Elsevier

Journal of Catalysis

Volume 229, Issue 2, 25 January 2005, Pages 446-458
Journal of Catalysis

Formation of subsurface oxygen species and its high activity toward CO oxidation over silver catalysts

https://doi.org/10.1016/j.jcat.2004.11.043Get rights and content

Abstract

Silver is well known to show peculiar catalytic activities in several oxidation reactions. In the present paper, we investigate the catalytic activity of silver catalysts toward CO-selective oxidation in H2. XRD, TEM, TPD, and in situ FTIR techniques were used to characterize the catalysts. The pretreatment of the catalysts was found to have great influence on their performance. The pretreatment in O2 improves the activity of the silver catalyst, whereas He pretreatment at 700 °C or direct hydrogen pretreatment shows an inverse effect. Silver catalysts undergo massive structural change during oxygen pretreatment at high temperatures (>500°C), and there is solid evidence for the formation of subsurface oxygen species. The existence of this silver-subsurface oxygen structure facilitates the formation of active sites on silver catalysts for CO oxidation, which are related to the size, morphology, and exposed crystal planes of the silver particles. Its formation requires a certain temperature, and a higher pretreatment temperature with oxygen is required for the silver catalyst with a smaller particle size. It is observed, for the first time, that adsorbed CO on the surface of silver particles can directly react with subsurface oxygen species at low temperatures (e.g., RT), and the surface oxygen can migrate into and refill these subsurface sites after the consumption of subsurface oxygen by the reaction with CO. This finding provides a new reaction pathway for CO oxidation on silver catalyst.

Introduction

Research interest in the catalytic oxidation of carbon monoxide has surged because of the possible uses for clean air, orbiting, closed-cycle CO2 lasers, and other remote sensing applications [1], [2], [3]. Recently, the preferential oxidation of CO in H2 has been studied for applications in polymer electrolyte-type fuel cells (PEFCs) to reduce the CO concentration in the produced fuel gases down to 10 ppm to prevent the fuel cell electrodes from being poisoned [4], [5]. The outstanding catalytic activities of Pt, Rh, and Au are widely recognized [6], [7], [8], [9], [10], [11]. A high activity for CO oxidation at higher reaction temperatures (150–250 °C) can be obtained on Pt catalysts; however, the competitive adsorption of CO and O2 decreases the low-temperature activity of the catalysts [8], [10]. The reaction mechanism has also been extensively investigated over these catalysts [10], [11]. For instance, a dominant reaction pathway proposed for Au catalysts supported on reducible transition metal oxides involved the adsorption of a mobile, molecular oxygen species on the support, dissociation at the interface (or lattice oxygen, which is produced by the adsorption of oxygen followed by dissociation immediately on the support), and reaction with CO adsorbed on the gold at the interface and/or on the gold particles after oxygen spillover onto the gold metal [9], [10].

However, silver has rarely been considered as the catalyst for CO-selective oxidation, although it has been recognized to show a high activity in several partial oxidation reactions, such as ethylene epoxidation, formaldehyde synthesis, etc. [12], [13]. Only relatively low activity of CO oxidation was observed over silver catalysts, and usually Ag2O was considered to be the active species for CO oxidation. It is known that Ag2O consumed by the reaction with CO is difficult to re-oxidize, and only the addition of a second active compound ((1): Co or Mn) or the use of reactive support (2) can increase the activity of the silver catalyst by the oxygen spillover from the second compound or reactive support onto the silver catalyst, as shown in Scheme 1 [14], [15], [16].

The interactions of silver with oxygen (e.g., the mechanism of oxygen activation, oxygen-induced reconstruction, and the incorporation of oxygen in the silver bulk) have been extensively studied in order to understand the catalytic behavior of silver catalysts. Various oxygen species were found on silver catalyst, for instance, the molecular, subsurface, and various forms of surface atomic oxygen [17], [18], [19]. The formation of subsurface oxygen (Oγ) was considered necessary for the activation of the silver catalysts for ethylene epoxidation and formaldehyde synthesis. Surface-bound atomic oxygen (Oα) preferentially led to the formation of complete oxidation products. The increase of the ratio of Oγ/Oα on the silver surface would result in an increase in direct dehydrogenation via the oxi-dehydrogenation pathway [20], [21]. More recent studies have shown that the Agsingle bondO interaction involving the formation of subsurface oxygen species exerts an important influence on the surface structure and, eventually, the catalytic properties of silver catalysts [22], [23], [24].

In this paper, we study the performance of Ag/SiO2 catalysts for the selective oxidation of CO in a gas mixture (1% CO, 0.5% O2, H2 balance) simulating an effluent of the fuel processing and the influence of the pretreatment conditions on the catalytic activity. The formation of subsurface oxygen is found to be critical for enhancing the activity of the silver catalyst toward CO-selective oxidation.

Section snippets

Experimental

We prepared the Ag/SiO2 catalysts by impregnating SiO2 granules (20–40 mesh, 495 m2/g) with an aqueous solution of silver nitrate, followed by drying at 120 °C for 12 h. The Ag/SiO2 catalysts were pretreated under different atmospheres at various temperatures before testing.

X-ray diffraction (XRD) measurements were made with a Rigaku D/max-rb X-ray diffractometer with a Cu-Kα X-ray source operating at 40 kV and 100 mA. The average crystallite size of silver was calculated with the Scherrer

Activity test

During the course of our experiments, we find that the silver catalyst deactivates significantly after a direct pretreatment with H2 at 500 °C for 2 h, whereas the treatment with oxygen (>400°C) (re)activates the catalyst, as shown in Fig. 2A. It can be seen from the TEM image (Fig. 2B) that silver particles do not sinter after H2 pretreatment. It has been known that the thermal stability of the silver particles is significantly lower under oxidizing conditions, and heating at higher

Conclusions

The catalytic activity for CO-selective oxidation over Ag/SiO2 is reported, and details of the effect of pretreatment on the structure and chemisorption properties of the silver catalyst have been clarified by XRD, TEM, TPD, FTIR and other methods.

A high activity for CO selective oxidation in H2 is obtained after the silver catalyst is pretreated with oxygen at high temperatures (>500°C), whereas a distinct reduction of the catalytic activity is observed after pretreatment with He at 700 °C or

Acknowledgments

This work was supported by the Chinese Ministry of Science and Technology and the Natural Science Foundation of China. We gratefully acknowledge Prof. Can Li and MSc Weicheng Wu (SKLC, DICP) for helpful discussions on the IR experiments.

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  • Cited by (0)

    1

    Present address: Institute of Physical Chemistry, University of Stuttgart, 70569 Stuttgart, Germany.

    2

    Present address: Department of Chemical Physics, University of Science and Technology of China, Hefei, 230026, China.

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