The Role of Organic Capping Layers of Platinum Nanoparticles in Catalytic Activity of CO Oxidation
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We report the catalytic activity of colloid platinum nanoparticles synthesized with different organic capping layers. On the molecular scale, the porous organic layers have open spaces that permit the reactant and product molecules to reach the metal surface. We carried out CO oxidation on several platinum nanoparticle systems capped with various organic molecules to investigate the role of the capping agent on catalytic activity. Platinum colloid nanoparticles with four types of capping layer have been used: TTAB (Tetradecyltrimethylammonium Bromide), HDA (hexadecylamine), HDT (hexadecylthiol), and PVP (poly(vinylpyrrolidone)). The reactivity of the Pt nanoparticles varied by 30%, with higher activity on TTAB coated nanoparticles and lower activity on HDT, while the activation energy remained between 27 and 28 kcal/mol. In separate experiments, the organic capping layers were partially removed using ultraviolet light-ozone generation techniques, which resulted in increased catalytic activity due to the removal of some of the organic layers. These results indicate that the nature of chemical bonding between organic capping layers and nanoparticle surfaces plays a role in determining the catalytic activity of platinum colloid nanoparticles for carbon monoxide oxidation.
Recent advances in colloidal chemistry have enabled catalytic nanoparticles to be synthesized with tunable particle sizes, shapes, and compositions [1, 2, 3, 4, 5]. Consequently, the model catalytic systems generated from colloidal nanoparticles enable quantitative investigations into the effects of these factors on catalytic activity and selectivity . The colloid nanoparticles are capped with stabilizing agents in order to prevent aggregation while in the liquid suspension. On the molecular scale, the porous organic capping layers have open spaces that permit reactant and product molecules to reach the metal catalysts. The degree of access of reactant and product molecules to the active sites in the metal catalyst depends on geometric factors such as length, compactness of the organic layers, and the nature of chemical bonding between the organic molecules and metal sites. It is also known that some elements, such as sulfur, block reactive sites when bonded to the catalyst, thus reducing the overall catalytic activity . Overall, the role of the capping agent in catalytic activity and selectivity is not yet well-understood in spite of its importance to model nanoparticle systems for studying heterogeneous catalysis.
One approach to address the influence of capping layers on the catalytic reaction involves the measurement of catalytic activity after the removal of capping layers. It was found that the organic capping molecules can be decomposed and removed using a UV light irradiation that generates ozone in air (or simply UV/ozone treatment) . Aliaga et al.  found that sum frequency generation (SFG) vibrational spectrum under ethylene hydrogenation measured on colloid Pt nanoparticles after UV-ozone treatment revealed the presence of adsorbed ethylidyne and di-σ-bonded reaction intermediate species, suggesting that UV-ozone effectively removes the capping layers.
In this study, we carried out CO oxidation on several platinum nanoparticle systems capped with various organic molecules to elucidate the role of the capping agent on catalytic activity. Platinum colloid nanoparticles with four types of capping layers have been used: TTAB (Tetradecyltrimethylammonium Bromide), HDA (hexadecylamine), HDT (hexadecylthiol), and PVP (poly(vinylpyrrolidone)). We show that the catalytic activity under CO oxidation with partial pressures of 100 Torr O2 and 40 Torr CO for UV-ozone treated Pt nanoparticles is higher compared to untreated nanoparticles, which suggests that some portion of the capping layers block the reactive sites and influence the catalytic turnover rate.
2.1 Synthesis of TTAB, HDA, HDT and PVP Capped Nanoparticles
In this study, we used platinum colloid nanoparticles with four types of capping layers: TTAB, HDA, HDT, and PVP. TTAB capped nanoparticles were synthesized as reported previously . Briefly, 1 mM aqueous K2PtCl4 in 100 mM TTAB was reduced by 30 mM NaBH4 at 50 °C. Excess H2 evolving from the reacting solution was released by inserting a needle into the septum. After 7 h, the reaction was allowed to cool to room temperature and left overnight to decompose the remaining NaBH4 in water. The Pt nanoparticles were collected and washed by repeated centrifugation and sonication. TTAB stabilized Pt nanoparticles have cubic shapes and an average size of 12.3 (±1.4) nm obtained from the size distribution of 500 nanoparticles . The organic capping layer was exchanged with HDA or HDT. About eight milliliters of TTAB capped nanoparticles were redispersed in 2 mL of deionized water after washing, then 10 mg of HDA or 20 μL of HDT was added to the washed nanoparticles. The solution was refluxed overnight at 50 °C, and the residual HDA or HDT was washed with ethanol. The nanoparticles were further washed by dispersing in chloroform and precipitating with hexane. Finally, the nanoparticles were dispersed in chloroform and deposited on a silicon wafer. Because the exchanging capping layers do not change the size and shape of nanoparticles, HDA and HDT capped nanoparticles had an average size of 12.3 (±1.4) nm with cubic shapes. For PVP capped Pt nanoparticles, 0.1 mmol of chloroplatinic acid hexahydrate (H2PtCl6, ACS reagent, Sigma–Aldrich), 4 mmol of tetramethylammonium bromide ((CH3)4NBr, >98%, Sigma–Aldrich), and 2 mmol of poly(vinylpyrrolidone) (PVP, Mw = 24,000, Sigma–Aldrich) (in terms of the repeating unit), were added to 20 mL of ethylene glycol (>98%, EMD) in a 50 mL three-necked flask at room temperature. PVP capped Pt nanoparticles have an average size of 9.5 (±0.8) nm obtained from the size distribution of 150 particles in TEM images .
2.2 Analytic Techniques for Characterization of Platinum Nanoparticles
2.3 Catalytic Reactor and UV-ozone Treatment
The reaction studies were performed in an ultrahigh vacuum chamber with a base pressure of 5.0 × 10−8 Torr. CO oxidation studies were carried out in a batch reaction system under 40 Torr CO, 100 Torr O2, and 620 Torr He. The gases were circulated through the reaction line by a Metal Bellows recirculation pump at a rate of 2 L/min. The volume of the reaction loop was 1.0 L. An HP Series II gas chromatograph equipped with a thermal conductivity detector and a 15′, 1/8′′ SS 60/80 Carboxen-1000 (Supelco) was used to separate the products for analysis. The measured reaction rates are reported as turnover frequencies (TOF) and are measured in units of product molecules of CO2 produced per metal surface site per second of reaction time. The irradiation of the nanoparticles by UV-ozone was conducted ex situ using a Bulbtronics 16 W low pressure mercury lamp emitting at 185 and 257 nm inside of a custom made metallic enclosure. The sample was positioned at a distance of 5 mm from the ultraviolet lamp’s quartz tube surface for a determined amount of time, and then removed from the UV chamber. SEM images reveal individual nanoparticles 12 nm in size and indicate that the nanoparticles do not agglomerate due to UV/ozone cleaning.
3 Results and Discussion
3.1 Catalytic Activity of Pt Nanoparticles with Different Capping Layers
Turnover rate measured (at 240 °C) and activation energy measured on Pt nanoparticles with capping layers of TTAB, PVP, HDA, and HDT
Types of capping layers
Turnover rate (/Pt site/s) at 240 °C
4.7 ± 0.3
4.0 ± 0.2
3.5 ± 0.2
3.2 ± 0.2
Activation energy (kcal/mol)
27.5 ± 0.9
26.7 ± 1.1
27.2 ± 0.8
28.1 ± 0.7
We now compare the catalytic activity of Pt nanoparticles with that of single crystal surfaces. Su et al.  carried out CO oxidation on Pt (111) surface at 100 Torr O2 and 40 Torr CO between 540 and 630 K. Below the ignition temperature, the reaction rate is governed by surface reaction kinetics and follows Langmuir–Hinshelwood kinetics [18, 19, 20, 21]. The activation energy for this reaction regime was found to be between 30  and 42 kcal/mol . From Su et al’s study, turnover frequency (TOF) for Pt (111) (at 513 K) is ~3 (CO2/metal site/second). This TOF value is lower than those of nanoparticles within a factor of two. The higher activity on Pt nanoparticles compared to the single crystal is most likely associated with the higher number of kinks and edge sites at nanoparticle surfaces, which are well known to be very reactive [23, 24].
3.2 XPS and Catalytic Activity of UV-ozone Cleaned Platinum Nanoparticles
After UV-ozone cleaning, the Pt surface exhibits higher oxidation states. Whether the oxidized Pt surface is catalytically active, however, is of some debate [29, 30]. In our study, the turnover rate of Pt nanoparticles after UV-ozone cleaning is higher than that of the bare Pt nanoparticle surface with capping layers, as well as clean Pt (111) surface. This result implies that the oxidized Pt surface is not a catalytically inactive species. This argument is also consistent with recent CO oxidation results on core-shell Pt/mesoporous silica nanoparticles which exhibited partial oxidation of Pt. The turnover rate of CO oxidation on these core-shell Pt/mesoporous silica nanoparticles was found to be as high as that on bare Pt nanoparticles . However, the role of Pt oxide in catalytic activity is beyond the scope of this study, and in addition, the removal of capping layers that occurs during the UV-ozone treatment makes a direct comparison of catalytic activity of Pt oxide with the bare Pt surface difficult. One approach to rule out the contribution of capping layers would be to synthesize Pt nanoparticles without capping layers and then compare their catalytic activity before and after UV-ozone treatment. The preparation of nanoclusters with UHV deposition  can be an approach to prepare capping layer-free nanoparticles and to study the role of platinum oxide on the catalytic activity exclusively.
Carbon monoxide oxidation was carried out on platinum nanoparticle systems capped with various organic molecules to study the influence of chemical bonding between the molecules and metal atoms on the catalytic activity. Four types of capping layers—TTAB, HDA, HDT, and PVP—were tested. The turnover rate of TTAB capped nanoparticles exhibited the highest activity presumably because of weak bonding between TTAB and the Pt atoms. Weak activity was observed using HDA and HDT capped nanoparticles, which can be attributed to strong NH2 and sulfur bonds. For all the capping layers, the activation energy remained the same within the error of the measurement. After partial removal of capping layers with UV-ozone treatment, the activity of CO oxidation increased by 60–100%, depending on the type of capping layer, while the activation energy remained the same.
This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geological and Biosciences and Division of Materials Sciences and Engineering of the US Department of Energy under contract No. DE-AC02-05CH11231.
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