Thin-layer Flow Research Articles

New Insights on Formic Acid Electrooxidation at a Polycrystalline Au Film Electrode: Simultaneous in Situ ATR-Ftirs and Online Dems Measurements

Formic acid oxidation to CO2 on a gold electrode was proposed recently to occur via dimers of adjacent bridge-bonded adsorbed formates [1]. A systematic theoretical study of different pathways for formic acid electro-oxidation at a Au(111) electrode [2] indicates that the reaction path depends on the potential and formate coverage (via single formate, direct path, complex of formate and formic acid, formate dimers), which is difficult to address experimentally.In the present work, formic acid oxidation on a polycrystalline Au thin film electrode was studied using simultaneous in situ ATR-FTIRS, online DEMS and electrochemical measurements [3] in both potentiodynamic and potentiostatic modes in a thin-layer flow cell configuration under well-defined mass transport conditions, and compared to the mass transport controlled bulk CO and oxalic acid oxidation. In addition, oxalic acid oxidation was studied as a model reaction for in situ sensing of adsorbed oxalates [4]. These may be plausible intermediates in the formic acid oxidation, resulting from the dimerization of neighboring adsorbed formate species. Simultaneous in situ ATR-FTIRS and online DEMS measurements were employed to address the relation between the oxidation current, the CO2 formation rate, and the appearance of the adsorbed formate during formic acid oxidation. The kinetic H/D isotope effect (kH/kD) in formic acid oxidation was studied by DEMS as a function of Au electrode potential to discriminate between adsorption of formate (C-H bond remains intact, kH/kD = 1), or oxidation to CO2 (C-H bond rupture, kH/kD > 1). A thin (~50 nm) Au film was electrolessly deposited on the top plane of a hemi-cylindrical Si prism, which was subsequently annealed in a butane flame in a N2 stream to improve the stability of the Au film. The IR spectra were acquired using p-polarized IR radiation at 1 s-1 time resolution and 4 cm-1 spectral resolution using a Cary 680 FTIR spectrometer (Agilent Technologies). A quadrupole mass spectrometer QMS 422 (Pfeiffer Vacuum) was used for online monitoring of CO2 at m/z = 44. Potentials were controlled and currents acquired by an AFRDE 5 (Pine Instruments) potentiostat.The onset of CO2 formation during formic acid oxidation during the potentiodynamic scan is significantly delayed compared to the onset of the Faradaic current, while in bulk CO oxidation and in oxalic electrooxidation they increase in parallel. In contrast, the bridge-bonded adsorbed formate species appear instantaneously with the current increase, indicating that at low potentials the Faradaic current results from adsorption of formate species under these conditions. At higher potentials (>0.9 V), the coverage of bridge-bonded formate still increases, while the overall Faradaic current saturates. A steep increase in formic acid oxidation rate at potentials positive of 1.25 V correlates with the appearance of a band at ca. 1250 cm-1, which was detected also in oxalic acid oxidation and assigned to adsorbed oxalate [4]. Accordingly, we postulate that at these potentials adsorbed oxalates are formed by dimerization of adsorbed formates. Although we cannot decide whether these species subsequently decompose to CO2, it is clear that at 1.25 V a new reaction pathway is activated. The kH/kD, found from the Faradaic currents in HCOOH and DCOOH potentiodynamic oxidation is close to one at the onset of formic acid oxidation, which indicates the C-H bond remains intact, as could be expected for the selective formic acid oxidation to adsorbed formate, in agreement with a low current efficiency for CO2 formation under these conditions. In contrast, the kH/kD, as found from the corresponding CO2 formation currents, is close to 5 at the onset of CO2 formation, approaching ca. 4 at potentials, where the current efficiency for CO2 formation is 100%, indicating that C-H bond splitting is the rate determining step of CO2 formation.Overall, the present findings demonstrate that at very low potentials formic acid oxidation starts by adsorption of formate species, followed by CO2 formation at potentials > 0.4 V. First spectroscopic evidence was provided for the dimerization of adsorbed formates to form adsorbed oxalate at high potentials. The present work shows the role of the adsorbed formate in formic acid oxidation to depend on the reaction conditions [2]. Acknowledgement. This work was supported by the Deutsche Forschungsgemeinschaft (Research unit FOR 1376, JU 2781/2-2) [1] A. Cuesta, G. Cabello, F.W. Hartl, M. Escudero-Escribano, C. Vaz-Domínguez, L.A. Kibler, M. Osawa, C. Gutiérrez, Cat. Tod. 202, 79 (2013). [2] W. Gao, E.H. Song, Q. Jiang, T. Jacob, Chem. Eur. J. 20, 11005 (2014).[3] M. Heinen, Y.X. Chen, Z. Jusys, R.J. Behm, Electrochim. Acta 52, 5634 (2007).[4] A. Berná, J.M. Delgado, J.M. Orts, A. Rodes, J.M. Feliu, Langmuir 22, 7192 (2006).

Horizontal mixing layer in shallow water flows

Horizontal-shear thin-layer homogeneous fluid flow in the open channel is considered. A one-dimensional mathematical model of the development and evolution of the horizontal mixing layer is derived within the framework of the three-layer scheme. The steady-state solutions of the equations of motion are constructed and investigated. In particular, supercritical (subcritical)-in-average flow concepts are introduced and the problem of the mixing layer structure is solved. The proposed model is verified on the basis of comparison with a numerical solution of two-dimensional equations of shallow water theory.

Inverse modeling of thin layer flow cells for detection of solubility, transport and reaction coefficients from experimental data

Thin layer flow cells are used in electrochemical research as experimental devices which allow to perform investigations of electrocatalytic surface reactions under controlled conditions using reasonably small electrolyte volumes. The paper introduces a general approach to simulate the complete cell using accurate numerical simulation of the coupled flow, transport and reaction processes in a flow cell. The approach is based on a mass conservative coupling of a divergence-free finite element method for fluid flow and a stable finite volume method for mass transport. It allows to perform stable and efficient forward simulations that comply with important physical constraints, namely mass conservation and maximum principles for the involved species. In this context, several recent approaches to obtain divergence-free velocity approximations from finite element simulations are discussed. In order to perform parameter identification, the forward simulation method is coupled to standard optimization tools. After an assessment of the inverse modeling approach using hypothetical, but realistic data, first results of the identification of solubility and transport data for O2 dissolved in organic electrolytes are presented. A plausibility study for a more complex situation with surface reactions concludes the paper and shows possible extensions of the presented numerical tools.

Symmetry in the problem of wave modes of thin viscous liquid layer flow

The equations in conservative form for modelling nonlinear waves on a liquid film flowing down a vertical plane have been investigated. It has been found out that the equations with boundary conditions are invariant under parity transformation in the extended computational domain. The steady-state travelling solutions are numerically shown to have the detected symmetry for moderate Reynolds numbers. The use of this symmetry for the numerical solution of the problem by Galerkin methods significantly increases the efficiency of calculations.

Using electric fields to induce patterning in leaky dielectric fluids in a rod-annular geometry

The stability and axisymmetric deformation of two immiscible, viscous, perfect or leaky dielectric fluids confined in the annulus between two concentric cylinders are studied in the presence of radial electric fields. The fields are set up by imposing a constant voltage potential difference between the inner and outer cylinders. We derive a set of equations for the interface in the long-wavelength approximation which retains the essential physics of the system and allows for interfacial deformations to be as large as the annular gap hence accounting for possible touchdown at the inner or outer electrode. The effects of the electric parameters are evaluated initially by performing a linear stability analysis which shows excellent agreement with the linear theory of the full axisymmetric problem in the appropriate long wavelength regime. The nonlinear interfacial dynamics are investigated by carrying out direct numerical simulations of the derived long wave models, both in the absence and presence of electric fields. For non-electrified thin layer flows (i.e. one of the layers thin relative to the other) the long-time dynamics agree with the lubrication approximation results found in literature. When the liquid layers have comparable thickness our results demonstrate the existence of both finite time and infinite time singularities (asymptotic touching solutions) in the system. It is shown that a two-side touching solution is possible for both the non-electrified and perfect dielectric cases, while only one-side touching is found in the case of leaky dielectric liquids, where the flattened interface shape resembles the pattern solutions found in literature. Meanwhile the finite-time singular solution agrees qualitatively with the experiments of~\cite{Reynolds1965}.

Determining Solubility and Diffusivity by Using a Flow Cell Coupled to a Mass Spectrometer.

One of the main challenges in metal-air batteries is the selection of a suitable electrolyte that is characterized by high oxygen solubility, low viscosity, a liquid state and low vapor pressure across a wide temperature range, and stability across a wide potential window. Herein, a new method based on a thin layer flow through cell coupled to a mass spectrometer through a porous Teflon membrane is described that allows the determination of the solubility of volatile species and their diffusion coefficients in aqueous and nonaqueous solutions. The method makes use of the fact that at low flow rates the rate of species entering the vacuum system, and thus the ion current, is proportional to the concentration times the flow rate (c⋅u) and independent of the diffusion coefficient. The limit at high flow rates is proportional to D2/3·c·u1/3 . Oxygen concentrations and diffusion coefficients in aqueous electrolytes that contain Li(+) and K(+) and organic solvents that contain Li(+) , K(+) , and Mg(2+) , such as propylene carbonate, dimethyl sulfoxide tetraglyme, and N-methyl-2-pyrrolidone, have been determined by using different flow rates in the range of 0.1 to 80 μL s(-1) . This method appears to be quite reliable, as can be seen by a comparison of the results obtained herein with available literature data. The solubility and diffusion coefficient values of O2 decrease as the concentration of salt in the electrolyte was increased due to a "salting out" effect.

Flavonoids electrochemical detection in fruit extracts and total antioxidant capacity evaluation

Phenolic compounds detection in fruit extracts from: açai, bacuri, buriti, blackberry, black mulberry, blueberry, juçara, physalis, raspberry, and tamarillo, have been performed by reverse-phase high performance liquid chromatography with electrochemical detection (RP-HPLC-EC), using two detectors in series: a wall-jet detector flow cell with a glassy carbon electrode, and a thin-layer flow cell detector with a boron doped diamond electrode. This methodology, in gradient elution mode, was successfully used to detect seventeen phenolic compounds in the fruit extracts. The total antioxidant capacity of the fruit extracts by the electrochemical quantitative index (EI) and the method of capture of diphenilpicrilhydrazil (DPPH●) free radical “efficient concentration” (EC50), was evaluated. A very good correlation between EI and EC50 assays has been obtained, the fruit with the highest total antioxidant capacity being blackberry, while physalis exhibited the lowest antioxidant power.

Численное исследование течения тонкого слоя жидкости с испарением

Численное исследование течения тонкого слоя жидкости с испарением

Multi-regime shallow free surface laminar flow models for quasi-Newtonian fluids

The mathematical modeling of thin free-surface laminar flows for quasi-Newtonian fluids (power-law rheology) is addressed with a particular attention to geophysical flows (e.g. ice or lava flows). Asymptotic thin-layer flow models (one-equation and two-equation models) consistent with various viscous regimes, corresponding to different basal boundary conditions (from adherence to pure slip), are derived. The challenge being to derive models consistent from slip to no-slip basal boundary condition, though at the price of balancing small friction by small mean slope. Starting from reference flows (the steady-state uniform ones) corresponding to different shear regimes, the exact expressions of all fields (σ,u,p) are calculated formally by a perturbation expansion method. The calculations are such that all field expressions remain valid for any laminar viscous regimes. The calculations are presented either in a mean slope coordinate system with local variations of the topography or in the Prandtl coordinate system, hence valid in presence of any non flat basal topography. Formal error estimates proving the consistency of the derivations are stated. An unified one-equation model (lubrication type in the depth variable h) is derived at order 1. Next, few unified two-equation models in variable (q,h) (shallow water type) are stated and discussed. The classical first order models from the literature are recovered if considering the corresponding particular cases (generally, flat bottom with a particular regime and/or specific basal boundary condition). Two one-dimensional numerical examples illustrate the robustness of these new multi-regime formulations (the change of flow regimes being due either to a sharp change of the mean-slope topography or to a sharp change of basal boundary condition).

Au/TiO2 Photo(electro)catalysis: The Role of the Au Cocatalyst in Photoelectrochemical Water Splitting and Photocatalytic H2 Evolution

Photoelectrochemical water oxidation and photocatalytic H2 evolution on planar nanocrystalline Au/TiO2 catalyst film electrodes were studied by combined electrochemistry and online mass spectrometry, allowing for the detection and quantification of the volatile reaction products O2 and H2, and thus, by comparison with the photocurrent, to determine selectivities and to map out contributions from side reactions. The catalysts were prepared by a photodeposition method and deposited on conductive glass substrates, and measurements were performed under defined mass transport conditions in a photoelectrochemical thin-layer flow cell. The role of Au nanoparticles as cocatalysts in these reactions was explored by varying the Au loading and the light intensity, respectively. On the basis of these data, it is demonstrated that the Au nanoparticles affect the above reactions in different ways, by (i) catalyzing the recombination of photogenerated holes and electrons as well as parasitic side reactions (O2 reduction...

Carbon Dioxide Electroreduction in Aqueous Media: Relation Between Electrochemical Surface Area and Product Selectivity on Copper-Based Catalysts

Carbon dioxide is a major contributor to global warming and, beyond the mandatory reduction of our emissions, the possibility of recycling this greenhouse gas is becoming increasingly attractive. The electrochemical reduction of CO2 is an interesting pathway, since a broad range of useful products can be formed such as methanol and formic acid (fuels for PEFC), methane and ethylene (reactants for synthesis or combustion process), CO and H2 (syngas), etc. [1]. Nevertheless, on top of the high overpotential required to drive this reaction, the electrochemical reduction of CO2 suffers from a poor yield/selectivity of valuable products [2, 3].Copper-based electrocatalysts seem to be the most efficient to overcome these kinetic and barriers, [3], but it has been shown that the size of the Cu (nano)particles envisaged for practical applications can strongly influence the reaction selectivity [4]. In order to clarify this particle size effect, one needs to determine the electrochemically active surface area (ECSA) of the catalysts of interest that, for Cu, can be derived in-situ by lead underpotential deposition (Pb-UPD) [5]. Additionally, the selectivity of the CO2-reduction reaction on these well-characterized materials needs to be determined with an appropriate technique. With this motivation, we have performed Pb-UPD measurement on polycrystalline Cu and on carbon-supported Cu nanoparticles (NPs) of various particle sizes in a 3-electrode glass cell filled with 0.01 M of HClO4 with 50nM of Pb(ClO4)2 and 100 nM of KCl. In these measurements, we have used rotating ring disk electrode (RRDE) voltammetry to quantify the amount of of Cl- (typically ≈ 10%) co-adsorbed with the Pb2+ [5]. An example of the results obtained with this approach, Figure 1 displays the disc and ring currents recorded in a shielding experiment recording a cyclic voltammogram on the polycrystalline Cu disk while applying a constant potential of -0.25V vs. RHE at the ring, at which the continuous Pb-plating ring current is affected by the adsorption/desorption of Pb2+ species at the disk. Having estimated the ECSA for various catalysts, we then used differential electrochemical mass spectrometry (DEMS) to quantify the yield of the reaction’s gaseous products, using an in-house constructed DEMS setup combined with a dual thin layer flow cell adapted from previous works [7, 8] and separated from the quadrupole mass spectrometer chambers by a PTFE membrane. In summary, this contribution will introduce an RRDE-based methodology for the determination of the ECSA of Cu-based electrocatalysts That we have applied to the characterization of polycrystalline Cu and carbon-supported Cu nanoparticles of different sizes,. These results will be complemented with the corresponding CO2-reduction yields, as quantified with our DEMS setup.

Electro-Oxidation of Ethanol at Low and Intermediate Temperature Investigated By on-Line Mass Spectrometry

Introduction The search for renewable energy sources is motivated by a decrease in the reserves of fossil fuels and rising environmental degradation. Researchers have studied the use of ethanol as a sustainable fuel, due to its several positive characteristics, such as low toxicity and wide availability. However, the ethanol oxidation reaction (EOR) has parallel reaction routes that release lower number of electrons than result from CO2 formation [1]. To address this challenge, ethanol electroxodiation at intermediate temperatures using solid acid electrolytes has been proposed [2]. In the present work, the EOR was investigated at low temperature, using aqueous H2SO4, and at intermediate temperature, using solid CsH2PO4, as electrolytes. The reaction products for the catalysts, carbon-supported combinations of Pt, Ru and Sn, were investigated by on-line mass spectrometry using a Dual Thin Layer Flow Cell for room temperature, and a tip inlet setup for the mass spectrometry measurements at intermediate temperature. Experimental The Pt3(SnOxRuOx)1/C electrocatalysts were synthesized via thermal decomposition of the precursor metal salts, adapting a previously published method [3]. Pt/C (E-TEK) was also studied for comparison. The atomic composition was estimated by X-ray energy dispersive spectroscopy (X-EDS) analysis, and structural features were investigated by X-ray diffraction (XRD) measurements. Synthesis of CsH2PO4 and fabrication of the CsH2PO4-based electrochemical cell, with Pt/C serving as the cathode, followed previously published procedures [2]. A dynamic hydrogen electrode was used as reference (humidified H2). The m/z signals of 22 (CO2), 29 (acetaldehyde), 44 (CO2 + acetaldehyde) and 15 (acetaldehyde and methane) was utilized during the DEMS measurements. Results and Discussion Fig. 1 presents the faradaic currents (a) and the mass signals (b) m/z = 22 and (c) m/z = 29 (insets) obtained during potentiostatic measurements of ethanol electro-oxidation (0.1 mol L-1) in the DEMS equipment, at room temperature, catalyzed by Pt3(SnOxRuOx)/C and Pt/C. One can be observed a deactivation of the currents (faradaic or ionic) as a function of the time, for both electrocatalysts, with the CO2 signal (m/z = 22) dropping to zero. The stripping of the adsorbed reaction fragments (Fig. 1(d)), revealed that the poisoning species are CO and CHx. These species may accumulate during the course of the ethanol oxidation reaction in this potential domain. Fig. 1: (a) Room temperature potentiostatic measurements presenting the faradaic and ionic currents (inset) obtained during on-line DEMS experiments (Dual Thin Layer Flow Cell) and (b) reaction fragment stripping obtained DEMS measurements. Ethanol: 0.1mol L-1 . Scan rate of 10 mVs-1. On the other hand, for the measurements conducted at 245oC, and at 0.5V and 0.8V, the faradaic current and the ionic signal for CO2 formation underwent no decay (Fig. 2). Additionally, the trimetallic electrocatalyst, presented much higher activity for the EOR than did Pt alone. While the calculation of the faradaic current efficiency for CO2 formation at intermediate temperature is still in progress, initial results showed much higher values for both electrocatalysts when compared to the values obtained at room temperature. Fig 2: (a) Elevated temperature potentiostatic measurements and (b) CO2 ionic signal (tip inlet DEMS setup) obtained during the EOR. Ethanol: 36 wt.% in H2O. Cell temperature = 245 oC. Acknowledgements The authors thank FAPESP, CNPq and LiOx Power, Inc.

Identification of the Molecular and Genetic Basis of PX2, a Glycosphingolipid Blood Group Antigen Lacking on Globoside-deficient Erythrocytes

The x2 glycosphingolipid is expressed on erythrocytes from individuals of all common blood group phenotypes and elevated on cells of the rare P/P1/P(k)-negative p blood group phenotype. Globoside or P antigen is synthesized by UDP-N-acetylgalactosamine:globotriaosyl-ceramide 3-β-N-acetylgalactosaminyltransferase encoded by B3GALNT1. It is the most abundant non-acid glycosphingolipid on erythrocytes and displays the same terminal disaccharide, GalNAcβ3Gal, as x2. We encountered a patient with mutations in B3GALNT1 causing the rare P-deficient P1 (k) phenotype and whose pretransfusion plasma was unexpectedly incompatible with p erythrocytes. The same phenomenon was also noted in seven other unrelated P-deficient individuals. Thin-layer chromatography, mass spectrometry, and flow cytometry were used to show that the naturally occurring antibodies made by p individuals recognize x2 and sialylated forms of x2, whereas x2 is lacking on P-deficient erythrocytes. Overexpression of B3GALNT1 resulted in synthesis of both P and x2. Knockdown experiments with siRNA against B3GALNT1 diminished x2 levels. We conclude that x2 fulfills blood group criteria and is synthesized by UDP-N-acetylgalactosamine: globotriaosylceramide 3-β-N-acetylgalactosaminyltransferase. Based on this linkage, we proposed that x2 joins P in the GLOB blood group system (ISBT 028) and is renamed PX2 (GLOB2). Thus, in the absence of a functional P synthase, neither P nor PX2 are formed. As a consequence, naturally occurring anti-P and anti-PX2 can be made. Until the clinical significance of anti-PX2 is known, we also recommend that rare P1 (k) or P2 (k) erythrocyte units are preferentially selected for transfusion to P(k) patients because p erythrocytes may pose a risk for hemolytic transfusion reactions due to their elevated PX2 levels.

The Effect of Anions on the Oxygen Reduction Reaction Activity and Selectivity on a Au Electrode: A Double Disk Electrode Flow Cell ATR-FTIR Spectroscopy Study

The oxygen reduction reaction (ORR) on Au electrode exhibits a sluggish kinetics and its selectivity is sensitive to adsorbed hydroxyl intermediates and the pH [1,2]. Adsorbed anions are known to strongly affect the ORR in the kinetic region via the occupation of surface sites required for the dissociative adsorption of O2 molecules. Also the ORR selectivity for the reduction to water instead to hydrogen peroxide is affected when going from weakly adsorbing to more strongly adsorbing anions [3,4]. Application of in situ Fourier transform infrared spectroscopy in an attenuated total reflection configuration (ATR-FTIRS) was demonstrated recently to offer important insights on the nature of adsorbed species / intermediates in the ORR [5,6], which was recently extended to ORR studies using a spectro-electrochemical flow cell [7]. However, the application of in situ ATR-FTIRS, which allows correlation of the ORR activity and the presence of adsorbed species, so far has not provided any direct information on the impact of the adsorbed anions on the ORR selectivity. To pursue this aim, a spectroelectro-chemical dual thin layer flow cell, used for combination of in situ ATR-FTIRS, online mass spectrometry and electrochemical measurements [8], was modified for simultaneous electrochemical detection of hydrogen peroxide at the second (Pt collector) electrode [9]. A thin (~30 nm) Au film was electrolessly deposited [10] onto the face plane of a hemi-cylindrical Si prism, and then carefully annealed in a butane flame in a N2 atmosphere to improve the film stability, while maintaining a high IR sensitivity (about three-fold higher linearly adsorbed CO band intensity at 0.0 V in CO saturated solution compared to the literature [5]). A homemade mirror accessory was used within the sample chamber of a Varian 670i FTIR spectrometer, equipped with a p-polarizer and a liquid nitrogen cooled mercury cadmium telluride detector. Measurements were performed at a spectral resolution of 4 cm-1 and a temporal resolution of 1 s-1 per spectrum (by co-adding 5 interferograms). The Pt collector was biased at 1.2 V (RHE) for hydrogen peroxide oxidation (collection efficiency of ca. 20%). O2 saturated solutions of perchloric, sulfuric acids (0.5 M) or sodium hydroxide (1.0 M) were used. The ORR starts at ca. 0.5, 0.6 and 1.0 V (RHE) in perchloric, sulfuric acids, and alkaline solution, respectively. It does not reach the mass transport limitation up to 0.0 V in acidic solutions, whereas in alkaline medium the mass transport limited ORR current is reached at about 0.5-0.6 V, and slightly decreases at lower potentials due to increasing hydrogen peroxide formation. The hydrogen peroxide yields of 60 to 80% were found at potentials 0.0 to 0.5 V in both perchloric and sulfuric acid solutions, in agreement with gradually increasing integrated band intensities of the corresponding anions. In contrast, in alkaline solution the hydrogen peroxide yields are close to 0% at the onset of the ORR on Au film at high potentials and gradually increase with decreasing potential to ca. 60 % at 0.0 V. In this case, IR spectroscopy is little helpful since it cannot discriminate between adsorbed water, hydroxyl or peroxyl. A correlation between the variation of the ORR activity and selectivity in alkaline solution and the OH adsorption is derived indirectly, by i) comparing the double layer current obtained on a Au film electrode in alkaline solution, and from ii) additional measurements in CO saturated 0.1 M NaOH solution, where the mass transport limited current is reached over a wide potential range (0.5 - 1.2 V). These results strongly support the presence of adsorbed OH in this potential range, which, however, does not favor the ORR to hydrogen peroxide. Acknowledgement. This work was supported by the Deutsche Forschungsgemeinschaft (Research unit FOR 1376, JU 2781/2-2) [1] V. Viswanathan, H.A. Hansen, J. Rossmeisl, J.K. Nørskov, J. Phys. Chem. Lett. 3, 2948 (2012). [2] P. Quaino, N.B. Luque, R. Nazmutdinov, E. Santos, W. Schmickler, Angew. Chem. Int. Ed. 51, 12997 (2012). [3] N.M. Markovic, P.N. Ross Jr., Surf. Sci. Rep., 45, 117 (2002). [4] J.X. Wang, N.M. Markovic, R.R. Adzic, J. Phys. Chem B 108, 4127 (2004) [5] M.H. Shao, R.R. Adzic, J. Phys. Chem B 109, 16563 (2005). [6] N. Ohta, K. Nomura, I. Yagi, J. Phys. Chem. C 116, 14390 (2012). [7] M. Nesselberger, S.J. Ashton, G.K.H. Wiberg, M. Arenz, Rev. Sci. Instr. 84, 074103 (2013). [8] M. Heinen, Y.X. Chen, Z. Jusys, R.J. Behm, Electrochim. Acta, 52, 5634 (2007). [9] Z. Jusys, J. Kaiser, R.J. Behm, Electrochim. Acta, 49, 1297 (2004). [10] K.-i. Ataka, T. Yotsuyanagi, M. Osawa, J. Phys. Chem. 100, 10664 (1996).

Formic Acid Electrooxidation on Au Electrode Studied By Potential Step and a Fast Scan ATR-FTIR Spectroscopy

The role of adsorbed formates in the electro-oxidation of formic acid on Pt electrodes was widely debated [1-3]; on a gold electrode formic acid oxidation was assumed to proceed via dimers of contiguously adsorbed bridge-bonded formates [4]. In the present work, formic acid oxidation on Au thin film electrode was studied by simultaneous in situ ATR-FTIRS and electrochemical measurements in both potentiodynamic and potential step mode with a fast IR spectra acquisition in a thin-layer flow cell configuration under well-defined mass transport conditions [2], to address the impact of adsorbed spectator species (perchlorate) and adsorbed bridge-bonded formate on the formic acid electrooxidation rate. 0.1 M HCOOH and DCOOH were used to i) assess the kinetic H/D isotope effect in formic acid oxidation on Au electrode; ii) identify the adsorbed species and follow their temporal development and the potential dependence. A thin (~30 nm) Au film was electrolessly deposited onto the top plane of a hemi-cylindrical Si prism, which was subsequently carefully annealed in a butane flame in nitrogen atmosphere to improve the adhesion and thus the stability of the Au film under enforced hydrodynamic conditions and large repetitive perturbations of the working electrode potential (from 0.0 up to 1.65 V vs. RHE). This gold thin film electrode exhibited an unprecedented sensitivity of DR/R ~ 0.03 for the linearly adsorbed CO band in CO saturated supporting electrolyte at 0.0 V (reference spectrum acquired in CO-free electrolyte at 0.0 V). The IR spectra were acquired applying p-polarized IR radiation at 0.025 s-1 time resolution, 16 cm-1 spectral resolution and 75 kHz frequency employing a Cary 680 FTIR spectrometer (Agilent Technologies) equipped with a step scan accessory, and operated in a dual scan / quad compute mode. A EG&G 273A (PAR) potentiostat was used for potentiodynamic (100 mV s-1) and multistep (from 0.0 V to desired oxidation potential) experiments. A base CV in 1M HClO4 supporting electrolyte revealed a clean Au film electrode as confirmed by in situ IR spectroscopy measurements, which exhibit only the potential dependent bipolar bands at 3610 / 3455 and 1654 / 1515 cm-1 related to adsorbed water, and a unipolar band at ca. 1125 cm-1 of adsorbed perchlorate. Formic acid oxidation starts at about 0.5 V, approaches a plateau from 0.9 to 1.2 V (HCOOH) and from 0.9 to 1.3 V (DCOOH), and then passes through a maximum at around 1.45 V, which is about three times higher for HCOOH than for DCOOH, due to kinetic isotope effects. The corresponding IR spectra show pronounced potential dependent HCOOad and DCOOad bands at 1326 and 1303 cm-1, respectively. Plots of the integrated adsorbed formate band intensity vs. the potential are qualitatively similar to those of the perchlorate band in supporting electrolyte. Perchlorate adsorption is suppressed by adsorbed formate, due to competitive adsorption of the more strongly adsorbing anion. When stepping the potential from 0.0 V to the respective oxidation potential, the integrated band intensities for both adsorbed bridge bonded formate and perchlorate increased instanta­neously (within 25 ms) and remained constant over the step duration (1 s). A similar behavior is observed for adsorbed perchlorate, a clear spectator species. The integrated intensity of adsorbed formate at constant potentials increases with the adsorption potential up to 1.0 V, similar to that of adsorbed perchlorate in pure supporting electrolyte. The development of adsorbed formate reduces the perchlorate adsorption at higher potentials, compared to pure supporting electrolyte, indicating a competitive adsorption of formate and perchlorate. Overall, the data reveal an adsorption behavior of the adsorbed bridge-bonded formate species, which is identical to that of other anions that are known to act as spectator species. Although the participation of adsorbed bridge-bonded formate as active intermediate in formic acid oxidation on a Au electrode [4] cannot be ruled out based on the present data, they are at least equally consistent with a model proposing reaction via coadsorbing formate anions and formic acid species [5]. Acknowledgement. This work was supported by the Deutsche Forschungsgemeinschaft (Research unit FOR 1376, JU 2781/2-2) [1] G. Samjeské, A. Miki, S. Ye, M. Osawa, J. Phys. Chem. B 110, 16559 (2006). [2] Y.X. Chen, M. Heinen, Z. Jusys, R.J. Behm, Angew. Chem. Int. Ed. 45, 981 (2006). [3] W. Gao, J.A. Keith, J. Anton, T. Jacob, J. Am. Chem. Soc. 132, 18378 (2010). [4] A. Cuesta, G. Cabello, F.W. Hartl, M. Escudero-Escribano, C. Vaz-Domínguez, L.A. Kibler, M. Osawa, C. Gutiérrez, Cat. Tod. 202, 79 (2013). [5] S. Brimaud, J. Solla-Gullón, I. Weber, J.M. Feliu, R.J. Behm, ChemElectroChem 1, 1075 (2014).

Electrochemical Atomic Layer Deposition and Characterization of CdTe and PbTe Thin Films

This study reports the cycle chemistries involved in depositing CdTe and PbTe nanofilms. An automated thin-layer flow cell electrodeposition system was used to deposit the films at room temperature. Cyclic voltammetry was used to study the Underpotential Deposition (UPD) of the compounds. The monolayer/cycle deposition rate was also monitored in order to insure that the film is depositing at a uniform rate. The chemical composition of the films was characterized using Energy-Dispersive X-ray Spectroscopy (EDS) on a Scanning Electron Microscope (SEM). The crystallinity of the films was studied using a glancing angle X-ray diffractometer. The bandgaps of the films were calculated using measured optical reflection data.

Height-resolved velocity measurement of the boundary flow during liquid impact on dry and wetted solid substrates

The impact of a droplet onto a dry or wet surface leads to a rapid formation of a shear flow at the boundary. We present a novel method to experimentally resolve this flow in time at different heights above the solid. The radial flow field close to the substrate is reconstructed by evaluation of streak images of fluorescent tracer particles in the liquid. By using a microscope objective with a narrow depth of field, it is possible to scan through the flow in thin horizontal layers of 5 μm thickness. We focus on the flow close (≤40 μm) to the boundary during the impact of elongated drops with diameters of 0.3–0.4 mm and speeds in the range of 2–3 m s−1. The spatial resolution is obtained from several individual events of the repeatable impact process and varying the focal plane. Fluorescent streaks formed by the suspended particles are recorded with high-speed photography at up to 20,000 frames per second. The impact of water and of ethanol is investigated both on dry glass and on glass covered with a thin film of the same liquid. Results are given as spatio-temporal maps of radial flow velocity at different heights, and the maximum shear stress at the substrate is evaluated. The implications of the results are discussed with respect to cleaning applications.

A novel photoelectrochemical flow cell with online mass spectrometric detection: oxidation of formic acid on a nanocrystalline TiO2 electrode.

A novel thin-layer photoelectrochemical flow cell allowing the online mass spectrometric detection of volatile reaction products during photoelectrocatalytic reactions has been developed and applied for separating the contributions from photoelectrochemical water splitting and photoelectrooxidation of formic acid to the overall photocurrent in formic acid containing aqueous solution, using a nanocrystalline TiO2 (P25) thin-film electrode. The data reveal a clear suppression of the water oxidation reaction to O2 in the presence of formic acid. Advantages of this flow cell design over conventional photoelectrochemical cells with stagnant electrolyte in terms of mass transport will be demonstrated and discussed.

Model for the initiation of atomization in a high-speed laminar liquid jet

AbstractA laminar water jet issuing at high speed from a short circular nozzle into air exhibits various instability features at different distances from the nozzle exit. Near the exit, the effects of gaseous friction and pressure are relatively weak. Deformation of the jet surface in this region is mainly due to the instability of a thin liquid shear layer flow, which relaxes from the velocity profile produced by the nozzle wall. In this paper, a model for this type of instability based on linear stability analysis is investigated to describe the process initiating the formation of liquid ligaments disintegrating into fine droplets near the nozzle exit. The modelling comprises identifying unstable waves excitable in the liquid shear layer and exploring a self-destabilizing mechanism by which unstable waves responsible for the formation of liquid ligaments are naturally reproduced from the upstream-propagating capillary waves produced by the growth of the unstable waves themselves. An expression for the location of ligament formation onset is derived that can be compared with experiments. The model also explains changes in jet instability features away from the nozzle exit and for very short nozzles.

Finite-difference lattice Boltzmann method with a block-structured adaptive-mesh-refinement technique

An adaptive-mesh-refinement (AMR) algorithm for the finite-difference lattice Boltzmann method (FDLBM) is presented in this study. The idea behind the proposed AMR is to remove the need for a tree-type data structure. Instead, pointer attributes are used to determine the neighbors of a certain block via appropriate adjustment of its children identifications. As a result, the memory and time required for tree traversal are completely eliminated, leaving us with an efficient algorithm that is easier to implement and use on parallel machines. To allow different mesh sizes at separate parts of the computational domain, the Eulerian formulation of the streaming process is invoked. As a result, there is no need for rescaling the distribution functions or using a temporal interpolation at the fine-coarse grid boundaries. The accuracy and efficiency of the proposed FDLBM AMR are extensively assessed by investigating a variety of vorticity-dominated flow fields, including Taylor-Green vortex flow, lid-driven cavity flow, thin shear layer flow, and the flow past a square cylinder.