Atlanta (September 12, 2005) — Researchers at the GeorgiaInstitute of Technology have uncovered important evidence that explainshow water, usually an inhibitor of catalytic reactions, can sometimespromote them. The findings could lead to fewer constraints on reactionconditions potentially leading to the development of lower costtechniques for certain industrially important catalytic reactions. Theresults appear in the September 6, 2005 issue of Physical ReviewLetters.
“Normally, in most catalytic reactions, water can stopthe reaction. It kills the catalyst,” said Uzi Landman, director of theCenter for Computational Materials Science, Regents’ and Instituteprofessor and Callaway chair of physics at Georgia Tech.
Andthat’s a big problem because ensuring that a reaction is water-free canadd to production costs. Many catalytic reactions occur at hightemperatures, which evaporates the water, said Landman. “However, anytime that the reaction temperature is lowered and there’s humidityunfavorable effects may occur. You hope that when you heat the reactionup that the adsorbed water will come off, but sometimes it doesn’t.Sometimes the adsorption of water leads to an irreversiblemodification, such as oxidation, and deactivation of the catalyst. It’spoison; it poisons the catalyst,” he said.
In the late 1980's,Japanese scientist Masatake Haruta discovered that small particles ofgold (which is chemically inert in bulk form and normally not acatalyst) are chemically very reactive. He also found that water canpromote this catalytic activity.
Since the late 1990's, Landman’sgroup has been using advanced quantum mechanical computational methodsto investigate how and why nanoclusters of gold act as chemicalcatalysts under dry conditions. This led to certain predictions thatwere verified experimentally by Ulrich Heiz’s group, who is now at theTechnical University of Munich.
Earlier this year, the two groupsco-authored a paper in the journal Science. It showed theoretical andexperimental evidence of the role of charging on the catalytic activityof gold nanoclusters made of eight atoms when they are bonded tonaturally occurring oxygen vacancy defects on a magnesia surface thatsupports the gold. In the recent Physical Review Letters paper, theGeorgia Tech group has made theoretical predictions on how a singlewater molecule can catalytically enhance a low-temperature reactionthat turns carbon monoxide into carbon dioxide.
Using computersimulations, Landman and post doctoral fellow Angelo Bongiorno, foundthat the water molecule enhances the binding of an oxygen molecule toan eight atom gold nanocluster, either free or supported on anundefective magnesia substrate. The water molecule catalyticallyactivates the aforementioned oxidation reaction of carbon monoxide. Inthe earlier studies on gold nanoclusters, defects in the supportsurface were required to give the gold a slight negative charge. Inthis latest study, the presence of a water molecule makes thatrequirement unnecessary.
Here’s how it works: the structure ofthe water molecule, H-O-H, is such that the end with the oxygen atomhas a slight negative charge, while the two hydrogen atoms arepositively charged. In the quantum molecular dynamics simulation, thenegatively charged oxygen side of the water molecule bonds to one ofthe gold atoms, leaving the positively charged hydrogens of the watermolecule dangling. Subsequently, an oxygen molecule (made of two oxygenatoms) binds favorably to a neighboring gold atom of the cluster andgets a slight negative charge in the process.
This results in anadsorbed slightly negatively charged oxygen molecule near one of thepositively charged hydrogen atoms of the adsorbed water molecule.Since, in chemistry, (as in love) opposites attract, the two gettogether. So the oxygen pulls a proton (a positively charged hydrogen)from the water molecule resulting in formation of a hydroperoxyl (OOH)group and a hydroxyl (OH).
Now, this relationship can’t lastbecause the addition of the hydrogen to the oxygen molecule to form OOHweakens the bond between the two oxygen atoms. All it takes to breakthat bond is a carbon monoxide molecule approaching from the gas phase,which bonds to one of the oxygens of the OOH to form carbon dioxide.This leaves the proton to return to the hydroxyl to reform the watermolecule. The product carbon dioxide desorbes readily from the surface,and the left over oxygen atom stays bonded to the gold. But this singleoxygen atom is very active (as singles often are) and is easily ledaway when another carbon monoxide comes along to bond with it to make asecond carbon dioxide molecule.
“This reaction opens the door toa completely new idea; that polar molecules, like water, or moleculesthat are good proton donors may show us new channels of reactivity,”said Landman. “We may be able to take other catalytic reactions and usewater as a promoter under some selective conditions,” added Bongiorno.
“Inthe future, we want to test the effect of multiple water molecules tosee if there is a limit to how many water molecules can enhancereactions. In this case, we used magnesium oxide as a substrate. We’dlike to know if the effect limited to that substrate or will it workwith others?,” the two researchers said.
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