Under normal, and at times not so normal, driving conditions your gasoline engine can vary from operating rich (less air than fuel) or lean (more air than fuel). When you accelerate or lift the throttle, the on-board diagnostics attempt to make corrections to compensate for these changes in the air/fuel ratio, but the adjustments are not necessarily fast enough to keep everything at the correct stoichiometric ratio. Situations where the engine runs rich can result in excess hydrocarbon emissions and where it runs lean, the engine can produce NOx. This is where we rely on modern catalytic convertor systems (and some exhaust recycling) to compensate for the changes in driving conditions to keep emissions as low as possible.
A key component to your catalytic convertor is a material that stores oxygen under lean conditions and releases it under rich. This helps balance the fluctuations in air/fuel ratio so the catalytic convertor operates at peak efficiency with respect to destruction of pollutants.
These materials are generally referred to as CZOs which is an acronym for a group of cerium zirconates. They have some really interesting oxygen storage / oxygen transport properties that when used with platinum group metals (PGMs) make them particularly useful for automotive applications – for the reasons I described above.
At a molecular level, the cerium cation in these oxides can oscillate between valence states of +4 and +3 depending on whether they reside in an oxidizing (lean) or reducing (rich) environment. Simplistically, the reaction can be described as: 2 Ce2Zr2O8 = 2 Ce2Zr2O7+ O2 As the cerium is reduced from +4 to +3, charge balance is maintained by the release of one mole of oxygen, just what we need when the engine is running momentarily rich, i.e. ensuring there is sufficient oxygen available within the catalytic convertor to fully oxidize the excess hydrocarbons.
At PCC, we are modifying non-PGM CZOs with two objectives in mind: (1) increase the oxygen storage capacity (OSC) of the CZOs in a way that exposes more of the cerium cations within the oxide and (2) reduce the temperature at which the cerium can be reduced. The first objective is related to surface area and the degree of crystallinity within the CZO and the second objective relates to modifying the surface free energy in a way that lowers barriers to cerium reduction.
Without getting into a lot of detail, we want the release of storage oxygen to happen very quickly and we want it to happen at a low temperature – like when your car first starts up and the catalytic convertor is cold. The first two minutes of a cold start of your car over its life, accounts for more than 70% of the total pollutants released into the atmosphere. So, the sooner your catalytic convertor becomes active the lower the total emissions.
Below I show the temperature programmed reduction (TPR) of a CZO where the ratio of Ce/Zr = 1. I include three examples, including one prepared by us (“PCC CZO”) and comparing it to commercially available CZOs (“Company 1 CZO”, and “Company 2 CZO”), that show a broad cerium reduction temperature centered around 600°C. We run 5 TPRs from ambient to 900°C with a 900°C oxidation step in between to ensure reproducibility and thermal stability. What I show below is the fifth scan which is consistent with previous scans, so we know these materials are thermal stable to 900°C under both oxidizing and reducing conditions.
The amount of cerium reduced can be calculated by integrating the area under the peak and comparing that to a theoretical value based on complete reduction of Ce2Zr2O8(from Ce+4to Ce+3). This value represents the total available oxygen storage capacity.
One of the interesting things about these oxides is we can substitute other cations in small amounts for either cerium or zirconium as long as we maintain charge balance within the oxide structure. There are some constraints on the size of the cations we substitute, but there are a fair number from which to choose.
Below is an example where we substitute a small amount of a secondary cation for cerium (“PCC CZO Promoted”). In this case we are trying to create nearest neighbor effects that influence the reduction temperature of cerium. We achieved the desired temperature reduction relative to our standard material, but at the expense of the fraction of Ce reduced. This is expected because the cation we chose to substitute is non-reducible. The narrower width of the “CZO Promoted” is interesting because it suggests a higher degree of cation ordering (or reduction site uniformity) within the oxide matrix.
These results pose the question: can we reduce the temperature of reduction without a loss in the oxygen storage capacity? As shown below, the answer is that we can come pretty close using both formulation chemistry and advanced manufacturing engineering (“PCC CZO AdManu”). This reduction temperature is by far the lowest we have seen in materials that do not contain a primary catalytic metal such as rhodium, platinum, or palladium.
Besides increasing OSC and reducing the reduction temperature, our focus is on reducing the temperature at which the catalyst becomes active for the complete conversion of gasoline engine exhaust.
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