Hydrogen Production via Hydrazine Decomposition on Model

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Hydrogen Production via Hydrazine Decomposition on Model Platinum-Nickel Nanocatalyst with Single (111) Facet Shirin Norooz Oliaee, Changlin Zhang, Sang Youp Hwang, Harry M. Cheung, and Zhenmeng Peng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00815 • Publication Date (Web): 21 Apr 2016 Downloaded from http://pubs.acs.org on April 22, 2016

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The Journal of Physical Chemistry

Hydrogen Production via Hydrazine Decomposition on Model Platinum-Nickel Nanocatalyst with Single (111) Facet Shirin Norooz Oliaee, Changlin Zhang, Sang Youp Hwang, Harry M. Cheung, and Zhenmeng Peng* Department of Chemical and Biomolecular Engineering, The University of Akron, Akron, Ohio 44325, United States

ABSTRACT: Model octahedral Pt-Ni/C nanocatalyst with different particle composition (0.5≤Pt/Ni≤4) was prepared and researched in hydrazine decomposition for hydrogen generation. The experiments discovered dependency of the catalyst activity, selectivity, durability, and stability on both the particle composition and morphology, with the octahedral PtNi/C being found the most active, 100% selective, and of superior durability and stability. Mechanistic studies and discussions on the reaction mechanism and kinetics were conducted by collecting the kinetic data and deriving the rate law. The finding suggests both synergistic effect between Pt and Ni active sites and morphology-originated geometric effect, which play crucial roles in determining the catalyst property.

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INTRODUCTION The demand for safe and efficient on-board hydrogen production for various applications, in particular hydrogen fuel cells, is increasing during the past decades1,2. Hydrazine is considered as one of the most promising hydrogen storage medium for this purpose. It contains 12.5 wt.% hydrogen in the molecule, which ensures high hydrogen storage capacity, and is liquefied under ambient condition, which makes it compatible with the current infrastructure for liquid fuels. The exothermic characteristic of hydrazine decomposition allows the reaction to occur at room temperature, thermodynamically speaking. In practice, catalyst materials are needed to overcome the reaction energy barrier and promote the kinetics. Hydrazine can decompose via two routes, which can be depicted as below and lead to different products3-5: → + 2

(1)

3 → + 4

(2)

The release of undesired ammonia via Eq. 2 causes less H2 generation, catalyst deactivation due to poisoning problem, and environmental pollution. Thus a favorable catalyst for the reaction should not only actively but also selectively decompose hydrazine into H2. Besides, excellent durability and stability are important metrics of a catalyst to meet the long-term operation requirement. Considerable research efforts were put forth to develop catalysts for the hydrazine decomposition reaction. For instance, several noble metals, including Ir, Rh, Ru, Pd, and Pt, were investigated and showed catalytic activity in decomposing hydrazine6,7. However their insufficient activity, low H2 selectivity, and their high prices make them unfavorable catalysts. Non-noble transition metals were also studied for the reaction, among which Ni was found the most promising catalyst for its satisfying activity and selectivity and thus received much research attention. Ni nanoparticles were prepared using different methods and deposited on various types


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of support materials, such as Al2O3, CeO2, SiO2, graphene, polymeric and metal-organic frameworks, and investigated for their hydrazine decomposition property8-18. However one common problem with Ni nanoparticle catalysts is their instability. Ni metal is susceptible to oxidation and quickly forms an oxide surface layer when being exposed to air, which deactivates the catalyst and raises a severe stability issue. To improve the stability of Ni catalyst, researchers alloyed Ni with noble metals and demonstrated the composition effect on the property. Among all Ni alloys, Pt-Ni nanoparticles were found promising group of catalyst and thus intensively studied19-33. For instance, Singh et al investigated the Pt-Ni bimetallic catalysts for hydrazine decomposition, which were found active and hydrogen selective when the Pt content was within the range of 7-31 mol.%. In comparison, the physical mixture of monometallic Ni and Pt were inactive and the Ni@Pt core-shell nanoparticles showed very poor activity, demonstrating synergistic effect between Pt and Ni for hydrazine decomposition catalysis20,23,33. Supported PtNi on graphene was also reported with high turnover frequency (TOF) of 133 h-1 and 100% H2 selectivity for hydrazine decomposition in alkaline environment14. However, many inconsistent results were reported regarding the catalytic property, including activity, selectivity, durability, stability, and even the rate law, which could be associated with their different particle parameters as well as the testing condition. Most recent studies on Pd-Ir nanocubes, Rh-Ni and Rh-Cu octahedral nanostructures revealed that hydrazine decomposition was altered in kinetics and selectivity with catalyst surface34-37. Considering the reaction is sensitive to both catalyst composition and surface, preparation of model Pt-Ni nanoparticle catalysts with control over both particle composition and morphology is thus necessary to study their effects on the catalytic property. Careful mechanistic experiments are also needed to study the reaction mechanism and understand the relationship between the particle parameters and the property.


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In the present study, we prepared both model octahedral Pt-Ni/C nanoparticle catalysts, which exposed exclusively the (111) surface, and their spherical counterparts, which contained a mixture of different low-index surfaces, to study the shape effect on hydrazine decomposition. The particle composition of the octahedral Pt-Ni/C was adjusted in a broad range of 0.5≤Pt/Ni≤4 to investigate the influence of particle composition on the catalyst property. Kinetic experiments were conducted to interpret the reaction mechanism on Pt-Ni and to understand the shape and composition effects. Our studies show that the dependency of the Pt-Ni property, including activity, selectivity, durability, and stability, on the particle parameters could be associated with the synergistic effect between Pt and Ni active sites as well as their arrangement in the surface.

EXPERIMENTAL SECTION Materials: Platinum acetylacetonate (Pt(acac)2, 97%), nickel acetylacetonate (Ni(acac)2, 95%) and anhydrous hydrazine (N2H4, 98%) were purchased from Sigma-Aldrich. Acetone (C3H6O, 99.8%), and chloroform (CHCl3, 99.9%) were purchased from Fisher Scientific. Carbon support (C, Vulcan® XC-72R) was purchased from Cabot. Hydrogen (H2, 99.999%), carbon monoxide (CO, 99.5%), and nitrogen (N2, 99.998%) gases were obtained from Praxair. Catalyst preparation: The octahedral Pt-Ni/C catalysts (0.5≤Pt/Ni≤4) were prepared using a modified solid-state chemistry method which involves impregnation of both metal precursors on a carbon support and then reduction in CO and H2 gas mixture. Typically, for preparing octahedral PtNi/C (10 wt.% Pt and Pt/Ni =1 ), C was pretreated in air at 300 oC overnight for removal of moisture. Pt(acac)2 (20 mg or 0.05 mmol) and Ni(acac)2 (13.0 mg or 0.05 mmol) were first dissolved in acetone (2 mL), and then added drop wisely onto the pretreated C support (90 mg). After the impregnation, the mixture was transferred to the furnace and was purged by


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N2 flow for 20 minutes. The mixtures were reduced by being heated at a ramping rate of 5 °C/min to 200 °C and maintaining at the temperature for 1 hour in H2/CO (5/150 cm3/min). The gas atmosphere was switched back to N2 and the product was cooled down to room temperature after the reaction. The octahedral Pt-Ni/C with different Pt/Ni molar ratios were prepared using a similar procedure but with different amounts of Pt(acac)2 and Ni(acac)2 precursors. The spherical Pt-Ni/C catalysts were prepared by using a similar method but the reduction was carried out in 150 cm3/min of H2 flow. For preparation of 10 wt.% Pt/C and 10 wt.% Ni/C, the precursor solution was first impregnated on carbon, followed by calcination at 300 oC for 1 hour and then reduction in H2 at 300 oC for 1 hour. Characterization: Transmission electron microscopy (TEM) images of catalyst samples were characterized by a JEOL JEM-1230 microscope operated at 120 kV. High-resolution TEM (HRTEM) of individual Pt-Ni/C nanoparticles was taken using a FEI Tecnai G2 F20 microscope operated at 200 kV. The X-ray diffraction (XRD) patterns were recorded on a Bruker AXS Dimension D8 X-Ray diffractometer with Cu Kα radiation source. Composition Analysis of the as-prepared Pt-Ni/C catalysts was conducted using EDX equipped on a JEOL-7401 field emission scanning electron microscope (FESEM) with an operating voltage at 25 kV. The accurate metal loading of Pt-Ni/C products was determined by heating the samples at 10 °C/min to 750 °C in a flow of air (60 cm3/min) and measuring weight of residue in a thermal gravimetric instrument (TA Instruments, Model Q500). Catalytic property test: The experiments were conducted in a semi-batch flask reactor containing 10 mL of water and 100 mg of catalyst. The reaction temperature was controlled using an oil bath. Under magnetic stirring of 450 rpm, the reaction was initiated by injecting designated amount of N2H4 into the reactor. The amount of generated gas was measured using an


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airtight gas burette after either passing or bypassing an ammonia trap. Hydrazine conversion (XN2H4) was determined by ratio of the reading from burette to the theoretical total volume of generated gas if the hydrazine is completely decomposed to N2 and H2. The mass activity (rʹN2H4) and Turnover Frequency (TOF) was calculated by the following equations:

− = − . =

!" ∙!$

(3)

(4)

Where N0 is initial mole of hydrazine, W is the mass of catalyst, dXN2H4/dt is obtained from the XN2H4-t plot, SA is the specific surface area calculated from the measured Pt-Ni loading and particle size, and SD is the surface density of atoms on (111) surface of Pt-Ni particles. The overall H2 selectivity value was determined based on the measured amount of gases using the following equation: +, ) , + ('() * , ) ,, . , ('() *

% =

(5)

Where Ngas is the total amount of gas measured without using trap, and NNH3 is obtained from the difference in the amount of gases measured with and without using ammonia trap. The gas effluent was also analyzed using an online gas chromatograph equipped with TCD detector (GC, Shimadzu GC-2014) by flowing Ar carrier gas at 100 sccm through the flask reactor, which carried the gas products to the GC. The H2 and N2 concentrations were calibrated using standard gas mixture with H2/N2.

RESULTS AND DISCUSSION The octahedral Pt-Ni/C (10 wt% Pt, 0.5≤Pt/Ni≤4) catalysts with different nanoparticle composition were prepared using a solid-state chemistry method developed in the lab38-40. Figure


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1a shows representative transmission electron microscopy (TEM) image of the as-prepared PtNi/C nanoparticles, most of which are in an octahedral morphology and have an average particle size of 6.5±2.2 nm. Figure 1b shows the high-resolution TEM (HRTEM) of one single octahedral PtNi particle with straight edges. Clear lattice fringes of 2.13Å are observed, indicating high crystallinity of the PtNi particle41. Powder X-ray diffraction (PXRD) of the sample, as shown in Figure 1c, exhibits one broad diffraction peak at 24.9o, which is associated with the carbon support, and three diffraction peaks for the PtNi, which can be assigned to (111), (200) and (220) planes of face-centered cubic (fcc) structure. The peaks are located between those for pure Pt and Ni, confirming the formation of alloy phase of Pt-Ni42. Quantitative energy dispersive X-ray (EDX) characterizations and thermos-gravimetric analyses (TGA) were performed for determining the actual particle composition and metal loading (Figure 1d and 1e). The octahedral PtNi nanoparticles contains 50.3 at.% of Ni and 49.7 at.% of Pt on average, which is close to the amount of metal precursors used for the sample preparation. The total metal loading of the octahedral PtNi/C was determined to be 12.93 wt.%. Octahedral Pt-Ni/C of other particle composition, together with spherical Pt-Ni/C, Pt/C, and Ni/C, were also prepared, with the TEM images shown in Figure S1 and sample information summarized in Table 1. The particle composition effect of the octahedral Pt-Ni/C catalysts was studied by testing and comparing the catalytic property in hydrazine decomposition under a same reaction condition. The experiments were conducted in a semi-batch flask reactor, which contained 100 mg catalyst in 10 mL of 0.05 M hydrazine aqueous solution and was connected to a burette for measuring the amount of generated gases. The temperature of the mixture was maintained at 50 °C during the course of reaction. Figure 2 shows the hydrazine conversion (XN2H4) as a function of reaction time using different catalysts. Positive XN2H4 values were obtained when using the Pt/C and the


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Ni/C as the catalysts, indicating both catalyze hydrazine decomposition. However, the increases in XN2H4 with reaction time were very slow and suggested low activity of the two catalysts. The octahedral Pt-Ni/C catalysts (denoted as O-Pt-Ni/C) exhibited varied rates in the reaction with the Pt-Ni composition, evidenced by the different slopes of their XN2H4-t curves. The results suggest a composition effect of the Pt-Ni on the catalytic activity. Both initial mass activity (-rʹN2H4,0) and turnover frequency (TOF), which represents intrinsic activity of single active site, values of the catalysts were calculated and plotted against the particle composition for quantitative comparison (Figure 3a and 3b). The Pt/C and the Ni/C showed low TOF of 13 and 16 h-1, respectively. All five octahedral Pt-Ni/C catalysts exhibited higher TOF values than pure Pt and Ni. The finding suggests a synergistic effect between Pt and Ni active sites, which promote each other and lead to enhanced hydrazine decomposition kinetics. The TOF values were significantly different with the particle composition, demonstrating a strong dependency of the Pt-Ni activity on this parameter. The highest TOF of 210 h-1 was obtained with the octahedral PtNi/C, indicating the catalyst has the optimal particle composition for the reaction. The H2 selectivity of the octahedral Pt-Ni/C catalysts was investigated by putting an ammonia trap before the burette and measuring any difference in the amount of gases generated during hydrazine decomposition. The two obtained conversion plots with and without using the trap overlapped well with each other when using the octahedral PtNi/C catalyst (Figure 3c), indicating little ammonia production and thus high selectivity towards H2 production during the reaction. The overall H2 selectivity value was determined to be 100% based on the difference in the amount of gases measured with and without using ammonia trap (Eq. 5). The excellent selectivity was further confirmed by analyzing the gas products using online GC (Figure 3d and


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S2). The octahedral Pt3Ni/C catalyst also exhibited 100% selectivity towards H2 generation. In comparison, the H2 selectivity of the spherical PtNi/C and Pt3Ni/C (denoted as S-PtNi/C and SPt3Ni/C) was limited to only about 72% and 75% respectively, indicating the generation of considerable amount of ammonia byproduct. Meanwhile a lower TOF was obtained for the octahedral Pt3Ni/C catalyst than that for its spherical counterpart. The experimental data suggest a significant shape effect on both Pt-Ni activity and selectivity, with the (111) surface favors for selective H2 generation but has a lower activity. The octahedral PtNi/C catalyst was tested at different reaction temperature for investigating the reaction kinetics. Figure 4a shows the obtained series of XN2H4-t plots in the temperature range from 20 to 60 °C. The XN2H4 values increased exponentially with the reaction temperature, a typical characteristic of reaction following the Arrhenius law. The activation energy was calculated to be 55.3 kJ.mol-1 from the Arrhenius plot in Figure 4b. This value is in decent agreement with reported values (~ 43.9-55.7 kJ.mol-1) for Pt-Ni bimetallic catalysts18,22,29,43. The influence of hydrazine concentration on its decomposition rate was studied by adjusting the initial concentration of hydrazine, CN2H4,0. Figure 5a shows the initial reaction rate, -rʹN2H4,0, which was calculated from the XN2H4-t data and exhibited a strong dependence on the CN2H4,0 values (Figure S3). The -rʹN2H4,0 was measured to be 12.1 µmol·s-1·g-1catalyst at 0.05 M hydrazine. The value increased to 28.3 µmol·s-1·g-1catalyst when a higher CN2H4,0 of 0.20 M was used, representing more than 2 times higher reaction rate. The experimental data suggest that the reaction kinetics on Pt-Ni is not zero-order over the hydrazine concentration as being claimed in some papers44-46. Possible reaction mechanisms were proposed for the catalytic hydrazine decomposition in previous studies47-55. It is generally believed that the reaction on metal surfaces initiates from


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adsorption of hydrazine molecules, which then dehydrogenate by breaking the N-H bonds and generate N2Hx (1≤x≤3) intermediates and adsorbed hydrogen species. A complete dehydrogenation of the N2Hx intermediates and recombination between adsorbed hydrogen atoms lead to the generation of final products, N2 and H2. The ammonia byproduct generation is associated with the N-N bond breakage in the N2Hx species. The hydrogen generation pathway on the octahedral Pt-Ni can be proposed as follows: /0 + % ⇌ ∙ % /0

K1= k1/k-1

(6)

/ ∙ % + % ⇌ ∙ % + ∙ % /

K2= k2/k-2

(7)

/ ∙ % + % ⇌ ∙ % + ∙ % /

K3= k3/k-3

(8)

/ ∙ % + % ⇌ ∙ % + ∙ % /

K4= k4/k-4

(9)

/2 ∙ % + % ⇌ + ∙ % /2

K5= k5/k-5

(10)

/3 ∙ % + ∙ % ⇌ + 2% /3

K6= k6/k-6

(11)

Where S represents active site on the catalyst surface, and ki, k-i, and Ki, are rate constants and equilibrium constants. Some mechanistic studies suggested the first deprotonation step (Eq. 7) is the rate-limiting step9,14,24. The rate law can thus be derived as below: − ′ =

5 67 8 89 ,/ 7/ 7/ : : : : : : : 0. 67 8 . . . . ,/

Hydrogen Production via Hydrazine Decomposition on Model

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