Subject: Catalytic properties of pgms
What intrinsic properties contribute to the catalytic activity of the platinum group metals?
It is difficult to answer this question in a perfectly general way, but some attempt can be made in the context of specific reaction types.
The types of reaction that are catalysed depend on how the reactants form intermediate species on the metal surface by a process called chemisorption. These ‘adsorption complexes’ bear some relation to the general chemistry of the elements; for example, there is a parallel between the way in which ethene (C2H4) is chemisorbed on the Group 10 metals (Ni, Pd, Pt) and the stability of the organometallic complexes that contain it (such as Zeise’s salt, KPtCl3(C2H4)).
Because of the difference in the electronic structures of palladium (4d 10) and platinum (5d 96s1), the ethene molecule is less strongly bound to palladium than to platinum. This explains why palladium is the catalyst of choice for the hydrogenation of alkynes: because, once the first two hydrogen atoms have been added, the resulting alkene is quickly released from the surface, so that a very selective partial hydrogenation is obtained. However, with platinum the alkene is reluctant to desorb and so risks being further reduced to the alkane.
Thus, the electronic structure of the element is ultimately responsible for its catalytic behaviour. This can be seen in another way.
As one moves to the left from Group 10 in the Periodic Classification, the number of valence electrons decreases (e.g. Rh, 4d 85s1; Ru, 4d 75s1) but, because the d-shell is limited to five pairs of electrons, the number of unpaired d-electrons increases (Pt, 10–9 = 1; Rh, 10–8 = 2; Ru, 10–7 = 3). This means that each surface atom can form more bonds to chemisorbed species, but for some reactions this is not desirable as it encourages fragmentation of the reactant molecule. The catalyst then becomes poisoned.
On the other hand where this is necessary to secure a desired product, as for example in the hydrogenolysis of ethane:
ruthenium and rhodium are much more active than platinum. Also, ruthenium with three unpaired d-electrons is able to retain the trivalent nitrogen atom, and hence is an excellent catalyst for ammonia synthesis, whereas rhodium and platinum are not.
Answer posted 16 May 2005
Submitted by: Ms Susan Ashton
Answered by: Professor Geoffrey Bond
Affiliation: Emeritus Professor (Brunel University)