Maxim Lyubovsky

CATALYTIC COMBUSTION OF METHANE OVER PALLADIUM-BASED CATALYSTS

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.

Catalyst microstructure and methane oxidation reactivity during the Pd

Abstract A 5xa0wt.% Pd/θ-Al 2 O 3 catalyst has been cycled in air at temperatures where the oxide PdO decomposes to Pd upon heating and reforms upon cooling. The microstructure of the Pd and PdO particles was studied using transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The extent of phase transformation was measured via thermogravimetric analysis (TGA). Our results confirm the observation reported previously by Farrauto et al. (Appl. Catal. A: General 81 (1992) 227) that the decomposition temperature ( T D ) of the PdO exceeds the oxide reformation temperature by a few hundred degrees K. TEM images provide insight into the morphology of the particles during the PdOxa0→xa0Pd phase transformation. This phase transformation is initiated at the surface and causes small domains of Pd metal to form on the surface of PdO. These small domains of Pd metal are easy to reoxidize upon cooling. However, complete transformation of the PdOxa0→xa0Pd at T xa0>xa01198xa0K yields single crystal metal particles that are harder to oxidize during cooling in air. Appreciable amounts of bulk oxide do not form on the transitional alumina supported Pd unless the sample is cooled below 873xa0K. The hysteresis in the reformation of oxide during cooling is related to strongly bound oxygen on the Pd surface that inhibits bulk oxidation. The relationship between bulk oxide formation and the reactivity for methane oxidation was also examined. It was found that reactivation of the catalyst occurred before significant bulk PdO had formed. Samples quenched during this reactivation process were examined by XPS and TEM, and no evidence was seen for any redispersion during the reoxidation of the Pd metal. Extensive surface roughening appears to result from bulk oxide formation, which may explain the higher reactivity seen after catalyst cool-down.

PdO transformation on alumina supports

Abstract The influence of the reaction parameters including temperature, oxygen concentration, and of in situ hydrogen reduction on the Pd catalyst activity towards complete methane oxidation is studied experimentally. Zero porosity α-alumina plates are used as a support for Pd catalyst. This lowers the influence of metal–support interaction on the catalyst state as confirmed by UV–visible spectroscopy. A plug flow reactor with a high linear gas velocity is used to measure the reaction rate. Overall conversion is kept low for most of the experiments so that the reaction is in the kinetically limited regime. The oxidation state of the catalyst before and after the reaction is determined using UV–visible reflectance spectroscopy of the plate surface. Changes in the catalyst activity with time are monitored after stepwise changes in the reaction parameters. Activity was found to decrease with time at low temperatures and high oxygen concentrations (condition when PdO phase is stable) and to increase with time at high temperatures and low oxygen concentrations (conditions when Pd is stable). A sharp increase in conversion was observed after the in situ hydrogen reduction of the sample. The experimental data is consistent with the reduced Pd form of the catalyst being more active towards methane oxidation than the oxidized PdO form at high temperatures. Possible particle size and morphology effects are discussed.

Complete methane oxidation over Pd catalyst supported on α-alumina. Influence of temperature and oxygen pressure on the catalyst activity

Abstract The effect of variations in temperature and oxygen partial pressure on the methane oxidation activity of α-alumina supported Pd catalysts was studied experimentally. It was found that after pretreatment at temperatures above 800°C, conditions where the metallic state of the catalyst is stable, “negative activation” occurred on cooling cycle leading to increase in conversion with decreasing temperature. The pretreatment at temperatures exceeding 800°C was found to be a necessary condition in order for this effect to take place. This phenomenon is attributed to appearance of highly dispersed PdO clusters in thermodynamic equilibrium with the previously formed metallic Pd surface. The contradictory results on the activity of Pd/PdO phase of the catalyst towards methane oxidation reported in the literature are explained by the proposed hypothesis.

Methane combustion over the α-alumina supported Pd catalyst: Activity of the mixed Pd/PdO state

Abstract Performance data are presented for methane oxidation on alumina-supported Pd, Pt, and Rh catalysts under both fuel-rich and fuel-lean conditions. Catalyst activity was measured in a micro-scale isothermal reactor at temperatures between 300 and 800xa0°C. Non-isothermal (near adiabatic) temperature and reaction data were obtained in a full-length (non-differential) sub-scale reactor operating at high pressure (0.9xa0MPa) and constant inlet temperature, simulating actual reactor operation in catalytic combustion applications. Under fuel-lean conditions, Pd catalyst was the most active, although deactivation occurred above 650xa0°C, with reactivation upon cooling. Rh catalyst also deactivated above 750xa0°C, but did not reactivate. Pt catalyst was active above 600xa0°C. Fuel-lean reaction products were CO 2 and H 2 O for all three catalysts. The same catalysts tested under fuel-rich conditions demonstrated much higher activity. In addition, a ‘lightoff’ temperature was found (between 450 and 600xa0°C), where a stepwise increase in reaction rate was observed. Following ‘lightoff’ partial oxidation products (CO, H 2 ) appeared in the mixture, and their concentration increased with increasing temperature. All three catalysts exhibited this behavior. High-pressure (0.9xa0MPa) sub-scale reactor and combustor data are shown, demonstrating the benefits of fuel-rich operation over the catalyst for ultra-low emissions combustion.

Catalytic combustion over platinum group catalysts: fuel-lean versus fuel-rich operation

The development of improved substrate properties for catalytic combustion has been an area of much interest in recent years. Towards this end, Precision Combustion Inc. has developed novel short channel length, high cell density substrates (trademarked Microlith ® ) and high surface area ceramic coatings for them. These substrates avoid substantial boundary layer buildup and greatly enhance heat and mass transfer rates in reactors. The high cell density of these substrates results in high amount of the catalyst per unit of reactor volume. In this paper we examine the performance of these substrates coated with precious metal catalysts for the catalytic combustion and reforming of methane. Under fuel-lean operating conditions the surface temperature of Pd-based catalyst supported on Microlith ® substrate and the temperature of the gas exiting the reactor remain stable at ∼800 ◦ C over a wide range of inlet conditions. This is attributed to combination of enhanced transport properties and characteristics of Pd–PdO transformation. Preheating of the gas mixture in the Microlith ® reactor was sufficient to stabilize a downstream premixed flame with NO x, CO, and UHC emissions in the single digit ppm range. Microlith ® substrates were also examined for partial oxidation of methane under fuel-rich conditions. The enhanced transport properties of the Microlith ® substrate allowed complete conversion of methane with surface temperature not exceeding material limits at 93% selectivity to partial oxidation products. High flow rate of reactants result in extremely high power densities and syngas output. The catalyst performance was observed to be stable over 500 h of operation.

Complete and partial catalytic oxidation of methane over substrates with enhanced transport properties

Use of catalytically coated short contact time (SCT) design approaches for application in mass transfer controlled reactors such as Auto Thermal Reformers (ATR’s) is an area of much recent interest. Precision Combustion, Inc. (PCI) has developed an efficient and compact ATR using ultra-short channel length, high cell density SCT substrates (Microlith ® ). PCI has also extended this Microlith technology to other fuel processor reactors that operate at lower temperatures and are not mass transfer limited. Namely, reactors for the Water Gas Shift (WGS) and Preferential Oxidation (PROX) of CO have been developed. Due to the higher surface area per unit volume of the Microlith substrate compared to conventional monoliths, size advantages have been observed for these reactions, which are more kinetically controlled. This results not only in shortened startup times and quick load following capability but also allows much smaller and lighter reactors – required attributes for automotive fuel cell applications. In this paper, experimental data on the performance of Microlith based ATR, WGSR and PROX reactors for reforming isooctane is presented. Transient and durability characteristics have also been included and compared to Department of Energy (DOE) targets.

Performance of Microlith Based Catalytic Reactors for an Isooctane Reforming System

Palladium (Pd) supported on alumina with various additives is the catalyst of choice for the catalytic combustion of methane. This catalyst is interesting in that it can change dramatically under reaction conditions and can chemically interact withhigh-surface-area (HSA) y -alumina supports. Much has been made of the observation that the apparent activity of these catalysts decreases in certain regimes as temperature increases, leading to the assertion that Pd metal is not active for methane oxidation, at least under fuel-leam conditons. This phenomenon has been suggested as a means to keep a catalyst from overheating in practical combustor designs. In this paper, we show that for Pd supported on low-surface-area (LSA) α-alumina, the methane oxidation activity increases as PdO is reduced to Pd at a fixed operating temperature, even for fuel-lean gas mixtures. This higher activity is significantly greater than the highest turnover frequencies reported by previous investigators when extrapolated back to 550 K and 2% methane in air. We propose that the differences in relative activity for methane oxidation between catalysts supported on different alminase are likely related to differences in morphology, the state of the Pd phases, differences in catalyst poisoning by water, and interactions between the Pd phases and the support. Most of these parameters were not directly monitored in previous studies.