On the optimum catalyst for structure sensitive heterogeneous catalytic reactions
- PDF / 1,109,314 Bytes
- 13 Pages / 439.37 x 666.142 pts Page_size
- 42 Downloads / 205 Views
On the optimum catalyst for structure sensitive heterogeneous catalytic reactions Dmitry Yu. Murzin1 Received: 26 June 2020 / Accepted: 7 August 2020 © The Author(s) 2020
Abstract Reaction rates in a two-step catalytic sequence, when plotted vs adsorption energy of the key or the most abundant surface intermediate, result in volcano shaped curves. In the current work, the optimal catalyst is discussed for structure sensitive reactions, which display dependence of activity on the cluster size of the active catalytic phase. An expression is derived relating the Gibbs energy for formation of the intermediate with the Gibbs energy changes in the overall reaction, difference in adsorption thermodynamics on edges and terraces and the cluster size. The kinetic expressions display dependence of activity vs the Gibbs energy of the adsorbed intermediate formation. Numerical analysis demonstrates that when the overall equilibrium constant K is high and the reaction is thermodynamically very favorable, the maxima in the rates vs the adsorption constant for the optimal catalyst are much broader being less dependent on the cluster size. When structure sensitivity is pronounced, there are smaller differences in the rates for the optimum and less optimal catalysts in comparison with reactions showing weak structure sensitivity. Keywords Structure sensitivity · Optimum catalyst · Cluster size
Introduction Selection of an optimum catalyst, i.e. a catalyst, which has a maximum reaction rate, has been of interest for decades starting from the classical works of Sabatier, Balandin and Temkin [1–3]. The Sabatier principle providing a conceptual framework for analysis of the optimum catalyst [4], relies on an intuitive concept of an intermediate binding strength of the reactants to the catalyst leading to different types of volcano curves, named after Balandin [2], Tamaru–Tanaka [5] or Sachtler–Fahrenfort [6].
* Dmitry Yu. Murzin [email protected] 1
Åbo Akademi University, Turku/Åbo, Finland
13
Vol.:(0123456789)
Reaction Kinetics, Mechanisms and Catalysis
Fig. 1 Two-step sequence, a reaction graph, b energy profile
Analysis of the optimal catalyst from a kinetic perspective was performed by Temkin [3] for a two-step sequence (Fig. 1), i.e. a mechanism with two kinetically significant steps [7, 8], and one most abundant surface intermediates
1. S + A1 ↔ I∕S (ad) + B1 2. I∕S(ad) + A2 ↔ S + B2 A1 + A2 = B1 + B2
(1)
Here A1, A2 are reactants, B1 and B2 are products, S is the surface site and I is an adsorbed intermediate. In [3], the linear free energy relationship was used, linking kinetics (e.g. rate constant, k) and thermodynamics (equilibrium constant, K) through for example the Brønsted equation:
k = gK 𝛼
(2)
Here g is a constant and α is the Polanyi parameter (0
Data Loading...
