Comparison of Partial Oxidation and Steam-CO2Mixed Reformingof CH4to Syngas on MgO-Supported Metals

Abstract

Partial oxidation (POX) and steam-CO2mixed reforming of CH4on MgO-supported noble metals were investigated at high spacevelocity (5.5×105h−1). Temperature-programmedreaction (TPR) and isotope transient techniques were used tostudy the mechanism of POX and mixed reforming. TPR profiles ofPOX and mixed reforming showed similar ignition reactionbehaviors, which implied that there are similar characteristicsin their mechanisms. Steam reforming and CO2reforming werefound to start at the same time in mixed reforming. TPR andCH4–D2exchange experiments indicated that CH4wasactivated at low temperature on Rh/MgO. POX showed much higheractivity than mixed reforming although their C, H, and O atomicconcentrations were the same at the beginning of each reaction.It is suggested that the lower rate of reaction in mixedreforming is due to the blocking of active sites for CH4activation by CO2and H2O. It seems that the coexistenceof CO2and H2O shows stronger inhibition than that ofCO2alone and H2O alone.Rh/MgO without previous reductiontreatment also showed a high reactivity for POX but with ahigher ignition temperature than a prereduced catalyst. Withregard to the H2/CO ratio, mixed reforming showed a changingratio with increasing temperature, which suggested that the ratefor CO2reforming increases faster than that of steamreforming. Anin situisotope-labelled13CO2transient experiment for mixed reforming indicated that carbonformed from CO2or CO decomposition was less active than(CHx)ad(x=0, 1, 2, and 3) formed from CH4decomposition.For POX, a small amount of steam had little effect on COformation rate for an active catalyst, e.g., Rh/MgO or Ru/MgO,but decreased the rate for less active catalyst Pt/MgO. All theresults indicated that steam reforming and CO2reforming inmixed reforming start simultaneously and have the same type ofreaction intermediate, adsorbed atomic oxygen. POX proceeds viaboth one-step and two-step mechanisms, the ratio for eachmechanism being dependent on the concentration and kinetics ofadsorbed atomic oxygen and gaseous atomic oxygen. Mechanismsfor POX and mixed reforming are suggested and the effect ofoxygen–metal bond strength on activity is discussed.