Insights into the mechanism of electrochemical ozone production via water splitting on the Ni and Sb Doped SnO2 catalyst

The H2O splitting mechanism is a very attractive alternative used in electrochemistry for the formation of O3. The most efficient catalysts employed for this reaction at room temperature are SnO2-based, in particular the Ni/Sb-SnO2 catalyst. In order to investigate the H2O splitting mechanism Density Functional Theory (DFT) was performed on a Ni/Sb-SnO2 surface with oxygen vacancies. By calculating different SnO2 facets, the (110) facet was deemed most stable, and further doped with Sb and Ni. On this surface, the H2O splitting mechanism was modelled paying particular attention to the final two steps, the formation of O2 and O3. Previous studies on β-PbO2 have shown that the final step in the reaction (the formation of O3) occurs via an Eley-Rideal style interaction where surface O2 desorbs before attacking surface O to form O3. It is revealed that for Ni/Sb-SnO2, although the overall reaction is the same the surface mechanism is different. The formation of O3 is found to occur through a Langmuir-Hinshelwood mechanism as opposed to Eley-Rideal. In addition to this the relevant adsorption energies (Eads), Gibb’s free energy (ΔGrxn) and activation barriers (Eact) for the final two steps modelled in the gas phase have been shown; providing the basis for a tool to develop new materials with higher current efficiencies

Effect of mass transport on the electrochemical oxidation of alcohols over electrodeposited film and carbon-supported pt electrodes

© 2018 The Author(s) Electrochemical oxidation of four different alcohol molecules (methanol, ethanol, n-butanol and 2-butanol) at electrodeposited Pt film and carbon-supported Pt catalyst film electrodes, as well as the effect of mass transport on the oxidation reaction, has been studied systematically using the rotating disk electrode (RDE) technique. It was shown that oxidation current decreased with an increase in the rotation rate (ω) for all alcohols studied over electrodeposited Pt film electrodes. In contrast, the oxidation current was found to increase with an increase in the ω for Pt/C in ethanol and n-butanol-containing solutions. The decrease was found to be nearly reversible for ethanol and n-butanol at the electrodeposited Pt film electrode ruling out the possibility of intermediate CO ads poisoning being the sole cause of the decrease and was attributed to the formation of soluble intermediate species which diffuse away from the electrode at higher ω. In contrast, an increase in the current with an increase in ω for the carbon supported catalyst may suggest that the increase in residence time of the soluble species within the catalyst layer, results in further oxidation of these species. Furthermore, the reversibility of the peak current on decreasing the ω could indicate that the surface state has not significantly changed due to the sluggish reaction kinetics of ethanol and n-butanol

Role of Water and Adsorbed Hydroxyls on Ethanol Electrochemistry on Pd: New Mechanism, Active Centers, and Energetics for Direct Ethanol Fuel Cell Running in Alkaline Medium

First principles calculations with molecular dynamics are utilized to simulate a simplified electrical double layer formed in the active electric potential region during the electrocatalytic oxidation of ethanol on Pd electrodes running in an alkaline electrolyte. Our simulations provide an atomic level insight into how ethanol oxidation occurs in fuel cells: New mechanisms in the presence of the simplified electrical double layer are found to be different from the traditional ones; through concerted-like dehydrogenation paths, both acetaldehyde and acetate are produced in such a way as to avoid a variety of intermediates, which is consistent with the experimental data obtained from <i>in situ</i> FTIR spectroscopy. Our work shows that adsorbed OH on the Pd electrode rather than Pd atoms is the active center for the reactions; the dissociation of the C–H bond is facilitated by the adsorption of an OH^– anion on the surface, resulting in the formation of water. Our calculations demonstrate that water dissociation rather than H desorption is the main channel through which electrical current is generated on the Pd electrode. The effects of the inner Helmholtz layer and the outer Helmholtz layer are decoupled, with only the inner Helmholtz layer being found to have a significant impact on the mechanistics of the reaction. Our results provide atomic level insight into the significance of the simplified electrical double layer in electrocatalysis, which may be of general importance

Effect of the Presence of MEA on the CO₂ Capture Ability of Superbase Ionic Liquids

The miscibility of monoethanolamine (MEA) in five superbase ionic liquids (ILs), namely the trihexyl-tetradecylphosphonium benzotriazolide ([P₆₆₆₁₄]­[Bentriz]), trihexyl-tetradecylphosphonium benzimidazolide ([P₆₆₆₁₄]­[Benzim]), trihexyl-tetradecylphosphonium 1,2,3-triazolide ([P₆₆₆₁₄]­[123Triz]), trihexyl-tetradecylphosphonium 1,2,4-triazolide ([P₆₆₆₁₄]­[124Triz]), and trihexyl-tetradecylphosphonium imidazolide ([P66614]­[Im]) was determined at 295.15 K using ¹H NMR spectroscopy. The solubility of carbon dioxide (CO₂) in equimolar (IL + MEA) mixtures was then studied experimentally using a gravimetric technique at 295.15 K and 0.1 MPa. The effect of MEA on the CO₂ capture ability of these ILs was investigated together with the viscosity of these systems in the presence or absence of CO₂ to evaluate their practical application in CO₂ capture processes. The effect of the presence of MEA on the rate of CO₂ uptake was also studied. The study showed that the MEA can enhance CO₂ absorption over the ideal values in the case of [P₆₆₆₁₄]­[123Triz] and [P₆₆₆₁₄]­[Bentriz] while in the other systems the mixtures behave ideally. A comparison of the effect of MEA addition with the addition of water to these superbase ILs showed that similar trends were observed in each case for the individual ILs studied

Understanding the Optimal Adsorption Energies for Catalyst Screening in Heterogeneous Catalysis

The fundamental understanding of the activity in heterogeneous catalysis has long been the major subject in chemistry. This paper shows the development of a two-step model to understand this activity. Using the theory of chemical potential kinetics with Brønsted–Evans–Polanyi relations, the general adsorption energy window is determined from volcano curves, using which the best catalysts can be searched. Significant insights into the reasons for catalytic activity are obtained

Mechanistic Study of 1,3-Butadiene Formation in Acetylene Hydrogenation over the Pd-Based Catalysts Using Density Functional Calculations

Green oil, which leads to the deactivation of the catalysts used for the selective hydrogenation of acetylene, has long been observed but its formation mechanism is not fully understood. In this work, the formation of 1,3-butadiene, known to be the precursor of green oil, on both Pd(111) and Pd(211) surfaces is examined using density functional theory calculations. The pathways containing C₂ + C₂ coupling reactions as well as the corresponding hydrogenation reactions are studied in detail. Three pathways for 1,3-butadiene production, namely coupling plus hydrogenation and further hydrogenation, hydrogenation plus coupling plus hydrogenation, and a two step hydrogenation followed by coupling, are determined. By comparing the effective barriers, we identify the favored pathway on both surfaces. A general understanding toward the deactivation process of the industrial catalysts is also provided. In addition, the effects of the formation of subsurface carbon atoms as well as the Ag alloying on the 1,3-butadiene formation on Pd-based catalysts are also investigated and compared with experimental results

A Comparative Study on the Thermophysical Properties for Two Bis[(trifluoromethyl)sulfonyl]imide-Based Ionic Liquids Containing the Trimethyl-Sulfonium or the Trimethyl-Ammonium Cation in Molecular Solvents

Herein, we present a comparative study of the thermophysical properties of two homologous ionic liquids, namely, trimethyl-sulfonium bis­[(trifluoromethyl)­sulfonyl]­imide, [S₁₁₁]­[TFSI], and trimethyl-ammonium bis­[(trifluoromethyl)­sulfonyl]­imide, [HN₁₁₁]­[TFSI], and their mixtures with propylene carbonate, acetonitrile, or gamma butyrolactone as a function of temperature and composition. The influence of solvent addition on the viscosity, conductivity, and thermal properties of IL solutions was studied as a function of the solvent mole fraction from the maximum solubility of IL, <i>x</i>_s, in each solvent to the pure solvent. In this case, <i>x</i>_s is the composition corresponding to the maximum salt solubility in each liquid solvent at a given temperature from 258.15 to 353.15 K. The effect of temperature on the transport properties of each binary mixture was then investigated by fitting the experimental data using Arrhenius’ law and the Vogel–Tamman–Fulcher (VTF) equation. The experimental data shows that the residual conductivity at low temperature, e.g., 263.15 K, of each binary mixture is exceptionally high. For example, conductivity values up to 35 and 42 mS·cm^–1 were observed in the case of the [S₁₁₁]­[TFSI] + ACN and [HN₁₁₁]­[TFSI] + ACN binary mixtures, respectively. Subsequently, a theoretical approach based on the conductivity and on the viscosity of electrolytes was formulated by treating the migration of ions as a dynamical process governed by ion–ion and solvent–ion interactions. Within this model, viscosity data sets were first analyzed using the Jones–Dole equation. Using this theoretical approach, excellent agreement was obtained between the experimental and calculated conductivities for the binary mixtures investigated at 298.15 K as a function of the composition up to the maximum solubility of the IL. Finally, the thermal characterization of the IL solutions, using DSC measurements, showed a number of features corresponding to different solid–solid phase transitions, <i>T</i>_S–S, with extremely low melting entropies, indicating a strong organizational structure by easy rotation of methyl group. These ILs can be classified as plastic crystal materials and are promising as ambient-temperature solid electrolytes

Origin of the Increase of Activity and Selectivity of Nickel Doped by Au, Ag, and Cu for Acetylene Hydrogenation

Activity and selectivity are both important issues in heterogeneous catalysis and recent experimental results have shown that Ni catalysts doped by gold exhibit high activity for the hydrogenation of acetylene with good selectivity of ethylene formation. To unravel the underlying mechanism for this observation, the general trend of activity and selectivity of Ni surfaces doped by Au, Ag, and Cu has been investigated using density functional theory calculations. Complete energy profiles from C₂H₂ to C₂H₄ on Ni(111), Au/Ni(111), Ag/Ni(111) and Cu/Ni(111) are obtained and their turnover frequencies (TOFs) are computed. The results show that acetylene adsorption on Ni catalyst is strong which leads to the low activity while the doping of Au, Ag, and Cu on the Ni catalyst weakens the acetylene adsorption, giving rise to the increase of activity. The selectivity of ethylene formation is also quantified by using the energy difference between the hydrogenation barriers and the absolute value of the adsorption energies of ethylene. It is found that the selectivity of ethylene formation increases by doping Au and Ag, while those of Cu/Ni and Ni are similar

Are Alkyl Sulfate-Based Protic and Aprotic Ionic Liquids Stable with Water and Alcohols? A Thermodynamic Approach

The knowledge of the chemical stability as a function of the temperature of ionic liquids (ILs) in the presence of other molecules such as water is crucial prior to developing any industrial application and process involving these novel materials. Fluid phase equilibria and density over a large range of temperature and composition can give basic information on IL purity and chemical stability. The IL scientific community requires accurate measurements accessed from reference data. In this work, the stability of different alkyl sulfate-based ILs in the presence of water and various alcohols (methanol, ethanol, 1-butanol, and 1-octanol) was investigated to understand their stability as a function of temperature up to 423.15 K over the hydrolysis and transesterification reactions, respectively. From this investigation, it was clear that methyl sulfate- and ethyl sulfate-based ILs are not stable in the presence of water, since hydrolysis of the methyl sulfate or ethyl sulfate anions to methanol or ethanol and hydrogenate anion is undoubtedly observed. Such observations could help to explain the differences observed for the physical properties published in the literature by various groups. Furthermore, it appears that a thermodynamic equilibrium process drives these hydrolysis reactions. In other words, these hydrolysis reactions are in fact reversible, providing the possibility to re-form the desired alkyl sulfate anions by a simple transesterification reaction between hydrogen sulfate-based ILs and the corresponding alcohol (methanol or ethanol). Additionally, butyl sulfate- and octyl sulfate-based ILs appear to follow this pattern but under more drastic conditions. In these systems, hydrolysis is observed in both cases after several months for temperatures up to 423 K in the presence of water. Therein, the partial miscibility of hydrogen sulfate-based ILs with long chain alcohols (1-butanol and 1-octanol) can help to explain the enhanced hydrolytic stability of the butyl sulfate- and octyl sulfate-based ILs compared with the methyl or ethyl sulfate systems. Additionally, rapid transesterification reactions are observed during liquid–liquid equilibrium studies as a function of temperature for binary systems of (hydrogen sulfate-based ionic liquids + 1-butanol) and of (hydrogen sulfate-based ionic liquids + 1-octanol). Finally, this atom-efficient catalyst-free transesterification reaction between hydrogen sulfate-based ILs and alcohol was then tested to provide a novel way to synthesize new ILs with various anion structures containing the alkyl sulfate group

Origin of low CO2 selectivity on platinum in the direct ethanol fuel cell

Calculated answer: First-principles calculations have been applied to calculate the energy barrier for the key step in CO formation on a Pt surface (see picture; Pt blue, Pt atoms on step edge yellow) to understand the low CO 2 selectivity in the direct ethanol fuel cell. The presence of surface oxidant species such as O (brown bar) and OH (red bar) led to an increase of the energy barrier and thus an inhibition of the key step