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Fe layered double hydroxides
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Experimental and theoretical investigations into the activity and mechanism of the wateregas shift reaction catalyzed by Au nanoparticles supported on ZneAl/Cr/Fe layered double hydroxides Yue Meng a, Yufu Chen b, Xiaobo Zhou c, Guoxiang Pan a, Shengjie Xia b,* a
School of Life Science, Huzhou University, 759 East Erhuan Road, Huzhou, 313000, PR China Department of Chemistry, College of Chemical Engineering, Zhejiang University of Technology, 18 Chaowang Road, Hangzhou, 310014, PR China c Entegris, Inc., 129 Concord Road, Billerica, MA, 01821, USA b
highlights
graphical abstract
Au NPs supported on LDHs were prepared as highly efficient and stable catalyst for WGSR. Incorporation of Au NPs significantly reduces Ea and enhances activity of LDHs for WGSR. The activity: Au/ZnCr-LDHs>Au/ ZnFe-LDHs>Au/ZnAl-LDHs>LDHs, the energy barrier is just opposite. Addition of Au alters the redox circles, which is a key intermediary
process
involved
in
the
catalysis. Redox mechanisms b is the most potential reaction pathway and perfectly supports in situ DRIFTS results.
article info
abstract
Article history:
Wateregas shift reaction (WGSR) is an industrialized reaction with numerous applications
Received 27 July 2019
concerning CO removal and H2 generation. Since this process is widely used and occurs at
Received in revised form
elevated temperatures, the development of high efficiency and stable catalysts for WGSR
26 September 2019
and investigating their catalytic mechanism is a hot topic. In this paper, we demonstrate
Accepted 21 October 2019
Au nanoparticles supported on different layered double hydroxides (LDHs) as highly effi-
Available online xxx
cient and stable catalysts for WGSR. The incorporation of Au nanoparticles significantly decreases the activation energy and enhances the catalytic activity of LDHs for WGSR, with Au/ZnCreLDHs exhibiting the best catalytic performance including: 79.4% CO conversion,
* Corresponding author. E-mail addresses: [email protected], [email protected] (S. Xia). https://doi.org/10.1016/j.ijhydene.2019.10.172 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Meng Y et al., Experimental and theoretical investigations into the activity and mechanism of the wateregas shift reaction catalyzed by Au nanoparticles supported on ZneAl/Cr/Fe layered double hydroxides, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.172
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Keywords: Layered double hydroxides Au supporting Water gas shift reaction (WGSR) Activity and reaction mechanism DFT calculation
1 of reaction rate, 1.01 s1 TOF values and 41.7 kJ mol1 of activation 102.1 mmol g1 cat s
energy. TPR experiments suggest that the addition of Au alters the redox cycle on the surface of the catalyst, a key intermediary step involved in the catalytic process. In situ DRIFTS shows that the production of CO2 during WGSR involves the reaction between CO and adsorbed O, which comes from the dissociation of OH species and not the decomposition of formates. DFT calculations indicate that Auebased catalysts can effectively lower the energy barrier of the kinetically relevant step of H2O dissociation, which is the most probable reason for the enhancement of activity. The calculated activation barriers coincide with the experimentally measured values with the order of Au/ZnCreLDHs
Introduction Clean energy can effectively solve the problem of energy shortage and environmental pollution. Of the many options, H2 is the most popular clean energy fuel candidate and has been intensively studied to try and address energy shortage and environmental protection considerations [1e3]. The water gas shift reaction (WGSR): H2O(g)þCO(g) ¼ CO2(g)þH2(g), is a powerful tool to convert industrial waste CO gas and at the same time generate H2. In addition, the other product CO2 is also a very important substance for industry application, such as solardriven photocatalytic conversion of CO2 to valuable fuels (CH4) [4], reverse water gas shift reaction to generate CO [5], synthesis of cyclic carbonates from CO2 and epoxides [6], and so on. This reaction gives the opportunity for both reducing pollution and generating a clean energy fuel [7e9]. The success of WGSR relies on finding a catalyst that can operate under moderate conditions. Traditional catalysts, such as FeeCr catalysts, require high operating temperatures higher than 300 C and have generally low activity [10,11]. CueZn and CoeMo based catalysts operate at lower reaction temperatures but tend to quickly lose activity [12,13]. Based on the disadvantages of currently used catalysts, developing new, more efficient WGSR catalysts has been drawing great attention. Au based catalysts have been one of the most popular research targets in recent years due to their high activity and low operating temperatures [14,15]. Despite these benefits, Au nanoparticles (NPs) have poor dispersive properties and are prone to aggregation, which causes the loss of its activity and selectivity and prevents this type of catalyst from being industrialized [16]. One solution for this issue is to support Au based catalyst with another carrier catalyst [17,18]. Gamboa et al. found that AueCo3O4/CeO2 has better reduction activity compared with Ce3O4/CeO2, and Au could greatly enhance WGSR activity [19]. Stere et al. used nonethermal plasma activation to enable lowetemperature wateregas shift over a type of Au/CeZrO4 catalysts. They found that Au/CeZrO4
showed high performance for WGSR at 100e250 C temperature range, with CO conversion reaching a maximum of 91% at 240 C [20]. Thuy et al. compared performance of catalysts with different Au/Pt ratios on CeO2 and TiO2 supports, and found that 90% CO could be converted under 300 C with 5% of both Au and Pt, achieving a good balance between reaction temperature and activity for WGSR [21]. Even with these impressive results, there still remains the need to understand the activity, selectivity and stability of these catalysts [22,23]. With these types of Au catalysts for WGSR, the challenge in the development of a good catalyst depends on the development of a good carrier support. Theoretical calculations can be used for determining the catalytically active species in a reaction, which helps guide the design of new catalysts. In recent years, the theoretical investigation of the WGSR mechanism has been studied intensively [24e26]. Saqlain et al. examined the WGSR at a CueAu bimetallic surface with density functional theory (DFT). Their results showed that the CueAu(100) surface has higher activity compared to Cu(100), and the reaction on the CueAu surface reduced the water dissociation energy [27]. Wu et al. found that a Co6@Au32 coreeshell nanoalloy has better electron activity that Au38. Their research showed that the core Co atoms can control both the surface geometry of the Au atoms and the electronic structure near the Fermi energy [28]. Lin et al. calculated the activation energy of the WGSR elemental reaction on Cu(111), Pt(111) and Au(111) surfaces, and concluded that the activation energy is reduced on all three catalysts with Au(111) having the lowered energy gap during the CO*þO* ¼ CO2*þ* step. The fast formation and desorption of CO2* made the reaction easier for reductioneoxidation to occur [29]. But, research on effective catalysts for the WGSR mechanism has been mostly been focused on crystal phases or particle clusters of single metal or bimetallic alloy systems. The study on Au NPs supported on high surface area materials is still an area to be investigated [30]. Our previous research showed that Zn based LDHs has high chemical stability, high specific surface area and
Please cite this article as: Meng Y et al., Experimental and theoretical investigations into the activity and mechanism of the wateregas shift reaction catalyzed by Au nanoparticles supported on ZneAl/Cr/Fe layered double hydroxides, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.172
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excellent catalytic activity [16,31], which can be used for supporting Au nanoparticles [32,33]. In this paper, we synthesized three ZnMeLDHs (M ¼ Al, Cr, Fe) as the base material to support Au NPs to study their efficacy as WGSR catalysts. The activity and reaction mechanism for WGSR on ZnMeLDHs and the Au supported ZnMeLDHs (M ¼ Al, Cr, Fe) are discussed. We found that the introduction of the Au NPs could effectively increase the activity and decrease the activation energy of WGSR. We also studied the relationship between the composition of the catalyst and its activity. In addition, the catalytic pathway is postulated using results from in situ DRIFTS and DFT calculations. The calculations also gave the most probable reaction pathway for WGSR catalyzed by Au/ZnMeLDHs (M ¼ Al, Cr, Fe) based on the comparison of reaction energy barriers.
Experimental procedures of water gas shift reaction (WGSR)
Experimental and theoretical calculations
The evaluation of catalytic activity was performed in a small fixedebed reactor under atmospheric pressure. The reactor was a microequartz glass tube with an interior diameter of 6 mm. Before evaluation, 100 mg of the catalyst was pre treated at 300 C for 2 h in a gaseous mixture of 10/90 vol% H2/ Ar with a flow rate of 20 mL min1, and then cooled to room temperature under an Ar atmosphere. Water was fed into the vaporizer (300 C) with a higheperformance liquid pump, and the resulting steam was mixed with reactant gas before entering the reactor. After that, a mixture of 3 vol % CO, 15 vol % H2O and Ar was introduced into the reactor with a flow rate of 150 mL min1, the corresponding space velocity being 1 90000 mL g1 cat h . The CO conversion was tested at a range of 150e300 C. The concentration of inlet (nin CO ) and outlet gases (nout CO ) was analyzed by an ineline gas chromatograph (GC2014C) with TDX01 column and TCD detector. The conversion of CO (XCO) can be calculated by equation (1):
Synthesis of Au/ZnMeLDHs (M¼Al, Cr, Fe)
XCO ¼
The detailed list of materials and instrumentation is given in Supporting Information (SI).
Turnover frequency (TOF) and specific reaction rate (r) for WGSR catalyzed by ZnMeLDHs or Au/ZnMeLDHs (M ¼ Al, Cr, Fe) catalysts at different temperatures were determined by decreasing the weight of catalyst (from 100 mg to 25 mg) as well as cutting down the reaction time to guarantee CO conversion below 20%. For each run at a specific temperature, the CO conversions were averaged at the steady state and used to obtain the specific reaction rates, which were calculated by equation (2):
Preparation of ZnMeLDHs (M¼Al, Cr, Fe) The synthetic procedure of ZnAleLDHs is similar with the references [34,35], which is given as follows: solution A and B were added with drop by drop into a 250 mL threeeneck flask containing 50 ml deionized water at room temperature, in which solution A contains 50 mL deionized water, 6.40 g of NaOH and 1.06 g of Na2CO3, and solution B has 50 mL deionized water, 17.82 g of Zn(NO3)2$6H2O (0.06 mol) and 7.50 g of Al(NO3)3$9H2O (0.02 mol). The pH of the reaction mixture was maintained at the range of 9.0e10.0 with the stirring rate of 300 rpm for 20 min. After that, the resulting slurry was held at 80 C for 18 h and then centrifuged for 5 min with 5000 rpm., The resulting solid was washed with deionized water until the pH reached 7.0. Finally, the sample was dried under vacuum at 60 C for 24 h. The catalyst powder was then ground and ready for use. When preparing ZnCreLDHs and ZnFeeLDHs, 8.00 g of Cr(NO3)3$9H2O (0.02 mol) or 8.10 g of Fe(NO3)3$9H2O (0.02 mol) were used in solution B, other steps are similar with the synthesis of ZnAleLDHs.
Preparation of Au/ZnMeLDHs (M ¼ Al, Cr, Fe) 2.0 g of asesynthesized ZnMeLDHs (M ¼ Al, Cr, Fe) was added into a 250 mL flask containing 120 ml of a 5 mM HAuCl4 aqueous solution. The mixture was then stirred with a rate of 300 rpm for 10 min. After that, 2 mL of aqueous NH3 (10%) was added into the reaction system and the resulting mixture was stirred at room temperature for 24 h. The resulting slurry was centrifugated for 5 min at 5000 rpm, washed with 50 ml deionized water for three times, and dried at room temperature under vacuum. Then, 1.0 g of the dried solid was dispersed into 100 ml deionized water and treated with 50 mg NaBH4 at room temperature for 2 h. After centrifugation, washing and drying, the final Au supported LDHs catalyst (Au/ ZnMeLDHs) was obtained.
rCO ¼
out nin CO nCO 100% in nCO
XCO ,fCO mAu
(1)
(2)
where f CO is the molar flow rate of CO in mol h1 and mAu is the mass of Au in the fixed bed. The value of turnover frequency (TOF) was calculated by equation (3) based on the specific reaction rates and dispersions of Au species. The dispersions of Au species were determined by CO chemisorption with the assumption of the stoichiometric ratio of adsorbed CO/Au¼1: TOF ¼
rCO ,MAu DAu
(3)
where MAu is the molar weight of Au, DAu is the dispersion of Au calculated from the results of CO pulse chemisorption by assuming that the stoichiometric ratio of adsorbed CO/Pt is 1. The apparent activation energy (Ea) of the catalysts was determined using the Arrhenius equation (4): Ea
k ¼ AeRT
(4)
Calculation methods and parameters of Au/ZnMeLDHs (M¼Al, Cr, Fe) Zn3M1/LDHs (M ¼ Al, Cr, Fe) is represented by the formula of [Zn12M4(OH)32]4þ, its structure has D3d symmetry with R3m space group. The Zn and M elements in the molecular cluster model have a structure of ZnM ¼ 3 in a single LDH layer, M is surrounded by six Zn to form a hexagon structure. The
Please cite this article as: Meng Y et al., Experimental and theoretical investigations into the activity and mechanism of the wateregas shift reaction catalyzed by Au nanoparticles supported on ZneAl/Cr/Fe layered double hydroxides, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.172
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Fig. 1 e Structural model of Au/ZnMeLDHs (M ¼ Al, Cr, Fe): (A) front view; (B) top view; XRD patterns of different samples (C): (a) ZnAleLDH, (b) Au/ZnAleLDH; (c) ZnCreLDH, (d) Au/ZnCreLDH; (e) ZnFeeLDH, (f) Au/ZnFeeLDH. octahedron unit has a positive charge since Zn2þ is replaced by M3þ which separates M ions apart from each other. The NO 3 anion serves to neutralize the positive charges at the surface of the hydrotalcite laminate. The optimized structure is expanded along the c direction to construct a 2 2 1 unit cell for the DFT simulation, with the unit cell having the formula [Zn12M4(OH)32](NO3)4. After optimization, an Au3 clustereloaded hydrotalcite laminate model was constructed based on the reason that Au elements tend to coordinate with three oxygen atoms anchored between the top of the metal element and the LDHs laminate (as shown in Fig. 1a and b) [36]. Dmol3 module in Materials Studio 8.0 was used for the geometric optimization and analysis with the GGA based PerdeweBurkeeErnzerhof (PBE) was used as the electron exchange functional. The inner layer electrons were simplified by frozen core approximation method with ECP, the valence electron wave function expansion used dual numerical polarization (DNP) function without further electron spin restriction. The smearing values was set at 0.005 hartree, SCF tolerance was 1 106 Ha, and the kepoint value of the Brillouin zone was 6 6 1. The criterion of the geometric
optimization is as follows: the energy difference should be lower than 1 105 hartrees, the force on each atom is lower A and the atomic displacement is lower than 2 103 hartree/ A [37]. than 5.00 103
Results and discussion Structural characteristics of Au/ZnMeLDHs (M ¼ Al, Cr, Fe) Fig. 1c shows the XRD patterns of ZnAleLDHs, ZnCreLDHs, ZnFeeLDHs, and Au/ZnMeLDHs (M ¼ Al, Cr, Fe). The characteristic peaks of three LDHs that corresponding to facets 003, 006, 009(012), 015, 018, 110 and 113, can be observed, typical of the LDHs structure [38e40]. In the case of Au supported samples, the XRD pattern of Au/ZnMeLDHs not only retains the specific peaks from LDHs, but new peaks also appear at about 38.0 (2q), 44.0 and 64.5 that represent the 111, 200 and 220 facets of Au NPs (JCPDS 04e0784), suggesting the successful loading of Au into the LDH structure [41,42]. There is no obvious separate peak of Au 111 in the Au/
Please cite this article as: Meng Y et al., Experimental and theoretical investigations into the activity and mechanism of the wateregas shift reaction catalyzed by Au nanoparticles supported on ZneAl/Cr/Fe layered double hydroxides, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.172
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Fig. 2 e TEM images of: (a) ZnAleLDH, (b) Au/ZnAleLDH; (c) ZnCreLDH, (d) Au/ZnCreLDH; (e) ZnFeeLDH, (f) Au/ZnFeeLDH.
Please cite this article as: Meng Y et al., Experimental and theoretical investigations into the activity and mechanism of the wateregas shift reaction catalyzed by Au nanoparticles supported on ZneAl/Cr/Fe layered double hydroxides, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.172
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Fig. 3 e Characterization of Au/ZnCreLDH: STEM image with elemental mapping (a), EDX result with weight and atomic percentage of elements (b), EDX intensity line profiles (c), HRTEM image (d), size distribution plots (e) and Electron diffraction pattern (SAED) (f).
ZnCreLDHs sample because of the overlap of the LDH 015 and Au 111, but the intensity of the peak increases significantly after the introduction of Au into LDHs. This increase in the peak intensity also supports the successful synthesis of Au/ ZnCreLDHs. TEM images of all LDHs and Au supported LDHs are shown in Fig. 2. Three LDH samples (Fig. 2a, c and e) display obvious lamellar crystal structures with crystallite sizes between 50 and 200 nm. TEM images of Au supported LDHs (Fig. 2b, d and f) reveal the presence of Au dark dots dispersed in LDH sheets. In order to further determine the structure of Au supporting LDHs, STEM, EDX and HRTEM were used for analysis of the
chosen Au/ZnCreLDH model catalyst. STEM images of Au/ ZnCreLDHs with elemental mapping and EDS intensity line profiles are shown in Fig. 3aec. The elemental mapping results clearly show the presence of Zn, Cr and Au elements in the samples. STEMeEDS intensity line profiles across the sample show that the selected area contains Au nanoparticles. These results can further indicate the successful synthesis of Au supported by on ZnCreLDH structure. Fig. 3def shows HRTEM images with particle size distributions and Selected Area Electron Diffraction (SAED) patterns of the Au/ZnCreLDH. The characteristic despacings of the (111) plane of Au NPs determined from the SAED patterns is
Please cite this article as: Meng Y et al., Experimental and theoretical investigations into the activity and mechanism of the wateregas shift reaction catalyzed by Au nanoparticles supported on ZneAl/Cr/Fe layered double hydroxides, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.172
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Fig. 4 e CO conversion as a function of temperature (a) and Arrhenius plots (b) for the WGSR over different catalysts. CO conversion as a function of time (c) and reutilization results (d) for different catalysts at 300 C. Reaction conditions: 100 mg catalyst dosage, a mixture of 3 vol % CO, 15 vol % H2O and Ar was introduced into the reactor with a flow rate of ¡1 . 150 mL min¡1, the corresponding space velocity is 90000 mL g¡1 cat h
0.244 nm, which is quite consistent with the reports from other references [43e45]. The particle size distribution gives an average size of the Au nanoparticles in the Au/ZnCreLDH sample of about 8.2 nm. SAED patterns of the polycrystalline diffraction rings including (111), (200), (220) and (311) planes were indexed, and the relative values of despacing were also calculated, giving spacings consistent with the XRD results.
Reaction activity, stability and recoverability for WGSR Fig. 4a lists all the CO conversion yields for various catalysts in a reaction temperature window of 150e300 C. The three LDH samples with no Au nanoparticles have very little activity for WGSR observed below 150 C, but their CO conversion effi ciency is increased to 18.7%, 23.9% and 21.3% at 300 C for ZnAleLDH, ZnCreLDH and ZnFeeLDH, respectively. After the incorporation of Au nanoparticles onto LDHs, the catalytic performance for WGSR is largely increased, with CO conver sion efficiencies reaching 65.9%, 79.4% and 74.5% at 300 C for Au/ZnAleLDHs, Au/ZnCreLDHs and Au/ZnFeeLDHs, respectively. The reactivity of various Au/LDH alloys follows the order of: Au/ZnCreLDH>Au/ZnFeeLDH>Au/ZnAleLDH. To further explore the mechanism and active sites of the different LDH and Au/LDH catalysts, the reaction rate and
turnover frequency (TOF) values were calculated at a low CO conversion (<20%), shown in Table 1. The apparent activation energies (Ea) were calculated using Arrhenius plots (as shown in Fig. 4b and Table 1). The reaction rate and Ea of the three 1 LDHs are 21.1e28.8 mmol g1 and 62.4e70.7 kJ mol1. Au/ cat s 1 LDHs show higher reaction rates (78.6e102.1 mmol g1 cat s ) and 1 lower activation energies (41.7e52.9 kJ mol ) than those of the original LDHs. Particularly, Au/ZnCreLDH has the best catalytic, with a WGSR reaction rate, TOF and activation en1 1 and 41.7 kJ mol1, ergy are 102.1 mmol g1 cat s , 1.01 s respectively. The activity and reaction conditions of other Au supported catalysts [46e52] were compared with Au/ ZnMeLDHs, with the results shown in Table 1. We can conclude that with similar Au loading and reaction temperature ranges, Au/ZnMeLDHs exhibit higher activity for WGSR than the other shown Au supported catalysts. In particular, Au/ZnCreLDH is one of the most active catalysts reported so far for the WGS reaction (see Table 2). As shown in Fig. 4c and d, we also studied the stability of supported LDHs. After 48 h of use, CO conversion decreases slightly for the LDHs and Au/ZnMeLDHs catalysts. Au/ ZnCreLDHs, Au/ZnFeeLDHs and Au/ZnAleLDHs maintain high CO conversion even after use, losing less than 5% overall conversion efficiency (75.0% from 79.4%, 69.7% from 74.5% to
Please cite this article as: Meng Y et al., Experimental and theoretical investigations into the activity and mechanism of the wateregas shift reaction catalyzed by Au nanoparticles supported on ZneAl/Cr/Fe layered double hydroxides, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.172
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Table 1 e Catalytic performance of various catalysts for WGSR. Catalyst ZnAl-LDHs ZnCr-LDHs ZnFe-LDHs Au/ZnAl-LDHs Au/ZnCr-LDHs Au/ZnFe-LDHs Au/TiO2-LPRD-3 Au/CeO2-FeOx/Al2O3 Au/a-MoC Au/FeOx Au/Mo2C Au/TiO2eY2O3 AueNa/MCM41
Au (wt%)
T (oC)
XCO (%)
1 Specific rate (mmol g1 cat s )
TOF (s1)
Ea (kJ mol1)
e e e 2.27 2.21 2.25 1.38 2.17 2.00 0.75 1.55 1.11 1.89
300 300 300 300 300 300 300 200 250 350 150 300 150
18.7 23.9 21.3 65.9 79.4 74.5 e e e e e e e
21.1 28.8 25.2 78.6 102.1 95.7 e 89.4 105 11 1.6 15.2 e
e e e 0.74 1.01 0.92 0.18a 1.31b 1.05c 0.31d 0.021e 0.32f 0.89g
70.7 62.4 65.2 52.9 41.7 45.6 e e 22.0 49.0 44 e 44.0
Ref. This This This This This This [46] [47] [48] [49] [50] [51] [52]
work work work work work work
Note: Au content is determined by ICP-AES. Reaction condition in this work: catalyst dosage is 100 mg, a mixture of 3 vol % CO, 15 vol % H2O and 1 Ar was introduced into the reactor with a flow rate of 150 mL min1, the corresponding space velocity is 90000 mL g1 cat h . The CO conversion was tested at a range of 150e300 C. The activity parameters recording in this table were tested at 300 C. a Feed composition: 4.76% CO, 10.06% CO2, 28.46% H2, 35.38% H2O, 21.34%, balance with N2. Reproduced from ref 46 with permission by Elsevier. b Feed composition: 9% CO, 30% H2O, 12% CO2 and 50%, balance with H2. Reproduced from ref 47 with permission by Elsevier. c Feed composition: 11% CO, 26% H2O, 26% H2, 7% CO2, balance with N2. Reproduced from ref 48 with permission by the American Association for the Advancement of Science (AAAS). d Feed composition: 11% CO, 26% H2O, 26% H2, 7% CO2, balance with He. Reproduced from ref 49 with permission by Springer. e Feed composition: 7% CO, 22% H2O, 8.5% CO2, 37% H2, and balance with Ar. Reproduced from ref 50 with permission by Elsevier. f Feed composition: 3% CO, 67% N2 (controlled by mass flow controllers) and 30% H2O (controlled by a Gilsons 307 pump) in a total flow of 50 mL 1 min1 was passed throughout the solid with a space velocity (SV) of 6000 cm3 g1 . Reproduced from ref 51 with permission by the Royal cat h Society of Chemistry (RSC Publications). g Feed composition: 11% CO, 26% H2O, 7% CO2, and 26% H2, balance with He. Reproduced from ref 52 with permission by the American Association for the Advancement of Science (AAAS).
62.1% from 65.9%, respectively). We further recycled the catalysts three times after a process of centrifuging, washing, drying and reduction by H2. The catalysts were used for WGSR again and the activity of Au/ZnCreLDHs, Au/ZnFeeLDHs and Au/ZnAleLDHs are 77.8%, 73.1% and 63.2%, with only a small decrease in activity. These Au/ZnMeLDHs are highly stable with reasonable catalytic activity and have the potential to be used and recycled for at least three times catalytic reactions.
Experimental investigation for WGSR mechanism The relationship between the activity catalyst composition In order to determine the relationship between the catalyst's activity and composition, the physiochemical properties of different catalysts were investigated. As shown in Table 1, BET
surface area of standalone LDHs are 75e81 m2 g1, while the surface area of Au/ZnMeLDHs increase to 88e94 m2 g1, showing the surface area of LDHs increases after the loading of Au NPs. The size of the Au nanoparticles can be determined based on TEM and XRD analysis. The results from the two methods are quite similar. The dispersity of Au on the surface of LDHs can be determined by CO chemisorption experiments. The relationship between dispersity of Au and the catalytic performance of different catalysts (TOF) is shown in Fig. 5a. Note that although the particle sizes differ between TEM and CO chemisorption measurements, they show qualitatively the same trend in terms of the relationship between Au dispersity and TOF, i.e., Au/ZnCreLDHs>Au/ZnFeeLDHs>Au/ZnAle LDHs. It shows that a higher dispersion of Au NPs may be beneficial for the activity of Au/ZnMeLDHs.
Table 2 e Physicochemical properties of different catalysts. Samples ZnAl-LDHs ZnCr-LDHs ZnFe-LDHs Au/ZnAl-LDHs Au/ZnCr-LDHs Au/ZnFe-LDHs a b c d e
BET surface area (m2 g1)
Au content (wt%)a
Au crystallite size (nm)b
Average Au particle size (nm)c
Au dispersion (%)d
Au dispersion (%)e
75 81 79 88 94 91
e e e 2.27 2.21 2.25
e e e 7.5 8.6 7.8
e e e 6.9 8.2 7.5
e e e 1.45 1.78 1.69
e e e 1.86 2.21 2.05
Au content was determined by ICPAES. Crystallite size was determined by XRD results based on the Scherrer equation. Average Au particle size was determined by TEM images. Au dispersion was calculated based on the results of CO pulse chemisorption (described in Supporting Information). Au dispersion was estimated from Au particle size measured by TEM (described in Supporting Information).
Please cite this article as: Meng Y et al., Experimental and theoretical investigations into the activity and mechanism of the wateregas shift reaction catalyzed by Au nanoparticles supported on ZneAl/Cr/Fe layered double hydroxides, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.172
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of a new reduction peak at 195 C, residing in the catalytically relevant temperature region. The TPR experiment manifests the ability of Au to enhance the reducibility of the catalyst’s surface and promote reactivity for WGSR.
Studies of the WGSR with in situ DRIFTS In situ DRIFTS measurements during the WGSR were carried out to track the adsorption, intermediates and reaction mechanism on Au/ZnMeLDHs (Au/ZnCreLDHs is used as the model catalyst). As shown in Fig. 5c, a sharp peak appears at 1648 cm1 with a broad peak centered around 3350 cm1 on catalyst at 100 C, which are attributed to the HOH bending vibration and hydrogenebonded OH stretching of adsorbed water molecules, respectively [53]. When the temperature reaches 200 C, these peaks disappear gradually while peaks at 3698, 3649, 3588 cm1 grow in. These peaks in the range of 3720 to 3520 cm1 are associated with adsorbed CO2 and OH groups on the catalyst [54]. The peaks are more obvious at 300 C, likely due to the promotion of H2O dissociation, which facilitates the formation of more CO2. The specific adsorption peaks at 1811 and 1745 cm1 are attributed to CO. The peak located at 1395 cm1 is attributed to carbonate species [54]. According to the literature, the production of CO2 during the WGSR originates from either the reaction of CO with dissociated O from OH species or the decomposition of formates from the reaction of OH with CO [55,56]. If the intermediates contain formates, in situ DRIFTS spectra generally have peaks around 2950 and 2850 cm1 which are attributed to CeH stretches of bridged and bidentate formates, respectively. In addition, it should also contain peaks around 1575 and 1355 cm1, which are attributed to the asymmetric and symmetric OCO stretches of bidentate formates [57]. In the collected DRIFTS spectra, there are no peaks in these mentioned regions, suggesting that the production of CO2 likely goes through the CO and adsorbed O intermediates on Au/ZnCreLDH, which comes from the dissociation of OH species.
DFT calculation of WGSR by Au/ZnMeLDHs (M ¼ Al, Cr, Fe)
Fig. 5 e The relationship between the TOF value and Au particle size or Au dispersion of Au/ZnCreLDH (a); H2eTPR profiles of ZnCreLDH and Au/ZnCreLDH (b); in situ DRIFTS spectra recorded during the WGSR on Au/ZnCreLDH at different temperatures from 100 to 300 C (c).
Since WGSR involves a redox cycle where CO* is oxidized by O*, we investigated the reducibility of the catalysts using temperature program reduction (TPR, Fig. 5b). Note that there is only one major peak in the standalone LDHs (ZnCreLDHs), with the reduction temperature at 385 C that originating from the calcination of LDHs in a reducing environment. Remarkably, the addition of Au nanoparticle leads to the appearance
In order to support the results from in situ DRIFTS and better understand the differences in catalytical performance for the catalyzed WGSR, we constructed structural models of Au/ ZnMeLDHs (M ¼ Al, Cr, Fe) (Fig. 1a) and investigated possible elementary pathways using DFT calculations. Based on the adsorption sites of Au/ZnMeLDHs (Fig. S1), the optimal adsorption configuration for each species (Fig. S2) were calculated and are shown in the supporting information (SI). Multiple reaction mechanisms for the WGSR have been proposed in the literature, including the redox mechanism, carboxyl mechanism and formic acid mechanism [55]. Since the formic acid mechanism generally requires a large activation energy with unstable reaction intermediates [56,58], we mainly focus on the redox and the carboxyl mechanisms shown in Table 3 and Fig. 6. Following adsorption of CO* and H2O* onto the catalyst surface, water dissiciation (or hydrolysis, H2Oþ* ¼ H*þOH*) is the first catalytic step in both the redox mechanism and carboxyl mechanism. It often has the highest energy gap and determines the overall WGSR reaction rate [55]. For ZnAleLDHs, this step has an energy barrier (Ea) of 2.46 eV and
Please cite this article as: Meng Y et al., Experimental and theoretical investigations into the activity and mechanism of the wateregas shift reaction catalyzed by Au nanoparticles supported on ZneAl/Cr/Fe layered double hydroxides, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.172
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Table 3 e Changes in activation energies and reaction energies for each reaction step in the Au/ZnMeLDHs (M ¼ Al, Cr, Fe) system.
Elemental reaction
Redox mechanism
Carboxyl mechanism
1:H2Oþ* ¼ H*þOH* 2a:OH* ¼ O*þH* 2b:OH*þOH* ¼ H2O*þO* 3:CO*þO* ¼ CO2*þ* 4:CO*þOH* ¼ COOH*þ* 5a:COOH*þ* ¼ H*þCO2* 5b:COOH*þOH* ¼ H2O*þCO2* 6:2H* ¼ H2*þ*
ZnAl-LDHs
Au/ZnAl
Au/ZnCr
Au/ZnFe
Ea/eV
DE/eV
Ea/eV
DE/eV
Ea/eV
DE/eV
Ea/eV
DE/eV
2.46 2.04 1.09 1.54 1.86 2.01 2.41 0.87
1.23 1.11 0.74 2.55 0.54 0.33 1.05 0.58
1.81 1.47 0.84 0.69 1.61 1.87 1.73 0.56
0.92 0.89 0.58 2.32 0.34 0.19 1.23 0.22
1.49 1.26 0.65 0.37 1.43 1.47 1.57 0.45
0.64 0.75 0.35 1.55 0.11 0.30 1.51 0.11
1.67 1.31 0.71 0.42 1.52 1.68 1.66 0.49
0.81 0.73 0.52 2.01 0.32 0.24 1.35 0.29
* indicates a vacant site. X* denotes an adsorbed X specie.
Fig. 6 e Reaction diagrams for the WGSR over different catalysts: redox mechanisms (a and b) and carboxyl mechanisms (c and d). Please cite this article as: Meng Y et al., Experimental and theoretical investigations into the activity and mechanism of the wateregas shift reaction catalyzed by Au nanoparticles supported on ZneAl/Cr/Fe layered double hydroxides, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.172
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the reaction is overall endothermic by 1.23 eV. In the case of Au/ZnMeLDHs, it is also an endothermic step, but the energy barrier decreased to 1.81 eV, 1.49 eV and 1.67 eV resectively for Au/ZnAleLDH, Au/ZnCreLDH and Au/ZnFeeLDH, suggesting that hydrolysis is more likely to occur on the surface of Au/ ZnMeLDHs and Au/ZnCreLDH may have the highest activity for this step in hydrolysis. After H2O* dissiciation, the hydroxyl reaction can be divided into two ways in the redox path. Redox path a involves the OH*, which would continue to dissociate (reaction 2ea, OH* ¼ O*þH*), redox path b involves the OH*, which would undergo disproportionation (reaciton 2eb, OH*þOH* ¼ H2O*þO*). As shown in Fig. 6a and b, the Ea significantly decreases after including Au NPs in the LDH structure, but all of Au/ZnMeLDHs still exhibit a relatively high energy barrier (>1.26 eV). The energy barrier in step 2eb is quite smaller than that of in step 2ea for all catalysts. This step for ZnAleLDHs overcomes the energy barrier (Ea) of 1.09 eV and is endothermic by 0.74 eV. In the case of Au/ZnMeLDHs, OH* is more prone to disproportionation, with a barrier of 0.65e0.84 eV and is exothermic by 0.35e0.58 eV. The Ea value for this elementary step follows the order: LDH>Au/ZnAleLDH>Au/ ZnFeeLDH>Au/ZnCreLDH. Based on the energy barrier and thermodynamics, the disproportionation (step 2eb) is more likely to occur on the surface of LDHs samples, especially for Au/ZnCreLDH. The CO* oxidation (step 3, CO*þO* ¼ CO2*þ*) by atomic O* is very easy to carry out on all catalysts based on thermodynamic analysis. The value of the energy barriers also follows the order of: LDH>Au/ZnAleLDH>Au/ZnFeeLDH>Au/ ZnCreLDH. The lowest energy configuration shows CO adsorbed at the BrieAu site, with the CO molecule lies vertically on the catalyst surface with the carbon side closest to the surface, as shown in Figure S2e1. The elementary reaction is exothermic on all LDHs, confirming that CO* is more easily oxidized under low temperature conditions. The H2* recombination reaction (step 6, 2H* ¼ H2*þ*) is endothermic, signifying that considerable H* coverage can be expected under typical lowetemperature WGSR conditions. H* is prone adsorption at BrieZn sites on the LDH or Au/ ZnMeLDH (M ¼ Al, Cr, Fe) surfaces, in which both H and Zn interact with oxygen at the BrieZn sites (Figure S2e5). After H* is recombined to form hydrogen, it is vertically adsorbed on the HcpeZn face of the catalysts, and the C and O bonds with the 3 oxygens around the Zn element in this adsorption process (Figure S2e7). The activation energy barrier for this step is relatively low, and the Ea is further decrease after Au supporting, from 0.87 eV to 0.45e0.56 eV. The energies of reaction intermediates for the WGSR over different catalysts following the carboxyl mechanism is shown in Fig. 6c and d. The first and last step in the carboxyl mechanism are the same as the redox mechanism and have been discussed above. In the carboxyl mechanism, the energy barrier of CO* oxidation by OH* (step 4, CO*þOH* ¼ COOH*þ*), COOH* dissociation (step 5ea, COOH*þ* ¼ H*þCO2*) and COOH* disproportionation (step 5eb, COOH*þOH* ¼ H2O*þCO2*) are quite larger than the Ea in step 2 and 3 in the redox mechanism. This indicates that the carboxyl mechanism is not the favorable reaction path for the WGSR on LDHs and Au/ZnMeLDHs. Comparing the activation energies for all
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the elementary reactions, we can conclude that the first reaction step generally has the highest energy gap and is the rate limiting step. Auebased catalysts can effectively lower the energy barrier of the kinetically relevant step of H2O dissociation, which is the reason for the high activity of Au/ ZnMeLDHs. Compared with the carboxyl mechanism, the redox mechanism has the lower energy barrier and is the more likely reaction pathway for WGSR reaction, especially for redox path b, 1e2ebe3e6. In most reaction steps, the required activation energy follows the following order: LDH>Au/ZnAleLDH>Au/ZnFeeLDH>Au/ZnCreLDH. Such calculations suggest that the activity for LDH samples are in the order of: Au/ZnCreLDH>Au/ZnFeeLDH>Au/ZnAleLDHs> LDHs, which is indeed the case in our experimental data (as shown in Fig. 4).
Conclusions In this paper, DFT calculations were employed to gain mechanistic insights into the adsorption energetics and activation barriers between the transitions of different elementary steps and routes in WGSR catalyzed by Au nanoparticles supported on different LDHs, Au/ZnMeLDHs (M ¼ Al, Cr, Fe). The calculations show that the introduction of Au NPs decreases the activation energy of important steps in the WGSR, especially the kinetically relevant step of H2O dissociation, and the apparent activation energy follows the following order: LDH>Au/ZnAleLDH>Au/ZnFeeLDH>Au/ ZnCreLDH. Combined with the insights obtained from our theoretical calculations, Au NPs supported on ZnMeLDHs were synthesized and their catalytic performances for WGSR were evaluated, yielding results consistent with our calculations. It was found that the addition of Au NPs changes the reaction mechanism by significantly lowering the activation barrier of the kinetically relevant step of H2O dissociation. Au/ZnCreLDH exhibits the best catalytic performance including 79.4% CO conversion, reaction rate of 1 1 TOF and an activation energy of 102.1 mmol g1 cat s , 1.01 s 41.7 kJ mol1. Also, TPR experiments suggest that the introduction of Au changes the electronic and geometric structure, which alters the redox behavior of the LDH involved in the WGSR. In situ DRIFTS shows that the production of CO2 during the WGSR process goes through the reaction between CO and adsorbed O, which comes from the dissociation of OH species. Such result can be supported by DFT calculations, which shows that the Redox mechanism b has the lowest energy barriers and is the most potential reaction pathway. Our study demonstrated a strategic method in the design of industrially relevant catalysts by integrating both theory and experiment, which allows one to extract mechanistic insights of a catalytic reaction and can be further applied in the engineering of more efficient catalyst.
Acknowledgments This work is supported by National Natural Science Foundation of China (21503188).
Please cite this article as: Meng Y et al., Experimental and theoretical investigations into the activity and mechanism of the wateregas shift reaction catalyzed by Au nanoparticles supported on ZneAl/Cr/Fe layered double hydroxides, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.172
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Appendix A. Supplementary data [15]
Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.10.172. [16]
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Please cite this article as: Meng Y et al., Experimental and theoretical investigations into the activity and mechanism of the wateregas shift reaction catalyzed by Au nanoparticles supported on ZneAl/Cr/Fe layered double hydroxides, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.172
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Experimental and theoretical investigations into the activity and mechanism of the wateregas shift reaction catalyzed by Au nanoparticles supported on ZneAl/Cr/Fe layered double hydroxides Yue Meng a, Yufu Chen b, Xiaobo Zhou c, Guoxiang Pan a, Shengjie Xia b,* a
School of Life Science, Huzhou University, 759 East Erhuan Road, Huzhou, 313000, PR China Department of Chemistry, College of Chemical Engineering, Zhejiang University of Technology, 18 Chaowang Road, Hangzhou, 310014, PR China c Entegris, Inc., 129 Concord Road, Billerica, MA, 01821, USA b
highlights
graphical abstract
Au NPs supported on LDHs were prepared as highly efficient and stable catalyst for WGSR. Incorporation of Au NPs significantly reduces Ea and enhances activity of LDHs for WGSR. The activity: Au/ZnCr-LDHs>Au/ ZnFe-LDHs>Au/ZnAl-LDHs>LDHs, the energy barrier is just opposite. Addition of Au alters the redox circles, which is a key intermediary
process
involved
in
the
catalysis. Redox mechanisms b is the most potential reaction pathway and perfectly supports in situ DRIFTS results.
article info
abstract
Article history:
Wateregas shift reaction (WGSR) is an industrialized reaction with numerous applications
Received 27 July 2019
concerning CO removal and H2 generation. Since this process is widely used and occurs at
Received in revised form
elevated temperatures, the development of high efficiency and stable catalysts for WGSR
26 September 2019
and investigating their catalytic mechanism is a hot topic. In this paper, we demonstrate
Accepted 21 October 2019
Au nanoparticles supported on different layered double hydroxides (LDHs) as highly effi-
Available online xxx
cient and stable catalysts for WGSR. The incorporation of Au nanoparticles significantly decreases the activation energy and enhances the catalytic activity of LDHs for WGSR, with Au/ZnCreLDHs exhibiting the best catalytic performance including: 79.4% CO conversion,
* Corresponding author. E-mail addresses: [email protected], [email protected] (S. Xia). https://doi.org/10.1016/j.ijhydene.2019.10.172 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Meng Y et al., Experimental and theoretical investigations into the activity and mechanism of the wateregas shift reaction catalyzed by Au nanoparticles supported on ZneAl/Cr/Fe layered double hydroxides, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.172
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Keywords: Layered double hydroxides Au supporting Water gas shift reaction (WGSR) Activity and reaction mechanism DFT calculation
1 of reaction rate, 1.01 s1 TOF values and 41.7 kJ mol1 of activation 102.1 mmol g1 cat s
energy. TPR experiments suggest that the addition of Au alters the redox cycle on the surface of the catalyst, a key intermediary step involved in the catalytic process. In situ DRIFTS shows that the production of CO2 during WGSR involves the reaction between CO and adsorbed O, which comes from the dissociation of OH species and not the decomposition of formates. DFT calculations indicate that Auebased catalysts can effectively lower the energy barrier of the kinetically relevant step of H2O dissociation, which is the most probable reason for the enhancement of activity. The calculated activation barriers coincide with the experimentally measured values with the order of Au/ZnCreLDHs
Introduction Clean energy can effectively solve the problem of energy shortage and environmental pollution. Of the many options, H2 is the most popular clean energy fuel candidate and has been intensively studied to try and address energy shortage and environmental protection considerations [1e3]. The water gas shift reaction (WGSR): H2O(g)þCO(g) ¼ CO2(g)þH2(g), is a powerful tool to convert industrial waste CO gas and at the same time generate H2. In addition, the other product CO2 is also a very important substance for industry application, such as solardriven photocatalytic conversion of CO2 to valuable fuels (CH4) [4], reverse water gas shift reaction to generate CO [5], synthesis of cyclic carbonates from CO2 and epoxides [6], and so on. This reaction gives the opportunity for both reducing pollution and generating a clean energy fuel [7e9]. The success of WGSR relies on finding a catalyst that can operate under moderate conditions. Traditional catalysts, such as FeeCr catalysts, require high operating temperatures higher than 300 C and have generally low activity [10,11]. CueZn and CoeMo based catalysts operate at lower reaction temperatures but tend to quickly lose activity [12,13]. Based on the disadvantages of currently used catalysts, developing new, more efficient WGSR catalysts has been drawing great attention. Au based catalysts have been one of the most popular research targets in recent years due to their high activity and low operating temperatures [14,15]. Despite these benefits, Au nanoparticles (NPs) have poor dispersive properties and are prone to aggregation, which causes the loss of its activity and selectivity and prevents this type of catalyst from being industrialized [16]. One solution for this issue is to support Au based catalyst with another carrier catalyst [17,18]. Gamboa et al. found that AueCo3O4/CeO2 has better reduction activity compared with Ce3O4/CeO2, and Au could greatly enhance WGSR activity [19]. Stere et al. used nonethermal plasma activation to enable lowetemperature wateregas shift over a type of Au/CeZrO4 catalysts. They found that Au/CeZrO4
showed high performance for WGSR at 100e250 C temperature range, with CO conversion reaching a maximum of 91% at 240 C [20]. Thuy et al. compared performance of catalysts with different Au/Pt ratios on CeO2 and TiO2 supports, and found that 90% CO could be converted under 300 C with 5% of both Au and Pt, achieving a good balance between reaction temperature and activity for WGSR [21]. Even with these impressive results, there still remains the need to understand the activity, selectivity and stability of these catalysts [22,23]. With these types of Au catalysts for WGSR, the challenge in the development of a good catalyst depends on the development of a good carrier support. Theoretical calculations can be used for determining the catalytically active species in a reaction, which helps guide the design of new catalysts. In recent years, the theoretical investigation of the WGSR mechanism has been studied intensively [24e26]. Saqlain et al. examined the WGSR at a CueAu bimetallic surface with density functional theory (DFT). Their results showed that the CueAu(100) surface has higher activity compared to Cu(100), and the reaction on the CueAu surface reduced the water dissociation energy [27]. Wu et al. found that a Co6@Au32 coreeshell nanoalloy has better electron activity that Au38. Their research showed that the core Co atoms can control both the surface geometry of the Au atoms and the electronic structure near the Fermi energy [28]. Lin et al. calculated the activation energy of the WGSR elemental reaction on Cu(111), Pt(111) and Au(111) surfaces, and concluded that the activation energy is reduced on all three catalysts with Au(111) having the lowered energy gap during the CO*þO* ¼ CO2*þ* step. The fast formation and desorption of CO2* made the reaction easier for reductioneoxidation to occur [29]. But, research on effective catalysts for the WGSR mechanism has been mostly been focused on crystal phases or particle clusters of single metal or bimetallic alloy systems. The study on Au NPs supported on high surface area materials is still an area to be investigated [30]. Our previous research showed that Zn based LDHs has high chemical stability, high specific surface area and
Please cite this article as: Meng Y et al., Experimental and theoretical investigations into the activity and mechanism of the wateregas shift reaction catalyzed by Au nanoparticles supported on ZneAl/Cr/Fe layered double hydroxides, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.172
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excellent catalytic activity [16,31], which can be used for supporting Au nanoparticles [32,33]. In this paper, we synthesized three ZnMeLDHs (M ¼ Al, Cr, Fe) as the base material to support Au NPs to study their efficacy as WGSR catalysts. The activity and reaction mechanism for WGSR on ZnMeLDHs and the Au supported ZnMeLDHs (M ¼ Al, Cr, Fe) are discussed. We found that the introduction of the Au NPs could effectively increase the activity and decrease the activation energy of WGSR. We also studied the relationship between the composition of the catalyst and its activity. In addition, the catalytic pathway is postulated using results from in situ DRIFTS and DFT calculations. The calculations also gave the most probable reaction pathway for WGSR catalyzed by Au/ZnMeLDHs (M ¼ Al, Cr, Fe) based on the comparison of reaction energy barriers.
Experimental procedures of water gas shift reaction (WGSR)
Experimental and theoretical calculations
The evaluation of catalytic activity was performed in a small fixedebed reactor under atmospheric pressure. The reactor was a microequartz glass tube with an interior diameter of 6 mm. Before evaluation, 100 mg of the catalyst was pre treated at 300 C for 2 h in a gaseous mixture of 10/90 vol% H2/ Ar with a flow rate of 20 mL min1, and then cooled to room temperature under an Ar atmosphere. Water was fed into the vaporizer (300 C) with a higheperformance liquid pump, and the resulting steam was mixed with reactant gas before entering the reactor. After that, a mixture of 3 vol % CO, 15 vol % H2O and Ar was introduced into the reactor with a flow rate of 150 mL min1, the corresponding space velocity being 1 90000 mL g1 cat h . The CO conversion was tested at a range of 150e300 C. The concentration of inlet (nin CO ) and outlet gases (nout CO ) was analyzed by an ineline gas chromatograph (GC2014C) with TDX01 column and TCD detector. The conversion of CO (XCO) can be calculated by equation (1):
Synthesis of Au/ZnMeLDHs (M¼Al, Cr, Fe)
XCO ¼
The detailed list of materials and instrumentation is given in Supporting Information (SI).
Turnover frequency (TOF) and specific reaction rate (r) for WGSR catalyzed by ZnMeLDHs or Au/ZnMeLDHs (M ¼ Al, Cr, Fe) catalysts at different temperatures were determined by decreasing the weight of catalyst (from 100 mg to 25 mg) as well as cutting down the reaction time to guarantee CO conversion below 20%. For each run at a specific temperature, the CO conversions were averaged at the steady state and used to obtain the specific reaction rates, which were calculated by equation (2):
Preparation of ZnMeLDHs (M¼Al, Cr, Fe) The synthetic procedure of ZnAleLDHs is similar with the references [34,35], which is given as follows: solution A and B were added with drop by drop into a 250 mL threeeneck flask containing 50 ml deionized water at room temperature, in which solution A contains 50 mL deionized water, 6.40 g of NaOH and 1.06 g of Na2CO3, and solution B has 50 mL deionized water, 17.82 g of Zn(NO3)2$6H2O (0.06 mol) and 7.50 g of Al(NO3)3$9H2O (0.02 mol). The pH of the reaction mixture was maintained at the range of 9.0e10.0 with the stirring rate of 300 rpm for 20 min. After that, the resulting slurry was held at 80 C for 18 h and then centrifuged for 5 min with 5000 rpm., The resulting solid was washed with deionized water until the pH reached 7.0. Finally, the sample was dried under vacuum at 60 C for 24 h. The catalyst powder was then ground and ready for use. When preparing ZnCreLDHs and ZnFeeLDHs, 8.00 g of Cr(NO3)3$9H2O (0.02 mol) or 8.10 g of Fe(NO3)3$9H2O (0.02 mol) were used in solution B, other steps are similar with the synthesis of ZnAleLDHs.
Preparation of Au/ZnMeLDHs (M ¼ Al, Cr, Fe) 2.0 g of asesynthesized ZnMeLDHs (M ¼ Al, Cr, Fe) was added into a 250 mL flask containing 120 ml of a 5 mM HAuCl4 aqueous solution. The mixture was then stirred with a rate of 300 rpm for 10 min. After that, 2 mL of aqueous NH3 (10%) was added into the reaction system and the resulting mixture was stirred at room temperature for 24 h. The resulting slurry was centrifugated for 5 min at 5000 rpm, washed with 50 ml deionized water for three times, and dried at room temperature under vacuum. Then, 1.0 g of the dried solid was dispersed into 100 ml deionized water and treated with 50 mg NaBH4 at room temperature for 2 h. After centrifugation, washing and drying, the final Au supported LDHs catalyst (Au/ ZnMeLDHs) was obtained.
rCO ¼
out nin CO nCO 100% in nCO
XCO ,fCO mAu
(1)
(2)
where f CO is the molar flow rate of CO in mol h1 and mAu is the mass of Au in the fixed bed. The value of turnover frequency (TOF) was calculated by equation (3) based on the specific reaction rates and dispersions of Au species. The dispersions of Au species were determined by CO chemisorption with the assumption of the stoichiometric ratio of adsorbed CO/Au¼1: TOF ¼
rCO ,MAu DAu
(3)
where MAu is the molar weight of Au, DAu is the dispersion of Au calculated from the results of CO pulse chemisorption by assuming that the stoichiometric ratio of adsorbed CO/Pt is 1. The apparent activation energy (Ea) of the catalysts was determined using the Arrhenius equation (4): Ea
k ¼ AeRT
(4)
Calculation methods and parameters of Au/ZnMeLDHs (M¼Al, Cr, Fe) Zn3M1/LDHs (M ¼ Al, Cr, Fe) is represented by the formula of [Zn12M4(OH)32]4þ, its structure has D3d symmetry with R3m space group. The Zn and M elements in the molecular cluster model have a structure of ZnM ¼ 3 in a single LDH layer, M is surrounded by six Zn to form a hexagon structure. The
Please cite this article as: Meng Y et al., Experimental and theoretical investigations into the activity and mechanism of the wateregas shift reaction catalyzed by Au nanoparticles supported on ZneAl/Cr/Fe layered double hydroxides, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.172
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Fig. 1 e Structural model of Au/ZnMeLDHs (M ¼ Al, Cr, Fe): (A) front view; (B) top view; XRD patterns of different samples (C): (a) ZnAleLDH, (b) Au/ZnAleLDH; (c) ZnCreLDH, (d) Au/ZnCreLDH; (e) ZnFeeLDH, (f) Au/ZnFeeLDH. octahedron unit has a positive charge since Zn2þ is replaced by M3þ which separates M ions apart from each other. The NO 3 anion serves to neutralize the positive charges at the surface of the hydrotalcite laminate. The optimized structure is expanded along the c direction to construct a 2 2 1 unit cell for the DFT simulation, with the unit cell having the formula [Zn12M4(OH)32](NO3)4. After optimization, an Au3 clustereloaded hydrotalcite laminate model was constructed based on the reason that Au elements tend to coordinate with three oxygen atoms anchored between the top of the metal element and the LDHs laminate (as shown in Fig. 1a and b) [36]. Dmol3 module in Materials Studio 8.0 was used for the geometric optimization and analysis with the GGA based PerdeweBurkeeErnzerhof (PBE) was used as the electron exchange functional. The inner layer electrons were simplified by frozen core approximation method with ECP, the valence electron wave function expansion used dual numerical polarization (DNP) function without further electron spin restriction. The smearing values was set at 0.005 hartree, SCF tolerance was 1 106 Ha, and the kepoint value of the Brillouin zone was 6 6 1. The criterion of the geometric
optimization is as follows: the energy difference should be lower than 1 105 hartrees, the force on each atom is lower A and the atomic displacement is lower than 2 103 hartree/ A [37]. than 5.00 103
Results and discussion Structural characteristics of Au/ZnMeLDHs (M ¼ Al, Cr, Fe) Fig. 1c shows the XRD patterns of ZnAleLDHs, ZnCreLDHs, ZnFeeLDHs, and Au/ZnMeLDHs (M ¼ Al, Cr, Fe). The characteristic peaks of three LDHs that corresponding to facets 003, 006, 009(012), 015, 018, 110 and 113, can be observed, typical of the LDHs structure [38e40]. In the case of Au supported samples, the XRD pattern of Au/ZnMeLDHs not only retains the specific peaks from LDHs, but new peaks also appear at about 38.0 (2q), 44.0 and 64.5 that represent the 111, 200 and 220 facets of Au NPs (JCPDS 04e0784), suggesting the successful loading of Au into the LDH structure [41,42]. There is no obvious separate peak of Au 111 in the Au/
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Fig. 2 e TEM images of: (a) ZnAleLDH, (b) Au/ZnAleLDH; (c) ZnCreLDH, (d) Au/ZnCreLDH; (e) ZnFeeLDH, (f) Au/ZnFeeLDH.
Please cite this article as: Meng Y et al., Experimental and theoretical investigations into the activity and mechanism of the wateregas shift reaction catalyzed by Au nanoparticles supported on ZneAl/Cr/Fe layered double hydroxides, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.172
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Fig. 3 e Characterization of Au/ZnCreLDH: STEM image with elemental mapping (a), EDX result with weight and atomic percentage of elements (b), EDX intensity line profiles (c), HRTEM image (d), size distribution plots (e) and Electron diffraction pattern (SAED) (f).
ZnCreLDHs sample because of the overlap of the LDH 015 and Au 111, but the intensity of the peak increases significantly after the introduction of Au into LDHs. This increase in the peak intensity also supports the successful synthesis of Au/ ZnCreLDHs. TEM images of all LDHs and Au supported LDHs are shown in Fig. 2. Three LDH samples (Fig. 2a, c and e) display obvious lamellar crystal structures with crystallite sizes between 50 and 200 nm. TEM images of Au supported LDHs (Fig. 2b, d and f) reveal the presence of Au dark dots dispersed in LDH sheets. In order to further determine the structure of Au supporting LDHs, STEM, EDX and HRTEM were used for analysis of the
chosen Au/ZnCreLDH model catalyst. STEM images of Au/ ZnCreLDHs with elemental mapping and EDS intensity line profiles are shown in Fig. 3aec. The elemental mapping results clearly show the presence of Zn, Cr and Au elements in the samples. STEMeEDS intensity line profiles across the sample show that the selected area contains Au nanoparticles. These results can further indicate the successful synthesis of Au supported by on ZnCreLDH structure. Fig. 3def shows HRTEM images with particle size distributions and Selected Area Electron Diffraction (SAED) patterns of the Au/ZnCreLDH. The characteristic despacings of the (111) plane of Au NPs determined from the SAED patterns is
Please cite this article as: Meng Y et al., Experimental and theoretical investigations into the activity and mechanism of the wateregas shift reaction catalyzed by Au nanoparticles supported on ZneAl/Cr/Fe layered double hydroxides, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.172
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Fig. 4 e CO conversion as a function of temperature (a) and Arrhenius plots (b) for the WGSR over different catalysts. CO conversion as a function of time (c) and reutilization results (d) for different catalysts at 300 C. Reaction conditions: 100 mg catalyst dosage, a mixture of 3 vol % CO, 15 vol % H2O and Ar was introduced into the reactor with a flow rate of ¡1 . 150 mL min¡1, the corresponding space velocity is 90000 mL g¡1 cat h
0.244 nm, which is quite consistent with the reports from other references [43e45]. The particle size distribution gives an average size of the Au nanoparticles in the Au/ZnCreLDH sample of about 8.2 nm. SAED patterns of the polycrystalline diffraction rings including (111), (200), (220) and (311) planes were indexed, and the relative values of despacing were also calculated, giving spacings consistent with the XRD results.
Reaction activity, stability and recoverability for WGSR Fig. 4a lists all the CO conversion yields for various catalysts in a reaction temperature window of 150e300 C. The three LDH samples with no Au nanoparticles have very little activity for WGSR observed below 150 C, but their CO conversion effi ciency is increased to 18.7%, 23.9% and 21.3% at 300 C for ZnAleLDH, ZnCreLDH and ZnFeeLDH, respectively. After the incorporation of Au nanoparticles onto LDHs, the catalytic performance for WGSR is largely increased, with CO conver sion efficiencies reaching 65.9%, 79.4% and 74.5% at 300 C for Au/ZnAleLDHs, Au/ZnCreLDHs and Au/ZnFeeLDHs, respectively. The reactivity of various Au/LDH alloys follows the order of: Au/ZnCreLDH>Au/ZnFeeLDH>Au/ZnAleLDH. To further explore the mechanism and active sites of the different LDH and Au/LDH catalysts, the reaction rate and
turnover frequency (TOF) values were calculated at a low CO conversion (<20%), shown in Table 1. The apparent activation energies (Ea) were calculated using Arrhenius plots (as shown in Fig. 4b and Table 1). The reaction rate and Ea of the three 1 LDHs are 21.1e28.8 mmol g1 and 62.4e70.7 kJ mol1. Au/ cat s 1 LDHs show higher reaction rates (78.6e102.1 mmol g1 cat s ) and 1 lower activation energies (41.7e52.9 kJ mol ) than those of the original LDHs. Particularly, Au/ZnCreLDH has the best catalytic, with a WGSR reaction rate, TOF and activation en1 1 and 41.7 kJ mol1, ergy are 102.1 mmol g1 cat s , 1.01 s respectively. The activity and reaction conditions of other Au supported catalysts [46e52] were compared with Au/ ZnMeLDHs, with the results shown in Table 1. We can conclude that with similar Au loading and reaction temperature ranges, Au/ZnMeLDHs exhibit higher activity for WGSR than the other shown Au supported catalysts. In particular, Au/ZnCreLDH is one of the most active catalysts reported so far for the WGS reaction (see Table 2). As shown in Fig. 4c and d, we also studied the stability of supported LDHs. After 48 h of use, CO conversion decreases slightly for the LDHs and Au/ZnMeLDHs catalysts. Au/ ZnCreLDHs, Au/ZnFeeLDHs and Au/ZnAleLDHs maintain high CO conversion even after use, losing less than 5% overall conversion efficiency (75.0% from 79.4%, 69.7% from 74.5% to
Please cite this article as: Meng Y et al., Experimental and theoretical investigations into the activity and mechanism of the wateregas shift reaction catalyzed by Au nanoparticles supported on ZneAl/Cr/Fe layered double hydroxides, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.172
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Table 1 e Catalytic performance of various catalysts for WGSR. Catalyst ZnAl-LDHs ZnCr-LDHs ZnFe-LDHs Au/ZnAl-LDHs Au/ZnCr-LDHs Au/ZnFe-LDHs Au/TiO2-LPRD-3 Au/CeO2-FeOx/Al2O3 Au/a-MoC Au/FeOx Au/Mo2C Au/TiO2eY2O3 AueNa/MCM41
Au (wt%)
T (oC)
XCO (%)
1 Specific rate (mmol g1 cat s )
TOF (s1)
Ea (kJ mol1)
e e e 2.27 2.21 2.25 1.38 2.17 2.00 0.75 1.55 1.11 1.89
300 300 300 300 300 300 300 200 250 350 150 300 150
18.7 23.9 21.3 65.9 79.4 74.5 e e e e e e e
21.1 28.8 25.2 78.6 102.1 95.7 e 89.4 105 11 1.6 15.2 e
e e e 0.74 1.01 0.92 0.18a 1.31b 1.05c 0.31d 0.021e 0.32f 0.89g
70.7 62.4 65.2 52.9 41.7 45.6 e e 22.0 49.0 44 e 44.0
Ref. This This This This This This [46] [47] [48] [49] [50] [51] [52]
work work work work work work
Note: Au content is determined by ICP-AES. Reaction condition in this work: catalyst dosage is 100 mg, a mixture of 3 vol % CO, 15 vol % H2O and 1 Ar was introduced into the reactor with a flow rate of 150 mL min1, the corresponding space velocity is 90000 mL g1 cat h . The CO conversion was tested at a range of 150e300 C. The activity parameters recording in this table were tested at 300 C. a Feed composition: 4.76% CO, 10.06% CO2, 28.46% H2, 35.38% H2O, 21.34%, balance with N2. Reproduced from ref 46 with permission by Elsevier. b Feed composition: 9% CO, 30% H2O, 12% CO2 and 50%, balance with H2. Reproduced from ref 47 with permission by Elsevier. c Feed composition: 11% CO, 26% H2O, 26% H2, 7% CO2, balance with N2. Reproduced from ref 48 with permission by the American Association for the Advancement of Science (AAAS). d Feed composition: 11% CO, 26% H2O, 26% H2, 7% CO2, balance with He. Reproduced from ref 49 with permission by Springer. e Feed composition: 7% CO, 22% H2O, 8.5% CO2, 37% H2, and balance with Ar. Reproduced from ref 50 with permission by Elsevier. f Feed composition: 3% CO, 67% N2 (controlled by mass flow controllers) and 30% H2O (controlled by a Gilsons 307 pump) in a total flow of 50 mL 1 min1 was passed throughout the solid with a space velocity (SV) of 6000 cm3 g1 . Reproduced from ref 51 with permission by the Royal cat h Society of Chemistry (RSC Publications). g Feed composition: 11% CO, 26% H2O, 7% CO2, and 26% H2, balance with He. Reproduced from ref 52 with permission by the American Association for the Advancement of Science (AAAS).
62.1% from 65.9%, respectively). We further recycled the catalysts three times after a process of centrifuging, washing, drying and reduction by H2. The catalysts were used for WGSR again and the activity of Au/ZnCreLDHs, Au/ZnFeeLDHs and Au/ZnAleLDHs are 77.8%, 73.1% and 63.2%, with only a small decrease in activity. These Au/ZnMeLDHs are highly stable with reasonable catalytic activity and have the potential to be used and recycled for at least three times catalytic reactions.
Experimental investigation for WGSR mechanism The relationship between the activity catalyst composition In order to determine the relationship between the catalyst's activity and composition, the physiochemical properties of different catalysts were investigated. As shown in Table 1, BET
surface area of standalone LDHs are 75e81 m2 g1, while the surface area of Au/ZnMeLDHs increase to 88e94 m2 g1, showing the surface area of LDHs increases after the loading of Au NPs. The size of the Au nanoparticles can be determined based on TEM and XRD analysis. The results from the two methods are quite similar. The dispersity of Au on the surface of LDHs can be determined by CO chemisorption experiments. The relationship between dispersity of Au and the catalytic performance of different catalysts (TOF) is shown in Fig. 5a. Note that although the particle sizes differ between TEM and CO chemisorption measurements, they show qualitatively the same trend in terms of the relationship between Au dispersity and TOF, i.e., Au/ZnCreLDHs>Au/ZnFeeLDHs>Au/ZnAle LDHs. It shows that a higher dispersion of Au NPs may be beneficial for the activity of Au/ZnMeLDHs.
Table 2 e Physicochemical properties of different catalysts. Samples ZnAl-LDHs ZnCr-LDHs ZnFe-LDHs Au/ZnAl-LDHs Au/ZnCr-LDHs Au/ZnFe-LDHs a b c d e
BET surface area (m2 g1)
Au content (wt%)a
Au crystallite size (nm)b
Average Au particle size (nm)c
Au dispersion (%)d
Au dispersion (%)e
75 81 79 88 94 91
e e e 2.27 2.21 2.25
e e e 7.5 8.6 7.8
e e e 6.9 8.2 7.5
e e e 1.45 1.78 1.69
e e e 1.86 2.21 2.05
Au content was determined by ICPAES. Crystallite size was determined by XRD results based on the Scherrer equation. Average Au particle size was determined by TEM images. Au dispersion was calculated based on the results of CO pulse chemisorption (described in Supporting Information). Au dispersion was estimated from Au particle size measured by TEM (described in Supporting Information).
Please cite this article as: Meng Y et al., Experimental and theoretical investigations into the activity and mechanism of the wateregas shift reaction catalyzed by Au nanoparticles supported on ZneAl/Cr/Fe layered double hydroxides, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.172
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of a new reduction peak at 195 C, residing in the catalytically relevant temperature region. The TPR experiment manifests the ability of Au to enhance the reducibility of the catalyst’s surface and promote reactivity for WGSR.
Studies of the WGSR with in situ DRIFTS In situ DRIFTS measurements during the WGSR were carried out to track the adsorption, intermediates and reaction mechanism on Au/ZnMeLDHs (Au/ZnCreLDHs is used as the model catalyst). As shown in Fig. 5c, a sharp peak appears at 1648 cm1 with a broad peak centered around 3350 cm1 on catalyst at 100 C, which are attributed to the HOH bending vibration and hydrogenebonded OH stretching of adsorbed water molecules, respectively [53]. When the temperature reaches 200 C, these peaks disappear gradually while peaks at 3698, 3649, 3588 cm1 grow in. These peaks in the range of 3720 to 3520 cm1 are associated with adsorbed CO2 and OH groups on the catalyst [54]. The peaks are more obvious at 300 C, likely due to the promotion of H2O dissociation, which facilitates the formation of more CO2. The specific adsorption peaks at 1811 and 1745 cm1 are attributed to CO. The peak located at 1395 cm1 is attributed to carbonate species [54]. According to the literature, the production of CO2 during the WGSR originates from either the reaction of CO with dissociated O from OH species or the decomposition of formates from the reaction of OH with CO [55,56]. If the intermediates contain formates, in situ DRIFTS spectra generally have peaks around 2950 and 2850 cm1 which are attributed to CeH stretches of bridged and bidentate formates, respectively. In addition, it should also contain peaks around 1575 and 1355 cm1, which are attributed to the asymmetric and symmetric OCO stretches of bidentate formates [57]. In the collected DRIFTS spectra, there are no peaks in these mentioned regions, suggesting that the production of CO2 likely goes through the CO and adsorbed O intermediates on Au/ZnCreLDH, which comes from the dissociation of OH species.
DFT calculation of WGSR by Au/ZnMeLDHs (M ¼ Al, Cr, Fe)
Fig. 5 e The relationship between the TOF value and Au particle size or Au dispersion of Au/ZnCreLDH (a); H2eTPR profiles of ZnCreLDH and Au/ZnCreLDH (b); in situ DRIFTS spectra recorded during the WGSR on Au/ZnCreLDH at different temperatures from 100 to 300 C (c).
Since WGSR involves a redox cycle where CO* is oxidized by O*, we investigated the reducibility of the catalysts using temperature program reduction (TPR, Fig. 5b). Note that there is only one major peak in the standalone LDHs (ZnCreLDHs), with the reduction temperature at 385 C that originating from the calcination of LDHs in a reducing environment. Remarkably, the addition of Au nanoparticle leads to the appearance
In order to support the results from in situ DRIFTS and better understand the differences in catalytical performance for the catalyzed WGSR, we constructed structural models of Au/ ZnMeLDHs (M ¼ Al, Cr, Fe) (Fig. 1a) and investigated possible elementary pathways using DFT calculations. Based on the adsorption sites of Au/ZnMeLDHs (Fig. S1), the optimal adsorption configuration for each species (Fig. S2) were calculated and are shown in the supporting information (SI). Multiple reaction mechanisms for the WGSR have been proposed in the literature, including the redox mechanism, carboxyl mechanism and formic acid mechanism [55]. Since the formic acid mechanism generally requires a large activation energy with unstable reaction intermediates [56,58], we mainly focus on the redox and the carboxyl mechanisms shown in Table 3 and Fig. 6. Following adsorption of CO* and H2O* onto the catalyst surface, water dissiciation (or hydrolysis, H2Oþ* ¼ H*þOH*) is the first catalytic step in both the redox mechanism and carboxyl mechanism. It often has the highest energy gap and determines the overall WGSR reaction rate [55]. For ZnAleLDHs, this step has an energy barrier (Ea) of 2.46 eV and
Please cite this article as: Meng Y et al., Experimental and theoretical investigations into the activity and mechanism of the wateregas shift reaction catalyzed by Au nanoparticles supported on ZneAl/Cr/Fe layered double hydroxides, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.172
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Table 3 e Changes in activation energies and reaction energies for each reaction step in the Au/ZnMeLDHs (M ¼ Al, Cr, Fe) system.
Elemental reaction
Redox mechanism
Carboxyl mechanism
1:H2Oþ* ¼ H*þOH* 2a:OH* ¼ O*þH* 2b:OH*þOH* ¼ H2O*þO* 3:CO*þO* ¼ CO2*þ* 4:CO*þOH* ¼ COOH*þ* 5a:COOH*þ* ¼ H*þCO2* 5b:COOH*þOH* ¼ H2O*þCO2* 6:2H* ¼ H2*þ*
ZnAl-LDHs
Au/ZnAl
Au/ZnCr
Au/ZnFe
Ea/eV
DE/eV
Ea/eV
DE/eV
Ea/eV
DE/eV
Ea/eV
DE/eV
2.46 2.04 1.09 1.54 1.86 2.01 2.41 0.87
1.23 1.11 0.74 2.55 0.54 0.33 1.05 0.58
1.81 1.47 0.84 0.69 1.61 1.87 1.73 0.56
0.92 0.89 0.58 2.32 0.34 0.19 1.23 0.22
1.49 1.26 0.65 0.37 1.43 1.47 1.57 0.45
0.64 0.75 0.35 1.55 0.11 0.30 1.51 0.11
1.67 1.31 0.71 0.42 1.52 1.68 1.66 0.49
0.81 0.73 0.52 2.01 0.32 0.24 1.35 0.29
* indicates a vacant site. X* denotes an adsorbed X specie.
Fig. 6 e Reaction diagrams for the WGSR over different catalysts: redox mechanisms (a and b) and carboxyl mechanisms (c and d). Please cite this article as: Meng Y et al., Experimental and theoretical investigations into the activity and mechanism of the wateregas shift reaction catalyzed by Au nanoparticles supported on ZneAl/Cr/Fe layered double hydroxides, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.172
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the reaction is overall endothermic by 1.23 eV. In the case of Au/ZnMeLDHs, it is also an endothermic step, but the energy barrier decreased to 1.81 eV, 1.49 eV and 1.67 eV resectively for Au/ZnAleLDH, Au/ZnCreLDH and Au/ZnFeeLDH, suggesting that hydrolysis is more likely to occur on the surface of Au/ ZnMeLDHs and Au/ZnCreLDH may have the highest activity for this step in hydrolysis. After H2O* dissiciation, the hydroxyl reaction can be divided into two ways in the redox path. Redox path a involves the OH*, which would continue to dissociate (reaction 2ea, OH* ¼ O*þH*), redox path b involves the OH*, which would undergo disproportionation (reaciton 2eb, OH*þOH* ¼ H2O*þO*). As shown in Fig. 6a and b, the Ea significantly decreases after including Au NPs in the LDH structure, but all of Au/ZnMeLDHs still exhibit a relatively high energy barrier (>1.26 eV). The energy barrier in step 2eb is quite smaller than that of in step 2ea for all catalysts. This step for ZnAleLDHs overcomes the energy barrier (Ea) of 1.09 eV and is endothermic by 0.74 eV. In the case of Au/ZnMeLDHs, OH* is more prone to disproportionation, with a barrier of 0.65e0.84 eV and is exothermic by 0.35e0.58 eV. The Ea value for this elementary step follows the order: LDH>Au/ZnAleLDH>Au/ ZnFeeLDH>Au/ZnCreLDH. Based on the energy barrier and thermodynamics, the disproportionation (step 2eb) is more likely to occur on the surface of LDHs samples, especially for Au/ZnCreLDH. The CO* oxidation (step 3, CO*þO* ¼ CO2*þ*) by atomic O* is very easy to carry out on all catalysts based on thermodynamic analysis. The value of the energy barriers also follows the order of: LDH>Au/ZnAleLDH>Au/ZnFeeLDH>Au/ ZnCreLDH. The lowest energy configuration shows CO adsorbed at the BrieAu site, with the CO molecule lies vertically on the catalyst surface with the carbon side closest to the surface, as shown in Figure S2e1. The elementary reaction is exothermic on all LDHs, confirming that CO* is more easily oxidized under low temperature conditions. The H2* recombination reaction (step 6, 2H* ¼ H2*þ*) is endothermic, signifying that considerable H* coverage can be expected under typical lowetemperature WGSR conditions. H* is prone adsorption at BrieZn sites on the LDH or Au/ ZnMeLDH (M ¼ Al, Cr, Fe) surfaces, in which both H and Zn interact with oxygen at the BrieZn sites (Figure S2e5). After H* is recombined to form hydrogen, it is vertically adsorbed on the HcpeZn face of the catalysts, and the C and O bonds with the 3 oxygens around the Zn element in this adsorption process (Figure S2e7). The activation energy barrier for this step is relatively low, and the Ea is further decrease after Au supporting, from 0.87 eV to 0.45e0.56 eV. The energies of reaction intermediates for the WGSR over different catalysts following the carboxyl mechanism is shown in Fig. 6c and d. The first and last step in the carboxyl mechanism are the same as the redox mechanism and have been discussed above. In the carboxyl mechanism, the energy barrier of CO* oxidation by OH* (step 4, CO*þOH* ¼ COOH*þ*), COOH* dissociation (step 5ea, COOH*þ* ¼ H*þCO2*) and COOH* disproportionation (step 5eb, COOH*þOH* ¼ H2O*þCO2*) are quite larger than the Ea in step 2 and 3 in the redox mechanism. This indicates that the carboxyl mechanism is not the favorable reaction path for the WGSR on LDHs and Au/ZnMeLDHs. Comparing the activation energies for all
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the elementary reactions, we can conclude that the first reaction step generally has the highest energy gap and is the rate limiting step. Auebased catalysts can effectively lower the energy barrier of the kinetically relevant step of H2O dissociation, which is the reason for the high activity of Au/ ZnMeLDHs. Compared with the carboxyl mechanism, the redox mechanism has the lower energy barrier and is the more likely reaction pathway for WGSR reaction, especially for redox path b, 1e2ebe3e6. In most reaction steps, the required activation energy follows the following order: LDH>Au/ZnAleLDH>Au/ZnFeeLDH>Au/ZnCreLDH. Such calculations suggest that the activity for LDH samples are in the order of: Au/ZnCreLDH>Au/ZnFeeLDH>Au/ZnAleLDHs> LDHs, which is indeed the case in our experimental data (as shown in Fig. 4).
Conclusions In this paper, DFT calculations were employed to gain mechanistic insights into the adsorption energetics and activation barriers between the transitions of different elementary steps and routes in WGSR catalyzed by Au nanoparticles supported on different LDHs, Au/ZnMeLDHs (M ¼ Al, Cr, Fe). The calculations show that the introduction of Au NPs decreases the activation energy of important steps in the WGSR, especially the kinetically relevant step of H2O dissociation, and the apparent activation energy follows the following order: LDH>Au/ZnAleLDH>Au/ZnFeeLDH>Au/ ZnCreLDH. Combined with the insights obtained from our theoretical calculations, Au NPs supported on ZnMeLDHs were synthesized and their catalytic performances for WGSR were evaluated, yielding results consistent with our calculations. It was found that the addition of Au NPs changes the reaction mechanism by significantly lowering the activation barrier of the kinetically relevant step of H2O dissociation. Au/ZnCreLDH exhibits the best catalytic performance including 79.4% CO conversion, reaction rate of 1 1 TOF and an activation energy of 102.1 mmol g1 cat s , 1.01 s 41.7 kJ mol1. Also, TPR experiments suggest that the introduction of Au changes the electronic and geometric structure, which alters the redox behavior of the LDH involved in the WGSR. In situ DRIFTS shows that the production of CO2 during the WGSR process goes through the reaction between CO and adsorbed O, which comes from the dissociation of OH species. Such result can be supported by DFT calculations, which shows that the Redox mechanism b has the lowest energy barriers and is the most potential reaction pathway. Our study demonstrated a strategic method in the design of industrially relevant catalysts by integrating both theory and experiment, which allows one to extract mechanistic insights of a catalytic reaction and can be further applied in the engineering of more efficient catalyst.
Acknowledgments This work is supported by National Natural Science Foundation of China (21503188).
Please cite this article as: Meng Y et al., Experimental and theoretical investigations into the activity and mechanism of the wateregas shift reaction catalyzed by Au nanoparticles supported on ZneAl/Cr/Fe layered double hydroxides, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.172
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Appendix A. Supplementary data [15]
Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.10.172. [16]
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Please cite this article as: Meng Y et al., Experimental and theoretical investigations into the activity and mechanism of the wateregas shift reaction catalyzed by Au nanoparticles supported on ZneAl/Cr/Fe layered double hydroxides, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.172