ФИЗИКОХИМИЯ ПОВЕРХНОСТИ И ЗАЩИТА МАТЕРИАЛОВ, 2009, том 45, № 1, с. 5-21
СОВРЕМЕННЫЕ ПРОБЛЕМЫ ФИЗИЧЕСКОЙ ХИМИИ ПОВЕРХНОСТИ, МАТЕРИАЛОВЕДЕНИЯ, ЗАЩИТЫ МАТЕРИАЛОВ
УДК 541.64:547.96
RECENT TRENDS IN EXPERIMENTAL AND THEORETICAL INVESTIGATIONS OF CHEMISORPTION ON METAL-ELECTROLYTE INTERFACE. I. IN SITU SPECTROSCOPIC STUDIES AND THE DENSITY FUNCTIONAL THEORY CALCULATIONS
© 2009 V. A. Marichev
Department of Chemistry, University of Western Ontario London, Ontario, Canada N6A 5B7. E-mail: vmarichev@mail.ru; marichev@rogers.com Поступила в редакцию 14.05.2007 г.
New trends in experimental and theoretical investigations of chemisorption on electrodes are considered on examples of in situ spectroscopic studies and the density functional theory calculations. The partial charge transfer during ionic and molecular adsorption from aqueous solutions on coinage and platinum metals and the thermo-dynamic uncertainty regarding the direction of the charge transfer are discussed.
PACS: 68.43.-h, 78.30.-j
1. INTRODUCTION
During the last decades, the results of novel experimental and theoretical methods have begun to clarify our understanding of ionic and molecular adsorption. The main contribution has been done by in situ structure sensitive techniques like scanning probe microscopy, X-ray scattering and spectroscopy using synchrotron radiation allowing the direct studies of the electrochemical interface on the atomic level. These methods provide detailed data especially on the structure of the ordered adlayers as a function of the potential, i.e., at the high enough coverage degree of adsorbate. Considerable progress has also been made in the theoretical descriptions, mainly in computer simulations, of the molecular adsorption on the initially clean surfaces in the ultra high vacuum (UHV) conditions, but the same could be hardly said in connection with anion adsorption on the electrochemical interfaces. At the beginning stage of adsorption (in the absence of stable adsorption structures), our experimental and theoretical possibilities stay far from being sufficient for the investigation of the partial charge transfer at the electrochemical interfaces.
Both experimental and theoretical approaches to the chemisorption on the electrodes are developed fast. The most important trend in this development appears to be a growing understanding of necessity to conjoint the theory and experiment. For example, we quote several phrases from two latest reviews [1, 2] devoted to the quantum chemical modeling of the charge transfer and catalysis and written by theoreticians who are familiar not only with numerical methods of investigations, but also the with electrochemical experiments. The first one: ".. .I strongly believe that only a combination of theory and experiment will lead to the understanding of electrocatalysis (and chemistry and catalysis in general) that transcends the phenomeno-
logical level. In particular, this is true for the application ab initio quantum chemical calculations, as these allow a relation to be established between experimental trends and the most fundamental chemical interactions" (Koper [1]), and the second one: "... Kinetics of the heterogeneous charge transfer belongs to the heart of electrochemistry and remains the most intriguing problem since the Tafel times. Some electrochemists-experimenters are still held captive by an illusion that they have already accumulated large amount of accurate data on electrode kinetics and that one can simply wait for the appearance of a novel theoretical interpretation. At the same time, some theoreticians are fully confident that many theoretical predictions come ahead of new experimental results that will be obtained in future. In the meanwhile, only an interdisciplinary correlation can lead to significant progress in this science." (Nazmutdinov et al. [2]).
Such ideas about "interdisciplinary correlation" are repeated from time to time mainly by theoreticians dealing with ab initio calculations in electrochemistry. They are absolutely correct, but are there some examples demonstrating the theory development basing on a new experimental approach during such cooperation or some non-trivial experiments planned and carried out following the theoretical predicaments? From the viewpoint of an experimenter, these quotations are the brief expert characteristics of the state of art in electrochemistry reflecting substantial difficulties of theoretical investigations of chemisorption and the heterogeneous charge transfer compared to the overall doubtless successes of general electrochemistry in last decades. A certain unbalance between theoretical and experimental studies of the charge transfer does exist, as well as the difficulties of combining them properly.
It is important to clarify that we do not review the concept of the partial charge transfer (PCT) itself since there were hundreds theoretical, experimental and review articles that formulated, justified and used this concept during last four decades. We will use the current state-of-art of this concept and try to follow development and application of both experimental and computational methods to evaluate the PCT value not pretending on innovations in the theoretical interpretation of this notion.
2. NEW EXPERIMENTAL AND THEORETICAL APPROACHES TO STUDY CHEMISORPTION AND CHARGE TRANSFER ON ELECTRODES
2.1. Ultra high vacuum (UHV) simulations of the metal-electrolyte interface
As being "new", we will consider the experimental and theoretical approaches proposed not only during the last few years but also some works published early if they have the interesting and productive continuation now days. In particular, data obtained by the UHV modeling of the electrochemical interface 15-20 years ago are often used now for a comparison with contemporary methods of ab initio adsorption simulation. Sass et al. [3-5] developed the concept of experimental simulating the metal-electrolyte interface by adsorption of solution components onto metal surfaces in UHV conditions. They investigated water adsorption in UHV, both on clean metal surfaces and in the presence of a coadsorbate and compared results with classical models of the electric double layer at a metal-electrolyte interface. Work function measurements, thermo-des-orption spectroscopy (TDS) and high-resolution electron energy loss spectroscopy (HREELS) have been used to characterize the dielectric screening by interfacial water. In Ref [3], three coadsorption systems, involving electro-chemically relevant ionic species interacting with water, were described to illustrate the detailed microscopic insight provided by this approach. In particular, the electrostatic potential drop in the inner layer for specifically adsorbed chloride and bromide on Ag(110) was shown to be very similar in situ and in UHV. Sass et al. [4, 5] considered a comparative energy scales based on the work function used in UHV-surface science and electrode potential used in electrochemistry. Basing on this approach they discussed the distinction between surface hydration and reaction processes in UHV studies of electrochemical phenomena [5].
They also studied coadsorption of H2O with Cs on the Cu(110) surface at low cesium coverage. Solvation of the adsorbed alkali is observed and above a critical coverage of 0Cs = 0.15 a surface reaction occurs forming hydroxide and hydrogen. TDS and HREELS measurements using isoto-pic substitution of hydrogen by deuterium have been used to investigate hydrogen adsorption on Cu(110) and Pt(111)
and the formation of H3O+d. It was shown that on copper, hydrogen is merely solvated by water, but on platinum, the production of hydrated protons occurred. The latter is connected with the higher work function of H2O-Pt system,
corresponding to a more positive electrode potential. Developing further this approach, Villegas and Weaver [6, 7] illustrated the utility of infrared reflection-absorption spec-troscopy (IRAS) for modeling the electrochemical interfaces in UHV experiments focusing on ionic and chemi-sorbate solvation at electrode surfaces. Their method consists of "synthesizing" electrochemical interfaces directly in UHV by dosing from gas phase the clean metal surface with the desired solute, solvent and double layer ionic components at low temperature (as a rule below 200 K) to avoid evaporation. They clearly demonstrated (for example, see Fig. 2 in [6]) the important changes in the vOD spectral response of the Pt(l11) surface predosed with 0.08 monolayers (ML) of K upon stepwise increasing D2O coverage used in this particular run as a solvent. Authors [6] concluded: "...Nevertheless, the net interactions triggered by the combined presence of chemisorbate, solvent, ions and electrons seem daunting (or even hopelessly complex) to UHV scientists, the inherently synergetic effects exerted by ion/solvent coadsorption can in principle be evaluated satisfactory on a molecular level."
As a rule, UHV modeling experiments [3-7] pursue only one sequence of adsorption of the solution components. At first, a small coverage (0 < 0.1 ML of K or CO) is dosed on the clean metal surface, than the solvent coverage (water, deuterium water, methanol or acetonitrile) is increased step by step in the range from 0 < 0.1 up to complete coverage. The letter results in the appearance and transformation of the solvent specific spectra and in the gradual disappearing of the predosed adsorbate spectra. Some equilibrium coverages of both components were created after each additional dose of solvent. In this way, the solvation and dissolution processes of the predosed K or CO on the platinum electrode surface are imitated in different solvents. However, an opposite sequence of adsorption events in low-pressure gaseous conditions would be also interesting and important to investigate.
Obviously, in real electrolyte, the solvent adsorption should occur at the electrode surface creating its
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