Elsevier

Catalysis Today

Volume 144, Issues 3–4, 30 June 2009, Pages 306-311
Catalysis Today

Bimetallic catalysts for the catalytic combustion of methane using microreactor technology

https://doi.org/10.1016/j.cattod.2008.10.053Get rights and content

Abstract

Pt–W and Pt–Mo based catalysts were evaluated for methane combustion using a sandwich-type microreactor. Alumina washcoated microchannels were impregnated with platinum in combination with and promoted with tungsten and molybdenum and compared with commercially available Pt/Al2O3 catalysts. Catalysts were tested in the range of 300–700 °C with flow rates adjusted to GHSV of 74,000 h−1 and WHSV of 316 L h−1 g−1. Catalysts containing tungsten were found to be the most active and the most stable possibly due to a metal interaction effect. A Pt–W/γ-Al2O3 containing 4.6 wt% Pt and 9 wt% W displayed the highest activity with full conversion at 600 °C and a selectivity to CO2 of 99%.

Introduction

For more than 30 years, many investigations have been conducted into elucidating the intrinsic details of the catalytic combustion of methane [1], [2], [3], [4], [5], [6]. The reasons for this are two-fold, namely for pollution abatement and for power generation. Methane is a by-product in many industrial processes and is also a pollutant from automobiles and gas power plants. Automotive applications require fuel cells with high power and efficiency, low operational costs, long durability of components, and compact dimensions. Proton exchange membrane (PEM) fuel cells are usually applied in cars because they are small and light, they operate at a relatively low temperature and they are operationally flexible. A fuel cell, which uses pure hydrogen as fuel, produces only steam with zero polluting emissions; however, for mobile applications, apart from the high cost of the on-board tanking, a major drawback which is hindering the application of the technology is the lack of infrastructure for pure hydrogen refueling. It is generally agreed that the only viable way to produce hydrogen, in large enough quantities to satisfy demand now and in the future, is via hydrocarbon conversion processes (usually petrol and diesel oils). Normally, this is achieved by autothermal reforming (ATR) and steam reforming (SR), resulting in hydrogen-rich gases.

Multifunctional reactors hosting catalytic combustion and steam reforming of fuel at the opposite sides of a heat-exchanger appear to be very promising for achieving maximum compactness, a very desirable property for an auxiliary power unit placed on-board road vehicles, yachts, and aircrafts. Microchannel reactors can satisfy this requirement because of their enhanced heat and mass transfer characteristics, the flow uniformity, the high surface area to volume ratio, safe control in explosive regimes and easier scale-up possibilities [7]. Microchannel fuel processors containing catalytic combustors have been recently developed [8], [9], [10]. Methane and diesel were used for combustion for preheating the future systems. The anode-off gases containing hydrogen and methane can be introduced into the combustor as a fuel during normal operation.

Many studies have been done into addressing both the design and manufacture of suitable catalysts for the combustion process and the fabrication of unique but efficient reactors so as to maximize the effectiveness of these active catalysts. Invariably, the majority of the most active combustion catalysts consist of an active constituent and a support. Perovskites [11] doped metal oxides [12] and hexaluminates [13] have all been utilized for catalytic combustion, but the majority of the studies have involved noble metal based catalysts [[14] and references therein]. Relative to oxide catalysts, precious metal catalysts possess a greater resilience against sintering and a higher resistance to sulfur poisoning, but more importantly a higher specific catalytic activity that renders them attractive as potential catalysts for catalytic combustion. They can be manufactured in a highly dispersed state on standard supports like silica and alumina, thus leading to improved activity.

With respect to Pt catalysts promoted with Mo and W, Yazawa et al. [15] showed that the activity of such catalysts for propane combustion was much enhanced, which was ascribed to the electronic properties of molybdenum and tungsten. The activity of Pt is mainly influenced by its oxidation state, with higher activities seen when Pt is less oxidized. Effectively, the electron deficiency of platinum varies with the electronegativity of additives, and the higher electronegativity enhances the electron deficiency of platinum, but depresses the oxidation of Pt. da Silva and Schmal [16] suggested that a synergy exists between Mo and Pt/Al2O3, which were studied for NO reduction by CO. Molybdenum helps to stabilize both the Pt and Al2O3 by enhancing the oxygen storage capacity and spillover effect, while on the other hand platinum maintains the surrounding molybdenum particles in its more active form. It has also been suggested [17] from density functional theory studies that MoO3 supported Pt catalysts can facilitate single C–H bond activation in methane.

Yatsimirskii et al. investigated the oxidation of H2 [18] and of CO [19] for 0.5 wt% Pt/WO3 catalysts and found that the enhancement in activity could be attributed to the formation of oxide bronzes, the subsequent interaction between these bronzes and the metallic Pt, and the appearance of oxygen vacancies.

Hua and Sommer [20] studied n-heptane isomerization over Al2O3-doped Pt/WOx/ZrO2 and found that the catalytic activity depends strongly on surface WOx loading and were attributed to an increase in the number of Brønsted acid sites. The maximum activity was observed at a WOx concentration of 10 wt% W, which was slightly higher than the theoretical monolayer capacity.

In this work, Pt/γ-Al2O3 catalysts promoted by W and Mo were prepared and assessed for their effectiveness for the catalytic combustion of methane. It is clear that the catalytic behavior of these systems is controlled by complex mechanisms, therefore characterization studies were also performed so as to develop a better perspective on the solid state behavior of these catalysts and concurrently to relate this to the catalytic activity.

Section snippets

Preparation and characterization of catalysts

The home-made catalyst coatings were based upon pure alumina carriers. The catalysts were prepared by washcoating alumina into the microchannels. This was achieved by manually filling the microchannels with alumina suspension. This was then followed by drying at room temperature and a calcination step at 600 °C. Incipient wetness impregnation was then performed in the following manner. The support was doubly impregnated, firstly with a certain amount of Pt (as tetramine platinum (II) nitrate)

Textural properties of catalysts

Textural properties and catalyst compositions are illustrated in Table 1. When commercial versus home-made catalysts are compared, the commercial based catalysts always have an enhanced surface area relative to the home based. When W versus Mo promoted catalysts are compared, it can be seen that Mo always has a larger surface area relative to W. Machida and co-workers [23] concluded from studies on Pt–M, co-exchanged hydrotalcite systems, M = W or Mo, that lower surface areas are a consequence of

Conclusion

Pt–W and Pt–Mo catalysts were evaluated for methane combustion using microreactor technology. W promoted catalysts show the most promise (of the two) for the catalytic combustion of methane. It is likely that the activity is a consequence of metal interaction effects between Pt° and W rather than the classical Pt°–Al2O3 interaction. For Pt–Mo catalysts, the extent of Pt dispersion appears to be the crucial factor. However, a detailed characterization study needs to be performed on both the sets

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