甲醇水蒸气重整制氢Pd/ZnO催化剂还原过程中Pd与ZnO的相互作用

Interaction between Pd and ZnO during Reduction of Pd/ZnO Catalyst for Steam Reforming of Methanol to Hydrogen

采用程序升温还原、程序升温脱附、程序升温电导及X射线衍射研究了并流共沉淀法制备的15.9%Pd/ZnO催化剂还原过程中结构和物种的变化及电荷的传递,考察了还原温度对该催化剂催化甲醇水蒸气重整制氢的影响. 结果表明,还原过程中Pd与ZnO间存在明显的相互作用,导致氢溢流,溢流氢促进了ZnO还原并使ZnO与Pd形成活性中心PdZn合金. 催化剂的还原历程为 PdO/ZnO → Pd/ZnO → PdZnO1-x/ZnO →无定形PdZn合金/ZnO →晶型PdZn合金/ZnO. 并流共沉淀法制备的Pd/ZnO催化剂Pd分散度高, PdZn合金形成温度低. 在523～573 K还原后,催化剂的PdZn合金粒径为5～14 nm, 此时催化剂对甲醇的转化率及二氧化碳选择性均达到最大值.

The changes in catalyst structure and species and charge transfer during the reduction of a co-precipitated Pd/ZnO catalyst with 15.9% Pd were investigated by temperature-programmed reduction, temperature- programmed desorption, temperature-programmed electronic conductivity, and X-ray diffraction. The effect of reduction temperature of the catalyst on the catalytic performance in steam reforming of methanol was also tested. The results showed that metallic Pd highly dispersed on ZnO over Pd/ZnO. The strong interaction between Pd and ZnO during the catalyst reduction with hydrogen led to hydrogen spillover from Pd to ZnO, which caused the reduction of ZnO near the metallic Pd and the formation of PdZn alloy. The reduction process proceeded as follows： PdO/ZnO → Pd/ZnO → PdZnO1-x/ZnO → amorphous PdZn alloy/ZnO → crystalline PdZn alloy/ ZnO. Metallic Pd was present in a finely dispersed and amorphous-like state in Pd/ZnO prepared by co-precipitation, and because of that PdZn alloy could be formed at lower temperature than reported in the literature. When catalysts were reduced in the temperature range from 523 K to 573 K, PdZn alloy with size of 5- 14 nm was obtained. At this moment, methanol conversion and CO2 selectivity reached maxima.