Spatio-temporal features of periodic oxidation of H2 and CO on Pt/CeO2/Al2O3

Infrared measurements reveal that cyclic shifts between lean and rich (H-2 and/or CO) feeds to a Pt/CeO2/Al2O3 monolith catalyst generate complex, spatio-temporal temperature features. A sharp temperature rise occurs in the upstream of the monolith shortly after the cyclic introduction of either H-2/CO to a pre-oxidized catalyst or O-2 to a pre-reduced catalyst. This initial upstream temperature rise following the reduction of oxygen stored on a pre-oxidized catalyst is higher than following the oxidation of either H-2 or CO (or their mixture) stored on the monolith. The upstream hot zone temperature decreases with time without forming a downstream moving temperature front. The intricate transient temperature gradients are caused by a competition between the chemical and transport rate process. However, the effluent concentrations do not reflect these complex interactions. Only about 20% of the total oxygen trapped during the pre-oxidation with a 5% O-2/N-2 mixture at 350 degrees C is strongly bound or chemisorbed. Most of the oxygen that reacts at high temperature is loosely-bound. The introduction of a nitrogen sweep flow between the lean and rich feeds removes a significant amount of the loosely held oxygen, leading to a much more uniform reduction. Inadequate resolution of the spatio-temporal phenomena may lead to a misinterpretation of the apparent kinetics. The spatial features of the thermal fronts of the two reductants (CO or H-2) are similar. The amplitude of the hot spot of the two reactants differ due to differences in the temperature dependencies of their oxidation rates. The transient oxidation of CO/H-2 mixtures reveal H-2 enhanced CO oxidation is likely due to a kinetic interaction as seen in previous steady-state studies. (C) 2011 Elsevier B.V. All rights reserved.