The Graphene Supported Copper-based Catalysts For The Reaction Of CO₂ Hydrogenation To Methanol

The methanol synthesis from CO2 hydrogenation is the most promising research direction in the utilization of CO2. Wherein the problem that copper-based catalysts used in the reaction have low activity, low methanol selectivity and instability influences industrial application. Reduce-graphene oxide (rGO) with large specific surface area, high hydrogen and carbon dioxide adsorption capacity was used as a new support of the catalyst for hydrogenation of carbon dioxide to methanol synthesis to improve the activity, methanol selectivity and stability. This research is significant for the improvement of catalytic performance and future industrial application.Firstly, thermodynamics model of methanol synthesis from the hydrogenation of carbon dioxide was established to simulate the influence of different temperature, pressure and H2/CO2 ratio on the carbon dioxide conversion and methanol selectivity by Matlab. The calculation results showed that it is favor of the production of methanol under high pressure and low temperature. Compared with experimental data obtained by other researchers, there has 10% space for the improvement of catbon dioxide conversion under the same reaction conditions. This demonstrated that the current reaction limit is not thermodynamics but kinetics. Therefore, the development of catalyst is crucial in the research.CuO-ZnO-ZrO2-Al2O3/rGO (CZZA/rGO) catalyst and CuO-ZnO-ZrO2-Al2O3 (CZZA) catalyst were prepared by coprecipitation method. Then the catalysts were characterized using BET, XRD, SEM, H2-TPR, H2-TPD and CO2-TPD techniques, and were evaluated by a fixed-bed reactor for methanol synthesis from the hydrogenation of carbon dioxide. Based on the research results of the effect on the catalytic performance by the reaction temperature, reaction pressure and reaction space velocity, the optimum reaction conditions for this experiment were determined as 513 K,2 MPa and 6075 h-1. Meanwhile, the structure, the reductibility, adsorption capacity and catalytic performance of CZZA/rGO and CZZA catalyst were tested and compared under the optimum reaction conditions. The characterization results showed that the CZZA/rGO catalyst has a large specific surface area of 125.56 m2·g-1, strong reducibility and good H2 and CO2 adsorption capacity, which are advantageous to reaction. The CO2 conversion and methanol yield over the CZZA/rGO catalyst was increased by 11.7% and 18.1% respectively compared with CZZA catalyst. The CO2 conversion between the experimental data and thermodynamic calculation is 8%. Moreover, CH3OH TOF value of CZZA/rGO catalyst is 0.0687 s-1, which is much higher than that of CZZA catalyst. The results mentioned above demonstrated CZZA/rGO catalyst has high activity.By urea hydrolysis method, new Cu-ZnO/rGO (CZ/rGO) and CuO-ZnO/Al2O3 (CZA) catalyst were prepared. The characteristic methods of BET. XRD. SEM, H2-TPR, H2-TPD and CO2-TPD have been used to test the micro-construction, the reducibility and adsorption properties of the catalyst. The catalyst has also been evaluated by a fixed bed reactor under the optimum reaction conditions. The effect of catalyst preparation hydrolysis temperature, content of active component and different carriers on the catalytic performance of the catalyst were studied. The researches disclosed the results that CZ/rGO catalysts have good reducibility and high CO2 and H2 adsorption capacity compared with CZ/A catalyst. As the active component content decreases, H2 and CO2 adsorption capacity increases, the active component of catalysts can maintain in Cu0 and Cu+ valence state. Evaluation results illustrated 25% CZ/75% rGO catalyst had the highest carbon dioxide conversion of 8.4% and methanol yield of 7.28%. The CH3OH TOF value of 25% CZ/75% rGO catalyst up to 0.0248 s-1,while the CH3OH TOF value of CZA catalyst is only 0.0029 s-1. The research above proved that rGO used as the support of catalysts lead to a high catalytic activity.Finally, the relationship between adsorption capacity, reducibility and catalytic performance was studied as well. The researches disclose CO2 and H2 adsorption capacity at lower temperature (<650 K) influence carbon dioxide conversion, while total H2 adsorption capacity influences methanol selectivity. The adsorption capacity of 25% CZ/75% rGO catalyst or 50% CZ/50% rGO catalyst is ten times of CZA catalyst. High H2 adsorption capacity supply a reduction atmosphere for catalyst, which keep the active center of catalysts at Cu0 or Cu+ valence state, further increase the methanol selectivity up to 90%. As a conclusion, the adsorption properties of a CZ/rGO catalyst with large surface area effect on the reductibility and catalytic performance of the catalyst.