Article Cite This: Ind. Eng. Chem. Res. 2018, 57, 12078−12086
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Structurally Strained Bimetallic PtFe Nanocatalysts Show Tunable Catalytic Selectivity in Aqueous Oxidation of Bio-Polyols to Dicarboxylic Acids Xin Jin,*,† Qi Xia,† Jie Ding,† Jian Shen,‡ Chaohe Yang,† and Raghunath V. Chaudhari§
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†
State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China University of Petroleum, Qingdao, Shandong 266580, China ‡ College of Environment and Resources, Xiangtan University, Xiangtan, Hunan 411105, China § Center for Environmentally Beneficial Catalysis, University of Kansas, Lawrence, Kansas 66047, United States S Supporting Information *
ABSTRACT: Rational control of catalytic selectivity still remains a grand challenge in the field of biomass conversion to valueadded chemicals. In this paper, we used PtFe catalyst as an example to understand fundamentals for lattice strain, electronic reconfiguration, reaction kinetics and catalytic selectivity for aqueous conversion biopolyols. Structurally strained face centered tetragonal (fct) PtFe crystals were synthesized and it was confirmed that such unique PtFe nanostructures display intriguing electronic coupling effect and partial charge distribution, as revealed by surface characterization using TEM, XRD and XPS, as well as DFT calculation. Bimetallic PtFe-fct catalysts exhibit a remarkable catalytic activity (TOF: 24 312 h−1 at 65 °C and 1 MPa O2), enhanced selectivity (tartronic acid: 54%) and improved stability for aqueous phase oxidation of biopolyols, in comparison with conventional fcc morphology. Kinetic modeling further indicates that relatively lower oxidation barrier and restrained decarboxylation reaction are key for improving selectivity on PtFe-fct catalysts.
1. INTRODUCTION Exploring morphological defects induced novel structures is known to be an effective strategy for enhancing catalytic activity.1−4 Lattice strain induced electron reconfiguration is key for tunable selectivity toward targeted products.5,6 In this context, structural strain is significantly important for manipulating interfacial lattice structures,7−13 thus catalytic selectivity of supported metal catalysts can be well tuned for specific reactions.14−17 In the field of biomass conversion to chemicals, improving catalytic selectivity still remains a central challenge. As far as we know, the influence of structural strain on catalytic selectivity and kinetic behaviors in aqueous biomass conversion is largely unexplored. Therefore, in this work, we used lattice distorted bimetallic PtFe crystal as a representative example, and demonstrated that lattice strain in PtFe facets induces tunable catalytic selectivity toward valueadded products during biopolyol oxidation in aqueous phase. Tartronic acid (TA) and derivatives are fundamental building blocks for manufacture of nylon and other important polymer materials.18−20 Catalytic conversion of cellulosic © 2018 American Chemical Society
biomass such as glycerol and glucose represents an economic way to produce such valuable product (Scheme 1). How to enhance selectivity toward TA still remains a grand challenge. To our best knowledge, however, only a few attempts have been made in literature on catalytic synthesis of TA from Scheme 1. Glycerol Oxidation to Tartronic Acid
Received: Revised: Accepted: Published: 12078
June 19, 2018 August 3, 2018 August 16, 2018 August 16, 2018 DOI: 10.1021/acs.iecr.8b02742 Ind. Eng. Chem. Res. 2018, 57, 12078−12086
Article
Industrial & Engineering Chemistry Research
bimetallic PtFe catalysts were treated under the same conditions (0.1 MPa H2 atmosphere at 400 °C); they are denoted as PtFex-fct. 2.3. Surface Characterization. Detailed procedures of transmission electron microscopy (TEM) were similar to that previously described.38,39 In particular, particle size was measured in STEM mode counting over 150 particles, while element mapping analysis was carried out to identify element distribution in selected region, which is similar to previously described.40,41 The specifics have also been described in our recent work. X-ray photoelectron spectroscopy (XPS) instrument: PHI 5000 Versa Probe II. X-ray source: Monochromated Al. Take-off angle: 45°. Beam Size: 100 μm. Beam Power: 25 W/15 kV. Pass Energy: 23.5 eV. eV Step: 0.2. Sweep: 20. 2.4. Oxidation Tests. In a typical run, about 0.05 g of solid catalyst powders was added to 25 mL of aqueous solution containing 0.22 kmol/m3 of glycerol. The reaction was carried out in a 50 mL of autoclave. The slurry was heated in jacket with precise temperature control before heated up to targeted reaction temperature (e.g., 80 °C). Once the liquid slurry was at reaction temperature, stirring rate was set at 1000 rpm, which signified the start of an experimental run. The starting pressure was set at 1 MPa of O2. Liquid samples were taken out of the reactor after a fixed period of time, then injected into HPLC (SHIMADZU with SH1011 column). The identification of each product has been reported previously. The concentration of glycerol and oxidation products were thus obtained for the calculation of conversion (X), selectivity (S), carbon balance (C%) and turnover frequency (TOF, in mol/ molPt·h). These have been clearly defined in our recent papers.42 Conversion is defined as the ratio of amount of glycerol converted to that initially charged. Selectivity toward a specific product is defined as the ratio of total amount of carbon atoms in this product generated during certain reaction time over that in converted glycerol. C% is defined as the ratio of total amount of carbon in all products to that of converted glycerol (or based on other substrates). TOF is defined as the amount of glycerol (substrate) converted over Pt content in the reaction system per time. External and internal mass transfer limitation was estimated to ensure its influence is negligible, which suggests that the reaction rates measured during experiments are intrinsic rates on catalyst surface. 2.5. DFT Calculation. The Pt(111) surface was modeled using a triple-layer p (4 × 4) slab comprising 3 metal layers with 16 atoms per layer and each slab was separated by a vacuum of 17 Å to guarantee the interaction between neighboring cells was negligible in the z direction. All DFT calculations were performed using the CASTEP code and the exchange-correlation effects were described with the generalized gradient approximation (GGA) using the PBE functional proposed by Perdew, Burke and Ernzerhof. The electron wave functional was expanded by a plane-wave basis set with a cutoff energy of 400 eV. A 3 × 3 × 2 Monkhorst−Pack k-point grid was used for the integrations of the Brillouin zone. Details were already provided in one of our previous work.
glycerol and glucose. It is well-known that oxidation of glycerol and glucose easily generates glyceric acid and lactic acid as monocarboxylic intermediates (selectivity: ∼90%).21−24 However, selective oxidation of glyceric acid to TA is very difficult due to intrinsically poor secondary oxidation selectivity. More importantly, decarboxylation tendency is significantly strong in the presence of noble metal catalysts.25−28 For example, monometallic Pd and Au catalysts display good selectivity of TA (∼80%), but poor activity (TOF < 100 h−1) restricts their industrial applications.29−31 Bimetallic alloyed PtM (M: Cu, Pd, Fe, Mn) catalysts show promising performances compared with monometallic ones (TOF: 2300−20 000 h−1, S = 12− 42%).32−37 Therefore, it is clear that, in most cases, bimetallic catalysts outperform monometallic ones in terms of catalytic activity. How to enhance catalytic selectivity toward TA is still a grand challenge in this area. Current efforts are primarily focused on exposing high index surfaces with more surface defects in novel catalytic materials, because it is known that defected sites are more catalytically active for oxidation reactions. However, such novel materials often unfavorably enhance side reactions (i.e., decarboxylation, CO2 selectivity ∼21%) even under mild conditions. Therefore, it is clear that there demand significant research efforts focusing on improving catalytic activity while restraining side reactions. There is a great impetus to improve catalytic selectivity by rational design of novel structured catalysts and fundamental understanding on reaction kinetics to achieve both good activity and selectivity for TA synthesis from biomass feedstocks. Therefore, in this work, we reported lattice strain induced fct PtFe nanocatalysts with distorted Pt−Fe lattice structures for enhanced selectivity of TA during oxidation of glycerol and other bioderived feedstocks in aqueous phase. Different from existing studies, we found that the remarkable structural strain within PtFe-fct crystal results in electronic exchange between Pt and Fe atoms. It is found that structurally strained PtFe-fct catalysts display 3-fold higher oxidation activity (TOF: 24 312 h−1 at 65 °C and 1 MPa O2) for glycerol conversion compared with disordered PtFe-fcc catalysts. More importantly, selectivity toward value-added TA can be enhanced by two folds, with almost negligible tendency toward decarboxylation reactions over PtFe-fct catalysts. Detailed kinetic modeling on glycerol oxidation further revealed that, high electron-rich and ordered Pt surface are key for enhanced selectivity toward dicarboxylic acid formation during aqueous oxidation of biopolyols.
2. EXPERIMENTAL SECTION 2.1. Materials. Chemicals used in this paper were purchased from Sigma-Aldrich and Aladdin. 2.2. Synthesis of Supported Pt, Fe and PtFe Catalysts. The growth of PtFex-fcc (x: Fe/Pt atomic ratio, x = 1.5, 2.5, 4, 6) nanocatalysts with disordered morphologies was achieved following a one pot wet chemistry approach in the presence of CeO2 support in aqueous medium. The Pt loading on CeO2 for all catalysts was 1 wt %, while the content of Fe varied. Typically, predetermined amounts of H2PtCl6 and Fe(NO3)3 were mixed with 1.2 g of CeO2 powder in 200 mL of deionized water (DI H2O) in a beaker. The slurry was stirred at room temperature for 30 min, followed by addition of a 100 mL of DI H2O containing 0.2 g of NaBH4. The mixture was aged for 8−10 h, after which the slurry was taken out, filtered and washed with 200 mL of DI H2O several times. All annealed
3. RESULTS AND DISCUSSION 3.1. Preliminary Results. Instead of manufacturing bimetallic nanocrystals in organic solvent via slow reduction kinetics as we reported before,43,44 in this work, we prepared PtFe-fcc catalysts using a simple in situ reduction method in aqueous medium. NaBH4 was added dropwise to an aqueous 12079
DOI: 10.1021/acs.iecr.8b02742 Ind. Eng. Chem. Res. 2018, 57, 12078−12086
Article
Industrial & Engineering Chemistry Research
between Pt and Fe crystals with average particle size of 17.5 ± 3.6 nm and lattice spacing of 0.21 nm, while Figure 1b confirms that, after thermal annealing, PtFe4-fct catalyst exhibits much more ordered shapes with 16.1 ± 3.4 nm in size and lattice spacing of 0.23 nm, suggesting a slight lattice expansion existing in ordered structure. XRD and XPS have been generally known to characterize plausible lattice distortion features in bimetallic catalysts. In this case, Pt content is very low compared with bulk CeO2 phases. It is found in XRD characterization for low Pt loading (1 wt %) only reflects bulk crystallinity of CeO2 support due to low metal loading. Therefore, we prepared PtFe4-fcc and PtFe4fct catalysts with higher Pt loading (5 wt %) to ensure more visible peaks for Pt species. It is found in Figure S1 that, while major diffraction peaks for (111), (200), (220), (311), (222) and (400) of CeO2 crystals are significant and unchanged for PtFe4-fcc and PtFe4-fct catalysts, we observe detectable but not significant chemical shift for Pt (111), (200) and (220) facet toward higher diffraction angle for PtFe4-fct catalyst. This observation further confirms the lattice distortion in fct structures. We also carried out preliminary XPS studies to understand possible Pt−Fe interaction in fcc and fct structures. As shown in Figure 1c, monometallic Pt catalyst shows characteristic peaks for Pt4f5/2 and Pt4f7/2 at 76.4 and 72.6 eV. Bimetallic PtFe samples, however, show significant red shift of binding energy toward lower levels. In particular, binding energies for PtFe4-fcc catalyst are 74.0 and 70.2 eV, respectively. Lower binding energies for PtFe4-fcc catalyst suggests that the addition of Fe alters Pt electronic binding capabilities by donating more electrons into Pt orbitals. This is different from what literature has reported for bimetallic PtFe nanocrystals. Possibly due to fast reduction kinetics in this case, Fe element tends to interact strongly with Pt lattices during reduction processes. It is observed in this case that PtFe4-fcc catalyst displays flowershaped morphologies with several small clusters agglomerating to form supper lattices during fast reduction of metal precursors. Such structures lead to lower Pt electron binding energies due to strong interaction between clusters. Surprisingly, PtFe4-fct catalyst shows a dramatic peak split around 76.4 and 72.6 eV. This observation suggests that fast reduction kinetics in aqueous phase medium are critical for eventual morphology of bimetallic PtFe structures, which is different from what we previously found using organic solvent as reducing agent.43,44 In previous case, slow reduction kinetics in organic solvent led to template growth of bimetallic PtFe clusters and anisotropic formation of disordered nanocrystals with high surface index numbers. However, in this case, fast reduction kinetics induce instantaneous formation of large amounts of small clusters, while surface redox of Fe3+−Pt0 and Pt2+−Fe0 is much minor. Therefore, fast reduction kinetics lead to formation of flower shaped disordered structures rather than pyramid morphologies, as observed in the case with slow reduction environment. Preliminary characterization by TEM, XRD and XPS indicates that fcc and fct structures display unique morphologies with varied local electronic configurations. These unique features motivated us to conduct activity and selectivity tests for these catalysts. Figure 1d presents the benchmark experimental results for glycerol oxidation in aqueous phase at 80 °C and 1.0 MPa. We observed that glycerol conversion is 35.2% over monometallic Pt catalysts after 6 h, with selectivity toward glyceric acid and TA being
solution containing H2PtCl6 and Fe(NO3)3 as metal precursors and CeO2 as support. After the drying process, PtFe-fcc catalysts were annealed in H2 environment at 400 °C to obtain PtFe-fct samples. PtFe-fcc and PtFe-fct catalysts with various Fe/Pt ratios were denoted as PtFex-fcc and PtFex-fct (Pt metal loading on CeO2: 1 wt %, atomic ratio x = 1.5, 2.5, 4, 6), respectively. Catalyst samples were characterized using TEM, XRD, XPS and chemisorption (Figures 1, S1 and 2, Table S1),
Figure 1. TEM images and morphological analysis for (a) PtFe4-fcc and (b) PtFe4-fct catalysts (yellow and white bars indicates 20 and 5 nm respectively). (c) XPS characterization for monometallic Pt, bimetallic PtFe4-fcc and PtFe4-fct catalysts. (d) Conversion/selectivity results of glycerol oxidation over the three catalysts after 6 h at 80 °C and 1.0 MPa.
while detailed lattice structures including Pt−Pt, Pt−Fe and Fe−Fe bond, partial charge distribution were obtained based on density functional theory (DFT) calculation (Figure 3). Aqueous phase oxidation of glycerol was conducted in an autoclave at 50−80 °C under 1.0 MPa O2 pressure, followed by detailed analysis on reaction results and kinetic modeling (Figures 4, S3 and S4). Surface characterization using TEM characterization clearly distinguish the morphologies of PtFe4-fcc and PtFe4-fct catalysts. It is shown in Figure 1a that PtFe4-fcc sample displays a disordered morphology due to lattice mismatch 12080
DOI: 10.1021/acs.iecr.8b02742 Ind. Eng. Chem. Res. 2018, 57, 12078−12086
Article
Industrial & Engineering Chemistry Research
Figure 2. Additional TEM images for (a) PtFe1.5-fcc, (b) PtFe2.5-fcc, (c) PtFe6-fcc and (d) PtFe2.5-fct catalysts (yellow and white bars indicates 20 and 5 nm, respectively), (e) surface Fe/Pt atomic ratios, (f) particle size analysis, (g) additional XPS characterization for PtFe1.5-fcc, PtFe6-fcc, PtFe1.5-fct and PtFe6-fct catalysts, (h,i) red shift for Pt4f5/2 and Pt4f7/2 orbitals.
Figure 3. DFT results on Pt, PtFe-fcc and PtFe-fct catalysts in terms of (a) DOS, (b−d) metal−metal spacing and (e,f) charge distribution.
(46%, after 6 h) compared with other catalysts. Side reactions are largely restrained as carbon loss is lower than 4% on PtFe4fct catalyst. Preliminary experimental studies clearly show that PtFe4-fct catalyst outperforms others in terms of conversion and selectivity toward TA. 3.2. Detailed Characterization on Bimetallic PtFe Nanostructures. We therefore carried out more detailed TEM and XPS characterization to understand the fundamentals of PtFe nanostructures on oxidation activity and selectivity toward TA in the following sections. TEM images presented in Figures 2, S2 and S3 present structural features and element distribution for bimetallic PtFe-
68.2% and 12.2%. Bimetallic PtFe catalysts display significantly higher conversion and improved selectivity. Specifically, PtFe4fcc catalyst shows 83.1% glycerol conversion and 14.5% selectivity to TA under similar reaction condition. Further analysis on product distribution also reveals that active PtFe4fcc catalyst unfavorably promote side decarboxylation reaction, producing CO2 as by product. Therefore, carbon loss is significant on this catalyst (∼19.2%). CO2 reacts with alkalis in the system and forms sodium carbonate. It cannot be detected by liquid chromatography, which is reflected by poor carbon balance in experimental results. We find glycerol conversion over PtFe4-fct catalyst is 100% with much higher TA selectivity 12081
DOI: 10.1021/acs.iecr.8b02742 Ind. Eng. Chem. Res. 2018, 57, 12078−12086
Article
Industrial & Engineering Chemistry Research
Figure 4. Experimental results on glycerol oxidation over bimetallic PtFe catalysts. (a) TOF vs Fe/Pt atomic ratio and TA yield (inset), (b) reaction pathways (1, glycerol; 2, glyceraldehyde; 3, glyceric acid; 4, glyceric aldehyde acid; 5, TA; 6, lactic acid; 7, glycolic acid), concentration time profiles on (c) PtFe4-fcc, (d) PtFe4-fct at 65 °C and 1 MPa O2. (e,f) Conversion/selectivity profiles at 80 °C and 1 MPa O2 over PtFe4-fcc and PtFe4-fct catalysts (see Figure S4 for more reaction profiles).
fcc and -fct catalysts with different Fe/Pt ratios. It is shown in Figure 2a−c that PtFex-fcc (x = 1.5, 2.5, 6) crystals clearly display disordered morphologies. They show average particle size of 17.3 ± 4.9 nm (Figure 2a), 18.9 ± 5.0 nm (Figure 2b) and 19.9 ± 5.5 nm (Figure 2c), respectively, suggesting the influence of Fe content on particle size is insignificant. Such mismatch creates strong surface tension thus regular growth of Pt and Fe monometallic crystals was disturbed, leading to anisotropic PtFe-fcc structures.45−48 EDX analysis (Figure S3) also shows that Pt and Fe elements are actually not evenly distributed within nanoparticles, suggesting fast and parallel growth of Pt and Fe crystals occur during NaBH4 reduction process. Compared with fcc structure, PtFe-fct samples show a clear size decrease after annealing processes. In particularly, we found that the average particle size for PtFex-fct (x = 1.5, 2.5, 6) is 14 ± 3.9 nm (Figure S2a), 14.1 ± 5.3 nm (Figure 2d) and 14.8 ± 3.6 nm (Figure S2b), respectively, which are slightly smaller compared with fcc counterparts (see particle size analysis in Figure 2f). Clearly, phase transformation from fcc to fct results in evolution from low to high structural orderness thus particle size becomes smaller.49,50 This hypothesis is also confirmed by element distribution by EDX mapping PtFe-fct catalysts for selected regions (Figure S3). Thus, it is clear that unexpected size changes during phase transformation from fcc to fct indicates metal−metal interaction and atomic alignment are critical for the morphology for bimetallic PtFe nanoparticles.51−54 To further understand the fundamentals underlying morphological evolution from fcc to fct structures caused by lattice mismatch, interfacial tension of Pt and Fe element was quantified by XPS characterization. XPS analysis reveals the surface Pt and Fe composition for fcc and fct structures (Figure 2e). We are surprised to find that surface Fe/Pt ratio is significantly higher than that of bulk ratio for all bimetallic catalysts. Specifically, Fe/Pt surface ratio in PtFe-fcc samples is in the range of 4 to 6, with increasing Fe/Pt bulk composition from 1.5 to 6. PtFe-fct catalysts exhibit slightly higher Fe/Pt
ratio compared with PtFe-fcc samples. Relatively higher Fe/Pt surface ratio in all catalyst samples results from slower reduction kinetic rates for Fe precursors during NaBH4 reduction processes, while further enhanced Fe/Pt surface composition in fct structures implies that there exists strong Pt−Fe interaction during annealing processes. Thus, we further investigated how binding energy changes with Fe/Pt compositions. Binding energies for all PtFe1.5, PtFe4 and PtFe6 catalysts (Figures 1c and 2g) show dramatic red shift to lower values in comparison with monometallic Pt catalyst, implying that the addition of Fe in Pt structures causes electronic reconfiguration in Pt-fcc crystals. Specifically, binding energy for both Pt4f5/2 and Pt4f7/2 in PtFe1.5-fcc, PtFe4-fcc and PtFe6-fcc samples exhibit downward shift up of approximately 2.4, 3.2 and 2.4 eV, respectively, in comparison with monometallic Pt catalyst (Figure 2h,i). This is possibly because insertion of Fe element with smaller lattice parameters cause structural changes in Pt facet during reduction of metal precursors.55 Local disturbance caused by such structural changes leads to lower binding energy of electron on Pt surface. It is further found that thermal annealing leads to significant lattice distortion with strong anisotropic properties for bimetallic PtFe crystals, which is evidenced by intriguing behaviors in terms of electron binding energy for PtFe-fct catalysts. While major red shift of two characteristic peaks is still significant, we also observe peak split for Pt4f5/2 and Pt4f7/2 orbitals (see arrows in Figures 1c and 2g). In particular, two new peaks start to emerge at 69.4 and 72.6 eV, the intensity of which is obviously increasing with more Fe content in PtFe composition. This unexpected result suggests that there exists significant lattice distortion induced by structural strain in fct crystal. It seems to be highly possible that transformation of fcc to fct crystals compresses Pt−Fe lattice distance due to strong tension at equatorial position in the cell (see insets in Figure 1c, details to be discussed in DFT study). Fe atomic layers between Pt layers alter the structural strain distribution in the cell, leading to facet contraction perpendicular to Fe layers. 12082
DOI: 10.1021/acs.iecr.8b02742 Ind. Eng. Chem. Res. 2018, 57, 12078−12086
Article
Industrial & Engineering Chemistry Research
Analysis on Fe atoms show a completely opposite trend as more electrons are believed to be pushed to Pt atoms. This result is consistent with XPS characterization, while binding energy for Pt displays a red shift in fct structures, suggesting more electrons on Pt sites in lattice distorted structures. Based on DFT calculation, it is very clear that, although DOS profiles does not show significant changes in Pt-based crystals, lattice strain in bimetallic PtFe not only alters metal−metal bonding but also significantly affects charge distribution, leading to several intriguing behaviors as revealed by XPS characterization.61−63 3.4. Reaction Profiles and Kinetic Modeling on PtFe4fcc and PtFe4-fct Catalysts. Because bimetallic fcc and fct nanocrystals display several unique features in electronic reconfiguration and lattice structures, we are very interested in how such structural novelty influences catalytic performances, particularly for selectivity for aqueous phase oxidation of glycerol. Our benchmark results already confirm that PtFe4-fct catalyst shows enhanced selectivity toward oxidation products, such as glyceric acid, TA, lactic acid and glycolic acid, while restraining decarboxylation, producing CO2 as the major byproduct to a large extent (
