Structure sensitivity in the nonscalable regime explored via catalysed ethylene hydrogenation on supported platinum nanoclusters

The sensitivity, or insensitivity, of catalysed reactions to catalyst structure is a commonly employed fundamental concept. Here we report on the nature of nano-catalysed ethylene hydrogenation, investigated through experiments on size-selected Ptn (n=8–15) clusters soft-landed on magnesia and first-principles simulations, yielding benchmark information about the validity of structure sensitivity/insensitivity at the bottom of the catalyst size range. Both ethylene-hydrogenation-to-ethane and the parallel hydrogenation–dehydrogenation ethylidyne-producing route are considered, uncovering that at the <1 nm size-scale the reaction exhibits characteristics consistent with structure sensitivity, in contrast to structure insensitivity found for larger particles. The onset of catalysed hydrogenation occurs for Ptn (n≥10) clusters at T>150 K, with maximum room temperature reactivity observed for Pt13. Structure insensitivity, inherent for specific cluster sizes, is induced in the more active Pt13 by a temperature increase up to 400 K leading to ethylidyne formation. Control of sub-nanometre particle size may be used for tuning catalysed hydrogenation activity and selectivity.


Supplementary Figure 14 | Optimal adsorption configurations and charge distribution
on bare Pt n / MgO (n= 9, 10) and co-adsorbed C 2 H 4 +H 2 on Pt 10 /MgO. In a and c, the blue and yellow contour hyper-surfaces correspond to excess (light blue) and deficient (yellow) charge distributions obtained as the difference between the total charges before and after adsorption of the clusters; these hypersurfaces are drawn such that the excess electronic (negative) charge inside the light blue hypersurface is 30% of the total electronic charge and the same for the positive charge inside the yellow hypersurfaces (for hypersurfaces corresponding to 50% of the excess negative (and positive) excess charges, see Instead the structure-insensitive hydrogenation of ethylene (occurring at higher hydrogen pressure and temperature) involves as reaction intermediates a weakly-bound π-bonded ethylene (in a near sp 2 hybridization) and the "half-hydrogenated" ethyl (C 2 H 5 ) molecule; for a proposed reaction scheme see Fig. 13

in 3 .
Real catalysis typically entails highly dispersed small particles supported on metal oxides or other high-surface-area substrates, whereas the results that were reviewed above were all obtained from investigations on extended single crystal metal surfaces. To bridge the so called "materialgap" attention has been shifted over the past decade to investigations involving finite particles on solid supports 4,5 . It is pertinent to remark here that the intrinsic size-effect of platinum particles supported on amorphous alumina in the hydrogenation of ethylene has been previously addressed 6 . In this investigation it was found that the reaction on Pt particles larger than 1.7 nm was structure insensitive and a turn over frequency (TOF) maximum was found for a particle size of ~ 0.6 nm (containing 10-20 atoms), with a similar result found for a Pt/SiO 2 system. From these results, it was suggested that underlying the apparent structure sensitivity at small sizes was "increased atom accessibility"; this interpretation was reached, in the absence of realistic quantitative estimates, based on analysis that considered rather idealized simple polyhedral models and heuristic arguments.
Additionally, the above experiment has been carried out on polydispersed particle samples, and a deconvolution of the effect of the particle-size distribution could not be made unambiguously. Several studies on the catalytic properties of Pt particles have been reported in the past few years, [see refs. 11-18 in Ref 7 ] finding structure insensitivity for ethylene hydrogenation on particles in the range of 1-11 nm, albeit using polydispersed particle samples.
To summarize: current opinion is that while the adsorption of ethylene is structure sensitive, the overall hydrogenation reaction is structure insensitive.

Supplementary Note 2 | The Dewar-Chatt-Duncanson model
In all cases (including hydrogenation of C 2 H 4 on Pt(111)) the microscopic reaction mechanism has been found to follow a frontier orbital description that find its origins in an adaptation of the Dewar-Chatt-Duncanson (DCD) model; here the addition of a hydrogen atom to the adsorbed molecule is described as an agnostic process (a term used to refer specifically to situations in which a hydrogen atom is covalently bonded to both a carbon and a transition metal atom 8 , with the change in the C-H distance affecting (increasing) the energy gap between the bonding (σ CH ) and antibonding (σ CH * ) states that shift away from the Fermi level as the C-H distance reduces (or equivalently the Pt-H distance increases). At the top of the activation barrier, the interaction between the σ CH and σ CH * orbitals with the s-, pand d-electrons of the Pt clusters brings about orbital mixing that may be described in term of the DCD donation and back-donation terms, culminating in attachment of the transferred H atoms (initially bonded to the Pt cluster) to the adsorbed molecule.