Catalyst

I've always heard the term catalyst and wondered how they work. The most common term people know is probably catalytic converter (a thing in your car that improves emissions) which uses Platinum and/or Palladium as catalysts. People used to steal them to try to extract the precious metals. Maybe they still do.

I generally understood what a catalyst is, but wanted to experiment and see them in action. I thought I would pick the very well-known and widely understood metal oxide added to hydrogen peroxide to speed its decomposition into water and oxygen.

    2H2O2 -> 2H2O + O2

It turns out that it is very well known, especially with MnO2, but remains an active area of research for the exact series of reactions involved. The best-looking answer I found on the internet initially for MnO2 was completely wrong as it left MnO as a product. A true catalyst speeds the reaction but is unchanged; it is not modified by the completed process even thought it may be temporarily modified during the reaction. You should be able to reuse the catalyst. Luckily, I don't have to take my car down to Jiffy Lube for a refill of Platinum.

For kicks, I had some Fe2O3 (hematite) laying around, and decided to bake-off the natural decomposition of H2O2 vs. with MnO2 vs. with FeO3. I had done no research on hematite as a catalyst for the reaction - I just thought it might work. In retrospect that was probably silly as Fe2O3 is abundant and cheaper than MnO2, so I would probably have found more on iron oxides as a catalyst than manganese oxides initially if it worked just as well.

OK, so I dumped some hydrogen peroxide into two beakers. Not much happened. There was an occasional bubble. Not enough for me to have even thought a reaction was occurring, but sloooowly. Then I dumped some MnO2 into one and it went crazy making oxygen gas. The beaker also became very warm. This is clearly an exothermic reaction! Next, I dumped a bit of red iron oxide into the control beaker. I didn't observe much. There was some bubbling, but I'd have to compare to video to try to determine if it was any faster than before.   

What is going on with the MnO2 reactions?? I was a bit disappointed to find no good reference on this on the internet. The ones I did find were vague or detailed but clearly wrong. A catalyst creates a series of reactions that end up with the catalyst unchanged, the same products (water and oxygen here), but has a lower activation energy so that the reaction can be done faster with the same energy or with less energy. In my case, I was adding no energy to the system beyond room temperature (which is what the hydrogen peroxide was stored at in the first place). I expected the rate of reaction to change.

Here is a handy artwork I made to show the H2O2 reactant, it's formation enthalpy, the normal activation energy, and the products with the exothermic release of energy (delta H) calculated from the formation enthalpy of liquid water. Since the activation energy with MnO2 is known, I charted that as well and tried to approximate a couple of reactions along the way in a second line on the diagram. But later I read that there could be five reactions involved.

After trying much math and good searches, I did feel better to finally find a recent academic paper which states that the exact series of reactions is unknown and proposes five reactions as a possible solution. When you are having trouble finding a solution, it is always nice to know what you thought should be easily discoverable is actually unknown to science. I feel less stupid. One thing I so know is the "slowest" of that series of reactions must have an activation energy of 58KJ/mol to match overall lab results for the catalyst. But I'm not going to repeat the research in detail here.

So what about my iron oxide (with Fe(III) ions instead of Mn(IV) ions)? Further research uncovered that this is also being actively studying for potential performance in different environments (temperature, pH level, etc.). It does seem that the effectiveness of iron oxide is not nearly as good as that of MnO2. I noticed what appears to be some bubbling at a greater rate than before, but nothing worth writing home about.

I once visited a vast field of manganese oxides in New Mexico. I have been to mines in California and other places. The ocean floor is littered with manganese oxide nodules. I would love to have collected some of these natural minerals (pyrolusite - MnO2 - among others) and try them out. But these things are mostly just ugly black and so I didn't bring much back or keep track of it. Maybe some day I'll run to the Quick-E-Mart and buy some hydrogen peroxide if in the areas again. Like taking ore to the coal mine, or Mohammad to the mountain, it seems that taking a bottle of liquid to the ore field is the easier task. I would pour it on the black ground and it should fizz wildly. In the process I water any plants and add oxygen to the New Mexico sky.

Thanks for reading,

Paul