Durability of WO 3/ZrO 2–CuO/CeO 2 catalysts for steam reforming of dimethyl ether

Abstract

Development of catalysts for steam reforming of dimethyl ether (DME) producing hydrogen was carried out with the aim of improving catalyst durability. The catalyst consisted of a mixture of solid acid catalysts for hydration of DME to methanol and 40 wt.% CuO/CeO 2 catalyst for steam reforming of methanol. Among various acid catalysts examined, 10 wt.% WO 3/ZrO 2 had the highest performance. WO 3/ZrO 2 showed a unique feature that the activity and durability of the combined catalysts (WO 3/ZrO 2 + 40 wt.% CuO/CeO 2) significantly changed depending upon the amounts of WO 3 loaded on ZrO 2 and the calcination temperatures of WO 3/ZrO 2. WO 3/ZrO 2 with low WO 3 loading (5 wt.% WO 3) combined with 40 wt.% CuO/CeO 2 had only low activity and those with high loadings (more than 15 wt.% WO 3) and calcined at 700 °C had high activity but they were deactivated in a few hours. Calcination at 800 °C prevented deactivation of the catalyst even with high W-loadings. Since hydration of DME proceeded even after deactivation, WO 3/ZrO 2 was not deactivated but deactivation occurred on the CuO/CeO 2 side. It was deduced that WO 3/ZrO 2 caused the deactivation of CuO/CeO 2: TPO measurements of the used catalysts showed that carbonaceous precursors formed on WO 3/ZrO 2 were transferred to CuO/CeO 2 and deposited as coke. Characterization of WO 3/ZrO 2 by BET, XRD and XPS measurements suggested that WO 3 formed a monolayer on ZrO 2 up to WO 3 loading of 12–13 wt.%. The excess WO 3 over monolayer loading formed carbonaceous precursors during the reaction and caused the deactivation of CuO/CeO 2. The excess surface WO 3 was not bound directly to ZrO 2 and was fragile against heat. Therefore, it changed into bulk WO 3 during high temperature calcination and lost its function to form carbonaceous precursors. Durability of the optimized catalyst consisting of 80 wt.% CuO/CeO 2 and 10 wt.% WO 3/ZrO 2 calcined at 800 °C was examined during 100 h reaction at 250 °C. This catalyst maintained high activity and no fatal deactivation occured. The used catalyst could be regenerated by treatment with oxygen at 300 °C.