Efficient removal of HCN through catalytic hydrolysis and oxidation on Cu/CoSPc/Ce metal-modified activated carbon under low oxygen conditions

Abstract

The hydrogen cyanide (HCN) removal efficiency of activated carbon modified with different metal was studied under low oxygen conditions. When activated carbon was modified with Cu(NO)₃, cobalt sulfonated phthalocyanine (CoSPc) and Ce(NO₃)₃·6H₂O, its catalytic efficiency in HCN removal was significantly enhanced with the optimal conditions of 400 °C as the calcination temperature, 10% relative humidity and 1% oxygen concentration. And the catalytic hydrolysis and oxidation efficiency of AC–Cu–CoSPc–Ce was more than 98% at 200–350 °C with the maximum selectivity to N₂ of 52.6% at 300 °C. Although both AC–Cu and AC–Cu–CoSPc–Ce catalysts were reliable and stable, AC–Cu–CoSPc–Ce had a better catalytic activity at lower temperature. According to BET and X-ray photoelectron spectroscopy (XPS) results, Cu was mainly in the form of CuO and Cu₂O, Co was present in the form of Co²⁺ and Co⁰, and Ce was mainly in the form of CeO₂ and Ce₂O₃ on the catalyst surface. The reaction pathways were proposed.

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1. Introduction

Hydrogen cyanide (HCN), as a colorless or light blue liquid or gas typically from car exhausts fumes, chemical processing and industrial tail gas, is extremely flammable and poisonous.^1–8 Due to the risk of exposure to HCN, the vapor in the air must be removed by a suitable device, such as gas mask filter or a filter for collective protection applications. With the presence of HCN, the purification and utilization of industrial waste gas becomes extremely difficult, and the environment and human health can be seriously damaged. Therefore, some HCN removal methods have been reported, including adsorption,^9–12 combustion,⁶ catalytic oxidation,^4,8 catalytic hydrolysis,^13,14 etc. Zhao et al.⁸ carried out investigation of the HCN oxidation reaction over a 0.5% Pt/Al₂O₃ catalyst. They found N₂, N₂O, NO, NO₂, CO₂ and H₂O as reaction products, but no NH₃ and CO due to the strongly oxidizing conditions. The maximum selectivity for N₂ was ca. 25% for the HCN + O₂ reaction at 250 °C. Ning et al.¹⁰ reported the adsorption removal of hydrogen cyanide with metal-modified zeolite. When metal (Cu, Co or Zn) was supported on ZSM-5 zeolite or Y, only Cu-modified zeolite molecular sieve significantly enhanced the HCN adsorption performance of zeolite. Oliver et al.⁹ synthesized an activated carbon containing copper from porous sulfonated styrene/divinylbenzene resin to remove hydrogen cyanide in the air as the following,

2CuO + 4HCN → 2CuCN + (CN)₂ + H₂O

2CuO + 2HCN → 2CuCN + H₂

2Cu₂O + 2HCN → 2CuCN + H₂O

Furthermore, Nickolov¹⁵ also studied the Cu–Zn–(Py;Cr)-impregnated carbons and porous material to remove HCN. The removal of HCN could be attributed mainly to the chemical interaction of CuO in the active phase:

2CuO + 4HCN → 2CuCN + (CN)₂ + H₂O

or to the reactions

Kroecher and co-workers⁷ studied the HCN hydrolysis and oxidation on different heterogeneous catalysts. Among the studied catalysts, titanium dioxide hydrolysis catalyst could convert HCN into CO and NH₃. The hydrolysis efficiency of alumina was only about 50% in the HCN removal. Cu-ZSM-5 had high selectivity to nitrogen, which reached up to 40% in the presence of O₂ and H₂O at 400 °C. They proposed a possible reaction process of HCN in oxygen and water-containing atmospheres over catalyst:

HCN + 0.5O₂ → HNCO

HNCO + H₂O → H₂NC(O)OH → NH₃ + H₂O

and/or

2HCN + 0.5O₂ → (CN)₂ + H₂O → HNCO + HCN

HNCO + H₂O → NH₃ + H₂O

NH₃-SCR was also an important process for improving N₂ selectivity. Wang et al.¹⁶ studied the cerium-stabilized Cu-SSZ-13 catalyst for the catalytic removal of NO_x by NH₃. Cu-SSZ-13 modified by Ce showed better NO_x removal efficiency and aging resistance. High-activity temperature windows of fresh samples with >85% NO_x conversion (the NO_x conversion rate was obtained through measurement of the rate of NO_x converted to N₂ using NH₃ as the reducing agent) were extended to 210–590 °C. The introduction of Ce enhanced the NO_x reduction rate especially in the fast SCR reaction, which contributed to the improvement of low-temperature activity.

In order to increase the conversion rate of hydrogen cyanide and N₂ selectivity and to broaden the activated carbon reaction temperature range, we presented a method of combination catalytic hydrolysis, oxidation and NH₃-SCR for HCN removal utilizing AC–Cu–CoSPc–Ce catalyst under low-oxygen conditions. Nitrogen oxides and ammonia from the byproduct of oxidation and hydrolysis of HCN could participate in the NH₃-SCR, resulting in the improvement of the N₂ selectivity. Containing copper catalyst showed better effects for HCN removal. Therefore, cobalt sulfonated phthalocyanine and cerium nitrate as adjuvants were added on a copper-based catalyst for performance enhancement, in which cobalt sulfonated phthalocyanine and cerium elements could store oxygen and promote the catalytic oxidation.^16–19 AC–Cu–CoSPc–Ce catalyst was synthesized from activated carbon impregnation.

2. Experimental

2.1 Experimental materials

Commercial coal-based activated carbon (Baota Activated Carbon Co., LTD, Ningxia, China) with a particle of 40 mesh was washed for 3 times with distilled water at 70 °C to remove dust and dried at 110 °C for 10 h. AC samples (5.0 ± 0.1 g) were impregnated with the reagents (Sinopharm Chemical reagent Co., Ltd, China): Cu(NO₃)₂ solution (0.1 mol L⁻¹, 50 mL), Ni(NO₃)₂ solution (0.1 mol L⁻¹, 50 mL), Co(NO₃)₂·6H₂O solution (0.1 mol L⁻¹, 50 mL), CoSPc solution (0.05 g CoSPc, 10% NaOH, 50 mL) and Ce(NO₃)₃·6H₂O (0.1 mol L⁻¹, 50 mL), respectively. AC was impregnated first with the solution of Cu(NO₃)₂ and then the CoSPc solution (0.05 g CoSPc, 10% NaOH, 50 mL) to fabricate AC–Cu–CoSPc. AC was impregnated first with the mixed solution of Cu(NO₃)₂ and Ce(NO₃)₃·6H₂O (0.1 mol L⁻¹ Cu(NO₃)₂, 0.01 mol L⁻¹ Ce(NO₃)₃·6H₂O, 50 mL) and then the CoSPc solution (0.05 g CoSPc, 10% NaOH, 50 mL) to fabricate AC–Cu–CoSPc–Ce. Multi-component load first need to complete a dipping, dry and then the next impregnation. Each impregnation was carried out under stirring for 24 h. After the impregnation, the solution was filtrated and adsorbents were dried at 120 °C for 12 h, followed by 6 h of calcination under atmospheric condition. The samples were marked as AC–Cu, AC–Ni, AC–Co, AC–CoSPc, AC–Ce, AC–Cu–CoSPc, AC–Cu–CoSPc–Ce, separately.

2.2 Experimental process

The reaction was performed in a quartz tube reactor with 150 mm in length and 8 mm in inner diameter with at 150–350 °C. The reaction was continued for 20 min at each temperature point. The dosage of catalyst was 0.2 g. Experimental devices were mainly composed of the following three parts: simulation of gas section, reactor (its heating rate was 10 °C min⁻¹) and analysis section. The simulation of gas stream was composed of nitrogen with 0–2% of O₂, 0–20% relative humidity and 100 ppm of HCN. The gas space velocity (GHSV) was controlled at 32 000 h⁻¹.

2.3 Sample analysis

Content of CO, CO₂, NO_x (NO, NO₂) and N₂O were analyzed on a gas chromatography (FULI GC-9790II, China) and flue gas analyzer (ecom EN2, Germany). Content of HCN was measured with the iso-nicotinic-acid-3-methyl-1-phenyl-5-pyrazolone spectrophotometric method. NH₃ was determined by sodium hypochlorite–salicylic acid spectrophotometry. N₂ content was calculated from the N-balance based on the remaining N-containing reaction products (eqn (1)).⁷

The selective generation of nitrogen-containing products was defined by the following equations,

The morphology of catalyst surface was observed by scanning electron microscopy (SEM). Surface area, pore size distribution and pore volume were measured through nitrogen sorption/desorption. Specific surface area was determined with the BET equation. The catalyst sample surface state was analyzed with X-ray photoelectron spectroscopy (Thermo ESCALAB 250XI, USA). Thermal stability of the catalyst was examined through thermogravimetry-differential thermal analysis (TG-DTA).

3. Results and discussion

3.1 Effects of modifiers on catalytic hydrolysis and oxidation of HCN

Seven adsorbents with different chemicals, including AC–Cu, AC–Ni, AC–Co, AC–CoSPc, AC–Ce, AC–Cu–CoSPc and AC–Cu–CoSPc–Ce, were tested as shown in Fig. 1. Purification ability of blank active carbon (AC) was also studied as a control. Among these adsorbents, the HCN removal efficiency of AC was the lowest. At a lower temperature, AC showed a relatively higher HCN removal efficiency (14.6%, 150 °C). With the increase of temperature, AC–Cu, AC–Ni, AC–Co, AC–CoSPc, AC–Ce, AC–Cu–CoSPc and AC–Cu–CoSPc–Ce showed significant improvement on the HCN removal efficiency, and the loading of CoSPc and Ce salts separately on the AC showed the inferior removal efficiency. After the loading of CoSPc and Ce on AC–Cu, the HCN removal efficiency was further improved, especially at 150–200 °C. Because cobalt ions could activate the molecular oxygen, cobalt modifier could improve the catalytic oxidation of HCN.^17,18 Rare earth metal Ce further could improved the performance of the oxygen storage of catalysts and reduce their initial activation temperature.¹⁹ Therefore, the combination of CoSPc and Ce could improve the performance of catalyst AC–Cu–CoSPc–Ce in the whole process. As shown in Fig. 1(b), HCN hydrolysis did not occur on blank activated carbon. Compared to the AC–Cu, both NH₃ production rates of AC–Cu–CoSPc and AC–Cu–CoSPc–Ce catalysts were reduced, and the decrease of NH₃ content might be due to its oxidative reaction with O₂ at higher temperatures, which once again showed the added CoSPc and Ce increased the catalytic oxidation activity of catalysts.

Fig. 1 (a) Catalytic performance of different catalysts; (b) the conversion of NH₃. Reaction conditions: GHSV = 32 000 h⁻¹, 100 ppm HCN, T(calcination) of 400 °C, 10% of H₂O and 0.5% O₂.

3.2 Effects of oxygen content

As shown in Fig. 2, the HCN removal efficiency of AC–Cu–CoSPc–Ce with 1.0% O₂ was the best compared to those with other oxygen contents. And anaerobic condition and 2% of oxygen content present the lower activity. HCN molecules could not be fully oxidized under the conditions with a low oxygen content. With the increase of oxygen content, HCN molecules could fully participate in the redox reaction in favor of catalytic oxidation to improve the HCN purification efficiency. Additionally, CoSPc and Ce led to more activated oxygen in the gas phase and rare earth element Ce improved the hydrothermal stability of the catalyst. Therefore, the modified activated carbon catalyst AC–Cu–CoSPc–Ce had high temperature aging properties. However, with too high oxygen concentration, the oxygen molecule or atom would compete with HCN to occupy a large number of active adsorption sites of AC, which would decrease the HCN purification efficiency.

Fig. 2 Catalytic performance of AC–Cu–CoSPc–Ce with different oxygen contents (GHSV = 32 000 h⁻¹, 100 ppm HCN, 400 °C calcination and 10% H₂O).

3.3 Effects of relative humidity

Fig. 3 shows the effects of different relative humidity for the HCN removal efficiency on AC–Cu–CoSPc–Ce catalyst. It could be found that the presence of water vapor in flue gas significantly affects the removal efficiency of HCN on AC–Cu–CoSPc–Ce catalyst. The HCN removal efficiency was enhanced when the relative humidity was increased from 0% to 10%. Introduction of water vapor improved HCN removal efficiency, indicating that a certain degree HCN hydrolysis occurred. A high removal efficiency of HCN was obtained at the whole experimental temperature range under the condition of 10% relative humidity. After the relative humidity above 10%, the HCN removal efficiency decreased obviously, presumably due to the formation of water clusters which blocked the micropores and impeded the diffusion of moisture into the pores.²⁰ Furthermore, it might due to competitive adsorption of water with HCN for catalysis sites on AC–Cu–CoSPc–Ce.²¹

Fig. 3 Effects of different relative humidity (GHSV = 32 000 h⁻¹, 100 ppm HCN, 400 °C calcination and 1% of O₂).

3.4 Effects of calcination temperature

As shown in Fig. 4, HCN removal efficiency of all AC–Cu–CoSPc–Ce was more than 95% at reaction temperature of over 200 °C. When AC–Cu–CoSPc–Ce was calcined at 350–400 °C, it had very high catalytic activity. However, the catalytic activities with calcining temperatures of 300 °C and 450 °C were relatively low. When calcination temperature was 300 °C, copper nitrate and cerium nitrate might be supported on activated carbon because the temperature was too low to decompose the less effective copper nitrate and cerium nitrate. A very high calcination temperature (450 °C) might cause the internal pores of AC were partially closed or collapsed, which resulted in the decrease of specific surface area and surface active sites, the reduced adsorption and desorption rate of reactant in the reaction process, and eventually the decrease of catalyst activity.⁸ As shown in Fig. 5, the SEM images further confirmed the effect of calcination temperature. After the 400 °C roasting, SEM micrograph (Fig. 5(c)) showed AC–Cu–CoSPc–Ce surface display of various sizes and shapes of the brighter areas. SEM image of activated carbon was no significant difference after 300 °C and 350 °C roasting. Due to insufficient roasting at lower temperatures, there was significant amount of load material had not been activated. Roasting at 400 °C, the surface of the metal material could be well activated to provide more active substances and active site for HCN removal. However, Fig. 5(d) can be seen in slight ashing, which may be due to the higher calcination temperatures cause partial sintering of the activated carbon. Therefore, the optimum calcination temperature was 400 °C.

Fig. 4 Effects of roasting temperature on the catalytic activity (GHSV = 32 000 h⁻¹, 100 ppm HCN, 400 °C calcination, 1.0% O₂ and 10% H₂O).

Fig. 5 SEM images of AC–Cu–CoSPc–Ce calcined at different temperatures: (a) 300 °C, (b) 350 °C, (c) 400 °C, (d) 450 °C.

3.5 Effects of temperature on nitrogen-containing products

As shown in Fig. 6, Tables 1 and 2, HCN was converted into NH₃ instead of N₂O and NO_x at 150 °C over AC–Cu–CoSPc–Ce, indicating that the catalytic hydrolysis and adsorption proceeded mainly at 150 °C. The overall conversion of HCN was increased to more than 98% on AC–Cu–CoSPc–Ce at 200–350 °C, and N₂ selectivity was increased to the maximum point of 52.6% when the reaction temperature was 300 °C. However, the maximum N₂ selectivity was 48.8% on AC–Cu at 300 °C. Ammonia conversion rate on AC–Cu–CoSPc–Ce was decreased, compared with that of AC–Cu, throughout the reaction temperature ranges. At 150 °C, the ammonia conversion over AC–Cu–CoSPc–Ce was only 14.1% with a hydrogen cyanide removal efficiency of 64.9%. However, the conversion of ammonia over AC–Cu was 35.8% with a hydrogen cyanide removal efficiency of 44.6%. These results suggested that the catalytic performance of AC–Cu–CoSPc–Ce was significantly improved due to the addition of CoSPc and Ce. At low temperatures (150–200 °C), AC–Cu–CoSPc–Ce catalyst also enhanced the catalytic properties of HCN, reducing the production rate of NH₃ and improving the selectivity of N₂.

Fig. 6 N-Containing products selectivity and HCN removing rate: (a) AC–Cu–CoSPc–Ce and (b) AC–Cu (GHSV = 32 000 h⁻¹, 100 ppm HCN, 400 °C calcination, 1.0% O₂ and 10% H₂O).

Table 1 The N-containing products content of the outlet over AC–Cu–CoSPc–Ce (GHSV = 32 000 h⁻¹, 100 ppm HCN, 400 °C calcination, 1.0% O₂ and 10% H₂O, each temperature point reaction duration 20 min)

Catalyst	AC–Cu–CoSPc–Ce
	150 °C	200 °C	250 °C	300 °C	350 °C
HCN (ppm)	35.10	1.70	0.60	0.60	1.20
NH3 (ppm)	9.15	36.67	38.17	25.25	17.39
NOx (ppm)	0	8.36	10.14	10.04	18.28
N2O (ppm)	0	6.73	6.01	5.91	6.97
N2 (ppm)	0	19.91	19.53	26.14	24.60

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Table 2 The N-containing products content of the outlet over AC–Cu (GHSV = 32 000 h⁻¹, 100 ppm HCN, 400 °C calcination, 1.0% O₂ and 10% H₂O, each temperature point reaction duration 20 min)

Catalyst	AC–Cu
	150 °C	200 °C	250 °C	300 °C	350 °C
HCN (ppm)	55.40	3.80	1.60	1.30	0.60
NH3 (ppm)	15.97	43.29	45.76	30.30	20.87
NOx (ppm)	0	4.33	5.12	9.48	16.70
N2O (ppm)	0	6.74	6.05	5.38	7.30
N2 (ppm)	0	17.56	17.71	24.08	23.65

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Because the representative products of NH₃ and NO_x were detected in exhaust gas, the hydrolysis and oxidation of HCN might occurred on the catalyst surface, respectively. As shown in Fig. 6, Tables 1 and 2, the observed major nitrogen-containing products were NH₃, N₂O and NO_x, with a small amount of incomplete reaction of HCN in the exhaust gas. The content of N₂ was calculated based on the N-balance. The decrease of NH₃ content might be attributed to oxidative reactions with O₂ as the following,

HCN + H₂O → NH₃ + CO

4NH₃ + 3O₂ → 2N₂ + 6H₂O

In order to prove that the formation of nitrogen might come from the ammonia oxidation process, we carried out an ammonia oxidation experiment. As shown in Fig. 7, the generation of nitrogen gas occur mainly at 300 °C and 350 °C, accompanied by the formation of nitrogen oxides, but the conversion rate of nitrogen was low. Additionally, a portion of N₂ might be from the following direct oxidation of hydrogen cyanide gas,

4HCN + 5O₂ → 4CO₂ + 2N₂ + 2H₂O

Fig. 7 NH₃ oxidation over AC–Cu–CoSPc–Ce (GHSV = 32 000 h⁻¹, 80 ppm NH₃, 1% O₂ and 10% H₂O).

NH₃, NO and NO₂ might continue to react with each other through the two most important reactions of SCR process (NO SCR reaction and NO/NO₂ SCR reaction) as the following,^7,16,29

4NH₃ + 4NO + O₂ → 4N₂ + 6H₂O

2NH₃ + NO + NO₂ → 2N₂ + 3H₂O

In the presence of H₂O, there were two possible reaction pathways regarding HCN decomposition. Firstly, the formation of NH₃ though hydrogen cyanide hydrolysis, and NH₃ was oxidized; second, HCN hydrolysis and oxidation process was carried out in parallel.²² A portion of the HCN was hydrolyzed to NH₃ and CO. Another part of the HCN was converted to cyanogen and isocyanic acid through oxidation reaction. The formed cyanogen hydrolyzes very easily with water to hydrocyanic acid and isocyanic acid. Then isocyanic acid was decomposed by oxidation and hydrolysis reactions. Cyanogen as an intermediate product was rapidly hydrolyzed to isocyanic acid (HNCO) and HCN in the presence of water, which was difficult to be present on the catalyst and detected.⁷ Therefore, cyanogen was ruled out during the assay in this study. According to the present results, HCN hydrolysis formation of NH₃ on AC–Cu–CoSPc–Ce was the main way of HCN removal. Under the condition of 10% relative humidity and 1% oxygen, HCN hydrolysis efficiency higher than that of HCN oxidation. While the presence of H₂O and O₂ in the gas, catalyzed hydrolysis and oxidation joint action in the process of removing hydrogen cyanide. It should be noted that the reaction generated a considerable amount of N₂O and NO_x, which was not conducive to the selectivity of N₂.

3.6 Thermal stability of catalyst

AC and AC–Cu–CoSPc–Ce catalyst stability under a nitrogen atmosphere at a heating rate of 10 °C min⁻¹ was analyzed by thermogravimetry. As shown in Fig. 8, there was a weight loss from 40 °C to 150 °C, which was mainly due to the removal of moisture and volatiles from activated carbon. After 550 °C, the weight of sample showed a slow downward trend, which should be the combustion of impurity gas on activated carbon surface. These results showed that AC–Cu–CoSPc–Ce had an excellent thermal stability in the experimental temperature range.

Fig. 8 TG-DTA curves: (a) AC and (b) AC–Cu–CoSPc–Ce.

3.7 Stability of catalyst

As shown in Fig. 9, at 250 °C, HCN removal efficiency was greater than 98% on AC–Cu. On AC–Cu–CoSPc–Ce, only at 200 °C, HCN removal efficiency was greater than 98%. In order to investigate catalytic lives and stabilities of these two catalysts with the same and high HCN removal efficiency of 98%, the reaction temperature was 250 °C for AC–Cu and 200 °C for AC–Cu–CoSPc–Ce were chosen to run HCN removal continuously for 48 hours. The results showed that, during the continuous 48 hours, both catalysts maintained high activities and HCN conversion rates were maintained at above 98%, indicating that these catalysts had excellent stabilities with high activities, but AC–Cu–CoSPc–Ce catalyst had more advantages in the HCN removal due to its lower required reaction temperature.

Fig. 9 Stability of catalyst (T(AC–Cu) = 250 °C, T(AC–Cu–CoSPc–Ce) = 200 °C) (GHSV = 32 000 h⁻¹, 100 ppm HCN, 400 °C calcination, 1.0% O₂ and 10% H₂O).

3.8 BET

As shown in Fig. 10, according to the IUPAC classification, AC–Cu–CoSPc and AC–Cu–CoSPc–Ce adsorption isotherms were type I, indicated that these catalysts were mainly composed of microporous and mesoporous as activated carbon.^23,24 These results illustrated the supported CoSPc and Ce, to some extent, affected the pore size distribution of activated carbon. The pore size distribution curves (Fig. 10(b)) showed that pore sizes of these three catalysts were mainly distributed in a range of 0.5–1 nm. The large specific surface area of catalyst was mainly from mesoporous and microporous pores, which provided a large amount of active sites for the reaction, thus to increase the efficiency of catalytic reaction.

Fig. 10 Nitrogen sorption/desorption isotherms (a) and pore size distributions (b) of AC–Cu, AC–Cu–CoSPc and AC–Cu–CoSPc–Ce.

As listed in Table 3, compared with AC–Cu, BET specific surface area, pore surface area, total pore volume and micropore volume of AC–Cu–CoSPc were decreased by 56.14%, 53.48%, 58.08% and 52.47%, respectively. BET specific surface area, pore surface area, total pore volume and micropore volume of AC–Cu–CoSPc–Ce were decreased by 70.58%, 68.75%, 70.67% and 67.96%, respectively. These results suggested that the added CoSPc and Ce affected the microporous structure of activated carbon significantly. According to the pore size distribution curve in Fig. 10(b), pore volume of the modified catalyst was reduced in the range of 0.5–1 nm. These results definitely indicated that the active component loaded on the activated carbon was mainly distributed on the surface of micropore, which could provide more active site to promoting the catalytic reaction.

Table 3 Porosity parameters of different activated carbon samples

Samples	SBET (m^2 g^−1)	Smicro (m^2 g^−1)	Vtotal (cm^3 g^−1)	Vmicro (cm^3 g^−1)	Raverage (nm)
AC–Cu	1238.2	316.2	0.1906	0.1233	1.4201
AC–Cu–CoSPc	543.1	147.1	0.0799	0.0586	1.3795
AC–Cu–CoSPc–Ce	364.3	98.9	0.0559	0.0395	1.4421

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3.9 XPS

XPS characterization was used to further explore the chemical state of AC–Cu–CoSPc–Ce catalyst. As shown in Fig. 11 and Table 4, Cu2p of AC–Cu–CoSPc–Ce had two main peaks at 933.03 and 952.97 eV, which corresponded to Cu2p_3/2 of CuO and Cu2p_1/2 of Cu₂O, separately,^25,26 with an atom ratio of 59.9 : 40.1, which indicated that Cu element in AC–Cu–CoSPc–Ce existed mainly with the forms of CuO and Cu₂O. Two Co2p_3/2 peaks at 779.81 and 778.30 eV were also identified as Co²⁺ and Co⁰ in AC–Cu–CoSPc–Ce, respectively,²⁶ with an atom ratio of 52.4 : 47.6. The surface CoSPc was the reactive oxygen species in favor of electron transfer in the catalytic oxidation process. Co²⁺ was a central metal of CoSPc, and which was catalytic oxidation of HCN active substance. The Ce2p of AC–Cu–CoSPc–Ce had two main peaks at 882.00 and 887.65 eV, which corresponded to Ce3d_5/2 of Ce⁴⁺ and Ce³⁺,^27,28 respectively, with an atom ratio of 50.86 : 49.14. Therefore, the added Ce element in AC–Cu–CoSPc–Ce catalyst was mainly in forms of CeO₂ and Ce₂O₃.

Fig. 11 XPS spectra of Cu2p, Co2p and Ce3d from AC–Cu–CoSPc–Ce.

Table 4 XPS analysis results of Cu2p, Co2p, Ce3d and O1s from AC–Cu–CoSPc–Ce

Element	Binding energy (eV)	Relative content (%)
Cu2p	933.03	59.99
	952.97	40.01
Co2p	779.81	52.37
	778.30	47.63
Ce3d	882.00	50.86
	887.65	49.14
O1s	531.10	89.25
	535.60	10.75

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As shown in Fig. 12, O1s XPS spectra of AC–Cu–CoSPc–Ce was similar to that of AC–Cu. The slight displacement of AC–Cu–CoSPc–Ce oxygen peak position was associated with CoSPc and Ce. The O1s of AC–Cu–CoSPc–Ce had two main peaks at 535.6 eV (peak I) and 531.1 eV (peak II). Peak I corresponded to C–O type oxygen atoms (10.75%) in C–OH and COOR groups, and peak II was from lattice oxide oxygen (O²⁻) and surface chemisorbed oxygen (O_2ads),^29–31 which both had strong oxidation activity to remove HCN, although the surface chemisorbed oxygen was considered to be highly active in catalytic oxidation reactions because of its higher mobility. The introduced oxygen replenished chemically adsorbed oxygen and lattice oxygen by adsorption dissociation during the oxidation process.^32,33

Fig. 12 XPS spectra of O1s from AC–Cu and AC–Cu–CoSPc–Ce.

3.10 Reaction pathway

As proposed in Fig. 13 of the HCN hydrolysis–oxidative reaction pathway, catalytic oxidation and hydrolysis of HCN occurred simultaneously in the presence of H₂O and O₂.

Fig. 13 The proposed reaction pathways of catalytic oxidation and hydrolysis of HCN. () oxidation, () hydrolysis, () SCR.

4. Conclusions

AC–Cu–CoSPc–Ce was found to have an excellent HCN removal efficiency at low temperature, in which oxygen content and calcination temperature played important roles. Compared with AC–Cu, the added CoSPc could activate oxygen molecular on the catalyst surface and improve the performance of catalytic oxidation of HCN. Rare earth elements Ce could further improve the oxygen storage capacity of the catalyst and reduce the catalyst light-off temperature. Therefore, HCN removal efficiency of AC–Cu–CoSPc–Ce was greater than 98% at 200–350 °C with N₂ selectivity of 52.6% at 300 °C. Although both AC–Cu and AC–Cu–CoSPc–Ce catalysts showed outstanding activity and stability, AC–Cu–CoSPc–Ce catalyst was a greater potential in HCN removal due to its better performance at low temperature. XPS results also suggested that Cu, Co and Ce were all active metals in the catalytic oxidation of HCN.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51268021, 51368026, 51568027).

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