CO selective oxidation using Co-promoted Pt/γ-Al2O3 catalysts

https://doi.org/10.1016/j.ijhydene.2016.08.170Get rights and content

Highlights

  • Activity of the PROX reaction reaches its maximum with a Co/Pt atomic ratio of 1.5.

  • The PtCo bimetallic catalysts presented a slight deactivation after several hours on stream.

  • The original activity is recovered by submitting the catalysts to a reduction process at 500 °C.

Abstract

In this work, alumina-supported platinum catalysts promoted with cobalt were analyzed in the preferential CO oxidation (PROX) reaction. The addition of Co represents a significant catalytic improvement with regard to the monometallic Pt catalyst. This improvement reaches its maximum with a Co/Pt atomic ratio of 1.5. The addition of a higher level of cobalt decreases the activity probably because it would produce the coverage of the Pt active sites, thus inhibiting the adsorption of CO. For all the bimetallic catalysts, the maximum conversion occurs at ca. 130 °C, a value which is within the acceptable working range for this process. A further temperature increase generates a decrease in the CO conversion. The studied catalysts presented a slight deactivation after several hours on stream. The original activity is recovered by submitting the catalysts to a reduction process at 500 °C. Results are explained in terms of TPR, DRS, EXAFS and XANES characterization of the catalysts.

Introduction

A variety of fuel cells have been developed having a wide field of application. For power generation both in mobile and stationary sources, proton exchange membrane fuel cells (PEMFC) appear as the most important. These cells produce electricity through an electrochemical reaction, so that their efficiency is not limited by the Carnot cycle. The anode of the cell is made of platinum, using hydrogen as fuel. They have advantages such as high efficiency, low operating temperature, quick start and mainly, they do not emit pollutants to the environment [1]. However, these same advantages are diminished when considering the fuel supply: pure hydrogen presents problems with its carriage and storage [2], besides, it has low energy per unit volume compared to other gaseous fuels and a high power is required to achieve its compression. A feasible alternative is to produce in situ the hydrogen to feed the cells.

Hydrogen can be generated through different processes: reforming, Water Gas Shift Reaction (WGSR), Kvaerner Process, coal gasification and fermentation of organic compounds. The most widely used method of production of hydrogen is the reforming, which generates a gas mixture (syngas) composed of variable amounts of carbon monoxide and hydrogen. The CO present in this stream must be eliminated because, when it is fed together with hydrogen to the fuel cell, is preferentially adsorbed on the platinum anode preventing the hydrogen oxidation reaction to occur [3]. According to current standards, the maximum amount of CO present in the feeding of PEMFC should be between 10 and 100 ppm [4]. The reforming of natural gas, methanol or gasoline generates a gas stream containing between 5 and 9% of CO [5]. After the reforming process, the gas mixture obtained is submitted to a two-steps WGSR: a first step at high temperature (HTS) between 350 and 600 °C, and a second step at low temperature (LTS), from 150 to 300 °C [6]. This reaction (Eq. (1)) allows to further reduce the content of CO to values between 0.2 and 2%.CO+H2OCO2+H2ΔH298=kJmol

The WGSR is effective for the conversion of CO to CO2, but because of its moderate exothermicity, is thermodynamically unfavorable at high temperature. An increase in temperature causes the conversion of CO to decrease [6].

There are several methods to further reduce the content of CO in H2-rich streams and achieve a mixture that could be fed to a PEM fuel cell, including: catalytic methanation of CO, selective diffusion using Pd membranes, cryogenic methods, adsorption through pressure changes and PROX. Selective diffusion, methanation and PROX are the most promising methods [4]. The preferential oxidation has as main advantage its low cost and simple implementation without involving significant loss of hydrogen [7].

The PROX reaction is a gas phase catalytic reaction through which CO is transformed into CO2, a product that does not interfere with the normal operation of the PEMFC. The PROX reaction is represented by Eq. (2):CO+12O2CO2

At the same time, the oxidation of hydrogen (Eq. (3)) must be avoided:H2+12O2H2O

At high temperatures undesired reactions may occur: Reverse Water Gas Shift Reaction and CO methanation. These reactions should be avoided because they consume one and three moles of H2 per mole of CO, respectively, representing a significant loss of H2 (Eqs. (4), (5)) [3]:CO2+H2CO+H2OCO+3H2CH4+2H2O

The main requirements to be fulfilled by the catalysts used in the PROX reaction are: high activity at low temperature, achieving conversions higher than 99%, high selectivity to the CO oxidation reaction and high resistance to deactivation caused by the presence of CO2 and H2O in the feed stream. Besides, the high activity should be obtained at temperatures between the operating temperature of the low temperature WGSR (150°C-300 °C) and that of the PEMFC (80°C-100 °C) [7], [8].

In the early 1960, the U.S. Company Engelhard Corporation, now belonging to BASF, developed and commercialized the first catalyst for the removal of CO used in the ammonia synthesis. This catalyst was supported on alumina and contained between 0.1 and 0.3 wt.% of platinum [9]. Currently, the catalysts used for the PROX reaction can be separated into three groups: gold-based catalysts, those containing other precious noble metals and those based on transition metal oxides. Although Au catalysts show higher activity and selectivity than Pt catalysts, especially in the low temperature range, they suffer deactivation due to sintering during PROX reaction. Another drawback of gold catalysts is that at high water content, the CO oxidation reaction is inhibited. Concerning transition metal oxide catalysts, their catalytic activity is usually superior to that of the noble metal catalysts at low temperature, but is necessary to improve their stability in the presence of H2O [3].

Platinum containing catalysts are the systems which have been more extensively studied. In 1963, Cohn proposed that alumina-supported Pt catalysts were an effective system for the conversion of CO in excess hydrogen. According to literature, this catalyst exhibited its maximum conversion at temperatures close to 200 °C, with oxygen concentrations just above the value given by the stoichiometry of the reaction and without formation of methane. Zeolitic Pt catalysts showed that they were able to oxidize CO more selectively than alumina-supported platinum catalyst, but it was necessary to add an excess of O2, with values of λ near 2, λ being the oxygen excess factor. In the presence of H2O and CO2, the following series of activity is observed for the selective oxidation of CO with Pt catalysts: Zeolite A > Mordenite > Zeolite X > γ Al2O3. The most interesting catalytic systems for the PROX reaction are those that combine the noble metal with some easily reducible second metal [10]. When metal promoters are added to the Pt/Al2O3 catalyst (either as a second metal or as an oxide), a significant increase is observed in the conversion of CO to CO2. Among the promoters that have been used to improve the activity of platinum-supported catalysts it can be mentioned the following: alkali metals, iron, manganese, nickel, niobium and cobalt. In our research group, Sn and Ge-modified bimetallic catalysts have been prepared through surface-controlled reactions and revealed higher catalytic activity compared to the conventional monometallic Pt catalysts [11], [12].

Among transition metals, cobalt has been described as the most effective promoter for the PROX reaction. In that sense, cobalt-modified platinum catalysts have shown good activity in this reaction due to the synergistic effect between cobalt and platinum.

In this investigation, it was studied the preferential CO oxidation in excess H2 over alumina-supported Co modified Pt catalytic systems. We examined the influence of the Co/Pt ratio on the catalytic performance, in order to find a catalyst that achieved a CO concentration in the outlet gas that were within the acceptable limits for feeding a PEMFC. In addition, a detailed characterization of the catalysts was carried out. Besides, in the present paper, we also report on the stability of the catalysts, demonstrating that they present a more than acceptable CO conversion in a realistic condition over a considerable period of time.

Section snippets

Catalyst preparation

A commercial γ-Al2O3 (Air Products) with a pore volume of 0.63 cm3/g and a surface area of 252 m2/g, was used as support. The solid was crushed and sieved to obtain particles in the range 60–100 mesh and before being used, it was submitted to a calcination process in air at 500 °C for 4 h.

Platinum was added to the support by ion exchange using an aqueous solution of hexachloroplatinic acid (H2PtCl6, Sigma Aldrich) of concentration such as to provide the catalysts a Pt loading of 1wt. %. The

Catalyst characterization

The XRD spectra of the calcined support and of 4Co and 4Co1Pt catalysts were recorded, and are shown in Fig. 1. The diffraction patterns consisted mostly of broad peaks of poorly crystallized γ-Al2O3. Even though the samples with the highest Co loadings were analyzed, cobalt signals could not be detected. Also, no diffraction peaks were observed for Pt in the 4Co1Pt sample. These results suggest that Pt and Co particles are either amorphous or are too small to be detected by XRD methods.

Conclusions

By adding cobalt to a Pt/γ-Al2O3 catalyst, a significant improvement in CO conversion in the PROX reaction is obtained. This improvement could be assigned to the existence of a strong interaction between cobalt and platinum, decreasing in this way the Pt-support interaction. Most likely a new Pt-Co phase with lower reduction temperature and high synergy is formed. The catalytic performance reaches its maximum value for a Co/Pt atomic ratio of 1.5. The addition of Co above this value causes a

Acknowledgements

This work was supported by Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) (PIP 0276 and PIP 1035), Universidad Nacional de La Plata (Projects X700 and I152) and Laboratório Nacional de Luz Síncrotron (LNLS, Brasil) (Project XAFS1-18859).

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