Surfaces are full of surprises, and of course mysteries. Ertl described the intricacies of ammonia formation on flat platin surfaces decades ago and won a Nobel for it, but what happens between real catalysts and the reactions they accelerate remains largely unknown. When it comes to the behavior of steps, kinks and other surface features under real conditions, we have hardly scratched the surface yet.
That is because usually surface interactions are observed by scientists, for practical reasons, in near vacuum and at low temperatures. Lots and lots of interesting things simply don’t happen in near vacuum at low temperatures. Nevertheless it turned out that even under these conditions adsorbates induced significant changes in surface morphology, especially on steps and other nonconformities. One should assume that the changes observed so far pale in comparison to what more extreme conditions will do to surfaces. As we read in this recent Science paper by Feng Tao et al, that is indeed the case.
At least for this system, a platinum(557)-surface with adsorbed carbon monoxide at different pressures. The researchers used scanning tunneling microscopy (STM) to image the surface directly and x-ray photoelectron spectroscopy to identify different surface positions. What they found was that the surface reconstruction at high pressure (1 Torr) differed markedly from that at near vacuum.
When the surface, which basically consists of rows after rows of straight steps of one atom height, is covered with CO at low pressure, the structure changes into two-atom-high terraces with wavy edges. Under high pressure though, the step structure breaks up into triangular nano-clusters.
Apparently the surface is forced into this structure by the electrostatic repulsion between adsorbed carbon monoxide molecules. Due to the high pressure a tight layer forms, in which every surface platinum atom is occupied by one CO. There are two different kinds of platinum atoms in this arrangement: Those in the terrace surface, who have many neighbors, and those at the edges, who are less constrained.
Density functional calculations reveal what happens then: At the edges the adsorbed CO molecules tilt away from their neighbors, thus reducing the mutual repulsion. These positions are energetically favorable, so the surface reorganizes accordingly – more edges, less flat surfaces. If the pressure decreases, edges are less favorable, so the low-pressure structure with its fairly straight edges reappears.
The structure of a catalyst’s surface determines its catalytic activity: not every surface atom is an active site. But apparently it’s not so much the static surface that determines activity, but the highly dynamic one that occurs during interaction with the reactants. Using those effects to design more effective catalyst systems is a major challenge for future catalysis chemistry – no less a task than Ertls original research during the 70s.
Tao, F., Dag, S., Wang, L., Liu, Z., Butcher, D., Bluhm, H., Salmeron, M., & Somorjai, G. (2010). Break-Up of Stepped Platinum Catalyst Surfaces by High CO Coverage Science, 327 (5967), 850-853 DOI: 10.1126/science.1182122