Surface reactivity, tools

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. 

Modern surface analytical tools make it possible to probe the physical structure as well as the chemical composition and reactivity of interfacial supramolecular assemblies with unprecedented precision and sensitivity. Therefore, Chapter 3 discusses the modern instrumental techniques used to probe the structure and reactivity of interfacial supramolecular assemblies. The discussion here is focused on techniques traditionally applied to the interrogation of interfaces, such as electrochemistry and scanning electron microscopy, as well as various microprobe techniques. In addition, some less common techniques, which will make an increasing contribution to supramolecular interfacial chemistry over the coming years, are considered. 

The SECM, which does not provide atomic level spatial resolution, cannot compete with STM or AFM as a tool for topographic imaging. However, SECM is well suited for high resolution mapping of surface reactivity. This can be done in either feedback or collection mode. The former can provide a spatial distribution of the rate of a redox reaction responsible for mediator regeneration at the substrate. By proper choice of solution components to control the tip... 

Experimental surface science is a meeting ground of chemistry, physics, and engineering.2 New spectroscopies have given us a wealth of information, be it sometimes fragmentary, on the ways that atoms and molecules interact with surfaces. The tools may come from physics, but the questions that are asked are very chemical, e.g., what is the structure and reactivity of surfaces by themselves, and of surfaces with molecules on them ... 

In the 1950-1970s, wet chemical and radiotracer methods were widely used to quantify additive incorporation in electrodeposits. More recently, modem surface analytical tools such as SIMS, AES, XPS and RBS analysis have been implemented. However, there have been few attempts to correlate electrochemical measurements with additive incorporation into the solid. Simultaneous monitoring of the quenching of reactivity and microstructural evolution of electrodes with pre-engineered... 

Increased capabilities of computational hardware have made possible impressive advances in the development and implementation of quantum-chemical computational tools. Current possibilities of studying computationally complex chemical systems with chemically relevant accuracy have made it an invaluable technique that complements experimental methods. In surface science and computational catalysis, this is in particular the case for calculations based on density functional theory (DFT) electronic structure calculations [1-5]. The large body of detailed experimental surface science results of adsorption and surface reactivity at molecular or atomic level [6, 7] has given a great impetus to the theoretical studies that we review. 

Rate constants can be estimated by means of transition-state theory. In principle all thermodynamic data can be deduced from the partion function. The molecular data necessary for the calculation of the partion function can be either obtained from quantum mechanical calculations or spectroscopic data. Many of those data can be found in tables (e.g. JANAF). A very powerful tool to study the kinetics of reactions in heterogeneous catalysis is the dynamic Monte-Carlo approach (DMC), sometimes called kinetic Monte-Carlo (KMC). Starting from a paper by Ziff et al. [16], several investigations were executed by this method. Lombardo and Bell [17] review many of these simulations. The solution of the problem of the relation between a Monte-Carlo step and real time has been advanced considerably by Jansen [18,19] and Lukkien et al. [20] (see also Jansen and Lukkien [21]). First principle quantum chemical methods have advanced to the stage where they can now offer quantitative predictions of structure and energetics for adsorbates on surfaces. Cluster and periodic density functional quantum chemical methods are used to analyze chemisorption and catalytic surface reactivity [see e.g. 24,25]. 

The SECM can also be used as an imaging device (see Section 12.3.3), as an electrochemical tool for studies of surface reactivity of thin films (see Section 12.4.4) and as a high-resolution fabrication tool (see Section 12.4.5). Finally, the SECM can be adapted to probe the transport activity of biological systems Uke single cells, the ion transport across channels and enzyme activity (see Section 12.4.6). 

Polymer Brushes a Mechanistic Tool for the Interrogation of Surface Reactivity... 

As for mineral surfaces, the matching of theoretical models to experimental data by molecular modeling methods is a powerful tool for the prediction of adsorption and surface reactivity on ceramics. The reviews by Henrich 167). Kun/. [68. Stoneham and Tasker (98, and Colboum and Mackrodt [63.99 discuss the applications of theoretical techniques and correlations with experimental results. 

During the 10 years since the first edition of this book, we have witnessed significant evolution of SECM for qualitative and quantitative surface reactivity characterization and patterning. It has also become an indispensible tool in electrochemistry. Evidence of this assertion is the increase in the number of chapters of the present book in comparison to the first edition. The new chapters contain further developments of previous SECM research areas that because of their relevance and progress merit an additional chapter. Other new areas have been opened recently thanks to extensions in instrumentation and new applications. One can expect in the next 10 years to see a continued growth of the SECM applications presented in this book and the anergence of new ones that will broaden even further the scope of this technique. 

Because of its simplicity of use and quantitative results, Scanning Electrochemical Microscopy (SECM) has become an indispensable tool for the study of surface reactivity. The fast expansion of the SECM field during the last several years has been fueled by the introduction of new probes, commercially available instrumentation, and new practical applications. Scanning Electrochemical Microscopy, Second Edition offers essential background and in-depth overviews of specific applications in self-contained chapters. 

In addition to its use in studying surface structure, surface reactivity, and molecular adsorption, STM is also finding use in nanofabrication, or the making of extremely small objects. One outstanding example of the use of STM as a fabrication tool is shown in the series of images in Figure 7, which depict the assembly of a ring of 48 iron atoms... 

In addition to the many applications of SERS, Raman spectroscopy is, in general, a usefiil analytical tool having many applications in surface science. One interesting example is that of carbon surfaces which do not support SERS. Raman spectroscopy of carbon surfaces provides insight into two important aspects. First, Raman spectral features correlate with the electrochemical reactivity of carbon surfaces this allows one to study surface oxidation [155]. Second, Raman spectroscopy can probe species at carbon surfaces which may account for the highly variable behaviour of carbon materials [155]. Another application to surfaces is the use... 

The preferable theoretical tools for the description of dynamical processes in systems of a few atoms are certainly quantum mechanical calculations. There is a large arsenal of powerful, well established methods for quantum mechanical computations of processes such as photoexcitation, photodissociation, inelastic scattering and reactive collisions for systems having, in the present state-of-the-art, up to three or four atoms, typically. " Both time-dependent and time-independent numerically exact algorithms are available for many of the processes, so in cases where potential surfaces of good accuracy are available, excellent quantitative agreement with experiment is generally obtained. In addition to the full quantum-mechanical methods, sophisticated semiclassical approximations have been developed that for many cases are essentially of near-quantitative accuracy and certainly at a level sufficient for the interpretation of most experiments.These methods also are com-... 

Molecular orbitals are useful tools for identifying reactive sites m a molecule For exam pie the positive charge m allyl cation is delocalized over the two terminal carbon atoms and both atoms can act as electron acceptors This is normally shown using two reso nance structures but a more compact way to see this is to look at the shape of the ion s LUMO (the LUMO is a molecule s electron acceptor orbital) Allyl cation s LUMO appears as four surfaces Two surfaces are positioned near each of the terminal carbon atoms and they identify allyl cation s electron acceptor sites... 

---