Low temperature catalysts reduce emissions

10 June 2019

CAR-EXHAUST

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A new catalytic converter technology is cleaning up car emissions...

Catalytic converters use chemical reactions to remove harmful emissions from the exhausts of cars and vehicles, but as engines become more efficient, exhaust temperatures are reducing and the high temperature chemistry catalysts rely on isn’t working as well. There is also the ‘cold start’ problem, when the catalyst hasn’t warmed up and isn’t working during the first 5-10 minutes of a journey.

If catalysts aren’t up to speed, carbon monoxide, hydrocarbons and nitrogen oxides pollute the air and lead to health issues. But scientists led by Yong Wang at Washington State University have created a system that removes 90% of carbon monoxide at lower temperatures and helps solve these issues.

A catalyst offers an easier route to get a reaction going. In this case, the reaction is combining harmful carbon monoxide with oxygen to make carbon dioxide, which happens much more easily when a platinum catalyst is present to help it along. Cerium oxide is used as a support, to hold the platinum in place.

Using smaller particles of platinum than has been possible before drives the reaction and makes the catalyst work better at lower temperatures. Extra improvement come from using a different preparation route to attach the platinum catalyst to the cerium oxide support.

Patrick Barrie, a catalyst expert from the University of Cambridge, read the paper and explains that researchers would like “to get a catalyst for the converter that will work nearer room temperature” and then “to actually understand what is happening at the molecular level, so that catalysts can be designed by scientific knowledge rather than trial and error”.

The hope is to use single atoms of platinum in the future, important because it’s a precious metal costing £20,000/kg. Only surface atoms of a particle can touch the carbon monoxide and convert it, so any atoms in the middle of a particle are being paid for but wasted. Barrie suggests however, “the problem with very small particles is they love to become big particles. Whenever you heat them up they like to become big particles and you’ve lost their advantage.” It is a difficult situation that this research has overcome.

“The importance of this work is that they've been able to tune the particle’s size. You want the platinum particles to be small, but not too small for the right reactivity and then to tune the state of the cerium oxide so it can easily supply oxygen atoms” says Barrie.

This combination is the result of two developments working together in chemical harmony. The first is making platinum particles of the right size, which happens once platinum is embedded into the cerium oxide support, initially keeping them apart as single atoms.

Single atoms should theoretically maximise the reactions that could happen, but in fact they are too effective, binding too tightly with the carbon monoxide and stopping carbon dioxide conversion happening. Instead Barrie says the researchers “had to heat up the platinum-embedded cerium oxide support in carbon monoxide at the right temperature and they found that 275°C was the magic temperature” to ‘activate’ this system.

They tried their activated catalyst when manufactured by a well established method and the results were promising, giving 90% conversion of carbon monoxide to carbon dioxide at 120°C. But the second development is to use their new activated catalyst, created by a different  manufacturing method they had developed previously. The results were even better than expected, with the big breakthrough being 90% conversion at only 65°C.

This low temperature conversion of carbon monoxide into carbon dioxide is supplied by oxygen atoms from the cerium oxide support, rather than it having to come from oxygen in the air. And to ensure the support doesn’t get used up over time, it then cleverly regenerates at a later point, itself using oxygen from the air instead.

This regeneration is essential for the catalysts to last a long time. As Barrie emphasises “they found their catalysts were stable and could do numerous cycles without deteriorating but they've only really tested over a matter of days, when you want a catalytic converter to work for a year of more to be practical”.

So before this technology can be found in vehicles of the future, Barrie also mentioned that “there are some things that still need to be explored, the cost of the preparation, whether they can do it on a large scale,the lifetime and the other things we want catalytic converters to do. Nitrogen oxides in car exhausts also need to be treated as well.”

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