Copper-Based Catalysts for Selective Hydrogenation of Acetylene Derived from Cu(OH)2.

Replacing precious metals with cheap metals in catalysts is a topic of interest in both industry and academia but challenging. Here, a selective hydrogenation catalyst was prepared by thermal treatment of Cu(OH)2 nanowires with acetylene-containing gas at 120 °C followed by hydrogen reduction at 150 °C. The characterization by means of transmission electron microscopy observation, X-ray diffraction, and X-ray photoelectron spectroscopy revealed that two crystallites were present in the resultant catalyst. One of the crystal phases was metal Cu, whereas the other crystal phase was ascribed to an interstitial copper carbide (Cu x C) phase. The reduction of freshly prepared copper (II) acetylide (CuC2) at 150 °C also afforded the formation of Cu and Cu x C crystallites, indicating that CuC2 was the precursor or an intermediate in the formation of Cu x C. The prepared catalysts consisting of Cu and Cu x C exhibited a considerably high hydrogenation activity at low temperatures in the selective hydrogenation of acetylene in the ethylene stream. In the presence of a large excess of ethylene, acetylene was completely converted at 110 °C and atmospheric pressure with an ethane selectivity of <15%, and the conversion and selectivity were constant in a 260 h run.

Introduction

Ethylene is mainly used for the production of polyethylene, the most widely used plastic. Ethylene feedstocks produced by steam cracking and alkane dehydrogenation always contain 0.5–2% acetylene, which is poisonous to the downstream polymerization catalysts and degrades the quality of polyethylene.[1−3] Therefore, the amount of acetylene in polymer-grade ethylene must be reduced to an acceptable level (<5 ppm).[4,5] An industrially used method of removing acetylene in the ethylene stream is by the selective hydrogenation of acetylene to ethylene.[5−7] Supported Pd catalysts promoted with Ag are commonly used in the petrochemical industry.[8−12] However, there is room for improving the conventional catalysts in terms of cost and product selectivity.

Design of noble metal-free catalysts from base metals with high hydrogenation activity and ethylene selectivity offers an effective way for removing the trace acetylene from ethylene. Among the base metals, Ni-based,[13−19] Cu-based,[20−24] and Fe-based[25,26] catalysts have been investigated for the selective hydrogenation of acetylene. Remarkably, some of them showed higher selectivity to ethylene than the Pd-based catalysts. For example, a high alkene selectivity is readily achieved in alkyne hydrogenation over Cu-based catalysts.[27−29] Nevertheless, higher reaction temperatures (>150 °C) are often required because of the lower ability of hydrogen dissociation for the base-metal catalysts. A high reaction temperature often leads to enhanced oligomerization of acetylene, and the produced green oil fouls the catalyst surface and deactivates the catalyst.[30] As a result, lowering the reaction temperature is critical to improve the catalytic performance of base-metal catalysts. Kyriakou et al.(31) reported that isolated Pd atoms on the Cu surface decreased the energy barrier of hydrogen uptake and desorption on the Cu surface. The H atoms dissociated on isolated Pd spilt over onto the Cu surface and facilitated the selective hydrogenation of acetylene, with improved selectivity to ethylene at lower temperatures. McCue et al. investigated the catalytic performance of a Cu/Al2O3 catalyst modified by different amounts of Pd.[32] They found that the optimal catalyst combined the properties of both Cu and Pd so as to achieve high ethylene selectivity with high activity at low reaction temperatures. The high hydrogenation activity was attributed to the H spillover from the Pd surface to the Cu surface.

In our previous study, a Cu2O-derived catalyst displayed much higher activity than the Cu catalyst in acetylene selective hydrogenation with excess ethylene.[33] The Cu2O-derived catalyst was in a core–shell structure, with the Cu core covered by a porous carbon shell in which Cu and copper carbide (CuC) particles were highly dispersed. The density functional theory (DFT) calculation results revealed that CuC was corresponding to Cu3C with a structure of rhombohedra-centered hexagonal, and the dissociation energy of H2 on Cu3C(0001) is extraordinary lower than that on Cu(111). Meanwhile, the barriers of acetylene hydrogenation to vinylidene and then to ethylene over Cu3C(0001) were higher than that on Cu(111). Therefore, CuC served as the sites for H2 adsorption and dissociation, and the Cu phase was the main site for acetylene hydrogenation. In the present study, Cu(OH)2 was used to prepare the acetylene selective hydrogenation catalyst using the same method. The prepared catalysts were characterized and tested in the selective hydrogenation of acetylene in the presence of a large excess of ethylene.

Results and Discussion

Catalyst Characterization

Figure a shows the X-ray diffraction (XRD) pattern of the synthesized Cu(OH)2. It indicates that pure Cu(OH)2 crystal phase was obtained. The scanning electron microscopy (SEM) image of Cu(OH)2 (Figure b) demonstrated that Cu(OH)2 was in bundles of nanowires. The transmission electron microscopy (TEM) observation indicated that the average diameter of the nanowires was ∼15 nm (Figure c,d). The high-resolution TEM (HRTEM) image indicated the presence of Cu(OH)2 polycrystallites in the nanowires (Figure e).

Figure 1

(a) XRD pattern, (b) SEM image, (c,d) TEM images, and (e) HRTEM image of the synthesized Cu(OH)2 nanowires.

When Cu(OH)2 was treated with an acetylene-containing gas (0.50% C2H2 and 99.50% Ar) at 120 °C for 2 h, the color changed from blue to black. As shown in the SEM image of the obtained material [Cu(OH)2(T120)] (Figure a), the thermal treatment with acetylene-containing gas led to a partial fracture of the nanowires, and the average diameter of the nanowires was increased probably due to carbon deposition on the external surface. After Cu(OH)2(T120) was subsequently reduced in H2 at 150 °C for 3 h, the morphology of the obtain catalyst Cu(OH)2(T120-R150) (Figure b) did not change significantly.

Figure 2

SEM images of (a) Cu(OH)2(T120) and (b) Cu(OH)2(T120-R150). (c) HRTEM image of Cu(OH)2(T120-R150).

The HRTEM image of Cu(OH)2(T120-R150) (Figure c) revealed the presence of two kinds of crystallites with different lattice spacings. The lattice spacing of 0.181 nm corresponded to the (200) plane of Cu crystallites, whereas the other one (0.238 nm) was different from those of Cu, CuO, and Cu(OH)2 and was consistent with the lattice spacing of the Cu3C(0001) plane determined by DFT calculation in the previous Cu2O-derived catalyst.[33]Figure a presents the XRD pattern of Cu(OH)2(T120-R150). The intense peaks at 2θ = 43.3, 50.4, and 74.1° were ascribed to the (111), (200), and (220) planes of metal Cu (PDF 04-0836). The diffraction peak at 2θ = 37.1° did not fit to the patterns of Cu(OH)2, Cu2O and CuO but was identical to that of the interstitial copper carbide (CuC) in the previous report.[33]

Figure 3

(a) XRD pattern of Cu(OH)2(T120-R150). (b) Cu 2p3/2 and (c) Cu LMM XPS spectra of Cu(OH)2(T120-R150) and the Cu catalyst. (d) C 1s spectra of Cu(OH)2(T120-R150).

Figure b–d illustrates the X-ray photoelectron spectroscopy (XPS) spectra of Cu(OH)2(T120-R150) in comparison with the metal Cu. In the Cu 2p2/3 region (Figure b), the absence of the peak at 933.2–934.6 eV and its satellite peak at around 945 eV indicated that no Cu2+ species were present in Cu(OH)2(T120-R150) and in the Cu catalyst,[21,34] suggesting that Cu2+ species had been reduced in the hydrogenation at 150 °C. The main peak in the Cu 2p3/2 spectrum of Cu(OH)2(T120-R150) could be deconvoluted to a peak characteristic of metal copper and a shoulder peak at 932.8 eV located between Cu2+ and Cu0/Cu+. It was due to the formation of Cu–C in CuC.[21] A shift to higher binding energy suggests the transfer of electron density from copper to carbon, resulting in a partial positive charge (δ+) of the new Cu species in CuC.[35] In the Cu LMM spectrum of Cu(OH)2(T120-R150) (Figure c), two peaks at 917.2 and 918.7 eV were observed, whereas there was only one peak at 918.7 eV in the Cu catalyst. The peak at 918.7 eV was attributed to Cu0 species,[36] whereas the kinetic energy (KE) at 917.2 eV was different from that of Cu+ (KE 916.8 eV). Because Cu2+ was absent (Figure b), the new peak at KE of 917.2 eV was tentatively assigned to the Cu–C bond in CuC. In the C 1s region (Figure d), the large peak at 284.6 eV was attributed to the C–C bond,[39] whereas the shoulder at 286.1 eV corresponded to the C–OH species.[37] The shoulder at 283.6 eV was attributable to the interstitial metal carbide carbon of CuC.[33,38] Kim et al. also confirmed that the simultaneous appearance of the two peaks at 932.8 eV in the Cu 2p2/3 spectrum and 283.6 eV in the C 1s spectrum was due to the formation of copper carbide.[39] Therefore, it suggests that the interstitial copper carbide (CuC) was present in the Cu(OH)2-derived catalyst.

In the Raman spectrum of Cu(OH)2(T120) (Figure S1), the two peaks at ∼1570 and ∼1370 cm–1 corresponding to the G band and D band of carbon, which are related to the structural order (sp2 carbons) and the imperfection present in the carbon lattice,[40] were observed, suggesting that carbon was formed in the acetylene treatment step, and the presence of G band and D band in Cu(OH)2(T120-R150) indicated that the formed carbon was stable in H2 reduction at 150 °C. The Brunauer–Emmett–Teller (BET) specific surface areas of Cu(OH)2, Cu(OH)2(T120), and Cu(OH)2(T120-R150) (Figure S2) increased from 70 to 103 and to 162 m2/g. The increased surface area may suggest that the carbon material, in which CuC and Cu particles were embedded, was porous in Cu(OH)2(T120) and Cu(OH)2(T120-R150).

In our previous study, it was found that the CuC phase was obtained by reducing copper (I) acetylide (Cu2C2) in H2.[33] Similarly, it is proposed that CuC in Cu(OH)2-derived catalyst originated from copper (II) acetylide (CuC2): (1) Cu(OH)2 reacts with acetylene to afford CuC2 at 120 °C; (2) CuC2 is partially reduced in hydrogen at 150 °C to form CuC. When Cu(OH)2 was treated with acetylene-containing gas at 120 °C, water drops was observed downstream on the inner wall of the quartz tube, and the color of the catalyst bed changed from blue to black. These phenomena indicated that Cu(OH)2 reacted with acetylene readily to produce CuC2.[41] Because CuC2 is highly unstable, it partially decomposed to Cu particles and carbonaceous materials as a parallel reaction in the thermal treatment at 120 °C and in the subsequent hydrogen reduction at 150 °C. The carbon deposition on the external surface of the nanowires might result from the decomposition of the in situ generated CuC2.[42]

In order to elucidate that the formation of CuC was from CuC2 by hydrogen reduction, bulk CuC2 (CAUTION: dry bulk CuCis explosive) was synthesized by bubbling acetylene-containing gas into a Cu(NO3)2 ammonia solution. The obtained CuC2 was treated in two different atmospheres at 150 °C for 3 h. The material obtained after thermal treatment in hydrogen was denoted as CuC2@150-H2 and that in vacuum as CuC2@150-vacuum. The HRTEM observation of CuC2@150-H2 indicated the presence of CuC and Cu crystallites, which were highly dispersed in the carbonaceous matrix (Figure a). The XPS characterization of CuC2@150-H2 (Figure S3) also suggested the presence of the Cu–C bond in CuC. In addition, the XRD pattern of CuC2@150-H2 revealed the presence of both CuC and Cu crystal phases as well (Figure b). As a result, it is evident that CuC was derived from CuC2 by hydrogen reduction at 150 °C. In contrast, the three diffraction peaks at 2θ = 43.3, 50.4, and 74.1° in the XRD pattern of CuC2@150-vacuum suggests that the thermal decomposition of CuC2 led to the formation of Cu particles and carbonaceous materials.[42] The BET specific surface areas of CuC2@150-H2 and CuC2@150-vacuum were 146 and 165 m2/g (Figure S4), respectively, comparable to that of Cu(OH)2(T120-R150).

Figure 4

(a) HRTEM image of the material obtained by reducing bulk CuC2 in hydrogen at 150 °C for 3 h. (b) XRD patterns of CuC2@150-H2 and CuC2@150-vacuum.

Catalytic Performance

In our previous investigation on the Cu2O-derived catalyst, it was found that the acetylene treatment conditions and subsequent hydrogenation conditions significantly affected the performance of the prepared catalysts in the selective hydrogenation of acetylene.

Figure shows the effect of thermal treatment temperature on the performance of the catalyst prepared by subsequent hydrogen reduction at 150 °C. All three catalysts showed significantly high hydrogenation activity with low ethane selectivity. Above 110 °C, acetylene in the ethylene stream was completely removed with ethane selectivity below 40%. In contrast, acetylene conversion over the Cu catalyst, which was obtained from the same precursor [Cu(OH)2] by hydrogen reduction at 400 °C for 5 h, was extremely low in the temperature range of 90–120 °C (Figure c). At 100 °C, the catalyst treated at 120 °C [Cu(OH)2(T120-R150)] exhibited the highest activity. The thermal treatment of Cu(OH)2 with acetylene leads to the formation of copper (II) acetylide (CuC2).[43] Since CuC2 is thermally unstable, it decomposes to yield Cu metal and amorphous carbon. Thermogravimetric (TG)–differential scanning calorimetry (DSC) curve of the prepared CuC2 (Figure S5) indicates that a strong exothermic peak appeared around 130 °C, suggesting that CuC2 decomposed markedly above 130 °C. As a result, the formation of CuC2 involves a consecutive reaction: the formation of CuC2 and the simultaneous thermal decomposition of CuC2. Therefore, an optimal treatment temperature exists.

Figure 5

(a) Acetylene conversion and (b) ethane selectivity as a function of reaction temperature over the materials obtained from Cu(OH)2 after thermal treatment at various temperatures followed by hydrogen reduction at 150 °C. (c) Acetylene conversion as a function of the reaction temperature over the Cu catalyst.

Figure displays the effect of treatment time at 120 °C on the performance of the prepared catalysts in the selective hydrogenation of acetylene. It is indicated that the optimal treatment time was 2 h. In the thermal treatment of Cu(OH)2 with acetylene, CuC2 acted like the desired product in a consecutive reaction, whose yield passed a maximum with increasing time. When treated for 3 h, the catalytic activity dropped dramatically, suggesting that the thermal decomposition of CuC2 predominated after 2 h.

Figure 6

(a) Acetylene conversion and (b) ethane selectivity as a function of the reaction temperature over the catalysts derived from Cu(OH)2 treated for different times at 120 °C followed by hydrogen reduction at 150 °C.

The effective conversion of the in situ generated CuC2 to CuC particles plays a determining role in the hydrogenation activity of the resultant catalyst. In the subsequent hydrogen reduction at higher temperatures, two parallel reactions, the reduction and the decomposition of CuC2, take place competitively. At optimal thermal treatment conditions (120 °C for 2 h), the effect of reduction temperature on the performance of the catalyst was then investigated. Figure shows the effect of the reduction temperature on acetylene conversion and ethane selectivity of the prepared catalysts. Reduction at 150 °C for 3 h resulted in the highest hydrogenation activity. When Cu(OH)2(T120) was reduced at 130 °C for 3 h, the acetylene conversion on the obtained catalyst was much lower, which might be attributable to the reduced production of CuC, probably due to the low rate of CuC2 reduction. On the other hand, at higher temperatures (180 and 210 °C), the enhanced thermal decomposition of CuC2 might lead to reduced production of CuC, lowering the catalytic activity.

Figure 7

(a) Acetylene conversion and (b) ethane selectivity as a function of the reaction temperature over the catalysts derived from Cu(OH)2 treated at 120 °C and reduced at different temperatures.

Figure presents the effect of reduction time at 150 °C on the acetylene hydrogenation performance of the resultant catalysts. It is indicated that the reduction time slightly affected the performance of the catalyst, probably because the reduction took place quickly at 150 °C.

Figure 8

(a) Acetylene conversion and (b) ethane selectivity as a function of the reaction temperature over the catalysts derived from Cu(OH)2 treated at 120 °C followed by hydrogen reduction at 150 °C for different times.

For practical application in removing the acetylene impurity from the ethylene product, there are many important factors which govern the performance and operation of the selective hydrogenation catalyst. Among them, the effect of hydrogen partial pressure and the catalyst stability are of paramount importance. In the industry, the Pd-based catalyst is sensitive to H2 partial pressure change, and therefore, the H2/C2H2 ratio is strictly controlled around 2.[22] Lower H2 partial pressure leads to decreased acetylene conversion and increased formation of green oil. Higher H2 partial pressure results in overhydrogenation of ethylene in the feed and increases the risk of thermal runaway of the catalyst bed because of the high exothermicity of ethylene hydrogenation. As a result, a complicated computer-aided control system is frequently constructed to monitor the variation of acetylene concentration and promptly respond to the change, so as to operate the reactor safely without net ethylene loss. In addition, the stability of the catalyst is an important concern because acetylene is prone to producing polymer and oligomer that foul the catalyst.

The ratio of hydrogen to acetylene (H2/C2H2) is strictly monitored in Pd-catalyzed selective hydrogenation for the removal of the acetylene impurity in the ethylene product in the industry. A high H2/C2H2 ratio often leads to the formation of an unselective palladium hydride phase and overhydrogenation to ethane, resulting in a net ethylene loss and increasing the risk of a thermal runaway due to the high exothermicity of ethylene hydrogenation. A low H2/C2H2 ratio decreases acetylene conversion and is favorable to the oligomerization of acetylene to produce green oil, leading to catalyst deactivation. Figure a displays the dependence of acetylene conversion and ethane selectivity on the H2/C2H2 ratio in the acetylene selective hydrogenation catalyzed by Cu(OH)2(T120-R150) at 120 °C and atmospheric pressure. In the range of 8–22, complete acetylene conversion was obtained, and the ethane selectivity increased insignificantly with the H2/C2H2 ratio. Increasing the partial pressure of H2 favors the adsorption of H2 on the catalyst surface, leading to increased surface coverage and hydrogenation rate. Remarkably, the selectivity to undesired ethane did not change dramatically with the H2/C2H2 ratio, and less than 40% ethane selectivity was obtained at the H2/C2H2 ratio of 22. Apparently, it is advantageous to operate the selective hydrogenation in a wide window of H2/C2H2 ratio with the complete removal of acetylene and without a net ethylene loss.

Figure 9

(a) Effect of H2/C2H2 ratio on acetylene conversion and ethane selectivity in acetylene hydrogenation over Cu(OH)2(T120-R150) at 120 °C and atmospheric pressure. (b) Variation of acetylene conversion and ethane selectivity with time on stream in acetylene hydrogenation over Cu(OH)2(T120-R150) at 110 °C.

Figure b illustrates the variation of acetylene conversion and ethane selectivity with time on stream in the acetylene selective hydrogenation catalyzed by Cu(OH)2(T120-R150) at 110 °C and atmospheric pressure. Cu(OH)2(T120-R150) showed constant acetylene conversion (100%) and low ethane selectivity (<15%) in the 260 h run. The Cu-based catalysts were reported to induce oligomerization and polymerization in acetylene hydrogenation at high temperatures.[30] The resultant oligomers (also known as green oils) and polymer foul the surface of the catalysts, thus leading to fast deactivation. In contrast, Cu(OH)2(T120-R150) showed substantially high stability on the stream. The improved stability of Cu(OH)2(T120-R150) might be attributed to the lowered reaction temperature and to the porous carbonaceous matrix, which may suppress the chain growth of linear hydrocarbons because of steric hindrance.

Conclusions

In summary, it was demonstrated that the interstitial copper carbide (CuC) phase with high hydrogenation activity was synthesized by thermal treatment of Cu(OH)2 with C2H2 at 120 °C followed by H2 reduction at 150 °C. The resultant catalyst exhibited outstanding performance and stability in the selective hydrogenation of acetylene in ethylene stream. CuC and Cu particles in the porous carbonaceous matrix served as the catalytic sites for hydrogen dissociation and acetylene hydrogenation, respectively. It is experimentally evident that copper (II) acetylide (CuC2) was the precursor or intermediate in the formation of the CuC crystal phase. The reduction of CuC2 afforded CuC crystallites, whereas the thermal decomposition of CuC2 led to the formation of Cu particles and carbonaceous material. Cu metal was highly selective, whereas the high hydrogenation activity of CuC enabled the effective hydrogenation of acetylene at low temperatures. The porous carbonaceous material might facilitate the diffusion of small molecules involved in the hydrogenation, whereas it might pose a steric hindrance to the chain growth of linear hydrocarbons. The prepared catalyst could be applied both in the front-end process and in the back-end process in removing the acetylene impurity in ethylene because the selectivity of ethane does not change markedly with hydrogen partial pressure in a wide range.

Experimental Section

Materials

Cu(NO3)2·3H2O, NaOH, and ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd, China. All reagents were used without further purification. Hydrogen gas and ethylene were from Dalian Guangming Gas Co., China. The reaction gas (0.80% methane, 0.50% acetylene, and 98.70% ethylene) and the treatment gas of 0.50% acetylene in argon were provided by Dalian Guangming Gas Co., China.

Preparation of Catalysts

As the precursor, Cu(OH)2 was synthesized as follows. 50 mL of NaOH aqueous solution (2.0 mol/L) was added dropwise into 200 mL of Cu(NO3)2 aqueous solution (0.1 mol/L) at 0 °C and stirred for 30 min. The blue Cu(OH)2 precipitate was collected by suction filtration, washed with deionized water and ethanol, and then dried at 30 °C in a vacuum oven for 12 h. The acetylene hydrogenation catalyst was prepared in situ from Cu(OH)2 by thermal treatment with acetylene-containing gas followed by hydrogen reduction, prior to the selective hydrogenation.

In order to investigate the formation of copper carbide, copper (II) acetylide (CuC2) was synthesized as reported in the literature.[43] 1.21 g of Cu(NO3)2·3H2O was dissolved in a 5.0% ammonia solution (100 mL). Ar flow (50 mL/min) was bubbled through the solution for 30 min and then shifted to a mixture gas (10.0% C2H2 and 90.0% Ar) regulated by two separate mass flow meters. After 5 h, the dark precipitate (CuC2) was collected through centrifugation and then washed with deionized water and ethanol and kept in water. (CAUTION: Dry CuCis highly explosive.)

When CuC2 was used as the precursor, a wet sample was carefully loaded in the quartz tubular reactor, which was heated in a hydrogen flow (50 mL/min) from room temperature to 150 °C at 3 °C/min and kept for 3 h. The obtained catalyst is denoted as CuC2@150-H2. In comparison, the wet CuC2 sample was annealed at 150 °C for 3 h in vacuum, and the resultant material is denoted as CuC2@150-vacuum. Both the obtained samples were nonexplosive and stable in air.

SEM observation was conducted on a SU8220 instrument with a beam energy of 5.0 kV. Powder XRD measurement was performed on a Rigaku SmartLab diffractometer with Cu Kα radiation at 40 kV and 100 mA with a scanning rate of 8°/min between 20 and 80°. TEM observation was performed on a FEI Tecnai G2 F30 instrument which was operated at 300 kV. The data of XPS was collected on a Thermo Fisher ESCALAB Xi+ electron spectrometer equipped with a monochromatic Al Kα excitation source (hν = 1486.6 eV). All binding energies were referenced to the C 1s peak (284.6 eV). Raman spectrum was recorded using a Thermo Scientific DXR Raman instrument with a 532 nm laser beam as the excitation source. Nitrogen sorption isotherms were measured at −196 °C on a Micromeritics TriStar II 3020 instrument. The specific surface areas were calculated according to the BET method. TG–DSC measurement was carried out on a TGA Q50 instrument in N2 flow (50 mL/min), and the heating rate was 3 °C/min.

The selective hydrogenation of acetylene in an ethylene stream was performed at ambient pressure in a quartz tubular fixed-bed reactor (10 mm i.d.). The synthesized Cu(OH)2 (0.1 g) was mixed with quartz sands (0.6 g, 60–80 mesh) and sandwiched in the middle of the reactor by quartz sands. The reactor was heated in the treatment gas (0.50% C2H2 and 99.50% Ar, at 30 mL/min) from room temperature to a set temperature (100–140 °C) at 3 °C/min and kept for 2 h and then cooled down to room temperature. The obtained material was denoted as Cu(OH)2(Tx) (x represents the temperature of the thermal treatment). At a low C2H2 concentration, no vigorous or explosive decomposition was observed. Then, Cu(OH)2(Tx) was heated in a hydrogen flow (50 mL/min H2) from room temperature to a set temperature (130–210 °C) at 3 °C/min and kept for 3 h and then cooled down to room temperature. The obtained catalyst is denoted as Cu(OH)2(Tx-Ry) (y stands for the reduction temperature). Cu(OH)2(Tx-Ry) was stable (not explosive) at high temperatures or on exposure to air. For comparison, a metal Cu catalyst was obtained by reducing Cu(OH)2 in H2 (50 mL/min) at 400 °C for 5 h.

In the selective hydrogenation of acetylene, a mixture gas (90.0% reaction gas and 10.0% H2) was fed at 10 mL/min, and the reactor was heated from room temperature to the reaction temperature (90–120 °C) at 3 °C/min. The gas composition at the reactor outlet was determined by an online gas chromatograph (GC A90, ECHROM) equipped with a flame ionization detector and a capillary column (Agilent HP-AL/S, 30 m × 0.535 mm × 15.00 μm). Methane was used as the internal standard for the gas chromatography (GC) analysis. Because it is difficult to accurately measure the concentration increment of ethylene by GC at a concentration as high as 98.7%, the selectivity to ethane was used to determine the selectivity performance of the catalyst. Acetylene conversion and ethane selectivity were calculated as follows.[44]where [C2H2]inlet and [C2H2]outlet are the acetylene concentrations at the inlet and at the outlet, respectively; [C2H6]outlet is the ethane concentration at the outlet.