C. G. Vayenas

Fundamental Modeling and Experimental Investigation of Concrete Carbonation

The physicochemical processes in concrete carbonation are presented and modeled mathematically. These processes include the diffusion of CO2, in the gas phase of concrete pores, its dissolution in the aqueous film of these pores, the dissolution of solid Ca(OH)2 in the water of the the pores, the diffusion of dissolved Ca(OH)2 in pore water, its ultimate reaction with the dissolved CO2, and the reaction of CO2 with CSH and with the yet unhydrated C3S and C2S. In addition, the parallel processes of production of materials susceptible to carbonation during the hydration and carbonation, are included in the model.

Physical and Chemical Characteristics Affecting the Durability of Concrete

The durability of reinforced concrete is influenced by those physical characteristics of concrete that control the diffusion of gases, such as CO2 and O2 or of liquids (mainly water) through its pores, and the diffusion of ions, such as Cl-, dissolved in the pore water. These physical characteristics depend on the composition of concrete, the chemical composition and type of cement, and the relative humidity and temperature of the environment. In the present paper, these characteristics of concrete are determined analytically and/or experimentally in terms of the composition parameters and environmental condtions; the molar concentration of those constituents that are susceptible to carbonation; the porosity and pre-size distribution; the degree of saturation of the pores; and effective diffusivity of gases through the concrete.

Experimental investigation and mathematical modeling of the concrete carbonation problem

Abstract Carbonation of concrete is the major time-limiting factor for the durability of reinforced concrete structures. The carbonation reaction between atmospheric CO 2 and Ca(OH) 2 of the concrete mass destroys the high pH environment of surrounding concrete which protects the steel bars of reinforced concrete from corrosion. In this paper we present experimental results obtained in an accelerated carbonation apparatus using a variety of techniques, including TGA, and we extend the mathematical model developed recently to include the entire range of ambient relative humidities.

Ammonia High Temperature Solid Electrolyte Fuel Cell

A study was made of the overpotential and product selectivity characteristics of the high temperature solid electrolyte fuel cellIt was found that is the primary oxidation product, although significant amounts of by‐product are also formed due to the catalytic reaction between and on Pt. The influence of this side reaction can be minimized under optimal operating conditions and yields of exceeding 60% can be achieved. Two dimensionless numbers have been identified which govern product selectivity and power output. The cell is a promising candidate for the cogeneration of electric energy and nitric acid.

Chemical Cogeneration in Solid Electrolyte Cells

The anodic oxidation of H2S was investigated in the solid electrolyte fuel cell H2S, Sx, SO2, Pt/ZrO2(8% Y~O3)/Pt, air operating at atmospheric pressure and temperatures 650 ~ to 800~ It was found that the fuel cell product selectivity crucially depends on the ratio M of the fluxes of oxygen anions 02_ and H2S reaching the porous Pt anode. When M < 0.33, elemental sulfur is the major product, and the anode is severely polarized. For higher M values, the product selectivity to SO2 exceeds 99% at H2S conversions as high as 99%. The cell appears to be a promising candidate for the cogeneration of electric energy and sulfur dioxide.

Solid electrolyte aided study of the mechanism of CO oxidation on polycrystalline platinum

The mechanism of CO oxidation on polycrystalline Pt at atmospheric pressure has been investigated by combining kinetic and simultaneous potentiometric studies in a gradientless reactor containing one or two polycrystalline Pt films supported on stabilized zirconia. The initial oxidation state of the catalyst was found to have an important effect both on the steady-state behavior and on the waveform of rate and emf oscillations. A simple kinetic model where both oxygen adsorption and surface reaction are rate limiting is found to describe semiquantitatively the steady-state kinetic and potentiometric results both on preoxidized and on prereduced surfaces. The oscillatory behavior of the system was studied in detail by simultaneous mass spectroscopic monitoring of the concentrations of O/sub 2/ and CO/sub 2/. The kinetic and potentiometric results suggest strongly that the oscillations are caused by periodic formation and consumption of surface PtO/sub 2/. The formation of PtO/sub 2/ is verified by a series of surface CO-O/sub 2/ titration experiments. The experiments with two polycrystalline films show that oscillation synchronization occurs via the gas phase as the two films exposed to the same gaseous environment exhibit synchronous oscillations in the surface oxygen activity.

Work function measurements on catalyst films subject to in situ electrochemical promotion

In order to investigate the origin of the recently found effect of Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA) a Kelvin probe was used to measure in situ the changes induced in the work function of Pt catalyst films deposited on doped ZrO2 and β″-Al2O3 solid electrolytes upon electrochemical supply or removal of O2− and Na+ to or from the catalyst surface. It was found that the change in catalyst work function equals eΔVWR where ΔVWR is the change in the ohmic-drop-free catalyst potential with respect to a reference electrode. The experimental procedure is described and the important consequences of this result both for the work-function-probing capability of solid electrolyte cells and also for the origin of NEMCA are briefly outlined.

Solid electrolyte cyclic voltammetry for in situ investigation of catalyst surfaces

Abstract The technique of cyclic linear potential sweep chronoamperometry, more commonly termed cyclic voltammetry, has been applied for the first time, in conjunction with on-line mass spectrometry, IR spectroscopy, and gas chromatography, to investigate the chemisorptive and catalytic properties of porous metal catalyst films also functioning as electrodes in solid electrolyte cells. The cases of O 2 adsorption and C 2 H 4 oxidation on Pt were examined. It was found that solid electrolyte cyclic voltammetry (SECV), which causes a cyclic variation in catalyst work function, provides useful in situ information about the coverage of adsorbed species and also about the occurence of non-Faradaic electrochemical modification of catalytic activity (NEMCA effect) on the catalyst surface. The technique also permits estimation of the “length” of the catalyst-solid electrolyte-gas three-phase boundaries.

Fermi level and potential distribution in solid electrolyte cells with and without ion spillover

Abstract The spatial distribution is discussed of the electrochemical potential of electrons and of the electrostatic potential in solid electrolyte cells without and with ion spillover on the gas-exposed electrode surfaces. In the latter case, where, over a wide temperature range, the spillover ions form an effective double layer at the electrode–gas interface, it is shown, both via an electrochemical and a surface science approach, that solid electrolyte cells are work function probes and work function controllers for their gas-exposed electrode surfaces, in agreement with experiment. Under these conditions the work function of the working electrode is fixed by the applied potential and not by the gas phase at the working electrode. This is related to the effect of electrochemical promotion (non-Faradaic electrochemical modification of catalytic activity, NEMCA) and allows for further clarification of the concept of absolute electrode potential in solid state electrochemistry.

Non-Faradaic Electrochemical Modification of Catalytic Activity in Solid Electrolyte Cells

The catalytic activity and selectivity of metal catalysts used as electrodes in high temperature solid electrolyte cells can be altered dramatically and in a reversible manner. This is accomplished by electrochemically supplying oxygen anions onto catalytic surfaces via polarized metal-solid electrolyte interfaces. Oxygen anions, forced electrochemically to adsorb on the metal catalyst surface, alter the catalyst work function in a predictable way and lead to reaction rate increases as high as 4000%. Changes in catalytic rates typically exceed the rate of O2− transport to or from the catalyst surface by 102-3 · 105. Significant changes in product selectivity have been also observed. The case of several catalytic reactions in which this new phenomenon has been observed is presented and the origin of the phenomenon is discussed.