Applied Catalysis B: Environmental 107 (2011) 205–209 Contents lists available at ScienceDirect Applied Catalysis B: Environmental jo ur n al homepage: www.elsevier.com/locate/apcatb New insights into the mechanism of photocatalytic reforming on Pd/TiO 2 Hasliza Bahruji a,b , Michael Bowker a , Philip R. Davies a,∗ , Fabien Pedrono a a Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, UK b Faculty Science and Mathematics, Universiti Pendidikan Sultan Idris, 35900 Tanjong Malim, Perak, Malaysia a r t i c l e i n f o Article history: Received 12 May 2011 Received in revised form 30 June 2011 Accepted 10 July 2011 Available online 20 July 2011 Keywords: Photocatalysis Methanol reforming TiO2 Alcohol Hydrogen production Water splitting a b s t r a c t Using sunlight to generate hydrogen from biomass is a promising and environmentally benign route to converting waste products into fuel but the practical application of the technology requires a photo- catalyst and the development of suitable materials for this task has been hampered by an incomplete understanding of the photocatalytic mechanism. By exploring the effect of molecular structure on the rate of hydrogen evolution from a variety of alcohols over Pd/TiO 2 catalysts, a few simple rules are derived that predict the relative rates of photocatalytic reforming and the dominant reaction products; the latter being confirmed by mass spectrometry. In general, for an alcohol C x H y OH, decarbonylation dominates with the formation of CO 2 and a hydrocarbon C x-1 . For diols and triols, alkyl fragments generally scav- enge hydrogen and desorb as alkanes but in cases where competition for hydrogen occurs between alkyl fragments, for example iso-propanol, some reaction of alkyl groups to CO 2 and H 2 is evident. Methylene groups are always oxidised. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Hydrogen generation from hydrocarbons is an important com- ponent in the global strategy for a hydrogen economy. At present, approximately 95% of the hydrogen produced commercially comes from the steam reforming of fossil fuels (Eq. (1)), an endother- mic process that requires a catalyst, temperatures up to 1000 ◦ C and pressures of up to 25 bar. The increasing drive to reduce our dependence on non-renewable sources and the increasing cost of fossil fuels is stimulating interest in alternative hydrogen sources: biomass represents a possible reforming feedstock with clear envi- ronmental benefits over the use of fossil fuels but, the hydrogen content of biomass is only ∼6.5% compared to ∼25% in natural gas, and therefore at present, biomass cannot compete with fossil fuels for standard reforming methods. However, if one could find a processing route for biomass reforming utilising milder condi- tions than steam reforming, biomass maybe able to compete more effectively with fossil fuels (Scheme 1). Photocatalytic reforming offers a route that takes place near to ambient conditions and, because it can utilise sunlight, it is partic- ularly attractive for areas of the world where high biomass supply coincides with high sunlight intensity. Furthermore, it is a rela- tively low tech method and therefore particularly useful in small production facilities. ∗ Corresponding author. Tel.: +44 2920874072. E-mail address: daviespr@cf.ac.uk (P.R. Davies). Many materials have been investigated as photocatalysts for the reforming reaction [1] but despite the relatively large band gap of ∼3 eV which limits the proportion of sunlight that can be utilised, titania remains the catalyst of choice [2] providing a combination of high chemical stability and activity at low cost. Considerable effort is now being expended to understand the mechanism of reactions on the TiO 2 surface inorder to provide a road map for devising alternate catalysts. A generally accepted mechanism for the pho- toreforming reaction on TiO 2 surfaces involves the oxidation of water molecules by photoinduced holes in the semiconductor pro- ducing • OH radicals which then attack the alcohol, abstracting an alpha hydrogen to create a • RCH 2 –OH radical. The radical is fur- ther oxidised to an aldehyde [3–5]. The photocatalytic activity of titania is significantly improved by the addition of low weight per- centages of a precious metal [6] and it is widely agreed that this enhancement is due to the metal nanoparticles trapping electrons and thereby extending the electron–hole pair lifetime. Whether, in the presence of the metal, the subsequent reaction steps follow the same mechanism proposed for pure TiO 2 remains a point of dis- cussion. On the basis of reactions observed on palladium and on palladium/TiO 2 model systems using surface science methods [7] we have proposed that the dominant alcohol oxidation step occurs on the metal or, at the metal/titania interface with the titania pro- viding the means of photon capture and the reduction of water while the oxidation of the alcohol occurs on the metal [8]. In the case of a palladium modified titania, chemisorption of the alcohol is followed by rapid reaction to form a strongly chemisorbed CO which inhibits further adsorption of the alcohol on the metal sur- face. Further reaction can then only take place after the oxidation of 0926-3373/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2011.07.015