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.
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