Infrared spectroscopy of model electrochemical interfaces in ultrahigh vacuum: interfacial cation solvation by ammonia on Pt(111)

Abstract

Infrared reflection-absorption spectroscopic (IRAS), along with work-function (Φ), measurements are reported for ammonia dosed onto potassium-predosed as well as clean Pt(111) in ultrahigh vacuum (UHV) with the objective of assessing the nature of interfacial cation solvation, which is of fundamental relevance to electrochemical systems. Ammonia was selected as a strongly coordinating as well as chemisorbing solvent, for comparison with other dipolar hydrogen-bonding media examined previously in this manner. In the absence of potassium, ammonia chemisorption at 90 K yields several vibrational features in the NH stretching ( v NH) and especially the symmetric HNH bending ( δ s HNH) regions: their identification with distinct chemisorbed, second-layer and multilayer ammonia states is aided by comparison of temperature-dependent spectra with earlier temperature-programmed desorption (TPD) data. These chemisorbate states were absent for ammonia dosed onto a saturated CO adlayer. In the presence of predosed potassium, distinctly different v NH and δ s HNH spectral features were obtained for initial ammonia dosages (corresponding to small NH 3 K stoichiometries, ≈1). Comparison with vibrational spectra for bulk-phase K +NH 3 species support the occurrence of K +-induced reorientation of ammonia, involving also NH 3-surface hydrogen bonding. Further ammonia exposure yielded frequency downshifts in the δ s HNH mode, ostensibly similar to that observed for second-shell solvation of gas-phase Na + and indicative of primarysecondary shell H bonding. However, completion of the K + primary solvation shell requires markedly higher ammonia exposures, reflecting further the competition between K +- and surface-ammonia interactions. In contrast to other solvents, Φ was observed to be essentially independent of the K coverage (≈3.0 eV) in the presence of ammonia multilayers. Comparison with solution-phase electrochemical data suggests strongly that solvated electrons along with K + are formed at the Pt(111)UHV interface under these conditions.