Hydrogen spillover effect

Hydrogen spillover effect over the oxide surfaces in supported nickel catalysts... 

Kusakari T, Tomishige K, Fujimoto K (2002) Hydrogen spillover effect on cumene cracking and n-pentane isomerization over catalysts prepared by physical mixture of H- with Pt/Si02. Appl Catal A Gen 224 219-228... 

Recently, the secondary hydrogen spillover effect was demonstrated also in MOFs [200]. 

The effect of spillover was observed for different species such as H,68 O69 n,70 NO64 or CO.69 Most of the research has been carried out with hydrogen spillover. 

A majority of publications available at the moment on the spillover effect, i.e., the effect in which active particles in heterogeneous systems flow from an activator (donor) to carrier (acceptor), is devoted to hydrogen. Among the systems considered are mainly metal-oxide ones, where this interesting effect has been observed for the first time [35]. 

Note that this method enables one to observe variation of electric conductivity of a sample due to adsorption of hydrogen atoms appearing as a result of the spillover effect, no more. In a S3rstem based on this effect it is rather difficult to estimate the flux intensity of active particles between the two phases (an activator and a carrier). The intensity value obtained from such an experiment is always somewhat lower due to the interference of two opposite processes in such a sample, namely, birth of active particles on an activator and their recombination. When using such a complicated system as a semiconductor sensor of molecular hydrogen (in the case under consideration), one should properly choose both the carrier and the activator, and take care of optimal coverage of the carrier surface with metal globules and effect of their size [36]. 

In order to develop more informative and direct method of studying the spillover effect of active particles, the authors of [37] suggested to use the sensor method of detecting migrating particles based on separation of sensor and emitter (donor) of active particles. The latter consists of small metal globules, or clusters (with a diameter of about 20-30 A) of Pt, Pd, Ni, etc. (activator) deposited on quartz or sapphire (AI2O3) plate in the form of a strip less than 1 cm wide. The sensor for detection of hydrogen atoms consisted of a zinc oxide strip (with a width of about 0.1 cm and thickness wlOO nm) deposited on the same plate at a distance of 0.03 or 0.6 cm (two versions) from the inner boundaries of activator strips [38]. 

By varying the temperature at which the experiments were conducted and the distance between the activator and the sensor, the data were obtained (Fig. 4.17) which allowed us to calculate the activation energy of migration of hydrogen adatoms (protium and deuterium) along the carrier surface and coefficients of lateral diffusion of hydrogen atoms appearing due to the spillover effect (see Table 4.2). 

It was initially believed that the promoter and Mo sulfides were individual crystallites in intimate contact (touching) and that the promoter aided hydrogen activation. Then it was proposed that the crystallites may not need be in direct contact as hydrogen spillover to the support could accomplish the same objective (see Fig. 16b). However, slowly it became apparent that the promoter was not effective as a separate sulfide crystallite but was actually only effective if it was present in some form on the surface of the MoS, crystallites (1-3). An early proposal suggested that the promoter is bonded to the support, which would lead to higher stability of a deposited MoS2 monolayer (50), as illustrated in Fig. 16c. However, the chemistry was subsequently found to be more subtle. 

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