Property of PtâAg Alloy Nanoparticle Catalysts in Carbon Monoxide

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Property of Pt-Ag Alloy Nanoparticle Catalyst in Carbon Monoxide Oxidation Sang Youp Hwang, Changlin Zhang, Eric Yurchekfrodl, and Zhenmeng Peng J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 22 Nov 2014 Downloaded from http://pubs.acs.org on November 23, 2014

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The Journal of Physical Chemistry

Property of Pt-Ag Alloy Nanoparticle Catalyst in Carbon Monoxide Oxidation Sang Youp Hwang, Changlin Zhang, Eric Yurchekfrodl, Zhenmeng Peng* Department of Chemical and Biomolecular Engineering, University of Akron, Akron, OH 44325, United States.

ABSTRACT.

Alumina-supported platinum-silver alloy nanoparticle (Pt-Ag/Al2O3) catalysts with different particle composition have been prepared using a wet chemistry method and studied for the property in carbon monoxide oxidation reaction. The Pt-Ag alloy catalyst exhibits significantly improved activity and largely decreased activation energy barrier comparing to both pure Pt and pure Ag catalysts in the room temperature range. The Langmuir-Hinshelwood mechanism, in which surface Pt active sites chemisorb CO and adjacent Ag sites activate O2, was proposed and discussed for explaining the promoted CO oxidation kinetics on the Pt-Ag alloy. Thermal stability of the Pt-Ag alloy catalyst under the reaction condition was also investigated using in situ Fourier transform infrared spectroscopy experiments and transmission electron microscopy characterizations.

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1. Introduction Catalytic CO oxidation at low temperature has received considerable interest for its practical significance in many applications, for instance air purification,1 closed-cycle CO2 laser,2-4 and CO preferential oxidation (PROX).5-7 Platinum nanoparticles serve as one important group of catalyst, with the Langmuir-Hinshelwood (L-H) mechanism being proposed for the catalytic reaction.8-10 However, Pt exhibits little activity in the room temperature range, which largely hinders the applications.11-12 It is discovered that CO molecules adsorb too strongly to Pt at low temperature, which inhibits O2 adsorption on Pt and causes large energy barrier for CO oxidation.9,

13-14

To promote the reaction at low temperature, many Pt alloy nanoparticles, for

instance Pt-Sn, Pt-Ni, and Pt-Rh, have been researched.13, 15-17 Ag nanoparticles were recently reported as an active catalyst for CO oxidation at low temperature.3, 18-19 Studies suggest that the reaction on Ag follows the Eley-Rideal (E-R) model, which differs from that on Pt.8-10, 20-21 Ag does not adsorb CO but can effectively activate O2,22-23 followed by reaction between activated O2 species and gas-phase CO. Given the experimental findings, Pt-Ag alloy nanoparticles could be excellent catalyst for CO oxidation. In such a catalytic structure, Pt active sites adsorb CO and adjacent Ag sites activate O2. The two species can react efficiently, leading to decreased energy barrier and promoted reaction kinetics. In this work, we report preparation and catalytic property study of Pt-Ag alloy nanoparticle catalyst in CO oxidation. The Pt-Ag alloy nanoparticles with different particle composition are synthesized using wet chemistry method and then put on alumina for making catalysts. The use of Al2O3 rather than reducible oxides as support is for studying the intrinsic property of Pt-Ag alloys by avoiding catalytic contribution from the support materials, because reducible oxide supports can involve in catalysis via the Mars-van Krevelen (MvK) mechanism.24-26 The


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catalytic study verifies the hypothesis and shows superior activity of Pt-Ag alloy than both pure Pt and pure Ag at low temperature. The Pt-Ag also exhibits interesting temperature dependence of activity, the mechanism behind which is carefully investigated using in-situ Fourier transform infrared spectroscopy (FTIR) experiments and high-resolution transmission electron microscopy (HRTEM) characterizations. 2. Experimental Section Materials: Platinum acetylacetonate (Pt(acac)2, 97%) and silver acetylacetonate (Ag(acac), 98%) were purchased from Sigma-Aldrich. Oleylamine (OAm, technical grade 70%), oleic acid (OLA, technical grade 90%), 1,2-hexadecandiol (HDD, 90%, technical grade), and diphenyl ether (DPE, 99%, ReagentPlus) were purchased from Sigma-Aldrich. Alumina nanopowders (Al2O3, Sigma-Aldrich) was used as support materials. Carbon monoxide (CO, 5% balanced Ar), oxygen (O2, 5% balanced Ar), hydrogen (H2, 99.9999%) and argon (Ar, 99.998%) gases were from Praxair. Synthesis of Pt-Ag alloy nanoparticles: All the synthetic experiments were conducted under continuous magnetic stirring and under argon atmosphere using a standard Schlenk line technique. In the synthesis of Pt1Ag2 alloy nanoparticles, Pt(acac)2 (79 mg or 0.20 mmol), Ag(acac) (41 mg or 0.40 mmol), and HDD (0.49 g or 1.9 mmol) were dissolved in a mixture of OLA (0.3 mL or 0.9 mmol), OAm (0.3 mL or 0.9 mmol), and DPE (5 mL or 31.5 mmol) in a 25mL three-neck round-bottom flask. The solution was then gradually heated at 5 °C/min to the refluxing temperature (around 270 °C) and was kept at the temperature for 1 hr. Pt1Ag1 and Pt1Ag3 alloy nanoparticles, as well as pure Pt and pure Ag particles, were synthesized using a similar procedure with different amounts of Pt(acac)2 and Ag(acac). The nanoparticles were washed and precipitated by adding excess amount of ethanol into the product mixture, followed


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by centrifugation at 5000 rpm for 5 min. The precipitated particles were re-dispersed in 10 mL of toluene. Preparation of Pt-Ag/Al2O3: The Pt-Ag alloy nanoparticles dispersion was mixed with designated amount of Al2O3 and was stirred for overnight. The mixture was then washed by adding ethanol and centrifugation at 5000 rpm for 5 min. The washing procedure was repeated for two more times before the sample was collected and dried at around 80 °C. The sample was then calcined for 2 hrs at 300 °C in air and reduced for 1 hr at 300 °C in H2 for preparing the PtAg/Al2O3 catalyst with clean particle surface. Characterizations: Transmission electron microscopy (TEM) images of the samples were taken using a JEOL JEM-1230 microscope operated at 120 kV. High-resolution TEM (HRTEM) images were taken using a FEI Tecnai G2 F20 microscope operated at 200 KV. Powder X-ray diffraction (PXRD) patterns were recorded by a Bruker AXS Dimension D8 X-Ray diffractometer with Cu Kα radiation source (λ = 1.5405 Å). The Pt-Ag particle composition and the metal loading of Pt-Ag/Al2O3 samples were quantitatively analyzed using energy dispersive X-ray (EDX) spectroscopy, which is equipped on a Hitachi TM3000 and operated at 15 kV. In Situ FTIR experiments: In-situ Fourier transform infrared spectroscopy (FTIR) experiments were carried out using a Thermo Scientific Nicolet 6700 spectrometer for studying CO adsorption to and interaction with Pt-Ag alloy nanoparticles under the reaction condition. The FTIR is equipped with a liquid nitrogen (LN2) cooled MCT detector and a diffuse reflectance infrared Fourier transform (DRIFT) system. The catalyst samples were transferred into the reaction chamber of a high temperature/high pressure cell (Praying MantisTM, Harrick Scientific Products, Inc.), which is equipped with KBr windows and a temperature controller.


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The reaction chamber was first purged with Ar for 10 min for removal of air and moisture. The gas was then switched to 100 sccm gas mixture containing 1% CO, 1% O2, and Ar for the balance. The catalyst was heated at 5 °C/min to 200 °C and held at the temperature for 30 min. Time-lapsed spectra were collected continuously every one minute by averaging 32 scans taken with 4 cm-1 resolution. CO oxidation reaction testing: The CO oxidation reaction was tested in a packed bed reactor (PBR). The catalyst powders were first pressed and crushed, and then sieved for preparing catalyst pellets in the 250 - 500 µm size range. 100 mg of catalyst pellets were calcined at 300 °C in air for overnight, and transferred into the PBR. Prior to the CO oxidation test, the catalysts were reduced at 300 °C for 1 hr in 20% H2/Ar for making clean metal nanoparticle surface. The reaction gases consisted of 1% CO, 1% O2, and Ar balance, and had a volumetric flow rate of 100 sccm. The gas reaction products were analyzed using an on-line gas chromatography (GC, Shimadzu GC-2014). The CO conversion (XCO) was calculated using the following equation: [େ୓ ]

(1)

మ X େ୓ = [େ୓]ା[େ୓ × 100% ] మ

where [CO2] and [CO] are concentrations of CO2 and CO in product. ᇱ The CO oxidation rate (−‫ݎ‬஼ை ), i.e. disappearance rate of CO per mass of catalyst, was

determined using the flowing equation:

ᇱ −‫ݎ‬஼ை =

ி಴ೀ,బ ∙௑಴ೀ ∆ௐ೎ೌ೟

=

[஼ை]బ ∙௩బ ∙௑಴ೀ ∆ௐ೎ೌ೟

௩

௩

௉

= ∆ௐబ ∙ [‫ܱܥ‬ଶ ] = ∆ௐబ ∙ ோ்೅ ∙ ‫ݕ‬஼ைమ ೎ೌ೟

(2)

೎ೌ೟


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where FCO,0 is the entrance molar flow rate of CO, ∆Wcat is the amount of catalyst, [CO]0 is the entrance concentration of CO, υ0 is the entrance volumetric flow rate, and [CO2] is the concentration of

CO2 in product. The disappearance rate of CO per unit area of metal

ᇱᇱ ᇱᇱ ᇱ nanoparticles (−‫ݎ‬஼ை ) was calculated using the equation (−‫ݎ‬஼ை = −‫ݎ‬஼ை /ܵ௠௘௧௔௟ ), where Smetal is

specific surface area of the Pt-Ag nanoparticle catalysts. The turn over frequency (TOF) for CO oxidation was calculated as below:

ܱܶ‫= ܨ‬

ᇲᇲ ି௥಴ೀ ∙ேಲ

(3)

ௌᇲ

where NA is the Avogadro’s number and Sʹ is the surface atom density of Pt-Ag nanoparticles. 3. Results and Discussion 3.1. Characterizations of Pt-Ag alloy nanoparticles Pt-Ag alloy nanoparticles were synthesized by simultaneously reducing both metal acetylacetonates in diphenyl ether (DPE) solvent. The particle composition was varied by adjusting the amount of metal precursors.27-28 Figures 1a, c, and e (left column) show TEM of the as-synthesized Pt-Ag nanoparticles with different particle composition. Monodisperse nanoparticles were obtained for Pt1Ag2 and Pt1Ag3 but not for Pt1Ag1, which contains a mixture of particles and worm-like nanostructures. The formation of Pt1Ag1 worm-like structure could be resultant of composition-dependent secondary particle growth.27-28 Statistical size analyses give an average particle diameter of 5.3 ± 0.8 nm for Pt1Ag2 and 5.5 ± 0.8 nm for Pt1Ag3 respectively. The Pt1Ag1 sample has an average diameter of 6.0 ± 1.3 nm for the particles and an average width of 4.0 ± 0.8 nm for the worm-like structure. The Pt-Ag nanoparticles were put on Al2O3 support and thermally treated for preparing Pt-Ag/Al2O3 catalyst. They were found with uniform distribution on Al2O3 and no significant size change comparing to their as-synthesized


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counterparts (Figure 1b, d, and e). It needs noted that the TEM of the Pt1Ag1/Al2O3 found no worm-like structure, which could turn into spherical particles during the thermal treatment. HRTEM characterizations show clear lattice fringes throughout the Pt-Ag particles, suggesting their single crystallinity and alloy formation (inset of Figure 1d). 3.9 ± 0.7 nm Pt (Figure 2 a) and 7.5 ± 1.5 nm Ag nanoparticles (Figure 2 c), as well as Pt/Al2O3 (Figure 2 b) and Ag/Al2O3 (Figure 2 d), were also produced using a similar procedure. Figure 3 shows powder X-ray diffraction (PXRD) patterns of all five catalysts, with information for pure Pt (PCPDFWIN #70-2431) and pure Ag (PCPDFWIN #87-0719) references being included for comparison. The diffraction peaks can be indexed to (111), (200), and (220) planes of face centered cubic (fcc) lattice. The three Pt-Ag/Al2O3 samples exhibit only one set of diffraction peaks and have the peaks positioned between those for pure Pt and Ag, indicating the Pt-Ag alloy formation. The diffractions shifted to lower angle with an increase in the Ag content (guided using the dash line), which agree with the Vegard’s law and further confirm the alloy formation.29 Both the Pt-Ag particle composition and the metal loading in the catalyst were determined using energy dispersive X-ray (EDX) quantitative analyses, with the data being summarized in Table 1. The Pt/Ag atomic ratios in the Pt-Ag particles are close to the ratios of the metal precursors, suggesting both precursors can be effectively reduced during the particles synthesis. The metal loadings of all the samples range from around 4 wt.% to 6 wt.%. 3.2. Catalytic Property of Pt-Ag/Al2O3 in CO oxidation The catalytic property of the Pt-Ag/Al2O3 in CO oxidation was tested at different temperature and comparing with that of Pt/Al2O3 and Ag/Al2O3. Prior to any catalytic test, the Pt-Ag/Al2O3 was first calcined under air at 300 °C and then reduced by H2 at 300 °C for 1 h for generating fresh Pt-Ag surface. HRTEM characterizations suggest the formation of surface Ag oxide layers


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on the particles after the calcination (Figure 4a) and their reduction back into metallic state after the consequent reduction (Figure 4b). The Pt-Ag nanoparticles show good single crystallinity and clean surfaces, suggesting their uniform alloy composition and no surface segregation after the two-step thermal treatment. Figure 5 shows the CO conversion-temperature (XCO-T) profiles for all five samples. The Pt-Ag/Al2O3 exhibited positive XCO at room temperature, indicating they are active catalysts for CO oxidation even under ambient condition. The XCO values varied with the Pt-Ag particle compositions, suggesting composition-dependent reaction kinetics. In comparison, the Pt/Al2O3 remained inactive until the temperature was above around 150 °C. The little activity of Pt at low temperature has been attributed to strong CO adsorption to the surface, which inhibits Pt from effective O2 adsorption and activation.13-14 The Ag/Al2O3 showed some CO oxidation activity at room temperature, which is in consistence with previous studies.3, 18-19 Turnover frequency (TOF) of the CO molecules was calculated for comparing the intrinsic activity (Figure 6). The Pt1Ag2/Al2O3 exhibited the highest TOF among the three Pt-Ag/Al2O3 catalysts at room temperature, suggesting an optimal surface composition for the reaction. The TOF was determined to be 1×10-2 1/s, which was significantly higher than both 8×10-4 1/s of the Pt/Al2O3 and 6×10-3 1/s of the Ag/Al2O3. The data validates the use of Pt-Ag alloys for promoting the CO oxidation kinetics. Dissimilar to the Pt/Al2O3 and the Ag/Al2O3 which showed monotonous increase in XCO with temperature, the XCO-T profile using the Pt1Ag2/Al2O3 can be divided into three segments (Figure 5). The XCO increased exponentially when the temperature was below 80 °C or above 180 °C, whereas the increase in XCO was gradual within the temperature range in between. The Pt1Ag1/Al2O3 and Pt1Ag3/Al2O3 behaved similarly to the Pt1Ag2/Al2O3, although there were some variations in their temperature windows. Interestingly, the Pt1Ag3/Al2O3 became the most


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active when the temperature was increased. It exhibited a TOF of 4.2×10-2 1/s at 100 °C (Figure 6), which was more than two times of that for the Pt1Ag2/Al2O3 (1.6×10-2 1/s) and about 50% higher than that for the Ag/Al2O3 (2.9×10-2 1/s). All three Pt-Ag/Al2O3 became less active than the Ag/Al2O3 with further increase in the temperature. The temperature-dependent catalytic behavior of the Pt-Ag/Al2O3 was likely associated with the particle surface compositions, which could be altered under the reaction condition and consequently affect the reaction kinetics. The activation energy (Ea) for CO oxidation was calculated from the Arrhenius plots (Figure 7). The Ea values using the Pt/Al2O3 and Ag/Al2O3 were determined to be 74.1 and 21.6 kJ/mol, in decent agreement with literatures.18,

30-32

In comparison, the Pt1Ag2/Al2O3 exhibited a

significant smaller Ea of 8.4 kJ/mol at around room temperature. The data was in consistence with higher activity of the Pt1Ag2/Al2O3 and confirmed improved reaction kinetics by alloying Pt and Ag. Previous studies have proposed different reaction pathways for CO oxidation on Pt and on Ag.9,

20

The reaction on Pt follows the L-H mechanism,8-10 in which both CO and O2

molecules chemisorb to the active sites prior to reaction. The little activity of Pt at low temperature is caused by strong CO adsorption, which inhibits effective O2 adsorption. The E-R mechanism has been suggested for CO oxidation on Ag.20-21 In the mechanism O2 first adsorb to Ag surface, which then react with gas phase CO molecules.20-21 The CO oxidation on Pt1Ag2 could follow a modified L-H mechanism, in which surface Pt sites adsorb CO and surface Ag sites adsorb O2. The two types of active sites are in close contact, which allows efficient reaction between adsorbed species and thus a low Ea. The Arrhenius plot for Pt1Ag2/Al2O3 contains three segments, which is in consistence with the XCO-T profile. The non-linear characteristic of the plot between 80 and 180 °C confirms a continuous change of the particle surface in the


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temperature range. The linear characteristic was recovered when the temperature went above 180 °C, suggesting completion of the surface change. 3.3. In-situ FTIR study of working Pt-Ag/Al2O3 catalyst Figure 8 shows the in-situ FTIR spectra of working Pt/Al2O3, Ag/Al2O3, and Pt1Ag2/Al2O3 catalysts under the reaction condition and at different temperature. The Pt/Al2O3 exhibited four absorbance bands at 2172, 2116, 2098, and 2073 cm-1 at around room temperature (Figure 8a). The first two peaks were associated with gas phase CO molecules.33 The other two peaks were caused by linear CO adsorption to different sites on Pt surface.34-36 According to previous studies, the band at 2098 cm-1 can be assigned to CO (L1 species) adsorbing to close packed terraces and the one at 2073 cm-1 is assignable to CO (L2 species) adsorbing to less densely packed sites.34-36 The Pt/Al2O3 exhibited low CO oxidation activity because both types of active sites were covered by CO, which inhibited O2 activation. A negative shift in the L1 peak with an increase in the temperature was observed (Figure 9), which has been attributed to decrease in the dipole-dipole interaction between adsorbed CO.34 Meanwhile, the L2 band shrank and finally disappeared above around 150 °C, indicating less CO adsorption to the less densely packed sites. Interestingly, the temperature was in good coincidence with the onset temperature for CO oxidation. It could be possible that these less densely packed Pt atoms provide actives sites for O2 activation. On the contrary, CO adsorption was not observed on the Ag/Al2O3 through the insitu FTIR experiments, with only gas phase CO absorbance being detected at 2164 and 2117 cm1

(Figure 8b). The data supports the proposed E-R mechanism for CO oxidation on Ag, in which

O2 is activated on Ag surface sites and reacts with gas phase CO.20-21 The in-situ FTIR spectra using the Pt1Ag2/Al2O3 differ from those using both Pt/Al2O3 and Ag/Al2O3 (Figure 8c). Besides the gas phase CO absorbance, only one peak associated with


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linear CO adsorption was observed. The peak was positioned at 2040 cm-1, which had a lower wavenumber comparing to both L1 and L2 bands on pure Pt. The negative peak shift indicates stronger CO adsorption,33 which is in agreement with theoretical calculations.37 The d-band center for surface Pt is calculated to shift up when being alloyed with Ag, which leads to more intense interaction with CO and thus stronger adsorption.38 Comparing to the L1 and L2 bands on pure Pt at room temperature, the CO adsorption peak on Pt1Ag2 was significantly less in intensity. It could be due to a lower content of Pt on the particle surface because of the presence of surface Ag. Comparing to pure Pt, the surface Ag of Pt1Ag2 can effectively activate O2, making the catalyst active even at low temperature. The higher activity of Pt1Ag2 comparing to pure Ag is probably caused by the difference in catalytic mechanism, wherein Pt1Ag2 likely follows the L-H path while pure Ag follows the E-R kinetics. The Pt1Ag2/Al2O3 also exhibited altered temperature-series CO adsorption comparing to the Pt/Al2O3. There was little variation in the peak position during the in-situ experiment (Figure 9), indicating little change in the CO adsorption mode within the temperature range. The peak intensity remained nearly constant at the low temperature range. It suggests that the particle surface remains a stable composition. However, there was a continuous increase in the intensity between around 80 °C and 160 °C, above which the increase became more gradual (Figure 10). The data suggests instability of the Pt1Ag2 and generation of more Pt surfaces in the temperature range, because the CO absorbance intensity is proportional to the amount of surface Pt sites. TEM of the reacted catalyst showed no obvious change in the size of Pt1Ag2 particles, indicating little sintering problem (Figure 11a). However, the particle surfaces became disordered and of lower imaging contrast comparing to the inner parts of the particles, suggesting structural and composition changes (Figure 11b). The elemental distributions of Pt and Ag were studied by


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HAADF-STEM and the corresponding EDX maps and line scans (Figure 11c-g). While both elements could be detected, the Ag map was slightly bigger than the Pt one. The Ag element line scans also showed broader signals than the Pt ones. The results indicate an Ag outmost layer, likely consisting of amorphous Ag oxides. These findings provide the explanation for segmental dependence of CO oxidation over the temperature. The Pt1Ag2/Al2O3 contains both metals in the surface and has an optimal surface composition for CO oxidation in the low temperature range. The two metal atoms cooperate to catalyze the reaction, leading to improved kinetics. The surface becomes unstable and segregates above 80 °C because of Ag oxidation, with amorphous Ag oxide layers being formed and encapsulating the particle surface. CO and O2 can still diffuse through these amorphous oxide layers and react on the beneath particle surface, in consistence with the observed catalyst activity at elevated temperature. However, the leaching out of Ag from the particles causes changes in particle surface composition and results in surface Pt enrichment beneath the Ag oxide layers, which leads to increased CO adsorption and altered activity dependence over the temperature. 4. Conclusion In summary, three Pt-Ag alloy nanoparticle catalysts, including Pt1Ag1/Al2O3, Pt1Ag2/Al2O3, and Pt1Ag3/Al2O3, have been prepared and studied for CO oxidation reaction. The Pt1Ag2/Al2O3 was found with the optimal particle composition and thus most active for the reaction at room temperature, with a significantly higher TOF of 1×10-2 1/s comparing to both Pt/Al2O3 (8×10-4 1/s) and Ag/Al2O3 (6×10-3 1/s). The improved reaction activity of Pt1Ag2/Al2O3 was attributed to a much decreased Ea value, which was determined to be 8.4 kJ/mol and was dramatically smaller than that for Pt/Al2O3 (74.1 kJ/mol) and Ag/Al2O3 (21.6 kJ/mol). The L-H mechanism, in which CO adsorbs to Pt active sites and O2 gets activated on adjacent Ag sites, was proposed for the Pt-


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Ag catalyst to explain the promoted CO oxidation kinetics. The Pt1Ag2 catalyst remained its superior activity comparing to pure Pt and Ag between room temperature and around 80 °C, above which deep oxidation of Ag in the near surface layers occurred and altered the reaction kinetics.

AUTHOR INFORMATION Corresponding Author *[email protected] ACKNOWLEDGMENT The research was supported by the UA start-up fund (Z.P.). The HRTEM data were obtained at the (cryo)TEM facility at the Liquid Crystal Institute, Kent State University, supported by the Ohio Research Scholars Program Research Cluster on Surfaces in Advanced Materials. The authors thank Dr. Min Gao for technical support with the TEM experiments.


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FIGURES

Figure 1. TEM of synthesized (a) Pt1Ag1, (c) Pt1Ag2, and (e) Pt1Ag3 nanoparticles, and (b, d, e) the correspondingly prepared Pt-Ag/Al2O3 catalysts.


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Figure 2. TEM of synthesized (a) Pt and (c) Ag nanoparticles, and the prepared (b) Pt/Al2O3 and (d) Ag/Al2O3 catalysts.

Figure 3. PXRD of Pt-Ag/Al2O3, Pt/Al2O3, and Ag/Al2O3 samples.


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Figure 4. HRTEM of the thermally treated Pt1Ag2/Al2O3: (a) after calcination in air at 300 °C for 1 h and (b) after consequent reduction in H2 at 300 °C for 1 h.

Figure 5. XCO-T profiles for CO oxidation using Pt-Ag/Al2O3, Pt/Al2O3, and Ag/Al2O3 catalysts.


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Figure 6. TOF for CO oxidation using Pt/Al2O3, Ag/Al2O3, and Pt-Ag/Al2O3 at room temperature (RT), 100 °C and 140 °C.

Figure 7. Arrhenius plots for CO oxidation using Pt/Al2O3, Ag/Al2O3, and Pt1Ag2/Al2O3 catalysts.


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Figure 8. In-situ FTIR spectra for working (a) Pt/Al2O3, (b) Ag/Al2O3, and (c) Pt1Ag2/Al2O3 catalysts by being heated at 5 °C/min to 200 °C in 100 sccm of 1% CO, 1% O2 and Ar balance.


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Figure 9. Peak position of CO absorbance on working Pt/Al2O3 and Pt1Ag2/Al2O3 catalyst at different temperature.

Figure 10. Intensity change of CO absorbance on working Pt1Ag2/Al2O3 catalyst at different temperature.


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Figure 11. (a) TEM, (b) HRTEM, (c) HAADF-STEM images, (d, e) elemental maps, and (f, g) elemental line scans for Pt and Ag of reacted Pt1Ag2/Al2O3 catalyst.

Table 1. Sample information of prepared Pt-Ag/Al2O3, Pt/Al2O3, and Ag/Al2O3 catalysts.

Sample

Particle size (nm)

Particle composition

Metal loading (wt.%)

(a) Pt

3.9 ± 0.7 nm

Pt

5.9

(b) Pt1Ag1

6.0 ± 1.3 nm

Pt52Ag48

6.3

(c) Pt1Ag2

5.3 ± 0.8 nm

Pt30Ag70

5.4

(d) Pt1Ag3

5.5 ± 0.8 nm

Pt22Ag78

6.1

(e) Ag

7.5 ± 1.5 nm

Ag

4.2


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Table of Contents Graphic and Synopsis


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REFERENCES 1.

Gardner, S. D.; Hoflund, G. B.; Schryer, D. R.; Schryer, J.; Upchurch, B. T.; Kielin, E. J.,

Catalytic Behavior of Noble Metal/Reducible Oxide Materials for Low-Temperature Carbon Monoxide Oxidation. 1. Comparison of Catalyst Performance. Langmuir 1991, 7, 2135-2139. 2.

Gardner, S. D.; Hoflund, G. B.; Upchurch, B. T.; Schryer, D. R.; Kielin, E. J.; Schryer, J.,

Comparison of the Performance Characteristics of Pt/SnOx and Au/MnOx Catalysts for LowTemperature CO Oxidation. J. Catal. 1991, 129, 114-120. 3.

Zhang, X.; Qu, Z.; Li, X.; Wen, M.; Quan, X.; Ma, D.; Wu, J., Studies of Silver Species

for Low-Temperature CO Oxidation on Ag/SiO2 Catalysts. Sep. Purif. Technol. 2010, 72, 395400. 4.

Stark, D. S.; Crocker, A.; Steward, G. J., A Sealed 100-Hz CO2 Tea Laser Using High

CO2 Concentrations and Ambient-Temperature Catalysts. J. Phys. E. 1983, 16, 158. 5.

Qayyum, E.; Castillo, V. A.; Warrington, K.; Barakat, M. A.; Kuhn, J. N., Methanol

Oxidation over Silica-Supported Pt and Ag Nanoparticles: Toward Selective Production of Hydrogen and Carbon Dioxide. Catal. Commun. 2012, 28, 128-133. 6.

Yung, M. M.; Zhao, Z.; Woods, M. P.; Ozkan, U. S., Preferential Oxidation of Carbon

Monoxide on CoOx/ZrO2. J. Mol. Catal. A: Chem. 2008, 279, 1-9. 7.

Alayoglu, S.; Nilekar, A. U.; Mavrikakis, M.; Eichhorn, B., Ru-Pt Core-Shell

Nanoparticles for Preferential Oxidation of Carbon Monoxide in Hydrogen. Nat. Mater. 2008, 7, 333-338.


22

Page 23 of 26

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


8.

Rothenberg, G., Catalysis; Wiley-VCH Verlag GambH & Co. KGaA: Weinheim, 2008.

9.

Wang, R.; He, H.; Wang, J.; Liu, L.; Dai, H., Shape-Regulation: An Effective Way to

Control CO Oxidation Activity over Noble Metal Catalysts. Catal. Today. 2013, 201, 68-78. 10. Chorkendorff, I.; Niemantsverdriet, J. W., Concepts of Mordern Catalysis and Kinetics, 2 ed.; Wiley-VCH: Weinheim, 2007. 11. Kahlich, M. J.; Gasteiger, H. A.; Behm, R. J., Kinetics of the Selective CO Oxidation in H2-Rich Gas on Pt/Al2O3. J. Catal. 1997, 171, 93-105. 12. Schubert, M. M.; Kahlich, M. J.; Gasteiger, H. A.; Behm, R. J., Correlation between CO Surface Coverage and Selectivity/Kinetics for the Preferential CO Oxidation over Pt/ɣ-Al2O3 and Au/α-Fe2O3: An in-Situ Drifts Study. J. Power Sources. 1999, 84, 175-182. 13. Michalak, W. D.; Krier, J. M.; Alayoglu, S.; Shin, J.-Y.; An, K.; Komvopoulos, K.; Liu, Z.; Somorjai, G. A., CO Oxidation on PtSn Nanoparticle Catalysts Occurs at the Interface of Pt and Sn Oxide Domains Formed under Reaction Conditions. J. Catal. 2014, 312, 17-25. 14. Fu, Q.; Li, W.-X.; Yao, Y.; Liu, H.; Su, H.-Y.; Ma, D.; Gu, X.-K.; Chen, L.; Wang, Z.; Zhang, H.; et al., Interface-Confined Ferrous Centers for Catalytic Oxidation. Science 2010, 328, 1141-1144. 15. Mu, R.; Fu, Q.; Xu, H.; Zhang, H.; Huang, Y.; Jiang, Z.; Zhang, S.; Tan, D.; Bao, X., Synergetic Effect of Surface and Subsurface Ni Species at Pt−Ni Bimetallic Catalysts for CO Oxidation. J. Am. Chem. Soc. 2011, 133, 1978-1986.


23


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 24 of 26

16. Alayoglu, S.; Eichhorn, B., Rh−Pt Bimetallic Catalysts: Synthesis, Characterization, and Catalysis of Core−Shell, Alloy, and Monometallic Nanoparticles. J. Am. Chem. Soc. 2008, 130, 17479-17486. 17. Zhang, H.; Jin, M.; Liu, H.; Wang, J.; Kim, M. J.; Yang, D.; Xie, Z.; Liu, J.; Xia, Y., Facile Synthesis of Pd–Pt Alloy Nanocages and Their Enhanced Performance for Preferential Oxidation of Co in Excess Hydrogen. ACS Nano 2011, 5, 8212-8222. 18. Qu, Z.; Cheng, M.; Huang, W.; Bao, X., Formation of Subsurface Oxygen Species and Its High Activity toward Co Oxidation over Silver Catalysts. J. Catal. 2005, 229, 446-458. 19. Liu, H.; Ma, D.; Blackley, R. A.; Zhou, W.; Bao, X., Highly Active Mesostructured Silica Hosted Silver Catalysts for CO Oxidation Using the One-Pot Synthesis Approach. Chem. Commun. 2008, 2677-2679. 20. Huber, H.; Ozin, G. A., Metal Atom Chemistry and Surface Chemistry. 2. Carbonylsilver(I) Superoxide (OC)Ag+,O2-, a Localized Bonding Model for Carbon Monoxide on an Oxidized Silver Surface. Inorg. Chem. 1977, 16, 64-67. 21. Keulks, G. W.; J. F. Outlaw, J., In Proceedings of the Fifth International Conference on Catalysis, North-Holland Publishing Co: Amsterdam, 1973; Vol. 65, p 945. 22. Wang, A.-Q.; Liu, J.-H.; Lin, S. D.; Lin, T.-S.; Mou, C.-Y., A Novel Efficient Au–Ag Alloy Catalyst System: Preparation, Activity, and Characterization. J. Catal. 2005, 233, 186-197. 23. Liu, J.-H.; Wang, A.-Q.; Chi, Y.-S.; Lin, H.-P.; Mou, C.-Y., Synergistic Effect in an Au−Ag Alloy Nanocatalyst: CO Oxidation. J. Phys. Chem. B. 2004, 109, 40-43.


24

Page 25 of 26

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


24. Heracleous, E.; Lemonidou, A. A., Ni–Nb–O Mixed Oxides as Highly Active and Selective Catalysts for Ethene Production Via Ethane Oxidative Dehydrogenation. Part II: Mechanistic Aspects and Kinetic Modeling. J. Catal. 2006, 237, 175-189. 25. Todorova, T. K.; Ganduglia-Pirovano, M. V.; Sauer, J., Vanadium Oxides on Aluminum Oxide Supports. 3. Metastable κ-Al2O3(001) Compared to α-Al2O3(0001). J. Phys. Chem. C. 2007, 111, 5141-5153. 26. Lewandowski, M.; Groot, I. M. N.; Shaikhutdinov, S.; Freund, H. J., Scanning Tunneling Microscopy Evidence for the Mars-Van Krevelen Type Mechanism of Low Temperature CO Oxidation on an FeO(111) Film on Pt(111). Catal. Today. 2012, 181, 52-55. 27. Peng, Z.; You, H.; Yang, H., Composition-Dependent Formation of Platinum Silver Nanowires. ACS Nano 2010, 4, 1501-1510. 28. Peng, Z.; Yang, H., Ag–Pt Alloy Nanoparticles with the Compositions in the Miscibility Gap. J. Solid State Chem. 2008, 181, 1546-1551. 29. Pan, A.; Yang, H.; Liu, R.; Yu, R.; Zou, B.; Wang, Z., Color-Tunable Photoluminescence of Alloyed Cdsxse1-X Nanobelts. J. Am. Chem. Soc. 2005, 127, 15692-15693. 30. Frey, K.; Iablokov, V.; Melaet, G.; Guczi, L.; Kruse, N., CO Oxidation Activity of Ag/TiO2 Catalysts Prepared Via Oxalate Co-Precipitation. Catal. Lett. 2008, 124, 74-79. 31. Hardacre, C.; Ormerod, R. M.; Lambert, R. M., Platinum-Promoted Catalysis by Ceria: A Study of Carbon Monoxide Oxidation over Pt(111)/CeO2. J. Phys. Chem. 1994, 98, 1090110905.


25


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 26 of 26

32. Haruta, M., Gold as a Novel Catalyst in the 21st Century: Preparation, Working Mechanism and Applications. Gold Bull. 2004, 37, 27-36. 33. Liu, D.; Que, G.-H.; Wang, Z.-X.; Yan, Z.-F., In Situ FT-IR Study of CO and H2 Adsorption on a Pt/Al2O3 Catalyst. Catal. Today. 2001, 68, 155-160. 34. Pillonel, P.; Derrouiche, S.; Bourane, A.; Gaillard, F.; Vernoux, P.; Bianchi, D., Impact of the Support on the Heat of Adsorption of the Linear CO Species on Pt-Containing Catalysts. Appl. Catal., A. 2005, 278, 223-231. 35. Tanaka, K.; White, J. M., Infrared Studies of CO Adsorption on Reduced and Oxidized Pt/TiO2. J. Catal. 1983, 79, 81-94. 36. Coloma, F.; Coronado, J.; Rochester, C.; Anderson, J., Infrared Study of Crotonaldehyde and CO Adsorption on a Pt/TiO2 Catalyst. Catal. Lett. 1998, 51, 155-162. 37. Nagai, N.; Hashimoto, H., FT-IR-ATR Study of Depth Profile of SiO2 Ultra-Thin Films. Appl. Surf. Sci. 2001, 172, 307-311. 38. Ruban, A.; Hammer, B.; Stoltze, P.; Skriver, H. L.; Nørskov, J. K., Surface Electronic Structure and Reactivity of Transition and Noble Metals. J. Mol. Catal. A: Chem. 1997, 115, 421-429.


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Property of PtâAg Alloy Nanoparticle Catalysts in Carbon Monoxide

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