Oxidizing sulfides to sulfoxides and sulfones is an essential transformation for pharmaceuticals, fine chemicals and industrial catalysis, but it usually requires strong oxidants and high temperatures. Metal oxides that generate ROS — singlet oxygen (1O2), superoxide (•O2−), hydroxyl (•OH) and hydroperoxyl (•OOH) radicals — offer a gentler route. α-Ag2WO4 is a striking example: earlier experiments showed that, with hydrogen peroxide as the oxidant, it converts sulfides completely into sulfones within an hour at 50 °C in the dark, despite a band gap of about 3.1 eV. Because ROS are extremely short-lived, however, the molecular mechanism behind this activity has remained unclear.
Recently, a research team led by Prof. Juan Andrés (Universitat Jaume I, Spain), in collaboration with Prof. Miguel A. San-Miguel (University of Campinas, Brazil), used density functional theory (DFT) to reconstruct the complete reaction pathway on the α-Ag2WO4 (110) surface — to the authors' knowledge, the first theoretical study of the full sulfide oxidation mechanism. The results, with Dr. Felipe Lipsky as first author, were published in Chinese Journal of Catalysis (DOI: 10.1016/S1872-2067(26)65115-7 ).
Under-coordinated surface silver atoms, assisted by neighboring lattice oxygen that accepts protons, act as active sites that split H2O2 and H2O with remarkably little effort: barriers of only 0.07 eV to convert H2O2 into •OOH and then 1O2, and 0.12 eV to dissociate water into •OH, so that 90% of adsorbed water splits within about 10 picoseconds at 50 °C. This is made possible by mid-gap electronic states that appear only in the outermost atomic layers. Like a small bank for electrons, these states can lend charge to an adsorbed molecule or take it back as each step requires, allowing the same surface to stabilize singlet oxygen, whose electrons are all paired, and radicals, which carry an unpaired electron.
For dimethyl sulfide (DMS), the first oxidation step, DMS to dimethyl sulfoxide (DMSO), proceeds with singlet oxygen over a 0.52 eV barrier. The second, DMSO to dimethyl sulfone (DMSO2), is the bottleneck: with singlet oxygen alone the barrier is 1.65 eV, but •OOH radicals from a second H2O2 molecule lower it to 0.86 eV, so the step proceeds on a timescale of seconds. This rationalizes why H2O2 is needed experimentally and identifies a clear design target: lowering this barrier by only 0.1 eV is projected to speed up turnover about 36-fold. The authors propose engineering more mid-gap states, e.g. via oxygen vacancies or N/S doping, aiming ultimately at oxidation with O2 alone. The same principles may guide the design of other metal-oxide catalysts for selective oxidation and environmental remediation.
About the journal
Chinese Journal of Catalysis is co-sponsored by Dalian Institute of Chemical Physics, Chinese Academy of Sciences and Chinese Chemical Society, and it is currently published by Elsevier group. This monthly journal publishes in English timely contributions of original and rigorously reviewed manuscripts covering all areas of catalysis. The journal publishes Reviews, Accounts, Communications, Articles, Highlights, Perspectives, and Viewpoints of highly scientific values that help understanding and defining of new concepts in both fundamental issues and practical applications of catalysis. Chinese Journal of Catalysis ranks among the top six journals in Applied Chemistry with a current SCI impact factor of 17.7.
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