PtMo Bimetallic Catalysts Synthesized by Controlled Surface

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PtMo Bimetallic Catalysts Synthesized by Controlled Surface Reactions for Water Gas Shift Canan Sener, Thejas S. Wesley, Ana C. Alba-Rubio, Mrunmayi D. Kumbhalkar, Sikander H Hakim, Fabio H Ribeiro, Jeffrey T Miller, and James A. Dumesic ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02028 • Publication Date (Web): 13 Jan 2016 Downloaded from http://pubs.acs.org on January 18, 2016

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PtMo Bimetallic Catalysts Synthesized by Controlled Surface Reactions for Water Gas Shift

Canan Senera, Thejas S. Wesleya, Ana C. Alba-Rubioa, Mrunmayi D. Kumbhalkara, Sikander H. Hakima, Fabio H. Ribeirob, Jeffrey T. Millerc, James A. Dumesica* a

Department of Chemical and Biological Engineering, University of Wisconsin-Madison, 1415

Engineering Drive, Madison, WI 53706, USA b

School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West Lafayette,

IN 47907-2100, USA c

Chemical Sciences and Energy Division, Argonne National Laboratory, 9700 S. Cass Ave,

Building 200, Argonne, IL 60439-4837, USA * Corresponding Author e-mail: [email protected]

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ABSTRACT Supported PtMo bimetallic catalysts were prepared by controlled surface reactions (CSR) and studied for water gas shift (WGS) at 543 K. Carbon and silica supports were used for the preparation of monometallic Pt catalysts, and Mo was deposited onto these catalysts by reaction with cycloheptatriene molybdenum tricarbonyl ((C7H8)Mo(CO)3). Catalysts were characterized by CO chemisorption, inductively coupled plasma-atomic emission spectroscopy (ICP-AES), STEM/EDS and XAS analysis. We report that carbon-supported Pt nanoparticles are saturated with Mo species at a Mo:Pt atomic ratio of 0.32. Molybdenum has a strong promotional effect in these catalysts, increasing the TOF by up to a factor of more than 4000. Silica-supported catalysts were found to be more active, but the TOF promotional effect of Mo was smaller than that of the carbon-supported catalysts at 15. EDS analyses and activity studies showed that the formation of bimetallic catalysts was therefore more efficient using the carbon support. The active sites for WGS are suggested to be at the interface between Pt atoms and Mo moieties that are possibly in an oxidized form.

KEYWORDS: bimetallic catalysts, controlled surface reactions, water gas shift, Platinum, Molybdenum, support effect, X-ray absorption spectroscopy, STEM/EDS


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1. Introduction The water-gas shift (WGS) reaction has been widely used for the production of hydrogen from synthesis gas, and its importance has increased with the use of gasification for power generation.1 Additionally, WGS is beneficial for the purification of hydrogen in mitigating CO poisoning of catalysts used in ammonia synthesis and fuel cell applications.2 Because WGS is equilibrium-limited at high temperatures, the reaction is commercially carried out in two steps: a high temperature step that exploits high catalytic activity over an iron-based catalyst at 623-673 K, followed by a low temperature step for high CO conversion over a copper-based catalyst at 463-503 K.3 These Cu-based catalysts are pyrophoric, deactivate by leaching, and are poisoned by impurities in the syngas feed.1 However, noble metals—platinum in particular—and their alloys have attracted attention due to their stability and high activity for low temperature WGS.48

The reactivity of Pt for WGS can be enhanced by introducing a metal oxide promoter. In this respect, ceria, zirconia, and titania supports have been found to show synergistic interactions with Pt.9-13 Schweitzer et al. synthesized Mo2C-supported Pt catalysts with close interaction of the metal precursor with the support, having higher WGS activity than oxide-supported Pt catalysts39. Very recently, the Ribeiro group studied the WGS activity of Pt on Mo2C40,41. They suggested that the active sites are formed by Pt-Mo alloy nanoparticles in contact with Mo2C. Water activation on Mo2C is suggested to be the cause for the higher WGS rate over Pt/Mo2C40,41. Similarly, the catalytic activities of Pt-based catalysts have been shown to increase with the addition of oxophilic promoter metals, such as molybdenum and rhenium.14, 3, 4, 10 In both cases, the role of the promoter is to stabilize adsorbed water and hydroxyl groups, thereby 3 ACS Paragon Plus Environment

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increasing their respective coverages substantially, since the dissociative adsorption of water on Pt(111) is endothermic by 0.33 eV.15 It follows that a Pt-promoter synergy arises from the coupling of this facile water activation with the irreversible binding of CO on Pt, giving rise to a two-site, carboxyl-mediated mechanism, which has been previously proposed.16,17 Therefore, a close interaction between Pt and the promoter metal is required to realize the full potential of these bimetallic catalysts. Preparing bimetallic catalysts with close interactions between the constituting metals is an important goal in catalysis research. Most commonly, co-impregnation and successive impregnation methods are applied to prepare bimetallic catalysts. In this approach, a monometallic parent catalyst is prepared and subsequently modified by the incorporation of a second metal. In general, this approach does not necessarily allow for control over the particle size and composition of the synthesized bimetallic catalyst. Accordingly, other techniques have been developed which allow for a higher degree of control over the particle size and composition of the bimetallic catalyst. These techniques include using surface redox reactions18,19, strong electrostatic adsorption (SEA)20, electroless deposition21, and atomic layer deposition (ALD).22,23 The Basset group has developed a synthetic route for bimetallic catalysts based on the use of surface organometallic chemistry in a one pot, surfactant-free procedure 24. In particular, they use organometallic precursors to deposit the second metal under conditions that favor controlled surface reactions. They first employed this surface organometallic chemistry in 1989 for the synthesis of M-Sn catalysts (M=Rh, Ru and Ni) by the reaction of tetra-n-butyl tin with M/SiO2. More recently, they prepared RhSn, NiSn and RuSn bimetallic catalysts by an organometallic route, and they studied the hydrogenolysis of ethyl acetate to ethanol. They identified the primary products of the reaction to elucidate a mechanistic explanation for the selective 4 ACS Paragon Plus Environment

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hydrogenation of ethyl acetate to ethanol 25. Controlled surface reactions of tetra-n-butyltin with a silica-supported rhodium catalyst were also used to prepare to a new bimetallic phase that is active and selective toward alcohol formation from ethyl acetate28. In general, the effectiveness of this approach is based on understanding the interactions between the organometallic precursor and the monometallic parent catalyst 25-32. In our previous study, we employed controlled surface reactions (CSR) to synthesize bimetallic catalysts having narrow particle size and composition distributions. This approach allowed us to prepare well-defined bimetallic catalysts to study the nature of the active sites.18 An advantage of this approach is the ability to accomplish controlled surface reactions in solution and without the need of high vacuum environments. In the present study, we present results for the WGS activity of PtMo catalysts prepared by CSR. The catalysts were characterized by CO chemisorption, inductively coupled plasmaatomic emission spectroscopy (ICP-AES), scanning transmission electron microscopy/energydispersive X-ray spectroscopy (STEM/EDS), and X-ray absorption spectroscopy (XAS). 2.Experimental 2.1.Catalyst Preparation 2.1.1. Materials Vulcan XC72 (Cabot) and Davisil Grade 646 (Sigma-Aldrich) were used as carbon and silica supports for the catalysts, respectively. Davisil was crushed and sieved to between 60 and 100 mesh (0.150-0.250 mm). Cycloheptatriene molybdenum tricarbonyl ((C7H8)Mo(CO)3, Strem Chemicals, 99%) and anhydrous n-pentane (Sigma-Aldrich) were used without further


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purification. The (C7H8)Mo(CO)3 compound and n-pentane were stored and handled inside a glove box filled with ultra-high purity argon (Airgas). Tetraammineplatinum(II) nitrate (Pt(NH3)4(NO3)2,

Sigma-Aldrich,

99.995%)

and

hydrochloroplatinic

acid

hydrate

(H2PtCl6.xH2O, Sigma-Aldrich, 99.995%) were used as Pt precursors. Ammonium hydroxide solution (ACS Reagent, 28.0-30.0% NH3, Sigma Aldrich) was used during the ion exchange of Pt onto silica. 2.1.2. Synthesis of Pt/C and Pt/SiO2 monometallic catalysts Pt-based monometallic catalysts were prepared by incipient wetness impregnation of H2PtCl6 on carbon and silica supports. The H2PtCl6 required to achieve a particular metal loading was weighed and dissolved in sufficient Milli-Q water to reach the wetness point of the support, which was found to be 1.7 mL (g carbon)-1 and 1.2 mL (g silica)-1. The dissolved precursor solution was then added to the support, and the impregnated support was dried at 383 K. The Pt/C and Pt/SiO2 catalysts were reduced at either 533 K or 723 K under flowing H2 for 4 hours (1 K/min ramp rate) and were then passivated with a mixture of 1% O2 in Ar at room temperature for one hour. 2.1.3. Synthesis of Pt/SiO2 monometallic catalysts by ion exchange The ion exchange procedure described by Williams, et al. was used to prepare a Pt/SiO2 catalyst.14 Briefly, Davisil was stirred in 0.1 M HNO3 to clean the silica surface prior to catalyst preparation, filtered, washed with Milli-Q water until the filtrate was neutral, and then dried at 383 K. Clean and dried Davisil was suspended in water and NH4OH was added to adjust the pH to 10.0. The precursor solution was prepared by dissolving the required quantity of Pt(NH3)4(NO3)2 in a solution of water and NH4OH. The precursor solution was added to the 6 ACS Paragon Plus Environment

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silica slurry followed by stirring for 15 minutes. The slurry was then filtered and washed with Milli-Q water. The catalyst was dried overnight, reduced at 533 K under hydrogen flow for 4 hours, and passivated with a mixture of 1% O2 in Ar at room temperature for 1 hour. 2.1.4. Synthesis of PtMo bimetallic catalysts by controlled surface reactions (CSR) PtMo bimetallic catalysts were prepared by controlled surface reaction techniques, as described elsewhere.18 Briefly, the passivated parent catalyst (Pt/C or Pt/SiO2) was reduced at 533 K (Pt/C and Pt/SiO2) or 723 K (Pt/SiO2) in a Schlenk tube prior to the CSR procedure. The reduced parent catalyst was cooled to room temperature and sealed. The required amount of molybdenum precursor, cycloheptatriene molybdenum tricarbonyl, was weighed and dissolved in n-pentane under an inert atmosphere (ultra-high purity Argon) using a glove box. The sealed Schlenk tube was placed in the glove box, unsealed and the precursor solution was added to the reduced parent catalyst. The resulting slurry was stirred for 1 hour. Molybdenum uptake by the parent catalyst caused a concomitant change in color of the precursor solution. The initially orange slurry became colorless as molybdenum was deposited from the precursor solution onto the parent catalyst (Figure S.1). Any solvent remaining after molybdenum uptake was evaporated in a Schlenk line. The dried catalyst was then reduced at 773 K under flowing hydrogen for 45 minutes and passivated with a mixture of 1% O2 in Ar at room temperature for 0.5 hour. To prepare catalysts with higher molybdenum content, the procedure described above was repeated multiple times (i.e., a multi-cycle synthesis). Catalysts were reduced at 673 K between each cycle and the final reduction was performed at 773 K. With each successive cycle, the total theoretical Mo:Pt atomic ratio was increased by 0.15. Using this procedure, PtMo/C catalysts were synthesized with Mo:Pt=0.15 (1 cycle), Mo:Pt=0.30 (2 cycles), Mo:Pt=0.45 (3


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cycles), Mo:Pt=0.60 (4 cycles), Mo:Pt=1:0.75 (5 cycles) and PtMo/SiO2 catalysts with Mo:Pt=0.15 (1 cycle), and Mo:Pt=0.30 (2 cycles). 2.2.Catalyst Characterization 2.2.1. CO Chemisorption Carbon monoxide chemisorption studies were carried out using a Micromeritics ASAP 2020C system. The catalysts were reduced under H2 flow at 473 K. After reduction, CO adsorption was performed at 308 K. The stoichiometry for adsorption of CO on surface Pt atoms was assumed to be 1:1. H2 chemisorption studies were carried out as described by Williams et al.14 2.2.2. Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) The Pt and Mo contents of the catalysts were determined by ICP analysis. A PerkinElmer 400 ICP Emission Spectrometer was used for the determination of the composition of the monometallic and bimetallic catalysts. Typically, 50 mg of catalyst samples was digested in 10 g of aqua regia by refluxing at 423 K for 14 hours. The post-digestion mixture was cooled to room temperature, diluted in water, centrifuged, and filtered. 2.2.3. Scanning Transmission Electron Microscopy/Energy Dispersive X-Ray Spectroscopy (STEM/EDS) A FEI Titan STEM with Cs probe aberration corrector operated at 200 kV with spatial resolution < 0.1 nm was used for scanning transmission electron microscopy (STEM) studies. A high-angle annular dark-field (HAADF) mode, with HAADF detector angle ranging from 54 to 270 mrad, probe convergence angle of 24.5 mrad, and probe current of ~25 pA was used to


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record the images. The energy-dispersive X-ray spectroscopy (EDS) results were obtained on the same microscope with convergence angle of 24.5 mrad and beam current of 640 pA, with spatial resolution 0.5 nm. The sample preparation procedure was carried out as described elsewhere.18, 19 2.2.4. X-ray Absorption Spectroscopy (XAS) Pt LIII-edge (11.564 keV) and Mo K-edge (20.000 keV) X-ray absorption measurements were conducted on the bending magnet beamline of the Materials Research Collaborative Access Team (MRCAT, 10-BM) at the Advanced Photon Source (APS) at Argonne National Laboratory. Measurements were made in transmission scan mode with steps from 250 eV before the edge and 1000 eV beyond the edge for both edges. Ionization chambers were optimized for linear response (ca. 1010 photons s-1) using a mixture of He, N2 and Ar gases for 10% and 70% absorption in Io and It respectively. The Pt or Mo foil spectrum was acquired simultaneously with each catalyst scan for energy calibration. The catalysts were treated in a continuous flow reactor, which consisted of a quartz tube (1 in. OD, 10 in. length) sealed with Kapton film windows by Ultra-Torr fittings at the ends. These fittings had ball valves welded onto them to allow gas to flow through. Catalysts were pressed into self-supporting wafers in a cylindrical holder consisting of six wells (six shooter), and the catalyst amount was calculated to give an absorbance (µx) of ~1.0. This six-shooter was placed inside the flow reactor in contact with a K type thermocouple (Omega), and the catalysts were reduced with flowing 3.5% H2/He. The catalysts were reduced at the desired temperature for 1 hour, purged with He for 15 min, and then cooled to room temperature in He. All XAS spectra were collected on reduced catalysts at room temperature under a static He environment.


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The normalized, energy calibrated spectra were obtained by standard protocol using the WinXAS 3.2 software. Edge energy was determined from the maximum in the first derivative of the X-ray absorption near edge structure (XANES) spectrum. Experimental phase shift and backscattering amplitudes functions were obtained from the Pt foil for Pt-Pt (12 at 2.77 Å), and Mo foil for Mo-Mo (8 at 2.76 Å). In addition to the foils, reference compounds, namely Mo2C (3 Mo-C at 2.09 Å and 12 Mo-Mo at 2.97 Å) and Na2MoO4 (4 Mo-O at 1.77 Å) were also used. Theoretical phase and amplitude functions for Pt-Mo and Mo-Pt were calculated from a scattering pair using the FEFF6 software. The EXAFS parameters were obtained by a least square fit in R-space of the k2-weighted Fourier transform (FT) data.

2.3. Catalytic Activity Studies A schematic diagram of the reactor system is given in Figure S.2. CO (99.9%, Airgas) was pretreated prior to reaction to remove iron carbonyls by flowing CO at atmospheric pressure over silica chips heated to 503 K. H2 and He (Industrial grade, Airgas) were used without further purification. Milli-Q grade water was used as a WGS feed component. All reactions were carried out in a ½ -inch OD stainless steel fixed-bed reactor operated in a down-flow configuration. 25-1000 mg of catalyst was diluted to a total mass of 2100 mg using inert, low surface area silica chips (Silicon Dioxide, fused, 4-20 mesh, Sigma Aldrich) crushed to –35 mesh (< 0.5 mm). The diluted catalyst was placed between two plugs of quartz wool (Grace) in the center of the reactor. Uncrushed silica chips filled the reactor upstream and downstream of the catalyst bed. The reactor was situated between two aluminum heating blocks within a clamshell furnace (Applied Test Systems) connected to a variable autotransformer and


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PID temperature controller (Love Controls Series 16A). Temperature was measured with a Ktype thermocouple in contact with the reactor at the catalyst bed height. Before reaction, the catalyst was reduced under 75 cm3(STP)/min of 35% H2/He at 573 K (2.3 K/min ramp) for 2 hours. Ion exchange Pt/SiO2 catalysts were reduced at 773 K. After reduction the reactor was cooled to the reaction temperature under the reducing mixture. All reactions were carried out at 543 K and atmospheric pressure. The feed composition was 10 mol% CO and 20 mol% H2O with balance He at a total flow rate of 100 cm3(STP)/min. Gas flows were regulated by thermal mass flow controllers (Brooks Instrument). A syringe pump (Harvard Apparatus PHD Ultra) was used to feed the water into a 373 K preheating zone directly upstream of the reactor. Unreacted water was condensed in a separator at 273 K, and the dry effluent flowed to an online Shimadzu GC-8A equipped with a TCD and Alltech HayeSep DB column, to quantify CO and CO2. Monometallic Pt catalysts exhibited initial deactivation, possibly due to surface saturation by CO, followed by stable reactivity. The steady-state rates are reported. PtMo catalysts also demonstrated initial deactivation followed by slow first-order deactivation (Figure S.3 and Table S.1). The reported PtMo rates were obtained by extrapolating the first-order deactivation regime following the initial deactivation period to time zero. Pt/C reactivity was below our detection limit at 543 K, and its reactivity at 543 K was extrapolated from rates measured at 573, 598, and 623 K. The error (95% confidence) in rate per mass of catalyst is estimated to be 10% based on repeated trials of selected catalysts. Internal mass transport limitations were predicted to be negligible for all catalysts according to the Weisz-Prater criterion. Details for calculation of Weisz-Prater numbers are presented in the Supporting Information (Table S.2).


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3. Results and Discussion 3.1.Characterization Results 3.1.1. Pt Based Monometallic Catalysts Pt supported on carbon was prepared by incipient wetness impregnation (Pt/C); Pt supported on silica was synthesized by incipient wetness impregnation (Pt/SiO2); and Pt supported on silica was prepared by ion exchange (Pt/SiO2-IE). For all monometallic parent catalysts, the theoretical Pt loading was set to be 5 wt%. The actual Pt loading was determined by ICP and average Pt particle sizes of the samples were determined by STEM. Pt dispersions were calculated from ICP and chemisorption data. Table 1 summarizes these characterization results for the Pt monometallic catalysts. The CO uptake of the Pt/C catalyst was measured to be 147 µmol/g with a Pt dispersion of 50%. The silica-supported Pt catalyst had a lower dispersion than the carbon supported sample (26%). This behavior is due to repulsive forces between the negatively charged support and H2PtCl6 at the pH of the impregnation procedure (20). The ion exchange procedure was followed to prepare a Pt based parent catalyst with smaller and more narrowly distributed Pt nanoparticles. The average metal particle size was smaller for the catalyst prepared by ion exchange, and the Pt particle size distribution was narrower compared to the incipient wetness impregnation procedure (Figure S.4). The average particle sizes calculated by using the formula d(nm)=110/D% (where D% is the percent dispersion) also support the STEM data (Table 1). As will be discussed later, the effectiveness of the CSR method on SiO2 is improved by increasing the temperature of the pre-synthesis parent reduction. Reduction of the Pt parent catalyst at higher temperature (723 K) did not significantly alter the Pt dispersion (Pt/SiO2-HRT).


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The CO uptake decreased by only 10%, from 74 µmol/g to 67 µmol/g. The Pt particle size increased slightly and the particle size distribution broadened (Figure S.4). In Figure S.5, slight sintering of the Pt nanoparticles can be observed by STEM. Table 1. Characterization results of Pt monometallic catalysts

Average Particle Size

Pt wt% Catalyst ID

Average Particle Size

CO uptake -1

Dispersion

(ICP)

(nm)*

(nm)**

(µmol g )

(%)

Pt/C a

5.7

-

2.2

147

50

Pt/SiO2 a

5.6

2.32

4.2

74

26

Pt/SiO2-IE b

4.3

1.85

2.2

111

50

Pt/SiO2-HRT c

5.0

2.95

4.2

67

26

a

Incipient Wetness Impregnation, reduced at 533 K

b

Ion Exchange, reduced at 533 K

c

Incipient Wetness Impregnation, reduced at 723 K

* Determined by STEM ** Calculated by CO chemisorption: d(nm)=110/D%

3.1.2. PtMo Bimetallic Catalsyts Characterization results for the PtMo bimetallic catalysts supported on carbon and silica are summarized in Table 2. The CO uptake of the Pt/C parent was 147 µmol/g. In a control experiment, the parent site density reduced to 91 µmol/g due to sintering after carrying out the CSR procedure in the absence of any Mo precursor. After incorporation of molybdenum by CSR with a Mo:Pt ratio of 0.15, the CO uptake decreased to 84 µmol/g (8% decrease with respect to the CSR control), suggesting that Pt sites were partially covered by Mo species. Addition of more Mo to produce Mo:Pt=0.30 by two cycles and Mo:Pt=0.45 with three cycles showed 13 ACS Paragon Plus Environment

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decreases of 16% and 43% in CO uptake, respectively. The CO uptake remained constant after the fourth and fifth cycles. Figure 1 shows that the bimetallic particle size distributions obtained by STEM analysis were largely unchanged with each successive cycle. Table 2. Characterization results of carbon-supported PtMo catalysts prepared by CSR

Catalyst ID

Atomic

Atomic

Mo:Pt

Mo:Pt

Pt wt%

Mo wt%

Atomi c Mo:Pt

CO uptake -1

Disp.

(Theo.)

(ICP)

(ICP)

(ICP)

(EDS)

(µmol g )

(%)

0

-

5.7

0

-

147

50

CSR-PtMo/C-1ba

0.15

0.17

4.4

0.4

0.11

84

37

CSR-PtMo/C-2ba

0.30

0.33

4.2

0.7

0.21

76

35

CSR-PtMo/C-3ba

0.45

0.47

4.4

1.0

0.34

52

23

CSR-PtMo/C-4ba

0.60

0.54

4.6

1.2

0.30

58

24

CSR-PtMo/C-5ba

0.75

0.61

4.5

1.4

0.32

63

27

0

-

5.6

0

-

74

26

CSR-PtMo/SiO2- 1c

0.15

0.14

5.0

0.3

-

56

22

CSR-PtMo/SiO2- 2c

0.30

0.21

4.7

0.5

0.14

35

15

0

-

4.3

0

-

111

50

0.30

0.21

4.7

0.5

Very small particles

85

35

0

-

5.0

0

-

67

26

CSR-PtMo/SiO2-HRT-2c

0.30

0.21

5.6

0.6

0.37

24

8

PtMo/SiO2b

1.3

1.17

2.0

1.1

-

18

Pt/Ca

Pt/SiO2

Pt/SiO2-IE CSR-PtMo/SiO2-IE- 2c Pt/SiO2-HRT

a

Catalysts described in Alba-Rubio et al.34

b

Catalyst prepared by the method of Williams et al.14

18


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The metal compositions of the catalysts were determined by ICP. These ICP values were found to be close to the theoretical values, especially for lower Mo loadings. Additionally, single nanoparticles were analyzed by EDS, and the results of these EDS measurements were in agreement with the ICP and theoretical values at lower Mo contents. The Mo atomic percentage obtained by EDS was slightly lower than the values obtained by ICP, indicating some deposition of Mo on the support instead of on the Pt nanoparticles. For 1-, 2- and 3-cycle catalysts, 62-71% of Mo is on Pt according to comparative EDS (number averaged values of 30-50 individual nanoparticles) and ICP (bulk catalyst) results. After the 4th cycle, the deposition of Mo onto Pt nanoparticles decreased to 56% and it decreased further to 52% for the 5-cycle catalyst. Moreover, the EDS Mo:Pt ratios of the 3-, 4-, and 5-cycle catalysts remain nearly constant at about Mo:Pt = 0.32, indicating that Pt nanoparticles become saturated with Mo at this ratio. This saturation occurred at a bulk (i.e., ICP) Mo:Pt ratio of 0.47. Figure 2 shows the variation in the number of surface Pt sites, the Mo:Pt ratio determined by EDS, and the Mo:Pt ratio determined by ICP versus the CSR cycle-number for the carbon-supported catalysts.


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Fraction

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Fraction

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

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(a)

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

Fraction

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1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

(b)

(c)

(d)

(e)

Page 16 of 48

Number of particles analyzed= 1309 Particle size= 1.26 ± 0.55 nm





Diameter (nm)

Figure 1. Particle size distributions of (a) CSR-PtMo/C-1b, (b) CSR-PtMo/C-2b, (c) CSRPtMo/C-3b, (d) CSR-PtMo/C-4b, (e) CSR-PtMo/C-5b.The errors reported are the standard deviation of the mean. 16 ACS Paragon Plus Environment

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ACS Catalysis

Figure 2. Characterization summary of carbon-supported PtMo catalysts by CSR (*Control experiment consisting of 1 CSR cycle performed without using the Mo precursor). Comparison of silica-supported PtMo bimetallics with carbon-supported catalysts shows that the silica-supported sample had less effective deposition of Mo onto Pt (Figure 3a, 3b) and larger Pt particles sizes (Figure 4a, 4b). This behavior might be caused by the smaller number of surface Pt sites on the silica support. As mentioned above, the average Pt particle size for the carbon-supported PtMo catalyst prepared by 2 cycles (CSR-PtMo/C-2b) is 1.31 nm, while that for the corresponding silica-supported PtMo catalyst (CSR-PtMo/SiO2-2c) is 3.39 nm. The lower selectivity of the CSR Mo precursor towards Pt on the silica-supported parent could also be due to reactive hydroxyl groups on silica surface. To partially remove such hydroxyl groups and therefore provide a more inert support surface, a Pt/SiO2 catalyst was reduced in a Schlenk tube at 723 K instead of 533 K prior to CSR synthesis. EDS analysis of CSR-PtMo/SiO2-HRT-2c shows a marked improvement in Mo uptake (Figure 3c) when 17 ACS Paragon Plus Environment

ACS Catalysis

compared to CSR-PtMo/SiO2-2c (Figure 3b). This result is in agreement with CO chemisorption results. The site densities for PtMo/SiO2 and PtMo/SiO2-HRT respectively constitute 53% and 66% decreases in CO uptakes in comparison to each corresponding parent, suggesting improved Mo uptake by Pt on the high-reduction catalyst.

(a)

20

Number of particles

25.3 at% Mo 15

10

5

0 0

10

20

30

40

50

60

70

80

90

100

60

70

80

90

100

60

70

80

90

100

at% Mo

(b)

20

Number of particles

17.8 at% Mo 15

10

5

0 0

10

20

30

40

50

at% Mo 15

(c)

17.9 at% Mo

Number of particles

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

Page 18 of 48

10

5

0 0

10

20

30

40

50

at% Mo

Figure 3. EDS determined compositions of (a) CSR-PtMo/C-2b, (b) CSR-PtMo/SiO2-2c, (c) CSR-PtMo/SiO2-HRT-2c.Vertical lines indicate the ICP-AES values. 18 ACS Paragon Plus Environment

Page 19 of 48

Fraction

(a)

Fraction

(b)

(c) Fraction

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ACS Catalysis

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0




Figure 4. Particle size distributions of (a) CSR-PtMo/C-2b, (b) CSR-PtMo/SiO2-2c, (c) CSRPtMo/SiO2-HRT-2c. The errors reported are the standard deviation of the mean.

X-ray absorption studies were carried out to probe the structure of the PtMo bimetallic catalysts after reduction pretreatment. Comparison of the EXAFS spectra of a Mo foil and PtMo/C indicates contributions of scattering from both metal-metal coordination and from 19 ACS Paragon Plus Environment

ACS Catalysis

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Page 20 of 48

coordination between the metal and a light element (e.g., O or C). The average bond lengths of Mo=O, Mo-O and Mo-Mo from reference compounds are 1.69 Å, 1.77 Å and 2.76 Å respectively.36 We were able to obtain good fits (Tables 3 and 4 for the Pt LIII-edge and Mo Kedge, respectively) with Mo-O bond distance of 2.06 Å, suggesting that not all Mo in the particles is in the form of MoO2 or MoO3, but rather is a highly reduced mixture of different Mo oxidation states. The Pt-Pt coordination from the Pt LIII-edge is almost constant, suggesting that the metal particle size does not change considerably over this range of particle composition. This result is in agreement with the STEM results in Figure 1. As the number of Mo addition cycles increased, the Pt-Mo coordination was observed to increase for the samples reduced at 673 K. The STEM particle size distribution showed that the silica supported PtMo catalysts had larger nanoparticles. This result was also observed in the EXAFS results shown in Figure 5, where the total Pt coordination was higher for the catalysts with silica as a support as compared to carbonsupported catalysts. Fits for the Mo K-edge showed lower total coordination for Mo as compared to the total Pt coordination. In addition, Mo-Mo interactions were not observed in any of the samples. As the Mo loading increased, the Mo-O coordination increased along with a decrease in the Mo-Pt coordination. Furthermore, the Mo-O coordination is lowered when the reduction temperature is increased from 473 to 673 K. In accordance with previous findings33, these results suggest that Mo initially favors entering the Pt nanoparticles, but is increasingly coordinated to a light element (i.e., present near the surface) as the Mo content increases and reduction temperature decreases. MoOX/C species may also contribute to the Mo-O coordination after saturation of the Pt nanoparticles with Mo. 20 ACS Paragon Plus Environment

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ACS Catalysis

Figure 5. Magnitude of k2-weighted Fourier transform of EXAFS data of Pt LIII-edge for CSRPtMo/C-2b (red), and CSR-PtMo/SiO2-2c (blue) (∆k = 2.7–12.0 Å-1 and ∆R = 1.8–3.2 Å-1)

The Mo K-edge XANES can be fit as a linear combination of contribution from Mo foil and MoO2; however, the edge energy is also in agreement with that of Mo-C in Mo2C. Since the measurements were obtained for catalysts for both the carbon supported and silica supported samples, the origin of the possible carbon species could be the organometallic precursor (cycloheptatriene molybdenum(0) tricarbonyl) used for Mo. The experimental XAS data, thus, were fit assuming the formation of molybdenum carbide (Tables S.3 and S.4). The fits for both the Pt LIII-edge and Mo K-edge assuming formation of molybdenum carbide have nearly the same trends as the fits assuming molybdenum oxide.


ACS Catalysis

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Table 3. EXAFS fits of Pt LIII-edge for PtMo/C catalysts reduced at varying reduction temperatures assuming formation of molybdenum oxide ∆σ2 x103 Treatment/Scan Sample Scatterer N R (Å) E0 (eV) (Å) condition CSR-PtMo/C-1b

H2 473 K/ He RT

H2 673 K/ He RT

CSR-PtMo/C-2b

H2 473 K/ He RT

H2 673 K/ He RT

CSR-PtMo/C-3b

H2 473 K/ He RT

H2 673 K/ He RT

CSR-PtMo/C-4b

H2 473 K/ He RT

H2 673 K/ He RT

CSR-PtMo/C-5b

H2 473 K/ He RT

H2 673 K/ He RT

CSR-PtMo/SiO2-1c

H2 473 K/ He RT

H2 673 K/ He RT

CSR-PtMo/SiO2-2c

H2 473 K/ He RT

H2 673 K/ He RT

Pt-Pt

7.8

2.71

2.0

-3.6

Pt-Mo

1.7

2.69

2.0

14.7

Pt-Pt

7.5

2.71

2.0

-3.8

Pt-Mo

1.5

2.69

2.0

10.8

Pt-Pt

7.0

2.71

2.0

-3.8

Pt-Mo

1.7

2.69

2.0

11.0

Pt-Pt

7.0

2.71

2.0

-3.9

Pt-Mo

1.9

2.69

2.0

8.9

Pt-Pt

7.2

2.71

2.0

-3.8

Pt-Mo

1.8

2.69

2.0

12.1

Pt-Pt

7.2

2.71

2.0

-4.0

Pt-Mo

2.0

2.69

2.0

9.3

Pt-Pt

7.3

2.71

2.0

-3.9

Pt-Mo

2.0

2.69

2.0

11.8

Pt-Pt

7.1

2.71

2.0

-4.2

Pt-Mo

2.3

2.69

2.0

8.9

Pt-Pt

7.1

2.71

2.0

-3.9

Pt-Mo

1.9

2.69

2.0

11.8

Pt-Pt

6.9

2.71

2.0

-4.2

Pt-Mo

2.4

2.69

2.0

8.1

Pt-Pt

10.2

2.74

2.0

-2.0

Pt-Mo

0.5

2.72

2.0

13.2

Pt-Pt

9.8

2.74

2.0

-1.5

Pt-Mo

0.8

2.72

2.0

12.8

Pt-Pt

10.0

2.74

2.0

-2.3

Pt-Mo

0.4

2.72

2.0

14.2

Pt-Pt

9.9

2.74

2.0

-2.1

Pt-Mo

1.3

2.72

2.0

12.0

The estimated uncertainties are: N, ±10%; R, ±0.02 Å


Page 23 of 48

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ACS Catalysis

Table 4. EXAFS fits of Mo K-edge for PtMo/C catalysts reduced at varying reduction temperatures assuming formation of molybdenum oxide ∆σ2 x103 Treatment/Scan Sample Scatterer N R (Å) E0 (eV) (Å) condition CSR-PtMo/C-1b

H2 473 K/ He RT

H2 673 K/ He RT

CSR-PtMo/C-2b

H2 473 K/ He RT

H2 673 K/ He RT

CSR-PtMo/C-3b

H2 473 K/ He RT

H2 673 K/ He RT

CSR-PtMo/C-4b

H2 473 K/ He RT

H2 673 K/ He RT

CSR-PtMo/C-5b

H2 473 K/ He RT

H2 673 K/ He RT

CSR-PtMo/SiO2-1c

H2 473 K/ He RT

H2 673 K/ He RT

CSR-PtMo/SiO2-2c

H2 473 K/ He RT

H2 673 K/ He RT

Mo-O

1.7

2.06

2.0

4.8

Mo-Pt

2.6

2.69

2.0

-0.6

Mo-O

0.9

2.06

2.0

6.6

Mo-Pt

4.2

2.69

2.0

1.3

Mo-O

1.5

2.06

2.0

5.5

Mo-Pt

2.2

2.69

2.0

0.2

Mo-O

1.2

2.06

2.0

5.3

Mo-Pt

3.6

2.69

2.0

-1.2

Mo-O

2.0

2.06

2.0

5.7

Mo-Pt

1.9

2.69

2.0

-2.6

Mo-O

1.6

2.06

2.0

4.1

Mo-Pt

3.0

2.69

2.0

-1.6

Mo-O

2.3

2.06

2.0

5.6

Mo-Pt

1.8

2.69

2.0

-6.1

Mo-O

1.9

2.06

2.0

3.8

Mo-Pt

3.0

2.69

2.0

-1.0

Mo-O

2.2

2.06

2.0

5.1

Mo-Pt

1.6

2.69

2.0

-4.0

Mo-O

1.9

2.06

2.0

3.5

Mo-Pt

3.2

2.69

2.0

-1.3

Mo-O

1.3

2.04

2.0

2.0

Mo-Pt

4.3

2.72

2.0

4.8

Mo-O

1.2

2.04

2.0

5.1

Mo-Pt

5.6

2.72

2.0

4.4

Mo-O

1.5

2.04

2.0

4.3

Mo-Pt

3.2

2.72

2.0

4.4

Mo-O

1.5

2.04

2.0

1.0

Mo-Pt

4.4

2.72

2.0

4.9

The estimated uncertainties are: N, ±10%; R, ±0.02 Å


ACS Catalysis

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Page 24 of 48

3.2.Reactivity Results 3.2.1. Activity of carbon-supported PtMo catalysts by CSR Table 5 summarizes the catalytic activities for WGS of carbon-supported catalysts prepared by CSR. It is apparent that the Pt/C monometallic parent catalyst has low activity for WGS. Incorporation of molybdenum onto the parent catalyst, with a Mo:Pt atomic ratio of 0.17, increases the rate per mass of catalyst by more than a factor of 600. Increasing the molybdenum content causes a nearly proportionate, further improvement in reactivity; the catalyst with the highest Mo loading is over 1500 times more reactive than Pt/C on a mass basis. The promotional factor in terms of the turnover frequency reaches a maximum of 4110 at an ICP ratio of Mo:Pt=0.47. For catalysts with higher ICP Mo:Pt ratios, a small decrease in TOF is observed. However, as discussed above, the saturation point for the effective deposition of Mo onto Pt by controlled surface reactions is at Mo:Pt =0.47 by ICP. Thus, the observation that the TOF remains relatively constant in Figure 6 as the ICP Mo:Pt ratio increases above 0.47 is at least partially caused by the non-selective deposition of Mo. Indeed, the catalytic activities of the bimetallic catalysts depend on the composition of the bimetallic nanoparticles rather than that of the bulk catalyst. Therefore, the EDS-determined compositions are potentially more useful than ICP compositions when searching for structure-reactivity relationships.


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ACS Catalysis

Table 5. Reactivity summary of carbon-supported PtMo catalysts

Atomic

CO Uptake

Rate

TOF

Mo:Pt (ICP)

(µmol g-1)

(µmol gcat-1 s-1)

(ks-1)

TOF Promotional Factor

0

147

0.0064

0.044

1

CSR-PtMo/C- 1b

0.17

84

3.9

47

1070

CSR-PtMo/C- 2b

0.33

76

7.9

104

2360

CSR-PtMo/C- 3b

0.47

52

9.4

181

4110

CSR-PtMo/C- 4b

0.54

58

8.8

151

3430

CSR-PtMo/C- 5b

0.61

63

10.0

158

3590

Catalyst ID Pt/C

Figure 6. WGS activities of carbon-supported PtMo catalysts prepared by CSR. Reactions were carried out at 543 K with a feed of 0.10 atm CO, 0.20 atm H2O, and 0.70 atm He.


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To further explore the implications of bimetallic composition on reactivity, we propose a simple model for the active site in the PtMo system. As discussed earlier, the WGS reaction likely proceeds via a two-site mechanism on these promoted catalysts. Thus, a reasonable active site for WGS might be the interface between Pt and Mo, i.e. any surface site with adjacent Pt and Mo species. Approximating the surface as a mean field, the rate per mass of catalyst would be

given by = ( ∏ ) , which predicts the TOF to be proportional to . Here, k is the rate constant; ∈ , , , ; SPt is the number of surface Pt atoms per mass of catalyst; N is the surface coordination of Pt; and is the mole fraction of Mo in the nanoparticle surface, given by =

!

atoms (# ), we find =

$ $ %

. Normalizing each to the total number of Pt

, where & = ⁄# and ( is the Pt dispersion. &

would be equal to the EDS Mo:Pt ratio if Mo were found only on the nanoparticle surface, however, the EXAFS results suggest that a fraction of the Mo is located within the Pt nanoparticle, especially at low loadings of Mo. Therefore, we define a parameter ) equal to the *+, number of sub-surface Mo atoms divided by the total number of Pt atoms, ) = /# , such .% − ). Optimizing the model (01 = 2 ) with respect to ) gives ) = 0.04, that & = &

suggesting partial migration of Mo into the Pt nanoparticle at low Mo loadings. This value of ) corresponds to roughly 90% of the Mo populating the nanoparticle surface when Mo saturates .% the nanoparticles. Figure 7 demonstrates a linear trend between TOF and & = & − ),

supporting the hypothesis of Pt-Mo interfaces as the active sites for WGS. Relevant calculated values are shown in Table S.5. The equilibrium constant of MoO3 reduction (MoO3 + H2 → MoO2 + H2O) is 6x107 at 7

7

673 K, while that of MoO2 reduction (MoO2 + H2 → Mo + H2O) is 2x10-2. Thus, if it is 26 ACS Paragon Plus Environment

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ACS Catalysis

assumed that all surface Mo moieties are coordinated to two oxygen atoms, and subsurface Mo is completely reduced, then we predict a Mo-O coordination number of 2(1 −

: ;< $

). These

predicted Mo-O coordination numbers agree with those values from EXAFS measurements, as seen in Table S.6.

Figure 7. Effect of surface molybdenum nanoparticle mole fraction ( ) on WGS turnover frequency. Reactions were carried out at 543 K with a feed of 0.10 atm CO, 0.20 atm H2O, and 0.70 atm He. As discussed earlier, the EXAFS data show that Mo is coordinated to a light scatterer, which can be either O forming a molybdenum oxide species, or C forming a molybdenum carbide species. If Mo is coordinated with C, then the active site for WGS could be a Mo-carbide species on the Pt surface. The CSR technique selectively deposits Mo onto Pt, thereby minimizing the formation of Mo2C on the support, especially for lower molybdenum loadings, as evidenced by EDS analysis (Figure 2). The linear increase in turnover frequency with increasing


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Mo loading (Figure 7) suggests that the active sites for WGS reaction under the conditions studied here are Pt and Mo moieties (i.e., MoOx or MoCx) in close contact. 3.2.2. Effect of the support To gauge the effect of support on the CSR synthesis and WGS activity, CSR PtMo catalysts were prepared on both carbon and silica supports. Table 6 summarizes the reactivities of silica-supported catalysts. The rate of WGS per mass of the Pt/SiO2 catalyst is 1600 times higher than that of the carbon-supported Pt catalyst. Additionally, the Pt/SiO2 and two-cycle PtMo/C (CSR-PtMo/C-2b) catalysts have similar activities for WGS, suggesting that SiO2 itself is capable of promoting the reaction. Comparison of support effects, however, is possibly complicated by different particle sizes of Pt on carbon and silica. Thus, to determine if the difference in activities is caused by a support effect or a particle size effect, WGS reactivity studies were carried out for Pt/SiO2 catalysts having different Pt particle sizes. Accordingly, a 1.61% Pt/SiO2 catalyst was prepared by ion exchange with two additional dispersions obtained by calcining the dried, unreduced catalyst at 573 K and 673 K. Dispersions were calculated based on CO uptake, and the particle sizes were estimated from dispersion using d(nm) = 1.1/D. The catalysts were reduced in situ at 773 K prior to reaction. The WGS turnover frequencies for these catalysts varied by only a factor of 2 (Figure 8), and the reaction was thus found to be structure insensitive for Pt/SiO2 catalysts. This result is in accord with experimental findings on Pt/Al2O3 and Pt/CeO2.9, 37 Kinetic Monte Carlo simulations also predict Pt to be structure insensitive under conditions similar to the present study.23 We note, however, that these simulations predict Pt(111) to be twice as active as Pt(211) for WGS. 28 ACS Paragon Plus Environment

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ACS Catalysis

In light of the structure insensitivity, we conclude that the difference in activity between the Pt/C and Pt/SiO2 catalysts is due to a support effect. The high activity of silica-supported catalysts is likely due to a facilitated, silica-mediated water activation process. A difference in TOF was observed between Pt/SiO2 and Pt/SiO2-IE, but steaming the Pt/SiO2 catalyst (673 K, 20 mol% H2O/He, 3 hours) did not change its reactivity, suggesting the absence of a chloride effect.

Figure 8. WGS turnover frequencies for Pt/SiO2 catalysts of varying particle size. Platinum nanoparticle diameters are estimated from dispersion. Reactions were carried out at 543 K with a feed of 0.10 atm CO, 0.20 atm H2O, and 0.70 atm He. The silica-mediated promotion diminishes the apparent Mo promotion by raising the activity of the Pt parent catalyst, but still only accounts for less than 10% activity for the silicasupported PtMo catalysts. The promotional factor for the PtMo/SiO2 catalyst with an ICP Mo:Pt atomic ratio of 0.21 is 12. Among the silica-supported catalysts, the highest promotional factor was obtained with the parent prepared by ion exchange. Since the dispersion is higher for the ion


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Page 30 of 48

exchange catalyst, there were more Pt sites for deposition of Mo, possibly leading to improved formation of bimetallic catalysts. Figure 9 summarizes reactivities of selected catalysts. We have shown that increasing the reduction temperature of the silica-supported parent improves the Mo uptake on Pt nanoparticles during CSR synthesis. The improved Mo uptake also leads to an improvement in reactivity. On a TOF basis, CSR-PtMo/SiO2-HRT-2c was found to be 25% more active than CSR-PtMo/SiO2-2c with a TOF of 2080 ks-1 (Table 6). This highreduction catalyst compares favorably with the PtMo/SiO2 catalysts synthesized by Williams et al.14 Replicating the synthesis of their most active catalyst resulted in a PtMo/SiO2 (1.9% Pt, 1.1% Mo) catalyst that gave a TOF of 1330 ks-1. It should be noted that the TOF values obtained here are higher than those reported by William et al.14, as the reactions reported here were conducted with no hydrogen in the feed, and hydrogen is known to be an inhibitor for the WGS reaction42. Thus, a control study was also performed with a PtMo/SiO2 (1.9% Pt, 1.1% Mo) catalyst using the same feed and chemisorption techniques as described in Williams, et al.14. These control measurements gave a TOF value of 236 ks-1, which is only 9% lower than their reported value. Herein, we report the total rates of product formation in a non-differential reactor (i.e., site time yields). In order to estimate the true TOF for the experimental conditions studied, we first calculate the rate constant using the rate expression reported in William et al.14 and the rates and conversion obtained from our experiments. We then use the calculated rate constant to further estimate the true TOF for the partial pressures corresponding to our experimental conditions. Using this analysis, we observe that our reported site time yields are within 5% of the true turnover frequencies.


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ACS Catalysis

As a final comparison, our PtMo/SiO2-HRT catalyst gives a rate per total Pt of 175 µmol CO2 (µmol Pt)-1 ks-1, whereas the highly active Pt/Mo2C catalyst synthesized in the Thompson group39 gives 2830 µmol CO2 (µmol Pt)-1 ks-1 under a complete WGS mixture. Table 6. Reactivity summary of carbon- and silica-supported PtMo catalysts

Atomic Pt:Mo Catalyst ID

CO Uptake -1

TOF Rate

TOF -1 -1

-1

Promotional

(ICP)

(µmol g )

(µmol gcat s )

(ks )

Factor

-

147

0.0064

0.044

1

0.33

76

7.9

104

2364

-

74

10.1

137

1

0.21

35

58

1670

12

67 b

1.9

28.0

1

0.21

85

55

652

23

-

67

9.2

137

1

CSR-PtMo/SiO2-HRT-2c

0.21

23

50

2080

15

PtMo/SiO2c

1.17

18

7.6

1330

Pt/C CSR-PtMo/C-2b Pt/SiO2 CSR-PtMo/SiO2-2c Pt/SiO2-IE a CSR-PtMo/SiO2-IE-2c Pt/SiO2-HRT

-

a

Reduced at 773 K prior to reaction

b

CO uptake after in situ reduction at 773 K

c

Catalyst prepared by the method of Williams et al.14

d

TOF promotional factor with respect to Pt/SiO2

9.7 d


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Figure 9. Turnover frequencies of the monometallic parents and bimetallic counterparts prepared by CSR with C and SiO2 supports (a) Pt/C (b) Pt/SiO2 (c) Pt/SiO2-HRT (d)Pt/SiO2-IE (e) CSR-PtMo/C-1b(Mo:Pt=0.17) (f) CSR-PtMo/C-2b(Mo:Pt=0.33) (g) CSR-PtMo/SiO22c(Mo:Pt=0.21)

(h)

CSR-PtMo/SiO2-HRT-2c(Mo:Pt=0.21)

(i)

CSR-PtMo/SiO2-IE-

2c(Mo:Pt=0.21).

4. Conclusion PtMo bimetallic catalysts were prepared by CSR on carbon and silica supports for WGS. EXAFS and reactivity results indicate that at low Mo loadings Mo preferentially migrates into the Pt nanoparticles and increasingly populates the surface with increasing Mo loading. Deposition of Mo onto Pt nanoparticles reaches saturation by Mo species at a nanoparticle composition of Mo:Pt=0.32. Beyond this Mo loading, bimetallic composition remains constant, and Mo is deposited onto the catalyst support. The turnover frequencies for WGS on these PtMo/C catalysts were found to scale linearly with EDS-based estimates of surface Mo mole


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fraction, suggesting surface Pt-Mo interfaces are the active sites. We show that the catalytic activities of silica-supported catalysts are higher than carbon-supported samples, but the promotional effect of Mo for WGS is higher on carbon support (i.e., promotion by a factor of more than 2000 for Pt/C compared to a factor of 15 for Pt/SiO2). Accordingly, formation of bimetallic nanoparticles and creation of active sites for WGS is more significant on the carbon support.

Associated Content The Supporting Information is available free of charge on the ACS Publications website. Figure S1-S5, Weisz-Prater number calculations, Tables S1-S6.


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Acknowledgements This material is based upon work supported by the U.S. Department of Energy, Office of Basic Energy Sciences (DE-FG02-84ER13183, and the Great Lakes Bioenergy Research Center (GLBRC) (PRJ-65UI). T.S.W. acknowledges support from the University of Wisconsin-Madison Holstrom Environmental Scholarship and Hilldale Undergraduate Research Fellowship. We are thankful for the use of the Advanced Photon Source, an Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory, supported by the U.S. DOE under Contract DE-AC02-06CH11357. The authors acknowledge use of facilities and instrumentation supported by the University of Wisconsin Materials Research Science and Engineering Center (DMR-1121288) and Nanoscale Science and Engineering Center (DMR0832760. The authors acknowledge Thomas J. Schwartz for insightful discussions, and also acknowledge Ali Hussain Motagamwala, Duygu Gerceker and Yifei Liu for their help in obtaining XAS data.


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Graphical abstract 84x38mm (96 x 96 DPI)

ACS Paragon Plus Environment

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Figure 1 112x261mm (96 x 96 DPI)


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Figure 5 235x180mm (300 x 300 DPI)


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PtMo Bimetallic Catalysts Synthesized by Controlled Surface

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