Single-site adsorption

Yan C, Jensen J A and Kummel A 1994 Large island formation versus single site adsorption for CI2 chemisorption onto Si(111 )-7 X 7 surfaces Phys. Rev. Lett. 72 4017-20... 

In the absence of transport limitations, the processes of adsorption, surface diffusion, surface reaction, and desorption can be treated via the transition state theory (Baetzold and Somorjai, 1976 Zhdanov et al, 1988). For example, the application of the TST to a single site adsorption process,... 

Only in one case (the hydration of 2-methylpropene over a H2S04—Si02 catalyst [278]) was the so-called Rideal mechanism proposed as a preferable model and expressed by the rate equation for single-site adsorption with retardation by the product alcohol, viz. 

Single-site adsorption. RDS surface he and H 2O reaction. E 27.5 kcal... 

Hypothesis for the atomic structure of the a and P states are obviously closely related to the suggested mechanism for the formation of these two phases. There appears, however, to be general acceptance of a description of the a phase in terms of a single-site adsorption complex bound to the surface via the carbon atom with a similar mechanism to that used to describe M—C O bonding in the binary carbonyls. Recently, good correlation between the shift in the infrared absorption bands measured by Blyholder and Allen 206) on the first-row transition metals from vanadium to copper has been found by them using a... 

In the absence of chemical reaction between adsorbed species, it is instructive to analyze adsorption/desorption equilibria via steps 3 and 6. The overall objective here is to develop expressions between the partial pressnie / a of gas A above a solid surface and the fraction of active sites a on the catalyst that are occnpied by this gas when it adsorbs. The phenomenon of chemisorption and the relation between pa and a apply to a unimolecular layer of adsorbed molecnles on the catalytic surface. This is typically referred to as a monolayer, where the intermolecular forces of attraction between adsorbed molecules and active snrface sites are characteristic of chemical bonds. When complete monolayer coverage of the surface exists, subsequent adsorption on this saturated surface corresponds to physisorption, which is analogous to condensation of a gas on a cold snbstrate. The enthalpy change for chemisorption is exothermic with valnes between 10 and 100 kcal/mol. The Langmuir adsorption isotherm, first proposed in 1918 (see Langmuir, 1918), is based on the following reversible elementary step that simulates single site adsorption on a catalytic surface when there is only one adsorbate (i.e., gas A) present ... 

SINGLE-SITE ADSORPTION OF EACH COMPONENT IN A MULTICOMPONENT MIXTURE... 

The fractional surface coverage by each component follows the Langmuir isotherm for single-site adsorption. Hence,... 

Reactants A and B reversibly produce C and D via gas-soUd kinetics. A and C experience single-site adsorption as characterized by Langmuir isotherms. Reactant B attacks adsorbed A from the gas phase or from a physisorbed layer. In other words, B does not occupy active sites on the catalyst, and neither does product D. Single-site chemical reaction on the surface is the slowest step in the mechanism. Hence,... 

All the Hongen-Watson models prior to this section have been presented analytically (i.e., in closed form) because solution of equation (14-121) for vacant-site fraction y is trivial when aU (Pi = I, which is consistent with the fact that each gas in the reactive mixture exhibits single-site adsorption. Numerical methods are required to calculate y if one or more gases adsorbs on several adjacent active sites without dissociation. 

Gases B, C, and D experience single-site adsorption on the catalyst, and surface coverage is described by the classic Langmuir isotherm 0, = KiPi v. Dual-site chemical reaction on the surface equilibrates on the time scale of the adsorption of reactant A. Hence,... 

As mentioned above, the Hougen-Watson kinetic rate law is based on single-site adsorption/desorption of reactant A (i.e., A - - ct o- Act) ... 

The total pressure dependence of the initial reactant product conversion rate near the inlet of a packed catalytic tubular reactor exhibits the following functional form when single-site adsorption of reactant A is the slowest step in the five-step mechanism given by (14-124). The feed stream is stoichiometric in reactants A and B ... 

The kinetic rate constant for adsorption of A is calculated from the intercept (i.e., 2/oo), and the adsorption/desorption equilibrium constant for reactant B is obtained from the ratio of slope to intercept (i.e 2ai/ao). If this procedure is successful, then one can be confident that the proposed five-step mechanism given by (14-124) and the choice of single-site adsorption of reactant A as the slow step are reasonable assumptions because the resulting Hougen-Watson model agrees with experimental data. 

The initial reactant product conversion rate should increase at higher temperature because kinetic rate constants for elementary steps, particularly the desorption of gas D, increase at higher temperature. In summary, there is no total pressure dependence of the initial reactant product conversion rate when (1) A -h B C -h D, (2) single-site adsorption is appropriate for each component, and (3) desorption of one of the products controls the Hougen-Watson kinetic rate law. 

If each component experiences single-site adsorption and dual-site chemical reaction is the slowest step, then the following four-step mechanism is reasonable ... 

The adsorption/desorption equilibrium constant for single-site adsorption of ethanol, with units of inverse atmospheres, is Fa = ai/ao-... 

Dissociative adsorption of reactant A2 on the catalytic surface is the rate-limiting step, and the stable reactive intermediate A2B occupies a significant fraction of surface sites. It is reasonable to assume that reactant B, intermediate A2B, and products C and D experience single-site adsorption. Express your final answer in terms of the partial pressures of... 

B and C undergo single-site adsorption on a heterogeneous catalyst, without dissociation. 

The feed stream is stoichiometric in terms of the two reactants. Diatomic A2 undergoes dissociative adsorption. Components B, C, and D experience single-site adsorption, and triple-site chemical reaction on the catalytic surface is the rate-controlling feature of the overall irreversible process. This Langmuir-Hinshelwood mechanism produces the following Hougen-Watson kinetic model for the rate of reaction with units of moles per area per time ... 

Diatomic gas A2 undergoes dissociative adsorption and gases B and C experience single-site adsorption. Chemical reaction on the catalytic surface is the slowest step. Stoichiometric proportions of A2 and B are present initially. 

At relatively low pressures, what dimensionless differential equations must be solved to generate basic information for the effectiveness factor vs. the intrapellet Damkohler number when an isothermal irreversible chemical reaction occurs within the internal pores of flat slab catalysts. Single-site adsorption is reasonable for each component, and dual-site reaction on the catalytic surface is the rate-limiting step for A -h B C -h D. Use the molar density of reactant A near the external surface of the catalytic particles as a characteristic quantity to make all of the molar densities dimensionless. Be sure to define the intrapellet Damkohler number. Include all the boundary conditions required to obtain a unique solution to these ordinary differential equations. 

Diatomic gas A2 undergoes dissociative adsorption, gas B does not require an active site on the catalytic surface because it attacks adsorbed atomic A from the gas phase, and gas C experiences single-site adsorption. Chemical reaction on the catalytic surface is rate limiting in the three-step mechanism. Stoichiometric proportions of A2 and B are present initially (i.e., a 1 2 feed of A2 and B). 

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