J. Cent. South Univ. (2018) 25: 691-700

DOI: https://doi.org/10.1007/s11771-018-3773-0

Preparation, characterization and catalytic performance of Cu nanowire catalyst for CO₂ hydrogenation

ZHANG Xiao-yan(张晓艳)^{1, 3}, WANG Ming-hua(王明华)¹, CHEN Zhong-yi(陈忠翼)¹,P. XIAO², P. WEBLEY², ZHAI Yu-chun(翟玉春)¹

1. School of Metallurgy, Northeastern University, Shenyang 110819, China;

2. Department of Chemical and Biomolecular Engineering, The University of Melbourne,Victoria 3010, Australia;

3. Library of Tongren College, Tongren 554300, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract: Pure Cu nanowires as catalyst were prepared by electrochemical deposition and were used in CO₂ hydrogenation to methanol. The active sites of the Cu based catalyst were discussed. The performance and structural development of the catalyst were observed during CO₂ hydrogenation. A mechanism for the deactivation of the catalyst was discussed. The key factors that affect the deactivation of the catalyst were found. Cu nanowire sample was characterized by SEM, EDS, XRD, and BET. The results show that Cu nanowires have very high sintering resistance and catalytic stability. This helps to develop high performance catalysts. The changes in the grain size, SEM morphology and catalytic properties of the sample during CO₂ hydrogenation show that the migration of the Cu atoms on the surface of the Cu nanowires can occur. Continuous migration of Cu atoms and sintering of Cu grains can lead to flow blockage in gas channels. The gas channel flow blockage or the sintering of Cu grains can lead to deactivation of the catalyst. However, the shape of catalytic performance curve indicates that the main reason for the deactivation of the catalyst is the gas channel flow blockage.

Key words: CO₂ hydrogenation; methanol; Cu nanowire; migration; sintering; flow blockage

Cite this article as: ZHANG Xiao-yan, WANG Ming-hua, CHEN Zhong-yi, P. XIAO, P. WEBLEY, ZHAI Yu-chun. Preparation, characterization and catalytic performance of Cu nanowire catalyst for CO₂ hydrogenation [J]. Journal of Central South University, 2018, 25(4): 691–700. DOI: https://doi.org/10.1007/s11771-018-3773-0.

1 Introduction

At present, there is still debate on active site of Cu based catalysts for CO₂ hydrogenation to methanol. One opinion is that Cu is main active site of the catalysts [1–5]. The activity of the catalysts is easily affected by the Cu specific surface area [4–10] or Cu grain size [5, 10, 11], and the catalytic activity of the catalysts with smaller grain size or larger specific surface area is usually higher. ZnO as additive may act important role in dispersing active component, inhibition of sintering, limiting poisoning of active site, and neutralizing the acidity of Al₂O₃ to prevent the formation of by-products, such as dimethyl ether [8, 12, 13]. ZnO is a good industrial fine desulfurization agent [14, 15]. The second opinion is that Cu–Zn site [16–19] may participate in CO₂ hydrogenation to methanol, and there is a strong interaction between Cu and ZnO [9, 20–25]. Cu must be alloyed with Zn to make methanol synthesis better and faster, and the interaction between single Cu and CO₂ is very poor [19].The previous research results showed that the activity of skeletal Cu based catalysts for methanol synthesis is higher than that of commercial Cu/ZnO/Al₂O₃ catalyst [26, 27]. The activity of pure Cu particles is very low for methanol synthesis, as reported in Ref. [18]. This is because the metal atoms have higher mobility [28], and they are easy to sinter in the absence of inert support. Copper micro-crystals can fuse even at a temperature less than 200 °C, and the activity decreases rapidly [29, 30]. The sintering characteristic of pure Cu particles has severely limited the study on pure Cu catalyst.

High concentration CO₂ can cause rapid deactivation of Cu-based catalysts [23]. The structure of common Cu based catalysts is more complex, and it is difficult to obtain the information about structural development or sintering mechanism during deactivation. One opinion is that Cu sintering is the main reason of catalyst deactivation [23, 31]. Researchers tried to improve the sintering resistance of Cu based catalysts by adding additive [25, 31–34] or changing the morphology [25, 32–34]. The second opinion is that the H₂O produced during CO₂ hydrogenation can promote the growth of metal oxide crystals [23, 35] or Cu grains [36], and can accelerate the sintering and deactivation of catalysts [23, 35, 36]. LI et al [2] mentioned that water is strong oxidant for metals such as copper at high temperature. When the reduction rate by H₂ or CO is not high enough, most of active site is oxidized to lose its activity. Researchers tried to improve the water resistance of Cu based catalysts by adding ZrO₂, SiO₂ or other additives [2, 36]. The information about the structural development or the sintering mechanism during deactivation is still currently scarce [37]. The study on the deactivation behavior has important practical significance for improving the performance of catalyst.

The methanol synthesis from CO₂ hydrogenation has structurally sensitive character [10, 11]. The morphology affects the performance of catalysts [25, 32–34, 38]. Nanowire has significant surface effect, quantum size effect and macroscopic quantum tunneling effect and exhibits unique structural, electronic and thermal properties, and it becomes an international frontier research hotspot recently. A few metal oxide catalysts for CO₂ hydrogenation were prepared to form a fibrous type or were loaded on one-dimensional carrier [25, 32–34]. AN et al [32, 33] reported that a better dispersion of Cu/Zn crystallites and the fibrous structure of catalyst were the main factors for the higher activity and stability. LEI et al [25] found that the activities of catalysts depended strongly on the morphology of ZnO, and CuO/ZnO catalyst prepared with filament-like ZnO exhibited the best activity.

Pure Cu nanowire catalyst was prepared by electrochemical deposition and was used the first time in CO₂ hydrogenation to methanol. The catalyst was characterized by various means. The performance of Cu nanowire catalyst was investigated. The mechanism of deactivation and active site of catalyst, which has attracted much attention, was discussed.

2 Experimental

2.1 Preparation of Cu nanowire catalyst

1060 aluminum plate was annealed at 400 °C for 4 h. Anodic aluminum oxide (AAO) template was prepared by one-step anodic oxidation method in 0.3 mol/L oxalic acid solution at 40 V for 8 h under water cooling.

Pure Cu nanowires were prepared by electrochemical deposition at room temperature using AAO template with aluminum substrate as cathode. Pure Cu sheet was used as anode. The distance between the two electrodes was about 3 cm. The electrolyte was comprised [39] of 60 g/L CuSO₄·5H₂O, 180 g/L H₂SO₄, 300 mg/L PEG,80 mg/L NaCl and 20 mg/L SPS. At initial stage of electro-deposition, current density was slowly increased to about 1.5 A/dm² and then kept constant for 20 min. The electrolyte was stirred by means of moving cathode with a frequency of about 60 min^–1. The AAO template with Cu nanowires was immersed into 0.75 mol/L NaOH solution with 3 g/L glucose until aluminum plate was completely exposed. After the aluminum plate was removed, remaining solution was stirred under ultrasonic at room temperature. After the AAO template was completely dissolved, the suspended solid particles in liquid were immediately filtered and washed with distilled water, and then red brown Cu nanowire sample was obtained after being washed with 95% ethanol. In order to prevent the Cu nanowires from oxidation in air, the prepared sample was sealed.

2.2 Characterization of catalyst

The morphology and surface composition of sample were observed with SEM (Model SSX-550, Shimadzu, Japan; Model S-3400N, Hitachi, Japan; Model Ultra Plus, Zeiss, Germany) with EDS. Crystal structure of sample was analyzed by XRD (Model PW3040/60, PANALYTICAL B.V, the Netherlands). The specific surface area and pore distribution of sample was tested using BET (Model Kubo X1000, Beijing builder electronic technology CO., LTD).

2.3 Catalytic performance test

The performance of Cu nanowire catalyst was tested by a hybrid instrument-combining reactor system, temperature-controlling system, flux- controlling system, and gas analysis system. The reactor was a stainless steel tubular fixed bed. The gas composition at the inlet and outlet of the reactor was detected by a gas chromatograph (Model GC-2060, Lunan Analytical Instrument CO., LTD). Because Cu nanowire sample mainly exists in metal state, therefore it was not pre-reduced before use and was directly used. Feed gas was composed of V(H₂)/V(CO₂)/V(N₂)≈3/1/1. Pressure was about 3 MPa. CO₂ conversion, selectivity of products was calculated by the following formulas:

where n_i is the number of carbon atoms in product i; C_i is the mole fraction of product i; C_CO2,0 is the mole fraction of CO₂ in feed gas.

3 Results and discussion

3.1 SEM morphologies of Cu nanowire sample

NaOH solution was used for dissolving AAO template, and a small amount of glucose was added in the NaOH solution to remove dissolved oxygen and to slow down Cu oxidation in alkaline solution. The standard electrode potential of glucose acid salt/glucose is –0.470 V, and the standard electrode potential of Cu₂O/Cu is –0.385 V [40]. Figure 1(a) shows that there are hemispherical particles and scattered Cu nanowires in the sample after AAO template was dissolved. Hemispherical particles with an average diameter of about 30 μm have regular shape. Figure 1(b) shows that the average diameter of scattered Cu nanowires is about 80 nm.

Figure 2 shows the SEM morphology of single hemispherical particle in sample at high magnification. Figure 2(a) shows that the Cu perpendicular to the plane of the hemispherical nanowires inside the hemispherical particle are particle and form an array, and the average diameter of the Cu nanowires is also about 80 nm. Figure 2(b) shows that there are pores on the spherical surface of the hemispherical particle, and the tops of some Cu nanowires are faintly visible (see Inset).

Figure 1 SEM morphologies of Cu nanowire sample (a) and scattered Cu nanowires (b) in sample

Figure 2 SEM morphologies of single hemispherical particle in sample at high magnification:

3.2 BET specific surface area and pore volume distribution of sample

The N₂ adsorption desorption isotherm and pore volume distribution of Cu nanowire sample are shown in Figures 3 and 4, respectively. The results show that the specific surface area of the sample is about 14.8 m²/g. The theoretical specific surface area of Cu nanowires with an average diameter of 80 nm is about 6 m²/g. The actual specific surface area of Cu nanowires is higher than the theoretical one, which is due to the fact that the surface of Cu nanowires is not smooth. Based on these findings and SEM morphologies of hemispherical particle it is speculated that the hemispherical particle is a Cu nanowire array.

Figure 3 N₂ adsorption–desorption isotherm of Cu nanowire sample

Figure 4 Pore volume distribution of Cu nanowire sample

Under the condition of the movement of moving cathode, the shear force generated by stirring is small, and the Cu nanowires deposited outside the template can still keep their own growth. When the Cu nanowires grow to a certain length, some adjacent Cu nanowires gather to form clusters and large numbers of clusters gather to form arrays.

3.3 EDS and XRD of Cu nanowire sample

There is no XRD peak of Cu oxide in Figure 5, but there is a weak EDS peak of oxygen (see Inset of Figure 5), which is because Cu nanowire surface has large number of dangling bonds, and the dangling bonds adsorbed the oxygen in environment form a protective layer of oxide film [41] on the Cu nanowire surface. These results indicate that Cu nanowires mainly exist in metal state (PDF card number 04–0836). The average grain size of Cu grains in the sample is about 26 nm by Jade 6 software.

Figure 5 XRD pattern of Cu nanowire sample (Inset shows EDS spectrum)

3.4 Initial activity of Cu nanowire catalyst

The main products of CO₂ hydrogenation are CO, methanol and H₂O over the Cu nanowire catalyst.

The suitable temperature for methanol synthesis on Cu based catalysts is generally 250 °C [8, 9, 27, 31]. Section I of Figure 6 shows that at 250 °C, CO₂ conversion rate is about 7.1%, CO selectivity is about 64.4%, methanol selectivity is about 35.5%, the space time yield (STY) of methanol is about 21.6 mg/(g_cat·h) and methanol specific yield is about 1.5 mg/(m²_Cu·h) on Cu nanowire catalyst of 1.0 g. The higher CO selectivity and lower methanol selectivity on the catalyst are in agreement with the performance characteristics of Cu based catalysts [1].

Figure 6 CO₂ conversion and product selectivity over Cu nanowire catalyst of 1.0 g:

3.5 Performance of catalyst after change in reaction temperature

The performance of Cu based catalysts is easier to be affected by temperature [1, 9, 31, 42]. The activity of the catalysts is higher at higher temperature and the selectivity for methanol is better at lower temperature generally. Section II of Figure 6 shows that, when the reaction temperature increased to 260 °C, the activity of the Cu nanowire catalyst obviously increased, CO₂ conversion rate and the STY of methanol reached 8.4% and 23.9 mg/(g_cat·h) respectively, and the selectivity for methanol slightly decreased to about 33.1%. But soon CO₂ conversion rate decreased significantly and later gently. It is speculated that some of the catalyst may be deactivated. Reaction temperature was decreased for continuing to react. When the temperature was reduced to 210 °C, the feed gas flow rate was increased from 3000 mL/h to 6000 mL/h in order to prevent lower boiling point product (such as methanol and H₂O) from staying. Section III of Figure 6 shows that at 210 °C, the activity of the catalyst was lower, CO₂ conversion rate was only about 0.4%, methanol selectivity was higher close to the 100% (CO was not detected), and the STY of methanol was about 6.9 mg/(g_cat·h). Section IV of Figure 6 shows that when reaction temperature increased to 240 °C again, the conversion of CO₂ and the STY of methanol significantly increased to 2.5% and 14.5 mg/(g_cat·h) respectively, and methanol selectivity slightly decreased to 67.5%. The change in performance of Cu nanowire catalyst with temperature is in agreement with the performance characteristic of Cu based catalysts. Combined with above phenomena, it is suggested that Cu may be the active site of catalyst.

3.6 Characterization of catalyst in intermediate stage of reaction

After 37 h reaction, the catalyst was taken out from reactor. Figure 7 shows that there are tiny Cu nano-particles on the surface of some scattered Cu nanowires. Figure 8 shows that the average grain size of sample is about 22 nm (by Jade 6 software), smaller than the original size of 26 nm. It is because FWHM of the diffraction peak of XRD was broadened because of the tiny Cu nanoparticles on the surface of some scattered Cu nanowires (see Figure 7), and therefore the average grain size of the sample became smaller by XRD analysis. Based on above phenomena it is speculated that more small Cu nanoparticles can be generated on the surface of scattered Cu nanowires during CO₂ hydrogenation.

Figure 7 Scattered Cu nanowires after 37 h reaction

Figure 8 XRD pattern of Cu nanowire catalyst after 37 h reaction

3.7 Separation and analysis of inactive parts of catalyst in intermediate stage of reaction

The catalyst was taken out and was dispersed in alkaline solution again, and it was found that some particles of the sample were easy to precipitate. The precipitate weighs about 0.3 g, and the conversion of CO₂ is only about 0.03% (trace amount of CO was detected, and no methanol was detected), indicating that this part of the sample is deactivated. This result is consistent with the obvious reduction of CO₂ conversion rate in Section II of Figure 6, which further confirms the above speculation that some of the catalysts are inactive. Figure 9 shows that Cu grains on surface of the deactivated catalyst have been sintered, and the average grain size is about 67 nm by XRD (its XRD pattern is not showed in this paper). Sintering may cause gas channel blockage. Gas channel blockage may impede gas flow and CO₂ hydrogenation. This leaded to deactivation of some of the catalyst. It is speculated that the deactivation may be attributed to the contact between some of the catalyst and higher temperature reactor wall.

Figure 9 Deactivated part of catalyst after 37 h reaction:

3.8 Separation and performance of active part of catalyst in intermediate stage of reaction

The suspended solid particles in above liquid were processed, and red brown solid powders, which comprise hemispherical Cu nanowire arrays and scattered Cu nanowires, were obtained. 0.5 g of the powders was taken out for performance test. Section I of Figure 10 shows that over the catalyst of 0.5 g, CO₂ conversion was about 3.4%, methanol selectivity was about 35.8%, and the STY of methanol was about 20.9 mg/(g_cat·h) at 250 °C. Compared with the results in Section I of Figure 6, the changes in methanol STY and methanol selectivity were very little, indicating that the catalyst of 0.5 g was not deactivated. In addition, the decrease in CO₂ conversion and methanol STY was very slow with reaction time from about 40 h to about 90 h, and the increase in methanol selectivity was also very slow, indicating that the performance of the catalyst is stable at 250 °C.

Figure 10 CO₂ conversion and product selectivity over Cu nanowire catalyst of 0.5 g at 250 °C:

3.9 Performance of catalyst after change in space velocity

Section I of Figure 10 shows that at around 90 h, the conversion of CO₂ was about 2.8%, methanol selectivity was about 40.7% and the STY of methanol was about 19.6 mg/(g_cat·h). The research of REN et al [1] showed that the STY of methanol over Cu based catalyst increases with increasing gas hourly space velocity (GHSV). The Section II of Figure 10 shows that when gas flow rate doubled, the conversion of CO₂ reduced to about 1.5%, methanol selectivity was almost unchanged and still about 40.6%, and the STY of methanol increased to about 20.9 mg/(g_cat·h). It is because increasing gas velocity while reducing contact time made CO₂ conversion rate decrease, but the amount of gas flowing through catalyst bed layer increased in unit time, which made the STY of methanol of the catalyst increase.

3.10 Stability of Cu nanowire catalyst

Section II of Figure 10 shows that at around 120 h, the conversion of CO₂ was about 1.4%, methanol selectivity was about 39.9%, and the STY of methanol was about 19.2 mg/(g_cat·h) and decreased by only 11.1% than 21.6 mg/(g_cat·h) over the fresh catalyst (see Section I of Figure 6). After a long-time running for up to 120 h, the performance of the Cu nanowire catalyst without oxide support still remained relatively stable in feed gas containing high concentration of CO₂, indicating that the Cu nanowires have very high catalytic stability. This helps to develop high performance catalysts.

3.11 Characterization and analysis of deactivated catalyst

Section II of Figure 10 shows that after 120 h reaction, the CO₂ conversion, methanol selectivity and methanol STY all quickly decreased, while CO selectivity quickly increased, speculating that the catalyst probably deactivated. Figure 11 shows that after 130 h reaction, the average grain size of sample is about 37 nm (by Jade 6 software), significantly greater than the original size of 26 nm, indicating that sintering for the catalyst may have happened. Literature reported that Cu sintering is the main reason of deactivation of Cu based catalyst [23, 31]. However, according to the shape of performance curve in Section II of Figure 10, it is speculated that the main cause of sudden deactivation of the Cu nanowire catalyst is probably gas channel flow blockage.

Figure 12 shows that the diameter of some scattered Cu nanowires in the deactivated catalyst became fine; however they still retained their nanowire shape that the average diameter is about 70 nm, and a large number of copper agglomerates appeared around these thinner Cu nanowires. Combining these phenomena with the SEM morphology of sample in Figure 7, it is speculated that the Cu atoms on surface of scattered Cu nanowires continuously migrate, which may be completed via species such as CuO [2], trans-COOH [43], CuCO or/and Cu₂HCOO [44], finally resulting in the formation of thinner Cu nanowires, while Cu nanoparticles formed during migration of Cu atoms are agglomerated and sintered continuously, which may be accelerated by H₂O [36, 45] produced during CO₂ hydrogenation via Ostwald ripening, finally resulting in the generation of copper agglomerate. The phenomenon of migration of Cu atoms in common Cu based catalysts [46, 47] is very difficult to be found, and so far there is still lack of relevant reports of migration of Cu atoms. It is speculated that the migration of Cu atoms and sintering of Cu nanoparticles may be avoided by increasing space velocity to remove generated liquid.

Figure 11 XRD pattern of Cu nanowire catalyst after 130 h reaction

Figure 12 Scattered Cu nanowires after 130 h reaction

In Section I of Figure 10, the phenomena (such as the slow decrease in both CO₂ conversion rate and methanol STY and the slow increase in methanol selectivity) may also be attributed to continuous migration of Cu atoms and sintering of Cu grains. Cu grains with smaller size formed during migration of Cu atoms can make catalytic activity, methanol selectivity and methanol STY increase, but Cu grains with larger size formed during sintering can make the catalytic activity, methanol selectivity and methanol STY decrease, and it may be that the cooperation of sintering and migration lead to the above phenomena. It is concluded that the performance of Cu nanowire catalyst can be improved by decreasing Cu grain size.

Figure 13 shows the SEM morphology of hemispherical Cu nanowire array in deactivated catalyst, and the inset shows that some Cu nanowires inside the array have the same average diameter as its original size. The morphology of the Cu nanowires has not obvious changed, which may be attributed to the partial gas channel blockage at the initial stage of CO₂ hydrogenation reaction or at the preparation stage of sample. Gas channel blockage may impede gas flow and CO₂ hydrogenation, and therefore the morphology of these Cu nanowires has not obvious changed. Figure 14 shows that Cu nanoparticles in inert atmosphere are quickly sintered at 250 °C for only 4 h. These findings indicate that the Cu nanowires have excellent sintering resistance.

Figure 13 Hemispherical Cu nanowire array after 130 h reaction (Inset shows enlarged view)

Figure 14 SEM morphologies of Cu nanoparticles (a) and their agglomerates after heat treatment at 250 °C for 4 h under N₂ (b)

4 Conclusions

1) Cu nanowires have very high sintering resistance and catalytic stability, and they may have potential application value in study on active site or in industrial application.

2) The changes in the grain size, SEM morphology and catalytic performance of sample indicate that migration of Cu atoms on the surface of Cu nanowires can occur during CO₂ hydrogenation.

3) Continuous migration of Cu atoms and sintering of Cu grains on surface of Cu nanowires can lead to flow blockage in gas channels. Gas channel flow blockage or sintering of Cu grains can lead to deactivation of the catalyst. However, the shape of catalytic performance curve indicates that the main reason for the deactivation of the catalyst is probably gas channel flow blockage. Further research is needed to improve the performance of Cu nanowire catalyst.

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(Edited by YANG Hua)

中文导读

CO₂加氢Cu纳米线催化剂的制备、表征和催化性能

摘要：采用电化学沉积方法制备纯Cu纳米线催化剂，并首次将其用于CO₂加氢合成甲醇反应，探讨Cu基催化剂的活性位，这有助于催化剂活性位的研究。观察CO₂加氢过程中催化剂的性能和结构变化，探讨催化剂的失活机理，找出影响催化剂失活的关键因素，这有利于提高催化剂的应用性能。通过SEM、EDS、XRD和BET等检测手段对Cu纳米线样品进行表征。结果发现，Cu纳米线具有非常高的抗烧结性能和催化稳定性，这有助于研制高性能催化剂。CO₂加氢过程中样品的晶粒尺寸、SEM形貌和催化性能的变化情况表明，Cu纳米线表面的Cu原子可能发生迁移。Cu原子不断的迁移和Cu晶粒不断的烧结可能导致气体通道堵塞。气体通道堵塞或Cu晶粒烧结均可能导致催化剂失活。然而，催化性能曲线的形状表明，该催化剂失活的主要原因是气体通道堵塞。

关键词：CO₂加氢; 甲醇; Cu纳米线; 迁移; 烧结; 流动阻塞

Foundation item: Project(51074205) supported by the National Natural Science Foundation of China

Received date: 2016-08-31; Accepted date: 2018-01-20

Corresponding author: ZHAI Yu-chun, PhD, Professor; Tel/Fax: +86-24-83687731; E-mail: zhaiyc@smm.ned.edu.cn; ORCID: 0000- 0002-1161-2390; WANG Ming-hua, PhD; E-mail: wangmh@smm.neu.edu.cn; ORCID: 0000-0001-6789-588x