Surface science modem

Studies of surfaces and surface properties can be traced to the early 1800s [1]. Processes that involved surfaces and surface chemistry, such as heterogeneous catalysis and Daguerre photography, were first discovered at that time. Since then, there has been a continual interest in catalysis, corrosion and other chemical reactions that involve surfaces. The modem era of surface science began in the late 1950s, when instmmentation that could be used to investigate surface processes on the molecular level started to become available. 

Essential to modem surface science teclmiques is the attaimnent and maintenance of ultrahigh vacuum... 

Woodruff, R.P. Delchar, T.A. (1994) Modem Techniques of Surface Science, Cambridge University Press, Cambridge. 

The examples introduced above refer to the characterization of the most common types of catalysts, usually supported metals or single, mixed, or supported metal oxides. Many other materials such as alloys [199,200], carbides [201-203], nitrides [204,205], and sulfides [206] are also frequently used in catalysis. Moreover, although modem surface science studies with model catalysts were only mentioned briefly toward the end of the review, this in no way suggests that these are of less significance. In fact, as the ultimate goal of catalyst characterization is to understand catalytic processes at a molecular level, surface studies on well-defined model catalysts is poised to be central in the future of the field [155,174], The reader is referred to the Chapter 10 in this book for more details on this topic. 

Carbon monoxide oxidation is a relatively simple reaction, and generally its structurally insensitive nature makes it an ideal model of heterogeneous catalytic reactions. Each of the important mechanistic steps of this reaction, such as reactant adsorption and desorption, surface reaction, and desorption of products, has been studied extensively using modem surface-science techniques.17 The structure insensitivity of this reaction is illustrated in Figure 10.4. Here, carbon dioxide turnover frequencies over Rh(l 11) and Rh(100) surfaces are compared with supported Rh catalysts.3 As with CO hydrogenation on nickel, it is readily apparent that, not only does the choice of surface plane matters, but also the size of the active species.18-21 Studies of this system also indicated that, under the reaction conditions of Figure 10.4, the rhodium surface was covered with CO. This means that the reaction is limited by the desorption of carbon monoxide and the adsorption of oxygen. 

The combined use of the modem tools of surface science should allow one to understand many fundamental questions in catalysis, at least for metals. These tools afford the experimentalist with an abundance of information on surface structure, surface composition, surface electronic structure, reaction mechanism, and reaction rate parameters for elementary steps. In combination they yield direct information on the effects of surface structure and composition on heterogeneous reactivity or, more accurately, surface reactivity. Consequently, the origin of well-known effects in catalysis such as structure sensitivity, selective poisoning, ligand and ensemble effects in alloy catalysis, catalytic promotion, chemical specificity, volcano effects, to name just a few, should be subject to study via surface science. In addition, mechanistic and kinetic studies can yield information helpful in unraveling results obtained in flow reactors under greatly different operating conditions. 

Fig. 6.20. Typical XPS spectrum taken using 1253-eV photons (MgKa). (Reprinted with permission from D. P. Woodruff and T. A. Delchar, Modem Techniques of Surface Science, Cambridge University Press, 1990, Fig. 3.8.)...

W. Jaegermann, in Modem Aspects of Electrochemistry, (R. E. White, B. E. Conway, J. O M. Bockris, eds.), pi36 (The semiconductor/electrolyte interface a surface science approach), Plenum, New York (1996). 

The four sections above followed the typical workflow of screening approaches of the early days. In recent years, it became evident that additional steps have to be added on top of the workflow. One such step is the analysis of the true kinetics and the interplay of its elemental reactions by modem surface-science techniques. 

D. P. Woodruff and T. A. Delchar Modem techniques of surface science... 

The development of new catalytic materials needs to be complemented with detailed studies of the surface chemistry of catalysis at the molecular level in order to better define the requirements for the catalytic active sites. The wide array of modem spectroscopies available to surface scientists today is ideally suited for this task (see Surfaces). Surface science studies on catalysis typically probe reaction intermediates on model metal samples under well controlled conditions. This kind of study is traditionally carried out in ultrahigh vacuum (UHV) systems such as that shown in Figure 10. Single crystals or other well-defined metal surfaces are cleaned and characterized in situ by physical and chemical means, and then probed using a battery of surface sensitive techniqnes snch as photoelectron (XPS and UPS), electron energy loss (ELS... 

The interaction of organic molecules with, and their subsequent reactivities at, electrode surfaces are among the more critical aspects of modem electrochemical surface science. However, the study of these processes is an exceedingly difficult proposition. In the past, experimental probes were limited to conventional electrochemical techniques. " But the information content of these methods is limited to the macroscopic properties of the electrodeelectrolyte interface. Consequently, results from surface studies based merely on ensemble thermodynamic and kinetic measurements can be rationalized only phenomenologically with little basis for interpretations at the molecular level." " Over the past few... 

The development of modem surface science techniques over the past decade has allowed for the investigation of numerous properties of adsorption at the atomic level. These techniques offer information about the electrode surface and the molecular identity of the species adsorbed at the electrode surface. These techniques have been used to study the electrode surface however, many require a gas phase or ex situ environment as summarized in Table 1. XAS including the two complimentary parts, x-ray absorption near edge stracture (XANES) and extended x-ray absorption fine stmc-ture (EXAFS) are in situ/operando spectroscopic techniques that can provide unique site-specific adsorbate bonding and particle size information. Further, these techniques do not suffer from the adsorption by the bulk electrolyte such as in the IR region. X-ray Diffraction (XRD) of course also provides detailed stractural... 

As far as TPB electrochemistry is concerned, modem surface science still unambiguously throws crossword puzzles at us. In these puzzles one discovered letter offers only the clue to other words while each empty square awaits its letter. Notwithstanding the brilliant advances of the recent years, many empty squares have yet to be filled before it would be possible to decipher the expression, Electrochemistry of gas-zirconia-semiconductor TPB. This book attempts to conquer this. If this crossword puzzle will be solved, we would acquire absolute power over the TPB reactions and the power to control them. 

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