Competitive Adsorption Of Hydrogen Research Articles

Electrochemical Stability of Thiolate Self-Assembled Monolayers on Au, Pt, and Cu.

Self-assembled monolayers (SAMs) of thiolates have increasingly been used for modification of metal surfaces in electrochemical applications including selective catalysis (e.g., CO2 reduction, nitrogen reduction) and chemical sensing. Here, the stable electrochemical potential window of thiolate SAMs on Au, Pt, and Cu electrodes is systematically studied for a variety of thiols in aqueous electrolyte systems. For fixed tail-group functionality, the reductive stability of thiolate SAMs is found to follow the trend Au < Pt < Cu; this can be understood by considering the combined influences of the binding strength of sulfur and competitive adsorption of hydrogen. The oxidative stability of thiolate SAMs is found to follow the order: Cu < Pt < Au, consistent with each surface's propensity toward surface oxide formation. The stable reductive and oxidative potential limits are both found to vary linearly with pH, except for reduction above pH ∼10, which is independent of pH for most thiol compositions. The electrochemical stability across different functionalized thiols is then revealed to depend on many different factors including SAM defects (accessible surface metal atom sites decrease stability), intermolecular interactions (hydrophilic groups reduce the stability), and SAM thickness (stability increases with alkanethiol carbon chain length) as well as factors such as SAM-induced surface reconstruction and the ability to directly oxidize or reduce the non-sulfur part of the SAM molecule.

Rhodium Sulfides (Rh x S y ) as a Halide-Resistant Nitrate Reduction Electrocatalyst for Wastewater Remediation

Excess nitrate (NO3 –) accumulation in the environment due to human activities has adverse environmental and health effects and requires intervention. Electrocatalytic nitrate reduction (NO3RR), where nitrate is reduced in aqueous solution on electrode surfaces, is a promising method for sustainable remediation of nitrate to value-added chemicals such as NH3 [1]. A challenge for NO3RR is that nitrate adsorbs weakly to many catalyst surfaces and must compete with hydrogen and reaction intermediates for active sites [2, 3]. Metal catalysts which adsorb nitrate strongly at low overpotentials and are active for NO3RR, such as Rh, can be expensive. Better understanding the relationship between nitrate and hydrogen adsorption energies and nitrate reduction activity for known catalysts would allow faster identification of new, less expensive, active catalysts. Additionally, competition of nitrate for active sites is exacerbated in real waste streams, where other anions such as chloride are often present (e.g., introduced via resin recovery in ion-exchange membranes) and can compete for active sites, lowering the NO3RR activity. Determining chloride adsorption energies for nitrate reduction catalysts affected by the presence of chloride would provide useful information for understanding which materials would be affected by chloride and act as a guide for selecting chloride-resistant NO3RR catalysts.We report the competitive adsorption of nitrate and hydrogen and the reaction mechanism of NO3RR. By using adsorption energies of nitrate and hydrogen as descriptors, we qualitatively understand many of the observed trends in NO3RR activity on metal surfaces through a Langmuir-Hinshelwood reaction mechanism [3]. We show the voltage dependence of NO3RR on platinum group metals, where competitive adsorption of hydrogen and nitrate or nitrate intermediates causes a maximum in NO3RR activity with potential. Identifying these activity descriptors allows rapid computational screening to identify new promising catalysts. Using cyclic voltammetry on Pt and Rh, we observe that the chloride adsorption voltage window overlaps with the maximum activity for NO3RR, due to the related adsorption energies of nitrate and chloride [4]. Using steady state current densities, we show that Rh is more active than Pt for NO3RR in acidic conditions but the addition of even 1 mM chloride lowers NO3RR activity by 30-60%, with Rh more affected by chloride than Pt. The lowering of activity is attributed to competitive adsorption between chloride and nitrate for active sites. Using DFT, we compute the chloride and nitrate adsorption energies on a series of metals and observe linear scaling relations, such that it is unlikely any transition metal binds chloride weakly while adsorbing nitrate strongly. To address chloride poisoning, we examine rhodium sulfide (Rh x S y ), which is an electrocatalyst with notable halide resistance. We show that Rh x S y is more active for NO3RR in acidic media with and without chloride than Pt or Rh and discuss plausible active sites.

Dynamic Reaction Mechanism of P-N-Switched H2-Sensing Performance on a Pt-Decorated TiO2 Surface.

Pt decoration is known to be one of the most promising strategies to enhance the performance of TiO2 hydrogen gas sensors, while the effect of Pt-decorating concentration on the sensing performance of TiO2 and the specific interaction between Pt and TiO2 have not been fully investigated. Here, a series of TiO2 nanoarray thin films with differing amounts of Pt decorated (Pt/TiO2) is fabricated, and the H2-sensing performance is evaluated. A switch in the response from P-type to N-type is observed with increasing Pt decoration. The response additionally depends on the H2 concentration: resistance increases in low H2 concentrations and decreases in hydrogen concentrations higher than 40 ppm. This is explained by the competitive adsorption of hydrogen between the Pt nanoparticles (Pt NPs) and the exposed TiO2 surface. The preference for H2 adsorption and splitting between Pt and TiO2 is established by DFT calculations. Humidity brings preferential adsorption of H2O on the surface of Pt, which affects the following adsorption and splitting of H2, thus resulting in a P-N switch of the sensing performance. The detailed dynamic reaction process is described according to the findings.

Hydrophobic Platinum Honeycomb Catalyst to Be Used for Tritium Oxidation Reactors

We have developed two types of hydrophobic platinum honeycomb catalyst to be used for tritium oxidation reactors. One is the hydrophobic platinum catalyst on a metal honeycomb. The other is the hydrophobic platinum catalyst on a ceramic honeycomb made of silicon carbide. The activity of these catalysts was evaluated with tritium. The effects of hydrogen concentration (0.02 to 1000 ppm) and water concentration (100 or 22000 ppm) in the gaseous feed on the activity were investigated. The fine platinum particles around a few nanometers significantly improve the catalytic activity for the oxidation tritium at a very small concentration. The hydrogen concentration in the gaseous feed slightly affects the overall reaction rate constant for hydrogen oxidation. Due to the competitive adsorption of hydrogen and water molecules on platinum surface, the overall reaction rate constant has the bottom value at the hydrogen concentration of 100 ppm with the dry feed gas. We have experimentally confirmed the activity of these honeycomb catalysts is as good as that of granular hydrophobic catalyst. The results support the hydrophobic honeycomb catalysts can be used for tritium oxidation reactors.

Product distribution analysis of the hydrogen peroxide direct synthesis in an isothermal batch reactor

The direct synthesis of hydrogen peroxide from molecular hydrogen and oxygen on a supported palladium catalyst was studied at 258–297K in a laboratory-scale batch reactor. The catalyst was in the form of finely dispersed slurry in methanol/CO2 to suppress the internal and external mass transfer resistances. Experiments carried out under kinetic control revealed that hydrogen peroxide was successfully formed on the catalyst surface, but it was hydrogenated as the reaction time was prolonged. The mass balances of the components were considered in detail and a reaction mechanism was proposed, based on the competitive adsorption of hydrogen and oxygen on the palladium surface. The surface reactions leading to the formation of hydrogen peroxide and water were assumed to be rate determining, and the rate equations describing direct synthesis, water formation as well as peroxide hydrogenation and decomposition were derived. A special kind of product distribution analysis was used to interpret the kinetic phenomena and to make the estimation of the kinetic parameters very robust. The parameters were estimated by nonlinear regression analysis and the model gave a good fit to the experimental data. The usefulness of the product distribution analysis was clearly demonstrated.

In situ scanning tunneling microscopy study of cobalt thin film electrodeposited on Pt(1 1 1) electrode

Thanks to the availability of single crystal electrodes and the advance of scanning probes, it is now possible to characterize the process of electrodeposition in real time and the atomic structure of ultrathin film. Reported here is an in situ scanning tunneling microscopy (STM) study of cobalt electrodeposition on an ordered Pt(111) electrode in a pH 3 chloride-containing medium. It was found to be a two-staged process, involving an underpotential deposition (UPD) stage and an overpotential deposition (OPD) occurring at potentials positive and negative of the Nernst potential of −0.57V (versus Ag/AgCl) calculated for 0.04M CoCl2. Time-dependent STM imaging revealed Co nucleated mainly in one atom high islands on terraces and at steps in the UPD stage, followed by quasi layer-by-layer growth to form a smooth thin film up to five layers in thickness. The UPD layer comprised Co atoms arranging in disarray, possibly resulting from competitive adsorption of hydrogen at the Pt electrode. Starting from the second layer, the Co film assumed ordered microstructures featuring a long range undulation of atomic height. This moire structure consisted of Co(0001) like plane stacked on the Pt(111) substrate with an in-plane interatomic spacing of 0.257nm in the bilayer film. Due to different lattice constants of Co and Pt, the Co/Pt(111) was strained, but the degree of this interfacial strain gradually decreased with the thickness of Co deposit, as revealed by the diminish of the moire pattern when the Co film grew to more than five layer in thickness. Meanwhile, the morphology of the Co deposit became rougher with thickness, which suggests a Stranski–Krastanov growth.

Competitive adsorption of hydrogen and bromide on Pt(100): Mean-field approximation vs. Monte Carlo simulations

The competitive adsorption of hydrogen and bromide on Pt(1 0 0) has been studied by modeling the experimental data by means of the mean-field approximation (Frumkin isotherm) and Monte Carlo simulations. This system is characterized by two kinds of species that adsorb at the same surface sites but whose saturation coverages are different (θH,max = 1, θBr,max = 0.5), as a result of different short range interactions. In a first step, hydrogen adsorption on Pt(1 0 0) has been modeled and it is found that the Frumkin isotherm and Monte Carlo simulations agree satisfactorily. The parameters obtained from fitting hydrogen adsorption on Pt(1 0 0) are then used to model the competitive adsorption of hydrogen and bromide. The competitive adsorption of H and Br causes a surface that is completely covered in the whole potential range. This leads to the existence of an extra degree of freedom in the fitting process that was checked by choosing two limiting values for the Br–Br interaction parameter. By doing so, the remaining interaction parameters and the energies of adsorption can be calculated. Both the approximate mean-field approach and the exact Monte Carlo simulations are able to give good fits of the experimental curve. However, comparison of the results given by the Frumkin isotherm and the Monte Carlo simulations demonstrates that the former is not adequate to model this system. The limitations of the mean-field approach are related to its inability to correctly deal with the presence of strong short range interactions.

Competitive Adsorption of Hydrogen, Calcium, Potassium, Magnesium, and Aluminum on Banana Roots: Experimental Data and Modeling

Aluminum (Al) adsorption on root exchange sites is one of the mechanisms that can be responsible for Al toxicity. Experimental measurements of calcium (Ca), magnesium (Mg), potassium (K), and Al adsorption on the main root axes of 5 banana cultivars (Grande Naine, Agbagba, Obino l'Ewaï, Kayinja, and Igitsiri) were carried out at pH 3.75 and 4.25. The data obtained were used to calibrate a mathematical model describing ion exchange as arising from Donnan effects and specific surface‐ion interactions. Adsorption of Ca, K, Mg, and Al ions increased with their concentration in solution and with increasing pH. The depletion rate of Ca, Mg, and K in the presence of the low Al concentration was greater at pH 4.25 than at pH 3.75. Therefore, any model based on constant exchange selectivity would fail to describe such complex equilibria. The calibration of the model involves to fix values for the intrinsic dissociation constants (KR−X ;) of cations (X) with root exchange sites (R−). Values from the literature were used for H‐surface (KR−H ;=10−3.2) and Ca‐surface (K R2– Ca=10−1.1) complexes. No specific ion‐surface interaction was assumed for K (KR−K ;=106) and a value of K R2– Mg=1 was found adequate for Mg‐surface complexes. Non‐linear regression led to K R3– Al=10(−3.78± 0.024). Introducing possible R– Al2+, R2– Al+, R–Al(OH)2, and R2–Al(OH) complexes did not improve the fitting. Using one single set of parameters for all experiments on the 5 banana cultivars resulted in good fitting of Al and Mg adsorption, and slight underestimation of Ca adsorption, although good estimate was obtained with pure Ca solutions. We conclude that Al is very efficient in competing with other cations for root exchange sites despite the low Al concentrations usually occurring in soil solutions. Magnesium was confirmed to form only little association with fixed charged groups, so that Al loading on roots occurs mainly to the detriment of Mg.

Temperature- and pH-dependence of albite dissolution rate at acid pH

Albite dissolution experiments performed in solutions at pH below neutral at 5°, 50° and 90°C combined with results from the literature for albite dissolution at other temperatures show that the pH- and temperature-dependence of dissolution can be modeled using the following equation for highly unsaturated (far-from-equilibrium) conditions: logr = −2.71 − 3410/T − 0.5 H where r is the dissolution rate in mol albite cm −2 s −1; and T is temperature in K. The above equation is valid between pH 1 and 5 and temperatures from 5° to 300°C. The activation energy of dissolution for albite for this temperature and pH range is 15.6 ± 0.8 kcal mol −1. However, in addition to pH, other species in solution also affect the feldspar dissolution rate: these variations may be modeled as a ΔG-effect or an ion-specific adsorption effect. Because our measurements were all completed for values of | ΔG| > 11 kcal mol −1, where the affinity effect should be small (assuming a linear model),we used an ion inhibition model to describe our data. Assuming feldspar dissolution is controlled by competitive adsorption of hydrogen and aluminium on the feldspar surface, we use a Langmuir competitive adsorption model to fit the data: r = k′[ K H { H +} (1 + K H { H +} + K Al { Al 3+}) ] 1 2 where k′ is the apparent rate constant (mol cm −2 s −1); K H is the proton adsorption equilibrium constant; K Al is the Al adsorption equilibrium constant; and {H +} and {Al 3+} are activities of H + and Al 3+ in solution, respectively. The temperature-dependent parameters ( k′, K H, K Al) are modeled using the Arrhenius and van't Hoff equations. The values of ΔH are assumed equal to 8 and −8 kcal mol −1 for Al 3+ and H +, respectively. A value of 10 −0.97 is used for K H at 25°C. The values of k′ and K Al at 25°C have been determined by non-linear curve fitting to be 1.7 × 10 −4 mol cm −2 s −1 and 2.0 × 10 3, respectively. The adsorption model fits the experimental data more closely than the simpler rate model, indicating that the model is consistent with the observed pH-, Al- and temperature-dependence of feldspar dissolution between 5° and 300°C. More data are needed to evaluate competitive effects of Na + or other ions, or the effect of ΔG for near-equilibrium solutions. This model emphasizes that the effect of inhibition by adsorbed cations should be greater at higher temperature (> 50°C), due to the positive value of the adsorption enthalpy of cation adsorption on oxide surfaces.

Hydrogenation of para-chloronitrobenzene over supported ruthenium-based catalysts

The catalytic hydrogenation of para-chloronitrobenzene to para-chloroaniline has been investigated over a series of ruthenium catalysts of widely varying dispersions and on supported bimetallics RuM ( M = Sn, Pb, Ge ). The kinetics follow a Langmuir-Hinshelwood mechanism with competitive adsorption of hydrogen and chloronitrobenzene at low or intermediate concentrations of substrate and a zero order at high concentrations. At saturation of the metal surface for the organic substrate, the dipolar interactions in the adsorbed phase then left free some sites for the adsorption of hydrogen. The rate of reaction expressed per surface metal atom is larger on poorly dispersed ruthenium catalysts, and increases upon alloying by tin, lead or germanium. These variations can be accounted for assuming that catalytic activity is larger on the dense planes of the ruthenium particles.

The kinetics and mechanism of the methanation of carbon monoxide on a nickel-silica catalyst

The kinetics of the methanation of carbon monoxide were measured on a 5% nickel on silica catalyst in a differential flow micro-reactor at temperatures from 460 up to 840 K, carbon monoxide pressures from 1 to 650 Torr and hydrogen pressures from 55 to 700 Torr. The rates observed covered 5 orders of magnitude. With increasing temperature from low to higher temperatures, the reaction order for carbon monoxide increased from −1 to almost +1 and the reaction order for hydrogen decreased from +1 to almost zero. Both reactants have an influence on the order of reaction for the other. A maximum was found in the reaction rate as a function of the carbon monoxide partial pressure. These facts indicate a competitive adsorption of hydrogen and carbon monoxide and suggest that a Langmuir-Hishelwood type of rate equation might describe the data. A number of mechanisms was tried. Those mechanisms which described the experimental data best, were based on dissociative adsorption of carbon monoxide and addition of a hydrogen atom to adsorbed CH X as the rate determining step. With x = 1 , an excellent description of the data is obtained.