Greetings folks! Total noob (and total chemistry layman) here, so apologies in advance for anything I screw up, and also for my lack of brevity, but this will unfortunately take a bit. Also apologies for not being able to make the subscript and italic functions work correctly.
I live in CA & drive an "older" vehicle (technically a 'classic'). CA has strict emissions requirements, which (on older vehicles) are tested bi-annually (& MUST be passed) prior to registration renewal. Smog test was due in March and it passed, but with the lowest possible score on one of the tests. In the months immediately preceding my smog test, my car's dreaded Check Engine Light (CEL) came on a few times intermittently; it has never otherwise come during the whole time I've owned this car. It has also re-illuminated a few times since the test. It ain't going away and it ain't going to fix itself.
So this is war! I’ve always wanted to better understand the auto emissions processes because knowledge is power and before some mechanic says, "Bro, you need new cat; 2 G’s My Friend," I want a second opinion, to understand the process, and what my options are going to be. Since the D-minus-minus smog score, I’ve set about to learning everything I can about emissions, how my car makes them, why it’s making them at the levels it’s currently making them @, catalytic converters, what causes them to fail, what can be done to improve my car’s emission output, how can I make it burn its fuel and air more efficiently, what are the ways a cat can go bad, and what can be done with a “BadCat” throwing a P0420 code.
If you don’t know, generally speaking, when your car’s engine experiences a problem, mishap et. al., its engine management computer (usually ECM) will signal a “code” which pertains to that specific mishap and thus illuminate the dreaded CEL. Plugging a scanner into your vehicle’s OBDII port, will tell the scanner what code (or problem) the engine is having. In my case, the code is P0420 which, when you look that up, means that my engine’s "Catalyst Is Operating Below It’s Intended Efficiency Threshold."
When you Google how to fix this, you get the phonebook. Could be a bad O2 sensor; could be a bad cat; could be a vacuum leak; could be a bad MAF sensor; could be a dirty throttle body; running too lean; too rich; clogged fuel injectors, leaky fuel injectors, & on & on & on… (If one is not careful, a situation such as this can easily become what some “car guys” call a 'parts cannon': one just keeps throwing replacement parts at a problem until something fixes it, without ever having correctly determined where the real problem(s) is/are.) So I give my trusty neighborhood mechanic 2 G’s to replace my dead cat, but whatever killed it may still be out there. Treating the symptom may alleviate the condition temporarily, but underlying problem(s) may in fact still be present, and my hypothetical new cat (& my 2 G’s) would then be next up on the proverbial chopping block.
It could just be that my car’s cat is 25 years old and things simply wear out after while, or it could be mechanically dead, (substrate has melted or broken apart) in which case I will have to pony up the 2 G’s, or it could simply be “chemically dead”, in which case (as you will shortly find out if you’re patient enough to continue reading) it may be able to be “chemically regenerated”.
In my quest to gain as much knowledge about the subject as I can, (it’s ongoing) I discovered research being conducted by some chemistry professors in Europe (1 in Sweden, 2 in Cyprus & 1 in Spain, although the Spaniard has recently passed away) with regard to chemically treating “heavily used” catalytic converter substrates (their internal ‘active ingredient’) in order to regenerate the catalyzing performance: essentially de-poisoning a chemically poisoned (i.e.: non-functioning, but otherwise intact) converter. And this research (though conducted scientifically in lab conditions (per the papers I read)) was ultimately geared toward performing this chemical regeneration in situ: that is, on actual practical catalytic converters that are presently part of an operational vehicle’s exhaust system. Wow! Fascinating! You have my complete attention!
I am not sure how to post / append .pdf files – if someone knows how to, please lmk. I had to jump through some gate-keeping hoops in order to download and read the full papers, but I have 3 of them locally on my own computer.
If interested, you can search for “Reactivation of an Aged Commercial Three-Way Catalyst by Oxalic and Citric Acid Washing” (2006), “Reactivation of severely aged commercial three-way catalysts by washing with weak EDTA and oxalic acid solutions” (2007), and “Regeneration of Aged Commercial Three-Way Catalysts Using CI-Containing Reagents” (2013). But I think all you will be able to read without first signing up are the abstracts / teasers from each.
I will try to link the papers I read here:
So what did these professors / researchers do and how did they do it? As mentioned, they did their work in labs, not in auto repair shops, so there’s going to be significant differences between the researcher’s applications and my own, and therein will be the chemistry questions I will ultimately come to in this post. But the results of their experiments are undeniable and astounding!
To paraphrase, they opened up severely used cats and removed the substrates. From the substrates, they cut out cubes of about 2 cm x 2cm x 2cm – sample pieces. They placed each of these “cubes” into a kind of laboratory “flow tube” enabling a solution to pass into the tube, over and thru the substrate cube and then back out of the tube. This was plumbed into a system consisting of a peristaltic (roller) pump, a heater, a supply vessel containing a 0.1 molar acid wash of (in the first case) EDTA (Ethylenediaminetetraacetic) acid solution, and (in the second case) a 0.1 molar Oxalic acid solution.
Their most dramatic results involved heating EDTA solution to 50º C and then flowing it at a rate of 600 ml / minute for 60 minutes. After which, the cubes were washed with water for 30 minutes at 600 ml / minute and then the second solution (also heated to 50º C) was flowed, again for 60 more minutes at 600 ml / minute. After the second solution flow completed, the cubes were again washed with water for 30 minutes at 600ml / minute and then thoroughly dried. It should be noted that the researchers flowed their solutions over the substrate cubes in the opposite direction of the normal flow of exhaust; but none of the papers I read explained why.
Before and after testing with real world exhaust gasses revealed astounding reductions in the amount of elements and compounds that had been “chemically poisoning” (& thus rendering inactive) the substrate cubes, the presence of which (prior to the acid washes) had rendered the Platinum, Palladium & Rhodium coatings of the substrates ineffective at both conversion and oxygen retention. These ‘poison’ reductions vis-a-vie the acid backwashes reactivated the catalyzing effect on the exhaust gasses significantly, in some cases 60%, 70%, 80%, 90% restoration (if I read correctly)!
As I understand it, a car’s exhaust gas is primarily composed of Hydro Carbons (HC) – unburned fuel, Carbon Monxide (CO) – the silent killer, & Nitrogen Oxides (NOx) – byproducts of lean fuel / air mixes, hot combustion temps, and are also smog forming particles. The way a properly functioning cat works is that its honeycomb substrate is coated with thin layers of the precious metals Pt, Pd & Rh, and when hot exhaust gasses pass in close proximity to these heated metals, the hot metals cause the hot HC, CO & NOx to transform into less harmful CO2, H2O, O2 & N2 where after most are free to exit the tailpipe. The O2 is temporarily retained within the substrate so it can be released during the formation of soon to be released (i.e.: the next batch of) CO2 & H2O.
However in a “chemically poisoned” cat, the Pt, Pd & Rh are “clogged” by layers of accumulated elements such as Ca, Cu, Fe, Pb, Mg, Ni, P, S, & Zn and are thus unable to effectively catalyze the exhaust gasses passing by them. This clogging also severely limits the ability of the substrate to temporarily “store” the O2 needed to form less harmful exhaust. How all these trace elements and the compounds composed of them accumulate in the cat and clog its substrate is a mildly interesting aside, but (in the name of one of the only bits of brevity I can afford here) not technically germane.
If I have understood correctly, the 0.1 m EDTA solution chemically removes significant quantities of the Ca, Cu, Fe, Pb, Mg, Ni, S & Zn contaminants embedded in the substrate. And the 0.1 m Oxalic solution chemically removes the P, as well as restoring the O2 retention capacity of the substrate.
I have been thinking through the "how-to" of performing these procedures, and have also been making notes because I want to attempt to backwash my own vehicle's catalytic converter in situ. And this layperson's thinking-through of a real-world application of these highly controlled laboratory procedures has prompted some basic questions. Following are notes I've made on the broad strokes of applying these procedures to a piece of exhaust pipe with a non-functioning (but likely intact) catalytic converter in the middle of it. Following those notes shall finally be my questions and the end of this post…
Catalytic Converter Two Acid Wash:
Remove cat portion of exhaust pipe then remove O2 sensors. Borescope both visible ends of cat looking for fragmentation & / or melting of substrate matrix. If any, cat is dead & this procedure will not help. However, assuming cat is structurally “intact”…
Prepare a 0.1 Molar solution of EDTA (Ethylenediaminetetraacetic acid) heated to 50º C and maintain temp while backwashing thru cat for 1 hour @ 600ml / minute. This flows a bit more than 9.5 US gallons of solution upstream thru the cat. If I have understood correctly, this procedure is supposed to chemically remove significant quantities of the Calcium, Copper, Iron, Lead, Magnesium, Nickel, Sulfur, and Zinc contaminants embedded in the substrate.
Rinse for 30 minutes @ 600 ml / minute. This will flow about 4.75 gallons of pure water over the surfaces to remove residues.
Prepare a 0.1 Molar solution of Oxalic acid heated to 50º C and maintain temp while backwashing thru cat for 1 hour @ 600 ml / minute. This also flows a bit more than 9.5 US gallons of solution upstream thru the cat. If I have understood correctly, this procedure is supposed to chemically remove significant quantities of Phosphorus contaminants embedded on the substrate as well as greatly improving the Oxygen Storage Capacities of the re-exposed, decontaminated noble metal catalysts (Palladium, Platinum, & Rhodium) embedded on the substrate’s surfaces.
Rinse for 30 minutes @ 600 ml / minute. This will flow about 4.75 gallons of pure water over the surfaces to remove residues.
Dry cat @ 120º C overnight, then an additional 2 hours @ 200º C. Install O2 sensors & reinstall assembly into exhaust system.
To do this I will need:
o New pre- & post-cat O2 sensors, studs, nuts, gaskets, exhaust pipe gaskets, penetrating oil, & appropriate tools.
o 30 Gallons of distilled water.
o Enough EDTA to make about 4.75 gallons of a 0.1 Molar solution.
o Enough Oxalic Acid to make about 4.75 gallons of a 0.1 Molar solution.
o 5-Gallon buckets that won’t melt while full of acid and while sitting on a heating plate @ 50º C for an hour.
o A heating plate able to heat and maintain a 5-gallon bucket full of acid sitting on it for an hour at 50º C.
o Bore-scope, thermometer & stir sticks.
o A peristaltic (roller) pump able to deliver a 600 ml / minute flow rate for 3 hours.
o Enough acid-resistant, pump-compatible flexible tubing to get from solution reservoir to pump, thru exhaust pipe, to cat.
o Miscellaneous rigging items to keep cat-portion of my exhaust assembly suspended in a 5-gallon bucket.
o White Vinegar (as neutralizing agent for after procedure and maybe as part of solution disposal).
Should I pre-wash or pre-soak cat to soften or remove any soot, grease, oils or other “dirt” prior to performing the chemical un-poisoning procedures? If yes, any advice? In the absence of advice, I would likely default to using a warm overnight bath (or perhaps backflow) of soapy dishwashing (Dawn) water or degreaser (Simple Green), followed by a thorough post-wash rinse.
Per the papers, the researchers experimented on portions of cat substrates of approximately 2 cm3 in enclosed tubes for their tests, and they were flowing 600 ml / minute solutions over relatively small 2 cm3 pieces of substrate placed inside lab flow tubes. So, do I need to seriously upscale my flow volume since I’m decontaminating the whole cat in situ, versus just little pieces of extracted substrate placed in lab flow tubes? I don’t know how to even derive the answer, but if it’s yes, I’m definitely going to require learned help to up-scale my flow conversions.
Are 5-gallon buckets made of HDPE (High Density Polyethylene) okay for mixing, heating & as catch basins for these solutions?
Is it acceptable to recycle these solutions back thru cat during these processes? Or should only “fresh” solution back-flow over cat?
The research papers list silicon as one of the contaminants these washes remove from the cat. It was surmised that the silicon got into the cat due to some silicone engine seal & gasket materials slowly degrading then making their ways thru combustion & out the exhaust stream. Peristaltic pumps are often plumbed with silicone hoses because of their flexibility and pliability. Is it OK to “plumb” my acid wash flow system with the silicone tubing used by this kind of pump? Or will the acid degrade the silicone tubing? If the answer is yes, what then would be a more appropriate tubing for pumping acid thru a ‘roller’ pump? Which acid solution removes the cat’s silicon contaminants?
Why were these washes flowed backwards thru the substrate, as opposed to the direction the exhaust would normally flow?
Since these procedures will be performed in situ, what can I expect regarding the solutions contacting the interior surface of my exhaust pipe?
Is white vinegar an acceptable neutralizer? If so, how much should I keep on hand? If not, what is an acceptable neutralizer (and how much of that should I have)?
Please characterize 0.1 Molar solutions of EDTA & Oxalic acids so as to give a layperson perspective of their relative corrosive strengths and better understand the dangers of what they’re dealing with.
Thank you all so very kindly for your time reading this post all the way thru, and also in advance for any additional outlay you may choose to spend responding – please be gentle with me; I’m neither a chemist nor an auto mechanic, but I love my old 4Runner and if I can successfully regenerate its cat instead of just buying a new one, it’ll make for one helluva good story, not to mention some immense personal satisfaction!!