Phase‐Contact Engineering in Mono‐ and Bimetallic Cu‐Ni Co‐catalysts for Hydrogen Photocatalytic Materials

Abstract

Understanding how a photocatalyst modulates its oxidation state, size, and structure during a photocatalytic reaction under operando conditions is strongly limited by the mismatch between (catalyst) volume sampled by light and, to date, the physicochemical techniques and probes employed to study them. A synchrotron micro‐beam X‐ray absorption spectroscopy study together with the computational simulation and analysis (at the X‐ray cell) of the light‐matter interaction occurring in powdered TiO2‐based monometallic Cu, Ni and bimetallic CuNi catalysts for hydrogen production from renewables was carried out. The combined information unveils an unexpected key catalytic role involving the phase contact between the reduced and oxidized non‐noble metal phases in all catalysts and, additionally, reveals the source of the synergistic Cu‐Ni interaction in the bimetallic material. The experimental method is applicable to operando studies of a wide variety of photocatalytic materials.

An Operando X-ray Absorption Spectroscopy Study of a NiCu−TiO2 Photocatalyst for H2 Evolution

“…However, as reported in the literature, 19,20,[27][28][29][30]35 Ni and Cu NPs undergo significant chemical transformations not only under ambient conditions (they are susceptible to oxidation by oxygen in the environment 50,51 ) but also when exposed to a reaction phase, in the dark or under illumination (e.g. during photocatalysis).…”

“…Differently, the oxidation of Cu 0 to Cu 2+ along with the formation of core-shell NPs (metallic core and oxidized Cu 2+ shell) was observed during gas phase photocatalytic H2 evolution. 35 Furthermore, Irie et al (and Dozzi and coworkers in follow up works) proposed that since the redox potential of the Cu 2+ /Cu + couple or the CB minima of crystalline CuO are less negative than the CB energy of TiO2, Cu 2+ species on the TiO2 surface may capture TiO2 CB electrons causing a consequent one-electron reduction of oxidized copper species (Cu 2+ to Cu + ) meanwhile enhancing the charge carrier separation efficiency. 36,37 This effect was reported to enable visible light activation, as electrons in the VB of TiO2 are directly transferred to the discrete energy levels of grafted Cu 2+ species.…”

“…In this scenario, the resulting metal NPs act as co-catalyst for the hydrogen evolution reaction (HER). However, an absence of chemical changes for Ni NPs, 35 or oxidation or disproportionation reactions forming both Ni and NiOx sites 32 or Ni/NiOx core-shell NPs 30 were also proposed. Such NiOx compounds or Ni/NiOx core-shell nanostructures are reported to form in the case of overall water splitting photocatalysts investigated in pure water.…”

Sunlight-Operated TiO2-Based Photocatalysts

“…FeO x -containing semiconductors are also actively investigated however the behavior of such systems combine frequently photocatalysis with photo-Fenton [ 81 ]. More complex formulations consider the combination of titania with, for example, heteropolyacids [ 82 , 83 ], vanadates such as BiVO 4 [ 84 , 85 ] or complex oxides such as CuNiOx [ 86 ] and FeAlO 3 [ 87 ]. Similar catalysts have been studied for ZnO-based materials.…”

“…( B ) toluene and styrene photo-catalytic elimination reaction rate as a function of the W/Ti atomic ratio of the samples. Reprinted with permission from [ 86 ]: Caudillo-Flores, U.; et al Appl. Catal.…”