Originally Posted By: Jim Allen
Originally Posted By: JHZR2
phosphorous and zinc can poison the catalyst. Clog is a term used incorrectly.
You need to understand heterogeneous catalysis first - chemical reactions that are facilitated by some kind of active surface (thus heterogeneous, as opposed to homogeneous where there is some kind of other propagation method for driving the reaction). So there are LOTS of surface sites that could possibly be used to catalyze a chemical reaction. Different sites may have differing accessibility, due to porosity and placement of the active component. Typically the active component, like a precious metal in a catalytic converter is dispersed and "supported" on a "support", like the corderite honeycomb inside the converter.
So you have these sites, some are inside pores of the support, some are on top. How well the reaction occurs depends upon how active the catalyst is, and how easy it is for compounds you are reacting to reach the catalytic sites.
So you can do two things - (1) chemically poison sites, through compounds that create non-catalytic versions of the active metal. Things like sulfides of platinum. Chemicals like sulfur and phosphorous are good at poisoning catalytic sites. Or, you can do (2) clog or cover the sites that are present so that they are no longer accessible for reaction. This could be due to a lot of oil, soot or other things inside the converter.
#1 is not reversible unless you can reduce the catalyst to bare metal at VERY high heat in hydrogen.
#2 some people claim to be able to fix to some extent by either washing with soap (degrease) or burning some compound (like lacquer thinner) that makes the car spit out lots of HCs, heat up the converter and try to burn it off. Both are risky because additional chemicals can create other non-active variants of the precious metals, like chlorides and phosphates, and excess temperature can either oxidize or sinter the metals, as well as crack the support, causing its efficacy to be poor.
Metallic adds in oils, if the oil is burned and sent through, can plug and coat catalyst sites and pores where active sites ma be present. Just like soot or hydrocarbons. But generally the concern is about poisoning.
You only theoretically need x catalyst to do the cleanup of a three-way converter. However there is an assumption of burned oil, life degradation, growth and agglomeration of catalyst particles, etc., so the converter is loaded with x+y amount of catalyst so that it will perform through warranty. THis is part of the reason why EPA/CARB approved converters cost more, since they have more metal... and why the $50 converters from mineke barely allow you to pass when they are brand new (poorly made catalyst, with minimal loaded content into the converter).
Hope this helps.
Great, complete answer! Enjoyed reading it.
