Structure sensitivity concept

O showed a profound difference in CO2 formation rate [M.J.P. Hopstaken and J.W. Niemantsverdriet, J. Chem. Phys. 113 (2000) 5457]. Hence, care should be taken to interpret apparent structure sensitivity found under normal operating conditions of high pressure and coverage in terms of the intrinsic reactivity of sites. From the theory of chemisorption and reaction discussed in Chapter 6 it is hard to imagine how the concept of structure insensitivity can be maintained on the level of individual sites on surfaces, as atoms in different geometries always possess different bonding characteristics. 

More directly, solid state physics contributed to the emergence of materials science, because of one of its foci. Spencer Weart identified three pillars on which solid state physics was erected First, X-ray diffraction techniques provided precise atomic picture of solids second, quantum mechanics provided the theoretical foundations for the description of solids and the third, more subtle pillar was the attempt to discriminate between properties depending on the idealized crystal pattern and properties dependent on accidents of either the inner arrangement or the surface of the solid. [13] This focus on structure-sensitive-properties can be seen as the main investigative pathway, to resume Frederic L. Holmes s concept, which lead to materials science. 

Since the late 1960s there has been some interest in the concept of a structure-sensitive reaction in heterogeneous catalysis (177, 178). In the case of supported metal catalysts, structure sensitivity is visualized as a dependence of metal particle size and catalytic behavior in a given reaction (activity and selectivity). Almost all of the possible kinds of relationships were reported in the past. Recently, Che and Bennett reviewed this problem (161). Our intention here is not to repeat most of their analysis, rather we shall try to present our view on the general characteristics of palladium versus other platinum metals. 

At sufficiently high temperatures, due to not too strong cohesion, the surface Pd atoms may acquire convenient positions to form a bond with reacting hydrocarbon molecule (189). This concept, called extractive chemisorption, was introduced by Burwell et al. (190, 191) as a possible cause of absence of steric hindrance in adsorption and reaction of some complex organic molecules. It was proposed that in chemisorption one or two metal atoms were displaced above the initial planar level, leading to increased bonding to the surface for low-dispersion catalysts. An extension of this concept to the problem of structure sensitivity allows one to explain several cases of the relatively mild (or absent) structure sensitivity in many reactions catalyzed by Pd catalysts. 

A new concept has emerged that distinguishes between structure-sensitive and structure-insensitive reactions the specific rate of the latter is independent of particle size. This is a useful concept with many implications. A complete independence from crystal size, if applicable to the transition from solid to atom would, however, seem incongruous in the light of any electronic theory-physical or chemical. We must realize that the metal particles, even at the low end of the range investigated, are still relatively large a 20-A metal crystallite contains 300 to 400 atoms. 

As a matter of convenience, it is useful to give a name to reactions that, on a given metal, exhibit a lack of sensitivity to details of surface structure. We have proposed to call these reactions facile. Another name for them would be structure-insensitive. The concept is probably as old as the concept of active centers and can be found in Taylor s 1925 paper in which he wrote There will be all extremes between the case in which all the atoms in the surface are active and that in which relatively few are so active and .. . the amount of surface which is catalytically active is determined by the reaction catalyzed (42). Similar ideas have been presented by Crawford et al. (43), who found that specific rates of ethylene hydrogenation on nickel evaporated films change only by a factor of 3 when, as a result of sintering, crystallite sizes change from 625 to 21,000 A. The authors concluded that studies of sintering should be conducted with more structure-sensitive mechanisms. ... 

The concept of structure-sensitivity was enunciated by Boudart,32,33 and has recently been summarised by Bond and Ponec.8 A reaction is... 

Boudart et al. for supported metal catalysts to indicate whether the rate of reaction is nonlinearly or linearly proportional to the total surface area of the metal, but the concept can be extended to other types of catalysts as well. Thus, for acid catalysts, the rate of formation of a particular species may depend upon the total number of acid sites, or upon the number of a particular type, e.g., a particular range of acid strengths. In general, the deactivation process may diminish the number of a particular type of active sites, while the rate of formation of each product may depend upon other types of active sites (which may include part or all, or none, of the first type). The interrelation between these types denotes whether the various reactions are structure-sensitive or structure-insensitive, and determines how the selectivities change with... 

Metal catalysed reactions are differentiated introducing the concept of facile and demanding reactions. In principle a single atom should be adequate for a facile (structure insensitive) reaction, while an ensemble of surface atoms is required to form a catalytic site adequate for demanding (structure sensitive) reactions. Consequently, there are reactions, which requires more than one species to form multiplets " or ensembles. In other words, some reactions depend on the surface geometry e.g. hydrogenolysis of hydrocarbons), while other may not e.g. hydrogenation of olefinic double bond). 

Important catalytic reaction concepts include structure sensitivity and insensitivity of reactions, mechanistic classifications (Langmuir-Hinshelwood, Eley-Rideal), the compensation effect, the presence of strongly chemisorbed overlayer, and the roles of structure and bonding modifier additives (promoters). 

A main field of activities is focused on structure and reactivity in two-dimensional adlayers at electrode surfaces. Significant new insights were obtained into the specific adsorption and phase formation of anions and organic monolayers as well as into the underpotential deposition of metal ions on foreign substrates. The in situ application of structure-sensitive methods with an atomic-scale spatial resolution, and a time resolution up to a few microseconds revealed rich, potential-dependent phase behavior. Randomly disordered phases, lattice gas adsorption, commensurate and incommensurate (compressible and/or rotated) stmctures were observed. Attempts have been developed, often on the basis of concepts of 2D surface physics, to rationalize the observed phase changes and transitions by competing lateral adsorbate-adsorbate and adsorbate-substrate interactions. 

For bulk materials, all techniques based on structure-insensitive properties, as described in this section and elsewhere, yield closely similar data. The crystallinity model is thus a valid defect concept to describe structure-insensitive properties of semicrystalline polymers. It breaks down for three-phase systems, consisting, for example, of a crystalline phase, a mobile amorphous phase, and a rigid-amorphous fraction (see Chap. 6). In addition, one does not expect valid answers for structure-sensitive properties. 

It was established that platinum single crystals served as excellent model catalysts LEED provided structural information, flat terraces could be prepared with steps one atom high, kink sites could be introduced and their catalytic activity recognized... The concept of structural sensitivity could be examined at the atomic level for the first time. Surface science has provided a remarkably detailed picture of catalysis on single metal crystals under conditions of high vacuum. 

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