Yanfei Pan,Hejian Li,Li Zheng,Xiufeng Xu
School of Chemistry and Chemical Engineering,Yantai University,Yantai 264005,China
Keywords: Greenhouse gas NF3 destructive sorption Sorbents Al2O3 Mn/Al2O3 Reactivity
ABSTRACT NF3 is commonly used as an etching and cleaning gas in semiconductor industry,however it is a strongly greenhouse gas.Therefore,the destruction of disposal NF3 is an urgent task to migrate the greenhouse effect.Among the technologies for NF3 abatement,the destructive sorption of NF3 over metal oxides sorbents is an effective way.Thus,the search for a highly reactive and utilized sorbent for NF3 destruction is in great demand.In this work,AlOOH supported on carbon-sphere (AlOOH/CS) as precursors were synthesized hydrothermally and heat-treated to prepare the Al2O3 sorbents.The influence of AlOOH/CS hydrothermal temperatures on the reactivity of derived Al2O3 sorbents for NF3 destruction was investigated,and it is shown that the Al2O3 from AlOOH/CS hydro-thermalized at 120 °C is superior to others.Subsequently,the optimized Al2O3 was covered by Mn(OH)x to prepare Mn/Al2O3 sorbents via changing hydrothermal temperatures and Mn loadings.The results show that the Mn/Al2O3 sorbents are more utilized than bare Al2O3 in NF3 destructive sorption due to the promotional effect of Mn2O3 as surface layer on the fluorination of Al2O3 as substrate,especially the optimal 5% Mn/Al2O3(160°C)exhibits a utilization percentage as high as 90.4%,and remarkably exceeds all the sorbents reported so far.These findings are beneficial to develop more efficient sorbents for the destruction of NF3.
Nitrogen tri-fluoride(NF3)is commonly used as an etching and cleaning gas in semiconductor industry.However,NF3is a highly greenhouse gas with global warming potential of 17200 and atmospheric lifetime of 740 years [1],its emission can cause serious environmental pollution and climate problems.Therefore,it is of great significance to destruct the disposal NF3before its emission into the atmosphere.As reported [2–4],catalytic hydrolysis is effective for NF3destruction (2NF3+3H2O=NO+NO2+6HF),but the HF produced is severely corrosive to the reactors.In contrast,NF3destructive sorption over metal oxides sorbents,such as 2NF3+Al2O3=NO+NO2+2AlF3,is a more factual method for its abatement without the formation of corrosive HF,furthermore the metal fluorides produced are valuable chemicals [5].Thus,the search for a highly reactive and utilized sorbent for NF3destruction is urgently demanded.
In our previous work,it is found that Al2O3was an reactive sorbent for NF3destruction in comparison with other single metal oxides such as Mn2O3,Fe2O3,Co3O4,NiO,however the utilization percentage of Al2O3in NF3reaction was as low as 53.1% [6,7].How to improve the reactivity of Al2O3in NF3sorption? It is proposed to prepare the Al2O3sorbent with larger surface area and pore volume using hydrothermal method rather than precipitation previously used in order to fasten the diffusion of NF3.
Recently,we found that the Al2O3covered by transition metal oxides such as V2O5,Fe2O3,Co3O4,NiO,especially Mn2O3,can be improved significantly for NF3sorption.However,the utilization percentage of Mn2O3-coated Al2O3(designated as Mn/Al2O3) sorbents prepared by the wetness impregnation of Mn(NO3)2solution onto Al2O3was less than 70%,and necessary to be improved [8,9].In reference to the literatures of hydrothermal synthesis for porous oxides [10–19],we suggested to prepare the Mn2O3-coated Al2O3sorbents by hydrothermal method instead of impregnation so as to enhance the interaction between Mn2O3as surface layer and Al2O3as substrate,and to deeply improve the reactivity for NF3sorption.
In this work,AlOOH supported on carbon-sphere (AlOOH/CS)was synthesized hydrothermallyviachanging the temperatures,and then heat-treated to prepare the Al2O3.The effect of hydrothermal temperatures of AlOOH/CS on the reactivity of derived Al2O3for NF3destruction was investigated.Subsequently,the optimized Al2O3was covered hydrothermally by Mn(OH)xat various temperatures to prepare Mn/Al2O3sorbents with different Mn loadings.
Different from our previous work[20],the novelty of this work lies in that(1)the urea other than hexamethylenetetramine(HMT)was used for the deposition of Mn(OH)xover Al2O3surface in order to investigate the effect of precipitant on the structure and reactivity of produced Mn/Al2O3sorbents,and (2) the concentration of NF3in feed gases was higher than before considering the more severe reaction conditions for NF3destruction in reality.Importantly,the utilization of Mn/Al2O3sorbent screened in this work was compared with that of the sorbents reported previously.
2.1.The preparation of sorbents
The preparation of carbon sphere(designated as CS)was carried out according to our previous works[21].For the subsequent Al2O3and Mn/Al2O3sorbents,the detailed preparation procedures are described in Supplementary Material.
2.2.NF3 destructive sorption
NF3destructive sorption was carried out in a home-made fixedbed reactor.The feeds consisting of 2.8%NF3/97.2%He with a total flow-rate of 50 ml∙min-1were passed over 2.0 g sorbent,and the reaction temperature was kept at 400 °C.The effluent products were monitored by a gas chromatography (GC950,Shanghai Haixin,China) with Hayesep D column and thermal conductivity detector.The reactivity of sorbents is expressed by NF3breakthrough curves,whereCandC0stand for the outlet and inlet concentrations of NF3,respectively,and the utilization percentage of each sorbent is calculated on the basis of actual time of NF3complete destruction.
2.3.The characterization of sorbents
The nitrogen adsorption–desorption isotherms were measured on an adsorption apparatus (ASAP2020,Micromeritics,USA) at 77 K.From the data,the surface area,pore volumes and pore size distributions were calculated.
X-ray diffraction(XRD)patterns were obtained at room temperature with a powder X-ray diffractometer (XRD-6100,Shimadzu,Japan) using Cu Kα radiation and a graphite monochromator.
The spent sorbents were etched by argon-ion with an etching rate of 120 nm per time,and the concentrations of fluorine and oxygen elements at different depths were analyzed by an X-ray photoelectron spectroscopy (XPS,ESCALAB Xi+,Thermo Scientific,USA) with Al Kα radiation as the excitation source.
Transmission electron microscopy (TEM) micrographs of sorbents were obtained on a Tecnai G2 F20 (FEI,USA) operating at 200 kV.The samples were ground and dispersed in ethanol by sonication,and the obtained suspension was supported and dried over a carbon-covered Cu grid to examine the morphology.
The morphology of sorbents was also examined by a scanning electron microscopy(SEM,S-4800,Hitachi,Japan),and the surface concentration of fluorine element was determined by an energy dispersive X-ray spectrometer (EDS,EX-350,Horiba,Japan).
3.1.Destructive sorption of NF3 over Al2O3 sorbents
In this part,AlOOH supported on carbon-sphere (AlOOH/CS)precursors were synthesized hydrothermally at various temperatures (110,120,or 130 °C),and their crystallographic structure was identified by XRD analysis.As shown in Fig.1,the hydrothermal temperatures have a prominent influence on the crystallinity of produced AlOOH/CS samples,in detail,the AlOOH synthesized at 110 °C is amorphous,while that synthesized at 120 °C has a orthorhombic boehmite phase with a strong diffraction peak of(0 2 0) crystal plane at 2θ=14.5° (PDF 74-1895),and with the increment in hydrothermal temperature to 130°C,the crystallinity of boehmite sample get enhanced.Fig.2 gives the same changing trend in crystallinity of Al2O3samples as that of their precursors,especially the Al2O3derived by calcinations of AlOOH/CS synthesized at 130°C exhibits a well-crystallized γ-Al2O3phase and bears a high degree of crystallinity.As calculated,the crystallinity of Al2O3(110 °C),Al2O3(120 °C),and Al2O3(130 °C) samples is 21.2%,25.4%,and 55.8%,respectively.
Fig.1.XRD patterns of AlOOH/CS precursors: (a) AlOOH/CS(110 °C),(b) AlOOH/CS(120 °C),(c) AlOOH/CS(130 °C).
Fig.2.XRD patterns of Al2O3 sorbents derived from AlOOH/CS precursors: (a)Al2O3(110 °C),(b) Al2O3(120 °C),(c) Al2O3(130 °C).
As calculated,a highly negative Gibbs free energy (ΔrG) of-493.9 kJ∙mol-1suggests the reaction between NF3and Al2O3at 400 °C is thermodynamically preferable,and the results are verified in Fig.3,where 460,480,320 min of NF3breakthrough times over the Al2O3(110 °C),Al2O3(120 °C),Al2O3(130 °C) sorbents are presented,respectively.After reaction,all the spent Al2O3sorbents are transformed into AlF3(Fig.4).Furthermore,the crystallographic structure of the formed AlF3is dependent upon the hydrothermal temperatures of alumina,that is the spent Al2-O3(110 °C) turns into AlF3with dominant rhombohedral phase and trace tetragonal phase,while only rhombohedral AlF3is formed in the spent Al2O3(120 °C) and Al2O3(130 °C) sorbents.
Fig.3.NF3 breakthrough curves on Al2O3 sorbents derived from AlOOH/CS precursors.
Fig.4.XRD patterns of spent Al2O3 sorbents: (a) Al2O3(110 °C),(b) Al2O3(120 °C),(c) Al2O3(130 °C).
Nitrogen adsorption–desorption measurement was conducted to characterize the textural properties of Al2O3sorbents.As shown in Figs.S1–S3 (in Supplementary Material),the hysteresis loops observed in the adsorption–desorption isotherms reveal a textural porosity,and the pore size distributions present bimodal peaks ascribing to the co-existence of micropores and mesopores in the Al2O3samples.After reaction,very few of micropores are remained due to the change of porous sorbents to dense fluorides.Table 1 lists the textural data of these sorbents.In contrast,the Al2O3(120 °C) sample presents a higher pore volume,while the Al2O3(130 °C) has a larger surface area.After reaction,the surface area and pore volume of all sorbents decreased largely due to the transformation into dense fluorides.
Table 1 Textural properties of fresh and spent Al2O3 sorbents
The morphology of Al2O3samples was explored by SEM and TEM measurements.Fig.5 presents the TEM images of Al2O3sorbents,it is shown that the fresh samples exhibit thin-shells for Al2-O3(110 °C) and Al2O3(120 °C),but thin-sheets morphology for Al2O3(130 °C).After reaction,the spent sorbents become significant agglomerates ascribing to dense AlF3.Fig.6 gives the SEM images of sorbents.For fresh samples,hydrothermal temperatures have an evident effect on their morphology,in detail,the broken shells appear in the Al2O3(110 °C) sample,clearly stating the hollow microspheres of boehmite,whereas the Al2O3(120 °C) reveals flower-like nanostructures constructed by the overlap of nanosheets,and the Al2O3(130 °C) sample seems willow-leaves.After reaction,the spent adsorbents become significant agglomerates as dense fluorides.
Fig.5.TEM images of fresh and spent Al2O3 sorbents:(a)fresh Al2O3(110°C),(a′)spent Al2O3(110°C),(b)fresh Al2O3(120°C),(b′)spent Al2O3(120°C),(c)fresh Al2O3(130°C),(c′) spent Al2O3(130 °C).
Fig.6.SEM images of fresh and spent of Al2O3 sorbents: (a) fresh Al2O3(110 °C),(a′) spent Al2O3(110 °C),(b) fresh Al2O3(120 °C),(b′) spent Al2O3(120 °C),(c) fresh Al2O3(130 °C),(c′) spent Al2O3(130 °C).
As a summary,the Al2O3sorbent derived from AlOOH/CS hydrothermally synthesized at 120°C is superior to others.As calculated,the utilization percentage of Al2O3(120°C)for NF3destruction reaches 70.0%.In order to deeply improve the reactivity and utilization of sorbents in NF3sorption,subsequently the Al2-O3(120°C)was covered by Mn(OH)xto prepare Mn/Al2O3sorbentsviachanging hydrothermal temperatures and Mn loadings.
3.2.NF3 destructive sorption over 5% Mn/Al2O3 sorbents from precursors synthesized at various temperatures
In this part,the Al2O3(120 °C) was covered hydrothermally by Mn(NO3)2solution at various temperatures to prepare the 5% Mn/Al2O3(T) sorbents,whereTstands for the hydrothermal temperatures.Fig.7 presents the XRD patterns of fresh Mn/Al2O3sorbents.The diffraction peaks match well with γ-Al2O3phase for bare Al2O3,while the Mn2O3-coated Al2O3exhibits a dominant phase of γ-Al2O3and weak phase of Mn2O3.With the increase in hydrothermal temperatures of precursors,the diffraction peaks of γ-Al2O3and Mn2O3phases in derived Mn/Al2O3sorbents become stronger,implying the higher crystallinity.
Fig.7.XRD patterns of fresh sorbents: (a) Al2O3(120 °C),(b) 5% Mn/Al2O3(120 °C),(c)5% Mn/Al2O3(140°C),(d)5% Mn/Al2O3(160°C),(e)5% Mn/Al2O3(180°C),(f)5% Mn/Al2O3(190 °C).
As shown in Fig.8,the NF3breakthrough times on Al2O3,5% Mn/Al2O3(120 °C),5% Mn/Al2O3(140 °C),5% Mn/Al2O3(160 °C),5% Mn/Al2O3(180 °C),and 5% Mn/Al2O3(190 °C) sorbents are 480,580,600,620,600,and 500 min,respectively.As listed in Table S1,it is interested that the utilization percentage of 5% Mn/Al2O3(160°C)as the most reactive sorbent is 90.4%,which is largely higher than that of all the sorbents reported in the previous works[6–9,20,21].
Fig.8.NF3 breakthrough curves on Al2O3 and 5% Mn/Al2O3 sorbents.
The XRD patterns in Fig.9 indicate that Al2O3and all the Mn/Al2O3sorbents are transformed into AlF3with rhombohedral phase after reaction.Among the spent sorbents,the EDS-mappings of Al2O3and 5% Mn/Al2O3(160°C)were measured for elemental analysis.As shown in Figs.S4 and S5,the mappings present a good dispersion of oxygen and fluorine elements in the particles,and obviously the signal of F element is largely stronger than that of O element,responding to the more fluorides formed and less oxide remained after NF3reaction.What is the distribution of oxygen and fluorine elements in spent sorbents? The used Al2O3and 5% Mn/Al2O3(160 °C) particles were etched by argon-ion and analyzed by XPS.On the basis of their binding energies,Mn and O elements in sorbents etched for 0–5 times are Mn3+and O2-,ascribing to MnF3formed and metal oxides remained,respectively.The concentrations of fluorine and oxygen elements at different depths are shown in Fig.10,where a trend is indicated that the concentration of fluorine element decreases while that of oxygen increases with the increase in etching depth,implying the solid product is less metal oxide coated by more metal fluoride.It can be noticed that the fluorine element in spent Mn/Al2O3is much more than that in spent Al2O3at the same depth while the contrast of oxygen concentration reveals a contrary trend,agreeing well with the higher fluorination and more utilization of Mn/Al2O3sorbents than bare Al2O3.
Fig.9.XRD patterns of spent sorbents: (a) Al2O3(120 °C),(b) 5% Mn/Al2O3(120 °C),(c)5% Mn/Al2O3(140°C),(d)5% Mn/Al2O3(160°C),(e)5% Mn/Al2O3(180°C),(f)5% Mn/Al2O3(190 °C).
Fig.10.The concentrations of fluorine and oxygen elements in spent sorbents etched by argon-ion and measured by XPS.
As shown in Figs.S6–S10,the pore size distributions reveal that the fresh sorbents consist of micro-pores and meso-pores together.After reaction,the meso-pores are sintered and changed into micro-pores because of the transformation of porous sorbents into dense products.Table 2 lists the textural parameters of the sorbents,it is noticed that the surface area and pore volume of the Mn2O3-coated Al2O3sorbents except for 5% Mn/Al2O3(180 °C) and 5% Mn/Al2O3(190 °C) through the second hydrothermal treatment are higher than that of bare Al2O3.As we know,larger surface area and pore volume of sorbents are conducive to the diffusion and reaction of NF3gas.As a clear difference,the textural parameters of all spent sorbents are declined largely,and it is impressive that the spent Mn/Al2O3sorbents descend more significantly due to the more fluorides formed,well matching their higher utilization than bare Al2O3in NF3destruction reaction.
Table 2 Textural properties of fresh and spent Mn/Al2O3 sorbents
The TEM images of 5% Mn/Al2O3(160 °C) sorbent as an example are shown in Fig.S11.It is observed that the fresh sample exhibits nano-sheet morphology,whereas the significant agglomerates appear in spent sorbent because of the formation of dense product.Fig.11 presents the SEM images of all sorbents,it can be seen that the fresh sorbents typically consist of irregular small and dispersive particles,whereas the spent ones become large agglomerates consisting of dense fluorides.Furthermore,the fluorine concentration of all the spent sorbents was measured by EDS,whose relationship with the breakthrough times of NF3was given.As shown in Fig.S12,a good linear line indicates that the longer time of NF3breakthrough produced more fluorides in spent sorbents.
Fig.11.SEM images of fresh and spent sorbents: (a)fresh Al2O3(120°C),(a′)spent Al2O3(120°C),(b)fresh 5% Mn/Al2O3(120°C),(b′)spent 5% Mn/Al2O3(120°C),(c)fresh 5% Mn/Al2O3(140°C),(c′)spent 5% Mn/Al2O3(140°C),(d)fresh 5% Mn/Al2O3(160°C),(d′)spent 5% Mn/Al2O3(160°C),(e)fresh 5% Mn/Al2O3(180°C),(e′)spent 5% Mn/Al2O3(180°C),(f) fresh 5% Mn/Al2O3(190 °C),(f′) spent 5% Mn/Al2O3(190 °C).
3.3.NF3 destructive sorption on Mn/Al2O3 sorbents with various Mn loadings
In order to screen the Mn loadings in Mn/Al2O3sorbents for NF3destructive sorption,the Al2O3(120 °C) as substrate was covered hydrothermally by Mn(NO3)2solution at 160 °C to prepare theyMn/Al2O3(160 °C) samples,whereyis the mass percentages of Mn atom to alumina (2%,3%,5%,or 8%).Fig.S13 presents the XRD patterns of fresh sorbents.TheyMn/Al2O3(160 ℃) sorbents with various Mn loadings exhibit a dominant phase of γ-Al2O3,while a weak peak ascribing to Mn2O3phase can be observed with the increase in Mn loadings.After reaction,the spent sorbents are transformed into AlF3with pure rhombohedral phase(Fig.S14).As shown in Fig.12,the NF3breakthrough times on 2% Mn/Al2-O3(160 °C),3% Mn/Al2O3(160 °C),5% Mn/Al2O3(160 °C),and 8% Mn/Al2O3(160 °C) sorbents are 480,520,620,and 580 min,respectively.Obviously,all theyMn/Al2O3(160 °C) sorbents are more reactive than bare Al2O3for NF3destruction.Furthermore,it is confirmed that the optimized loading of Mn in Mn2O3-coated Al2O3sorbents is 5%,and the corresponding sorbent of 5% Mn/Al2-O3(160 °C) exhibits a high utilization percentage of 90.4%.
Fig.12.NF3 breakthrough curves on Mn/Al2O3 sorbents with various Mn loadings.
3.4.A possible pathway of NF3 destructive sorption on Mn/Al2O3 sorbents
The above results show that for most of Mn2O3-coated Al2O3sorbents,the increase in specific surface area and pore volume parallels to the enhancement of their reactivity for NF3sorption.However,the textural property of 5% Mn/Al2O3(180 °C) and 5% Mn/Al2O3(190 °C) sorbents is lower,impressively they still exhibit a higher reactivity than bare Al2O3.Hereby,a promotional effect of Mn2O3as surface layer on the fluorination of Al2O3as substrate in Mn/Al2O3sorbents for NF3sorption is suggested and shown in Fig.13,which is an important factor influencing on the reactivity improvement of Mn/Al2O3sorbents for NF3destruction.
Fig.13.The illustration of promotional effect of Mn2O3 as surface layer on the fluorination of Al2O3 as substrate in Mn/Al2O3 sorbents for NF3 sorption.
In this work,AlOOH/CS precursors were synthesized hydrothermally at different temperatures,and then heated-treated to prepare the Al2O3sorbents,where the one derived from AlOOH/CS synthesized at 120°C is superior to others for NF3sorption.Subsequently,the optimized Al2O3was covered by Mn(OH)xto prepare Mn/Al2O3sorbentsviachanging hydrothermal temperatures and Mn loadings.It is shown that the Mn/Al2O3sorbents are more reactive and utilized than bare Al2O3in NF3destructive reaction due to the promotional influence of Mn2O3as surface layer upon the fluorination of Al2O3as substrate,especially the 5% Mn/Al2O3(160 °C)exhibits a utilization percentage as high as 90.4%,which remarkably exceeds all the sorbents reported so far.These findings are beneficial to develop more efficient sorbents for the destruction of NF3as a novel greenhouse gas.
Data Availability
Data will be made available on request.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The financial support from the Natural Science Foundation of Shandong Province (ZR2020KB003) is gratefully acknowledged.
Supplementary Material
Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2023.07.005.