Iridium, surface structures

Engstrom J R, Goodman D Wand Weinberg W H 1986 Hydrogenolysis of n-butane over the (111) and (110)-(1 2) surfaces of iridium a direct correlation between catalytic selectivity and surface structure J. Am. Chem. Soc. 108 4653... 

The chemisorption of acetylene, ethylene, benzene, and cyclohexane were also studied on the Ir(lll) and stepped Ir[6(111) x (100)] crystal surfaces (30). Chemisorption characteristics of the Ir(lll) and Pt(lll) surface are markedly different. Also, the chemisorption characteristics of the low Miller index Ir(l 11) surface and the stepped Ir[6(l 11) x (100)] surface are markedly different for each of the molecules studied. The hydrocarbon molecules form only poorly ordered surface structures on either the Ir(l 11) or stepped iridium surfaces. Acetylene and ethylene (C2H2 and C2H4) form surface structures that are somewhat better ordered on the stepped iridium than on the low Miller index Ir(l 11) metal surface. The lack of ordering on iridium surfaces as compared to the excellent ordering characteristics of these molecules on... 

It appears that the stronger metal-carbon interaction on iridium surfaces imposes the periodicity on the carbon atoms in the overlayer, while the structure of the graphite overlayer on the Pt( III) face is independent of the substrate periodicity and rotational symmetry. Ordering of the dehydrogenated carbonaceous residue on the stepped iridium surface is absent when the surface is heated to above 1100 K. Atomic steps of (100) orientation appear to prevent the formation of ordered domains that are predominant on the Ir(lll) crystal face. The reasons for this are not clear. Perhaps the rate of C-C bond breaking on account of the steps is too rapid to allow nucleation and growth of the ordered overlayer. On the (111) face, the slower dehydro-... 

Figure A3.10.22 Relationship between selectivity and surface structure for w-butane hydrogenolysis on iridium, (a) Illustrations of the Ir(l 10)-(1 x 2) and Ir(l 11) surfaces. The z-axis is perpendicular to the plane of the surface, (b) Selectivity for 2 production (mol% total products) for w-butane hydrogenolysis on both Ni single crystals and supported catalysts at 475 K. The effective particle size for the single crystal surfaces is based on the specified geometric shapes [43]. A Ir/Al203 Ir/Si02-...

Field ion microscopy, or FIM, is a classical technique for the study of surface structure that can yield a structural image with a magnification factor on the order of 10. About 10,000 V are applied between the sample, shaped into a pointed tip (positive pole), and a fluorescent screen in a low-pressure atmosphere of helium. The helium atoms that approach the sample tip loose an electron due to the strong eleelrie field and the resulting He ions are accelerated towards the fluorescent sereen where they form an image. The ionization probability depends on the local intensity of the electric field at the sample, and therefore on the atomic structure. The method is limited to refractory metals such as tungsten, tantalum or iridium. 

Gold, platinum and iridium (100) crystal faces all show reconstruction [1]. Figure 10 shows the stucture of the reconstructed silicon (100) crystal face. In this surface, the silicon atoms form a dimer-like surface structure and there is a relaxation or contraction at... 

From results on interatomic distances derived from analysis of EXAFS data, one can draw some conclusions about the structure of platinum-iridium clusters (13,17). If the clusters were truly homogeneous, the interatomic distance characteristic of the platinum EXAFS should be identical to that characteristic of the iridium EXAFS. When we analyze EXAFS data on the clusters, however, we do not find this simple result. We find in general that the distances are not equal. The data indicate that the clusters are not homogeneous in other words,the environments about the platinum and iridium are different. We conclude that the platinum concentrates at the surface or boundary of the clusters. In the case of very highly dispersed platinum-iridium clusters on alumina, the clusters may well have "raft-like" two dimensional structures, with platinum... 

Van Hove MA, Koestner RJ, Stair PC, Biberian IP Kesmodel LL, Bartos I, Somorjai GA. 1981. The surface reconstructions of the (100) crystal faces of iridium, platinum and gold, 1. Experimental-observations and possible structural models. Surf Sci 103 189-217. 

Whereas determination of chemisorption isotherms, e.g., of hydrogen on metals, is a means for calculating the size of the metallic surface area, our results clearly demonstrate that IR studies on the adsorption of nitrogen and carbon monoxide can give valuable information about the structure of the metal surface. The adsorption of nitrogen enables us to determine the number of B5 sites per unit of metal surface area, not only on nickel, but also on palladium, platinum, and iridium. Once the number of B5 sites is known, it is possible to look for other phenomena that require the presence of these sites. One has already been found, viz, the dissociative chemisorption of carbon dioxide on nickel. 

Pellistors are used to detect flammable gases like CO, NH3, CH4 or natural gas. Some flammable gases, their upper and lower explosion limits and the corresponding self-ignition temperatures are listed in Tab. 5.1. This kind of gas sensor uses the exothermicity of gas combustion on a catalytic surface. As the combustion process is activated at higher temperatures, a pellistor is equipped with a heater coil which heats up the active catalytic surface to an operative temperature of about 500 °C. Usually a Platinum coil is used as heater, embedded in an inert support structure which itself is covered by the active catalyst (see Fig. 5.33). The most frequently used catalysts are platinum, palladium, iridium and rhodium. 

Field emission microscopy was the first technique capable of imaging surfaces at resolution close to atomic dimensions. The pioneer in this area was E.W. Muller, who published the field emission microscope in 1936 and later the field ion microscope in 1951 [23]. Both techniques are limited to sharp tips of high melting metals (tungsten, rhenium, rhodium, iridium, and platinum), but have been extremely useful in exploring and understanding the properties of metal surfaces. We mention the structure of clean metal surfaces, defects, order/disorder phenomena,... 

Stepped surfaces withstand cyclic oxidation-reduction treatments (146) like [111] and some other low-index planes. Steps have either [311] or [110] structures. They are claimed to be the only places where orbital hybridization does not take place (136). No wonder that such platinum (138) and iridium (147) surfaces have enhanced activity in Cg dehydrocyclization of n-heptane. 

XPS measurements showed clearly the interachons of the IL with the metal surface, that occurs through F (for BF4 and PF i) or O (for CF3SO1) of the anions, demonstrating the formation of an IL protechve layer surrounding the iridium nanoparticles. Additional extended X-ray absorption fine structure (EXAFS) analyses also provided evidence for interaction of the IL liquid with the metal surface. 

Ir4(CO)i2 reacts with the surface of MgO to generate surface species in which the tetrahedral metal framework is preserved. The structures obtained after decar-bonylation under H2 at 573 K depend on the degree of hydroxylation of the support The iridium cluster nuclearity of 4 was maintained for a low degree of MgO hydroxylation (MgO pretreated at 973 K), but it increased to 6 when the MgO was highly hydroxylated (MgO pretreated at 573 K) [206, 207]. The activity in propane hydrogenolysis of the tailored catalyst is two orders of magnitude less than that of the conventional catalyst at atmospheric pressure and 200 °C. 

Fig. 4. 54 Helium field ion images showing formation of iridium silicide layers of two different atomic structures on the Ir (001) surface. The first micrograph in (a) show s the Ir surface before formation of an IrSi layer.

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