Mechanism and kinetics of the electrosorption of sulphate anions on the (111) face of silver

Abstract

The processes of adsorption of sulphate anions onto the (111) face of silver have been investigated by analysis of the cyclic voltammetry and potcntiostatic pulse results. It was shown, by fitting the experimentally recorded θ - E dependences, that the adsorption of anions follows Frumkin adsorption isotherm. The adsorbed structure was found to be less dense than expected for the dense packed anion monolayer and the process was characterized by high value of the interaction parameter and the Gibbs energy of adsorption (f = 16.5 and ΔGoads.θ→0 = - 64 kJ mol 1). From the slope of the Ep.a - log (c0) dependence it was concluded that complete charge transfer between the adsorbed anions and the silver substrate occurs. This is supported by the Ep.a - log v linear dependence in the region of high sweep rates (v > 10 V s 1) with rational value of line slope (≈ 60 mV dec 1) which is possible only if the complete charge transfer reaction occurs. The elcctrosorption process of sulphate anions was found to be complex electrochemical reaction, taking place through three steps. It was proposed that the rate-determining step is the second step, i.e., the electrochemical formation of (SO4)ads species.

Proučavan je proces adsorpcije sulfatnih anjona na (111) pljosni monokristala srebra metodama ciklične voltametrije i potenciostatskih pulseva. Na osnovu eksperimentalno određenih q–E zavisnosti pokazano je da se na adsorpciju može primeniti Frumkinova adsorpciona izoterma. Nađeno je da je gustina adsorbovanog sloja manja nego kada bi sloj bio maksimalno moguće gusto pakovan kao i da adsorpciju karakterišu visoke vrednosti Frumkinovog parametra interakcije (f = 16,5) i Gibsove energije adosrpcije (DGºads,q®0 = –64 kJ mol). Na osnovu nagiba zavisnosti Ep,a –log (c0) zaključeno je da se pri elektrosorpciji odigrava i potpuna izmena naboja anjona sa supstratom. Ovaj zaključak podržava dobijena linearna zavisnost Ep,a – log v u oblasti velikih brzina promene potencijala (v>10 V s–1) sa racionalnom vrednošću nagiba (»60 mV dek–1), a koja se može dobiti samo pri odigravanju potpune razmene naboja. Utvrđeno je da je elektrosorpcija sulfatnih anjona složena elektrohemijska reakcija koja se odigrava u tri stupnja. Stupanj koji određuje brzinu ukupne reakcije je drugi stupanj, elektrohemijsko stvaranje intermedijera (SO4)ads.