Comments on the paper by M.-S. Zheng and S.-G. Sun entitled ‘In situ FTIR spectroscopic studies of CO adsorption on electrodes with nanometer-scale thin films of ruthenium in sulfuric acid solutions’ [J. Electroanal. Chem. 500 (2001) 223]

Surface Sensitive Infrared Spectroelectrochemistry using Palladium Electrodeposited on ITO-Modified Internal Reflection Elements

Palladium nanoparticles have been electrodeposited on the surfaces of conductive indium tin oxide (ITO) modified silicon internal reflection elements. The resulting films are shown to be excellent platforms for attenuated...

Probing Heterogeneity in Attenuated Total Reflection Surface-Enhanced Infrared Absorption Spectroscopy (ATR-SEIRAS) Response with Synchrotron Infrared Microspectroscopy

The heterogeneity of metal island films electrodeposited on conductive metal oxide modified internal reflection elements is shown to provide a variable attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) response. A self-assembled monolayer of a ferrocene-terminated thiol monolayer (FcC₁₁SH) was formed on the gold islands covering a single substrate, which was measured using both a conventional spectrometer and a custom-built horizontal microscope. Cyclic voltammetry and ATR-SEIRAS results reveal that the FcC₁₁SH-modified substrate undergoes a reversible electron transfer and an associated re-orientation of both the ferrocene/ferrocenium headgroup and the hydrocarbon backbone. The magnitude of the absorption signal arising from the redox changes in the monolayer, as well as the IR signature arising from the ingress/egress of the perchlorate counterions, is shown to depend significantly on the size of the infrared beam spot when using a conventional Fourier transform infrared spectrometer. By performing equivalent measurements on a horizontal microscope, the primary cause of the differences in the signal level is found to be the heterogeneity in the density of gold islands on the conductive metal oxide.

Spontaneous Electric Fields Play a Key Role in Thermochemical Catalysis at Metal-Liquid Interfaces

Large oriented electric fields spontaneously arise at all solid-liquid interfaces via the exchange of ions and/or electrons with the solution. Although intrinsic electric fields are known to play an important role in molecular and biological catalysis, the role of spontaneous polarization in heterogeneous thermocatalysis remains unclear because the catalysts employed are typically disconnected from an external circuit, which makes it difficult to monitor or control the degree of electrical polarization of the surface. Here, we address this knowledge gap by developing general methods for wirelessly monitoring and controlling spontaneous electrical polarization at conductive catalysts dispersed in liquid media. By combining electrochemical and spectroscopic measurements, we demonstrate that proton and electron transfer from solution controllably, spontaneously, and wirelessly polarize Pt surfaces during thermochemical catalysis. We employ liquid-phase ethylene hydrogenation on a Pt/C catalyst as a thermochemical probe reaction and observe that the rate of this nonpolar hydrogenation reaction is significantly influenced by spontaneous electric fields generated by both interfacial proton transfer in water and interfacial electron transfer from organometallic redox buffers in a polar aprotic ortho-difluorobenzene solvent. Across these vastly disparate reaction media, we observe quantitatively similar scaling of ethylene hydrogenation rates with the Pt open-circuit electrochemical potential (E OCP). These results isolate the role of interfacial electrostatic effects from medium-specific chemical interactions and establish that spontaneous interfacial electric fields play a critical role in liquid-phase heterogeneous catalysis. Consequently, E OCP-a generally overlooked parameter in heterogeneous catalysis-warrants consideration in mechanistic studies of thermochemical reactions at solid-liquid interfaces, alongside chemical factors such as temperature, reactant activities, and catalyst structure. Indeed, this work establishes the experimental and conceptual foundation for harnessing electric fields to both elucidate surface chemistry and manipulate preparative thermochemical catalysis.

In-Situ Infrared Spectroscopy Applied to the Study of the Electrocatalytic Reduction of CO2 : Theory, Practice and Challenges

The field of electrochemical CO₂ conversion is undergoing significant growth in terms of the number of publications and worldwide research groups involved. Despite improvements of the catalytic performance, the complex reaction mechanisms and solution chemistry of CO₂ have resulted in a considerable amount of discrepancies between theoretical and experimental studies. A clear identification of the reaction mechanism and the catalytic sites are of key importance in order to allow for a qualitative breakthrough and, from an experimental perspective, calls for the use of in-situ or operando spectroscopic techniques. In-situ infrared spectroscopy can provide information on the nature of intermediate species and products in real time and, in some cases, with relatively high time resolution. In this contribution, we review key theoretical aspects of infrared reflection spectroscopy, followed by considerations of practical implementation. Finally, recent applications to the electrocatalytic reduction of CO₂ are reviewed, including challenges associated with the detection of reaction intermediates.

Tuning the Electrocatalytic Oxygen Reduction Reaction Activity and Stability of Shape-Controlled Pt-Ni Nanoparticles by Thermal Annealing - Elucidating the Surface Atomic Structural and Compositional Changes

Shape-controlled octahedral Pt-Ni alloy nanoparticles exhibit remarkably high activities for the electroreduction of molecular oxygen (oxygen reduction reaction, ORR), which makes them fuel-cell cathode catalysts with exceptional potential. To unfold their full and optimized catalytic activity and stability, however, the nano-octahedra require post-synthesis thermal treatments, which alter the surface atomic structure and composition of the crystal facets. Here, we address and strive to elucidate the underlying surface chemical processes using a combination of ex situ analytical techniques with in situ transmission electron microscopy (TEM), in situ X-ray diffraction (XRD), and in situ electrochemical Fourier transformed infrared (FTIR) experiments. We present a robust fundamental correlation between annealing temperature and catalytic activity, where a ∼25 times higher ORR activity than for commercial Pt/C (2.7 A mg_Pt^-1 at 0.9 V_RHE) was reproducibly observed upon annealing at 300 °C. The electrochemical stability, however, peaked out at the most severe heat treatments at 500 °C. Aberration-corrected scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy (EDX) in combination with in situ electrochemical CO stripping/FTIR data revealed subtle, but important, differences in the formation and chemical nature of Pt-rich and Ni-rich surface domains in the octahedral (111) facets. Estimating trends in surface chemisorption energies from in situ electrochemical CO/FTIR investigations suggested that balanced annealing generates an optimal degree of Pt surface enrichment, while the others exhibited mostly Ni-rich facets. The insights from our study are quite generally valid and aid in developing suitable post-synthesis thermal treatments for other alloy nanocatalysts as well.

Electrochemical CO Oxidation at Platinum on Carbon Studied through Analysis of Anomalous in Situ IR Spectra

The oxidation of adsorbed CO is a key reaction in electrocatalysis. It has been studied extensively on both extended model surfaces and on nanoparticles; however, correlation between the two is far from simple. Molecular insight into the reaction is often provided using in situ IR spectroscopy; however, practical challenges mean in situ studies on nanoparticles have yet to provide the same level of detail as those on model surfaces. Here we use a new approach to in situ IR spectroscopy to study the mechanism of CO adlayer oxidation on a commercial carbon-supported Pt catalyst. We observe bipolar IR absorption bands but develop a simple model to enable fitting. Quantitative analysis of band behavior during the oxidation prepeak using the model agrees well with previous analysis based on conventional absorption bands. A second linear CO band is observed during the main oxidation region and is assigned to the distinct contribution of CO on step as opposed to terrace sites. Analysis of the step and terrace CO bands during oxidation shows that oxidation begins on the terraces of the nanoparticles before CO on steps is removed. Further correlation of this behavior with the current shows that step CO is only lost in the first of the two main oxidation peaks.

Formate adsorption on Pt nanoparticles during formic acid electro-oxidation: insights from in situ infrared spectroscopy

Adsorbed formate is observed on a supported Pt nanoparticle for the first time during formic acid electro-oxidation. Bands assigned to OCO stretching and CH bending reveal some OCO but little CH bond weakening on adsorption compared to the free anion. The formate potential dependence is similar to polycrystalline electrodes while adsorbed CO persists up to +1.2 V, 0.5 V higher than on polycrystalline Pt.

Adsorption and oxidation of ethanol on colloid-based Pt/C, PtRu/C and Pt3Sn/C catalysts: In situ FTIR spectroscopy and on-line DEMS studies

The interaction of colloid-based, carbon supported Pt/C (40 wt%), PtRu/C (45 wt%) and Pt3Sn/C (24 wt%) catalysts with ethanol and their performance for ethanol electrooxidation were investigated in model studies by electrochemical, in situ infrared spectroscopy and on-line differential electrochemical mass spectrometry measurements. The combined application of in situ spectroscopic techniques on realistic catalysts and under realistic reaction (DEMS, IR) and transport conditions (DEMS) yields new insight on mechanistic details of the reaction on these catalysts under the above reaction and transport conditions. Based on these results, the addition of Sn or Ru, though beneficial for the overall activity for ethanol oxidation, does not enhance the activity for C-C bond breaking. Dissociative adsorption of ethanol to form CO2 is more facile on the Pt/C catalyst than on PtRu/C and Pt3Sn/C catalysts within the potential range of technical interests (<0.6 V), but Pt/C is rapidly blocked by an inhibiting CO adlayer. In all cases acetaldehyde and acetic acid are dominant products, CO2 formation contributes less than 2% to the total current. The higher ethanol oxidation current density on the Pt3Sn/C catalyst at these potentials results from higher yields of C2 products, not from an improved complete ethanol oxidation to CO2.

Extending in Situ Attenuated-Total-Reflection Surface-Enhanced Infrared Absorption Spectroscopy to Ni Electrodes

Surface-enhanced infrared absorption spectroscopy (SEIRAS) in the attenuated-total-reflection configuration (ATR-SEIRAS) has been applied for the first time to Ni electrodes. SEIRA-active Ni electrodes were obtained through initial chemical deposition of a 60-nm-thick Au underlayer on the reflecting plane of an ATR Si prism followed by potentiostatic electrodeposition of a 40-nm-thick Ni overlayer in a modified Watt's electrolyte. The Ni nanoparticle film thus obtained exhibited exceptionally enhanced IR absorption for the surface probe molecule CO while maintaining unipolar and normally directed bands. With the advantages of ATR-SEIRAS, free H2O molecules coadsorbed with CO at the Ni electrode were revealed, and their role in the electrooxidation of the CO adlayer at the Ni electrode is discussed. In addition, the conversion of bridge to linearly bonded CO at Ni electrode in a neutral solution was clearly identified upon electrooxidation of the CO adlayer. ATR-SEIRAS was also used to characterize the adsorption configuration of a pyridine adlayer at the Ni electrode. Both A1 and B1 modes of adsorbed pyridine were detected with comparably large intensities, essentially maintaining the spectral feature of pyridine molecules rather than that of "alpha-pyridyl species", which strongly suggests an "edge-tilted pyridine" configuration present at the Ni electrode, a configuration intermediate between the "end-on pyridine" and "edge-on alpha-pyridyl" adsorption modes reported in the literature.

Ubiquitous Strategy for Probing ATR Surface-Enhanced Infrared Absorption at Platinum Group Metal−Electrolyte Interfaces

A versatile two-step wet process to fabricate Pt, Pd, Rh, and Ru nanoparticle films (simplified as nanofilms hereafter) for in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) study of electrochemical interfaces is presented, which incorporates an initial chemical deposition of a gold nanofilm on the basal plane of a silicon prism with the subsequent electrodepostion of desired platinum group metal overlayers. Galvanostatic electrodeposition of Pt, Rh, and Pd from phosphate or perchloric acid electrolytes, or potentiostatic electrodeposition of Ru from a sulfuric acid electrolyte, yields sufficiently "pinhole-free" overlayers as evidenced by electrochemical and spectroscopic characterizations. The Pt group metal nanofilms thus obtained exhibit strongly enhanced IR absorption. In contrast to the corresponding metal films electrochemically deposited directly on glassy carbon and bulk metal electrodes, the observed enhanced absorption for the probe molecule CO exhibits normal unipolar band shapes. Scanning tunneling microscopic (STM) images reveal that fine nanoparticles of Pt group metals are deposited around wavy and stepped bunches of Au nanoparticles of relatively large sizes. This ubiquitous strategy is expected to open a wide avenue for extending ATR surface-enhanced IR absorption spectroscopy to explore molecular adsorption and reactions on technologically important transition metals, as exemplified by successful real-time spectroscopic and electrochemical monitoring of the oxidation of CO at Pd and that of methanol at Pt nanofilm electrodes. The spectral features of free water molecules coadsorbed with CO on Pt, Pd, Rh, and Ru are also discussed.