Modelled kinetics of the rate of hydrogen evolution as a function of metal catalyst loading in the photocatalysed reforming of methanol by Pt (or Pd)/TiO2

Abstract

The rate of hydrogen evolution, r(H 2), due to the photocatalysed reforming of methanol, MeOH, is studied as a function of Pt loading on P25 TiO 2 both in the aqueous (at room temperature) and gas phase (at 100 o C). A similar study, using Pd on P25, has been reported earlier [M. Bowker et al. J. Catalysis, 2003, 217, 427-433] and a perimeter-based Metal-Support Interface (MSI) kinetic model used to interpret the results. Here a new kinetic model is introduced which appears to provide a better fit to all the r(H 2) vs wt% metal data sets, in which the rate of reaction is proportional to an extended area around each metal island, rather than its perimeter. A simple theoretical rationale is provided for this expanding photocatalytic area and overlap (EPAO) kinetic model in which each metal island forms an electric field with the surrounding TiO 2 so as to act as a sink for electrons photogenerated in the surrounding TiO 2 film, thereby allowing it to effect the reduction of water, leaving the remaining photogenerated holes to oxidise the methanol adsorbed on the TiO 2. Some support for this very simple kinetic and theoretical model is provided by the results of a brief study of the oxidation of soot deposited on and around a Pt 'dot' on a sol-gel TiO 2 film, in that, upon irradiation of this system a zone of activity is visibly revealed by the gradual disappearance of the soot around the Pt 'dot', the radius of which appears to be proportional to the radius of the metal 'dot'.

Photonic efficiency and selectivity study of M (M=Pt, Pd, Au and Ag)/TiO2 photocatalysts for methanol reforming in the gas phase

“…(2) to be consistent with a previous study which examined in detail the effect of Pt metal loading on the kinetics of the TiO 2 photocatalysed reaction (1) and (2) [26]. In addition, other work, carried out < 100 o C by other [18], suggests production of formic acid and formaldehyde production is an issue, which does not appear to be the case at 100 o C, vide infra.…”

“…The TEMs of the photocatalysts before and after use showed no obvious change, with the metal particle sizes remaining unchanged. Note that in a previous paper [23], using ostensibly the same Pt/TiO 2 photocatalyst, aggregated Pt particles of 2-3 nm were observed using HAADF STEM and this might appear to suggest some lack or reliability in the method of production, the incipient wetness method -which would be surprising given it widespread use [23,[26][27][28][29]. However, a closer inspection of the HAADF STEM image reveals that whilst some aggregated particles were present, the bulk of the Pt particles appeared to be ca.…”

“…All M/TiO 2 photocatalysts, whereby M = Pt, Pd, Au and Ag, were prepared using the incipient wetness method [9,23,25,26]. Thus, for the typical photocatalyst Pt/TiO 2 (0.2 wt% Pt), 58 mg of the metal salt (H 2 PtCl 6 .6H 2 O) were dissolved in 80 cm 3 of water and the resulting solution then added dropwise to 10 g of the P25 TiO 2 powder which was agitated continuously.…”

“…The powder was subsequently reduced at 200 o C for 2 h in a tubular furnace under a steady flow of pure H 2 (50 cm 3 min -1 ), thereby yielding the 0.2 wt% Pt/TiO 2 photocatalyst. Note: the incipient wetness method, with its final reduction step, is often used by the catalyst community to produce nanoparticulate metal dispersions on a wide variety of inert substrates [23,[26][27][28][29]. In this work, as noted elsewhere, the conversion of the cream colour PtO 2 coated TiO 2 powder particles by the hydrogen produces a striking and rapid colour change to a grey powder, commonly assumed to be due to the reduction of the metal oxide to the metal, as illustrated by the before and after photographic images illustrated in the figure S2 in the SI section; no further colour change is effected by increasing the temperature of reduction reactor temperature even to 500 o C. Temperature programme reduction studies of the Pt/TiO 2 catalyst produced using this technique show that the reduction is readily effected at 200 o C [27,29].…”

Photocatalytic production of H2 is a multi-criteria optimization problem: Case study of RuS2/TiO2

“…The decrease of PHER activity in methanol on Pt and Pd -loaded TiO 2 was previously attributed to the overlapping of surface zones where the particles provide optimal electrons harvesting. As far as such zones begin to overlap, the conditions deteriorate in the overlapping areas, as the electronic properties of TiO 2 are somehow unfavorably modified [83].…”