Photocatalytic reduction of carbon dioxide
K. Kočí1. L. Matejová1, L. Čapek2, L. Obalová1
1Institute of Environmental Technology , 2Faculty of Metallurgy and Materials Engineering, VŠB – Technical University of Ostrava 17. listopadu 15, 708 33 Ostrava, Czech Republic
2University of Pardubice, Faculty of Chemical Technology, Studentska 95, Pardubice, Czech Republic
Tackling global warming and fuel crisis is one of the key challenges in nowadays. C02 is a relatively inert and stable compound and its reduction is quite challenging, high-temperature and/or pressure are necessary to achieve it. Conversely, photocatalytic reduction of C02 in the presence of water occurs under relatively mild conditions, especially when the reaction is activated by solar energy. This technology not only offered an alternative method to produce the sustainable fuels, but also transform the waste C02 into valuable chemicals since methane, hydrogen, methanol are the possible products of reaction
In photocatalytic reduction of C02, several key factors are imperative to be considered for achieving higher conversion and for improving the efficiency of photocatalytic reduction of C02. The first key factor is the selection of semiconductor photocatalyst having relevant band-gap energies. The appropriate structure of photocatalyst participates significantly in improving the conversion of C02 and selectivity to desired products. The photocatalyst most widely employed, Ti02, has in comparison with other semiconductor photocatalysts many advantages: suitable optical and electron properties, chemical stability, corrosion resistance, and non-toxicity. However, TiO2 has a relatively broad band gap which allows absorbing only about 4 % of solar energy resulting in low quanta efficiencies. Therefore, effort is focused on broadening the spectrum range, in which Ti02 is capable to absorb light. This can be achieved by e.g. doping titanium dioxide with other elements (rare metals, rare-earth metals). The metal doped catalysts could help to improve recombination times of charges particles.
Suitable design of a photocatalytic reactor is the second important factor. The design of photoreactor is different from the traditional chemical reactors. The construction of photocatalytic reactor is effective only if there is a good interaction between all phases of reaction system and simultaneously, photocatalyst is irradiated effectively. The distribution of photocatalyst and irradiation within photocatalytic system are significant to increase conversion and yield rates.
At last, optimizations of the operating conditions are important for engineering intent. For that reason, it is significant to take into account all operating parameters such as light intensity, C02 concentration, reaction time, temperature, system pressure and pH.
The C02 photocatalytic reduction over the semiconductors is not yet ready to be carried out in real-life solar applications. Nevertheless, with the present rate of developments in this field, should bring the promise held by this process much closer to accomplishment in the near future.