Understanding the competition between hydrogen evolution and CO2 reduction is of fundamental importance to increase the faradaic efficiency for electrocatalytic CO2 reduction in aqueous electrolytes. ...Here, by using a copper rotating disc electrode, we find that the major hydrogen evolution pathway competing with CO2 reduction is water reduction, even in a relatively acidic electrolyte (pH 2.5). The mass-transport-limited reduction of protons takes place at potentials for which there is no significant competition with CO2 reduction. This selective inhibitory effect of CO2 on water reduction, as well as the difference in onset potential even after correction for local pH changes, highlights the importance of differentiating between water reduction and proton reduction pathways for hydrogen evolution. In-situ FTIR spectroscopy indicates that the adsorbed CO formed during CO2 reduction is the primary intermediate responsible for inhibiting the water reduction process, which may be one of the main mechanisms by which copper maintains a high faradaic efficiency for CO2 reduction in neutral media.
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Gold is one of the most selective catalysts for the electrochemical reduction of CO2 (CO2RR) to CO. However, the concomitant hydrogen evolution reaction (HER) remains unavoidable under aqueous ...conditions. In this work, a rotating ring disk electrode (RRDE) setup has been developed to study quantitatively the role of mass transport in the competition between these two reactions on the Au surface in 0.1 M bicarbonate electrolyte. Interestingly, while the faradaic selectivity for CO formation was found to increase with enhanced mass transport (from 67% to 83%), this effect is not due to an enhancement of the CO2RR rate. Remarkably, the inhibition of the competing HER from water reduction with increasing disk rotation rate is responsible for the enhanced CO2RR selectivity. This can be explained by the observation that, on the Au electrode, water reduction improves with more alkaline pH. As a result, the decrease in the local alkalinity near the electrode surface with enhanced mass transport suppresses HER due to the water reduction. Our study shows that controlling the local pH by mass transport conditions can tune the HER rate, in turn regulating the CO2RR and HER competition in the general operating potential window for CO2RR (−0.4 to −1 V vs RHE).
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In this work we study the role of alkali metal cation concentration and electrolyte pH in altering the kinetics of the hydrogen evolution reaction (HER) at gold (Au) electrodes. We show that at ...moderately alkaline pH (pH 11), increasing the cation concentration significantly enhances the HER activity on Au electrodes (with a reaction order ≈0.5). Based on these results we suggest that cations play a central role in stabilizing the transition state of the rate‐determining Volmer step by favorably interacting with the dissociating water molecule (*H–OHδ−–cat+). Moreover, we show that increasing electrolyte pH (pH 10 to pH 13) tunes the local field strength, which in turn indirectly enhances the activity of HER by tuning the near‐surface cation concentration. Interestingly, a too high near‐surface cation concentration (at high pH and high cation concentration) leads to a lowering of the HER activity, which we ascribe to a blockage of the surface by near‐surface cations.
The kinetics of the hydrogen evolution reaction (HER) at gold electrodes is affected by the concentration of alkali metal cations and electrolyte pH. At moderately alkaline pH, increasing the cation concentration greatly enhances the HER activity. Cations are proposed to help stabilize the transition state of the rate‐determining Volmer step by a favorable interaction with the dissociating water molecule (*H–OHδ−–cat+).
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Ammonia is an important nutrient for the growth of plants. In industry, ammonia is produced by the energy expensive Haber-Bosch process where dihydrogen and dinitrogen form ammonia at a very high ...pressure and temperature. In principle one could also reduce dinitrogen upon addition of protons and electrons similar to the mechanism of ammonia production by nitrogenases. Recently, major breakthroughs have taken place in our understanding of biological fixation of dinitrogen, of molecular model systems that can reduce dinitrogen, and in the electrochemical reduction of dinitrogen at heterogeneous surfaces. Yet for efficient reduction of dinitrogen with protons and electrons major hurdles still have to be overcome. In this tutorial review we give an overview of the different catalytic systems, highlight the recent breakthroughs, pinpoint common grounds and discuss the bottlenecks and challenges in catalytic reduction of dinitrogen.
Catalytic reduction of dinitrogen with protons and electrons is a very challenging alternative to the energy expensive Haber-Bosch reaction.
The search for improved heterogeneous catalysts is an important but difficult task. Scaling relations between the adsorption energies of reaction intermediates greatly facilitate the computational ...design of catalysts. However, this methodology does not currently incorporate structure sensitivity and hence cannot describe adequately the overall activity of realistic catalyst particles and extended surfaces with several facets, edges and apices. Here, we generalize scaling relations by examining twelve different low-index, stepped and kinked surfaces of nine transition metals. This allows us to quantify the effect of the adsorption-site geometry on these relations, ensures a full prediction of their parameters, and helps in identifying intrinsic thermodynamic restrictions to the performance of catalysts. The resulting fully predictable, structure-sensitive scaling relations are a step towards the long-sought rational design of multifaceted catalytic particles. Such a design can now target not only the chemical nature of active materials but also the actual geometry of their active sites.
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CO‐products: DFT calculations are used to construct a mechanism for the electrochemical reduction of CO on Cu(100) that agrees with the experimental observations (see picture) of pH independence in ...the formation of C2 species. The rate‐determining step is an electron‐transfer‐mediated CO dimerization. Ethylene, acetaldehyde, and ethanol are formed through a common pathway, and adsorbed ethylene oxide is one of the reaction intermediates.
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Nickel oxyhydroxide (NiOOH) is extensively used for energy storage and it is a very promising catalyst for the oxygen evolution reaction (OER). However, the processes occurring on the NiOOH surface ...during charge accumulation and OER are not well understood. This work presents an
Surface Enhanced Raman Spectroscopy (SERS) study of the pH dependent interfacial changes of the NiOOH catalyst under the working conditions used for OER. We demonstrate the important effect of the electrolyte pH on the degree of surface deprotonation of NiOOH, which crucially affects its OER activity. Our results show that the deprotonation of NiOOH produces negatively charged (or proton-deficient) surface species, which are responsible for the enhanced OER activity of NiOOH in highly alkaline pH. Moreover, we provide spectroscopic evidence obtained in an
O-labeled electrolyte that allows us to assign this surface species to a superoxo-type species (Ni-OO
). Furthermore, we propose a mechanism for the OER on NiOOH which is consistent with the observed pH-sensitivity, and that also explains why NiOOH is not a suitable catalyst for applications in neutral or moderately alkaline pH (in the range 7-11), apart from the lower stability of the catalyst under these conditions.
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Imbalances in the nitrogen cycle caused by human activities (combustion, fertiliser-intensive agriculture) have resulted in alarmingly increased levels of nitrate in groundwater and other water ...bodies, with potentially health-threatening consequences. The electrocatalytic removal of nitrate from polluted water is a promising alternative to bacterial denitrification, provided that full selectivity to harmless N
2
, which can be returned to the atmosphere, is achieved. This perspective article discusses the state-of-the-art of research on electrocatalytic denitrification, critically evaluating the obstacles still hampering large-scale application of this technique. The milestones of fundamental research focussing on the cathode reaction will first be dealt with, followed by their translation into electrochemical reactors of practical interest. Finally, a short foray into the novel field of bioelectrochemical reactors will close the article. Challenges and opportunities pertaining to these three topics will be analysed.
Understanding the fundamental underpinnings of nitrate reduction to N
2
is central to the implementation of applied electrocatalytic denitrification.
Electrocatalysis on gold Rodriguez, Paramaconi; Koper, Marc T. M
Physical chemistry chemical physics : PCCP,
01/2014, Volume:
16, Issue:
27
Journal Article
Peer reviewed
This perspective article reviews recent advances in the study of important catalytic reactions on gold electrodes. The paper discusses both oxidation and reduction reactions: the oxidation of carbon ...monoxide and alcohols as well as the oxygen reduction reaction on gold electrodes and also a brief discussion of other interesting reactions on gold electrodes such as the amine borane oxidation and the CO
2
reduction. A common theme in electrocatalysis on gold is the sensitive dependence of various reaction rates on pH and gold surface structure. The electrocatalysis of redox reactions on gold is highly pH dependent, often preferring alkaline media, due to the prominent role of negatively charged reaction intermediates related to the fact that gold does not bind the neutral intermediates strongly enough. Gold also tends to be a selective catalyst, again due to its weak adsorption properties, as on gold the reaction often stops when a difficult bond breaking or making event will be the necessary next step.
This perspective article reviews recent advances in the study of important catalytic reactions on gold electrodes.
This perspective article outlines a simple but general theoretical analysis for multiple proton-electron transfer reactions, based on the microscopic theory of proton-coupled electron transfer ...reactions, recent developments in the thermodynamic theory of multi-step electron transfer reactions, and the experimental realization that many multiple proton-coupled electron transfer reactions feature decoupled proton-electron steps in their mechanism. It is shown that decoupling of proton and electron transfer leads to a strong pH dependence of the overall catalytic reaction, implying an optimal pH for high catalytic turnover, and an associated optimal catalyst at the optimal pH. When more than one catalytic intermediate is involved, scaling relationships between intermediates may dictate the optimal catalyst and limit the extent of reversibility that may be achievable for a multiple proton-electron transfer reaction. The theory is discussed in relation to the experimental results for a number of redox reactions that are of importance for sustainable energy conversion, primarily focusing on their pH dependence.
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