Reactivity of Bismuth Molybdates for Selective Oxidation of Propylene

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Reactivity of bismuth molybdates for selective oxidation of propylene probed by correlative operando spectroscopies Paul Sprenger, Matthias Stehle, Abhijeet Gaur, Andreas Gänzler, Daria Gashnikova, Wolfgang Kleist, and Jan-Dierk Grunwaldt ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00696 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018

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ACS Catalysis

Reactivity of bismuth molybdates for selective oxidation of propylene probed by correlative operando spectroscopies

Paul Sprenger,1 Matthias Stehle,1 Abhijeet Gaur,1 Andreas Martin Gänzler,1 Daria Gashnikova,1 Wolfgang Kleist1,2,3 and Jan-Dierk Grunwaldt1,2,* 1

Institute for Chemical Technology and Polymer Chemistry, Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe, Germany

2

Institute of Catalysis Research and Technology, Karlsruhe Institute of Technology (KIT), 76344 Eggenstein-Leopoldshafen, Germany 3

Present address: Industrial Chemistry - Nanostructured Catalyst Materials, Ruhr-University Bochum, 44801 Bochum, Germany *Corresponding author: [email protected]

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Abstract α-Bi2Mo3O12, β-Bi2Mo2O9 and γ-Bi2MoO6 as target bismuth molybdate phases were prepared by hydrothermal synthesis and flame spray pyrolysis and tested for their catalytic performance in the selective oxidation of propylene. Their structure and reactivity during temperature programmed reaction (TPR) and under reaction conditions was investigated by in situ and operando X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD) and Raman spectroscopy. To gain insight into amorphous and crystalline structures at the same time, XAS and XRD as well as XAS and Raman spectroscopy were combined in one experiment. TPR in propylene revealed that the reduction of Mo6+ to Mo4+ occurred at lower temperatures than from Bi3+ to Bi0 in scheelite-structured systems. In a reaction cycle, mainly reduction of molybdenum was observed and EXAFS fitting confirmed the removal of oxygen from MoO42- entities. Minor structural transformations were detected by XRD and Raman spectroscopy. The catalytic performance of aurivillius-structured systems was more diverse than for scheelite based ones, and ranged from highest to lowest observed acrolein yield, probably due to a synergy effect of two or more bismuth molybdate phases. For phase pure systems, bismuth was easier reduced than molybdenum. In contrast, aurivillius structures with additional phases showed reduction and oxygen removal from both metal centers under steady-state conditions but molybdenum was in most cases easier reduced. A high catalytic performance mostly coincided with low reduction temperatures, except for the unselective pure γ-Bi2MoO6 that showed a facilitated reduction of bismuth compared to molybdenum. Hence, the combination of operando methods led to an understanding of the redox behavior of bismuth and molybdenum and their influence on the catalytic performance. Keywords: Propylene oxidation, acrolein, Raman spectroscopy, X-ray diffraction, X-ray absorption spectroscopy, temperature-programmed reduction


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1

Introduction

Catalysts used for the selective oxidation of propylene to acrolein are typically multicomponent systems based on bismuth molybdates.1-3 Since acrolein is an important precursor for the large scale production of the amino acid methionine or acrylic acid as polymer building unit,4 a thorough understanding of the catalysts active species is highly desired and led to a recently renewed interest.5 Depending on the Bi/Mo ratio, three main bismuth molybdate phases with different structures are formed: α-Bi2Mo3O12, β-Bi2Mo2O9 and γ-Bi2MoO6. The former two represent defect and scheelite-like structures with distorted and regular MoO4 tetrahedra, respectively. In contrast, γ-Bi2MoO6 exhibits a layered aurivillius structure and consists of (Bi2O2)2+ layers between elongated MoO6 octahedra.6-8 In literature, there is an ongoing discussion about the relative stability of these phases and their activity in the selective propylene oxidation.2,

9-12

Scheelite-type phases are often considered catalytically more

attractive.13 Bismuth molybdate catalysts are typically prepared by solid-state reaction,14 coprecipitation15-16 or spray-drying.17-18 Recently applied synthesis methods, such as hydrothermal synthesis19-20 or flame spray pyrolysis,21-22 indicate that the catalytic performance further depends on the preparation technique and the calcination temperature.23 In order to describe the diverse catalytic performance, different material properties need to be taken into account. Bell and co-workers24-25 suggested the band gap as a descriptor, as it correlates with the apparent activation energy for propylene oxidation over scheelite-structured α-Bi2Mo3O12 and aurivillius-structured γ-Bi2MoO6. Hence, the higher activity of scheelitestructured phases was traced back to a 6.3 kJ mol-1 lower apparent activation energy24-25. Furthermore, the mobility of lattice oxygen and anionic vacancies is considered a key parameter. Although the catalytic performance of γ-Bi2MoO6 correlates with oxygen mobility,26-27 its


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validity as a stand-alone descriptor for the catalytic activity is limited.28 Moreover, a synergy effect, which occurs if mixtures of γ-Bi2MoO6 and scheelite-structured phases are present and leads to higher catalytic activity, has been reported and can be traced back to the combination of beneficial properties of the individual phases and an increased material conductivity.10, 29-32 For bulk metal oxide catalysts under reaction conditions, the possibility to remove lattice oxygen is connected to the reduction of bismuth and molybdenum centers within the mechanistic cycle. In this regard, Grasselli et al.2, 33 stated that during propylene oxidation, Bi3+ and Mo6+ are reduced to Bi2+ and Mo3+ respectively.34-35 However, using a combination of DFT and kinetic studies on α-Bi2Mo3O12, Bell and co-workers36-37 claimed that two molybdenum centers are reduced to Mo4+ while Bi3+ remains unchanged. This is further supported by XPS38-40 and XANES25,

37

studies showing the easier reduction of Mo6+ compared to Bi3+. This reduction

process is linked to the removal of certain oxygen species. For scheelite-structured phases, this is considered to be equatorial Mo=O36 and, for aurivillius-structured phases, oxygen from the top and bottom positions of MoO6 octahedra.24 The removal of oxygen from (Bi2O2)2+ entities, however, is considered to require a higher activation energy as estimated by Dadyburjor et al.41 The aim of the present study is to probe the reducibility and reactivity of α-Bi2Mo3O12, βBi2Mo2O9 and γ-Bi2MoO6 prepared by hydrothermal synthesis and flame spray pyrolysis. Thus, a combination of X-ray absorption spectroscopy (XAS), Raman spectroscopy and X-ray diffraction (XRD) was applied to identify crystalline and amorphous phases, local structure and oxidation states of bismuth and molybdenum during temperature-programmed reduction in propylene (TPR) and under reaction conditions in a spectroscopic fixed-bed microreactor. The combination of these complementary spectroscopic techniques were used to gain insights into structure-selectivity/activity relationships.


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2

Experimental Section

2.1

Catalyst synthesis

Samples of the nominal composition of α-Bi2Mo3O12, β-Bi2Mo2O9 and γ-Bi2MoO6 were synthesized by hydrothermal synthesis (HS) and flame spray pyrolysis (FSP) based on the procedures by Li et al.7 and Schuh et al.,21 respectively. For HS, bismuth(III) nitrate pentahydrate (Alfa Aesar) was dissolved in 2 M nitric acid, and stoichiometric amounts of ammonium heptamolybdate tetrahydrate (H. C. Starck) were dissolved in deionized water. After combining both solutions in a Teflon® inlay, the pH value was adjusted to 1 (HS-a, HS-b) and 6 (HS-c), respectively, by adding dropwise a 25 % ammonia solution. The Teflon® inlay was sealed in a stainless steel autoclave (Berghof) and heated to 180 °C for 24 h. After cooling to RT, the product was filtered off and washed. For FSP, stoichiometric amounts of bismuth(III) 2ethylhexanoate (Alfa Aesar) and molybdenum(IV) 2-ethylhexanoate (STREM Chemicals) precursors were dissolved in xylene, giving a total metal concentration of 0.25 M. The solutions were transferred to syringes and sprayed through a flame as described elsewhere.21-22 The product particles were collected on a cooled glass fiber filter positioned above the flame. Note that molybdenum(IV) 2-ethylhexanoate was received in two different batches from STREM. Due to a higher sodium content in the second batch, which was detected via ICP-OES, we denoted the samples as FSP-b and FSP-b2. Further details on the synthesis using HS and FSP are given in the electronic supporting information (ESI). Apart from the hydrothermally synthesized HS-b, which was calcined for 5 560 °C, all as-prepared catalysts were calcined for 5 h at 320 °C.

2.2

Ex situ catalyst characterization

Bulk metal composition was determined by inductively coupled plasma optical emission


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spectrometry (ICP-OES) using a 720/725-ES spectrometer (Agilent). For this, 50 mg grounded sample were dissolved in 6 mL hydrochloric acid, 2 mL nitric acid and 1 mL hydrogen peroxide by heating for 45 min in a microwave at 600 W. Specific surface area was determined by N2 physisorption at -196 °C with a BELSORP-mini II (Rubotherm) and calculated via the BrunauerEmmett-Teller (BET) method in the p/p0 = 0.05–0.3 range.42 Ex situ powder XRD was performed with a D8 Advance (Bruker) diffractometer equipped with a Cu Kα source at 0.15418 nm and a Ni filter. PXRD patterns were recorded in the 2θ range of 8 to 80° with a step size of 0.0165° and a dwell time of 2 s per step. Ex situ Raman spectroscopy was performed with an inVia Reflex Spectrometer System (Renishaw) equipped with a helium-neon laser (633 nm, 17 mW). All measurements were performed with a 20x objective at multiple positions on the samples. For each acquisition in the spectral range from 80 to 1300 cm-1, the laser intensity was set to 10 % and the acquisition time was varied between 2 and 90 s per point.

2.3

Catalytic testing

The catalyst was pressed and sieved to give a sieve fraction 300-450 µm and 1600 mg of the catalyst were placed in a quartz tubular reactor with 6 mm inner diameter resulting in a bed length of 45 mm. The reactor was heated by a vertically adjusted oven (HMT Reetz). Nitrogen (N50, Air Liquide), oxygen (N48, Air Liquide), and propylene (N25, Air Liquide) were supplied via mass flow controllers (Bronkhorst) and water was dosed through a CEM (Bronkhorst). The catalysts were heated to 180 °C (5 K min-1) in synthetic air (N2/O2=80/20, 100 NmL min-1) for preconditioning, and then in sequential ramp steps to T1, oven=400

oven=345,

T2,

catalyst=380

and T3,

°C (2 K min-1) under reaction conditions (C3H6/O2/H2O/N2=8/14/8/70). At each

temperature step, the total flow was set to 100, 150, 200 and 300 NmL min-1 in order to probe different weight hourly space velocities (WHSV). Hence, the WHSV, defined as propylene mass


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ACS Catalysis

flow per catalyst mass, was varied from 0.57 h-1 to 1.71 h-1. The catalyst/oven temperature was held at each condition for at least 3 h until the conversion was constant as monitored by an online oxygen sensor PAROX 1200 H (MBE AG). Reaction products were transferred via heated lines (180 °C) and analyzed via on-line gas chromatography (GC) using a 7890B (Agilent) positioned at the reactor outlet. The GC was equipped with two micropacked Hayesep Q columns for CO2 retention and a micropacked molsieve 5Å column for a separation of N2, O2 and CO, which were all detected by a thermal conductivity detector. A HP-FFAP column for separation of organic species (e.g. acrolein and acrylic acid) was connected to a flame ionization detector. Peak areas obtained from chromatograms were calibrated by means of calibration gases giving a conversion factor for each substance in µmol per area unit. Thus, the selectivity and conversion could be calculated based on the molar amounts of substrates and products. Propylene conversion was calculated by (1). 𝑋𝑋𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃 =

𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵

𝑛𝑛𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 −𝑛𝑛𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝

(1)

𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵

𝑛𝑛𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝

Acrolein selectivity was calculated based on the carbon atom number of each main-product

species as given in (2). 𝑆𝑆𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 = 𝑛𝑛

3 𝑛𝑛𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴

𝐶𝐶𝐶𝐶 +𝑛𝑛𝐶𝐶𝑂𝑂2 +3 𝑛𝑛𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃 +3 𝑛𝑛𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 +3 𝑛𝑛𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎

2.4

(2)

Operando and in situ spectroscopy

Operando and in situ spectroscopy was performed by means of a fixed-bed microreactor based on quartz glass capillaries.43 Raman spectroscopy, XAS and XRD were used in different arrangements, as illustrated in Figure 1. In general, XAS was performed at the Bi L3 (13.419 keV) and Mo K (20.000 keV) edges in transmission mode quasi-simultaneously by using the fast edge changing possibilities at dedicated beamlines.


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a) Combined operando XAS/XRD

b) Combined operando XAS/Raman spectroscopy

c) Operando Raman spectroscopy

Synchrotron radiation SOLEIL

Synchrotron radiation ESRF

Feed dosage

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MS

Feed dosage

MS MS

Feed dosage

Figure 1: Arrangement of spectroscopic devices around the fixed-bed during combined operando XAS/XRD (a, at BM31 beamline, ESRF), XAS/Raman (b, at ROCK beamline, SOLEIL) and Raman spectroscopy (c, at KIT) measurements (I0, I1 and I2 = ionization chambers, IRC=IR camera, MS=mass spectrometer). A detailed sequence of performed experiments is given in the ESI (Table S3). For combined XAS/XRD and XAS/Raman spectroscopic studies (Figure 1a-b), the bismuth molybdate samples were diluted and pestled with α-Al2O3 (tempered at 1200 °C for 5 h), in a ratio of 1:2. For exclusive Raman spectroscopy studies (Figure 1c), pure catalysts were used. Ca. 10 mg of a sieve fraction of 100-200 µm of the diluted or pure catalyst sample were filled in quartz capillaries of 1 mm diameter (Hilgenberg). The capillary was heated with a gas blower (FMB Oxford) and the catalyst bed was centered in the laser and X-ray beam, respectively. Furthermore, the microreactor was connected to feed gas supply and product gas analysis via tubes heated to 160 °C. Propylene (N35, Air Liquide), oxygen (N45, Air Liquide) and helium (N50, Air Liquide) were dosed through mass flow controllers (Bronkhorst) and water was dosed via a self-built heated saturator. In general, the feed gas flow was set to 10 mL min-1 and the ratios of He/O2/H2O/C3H6 gas compositions applied for each experiment are listed in Table S3 in the ESI. In order to allow a fast switching between different gas mixtures, a 4-way VICI® valve


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ACS Catalysis

(Valco Instruments) was implemented. The product gas was analyzed by means of a mass spectrometer OMNI Star (Pfeiffer Vacuum). A more detailed flowsheet of the in situ set-up is given in the ESI (Figure S2). Combined XAS/XRD (Figure 1a) for propylene temperature programmed reduction (TPR) experiments were conducted at the recently built BM31 beamline (previously BM01B,44 ESRF, Grenoble, France). A molybdenum foil was measured simultaneously as a reference compound for energy calibration. Switching between energies took approx. 30 s. XRD patterns were recorded with a CMOS-Dexela 2D detector at an energy of 0.050208±0.00001 nm. For each diffractogram, ten scans and ten dark images for background subtraction were recorded in the 2θ range of 2-35° with a resolution of approximately 0.01° and averaged. The sample to detector distance was calibrated with a LaB6 reference. For XRD data treatment, diluent α-Al2O3 was defined as internal standard. After each XRD scan, three alternating XAS measurements of the Mo K edge and the Bi L3 edge were performed, followed by another XRD, and so on. Combined XAS/Raman studies (Figure 1b) were conducted at ROCK beamline (SOLEIL, Saint-Aubin, France). The layout is described elsewhere.45 For both Mo K and Bi L3 edges, scans were taken with 1 Hz and 100 scans were averaged for one spectrum. Switching between both edges took ca. 1 min and a different set of ionization chambers was used per edge. Thus, a bismuth and a molybdenum foil were measured simultaneously for energy calibration. For Raman spectroscopy, a Raman RXN System (Kaiser Optical Instruments) equipped with a 532 nm laser and an optical fiber probe with a long-range objective of 7.5 cm focal length were used. The objective was placed orthogonal to the X-ray beam. Thus, by placing the sample capillary by an angle of 45° relative to both, the 532 nm laser beam and the X-ray beam, it was probed by both simultaneously. Furthermore, a NG 7 filter (Altechna) was used to reduce the


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laser power to ca. 30 mW, enabling acquisition times of 30 s. The temperature profile of the catalyst bed within the capillary was recorded with an ImageIR 8300 (InfraTec), equipped with a 55 mm objective. The images were taken with a frequency of 1 Hz in a calibrated range from 175-400 °C at an integration time of 47 µs. The software IRBIS 3 plus was used for thermography data treatment. The XAS data were treated with the software package IFEFFIT.46 The spectra were energy calibrated, normalized and background subtracted. Linear combination analysis (LCA) of XANES spectra was performed in a range of -20 eV to +50 eV at the Mo K and Bi L3 edges, respectively. For TPR experiments, the first and last spectra of the TPR series were used as LCA standards. For LCA treatment of the reaction cycle data, standards were obtained by recording spectra of reference pellets of hydrothermally synthesized α-Bi2Mo3O12, β-Bi2Mo2O9 and γ-Bi2MoO6 as well as MoO2 (99 %, Sigma Aldrich) and Bi2O3 (99.999 %, Sigma Aldrich). For EXAFS-analysis, the k3-weighted EXAFS spectra were Fourier transformed using k-ranges of 2.7-12.0 Å-1 at the Mo K-edge and 2.3-10.2 Å-1 at the Bi L3-edge. Additional in situ Raman spectroscopic studies (Figure 1c) were performed on non-diluted samples with an inVia Reflex Spectrometer System (Renishaw) equipped with a helium-neon laser (633 nm, 17 mW). Typically, the data were collected in a spectral range from 50 to 4600 cm-1 with a grating of 600 lines mm-1 resolution. The laser beam was focused on the quartz glass capillary via Renishaw’s video fiber optics probe with a long distance objective (focal length of 70 mm). The laser spot size on the catalyst bed was approximately 70 µm in diameter. Raman spectra were recorded continuously at 10-50 % laser power with 10-30 s acquisition time per individual spectrum. The data treatment was performed with WiRE 4.2 from Renishaw.


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3

Results and discussion

3.1

Material properties and catalytic activity of bismuth molydates

Different bismuth molybdate phases were prepared by the advanced methods of hydrothermal synthesis (HS) and flame spray pyrolysis (FSP). The obtained HS samples HS-a, HS-b and HS-c showed characteristic XRD patterns of α-Bi2Mo3O12, β-Bi2Mo2O9 and γ-Bi2MoO6, respectively, with the main reflections at 28.0°, 27.9° and 28.3° (Figure 2a).8,

47-48

Furthermore, Raman

spectroscopy gave characteristic spectra with main bands at 902, 887 and 799 cm-1 corresponding to the three pure phases (Figure 2b).49-50 For HS-a and HS-b, XANES spectra at the Mo K edge showed a strong pre-edge feature ‘A’ at ca. 20.005 keV, which can be assigned to a 1s-4d transition of tetrahedrally coordinated Mo6+, typical for scheelite-like structures (Figure 2c).51-52 In contrast, HS-c showed only a weak pre-edge feature, but a more pronounced feature ‘B’ at ca. 20.024 keV corresponding to a 1s-5p transition, which is characteristic for octahedrally coordinated Mo6+, present in aurivillius phases.51-52 XANES spectra taken at the Bi L3 edge showed a more pronounced absorption maximum for HS-c than for HS-a and HS-b (Figure 2d).51,

53

Overall, ex situ spectroscopic characterization supported the successful hydrothermal

synthesis of pure α-Bi2Mo3O12 (HS-a) β-Bi2Mo2O9 (HS-b) and γ-Bi2MoO6 (HS-c). The elemental analyses of the samples further confirmed the expected Mo/Bi ratios and BET analysis gave rather low specific surface areas of 1-6 m2 g-1 (Table 1). In contrast, flame-made catalysts FSP-a, FSP-b, FSP-b2 and FSP-c exhibited a larger surface area (14-26 m2 g-1) compared to hydrothermally synthesized samples. In general, phase characterization of flame-made catalysts was more difficult due to their small crystallite size, which resulted in broad diffraction reflections and Raman bands (cf. ESI, Figure S3). For FSP-a and FSP-b2, Raman spectroscopy revealed a broad band around 910 cm-1 with a shoulder at lower wavenumbers, which is


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ACS Catalysis

characteristic for isolated monomolybdate MoO42- entities.54 In general, FSP-a, FSP-b and FSP-c contained α-Bi2Mo3O12, β-Bi2Mo2O9 and γ-Bi2MoO6 phases, respectively. However, Raman spectroscopy (Table 1) as well as LCA of XANES spectra (cf. section 3.3) revealed the presence of additional amorphous phases and, thus, flame-made catalysts were more amorphous and not as phase pure as hydrothermally synthesized catalysts. As reported previously by Beale and Sankar52 as well as Li et al.,7 pure bismuth molybdate phases were obtained by HS, whereas a calcination step at 560 °C was necessary to obtain the metastable β-Bi2Mo2O9 phase.7, 52 FSP, on the other hand, gave access to all phases in a single step, as reported by Schuh et al.21 Flame-made catalysts generally showed a larger surface area due to their nano-particle character, which makes them especially attractive for operando studies with bulk-sensitive techniques due to the higher number of accessible active sites. γ

Normalized intensity

γ β

HS-b β

β

β

ββ

α α

847 714 887

HS-b 752 768 902

HS-a α

α

30

α

799

HS-c

γ

γ

15

b)

HS-c

γ


a)

840

HS-a

α

816 858 926 958

651

45

60

100

75

300

2θ / °

500

700

900

1100

Raman shift / cm-1

d) 1.2

c)

Mo K edge

B

1.2

0.8

A 0.4

MoO2 HS-a (α-Bi2Mo3O12) HS-b (β-Bi2Mo2O9) HS-c (γ-Bi2MoO6)

0.0 20.00

20.05

Normalized absorption / a.u.


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20.10

Bi L3 edge

0.8

0.4 Bi foil HS-a (α-Bi2Mo3O12) HS-b (β-Bi2Mo2O9) HS-c (γ-Bi2MoO6)

0.0 13.400

13.425

Energy / keV

13.450

13.475

Energy / keV

Figure 2: XRD patterns (a), Raman spectra (b), Mo K edge XANES (c) and Bi L3 edge XANES (d) of hydrothermally synthesized samples HS-a, HS-b and HS-c. Corresponding EXAFS data are given in Figure S4.


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Table 1: Characterization of catalysts prepared by HS and FSP with different Bi/Mo ratios before (fresh) and after catalytic testing (used) as determined by ICP-OES, N2 physisorption (BET), PXRD and Raman spectroscopy (main phases in bold letters). XRD patterns and Raman spectra are given in Figure S3. Mo/Bi ratio by Catalyst

ICP-OES (theoretical)

Specific surface area (BET) / m2 g-1 Fresh

Used

Phases (Ex situ PXRD)8, 47-48

Phases (Ex situ Raman spectroscopy)49-50

Fresh

Used

Fresh

Used

HS-a

1.4 (1.5)

5

α-Bi2Mo3O12

α-Bi2Mo3O12

α-Bi2Mo3O12

α-Bi2Mo3O12

HS-b

1.0 (1.0)

1

β-Bi2Mo2O9

β-Bi2Mo2O9

β-Bi2Mo2O9

β-Bi2Mo2O9

HS-c

0.5 (0.5)

6

γ-Bi2MoO6

γ-Bi2MoO6

γ-Bi2MoO6

γ-Bi2MoO6

FSP-a

1.3 (1.5)

14

α-Bi2Mo3O12

α-Bi2Mo3O12

MoO42- entities

α-Bi2Mo3O12

FSP-b

1.0 (1.0)

15

FSP-b2

1.0 (1.0)

11

FSP-c

0.5 (0.5)

26

4

β-Bi2Mo2O9

2

4

β-Bi2Mo2O9

β-Bi2Mo2O9

β-Bi2Mo2O9

γ-Bi2MoO6

γ-Bi2MoO6

α-Bi2Mo3O12, MoO42entities, MoO3 γ-Bi2MoO6, β-Bi2Mo2O9

α-Bi2Mo3O12, MoO3 γ-Bi2MoO6, β-Bi2Mo2O9, α-Bi2Mo3O12


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Page 14 of 41

Notably, FSP-b2 contained phases other than β-Bi2Mo2O9 according to Raman spectroscopy (Table 1) due to the two different precursor deliveries (cf. section 2.1). The catalytic activity and selectivity of bismuth molybdates was studied in a dedicated testing unit for the selective oxidation of propylene. The catalysts were tested in a feed flow of N2/O2/H2O/C3H6 = 70/14/8/8. The results are shown in Figure 3. Bismuth molybdates prepared by HS and FSP differed significantly in propylene conversion and product selectivity. In general, higher WHSVs led to lower propylene conversion, but higher acrolein selectivity and, thus, lower CO and CO2 selectivity. Minor by-products, such as acrylic acid, acetic acid and propane were detected, but are not shown here. For hydrothermally synthesized catalysts, HS-a (αBi2Mo3O12) showed the highest propylene conversion (up to 39 % at 0.57 h-1). Like HS-b (βBi2Mo2O9), HS-a showed a high acrolein selectivity of ca. 91 %, when compared at similar conversions (ca. 15 %). In contrast, γ-Bi2MoO6 (HS-c) showed generally a high selectivity towards CO and CO2 (up to 47 % at 0.57 h-1). Flame-made γ-Bi2MoO6 (FSP-c), on the other hand, combined both high propylene conversion and acrolein selectivity (54 % and 81 % at 0.57 h-1). Compared to FSP-c, FSP-a showed similar acrolein selectivity, but a much lower propylene conversion (only 9 % at 0.57 h-1). FSP-b2 showed a good propylene conversion and selectivity, similar to HS-a. In general, the same activity trends were observed at 345 °C and 400 °C oven temperature (Figure S1). After catalytic testing and ca. 4 d on stream, phase transformations were observed for flame-made catalysts, as listed in Table 1. Hot-spots within the catalyst bed and, thus, temperature-induced phase transformations55 or MoO3 depletion56 may have occurred.


14

Page 15 of 41

b) 50

a) 100

0.57 h-1

(Synergy promoted)

Aurivillius structure

90 1.71 h-1

80

CO and CO2 selectivity / %

Acrolein selectivity / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Scheelite structure

1.14 h-1

70 0.86 h-1

Aurivillius sutructure

60

50

0.57 h-1

0

15

30

45

60

HS-a HS-b HS-c FSP-a FSP-b2 FSP-c

40 0.86 h-1

30 1.14 h-1

20

1.71 h-1

10

0

0

Propylene conversion / %

15

30

45

60

Propylene conversion / %

Figure 3: Acrolein selectivity over propylene conversion (a) as well as CO and CO2 selectivity over propylene conversion (b) for bismuth molybdates. An aurivillius phase mixed with additional phases showed a so-called phase synergy effect and was, thus, denoted as synergy promoted. Tested in a lab-reactor at 380 °C catalyst bed temperature at WHSVs in a range of 0.57-1.71 h-1.

Based on ex situ characterization results, it is possible to correlate the observed catalytic performances with the crystal structure, as highlighted in Figure 3. While scheelite-structured FSP and HS systems generally combined sufficient acrolein selectivity and propylene conversion, aurivillius-structured catalysts seemed more versatile. HS-c showed a high selectivity towards undesired by-products CO and CO2 and, in contrast, FSP-c was best performing in terms of acrolein formation. As summarized by Carson et al.,10 literature studies often ascribe the role of either the best or the worst performing catalyst to γ-Bi2MoO6 when compared to scheelite-structured systems. The unselective product formation, observed for HS-c (γ-Bi2MoO6), can be explained with the site-isolation principle of Grasselli57 and the high availability of lattice oxygen around the active center of aurivillius-structured catalysts. Notably, the formation of CO and CO2 may take place via a different reaction mechanism. After the 2nd Habstraction, the allyl species most likely reacts with adsorbed oxygen, leading to CO, CO2 or acetaldehyde instead of acrolein.37,

58

In the here presented case, either lattice oxygen from


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Page 16 of 41

(Bi2O2)2+ layers may act similar to the adsorbed oxygen, or these lattice vacancies facilitate the adsorption of gas phase oxygen, which then lead to overoxidation. Furthermore, Zhai et al.24 found that aurivillius-structured phases form unselective reaction products if the band gap of the material is below 2.1 eV. Thus, the different catalytic behavior observed for HS-c and FSP-c may have originated from a synergy effect, caused by additional phases in FSP-c, eventually preventing the unselective reaction pathway. The synergy effect, an effect of phase cooperation that has been described earlier for bismuth molybdates in literature,9-10, 30-31 was observed for γBi2MoO6 when mixed with additional bismuth molybdate phases and is denoted as synergypromoted in the following. In summary, the observed performance of scheelite-structured, aurivillius-structured and synergy-promoted aurivillius-structured phases is strikingly different. Thus, the broad range of catalytic activity and material properties that were observed makes hydrothermally synthesized and flame-made bismuth molybdates highly attractive for operando studies.

3.2

In situ XAS, XRD and Raman spectroscopy during temperature-programmed reduction with propylene

In order to probe the reducibility of bismuth molybdates, temperature-programmed reduction (TPR) in propylene/helium atmosphere was performed, similar to earlier studies on MoO3,59 and monitored by combined in situ XAS/XRD and additionally by in situ Raman spectroscopy. Based on the spectroscopic analysis presented in the previous section, oxidation states of bismuth and molybdenum during TPR were directly accessible via XANES spectra, which were recorded almost simultaneously within the same heating cycle and are, thus, directly comparable. For TPR studies, samples of HS-a, HS-b, HS-c and FSP-b were heated to 700 °C at a ramp rate of 2 °C min-1 under a propylene/helium = 20/80 feed. During the course of all XAS-TPR


16

Page 17 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

experiments, the Mo K edge clearly shifted to ca. 20.010 keV, which corresponded to the reduced species MoO2 with a well pronounced feature ‘B’ (cf. Figure 2c) attributed to its pseudooctahedral Mo4+ coordination.60 Furthermore, the Bi L3-edge absorption edge shifted from ca. 13.426 keV for Bi3+ to ca. 13.422 keV corresponding to elemental Bi0. An example is shown in Figure 4a-b for HS-c and in the ESI for HS-a and HS-b (Figure S5). By using the first and last XANES spectrum of each TPR series for linear combination analysis (LCA), fractions of reduced and oxidized molybdenum and bismuth species were determined and plotted as function of temperature. The data are presented in Figure 4c-f. For α-Bi2Mo3O12 (HS-a) and β-Bi2Mo2O9, (HS-b) the reduction of Mo6+ to Mo4+ started slightly above 500 °C. At slightly higher temperatures, the reduction of bismuth from Bi3+ to Bi0 was observed. The reduction reached 50 % at 540 °C for molybdenum and 567 °C for bismuth in the case of HS-a and, similarly, at 580 °C and 612 °C the case of HS-b (cf. Table S4). In contrast, γ-Bi2MoO6 (HS-c) behaved strikingly different. Bismuth reduction started already at 559 °C and was reduced by 50 % at a 43 °C lower temperature compared to molybdenum (602 °C). The same order of reduction temperatures becomes evident by only comparing the half edge energy shift of bismuth and molybdenum (Figure S6). For scheelite-structured catalysts, the catalytically more active sample (HS-a) was reduced at lower temperatures (compared to HS-b). Furthermore, the general observation that molybdenum is easier reduced compared to bismuth is in agreement with XPS studies38-40 and the assumption of Zhai et al.37 A similar reducibility was assumed for aurivillius-structured systems.24 However, here we observed the opposite trend. The reduction of bismuth is likely attributed to an easy removal of bismuth-bound oxygen from (Bi2O2)2+ layers, which is in contrast to early calculations by Dadyburjor et al.41 Such a more facile reduction of Bi3+ was previously reported


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Page 18 of 41

for Bi-Mo-Ti-O catalysts.61 Keeping in mind that only aurivillius-structured HS-c showed a high selectivity to the overoxidation products CO and CO2 (section 3.1), the reduction of (Bi2O2)2+ layers may be linked to a new unselective reaction pathway. Haber and Turek62 showed that, while the selective oxidation is generally conducted by nucleophilic O2-, the total oxidation requires electrophilic O2- or O-. Thus, an electron transfer and the reduction of bismuth may have occurred prior to the release of oxygen from the lattice. This represents a new pathway for the unselective oxidation reaction with a redox role attributed to bismuth. Nevertheless, the main product formed for HS-c was still acrolein, most likely using nucleophilic O2- from MoO6 octahedra as oxidizing species, as discussed by Zhai et al.24 Furthermore, as compared in Figure 4d and Figure 4f, the reduction temperatures seemed to depend also on the preparation technique. Both HS-b and FSP-b consisted of phase-pure βBi2Mo2O9 according to ex situ XRD and Raman spectroscopy (Table 1). HS-b reached 50 % reduction of molybdenum at ca. 580 °C and FSP-b at ca. 545 °C, whereas for bismuth this temperature was 32 °C and 15 °C higher, respectively (cf. Table S4). This may be explained with the different material properties. The specific surface area is strongly different (1 m2 g-1 compared to 15 m2 g-1) and may originate from various amounts of amorphous and crystalline βBi2Mo2O9 and, thus, different accessibility of lattice oxygen. In general, the reductive behavior is determined by the type of phases and material properties, which is in line with the diverse catalytic activity observed for these catalysts. For all tested catalysts, a high acrolein yield (YAcrolein at 380 °C and 0.57 h-1) did correlate with a low molybdenum reduction temperature (T50 %, cf. Table S4): YAcrolein = 29 % and T50 % = 540 °C (HS-a), 23 % and 545 °C (FSP-b2), 15 % and 579 °C (HS-b) as well as 14 % and 602 °C (HS-c).


18

Page 19 of 41

b) 1.2

a)

Bi L3 edge



Mo K edge 1.2

0.8

0.4

0.8

0.4

Propylene TPR HS-c (γ-Bi2MoO6)

0.0 20.00

20.05

Propylene TPR HS-c (γ-Bi2MoO6)

0.0

20.10

13.400

13.425

d)

c) 100

60

HS-a (α-Bi2Mo3O12) Bi3+ Bi0 Mo6+ Mo4+

40

20

80

60

HS-b (β-Bi2Mo2O9) Bi3+ Bi0 Mo6+ Mo4+

40

20

0 100

0 200

300

400

500

600

700

100

200

f)

100

60

HS-c (γ-Bi2MoO6) Bi3+ Bi0 Mo6+ Mo4+

40

20

0 100

400

500

600

700

600

700

100

Fraction by LCA / %

80

300

Temperature / °C

Temperature / °C

e)

13.475

100

Fraction by LCA / %

Fraction by LCA / %

80

13.450

Energy / keV

Energy / keV

Fraction by LCA / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

80

FSP-b (β-Bi2Mo2O9) Bi3+ Bi0 Mo6+ Mo4+

60

40

20

0 200

300

400

500

600

700

100

200

Temperature / °C

300

400

500

Temperature / °C

Figure 4: Series of in situ XANES spectra of HS-c, showing a shift of the adsorption edge towards MoO2 (a) and metallic Bi0 (b) during TPR in propylene (C3H6/He = 20/80; 2 °C min-1). LCA plots as function of the temperature show the reduction progress of HS-a (c), HS-b (d), HSc (e) and FSP-b (f). During TPR in propylene, bismuth molybdate phases were reduced to Bi0 and MoO2, as shown by XAS. In all cases, the final spectrum of the TPR series taken at the Mo K edge corresponded


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Page 20 of 41

to MoO2, as shown in Figure S7 for the XANES and the EXAFS region. This was further supported by simultaneously recorded XRD patterns. As shown in Figure 5, HS-c showed an XRD pattern characteristic for γ-Bi2MoO6 at the beginning of the TPR, which disappeared almost completely at 688 °C. Instead, reflections corresponding to MoO2 appeared and a very weak reflection around 2θ of 8.8° indicated the presence of Bi0.63 As the reflection ranged from 7.9° to 10.6°, the particle size was rather low. The same was observed for HS-a, HS-b and FSP-b (Figure S8). In order to probe the stability of the oxygen-deficient lattice, a reoxidation step at 700 °C was performed after TPR by switching to an oxygen/helium atmosphere. XAS and XRD were recorded after 10 min and again after cooling down to ca. 100 °C. As shown in Figure 5, a phase transformation of γ-Bi2MoO6 (HS-c) to the high temperature phase γ-Bi2MoO6 (HT), which typically forms above 600 °C,55 was observed. In all other cases, only minor phase transformations were observed as listed in Table 2. Thus, despite of the lack of lattice oxygen in the bulk structure at the end of the TPR experiment, bismuth molybdate phases were able to retain their structure. In situ XAS/XRD TPR studies were supported by in situ Raman spectroscopy data, which are presented in detail in the ESI, section 6. Under the reducing conditions of the propylene/He atmosphere, the formation of amorphous carbon was observed as evident from intense bands originating from ideal and disordered graphitic lattices (G and D1 band at 1577 cm-1 and 1343 cm-1).64-65 Upon further heating, Raman bands corresponding to Mo-O vibrations disappeared, while mass spectrometry revealed the formation of CO, CO2 and acrolein due to the oxidation with lattice oxygen. After a reoxidation of the reduced catalysts, Raman bands corresponding to Mo-O vibrations reappeared. Similar to XRD, TPR-induced phase


20

Page 21 of 41

700 °C (Reoxi) 700 °C (TPR) 688 °C (TPR)

(HT)

100 °C (Reoxi)

MoO2 γ-Bi2MoO6

transformations were observed via Raman spectroscopy, as listed in Table 2.


611 °C (TPR) 534 °C (TPR) 456 °C (TPR) 379 °C (TPR) 302 °C (TPR)

γ-Bi2MoO6

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

225 °C (TPR) 147 °C (TPR)

5

10

15

20

25

2θ / °

Figure 5: XRD patterns of HS-c during TPR in propylene as recorded via in situ XAS/XRD, whereas the corresponding XAS data are shown in Figure 4e. The dotted lines refer to α-Al2O3 used as diluent (TPR=reducing conditions; Reoxi=reoxidation; λ=0.050208 nm).


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Page 22 of 41

Table 2: Phase composition of bismuth molybdates before (first spectra of TPR series) and after (last spectra after catalyst reoxidation) the TPR experiments (main phases in bold). Combined in situ XAS/XRD In situ Raman spectroscopy Sample

XAS Before

HS-a

HS-b

HS-c

α-Bi2Mo3O12

β-Bi2Mo2O9

γ-Bi2MoO6

XRD After α-Bi2Mo3O12, β-Bi2Mo2O9

Before

After

α-Bi2Mo3O12

γ-Bi2MoO6

Before a

α-Bi2Mo3O12

After α-Bi2Mo3O12, β-Bi2Mo2O9

β-Bi2Mo2O9,

β-Bi2Mo2O9,

β-Bi2Mo2O9

α-Bi2Mo3O12

γ-Bi2MoO6 (HT),

β-Bi2Mo2O9

β-Bi2Mo2O9

γ-Bi2MoO6

γ-Bi2MoO6 (HT)

γ-Bi2MoO6 γ-Bi2MoO6

b

γ-Bi2MoO6 (HT) β-Bi2Mo2O9,

FSP-b

β-Bi2Mo2O9

β-Bi2Mo2O9

β-Bi2Mo2O9

γ-Bi2MoO6 (HT), (γ-Bi2MoO6)

a

At 700 °C; Reoxidation was probably not complete

b

Fitting was not possible


22

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ACS Catalysis

3.3

Reaction behavior during operando XAS/Raman spectroscopic studies

In order to elucidate the behavior of bismuth molybdates under working conditions, the catalysts were treated under various conditions and studied via combined operando XAS/Raman spectroscopy. As illustrated in Figure 6, a reaction cycle with a sequence of selected gas compositions was performed under steady state conditions. XAS at the Bi L3 and Mo K edges was measured alternately, while Raman spectroscopy was recorded continuously. For this experiment, flame-made bismuth molybdate phases FSP-a, FSP-b and FSP-c were selected because of their comparatively large specific surface area (14-26 m2 g-1), their higher number of surface sites facilitates the observation of subtle changes despite of the applied bulk-sensitive techniques. a)

Start

80/12/8/0

Reaction conditions 72/12/8/8

Oxidizing conditions

Reducing conditions with water

Reducing conditions

Reaction conditions

End

80/12/8/0

84/0/8/8

92/0/0/8

72/12/8/8

80/12/8/0

400 °C 140 °C

140 °C

Figure 6: During the operando XAS/Raman study, flame-made bismuth molybdate catalysts were treated with a sequence of gas mixtures and temperature conditions (a). The given feed gas ratios refer to a mixture of He/O2/H2O/C3H6 and the given temperatures to the set gas blower temperatures. Series of XANES spectra of FSP-c recorded simultaneously at the Mo K (b) and Bi L3 (c) edges during the reaction cycle.


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Page 24 of 41

During the reaction cycle, Mo K edge and Bi L3 edge XANES of FSP-a, FSP-b and FSP-c showed various changes, depending on the reaction conditions, as shown for FSP-c (Figure 6) as an example (FSP-a and FSP-b given in Figure S13). The spectra were analyzed by linear combination analysis (LCA) using the reference spectra shown in Figure 2c. As discussed in section 3.1, pure bismuth molybdate phases exhibit different features at the Mo K edge due to a characteristic oxygen coordination geometry at the molybdenum centers. However, near-edge features at the Bi L3 edge are generally less distinct and LCA was only applied for the determination of the bismuth oxidation state. In the following, LCA results of averaged XANES spectra after ca. 30 min equilibration time at each condition, which were extracted from the series of spectra (Figure 6, Figure S13), will be discussed. As shown in Figure 7a, FSP-a initially showed a main contribution of α-Bi2Mo3O12. However, a minor contribution of β-Bi2Mo2O9 was found. As already indicated by Raman spectroscopy and discussed in section 3.1, flame-made catalysts contained additional amorphous phases, which could be further confirmed via XAS, since this technique is sensitive to both crystalline and amorphous phases regardless of the crystallite size. Note that reference spectra and operando spectroscopic data were taken at different temperatures (RT vs. 140 °C/400 °C). Both FSP-a (Figure 7a) and FSP-b (Figure 7b) showed an increasing contribution of β-Bi2Mo2O9-like structures under reaction conditions (33 % and 75 %), compared to their initial state (21 % and 65 %) and under oxidizing conditions (29 % and 73 %). Furthermore, both FSPa and FSP-b showed a significant contribution of reduced MoO2 species under reducing reaction conditions in the presence of water (8 % and 24 %) and even more in its absence (14 % and 30 %). This supports the hypothesis of an oxidizing role of water and its provision of lattice oxygen.66 For FSP-b, the phase composition determined by LCA of the XANES data revealed


24

Page 25 of 41

that mainly α-Bi2Mo3O12 and γ-Bi2MoO6 were present after applying reducing conditions in the presence of water. This is surprising, since no complete decomposition of FSP-b was observed under any other conditions, not even during TPR in propylene, where the catalyst was fully reduced (see section 3.2). MoO2

α-Bi2Mo3O12

Fraction as determined by LCA / %

a) 100

80

33%

29%

67%

71%

γ-Bi2MoO6

14%

15%

14%

86%

85%

85%

60 92% 40

79%

20

Start

b) 100 Fraction as determined by LCA / %

β-Bi2Mo2O9 8%

21%

0

80

Oxidizing Reducing End Reaction Reaction Reducing with water 8% 15% 24% 30% 21% 19%

65% 75%

60

73%

40

20

76%

35% 25%

0

Start

c) 100 Fraction as determined by LCA / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

60

33% 43%

27%

36%

52%

54%

23%

40

0

66%

Reducing End Oxidizing Reaction Reaction Reducing with water

12%

20

71%

27%

27% 80

70%

32%

37%

16%

16% 50%

48%

45% 32%

Start

15%

34%

36%

31%

Reducing End Oxidizing Reaction Reaction Reducing with water

Figure 7: Fraction of bismuth molybdate phases in FSP-a (a), FSP-b (b) and FSP-c (c) as determined by LCA of Mo K edge XANES spectra, which are shown in Figure S14 and were extracted from the data shown in Figure 6 and Figure S13.


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Page 26 of 41

FSP-c, on the other hand, showed a high contribution of all three bismuth molybdate phases in its initial state (cf. Table 1) and, according to LCA of the XANES data, only 27 % of γ-Bi2MoO6 (Figure 7c). Additional phases were already expected from ex situ Raman spectroscopy (Table 1) and are expected to be the origin of the high catalytic activity of FSP-c due to phase synergy. Increased fractions of γ-Bi2MoO6 were observed under oxidizing conditions (52 %) but less under reaction conditions (43 % and 36 %). Under reducing conditions, an extensive formation of MoO2 was observed (up to 33 %). Comparing the phase composition of FSP-b and FSP-c at the start and end of the reaction cycle, phase transformations occurred. Presumably, this was caused by the interaction of flamemade nanoparticles containing crystalline as well as amorphous phases at elevated temperatures and different feed gas compositions. Thus, particle sintering under such conditions, as evident from a decrease in surface area (cf. Table 1), was linked to the formation of larger crystalline domains under the intermixing of involved spheres as indicated in the phase diagram.55 According to the absorption edges in the XANES spectra, the Bi/Mo ratio was not affected during the reaction cycle. A time resolved consideration of LCA allows a detailed view on the reduction of flame-made bismuth molybdates under steady state conditions at a constant temperature. As shown in Figure 8, after switching to reducing conditions, the contribution of β-Bi2Mo2O9 decreased instantly. The reduction of Mo6+ and the formation of MoO2 started thereon. For FSP-b, the reduction of Mo6+ started almost at the same time as the reduction of Bi3+. This is in line with the close TPR reduction temperatures of both metals reported in section 3.2 for FSP-b. FSP-c, on the other hand, exhibited a much later starting point of Bi3+ reduction, which indicates that molybdenum is much easier reduced. After switching from reducing to reaction conditions, the catalysts were


26

Page 27 of 41

quickly reoxidized. Thus, the saw tooth-like curve shape in Figure 8 illustrates that the reduction was a reversible process and the reoxidation was much faster than the reduction of bismuth molybdates.

Bi3+

Fraction of Bi species / %

50 60 70

Bi0

MoO2

FSP-a Reducing +water

β-Bi2Mo2O9

α-Bi2Mo3O12

γ-Bi2MoO6

FSP-b Reducing

Reducing +water

FSP-c Reducing

Reducing +water

Reducing

80 90 100

Fraction of Mo species / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

80 60 40 20 0

250

300

Time / min

250

300

Time / min

300

350

400

Time / min

Figure 8: LCA of Mo K edge and Bi L3 edge XANES spectra of flame-made bismuth molybdates focussed on the reduction sequences of the reaction cycle. Red circles represent the recorded spectra. Analysis is based on data presented in Figure 6 and Figure S13. Furthermore, Mo K edge EXAFS spectra (Figure S14) were evaluated and variations in the coordination number (CN) of the Mo-O1 shell (at 1.8 Å) were observed under different conditions, as shown in Figure 9 and listed in Table S7. FSP-a showed only minor phase transformations and comparatively small changes in the CN (2.5-4) throughout the reaction cycle. This indicates a high structural stability and resistance to form oxygen vacancies, which may explain the lower reactivity. In contrast, FSP-b revealed a significantly lower CN under reducing conditions (2.0) compared to reaction conditions (4.6). This can be attributed to the loss of lattice oxygen and the presence of pseudo-octahedral MoO2 with longer axial bonds. After switching to oxidizing conditions, the CNs of FSP-a and FSP-b basically went back to their


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initial value (~4). For FSP-c, the CN of Mo-O1 decreased under reducing conditions (from 4.3 to 2.0) but was restored under reaction conditions. Changes of the XANES at the Bi L3 edge were observed only for FSP-b and FSP-c. LCA of Bi L3 edge XANES revealed a partial reduction of bismuth under reducing conditions with and without water (Figure 8). Thus, fitting of the k3-weighted Fourier transformed EXAFS spectra was performed in R-space and CNs of the Bi-O1 path were determined, as shown in Figure 9c and listed in Table S7. For scheelite-structured FSP-a and FSP-b, the oxygen coordination remained almost constant throughout the cycle. In contrast, aurivillius-structured FSP-c exhibited a lower oxygen coordination of bismuth under reducing conditions compared to reaction conditions (1.8 compared to 0.75). Thus, both XANES and EXAFS results support a partial reduction of bismuth in FSP-c. In summary, scheelite-structured FSP-a and FSP-b showed similar behavior. Under reaction conditions, β-Bi2Mo2O9 fraction was enhanced, but no reduction was observed, since the reoxidation was much faster than the removal of lattice oxygen.67 Thus, MoO2 was only formed under reducing conditions. The extent of reduction correlated with the catalytic activity, similar to what was described in section 3.2. While XANES indicated also a reduction of bismuth for FSP-b, EXAFS only confirmed a reduction of molybdenum under reducing conditions. Overall, at reaction temperature as well as during TPR experiments (cf. section 3.2), a more facile reduction of molybdenum was observed under propylene-rich feed for scheelite-structured phases, which is in agreement with the predictions by Bell and co-workers.36-37 Aurivillius-structured FSP-c, which contained additional phases and was, thus, promoted by a synergy effect, showed reduction of both bismuth and molybdenum as confirmed by XANES and EXAFS. The high catalytic activity, which was reported in section 3.1, could be connected


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to an easy reduction and, thus, a removal of lattice oxygen from both metallic centers. The synergy effected aurivillius-structured FSP-c showed an easier reduction of molybdenum at reaction temperature, unlike it was observed for the aurivillius-structured HS-c during TPR experiments (cf. section 3.2). Thus, the behavior of FSP-c was more similar to scheelitestructured systems. The catalytically unselective HS-c material was the only catalysts for which an easier reduction of bismuth was observed. The synergy effect, which was observed for FSP-c, directly affected the bulk behavior of γ-Bi2MoO6 and may be more than just a surface effect. In conclusion, the reduction behavior of aurivillius-structured phases, depended on the purity of the phases and presence of a synergy effect. Regarding the reducibility of bismuth and molybdenum of aurivillius-structured phases, the predictions of Bell and co-workers24 could not be fully confirmed. While FSP-c showed an earlier reduction at reaction temperature, HS-c showed a reduction of Bi3+ at lower temperatures compared to Mo6+ during TPR experiments. Simultaneously recorded mass spectrometric and operando Raman spectroscopic data supported the observations by XAS. As shown in Figure 10a, the formation of CO2 (m/z 44) and acrolein (m/z 56) was observed for FSP-b under reaction conditions. As expected, almost no acrolein was detected under oxidizing conditions. However, due to propylene oxidation with lattice oxygen, small amounts of acrolein and CO2 were formed under reducing conditions with and without water. MS data were similar for all catalysts. Furthermore, operando Raman spectroscopy revealed the formation of phases. As shown in Figure 10b for FSP-b, characteristic Raman bands of β-Bi2Mo2O9, including the main band at 887 cm-1, confirmed a phase pure system under reaction and oxidizing conditions. After switching to reducing conditions, Raman bands attributed to Mo-O vibrations diminished due to the lack of lattice oxygen, as already shown by EXAFS fitting. The structural collapse of the β-Bi2Mo2O9 phase, which was expected


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according to the LCA of Mo K edge XANES data, was confirmed by Raman spectroscopy: After treating the sample with propylene and water, two new main bands appeared at 800 and 901 cm-1 that can be assigned to α-Bi2Mo3O12 and γ-Bi2MoO6, respectively. Hence, a feed of propylene and water, seems to facilitate phase decomposition.

Magnitude of FT

a) 12

Start Reaction cond. Oxidizing cond. Reducing with water Reducing cond. Reaction cond. End

8

4

0 0

2

Coordination number of Mo-O1 path

4

6

R/Å

b) 6 FSP-a

FSP-b

FSP-c

4

2

0

Start

Oxidizing Reducing End Reaction Reducing Reaction with water

Start

Oxidizing Reducing End Reaction Reducing Reaction with water

c) 4 Coordination number of Bi-O1 path

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3

2

1

0

Figure 9: Fourier transformed EXAFS of FSP-c at the Mo K edge during the reaction cycle (a) and CNs of Mo-O1 (b) and Bi-O1 (c) shells as determined via EXAFS fitting of Mo K and Bi L3 edge data, respectively. Fitting results are listed in Table S7.


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As further shown in Figure 10b for FSP-b, Raman bands attributed to carbon depositions were observed in the range of 1200-1600 cm-1. Remarkably, even under partially oxidative reaction conditions, carbon deposited possibly due to the special properties of flame-made metal oxide nanoparticles (e.g. high density of active sites and surface species). After the above described phase decomposition and particle sintering, carbon depositions were only found under reducing conditions. Furthermore, IR thermography revealed that a temperature hot-spot of >20 K occurred at the outlet of the catalyst bed, when switching to reaction conditions due to the removal of carbon (Figure S15). For all samples, especially amorphous carbon was found with a dominant and broad disordered graphitic lattice band and a D1 to G band intensity ratio of ID1/IG > 1.3. Again, the observed carbon formation was highest for those catalysts that featured a large surface area. For aurivillius-structured FSP-c, operando Raman spectroscopy confirmed an increased crystallinity of the γ-Bi2MoO6 phase, which was observed by LCA of Mo K edge XANES. Operando Raman spectroscopy results of FSP-a, HS-a, HS-b and HS-c are summarized in Table S8. In general, phase transformations were more extended for flame-made catalysts compared to hydrothermally synthesized catalysts. Under reducing conditions, Raman bands attributed to MoO vibrations did not vanish for hydrothermally synthesized catalysts. Thus, the exchange-ability of lattice oxygen and the formation of oxidation products was not as high as observed for flamemade catalysts. This further supports the claim that flame-made catalysts were more easily reduced and more prone to the loss of lattice oxygen, possibly due to their smaller crystallite size and larger number of surface sites.


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Figure 10: MS data (a) and normalized Raman intensity map over time (b) as recorded during operando XAS/Raman measurement of FSP-b during the reaction cycle (Rct = reaction, Ox = oxidizing, Red+w = reducing with water, Red = reducing conditions; m/z 28 = CO and nitrogen, m/z 32 = oxygen, m/z 42 = propylene, m/z 44 = CO2, m/z 56 = acrolein).


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4

Conclusions

Various bismuth molybdates were synthesized via hydrothermal synthesis and flame spray pyrolysis, tested in a lab reactor and studied during in situ TPR-XAS/XRD, TPR-Raman spectroscopy and combined operando XAS/Raman spectroscopy with on-line product analysis. While phases with scheelite-structure exhibited good propylene conversion and acrolein selectivity, the catalytic performance of aurivillius-structured systems differed a lot depending on the presence of additional phases. In situ TPR studies in propylene atmosphere revealed for scheelite-structured α-Bi2Mo3O12 and β-Bi2Mo2O9 that molybdenum is reduced more easily from Mo6+ to Mo4+ than bismuth from Bi3+ to Bi0. In contrast, aurivillius-structured γ-Bi2MoO6 showed a more facile reduction of bismuth, which is most likely linked to a removal of oxygen from (Bi2O2)2+ layers. Notably, CO and CO2 formation was most pronounced for the pure γ-Bi2MoO6 phase and, hence, oxygen coordinated to bismuth may enable an unselective reaction pathway. For all systems, high acrolein yields correlated with low molybdenum reduction temperatures. Consequently, a more dynamic reduction/oxidation of Mo entities led to an increased catalytic activity. Furthermore, operando studies under steady-state-conditions revealed an increase of the β-Bi2Mo2O9 fraction under reaction conditions and showed a partial reduction of molybdenum under reducing conditions. Flame-made γ-Bi2MoO6 showed the highest catalytic performance probably due to a synergy effect of additional phases. In contrast to scheelite-structured phases, reduction and depletion of lattice oxygen from both metal centers, bismuth and molybdenum, was observed. However, aurivillius phases promoted by additional bismuth molybdates showed an easier reduction of molybdenum, unlike the pure aurivillius phase. Hence, pure γ-Bi2MoO6 with an intense CO and CO2 formation was the only system, where an easier reduction of bismuth was observed. Finally, the formation of amorphous carbon


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was observed especially under reducing conditions. The amount of carbon increased with higher specific surface area. Another deactivation pathway was the decomposition of flame-made β-Bi2Mo2O9 in propylene/water atmosphere. Since the observed phase changes of bulk catalysts under reaction conditions were rather small, more sensitive techniques, like modulation-excitation XAS or Raman spectroscopy, should be used in future. In summary, complementary in situ and operando XAS, XRD and Raman spectroscopy were required to detect and identify the transformation of amorphous and crystalline phases and, thus, provided fundamental insight on the reactivity of various bismuth molybdates.


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AUTHOR INFORMATION Corresponding Author *E-Mail: [email protected] Present address: WK: Laboratory of Industrial Chemistry, Ruhr-University Bochum, 44801 Bochum, Germany Notes The authors declare no competing financial interest.

ASSOCIATED CONTENT Supporting Information. Catalyst synthesis; calculation of conversion and selectivity; catalytic data; stability study; additional XAS, Raman and XRD spectra; fitting parameters and fitting results of Raman, XANES and F.T. EXAFS; IR thermography results


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ACKNOWLEDGMENT Combined XAS/XRD studies were performed at the Swiss-Norwegian-beamline (BM31, ESRF) with the support of Dr. Michela Brunelli. Combined XAS/Raman studies were conducted at the ROCK beamline (SOLEIL) and supported by a public grant, overseen by the French National Research Agency (ANR) as part of the "Investissements d'Avenir" program (reference: ANR-10EQPX-45). We thank Dr. Stéphanie Belin and Dr. Valérie Briois for their support during beamtime as well as Gülperi Cavusoglu, Dr. Thomas Sheppard and Marc-André Serrer (KIT). We gratefully acknowledge KIT and DFG for financing the Renishaw inVia Reflex Spectrometer System (INST 121384/73-1). We thank Angela Beilmann (ITCP, KIT) for BET analysis and Hermann Köhler (IKFT, KIT) for ICP-OES measurements.


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Reactivity of Bismuth Molybdates for Selective Oxidation of Propylene

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