The highly efficient removal of HCN over Cu₈Mn₂/CeO₂ catalytic material

Abstract

In this work, porous CeO₂ flower-like spheres loaded with bimetal oxides were prepared to achieve effective removal of HCN in the lower temperature region of 30–150 °C. Among all samples, the CeO₂ loaded with copper and manganese oxides at the mass ratio of 8/2 (Cu₈Mn₂/CeO₂) exhibited the highest catalytic activity: the HCN removal rate was nearly 100% at 90 °C at the conditions of 120 000 h⁻¹ and 5 vol% H₂O, the catalytic activity of which was higher than for other reported catalysts. The introduction of MnO_x could improve the dispersion of CuO particles and increase the total acid sites of the prepared samples. It was proved that the synergy between CuO and MnO_x, the chemisorption oxygen, the oxygen vacancies, the Cu²⁺ and Mn⁴⁺ all played an important role in determining the good catalytic activity of the prepared samples. NH₃-TPD analysis indicated the introduction of MnO_x promoted the conversion of NH₃ and N₂ selectivity by increasing the acid sites of the sample. According to the C, N balance data and FT-IR results, when the temperature was below 30 °C, the removal of HCN over Cu₈Mn₂/CeO₂ was mainly by chemisorption and the HCN breakthrough behaviors corresponded to the Yoon and Nelson's model. When temperature was above 120 °C, the HCN was totally removed by catalytic hydrolysis and catalytic oxidation.

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1 Introduction

Hydrogen cyanide (HCN) is a kind of highly toxic and volatile compound, with a boiling point of 26 °C, which is widely used in the laboratory and chemical industry.¹ HCN could inhibit the enzyme cytochrome oxidase, and interfere with aerobic respiration at cellular level, which poses a threat to human health.² Significantly, as the precursor of nitrogen oxides such as NO, NO₂ and N₂O, HCN could also lead to the greenhouse effect and generation of photochemical fumes.³ In addition to laboratory synthesis, there is a fair chance to produce HCN including the processing of cyanide-containing chemicals,⁴ processing tail gas of calcium and yellow phosphorus,⁵ fossil fuels and biomass combustion,⁶ selective catalytic reduction of NO_x as well as the well-established three-way-catalyst process.⁷ To relieve or reduce the negative effect on both the human body and the environment caused by HCN, it is essential to develop highly efficient dicyanide methods. Based on the above discussion, the removal of HCN has been paid more and more attention and the treatment of HCN has important prospects.

Diverse strategies have been reported to remove HCN from gas which involves absorption, adsorption, combustion, catalytic hydrolysis and catalytic oxidation.⁸

Liu et al.⁹ studied the catalytic combustion of HCN over ZSM-5 catalysts loaded with Cu, Co, Fe, Mn, Ni species. Among all samples, Cu-ZSM-5 exhibited excellent HCN conversion activity (T₉₀ = 350 °C) and high N₂ yield (>95%). Nickolov et al.¹⁰ reported the adsorption characteristics of modified activate carbons (AC) toward HCN. When loaded with Cu (5.6 wt%), Zn (5.24 wt%), Ag (0.61 wt%), the AC material (VSZC) showed enhanced HCN removal ability, which was proved to be comparable with the standard ASC Whetlerite carbon. The Zn phase deposited near the external surface of the sample was consumed first to reduce the production of cyanogen. It has also been proved that the addition of negligible amount of Cr⁶⁺ (3.5 times lower than that in ASC Whetlerite carbon) could enhance the conversion of cyanogen. Hudson et al.¹¹ synthesized an SBA-15 material modified with the BBOP and equilibrated with copper (II) nitrate. Owing to the greater accessibility of the functional groups, the doddered nature of the interconnected porous network and large mesopores, The SBA-15 material showed an improved breakthrough time for HCN (36 min) which was closed to the reference AC adsorbents. Ma et al.¹² investigated the Nb/La–TiO_x catalyst's activity for the hydrolysis of HCN, the conversion of HCN over which could reach 100% accompanied with more than 90% NH₃ selectivity. They proposed following mechanism of HCN hydrolysis over Nb/La–TiO_x catalyst: HCN was first converted to intermediated CHONH₂ under the function of catalyst, subsequently, the CHONH₂ was continually hydrolyzed to NH₄⁺ and HCOOH with the participation of acid sites of the catalyst. Finally, the NH₄⁺ was converted into NH₃·H₂O, and simultaneous HCOOH was decomposed to CO and H₂O or CO₂ and H₂O. Hu¹³ reported the metal oxides (Fe, Cu) modified HZSM-5 catalysts synthesized by incipient wetness method showed improved performance toward HCN removal (100% conversion of HCN and 80% selectivity of N₂), the high catalytic activity of which could be attributed to the highly dispersed iron and copper composites on the surface of the catalyst support, the excellent redox ability and regulated acid properties. Wang et al.¹⁴ carried out a detail investigation on coupling catalytic hydrolysis and oxidation of HCN over a serious MnO_x/TiO₂–Al₂O₃ prepared via sol–gel method. 15 wt% MnO_x/TiO₂–Al₂O₃ showed the best performance on HCN oxidation, and nearly 100% HCN conversion and 70% N₂ yield were obtained at 200 °C.

From these above statements, it could be observed the catalytic hydrolysis and oxidation based on these catalysts require relatively higher temperature (>200 °C) to achieve deep purification of HCN, which might cost a lot of energy. Adsorbent materials showed excellent removal efficiency toward HCN at room temperature while the incomplete conversion of HCN and the secondary pollution caused by the desorption products limit the further application of this method. Therefore, it is necessary to develop novel catalytic materials to obtain high HCN removal rate at lower temperature range (30–150 °C).

CeO₂ has been widely used in various fields including low-temperature water-gas shift reaction, fuel cells, phosphors, catalysts, cocatalysts, catalyst supports as well as hydrogen storage materials due to the high oxygen storage/release capacity (OSC) and high oxygen mobility arising from facile redox cycle between Ce⁴⁺ and Ce³⁺.^15,16 In recent reports, CeO₂ materials with different structure and morphologies has been prepared and evidenced to exhibit unique properties differing from common CeO₂.^17,18 The porous CeO₂ flowerlike sphere has attracted lots of interest for its larger pore volume, higher surface area and marked hydrothermal stability when used as supports.^19,20 Tan et al.²¹ conducted the oxidation of styrene on the porous CeO₂ flowerlike sphere loaded with CuO, La₂O₃ and Fe₂O₃, and found the CuO/CeO₂ showed the highest activity. Sun et al.²² examined the catalytic activities and hydrothermal stability of Cu/CeO₂ for ethanol stream reforming. At 300 °C, the H₂ selectivity could reach 74.9% while the CO was below the detection line.

In this work, the porous CeO₂ flowerlike spheres modified with a series of bimetallic oxides (Cu–Ag, Cu–Mn, Cu–Zn, Cu–Ni, Cu–Co, Cu–Fe) were prepared and their catalytic activity for HCN removal were evaluated under lower temperature region (30–150 °C). The reaction kinetics and the mechanism of HCN removal over the synthesized samples were analyzed.

2 Experimental

2.1 Catalytic materials preparation

2.1.1 Porous CeO₂ flowerlike spheres preparation. The synthesis of porous CeO₂ flowerlike spheres was according to our previous method, which were reported in the literature.²²

2.1.2 Catalytic materials preparation. CeO₂ loaded with copper and manganese oxides were prepared by precipitation method. The CeO₂ sample was dissolved into deionized water with magnetic stirring, followed by the addition of Cu(NO₃)₂·3H₂O and Mn(NO₃)₂·4H₂O at the mass ratio of 8 : 2. The total content of Cu and Mn oxides always accounted for 10 wt% of the CeO₂ support. Then the Na₂CO₃ solution (0.5 mol L⁻¹) was added at the speed of 1drop/s until the pH value of the solution reached 9. After being stirred for 4 h, the suspension and precipitate were separated by centrifugation and the precipitate was washed with deionized water and ethanol for three times respectively. Finally, the samples donated as Cu₈Mn₂/CeO₂ were obtained after undergoing such steps: calcination, grinding, pelletizing and sieving. The total metal mass percentage was calculated by the equation (M/(M + CeO₂), M = Cu, Mn), in which wt% means the mass percentage. The Cu₈Ag₂/CeO₂, Cu₈Ni₂/CeO₂, Cu₈Co₂/CeO₂, Cu₈Zn₂/CeO₂ and Cu₈Fe₂/CeO₂ samples were also prepared by the same method by changing the nitrate solution of metal species.

2.2 Experimental setup

The catalytic activity test of the prepared samples was evaluated in a fixed-bed reactor, and the reaction scheme was shown in Fig. S1.† The prepared sample (0.25 g) was placed in a quartz tube (with an inner diameter of 5 mm) for each reaction. The mixed gases containing 400 mg m⁻³ HCN and 5 vol% H₂O were introduced into the fixed-bed quart tubular reactor at a total flow rate of 400 mL min⁻¹ (GHSV = 120 000 h⁻¹). The exhaust gas from the sample bed was first analyzed (as described below) and then absorbed and discharged.

2.3 Detection and analysis of reacted products

The outlet gas from the sample bed was first analyzed and then absorbed by the NaOH solution (1 mol L⁻¹). The HCN and NH₃ were 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⁻¹ and 0.0025 mg L⁻¹ respectively. The concentration of CO, CO₂, NO and NO₂ (mg m⁻³) were measured using a flue gas analyzed (DeTu, 350, German) after the gas stabilized for 80 min at the corresponding temperature. The HCN removal rate and reaction products selectivity were calculated according to the eqn (S1)–(S8).†

2.4 Characterization

The related characterization information was detailed in the ESI.†

3 Results and discussion

3.1 Catalytic activity of CeO₂ loaded with copper and manganese oxides

In order to evaluate the catalytic activity of CeO₂ and the CeO₂ loaded with copper and manganese oxides, the four prepared samples including CeO₂, CuO/CeO₂, MnO_x/CeO₂ as well as Cu₈Mn₂/CeO₂ were tested for the HCN catalytic removal and the results were shown in Fig. 1. It could be observed the HCN removal rate of all samples increased with increasing the reaction temperature from 30 °C to 150 °C. The CeO₂ support showed the lowest activity toward HCN, even at 150 °C, the HCN removal rate was still below 70%. The HCN removal rate over CuO/CeO₂ were higher than MnO_x/CeO₂ throughout the reaction temperature region, indicating the CuO phase on the CeO₂ played an important role in the HCN catalytic removal. The reason why CuO exhibited good catalytic performance could be explained by the interaction between CuO and HCN which was demonstrated in following equations.^23,24

2CuO + 4HCN → 2CuCN + (CN)₂ + 2H₂O (1)

Fig. 1 The HCN removal rate over CeO₂, CuO/CeO₂, MnO_x/CeO₂ and Cu₈Mn₂/CeO₂ samples. Reaction condition: 120 000 h⁻¹, 5 vol% H₂O.

The highest HCN removal rate was obtained for Cu₈Mn₂/CeO₂: it was equal to 73% at 30 °C and reached nearly 100% at 90 °C. To determine the optimal Cu/Mn mass ratio, a serious of Cu_xMn_y/CeO₂ materials including Cu₈Mn₂/CeO₂, Cu₆Mn₄/CeO₂, Cu₄Mn₆/CeO₂ and Cu₂Mn₈/CeO₂ were prepared and the catalytic activity of these samples were compared with CuO/CeO₂ and MnO_x/CeO₂. As presented in Fig. S2,† the Cu₈Mn₂/CeO₂ still exhibited the highest activity and the optimal Cu/Mn ratio was 8 : 2. The addition of a small amount of MnO_x could enhanced the catalytic activity of CuO/CeO₂ and the results might be attributed to the synergy between copper and manganese oxides, which was in accordance with the study of Li and Njagi et al. who studied the catalytic activity of Cu–Mn–O catalysts toward HCN and CO respectively.^25,26

Cu²⁺ + Mn³⁺ ↔ Cu⁺ + Mn⁴⁺ (4)

3.2 Catalytic activity of CeO₂ loaded with different bimetal oxides

Fig. 2 investigated the HCN catalytic removal rate over CeO₂ loaded with different bimetal oxides including Cu₈Fe₂/CeO₂, Cu₈Ni₂/CeO₂, Cu₈Co₂/CeO₂, Cu₈Ag₂/CeO₂ and Cu₈Zn₂/CeO₂ samples. All samples exhibited the same HCN removal trend as Cu₈Mn₂/CeO₂ at the temperature region of 30–150 °C: the HCN removal rate increased as temperature improving. Cu₈Ni₂/CeO₂ and Cu₈Fe₂/CeO₂ showed the similar catalytic performance and their activity were both stronger than CuO/CeO₂. The results indicated the introduction of iron and nickel species could generated good synergy with copper oxide species. Significantly, the addition of iron species could strengthen the redox properties of catalysts and increase the dispersion of copper species by inhabiting the particles agglomeration.²⁷ For Cu₈Co₂/CeO₂, it exhibited the same catalytic activity toward HCN as CuO/CeO₂. Noteworthy, when cobalt and sliver oxides were introduced, the activity of two samples were lower than CuO/CeO₂, this might be ascribed to the weaker synergy and lower CuO active sites on the support surface. Among all samples, the Cu₈Mn₂/CeO₂ still showed the most remarkable catalytic performance toward HCN, implying the synergy between copper and manganese was stronger than other bimetal oxides. Table S1† showed the comparison of HCN catalytic removal activity over various catalysts, indicating the Cu₈Mn₂/CeO₂ exhibited the lowest T_100% temperature, at which HCN could be totally removed.

Fig. 2 The HCN removal rate over CeO₂ loaded with different bimetal oxides. Reaction condition: 120 000 h⁻¹, 5 vol% H₂O.

In order to investigate the thermal stability of Cu₈Mn₂/CeO₂, the Cu₈Mn₂/CeO₂ pretreated at 200 °C, 400 °C and 600 °C were tested for the HCN removal and the results shown in Fig. S3† indicated the catalytic activity of Cu₈Mn₂/CeO₂ were not influenced when the pretreated temperature varied. Further TG-DCS analysis shown in Fig. S4† also proved the Cu₈Mn₂/CeO₂ showed high thermal stability.

It was necessary to know whether the Cu₈Mn₂/CeO₂ could be recycled. The regeneration of the Cu₈Mn₂/CeO₂ were performed by the calcination of the samples at 450 °C for 4 h. Four recycling runs for the Cu₈Mn₂/CeO₂ were carried out, the results shown in Fig. S5† indicated the Cu₈Mn₂/CeO₂ could still achieve nearly 80% removal rate toward HCN even after 4 runs.

3.3 Analysis of HCN reaction products over Cu₈Mn₂/CeO₂

The HCN removal rate and the reaction products over Cu₈Mn₂/CeO₂ were investigated at the temperature region of 30–150 °C and the results were shown in Fig. 3. The products detected in the outlet gas involved CO, CO₂, NH₃, NO and NO₂. The selectivity of CN⁻ and N₂ were calculated according to the C-balance N-balance data and the concentration of all reaction products were listed in Table S2.†

Fig. 3 The removal rate of HCN and selectivity of reacted products over Cu₈Mn₂/CeO₂ at different temperature. Reaction condition: 120 000 h⁻¹, 5 vol% H₂O.

At 30 °C, the selectivity of CO and CO₂ were not in line with the HCN removed, which implied most HCN were converted to CN⁻ due to the chemical interaction between CuO and MnO_x phase on the CeO₂ support (eqn (1)) and the HCN was mainly removed by chemisorption. The small amount of CO and NH₃ might be attributed to the catalytic hydrolysis reaction of HCN (shown in eqn (5)), while the CO₂ were assigned to the catalytic oxidation (shown in eqn (6)). In addition, the oxidation of CO could also generate CO₂.

HCN + H₂O → NH₃ + CO (5)

4HCN + 5O₂ → 4CO₂ + 2N₂ + 2H₂O (6)

The catalytic hydrolysis of HCN over Cu₈Mn₂/CeO₂ showed low activity at the temperature region of 30 °C–60 °C, the NH₃ yield changed little with temperature increasing. However, the high CO₂ selectivity indicated the catalytic oxidation of HCN was enhanced, the CN⁻ was oxidized to CO₂ and released more active sites, which could strengthen the HCN removal rate.

The selectivity of NH₃ and CO₂ increased significantly from 60–150 °C as results of catalytic hydrolysis and catalytic oxidation of HCN showing high activity. The generated NO and NO₂ might be attributed to the byproducts of catalytic oxidation of HCN. Compared with NH₃ generated by catalytic hydrolysis, the amount of CO disappeared with temperature increasing due to the oxidization of CO to CO₂. When the temperature reached 120 °C and increased further, the CO₂ selectivity stayed 100%, implying the HCN was completely removed by catalytic hydrolysis and oxidation.

More significantly, the NH₃ selectivity generated by catalytic hydrolysis was below than 20% at the whole reaction temperature, which were lower than the reported literature.^12,13 It might be attributed to the increase of acid sites on the Cu₈Mn₂/CeO₂ after MnO_x introduced and the generated NH₃ could be absorbed on the Cu₈Mn₂/CeO₂ samples and converted into N₂.

3.4 Breakthrough behaviors of HCN over Cu₈Mn₂/CeO₂

By analyzing the reaction products of HCN, it was concluded that the HCN was mainly removed by chemisorption at 30 °C. To study the catalytic materials' adsorption performance, the breakthrough behaviour of HCN over Cu₈Mn₂/CeO₂ was investigated and the breakthrough curves were shown in Fig. S6.†

The Yoon and Nelson's equation was introduced to describe the breakthrough behaviour of HCN over Cu₈Mn₂/CeO₂.²⁸

where C₀ was the inlet concentration of HCN, mg m⁻³; C referred to the breakthrough concentration of HCN, mg m⁻³; t was the breakthrough time, min; τ was the time required to obtain 50% breakthrough, min⁻¹; k was the lumped shape factor of breakthrough, min⁻¹.

Fig. 4 demonstrated the experimental results of the and t curves, the values of k and τ at different temperature were calculated and the results were listed in Table S3.† With temperature increased, the thermal motion of HCN became intense, and the number of activated molecules increased, which accelerated the saturation of activated sites on the Cu₈Mn₂/CeO₂. HCN was absorbed on the samples' surface in the form of CN⁻ and could not be converted further. The activated sites could not be released, as a result, the breakthrough curve became steeper and breakthrough time became shorter as temperature increased.

Fig. 4 Plot of –t for Cu₈Mn₂/CeO₂ bed at temperature region of 10–30 °C. Reaction condition: 120 000 h⁻¹, 5 vol% H₂O.

3.5 SEM and TEM analysis

The morphological properties of samples were characterized by SEM analysis. As presented in Fig. 5, the CeO₂ support exhibited flowerlike mesoporous microspheres with average diameter of 2–4 μm and dispersed evenly. There were many petal-like nanosheets on the microsphere surface and these nanosheets interweaved together to form the porous structure.²⁹ Fig. 6 showed the morphology and size of Cu₈Mn₂/CeO₂ materials, it could be observed the metal oxide particles were loaded dispersively on the CeO₂ supports and the porous structure were covered after metal oxide species loaded. EDS analysis shown in Fig. S6† implied the Cu and Mn elements had uniform distribution on the CeO₂ surface. The SEM images for Cu₈Zn₂/CeO₂, Cu₈Ni₂/CeO₂, Cu₈Co₂/CeO₂, Cu₈Fe₂/CeO₂ and Cu₈Ag₂/CeO₂ were shown in Fig. S7.† The actual metal oxides loading of all samples were calculated according to EDS analysis shown in Fig. S8† and the results were listed in Table 1, which demonstrated the actual metal oxides loading were in accordance with theoretical value.

Fig. 5 The SEM images of the CeO₂. (1) and (2) Low and high magnification SEM images of CeO₂.

Fig. 6 The SEM images of the Cu₈Mn₂/CeO₂ and the corresponding elements (O, Cu, Ce, Mn) mapping. (1) and (2) Low and high magnification SEM images of Cu₈Mn₂/CeO₂.

Table 1 The structural parameters of the CeO₂ loaded with different bimetal oxides

Samples	CuO (wt%)	MxOy (wt%)	Crystal size^a
a Crystallite sizes calculated from the line broadening of the (002) plane of CuO.
Cu8Mn2/CeO2	9.7	1.8	—
Cu8Zn2/CeO2	9.3	2.1	30
Cu8Ni2/CeO2	9.5	2.3	17
Cu8Co2/CeO2	9.3	2.3	22
Cu8Fe2/CeO2	10.7	2.0	—
Cu8Ag2/CeO2	9.4	1.7	41

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In order to analyze the microstructure and crystallinity of the Cu₈Mn₂/CeO₂ samples, the HR-TEM characterization was carried out and the results were shown in Fig. 7. As shown in Fig. 7(1), it could be seen the Cu₈Mn₂/CeO₂ exhibited a sphere structure with diameter of 2–3 μm, which was consistent with SEM characterization. Fig. 7(2) indicated the Cu₈Mn₂/CeO₂ was polycrystalline structure composed of many nanocrystals at different orientations.³⁰ The nanocrystals marked in Fig. 7(4) was assigned to CuO species loaded on CeO₂ support, the crystal size of which were around 10–15 nm. Three interplanar spacings of the ordered stripes marked in Fig. 7(5) were 0.27 nm, 0.31 nm and 0.23 nm, corresponding to the (200) and (111) lattice planes of CeO₂ and (111) lattice planes of CuO.³¹ However, there no MnO_x phase observed in Fig. 7(5), this could be explained by the high dispersion of MnO_x due to the low loading. Fig. 7(6) displayed the TEM image for CeO₂, it could be concluded the structure of CeO₂ was not changed after metal oxides loaded and still exposed (200) and (111) lattice planes.

Fig. 7 The HR-TEM images of the Cu₈Mn₂/CeO₂ at different magnification. (1) The overall Cu₈Mn₂/CeO₂ morphology; (2) and (3) the polycrystalline structure of Cu₈Mn₂/CeO₂; (4) the CuO phases; (5) the crystal planes of Cu₈Mn₂/CeO₂; (6) the crystal planes of CeO₂.

3.6 XRD analysis

Fig. 8 showed the XRD patterns of CeO₂ loaded with different bimetal oxides. The main diffraction peaks at 28.8°, 33.1°, 47.8°, 56.5°, 59.5°, 69.3°, 77.1° and 79.2° were ascribed to the cubic fluorite cerium oxide and the peaks at 35.4° and 38.7° were attributed to the CuO (JCPDS 34-0394).³² Except Cu₈Ag₂/CeO₂ exhibited the Ag₂O diffraction peak at 38.2°, no diffraction peaks derived from other metal or metal oxides (Mn, Zn. Ni, Co, Fe) were observed in the XRD patterns.³³ The results implied these metal oxides were well dispersed on the CeO₂ support due to theirs low loading. The crystal size of CuO loaded on CeO₂ in all samples was calculated according to the Scherrer equation and the results were listed in Table 1. It could be seen the introduction of ZnO or Ag₂O promoted the agglomeration of CuO on the surface of supports and increased the crystal size. For NiO or Co₃O₄, they showed little effect toward the crystal size of CuO. When introduced MnO_x or Fe₂O₃, the diffraction peaks of CuO disappeared, indicating the CuO species were dispersed well as amorphous oxides or some Cu species doped into the lattice of CeO₂ and formed Cu–Ce–O solid solution.³¹ CuO and MnO_x species could strengthen the synergy between CuO and CeO₂ support to restrain the growth of CuO particles. Combined with catalytic activity test results, it could be deduced the bimetal synergistic effect could improve samples' activity by strengthening the interaction between metal oxides and supports. The XRD patterns of CeO₂, CuO/CeO₂ and MnO_x/CeO₂ were shown in Fig. S9,† and the crystal size of metal oxide particles calculated were also listed in Table 2.

Fig. 8 The XRD patterns of CeO₂ loaded with different bimetal oxides.

Table 2 The textural data and crystallite size of the CeO₂, CuO/CeO₂, MnO_x/CeO₂, Cu₈Mn₂/CeO₂

Samples	Surface area/(m^2 g^−1)	Pore volume/(cm^3 g^−1)	Pore size/nm
CeO2	104.4	0.12	4.6
CuO/CeO2	59.1	0.23	14.3
MnOx/CeO2	96.3	0.16	6.5
Cu8Mn2/CeO2	40.1	0.22	24

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3.7 H₂-TPR

The reduction properties of CeO₂, CuO/CeO₂, MnO_x/CeO₂ and Cu₈Mn₂/CeO₂ were investigated by H₂-TPR analysis and the results were shown in Fig. 9. The reduction peaks of CeO₂ have been detailed discussed in our previous work.⁸

After peak fitting process, three reduction peaks were observed in the CuO/CeO₂ spectra. The first peak at 158 °C were attributed to the reduction of finely dispersed CuO species on the CeO₂ supports, the second peaks at 181 °C came from the reduction of CuO with moderate crystal size which interacted with the oxygen vacancies of CeO₂ and the third peak at 212 °C was related to the reduction of a part of the surface ceria, the copper oxide incorporated into the ceria lattice and the large copper oxide phase. Compared with CeO₂, the surface ceria reduction peak shifted to lower temperature, which could be assigned to the presence of hydrogen spillover process from metal Cu surface and the interaction between CuO and CeO₂.³⁴

For MnO_x/CeO₂, the first reduction peak at 250 °C was assigned to the reduction of MnO₂ or Mn₂O₃ to Mn₃O₄, the second peak at 410 °C was related to the reduction of Mn₃O₄ to MnO and surface Ce⁴⁺ to Ce³⁺.³⁵ The results indicated the interaction of MnO_x and CeO₂ were weaker than CuO and CeO₂, which might help explain why CuO/CeO₂ showed high catalytic activity than MnO_x/CeO₂. When loaded with MnO_x, the Cu₈Mn₂/CeO₂ not only exhibited the three reduction peaks corresponding to CuO/CeO₂ but also showed the reduction peak of MnO₂ or Mn₂O₃ to Mn₃O₄, while the reduction peak of Mn₃O₄ to MnO was not observed, implying the Mn⁴⁺ were more actively than Mn³⁺ or Mn⁴⁺. Combing with the XRD analysis, the MnO_x had a high dispersion on the CeO₂ support due its low content and the interaction between MnO_x and CeO₂ prevented the reduction of Mn₃O₄ to MnO. Compared with CuO/CeO₂, the peak intensity of Cu₈Mn₂/CeO₂ was improved and the H₂ consumption also increased, which implied the Cu₈Mn₂/CeO₂ showed high catalytic activity. The TPR peaks and H₂ consumption of CuO/CeO₂ and Cu₈Mn₂/CeO₂ were summarized in Table S4.† In addition, the reduction peaks of Cu₈Mn₂/CeO₂ were shifted to higher temperature region, which might be attributed to the formation of polymeride.³⁶

Fig. 9 The H₂-TPR profiles of the prepared CeO₂, CuO/CeO₂, MnO_x/CeO₂ and Cu₈Mn₂/CeO₂ samples.

3.8 NH₃-TPD

In order to probe the acidity of CeO₂, CuO/CeO₂, MnO_x/CeO₂ and Cu₈Mn₂/CeO₂ samples, the NH₃-TPD analysis were carried out. As presented in Fig. 10, the NH₃ desorption peaks could be divided into three parts: peaks at 100–200 °C were corresponded to the desorption of physically absorbed NH₃ and NH₃ absorbed on the weak Brønsted acid sites, the peaks at 200–500 °C were attributed to the desorption of NH₄⁺ absorbed on the strong Brønsted acid sites and the peaks at above 500 °C were related to the desorption of NH₃ absorbed on the weak and strong Lewis acid sites.^37,38 The area of the NH₃ desorption peaks directly related to the amount of acidic sites and their acidic strength, respectively. Quantitative analysis of NH₃ desorption at corresponding temperature over all samples were calculated and the results were listed in Table S5.† It could be observed the total amount of acid sites of Cu₈Mn₂/CeO₂ at lower temperature were improved and a desorption peak at above 500 °C region was observed, indicating the acidic strength of samples was enhanced when MnO_x was introduced. Significantly, the NH₃ desorption amount of Cu₈Mn₂/CeO₂ was higher than CuO/CeO₂ at 100–200 °C region, which implied the Cu₈Mn₂/CeO₂ could bind more NH₃ at lower temperature than CuO/CeO₂. The stronger NH₃ adsorption capacity of samples could enhance the absorption ability toward NH₃ generated by catalytic hydrolysis and promote the conversion of NH₃ to N₂ which was consistent with the results of Fig. 3.

Fig. 10 The NH₃-TPD profiles of CeO₂, CuO/CeO₂, MnO_x/CeO₂ and Cu₈Mn₂/CeO₂ samples.

3.9 BET analysis

The N₂ desorption–desorption isotherms of CeO₂, CuO/CeO₂, MnO_x/CeO₂ as well as Cu₈Mn₂/CeO₂ were displayed in Fig. 11. The adsorption–desorption isotherms of all samples showed a type IV trend with a H₂-type hysteresis loop, implying the typical mesoporous structure of CeO₂ was not influenced after metal oxides loaded.²⁹ The pore size distribution of three catalytic materials were calculated and the results were shown as the insert part in Fig. 11. The textural properties including the specific surface area, pore volumes, and pore size were listed in Table 2. The specific surface area of three catalytic materials decreased while the pore volumes and pore size increased and Cu₈Mn₂/CeO₂ showed the smallest specific surface area 40.1 m² g⁻¹ and largest pore size 24 nm respectively. These might be assigned to the fact that copper and manganese oxides covered the external surface of CeO₂, and block the micropores structure, which was consistent with SEM analysis.¹³

Fig. 11 The nitrogen adsorption–desorption isotherms and pore size distributions of CeO₂, CuO/CeO₂, MnO_x/CeO₂ and Cu₈Mn₂/CeO₂ samples.

3.10 XPS analysis

By analyzing the reaction products, it could be obtained the HCN was mainly removed by catalytic hydrolysis and catalytic oxidation when the reaction temperature was higher than 90 °C. To further investigate the reaction mechanism between HCN and Cu₈Mn₂/CeO₂, the XPS characterization was carried out to analyze the chemical state, surface atomic concentration and the dispersion of main elements of the samples before and after reaction and the results were summarized in the Table 3.

Table 3 The surface atom types and atomic ratio among Cu₈Mn₂/CeO₂ samples

Samples	O (100%)			Cu (100%)		Ce (100%)		Mn (100%)
	Oα	Oβ	Oγ	Cu^2+	Cu^+	Ce^4+	Ce^3+	Mn^4+	Mn^3+	Mn^2+
Fresh	54	35	11	31	44	73	27	21	54	0
30 °C	47	31	22	25	52	68	32	18	33	19
90 °C	65	26	9	19	58	65	35	14	48	11

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Fig. 12 showed the O 1s XPS spectra of the fresh samples and samples reacted at 30 °C and 90 °C. It could be observed an asymmetrical peak at the bonding energy range of 528–536 eV, which could be resolved into three distinct peaks: peak located at 529.5 eV (denoted as O_α) was ascribed to the lattice oxygen, peak located at 531.1 eV (denoted as O_β) was corresponded to the surface chemisorption oxygen and the peak at 532.2 eV (denoted as O_γ) was assigned to the adsorbed water or surface OH species on the samples.³⁹ The content of the O_α, O_β and O_γ were calculated and listed in the Table 3. As presented in Table 3, after reacted with HCN at 30 °C, the O_β content of Cu₈Mn₂/CeO₂ decreased from 35% to 31% which indicated the O_β took part in the HCN catalytic removal over the samples due to its higher mobility. The increase of O_γ content was attributed to the H₂O in the reaction system absorbed on the surface of samples. With temperature increased further, the O_β content decreased continuously and O_γ content also decreased, implying the catalytic hydrolysis activity enhanced, the O_β and H₂O absorbed worked together to contribute to the HCN catalytic removal over Cu₈Mn₂/CeO₂.

Fig. 12 The O 1s spectra of Cu₈Mn₂/CeO₂ samples.

Ce 3d XPS spectra of Cu₈Mn₂/CeO₂ were displayed in Fig. 13. After peak fitting process, it could be observed eight binding energy peaks at the bonding energy region of 875–925 eV. The binding energy peaks of u′ and v′ were assigned to the Ce³⁺ species while others were related to Ce⁴⁺ ions and the content of Ce³⁺ and Ce⁴⁺ were listed in the Table 3.⁴⁰ With reaction temperature increased from 30 to 90 °C, the Ce³⁺ content increased from 27% to 35%, while the Ce⁴⁺ content decreased from 73% to 65%, owing to the synergy between Cu²⁺ and Cu³⁺ shown as following equation.⁴¹

Ce⁴⁺ + Cu⁺ ⇌ Ce³⁺ + Cu²⁺ (8)

Fig. 13 The Ce 3d spectra of Cu₈Mn₂/CeO₂ samples.

Oxygen vacancies could be generated as a function of surface Ce³⁺ on the samples, which could avoid the CuO particles agglomeration and improve its dispersion on CeO₂ support by enhancing the synergy between CuO and CeO₂.⁴⁰ This could help explain why no diffraction peak of CuO was observed in the XRD patterns of Cu₈Mn₂/CeO₂.

Cu 2p spectra were fitted with four energy peaks and the results were shown in Fig. 14, the bonding energy peaks m⁰, n⁰ represented Cu⁺ while m′, n′ peaks were related to Cu²⁺. After reacted with HCN, the Cu²⁺ decreased from 31% to 19% while Cu⁺ increased from 44% to 58%.⁴² It might be ascribed to the chemical interaction between CuO phase and HCN: HCN bonded with Cu²⁺ and generated CuCN due to chemisorption, and promote the reduction of Cu²⁺to Cu⁺, which implied the Cu²⁺ played an important role in the HCN catalytic removal over Cu₈Mn₂/CeO₂.

Fig. 14 The Cu 2p spectra of Cu₈Mn₂/CeO₂ samples.

Fig. 15 demonstrated the Mn 2p spectra of all samples, there were two main peaks at 642 eV and 653 eV, which were ascribed to the Mn 2p_3/2 and Mn 2p_1/2 respectively. The Mn 2p_3/2 spectra could be decomposed into four bonding energy peaks: 640.3, 641.6, 643.2 and 646.2 eV, corresponding to Mn²⁺, Mn³⁺, Mn⁴⁺ and manganese nitrate.⁴³ For the fresh Cu₈Mn₂/CeO₂ samples, Mn³⁺ and Mn⁴⁺ peaks were observed while Mn²⁺ peak was not founded. When reacted with HCN at 30 °C, the Mn²⁺ content increased to 19% while Mn³⁺ and Mn⁴⁺ content all decreased, indicating the Mn³⁺ and Mn⁴⁺ participated in the HCN catalytic removal over Cu₈Mn₂/CeO₂, and the reaction system referred the reduction of Mn⁴⁺ to Mn³⁺ and Mn³⁺ to Mn²⁺. With reaction temperature increased to 90 °C, the Mn³⁺ content still increased, while Mn³⁺ and Mn²⁺ content decreased, which demonstrated the HCN catalytic removal at 90 °C was mainly attributed to the reduction of Mn⁴⁺ to Mn³⁺. It could be deduced that Mn⁴⁺ played an important role in the investigated reactions, and the results were consistent with the H₂-TPR analysis. Besides, the redox couple shown in following equation could also strengthen the samples' activity toward HCN.⁴⁴

Cu²⁺ + Mn³⁺ = Cu⁺ + Mn⁴⁺ (9)

Fig. 15 The Mn 2p spectra of Cu₈Mn₂/CeO₂ samples.

3.11 The results of FT-IR

The FT-IR spectra of fresh and used Cu₈Mn₂/CeO₂ which reacted with HCN at 30 °C, 90 °C and 120 °C were analyzed to investigate the reaction mechanism of HCN over prepared samples. As presented in Fig. 16, all samples exhibited two characteristic bonds at 3416.3 cm⁻¹ and 1650 cm⁻¹ corresponding to the stretching vibration of surface OH groups and the bending vibration of H₂O adsorbed, which were beneficial to the HCN catalytic hydrolysis and oxidation.⁴⁵ The absorption peak at 774.2 cm⁻¹ was assigned to the Ce–O asymmetric vibrational stretch and peak at 1319.2 cm⁻¹ might be attributed to the CO₃²⁻ vibrational stretch.²⁹ After reacted at 30 °C, a new peak appeared at 2148.5 cm⁻¹ which was ascribed to the CN⁻ vibrational stretch, indicating the HCN bonded with Cu²⁺ and generated CuCN due to chemisorption.⁴⁶ When the reaction temperature increased to 90 °C, the peak at 2148.5 cm⁻¹ could still be observed, which implied the chemisorption were still involved in the reaction system for removal of HCN. With temperature increased further, the peak corresponding to C=N vibrational stretch disappeared while other peaks were not changed, indicating the Cu₈Mn₂/CeO₂ exhibited high stability in the reaction system, and the results were consistent with the XRD and ICP analysis shown in Fig. S10 and Table S6† respectively. The CN⁻ absorbed might be converted into N₂, NH₃ and CO₂ with the participation of O₂, H₂O and OH species. As a result, the active site on the Cu₈Mn₂/CeO₂ was supplemented, which could capture more HCN molecular to strengthen the catalytic activity of the samples.

Fig. 16 The FTIR spectra of Cu₈Mn₂/CeO₂ samples.

4 Discussion

Based activity test results, the Cu₈Mn₂/CeO₂ was chosen as the ideal catalytic material. It was believed the CeO₂ still played an important role in the HCN catalytic removal over Cu₈Mn₂/CeO₂. Combing the SEM and TEM analysis, the CeO₂ exhibited a 3D porous structure which was beneficial for gas transport.²⁹ The flowerlike sphere structure of CeO₂ could also improve the dispersion of metal species.²¹ Besides, the addition of MnO_x could restrain the growth of CuO particles and improve their dispersion on the CeO₂ by the synergistic effect according to the XRD results, which promoted the removal of HCN over Cu₈Mn₂/CeO₂ sample. H₂-TPR analysis indicated the redox ability of CeO₂ was enhanced after CuO and MnO_x loaded, as reported, the excellent activity of CeO₂ based catalysts derived from the revisable Ce³⁺–Ce⁴⁺ transition in CeO₂ support associated with oxygen vacancy formation and migration which might strengthen the interaction between CuO, MnO_x and CeO₂ support and promoted its redox ability.⁴⁷

The CuO phase on the CeO₂ played an important role in the HCN removal process, the HCN was absorbed on the Cu₈Mn₂/CeO₂ to generate CuCN by the reduction of Cu²⁺ to Cu⁺. The reaction products of HCN over Cu₈Mn₂/CeO₂ at the temperature region of 30–150 °C indicated the catalytic hydrolysis and catalytic oxidation improved as temperature increasing. Combing the XPS analysis, the chemisorption oxygen, the oxygen vacancies, the Cu²⁺ and Mn⁴⁺ all played an important role in determining the excellent HCN removal performance of Cu₈Mn₂/CeO₂. NH₃-TPD characterization indicated the addition of MnO_x could increase the acid sites of Cu₈Mn₂/CeO₂, which could strengthen the absorption ability of NH₃ and promoted its conversion.

Based the FT-IR results, the CN⁻ absorbed on the samples disappeared when the reaction temperature was above 120 °C, this could be explained by the totally conversion of CN⁻. At this moment, the HCN removal was completely in the form of catalytic hydrolysis and catalytic oxidation. When the temperature was lower than 30 °C, the removal of HCN was mainly attributed to the sample's chemisorption. The HCN breakthrough behavior on the catalytic materials bed was corresponded to the Yoon and Nelson's model.

Based above analysis, the HCN reaction mechanism over Cu₈Mn₂/CeO₂ at 30–150 °C was described as follows: HCN was first absorbed and then decomposed into CN⁻ on the surface of Cu₈Mn₂/CeO₂ due to the chemisorption of bimetal oxides on the CeO₂. With reaction temperature increased, the CN⁻ could be oxidized into –NCO with the participation of O_β, Cu²⁺, Mn⁴⁺ and oxygen vacancies.^10,12 The formed –NCO were further converted into NH₃ and CO by catalytic hydrolysis. On the other hand, the generated –NCO species were oxidized to form CO₂, N₂ and byproducts such as NO and NO₂ by catalytic oxidation. The schematical diagram of the HCN reaction mechanism over Cu₈Mn₂/CeO₂ was shown in Fig. S11.†

5 Conclusions

The Cu_xMn_y/CeO₂ catalytic material was synthesized by precipitation method to achieve deep purification of HCN at the lower temperature region of 30–150 °C. By investigating the effect of different metal oxide species and the Cu/Mn mass ratio on the sample's catalytic activity, the Cu₈Mn₂/CeO₂ sample was chosen as the ideal catalytic material, which could obtain almost 100% removal rate toward 400 mg m⁻³ HCN at 90 °C. The introduction of MnO_x could improve the dispersion of CuO particles and increase the total acid sites of the prepared samples. It was proved the synergy between CuO and MnO_x, the chemisorption oxygen, the oxygen vacancies, the Cu²⁺ and Mn⁴⁺ all played an important role in determining the good catalytic activity of the prepared samples.

Author contributions

Z. H. Yi: conceptualization, formal analysis, investigation, writing original draft. J. Sun: funding acquisition, formal analysis, Rresources, writing review and editing. Y. L. Yang: data curation. J. G. Li: investigation. T. Zhou: investigation. S. P. Wei: investigation. A. N. Zhu: investigation.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge the Scientific Research Program (Zhuangzong [2018] No. 635) foundation and the Chinese Civil Air Defense Office [2014] No. 251-61 for the financial support of the current study.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra10177j

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