Insights into the role of transition and noble metals mediating photochemical vapor generation†

The recent expansion of the suite of elements amenable to photochemical vapor generation (PVG) is primarily linked to the addition of mg L⁻¹ concentrations of selected transition metals (TMs) to the photolysis medium, principally Fe, Cd, Co, Ni and Cu. Their presence enhances synthesis yields of several analytical targets, particularly carbonylated species, in some cases by orders of magnitude. A consideration of curated analytical PVG literature reveals substantial inconsistencies with the current use of generalized ligand-to-metal charge transfer processes to mechanistically account for these so-called TM “sensitizer” effects via their enhancement in free radical populations participating in the PVG synthesis routes. In this study, a novel approach utilizes an independent window for evaluation of the effects of added TMs on radical production based on an examination of the altered concentration profiles of H₂, CO, CH₄ and CO₂ generated in formic and acetic acid media, whose origins lie with the precursor free radicals responsible for the analytical PVG process. A photocatalytic mechanism induced by homogeneous co-generation of TM nanoparticles is proposed which more reasonably accounts for both the altered gas profiles and their notable selectivity evident with improved PVG efficiencies of specific analytes. A tutorial approach to the topic has been adopted in an effort to provide a balanced framework within which the various processes are comprehensively discussed with relevance to state-of-the-art PVG techniques and current literature.