Surface reaction

Product state analysis offers a flexible way to obtain detailed state resolved information on simple surface reactions and to explore how their dynamics differ from the behaviour observed for H2 desorption [7]. In this chapter, we will discuss some simple surface reactions for which detailed product state distributions are available. We will concentrate on N2 formation in systems where the product desorbs back into the gas phase promptly carrying information about the dynamics of reaction. Different experimental techniques are discussed, emphasising those which give fully quantum state resolved translational energy distributions. The use of detailed balance to relate recombinative desorption measurements to the reverse, dissociation process is outlined and the influence of the surface temperature on the product state distributions discussed. Simple low dimensional models which provide a reference point for discussing the product energy disposal are described and then results for some surface reactions which form N2 are discussed in detail, emphasising differences with the behaviour of H2. 

Product state measurements have been applied to surface reactions run under a variety of conditions. Desorbing molecules can be supplied by permeation through a film, by steady state catalytic reaction, molecular beam deposition or by temperature programmed reaction (TPR). The individual 

Because of the inherently destructive nature of ion bombardment, the use of SSIMS alone in the study of the reactions of surfaces with gases and vapor must be viewed with caution, but in combination with other surface techniques it can provide valuable additional information. The parallel techniques are most often XPS,TDS, and LEED, and the complementary information required from SSIMS normally refers to the nature of molecules on surfaces and with which other atoms, if any, they are combined. 

A typical SSIMS spectrum of an organic molecule adsorbed on a surface is that of thiophene on ruthenium at 95 K, shown in Eig. 3.14 (from the study of Cocco and Tatarchuk [3.28]). Exposure was 0.5 Langmuir only (i.e. 5 x 10 torr s = 37 Pa s), and the principal positive ion peaks are those from ruthenium, consisting of a series of seven isotopic peaks around 102 amu. Ruthenium-thiophene complex fragments are, however, found at ca. 186 and 160 amu each has the same complicated isotopic pattern, indicating that interaction between the metal and the thiophene occurred even at 95 K. In addition, thiophene and protonated thiophene peaks are observed at 84 and 85 amu, respectively, with the implication that no dissociation of the thiophene had occurred. The smaller masses are those of hydrocarbon fragments of different chain length. 

SSIMS has also been used to study the adsorption of propene on ruthenium [3.29], the decomposition of ammonia on silicon [3.30], and the decomposition of methane thiol on nickel [3.31]. 

SSIMS has been used in the TOP SSIMS imaging mode to study very thin layers of organic materials [3.32-3.36], polymeric insulating materials [3.37], and carbon fiber and composite fracture surfaces [3.38]. In these studies a spatial resolution of ca. 80 nm in mass-resolved images was achieved. 

Some of the entries under this heading in Table 1 will be illustrated with particular examples. The variety of surface reactions of minerals, and the techniques used for their study, is far too broad for all aspects to be included in this chapter. Several monographs [1-3,15, 67] and a review [4] provide many relevant ca.se studies. 

Surface oxidation of sulfide minerals has been reviewed recently by Smart et al. [78]. Studies of the physical and chemical forms of oxidation products by SAM, XPS, STM. AFM, SEM and ToF-SIMS have revealed several different proces.ses of oxidation. The seminal work of Buckley et al. (e.g., [79,80)) was the first to identify the process of formation of oxyhydroxide products on underlying metal-deficient, sulfur-rich layers of similar forms to those described in Section 4.3. Other oxidation products have been observed directly, such as polysulfides, elemental sulfur, oxidized fine sulfide particles, colloidal hydroxide particles and flocculated aggregates, as well as continuous surface layers of 

An example of surface phase transformation can be found in the work of Jones et al. 85. Pyrrhotite (Fe., S) surfaces in acid solution restructured to a crystalline, defective, tetragonal Fc Si. surface phase due to loss of Fe to solution. The metal-deficient, S-rich product was identified by XRD in combination with XPS. Linear chains of sulfur with a nearest-neighbor distance similar to that of elemental S were observed, but with an S 2p binding energy 0.2-0.6 eV less than that of Ss- As a result of the oxidation process, hydrophilic iron oxides were found at the surface. 

Two examples of surface modification of minerals will be discussed in order to illustrate the merits of a combined-technique approach. The breadth of the subject constrains the discussion to that dealing with strategies for gathering information and for choosing the most relevant surface analytical techniques. 

The heterogeneously catalyzed reaction of starting materials A and B can follow different mechanisms [Claus 1996, Ertl 1990]. The models of Langmuir-Hinshelwood, Eley-Rideal, and Mars-van Krevelen are widely used in practice. 

Type I Microporous materials and chemisorption. In micropores the adsorption potentials ofthe two walls overlap, and this leads to amplification ofthe adsorption potential [Everett 1976]. This results in increased adsorption energy at very small relative pressures. 

Types IVand V Capillary condensation in mesopores. Initially, a multilayer is adsorbed on the capilla walls. When the pressure is further increased, liquids droplets form preferentially at sites where the curvature fulfills the Kelvin equation. When two opposing droplets touch, the pore is filled. On desorption, pores whose radius is smaller than the Kelvin radius are emptied. The adsorption branch indicates the extent ofthe pores, and the desorption branch the size ofthe pore openings [Evertt 1976]. 

Type VI Stepped isotherms, for example, nitrogen on some activated carbons. 

With Equation (2.1.33) this gives the classical Langmuir-Hinshelwood expression (Equation 2.1-40). 

Most laboratory studies of solids that have oxidizing activity have been performed using metal oxides such as those of silver, manganese, chromium, mercury, and lead (Pickering, 1966). In the environment, however, from the limited number of studies that have been performed, it appears that iron and manganese oxides are probably the most important naturally occurring catalysts. 

Similar mechanisms appear to be predominant in the oxidative oligomerization or polymerization of other types of aromatic compounds (hydrocarbons, ethers, phenols, etc.) on smectites containing Fe(III) or Cu(II) (Mortland and Halloran, 1976 Soma and Soma, 1983 Sawnhey et al., 1984). Electron transfer to Cu(ll) smectite by 

4-chloroanisole (24) results in a radical cation intermediate that undergoes dechlorination to chloride ion and the coupling product, 4,4 -dimethoxybiphenyl (Govin-daraj et al., 1987). Even dibenzodioxin (unchlorinated) forms a radical cation on Cu(ll)-smectite and polymerizes (Boyd and Mortland, 1985). 

sediments, and soil dusts promote the oxidation of various reactive organic species such as catechol (25 Larson and Hufnal, 1980), pyrogallol (26 Wang et al., 

1983) and parathion (27 Spencer et al., 1980). In the case of parathion, the reaction product was the highly toxic derivative, paraoxon (28). Kaolinite clays were more reactive than montmorillonite in this reaction, which may also have involved atmospheric ozone. 

The MEP between two stationary points on the PES can more formally be defined as the continuous and smooth path among all possible paths connecting the two stationary points, which satisfies the two following properties  

It is a path of least action At any point along the path, the gradient of the potential has no component perpendicular to the path. 

The highest potential energy along the MEP is equal to or lower than the highest potential energy along all stationary paths. 

One can combine a series of elementary reactions steps into a PED for a complete catalytic process. Let us take the ammonia synthesis reaction as an example. The overall reaction can be written as 

FIGURE 2.4 lUnstration of the elementary reaction steps on surfaces. See insert for color representation of the figure.) 

---

The physical chemist is very interested in kinetics—in the mechanisms of chemical reactions, the rates of adsorption, dissolution or evaporation, and generally, in time as a variable. As may be imagined, there is a wide spectrum of rate phenomena and in the sophistication achieved in dealing wifli them. In some cases changes in area or in amounts of phases are involved, as in rates of evaporation, condensation, dissolution, precipitation, flocculation, and adsorption and desorption. In other cases surface composition is changing as with reaction in monolayers. The field of catalysis is focused largely on the study of surface reaction mechanisms. Thus, throughout this book, the kinetic aspects of interfacial phenomena are discussed in concert with the associated thermodynamic properties. 

Studies of inelastic scattering are of considerable interest in heterogeneous catalysis. The degree to which molecules are scattered specularly gives information about their residence time on the surface. Often new chemical species appear, whose trajectory from the surface correlates to some degree with that of the incident beam of molecules. The study of such reactive scattering gives mechanistic information about surface reactions. 

INS Ion neutralization An inert gas hitting surface is spectroscopy [147] neutralized with the ejection of an Auger electron from a surface atom Spectroscopy of Emitted Ions or Molecules Kinetics of surface reactions chemisorption... 

MBRS Molecular beam spectroscopy [158] A modulated molecular beam hits the surface and the time lag for reaction products is measured Kinetics of surface reactions chemisorption... 

The silanization reaction has been used for some time to alter the wetting characteristics of glass, metal oxides, and metals [44]. While it is known that trichlorosilanes polymerize in solution, only very recent work has elucidated the mechanism for surface reaction. A novel FTIR approach allowed Tripp and Hair to prove that octadecyl trichlorosilane (OTS) does not react with dry silica. 

Perxanthate ion may also be implicated [59]. Even today, the exact nature of the surface reaction is clouded [59, 79-81], although Gaudin [82] notes that the role of oxygen is very determinative in the chemistry of the mineral-collector interaction. 

As on previous occasions, the reader is reminded that no very extensive coverage of the literature is possible in a textbook such as this one and that the emphasis is primarily on principles and their illustration. Several monographs are available for more detailed information (see General References). Useful reviews are on future directions and anunonia synthesis [2], surface analysis [3], surface mechanisms [4], dynamics of surface reactions [5], single-crystal versus actual catalysts [6], oscillatory kinetics [7], fractals [8], surface electrochemistry [9], particle size effects [10], and supported metals [11, 12]. 

Figure XVIII-2 shows how a surface reaction may be followed by STM, in this case the reaction on a Ni(llO) surface O(surface) + H2S(g) = H20(g) + S(surface). Figure XVIII-2a shows the oxygen atom covered surface before any reaction, and Fig. XVIII-2h, the surface after exposure to 3 of H2S during which Ni islands and troughs have formed on which sulfur chemisorbs. The technique is powerful in the wealth of detail provided on the other hand, there is so much detail that it is difficult to relate it to macroscopic observation (such as the kinetics of the reaction).

Fig. XVIII-22. Schematic illustration of the steps that may be involved in a surface-mediated reaction initial adsorption, subsequent thermalization, diffusion and surface reaction, and desorption. (From Ref. 199 copyright 1984 by the AAAS.)...

The above situation led to the proposal by Rideal [202] of what has become an important alternative mechanism for surface reactions, illustrated by Eq. XVIII-33. Here, reaction takes place between chemisorbed atoms and a colliding or physical adsorbed molecule (see Ref. 203). 

The Langmuir-Hinshelwood picture is essentially that of Fig. XVIII-14. If the process is unimolecular, the species meanders around on the surface until it receives the activation energy to go over to product(s), which then desorb. If the process is bimolecular, two species diffuse around until a reactive encounter occurs. The reaction will be diffusion controlled if it occurs on every encounter (see Ref. 211) the theory of surface diffusional encounters has been treated (see Ref. 212) the subject may also be approached by means of Monte Carlo/molecular dynamics techniques [213]. In the case of activated bimolecular reactions, however, there will in general be many encounters before the reactive one, and the rate law for the surface reaction is generally written by analogy to the mass action law for solutions. That is, for a bimolecular process, the rate is taken to be proportional to the product of the two surface concentrations. It is interesting, however, that essentially the same rate law is obtained if the adsorption is strictly localized and species react only if they happen to adsorb on adjacent sites (note Ref. 214). (The apparent rate law, that is, the rate law in terms of gas pressures, depends on the form of the adsorption isotherm, as discussed in the next section.)... 

The course of a surface reaction can in principle be followed directly with the use of various surface spectroscopic techniques plus equipment allowing the rapid transfer of the surface from reaction to high-vacuum conditions see Campbell [232]. More often, however, the experimental observables are the changes with time of the concentrations of reactants and products in the gas phase. The rate law in terms of surface concentrations might be called the true rate law and the one analogous to that for a homogeneous system. What is observed, however, is an apparent rate law giving the dependence of the rate on the various gas pressures. The true and the apparent rate laws can be related if one assumes that adsorption equilibrium is rapid compared to the surface reaction. 

If a surface reaction is bimolecular in species A and B, the assumption is that the rate is proportional to 5a x 5b- We now proceed to apply this interpretation to a few special cases. 

A. Unimolecular Surface Reactions We suppose the type reaction to be... 

The apparent activation energy is then less than the actual one for the surface reaction per se by the heat of adsorption. Most of the algebraic forms cited are complicated by having a composite denominator, itself temperature dependent, which must be allowed for in obtaining k from the experimental data. However, Eq. XVIII-47 would apply directly to the low-pressure limiting form of Eq. XVIII-38. Another limiting form of interest results if one product dominates the adsorption so that the rate law becomes... 

R. J. Madix, ed.. Surface Reactions. Springer-Verlag, New York, 1994. 

The surface work fiincdon is fonnally defined as the minimum energy needed m order to remove an electron from a solid. It is often described as being the difference in energy between the Fenni level and the vacuum level of a solid. The work ftmction is a sensitive measure of the surface electronic structure, and can be measured in a number of ways, as described in section B 1.26.4. Many processes, such as catalytic surface reactions or resonant charge transfer between ions and surfaces, are critically dependent on the work ftmction. 

Prybyla J A, Tom H W K and Aumiller G D 1992 Femtosecond time-resolved surface reaction desorption of Co from Cu(111) in <325. fsec Phys. Rev. Lett. 68 503... 

Harris J and Kasemo B 1981 On precursor mechanisms for surface reactions Surf. Sc/. 105 L281... 

Weinberg W H 1991 Kinetics of surface reactions Dynamics of Gas-Surface Interactions ed C T Rettner and M N R Ashfold (London Royal Society of Chemistry)... 

Rettner C T and Auerbach D J 1994 Distinguishing the direct and indirect products of a gas-surface reaction Science 263 365... 

Surface science has tlirived in recent years primarily because of its success at providing answers to frmdamental questions. One objective of such studies is to elucidate the basic mechanisms that control surface reactions. For example, a goal could be to detennine if CO dissociation occurs prior to oxidation over Pt catalysts. A second objective is then to extrapolate this microscopic view of surface reactions to the... 

How are fiindamental aspects of surface reactions studied The surface science approach uses a simplified system to model the more complicated real-world systems. At the heart of this simplified system is the use of well defined surfaces, typically in the fonn of oriented single crystals. A thorough description of these surfaces should include composition, electronic structure and geometric structure measurements, as well as an evaluation of reactivity towards different adsorbates. Furthemiore, the system should be constructed such that it can be made increasingly more complex to more closely mimic macroscopic systems. However, relating surface science results to the corresponding real-world problems often proves to be a stumbling block because of the sheer complexity of these real-world systems. 

The importance of low pressures has already been stressed as a criterion for surface science studies. However, it is also a limitation because real-world phenomena do not occur in a controlled vacuum. Instead, they occur at atmospheric pressures or higher, often at elevated temperatures, and in conditions of humidity or even contamination. Hence, a major tlmist in surface science has been to modify existmg techniques and equipment to pemiit detailed surface analysis under conditions that are less than ideal. The scamiing tunnelling microscope (STM) is a recent addition to the surface science arsenal and has the capability of providing atomic-scale infomiation at ambient pressures and elevated temperatures. Incredible insight into the nature of surface reactions has been achieved by means of the STM and other in situ teclmiques. 

This chapter will explore surface reactions at the atomic level. A brief discussion of corrosion reactions is followed by a more detailed look at growth and etchmg reactions. Finally, catalytic reactions will be considered, with a strong emphasis on the surface science approach to catalysis. 

Surface science studies of corrosion phenomena are excellent examples of in situ characterization of surface reactions. In particular, the investigation of corrosion reactions with STM is promising because not only can it be used to study solid-gas interfaces, but also solid-liquid interfaces. 

The characterization of surfaces undergoing corrosion phenomena at liquid-solid and gas-solid interfaces remains a challenging task. The use of STM for in situ studies of corrosion reactions will continue to shape the atomic-level understanding of such surface reactions. 

On the atomic level, etching is composed of several steps diflfiision of the etch molecules to the surface, adsorption to the surface, subsequent reaction with the surface and, finally, removal of the reaction products. The third step, that of reaction between the etchant and the surface, is of considerable interest to the understanding of surface reactions on an atomic scale. In recent years, STM has given considerable insight into the nature of etching reactions at surfaces. The following discussion will focus on the etching of silicon surfaces [28]. 

---