Mo Catalysts Prepared by Atomic

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Synthesis Gas Conversion over Rh/Mo Catalysts Prepared by Atomic Layer Deposition Lifeng Zhang, Madelyn R. Ball, Yifei Liu, Thomas F. Kuech, George Huber, Manos Mavrikakis, Ive Hermans, and James A. Dumesic ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04649 • Publication Date (Web): 16 Jan 2019 Downloaded from http://pubs.acs.org on January 16, 2019

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ACS Catalysis

Synthesis Gas Conversion over Rh/Mo Catalysts Prepared by Atomic Layer Deposition Lifeng Zhanga, Madelyn R. Balla, Yifei Liua, Thomas F. Kuecha, George W. Hubera, Manos Mavrikakisa, Ive Hermansa,b and James A. Dumesica,* a

Department of Chemical and Biological Engineering, University of Wisconsin-Madison, 1415 Engineering Drive, Madison, WI 53706, United States b Department

of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, WI 53706, United States Corresponding Author *Prof. James A. Dumesic, Email: [email protected] Abstract Conversion of synthesis gas towards value-added products, including oxygenates and C2+ hydrocarbons, was studied at 523 K, 580 psi and CO/H2 = 1/1 over Rh catalysts on catalyst supports prepared by atomic layer deposition (ALD) of molybdenum and tungsten species on silica. The reactivity measurements showed that coating the silica support with molybdenum and tungsten species helped to suppress the methane selectivity and promote the overall conversion rate. Upon coating the silica support with 5 cycles of β-Mo2C, the methane selectivity decreased from 32% (1%Rh/SiO2) to 13% (1%Rh/5c-Mo2C/SiO2), and the overall product rate increased 33 times from 0.4 mmol/min/g Rh to 12.7 mmol/min/g Rh. CO-FTIR results showed that supporting Rh on silica led to the formation of Rh(211) facets, which favored formation of methane and had higher CO conversion rate. Rh on a MoO3/SiO2 support prepared by ALD was found to be the most active catalyst while maintaining the suppression of methane selectivity, showing around 60 times higher overall rate than 1%Rh/SiO2. A reaction pathway is proposed, in which hydrogenation steps are promoted most significantly by Mo and W species, followed by promotion of CO insertion steps for ethanol synthesis and C-C coupling steps for hydrocarbons formation. CO-FTIR

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results showed that 1%Rh/MoO3/SiO2 has the highest proportion of gem-dicarbonyl adsorption sites and the lowest proportion of bridge-bonded CO adsorption sites. The rate of methanol formation shows a positive correlation with the number of sites that form gem-dicarbonyl species. Keywords: Synthesis Gas Conversion, Rhodium, Molybdenum, Atomic Layer Deposition (ALD), Metal Carbides Introduction The catalytic conversion of synthesis gas (syngas) to oxygenates is a promising process to transform coal, natural gas and biomass into high value chemicals and transportation fuels1. However, development of active catalysts with satisfactory selectivity remains challenging and a clear understanding of structure-reactivity relation remains to be explored1,2. Rh is reported to be a good candidate for direct higher alcohol synthesis from synthesis gas as it is able to catalyze both CO dissociation and CO insertion steps2,3. Methanol4,5,6, ethanol7,8, acetaldehyde9, acetic acid9 and other oxygenates9,10 have been reported to be produced from syngas conversion over supported Rh catalysts modified with various promoters. Yang et al. reported the Rh stepped (211) surface to be more active than the Rh terrace(111) but highly selective toward methane, while the terrace surface is intrinsically selective toward acetaldehyde3. Molybdenum containing compounds are also potential candidates for higher alcohol synthesis from syngas. Patents from the Dow Chemical Company showed that bulk molybdenum carbide and molybdenum disulfide were able to produce alcohols from synthesis gas11,12. Co and Ni promoted MoS2 catalysts have been reported to enhance overall alcohol yield as well as higher alcohol selectivity13. Xiang et al. reported that K-doped Mo2C was able to promote the formation of alcohol from syngas conversion14. They also pointed out that the existence of Mo(IV) species improved the alcohol selectivity while Mo(II) species were responsible for hydrocarbon formation. Patents from the Union Carbide Corporation


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investigated the effect of addition of Mo and W to Rh catalysts in synthesis gas conversion15,16. Their results suggested that Mo and W were able to promote the selectivity towards methanol. Atomic layer deposition (ALD) has emerged as a useful method for the synthesis of catalytic materials 17. The self-limiting property of ALD allows the control of metal and metal oxide sites and improvement of catalytic activity, selectivity and longevity18,19. Yang et al. showed that ALD could provide surface modification to the catalyst support without altering the structural characteristics, thus eliminating variations from support structure and producing chemically modified catalyst supports with high surface area20. Our previous study showed that coating a silica support with tungsten carbide by ALD was effective in suppressing CH4 selectivity, which is the major undesirable product in syngas conversion, and promoting the overall production rate over Rh-based catalysts6. Herein, we report a method to coat a silica support with molybdenum oxide and carbides and we use these materials as supports of Rh nanoparticles for synthesis gas conversion. By combining reactivity measurements, CO-FTIR, XPS, STEM and XRD, we demonstrate that molybdenum coatings are able to suppress the selectivity of CH4 and promote methanol and ethanol formation rate by four and two orders of magnitude, respectively. A positive correlation between the number of Rh+ sites and the methanol synthesis rate is suggested. Experiment and Methods Catalyst Preparation SiO2(Davisil 646, Sigma-Aldrich) was first crushed and sieved between 60 and 80 mesh. For acid washed silica, the silica support was further washed using dilute nitric acid (Sigma-Aldrich) at room temperature for 3 hours and rinsed with milliQ water until the water became neutral. It should


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be noted that all the silica supports were not washed, unless they are specified with the subscript saying washed. RhCl3·xH2O (Strem Chemicals, 40% Rh) was used as precursor for impregnation to prepare Rh-based catalysts. The sample was then dried at 383 K for 3 h and reduced at 723 K(1 K/min, 2h). The reduced catalyst was passivated using 1% O2 in He at room temperature. A sample consisting of Mo impregnated onto silica was prepared with ammonium molybdate (Sigma Aldrich, 99.98%) and is denoted as IWI-MoO3/SiO2. Atomic layer deposition was performed in a fluidized bed reactor described elsewhere21. The methodology of performing atomic layer deposition in a fluidized bed reactor was adopted from previously reported literature19,21. Bis(tert-butylimino)bis(dimethylamino) molybdenum (Strem Chemicals) and Bis(tert-butylimino)bis(dimethylamino) tungsten (Strem Chemicals) were used as precursors for Mo and W, respectively. Ultra high purity (UHP) N2 was used as carrier gas and miliQ water was used as oxygen source. All depositions were conducted at 623 K and a pressure of 6 Torr. Before each deposition, the silica support was held at 523 K overnight under N2 flow to remove any moisture. The reactor effluent was analyzed by a residual gas analyzer 300 quadropole mass spectrometer (Stanford Research Systems) to monitor the deposition process. The as-synthesized MoOx/SiO2 powder was calcined at 623 K under air flow for 3 h to produce MoO3/SiO2. To produce β-Mo2C/SiO2, the as-synthesized MoOx/SiO2 powder was carburized under 100 cm3(STP)/min of 15%/85% CH4/H2 (vol/vol) mixture flow at 973 K for 3 h, then switched to 85 cm3(STP)/min of H2 flow at 973 K for 1 h to remove surface polymetric carbon22. To produce α-MoC/SiO2, the as-synthesized MoOx/SiO2 powder was first treated under NH3 flow under 973 K for 2 h and then switched to 15%/85% CH4/H2 (vol/vol) at 973 K for another 4 h. To produce W2C/SiO2, the as-synthesized WOx/SiO2 powder was carburized under 100 cm3(STP)/min of 15%/85% CH4/H2 (vol/vol) mixture flow at 1173 K for 3.5 h, then switched to


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ACS Catalysis

85 cm3(STP)/min of H2 flow at 1173 K for 0.5 h. The as-synthesized carbides were passivated by diffusing air into the cell. The as-synthesized MoO3/SiO2, α-MoC/SiO2, β-Mo2C/SiO2 and W2C/SiO2 were used as supports for Rh impregnation without further treatment. Reactivity Measurements Studies of syngas conversion were performed by feeding CO and H2 with 1/1 molar ratio to a halfinch OD fixed-bed reactor at 523 K and total pressure of 580 psi. Iron carbonyl impurity from the CO feed was removed by a purifier which was filled with silica chips (SiO2, fused,4-20 mesh, Sigma-Aldrich) at 623 K. The catalyst bed was positioned between plugs of quartz wool in the center of the reactor. The upstream and downstream parts of the reactor were filled with silica chips. A K-type thermocouple was attached to the reactor wall for temperature measurement and a PID controller (Love Controls Series 16A) was used for temperature control. The catalyst was reduced under 100 cm3(STP)/min of 20%/80% H2/He (vol/vol) flow at 723 K (1K/min, 2 h) and cooled to 523 K for reaction. The outlet of the reactor was connected to an online Agilent GC-FID with Rtx column for the detection of oxygenates and hydrocarbons, and to a Shimadzu 2014 GC-TCD with HaySep DB 100/120 column for the detection of CO and CO2. All the products were maintained in the gas phase by heating the pipelines. All experiments were performed at low conversion (e.g., near 1% conversion). CO conversion is calculated based on products detected. Selectivity is defined as:

𝑆𝑒𝑙𝑒𝑐𝑡𝑖𝑣𝑖𝑡𝑦 =

𝑛𝑖 𝑀𝑖 ∑𝑛𝑖 𝑀𝑖

× 100%


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where ni is the number of carbon and Mi is the molar amount of product i detected. The factor of promotion is defined as the rate ratio between a sample catalyst and the Rh/SiO2 with same Rh loading. For instance, the factor of promotion of 1%Rh/MoO3/SiO2 is defined as:

𝐹𝑎𝑐𝑡𝑜𝑟 𝑜𝑓 𝑃𝑟𝑜𝑚𝑜𝑡𝑖𝑜𝑛 =

𝑟𝑖,

1%𝑅ℎ/𝑀𝑜𝑂3/𝑆𝑖𝑂2

𝑟𝑖,

1%𝑅ℎ/𝑆𝑖𝑂2

where i represents different products and r represents the formation rate(mmol/min/g Rh). Error estimates for reaction rates were based on all measurement errors, including GC area integration, mass measurement and flow measurement. Catalyst Characterization Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) The metal loadings of Rh and Mo were determined using Varian Vista-MPX CCD Simultaneous ICP-OES. To determine the loading of Rh and Mo, samples (~20 mg) were digested using a mixture of 7.5 mL of hydrochloride acid and 2.5 mL of nitric acid at 423 K overnight below a water-cooled reflux column. The mixture was cooled to room temperature and diluted to the appropriate concentration. Rh and Mo standard solutions were prepared from Rh and Mo standards for ICP (Fluka).

CO Chemisorption The number of Rh surface sites was determined by CO chemisorption. Typically, 100 mg of catalyst was loaded into a glass cell and reduced at 723 K(1 K/min, 2 h). The cell was cooled to room temperature and vacuumed to 10-4 Torr after reduction. CO was introduced for chemisorption


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ACS Catalysis

at room temperature. The amount of CO chemisorbed on Rh was quantified by monitoring the CO pressure using a gas handling system and a volumetric system employing Baratron capacitance manometers for precision pressure measurement. Detailed information regarding the gas handling system is described elsewhere23. X-ray Photoelectron Spectroscopy (XPS) A K-alpha XPS (Thermo Scientific) instrument with a micro-focused monochromatic Al Kα Xray source was used to measure the surface composition and chemical state of both the passivated and reduced catalysts. Typically, 100 mg of sample was loaded into a Schlenk tube and reduced at 723 K(2 h, 1 K/min). The tube was transferred to glovebox and the sample was mounted on a transfer vessel (Transfer Vessel K-Alpha), which was then transferred to the XPS chamber. The entire procedure was operated without exposing the catalyst to air or moisture. The spectra in the Rh 3d, Mo 3d, C 1s, O 1s and Si 2p regions were collected with 10 scans, 100 ms dwell time, and 0.02 eV energy step size. The binding energy scale was calibrated using the Si 2p feature at 103.3 eV 24. The experimental spectra were fitted to Gaussian/Lorentzian lines after removal of an Sshaped background. X-ray Diffraction (XRD) Powder X-ray diffraction (XRD) patterns of the catalysts were collected from 2θ range of 20o to 80o using Bruker D8 Discover Diffractometer with Cu-Kα radiation.

Fourier Transform Infrared Spectroscopy (FT-IR)


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FT-IR (Nicolet 6700) spectra of CO absorbed on Rh were obtained in transmission mode. About 10 mg of catalyst sample was pressed into a self-supporting pellet and mounted in the sample holder of a transmission cell. Prior to measurement, the pellet was reduced at 573 K(1 K/min, 2 h). The cell was vacuumed to 10-4 Torr at room temperature after the reduction was complete. A background spectrum was taken under vacuum. 800 Torr of 1% CO in He was dosed into the cell and allowed to equilibrate for 5 min before the first spectrum was recorded. Weakly adsorbed CO was removed by flowing He before the final spectrum was recorded. Traces of O2 and moisture in He flow were removed by an oxygen trap (Alltech Oxy-Purge N). All spectra were recorded by averaging 256 scans with a resolution of 4 cm-1. To obtain the proportions corresponding to different states of absorbed CO, spectral deconvolutions were performed using OMNICTM. In general, four peak ranges were used for deconvolution. Peaks at 2090 cm-1 to 2100 cm-1 and from 2020 cm-1 to 2040cm-1 were assigned as gem-dicarbonyl adsorption. Peaks near 2050 cm-1 and 1880 cm-1 were assigned as linear and bridge adsorption, respectively25. Prior to studies of the hydroxyl group region, the catalyst pellet was treated under He flow at 423 K for 90 min in the IR chamber to remove absorbed moisture.

Scanning Transmission Electron Microscopy/Energy Dispersive X-ray Spectroscopy (STEM/EDS) A FEI Titan STEM with a Cs probe aberration corrector operated at 200 kV with spatial resolution < 0.1 nm was used for STEM studies. High-angle annular dark-field (HAADF) mode was applied to record images, with HAADF detector angle ranging from 54 to 270 mrad, probe convergence angle of 24.5 mrad, and probe current of approximately 25 pA. EDS data were recorded using the same microscope with an EDAX SiLi detector. Samples were prepared by dropping the passivated


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ACS Catalysis

catalyst, suspended in ethanol, on a holey carbon coated Cu grid. The samples were plasma cleaned before loading in the microscope. Results and Discussion 1. Catalytic Performance of Syngas Conversion Reaction kinetics studies of synthesis gas conversion were performed at 523 K, 580 psi and CO:H2 = 1:1 (vol:vol). The products detected include CH4, hydrocarbons (C2+ HC), CO2, methanol, ethanol, acetic acid, acetaldehyde, methyl acetate and ethyl acetate. All the reaction kinetics measurements were conducted at near 1% CO conversion and data were collected at steady state. A plot of rate and selectivity versus time on stream (Figure S1) indicated that the catalyst reached steady state after 4 hours. An effect of the SiO2 washing step on catalytic performance was observed. As shown in Table 1, Rh supported on unwashed silica leads to formation of C2+ oxygenates, in which acetic acid is the major product with selectivity of 45%, followed by CH4 with selectivity of 17%. Washing the support with diluted nitric acid leads to suppression of acetic acid (20%) and enhancement of CH4 (32%) and acetaldehyde(17%). Almost no CO2 is produced from either of the catalysts. The washing step is also found to improve activity. Rh on washed silica has nearly 3 times higher overall production rate than that on unwashed silica (i.e., 0.4 mmol/min/g Rh versus 0.1 mmol/min/g Rh). Our previous results showed that after acid wash, the Na concentration on silica surface decreased6. Yang et al. showed that Na species could selectively block Rh step and defects sites26, leading to higher C2+ oxygenates production and lower activity, which is consistent with our current results. Overcoating silica with molybdenum carbide by ALD is able to eliminate the effect of surface Na species on catalytic performance. Rh supported on 5 cycles of Mo2C/SiO2


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washed

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has nearly the same selectivity profile and activity as Rh supported on 5 cycles of Mo2C/SiO2

unwashed (Table

1). Our previous study showed that after ALD coating, the surface concentration of

Na decreases compared to unwashed silica6. Table 1. Reactivity of Supported 1%Rh Catalysts for Syngas Conversion Catalyst Supports

SiO2 washed

SiO2 *

W2C/SiO2

β-Mo2C/SiO2

β-Mo2C/SiO2 washed

α-MoC/SiO2

MoO3/SiO2

CO2

0%

2%

7%

5%

4%

4%

5%

Methane

32%

17%

13%

9%

13%

10%

11%

C2+ Oxygenates**

62%

73%

17%

19%

17%

12%

13%

Methanol

0%

2%

54%

57%

54%

64%

61%

Acetic Acid

20%

45%

2%

1%

1%

1%

0%

Acetaldehyde

17%

3%

0%

0%

0%

0%

0%

Alkane

4%

5%

6%

6%

6%

6%

6%

Alkene

2%

2%

3%

5%

4%

4%

4%

Alkene/Alkane

0.5

0.4

0.6

0.8

0.6

0.6

0.7

1.3%

1.0%

1.3%

1.5%

1.3%

1.5%

1.4%

0.4±0.1 0.1±0.02

4.3±0.4

13.2±1.3

12.7±1.3

12.9±1.3

23.4±2.3

37.8±3.7

134.2±13.4

100.6±10.1

Conversion Overall Rate (mmol/min/g Rh) Overall Rate (umol/min/g catalyst)

3.2±0.3

0.9±0.1

118.9±11.9 215.8±21.6

* Silica support is not washed with nitric acid unless specified otherwise. ** C2+ oxygenates include ethanol, acetic acid, acetaldehyde, methyl acetate and ethyl acetate. Molybdenum carbide and molybdenum oxide supports produced by ALD are active for syngas conversion (Table 2). -Mo2C/SiO2 and MoO3/SiO2 have similar selectivity profiles, where C2+ hydrocarbons are the major products (31% and 37%, respectively), followed by CO2 (20% and 21%, respectively) and methanol (19% and 18%, respectively). The high selectivity towards CO2


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ACS Catalysis

suggests that these catalysts have a high activity of water-gas shift reaction, which is in agreement with a previous study27. Table 2. Reactivity of 5 cycles of β-Mo2C and MoO3 supported on silica β-Mo2C/SiO2 MoO3/SiO2

Catalyst CO2

20%

21%

Methane

13%

18%

C2+ Oxygenates

17%

7%

Methanol

19%

18%

Alkane

16%

23%

Alkene

14%

14%

0.9

0.6

0.9%

0.5%

12.3±1.2

7.1±0.7

Alkene/Alkane Conversion Overall Rate (umol/min/g cat)

Overcoating the silica support with either tungsten or molybdenum carbides decreases the selectivity of CH4 and promotes the formation of methanol (Table 1). For instance, CH4 selectivity decreased from 32% to 13% upon coating washed silica with 5 cycles of β-Mo2C, while methanol selectivity increased from 0.3% to 54%. All of the carbide-supported catalysts, including 1%Rh/W2C, 1%Rh/α-MoC and 1%Rh/β-Mo2C have similar selectivity profiles, suppressing CH4 to nearly 10% and promoting value-added products, including oxygenates and C2+ hydrocarbons, to above 80%. The activity of Rh nanoparticles on these carbide supports is orders of magnitude higher compared to Rh supported on silica. 1%Rh/5c-W2C has 11 times higher overall product rate than 1%Rh/SiO2 washed. Changing from W2C to Mo2C further increased the rate. The rate of 1%Rh/β-Mo2C is 3 times higher than 1%Rh/W2C and 33 times higher than 1%Rh/SiO2. Both αMoC and β-Mo2C supported catalysts show similar selectivity profiles as well as reaction rates (Table 1).


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The effect of carburization is studied by comparing the performance of Rh supported on molybdenum oxide and molybdenum carbides (Figure 1). Both 1%Rh/5c-MoO3 and 1%Rh/5c-βMo2C display similar selectivity profiles, promoting the formation of methanol to above 55% and suppressing the formation of CH4 to near 10%. The overall selectivity to value-added products selectivity is similar as well (i.e., 86% versus 84%). 1%Rh/MoO3 catalyst has a higher activity than 1%Rh/β-Mo2C (i.e., 23.4 mmol/min/g Rh versus 13.2 mmol/min/g Rh). Lowering the Rh loading on β-Mo2C does not lead to a significant change of reactivity. Both 0.2% Rh/β-Mo2C and 1% Rh/β-Mo2C have CH4 selectivities near 10% and above 80% selectivity of value-added products, among which methanol is the major product. These two catalysts also have similar rate (i.e., 15.7 mmol/min/g Rh versus 13.2 mmol/min/g Rh, Figure 1). In contrast, lowering the Rh loading from 1.0% to 0.2% on the 5c-MoO3 support leads to lower methanol selectivity (61% versus 39%), lower selectivity for total value-added products (84% versus 70%), and higher selectivity to CO2 (5% versus 17%) and C2+ hydrocarbons (11% versus 22%) (Figure 1). Furthermore, the activity is lower for 0.2%Rh/5c-MoO3 compared to 1%Rh/5c-MoO3 (i.e., 7.4 mmol/min/g Rh versus 23.4 mmol/min/g Rh). The higher water-gas shift activity and higher selectivity towards C2+ hydrocarbons, which are also observed with MoO3 support solely as catalyst (Table 2), suggest a nontrivial contribution from the 5c-MoO3 support itself to the reactivity for the 0.2%Rh/5c-MoO3 catalyst. This behavior could be attributed to strong metal support interactions (SMSI) caused by reducible MoO3 species25, where a higher portion of Rh surface may be covered by MoOx species after high temperature treatment compared to the higher Rh loading catalysts, leading to similar selectivity between 0.2%Rh/MoO3 and the MoO3 support. A comparison of the reactivity of 1%Rh/5c-β-Mo2C/SiO2 and 1%Rh/10c- β-Mo2C /SiO2 is presented in Table S1. No significant difference is observed between these catalysts.


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ACS Catalysis

Figure 1. Selectivity and activity of Rh supported on β-Mo2C and MoO3. Table 3 lists factors of promotion of molybdenum and tungsten species supported Rh compared to silica supported Rh catalysts with the same Rh loading. For 1%Rh/W2C catalysts, hydrogenation towards methanol is promoted by a factor of near 1900 while the insertion step for ethanol formation is promoted by a factor of 35. The total rate of formation of coupling products, including all C2+ oxygenates and hydrocarbons, is promoted by a factor of 4. Changing from W species to Mo species further promotes the formation rates. 1%Rh/MoO3 has the highest effect in terms of promotion with respect to Rh/SiO2, where hydrogenation towards methanol is promoted by 4 orders of magnitudes and the ethanol formation is promoted by a factor of 214. Our previous study indicated a similar trend of promotion, where W2C-supported Rh promoted the formation of methanol, ethanol and other C2+ products. Adding Mn to the 2% Rh/W2C lowered the total rate,


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which might be attributed to blockage of Rh surface by Mn species, but promoted the coupling products, in which acetic acid was promoted the most significantly. Table 3. Rate of product formation on Mo and W supported Rh catalysts normalized by the rate on silica-supported Rh catalysts

Factor of promotion* CH4 CH3OH C2H5OH CH3COOH C2+ HC

C2+ C2+ Total Oxygenates Product

Ref

1%Rh/Mo2C

9

6480

155

2

47

10

13

33

This work

1%Rh/MoO3

20

11900

214

0

96

12

19

59

This work

1%Rh/W2C

4

1900

35

1

16

3

4

11

This work

2%Rh/W2C

1

246

11

0

2

2

2

3

Ref. 6

2%RhMn/W2C

1

39

11

3

3

3

3

3

Ref. 6

*: Factors of promotion are calculated based on rate per gram Rh (mmol/min/g Rh). Based on the reactivity analyses above, we propose a reactivity pathway in Figure 2. Methanol is formed by hydrogenation of non-dissociatively absorbed CO28,29. Ethanol is formed by an insertion mechanism between CO* and CHx* species29. Hydrocarbons are formed via C-C coupling of alkyl species. Molybdenum and tungsten species have the largest effect of promotion for hydrogenation steps, followed by the CO insertion steps for C2+ oxygenates formation and C-C coupling steps for C2+ hydrocarbons. Adding Mn to supported Rh promotes the pathway towards acetic acid10,29.


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ACS Catalysis

Figure 2. Proposed reaction pathways for Rh/Mo and Rh/W catalysts. 2. Catalyst Characterization a. Composition and morphology of catalysts Table 4 lists metal loading, surface sites and dispersions calculated from CO chemisorption of supported Rh catalysts. 1% Rh supported on silica has the highest Rh dispersion, followed by carbide-supported Rh catalysts and then MoO3-supported Rh. This behavior can be attributed to the SMSI effect of MoOx25, which may cover the Rh surface after high temperature reduction, leading to lower CO chemisorption uptake. Figure 3 shows STEM images of 1% Rh on SiO2, αMoC, β-Mo2C and MoO3. The 1% Rh/MoO3 catalyst showed the smallest particles size (1.87 ± 0.7 nm), followed by 1%Rh/β-Mo2C (1.93 ± 0.66 nm), 1%Rh/α-MoC(2.10 ± 0.68 nm) and 1%Rh/SiO2(2.73 ± 0.64 nm)(Figure S2). All Rh particle sizes were analyzed with more than 1000 particles. STEM-EDS mapping results indicated uniform Mo coverage over silica and distinct particles of Rh (Figure 4). Figure 5 shows XRD patterns of 5c--MoC, 5c--Mo2C and 5c-MoO3.


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Peaks in the β-Mo2C sample at 34.7o, 37.9o, 39.5o, 61.8o and 69.4o are assigned to β-Mo2C with (100), (002), (101),(110) and (103) facets (JCPDS No.35-0787). Peaks at 37.2o, 43.3o, 62.5o and 75.4o are assigned to α-MoC (JCPDS No.65-0280). Peaks at 23.6 o, 25.6 o, 27.6 o ,34 o, 39.1 o, 46.2o, 49.1o and 55.5o are assigned to α-MoO3 with (110), (040), (021), (111), (150), (200), (002) and (112) facets (JCPDS No.05-0508). XRD analysis of spent catalysts indicated that the carbide structure was retained after reaction (Figure S3). Table 4. Summary of ICP and CO chemisorption results for Rh catalysts Rh Loading(%)

Mo or W Loading(%)

1%Rh/SiO2 washed

0.81

-

-

-

1%Rh/SiO2

0.79

-

32.2

42%

1%Rh/W2C

0.88

8.70

18.3

22%

1%Rh/β-Mo2C

1.01

3.40

19.6

21%

1%Rh/α-MoC

0.92

4.77

25.0

29%

0.2%Rh/β-Mo2C

0.19

3.70

2.7

15%

1%Rh/MoO3

0.92

3.50

8.1

9%


CO Sites(µmol/g) Dispersion

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ACS Catalysis

Figure 3. STEM images of supported Rh catalysts. (A:1%Rh/SiO2, B:1%Rh/α-MoC, C:1%Rh/βMo2C, D: 1%Rh/MoO3) All the catalysts were characterized in passivated form.

Figure 4. STEM-EDS mapping of 1%Rh/MoO3. Red: Mo, green:Rh.


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Figure 5. XRD patterns of 5 cycles of MoO3, α-MoC and β-Mo2C on SiO2. All catalysts were characterized in passivated form. b. Surface composition and characterization Figure 6 shows XPS spectra of the passivated and reduced 1%Rh/β-Mo2C/SiO2. The Rh peak at 307.2 eV in both passivated and reduced spectra shows metallic Rh, and the shoulder at 308.5 eV in the passivated sample indicates the existence of oxidized species (i.e., Rh2O3). Deconvolution of the Rh spectrum indicated that approximately 16% of Rh existed as oxidized species (Figure S4). After reduction, the Rh3+ shoulder decreases and only Rh0 is observed (Figure 6 and Figure S4). In the Mo spectra, the passivated sample displays a peak of metallic Mo or Mo2C at 228.0 eV and shoulders of Mo4+ at 229.1 eV and Mo6+ at 232.5 eV. In the reduced sample, shoulders of Mo4+ and Mo6+ decrease, indicating that the major species of Mo is metallic Mo or Mo2C with


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ACS Catalysis

partially oxidized Mo species. As Mo0 and Mo2C are too close to be distinguished, we confirm the existence of carbide species by C 1s spectra displayed in Figure 6.C. Both two spectra display the carbidic C1s feature at 282.7 eV. Peaks at 284.3 eV are attributed to organics originated from pump oil. Figure 7 shows the XPS spectra of passivated and reduced 1%Rh/MoO3/SiO2. Similar to 1%Rh/Mo2C/SiO2, the Rh3+ signal in the passivated sample decreases after reduction and metallic Rh is the only species observed. Deconvolution results indicated that approximately 26% of Rh existed as oxides in the passivated sample (Figure S4). Most of oxidized molybdenum species are reduced to metallic Mo upon reduction at 623 K. A comparison between the XPS signals of ALD-MoO3/SiO2 and IWI-MoO3/SiO2 with similar Mo loading is displayed in Figure S5. The atomic ratio of Mo:Si for ALD-MoO3/SiO2 is 0.09 while this ratio is equal to 0.01 for IWI-MoO3/SiO2, indicating a higher dispersion of Mo for the ALD sample30. FTIR studies of the hydroxyl group region (Figure S6) indicated that approximately 30% of the hydroxyl groups on silica was consumed in 5 cycles of ALD MoO3/SiO2.

Rh3+ Rh0

A

Reduced

Passivated

320

310

Binding Energy(eV)


300

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Mo4+ Mo2C

Mo6+ B

Reduced

Passivated

235

230

225

Binding Energy(eV)

C-C

Carbide

C

Reduced

Passivated

300

295

290

285

280

275

Binding Energy(eV)

Figure 6. XPS spectra of passivated and reduced 1%Rh/β-Mo2C/SiO2. A: Rh 3d B: Mo 3d C: C 1s.


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ACS Catalysis

Rh3+

A

Rh0

Reduced

Passivated

320

310

300

Binding Energy (eV)

4+ Mo6+ Mo Mo0

B

Reduced

Passivated

250

245

240

235

230

225

220

Binding Energy (eV)

Figure 7. XPS spectra of passivated and reduced 1%Rh/MoO3/SiO2. A: Rh 3d B: Mo 3d. Figure 8 shows the CO stretching region of 1%Rh over washed and unwashed silica after reduction at 573 K under H2 flow. Both samples display symmetric and anti-symmetric stretching of gemdicarbonyl Rh-(CO)2 and linear Rh-CO stretching at near 2090 cm-1, 2020 cm-1 and 2048 cm-1,


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respectively. In 1%Rh/SiO2

unwashed,

the major bridge peak is located at 1861 cm-1, while in

1%Rh/washed SiO2, the major bridge peak is shifted to 1882 cm-1. Previous DFT calculations showed that the CO bridge-bonded adsorption peak at 1864 cm-1 was assigned to Rh terrace sites (111), which shifts to 1887 cm-1 when adsorbed on Rh step sites (211)10. Thus, the washing step here leads to the preferably formation of the Rh(211) facet over Rh(111) 10. As Rh(211) has been reported to favor the formation of CH4 in syngas conversion and have higher CO conversion rate3, this behavior is consistent with higher CH4 selectivity and overall production rate observed for 1%Rh/washed SiO2 washed compared to 1%Rh/SiO2 unwashed.

Figure 8. CO-FTIR spectra of 1%Rh supported on washed and unwashed SiO2. Figure 9 shows the CO stretching region of 1% Rh over washed-SiO2, α-MoC, β-Mo2C and MoO3. All four samples display gem-dicarbonyl adsorption at near 2092 cm-1 and 2020 cm-1, linear adsorption at 2050 cm-1, and bridge-bonded adsorption at 1850-1870 cm-1. Deconvolution of the


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ACS Catalysis

FTIR spectra (Figure S7) shows that 1%Rh/MoO3 sample has the highest proportion of gemdicarbonyl adsorption and the lowest proportion of bridge-bonded adsorption, followed by the two carbides supported samples, 1%Rh/α-MoC and 1%Rh/β-Mo2C. 1%Rh/SiO2 has the lowest proportion of gem-dicarbonyl adsorption and the highest proportion of bridge-bonded adsorption. This trend is consistent with our STEM particle size analysis, where 1%Rh/MoO3 contains the smallest Rh particle size and 1%Rh/SiO2 has the largest particle size. Besides the effect of particle size, it was reported that surface hydroxyl groups can react with Rh0-CO to generate gemdicarbonyl sites31. Molybdenum oxycarbide and molybdenum oxide were reported to have surface acidity32, which we suggest might also lead to the formation of gem-dicarbonyl species.

Figure 9. CO-FTIR spectra of 1%Rh supported on washed SiO2, α-MoC, β-Mo2C and MoO3


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Figure 10 depicts a relation between the rate of methanol formation with respect to the proportion of surface gem-dicarbonyl adsorption sites deconvoluted from CO-FTIR spectra. The formation rate of methanol is found to increase along with an increase in the proportion of gem-dicarbonyl adsorption sites. We speculate that positively charged RhI(CO)2 sites might be responsible for methanol production from syngas conversion. Previous studies have shown that noble metal cations are able to activate CO towards methanol formation, with metallic sites providing H atoms for hydrogenation, which is consistent with our observations in the current work25,33,34.

Figure 10. Relation between methanol formation rate and the proportion of gem-dicarbonyl sites.


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ACS Catalysis

Conclusion Overcoating a silica support with MoO3 and Mo2C via ALD leads to supported Rh catalysts that display suppressed CH4 selectivity, enhanced methanol formation, and higher overall production rate of CO conversion. The CH4 selectivity decreases from 32% (1%Rh/SiO2

washed)

to

13%(1%Rh/β-Mo2C/SiO2 washed) upon overcoating the silica support with 5 cycles of β-Mo2C over washed SiO2.The selectivity towards value-added products, including oxygenates and C2+ hydrocarbons, reaches 86% over 1%Rh/β-Mo2C, while it is 68% over 1%Rh/SiO2 washed. The rate of syngas conversion increases 33 times upon overcoating with Mo (i.e., 12.7 mmol/min/g Rh over 1%Rh/β-Mo2C/SiO2 washed versus 0.4 mmol/min/g Rh over 1%Rh/SiO2 washed). Washing the SiO2 support is found to influence the catalytic performance of Rh supported on silica. 1%Rh/SiO2 unwashed

has lower CH4 selectivity and higher acetic acid selectivity, while 1%Rh/SiO2 washed has

higher activity. ALD coating is effective to eliminate the influence of Na species from the SiO2 support toward reactivity. 1%Rh/5c-β-Mo2C/SiO2 washed and 1%Rh/5c-Mo2C/SiO2 unwashed displays nearly identical selectivity profile and overall production rate. Changing the nature of the ALD overcoat from Mo2C to MoO3 does not lead to a significant change in the selectivity profile; however, the activity is higher by a factor of 2 on the oxidized support. The rate of methanol formation is promoted by a factor of near 12000, ethanol by a factor of near 210, and C2+ hydrocarbons by a factor of 96 for MoO3 supported Rh catalysts compared to silica supported catalysts. A reaction network is proposed, in which hydrogenation steps are promoted to the largest extent by Mo and W species, followed by the CO insertion steps for ethanol synthesis and C-C coupling steps for hydrocarbon formation. Here, we report that the best performing catalyst, 1%Rh/MoO3 has 11% selectivity towards CH4 and 84% selectivity towards value-added products,


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and an overall production rate of 23.4 mmol/min/g Rh, while for 1%Rh/SiO2 washed these values are 32%, 68% and 0.4 mmol/min/g Rh, respectively. XPS results indicate that all the passivated catalysts based on molybdenum carbide supports have oxidized Mo species over the surface and are reduced to molybdenum carbide after reduction under H2 at 723 K. The Mo oxides species on 1%Rh/MoO3 are reduced to metallic Mo under H2 at 723 K. CO-FTIR spectra show that the major bridge-bonded adsorption peak is at 1882 cm-1 for the 1%Rh/SiO2 washed sample, which is assigned to Rh(211) step sites, while this peak is at 1861 cm-1 for the 1%Rh/SiO2 unwashed sample, which is assigned to Rh(111) terrace sites. This difference in facet can contribute to the differences in selectivity and activity between these two catalysts, where Rh(211) favors formation of CH4 and has higher activity. Deconvolution of CO-FTIR spectra shows that the 1%Rh/MoO3 catalyst has highest proportion of gem-dicarbonyl adsorption sites and lowest proportion of bridge-bonded adsorption sites, followed by α-MoC and β-Mo2C supported Rh catalysts, and then the silica supported catalyst. This trend, combined with results from reactivity measurements, shows a positive correlation between the proportion of gem-dicarbonyl adsorption sites and the methanol formation rate, suggesting that positively charged Rh+ sites may be responsible for methanol synthesis from synthesis gas on these catalysts.

Associated Contents Supporting Information. Plot of rate and selectivity versus time on stream for 1%Rh/β-Mo2C. Particle size distribution of 1%Rh/SiO2, 1%Rh/-MoC, 1%Rh/-Mo2C and 1%Rh/MoO3. XRD patterns of spent 1%Rh/-Mo2C and 1%Rh/-MoC. Deconvolution of Rh 3d peaks of 1%Rh/Mo2C and 1%Rh/MoO3 from XPS. Mo 3d peaks of ALD-MoO3/SiO2 and IWI-MoO3/SiO2 from


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ACS Catalysis

XPS. Hydroxyl group region in FTIR for 1%Rh/5c-MoO3/SiO2 and blank SiO2 support. Deconvolution of CO-FTIR spectra of 1%Rh/SiO2, 1%Rh/-Mo2C, 1%Rh/-MoC, 0.2%Rh/Mo2C, 1%Rh/MoO3 and 1.5%Rh/MoO3. Reactivity of 1%Rh over 5 and 10 cycles of β-Mo2C. This material is available free of charge via the Internet at http://pubs.acs.org. Author Information Corresponding Author *Email: [email protected] ORCID Manos Mavrikakis: 0000-0002-5293-5356 Ive Hermans: 0000-0001-6228-9928 James A. Dumesic: 0000-0001-6542-0856 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. Acknowledgement This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division under Award Number DE-SC0014058. We also acknowledge support of this research by the


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TOC:


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Figure 1. Selectivity and activity of Rh supported on β-Mo2C and MoO3. 165x101mm (96 x 96 DPI)


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Figure 2. Proposed reaction pathways for Rh/Mo and Rh/W catalysts. 508x285mm (96 x 96 DPI)


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Figure 3. STEM images of supported Rh catalysts. (A:1%Rh/SiO2, B:1%Rh/α-MoC, C:1%Rh/β-Mo2C, D: 1%Rh/MoO3) 111x111mm (96 x 96 DPI)


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ACS Catalysis

Figure 4. STEM-EDS mapping of 1%Rh/MoO3. Red: Mo, green:Rh. 338x190mm (96 x 96 DPI)


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Figure 5. XRD patterns of 5 cycles of MoO3, α-MoC and β-Mo2C on SiO2. All catalysts were characterized in passivated form. 508x285mm (96 x 96 DPI)


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ACS Catalysis

Figure 6. XPS spectra of passivated and reduced 1%Rh/β-Mo2C/SiO2. A: Rh 3d B: Mo 3d C: C 1s. 111x287mm (96 x 96 DPI)


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Figure 7. XPS spectra of passivated and reduced 1%Rh/MoO3/SiO2. A: Rh 3d B: Mo 3d. 145x260mm (96 x 96 DPI)


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ACS Catalysis

Figure 8. CO-FTIR spectra of 1%Rh supported on washed and unwashed SiO2 116x98mm (96 x 96 DPI)


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Figure 9. CO-FTIR spectra of 1%Rh supported on washed SiO2, α-MoC, β-Mo2C and MoO3 111x111mm (96 x 96 DPI)


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ACS Catalysis

Figure 10. Relation between methanol formation rate and the proportion of gem-dicarbonyl sites. 338x190mm (96 x 96 DPI)


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TOC 338x190mm (96 x 96 DPI)


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Mo Catalysts Prepared by Atomic

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