Support particulate metal studies

The literature of the vibrational spectra of adsorbed alkynes (acetylene and alkyl-substituted acetylenes) is very much in favor of single-crystal studies, with fewer reported investigations of adsorption on oxide-supported metal catalysts. Fewer studies still have been made of the particulate metals under the more advantageous experimental conditions for spectral interpretation, namely, at low temperatures and on alumina as the support. (The latter has a wide transmittance range down to ca. 1100 cm-1.) A similar number of different single-crystal metal surfaces have been studied for ethyne as for ethene adsorption. We shall review in more detail the low-temperature work which usually leads to HCCH nondissociatively adsorbed surface structures. Only salient features will be discussed for higher temperature ethyne adsorption that often leads to dissociative chemisorption. Many of the latter species are those already identified in Part I from the decomposition of adsorbed ethene. 

In a few cases, adsorption on particulate nickel has been studied other than in the form of the conventional oxide-supported metal catalysts. Nash and De Sieno (73) exploded nickel wires in a rare-gas atmosphere to give Ni particles of ca. 20-nm diameter. Results were reported (but not illustrated) characterizing adsorption of ethyne they are similar to those found by Eischens and Pliskin. 

The development of these catalysts occurred in an atmosphere of tight secrecy, and little has been published on those efforts. All three were of the particulate type one contained noble metal(s) while the other two were noble-metal promoted base-metal catalysts (16). The UOP noble metal catalyst was provided several years later to GM and to Ford for evaluation with unleaded fuel and for other studies, and it can be deduced that it was supported on low-density (about 0.32 g/mL apparent bulk density) 1/8 inch spheres (17) and may have contained about 0.47 troy ounces of platinum per cubic foot (ca. 0.16 weight percent), with the platinum concentrated in a subsurface shell some distance below the exterior surfaces of the spheres for improved poison resistance (18) The Cyanamid catalyst was later studied by Ford (16, 19) it apparently was an extrudate (1/8" diameter x 1/8" long) of about 0.67 g/mL ABD, with about 125 ppm (weight) of palladium and 5 weight % each of CuO and 2 5 Si02 95% AI2O3 support of about... 

Physical processes may also be used to deposit Pt onto various types of supports. An example of this type of approach is the preparation of Pt-metal monolayers supported on low-cost transition metal carbides, prepared by magnetron sputtering of Pt onto thin films of W and Mo carbides. While Pt monolayers were achieved on the thin-film electrode geometry used in this study, uniform deposition of Pt onto high surface area particulate materials or mesoporous structures by these methods remains challenging [48]. 

Catalytic materials for MRs have some particular requirements compared to a conventional tube flow reactor. The catalytic material should be in a form that can be inserted easily into the membrane reactor, and the catalyst should not have any mechanical failure or properties which are not suitable for a MR. Very hne powder form catalysts cannot be used, as the small particulates may block the pores of the membrane however, small particulates (>0.2 mm) have been considered (e.g., Li et al., 2010). Thus, in many cases, the catalysts generally used in MRs are pellets, extrudates or tablets. In addition to these forms, novel hbre type or foam catalysts have been studied as support materials for active metals. Li et al., (2010) have presented in their study one kind of a method of encapsulating the catalyst particles (diameter 0.2-1.7 mm) which combines a catalyst particulate with a membrane layer. This has been reported to increase the selectivity of the reaction, and thus the separation process is much easier. 

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