Graphene exhibits catalytic properties

Pinning yet another magical property on the coat of the wonder material known as graphene begs the question – is there anything that graphene can't do?

A team of researchers from the National Graphene Institute at the University of Manchester has discovered that imperfect graphene can exhibit qualities much like a catalyst, which is contrary to the general expectation that the carbon sheet is as chemically inert as the bulk graphite from which it is obtained.

Published in the "Proceedings of the National Academy of Sciences," the research has shown that graphene with nanoscale corrugations or nanoripples along its surface can accelerate hydrogen splitting as well as the best metallic-based catalysts.

The researchers postulate that this unexpected effect is likely to be present in all two-dimensional materials, which are inherently non-flat.

Catalysts for everyday products

Catalysis is the process of creating or increasing the rate of a chemical reaction by adding a substance known as a catalyst. This process is applied for the manufacture of various chemicals and products that are used in everyday life.

For example, iron is one of the most commonly used catalysts, usually for producing ammonia, which in turn is used in countless chemical and biological goods.

Another important catalyst, although this may be affected due to growing demand, is nickel. Used in the production of synthesis gas, also known as syngas – a developing fuel source that consists of hydrogen, carbon monoxide, and in some cases, carbon dioxide. Nickel has been prized for its use as a catalyst thanks to its relatively low cost.

Sharing the list of common catalysts are vanadium, a catalyst in sulfuric acid production for over 100 years, platinum metals used to scrub emissions from auto and industrial exhaust, and aluminum, which comprise a bifunctional catalyst that creates an important chemical intermediary.

Essentially, catalysts speed up a chemical reaction by lowering the amount of energy required to turn raw materials into useful products like plastics and many other manufactured items.

Corrugated 2D catalysts

The Manchester team, in collaboration with researchers from China and the United States, conducted a series of experiments to show that the non-flatness of graphene makes it a strong catalyst.

First, using ultrasensitive gas flow measurements and Raman spectroscopy (a non-destructive chemical analysis technique), the team demonstrated that graphene's nanoscale corrugations were linked to its chemical reactivity with molecular hydrogen (H2) and that the activation energy for its dissociation into atomic hydrogen (H) was relatively small.

The team evaluated whether this reactivity would be enough to make the lumpy graphene an efficient catalyst.

To this end, the scientists used a mixture of hydrogen and deuterium (D2) gases and found that graphene indeed behaved as a powerful catalyst, converting H2 and D2 into hydrogen deuteride (HD).

This was in stark contrast to the behavior of graphite and other carbon-based materials under the same conditions.

The gas analyses revealed that the amount of HD generated by monolayer graphene was approximately the same as for the known hydrogen catalysts, such as zirconia, magnesium oxide and copper, but graphene was required only in tiny quantities, less than 100 times the latter catalysts.

"Our paper shows that freestanding graphene is quite different from both graphite and atomically flat graphene that are chemically extremely inert," said Pengzhan Sun, first author of the paper. "We have also proved that nanoscale corrugations are more important for catalysis than the 'usual suspects' such as vacancies, edges and other defects on graphene's surface."

Andre Geim, lead author of the paper, added, "As nanorippling naturally occurs in all atomically thin crystals, because of thermal fluctuations and unavoidable local mechanical strain, other 2D materials may also show similarly enhanced reactivity. As for graphene, we can certainly expect it to be catalytically and chemically active in other reactions, not only those involving hydrogen.

"2D materials are most often perceived as atomically flat sheets, and effects caused by unavoidable nanoscale corrugations have so far been overlooked. Our work shows that those effects can be dramatic, which has important implications for the use of 2D materials. For example, bulk molybdenum sulphide and other chalcogenides are often employed as 3D catalysts. Now we should wonder if they could be even more active in their 2D form."

With the abundance of graphite on Earth, the possibilities for exploration into graphene and other 2D materials exhibiting catalytic properties open the way for a new path of research into these wonderous and mysterious two-dimensional materials.