| The thermal stability of noble metal single-atom catalyst loaded on reducible supports is relatively good,but it is poor on non-reducible support.In part of this study,we choose a widely used industrial La-Al2O3 to support the single-atom Pt species,and find that the atomically dispersed Pt1(II)-Ox-species,rather than the large metal particles,are the actual catalytic sites for the CO and C3H6 oxidation reactions.Unfortunately,the La dopants can't hinder the sintering of Pt single atoms during hydrothermal ageing.As a natural next step,the Ba-Ox-species are introduced to specifically stabilize the single-atom Pt on the La-Al2O3 support.Then we find that the atomically dispersed Pt retains the original full dispersion on Pt-Ba/La-Al2O3catalyst even after 650°C hydrothermal aging.Intriguingly,with or without the barium additives and/or sintered platinum particles in the catalysts,the atomically dispersed Pt atoms are the active sites and the intrinsic activity per Pt atom stays intact.Along with the development of noble metal single-atom catalyst,the activity of single atom has been questioned currently from the initial high praise.In another part of this study,we firstly prepare a series of ceria support with different redox property,and load Pt1 single atoms on them by a strong electrostatic adsorption method.After a mild activation,these isolated Pt1 atoms on various ceria supports are used as“seeds”to assemble stable cross-linked Pt-O-Pt structures with the size of?1 nm,which we named as Pt-O-Pt/Ce O2.We find that each Pt atom in Pt-O-Pt/Ce O2 is 100-1000 times more active than their single-atom Pt1/Ce O2 parent in catalyzing the CO oxidation from 80 to 150 oC.Without introducing experimental constraints on how the Pt atoms should be structured as“boundary conditions”,our DFT calculations identified the stable Pt structures for the Pt1/Ce O2 and Pt-O-Pt/Ce O2 catalysts as isolated Pt1 atom which substitutes the Ce atom and Pt8O14,respectively.Our computational and experimental characterizations consistently support each other both on structure and CO adsorption.Afterwards,DFT calculations combined with microkinetics simulations are conducted to study the CO oxidation mechanism on Pt1/Ce O2 and Pt8O14/Ce O2.It turned out that CO oxidation proceed at the interface of Pt1-Ce O2,Ce O2 is involved in the reaction process,and O2 dissociation is the rate-determining step.However,interfacial reaction is not the optimal path on Pt8O14/Ce O2.The new catalytic unit is the Pt-O-Pt itself without participation of oxygen from the ceria supports,it produces a similar apparent activation energy with our experimental measurements,and the rate-determining step is the oxygen atom migration at the Pt-O-Pt ensemble.Meanwhile,the reducible oxygen from ceria has no contribution to the low temperature CO oxidation on Pt-O-Pt/Ce O2,according to our experimental test.This consists well with the result from theory.We can conclude from the above study of Pt/Ce O2 system:The key difference in reactivity between the Pt1/Ce O2 and Pt-O-Pt/Ce O2 samples is therefore the O2activation pathway with or without platinum-ceria metal-support interaction,depending on the design of Pt atoms as either isolated or cross-linked Pt-O-Pt species. |
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