Back-spillover

Note that upon increasing the time of 02 desorption, denoted tHe, the peak A, corresponding to chemisorbed O, decreases much faster than the back-spillover oxygen peak B. Similarly upon increasing the holding time, tH, of positive potential application (UWr=0.2 V) peak A reaches saturation first, followed by a gradual approach to saturation of the backspillover oxygen peak B. 

L. Basini, C.A. Cavalca, and G.L. Haller, Electrochemical Promotion of Oxygen Atom Back-Spillover from Yttria-Stabilized Zirconia onto a Porous Platinum Electrode Detection of SERS Signals,/. Phys. Chem. 98, 10853-10856 (1994). 

Nevertheless both transient rate analysis24,71 and XPS24 have shown that in both cases the electrochemical promotion mechanism is identical with that obtained with YSZ, i.e. electrochemically controlled migration (back-spillover) ofO2 onto the gas-exposed catalyst-electrode surface.24,71... 

XPS has shown that no bulk or surface PtC>2 forms even under severe and prolonged anodic polarization. The back-spillover O2 species is a surface species, not a bulk oxide. 

Fig. 7.187. Schematic representations of a metal electrode deposited on an ( -conducting and on an Na+-conducting solid electrolyte, showing the location of the metal-electrolyte double layer and of the effective double layer created at the metal/gas interface due to potential-controlled ion migration (back-spillover). (Reprinted with permission from S. Bebelis, I. V. Yantekakis, and H. G. Lintz, Catalysts Today 11 303,1992.)...

Reverse spillover or back-spillover is observed to proceed by surface migration of the spiltover species from the accepting sites to the metal, where it desorbs as H2 molecules or reacts with another hydrogen acceptor such as 02, pentene, ethylene, etc. Reverse or back-spillover (primary as well as secondary) is hindered by H20 (11), whereas secondary spillover is promoted by H20 (case B in Fig. 1). Hydrogen spillover depends on the acceptor surface it is thought to be easier on silica than on alumina (45) for hydrogen-molybdenum-bronze preparation. 

Associated with the presence of the dispersed platinum phase, a through>the-metal, back-spillover, mechanism, much faster than the direct recombination of the hydrogen species chemisorbed on the support, would mainly govern the desorption reaction. Also remarkable is the close analogy between trace B3 in Figure 4.7, and diagram reported in ref (204) for a Rh/Ce02 catalyst reduced at 773 K and fiirther treated with H2 in the same way as indicated above... 

The effect of EP, or NEMCA, is due to the controlled migration (back-spillover) of ions from the solid electrolyte to the gas-exposed catalyst-electrode surface under the influence of the applied current or potential. ... 

It was found that the catalytic activity of three-way catalysts can be altered significantly by using different supports for each noble metal component. This may be due to electronic interactions between the metal and the support [9,10] and/or due to oxygen back spillover from the supports to the catalyst as in the NEMCA experiments [16-18,28],... 

Figure 5. Schematic representation of the mechanism of electrochemical promotion under anodic current application via back-spillover of charged promoting species (0 )-...

The semicircle labeled Ci is the back-spillover oxygen semicircle . It appears only at positive imposed (7wr values, that is, when 0 is supplied to the catalyst surface. It corresponds to a capacitance Q 2 again computed from Q 2 = l/2 r/m,2f 2 and gives a Q 2 value of 200pFcm . It is due to the charge-transfer reaction (17) now taking place over the entire gas-exposed electrode surface area. The dependency of Qj, and Q,2 potential is shown in Fig. 20(b). 

It is valid for any electrochemical cell [1], where pT is the electrochemical potential of electrons in the catalyst electrode, Ep (= p) is the Fermi level of the catalyst-electrode and P is the outer (Volta) potential of the metal catalyst-electrode in the gas outside the metal/gas interface. The latter vanishes (T = 0, AO = 0) when no net charge resides at the metal/gas interface [1, 25]. Thus, the experimental Eq. 4 manifests the formation of a neutral double layer, termed effective double layer, at the metal/gas interface (Fig. 2). At the molecular level, the stability of the effective double layer, and thus the validity of Eq. 4, requires that the migration (back-spillover) of the promoting ion (O , Na " ) is fast relative to its desorption or catalytic consumption. When this condition is not met (e.g., high T or non-porous electrodes), or also when the limits of zero or saturation coverage of the promoting ion are reached (at very positive or negative AUwr), then deviations from Eq. 1 are observed [1, 25]. 

The origin of NEMCA effects is also studied, and it is reported that the surface modification of reactant by ion back spillover of an effective double layer at the metal-gas interface is strongly related with unique improvement in reaction rate and selectivity [5, 7], In situ work function measurements are performed by the Kelvin probe technique [7] or UPS [8] for explanation of NEMCA effects. These measurements show that over a wide range of temperatures, work function of metal catalyst linearly changes with increasing potential and the supplied ion species like oxide ion, proton, or Na" spiU over [9] on the metal catalyst to form electric double layer. Schematic image of electrochemical modification is shown in Fig. 3 for the case of oxide ion conductor, in which 8 value is not still determined yet [5]. 

We note in passing that for dissociation of many gases the adsorption of molecules from the gas phase directly upon the surface of catalyst nanoparticles is not necessary. Tsu and Boudart (1961) and others (Henry et al. 1991 Bowker 1996) showed that the molecules can first adsorb onto the oxide support and diffuse to a catalyst particle. This means that the effective capture radius of a catalyst nanoparticle (Pd, Pt, etc.) can be much greater than the nanoparticle s physical radius. As with the spillover zones, the molecule-collection zones (Tsu and Boudart 1961) overlap when the coverage of catalyst nanoparticles exceeds some threshold value, effectively converting the entire surface of the nanostructure into a molecular delivery system for the metal catalyst nanoparticles. This so-called back-spillover effect further increases the likelihood of molecular dissociation and ionosorption on the metal oxide surface. 

XPS Spectroscopic and Voltammetric Identification of Back-Spillover Ions as the Cause of NEMCA... 

II. NEMCA is due to an electrochemically driven and controlled back-spillover of ions from the sohd electrolyte onto the gas-exposed electrode surface. These back-spillover ions establish an effective electrochemical double layer and act as promoters for catalytic reactions. This interfacing of electrochemistiy and catalysis offers several exciting theoretical and technological possibihties. We note that in the catalytic literature, the term spillover usually denotes migration of a species from a metal to a support, while the term back-spillover denotes migration in the opposite direction and is thus more appropriate here. 

This observation shows that NEMCA is a catalytic effect, i.e., it takes place over the entire gas-exposed catalyst surface, and is not an electrocatalytic effect localized at the tpb metal/solid electrolyte/gas. This is because 2FN/I is the time required to form a monolayer of an oxygen species on a surface with N sites when it is supplied at a rate I/2F. The fact that X is found to be smaller than 2FN/1, but of the same order of magnitude, shows that only a fraction of the surface is occupied by oxygen back-spillover species, as discussed in detail elsewhere. It is worth noting that if NEMCA were restricted to the tpb, i.e., if the observed rate increase were due to an electrocatalytic reaction, then x would be practically zero during galvanostatic transients. 

Therefore, by applying currents or potentials in NEMCA experiments and thus by varying VwR, one also varies the average catalyst surface work function e (Equation 13.55). Positive currents increase e and negative currents decrease it. Physically, the variation in e is primarily due to back-spillover of ions to or from the catalyst surface. 

The first XPS investigation of Ag electrodes on YSZ under 0 pumping conditions was published in 1983.That study provided direct evidence for the creation of back-spillover oxide ions on Ag (Ols at 529.2 eV) upon applying positive currents. More recently, Gdpel and co-workers have used XPS, UPS, and EELS to study Ag/YSZ catalyst surfaces under NEMCA conditions. Their XPS spectra are similar to those in Reference 113. 

I. Back-spillover oxide ions (Ols at 528.8 eV) are generated on the gas-exposed electrode surface upon positive current appUcation (peak 8 in Figure 13.17, top). 

Oxidic back-spillover oxygen (8 state) is less reactive than normally chemisorbed atomic oxygen (y state) with the reducing (Hj and CO) UHV background. ... 

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