A series of Ag-Pd/TiO2 catalysts have been prepared, characterized and tested for H2 production activities from water in the presence of organic sacrificial agents. The synergistic effect of metallic properties (plasmonic and Schottky mechanisms) was investigated. XPS results indicated that silver is present in the form of its oxides (Ag2O and AgO) at 0.2-0.4 wt. % loading while palladium is present as PdO and Pd metal at similar loading. However, metallic character for silver particles increases while that of palladium metal particles decreases with increasing their % in the investigated range (0-1 wt. %). HRTEM results coupled with EDX analyses indicated the presence of two types of Ag containing particles (large ones with about 4-6 nm and smaller ones with ca. 1nm in size). Palladium was only found forming Ag-Pd alloy/composite with a wide size distribution range between 10-60 nm. Both particles are composed of silver and palladium, however. Optimal photocatalytic H2 production rates were obtained for catalysts with a palladium to silver ratios between 4 and 1.5 in the case of bimetallic catalysts. In addition, H2 production rates showed linear dependency on plasmonic response of Ag. The study demonstrates that increased H2 production rates can be achieved from an understanding of plasmonic and Schottky properties of metals loaded on top of the semiconductor.
In this work we present an overview of the photo-reaction of ethanol over the surface of TiO2
(110) single crystal under photo-excitation and compare it to that over Au/TiO2 nano-particle.
Over rutile TiO2(110) surface ethanol is present mainly in ethoxide (CH3CH2O(a)) form at 300K
as evidenced by the presence of XPS C1s peak at 286.5 eV due to the -CH2-O(a) function; (a) for
adsorbed. DFT computation of the same system indicated that the surface coverage is 50% or
less in line with previous experimental results [1]. Exposing a pre-dosed surface to UV light in
presence of oxygen resulted in the formation of acetaldehyde (CH3CHO(g); (g) for gas phase)
with the extent of reaction depending on the square root of the O2 pressure in the 10-10 - 10-6torr range. Over the Au/TiO2 powder system we have focused the attention on the production
of hydrogen as the oxidation of ethanol of ethanol to acetaldehyde has been previously studied
[2]. The reaction is found to be sensitive to the polymorph nature of TiO2 with anatase showing
two orders of magnitudes higher activity than rutile. We have also addressed the TiO2 particle
size effect on the reaction and found that the TiO2 particles, in the 150 to 10 nm range, to have
the same reactivity.
The work presents the photo-catalytic reactions of ethanol over Au particles deposited on
TiO2 anatase nano- (≤ 10 nm) and micro- (ca. 0.15 μm) particle catalysts. The Au particles
are of uniform and similar dimension (mean particle size = ca. 5 nm and 7 nm on the microand
nano-sized TiO2, respectively). XPS Au4f indicated that in both cases Au particles are
present in their metallic state with no evidence of charge transfer to (or from) the
semiconductor. Liquid slurry photoreaction indicated the production of hydrogen with a rate
≈ 2 L/kgCatal.min on 2 wt. % Au/TiO2 anatase nano-particles under UV photo irradiation of
comparable intensity to solar radiation. While the reaction rate per unit mass was lower on
the micro-sized Au/TiO2 it simply scaled up to an equivalent rate for the nano-sized Au/TiO2
catalyst when normalised by unit area.
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