TiO2 catalysts at low temperatures

Microporous and Mesoporous Materials 282 (2019) 260–268

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Catalytic oxidation and hydrolysis of HCN over LaxCuy/TiO2 catalysts at low temperatures

T

Qi Wang, Xueqian Wang∗, Langlang Wang, Yanan Hu, Ping Ning, Yixing Ma, Liming Tan Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming, 650500, China

A R T I C LE I N FO

A B S T R A C T

Keywords: HCN N2 selectivity LaxCuy/TiO2 Catalytic hydrolysis and oxidation NH3-SCR

A series of LaxCuy/TiO2 catalysts were prepared by sol-gel method to remove HCN under the conditions of 1% O2 and 10% relative humidity. Samples were characterized by BET, XRD, XPS, H2-TPR, NH3/CO2-TPD and FT-IR. NH3-SCR reactions were carried out to verify the results of the catalyst activity tests and explore N2 formation mechanism. La/Cu ratios had a prominent impact on the catalyst performance. La1Cu9/TiO2 had the best catalytic activity: its conversion of HCN and N2 selectivity were 100% and 62.24% at 150 °C, respectively. The characterization results indicated interactions between La and Cu, catalytic redox activity and existence of abundant acidic and basic sites on the catalyst surface. Low content of La significantly improved catalytic redox ability and total acidity as well bring about the catalyst medium strong acid sites with a lower temperature. Synergistic effects of La and Cu oxides provided La1Cu9/TiO2 catalysts with remarkable activity at low temperatures.

1. Introduction HCN (Hydrogen cyanide) is highly toxic [1,2] and is a component of many exhaust gases, which include coal cracking and coal industry [3] as well as denitrification processes by selective catalytic reduction (SCR) of nitrogen oxides by hydrocarbons or ammonia [4,5]. Other sources contributing to HCN emissions include fabrication of PANbased carbon fibers, automobile exhausts [6,7], yellow phosphorus tail gases, burning of biomass fuels and nitrogenous substances [4,8,9], calcium carbide furnace exhausts [10], etc. HCN can directly penetrate into the human body through the skin and respiratory organs, then react with the cell components and cause intracellular asphyxia hypoxia. High concentrations of HCN can cause not only acute poisoning but also death [11,12]. Current methods of removing HCN from exhaust include combustion [13], adsorption [14], catalytic hydrolysis and catalytic oxidation [11]. Drawbacks of combustion and adsorption methods are incomplete conversion of HCN and potential secondary pollution [15]. Thus, current methods of removing HCN mainly focus on catalytic hydrolysis and catalytic oxidation because these methods provide complete conversion of HCN and result in non-toxic or low toxicity end- and by-products. Miyadera and Zhao [15,16] found it easy to convert HCN to NOx by Pt/ TiO2 and noble metals (Pt, Pd, Ru, Ir) supported on alumina under

oxygen-enriched conditions, but they could not further convert NOx to non-toxic N2. They reported the maximum conversion of 75% at 375 °C. Kröcher et al. [11] demonstrated Pd/Al2O3 and Pt/Al2O3 catalytic systems with higher activities, but only above 300 °C at conditions of 5% H2O and 10% O2. However, not only the high cost limit its industrialization, but also the low nitrogen selectivity at low temperatures is unfriendly for the environment due to the generation of a considerable amount of undesired nitrogen oxides, such as N2O, NO and NO2 [17]. Thus, one of the solutions is to replace noble-metal catalyst with transition metal oxides. Titanium dioxide demonstrated excellent hydrolysis activity according to the previous reports. However, hydrolytic activities of WO3eTiO2 and V2O5/WO3eTiO2 were lower than that of pure TiO2 under NO or NO2 atmosphere [11]. Ma et al. [18] explored hydrolytic activity of catalysts X-TiOx, such as Fe-TiOx, Ni-TiOx, Nb-TiOx Co-TiOx and Nb/La-TiOx, and reported the highest HCN and NH3 conversion rates (97.6% and 96% at 250 °C, respectively). Miyadera et al. [15] also covered that HCN could be removed efficiently by using it as a reducing agent in the selective catalytic reaction of NOx over CuSO4/TiO2 catalyst. Zeolite-based catalysts are also used successfully to purify nitrogenous substances [19]. However, few studies of HCN removal have been conducted to increase the N2 selectivity. In our previous studies, MnOx/TiO2eAl2O3 with excellently performance on HCN catalytic

∗ Corresponding author. NO. 727, Jinming South Road, Kunming University of Science and Technology, Chenggong new District, Kunming, 650500, Yunnan, China. E-mail addresses: [email protected], [email protected] (X. Wang).

https://doi.org/10.1016/j.micromeso.2019.02.025 Received 22 November 2018; Received in revised form 29 January 2019; Accepted 13 February 2019 Available online 25 February 2019 1387-1811/ © 2019 Published by Elsevier Inc.

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humidity of the inlet gas was controlled by varying the fraction of the N2 stream bubbling through the water prior to entering the system. All the water vapor content mentioned in this work refers to the relative humidity of 10%. Gas flow was controlled by flow meters. 1% O2 and 100 ppm HCN were introduced since many HCN-containing exhausts contain traces of O2 and the HCN is usually in the range of a few ppm to 200 ppm. These three gases were thoroughly mixed in the mixing chamber. The mixed gas was first analyzed by a moisture detector and then passed through the quartz tube containing catalysts at a gas hourly space velocity equal to 32,000 h−1. Exhaust gas from the bed was first analyzed (as described below) and then absorbed by the liquid and discharged.

oxidation were studied by Wang et al. [20], and it obtained nearly 100% HCN conversion and 70% N2 selectivity at 200 °C. Hu et al. [21] reported that Cue, Fee and CueFe/ZSM-5 composites achieved nearly 100% HCN conversion and 80% N2 selectivity at about 250 °C. However, all these studies are focused on reactions at high or medium temperatures, which are not very favorable for practical applications. Less attention have been directed to the method of efficient purification of HCN and high N2 selectivity at low temperatures. Better and more energy-efficient and environmentally-friendly methods are still needed. Based on the above studies and our previous work, in order to save energy and further reduce the environmental and human health hazards caused by HCN and NOx emissions, it is very important to develop a new catalyst that offers both high conversion of HCN and selectively of nitrogen at low-temperatures. In this study, titanium dioxide was selected as a catalyst carrier because of its excellent hydrolytic activity and promising De NOx properties [15]. Copper oxide has excellent oxidation activity during SCR of nitrous oxides (NOx) with ammonia presence (NH3-SCR). Lanthanum oxide possesses outstanding hydrolysis activity relative to HCN [18,22], and La2O3 supported on TiO2 shows a much higher activity for the hydrolysis of HCN than pure TiO2 [11]. Therefore, we designed and synthesized a composite catalyst LaxCuy/TiO2 for HCN removal at low temperatures, which combines both catalytic hydrolysis and catalytic oxidation properties. Performance of LaxCuy/TiO2 catalyst prepared by sol-gel method was systematically studied to determine the best conditions for nitrogen selectivity and HCN removal rate at 10% relative humidity and 1% O2. The main factors positively attributing to the performance of the catalysts were active component content, reaction and calcination temperatures. Catalysts were characterized by BET, XRD, XPS, H2-TPR, CO2/NH3-TPD and FI-TR. NH3-SCR reactions were also explored to study formation mechanism of N2 and other possible reaction paths of this catalytic system.

2.3. Detection and analysis of reaction products HCN and NH3 in tail gas were determined by isonicotinic acidpyrazolone spectrophotometry and sodium hypochlorite-salicylic acid spectrophotometry, respectively. CO, CO2, N2O, NO and NO2 were measured using gas chromatography (FULI, 9790II, China) and a flue gas analyzer (LaoYing, 3022, China). Concentration of N2 was calculated from the N-balance of the nitrogenous materials and the C-balance of the carbonaceous materials in the reactants and products. Analysis was performed after the gas stabilized for 60 min at the corresponding temperature. Calculations of HCN removal rate and N2 yield were performed using the following equations, respectively:

CHCN =

CN2 =

CHCN − in − CHCN − out CHCN − in

(1)

CHCN − in − CHCN − out − CNH3− out − CNO2 − out − CNO − out − CN2O − out 2 (2)

2. Experimental 2.4. Characterization 2.1. Materials Specific surface area was determined by a BET method using 3H2000PM. Pores volume and diameters were calculated using BJH method. XRD patterns were collected using XRD-6100 system with Cu radiation, in the 10–90° 2θ range and with 2°/min scanning speed. H2 temperature-programmed reduction (TPR) experiments were recorded using FineSorb 3010 with N2 gas containing 5% of H2. After flushing the system with samples for 30 min, it was then continuously heated from 373 to 1073 K under the same gas flow. CO2/NH3 temperatureprogrammed desorption curves were obtained for 20 mg samples using Chembet Pulsar TPR/TPD (equipped with a thermal conductivity detector) with CO2/He or NH3/He gases flowing at 15 °C min−1 rate. FTIR analysis was performed by a Nicolet Impact 400 spectrometer with a TGS detector using 15 mg sample in the 4000-400 cm−1 range. XPS was performed using PHI 5000 Versa Probe II analyzer with aluminum Kα radiation (1486.6 eV) at 10−9 Pa.

Catalysts consisting of La and Cu complex meal oxides supported on TiO2 were prepared using sol-gel method. The precursors were tetrabutyl titanate, La(NO3)3·6H2O and Cu(NO3)2·3H2O. Concentrations of the precursors were chosen in such way that total content of La and Cu oxides always accounted for 10 wt% of the catalyst. Thus, specific stoichiometric amounts of La(NO3)3·6H2O and Cu(NO3)2·3H2O were dissolved in deionized water. Anhydrous citric acid acting as a chelating agent was added to anhydrous ethanol at 1:1 ratio with the later added La and Cu nitrate solutions. Tetrabutyl titanate was then added to anhydrous ethanol under constant stirring at 30 °C, after which glacial acetic acid was added. Keep stirring for about 1 h, then the metal salt solution was added to this mixture. After the mixture was thoroughly stirred, we added nitric acid as a hydrolysis catalyst and a chelating agent. Finally, the pH value was adjusted by aqueous ammonia. The temperature of the constantly stirred solution was raised by 10 °C every 4 h until it became sol-gel at 70 °C, after which the gel was removed from the solution and dried at 80–120 °C. The catalysts powder underwent several additional preparation steps to achieve good dispersibility: calcination, grinding, pelletizing and sieving. Samples, not specifically mentioned in this paper, were calcined at 450 °C.

3. Results and discussion 3.1. Activity measurements over catalyst compositions To optimize the ratio of the active ingredients in the catalyst, we prepared series of LaxCuy/TiO2 catalysts with different La/Cu ratios: La1Cu9/TiO2, La3Cu7/TiO2, La5Cu5/TiO2, La7Cu3/TiO2 and La9Cu1/ TiO2. La10/TiO2 and Cu10/TiO2 were also prepared for comparison. Fig. 2 shows HCN removal difference between catalysts with different La/Cu ratios from 100 °C to 400 °C at 10% relative humidity and 1% of O2. Data was recorded after the adsorption reached a steady state. HCN removal rate improved as the temperature increased. HCN conversion of Cu10/TiO2 was significantly higher than that of the La10/ TiO2. The highest HCN conversion was obtained for the La1Cu9/TiO2

2.2. Experimental setup Experimental setup used in this work to evaluate catalyst efficiency of removing HCN is shown in Fig. 1. All experiments were carried out in a fixed-bed equipped with a thermocouple. 0.2 g catalyst was packed in a U-shaped quartz tube (8 mm in inner diameter and 140 mm in length). The conversion of HCN for different quality La1Cu9/TiO2 samples were show in Fig. S1 (as displayed in supplementary material). Relative 261

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Fig. 1. HCN catalytic reaction flow chart.

Fig. 3. HCN conversion rates and product yields over La1Cu9/TiO2 catalyst. Fig. 2. HCN conversion over catalysts with different La/Cu ratios.

HCN, and its content first increased below 200 °C but then decreased because of its oxidation to nitrogen oxides. NH3 was also consumed by NH3-SCR. Thus, N2O, NO and NO2 formed as a result of NH3 and HCN oxidation. Some N2O oxidized to NO and its content gradually decreased at high temperatures. According to the nitrogen and carbon balance, maximum N2 yield was 62.24% (at 62.24% of N2 selectivity) at 150 °C. The yield of N2 decreased because of nitrogen oxides formation. View from the variation tendency of products: hydrolysis reactions and adsorption by the catalyst were the dominant processes at low temperatures. Catalytic oxidation and hydrolysis of HCN, running in parallel, were dominant at higher temperatures. These conclusions are consistent with previous reports [23].

catalyst: it was equal to 60.22% at 100 °C and reached almost 100% at 150 °C. As La/Cu ratios increased from 1:9 to 9:1, conversions of HCN gradually decreased from 60.22% to 29.13% (at 100 °C) indicating that excess La2O3 was detrimental to the activity of the catalyst. When La:Cu was above 5:5, complete conversion of HCN was achieved only above 200 °C. La10/TiO2 demonstrated the worst catalytic activity. Thus, addition of just a small amount of La to Cu/TiO2 greatly promoted the catalyst activity. Based on the above activity experiments, we have explored the deactivation of the La1Cu9/TiO2 catalyst, and the results are shown in Fig. S2 (as displayed in supplementary material).

3.2.2. NH3-SCR reactions Since NH3, NO, NO2 were generated during the process, and the conditions were coincident with the NH3-SCR system, we believe catalytic reactions of HCN were accompanied by NH3-SCR reactions. To further explore the reaction pathway and understand the origin of each product, two tests of NH3-SCR were conducted, which might involve two main reactions:

3.2. Evaluation of the activity of the catalyst 3.2.1. Reaction products of the catalyst-assisted HCN reaction Fig. 3 shows HCN conversion and distribution of N and C-containing products over La1Cu9/TiO2 catalyst under 1% O2 and 10% H2O conditions from 100 to 400 °C. Since CO, NH3, N2, and CO2 were detected in products, we speculated that the following reactions occurred: hydrolysis reaction of HCN with H2O generated CO and NH3, while oxidation reaction of HCN with O2 formed CO2, N2 and H2O. HCN + H2O = NH3 + CO

(3)

4HCN + 5O2 = 4CO2 +2N2 + 2H2O

(4)

8NH3 + 6NO2 = 7N2 + 12H2O

(5)

4NH3 + 4NO + O2 = 4N2 + 6H2O

(6)

Fig. 4(a) and (b) show the results of these two NH3-SCR reactions, respectively. N2 production first increased but then decreased as temperature increased. Total concentration of NO and NO2 in the exhaust gas had the opposite trend: it first decreased but then increased with temperature going up. NH3 content gradually decreased until it was undetectable, which agrees with results of the catalytic activity tests.

CO yield gradually decreased (because of its oxidation to CO2) as temperature increased and was 0 at 200 °C. CO2 yield, resulted from catalytic oxidation of HCN as well as CO, increased to almost 100% as the temperature increased. NH3 formed from hydrolysis reaction of 262

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Fig. 5. N2 sorption-desorption isotherms of La1Cu9/TiO2 catalysts for different calcination temperatures.

at 77 K for powder catalyst samples. As observed in Fig. 5, the N2 adsorption-desorption isotherms of samples calcined at 350, 450 and 550 °C exhibites hysteresis loops, which can be classified as type IV according to IUPAC classification [24,25]. Based on the relative pressure range of hysteresis loops of samples calcined at 350 °C and at 550 °C, well developed textural mesopores were proved. The shift in the hysteresis position towards higher relative pressures further indicate enlargement of the mesopores with the increase in calcination temperature. This was further verified by the adsorption pore size distribution (PSDs) calculated according to the BJH method in Fig. 6. The sample prepared at 450 °C had an isotherm that indicated collapse of the narrower mesopores, which then yielded larger pores after heating at 550 °C. This transformation was most likely due to particle sintering. The sample obtained at 650 °C had a type II isotherm, manifesting that the mesopores collapsed due to crystallite growth [26]. The small amount of gas uptake provides additional evidence for that. According to PSDs, the pore size of the catalyst calcined at 450 °C is mainly distributed at 1–6 nm, with a more concentrated pore size distribution and smaller pore size than other catalysts. The results show that the catalysts calcined at 450 °C were mainly composed of mesopores and micropores. Table 1 shows BET surface areas, pore volume and pore size for the La1Cu9/TiO2 catalysts. Specific surface area first increased and then decreased as the calcination temperature increased. The largest surface area was 46.445 m2/g obtained for the La1Cu9/TiO2 catalyst calcined at 450 °C. The same trend was observed for the pore volume. Inner pores of the samples calcined at 350 °C were not sufficiently activated due to incomplete calcination, which resulted in small surface area, low pore

Fig. 4. NH3-SCR over La1Cu9/TiO2: (a) NH3 oxidation with NO2 (100 ppm of NH3 and 100 ppm of NO2); (b) NH3 oxidation with NO (100 ppm of NH3, 100 ppm of NO and 1% of O2).

Variation of NO concentration (shown in Fig. 4(a)) implies that it was generated during the reaction between NH3 and NO2. Thus, a possible reaction mechanism of this system could be presented by the following reactions: 2NH3 + 3NO2 = 3NO + N2 + 3H2O

(7)

2NH3 + 5NO2 = 7NO + 3H2O

(8)

NO was partially consumed by its reaction with NH3 and NO2: 2NH3 + NO + NO2 = 2N2 + 3H2O

(9)

The highest N2 yield in the NH3-SCR occurred at 200 or 250 °C, while during the activity tests the highest yield was obtained at 150 °C. We believe that it is because HCN was catalytically converted to NH3 and N2 below 200 °C with minimum NOx formation. Even if small amounts of nitrogen oxides were generated, they transformed into N2 via the NH3-SCR. Thus, NH3-SCR experiments further validated activity tests results and demonstrated that sources of N2 were products of catalytic conversion of HCN and products of the reaction between NH3 and NOx. 3.3. Characterization 3.3.1. BET results To explore the surface physical properties of catalysts calcined at different temperatures, nitrogen adsorption-desorption were measured

Fig. 6. Pore-size distributions curves according to the BJH equation of La1Cu9/ TiO2 samples for different calcination temperatures. 263

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Table 1 Surface area, pore volume, and pore size of La1Cu9/TiO2 samples for different calcination temperatures. Samples

BET surface area (m2/g)

Pore Volume (cm3/g)

Pore diameter (nm)

350 °C 450 °C 550 °C 650 °C

23.334 46.445 21.098 5.729

0.062 0.103 0.180 0.063

3.055 4.289 6.205 7.420

Fig. 7. XRD patterns of (a) TiO2, (b) La10/TiO2, (c) Cu10/TiO2 and La1Cu9/TiO2 (fresh (d) and (used (e)).

volume and pore diameters. At 550 °C, pore sizes and volume increased, while the specific surface area decreased. Pore sizes and volume further decreased above 650 °C. The larger the specific surface area of the catalyst, the more favorable the adsorption of gas [27]. Thus, because catalyst calcined at 450 °C exhibited the largest specific surface area and relatively small average pore diameters, we selected 450 °C as the optimal activation temperature. All further experiments were performed with the catalysts calcined at 450 °C. 3.3.2. XRD results The results of XRD are shown in Fig. 7. All peaks of the five samples are characteristic peaks of TiO2 [28–30]. As observed, titanium dioxide contains two crystal forms, anatase (at ∼25.8°) and rutile (at ∼27.8°). No obvious diffraction peaks corresponding to any La and/or Cu phases could be observed suggesting that La and Cu were homogeneously distributed [31,32]. Even if very small crystals formed, they were under XRD detection limit. Cu-containing samples demonstrated rutile peaks with only one small additional peak belonging to anatase. The peak of La10/TiO2 is wider than that of Cu10/TiO2 and La1Cu9/TiO2, indicating that the crystal size is larger and the crystallinity is higher of Cu10/TiO2 and La1Cu9/TiO2 catalysts. Small rutile diffraction peak appeared in La10/TiO2. From a thermodynamic point of view, polymorphic transformation from anatase to rutile is irreversible. Presence of both rutile and anatase might be explained by the fact that the copper oxide catalyzed the mass transport to the nucleation region of rutile phase (which has higher mass density) during the temperature increase, thus, promoting rutile nuclei growth. Therefore, presence of Cu in our catalysts facilitated anatase to rutile phase transition.

Fig. 8. XPS spectra of (a) La10/TiO2, (b) Cu10/TiO2 and La1Cu9/TiO2 (fresh (c) and used (d)) over the spectral regions of (A) O1s, (B) La 3d and (C) Cu 2p.

3.3.3. XPS analysis XPS revealed presence of Cu (Cu 2p1/2, Cu 2p3/2), La (La 3d3/2, La 3d5/2) and O (O 1s) in the 0–1000 eV binding energy range, and the results are displayed in Fig. 8. Three different peaks of O confirmed presence of different oxygen morphologies as diagramed in Fig. 8(A). Peak O α located at 529.6–529.9 eV range for all samples could be attributed to the lattice oxygen [33,34]. Peak O β located at

531.2–531.8 eV range was assigned to the chemisorbed oxygen and weakly bonded oxygen species [35,36]. Peak O γ at about 532.9 eV was observed only for the La10/TiO2 catalyst. This peaks could be attributed to the hydroxyl oxygen and surface adsorbed water [37]. The ability of adsorbing water molecules in the catalyst surface is beneficial to the catalytic hydrolysis reaction. This observation is consistent with 264

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Table 2 Surface atom types and atomic ratio among different catalysts. Catalyst

La10/TiO2 Cu10/TiO2 La1Cu9/TiO2(fresh) La1Cu9/TiO2(used)

O (%)

Cu (%)

La (%)

Oα

Oβ

Oγ

Cu 2p 3/2

Cu 2p 1/2

La 3d 5/2

La 3d 3/2

57.13 77.00 74.77 74.36

14.53 23.00 25.23 25.64

28.31 0 0 0

– 50.91 53.82 51.06

– 20.30 19.06 19.40

57.12 – 56.27 57.12

42.88 – 43.73 42.88

previously report that excellent hydrolytic activity of La compounds [38]. Data in Table 2 shows that O α peak is much larger than O β peak. Oxygen content calculated from the O β peak changes in the following order: La1Cu9/TiO2 > Cu10/TiO2 > La10/TiO2. Cu10/TiO2 has a higher O β content than La10/TiO2, and the O β content is further increased after loading La. Chemisorbed oxygen is the most active oxygen form and it is the first one consumed during the catalytic reactions [18,39], which was also confirmed in our catalytic activity tests. Peaks at 529.6 eV and 531.2 eV of La1Cu9/TiO2 shifted towards higher binding energy compared to La10/TiO2, indicating electrons transmission from the metal atoms to O atoms in metal oxides [35,36] and the existence of oxygen vacancy after Cu was added to La10/TiO2. At the same time, this shift demonstrated that the La1Cu9/TiO2 catalysts had stronger oxidation properties [35] than La10/TiO2. There was almost no evident changes in the oxygen content of catalysts before and after the reaction, indicating the catalytic action of La1Cu9/TiO2. For investigating the chemical state of metal oxides on the catalyst surface, we compared Cu 2p and La 3d XPS peaks of La10/TiO2 as well as of fresh and used La1Cu9/TiO2 catalysts. La XPS signal consisted of two peaks corresponding to La 3d 5/2 and La 3d 3/2 in Fig. 8(B). All catalyst showed La XPS signals at almost the same energy. Peaks at ∼834.9 and 838.5 eV correspond to La 3d 5/2 while peaks at ∼851.6 and 855.3 eV belong to La 3d 3/2 [40]. All these peaks corresponded to the three-valent La. Both (fresh and used) La1Cu9/TiO2 samples showed two peaks at ∼953.4 and 933.5 eV in Fig. 8(C), which correspond to Cu 2p 1/2 and Cu 2p 3/2, respectively, confirming existence of CuO in the catalysts [41,42]. Peaks at ∼941.3 and 943.7 eV are the characteristic satellite peaks [43,44] belonging to cupric oxide further proving presence of two-valent copper species. In Fig. 8(B), the peaks of the La1Cu9/TiO2 catalyst shifted to the higher binding energy compared to the La10/TiO2 catalyst. This shifting demonstrate interaction between different metal oxides, which resulted in abundant oxygen vacancies, thus promoted oxygen adsorption (resulting in increase of the O β peak in La1Cu9/TiO2) and surface oxygen conversion into O α and O β after Cu was added to La10/TiO2 sample. Since oxygen vacancies excess accelerated transformation of anatase to rutile as well rutile crystal growth [45], this can explain the analysis of XRD well. Superoxide ions formed [46,47] after Cu addition because of the reaction between adsorbed oxygen and the oxygen holes produced by the interaction between highly dispersed metal oxides. This was the reason that the catalysts had stronger oxidation properties after Cu oxides were added to La/TiO2 in O 1s analysis. In addition, interactions between metal oxides promoted the metal oxide to penetrate into TiO2, resulting in a uniform distribution, which is inline with the results of XRD.

Fig. 9. H2-TPR profiles for the La10/TiO2, Cu10/TiO2 and La1Cu9/TiO2 catalysts.

the Cu10/TiO2 sample, a broader and more intense peak was obtained for the La1Cu9/TiO2 sample, indicating the total H2 consumption increased. It demonstrated that La1Cu9/TiO2 catalyst possessed stronger redox ability than Cu10/TiO2 [20,52]. TPR peaks and H2 consumption of Cu species were summarized in Table S1 (as displayed in supplementary material). In addition, La inclusion shifted the reduction temperature to higher values. Based on the XPS results of interactions between La and Cu species, this might be due to the formation of the polymeride, resulting in the enhancement of thermal stability after La was added [53].

3.3.5. TPD results Properties of the catalyst surface directly correlate with its activity and selectivity of nitrogen [54]. Therefore, NH3-TPD and CO2-TPD were carried out to investigate the acidity and basicity of different catalysts. As observed in Fig. 10, acidity tests showed desorption peaks that could be divided into three parts: centered at 1) 100–150 °C, 2) 200–400 °C and 3) above 400 °C. These three peaks correspond to 1)

3.3.4. H2-TPR results Redox property tests were performed about three catalyst samples by a H2-TPR, and the results are shown in Fig. 9. Since pure TiO2 is difficult to reduce, no evident TPR peaks were observed at 50–750 °C of TiO2. According to the H2-TPR profile, the small reduction peak at 150 °C is attributed to the reduction of copper oxide coarser clusters [48,49] and the strongest reduction peak observed at 150–300 °C for Cu10/TiO2 and La1Cu9/TiO2 samples is associated with the reduction of finely dispersed copper oxide to metallic copper [50,51]. Comparing to

Fig. 10. NH3-TPD profiles for La10/TiO2, Cu10/TiO2 and La1Cu9/TiO2 catalysts. 265

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Table 3 NH3-TPD: Temperature of desorption peaks position and amounts of desorbed NH3. Samples

La10/TiO2 Cu10/TiO2 La1Cu9/TiO2

Region (°C) and NH3 desorption (cm3/g) 100–150

200–400

Above 400

0.2728 0.8253 0.5613

8.3377 8.0061 7.8154

0.6245 0.0726 0.4472

Total desorption (cm3/g)

9.2350 8.9040 8.8239

desorption of physically adsorbed ammonia adsorbed on weak Brønsted acid sites [55], 2) desorption of NH4+ bounded to strong Brønsted acid sites and 3) desorption of NH3 adsorbed on weak and strong Lewis acid sites [56,57]. Thus, acidic sites provided active sites for NH3 adsorption produced by the hydrolysis reaction. Temperature of desorption peaks position and amounts of desorbed NH3 are summarize in Table 3. The desorption amount of total NH3 of catalysts is in the order: La10/ TiO2 > La1Cu9/TiO2 > Cu10/TiO2. It is indicated that the total acid sites obtained of the Cu10/TiO2 catalyst are relatively small, and the total acid sites are increased after La was added. Besides, desorption peaks moves to a lower temperature, indicating that the La1Cu9/TiO2 catalyst could bind NH3 at a lower temperature than Cu10/TiO2 catalyst. Strong ammonia adsorption capacity of a catalyst played an important role in promoting conversion of NOx to N2 in the NH3-SCR system, which was also the reason for high N2 selectivity. Besides the acidic sites on the catalysts, we also conducted CO2-TPD to explore the basic sites. The desorption amount of CO2 is divided into three parts according to the temperature corresponding to the peak and were summarize in Table 4. Peaks range from 100 to 150 °C corresponding to a slightly alkaline desorption peak. Peaks in the 220–400 °C range was attributed to the sites with medium basicity. Desorption peaks above 400 °C were attributed to strongly basic sites [58,59]. While for the desorption amount of CO2-TPD, it is opposite to that of NH3-TPD. The distribution of total CO2 desorption is as follows: Cu10/ TiO2 > La1Cu9/TiO2 > La10/TiO2. Cu10/TiO2 catalyst has more alkaline sites, and after adding La, the alkaline sites are reduced. Since HCN gas is acidic, basic sites on the catalyst surface play a very important role in adsorbing HCN, thus they are directly responsible for good catalytic activity. Moreover, the synergy between acidity and alkalinity of the support catalyst further promoted formation of highly dispersed CuO [60,61] identified by XRD.

Fig. 11. FT-IR spectra for La10/TiO2, Cu10/TiO2 and La1Cu9/TiO2 catalysts.

hydrolysis and oxidation processes. OH group was formed during the nitrate reduction process, and OH group and the adsorbed water contributes sites. The adsorbed water would partly lose when samples were treated under 200–450 °C, while de-OH would occur and adsorbed water disappeared at higher temperatures [66,67], which was consistent with a significant decrease in BET surface area.

4. Discussions A mechanism over La1Cu9/TiO2 catalyst on the selective catalytic oxidation of HCN at low temperature under the conditions of 1% O2 and 10% relative humidity was proposed. Overall reaction pathways for catalytic hydrolysis and oxidation of HCN as well NH3-SCR associated with HCN are proposed as shown in Fig. 12. Based on the results from HCN activity test and NH3-SCR test as well as former studies [7,9,66], HCN was partially catalyzed into NH3 and CO by H2O through the hydrolysis reaction at low temperature. On the other hand, HCN was catalyzed to produce intermediates (such as eCNO and eCN) by oxidation reaction. -CN groups probably transformed to -CNO [4,20]. -CNO groups continued to hydrolyze and oxidize yielding NH3 and N2. Subsequently, NH3 reacted with NOx (by-product of HCN oxidation) to form N2. It has been proved by SCR experiments that the catalyst has

3.3.6. FT-IR results As presented in Fig. 11, no noticeable differences were observed in the FT-IR spectra of La10/TiO2, Cu10/TiO2 and La1Cu9/TiO2 catalysts. Main absorption bands were at 1430, 1632, 2361 and 3448 cm−1. Absorption band at 1430 cm−1 was attributed to NH4+ [62], indicating the presence of strong Brønsted acid sites or weak Lewis acid sites, which is consistent with the results of NH3-TPD. The band centered at 2361 cm−1 can be assigned to the adsorbed atmospheric CO2 [28,63]. Absorption bands at 1632 and 3448 cm−1 correspond to deformation vibration mode of the HeO bond in physisorbed H2O and to vibrational mode of OH groups on the catalyst surface [64,65], respectively. OH groups and H2O play an important role during the HCN catalytic Table 4 CO2-TPD: Temperature of desorption peaks position and amounts of desorbed CO2. Samples

La10/TiO2 Cu10/TiO2 La1Cu9/TiO2

Region (°C) and CO2 desorption (cm3/g) 100–150

220–400

Above 400

0.00131 0.06934 0.05529

0.40017 0.043522 0.42734

0.00085 0.03028 0.01496

Total desorption (cm3/g)

0.40233 0.53484 0.49759

Fig. 12. Possible reaction pathways of HCN over La1Cu9/TiO2 catalyst. 266

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good activity for denitrification, which was also the reason why La1Cu2/TiO2 had a high N2 selectivity.

[15]

5. Conclusions [16]

We synthesized LaxCuy/TiO2 catalyst using sol-gel method to study catalytic hydrolysis and oxidation of HCN at 100–400 °C. Several catalysts with different La/Cu ratios were tested. La1Cu9/TiO2 calcined at 450 °C exhibited the highest catalytic activity with almost 100% of HCN conversion and 62.24% of N2 selectivity without NOx as a by-product at 150 °C. XRD and XPS analysis indicated that metals in the La1Cu9/TiO2 catalyst were highly dispersed. XPS results proved interaction between La and Cu oxides as well as the existence of oxygen vacancy, thus promoted the oxidability. A small amounts of La oxide participated the formation of strong interaction with Cu oxide and synergistic effects, and promoted the redox ability of the catalyst according to H2-TPR and XPS. In TPD analyses, abundant acid and basic sites presented on the surface of La1Cu9/TiO2. Besides, La increased the total acidity and bring about the catalyst medium strong acid sites with a lower temperature. NH3-SCR experiments confirmed activity test results as well the fact that oxidation of nitrogen oxides and catalytic oxidation of HCN were the main sources of N2.

[17]

[18] [19]

[20]

[21]

[22]

[23]

[24] [25]

Acknowledgements [26]

This work was supported by the National Natural Science Foundation of China (No. 51868030, No. 51568027), Candidates of the Young and Middle Aged Academic Leaders of Yunnan Province (2015HB012) and National Key Research and Development Program of China (2017YFC0210500).

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[28]

Appendix A. Supplementary data [29]

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.micromeso.2019.02.025.

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