High-Performance PdNi Nanoalloy Catalyst in Situ Structured on Ni

Research Article pubs.acs.org/acscatalysis

High-Performance PdNi Nanoalloy Catalyst in Situ Structured on Ni Foam for Catalytic Deoxygenation of Coalbed Methane: Experimental and DFT Studies Qiaofei Zhang,†,§ Xin-Ping Wu,‡,§ Yakun Li,† Ruijuan Chai,† Guofeng Zhao,*,† Chunzheng Wang,† Xue-Qing Gong,*,‡ Ye Liu,† and Yong Lu*,† †

Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062, People’s Republic of China ‡ Key Laboratory for Advanced Materials, Centre for Computational Chemistry and Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai 200237, People’s Republic of China S Supporting Information *

ABSTRACT: A Ni-foam-structured PdNi nanoalloy catalyst engineered from nano- to macro-scales has been successfully fabricated for the catalytic deoxygenation of coalbed methane (CBM). The catalyst was obtainable by embedment of Pd nanoparticles onto Ni-foam substrate via a galvanic exchange reaction method and subsequent in situ activation in the reaction, which was active at low temperature, selective (no CO formation), and oscillation free in this CH4-rich catalytic combustion process. Special Pd@NiO (Pd nanoparticles partially wrapped by tiny NiO fragments) ensembles were formed in the galvanic deposition stage and could merely be transformed into PdNi nanoalloys in the real reaction stream at elevated temperatures (e.g., 450 °C or higher). Density functional theory (DFT) calculations were carried out to reveal the role of Ni decoration at Pd in PdNi nanoalloy catalyst for the CBM deoxygenation. By nature, the Pd−Ni alloying modified the electronic structure of surface Pd and led to a decrease in the O adsorption energy, which can be taken as the activity descriptor for the CBM deoxygenation. A reaction kinetic study indicated that the Ni decoration at Pd by Pd−Ni alloying lowered the apparent activation energy in comparison to the pristine Pd catalyst, while leading to an increase of the reaction order of O2 from −0.6 at Pd catalyst to −0.3. The foam-structured PdNi nanoalloy catalyst thus offered enhanced low-temperature activity and the elimination of oscillating phenomena as the result of a transient balance obtained between the cycles of O2 adsorption/activation and CH4 oxidation. KEYWORDS: structured catalyst, nanoalloy catalysis, foam, coalbed methane, catalytic combustion, reaction kinetics, DFT calculations

1. INTRODUCTION Coalbed methane (CBM) drained from a typical gassy coal mine usually contains 30% to more than 95% methane.1 The upgradation of CBM to pipeline-quality gas as a high-valueadded energy source is imperative due to the worldwide exploration of the unconventional energy sources with the increasing depletion of conventional energy resources.2 Since a certain amount of oxygen in CBM would pose a potential safety hazard,3 it is absolutely critical for newly extracted CBM to have an oxygen content below 0.1% through catalytic deoxygenation.1,3 In principle, CBM deoxygenation can be achieved by catalytic combustion in an O2-lean (or CH4-rich) environment, which has not been well studied. An Er-doped lanthanum cobaltite perovskite (La1−xErxCoO3) has been applied in CBM deoxygenation due to its high O2 migration capability and outstanding thermal/hydrothermal stability.4 However, such a catalyst works only at high temperature, delivering a complete O2 conversion at 450 °C using a low © XXXX American Chemical Society

space velocity. In particular, Pd-based catalysts are generally recognized as the most active systems for application in the catalytic combustion of CH4 owing to its higher activity, lower ignition temperatures, and better poison resistance.5,6 Highly efficient Pd-based catalysts have been designed for O2-rich CH4 combustion through controlling the particle sizes and shapes, tuning the electronic states of Pd, and modulating metal− support interactions.7−9 High-performance Pd-based catalysts for CBM deoxygenation are presently under development in a similar way. In addition, oscillation is a common and undesirable behavior present in the Pd-based catalysts for CH4 combustion under CH4-rich conditions: e.g., CBM deoxygenation.10−15 Although the oscillation generally originates from chemical mechanisms, Received: May 1, 2016 Revised: August 5, 2016

6236

DOI: 10.1021/acscatal.6b01226 ACS Catal. 2016, 6, 6236−6245

Research Article

ACS Catalysis

foam chips (as support, 8.0 mm diameter) from their original sheet. At first, NiO on the Ni-foam supports was removed by immersing them in a 1 wt % HNO3 solution at room temperature for several minutes. Then Pd was placed onto the Ni-foam supports by wetting them with an aqueous Pd(NO3)2 solution at room temperature because the galvanic exchange reaction could proceed spontaneously owing to the large electrode potential difference between the Ni2+/Ni0 (−0.26 V) and Pd2+/Pd0 (0.95 V) pairs. After it was washed thoroughly with deionized water, dried at 100 °C, and calcined at 450 °C for 2 h in air, a fresh Pd-PdO-NiO/Ni-foam sample was obtained. We also prepared several reference catalyst samples for comparative studies. First, the reference catalyst of Pd-PdONiO/Ni-foam-NW was obtained by following a similar preparation procedure, only omitting the washing step after the galvanic exchange reaction. Second, a reference catalyst sample with 1 wt % Pd supported on a NiO-Ni-foam (hereafter referred to as Pd/NiO-Ni-foam-IWI) was prepared by the incipient wetness impregnation method with a solution of palladium nitrate dissolved in acetone (to avoid occurrence of a galvanic reaction in aqueous solution), followed by calcination at 450 °C in air for 2 h. The NiO-Ni-foam support with a rough NiO surface was obtained by immersing Ni-foam in an aqueous solution of 5 wt % nitric acid for 10 min, followed by calcining at 450 °C in air for 2 h. In order to highlight the advantages of foam-structured catalysts, a reference catalyst of particulate PdNi(alloy)/SiO2 (1 wt % Pd and 10 wt % Ni) was prepared with 0.2 mm SiO2 as support (89 m2 g−1, Alfa Aesar). First, SiO2 was incipiently impregnated with a Ni(NO3)2 aqueous solution, followed by calcining in air at 450 °C for 2 h and reducing under an H2 atmosphere at 400 °C for 2 h. Palladium was then placed onto the Ni/SiO2 sample via a galvanic exchange reaction between Ni and Pd2+ by immersing the Ni/SiO2 sample in a Pd(NO3)2 aqueous solution. After drying at 100 °C overnight and calcining at 450 °C for 2 h, the fresh Pd-PdO-NiO/SiO2 sample was obtained. 2.2. Catalyst Characterization. The catalyst samples were characterized by inductively coupled plasma atomic emission spectrometry (ICP-AES; ICP Thermo IRIS Intrepid II XSP, USA), scanning electron microscopy (SEM; Hitachi S-4800) equipped with an energy dispersive X-ray fluoresence spectrometer (EDX; Oxford, U.K.), transmission electron microscopy (TEM; FEI-Tecnai G2F30) equipped with selected area electron diffraction (SAED), an N2 adsorption isotherm using standard Brunauer−Emmett−Teller (BET) theory on a Quantachrome Autosorb-3B apparatus, X-ray diffraction (XRD; Rigaku Ultra IV diffractometer), X-ray photoelectron spectroscopy (XPS; Escalab 250xi spectrometer, Al Kα, adventitious C 1s line (284.8 eV) as the reference), and temperatureprogrammed reduction with hydrogen (H2-TPR) and CO pulse adsorption on a Quantachrome ChemBET-3000 chemisorption instrument connected with an online mass spectrometer (Proline Dycor, AMETEK Process Instrument, USA). In each H2-TPR experiment, the sample (100 mg) purged by He at 300 °C for 1 h in advance was heated from 20 to 600 °C in a gas mixture of 10% H2 in N2 (50 mL min−1) at a heating ramp of 10 °C min−1. CO pulse adsorption was employed to estimate the number of surface Pd sites per unit mass of catalyst on the basis of the adsorption stoichiometry of 1/1 CO/Pd at 20 °C.39,40 After being degassed under an He flow at 300 °C for 1 h, the sample was reduced at 150 °C for 1

such as the alternating oxidation−reduction of the catalyst surface,10,15−17 reconstruction of the active surface,11,13 and the strong adsorption of reactants, intermediates, or products,12,14,15 it can be modified by the enhancement of heat transfer and diffusion in the catalyst bed.18−20 It has been reported that CeO2 modification of Pd/Al2O3 (denoted as PdCeO2/Al2O3) weakened the undesirable reaction oscillation as a result of the stabilization of the Pd chemical state due to the oxygen storage capability of CeO2.18 The Pd@CeO2 core−shell structure maximizes the active interfacial area between Pd and CeO2, thereby further suppressing the oscillatory behavior in the CBM deoxygenation via methane combustion. However, last but not least, the strong exothermicity of CBM deoxygenation leads to a high local temperature of active interfacial area on Pd-CeO2/Al2O3 and Pd@CeO2, which would inevitably provoke their tendency to rapid agglomeration and deactivation. The demand for rapidly dissipating reaction heat has provided particular impetus for research on a novel catalyst system with high thermal conductivity. Bimetallic nanoparticles (NPs) often show unique electronic and chemical properties that are distinct from those of their monometallic counterparts, rendering profoundly enhanced catalytic activity and high thermal stability as well as excellent heat conductivity.21 Pd-based bimetallic catalysts have thus played an extraordinary role in various important technological areas,22,23 including chemical synthesis, energy conversion, and environmental protection.24−31 Under a reducing atmosphere, Pd usually is enriched on the Pd-based bimetallic catalyst surface over a wide temperature range,32−37 which can effectively combine the alloy-catalysis effect with higher surface area to volume ratio. In our previous work, PdNi alloy NPs firmly embedded onto a three-dimensional geometry open-cell Ni-foam were triumphantly developed by a galvanic deposition (GD) method and subsequent in situ reaction activation at elevated temperature.38 This Ni-foam-structured PdNi nanoalloy catalyst with high heat conductivity exhibited extraordinary performance for CBM deoxygenation, opening up further prospects for the catalytic functionalization of monolithic substrates.38 Although our previous studies have clearly showed the PdNi nanoalloy catalysis with significant activity/selectivity improvement and oscillation suppression, therein still lie the questions of what is the real and ideal precursor nanostructure ultimately orientated toward the formation of PdNi nanoalloy, as well as the optimal preparation and pretreatment conditions, and how and why can the PdNi nanoalloy tune the underlying catalytic reaction pathways to enhance activity and suppress oscillation. To clarify the formation dependence of PdNi alloy, a series of reference catalysts were prepared, characterized, and evaluated in the CBM deoxygenation reaction. Density functional theory (DFT) calculations were performed to understand the reaction network of CBM deoxygenation and to obtain the O adsorption energy over PdNi alloy. Furthermore, kinetics parameters were also obtained to evidence the nature of high activity and oscillation-suppression properties of PdNi alloy.

2. EXPERIMENTAL SECTION 2.1. Catalyst. The fresh Pd-PdO-NiO/Ni-foam catalyst (nominal Pd loading of 1 wt %) was obtained from a galvanic exchange reaction between Ni-foam (100 PPI, 2 mm thickness; Changsha Lyrun Material Co., Ltd., People’s Republic of China) and a palladium nitrate (Pd(NO3)2, A.R., Aladdin) solution. Laser cutting was employed to obtain the circular Ni6237


Research Article

ACS Catalysis

(denoted as in situ reaction activation).38,41 During the catalyst preparation, the galvanic exchange reaction took place spontaneously once Ni-foam was immersed into the Pd(NO3)2 aqueous solution, because the standard reduction potential of the Pd2+/Pd pair (0.95 V) is much higher than that of the Ni2+/ Ni pair (−0.26 V). As illustrated in Figure 1A, Ni atoms on the

h using a 10% (v/v) mixture of H2/N2. Given that CO may adsorb on the surface Ni atoms competitively, which will cause error in the calculation of surface Pd sites, over the NiO-Nifoam support prereduced at 480 °C using a 10% (v/v) mixture of H2/N2, the CO adsorption was also performed and delivered an ignorable amount of 1.5 × 10−7 mol g−1. 2.3. Reactivity Tests. Ni-foam-structured catalyst chips (0.25 g) were packed layer-up-layer into a quartz tube reactor (i.d. 8 mm; Figure S1 in the Supporting Information) and were examined in the catalytic CBM deoxygenation for a feed of CH4/O2/N2 (40/3/57, vol %) under atmospheric pressure, using a gas hourly space velocity (GHSV) set to 12000 mL g−1 h−1. Any further information on the catalyst bed packing as well as the effluent gas analyses and catalyst activity definition is described in detail elsewhere.41 2.4. TOF Calculations. The turnover frequency (TOF) was calculated to investigate the intrinsic activity of the catalysts, as described in detail elsewhere.41 Here TOF is defined as the quantity of CO2 produced per surface Pd atom per hour and is calculated by the equation TOF =

product molecules Fα = VmN active sites × time

where F is the flow rate of O2 (L s−1), α the yield of CO2, Vm the molar volume of gas (L mol−1, assuming ideal behavior at 25 °C), and N the number of active sites (mol g−1, i.e., surface Pd atoms determined by CO pulse chemisorption). 2.5. DFT Calculations. The PBE functional was employed in the spin-polarized DFT calculations by using the Vienna ab initio simulation package (VASP).42,43 The core−valence electron interactions were described by using the projectaugmented wave (PAW)44 method. Calculations were all based on a kinetic energy cutoff of 400 eV and 5 × 5 × 1 k-point mesh. For structure optimization, calculations were performed until the Hellman−Feynman forces on each ion were converged to within 0.02 eV/Å. The calculated lattice parameter of bulk Pd was 3.95 Å, which is very close to the values reported previously.45,46 We used a periodic slab to model the clean and Ni-decorated Pd(111) surfaces, which were repeated in a 4 × 4 surface unit cell while having four atomic layers and a vacuum gap of >10 Å. We fixed the bottom Pd layer but allowed other layers to relax in all calculations. To calculate the adsorption energies (Eads) of adsorbates, we used the following expression:

Figure 1. (A) Schematic image illustrating the possible formation mechanism of the GD method during the preparation of Pd-PdONiO/Ni-foam catalyst. The blue cuboid represents the Ni-foam surface; yellow and blue spheres are Pd/Pd2+ and Ni/Ni2+ ions. (B) XRD patterns for the pristine Ni-foam, Pd-PdO-NiO/Ni-foam, and PdNi(alloy)/Ni-foam samples. (C) H2-TPR results for the NiO-Nifoam, Pd-PdO-NiO/Ni-foam, and PdNi(alloy)/Ni-foam samples. NiO loading was calculated according to the standard working curve of H2reduction peak area against NiO content in Figure S3 in the Supporting Information.

foam surface served as sacrificial templates, from which some Ni atoms were oxidized and transferred into the solution concurrently with the reduction and deposition of Pd0. As a result, the foam surface became rough in comparison with the pristine surface, in association with the clear observation of Pd, PdO, and NiO phases on the as-prepared Pd-PdO-NiO/Nifoam (Figure 1B). Moreover, SEM/EDX mapping images indicated that a uniform layer consisting of such Pd, PdO, and NiO phases was foam across the formed struts (Figure S2 in the Supporting Information). Meanwhile, the H2-TPR profile of the Pd-PdO-NiO/Ni-foam exhibited two reduction peaks: the strong peak at low temperature (∼275 °C, NiO content 1.0 wt %) was assigned to the reduction of Ni2+ at the interface between NiO and Pd, while that at high temperature (∼310 °C, NiO content 0.6 wt %) was the reduction of NiO weakly interacting with Pd (Figure 1C). Comparatively, the reference H2-TPR for the NiO-Ni-foam without Pd only displayed a high-temperature reduction peak of bulk NiO species (NiO content 5.5 wt %). These observations confirmed that the Pd nanoparticles strongly interacted with the NiO species over the Pd-PdO-NiO/Ni-foam, which was formed through the pinhole diffusion of the GD method.48 To further address the interaction effect between Pd and NiO, we prepared a reference Pd/NiO-Ni-foam-IWI catalyst using conventional incipient wetness impregnation (IWI) at 1 wt % Pd loading and calcination conditions same as the PdPdO-NiO/Ni-foam obtained by the GD method. A CO chemisorption experiment confirmed similar Pd site accessibil-

Eads = Ead + Esub − Ead/sub

where Ead, Esub, and Ead/sub are the DFT total energies of the gas-phase adsorbate (note: Ead is equal to 0.5EO2 in calculating the adsorption energies of O; EO2 is the DFT total energy of an O2 molecule), the substrate, and the adsorption complex, respectively. Transition states were searched with a constrained optimization scheme.47

3. RESULTS AND DISCUSSION 3.1. Interaction, Nanostructure, and Reduction Conditions Dependent Pd−Ni Alloying from Pd−NiO Composites. 3.1.1. Pd−NiO Interaction. As previously noted, the low-temperature-active and oscillation-free PdNi(alloy)/Ni-foam was obtainable after the as-prepared Pd-PdONiO/Ni-foam catalyst underwent reaction at 480 °C for 1 h 6238


Research Article

ACS Catalysis Table 1. CBM Deoxygenation Catalyzed by Different Catalysts Pd loadingd (wt %)

catalyst a

Pd-PdO-NiO/Ni-foam Pd-PdO-NiO/Ni-foam-NWa Pd-PdO-NiO/Ni-foam-NWb PdNi(alloy)/Ni-foamb Pd/NiO-Ni-foam-IWIb PdNi-H2/Ni-foamc

0.9 0.9 0.9 0.9 0.9 0.9

SBET (m2 g−1) 7.9 6.7 9.5 27.4 7.1 3.9

T10 (°C) 305 316 273 258 261 291

T50e (°C) 330 354 318 295 305 360

surface Pdf (mol g−1) 4.0 9.7 6.8 2.2 2.0 7.9

× × × × × ×

−6

10 10−7 10−6 10−5 10−5 10−6

TOF (h−1)g 57 150 292 76 46

Freshly obtained sample. Pd-PdO-NiO/Ni-foam was tested directly from 220 to 480 °C without in situ activation. bIn situ activation was performed before reaction test. cH2 reduction was performed before reaction test. dMeasured by ICP-AES. eT10 and T50 represent the reaction temperatures for oxygen conversions of 10% and 50%, respectively (0.25 g of cat., CH4/O2/N2 = 40/3/57 (vol %), GHSV = 12000 mL gcat.−1 h−1). fDetermined by CO pulse absorption. gTurnover frequency (TOF, h−1) at 280 °C based on CO2 yield and surface Pd. a

As shown in Figure 2A, the as-prepared Pd-PdO-NiO/Nifoam-NW and Pd-PdO-NiO/Ni-foam samples contained

ities over these two in situ activated catalysts, because of their equivalent amounts of surface Pd atoms (2.2 × 10−5 mol g−1 for PdNi(alloy)/Ni-foam vs 2.0 × 10−5 mol g−1 for Pd/NiO-Nifoam-IWI; Table 1). XRD patterns of the Pd/NiO-Ni-foamIWI revealed that no Pd phase was detected except for Ni and NiO, further confirming the high dispersion of Pd NPs (Figure S4 in the Supporting Information). Figure S5 in the Supporting Information shows that Pd NPs were uniformly dispersed on the NiO support, while the ring patterns of an SAED micrograph were consistent with the planes corresponding to the face-centered-cubic form of nickel oxide. Unfortunately, the Pd/NiO-Ni-foam-IWI showed poor catalytic activity associated with obvious oscillatory behavior in the catalytic CBM deoxygenation reaction (Figure S6 in the Supporting Information). In comparison with the Pd-PdONiO/Ni-foam prepared by the GD method, the Pd/NiO-Nifoam-IWI exhibited quite different surface morphology but similar specific surface area (Figure S7 in the Supporting Information and Table 1). This observation excluded the possibility of a variation in active surface area as the main cause for their marked difference in catalytic performance. As shown in Figure S8 in the Supporting Information, a weak interaction was formed between Pd and NiO species over the Pd/NiO-Nifoam-IWI sample, evidenced by the weak peak at lower temperature (∼275 °C) assigned to a strong Pd−NiO interaction, which restrained the Pd-catalyzed reduction of NiO species as well as Pd−Ni alloying. Not surprisingly, as confirmed by XPS analysis, the Pd0 binding energy (BE) of the activated Pd/NiO-Ni-foam-IWI was 335.5 eV, with a negligibly small increment of only 0.4 eV relative to that (335.1 eV) on the fresh Pd-PdO-NiO/Ni-foam (Figure S9 and Table S1 in the Supporting Information), showing no occurrence of Pd−Ni alloying while offering a low TOF of 76 h −1 (almost a fourth of that (292 h−1) for the PdNi(alloy)/Ni-foam; Table 1). Therefore, a strong Pd−NiO interaction was paramount for Pd−Ni alloying under the in situ reaction activation conditions. However, the real Pd−NiO nanostructure with enhanced Pd− NiO interaction for facilitating Pd−Ni alloying is not yet clear. 3.1.2. Nanostructure: Pd@NiO (Pd Partially Wrapped by NiO) Ensembles. During the catalyst preparation process, the galvanic exchange reaction between Pd(NO3)2 solution and Nifoam could generate metallic Pd NPs as well as Ni(NO3)2. Intriguingly, if the sample was not washed thoroughly with distilled water to remove Ni(NO3)2 after the galvanic exchange reaction and was directly calcined at 450 °C in air, the corresponding catalyst denoted as Pd-PdO-NiO/Ni-foam-NW delivered entirely different surface morphology and catalytic performance for the CBM deoxygenation, in comparison with the Pd-PdO-NiO/Ni-foam (washed after the GD process).

Figure 2. Structural features and catalytic performance of fresh PdPdO-NiO/Ni-foam-NW and Pd-PdO-NiO/Ni-foam samples (0.9 wt % Pd, determined by ICP-AES): (A) XRD patterns; SEM images of (B) Pd-PdO-NiO/Ni-foam-NW and (C) Pd-PdO-NiO/Ni-foam; (D) O2 conversion for temperature-dependent catalytic CBM deoxygenation. The inset in (D) is O2 conversion at 400 °C vs time on stream to show the oscillatory behavior. Legend: (black ●) Pd-PdO-NiO/Nifoam-NW; (red ●) Pd-PdO-NiO/Ni-foam. Both of the samples underwent in situ activation at 480 °C for 1 h. Reaction conditions: 0.25 g of cat., CH4/O2/N2 = 40/3/57 (vol %), GHSV = 12000 mL gcat.−1 h−1.

similar phase compositions such as Ni, Pd, PdO, and NiO. Their SEM images showed a considerable amount of lamellarshaped NiO formed on the former sample surface but a uniform burrlike-particle coverage across the latter surface (Figure 2B,C). As indicated by the CO chemisorption results in Table 1, the Pd-PdO-NiO/Ni-foam-NW provided a very small amount of surface Pd of 9.7 × 10−7 mol g−1, only about onefourth of that (4.0 × 10−6 mol g−1) on the Pd-PdO-NiO/Nifoam. Furthermore, the H2-TPR profile of fresh Pd-PdO-NiO/ Ni-foam-NW showed a very large H2 consumption peak at a high temperature of ∼310 °C and a very weak peak at a low temperature of ∼275 °C in comparison with the Pd-PdO-NiO/ Ni-foam, clearly manifesting the existence of an excess amount of NiO species as well as a weak Pd−NiO interaction (Figure S10A in the Supporting Information). Notably, the dramatic activity promotion and oscillation suppression observed on the activated Pd-PdO-NiO/Ni-foam (i.e., PdNi(alloy)/Ni-foam) did not appear at all on the in situ reaction activated Pd-PdO-NiO/Ni-foam-NW (Figure 2D). No XRD peaks of Pd species were detectable over such activated Pd-PdO-NiO/Ni-foam-NW, while the surface Pd atoms were determined by CO pulse chemisorption to be only one-third of 6239


Research Article

ACS Catalysis those on the PdNi(alloy)/Ni-foam (Figure S10B in the Supporting Information and Table 1). The presence of large amounts of NiO as indicated by H2-TPR was the primary cause for the low accessibility of the Pd phase on the activated PdPdO-NiO/Ni-foam-NW, due to the encapsulation of Pd NPs within excess NiO (NiO loading 4.0 wt %) on its corresponding fresh sample (Figure S10A). In contrast, an HRTEM image of the fresh Pd-PdO-NiO/Ni-foam with 1.6 wt % NiO loading showed that the Pd nanoparticle was partially covered with tiny amorphous NiO, denoted as Pd@NiO (Figure S11 in the Supporting Information). In addition, the optimal NiO content in the Pd@NiO over the Pd-PdO-NiO/ Ni-foam can be determined on the basis of the standard working curve of H2-reduction peak area against NiO content deduced from the H2-TPR results for the catalysts calcined at various temperatures (Figure S12 in the Supporting Information). As reported in our previous work, Pd−Ni alloying behavior was susceptible to the catalyst calcination temperature,41 by nature, due to the different NiO coverages in the Pd@NiO ensembles. With an increase in the calcination temperature, NiO contents progressively increased from 0.4 wt % on the catalyst calcined at 300 °C to 14.1 wt % on that calcined at 600 °C. The exorbitant calcination temperatures (at and above 500 °C) would cause the Pd NPs to be heavily covered with excessively formed NiO species. In combination with the results that only the catalysts calcined at below 500 °C can form PdNi alloy after in situ reaction activation, it is thus believed that such special Pd@NiO ensembles with an NiO content of below 3.8 wt % were indispensable for a strong Pd− NiO interaction which was paramount for Pd−Ni alloying under in situ reaction activation treatment. 3.1.3. In Situ Reaction Activation vs Hydrogen Prereduction. For fresh Pd-PdO-NiO/Ni-foam, in situ reaction activation at 480 °C could cause metal oxide species of NiO and PdO to be reduced to metal and subsequently form PdNi nanoalloy.38 We wonder whether the PdNi alloy could also be generated when the Pd-PdO-NiO/Ni-foam underwent H2 prereduction (hereafter referred to as PdNi-H2/Ni-foam). XRD and H2-TPR results in Figure S13 in the Supporting Information indicated that the pretreatment in H2 at 500 °C could reduce the metal oxides to metal, but the diffraction peaks of metallic Pd could not be observed. SEM/EDX images showed that surface Ni aggregated together in clumps with only a small amount of detectable Pd species instead of the formation of PdNi alloy (Figure S14 in the Supporting Information). CO chemisorption experiments corroborated that the PdNi-H2/Ni-foam provided much fewer surface Pd sites in comparison to PdNi(alloy)/Ni-foam (Table 1). This PdNi-H2/Ni-foam catalyst exhibited inferior catalytic performance with only 65% conversion of O2 at 460 °C, accompanied by obvious oscillatory behavior over a wide range of temperature (380−450 °C, Figure S15 in the Supporting Information). In addition, Figure 3 and Table S1 in the Supporting Information also showed that the Pd0 BE was 335.4 eV on the used PdNi-H2/Ni-foam, with a negligibly small increment of only 0.3 eV relative to that (335.1 eV) on the fresh Pd-PdONiO/Ni-foam. In contrast, the Pd0 BE increased to 335.9 eV on the PdNi(alloy)/Ni-foam as a result of Pd−Ni alloying, with an obvious increment of 0.8 eV from 335.1 eV on its corresponding fresh sample (i.e., Pd-PdO-NiO/Ni-foam).38 Clearly, Pd−Ni alloying could not proceed during the prereduction of the Pd-PdO-NiO/Ni-foam using pure H2,

Figure 3. XPS spectra in the (A) Pd 3d and (B) Ni 2p regions on the surfaces of the fresh Pd-PdO-NiO/Ni-foam, the activated PdNi(alloy)/Ni-foam, and the PdNi-H2/Ni-foam after reaction for 1 h.

which thus resulted in its poor intrinsic activity with a low TOF of only 46 h−1. It is generally known that different operating conditions (e.g., pressure, reactant composition, temperature) will lead to different changes on the catalyst surface. H2 prereduction likely induced the segregation and aggregation of Ni and Pd particles, thereby significantly suppressing the Pd−Ni alloying. In the case of in situ reaction activation, reactant-induced surface reconstruction might be the most eligible route to obtain the optimal surface structure (i.e., PdNi alloy) for CBM deoxygenation. 3.2. Evolution of PdNi Nanoalloy against Reaction Temperature. The PdNi(alloy)/Ni-foam samples after experiencing CBM deoxygenation at various temperatures were investigated by means of XRD and H2-TPR, and the results are shown in Figure 4 and Figure S16 in the Supporting

Figure 4. XRD patterns for the Pd-PdO-NiO/Ni-foam catalyst used at different temperatures. The temperatures marked from bottom to top in this figure followed the tracks of increasing reaction temperature from 350 to 480 °C, then decreasing temperature gradually from 480 to 220 °C, and increasing temperature again from 220 to 400 °C.

Information. Intriguingly, the PdNi nanoalloy particles did not hold still under reaction conditions, exhibiting obvious structure and composition changes against the reaction temperature. After reaction at 480 °C for 1 h (e.g., reaction activation), H2-TPR peaks of the nickel oxide on the Pd-PdONiO/Ni-foam disappeared while the Pd(111) XRD peak was shifted to a higher angle, solidly indicating that the NiO species was reduced and subsequently diffused into Pd NPs to form PdNi nanoalloy. On this PdNi(alloy)/Ni-foam catalyst (i.e., the in situ reaction activated Pd-PdO-NiO/Ni-foam), the PdNi nanoalloy was estimated to be Pd93Ni7 on the basis of Vegard’s law (Table S2 in the Supporting Information).49−51 The formed PdNi nanoalloy could remain stable but the Ni content in the alloy changed until the reaction temperature was below 220 °C: for example, 12 atom % Ni in nanoalloys at 400 °C but 5% Ni at 6240


Research Article

ACS Catalysis

Figure 5. Calculated structures of Pd(111) with different degrees of Ni decoration (i.e., 0, 0.5, and 1 ML Ni): (top panels) top views; (bottom panels) side views. Pd and Ni atoms are shown in cyan and yellow, respectively.

Figure 6. Reaction network of O2 elimination through H2O and CO2 formation by surface adsorbed H and CH species. Surface adsorbates are marked with asterisks.

220 °C. With a decrease in the reaction temperature, some alloyed Ni atoms segregated from the bulk to the surface and formed NiO at low reaction temperatures, associated with the incomplete oxygen conversion (for example, 2% O2 conversion at 220 °C). Nevertheless, the H2-TPR results in Figure S16A in the Supporting Information clearly indicated that the surfacesegregated Ni species existing in the form of NiO could be realloyed reversibly when the reaction temperature was increased again. One may raise doubts about the stability maintenance of PdNi nanoalloy during a long-term run at a low temperature (e.g., 220 °C). Our testing results showed that the PdNi nanoalloy of Pd95Ni5 remained unchanged for at least 100 h in the CBM deoxygenation process at 220 °C, with a constant amount of surface NiO as well as excellent maintenance of oscillation-free O2 conversion (Figures S16 and S17 in the Supporting Information). 3.3. Advantages of Ni Foam Structured Catalyst. In order to highlight the advantages of Ni-foam-structured PdNi nanocatalyst, a reference catalyst of Pd-PdO-NiO/SiO2 was also prepared via a galvanic exchange reaction method (see the Experimental Section for preparation details). As shown in Figure S18A in the Supporting Information, PdNi alloy was clearly formed over the Pd-PdO-NiO/SiO2

particulate catalyst after in situ activation. As expected, the temperature-dependent activities of CBM deoxygenation over both the PdNi(alloy)/Ni-foam and PdNi(alloy)/SiO2 catalysts were quite similar on a laboratory scale in the reaction temperature range from 220 to 400 °C (Figure S18B,C). Notably, the O2 conversions for heating and cooling curves almost overlapped over the PdNi(alloy)/Ni-foam while an obvious hysteresis loop was observed over the PdNi(alloy)/ SiO2 catalyst. The poor thermal conductivity of the PdNi(alloy)/SiO2 catalyst would undoubtedly trap the reaction heat in the reactor bed during temperature cool-down for testing, thereby leading to a higher O2 conversion. In contrast, this phenomenon could be avoided because of the high thermal conductivity of our Ni-foam-structured catalyst. Indeed, the high permeability and enhanced heat transfer of such Ni-foamstructured catalyst are solidly confirmed by the pressure drop measurements and computational fluid dynamics (CFD) simulations, as noted in our previous studies.41 3.4. DFT Calculations. 3.4.1. Models. The obtained results clearly indicated that PdNi nanoalloys were responsible for the dramatic activity improvement and elimination of reaction oscillation and revealed that the special Pd@NiO ensembles were effective nanostructures for Pd−Ni alloying only under a 6241


Research Article

ACS Catalysis

Table 2. Calculated Reaction Barriers (Ea) and Energies (ΔE) of the Key Reaction Steps at Three Model Surfacesa ΔE (eV)

Ea (eV) reaction step

Pd(111)

Pd/PdNi/Pd(111)

Pd/Ni/Pd(111)

Pd(111)

Pd/PdNi/Pd(111)

Pd/Ni/Pd(111)

O* + H* → OH* O2* + H* → OOH* OOH* + H* → HOOH* OH* + H* → H2O* O* + CH* → HCO* CO* + O* → CO2*

1.07 0.88 1.20 0.78 1.40 1.67

0.98 0.80 0.96 0.71 1.26 1.40

0.85 0.69 0.80 0.68 1.00 1.23

−0.03 0.16 0.12 −0.37 −0.52 −0.05

−0.26 −0.04 −0.06 −0.50 −0.89 −0.43

−0.51 −0.29 −0.18 −0.64 −1.29 −0.90

a

Calculated structures of surface species at Pd(111) can be found in Figure S21 in the Supporting Information. Note that the optimized structures of each surface species at the three model surfaces are nearly identical.

real reaction stream at high temperature (e.g., 480 °C). To help better understand the PdNi alloy catalysis in the CBM deoxygenation, the chemical effect of Ni decoration at Pd was explored using DFT-based first-principles calculations. Our calculations were carried out on the (111) surface of Pd, since the Pd NPs mainly exposed the most stable (111) facets, as evidenced by XRD results in Figure 4. First, we constructed the pure Pd(111), which represents the fresh Pd-PdO-NiO/Nifoam, and two PdNi alloy models, i.e., Pd(111) with 0.5 and 1 ML Ni decoration; the former has the zigzag Pd/PdNi/ Pd(111) structure (Figure 5, see Figure S19 in the Supporting Information for details), and the latter has the Pd/Ni/Pd(111) sandwich structure (Figure 5). 3.4.2. Role of Ni Decoration. Since methane is the dominant gas species in CBM (40 vol %), the catalyst surfaces should be largely covered by the main dissociation species of methane: i.e., H and CH species.38,52,53 Therefore, in CBM deoxygenation, we mainly focus on the key reactions: i.e., surface adsorbed oxygen species (confirmed by O2-TPD results as shown in Figure S20 in the Supporting Information) from O2 dissociation eliminated by surface adsorbed H and CH species from methane dissociation. Figure 6 represents the proposed reaction network, which evolves gas-phase O2 to H2O and CO2. Specifically, the reaction network involves the following elementary steps: O2 (g) → O2 *

(1)

O2 * → 2O*

(2)

O* + H* → OH*

(3)

O2 * + H* → OOH*

(4)

OOH* → O* + OH*

(5)

OOH* + H* → HOOH*

(6)

HOOH* → 2OH*

(7)

OH* + H* → H 2O*

(8)

H 2O* → H 2O(g)

(9)

O* + CH* → HCO*

(10)

HCO* → CO* + H*

(11)

CO* + O* → CO2 *

(12)

CO2 * → CO2 (g)

(13)

(highlighted in red, Figure 6).38 Therefore, we mainly focused on the key (slow) reaction steps: i.e., eqs 3, 4, 6, 8, 10, and 12. The calculated reaction barriers and energies of these steps at three model surfaces were summarized in Table 2. Interestingly, it can be found that reaction barriers and energies of all reaction steps decrease with an increase of Ni content, indicating that Ni decoration promotes the activity of CBM deoxygenation at the Pd-based catalyst. We can also expect that the fast O2 conversion at PdNi alloy will lead to the dramatic reduction of surface O* coverage, thus facilitating the methane activation and suppressing the oscillatory behavior induced by accumulated surface O* as well.38 3.4.3. O Adsorption Energy as an Activity Descriptor for CBM Deoxygenation. We also studied the O adsorption energy at the three model surfaces and found that Ni decoration can weaken the interaction between O and surface Pd, with calculated O adsorption energies of 1.26, 1.05, and 0.85 eV at Pd(111), Pd/PdNi/Pd(111), and Pd/Ni/Pd(111), respectively. Detailed analyses showed that Ni decoration at Pd actually modifies the electronic structure of surface Pd with a decrease in the surface palladium d band center with an increase in Ni content, giving rise to a decrease in O adsorption energy with Ni decoration (Figure 7).

Figure 7. Calculated d band center of surface Pd with the reference of Fermi level (Ed − EF, green) and O adsorption energy (Eads[O], violet) as a function of Ni content at Pd(111).

Interestingly, Figure 8 shows very good linear relations between reaction barrier/energy and O adsorption energy for all key reaction steps, indicating that O adsorption plays a critical role in these steps. Therefore, O adsorption energy can be taken as the activity descriptor for CBM deoxygenation. 3.5. Kinetic Experiments for CBM Deoxygenation. The kinetics of CH4 combustion in the presence of excess O2 have been extensively investigated over the supported Pd catalysts. In this case, O2 concentration basically has no influence on the

It should be noted that the four dissociation steps (highlighted in green, Figure 6) have been shown to be significantly more facile than other surface reaction steps 6242


Research Article

ACS Catalysis

Figure 8. Calculated (A) reaction barriers (Ea) and (B) energies (ΔE) of the key reaction steps in O2 elimination as a function of O adsorption energy (Eads[O]).

Figure 9. (A, C) Variation of fractional conversion of O2 with W/FO2 and (B, D) reaction rate as a function of temperature along with the Arrhenius plots in the insets, respectively: (A, B) fresh Pd-PdO-NiO/Ni-foam catalyst; (C, D) PdNi(alloy)/Ni-foam catalyst. F is the flow of the gas (mol s−1), and W is the weight of the catalyst (g).

oxidation rate of methane because of the large coverage of O2 over the Pd surface. However, the CBM deoxygenation with CH4 combustion proceeded with the lack of O2, the reaction kinetic model of which may be disparate from that under O2rich conditions.54 Herein a kinetics investigation for the CBM deoxygenation was carried out over both the fresh Pd-PdONiO/Ni-foam and the activated PdNi(alloy)/Ni-foam catalysts to obtain data for the reaction orders and apparent activation energies. In accordance with the empirical kinetic expression and given that the CO2 and H2O concentrations in the feed gases are almost constant, the rate equation of the CBM deoxygenation can be simplified as4,55,56 ⎛ E ⎞ ln r = a ln PCH4 + b ln PO2 + ln⎜ − a ⎟ ⎝ RT ⎠

rate (r ) =

Fx x = W W /F

(2a)

where F is the flow of reaction gas (mol s−1), W the weight of catalyst (g), and x the fractional O2 conversion. So once two of the parameters are fixed on the right side of eq 1a, the remaining parameter can be calculated. When the feed rates of CH4 and O2 are fixed but the reaction temperature is changed over a small range, the Ea values of Pd-PdO-NiO/ Ni-foam and PdNi(alloy)/Ni-foam could be calculated and were finally determined to be 154 and 100 kJ mol−1, respectively (Figure 9). Note that the CBM deoxygenation reaction was performed under a kinetic-limiting region by controlling the temperature and GHSV. The lower activation energy of PdNi(alloy)/Ni-foam indicated that the Ni decoration at Pd by Pd−Ni alloying could lower the apparent activation energy in comparison to the pristine Pd catalyst, confirming the enhanced catalytic activity of PdNi alloy. When the O2 concentration was fixed (5 vol %) but the concentration of CH4 (40−60 vol %) was changed at a certain

(1a)

At the same time, the reaction rate can also be defined by the fractional O2 conversion as shown in eq 2a: 6243


Research Article

ACS Catalysis Author Contributions

temperature, the reaction orders for CH4 (i.e., slope of the straight line) over the Pd-PdO-NiO/Ni-foam and PdNi(alloy)/ Ni-foam were determined to be 1.05 and 1.16 (Figure S22 in the Supporting Information), in good agreement with the reported first order under oxygen-rich condition.4,57−60 Similarly, when feed gas consisting of 50 vol % CH4 (i.e., fixed CH4 concentration) and 3−9 vol % O2 was used, the reaction order for O2 could be determined by plotting ln r against ln PO2 (Figure S22 in the Supporting Information). Unlike the reported zero order in the case of oxygen-rich combustion,57−60 a negative reaction order for O2 was obtained: −0.67 for Pd-PdO-NiO/Ni-foam and −0.32 for PdNi(alloy)/Ni-foam. The results indicated that the increase in O2 concentration caused a decrease in the reaction rate, directly revealing the origin of oscillatory behavior during the CBM deoxygenation via methane catalytic combustion. Most notably, the Ni decoration at Pd by Pd−Ni alloying resulted in an increase of O2 reaction order from −0.67 for the pristine Pd catalyst to −0.32 for the PdNi alloy. As a result, the Ni-foamstructured PdNi nanoalloy catalyst provided the ability to transiently balance the O2-adsorption/activation with the CH4 oxidation, thereby eliminating the oscillatory phenomenon.38

§

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by the “973 program” (2011CB201403) from the MOST of China and the NSF of China (21473057, 21273075, 21322307, U1462129).

■

(1) Karacan, C. Ö .; Ruiz, F. A.; Cotè, M.; Phipps, S. Int. J. Coal Geol. 2011, 86, 121−156. (2) Rightmire, C. T.; Eddy, G. E.; Kirr, J. N. Coalbed methane resources of the United States; Tulsa, OK, 1984. (3) Olajossy, A.; Gawdzik, A.; Budner, Z.; Dula, J. Chem. Eng. Res. Des. 2003, 81, 474−482. (4) Liu, Z. C.; Lu, G. Z.; Guo, Y.; Wang, Y. Q.; Guo, Y. L. Catal. Sci. Technol. 2011, 1, 1006−1012. (5) Zarur, A. J.; Ying, J. Y. Nature 2000, 403, 65−67. (6) Lee, J. H.; Trimm, D. L. Fuel Process. Technol. 1995, 42, 339−359. (7) Cargnello, M.; Jaén, J. J. D.; Garrido, J. C. H.; Bakhmutsky, K.; Montini, T.; Gámez, J. J. C.; Gorte, R. J.; Fornasiero, P. Science 2012, 337, 713−717. (8) Colussi, S.; Gayen, A.; Camellone, M. F.; Boaro, M.; Llorca, J.; Fabris, S.; Trovarelli, A. Angew. Chem., Int. Ed. 2009, 48, 8481−8484. (9) Chen, C.; Yeh, Y.-H.; Cargnello, M.; Murray, C. B.; Fornasiero, P.; Gorte, R. J. ACS Catal. 2014, 4, 3902−3909. (10) Slinko, M. M.; Korchak, V. N.; Peskov, N. V. Appl. Catal., A 2006, 303, 258−267. (11) Vendelbo, S. B.; Elkjær, C. F.; Falsig, H.; Puspitasari, I.; Dona, P.; Mele, L.; Morana, B.; Nelissen, B. J.; van Rijn, R.; Creemer, J. F.; Kooyman, P. J.; Helveg, S. Nat. Mater. 2014, 13, 884−890. (12) Figueroa, S. J. A.; Newton, M. A. J. Catal. 2014, 312, 69−77. (13) Graham, G. W.; König, D.; Poindexter, B. D.; Remillard, J. T.; Weber, W. H. Top. Catal. 1999, 8, 35−43. (14) Bychkov, V. Y.; Tyulenin, Y. P.; Gorenberg, A. Y.; Sokolov, S.; Korchak, V. N. Appl. Catal., A 2014, 485, 1−9. (15) Bychkov, V. Y.; Tyulenin, Y. P.; Slinko, M. M.; Shashkin, D. P.; Korchak, V. N. J. Catal. 2009, 267, 181−187. (16) Zhang, X.; Lee, C. S. M.; Hayward, D. O.; Mingos, D. M. P. Catal. Today 2005, 105, 283−294. (17) Deng, Y. Q.; Nevell, T. G. J. Mol. Catal. A: Chem. 1999, 142, 51−60. (18) Deng, Y. Q.; Nevell, T. G. Faraday Discuss. 1996, 105, 33−46. (19) Schüth, F.; Song, X.; Schmidt, L. D.; Wicke, E. J. Chem. Phys. 1990, 92, 745−756. (20) Jensen, K. F.; Ray, W. H. Chem. Eng. Sci. 1982, 37, 1387−1410. (21) Greer, J. R. Science 2014, 343, 1319−1320. (22) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. Rev. 2005, 105, 1025−1102. (23) Christensen, A.; Stoltze, P.; Norskov, J. K. J. Phys.: Condens. Matter 1995, 7, 1047−1057. (24) Wan, Y.; Wang, H.; Zhao, Q.; Klingstedt, M.; Terasaki, O.; Zhao, D. J. Am. Chem. Soc. 2009, 131, 4541−4550. (25) Xiong, G.; Luo, L.; Li, C.; Yang, X. Energy Fuels 2009, 23, 1342− 1346. (26) Felpin, F.-X.; Ibarguren, O.; Nassar-Hardy, L.; Fouquet, E. J. Org. Chem. 2009, 74, 1349−1352. (27) Shao, M.-H.; Sasaki, K.; Adzic, R. R. J. Am. Chem. Soc. 2006, 128, 3526−3527. (28) Fors, B. P.; Watson, D. A.; Biscoe, M. R.; Buchwald, S. L. J. Am. Chem. Soc. 2008, 130, 13552−13554. (29) Bonarowska, M.; Burda, B.; Juszczyk, W.; Pielaszek, J.; Kowalczyk, Z.; Karpiński, Z. Appl. Catal., B 2001, 35, 13−20. (30) Cárdenas-Lizana, F.; Gómez-Quero, S.; Hugon, A.; Delannoy, L.; Louis, C.; Keane, M. A. J. Catal. 2009, 262, 235−243.

4. CONCLUSION A high-performance PdNi nanoalloy supported on Ni-foam to be used in catalytic CBM deoxygenation has been successfully developed by a galvanic deposition method in association with in situ reaction activation. Detailed investigations suggest that special Pd@NiO ensembles (Pd nanoparticle partially wrapped with tiny NiO fragments) formed in the galvanic deposition stage of catalyst preparation can merely be transformed into PdNi nanoalloys in the real reaction stream at elevated temperatures (e.g., 450 °C or higher). In addition, the effect of Ni decoration at Pd has been confirmed by DFT calculations. It is found that the O adsorption energy can be taken as the descriptor for CBM deoxygenation. Ni decoration at Pd can also modify the electronic structure of surface Pd and thus influence the O adsorption energy. Specifically, the interaction between O and surface Pd is decreased with Ni decoration, thereby making the adsorbed O species more active, which thus promotes the activity and suppresses the oscillatory phenomena. The kinetic studies further confirm that Ni decoration at Pd can not only lower the apparent activation energy but also result in an increase in the negative reaction order for O2 partial pressure.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b01226. Tables S1 and S2 and Figures S1−S22 as described in the text (PDF)

■

REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*G.Z.: tel and fax, (+86) 21-62233882; e-mail, gfzhao@chem. ecnu.edu.cn. *X.-Q.G.: tel and fax, (+86) 21-64251101; e-mail, xgong@ ecust.edu.cn. *Y.L.:tel and fax, (+86) 21-62233424; e-mail, [email protected]. edu.cn. 6244


Research Article

ACS Catalysis (31) Wen, F.; Zhang, W.; Wei, G.; Wang, Y.; Zhang, J.; Zhang, M.; Shi, L. Chem. Mater. 2008, 20, 2144−2150. (32) Badini, C.; Saracco, G.; Russo, N.; Specchia, V. Catal. Lett. 2000, 69, 207−215. (33) Moss, R. L.; Gibbens, H. R. J. Catal. 1972, 24, 48−56. (34) Bertolini, J.; Miegge, P.; Hermann, P.; Rousset, J.; Tardy, B. Surf. Sci. 1995, 331-333, 651−658. (35) Del Angel, G.; Coq, B.; Figueras, F. J. Catal. 1985, 95, 167−180. (36) Maillet, T.; Barbier, J., Jr; Gelin, P.; Praliaud, H.; Duprez, D. J. Catal. 2001, 202, 367−378. (37) Tao, F.; Grass, M. E.; Zhang, Y.; Butcher, D. R.; Renzas, J. R.; Liu, Z.; Chung, J. Y.; Mun, B. S.; Salmeron, M.; Somorjai, G. A. Science 2008, 322, 932−934. (38) Zhang, Q. F.; Wu, X.-P.; Zhao, G. F.; Li, Y. K.; Wang, C. Z.; Liu, Y.; Gong, X.-Q.; Lu, Y. Chem. Commun. 2015, 51, 12613−12616. (39) Hattori, T.; Matsumoto, H.; Murakami, Y. In Preparation of catalysts IV; Elsevier: Amsterdam, 1987. (40) Li, J.; Ma, L.; Li, X.; Lu, C.; Liu, H. Ind. Eng. Chem. Res. 2005, 44, 5478−5482. (41) Zhang, Q. F.; Li, Y. K.; Chai, R. J.; Zhao, G. F.; Liu, Y.; Lu, Y. Appl. Catal., B 2016, 187, 238−248. (42) Kresse, G.; Hafner, J. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 49, 14251−14269. (43) Kresse, G.; Furthmüller, J. Comput. Mater. Sci. 1996, 6, 15−50. (44) Blöchl, P. E. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953−17979. (45) Padama, A. A. B.; Kasai, H. J. Chem. Phys. 2014, 140, 244707. (46) Johnson, R. S.; DeLaRiva, A.; Ashbacher, V.; Halevi, B.; Villanueva, C. J.; Smith, G. K.; Lin, S.; Datye, A. K.; Guo, H. Phys. Chem. Chem. Phys. 2013, 15, 7768−7776. (47) Alavi, A.; Hu, P.; Deutsch, T.; Silvestrelli, P. L.; Hutter, J. Phys. Rev. Lett. 1998, 80, 3650−3653. (48) Anderson, B. D.; Tracy, J. B. Nanoscale 2014, 6, 12195−12216. (49) Denton, A. R.; Ashcroft, N. W. Phys. Rev. A: At., Mol., Opt. Phys. 1991, 43, 3161−3164. (50) Antolini, E.; Cardellini, F. J. Alloys Compd. 2001, 315, 118−122. (51) Radmilovic, V.; Gasteiger, H. A.; Ross, P. N. J. Catal. 1995, 154, 98−106. (52) Mayernick, A. D.; Janik, M. J. J. Catal. 2011, 278, 16−25. (53) Trinchero, A.; Hellman, A.; Grönbeck, H. Surf. Sci. 2013, 616, 206−213. (54) Ortloff, F.; Bohnau, J.; Graf, F.; Kolb, T. Appl. Catal., B 2016, 182, 375−384. (55) Chen, W.; Sheng, W.; Cao, F.; Lu, Y. Int. J. Hydrogen Energy 2012, 37, 18021−18030. (56) Shinde, V. M.; Madras, G. AIChE J. 2014, 60, 1027−1035. (57) Cortés, J.; Valencia, E.; Araya, P. Catal. Lett. 2006, 112, 121− 128. (58) Wang, S.; Gao, D. N.; Zhang, C. X.; Yuan, Z. S.; Zhang, P.; Wang, S. D. Prog. Chem. 2008, 20, 789−797. (59) Deng, J. G.; Zhang, L.; Dai, H. X.; He, H.; Au, C. T. Appl. Catal., B 2009, 89, 87−96. (60) Bukhtiyarova, M. V.; Ivanova, A. S.; Slavinskaya, E. M.; Plyasova, L. M.; Rogov, V. A.; Kaichev, V. V.; Noskov, A. S. Fuel 2011, 90, 1245−1256.

6245


High-Performance PdNi Nanoalloy Catalyst in Situ Structured on Ni

Recommend Documents