H2 oxidation as criterion for PrOx catalyst selection: Examples based on Au–CoOx-supported systems

doi:10.1016/j.jcat.2015.03.015

Journal of Catalysis

Volume 326, June 2015, Pages 161-171

https://doi.org/10.1016/j.jcat.2015.03.015 Get rights and content

Highlights

•
A new approach for understanding PrOx reaction over gold- and cobalt-based catalysts.
•
Splitting CO and H₂ oxidation to study the selectivity of the CO-PrOx reaction.
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Emphasizing the fundamental role of the support in the H₂ combustion and its effect in the overall selectivity.
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Fine tune of the oxygen storage capacity of the catalysts to achieve the best activity/selectivity balance.

Abstract

A new approach for understanding PrOx reaction over gold catalysts is proposed in this work. The competition between H₂ and CO oxidation has been studied over a series of Au/MO_x/Al₂O₃ (M = Ce and Co) catalysts in simulated post-reforming gas stream, containing H₂O and CO₂ for H₂ cleanup goals. The catalysts’ behavior is correlated to their oxygen storage capacity, redox behavior, and oxidation ability. The estimation of the reaction rates reveals that in these solids the H₂ combustion, the selectivity limiting factor in the PrOx process, is mainly controlled by the support and not by the gold presence. The possible use of the hydrogen oxidation reaction as a catalyst selection criterion is discussed.

Graphical abstract

Introduction

One of the most important issues related to the hydrogen technology is the serious difficulty encountered for onboard H₂ storage as well as for distribution from a centralized production facility [1]. As a result, many efforts have been focused on the conversion of more readily available fuels to hydrogen, either for on-board or for stationary applications. Hydrogen is mainly produced by hydrocarbons- or alcohols-reforming reactions [2] in which the hydrogen stream is often accompanied by relatively high levels of carbon oxides and steam [1], [3].

An essential requisite for fueling the polyelectrolyte membrane fuel cells (PEMFC) with hydrogen is the absence of CO or at least its presence within the ultra-low trace levels, preferably less than 10 ppm. Therefore, CO abatement processes must be applied to reduce its level within the H₂ stream. The CO purification processes are commonly composed by a combination of high- and low-temperature water–gas shift reactions, allowing CO decrease to 1–2 vol.% level [4], [5], [6] and the preferential oxidation with air of the pre-cleaned reformate (PrOx) to reduce the CO to ultra-low level. The PrOx reaction is often preferred as a cheap and effective solution for the final purification step, since the range of working temperatures matches that of the PEM fuel cells operation [7].

For PrOx reaction, the gold-based catalysts are potentially advantageous [8], [9]. Earlier studies on Au/ceria catalysts found high activity and good selectivity of those systems in the 70–120 °C temperature range, caused by the active participation of ceria in the oxidation process governed by gold [10], [11], [12]. It is often reported that the presence of structural defects in the ceria lattice, for example, oxygen vacancies, potentiates the activity of the gold-based catalyst in the CO oxidation reaction [9], [13]. As a common strategy, the inclusion of aliovalent cations in the ceria lattice is employed to increase the number of those defects [13], [14], [15], [16]. The CO oxidation activity is then directly related to the concentration of those defects in such a way that higher the number of defects, greater the oxygen mobility and higher the oxidation activity.

However, those systems, together with the high efficiency in the CO oxidation, in terms of activity show limited selectivity due to their ability to burn H₂ with the same facility. As a result, all future applications of those systems are questioned more by the selectivity than by its activity. Then it is essential to study the selectivity of the PrOx gold-based catalysts with the consideration of the H₂ oxidation, as a separate process. Several authors have studied H₂ oxidation and also the effect of H₂ presence on the selective CO oxidation on gold catalysts with contradictory results [17], [18], [19], [20]. An enhancement of the CO oxidation activity in the presence of hydrogen was found in some studies [17], [19], and negative effect was reported by others [18], [20]. In addition, numerous reports comparing the PROX activity and selectivity of a variety of supported gold catalysts in dry and post-reformate conditions could be found in the literature [21], [22], [23], [24]. Notwithstanding the valuable information that can be extracted from these works, the influence of the realistic reforming streams, for example, the presence of H₂O and CO₂ and its effect on the hydrogen combustion, was obviated. This major topic and some other questions, such as, which is the role of the support in the H oxidation? Is there a support-dependent hydrogen effect on the CO oxidation? Are the gold nanoparticles the only responsible for the hydrogen oxidation? remain unclear.

In order to address these issues, the main goal of this work was to study the hydrogen combustion and PrOx activity in presence of CO₂ and H₂O over a series of gold–cobalt-based catalysts. The H₂ oxidation has been discussed as a criterion for providing clues focused toward proper catalyst design and selection.

Section snippets

Support preparation

The supports were synthesized by a conventional impregnation method. The necessary amounts of metal precursor (cobalt and/or cerium nitrate, Aldrich) were impregnated on γ-alumina powder (Sasol). The impregnation was carried out in 50 mL of ethanol in rotary evaporator at reduced pressure and temperature of 50 °C. The obtained dry solid was treated with NH₃ solution (10 mol L⁻¹) during 30 min in order to assure the full conversion of the nitrate precursors to hydroxides. The supports were then

Results and discussion

Careful analysis of the catalyst properties often starts with an exhaustive analysis of its electronic and redox properties. Regarding the complexity of the series of catalysts chosen in this study, a separation between bi- and tri-component catalysts should be considered for better description of the systems. In all cases, the alumina component will be neglected pointing to its principal role of inert metal oxide used for “active oxide component” (CeO₂, CoO_x_, or mixed) dispersion and

Conclusions

A series of effective CO-PrOx catalysts based on Au/CeO₂/Al₂O₃ formulation have been developed. The inclusion of small amounts of cobalt oxide on the catalyst formula appreciably improves the CO-PrOx oxidation activity in a real post-reforming stream, being the Ce–Co mixed samples the most performing ones. The Ce–Co contact results in satisfactory electronic and redox properties and especially enhanced OSC that benefits the catalytic activity.

The combustion of H₂ mainly occurs on the support.

Acknowledgments

T.R. Reina acknowledges CSIC for his JAE-Predoc fellowship. The Spanish Ministerio de Ciencia e Innovación project (ENE2012-374301-C03-01) and Junta de Andalucía project (TEP-8196) provide financial support for this work, both programs being co-funded by the European Union FEDER. Dr. J.J. Delgado is also kindly acknowledged for the TEM experiments and Sasol for providing the γ-Al₂O₃.

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¹: Departamento de Química, Facultad de Ciencias, Pontificia Universidad Javeriana, Bogotá D.C., Colombia.

View full text

H2 oxidation as criterion for PrOx catalyst selection: Examples based on Au–CoOx-supported systems

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Support preparation

Results and discussion

Conclusions

Acknowledgments

Appl. Catal. B: Environ.

J. Power Sources

Catal. Today

Catal. Commun.

Catal. Today

Int. J. Hydrogen Energy

Catal. Today

App. Catal. B: Environ.

Appl. Catal. A

Catal. Today

J. Catal.

J. Catal.

J. Mol. Catal. A

J. Catal.

J. Catal.

Catal. Today

Catal. Today

J. Catal.

J. Catal.

Appl. Catal. A: Gen.

Appl. Catal. A: Gen.

J. Solid State Chem.

H₂ oxidation as criterion for PrOx catalyst selection: Examples based on Au–CoO_x-supported systems