Understanding catalysis

Abstract

The large majority of chemical compounds underwent at least one catalytic step during synthesis. While it is common knowledge that catalysts enhance reaction rates by lowering the activation energy it is often obscure how catalysts achieve this. This tutorial review explains some fundamental principles of catalysis and how the mechanisms are studied. The dissociation of formic acid into H2 and CO2 serves to demonstrate how a water molecule can open a new reaction path at lower energy, how immersion in liquid water can influence the charge distribution and energetics, and how catalysis at metal surfaces differs from that in the gas phase. The reversibility of catalytic reactions, the influence of an adsorption pre-equilibrium and the compensating effects of adsorption entropy and enthalpy on the Arrhenius parameters are discussed. It is shown that flexibility around the catalytic centre and residual substrate dynamics on the surface affect these parameters. Sabatier's principle of optimum substrate adsorption, shape selectivity in the pores of molecular sieves and the polarisation effect at the metal-support interface are explained. Finally, it is shown that the application of a bias voltage in electrochemistry offers an additional parameter to promote or inhibit a reaction.

A Study of Low-Temperature CO Oxidation over Mesoporous CuO-TiO2 Nanotube Catalysts

“…Catalysts 2017, 7, 129 12 of 18 energy and the heat of adsorption of the reactants [60]. Therefore, the high values of the apparent activation energies might be due to the strong CO adsorption bond energy to the catalyst surface which increases with increasing Cu loading and is not actually due to a hindered surface reaction, as the order of the apparent activation energy may indicate [61].…”

“…The higher activation energy of the 50CuO-TiO 2 NT catalyst might suggest that its catalytic activity is lower than that of the 20CuO-TiO 2 NT or 2CuO-TiO 2 NT catalyst, which contradicts the order of the CO oxidation reaction rates where the 50CuO-TiO 2 NT catalyst demonstrated the highest reaction rate of 36 µmole s −1 g −1 compared to 30 µmole s −1 g −1 and 4.8 µmole s −1 g −1 for the 20CuO-TiO 2 NT and 2CuO-TiO 2 NT catalysts, respectively. However, the experimentally-determined Arrhenius parameters and activation energies represent the apparent values and the apparent activation energy for a bimolecular catalyzed reaction that does not only depend on the true surface activation energy and the heat of adsorption of the reactants [60]. Therefore, the high values of the apparent activation energies might be due to the strong CO adsorption bond energy to the catalyst surface which increases with increasing Cu loading and is not actually due to a hindered surface reaction, as the order of the apparent activation energy may indicate [61].…”

Tailoring the reducibility and catalytic activity of CuO nanoparticles for low temperature CO oxidation

“…The experimentally-determined Arrhenius parameters and activation energies represent the apparent values and the apparent activation energy for a bimolecular catalyzed reaction depends on the true surface activation energy and the heat of adsorption of the reactants. 61 Therefore, the high values of the apparent activation energies might be due to the strong CO adsorption bond energy to the CuO catalyst surface.…”

Evolution of the PVP–Pd Surface Interaction in Nanoparticles through the Case Study of Formic Acid Decomposition

“…30 Some noble metal-based catalysts (Ag, Pt and Pd) are the most selective towards H 2 production instead of CO and water formation (the undesired possible reaction). [31][32][33] Among them, Pd is one of the metals with the lowest activation energy and most promising TOF values. 34,35 The incorporation of Pd NPs on different supports (carbon, zeolites, MOFs or silica) has also been studied in the literature with different results depending on the nature of the support and the active phases features.…”