Microkinetic Modeling of CO Oxidation over FePt-Decorated Graphene

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Microkinetic modelling of CO oxidation over FePt decorated GO Ravi Kiran mandapaka, Saiphaneendra Bachu, Chandan Srivastava, and Giridhar Madras Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01935 • Publication Date (Web): 05 Jul 2017 Downloaded from http://pubs.acs.org on July 10, 2017

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Industrial & Engineering Chemistry Research

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Microkinetic modelling of CO oxidation over FePt decorated GO Ravikiran Mandapaka1, Saiphaneendra Bachu2, Chandan Srivastava2, Giridhar Madras1*, 1. Department of Chemical Engineering, 2. Department of Materials Engineering, Indian Institute of Science, Bangalore 560012.

*Corresponding author. Tel.: +91-80-22932321; Fax: +91-80-23601310. E-mail: [email protected]

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Abstract This study presents the synthesis and activity of Pt-Fe nanoparticles decorated graphene oxide (GO) for CO oxidation. As compared to conventional methods of synthesizing noble metal impregnated catalysts in this study, Fe -Pt nanoparticles were decorated onto GO using ultrasonication. The obtained GO/Fe-Pt were characterized using XRD, XPS, Raman and TEM analysis confirming the formation of GO and the presence of Fe-Pt on GO. GO/Fe-Pt was then tested for its activity for CO oxidation. Different catalyst loadings were taken and differential reactor approach was used to obtain the intrinsic rate of reaction at different experimental temperatures. The active site concentration on the catalyst was obtained using CO chemisorption and this was incorporated in the kinetic model to propose a dual site microkinetic model. The kinetic parameters developed in the model were validated against the experimental turnover frequency (TOF) values obtained. As evident from the model predictions against the experimental TOF, it can be observed that the model developed in this study fits the experimental results to reasonable accuracy within the differential proximity limit. Keywords: GO; CO oxidation; dispersion; ultrasonication; microkinetics.


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1. Introduction Due to the exponential increase in the automobile and industrial emissions, carbon monoxide abatement is the need of the hour. Noble metal based catalysts are known for their activity for CO oxidation 1. Therefore, noble metals such as Pt, Pd and Rh are supported on reducible supports such as TiO2, CeO2 etc. and non-reducible supports such as silica, alumina etc, for CO oxidation2. The primary reason for the usage of noble metal nanoparticles is that the nanoparticles provide active surface sites for CO adsorption facilitating effective catalytic CO oxidation. The catalytic activity can be further improved by dispersion of noble metals on support materials such as silica, zirconia and alumina. In recent years, there has been intense search for viable replacements of

noble metals as well as the support materials for low

temperature CO oxidation. Combining noble metals with cheaper metals such as Fe, Co, Cu and Ni can reduce the cost 3. This includes synthesis of bi-metallic catalysts primarily by alloying or by formation of bimetallic core shell structures 4. The noble metal alloys as nanostructures facilitate higher surface area for the catalytic activity. Among different noble metal alloys, FePt nanoparticles are popular, having a wide variety of applications in the fields of bio-medical imaging, drug delivery and media recording etc. The active metal concentration for catalytic activity can be further improved by using highly porous support materials such as activated carbon, graphene and graphene oxide, etc. 5-7 Moreover, in contrast to the conventionally used supports, carbon based supports are feasible for low temperature gas phase reactions 8. Graphene, a 2-dimensional single atomic layer thick graphite with sp2 hybridization carbon atoms9, has diverse applications in the field of sensors 10, supercapacitors 11 and solar cells 12. This interest in the use of graphene can be attributed to its prominent catalytic properties, good electrical and thermal conductivity and excellent mechanical properties

13

. Graphene

Oxide (GO), the oxidized form of graphene, contains functional groups such as carbonyl,



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hydroxyl and epoxy groups attached to graphene sheet

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that can act as potential nucleation

sites for nanoparticles 15. These functional groups are also effective as traps for anchoring the nanoparticles onto the graphene sheet and preventing them from migrating at ambient temperatures

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. This unique nature of GO facilitates the effective impregnation and

dispersion of metal nanoparticles on graphene contributing to the increase in the catalytic performance

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. Thus, it is important that graphene is present in its oxidized form in the

graphene-nanoparticle composites to firmly hold onto the nanoparticles. However, it is a challenge to retain the GO in one-pot in situ reactions because these procedures often involve reducing agents that reduce the GO to graphene while forming the composites. To retain GO in its oxidized form, we have chosen an ex situ methodology, which facilitates the incorporation of FePt nanoparticles onto GO using the ultrasonication method without involving any reducing agents. The GO/ reduced graphene oxide (rGO) supported FePt has been used for a wide range of catalytic reactions such as methanol and formic acid oxidation

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, photocatalytic hydrogen

production 19 and oxygen reduction for fuel cells 20. While the studies on those reactions are abundant, reports on its usage for CO oxidation are not available. In this study, we report the synthesis, characterization and catalytic activity of GO supported FePt (GO/FePt) for CO oxidation. The GO/FePt in this study was synthesized in an ex situ manner, wherein pre-synthesized FePt particles were loaded onto GO using sonication. The obtained catalytic material was characterized using XRD, XPS, Raman spectroscopy, TGA, VSM, SEM and TEM. To probe the intrinsic kinetic and mechanistic aspects of CO oxidation reaction over the catalyst, a dual site microkinetic model involving the utilization of FePt sites has been proposed. The kinetic parameters, developed in this study are then validated against the experimental data.


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2. Experimental Section 2.1 Catalyst preparation Hummers method21 was employed to synthesize graphene oxide (GO). This process involves oxidation of graphite powder using strong oxidizing agents such as sulfuric acid and phosphoric acid. The FePt alloy, i.e. Fe50Pt50 nanoparticles were synthesized using the superhydride method 22. The typical procedure involves dissolving 0.23 g of FeCl2.4H2O and 0.46 g of Pt (acac)2 in 75 mL of benzyl ether. The precursor solution was then heated to 100°C under Ar atmosphere. To the resultant mixture, 0.51 mL of oleic acid and oleylamine were added and heated further. After the reaction mixture reached a temperature of 200°C, 6 mL of superhydride was slowly injected into the mixture. Prior to cooling the reaction mixture to room temperature, the reaction mixture was refluxed at 200°C for 45 min. The cooled reaction mixture was then transferred into a beaker and ethanol was added to the reaction mixture to facilitate the settling process. The settled precipitate containing FePt nanoparticles was then centrifuged and dispersed in hexane. As prepared FePt nanoparticles were then assembled/decorated onto GO using the procedure as described by Guo et al. 20 with some alterations. Briefly, the synthesis procedure involves dispersing 90 mg of as-synthesized GO in dimethylformamide (DMF) by ultrasonication. Then 10 mg of FePt nanoparticles dispersed in 20 mL of hexane, prepared as described in previous paragraph, was added to the GO DMF dispersion mixture. The mixture was then sonicated for 3 h followed by the addition of ethanol to facilitate swift settling of the precipitate. The precipitate thus obtained was then centrifuged at 9500 rpm for 10 min and the resultant mixture was dried to evaporate solvents in the dispersion mixture. Note that the FePt nanoparticle assembled GO will be hereafter referred to as GO/FePt.



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2.2 CO oxidation activity The synthesized catalyst was tested for its activity for CO oxidation. For this, different weights i.e., 10 mg, 25 mg, 50 mg of catalyst was diluted with silica gel and was packed inside a quartz reactor (i.d. 4 mm) to make a catalyst bed length of 1 mm. The reactant gases, 2.4 vol% CO (5%CO+95% N2 supplied by Chemix®), 2.4 vol% O2 (Zero air i.e.79 vol% N2 and 2 vol %O2 supplied by Noble gases®) and rest N2 (99.9 % pure supplied by Noble gases®) were passed through the catalyst bed at 25 mL/min using a manifold with mass flow controllers maintaining the individual reactant gas flow rates constant. The exit gas composition was determined by gas chromatograph (Mayura Analyticals ltd., Bangalore) equipped with a flame ionization detector (FID). The temperature of the catalyst bed was varied using a PID controlled furnace equipped with a thermocouple. In order to obtain the intrinsic rate of reaction, the differential reactor approach has been adopted. 3. Catalyst characterization The X-ray diffraction (XRD) patterns of the as-synthesized materials were obtained from X’Pert PRO, PANalytical using Cu Kα (λ = 1.5406 Å) radiation. The constituent composition of the as-prepared FePt nanoparticles was determined by 20 kV scanning electron microscope (ESEM QUANTA 200, FEI) using the X-ray spectroscopy (EDS) detector. A 300 kV field emission FEI Tecnai F-30 transmission electron microscope (TEM) was used for obtaining TEM bright field images of as-synthesized materials. X-ray photoelectron spectroscopy (XPS) profiles were collected from AXIS ULTRA instrument with Al Kα source. The Raman spectra were recorded using microscope setup (HORIBA JOBIN YVON, Lab RAM HR) consisting of Diode-pumped solid-state laser operating at 532 nm with a charge coupled detector. Thermogravimetric analysis (TGA) experiments were carried out using Netzsch STA 409 PC thermogravimetry analyzer in air atmosphere at a heating rate of 10°C/min. The


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magnetic hysteresis loops were generated using Lakeshore vibrating sample magnetometer (VSM). The active metal site concentration on the catalyst was measured by injecting pulses of CO onto 10 mg of catalyst at room temperature using helium gas as carrier and the response obtained was measured using a TCD detector (Mayura Analyticals Ltd., Bangalore). 4. Results and Discussion 4.1 XRD Fig. 1 shows the XRD patterns of as-synthesized materials. GO shows a strong peak at 10.4°, which corresponds to (002) graphitic basal planes with an interplanar spacing of 8.493 Å. The increase of interplanar spacing from 3.395 Å (2θ = 26.2°) to 8.493 Å indicates the successful oxidation of graphite to form graphene oxide

9, 23

. XRD pattern of FePt nanoparticles shows

broad peaks at 39.94°, 46.43° and 67.8° indicating the formation of disordered fcc structure24. Those reflections correspond to (111), (200) and (220) planes, respectively. The particle size of FePt estimated from (111) peak using the Debye-Scherrer equation is approximately 3 nm. XRD pattern of GO/FePt does not show any reflections corresponding to FePt. This can be attributed to the effective dispersion of metal nanoparticles on GO and lower i.e.10 % loading of FePt nanoparticles on GO. 4.2 SEM-EDS and TEM analysis SEM was used to analyze the composition of as prepared FePt nanoparticles, using the EDS technique. TEM was used to further characterize the morphology and size distribution of the FePt particles in as prepared condition and after the GO/FePt formation. The SEM-EDS spectrum obtained is presented in Fig. 2. The quantitative analysis estimated the average composition of FePt nanoparticles to be 47.4 to 52.6 atomic ratio of Fe and Pt, respectively, which is close to the intended 50 to 50 ratio. The particle size distribution was estimated from



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the TEM micrograph of the as prepared FePt nanoparticles both of which are shown together in Fig. 3. It is evident from the size distribution analysis that most of the nanoparticles are in the range of 2 to 4 nm with an average size of 2.9 nm. Fig. 4A shows a bright field image of GO/FePt. The arrow marks indicate the graphene sheet (G) and nanoparticles (N) present in the sample. It can be seen that the loading of FePt nanoparticles on GO is fairly uniform with some heavily loaded regions. One such heavily loaded region is shown in Fig 4B. The nanoparticles tend to agglomerate due to their extremely small size. 4.3 XPS XPS analysis was carried out to further confirm the chemical nature of GO and FePt nanoparticles in GO/FePt. The wide scan XPS spectra of GO and GO/FePt are shown in Fig. 5A. Peaks of higher intensity can be seen at ~ 285 and 532 eV corresponding to C 1s and O 1s core spectra, respectively. GO/FePt shows additional smaller peaks at ~ 71 eV and ~ 711 eV that correspond to Pt 4f and Fe 2p, spectra respectively, thus confirming the presence of FePt nanoparticles on GO. The high resolution C 1s, Pt 4f and Fe 2p spectra were charge corrected with respect to the adventitious carbon peak at 284.6 eV. Thus corrected C 1s spectra were deconvoluted and shown in Fig. 5B. C 1s spectra of GO and GO/FePt were deconvoluted into four peaks: C = C sp2 (284.6 eV), C-OH/C-O-C (286.6 eV), C = O (288.4 eV) and O-C = O (289.6 eV) 9, 25, 26. It is clear from the intensities of the deconvoluted peaks that the oxygen containing groups on GO were well retained after the ex situ reaction. This was desired as this would facilitate good anchoring of nanoparticles on the graphene oxide. The high resolution Pt 4f and Fe 2p spectra are shown in Fig. 5C and Fig. 5D respectively. Pt 4f spectrum shows two peaks at 71.4 eV and 74.6 eV corresponding to Pt 4f7/2 and Pt 4f5/2 respectively. The asymmetric nature of Pt 4f7/2 and Pt 4f5/2 peaks and the presence of Pt 4f7/2 peak at ~71.7 eV confirm the metallic nature of Pt in the catalyst 27. Fe 2p spectrum exhibits


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two peaks at ~711 eV (Fe 2p3/2) and ~ 725 eV (Fe 2p1/2),which suggests that the iron is present in its oxidized form as opposed to metallic form. This is possible as the iron present in the exterior layer of FePt nanoparticles tend to oxidize in air, particularly when the nanoparticles are lower than 5 nm in size 28. 4.4 Raman spectroscopy Graphene present in GO and GO/FePt was further characterized using Raman spectroscopy and the spectra obtained are shown in Fig. 6. GO and GO/FePt show two peaks namely D and G bands at ~ 1350 and ~1600 cm-1, respectively, which are characteristic of graphene 29. D band and G band originate from sp3 defects and in-plane vibrations of sp2 carbon atoms present in graphene, respectively. The change in ratio of intensities of D and G bands (ID/IG), an indication of creation of additional sp2 domains in graphene lattice

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, is not significant

between GO and GO/FePt. This further indicates very little to no reduction of GO has occurred during the ex situ process. 4.5 TGA TGA was carried out to study the thermal behaviour of GO/FePt and to compare it with that of GO, as shown in Fig. 7. As evident from the TGA curves, the base GO disintegrates to higher extent in the experimental temperature range (30 to 220 °C) compared to GO/FePt confirming the effective bonding between FePt nanoparticles and graphene sheet in the catalyst. The thermal stability exhibited by the catalyst can be useful in the applicability of these catalysts for low temperature CO oxidation. 4.6 VSM Magnetic hysteresis loops of FePt nanoparticles and GO/FePt, obtained at 300 K with the applied magnetic field varied between -15 kOe and 15 kOe, are shown in Fig. 8. They both



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exhibit superparamagnetism, as expected from ultra-fine FePt nanoparticles. It means that the thermal energy at 300 K is sufficient to overcome the anisotropy energy to make the net magnetization zero when no external field is applied. The magnetization value of GO/FePt at 15 kOe is 0.03 emu/g, which is one order of magnitude less when compared to that of FePt nanoparticles (0.22 emu/g). This roughly reflects the 10 % loading of FePt used in preparing the catalyst. 4.7 CO oxidation activity As evident from the W/FCO versus fractional CO conversion plot shown in Fig. 9, it can be observed that the differential reactor approach can be used till fractional CO conversion of 0.4. The Arrhenius plot of experimental rate shows the apparent activation energy of 47.2 kJ/mol. The catalytic activity of the material developed in this study is compared against the activation energies reported for different impregnated catalysts presented in the literature, in table 1. From the conversion versus experimental temperature plot shown in Fig. 10, it can also be seen that the catalysts exhibit light off for different loadings. 4.8 Active metal-site concentration The amount of active metal concentration of FePt on GO was determined using CO adsorption at room temperature. The uptake of CO on the catalyst was 22.3 µmol/g at room temperature. This result inferred is in good accord with the CO uptake of 20 µmol/g reported for Ir-Fe nano-alloys 31. 5. Mechanism and microkinetic modeling for CO oxidation Most of the kinetic models developed for noble metal impregnated on graphene/GO, propose the Langmuir-Hinshelwood (LH) mechanism to be operational for CO oxidation

6, 7, 10

. This

mechanism primarily constitutes the competitive adsorption of reactants CO and O2 on noble


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metal site followed by the surface reaction to form carbonate type surface intermediates and the desorption of product CO2. The graphene present in the catalysts was found to be inert to the catalytic reaction contributing only as a support. This is also attributed to GO which acts as inert support for the active nanomaterial and does not participate in the reaction mechanism. However, the mechanism for CO oxidation is complex in the case of noble metal alloy nano particles on inert support such as GO, which involves simultaneous utilization of surface sites on FeOx species and metallic site i.e. different adsorption sites exist for reactants i.e., CO and O2. In case of noble metal-iron alloy nanoparticles, the adsorption of CO takes place on the noble metal site and O2 adsorbs on the FeOx site

31

. This assumption of

considering the FeOx species in the mechanism is explained by the fact of Fe being present in the oxygen atmosphere in reacting conditions. This inference is also reflected from the XPS of Fe, which shows the presence of surface Fe in oxidized Fe form. In case of Pt-FeOx catalysts developed in this study, CO is assumed to adsorb reversibly on Pt sites denoted by ‘*’. This assumption is in good agreement with the propositions of Qiao et al.32 that indicated CO adsorbs on the Pt site of Pt/FeOx. O2 is assumed to proceed through reversible dissociative adsorption on FeOx sites, denoted by ‘×’. The adsorbed surface species of CO, denoted by CO* then react with adsorbed oxygen surface species on FeOx sites, denoted by [O] to give CO2 completing the catalytic cycle. The proposed mechanism is shown in equations 1-3.

CO + * ⇔ CO*

(1)

O2 + 2× ⇔ 2[O]

(2)

CO* +[O] ⇔ CO2 +×+ *


(3)


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In order to develop the rate expression for the reaction and to identify the rate determining step of the reaction mechanism, reaction route (RR) analysis has been adopted.33 The procedure and terminology adopted for the development of rate expression can be seen from our previous studies34, 35 . In brief, the rate expression is developed on the similar notion of Ohm’s law application to electrical circuit. The overall step reversibility is the driving force term, kinetic resistance is the resistance term and rate of reaction is the current flowing through the circuit. The overall kinetic resistance is the net contribution of the individual step resistances. The individual step in the analogy is developed based on the Langmuir Hinshelwood Hougen Watson (LHHW) terminology33of obtaining rate expressions, where the step resistance is the inverse of the forward rate of the elementary reaction assuming the elementary step to be rate determining step (RDS). According to the reaction mechanism presented for CO oxidation, three step resistances corresponding to the 3 elementary reactions in the mechanism can be obtained. The analytical expression for the step resistances are presented in the equations 4-6.

1+ R1 =

R2 =

PCO

2

K3 K 2PO 2 k1PCO

 PCO  2 1 +   K 3K1PCO   

(4)

2

(5)

k 2Po

2

R3 =

(1+ K1PCO ) 1+



K 2PO  2

k3K1PCO K 2PO



2


(6)

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Note that in the equations presented in 4-6, i’ represents the forward reaction of the ith surface reaction and ‘-i’ represents the backward reaction of ith surface reaction. ki represents the rate constant of the ith surface reaction and Ki is the equilibrium constant of ith surface reaction given as Ki= ki/k-i . The rate constant ki is defined as shown in equation 7.

k i = Ai

− Ei × e RT

(7)

Note that in equation 7, Ai represents the pre-exponential and Ei represents the activation energy of ith surface reaction. The values of pre-exponential factors (Ai) and activation energy (Ei) were taken from literature, as explained below. 5.1 Microkinetic model for CO oxidation over Pt-Fe 5.1.1 CO adsorption on metallic Pt site of Pt/FeOx The activation energy corresponding to adsorption of CO on PtFe site has been proposed to be 189.1 kJ/mol32. Accordingly, the desorption energy has been taken to be 189.1 kJ/mol. The adsorption pre–exponential has been considered to be 101 Pa-1s-1

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according to the

transition state estimates of molecular adsorption. The desorption pre-exponential is taken to be 1016 s-1 37 which is in good accord for reversible CO adsorption on Pt site. 5.1.2 O2 adsorption on FeOx species As proposed in the previous section, O2 is assumed to adsorb dissociatively in the vicinity of FeOx surface site of Pt/FeOX species with an activation energy of 132.2 kJ/mol38. Therefore, the desorption energy for this reaction has been considered to be 132.2 kJ/mol. The adsorption pre-exponential has been taken to be 103 Pa-1s-1 pertaining to the transition state estimates for dissociative adsorption. The desorption pre exponential has been considered to be 1013 s-1.


36


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5.1.3 Surface reaction of CO and O surface species to give product CO2 Based on the experimental data, the activation energy for surface CO reaction is taken to be 47.2 kJ/mol and the pre-exponential has been taken to be 2.2 ×104 s-1. This activation energy is consistent with energy barrier of 47.23 kJ/mol, reported by DFT studies

32,39

. The

activation energy for backward reaction is considered to be 75.0 kJ/mol, owing to the overall heat of reaction of -283 kJ/mol for CO oxidation. The pre-exponential for backward surface reaction has been taken to be 1×105 s-1. The values of these pre-exponentials are in good accord with the kinetic parameters obtained for the surface reaction proposed in recent studies for CO oxidation40, 41. The activation energies proposed in this study, for the individual elementary steps of the reaction mechanism yields the heat of reaction ( ∆ H ) to be -283 kJ/mol, which is in good accord with the heat of reaction of CO oxidation. The microkinetic model thus developed in this study for the FePt bimetallic catalyst is shown on table 2. In order to identify the rate determining step (RDS) of the mechanism, the individual step resistances are plotted across the experimental temperature range using the kinetic parameters presented in Table.2. For this purpose, Matlab® code was used taking the inlet mole fractions and temperatures as input parameters. The individual step resistances across the experimental conditions presented in Fig.11 are obtained. Fig.11 shows the variation of step resistances presented in equations 4-7, across the experimental temperature range of 140-230 °C. As seen from the figure, the step resistance pertaining to formation of gaseous CO2 from surface reaction of adsorbed species is the RDS of the system. Therefore, the rate of the reaction is given as presented in equation 8,


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rCO =

 PCO2 1−   K1K 2 K 3PCO PO 2 

(1+ K1PCO ) (1+

  

K 2PO2

)

(8)

k3K1PCO K 2 PO2 In the expression presented in equation 8, numerator denotes the overall step reversibility and the denominator denotes the dominating step resistance, which in present mechanism is

PCO2

resistance 3.For current experimental conditions, the term

K1K 2 K 3PCO PO2

Microkinetic Modeling of CO Oxidation over FePt-Decorated Graphene

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