Selective Photocatalytic Conversion of Methane into Carbon Monoxide over Zinc-Heteropolyacid-Titania Nanocomposites

Selective photocatalytic conversion of methane into carbon monoxide over zinc-heteropolyacid-titania nanocomposites

Xiang Yu, Vitaly Ordomsky, Andrei Khodakov

 Univ. Lille, CNRS, Centrale Lille, ENSCL, Univ. Artois, UMR 8181 - UCCS - Unité de Catalyse et Chimie du Solide, F-59000 Lille, France

In recent years, methane has become increasingly abundant due to the development of shale gas fields and other cost efficient feedstocks. Methane is also considered as one of the greenhouse gases with a global warming potential 50 times higher than carbon dioxide [1]. Methane activation is therefore a formidable challenge for catalysis [2,3]. High reaction temperatures (>700°C), low selectivity to the target products and often high CO₂ production are major drawbacks of the conventional thermocatalytic approach.

Photocatalysis, which uses sunlight, has been shown very promising for water decomposition and environmental remediation. Photocatalysis has been also considered as one of pathways to break the thermodynamic barrier [4]. Only a limited number of examples of methane photocatalytic conversion have been available in the literature. Carbon monoxide is a very important compound and a building block in chemical industry. It is utilized as a feedstock in the production of chemicals ranging from acetic acid to polycarbonates and polyurethane intermediates. Syngas, which is a mixture of carbon monoxide and hydrogen, is a valuable feedstock for manufacturing methanol, hydrocarbon fuels, numerous oxo-alcohols and aldehydes. CO is also an important reducing agent and it is used for manufacturing pure metals and in particular iron, cobalt and nickel.

Chemical utilization of vast fossil and renewable feedstocks of methane remains one the most important challenges of modern chemistry. Herein, we report direct selective photocatalytic conversion of methane into carbon monoxide under ambient conditions with the selectivity to CO higher than 80% and only marginal CO₂ production: 2CH₄ + 3O₂ → 2CO + 4H₂O

A series of catalysts were developed on the basis of metals, H₃PW₁₂O₄₀ heteropolyacids (HPW) and TiO₂ (P25). The Zn-HPW/TiO₂ system exhibited exceptional photocatalytic activity in selective carbon monoxide production from methane (Figure 1). In the composite catalysts tungstophosphoric acid constitutes a thin layer of 1-2 nm over titanium oxide, while zinc species which are involved in methane photocatalytic oxidation, are highly dispersed on the tungstophosphoric layer. Zinc species seem to play an important role in methane activation [5,6]. The role of the TiO₂ semiconductor is primarily assigned to the absorption of light with generation of electron and hole pairs. The electron and hole pairs can then migrate through HPW, which also has properties of semiconductor, to the zinc species, which are active in methane selective photooxidation. Another important role of HPW is relevant to enhancing Zn dispersion and mobility in the catalysts. Importantly, methane activation and reaction are carried out at ambient temperature. High carbon monoxide yields (up to 3-4% in a single batch experiment) and extended catalyst stability achieved in this process make it practical for industrial applications. To the best of our knowledge, the present work presents the first example of utilizing photocatalysis for methane selective oxidation into carbon monoxide.

The conducted in-situ investigation of the reaction mechanism is indicative of the zinc reduction by methane with important modifications of the catalyst structure. Exposure to oxygen leads to subsequent regeneration of the composite catalyst according to Mars-Van Krevelen mechanism [6,7]. And we found that the process involves formation of the surface methoxy-carbonate as reaction intermediate (Figure 2).

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