View Full Version : Electrically charged Catalytic converters
10-26-2007, 01:15 PM
Thought I may have posted this thread in the past , but can't find it, so here we go again... :rolleyes:
Suppose we had a 3-way catalytic converter. The exhaust first flows through a reducing monolith to break up NOx into nitrogen and oxygen. Then onto an oxidizing monolith to burn up CO and HC into H20 and CO2.
For the sake of this arguement let the monoliths be constructed out of stainless steel instead of the usual ceramic.
Electrically isolate each monolith from the other and the catcon casing itself.
Apply a negative charge to the reducing monolith upstream and a positive carge to the oxidizing monolith downstream.
Will the efficiency of the catcon get better, worse, or stay the same?
10-26-2007, 08:21 PM
While these reactions are oxidation / reduction reactions, the electrons or "electron holes" (for lack of a better word) are not supplied by the catalyst, they are supplied by the reactants. The reaction is already a downhill process. Raising the potential at the surface would raise the potential of both the reactants and the product so would be unlikely to have any net effect on the reaction barrier.
That said, surface chemistry reactions are complicated multistep systems. It is not a simple CO + 1/2O2 -> CO2. The first step involves one or more of the reactants adsorbing on to the surface. Once on the surface the molecules may undergo structural changes, followed by reacting to form a surface adsorbed product. Finally the product releases. There can be many intermediate steps in the reaction, but the only one that matters is the rate determining step. If the rate determining step is effected, then the reaction rate will change.
Whether or not a positive or negative voltage will increase or decrease the reaction rate is irrelevent to either reaction. In the case of oxidizing CO the CO is being oxidized, but the O2 is being reduced. Likewise for the case of reducing NOx.
10-27-2007, 12:08 PM
I felt that charging the monoliths would make certain molecules "stick" to their respective cats better than ordinary.
If, instead of a standard 2-monolith cat, a multi monolith cat were substituted...with negatively charged reducing cats and positively charged oxidizing cats placed in sequence...the exhaust stream would be ionized and pollutants would adhere to their respective cats, like the Ionic Breeze.
(yeah, I know, the IB is just a joke BUT the way it works DOES work in hospital ORs and electronic Clean Rooms).
10-27-2007, 10:52 PM
I'm not familiar with some of the terminology you are using. I am not certain exactly how air ionisers work. I am relatively (I'm no nobel prize winner ;) psst ... search nobel prize chemistry 2007) familiar with the chemistry you are playing with.
As I pointed out before, why do you suppose that the "reducing" cats would be best if they were negative. They are called "reducing" cats because the nitrogen is being reduced, but what do you think is happening to the oxygen? The oxygen is being oxidized.
The same applies to the "oxidizing" cats. The carbon is being oxidized, but the oxygen is being reduced from zero to minus two. (You'll notice I fret at naming oxidation states to these compounds. More on oxidation states later.)
Also, if your "reducing" cat is negatively charged, it is going to be producing negatively charged molecules. Will these negatively charged molecules be attracted to or repelled from the negatively charged catalyst?
On oxidation states: While many chemists, and even respected professors, forget this, oxidation state is a formality. It is a useful formality that can help predict reactivity, but none-the-less it doesn't exist except in a vacuum. When you are talking about the oxidation of CO into CO2, you are talking about the formal oxidation of carbon two to carbon four in most peoples minds, but have you ever tried to draw the lewis structure (another formality) of CO? Oxidation states depend completely on how we define the bonds between the atoms in a molecule. These atoms do not actually posses the charges that you are assigning them. An atom with a plus four charge doesn't exist anyplace except a vacuum.
You are defining the "reducing" catalyst as the "reducing" catalyst because you have decided that the formal oxidation state of the nitrogen is what is important. If you looked at the formal oxidation state of the oxygen, the same catalyst would be called the "oxidizing" catalyst.
10-28-2007, 01:04 AM
I'm certainly no chemist. I park cars for a living. :(
I DO remember that chemical bonding and catalytic reactions are the domain of electromagnetizm, and assumed that an electric charge on the catalytic substrates may have some influence on the efficiency of the reaction.
If a catcon could be made to clean up more NOx, for example, we could have better lean-burn engines.
Also, if the catcon could adsorb and burn up soot better, more diesel cars could be marketed in the US.
10-28-2007, 09:52 AM
I'm certainly no chemist. I park cars for a living. :(
And I'm not terribly familiar with how these air ionisers work. Charged particles could poison the catalyst though. The catalysts are designed to work on CO and O2 not CO+ and O2+ so it may make the problem worse.
You did get one thing right though. Generally the rate limiting step in these reactions is adsorption of the substrates, so increasing contact with the substrate should improve performance.
I DO remember that chemical bonding and catalytic reactions are the domain of electromagnetizm,
This is wrong. Catalytic converters work without any electric charge. All the catalyst does is lower the activation barrier for a reaction that is already favorable.
Gerhardt Ertl is this years winner of the Nobel Prize in Chemistry. The catalytic converter pretty much was his research. With the amount of research and engineering that has gone into this device it would be suprising if something like this hasn't been considered.
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