Surface concentration of hydrogen

THE SURFACE CONCENTRATION OF HYDROGEN ON IRON DURING HYDROGEN EVOLUTION... 

The yield maximum as a function of temperature is probably due to two effects. At low temperatures the reaction rate of the chemical reaction increases with temperature. At high temperatures the recombination of to with subsequent desorption leads to a lowering of the surface concentration of hydrogen and to a lower methane production rate. 

Proton catalysis of the dissolution rate of aluminum oxide. The logarithm of the dissolution rate, Rh, is plotted as a function of pH (A) and the logarithm of the surface concentration of hydrogen ion,... 

A diffusion coefficient can be derived from the variation of the peak height or the surface concentration of hydrogen (deuteron) with time. The deuteron concentration Cp at the back surface of the H2O ice layer shows an exponential increase with time, as expected from Eq. (24.2), as displayed in Fig. 24.8. It rises abruptly after a small time lag of several hours (not able to be seen in Fig. 24.8 plotted with a full time-scale of 1200 h) and approaches a steady value of 0.52. The fitting of the experimental results yields a diffusion coefficient of 6.2 x lO i m s i. This... 

A recurring self-imposed task of many of the publications has been to identify the sites responsible for each type of reaction, so that structure-sensitivity has been a dominant theme. What has received much less attention is the possibility that, for example, particle size might determine the strength of hydrogen chemisorption, so that the use of constant operating conditions on a series of catalysts might produce results mainly decided by the surface concentration of hydrogen atoms. The dependence of kinetic parameters on particle size or other catalyst feature has been rarely examined. 

Strictly speaking, AS is not a truly thermod ynaniic quantity, since it corresponds to a situation where charge-transfer adsorption processes are not in equilibrium. Therefore, in order to derive the thermodynamic relationships for AS, the equihbrium conditions for hydrogen and OH adsorption (Eqs. 38 and 39) should not be employed. Instead, under conditions of constant surface concentration of hydrogen and OH species (i.e., and F j constant),... 

The above equation provides a method of evaluation of the difference in the surface concentration of hydrogen and OH species. First, values of the surface energy, 7, are obtained, up to a constant of integration, from ... 

Consider, for instance, evolution of hydrogen involving the path discharge-electrochemical desorption. If the process is stationary, the rate of formation of the adsorbed hydrogen (B) [i.e., the number of acts of reaction (B) per second per cm of the electrode] Vb is equal to the rate of its removal through D, Vd otherwise, the surface concentration of hydrogen, hence, the reaction rate would vary in time, i.e., the process would not be stationary. Each of the steps may proceed in a forward as well as a backward direction, so that the total rate, Vi, is the difference of the two rates V and V, With this in view, the steady state condition is written as follows ... 

In the low-pressure region the effective surface concentration of hydrogen is zero and the hydriding rate is determined by the impingement of Hj on the surface. According to kinetic theory the molecular collision frequency is proportional to The inverse... 

Figure 4 Isosteric heat of adsorption (kcal/mol) for hydrogen on Ni(lOO) surface as a function of hydrogen-induced work function change (which is a direct measure of the surface concentration of hydrogen). (From Ref 10.)...

The TOF-ESD measurement was carried out in the TOF-ESD protoscope analyzer (Fig. 7.1). Details of the protoscope were previously reported [16, 17], In the present study, the off-axis electron gun (LEED gun, spot size, 100 pm) was mainly used for the TOF-ESD measurements on the submillimeter scale to roughly estimate the change in the surface concentration of hydrogen at the diamond surfaces. A second, pencil-type. 

Reaction rate equations get their final forms after inserting the surface concentrations of hydrogen. For practical purposes, the rate expressions can be compressed to... 

It should also be emphasized that the overpotential independence of Ch30 and the double layer structure in the case of barrierless discharge does not contradict the idea that it is the slow discharge that determines the reaction rate. Indeed, if we compare the rates of evolution of hydrogen at a fixed potential, we shall find that it is proportional to the concentration of HaO ions. On the other hand, if we compare overpotentials for different solutions with the same surface concentration of hydrogen ions, the effect of the i i-potential would become clear. 

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