Structure-sensitivity

The hydrogenation of alkenes has long been recognised as being structure-insensitive , by which is meant rates per exposed metal atom (i.e. TOFs) are essentially independent of particle size, the number of such atoms being estimated before reaction, and usually by hydrogen chemisorption. The general truth of this view is not in doubt, but it is necessary to test its veracity and to examine apparent exceptions. 

There are some indications of particle size dependence with propene hydrogenation with Pt/Si02, rates at 220 K increased less than two-fold as dispersion rose from 5 to 80%, but they also depended on the type of pre-treatment applied. With Pd/Si02, rates passed through a maximum at about 60% 

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It is applied along with traditional methods to test strength properties, hardness, to determine standardized characteristics of stamping, grain size and other structural-sensitive characteristics. 

There has been considerable elaboration of the simple Girifalco and Good relationship, Eq. XII-22. As noted in Sections IV-2A and X-6B, the surface ftee energies that appear under the square root sign may be supposed to be expressible as a sum of dispersion, polar, and so on, components. This type of approach has been developed by Dann [70] and Kaelble [71] as well as by Schonhom and co-workers (see Ref. 72). Good (see Ref. 73) has preferred to introduce polar interactions into a detailed analysis of the meaning of in Eq. IV-7. While there is no doubt that polar interactions are important, these are orientation dependent and hence structure sensitive. 

R. Hicks and co-workers, Structure Sensitivity of Methane Oxidation overl latinum and Palladium J. Catal, 280—306 (1990). 

List five different structure-sensitive properties. 

Answers Structure-sensitive properties yield strength, hardness, tensile strength, ductility, fracture toughness, fatigue strength, creep strength, corrosion resistance. 

There are, of course, many more ceramics available than those listed here alumina is available in many densities, silicon carbide in many qualities. As before, the structure-insensitive properties (density, modulus and melting point) depend little on quality -they do not vary by more than 10%. But the structure-sensitive properties (fracture toughness, modulus of rupture and some thermal properties including expansion) are much more variable. For these, it is essential to consult manufacturers data sheets or conduct your own tests. 

The systematic use of classical catalytic kinetics is always a useful approach in modeling (Boudart 1986). Even if these models do not reflect the true mechanism in the case of structure-sensitive catalysts, they are a formally correct representation of the observed facts. As Boudart sees it in the case of structure-insensitive reactions, it can also be the real thing. 

Very recently, considerable effort has been devoted to the simulation of the oscillatory behavior which has been observed experimentally in various surface reactions. So far, the most studied reaction is the catalytic oxidation of carbon monoxide, where it is well known that oscillations are coupled to reversible reconstructions of the surface via structure-sensitive sticking coefficients of the reactants. A careful evaluation of the simulation results is necessary in order to ensure that oscillations remain in the thermodynamic limit. The roles of surface diffusion of the reactants versus direct adsorption from the gas phase, at the onset of selforganization and synchronized behavior, is a topic which merits further investigation. 

Reduction of enynones to dienones is structure sensitive and is often unsatisfactory if the acetylenic bond is attached directly to the carbonyl 30J 1,52). Selectivity is improved if the acetylenic bond is terminal 52,70,71). 

A novel nondestructive method for the determination of total charges and hence of EQ=Qy that is based on the CO displacement experiments has been worked out.795,796 This method has been applied to Pt(lll) and Pt(l 10) electrodes in contact with solutions at different pHs. For both Pt faces, the potential-of-zero total charge lies in a potential region similar to that forpc-Pt.8,10,11 It was found that thepztc depends on pH in different ways for Pt(l 11) and Pt(l 10), which demonstrates that not only is the pztc structure sensitive, but also that it varies with pH.795 The value of pztc for Pt(l 11) is more positive than that for Pt(l 10), and df j/dpHis higher for Pt( 111) than for Pt( 110). 

S. Tsubota, D.A.H. Cunningham, Y. Bando, and M. Haruta, Preparation of nanometer gold strongly interacted with Ti02 and the structure sensitivity in low-temperature oxidation of CO, in Preparation of catalysts VI, G. Ponchelet, ed. (1995), pp. 227-235. 

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. 

The undoubtedly structure-sensitive reaction NO -r CO has a rate that varies with rhodium surface structure. A temperature-programmed analysis (Fig. 10.8) gives a good impression of the individual reaction steps CO and NO adsorbed in relatively similar amounts on Rh(lll) and Rh(lOO) give rise to the evolution of CO, CO2, and N2, whereas desorption of NO is not observed at these coverages. Hence, the TPRS experiment of Fig. 10.8 suggests the following elementary steps ... 

The NO + CO reaction is only partially described by the reactions (2)-(7), as there should also be steps to account for the formation of N2O, particularly at lower reaction temperatures. Figure 10.9 shows the rates of CO2, N2O and N2 formation on the (111) surface of rhodium in the form of Arrhenius plots. Comparison with similar measurements on the more open Rh(llO) surface confirms again that the reaction is strongly structure sensitive. As N2O is undesirable, it is important to know under what conditions its formation is minimized. First, the selectivity to N2O, expressed as the ratio given in Eq. (7), decreases drastically at the higher temperatures where the catalyst operates. Secondly, real three-way catalysts contain rhodium particles in the presence of CeO promoters, and these appear to suppress N2O formation [S.H. Oh, J. Catal. 124 (1990) 477]. Finally, N2O undergoes further reaction with CO to give N2 and CO2, which is also catalyzed by rhodium. 

Structure Sensitivity over Pe. Table II presents the rates of ammonia synthesis over each of the low Miller index planes of Pe. 

Structure Sensitivity over Re. As in the case of the Fe catalysts the rate of ammonia synthesis varies greatly over Re single crystal surfaces of different orientations. This phenomenon has been studied over the (0001), (loTo), (1120) and 0121) planes in a 3 1 Hp/N mixture at a total pressure of 20 atm. and a temperature or 870 K. Under these conditions these surfaces catalyze the reaction with relative rates of 1 94 920 2820 respectively, showing a range of activities even greater than that observed on Fe. 

The studies of ammonia synthesis over Fe and Re and the hydrodesulfurization of thiophene over Mo, described above, illustrate the importance and success of our approach of studying catalysis over single crystal samples at high pressures. The use of surfaces having a variety of orientations allows the study of reactions that are surface structure sensitive 6Uid provides insight into the nature of the catalytic site. Here we have shown that the ammonia synthesis... 

These are examples of structure Insensitive (facile) (Reactions 1-4) and structure sensitive (demanding) reactions (Reactions 3-7). 

Kinetics of Structure Sensitive Reactions Over Clean Single Crystal Surfaces... 

We have studied the steady-state kinetics and selectivity of this reaction on clean, well-characterized sinxle-crystal surfaces of silver by usinx a special apparatus which allows rapid ( 20 s) transfer between a hixh-pressure catalytic microreactor and an ultra-hixh vacuum surface analysis (AES, XPS, LEED, TDS) chamber. The results of some of our recent studies of this reaction will be reviewed. These sinxle-crystal studies have provided considerable new insixht into the reaction pathway throuxh molecularly adsorbed O2 and C2H4, the structural sensitivity of real silver catalysts, and the role of chlorine adatoms in pro-motinx catalyst selectivity via an ensemble effect. 

Structural Sensitivity. Figure 1 shows the steady-state rates of ethylene oxide (EtO) and CO2 production as a function of temperature, in Arrhenius form, at an ethylene pressure (P-.) of 20 torr and P. 

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