Photo-catalytic reduction of carbon dioxide to methane using TiO2 as suspension in water

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Abstract

Photo-catalytic reduction of CO2 to methane was studied in CO2-saturated aqueous solution in presence of TiO2 photo-catalyst (0.1%, w/v) as a suspension using 350 nm light. CO2 methanation rate was very much enhanced in the presence of 2-propanol as a hole scavenger. In addition to CO2 reduction, photo-catalytic reduction of methanol in N2-purged system was also tried but no methane was generated in presence of TiO2 without 2-propanol. The yield of methane was quite low even in presence of 2-propanol in this system. In the methanol photolysis CO2 was found to be the main product with a yield of 60×10−8mol to 200×10−8mol. This shows that the generation of CH4 from CO2 does not proceed via methanol as an intermediate under these conditions. In the aerated system also, methane was produced during photolysis of TiO2 suspension in presence of 2-propanol and its yield was comparable to that in CO2-saturated system. This suggests that surface-adsorbed as well as in situ generated CO2 are equally responsible for methane formation through photo-reduction in presence of TiO2. In the aerated system, 45 μmol CO2 was produced by photo-degradation of 2-propanol. In O2-saturated system, the methane yield was lower as compared to that in aerated system whereas CO2 yield was higher. Overall, the yield of methane was quite high if extrapolated to per gram of TiO2.

Introduction

Use of solar energy in solving environmental problems has become very important [1], [2], [3], [4], [5], [6], [7]. Insecticides, herbicides and pesticides used in farming with subsequent run-off to catchments areas lead to organic contamination of soil and water. Other common examples of pollution are accidental leakages and spills and continuous discharge of waste by-products in effluent streams from essential industries. This water eventually contaminates ground and surface water, which is the primary source of human contact with toxic chemicals. In order to tackle this problem, extensive researches are being carried out to develop advanced physicochemical methodologies for the elimination of hazardous chemicals from air, soil and water. Purification procedures based on photochemical reactions are potentially useful for this purpose. Photochemical means to destroy organic pollutants through oxidation as well as to convert them to useful material by means of reduction are the best ways to tackle this problem. The use of sunlight to convert potentially harmful organic chemicals to harmless mineral products or useful organic chemicals at accelerated rates by the use of a comparatively cheap and non-toxic photo-catalyst is an attractive alternative and is aimed at restoration of the environment. Various photo-catalysts such as TiO2, ZnO, WO3, CdS, ZnS, SnO2 and Fe3O4 have been tried independently or in the mixed or metal-doped forms for this purpose [4], [5]. The initial step in TiO2 photo-catalysed oxidation is believed to be the formation of both hydroxyl radicals and super oxide radical anions [2]:TiO2e+h+h++H2OOH+H+In presence of air,e+O2O2Thus two species, OH (E°=2.72V) [8] a strong oxidant and O2 (E°=−0.33V) [9] a weak reductant, are generated from the above photo-catalysed oxidation. In the absence of oxygen, OH radical, a strong oxidant, and e, a strong reductant, are the reactive species generated on photolysis of TiO2 in aqueous medium [10], [11], [12]. In the presence of an OH or an h+ scavenger, it is possible to use TiO2 for photo-reduction processes.

Conversion of CO2 into methane or other organic substances is of importance not only for the development of alternative fuel sources, but also for the prevention of rise in temperature due to green house effect. Photo-catalytic reduction of CO2 using photosensitive semiconductor powders has been studied in aqueous solutions [13], liquid CO2 [14] and high pressure CO2 [15]. Electrochemical reduction of CO2 has also been extensively studied using various kinds of metals as cathode, producing CO or formic acid as the main product depending on the kind of cathode materials [16], [17] and the route for the electrochemical reduction reaction has been established. Kaneco et al. [15] have reported that CO2 reduction does not take place in the absence of electron donors. They tried 2-propanol as a hole scavenger during photo-catalytic reduction of high pressure CO2 using TiO2 suspensions in water and reported the formation of methane. In the work on photo-reduction of liquid CO2, no gaseous products were observed; only formic acid was reported as a reduction product [14]. Methanol, formaldehyde and traces of formic acid in liquid phase and CO in the gas phase were reported as the products for photo-catalysed reduction of CO2 in the TiO2 suspension containing copper powder [18]. Low yield of methane was observed after a long illumination time when KHCO3 was used in the system.

We have studied the photo-reduction of CO2, and methanol in the presence of TiO2 semiconductor in aqueous medium at room temperature under normal pressure conditions and observed the formation of methane in presence of 2-propanol. The main objective of this work is to elucidate the mechanism of CO2 methanation in the presence of 2-propanol as a hole scavenger at ambient conditions and also to compare the results with deoxygenated and oxygenated systems where methane was also produced. The results show methane can be produced under these experimental conditions with good yield.

Section snippets

Experimental

TiO2 (anatase, −325 mesh) was purchased from Aldrich and used as received. A quartz cell of 16.5 ml capacity having a gas purging inlet and a gas sampling port was used for photo-irradiation with 350 nm light using Rayonet photoreactor having photon flux 4×1015 photons cm−2 s−1. The amount of solution used for photolysis experiments was 5 ml. The ambient gas after photolysis of sample was analysed by gas chromatography (GC) using a poropak Q-S column of 2 m length and diameter 1/8 in. Helium was used

Results

The yield of methane obtained from the photolysis of different solutions containing various concentrations of 2-propanol in the presence of 0.1% (w/v) TiO2 is given in Table 1. The amount of solution used for photolysis experiments was 5 ml and it contained 5 mg of TiO2. In the CO2-saturated system, the yield of methane observed after 5 h photolysis was 5.4×10−8mol and was found to increase with the time of photolysis as shown in Fig. 1. It may be mentioned here that this yield of methane was

Discussion

It is clear from Table 1 and Fig. 2 that the amount of semiconductor catalyst and the ambient are very important for the reduction of CO2. If we compare the yield of methane in CO2, N2 and aerated systems we find that a higher yield of methane was obtained in both aerated and CO2-saturated systems whereas in N2-purged system the yield was lower. On photo-irradiation, hole (h+) and e are generated as explained in reaction (1). The presence of 0.5 M 2-propanol is enough to scavenge h+ an

Conclusions

Photo-catalytic reduction of CO2 using TiO2 suspension in aqueous solutions containing 2-propanol as a hole scavenger leads to the formation of methane. CO2 reduction is not possible without light or without a positive hole scavenger such as 2-propanol. CO2 reduction on TiO2 was found to be very less in the absence of a hole scavenger. Methane was also produced on photolysis of aerated 2-propanol solutions containing TiO2. The methane formed in this system could be explained as due to the

Acknowledgements

The authors are thankful to the referees for their useful suggestions. The authors also thank to Dr. D.B. Naik for useful discussions, Dr. T. Mukherjee, Head RC & CDD and Dr. N.M. Gupta, Head ApCD for their encouragement.

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