Diffusion of H atoms

Plane oxide sensors were used to study lateral diffusion of H-atoms and spill-over effect. 

The results obtained for the stochastic model show that surface reactions are well-suited for a description in terms of the master equations. Since this infinite set of equations cannot be solved analytically, numerical methods must be used for solving it. In previous Sections we have studied the catalytic oxidation of CO over a metal surface with the help of a similar stochastic model. The results are in good agreement with MC and CA simulations. In this Section we have introduced a much more complex system which takes into account the state of catalyst sites and the diffusion of H atoms. Due to this complicated model, MC and in some respect CA simulations cannot be used to study this system in detail because of the tremendous amount of required computer time. However, the stochastic ansatz permits to study very complex systems including the distribution of special surface sites and correlated initial conditions for the surface and the coverages of particles. This model can be easily extended to more realistic models by introducing more aspects of the reaction mechanism. Moreover, other systems can be represented by this ansatz. Therefore, this stochastic model represents an elegant alternative to the simulation of surface reaction systems via MC or CA simulations. 

These reactions have already been discussed in Part VIII. This mechanism would lead to only H202 as the product and a quantum yield of unity. The high percentage yields of peroxide obtained by both Volman and Watson tend to confirm the mechanism. The quantum yields of 0.5 to 0.6 are somewhat low. However all the authors point out that diffusion of H atoms to the wall could reduce the yield by eventually recombining to give hydrogen, and they all give the reaction... 

The properties of metallic hydrides depend on their composition, which is a function of the partial pressure of H2 gas in the surroundings. For example, PdH behaves as a metallic conductor for small values of x but becomes a semiconductor when x reaches about 0.5. (Semiconductors are discussed in Section 21.5.) The H atoms in PdH are highly mobile, and H2 can pass through a membrane of palladium metal. The process probably involves dissociation of H2 into H atoms on one surface of the membrane, diffusion of H atoms through the membrane as they jump from one interstice to another, and recombination to form H2 on the opposite surface of the membrane. Because other gases don t penetrate palladium, this process can be used to separate H2 or D2 from other components of gas mixtures. 

Fig. 20. The dependence on temperature of the coefficient of diffusion of H atoms in H2 molecules at 1 atm ( 2 ) D> Sancier and Wise [79] Wise [82] X, Browning and Fox [83] , Cheng and Blackshear [84] +, Schiff and co-workers [85] I, Chery and Villermaux [67] , Lede and Villermaux [68]. (Courtesy Lede and Villermaux [68].)...

The efficiencies of various metals for the recombination of H atoms are given in Table 8. In all cases, the recombination is first-order and the activation energy is less than 5 kJ mole 1. The fact that the state of anneal of the metal can alter 7 by a factor of ten [14] suggests that the reaction may occur on minority sites, rather than on a well-defined surface characterised by the intrinsic properties of the metal. This is certainly the explanation for the wide range of values reported for palladium. The extent of diffusion of H atoms into the bulk is also a complicating factor, making 7 dependent on the time of exposure to H atoms for palladium, this effect can be so gross as to result in distortion of the sample. 

The basic principle of H2 sensitivity is connected with the dissociation of H2 molecules on the catalytic metal surface and on the subsequent adsorption and diffusion of H atoms throughout the metal itself. This process is reversible in the presence of 02. In fact, at the metal surface, chemical reactions between 02 and H2 take place with H20, OH, H202 formation until all the H present in the Pd film is completely removed (Lundstrom, 1981). 

A possible explanation of these results is that most of the surface of the Pd is contaminated after reduction at 773 K, with only a few sites remaining where H2 can be dissociated and then transferred to sub-surface positions. On a normal, clean Pd surface almost complete monolayer coverage occurs before any sub-surface can be formed because of the depth of the potential well for adsorbed H2 and the activation energy to diffusion. However, it is common knowledge that contaminated Pd wire or foil will not absorb H2 at room temperature, but that mechanical cleaning of the surface allows normal absorption to occur at a rate which is determined by the rate of diffusion of H atoms from surface portholes into the bulk metal. In contrast to the foregoing observations, an Exxon patent claims that Pd/Ti02 in the SMSl states does not form the 3-hydride phase. 

The process can be stimulated by vibrationally excited H2 molecules through their surface dissociation and diffusion of H atoms into the crystal structure. The process can be considered to be gasification of oxygen from the crystal structure by vibrationally excited hydrogen (Legasov et al., 1978c). Metal reduction starts from the surface. The front of metal formation propagates from the surface into the solid body. The depth of the reduction layer is limited by the recombination of H atoms ... 

In most practical implementations, the rate of diffusion of H atoms through the bulk is significantly lower than the rate of molecular dissociation on the surface. Therefore,... 

In this expression, /h is the flux of H atoms through the membrane (atoms/area/ time) is the coefficient of diffusion of H atoms in the Pd lattice and C and C2 are the concentrations of H atoms in the membrane at its upstream ( retentate ) and downstream ( permeate ) faces. Xpmembrane membrane thickness appears in the denominator of this expression, meaning that flux decreases with increasing thickness. For this reason, dense metal layers in membrane systems are always designed to be as thin as possible - we will return to this issue later. 

When the diffusion of H atoms through the membrane bulk is the rate-limiting step. Pick s first law can be used to describe the H2 permeation flux ... 

Apart from the La(Ni, Al)5 ternaries, diffusion of H atoms was also studied in several La(Ni, Cu)s ternaries and in LaCus. Shinar et al. (1980) found that increasing Cu concentration leads to an increase in the activation energy Ef, the value in LaCujHi j being almost twice as high as in LaNijHj. 

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