Interactions metal-support

A well defined epitaxial relationship between the ceria support and the metal (Rh [62, 109-113], Pt [114, 115] and Pd [116]) was detected by HREM and electron diffraction. This preferential orientation of the noble metal crystallites on ceria is probably due to metal-support interaction. For reduction temperatures up to 773 K, no significant nanostructural changes could be deduced from the HREM information. HREM also excludes any metal decoration and alloying phenomena as relevant factors for the chemical behavior of noble metals (NM) on Ce02 reduced at 773 K or lower temperatures. In contrast, the occurrence of decoration phenomena was well established for reduction at higher temperature [117]. 

HREM also provided interesting information about nanostructural effects induced by reoxidation. Reoxidation temperatures ranging from 373 to 1173 K have been investigated, as well as samples pre-reduced at temperatures up to 1173 K. For catalysts reduced at 773 K, reoxidation up to 773 K led only to minor nanostructural changes in the catalysts. Reoxidation at 773 K does not allow any recovery from decoration or alloying phenomena for NM/Ce02 catalysts. Much higher reoxidation temperatures are needed to achieve this objective [117]. 

The synthesis of nano-sized Ceo.3Zro7Bao.i02.i mixed oxide is possible by modifying the surface with macromolecules. High surface area can be obtained, with novel catalytic activity and thermal stability of Ceo,3Zro,7Bao.i02.i. TEM showed nano-sized particles ranging from 30 to 50 nm in the fresh sample calcined at 600 °C for 4h. After treatment at 1000 °C for 4h the powder stays in particles of 50-80 nm in diameter [119]. 

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Electrochemical promotion or NEMCA is the main concept discussed in this book whereby application of a small current (1-104 pA/cm2) or potential ( 2 V) to a catalyst, also serving as an electrode (electrocatalyst) in a solid electrolyte cell, enhances its catalytic performance. The phenomenology, origin and potential practical applications of electrochemical promotion, as well as its similarities and differences with classical promotion and metal-support interactions, is the main subject of this book. 

Chapter 11 analyzes the recently discovered mechanistic equivalence of electrochemical promotion and metal-support interactions on ionic and mixed conducting supports containing Zr02, Ce02 or Ti02. The analysis focuses on the functional identity and operational differences of promotion, electrochemical promotion and metal support interactions. 

It is now well established that spillover-backspillover phenomena play an important role in numerous catalytic systems. It is worth reminding that the effect of strong-metal-support interactions (SMSI), which was discovered by Tauster74 and attracted the intense interest of the catalytic community for the least a decade75 was eventually shown to be due to backspillover of ionic species from the Ti02 support onto the supported metal surfaces. 

S.J. Tauster, S.C. Fung, and R.L. Garten, Strong metal-support interactions. Group 8 noble metals supported on T1O2, JACS 100, 170-175 (1978). 

G.L. Haller, and D.E. Resasco, Metal-Support Interaction Group VIII Metals and Reducible Oxides, Advances in Catalysis 36, 173-235 (1989). 

J. Nicole, D. Tsiplakides, C. Pliangos, X.E. Verykios, C. Comninellis, and C.G. Vayenas, Electrochemical Promotion and Metal-support interactions, J. Catal., in press (2001). 

C.G. Vayenas, and G. Pitselis, Mathematical Modeling of Electrochemical Promotion and of Metal-Support Interactions, I EC Research 40(20), 4209-4215 (2001). 

It will also be shown that the absolute electrode potential is not a property of the electrode but is a property of the electrolyte, aqueous or solid, and of the gaseous composition. It expresses the energy of solvation of an electron at the Fermi level of the electrolyte. As such it is a very important property of the electrolyte or mixed conductor. Since several solid electrolytes or mixed conductors based on ZrC>2, CeC>2 or TiC>2 are used as conventional catalyst supports in commercial dispersed catalysts, it follows that the concept of absolute potential is a very important one not only for further enhancing and quantifying our understanding of electrochemical promotion (NEMCA) but also for understanding the effect of metal-support interaction on commercial supported catalysts. 

Consequently the absolute potential is a material property which can be used to characterize solid electrolyte materials, several of which, as discussed in Chapter 11, are used increasingly in recent years as high surface area catalyst supports. This in turn implies that the Fermi level of dispersed metal catalysts supported on such carriers will be pinned to the Fermi level (or absolute potential) of the carrier (support). As discussed in Chapter 11 this is intimately related to the effect of metal-support interactions, which is of central importance in heterogeneous catalysis. 

C2H4 Oxidation on Ir02, Ru02 and Ir02-Ti02 Mixtures Equivalence of Metal-Support Interaction and NEMCA... 

Equivalence of Metal-Support Interaction and Electrochemical Promotion... 

The second phenomenon, i.e. the change in catalytic activity or selectivity of the active phase with varying catalyst support, is usually termed metal-support interaction. It manifests itself even when the active phase has the same dispersion or average crystallite size on different... 

This implies that Electrochemical Promotion or NEMCA is an electrochemically controlled metal-support interaction. It also implies that metal-support interactions on these supports can be viewed as a self-driven wireless NEMCA system, such as the one explored by Cavalca, Haller and Vayenas for the CH3OH oxidation system under catalyst-counter electrode short-circuit conditions where gaseous 02 replenishes O2 in the YSZ support at the vicinity of the counter electrode.24... 

EXPERIMENTAL CONFIRMATION OF THE MECHANISTIC EQUIVALENCE OF NEMCA AND METAL-SUPPORT INTERACTIONS... 

Three independent systems were used by Nicole, Tsiplakides, Pliangos, Verykios, Comninellis and Vayenas22 to show the mechanistic equivalence of NEMCA and metal-support interactions (Fig. 11.3). 

The inset of Figure 11.8 shows the rate dependence on P02 (at the same PC2H4 and T) for the Rh film deposited on YSZ at various imposed potentials Uwr. The similarity between Figure 11.8 and the inset of Figure 11.8 is striking and underlines the equivalence of metal-support interactions and electrochemical promotion For low po2 values the rate is first order in P02 followed by a sharp decrease at a characteristic po2 value denoted by P02 ( Uwr ) which depends on the support (Fig. 11.8) or on the potential (inset of Fig. 11.8). Thereafter the rate becomes very low and negative order... 

The good qualitative agreement between eUwR variation and O0 variation shown in Figure 11.11 for the various supports used, underlines again the common promotional mechanism of electrochemically promoted and metal-support interaction promoted metal catalysts. 

In dispersed metal-support systems (Fig. 11.2 right), one can vary pe(M) - M-e(S) by varying the support or by doping the support with aliovalent cations. This is known in the literature as dopant-induced metal-support interactions (DIMSI).8,11,41,42 Thus one can again vary the electrochemical potential and thus the coverage of backspillover O2 on the supported catalyst surface. 

An important question frequently raised in electrochemical promotion studies is the following How thick can a porous metal-electrode deposited on a solid electrolyte be in order to maintain the electrochemical promotion (NEMCA) effect The same type of analysis is applicable regarding the size of nanoparticle catalysts supported on commercial supports such as Zr02, Ti02, YSZ, Ce02 and doped Zr02 or Ti02. What is the maximum allowable size of supported metal catalyst nanoparticles in order for the above NEMCA-type metal-support interaction mechanism to be fully operative ... 

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