High efficient removal of HCN over porous CuO/CeO2 micro-nano spheres at lower temperature range

Zhihao Yi,Jie Sun,Jigang Li,Tian Zhou,Shouping Wei,Hongjia Xie,Yulin Yang

Department of Chemistry Defense,Institute of NBC Defense,Beijing 102205,China

Keywords:Rice (Oryza sativa L.)Saline–alkaline stress Abscisic acid (ABA)OsABA8ox1-kd Endogenous ABA levels

ABSTRACT The porous CeO2 flowerlike micro-nano spheres based materials were prepared to remove HCN effectively at lower temperature range.The CeO2 and a serious of porous flowerlike ceria based materials loaded with metal species including Cu,Ag,Ni,Co and Fe were synthesized by hydrothermal method and precipitation method respectively.The physicochemical properties were probed by means of XRD,H2-TPR,BET,SEM and XPS.The removal ability for 130 mg.m-3 HCN over CuO/CeO2 showed the highest activity,the breakthrough time of which was more than 70 min at the condition of 30°C,120,000 h-1 and 5% (volume) H2O,owe to a higher relative atomic ratio of oxygen vacancies and Oβ,the stronger interaction between metal particle and support,the optimum redox properties.The reaction mechanism was speculated by detecting the reaction products selectivity at different reaction temperature.It was shown that the reaction system for removal of HCN over CuO/CeO2 catalytic material involved chemisorption,catalytic hydrolysis,catalytic oxidation as well as NH3-SCR reactions.

1.Introduction

Hydrogen cyanide (HCN) is a kind of highly toxic and volatile compound,with boiling point of 26°C [1].As a toxic precursor for many laboratory and industrial syntheses,it was ever used as a chemical war-fare agent [2].Low concentration HCN can cause severe symptoms such as nausea,vomiting,dyspnea,and difficulty breathing.Unprotected people might be slightly poisoned after several hours of exposure to 240 mg.m-3HCN and will be at risk of life due to physical and mental disorders after staying in 600 mg.m-3HCN more than 1 h.When the concentration of HCN in the air reaches 3600 mg.m-3,it would lead to rapid death [3].

In spite of the prohibition of HCN as a chemical war-fare agent,there is a fair chance to produce HCN including the combustion of biological and fossil fuel [4],selective catalytic reduction (SCR) of nitrogen oxides by hydrocarbons or ammonia [5],the fabrication of PAN based carbon fibers as well as electronic furnace exhausts[6].More significantly,HCN is regarded as the main intermediate for NOx,such as N2O,NO2,NO,which can arise from greenhouse effect and generation of photochemical fumes [7].

During the past decades,various methods of HCN removal has been studied including absorption[8],adsorption[9–11],combustion [12],catalytic hydrolysis [13] and catalytic oxidation [12,14].Absorption is the earliest and most mature method applied in the industrial field,HCN is converted into CN-in the alkaline solution,however,CN-solution also has high toxicity and still needs to be treated in future.As for the combustion,it usually cost a lot of energy as the high temperature is essential.The adsorption method refers to physically or chemically adsorbing toxic gases by using a porous adsorbent such as activated carbon or molecular sieve,the materials' capacity play an important role in achieving deep purification of HCN.Catalytic oxidation and catalytic hydrolysis methods are widely used to achieve complete conversion of HCN with low concentration due to their high efficiency,low energy consumption as well as low side reaction,the reaction mechanism of which are shown as follows:

The absorbents and catalyst materials for HCN elimination were usually composed of supported materials (Al2O3,TiO2,activated carbon,Molecular sieve,Hydrotalcite-like compound) and metal oxides (Cu,Ag,Co,Ni,Fe) [15].

Kr?cher and Elsener[16]investigated the elimination of HCN at a concentration of 60 mg.m-3over heterogeneous catalysts.They found when TiO2was chosen as the catalyst support,its catalytic activity for hydrolysis was the highest,as for the Al2O3which usually employed as hydrolysis catalyst,its HCN removal activity was only half of that of TiO2.Wang [17] demonstrated La1Cuy9/TiO2exhibits the best elimination performance for HCN which could achieve 100% removal of 120 mg.m-3HCN at 150°C.The incorporation of La improves the reduction performance of the catalyst and the total acid sites on the catalyst surface simultaneously,which enhances the adsorption performance of NH3.The strong ammonia adsorption capacity of a catalyst played an important role in promoting conversion of NOxto N2in the NH3-SCR system.T.Miyadera[18]reported the conversion of HCN over various transition metal catalyst materials including V2O5/TiO2,WO3/TiO2,CuSO4/TiO2and MnOx-Nb2O5-CeO2,among which the CuSO4/TiO2catalyst showed the highest catalytic activity (the removal rate of HCN was 100% at 300°C) while V2O5/TiO2and WO3/TiO2showed the lowest activity.

From these above statements,it could be observed the prepared catalysts require relatively high reaction temperature(150–300°C)to achieve deep purification of HCN.Therefore,it is necessary to develop novel catalysts showing high catalytic activity toward HCN at lower temperature range (30–150°C),especially those could achieve high removal rate of HCN at 30°C to meet the technical indicators of gas masks.

CeO2has attracted extensive interest due to its excellent oxygen storage capacity and low redox potential between Ce3+and Ce4+[19].While used as a support,it has an advantage to increase the dispersion of active metal components.Besides,the synergy between CeO2and supported metal species can increase the activity,selectivity and stability of the catalysts [20].As reported,the porous CeO2flowerlike sphere prepared by hydrothermal method has larger pore volumes and higher surface areas compared with common CeO2,in addition,the oxygen storage/release capacity(OSC) was also improved [21].Zhang reported the flower-shaped unique petal structure of CeO2can fully expose the active site of Au species and improve the dispersion on Au on the surface of CeO2.At room temperature,the conversion of CO over Au/CeO2can reach 99% [22].Recently,porous CeO2flowerlike sphere has been proved a promising support material for catalysts used in steam reforming ethanol for producing H2[23],the catalytic combustion of methane [24],and CO removal [25].

However,to the best of our knowledge,there has been no report on the removal of HCN over porous CeO2flowerlike sphere.

Herein,the porous CeO2flowerlike sphere and a serious ceria based materials loaded with metal species including Cu,Ag,Ni,Co and Fe were synthesized by hydrothermal method and precipitation method respectively,and were applied for the effective removal of HCN under lower temperature.The physicochemical properties were probed by means of XRD,H2-TPR,BET,SEM and XPS.The effects of key parameters,including reaction temperature and calcination temperature on the removal of HCN over the prepared samples were investigated.Moreover,the reaction mechanism was speculated by analyzing the reaction products selectivity at different reaction temperature.

2.Experimental

2.1.Catalyst materials preparation

2.1.1.CeO2preparation

All chemicals were purchased from Beijing chemical reagents company and used without further purification.Porous CeO2flowerlike sphere was prepared by a hydrothermal method.In a typical experiment,the glucose,acrylamide and hydrated cerium(III)nitrate were dissolved in the 180 ml aqueous solution with magnetic stirring at the mass ratio of 1:1:2.Then the ammonia solution(25%,mass)was added to the solution dropwise until the pH of the solution reached 10.After stirring for 5 h,the solution mixture was transferred into the Teflon-lined autoclave,sealed in an oven and kept at 180°C for 72 h,the orange suspension and precipitate were separated by centrifugation after the autoclave was cooled to room temperature naturally.Then the precipitate was washed with deionized water and ethanol for three times,respectively and then dried at 80°C in the electric oven for 8 h.The porous CeO2flowerlike sphere could be obtained after the precipitate was calcined in air at 450°C.

2.1.2.MxOy/CeO2preparation

MxOy/CeO2(M=Cu,Ag,Co,Ni,Fe) were prepared by precipitation method.The CeO2were dissolved into deionized water by magnetic stirring.Then a certain amount of nitrate solution of various metal species (Cu,Ag,Co,Ni,Fe) and Na2CO3(0.05 mol.L-1)solution were added into the solution at the speed of 1 drop.s-1until the pH reached 9.After stirring for 4 h,the suspension and precipitate were separated by centrifugation.The precipitate was washed with deionized water and ethanol for three times respectively and then dried at 80°C in the electric oven for 12 h.Then the 10% (mass) MxOy/CeO2can be obtained after the precipitate was calcined in air at 450°C,which were marked as Ce-Cu,Ce-Ag,Ce-Co Ce-Ni and Ce-Fe.The total metal mass percentage was calculated by the equation (M/(M+CeO2,M=Cu,Ag,Co,Ni,Fe).

2.2.Catalytic material characterization

X-ray diffraction(XRD) patterns of all samples were performed on a Rigaku D/max2000 PCX diffractometer using CuKα radiation(λ=0.15406 nm,operated at 40 kV and 100 mA).Nitrogen adsorption–desorption isotherms were measured at 77 K using a TriStarII3020 analyzer.Before the measurements,all the samples were outgassed to remove the moisture and impurities at 150°C under vacuum for 3 h.The adsorption capacity was tested at-196°C using liquid nitrogen as the adsorption medium.The specific surface areas of samples were estimated by BET and BJH method respectively.The morphology of the samples was observed by a scanning electron microscope(SEM,XL30s-FEG,10 kV).The Xray photoelectron spectroscopy (XPS) of the samples was performed using EACALAB 250Xi system with monochromatic AlKα radiation (hv=1486.6 eV).The sample charging effect was compensated by calibrating the binding energy with adventitious C1s peak at 284.8 eV.H2-TPR experiments were performed using a multifunction chemisorption analyzer (PX200) in a H2-Ar mixture(10%H2,40%Ar,by volume,40 ml.min-1)as the reductant.Before the reduction,50 mg catalyst was pretreated with N2at 300°C for 1 h,and then cooled to ambient temperature.After that the H2-Ar mixture gas was introduced,and the H2-TPR profile collected from 100–600°C at a rate of 10°C.min-1.Fourier-transform infrared spectra(FT-IR)of the samples were carried out using the KBr pellet technique on a Bruker VERTEX 70 spectrometer over the 4000–400 cm-1wave-number range.

2.3.Experimental setup and analysis

As shown in Fig.S1,the catalytic activity tests of the prepared catalytic materials were carried out in a quartz tube(with an inner diameter of 5 mm) on a fixed bed loaded with 0.25 g samples.When the removal rate of HCN was less than 100%,the catalytic activity was evaluated according to the breakthrough time.

The whole device process was consisted of the following sections:mixing section,reactor section and analysis section.The air and HCN with different flow rates were obtained by setting the parameters of the mass flow controller.The water vapor content of the mixed HCN gas was adjusted by changing the flow rate of the air and the temperature of the water vapor generator.After mixed in the mixing chamber,a 0.25 g typical sample and 130 mg.m-3HCN with 5% (volume) H2O were introduced into the fixed-bed quart tubular reactor with a gas hourly space velocity(GHSV) of 120,000 h-1.The exhaust gas from the fixed-bed quart tubular reactor was first analyzed and then absorbed by the NaOH solution (1 mol.L-1).The concentration of HCN and NH3were determined by iso-nicotinic-acid-3-methy-1-phenyl-5-pyrazolone spectrophotometric method and sodium hypochlorite-salicylic acid spectrophotometric method respectively,the detection limits of which were 0.005 mg.L-1and 0.0025 mg.L-1.The concentration of CO,CO2,NO and NO2(mg.m-3) were measured using a flue gas analyzer(DeTu,350,German)after the gas stabilized for 120 min at the corresponding temperature.The removal rate of HCN and the selectivity of reaction products were calculated as following Eqs.(1)–(6)where C0and C referred to the inlet and outlet concentration of HCN (mg.m-3) respectively.

3.Results and Discussion

3.1.SEM analysis

The size and morphology of all samples were investigated by scanning electron microscopy (SEM).Images from SEM for CeO2were shown in Fig.1,which further proved the porous CeO2flowerlike sphere were synthesized successfully.It could be observed the CeO2appeared as three-dimensional flowerlike mesoporous spheres with a diameter of 2–4 μm,which were consisted of several interweaved petal-like nanosheets with 40–60 nm thickness.Fig.2 showed the SEM images for Ce-Cu,Ce-Co Ce-Ni Ce-Ag Ce-Fe,although the metal elements on the support varied,however,the CeO2still exhibited a microsphere with diameter of 2–4 μm,which indicated the loading of metal species wouldn't change the structure of the CeO2support.The porous structure of all samples except Ce-Fe could be still observed after metal species loaded implying the CuO,Ag2O,NiO as well as Co3O4had a high dispersion on the support.Ce-Fe sample still appeared as three-dimensional sphere while the interweaved petal-like nanosheets structure could not be seen for the surface of CeO2was filled with Fe2O3species,some of which might even doped into the lattice of CeO2.

Fig.1.The SEM images of the CeO2.

Fig.2.The SEM images of the catalytic materials.

3.2.XRD analysis

The XRD patterns of all prepared samples were shown in Fig.3.It could be observed that all samples exhibited main reflections at 28.8°,33.1°,47.8°,56.5°,59.5°,69.3°,77.1° and 79.2° in the XRD patterns corresponding to the distinct cubic fluorite diffraction pattern of CeO2(JCPDS 34-0394),which indicated the structure of CeO2had not been changed significantly after metal species loaded [26].In addition,the XRD patterns of some catalytic materials also showed the characteristic diffraction peaks of the metal species,which proved that the metal species were successfully loaded on the CeO2support.For Ce-Ag,the three diffraction peaks exhibited at 38.2°,44.4°,and 64.2° correspond to the (111),(200),and(220)crystal planes of Ag2O[27].The diffraction peak of Ce-Co material at 2θ=36.7° was ascribed to the (311) crystal plane of Co3O4[28].It could be observed two characteristic peaks of NiO at 36.2° and 43.2°,which was attributed to the (111) and (200)crystal planes respectively [11].The two obvious characteristic diffraction peaks of CuO at 35.4° and 38.7° belonged to the (002),(311) crystal planes [29].However,the characteristic peaks of Fe elements were not seen on the XRD patterns of Ce-Fe which might be ascribed to the Fe2O3species doped into the lattice of CeO2,the results were consistent with the SEM analysis [30].The crystallite size of the metal species loaded on the CeO2were calculated according to the Scherrer equation and listed in Table 1.Among all samples,CuO species had the smallest crystallite size(18.3 nm) which demonstrated the interaction between CeO2and CuO was stronger than others to restrain the growth of the CuO species.

Fig.3.The XRD diagram of the prepared samples.

3.3.XPS analysis

It is well known that the adsorption and activation of reactant molecules are influenced greatly by the materials' surface properties.Therefore,XPS characterization was conducted to investigate the chemical state,surface atomic concentration and the dispersion of main element for the prepared samples,and the results were summarized in Table 1.

Fig.4 demonstrated the O 1s spectra of the five catalytic materials,it was observed that the spectra of the O1s could be fitted into three peaks at about 529.5 eV,531.05 eV and 532.4 eV,corresponding to the lattice oxygen (denoted as Oα),the chemisorbed oxygen(denoted as Oβ)and the adsorbed water or surface OH species(denoted as Oγ)on the surface of the samples[31].To our bestknowledge,Oβwas considered to be the most active oxygen form and always consumed first during the catalytic reactions due to its high mobility [32].The Oβcontent of the samples were calculated and listed in Table 1,which implied that Ce-Cu exhibit the highest Oβratio.

Table 1Crystallite size,surface composition and atomic ratios of the catalytic materials calculated from XPS

Fig.4.The XPS spectra of O1s.

Fig.5.The XPS spectra of Ce3d.

Fig.6.The XPS spectra of metal elements on CeO2.

Fig.5 showed the Ce 3d spectra of the five samples,The Ce 3d5/2spectra could be decomposed into four binding energy peaks at 882.4 eV,884.4 eV,888.7 eV and 898.5 eV (denoted as u0,u′,u′′,u′′′)while the Ce 3d3/2spectra was fitted with another four binding energy peaks at 901.2 eV,903.7 eV,907.9 eV,916.7 eV(denoted as v0,v′,v′′,v′′′).The binding energy peaks of u′and v′were attributed to the Ce3+species while others were related to Ce4+ions[33].Oxygen vacancies were usually generated as a function of surface Ce3+in ceria-based catalytic materials,which could enhance the catalytic activity by strengthen the synergy between metal active species and supports [21].Besides,as reported,oxygen vacancies could also weaken N–O bonds to promote the dissociation of NO[34].The Ce3+content was calculated and the results were listed in Table 1.Among all samples,the Ce-Cu exhibited the highest Ce3+ratio.

The spectra of five samples loaded with different metal species were shown in Fig.6.It could be seen all samples except Ce-Ag were fitted with four main binding energy peaks (denoted as,m0,m′,and n0,n′respectively),in which m0,n0represented the metal species with lower chemical valence and m′,n′were attributed to the metal species with higher chemical valence[31].For Ce-Ag,the binding energy peaks of Ag 3d5/2and Ag 3d3/2were centered at 368.3 and 374.3 eV respectively,which could be attributed to the Ag+.Taking Ce-Cu catalytic material for example,the synergy between Cu and Ce could be explained by following equation:which can help explain why the Ce3+ratio among the five materials varies [29].Noteworthy,the increase of Cu2+,Co3+or Ni2+were proved to enhance the removal activity toward HCN [15,35].The ratio of metal species with high chemical state was calculated and listed in Table 1.

3.4.H2-TPR analysis

In order to characterize the redox properties of the porous flowerlike ceria-based materials,the H2-TPR characterization was conducted and the results were displayed in Fig.7.As reported,the reduction of CeO2proceeded in two steps (Fig.7) [36].The first reduction peak appearing at 340°C and reaching a maximum at about 490°C could be assigned to the reduction of surface capping oxygen species of ceria which were generally considered as the source of active oxygen in low-temperature oxidations.The second peak would increase gradually above 600°C,which was attributed to the reduction of bulk material,however,due to the limit of reaction temperature,the second reduction peak was not shown in the spectra of CeO2[25].In addition,the new peak at 250°C came from the reduction of absorbed species on the surface of CeO2,which might be the absorbed oxygen or OH groups that could be reduced at lower temperature [37].

Fig.7.The H2-TPR profiles of the prepared samples.

There were three reduction peaks observed in the spectra of Ce-Cu,the small peak around 142°C(donated as α)and a pair of overlapping reduction peaks at 152°C (donated as β) and 174°C (donated as γ).The α peak came from the reduction of finely dispersed CuO species,the β peak was attributed to the reduction of CuO with moderate crystal size which interacted with the oxygen vacancies of CeO2and the γ peak was due to the reduction of a part of the surface ceria,the copper oxide incorporated into the ceria lattice as well as the large copper oxide phase[38].The lower reduction temperature of Ce-Cu owed to the reduction of easily reduced metal species and the interaction between CuO species and CeO2support,in addition,the results also indicated the Ce-Cu material had higher oxygen mobility and more surface active oxygen,which were consistent with XPS characterization [39].

The spectra of Ce-Ag exhibited two reduction peak,one peak at 146°C was related to the reduction of Ag+to Ag0and the other peak at 360°C was ascribed to the reduction of Ag2O aggregated on the surface of CeO2support [40].It was noteworthy that the broad H2consumption band in the 180–490°C range for CeO2in the spectra of Ce-Cu and Ce-Ag became weak,indicating that most of the hydrogen was used to remove the readily reducible oxygen in the surface layer of ceria.

The TPR spectra of Ce-Fe was composed of two peaks which were assigned to the reduction of Fe3+and/or a-Fe2O3to Fe(3-δ)+with intermediate valence as that in Fe3O4and then to Fe2+and the partial reduction of Fe2+to Fe0respectively [30].

The TPR spectra of Ce-Co were also consisted of two reduction peaks.The peak at 340°C was attributed to the reduction of Co3+to Co2+,while peak at 510°C represented the reduction of Co2+to Co [41].It was observed three peaks in the TPR spectra of Ce-Ni,the peak at 250°C was assigned to the reduction of welldispersed NiO species interacting strongly with the CeO2,another two weak peaks at 350°C and 420°C were attributed to the reduction of bulk NiO and crystallized NiO respectively [42].

Compared to the CeO2,the reduction temperatures corresponding to the reduction peaks of Ceria-based materials loaded with metal elements were significantly lower (Fig.7),which indicated the reduction behavior of the materials were enhanced.Among all samples,the Ce-Cu and Ce-Ag showed better reduction ability than other three samples for the reduction temperatures of which were much lower.

3.5.Effect of metal species

The prepared samples loaded with different metal species were tested for the removal of HCN at the condition of 30°C,120,000 h-1and 5% (volume) H2O,the results were illustrated in the Fig.8.It was observed the removal ability of HCN followed this order:Ce-Cu>Ce-Ag>Ce-Ni>Ce-Co>Ce-Fe>CeO2.The CeO2showed the lowest activity toward HCN while the Ce-Cu showed excellent HCN removal performance:the reaction time range for deep purification of HCN (100% removal rate) exceeded 70 min.The removal rate of HCN over prepared samples at different reaction temperature was also studied and the results were shown in Fig.S2 (Supplementary Material),which demonstrated the Ce-Cu still showed the highest activity among all samples.

The high activity of Ce-Cu toward HCN could be ascribed mainly to the chemical interaction of CuO in the active phase,which was shown in the following equation [43].

Fig.8.The removal rate of HCN over catalytic materialsReaction condition:30°C,120,000 h-1,5% (volume) H2O.

As reported,the HCN could also be converted directly to HOCN with the participation of O2[35].

In order to analysis the interaction of HCN and CuO,the FT-IR characterization was conducted and the results were listed in Fig.S3.The peak observed at 2142.5 cm-1confirmed the presence of CuCN on the surface of Ce-Cu catalytic material [44].

In addition,the atomic ratio of Oβand Ce3+on the surface of catalytic materials (Table 1) and the redox properties of samples(Fig.7) also played an important role in the HCN removal process.The chemisorbed oxygen (Oβ) was considered to be an active oxygen which generally played a positive role in the reaction system.Oxygen vacancies were usually generated as a function of surface Ce3+in ceria-based catalytic materials,which could enhance the catalytic activity by strengthen the synergy between metal active species and supports [45].It was noteworthy that the redox properties should not be seen as a key parameter influencing the performance of the samples on HCN removal,because the reduction temperature of Ce-Cu was higher than Ce-Ag in the Fig.7 while the removal ability of HCN over Ce-Cu was stronger.

The removal rate of HCN began to decrease obviously as the reaction time exceeded 70 min.At 30°C,the removal of HCN was mainly due to chemisorption on the surface of catalytic material.With reaction time proceeded,the adsorption sites on the Cu-Ce material began to saturate,the adsorption of HCN on other sites decreased and the adsorbed HCN could also occur desorption,which contributed the decrease of HCN removal rate [46].

3.6.Effect of calcination temperature

The influence of calcination temperature on the removal of HCN over CuO/CeO2were investigated at the reaction condition of 30°C,120,000 h-1and 5% (volume) H2O and the results were shown in Fig.9.As presented,the CuO/CeO2sample calcinated at 350°C exhibited the lowest catalytic activity as a result of the incompletely conversion of Cu2(OH)2CO3to CuO,as reported,the removal of HCN was mainly attributed to the chemical interaction of CuO in the active phase.Among all samples,the CuO/CeO2calcinated at the temperature of 450°C showed the highest catalytic activity(the breakthrough time was more than 70 min) which might be ascribed to the highly dispersion of CuO species on the surface of the support.However,as the calcination temperature increased further,the removal rate of HCN decreased instead,which might be assigned to the low surface area of the CuO/CeO2.The BET specific area,pore volume and pore size of the CuO/CeO2at different calcination temperatures were listed in Table 2.Higher calcination temperature could not only contribute to the structure collapse and an increase of the dimensions of metal-oxide crystalline which could further cause less surface chemical adsorption and fewer active catalytic sites of HCN but also decrease the amounts of functional OH groups and adsorbed H2O on the surface of the sample,which played an important roles in the catalytic activity of the CuO/CeO2[47].Based on the above analysis,the calcination temperature was considered as a key parameter influencing the performance of the sample on HCN purification.

Fig.9.The removal rate of HCN over CuO/CeO2 at different calcination temperature.Reaction condition:30°C,120,000 h-1,5% (volume) H2O.

3.7.BET results

The textural properties of the CuO/CeO2calcinated at different temperature including 350°C,450°C,550°C and 650°C were characterized by BET measurements,and the BET specific area,pore volume and pore size of the CuO/CeO2at different calcination temperatures were listed in Table 2.We observed that the BET specific surface area and pore volume of the CuO/CeO2increased first and then decreased while the pore size continued to increase as the increase of the calcination temperature.The results were reasonable because when the calcination temperature was lower than 450°C,the Cu2(OH)2CO3might have not been completely converted into CuO species,which leaded to many large particles of active species were supported on the surface of the support[48].As the calcination temperature increased,the CuO species on the surface of the support were dispersed uniformly,the space between the particles was reduced due to the particles becoming dense,thereby increasing the specific surface area and pore volume of CuO/CeO2.When the calcination temperature reached 450°C and increased further,the sintering of the support occurred,the CuO species might agglomerate on the support owing to the reduction of the active sites,which was consistent with the results of XRD characterization shown in Fig.S4 [49].Consequently,the BET specific area decreased sharply while the pore size increased.More significantly,the amounts of OH groups or H2O would also decrease gradually which were believed to play an important role in the removal of HCN under the low oxygen condition [47].

Table 2Surface area,pore volume,and pore volume of CuO/CeO2 at different calcination temperature

Fig.10.The Nitrogen adsorption/desorption isotherms of CuO/CeO2 samples at different calcination temperatures.

Fig.10 showed the N2adsorption–desorption isotherms for CuO/CeO2prepared under different calcination temperatures.The isotherms of all the samples followed the type IV trend with a H2-type hysteresis loop [25],which indicated the typical mesoporous structure was not influenced by the calcination temperature.With the increase of calcination temperature,the hysteresis loop of CuO/CeO2shifted in the direction of relatively high pressure,indicating the pore size of CuO/CeO2increased which was consistent with the above analysis [50].

3.8.Effect of reaction temperature

The effect of the reaction temperature on the removal rate of HCN over CuO/CeO2was investigated and the results were shown in Fig.11.As presented,the catalytic ability of CuO/CeO2increased with increasing the reaction temperature from 15°C to 150°C and improved significantly at the temperature region of 60–120°C,the breakthrough time of which could exceeded 240 min at the temperature of 90°C.Even the reaction time reached 300 min,the removal rate of HCN over CuO/CeO2could still attain 98.9% at 90°C while maintain 100% at 120°C and 150°C.However,the increase of reaction temperature had little effect on the catalytic activity of the CuO/CeO2while the temperature was lower than 60°C,for instance,the breakthrough time only prolonged 40 min(from 40 min to 80 min) when the temperature rose from 15°C to 60°C.

The reaction mechanism of the removal of HCN over CuO/CeO2at different reaction temperature was analyzed by detecting the products in the reaction system and the selectivity was listed in Fig.12.It could be observed the N-containing products involved NH3,NO,NO2and the C-containing products were composed of CO and CO2.The concentration of CN-and N2not detected were calculated from the C-balance and N-balance which were shown in Eqs.S(1)–S(2).The balance data of C and N elements were listed in Table S1.The ratio of methods of catalytic removal and chemisorption at different reaction temperature was calculated according to the C-balance and N-balance and listed in Table S2.

Fig.11.The removal rate of HCN over CuO/CeO2 at different reaction temperature.Reaction condition:120,000 h-1,5% (volume) H2O.

Fig.12.The removal rate of HCN and selectivity of reaction products over CuO/CeO2 at different reaction temperature.Reaction condition:120,000 h-1,5%(volume) H2O.

As presented in Fig.12 and Table S1,it was observed there were no N-containing products or C-containing products detected in the reaction system at the temperature of 15°C,which implied the HCN was adsorbed on the surface of CuO/CeO2in the presence of CN-due to the chemical interaction of CuO.When the reaction temperature rose to 30°C,the NH3and CO was detected which could be related to the hydrolysis of HCN by the following equation[16].

It was important to note that at the temperature of 30°C,the total amount of CO and CO2was not in line with that of removed HCN according to the C-balance,which might be the result of chemical adsorption by the CuO/CeO2.Besides,the generation of CO2indicated catalytic oxidation reaction occurred (as shown in the following equation)on the CuO/CeO2,however,the chemisorption still dominated in the reaction temperature [14].

The selectivity of CO2was improved dramatically with the increase of reaction temperature and reached 62.8%at 60°C,however,NH3was still lower than 10%,which implied catalytic oxidation began to play an important role while catalytic hydrolysis still showed low activity when the temperature was below 60°C.According to the Table S2,the removal of HCN was mainly attributed to the catalytic oxidation and chemisorption.

It could be observed both the selectivity of CO2and NH3were enhanced significantly when the temperature rose to 90°C as results of catalytic oxidation and catalytic hydrolysis dominated in the whole reaction system,which could help explain why the CuO/CeO2show excellent activity for the removal of HCN at 90°C.

The selectivity of NH3further increased and reached the maximum of 23.3%at 120°C implying the catalytic hydrolysis exhibited the highest activity,the decrease at higher temperature was probably due to the NH3oxidation shown as Eq.(13).Compared to the NH3formed from hydrolysis of HCN,only few amounts of CO could be measured which might be ascribed to the further oxidation of CO to CO2[51].

At the temperature region of 60–150°C,the NO selectivity decreased while NO2increased with the temperature going up as results of NO converted to NO2,however,both of which were still less than 10% over the whole reaction temperature range.The results were reasonably because in the presence of NO,NO2and NH3,N2could be generated by the NH3-SCR reaction on CuO/CeO2as shown in Eq.(14),which resulted the low concentration of NO and NO2[17].

4.Discussion

The reason why CuO/CeO2showed high activity for the removal of HCN among all samples was analyzed based the experimental results and the XRD,XPS as well as H2-TPR analysis.It was proved that the chemical interaction of CuO and HCN,the interaction between CuO and CeO2supports,the atomic ratio of Oβand Ce3+on the surface of CuO/CeO2and the redox properties of samples played an important role in determining the good activity for the removal of HCN.The XRD results demonstrated the CuO species had the smallest crystallite size(18.32 nm)as results of the stronger interaction between CuO and CeO2.In addition,the large amount of oxygen vacancies on the surface of CuO/CeO2which could strengthen the synergy between metal active species and supports (XPS analysis) also confirmed the XRD results.The XPS analysis illustrated the surface chemisorbed oxygen of CuO/CeO2considered to be highly active due to its higher mobility was the highest among all samples The lower reduction temperature of CuO/CeO2in the H2-TPR spectra attributed to the reduction of easily reduced metal species and the interaction between CuO and CeO2.More significantly,the H2-TPR analysis also indicated the CuO/CeO2had higher oxygen mobility and more surface active oxygen,which was consistent with XPS characterization.

The effect of calcination temperature on the removal of HCN over CuO/CeO2was investigated which implied the sample calcinated at 450°C exhibited the best HCN removal ability.The BET results showed the CuO/CeO2calcinated at 450°C possessed the largest specific surface area of 61.478 m2.g-1,which could enhance the sample's removal ability of HCN by promoting the highly dispersion of CuO species on the surface of the support.

The mechanism of the removal of HCN over CuO/CeO2material at different reaction temperature was analyzed which showed the chemisorption,catalytic hydrolysis,catalytic oxidation and NH3-SCR worked together to achieve the deep purification of HCN at the temperature region of 15–150°C.Based the results,a possible mechanism over CuO/CeO2sample on the removal of HCN under lower temperature at the conditions of 120,000 h-1and 5% (volume) H2O was proposed.

Fig.13.The possible reaction mechanism for the removal of HCN over CuO/CeO2.

HCN was first decomposed into CN-on the surface of CuO/CeO2due to the chemisorption of CuO,and then CN-were oxidized to NCO by H2O and O2under the catalysis of Cu2+[35].The formed NCO could further be converted to NH3and CO by the catalytic hydrolysis.On the other hand,the generated NCO species were oxidized to form N2,NO and NO2directly by catalytic oxidation [52]Besides,NH3reacted with NO (byproduct of HCN oxidation) to form N2.The brief mechanism diagram and the reaction process was showed in Fig.13.

5.Conclusions

In this paper,porous flowerlike ceria-based materials loaded with different metal species were synthesized for the efficiency removal of HCN,which demonstrated that CuO/CeO2showed the highest activity at the calcination temperature of 450°C.Based the SEM,XRD,XPS,H2-TPR and BET analysis,it was proved the interaction between CuO and CeO2,the atomic ratio of Oβand Ce3+as well as the redox properties of samples played an important role in determining the excellent activity for the removal of HCN.The mechanism of the removal of HCN over CuO/CeO2material was analyzed by examining the selectivity of the reaction products at different reaction temperature.It could be concluded the removal of HCN was attributed to the chemisorption,catalytic oxidation,catalytic hydrolysis as well as NH3-SCR reaction at the temperature region of 15–150°C.

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

This work is supported by the Chinese Civil Air Defense Office([2014] No.251-61) and the Military Scientific Research Program(Zhuangzong [2018] No.635).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.08.029.