An inside look at a catalyst surface

	What makes something wet? Salmeron’s team has also developed a new technique capable of imaging liquid surfaces with nanometer-scale resolution. Called Scanning Polarization Force Microscopy (SPFM), the technique captures the condensation and evaporation of water on surfaces at the atomic scale. Using SPFM, Salmeron and his colleagues have seen strings of potassium hydroxide droplets persist along graphite ledges only a few atoms high, even after the graphite has apparently been blown "dry." They have seen fresh-cleaved sheets of mica, exposed to air, exhibit increasingly branching patterns of wetting. They have also observed ice crystals coated with a layer of liquid water, even at temperatures below freezing. These phenomena have never been imaged at these scales before. That’s because optical microscopes lack the resolution to image at nanometer resolutions. Electron microscopes, which operate in a vacuum, destroy such types of samples, and conventional atomic force microscopes brush away or wick up liquid or weakly bound samples. Only SPFM offers the technology needed to explore such phenomena.

Catalyst surfaces supply chemical reactions with atoms. In a hydrogen fuel cell, for example, a metal surface adsorbs hydrogen atoms from a gas such as methane, and supplies these atoms to an electrochemical reaction that generates electrical power. But although adsorption — in which a molecule breaks into atoms, and these atoms adhere to a surface — is a cornerstone of hundreds of catalytic processes, it is only partially understood. Researchers know that in order for a molecule to adsorb onto a catalyst surface, it must find a home among the surface’s constantly fluctuating mix of atoms. If the molecule can’t find this home, or active site, it bounces off.

Researchers also know that in many cases these active sites are composed of atom-free vacancies. Called atomic adsorption sites, these vacancies jitterbug across a catalyst surface, sometimes moving alone, sometimes forming short-lived clusters of two, three, or more. Every so often, this random dance brings together enough vacancies to create a home for a hydrogen molecule — a phenomenon at the heart of how catalyst surfaces work, and also where scientists’ understanding becomes hazy. For years, they believed an active site needs only as many vacancies as the number of atoms in a molecule. This assumption is repeated in many textbooks, and it makes sense — one vacancy for each atom — but it’s wrong.

Hydrogen atoms don't favor conduction, so they appear much smaller than vacancies under a scanning tunneling microscope.

The truth didn’t reveal itself until Salmeron and colleagues trained a scanning tunneling microscope onto a catalyst surface made of palladium. Salmeron’s team used this special microscope to observe the formation of sites that adsorb hydrogen. They cooled a palladium surface to an extremely low temperature, which slows its atomic movement, and exposed it to hydrogen molecules. They then watched what happened. Like a bird’s eye view of people mish-mashing their way through a crowded train station, the vacant atomic adsorption sites skitter across the surface. When two vacancies bump together, creating the condition scientists long believed is ripe for hydrogen adsorption, nothing happens. The two vacancies merely dance together for a fraction of a second, then part ways.

“Although the pair of vacancies are constantly bombarded with hydrogen molecules, they never adsorb hydrogen atoms,” Salmeron says. “This proves that two vacancies don’t work.”

But when three vacancies cluster together, two vacancies in the group quickly accept a hydrogen atom. This sequence of images, from a cluster of three vacancies to two adsorbed hydrogen atoms and one remaining vacancy, marks the first time the formation and function of an active site has been observed. Based on these images, the Berkeley Lab team concluded hydrogen adsorption requires active sites containing three or more vacancies.