Alkali promoted tungsten carbide as a selective catalyst for the reverse water gas shift reaction

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Alkali promoted tungsten carbide as a selective catalyst for the reverse water gas shift reaction ⁎

James R. Morsea, Mitchell Juneaub, Jeffery W. Baldwinc, Marc D. Porosoffb, , ⁎ Heather D. Willauera, a

6300. Materials Science and Technology Division, Naval Research Laboratory, 4555, Overlook Avenue, Washington DC, 20375, United States Department of Chemical Engineering, Rochester University, 4305 Wegmans Hall, Rochester, NY, 14627, United States c 7100. Acoustics Division, Naval Research Laboratory, 4555 Overlook Avenue, Washington DC, 20375, United States b

A R T I C LE I N FO

A B S T R A C T

Keywords: Catalyst Supported Catalyst Tungsten Carbide Alkali Promoter Reverse Water Gas Shift Catalyst Promoter

The conversion of CO2 to CO via the reverse water gas shift (RWGS) reaction is often envisioned as the first step in the large scale transformation of CO2 into value added hydrocarbons. Accordingly, low-cost and highly selective RWGS catalysts are desirable materials for sustainable CO2 utilization. Here, we describe synthesis, characterization and performance evaluation of alkali metal promoted tungsten carbide (WC) RWGS catalysts, supported on porous γ-Al2O3. Scanning electron microscopy reveals significant differences in the morphology of the catalysts, while X-ray photoelectron spectroscopy indicates a greater extent of carburization of the alkalipromoted catalysts, suggesting that alkali metals act as structural and electronic promoters, respectively. Performance data for the RWGS reaction at 300 and 350 °C shows all catalysts are active, with the alkali promoted catalysts demonstrating improved CO selectivity. Of the catalysts tested, the highest CO yield is achieved using K-promoted WC, demonstrating K-WC/γ-Al2O3 is a low-cost, active and selective RWGS catalyst.

1. Introduction The depletion of fossil fuels and corresponding increases in atmospheric and oceanic CO2 concentrations have motivated research in the field of alternative energy and inspired the development of technologies capable of sequestering and transforming CO2 into fungible fuels. [1–5] With CO2 concentrations on the order of 100 mg L−1 (140 times greater than atmospheric levels), [6] the World’s oceans offer a natural source of CO2 which could be leveraged as a carbon feedstock. A recently patented process by the U.S. Navy realizes this concept, by demonstrating CO2 and H2 can be simultaneously extracted from seawater, [7–13] and subsequently used as reactants in the thermochemical hydrogenation of CO2 into more valuable chemicals and fuels [14–20]. The thermochemical hydrogenation of CO2 to liquid hydrocarbon fuels (C6+) can be achieved using a modified Fischer-Tropsch synthesis (FTS) process, by replacing CO with CO2 (CO2-FT). [21–25] In a commonly proposed pathway, CO2 is first converted to CO through the endothermic reverse water gas shift (RWGS) reaction, followed by hydrogenation of CO via exothermic FTS. [26–30] To increase the yield toward C6+ hydrocarbon products, it is ideal to operate RWGS at high conversion, while simultaneously minimizing CH4 formation via the undesirable Sabatier reaction. [26,31,32] Although performing RWGS ⁎

at higher temperatures will undoubtedly improve CO yields, many RWGS catalysts exhibit decreased catalyst lifetimes at elevated temperatures [33–35]. Alternatively, at FTS temperatures (200–350 °C), the thermodynamic limitations of RWGS can be offset by operating the two-step process autothermally, by utilizing heat from the exothermic FTS reaction. Furthermore, the CO2 conversion during RWGS can be pushed beyond equilibrium (23% at 300 °C for a 3:1 H2:CO2 ratio), by removing the produced water and subsequently recycling the reactor effluent. Accordingly, coupling RWGS with FTS requires RWGS catalysts which can function under FTS conditions with high activity and selectivity toward the desired product, CO. Platinum catalysts dispersed on metal oxide supports have shown reasonable activity and selectivity for the RWGS reaction at relatively low temperatures (300 °C), [36–38] but the cost of these catalysts make them non-ideal for practical applications. Transition metal carbides (TMCs) have received significant attention as alternatives to precious metal catalysts for their comparable catalytic activities for various reactions [39–41], including RWGS [42]. The addition of promoting species to TMCs can further improve performance by attenuating the structural and electronic properties of the catalyst. [43] In particular, alkali promoters reduce CH4 selectivity in TMCs, [44,45] and recent

Corresponding authors. E-mail addresses: marc.porosoff@rochester.edu (M.D. Porosoff), [email protected] (H.D. Willauer).

https://doi.org/10.1016/j.jcou.2019.08.024 Received 29 July 2019; Received in revised form 20 August 2019; Accepted 29 August 2019 2212-9820/ © 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: James R. Morse, et al., Journal of CO₂ Utilization, https://doi.org/10.1016/j.jcou.2019.08.024

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2.3. Removal of polymeric carbon

studies by our group demonstrate the selectivity toward CO increases from ˜75% to 95% for the low temperature RWGS reaction when potassium is incorporated into a Mo2C-based catalyst. [46] These findings indicate that promoted TMCs are selective and low-cost alternative catalysts for the low temperature RWGS reaction. Of the TMC family, WC is frequently compared to Mo2C in terms of catalytic properties, with similar activities observed for deoxygenation, [47] hydrogenation [48–50], and reforming reactions [51–53]. A recent comparison of the RWGS activity of various bulk TMC catalysts demonstrates oxygen binding energy is a descriptor of activity, with WC and Mo2C exhibiting the highest CO2 conversions. [42] These trends suggest WC can be optimized to function as an effective low-temperature RWGS catalyst, similar to Mo2C. [46] However, WC has not yet been further investigated as a catalyst for the RWGS reaction. In this paper, unpromoted and alkali metal promoted WC supported on high surface area γ-Al2O3(WC/γ-Al2O3) are studied as low-cost catalysts for the low-temperature RWGS reaction. Characterization with X-ray diffraction (XRD), pulsed chemisorption, N2 physisorption, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) demonstrate that alkali metals act as structural and electronic promoters, attenuating surface chemistry and catalyst dispersion, while increasing the extent of tungsten carburization. Reactor studies indicate that the alkali metals increase CO selectivity, with the K promoter exhibiting the highest increase in overall CO yield. This study extends the library of low-cost, low-temperature RWGS catalysts, and further demonstrates that alkali promoters are an effective means to tune RWGS selectivity.

To remove polymeric carbon deposited during the carburization step, the catalysts were reduced under an H2 atmosphere using a slightly modified version of a previously published procedure. [54] Briefly, the carburized catalysts were heated to 750 °C under H2 at a flow rate of 100 mL min−1for 3.5 h. The reduction of polymeric carbon was characterized by the evolution of CH4 using gas chromatography. Following this, H2 flow was cut off, a flow of He gas (UHP grade) was introduced at 100 mL min−1, and the catalysts were heated to 800 °C for 0.5 h to promote equilibration of the bulk and surface compositions. Once cooled to room temperature, the catalysts were again passivated with a 1% O2/N2 blend overnight. These catalysts are subsequently referred to as “H2treated”. 2.4. Temperature programmed carburization (TPC) TPC was performed using a Micromeritics AutoChem II to qualitatively compare the consumption of H2 and CH4 under carburization conditions. Under a 20% CH4/H2 blend flowing at 50 mL min−1, 100 mg of the WO3/γ-Al2O3 precursor was heated to 950 °C at a ramp rate of 10 °C min−1. The effluent gases were measured as a function of temperature via the in-line TCD. 2.5. X-Ray diffraction (XRD) Measurements were performed on a Rigaku Smartlab X-ray diffractometer using Cu Kα monochromatized radiation operated at 44 kV and 40 mA at room temperature over the range of 20-90° 2θ. X-ray diffraction patterns for the catalysts were referenced to reported patterns from multiple databases and were found to correspond to WC (PDF: 01-077-4389) and WC1-x (PDF: 00-020-1316) crystal structures. Average crystallite size was estimated by performing Scherrer analysis using the (101) reflection of WC at 48.2° 2θ.

2. Experimental 2.1. Materials Ammonium metatungstate hydrate ((NH4)6H2W12O40 · xH2O) 99.99% trace metals basis, sodium carbonate (Na2CO3) > 99%, and potassium carbonate (K2CO3) > 99% were purchased from Sigma Aldrich. γ-Al2O3 powder (> 97%) was purchased from STREM Chemicals. All gases used for catalyst synthesis and testing were supplied from Earlbeck Gases & Technologies, while the gases used for pulse chemisorption were research grade purity and purchased from Airgas.

2.6. Pulse chemisorption All pulse chemisorption experiments were performed at 30 °C using a Micromeritics AutoChem II. Prior to each measurement, the catalyst was reduced in H2 for 2 h at 350 °C. The analysis gas was then pulsed over the catalyst ten times, or until the amount of analyte adsorbed by the catalyst was equal to zero. Because pure CO2 was used for the pulse CO2 adsorption experiments, CO2 was only pulsed over the catalyst five times. Pulse chemisorption experiments were performed for CO, CO2 and H2 to determine the catalytically active surface area with respect to each adsorbate. The concentration of the active gases were 5% CO in He, 100% CO2 and 10% H2 in Ar, and were certified mixtures and/or research grade gases obtained from Airgas.

2.2. Catalyst preparation The WC/γ-Al2O3 catalysts were synthesized by the evaporation deposition method. Briefly, 20.2 g ammonium metatungstate hydrate was dissolved in 300 mL deionized water using a magnetic stir bar. Five minutes later, 30.0 g γ-Al2O3 powder was mixed into the solution. The excess water was evaporated over the course of 72 h under gentle stirring at 60 °C. For the Na and K promoted catalysts, identical procedures were followed, however, 1.14 g of Na2CO3 or 1.48 g of K2CO3 were added with the tungsten precursor. These precursor amounts correspond to 1:4:15 X/W/γ-Al2O3 molar ratios (X = K, or Na), which are commensurate to the ratios described for our previously reported KMo2C/γ-Al2O3 catalyst.[46] Once dry, the impregnated supports were ground with a mortar and pestle, and then calcined at 600 °C for 12 h. The catalysts were pelletized under a force of 1 ton for 10 min, then ground and sieved to a particle size of 200–350 μm prior to carburization to improve mass transfer. 5 g of each catalyst was then individually carburized under a 20% CH4/H2 blend at 300 mL min−1for 20 h at 825 °C. The catalysts were then cooled to room temperature under the carburizing atmosphere and finally passivated with a 1% O2/N2 blend at 10 mL min−1 overnight. After passivation, the catalysts were exposed to ambient atmosphere and a portion of each carburized catalyst was saved for analysis and future testing. These catalysts are subsequently referred to as “as synthesized”.

2.7. BET surface area analysis Nitrogen adsorption analysis was performed at −196 °C, using a Beckman-Coulter S.A. 3100 Surface Area Analyzer. Prior to the measurement, the catalyst was degassed at 300 °C for 60 min under 4 μm Hg vacuum. To account for any temperature induced changes to the catalyst support, the bare γ-Al2O3 was calcined, pelletized and heated under the same carburizing atmosphere as the WC catalysts before BET characterization. 2.8. Scanning Electron Microscopy (SEM) SEM characterization was performed using a Carl Zeiss Supra 55 Schottky thermal field-emitting microscope with a 30 μm aperture 10 kV accelerating voltage. For EDS measurements, an Oxford Instruments X-Max was inserted into the SEM chamber using an accelerating voltage of 5 kV and working distance of 8.5 mm. 2

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During the electron microscopy measurements, many areas of the supported catalyst were surveyed, and no significantly different structures or elemental distributions were observed other than what is presented in this manuscript. 2.9. X-ray photoelectron spectroscopy (XPS) Measurements were performed over the “as synthesized” and “H2treated” catalysts using a commercial XPS system (Thermo Scientific Kα) equipped with a monochromatic AlKα source and an 1808 double focusing hemispherical analyzer with 128-channel detector. The nominal XPS spot size was 400 mm2. Settings for data collection were: Pass Energy = 20 eV, dwell time = 100 ms, energy step size = 0.15 eV over 40 averaged scans. Charge compensation was necessary. Calibration of the instrument was confirmed using the C1s peak of a bare γ-Al2O3 powder at 284.8 eV. W4f signals were quantitatively fitted using the Thermo Avantage software package. The following values were used to deconvolute the XPS data: WO3 – 35.6 eV, WO2– 32.8 eV, WC – 31.9 eV and W2C – 33.6 eV. [55–57]

Fig. 1. X-ray diffraction patterns corresponding to the bare γ-Al2O3 support, along with the “as synthesized”unpromoted (blue), Na-promoted (red), and Kpromoted (green) WC/γ-Al2O3. Reference patterns corresponding to WC and WC1-x crystal structures are also included.

corresponding to the cubic WC1-x phase are also present for each of the characterized catalysts, with the unpromoted WC/γ-Al2O3 exhibiting the strongest reflections. This carbon deficient phase has been observed previously in the temperature programmed carburization of tungsten species, and is believed to be a meta-stable intermediate resulting from incomplete carburization. [58,59] The observation that this phase constitutes a greater fraction of the unpromoted catalyst is in line with the TPC experiments, suggesting alkali promoters facilitate tungsten carburization. Scherrer analysis to estimate the crystallite size of the catalysts is provided in Table 1, along with corresponding surface areas of the catalysts as measured by N2 physisorption. The estimated crystallite sizes of the catalysts are also nearly identical, about 11–12 nm, which is comparable to WC crystallite sizes reported by previous authors using similar synthetic routes. [54,59] To directly visualize the distribution of the tungsten species across the γ-Al2O3 support, the three “H2treated” catalysts were imaged with scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDS). For each sample, multiple micrographs were collected at various positions across the catalysts. Fig. 2 displays SEM-EDS images that can be considered representative for the three catalysts (additional SEM-EDS images are provided in Figures S2-4 of the Supplemental Information). The unpromoted WC/γ-Al2O3 consists of localized tungsten distributions on the order of 1 μm in diameter, and clear segregation between tungstenrich regions and the γ-Al2O3 support can be observed. On the other hand, the Na and K-promoted catalysts are composed of much smaller, better dispersed tungsten-rich regions, which appear to be localized to domains on the order ˜50-200 nm. While discrete nanoscale WC particles most likely exist below the spatial resolution of SEM, these data demonstrate clear structural differences amongst the catalyst samples,

2.10. Reactor studies For a typical experiment, 1 g of catalyst pellets were loaded into a 0.25 in stainless-steel reactor. The catalyst bed was reduced under H2 at 350 °C and 0.5 MPa at a flow rate of 50 mL min−1 for a total of 2.5 h. The catalyst bed was then isolated as the rest of the reactor system was pressurized to 2.0 MPa with a blend of N2, CO2 and H2 gases at ratios commensurate with the subsequent experimental flowrates. Once pressurized, the reactant gases were introduced to the catalyst bed at a 3:1 H2/CO2 ratio, with flowrates corresponding to the reported weight hourly space velocities (WHSV). N2 was used as an internal standard to quantify CO2 conversion and CO yield, and represented 16% of the reagent gas blend. Reactant flowrates were controlled using programmable mass flow controllers (Brooks SLA5850). Reactions were run for 16 h and monitored by gas chromatography to ensure a steady state had been achieved. Unless otherwise noted, all reported catalytic data refers to the experiments after 16 h on stream. To prevent deterioration of the GC columns by water, all effluent reaction gas was passed through a cold trap (5 °C) before characterization by gas chromatography (Agilent 7890A). 3. Results and discussion 3.1. Catalyst synthesis and characterization The optimal experimental conditions for synthesizing the WC/γAl2O3 catalyst were determined by temperature programmed carburization (TPC) experiments of the unpromoted, Na-promoted and Kpromoted catalyst precursors. Further details and experimental results are provided in the Supplemental Information of this manuscript. From these experiments, it was found that temperatures of 825 °C were sufficient to carburize all three precursor samples, and carburization occurred more readily for the alkali promoted samples relative to the unpromoted tungsten precursor. In an attempt to remove any polymeric carbon that may have deposited on the carbide surface during synthesis, a portion of the “as-synthesized” WC catalysts were further treated under an H2 atmosphere at 750 °C for 3.5 h. The resulting catalysts are subsequently referred to as “H2treated”. XRD patterns for the three “as synthesized” WC catalysts, along with a reference pattern for the γ-Al2O3 support are shown in Fig. 1. All three patterns are nearly identical, with clear reflections corresponding to the hexagonal WC phase present in each catalyst. The “as synthesized” catalysts exhibit identical X-ray diffraction patterns to the corresponding “H2treated” catalysts (not shown), indicating the H2 treatments did not alter the bulk crystal structure. Much weaker reflections

Table 1 Calculated crystallite size and BET surface area for the WC/γ-Al2O3 catalysts.

γ-Al2O3 “As Synthesized” γ-Al2O3– Unpromoted WC “H2Treated” γ-Al2O3– Unpromoted WC “As Synthesized” γ-Al2O3 – Na-WC “H2Treated” γ-Al2O3– Na-WC “As Synthesized” γ-Al2O3– K- WC “H2Treated” γ-Al2O3 – K- WC a

3

WC Crystallite Size (nm)a

BET Surface Area (m2 g−1)

N.A. 12

160 122

12

136

11 11 11 11

93 97 83 85

Crystal size estimated using the Scherrer formula.

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Fig. 2. SEM images and corresponding EDS maps for the “H2treated” a) unpromoted, b) Na-promoted, and c) K-promoted WC/γ-Al2O3. Scale bar in the inset of image c) corresponds to 500 nm.

the unpromoted WC/γ-Al2O3, which likely promotes improved CO selectivity for the RWGS reaction. The promotional effects of the alkali species on WC are further characterized by the surface sensitive XPS measurements in Fig. 3. Fig. 3a shows the XPS signals corresponding to the W4f region for the three “as synthesized” WC catalysts, while Fig. 3b shows the same data set for the “H2treated” catalysts. Prior to H2 treatment, all catalysts display a single pair of peaks, corresponding to the W4f7/2 and W4f5/2 signatures of WC, respectively. [64,65] After H2 treatment, the same carbide signatures are present, along with newly developed shoulders at slightly higher binding energies, representative of tungsten oxide species. [65] The fact that these oxide signals are not noticeably present in the “as synthesized” catalysts indicates polymeric surface carbon deposited during the synthesis of the catalysts is significant enough to envelop the catalyst surface, preventing oxidation. These results confirm that the H2 treatment removes the polymeric carbon from the catalyst, allowing the catalyst surface to be exposed and subsequently oxidized during passivation. Additionally, to better understand the effect of alkali metal promoters on catalytic performance, the quantitative composition of each catalyst from the deconvoluted XPS spectra can be found in Fig. 4 and Table 3, respectively. The following values were used to fit the XPS data: WO3 – 35.6 eV, WO2– 32.8 eV, WC – 31.9 eV and W2C – 33.6 eV. [55–57] Clearly, as seen in the table, the alkali promoted catalysts are reduced to a greater extent than the unpromoted WC/γ‐Al2O3, which is also indicated in Fig. 4 by a shift toward lower binding energies. Furthermore, the greater extent of carburization of the alkali-promoted catalysts indicate that the addition of K and Na to WC results in more facile carburization of the WOx precursors, which is consistent with the TPC experiments in Figure S1 of the Supplemental Information. These

indicating Na and K serve as structural promoters in the synthesis of WC/γ-Al2O3. Further evidence of the structural effect of alkali promoters are shown by the BET data in Table 1. As shown in the table, the surface area of the porous γ-Al2O3 support is 160 m2 g−1, which is greater than any of the WC/γ-Al2O3 catalysts. Of the three catalysts, the unpromoted WC/γ-Al2O3 has the highest surface area. Both the Na and K-promoted WC exhibit significantly lower surface areas, which is due to either (1) blockage of the γ-Al2O3 micropores due to the better dispersion of the alkali promoted-tungsten species across the high surface area support; or (2) deposition of a relatively high molecular weight, low surface area material (alkali-promoted WC) onto a low molecular weight, high surface area support (γ-Al2O3).[60,61] After the catalysts are treated with H2, an increase in surface area over all three samples is observed, likely from the removal of polymeric surface carbon. In addition to structural effects, previous authors have directly observed electronic promotion via alkali metals, which strengthen CO and CO2 adsorption, while weakening H2 adsorption. Addition of alkali metals is therefore hypothesized to result in a lower H2/COx ratio on the catalyst surface, thus inhibiting hydrogenation activity, and reducing the generation of CH4 [4,60,62,63]. To compare the adsorption of the relevant chemical species for the RWGS reaction, CO, CO2, and H2 uptake experiments on the three “H2treated” WC-based catalysts are shown in Table 2. CO adsorption is about an order of magnitude greater for Na-WC/γ-Al2O3 relative to the K-promoted and unpromoted WC/γAl2O3, while the CO2 uptake experiments demonstrate both Na and Kpromoted samples adsorb significantly more CO2 than the unpromoted catalyst. The H2 uptake data, however, are of comparable magnitude for all three catalysts. Taken together, these data suggest a shift to lower H2/CO2 ratios on the surface of the alkali promoted catalysts, relative to Table 2 CO, CO2 and H2 uptake for the WC/γ-Al2O3 catalysts.

Unpromoted-WC “H2Treated” K-WC "H2 Treated" Na-WC "H2 Treated"

CO uptake (μmol/g)

CO2 Uptake (μmol/g)

H2Uptake

1.1 2.1 16.3

3.8 21.8 79.9

213.4 324.6 202.7

4

(μmol/g)

H2/CO2 ratio 56.2 14.9 2.5

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Fig. 3. W4f spectra for unpromoted, Na-promoted, and K-promoted WC/γ-Al2O3 a) “as synthesized” and b)“H2treated”.

results correlate well with previous studies, which report potassium reduces the oxidation state of Fe and Mo-based catalysts, characteristic of electronic promotional effects, that can in turn, alter the catalytic activity. [66–68]

Table 3 Composition of deconvoluted XPS spectra for WC, K-WC and Na-WC supported on γ‐Al2O3 after treatment in H2 at 750 °C. Catalyst

Composition (Atomic %) WO3

WO2

WC

W2 C

29.6 28.5 29.4

30.2 18.6 16.3

38.4 49.8 47.0

1.8 3.1 7.3

3.2. RWGS performance WC K-WC Na-WC

The activity and selectivity for the three “as synthesized” and “H2treated” WC/γ-Al2O3 catalysts were tested for the RWGS at 300 and 350 °C, respectively, with a weight hourly space velocity (WHSV) of − 1.01 mL g−1 s 1 and an H2:CO2 ratio of 3:1 at a pressure of 2.0 MPa. The results are tabulated in Table 4, along with our previously reported data for the Mo2C/γ-Al2O3 catalyst. From these data, it is clear WC is active for the RWGS reaction, however, appreciable activity is only observed for the “H2treated” catalysts. For comparison, the activities reported for other notable low temperature RWGS catalysts are tabulated in Table S1 of the Supplemental Information of this manuscript. At 300 °C, the CO2 conversion for all of the WC catalysts is lower compared to the previously reported Mo2C catalyst under identical conditions. [46] However, when the reaction temperature is increased to 350 °C, the CO2 conversion over the WC-based catalysts increases

substantially. Previous studies comparing Mo2C and WC catalysts have also noted greater activity for Mo2C under equivalent conditions. [47,48,53] This is typically attributed to the increased surface area of Mo2C, as the higher carburization temperatures required to synthesize the WC phase may cause sintering of the WC domains, thereby lowering the catalytic activity. [47,53] Although such sintering may be partially responsible for the lower CO2 conversion observed in this work, it is also possible Mo2C is intrinsically more active than WC for the RWGS reaction. To characterize the intrinsic activity of the WC catalysts, apparent

Fig. 4. Deconvoluted XPS spectra for the W4f region of the “H2treated” WC/γ-Al2O3. 5

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Table 4 Comparison of catalyst activity, selectivity and experimentally determined activation energies. Conversion and selectivity values reported for WC/γ-Al2O3 were − collected after 16 h on stream, using a 3:1 H2/CO2 ratio at a WHSV of 1.01 mL g−1s 1 and a total pressure of 2.0 MPa. 300 °C

Mo2C[46] 2 wt% K-Mo2C [46] Unpromoted-WC “as synthesized” K-WC “as synthesized” Na-WC “as synthesized” Unpromoted-WC “H2Treated” K-WC “H2Treated” Na-WC “H2Treated”

350 °C

CO2 Conversion (%)

CO Selectivity (%)

CO2 Conversion (%)

CO Selectivity (%)

EA (Kcal mol−1)*

19.9 18.1 3.8 1.4 1.7 8.8 4.8 1.9

73.5 95.9 90.0 100 100 89.9 100 100

N.A. N.A. 14.0 5.3 3.7 24.3 20.3 13.6

N.A. N.A. 96.4 100 100 88.0 98.1 100

14.0 11.4 N.A. N.A. N.A. 17.7 15.0 14.9

* Activation energies were determined experimentally using Arrhenius plots.

intermediates, thus affecting catalytic activity and selectivity. Electronic effects are primarily observed over the WC catalysts via the greater extent of carburization of the W4f XPS spectrum upon the addition of alkali metal promoters. It is also notable that the Na-promoted WC demonstrates the highest CO2 and lowest H2/CO2 uptake ratio in Table 2, and subsequently, the highest CO selectivity of all the conditions tested. However, Na-WC also exhibits the highest CO uptake of all three catalysts, possibly due to an increased CO binding energy, which can explain the lowest CO yield observed over Na-WC in Fig. 6. Although it may be difficult to precisely decouple the structural and

Fig. 5. Arrhenius plots for “H2treated” unpromoted (blue), Na-promoted (red), and K- promoted (green) WC/γ-Al2O3. The dashed line corresponds to the thermodynamic maximum CO yield under the conditions tested.

activation energies for the “H2treated” catalysts using Arrhenius plots are provided in Fig. 5. These values are also included in Table 4, along with the apparent activation energies previously reported for the Mo2Cbased catalyst. The supported WC catalysts do indeed possess higher activation energies, between 14.9 and 17.7 kcal mol−1, than those reported for the Mo2C catalysts, which are between 11.4 and 14.0 kcal mol−1. The calculated activation energies for the Na and Kpromoted WC/γ-Al2O3 are also lower than the unpromoted WC/γ-Al2O3 catalyst, which is in agreement with the previously observed trends for the Mo2C-based RWGS catalysts. [46] With the Na and K-promoted WC catalysts exhibiting lower activation energies and a greater extent of CO2 adsorption relative to the unpromoted WC catalyst, it might be anticipated that the alkali-promoted WC catalysts would exhibit higher activity. Under the conditions described for Table 4, however, CO2 conversion at 350 °C for the “H2treated” Na and K-promoted catalysts are 13.6 and 20.3%, respectively, while the unpromoted “H2treated” catalyst achieves a CO2 conversion of 24.3%. A similar decrease in RWGS activity was observed with the addition of K to Mo2C in our previous work, [46] and various other authors have also described decreased carbide activity due to the addition of alkali promoters for other catalytic reactions [67,69,70]. Such changes to catalyst performance may be rationalized by structural and electronic effects from addition of alkali metal promoters. For example, structural promoters may alter the geometry and crystallographic orientation of the catalyst, which in turn can affect the availability and activity of surface sites. [71] These structural changes are observed for the WC-based catalysts upon alkali metal addition via lower BET surface areas, and increased tungsten distributions. Electronic promotion, on the other hand, results in greater extent of carburization and attenuated chemisorption of reactants and

Fig. 6. a) CO selectivity as a function of CO2 conversion at 350 °C for “H2treated” unpromoted (blue), Na-promoted (red), and K-promoted (green) WC/γ-Al2O3. b) CO yield at 350 °C as a function of WHSV for “H2treated”unpromoted (blue), Na-promoted (red), and K-promoted (green) WC/γ-Al2O3. Tests were performed at 350 °C using varying WHSVs and a 3:1 H2:CO2 ratio at a total pressure of 2.0 MPa. 6

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4. Conclusions The effects of the alkali metal promoters on WC for the RWGS reaction have been well characterized with several analytical techniques, which demonstrate introduction of Na and K species act as structural and electronic promoters. The Na and K promoters improve the selectivity of WC for the RWGS reaction at low temperatures (300–350 °C), although their presence reduces the overall CO2 conversion, with Na having more of an inhibiting effect than K. For all WC samples, significant improvement in catalytic performance is realized upon treatment of the catalysts under H2 at 750 °C, which removes polymeric carbon that is deposited during TPC of the catalyst precursors. The CO2 conversion for the K-promoted WC catalyst is comparable to the unpromoted WC catalyst under identical conditions, and provides the highest CO yield of 26.7% at 350 °C. This work demonstrates the efficacy of WC/γ-Al2O3 as a low temperature, RWGS catalyst, and further establishes the role of alkali promoters as a means to tune catalyst selectivity toward CO.

Fig. 7. CO yield versus time on stream for “H2treated” unpromoted (blue), Napromoted (red), and K-promoted (green) WC/γ-Al2O3. Catalysts were tested at − 350 °C using a WHSV of 1.01 mL g−1 s 1 and a 3:1 H2:CO2 ratio at a total pressure of 2.0 MPa.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

electronic effects of promoters in the current study, the addition of alkali metal promoters to WC/γ-Al2O3 clearly results in increased CO selectivity, which is consistent with previous studies by the group over K-Mo2C. [46,72] To test if the CO selectivities of the alkali promoted catalysts could be maintained at greater CO2 conversions, the catalysts were tested at 350 °C under identical conditions to those described for Table 4, but at various reactant flow rates. Fig. 6a plots the CO selectivity as a function of CO2 conversion for the three “H2treated” catalysts at varying WHSVs, while Fig. 6b plots the CO yield (CO selectivity times conversion) as a function of reactant flow rate for the same data set. For reference, the CO yield corresponding to the thermodynamic maximum of the RWGS at this temperature is indicated as a dashed line in Fig. 6b. − When the WHSV is lowered to 0.50 mL g−1 s 1, CO2 conversions for the Na and K-promoted catalysts are increased to 17.9 and 25.3%, respectively, yet they still exhibit CO selectivities > 95%, demonstrating that even at similar CO2 conversions, CO selectivities are greater for the alkali promoted catalysts. As displayed in Fig. 6b, maintaining such high selectivities under the greater CO2 conversions is especially notable for the K-promoted catalyst, which displays the greatest CO yields − of the three catalysts when WHSVs are lowered to 0.50 mL g−1 s 1 or below. For all of the conditions tested, the highest CO yield is measured − at 26.7% using the K-promoted WC catalyst at a WHSV 0.28 mL g−1 s 1, corresponding to roughly 89% of the equilibrium value at 350 °C. [73] Finally, to characterize the stability of the WC catalysts, the “H2treated” samples were tested at 350 °C, under the conditions described in Table 4 for a total of 120 h. Product selectivity remained constant over the entirety of the experiment, but CO2 conversion decreased slightly with time. Fig. 7 plots the CO yields recorded for the three catalysts over the course of this experiment. Both the unpromoted and K-promoted WC catalysts demonstrate moderate deactivation with time, with CO yields 3.1 and 7.0% lower after 120 h on stream, respectively. The drop in activity for these two catalysts appears to be greatest during the initial 52 h on stream, with the rate of deactivation decreasing for both catalysts as a function of time on stream. Interestingly, the Na-promoted WC catalyst demonstrates no observable decrease in CO yield over the entirety of the experiment, although the overall yield is ˜75% of that of K-promoted WC. For the unpromoted and K-promoted WC catalysts, the decrease in catalytic activity over the course of this experiment is on the order of our previously reported Mo2C-based catalysts. [46] Therefore, the supported WC-based catalysts are active, selective and stable for the low temperature RWGS reaction.

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