Hydrogen Cyanide Removal over Ti-Al 2 O 3 Catalyst: Activity at Low Temperature

Abstract

Catalytic activity of γ-Al₂O₃ catalyst modified with Ti (Ti-Al₂O₃) for hydrogen cyanide (HCN) removal was studied, and the TiO₂ and γ-Al₂O₃ were tested as comparisons. It was found that Ti-Al₂O₃ showed a remarkable composite effect with superior HCN removal activity compared to TiO₂ and γ-Al₂O₃, especially at low temperature range. HCN conversion efficiency improved with the increase of Ti content. Characterization results show that there is a strong interaction between TiO₂ and Al₂O₃, and TiO₂-anatase is well dispersed on γ-Al₂O₃, which gives more active site for HCN. The high catalytic activity was attributed to the combined action of high pore volumes and surface area, as well as abundant hydroxyl groups of composite catalyst (Ti-Al₂O₃).

Introduction

Hydrogen cyanide (HCN) is a highly toxic gas, which may lead to suffocation because tissue cells that cannot utilize oxygen for the CN⁻ will combine with Fe³⁺ in the cytochrome oxidase and block the electron transfer of Fe³⁺ in the process of oxidation. One minute exposure to concentrations of ca. 300 ppm is lethal (Osińska, 2013). HCN usually exists in reductive gas such as yellow phosphorus tail gas (Wang et al., 2011), closed calcium carbide furnace tail gas (Jiang et al., 2014; Zhou et al., 2014), coal gas (Xu et al., 2016), and petroleum refineries (Wei, 2015). Besides, HCN in these gases usually convert to NO_x in the combustion process or after emission. For its high toxicity and a major precursor of nitrous oxides, more and more attention of HCN treatment was attracted.

Methods of absorption, adsorption, combustion, and catalysis were investigated for removing HCN (Nanba et al., 2000; Ning et al., 2013; Zhao et al., 2015; Hsiao et al., 2016). Catalysis shows superiority for it can reduce toxicity and has less secondary pollution. Catalysts based on carbon (Wang et al., 2016), molecular sieve (Song et al., 2016; Hu et al., 2018), aluminum, and titanium (Ma et al., 2017) materials were found effective to remove HCN. TiO₂-anatase had a high HCN hydrolysis activity and can remove HCN 90% above 400°C (Kröcher and Elsener, 2009). Marsh et al. (1952) investigated the hydrolysis of HCN on γ-Al₂O₃, in which the conversion of HCN was about 80% at 300°C (Marsh et al., 1952). Nanba removed HCN over H-ferrierite (SiO₂/Al₂O₃ = 17) with conversion efficiency of ∼80% at 500°C (Nanba et al., 2000). Pt/Al₂O₃ for HCN removal catalyst was studied by fractional factorial design and was found that temperature and gas hourly space velocity (GHSV) had the most significant effects on HCN conversion (Zhao et al., 2006). In our preliminary work, HCN can be converted completely with Al-Ti-O_x at 250°C. It has been proved that both TiO₂ and Al₂O₃ catalysts are effective for removing HCN. While there could be room for improvement of conversion efficiency, the operational temperature could be lowered to reduce the energy consumption.

In this article, γ-Al₂O₃ was modified with Ti by our own created methods to prepare catalysts for removing HCN. The activity and characterization of the materials were examined and discussed.

Experimental Section

Preparation of catalyst

Al₂O₃ used in the experiment is γ-Al₂O₃ in the market (Shandong Zibo Bell Chemical Technology Co., Ltd.). TiO₂ is prepared by hydration and roasting of titanium butoxide (Tianjin Kemiou Chemical Reagent Co.). The method is as follows: an appropriation of titanium butoxide was added to absolute ethanol and with magnetic stirring for 30 min. Then water was added into the mixture slowly and drop by drop, accompanied with stirring. The product was hydrated for 1 h at room temperature and then dried in an oven at 100°C for 12 h. The following calcination in a furnace at 500°C for 3 h afforded the catalyst. Ti-Al₂O₃ was prepared by the following steps: an appropriation of titanium butoxide was added to absolute ethanol and with magnetic stirring for 30 min, followed with an addition of γ-Al₂O₃. Then the mixture was mixed well and let to sit for 4 h. Subsequently, water was added into the mixture drop by drop and accompanied with stirring. The product was hydrated for 1 h at room temperature and then dried in an oven at 100°C for 12 h. The following calcination in the furnace was at 500°C for 3 h to gain the Ti-Al₂O₃.

Characterization

To study the characterization of materials, a series of measurements were used, including N₂ adsorption/desorption, X-ray diffraction (XRD), Ramon, high-resolution transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS), and in situ diffuse reflectance infrared fourier transform spectroscopy (in situ DRIFTS) analysis of hydroxy content with NH₃ as the probe molecule.

N₂ adsorption/desorption was applied to determine the surface area, pore size distribution, and total pore volume with a TriStar II 3020 3.02 (Micromeritics Instrument Corporation). The specific surface areas were determined using the Brunauer-Emmett-Teller equation. The pore size distribution was calculated with the Barrett-Joyner-Halenda method. XRD measurements were performed by a Bruker D8 Advance (Bruker) operated at 40 mA and 40 kV. The angle range scanned was 5–90° with a counting step of 0.05°. Raman spectrum spectroscopy was performed by LabRAM HR800 Raman microscope and spectrometer (HORIBA Scientific) using a 325 nm diode laser excitation. HR-TEM was analyzed on an FEI Tecnai G20 (FEI) operated at 200 kV. XPS analyses were conducted on a Thermo ESCALAB 250XI spectrometer (Thermo Fisher Scientific) using an Al-Kα (hv 1486.6 eV) at 150 W. XPS spectra were calibrated using the main C 1s feature resulting from adventitious C at 284.8 eV. In situ DRIFTS analysis of hydroxy content with NH₃ as the probe molecule was investigated by NEXUS 670-FTIR instrument (Nicolet) equipped with a smart collector and a MCT/A detector cooled with liquid nitrogen. Thirty micrograms of catalysts were put in the in-situ tank reactor and pretreated at 400°C for 30 min in the air. When the temperature decreased to 30°C, the catalysts were purged with high purity N₂ for 30 min and collected in the background with Fourier-transform infrared spectroscopy. Then gas of NH₃ (500 ppm) balanced with nitrogen balance was led to the catalysts for adsorption for 1 h to saturation. After that, the catalysts were purged with high purity N₂ for 30 min again. All spectra were collected in the 4,000–400 cm⁻¹ frequency ranges at a resolution of 4 cm⁻¹. A total of 100 scans were averaged for each spectrum.

Catalyst activity tests

Catalyst tests were carried out in a fixed-bed flow reactor. The reaction gas was composed of 100 ppm HCN with nitrogen balance. Water was added into the gas according to part of nitrogen flowed through water setup, and gas achieved moisture of 0.024% finally. The mix gas was introduced to the catalyst bed at a flow rate of 200 mL/min. The reaction occurred in a quartz column reactor with 7 mm inner diameter with temperature controlling equipment. Specifically, 200 mg of each catalyst was used in the catalyst test. The GHSV was ∼42,000 h⁻¹. HCN and NH₃ were measured by a mass spectrometer (MAX300-LG; Extrel). N₂O, NO_x, CO, and CO₂ were measured by gas chromatography (GC-97902; Fuli Instruments). To guarantee a steady gas flow rate and to avoid adsorption disturbance, the reaction was run 1 h before detected gas.

Results and Discussion

Results of activity test

Conversion efficiency and products of HCN treated by TiO₂, γ-Al₂O₃, and Ti-Al₂O₃ (the content of TiO₂ was controlled as 25% in the experiment) were tested, which are presented in Fig. 1.

FIG. 1.

Results of activity tests, (A) conversion of HCN with different materials, (B) products of HCN with TiO₂, (C) products of HCN with γ-Al₂O₃, (D) products of HCN with Ti-Al₂O₃. HCN, hydrogen cyanide.

From Fig. 1A, all the materials are effective for HCN conversion, and the conversion efficiency of HCN increases with temperature. It also can be seen that Ti-Al₂O₃ is superior to TiO₂ and γ-Al₂O₃ for HCN conversion, especially at low temperature. The conversion efficiency of HCN with Ti-Al₂O₃ reaches 82%, which are 15% and 22% for TiO₂ and γ-Al₂O₃ at 150°C. When the temperature went up to 250°C, HCN was converted completely by Ti-Al₂O₃. Conversion efficiencies of HCN treated by TiO₂ and γ-Al₂O₃ are under 70% at the same temperature. The reaction temperature should be above 350°C to achieve 100% conversion when HCN was treated by TiO₂ and γ-Al₂O₃. The Ti-Al₂O₃ showed higher activity for HCN at lower temperature.

NH₃, NO_x, CO, and CO₂ can be detected in outlet gas of reactor, the concentrations of them are showed in Fig. 1A–C. Combined with conversion of HCN, it can prove that the C of HCN was converted to CO and CO₂ no matter with which material. There was no CO₂ generated in the reaction on TiO₂, that is, all the C of HCN was converted to CO. When HCN was treated by Ti-Al₂O₃ and γ-Al₂O₃, both CO and CO₂ can be found, but CO₂ began to generate only when the temperature was above 250°C and 300°C, respectively. And higher the temperature was, the more the CO₂ was generated. However, production of CO occupied an absolutely dominant position. N in the HCN was converted to NH₃, NO_x, and N₂, which are hard to detect. NH₃ and NO_x can be measured in every reaction of HCN with TiO₂, γ-Al₂O₃, and Ti-Al₂O₃; the masses increased with the increase of temperature, and NO_x topped out at 8 ppm. The changes of CO₂ and NO_x with temperature indicate that stronger oxidizing reaction happened at a higher temperature.

Characterization

N₂ adsorption–desorption isotherms and corresponding pore size distribution of TiO₂, γ-Al₂O₃, and Ti-Al₂O₃ are illustrated in Fig. 2A and B. And related porosity parameters are gathered in Table 1. The N₂ adsorption–desorption isotherms of γ-Al₂O₃ and Ti-Al₂O₃ belong to the type-IV isotherms with hysteresis loops of H₃ type, which shows that microstructures with plate-like particles forming slit-like pores are the main pore structure of them (Kruk and Jaroniec, 2001). The nitrogen adsorption–desorption isotherms of TiO₂ present the feature of type-IV isotherms with hysteresis loops of H₄, which indicates that TiO₂ own narrow slit-like pores collapse of lamellar structures (And et al., 2000). Pore size distribution of γ-Al₂O₃ and Ti-Al₂O₃ ranged from 0.6 to 50 nm with large mass; the peaks centered around 3 and 4 nm, respectively. Pore size of TiO₂ mainly distributes between 2 and 6 nm, and the mass is very low. This meets the N₂ adsorption–desorption isotherms in Fig. 2A and the dates in Table 1. It is clear that γ-Al₂O₃ and Ti-Al₂O₃ have high surface areas with 328.7 and 263.2 m²/g, while the surface areas of TiO₂ are fairly poor. The situation of total pore volumes for TiO₂, γ-Al₂O₃, and Ti-Al₂O₃ are exactly analogous to surface areas. The average diameters of them are close to around 4.5 nm.

FIG. 2.

N₂ adsorption–desorption isotherms (A) and pore size (B) of TiO₂, γ-Al₂O₃, and Ti-Al₂O₃.

Table 1.

Porosity Parameters of Different Samples

Sample	S_BET (m ² /g)	V_total (cm ³ /g)	D_average (nm)
TiO₂	5.5	0.048	4.55
Al₂O₃	328.7	0.470	4.49
Ti-Al₂O₃	263.2	0.456	4.48

From the results of N₂ adsorption/desorption, it can be learned that the pore structure of pure TiO₂ is not abundant, which is very different from γ-Al₂O₃ and Ti-Al₂O₃. Modifying Ti on γ-Al₂O₃ by the prepared method will block some pores and lead to decrease of the surface areas and pore volumes to a certain extent, but Ti-Al₂O₃ still keeps high surface areas and pore volume. γ-Al₂O₃ provides rich surface areas and pore volume for active component Ti, which benefit adsorption and reaction.

Figure 3 shows the XRD patterns of TiO₂, γ-Al₂O₃, and Ti-Al₂O₃. It can be seen that a strong peak appears at 25.48° 2θ, a minor peak at 48.25° 2θ, and other several clear peaks on XRD patterns of TiO₂ (Khalilzadeh and Fatemi, 2016). These peaks are characteristic of TiO₂-anatase, and there are no peaks corresponding to TiO₂ rutile or brookite, that is, pure TiO₂-anatase is formed by the prepared method. The pattern of γ-Al₂O₃ shows no sharp peaks and a grossly amorphous line with broad maxima around the positions of the diffraction lines for γ-Al₂O₃ (Öhman and Paul, 2002). For Ti-Al₂O₃, the peaks that belong to TiO₂-anatase and Al₂O₃ appear, but the crystalline degrees decrease. Another, the full width at half maxima of TiO₂ increases and decreases for Al₂O₃, that is, the grain size of TiO₂ decreases and the grain size of Al₂O₃ increases in the Ti-Al₂O₃ compared with simple TiO₂ and γ-Al₂O₃. The decrease of TiO₂ grain size demonstrates that TiO₂-anatase is well dispersed on γ-Al₂O₃, which gives more active site for HCN conversion. The increase of Al₂O₃ grain size in the Ti-Al₂O₃ resulted from recrystallization in the calcination process.

FIG. 3.

X-ray diffraction patterns of TiO₂, γ-Al₂O₃, and Ti-Al₂O₃.

According to Fig. 4 five bands are observed for TiO₂ at E_g (144 cm⁻¹), E_g (195 cm⁻¹), B_1g (400 m⁻¹), A_1g (516 cm⁻¹), and E_g (638 cm⁻¹) corresponding to the active modes of the TiO₂-anatase phase (Fagan et al., 2016; Ledesma et al., 2016). Peaks appeared on the patent of Ti-Al₂O₃ at the same position, but the intensities of them are much weaker. There is no peak found on the patent of γ-Al₂O₃ because its grossly amorphous. The results of Raman spectra well meet the results of XRD.

FIG. 4.

Raman spectra of samples.

HR-TEM images of TiO₂, γ-Al₂O₃, and Ti-Al₂O₃ are shown in Fig. 5. TiO₂ is in a larger particle size and with serious aggregation when comparing Fig. 5A with Fig. 5B and C. These may be the reason for TiO₂ to have low pore volume and surface area. In the Fig. 5B, image of γ-Al₂O₃ presents dispersiveness, which is the important factor for own high pore volume and surface area in above measure. It can be observed that the TiO₂ particles are well distributed on the γ-Al₂O₃, and the particle size decreases dramatically as shown in Fig. 5C. The introducing of TiO₂ in γ-Al₂O₃ leads to decrease of pore volume and surface area, and the decreasing amplitude is small because of TiO₂ well distribution so that Ti-Al₂O₃ not only kept high pore volume and surface area but also achieved TiO₂ well distribution.

FIG. 5.

High-resolution transmission electron microscopy of samples, (A) TiO₂, (B) γ-Al₂O₃, and (C) Ti-Al₂O₃.

The XPS spectra of Al 2p, Ti 2p, and O1s were conducted to study the binding energy, as showed in Fig. 6. The Al 2p peaks were found at 74.25 and 74.55 eV on patent of γ-Al₂O₃ and Ti-Al₂O₃, which are characteristic for oxidized aluminum (Xu et al., 2016; Bing et al., 2017). A positive shift in Al 2p was observed on Ti-Al₂O₃ indicating electron transfer between the Ti and Al. As can be seen from Fig. 6B, two peaks attributed to TiO₂ are detected between 457 and 460 eV (Ti2p 3/2) and 462–467 eV (Ti2p 1/2) both on TiO₂ and Ti-Al₂O₃ (Moakhar et al., 2017). But it is obvious that peak center of each region on Ti-Al₂O₃ move to lower binding energy. From XPS spectra results of O1s in Fig. 6C, peaks at 531.05 eV and a minor peak at 531.35 eV in the O1s spectra of TiO₂ and Ti-Al₂O₃ corresponded to the lattice oxygen of TiO₂ (Su et al., 2013; Santos et al., 2015) and Al₂O₃ (Xu et al., 2016), respectively. XPS spectra of O1s on the Ti-Al₂O₃ show two binding energies at 529.62 and 531.35 eV assigned to TiO₂ and Al₂O₃. There is a remarkable negative shift for Ti-O-Ti on Ti-Al₂O₃. From all the results of XPS spectra, positive or negative binding energy shift occurred in Al 2p, Ti 2p, and O1s for Ti-Al₂O₃ compared with TiO₂ and γ-Al₂O₃, which infer that strong interaction happened between Al and Ti, and electron density had changed in the preparation process.

FIG. 6.

X-ray photoelectron spectroscopy spectra of TiO₂, γ-Al₂O₃, and Ti-Al₂O₃, (A) Al 2p, (B) Ti 2p, (C) O 1s.

Hydroxy contents of the catalysts were investigated by in situ DRIFTS analysis with NH₃ as the probe molecule. The results of NH₃ adsorption are showed in Fig. 7. The negative peaks round 3,750, 3,680, and 3,620 are hydroxyl consumption, which were the results of interacting between NH₃ and surface hydroxyl groups (Primet, 1971; Hernández-Alonso et al., 2013; Zhang et al., 2017). The area of peaks mirrors the mass of hydroxyl; it is obvious that Ti-Al₂O₃ owns highest hydroxyl with OH area of 9.85, the next is γ-Al₂O₃, and TiO₂ is least. From this knowledge, the modification of Ti to γ-Al₂O₃ could add surface hydroxyl even though it decreases the surface area. The strong interaction that happened between Al and Ti in Ti-Al₂O₃ (results of XPS) would lead to lattice imperfection, which may benefit to generate surface hydroxyl (Orge et al., 2011).

FIG. 7.

In situ diffuse reflectance infrared fourier transform spectroscopy analysis of hydroxy content.

Activity of catalysts in different Ti contents

According to the results above, it could be preliminarily inferred that the addition of TiO₂ would affect the characteristic of γ-Al₂O₃ and it may be the key active component. So Ti-Al₂O₃ with different contents of TiO₂ was tested for HCN conversion, including 10%, 20%, 30%, and 40%. The results are showed in Fig. 8.

FIG. 8.

Conversion of HCN with different Ti contents.

Whatever the TiO₂ content is, the materials of Ti-Al₂O₃ showed significantly higher conversion efficiency compared with γ-Al₂O₃. At the temperature of 150°C, HCN conversion efficiency by γ-Al₂O₃ with 10% TiO₂ reached to 71%, which is only 22% for γ-Al₂O₃ without TiO₂. The higher the TiO₂ content was, the higher HCN conversion efficiency was achieved. When the TiO₂ content was 40%, the HCN conversion efficiency was above 90% at 150°C, which is a gratifying result. The results also illustrate the key role of Ti in HCN conversion.

Discussion

Based on the experiment and characterization of catalysts, conversion ways of HCN may be as follows: \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} { \rm{HCN}} + {{ \rm{H}}_2}{ \rm{O}} \to { \rm{N}}{{ \rm{H}}_{3 }} + { \rm{ CO}} \tag{1} \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} { \rm{HCN}} + { \rm{OH}} \to {{ \rm{H}}_2}{ \rm{O}} + { \rm{CN }} \\ \ {\rm CN} + { \rm{OH}} \to { \rm{NCO}} + { \rm{H }} \quad 2{ \rm{NCO}} \to {{ \rm{N}}_{2 }} + { \rm{CO}} \tag{2} \end{align*} \end{document}

\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} { \rm{HCN}} \; + \;{{ \rm{O}}_{2 \;}} \to { \rm{C}}{{ \rm{O}}_{2 \;}} + \;{ \rm{N}}{{ \rm{O}}_x} \tag{3} \end{align*} \end{document}

According to section “Results of activity test”, CO and NH₃ are the dominant products of HCN. It can be inferred that Equations (1) and (2) are main conversion way of HCN conversion; CO₂ and NO_x were formed at higher temperature.

From the experiment result, both the TiO₂ and γ-Al₂O₃ can convert HCN effectively as long as the reaction temperature is high enough. But the HCN conversion improved dramatically at lower temperature when adding Ti to γ-Al₂O₃ by the preparation method. Ti plays the crucial role in the conversion of HCN. Based on the results of XRD and Raman, TiO₂-anatase was formed, which often achieved higher catalytic activity than rutile TiO₂ and brookite TiO₂ (She et al., 2016; Zhao et al., 2016). The porous structure of pure TiO₂ is very poor with low total pore volumes and surface area. That may give few exposure opportunities for HCN, thereby hindering the conversion of HCN on TiO₂. For Ti-Al₂O₃, Ti was loaded on γ-Al₂O₃, which own abundant pore volumes and surface area. It can be confirmed that TiO₂ particles were well distributed on the γ-Al₂O₃ according to the HR-TEM images, and the TiO₂ particle size decreases dramatically. The introduction of TiO₂ would decrease pore volumes and surface area, but not too much. So the contact area and the conversion probability were significantly enhanced that benefitted improving the activity of Ti-Al₂O₃.

Water and hydroxyl groups are important in the conversion process of HCN. TiO₂ has strong hydrophilicity, so H₂O can be attached on TiO₂ in terms of molecule and dissociation. From the results of in situ DRIFTS analysis with NH₃, Ti-Al₂O₃ own abundant hydroxyl groups, which may have resulted from strong interaction that happened between Al and Ti in Ti-Al₂O₃ from the XPS analysis. In addition, hydroxyl groups can promote the adsorption of HCN as the first step of reaction plus the higher surface area and well distribution of Ti; Ti-Al₂O₃ showed that high activity at lower temperature resulted from its comprehensive characteristic.

Conclusion

γ-Al₂O₃ modified with Ti was prepared for HCN conversion in this study. Ti-Al₂O₃ achieved high pore volumes and surface area with TiO₂-anatase being well distributed. More surface hydroxyl was formed because of strong interaction that happened between Al and Ti in Ti-Al₂O₃. By the contribution of these characteristics, HCN was converted by Ti-Al₂O₃ effectively. The conversion efficiency of HCN with Ti-Al₂O₃ reached 82% at 150°C and 100% at 250°C. In the reaction, HCN was converted to NH₃, NO_x, CO, and CO₂; more NO_x and CO₂ were generated at a higher temperature. The content of Ti has a remarkable impact on HCN conversion that the higher the Ti content was, the higher HCN conversion efficiency was achieved. The HCN conversion efficiency could be above 90% at 150°C when the Ti content was 40%. The selectivity of products and the controlment of reaction path were worth being investigated further for drastically hazard-free treatment of HCN.

Footnotes

Acknowledgments

This work was supported by the National Natural Science Foundation of China (No. 21876071, 51868030) and National Key R&D Program of China (No. 2018YFC1900200, 2017YFC0210503).

Author Disclosure Statement

No competing financial interests exist.

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