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Article

Enhancing the Ethynylation Performance of CuO-Bi2O3 Nanocatalysts by Tuning Cu-Bi Interactions and Phase Structures

by
Zhipeng Wang
1,
Zhuzhu Niu
1,
Quanai Hao
1,
Lijun Ban
1,
Haitao Li
1,*,
Yongxiang Zhao
1,* and
Zheng Jiang
1,2
1
Engineering Research Center of Ministry of Education for Fine Chemicals, School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, China
2
Faculty of Engineering and the Environment, University of Southampton, Highfield, Southampton SO17 1BJ, UK
*
Authors to whom correspondence should be addressed.
Catalysts 2019, 9(1), 35; https://doi.org/10.3390/catal9010035
Submission received: 29 November 2018 / Revised: 25 December 2018 / Accepted: 25 December 2018 / Published: 2 January 2019
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Abstract

:
Catalytic systems consisting of copper oxide and bismuth oxide are commonly employed for the industrial production of 1,4-butynediol (BD) through ethynylation. However, few studies have investigated the influence mechanism of Bi for these Cu-based catalysts. Herein, a series of nanostructured CuO-Bi2O3 catalysts were prepared by co-precipitation followed by calcination at different temperatures. The obtained catalysts were applied to the ethynylation reaction. The textural and crystal properties of the catalysts, their reduction behavior, and the interactions between copper and bismuth species, were found to strongly depend on temperature. When calcined at 600 °C, strong interactions between Cu and Bi in the CuO phase facilitated the formation of highly dispersed active cuprous sites and stabilized the Cu+ valency, resulting in the highest BD yield. Bi2O3 was completely absent when calcined at 700 °C, having been converted into the spinel CuBi2O4 phase. Spinel Cu2+ was released gradually to form active Cu+ species over eight catalytic cycles, which continuously replenished the decreasing activity resulting from the formation of metallic Cu and enhanced catalytic stability. Moreover, the positive correlation between the in-situ-formed surface Cu+ ions and BD yield suggests that the amount of Cu+ ions is the key factor for ethynylation of formaldehyde to BD on the as prepared CuO-Bi2O3 catalysts. Based on these results and the literature, we propose an ethynylation reaction mechanism for CuO-based catalysts and provide a simple design strategy for highly efficient catalytic CuO-Bi2O3 systems, which has considerable potential for industrial applications.

1. Introduction

In the early 1900s, Alexei Favorskii first reported the addition of alkynes to carbonyl compounds in basic media to form alkinols, which was then further developed into a known reaction process by Reppe [1]. Today, the Reppe process is the major route for the commercial manufacture of 1,4-butynediol (BD) [2,3]. Due to the numerous applications of chemicals derived from BD, such as 1,4-butanediol (BDO), 3-butene-1-ol (BTO), tetrahydrofuran (THF), poly(tetramethylene ether) glycol (PTMEG), γ-butyrolactone (GBL), and poly(butylene succinate) (PBS), among others [4,5,6], which are in high demand in the chemical, polymer, pharmaceuticals, and textiles industries [7,8,9,10,11], the design of highly efficient ethynylation catalysts for the synthesis of BD is of great significance.
To date, catalyst systems for the Reppe process have been reported to mainly include organic-Li, Lewis acids, and CuO-based catalysts [1,12,13]. Among them, CuO and the corresponding compounds have been widely employed due to their low costs, high activities, ease of separation from products, and their low environmental toxicities. However, active cuprous species (cuprous acetylides) generated in situ during the reaction process actually catalyze the reaction [1]. The phase transformation from CuO to the active cuprous species involves reduction and carbonization processes [14,15]. During the phase transformation, the selective reduction of Cu2+ to Cu+ without the formation of inactive metallic Cu while maintaining highly dispersed cuprous species is a challenging task, the outcome of which have a critical effect on the catalytic performance [16,17,18,19,20]. From the perspective of developing highly efficient catalysts, researchers in early studies screened catalyst components and found that CuO-based catalysts that use Bi as a promoter exhibited high activities and stabilities. Thereafter, catalysts supported by diatomite, silica gel, and SiO2-MgO complexes and so forth, as well as synthetic malachite catalysts based on the Cu–Bi catalytic system have often been reported in the literature [21,22,23,24]. Studies have shown that the introduction of supports and/or promoters is an effective strategy for improving the ethynylation activity of Cu-based catalysts [16,17,18,19,20,21,22,23,24]. Despite considerable effort toward the development of Cu-based ethynylation catalysts, to the best of our knowledge, very little work has been carried out to unfold the promotion mechanism of Bi species in the ethynylation reaction and to establish the structure-activity relationships of the catalysts.
In recent years, our research group has carried out in-depth studies into the design and development of new Cu-based ethynylation catalysts, such as the CuO-Bi2O3/SiO2-MgO aerogel catalyst, the CuO-Bi2O3/Fe3O4-SiO2-MgO magnetic catalyst, and the CuO-Bi2O3@SiO2 core-shell catalyst [16,17,18,19]. We found that doping with specific amounts of bismuth oxide not only disperses the cuprous species, but also effectively restricts the formation of metallic Cu [2,3,12,18,19], which is consistent with previous literature reports. We speculated that interactions between Cu and Bi influence the phase transformation of CuO and, as a consequence, the catalytic performance. However, this hypothesis lacks detailed experimental evidence, especially the support of characterization data. In addition, catalytic systems constructed following previous reports or in our group are relatively complex and often include multiple components in addition to Cu and Bi, which are not conducive to clarifying the specific mechanism involving Bi on Cu. Therefore, it is necessary to simplify this complex multiple-component system to a binary Cu–Bi model catalyst.
It is important to effectively tune the structure and Cu–Bi interactions of a binary catalyst in order to systematically study the effects of Bi in Cu-based ethynylation catalysts. Numerous studies over many years have investigated the structures and performance of copper-based catalysts for methanol steam reforming, CO oxidation, carbonyl hydrogenation, etc. [25,26,27,28]. The preparation process, especially calcination temperature, has been shown to greatly impact particle size, the crystalline phase, and the interactions between different components in the catalyst, which can substantially affect catalytic performance [25,26,27,28]. Therefore, it is reasonable to expect that the calcination temperature influences the structure and the interactions between Cu and Bi in a Cu/Bi-based catalyst.
With the above discussion in mind and aided by simple calcination treatments, in the present study we investigated the structure, texture, phase transformation, and catalytic performance of binary CuO-Bi2O3. The relationship between structure and catalytic performance of the CuO-Bi2O3 catalysts is discussed based on characterization studies that include Brunauer–Emmett–Teller (BET) surface-area experiments, X-ray diffraction (XRD), Raman spectroscopy, temperature-programmed reduction (H2-TPR) studies, X-ray photoelectron spectroscopy (XPS), and CO-adsorbed infrared (CO-IR) spectroscopy. The results revealed that there are various degrees of interaction between the copper and bismuth species among catalysts calcined at different temperatures, and the formation of a spinel CuBi2O4 phase was observed, which greatly influences catalytic activity and stability, and supplements the theoretical basis for the development of highly efficient ethynylation catalysts.

2. Results

2.1. Characterizing Fresh Catalysts

2.1.1. Structure, Texture, and Morphology

To demonstrate the structural evolution, XRD patterns of fresh catalysts are displayed in Figure 1, along with that of pure CuO-600 for comparison. Here “CB” refers to the copper–bismuth catalyst and the number refers to the calcination temperature. Peaks observed at 32.6°, 35.6°, 38.7°, 48.8°, 53.5°, 58.3°, 61.5°, 66.2°, and 68.1° in the XRD pattern of CuO are assigned to CuO (110), (002), (111), (−202), (020), (202), (−113), (−311), (220), and (004) diffractions, respectively (JCPDS45-0937). In addition, a small peak at 27.7°, which is attributed to Bi2O3 (121) diffraction (JCPDS41-1449), is also observed in the case of CB-500 [29]. Calcination at 600 °C resulted in dramatic increases in the intensities of the CuO and Bi2O3 XRD peaks. In addition, the weak peaks observed at 28.1° and 33.2° are assigned to CuBi2O4 (211) and (102) diffractions (JCPDS48-1886), which is indicative of the formation of the spinel CuBi2O4 phase mainly due to solid–solid interactions between CuO and Bi2O3 [30,31]. The diffraction peaks corresponding to Bi2O3 were entirely absent in the XRD pattern of the material calcined at 700 °C, while the CuBi2O4 diffraction peaks were noticeably more intense.
The CuO crystallite sizes, calculated from the line broadening of the most intense XRD reflections using the Scherrer equation, are listed in Table 1. As the calcination temperature was increased from 500 to 600 °C, the CuO-particle size sharply increased from 17.2 to 33.0 nm. However, elevating the temperature to 700 °C resulted in only a further slight increase in the CuO particle size, to 39.4 nm, indicating that the gradual formation of CuBi2O4 inhibits the further growth of CuO crystals to some extent.
Taking the CuO(111) diffraction peak at about 38.9° as an example, this peak in the patterns of the CB-500 and CB-600 samples is shifted to lower diffraction angles compared to that of pure CuO-600, to 2θ values of about 38.8° and 38.7°, respectively. These observations indicate that Bi3+ ions thermally diffuse into the CuO crystal lattice during calcination at 500 °C and 600 °C. Based on their ionic radii (Bi3+ 0.96 Å, Cu2+ 0.72 Å), we infer that the Bi3+ ions doped into the CuO crystal structure in two ways, namely through substitutional and interstitial doping. Both ways expand the cell volume of the CuO crystal [32], which suggests that Cu and Bi interact. The XRD patterns reveal that the CuO in CB-600 exhibits the maximum cell volume, which demonstrates the existence of stronger Cu–Bi interactions in the CuO of the CB-600 catalyst compared to the others. Interestingly, the CuO(111) peak shifted to the same position as that observed for pure CuO in the case of CB-700, suggestive of a lack of interaction between the Cu and Bi in the CuO phase in this catalyst.
Figure 2 shows Raman spectra of fresh CuO-Bi2O3 catalysts and pure CuO-600. Three peaks—at about 293.3 cm−1 (Ag mode), 345.4 cm−1 (Bg mode), and 626.4 cm−1 (Bg mode)—in pure CuO-600 are attributed to the vibrations of the oxygen atoms in the lattice (Cu-O and Cu-O-Cu bonds) [33,34]. Compared with pure CuO, the Raman peaks of CB-500 and CB-600 were broader and shifted to lower frequencies; the main peak at 293.3 cm−1 observed for pure CuO shifted to 293.0 cm−1 for CB-500, and to 284.5 cm−1 for CB-600. The red-shifting and broadening of the peaks in the Raman spectra are mainly ascribable to defects in CuO originating from interactions between Cu and Bi [35,36]. Combining this information with the XRD results, we ascertain that Bi3+ ions diffuse into the CuO lattice to distort the CuO structure. Since a charge difference exists between Bi3+ and Cu2+, the introduction of Bi generates oxygen vacancies in the CuO lattice to balance the charge [35,36]. Compared to CB-500, the CuO peaks in the spectrum of CB-600 were observed at lower frequencies, indicating the existence of stronger interactions in the CB-600 catalyst. No obvious Raman features corresponding to Bi2O3 were observed in the CB-500 and CB-600 spectra, possibly due to its lower content and crystallinity. In addition, four distinct vibrational peaks—at 130.9, 261.8, 402.8, and 586.6 cm−1—were observed for the CB-700 sample, which corresponds to the presence of CuBi2O4 [30,31].
Table 1 presents the BET specific surface areas of the fresh catalysts. CB-500 exhibits the largest surface area (23.2 m2 g−1); however, a significantly reduced surface area (8.7 m2 g−1) was obtained for the sample calcined at 700 °C. This decrease in surface area is normally associated with the growth of larger particles [27].
The morphologies and microstructures of the fresh catalysts were investigated by transmission electron microscopy (TEM). As shown in Figure 3, the catalysts have nearly spherical or irregular polyhedral morphologies. Elevated calcination temperatures lead to larger catalyst particles that are randomly distributed, which is ascribable to metal oxide sintering at high temperatures.

2.1.2. X-ray Photoelectron Spectroscopy

The fresh CuO-Bi2O3 catalysts were subjected to XPS in order to identify the surface chemical states of their copper species; the Cu2p XPS spectra are displayed in Figure 4. All catalysts displayed two peaks, one centered at about 933.1 eV and the other at about 955.9 eV, which are attributed to Cu2p3/2 and Cu2p1/2, respectively. The main Cu2p3/2 peak at 933.1 eV and its satellite at about 940–945 eV are characteristic of Cu2+ species [37,38]. Major Cu2p3/2 peaks are observed at around 933.1 eV in the spectra of CB-500 and CB-700; however, this peak was observed at 933.9 eV in the spectrum of CB-600. The positive binding-energy (BE) shifts suggest charge transfer from the metal ions to other components, which is indicative of strong interactions between Cu and Bi [39,40]. In addition, the shoulder peak at 936.0 was observed to gradually increase with increasing calcination temperature, which is attributed to spinel Cu2+ species [26].

2.1.3. Reduction Behavior

To investigate the interactions between the different components, we employed H2-TPR experiments. As shown in Figure 5, the reduction profiles can be divided into α (around 220–450 °C), β (around 450–550 °C), and γ (around 550–700 °C) peaks. The α peak is attributed to the reduction of CuO. The stepwise reduction of CuO, i.e., Cu2+ to Cu+ (low temperature) and Cu+ to Cu0 (high temperature) [41,42], can be discounted because of the fast H2-flow rate and deviations from the ideal pattern (the area of the low-temperature reduction peak is ideally equal to that of the high-temperature peak). The α peak was observed to shift toward higher temperatures with increasing calcination temperature. In the present experiment, both increases in CuO-particle size and interactions between copper and bismuth species improve the reduction temperature [43,44]. To identify the influence of Cu–Bi interactions, we also examined the H2-TPR of pure CuO samples (Figure 6) that were calcined at different temperatures to produce CuO particles of equivalent sizes to those of the corresponding CuO-Bi2O3. Symmetrical reduction peaks were observed for all pure CuO samples, with reduction temperatures of 257, 326, and 390 °C. By comparing the shapes and temperatures of the reduction peaks for the CB-500 and CB-600 catalysts, we ascribe the first peak—centered at about 266 °C or 339 °C—to the reduction of isolated bulk CuO and/or weak interactions between the CuO and Bi species [39], and the second peak—centered at about 334 °C or 385 °C—to the reduction of CuO that strongly interacts with Bi2O3 [43,44]. Clearly, the peak area corresponding to strong CuO-Bi2O3 interactions is larger in the CB-600 profile than the CB-500 profile, suggesting that stronger interaction exist within CB-600. The reduction profile of CuO-700 is very close to that of the CB-700 catalyst. Moreover, the XRD and Raman results reveal that large CuO particles devoid of interactions are observed in the CB-700 sample. Hence, the single symmetrical peak observed at 392 °C in the profile of the CB-700 catalyst is attributable to the reduction of large CuO particles. These observations suggest that calcination temperature affects the interactions between Cu and Bi and, as a consequence, the reduction behavior of the Cu species. This also illustrates that the reducibility of copper species is hindered by proper interactions between the Cu and Bi species, which might protect the copper species against further reduction by HCHO in the ethynylation reaction.
These reduction properties may be related to the coordination structures and electron charge densities (ECDs) of the copper species [45,46]. The partial incorporation of Bi3+ into the CuO lattice (the Cu2+ ions are at centers of inversion symmetry in a single four-fold 4c site [47]) results in structural distortion, based on the above XRD and Raman results; hence, the Cu2+-coordination structures will also be distorted [32,33]. Moreover, the XPS results reveal that copper species that interact with the bismuth species exhibite higher BEs, indicating that the copper species have low outer-shell electron densities. This result may explain why the copper species that interact with bismuth species are more difficult to reduce than bulk CuO under identical conditions. The above two points may be related to the intrinsic nature of Cu–Bi interactions.
In the higher-temperature region (>450 °C), the β and γ peaks mark the reductions of Bi2O3 and CuBi2O4, respectively [29,30,31]. Compared to that of the CB-500 sample, the β-reduction-peak area of CB-600 was lower and a γ peak at 578 °C emerged due to the partial transformation of Bi2O3 into CuBi2O4. The γ peak was even larger and shifted to a slightly higher temperature (608 °C) in the profile of CB-700, while the β peak was absent, suggesting that Bi2O3 had completely transformed into CuBi2O4, with larger particles, which is consistent with the XRD results. Clearly, the Cu2+ species in the spinel matrix are less reducible than those in the non-spinel CuO phase. Previous authors have also observed similar phenomena in Cu–Al spinel catalysts [26,48].
In comparison to CB-500 and CB-700, CB-600 contains more copper species that interact with bismuth, which may facilitate the effective conversion of CuO into active cuprous species, thereby enhancing its catalytic performance. It should be noted that CB-700 contains more spinel copper species that seem to play crucial roles in the catalytic stability, as discussed below.

2.2. Characterizing the Used Catalysts

2.2.1. Structure and Texture

Cuprous species (cuprous acetylides) are considered to be the active species in the ethynylation reaction, which are formed in situ from CuO during the reaction [1]. These species undergo phase transformations including reduction from Cu2+ to Cu+ followed by coordination to acetylene. Hence, we employed XRD, BET, and CO-IR techniques to trace the changes in the structures, textures, and surface valencies of the used catalysts. Herein, we characterize the CuO-Bi2O3 catalysts after 15 h of evaluation.
The XRD patterns of the used samples shown in Figure 7 exhibit distinct diffraction-peak differences. No diffraction peaks related to CuO are observed in the pattern of CB-600. Meanwhile, weak broad peaks at about 28.1°, 33.0°, and 46.9° emerged, which may belong to amorphous active cuprous species, which indicates that these active cuprous species form during the complete transformation of CuO and the less-crystalline CuBi2O4 during ethynylation, which is beneficial for increased activity [16,17]. With the exception of the weak broad peaks, a diffraction peak of metallic Cu (JCPDS04-0836) was detected at a 2θ value of 43.3° for CB-500, indicating that Cu2+ ions were partially reduced to metallic Cu by HCHO in the ethynylation reaction [16,17]. Based on the H2-TPR profiles of the fresh catalysts, the appearance of metallic Cu can be ascribed to noninteracting copper species, which are easily reduced in the CB-500 catalyst. In contrast, some of the CuO and CuBi2O4 XRD peaks remained in the pattern of the CB-700 sample, suggesting that this catalyst was not completely transformed into active cuprous species because of the weak reducibility of the copper species in the large CuO particles and spinel matrices.
BET data for the used catalysts are listed in Table 1. Compared to the corresponding fresh catalysts, the surface areas of the used catalysts were all lower to various extents. This observation is possibly ascribable to the collapse of porous channels and shrinkage of the frameworks of copper oxide species during phase transformation [14,15]. The used CB-600 catalyst shows a minimal decrease in SBET, suggesting that strong Cu–Bi interactions effectively hinder the migration and aggregation of catalyst particles, and retain the original framework of the fresh catalysts to an extent. Moreover, the used CB-600 also exhibits the maximal SBET, which is beneficial for exposing active sites.

2.2.2. CO-IR Spectroscopy

To further determine the valencies and chemical environments of the surface copper species, CO-IR spectra were recorded and are shown in Figure 8. As detailed previously, the linear adsorption of CO on Cu+ species gives rise to peaks with vibrational frequencies at about 2160–2110 cm−1 [49,50,51,52,53,54]. However, Cu0−CO and Cu2+−CO species are usually unstable and do not exist at room temperature, especially following the evacuation process. Thus, based on our experiment conditions and literature data, we attribute the strong peak at 2110 cm−1 to linearly adsorbed CO on Cu+. In addition, the intensity of this peak followed the order: CB-600 > CB-500 > CB-700, which corresponds to the ordering of the BET specific surface areas of the used catalysts. The results of the integrated areas of the Cu1+−CO band are listed in Table 1, which reveals that the used CB-600 catalyst has the highest number of exposed active Cu+ sites, which is due to strong interactions between copper and bismuth species. On the basis of the XRD and Raman results from the calcined catalysts, we conclude that Bi3+ ions diffuse into the CuO lattice and distort the CuO structure. Doping with bismuth species may play a role in diluting the copper species. During phase evolution, the strong interactions between the Cu and Bi species hinder the migration and aggregation of copper species to retain the framework of the fresh catalysts to some extent. The XPS and H2-TPR results confirm that the outer shell electron configurations and the reduction behavior of Cu2+ are affected by bismuth-species doping regulated by calcination temperature. The XRD results for the used catalysts further confirm that the over-reduction of Cu2+ to metallic Cu can be effectively restrained and that the Cu+ valency is stabilized in the used CB-600 catalyst. Similar observations have been reported by Di et al. and Wen et al. [44,55], who noted that the reduction behavior and the dispersion of copper species can be regulated by altering interactions between copper species and promoters or supports.

2.3. Catalytic Performance

The main reaction involved in the ethynylation of formaldehyde can be written as follows:
HC≡CH + 2HCHO → OH-CH2C≡CCH2-OH
Firstly, we investigated the influence of reaction temperature on the catalytic activities of the CuO-Bi2O3 and CuO catalysts in a simulated slurry reactor at 90 °C and under atmospheric pressure. The main product from the ethynylation reaction is BD; by-products, such as propargyl alcohol, formic acid, and polyformaldehyde are not shown because of their low selectivities. The yield of BD over 15 h with different reaction temperatures are plotted in Figure 9a. The reaction temperature was found to have a great influence on the catalysts performance. The yield of BD could be greatly improved with the temperature elevated. When the reaction temperature was 90 °C or 100 °C, all the catalysts displayed relatively high activity. Figure 9b displays the ethynylation activities of the CuO-Bi2O3 and CuO catalysts as functions of time over 15 h at 90 °C. The CuO-based catalysts gradually became catalytic active after an induction period of around 2 h; this induction period possibly correlates with phase evolution in the catalysts. It is widely believed that cuprous acetylide is the actual active phase involved in the ethynylation reaction, rather than CuO. With extended reaction times, all catalysts showed increased activities. However, the BD yield first increased and then decreased with increasing calcination temperature. As shown in Table 1, the BD yield over CB-600 was 92.4% after 15 h. In contrast, the BD yields over CB-500 and CB-700 were 84% and 74.6% under the same conditions, respectively. Figure 10 reveals that the integrated areas of the CO-IR peaks correlate with reaction yield, which indicates that catalytic activity is proportional to the Cu+ surface area and that surface cuprous species are responsible for the formation of BD.
The stabilities of the CuO-Bi2O3 catalysts were examined by filtering off the catalysts after 15 h of reaction time and reusing them in another seven cycles under the reaction conditions of 90 °C and atmospheric pressure (Figure 11). The yields of BD after recycling are listed in Table 2. During the eight runs, marginal decreases in BD yield were observed for CB-600 (92.5% to 62.2%), while larger decreases were observed for CB-500 (86% to 32.8%). However, CB-700 showed optimal catalytic stability (72.5% to 65.1%), despite its lower activity. The formation of metallic Cu may be the predominant factor responsible for catalyst deactivation in this work. Hence, the CuO-Bi2O3 catalysts were examined by XRD after 1, 4, and 8 cycles in order to investigate their phase evolution (Figure 12). The CB-600 and CB-500 catalysts exhibited the formation and growth of metallic Cu that positively correlates with the observed decreases in catalyst stabilities. However, metallic Cu particles were observed to grow on the CB-700 catalyst, while a decline in the CuBi2O4 phase was observed after the eight runs. Hence, active cuprous species gradually formed in situ from the spinel phase, which replenished the decline in activity resulting from a decrease in Cu+ species. According to the H2-TPR results, spinel Cu2+ species are more difficult to reduce than non-spinel Cu2+ species under the same conditions; hence, the formation of active cuprous species from the CuBi2O4 phase will be somewhat slower than that from the CuO phase. From the above discussion, we conclude that the excellent catalytic stability of CB-700 is due to synergism between the CuO and CuBi2O4 phases. During ethynylation, the CB-700 catalyst is first activated by the formation of active cuprous species from non-spinel Cu2+, followed by the formation of active cuprous species through the gradual release of Cu2+ from the spinel structure, as displayed in Figure 13.
Based on the above analysis, we conclude that the temperature at which the CuO-Bi2O3 catalyst is calcined greatly influences its phase structure, texture, and the interactions between the metal oxide precursors, as well as the transformations of active cuprous species and the corresponding catalytic performance. There is a range of calcination temperatures at which the catalyst exhibited the best activity. The excellent catalytic performance of Cu/Bi-based catalysts is ascribable to strong interactions between Cu and Bi. On the one hand, these strong interactions stabilize the Cu+ valency and inhibit the formation of metallic Cu, which facilitates the efficient transformation of CuO into active cuprous species. On the other hand, this strong interaction retains the catalyst framework and hinders the migration and aggregation of catalyst particles, resulting in more active sites exposed on the catalyst surface. In addition, the gradual release of copper species from spinel structures formed at high temperatures sustainably replenishes catalytic activity [26,56], resulting in improved catalytic stability.

2.4. Reaction Mechanism

Given our present observations, phase transformation appears to be important during the reaction, in which the catalysts undergo reduction and carbonization by HCHO and C2H2; this phase change can be expressed as: CuO → active cuprous species (Cu2C2) → Cu. The first phase transformation results in induction behavior that activates the catalyst. The dispersion and valence stability of Cu+ species in the activated catalyst are responsible for catalytic performance. The second step deactivates the CuO-Bi2O3 catalyst. The active cuprous species formed in situ are sufficiently nucleophilic to react with formaldehyde. Hübner et al. [57] also observed net electron transfer from Cu+ ions in zeolites that act as electron donors to acetylene by IR spectroscopy and by quantum-chemical calculations, which demonstrated that the acetylene molecules adsorbed on the Cu+ ions are nucleophilic. In contrast, it has been suggested that Cu+ acts at electrophilic or Lewis acidic sites, thereby improving the reactivity of the C=O bond [39,58,59]. The carbonyl group is coordinated to Cu+ through a lone pair of electrons on oxygen, leading to a more-electrophilic carbonyl carbon atom [59]. This assumption was confirmed by the hydrogenation of C=O bonds on copper-based catalysts [34]. According to the above discussion and experimental observations, we have reason to believe that the carbonyl and alkynyl are simultaneously activated by Cu+ and, consequently, propose a plausible mechanism to account for the CuO-catalyzed ethynylation (Figure 14). This mechanism indirectly supports the study of Chu et al. who reported that the adsorption and activation of HCHO and C2H2 molecules on catalyst surfaces are critical to the intrinsic reaction rates [60].

3. Materials and Methods

3.1. Preparation of the Catalysts

All chemical reagents, namely Cu(NO3)2, Bi(NO3)3, PEG-400, NaOH, were of analytical grade and were used as received. The CuO-Bi2O3 catalyst was prepared by the coprecipitation method. In a typical synthesis procedure, urea (100 g) and PEG-400 (100 g) were dissolved in 200 mL of a solution of Cu(NO3)2 (0.5 M) and Bi(NO3)3 (0.1 M) at room temperature. A solution of NaOH (200 mL, 2 M) was then added dropwise to the above solution at 80 °C with stirring. Following reaction, the precipitate was separated by centrifugation and washed three times (successively) with H2O and EtOH. The final product was dried at 80 °C for 12 h and calcined at 500, 600, or 700 °C for 3 h. The powders prepared in this manner are referred to as “CB-T”, where T indicates the calcination temperature. In addition, the used catalysts were washed, filtered, and dried under vacuum at 60 °C for 48 h. Pure CuO was prepared in the same way as the CuO-Bi2O3 samples. As summarized in Table 3, pure CuO was calcined at different temperatures to obtain samples with essentially equivalent CuO particle sizes to those of CB-500, CB-600, and CB-700. For ease of reference, these samples are referred to as “CuO-500”, “CuO-600”, and “CuO-700”, respectively.

3.2. Characterization

XRD patterns were collected on a Bruker D8 Advance spectrometer (Bruker, Billerica, MA, USA) with a Cu Kα target (λ = 0.15418 nm) operating at 40 kV and 40 mA, at a scanning rate of 2.4°/min and over the 10°–80° 2θ range.
N2-Physisorption analyses were performed using a Micromeritics ASAP-2020 apparatus (Norcross, GA, USA). The fresh and used samples were degassed at 150 °C for 5 h and 60 °C for 24 h respectively prior to adsorption testing. Specific surface areas were obtained using the multi-point BET procedure.
H2-TPR experiments were performed using a Micromeritics AutoChemII 2920 instrument (Norcross, GA, USA). A 30 mg sample was loaded into the instrument. A 5 vol% mixture of H2 in N2 (30 mL/min) was introduced and the temperature was raised from room temperature to 700 °C at a rate of 10 °C/min. The consumption of H2 was determined using a thermal conductivity detector, and the end gas was analyzed using an on-line quadrupole mass spectrometer (HIDEN HPR-20 QIC) (Beijing Henven Scientific Instrument Co., Ltd., Beijing, China) with a minimum dwell time of 3 ms.
Raman spectra were recorded with a LabRAM HR Evolution instrument (HORIBA, Tokyo, Japan) equipped with a CCD detector at room temperature. A 532 nm Ventus laser operating at 10 MW was used as the excitation source.
CO-IR spectra were recorded on a Bruker Tensor 27 Fourier-transform infrared spectrometer (Bruker, Billerica, MA, USA) equipped with a highly sensitive MCT detector cooled by liquid N2. The used samples were evacuated at 60 °C for 3 h.
XPS experiments were performed in an ultra-high vacuum chamber (THERMO Scientific K-Alpha spectrometer) (Thermo Fisher Scientific, Waltham, MA, USA) at a base pressure of −4 × 10−10 Torr. The XPS spectrometer was equipped with a high-resolution hemispherical electron analyzer (279.4 mm diameter with 25 meV resolution) and a Mg Kα (hν = 1253.6 eV) X-ray excitation source. All spectra were referenced to adventitious C (1s) at a BE of 284.5 eV.
TEM experiments were performed using a JEOL JEM-2100 transmission electron microscope operating at 200 kV (JEOL, Tokyo, Japan). TEM samples were dispersed by sonication in ethanol followed by deposition of the suspension onto a standard Cu grid covered with a porous carbon film.

3.3. Catalyst Test

A 0.5 g sample of the catalyst and 10 mL of aqueous formaldehyde (30 vol%) were sequentially added to a 100-mL round bottom three-port flask equipped with a thermometer and a condenser tube. The flask was purged with N2 and the system was heated to 90 °C in an oil bath with stirring, after which C2H2 gas was introduced for 15 h. The mixture was cooled to room temperature following the reaction. The solid catalyst was removed by centrifugation and quantitatively analyzed on an Agilent 7890A gas chromatograph fitted with a DB-5 capillary column (0.32 mm × 50 m) and an FID detector. 1,4-Butanediol was used as the internal standard. The unconverted formaldehyde in solution was determined by iodimetry.

4. Conclusions

In the present study, a series of nanostructured CuO-Bi2O3 catalysts was prepared using simple coprecipitation methods followed by calcination at different temperatures, after which they were applied to the ethynylation of formaldehyde to produce BD. We reveal that the calcination temperature greatly influences the crystal phase structures of the CuO-Bi2O3 catalysts and interactions between the components. The CB-600 catalysts demonstrated strong interactions between CuO and Bi2O3, which improves the stability of the Cu+ valence state and maintains a higher degree of dispersion of the active cuprous species during the transformation from copper oxide to cuprous acetylide, resulting in enhanced catalytic activity. The spinel CuBi2O4 phase formed at high temperatures (≥600 °C) gradually releases copper species to form active cuprous species over multiple cycles, which maintains catalytic stability. Furthermore, we found a linear relationship between catalytic activity and the Cu+ area, which indicates that Cu+ is the actual active species that simultaneously activates C2H2 and HCHO. Hence, the present observations and the literature allow us to propose a plausible ethynylation mechanism for CuO-based catalysis. Our work highlights a simple method for the rational design of highly active and stable CuO-based catalysts that simultaneously solves the problem of over-reduction and the aggregation of copper species and maintains sustainable catalysis during the phase transformation of copper oxides, thereby demonstrating important academic and industrial value.

Author Contributions

The idea was conceived by H.L. and Z.W. Z.W. performed the experiments and drafted the paper under the supervision of Y.Z., Z.J. and H.L. Z.N., Q.H. and L.B. helped to collect and analyse some characterization data. The manuscript was revised through the comments of all authors. All authors have given approval for the final version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (No. U1710221, 21503124) and the International Scientific and Technological Cooperation Project of Shanxi Province, China (No. 201703D421034).

Acknowledgments

The authors gratefully thank the financial supports of the National Natural Science Foundation of China (No. U1710221, 21503124) and the International Scientific and Technological Cooperation Project of Shanxi Province, China (No. 201703D421034).

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Trotuş, I.T.; Zimmermann, T.; Schüth, F. Catalytic reactions of acetylene: A feedstock for the chemical industry revisited. Chem. Rev. 2014, 114, 1761–1782. [Google Scholar] [CrossRef] [PubMed]
  2. Reppe, W.; Keyssner, E. Production of Alkinols. U.S. Patent 2,232,867, 25 February 1941. [Google Scholar]
  3. Keyssner, E.; Eichler, E. Production of Dihydric Alcohols of the Acetylene Series. U.S. Patent 2,238,471, 15 April 1941. [Google Scholar]
  4. Tanielyan, S.; Schmidt, S.; Marin, N.; Alvez, G.; Augustine, R. Selective hydrogenation of 2-Butyne-1,4-diol to 1,4-Butanediol over particulate Raney® Nickel catalysts. Top. Catal. 2010, 53, 1145–1149. [Google Scholar] [CrossRef]
  5. Nadgeri, J.M.; Telkar, M.M.; Rode, C.V. Hydrogenation activity and selectivity behavior of supported palladium nanoparticles. Catal. Commun. 2008, 9, 441–446. [Google Scholar] [CrossRef]
  6. Kriaa, K.; Serin, J.P.; Contamine, F.; Cézac, P.; Mercadier, J. 2-Butyne-1,4-diol hydrogenation in supercritical CO2: Effect of hydrogen concentration. J. Supercrit. Fluids 2009, 49, 227–232. [Google Scholar] [CrossRef]
  7. Li, H.T.; Zhao, Y.X.; Gao, C.G.; Wang, Y.Z.; Sun, Z.J.; Liang, X.Y. Study on deactivation of Ni/Al2O3 catalyst for liquid phase hydrogenation of crude 1,4-butanediol aqueous solution. Chem. Eng. J. 2012, 181–182, 501–507. [Google Scholar] [CrossRef]
  8. Hari Prasad Reddy, K.; Anand, N.; Sai Prasad, P.S.; Rama Rao, K.S.; David Raju, B. Influence of method of preparation of Co-Cu/MgO catalyst on dehydrogenation/dehydration reaction pathway of 1, 4-butanediol. Catal. Commun. 2011, 12, 866–869. [Google Scholar] [CrossRef]
  9. Zhang, Q.; Zhang, Y.; Li, H.T.; Gao, C.G.; Zhao, Y.X. Heterogeneous CaO-ZrO2 acid–base bifunctional catalysts for vapor-phase selective dehydration of 1,4-butanediol to 3-buten-1-ol. Appl. Catal. A Gen. 2013, 466, 233–239. [Google Scholar] [CrossRef]
  10. Kim, M.N.; Kim, K.H.; Jin, H.J.; Park, J.K.; Yoon, J.S. Biodegradability of ethyl and n-octyl branched poly(ethylene adipate) and poly(butylene succinate). Eur. Polym. J. 2001, 37, 1843–1847. [Google Scholar] [CrossRef]
  11. Zhao, F.Y.; Ikushima, Y.; Arai, M. Hydrogenation of 2-butyne-1,4-diol in supercritical carbon dioxide promoted by stainless steel reactor wall. Catal. Today 2004, 93–95, 439–443. [Google Scholar] [CrossRef]
  12. Bonrath, W.; Bernd, E.; Reinhard, K.; Schneider, M. Ethynylation Process. U.S. Patent 6,949,685, 27 September 2005. [Google Scholar]
  13. Frantz, D.E.; Fässler, R.; Carreira, E.M. Facile Enantioselective Synthesis of Propargylic Alcohols by Direct Addition of Terminal Alkynes to Aldehydes. J. Am. Chem. Soc. 2000, 122, 1806–1807. [Google Scholar] [CrossRef]
  14. Bukur, D.B.; Okabe, K.; Rosynek, M.P.; Li, C.P.; Wang, D.J.; Rao, K.R.P.M.; Huffman, G.P. Activation studies with a precipitated iron catalyst for Fischer-Tropsch synthesis. J. Catal. 1995, 155, 353–365. [Google Scholar] [CrossRef]
  15. Gao, F.F.; Wang, H.; Qing, M.; Yang, Y.; Li, Y.W. Controlling the phase transformations and performance of iron-based catalysts in the Fischer-Tropsch synthesis. Chin. J. Catal. 2013, 34, 1312–1325. [Google Scholar] [CrossRef]
  16. Wang, J.J.; Li, H.T.; Ma, Z.Q.; Wang, Z.P.; Guo, J.Y.; Zhao, Y.X. Preparation of magnetic CuO-Bi2O3/Fe3O4-SiO2-MgO catalyst and its catalytic performance for formaldehyde ethynylation. J. Chem. Ind. Eng. 2015, 66, 2098–2104. [Google Scholar]
  17. Zheng, Y.; Sun, Z.J.; Wang, Y.Z.; Li, H.T.; Wang, S.A.; Luo, M.; Zhao, J.L.; Zhao, Y.X. Preparation of CuO-Bi2O3/SiO2-MgO catalyst and its ethynylation performance. J. Mol. Catal. 2012, 26, 233–238. [Google Scholar]
  18. Yang, G.F.; Li, H.T.; Zhang, H.X.; Wang, Z.P.; Liu, L.L.; Zhao, Y.X. Effect of NaOH concentration on structure and catalytic performance of Cu2O for formaldehyde ethynylation. J. Mol. Catal. 2016, 30, 540–546. [Google Scholar]
  19. Li, H.T.; Niu, Z.Z.; Yang, G.F.; Zhang, H.X.; Wang, Z.P.; Zhao, Y.X. Support effect of Cu2O/TiO2 employed in formaldehyde ethynylation. J. Chem. Ind. Eng. 2018, 69, 2512–2518. [Google Scholar]
  20. Reppe, W.; Steinhofer, A.; Spaenig, H. Production of Alkinols. U.S. Patent 2,300,969, 3 November 1942. [Google Scholar]
  21. Codignola, F.; Aires, B. Process for the Catalytic Hydrogenation of 1,4-Butynediol to 1,4-Butanediol. U.S. Patent 4,438,285, 20 March 1984. [Google Scholar]
  22. Hort, E.V.; Piscataway, N.J. Ethynylation Catalyst and Method of Production Alkynols by Low Pressure Reactions. U.S. Patent 3,920,759, 18 November 1975. [Google Scholar]
  23. Fremont, J.M. Malachite Preparation. U.S. Patent 4,107,082, 15 August 1978. [Google Scholar]
  24. Fremont, J.M. Preparation of Butynediol Using Bismuth Modified Spheroidal Malachite. U.S. Patent 4,127,734, 28 November 1978. [Google Scholar]
  25. Luo, M.F.; Fang, P.; He, M.; Xie, Y.L. In situ XRD, Raman, and TPR studies of CuO/Al2O3catalysts for CO oxidation. J. Mol. Catal. A Chem. 2005, 239, 243–248. [Google Scholar] [CrossRef]
  26. Liu, Y.J.; Qing, S.J.; Hou, X.N.; Qin, F.J.; Wang, X.; Gao, Z.X.; Xiang, H.W. Temperature dependence of Cu–Al spinel formation and its catalytic performance in methanol steam reforming. Catal. Sci. Technol. 2017, 7, 5069–5078. [Google Scholar] [CrossRef]
  27. Zhu, Y.F.; Kong, X.; Cao, D.B.; Cui, J.L.; Zhu, Y.L.; Li, Y.W. The rise of calcination temperature enhances the performance of Cu catalysts: Contributions of support. ACS Catal. 2014, 4, 3675–3681. [Google Scholar] [CrossRef]
  28. Chen, C.; Cao, J.J.; Cargnello, M.; Fornasiero, P.; Gorte, R.J. High-temperature calcination improves the catalytic properties of alumina-supported Pd@ceria prepared by self assembly. J. Catal. 2013, 306, 109–115. [Google Scholar] [CrossRef]
  29. Abdulkarem, A.M.; Aref, A.A.; Abdulhabeeb, A.; Li, Y.F.; Yu, Y. Synthesis of Bi2O3/Cu2O nanoflowers by hydrothermal method and its photocatalytic activity enhancement under simulated sunlight. J. Alloys Compd. 2013, 560, 132–141. [Google Scholar] [CrossRef]
  30. Muthukrishnaraj, A.; Vadivel, S.; Made Joni, I.; Balasubramanian, N. Development of reduced graphene oxide/CuBi2O4 hybrid for enhanced photocatalytic behavior under visible light irradiation. Ceram. Int. 2015, 41, 6164–6168. [Google Scholar] [CrossRef]
  31. Abdulkarem, A.M.; Li, J.L.; Aref, A.A.; Ren, L.; Elssfah, E.M.; Wang, H.; Ge, Y.K.; Yu, Y. CuBi2O4 single crystal nanorods prepared by hydrothermal method: Growth mechanism and optical properties. Mater. Res. Bull. 2011, 46, 1443–1450. [Google Scholar] [CrossRef]
  32. Qin, W.; Qi, J.; Wu, X.H. Photocatalytic property of Cu2+-doped Bi2O3 films under visible light prepared by the solegel method. Vacuum 2014, 107, 204–207. [Google Scholar] [CrossRef]
  33. Xu, J.F.; Ji, W.; Shen, Z.X.; Li, W.S.; Tang, S.H.; Ye, X.R.; Jia, D.Z.; Xin, X.Q. Raman spectra of CuO nanocrystals. J. Raman Spectrosc. 1999, 30, 413–415. [Google Scholar] [CrossRef]
  34. Zhu, Y.F.; Zhu, Y.L.; Ding, G.Q.; Zhu, S.H.; Zheng, H.Y.; Li, Y.W. Highly selective synthesis of ethylene glycol and ethanol via hydrogenation of dimethyl oxalate on Cu catalysts: Influence of support. Appl. Catal. A 2013, 468, 296–304. [Google Scholar] [CrossRef]
  35. Águila, G.; Gracia, F.; Araya, P. CuO and CeO2 catalysts supported on Al2O3, ZrO2, and SiO2 in the oxidation of CO at low temperature. Appl. Catal. A 2008, 343, 16–24. [Google Scholar] [CrossRef]
  36. Águila, G.; Gracia, F.; Cortés, J.; Araya, P. Effect of copper species and the presence of reaction products on the activity of methane oxidation on supported CuO catalysts. Appl. Catal. B 2008, 77, 325–338. [Google Scholar] [CrossRef]
  37. Sato, A.G.; Volanti, D.P.; Meira, D.M. Effect of the ZrO2 phase on the structure and behavior of supported Cu catalysts for ethanol conversion. J. Catal. 2013, 307, 1–17. [Google Scholar] [CrossRef]
  38. Zhu, S.H.; Gao, X.Q.; Zhu, Y.L.; Zhu, Y.F.; Zheng, H.Y.; Li, Y.W. Promoting effect of boron oxide on Cu/SiO2 catalyst for glycerol hydrogenolysis to 1,2-propanediol. J. Catal. 2013, 303, 70–79. [Google Scholar] [CrossRef]
  39. Yin, A.Y.; Guo, X.Y.; Dai, W.L.; Fan, K.N. The nature of active copper species in Cu-HMS catalyst for hydrogenation of dimethyl oxalate to ethylene glycol: New insights on the synergetic effect between Cu0 and Cu+. J. Phys. Chem. C 2009, 113, 11003–11013. [Google Scholar] [CrossRef]
  40. Zhang, B.; Zhu, Y.L.; Ding, G.Q.; Zheng, H.Y.; Li, Y.W. Modification of the supported Cu/SiO2 catalyst by alkaline earth metals in the selective conversion of 1, 4-butanediol to γ-butyrolactone. Appl. Catal. A 2012, 443–444, 191–201. [Google Scholar] [CrossRef]
  41. Xue, J.J.; Wang, X.Q.; Qi, G.S.; Wang, J.; Shen, M.Q.; Li, W. Characterization of copper species over Cu/SAPO-34 in selective catalytic reduction of NOx with ammonia: Relationships between active Cu sites and de-NOx performance at low temperature. J. Catal. 2013, 297, 56–64. [Google Scholar] [CrossRef]
  42. Bienholz, A.; Blume, R.; Knop-Gericke, A.; Girgsdies, F.; Behrens, M.; Claus, P. Prevention of catalyst deactivation in the hydrogenolysis of glycerol by Ga2O3-modified copper/zinc oxide Catalysts. J. Phys. Chem. C 2011, 115, 999–1005. [Google Scholar] [CrossRef]
  43. Hu, Z.P.; Zhu, Y.P.; Gao, Z.M.; Wang, G.X.; Liu, Y.P.; Yuan, Z.Y. CuO catalysts supported on activated red mud for efficient catalytic carbon monoxide oxidation. Chem. Eng. J. 2016, 302, 23–32. [Google Scholar] [CrossRef]
  44. Wen, C.; Yin, A.Y.; Cui, Y.Y.; Yang, X.L.; Dai, W.L.; Fan, K.N. Enhanced catalytic performance for SiO2–TiO2 binary oxide supported Cu-based catalyst in the hydrogenation of dimethyloxalate. Appl. Catal. A Gen. 2013, 458, 82–89. [Google Scholar] [CrossRef]
  45. Sun, C.Z.; Zhu, J.; Lv, Y.Y.; Qi, L.; Liu, B.; Gao, F.; Sun, K.Q.; Dong, L.; Chen, Y. Dispersion, reduction and catalytic performance of CuO supported on ZrO2-doped TiO2 for NO removal by CO. Appl. Catal. B 2011, 103, 206–220. [Google Scholar] [CrossRef]
  46. Hu, Y.H.; Dong, L.; Wang, J.; Ding, W.P.; Chen, Y. Activities of supported copper oxide catalysts in the NO + CO reaction at low temperatures. J. Mol. Catal. A Chem. 2000, 162, 307–316. [Google Scholar] [CrossRef]
  47. Zhang, Q.B.; Zhang, K.L.; Xu, D.G. CuO nanostructures: Synthesis, characterization, growth mechanisms, fundamental properties, and applications. Prog. Mater. Sci. 2014, 60, 208–337. [Google Scholar] [CrossRef]
  48. Li, G.J.; Gu, C.T.; Zhu, W.B.; Wang, X.F.; Yuan, X.F.; Cui, Z.J.; Wang, H.L.; Gao, Z.X. Hydrogen production from methanol decomposition using Cu-Al spinel catalysts. J. Clean. Prod. 2018, 183, 415–423. [Google Scholar] [CrossRef]
  49. Kefirov, R.; Penkova, A.; Hadjiivanov, K.; Dzwigaj, S.; Che, M. Stabilization of Cu+ ions in BEA zeolite: Study by FTIR spectroscopy of adsorbed CO and TPR. Micropor. Mesopor. Mater. 2008, 116, 180–187. [Google Scholar] [CrossRef]
  50. Hadjiivanov, K.; Vayssilov, G. Characterization of oxide surfaces and zeolites by carbon monoxide as an IR probe molecule. Adv. Catal. 2002, 47, 307–511. [Google Scholar]
  51. Tang, Y.X.; Dong, L.H.; Deng, C.S.; Huang, M.N.; Li, B.; Zhang, H.L. In situ FT-IR investigation of CO oxidation on CuO/TiO2 catalysts. Catal. Commun. 2016, 78, 33–36. [Google Scholar] [CrossRef]
  52. Broclawik, E.; Kozyra, P.; Datka, J. IR studies and DFT quantum chemical calculations concerning interaction of some organic molecules with Cu+ sites in zeolites. C. R. Chim. 2005, 8, 491–508. [Google Scholar] [CrossRef]
  53. Datka, J.; Kukulska-Zając, E.; Kobyzew, W. IR studies of coadsorption of organic molecules and CO on Cu+ cations in zeolites. Catal. Today 2006, 114, 169–173. [Google Scholar] [CrossRef]
  54. Xiong, Y.; Yao, X.J.; Tang, C.J.; Zhang, L.; Cao, Y.; Deng, Y.; Gao, F.; Dong, L. Effect of CO-pretreatment on the CuO–V2O5/γ-Al2O3 catalyst for NO reduction by CO. Catal. Sci. Technol. 2014, 4, 4416–4425. [Google Scholar] [CrossRef]
  55. Di, W.; Cheng, J.H.; Tian, S.X.; Li, J.; Chen, J.Y.; Sun, Q. Synthesis and characterization of supported copper phyllosilicate catalysts for acetic ester hydrogenation to ethanol. Appl. Catal. A 2016, 510, 244–259. [Google Scholar] [CrossRef]
  56. Shokouhimehr, M. Magnetically separable and sustainable nanostructured catalysts for heterogeneous reduction of nitroaromatics. Catalysts 2015, 5, 534–560. [Google Scholar] [CrossRef]
  57. Hübner, G.; Rauhut, G.; Stoll, H.; Roduner, E. Ethyne adsorbed on CuNaY zeolite:  FTIR spectra and quantum chemical calculations. J. Phys. Chem. B 2003, 107, 8568. [Google Scholar] [CrossRef]
  58. Poels, E.K.; Brands, D.S. Modification of Cu/ZnO/SiO2 catalysts by high temperature reduction. Appl. Catal. A 2000, 191, 83–96. [Google Scholar] [CrossRef]
  59. Gong, J.L.; Yue, H.R.; Zhao, Y.J.; Zhao, S.; Zhao, L.; Lv, J.; Wang, S.P.; Ma, X.B. Synthesis of ethanol via syngas on Cu/SiO2 catalysts with balanced Cu0-Cu+ Sites. J. Am. Chem. Soc. 2012, 134, 13922–13925. [Google Scholar] [CrossRef] [PubMed]
  60. Chu, J.J.; Li, K.T.; Wang, I. Kinetics of the synthesis of 1,4-butynediol over a copper-bismuth/magnesium silicate catalyst. Appl. Catal. A 1993, 97, 123–132. [Google Scholar] [CrossRef]
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Figure 1. X-ray diffraction patterns (XRD) of the catalysts in this study.
Figure 1. X-ray diffraction patterns (XRD) of the catalysts in this study.
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Figure 2. Raman spectra of the CuO-Bi2O3 catalysts in this study.
Figure 2. Raman spectra of the CuO-Bi2O3 catalysts in this study.
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Figure 3. Transmission electron microscopy (TEM) images of the CuO-Bi2O3 catalysts. (A) CB-500, (B) CB-600, and (C) CB-700.
Figure 3. Transmission electron microscopy (TEM) images of the CuO-Bi2O3 catalysts. (A) CB-500, (B) CB-600, and (C) CB-700.
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Figure 4. X-ray photoelectron spectroscopy (XPS) spectra of fresh and used CB-600 catalysts.
Figure 4. X-ray photoelectron spectroscopy (XPS) spectra of fresh and used CB-600 catalysts.
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Figure 5. H2-TPR profiles of CuO-Bi2O3 catalysts.
Figure 5. H2-TPR profiles of CuO-Bi2O3 catalysts.
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Figure 6. H2-TPR profiles of pure CuO catalysts.
Figure 6. H2-TPR profiles of pure CuO catalysts.
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Figure 7. XRD pattern of used CuO-Bi2O3 catalysts.
Figure 7. XRD pattern of used CuO-Bi2O3 catalysts.
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Figure 8. CO-IR spectra of used CuO-Bi2O3 catalysts.
Figure 8. CO-IR spectra of used CuO-Bi2O3 catalysts.
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Figure 9. Yields of 1,4-butynediol (BD) over various CuO-Bi2O3 and CuO catalysts as functions of temperature (a) and time (b).
Figure 9. Yields of 1,4-butynediol (BD) over various CuO-Bi2O3 and CuO catalysts as functions of temperature (a) and time (b).
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Figure 10. BD yield after 15 h at 90 °C as a function of the integrated area of the CO-IR peak.
Figure 10. BD yield after 15 h at 90 °C as a function of the integrated area of the CO-IR peak.
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Figure 11. BD yields as functions of cycle number during recycling studies involving the CuO-Bi2O3 and CuO catalysts.
Figure 11. BD yields as functions of cycle number during recycling studies involving the CuO-Bi2O3 and CuO catalysts.
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Figure 12. XRD patterns of the (a) CB-500, (b) CB-600, (c) CB-700 catalysts after 1, 4 and 8 recycles evaluation.
Figure 12. XRD patterns of the (a) CB-500, (b) CB-600, (c) CB-700 catalysts after 1, 4 and 8 recycles evaluation.
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Figure 13. Activation process scheme of CuO and CuBi2O4.
Figure 13. Activation process scheme of CuO and CuBi2O4.
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Figure 14. Plausible reaction mechanism for catalytic reactions involving CuO-Bi2O3.
Figure 14. Plausible reaction mechanism for catalytic reactions involving CuO-Bi2O3.
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Table
Table 1. Structural and textural data for the CuO-Bi2O3 catalysts.
Table 1. Structural and textural data for the CuO-Bi2O3 catalysts.
SampledCuO (nm)SBET (m2 g−1)ACu+ (a.u.)
FreshUsed
CB-50017.234.211.22.5
CB-60033.020.113.92.8
CB-70039.48.75.91
Note: dCuO values were calculated from CuO(111) XRD reflections by the Scherrer equation; SBET values for the fresh and used catalysts were calculated by the Brunauer–Emmett–Teller (BET) method; and ACu+ values were calculated using the integrated areas of Cu1+-CO IR bands.
Table
Table 2. BD yields after recycling for the CuO-Bi2O3 and CuO catalysts.
Table 2. BD yields after recycling for the CuO-Bi2O3 and CuO catalysts.
SampleYBD1YBD2YBD3YBD4YBD5YBD6YBD7YBD8
CB-50086.075.568.762.354.848.74132.8
CB-60092.490.985.782.680.875.667.662.2
CB-70072.576.972.471.773.070.669.565.1
CuO67.856.645.237.231.915.72.40
Note: The amount of the catalyst used was 0.5 g; the concentration of the formaldehyde solution was 35 vol%; the consumption of the formaldehyde solution was 10 mL; the reaction temperature was 90 °C; the reaction time was 15 h.
Table
Table 3. Properties of pure CuO.
Table 3. Properties of pure CuO.
SampleCalcination Temperature (°C)dCuO (nm)
CuO-50042016.5
CuO-60054035.1
CuO-70060040.6
Note: dCuO was calculated from CuO(111) XRD reflections by the Scherrer equation.

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Wang, Z.; Niu, Z.; Hao, Q.; Ban, L.; Li, H.; Zhao, Y.; Jiang, Z. Enhancing the Ethynylation Performance of CuO-Bi2O3 Nanocatalysts by Tuning Cu-Bi Interactions and Phase Structures. Catalysts 2019, 9, 35. https://0-doi-org.brum.beds.ac.uk/10.3390/catal9010035

AMA Style

Wang Z, Niu Z, Hao Q, Ban L, Li H, Zhao Y, Jiang Z. Enhancing the Ethynylation Performance of CuO-Bi2O3 Nanocatalysts by Tuning Cu-Bi Interactions and Phase Structures. Catalysts. 2019; 9(1):35. https://0-doi-org.brum.beds.ac.uk/10.3390/catal9010035

Chicago/Turabian Style

Wang, Zhipeng, Zhuzhu Niu, Quanai Hao, Lijun Ban, Haitao Li, Yongxiang Zhao, and Zheng Jiang. 2019. "Enhancing the Ethynylation Performance of CuO-Bi2O3 Nanocatalysts by Tuning Cu-Bi Interactions and Phase Structures" Catalysts 9, no. 1: 35. https://0-doi-org.brum.beds.ac.uk/10.3390/catal9010035

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