Report
Exploring Electrochemical Interfaces
I
n this Report we review recent progress in the use of nuclear magnetic resonance (NMR) spectroscopy to study static, dynamic, and electronic structure at the solid-liquid electrochemical interface. NMR is an extremely powerful analytical tool for many areas of research, but its low sensitivity makes applications to surface science rather challenging. Nevertheless, since the early 1980s its use in surface science, especially for heterogeneous catalysis, has been extensively developed (1-9). More recently these solid-state NMR techniques have been further developed for use in electrochemical systems (10-17) which is the focus of this Report To appreciate the difficulties with sensitivity encountered in surface NMR measurements, it should be noted that a typical high-field NMR instrument needs around 1018-1019 NMR active atoms, for example 13 C spins, to make a signal detectable within a reasonable period. However, 1 cm2 of a single crystal metal surface contains only about 1015 atoms. Therefore, at least 1 m 2 surface area of NMR active atoms is needed to meet the sensitivity requirement. This can be met by working with large surface area samples, such as those of supported heterogeneous metal catalysts (2 3 9) but generally at the expense of broad linewidths caused by heterogeneity Despite these difficulties work with dry catalyst surfaces has proven to be very fruitful (i-9) Forexample NMR has yielded uniaue information on the Fermi-level density of states in which the Fermi IPVPI is
defined as the electronic enerov level that separates the ocnipierl and unomipied elec 518 A
Metal and adsorbate NMR spectra are recorded in an electrochemical environment. tronic orbitals within the conduction band of a metal at 0 K, as well la so surface eonding and molecular structure of adsorbates, and on the dynamics at interfaces. NMR studies can be carried out under experimental conditions that are very close to those encountered with real-world heterogeneous catalysts, without any perturbation of the system caused by the very small probing quanta. .I has been only in the past few years, however, that solid-state NMR techniques have been developed for investigating electrochemical interfaces in which the ability to control in situ the potential of working catalysts can offer major new insights into the molecular mechanisms of electrocatalyst structure (static dynamic and electronic) and function (10-17)
cules on both sides of the interface, that is, the adsorbate and the substrate. All organic reactants contain carbon atoms, which can be selectively labeled with 13C, although platinum is the most frequently used metal ingredient in heterogeneous catalysts. Also, both 13C and 195Pt possess reasonable gyromagnetic ratios, ensuring adequate sensitivity. They also have a nuclear spin / = 1/2, which generally simplifies data interpretation because of fhe absence of quadrupolar effects. Indeed, 13C NMR of submonolayer chemisorbed small organic molecules has provided numerous textbook examples of how NMR techniques can be used to study the chemical and physical properties of adsorbates (1-7). In contrast, 195Pt NMR of catalyst samples, usually in the form of small metallic particles supported on oxides, is a little more challenging because one has to solve the problem "How can a signal from surface atoms be distinguished from that of atoms inside the particles?"
Halperin and his co-workers were the first to pioneer the use of 195Pt NMR for studying small self-supported platinum particles, ranging in size from 3.3 to 20 nm (18). They did not, however, observe a distinguishable surface signal. A couple of years Within the realm of heterogeneous catallater, a seminal paper by Slichter and coysis, 13C and 195Pt NMR appear to be an workers (19) clearly demonstrated several ideal pair of noninvasive microscopic probes unique features of the 195Pt NMR of oxidewith which to investigate atoms and molesupported small platinum particles. They found that the overall 195Pt NMR lineshape was extremely broad, extending downfield YuYe Tong some 4 kG from the position of bulk platiEric Oldfield num (1.138 G/kHz) and containing a feature Andrzej Wieckowski on the low-field side (1089 G/kHz) arising University of lllinois st Urbana-Champaign
Analytical Chemistry News & Features, August 1, 1998
with Solid-State NMR
from surface atoms. Van der Hink and coworkers (20-22) later confirmed these observations, finding that the signal from cleansurface atoms was centered at 1.100 G/kHz, a position clearly very different from the bulk position (the frequency difference is magnetic field-dependent and is about 2.5 MHz in a field of 8.5 T). Ab initio calculations on a five-layer Pt(001) cluster demonstrated that the surface shift must be caused by a gradual drop in the d-like Fermi-level local density of states upon moving from the
inside of the particle to the surface (23). It is this distinguishable surface signal that makes 195Pt NMR unique in the investigation of the surface physics and chemistry of nanoscale platinum particles. Solid-state NMR studies at an electrochemical interface, composed of submonolayer adsorbates and electrocatalyst and in an electrochemical environment, are even more challenging. This is because the presence of conducting material in the NMR detection coil degrades considerably the so-
called quality factor of the probe, which decreases sensitivity. Nevertheless, the advantages of interfacing in situ solid-state NMR with electrochemistry for investigating the electrochemical interface are many. For example, NMR directly probes the surface dynamics and the Fermi-level local densities of states of adsorbates and substrates. Moreover, the structure and bonding in the electrochemical environment can be investigated using various NMR interactions and the
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Report For room-temperature electrode potentialdependence studies, we incorporated an electrochemical cell inside the NMR probe. This permits running NMR measurements while an external electrode potential is applied and varied. Figure 1 shows a schematic diagram of the setup. This cell design permits voltammetry, NMR data acquisition, and potential control in the probe. No sample transfer from a preparative electrochemical cell ls required. While the potentiostat is on, there is considerable "noise" injected into the NMR probe. This is removed by extensive electronic filtering of the electrochemical leads entering the probe. The low-pass filter circuit elements L t C » L ,C , and V.W11. v.i\.iiiv.in.o ^counter^counter. ^ref. ^ref,
Figure 1. Schematic of the NMRE probe circuitry showing the interface between NMR and electrochemistry.
effects of potential control can be studied in working electrochemical systems—even where the adsorbates (such as CN" on platinum) would not be amenable to investigation in a conventional gas/solid interfacial system. Although very promising, NMR/electrochemistry is still in its early stage of development. Consequently, published data are primarily from our lab and the Berkeley group (27). Nevertheless, in this Report, we can show that it is possible to record metal and adsorbate NMR spectra in an electrochemical environment; that spectra may be obtained over a wide range of temperatures; that both metal and adsorbate relaxation times (T, and T2), surface diffusion rates, and activation energies for surface diffusion Ccin be determined; that D3.rti.cle sizes c
