Panel (A): an isolated carbon monoxide molecule adsorbed on silver surface. Panel (B): a pair of oxygen atoms on the surface. Panel (C): CO and two O atoms before reacting. Panel (E): an intermediate O-CO-O complex formed to split into a CO2 molecule and an oxygen atom. Panels (D) and (F) show the adsorption of oxygen and CO for panels (C) and (E), respectively.
Panel (A): a CO molecule is on the STM tip. The image shows two O atoms. Panel (B): the STM tunneling current during the time in which a voltage is applied to make the CO molecule leave the tip and react with an oxygen atom to form a CO2 molecule. Panel (C) shows an image of the area after the reaction. Panels (D), (E), and (F) are the arrangements for (A), (B), and (C), respectively.
This is the first microscopic understanding of a model catalytic oxidation of carbon monoxide on a metal surface. A reaction that is widely used in vehicle emission control, air purification, and instrumentation is seen here through a scanning tunneling microscope (STM) while a CO molecule and an O atom react to make a CO2 molecule. Hahn and Ho from University of California (Irvine) accomplished the reaction steps in color including the intermediate O-CO-O complex that splits to release the CO2. In the lower panel (A) the asterisk refers to one of the two oxygen atoms that is under the CO-terminated tip in panel D. The linear molecule in red-black-red is the desorbed product, or the newly-born CO2 molecule. See lower panel (E). The red and black circles are the oxygen and carbon atoms and the grey is for the silver (Ag) atoms.
As far as CO occurrence is concerned, we have both CO and CO2 in our galaxy and all over the universe. The main sources for CO in the Earth's atmosphere are the volcanoes, natural and man-made fires, and the exhaust of internal combustion engines. Below is another first observation, the first ever view of the carbon monoxide spreading through the Earth’s atmosphere as assembled by the MOPITT sensor aboard the NASA Terra satellite. The false colors from dark brown to red and blue represent levels of carbon monoxide concentration in the lower atmosphere, from 390 parts per billion to 220 parts per billion and 50 parts per billion, respectively. The observation dates are April 30 (top) and October 30 (bottom) in 2000. Atmospheric CO is short-lived to become CO2 eventually but it has an indirect (and increasing) effect on methane and ozone. Please note the differences in concentrations and locations in the two satellite images that are just six months apart.
MOPITT aboard the NASA Terra satellite for the first view of CO spreading in the atmosphere. (Animation by NASA GSFC Scientific Visualization Studio, data from the MOPITT Team) Click here for an animation (a new window).
Image Credit: aip.org and nasa.gov.
J.R. Hahn and Wilson Ho, Physical Review Letters, 15 October 2001.