Electron transfer reactions in the photocatalytic hydrogen production rely on the presence of metals of cluster or nanoparticle nature dispersed on top of a semiconductor. Among the most promising methods of photo-catalytic water splitting are those involving modified ultra-high concentrated solar cells. At very high photon flux, the kinetics of the reactions are expected to be different because of possible cluster sintering and changes in electron transfer rates. The complexity of multi-component photo-catalysts hinders accurate measurements dictating the use of simplified methods. In order to explore part of this complex kinetics, H2 production rates of an electron donor, such as ethanol, over Au clusters with different sizes and coverage deposited on single crystal rutile TiO2(110) were studied by scanning tunneling microscopy, online mass spectrometry and complemented by femto second pump probe spectroscopy. It was also found that there is a non-linear increase of the H2 production rate with increasing gold coverage. The key determining factor appears to be the Au inter-particle distance. Increasing this distance resulted in an increase in the normalized reaction rate. These results are explained in terms of competition between particles for excited electrons to reduce H+ (of surface OH groups) to H2. The fact that metal inter-particle distances directly affect the reaction rate indicates that nanostructured synthesis is needed in photocatalyst manufacturing for future technologies.

An update of selected results for hydrogen production from pure water at high sun concentrations using catalysts based on multijunction solar cells and combined multijunction cells with electrocatalysts will be presented.

Selected recent Literature Gold Cluster Coverage Effect on H2 Production over Rutile TiO2(110). K. Katsiev, G. Harrison, G. Thornton and H. Idriss, ACS Catalysis, 9, 8294-8305 (2019) Metal particle size effects on the photocatalytic hydrogen ion reduction, Z. H. N. Al-Azri, Al-Oufi, A. Chan, G. I.N. Waterhouse, H. Idriss ACS Catalysis, 9, 3946−3958 (2019) Importance of O2 Measurements During Photoelectrochemical Water-Splitting Reactions. M. A. Khan, P. Varadhan, V. Ramalingam, H. C. Fu, H. Idriss, J-H. He, ACS Energy Letters, https://doi.org/10.1021/acsenergylett.9b02151 (2019) Methanol Production by H2 Generated from Water Using Integrated Ultra High Concentrated Solar Cells-Electrolysis and Captured CO2: A Process Development and Techno-Economy Analysis, T. T. Isimjan, S. Al-Sayegh, R. Varjian, H. Idriss, ACS. Energy Lett. Accepted