Carbon formation during carbon-dioxide reforming of methane.

摘要

Present energy and environment situations have renewed interest in using methane and carbon dioxide, the cheapest sources of carbon and two major greenhouse gases. Fortunately, methane can be reformed by carbon dioxide to produce syngas, a mixture of carbon monoxide and hydrogen, and a feedstock for many chemical processes. This process is of special interested when carbon monoxide or syngas with a high carbon monoxide to hydrogen ratio is desired, or carbon dioxide coexists in natural gas.; Previous work finished in our group found that platinum-rhenium, bimetallic catalysts have excellent stability and activity for this process at temperature above 800°C, but dramatically lose the superiority at low temperatures. The goals of this work are to understand the catalytic performance of bimetallic catalysts and to screen catalysts that are promising at low temperature reforming.; Carbon formation in reforming on the catalyst is minimized when Re is added to Pt to form bimetallic Pt-Re catalysts due to an ensemble effect. Rhenium divides Pt into smaller ensembles, which are big enough to catalyze reforming and small enough to prohibit carbon formation, which requires ensembles containing more active sites. There is no carbon formation in reforming for Re-rich bimetallic catalysts. The deactivation and low activity of these catalysts at low temperature are not due to carbon deposition, but to the interaction between carbon dioxide and Re. Carbon dioxide poisons the catalysts at low temperatures.; Yttria-stabilized zirconia (YSZ)-supported catalysts are better than alumina-supported catalysts for the process because of lower acidity (therefore, lower carbon deposition) and possibly oxygen mobility. Most YSZ-supported catalysts show very high activity and stability. The sequences of activity are Rh/Al2O3 > Pt/Al2O3 > Pd/Al 2O3 > Ir/Al2O3 > Ru/Al2O 3 Re/Al2O3, and Pt/YSZ > Rh/YSZ > Ru/YSZ > Ir/YSZ > Pd/YSZ Re/YSZ. The deactivation sequences are Pt/Al2O 3 > Pd/Al2O3 > Ir/Al2O3 > Rh/Al2O3, Ru/Al2O3, and Re/Al 2O3, and Pd/YSZ Pt/YSZ > Ir/YSZ, Rh/YSZ, Ru/YSZ, and Re/YSZ. Combining stability, activity, and current price, Ru/YSZ is the most promising catalyst. Other candidates are Rh/YSZ, Ir/YSZ, Pt/YSZ, Rh/Al2O 3 and Ru/Al2O3. These catalysts work well at all temperature ranges.