Surface Catalysts

In simplest terms - surface catalyst force orientation.

Said more correctly, but not as simply:

Surface catalysts work by increasing the percentage of 

collisions that have correct orientation. 

In other words, if 10% of collisions have correct orientation initially and, by adding something, I can increase that to 30% , the rate will triple (assuming that all other conditions stay the same).

The question, of course, is how does that work? We will use two examples to explain

Catalytic Converters

In the engine of a car, oxygen and gasoline are mixed, ignited and then shoved out into the exhaust system. This happens incredibly quickly, so quickly in fact, that there is often not enough time for the reaction to go to completion.

This can result in production of carbon monoxide, a toxic gas.

(There are also nitrogen compounds formed and un-burned fuel residue, but we can understand the working of a surface catalyst just focusing on CO.)

This carbon monoxide moves through the tailpipe along with un-reacted oxygen.

These molecules bump together and are certainly hot enough to react, but most collisions that occur do not result in a further reaction (producing carbon dioxide) because correctly oriented collisions are extremely rare.

In order to deal with this problem, cars have a catalytic converter installed as part of their exhaust system.



The catalytic converter contains 2 ceramic blocks. These blocks are not solid, rather they are filled with "micro-ducts" - little passages or tubes that the gases can move through.




Since the passages are so skinny, we know that the gas particles moving through them will collide with the walls a great deal. Because there are so many of these passages for the gases to move through, the total surface area is huge - roughly the area of a football field.


These passages are coated with a mixture of metals - specifically platinum, palladium and rhodium. Remember that metals (like everything else) are made of atoms, so that a thin, seemingly-smooth layer of metal is actually quite bumpy at the atomic scale.

These particular metals (although QUITE expensive) were chosen because the spacing of the bumps in these metal surfaces matches the spacing of the atoms in the oxygen molecule and in CO quite well. So, when a CO or O2 bumps into the surface it may stick.

That means that the atoms are held in place near each other. This means that correct orientation is MUCH more likely.


Carbon dioxide does not fit into the metal surface as well, so the product is released leaving the metal surface unchanged and able to help more reactions occur.

This is the classic demonstration of a catalyst: it helps a reaction occur, but is unchanged itself in the process.

In this way, the catalytic converter (and the catalysts it contains—the Pt, Pd and Rh) help to produce more carbon dioxide and less carbon monoxide, making the exhaust fumes less toxic. (At the same time the catalytic converter also breaks down nitrogen oxides into nitrogen and helps "finish" burning any un-burnt fuel residue that escapes the engine.)

As a side note, the introduction of the catalytic converter is one reason that leaded fuels were phased out. Certain lead compounds had been added to gasoline to make them burn more efficiently, but it was found that the lead coated the surfaces in the catalytic converter rendering them useless.

All gasoline sold in the United States are now unleaded, but for a number of years, the choices at the gas station were not regular and premium, but rather regular or unleaded.

Enzymes

If you have already taken biology, you may remember the "Lock and Key Theory of Enzymatic Action." This was the idea that your body contains enzymes (certain proteins) that have "active sites" where molecules that need to react fit like keys in a lock.

This "fitting" of molecules puts them in proper position in order react.