铬助剂对Cu/ZrO_2/CNTs-NH_2催化剂催化CO_2加氢合成甲醇性能的影响

采用并流共沉淀方法制备了一系列不同铬含量的Cu/ZrO2/CNTs-NH2催化剂,在固定床反应器上考察铬对催化剂催化CO2加氢合成甲醇反应性能的影响.当铬含量为1%(w),反应温度为260°C,压力为3.0MPa,原料气组成为V(H2):V(CO2):V(N2)=69:23:8,空速为3600 mL·h-1·g-1时,催化剂的促进效果最显著,甲醇收率达7.78%.氮吸附、粉末X射线衍射(XRD)、氢气程序升温脱附(H2-TPR)、X射线光电子能谱(XPS)、二氧化碳程序升温脱附(CO2-TPD)、差热分析(DTA)以及扫描电子显微镜(SEM)等表征结果表明,随着铬含量的增加,铜颗粒的粒径减小,催化剂的比表面积增大.铬的加入一方面提高了铜的分散性,抑制了ZrO2的相变和活性组分的烧结;另一方面提高了CO2的吸附量并促进CO2由弱吸附向强吸附转化,从而提高甲醇的收率;但是当铬含量大于1%时,催化剂表面Cu、Zr的总含量明显下降,降低了CO2的吸附量并且形成了超强CO2吸附物种,抑制了CO2及其中间产物的转化,从而降低了甲醇收率.

Effect of Chromium Doping on the Catalytic Behavior of Cu/ZrO2/CNTs-NH2 for the Synthesis of Methanol from Carbon Dioxide Hydrogenation

A series of Cu/ZrO₂/CNTs-NH₂ catalysts with various chromium dopings were prepared using a coprecipitation method for the synthesis of methanol by the hydrogenation of CO₂. The impact of the addition of chromium on the catalytic performance of the Cu/ZrO₂/CNTs-NH₂ catalyst was investigated in a fixed-bed plug flow reactor. When the chromium loading was set to 1% of the total amount of Cu²⁺ and Zr⁴⁺, the methanol yield increased to a maximum of 7.78% (reaction conditions: 3.0 MPa, 260 ℃, V(H₂):V(CO₂):V(N₂)=69:23:8 and gaseous hourly space velocity (GHSV)=3600 mL·h^-1·g^-1). The catalysts were characterized by N₂ physisorption, X-ray diffraction (XRD), temperature-programmed desorption of H₂ (H₂-TPD), X-ray photoelectron spectroscopy (XPS), temperature- programmed desorption of CO₂ (CO₂-TPD), differential thermal analysis (DTA), and scanning electron microscopy (SEM). The results of these analyses indicated that the introduction of chromium reduced the size of the Cu nanoparticles, enhanced the dispersion of the Cu species, inhibited the phase transformation and sintering of ZrO₂, increased the specific surface area, enhanced the amount of CO₂ adsorbed, and promoted the conversion of weakly adsorbed CO₂ species to strongly adsorbed CO₂ species. Taken together, these factors lead to a high methanol yield. However, when the chromium loading was greater than 1%, the amount Cu and Zr on the surface, as well as the size of the Cu nanoparticle reduced considerably, which led to a significant reduction in the adsorption of CO₂ species. This effect also facilitated the formation of strongly adsorbed CO₂ species, leading to lower methanol yields.