Preparation and Catalytic Performance of Single Atomic Cluster Catalysts Based on MOFs and Phosphorus Nitride Matrix

Author:Yang Jian

Supervisor:wu yu en li ya dong


Degree Year:2019





Since the 18th to the 21st century,humans have experienced four industrial revolutions.Their logos are the first use of Watt’s steam engine,the second power application,the third atomic energy,electronic computer and space.The widespread use of technology and new energy era had led to the last energy and environmental crisis.Through these four industrial revolutions,the productivity of human society has been liberated unprecedentedly,and the material and spiritual civilization of society has been pushed to a new height unpredictable by predecessors.However,after such hundreds of years of rough development,a series of very difficult social problems have emerged.For example,a large amount of fossil energy consumption not only causes the depletion of precious resources,but also causes serious air pollution,excessive carbon dioxide emissions,global temperature rise,etc.This series of problems is gradually threatening the survival of mankind.As a chemical researcher,how to achieve a mild reaction condition and a highly efficient catalyst by changing the catalytic path and reducing the activation energy barrier of the reaction is a challenge we face.As the most basic raw material for synthetic artificial fertilizer,ammonia gas plays an important role in the development of human society,especially in population growth.However,in the synthesis of ammonia,the Haber-Bosch method is currently widely used.And the catalytic conditions require for the method are harsh conditions of 150 to 350 atmospheres while requiring a high temperature of 550℃.To create such extreme conditions,it will inevitably consume a lot of precious energy source.According to statistics,in the ammonia industry alone,the energy consumes per year accounts for about 1-3%of the world’s total energy,and greenhouse gas carbon dioxide emissions also contribute to the global total.The emission is about 1.6-3%.Therefore,it is of great significance to seek efficient catalysts to catalyze this reaction at low temperatures and pressures.One way to avoid the consumption of fossil energy is to use alternative clean energy.Molecular hydrogen fuels are promising alternatives due to their high energy density and clean combustion characteristics.Hydrogen produced by electrolyzed water is considered to be the main route for future hydrogen production due to its more economical,sustainable and cleanliness compared to hydrogen from methane reforming that consumes fossil energy.However,the main problems in this field are as follows:1.The hydrogen evolution potential is too high,resulting in excessive power consumption.2.The kinetics are slow,that is,the Tafel slope is too large.Although the ideal catalyst platinum exhibits amazing catalytic efficiency,its price severely restricts its industrialization process.Therefore,the design price is relatively low,and the high efficiency catalytic cathode hydrogen evolution catalyst is particularly important.Carbon dioxide gas is considered to be the main cause of the increasing in the temperature of the Earth’s environment and is called greenhouse gas.Due to the symmetrical structure of carbon dioxide molecules and the extremely high carbon-oxygen bond energy,it is extremely difficult to directly activate carbon dioxide molecules without catalyst.Recently,the synthesis of useful small molecules by electrochemical reduction of carbon dioxide is considered to be one of the feasible methods.On the one hand,this method can eliminate carbon dioxide in the air,and at the same time further recycle carbon dioxide for reuse.However,as in the aqueous phase,the hydrogen evolution reaction is often the largest competitive reaction,which will greatly reduce the electroreduction efficiency of carbon dioxide.Therefore,designing an efficient,highly selective carbon dioxide electroreduction cathode catalyst is a critical step in the overall catalysis.Herein,the main contents of this paper including the synthesis of highly active ruthenium-cobalt clusters,ruthenium and nickel single atomic catalysts,which exhibit excellent performance in ammonia synthesis,electrochemical hydrogen evolution and electrochemical carbon dioxide electroreduction,respectively.1.The application of RuCo clusters catalyst in the synthesis of ammonia and the catalytic decomposition of ammonia to hydrogen.Since the individual atoms tend to aggregate and rearrange,the resulting thermodynamically stable nanoparticles are eventually formed.Therefore,the precise control of sub-nanostructures such as clusters is extremely challenging.In order to achieve the preparation of sub-nano-based catalysts modulated on an atomic scale,the main consideration is to avoid excessive growth and migration of atoms.Here,we have prepared a series of ruthenium-based cluster catalysts encapsulated in porous carbon with the aid of zeolite imidazole MOF.The confinement strategy is similar to molecular sieves,that is,using a triatomic carbonyl ruthenium small molecule cluster having a certain structure as a precursor,introduced into the pore cavity when synthesizing the MOF in situ,and at the same time,due to the coordination saturation of the ruthenium.The integrity of the skeleton of the in situ synthesized MOF is maintained.The high temperature annealing in a subsequent inert atmosphere forms the ruthenium cluster species,and if a cobalt node is further introduced into the framework of the MOF,it will be bonded in situ to the cobalt node to form a bimetallic ruthenium cobalt cluster.After that,we used the synthesized ruthenium-based catalyst for the catalysis of ammonia decomposition.Compared to ruthenium cobalt nanoparticles,ruthenium particles and ruthenium clusters,ruthenium cobalt clusters exhibit significant ammonia decomposition catalytic activity.At 798,823 and 848 K temperatures,the ruthenium-cobalt catalysts have TOFs as high as 4,600,7070 and 10,750.Since the ammonia decomposition reaction is a reverse reaction of ammonia synthesis,it is also possible to have a good ammonia synthesis activity for a highly active ammonia decomposition catalyst.In the subsequent low-temperature ammonia performance test,the rate of ammonia synthesis was up to 40.2 umol NH3h-1Rumg-1 under the promotion of alkali.2.Ru single atom supported on a phosphorus nitride matrix catalyst and its application in hydrogen evolution reaction under acidic conditions.The role of the support in catalysis is much more important than providing a large specific surface area,regulating the geometric and electronic structure of the active site.The carrier effect is more obviously for single atomic catalysts.Here,we utilize an amorphous non-carbonitride nitride nanotube carrier as a novel substrate material for stabilizing single atomic metal sites,through the d-orbitals of germanium atoms and the lone pair of electrons in the nitride framework.The interaction of the Ru atom can be well anchored on the phosphorus nitride carrier.More importantly,we find that the catalyst can be used as an excellent electrocatalyst in an acidic hydrogen evolution reaction.At a current density of 10 mA/cm2,the required overpotential is only 24 mV and the Tafel slope is 38 mV/dec.this single atom catalytic performance is almost close to commercial platinum carbon.Moreover,in the subsequent i-t mode,it can be stabilized for more than 24 hours at a large current of 160 mA/cm2.The single atom catalyst can be stabilized for more than 5,000 cycles in the CV cyclic voltammetry scan mode.In addition,the single atomic catalyst exhibits a very high TOF value,with hydrogen production rates of 1.67 and 4.29 H2 s-1 at a single active site at overvoltages of 25 and 50 mV,which is superior to most reported acidic hydrogen evolution catalysts.Density functional theory calculations further demonstrate that the Gibbs free energy of H*adsorption on phosphorus nitride supports is closer to that of Ru on other supports such as activated carbon and carbon nitride.3.Application of surface-enriched Ni sinle atom catalysts in the electroreduction of carbon dioxide.For an ideal heterogeneous catalyst,the carriers contain highly dispersed active materials and sufficient accessible surfaces to avoid mass transfer problems which are two basic requirements.Therefore,by selecting the porous carrier,not only the reaction site can be stabilized,but also rapid adsorption and desorption of the reactants and products can be ensured.Recently,single atom catalysts have attracted attention due to their CO2 electroreduction performance and atomic economy.However,most single atom catalyst synthesis methods are based on a bottom-up strategy in which metal ions are adsorbed onto a substrate containing defects and then reduced throughout the support to form a single atom.Based on this bottom-up approach,porous supports do not limit most atomic metal species to the surface because it is difficult to avoid migration of the metal within the carrier matrix such that the single metal sites are uniform throughout the matrix rather than on the surface distribution and result in a large number of mass transfer problems and the deactivation of the catalytic process.Therefore,it is important to find a suitable synthesis method so that the single atom metal can be distributed as much as possible on the surface of the carrier.We propose a strategy for the synthesis of single atom catalysts through top-down method.Nickel particles can be uniformly distributed on the surface of the carrier nitrogen by thermal diffusion in situ on the surface of the support.With the aid of nitrogen-doped carbon with abundant defects,the above synthesis process not only converts the nanoparticles into single atoms,but also produces a number of pores to promote the contact of dissolved CO2 with a single nickel site.The CO2 electroreduction test shows that the surface enriched in nickel single atoms has better performance than the supported nickel nanoparticles and other similar catalysts.When the potential is-1.0 V relative to RHE,the Faraday efficiency and the TOF of the reaction are as high as 88%and 47805 h-1,respectively,far exceeding the performance of the nickel single atomic distribution over the entire support.Its ultra-high single atomic activity further confirms the superiority of our proposed surface enrichment strategy.In summary,through the above experimental results,we systematically explored the application of Ru clusters,single atoms and nickel-based catalysts in the synthesis of ammonia,hydrogen evolution reaction and carbon dioxide electroreduction.These results show that sub-nano structures such as single atoms and clusters have broad application prospects in catalysis and have taken an important step for the subsequent industrialization.