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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2024 , Vol. 36 >Issue 5: 783 - 797

DOI: https://doi.org/10.7536/PC230816

Review

Application of Copper-Based Catalysts for the Selective Catalytic Combustion of Ammonia

  • Qian Zhang , 1, 3, * ,
  • Runze Tao 1, 2 ,
  • Yu Huang 1 ,
  • Houzhang Tan , 2, * ,
  • Baochong Cui 2
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  • 1 Key Laboratory of Aerosol Chemistry and Physics, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China
  • 2 MOE Key Laboratory of Thermo-Fluid Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, China
  • 3 Environmental Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, China
* e-mail: (Qian Zhang);
(Houzhang Tan)

Received date: 2023-08-17

  Revised date: 2023-12-14

  Online published: 2024-03-15

Supported by

Youth Innovation Promotion Association, Chinese Academy of Sciences(2021413)

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Abstract

Ammonia is a promising new energy carrier that has the potential to replace traditional fossil fuels due to its high calorific value,CO2emission-free and renewable properties,and low storage and transportation costs.However,ammonia combustion faces several challenges,such as high NOxemission,slow combustion rate,and ignition difficulties.To address these challenges,selective catalytic combustion is an efficient and promising way that can decrease combustion temperature,enhance combustion stability,improve fuel conversion rate and reduce NOxemission while requiring less auxiliary fuel and consuming less energy.Copper-based catalysts are cost-effective and they are demonstrated to exhibit excellent performance in ammonia catalytic combustion with high N2selectivity.In this review,the reaction experimental setup and primary mechanisms,namely NH-HNO,N2H4and i-SCR pathways,are presented first;then,the recent research progress on copper-based supported catalysts for ammonia combustion,including the effect of preparation methods,types of support materials,metal-support interactions,and Cu-based single or multiple active components on catalytic performance are reviewed in detail.Finally,the review is concluded by summarizing the research achievements,current drawbacks and proposing the future developments 。

Contents

1 Introduction

2 Catalytic NH3combustion experimental setup and mechanisms

2.1 Experimental setup and performance indicators

2.2 Mechanism and research techniques

3 Copper-based catalysts for NH3combustion

3.1 The effect of preparation methods on performance

3.2 The effect of support on performance

3.3 The effect of active component on performance

4 Conclusion and outlook

Key words： new energy carrier; ammonia; copper based catalysts; selective catalytic combustion; NOx emission

Cite this article

Qian Zhang , Runze Tao , Yu Huang , Houzhang Tan , Baochong Cui . Application of Copper-Based Catalysts for the Selective Catalytic Combustion of Ammonia[J]. Progress in Chemistry, 2024 , 36(5) : 783 -797 . DOI: 10.7536/PC230816

1 Introduction

The accelerating process of urbanization and industrialization in China has caused tremendous damage to the ecological environment on which human beings depend for survival.Fossil fuel combustion produces a large number of polluting gases such as SO2,NOxand many kinds of particulate matter;At the same time,it emits a large amount of CO2greenhouse gases.In recent years,social problems such as global warming,glacier melting,sea level rise,frequent extreme weather,poverty and disasters have seriously affected the sustainable development of the global society[1]。 Under the severe situation,China has formulated a double-carbon target plan,and energy conservation and emission reduction are imminent。
China is rich in coal resources,but the combustion of coal has caused great pressure on China's emission reduction.According to the data of China Statistical Yearbook in 2021,China's total energy consumption reached 4.98×109tstandard coal in 2020,and coal accounted for 56.8%of the total energy consumption,as shown in Figure 1A[2]。 China's carbon emissions account for about 40%of the global carbon emissions,and the carbon emissions of major industries/fields account for 40%,as shown in Figure 1b.The carbon emissions of the power industry account for up to 40%,of which the carbon emissions generated by coal in the full utilization process account for 70%to 80%of China's total carbon emissions,and the average carbon emissions of the coal industry in 2020 are about 6.05×1013t[3][4][5]。 Therefore,it is imperative to study how to efficiently use clean energy that can replace traditional fossil energy combustion.the earlier and faster the energy transformation,the safer the future energy supply and global climate will be。
图1 2020年我国能源消费占比(a)与主要行业/领域碳排放占比图(b)

Fig. 1 The ratio of energy consumption (a) and the carbon emission ratio in major industries/fields (b) by 2020

Ammonia is an important new energy carrier with high energy density(3.16 kW H/L),which is about 1.5 times that of hydrogen,and can achieve efficient and clean combustion,so it has great application potential in gas turbines,engines,boilers,fuel cells and other fields[6]。 in this paper,the physical properties,combustion characteristics and production,storage and transportation costs of ammonia,hydrogen and natural gas are compared,as shown In Table 1[7⇓⇓⇓~11]。 hydrogen is recognized as the cleanest fuel because of its high calorific value and water as its combustion product.However,hydrogen is extremely difficult to liquefy(boiling point is about-252.77℃),which greatly increases the cost of storage and transportation,and to some extent restricts the development of hydrogen as an alternative to fossil fuels.In contrast,ammonia has a high boiling point(-33.34℃),is easy to liquefy and store,and has low transportation cost,so it can be considered to replace hydrogen for combustion.Wang Yuegu,a domestic scholar,analyzed the feasibility and low-carbon economy of ammonia as an alternative energy to oil and natural gas to improve China's energy structure from four aspects of economy,safety,environmental protection and climate protection[12]。 Studies have shown that China has abundant reserves of synthetic ammonia,and its output of synthetic ammonia in 2020 is about 58.84 million tons,accounting for about 1/3 of the global output[13]。 If the output of synthetic ammonia is used to replace natural gas,China's dependence on foreign natural gas can be reduced by 20%;the liquid ammonia can be directly transported by slightly modifying the connection of the existing natural gas pipeline,and the energy carried by the liquid ammonia is 1.5 times that of the natural gas when the liquid ammonia is transported in the same pipeline.More importantly,as a carbon-free fuel,the combustion process of ammonia does not emit greenhouse gases,and can use the trough power of renewable energy to synthesize ammonia through electrochemical methods to achieve zero carbon emissions in the whole life cycle of ammonia。
表1 Comparison of Three New Energy Carriers: Ammonia, Hydrogen and Natural Gas[7⇓⇓⇓~11]

Table 1 Comparison of three new energy carriers-ammonia,hydrogen and natural gas[7⇓⇓⇓-11]

Fuel Boiling point
(1 atm, ℃)		 Liquefaction pressure
(25 ℃, MPa)		 Lower calorific value (MJ/m3) Minimum ignition energy
(MJ)		 Flame speed
(cm/s)		 Production cost (RMB Yuan/t） Transportation
cost (RMB Yuan/t, 100 km）
NH3 -33.34 1.03 22.5 11.3 10 4900 150
H2 -252.77 70 142 3.75 280~320 27000~29500 8660
Natural gas -161 0.3 55 0.31 38 4000 100
Ammonia combustion usually has the following four reactions(1~4),which can realize the complete reaction of NH3to produce N2（1）in theory,but has the disadvantage of excessive oxidation,and NH3combustion will produce more NOx(x=1 and 2)and N2O（2~4）in practical application.These by-product gases can cause ecological and environmental problems such as haze and ozone pollution,as well as human health problems such as respiratory infections.Many scholars have studied the combustion mechanism of NH3,but the research on the ignition characteristics of ammonia fuel,how to reduce the combustion temperature,stable combustion and reduce the formation of NOxis not very clear,which to some extent restricts the practical application of ammonia combustion technology[9,14⇓~16]。
4NH3+3O2→2N2+6H2O+1266 kJ
4NH3+5O2→4NO+6H2O+904 kJ
4NH3+7O2→2NO2+6H2O+789 kJ
4NH3+4O2→2N2O+6H2O+2240 kJ
Selective catalytic combustion of ammonia(SCR of ammonia)is an efficient and potential combustion technology,which requires less auxiliary fuel and low energy consumption.It can reduce the combustion reaction temperature and reaction activation energy,enhance combustion stability,improve fuel conversion rate,and reduce Selective catalytic combustion of ammonia emissions.in recent years,the Hinokuma team In Japan has continued to carry out basic research on ammonia catalytic combustion[17⇓⇓⇓⇓~22]。 The development of suitable catalysts to improve the efficient conversion of NH3to N2and H2O,while reducing NOxemissions,is the research focus of ammonia catalytic combustion technology 。
Ammonia combustion catalysts mainly include two kinds:one is the catalysts with noble metals such as Pt,Ru,Au,Pd,Ag as active components,such as Ag/Al2O3and Ag/SiO2-TiO2supported catalysts developed by Wang et al.The effects of the types of acid sites on the support,the dispersion of Ag on the surface of the support and different supports on the adsorption and activation of NH3were investigated in detail[23⇓~25]。 The advantage of noble metal catalysts lies in their excellent low-temperature oxidation ability,but NH3are often easily over-oxidized to NOx,which reduces the selectivity of N2.Therefore,Lin et al.Developed Pt and Pd-based supported catalysts with Nb2O5as the support,and utilized the abundant Brønsted and Lewis acid sites on the surface of Nb2O5support to improve the NH3adsorption and N2selectivity[26]。
the other is the catalyst with Mn,Cu,Fe,Co and other non-precious metals as active components,which can be a single component,or two or more active components loaded on the catalyst carrier at the same time[27]。 Compared with the characteristics of low selectivity and high price of noble metal N2,the non-noble metal is abundant in nature,low in price and easy to be applied in industry on a large scale;The Cu-based catalyst is a typical representative,which has high N2selectivity,but its catalytic activity at low temperature needs to be improved.Hinokuma et al.Showed that the supported Cu-based catalyst can effectively inhibit the generation of N2O/NO in the combustion decomposition process of NH3[21,28⇓~30][6,17⇓⇓⇓⇓~22]。
There are many kinds of Cu-based catalysts,and the performance of Cu-based catalysts in ammonia combustion reaction is affected by many factors,such as preparation method,support type,composition and content of active component,etc.,so the action mechanism of Cubased catalysts prepared by different types or methods in ammonia catalytic combustion is also different.based on this,this paper first introduces the reaction device and reaction mechanism of ammonia catalytic combustion,and then discusses in detail the preparation methods,support types,metal-support interaction,and the effects of single or multi-component Cu-based supported catalysts on catalytic performance.Finally,the development direction and challenges of Cu-based catalysts with application prospects in ammonia combustion were prospected。

2 Ammonia catalytic combustion reactor and its reaction mechanism

2.1 Reaction Device and Performance Evaluation Index

The performance of ammonia combustion catalyst is usually evaluated in a fixed-bed quartz reactor with continuous gas flow at atmospheric pressure.The reaction device consists of a gas distribution system,a reactor and a detection system.The gas composition of the gas distribution system is NH3,O2and balance gas（N2or Ar),the reaction gas with a certain concentration and different NH3/O2ratios are allocated through the flow rate control of the mass flowmeter,and the postposition gas mixer ensures the uniformity of the allocated gas;The reactor consists of a split heating furnace and a quartz tube,a thermocouple is arranged in the heating furnace to control the reaction temperature,the temperature range is generally 100-900 deg C,the heating rate is kept at 5 or 10 deg C/min,a catalyst sample(40-60 meshes)is placed in a constant temperature zone of the quartz reactor,and quartz wool is filled above and below the quartz reactor to support the catalyst powder[6][31]。 A sample of that catalyst may be pretreat in an inert atmosphere at about 120.degree.C.prior to the reaction to remove excess moisture from the sample[20]。 The concentration of NH3,NOxand N2O in the system is generally detected by infrared gas analysis method such as Fourier transform infrared spectrometer,gas chromatography,chemiluminescence method and mass spectrometer,and the sampling part and pipeline are heated in the whole process[28][28][20,32][28,31,33]。
图2 氨燃烧催化剂性能评价装置

Fig. 2 The performance evaluation device for ammonia catalytic combustion catalysts

Ammonia catalytic combustion performance is usually reflected by NH3conversion (${{X}_{N{{H}_{3}}}}$ )and N2selectivity (${{S}_{{{N}_{2}}}}$ ),and some scholars combine other indicators to evaluate the performance of the catalyst.The activity of the catalyst was evaluated by using the light-off temperature T10and the conversion temperature T90（,that is,the temperature at which the conversion of NH3reaches 10%and 90%,respectively[20]。 The calculation formula of NH3conversion rate and N2selectivity evaluation index is as follows(to calculate the conversion rate and selectivity,the parameters in the steady state should be selected to eliminate the influence of physical adsorption of NH3on catalyst performance):[31,34]
${{X}_{\text{N}{{\text{H}}_{3}}}}=\left( 1-\frac{{{\left[ \text{N}{{\text{H}}_{\text{3}}} \right]}_{\text{out}}}}{{{\left[ \text{N}{{\text{H}}_{\text{3}}} \right]}_{\text{in}}}} \right)\times 100\%$
${{S}_{{{\text{N}}_{\text{2}}}}}=\frac{{{\left[ \text{N}{{\text{H}}_{\text{3}}} \right]}_{\text{in}}}-{{\left[ \text{N}{{\text{H}}_{\text{3}}} \right]}_{\text{out}}}-2{{\left[ {{\text{N}}_{2}}\text{O} \right]}_{\text{out}}}-{{\left[ \text{N}{{\text{O}}_{\text{2}}} \right]}_{\text{out}}}-{{\left[ \text{NO} \right]}_{\text{out}}}}{{{\left[ \text{N}{{\text{H}}_{\text{3}}} \right]}_{\text{in}}}-{{\left[ \text{N}{{\text{H}}_{\text{3}}} \right]}_{\text{out}}}}\times 100\%$

2.2 Reaction Mechanism and Research Means

The essence of ammonia combustion reaction is ammonia oxidation reaction.Therefore,it is of great significance to study the mechanism of ammonia catalytic oxidation in the presence of different catalysts to improve the performance of catalysts,which is helpful to better understand the surface chemical properties of catalysts and the interaction between catalysts and NH3,so as to screen out catalysts with high activity,N2selectivity and stability.Scholars have proposed different reaction mechanisms for different types of catalysts and different reaction conditions(such as reaction temperature,gas partial pressure,etc.),but all agree that NH3adsorption is the first step of the reaction,and the type of acid sites(Brønsted or Lewis acid)for the adsorption of NH3molecules depends on the type of catalyst used.After that,the NH3molecule is dehydrogenated to-NH2,-NH,-N and other intermediates,which are converted to N2,NO,NO2or N2O through different reaction paths in the presence of oxygen.At present,there are three mechanisms widely recognized by scholars:(1)NH-HNO reaction mechanism;(2）N2H4reaction mechanism;(3)internal-SCR(i-SCR)reaction mechanism.Fig.3 is a schematic diagram of ammonia catalytic oxidation reaction paths under three different reaction mechanisms 。
图3 三种不同反应机理下氨催化氧化反应路径示意图

Fig. 3 Diagram of reaction pathways in the catalytic oxidation of ammonia

(1)NH-HNO reaction mechanism.In 1950,Zawadzki proposed the NH-HNO reaction mechanism based on the experimental results of ammonia oxidation by himself and other scholars,and then Offermans et al speculated the reaction mechanism through Density functional theory(DFT)calculation[35][36⇓~38]。 The reaction mechanism is mainly aimed at noble metal catalysts such as Pt and Rh.According to the NH-HNO mechanism,the reaction starts from the adsorption of NH3,and NH3is gradually dissociated under the action of active[O]to form-NHx（x=2,1）intermediates(Reactions 5 and 6),-NH continues to react with active[O]to form intermediate HNO(Reaction 7),HNO continues to react with-NH to form N2（Reaction 8 a),and another part of HNO reacts with itself to form N2O by-products(Reaction 8 B),in which HNO is an important reaction intermediate.Therefore,in this mechanism,the active[O]formed by dissociative or non-dissociative adsorption of O2molecules on noble metal catalysts is essential for the further activation of NH3,and the activation of O2molecules is highly dependent on the physicochemical properties and surface coordination structure of the catalysts 。
NH3+[O]→NH2+OH
-NH2+[O]→-NH+H2O
-NH+[O]→-HNO
HNO+NH→N2+H2O
2HNO→N2O+H2O
（2）N2H4reaction mechanism.When the O2content is low,the ammonia catalytic oxidation may follow the N2H4reaction path,in which case the lattice oxygen（O2−）on the surface of the transition metal oxide participates in the reaction[39]。 The results show that the reaction mechanism of Cu-based catalyst is N2H4[40,41]。 Similar to the NH-HNO mechanism,this mechanism also considers that the reaction starts from the adsorption of NH3,and the dehydrogenation of NH3generates-NH2（reaction 9).In the absence of active[O],the NH2formed on the surface recombines to form a NH2=NH2intermediate(reaction 10a).It is then oxidized by O2to form N2（reaction 11 a)or N2O（reaction 11 B),and another part of the NH2adsorbed on the surface of the catalyst reacts with O2in the gas phase to form NO by-product(reaction 10 B )。
NH3+[O]→-NH2+OH
NH2+NH2→N2H4
NH2+O2→NO+H2O
N2H4+O2→N2+H2O
2N2H4+3O2→2N2O+4H2O
He et al.Combined with the characterization results of TPR,TPD and Fourier transform infrared spectroscopy(DRIFTS)of the catalyst,it was found that the-NH2intermediate appeared on the surface of Cu/TiO2without the existence of NOxspecies,so the explanation of the reaction mechanism of N2H4was more reasonable[30]。 Both Williamson et al.And Bagnasco et al.Found that the surface-NH2formed by the oxidation of NH3on the Cu2+site was the key intermediate for the catalytic oxidation of ammonia to N2on Cu-based molecular sieve catalysts[41][42]。 Compared with noble metals,the oxygen activation ability of Cu is low,which leads to the slow dehydrogenation process of chemisorbed NH3molecules,and there are a large number of-NH2species on the surface of the catalyst,which is conducive to the N2H4reaction mechanism[43]。 In addition to Cu-based catalysts,the N2H4reaction mechanism has been further confirmed on some other transition metal oxide catalysts,such as V2O5/TiO2,V2O5-WO3/TiO2,as well as Fe2O3/TiO2[40,44]。
(3)i-SCR reaction mechanism,also known as internal selective catalytic reduction reaction mechanism.In the ammonia catalytic oxidation reaction,part of NH3is oxidized to produce NO(Reaction 12),and according to the principle of SCR reaction,the produced NO will react with part of NH3to produce the main products N2and H2O（Reaction 13 a),and at the same time,the by-product N2O will be produced(Reaction 13 B )。
4NH3+5O2→4NO+6H2O
4NO+4NH3+O2→4N2+6H2O
4NO+4NH3+3O2→4N2O+6H2O
The i-SCR mechanism has been proposed in numerous catalyst systems for ammonia catalytic oxidation.Uch as single component supported catalyst Ag/Al2O3,V2O5/TiO2,(Pt/Rh/Pd)/Al2O3,Ag/SiO2,CuO/Al2O3,Ni/Al2O3,etc.And composite metal oxide catalyst Cu-Mg-Fe and(Pt/Rh/Pt)/Cu-Mg-Al[45][46][47][48][31][43]。 Jab Jabłońska et al combined the NH3-TPD and XPS results and found that there was a strong interaction between Cu and Ce,which enhanced the adsorption of NH3molecules,and NOxspecies were found in the reaction process,which was more consistent with the i-SCR reaction mechanism[31]。
For the above three reaction mechanisms,their applicability depends not only on the type of catalyst used(such as precious metal,non-precious metal,alloy,etc.),but also on the reaction conditions(such as reaction temperature,gas pressure and ratio,reaction space velocity,etc.).For example,when the reaction is carried out at a low space velocity,the reaction gas has sufficient contact time with the catalyst surface,and the NO generated by the oxidation of NH3can be reduced to N2by the subsequent unreacted NH3.However,when the space velocity increases,the contact time is too short for NH3to effectively reduce NO,so when the space velocity increases,it will lead to an increase in the selectivity for NO and a decrease in the selectivity for N2.The effect of temperature is that when the temperature is low,the activation rate is slow,which leads to the formation of N2O,while the dehydrogenation rate of NH is fast at high temperature,which leads to the rapid production of NO.Therefore,in the actual reaction process,different mechanisms will be transformed into each other,and the same catalyst may have two or more reaction mechanisms at the same time 。
in order to study the mechanism of ammonia catalytic oxidation,scholars often use in situ infrared spectroscopy(in situ FTIR)to study the changes of surface species of catalysts in the reaction process,and to observe the intermediate products produced by different kinds of catalysts at different temperatures[20,49]; The acid sites on the catalyst surface and the adsorption of O2were investigated by ammonia temperature programmed desorption(TPD)（NH3-TPD）and O2chemisorption（O2-TPD）.The mechanism of the reaction between ammonia and oxygen adsorbed on the catalyst was analyzed by temperature programmed surface reaction（O2-TPSR）[49,50][51]; The reduction properties of the active species in the catalyst were investigated by temperature programmed reduction（H2-TPR）technique[33,52]; in situ X-ray photoelectron spectroscopy(XPS)and In situ electron paramagnetic resonance(EPR)were used to investigate the change of valence state of catalyst elements and the possible formation of lattice defects[44,52,53]。 Ivashenko et al.Demonstrated how the selectivity of the products of NH3catalytic oxidation on PtRh alloy surface varied with reaction temperature,pressure,NH3/O2ratio and PtRh alloy ratio by in situ XPS and mass spectrometry[54]。 Decarolis et al.Used mass spectrometry and X-ray absorption spectroscopy to study the change of Pd particles at different catalyst bed positions with temperature and the selectivity distribution of products during the reaction.On the one hand,they proved that the structure of PdNxwas highly correlated with the selectivity of N2[55]; On the other hand,it is revealed that above 400°C,the two-step reaction of NH3on the surface of PdO to form NOxand the subsequent reduction by NH3to form N2exists simultaneously 。
In addition to experimental methods,density functional theory(DFT)is often used to calculate the adsorption and dissociation States of NH3and O2on the surface of catalysts with specific crystal plane structure,the number of electron transfer and the energy barrier of reaction steps,and then to provide a scientific basis for speculating the mechanism of ammonia catalytic oxidation and optimizing the design of catalyst active sites combined with experimental methods[36⇓⇓⇓~40]。 Ma et al.Used DFT theoretical calculation to compare the NH3reaction activity and product selectivity of Pt(211)and Pt(111)surface step sites,and revealed that the NH3oxidation reaction rate showed structural sensitivity at low temperature,that is,exposing more step sites was beneficial to improve the ammonia oxidation reaction rate at low temperature[56]。 At high temperature,the reaction step of ammonia activation by O*at the step site limits the overall reaction rate,and reducing the energy barrier of ammonia activation can effectively promote the overall performance of the catalyst.They further analyzed that the product selectivity was controlled by kinetic process,and the transition temperature of N2to NO on Pt(211)surface was higher than that on Pt(111)surface,which meant that more step sites on Pt(211)crystal surface were beneficial to improve the selectivity of N2.Shojaee et al.Calculated the B-type termination surface(exposed O and Co sites are octahedral coordination environment)on the crystal face of Co3O4(110),that is,the NH3adsorption and reaction path on Co3O4(110)-B by DFT[57]。 The energy barrier for the dehydrogenation of NH3to NH2,NH is approximately at 29~67 kJ·mol−1,indicating that the Co3O4(110)-B surface is favorable for the dehydrogenation reaction to occur,which is mainly determined by the low oxygen coordination number of the Co3O4(110)crystal face.The formation of products NO,N2and N2O depends on their desorption energy,and the desorption energy of Co3O4(110)-B is lower because of the existence of surface oxygen;However,the formation of N2and N2O involves the participation of lattice vacancies and has high desorption energy,so they are not easy to form.To sum up,the experimental characterization technique combined with DFT theoretical calculation provides a scientific basis for clarifying the ammoxidation activity and product selectivity of the catalyst at different temperatures,understanding the elementary reaction steps,and then regulating the catalyst synthesis and optimizing the reaction performance 。

3 Ammonia combustion performance of Cu-based catalyst

3.1 Influence of preparation method

the preparation method has an important influence on the performance of the catalyst.Generally speaking,the supported catalyst is obtained by uniformly dispersing the active component on a specific carrier,which can select the appropriate carrier according to the needs,and is easy to support multiple components at the same time.the preparation methods mainly include impregnation,precipitation,sol-gel,ion exchange,vapor deposition and hydrothermal methods,among which the supported catalysts for ammonia catalytic combustion are usually prepared by co-precipitation,sol-gel and chemical impregnation[58][17,33,59,60]。
The coprecipitation method is usually to mix different metal precursors in solution,add appropriate precipitant to the mixed solution to prepare the precursor precipitate,and then dry or calcine the precipitate to prepare the corresponding catalyst powder.Coprecipitation method is an important method to prepare catalyst ultrafine powder containing two or more metal elements.Guan et al.Prepared 1 wt%CuO-Fe2O3by coprecipitation method,and combined with backscattering scanning transmission microscopy,X-ray absorption spectroscopy and DFT partial wave density of States calculation,it was shown that Cu dispersed more uniformly on the surface of Fe2O3and formed Cu2+-O-Fe3+sites of atomic structure[59]; The Cu2+in the sample prepared by the impregnation method does not occupy the Fe3+sites,but has a tendency to form Cu agglomerates on the surface of Fe2O3.In the ammonia catalytic reaction,the sample prepared by the coprecipitation method performs better in terms of NH3conversion and N2selectivity,which is due to the formation of Cu-O-Fe sites that enhance NO adsorption and facilitate N-N bond coupling to form N2 。
Sol-gel method uses inorganic salt or metal alkoxide as precursor,through hydrolysis(alcoholysis)and polycondensation reaction,the sol is gradually formed into gel,and the required catalyst is obtained after aging,drying,calcination and crushing.The catalyst with high specific surface area and pore structure can be prepared by reasonably adjusting and controlling the type of metal precursor,solution pH,reaction temperature,sol concentration and time.Wang et al.Studied the ammonia catalytic combustion performance of CuO-Fe2O3catalysts prepared by sol-gel method at different heat treatment temperatures[60]。 Because the low temperature heat treatment can better retain the active oxygen species and more acid sites on the surface of the catalyst,the ammonia conversion efficiency is higher than that of the high temperature heat treatment,and the CuO-Fe2O3catalyst with a heat treatment temperature of 225℃can achieve complete conversion of NH3.Zhang et al.Compared the performance of Cu-Ce-Zr catalysts prepared by citric acid sol-gel method,homogeneous precipitation and chemical impregnation in ammonia catalytic oxidation[61]。 The Cu-Ce-Zr catalyst prepared by the sol-gel method has the best catalytic performance for ammonia oxidation due to better NH3adsorption performance,highly dispersed CuO nanoparticles and the formation of Cu-Ce-Zr solid solution;However,the catalysts prepared by the other two methods had poor catalytic performance due to the stable Brønsted acid sites and large CuO particles 。
the Chemical impregnation method is usually to contact the carrier with the aqueous solution of metal salt,so that the aqueous solution of metal salt is adsorbed or stored in the capillary of the carrier,and the excess solution is removed,and then the catalyst is prepared by drying and calcination.chemical impregnation can be divided into excess impregnation and equal volume impregnation,and can also be divided into single-step impregnation and multi-step impregnation according to different processes[54]。 The active component is dispersed evenly by excessive impregnation,but the loading of active component can not be controlled;The catalyst prepared by incipient wetness impregnation has poor dispersion of the active component,but it is convenient to control the loading of the active component.Taking the preparation of ammonia combustion catalyst Cu/Al2O3by excessive impregnation method as an example,a certain mass of carrier Al2O3is added into a certain concentration of Cu(NO3)2,Cu(AC)2,Cu(SO4)2or other kinds of Cu precursor aqueous solution,and continuously stirred at room temperature.The obtained mixed liquid is subjected to rotary evaporation at a certain temperature(usually 50 to 80 deg C)to remove water,or is directly placed in a drying oven at about 100 deg C for overnight drying,and then is placed in a muffle furnace for calcination at a temperature of usually 400 to 900 deg C[30][20,52,62]。 There are many studies on the preparation of ammonia catalytic oxidation catalysts by chemical impregnation。
The addition method and addition sequence of active components in the impregnation method have a direct impact on the performance of the catalyst.Cui et al.Used the co-impregnation method to prepare RuO2CuO/Al-ZrO2.Through energy dispersive X-ray spectroscopy(EDX)and TEM characterization,it was found that RuO2and CuO nanoparticles were uniformly dispersed on the surface of Al-ZrO2porous support and in the pore structure,and the performance of the catalyst was also significantly improved[63]。 Hinokuma et al.Compared the CuOx-Ag/Al2O3catalyst prepared by co-impregnation method and the catalyst prepared by physical and mechanical mixing method,as shown in Fig.4,the active components Cu and Ag of the sample prepared by co-impregnation method were distributed uniformly,while the active components of the sample prepared by physical mixing method were distributed unevenly,and partial agglomeration occurred[17]。 The results of NH3combustion test showed that the CuOx-Ag/Al2O3catalyst prepared by co-impregnation method with uniform active component distribution exhibited higher catalytic activity and N2selectivity,which was due to the high dispersion and close proximity of CuOxand Ag nanoparticles in the catalyst prepared by impregnation method[17,64]。
图4 CuOx-Ag/Al2O3 (a)和CuOx/Al2O3+Ag/Al2O3 (b)的HAADF-STEM和EDX-mapping图[17]

Fig. 4 HAADF-STEM and EDX-mapping diagrams for (a) CuOx-Ag/Al2O3 and (b) CuOx/Al2O3+Ag/Al2O3[17], Copyright 2017, American Chemical Society

The heat treatment method can also affect the crystallization state,valence state,surface group and gas adsorption state of the catalyst.Lan et al.Calcined the CuOx/γ-Al2O3catalyst prepared by the impregnation method in H2atmosphere.Compared with the CuO/γ-Al2O3calcined in air,the former sample had better particle dispersion and uniformity,and more-OH groups were retained in both the support and active component during the heat treatment.The existence of-OH was proved to be more conducive to the adsorption of NH3molecules by in situ infrared spectroscopy[34]。 In the NH3catalytic oxidation experiment,the CuOx/γ-Al2O3catalyst calcined under H2atmosphere exhibited higher ammonia conversion performance as well as lower conversion temperature,while maintaining better N2selectivity.Lan et al.Proposed a"fast i-SCR"reaction mechanism,that is,the CuOx-OH sites on the CuOx/γ-Al2O3catalyst calcined under H2atmosphere can provide Lewis and Brønsted acid sites to adsorb more NH3molecules and promote the dehydrogenation reaction[34]; At the same time,the CuOx-OH site promotes the activation of oxygen adsorption and promotes the generation of NO2.It accelerates the reaction of-NH2with NO2to produce N2and H2O,as shown in Fig.5.On the contrary,the samples calcined in air showed poor performance in terms of NH3adsorption and O2activation due to the lack of Cu+and-OH sites,which restricted the improvement of ammonia catalytic oxidation performance 。
图5 (a) CuOx/Al2O3-H和CuO/Al2O3-A两种催化剂的H2-TPR曲线；(b) 不同催化剂表面NH3吸附随时间变化的原位漫反射红外光谱；(c, d) CuOx/Al2O3-H和CuO/Al2O3-A催化剂表面NH3催化氧化性能的NH3转化率和N2选择性，反应条件：气体组成为500 ppm NH3, 5 vol% O2和N2平衡气，空速GHSV为60 000 h-1；(e) CuOx/Al2O3-H催化剂表面“快速i-SCR”反应机制[34]

Fig. 5 (a) H2-TPR profiles of (a) CuOx/Al2O3-H and (b) CuO/Al2O3-A catalysts. (b) In situ DRIFTs recorded during NH3 adsorption as a function of time upon different samples. (c) NH3-SCO light-off curves and (d) plots of N2 selectivity as a function of NH3 conversion of CuOx/Al2O3-H and CuO/Al2O3-A catalysts. Reaction conditions: 500 ppm NH3, 5 vol % O2, N2 as the balance, and GHSV of 60 000 h-1. (e) Reaction pathways in the proposed ′′fast i-SCR′′ mechanism over the CuOx/Al2O3-H catalyst [34]. Copyright 2022, American Chemical Society

Jab Jabłonska et al.compared two catalyst samples of Cu/Al2O3prepared by impregnation method and Cu-Mg-Al-Oxprepared by coprecipitation method respectively,and tested the ammonia catalytic oxidation performance（NH3/O2/CO2/H2O/N2）under actual flue gas conditions[65]。 The Cu species obtained by different preparation methods are different,and the CuO and CuAl2O4species formed on the surface of Cu/Al2O3are more conducive to improving the ammonia conversion efficiency of the catalyst under actual flue gas conditions 。
Han et al.Proposed the"Trojan Horse"strategy to uniformly deposit CuO nanoclusters inside the pore structure of zeolite SAPO-34 molecular sieve,compared with the traditional impregnation method,CuO nanoparticles can only agglomerate and disperse outside the pore structure of molecular sieve.The catalyst prepared by the new method showed higher reaction rate and N2selectivity in ammonia catalytic oxidation performance,and the catalytic performance continued to improve with the increase of Cu loading[66]。 Superoxide species formed on the surface of CuO clusters were proved to be the active sites for ammonia catalysis by in situ Raman,in situ infrared spectroscopy,and characterization methods such as CO-TPR,O2-TPD,and TPSR 。
In general,the preparation method significantly affects the dispersion of the active component on the catalyst support,the specific surface area and pore structure,the active oxygen species and the type of acid sites,thereby affecting the performance of the catalyst.For a specific catalyst,it is very important to find a suitable preparation method to improve the performance of the catalyst。

3.2 Support effect and metal-support interaction

The support is the skeleton of the supported catalyst,which is usually porous,and the uniform distribution of the active component on the surface of the support can significantly improve the activity of the catalyst.The interaction between different supports and active components is different,the specific surface area of supports is different,and the dispersion and particle size of active components on the surface of supports are also different,which affect the performance of catalysts.Therefore,the support plays an important role in the catalytic combustion reaction of ammonia.Metal oxides such as Al2O3,SiO2,TiO2,CeO2and MgO,and zeolite molecular sieves are common catalyst supports[66⇓~68]; In addition,some new supports,such as Nb2O5and perovskite LaFeO3,have been reported to support active components for ammonia catalytic oxidation[26,69]。 Abundant Brønsted and Lewis acid sites on the surface of Nb2O5are favorable for NH3adsorption;Fe-based perovskites have excellent element doping properties,and the oxygen migration and N2selectivity can be improved by doping Cu or Pd 。
The strong interaction between the support and the active component can improve the anti-poisoning performance of the catalyst,increase the oxygen mobility or reduce the oxygen binding strength of the support,which has an important impact on the performance of the catalyst.Zhang Liming et al.Studied the effects of MgO,α-Al2O3andγ-Al2O3as carriers on the activity of ammonia decomposition,and found that the activity ofγ-Al2O3was the highest when the temperature was higher than 700℃[67]。 When these supports were further loaded with Ni-based active components,NiO/MgO had the best catalytic activity,because NiO could form stronger interaction with the active sites on the surface of MgO,which made the catalyst have stronger resistance to sulfur poisoning at high temperature。
Fang et al.Further revealed that the surface of MgO(111)support is conducive to loading atomically dispersed Ru species by means of scanning transmission microscopy,synchrotron radiation X-ray absorption spectroscopy,in situ infrared spectroscopy and DFT calculation.The strong metal-support interaction formed between monatomic Ru and MgO(111)(Ru2+-O2-ion pair at the interface)is beneficial to the N—H bond cleavage and N—N bond formation of NH3molecules,which greatly improves the decomposition performance of NH3[68]。 Gang et al.Supported Ag onγ-Al2O3and SiO2respectively,compared with the unsupported Ag catalyst,and found that the performance of Ag catalyst was the best due to the interaction between Ag andγ-Al2O3[46]; Adsorbed molecular oxygen,adsorbed atomic oxygen,subsurface oxygen and bulk oxygen are the four main active oxygen species.The surface of Al2O3and TiO2can interact with the active component during the loading process because they contain more–OH groups,so the catalyst can usually obtain better performance[70]。 He et al.Compared Cu/Al2O3with Cu/TiO2with the same loading and found that the 10 wt%Cu/TiO2catalyst could completely convert ammonia at 250°C,and the selectivity of N2reached 95%[30]。 They suggested that TiO2has higher oxygen mobility and lower oxygen binding strength than Al2O3,making TiO2more suitable as a support for Cu-based catalysts than Al2O3 。
In addition,the specific surface area of the support,the dispersion of the active component on the surface of the support,and the particle size also have a significant effect on the catalyst performance.Al2O3,SiO2,NaY and TiO24 were used to evaluate the formation of different Ag species and their effects on the catalytic oxidation of ammonia[51]。 The dispersion effect of Ag on the surface of different supports is different,and the size of Ag species formed is also different.The 10 wt%Ag/Al2O3catalyst has smaller Ag0particles and better dispersion,so it has better catalytic performance at low temperature.It can completely convert ammonia at 180℃,and the selectivity of N2is 89%.Al2O3has been used as a support for ammonia catalytic oxidation catalyst in many studies because of its large specific surface area and the required pore structure of the catalyst[28,49,52]。 Hinokuma et al.compared the active component dispersion of Cu supported on Al2O3and 10A2B carrier respectively,and it can be clearly observed from the backscattered electron image(BSE)of the catalyst in Fig.6 that the CuOxis uniformly dispersed when Al2O3is used as the carrier,while the CuOxis very unevenly distributed on 10A2B carrier[22]。
图6 CuOx/Al2O3和CuOx/10A2B催化剂的BSE和X射线波谱仪(WDX)图[22]

Fig. 6 BSE and WDX diagrams of CuOx/Al2O3 and CuOx/10A2B catalysts[22]. Copyright 2020, Japan Petroleum Inst

Hinokuma et al.Studied the ammonia catalytic combustion performance of active component Cu supported on different metal oxide supports in detail,as shown in Table 2[20]。 It can be seen that the low-temperature ammonia catalytic activity of Al2O3and CeO2is better,and the corresponding specific surface area（SBET）and copper dispersion（DCu）are also higher.Therefore,the activity of the catalyst is closely related to the specific surface area of the support and the dispersion of the active component 。
表2 Effect of Different Kinds of Metal Oxide Supports on the Performance of Cu-based Catalysts[20]

Table 2 Effect of different types of metal oxide supports on the performance of copper based catalysts[20]。 Copyright 2018,Elsevier

Supports T10/℃ T90/℃ Selectivity（T90） SBET/m2·g−1 DCu/% Cu Particle size/nm
N2 N2O NO
MgO 360 487 83 1 16 33 29 3.6
Al2O3 303 476 92 6 2 149 43 2.4
10A2B 307 474 97 2 1 77 27 3.9
AlPO4 351 501 95 3 2 64 12 8.6
SiO2 334 490 97 1 2 177 11 9.4
TiO2 340 481 73 4 23 9 19 5.3
ZrO2 306 438 84 2 14 11 25 4.1
La2O3 290 440 57 6 37 13 30 3.4
CeO2 301 476 88 1 11 137 42 2.5

3.3 Effect of active component

3.3.1 Ingle-component supported Cu-based catalyst

The performance of single-component supported Cu-based catalysts is affected by many factors,such as the type of surface copper,the type of precursor,the calcination temperature,the loading of active component and the reaction conditions.Table 3 summarizes the ammoxidation performance of the single component supported Cu-based catalysts.As mentioned above,Cu-based catalysts supported on Al2O3have been widely studied due to the large specific surface area of Al2O3and the required pore structure of the catalyst 。
表3 Performance of Single Component Supported Cu-based Catalysts for Ammonia Oxidation

Table 3 Ammonia oxidation performance of single component copper-based catalysts

Catalyst types Preparation method Reaction conditions
（Gas composition and
space velocity）		 Reaction temperature (℃) Performance Ref
NH3 Conversion (%) N2 Selectivity (%)
CuOx/Al2O3-600
CuOx/Al2O3-900		 Wet impregnation NH3 (0.6%~1.0%)+O2 (1.5%~3.75%) + He
W/F = 5.0×10−4 g·min·cm−3		 476
450		 90
90		 92
91		 17
CuOx/10A2B Wet impregnation NH3 (0.6%~1.0%)+O2 (1.5%~3.75%) + He
W/F = 5.0×10−4 g·min·cm−3		 474 90 97 19
CuOx/3A2S
CuOx/Al2O3
CuOx/SiO2		 Wet impregnation NH3 (0.6%~1.0%)+O2 (1.5%~3.75%) + He
W/F = 5.0×10−4 g·min·cm−3		 484
476
490		 90
90
90		 97
92
97		 20
10 wt%Cu/TiO2 Wet
impregnation		 0.04%NH3+10%O2 + He
Space velocity: 50 000 h−1		 250 95 95 30
CuO-Fe2O3 Sol-gel 0.08%NH3+0.01%SO2+10%H2O+5%O2+Ar
Space velocity: 60 000 h−1		 225 100 - 60
5 wt%Cu/Al2O3
10 wt%Cu/Al2O3
15 wt%Cu/Al2O3		 Wet impregnation 1.14%NH3+8.21%O2+ He 350
350
350		 75
100
100		 96
90
94		 28
3.7 wt%Cu/NaY
8.4 wt%Cu/NaY		 Wet impregnation 1.14%NH3+8.21%O2+ He 350
350		 100
100		 97
98		 28
1.2 wt%Cu/beta
3.0 wt%Cu/beta
6.6 wt%Cu/beta
4.3 wt%CuO/Al2O3		 Wet
impregnation		 0.54%NH3+8%O2+ He
W/F = 0.015 g·s·mL−1		 400
400
400
400		 90
100
100
100		 98
98
98
51		 29
10 wt%Cu(S)/γ-Al2O3
10 wt%Cu(AC)/γ-Al2O3
10 wt%Cu(N)/γ-Al2O3		 Wet
impregnation		 0.1%NH3+10%O2+He
Space velocity: 50 000 h−1		 350
350
350		 95
85
100		 99
93
100		 52
1 wt%Cu/Al2O3
5 wt%Cu/Al2O3
10 wt%Cu/Al2O3
15 wt%Cu/Al2O3		 Wet
impregnation		 0.5%NH3+2.5%O2+ 97%Ar
WHSV=24 000 mL/h/g		 379
450
425
425		 50
100
100
100		 96
81
85
83		 49
10 wt%Cu/Al2O3 Wet
impregnation		 1%NH3+10%O2+ 89%He 350 92 96 77
10 wt%Cu/Al2O3 Wet
impregnation		 1.14%NH3+8.21%O2+ 89%He
WHSV=22410 NmL/h/g		 350 100 90 78
1 wt%Cu/TiO2 Wet
impregnation		 0.5%NH3+2.5%O2+ 97%He 425 100 54 33
1 wt%Cu/Al2O3 Wet
impregnation		 0.5%NH3+2.5%O2+ 97%He 500 100 96 33
However,the active sites on the Cu/Al2O3catalyst are still unclear.On various types of Cu/Al2O3catalysts,a mixture of CuO and CuAl2O4is formed[51]; CuAl2O4is formed by solid-solid reaction：CuO+Al2O3→CuAl2O4,which is similar to the formation of NiAl2O4spinel[71]。 The type of copper generated after loading and its dispersion have a significant impact on the performance of the catalyst.Gang et al.Studied the type distribution of Cu from the perspective of Cu loading,and found that the catalytic performance of CuAl2O4as an active center was better than that of CuO[28]。
Different precursor species can also affect the distribution of Cu species,and then affect the performance of the catalyst.Park and his team found that for the Cu/γ-Al2O3catalyst,the catalyst prepared by the precursor of Cu(NO3)2produced more crystalline CuO active phase than the catalyst prepared by the precursor of DETA copper ethanol solution[72]。 Lu et al.Also found in their study that different precursors would generate different active phases,only one spherical active phase could be observed when using Cu(NO3)2or CuSO4as the precursor,while two active phases could be observed when using(CH3COO)2Cu as the precursor,one was spherical and the other was cubic,and the catalyst activity was the best when using(CH3COO)2Cu as the precursor[73]。 Liang et al.Tried to use different kinds of copper precursors Cu(NO3)2,CuSO4and(CH3COO)2Cu to load on the Al2O3respectively,and found that,The type of copper precursor affects the formation of active sites on the support.The：(CH3COO)2Cu precursor is more conducive to the formation of CuO,while the CuSO4precursor is more likely to produce CuAl2O4species[52]。 Different from the results of Gang et al.,the results of Liang et al.Showed that the performance of CuO active phase was better than that of CuAl2O4active phase in ammonia catalytic oxidation reaction[28][52]。 It can be seen that the active sites on Cu-based catalysts for ammonia catalytic reaction are still controversial。
Calcination at high temperature can improve the firmness of the active component on the surface of the support,and the calcination temperature can also affect the formation and distribution of different Cu species on the surface of the catalyst,thus affecting the performance of the catalyst.Kim et al.Studied the Cu/γ-Al2O3catalyst calcined at different temperatures,and found that the 10 wt%Cu/γ-Al2O3catalyst had higher Cu dispersion and more CuO species on the catalyst surface when calcined below 700℃,while more CuAl2O4species were generated when calcined at 800℃[74]。 Jin et al.And Luo et al.Also obtained similar results,that is,lower calcination temperature is beneficial to the formation of CuO phase,and higher calcination temperature is beneficial to the formation of CuAl2O4phase[75][76]。
In addition,the loading of active component also has a significant effect on the performance of the catalyst.Jab Jabłońska et al.Found through experiments that it was difficult to achieve complete conversion of NH3when the Cu loading was 1 wt%,and the effect of Cu/Al2O3with 10 wt%loading was the best,the temperature corresponding to 50%and 100%conversion of T50and T100（NH3was the lowest,and the selectivity of N2was also the best[49]。 Gang et al.Prepared CuO/Al2O3catalyst using Cu(NO3)2precursor,and studied the effect of copper loading on the catalyst effect when the copper loading was 5 wt%,10 wt%and 15 wt%,respectively[28]。 Similar to the results of Jab Jabłońska et al.,when the Cu loading was 10 wt%,the ammonia catalysis was the best,and the NH3conversion reached 90%at 300°C,at which the N2selectivity was 97%.However,the samples with too high or too low Cu loading showed a significant decrease in catalytic performance,which may be due to the formation of more CuAl2O4like species in the 5 wt%and 10 wt%catalysts.When the Cu loading increased to 15 wt%,more bulk CuO was formed,and the activity of CuO was lower than that of CuAl2O4 。
In general,for single-component Cu-based catalysts,many other factors,such as precursor type,calcination temperature,active component loading and reaction conditions,will affect the performance of the catalyst by affecting the composition and distribution of the active phase in the catalyst.For Cu-based Al2O3catalysts,CuO and CuAl2O4are the main active sites for the reaction,but which active phase is superior between them is still controversial.In the future,further studies on the active sites on Cu-based catalysts should be the focus of research 。

3.3.2 Multi-component supported Cu-based catalyst

Multi-component supported Cu-based catalyst is a kind of catalyst formed by Cu and other(one or more)metals supported on a certain carrier through composite action,which aims to improve the catalytic performance through the interaction between multi-components.For a single Cu-based catalyst,although it has high N2selectivity,it often has poor activity at low temperature,and it usually needs a higher temperature(375–400°C)to completely convert NH3,so some scholars consider introducing other metals on the basis of Cu-based catalyst to improve the catalytic performance of the catalyst[31,49,62,77,79,80]。 Table 4 summarizes the ammoxidation performance of some multi-component supported Cu-based catalysts。
表4 Ammoxidation performance of multi-component supported Cu-based catalyst

Table 4 Ammonia oxidation performance of multi-component supported copper-based catalysts

Catalyst types Preparation method Reaction conditions
(Gas composition and
space velocity)		 Reaction temperature (℃) Performance Ref
NH3 Conversion （%） N2 Selectivity (%)
6 wt%CuOx/10 wt%Ag/Al2O3
10 wt%Ag/6 wt%CuOx/Al2O3		 Wet
impregnation		 NH3 (0.6%~1.0%) + O2 (1.5%~3.75%) + He;
W/F = 5.0 × 10-4 g·min·cm-3		 370
365		 90
90		 98
99		 17
1 wt%Pt/20 wt%CuO/Al2O3
4 wt%Pt/20 wt%CuO/Al2O3		 Wet
impregnation		 0.07%NH3+8%O2+ N2
Space velocity: 3600 mL/(min·gcat)		 235
235		 100
100		 79
84		 62
1 wt%Pt/20 wt%CuO/Al2O3
1 wt%Pt/20 wt%CuO/Al2O3		 Wet
impregnation		 0.7%NH3 + 8%O2 + N2
Space velocity: 3600 mL/(min·gcat)		 235
235		 100
100		 98
98		 79
1.5 wt%Ru/1.5 wt%Cu/C Reduction 0.1% NH3 180 95 92 80
1 wt% Ag/1 wt%Cu/Al2O3
1 wt% Ag/10 wt%Cu/Al2O3
1.5 wt% Ag/10 wt%Cu/Al2O3
5 wt% Ag/5 wt%Cu/Al2O3		 Wet
impregnation		 0.5%NH3 + 2.5%O2 + 97%Ar
WHSV = 24000 mL·h-1·g-1		 475
375
375
275		 100
100
100
100		 87
92
94
79		 49
5 wt%Cu/5 wt%Ag/Al2O3 Wet
impregnation		 1%NH3 + 10%O2 + 89%He
WHSV=30000 mL·h-1·g-1		 320 100 95 81
5 wt%Cu/5 wt%Ag/Al2O3
10 wt%Cu/10 wt%Ag/Al2O3		 Wet
impregnation		 1%NH3 + 10%O2 + 89%He 320
280		 100
100		 96
78		 77
1 wt%Cu/0.5 wt%Pt/TiO2 Equal volume
impregnation		 0.5%NH3 + 2.5%O2 + 97%He 325 100 90 33
The binary or multi-component composite systems composed of Cu and other metals,such as Cu-Ag,Cu-Pt,Cu-Ru,Cu-Ce and other composite metals supported on Al2O3and TiO2carriers have shown good catalytic performance for ammonia oxidation to N2[17,20,49,77][57,79,82][80][31]。 Especially when Cu and noble metal are loaded at the same time,the temperature of ammonia catalytic oxidation is significantly reduced[33]。 Hinokuma et al.Showed that the catalyst loaded with both Cu and Pt had high N2（,low N2O/NO）selectivity and ammonia combustion decomposition activity[6]。 Among all the composite catalysts of Cu-metal or CuOx-metal supported on Al2O3,Cu-Ag/Al2O3showed higher NH3conversion and N2selectivity[49,77,78]。 Wu et al.Prepared the Cu-Ag/Al2O3composite catalyst,and the experimental results showed that the NH3conversion of 5 wt%Cu-5 wt%Ag/Al2O3was>96%and the N2selectivity was more than 95%at 280°C[79]。 That is to say,in a certain range of loading ratio,the Cu-Ag/Al2O3binary composite catalyst realizes high oxidation activity and high N2selectivity at the same time.Through the comparative experiments of Jab Jabłońska et al.,it was found that the overall performance of the catalyst was significantly improved by adding a proper amount of Ag to the Cu/Al2O3catalyst[49]。 Further investigation showed that the formation of Ag2O particles on the catalyst surface and their size are important factors affecting the selectivity of N2 。
Cui et al.Loaded RuO2-CuO active component on the surface of porous Al-ZrO2carrier by impregnation method,and obtained the best performance of 100%NH3conversion at 195°C when the loading ratio of RuO2-CuO was 1:1,and maintained nearly 100%N2selectivity in the range of 195–300°C[63]。 Increasing or decreasing the ratio of RuO2will lead to a decrease in catalytic performance.Through XPS,H2-TPR and other characterization analysis,it is believed that the improvement of catalytic performance is mainly due to the synergistic effect of bimetallic and support,that is,the coordinated unsaturated Ru atoms on the surface of RuO2provide a large number of adsorption sites for NH3and O2.Imultaneously,a large amount of oxygen vacancy formed on that surface of the support by the Cu2+,the Cu+and the Al dope are beneficial to the adsorption and dissociation of the O2and the generation of surface active oxygen O*,Subsequently,the adsorbed NH3undergoes dehydrogenation in the presence of O*and finally generates N2.The presence of CuO also provides an active site for the conversion of NO to N2 。
Excessive loading usually leads to the agglomeration of active components and the decline of catalytic performance,so there is often an optimal metal loading and loading ratio.Yang et al.Found that 5 wt%Cu-5 wt%Ag loaded in equal proportion showed relatively good low temperature conversion(<320℃)and N2selectivity(<95% )[81]。 Jab Jabłońska et Al.Prepared Cu-Mg-Al multicomponent mixed metal oxide catalyst by co-precipitation method,and found that different kinds of highly dispersed copper oxides were formed on the surface of Cu-Mg-Al-Ox,and the high concentration and dispersion of CuOxplayed a vital role in the high activity,selectivity and stability of the catalyst[31]。 However,the catalyst can be further activated by adding Ce,especially at low temperature,and it is suitable for both NH3-SCO and NH3-SCR[63]。 Subsequently,they changed the performance of the catalyst by changing the content of Ag(Ce or Ga)added,and finally screened out two catalysts with the best performance,Ga0.25-Cu-Mg-Al-Oxand Ag0.25-Cu-Mg-Al-Ox.Wu et al.Compared 5 wt%Cu-5 wt%Ag/Al2O3with 10 wt%Cu-10 wt%Ag/Al2O3catalyst,and found that when the loading of active component increased,the selectivity of N2decreased significantly,and 5 wt%Cu-5 wt%Ag/Al2O3was the best catalyst[75]。
Combining the active component Cu with the noble metal will also improve the N2selectivity of the noble metal at higher temperature.Pt usually over-oxidizes NH3to produce by-products such as N2O,NO and NO2.To solve this problem,Ghosh et al.Designed and synthesized Pt/Al2O3@Cu/ZSM-5 core-shell catalyst.Cu/ZSM-5 was used to provide active sites to promote the reaction of N2O,NOxand NH3to produce N2,which realized the excellent performance of the catalyst with N2selectivity>90%at higher temperature(200~450℃ )[83]。 Compared with the physically and mechanically mixed Pt/Al2O3and Cu/ZSM-5 catalyst,the performance is significantly improved(Fig.7),which is mainly due to the shell structure stabilizing the Pt site,proving the rationality of the core-shell structure design 。
图7 (a) 核-壳型Pt/Al2O3@Cu/ZSM-5催化剂多步合成策略；(b) CS, P(0.05)-A和P(0.15)-A三种催化剂表面NH3催化氧化性能，其中(a，b)分别为NH3转化率和N2产率；(c) P(0.05)-A, P(0.05)-AS, P(0.05)-AS-S1和CS 4种催化剂表面NH3催化氧化性能，(a, b)分别为NH3转化率和N2产率，测试条件：催化剂0.18 g，反应气体为500 ppm NH3，5% O2，其余为平衡气体Ar，流速1000 sccm[83]

Fig. 7 (a) Multiple synthesis steps involved in the preparation of core−shell (CS) catalysts with a Cu/ZSM-5 shell (SCR catalyst) and a Pt/Al2O3 core (PGM catalyst). (b) Steady-state NH3 oxidation over three catalysts: CS, P(0.05)-A, and P(0.15)-A with 0.05, 0.15, and 0.05 wt% Pt, respectively. Comparisons are made for the (a) NH3 conversion and (b) N2 yield. For each experiment, 0.18 g of the catalyst was used in the fixed bed. Feed conditions: 500 ppm NH3, 5% O2, balance Ar, 1000 sccm. (c) Steady-state NH3 oxidation over four catalysts: P(0.05)-A, P(0.05)-AS, P(0.05)-AS-S1, and CS. Each sample contains 0.05 wt% Pt and represents one of the four different stages of catalyst synthesis (Figure 7a). Reaction data are plotted for (a) NH3 conversion and (b) N2 yield. Feed conditions: 500 ppm NH3, 5% O2, balance Ar, 1000 sccm, and 0.18 g of the catalyst[83]. Copyright 2020, American Chemical Society

In addition,the pretreatment of catalyst will also have an important impact on its ammonia catalytic oxidation performance.Olofsson et al.Investigated the ammonia catalytic oxidation performance of the Pt-CuO/Al2O3catalyst pretreated in 0.01%SO2atmosphere for 30 min and that of the unpretreated catalyst,and found that after SO2pretreatment,the catalyst had little effect on the conversion of NH3,which remained 100%,but the selectivity of N2was significantly improved,from 79%~84%to 98%[79]。

4 Conclusion and prospect

Selective catalytic combustion(SCR)is a very promising technology in the field of ammonia combustion,which can reduce light-off temperature,enhance combustion stability,improve fuel conversion rate and reduce NOxemissions.In this paper,the reaction equipment and main mechanism of ammonia catalytic oxidation are listed firstly;The preparation method of the Cu-based supported catalyst,the selection of the support and the metal-support interaction,the single-component Cu-based supported catalyst(precursor type,calcination temperature,The type of surface copper,the loading of active component and the reaction conditions)and the effects of multi-component supported Cu-based catalysts on the catalytic oxidation of ammonia were reviewed in detail 。
The catalyst preparation method and the type of support have a significant effect on the performance of the catalyst.The catalyst prepared by chemical impregnation method often has a good dispersion of the active component,and metal oxides such as Al2O3usually have a good effect as the support.The type of precursor,calcination temperature,the type of surface copper,the loading of active component and reaction conditions can affect the performance of the catalyst.For Cu-based catalysts,CuO and CuAl2O4are the main reactive sites.The introduction of noble metals or other metals into Cu-based catalysts can significantly improve the NH3conversion and N2selectivity of the catalysts,and there is often an optimal value for the proportion and content of the active components loaded.At present,the main reaction mechanisms of ammonia catalytic oxidation are NH-HNO,N2H4and i-SCR.Different catalytic systems may follow different reaction routes,and Cu-based catalysts are more likely to occur N2H4and i-SCR reaction mechanisms.In general,the type of active species,the dispersion of active components and the redox performance of catalysts have an important impact on the performance of catalysts,and the study of the best active species on Cu-based catalysts should be the core of future research;The surface acid sites of the catalyst also affect the selectivity of the N2of the catalyst,and the modification of the support of Cu-based catalyst can become the focus of future research to make it have better dispersion ability 。
Although there have been many studies on the preparation,modification,performance evaluation and mechanism of catalysts for ammonia catalytic oxidation,most of the current studies focus on the catalytic oxidation of low concentration NH3at low temperature,and there are few studies on the performance of catalysts at high temperature during actual combustion.For ammonia combustion,the core of the research is to improve the stability of the catalyst and improve the N2selectivity of the catalyst in the high temperature region,so as to ensure the stable combustion of NH3and reduce or eliminate the content of NOxproduced in the combustion process.In addition,the intrinsic reaction mechanism of ammonia catalytic oxidation is not very clear at present,so it is necessary to further explore the reaction mechanism of representative catalysts in ammonia catalytic oxidation reaction system by in-situ FTIR,TPD,TPR and other characterization methods combined with DFT calculation,and further study the possible intermediate reaction species[84]。 In addition,in the actual ammonia combustion process,monolithic catalysts are often needed,such as Al2O3as the coating material,and the active component Cu or other metal components are impregnated on the wire mesh burner.Machida et Al.Developed a pulse cathodic arc plasma deposition method to deposit a nano-thin Pt(111)catalyst layer on the surface of Fe-Cr-Al metal foil,and achieved ammonia catalytic performance superior to that of powdered catalyst Pt/Al2O3[85]。 Therefore,the research and development of monolithic catalysts for NH3combustion is also very important.How to prepare monolithic catalysts for practical use,such as loading methods,high temperature thermal stability,mechanical properties and diffusion resistance,needs further study 。

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