DFT Mechenism Study Of Pom-supported Single Atom Catalyst For CO Oxidation

The discrete anionic structure of polyoxometalates?POMs?in the interface is more like a separate small“island”,which effectively prevents the diffusion of single atoms and prohibits agglomeration and generation of metal particles,and thus can enhance sintering–resistant behavior and increase metal loading on the surface of single–atom catalysts?SACs?.In order to explore the catalytic performance of POM–supported SACs for CO oxidation,we employed density functional theory?DFT?calculations to gain an understanding of the some important aspects of the system,including CO adsorption,formation of oxygen vacancy,and activity of surface oxygen species.Distinct from previous theoretical studies,which probes the catalytic behavior of POMs based on the anionic unit with high negative charge,here,we construct a model of the PTA–supported SACs which are the neural species.Our DFT calculations indicate that:?1?We here present results of DFT calculations on the neutral phosphotungstic acid?PTA?supported Rh SAC?Rh₁/PTA?,aiming at the identification of an Rh–assisted MvK mechanism.Our DFT calculations show that two CO molecules adsorb simultaneously at the Rh site with considerable adsorption energy,which inhibits the adsorptions of O₂.The difference between O_b and O_c atoms of PTA surface is remarkable,the calculated barrier for vacancy formation by reaction with adsorbed CO and Ob atom is lower than that of Oc atom?25 kcal/mol vs 40 kcal/mol?.However,previous experimental and theoretical studies neglect this remarkable difference of the Ob and Oc atoms in CO oxidation,which were treated indiscriminately.The dissociation of the adsorbed O₂ molecule is the rate–determining step for the whole catalytic cycle.?2?Anchoring of single metal atom on the PTA surface leads to the four key surface oxygen atoms were lifted from the PTA surface to form a new interface in the series of SACs studied here,and thus the surface oxygen species have been activated.CO adsorbs more strongly on Ir,Os,Rh,Pt,and Ru sites than on the Fe,Mn,and Co sites in the series of SACs studied here.The Pt system is easy to form an oxygen vacancy on the PTA surface when compared with others;and the Oc atom at the catalyst interface has high reactivity for CO oxidation relevant to O_b atom in Pt system studied here.?3?In general,reduction of N₂O by CO is first performed by N₂O decomposition over catalyst surface to release N₂ and form an active oxygen species,and subsequently CO is oxidized by the active oxygen species to produce CO₂.However,the strong adsorption behavior of CO on catalyst surface usually inhibits adsorption and decomposition of N₂O,which leads to a low activity or poisoning of catalysts.In the present paper,a Mans–van Krevelen?MvK?mechanism has been probed based on a series of phosphotungstic acid?PTA?supported single–atom catalysts?SACs?,M₁/PTA?M=Fe,Co,Mn,Rh,Ru,Ir,Os,Pt,and Pd?.Although the calculated adsorption energy of CO is exceedingly higher than N₂O for our studied systems,the adsorbed CO could react with the surface oxygen atom of PTA support through the MvK mechanism to form oxygen vacancy on the PTA surface.The free energy profiles show that the catalytic activity of Pd₁/PTA,Rh₁/PTA,and Pt₁/PTA SACs is quite high,especially for Pt₁/PTA system,which has the lowest rate–determining barrier of 16.25 kcal/mol.Meanwhile,molecular geometry and electronic structure analysis along the favorable reaction pathway indicates that the metal single atom not only plays the role of adsorbing CO and activating surface atoms of PTA support,but also works as an electron transfer media in the whole reaction process.