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Identifying Size Effects of Pt as Single-Atoms and Nanoparticles Supported on FeOx for Water Gas Shift Reaction Yang Chen, Jian Lin, Lin Li, Botao Qiao, Jingyue Liu, Yang Su, and Xiaodong Wang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02751 • Publication Date (Web): 13 Dec 2017 Downloaded from http://pubs.acs.org on December 13, 2017
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Identifying Size Effects of Pt as Single-Atoms and Nanoparticles Supported on FeOx for Water Gas Shift Reaction Yang Chen,a,b Jian Lin,a* Lin Li,a Botao Qiao,a Jingyue Liu,c Yang Su,a Xiaodong Wanga* a
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, Dalian 116023, P. R. China. b c
University of Chinese Academy of Sciences, Beijing 100049, P. R. China
Department of Physics, Arizona State University, Tempe 85287, USA
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Abstract: Identification of size effects at an atomic level is essential for designing high performance metal-based catalysts. Here, the performance of a series of FeOx supported Pt catalysts with Pt as nanoparticles (Pt-NP) or single-atoms (Pt-SAC) are compared for low-temperature water gas shift (WGS) reaction. A variety of characterization methods such as adsorption microcalorimetry, H2-TPR, in situ DRIFTS and transient analysis of product tests were used to demonstrate that Pt nanoparticles exhibit much higher adsorption strength of CO; the adsorbed CO reacts with the OH groups, which are generated from activated H2O, to form intermediate formates that subsequently decompose to produce CO2 and H2 simultaneously. On the other hand, Pt single-atoms promote the formation of oxygen vacancies on FeOx which dissociate H2O to H2 and adsorbed O that then combines with the weakly adsorbed CO on these Pt sites to produce CO2. The activation energy for the WGS reaction decreases with the downsizing of Pt species and Pt-SAC possesses the lowest value of 33 kJ/mol. As a result, Pt-SAC exhibits one order of magnitude higher specific activity than Pt-NP. With a loading of only 0.05 wt% the Pt-SAC can achieve ~65% CO conversion at 300 oC, representing one of the most active catalysts reported so far. Keywords: Water gas shift, Size effect, Pt, Single-atoms, Nanoparticles, Mechanism
1. Introduction Hydrogen plays an important role in industrial application as a raw material in process of hydrogenation or ammonia synthesis1,2 and also acts as a clean energy carrier for polymer electrolyte membrane fuel cells (PEMFCs) to provide clean power for automobiles.3,4 Currently, the state-of-the-art strategy for H2 production is based on steam reforming of fossil fuels such as CH4, hydrocarbons and coal.1,5 Particularly, this established technology is more and more recognized as a leading player with the increasing production capacity of natural/shale gas recently. One of the drawbacks by this reforming technology is coproduction of a large amount of poisonous CO.6 The water gas shift (WGS) reaction (CO + H2O ↔ CO2 + H2, ∆Ho = -41.2 kJ/mol) is a key step to eliminate the residual CO and simultaneously increase the H2 yield.7 Moreover, WGS is a moderately exothermic reaction, to which the low CO concentration level can be achieved favorably at lower temperatures. Pt-group-metal and Au based catalysts have been under intense development during 2
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the past decades for low-temperature WGS reaction with good stability and activity.8-10 Haruta el al. found that the Au-based catalysts with particle sizes of 2~5 nm were active not only for CO oxidation but also for low-temperature WGS.11 Andreeva et al. postulated that the ionic Au species at the interfaces between Au NP and support were the active sites.12 Flytzani-Stephanopoulos et al. reported that metallic Au acted as spectator and that the active sites were in the form of Au-O species.13 Furthermore, they confirmed that atomically dispersed Au species stabilized by alkali ions were active regardless of chemical properties of the supports. Such a process was not limited to Au but also worked for Pt metal.14-17 Our group reported that Pt-group-metal single-atoms such as Ir, Pd, Rh acted as dominant sites for WGS reaction with much higher activity and good selectivity to avoid hydrogenation of CO or CO2.18-20 Rodriguez et al., however, found that when ionic Au species were completely transformed into metallic Au, the WGS activity was increased.21 Ma et al. synthesized flat Au islands with size around 2 nm on α-MoC catalyst, which can realize the equilibrium conversion of CO at 150 oC due to the facile activation of H2O on MoC to provide OH species which reacted with adsorbed CO on adjacent Au sites.22 Similarly, Pt raft-like particles on Mo2C also possessed much higher activity than other types of Pt-based catalysts.23 This inconsistency is even more severe on the role of alkali cations. Different from the reported improvement in metal dispersion, Zhu et al. found that the alkali cations kept Pt positively charged without modifying the size distribution of the Pt nanoparticles.24 While Pazmiño et al. proposed that the promotional effect was caused by the modification of the properties of the support and that the active Pt species remained in a metallic state.25 The identification of active species for a specific reaction such as the WGS reaction has attracted extensive attention and need further clarification. As for the reaction mechanism of WGS, two general schemes of either “redox” or “associative” process have been proposed with the participation of the dispersed metal or/and the support. Recently, Heyden et al. tried to discriminate the nature of active sites and reaction mechanism by using a combination of DFT and microkinetic modeling, which showed a redox pathway on both Pt single cation and Pt clusters for WGS reaction on Pt/TiO2.26 It was also claimed that the second CO molecule adsorbed on Pt single-atoms was much weaker, which can facilely react with the neighboring O species to form CO2, thus explaining the non-disappearance of the Pt single-atom related CO adsorption peak.27 On the other hand, the role of a reducible 3
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oxide support should be considered with the change between “redox” mechanism and “associative” mechanism depending on the specific reaction conditions.28,29 The detailed comparison of reducible oxide supported metal single-atoms and nanoparticles for WGS reaction is still needed. In this work, we specifically constructed two types of Pt-based catalysts with exclusive Pt single-atoms or nanoparticles dispersed on the same Fe3O4 support in order to probe the effect of Pt particle size on the kinetic and mechanistic aspects for WGS reaction. The selection of Pt was based on extensive interest in low-temperature WGS and FeOx as support due to the stabilization of highly dispersed metal species but also itself as active sites.8,16,30,31 A series of characterizations such as adsorption microcalormetry, in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) and transient analysis of surface reaction, coupled with kinetic measurements were employed to establish relationship between particle size and adsorption/reaction performance of these Pt/FeOx catalysts. The reaction mechanism of WGS was then identified with a “redox” process on the Pt/FeOx single-atom catalyst while on the Pt nanoparticle catalyst the “associative” process dominated.
2. Experimental 2.1 Catalyst preparation All chemical reagents used in this experiment were of analytical grade. FeOx supported Pt catalysts were prepared by co-precipitation methods as reported previously.32 Taking 1.15 wt% Pt/FeOx as an example, 3.78 g of Fe(NO3)3·9H2O and 3.7 mg mL-1 H2PtCl6·6H2O (3.24 mL) were mixed in 100 mL deionized water. This aqueous solution was added dropwise (3 mL min-1) to a 0.28 M NaOH solution with the pH value of the resulting solution controlled to ~8. After stirring for 3 h and aging for 1 h at 80 oC, the resulting precipitation was filtered and washed with 1 L hot ultrapure water, then dried at 80 oC overnight and calcined at 400 oC for 6 h. The other samples with different loadings were prepared by the same method but changing the used amount of H2PtCl6·6H2O. The resulted samples were denoted as Pt/Fe-n with n as the loading amount of Pt. The FeOx support was prepared by precipitation from Fe(NO3)3·9H2O under similar conditions. FeOx supported Pt nanoparticles catalyst, denoted as PtNP/Fe-n was prepared by the colloid-deposition method. The Pt nanoparticles was synthesized by modifying the method reported in literature.33 For a typical synthesis, 1 g of H2PtCl6·6H2O was 4
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dissolved in 50 mL glycol solution, then a glycol solution of NaOH (50 mL, 0.5 M) was added with stirring to obtain a yellow Pt colloidal solution. This solution was heated at 140 oC for 3 h under a flow of inert atmosphere of Ar. Then dark-brown colloidal Pt nanoparticles were obtained. Afterwards, a certain amount of colloidal Pt nanoparticles was deposited onto the FeOx support under stirring at 80 oC. The resulting precipitation was filtered and washed with 1 L hot ultrapure water, then dried at 80 oC overnight and calcined at 400 oC for 6 h. 2.2 Catalyst performance test The catalytic activity tests were carried out in a fixed-bed reactor under atmospheric pressure. Before evaluation, 100 mg catalyst was reduced at 300 oC in a flow of 20 mL min-1 of 10 vol% H2/He. A mixture of 2 vol% CO + 10 vol% H2O + He were used for WGS reaction with a flow rate of 30 mL min-1, which resulted in a space velocity of 18000 mL gcat-1 h-1. The concentration of water was controlled by bubbling He through a water reservoir at controlled temperature. The concentrations of reactants and products in the inlet and outlet of the reactor were analyzed by an on-line gas chromatograph (Agilent 7890) with TDX-01 column using He as carrier gas. CO conversion (XCO) was calculated by equation (1): X CO =
ncoin − ncoout ×100% ncoin
(1)
Specific reaction rates and turnover frequency (TOF) for WGS over the Pt/FeOx catalysts at different temperatures were evaluated by decreasing the weight of catalysts from 100 mg to 3 mg to guarantee that the CO conversion is below 15%. For each run at a specific temperature, the CO conversions were averaged at the steady state and used to obtain the specific rates, which was calculated by equation (2): rCO =
X co ⋅ fco mPt
(2)
Where fco was molar flow rate of CO in mol h-1, and mPt was the mass of Pt in the fixed bed. TOF was calculated according to equation (3) based on the specific rates and dispersions of Pt species, which were determined by CO chemisorption with the assumption of the stoichiometric ratio of adsorbed CO/Pt=1. For Pt-SAC, Pt dispersion can be considered as 100%.34,35 5
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TOF =
rCO ⋅ MPt DPt
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(3)
Where the MPt was the molar weight of Pt and DPt was the dispersion of Pt calculated from the results of CO pulse chemisorption by assuming the stoichiometric ratio of adsorbed CO/Pt is 1. The apparent activation energies of these catalysts were determined by the Arrhenius equation (4).
k = Ae− Ea / RT
(4)
2.3 Characterization techniques Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and high resolution transmission electron microscopy (HRTEM) images were obtained on a JEOL JEM-ARM 200F equipped with a CEOS probe corrector, with a guaranteed resolution of 0.08 nm. Before microscopy examination, the samples were reduced by 10 vol% H2/He at 300 oC for 30 min. After that, the samples were suspended in ethanol with ultrasonic dispersion for 5-10 min and then a drop of the solution was put onto a TEM copper grid coated with a thin holey carbon film. The in situ DRIFT spectra were collected with a Bruker Equinox 70 spectrometer, equipped with a MCT detector and operated at a resolution of 4 cm-1 for 128 scans. There were three sets of experiments. Firstly, before CO adsorption, the samples were reduced in situ with 10 vol% H2/Ar at a specific temperature according to the TPR results. Then the samples were cooled to 100 oC, the background spectrum was recorded. After that, 1 vol% CO/He was introduced to the sample successively and then switched to He to flush the gaseous CO. The spectra were collected till the steady state. Secondly, as for the in situ study of WGS, the reduced sample was submitted to 1 vol% CO + 3 vol% H2O with temperature rising from 100 to 250 oC. The DRIFTS spectra at steady state were recorded as a function of temperature. Finally, the fresh samples were submitted to He at 250 oC and the background spectrum was recorded. After that, CO was introduced. Then CO was switched off and 3 vol% H2O was introduced. After that, CO was introduced again after the stop of H2O. The DRIFT spectra were obtained as a function of time until saturation at each step. CO adsorption microcalorimetry was performed using a BT 2.15 heat-flux calorimeter (Seteram, France), of which the detailed procedures have been described earlier.36 The calorimeter was connected to a gas handling and a volumetric system 6
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employing MKS 698A Baratron Capacitance Manometers for precision pressure measurement (±1.33x10-2 Pa). Prior to the adsorption, the sample was pre-reduced by H2 in a special treatment cell, followed by evacuation at 310 oC for 30 min. The adsorption experiment was conducted at 40 oC. The transient analysis of surface reaction at a specific temperature was carried out with on-line mass spectroscopy (MS) analysis. The gate time for MS was 0.1 s for each detected component (CO, CO2, H2, H2O), equivalent to the acquisition of 2 data point per second. Approximately 100 mg of Pt/Fe catalysts were loaded into a fix-bed reactor and reduced in situ with 10 vol% H2/He at 300 oC for 30 min, then purged with He for 1 h. The reduced samples were submitted to the WGS reactants consisted of 2 vol% CO+10 vol% H2O at a flow rate of 30 mL min-1 at a specific temperature. The MS recorded the gas components until the steady state. More characterization techniques such as ICP, BET, XRD, Raman, H2-TPR, CO pulse chemisorption can be found in the supporting information.
3. Results 3.1 Evaluation of catalyst performance The CO conversions with temperature during WGS over these Pt/FeOx catalysts are presented in Fig. 1. On the pure FeOx support, the CO conversion is negligible in the temperature range of interest,
