Read an Excerpt
Catalysis Volume 14
A Review of Recent Literature
By James J. Spivey
The Royal Society of ChemistryCopyright © 1999 The Royal Society of Chemistry
All rights reserved.
Catalytic Oxidation of Methane on Supported Palladium Under Lean Conditions: Kinetics, Structure and Properties
BY YA-HUEI CHIN AND DANIEL E. RESASCO
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 when the comparison is made with catalysts that have been pre-oxidized before the reaction. However, most studies seem to agree that the TOF increases with particle size. For example, Hicks et al., working at 335°C on pre-reduced Pd/Al2O3 and Pd/Y-ZrO2 samples, observed a steady state TOF of about 1.3 s-1 for large particles and 0.02 s-1 for small particles. Similarly, Muller et al. examined the effect of crystallite size on Pd/ZrO2 catalysts. The crystallite size, as measured by XRD, was varied by using different reduction temperatures. This study showed that the intrinsic activity increased with Pd crystallite size. At 330°C, the TOF went from 0.035 to 0.17 s- 1 as the crystallite size was varied from 6 to 12 nm.
By contrast, some investigations have found no clear correlation between particle size and TOF. For example, experiments conducted by Baldwin and Burch20 on Pd/AL2O3 catalysts calcined in air at different temperatures and time periods showed a wide variation in activity without any clear relationship between the TOF and particle size. Similar data with little correlation with particle size was reported by Muto et al. Ribeiro et al. have found relatively modest increases in TOF as the particle size was varied. They observed that when the particle size increased from 2 to 110 nm, the TOF increased from 2 x 10-2 s-1 to 8 x 10-2 s-1 (see Figure 2). Despite this increase, the authors have suggested that methane oxidation is structure insensitive and the observed changes in TOF should not be ascribed to particle size effects.
Fujimoto et al. have identified several possible causes for the wide range of TOF values and, in some cases, the lack of correlation found in the literature. In the first place, they have pointed out that, in most studies, the TOF values are based on H2 or CO chemisorption measurements conducted on reduced catalysts. However, these initial dispersion measurements may not represent the actual dispersion present under reaction conditions. Both oxidation state and morphology of the Pd particles may be altered in the presence of the reaction mixture. The second important consideration necessary to analyse the wide variation of TOF values is the role of product inhibition, which, obviously, would greatly vary as the conversion varies. To make a proper comparison one needs to operate at similar conversions, and this aspect has not been considered in many of the studies. A third complication in comparing TOF values is the presence of long induction and activation periods exhibited by some of these catalysts. In most cases, the initial activity is very different from that at steady state and, as described in the next section, in some cases, steady state conversions are only achieved after several hours. Therefore, in making a comparison of activity data, one must be sure that steady state values are used. When all of these conditions were carefully taken into consideration, 18 a clear correlation between TOF and particle size was obtained, showing that the TOF indeed increases with particle size, at least for particles smaller than about 15 nm.
2.2 Time-dependent Catalytic Activity - The strong variation of activity as a function of time on stream is a typical feature of the methane combustion reaction on most Pd catalysts. Several different transient phenomena have been reported. In some cases, the activity is initially low, or even zero, but then it increases with time on stream. In other cases, the activity starts high but then it drops to a lower steady state value. The approach to the steady state has also been found to vary greatly. In some cases, it reaches a relatively constant value in a few minutes. In other cases, the activity still changes after several hours on stream.
It has been proposed that the increases in activity as a function of time on stream may be due to the decomposition of inactive Pd oxy-chloride species, which attenuate the oxygen adsorption capacity. In line with this concept, previous investigations have shown that the presence of halogenated hydrocarbons and organosiloxanes greatly inhibited the methane oxidation reaction. It was found that, above 350°C, the presence of chlorine- or bromine-containing compounds has a strong deactivating effect, and thus the deactivation could be ascribed to the adsorption of halogens, which block sites needed for oxygen activation. Similarly, Simone et al. observed higher activity on Pd catalysts prepared from nitrate than on those prepared from chlorides.
However, the presence of halogenated compounds may not be the only cause for the low initial activity of Pd catalysts, since halogen-free catalysts also exhibit time-dependent activity increases. Peri and Lund have made a clear distinction between induction periods (i.e. time during which no activity is observed after exposing the catalyst to reaction conditions) and activation periods (i.e. the activity steadily increases with time on stream). Figure 3 illustrates these two types of transient behavior. According to Peri and Lund, the induction periods are related to the presence of residual chlorine, but the activation periods should not necessarily be linked to chlorine. They arrived at this conclusion from their observation that induction periods were observed only on samples containing chlorine. Those samples, from which chlorine had been eliminated by either washing the sample or pre-treating it under reaction conditions, did not exhibit induction periods, but they did exhibit activation periods. Similarly, chlorine-free samples prepared from nitrates and employed in other studies did not exhibit an induction period, while they still presented activation periods.
If the presence of impurities is not the cause of the observed activation periods, a reconstruction or morphological change of the Pd particles must be responsible for the observed transients. In a combined characterization study, using FTIR, TEM, and nano-diffraction techniques, Garbowski et al. observed clear reconstruction of the Pd particles supported on Al203. Before the methane combustion period, the reduced catalyst had metal particles in epitaxy with the support and exhibited mainly Pd (111) planes. After exposure to the reaction mixture, the ( 111) planes were exchanged by ( 100) and ( 110) planes. These authors speculated that these more open structures, generated during the reaction, are able to accommodate oxygen and form the PdO needed for the reaction, lowering the activation energy for the whole process.
The activity transients are also affected by the type of support used. As shown in Figure 4, the activation periods were very different for a series of 0.5 wt% Pd catalysts on various supports. It was found that the activity of the alumina-supported catalyst first increased, reached a maximum, and then decreased with time on stream. By contrast, the silica- and silica-alumina-supported catalysts did not have an activation period and the conversion decreased with time from the start. Finally, with the mordenite support, the activity slowly increased and did not reach a maximum during the period of the study, about 7 h.
2.3 Effects of Thermal Pretreatments - The transient and steady state activities of Pd catalysts are highly sensitive to the pretreatment procedures. For example, the activities of Pd/Al2O3 samples were compared after different pretreatments with dry or wet air, H2, N2, or methane. Table 1 shows the dispersion, the initial and steady state conversions on a 4% Pd/Al2O3 catalyst, preconditioned under various environments during either 1 h or 16 h. Although the dispersion was relatively constant for all samples, ranging from 18 to 25%, a very broad range of time-dependent conversions was obtained for the pretreatments conducted under different gases or for different periods of time. For example, the catalyst pretreated in H2 for l h exhibited a low initial activity, but it increased by a factor of about 4.5 before reaching the steady state activity. On the other hand, the samples pretreated in dry air or N2 only exhibited modest changes.
Several studies have reported that catalysts pretreated under reducing conditions have a higher steady state activity than those pretreated under oxidizing atmospheres. In fact, as shown in Table l , the steady state activity of the catalyst reduced in H2 was the highest among the various samples. In a similar study, Cullis and Willatt 30 investigated the activity of Pd/Al203 after pretreatments H2+He, He, or O2 in pulse measurements. In every pulse (except for the first one), the catalyst conditioned under H2+He exhibited higher transient activities than those pretreated in He or O2.
Although in Table 1, the samples activated in 1% CH4/air show a relatively low activity, most studies have found that the methane combustion mixture is a better activating environment than other oxidizing pretreatments. For example, important differences have been found in comparing the activity of a catalyst after 14 h at 600 °C on stream in a 1% CH4/4% O2/N2 environment, with that of the same catalyst pretreated at the same temperature for the same period of time, but without CH4. The catalyst aged under reaction conditions was significantly more active. Similarly, to study the variation in activity due to different pretreatments while keeping the crystallite size constant, Baldwin and Burch 20 compared the activities of two Pd/Al2O3 samples with the same particle size, conditioned either in air or in a 1% CH4/air mixture. Comparison of activity at 350°C showed that the sample pretreated in air at 500°C for 16 h and subsequently treated in a reaction mixture at 405°C for 60 h was two orders of magnitude more active than another sample only exposed to air at 600°C for 16 h.
Excerpted from Catalysis Volume 14 by James J. Spivey. Copyright © 1999 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.