Mechanisms of heterogeneous catalysis

Heterogeneous catalysis normally depends on at least one reactant being adsorbed (usually chemisorbed) and modified to a form in which it readily undergoes reaction. This modification often takes the form of a fragmentation of the reactant molecules. In practice, the active phase is dispersed as very small particles of linear dimension less than 2 nm on a porous oxide support. Shape-selective catalysts, such as the zeolites (ImpactI25.2), which have a pore size that can distinguish shapes and sizes at a molecular scale, have high internal specific surface areas, in the range of 100–500 m² g⁻¹. The decomposition of phosphine (PH3) on tungsten is first-order at low pressures and zeroth-order at high pressures. To account for these observations, we write down a plausible rate law in terms of an adsorption isotherm and explore its form in the limits of high and low pressure. If the rate is supposed to be proportional to the surface coverage and we suppose that θ is given by the Langmuir isotherm, we would write v =kθ= where p is the pressure of phosphine. When the pressure is so low that Kp << 1, we can neglect Kp in the denominator and obtain v =kKp and the decomposition is first-order. When Kp >> 1, we can neglect the 1 in the denominator, whereupon the Kp terms cancel and we are left with v =k and the decomposition is zeroth-order.

In the Langmuir–Hinshelwood mechanism (LH mechanism) of surface-catalysed reactions, the reaction takes place by encounters between molecular fragments and atoms adsorbed on the surface. We therefore expect the rate law to be second-order in the extent of surface coverage: A+B→P    v=kθAθB Insertion of the appropriate isotherms for A and B then gives the reaction rate in terms of the partial pressures of the reactants. For example, if A and B follow Langmuir isotherms, and adsorb without dissociation, so that θA = θA = then it follows that the rate law is v = The parameters K in the isotherms and the rate constant k are all temperature dependent, so the overall temperature dependence of the rate may be strongly non-Arrhenius (in the sense that the reaction rate is unlikely to be proportional to e^−Ea/RT). The Langmuir–Hinshelwood mechanism is dominant for the catalytic oxidation of CO to CO₂. In the Eley–Rideal mechanism (ER mechanism) of a surface-catalysed reaction, a gas-phase molecule collides with another molecule already adsorbed on the surface. The rate of formation of product is expected to be proportional to the partial pressure, pB, of the non-adsorbed gas B and the extent of surface coverage, θA, of the adsorbed gas A. It follows that the rate law should be A+B→P    v=kp_Bθ_A The rate constant, k, might be much larger than for the uncatalysed gas-phase reaction because the reaction on the surface has a low activation energy and the adsorption itself is often not activated. If we know the adsorption isotherm for A, we can express the rate law in terms of its partial pressure, pA. For example, if the adsorption of A follows a Langmuir isotherm in the pressure range of interest, then the rate law would be v = If A were a diatomic molecule that adsorbed as atoms, we would substitute the isotherm given in eqn 25.6 instead. According to eqn 25.27, when the partial pressure of A is high (in the sense KpA>> 1) there is almost complete surface coverage, and the rate is equal to kpB. Now the rate determining step is the collision of B with the adsorbed fragments. When the pressure of A is low (KpA << 1), perhaps because of its reaction, the rate is equal to kKpApB; now the extent of surface coverage is important in the determination of the rate. Almost all thermal surface-catalysed reactions are thought to take place by the LH mechanism, but a number of reactions with an ER mechanism have also been identified from molecular beam investigations. For example, the reaction between H(g) and D(ad) to form HD(g) is thought to be by an ER mechanism involving the direct collision and pick-up of the adsorbed D atom by the incident H atom. However, the two mechanisms should really be thought of as ideal limits, and all reactions lie somewhere between the two and show features of each one.

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مواضيع ذات صلة

Units

Names of quantities

Working galvanic cells

Electrolysis

Experimental techniques

The rate laws

The electric potential at the interface

The structure of the interface

Catalytic activity at surfaces

Mobility on surfaces

The rate of desorption

The rate of adsorption