investigation on electron transfer from semiconductor to metal in photocatalytic h2 production
Résumé
Introduction
The modification of TiO2 with noble-metal cocatalyst can enhance the overall photocatalytic efficiency by acting like an electron sink to separate the electrons and holes, resulting in increasing the overall photoredox reaction [1].
Therefore, the metal photodeposition is considered for preparing the heterogeneous catalyst. Metal-ions are reduced by photogenerated electrons on the semiconductor. This method can be enhanced by adding aqueous alcohol as sacrificial electron donor. Most of research have been focused on the net photoefficiency or the effect of physical properties for the photoactivity [2,3]. However, the metal behavior for H2 formation on alcohol photooxidation is not well understood.
Here, a metal-photodeposition method was proposed to access the effective noble-metal nanoparticles through monitoring the amount of evolved H2. Metal precursors including Ag, Au, Cu, Ir, Ni, Pd, Pt, Ru and Rh were applied on TiO2 anatase to study photocatalytic H2 generation under UV irradiation with 2-propanol as sacrificial agent. The generation of carbon products, such as propane and propene was also evaluated to categorize the role of metals.
Experimental
Photodeposition of metal on TiO2 anatase was performed on a photoreactor. H2Cl6Pt.6H2O, PdCl2, RhCl3, IrCl3, HAuCl4, Cu(NO3)2.3H2O , and Ni(NO3)2.6H2O were used as metal precursor of Pt, Pd, Ir, Rh, Au, Cu and Ni, respectively. 200 mg TiO2 was dispersed in 50 mL of aqueous 2-propanol solution (50 vol%) in the reactor and purged with Ar. Addition of metal solution was gradually injected into catalyst suspension. Subsequently, the photocatalytic reaction was carried out by illuminating the suspension with a 125-W mercury lamp while continuous stirring. Analysis of the H2 and oxidized products evolution were analyzed by Gas Chromatography with Thermal Conductivity Detector. The precipitate was centrifuged and washed repeatedly with distilled water and then dried at 110°C overnight under air.
Results and Discussions
Effect of metallic type-TiO2 on photooxidation of 2-propanol
Fig. 1 shows the products, including H2, propane and propene produced from the photoreaction.
Fig. 1 The metallic behavior on photo-oxidation of 2-propanol for H2 generation
The results exhibit that Pt/TiO2 produced the highest amount of H2 rate following by Pd, Rh and Ir, respectively. Propane was produced only with Pt at about 1.0 µmol hr-1. On the other hand, propene was mainly generated over Au, Cu and Ag. However, propane and propene were also observed in other metals with very small quantity. As Norskov principal, Pt, Pd, Rh and Ir have a thermal-neutral of H2 chemisorption energy (ΔEH), resulting in produce high amount of H2 as shown in Fig. 2. The Classification of metal behavior was demonstrated in Table 1. Propene was produced over Ni with strong M-H bonding and Cu, Ag, Au as unreactive metals with weakness of M-H bonding [4].
Fig. 2 H2 rate over different metals plotted as a function of ΔEH.
Table 1 : Classification of metal behavior on photooxidation of 2-propanol for H2 photogeneration
Relation between metal workfunction and photocatalytic activity for H2 production
Fig. 3 shows the variation of H2 production rate as a function of the workfunction of the metal photodeposited onto three different TiO2.
Fig. 3 : rate of H2 production as a function of workfunction of metal cocatalyst.
Considering minimum and maximum workfunction of the different metals, the trend seems to obey to a threshold function ie below a value, the metals have a low activity and above this value the activity increases rapidly with the increase of the work function. Considering the following reaction mechanism
we assume that the electron transfer to metal (step 3) is the limiting step. By using Fermi-Dirac statistics, it was possible to predict that the fraction of photogenerated electrons from TiO2 to metal nanoparticles is governed by the difference between Fermi level of TiO2 and workfunction of the metal. This observation has been confirm by UV photoemission spectroscopy allowing to characterize electronic structure at metal/TiO2 interface.
Conclusions
The H2 chemisorption energy (ΔEH) has influence in H2 production and propene hydrogenation reaction. The metals with balancing of ΔEH (Pt, Pd, Rh and Ir) reveal high amount of H2. The photooxidation of 2-propanol for H2 evolution over different metallic types can be classified for three proposed mechanism pathway. Propene generation was mainly found in less reactive metals, Au, Cu, Ag. But moreover we demonstrate that the photocatalytic activity of M/TiO2 is governed by the difference between Fermi level of TiO2 and metal workfunction. This fundamental can be applied to be effective for metal with low work function such as Ni or Sn, by using semiconductor with sufficient higher Fermi level.
Acknowledgements. The authors would like to thank Thailand Research Fund through the Royal Golden Jubilee Ph.D. Program (Grant no. PHD/0371/2552), The french government and Centre National de la Recherche Scientifique (CNRS), France for their financial supports.
References
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