The Photocatalytic Window: Photo-Reforming of Organics and Water Splitting for Sustainable Hydrogen Production
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Precious metal-titania materials make good catalysts for hydrogen production from a variety of organic substrates using sunlight. These substrates essentially act as reductants for water, by intercepting electrophilic oxygen species generated by electron–hole excitation resulting from photon absorption in the titania support. As a result, the hydrogen produced comes partly from water splitting and partly from dehydrogenation of the organic substrate. Why only precious metals work for the reaction is discussed, together with the mechanism of these reactions. The oxygenate substrates are decarbonylated to produce adsorbed CO, which is removed in the presence of light by the electrophilic oxygen as CO2, but the level of CO2 detected is strongly affected by the amount of liquid water present, due to absorption and reaction to form carbonic acid. The possibilities for application of this technology in the domestic environment, the ‘Photocatalytic Window’ is considered.
KeywordsPhoto-catalysis Photo-reforming Water splitting Hydrogen production Methanol
1 Setting the Scene
The consequences of this geologically fast increase in CO2 levels can be debated, but what is absolutely certain is that, as sentient beings, humans should not be playing with dice by abnormal perturbance of natural equilibrium, over a geologically fast timescale. The results are not likely to be positive for the planet, except in the sense that it might help reduce the human population.
Thus there is an urgent need to stop the CO2 increase and to find new sustainable ways of fuelling our future. Of course we have a number of successful technologies in place which are being applied and made ever more efficient technologically and economically. Wind power and solar power are mainstays of sustainable energy production, but are geographically very variable in efficiency and are often in very variable time-dependence of production, due to often chaotic weather patterns. For that reason storage of such power production in peak times, for use in slack times, is essential. It is recently being appreciated that perhaps the only efficient way of doing this is to store such energy in the form of chemical energy. The way at present being considered is hydrogen production by electrolysis, followed by conversion into a transportable form such as methanol or ammonia .
The point of this article is to describe current research in our group and others concerning an alternative way of producing hydrogen that is also direct, namely photocatalytic hydrogen production using sunlight energy and to consider the possibility of utilising such energy in the domestic environment.
2 Basics: Kinetics and Thermodynamics
As always should be the case for determining the feasibility of catalytic reactions we must consider the thermodynamics, which are particularly simple for water splitting: the conclusion from this is that it is an extremely difficult reaction since it is endergonic by ~240 kJ mol−1, as discussed in an earlier review in this journal  and elsewhere  in more detail.
Thus dissociation can be obtained with reasonable equilibrium yields but with very high thermal input at high temperature. The alternative is to provide the thermodynamic energy with light, in which case we need light of ~500 nm wavelength. Such light is abundant on earth, but fortunately for the biota, water is stable because the kinetic barrier to split off the first hydrogen atom from water is much more energy demanding, needing ~500 kJ mol−1, or light of at maximum wavelength 250 nm. Hence water is stable at the earth’s surface—such wavelengths are cut out in the stratosphere before reaching ground level.
3 Electronic Transitions in Titania
The approach to storing sunlight to enable water splitting has been to utilize solid state chemistry and the properties of semiconductors. This is because sunlight can excite a ground state electron across the band gap and thereby store energy which may be used to split water. Although many such materials have been made and utilized a crucial factor in application is the photo-hydro-stability of the material. For example, CdS could be a much better material for water splitting because it has a band gap in the visible (~2.4 eV), but it oxidises in the presence of water and light  and the band positions are not properly aligned for water splitting. Titania has been shown to be generally photo-hydrostable and so is the focus here, but it is worth noting that other, less stable semiconductors have been proposed for use when coated with a fairly thick layer of titania for protection .
However, a number of workers [17, 18, 19] claim that a mix of the two morphologies gives higher yields than either individually due to positive interfacial effects between the two, and indeed the material often used as choice for the reaction is commercially available P25 TiO2, which is an approximately 4:1 mix of anatase and rutile. A number of reasons have been postulated for the advantage of the mixture, the most obvious perhaps being that excitation can occur in anatase, but excited electrons transfer to the rutile, extending the lifetime if the e–h pair significantly.
Nonetheless the picture is not as simple as this—some anatase titanias perform better than P25 for the reactions described and rutile titania can be a good catalyst under the right conditions.
4 Reactivity of Titania Under Anaerobic Conditions
Notwithstanding our discussion in the section above, the reactivity of titania alone is low in the absence of gas phase oxygen. In the presence of oxygen it is widely applied for the demineralisation of water, and in academic studies for the decolouration of dyes, but in terms of the hydrogen production we are discussing in this paper, it is important to note that it has good activity for recombining oxygen and hydrogen . Hence it is essential to carry out hydrogen production in an anaerobic environment, but is also important to note that titania is very inactive for hydrogen producing reactions by reforming of simple organics, such as methanol. There is some activity, but it is very much lower than the activity of the metal-loaded catalysts described below under the same conditions.
5 The Specifics: Reactivity
6 Why Metal is Important for Steady State Photocatalysis
For most of the reactions involved (for example methanol reforming with water) addition of metal to the surface of titania enormously improves the hydrogen yield, as shown above. So why is this? What is the role of the metal? There are a number of roles that have been proposed in the literature. One is that the metal acts to improve the lifetime of the electron–hole pair by trapping the itinerant electron [17, 18, 19]. Another is that the metal is essential for the initial steps of the photocatalysis, that is, the reaction of the organic with the surface, usually involving dehydrogenation/decarbonylation reactions [25, 26]. Similarly, Joo et al. recently dismissed the role of the metal as a centre for electron trapping and also postulated that the metal plays an important chemical role, that is, as a centre for hydrogen atom recombination . Such reactions are much easier on precious metals than on titania. So, for instance, methanol and other oxygenates simply decarbonylate on pure Pd to CO and H2, even at room temperature [25, 26, 28], whereas on titania it follows a much more complex and energetically demanding route, involving dehydration and dehydrogenation reactions, occuring above 600 K [29, 30].
7 Substrate Structure Effects
8 CO2 Production
9 So What? The Photocatalytic Window
Besides use in this way such a device may be more appropriately applied in communities in the world who have exhausted wood supplies for cooking and don’t have easy access to fossil fuels, but who have high sunlight levels. It is likely that they can ferment alcohol products which could then be used to generate useable gas, particularly appropriate, perhaps, in communities where consumption of the alcohol is forbidden!
Of course, much of this is surmise at present and such a device has not yet been produced. It is feasible, but for domestic application safety is of major concern, and storage/monitorring/control of the gas would have to be arranged most carefully.