To the best of my knowledge, this is the first report about the synthesis and the catalysis application of ZrO2 mesocrystal.The whole dissertation focuses on the modification of CuO/ZrO2 catalyst aiming to improve its activity and stability in the WGS reaction. A lot of meaningful results have been achieved. Particularly, the new CuO/ZrO2-Al2O3 and CuO/ZrO2-Y2O3 catalysts should be highlighted. Because they show the comparable or higher catalytic activity to or than the commercial Cu-Zn-Al catalyst, even they have a much lower Cu loading (8.2 wt%) than the latter (-25 wt%). Therefore, these catalysts show a good potential in the fuel cell application.