CO₂ Production at the Single-Molecule Level

CO₂ production at the single-molecule level has been observed for a specific chemical reaction. Rather than micromanage (or nanomanage) compliance of the Kyoto Treaty, this new accomplishment instead offers fundamental chemical details of CO₂ formation which may bring improvements to automobile emission control, air purification, and chemical sensing.

Using a scanning tunneling microscope (STM) as a "nanoreactor," researchers (Wilson Ho, UC-Irvine, 949-824-5234, wilsonho@uci.edu) studied the oxidation of a single carbon monoxide (CO) molecule on a metal surface. In this "catalytic oxidation reaction," CO combines with oxygen (O) on the surface to form CO₂.

Putting CO next to two oxygen atoms on a silver surface, the researchers found that the CO and O species must be very close to each other to react. When the CO is at the closest possible distance to an oxygen atom on the surface, at just 1.78 angstroms away, the researchers saw that the reactants form an intermediate O-CO-O complex, which has not been observed to date (see images). Then, tunneling electrons from the STM tip flow through the CO and O species to bring the O-CO-O complex to a higher energy state and enable it to transform into CO₂ and a lone oxygen atom. (In a real-world situation, heat rather than tunneling electrons would spur this transformation.)

In separate experiments investigating another pathway for the same chemical reaction, the researchers put a CO molecule on the STM tip, and then positioned it close to an O atom on a silver surface. In this case, the CO and O loosen their respective bonds to the STM tip and the silver surface. Eventually, both species are on the surface and they interact to form CO₂. In demonstrating this, they showed that CO and O don't initially have to be on the surface to form CO₂; the CO can come from above. The researchers also ruled out the idea that CO reacts with molecular oxygen (O₂) on the surface to form CO₂; instead it only reacts with atomic oxygen. (Hahn and Ho, Physical Review Letters, 15 October 2001.)