Platinum negative

An ionic EAP based actuator (3.16 g), with an embedded spiral stainless steel wire as the positive electrode and an external platinum negative electrode, was mechanically tested isometrically with a 587.7 mg counterweight. The experimental design used a Mettlar analytical balance and a pulley system (Fig. 4.24). The isometric mechanical testing of an activation-relaxation cycle is shown in Fig. 4.25. This was a fairly large actuator. We have observed that the smaller the actuator, the faster and more pronounced the contraction. 

Ghilane, J., C. Lagrost, M. GuiUoux-Viry, J. Simonet, M. Delamar, C. Mangeney, and P. Hapiot. Spectroscopic evidence of platinum negative oxidation states at elec-trochemicaUy reduced surfaces. J. Phys. Chem. C 111 (2007) 5701. 

In these equations the electrostatic potential i might be thought to be the potential at the actual electrodes, the platinum on the left and the silver on the right. However, electrons are not the hypothetical test particles of physics, and the electrostatic potential difference at a junction between two metals is nnmeasurable. Wliat is measurable is the difference in the electrochemical potential p of the electron, which at equilibrium must be the same in any two wires that are in electrical contact. One assumes that the electrochemical potential can be written as the combination of two tenns, a chemical potential minus the electrical potential (- / because of the negative charge on the electron). Wlien two copper wires are connected to the two electrodes, the... 

Ultraviolet photoelectron spectroscopy (UPS) results have provided detailed infomiation about CO adsorption on many surfaces. Figure A3.10.24 shows UPS results for CO adsorption on Pd(l 10) [58] that are representative of molecular CO adsorption on platinum surfaces. The difference result in (c) between the clean surface and the CO-covered surface shows a strong negative feature just below the Femii level ( p), and two positive features at 8 and 11 eV below E. The negative feature is due to suppression of emission from the metal d states as a result of an anti-resonance phenomenon. The positive features can be attributed to the 4a molecular orbital of CO and the overlap of tire 5a and 1 k molecular orbitals. The observation of features due to CO molecular orbitals clearly indicates that CO molecularly adsorbs. The overlap of the 5a and 1 ti levels is caused by a stabilization of the 5 a molecular orbital as a consequence of fomiing the surface-CO chemisorption bond. 

The electrolyte thus formed can conduct electric current by the movement of ions under the influence of an electric field. A cell using an electrolyte as a conductor and a positive and a negative electrode is called an electrolysis cell. If a direct-current voltage is appHed to a cell having inert electrode material such as platinum, the hydrogen ions (cations) migrate to the cathode where they first accept an electron and then form molecular hydrogen. The ions... 

Thus titanium by itself cannot function as an efficient anode for the passage of positive direct current into an electrolyte. The surface film of oxide formed upon the titanium has, however, a most useful property while it will not pass positive direct current into an electrolyte (more correctly, while it will not accept electrons from negatively charged ions in solution), it will accept electrons from, or pass positive current to, another metal pressed on to it. Hence a piece of titanium which has pressed on to its surface a small piece of platinum will pass positive direct current into brine and into many electrolytes, at a high current density, via the platinum, without undue potential rise, and without breakdown of the supporting titanium . ... 

The kinetic activities of noble metals and of base metal oxides are complementary, so that a mixture of the two would perform better than each class of material alone. We have already observed in Fig. 16 that noble metals have superior activity at high temperatures but base metal oxides have superior activity at low temperatures. Since the CO oxidation kinetics is negative first order with respect to CO over platinum but first order with respect to CO over copper chromite, the rates must be faster over platinum at low CO concentration but the reverse is true at high CO concentrations, as shown in Fig. 19. 

Oxidation kinetics over platinum proceeds at a negative first order at high concentrations of CO, and reverts to a first-order dependency at very low concentrations. As the CO concentration falls towards the center of a porous catalyst, the rate of reaction increases in a reciprocal fashion, so that the effectiveness factor may be greater than one. This effectiveness factor has been discussed by Roberts and Satterfield (106), and in a paper to be published by Wei and Becker. A reversal of the conventional wisdom is sometimes warranted. When the reaction kinetics has a negative order, and when the catalyst poisons are deposited in a thin layer near the surface, the optimum distribution of active catalytic material is away from the surface to form an egg yolk catalyst. 

Many elements of a mathematical model of the catalytic converter are available in the classical chemical reactor engineering literature. There are also many novel features in the automotive catalytic converter that need further analysis or even new formulations the transient analysis of catalytic beds, the shallow pellet bed, the monolith and the stacked and rolled screens, the negative order kinetics of CO oxidation over platinum,... 

For example, Pt/Si02 catalysts are conveniently made by impregnating a silica support vith a basic solution (pH 8-9) of platinum tetraammine ions, Pt(NH3)4 (dissolved as chloride). As the silica surface is negatively charged, the Pt-containing ions attach to the SiO entities and disperse over the surface. The pH should be kept belo v 9 because other vise the silica surface starts to dissolve. 

The electrochemical oxidation of methanol occurs on the anode electrocatalyst (e.g., dispersed platinum), which constitutes the negative electrode of the cell ... 

Adsorption of Reaction Components In many cases, adsorption of a reactant is one of the hrst steps in the electrochemical reaction, and precedes charge transfer and/or other steps of the reaction. In many cases, intermediate reaction products are also adsorbed on the electrode s snrface. Equally, the adsorption of reaction products is possible. The example of the adsorption of molecular hydrogen on platinum had been given earlier. Hydrogen adsorption is possible on the platinum electrode in aqueons solntions even when there is no molecular hydrogen in the initial system at potentials more negative than 0.3 V (RHE), the electrochemical reaction... 

The surface of the platinum electrode can be studied conveniently in the potential range between 0 and 1.7 V (RHE), where in inert solutions (not containing substances able to be oxidized or reduced), the surface is ideally polarizable. At a more negative potential, cathodic hydrogen evolution starts, whereas at more positive potentials, oxygen is evolved anodicaUy. 

The galvanostatic and potentiodynamic charging curves of platinum electrodes shift approximately 60 mV in the negative direction when the solution pH is raised by 1 unit. This implies that when potentials which refer to the equilibrium potential of a hydrogen electrode in the same solution (RHE) are used, these curves remain practically at the same place within a wide range of solution pH. Hence, we shall use this scale while analyzing these curves. 

Oxygen adsorption that occurs at platinum at potentials more positive than 0.9 to 1.0 V is irreversible, in contrast to hydrogen adsorption. Oxygen can be removed from the surface by cathodic current, but the curves obtained in the anodic and cathodic scan do not coincide cathodic oxygen desorption occurs within a narrower region of potentials, and these potentials are more negative than the region where the... 

At the platinum electrode the individual steps of the four-electron reaction cannot be studied separately. Slope b has its usual value of about 0.12 V, but in contrast to what is seen at the mercury electrode, the polarization is practically independent of solution pH (i.e., the potential at a given current density shifts by 0.06 V in the negative direction when the pH is raised by a unit). It follows that the reaction rate depends on hydrogen ion concentration. The step in which an electron and a proton are transferred while the 0-0 bond is broken is probably the ratedetermining step. 

For the cathodic reduction of organic substances, electrodes of two types are used the platinum and the mercury type. Those of the first type (platinum metals, and in alkaline solutions nickel) exfiibit low polarization in hydrogen evolution their potential can be pushed in the negative direction no further than to -0.3 V (RHE). Hydrogen readily adsorbs on these electrodes, which is favorable for reduction... 

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