甲醇水蒸气重整制氢Pd/ZnO催化剂的研究

Pd/ZnO catalysts with Pd loadings between 5.2% and 21.9% for steam reforming of methanol to hydrogen have been prepared by co-precipitation. The effects of Pd loadings and the reduction temperatures on the catalytic performance have been investigated in a stainless steel reactor at 1 bar, 523 K, 17.2 h-1～57.2 h-1 (WHSV) and H2O/CH3OH(mol ratio)=1.0. The 15.9% Pd/ZnO catalyst at a reduction temperature of 523 K～573 K has the best methanol conversion. 41% of methanol conversion, 94% of selectivity for CO2 and 0.65 mol/(gcat·h) of hydrogen yield are obtained at 1 bar, 523 K and 17.2 h-1 over 15.9% Pd/ZnO reduced at 573K. The temperature programmed reduction (TPR) results show that PdO can be reduced to Pd at room temperature, and some of ZnO can be initially reduced at about 440 K. Introduction of Pd facilitates the reduction of ZnO. The X-ray diffraction (XRD) results of various Pd loading catalysts, reduced at 573 K, confirm the existence of hexagonal ZnO and tetragonal PdZn alloy. Moreover, minor cubic Pd2Zn on 21.9% Pd/ZnO is also observed. PdZn alloy may be the active site for the titled reaction, while Pd2Zn on 21.9% Pd/ZnO results in low activity. XRD results of 15.9% Pd/ZnO reduced at different temperatures show a peak ascribed to PdZn alloy appeared progressively, which becomes remarkably intense and sharp with the increasing reduction temperature. A marked PdZn alloy occurs when the reduction temperature rises up to 573 K, which suggests that PdZn alloy forms easily over Pd/ZnO catalysts prepared by co-precipitation. The crystal size of Pd-Zn alloy (111) grows up from 14.4 nm to 27 nm while the reduction temperature rises from 573 K to 773 K. XRD of 15.9% Pd/ZnO, subjected to methanol reforming at 523 K after reduction at room temperature, also shows the peak ascribed to PdZn alloy. Though the hydrogen product can reduce the catalyst, the best reduction atmosphere is under pure hydrogen conditions. The initial stability tests over 15.9% Pd/ZnO and the commercial CuO/ZnO/Al2O3 catalyst are operated at 573 K. The results show that the methanol conversion over 15.9% Pd/ZnO remains a constant value for 8 h without deactivation. On the contrary, the methanol conversion over CuO/ZnO/Al2O3 catalyst gradually decreases with time on stream.