Scientists from the Faculty of Chemical and Process Engineering of the Warsaw University of Technology, the Institute of Physical Chemistry of the Polish Academy of Sciences, Fuzhou University, Universidad Cooperativa de Colombia, and the SOLARIS National Synchrotron Radiation Centre have published a paper in the Chemical Engineering Journal in which they present a Cu/TiO2 nano photocatalyst enabling experimentally confirmed photocatalytic hydrogen production without causing greenhouse gas emissions. They explained the results of the photoreforming reaction of an aqueous methanol solution under UV LED illumination. To investigate the degree of copper oxidation on the surface of titanium dioxide (TiO2) nanoparticles, X-ray absorption spectroscopy measurements were carried out using structure data close to the X-ray absorption edge (XANES technique) at the ASTRA beamline.

Hydrogen production by photoreforming methanol is one of the most intensively researched photochemical transformations in pursuing a sustainable, circular economy. In the published work, the authors describe atomic-dispersive copper-decorated TiO₂ nanoparticles synthesized by wet impregnation, which are highly active and 100% selective in hydrogen production in the photoreforming reaction of methanol solution and exhibit an apparent quantum yield of 10% at a wavelength of 365 nm. The only gaseous product found is hydrogen, while all the carbon is retained in the liquid phase of a mixture of formaldehyde and formic acid, making the process fully sustainable and emission-free. Characterization of the Cu/TiO₂ photocatalyst to identify the features underlying this unique behavior was achieved by combining ab initio simulations using density functional theory (DFT) and X-ray photoelectron spectroscopy (XPS). This allowed the identification of the Cu⁺→Cu⁰→Cu⁺ redox cycle under the reaction conditions. Bypassing the Cu²⁺ oxidation state during illumination is crucial to keep the oxidation potential of photogenerated holes low enough to prevent CO₂ production and keep all carbon in the liquid phase. Synchrotron measurements complemented the research methodology, enabling the identification of the oxidation state of copper in samples before and after the reaction and confirmation of the atomic dispersion of Cu clusters on the TiO₂ surface.

Picture 1. Scheme of the mechanism of zero-emission hydrogen generation over Cu/TiO₂ nanocatalyst.

Picture 2. HPLC chromatogram of the products formed during photoreforming of methanol (a), normalized Cu K edge absorption coefficient μ(E) for metallic Cu (black), Cu₂O (red), and CuO (blue) reference samples (b).

Written by: Karol Ćwieka

Link to the publication:  K. Ćwieka, Z. Bojarska, K. Czelej, D. Łomot, P. Dziegielewski, A. Maximenko, K. Nikiforow, L. Gradoń, M. Qi, Y. Xu, J. C. Colmenares, Zero carbon footprint hydrogen generation by photoreforming of methanol over Cu/TiO₂ nanocatalyst, Chemical Engineering Journal, 474, 145687(2023) doi: 10.1016/j.cej.2023.145687