COMPARISON OF PARTIAL OXIDATION AND STEAM-CO2 MIXED REFORMING OF CH4 TO SYNGAS ON MGO-SUPPORTED METALS

Partial oxidation (POX) and steam-CO2 mixed reforming of CH4 on MgO-su pported noble metals were investigated at high space velocity (5.5 x 1 0(5) h(-1)). Temperature-programmed reaction (TPR) and isotope transie nt techniques were used to study the mechanism of POX and mixed reform ing. TPR profiles of POX and mixed reforming showed similar ignition r eaction behaviors, which implied that there are similar characteristic s in their mechanisms. Steam reforming and CO2 reforming were found to start at the same time in mixed reforming. TPR and CH4-D-2 exchange e xperiments indicated that CH4 was activated at low temperature on Rh/M gO. POX showed much higher activity than mixed reforming although thei r C, H, and O atomic concentrations were the same at the beginning of each reaction. It is suggested that the lower rate of reaction in mixe d reforming is due to the blocking of active sites for CH4 activation by CO2 and H2O. It seems that the coexistence of CO2 and H2O shows str onger inhibition than that of CO2 alone and H2O alone. Rh/MgO without previous reduction treatment also showed a high reactivity for POX but with a higher ignition temperature than a prereduced catalyst. With r egard to the H-2/CO ratio, mixed reforming showed a changing ratio wit h increasing temperature, which suggested that the rate for CO2 reform ing increases faster than that of steam reforming. An in situ isotope- labelled (CO2)-C-13 transient experiment for mixed reforming indicated that carbon formed from CO2 or CO decomposition was less active than (CHx)(ad) (x = 0, 1, 2, and 3) formed from CH4 decomposition. For POX, a small amount of steam had little effect on CO formation rate for an active catalyst, e.g., Rh/MgO or Ru/MgO, but decreased the rate for l ess active catalyst Pt/MgO. All the results indicated that steam refor ming and CO2 reforming in mixed reforming start simultaneously and hav e the same type of reaction intermediate, adsorbed atomic oxygen. POX proceeds via both one-step and two-step mechanisms, the ratio for each mechanism being dependent on the concentration and kinetics of adsorb ed atomic oxygen and gaseous atomic oxygen. Mechanisms for POX and mix ed reforming are suggested acid the effect of oxygen-metal-bond streng th on activity is discussed. (C) 1996 Academic Press, Inc.