Formation of hierarchically-ordered nanoporous silver foam and its electrocatalytic properties in reductive dehalogenation of organic compounds

Abstract

Nanoporous silver foam on a glassy carbon electrode (AgNF/GC) was formed by cathodic deposition of silver from an acidic solution of AgBF₄ at high current density. The material can be described (based on results of optical microscopy, SEM, TEM and XRD investigation) as hierarchically (micro/nano) porous matter with ca. 40 nm silver crystals assembled in irregular ca. 70–300 nm thick filaments arranged in a foam-like structure with ca. 20 μm cavities and ca. 7 μm walls. The AgNF surface area was estimated by pseudocapacitance measurement as ca. 12 times higher than the geometrical area. Reduction of bromobenzene and other aryl bromides containing redox-inactive (F-, CH₃-, CH₃O-) or redox-active (–NO₂, –CN and CH₃CO-) substituents, as well as alkyl bromides (CF₃CHClBr, CF₂Br₂) on AgNF/GC was studied. The peak potentials of the processes assigned to debromination of the organic halides were less negative (up to +345 mV) than on smooth silver, indicating superior electrocatalytic properties of the nanoporous silver foam. Comparison of the CV peak currents of the processes on AgNF/GC and smooth silver, as well as their changes in cycling in different regimes allowed formation of a few kinds of electrocatalytically active sites on the AgNF surface to be suggested. Electrolysis of 1-bromo-4-fluorobenzene on the AgNF electrode led to fluorobenzene, and less than 1 mg of AgNF revealed performance comparable to 500 mg of smooth silver wire. The results can be utilized for creation of electrochemical sensors as well as for preparative detoxification of halogen-containing persistent organic pollutants.