Carbon Allotropes Accelerate Hydrogenation via Spillover Mechanism

Oct 20, 2014 - Efrat Ruse,. †,‡ and Oren Regev*. ,‡. †. Department of Chemistry, Nuclear Research Center Negev, P.O. Box 9001, 84190 Beer Shev...
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Carbon Allotropes Accelerate Hydrogenation via Spillover Mechanism Svetlana Pevzner,*,† Ilan Pri-Bar,‡ Itay Lutzky,‡ Eyal Ben-Yehuda,† Efrat Ruse,†,‡ and Oren Regev*,‡ †

Department of Chemistry, Nuclear Research Center Negev, P.O. Box 9001, 84190 Beer Sheva, Israel Department of Chemical Engineering, Ben-Gurion University of the Negev, 84105 Beer Sheva, Israel

‡

S Supporting Information *

ABSTRACT: Solid-phase hydrogenation kinetics can be substantially increased by utilizing hydrogen spillover phenomenon. Carbonaceous allotropes are considered as promising spillover agents (SOAs) for improved hydrogen transport rate. We studied the effect of carbon-based SOA properties on irreversible hydrogenation. We divided the reaction into two major stages, near- and far-field hydrogenation (with respect to a catalyst), and determined their rate-limiting steps. The hydrogenation kinetics was analyzed for hydrogen originating from either catalyst on activated carbon or catalyst-decorated carbon nanotubes. The far-field hydrogenation is investigated for three types of loaded nanocarbons: 1D (nanotubes), 2D (graphene), and 3D (activated carbon). We found that the kinetics acceleration is strongly correlated with the nanocarbon dimension, 1D > 2D > 3D, and could reach almost 2 orders of magnitude. These findings are useful for the study of reversible hydrogen storage applications.

1. INTRODUCTION Hydrogen storage is one of the key enabling technologies for hydrogen economy realization. The development of an efficient solid-state hydrogen storage1−4 is essential for the commercialization of hydrogen-fueled engines and proton-exchange membrane fuel cells.5 Solid-phase catalyzed hydrogenation kinetics relies on hydrogen atoms transport rate into an acceptor.1,4,6,7 Key processes in such reactions are surface diffusion and hydrogen spillover.2,7 The spillover phenomenon describes the transport of activated species, sorbed on one surface, onto another, which is not capable of producing such species.6 Hydrogen spillover (HSO) refers to a process in which H atoms, produced by H2 dissociation on the surface of a reactive metal catalyst (MC), migrate to another admixed phase termed spillover agent (SOA). The SOA assists in a hydrogen atom transport to a solid acceptor, where it is finally stored.2,7 Carbonaceous materials are employed both as hydrogen acceptors2,4 and as efficient SOAs due to their high surface area;2,4,7a few examples are activated carbon (AC),8−12 carbon nanotubes (NTs),12−14 graphite,15 and graphene nanoplatelets (GNPs).16,17The kinetics of hydrogen spillover in solid composites on carbonaceous materials has not been elucidated yet. In powder composites consisting of carbon-supported MC and SOA, H atoms migration can be divided into primary and secondary spillover paths (Figure 1). The former relates to H atom transfer from the MC surface to the AC or NT support, and the latter relates to further migration to another SOA (e.g., NT, GNP).4,12,18 From there, H atoms finally absorb into a solid-phase hydrogen acceptor.19 Being a surface phenomenon, hydrogen spillover kinetics strongly depends on the carbonaceous SOA’s specific surface © 2014 American Chemical Society

Figure 1. Schematic view of hydrogen spillover (HSO) in composites consisting of hydrogen acceptor, Pd on activated carbon (AC) with pristine NT (composite A), and hydrogen acceptor, Pd-decorated NT with pristine NT (composite B).The initial dissociation of hydrogen molecule on Pd nanoparticle is followed by hydrogen spillover (HSO) to AC or NT (1a and 1b), termed primary spillover. The secondary HSO is conducted either between AC to NT (2a), AC to AC (2b), or NT to NT (2c). The termination step is a nonreversible HSO to the unsaturated hydrogen acceptor (3a and 3b).

area, morphology, impurities concentration, and defects density.1,6,7,14,18,20 In the present work we focus on the effect of the SOA properties on hydrogen spillover kinetics in an irreversible hydrogenation of model composites. Our model system consists of solid unsaturated hydrogen acceptor, supported MC, and SOA. The understanding of hydrogen spillover mechanism and identifying the properties affecting the rate acceleration will be useful in designing a reversible hydrogen Received: August 29, 2014 Revised: October 18, 2014 Published: October 20, 2014 27164

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Figure 2. TEM and SEM micrographs of Pd nanoparticles on NT: (a) Pd/NC7000 (TEM); (b) size distribution of Pd/NC7000 and CT < 8, based on 100 NPs and extracted from TEM imaging; (c) Pd/NC7000 (SEM); (d) Pd/NC7000 cluster (SEM); (e, f) SEM/EDS elemental analysis of the cluster in (d): light blue, carbon (e) and red, Pd (f).

HHTFO). The reaction mixture is intermittently microwaved (90 W; 10 s pulses separated by 60 s intervals) until a temperature of 150 °C is reached. The resulting Pd-decorated NT (Pd/NT) reaction mixture is centrifuged (20 min, 4000 rpm), followed by filtering through Fluoropore membrane (Merck Millipore type GVWP, 0.2 μm pore), rinsing with 75 mL of DI water, and overnight drying at 70 °C. 2.3. Composite Preparation. Pd/AC (for composite A) or Pd/NT (for composite B), carbonaceous additive, and DPA are mixed for 15 min at 600 rpm in a planetary ball-milling machine (Fritsch Pulverisette 6) equipped with agate grinding bowl (80 mL) and five agate 10 mm balls (powder-to-balls weight ratio is 0.2). This procedure was applied for all composites. 2.4. Composite Characterization. The kinetics of the hydrogenation is determined by a homemade computercontrolled volumetric pressure measurement system (Sievertstype apparatus).24 A multistage pulse hydrogenation is performed at ambient temperature. The portion of hydrogen introduced at each pulse is 5% of the theoretical hydrogen capacity of the acceptor (calculated from sample weight and composition). The total number of portions is 10. A convenient measure of the reaction rate is the time needed to complete a given fraction of the theoretical acceptor capacity. The cumulative time needed to reach this fraction (e.g., 50%) is accordingly termed t50. (Figure S1 of the Supporting Information demonstrates the kinetic profile of the pulse hydrogenation reaction.) Thermal gravimetric analysis (TGA, Mettler Toledo Star, STDA85) of the dried Pd/NT is performed under an air flow rate of 50 mL·min−1, to ensure a complete oxidation of the carbonaceous ingredients, leaving only deposited palladium residue and other inorganic impurities. The sample is heated (10 °C·min−1 rate) from 40 to 700 °C, at which it remains for 30 min.

storage system, as defined by the U.S. Department of Energy (DOE)21 (i.e., charge/discharge rate, capacity, and life cycle). We have recently found7 that the hydrogenation kinetics depends on the properties of the SOA (namely, NT). In this article we explore the dimensionality effect, where NT is onedimensional (1D), GNP is 2D, and AC is 3D. We also focus on the effect of NTs decoration by Pd (composite B, Figure 1) on the hydrogenation kinetics in comparison to nondecorated NT (composite A, Figure 1). By comparing two types of composites, we analyze the kinetics of different spillover stages to establish the rate-limiting step in the reactions studied.

2. EXPERIMENTAL SECTION 2.1. Materials. Activated carbon M-2184 (64365-11-3) (AC-1) was purchased from Merck. Palladium on activated carbon−5 wt % Pd (Pd/AC), 1,2-diphenyl acetylene (501-655) (DPA), ethylene glycol (107-21-1) (EG), N-methyl pyrolydinon 99% (872-50-4) (NMP), activated carbon C4386 (AC-2), and potassium tetrachloropalladate (10025-98-6) were purchased from Aldrich. Multiwalled carbon nanotubes (MWCNTs), NC7000, were purchased from Nanocyl; CT > 50, CT20−30, CT8−15, CT10−20, CT < 8 were purchased from Cheaptube. Graphene nanoplatelets (GNPs), M-15 (GNP-1), C-500 (GNP-2), and C-750 (GNP-3), were purchased from XG Sciences; GNP grade 3 were purchased from Cheaptube (GNP-4). Deionized (DI) water with resistivity of 18.2 MΩ was used. All materials were used as received. 2.2. Preparation of Palladium-Decorated MWCNT (Pd/ NT). Potassium tetrachloropalladate (20 mg) is dissolved in DI water (2 mL) and EG (5 mL), followed by 5 min of stirring at ambient temperature.22,23 This solution is added to a premixed mixture of pristine NT (35 mg) and NMP (18 mL) and stirred for an additional 15 min. Then it is transferred to a 50 mL Teflon microwave reaction vessel whose temperature is monitored by a fiber optic thermometer (Omega model 27165

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Scanning electron microscope (SEM) imaging and energydispersive X-ray spectrometry (EDS) of MWCNTs and Pddecorated MWCNTs are conducted using a high-resolution field emission gun−SEM (JSM-7400F, JEOL) mounted with EDS Noran system 6 (Thermo) allowing elemental analysis. Transmission electron microscope (TEM) FEI Tecnai 12 G2 TWIN is operated at 120 kV and mounted with Gatan 794 charge-coupled device (CCD) camera. The micrographs are analyzed by Digital Micrograph 3.6 software. Surface area is measured in a homemade calibrated system where liquid nitrogen is introduced in 10 portions (6.7 kPa each) to 80−90 mg samples. The liquid nitrogen pressure decrease is measured and fitted to the Brunauer−Emmett− Teller (BET) model.25 2.5. Characterization of the Palladium-Decorated MWCNTs (Pd/NT). The yield of Pd nanoparticles precipitated on the NT is analyzed by TGA and found to be 90 wt % of the initial Pd weight. Pd/NT is also characterized by TEM and SEM (Figure 2a and c, respectively). The size distribution of the Pd nanoparticles is measured from the TEM micrographs (Figure 2b). Elemental analysis by EDS indicates a nonhomogeneous distribution of Pd nanoparticles deposited on the NT surface (Figure 2 d−f).

Figure 3. Hydrogenation rate (represented by t50) as a function of SSA of various carbonaceous additives to DPA and Pd/AC (0.8 wt % of Pd, 16 wt % AC) for composite A. The additive concentrations are 3 wt % (hollow symbols) and 10 wt % (full symbols). Dimensionality: sphere, square, and star refer to 1D, 2D, and 3D, respectively.

3. RESULTS Our kinetic analysis is based on comparison of the time required to obtain 50% hydrogenation of the hydrogen acceptor (DPA) (termed t50, see Experimental Section). A detailed example of kinetic curves showing the hydrogenation of DPA with different types of NT is shown in Figure S1 of the Supporting Information. 3.1. Hydrogenation Kinetics with Pd/AC, Composites A. Different types of carbon allotropes (NT, GNP, and AC) are added to Pd/AC−DPA composite A (see Figure 1) at constant Pd concentration in order to explore their effect on the hydrogenation kinetics. The properties of the allotropes, such as length, diameter, and specific surface area (SSA), are summarized in Table S1 of the Supporting Information. We studied six types (n = 6) of NTs and found that two parameters have a major effect on the hydrogenation kinetics: the SSA and the carbon allotrope type (Figure 3 and Table S1, Supporting Information). The Cheaptube NTs (CT < 8, CT20−30, etc.) differ in their outer diameter and SSA but have the same length (>10 μm), metal oxide content, and defect concentration. The Nanocyl NT (NC7000) are 1 order of magnitude shorter (Figure 3, red) and have higher metal oxides and defect concentrations. As expected, carbonaceous additives with higher SSAs result in faster hydrogenation kinetics, while the NC7000 stands out with superior kinetic performance despite its relatively lower SSA (Figure 3). Interestingly, NTs of similar SSAs (∼180 m2/ g) but with different outer diameters (8−15, 10−20, 20−30 nm) show similar kinetics. The rate of hydrogenation increases (lower t50) with NT concentration for all NT types (Figure 3). The kinetics of GNP-based composites (n = 4) shows similar SSA dependence as the NT-based ones (Figure 3) with slightly slower hydrogenation rate. Although AC (n = 2) has the highest SSA among the studied additives, its kinetics is the slowest (Figure 3) with weak dependence on SSA. 3.2. Hydrogenation Kinetics with Pd/NT, Composites B. So far, we have demonstrated the effect of carbon additives on Pd/AC-based composites (composite A) in which the Pd

catalyst is remote and not in intimate contact with the additive (e.g., NT). We now turn to explore the effect of Pd-to-additive proximity. Therefore, we replace the AC by NT as a catalyst support and decorate the NT with Pd nanoparticles (termed Pd/NT) via Polyol synthesis (see Experimental Section). Here, the NT serves both as catalyst support and as a spillover agent. We focus on the effect of the intimate Pd/NT contact on the hydrogenation kinetics. We follow the hydrogenation kinetics of three types of decorated NT, namely, NC7000, CT < 8, and CT20−30 at various NT and Pd concentrations (Figure 4). CT20−30 induces the slowest kinetics, most probably due to its low SSA.

Figure 4. t50 of composites B (containing Pd/NTs) as a function of Pd concentration in the composite. The NTs are examined at concentrations of 4 (empty symbols) and 10 wt % (full symbols).

The overall Pd concentration effect on the kinetics is surprisingly weak, while that of the NT is very strong (Figure 5, reduction of 2 orders of magnitude at 12 wt % NT) for all NT types at all Pd concentrations. In other words, we find that the concentration of the spillover agent is more important than that of the metal catalyst. 27166

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reactivity.26 The shorter hydrogen migration distance derived from the short NT length could be compensated by the alignment of NC7000 in well-organized bundles that resemble combed yarn.27 The high defect density of NC7000 (D/G = 1.2 in Raman spectrum, see Table S1 of the Supporting Information) could stem from the high concentration of endcaps, characterized by intrinsic structural defects.28 2D carbonaceous additives (GNP) in composites type A also accelerate the hydrogenation: with 10 wt % GNPs additive, t50 values of 25−105 min were found, compared to 15−170 min for 1D additive (MWCNT). GNPs with higher SSAs yield faster hydrogenation, similar to the NTs (Figure 3). Interestingly, GNP-3 and GNP-4 have similar hydrogenation rates (Figure 3), although GNP-3 is 4 times thinner than GNP4. This indicates less GNP-4 particles in the composite compared to GNP-3 and suggests that the rate depends neither on GNP thickness nor on the number of the particles per specified weight. Loading of 3D additive (AC) to type A composites has a very weak effect on the hydrogenation rate: 2-fold only, compared to 60- and 100-fold for GNP and NT, respectively (Figure 3). A similar trend is found for composite B (not shown). 4.2. Hydrogen Spillover Steps Analysis in Composites. The solid-phase reaction could be divided into two main stages: (1). Near-Field Hydrogenation. Near-field hydrogenation occurs when the hydrogen atoms, produced on the Pd surface, need to travel only short distances to reach the acceptor molecule (DPA). This takes place upon the first exposures of the composite to hydrogen (t5−t10). (2). Far-Field Hydrogenation. Far-field hydrogenation occurs when the acceptor molecules in the vicinity of the Pd particles are already hydrogenated, and the hydrogen atoms need to migrate longer distance to the nearest available nonhydrogenated acceptor molecule. The HSO steps for both near- and far-field stages are summarized in Table 1 for composites A and B.

Figure 5. t50 of composites B (containing Pd/NT) as a function of NT concentration at Pd concentrations of 0.3 (thin symbols), 0.6 (bold symbols), and 0.8 wt % (full symbols). Separate curves for 0.3 and 0.8 Pd wt % are shown in Figures S2 and S3 of the Supporting Information.

In summary, the shortest NC7000 NT presents the best hydrogenation kinetics for both types of composites (A and B). Of minor importance are parameters such as SSA and defect concentration, while NT diameter has no effect at all.

4. DISCUSSION 4.1. Parameters Influencing Composites Hydrogenation Rate. NT-containing composites present substantially faster kinetics compared to NT-free composites with the same Pd loading (up to 100 times faster upon loading of 10 wt % of pristine NT in composites A, Figure 3). When comparing composites B to composites A without NT addition (Figures 4 and 5 and a star symbol in Figure 3), one can observe that the reaction rate with Pd/NT is up to 30 times faster than the one with Pd/AC despite the roughly similar Pd particle sizes (Figure 2 for the Pd/NT and Figure 4c in ref 7 for Pd/AC). In type A composites, the dimensionality of the carbon additives substantially affects the hydrogenation rates, where NT (1 dimension, 1D) and GNP (2D) enhance the kinetics much more than AC (3D), despite the higher SSA of the latter. However, at a given dimensionality SSA is usually a dominant factor on the kinetics (Figure 3). For all dimensionalities, the rate of reaction increases with the additive concentration. For 1D additives, a similar trend in hydrogenation rates for various NT types is demonstrated for both composites A (Figure 3) and B (Figures 4 and 5) configurations: (NC7000) > (CT < 8) > (CT8−15, CT10−20, CT20−30) > (CT > 50). Because three NT types with different outer diameters (namely, CT8−15, CT10−20, and CT20−30) have a very similar hydrogenation rate (Figure 3), we conclude that the diameter is not a critical parameter in determining the rate of hydrogenation. NC7000 stands out with the highest reaction rate (Figures 3−5), most probably due to its different properties (Supporting Information, Table S1), namely, highest NT defect density and Al2O3 concentration, and significantly shorter length than all other additives. We ask which of these properties has a critical effect on the hydrogenation rate. It was found that Al2O3 impurities have only negligible effect on spillover in the system studied.7 The shorter NC7000 length (1.5 vs 10 μm for the other NT studied) may contribute to its reactivity. The addition of short NTs results in higher concentration of NTs’ end-cap with intrinsic chemical

Table 1. Hydrogen Spillover Mechanism in Composites A and B; X ↔ Y Represents Reversible Migration of Hydrogen Atoms between X and Y Species composite A primary HSO secondary HSO

terminationa a

composite B

Pd ↔ AC (1a) Pd ↔ NT (1b) AC ↔ NT (2a) AC ↔ AC (2b) NT ↔ NT (2c) AC → DPA (3a) NT → DPA (3b)

Only the termination step is irreversible.

The hydrogenation rate in the near-field (e.g., t5) strongly depends on the Pd concentration (Supporting Information, Figure S4). Therefore, the steps in which Pd participates, namely, steps 1a and 1b (Table 1), are rate-limiting. In the farfield hydrogenation, the Pd concentration has only a minor effect on t50 (Figure 4). The time needed for each forthcoming individual portion (Supporting Information, Figure S5) lengthens (with the exception of NC7000) because the hydrogen atoms need to travel longer distances, suggesting that the secondary spillover (steps 2 in Table 1) is a rate-limiting step. 27167

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We now turn to discuss the various secondary HSO steps (2a−2c). The NT concentration was found to substantially affect the hydrogenation rate (Supporting Information, Figure S2), and therefore, step 2a (AC ↔ NT) is faster than step 2b (AC ↔ AC). When the composite contains NT with no AC (composite B), the only secondary spillover occurs between NTs (step 2c). Interestingly, the kinetics of such a system is much faster than composites containing AC alone (40 and 1600 min, respectively; Figures 3 and 4). Therefore, step 2c (NT ↔ NT) is faster than step 2b (AC ↔ AC). These findings show that NTs demonstrate superior hydrogen kinetics over AC for all systems studied. We then investigate the NT parameters (e.g., SSA and length), which optimize NT choice even further. These results could be employed in judicious selection of materials for composites used in hydrogen storage.

5. CONCLUSIONS Different types of carbon allotropes enhance the kinetics of the catalytic hydrogenation in the solid state, where 1D allotropes (NT) are superior to GNP (2D) or AC (3D). We found that Pd-decorated NTs enhance the hydrogenation rate better than Pd on AC. The NT nature could be employed for further tuning the reaction rate: the most effective properties are the SSA of the NT and their length. We found that higher SSA and shorter NT yield faster kinetics. The hydrogenation mechanism in the presence of carbon allotrope could be discussed in terms of hydrogen spillover: the near-field hydrogenation kinetics is controlled by the primary spillover, while the far-field rate by the secondary spillover. When the composite contains both AC and NT (composites A), the slowest step of hydrogen spillover is AC to AC.

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ASSOCIATED CONTENT

S Supporting Information *

Table summarizing the properties of the carbon allotropes we used, kinetic profiles of the hydrogenation reactions, and figures describing the catalyst and the SOA effect on the hydrogenation kinetics. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to acknowledge the Prof. A. Pazy IAEC-UPBC Joint Research Foundation for the financial support of this research. Dror Cohen is kindly acknowledged for designing and programming the Sieverts-type apparatus.

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REFERENCES

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