Single-crystal surfaces catalytic reactions

The effect of alkali presence on the adsorption of oxygen on metal surfaces has been extensively studied in the literature, as alkali promoters are used in catalytic reactions of technological interest where oxygen participates either directly as a reactant (e.g. ethylene epoxidation on silver) or as an intermediate (e.g. NO+CO reaction in automotive exhaust catalytic converters). A large number of model studies has addressed the oxygen interaction with alkali modified single crystal surfaces of Ag, Cu, Pt, Pd, Ni, Ru, Fe, Mo, W and Au.6... 

While experiment and theory have made tremendous advances over the past few decades in elucidating the molecular processes and transformations that occur over ideal single-crystal surfaces, the application to aqueous phase catalytic systems has been quite limited owing to the challenges associated with following the stmcture and dynamics of the solution phase over metal substrates. Even in the case of a submersed ideal single-crystal surface, there are a number of important issues that have obscured our ability to elucidate the important surface intermediates and follow the elementary physicochemical surface processes. The ability to spectroscopically isolate and resolve reaction intermediates at the aqueous/metal interface has made it difficult to experimentally estabhsh the surface chemistry. In addition, theoretical advances and CPU limitations have restricted ab initio efforts to very small and idealized model systems. 

The presence of solution at a metal surface, as has been discussed, can significantly influence the pathways and energetics of a variety of catalytic reactions, especially electrocatalytic reactions that have the additional complexity of electrode potential. We describe here how the presence of a solution and an electrochemical potential influence the reaction pathways and the reaction mechanism for methanol dehydrogenation over ideal single-crystal surfaces. 

Figure 3.6. Example of the type of kinetic information available for the catalytic reduction of NO on rhodium single-crystal surfaces under atmospheric conditions. The data in this figure correspond to specific rates for C02, N20, and N2 formation over Rh(l 11) as a function of inverse temperature for two NO + CO mixtures PNO = 0.6 mbar and Pco — 3 mbar (A), and Pno — Pco = 4 mbar (B) [55]. The selectivity of the reaction in this case proved to be approximately constant independent of surface temperature at high NO pressures, but to change significantly below Pno 1 mbar. This highlights the dangers of extrapolating data from experiments under vacuum to more realistic pressure conditions. (Reproduced with permission from the American Chemical Society, Copyright 1995).

Figure 3.7. In-situ reflection-absorption infrared (RAIRS) spectra as a function of catalyst temperature from a Pd(lll) single-crystal surface in the presence of a NO + CO gas mixture (240mbar, Pco/Pno = 1-5) [66]. The data clearly show the appearance of an isocyanate-related band at 2256 cm-1 at temperatures above 500 K. In-situ spectroscopic experiments such as these have proven indispensable to detect and identify key reaction intermediates for the catalytic reduction of NO on metal surfaces. (Figure provided by Professor Goodman and reproduced with permission from the American Chemical Society, Copyright 2003).

Such a possibility has been recognized by early workers,9 but in spite of this intriguing possibility, only recently has such a metal surface been created. Chiral kink sites were created on Ag single crystal surfaces to produce the enantiomeric surfaces Ag(643)s and Ag(643)R however, no differences between (R)- and (S)-2-butanol were observed for either the temperature-programmed desorption from the clean surfaces or the dehydrogenation (to 2-butanone) from preoxidized surfaces.10 Unfortunately, Ag exhibits few catalytic properties, so only a limited array of test reactions is available to probe enantioselectivity over this metal. It would be good if this technique were applied to a more catalytically active metal such as Pt. 

The aim of this chapter is to document how catalytic reactions can be promoted and regulated by the presence of coadsorbed species and by the design of active structures at catalyst surfaces. To clarify these issues, this chapter examines two classes of well-defined catalyst surfaces chemically designed surfaces and single-crystal surfaces. 

The occurrence of kinetic instabilities as well as oscillatory and even chaotic temporal behavior of a catalytic reaction under steady-state flow conditions can be traced back to the nonlinear character of the differential equations describing the kinetics coupled to transport processes (diffusion and heat conductance). Studies with single crystal surfaces revealed the formation of a large wealth of concentration patterns of the adsorbates on mesoscopic (say pm) length scales which can be studied experimentally by suitable tools and theoretically within the framework of nonlinear dynamics. [31]... 

The key to understanding such processes lies in our ability to dissect the catalytic event into its separate components. Numerous ingenious experiments have been performed by workers in the field of catalysis for many years, and it is not the intent of this article to review these contributions. It is important to note that such studies have advanced the field of catalysis to a refined science and that a number of general observations have been developed which serve as guidelines for the development and improvement of catalytic materials. Insofar as surface science and the study of reactions on macroscopic single crystal surfaces is related to catalysis, its purpose should therefore be to contribute a more exact and, thereby, a more general understanding of the basic phenomena involved. 

The studies of well defined systems consists of spectroscopic studies of individual molecules and measurements of the rate of catalytic reactions on single crystal surfaces, as well as structure and reactivity of well-defined catalyst models. 

The link between the microscopic description of the reaction dynamics and the macroscopic kinetics that can be measured in a catalytic reactor is a micro-kinetic model. Such a model will start from binding energies and reaction rate constants deduced from surface science experiments on well defined single crystal surfaces and relate this to the macroscopic kinetics of the reaction. 

Our article has concentrated on the relationships between vibrational spectra and the structures of hydrocarbon species adsorbed on metals. Some aspects of reactivities have also been covered, such as the thermal evolution of species on single-crystal surfaces under the UHV conditions necessary for VEELS, the most widely used technique. Wider aspects of reactivity include the important subject of catalytic activity. In catalytic studies, vibrational spectroscopy can also play an important role, but in smaller proportion than in the study of chemisorption. For this reason, it would not be appropriate for us to cover a large fraction of such work in this article. Furthermore, an excellent outline of this broader subject has recently been presented by Zaera (362). Instead, we present a summary account of the kinetic aspects of perhaps the most studied system, namely, the interreactions of ethene and related C2 species, and their hydrogenations, on platinum surfaces. We consider such reactions occurring on both single-crystal faces and metal oxide-supported finely divided catalysts. 

The catalytic oxidation of CO on iridium has not been as extensively studied as with palladium and platinum. However, as for these metals, both steady-state (40, 54, 124, 199-202) and nonsteady-state investigation (124, 200-203) have been carried out on both polycrystalline and single crystal surfaces. As the results are for the most part very similar to those obtained on palladium and platinum surfaces, only those results that shed additional light on the kinetics and mechanism basic to the reaction will be emphasized here. 

Fig. 16. Schematic of the experimental apparatus to carry out catalytic reaction rate studies on single-crystal surfaces at low and high pressures in the range 10 7-104 Torr.

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