Effects To Hydrogenation Derived From The Catalytic Surface Structure And Environment

Catalytic hydrogenation is among the pillars of the chemical industry.It contributes to ca.8%of the world’s GDP.However,harmful or inefficient catalysts have caused serious pollution and other environment issues.Developing new effective catalysts or redesigning the commercial ones is of great urgency.In this thesis,theoretical calculations in first principle,combined with experimental studies,are applied to study the effects of the catalytic surface structure（geometrical and electronic）and the environment nearby to catalytic hydrogenation reactions,hoping to be helpful for the rational catalyst design.Hydrogenation of the biological platform molecule to the bulk chemicals is an important green route.Hydrogenation of phenol to cyclohexanone by "one step" is just among one of them.Though some effective noble metal supported catalysts have been developed,the origin of the differential selectivity to cyclohexanone on different noble metals is still unclear.Herein the hydrogenation mechanism of phenol is investigated.Results suggest that the reaction pathways on different noble metals is not the main factor.Because the all the reaction pathways assembled to cyclohexanone firstly.The true key factor that matters is the hydrogenation activity to cyclohexanone on the noble metal catalysts,which is also testified by experiments.Then this factor is quantified by a predictive descriptor of Eb（one/pl）/Ea,in which Ea has been correlated to an electronic descriptor of the d band,i.e.the d band center（εd）.According to this discreptor,a good selectivity should be abtained with the value below 0.6.Owing to the influence of the electronic factor to the performance of the catalysts,two strategies are applied to modify the electronic structure of the active component.The first strategy is alloying.Through the induce of second metal,the modification of the distribution of the hydrodeoxygenation product of phenol on a series of Ru-based catalysts was realized.The results implied that the introduction of the second metal component is not helpful to the enhancement of the activity in most cases.However,the modification of the selectivity is effective.While to the hydrogenation of benzoic acid（BA）,the alloy of Pd enhanced the activity of the Ru-based catalyst through an improved adsorption of the reagent.The second strategy is to adjust the interaction between the supports and the active components.Plenty of research work has revealed that the modification of the carbon support by nitrogen doping can significantly tune the catalytic performance of supported noble metal nano particles（NPs）.Then the interaction between the noble metal NPs and the carbon-based supports was explored,especially the modification mechanism to the electronic structure of the active noble metal components.Results showed that supported NPs cannot be as stable as the bulk ones with spin density of zero when pure carbon material was served as support,indicating the weak interaction between the noble metal NPs and the pure carbon support.In addition,the electron deficiency of the supported noble metal NPs increased the trend to be oxidized.When the carbon support was doped with nitrogen,graphitic or pyridinic,the spin density on the noble metal NPs decreased remarkably,and εd also moved to a deeper energy level concomitantly.All these are helpful to the resisitance of oxidation on the active component.Meanwhile,the movement of εa indicates a stronger interaction between the noble metal NPs and the support.The above conclusions are consistent with the phenomena already observed in experiments.Therefor here we recommend a model with clearer physical significance for carbon supported noble metal catalysts as the basis for the deconvolution of the X-ray photoelectron spectroscopy（XPS）.Besides the electronic structure,the topological structures of the active sites also always have an effect on the reactions,especially the structural sensitive ones.Then the impacts on two hydrogenation reactions respectively are expored on two types of active sites accordingly.One kind is supported Pd-based catalyst with continuous active sites.Results showed that the real reason for the improved selectivity for the 2-methyl-3-butyn-2-ol（MBY）semi-hydrogenation is the overlay of Zn at low coordinated Pd active sites.It is actually unfavorable to the selectivity to break the Pd-Pd bonds on the planes by the alloy of Zn.On the contrary,it indeed decreases the activity.Consequently,the rational catalyst design principle of this kind of reaction should be the selective covering of the active sites with low-coordination,such as edges and corners,while keeping the continuity of the active sites in planes,if one aims to obtain both high selectivity and activity.Another is the CoS2 catalyst with atomically monodispersed and face to face distributed C03-Co4 active site pairs.The synergic catalytic effect induced by the C03-Co4 active site pairs and the strong adsorption of the reagents and intermediates on the active sites together facilitate the localization of the reaction.Thus,the coupling or condensation of the intermediates was interrupted,making the reaction proceed through a direct route.Owing to which,the acitivity of the catalyst increased.The reaction environment such as the solvent also affects the reaction.Regarding the common solvent systems,the promotional effects of H₂O to the hydrogenations was chosen to invstigated.In the aqueous hydrogenation of phenol,H₂O plays different roles on different noble metal catalysts.On Pt,the over-hydrogenation was promoted by inhibiting the formation of cyclohexanone throuth H-bonding.While on Pd,the effect is opposite.However,H₂O served as a co-catalyst promoting the over-hydrogenation of cyclohexanone.In the hydrogenation of BA,the activity was then tuned by the competitive adsorption between H₂O and H₂ on different catalysts.According to the as-obtained volcano curve,the best activity was achieved when the difference value of the adsorption/dissociation energy between H₂ and H₂O was ca.-0.6 eV.