Surface reaction limiting forms

Table 10-5 gives the forms of rate laws for different reaction mechanit that are irreversible and surface-reaction-limited. 

The rate equation for any specific situation is easily assembled using these tables. Surface reactions of molecularity greater than two are not known. Since the surface reaction limiting case is the most important for industrial-scale reactions, the specific terms and exponent n values corresponding to this particular case are formulated in Table 2.1 for various reactions and mechanisms. The surface reaction rate constants (,ksr) appearing in the kinetic terms of the various cases are lumped parameters including the total number of active sites So or the number of adjacent sites in some form, since the latter is generally unknown or not independently measurable. 

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... 

Determine the form of the pseudohomogeneous, intrinsic kinetics for each of these cases. Assume that the surface reaction step, as shown above, is rate limiting. 

In a similar way, electrochemistry may provide an atomic level control over the deposit, using electric potential (rather than temperature) to restrict deposition of elements. A surface electrochemical reaction limited in this manner is merely underpotential deposition (UPD see Sect. 4.3 for a detailed discussion). In ECALE, thin films of chemical compounds are formed, an atomic layer at a time, by using UPD, in a cycle thus, the formation of a binary compound involves the oxidative UPD of one element and the reductive UPD of another. The potential for the former should be negative of that used for the latter in order for the deposit to remain stable while the other component elements are being deposited. Practically, this sequential deposition is implemented by using a dual bath system or a flow cell, so as to alternately expose an electrode surface to different electrolytes. When conditions are well defined, the electrolytic layers are prone to grow two dimensionally rather than three dimensionally. ECALE requires the definition of precise experimental conditions, such as potentials, reactants, concentration, pH, charge-time, which are strictly dependent on the particular compound one wants to form, and the substrate as well. The problems with this technique are that the electrode is required to be rinsed after each UPD deposition, which may result in loss of potential control, deposit reproducibility problems, and waste of time and solution. Automated deposition systems have been developed as an attempt to overcome these problems. 

Metal clusters on supports are typically synthesized from organometallic precursors and often from metal carbonyls, as follows (1) The precursor metal cluster may be deposited onto a support surface from solution or (2) a mononuclear metal complex may react with the support to form an adsorbed metal complex that is treated to convert it into an adsorbed metal carbonyl cluster or (3) a mononuclear metal complex precursor may react with the support in a single reaction to form a metal carbonyl cluster bonded to the support. In a subsequent synthesis step, metal carbonyl clusters on a support may be treated to remove the carbonyl ligands, because these occupy bonding positions that limit the catalytic activity. 

This relation is plotted as curve Bin Figure 12.11. Smith (66) has shown that the same limiting forms for are observed using the concept of effective dififusivities and spherical catalyst pellets. Curve B indicates that, for fast reactions on catalyst surfaces where the poisoned sites are uniformly distributed over the pore surface, the apparent activity of the catalyst declines much less rapidly than for the case where catalyst effectiveness factors approach unity. Under these circumstances, the catalyst effectiveness factors are considerably less than unity, and the effects of the portion of the poison adsorbed near the closed end of the pore are not as apparent as in the earlier case for small hr. With poisoning, the Thiele modulus hp decreases, and the reaction merely penetrates deeper into the pore. 

At least for ethylene hydrogenation, catalysis appears to be simpler over oxides than over metals. Even if we were to assume that Eqs. (1) and (2) told the whole story, this would be true. In these terms over oxides the hydrocarbon surface species in the addition of deuterium to ethylene would be limited to C2H4 and C2H4D, whereas over metals a multiplicity of species of the form CzH D and CsHs-jD, would be expected. Adsorption (18) and IR studies (19) reveal that even with ethylene alone, metals are complex. When a metal surface is exposed to ethylene, selfhydrogenation and dimerization occur. These are surface reactions, not catalysis in other words, the extent of these reactions is determined by the amount of surface available as a reactant. The over-all result is that a metal surface exposed to an olefin forms a variety of carbonaceous species of variable stoichiometry. The presence of this variety of relatively inert species confounds attempts to use physical techniques such as IR to char-... 

Historically, EC-ALE has been developed by analogy with atomic layer epitaxy (ALE) [76-82], ALE is a methodology used initially to improve epitaxy in the growth of thin-films by MBE and VPE. The principle of ALE is to use surface limited reactions to form each atomic layer of a deposit. If no more than an atomic layer is ever deposited, the growth will be 2-D, layer by layer, epitaxial. Surface limited reactions are developed for the deposition of each component element, and a cycle is formed with them. With each cycle, a compound monolayer is formed, and the deposit thickness is controlled by the number of cycles. 

TPD and EELS experiments have shown coadsorbed NO/NH3 to form a surface stabilized complex on Pt(lll). This assertion was based in part on the very large vibrational shifts observed for the neat (NO or NH3) systems compared to the mixed system, and in part on the simultaneous desorption of NO and NH3 from the mixed system at values of Tg higher than typical of desorption from the neat systems at comparable initial coverage Kinetit y, desorption from this system has been characterized as reaction limited,... 

Formic acid adsorbed with near unit sticking probability on clean Fe(lOO). The reaction product spectrum for HCOOH on Fe(lOO) is shown in Fig. 17 (95). As with Cu( 110) the reaction proceeded via two steps, one which evolved Hj at 350 K and the other which formed, CO2, and CO at 490 K. A small amount of CO was evolved at 800 K due to the reaction of residual carbon and oxygen atoms on the surface. From these results it was concluded that CO, H2, and CO2 were formed by a common rate-limiting step at 490 K. Since the H2/H2 TPD peak appears at 400 K and below, this step was determined to be a surface reaction. No water was formed. Evidently the reaction proceeded by the pathways... 

Chemical/Physical. Matheson and Tratnyek (1994) studied the reaction of fine-grained iron metal in an anaerobic aqueous solution (15 °C) containing chloroform (107 pM). Initially, chloroform underwent rapid dehydrochlorination forming methylene chloride and chloride ions. As the concentration of methylene chloride increased, the rate of reaction appeared to decrease. After 140 h, no additional products were identified. The authors reported that reductive dehalogenation of chloroform and other chlorinated hydrocarbons used in this study appears to take place in conjunction with the oxidative dissolution or corrosion of the iron metal through a diffusion-limited surface reaction. 

The similarity between the measured activation energies for the reaction-limited production of acrolein over Cu20(100) from allyl alcohol in UHV or propene following a 1 atm. exposure gives a clear indication that these reactions involve the same surface intermediate, an allyloxy. This similarity also suggests that the surface intermediates formed by these two routes behave in a chemically similar fashion. For the (100) surface, the Cu -alkoxide surface complex is similar regardless of whether oxygen from... 

Platinum electrodes are widely used as an inert electrode in redox reactions because the metal is most stable in aqueous and nonaqueous solutions in the absence of complexing agents, as well as because of its electrocatalytic activity. The inertness of the metal does not mean that no surface layers are formed. The true doublelayer (ideal polarized electrode) behavior is limited to ca. 200-300 mV potential interval depending on the crystal structure and the actual state of the metal surface, while at low and high potentials, hydrogen and oxygen adsorption (oxide formation) respectively, occur. 

A potential limitation in the application of MS to near-surface measurements is the tremendous number of compounds in the atmosphere, particularly organics, and hence the increased complexity of interpretation of the single mass spectrum. In the MS ion source, the use of particular ion-molecule reactions to form the ions of interest or the ionization of one selected com-... 

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