Au@AuPd Core-Alloyed Shell Nanoparticles for Enhanced Electrocatalytic Activity and Selectivity under Visible Light Excitation.

Plasmonic catalysis has been employed to enhance molecular transformations under visible light excitation, leveraging the localized surface plasmon resonance (LSPR) in plasmonic nanoparticles. While plasmonic catalysis has been employed for accelerating reaction rates, achieving control over the reaction selectivity has remained a challenge. In addition, the incorporation of catalytic components into traditional plasmonic-catalytic antenna-reactor nanoparticles often leads to a decrease in optical absorption. To address these issues, this study focuses on the synthesis of bimetallic core@shell Au@AuPd nanoparticles (NPs) with ultralow loadings of palladium (Pd) into gold (Au) NPs. The goal is to achieve NPs with an Au core and a dilute alloyed shell containing both Au and Pd, with a low Pd content of around 10 atom %. By employing the (photo)electrocatalytic nitrite reduction reaction (NO2RR) as a model transformation, experimental and theoretical analyses show that this design enables enhanced catalytic activity and selectivity under visible light illumination. We found that the optimized Pd distribution in the alloyed shell allowed for stronger interaction with key adsorbed species, leading to improved catalytic activity and selectivity, both under no illumination and under visible light excitation conditions. The findings provide valuable insights for the rational design of antenna-reactor plasmonic-catalytic NPs with controlled activities and selectivity under visible light irradiation, addressing critical challenges to enable sustainable molecular transformations.

Ultralow Catalytic Loading for Optimised Electrocatalytic Performance of AuPt Nanoparticles to Produce Hydrogen and Ammonia.

The hydrogen evolution and nitrite reduction reactions are key to producing green hydrogen and ammonia. Antenna-reactor nanoparticles hold promise to improve the performances of these transformations under visible-light excitation, by combining plasmonic and catalytic materials. However, current materials involve compromising either on the catalytic activity or the plasmonic enhancement and also lack control of reaction selectivity. Here, we demonstrate that ultralow loadings and non-uniform surface segregation of the catalytic component optimize catalytic activity and selectivity under visible-light irradiation. Taking Pt-Au as an example we find that fine-tuning the Pt content produces a 6-fold increase in the hydrogen evolution compared to commercial Pt/C as well as a 6.5-fold increase in the nitrite reduction and a 2.5-fold increase in the selectivity for producing ammonia under visible light excitation relative to dark conditions. Density functional theory suggests that the catalytic reactions are accelerated by the intimate contact between nanoscale Pt-rich and Au-rich regions at the surface, which facilitates the formation of electron-rich hot-carrier puddles associated with the Pt-based active sites. The results provide exciting opportunities to design new materials with improved photocatalytic performance for sustainable energy applications.

Open Circuit Interaction Between Ethanol or 2-Propanol and Oxidized Platinum Surfaces.

The interaction between organic molecules and oxidized catalyst surfaces has frequently been used to study the fuel crossover from the anode to the cathode in direct liquid fuel cells. In such experiments, the oxidized surface is put in contact with the fuel under open circuit conditions, and the evolution of the potential is registered. The open circuit potential (OCP) vs. time features can then inform on the reactivity of the fuel with the oxidized surface and provide valuable information not only to applications in fuel cells but also to the electrochemical reform of those molecules to produce clean hydrogen. In this paper, we present an experimental investigation of the open circuit interaction between ethanol or 2-propanol with oxidized platinum surfaces. Besides the OCP time traces, we have also employed cyclic voltammetry and fast oxide reduction sweep in the presence of the alcohols. Comparable reaction currents are obtained in the cyclic voltammogram, but the electro-oxidation of 2-propanol sets in at considerably lower overpotentials than that of ethanol. At the high potential region, both the magnitude and the potential of the current peak are nearly identical in both cases. In contrast, under open circuit conditions, the interaction of ethanol with the oxidized platinum surface is more pronounced than that found for 2-propanol, and these results are corroborated by the facile reduction of the platinum oxides along the fast backward sweep for the case of the latter.

Oscillatory dynamics during the methanol electrooxidation reaction on Pt(111).

Despite several papers describing the oscillatory methanol electrooxidation reaction (OMOR) catalyzed by polycrystalline Pt, these dynamic instabilities are less explored on single crystalline surfaces. Herein, we observed and mapped for the first time the OMOR on Pt(111) in non-adsorbing anion solutions as well as in the presence of small amounts of sulfate anions. Period 1 oscillations with oscillation frequencies from 1.2 to 2.0 Hz were observed for methanol concentrations higher than 1.0 M, with no evolution to more complex patterns. These oscillations occur in the potential range in which PtOH is partially covering the surface without irreversible oxidation processes. Small changes in both the mean potential (Em) and the poisoning rate along the time-series were observed, the so-called drift, and were explained in terms of the accumulation of intermediates at the interface. The presence of sulfate strongly inhibits the OMOR, and the results are discussed in terms of sulfate adlayer formation on {111} domains.

Tuning Electrochemical Bistability by Surface Area Blocking in the Cathodic Deposition of Copper.

We report herein a precise control of the electrochemical bistability induced by surface area changes during the cathodic deposition of copper. Small additions of 1,10-phenanthroline (Phen) in the reaction media present an inhibiting effect on the global rate mainly due to the adsorption of protonated Phen. The increase of its concentration favors a shrinkage of the bifurcation (saddle-node) diagram and shifts it to less negative potentials. The dynamic instability is verified by impedance measurements, and a negative impedance is clearly found. We calculated the apparent molar mass of the adsorbents using in situ gravimetric monitoring in the electrochemical experiments, and the results indicate that mass changes occur mainly due to the reduction of copper from bivalent ions dissolved in the reaction media. Importantly, the adsorption of protonated Phen molecules does not show a considerable contribution in mass variations but prevents the formation of a copper course grained morphology over the surface. Imaging analysis indicates finer nodulations at the lower branch compared to the upper branch in the bistability domain. On the basis of these observations, a kinetic mechanism is proposed and a good agreement is obtained between the apparent molar mass extracted from experiments and the theoretical values. Altogether, our results contribute to a detailed physical chemical description of the nonlinear behavior, bringing new insights about this reaction and pointing out the possibility to design switchable surface electrodes by taking advantage of the bistable behavior.

The electro-oxidation of ethylene glycol on platinum over a wide pH range: oscillations and temperature effects.

We report a comprehensive study of the electro-oxidation of ethylene glycol (EG) on platinum with emphasis on the effects exerted by the electrolyte pH, the EG concentration, and temperature, under both regular and oscillatory conditions. We extracted and discussed parameters such as voltammetric activity, reaction orders (with respect to [EG]), oscillation's amplitude, frequency and waveform, and the evolution of the mean electrode potential at six pH values from 0 to 14. In addition, we obtained the apparent activation energies under several different conditions. Overall, we observed that increasing the electrolyte pH results in a discontinuous transition in most properties studied under both voltammetric and oscillatory regimes. As a relevant result in this direction, we found that the increase in the reaction order with pH is mediated by a minimum (~ 0) at pH = 12. Furthermore, the solution pH strongly affects all features investigated, c.f. the considerable increase in the oscillatory frequency and the decrease in the, oscillatory, activation energy as the pH increase. We suggest that adsorbed CO is probably the main surface-blocking species at low pH, and its absence at high pH is likely to be the main reason behind the differences observed. The size of the parameter region investigated and the amount of comparable parameters and properties presented in this study, as well as the discussion that followed illustrate the strategy of combining investigations under conventional and oscillatory regimes of electrocatalytic systems.

Mechanistic aspects of the linear stabilization of non-stationary electrochemical oscillations.

The problem of non-stationarity in experimentally recorded time-series is common in many (electro)chemical systems. Underlying this non-stationarity is the slow drift in some uncontrollable parameter, and it occurs in spite of the fact that all controllable parameters are kept constant. Particularly for electrochemical systems, some of us have recently suggested [J. Phys. Chem. C, 144, (2010), 22262-22268] an empirical method to stabilize experimental time-series. The method was exemplified for the electro-oxidation of methanol and different patterns were satisfactorily stabilized. In this paper we further elaborate some mechanistic aspects of this method and test it for the electro-oxidation of formaldehyde, a system that has some resemblance with the electro-oxidation of methanol, but produces a richer dynamics. In terms of the reaction mechanism, we were able to describe the coupling and to separate the surface processes of the two sub-systems: the fast one (or the core-oscillator) and the slow one, responsible for the drift.

The impact of the alkali cation on the mechanism of the electro-oxidation of ethylene glycol on Pt.

By means of in situ IR spectroscopy we investigate the effect of dissolved alkali cations on the electro-oxidation of ethylene glycol on platinum in alkaline media. The results revealed that the increase in the oxidation currents (Li(+) < Na(+) < K(+)) is reflected in the increase in the ratio between carbonate and oxalate produced.

Complex kinetics, high frequency oscillations and temperature compensation in the electro-oxidation of ethylene glycol on platinum.

Despite the fact that the majority of the catalytic electro-oxidation of small organic molecules presents oscillatory kinetics under certain conditions, there are few systematic studies concerning the influence of experimental parameters on the oscillatory dynamics. Of the studies available, most are devoted to C1 molecules and just some scattered data are available for C2 molecules. We present in this work a comprehensive study of the electro-oxidation of ethylene glycol on polycrystalline platinum surfaces and in alkaline media. The system was studied by means of electrochemical impedance spectroscopy, cyclic voltammetry, and chronoamperometry, and the impact of parameters such as applied current, ethylene glycol concentration, and temperature were investigated. As in the case of other parent systems, the instabilities in this system were associated with a hidden negative differential resistance, as identified by impedance data. Very rich and robust dynamics were observed, including the presence of harmonic and mixed mode oscillations and chaotic states, in some parameter region. Oscillation frequencies of about 16 Hz characterized the fastest oscillations ever reported for the electro-oxidation of small organic molecules. Those high frequencies were strongly influenced by the electrolyte pH and far less affected by the EG concentration. The system was regularly dependent on temperature under voltammetric conditions but rather independent within the oscillatory regime.

Autocatalysis in the open circuit interaction of alcohol molecules with oxidized Pt surfaces.

We studied the open circuit interaction of methanol and ethanol with oxidized platinum electrodes using in situ infrared spectroscopy. For methanol, it was found that formic acid is the main species formed in the initial region of the transient and that the steep decrease of the open circuit potential coincides with an explosive increase in the CO2 production, which is followed by an increase in the coverage of adsorbed CO. For ethanol, acetaldehyde was the main product detected and only traces of dissolved CO2 and adsorbed CO were found after the steep potential decay. In both cases, the transients were interpreted in terms of (a) the emergence of sub-surface oxygen in the beginning of the transient, where the oxide content is high, and (b) the autocatalytic production of free platinum sites for lower oxide content during the steep decay of the open circuit potential.