Kinetics of Surface Reactions on Rotating Disk Electrodes

The diffusive-convective equation under the hydrodynamic conditions of rotating disk electrodes was solved for electrochemical reactions following the Langmuir-Hinshelwood kinetic mechanism, and an expression has been obtained for the current (reaction rate) as a function of the rotation rate, considering adsorption-desorption equilibrium and slow reaction of adsorbed species on the electrode surface. This expression has been applied to the experimental evaluation of electrochemical oxygen transfer to p-methoxyphenol and p-nitrophenol on lead dioxide electrodes. The Langmuir equilibrium constants and kinetic reaction rate constants obtained with the rotating disk electrode compare well with the corresponding values determined from independent adsorption isotherm and electrolysis studies, respectively, demonstrating that the approach presented appropriately describes the kinetic couplings between mass transport, adsorption-desorption equilibrium, and surface reactions, occurring in processes following the Langmuir-Hinshelwood mechanism.

Trabajo publicado:
Electrochimica Acta, Vol. 80, 2012, Pages 326-333
Ronald Vargas, Carlos Borrás, Jorge Mostany, Benjamin R. Scharifker

Electrocatalysis of the anodic oxygen transfer reaction

After introducing the general aspects of the electrochemical oxygen transfer reaction of organic compounds in aqueous solutions, we discuss the fundamental and mechanistic theory of these reactions. Commonly used electrode materials and its characteristics are described considering their electrocatalytic performance during the oxidation of some organic compounds. Additionally, kinetic models based on mass transport of electroactive species described in the literature are compared with our proposed formalism, were both mass transport and kinetics of surface reactions are considered. Finally, we show the importance of predicting reaction rates, and the physicochemical parameters that describe the chemical interactions during the electrolysis of organic compounds in the understanding, design, operation and control of these reactive systems.

Trabajo publicado:
Boletin de la Academia de Ciencias Físicas, Matemáticas y Naturales, Vol. 71, no. 2, 2011, Pages 37-56
Ronald Vargas, Carlos Borrás, Jorge Mostany, Benjamin R. Scharifker

Photocatalysis of p-nitrophenol on nanostructure TiO2 flims

The photocatalytic activity of supported TiO2 nanostructures (nanotubes and nanoparticles) was evaluated kinetically during the p-nitrophenol oxidation. The results were compared with those obtained using nanoparticles in suspension and a thin film of catalyst respectively. The nanotubes were synthesized electrochemically by titanium anodization in HF electrolyte; the nanoparticles were synthesized in microemulsion by TiCl4 chemical oxidation. The degradation was carried out in a flow reaction system with recirculation in presence of UV light provided by a solar simulator. It was found that both nanostructures degrade the organic matter efficiently following the Langmuir-Hinshelwood kinetic model; supported nanoparticles have a maximum reaction rate of 3.3 times faster than the thin film, the nanotubes are 2.5 times faster than the supported nanoparticles, and nanoparticles in suspension are 1.3 times faster than the nanotubes.

Trabajo Publicado:
Avances en Ciencias e Ingeniería, Volume 2, Issue 4, 2011, Pages 47-58
Damelis López, et al.

Kinetic study of the electrochemical mineralization of phenols in thin-layer condition

Electrochemical oxidation of p-nitrophenol (PNP) and p-methoxyphenol (PMP) at Bi-doped PbO2 electrodes under thin-layer condition was studied by Fourier transform infrared spectroelectrochemistry (FTIR) as a function of electrode potential and the initial concentration of organic compound. The results show that complete mineralization of the organic species was achieved at potentials more positive than 1.4 V vs. SCE and the mineralization process was not determined by the concentration of phenols. From the electric charge transferred during thin-layer controlled potential electrolysis, we found that PNP and PMP electrooxidation follow Langmuir–Hinshelwood kinetics.

Trabajo Publicado:
Electrochimica Acta, Volume 44, Issue 3, February 2010, Pages 911-917
Ronald Vargas, Carlos Borrás, Daniela Plana, Jorge Mostany, Benjamín R. Scharifker

Photocatalytic degradation of oil industry hydrocarbons models at laboratory and at pilot-plant scale

Photodegradation/mineralization (TiO2/UV Light) of the hydrocarbons: p-nitrophenol (PNP), naphthalene (NP) and dibenzothiophene (DBT) at three different reactors: batch bench reactor (BBR), tubular bench reactor (TBR) and tubular pilot plant (TPP) were kinetically monitored at pH = 3, 6 and 10, and the results compared using normalized UV light exposition times. The results fit the Langmuir-Hinshelwood (LH) model; therefore, LH adsorption equilibrium constants (K) and apparent rate constants (k) are reported as well as the apparent pseudo-first-order rate constants, k´obs = kK/(1+Kcr). The batch bench reactor is the most selective reactor toward compound and pH changes in which the reactivity order is: NP > DBT> PNP, however, the catalyst adsorption (K) order is: DBT > NP > PNP at the three pH used but NP has the highest k values. The tubular pilot plant (TPP) is the most efficient of the three reactors tested. Compound and pH photodegradation/mineralization selectivity is partially lost at the pilot plant where DBT and NP reaches ca. 90 % mineralization at the pH used, meanwhile, PNP reaches only 40 %. The real time, in which these mineralization occur are: 180 min for PNP and 60 min for NP and DBT. The mineralization results at the TPP indicate that for the three compounds, the rate limiting step is the same as the degradation one. So that, there is not any stable intermediate that may accumulate during the photocatalytic treatment.

Trabajo publicado:
Solar Energy, Volume 84, 2010, Pages 345-351
Ronald Vargas, Oswaldo Núñez

Measurement of Phenols Dearomatization Via Electrolysis: The UV-Vis Solid Phase Extraction Method

Dearomatization levels during electrochemical oxidation of p-methoxyphenol (PMP) and p-nitrophenol (PNP) have been determined through UV-Vis spectroscopy using solid-phase extraction (UV-Vis/SPE). The results show that the method is satisfactory to determine the ratio between aromatic and aliphatic compounds and reaction kinetics parameters during treatment of wastewater, in agreement with results obtained from numerical deconvolution of UV-Vis spectra. Analysis of solutions obtained from electrolysis of substituted phenols on antimony-doped tin oxide (SnO2-Sb) showed that an electron acceptor substituting group favored the aromatic ring opening reaction, preventing formation of intermediate quinone during oxidation.

Trabajo publicado:
Water Research, Volume 44, 2010, Pages 911-917
Ronald Vargas, Carlos Borrás, Jorge Mostany, Benjamin R. Scharifker

Hydrogen bond interactions at the TiO2 surface: Their contribution to the pH dependent photo-catalytic degradation of p-nitrophenol

We have obtained pKa values of p-nitrophenol–TiO2 by measuring the adsorption equilibrium constants of p-nitrophenol (PNP) on the TiO2 surface at different pH values. These values have been obtained from Langmuir isotherms and from a plot of 1/rate vs. 1/[PNP]o obtained during TiO2 catalyzed solar light photodegradation of PNP. Two limit equilibrium constants are readily obtained depending on the solution pH: at pH 5 at which the TiO2 surface is mainly positively charged and at pH 8 when it is negatively charged. With these and other adsorption equilibrium constants and the PNP pKa value in solution, thermodynamic cycles are established in order to obtain the PNP pKa when it is adsorbed on positively charged, neutral and negatively charged TiO2 surfaces. From these pKa values useful information on the PNP–TiO2 interaction is readily obtained. For instance, the PNP nitro group interacts with the TiO2 surface via a hydrogen bond, arising from the complex of water molecules with the Ti4+ ions on its surface. The weaker the hydrogen bond donor, the stronger the oxygen nitro group basicity. Therefore, pKa changes on the phenolic hydroxyl group result from these interactions. Linear free energy correlations, maximum PNP adsorption capacity values (QL) and FTIR-ATR, spectrum support this proposal. A kobs vs.pH degradation profile of p-nitrophenol is also provided.

Trabajo publicado:
Journal of Molecular Catalysis A: Chemical, Volume 300, Issues 1-2, 2 March 2009, Pages 65-71
Ronald Vargas, Oswaldo Núñez

The photocatalytic oxidation of dibenzothiophene (DBT)

TiO2/UV solar light degradation of dibenzothiophene (DBT) aqueous solutions readily occurs in neutral and acid media. For instance, at neutral pH, t1/2 = 30 min are found for DBT degradation and mineralization. In neutral and acid media the rate limiting step for mineralization is the same as for degradation and corresponds to the DBT sulfone formation. Benzaldehyde is the major reaction product. In addition, it also acts as an intermediate in the mineralization to CO2 +water + sulfate. The results support the Langmuir–Hinshelwood degradation mechanism. Therefore, instead of pseudo-first-order rate constants, apparent rate constants that depend on concentration are obtained. Only at basic pH these apparent rate constant are independent of the [DBT]0. The solubility of DBT in water is increased by one-order of magnitude when solutions of the surfactant Triton X-100 is used at [Triton X-100] >CMC (critical micelle concentration). Under these conditions DBT is readily degraded without degrading Triton X-100 at t1/2 ca. 120 min. DBT is solubilized into Triton X-100 micelles from where it exchanges with water and becomes available for degradation. Triton X-100 monomers slightly compete with DBT for the TiO2 catalytic site. The degradation method established in this work might be used, for instance, to decontaminate soil contaminated with sulphur-rich heavy oil following a pre-treatment consisting of washing out the solid with Triton X-100 solutions, prior to photolysis.

Trabajo publicado:
Journal of Molecular Catalysis A: Chemical, Volume 294, Issues 1-2, 15 October 2008, Pages 74-81
Ronald Vargas, Oswaldo Núñez

Photocatalytic TiO2 – assisted decomposition of Triton X-100: inhibition of p-nitrophenol degradation

A decrease in the apparent pseudo first-order rate constant is observed in the photocatalyzed (TiO2) degradation of surfactant Triton X-100 (Triton) when its concentration is increased. The measured rate versus the concentration profile is consistent with a hyperbolic form (rate increases with concentration) as described by the Langmuir–Hinshelwood (LH) model. The rate is then given by the expression: r=kK[Triton]/(1+K[Triton]) but the apparent rate constant by kapp=kK/(1+K[Triton]o), where k=0.66 mg/(L min) and K=0.037 L/mg. Therefore, at low [Triton]o, kapp=kK but at high [Triton]o, kapp=k/[Triton]o, that is, an inverse function of the reactant concentration. Although, in the latter case the reaction does not follow first-order kinetics, its pseudo first-order deviation is not easily noticeable. Therefore, this decrease in kapp with reactant concentration may limit its use when rate constants are compared to evaluate degradation efficiency or when it is used to show reaction inhibition. However, we have detected p-nitrophenol inhibition induced by Triton using kapp values. Inhibition is observed at [Triton]o > CMC. These inhibitions are consistent with the LH model given by the expression: r=k’K’[phenol]/(1+K’[phenol]o+K([Triton]o), where [phenol] is equal during all kinetic runs.

Trabajo publicado:
Journal of Physical Organic Chemistry, Volume 21, Issue 12 (December 2008), pp 1072-1078
Gloria Pardo, Ronald Vargas, Oswaldo Núñez