Catalytic Oxidation of Methane

1. Introduction

Methane (the major component of natural gas) is one of the main energy sources for gas-powered turbines for power generation, and transport vehicles. Methane is 84 times more potent than carbon dioxide as a greenhouse gas and is emitted from power generation, diesel and compressed natural gas engines, gas wells, etc. Catalytic oxidation of methane is an important area for both academic research and industrial applications. The three-way catalytic converters work well at higher temperatures (above 600 °C) but are not very effective for lean-burn engines and “idling”. Even though a good amount of research has been reported on the catalytic oxidation of methane, the search is still on for the development of low-temperature catalysts [1]. Two recent reviews [2,3] summarized the advances made on development of catalysts, including identification of the active and surface species and the reaction mechanisms during the catalytic combustion of methane. Catalytic oxidation of methane over alumina and ceria supports have been reported to be promising [4,5,6]. Additionally, particle size, chemical composition, surface species, and metal–support interactions could significantly influence the properties and activities of metal catalysts [7,8,9,10,11].

2. The Special Issue

The focus of this Special Issue was on the synthesis, characterization, and activity of catalysts on various supports (alumina, ceria, silica), methane combustion over composite catalysts, reactor design and the combustion characteristics in small-scale systems. Liu et al. [12] reported the development of Pd/CeO₂ catalysts on rice husk silica support for the catalytic methane combustion in the temperature range of 150–500 °C, under methane lean conditions. The incorporation of Pd-CeO₂ into rice husk silica support improved the water-resistance. Khader et al. [13] synthesized palladium/ceria nano-catalysts supported on alumina and prepared via a one-step solution-combustion synthesis (SCS). High-resolution transmission electron microscopy showed bigger Pd particles (5 nm and more) were surrounded by CeO₂, resembling a core shell structure. The results indicated that the Pd-SCS nano-catalysts were exceptionally more active and stable than conventional catalysts. A PdO-PdO_x/γAl₂O₃ catalyst synthesized by a vortex-assisted incipient wetness method exhibited exceptional low-temperature activities, with 90–94% methane conversion at 300–320 °C. X-ray photoelectron spectroscopy established that the active phase, PdO_x, originated from the interaction of PdO with the alumina support during the calcination process [14]. Banerjee et al. [15] compared the relative efficiencies of Pd/alumina catalysts prepared by the vortex and incipient wetness methods. The catalyst synthesized by the vortex method produced smaller PdO/PdOx nanoparticles (2–5 nm) and converted 90% methane at 325 °C. Li et al. [16] reported nitrogen-modified perovskite type composite catalysts prepared by a hydrothermal method for catalytic oxidation of methane. The surface reaction mechanism was investigated using in-situ diffuse reflectance infrared Fourier transform spectroscopy. LaMnO₃ catalysts synthesized by sol–gel methods and characterized by a variety of techniques displayed good catalytic activities for hydrocarbon oxidation [17]. Sang et al. [18] designed a novel rotary regenerator-type catalytic combustion reactor and found that the performance of the reactor was more sensitive to the increase of velocity and the decrease in methane concentration. Chen et al. investigated the catalytic combustion characteristics of methane-air mixtures in small-scale systems, using computational fluid dynamics simulations and chemical kinetic mechanisms. The results indicated that the distribution of oxidized products depended critically on the feed composition, dimension, temperature, and pressure.