Design Of Catalysts For Selective Hydrogenation Of Acetylene Based On Descriptors

The ethylene from naphtha crackers contains trace acetylene,which can poison the polymerization catalysts and reduce the quality of ethylene polymers,so it is essential to remove acetylene from ethylene gas.The selective hydrogenation of acetylene is a widely-used method to remove minor acetylene in excess ethylene,Pd is the commonly used catalyst for the acetylene thermos-catalytic selective hydrogenation reaction,and Cu is the popular catalyst for the acetylene electro-catalytic selective hydrogenation in recent years.However,at present,the research and development of new catalysts for selective hydrogenation of acetylene is still mainly based on the"trial and error method",and the research and development efficiency of catalysts is low.In this paper,by combining density functional theory calculations and microkinetic modelling,a series of theoretical studies were carried out in the field of acetylene selective hydrogenation,including thermos-catalysis and electro-catalysis,revealed the mechanism of acetylene selective hydrogenation,and provided theoretical basis for the metal catalyst design in the selective hydrogenation acetylene.The main research contents are as follows:1.The relation between“surface structure-catalytic performance”of PdAg catalysts was illustrated based on“structure descriptor”.Combining density functional theory calculations and microkinetic modelling,a total of 21 PdAg alloy catalyst models with different configurations and proportions including Pd and Ag were calculated,and it was found that the catalytic performance of PdAg catalysts with different configurations showed a volcanic trend with composition,catalysts with the Pd single atomic distribution presented the highest performance.The free energies of adsorbed methyl and hydrogen correlate energies of intermediates and transition states in the selective hydrogenation of acetylene.The structure descriptors were constructed by using parameters such as valence electrons,electronegativity,and atomic radius,to quantitatively describe free energies of adsorbed methyl and hydrogen.The descriptor-based screening strategy identified Ag₇@Pd,Pd₁@Ag and Pd₁Ag₃ alloy as the promising catalysts for high ethylene yield.Microkinetic analysis further confirmed that C₂H₄ yield on the identified Pd₁Ag₃ alloy was superior to those on PdAg bimetals with other surface structures under industrial reaction conditions,which has great industrial application prospects.2.The relation between“surface structure-catalytic performance”of Pd-based alloy catalysts was illustrated based on“structure descriptor”.19 Pd-based bimetallic alloy catalysts including Pd were calculated,the selective hydrogenation of acetylene on clean and hydrogen-adsorbed stable surfaces were studied,and it was found that there was a linear relationship between the state energies of each elementary step on the clean surfaces and hydrogen-adsorbed stable surfaces,Rh@Pd and Pd₁Au₃ exhibit highest activity among core-shell and ordered alloy models,respectively.Although Pd₁Ag₃ is less active than Pd₁Au₃,it displays the highest selectivity.The binding energies of methyl and hydrogen correlate energies of intermediates and transition states in the selective hydrogenation of acetylene on Pd-based bimetal catalyst surfaces,electronic effects and geometric effects were analyzed by using the parameters such as valence electrons,electronegativity,surface atomic distance,and coordination number,and a structural descriptor that could quantitatively describe the methyl and hydrogen binding energies on the clean surface was constructed.Aided by structure descriptors,a high-throughput screening protocol efficiently evaluated 489 Pd-based bimetallic alloys and identified promising ones（Pd₁Nb₃ and Pd₁Hf₃）with more yield of ethylene and inexpensive cost comparable to Pd₁Ag₃,finally validated by our experimental studies.3.Based on“descriptor”,the facet effect of Pd nanoparticles in reaction conditions was studied to find high-performance facets.The activity and selectivity on Pd（100）,Pd（110）,Pd（111）and Pd（211）for selective hydrogenation of acetylene were analyzed,and there were huge differences of activity and selectivity between clean Pd facets and stable Pd facets.To attain more accurate activity and selectivity on Pd facets,we took the hydrogen coverage as descriptors to relate reaction energies,energy barriers of elementary steps and adsorption energies of related substances.According to the iterative hydrogen coverage self-consistent microkinetic modelling in hydrogenation conditions,reaction rates on different Pd facets were evaluated,and Pd（211）had the highest activity and selectivity at the same time.4.The influence of work potential on reaction pathways in the acetylene electro-catalytic selective hydrogenation was discussed,which build a theoretical foundation for the subsequent construction of“descriptor”for the catalyst design.The mechanism of acetylene electro-catalytic selective hydrogenation on Cu（100）surfaces in alkaline conditions were calculated based on the density functional theory,the thermodynamic and kinetic analysis shows that the acetylene selective hydrogenation follows Eley-Rideal（ER）mechanism in the whole potential range,the rate controlling step is the first step of acetylene hydrogenation when potential is less than-1.0 V,but the rate controlling step changes to the second step of acetylene hydrogenation when potential is higher than-1.0 V,so the potential influences the catalytic activity and selectivity by changing the rate controlling step of acetylene selective hydrogenation and hydrogen evolution reactions.