Spillover Effects

Volume 77 New Aspects of Spillover Effect in Catalysis. For Development of Highly Active... 

The spillover effect can be described as the mobility of sorbed species from one phase on which they easily adsorb (donor) to another phase where they do not directly adsorb (acceptor). In this way a seemingly inert material can acquire catalytic activity. In some cases, the acceptor can remain active even after separation from the donor. Also, quite often, as shown by Delmon and coworkers,65 67 simple mechanical mixing of the donor and acceptor phases is sufficient for spillover to occur and influence catalytic kinetics leading to a Remote Control mechanism, a term first introduced by Delmon.65 Spillover may lead, not only to an improvement of catalytic activity and selectivity but also to an increase in lifetime and regenerability of catalysts. 

O.A. Mar ina, V.A. Sobyanin, V.D. Belyaev, and V.N. Parmon, The effect of electrochemical pumping of oxygen on catalytic behaviour of metal electrodes in methane oxidation, in New Aspects of Spillover Effect in Catalysis for Development of Highly Active Catalysts, Stud. Surf. Sci. Catal. 77 (T. Inui, K. Fujimoto, T. Uchijima,... 

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). 

New Aspects of Spillover Effect in Catalysis. For Development of Highly Active Catalysts. Proceedings of the Third International Conference on Spillover, Kyoto, Japan, August 17-20,1993... 

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