Atomically Dispersed Rhodium on Self-Assembled Phosphotungstic

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Atomically Dispersed Rhodium on Self-Assembled Phosphotungstic Acid: Structural Features and Catalytic CO Oxidation Properties Bin Zhang, Hiroyuki Asakura, and Ning Yan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b00376 • Publication Date (Web): 09 Mar 2017 Downloaded from http://pubs.acs.org on March 12, 2017

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Industrial & Engineering Chemistry Research

Atomically Dispersed Rhodium on Self-Assembled Phosphotungstic Acid: Structural Features and Catalytic CO Oxidation Properties Bin Zhang†, Hiroyuki Asakura‡, §, and Ning Yan*,† †

Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117585 Singapore ‡Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan §Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, Kyoto 615-8245, Japan

KEYWORDS: Atomically dispersed catalyst, heteropoly acid, self-assembly, CO oxidation

ABSTRACT: In this study, atomically dispersed Rh catalysts supported on phosphotungstic acid (PTA) with Rh loading up to 0.9 wt% were prepared through a self-assembly method. Rh stays exclusively as single atom species coordinated to 6 oxygen atoms, plausibly located at the 4-fold hollow site on one phosphotungstic acid (PTA) molecule together with a chemically adsorbed O2 molecule. The catalyst is active in CO oxidation affording turn over frequencies between 0.2 and 1.7 s-1 from 165 to 195 oC, with an apparent activation energy of 127 kJ/mol. The catalyst is highly stable, well maintaining the Keggin structure of PTA as well as the single-atom identity of Rh after three catalytic cycles (50 to 400 oC). CO activation is mainly achieved on Rh via the formation of dicarbonyl species, while oxygen activation and transfer mainly occur through PTA. The proposed catalytic cycle consists of alternation of Rh(CO)23+ species and Rh(CO)21+ species on PTA, during which CO transforms into CO2 with oxygen activation being rate-determining.

1. INTRODUCTION Heteropoly acids (HPAs) are a class of oxide clusters with unique chemical properties, widely applied in various fields especially catalysis.1, 2 HPAs and their metal salts bearing Keggin structures are most extensively evaluated, since they are efficient in a series of organic reactions (e.g. acidic reactions, oxidation reactions) both in lab-scale and large-scale processes.3, 4 Considering that the oxygenenriched Keggin structure endows different anchoring sites for metal atoms, the combination of catalytically active metals (mainly noble metals) and HPAs has been explored in depth in which homogeneous metal complexes, nanoparticles (NPs) and nanoclusters (NCs) are loaded on HPAs.5 The well-defined Keggin structure also benefits the construction of metal single-atom catalysts (SACs)6-17 with precise anchoring location of isolated metal atoms.18, 19

Nevertheless, pure HPAs usually have limited active sites due to low surface area (< 10 m2/g),20 low melting points and difficulties for separation in solution phase reactions, thus limiting their catalytic application. Two strategies have been commonly utilized to increase the dispersion of active sites: a) anchoring HPAs onto a porous support

through electrostatic interactions prior to the introduction of metal precursors; (b) formation of insoluble porous salts by the self-assembly between HPAs and transitional metal precursors. Dramatic enhancement in catalytic performance has been demonstrated through these two approaches.21 For the first approach, various metalHPA/support systems have been developed, in which metals exist as isolated atoms, organometallic complexes, NPs or NCs.18, 22-25 Although these hybrid catalytic materials have many intriguing advantages, the system adds complexity in understanding the structure of the catalyst (e.g., whether metals are loaded on HPAs or directly on the support) and the reaction mechanism (i.e., the interplay among metals, HAPs and the support, and the consequence on catalytic properties). The second approach, self-assembly process without using an external porous support, excels in this aspect. Porous, unidirectionally oriented crystalline HPA salts can be synthesized through simple procedures, preserving the original Keggin structure.19 This approach also provides a potential way to prepare SACs, since a metal element of interest (such as Rh) can be co-fed during the self-assembly procedure leading to isolated metal species well dispersed within the crystal-

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line structures, thanks to the strong electrostatic interactions between metal cations and HPAs. Currently, K+ Cs+ and NH4+ are the most utilized inorganic cations to form the skeleton of HPA salts with a specific surface of around 60 to 200 m2/g (determined by N2 adsorption), which is comparable to many common supports.26, 27 Among transitional metals, Ag, Pd and Pt incorporated within the crystalline structures have been extensively examined in a number of catalytic reactions,28, 29 including alkane isomerization,30 alkene oxidation31 as well as the esterification reactions.32 In many cases, researchers treat the pre-catalysts under severe conditions to obtain metal NPs or NCs as the active sites.33, 34 Rh has also been examined in self-assembled HPA porous materials, but only a few studies were performed and the application was limited.32, 35, 36 For instance, carbonization of dimethyl ether in the presence of CO has been illustrated over Rh/Cs2PW12O40, but the structure of the catalysts was not thoroughly characterized.35 It is not clear whether Rh cations are transformed into NPs or NCs during catalysis, which Rh species is the active site and how the reaction occurs over the active site. Herein, in this study, the porous ammonia salt of phosphotungstic acid (NPTA, (NH4)xPW12O40) was employed as the skeleton to form a unidirectional structure with atomically dispersed Rh species coordinated with O atoms. Thorough characterizations were conducted to unveil the key structural features of the Rh catalysts, following which CO oxidation, a commonly used model reaction for Rh catalysts,37-39 was performed and the reaction kinetics was examined. In-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was adopted to probe CO adsorption behavior over Rh at varying conditions providing insights into reaction mechanism.40-42

2. EXPERIMENTAL SECTION 2.1 Materials and Chemicals Ammonium nitrate (NH4NO3, ACS reagent, ≥98%) and phosphotungstic acid hydrate (PTA, H3PW12O40·xH2O, reagent grade) were purchased from Sigma Aldrich. Rhodium nitrate solution (Rh(NO3)3, Rh 10~15% w/w) was from Alfa Aesar. Chemicals were used as received. 2.2 Preparation of (NH4)3PW12O40, 0.2 wt% Rh/(NH4)3PTA and 0.9 wt% Rh/(NH4)3PTA. (NH4)3PW12O40 was synthesized following a literature method with modifications43: In an ice bath, an aqueous solution of NH4NO3 (30 cm3, 0.075 mol/L) was added dropwise to an aqueous solution of H3PW12O40 (30 cm3, 0.025 mol/L) under vigorous stirring (1200 rpm) to form a white colloidal solution, forming well-dispersed (NH4)3PW12O40 precipitates. The rate of NH4NO3 addition was 1 mL/min. The resulted solution was aged for 30 min in the ice bath and then centrifuged (4 min, 8000 rpm) to obtain the precipitate. The product was washed with water three times, followed by centrifugation (4 min, 8000 rpm) and lyophilized for 24 h. 1.8 g of dry powder was obtained, denoted as NPTA for short.

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As for the preparation of Rh/(NH4)3PTA, stoichiometric amount of Rh(NO3)3 solution was mixed with NH4NO3 solution before adding together into the PTA solution to get Rh/NPTA with 0.2 wt% Rh (abbreviated as 0.2Rh/NPTA) and 0.9 wt% Rh (0.9Rh/NPTA). The synthetic and post-treatment procedure was identical as that for NPTA. Prior to tests, the samples were treated in air at 250oC for 30 min (40 mL/min).

2.3 Materials Characterization Electrospray ionization time-of-flight mass (ESI-TOF-MS) spectra were obtained from a Bruker MicroTOF-Q system. The samples were directly injected into the chamber at 20 μL·min-1. Typical instrument parameters: capillary voltage, 4 kV; nebulizer, 0.4 bars; dry gas, 2 L·min-1 at 120 °C; m/z range, 50 – 3000. Metal content in the catalyst was determined by iCAP 6000 series inductively coupled plasma optical emission spectrometry (ICP-OES). The catalysts were digested in aqua regia (HCl/HNO3 = 3:1) at 353 K for 4 h and then diluted with deionized water to a certain volume before analysis. To measure the dispersion of the Rh catalyst, CO pulse titration was conducted on a ChemBET Pulsar TPR/TPD (Quantachrome). Typically, 100 mg of each freshly prepared catalyst was heated up to 100 °C in Helium (100 mL/min). Subsequently, successive injections of CO gas were introduced into He stream via a calibrated injection valve (159 μL CO pulse−1) at 200 °C. The titration would end when the peak areas of three injections in a row keeps constant. In principle, the titration should follow this equation: Rh (available sites) + 2 CO = Rh(CO)2 Rh dispersion (dRh) was calculated using the formula: dRh = 1/2 n (CO)/ntotal(Rh) Rh K-edge x-ray absorption spectra of the Rh/NPTA, RERh/NPTA and reference samples (Rh foil, and Rh2O3) were recorded at the BL01B1 beamline at the SPring-8 (Japan Synchrotron Radiation Research Institute, Hyogo, Japan) in the transmission mode at ambient temperature. A Si(3 1 1) double crystal monochromator was used to obtain a monochromatic X-ray beam. The monochromator was calibrated at the inflection point of the X-ray absorption near edge structure (XANES) spectrum of the Rh foil. Higher harmonics were removed by changing glancing angles of collimation and focusing mirrors. Data reduction was carried out with Athena and Artemis included in the Ifeffit and Demeter package. Thermogravimetric analysis (TGA) was conducted over an EXSTAR TG/DTA 6300 (Seiko Instruments). 12 mg of sample was heated in air (flow rate: 50 mL/min) with a standard heating program: temperature increased to 70 oC at 10 oC/min and was held for 10 min; then the temperature was further increased to 600 oC at the same heating rate. Nitrogen adsorption−desorption isotherms were obtained on Quantachrome NOVA-3000 system at 77 K. Prior to the measurements, samples were degassed at 100 °C. The Brunauer−Emmett−Teller (BET) speciﬁc surface areas were calculated using the adsorption data in the relative pressure (P/P0) range of 0.05−0.35. The total pore volumes were estimated from the amount absorbed at a rela-


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tive pressure (P/P0) of 0.98. Surface area of the materials was measured by the Brunauer−Emmet−Teller (BET) method. Transmission electron microscopy (TEM) was performed on a JEM 2100F (JEOL, Japan) microscope operated at 200 kV. X-ray diffraction (XRD) analysis was carried out using a Bruker D8 Advance X-Ray Diffractometer, at a scan rate of 2 omin−1. It was operated at 40 kV applying a potential current of 30 mA. X-ray photoelectron spectra (XPS) were recorded on a VG Escalab MKII spectrometer, using a mono Al Kα X-ray source (hν = 1486.71 eV, 5 mA, 15 kV), and the calibration was done by setting the C1s peak at 284.5 eV. Attenuated total reflectance infrared (ATR-IR) spectroscopy analysis was carried out on a Thermo Scitific Nicolet iS50 FT-IR spectrometer integrated with a diamond ATR accessory. The IR spectra (attenuated total reflectance mode, ATR) were collected in the spectral range 4000–525 cm-1 with a resolution of 4 cm−1 and scan number of 32. Diffuse reflectance infrared fourier transform spectroscopy (DRIFT) study of CO adsorption was also carried out over the iS50 FT-IR spectrometer with a mercury-cadmium-telluride (MCT) detector. A powder sample was put into a reaction cell (Harricks HV-DR2) and pre-treated with N2. Background spectra were collected afterwards at different temperatures, and the sample spectra were collected at corresponding temperatures after different treatment. All spectra were taken after 32 scans with a resolution of 4 cm−1.

3. RESULTS AND DISCUSSION 3+

3.1 The interaction between Rh species and PTA in solution phase In this work, ammonia nitrate (NH4NO3) and phosphotungstic acid (H3PW12O40, PTA) are utilized as the skeleton to form a unidirectional structure. Rh(NO3)3 is employed as the precursor because Rh exists in the solution as H2O coordinated Rh3+ species, which we expect to have strong interaction with PTA3- in a one to one manner, forming a Rh-PTA neutral species. ESI-MS technique was used to prove this assumption. In Figure 1, strong mass signals were detected for both pure PTA solution and Rh(NO3)3 solution at the same concentration. After mixing the two, a darker solution was obtained indicating the formation of new species. Meanwhile, MS signals in ESIMS spectrum can no longer be observed, suggesting the new species, indeed, is a neutral Rh1PTA1 complex. Several peaks with very weak intensities were observed. Enlarged MS spectrum and assignments were provided in

2.4 Catalytic CO oxidation reaction CO oxidation was performed in a flow steel tube reactor, and the gases (CO, O2, and CO2) were analyzed on an Agilent 7890B GC utilizing a TCD detector. The samples of as-prepared catalysts were tested in the reactor using 100 mg 0.9 wt% Rh sample or 0.2 wt% Rh sample for each experiment. The samples were packed into a reactor which was mounted inside a tube furnace. The powder samples were packed between quartz wool with a thermocouple placed touching the position of the sample outside the reactor. The composition of the introduced mixed gas is 2.5% CO, 2.5% O2 and 95% Ar, and the total flow rate is 80 mL/min. The reaction temperature was increased from room temperature to different desired temperatures with a ramp rate of 10 °C/min, and held at each desired temperature for 30 min before the sampling was performed. For the estimation of reaction orders with respect to the reagents, 250 oC was selected and 23 mg of 0.9wt% Rh catalyst was utilized. To measure the relationship between the reaction rate with O2 partial pressure, CO partial pressure was maintained as 0.125 bar and that of O2 was increased gradually from 0.005 bar to 0.025 bar. In the case of CO, the partial pressure of O2 was kept as 0.005 bar while that of CO was increased from 0.015 bar to 0.045 bar. As to the calculation of activation energy, 50 mg of catalyst was utilized with the same gas mixtures and flow rate as the recycling test experiment, and the reaction temperature ranges from 165 to 195 oC.

Figure 1. ESI mass spectra (negative mode) of PTA solution (0.5 mmol/L), Rh(NO3)3 solution (0.5 mmol/L) and a mixture of the two solutions (1 mL + 1 mL).

3.2 Structural features of the self-assembled, porous Rh/NPTA The samples were prepared through a simple selfassembly process by adding NH4NO3 (or NH4NO3 + Rh(NO3)3) solution to PTA solution. For illustration purpose, the sample x% Rh-(NH4)yPW12O40 was denoted as xRh/NPTA. After purification and drying, the Rh content was confirmed as 0.9 wt% for 0.9Rh/NPTA and 0.2 wt% for 0.2Rh/NPTA by ICP-OES analysis. The molar ratio between Rh and PTA is 1:4 and 1:18 for 0.9Rh/NPTA and 0.2Rh/NPTA, respectively. The morphology and elemental distribution of the catalysts were investigated with SEM, TEM and EDS mapping techniques. As shown in Figure 2a, the well-assembled polyhedral nanocrystals with a size of two to three hundred nanometers were formed in 0.9Rh/NPTA, with Rh and W distributed evenly throughout the material (Figure



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2b-d). Similar result was obtained for 0.2Rh/NPTA (Figure S2). N2 adsorption-desorption isotherms of NPTA, 0.9Rh/NPTA and 0.2Rh/NPTA were recorded (Figure S3), demonstrating that these materials are microporous. The physical properties of the three catalysts based on BET analysis are listed in Table 1. The addition of 0.9 wt% Rh slightly decreased the BET surface area by 10%, but increased the average pore size from 2.4 to 3.4 nm, which is beneficial for the internal diffusion of reagents and products during catalytic reactions.

Figure 2. SEM (a) and EDX mapping images of 0.9Rh/NPTA (b-d).

Table 1. BET results of NPTA, 0.2Rh/NPTA and 0.9Rh/NPTA. Sample

BET surface 2 area (m /g)

Pore volume 3 (cm /g)

Average pore size (nm)

NPTA

102

0.061

2.4

0.2Rh/NPTA

98

0.071

2.7

0.9Rh/NPTA

92

0.077

3.4

In ATR-IR spectra (Figure 3a), the characteristic peaks for the PTA Keggin anion were observed at 1080, 990, 880 cm−1 and 770 cm-1, corresponding to νas(P–Oa), νas(W–Ob), νas(W–Oc–W) and νas (W–Od–W), respectively.44 This indicates that the structure of the Keggin anion is well preserved in all samples. The peaks corresponding to the vibrations of NH4 located at 3220 and 1406 cm-1 were also observed for NH4-containing samples. XRD patterns of NPTA and Rh/NPTA are shown in Figure 3b, which are fully consistent with the reported cubic structure with a lattice constant resembling H3PW12O40·6H2O, despite slightly increased line widths. Therefore, the micropores in the material are not formed within individual crystals but originate from the irregular and non-condensed packing of nanocrystals. Commercial PTA hydrate provided a dramatically different XRD pattern, suggesting that PTA

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molecules are arranged in various orientations in the sample.

Figure 3. a) ATR-IR spectra and b) XRD patterns of PTA hydrate, NPTA and Rh/NPTA.

Fourier transformed extended X-ray absorption fine structure (EXAFS) spectra for the as-prepared samples were provided in Figure 4a. In both Rh/NPTA samples, no appreciable Rh-Rh contribution at 2.4 Å was observed.45, 46 Instead, a prominent peak at ~1.5 Å was detected, and the intensities are quite similar to that of Rh2O3. Since Rh could only coordinate with O and Rh atoms in the catalysts, a double shell fitting considering both Rh-O and Rh-Rh contributions was conducted. As listed in Table S1, the Rh-Rh coordinator number is negligible for both 0.2Rh/NPTA and 0.9Rh/NPTA, proving that predominant Rh species exists as isolated atoms instead of aggregated Rh NCs or NPs. A coordination number of around 6 (5.6 for 0.9Rh/NPTA and 6.7 for 0.2 Rh/NPTA) was determined for Rh-O bond. Heteropolyacid bearing Keggin structure furnishes with several different coordinating sites for metals. In a previously study, the 4-fold hollow site was identified as the strongest anchoring site for single Pt atoms.18 Based on the ESI-MS result, Rh3+ prefers to form a 1:1 complex with PTA. As such, it is plausible that Rh atom is anchored on one PTA molecule in the 4-fold coordinating with 4 O atoms. O2 has good affinity to Rh species at room temperature (verified by in-situ DRIFT analysis, vide infra), which accounts for the remaining two coordination numbers with oxygen (Figure 4b). Analysis on the valent state of Rh and W provides further evidence of the “one to one” binding mode between Rh and PTA, as well as insights into their electronic interac-


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tions. The binding energy of Rh 3d electron is 309.8 eV for 0.9Rh/NPTA which is a typical value for positively charged Rh (Figure S4), while the signal was not welldistinguished for 0.2Rh/NPTA due to the low Rh loading. The weak signal of Rh suggest that Rh was well dispersed within the skeleton of the catalyst instead of accumulating on the exterior surface of the support. In the X-ray absorption near-edge structure (XANES) spectra, the white line intensities of both catalysts at 23240 eV confirm that Rh exists as positively charged species with an estimated charge close to +3 (Figure 4c). Nevertheless, the white line intensity is slightly less than that in Rh2O3 suggesting Rh becomes less positively charged upon interacting with PTA (i.e., electrons transfer from PTA to Rh).

Figure 4. a) k2-weighted Rh K-edge EXAFS spectra of Rh foil, -1 -1 Rh2O3 Rh/NPTA. The k-range from 3 Å to 12 Å was used in all Fourier transforms; b) a proposed configuration of Rh on PTA in Rh/NPTA; c) normalized XANES spectra at the Rh K edge of Rh/NPTA, Rh2O3 and Rh foil. The intensities of white-line at 23240 eV are as follows: Rh foil (1.00), Rh2O3 (1.22), 0.9Rh/NPTA (1.18), 0.2Rh/NPTA (1.18). The numbers in the parenthesis refer to the normalized absorption height for different samples; d) W 4f XPS spectra of PTA, NPTA and Rh/NPTA (solid lines refer to the experimental spectra and the dash lines refer to the deconvoluted fitting spectra). The area ratio of two sets of fitting curves for 0.2Rh/NPTA is 12 (lower binding energy) : 1 (higher binding energy), and that for 0.9Rh/NPTA is 2.5 : 1.

Electronic interactions between Rh3+ and PTA were consolidated by W 4f XPS spectra of 0.9Rh/NPTA and 0.2Rh/NPTA (Figure 4d). Only one set of peaks corresponding to W(VI) (binding energy: 35.9 eV) was observed in the spectra of PTA or NPTA, while a new set of peaks located at even higher binding energy (37.0 eV) was observed for 0.9Rh/NPTA and 0.2Rh/NPTA. This set of new peaks was only observed after Rh incorporation, and the intensity is proportional to Rh content, thus it reflects the electronic interactions between Rh and PTA. The ratio between the area of the new peaks and the total area is 1/3.5 for 0.9Rh/NPTA and 1:13 for 0.2Rh/NPTA, respective-

ly, which nicely match with ICP-OES result where the molar ratio of Rh to PTA is 1:4 in 0.9Rh/NPTA and 1:18 in 0.2Rh/NPTA. As such, XPS analysis provided strong evidence that each Rh species interact with one PTA, corroborating with ESI-MS and EXAFS data.

3.3 Catalytic performance of Rh/NPTA in CO oxidation Heteropoly acids including PTA have quite low melting point (

Atomically Dispersed Rhodium on Self-Assembled Phosphotungstic

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