Size-Dependent Reaction Mechanism and Kinetics for Propane

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Research Article pubs.acs.org/acscatalysis

Size-Dependent Reaction Mechanism and Kinetics for Propane Dehydrogenation over Pt Catalysts Jun Zhu,† Ming-Lei Yang,‡ Yingda Yu,§ Yi-An Zhu,*,‡ Zhi-Jun Sui,‡ Xing-Gui Zhou,‡ Anders Holmen,† and De Chen*,† †

Department of Chemical Engineering, Norwegian University of Science and Technology, N-7491 Trondheim, Norway UNILAB, State Key Laboratory of Chemical Engineering, Shanghai Key Laboratory of Multiphase Materials Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China § Department of Materials Science and Engineering, Norwegian University of Science and Technology, N-7491 Trondheim, Norway ‡

S Supporting Information *

ABSTRACT: Platinum cluster size has a significant influence on the activity, selectivity, and stability as well as the reaction mechanism during propane dehydrogenation (PDH). Wellcontrolled platinum catalysts of different cluster sizes are prepared by a seed growth method and supported on calcined hydrotalcite. The Pt catalysts show strong structure-sensitive behavior both in the C−H bond activation of propane and in the C−C bond activation to yield ethylene, methane, and coke. The Pt clusters of small cluster sizes, with (211) dominating on the surface, have a lower dehydrogenation energy barrier and thus higher activity. However, large Pt clusters with Pt(111) dominating result in a weakened binding strength of propylene and an increased energy barrier for the activation of C−H bonds in propylene, which leads to higher selectivity toward propylene by lowering the possibility of deep dehydrogenation. Kinetic analysis illustrates that the reaction order in hydrogen decreases and activation energy increases with an increasing Pt cluster size. Combined with density functional theory calculations and isotope effect experiments, it gives strong evidence of the change in reaction mechanism with Pt cluster size. It suggests that on small Pt clusters that are mostly surrounded by undercoordinated surface sites, the first C−H bond activation is likely to be the rate-determining step, while the second C−H bond activation is kinetically relevant on large Pt particles with terrace sites dominating. KEYWORDS: propane dehydrogenation, size effects, reaction mechanism, kinetic isotope effect, DFT simulation, platinum catalyst

1. INTRODUCTION

Recent progress in nanocatalysis by sizing and shaping nanoparticles provides new approaches for tuning the catalyst activity, selectivity, and stability.13,14 Atomic steps, kinks, and sites with special coordination properties can selectively break H−H, C−H, C−C, CO, and OO bonds.15 Platinum catalysts are widely used in hydrogenation and dehydrogenation of hydrocarbons. Many of these reactions are known to be structure-sensitive,16 which has been intensively studied for supported metal catalysts.17,18 The population of atoms on steps is inversely proportional to the size of Pt particles, and it has a significant effect on the catalytic behavior.15,19,20 However, the effect of cluster size on propane dehydrogenation has very rarely been reported. Our previous work has shown that Pt cluster size has a significant effect on both activity and selectivity toward methane and coke formation. On Pt nanoclusters in a range of 1−5 nm, propane mainly

1−3

The demand for propylene is growing rapidly. There has been a large increase in the demand for polypropylene, acrylonitrile, propylene oxide, cumene, phenol, isopropyl alcohol, and many other propylene derivatives. The propylene obtained as a coproduct from steam crackers or FCC units does not satisfy the growing demand. Developing alternative routes of propylene production is therefore of great interest. Propane dehydrogenation (PDH) is believed to have great potential for intentional propylene production, and as a result, it has been extensively investigated. However, a complete understanding of the reactions, including coke formation and deactivation, at an elementary step level is lacking. Most of the reported propane or ethane reaction order is identical to 1, and the hydrogen reaction order changes from −1.9 to 1, depending on the active metals, bimetallic structures, and supports as well as the reaction conditions.4−12 However, few of these studies have discussed the mechanism for the change in reaction order with respect to hydrogen. © XXXX American Chemical Society

Received: July 8, 2015 Revised: August 31, 2015

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condenser, and a mechanical stirrer. Then, solution B was added dropwise to solution A while it was being stirred, with an injection rate of 3.3 cm3 s−1. After that, the pH of the mixture was adjusted to 8−9 and the mixture was heated for 15 h at 353 K. After being cooled to room temperature, the resultant gel was filtered and thoroughly washed with 2 L of deionized water. The catalysts were dried overnight in vacuum at 343 K and calcined under a N2 (Yara Praxair, 99.999%, approximately 1 cm3 s−1) flow at 873 K for 6 h. Pt nanoparticles of varied cluster sizes were dissolved in the proper amount of ethanol and ultrasonically treated for 10 min to ensure a homogeneous dispersion of metal clusters. Then the Pt nanoparticles were supported on Mg(Al)O by the incipient wetness deposition method. The slurry was dried under ambient conditions for 16 h and then dried in an oven under N2 purging at 383 K for 10 h. Then, the catalyst was recalcined at 873 K for 3 h. The final Pt loading is 1.0 wt %. 2.2. Characterization. The Pt nanoparticles were investigated by transmission electron microscopy (TEM) at 200 kV (JEOL 2010). The samples were prepared by dispersing the Pt colloid in ethanol with ultrasonic treatment for 15 min. Then a drop of this suspension was placed on a holey carbon film on a copper grid and dried under ambient conditions for 20 min. Up to 200 individual metal particles were counted for each catalyst to determine the surface-averaged Pt cluster diameters (dp):

decomposes into methane and coke instead of undergoing dehydrogenation.3 It reveals that the coordinatively unsaturated Pt atoms on steps and corners have high activity toward cracking. As a result, the study of the effects of the cluster size of platinum on propane dehydrogenation is of great importance as it can provide fundamental knowledge about mechanisms of dehydrogenation and coke formation and thus contributes to the design of better industrial catalysts. Normally, noble metal nanoparticles are synthesized by reduction of metal halides or anionic metal chloride precursors with alcohols or reductants (such as NaBH4 or H2) in the presence of surface stabilizers such as poly(vinylpyrrolidone) (PVP), polyacrylate, etc. Homogeneously dispersed nanoparticles can be synthesized. However, preparation of a series of nanoparticles within the subnanometer range is still a challenge. In this work, Pt catalysts of well-controlled cluster sizes (3−9 nm) were prepared. The effects of Pt cluster size on catalytic performance in propane dehydrogenation in terms of activity, selectivity, coke formation, and stability as well as reaction mechanism are investigated by kinetic and isotope effect experiments and density functional theory (DFT) investigations.

2. EXPERIMENTAL SECTION 2.1. Preparation of Size-Controlled Pt Catalysts. The platinum nanoparticles reported here were synthesized using the seeded growth method.21 First, 6 mL of 6 mM H2PtCl6· 6H2O (Aldrich, ≥99.5%) in the presence of 1 g of Pluronic L64 (EO13PO30EO13, Sigma-Aldrich) triblock copolymer (60 μmol) was dissolved in 50 mL of H2O and ethylene glycol (EG, Fluka, ≥99.5%) solution (50% EG by volume). The mixture was placed in a 100 mL three-neck flask and heated in a preheated oil bath at 160 °C for 10 min. The fast reduction of the Pt precursor salt and the large excess of the block copolymer capping agent resulted in a homogeneous solution of small Pt seeds (3 nm), which was used as the starting solution toward synthesizing bigger particles. Then, larger nanoparticles (5, 7, and 9 nm) were synthesized by a subsequent addition of 3, 6, or 12 mL of a 6 mM H2PtCl6 solution over time (10, 20, or 30 min) to the Pt seed solution while H2 was passing through the reaction vessel. H2 was bubbled for an additional 5 min after addition of the platinum precursor. The product was centrifuged at 5000 rpm for 15 min. The supernatant was separated and precipitated by adding a triple volume of acetone, followed by centrifugation at 3000 rpm for 5 min. The precipitate was collected and redispersed in 10 mL of ethanol (Sigma-Aldrich, ≥99.5%) with sonication. A volume of 10 mL of hexane (Sigma-Aldrich, 95%) was added to the dispersion, and the solution was centrifuged at 3000 rpm for 5 min. The precipitate was washed twice with the same solvent mixture. Finally, the precipitate was dispersed in 3 mL of ethanol. Mg(Al)O, a hydrotalcite-type support with the Mg/Al atomic ratio fixed at 2, was prepared by coprecipitation of Mg(NO3)2·6H2O (Sigma-Aldrich, 99%) and Al(NO3)3·9H2O (Sigma-Aldrich, ≥98%) with a basic solution.22 The samples were prepared according to the following procedure. Solution A contains a Mg, Al precursor, with a calculated amount of precursor salts mixed with 400 cm3 of deionized water. Solution B contains calculated amounts of Na2CO3 (Sigma-Aldrich, ≥99.5%) and NaOH (Sigma-Aldrich, ≥97%) mixed with 400 cm3 of deionized water. Then, solution B was transferred into a 1 L three-neck vessel equipped with a thermometer, a reflux

dp =

∑i nidi 3 ∑i nidi 2

(2-1)

in which ni is the number of particles with diameter di, using at least 200 clusters for each sample. The BET surface areas and the pore sizes of the Pt catalysts were measured on a TriStar 3000 instrument. The surface area was calculated with the Brunauer−Emmett−Teller (BET) equation. The pore size distribution and the pore volume of all the catalysts samples were determined by using the adsorption method based on the Barrett−Joyner−Halenda (BJH) model. Hydrogen chemisorption was performed on a Micromeritics ASAP 2020 unit to determine the Pt metal dispersion. The fresh catalyst samples, Mg(Al)O-supported Pt nanoparticles of different cluster sizes, were reduced in situ with 2 cm3 of H2 g−1 s−1 (Yara Praxair, 99.999%) at 773 K using a heating rate of 1 K min−1. After reduction, the samples were first cooled to 603 K and evacuated for 1 h, then cooled to 373 K and evacuated for 30 min, and finally cooled to 313 K. The adsorption isotherms were recorded at pressure intervals ranging from 2.7 to 68 kPa. The amount of hydrogen chemisorbed on the catalyst was determined by extrapolating the straight line of the isotherm to zero pressure. Pt catalysts after reaction were also characterized via hydrogen chemisorption to examine the cluster size change and possible sintering. They were treated in situ with a mixture of 0.5 cm3 of O2 g−1 s−1 (Yara Praxair, 99.999%) and 4.5 cm3 of Ar g−1 s−1 (Yara Praxair, 99.999%) at 673 K for 0.5 h to remove the possible coke on them, and then these catalysts were characterized with the same procedure as the fresh ones. The Pt metal dispersion was calculated on the basis of the assumption that two Pt surface atoms were covered by one hydrogen molecule. The total amount of coke formed on the used catalysts was determined from temperature-programmed oxidation (TPO), using a PerkinElmer thermogravimetric analyzer (TGA). The samples were placed in a sample holder and purged with Ar (1 6311

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ACS Catalysis cm3 s−1, Yara Praxair, 99.999%) for 30 min at 393 K. Then the samples were heated at a rate of 0.167 K s−1 from 298 to 1173 K in 1.5 cm3 of flowing air s−1 (Yara Praxair, 99.999%). 2.3. Kinetic Study. The Pt catalysts were reduced in a quartz reactor with 2 cm3 of H2 g−1 s−1 (Yara Praxair, 99.999%) at 793 K for 2 h before being used for propane dehydrogenation, which was conducted under isothermal conditions at a temperature between 723 and 813 K and atmospheric pressure. Typically, 0.05 g of catalyst (1.0 wt % Pt) was loaded in the quartz reactor. The feed consisted of 1−8 kPa of C3H8 and 1− 10 kPa of H2 (Yara Praxair, 99.999%), balanced with He (Yara Praxair, 99.999%). Reactants and products were analyzed online by MicroGC 3000. The diffusion free operation was checked experimentally by varying space velocities and catalyst particle sizes. All the experiments were performed in a kinetic regime and with relatively low propane conversions. The surfacespecific activities, including turnover frequency (TOF) for propylene and ethylene production, were calculated by normalizing the mass activity to the number of surface Pt atoms (Pts) determined by H2 chemisorption. The selectivity of propylene was calculated on the basis of carbon mass balance. Preliminary experimental results showed that, under these conditions, the influences of internal and external diffusion were safely excluded. For the kinetic analysis, the TOFpropylene was corrected for approach to equilibrium (η) from thermodynamic data23 and the prevalent pressures of reactants and products: TOFC3H6 = TOFn /(1 − η) η=

by the growth of the particles is not exceeded by the rate of addition of the precursor to the solution, no new nuclei are formed.34 In this work, 3 nm Pt seeds were first synthesized by reduction of H2PtCl6·6H2O by EG at 160 °C in the presence of a capping agent, Pluronic L64. Then, a certain amount of Pt precursor was calculated and added to the solution slowly to make sure that the Pt seeds grow to the controlled size range and avoid the formation of new nuclei. After addition of a platinum precursor, H2 was used for an additional 5 min to ensure full reduction of the Pt precursor. The prepared Pt colloid was treated with acetone, ethanol, and hexane to remove the capping agent, Pluronic L64, which is used in the preparation procedure. The Pt cluster size was measured by TEM. As shown in Figure 1A−D, differently sized Pt colloids (∼3 to 9 nm) with a

(2-2)

PC3H6PH2 PC3H8Keq

(2-3)

where TOFn is the net formation rate of propylene and Keq is the equilibrium constant of propane dehydrogenation. 2.4. Kinetic Isotope Experiment. C3H8/C3D8 isotope effects were measured using H2/CO/Ar (7.5/2.5/40 N mL/ min) and D2/CO/Ar (7.5/2.5/40 N mL/min) reactant mixtures. The following gases have been used: H2 (Yara, 99.999%), C3H8 (Yara, 99.9%), C3D8 (Cambridge Isotope Laboratories, 98.6%), and Ar (Yara, 99.999%). The reactant gases were introduced separately and mixed before entering the reactor. The measurements were performed on an in situ reduced catalyst. The reduction and reaction procedures are the same as in section 2.3. The kinetic isotope effect (KIE) measurements were performed after the pseudosteady state had been reached, ∼2 h time on stream. 2.5. Theoretical Calculation. DFT calculations were conducted by using the VASP package with a GGA-PBE functional.24−26 The Pt(100) and Pt(111) surfaces were modeled by p(3 × 3) supercells, and the Pt(211) surface was represented as a p(4 × 1) supercell (see the details in the Supporting Information).27,28

Figure 1. TEM images of Pt nanocolloids: (A) 3.1, (B) 5.3, (C) 7.0, and (D) 9.1 nm.

narrow size distribution have been synthesized. The Pt nanoclusters take an almost spherical shape, and they are assumed to be spherical particles in the following study. Therefore, the Pt nanoparticles are dispersed in ethanol and then supported on calcined hydrotalcite, Mg(Al)O, by the incipient wetness impregnation method. The benefit of using calcined hydrotalcite is that it not only has a strong interaction with Pt clusters but also contains a large external surface.35−37 Moreover, its basic surface property could minimize coke formation on the support itself by suppressing the acidcatalyzed polymerization and aromatization, which makes it easier to study the properties of the supported metal by avoiding the obstruction from the support. The BET surface area of Mg(Al)O is 165 m2/g with a pore volume of 0.55 cm3/ g. After the Pt nanoparticles had been loaded, the catalysts were recalcined at 873 K. The structural properties of the catalysts were tested by BET. The average Pt cluster size on the supported Pt catalysts was measured by H2 chemisorption. The characterization results are summarized in Table 1. After the Pt nanoparticles had been loaded, there was only a very small

3. RESULTS AND DISCUSSION 3.1. Catalyst Preparation and Characterization. Seeded growth methods were developed to control the size of transition metal nanoparticles.21,29−33 Small nanoparticles are synthesized first as seeds, and then additional precursor is added to grow larger-sized particles. For seeded growth, the rate of addition of metal ions and their concentration are key parameters in controlling the size distribution of the final material. As long as the consumption of the metal ion reactants 6312

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ACS Catalysis Table 1. Structural Properties of the Catalysts Characterized by BET and Comparison of Cluster Sizes Obtained from TEM and H2 Chemisorption

Mg(Al)O Cat Id Cat II Cat III Cat IV Cat V Cat VIe

BET SA (m2/g)

pore volume (m3/g)

cluster sizea (nm)

cluster sizeb (nm)

cluster sizec (nm)

165 − 161 157 160 159 155

0.55 − 0.52 0.50 0.52 0.51 0.50

− − 3.1 5.3 7 9.1 −

− 1.0 3.0 5.0 7.5 9.3 4.9

− 1.9 3.2 5.5 7.7 9.5 5.5

a

Pt colloids, characterized by TEM and surface-averaged Pt cluster diameters estimated by eq 2-1. bSupported Pt catalysts, before PDH reaction, calculated on the basis of H2 chemisorption, assuming a Pt spherical shape. cSupported Pt catalysts, after PDH reaction, calculated on the basis of H2 chemisorption, assuming a Pt spherical shape. dPt/ SBA-15 provided by G. A. Somorjai’s group. ePrepared by the incipient wetness impregnation method.

Figure 2. Comparison of catalytic performance: (□ and ■) 5.0 nm Pt catalyst by the colloid method and (○ and ●) 4.9 nm Pt catalyst prepared by the incipient wetness impregnation method (793 K, 10 kPa C3H8, 5 kPa H2, and Ar used as a balance).

change in surface area and pore volume. The changes in BET surface area (157−165 m2/g) and pore volume (0.50−0.55 m3/ g) are within 5 and 10%, respectively. It indicates that most of the Pt nanoparticles are possibly located on the external surface of the supports and that they are free to be accessed by the reactants. As shown in Table 1, the supported Pt cluster size calculated on the basis of H2 chemisorption was found to be consistent with the size of Pt colloids, measured by TEM. No obvious changes in Pt cluster size were found during the supporting procedure. Moreover, the average Pt cluster size on the catalysts after propane dehydrogenation reaction was also tested by H2 chemisorption (Table 1) and TEM (Figure S1). The Pt cluster size changes little, and it is clear that no sintering happened on these Pt catalysts under the reaction conditions. The good interaction between the calcined hydrotalcite and the Pt nanoparticles enhances their stability during calcination and PDH reaction.37,38 3.2. Effect of Pt Cluster Size on Catalytic Performance. 3.2.1. Effects on Activity and Selectivity. Although a large amount of solvent has been used to wash away the capping agent, it is still not sufficient to ensure that they are thoroughly cleaned. The surface protective agents have been reported to have effects on the catalytic performances.39−41 To examine the possible effects of the capping agent, one catalyst prepared by incipient wetness impregnation without any capping agent, with an average cluster size of ∼4.9 nm, was compared with the Pt catalyst of a similar cluster size prepared by the colloid method with the capping agent under the same reaction conditions, as shown in Figure 2. Similar activity and selectivity as well as stability for both catalysts confirm that there is no significant effect of the capping agent used in catalyst preparation on the catalyst properties used in this study. The catalytic performance of differently sized Pt catalysts is plotted in Figure 3A. As the Pt particle size remained constant during the reaction (Table 1), the activities at different times on stream (TOS) are all normalized using the initial Pt surface atom number. In general, the smaller sized Pt clusters have higher TOFs, lower selectivities for propylene, and higher deactivation rates. The initial TOF, selectivity, and propylene yield of differently sized Pt clusters are plotted versus Pt cluster size in Figure 3B. Catalyst I, with the smallest Pt cluster size (∼1 nm), is the most reactive for propane conversion

Figure 3. (A) Comparison of the activity and selectivity of catalysts of different Pt cluster sizes, where the TOFs at different TOSs were estimated by using the initial Pt surface area: (◆ and ◇) 1 nm, (■ and □) 3 nm, (● and ○) 5 nm, (▲ and △) 7 nm, and (▼ and ▽) 9 nm. Empty and filled symbols are for selectivity and TOF, respectively. (B) Comparison of the initial TOF and selectivity of catalysts of different Pt cluster sizes: (■) initial propane TOF, (▲) initial propylene TOF, and (●) selectivity for propylene (793 K, 3 kPa C3H8, 3 kPa H2, Ar used as a balance, and WHSV = 23.6 h−1).

(TOFpropane = 1.28 s−1); however, the selectivity for propylene is the lowest (51.9%), and the main byproducts are C−C bond scission products, i.e., methane, ethylene, and ethane. As the Pt cluster size increases, the activity of propane conversion decreases and the selectivity for propylene increases. TOFpropane decreases to 0.45 s−1, and the propylene selectivity increases to 95.8% when the cluster size increases to 9 nm (catalyst V). The opposite changing trend of TOFpropane and propylene selectivity results in a volcano curve for propylene formation rate as a function of Pt particle size, and the Pt catalyst 3 nm in size has the highest TOFpropylene. The fact that Pt cluster size has a significant effect on both activity and selectivity toward methane and coke formation in propane dehydrogenation is in good agreement with the results of our previous experiments.3 It reveals that the smaller Pt clusters with more coordinatively unsaturated Pt atoms on steps and corners have higher activity toward cracking. Thus, the catalyst with smaller Pt clusters has a lower selectivity for propylene. However, these coordinatively unsaturated sites might be poisoned by coke and cause the shift in the selectivity with time on stream. 6313

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DFT results that are essential to the kinetic study are listed and summarized in Table 3, including some previously published

The amount of steps or edges is estimated according to the model proposed by van Hardeveld and Hartog, as given in Table 2.42 The Pt clusters are assumed to be cuboctahedral in

Table 3. Activation Energies and Reaction Energies for Elementary Steps Involved in Scheme 1 on Different Pt Surfacesa (kilojoules per mole)

Table 2. Theoretically Estimated Fractions of Facets and Experimentally Measured Coke Formation Rates on Different Pt Catalysts

Cat Cat Cat Cat Cat

I II III IV V

size (nm)

S(211) (%)

S(100) (%)

S(111) (%)

coke formation rate [mg (g of Pt)−1 ks−1]

1.0 3.0 5.0 7.5 9.1

43 16 9 7 5

7 15 17 18 17

50 69 74 76 78

14.0 7.3 5.3 4.2 3.9

Pt(211) ΔH1 ΔH2c −ΔH5d E2e E3e Eaefff first H second H b

shape, consisting of Pt(111), Pt(100), and steps/edges. Here the coordinatively unsaturated Pt atoms on steps, corners, and edges are estimated as Pt(211) facets. The smallest Pt nanoparticles 1 nm in size are dominated by Pt(211) facets, 43% of the surface Pt atoms. With an increasing Pt cluster size, the fraction of Pt(211) facets decreases while the fraction of Pt(111) increases. The Pt catalyst 9 nm in size is dominated by Pt(111) facets, 78% of the surface Pt atoms. The fraction of Pt(100) also increases slightly. However, it is not dominant on spherical Pt nanoparticles. To understand the effects of different facets on the catalytic performances, DFT calculations were performed to obtain the adsorption energies of intermediates and energy barriers for elementary steps involved in propane dehydrogenation. The kinetics for this reaction has been previously studied both theoretically and experimentally,4−11 and the well-accepted reaction mechanism is shown in Scheme 1.23 It is assumed that

Pt(100)

Pt(111)

clean

C-adg

clean

C-adg

clean

C-adg

−3.9 −46.3 −127.4 30.9 32.8

−1.0 25.0 −58.4 86.9 80.1

−7.7 −29.6 −123.7 41.5 37.6

−1.6 8.2 −78.0 60.4 58.5

−3.9 −6.8 −90.7 67.6 67.6

−1.3 43.1 −11.5 104.3 98.7

27.0 46.3

85.9 133.3

33.8 62.2

58.8 104.1

63.7 102.3

103 146.3

a

The energy of hydrogen adsorption and the data on C-adsorbed surfaces at an equivalent C surface coverage of 1/3 ML were obtained in this work. The other data were reported in our previous work.28,43 b ΔH1 is the energy of propane adsorption. cΔH2 represents the enthalpy change of the first C−H bond activation. d−ΔH5 is the energy of hydrogen adsorption. eE2 and E3 represent the activation energies for the first and second C−H bond activations, respectively. f Effective activation energy on each facet, predicted by eqs 3-8 and 3-9. g Carbon preadsorbed surfaces with a carbon cite coverage assumed to be 33%.

data.28,43 Detailed energetics for the elementary steps involved in Scheme 1 and geometries of the transition states are given in the Supporting Information. On the basis of our DFT calculations, the dehydrogenation activation energies for the first and second C−H bond activations (E 2 and E 3 , respectively) follow the same order: Pt(211) < Pt(100) < Pt(111). It indicates that the catalyst with a higher fraction of Pt(211) on the surface would give rise to a lower dehydrogenation activation energy and thus a higher activity. It is consistent with the experimental findings that smaller Pt clusters with a higher Pt(211) fraction on the surface have a higher dehydrogenation activity and the propane conversion decreases with increasing Pt cluster size. In addition, an attempt has been made to correlate the changes in experimentally observed propylene selectivity with DFT-calculated results. The cracking of C3 derivatives gives rise to the formation of undesired side products, and it significantly lowers the selectivity toward propylene. Our previous DFT calculation determined that the activation energy difference between propylene dehydrogenation and propylene desorption can be the selectivity descriptor.28 The deeply dehydrogenated intermediates of propylene are the precursors for C−C bond activation. Propylene favors adsorption through the di-σ mode on Pt(211). The desorption barrier on Pt(211) is higher than the activation energy for propylene dehydrogenation, which leads to a strong preference for the formation of deeply dehydrogenated intermediates. On the Pt(111) surface, however, the difference in activation energy between propylene dehydrogenation and propylene desorption is much less negative than that on Pt(211). As a result, the selectivity toward propylene is predicted to be higher on Pt(111) than on Pt(211). It is consistent with the experimental observations (see Figure 3) that Pt catalysts dominated by Pt(211) have propylene selectivities much lower than those of catalysts dominated by Pt(111).

Scheme 1. Reaction Pathway of Propane Dehydrogenation23 (S stands for a Pt site)

propane is first physisorbed on the Pt surface, followed by the surface reaction of the first and second C−H activation, and desorption of propylene and hydrogen molecules. Because the activation energies for C−H bond cleavage at the -CH3 and -CH2- groups are very close,28,43 no special preference for the activation of C−H bonds in propane is found. Here the elementary step with a higher energy barrier is taken into account on each facet; that is, the first C−H bond activation refers to the activation of the -CH2- group on Pt(111), while on Pt(211) and Pt(100), the C−H bond at the -CH3 group is first activated. The DFT calculations have been performed on different facets, including Pt(211), Pt(100), and Pt(111). The clean facets together with the facets with carbon preadsorbed were employed to account for the effect of adsorbate−adsorbate interaction on the reaction kinetics. For the sake of clarity, the 6314

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ACS Catalysis 3.2.2. Effects on Coke Formation and Stability. The amount of coke was measured by TPO after the reaction. Our previous study using a tapered element oscillating microbalance (TEOM) revealed a nearly linear increase in coke content with time on stream during propane dehydrogenation on Pt catalysts.44 In this work, the coking rates could be reasonably well estimated directly from the coke content and time on stream based on the assumption of a constant coking rate. As shown in Table 2, the rates of coke formation were estimated to be 3.9−14.0 mg (g of catalyst)−1 ks−1 on different Pt catalyst nanoparticles (1.0−9.1 nm). As shown in Figure 4A, the rate of

energy of propene adsorption (Table 3), the site coverage of *C3H6 follows the order Pt(211) > Pt(100) > Pt(111). In contrast, C−C bond formation between the surface *C3H6 intermediates is expected to follow the order Pt(211) < Pt(100) < Pt(111). The compensation between the opposite dependence of the site coverage of *C3H6 and the rate constant on the Pt facets seem to lead to a weak dependence of the coking rate on Pt cluster size. Figure 3A shows the stability of the catalysts of different Pt cluster sizes. The catalytic activity decreases with time on stream on all the Pt catalysts, but the activity decreases more rapidly on smaller clusters. One of the possible reasons is that the coke preferentially covers the undercoordinated active sites, and coke formation is the main cause of catalyst deactivation.45 Faster coke formation on smaller Pt particles results in faster deactivation. 3.3. Kinetic Study. 3.3.1. Reaction Order in Propane and Hydrogen. Detailed kinetic studies were performed on these catalysts at relatively low propane and hydrogen partial pressures (from 1 to 9 kPa). The initial reaction rates were obtained by extrapolation to zero time. TOFpropane is calculated according to the method described in section 2.3. The reaction orders in propane and hydrogen on different catalysts are calculated as slopes of ln(TOF) versus ln(P) plots, as shown in Figure 5. The propane reaction order remains almost constant

Figure 4. Coke formation rate as a function of (A) platinum cluster size and (B) platinum surface area (793 K, 3 kPa C3H8, 3 kPa H2, 8 h, Ar used as a balance, and WHSV = 23.6 h−1).

coke formation decreases as Pt cluster size increases. Moreover, the coke formation rate scales linearly with Pt surface area, as shown in Figure 4B. It indicates that coke formation is not structure-sensitive, and all Pt facets contribute to coke formation. The higher coking rate on small Pt clusters is a result of a larger Pt surface area. However, it should be noted that the intercept of the coke formation rate curve is not zero at the zero surface area of Pt. A small contribution of the support to coke formation would probably explain this. Another reason might be due to fast coking on Pt surfaces, which is not a linear increment as a function of time. To understand the mechanism leading to structureinsensitive coke formation on Pt during propane dehydrogenation, a better understanding of the nature of the coke and coke formation mechanism is essential. The nature of coke formed and the mechanism for coke formation on supported Pt catalysts were investigated by detailed characterization and kinetic study in our previous work.45 It was revealed that polymer-like coke was formed on Pt surfaces while coke with a high graphitic degree was formed on Al2O3 surfaces caused by acid-catalyzed reaction. The basic supports, namely calcinated hydrotalcites, were used as supports in this work for the purpose of avoiding coke formation on supports.11 There is only one peak in TPO derivative curves, suggesting that coke is mostly formed on Pt surfaces instead of on calcinated hydrotalcites. In addition, the previous kinetic study suggested C−C bond formation between the surface *C3H6 groups to be the ratedetermining step (RDS) in the formation of coke through polymerization.45 The coking rate is then expected to be a function of the site coverage of *C3H6 and the rate constant for C−C bond formation between the two *C3H6 groups. The site coverage of *C3H6 on the surface is governed by the heat of adsorption of propene. On the basis of the DFT-predicted

Figure 5. TOF of propane conversion as a function of (A) propane partial pressure and (B) hydrogen partial pressure on Pt catalysts of different cluster sizes: (■) 3, (●) 5, (▲) 7, and (▼) 9 nm (723 K, 1− 9 kPa propane, 3−9 kPa hydrogen, Ar used as a balance, and WHSV = 23.6 h−1).

(∼1.0), regardless of the size of Pt clusters, while the hydrogen reaction order differs greatly, changing from −0.07 to −0.51 with the Pt cluster size decreasing from 3 to 9 nm. There could be two possible explanations for the large difference in hydrogen reaction order in dehydrogenation of propane on Pt catalysts. One could be a result of a different adsorption strength, and the other could be different reaction mechanisms on differently sized Pt clusters. The decrease in the reaction order with respect to hydrogen with an increasing Pt cluster size possibly suggests that the mechanism changes. Therefore, a detailed kinetic analysis was performed to provide insights into the reaction mechanism. The elementary steps involved in propane dehydrogenation are shown in Scheme 1. Equation 3-1 or 3-2 shows the forward dehydrogenation reaction rate, assuming that the first or second C−H bond activation is the RDS, respectively. The detailed derivation of kinetic equations is included in the Supporting Information. 6315

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ACS Catalysis rC3H8 ,1st =

k 2K1pC H

thermodynamic properties derived by DFT calculations. The calculated results indicate that the H surface coverage is rather low and can be neglected on all three facets [e.g., 1.08 × 10−3 and 1.24 × 10−2 on Pt(111) and Pt(211), respectively, at 793 K and a H2 partial pressure of 3 kPa]. As the reaction order in hydrogen is close to zero on Pt nanoparticles 3 nm in size (see Figure 5B), the first C−H bond activation is most likely the RDS on the small Pt catalysts. On the other hand, the hydrogen reaction order is approximately −0.51 on Pt nanoparticles 9 nm in size, which indicates that the RDS on the large Pt nanoparticles is most likely the second C−H bond activation. For the Pt nanoparticles with a particle size between 3 and 9 nm, no obvious RDS can be identified, and it seems that the first and second C−H bond activations would jointly dominate the overall reaction rate. These results are also in accordance with the results obtained from DFT calculations, as shown in Table 3. On Pt(211), the activation energy for the first C−H bond activation is much higher than that of the second C−H bond activation, and the potential energy diagram going from propane to propylene is apparently downhill; therefore, the first C−H bond activation is most likely the RDS. On Pt(111), however, the activation energies are quite close for both C−H activations. Considering the low surface coverage of C3H7 at very low C3H8 partial pressures,44,45 it is reasonable to expect that the second C−H bond activation is the RDS. 3.3.2. Activation Energy. The significant change in the observed activation energy for dehydrogenation provides another proof of the possible change in the reaction mechanism. The observed activation energy on Pt catalysts of different cluster sizes was measured from 723 to 823 K. The initial observed activation energy (Eobs,0) was calculated on the basis of the TOF at a TOS equal to 0, while the observed activation energy at steady state (Eobs,s) was calculated on the basis of the TOF after the reaction had reached steady state. As plotted in Figure 6, the observed activation energies for dehydrogenation increase with increasing Pt cluster size, but the increase in activation energy at steady state (Eobs,s) is much less dramatic than that of the initial activation energy (Eobs,0).

3 8

0.5

(1 + K1pC H + pH 3 8

K3−1K4 −1K5

2

K5

−0.5

+ pC H K4

−1

3 6

+ pC H pH 3 6

0.5

2

−0.5 2

)

(3-1) −0.5

0.5

rC3H8,2nd =

k 3K1K 2K5 pC H pH 3

8

2

(1 + K1pC H + pC H K4 −1 + pH2 0.5 K5−0.5 3

8

3

6

−0.5 2

0.5

+ K1K 2K5 pC H pH 3

8

2

)

(3-2)

where ki and Ki (i = 1−5) are the forward reaction rate constants and equilibrium constants of step i shown in Scheme 1, respectively. Given the fact that the observed reaction order in propane remains at 1, the site coverages of propane and C3H7 should be low, so that the corresponding equilibrium constants for propane adsorption and propane dehydrogenation are small as well and can be neglected in eqs 3-1 and 3-2. Moreover, the experiments were conducted under rather low propane conversions and at low propane pressures (1−9 kPa C3H8), and therefore, the site coverage (and hence the partial pressure) of propylene is expected to be low. As a result, the forward dehydrogenation reaction rates (i.e., eqs 3-1 and 3-2) can be simplified into eqs 3-3 and 3-4, respectively. It should be noted that these two simplified rate expressions are valid only under the conditions employed in this work. rC3H8,1st =

rC3H8,2nd =

k 2K1pC H 3

(1 + pH

0.5 2

8

K5−0.5)2

(3-3)

k 3K1K 2K50.5pC H pH −0.5 3

(1 + pH

0.5 2

8

2

K5−0.5)2

(3-4)

Furthermore, we can obtain the relationship between reaction orders and the site coverage of hydrogen by eqs 3-5 and 3-6 and eqs 3-5 and 3-7 with the first and second C−H bond activations assumed to be the RDS, respectively. Detailed derivation of these equations is included in the Supporting Information. nC3H8 = 1

(3-5)

n H 2 = − θH

(3-6)

n H2 = −0.5 − θH

(3-7)

The analysis described above suggests that the reaction order in propane remains at 1 at the low site coverages of the intermediates, no matter which dehydrogenation step is the RDS. It is in agreement with our experimental results for propane reaction order, as shown in Figure 5A. The reaction order in hydrogen, however, varies from −θH to −0.5 − θH when the assumed RDS is changed from the first C−H bond activation to the second C−H bond activation. Therefore, eqs 3-6 and 3-7, in combination with the corresponding experimental data given in Figure 5B, might be used to determine the RDS for propane dehydrogenation on both small and large Pt particles. Because no matter which C− H bond activation step is assumed to be the RDS, step (5) in Scheme 1 is in equilibrium, so that the H surface coverage can be computed with the Langmuir adsorption isotherm and the

Figure 6. Observed activation energies for propane conversion on catalysts of different Pt cluster sizes, (● and ■) initial and steady state apparent activation energies calculated using experimental data, and apparent activation energies predicted by DFT calculations through eq 3-10: (···) on clean facets, (-·-) on clean Pt(111) together with carbon preadsorbed Pt(211)/Pt(100) (case I), and (---) on clean Pt(111) together with deactivated Pt(211)/Pt(100) (case II) (723−823 K, 3 kPa propane, 3 kPa hydrogen, Ar used as a balance, and WHSV = 23.6 h−1). 6316

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As shown in Figure 3A, coke formation caused a decrease in the reaction rate. Coke formed on the Pt surface has two effects, namely, blockage of active sites as well as electronic modification of Pt surfaces by adsorbed carbonaceous species. DFT calculation revealed that Pt(211) and Pt(100) have higher potential for deep dehydrogenation of propylene and subsequent cracking than Pt(111). It is possible to form atomic carbon by C−C bond cleavage of highly unsaturated intermediates. In addition, previous theoretical work suggested that adsorption of C on the surface changes significantly the electronic properties and thus the adsorption energy as well as the activation energy.47,48 Therefore, in this work, the elementary steps were also investigated on the Pt surfaces with C atoms preadsorbed. On Pt(211), all the Pt steps were blocked by C atoms, giving rise to a C step-edge coverage of 1 ML. On Pt(100), an equivalent C surface coverage (1/3 ML) was used. The calculated kinetic and thermodynamic parameters, as summarized in Table 3, are also used to predict the apparent activation energy on Pt catalysts. To compare the observed activation energies at steady state with the DFT-calculated apparent energy barriers, two extreme cases were considered and the calculated activation energies are given in Figure 6B, as well. In one case (case I), carbon partially blocks Pt(211) and Pt(100), and these two surfaces, together with clean Pt(111), are active for PDH. In the other case (case II), Pt(211) and Pt(100) are completely deactivated, and only clean Pt(111) facets are active for PDH. The multifaceted model was also used to estimate the apparent activation energy on multifaceted Pt catalysts. As shown in Figure 6B, the experimental Eobs,s lies between these two extreme conditions. As the multifaceted kinetic model succeeds in predicting the apparent activation energies at both the initial and the steady states, the assumption that the first C−H bond activation is the RDS on Pt(211) and Pt(100) and on Pt(111) the second C−H bond activation dominates the overall kinetics is reasonable. In addition, the computation suggests that deactivation is faster on Pt(211) and Pt(100) than on Pt(111), and carbon on Pt(211) and Pt(111) not only blocks the sites but also modifies the local electronic structures of Pt. 3.4. Kinetic Isotope Effect. The kinetic isotope effect (KIE) is a useful method for studying reaction mechanism. In our study, the KIE experiment was performed to examine the dehydrogenation reaction mechanism change on Pt catalysts of different Pt cluster sizes. KIE in oxidative propane dehydrogenation has been studied by Iglesia and co-workers.48 The contributions to KIE come from four different parts: translational, rotational, vibrational, and electronic partition functions. In the propane dehydrogenation reaction, the KIE can be expressed as eq 3-11 or 3-12 if the first C−H or second C−H bond activation is assumed to be the RDS, respectively. The detailed derivation is included in the Supporting Information.

To identify the RDS on each Pt facet by taking into account the change in the observed activation energy, a multifaceted kinetic model was employed. Taking the partial derivative of ln(r) with respect to the reciprocal temperature allows for the calculation of the apparent activation energy for propane dehydrogenation on multifaceted catalyst particles.46 First, the apparent activation energy on each Pt facet was calculated through eqs 3-8 and 3-9 with the first and second C−H bond activations assumed to be the RDS, respectively, and the calculated data are summarized in Table 3. Detailed derivation of the equations is included in the Supporting Information. Ea,obs = E2 + ΔH1 + θHΔH5

(3-8)

Ea,obs = E3 + ΔH1 + ΔH2 + (0.5 + θH)ΔH5

(3-9)

where E2 and E3 represent the activation energies for the first and second C−H bond activations, respectively, ΔH 1 represents the energy of propane adsorption, ΔH2 represents the enthalpy change for the first C−H bond activation, −ΔH5 represents the energy of propylene adsorption, and θH represents the hydrogen site coverage. Considering the conclusion that has been reached in section 3.3.1, it is reasonable to assume that on Pt(211) and Pt(100) the first C−H bond activation is the RDS, while on Pt(111) the second C−H bond activation dominates the overall kinetics. Then, the multifaceted kinetic model was employed to take into account the contributions from different facets on the real Pt catalyst to kinetics.46 Here we made two assumptions to simplify the kinetic analysis. The first is that the mobility of adsorbed intermediates and transition state complexes (and hence pre-exponential factor ki) is similar on different facets. The second is that surface diffusion is much faster than reaction rate, giving rise to similar site coverage of adsorbates on different facets. By taking into account these assumptions, we can determine the resulting apparent activation energy, Ea,app, for propane dehydrogenation on multifaceted catalysts via N

Ea,app =

N ∑i =facets 1

−Ea, i

( )E ϕ exp( )ϕ

∑i =facets exp 1

RT

a, i i

−Ea, i RT

i

(3-10)

where Nfacet is equal to 3, accounting for (211), (100), and (111) facets, Ea,i is the apparent activation energy on different facets, and ϕi is the mole fraction of each facet on the Pt surface. The apparent activation energies over Pt catalysts of different sizes were estimated with eq 3-10 using the DFT-calculated results (see Table 3) and the mole fraction of different facets given in Table 2. Figure 6A shows the predicted apparent activation energies at zero H surface coverage on different Pt catalysts. The multifaceted model successfully predicted the tendency of the increase in Eobs,0 with an increasing number of Pt clusters. All three facets, including Pt(211), Pt(100), and Pt(100), contribute to the reaction. However, it should be noted that there is a deviation between the predicted activation energy and the experimental data, and that the deviation is greater on the larger particles. This can be explained by the blockage of step sites even at the initial state, and the kinetics on larger particles with less step sites are affected more dramatically. In addition, the co-adsorption of hydrogen and other intermediates could cause an increase in the dehydrogenation barrier, which is neglected in this work.

Ξ

Ξ

KIE = 2.23e 0.3(EC−H − EC−H − E H−Pt)/ RT Ξ

Ξ

KIE = 2.17e 0.3(2EC−H − EC−H − E H−Pt)/ RT

(3-11) (3-12)

in which EC−H is the energy of the C−H bond in propane and EΞC−H and EΞH−Pt are the energies of partially formed C−H and H−Pt bonds in the transition state, respectively. The 0.3EC−H term (=4.83 kJ) can be obtained from the literature.49 The energy of EΞC−H and EΞH−Pt is not available from theoretical calculations but can be obtained from the isotope experimental results. 6317

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■

We further studied the Pt cluster size effect with isotopic experiments by switching C3H8 to C3D8 after reaching the steady state. As shown in Figure 7, a normal kinetic isotope effect of 1.5 (rD/rH) has been found on Pt catalysts 3 nm in size, while KIE equals 2.5 (rD/rH) for 9 nm Pt catalysts.

Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.5b01423. Computational details, theoretical treatment of kinetic isotope effect, and derivation of kinetic equations (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The work is supported by the Norwegian Research Council and Natural Science Foundation of China (21003046 and 21473053). The computational time provided by the Notur project is gratefully acknowledged.

Figure 7. KIEs of propane dehydrogenation on (A) 3 and (B) 9 nm Pt catalysts.

■

REFERENCES

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According to the KIE of 1.5 on 3 nm Pt catalysts, the energy of EΞC−H + EΞH−Pt is calculated to be 24.8 kJ. If it is applied to eq 3-12, the KIE on 9 nm Pt catalysts can be calculated to be 3.0, which is close to the experimental result of 2.5. Thus, the change in KIE confirms the result obtained from kinetic analysis that the RDS in the reaction mechanism changes from the first C−H bond activation on 3 nm Pt catalysts to the second C−H bond activation on 9 nm Pt catalysts.

4. CONCLUSION Calcined hydrotalcite-supported platinum catalysts of different cluster sizes (3−9 nm) were prepared by the stepwise growth method. The Pt colloids were homogeneously distributed with nearly monodispersion. It has been shown that dehydrogenation of propane on a Pt catalyst is highly structure-sensitive. The activity of propane conversion decreases with an increasing Pt cluster size, while the selectivity to propylene performs in an inverse way. The catalyst with smaller cluster sizes, with (211) dominating on the surface, has a lower dehydrogenation energy barrier and thus a higher activity. However, the weakened binding strength of propylene and increased energy barrier for propylene deep dehydrogenation on Pt(111) contribute to the high selectivity toward propylene and better stability on the Pt catalysts of larger cluster sizes, on which (111) is dominant on the surface. While the reaction order in propane is identified as 1 for all the catalysts, the reaction order in hydrogen is found to change from 0 on a catalyst with a Pt cluster size of 3 nm to −0.5 on a catalyst with a Pt cluster size of 9 nm. In addition, the observed activation energies also varied from 80 to 96 kJ/mol with an increasing Pt particle size. Through kinetic analysis, including a multifaceted kinetic model coupled with DFT calculations and kinetic isotope effect experiments, the first C−H bond activation is identified as the RDS on Pt(211) and Pt(100) while the second C−H bond activation is found to be kinetically relevant on Pt(111). Considering the different mole fractions of each facet on Pt clusters of different sizes, our combined experimental and theoretical work provides a rational interpretation of the size-dependent reaction mechanism and kinetics for propane dehydrogenation. 6318

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