Preferential CO oxidation over Cu/CeO2−x catalyst: Internal mass transport limitation

doi:10.1016/j.cej.2011.08.081

Chemical Engineering Journal

Volumes 176–177, 1 December 2011, Pages 165-171

https://doi.org/10.1016/j.cej.2011.08.081 Get rights and content

Abstract

The effect of internal mass transport limitation on the preferential CO oxidation in hydrogen-rich mixture over copper-cerium oxide catalyst in a form of pellets and washcoat in microchannel reactor is estimated. Internal effectiveness factor η_СО >0.8 in the optimum interval of reaction temperature (170–230 °C) is reached if the pellet diameter and washcoat thickness do not exceed 100 and 20 μm, respectively. Compared to conventional packed-bed reactor with catalyst pellets, microchannel catalytic washcoated reactor is more appropriate for practical use.

Highlights

► The internal mass transfer limitation could decrease the CO PrOx selectivity of Cu/CeO₂ catalyst. ► The pellet diameter and coating thickness should not exceed 100 and 20 μm for high effectiveness factor. ► Microchannel catalytic washcoated reactor is more appropriate for CO PrOx than conventional packed-bed reactor.

Introduction

Approaches to increase efficiency of CO removal from hydrogen rich gas mixtures have been attracting considerable attention for a long time. In recent years, the interest to this problem became even stronger in view of hydrogen-rich mixtures application for feeding proton-exchange membrane fuel cells (PEM FC). One of the promising methods of hydrogen production for PEM FC applications is the on-board multi-stage process including reforming of fuels such as hydrocarbons, alcohols or ethers, followed by the CO water–gas shift (WGS) reaction. Typically, the obtained hydrogen-rich gas contains 0.5–1.0 vol.% CO, in some applications (for example, after methanol or DME steam reforming) the low level of the outlet CO concentration (up to 2–3 vol.%) can be directly attained without WGS stage. Since carbon monoxide poisons PEM FC anode catalyst, its concentration in the hydrogen-rich gas mixture must be reduced to at least 100 ppm, or still better to 10 ppm [1], [2], [3], [4], [5].

Among the approaches investigated to remove the trace amount of CO in H₂-rich stream [1], [2], [3], [4], [5], the CO preferential oxidation (CO PrOx) has been considered to be suitable for sufficient CO removal. CO PrOx includes two reactions:2CO + O₂ = 2CO₂2H₂ + O₂ = 2H₂O

Hydrogen oxidation via reaction (2) spends a part of PEM FC fuel and thus decreases the process efficiency; the contribution of this reaction must be minimized.

Supported Au, Pt, Ru, Rh, Co, and Cu catalysts demonstrated good performance for the CO PrOx reaction [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31]. Among them, copper-cerium oxide systems appeared sufficiently active, most selective [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29] and less expensive that the catalysts based on Au and Pt group metals and therefore seem to be quite promising for practical applications.

Although considerable progress has been made towards the development of active and selective catalysts, the removal of CO to a level below 10 ppm and the simultaneous minimization of hydrogen losses are achievable only in a narrow temperature interval. For example, the suitable temperature window on platinum, gold or copper-cerium oxide based catalysts is usually less than 40 °C.

Most CO PrOx studies have been based on packed-bed reactors (PBR) with diameters lager than 3 mm, which, because of the large heat generation from two oxidation reactions (see Eqs. (1), (2)), are susceptible to mass and heat-transport limitations. Therefore, thermal and mass transport management is a key issue for practical PrOx reactor designs and efficient reactor operation in terms of highly selective CO reduction to levels acceptable for PEM FC application.

According to Kolb et al. [32], CO PrOx in microchannel reactors is the most appropriate technology for designing of compact fuel processors for portable PEM FC applications. Advantages of microchannel reactors for running highly exothermic CO PrOx reaction over conventional PBRs were demonstrated in a number of reports [28], [29], [30], [31], [33]. In a microreactor, the catalyst layer is deposited inside microchannel as a thin wall coating. Such a structure allows the achievement of high heat- and mass-transfer rates and nearly isothermal operation regime that helps to prevent side reactions (hydrogen oxidation, reverse water–gas shift, CO methanation) and provides the required level of CO concentration in a wide temperature interval [29], [30], [31].

Thin catalyst coatings help also to minimize internal diffusion resistance. According to [31], the effect of internal mass-transport limitation on preferential oxidation of CO in microchannel reactor with 2–5 μm thick, Pt/γ-Al₂O₃ catalyst coating is negligible. However, even for small Pt/γ-Al₂O₃ particles (360 μm diameter) the internal efficiency factor reduced to 0.5 at 250 °C [9]. Obviously, the use of ∼5 μm catalyst particles in a packed-bed reactor is hardly possibly because of high pressure drop, while the use of “crusted” catalysts will lead to inevitable reactor enlargement.

In general, one can agree with the conclusions made in report [33], which presents comparative analysis of system designs of the PrOx device (with Pt-Ru/α-Al₂O₃ catalyst) for conventional packed-bed reactor technology and microreactor technology on 0.1 and 5 kW_e power output scales. It was proved that оn both power output scales studied, the microreactor designs outperform the conventional designs, leading to lower reactor volumes and weights. However, the scaling factors of the reactor volume and weight were larger for the microreactor systems as for the conventional systems, indicating that at larger scales packed-bed reactors will ultimately outperform the microreactor designs.

Thus, to make valid choice between the conventional packed-bed reactor technology and microreactor technology, we should know their optimum operation parameters to provide the best process efficiency. In particular, it is necessary to determine the optimum catalyst pellet diameter and catalytic coating thickness that allow minimized mass-transfer limitations. The solving of this complex problem depends on a number of factors such as catalysts nature, morphology and texture, target and side reactions, etc. To the best of our knowledge, there are no reports in the literature on detailed studies of transport properties of microreactors and conventional packed-bed reactors with copper-cerium oxide catalyst for CO PrOx.

In our earlier work [29], the effect of internal mass-transfer limitations on CO PrOx over copper-cerium oxide catalysts was estimated using the Thiele–Zeldovich modulus. It was shown that the adverse effect of internal diffusion may appear at temperatures of ∼200 °C for coating thickness (or pellet radius) of ∼100 μm. More detail calculations require the knowledge on the catalyst texture and reaction kinetics.

In the present work, the data on the porous structure of copper-cerium oxide catalyst and reaction kinetics were used to study comparatively how internal mass transport limitations affect the performance of CO PrOx reaction both in thin catalytic coatings inside microchannel reactor, and in catalyst pellets. The study aims at the CO PrOx process optimization with respect to both conventional packed-bed reactor and microreactor technologies.

Section snippets

Catalyst preparation

The 5 wt.% Cu/CeO_2−x coatings were prepared as described elsewhere [28], [29]. Stainless steel platelets were manufactured using a commercial photoetching technique based on wet chemical etching with an aqueous iron trichloride solution. The platelets each having 14 semicylindrical channels of 500 μm width, 250 μm depth, and 25 mm length were first cleaned with isopropanol for 10 min in an ultrasonic bath and after drying calcined at 800 °C for 2 h. This treatment presumably generated a thin oxide

Experiments with catalyst pellets

To illustrate quite strong effect of mass transfer limitation on CO PrOx reaction, Fig. 1 presents the temperature dependencies of CO conversion for two CuCe(I) catalyst fractions. In both cases, the CO conversion rapidly increases with increasing temperature, but at temperatures above 70 °C the difference in the CO conversion curves is observed that indicates increasing effect of mass transfer limitations in larger pellets. This experiment represents only a qualitative proof of the effect of

Conclusions

The effect of internal mass transport limitation on the preferential CO oxidation in hydrogen-rich mixture over 5 wt.% Cu/CeO_2−x catalyst in a form of pellets and washcoat in microchannel reactor is estimated. It is shown that the internal effectiveness factor η_СО >0.8 in the optimum temperature interval 170–230 °C is reached if the pellet diameter and coating thickness do not exceed 100 and 20 μm, respectively. Practical use of so small particles on a larger scale is hardly possible because of

Acknowledgements

The authors are grateful to Dr. M.S. Melgunov, Prof. V.B. Fenelonov, Mr. A.V. Ishchenko, Dr. S.I. Reshetnikov for help in running some experiments and fruitful discussions.

The work was partially supported by RF President Grant 02.120.11.5602-MK, Government Contract No. P1015 of 27.05.10 in the framework of Federal Targeted Program: “Scientific and Research-Educational Staff for Innovative Russia 2009–2013”, and GK No. 16.516.11.6006.

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Preferential CO oxidation over Cu/CeO2−x catalyst: Internal mass transport limitation

Abstract

Highlights

Introduction

Section snippets

Catalyst preparation

Experiments with catalyst pellets

Conclusions

Acknowledgements

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J. Mol. Catal. A: Chem.

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Preferential CO oxidation over Cu/CeO_2−x catalyst: Internal mass transport limitation