Probing the size dependent chemical properties of metals in reduced dimension

Heterogeneously catalyzed reactions typically start with adsorption and dissociation of reactant molecules on the surface of a solid catalyst. In many instances, this is followed by surface diffusion of the adsorbed species, chemical reaction, and removal of the product molecule. According to the Sabatier principle, optimal catalytic performance requires that the bonding between the adsorbate molecule and the surface should neither be too strong nor too weak. This bonding strength is directly related to the catalyst’s surface electronic structure and hence, electronic structure modification would seem a promising approach for tuning catalytic activity.

There have been many studies along this line, including electronic structure modifica- tion via surface alloying, introduction of ’active sites’, size control, and charge transfer between the catalyst and its support. The underlying physics is often expressed within the context of the d-band model by Norskov. Specifically, the bonding strength of adsorbate molecules on transition metal surfaces is strongly influenced by the interaction between the molecular orbitals and the metal d states, which can be parameterized by the location of the d-band center relative to the Fermi level. This model has been successful in explaining trends in catalytic activity of transition metal surfaces but there are exceptions, presumably to competing factors that are structure or element specific, and that are not considered in the model. To firmly establish the validity of the model, we investigated ultrathin Pd and Ru films and tuned the location of the d-band center by changing the film thickness one atomic layer at a time, while keeping all other variables unchanged. Interestingly, while bulk Pd is reactive towards oxygen, Pd(111) films below five monolayer, grown on Ru(0001), are surprisingly inert to oxygen. This trend is fully in line with the d-band model prediction. Here, the shift of the d-band center is associated with the increased band width of the 4d_xz [4dxz] and 4d_yz [4dyz] orbitals. On the other hand, Ru(0001) films on Pd(111) reveal a more complex behavior which can be attributed to Pd segregation. This study provides an in-depth look at orbital specific contributions to the chemical reactivity, providing new knowledge that could be useful in surface catalysis.