Pressurized Reduction of Cu-ZnO Water-Gas Shift Catalysts in Presence of Water
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CuO/ZnO/Al2O3 is a commonly used catalyst for the commercialized water gas shift reaction. Prior to industrial use, CuO has to be reduced to the active metallic copper phase. Yara has experienced major issues as a consequence of this reduction, and the issues are believed to originate from the presence of water vapor in the pressurized reactor during reduction. However, this needs further investigation. A CuO/ZnO/Al2O3 (weight% 60/30/10) catalyst was synthesized by the co-precipitation method, and was promoted with cesium by incipient wetness impregnation. The chemical composition was analyzed by x-ray fluorescence (XRF), and the catalyst was reduced with a temperature programmed reduction apparatus (TPR). BET, x-ray diffraction (XRD), thermogravimetric analysis (TGA) and TPR were used to analyze the effects of high pressure (20 bara), water vapor (12 vol%) and cesium promotion on the reduction of the catalyst. TGA and XRD demonstrated that the catalyst precursor mainly consisted of zincian malachite and hydrotalcite before calcination, while XRF confirmed the chemical composition of the catalyst after synthesis. TPR showed that the heating rate greatly affected the reduction temperature of pressurized samples, and displayed a superficial reoxidation of reduced catalysts when exposed to air. Non-promoted catalysts reduced under high pressure contained a high amount of Cu2O, while the promoted catalyst mainly contained pure metallic copper. The cause is debated, but is believed to stem from either pore condensation or reoxidation. Cesium worked as a mineralizer for samples treated with water vapor, resulting in an increased crystallinity. Water and cesium may have formed a complex initiating the mineralizing effect, in addition to the waters direct effect of increasing surface atom mobility. Cesium also prevented Cu2O formation, which was an effect of either mineralization or increased reduction rate. The combination of high pressure and water vapor resulted in the contradictory increase in both crystallite size and specific surface area, and thus appeared to restructure the pore network. This occurred for unknown reasons, but could be an effect of amorphous alumina and metal-support interactions.