A general one-pot solvothermal process was explored to prepare BiOX (X = Cl, Br, I) powders by employing ethylene glycol as the solvent. The as-prepared BiOX powders were characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, UV−vis diffuse reflectance spectroscopy, and nitrogen sorption. The resulting BiOX samples were phase-pure and of hierarchical microspheres consisting of nanoplates. The band gaps of the as-prepared powders were estimated to about 3.22, 2.64, and 1.77 eV for BiOCl, BiOBr, and BiOI, respectively. On the basis of characterization results, we proposed a possible process for the growth of hierarchical BiOX nanoplate microspheres. Moreover, we evaluated their photocatalytic activities on the degradation of methyl orange and compared them with TiO2 (Degussa, P25) under UV−vis light irradiation and C-doped TiO2 under visible light (λ > 420 nm) irradiation, respectively. It was found that al...

Generalized One-Pot Synthesis, Characterization, and Photocatalytic Activity of Hierarchical BiOX (X = Cl, Br, I) Nanoplate Microspheres

This review examines recent developments in the complete oxidation of methane at low temperature over noble metal based catalysts in patents and open literature. The abatement of natural gas vehicle (NGV) methane emissions is taken as one example among possible applications. The review develops current ideas about the properties of palladium and platinum catalysts supported on silica and alumina supports in the complete oxidation of methane under oxidising conditions, focusing on low-temperature reaction conditions. The influence of residual chloride ions on the catalytic activity, the kinetic aspects of the oxidation of methane over these catalysts, the nature of the active sites, the influence of metal particle size and reaction products on the activity, the observed changes in catalytic activity with reaction time and the effect of sulphur containing compounds are examined. The latest studies concerned with improved palladium and platinum supported catalysts which would exhibit enhanced and stable catalytic activity at low temperature in the presence of water and sulphur containing compounds are reported. Possible routes for preparing catalysts able to meet future regulations concerning methane emissions from lean-burn NGV vehicles are discussed.

Complete oxidation of methane at low temperature over noble metal based catalysts: a review

The important greenhouse effect of methane (more than an order of magnitude greater than CO2) makes it essential to eliminate/control the methane emission from natural gas engines/power plants and petroleum industries. Catalytic combustion of methane is favored over homogeneous combustion, because the former greatly facilitates the oxidative destruction of methane. Moreover, use of catalysts for methane combustion in gas-turbines affords lower working temperatures (as compared to gas-fired turbines) and thermodynamically limits NOx (which is an extremely harmful environmental pollutant) emission. A large amount of work has been undertaken to develop catalysts both for controlling methane emission as well as for generating power in high temperature natural gas-turbines. This review will address the different issues related to the variety of catalysts which have been employed for methane/lower alkane combustion. Although all the related important aspects of the combustion catalysts will be addressed, greater emphasis will be placed on recent work in this field.

Catalysts for combustion of methane and lower alkanes

Palladium-based catalysts are widely applied in exhaust catalytic converter and catalytic combustion systems. The mechanism for methane oxidation on a Pd-based catalyst is complex. Catalyst activity is influenced by variations in the process pressure and temperature, by the gas mixture composition, by the type of support and various additives, and by pretreatment under reducing or oxidizing atmospheres. In this paper, we review the literature on supported Pd catalysts for combustion of methane. The mechanisms involved are discussed taking into consideration the oxidation/reduction mechanisms for supported palladium, poisoning, restructuring, the form of oxygen on the surface, methane activation over Pd and PdO phases, and transient behavior. Our review helps explain the array of experimental results reported in the literature.

Catalytic combustion of methane over palladium-based catalysts

Heterogeneous catalysis is one of the most important chemical processes of various industries performed on catalyst nanoparticles with different sizes or/and shapes. In the past two decades, the catalytic performances of different catalytic reactions on nanoparticles of metals and oxides with well controlled sizes or shapes have been extensively studied thanks to the spectacular advances in syntheses of nanomaterials of metals and oxides. This review discussed the size and shape effects of catalyst particles on catalytic activity and selectivity of reactions performed at solid–gas or solid–liquid interfaces with a purpose of establishing correlations of size- and shape-dependent chemical and structural factors of surface of a catalyst with the corresponding catalytic performances toward understanding of catalysis at a molecular level.

Size- and shape-dependent catalytic performances of oxidation and reduction reactions on nanocatalysts.

The influence of water and carbon dioxide on the activity of a series of palladium catalysts for the combustion of methane has been studied. It has been found that both compounds have an inhibitory effect but the extent of this depends on the experimental conditions. When both are present in the feedstream, the effect of carbon dioxide is negligible, the total decrease in activity only being equal to that of water alone. It is concluded that both water and carbon dioxide poison the same active sites on the palladium oxide with water able to displace carbon dioxide to form an inactive surface hydroxide. The magnitude of the effect due to both additives decreases with increasing reaction temperature and conversion. This is due both to the increased amount of water producted in the reaction reducing the effect of added water or carbon dioxide, and to the decreasing stability of the surface hydroxide at higher temperatures. In relation to practical applications, it is concluded that carbon dioxide will have no effect on catalytic activity under normal working conditions but water present in the feedstream may reduce the activity of the catalysts very significantly at lower temperatures. However, above about 450°C the effect of water is also expected to be very small.

Methane combustion over palladium catalysts: The effect of carbon dioxide and water on activity

Possible relationships between the structure and activity of supported palladium catalysts for the oxidation of methane have been explored. Samples have been exposed to a wide range of oxidation and reduction pretreatments and the activity for methane oxidation has been determined as a function of time-on-stream, both for pre-oxidised and for pre-reduced samples. Parallel experiments on the rate and extent of oxidation at 300°C have been performed. The results show that the rate of oxidation of pre-reduced palladium samples is very sensitive to the previous thermal history and samples retain a ‘memory’ of their treatment. In all cases a monolayer of adsorbed oxygen is taken up very rapidly but further oxidation is slower, although the samples approach the bulk oxide state within a few minutes. There are no obvious correlations between the catalytic activity for methane oxidation and the previous treatment of the samples, suggesting a sensitive dependence of activity on the morphology of the samples. It is clear, however, that metallic palladium is not active for methane oxidation. Similarly, chemisorbed oxygen is not as active as oxidised palladium. Moreover, it is found that there is no loss of activity when the palladium is fully oxidised, suggesting that previously reported changes in activity may reflect long-term morphological effects rather than simple over-oxidation of palladium particles.

Investigation of the active state of supported palladium catalysts in the combustion of methane

The formation of ethene from methane on a range of oxychloride catalysts has been investigated using a pulse technique which allows the characteristic properties of inherently unstable materials to be examined. The results show that the selectivity of the catalysts for the formation of ethene is sensitive to the choice of experimental conditions. The reactions of ethane and ethene with air have been investigated over these catalysts and the results compared with those obtained on an active oxide catalyst. The results suggest that the very high selectivity of oxychloride catalysts for the formation of ethene from methane is associated with the relative inability of such catalysts to oxidise totally any of the hydrocarbon species introduced, or formed, in the reaction. In particular, it is shown that the oxidation of ethane to CO x is less rapid on chlorine-containing catalysts. The effect of chlorine as a gas-phase promoter and as a catalyst modifier is discussed.

Investigation of ethene selectivity in the methane coupling reaction on chlorine-containing catalysts

Deposition of tin-oxygen species on mesoporous silica (MCM-41) was achieved via chemical reaction of SnEt 4 vapour with surface silanol groups followed by heat treatment in air. Evidence suggested that highly reactive tin-oxygen overlayers were formed on the internal surface of the MCM-41 silica. These tin-oxygen modified silicas, after H 2 pre-treatment, showed excellent activity in catalysing reduction of crotonaldehyde to crotyl alcohol using propan-2-ol as the hydrogen transfer agent (acetone was formed as the by-product). It was also found that the catalytic activity per gram of tin increased dramatically as the tin loading decreased. The increase in the specific activity with the decrease in tin loading was attributed to the improved dispersion of the tin-oxygen overlayers.

R. Burch

Papers

Methane combustion over palladium catalysts: The effect of carbon dioxide and water on activity

Investigation of the active state of supported palladium catalysts in the combustion of methane

Investigation of ethene selectivity in the methane coupling reaction on chlorine-containing catalysts

Nanoscopic tin-oxygen linings on mesoporous silica as a novel catalyst for organic hydrogen transfer reaction