Why Spread a Metal Over Something Else? Surface Is What Counts
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Reactions on a solid catalyst happen only at the surface, so the interior of a lump of expensive metal does nothing at all. Spreading that metal thinly over a porous material solves the problem and creates others.
Why the interior is wasted
A reaction catalysed by a solid takes place where reacting molecules meet the surface, so the atoms inside a particle never participate. A solid lump of platinum therefore uses a minute fraction of the metal present, which matters enormously given the price. Dividing the metal into smaller particles raises the proportion at the surface sharply, since surface area relative to volume rises as particles shrink, and at a few nanometres a substantial share of the atoms sit on the outside. Fine particles clump together and sinter into larger ones when heated, which destroys the advantage, so they must be held apart, and that is what a support does.
What the support provides
The carrier material does several jobs beyond holding the metal:
- •An enormous internal surface area, with a gram of some supports offering hundreds of square metres
- •Physical separation preventing metal particles from merging
- •Mechanical strength and a shape that gas or liquid can flow through
- •Thermal stability at operating temperature
- •Heat conduction, spreading the heat a reaction releases
- •In many cases chemical participation of its own, since the support is not always inert
How they are made
Preparing a supported catalyst is a controlled process with several standard routes. Impregnation soaks the porous support in a solution of a metal compound, fills the pores, dries it and then reduces the compound to metal, with the distribution depending on how much solution was used and how it was dried. Precipitation forms the metal compound in the presence of the support so it deposits on the surface. Ion exchange attaches metal ions to sites on the support chemically. Each route produces a different distribution of particle sizes and positions, and the performance depends on that distribution more than on the quantity of metal, which is why preparation is a speciality and why recipes are closely held commercially.
When the support does the chemistry
Treating the carrier as a passive scaffold is a simplification that fails in several important cases. Some supports are acidic and catalyse reactions themselves, which is exploited deliberately in refining where a metal and an acidic support perform different steps of the same overall transformation, one rearranging the molecule and the other adding or removing hydrogen. Some interact strongly with the metal, changing its electronic properties and therefore how tightly it binds reacting species, which shifts activity and selectivity substantially and is an active research area. Some encapsulate the metal at high temperature, reducing activity in a way that reverses on treatment. The practical consequence is that changing the support changes the catalyst, so the two are developed together rather than chosen separately.
Where they are used
Supported catalysts are behind a large share of industrial chemistry and of emissions control. Refining processes use them to reform, crack and treat petroleum fractions, with platinum on alumina among the most important systems. Ammonia synthesis uses iron on a support with additives. Vehicle exhaust converters use platinum, palladium and rhodium spread over a washcoat on a ceramic honeycomb, with the honeycomb chosen for surface area and low resistance to flow and the metal loading kept as low as performance allows because of cost. Fuel cells use platinum on carbon. Hydrogenation of fats and of chemical intermediates uses nickel on various supports. In every case the design question is the same, being how to present the most active surface for the least metal.
The takeaway
Only surface atoms take part, so a lump of catalyst wastes nearly all of it, and dividing the metal into nanometre particles puts a large share of the atoms on the outside. Those particles merge when heated, so a porous support holds them apart and supplies hundreds of square metres of area per gram. Vehicle exhaust converters spread precious metals over a ceramic honeycomb for exactly this reason.