Total Reflection Surface-enhanced Infrared Absorption Research Articles

In situ surface-enhanced infrared absorption analysis of the excited carrier transfer from n-type Si photoelectrode to Pt oxygen evolution cocatalyst by probing adsorbed CO molecules

BackgroundThe development of efficient photoelectrode material for water-splitting reactions has received considerable attention as it produces hydrogen gas from water using solar energy. Photoexcited holes migrate to the surface of an n-type photoelectrode oxidizing water to oxygen, and excited electrons migrate to the counter electrode reducing protons to hydrogen. However, there is limited knowledge of excited carrier transfer. MethodsTherefore, in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) has been used to investigate the excited hole migration from n-type Si photoelectrode to Pt oxygen evolution cocatalysts by monitoring the frequency of CO adsorbed on the Pt cocatalysts. Significant findingsAt the positive potential of Si photoelectrode under UV irradiation, the SEIRA spectrum of the CO adsorbed on the Pt cocatalyst shifted to higher wavenumbers owing to the excited hole transfer from n-type Si photoelectrode to the Pt cocatalyst attributed to band bending at the interface between the Si photoelectrode and Pt cocatalysts. It was successfully demonstrated that the regulation of photoexcited carriers was crucial for enhancing the photoelectrochemical activity of water splitting.

Role of Electrolyte-Adsorbates Interaction in Determining CO2 Reduction Reaction Pathways

Electrochemical CO2 reduction reaction (CO2RR) is a promising strategy to convert atmospheric CO2 into valuable fuels and chemicals. Although the optimization of the catalyst-adsorbates interaction via tuning the binding energies of electrocatalysts has been extensively studied, activity and product selectivity remain a challenge. Recently, there has been a growing recognition that cations in electrolytes influence the activity of surface electrocatalytic reactions.1 A recent density functional theory study suggested that partially desolvated metal cations stabilize CO2RR intermediates via electrostatic interactions.2 Therefore, an experimental understanding of how the cations (and surrounding water molecules) control the intermediates present on the surface and govern the reaction pathway is critical to establish further insights into the design of electrolytes for CO2RR.In this talk, the role of alkali metal cations and water molecules on the CO2RR is revealed by tracking the surface-bound intermediates using operando attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS). We will also integrate the operando surface analysis with final product selectivity, further strengthening the understanding of the reaction mechanism. It will also be demonstrated that the holistic information about the electrolyte-adsorbates interaction can provide additional knobs to tune the energetics of key reaction intermediates, leading to further improvements in electrocatalytic activity for the CO2RR.[Reference]1 Ringe, S. et al., Energy Environ. Sci. 2019, 12, 3001–3014.2 Monteiro, MCO. et al., Nat. Catal. 2021, 4, 654–662.

Achieving Efficient CO2 Electrolysis to CO by Local Coordination Manipulation of Nickel Single-Atom Catalysts.

Selective electroreduction of CO2 to C1 feed gas provides an attractive avenue to store intermittent renewable energy. However, most of the CO2-to-CO catalysts are designed from the perspective of structural reconstruction, and it is challenging to precisely design a meaningful confining microenvironment for active sites on the support. Herein, we report a local sulfur doping method to precisely tune the electronic structure of an isolated asymmetric nickel-nitrogen-sulfur motif (Ni1-NSC). Our Ni1-NSC catalyst presents >99% faradaic efficiency for CO2-to-CO under a high current density of -320 mA cm-2. In situ attenuated total reflection surface-enhanced infrared absorption spectroscopy and differential electrochemical mass spectrometry indicated that the asymmetric sites show a significantly weaker binding strength of *CO and a lower kinetic overpotential for CO2-to-CO. Further theoretical analysis revealed that the enhanced CO2 reduction reaction performance of Ni1-NSC was mainly due to the effectively decreased intermediate activation energy.

Reaction-induced iodine adsorption on Cu surfaces facilitates electrocatalytic CO2 reduction.

The electrolyte effect has been key to the electrochemical CO2 reduction reaction (CO2RR) and has received extensive attention in recent years. Here we combined atomic force microscopy, quasi-in situ X-ray photoelectron spectroscopy, and in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) to study the effect of iodine anions on Cu-catalyzed CO2RR in the absence or presence of KI in the KHCO3 solution. Our results suggested that iodine adsorption caused coarsening of the Cu surface and altered its intrinsic activity for CO2RR. As the potential of the Cu catalyst became more negative, there was an increase in surface iodine anion concentration ([I-]), which could be connected to the reaction-enhanced adsorption of I- ions accompanying the increase in CO2RR activity. A linear relationship was observed between [I-] and current density. SEIRAS results further suggested that the presence of KI in the electrolyte strengthened the Cu-CO bond and facilitated the hydrogenation process, enhancing the production of CH4. Our results have thus provided insight into the role of halogen anions and aided in the design of an efficient CO2RR process.

A Spectroscopic Study on Nitrogen Electrooxidation to Nitrate.

As a potential substitute technique for conventional nitrate production, electrocatalytic nitrogen oxidation reaction (NOR) is gaining more and more attention. But, the pathway of this reaction is still unknown owing to the lack of understanding on key reaction intermediates. Herein, electrochemical in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) and isotope-labeled online differential electrochemical mass spectrometry (DEMS) are employed to study the NOR mechanism over a Rh catalyst. Based on the detected asymmetric NO2- bending, NO3- vibration, N=O stretching, and N-N stretching as well as isotope-labeled mass signals of N2O and NO, it can be deduced that the NOR undergoes an associative mechanism (distal approach) and the strong N≡N bond in N2 prefers to break concurrently with the hydroxyl addition in distal N.

Charge-acquired Pb2+ boosts actively and selectively electrochemical carbon dioxide reduction reaction to formate

For lead carbonate-based (PbCO3) carbon dioxide reduction reaction (CO2RR) electrocatalysts, the electron-deficiency of Pb2+ active sites adverse to activate *CO2 and generate HCOO* intermediates hinder the active and selective CO2RR to formate. In this work, the Co modified (heterojunction and doping) PbCO3 electrocatalyst is constructed with prominent catalytic activity and high formate selectivity (Faradaic efficiency about 98.15% at −0.70 V vs. RHE). The in-depth experimental and theoretical analyses reveal that the Co-PbCO3 interaction can enable the electron of Co transfer to Pb2+, leading to the charge-acquired Pb2+ (CA-Pb2+). Its highly catalytic activity and selectivity of formate is attributed to the reduced formation energy barrier and enhanced adsorption behavior of HCOO* intermediate. The in-situ attenuated total reflection surface-enhanced infrared absorption spectroscopy demonstrates HCOO* intermediate is more likely to generate and adsorb on CA-Pb2+ than Pb2+, verifying the charge acquisition of Pb2+ enhances the activity and selectivity of formate product.

Unraveling the electrocatalytic reduction mechanism of enols on copper in aqueous media

Deoxygenation of aldehydes and their tautomers to alkenes and alkanes has implications in refining biomass-derived fuels for use as transportation fuel. Electrochemical deoxygenation in ambient, aqueous solution is also a potential green synthesis strategy for terminal olefins. In this manuscript, direct electrochemical conversion of vinyl alcohol and acetaldehyde on polycrystalline Cu to ethanol, ethylene and ethane; and propenol and propionaldehyde to propanol, propene and propane is reported. Sensitive detection was achieved using a rotating disk electrode coupled with gas chromatography-mass spectrometry. In-situ attenuated total reflection surface-enhanced infrared absorption spectroscopy, and in-situ Raman spectroscopy confirmed the adsorption of the vinyl alcohol. Calculations using canonical and grand-canonical density functional theory and experimental findings suggest that the rate-determining step for ethylene and ethane formation is an electron transfer step to the adsorbed vinyl alcohol. Finally, we extend our conclusions to the enol reaction from higher-order soluble aldehyde and ketone. The products observed from the reduction reaction also sheds insights into plausible reaction pathways of CO2 to C2 and C3 products.

In Situ Wide-Frequency Surface-Enhanced Infrared Absorption Spectroscopy Enables One to Decipher the Interfacial Structure of a Cu Plating Additive.

In situ spectroscopic characterization of the interfacial structure of an organic additive at a Cu electrode is essential for a mechanistic understanding of Cu superfilling at the molecular level. In this work, we demonstrate wide-frequency attenuated total reflection surface-enhanced infrared absorption spectroscopy (wf-ATR-SEIRAS) to elucidate the dissociative adsorption of bis(sodium sulfopropyl)-disulfide (a typical accelerator) on a Cu electrode in conjunction with the electrochemical quartz crystal microbalance measurement and modeling calculations. The wf-ATR-SEIRAS clearly identifies the peaks featuring the sulfonate and methylene groups as well as the C-Ssulfonate and C-Sthiol vibrations of the adsorbate. Analysis of relative peak intensities from 1100 to 650 cm-1 reveals a more tilted alkyl chain axis for the thiolate on Cu than that on Au, which is supported by comparative density functional theory calculations. This work opens a new avenue for the wf-ATR-SEIRAS to study interfacial structures of electroplating additives related to advanced microelectronics manufacture.

Electrochemical Upgrading of Formic Acid to Formamide via Coupling Nitrite Co-Reduction.

Formic acid (HCOOH) can be exclusively prepared through CO2 electroreduction at an industrial current density (0.5 A cm-2). However, the global annual demand for formic acid is only ∼1 million tons, far less than the current CO2 emission scale. The exploration of an economical and green approach to upgrading CO2-derived formic acid is significant. Here, we report an electrochemical process to convert formic acid and nitrite into high-valued formamide over a copper catalyst under ambient conditions, which offers the selectivity from formic acid to formamide up to 90.0%. Isotope-labeled in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy and quasi in situ electron paramagnetic resonance results reveal the key C-N bond formation through coupling *CHO and *NH2 intermediates. This work offers an electrochemical strategy to upgrade CO2-derived formic acid into high-value formamide.

The PdHx metallene with vacancies for synergistically enhancing electrocatalytic N2 fixation

It is of great significance to controlled synthesis of high activity, selectivity and durable electrocatalysts. In this work, for the first time, the ultrathin (0.9 nm) of interstitial metallene with controlled vacancies and lattice hydrogen atoms are successfully gained. With the optimal lattice hydrogen atoms and composition, the interstitial VPd-PdH0.41 metallene exhibits the great nitrogen reduction (NRR) activity in Li2SO4 solution, among interstitial PdHx with different extent of vacancies, interstitial PdH0.41 without vacancies and Pd nansheets. The density functional theory (DFT), in situ Raman and Attenuated total reflection surface-enhanced infrared absorption spectrum (ATR-SEIRAS) tests further display that the VPd on the interstitial VPd-PdH0.41 metallene can urges the adsorption and arousal of N2, while lattice hydrogen atoms as active hydrogen source to follow an association mechanism rather than a dissociation mechanism, showing nice NRR performance at low overpotential.

Adjusting Local CO Confinement in Porous-Shell Ag@Cu Catalysts for Enhancing C-C Coupling toward CO2 Eletroreduction.

Tuning the local confinement of reaction intermediates is of pivotal significance to promote C-C coupling for enhancing the selectivity for multicarbon (C2+) products toward CO2 electroreduction. Herein, we have gained insights into the confinement effect of local CO concentration for enhanced C-C coupling over core-shell Ag@Cu catalysts by tuning the pore diameters within porous Cu shells. During CO2 electroreduction, the core-shell Ag@Cu catalysts with an average pore diameter of 4.9 nm within the Cu shells (Ag@Cu-p4.9) exhibited the highest Faradaic efficiency of 73.7% for C2+ products at 300 mA cm-2 among the three Ag@Cu catalysts. Finite-element-method simulations revealed that the pores with a diameter of 4.9 nm in Cu conspicuously enhanced the local CO concentration. On the basis of in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy measurements, Ag@Cu-p4.9 exhibited the highest surface coverage of adsorbed CO intermediates with a linear adsorption configuration due to the confinement effect, thus facilitating C-C coupling.

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.

Optimizing CO Coverage on Rough Copper Electrodes: Effect of the Partial Pressure of CO and Electrolyte Anions (pH) on Selectivity toward Ethylene

The conversion of the initial intermediate CO in the electrochemical reduction reaction of CO2 on the surface of oxide-derived Cu electrodes has been investigated as a function of partial pressure and pH, manipulated by the composition of the electrolyte. We show that in inert gas, an increase in partial pressure of CO results in a continuous increase in Faradaic efficiency (FE) for ethylene, at various potentials ranging from −0.7 to −1.1 V versus RHE, with the highest FE of ∼28% obtained using 1 bar CO at −0.8 V. When the partial pressure of CO is increased in a mixture of CO and CO2, an optimum in the ethylene FE was found for the partial pressure of CO in the range from 0.5 bar (at −1.1 V, FE is ∼45%) to 0.8 bar (at −0.9 V, FE is ∼35%). At lower negative potentials (−0.8 to −0.7 V), the presence of CO2 has negligible influence, and similar data to reduction of CO in inert gas were obtained. Variation of the anion in solution (0.1 M concentration) shows that the optimized FE toward ethylene increases from 5.2% in KH2PO4 to 43.2% in KOH. The observed differences in selectivity are attributed to anion buffering capacity and the associated local pH near the surface of the electrode. Using in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS), it was determined that the CO coverage increases as a function of increasing pH, confirming that CO coverage and pH correlate. Collectively, the data herein outline the critical role of reactant partial pressures and the significant effect of anion composition (pH) on the surface coverage of CO and concomitant selectivity in electrochemical reduction of CO2 to ethylene.

The rational adjusting of proton-feeding by Pt-doped FeP/C hollow nanorod for promoting nitrogen reduction kinetics

Following the guidance of nitrogenase, the natural iron-based catalysts with remarkably high activity in nitrogen reduction reaction (NRR) may be a talent option to develop artificial molecular mimics. However, the electrochemical NRR process of iron-based catalysts still suffers from the high overpotential, poor lifespan, and slow reaction kinetics of the hydrogenation process. Through a controllable etching and in-situ carbonating process, a Pt-doped FeP/C hollow nanorod catalysts was fabricated. Driven by the doped Pt atom, the catalyst solves the key kinetic problem about insufficient proton feeding in the process of nitrogen reduction, and actively transfers protons to intermediates of the hydrogenation steps in NRR. Moreover, the as-prepared catalyst presents the highest NH3 yield value of 10.22 μg h−1 cm−2 among multitudinous Fe based catalysts at lower potential of −0.05 V (vs. RHE) with an excellent Faraday efficiency (FE) of 15.3 %, which is higher than FeP/C without the Pt-driven proton-feeding effect, and the cathodic current density presents no significant change within 50 h. In addition, the density functional theory (DFT) and Attenuated total reflection surface-enhanced infrared absorption spectrum (ATR-SEIRAS) results verify the reaction path and protons-feeding kinetic process.

Efficient Electrocatalytic CO2 Reduction to C2+ Alcohols at Defect-Site-Rich Cu Surface

Summary Electrochemical CO2 reduction is a promising approach for upgrading excessive CO2 into value-added chemicals, while the exquisite control of the catalyst atomic structures to obtain high C2+ alcohol selectivity has remained challenging due to the intrinsically favored ethylene pathways at Cu surface. Herein, we demonstrate a rational strategy to achieve ∼70% faradaic efficiency toward C2+ alcohols. We utilized a CO-rich environment to construct Cu catalysts with stepped sites that enabled high surface coverages of ∗CO intermediates and the bridge-bound ∗CO adsorption, which allowed to trigger CO2 reduction pathways toward the formation alcohols. Using this defect-site-rich Cu catalyst, we achieved C2+ alcohols with partial current densities of > 100 mA·cm−2 in both a flow-cell electrolyzer and a membrane electrode assembly (MEA) electrolyzer. A stable alcohol faradaic efficiency of ∼60% was also obtained, with ∼500 mg C2+ alcohol production per cm2 catalyst during a continuous 30-h operation.

Revisiting the Acetaldehyde Oxidation Reaction on a Pt Electrode by High-Sensitivity and Wide-Frequency Infrared Spectroscopy.

High-sensitivity and wide-frequency attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) is highly demanded in unraveling electrocatalytic processes at the molecular level. In this work, an in situ ATR-SEIRAS technique incorporating a micromachined Si wafer window, p-polarized infrared radiation, and isotope labeling is extended to revisit the acetaldehyde oxidation reaction (AOR) on a Pt electrode in an acidic medium. New spectral features in the fingerprint region are detected, including ω(C-H) at 1078 cm-1 and νas(C-C-O) at 919 cm-1 for adsorbed acetaldehyde and δ(O-C-O) at 689 cm-1 for adsorbed acetate, besides the other enhanced and clearly discriminated spectral signals at higher frequencies. Time-evolved and potential-dependent ATR-SEIRAS measurements together with advanced density functional theory calculations considering the coadsorption of CO and C2 species enable clarification of the structures and roles of surface C2 intermediates (η1(C)-acetyl and η1(H)-acetaldehyde), as reflected by the two bands at 1630 and 1663 cm-1, respectively, leading to updated pathways for the AOR on a Pt electrode.

Concentrating and activating carbon dioxide over AuCu aerogel grain boundaries

Alloys are active in CO2 electroreduction due to their unique electronic and geometric structures. Nevertheless, CO2 reduction selectivity is still low due to the low concentration of CO2 near the catalyst surface and the high energy barrier for CO2 activation. This paper describes an AuCu nanochain aerogel (NC-AuCu) with abundant grain boundaries (GBs) that promote the accumulation and activation of CO2 for further electrochemical reduction, employing in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy and density functional theory calculations. GBs can induce a strong local electric field to concentrate the electrolyte cations and thus accumulate CO2 near the catalyst surface. NC-AuCu exhibits a superior Faradaic efficiency of close to 100% for CO2 electroreduction to CO at an extremely low overpotential of 110 mV with a high CO partial current density of 28.6 mA cm-2 in a flow cell. Coupling with a Si solar cell to convert solar energy to CO, a very high conversion efficiency of ∼13.0% is achieved. It potentially provides broad interest for further academic research and industry applications.

Grain-Boundary-Rich Copper for Efficient Solar-Driven Electrochemical CO2 Reduction to Ethylene and Ethanol.

The grain boundary in copper-based electrocatalysts has been demonstrated to improve the selectivity of solar-driven electrochemical CO2 reduction toward multicarbon products. However, the approach to form grain boundaries in copper is still limited. This paper describes a controllable grain growth of copper electrodeposition via poly(vinylpyrrolidone) used as an additive. A grain-boundary-rich metallic copper could be obtained to convert CO2 into ethylene and ethanol with a high selectivity of 70% over a wide potential range. In situ attenuated total reflection surface-enhanced infrared absorption spectroscopy unveils that the existence of grain boundaries enhances the adsorption of the key intermediate (*CO) on the copper surface to boost the further CO2 reduction. When coupling with a commercially available Si solar cell, the device achieves a remarkable solar-to-C2-products conversion efficiency of 3.88% at a large current density of 52 mA·cm-2. This low-cost and efficient device is promising for large-scale application of solar-driven CO2 reduction.

Optimization of a Commercial Variable Angle Accessory for Entry Level Users of Electrochemical Attenuated Total Reflection Surface-Enhanced Infrared Absorption Spectroscopy (ATR-SEIRAS).

An evaluation of several experimental aspects that can optimize electrochemical attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) performance using a commercially available, specular reflection accessory is provided. A comparison of different silicon single-bounce internal reflection elements (IREs) is made with emphasis on different face-angled crystal (FAC) options. Selection of optimal angle of incidence for maximizing signal and minimizing noise is shown to require consideration of the optical throughput of the accessory, reflection losses at the crystal surfaces, and polarization effects. The benefits of wire-grid polarizers and antireflective (AR) coatings on the IREs is discussed. High signal-to-noise ratios can be achieved by omitting polarizers, using an AR-coated FAC with a larger face angle, and working at angles of incidence close to the maximum throughput angle of the accessory.

Antenna array-enhanced attenuated total reflection IR analysis in an aqueous solution.

Attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) is a powerful technique that provides structural and functional information during dynamic reactions in aqueous solutions. One existing limitation is the sensitivity to extract the signals of trace-level analytes from the background water in situ and in real time. Here, we proposed a novel ATR-SEIRAS platform that integrated a large-scale triangle gold antenna array onto a conventional ATR-IR platform to increase the sensitivity of this analytical technique. A square centimeter level well-ordered gold antenna array was fabricated onto an Si prism via nanosphere lithography. The size-dependent antenna array resonance had weak correlation with the incident polarization and antenna orientation, allowing antenna array-enhanced IR detection without the requirement of a microscope. In addition, the antenna resonance shift that occurred due to analyte adsorption-induced refractive index variation could be minimized benefiting from the high refractive index of Si (3.4). As a demonstration, we dynamically monitored the adsorption of the trace levels of proteins on top of the antenna array with a real signal enhancement factor larger than 300. Our platform opens an avenue to apply antenna array-enhanced IR spectroscopy in an aqueous environment measured via commercial IR instruments, which is extremely promising for the interfacial applications that require signal enhancement.