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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 CO 2 , but the level of CO 2 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.Keywords Photo-catalysis Á Photo-reforming Á Water splitting Á Hydrogen production Á Methanol Setting the SceneThe human race is in a period of fast transition in all sorts of aspects of life. Perhaps the most important of these is the changing energy scene. Because of the increase in population and increased industrialisation the most convenient forms of useable energy that are available in large quantity, fossil fuels, have been utilized for energy supply. These are firstly coal, used originally for driving steam generation for the new steam engines invented and widely introduced for manufacturing and transport in England from the late 18th century. Following from this came the widespread use of crude oil for transport, in the form of refined petroleum spirit, and in more recent times increasing use of gas powered electrical power generation and domestic combustion. As a result there has been an increase of CO 2 in the atmosphere-increasing from the geologically steady value of about 280 ppm, now just breaking the 400 ppm level, a 42 % increase. The source of these increases can be traced back to the original industrialisation of the early-mid 18th century. Figure 1 illustrates this in terms of the CO 2 level in the atmosphere, combined with a derivative which traces the start of this increase to the period just after the development of more efficient forms of steam engines for mechanical power generation see Ref. [1].The consequences of this geologically fast increase in CO 2 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 CO 2 increase and to find new sustainable ways of fuelling our future. Of course we have a number of successful technologies in Electronic supplementary material The online version of this article

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A B S T R A C TWe have studied hydrogen gas production using photocatalysis from C2-C5 carbon chain polyols, cyclic alcohols and mono and di-saccharides using palladium nanoparticles supported on a TiO 2 catalyst. For many of the polyols the hydrogen evolution rate is found to be dictated by the number of hydroxyl groups and available a-hydrogens in the structure. However the rule only applies to polyols and cyclic alcohols, while the sugar activity is limited by the bulky structure of those molecules. There was also evidence of ring opening in photocatalytic reforming of cyclic alcohols that involved dehydrogenation and decarbonylation of a CÀ ÀC bond.

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We have examined the reforming of methanol and CO on Pd/P25 TiO2 catalysts for hydrogen production, and compared it with rates for similarly supported Au and Cu catalysts. Both reactions proceed, but the photocatalytic water–gas shift (WGS) reaction is much slower than for methanol reforming. CO2 is evolved as expected, but the yields can be much lower than for the expected stoichiometry (CH3OH + H2O → CO2 + 3H2). We show that this is due to dissolution of the carbon dioxide into the aqueous phase. We have also carried out both reactions in the gas phase. Both proceed at a higher rate in the gas phase, and for methanol reforming, there is some CO evolution. In H2 + CO2 reactions, there is little sign of the reverse WGS reaction, but some photo-methanation does occur. Of the three catalysts Pd is the best for the methanol reforming reaction, while Au is best for the water–gas shift. Nonetheless, Cu works reasonably well for methanol reforming and makes a much cheaper, earth-abundant catalyst.