Mechanism Of Ethanol Oxidation Research Articles

Effect of SnO2 and Rh modifications on CO-stripping kinetics and ethanol oxidation mechanism of Pt electrode

Effect of SnO2 and Rh modifications on CO-stripping kinetics and ethanol oxidation mechanism of Pt electrode

New and Ultra‐Rapid Approach for Electrosynthesis of Highly Efficient Catalysts of Poly(para‐phenylenediamine) Supported Copper Oxide for Ethanol Oxidation in Alkaline Medium

AbstractHerein, a new and simple strategy based on electrochemical cascade reactions was used to prepare copper/poly(para‐phenylenediamine). The catalysts were prepared by a facile one‐pot synthesis via an electrochemical method in a solution containing monomer and copper in a suitable concentration using potentiostatic or galvanostatic mode. Using galvanostatic mode leads to the formation of nanodendritic shape of copper hydroxide‐copper oxide in contrast with potentiostatic mode which leads to a spherical form. The response toward ethanol oxidation was greatly affected by the morphology of the obtained nanocomposite. Indeed, the current density obtained when using potentiostatic mode is about 26.55 mA cm−2 at 1.05 V vs Ag/AgCl. This performance is about 2.4 times higher than of the electrocatalyst prepared by galvanostatic mode (11.09 mA cm−2 at 0.85 V). The durability and long‐term stability of the obtained electrocatalysts were investigated using chronoamperometry and cyclic voltammetry. The decrease in current density is about 19 % and 49 % for electrocatalysts prepared by galvanostatic and potentiostatic mode respectively, after 700 scans. Finally, ex–situ FTIR spectroscopy was conducted in order to understand the reaction mechanism of ethanol oxidation in alkaline medium. It demonstrates that Cu(OH)2−Cu2O@PpPD can perform the complete oxidation of ethanol molecules, leading to the formation of CO2 molecule as the final product. This study highlighted a new simple and facile approach for synthesizing electrocatalysts based on Cu(OH)2−Cu2O@PpPD for alcohol oxidation applications.

Rate Equations of Structure-Sensitive Catalytic Reactions with Arbitrary Kinetics

A general mathematical framework for the quantitative description of the cluster size dependence in heterogeneous catalytic reactions has been developed based on an analysis of the Gibbs energy of elementary reactions. The methodology was illustrated for a generic linear sequence of elementary reactions with three steps, a multi-step mechanism of ethanol oxidation comprising linear, nonlinear and quasi-equilibria steps and a network of parallel reactions in transformations of furfural.

IR exciton activation mechanism of ethanol oxidation by human alcohol dehydrogenase (ADH) 1A enzyme

Low-energy (2500 cm−1) exciton transfer was explored in Müller cell (MC) intermediate filaments (IFs) isolated from porcine retina and filling a capillary matrix. Excitons were generated by absorption of IR radiation at 4 μm. The effects of these excitons on ethanol oxidation by human alcohol dehydrogenase (ADH1A) enzyme were quantified. It was found that IR excitons transferred to the enzyme accelerated alcohol oxidation rate, which increased by the factor of 2.76 when exciting the IFs with 2.39 μW/cm2 of 4 μm IR light. Power dependence of the oxidation rate was also explored. These results show that IFs may be transmitting energy to enzyme molecules in vivo, facilitating enzymatic reactions. The required excitons may be produced by cells at the cost of adenosine triphosphate (ATP) hydrolysis energy. Therefore, such control mechanism for enzymatic reactions may be operational in living systems. Direct activation of the enzyme by IR radiation with 4 μm wavelength did not occur; instead, indirect activation of the IF…ADH1A…NAD (nicotinamide adenine dinucleotide)…EtOH complex occurred by energy transfer of the IR exciton to the ADH1A molecule of this complex. Considering that every living cell has a network of IFs, a similar reaction control mechanism may be operational in vivo, providing a much faster energy supply redirection within the cell than ATP diffusion, and justifying a closer inquiry.

Effect of electrochemical activation on the performance and stability of hybrid (PPy/Cu2O nanodendrites) for efficient ethanol oxidation in alkaline medium

Abstract One of the most challenges in Direct Ethanol Fuel Cell (DEFC) is to prevent the poisoning of the surface of the electrode due to the slow kinetics of ethanol oxidation reaction. Herein, we report the development of a new catalysts for ethanol oxidation using a well-defined Cu2O nanodendrites (Cu2O-NDs) supported on polypyrrole film (PPy) using a simple electrochemical approach. The effect of regeneration of the electrocatalyst in acidic medium on the performance of ethanol electro-oxidation was reported for the first time. Fourier Transform Infrared (FTIR), X-ray diffraction (XRD), Transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) were used to characterize the new catalysts. The SEM and TEM images confirm that the octahedral shape of Copper oxide is converted to a dendritic one after the regeneration of the catalyst. The electrochemical behavior of the prepared catalysts towards the oxidation of ethanol were investigated by Cyclic voltammetry (CV), Chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS). The current obtained by CV measurements on Cu2O-NDs/PPy/CPE was about 4 mA cm−2 which is 1.77 time higher than Cu2O/PPy/CPE catalyst (2.25 mA cm−2) with a shift of 60 mV and 150 mV in anodic and onset potential respectively demonstrating a good catalytic performance for ethanol oxidation. The stability test shows that the Cu2O-NDs/PPy/CPE loses only 7.25% of its initial current after 1000 cycles. Finally, Ex–situ FTIR spectroscopy was conducted in order to understand the reaction mechanism. Interestingly, our ex-situ FTIR results demonstrated that the ethanol oxidation mechanism displayed shape dependent behavior of copper oxide, indicating that the ethanol molecules are totally oxidized on the Cu2O-NDs leading to the formation of CO2 molecule as final product.

Easy approach for decorating of poly 4-aminithiophenol with Pd nanoparticles: an efficient electrocatalyst for ethanol oxidation in alkaline media

In this work, a polymer composite based on poly 4-aminothiophenol (p-ATP) is prepared via a facile electrochemical route and is used as an appropriate support for Pd nanoparticles. The morphological studies reveal that a nanofiber structure patterned on the multiwalled carbon nanotubes (MWCNT) structure is obtained for the synthesized catalyst viz. Pd/p-ATP/MWCNT. The Pd/p-ATP/MWCNT catalyst delivers a mass activity of 3245 $$ \mathrm{mA}\ {\mathrm{mg}}_{\mathrm{Pd}}^{-1} $$ manifesting its superb electrocatalytic activity for ethanol oxidation in the alkaline media. The vital role of the polymeric substrate on the electrocatalytic activity of Pd nanoparticles is established by comparing the catalytic performance of Pd/p-ATP/MWCNT with Pd/MWCNT. Finally, the superb activity of the synthesized catalyst is explicated according to the mechanism of ethanol oxidation on the synthesized catalyst.

Accelerating Laminar Flame Speed of Hydrous Ethanol via Oxygen-Rich Combustion

To investigate the laminar flame characteristics of hydrous ethanol, experiments were conducted in a constant volume combustion chamber. The laminar burning velocity and Markstein length of ethanol with water content from 0 to 20% by volume were measured over a wide range of equivalence ratios from 0.7 to 1.4 under the initial condition of 388 K and 0.1 MPa. Calculations of the laminar burning velocity with the constant pressure method (CPM) and constant volume method (CVM) were compared and discussed. Apart from experiments, numerical studies were carried out in CHEMKIN based on the Marinov’s and Olm’s ethanol oxidation mechanism. Moreover, the combustion of hydrous ethanol with 20% water by volume in oxygen-enriched air (O2 = 23% by volume) was also studied. Results indicated that the overall trends of laminar burning velocity versus the equivalent ratio were basically consistent between the experiment and the simulation, but the results obtained by CVM were larger than those obtained by CPM and closer to the simulation results. The peak value of laminar burning velocity decreased from 0.638 to 0.591 m/s when the water content increased from 0 to 20% in CVM calculation, but the peak laminar burning velocity of hydrous ethanol with 20% water in oxygen-enriched air enhanced to 0.771 m/s, meaning that the oxygen-enriched air can effectively offset the negative impacts of water. Moreover, the Markstein length of hydrous ethanol decreases with increasing equivalence ratios.

Ethanol Metabolism Dynamics in Clostridium ljungdahlii Grown on Carbon Monoxide.

Bioethanol production from syngas using acetogenic bacteria has attracted considerable attention in recent years. However, low ethanol yield is the biggest challenge that prevents the commercialization of syngas fermentation into biofuels using microbial catalysts. The present study demonstrated that ethanol metabolism plays an important role in recycling NADH/NAD+ during autotrophic growth. Deletion of bifunctional aldehyde/alcohol dehydrogenase (adhE) genes leads to significant growth deficiencies in gas fermentation. Using specific fermentation technology in which the gas pressure and pH were constantly controlled at 0.1 MPa and 6.0, respectively, we revealed that ethanol was formed during the exponential phase, closely accompanied by biomass production. Then, ethanol was oxidized to acetate via the aldehyde ferredoxin oxidoreductase pathway in Clostridium ljungdahlii A metabolic experiment using 13C-labeled ethanol and acetate, redox balance analysis, and comparative transcriptomic analysis demonstrated that ethanol production and reuse shared the metabolic pathway but occurred at different growth phases.IMPORTANCE Ethanol production from carbon monoxide (CO) as a carbon and energy source by Clostridium ljungdahlii and "Clostridium autoethanogenum" is currently being commercialized. During gas fermentation, ethanol synthesis is NADH-dependent. However, ethanol oxidation and its regulatory mechanism remain incompletely understood. Energy metabolism analysis demonstrated that reduced ferredoxin is the sole source of NADH formation by the Rnf-ATPase system, which provides ATP for cell growth during CO fermentation. Therefore, ethanol production is tightly linked to biomass production (ATP production). Clarification of the mechanism of ethanol oxidation and biosynthesis can provide an important reference for generating high-ethanol-yield strains of C. ljungdahlii in the future.

Boosting multiple photo-assisted and temperature controlled reactions with a single redox-switchable catalyst: Solvents as internal substrates and reducing agent

An alternative and economically viable process for the synthesis of β-aryl enals, enones and the aryl amines has been developed by partial oxidation of ethanol, isopropanol and N, N-dimethyl formamide (DMF). The formation of β-aryl enals, enones and the aryl amines was catalyzed by a mixed metal oxides layer of cobalt and chromium supported on halloysite nanotubes, designated as CoCr2O4-HNT. The CC and CN bond formation reactions were found to be influenced by temperature and the nature of base. The condensation of aldehyde with in situ generated acetaldehyde by ethanol oxidation forming β-aryl enals occurred selectively at 120 °C. The partial oxidation of isopropanol to acetone and its condensation with aldehydes forming β-aryl enones occurred at room temperature. Increase in temperature caused the liberation of hydrogen gas from isopropanol and allowed the reversible reduction of aldehydes to alcohols. Increase in temperature in isopropanol and increase in base concentration in ethanol causes the selective reduction of aldehydes to alcohols. Besides being active for the Claisen-Schmidt type of reactions and the aryl halides amination process, the synthesized catalyst was also found to be highly active for the photocatalytic oxidation of benzyl alcohols in absence of any external oxidizing agent. The positive holes (h+) generated at the Co(II) site as evident from EPR analysis was considered to be responsible for high photocatalytic activity of the material reducing the recombination rate of holes and electrons (e−). Density Functional Theory calculations were performed to understand the mechanism of ethanol oxidation to acetaldehyde.

Highly active Pt3Rh/C nanoparticles towards ethanol electrooxidation. Influence of the catalyst structure

The electrochemical oxidation of ethanol results in the formation of strongly adsorbed intermediates. Pt–Rh catalysts are proposed as alternatives since they easy the CC bond breaking. However, the effect of the Pt–Rh structure on the catalytic activity and selectivity to CO2 is not well understood. Here, we synthesised Pt/C and two different Pt–Rh/C catalyst architectures, an alloy (Pt3Rh/C) and a bimetallic mixture (Pt3–Rh/C) to study the effect of catalyst structure on its catalytic activity and on the products formed during the ethanol oxidation in acid media. The nanoparticles were prepared by a modified polyol reduction method using ethylene glycol as a co-reducing agent and Pb as a material of sacrifice, to obtain very small and well-dispersed nanoparticles on the carbon support. Fourier transform infrared spectroscopy and derivative voltammetry was used to give insights about the ethanol oxidation mechanism occurring at the developed catalysts. The samples characterised by X-ray diffraction analysis showed distortions in the Pt lattice parameters for the Pt-Rh alloy structure due to the presence of Rh in the catalyst’s composition. Transmission electron microscopy analyses indicate that nanoparticles were well-dispersed on a carbon support, with spherical shapes and small particle sizes (2–3 nm). in situ X-ray absorption spectroscopy data evidence that Pt–Rh interactions produce changes in the Pt 5d band vacancy. The electronic effect is maximized when Pt forms an alloy with Rh, resulting in the highest d-band vacancy of the Pt3Rh/C. The Pt3Rh/C catalyst showed the highest activity towards ethanol oxidation, presenting current densities in a quasi-steady-state condition (measured at 600 mV) around 5.2 times higher than the commercial Pt/C (Alfa Aesar). Moreover, the onset potential for ethanol oxidation shifts to more negative potentials (110 mV lower taken at 1 mA cm–2) was also observed. In situ FTIR data revealed that Pt/C catalyst favours the formation of acetic acid. The synergistic effect between Rh and the alloy structure results in an easier CC bond breaking for Pt3Rh/C, in comparison to Pt3–Rh mixture, thus favouring CO2 formation at lower potentials.

AuPd/C core–shell and alloy nanoparticles with enhanced catalytic activity toward the electro-oxidation of ethanol in alkaline media

Carbon-supported Pd, Au@Pd core–shell and Au1–xPdx-alloyed nanoparticles were prepared by a chemical reduction method and characterized by different experimental techniques, including X-ray powder diffraction, transmission electron microscopy, scanning-transmission electron microscopy using bright-field and high-angle annular dark field detectors and X-ray energy dispersive spectroscopy. The catalytic mass activity toward ethanol oxidation was assessed by cyclic voltammetry and chronoamperometry at room temperature. The measurements showed that the addition of Au enhances remarkably the electrocatalytic activity of the material, due to the bifunctional effect of Au1–xPdx/C alloys, and the synergetic effect on Au@Pd/C, resulting in a dissolution resistance of core–shell catalysts at potentials of 1.5 V versus reversible hydrogen electrode. In situ Fourier transform infrared spectroscopy measurements showed that the mechanism for ethanol oxidation depends on the electrocatalyst structure and morphology. Acetate was identified as the main product of ethanol electro-oxidation on the studied electrocatalysts. However, the presence of a core–shell structure on Au@Pd/C resulted in enhanced ethanol oxidation selectivity toward CO2. The improvement of activity is attributed to the interaction between Pd shell and Au core.

The catalytic and radical mechanism for ethanol oxidation to acetic acid.

Au/TiO2 is a much-used catalyst for the conversion of ethanol to acetic acid. The proposed mechanism speaks of two essential reaction steps on the catalytic surface. The first is the ethanol to acetaldehyde and the second the acetaldehyde to acetic acid. When operating in the gas phase, acetic acid is usually absent. This work focuses on determining what triggers the second step by comparing the ethanol with acetaldehyde oxidation and the liquid with gas-phase reaction. We propose an updated reaction mechanism: acetaldehyde autoxidises non-catalytically to acetic acid, likely driven by radicals. The requirement for the autoxidation is the presence of oxygen and water in the liquid-phase. The understanding of the interplay between the catalytic ethanol to acetaldehyde and the following non-catalytic reaction step provides guiding principles for the design of new and more selective alcohol oxidation catalysts.

Investigation of ethanol oxidation over aluminum nanoparticle using ReaxFF molecular dynamics simulation

Aluminum nanoparticles are an effective and economical additive for producing energetic fuels. In the present study, the state of the art ReaxFF molecular dynamics (MD) simulation has been used to uncover the detailed mechanisms of ethanol oxidation over aluminum nanoparticles with different oxidation states. The MD results reveal the dynamics process of ethanol oxidation reactions at nanoscales. The presence of aluminum nanoparticles is found to reduce the initial temperature of ethanol oxidation to 324 K. It is also found that compared to ethanol, oxygen molecules are more easily adsorbed on aluminum surfaces. Moreover, different oxidation states of aluminum nanoparticles influence the initial ethanol reactions on the nanoparticles’ surfaces. OH-abstraction is more commonly observed on pure aluminum nanoparticles while H-abstraction prevails on aluminum nanoparticles with oxide. The separated H atom from hydroxyl forms bonds with Al and O atom on aluminum nanoparticles surrounded by thin and thick oxide layers, respectively. Adsorptive dissociation of ethanol is hindered by the oxide layer surrounding the aluminum nanoparticle. Gas products like H2O and CO resulting from ethanol oxidation on aluminum nanoparticles with the thick oxide layer are observed while almost all the C, H and O atoms in ethanol diffuse into the nanoparticles without or with the thin oxide layer. For ethanol dissociation, a higher temperature is required than adsorption. In addition, the rate of ethanol dissociation increases with rising reaction temperatures. The activation energy for ethanol adsorptive dissociation is found to be 4.58 kcal/mol on the aluminum nanoparticle with the thin oxide layer, which is consistent with results from much more expensive DFT calculations.

High-pressure ethanol oxidation and its interaction with NO

Ethanol has become a promising biofuel, widely used as a renewable fuel and gasoline additive. Describing the oxidation kinetics of ethanol with high accuracy is required for the development of future efficient combustion devices with lower pollutant emissions. The oxidation process of ethanol, from reducing to oxidizing conditions, and its pressure dependence (20, 40 and 60 bar) has been analyzed in the 500–1100 K temperature range, in a tubular flow reactor under well controlled conditions. The effect of the presence of NO has been also investigated. The experimental results have been interpreted in terms of a detailed chemical kinetic mechanism with the GADM mechanism (Glarborg P, Alzueta MU, Dam-Johansen K and Miller JA, 1998) as a base mechanism but updated, validated, extended by our research group with reactions added from the ethanol oxidation mechanism of Alzueta and Hernández (Alzueta MU and Hernández JM, 2002), and revised according to the present high-pressure conditions and the presence of NO. The final mechanism is able to reproduce the experimental trends observed on the reactants consumption and main products formation during the ethanol oxidation under the conditions studied in this work. The results show that the oxygen availability in the reactant mixture has an almost imperceptible effect on the temperature for the onset of ethanol consumption at a constant pressure, but this consumption is faster for the highest value of air excess ratio (λ) analyzed. Moreover, as the pressure becomes higher, the oxidation of ethanol starts at lower temperatures. The presence of NO promotes ethanol oxidation, due to the increased relevance of the interactions of CH3 radicals and NO2 (from the conversion of NO to NO2 at high pressures and in presence of O2) and the increased concentration of OH radicals from the interaction of NO2 and water.

Laminar flame characteristcs of ethanol-air mixture: Experimental and simulation study

Experimental test for laminar combustion of ethanol-air mixture was investigated in a constant volume combustion bomb. The laminar burning velocity and Markstein length were determined over an extensive range of equivalence ratios from 0.7-1.6 under an initial condition of 0.1 MPa pressure and 358 K temperature, with high-speed Schlieren system. The methods of linear extrapolation and non-linear extrapolation are compared and discussed. Apart from experiments, simulation was carried out in Chemkin by using the Marinov ethanol oxidation mechanism. Results indicate that non-linear extrapolation is more suitable to calculate the laminar burning velocity of ethanol-air mixture. The overall trends of laminar burning velocity vs. equivalence ratio are consistent between the experiment and simulation. The peak values of the laminar burning velocity from present experiment and simulation are 531.2 mm/s and 565.3 mm/s, both appearing at the equivalence ratio of 1.1. Moreover, the Markstein length of ethanol-air mixtures generally decreases with increasing equivalence ratio.

Preparation of Ni/NiO-C catalyst with NiO crystal: catalytic performance and mechanism for ethanol oxidation in alkaline solution

Highly active and durable catalysts are imperative for the development of alcohol oxidation research. Herein, a Ni/NiO-C catalyst with hollow NiO crystal can be used in ethanol oxidation. The hollow NiO nanoparticles are absolutely coated in the amorphous carbon and the generated NiO crystal remarkably raises the electrocatalytic performances of catalysts. Among all samples, the Ni/NiO-C3 catalyst shows the highest current density in 0.5 M ethanol solution. This is mainly derived from the synergistic effect of the crystallographic form, carbon coating, and hollow structure. Besides, the reaction mechanism of Ni/NiO-C3 electrode is controlled by charge transfer in high-concentration ethanol. With the concentration of ethanol decreased, a diffusion control mechanism is observed on Ni/NiO-C3 electrode. The attenuation rate of current density is only 0.1% after 3600 s by chronoamperometry in 0.1 M ethanol aqueous solution, indicating that the Ni/NiO-C3 electrode has a superior stability. Moreover, Ni/NiO-C3 electrode exhibits an rapid current response in 0.1 mM ethanol solution and a linear relationship between 0.69 and 40.17 mM, which provides a new idea for the research of fuel cell type ethanol sensor.

Mechanism of Ethanol Oxidation on Unsupported Noble Metal Nanoalloys Studied By Differential Electrochemical Mass Spectrometry

Electrooxidation of ethanol on noble metals is of particular interest due to its potential application in low temperature direct ethanol fuel cells (DEFCs). However there is no known ethanol electrooxidation reaction (EOR) catalyst, allowing for complete electrooxidation of ethanol to CO2. Even for the most active systems, incomplete and slow EOR renders the DEFCs not economically viable. As a result significant effort has been devoted to understand the reasons of low activity of known catalysts and to finding better, more active and selective systems. It is widely accepted that systems consisting of platinum, and platinum alloyed with other metals are most active towards ethanol electrooxidation. Pt-Ru and Pt-Sn systems are often reported as those with the highest current density in acidic media. However, despite the higher current, addition of Ru or Sn to Pt further decrease the already small amount of CO2, produced in EOR on pure Pt. Selectivity of a given catalyst towards CO2 in EOR is especially important, due to the facts, that CO2 is the product of complete, 12-electron oxidation of ethanol molecule, where acetaldehyde and acetic acid (other possible products) yields respectively 2- or 4-electrons. Additionally acetic acid cannot be further oxidized in a working fuel cell, thus it is effectively a dead-end of EOR in a working low temperature DEFC. As a result other catalysts, more active and selective toward ethanol electrooxidation to CO2 are needed for a commercially viable DEFC. In this report we used unsupported ultra-pure platinum-containing nanoalloys as a catalyst for EOR, to determine the possible changes in mechanism of EOR (based on product distribution determined by DEMS) as a function of nanoalloy composition. We focused on possible changes in surface morphology and changes in electronic properties due to alloying and how those factor impact electrode reaction mechanisms. Using DEMS experiments we determined how the amounts of products like carbon dioxide, acetaldehyde and acetic acid per active surface area changed as a function of nanoparticle composition and how it impacted the onset potential for particular products. As a result we were able to comment on the selectivity and activity of unsupported Pt-containing nanoalloys in EOR as well as on the activity of this material in C-C bond scission.

Impedance Spectroscopy of Direct Ethanol Fuel Cells with Anodes Containing Pt Group Metals

Alcohols might be a promising alternative to gaseous hydrogen in fuel cells technologies. In this study ethanol electrooxidation and oxygen electroreduction were studied by using classical electrochemical methods and electrochemical impedance spectroscopy (EIS). Studies have been carried out for both Pt and Pt-Ru catalysts for the cathode and anode separately in 0.5 M sulfuric acid solutions and for the assembled fuel cells with Nafion membranes. EIS investigations for anodes were carried out in the potentiodynamic conditions because otherwise a stationary state necessary for impedance measurements does not occur [1]. Impedance of the ethanol electrooxidation was recorded in voltammetric experiments with a sinusoidal perturbation of single frequency and small amplitude additionally applied at the electrode. After several voltammetric runs impedance spectra were built for selected electrode potentials. Thus obtained time and potential resolved impedance spectra were characterised by good reproducibility and their validity was confirmed in Kramers–Kronig transformations. The impedance spectra were qualitatively similar to those reported earlier and obtained by using Dynamic EIS (DEIS) experiments [1]. Complex mechanism of ethanol oxidation manifest itself as a negative resistance of the charge transfer, which is related to decrease in the rate of ethanol electrooxidation with increasing anodic potential. This effect is also accompanied by a significant drop in the capacity of the double layer and can be ascribed to the accumulation of carbon monoxide at the electrode surface. Impedance measurements were also performed for assembled fuel cells under different loads with current perturbation equal to 5% of the overall current density. Since the stationary state was assured by a constant flow of the reactants, in this configuration impedance was recorded in a broader frequency range and for much lower frequencies than in ex situ experiments. Equivalent circuit were proposed to model experimentally recoded spectra. The resistance of the charge transfer and the capacity of the double layer at the cathode were virtually independent from the oxygen partial pressure within studied range. This effect, observed particularly for current densities greater than 4 mA cm-2, suggest nearly constant potential of the cathode. Ethanol concentration largely affects impedance of the anode. For higher ethanol concentration a decrease in the double layer capacity accompanied by an increase in the absolute values of the negative resistance were observed. These effects reflect a strong poisoning of the electrode surface. Results points out that the current efficiency is strongly limited by the electrooxidation process at the anode. The influence of other experimental conditions such as temperature and fuel to oxygen stoichiometry on the impedance of direct ethanol fuel cells will be also discussed. [1] E.Desilets, A. Lasia, Electrochimica Acta 78 (2012) 286-293

Catalytic Activity of Unsupported Pd-Pt Nanoalloys Towards Formic Acid and Ethanol Electrooxidation

Formic acid and ethanol oxidation reactions on noble metals were extensively studied in the past. However it is still unknown how the catalytic activity of Pd-Pt alloys towards formic acid and ethanol scale with composition. In case of formic acid oxidation, palladium is active at low electrode potentials, where platinum is quickly poisoned by strongly adsorbed CO. On the contrary, for ethanol oxidation, only platinum is active. Thus the activity of the alloys should scale with composition. However multitude of effects, such as surface morphology, segregation and changes in electronic properties make the relationship between composition of Pt-Pd alloys and catalytic activity a complicated one. Oxidation of formic acid on Pt, Pd and Pd-Pt alloys follows a so-called “dual pathway” mechanism, namely formic acid can undergo dehydrogenation leading directly to CO2 (not involving strongly adsorbed CO, COads, as an intermediate) or dehydration leading to formation of COads and its possible further oxidation to CO2. Direct path (dehydrogenation) is operative at relatively low potentials but, in case of Pt, strongly adsorbed COads quickly blocks the surface sites, causing overall low activity of Pt in formic acid oxidation at low potentials. On the contrary for Pd the poisoning by COads at low potentials is much slower and the direct path (dehydrogenation) is operative to a much large extend. As a result Pd is significantly more active than Pt towards formic acid oxidation at low potential. In case of Pt-Pd alloys additional factors, related to surface morphology, must be considered: the number of adjacent Pd atoms required to adsorb formic acid molecule and number of adjacent Pt atoms required to form CO from formic acid. In particular is known that COads cannot be formed from formic acid on isolated Pt atoms, because at least two adjacent Pt atoms are required to dehydrate formic acid molecule and form COads, a phenomenon known as the “ensemble effect”. As a result such isolated Pt atoms can be much more active towards formic acid oxidation directly to CO2, and low Pt concentration Pd-Pt nanoalloys favor formation of such sites, a so called “third body effect”. Also changes in electronic properties, due to Pt-Pd interactions, cannot be excluded. In particular we observed changes in onset potential of formic acid and in the catalytic current density. The changes in surface morphology can to a large extend explain the changes in catalytic currents, and the only observation tentatively suggesting the changes in electronic properties are the difference between Pd and Pd-Pt nanoalloys catalytic activity observed in the cathodic scan. However the changes in formic acid oxidation potential cannot be fully explained by surface morphology. As a result we explain the changes in formic acid oxidation potential based on changes in the electronic properties of the Pd-Pt surface due to interactions between Pt and Pd. In case of ethanol electrooxidation on Pt-Pd alloys similar factors, as mentioned above, must be considered. Additionally we used Differential Electrochemical Mass Spectrometry to distinguish between the possible ethanol oxidation products at different electrode potentials. Most common products of ethanol oxidation on Pt and Pt-Pd is carbon dioxide, acetic acid and acetaldehyde. Although it is known that all of these products are produced during the process of ethanol oxidation, the details of this reaction are still unknown, and how the ethanol oxidation mechanism change with Pt-Pd alloy composition. The use of DEMS allowed us to address those issues and to elucidate the changes in reaction mechanism when Pt is diluted by Pd. Better understanding of the mechanism of ethanol electrooxidation can help to develop new, more active catalysts for low temperature direct ethanol fuel cells. This project was funded from Polish National Science Centre budget based on decision number DEC-2013/09/B/ST4/00099

Numerical investigation of ethanol fuelled HCCI engine using stochastic reactor model. Part 2: Parametric study of performance and emissions characteristics using new reduced ethanol oxidation mechanism

Ethanol fuelled homogenous charge compression ignition engine offers a better alternative to tackle the problems of achieving higher engine efficiency and lower emissions using renewable fuel. Present study computationally investigates the HCCI operating range of ethanol at different compression ratios by varying inlet air temperature and engine speed using stochastic reactor model. A newly developed reduced ethanol oxidation mechanism with NOx having 47 species and 272 reactions is used for simulation. HCCI operating range for compression ratios 17, 19 and 21 are investigated and found to be increasing with compression ratio. Simulations are conducted for engine speeds ranging from 1000 to 3000rpm at different intake temperatures (range 365–465K). Parametric study of combustion and emission characteristics is conducted and engine maps are developed at most efficient inlet temperatures. HCCI operating range is defined using combustion efficiency (>85%) and maximum pressure rise rate (<5MPa/ms). In HCCI operating range, higher efficiency is found at higher engine loads and lower engine speeds. Emission characteristics of species (NOx, N2O, CO, CH4, C2H4, C2H6, CH3CHO, and HCHO) found in significant amount is also analysed for ethanol fulled HCCI engine. Emission maps for different species are presented and discussed for wide range of speed and load conditions. Some of unregulated species such as aldehydes are emitted in significantly higher quantities from ethanol fuelled HCCI engine at higher load and speed conditions.