chemical vignette/ Beauty Is Only Skin Deep Ever think of going into the gold-brick business? Just take a big hunk of gold and a hacksaw and you've got a goodlooking hrick with a nice heft. Unfortunately, one such brick and you're talking $140-150,000! There's no room for markup. But suppose you get an ordinary hrick (wholesale in South Jersey, 17P) and just coat the surface with gold-the cost will come down alot. And you'll have a beautiful brickwell, at least "skin-deep." So how much would such asurface coating cost? For openers, put a one-atom-thick layer of gold atoms over the entire surface of the brick. Let's see, 2 inches by 4 inches by 8 inches-gold a t $320 an ounce-one atom thick-that'll be . . . 0.36 worth of gold. Wow! There, we've got an attractive product a t a total material cost of 17.36 (not including packaging). That's pretty impressive. I t means that the outermost layer (the surface) of a $150,000 piece of gold involves so few atoms that they would cost less than a cent. Yet that miniscule fraction of atoms on the surface of a piece of metal controls the chemistry of that piece. For instance, these surface atoms are the ones that determine whether the metal surface acts as a catalyst or not. And catalysts account, one way or another, for about 20 percent of our gross national product. So what is acatalyst? It's a chemical substance that speeds up a chemical reaction without itself getting into the act (i.e., it is not consumed while doing its thing). A solid catalyst merely furnishes its surface as a meetine".lace for easeous molecules. For instance, when a molecule of methanol lands on a rhodium catalyst surface, it usually sticks for a while (becomes adsorbed). Now, if a carbon monoxide molecule happens to arrive, zingo, i t reacts with an adsorbedmethanol molecule and they leave the surface as acetic acid. When methanol and carbon monoxide meet in the gas phase, they won't even give each other the time of day. But because of the special environment provided by that thin layer of surface atoms on the rhodium catalyst, methanol snd carbon monoxide react so rapidly that 500,000 tons of commercial acetic acid are made every year this way! This kind of speed-
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up might he anywhere from a thousand-fold to a million-fold when things are working. Because of such successes, chemists care a lot about how these catalytic gold bricks do their job. What actually happens to that thin layer of adsorbed molecules as they come and go on a catalytic metal surface? Unfortunately, that's where the skin-deep principle works against us. If there isn't much on the surface, there isn't much to see. But nowadays, we have several powerful instruments with which we can learn about the special properties of the skin of a metal. These instruments also let us watch molecules as they lodge on the surfaces of catalysts like platinum and rhodium and many others. We can see how the molecules are chemically changed by the metallic skin to make them more reactive when a suitable reaction partner comes along. So chemists are beginning t o understand how to design these catalytic gold bricks to do whatever we want. Right now, every gallon of your gasoline began as a bunch of molecules sure to make your engine knock and then some chemist catalytically converted them into other molecules that make your engine purr. But now we are looking ahead to new energy feedstocks with more sulfur and metallic contaminants that will require much better catalysts so that we can keep your engine purring and the air clean a t the same time. We'll do it by learning how those catalytic gold bricks work so we can tailor them to our needs. This is a case where skindeep beauty really pays off! These short vignettes are taken t o m "Oppammities In Chemistry", copyright 1985 by the National Academy of Sciences and r e p m duced with its permls3on. This comprehensive report on future directions and goals for chemical research contains much informationboth on chemistry and manpower and education that is of use to the teacher: these vignettes are being reproduced because of their obvious wide utility in the dassroorn. The full repan can be purchased for $28.50 clothbound and $18.50 paperbound hom: National Academy Press. 2101 Constitution Avenue, NW, Washington. DC 20418. A quantity discount of 15% for 5-24 capiesand 25% for 25-499 copies is available: all orders must be prepaid.
Volume 63
Number 12
December 1986
1037
