Effects of electron charge density and particle size of alkali metal titanate nanotube-supported Pt photocatalysts on production of H2 from neat alcohol

Abstract

Pt nanoparticles (PtNPs) in the range of 1.0–3.0 nm were deposited on alkali titanate nanotubes (MTNTs = M_2−xH_xTi₃O₇, M = Li⁺, Na⁺, K⁺ and Cs⁺) by wet impregnation. While most of the physical properties of Pt/MTNTs remained almost constant, the oxidation state and size of PtNPs varied systematically with the size of the cations of MTNTs. XPS indicated that the binding energy of Pt in Pt/MTNTs was reduced to a lower value than that of Pt⁰, yielding a Pt^δ− oxidation state. Diffuse-reflectance infrared Fourier transform spectroscopy coupling with CO adsorption studies confirmed the formation of the Pt^δ− state in Pt/MTNTs. Thus, electrons were transferred from MTNTs to PtNPs establishing an electric double layer at the interface between PtNP and MTNT supports, and the degree of electron transfer increased with the size of the cations in MTNTs. HRTEM revealed that the mean sizes of PtNPs followed the order, Pt/LiTNTs < Pt/NaTNTs < Pt/KTNTs < Pt/CsTNTs. TPR showed that the reducibility of PtO_x/MTNTs determined the order of PtNPs size. In the photocatalytic production of H₂ (2H⁺ + 2e⁻ → H₂), since H₂ is produced at the interfacial Pt sites, the electron charge density and the particle size of PtNPs are the two competing factors in producing H₂. Photoluminescence studies revealed that the initial increase in electron density on PtNPs reduced the recombination of h⁺–e⁻ pairs and increased H₂ yields, but a further increase in charge density enhanced the recombination of h⁺–e⁻ pairs and lowered the H₂ yield. PtNPs in Pt/KTNTs had a moderate charge density and a moderate particle size, and so, produced a maximum amount of H₂ among Pt/MTNTs.