Several Catalytic Processes On Transition Metal-Embedded Graphene Sheet: A First Principles Study

In recent years, due to the rapid development of global economy and growingenvironmental pollution, how to improve the living enviroment have been receivedconsiderable attention. CO is atomospheric pollutant, chiefly from the exhaust of internalcombustion engines, but also from incomplete conbusion of various other fuels. COoxidation plays an important role in solving the growing environmental problems. The highgreenhouse potential of N₂O with a yearly increasing rate of about0.2%as an importantenvironment problem （310and21times more powerful than CO₂and CH₄, respectively） hasbeen attracting considerable attention. The emission of N₂O is regard as the most importantozone-depleting substance and is thereby expected to remain throughout the21^stcentury. Inorder to eliminate CO and N₂O, it is the most effective way to catalytic CO oxidation andN₂O decomposition. Earlier investigations, both experimentally and theoretically, have beenmade to lower the energy barrier Ervalues for CO oxidation and N₂O decomposition onmetallic surfaces. For examples, some noble metals of Pd, Pt, Rh, etc., can effectivelycatalyze CO oxidation and N₂O decomposition. They are however costly and require highreaction temperatures for efficient operations. Especially, because of the strong adsorption ofnoble metals toward O, the desorption of O adatoms from metal surfaces is cumbersome andis usually considered as a rate-determining step, which poisons metal surfaces and limits thelower-temperature activity. In order to conquer this bottleneck, recent studies address novlecataysts such as alloys, clusters, metal oxides, and even metallic nanotubes. Obiviously，catalyst with higher activity and lower cost are desirable for wide applications.Solid-state gas sensors are renowned for their high sensitivity, which in combinationwith low production costs and miniature sizes have made them unverally and widely used inmany applicaitions. Resently, carbon naonotues and semiconductor nanowires as gas sensorshave fast response time and high sensitivity at room temperature to detect toxic gases at theconcentration range of ppb （parts per10⁹） in inducstrial, environmental and militarymonitoring. In order to search for a gas sensor to detect H2CO and HF, the adsorption ofH2CO and HF on transition metal-embedded graphene （TM/graphene） is investigated bydensity functional theory calculations. In this thesis, utilizing the first-principle calculations based on density functional theory,the mechanisms of CO oxidation and N₂O decomposition on TM/graphene are investigated,and the high sensitivity of TM/graphene for H2CO and HF is explored as gas sensors. Themain results are divided into four parts as following:Firstly, the mechanisms of CO catalytic oxidation are studied on Cu/graphene. Thereaction proceeds via a two-step mechanism of CO+O₂â†'OOCOâ†'CO₂+O and CO+Oâ†'CO₂. The energy barriers of the former are0.25and0.54eV, respectively, while the latteris a process with energetic drop. The high activity of Cu-embedded graphene may beattributed to the electrionic resonace among electronic states of CO, O₂and the Cu atom,particulary among Cu-3d, CO-2, and O₂-2orbitals. This good catalytic activity opens anew avenue to fabricate carbon-based catalysts for CO oxidation with lower cost and higheractivity.Secondly, the mechanisms of N₂O catalytic decompostion on Mn/graphene underexternal electric field are studied. The simulations demonstrate that Mn/graphene has a betteradsorption ability than the corresponding typical catalyst, such as platinum group metals,while the appropriate positive F can make the N₂O decomposition spontaneously occur via atwo-step mechanism of N₂Oâ†'N₂+O and N₂O+Oâ†'N₂+O₂. In addition, Fsimultaneously facilitates O₂desorption and regeneration of the Mn/graphene system,completing the whole catalytic cycle. The high synergetic catalytic effect may be attributedto that F induces an enhancement of the charge transfer between N₂O and Mn/graphene.Thus, the Mn/graphene system together with the synergy of F is a good candidate for N₂Oadsorptive decomposition.Thirdly, the adsorption/desorption behaviors of H2CO on Mn/graphene under externalelectric field F are investigated in order to find new sensors for detecting H2CO. It is foundthat Mn/graphene has a better adsorption ability than other TM/graphene while a negative Fleads to the H2CO desorption and regeneration of Mn/graphene system, completing thewhole detection reversibility. The high synergetic effect may be attributed to surfacereconstruction and charge transfer between H2CO and Mn/graphene system induced by anegative F. Thus, this newly developed Mn/graphene system together with the synergy of Fwould be an excellent candidate for sensing H2CO with lower cost and higher sensitivity.Lastly, the adsorption of HF on TM/graphene is investigated in order to search for a gassensor to detect HF. Compared with the relatively weak adsorption on other TM/graphenesysterms, HF molecule tends to be adsorbed on Ti/graphene with appreciable adsorption energy. Based on these calculations, two gas sensing mechanisms are proposed and revealedthat both surface reconstruction and charge transfer result in a change of electronicconductance of Ti/graphene. Thus, this developed Ti/graphene would be an excellentcandidate for sensing HF with lower cost and higher activity.Based on first-principle calculations, the mechanisms of TM/graphene have beeninvestigated as novel catalysts and sensors, which provide the theoretical base for fabricatingnew instruments and tuning the relevant properties.