Promotion experiments

Appendix B answers the basic question What materials and instruments are needed to start electrochemical promotion experiments ... 

A difference between the two systems is that in NEMCA experiments the spillover-backspillover rate can be accurately measured and controlled by simply measuring the imposed current or potential. Another difference is that in electrochemical promotion experiments backspillover provides a promoting species, not an active site, to the catalyst surface. This latter difference can however be accommodated by a broader definition of the active site . 

A typical electrochemical promotion experiment utilizing (3"-Al203, a Na+ conductor, as the promoter donor is shown in Fig. 4.15. The reaction under study is the oxidation of CO on Pt.51... 

The common feature of galvanostatic electrochemical promotion experiments is that, both in the case of O2 and Na+-conductors, one obtains pronounced changes in catalytic rate which are orders of magnitude larger than the rate of supply of ions onto the catalyst surface. 

C.G. Vayenas, and S. Brosda, Electrochemical promotion Experiment, rules and mathematical modeling, Solid State Ionics, submitted (2001). 

The electrochemically induced creation of the Pt(lll)-(12xl2)-Na adlayer, manifest by STM at low Na coverages, is strongly corroborated by the corresponding catalyst potential Uwr and work function O response to galvanostatic transients in electrochemical promotion experiments utilizing polycrystalline Pt films exposed to air and deposited on (T -AbCb. 3637 Early exploratory STM studies had shown that the surface of these films is largely composed of low Miller index Pt(lll) planes.5... 

In the previous section we have assumed that AO, thus n, is an independently controllable variable, such as pj. This is true both in electrochemical promotion experiments, since AO=eAUWR and in classical promotion experiments where AO can largely controlled, albeit not in situ, by the amount of promoter species deposited on the catalyst surface. 

In electrochemical promotion experiments (Fig. 11.2 left) one can vary jjiJe(M) = Fe(S)by varying Uwr and thus also (Eq. 6.11) electrochemical potential and thus coverage of backspillover O2 on the catalyst-electrode surface. 

The only physical difference is that here the current, I, is not directly measurable and thus the dimensionless current density, J, is not directly computable. This difficulty can, however, be overcome if the ratio of the reactivities, A, of normally adsorbed and backspillover oxygen is known (e.g. from electrochemical promotion experiments, where A, as already noted, also expresses the Faradaic efficiency). Thus in this case upon combining the definition of A with equation (11.23) one obtains the following expression for J ... 

In order to estimate T P in actual electrochemical promotion experiments we use here typical values23 of the operating parameters (Table 11.2) to calculate J and galvanostatic transients which show that the lifetime of the promoting O5 species on the catalyst surface is typically 102 s at temperatures 350°-400°C. 

For any practical classical promotion application in a fixed bed catalytic reactor, tPr must be longer than a year ( 3-107 s). But even for lab scale classical promotion experiments tPR values in excess of 106 s are required (Fig. 11.15). 

As in aqueous electrochemistry it appears that application of a potential between the two terminal (Au) electrodes leads to charge separation on the Pt film so that half of it is charged positively and half negatively8 thus establishing two individual galvanic cells. The Pt film becomes a bipolar electrode and half of it is polarized anodically while the other half is polarized cathodically. The fact that p is smaller (roughly half) than that obtained upon anodic polarization in a classical electrochemical promotion experiment can be then easily explained. 

MATERIALS AND INSTRUMENTATION FOR STARTING ELECTROCHEMICAL PROMOTION EXPERIMENTS... 

There are no specific requirements for the solid electrolytes (pellets or tubes) used in electrochemical promotion experiments. However they should be stable under the conditions of the experimental study. Also one should know the type of ionic conductivity and the possibility of appearance of mixed ionic-electronic conductivity under the conditions of electrochemical promotion. This is quite essential for the correct interpretation of results. Addresses of suppliers of solid electrolytes included in Table B.l are presented below ... 

A typical apparatus for electrochemical promotion experiments consists of three parts (a) The gas feed and mixing system (b) the reactor and (c) the analysis and electrochemical measurements system. A detailed schematic of the experimental apparatus is shown in Figure B.l, where the three parts are clearly shown. 

Two types of continuous flow solid oxide cell reactors are typically used in electrochemical promotion experiments. The single chamber reactor depicted in Fig. B.l is made of a quartz tube closed at one end. The open end of the tube is mounted on a stainless steel cap, which has provisions for the introduction of reactants and removal of products as well as for the insertion of a thermocouple and connecting wires to the electrodes of the cell. A solid electrolyte disk, with three porous electrodes deposited on it, is appropriately clamped inside the reactor. Au wires are normally used to connect the catalyst-working electrode as well as the two Au auxiliary electrodes with the external circuit. These wires are mechanically pressed onto the corresponding electrodes, using an appropriate ceramic holder. A thermocouple, inserted in a closed-end quartz tube is used to measure the temperature of the solid electrolyte pellet. 

Before starting an electrochemical promotion experiment, one should check carefully that the catalytic reaction under study is not subject to external or internal mass transfer limitations within the desired operating temperature range, which can obscure or even completely hide the electrochemical promotion effect. 

Besides mass transfer limitations, it is very important in electrochemical promotion experiments to compute the maximum mass-balance allowable rate enhancement. This is intimately related to the conversion of the limiting reactant under open circuit conditions, as the conversion of the latter cannot exceed 100%. In this respect keeping the open circuit conversion as low as possible (normally by using a small amount of catalyst) allows the system to exhibit a pronounced rate enhancement ratio. 

A typical electrochemical promotion experiment includes kinetic measurements under open and closed circuit conditions as well as study of the effect of catalyst potential or work function on catalytic rate and selectivity under steady state and transient conditions. In kinetic measurements one should change the partial pressure of each reactant while... 

Zajdela F, Croisy A, Barbin A, et al. 1980. Carcinogenicity of chloroethylene oxide, an ultimate reactive metabolite of vinyl chloride, and bis(chloromethyl) ether after subcutaneous administration and in initiation-promotion experiments in mice. Cancer Res 40 352-356. 

In five of six nondietary tumor-promotion experiments, sodium selenide significantly reduced the number of mice with tumors induced by 7,12-dimethyl-benzanthracene (DMBA)-croton oil (1). In these experiments, sodium selenide was applied concomitantly along with croton oil to female Swiss albino mice initiated with DMBA. Riley has also observed a reduction in DMBA-phorbol ester carcinogenesis by sodium selenide (2). The effect of selenium-deficient and selenium-adequate diets on DMBA-croton oil and benzopyrene skin carcinogenesis has also been studied. Supplemental dietary selenium inhibited both types of carcinogenesis. 

In order to estimate t]p in actual electrochemical promotion experiments we use here typical values [138] of the operating parameters (Table 4) to calculate J and The... 

The difficulty inherent in extrapolating initiation-promotion experiments to human exposure precludes their being used as the basis for human cancer effect levels. Since PAHs occur in complex mixtures of chemicals that may include tumor promoters, their activity as initiating agents is noteworthy. Thus, it is possible that humans dermally exposed to PAHs that are initiating agents, concomitantly with other chemicals that may be active as tumor promoters (including other PAHs) found at nearby hazardous waste sites, may have an increased risk of skin cancer. 

In an electrochemical promotion experiment a potential step is applied to the electrochemical cell. In the ideal case of a perfect reference electrode having invariant potential, the applied change in the ohmic drop free potential difference between the catalyst (working electrode) and the reference electrode, I wr. is equal to the change in the inner (Galvani) potential of the catalyst, ( > ... 

In order to characterize the gold reference electrode and to estimate the potential distribution in the single-pellet cell, an electrochemical cell identical to that shown in Figure 1, but having all three electrodes made of gold, was used. A good reference electrode for the purposes of electrochemical promotion experiments must be catalytically inert in all experimental conditions, and it must have an invariant potential defined by a (preferably reversible) redox couple. Nevertheless, a... 

It may be concluded that the use of a continuous, porous gold film as reference electrode in the given solid oxide electrochemical cell is an appropriate choice. It was shown to be catalytically inert and to have a reasonably stable potential. The estimated error of the latter is less than 20 mV under typical conditions of electro-chemical promotion experiments. The potential distribution in the solid electrolyte was shown to be highly sensitive to the exact position of the electrodes. Nevertheless, estimation of the catalyst potential remains reliable because the ohmic drop correction is almost negligible, e.g., at 375°C it is in the order of 1 mV pA . ... 

The observation that remote water materia prima) only comes from the atmosphere (atmospheric water) certainly promoted experiments to derive the philosopher s stone from it. Despite much progress in science at the beginning of the seventeenth century, the belief of convertibility between air and water, and water and soil (and vice versa) was widely accepted until the chemical composition of the air and the structure of water was discovered by Cavendish, Scheele, Priestley, Lavoisier and others after 1770. 

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