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Characterization and catalytic performance of Cu/ZnO/Al2O3 watergas shift catalysts derived from Cu-Zn-Al layered double hydroxides Dalin Li, Shuping Xu, Yunbing Cai, Chongqi Chen, Yingying Zhan, and Lilong Jiang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04337 • Publication Date (Web): 07 Mar 2017 Downloaded from http://pubs.acs.org on March 9, 2017

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Industrial & Engineering Chemistry Research

Characterization and catalytic performance of Cu/ZnO/Al2O3 water-gas shift catalysts derived from Cu-Zn-Al layered double hydroxides Dalin Li, Shuping Xu, Yunbing Cai, Chongqi Chen, Yingying Zhan, Lilong Jiang* National Engineering Research Center of Chemical Fertilizer Catalyst (NERC-CFC) School of Chemical Engineering, Fuzhou University, Gongye Road No.523, Fuzhou 350002, Fujian, P. R. China *Corresponding author: Prof. Lilong Jiang, e-mail: [email protected] Tel: +86 595 83731234, Fax: +86 591 83707796 Abstract Cu/ZnO/Al2O3 catalysts with different compositions were prepared from Cu-Zn-Al layered double hydroxides (LDHs) and tested for water-gas shift reaction. LDHs were synthesized by co-precipitation method and Cu-Zn-Al LDHs or Cu-Al LDHs could be formed depending on the (Cu + Zn)/Al atomic ratio. Upon calcination, LDHs decomposed to form mixed metal oxides consisted of CuO, ZnO, ZnAl2O4, CuAl2O4, and/or amorphous Al2O3. After reduction, well dispersed Cu metal particles with 18-48% dispersion and 2-6 nm size were formed. It was observed that the initial activity of Cu/ZnO/Al2O3 catalysts was proportional to the number of surface Cu0 atoms and the 30%Cu/Zn1Al catalyst showed the highest activity. Moreover, this optimum catalyst exhibited superior activity, thermal stability, and long-term stability than a commercial Cu/ZnO/Al2O3 catalyst. It was considered that a synergetic effect between Cu metal and ZnAl2O4 spinel might exist and play a key role for the high catalytic performance. Keywords: Hydrogen production; Water-gas shift reaction; Copper-zinc-aluminum catalyst; 1

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Layered double hydroxides; 1. Introduction Water-gas shift (WGS) (eq.1) is an important industrial process in the hydrogen production from coal gasification and steam reforming of natural gas and naphtha, and has been widely applied in ammonia synthesis and petroleum refinery.1,

2

Since WGS is a

moderately exothermic reverse reaction, high CO conversion is thermodynamically favored at low temperatures. However, despite the thermodynamic favorability at low temperatures, the reaction is kinetically favored at high temperatures. In order to take advantage of both the thermodynamics and kinetics of the reaction, WGS is usually operated in two-stages consisting of a high-temperature shift (HTS, 573-723 K) followed by a low-temperature shift (LTS, 443-523 K). Typically, iron-based and copper-based catalysts are used for HTS and LTS, respectively. By combination of HTS and LTS, the CO concentration can be reduced to below 0.3%. CO + H2O ↔ CO2 + H2, ∆H298K = -41.2 kJ mol-1

(1)

Copper is one of the most active components for WGS and Cu/ZnO/Al2O3 catalysts have been used as industrial LTS catalysts since 1960s. Owing to the low melting point of copper metal (1356 K), copper has low Tammam and Huttig temperatures so that small Cu crystallites are susceptible to sintering. When heated above 573 K, Cu/ZnO/Al2O3 catalysts sinter and lose copper surface area and consequently catalytic activity. For this sake, Cu/ZnO/Al2O3 catalysts are required to activate in a well-controlled way and operate at sufficiently low temperatures. It was estimated that a rise of 0.1% CO in the LTS converter

2


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effluent would cause a production loss of ~30 T/day in a 3000 T/day of ammonia plant.2 From this point of view, the development of active and stable Cu catalyst is critically important. Although Cu/ZnO/Al2O3 catalysts have been commercialized for decades, the catalyst preparation and modification is still a subject of interest.3-15 Cu/ZnO/Al2O3 catalysts are usually prepared by co-precipitation method to achieve higher Cu dispersion and higher catalytic activity. Depending on the metal composition and preparation condition, different hydroxycarbonate precursors including malachite [Cu2(OH)2CO3], hydrozincite [Zn5(CO3)2(O H)6], rosasite [(Cu, Zn)2(OH)2CO3), aurichalcite [(Cu, Zn)5(CO3)2(OH)6], and/or hydrotalcite [(Cu, Zn)6Al2CO3(OH)16·4H2O] can be formed. Among these precursors, hydrotalcite appears a promising precursor.13,

14

Hydrotalcite-like compounds (HTlcs), also known as layered

double hydroxides (LDHs), are a class of anionic clays consisting of positively charged brucite-like layers and interlayer charge-balancing anions and water molecules, which can be expressed by the formula [M2+1-xM3+x(OH)2]x+(An-)x/n·mH2O.16, 17 A key structural feature of LDHs is that the M2+/M3+ cations are homogeneously distributed in the brucite-like layers and a variety of M2+/M3+ cations can be incorporated into the structure in a wide composition range. This unique property makes it possible to synthesize a variety of LDHs to prepare various mixed metal oxides and supported metal catalysts.18-25 The preparation of Cu/ZnO/Al2O3 catalysts from Cu-Zn-Al LDHs and their catalytic applications in WGS13, 14 and steam reforming of methanol26, 27 and ethanol28 have been reported. Gines et al.13 reported that the Cu dispersion and WGS activity of Cu/ZnO/Al2O3 catalysts were related to the amount of Cu-Zn-Al LDHs contained in the hydroxycarbonate precursor. The LDHs-derived Cu/ZnO/Al2O3 catalyst exhibited significantly higher WGS activity than the Cu/ZnO catalyst 3



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obtained from aurichalcite. However, the resulting Cu metal dispersion was quite low. In another study, Souza et al.14 reported that the Cu/ZnO/Al2O3 catalyst prepared from LDHs showed higher steady-state stability than the commercial Cu catalyst, which was attributed to the higher resistance to Cu sintering due to the strong interaction between Cu and ZnO. Nevertheless, the initial activity of the prepared catalyst was relatively lower as compared to that of commercial catalyst. As far as we known, no LDHs-derived Cu/ZnO/Al2O3 catalysts have been reported to show both high activity and stability for WGS reaction. A systemic study on the preparation, characterization, and catalytic performance of Cu/ZnO/Al2O3 catalysts would be desirable in order to achieve a high-performance Cu catalyst and to better understand the structure-performance relationship. In this work, the influence of chemical compositions of Cu-Zn-Al LDHs on the physicochemical and catalytic properties of Cu/ZnO/Al2O3 catalysts has been studied in detail, with an aim to optimize the catalyst composition to develop an active and stable Cu WGS catalyst. By adjusting the layer composition, a series of Cu-Zn-Al LDHs with different Cu contents and (Cu + Zn)/Al atomic ratios were synthesized. The prepared catalysts were characterized by ICP, N2 adsorption, SEM, XRD, H2-TPR, N2O chemisorption, and their catalytic performances including initial activity, thermal stability, and long-term stability were tested and compared with those of commercial catalyst. 2．．Experimental Section 2.1. Catalyst Preparation All chemicals including Cu(NO3)2·3H2O, Zn(NO3)2·6H2O, Al(NO3)3·9H2O, Na2CO3, and NaOH were analytical grade. Cu-Zn-Al LDHs were synthesized by co-precipitation method 4


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as reported previously.24 The nominal compositions of starting materials are given in Table S1 (Supporting Information). Typically, a 100 mL aqueous solution containing the desired amounts of Cu(NO3)2·3H2O, Zn(NO3)2·6H2O, and Al(NO3)3·9H2O was added drop-by-drop into a 100 mL aqueous solution of Na2CO3 under stirring at room temperature. During the precipitation, the pH of the solution was measured with a pH electrode and kept constant at pH = 10 ± 0.5 by adding an aqueous solution of NaOH (2M) with a peristaltic pump. When the precipitation was completed, the resulting precipitate was kept at ambient temperature for 24 h, and then filtered, washed with 1L de-ionized water to remove Na+ ions, and dried at 373 K for 12 h. The precipitate was ground to fine powders and then heated in air from ambient temperature to 773 K by 3 K min-1 and kept at 773 K for 5 h. The calcined product was pressed to a disk, and then crushed and sieved to particles with 20-40 mesh size (diameter 0.4-0.8 mm). The prepared catalysts are denoted as xCu/ZnyAl, where x is the weight percentage of metallic Cu in the reduced catalyst and y is the (Cu + Zn)/Al atomic ratio. For the sake of comparison, a commercial Cu/ZnO/Al2O3 catalyst (B207) was chosen, which contained ~43.2% CuO, 38.0% ZnO, and 16.6% Al2O3 (determined by XRF). The commercial catalyst was crushed and sieved to the desired particle size and used as received without any further treatment. 2.2. Catalyst Characterization The metal contents of the as-synthesized precipitates were measured by inductively coupled plasma (ICP) on OPTIMA 8000 after the sample was dissolved with diluted nitric acid. The BET specific surface area was measured by N2 physical adsorption at 77 K on Micromeritics ASAP 2020 instrument. The sample was pretreated at 453 K for 4 h in vacuum 5



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before N2 adsorption. Field emission scanning electron microscopy (FE-SEM) was carried out on a Hitachi-S4800 operated at 5.0 kV. Powder X-ray diffraction (XRD) patterns were recorded on PANalytical X’PertPro diffractometer using Co Kα radiation (λ = 0.1789 nm) operating at 40 kV and 40 mA. H2 temperature-programmed reduction (H2-TPR) was performed on Micromeritics AutoChem2920 instrument equipped with a thermal conductivity detector. 50 mg of sample was used and the H2-TPR profile was recorded in a 10% H2/Ar mixed gas (30 mL min-1) from ambient temperature to 873 K by 10 K min-1. N2O chemisorption was performed on Micromeritics AutoChem2920 instrument. 50 mg of sample was pre-treated with He gas flow at 573 K for 0.5 h, and then cooled to ambient temperature. A 3.35% N2O/He mixed gas was introduced to the sample and the temperature was raised to 363 K and kept for 0.5 h. After N2O chemisorption, the sample was purged with He and cooled to ambient temperature, and then the second H2-TPR was carried out. The H2 consumption in the second H2-TPR was used to calculate the amount of N2O chemisorption and the number of surface Cu0 atoms by assuming a stoichiometry of N2O/Cu0 = 0.5.29 The Cu dispersion was calculated by the ratio of the H2 consumption in the two H2-TPR measurements, and the Cu metal particle size was calculated by the equation: d (nm) = 1.1/dispersion.30 2.3. Catalytic Reaction The WGS reaction was performed on a fixed bed reactor under atmospheric pressure. 0.5 g of catalyst was placed between two layers of quartz granules inside a stainless steel tube (inner diameter 8 mm). The reaction temperature was measured with a thermocouple inserted into the center position of the catalyst bed. The activity of catalyst was tested at elevated 6


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temperatures from 453 to 623 K and evaluated at each temperature for 3 h. Prior to the reaction, the catalyst was pre-reduced with pure H2 (30 mL min-1) at 573 K for 0.5 h. After cooling to 453 K, the reaction was started by introducing the feed gas into the reactor. The composition of dry feed gas was 15%CO/55% H2/7%CO2/N2 and the flow rate was 66.7 mL min-1, corresponding to a space velocity of 8,000 mL g-1 h-1. The ratio of vapor to dry feed gas was 1 or 0.4. The reactor exit gas was passed through an iced water condenser to remove water and then analyzed with an online TCD-gas chromatography (Shimadzu GC-8A). The CO conversion is calculated by the equation: XCO (%) = (1-V'CO/VCO)/(1+V'CO) × 100%, where VCO is the CO content in the dry feed gas and V'CO is the CO content in the dry exit gas. The thermal stability of catalyst was tested as follows. When the initial activity test at 623 K was finished, the catalyst was further subjected to thermal aging at 623 K for 10 h under the reaction atmosphere; after that, the temperature was decreased to 453 K and then the activity was tested again from 453 K to 623 K. The long-term stability of catalyst was tested at 473 K for 100 h. 3. Results and Discussion 3.1. Characterizations of the LDHs-Cu/ZnAl catalysts 3.1.1 XRD measurements The structure of the as-synthesized precipitates and their evolution during the preparation steps were investigated by XRD measurements. Figure 1 shows the XRD patterns of the as-synthesized precipitates and their metal compositions measured by ICP are given in Table 1. In all cases, the metal compositions of the as-synthesized precipitates were similar to the nominal compositions (Table S1), suggesting that all of the Cu2+, Zn2+, and Al3+ cations 7



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were almost completely precipitated. The characteristic XRD pattern of LDHs was found on all precipitates. In the case of (Cu + Zn)/Al ≥ 1, the diffraction peaks could be indexed to Cu-Zn-Al LDHs (JCPDS 37-0629), while in the case of (Cu + Zn)/Al = 0.5 the diffraction peaks might be assigned to Cu-Al LDH (JCPDS 46-0099). No diffraction peaks associated with other hydroxycarbonates such as malachite, hydrozincite, rosasite, or aurichalcite were detected. Nevertheless, a few Al(OH)3 species (JCPDS 07-0324) were detected on the 30% Cu/Zn0.5Al and 30%Cu/Zn1Al precipitates. This is in agreement with the report that a M2+/M3+ atomic ratio equal or larger than 2 is needed to obtain pure LDHs.16 For M2+/M3+ ratios higher than 2, the Al octahedra remain distant one from the other because of the repulsion of positive charges, whereas for lower M2+/M3+ values the increased number of neighbouring Al octahedra leads to the formation of Al(OH)3. The lattice parameters of LDHs are given in Table 1. As the Cu content increased, a0 remained unchanged or decreased slightly. This is expected since the ionic radii of Cu2+ and Zn2+ were similar (0.073 nm for Cu2+ and 0.074 nm for Zn2+).31 As the (Cu + Zn)/Al atomic ratio varied from 0.5 to 4, both a0 and c0 increased. The increase of a0 was attributed to the larger ionic radius of Zn2+ than Al3+ (0.054 nm),31 while the increase of c0 might be attributed to the decreased electrostatic interaction between brucite-like layer and anionic interlayer as the excess positive charge induced by Al3+ was reduced. The formation of LDHs was also evidenced by SEM images, which showed the typical plate-like morphology of LDHs (Supporting Information, Fig. S1). Figure 2 shows the XRD patterns of the Cu-Zn-Al mixed oxides after calcination of the precipitates at 773 K. After calcination, the diffraction peaks of LDHs disappeared and those of ZnO (JCPDS 36-1451), CuO (JCPDS 48-1548), and spinel such as ZnAl2O4 (JCPDS 8


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01-074-1138) and/or CuAl2O4 (JCPDS 33-0448) appeared depending on the catalyst composition. For those samples with (Cu + Zn)/Al atomic ratio of 3, the peak intensity of ZnO decreased with increasing the Cu content, and the diffraction peaks of CuO appeared and became stronger at Cu content ≥30%, suggesting the formation of CuO crystallites at high Cu contents. On the other hand, for those samples with the same 30% Cu content, the phase composition changed significantly with the (Cu + Zn)/Al ratio. At (Cu + Zn)/Al ratio of 0.5, crystalline CuO was observed together with minor CuAl2O4, whereas at (Cu + Zn)/Al ratio of 1 only poorly crystalline CuO and ZnAl2O4 were detected. It was difficult to discriminate the CuAl2O4 and ZnAl2O4 spinels from XRD because of their similar lattice parameters and poor intensities, but the results of H2-TPR and XRD patterns of the reduced catalysts (vide infra) revealed great difference between CuAl2O4 and ZnAl2O4, that is, CuAl2O4 showed the reduction peak in H2-TPR and the disappearance of spinel XRD peaks after reduction, both of which did not occur for ZnAl2O4. Therefore, the spinel was assigned to CuAl2O4 for (Cu + Zn)/Al = 0.5 and ZnAl2O4 for (Cu + Zn)/Al = 1, respectively. By raising the (Cu + Zn)/Al ratio ≥2, the diffraction peaks of ZnO appeared and the peak intensities of ZnO and CuO increased with increasing the (Cu + Zn)/Al ratio. Particularly, the peak intensity of CuO was much higher on the 30%Cu/Zn4Al sample than on the other samples, suggesting the formation of larger CuO crystallites at low Al content. This is similar with the reports4, 13 that the addition of Al improved the CuO dispersion. Nevertheless, at higher Al content, i.e., (Cu + Zn)/Al = 0.5, large CuO crystallites were also formed. This might be related to the formation of Cu-Al LDH precursor, which had lower thermal stability due to the Jahn-Teller effect of Cu2+ ion.32, 33 From the XRD result, it seems that the (Cu + 9



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Zn)/Al ratio of 1 gave good dispersion of copper oxide. The calcined samples were reduced at 573 K with pure H2 and the XRD patterns of the reduced catalysts are presented in Figure 3. The diffraction peaks of CuO disappeared and those of metallic Cu (JCPDS 04-0836) were observed, indicating the reduction of copper oxide to Cu metal. Besides Cu metal, several broad and weak peaks attributable to Cu2O (JCPDS 01-077-0199) were also detected on the catalysts with higher Cu content (≥30%) and (Cu + Zn)/Al ratio (≥2). The presence of Cu2O was probably due to the partial oxidation of metallic Cu by contact with air. It was also possible that a fraction of Cu2+ species were partially reduced to Cu+ as a result of strong metal-support interaction (vide infra). 3.1.2. H2-TPR H2-TPR was performed to explore the reduction behavior of copper oxides and the strength of metal-support interaction. Figure 4 shows the H2-TPR profiles of LDHs-Cu/ZnAl and com-Cu/ZnAl as a reference. The H2 consumption was mainly observed below 573 K with the maximum at 470-500 K, attributable to the reduction of copper oxide. There was a high-temperature reduction peak at 532 K in sample 30%Cu/Zn0.5Al, which might be assigned to the Cu2+ species strongly interacted with Al2O3 forming a surface nonstoichiometric copper aluminate phase.27 This is in agreement with the presence of CuAl2O4-type spinel in the calcined sample and its disappearance after reduction (Fig. 2e and Fig. 3e). The amount of H2 consumption integrated from H2-TPR below 573 K and the calculated Cu reduction degree are given in Table 2. The Cu reduction degree was similar for the LDHs-Cu/ZnAl catalysts, which ranged from 71% to 85% assuming that Cu2+ was completely reduced to Cu0. This shows that a major amount of Cu could be reduced with H2, 10


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while a fraction of oxidized Cu was not available for reduction. Similar result has been reported by Behrens et al.34 They ascribed the unreduced Cu2+ species to those located at the grain boundaries between the Cu metal particles and the support oxides forming Cu-O bonds across the interface. Another possible explanation is the partial reduction of Cu2+ to Cu+. Shishido et al.3 have reported the presence of Cu+ together with Cu0 on the reduced Cu/ZnO and Cu/ZnO/Al2O3 catalysts. It is worth noting that the LDHs-Cu/ZnAl catalysts showed higher reduction temperature than the com-Cu/ZnAl catalyst, indicative of stronger metal-support interaction between Cu-Zn-Al mixed oxides. Since the Cu2+, Zn2+, and Al3+ cations were homogeneously distributed in the brucite-like layers of LDHs, calcination of LDHs would produce CuO/ZnO/Al2O3 mixed oxides in good contact with each other. Thus, a strong interaction between the CuO/ZnO/Al2O3 mixed oxides can be expected. 3.1.3. N2 adsorption and N2O chemisorption The BET specific surface areas of the calcined samples are shown in Table 2. The N2 adsorption-desorption isothermals and the pore size distribution curves are given in Fig. S2 (Supporting Information), which showed the type-IV isotherms and average pore diameter of 10-20 nm. The change of Cu content had no significant effect on the BET area. On the other hand, the BET area decreased remarkably with the increase of (Cu + Zn)/Al ratio, i.e., the BET area increased with the Al addition, evidencing the role of aluminum as a textural promoter. Both 30%Cu/Zn0.5Al and 30%Cu/Zn1Al samples showed much higher BET area than the other samples, which was consistent with the poor crystallinity of Cu-Zn-Al mixed oxides on these two samples. N2O chemisorption was performed to measure the number of surface Cu0 atoms and to 11



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calculate the Cu dispersion and Cu particle size, as shown in Table 2. Generally, the number of surface Cu0 atoms increased with increasing the Cu content except for 40%Cu/Zn3Al, whereas the Cu dispersion decreased and the Cu particle size increased. This is expected since Cu sintering tended to occur at higher Cu contents. On the other hand, the number of surface Cu0 atoms increased with increasing the (Cu + Zn)/Al ratio and reached the maximum at (Cu + Zn)/Al = 1; further increasing the (Cu + Zn)/Al ratio caused a decrease of surface Cu0 atoms. The Cu dispersion followed a similar tendency, while the Cu particle size showed a reverse tendency. Among the prepared catalysts, 30%Cu/Zn1Al exhibited the largest number of surface Cu0 atoms, which also showed the highest Cu dispersion among the catalysts with different (Cu + Zn)/Al ratios. It is worth noting that most of the LDHs-Cu/ZnAl catalysts showed higher Cu dispersion than the com-Cu/ZnAl catalyst, demonstrating the advantage of using Cu-Zn-Al LDHs as catalyst precursor. It should also be noted that the Cu dispersion of our LDHs-Cu/ZnAl catalysts was much higher than those ( 40%Cu/Zn3Al > 20%Cu/Zn3Al > 10%Cu/Zn3Al. By fixing the Cu content at 30% and varying the (Cu + Zn)/Al ratio from 0.5 to 4, the CO conversion increased with the increase of (Cu + Zn)/Al ratio reaching the maximum at (Cu + Zn)/Al = 1 and then decreased with further increasing the (Cu + Zn)/Al ratio (Fig. 5b). The catalyst activity was 30%Cu/Zn1Al > 30%Cu/Zn2Al > 30%Cu/Zn0.5Al > 30%Cu/Zn4Al. Among the prepared catalysts, the 30%Cu/Zn1Al catalyst showed the highest activity. The composition of the optimum catalyst was different with that reported by Gines et al,13 which might be related to the different catalyst preparation conditions. In Fig. 5c, the CO conversions were plotted as a function of the number of surface Cu0 atoms. It was observed that the CO conversion increased linearly with the number of surface Cu0 atoms. Based on the CO conversion and the number of surface Cu0 atoms, the turnover frequency (TOF) was calculated. Figure 5d shows that the TOF values were essentially constant, i.e., ~0.009 s-1 at 453 K and ~0.012 s-1 at 473 K, when the Cu particle size varied from 2.3 nm to 6.0 nm. This is in agreement with the result reported by Gines et al.,13 who concluded that the WGS reaction was a structure-insensitive reaction on Cu/ZnO/Al2O3 catalysts and the reaction rate was proportional to the Cu metal surface area. On the other hand, it should be noted that the optimum LDHs-30%Cu/Zn1Al catalyst showed much higher activity than the previously reported LDHs-30%Cu/Mg2Al catalyst under the same reaction conditions, although the latter had higher Cu dispersion (37.8%) and larger number of surface Cu0 atoms (1.3 mmol 13



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gcat-1).25 For LDHs-30%Cu/Mg2Al, TOF was calculated to be ~0.004 s-1 at 453 K and ~0.006 s-1 at 473 K, respectively, which was only about 0.5 times that of LDHs-30%Cu/Zn1Al. Similar low TOF value was also shown for the previously reported 30%Cu/Al2O3 catalyst,24 which was ~0.004 s-1 at 453 K and ~0.005 s-1 at 473 K, respectively. This suggests that not only the number of surface Cu0 atoms and Cu dispersion but also the interaction between Cu metal and ZnAl2O4 spinel might play an important role in determining the activity. To further demonstrate the efficiency of the LDHs-Cu/ZnAl catalysts, the catalytic performance of the optimum 30%Cu/Zn1Al catalyst including thermal stability and long-term stability was tested and compared with that of com-Cu/ZnAl catalyst. 3.2.2. Thermal stability Since Cu crystallites easily suffer from sintering at high temperatures leading to activity decrease, the thermal stability of the 30%Cu/Zn1Al catalyst was investigated. To accelerate the Cu sintering, the catalyst after the initial activity test was subject to a thermal aging at 623 K for 10 h under the reaction atmosphere. After the thermal aging, the activity was tested again and compared with the initial activity. Figure 6 shows the results obtained on the LDHs-30%Cu/Zn1Al and com-Cu/ZnAl catalysts. It can be seen that the initial activity of LDHs-30%Cu/Zn1Al was much higher than that of com-Cu/ZnAl. For com-Cu/ZnAl, the TOF value was calculated to be ~0.008 s-1 at 453 K and ~0.01 s-1 at 473 K, slightly lower than those of LDHs-30%Cu/Zn1Al. This might be related to the stronger metal-support interaction for LDHs-30%Cu/Zn1Al than for com-Cu/ZnAl as indicated from H2-TPR. Moreover, the activity of LDHs-30%Cu/Zn1Al remained almost unchanged after the thermal aging. In contrast, a significant decrease of activity was observed on the com-Cu/ZnAl 14


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catalyst, where the CO conversions at 453-573 K decreased ~8%-10%. This result clearly shows the superior thermal stability of 30%Cu/Zn1Al than com-Cu/ZnAl. 3.2.3. Long-term stability Figure 7 shows the long-term stabilities of 30%Cu/Zn1Al and com-Cu/ZnAl catalysts, which were tested at 473 K for 100 h. It was found that the 30%Cu/Zn1Al catalyst maintained a high level of CO conversion (~86%) during 100 h time-on-stream. In contrast, the com-Cu/ZnAl catalyst gradually lost its activity with time-on-stream; the CO conversion decreased from initial 60% to 51% after 100 h time-on-stream. This result well demonstrates the superior long-term stability of LDHs-30%Cu/Zn1Al than com-Cu/ZnAl. By the way, commercial Cu/ZnO/Al2O3 catalyst is relatively stable under standard LTS conditions, which is different with the result shown here. Nevertheless, it should be noted that commercial Cu/ZnO/Al2O3 catalyst is very sensitive toward the reduction treatments. In practical operation, the catalyst is reduced in a well controlled way, e.g., using low concentration H2 (≤2%) and keeping the reduction temperature below 493 K, to obtain finely dispersed Cu crystallites. However, in this study, the catalyst was reduced with pure H2 at temperature as high as 573 K. Such a severe reduction treatment might lead to larger Cu crystallites and weaken the interaction of Cu crystallites with ZnO and Al2O3 and consequently decrease the catalytic stability. For a reference, the commercial catalyst was reduced at 493 K with 10%H2/N2 and then tested for the WGS reaction. As shown in Fig. S3 (Supporting Information), the decrease of reduction temperature and H2 concentration enhanced the catalytic activity and stability to some degrees, although the catalyst was still deactivated with time on stream. Another point should be noted is that the LDHs-30%Cu/Zn3Al catalyst, 15



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which had similar physicochemical properties including XRD phases with the com-CuZnAl catalyst, was also deactivated under the same reaction conditions, where the CO conversion decreased from initial 71% to 62% after 10 h time on stream.24 This further demonstrates that the formation of ZnAl2O4 is critical for metal-oxide interaction for sustained activity. Figure 8 shows the XRD patterns of the two catalysts after different reaction tests. Different with the freshly reduced LDHs-30%Cu/Zn1Al, where poorly crystalline ZnAl2O4 spinel was formed, com-Cu/ZnAl was mainly consisted of relatively well crystallized ZnO as well as amorphous Al2O3 considering the catalyst composition. The crystalline structure of the spent catalysts was almost identical to that of the freshly reduced catalysts, but catalyst sintering occurred to some degree on both catalysts. The crystallite sizes of Cu metal and Zn-Al oxides calculated by the Scherrer equation are listed in Table 3. For com-Cu/ZnAl, the crystallite size of ZnO remained basically unchanged, whereas the crystallite size of Cu metal increased from 6.1 nm to 8.7-10.7 nm. Thus, sintering of Cu crystallites was considered to be mainly responsible for the catalyst deactivation. For LDHs-30%Cu/Zn1Al, a larger extent of sintering of Cu crystallites and ZnAl2O4 spinel took place. Surprisingly, in spite of the serious sintering, this catalyst remained quite stable activity, as shown in Figs. 6 and 7. Although the reason was not clear at this moment, we considered that the interaction between Cu metal and ZnAl2O4 might play an important role. This was in part supported by our result that the LDHs-30%Cu/Zn1Al catalyst showed much higher activity than the LDHs-30%Cu/Mg2Al catalyst despite its Cu dispersion was relatively lower. In addition, the results of XRD and H2-TPR showed the possibility of the presence of Cu+ species on the reduced catalysts. Studies on Cu/ZnO-based catalysts indicate that Cu+ species may form at the boundary 16


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between Cu metal particles and ZnO particles acting as the active sites for methanol synthesis and WGS reaction, in which ZnO may play an active role either by stabilizing active Cu+ species in the ZnO matrix or by creating synergetic effects with metal Cu sites.35-37 To clarify the interaction between Cu metal and ZnAl2O4, further study including spectra measurements is necessary. 4. Conclusions (1) The structural and physicochemical properties of LDHs precursors and resulting Cu/ZnO/Al2O3 catalysts were significantly affected by the catalyst composition. Depending on the (Cu + Zn)/Al atomic ratio, Cu-Zn-Al LDHs or Cu-Al LDHs could be formed. Upon calcination, these LDHs gave rise to mixed metal oxides consisted of CuO, ZnO, ZnAl2O4, CuAl2O4, and/or amorphous Al2O3, and after reduction well dispersed Cu metal particles with 18-48% dispersion and 2-6 nm size were formed. For the given Cu content (30%), (Cu + Zn)/Al ratio of 1 provided higher Cu dispersion, at which ZnAl2O4 spinel supported ~4.0 nm Cu metal particles were obtained. (2) The initial WGS activity of Cu/ZnO/Al2O3 catalysts was proportional to the number of surface Cu0 atoms and the highest activity was obtained on the 30%Cu/Zn1Al catalyst. The activity of LDHs-30%Cu/Zn1Al was also higher than that of LDHs-30%Cu/Mg2Al despite its Cu dispersion was relatively lower. This suggests that a synergetic effect between Cu metal and ZnAl2O4 spinel might exist and play an important role in determining the activity. (3) The optimum LDHs-30%Cu/Zn1Al catalyst exhibited superior catalytic performance than the commercial Cu/ZnO/Al2O3 catalyst. Not only the initial activity but also the thermal stability and long-term stability were much higher for LDHs-30%Cu/Zn1Al. Interestingly, 17



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this catalyst remained quite stable activity even the catalyst sintering occurred. It is likely that the interaction between Cu metal and ZnAl2O4 spinel was a key for the high catalytic performance. Acknowledgements This work was financially supported by National High Technology Research and Development Program of China (No.2015AA03A402), National Nature Science Foundation of China (No. 21576052), and Natural Science Foundation of Fujian Province (No. 2015J01050). Supporting Information Staring composition of the samples; SEM images of the as-synthesized precursors; N2 adsorption-desorption isothermals and pore size distribution curves of the calcined samples; WGS long-term stability of commercial Cu/ZnO/Al2O3 catalyst reduced at 493 K with 10%H2/N2. References (1) LeValley, T. L.; Richard, A. R.; Fan, M. The progress in water gas shift and steam reforming hydrogen production technologies–A review. Int. J. Hydrogen Energy 2014, 39 (30), 16983-17000. (2) Ratnasamy, C.; Wagner, J. P. Water gas shift catalysis. Catal. Rev. Sci. Eng. 2009, 51, 325-440. (3) Shishido, T.; Yamamoto, M.; Li, D.; Tian, Y.; Morioka, H.; Honda, M.; Sano, T.; Takehira, K. Water-gas shift reaction over Cu/ZnO and Cu/ZnO/Al2O3 catalysts prepared by homogeneous precipitation. Appl. Catal. A: Gen. 2006, 303 (1), 62-71. 18


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(4) Atake, I.; Nishida, K.; Li, D.; Shishido, T.; Oumi, Y.; Sano, T.; Takehira, K. Catalytic behavior of ternary Cu/ZnO/Al2O3 systems prepared by homogeneous precipitation in water-gas shift reaction. J. Mol. Catal. A: Chem. 2007, 275 (1-2), 130-138. (5) Nishida, K.; Atake, I.; Li, D.; Shishido, T.; Oumi, Y.; Sano, T.; Takehira, K. Effects of noble metal-doping on Cu/ZnO/Al2O3 catalysts for water-gas shift reaction. Appl. Catal. A: Gen. 2008, 337, 48-57. (6) Nishida, K.; Li, D.; Zhan, Y.; Shishido, T.; Oumi, Y.; Sano, T.; Takehira, K. Effective MgO surface doping of Cu/Zn/Al oxides as water-gas shift catalysts. Appl. Clay Sci. 2009, 44, 211-217. (7) Tanaka, Y.; Utaka, T.; Kikuchi, R.; Sasaki, K.; Eguchi, K. CO removal from reformed fuel over Cu/ZnO/Al2O3 catalysts prepared by impregnation and coprecipitation methods. Appl. Catal. A: Gen. 2003, 238 (1), 11-18. (8) Guo, P.; Chen, L.; Yang, Q.; Qiao, M.; Li, H.; Li, H.; Xu, H.; Fan, K. Cu/ZnO/Al2O3 water-gas shift catalysts for practical fuel cell applications: the performance in shut-down/start-up operation. Int. J. Hydrogen Energy 2009, 34, 2361-2368. (9) Figueiredo, R. T.; Ramos, A. L. D.; Andrade, H. M. C.; Fierro, J. L. G. Effect of low steam/carbon ratio on water gas shift reaction. Catal. Today 2005, 107–108, 671-675. (10) Figueiredo, R. T.; Andrade, H. M. C.; Fierro, J. L. G. Influence of the preparation methods and redox properties of Cu/ZnO/Al2O3 catalysts for the water gas shift reaction. J. Mol. Catal. A: Chem. 2010, 318, 15-20. (11) Figueiredo, R. T.; Santos, M. S.; Andrade, H. M. C.; Fierro, J. L. G. Effect of alkali cations on the CuZnOAl2O3 low temperature water gas-shift catalyst. Catal. Today 2011, 172, 19



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166-170. (12) Fu, W.; Bao, Z. H.; Ding, W. Z.; Chou, K. C.; Li, Q. The synergistic effect of the structural precursors of Cu/ZnO/Al2O3 catalysts for water-gas shift reaction. Catal. Commun. 2011, 12, 505-509. (13) Gines, M. J. L.; Amadeo, N.; Laborde, M.; Apesteguia, C. R. ACTIVITY AND STRUCTURE-SENSITIVITY OF THE WATER-GAS SHIFT REACTION

OVER

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(27) Murcia-Mascarós, S.; Navarro, R. M.; Gómez-Sainero, L.; Costantino, U.; Nocchetti, M.; Fierro, J. L. G. Oxidative Methanol Reforming Reactions on CuZnAl Catalysts Derived from Hydrotalcite-like Precursors. J. Catal. 2001, 198 (2), 338-347. (28) Cunha, A. F.; Wu, Y. J.; Santos, J. C.; Rodrigues, A. E. Steam Reforming of Ethanol on Copper Catalysts Derived from Hydrotalcite-like Materials. Ind. Eng. Chem. Res. 2012, 51, 13132-13143. (29) Chen, C. S.; Lai, Y. T.; Lai, T. W.; Wu, J. H.; Chen, C. H.; Lee, J. F.; Kao, H. M. Formation of Cu nanoparticles in SBA-15 functionalized with carboxylic acid groups and their application in the water-gas shift reaction. ACS Catal. 2013, 3, 667-677. (30) Dandekar, A.; Vannice, M. A. Determination of the dispersion and surface oxidation states of supported Cu catalysts. J. Catal. 1998, 178 (2), 621-639. (31) Shannon, R. D. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta. Crystallogr. A: Found. Crystallogr. 1976, 32, 751-767. (32) Alejandre, A.; Medina, F.; Salagre, P.; Correig, X.; Sueiras, J. E. Preparation and study of Cu-Al mixed oxides via hydrotalcite-like precursors. Chem. Mater. 1999, 11, 939-948. (33) Britto, S.; Vishnu, K. P. Thermal, solution and reductive decomposition of Cu-Al layered double hydroxides into oxide products. J. Solid State Chem. 2009, 182, 1193-1199. (34) Behrens, M.; Kasatkin, I.; Kuehl, S.; Weinberg, G. Phase-Pure Cu,Zn,Al Hydrotalcite-like Materials as Precursors for Copper rich Cu/ZnO/Al2O3 Catalysts. Chem. Mater. 2010, 22, 386-397. (35) Kanai, Y.; Watanabe, T.; Fujitani, T.; Uchijima, T.; Nakamura, J. The synergy between 22


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Cu and ZnO in methanol synthesis catalysts. Catal. Lett. 1996, 38 (3-4), 157-163. (36) Jansen, W. P. A.; Beckers, J.; Heuvel, J. C.; Denier Gon, A. W.; Bliek, A.; Brongersma, H. H. Dynamic behavior of the surface structure of Cu/ZnO/SiO2 catalysts. J. Catal. 2002, 210 (1), 229-236. (37) Naumann d'Alnoncourt, R.; Kurtz, M.; Wilmer, H.; Löffler, E.; Hagen, V.; Shen, J.; Muhler, M. The influence of ZnO on the differential heat of adsorption of CO on Cu catalysts: a microcalorimetric study. J. Catal. 2003, 220 (1), 249-253.

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Table 1 Chemical compositions and structural properties of the as-synthesized precursors Metal composition of the precursors a (at.%)

(Cu + Zn)/Al

Cu

Zn

Al

atomic ratio a

10%Cu/Zn3Al

12.0

63.6

24.4

3.1

20%Cu/Zn3Al

23.2

52.2

24.6

30%Cu/Zn3Al

34.5

41.0

40%Cu/Zn3Al

46.3

30%Cu/Zn0.5Al

Catalysts

Phases detected by XRD

Lattice parameters of LDHs (nm) a0 b

c0 c

Cu-Zn-Al LDH

0.3072

2.2758

3.1

Cu-Zn-Al LDH

0.3072

2.2758

24.5

3.1

Cu-Zn-Al LDH

0.3070

2.2756

29.1

24.6

3.1

Cu-Zn-Al LDH

0.3066

2.2756

29.6

4.8

65.6

0.6

Cu-Al LDH, poorly crystalline Al(OH)3

0.2936

2.2398

30%Cu/Zn1Al

31.4

19.5

49.1

1.0

Cu-Zn-Al LDH, poorly crystalline Al(OH)3

0.3050

2.2425

30%Cu/Zn2Al

34.0

33.5

32.5

2.1

Cu-Zn-Al LDH

0.3068

2.2518

30%Cu/Zn4Al

34.8

44.8

20.3

3.9

Cu-Zn-Al LDH

0.3078

2.2707

a

Determined by ICP analysis; b a0 = 2d110; c c0 = 3d003 (Fig. 1);

24


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Table 2 Physicochemical property of the Cu/ZnO/Al2O3 catalysts Catalyst

Cu content a

Surface area

H2 consumption b

Reduction degree

Number of surface Cu0

Cu dispersion e

Particle size f

(mmol gcat−1)

(m2 gcat-1)

(mmol g−1)

of Cu c (%)

atoms d (mmol gcat−1)

(%)

(nm)

10%Cu/Zn3Al

1.64

76

1.17

71.3

0.56

47.9

2.3

20%Cu/Zn3Al

3.16

73

2.51

79.4

0.80

31.9

3.4

30%Cu/Zn3Al

4.69

65

3.75

79.9

0.88

23.5

4.7

40%Cu/Zn3Al

6.32

74

4.70

74.4

0.86

18.3

6.0

30%Cu/Zn0.5Al

5.25

150

3.95

75.2

0.84

21.3

5.2

30%Cu/Zn1Al

5.13

107

4.05

78.9

1.10

27.2

4.0

30%Cu/Zn2Al

4.76

72

4.04

84.9

0.90

22.3

4.9

30%Cu/Zn4Al

4.67

63

3.76

80.5

0.74

19.7

5.6

com-Cu/ZnAl

5.43

65

3.78

69.6

0.82

21.7

5.1

a

Calculated in the calcined mixed oxides based on the metal compositions determined by ICP analysis;

b

H2 consumption in the TPR profiles below 573 K;

c

Calculated by the equation: reduction degree (%) = H2 consumption/Cu content × 100%;

d

Measured by N2O chemisorption and H2-TPR;

e

Calculated by the equation: dispersion (%) = number of surface Cu0 atoms/H2 consumption × 100%;

f

Calculated by the equation: particle size (nm) = 1.1/ dispersion (%)[30];

25



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Table 3 Crystallite sizes of Cu metal and Zn-Al oxides on the catalysts after different reaction tests Catalyst state

After reduction

LDHs-30%Cu/Zn1Al

com-Cu/ZnAl

Cu a (nm)

Cu a (nm)

ZnAl2O4 b (nm)

ZnO c (nm)

6.3

4.4

6.1

11.4

After initial activity test

14.8

7.8

10.7

11.9

After thermal stability test

14.8

9.3

10.0

12.3

9.4

8.5

8.7

11.2

After long-term stability test a

Calculated based on the diffraction peak of Cu metal at 2θ = 50.7º;

b

Calculated based on the diffraction peak of ZnAl2O4 at 2θ = 42.8º;

c

Calculated based on the diffraction peak of ZnO at 2θ = 37.0º.

26


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Figure 1 XRD patterns of the as-synthesized Cu-Zn-Al precipitates: (a) 10%Cu/Zn3Al, (b) 20%Cu/Zn3Al, (c) 30%Cu/Zn3Al, (d) 40%Cu/Zn3Al, (e) 30%Cu/Zn0.5Al, (f) 30%Cu/Zn1Al, (g) 30%Cu/Zn2Al, and (h) 30%Cu/Zn4Al. Crystalline phases: (●) LDH and (○) Al(OH)3.

27 ACS Paragon Plus Environment


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Figure 2 XRD patterns of the Cu-Zn-Al mixed oxides after calcination: (a) 10%Cu/Zn3Al, (b) 20%Cu/Zn3Al, (c) 30%Cu/Zn3Al, (d) 40%Cu/Zn3Al, (e) 30%Cu/Zn0.5Al, (f) 30%Cu/Zn1Al, (g) 30%Cu/Zn2Al, and (h) 30%Cu/Zn4Al. Crystalline phases: (*) ZnO, (▲) CuO, (□) CuAl2O4, and (◊) ZnAl2O4.


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Figure 3 XRD patterns of the Cu-Zn-Al catalysts after reduction: (a) 10%Cu/Zn3Al, (b) 20%Cu/Zn3Al, (c) 30%Cu/Zn3Al, (d) 40%Cu/Zn3Al, (e) 30%Cu/Zn0.5Al, (f) 30%Cu/Zn1Al, (g) 30%Cu/Zn2Al, and (h) 30%Cu/Zn4Al. Crystalline phases: (*) ZnO, (□) γ-Al2O3, (◊) ZnAl2O4, (■) Cu metal, and (∆) Cu2O.



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Figure 4 H2-TPR profiles of the calcined samples: (a) 10%Cu/Zn3Al, (b) 20%Cu/Zn3Al, (c) 30%Cu/Zn3Al, (d) 40%Cu/Zn3Al, (e) 30%Cu/Zn0.5Al, (f) 30%Cu/Zn1Al, (g) 30%Cu/Zn2Al, (h) 30%Cu/Zn4Al, and (i) com-Cu/ZnAl.


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Figure 5 Influence of (a) Cu content and (b) (Cu + Zn)/Al ratio on the initial WGS activity of Cu/ZnO/Al2O3 catalysts, (c) CO conversion as a function of surface Cu0 atoms, (d) dependence of TOF on Cu particle size. Reaction conditions: 15%CO/55%H2/7%CO2/N2, WHSV = 8,000 mL g-1 h-1 (dry-gas base), vapor/gas = 1; catalyst, 0.5 g, pre-reduced with H2 at 573 K for 0.5 h.



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Figure 6 Thermal stabilities of (○, ●) LDHs-30%Cu/Zn1Al and (□, ■) com-Cu/ZnAl catalysts: (open) freshly reduced, (solid) after thermal aging at 623 K for 10 h. Reaction conditions were the same with those in Fig. 5.


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Figure 7 Long-term stabilities of (○) LDHs-30%Cu/Zn1Al and (□) com-Cu/ZnAl catalysts for WGS at 473 K. The other conditions were the same with those in Fig. 5.



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Figure 8 XRD patterns of (A) LDHs-30%Cu/Zn1Al and (B) com-Cu/ZnAl catalysts: (a) after reduction, (b) after initial activity test, (c) after thermal stability test, and (d) after long-term stability test. Crystalline phases: (■) Cu metal, (◊) ZnAl2O4, (*) ZnO, and (+) SiO2.


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For Table of Contents Only Cu-Zn-Al-OH Calcination Cu-Zn-Al-OH Reduction Cu-Zn-Al-OH Cu-Zn-Al LDHs

Cu NPs@ZnAl2O4


Al2O3 Water

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