In Situ Formation of FeRh Nanoalloys for Oxygenate Synthesis - ACS

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In-Situ Formation of FeRh Nanoalloys for Oxygenate Synthesis Pamela Carrillo, Rui Shi, Krishani Teeluck, Sanjaya D. Senanayake, and Michael G White ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02235 • Publication Date (Web): 27 Jun 2018 Downloaded from http://pubs.acs.org on July 1, 2018

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In-Situ Formation of FeRh Nanoalloys for Oxygenate Synthesis Pamela Carrillo,[b] +Rui Shi,[b] Ω Krishani Teeluck,[b] § Sanjaya D. Senanayake[a] ‡ and Michael G. White*[a,b] a

b

Chemistry Division, Brookhaven National Laboratory, Upton, New York 11973

Department of Chemistry, Stony Brook University, Stony Brook, New York 11794 * [email protected] ABSTRACT

Early and late transition metals are often combined as a strategy to tune the selectivity of catalysts for the conversion of syngas (CO/H2) to C2+ oxygenates, such as ethanol. Here we show how the use of a highly reducible Fe2O3 support for Rh leads to the in-situ formation of supported FeRh nanoalloy catalysts that exhibit high selectivity for ethanol synthesis. In-situ characterizations by x-ray diffraction (XRD) and x-ray absorption spectroscopy (XAS) reveal the coexistence of iron oxide, iron carbide, metallic iron and FeRh alloy phases depending on reaction conditions and Rh loading. Structural analysis coupled with catalytic testing indicate that oxygenate formation is correlated with the presence of FeRh alloys, while the iron oxide and carbide phases lead mainly to hydrocarbons. The formation of nanoalloys by in-situ reduction of a metal oxide support under working conditions represents a simple approach for the preparation bi-metallic catalysts with enhanced catalytic properties. KEYWORDS heterogeneous catalysis • supported catalysts • in-situ characterization • CO hydrogenation • oxygenate synthesis • iron rhodium alloy

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1. INTRODUCTION The detrimental effects of fossil fuel combustion coupled with the depletion of petroleum sources have spurred the development of sustainable energy technologies. One attractive alternative for fuel synthesis is ethanol produced from syngas (CO/CO2 + H2) due to its high energy density and compatibility with the existing liquid fuel infrastructure. Moreover, syngas from renewable sources such as biomass could result in an overall carbon-neutral process. Typical catalysts for syngas conversion to ethanol and other C2+ oxygenates are composed of metallic nanoparticles supported on high surface area metal oxides, e.g., Rh/Al2O3, yet all known catalysts suffer from low activity, low ethanol selectivity or both.1-3 Four types of catalysts for syngas conversion to C2+ oxygenates have been reported: modified methanol (typically Cubased)4, modified Fischer-Tropsch (FT), Mo-based and Rh-based catalysts.1-3 Amongst these, the most effective to date are Rh supported on metal oxides due to the ability of Rh to promote C-C coupling, an important step for higher oxygenate synthesis.5 Nonetheless, the selectivity to oxygenates is constrained by the thermodynamically preferred production of methane.5,6 To kinetically enhance oxygenate selectivity, a wide variety of promoters such as alkalis, rare-earth elements or transition metals can be added to supported Rh catalysts.1-3 Of the transition metals, Fe has been shown to be very effective as a modifier by suppressing methane formation and enhancing oxygenate selectivity.7-12 Previous studies on Fe-promoted Rh catalysts, suggest that improvements in ethanol selectivity can be attributed to the presence of a FeRh alloy, but direct evidence for its presence under reaction conditions has been lacking.8,13-15 In our earlier study of Fe-promoted Rh/TiO2 catalysts, Rietveld refinements of in-situ x-ray diffraction (XRD) data suggested the presence of FexRh1-x alloy phases, but the analyses were complicated by overlapping diffraction peaks from


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the TiO2 support.8 A small FeOx phase was also detected under reaction conditions, but its catalytic role could not be determined. In this work, we investigated the possibility of using iron (III) oxide as both the support and Fe source for the in-situ formation of FeRh alloys. We take advantage of the reducibility of iron oxide coupled with the high surface area of a nano-Fe2O3 support to maximize Rh-Fe2O3 interfaces and promote FeRh alloy formation under reducing conditions, i.e., exposure to H2 or H2 + CO mixture. As shown below, the Rh/nFe2O3 system undergoes dramatic phase changes under reaction conditions including the complete transformation of the iron oxide support to carbide (ε’-Fe2.2C). In-situ x-ray characterization methods also provide definitive evidence for the formation of FeRh alloy phases whose presence can be correlated with significant increases in selectivity for C2+ oxygenates. 2. EXPERIMENTAL DETAILS 2.1 Catalyst Synthesis Several weight loadings of Rh (1, 2, 5, and 7wt. %) were added via incipient wetness impregnation (IWI) to iron (III) oxide nanopowders (Sigma Aldrich). Stoichiometric amounts of rhodium (III) nitrate hydrate (Rh(NO3)3∗xH2O) were dissolved in deionized water and added in a drop-wise fashion to iron (III) oxide powders supports until forming a paste. These were dried at 120°C overnight, calcined at 450°C for 4 hours and grinded until forming a powder. A sample of nFe2O3 was treated in the same manner as the rhodium-containing catalysts to account for any changes during synthesis. The samples are referred to hereafter as 1Rh, 2Rh, 5Rh and 7Rh depending on their rhodium content, and as nFe2O3 for the bare support in the manuscript and as Fe in the figures. The synthesized catalysts with their expected and actual rhodium content determined by inductively coupled plasma mass spectroscopy (ICP-MS) are given in Table S1.


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The ICP-MS measurements were performed at the Spectroscopy and Biophysics Core of the University of Nebraska (Lincoln). 2.2 Catalytic Measurements The synthesized catalysts were tested using a flow-plug reactor based on the design of Chupas et. al.16 This reactor was used to ensure consistent results between lab-based reactivity and synchrotron-based characterization measurements that used the same reactor set-up but with different capillaries depending on application. For activity measurements, approximately 40 mg of catalyst were loaded into quartz capillaries (1/8”OD) using glass wool as plugs and then placed in the reactor. Heating was delivered to the sample by a filament around the capillary connected to a power supply. Temperature was monitored with a thermocouple inserted inside the quartz capillary with the loaded sample. The gases, CO and H2, used were UHP grade and controlled by mass flow controllers (MKS instruments). Reduction was carried out using H2 at 9 ml/min (gas space hourly velocity, GHSV= 1680 h-1), at 300°C and 1.0-1.3 bar pressure for 30 min. The samples were then cooled down and the gas flow was changed to a mixture of CO (6 ml/min) and H2 (3 ml/min), GHSV= 1140 h-1, with a H2 to CO ratio of 2:1 at 1.0-1.3 bar pressure. Reaction measurements were performed at 240°C and products were analyzed using a gas chromatograph (Agilent © 3000 Micro GC) after running for approximately for three hours. The product distributions reached stable values after approximately one hour of reaction and did not exhibit changes for up to three hours of on-stream testing. The GC was connected to the output of the reactor via heated a 1/8”OD stainless steel tubing to avoid condensation of the liquid products and was equipped with three micro columns for detection of carbon monoxide and methane, C2+ hydrocarbons and oxygenates. Quantitative product concentrations were determined by calibration curves for each compound measured separately.8


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The CO conversion was calculated from the expression (%) = ∑

∙

⋅ 100%

where ni is the number of carbon atoms in product i, Mi is the amounts of product i detected, and MCO is the amounts of carbon monoxide in the gas feed. The product selectivity is based on the total number of carbon atoms and is defined as ∙ ∙

(%) = ∑

⋅ 100%

where Si is the selectivity for product i. 2.3 In-situ Characterization In-situ powder x-ray diffraction (XRD) measurements were performed at the 11-ID-B beamline at the Advance Photon Source located at Argonne National Laboratory. The photon energy was 55.6 keV (0.211 Å). A Perkin Elmer amorphous silicon 2-D detector was used at a 600 mm distance from the sample. Calibration was performed using a cerium dioxide standard. The samples were loaded into 1.2 mm (OD) Kapton ® capillaries and mounted into the same plugflow reactor used for reactivity studies. Data was collected for the samples as-synthesized under Ar flow at room temperature, under H2 reducing conditions at 300°C after 30 min and CO hydrogenation conditions at 240°C after one hour. XRD data was also collected for the bare nFe2O3 support prepared using the same procedure as the Rh-containing samples. The XRD diffraction images were converted into 1-D, 2-theta scans and Rietveld refinements performed with the use of the GSAS-II software.17 Refinements used published crystallographic data to simulate the XRD patterns and quantify the phases present. Lattice constants, instrument parameters, atomic coordinates, thermal parameters, and peak profile functions were varied in


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order to achieve a simulated diffraction pattern that matched the experimental data. The completed refinements provided phase quantification and lattice parameter determination. In-situ x-ray absorption spectroscopy (XAS) spectra at the Rh and Fe K-edges were collected at the 5-BM-D at the Advance Photon Source at Argonne National Laboratory under both reducing and reaction conditions. Both near-edge (XANES) and above-edge extended fine structure data (EXAFS) were obtained to extract chemical state and local structural information, respectively. The samples were loaded into Kapton ® capillaries, and mounted into the same plug-flow reactor used for the in-house activity measurements. Rhodium and iron reference foil data were collected simultaneously with the catalyst samples to provide energy calibration after 30 min of reduction and reaction conditions. This set point was determined via the reactivity measurements when catalysts became active for oxygenate production. Data was collected using x-ray fluorescence detected by a 13-element germanium multichannel detector. Extended fine structure data processing and curve fit analysis were performed using the Athena and Artemis package that are based on IFFEFIT.18 See SI for more detailed descriptions of XRD, XANES and EXAFS data collection and analyses. 3.0 RESULTS 3.1 Catalytic Activity and Selectivity The normalized product selectivity and CO conversion versus Rh weight loading from the catalytic tests are shown in Figure 1a and 1b, respectively. The values for the product distribution and CO conversion can be found in Table 1. Oxygenate selectivity increases with Rh loading up to ∼5 wt %, above which the selectivity plateaus at ~32% for the highest Rh loading (7 wt %). Ethanol is the largest fraction of oxygenates, with a total selectivity of 27% for 7Rh. The increase in C2+ oxygenate selectivity is mirrored by a nearly equivalent decrease in C2+


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Figure 1. (a) Selectivity for various reaction products from CO hydrogenation and (b) CO conversion versus Rh weight loading for Rh/nFe2O3 catalyst samples. Hydrocarbons include ethylene, ethane, propane and butane. Oxygenates include acetaldehyde, ethanol and methyl acetate. Reaction conditions: 2:1 of H2:CO gas mix at 1.01.3 bar total pressure and 240ºC.

hydrocarbons, suggesting that oxygenates are formed at the expense of the hydrocarbons when Rh is introduced to the support. By comparison, selectivity for methane stays roughly constant (~35%) for all Rh loadings. The CO conversion (Fig. 1b) rises to a maximum (2.4 %) for 2Rh and is roughly constant (within experimental uncertainty) for higher Rh loadings. Compared to unpromoted Rh on TiO2 at the same Rh loading (2 wt. %) and reaction conditions, the 2Rh catalyst on iron oxide exhibits a ∼4 fold increase in ethanol and oxygenate selectivity.8 At much higher reactor pressures (20 bar), unpromoted Rh (2 wt %) on TiO2 exhibits similar ethanol selectivity,10 whereas for SiO210,19 and Al2O39 supports, the total oxygenate selectivity is very low (< 2%, see Table S2). These comparisons suggest that the observed oxygenate activity of the Rh/nFe2O3 catalysts is a result of promotion by the nFe2O3 support. A hint as to its role comes from the observation of hydrocarbon activity for the bare nFe2O3 support under reaction conditions (Fig. 1a). This activity can be attributed to the reduction of Fe2O3 by exposure to CO + H2 at elevated temperatures, which is known to result in iron carbide and metallic Fe mixtures. The latter can 7 Environment ACS Paragon Plus

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act as a FT catalyst for hydrocarbon synthesis.20 For the Rh/nFe2O3 catalysts studied here, reduction of the nFe2O3 support is likely to result in Fe metal or iron carbides at the Rh interface. These could lead to FeRh alloy formation and/or iron-carbide-Rh interactions that could be responsible for increased oxygenate selectivity.8,13,21 To elucidate the structural changes of the Rh/nFe2O3 catalysts under working conditions, in-situ structural characterizations were performed using XRD and XAS, including extended x-ray fine structure, under H2 reduction followed by CO hydrogenation conditions.

Table 1. CO conversion (%) and product selectivity (%) from CO hydrogenation on Rh/nFe2O3 catalysts studied in this work.a Sampleb

CO Conv.

CH4

C2H6

C2H4

C2+ HCsc

C2H5OH

CH3CHO

CH3COOCH3

C2+ Oxyd

CH3OH

nFe2O3

1.2

37.6

27.3

20.1

62.4

-

-

-

-

-

1Rh

1.6

35.3

21.7

15.6

47.4

10.8

1.5

1.0

13.4

3.9

2Rh

2.4

37.2

10.9

18.5

41.6

14.4

2.2

1.9

18.5

2.7

5Rh

2.3

35.3

12.2

20.4

32.6

24.8

3.3

1.6

29.7

2.2

7Rh

2.0

31.0

11.4

22.5

33.9

27.3

3.0

2.2

32.5

2.7

(a) Reaction conditions: 240°C, 1 bar, CO:H2=2:1. (b) Integers related to approximate Rh % metal weight loading of catalyst sample. (c) C2+ hydrocarbons include ethylene, ethane, propane and butane. (d) C2+ oxygenates include ethanol, methyl acetate and acetaldehyde.

3.2 Metal Oxidation States from in-situ XANES Figure 2 shows the in-situ Fe and Rh K-edge XANES spectra of 2Rh under reduction and reaction conditions. Comparing the white line intensities22 and positions of the absorption K-


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edges for the Rh foil standard and 2Rh sample indicate that Rh is present as metallic Rh0 under both reduction and reaction conditions (Fig. 2a). The broadening of the nearedge peaks for 2Rh can be attributed to the small particle size compared to

Figure 2. XANES spectra of 2Rh catalyst under reduction and

bulk Rh metal.23 Figure 2b shows the

CO hydrogenation conditions: (a) Rh K-edge and (b) Fe K-edge.

corresponding Fe K-edge spectra for

H2:CO gas mix at 1 bar total pressure and 240ºC.

Reduction conditions: H2 flow at 300ºC. Reaction conditions: 2:1

2Rh along with those for α-Fe2O3, Fe3O4, and a Fe metal foil. The Fe K-edge spectra during reduction and reaction conditions have the same edge energy position (7112 eV) as the Fe metal foil indicating that the nFe2O3 support is mostly reduced to Fe metal. Differences in intensities of the pre-edge features at ~7111 eV and the continuum resonances at higher energies (≥ 7120 eV) suggest that the local geometry of the Fe species in 2Rh differ from reduction to reaction conditions.24,25 Overall, the K-edge spectra show that both Fe and Rh are present in their metallic states under catalytic conditions, but also indicate a change in the Fe chemical environment when going from reduction to CO hydrogenation conditions. As shown below, this is due to different phase transformations of the nFe2O3 support under reducing and reaction conditions. 3.3 Chemical Phase Analyses from in-situ XRD 3.3.1 Catalysts under H2 Reduction Figure 3 shows in-situ XRD patterns for all the catalyst samples and the bare nFe2O3 support under H2 reduction at 300°C. Rietveld refinement was performed using published crystallographic data and the GSAS-II software.17 Under reducing conditions (Fig. 3a), the


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Figure 3. XRD patterns of Rh/nFe2O3 catalysts under hydrogen reduction conditions: (a) full diffraction patterns, (b) expanded XRD regions associated with the Fe(110) scattering plane. Line color: brown=bare nFe2O3; blue=2Rh; purple=5Rh; green=7Rh.

diffraction pattern for the bare nFe2O3 support shifts to Fe3O4 with a minor fraction of metallic α-Fe (bcc). Introduction of Rh onto the support causes a dramatic drop in the Fe3O4 content that almost disappears in 5Rh and 7Rh. At the same time, diffraction peaks associated with α-Fe dominate the XRD patterns as Rh loading is increased. The near complete reduction of the nFe2O3 support to metallic Fe is mostly likely due to hydrogen-atom spillover26 from the dissociation of H2 on Rh surfaces which causes reduction of the oxide by the removal of Oatoms via water formation. The (110) diffraction peaks of α-Fe are shown on an expanded scale in Figure 3b. The asymmetric tails to lower scattering angles when Rh is added to the support can be attributed to lattice expansion resulting from Rh incorporation into the α-Fe bcc unit cell via formation of FeRh alloy phases. This was confirmed by Rietveld refinements that included Fe1-xRhx alloy phases with Rh contents of 0.06 ≤ x ≤ 0.5 (see Fig. S6). Although the Rietveld refinements required only four different Fe1-xRhx phases (x= 0.06, 0.1, 0.2, 0.3, 0.5) to adequately reproduce the data,


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particle size and microstrain parameters were tuned to reproduce the smooth profile of the tail (see SI). The range of alloy compositions is likely to depend on the heterogeneity of the Rh-Fe interfaces and Rh particle sizes, and the smooth tail may actually indicate a near continuous range of Fe1-xRhx phases up to x ≈ 0.5. 3.3.2 Catalysts under Reaction Conditions (CO + H2) Figure 4 shows an expanded region of the XRD patterns for the Rh/nFe2O3 catalysts under reaction conditions (following H2 reduction) along with peak assignments for the Fe phases obtained from Rietveld analyses (see Fig. S5 for the full patterns). In the processes of reduction followed by reaction, the nFe2O3 support for the Rh containing catalysts is completely converted to metallic Fe and carbide phases. By contrast, the bare nFe2O3 support remains mostly Fe3O4 (83%) with a small fraction of carbide (11.4%; see Table S4). The main carbide phase of the Rh containing catalysts can be assigned to ε’-Fe2.2C

Figure 4. XRD patterns of Rh/nFe2O3 catalysts under

CO

hydrogenation

conditions

and

the

reflections corresponding to the phases determined

based on the extracted cell parameters (see Table

by Rietveld refinement. Dotted lines represent the

S4).27,28 Small amounts of Hägg carbide (χ-

diffraction peaks associated with different Fe phases:

Fe5C5) are also evident which could result from

pink: χ-Fe5C5. *Identifies peaks associated with

green: ε’-Fe2.2C; blue: FeRh alloys, red: α-Fe and

the ε → χ phase transition which is close to the reaction temperature used (∼240°C).25 As in the H2 reduced samples, diffraction features resulting from the metallic Fe1-xRhx alloy phases are


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broad and mostly responsible for the scattering intensity between the α-Fe reflections and those associated with the ε’ and χ carbide phases (see also Table S5 and Figure S7). The Fe phase fractions extracted from Rietveld refinements can also be used to obtain estimates of the percentages of the total Rh loading that is incorporated into FeRh alloys. This was done by first quantifying the total amount of Fe and Rh found in the all the FexRhx-1 alloys using the stoichiometry of each alloy (x) and the Fe phase fractions involved in each from

Figure 5. Molar fractions of Rh incorporated into FexRh1-x alloy phases extracted from XRD Rietveld refinements and ICP measurements. See text for details.

the Rietveld refinements (Table S4). The total amount of Rh in the alloys could then be calculated and the sum compared to the total amounts of Rh present as determined by ICP analysis (Table S1). The mole fraction of Rh in all the FexRhx-1 alloy phases is then given by their ratio, i.e., ∑ () ⁄( , "#). The Rh molar fractions calculated in this way are shown in Figure 5 for the 2Rh, 5Rh and 7Rh samples. It is seen that as more than 80% of the available Rh is found as FeRh alloy phases for 2Rh and about 60% in the catalysts with the highest Rh loadings studied here. At the higher Rh loadings (5Rh and 7Rh), the amount of Rh incorporated into alloy phases may be limited by the kinetics of alloy formation at the relatively low temperatures used for reduction (300°C) and reaction (240°C). The observation of a nearly continuous range of alloy phases found in the catalysts under working conditions (Figure 4 and Table S4) is also consistent with non-


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equilibrium mixing. Nonetheless, the total amounts of the Fe-Rh alloy phases increase with Rh loading (Table S4), and these are likely to be the most important phases for oxygenate formation. 3.4 Identification of FeRh Nanoalloys via in-situ EXAFS Additional evidence for the presence of FeRh alloy phases under reaction conditions is provided by Rh K-edge EXAFS measurements shown in Figure 6. The radial distributions extracted from the EXAFS spectral regions (k3-weighted Fourier transforms) along with model fits are shown for 5Rh and 2Rh under CO hydrogenation conditions in the upper two panels, and the theoretical Fe-Rh and Rh-Rh scattering paths produced with Artemis using the FEFF8.2 code are shown in the bottom panel.29 Extracted bond lengths and coordination numbers are given in Table 2. Both the 2Rh and 5Rh samples exhibit peaks centered

Figure 6. Top panels: EXAFS k3-weighted Fourier transform magnitudes for 2Rh and 5Rh

at ~2.15Å and an asymmetry to longer distances that can be well described by a model fit that

samples; bottom panel: theoretical EXAFS scattering paths for Rh-Fe and Rh-Rh.

includes both Fe-Rh and Rh-Rh scattering paths centered at 2.20 Å and 2.42 Å. The Fe-Rh radial distance is consistent with the value (2.15 Å) determined in a previous EXAFS study of the chemically ordered FeRh alloy.30 The Rh-Rh contribution to the 2Rh distribution is less perceptible suggesting a lower Rh-Rh coordination compared to 5Rh. The extracted CN for RhFe are the same within experimental error for both 2Rh (3.3 ± 0.2) and 5Rh (2.8 ± 0.4), whereas


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the Rh-Rh CN for 2Rh (0.7 ± 0.2) is about half that of 5Rh (1.7 ± 0.2). The changes in Rh-Rh coordination for the 2Rh and 5Rh samples are consistent with our estimates for the Rh content in FeRh alloy phases, which decrease from 80% for 2Rh to 60% for 5Rh (See Figure 5). The much smaller CN’s between Rh and neighboring atoms versus bulk Rh (CN=12) and FeRh (CN=8) suggests that the Rh phases exist as small nanoparticles.31 Table 2: Results of the EXAFS curve fitting for the Rh K-edge spectra for the 2Rh and 5Rh catalysts under reduction and reaction conditions. Sample

Conditions

Shell

CN

R (Å)

∆σ2 (10-3 Å2)

Rh-Fe

4.3±0.8

2.54±0.01

6.0±1.9

Reduction

∆E° (eV)

0.54 Rh-Rh

1.4±0.5

2.67±0.01

3.1±2.0

Rh-Fe

3.3±0.2

2.54±0.01

6.0±1.9

2Rh Reaction

-3.44 Rh-Rh

0.7±0.2

2.66±0.01

3.1±2.0

Rh-Fe

3.0±0.3

2.55±0.01

6.0±1.9

Reduction

-0.23 Rh-Rh

2.0±0.2

2.67±0.01

3.1±2.0

Rh-Fe

2.8±0.4

2.55±0.01

6.0±1.9

Rh-Rh

1.7±0.2

2.67±0.01

3.1±2.0

5Rh Reaction

-1.02

CN= coordination number; R= interatomic distance; ∆σ2= Debye-Waller factor; ∆E°= inner potential correction The EXAFS results can be compared to earlier studies of Fe promoted Rh catalysts supported on oxides where Fe-Rh interactions were also implicated in enhancing oxygenate selectivity. Ichikawa, et al. examined the Fe and Rh K-edge EXAFS for Fe-Rh/SiO2 catalysts after reduction (ex-situ).13 A broad first shell scattering peak was observed in the Fe K-edge


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EXAFS from which a Fe-Rh bond distance (2.62 Å) was extracted that is larger than the found here (2.55 Å)32, but they were unable to observe any features associated with Rh-Fe bonds in the Rh K- edge EXAFS. In a more recent in-situ EXAFS study of Fe-Rh/TiO2 catalysts, Gogate, et al. were unable to detect the presence of Fe-Rh alloy in either the Fe or Rh K-edge EXAFS under reaction conditions.21 Finally, assignments to FeRh alloys in our recent study of Fe-Rh/TiO2 catalysts using in-situ XRD measurements were complicated by overlapping diffraction lines from the TiO2 support.8 Here, the large portion of Rh alloyed with Fe simplifies the analyses of the in-situ XRD and EXAFS data and provides clear evidence for the presence of metallic Fe-Rh alloy phases in an Fe-promoted Rh catalyst under reaction conditions. 4. Discussion To derive structure-activity relationships, the quantitative phase information from in-situ XRD (Fig. 4) and the catalytic selectivity results (Fig. 1a) are combined in Figure 7. The Fe metal and FeRh alloy phase fractions qualitatively follow oxygenate selectivity with Rh loading. The apparent correlation between FeRh alloy phase fraction and oxygenate selectivity has also been noted in previous studies.8,13-15 The fact that both FeRh and Fe metal track together is also not surprising since the latter is the Fe source for alloy formation. The catalytic role of Rh which is not involved in alloy formation in the 5Rh and 7Rh samples (see Figure 5) is not evident from Figure 5, but is likely to behave as other unpromoted Rh catalysts on oxide supports for which the main products are methane and C2+ hydrocarbon, with low selectivity for oxygenates (except at much higher pressures than used here; see Table S2). As for the oxide or carbide phase fractions, neither exhibits a definitive correlation with oxygenate selectivity indicating that these phases have a minimal effect on oxygenate formation. Moreover, the rapid change over from the mostly iron oxide to the carbide phase with the introduction of Rh is accompanied by only


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gradual changes in C2+ hydrocarbon and methane

selectivity

as

Rh

loading

increases. Overall, these trends suggest that the oxide and carbide phases, as well as any un-alloyed Rh, are active for hydrocarbon

and

methane

formation,

whereas the metallic FeRh alloy phases are mostly responsible for oxygenates. Whether the carbide influences the Figure 7: Oxygenate selectivity (right) and iron phase

activity of the FeRh alloys or the remaining

monometallic

Rh

through

content extracted from Rietveld refinements (left) versus Rh loading for Rh/nFe2O3 catalysts studied under CO hydrogenation conditions after hydrogen reduction.

support interactions is an open question. Recent DFT calculations on a similar catalyst, Cu/χ-Fe5C2, suggest that CO dissociation and CHx coupling to hydrocarbons are energetically favoured at Fe sites on the carbide surface, while CO insertion into CnHx species leading to oxygenates occurs at the Cu-carbide interface.33 Here, we might expect the ε’-Fe2.2C carbide to be similarly active for hydrocarbon formation, consistent with our reactivity results. By contrast to Cu, the calculations of Yang and Liu show that CHx binding and CO insertion can occur directly on the FeRh alloy surfaces without participation of the support.7 Hence, the ε’-Fe2.2C carbide support in this work is likely to play an indirect role in oxygenate formation, e.g., through electronic support interactions or controlling the extent of alloy formation. Compared to other Fe-promoted Rh catalysts for syngas conversion to ethanol, the optimized 7Rh catalyst exhibits an ethanol selectivity (∼30%) that is lower than our previous best


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Fe/Rh/TiO2 catalyst (48%) tested under the same reaction conditions, but is generally comparable to other published results for Fe/Rh catalysts supported on SiO2 and Al2O3 (see Table S3). Although the latter were tested at similar temperatures (250-270°C) and have comparable CO conversions, the pressure of reactants was much higher, 10-40 bar, which could strongly influence kinetics of the competing reaction pathways and affect product selectivity. It is likely that for all these catalysts, the formation of FeRh alloys is key to enhanced selectivity for ethanol, and the Rh/nFe2O3 catalysts studied here demonstrate a unique approach to alloy formation and also allowed unambiguous identification of the FeRh alloys via x-ray characterization. The absolute catalytic performance of the Rh/nFe2O3 catalysts studied here can be further enhanced by the addition of a second promoter such as Mn,15,34 and/or through modification of reaction conditions. 5. CONCLUSIONS The main findings of this work can be summarized as follows: (1) The nFe2O3 support is reduced and converted primarily to iron carbide under CO hydrogenation conditions. (2) Reduction of the oxide support produces metallic Fe, part of which is incorporated into FeRh alloy phases, which include 60-80% of the available Rh. (3) Ethanol and other oxygenates synthesis correlates with the presence of FeRh alloys. (4) Iron oxide and carbide phases contribute mostly to hydrocarbon and methane formation. From a broader perspective, this work presents a novel and simple way to prepare bimetallic catalysts by exploiting the reducibility of the metal oxide support and also the possibility of using metal carbides as supports as alternatives to metal oxides. The extent of


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reduction can be greatly enhanced by the ability of the supported metal to induce hydrogen spillover. Similar strategies have been used to successfully produce FePt35, FeRh36,37 and FePd37 alloy nanoparticles. Finally, this work stresses the importance of in-situ measurements for identifying the active phases responsible for catalytic performance. Indeed, the sensitivity of the Rh/nFe2O3 catalysts to re-oxidation made in-situ measurements necessary in order to identify the active phases for oxygenate and hydrocarbon formation which strongly depend on reaction conditions. ASSOCIATED CONTENT Supporting information is provided that includes additional details concerning sample compositions, comparisons of product selectivity for catalysts studied in this work and those in the literature, ex situ transmission electron microscopy images, full XRD patterns under reduction and reaction conditions, comparisons of XRD data with Rietveld refinement fits, summaries of quantitative phase information from Rietveld refinements, and details on EXAFS data fitting. ACKNOWLEDGMENT This research was carried out at Brookhaven National Laboratory under contract DE-SC0012704 with the US Department of Energy, Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences. S. D. Senanayake is supported by a DOE Early Career Award. XAS and XRD data were collected at beamlines 5-BM-D and 11-ID-B with the help of scientists Q. Ma, K. W. Chapman, K. A. Bayer and O. J. Borkiewicz at the Advanced Photon Source at Argonne National Laboratory, a DOE Office of Science User Facility.


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ABBREVIATIONS Syngas, synthetic gas; XRD, x-ray diffraction; XAS, x-ray absorption spectroscopy; EXAFS, extended x-ray absorption fine structure; FT, Fischer-Tropsch; ICP-MS, inductively coupled plasma mass spectroscopy; UHP, ultra-high purity; GHSV, gas space hourly velocity; GC, gas chromatograph; TCD, thermal conductivity detector; CN, coordination number; XANES, x-ray absorption near edge structure. AUTHOR INFORMATION Corresponding Author *[email protected] Present Addresses §

Department of Chemistry and Chemical Biology, Rutgers University, New Brunswick, NJ

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. +Main contributor in charge of writing the original draft, data acquisition and analyses of synchrotron (XANES and XRD) and catalytic activity data.

Ω

Contributed to data acquisition of synchrotron (XRD and XANES), and TEM data. §Performed preliminary studies on catalytic activity and reactivity, and synthesis of the catalysts. ‡Contributed to data analyses and final manuscript preparation. *Project leader, contributed to data interpretation and analyses, and final manuscript preparation. Funding Sources U.S. Department of Energy


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Surface Controls PtFe Nanoparticle Growth and Catalytic Properties. Nanoscale 2013, 5, 29212927. (36) Thalinger, R.; Götsch, T.; Zhuo, C.; Hetaba, W.; Wallisch, W.; Stöger‐Pollach, M.; Schmidmair, D.; Klötzer, B.; Penner, S. Rhodium‐Catalyzed Methanation and Methane Steam Reforming

Reactions

on

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Systems:

Metal–Support

Interaction.

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TABLE OF CONTENTS FIGURE


In Situ Formation of FeRh Nanoalloys for Oxygenate Synthesis - ACS

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