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Rh-MnO Interface Sites Formed by Atomic Layer Deposition Promote Syngas Conversion to Higher Oxygenates Nuoya Yang, Jong Suk Yoo, Julia Schumann, Pallavi Bothra, Joseph A. Singh, Eduardo Valle, Frank Abild-Pedersen, Jens K. Norskov, and Stacey F. Bent ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01851 • Publication Date (Web): 13 Jul 2017 Downloaded from http://pubs.acs.org on July 13, 2017

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ACS Catalysis

Rh-MnO Interface Sites Formed by Atomic Layer Deposition Promote Syngas Conversion to Higher Oxygenates Nuoya Yanga, Jong Suk Yoob, Julia Schumannb,c, Pallavi Bothrab,c, Joseph A. Singhd, Eduardo Valleb, Frank Abild-Pedersenb,c, Jens K. Nørskovb,c, Stacey F. Bentb* a

Department of Materials Science and Engineering, Stanford University, 496 Lomita Mall,

Stanford, California, 94305, United States b

Department of Chemical Engineering, Stanford University, 443 Via Ortega, Stanford,

California, 94305, United States c

SLAC National Accelerator Laboratory, SUNCAT Center for Interface Science and Catalysis,

2575 Sand Hill Road, Menlo Park, California, 94025, United States d

Department of Chemistry, Stanford University, 333 Campus Drive, Stanford, CA 94305, United

States * Corresponding Author: [email protected]

Abstract

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Rhodium (Rh) catalysts are among the major candidates for syngas conversion to higher oxygenates (C2+oxy), with manganese (Mn) a commonly used promoter for enhancing the activity and selectivity towards C2+oxy. In this study, we use atomic layer deposition (ALD) to controllably modify Rh catalysts with MnO, by depositing manganese oxide as a support layer or an overlayer, in order to identify the function of the Mn promoter. We also compare the ALDmodified catalysts with those prepared by co-impregnation. An ultrathin MnO support layer shows the most effective enhancement for C2+oxy production. Transmission electron microscopy, temperature programmed reduction, and diffuse reflectance infrared Fourier transform spectroscopy characterization indicates that formation of Rh-MnO interface sites is responsible for the observed activity and selectivity improvements, while ruling out Rh nanoparticle size and alloy or mixed oxide formation as significant contributors. MnO overlayers on Rh appear to suffer from poor stability upon CO adsorption and are less effective than a MnO support layer. Density functional theory (DFT) calculations show that MnO species on the Rh(111) surface lower the transition state energy for CO bond dissociation and stabilize the key transition state for C2+oxy synthesis more significantly than that for methane synthesis, leading to enhanced activity and C2+oxy selectivity. Keywords: atomic layer deposition, syngas conversion, higher oxygenates synthesis, Rh catalyst, catalytic promoter 1. Introduction: The direct synthesis of higher oxygenates (C2+oxy) from syngas (CO+H2) has great potential in transforming methane, coal, biomass and organic waste into clean liquid transportation fuels and high-value chemicals1–4. Supported Rh catalysts have shown relatively high activity and selectivity towards higher oxygenates compared to other syngas conversion catalysts5. Previous


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studies have shown that the terrace sites of Rh are preferred over the stepped sites for achieving selective production of higher oxygenates6,7. It has also been found that appropriate choices of Rh support materials8–12 and promoters13–17 are critical to achieving high C2+oxy yield. Among more than sixty different elements tested as promoters18,19, several early transition metals20–23 (manganese, iron), rare earth metals16,17 (lanthanum, vanadium, zirconium, cerium) and alkali metals24,25 (lithium) were found to be effective in promoting syngas conversion activity, higher oxygenate selectivity, or both. As a non-toxic, earth abundant element, Mn has shown superior activity and selectivity improvement and thus it has been widely studied as either a single promoter or combined with other promoters. The synthesis procedure for adding Mn was also shown to influence C2+oxy production18,26. While co-impregnation has been widely used to synthesize Mn-promoted Rh catalysts, impregnation of Mn onto the support prior to Rh deposition was reported to yield higher C2+oxy production than co-impregnation18,27. The promotion functions of Mn have been investigated by both experimental and theoretical approaches13,20,28–31. On Rh catalysts, C-O bond dissociation of CHxOH* surface species is usually considered to be the overall rate determining step and CO* insertion to CHx* to form CHxCO* and the following hydrogenation of CHxCO* determine C2+oxy selectivity, respectively, on Rh. Previous studies have proposed that Mn could influence either or both of these two critical reaction steps in the following ways: (1) increasing Rh dispersion and stabilizing partially oxidized Rh clusters32–35; (2) modifying the electronic structure of Rh 20,30,32; (3) forming mixed oxides or a metallic alloy15,20,34,36; and (4) forming interface sites between Rh and MnO26,36–39, all of which may stabilize/destabilize reaction intermediates40. However, these different mechanisms have been the subject of conflicting reports in the literature and are still being debated. A major difficulty of understanding the promotion effect of Mn as well as other


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promoters lies in controlling the structural, chemical and electronic properties of the promoted catalysts. Variations in the commonly used impregnation synthesis method often lead to complex changes that are not well defined and controlled, including differences in particle size or the ratio of various species on the catalyst surface, as well as differences in the degree of physical site blocking, metal-support interaction, mixed oxides/alloy formation and electronic effects. Although some of these factors are correlated, better control of the catalyst structure can help deconvolute different promotion effects and improve the understanding of the active structure and promotion mechanism for enhanced C2+ oxy production. In this work, we apply atomic layer deposition (ALD) to deposit ultrathin manganese oxide (MnO) layers to modify the surface of Rh supported on silica. Based on a self-saturating, layerby-layer growth model, ALD has the capability to achieve uniform coatings on high surface area substrates 41–45 and hence enables controllable design and synthesis of heterogeneous catalysts46– 51

. While previous studies typically involve multiple changes in the catalyst structure and

composition and thus make the effect of Rh-MnO interface more difficult to identify, this study helps confirm the promotion effect of Rh-MnO by introducing MnO films through ALD to separate the interface effect from changes in the size and bulk phase composition of Rh nanoparticles. Compared with the commonly used synthesis method of impregnation, the introduction of MnO by ALD allows better control over the spatial distribution and surface concentration of Rh versus Mn. It also allows for the separation of effects based on particle size, physical blocking of surface sites, and metal-support interactions. Two types of MnO-promoted Rh catalyst are synthesized using ALD: MnO as a support modification layer and MnO as an overlayer, as illustrated in Figure 1. MnO-promoted Rh catalysts are also synthesized by coimpregnation to compare the different synthesis methods. The Rh catalysts were evaluated as


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syngas conversion catalysts and characterized by transmission electron microscope (TEM), temperature programmed reduction (TPR), CO chemisorption, X-ray photoelectron spectroscopy (XPS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), as well as density functional theory (DFT) calculations.

Figure 1. Modification of Rh catalysts by ALD. MnO was deposited by ALD onto silica as a support layer, followed by Rh incipient wetness impregnation, or onto calcined Rh/SiO2 as an overlayer. MnO generally increased both the overall activity and the C2+oxy selectivity. Comparing unpromoted Rh catalysts with MnO-promoted Rh produced by either ALD modification or impregnation, we show that the improved C2+oxy production can be attributed to Rh-MnO interface sites that are present between Rh nanoparticles and the MnO support. The effect is significant with MnO as a support layer. However, a MnO overlayer on Rh is less effective in enhancing activity and C2+oxy selectivity, possibly due to the migration of MnO upon CO adsorption. DRIFTS characterization shows a lower wavenumber “tilted” CO species on samples with MnO as a support layer, which is likely to facilitate CO dissociation and enhance activity. We also applied DFT calculation to fundamentally understand the effect of MnO on the reaction energy diagram. Previous DFT studies have focused on ultra-small Rh-Mn clusters20, Mn atoms29 or a single layer28 on Rh single crystal surface; however, this study considers the


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oxide phase, MnO,

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which has been suggested as the stable phase of Mn under reaction

conditions. The reaction pathways towards methane, acetaldehyde and ethanol have also been studied in this work. We show that the presence of MnO on the Rh surface lowers the transition state energy for CO bond dissociation, leading to increased overall activity. In addition, MnO stabilizes the key transition state for acetaldehyde production more significantly than that for methane production, therefore improving C2+oxy selectivity. In addition, our results suggest that C2+ oxy production can be improved without the need for increased Rh dispersion, oxidized Rh species, mixed oxides or Rh-Mn alloy formation, although such effects cannot be excluded for Rh-Mn catalysts synthesized by co-impregnation.

2. Experimental and theoretical methods 2.1 Catalyst preparation SiO2 (Davisil grade 643, Sigma Aldrich) was used as the basic substrate for all catalysts studied in this work. The silica was washed in 2M nitric acid aqueous solution under 80℃ for 2 hours in order to remove iron and sodium impurities52. Then the silica was washed in deionized water to remove residual nitric acid and dried at 120℃ for 24 hours before use in Rh nanoparticle synthesis or ALD. Rh was deposited by incipient wetness impregnation (IWI) onto SiO2 with or without ALD-MnO deposition. For IWI synthesis, an appropriate amount of RhCl3·xH2O (Sigma-Aldrich, 39 wt% Rh) was dissolved in deionized water so that the Rh metal loading was 5 wt% and the solution volume equaled the total pore volume of the silica. This solution was added onto the silica gel in a dropwise fashion. After drying at room temperature for 24 hours, the Rh catalysts were calcined in static air at 500 ºC for 4 hours. Samples of Rh supported on unmodified SiO2 and MnO-modified SiO2 are designated as Rh/SiO2 and Rh/MnO/SiO2,


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respectively. Co-impregnation of Rh and Mn was carried out by mixing MnCl2·4H2O (Sigma Aldrich 99.99%) with RhCl3 into the impregnation solution; the metal loading of Rh was maintained at 5 wt% and that of Mn at 1.52 wt%. The co-impregnated catalyst is designated as Rh-Mn/SiO2. 2.2 MnO deposition by ALD As shown in Figure 1, MnO was deposited by ALD onto two types of samples: on the washed SiO2 substrate as a support modification layer, and on calcined Rh/SiO2 as an overlayer. Rh/SiO2 with ALD-MnO as an overlayer is designated as MnO/Rh/SiO2. Manganese monoxide (MnO) has been reported to be the stable phase grown by ALD53 as well as the stable phase under reaction conditions due to the difficulty of reducing Mn2+ 54,33,36,39. The details of the ALD process for MnO deposition have been reported previously55. In brief, MnO was deposited in a custom-built, hot-wall ALD reactor. SiO2 or Rh/SiO2 powders were contained in a customized particle holder following the design reported by Libera et al.56. Bis(ethylcyclopentadienyl) manganese (Mn(EtCp)2, Strem Chemicals, 98+%) and deionized water were alternately exposed to the substrate. Nitrogen was used as the carrier and purge gas. The standard experimental sequence was a 6 second exposure of Mn(EtCp)2 and a 6 second exposure of H2O with at least 180 second purge between pulses. The precursor Mn(EtCp)2 was kept at 65ºC and the water was held at room temperature. The stage temperature was maintained at 175ºC. Catalysts with various numbers of ALD-MnO cycles (3, 5, 10, 20) were tested in syngas conversion. Samples with 5 cycles of ALD-MnO show the highest C2+oxy production so in this work we focus on catalysts promoted by 5 cycles of ALD-MnO. Inductively coupled plasma


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optical emission spectroscopy (ICP-OES) quantification showed ~0.33 wt% Mn loading after 5 cycles of ALD-MnO deposition on silica. 2.3. Reaction testing Syngas conversion reactions were carried out in a tubular fixed bed reactor (Altamira Instruments, glass-lined stainless steel reactor, 30 cm length, 4 mm internal diameter). A gas purifier (Pall GASKLEEN ST) was used to remove nickel and iron carbonyl from CO (Matheson, 99.999%) before the gas stream enters the reactor. Usually, 100 mg catalyst was loaded into the reactor and reduced in-situ at atmospheric pressure at 250ºC for 2 hours in a H2 (Airgas, 99.9999%) /He mixture (20sccm H2, 80sccm He). After reduction, the pressure was increased to 20 bar while the temperature remained at 250ºC for the syngas conversion reaction. The total gas flow rate during reaction was 90 sccm with a H2:CO ratio of 2. For the Rh/MnO/SiO2 and Rh-Mn/SiO2 samples, only 20 mg of catalyst was used due to the high activity of these two types of catalysts. These flow rate and catalyst charge values were chosen to maintain CO conversion below 5% in order to approach differential condition and minimize secondary reactions57. All the products were analyzed in-line using gas chromatography (SRI 8610C) equipped with a capillary column and a HaysepD column, and the products were detected by two flame ionization detectors (FID). The selectivity was calculated on a carbon basis, i.e. S =

∑

where Si is the selectivity to species i, ci is the number of carbon atoms in

species i and ri is the production rate of species i. In this work, C2+oxy includes acetaldehyde, ethanol and small amounts of acetone and acetic acid. 2.4. Catalyst characterization Catalysts were characterized by a combination of temperature programmed reduction (TPR), CO chemisorption, X-ray photoelectron spectroscopy (XPS), transmission electron microscope


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(TEM) measurements, and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). For the TPR and CO chemisorption experiments, typically 80mg of Rh catalysts was loaded into a quartz U-tube. Prior to TPR, the catalyst was treated in Ar at 300ºC to remove moisture and surface contamination. After cooling to 30~40ºC in Ar, the gas stream was switched to 50 sccm of 10% H2/Ar and the temperature was increased at 10ºC/min to 500℃. The thermal conductivity detector (TCD) signal was recorded to track H2 consumption. For CO chemisorption, the catalysts were reduced in 10% H2/Ar at 250ºC for 1 hr. After the catalyst was cooled to room temperature, CO was pulsed into the U-tube and CO consumption was recorded by TCD. The CO pulse was maintained until full saturation was reached as measured by a constant CO signal. XPS measurements to determine surface composition of the catalysts were performed on a PHI VersaProbe Scanning XPS Microprobe with Al(Kα) radiation (1486 eV). Rh nanoparticle size was analyzed by an FEI Tecnai G2 F20 X-TWIN TEM system operated at 200KV. Rh catalysts were reduced in 20% H2/He at 250ºC for 2 hours before TEM characterization. DRIFTS was performed in a Bruker Vertex 70 infrared spectrometer with a closed reaction cell (Harrick Scientific) and an MCT detector. All spectra were collected at 2 cm-1 resolution over 200 scans. During DRIFTS measurements, ultrahigh purity H2 (99.999%, Matheson) and CO (99.99%, Praxair) were used and a heated stainless steel cylinder filled with ϒ-Al2O3 was employed to remove metal carbonyl impurities from the CO source. 20 mg of catalyst powder was loaded into the sample cup and evacuated by a mechanical vacuum pump to reach pressures around 15 mTorr. Prior to CO adsorption, the catalyst sample was reduced under flowing H2 at 300ºC for 2 hours and then H2 was evacuated at 300ºC for 15 minutes before cooling down to room temperature. A background scan was taken before introducing CO. Then CO was dosed


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into the cell and the sample surface was saturated. After excess CO was evacuated and the absorption spectrum was stabilized, room temperature CO absorption spectra were collected. To investigate the hydrogenation of adsorbed CO species, H2 was flowed into the cell and temperature was increased stepwise to175ºC, 225ºC and 250ºC. At each elevated temperature, a spectrum was taken after allowing the system to stabilize for 5 minutes. 2.5 Calculational method Periodic DFT calculations were carried out using the Quantum Espresso code58 in connection with the Atomic Simulation Environment59. The BEEF-vdW exchange correlation functional was used as it is specifically developed to describe both chemisorption and physisorption properties of adsorbates60. A kinetic energy cut-off of 500 eV and a density energy cut-off of 5000 eV were used for all calculations. The ionic cores were described using Vanderbilt ultrasoft pseudopotentials61. The slab models were based on 3×3×4 supercells, which were separated by more than 13 Å of vacuum space in the direction perpendicular to the surface plane. The top two atomic layers of the slab models were allowed to relax whereas the bottom two were fixed at their bulk positions. The Brillouin zones were sampled using a 4×4×1 Monkhorst–Pack k-point mesh62.The convergence criterion for the energy optimization was a maximum force of 0.05 eV/Å per atom. Transition states were calculated using the nudged elastic band (NEB) method63. The MnO promotion effect was studied by introducing a MnO monomer onto the Rh(111) surface. This structure serves as a simple model to explain the observed trends of MnO addition to a Rh catalyst. 15 reaction intermediates and 13 transition states were calculated for a full investigation of the reaction pathway. Hydroxylation of the MnO monomer was not explicitly considered, except for the C-O bond breaking step of CHOH. In addition, selected adsorbates and one transition state were calculated on a more complex model, including a MnO stripe with


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4 Mn atoms in the unit cell of a 2x5 Rh(111) supercell; the results on this stripe model confirmed the trends observed for the simple MnO monomer model. To obtain a semi-quantitative relationship between C2+oxy selectivity and activity with varying MnO concentration on Rh, we start from a certain step vs. terrace ratio of Rh and replace Rh step sites with Rh/MnO sites. This simulation predicts the activity and selectivity properties of MnO prmoted Rh catalysts with changing MnO coverage. 3. Results and Discussion: 3.1. The effect of Mn promoter on Rh nanoparticle dispersion Some prior studies have reported that Mn promotion increases Rh nanoparticle dispersion and therefore particle size effects could contribute to the enhanced C2+oxy production33,34, while an absence of change in Rh nanoparticle size or morphology has also been reported32. We analyzed the size distributions of the Rh nanoparticles with and without Mn promoters by TEM (Fig.2). Between 250 to 350 particles were analyzed for each catalyst sample. As shown in Table 1, both Rh/SiO2 and Rh/MnO/SiO2 have a very similar Rh nanoparticle size distribution, with average diameters of 3.1-3.4 nm. However, the average diameter of the co-impregnated Rh-Mn/SiO2 is around 2.3 nm, ~1 nm smaller than that of Rh/SiO2 and Rh/MnO/SiO2. Such a difference in the Rh nanoparticle size distribution between Rh/MnO/SiO2 and Rh-Mn/SiO2 is consistent with literature, in which smaller Rh particles were observed on promoted Rh catalysts prepared by coimpregnation64,65. On the contrary, the ALD-MnO support modification layer does not change the dispersion of Rh nanoparticles and therefore the effect of the MnO support on the catalytic activity and selectivity should not result from changing the Rh nanoparticle size.


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Figure 2. TEM images of Rh/SiO2 (a), Rh/MnO/SiO2 (b) and Rh-Mn/SiO2 (c) Table 1. Average diameter of Rh nanoparticles characterized by TEM sample

Average diameter (nm) Standard deviation (nm)

Rh/SiO2

3.4

1.5

Rh/MnO/SiO2

3.1

1.3

Rh-Mn/SiO2

2.3

0.8

3.2. Temperature programmed reduction (TPR) We performed temperature programmed reduction to examine the interaction between Rh and the Mn promoter. Previous work has suggested that strong interaction between Rh and Mn results in decreased reducibility of Rh and hence a higher reduction temperature32,36,64,66. The TPR profile of the Rh/MnO/SiO2 sample closely resembles that of Rh/SiO2 (Fig. 3). The unchanged reduction temperature suggests a lack of strong interaction between Rh and Mn and a similar Rh nanoparticle dispersion as for un-promoted Rh/SiO2. This is consistent with TEM characterization, which shows a similar Rh nanoparticle size distribution between Rh/MnO/SiO2 and Rh/SiO2. The co-impregnated Rh-Mn/SiO2 catalyst shows a 15 ºC increase in reduction temperature compared to Rh/SiO2 (Fig. 3) This indicates that strong interaction between Rh and the Mn promoter may exist in the bulk phase of the Rh nanoparticle and that the formation of


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mixed oxides or a Rh-Mn alloy is possible in co-impregnated Rh-Mn/SiO2, but not likely on Rh/MnO/SiO2. On MnO/Rh/SiO2, two reduction peaks are observed at 80 ºC and 224 ºC. Reduction of MnO2 to Mn3O4 and MnO is not likely, since these processes occur at temperatures above 330 ºC

32,67

,

100 ºC higher than the second reduction peak. Therefore, these two peaks may result from the reduction of bare Rh nanoparticles and those overcoated by MnO, respectively. The physical presence of MnO on the Rh surface could increase the reduction temperature of the underlying Rh nanoparticles, corresponding to the higher temperature peak. The low temperature peak, which is 10 ºC higher than the un-promoted Rh/SiO2, likely comes from the reduction of bare Rh nanoparticles, without MnO overcoating.

Figure 3. Temperature programmed reduction (TPR) profiles of Rh/SiO2 (black), Rh/MnO/SiO2 (red), MnO/Rh/SiO2 (green) and Rh-Mn/SiO2 (blue). Signal intensities have been normalized. 3.3. Syngas conversion Figure 4 and Table 2 show the syngas conversion activity and selectivity for promoted and unpromoted Rh catalysts. All catalysts that contain Mn show improved overall activity and selectivity towards C2+ oxy. Across the various catalysts, acetaldehyde is always the major C2+oxy product, with a molar ratio of acetaldehyde to ethanol (AcH: EtOH) of approximately 10:1. The presence of the Mn promoter did not improve the percentage of ethanol in the total


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C2+oxy in any of the cases that we studied. Some previous studies show Mn increases ethanol selectivity26,66, and we suggest that this discrepancy with the literature reports may result from the presence of impurities (e.g. Fe)52 in the catalysts or from secondary reactions in those studies, since ethanol can be produced by reduction of acetaldehyde. The Rh/MnO/SiO2 catalyst shows the highest activity (~ 4 times higher than Rh/SiO2) and the highest total C2+oxy yield. Both Rh-Mn/SiO2 and Rh/MnO/SiO2 show increased TOF from 0.02 1/s to 0.08 1/s. Both Rh/MnO/SiO2 and Rh-Mn/SiO2 significantly increase C2+oxy selectivity, raising it from ~30% in the case of the Rh/SiO2 catalyst to ~60%, and both decrease methane selectivity while leaving the selectivity towards higher hydrocarbons roughly unchanged. C-O bond dissociation through a CHxOH* intermediate is commonly recognized as the rate-limiting step in syngas conversion. On the other hand, selectivity is generally determined by the difference in energy barriers between hydrogenation of CHx* species, which produces methane, and CO* insertion into CHx* species to form CHxCO* and the following hydrogenation of CHxCO*6,68, which leads to C2+oxy. The higher activity of the MnO-promoted catalysts indicates enhanced C-O bond dissociation, and the data show that with MnO as a support layer, such enhancement is more effective than for other geometries. The substantially increased C2+oxy selectivity and decreased methane selectivity of Rh/MnO/SiO2 and Rh-Mn/SiO2 further suggest that CO* insertion into CHx* species becomes more competitive than hydrogenation of CHx* into methane. The similar activity and C2+oxy selectivity from both Rh/MnO/SiO2 and Rh-Mn/SiO2 and the lack of alloy or mixed oxide formation in Rh/MnO/SiO2 indicated by TEM and TPR characterizations suggest that neither a Rh-Mn alloy nor mixed oxide formation is required to promote catalytic activity and selectivity of Rh. Rather, the data indicate that it is the presence of the Rh-MnO interface


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sites that are highly effective in facilitating both C-O bond dissociation and C2+oxy formation. We will discuss this in more detail based on density functional theory calculations in Section 3.6.

Figure 4. Total CO conversion and C2+oxy production (a) and selectivity towards methane, higher hydrocarbons and C2+oxy (b) for the different Rh catalysts studied in this work. C2+HC represents higher hydrocarbon products. Reaction conditions: T= 250 ºC, P=20 bar, flow rate 90 sccm (H2/CO=2) Table 2. Summary of the syngas conversion activity and selectivity for the Rh catalysts studied in this work. CO conversion rate* (µmol/gcat/s)

S(CH ) %

S(C2+HC **) %

S(AcH** )%

S(EtOH** )%

S(total C2+oxy) %

TOF*** (1/s)

Rh/SiO2

3.29

50.38

17.66

26.82

3.08

30.3

0.018

Rh-Mn/SiO2

10.77

27.59

12.65

51.87

7.37

58.76

0.087

Rh/MnO/SiO2

13.08

26.06

17.49

55.37

4.19

56.12

0.080

4


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MnO/Rh/SiO2

9.06

30.36

28.68

35.01

4.16

40.50

MnO/Rh/SiO2

7.38

38.74

24.57

31.76

3.57

36.30

0.051

Calcined again

* Reaction conditions: T= 250ºC, P=20 bar, flow rate 90 sccm (H2/CO=2) ** AcH = acetaldehyde; EtOH = ethanol; C2+HC = higher hydrocarbons. Methanol production is negligible for all catalysts tested. ***TOF is calculated based on CO chemisorption measurements.

The existence of Mn species atop the Rh surface and subsequently the interface sites formed between Rh and Mn has been speculated as a possible reason for promoting C2+oxy production15,28,32,36. We examine this effect by depositing MnO directly onto Rh/SiO2 using ALD at T = 175 oC so that the surface of Rh will be partially overcoated by MnO without forming a Rh-Mn alloy. As shown in Fig. 4 and Table 2, selectivity towards higher hydrocarbons nearly doubles, implying that MnO present on top of Rh nanoparticle may facilitate coupling of the surface CHx* species69. The MnO/Rh/SiO2 catalyst also shows higher activity, higher TOF and C2+oxy selectivity than un-promoted Rh/SiO2, suggesting that both C-O bond dissociation and CO* insertion into CHx* to form CHxCO* can be enhanced by simply having MnO on top of the Rh surface. However, the enhancement is much lower than that of the Rh/MnO/SiO2 and RhMn/SiO2 catalysts. We offer two possible explanations: that MnO in MnO/Rh/SiO2 may adversely block active Rh surface sites thus resulting in less activity enhancement; or, that the ALD-deposited MnO overlayer may not be stable under reaction conditions. Decreasing the number of ALD-MnO cycles failed to improve the activity and C2+oxy selectivity, compared to the 5 cycles of ALD-MnO reported here (data not shown). Therefore, blocking Rh active sites may not be the primary cause. We also attempted to stabilize the MnO layer by calcining MnO/Rh/SiO2 at 500ºC for 4 hrs. However, this treatment did not lead to any improvement in


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either activity or selectivity (Table 2). In addition, to examine these hypotheses, we performed XPS and CO chemisorption to investigate the Rh surface exposure and CO absorption capability after ALD MnO deposition, as described below. 3.4. XPS and CO chemisorption Figure 5a shows ex-situ XPS spectra of the Rh/SiO2 catalyst before and after MnO ALD. A decreased Rh signal and a strong Mn signal are observed on MnO/ Rh/SiO2, indicating effective MnO coverage on the Rh nanoparticles. After reduction, the surface composition of Rh and Mn remains unaffected (Fig. 5a), suggesting that the MnO overlayer remains stable. Figure 5b shows the change in reduced MnO/Rh/SiO2 surface composition before and after CO chemisorption. The relative intensity of Mn decreases (Fig. 5b), with the ratio of Mn/Rh reduced from 1.95 to 1.15 after CO chemisorption, suggesting that MnO may segregate away from the Rh surface under CO exposure, thus decreasing the amount of the Rh-MnO interface sites. Such MnO layer reconstruction has also been observed by Stevenson et al70 and the authors also pointed out that the surface coverage of MnO on Rh could be influenced by gas phase composition and temperature. It is possible that under syngas conversion condition, high CO pressure and high temperature may drive MnO reconstruction to an even larger degree than that observed in Figure 5b. Such segregation may explain why the activity and C2+oxy selectivity enhancement of MnO/Rh/SiO2 are inferior compared with Rh/MnO/SiO2 and Rh-Mn/SiO2. It is worth noting that MnO/Rh/SiO2 still shows better activity and C2+oxy selectivity than unpromoted Rh/SiO2, suggesting that some interface between Rh and MnO still exists after MnO migration. We speculate that this could occur from MnO that remains around the perimeter of the Rh nanoparticles. Future studies using in-situ TEM may provide valuable information on the migration of MnO layers under the reactive environment.


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Figure 5. XPS spectra of as synthesized Rh/SiO2, as synthesized MnO/Rh/SiO2 and reduced MnO/Rh/SiO2:(a) showing effective MnO coverage on Rh surface and the MnO overlayer remains stable after reduction. Reduced MnO/Rh/SiO2 before and after CO chemisorption. (b) shows Mn signal decreases after CO chemisorption. (c) Deconvolution of Mn and Rh peaks for reduced MnO/Rh/SiO2 before and after CO chemisorption. Deconvolution of the Mn and Rh XPS peaks provides further information on the change in chemical environment after CO chemisorption. Mn signals are mainly composed of two peaks at 642 and 644.6 eV71, representing the Mn(II) and Mn (IV) component, respectively. After CO chemisorption, the relative intensity of the lower binding energy component decreases. MnO/SiO2 did not adsorb CO in the chemisorption experiment (data not shown), and CO is expected to adsorb on the Rh surface due to the strong binding energy. Therefore, the change in


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Mn binding energy may result from its migration away from the Rh surface, which may lead to a configuration in which Mn is surrounded by more O. The Rh 3d5 signal mainly contains two components at 307 and 308.5 eV before CO chemisorption, representing Rh0 and Rh(III) species31. The high Rh (III) signal from the reduced sample could indicate that the coverage on Rh surface hinders reduction of Rh. After interacting with CO, the 308.5 eV component shifts to 309.5 e V, which is likely induced by CO, as shown in previous study. We also performed CO chemisorption measurements to study the exposed surface area and CO adsorption capability of the catalysts. According to Table 3, the CO chemisorption capability of MnO/Rh/SiO2 is very similar to that of Rh/SiO2, indicating that the active Rh sites for CO adsorption are not significantly affected by the MnO overlayer. (MnO/SiO2 itself does not adsorb CO under such conditions, data not shown.) This result is consistent with the XPS characterization, which shows that segregation of MnO away from the Rh surface may take place upon CO adsorption. In addition, the amount of MnO deposited is low (~0.33 wt%), so the portion of the Rh surface overcoated by MnO may also be low and hence there may not be sufficient MnO deposited to significantly affect the CO chemisorption capability. Liu et al.39 also reported that Mn does not reduce the available surface area for CO adsorption on the Rh surface, consistent with our observations. Table 3. CO chemisorption at room temperature on different Rh catalysts studied in this work sample

Rh/SiO2 MnO/Rh/ SiO2

Rh/MnO/ SiO2

Rh-Mn-coIMP/ SiO2

CO chemisorption (µmol/gcat)

178.58

177.92

164.52

124.13

36.6

33.9

25.6

Dispersion* (%) 36.8

*Dispersion defined as the ratio between the number of surface Rh atoms and total Rh atoms


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CO chemisorption on Rh/MnO/SiO2 slightly decreased compared to un-promoted Rh/SiO2. However, co-impregnated Rh-Mn/SiO2 showed 30% less CO chemisorption than Rh/SiO2. Such a phenomenon has been reported in some previous studies15,36,70 and Mn was hypothesized to reduce the heat of CO chemisorption15. As shown in the TEM and TPR characterizations, coimpregnation results in a larger degree of intermixing between Rh and the Mn promoter so the Mn promoter may be able to reduce the CO chemisorption capability of Rh. 3.5. DRIFTS characterization of CO adsorption on Rh catalysts 3.5.1 CO adsorption at room temperature The CO adsorption properties on promoted and un-promoted Rh catalysts were investigated using DRIFTS at room temperature. As shown in Figure 6, on un-promoted Rh/SiO2, the major adsorption peak, usually assigned to linear CO adsorption14,72, appears around 2059~2065 cm-1; another peak present at lower wavenumber (~1929 cm-1) corresponds to bridged CO that is coadsorbed between two or more Rh atoms14,70,72–74. Upon adding Mn--either through ALD or coimpregnation--the linear CO peak positions are not significantly shifted. The weak shoulder around 2103 cm-1 can be assigned to gem-dicarbonyl adsorption species75,74, with the other peak associated with this species (typically around 2030 cm-1) likely to be buried under the strong linear CO peak. Gem-dicarbonyl species are associated with partially-oxidized, ultrasmall Rh clusters smaller than 1 nm76. All catalysts tested in this study show only weak gem-dicarbonyl peaks (Fig. 6), consistent with the Rh size distribution, ranging from 2.5-3.5 nm, obtained from TEM. The ratio between the gem-dicarbonyl peak and linear peak slightly changes for different samples. However, the variation does not correlate with C2+oxy selectivity: relative intensity of the gem-dicarbonyl peak are the highest and lowest for the two most selective catalysts (Rh/MnO/SiO2 and Rh-Mn/SiO2) and that of the other three catalysts are very similar. Therefore,


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the presence of the Mn promoter does not strongly induce oxidized small Rh clusters and thus the increased C2+oxy selectivity does not seem to correlate to these Rh clusters. The major differences in the DRIFTS spectra between samples lie in the bridged CO peak region. On Rh-Mn/SiO2, the bridged CO peak centered near 1920 cm-1 becomes small, while a strong, lower wavenumber peak around 1704 cm-1 can be observed with a shoulder at 1586 cm-1. This lower wavenumber peak around 1700 cm-1 has been commonly assigned to “tilted” CO species adsorbed at Rh-Mn interface sites, with carbon bonded to Rh and O bonded to the oxophilic Mn2+ cation64,70. The nearly complete disappearance of the bridged CO peak on RhMn/SiO2 may indicate highly intermixed Rh and Mn, in which Mn results in a strong disruption in the Rh surface structure. This is in agreement with the smaller particle size characterized by TEM compared to un-promoted and ALD modified catalysts. This effective mixing of Rh and Mn is also consistent with the increased reduction temperature in TPR. The Rh/MnO/SiO2 catalyst also exhibits a low frequency CO adsorption band, in this case near 1740 cm-1. This band may also be assigned to tilted CO absorbed at Rh-Mn interface sites, although the CO vibration frequency is not decreased as much as that on co-impregnated RhMn/SiO2. Such interface sites could be formed in two possible ways: they exist around the contact perimeter present between the Rh nanoparticles and the MnO-modified support; and, trace amount of the MnO may have transferred onto the Rh surface as dissolved Mn2+ cation during the incipient wetness impregnation process52. However, on Rh/MnO/SiO2, the relative intensity of the tilted CO peak compared to the linear CO peak is much lower than that on RhMn/SiO2, and the bridged CO peak remains similar to that on un-promoted Rh/SiO2. This implies that on Rh/MnO/SiO2, the extended Rh surface stays mostly undisrupted.


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The DRIFTS spectrum of MnO/Rh/SiO2 largely resembles that of un-promoted Rh/SiO2, although the tail around 1700-1800 cm-1 may indicate a small amount of tilted CO species formation or CO bond weakening by Mn existing on the Rh surface. The similarity of the DRIFTS spectra of MnO/Rh/SiO2 and the unpromoted Rh/SiO2 indicates that ALD-MnO deposited on Rh/SiO2 does not strongly influence CO adsorption properties on Rh surface (Fig. 6). The oxophilic property of the Mn promoter and formation of “tilted” CO, which has a weaker C-O bond than other CO adsorption species, have been proposed to lower the C-O bond dissociation barrier and increase syngas conversion activity13,70. The Rh/MnO/SiO2 and RhMn/SiO2 catalysts clearly exhibit the presence of such tilted CO species, which is likely to account for the significantly improved activity of these two catalysts.

Figure 6. Room temperature DRIFTS spectra of adsorbed CO on the SiO2-supported Rh catalysts studied in this work. Spectra have been normalized for comparison. 3.5.2 Hydrogenation and desorption of adsorbed CO under H2 flow


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Figure 7. DRIFTS spectra of adsorbed CO on the SiO2-supported Rh catalysts with flowing H2 and increasing temperatures To compare the reactivity of different CO adsorption species, DRIFTS experiments were carried out in the presence of H2. After CO adsorption was stabilized at room temperature, H2 flow was introduced to the cell and the temperature was increased stepwise. The results are shown in Figure 7. The red shift of the linear CO peak under H2 flow and increasing temperature is attributed to decreased CO coverage, which results in reduced dipole moment coupling and more back donation of electrons from Rh to the CO anti-bonding orbitals77. The gem-dicarbonyl species vanish under H2 flow at higher temperature, which has also been observed in previous studies25,78. The overall rate of decrease of the CO adsorption peaks with increasing temperature under H2 flow generally agrees with the activity of these Rh catalysts in syngas conversion: the Rh/MnO/SiO2 catalyst exhibits the highest rate of decrease of the CO peak (Fig.7d), followed by


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the Rh-Mn/SiO2 sample (Fig.7c), while the CO adsorption species appear more stable on MnO/Rh/SiO2 (Fig.7b) and Rh/SiO2 (Fig.7a). More mechanistic information can be extracted from the details of the spectral changes. For Rh/MnO/SiO2 and Rh-Mn/SiO2 samples, the bridged and tilted CO peaks decrease more rapidly than the linear CO peak and this could indicate that the bridged and tilted CO species have higher activity towards hydrogenation. Linear CO can potentially transfer to these highly active interface sites and subsequently be hydrogenated, leading to a fast decrease of all CO species. In contrast, the MnO/Rh/SiO2 catalyst does not form stable interface sites upon CO adsorption as indicated by XPS characterization, in turn generating fewer of the more active tilted CO species, and therefore its activity enhancement is only moderate. 3.6 DFT calculations on the effect of MnO on syngas conversion selectivity Periodic DFT calculations were employed to investigate the effect of MnO on the catalytic activity and selectivity of Rh catalysts. First, we found that a MnO cluster prefers to decorate the stepped site of Rh(211) over the terrace site of Rh(111) by 0.93 eV. This suggests that during MnO ALD or impregnation processes, the stepped sites of Rh catalysts, which are more reactive towards production of methane than acetaldehyde7, could become relatively inactive due to the preferential binding of MnO species on such under-coordinated sites. Thus, C2+oxy selectivity can be improved. Previously, we have shown that CHOH* CH* + OH* with CH-OH* as the transition state (TS) is the overall activity limiting step, whereas CH2* + H* CH3* (TS: CH2-H*) and CH2CO* + H* CH3CO* (TS: H-CH2CO*) are the selectivity determining steps for methane and acetaldehyde production, respectively, on the Rh(111) surface7. Assuming that the reaction mechanism on Mn-promoted Rh catalysts is the same as that on Rh(111), we have calculated the


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full reaction pathway on Rh(111) with and without MnO* monomer adsorbed on the Rh surface. (Energy diagram of syngas conversion on Rh with and without MnO promoter can be found in SI, section 2.) This constructed model serves as a simple model to explain the observed trends of MnO addition to a Rh catalyst. We believe that the types of interfacial sites and blocked step sites should remain the same, whether MnO is used as a support or overlayer. In addition, calculation of key intermediate and transition state energies on MnO stripe decorated Rh (111) confirmed the same trend (data not shown). Using the energies calculated by DFT we solved the micro-kinetic model for the MnO promoted Rh surface (SI Figure S3). The results for the three rate and selectivity determining steps are shown in Table 4. Here, it can be seen that the presence of MnO* lowers the energy of CH-OH* and H-CH2CO* by 0.22 eV and 0.33 eV, respectively. The energy of CH2-H* was only lowered by 0.08eV. The significant stabilization of the CH-OH* and H-CH2CO* transition states may result from the oxophilic nature of the Mnδ+ cation that selectively interacts with the Oδ- atom of a hydroxyl or carbonyl group via Coulombic attraction. A lower energy of the CH-OH* transition state leads to a reduced CO bond dissociation barrier and increased overall activity. With the presence of MnO, H-CH2CO* becomes 0.31eV more stable than CH2-H*. Thus, in addition to the selective decoration of MnO* on the stepped sites of Rh particles, we attribute the enhanced acetaldehyde selectivity of the Mn-promoted Rh catalysts also to MnO* stabilizing the transition states involved in acetaldehyde production to a larger extent than that involved in methane production. Therefore, our DFT calculations provide good explanations for both the increased activity and the C2+oxy selectivity achieved with MnO introduced by ALD and co-impregnation. Rh/MnO/SiO2 and Rh-Mn/SiO2 effectively form RhMnO interface sites, as indicated by DRIFTS measurements, and hence the improvements in activity and C2+oxy selectivity are more significant compared to MnO/Rh/SiO2.


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Table 4. The calculated transition-state free energies (eV) on Rh(111) and Rh(111) with MnO clusters adsorbed on the Rh surface (MnO*/Rh(111)) relative to gas phase CO, H2 and H2O at T = 523 K and P = 20 bar (H2 : CO = 2 : 1). Transition state

CH-OH*

Surface

Rh

MnO*/

Rh

MnO*/

Rh

MnO*/

(111)

Rh(111)

(111)

Rh(111)

(111)

Rh(111)

1.37

1.15

0.3

0.22

0.24

-0.09

Free Energy (eV)

CH2-H*

H-CH2CO*

* Represents species adsorbed on Rh surface 4. Proposed mechanism of MnO promotion Based on the results from the syngas conversion reaction and other characterizations on Rh/MnO/SiO2, the ultrathin MnO support layer deposited by ALD greatly increases both overall CO conversion activity and selectivity towards C2+oxy. As discussed previously, the experimental results show that this MnO support layer does not affect the size nor the reducibility of the Rh nanoparticles. Therefore, we conclude that the activity and selectivity enhancements of this structure mainly result from Rh-MnO interface sites. In support of this conclusion, the tilted CO species on Rh/Mn/SiO2 observed in DRIFTS indicates effective interface site formation between Rh and the ALD-MnO support layer. As shown in Figure 8, with the presence of interface sites, a weakened C-O bond at the Rh-MnO interface such as that associated with the tilted species likely leads to faster CO dissociation and hence higher activity. Our DFT calculations also show that the presence of MnO on the Rh surface lowers the transition state energy for CO hydrogenation and CO bond dissociation and thus provides a reduced energy barrier for the rate determining step. In terms of selectivity, MnO reduces the


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energy barriers for the acetaldehyde pathway more than the methane pathway, leading to increased C2+oxy selectivity (Figure S3). The main effect is a strengthening of the oxygen binding energy at Rh sties close to the MnO species, which lowers the adsorption energy of oxygen-containing intermediates and transition states. Furthermore, our calculations and experimental data suggest that the promotion effect of MnO is primarily electronic rather than a bifunctional effect79 for the following reasons. First, the ALD-MnO/SiO2 catalysts did not exhibit any syngas conversion activity or CO chemisorption capacity. Moreover, with Rh present, CO does not absorb on MnO. H2 splitting is also generally accepted to take place on transition metal surfaces, rather than on oxides. The only evidence for potential bifunctional effect is that that H2O formation could occur on MnO, as this step has a lower barrier on MnO than on Rh (111). However, similar activity and selectivity should be expected from MnO/Rh/SiO2 and Rh/MnO/SiO2 if the bifunctional effect were dominant. Instead, the significantly different reaction performance, especially selectivity, between MnO/Rh/SiO2 and Rh/MnO/SiO2, combined with the change in syngas conversion energy diagram induced by MnO, points to a primarily electronic effect from MnO promotion. We also tested Rh of different loadings on MnO/SiO2 (Figure S1). With increasing Rh loading of 0.5%, 2% and 5%, C2+oxy selectivity increased from 43% to 56%, while activity per amount of Rh increased more than three times. As we expect a higher contact area between Rh and the MnO/SiO2 support with increasing Rh loading (SI Section 1), the increased activity per amount of Rh indicates that larger contact between Rh and MnO enhances the promotion. Besides using ALD to deposit MnO film, we also synthesized MnO2/Rh/SiO2 by depositing MnO2 on calcined Rh/SiO2 using incipient wetness impregnation. Comparison between the two catalysts suggests


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that a more evenly distributed MnO film leads to better contact with Rh and therefore improves selectivity more than MnO particles (SI Section 3). It is also possible that a trace amount of Mn is transferred onto the Rh surface during impregnation, as illustrated in Figure 8, preferentially blocking stepped/defect sites and contributing to the reduced selectivity towards CH4. Future work will focus on investigating such effects.

Figure 8. Proposed mechanism for the promotion effect by ALD-MnO support modification. A trace amount of MnO may be transferred to the Rh surface, blocking step sites and reducing methane production. Rh-MnO interface may increase activity by forming tilted CO species. The Rh-MnO interface sites can stabilize key transition states for C2+oxy formation and therefore improve C2+oxy selectivity. The presence of Rh-MnO interfaces also improves selectivity for the Rh/MnO/SiO2 catalyst. DFT calculations show that the presence of MnO on the Rh surface stabilizes the key transition state for acetaldehyde formation (H-CH2CO*) and hence increases C2+oxy selectivity. In comparison, such stabilization is not as significant for the key transition state for methane formation (CH2-H*). Moreover, based on our recent work7, the activity and selectivity of Rh is highly structure-sensitive and depends upon the ratio between stepped/defect sites and terrace sites. We therefore extended our previous model on the structure sensitivity of Rh7 to include replacement of 211 sites by MnO-promoted sites. This model shown in Figure 9 gives an


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explanation for the observed increased rate and C2+oxy selectivity. In this model, starting from a certain step vs. terrace ratio, we replace highly active (but methane-selective) Rh step sites with slightly less active but extremely acetaldehyde selective Rh/MnO sites. This will lead to comparable selectivities as in the unpromoted case, while improving the observed rate significantly. This relationship could be used to approximately predict the activity and C2+oxy selectivity with varying MnO coverage on Rh.

Figure 9. Proposed semi-quantitative relationship between C2+oxy selectivity and TOF on MnO promoted Rh (black) and unpromoted Rh (green). Red square and red dots represent experimental results from this work. Green dots represent experimental data reported in our previous publication7. The simulation curve and experimental data on unpromoted Rh/SiO2 (green curve and green squares) are adapted with permission from Ref.7 Copyright (2016) American Chemical Society. The co-impregnated Rh-Mn/SiO2 catalyst also shows an increase in CO conversion activity and selectivity towards C2+oxy. The structural difference between co-impregnated Rh-Mn/SiO2 and Rh/MnO/SiO2 suggests that although some of the same effects may play a role in the activity and selectivity in both systems, the Mn in the co-impregnated Rh-Mn/SiO2 may promote C2+oxy production through additional effects. The smaller nanoparticle size, decreased reducibility of Rh


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and disappearance of the bridged CO peak all indicate better Rh-Mn mixing and therefore mixed oxide or alloy formation could be possible. Mei et al.20 conducted DFT calculations to show that Rh and Mn could form a metallic alloy under reaction conditions and the presence of Mn on a Rh cluster could lower the CO insertion barrier through electronic effects. They concluded that a high concentration of Mn on the Rh surface is necessary for lowering the CO insertion barrier. Such an effect could account for the higher C2+oxy production for the co-impregnated RhMn/SiO2 in this study. However, for ALD-modified Rh/MnO/SiO2, particle size effects and the formation of mixed oxides or alloys do not seem to play a strong role in the improved activity and C2+oxy selectivity. Comparing the two catalyst structures prepared by ALD (MnO/Rh/SiO2 and Rh/MnO/SiO2), the data shows that the presence of MnO deposited onto the Rh surface is less effective in promoting activity and C2+oxy selectivity than MnO used as a support layer. Based on the XPS and CO chemisorption results, the MnO overlayer is not stable upon CO adsorption and hence we propose that the reason that the catalysts created by post-deposition of MnO onto the Rh are less effective is that the MnO migrates away from the Rh surface. 5. Conclusions We apply ALD to modify the surface and interface structure of Rh catalysts supported on silica in a controlled fashion to investigate the promotion effect of MnO for higher oxygenate synthesis. We find that support surface modification by ALD-MnO can achieve large enhancements in overall activity and C2+oxy selectivity. Facilitating CO dissociation at Rh-MnO interface sites and stabilizing the transition state for C2+oxy formation are proposed to be the primary contributions for improved C2+oxy production. A physical overcoating layer of MnO on the Rh surface leads to increased activity and higher hydrocarbon selectivity, but it is less


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effective in improving C2+oxy selectivity. Neither particle size effects or the formation of mixed oxides or alloys appear to be the major promotion factors in the ALD-modified catalysts; however, these could account for the increased C2+oxy production of Rh-Mn/SiO2 catalysts synthesized by co-impregnation. This study’s methodology for controllable modification of catalyst structure and separation of various support or promoter effects could also be applied to other heterogeneous catalysis problems. This method could provide improved understanding of the active reaction sites in supported metal catalysts and be applied to synthesize highly controllable heterogeneous catalysts.

Associated content Supporting information Syngas conversion activity and selectivity of Rh/MnO/SiO2 catalysts with different Rh loading; Structural illustration of MnO promoted Rh surface models used in DFT calculation; Free energy diagram of syngas conversion towards methane, acetaldehyde and ethanol on unpromoted and MnO promoted Rh (111) surface; Comparison between ALD and incipient wetness impregnation deposited MnO/Rh/SiO2 Author Information Corresponding Author *E-mail: [email protected] (S. F. Bent) Acknowledgements


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We gratefully acknowledge the support from the Global Climate and Energy Project (GCEP) at Stanford and the U.S. Department of Energy, Office of Basic Energy Sciences to the SUNCAT Center for Interface Science and Catalysis through the SUNCAT-FWP. JSY acknowledges the support from U.S. Department of State via the International Fulbright Science and Technology award program. A part of this work was supported by a postdoc fellowship of the German Academic Exchange Service (DAAD) to Julia Schumann. Nuoya Yang would like to thank Professor Thomas F. Jaramillo for helpful discussion. References (1)

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