Understanding the Roles of Promoters of Heterogeneous Catalysts

Author:Wang Pei

Supervisor:wan hui lin fu gang


Degree Year:2017





Almost 90%of all commercially produced chemicals utilize the catalyst at some stage in the production process,which make the manufacture of catalysts alone account for over USD 10 billion in sales revenue.The heterogeneous catalysts would be more industrial attractive since it has good stability,more convenient to be separated and recycled.The catalyst often composed by three intergredients,which was the active components,promoters and supports.The performance of catalysts is often adjusted by promoters,including increased activity,increased selectivity and modified reaction pathway and so on,understanding the role of promoters have been a subject in heterogeneous catalysis for decades.Based on the density functional theory(DFT)calculations,this thesis usedsystematic studied focus on the influences of promoters,including the surface organic,alloy and oxide for metal and metal oxides system.The results are showed as follows:Chapter 1:In terms of promoters,the definition,species and characteristics have been introduced.And focusing on discussing the influence of promoters for metal system mainly the Pt group metal,and metal oxides especially vanadium based systems,and finally elaborated the research contents of this thesis.Chapter 2:The promotion mechanism of sodium formate(HCOONa)addition for the decomposition of formic acid in solution over Pd(111)have been investigated.With comparison the reactivity between HCOONa co-adsorption and no promoter,and also HCOO co-adsorption,HCOO-anion coadsorption and 1e-inject to Pd(111)surface,it is clear that the neutral HCOO co-adsorption behaves closely to the non-promoted situation while the HCOO-anion and the negatively charged(1e-)surface both mimic the presence of HCOONa;And the energetic span has decreased about 0.3 eV for dehydrogenation pathways thus enhanced the activity,while the energy gap of the transition state between the dehydration pathway and dehydrogenation pathway has increased and enhanced the selectivity;Hence,the promotion effect originates in the electron donation ability of HCOONa towards the Pd surface.Further investigation indicated that HCOONa co-adsorption generated electric field which interacted with the dipole moment produced by the adsorbate,and influenced the stability of those intermediates and finally influence the reactivity and selectivity.In addition,the promotion is well captured by modelling the anion by a negatively charged surface.This promotional effect can be traced back to the modulation of the electric field at the catalyst surface,with a strongly contrasted action on the energy of the various species along the competing pathways through the electrostatic interaction between the electric field and the surface dipole moment.As a result,both the reaction kinetics and selectivity are markedly improved.Chapter 3:The promotion of Ag for Pd catalyzing formic acid decomposition have been inverstigated.It was found that the PdAg/Pd(111)alloy catalyst exhibited high selectivity for formic acid dehydrogenation than Pd(111)surface,which was depending on the top surface Pd atom density and distribution.The rate determinging step of formic acid decomposition was calculated based on PdiAg8/Pd(111),two inequivalance Pd2Ag7/Pd(111)and five inequivalance Pd3Ag6/Pd(111)structures.It was found that the surface with three Pd atoms and in isolated atom distribution,exhibited highest activity and selectivity,and higher than Pd(111)surface.This was because of the ligand effect and synergetic effect of Ag atom.Chapter 4:The promotion of phosphrus oxide for vanadium oxide(vanadium phosphorus oxides,VPO)catalyzing n-butane activation have been studied.VPO is the effective commercial catalyst for the selective oxidation of butane to malaic anhydride,which the n-butane methylene C-H bond cleavage was the rate determining step,however the reaction mechanism and active site are still under debate.Through the comparation of H combination energy,it turns out that the V5+=O was need,based on this the VPi5O40 and VP7O20 model have been constructed.It reveals that the C-H bond cleavage is a proton coupled electron transfer(PCET)process,and some of the nearest-neighbour P=O group compaing wth the V5+=O group to be the active site,and the reactivity is able to be spread over all P=O sites through-OP-chain(s).With increasing the separated block(s),the reactivity is gradually decayed.We demonstrate that such a tendency can be quantitatively described by the center of P=O lone-pair band(εlp).To our surprise,some of the next-nearest-neighbor P=O group could be a potential active site.