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Nature communications(IF12.353),published in August 2018,reported a paper called “Atomic engineering of high-density isolated Co atoms on graphene with proximal-atom controlled reaction selectivity”written by professor su chenliang's research group from Shenzhen university.

Controllable synthesis of single atom catalysts (SACs) with high loading remains challenging due to the aggregation tendency of metal atoms as the surface coverage increases.They synthesize the graphene supported cobalt SACs (Co1/G) with a tuneable high loading by atomic layer deposition.

In contrast to solution-phase deposition,self-limiting surface reactions (Fig.1)of ALD ensure that each Co precursor molecule is anchored on a single active site of the graphene support.Interestingly, the active sites on graphene can be re-generated by ozone treatment in the second pulse of each ALD cycle, allowing for the loading of another batch of Co single atoms.As a result,the loading of Co1 single atoms can be precisely tuned by controlling the number of Co ALD cycles as illustrated in the Fig.1.In the selective hydrogenation of nitrobenzene,all the Co1/G SACs prepared show outstanding activity and remarkable selectivity to azoxy compounds.The mechanistic studies show that the electronic coupling of Co atoms with adjacent oxygen atoms results in more positively charged Co1 catalytic center,which helps to reduce its binding strength to azoxy compounds.Such an electronic coupling between the Co atom and its neighboring oxygen atoms prevents the full hydrogenation of nitroarenes,leading to a remarkably high selectivity towards the partially hydrogenated product.

In their study, reduced graphene oxide was selected as the support for the preparation of Co1/G SACs due to the following figures of merit:(i)chemically derived graphene offers an ideal low-cost platform for the anchoring of individual Co ALD precursors to the oxygen-decorated carbon sites;(ii)the density of anchoring sites on graphene can be tuned by controlling the pretreatment conditions. 

They performed X-ray photoemission spectroscopy (XPS) measurements to investigate the evolution of the amount of oxygen-containing groups on graphene exposed to O3 at 150°C (Supplementary Figure 2).It was found that ozonation of graphene at 150°C creates predominantly epoxy groups as confirmed by the observation of a strong O1s peak at 532.08eV,the increase in the amount of epoxy groups on the graphene support is approximately linear in the first five cycles of ozone pretreatment but tends to plateau as the number of ozone pretreatment cycles further increases (Fig.2g).They managed to synthesize a series of Co1/G catalysts with Co loadings of 0.4, 0.8, 1.3, 2.0, and 2.5wt% (designated as Co1/G-0.4, Co1/G-0.8, Co1/G-1.3, Co1/G-2.0, and Co1/G-2.5) by performing 1, 2, 3, 4, 5 cycles of Co ALD respectively.

In conclusion,they have developed a stepwise approach to fabricate a series of Co1/G SACs with high and precisely tunable loadings.The results reveal that the ozone treatment of a graphene support at mild ALD conditions not only burns off metal ligands,but also recreates active sites for the subsequent anchoring of another batch of Co atoms.This unique approach allows them to precisely tune the density of the supported Co atoms from 0.4% up to 2.5% without formation of any Co nanoparticle or clusters.As compared to conventional Co nanoparticles and precious Pt/carbon catalysts,all the Co1/G SACs exhibit remarkably high selectivity towards azoxy compounds in the hydrogenation of nitrobenzene aromatics.This can be attributed to the electronic coupling between Co atoms and adjacent oxygen atoms that results in a positively charged catalytic center.Consequently,the adsorption of electron deficient azoxy compounds is weaker and thus the full hydrogenation of nitroarenes is prevented.These findings have opened up an unprecedented avenue to precisely control the loading of single metal atoms in a wide range of SACs for industrially important chemical transformations.

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