Probing Site Activity of Monodisperse Pt ... - ACS Publications

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Probing Site Activity of Monodisperse Pt Nanoparticle Catalysts Using Steam Reforming of Methane Yongquan Qu, Alexander M. Sutherland, Jennifer Lien, George D. Suarez, and Ting Guo* Department of Chemistry, University of California, Davis, California 95616

ABSTRACT Monodisperse platinum nanoparticles between 2 and 8 nanometers were synthesized to help quantitatively investigate the size dependency and the activity of surface sites for steam reforming of methane reaction to produce hydrogen gas. It was observed that these monodisperse nanoparticles aggregated to almost double the size of nanoparticles after a few hours of steam reforming reactions at high temperatures. Using the size of nanoparticles determined by transmission electron microscopy and the measured turnover frequencies for these monodisperse nanoparticles, it was found, for the first time, that the activity of these nanoparticles of different sizes for steam reforming of methane can be described mathematically using only two variables of the type and the coordination number of the surface atoms. SECTION Surfaces, Interfaces, Catalysis

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single atoms as demonstrated by extensive work carried out by Iglesia et al. and Nørskov et al. and if no substrate or promoter effects exist, then the surface area and site arrangement such as coordination number are the two key parameters.11,12 In general, smaller nanoparticles have larger surface areas, and their surface atoms have fewer neighbors, making smaller nanoparticle catalysts more active. The coordination number argument can also be inferred from the results obtained in the field of surface science, which clearly suggested that steps, kinks, adatoms, and other nonflat, irregular structures may possess higher catalytic activity, defined as the average number of reactions catalyzed per surface site per unit time.13-15 For example, surface cracks on bulk Ni surfaces have been shown to be more active toward decomposition of ethylene by Vang et al.16 In another case, Yang et al. have shown that the edges of shaped nanoparticles are more active in methanol fuel cells.17 Other factors such as substrate and alloying effects may complicate this simple physical picture. For example, Haruta and Goodman et al. have independently proven that smaller nanoparticles (down to ∼1-2 nm) are more active than larger ones because the former exhibit unique electronic properties quite different from those of the bulk due to the interference from the substrate.18-20 More precisely, charge transfer between the support (TiO2 in this case) and nanoparticle catalysts (Au nanoparticles) is considered as one of the contributing factors for the enhanced activity.21,22 Furthermore, substrates may help control the morphology of nanoparticle catalysts, as found by Chen et al.23 Changing

eterogeneous catalysis of steam reforming of methane (SRM) is one of the most popular methods to commercially produce hydrogen gas.1-5 SRM also produces carbon monoxide, which together with hydrogen can be converted into liquid fuels. Due to the prominent role hydrogen may play in the new infrastructure of the clean energy paradigm, it is important to find ways to enhance the overall activity of the catalysts, which is generally dependent on at least two intimately connected parameters: (1) the number of available surface sites per unit mass and (2) the activity of each such site. The number of surface sites per unit mass can be drastically increased with the use of small nanoparticles, so, to no surprise, researchers in the field of catalysis have been employing small nanoparticles for decades, so long as the size is greater than the minimum dimension required to catalyze a specific reaction.6 Although it is possible to employ small catalysts all the way down to clusters of a few atoms,7 too small a nanoparticle may not always be better in terms of the surface sites having higher activities or practically supporting a high density of very small nanoparticles on a substrate, especially at high temperatures. The minimum number of atoms needed to function as an efficient catalytic site ranges from a single atom to a cluster of many atoms.8 The activity of a catalytic site is generally determined by the electronic structure of the site for a specific reaction to be catalyzed, and the electronic structure is controlled by at least the geometric structure (e.g., size, local morphology and surface structure) of the catalyst. As a result, the arrangement of atoms in nanoparticles to a large extent controls the activity. In addition to the catalyst itself, other factors such as the influence from the electronic structure of the substrate and promoters should also be considered, especially when the catalysts are below 2 nm in diameter.9,10 For reactions such as reforming of methane that relies on

r 2009 American Chemical Society

Received Date: October 6, 2009 Accepted Date: November 16, 2009 Published on Web Date: November 24, 2009

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DOI: 10.1021/jz900098x |J. Phys. Chem. Lett. 2010, 1, 254–259

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Figure 1. TEM images of as-made Pt nanoparticles, after treatment, and after reactions. Only 2.5 nm as-made nanoparticles are shown here. The average size is shown as well. Table 1. Calculated TOR and TOF for Four Sizes of Nanoparticlesa

the composition of nanoparticles offers an additional leverage to tuning the electronic structure of nanoparticle catalysts. For instance, bimetallic metal nanoparticles produced by Sinfelt, as well as those recently created by Somorjai and Guo et al., have suggested that the surfaces of bimetallic small nanoparticles may possess unique electronic properties that may significantly increase the activity of surface sites.24-26 In another example of bimetallic bulk shown by Stamenkovic et al., the added nickel atoms beneath the surface platinum atoms in the bulk render the new surface 10 more active than pure platinum surfaces because bimetallic catalysts provide a more suitable electronic structure for catalyzing the oxygen reduction reaction in the polymer electrolyte membrane fuel cells.27 In order to quantitatively study the size-dependent catalytic activity for SRM while avoiding complicated issues such as the substrate effect, it is best to employ nanoparticles greater than 2 nm in diameter. SRM is a perfect candidate for studying the size dependency because CH4 is considered to be activated on single Pt atoms through σ bond activation.12 However, the role of different surface sites has not been investigated quantitatively, possibly because the size distribution of nanoparticles is too large when nanoparticles are made via conventional methods such as the wet impregnation method, therefore obscuring the true size dependency. In addition, the normally useful dispersion measurements developed and widely used at room or slightly elevated temperatures to date often does not provide the true size distribution information or does not emphasize the importance of different surface sites, making assessing the sizestructure-activity relationship difficult. Therefore, it is important to use monodisperse nanoparticles of different sizes with narrow size distributions to help understand the activity of each surface site of a nanoparticle catalyst. In this work, premade Pt nanoparticles of varying sizes and size distributions were used to elucidate the size dependency and surface site dependency of Pt nanoparticles using the catalytic activity for SRM at relatively high temperatures. A quantitative relationship is obtained for the first time to describe the activity of SRM as a function of size of the nanoparticles. The synthesized Pt nanoparticles were uniform in size with narrow size distributions, as shown in Figure 1 (see Supporting Information (SI) for details). The average sizes are given as well. The sizes were approximately the same as those reported. After the high temperature treatment and SRM


size (nm)

750 °C reaction

TOR (mole g-1 hr-1)

TOF (s-1)

2.5

4.9 ( 1.7

25

3.7

3.3

5.9 ( 2.7

12

3.3

5.2

8.3 ( 2.0

8.0

2.6

9.0

18.0 ( 8.0

2.0

1.5

a

The sizes after reactions were used in the calculations.

reactions, the original nanoparticles sintered into larger nanoparticles with broader size distributions (see SI for details). The size almost doubled for nanoparticles of all four sizes, although this sintering process appeared to treat nanoparticles of different size equally. For Pt nanoparticles on alumina supports, the surface area calculated from the size measured from transmission electron microscopy (TEM) has been close to that determined by chemical measurements such as sulfur attachment.11 Therefore, TEM size measurements are considered to be reliable in determining the size and surface area of nanoparticles. The activity of four differently sized catalysts was measured based on the percentage of methane conversion for SRM. On the basis of the discussion given above, Pt nanoparticles aggregated to become larger nanoparticles during or after high-temperature SRM reaction, so the average size measured after the reaction was used in calculating the turnover rate or TOR (mol-CH4 conversion per gram of Pt per hour). TOR is a general gauge of the activity of a catalyst, and has the surface area information embedded in it. Since smaller nanoparticles contain a larger number of surface sites per unit mass, they are expected to have higher TOR values. If the surface atoms behave similarly regardless of the size of nanoparticles, then the TOR values should simply be linearly proportional to the surface area or total number of active surface sites. Average turnover frequency per surface atom (TOF) can be calculated from TOR and surface area. Both TOR and TOF are listed in Table 1. It clearly shows that TOF is higher for the surface atoms in smaller nanoparticles. This strongly suggests enhanced activity for surface atoms in smaller nanoparticles. The highest TOF was obtained for the smallest Pt nanoparticles of nearly 5 nm in diameter (measured after the SRM reaction), and the value was approximately 4.0 s-1, which is within a factor of 4 of the highest values available in the literature.11 The TOF for the largest Pt

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Figure 3. A 147-atom cuboctahedral nanoparticle. Four types of atoms are shown: vortex (5), edge (7), square face (8) and triangle face (9). The coordination numbers are given as well (in parentheses). Figure 2. Results of TOR values for SRM reaction using four differently sized Pt nanoparticles. The reaction temperature was 750 °C. The surface areas of the nanoparticles and calculated TOR values using the 9.0 nm nanoparticle and the surface areas are given. As the particle size becomes smaller, the measured TOR values increase more than those predicted using the surface areas and the TOR value of the largest Pt nanoparticles, suggesting an enhanced activity for smaller nanoparticles.

nanoparticles in this case was 1.5 s-1. The results are also shown in Figure 2, along with the calculated surface areas of the four differently sized nanoparticles measured using TEM and the calculated TOR for these four sizes using the measured TOR value for 18 nm nanoparticles. Because SRM reaction is catalyzed by single Pt atoms, SRM activity should be to a large extent dependent only on the size and shape of relatively large Pt nanoparticles (>2 nm) that help exclude the substrate, bimetallic, and other effects. However, no quantitative relationships between the activity and the size or surface morphology have been established. Since smaller Pt nanoparticles are generally more active, it is reasonable to assume that the more exposed surface Pt atoms should be more active. Assuming that activation energy is about the same for all surface atom, it is possible to simplify this complex function through the following mathematical operation: For a given shape and size, the activity of a nanoparticle can be expressed analytically with a formula that includes all the atomic surface sites. Because these surface sites can be best described by the number of nearest neighbors or coordination numbers, we can use the nearest neighbors to construct this formula. For example, when the shape of the nanoparticles is cuboctahedral, the activity can be expressed as ( 9 þ # of atoms ðedgeÞ KðsizeÞ ¼ K0 12ðvertexÞ 5 9 9 þ # of atoms ðsquareÞ þ 7 8 ) # of atoms ðtriangleÞ

Figure 4. Activity as a function of the size of nanoparticles. The activities for nanoparticles of different sizes (þ) are calculated based on eq 1. It is normalized to TOF data (2) for 18 nm nanoparticles measured here. Experimental data by Nørskov et al. (() and Iglesia et al. (9) are also shown.11,12 The general trends are similar, although these three sets of data differ quantitatively.

two assumptions link activity to the solid angle subtended by the approaching reactants toward the catalytic Pt atoms. The type of surface atoms is chosen based on the cuboctaheral shaped nanoparticles, each having six square faces and eight triangle faces.28 There are 12 vertex position atoms, each having five nearest neighbors, as shown in Figure 3. There are three other types of surface atoms: edge (7), square (8), and triangle (9). The numbers of nearest neighbors or coordination numbers are indicated in the parentheses. Once these assumptions are in place, the number of atoms of each surface type is a simple function of the size of the nanoparticles. For more complicated reactions or shapes, this formula may be different. Nonetheless, this is the first time where activity is quantitatively described based on the surface atoms of nanoparticles. Figure 4 shows the formula as a function of the size of the nanoparticles. As the size of nanoparticles becomes smaller, more atoms on the surface will have fewer nearest neighbors. Although in cuboctahedral shaped nanoparticles the smallest coordination number is 5, it is possible that nanoparticles of irregular shapes may have even fewer nearest neighbors, therefore exhibiting higher activities. Figure 4 also shows our experimental data using the sizes of nanoparticles after reactions to more accurately express the size, and the data agrees well with the trend

ð1Þ

K0 is a constant. This formula assumes that the activity of each site is inversely proportional to the number of nearest neighbors or coordination numbers, and the total activity of a nanoparticle is the sum of all surface atoms with the activity of each site weighted by the coordination number factor. These


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steeply than either function as the size decreases. All three curves are plotted in Figure 5. The multiplication process may help explain other results. For instance, even if the activity per site is lower for smaller nanoparticles for certain reaction, the overall TOR may still be higher or leveled off because the increase in surface area for smaller nanoparticles may offset the decrease in activity as observed in other works.8 On the basis of the agreement between the experimental data in this work and the proposed activity model, it seems that the origin of the enhanced activity as a function of size is mainly attributed to the accessibility of the nanoparticles by the gaseous reactants, at least true for single-atom catalytic sites such as Pt atoms. Since the pore size of Al2O3 substrate is of the order of less than 20 nm, we also investigated whether they could block the pores if the nanoparticles were too large. Control experiments were performed in which spherical Al2O3 nanoparticles were used to eliminate the possible clogging problem. The reforming experiments showed similar results, suggesting that either the largest nanoparticles were still smaller than the average pore size, or that the majority of nanoparticle catalysts were deposited near or outside the entrance of the pores, thus limiting or eliminating the clogging effect. In the size range of 2-18 nm (including both the as-made and the aggregated nanoparticles), the substrate influence on the activity of these nanoparticles were at most minimal. Recent work by Goodman et al. suggested that substrates may be an integral part of the catalysts, and the influence of the substrate can be extremely important, especially with small nanoparticles, a few atomic layers across the nanoparticles.20 As a result, we do not believe that the observed enhancement in activity for smaller nanoparticles is caused by the substrate effect, as it was attributed to playing a significant role in causing the enhanced activity for gold nanoparticles in oxidation of carbon monoxide.16 We also attempted to address this question by using other substrates such as carbon black. However, because SRM can effectively remove amorphous carbon, carbon black substrates did not work. Therefore, we could not completely exclude the possibility of the substrate effect causing at least part of the enhancements. Future experiments are required to fully address this issue. It is entirely possible that both substrate and small size play important roles in enhancing the catalytic activity. The formula shown here suggests that the types of surface atoms can be investigated with reactions that are sensitive to them. It also shows that the activity should rise sharply as the size approaches 1 nm or smaller, and it becomes unnecessary to use the shape to control the activity as a result of the lack of triangle face atoms with nine coordination numbers. On the other hand, shape control is more critical for larger nanoparticles with sizes over several nanometers, and altering the shape will lead to changes in the percentage of edge and corner atoms as well as their local and global electronic structures.30-33 A practical consideration for using Pt nanoparticles as catalysts is the cost. As mentioned at the beginning, other metals make more practical and effective SRM catalysts in reality because Pt is neither the most active metal nor the most inexpensive one. If other small metal nanoparticles can be made while remaining small during reactions, as predicted

Figure 5. Total catalytic activity per mass of a material as a function of the size of nanoparticles. It is the product of the fraction of surface atoms and the activity defined by eq 1. The percentage of surface atoms (blue line) and averaged TOF (per surface site) (green line) are also plotted as a function of size.

predicted by eq 1, which is normalized to the activity of the 18-nm Pt nanoparticles. When other experimental data is plotted in this framework, the same general trend is observed. For example, two sets of the SRM data obtained with the wet impregnation methods are given in Figure 4.11,12 Although the trend of these two set of data is the same as that of this work, both quantitatively deviate from the proposed model, especially when the nanoparticles are small, approaching 1-2 nm using the dispersion measurements. The absolute TOF (number of reactions per surface site per second, not the forward TOR given in ref 12) measured here for ∼5 nm nanoparticles was ∼4.0 s-1, which is 8 times that reported by Nørskov et al. using 1 wt % Pt loading and 3.9 nm nanoparticles measured with the dispersion method,11 but only 25% of that reported by Iglesia et al. at 1.5 wt % Pt loading for the same nanoparticles.12 Both reports used ZrO2 as the substrate. The discrepancies could be originated from the differences in procedures in which the samples were prepared and reactions were studied, especially when the size of nanoparticles become smaller than 2 nm to invoke other effects. It is also possible that the dispersion measurements overestimated the surface sites as discussed in many publications,29 although the TEM measurements may also overestimate the total accessible surface sites. It emphasizes that precision synthesis of nanoparticles with narrow size distributions is critical to accurately modeling the surface site activity of these nanoparticles. A direct implication of this quantitative description of the activity as a function of size is that it can be used to examine and predict the activity of nanoparticles, at least for singleatom catalyzed reactions such as SRM and carbon dioxide reforming of methane (CRM). The formula quantitatively predicts how much higher the activity can reach for smaller nanoparticles. For example, on average, each surface atom on 2-nm nanoparticles is 1.8 as active as that on 4-nm nanoparticles. Multiplying this average activity with the increased percentage of surface atoms, which is equivalent to the number of surface sites per unit mass mentioned earlier, we have the overall activity of the nanoparticles increasing more


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by eq 1, then they can replace Pt nanoparticles. An ideal catalyst would exhibit high activity and long lifetime with relatively low production costs. Immobilized catalysts will help to increase the lifetime, as shown in the literature.34 Yet another way to reduce costs and increase activity is to use alloys such as Pt/Ni nanoparticles. In summary, we have shown here that it is possible to analytically model the activity of Pt nanoparticle catalysts for SRM because SRM reaction relies on single Pt atomic catalytic sites. This modeling is enabled by the synthesis of monodisperse Pt nanoparticles between 2 and 8 nanometers. The enhanced activity is mainly attributed to the decreased number of nearest neighbors or coordination numbers. This new analytical formula helps explain and predict the activity of smaller nanoparticles, and help elucidate the influence of other effects when the predicted activity deviates from the measured ones. The experimental data obtained in this work agreed with the trend predicted by this formula. This finding may directly influence the production of energy carriers such as hydrogen and energy products such as petroleum.

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SUPPORTING INFORMATION AVAILABLE Synthesis and characterization of Pt nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: tguo@ ucdavis.edu. Tel: (530) 754-5283. Fax: (530) 752-8995.

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ACKNOWLEDGMENT The authors thank Mr. D. J. Masiel and Professor N. D. Browning for their experimental assistance. Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research.

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Probing Site Activity of Monodisperse Pt ... - ACS Publications

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