EGEE 101
Energy and the Environment

Catalytic Converters & Sulphur

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an open catalytic converter.
This catalytic converter, used to reduce NOx emissions at power plants, is nevertheless very similar to the one in your vehicle in terms of shape, basic components, and functionality. Engine gases enter one end, pass through a number of catalysts, and emerge from the other end.
Credit: Sandia Nat'l Labs

So far, we have looked at efficiencies, conservation, changing the fuel, and now we have come to the treatment(s) of the products of combustion before they leave the tailpipe. Therefore, in a way we have covered abatement: before you get in the car, reduction strategies while driving, and now hot-gas cleanup while the engine is running. Recall that we have already discussed the issue with lead, now we need to discuss the catalytic converter.

We are required by law to have catalytic converters on our gasoline cars. We do not have them (yet) on diesel engines. They do "rob" the vehicle of a few horses (horsepower) and so were not particularly popular when first introduced but there are fines and penalties for those that disable their catalytic converter, so don't do it! The job of the catalytic converter is to lower the emissions of three gases: CO, NOx, and hydrocarbons (uncombusted fuel-coming out of the tailpipe, fuel escapes from other locations in the car too).

Step 1: CO → CO2 (Oxidation)

Step 2: NOx → N2 (Reduction)

Step 3: Hydrocarbons → CO2 and H2O (Oxidation)

The ceramic monolith which is the honey-comb like insides of a catalytic converter.
This ceramic monolith, which would be found inside the catalytic converter, has many channels, so there is a high surface area for the catalysts to be exposed to the hot products of combustion. They tend to be situated either close to the muffler or underneath the passenger seat to be closer to the engine (quicker warm-up).
Credit: ORNL

Therefore, we have a system that can oxidize and reduce at the same time! It is a tall order, but we can do it because we use three catalysts and we operate the vehicle so the air to fuel ratio produces the correct oxygen concentrations in the flue gas. That is why we have an oxygen sensor in the car (anyone had it replaced?) We are required to have monitoring equipment so that we know the pollution control devices are working. In the more polluted parts of the country, the standard vehicle inspection also requires an emission inspection, where they monitor the emissions from the tailpipe.

The three catalysts that are used are Rhodium (Rh), Platinum (Pt), and Palladium (Pd). These are very expensive metals and so we use as little catalyst as possible. As only the surface of the catalyst is used to oxidize (or reduce) the gases, we spread it very thinly (just like marmite) (text version).

We are trying to achieve a high active surface area so the inside of the catalytic converter has a ceramic honeycomb-type series of channels. The actual surface is about a couple of American football fields. The catalysts are well dispersed throughout the channels.

In this diesel engine, you can see how the fuel injection process produces a fine spray that enhances the mixing of the liquid (now atomized into small droplets) and the oxygen from the air. Nitrogen from the air is also present and an unwanted side reaction is the formation of NOx. Some of the nitrogen will also be present in the fuel as well.

An up-close-and-personal view, via high-speed digital camera, looking up into the piston housing of a diesel fuel injector, without the piston and combustion process.
Credit: Jim Szybist (The Energy Institute)
Click for the transcript.

[Video opens with a close-up view of a diesel engine. The injector is spraying fuel.] Dr. Mathews: This is what the diesel injection process looks like. This is the top of the cylinder head. You can see the various sprays coming in the very fine atomization to enable the mixing with the air. And then if there was a cylinder in place, it would come up, compress the gases, and self-ignite the system. What you are seeing here is a number of injections of the mist. [Video ends]

Chemistry of the Process

Graphs of the endothermic and exothermic energy diagrams. Described below.
Examples of endothermic and exothermic energy diagrams
Credit: JPM

In the energy diagrams above you can see that when compounds or elements react, the process is either endothermic (requires energy) or exothermic (produces energy). This is a result of the conservation of energy (First Law of thermodynamics) that we encountered in earlier lessons (remember?). A common assumption is that the larger the energy gap, the quicker the reaction - but this is not the case. This delves into the realm of kinetics: the rates at which chemical reaction occurs. A reaction might be thermodynamically favorable, but the kinetics might be very slow (or very rapid). So often, the kinetics will control whether we observe a reaction or not. We have seen the equilibrium symbol already (insert equilibrium symbol) what this actually means is that the forward reaction is equal to the reverse reaction. So in the case of CO2 in the atmosphere being in equilibrium with the CO2 in the oceans, it does not mean there is no exchange taking place, that occurs all the time, it is just that as many molecules enter the atmosphere from the ocean as enter the ocean from the atmosphere.

Graph of the enothermic level rising with the activation energy. Described in text.

Often there is a special excited state of the molecule or compound that needs to form before the reactions can occur. This requires that the molecule reaches an activation energy before it can complete the reaction. A catalyst works by offering an alternative route to achieving the excited state. So at the same temperature more of the gas molecules can achieve the excited energy state and so the reaction proceeds at a quicker pace. We could achieve the same results by increasing the temperature and the pressure but adding a catalyst is the lower-cost option. So, a catalyst is any material that changes the activation energy of a reaction. It can be used to slow reactions down, but mostly we use catalysts to speed reactions up.

Why does increasing the temperature increase the rate?

Graph of the endothermic level rising with the activation energy with catalyst. Described in text.

We use catalysts to help chemical reactions and transformations in the chemical industry and the petrochemical industry all the time. We also have very large devices that are similar to catalytic converters to power stations to reduce NOx emissions. So catalysts are very useful in controlling emissions. More in Unit III.

Examples of some of our initial "live snippets" efforts can be viewed by clicking below the following images.

link to animation about the endothermic and endothermic reactions
Click for an explanation (1:53) of Endothermic vs. Exothermic.
Credit: JPM
link to an animation about catalytic converters
Click for an explanation (2:15) of Catalytic Converters.

The problem with Sulphur (S)

In coal, the percentage of S is between 0 and a few percent (by weight). Thus, when we burn coal, there are lbs of Sulphur dioxide (SO2) released for every million Btu's of thermal energy. We will discuss crude oil in the next lesson and will discover that like coal, the quality (and hence the value), of the oil is influenced by the S content. But the S in gasoline, at 300 PPM, is very low already. We have to remove the S from the compounds in the oil ($$$) because S is a catalyst poison. Much like the CO and hemoglobin example, S will bond with the catalyst and that portion of the catalyst surface will no longer function. We are in the process of reducing S content even lower (as California has already done) so that new catalyst technologies can be employed.

S emissions also contribute to regional haze, and to acid deposition. More in Unit III.