The Photocatalytic Window: Photo-Reforming of Organics and Water Splitting for Sustainable Hydrogen Production

Abstract

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

The roles of metal co-catalysts and reaction media in photocatalytic hydrogen production: Performance evaluation of M/TiO2 photocatalysts (M = Pd, Pt, Au) in different alcohol–water mixtures

“…Fundamental understanding of these factors, and their individual impact on photocatalytic hydrogen production, still requires considerable work. Sacrificial hole scavengers, such as alcohols, further enhance charge-separation [18][19][20][21][22][23]. Alcohol-photoreforming is energetically facile compared to water splitting, typically affording hydrogen production rates 1-2 orders of magnitude higher than those achieved in pure water.…”

“…Alcohol-photoreforming is energetically facile compared to water splitting, typically affording hydrogen production rates 1-2 orders of magnitude higher than those achieved in pure water. H 2 production rates are strongly dependent on the hole scavenger used (typically primary alcohols, diols or triols) [18][19][20][21][22][23], though little work has been reported examining the effect of alcohol concentration on H 2 production rates.…”

“…Only a limited number of such studies have been reported. Bowker and co-workers [18][19][20][21][22] studied hydrogen production rates over Pd/P25 TiO 2 in different alcohol-water mixtures, and concluded that the presence of a-hydrogens on the alcohol was critical to achieving high H 2 production rates. H 2 production tests using 20 different sacrificial reagents established the reactivity series triols > diols > 2°alcohols > 1°alcohols > 3°alcohols [18].…”

Evidence of Plasmonic Induced Photocatalytic Hydrogen Production on Pd/TiO2 Upon Deposition on Thin Films of Gold

“…external diameter) at 45 •C and N 2 was used as a carrier gas (Flow rate of 20mL/min). We, and others, have studied the reaction products during photocatalytic reforming of alcohols which ultimately results in hydrogen and CO 2 formation as the final reaction products [10][11][12][13][14][15].…”

Novel Au/TiO2 photocatalysts for hydrogen production in alcohol–water mixtures based on hydrogen titanate nanotube precursors

“…It is not clear at all from current literature what the optimum alcohol concentration is for H 2 production in the different M/TiO 2 systems, and whether the optimum concentration varies from alcohol to alcohol (alcohols typically used as sacrificial agents are methanol and ethanol, and to a lesser extent ethylene glycol and glycerol). Bowker and co-workers have made the most important contributions in this area to date [10,[26][27][28][29][30][31][32], reporting hydrogen production rates for the photoreforming of a variety of bio-derivable feedstocks over Pd/P25 and Au/P25 under UV excitation. In the Pd/P25 system, H 2 production rates at an alcohol concentration of 1 vol.% were observed to follow the order triols > diols > 2°alcohols > 1°alcohols > 3°alcohols [26,[28][29][30].…”