A Theoretical Investigation On The Evolution Of Pd₁/Graphene And Ir₁/Graphdiyne Catalysts In Reaction Conditions

People have been fabricating single transition metal atom catalysts through the interaction between metal atoms and the support for maximized usage of transition metals and better catalytic performance.Though considerable achievements have been made on transition metal single-atom catalysts,the origin of the superior catalytic performance,the nature of the active sites,the reaction pathways,etc are still subjects of debate and call for further investigations combining experimental and theoretical efforts.By taking low temperature CO oxidation as a model reaction,we investigated the catalytic performance of monodispersed Pd atom on graphene and evolution of Ir atoms supported on graphdiyne in reaction conditions,the key findings are as the following:On one hand,graphene was commonly used as catalyst support for its superior surface area,excellent mechanical properties and chemical stability,etc,while Pd is considered the most Pt like metal that can withstand CO poisoning.Combining their strength,we investigated the performance of Pd atoms mono-dispersed on graphene?PdGr?for CO oxidation.With first-principles-based thermodynamics calculations and microkinetics modeling,we showed that the positively charged PdGr can exhibit a rather high low-temperature activity for the CO oxidation(TOF=0.05 s^-11 at 298 K and 0.11 s^-11 at 350 K,P_CO/P_O2=1:20 and P_CO=0.01 atm;TOF=1.97 s^-11 at 325 K,P_CO/P_O2=1:1 and P_CO=0.20 atm),Under reaction conditions,O₂ binds strongly with the Pd acting as the major surface species to convert CO.CO oxidation over PdGr mainly proceeds with the revised Langmuir-Hinshelwood pathway,with the dissociation of the peroxide species as the rate-limiting step.The reaction rate is strongly dependent on the thermostability of the reaction species.The reaction rates along some previously proposed pathways with even lower activation energy are significantly slower as compared with rLH pathway,due to the low surface coverage of the involved species originated from their poor thermostability.The predicted catalytic performance of PdGr can be attributed to the specific electronic structure of PdGr originated from the Pd-C interactions with positively charged Pd that is capable to stabilize the reaction species on the revised Langmuir-Hinshelwood pathway.We expect that these findings would help to understand the performance of SACs and to rationalize the design and fabrication of SACs with superior catalytic performance.Compared with graphene,graphdiyne?GDY?is a new 2-dimentional allotrope of carbon with both delocalized?electronic structure and well-defined 18-membered ring structures,composed by 6-member rings of sp² carbon atoms interconnected by diynene formed by sp carbon atoms.The coexistence of both sp and sp² carbon atoms in GDY makes the bonding with transition metal atoms quite flexible.We investigated the electronic structure of small Ir_n clusters supported on GDY with extensive first-principles based calculations and investigated the structural evolution these clusters in reaction conditions,we found that:?1?The binding of monodispersed Ir atoms onto the corners of the 18-membered ring by forming 4 Ir-C bonds at H2 site is the most plausible;the calculated binding energy is-4.10 eV and the accompanied charge transfer is 0.19|e|;?2?The diffusion of Ir atom within the same 18-membered ring requires breaking of Ir-C bonds and the highest diffusion barrier is 1.69 eV;?3?In reaction conditions,the adsorption of CO on Ir/GDY is free-energy driven and each Ir atom can bind up-to 4 CO and leave GDY as Ir?CO?₄;?4?The formation of Ir-Ir and Ir-C bonds are strongly exothermic and Ir atoms can aggregate to form small Ir_n clusters inside the 18-membered rings;?5?The impact of reaction condition to Ir diffusion on GDY was also investigated with Ir?CO?₂/GDY;CO adsorption reduces Ir diffusion barrier to 0.60 eV and promotes the diffusion and aggregation;?6?CO adsorption is a simultaneous process and may affect both the electronic structure and the affinity of Ir_n to GDY.The sequential adsorption of CO leads to formation of carbonyl complexes which leave the GDY and result in leaching of Ir and deactivation of the catalyst.The findings would be helpful to understand the mechanism of structure evolution and leaching of Ir on GDY and may promote the controlled fabrication and application of GDY-based single-atom catalysts.