Study On Cu-Based Catalysts Used For Steam Reforming Of Dimethyl Ether To Produce Hydrogen

The rapid consumption of fossil fuels and as-induced many kinds of environmentpollution have driven extensive exploration of new energy alternatives. Hydrogen is akind of clean and renewable energy, which is more suitable for future energydevelopment. Dimethyl ether steam reforming （DME SR） is an effective way toproduce hydrogen. For the production of hydrogen via DME SR, the development ofhighly efficient catalysts is crucial. Currently, the greatest challenge is to improve thecatalytic performance of the catalysts, including low-temperature activity andselectivity. In this work, a kind of complex catalysts consisting of metal （CuZnAl orCuFe₂O₄） and solid acid （γ-Al₂O₃） is employed for DME SR reaction, andsystematically studied from several aspects, including the effect of calcinationtemperature, contents of active components, reaction conditions and active phasemodifications. Multiple techniques such as BET, XRD, H₂-TPR, N₂O chemisorptionand XAFS （including XANES and EXAFS） were employed for catalystcharacterization. The structures and the properties of the catalysts are well correlated.Firstly, we performed thermodynamics analysis and drew a conclusion that DMESR reaction is not controlled by thermodynamics equilibrium and DME can beconverted totally when reaction temperature is higher than200oC and S/C>1.5.Besides, CH₄and C are thermodynamically favorable products; however, theappearance of them will decrease the hydrogen yield. Therefore, high selectivity isrequired for the catalysts to inhibit the formation of CH₄and C.Coprecipitation method was employed to prepare CuZnAl catalysts viahydrotalcite precursor, the effect of calcination temperature and contents of activecomponents are investigated based upon catalytic test and characterization results.According to XRD and TPR results, the sample calcined at500oC which is moreactive has a proper interaction between Cu and carrier, besides, no CuAl₂O₄wasidentified and the CuO species in it can be easily reduced to active spieces （Cu⁰andCu⁺）. When CuO content reached40wt.%, hydrotalcite precursor was well formed,and the particle size of CuO and ZnO in the calcined catalyst is quite small from theXRD results, as a result, the catalyst shows the highest DME conversion.Metal catalysts are often modified to achieve better catalytic activity andselectivity. In the first step, Al was partially substituted by Zr. The partial substitution of Al by Zr can decrease the Cu-Al ineration and improve the reducibility of Cuspeices. But the presence of excessive amount of Zr results in the aggregation of Cuspecies, decreasing the dispersion of copper species and the catalytic performance ofthe catalysts. The optimal content of Zr is20wt%according to ZrO₂/（Al₂O₃+ZrO₂）ratio. The catalyst CuZnAl_0.8Zr_0.2O with the highest Cu dispersion and smallest Cucrystallite size exhibits the best performance for DME SR reaction. In addition, it isrevealed by XANES that among all catalyst CuZnAl0.8Zr0.2O contains the highestamount of Cu⁺species, which effectively inhibits the formation of CO and increasesthe selectivity of CO₂during dimethyl ether steam reforming. In the second step, Naand K were introduced to CuZnAl catalyst, neutralized a part of acid sites on catalystsurface and improved CO₂selectivity. From the characterization results, theintroduction of K, decreased the Cu-Al interation, made Cu speices more dispersed;however, excessive amount of K would neutralize more acid sites, decrease the activesites of DME hydrolysis, and then decrease DME conversion.Finally, a study on the complex catalyst of CuFe₂O₄and Al₂O₃for DME SR wascarried out. Too long time or too high temperature of reduction will transform Fe₃O₄to metallic Fe and decrease the interation between Cu and Fe₃O₄, leading to increaseof Cu particles size and decrease of DME conversion. Meanwhile, the generation ofmetallic Fe will promote methanol decomposition reaction and increase CO selectivity.When the catalyst was doped with Co, the DME conversion increased due to thepromotion of methanol decomposition reaction. Although DME conversion on thecatalyst doped with Co is higher than that on the undoped one, H₂yield of the twocatalysts does not show much difference. Ni as the dopant has a negative effect onDME conversion and CO₂selectivity. The results of long-term DME SR reaction test（45h） shows that the complex catalyst of CuFe₂O₄and Al₂O₃exhibited high stability.