Methane oxidation over Pt/Al2O3 and Pd/Al2O3 catalysts under transient conditions

Abstract

We communicate experimental results for the oxidation of methane by oxygen over alumina supported Pd and Pt monolith catalysts under transient conditions. Temperature programmed reaction (TPReaction) and reactant pulse-response (PR) experiments have been performed, using a continuous gas-flow reactor equipped with a downstream mass spectrometer for gas phase analysis. Special attention was paid to the influence of gas composition changes, i.e., O 2 and H 2 pulsing, respectively, on the methane conversion. For Pt/Al 2 O 3 oxygen pulsing can significantly increase the methane conversion which can be even further improved by pulsing hydrogen instead. Such transient effects are not observed for the Pd/Al 2 O 3 catalyst for which instead constantly lean conditions is beneficial. Our results suggest that under lean conditions Pd and Pt crystallites may undergo bulk-and partial (surface oxide formation) oxidation, respectively, which for Pd results in more active surfaces, while for Pt the activity is reduced. The latter seems to connect to a lowering of the ability to dissociate methane.

In Situ Spectroscopic Investigation of Low-Temperature Oxidation of Methane over Alumina-Supported Platinum during Periodic Operation

“…The minimum in CH 4 concentration observed here is not as pronounced in terms of absolute levels and duration as previously observed. 12 The reason for this is mainly due to the higher feed of oxygen in relation to both the concentration of CH 4 and amount of catalyst in the present study, leading to a more rapid change of the surface conditions, i.e., oxygen poisoning. The transient high activity can be elaborated on by varying the gas flow and composition as well as pulse duration/frequency.…”

“…These observations indicate, in line with previous studies, 8,30 a sensitivity towards oxygen poisoning which effectively can be reduced through cycling of the feed gas composition. 12,13,15,31 In terms of surface kinetics, the rate-determining step for methane oxidation is generally considered to be the dissociative adsorption of methane. 10 This activated process depends not only on the catalyst surface, for example the initial sticking probability is higher on the low-coordinated Pt[533] plane as compared to the close-packed Pt[111] surface, 32 but also on the actual coverage of reaction intermediates on the surface.…”

“…12,13 As a means to overcome such inhibition effects, periodic operation, i.e., alternating between net-oxidizing and net-reducing characteristics of the feed gas, can be employed. 12 Notably in such experiments, the conversion of methane is surprisingly high just after the switches.…”

Determination of Kinetic Parameters for Methane Oxidation over Pt/-Al2O3 in a Fixed-Bed Reactor

“…Dynamic operation can drive the catalyst to achieve its optimum condition, which cannot be achieved with steadystate operation. For Pt/γ-Al 2 O 3 , the optimum condition occurs when the active site is neither completely reduced nor fully oxidized [7]. This state induces the reaction kinetic parameters determined through a power law model become hard to apply to systems in dynamic conditions.…”

Unsteady-state operation of supported platinum catalysts for high conversion of methane

“…In this case the benefit with periodic operation is mainly due to a lowering of the CO self-poisoning, which has previously been studied in detail [8,9,10,11]. Unsteady-state operation of the exhaust composition has been shown beneficial also for total oxidation of saturated hydrocarbons, e.g., methane [12,13,14,15] and propane [16]. In these cases temporary high activity is observed during the switch from net-reducing to net-oxidising conditions and vice versa.…”