Catalytic Oxidation of Methane on Supported Palladium Under Lean Conditions: Kinetics, Structure and Properties

BY YA-HUEI CHIN AND DANIEL E. RESASCO

1 Introduction

The catalytic total oxidation of hydrocarbons is generally considered as an effective method to generate power and reduce emissions. In recent years, the interest towards catalytic combustion of methane has increased considerably. Methane has several advantages as an energy source. It has a high H/C ratio, and therefore the heat of combustion per mole of 'greenhouse' CO2 generated is significantly higher for methane than that for other fuels. For example, while the combustion of methane generates 890 kJ mo1-1 of CO2 produced, the corresponding values for n-decane and coal are 680 and 390 kJ, respectively. At the same time, the level of sulfur and nitrogen impurities in natural gas is much lower than in other fuel sources. The two main applications of the catalytic combustion of methane are:

(1) catalytic combustion as an alternative to conventional thermal combustion in gas turbine combustors used for power generation;

(2) abatement of methane emissions from compressed natural-gas vehicles (NGVs).

In the first application, the use of a catalyst results in minimization of NOx emissions. Owing to the presence of the catalyst, the combustor can operate at air/fuel ratios higher than those of a flammable mixture. The role of the catalyst is to initiate the reaction at the relatively low temperatures typical of the inlet of the combustor. As the exothermic combustion accelerates, the temperature along the combustor rises until the mass transfer limitation conditions are reached. At about this point, homogeneous gas phase reaction occurs, completing the combustion process.

The maximum temperature attainable in the combustor can be controlled by varying the air/fuel ratio. This is a unique feature of the catalytic combustor, since without a catalyst a flame can only be sustained in a narrow air/fuel ratio range. By using an appropriate catalyst, instead of operating at the typical temperature of conventional flame combustion (i.e. 1500°C), the combustor can operate under flameless conditions below 1300°C. It is well known that the emissions of nitrogen oxides can be minimized by reduction of the average temperature at which the combustion takes place and by elimination of hot spots (1700-1800°C) that are sites of rapid NOx production. In a typical combustion process, 95% of NOx is generated via the Zeldovich radical chain mechanism, 5 which is kinetically limited below 1500°C. Above 1500°C, the thermal NOx production is doubled every time the temperature increases by 40°C. The use of a catalytic combustor not only increases the fuel efficiency, but also eliminates the possibility of local hot spots. In this way, most of the 'thermal' NOx3 is eliminated, resulting in a dramatic decrease in the NOx emissions. While a standard diffusion flame combustion turbine produces exhausts with NOx concentrations higher than 150 ppm, a flameless catalytic combustor achieves concentrations of the order of 3 ppm. The search for efficient catalysts, active over a wide temperature range, and able to withstand the severe conditions under which combustion takes place has generated a large body of information on catalytic materials and the mechanisms by which they operate.

In the second type of application, the intrinsic knock resistance of natural gas as a motor vehicle fuel makes it very attractive, particularly because this property is maintained over a wide range of air/fuel ratios. Therefore, natural-gas engines can operate under lean conditions, thus increasing their fuel efficiency and minimizing the typical products of incomplete combustion, such as soot, CO, and volatile organic compounds (VOCs). In addition to this advantage, the lower cost and lower emissions associated with natural gas are important benefits that have greatly increased the potential of natural-gas vehicles. However, one of the concerns about the use of methane as a fuel is that it is a greenhouse gas with a global warming potential much higher than that of CO2. Thus, large emissions of unburned methane would become an environmental problem. Therefore, there has been considerable interest in the study of catalytic materials for methane combustion under typical exhaust conditions, with low CH4 concentrations and in the presence of varying concentrations of O2, H2O, CO2, SO2, and NOx. In this case, the desired characteristic is high activity in the presence of these components and at temperatures typical of exhausts (i.e.<500°C).

Among a large number of formulations investigated, the superiority of Pd-based catalysts has been widely recognized. In this contribution, we will review the current ideas about the nature of the active sites and the effects of particle size, state of Pd, and metal-support interactions on the catalytic properties. We will analyse these effects for two different regimes. The low-temperature region (below 800°C) is most relevant for combustion catalysts employed in catalytic converters for the abatement of unburned methane in exhausts. Pd-based catalysts can also be potentially useful for the simultaneous elimination of unburned CH4 and NOx. At the same time, low temperature combustion studies are also important to light-off the gas feed in catalytic combustors. The high temperature region usually refers to reactions carried out at or above the temperature of PdO decomposition in air (i.e. 800°C). This region is particularly important for applications in gas turbine combustors. There are significant differences between the two temperature regimes. Therefore, we will analyse them separately.

2 Catalytic Combustion of Methane at Low Temperatures (below 800°C)

2.1 Effects of Particle Size - The question of whether the methane oxidation is structure sensitive or structure insensitive has generated some controversy in the scientific literature. Some studies have demonstrated a strong dependence of activity on particle size. For example, Figure 1 summarizes literature data of the variation of turnover frequency (TOF) with Pd dispersion for catalysts preconditioned in He or H2. Despite some scattering in the data, a clear correlation is observed, indicating that the specific activity of Pd does in fact increase with particle size.

The correlation is not so clear...