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

Infrared measurements reveal that cyclic shifts between lean and rich (H2 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 H2/CO to a pre-oxidized catalyst or O2 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 H2 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% O2/N2 mixture at 350 °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 H2) 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/H2 mixtures reveal H2 enhanced CO oxidation is likely due to a kinetic interaction as seen in previous steady-state studies.

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► Infrared imaging detects complex spatio-temporal patterns during the H2 and CO reduction of Pt/CeO2/alumina monolith catalyst.
► Reliance on the effluent concentration of reacting species does not capture the rich dynamics.
► Transient oxidation of CO is enhanced by H2.