The effect of temperature upon metal-modified TiO2 meterials in methanol photo-steam reforming reaction

In this work, we studied the effect of thermal treatments and of reaction temperature upon the photocatalytic activity of Pt, Cu or Cu/Pt-modified TiO2 materials, prepared by means of two different techniques, i.e. flame spray pyrolysis1 (FSP series) and impregnation method2 (G series). While the loading of Pt nanoparticles (NPs) corresponded to 0.5 wt.%, the Cu content was previously optimized, thus being fixed at 0.05 wt.% and 0.1 wt.% in the case of FSP and G samples, respectively. The home-made materials were characterised by UV-vis DRS, XRPD and BET analyses and their performance was investigated in a thermodynamic up-hill reaction for solar fuels production, i.e. hydrogen evolution from methanol photo-steam reforming3, according to the reaction CH3OH + H2O → 3H2 + CO2. The materials efficiency, expressed in terms of both H2 production rate (rH2) and selectivity towards CO and CO2 products, has been studied by carrying out the test reaction at 40 °C in the presence of each sample either as-prepared or after performing its in-situ thermal reduction in a 10% H2/N2 atmosphere (250 °C for 30 minutes). Moreover, in the case of pre-reduced samples, i.e. showing Pt and/or Cu NPs in pure metallic form, the effects induced by increasing the reaction temperature up to 200 °C were also investigated. Firstly, among the as-prepared materials, the Cu/Pt co-modified TiO2 sample of G series showed the best performance, with a rH2 of 32 mmol g-1 h-1. The thermal reductive treatment produced interesting results, not only related to the nature of the metallic co-catalyst but also to the set temperature of CH3OH steam reforming. In fact, while reduced Pt/TiO2 samples showed a dramatic increase of rH2 at 40 °C with respect to the corresponding as-prepared materials, they exhibited a not-stable hydrogen production, accompanied by a larger selectivity towards CO when the photocatalytic experiment was performed at 200 °C. Differently, Cu/TiO2 samples demonstrated an intriguing behaviour, i.e. their efficiency being hindered at 40 °C due to thermal pre-reduction and restored to competitive value with notable decrease in CO selectivity only after increasing the reaction temperature up to 200 °C. Intermediate performances were attained by Cu and Pt co-modified TiO2 photocatalysts. The here reported results, clearly showing specific effects induced by pre-reduction and test reaction conditions on both activity and working mechanism (together with stability) of Pt and/or Cu modified TiO2 materials, will be presented.