Role of Platinum Deposits on Titanium(IV) Oxide Particles:  Structural and Kinetic Analyses of Photocatalytic Reaction in Aqueous Alcohol and Amino Acid Solutions

Abstract

Photocatalytic reaction at 298 K by platinum-loaded titanium(IV) oxide (TiO 2 -Pt) particles suspended in deaerated aqueous solutions of 2-propanol or (S)-lysine (Lys) was investigated. The TiO 2 catalysts with various amounts of Pt loadings were prepared by impregnation from aqueous chloroplatinic acid solution onto a commercial TiO 2 (Degussa P-25) followed by hydrogen reduction at 753 K. The physical properties of deposited Pt, e.g., particle size, surface area, and electronic state, were studied respectively by transmission electron microscopy, volumetric gas adsorption measurement, and X-ray photoelectron spectroscopy as well as infrared spectroscopy of adsorbed carbon monoxide. The increase in Pt amount mainly resulted in an increase of the number of Pt deposits, not of their size. The catalysts were suspended in the aqueous solutions and photoirradiated at a wavelength >300 nm under an argon (Ar) atmosphere. The overall rate of photocatalytic reactions for both 2-propanol and Lys, corresponding to the rate of consumption of these substrates, was negligible without Pt loading, increased drastically with the loading up to ca. 0.3%, and was almost constant or a little decreased by the further loadings. However, the rate of formation of pipecolinic acid (PCA) from Lys was improved gradually with a increase of Pt loading up to ca. 2 wt %. These dependences were discussed as a function of Pt surface area, which is employed as a measure that includes the properties of both number and size of Pt deposits. For the photocatalytic dehydrogenation of 2-propanol, the rate dependence could be interpreted semiquantitatively with the model that only the TiO 2 particles loaded with at least one Pt deposit can photocatalyze, but the reaction rate is independent of the number of Pt deposits. Therefore, the overall rate is proportional to the number of Pt-loaded TiO 2 particles. On the other hand, for the interpretation of the rate of PCA and H 2 productions, the number of Pt deposits on each TiO 2 particle had to be taken into account. The efficient production of PCA at higher Pt loadings was attributed to the reduction of a Schiff base intermediate produced via oxidation of Lys with positive holes and subsequent intramolecular condensation at the Pt deposit that is close to the site for the oxidation. Otherwise, photoexcited electrons are consumed for H 2 production and the intermediate remains unreduced or undergoes further oxidation. It was suggested that the intermediate produced at the TiO 2 surface sites within a distance of several nanometers from the Pt deposit undergoes efficient reduction to PCA. Thus, the importance of the distribution of Pt deposits for the preparation of highly active and selective TiO 2 -Pt photocatalyst has been clearly demonstrated.

Decahedral-shaped anatase titania photocatalyst particles: Synthesis in a newly developed coaxial-flow gas-phase reactor

“…The presence of platinum as a co-catalyst is a necessary condition for H 2 evolution. In the H 2 system, methanol plays the role of a hole scavenger [10,51]. Time courses of CO 2 /H 2 gas evolution were linear as shown in Figure S7, supplementary data.…”

“…Many studies have been performed to obtain visible light activity by modification of titania with metal and non-metal elements [4][5][6][7]. The second limitation can be overcome by surface modification of titania with noble metals and their compounds that inhibit the charge recombination by increasing the transfer of photoexcited electrons from titania to substrates [5,[8][9][10]. Another approach is the preparation of p-n junction photocatalysts (e.g., nickel oxide/titania) to induce an internal electric field in the contact between p-type and n-type materials, which can increase e --h + separation [11].…”

Correlation between Some Physical Properties of Titanium Dioxide Particles and Their Photocatalytic Activity for Some Probe Reactions in Aqueous Systems

“…In a series of studies on TiO 2 photocatalytic reactions, [48][49][50][51][52][53] we have shown that the overall kinetics depends on both the rates of reduction and oxidation by a photoexcited electron (e -) or positive hole (h + ), respectively, and the rate of recombination of e -and h + . To obtain a highly active TiO 2 photocatalyst, therefore, it is necessary to synthesize a TiO 2 powder that has contradictory properties, that is, large surface area to adsorb substrates and high crystallinity (or lesser surface and bulk defects) to diminish the e --h + recombination.…”

Layered Fe(III) doped TiO2 thin-film electrodes for the photoelectrocatalytic oxidation of glucose and potassium hydrogen phthalate

“…In Figure 6, with increasing KHP concentration, i net for TT, FT and FF decreased, probably due to poisoning of the TiO 2 photocatalyst by the high KHP concentration [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29]. An i net decrease for TF was not observed, and i net further increased with increasing KHP concentration.…”

“…anatase/rutile) may possess higher activity in *Corresponding author (email: s.zhang@griffith.edu.au) the photoelectrocatalytic degradation of organic compounds than pure anatase electrodes because the electron-transfer pathway from anatase phase to rutile phase facilitates the separation of photoelectron and photohole, extending the lifetime of the photoelectron and photohole [14]. Other TiO 2 modifications such as Pt 15 and Ag 16 doping have also been shown to improve organic degradation efficiency in photoelectrocatalytic processes, perhaps due to the Pt or Ag nanoparticles acting as electron traps and preventing exciton recombination.…”