Synthesis and characterisation of Cu2O/Pt/TiO2 hybrid materials for photocatalytic valorization of CO2
Résumé
1Introduction
In the last decades, the increase of atmospheric concentration of CO2, identified as the principal greenhouse gas in the atmosphere, seems to be one of our major environmental concerns. In May 2013, the concentration of CO2 measured at Mauna Loa station, has been measured as 400ppm for the first time[1] representing an important turning point for the climate changes. For our energy-hungry civilization the reduction of CO2 emissions seems to be a difficult task. Thus, the valorization of this greenhouse gas appears as the alternative solution to reduce its atmospheric concentration.
In the context of renewable energy use, it could be attractive to employ sunlight energy to transform the CO2 into hydrocarbon molecules. Nature works wonders: indeed the photosynthesis process is able to convert CO2 and H2O in hydrocarbon compounds by using the sunlight. This capture and transformation of light energy by plants is based on a Z-scheme, first described by Robin Hill and Fay Bendall [2]. According to this model, the light reactions consist of two separate photosystems operating in tandem, each activated by slightly different wavelengths of light. One photosystem oxidizes water and reduces cytochrome f, while the other oxidizes cytochrome f and reduces NADP+. Inspired by these mechanisms, we propose a photocatalytic strategy using inorganic hybrid semiconductors with heterojunctions to convert CO2 into various value added compounds. Indeed, the formation of heterojunctions leads to an efficient separation of electron-hole pairs to minimize the energy-wasteful electron-hole recombination, which enhancing photocatalytic activity[3]. The objective of this research is to synthesize a hybrid material composed of two different semiconductors and one metal to create SC2-M-SC1 heterojonction. This photocatalyst, by reaching the suitable potentials, should be able to convert CO2 and H2O under artificial or natural light as described on the Figure 1.
Fig. 1. Reactionnal mechanism and photocatalyst structure considered
The first efforts were focused on the synthesis of the hybrid material Cu2O/Pt/TiO2 according to the electronic properties of its different components apart. The main challenge is to cover selectively platinum nanoparticles deposited on TiO2 by Cu2O.
2Experimental/methodology
This kind of hybrid materials were already synthesised by various techniques such as co-precipitation, sol-gel method, impregnation...[4], [5].However, the selective covering of metal does not seem to be really proved. To achieve a selective covering of metallic particles and to place them on the electrons courses, we build our material by successive photodeposition of Pt and Cu2O. The first step is the photoreduction of platinum salt on TiO2 P25 and the second step is to photoreduce copper nitrate precursor on this metallic surface. Indeed, electrons might be preferentially located in metal nanoparticles under irradiation of Pt/TiO2 which might enhance copper ions adsorption on Pt particles and also favour reduction process on them[6]. The synthesis is carried out in an aqueous isopropanol media to follow the H2 photocatalytic production and so the covering of Pt particles which are the major HER sites on Pt/TiO2.
3Results and discussion
Various characterizations were conducted to verify specific architecture of obtained hybrid materials. XPS analysis confirms the reduction of 60% of Cu +II into Cu +I on the catalyst surface. TEM images certifies the presence of Cu2O-Pt-TiO2 heterojunctions as shown in the Figure 2. Different parameters such as sacrificial agent, light flux, Pt content were optimized to obtained the targeted material. A tentative mechanism has been established and will be discussed.
Fig. 2. TEM image obtained for hybrid material Cu2O/Pt/TiO2
This material was tested in photocatalytic reduction of carbonates in liquid phase. The comparison with the blanks Pt/TiO2 and TiO2 tends to admit that our material has a specific activity toward photocatalytic transformation of carbonates in different carboxylic acids. The presence of heterojunctions, apparently responsible for the specific activity, will be discussed.
4Conclusions
A specific strategy was implemented to build suitable photocatalysts to convert CO2. The recombination process must be limited by heterojunctions and proper potentials must be achieved to reduce CO2 and to oxidize H2O or H2 simultaneously. Specific syntheses were carried out to obtain the catalyst Cu2O/Pt/TiO2 with selective covering of Pt by Cu2O. This catalyst seems to present a specific activity towards photocatalytic reduction of CO2 in liquid phase compared to Pt/ TiO2 and TiO2. We are continuing the research to understand better the mechanism, to im-prove our catalyst and to synthesize others.
Acknowledgements