Anomalous steady-state and spatio-temporal features of methane oxidation on Pt/Pd/Al2O3 monolith spanning lean and rich conditions

The steady-state and transient features of methane oxidation on a Pt/Pd/Al2O3 washcoated monolith are reported for a wide range of temperatures and feed gas compositions. Above 400 degrees C the methane conversion dependence on the O-2 feed concentration exhibits a multiplicity of states. With increasing temperature the multiplicity, in the form of a clockwise hysteresis in the rich regime, expands over a broader O-2 concentration range. At the highest temperature considered (538 degrees C) extinction occurs near the rich-lean transition (O-2/CH4 = 1.9), while ignition occurs at O-2/CH4 similar to 0.6. A low conversion state exists for all O-2 concentrations exceeding the ignition concentration at this temperature. In the rich regime, multiple high conversion states are encountered, all of which produce a mixture of CO, CO2, H-2, and H2O. Only the lowest conversion branch of this group is a steady-state. Another high conversion branch is encountered for O-2 concentrations spanning a wide lean regime (2 < O-2/CH4 < 125). Spatially resolved capillary-inlet mass spectrometry (SpaciMS) measurements reveal two primary zones within the monolith. In the front zone, complete oxidation of methane to CO2 is dominant until the O-2 concentration decreases downstream below a critical value. Beyond that point both CO and H-2 are produced, revealing the emergence of methane partial oxidation, steam reforming, and water gas shift (WGS) reactions. Transmission electron microscopy and energy dispersive spectroscopy reveal particles having a distribution of sizes and compositions, Pt/Pd ratios ranging from 10 (Pt-rich) to 0.2 (Pd-rich), and particles having an average size of similar to 15-20 nm. Accordingly, the conversion trends and spatiotemporal data are interpreted with a mechanistic model accounting for contributions by Pt, Pd, and PdO phases. For example, methane conversion in the lean regime is likely dominated by an active PdO phase, while the rich oxidation behavior is strongly affected by the oxygen self-poisoning on metallic Pd and Pt sites. The low conversion branch is attributed to inhibition by O-2 adsorption occupying the metallic catalyst sites, although the state is unstable in the lean regime as the more active PdO phase forms. The multiple high conversion state branches in the rich regime and the associated slow transient approach to a single stable high conversion branch are attributed to the supply of oxygen from the underlying bulk of PdO which favors methane partial oxidation. (C) 2014 Elsevier B.V. All rights reserved.