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Phase Transformed PtFe Nanocomposites Show Enhanced Catalytic Performances in Oxidation of Glycerol to Tartronic Acid Xin Jin, Hao Yan, Chun Zeng, Prem S. Thapa, Bala Subramaniam, and Raghunath V. Chaudhari Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01473 • Publication Date (Web): 03 Aug 2017 Downloaded from http://pubs.acs.org on August 4, 2017

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Industrial & Engineering Chemistry Research

Phase Transformed PtFe Nanocomposites Show Enhanced Catalytic Performances in Oxidation of Glycerol to Tartronic Acid Xin Jin,1,2 Hao Yan,1 Chun Zeng,2 Prem S. Thapa,3 Bala Subramaniam,2 Raghunath V. Chaudhari2*

1

State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China

University of Petroleum, Qingdao, Shandong 266580, China 2

Center for Environmentally Beneficial Catalysis, Department of Chemical and

Petroleum Engineering, University of Kansas, 1501 Wakarusa Drive, Lawrence, Kansas 66047, USA 3

Microscopy and Analytical Imaging Laboratory, Haworth Hall, 1200 Sunnyside Ave,

University of Kansas, Lawrence, Kansas 66045, USA

*

Corresponding author: [email protected]

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Abstract Phase transformation in anisotropic nanocrystals is critical for enhanced surface properties of bimetallic catalysts. In this work, we report a series of phase transformed PtFe nanocomposites as catalysts for facile oxidation of glycerol to value-added tartronic acid at mild operating conditions. The lattice mismatch between Pt and Fe intrinsically drive structure reconstruction from face centered cubic (fcc) to ordered and lattice distorted face centered tetragonal (fct) phase. Surface characterization reveals that there exists an interdiffusion of Pt and Fe during phase transformation and a downward shift of binding energy for Pt species. Such transformation induces the formation of strained PtFe nanocomposites and enhanced structural coherence leading to three-fold enhancement in catalytic activity for one pot glycerol oxidation to tartronic acid compared with the fcc structure. Key words: phase transformation, lattice strain, bimetallic nanocrystals, catalysis, biomass conversion

Introduction Phase transformation of bimetallic nanoparticles from one particular crystallinity to another often leads to both structural and surface atomic reconstruction. These evolutions are often accompanied with significant changes in physical and chemical properties. Understanding how to engineer anisotropic transformation and lattice reconfiguration to rearrange nanocrystals at an atomic level thus provides an attractive avenue to design novel catalysts. In this work, we use PtFe nanocomposites as an example to demonstrate 2 ACS Paragon Plus Environment

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how phase transformation and anisotropic segregation significantly influence the surface catalytic properties of bimetallic crystals during liquid phase oxidation of polyols. Bimetallic nanocrystals often show enhanced catalytic performance in various energy and environmental applications compared with monometallic ones.1 It is generally established that catalytic properties of bimetallic crystals can be fine-tuned by tailoring atom arrangement during catalyst synthesis.2 Conventional “bottom up” synthesis procedures make use of “external forces” to template the growth of metallic species in homogeneous medium. The external forces/templates include specially designed ligands/polymers (e.g. PVP, CTAB)3,4 or organic solvents with electron dense functional groups such as oleylamine,5 DMF etc.6 This methodology has been successfully developed to tailor the morphologies for PtCu,7-11 PtAu,12 PdCu,13,14 PdPt,15-18 PdAg19 and PdAu20 nanoparticles. Extensive studies have also confirmed that these nanocomposites exhibit remarkable catalytic activity in various applications such as hydrogenation and oxygen reduction reaction.3,4,21-24 Fe, Co and Ni-based nanocomposites have been attracting extensive research interest due to their tunable morphologies and ability to undergo facile phase transformation. Examples include PdCo,25 PtFe,26 PtCo27 and PdNi19 nanocomposites that have been shown to be effective catalyts in energy conversion applications. However, the controlled synthesis of such alloy particles via bottom up methodologies is very difficult due to differences in the reduction kinetics of the metal precursors.28 Thus, the precise control of the electronic configuration in these bimetallic nanocrystals to achieve desired catalytic performance still remains a grand challenge. Recent work has demonstrated that magnetocrystals can undergo phase transformation via interdiffusion from fcc disordered 3 ACS Paragon Plus Environment


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morphology and L10 ordered fct structure in a simple one-pot annealing process.29 Significant structural evolution of bimetallic nanocomposites has been observed due to anisotropic transformation. The mechanism underlying such phase transformation and its impact on the catalytic properties of the resulting bimetallic crystals are largely unexplored and require detailed investigations. In this work, we report on the observation of interdiffused fct PtFe nanocrystals that display 3-fold higher activity than fcc PtFe catalysts for aqueous phase oxidation reactions. Plausible mechanism for the phase transformation from fcc to fct structure is proposed by linking interfacial strain and surface electronic properties with intrinsic catalytic activity during aqueous phase oxidation of bio-derived polyols. The insights gained from this work advance our fundamental understanding of how phase transformation of bimetallic composites can be exploited to engineer electronic coupling and improve catalytic properties.

Experimental Chemicals. Chemicals used in this paper were purchased from Sigma Aldrich and Fisher Scientific. Synthesis of supported Pt, Fe and PtFe catalysts. The growth of PtFex (x: Fe/Pt atomic ratio) nanocatalysts with disordered morphologies was achieved following a one pot wet chemistry approach in the presence of CeO2 support in dimethyl formamide (DMF) medium. The Pt loading on CeO2 for all catalysts was 1wt%, while the content of Fe was varied. Typically, predetermined amounts of Pt(acac)2 and Fe(acac)3 were mixed with 1.2 g of CeO2 powder in 20 mL of DMF solution, which was then transferred to an autoclave


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with pressure and temperature monitoring. The slurry was flushed with nitrogen thrice and sealed with 10 bar pressure at room temperature before heating to 200 °C. The mixture was refluxed under such conditions for 12 h after which the autoclave was cooled down.

Following this step, the catalyst powders were taken out and washed with

ethanol/water (vol/vol: 2/1) solution for at least seven times.30 All annealed bimetallic PtFe catalysts were treated under the same conditions (pure H2 at 400 °C), and denoted as PtFex-a. It is important to mention that, during initial experiments, it was observed that the stability of CeO2 support is very important for successful preparation of PtFe catalysts. Fresh CeO2 without pretreatment was found to undergo agglomeration during annealing process, which adversely influences the phase changes of bimetallic PtFe clusters. To ensure the stability of CeO2 during annealing of PtFe nanoparticles, CeO2 support was first calcined under air at 400 °C before catalyst preparation. Annealing of bimetallic PtFe catalysts. The as-prepared solid catalysts from solvothermal synthesis were dried in the oven overnight before annealed in a tubular setup. Catalyst annealing was carried out under pure H2 atmosphere for 5 h. Specifically, catalyst samples first underwent thermal treatment under N2 (at a flow rate of std 100 mL/min) for 0.5 h before the temperature increased from ambient to 150 °C, after which N2 source was switched off and H2 was introduced at the same flow rate. The temperature was increased to 400 °C and maintained for 5 h. Then the system was cooled naturally to room temperature. H2 flow was shut off and N2 gas was introduced when the system temperature was approximately 150 °C. Catalyst samples were collected and tested without any further treatment. 5 ACS Paragon Plus Environment


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Surface Characterization. Procedure for transmission electron microscopy (TEM) characterization was similar to that previously described.31 Scanning electron microscopy (SEM): A Versa 3D dual beam Scanning Electron Microscope/ Focused Ion Beam (FEI, Hillsboro, OR, USA) with a silicon drift EDX detector (Oxford Instruments, X-Max, UK) was used to measure the surface morphology, elemental composition and distribution of metals. The specifics have also been described in our recent work.11 Detail of X-ray photoelectron spectroscopy (XPS) are: Instrument: PHI 5000 Versa Probe II. X-ray Source: Monochromated Al. Take-off angle: 45 degree. Beam Size: 100 micron. Beam Power: 25 W/15kV. Pass Energy: 23.5eV. eV Step: 0.2. Sweep: 20. Density functional theory (DFT) calculation. Pt (111) surface were modeled using a triple-layer p (3×4) slab comprising 3 metal layers with 12 atoms per layer and each slab was separated by a vacuum of 15˚A 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. The performance of the three-layer slab model and the calculation parameters, including the p(3×4) supercell, 3×3×2 k-point, 15 A vacuum zone, and the energy cutoff of 400 eV, have been tested extensively in previous studies. Oxidation Tests. In a typical run, about 0.10 g of solid catalyst powder was added to 25 mL aqueous solution containing glycerol (1.0 g) and NaOH (1.7 g). The reaction was carried out in a 100 mL three-neck flask. The slurry was heated up to the reaction 6 ACS Paragon Plus Environment

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temperature (e.g., 70 °C) in an oil bath with precise temperature control. Once the liquid slurry was at reaction temperature, stirring rate was set at 1000 RPM, and air was introduced into the liquid by bubbling, signifying the start of reaction. Approximately 1 mL of liquid sample was taken during batch studies and acidified with H2SO4 solution before injecting into HPLC (SHIMADZU with SH1011 column). The concentrations of glycerol and oxidation products were thus obtained for estimating substrate conversion (X), selectivity (S), carbon balance (C%) and turnover frequency (TOF, in mol/molPt.h). These metrics are as defined in recent publications.11,31,32 Conversion is defined as the ratio of amount of glycerol converted to that initially charged. Selectivity towards a specific product is defined as the ratio of the total amount of carbon atoms in this product to that in converted glycerol. Carbon balance is defined as the ratio of total amount of carbon in all liquid phase products to that of converted glycerol. TOF is estimated at conversions less than 28% and defined as the amount of glycerol converted per mole Pt per time. Estimates of external and internal mass transfer limitations in our previous studies already confirm that the reaction rates measured during experiments are controlled by intrinsic rates on the catalyst surface.30,33 Recycle studies were carried out on selected PtFe catalyst (PtFe4.5-a). After each batch run of a fixed duration, the reaction slurry containing the catalyst was collected and filtered. The solid catalyst powder was washed with DI water for several times and added directly with fresh glycerol solution to the next batch run without any further treatment. The experimental procedure was similar to that previously reported.30

Results and Discussion Structure identification of PtFe catalysts 7 ACS Paragon Plus Environment


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Surface characterization was carried out for fresh and phase-transformed PtFe nanoclusters using TEM and XPS characterization. Figures 1a-1c present TEM images of CeO2 supported Pt, PtFe1 and PtFe3 catalyst samples, respectively. As shown in Figure 1a, monometallic Pt sample exhibits ordered (111) and (100) lattice facets on the surface CeO2 support. This observation suggests that isotropic growth of Pt nanocrystals is favored in presence of DMF as a soft reducing agent (see experimental for details) under solvothermal conditions.24,26,30,34,35 When Fe element is present along with Pt , lattice mismatch enables anisotropic growth of bimetallic PtFe clusters.30 In particular, the growth of ordered Pt lattice planes is clearly disturbed by Fe presence due to large lattice constant mismatch between the two elements (Pt: 0.39 nm; Fe: 0.29 nm). Although the presence of Fe3+ in DMF medium alters the kinetics of reduction during catalyst preparation via galvanic displacement, anisotropy is still the dominant factor determining the final morphology of bimetallic PtFe clusters.30 Therefore, highly disordered PtFe structures were achieved after solvothermal preparation. When even more Fe is added to PtFe system (PtFe3 in Figures 1c), TEM images still show the presence of such disordered structures in these samples. However, Fe species tend to coat on the surface of PtFe structure (see EDX mapping in Figure 1d for details). Two possible factors might contribute to this observation: (a) galvanic displacement of Fe3+-Pt and Fe-Pt2+ significantly slowing the reduction rate of Fe species on nanocrystals; (b) negative segregation energy of Fe in PtFe solid solution favoring more Fe on surface (also confirmed later by XPS characterization).36 Rather than being incorporated with Pt to form bimetallic particles, a large fraction of the Fe species are directly deposited on the 8 ACS Paragon Plus Environment

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CeO2 surface forming very fine particles (green dots in Figure 1d). This finding implies that there exists a parallel Fe deposition process caused by the presence of excess Fe species (than Pt) in these samples. Based on recent theoretical calculations for PtFe system, the maximum (atomic) bulk composition of Fe in fcc PtFe solid solution might be as low as 50%.37 This limits the maximum level of incorporation of Fe species in fcc PtFe alloy system. In order to understand how phase transformation of PtFe crystals occurred during the annealing process, and more importantly affect surface catalytic properties, we systematically conducted TEM characterizations of the annealed catalyst samples. HRTEM image of PtFe1-a reveals significant morphological changes after thermal treatment at 400 °C, from a disordered flower shaped heterocluster structure to ordered layer-bylayer structure. Surprisingly, the particle size of bimetallic PtFe1-a sample actually decreases after the annealing process in comparison with the fresh PtFe1 sample, which is also seen with PtFe3-a and PtFe4.5-a samples. This observation suggests that interdiffusion of Pt and Fe atoms during annealing results in structural changes, which leads to the transformation of heteroclusters to more condensed particles. Usually, phase transformation of PtFe solid solution under harsh conditions (~ 500 °C) is usually accompanied by particle sintering.38 The presence of H2, however, could reduce the critical temperature (Curie Temperature),28 and minimize agglomeration of PtFe clusters due to Oswald ripening effect under such condition.39 Thus, the annealing process for PtFe samples mainly contributes to structural reconstruction rather than Oswald ripening. Elemental mapping of fresh and transformed PtFe nanocrystals further reveals the nature of interdiffusion processes of Pt and Fe atoms. For fresh PtFe catalyst samples, (see 9 ACS Paragon Plus Environment


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Figure 1d) we find that although Pt and Fe form bimetallic clusters in selected regions, a significant fraction of Fe species is dispersed on the surface of PtFe clusters. This observation suggests that relatively higher Fe/Pt atomic ratios (> 1/1) may facilitate formation of core-shell structures.40 Such deposition results in Fe-rich shell with PtFe core structures. After annealing, TEM images show that PtFex samples were transformed from disordered fcc structures to ordered fct ones. EDX mapping of PtFe3-a sample in Figure 2d confirm the reconstruction processes. It is possible that the intermetallic (layerby-layer Pt and Fe atomic arrangement) structures in fct PtFe composites have relatively lower surface energy compared with Fe, rendering it more favorable to have both Pt and Fe in the shell compared with Fe shell in fct structures. Electronic properties and interfacial strain Understanding the possible Pt and Fe interactions in fresh and annealed samples should provide important information on optimal bimetallic structures for catalysis. Therefore, XPS characterization for selected samples (Pt, PtFe4.5 and PtFe4.5-a) was carried out to reveal possible metal-metal interaction (see Figure 3). XPS spectra for Pt in monometallic Pt/CeO2 sample are used as reference for interpretation of the spectra obtained with bimetallic samples. For annealed bimetallic sample (PtFe4.5-a), we observe that 4f5/2 and 4f7/2 peaks are further split and exhibit a red shift towards lower binding energy. The higher binding energies (4f5/2 and 4f7/2 components) are associated with the bulk Pt metal atom while the lower binding energies are assigned to the reconstructed surface of Pt atoms. When more Fe is present, the lower binding energy components become more significant. The split orbitals suggest that there exists accumulation of compressive strain at the surface plane.41 10 ACS Paragon Plus Environment

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Such downward shift of binding energy and split observed in this work is believed to be induced from fct distortion due to strained Pt and Fe lattices in annealed catalyst samples.41 This is due to the existence of two competitive processes between surface migration of Fe in PtFe systems42 and the Kirkendall effect43,44 caused by different diffusion rates for Pt and Fe species. The former should yield Fe-rich shells while the latter leading to hollow core-shell structures. We find that it is difficult to use the former mechanism to explain the possible annealing effects, because with more Fe content present in PtFe system, annealing should lead to even more Fe content on the catalyst surface. However, it is found that most annealed catalyst samples exhibit enhanced catalytic performances (see Figure 4a for details), which obviously cannot occur with increasing Fe content on catalyst surface. Interestingly, we did not observe any hollow structures for any of the annealed catalysts. This is possibly because hollow core-shells formed via Kirkendall effect are often generated from clusters with single crystal structures.43 In other words, such interdiffusion usually takes place in certain direction, which facilitates creating uniform and symmetric hollow structures. In our case, however, existing PtFe clusters are poly-crystallized with different types of high index facets. The Kirkendall interdiffusion during annealing is disoriented and only favored the formation of intermetallic structures rather than uniform hollow structure. Therefore, the phase transformation mainly contributes to the structural reconstruction from disordered to ordered fashion rather than segregating PtFe cluster-in-cluster structures. We also conducted detailed inspection of metal-support interaction based on XPS spectra. It is found that for monometallic Pt/CeO2 catalyst, binding energies for Pt and Ce are similar to corresponding reference values in the literature. More importantly, the binding



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energies of Pt Fe and Ce remain almost unchanged for bimetallic PtFe catalysts before and after annealing processes, suggesting an insignificant metal-support interaction on CeO2 surface DFT calculations on disordered and ordered PtFe catalysts were also conducted to understand plausible electronic interactions between Pt and Fe atoms in different structures (Figure 4). First, it is found that the d-band center of Pt sites shifts towards the Femi level when Fe atoms are present in Pt crystals. The characteristic peak for Fe atom disappears in the disordered PtFe system even though both Pt and Fe atoms are present on the surface. However, it is surprising that the characteristic Fe peak is very strong when intermetallic structures of PtFe are formed (ordered structures), even though Fe atoms are not present on the surface of the nanocrystals. The DFT calculations further demonstrate that the interaction between Pt and Fe is critical for surface electronic configurations, which will significantly affect surface catalytic properties Oxidation activity of PtFe catalysts for biomass conversion. The results in this section help us understand how compositions and electronic properties of bimetallic PtFe crystals and energy splits of Pt element influence surface catalytic properties during the aqueous phase oxidation of biomass-derived substrates to valueadded carboxylic acids (e.g. tartronic acid, see Scheme 1). Specifically, glycerol, a C3 polyol was chosen as the model compound to investigate the relationship between bimetallic structures and the activation C-H and C-O bond in biomass feedstocks. We first studied the oxidation activity of monometallic Pt and bimetallic PtFe at 70 °C under 1 atm O2 pressure. It is found (see Figure 5a) that the addition of Fe to Pt system (fresh PtFe1) remarkably enhances catalytic activity (TOF ~ 20,100 h-1) for glycerol oxidation 12 ACS Paragon Plus Environment

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(blue bar) in comparison with monometallic Pt (TOF: 2,677 h-1 reported previously30) and Fe catalysts (TOF: 47.7 h-1). However, increasing Fe content (Fe/Pt = 2, 3, 4.5) does not lead to higher activity. Rather, with increasing Fe content (PtFe2 to PtFe4.5), the TOF of PtFe catalysts decreases to 8,126 h-1. Interestingly, we find that the activity trend for annealed PtFe catalysts is different from the fresh ones (gray bars). Although overall activities of PtFex-a samples are lower than fresh ones for low Fe/Pt ratios, the activity is further enhanced to approximately 26,340 h-1 when more Fe content is present in the annealed PtFe samples. Similar trends are also observed for tartronic acid yields. It is found that PtFe4.5-a sample exhibits the highest tartronic acid yield of 58.6% (see Figure 5b), in comparison with other fresh and annealed catalysts. Thus, it is clear that that annealing process can significantly change catalytic activity and selectivity of PtFe catalysts during aqueous phase oxidation of glycerol. In addition, Figure 5c presents concentration-time profiles for glycerol oxidation in the presence of PtFe4.5-a catalyst. It is found that glyceric acid is the primary product initially, with the formation of tartronic acid as well as other co-products (e.g. lactic acid, formic acid) being enhanced at prolonged reaction time. In other words, the secondary oxidation of glyceric acid is dominant after glycerol is completely consumed.30 Based on the foregoing observations, we propose the following surface reaction rearrangement mechanism: (a) Segregation of surface and sub-layered Pt atoms may result in dislocation of surface Pt ones and cause electron depletion from disordered fcc structures.41 (b) XPS characterization results on the chemical shift of Pt element suggests that the electron reconfiguration occurs within the surface and sub-layered Pt atoms.45 (c) The red shift suggests that surface Pt atoms are more negatively charged after annealing.



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In other words, there exists a charge transfer from sub-layers to surface Pt after annealing processes. (d) This type of structure can facilitate 2π electron transfer from O2.46 Such red shift and more negatively charged Pt surface mean a relatively higher electron density in the d-band, which could be favorable for C-H bond activation.45 (e) The intermetallic structures of fct PtFe catalysts indicate that there exist more Pt-Fe interfaces. The presence of Fe near Pt atoms is believed to decrease the binding energy between the Pt metal surface and C=O groups in oxidation intermediates,30,47 which may assist O2 transfer from metal surface to molecules, which enhances both primary and secondary oxidation rates. Based on the foregoing explanations, a plausible surface reaction mechanism is proposed as follows (see Figure 6). (i) Strained surface Pt atoms catalytically activate C-H bond cleavage in glycerol molecule, while Fe atoms in intermetallic structures facilitate O2 spillovers on catalyst surface and the formation of C=O bond during oxidation. (ii) -OH group in glycerol tends to interact with surface oxygen on -Pt-Fe- surface. (iii) Although the presence of Fe in disordered PtFe structures is known to decrease the binding energies with C=O functional groups, lactic acid is often generated as a by-product due to poor secondary oxidation rate on fcc surface when temperature increases.30 In contrast, secondary oxidation of C-H bond in glyceric acid is more favorable on the surface of intermetallic PtFe catalysts. Hence tartronic acid yield is higher on fct compared with fcc structure. We further carried out recycle studies for PtFe4.5-a catalyst for glycerol oxidation. While it was previously found that disordered PtFe catalysts undergo significant Fe leaching (~ 40%) but with only slight decrease in activity after 3rd recycle for phase transformed 14 ACS Paragon Plus Environment

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PtFe4.5-a catalyst, only negligible leaching of Pt (< 0.03%) and Fe (< 2.4%) was observed after 3rd recycle. For example, the yield of tartronic acid (after 24 h at 70 oC) was found to be 58%, 59.3%, 54% and 57% for fresh, 1st, 2nd and 3rd recycled catalysts, respectively. Recycle studies clearly demonstrate improved activity, selectivity and stability of phase transformed PtFe catalysts for glycerol oxidation.

Conclusion We have demonstrated that thermal annealing of PtFe bimetallic nanocrystals leads to significant phase transformation from disordered fcc to ordered fct structure. The phase transformation is induced by interdiffusion assisted coupling of Pt and Fe atoms in bimetallic clusters. While changes in surface composition of the Pt and Fe elements are insignificant before and after annealing, the catalytic activity, selectivity and stability of bimetallic PtFe catalysts are enhanced with fct structures during aqueous phase oxidation of biomass substrates to dicarboxylic acid. Based on extensive surface characterization using TEM and XPS, it is believed that Kirkendall effect is the major contributing factor that governs the asymmetric interdiffusion. A rare behavior of surface and sub-layered Pt lattice segregation is thus observed for annealed bimetallic crystals. The detailed experimental studies presented in this work provide insights and guidance for the rational design of bimetallic nanocatatalysts with improved activity, selectivity and stability for targeted applications.

Acknowledgement X. J. and R. V. C. acknowledge partial support from NSF/EPA Grant CHE-1339661. This work is partially supported by “the Fundamental Research Funds for the Central



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Universities” (17CX02017A) and Fundamental Research Funding from Qingdao City, China (17-1-1-80-jch). The experimental part was conducted at University of Kansas, while DFT calculation and further interpretation were carried out at China University of Petroleum.

Supporting Information. Additional results including surface characterization and data comparison supplied as Supporting Information.

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Figures and Table

Figure 1. TEM and EDX images of fresh (a) Pt, (b) PtFe1 and (c, d) PtFe3 samples (yellow bars indicate 20 nm)



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Figure 2. TEM and EDX images of annealed (a) PtFe1, (b, d) PtFe3 and (c) PtFe4.5 samples (yellow bars indicate 20 nm)


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Figure 3. XPS spectra for Pt and PtFe catalysts



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Figure 4. Density functional theory calculation on disordered and ordered PtFe crystals


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Figure 5. Effects of Fe/Pt ratio on (a) oxidation activity, (b) product yield for glycerol oxidation in the presence of annealed PtFe catalysts and (c) concentration-time profiles on PtFe4.5-a catalyst (GLY: glycerol, GLYA: glyceric acid, TAR: tartronic acid, Others: lactic acid, formic acid, etc)


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Figure 6. Reaction schemes of secondary oxidation on monometallic Pt, disordered PtFe and intermetallic PtFe catalysts



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Scheme 1. Overall reaction pathways for glycerol oxidation to tartronic acid


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Table of Contents Graph


Phase Transformed PtFe Nanocomposites Show ... - ACS Publications

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