Self-assembly And Reaction Of Normal Alkanes(Derivatives) On Single Crystal Metal Surfaces

Alkanes,as a natural petrochemical resource with abundant reserves and wide distribution,have a broad prospect of applications.However,the inert chemical properties of alkanes substantially limit the efficiency of resource utilization.The conversion of alkanes in homogeneous systems holds many problems such as solution dependence,product separation,and high cost of metal catalysts,which hinder its industrialization.The researches of alkane transformation in heterogeneous catalysis show great potential for industrial applications.The concept of heterogeneous catalysis subjects to surface catalysis,whereas,it is significant to understand the interfacial catalysis from atomic and molecular views and take interfacial control over catalytic reactions.On-surface chemistry is a newly developed interdisciplinary research field.It mainly uses metal single crystals as the place where the reactions occur to construct model catalytic systems,which shows great advantages in developing new reaction systems and synthesizing novel functional molecular materials.The combination of advanced microscopic and spectroscopic characterization methods and theoretical calculation facilitates the elucidation of reaction pathways,providing solutions to the longstanding problems,such as elusive reaction mechanism,in heterogeneous catalysis.In this thesis,we systematically investigated the polymerization,crack,and dehydrogenation transformation into alkenes of normal alkanes and the self-assembly behavior of alkane(derivatives)on metal and partially oxidized mental surfaces.Some experimental results and experience in model catalysis were obtained,providing a meaningful input for alkanes conversion.The main contents are as follows:(1)Study of the high selective polymerization of n-dotriacontane(n-C32H66)molecules on the step edges of Cu(100)surfaces.n-Dotriacontane molecules can be activated at mild temperature to polymerize into linear polymers on the step edges of Cu(100)surfaces.High resolution scanning tunnelling microscope(STM)images,show the evidence of head-to-tail connections in terminal methyl groups between adjacent monomers,demonstrate the high selectivity of intermolecular coupling.Density functional theory(DFT)calculation uncloses the underlying mechanism of reaction:Low coordination Cu atoms at the step edges hold great catalytic dehydrogenation ability for n-dotriacontane molecules.Control experiments on different metal crystal surfaces illustrate the universality of step edge assisted polymerization of linear alkane regardless of the metal catalytic effects.(2)Study of the low temperature conversion of normal alkanes on oxidized Cu(110)surfaces into unsaturated hydrocarbons.Normal alkanes immigrate spontaneously from Cu(110)surface to Cu(110)-(2×1)O surface accompanied with the molecular axis rotating 90° at low temperature(ca.200 K).The possible reaction route of oxygen assisted dehydrogenation of alkanes into unsaturated hydrocarbons has been proposed after the characterization of IRRAS,XPS,and TPD etc.A set of comparison experiments have been performed to verify the hypothesis.(3)Study of the low temperature crack of polyene on oxidized Cu(110)surfaces.After low temperature annealing,n-dotriacontane molecules on oxidized Cu(110)surfaces convert into products with the characters of polyene.During the annealing process,TPD detected the formation of CH4,CO,and CO2.It is preliminarily confirmed the reaction route,in which n-dotriacontane molecules undergo dehydrogenation into polyene then crack into small molecular compounds.Additionally,we made polyene monomers polymerize on bare Cu(110)surface firstly,then adsorbed oxygen on surface at room temperature and annealed the surface at 453 K.Such comparison experiments verify the crack of polyene polymers and prove the different catalytic activities of oxygen species in different adsorption states on Cu(110)surface.(4)Study of the polymerization of polyene monomers on bare Cu(110)surface and regulation of polymers linkages by means of controlling initial molecular coverage.On bare Cu(110)surface,n-dotriacontane molecules undergo the initial transformation into disperse polyene monomers,then polymerize into longer polyene when lifting the temperature with the formation of different kinds of linkages in the polymers.Statistic data certifies the good government over the ratio of different linkages.(5)Study the self-assembly regulation of n-dotriacontane molecules on Cu(100)surface and the self-assembly phases transition of TDPB molecules on Au(111)and Cu(111)surfaces.The self-assembly of n-tridodecane molecules on clean Cu(100)surface exhibit two kinds of alignments with long axis vertical.1D linear polymers produced via direct alkane polymerization on step edge induce the uniaxial arrangement of ntridodecane monomers.Different self-assembly structures can be obtained by depositing TDPB molecules on Au(111)and Cu(111)surfaces under the same preparation conditions,the increase of molecular coverage lead to self-assembled phases transition.It is demonstrated the molecule-substrate interaction plays an important role in the appearance of various self-assembly structures.