NixCoy Nanocatalyst Supported by ZrO2 Hollow Sphere for Dry

Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 24078−24087

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NixCoy Nanocatalyst Supported by ZrO2 Hollow Sphere for Dry Reforming of Methane: Synergetic Catalysis by Ni and Co in Alloy Kefa Sheng,† Dong Luan,‡ Hong Jiang,*,‡ Fang Zeng,† Bo Wei,† Fei Pang,† and Jianping Ge*,† †

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Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062, China ‡ Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: NixCoy/H-ZrO2 catalysts composed of highly dispersed NixCoy nanoparticles supported by mesoporous ZrO2 hollow sphere are synthesized by templating and impregnation processes. According to thermogravimetric analysis, X-ray photoelectron spectroscopy, and dry reforming results, a synergetic reaction mechanism is proposed to explain the better performance of alloy catalysts compared to Ni/HZrO2 or Co/H-ZrO2. In dry reforming of methane (DRM) reaction, Ni and Co act as catalysts for CH4 cracking and CO2 reduction, respectively, and the induced carbon deposits on Ni can be oxidized by the active oxygen left on Co, which regenerate the metal surface for the following reaction. Among all the alloy catalysts, the Ni0.8Co0.2/H-ZrO2 catalyst presents the highest activity and stability because the strong metal−support interaction prevents the sintering of nanocatalysts at high temperature and the hollow structure enhances the mass transportation of reactants and products. More importantly, Ni and Co can synergistically balance the speed of CH4 cracking and CO2 reduction, which effectively avoid coke accumulation/catalyst oxidation and ensure fast and stable conversion for DRM reaction. KEYWORDS: NixCoy alloy, ZrO2 hollow sphere, nanocatalyst, dry reforming, synergetic effect

1. INTRODUCTION Greenhouse effect caused by the excessive usage of fossil fuels including coal, natural gas, and petroleum oil has become an inevitable challenge in human society.1−3 CO2 from fuel combustion contributes 84% of greenhouse gas emission in the world.4,5 Meanwhile, CH4 produced from petroleum reserves6 and landfill gas7,8 is also found to be a major contributor to the greenhouse gases. The conversion of CO2 and CH4 into chemical products with high economic values is one of the best ways to alleviate the greenhouse effect, and the related research certainly becomes a hot topic in the green carbon science. Although various reforming technologies including autothermal reforming, partial oxidation, and steam reforming have been developed to achieve the above conversion, dry reforming of methane (DRM) still has its own advantages,2 such as 20% lower operating cost compared to others9 and synchronized conversion of CH4 and CO2 via a single process.10,11 More importantly, it provides a promising route for direct synthesis of high-purity syngas, which is broadly used as the raw material for the synthesis of downstream products via methanol synthesis, hydrogenation, and Fischer−Tropsch reaction.12 During the past decades, large amounts of metal catalysts have been synthesized and investigated in order to realize highly active and long-life conversion of CH4 and CO2 in the DRM reaction. Noble metal catalysts,13−17 particularly Ru and Rh,18 are found to be good catalysts because of their strong © 2019 American Chemical Society

resistance to carbon depositions in the DRM reaction. However, the high cost of noble metals limits their application in a large scale. As an economic substitute, Ni-based catalysts quickly gained peoples’ interests, but the severe deactivation due to carbon deposition and sintering at high temperature are still big challenges in industrial applications.19−23 Great efforts have been made to overcome the aforementioned problems of Ni-based catalysts. From the consideration of supporting materials, the Ni nanoparticles are designed to be loaded to Lewis base oxides, including CaO,24 ZrO2,25,26 BaO,27 MgO,28 La2O3,29,30 CeO2,31−34 and so forth, which promote the adsorption of CO2 and accelerate the elimination of carbon deposit through the reaction between CO2 and C.35 Rare earth metal oxides, such as La2O3 and CeO2, are also proposed to be the supporting materials because their high surface oxygen mobility is beneficial to the removal of carbon deposition.36−40 In the aspect of metal catalysts, metal alloys are used to adjust the speed of CH4 cracking and CO2 reduction, so that carbon residuals can be removed in time. The active Ni nanoparticles can also be encapsulated in mesoporous silica to form Ni@SiO241,42 core−shell structures. For example, M-Ni@SiO2 multiple-cores@shell nanocatalysts Received: April 3, 2019 Accepted: June 13, 2019 Published: June 13, 2019 24078

DOI: 10.1021/acsami.9b05822 ACS Appl. Mater. Interfaces 2019, 11, 24078−24087

Research Article

ACS Applied Materials & Interfaces

with the aqueous solution of NaOH (13.2 mL, 2.5 M) to form a homogeneous solution. Then, the mixture was heated to 70 °C and maintained at the same temperature for 6 h to ensure the complete etching of the SiO2 core. After being cooled down to room temperature, the hollow spheres were centrifuged, washed, and dried for the loading of metal catalysts. 2.4. Synthesis of NixCoy/H-ZrO2 Nanocatalysts. A series of NixCoy alloy catalysts with different Ni/Co molar ratios, including Ni/H-ZrO2, Ni0.8Co0.2/H-ZrO2, Ni0.6Co0.4/H-ZrO2, Ni0.4Co0.6/HZrO2, Ni0.2Co0.8/H-ZrO2, and Co/H-ZrO2, were prepared by the impregnation method. The total controlled amount of metal, including Ni and Co, was estimated to be 4.76% in weight for all NixCoy/H-ZrO2 catalysts. The Ni/Co ratio was controlled by the dosage of Ni(NO3)2·6H2O and Co(NO3)2·6H2O during the synthesis. The practical amount of Ni and Co was determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES) and listed in Table 1. Taking the preparation of the Ni0.8Co0.2/H-

showed superior long-term durability in the DRM reaction with the CH4 and CO2 conversion close to the thermodynamic equilibrium and the H2/CO ratio near 1.43 The confinement effect from mesoporous materials can effectively solve the sintering problem of nanoparticle, which helps to maintain the high activity in long-term reaction.24,44−46 In addition to the traditional supported catalysts, crystalline oxide catalysts with metal-active sites in the lattice, including perovskites, pyrochlores, fluorites, and hexaaluminates, have also been used for the DRM reaction because of their high thermal stability.47−51 For instance, well-dispersed Ni particles reduced from the crystalline La2Zr2−xNixO7 and La2NiZrO6 showed excellent activity for DRM at temperature as low as 600 °C and good stability over 350 h.52 Although many research works about the modification of catalysts have been reported recently, more active and long-life catalysts with good resistance to carbon deposits and sintering are still highly desired in the current stage. In this work, NixCoy/H-ZrO2 alloy nanocatalysts with precise control of Ni/Co ratios were synthesized by coating of mesoporous ZrO2 on SiO2 colloids, etching of SiO2 core with alkaline solution, and loading of metal nanocatalysts via the impregnation method. Among all the alloy catalysts, the Ni0.8Co0.2/H-ZrO2 catalyst presented the highest activity and long-term stability in the DRM reaction. On the basis of thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS) of the NixCoy/H-ZrO2 alloy nanocatalysts after the reaction, a synergistic mechanism was proposed to describe the kinetics for the DRM reaction, in which Ni and Co act as catalysts for CH4 cracking and CO2 reduction, respectively, and the induced carbon deposits and active oxygen combined to release CO, thus regenerating the metal surface for the next-round reaction. The Ni0.8Co0.2 catalyst supported by the mesoporous shell of a hollow ZrO 2 nanosphere showed higher activity and better stability because of the strong metal−support interactions and the effective mass transportation of reactants and products. More importantly, the coexistence of Ni and Co helped to balance the number of carbon species derived from CH4 decomposition and oxygen species from the reduction of CO2, which effectively avoided coke accumulation and catalyst oxidation.

Table 1. Grain Size of Metal Particles, Weight Percentage of Metals, Surface Area, Pore Volume, and Pore Diameter for SiO2@ZrO2 Particles, H-ZrO2 Hollow Particles, Ni/SiO2@ ZrO2 Catalysts, and NixCoy/H-ZrO2 Alloy Catalysts sample SiO2@ZrO2 H-ZrO2 Ni/SiO2@ZrO2 Ni/H-ZrO2 Ni0.8Co0.2/HZrO2 Ni0.6Co0.4/HZrO2 Ni0.4Co0.6/HZrO2 Ni0.2Co0.8/HZrO2 Co/H-ZrO2

dmetal (nm)

Ni (%)

Co (%)

SBET (m2/g)

VP (cm3/g)

DP (nm)

0.0767 0.159 0.056 0.174 0.164

3.26 3.92 3.46 3.15 3.39

9.4 9.1 9.5

4.93 4.82 3.83

0.89

29.0 42.6 29.0 44.0 41.9

8.0

2.83

1.78

44.1

0.172

3.15

9.6

1.9

2.82

43.5

0.176

3.28

8.0

0.87

3.85

43.4

0.175

3.28

4.92

43.9

0.173

3.40

8.2

ZrO2 catalyst as an example, Ni(NO3)2·6H2O (0.04 g) and Co(NO3)2·6H2O (0.01 g) were dissolved in ethanol (10 mL) to form a homogeneous solution. H-ZrO2 nanoparticles (0.2 g) were dispersed in the abovementioned solution, which allowed the adsorption of the metal precursor into the mesopores of the supporting materials. After the mixture was dried and calcined at 500 °C in air for 2 h, the Ni0.8Co0.2/H-ZrO2 catalysts with a practical metal loading of 4.76% (3.83% Ni, 0.89% Co) could be obtained, which was consistent with the estimated value. 2.5. Characterization. The morphologies of nanocatalysts were characterized by a Hitachi S4800 scanning electron microscope and an FEI Tecnai G2 F30 transmission electron microscope. The energydispersive spectroscopy (EDS) mapping of Ni0.8Co0.2/H-ZrO2 catalysts was also measured by Tecnai G2 F30 under high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) mode. Nitrogen adsorption−desorption (AD) isotherms were measured at 77 K by a Belsorp-Max analyzer to determine the BET (Brunauer−Emmett−Teller) surface areas, Barret−Joyner−Halenda pore diameters, and total pore volume of the nanocatalysts. Before each measurement, the samples were pretreated at 300 °C for 3 h to remove the adsorbed gases. Powder Xray diffraction (XRD) patterns were scanned from 10° to 90° with a speed of 30°/min by a Rigaku Ultima IV X-ray diffractometer, which was operated at 35 kV and 40 mA with Ni-filtered Cu Kα radiation as the X-ray beam source. The mass percentage of Ni or Co content of the H-ZrO2-supported alloy catalysts was measured by a Thermo IRIS Intrepid II XSP ICP-AES. TGA was performed to determine the amount of carbon deposition in dry reformation reaction. XPS for surface analysis of samples was performed on an AXIS Ultra DLD.

2. EXPERIMENTAL SECTION 2.1. Materials. Zirconium(IV) butoxide (ZrB, 80 wt % in 1butanol), hexadecylamine (HDA, 90%), nickel(II) nitrate hexahydrate (Ni(NO3)2·6H2O, 99%), and cobalt(II) nitrate hexahydrate (Co(NO3)2·6H2O, 99%) were purchased from Sigma-Aldrich. Tetraethylorthosilicate (98%), aqueous ammonia (NH3·H2O, 28%), ethanol (99.7%), and sodium hydroxide (NaOH, 96%) were purchased from Sinopharm Chemical Reagent Co. Ltd. 2.2. Synthesis of SiO2@ZrO2 Nanoparticles. Monodisperse SiO2 particles were first synthesized by the Stöber method. They were coated by zirconium oxide through hydrolysis of ZrB in ethanol to produce SiO2@ZrO2 nanoparticles.53 In a typical process, SiO2 nanoparticles (1 g) and HDA (1.2 g) were fully dispersed in ethanol (200 mL) by sonication, and NH3·H2O (4 mL) was added to the above solution under continuous stirring for 30 min. Then, ZrB (3.0 mL) was gradually added, and the solution was stirred for another 4 h to produce SiO2@ZrO2 nanoparticles. These core−shell particles were centrifuged from the solution, washed with ethanol, dried in an oven at 90 °C, and calcined at 600 °C in air for 4 h to remove HDA. 2.3. Synthesis of H-ZrO2 Nanoparticles. The H-ZrO2 nanoparticles were prepared by etching of the SiO2 core with an aqueous solution of NaOH. In detail, the calcined SiO2@ZrO2 nanoparticles (2 g) were dispersed in deionized water (200 mL), which was mixed 24079


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Figure 1. (a) Schematic illustration of the synthesis of NixCoy/H-ZrO2 nanocatalysts and SEM images of particles in each step, including (b) SiO2, (c) SiO2@ZrO2, (d) H-ZrO2, and (e) Ni/H-ZrO2 particles. (f) XRD patterns, (g) N2 AD isotherms, and (h) pore size distributions of SiO2@ZrO2, H-ZrO2, and Ni/H-ZrO2 particles. [r(CH4)in − r(CH4)out]/r(CH4)in and Conv.(CO2) = [r(CO2)in − r(CO2)out]/r(CO2)in. Turnover frequency (TOF) is originally defined as the number of substrate molecules turned over by the number of active centers in unit time, and it should be calculated by the number of exposed metal atoms in heterogeneous catalysis. In order to compare with the catalysts mentioned in previous works (Table S2), TOF values are calculated by the total number of metal atoms mentioned in this work. The TOF values of CH4 and CO2 were calculated according to the following formulas, where the molar amount of gas (n) was calculated from its initial flow rate and conversion and the molar amount of NixCoy alloy was calculated by the mass of catalyst, the weight percentage of NixCoy alloy (wt %) from ICP-AES, and its molecular weight (M): TOF (CH4) = n(CH4)/[n(NixCoy) × t)] = [r(CH4)in × Conv.(CH4) × t/22.4]/ [m(cat) × wt % (NixCoy)/M(NixCoy) × t]. TOF (CO2) = n(CO2)/ [n(NixCoy) × t)] = [r(CO2)in × Conv.(CO2) × t/22.4]/[m(cat) × wt % (NixCoy)/M(NixCoy) × t]. The H2/CO ratio was calculated by the flow rate of H2 and CO in the products. H2/CO ratio = r(H2)out]/ r(CO)out.

Temperature-programmed reduction (TPR) of the calcined NixCoy/H-ZrO2 catalysts by hydrogen (H2-TPR) was performed on a TP5080 chemical adsorption analyzer equipped with a thermal conductivity detector (TCD). Typically, the oxidized catalysts were reduced under the flow of H2/N2 gas (5%) with a flowing rate of 30 mL/min, where the temperature was raised from 20 to 900 °C with a heating rate of 10 °C/min. Temperature-programmed desorption of CO2 adsorbed on the catalyst (CO2-TPD) was also performed on a TP5080 chemical adsorption analyzer. The H2-reduced Ni/H-ZrO2 and Co/H-ZrO2 catalysts were placed under the flow of CO2 stream (30 mL/min) for 60 min at room temperature to achieve saturated adsorption of CO2. After purging the unadsorbed CO2, the CO2 desorption was recorded as the temperature raised from 25 to 1000 °C with a heating rate of 10 °C/min. In situ Fourier transform infrared (FTIR) spectra of CO2 in a sealed chamber with the presence of Ni/H-ZrO2 and Co/H-ZrO2 catalysts at 550 °C were recorded to monitor the possible conversion of CO2 to CO. The catalysts were reduced by 5% H2/Ar with a flow rate of 15 mL/min for 1 h and purged with He (15 mL/min) for an additional 0.5 h at 550 °C. After purging the IR cell with 10% CO2/He for 3 s, the cell was sealed and the IR spectra were recorded at 5, 15, and 30 min. 2.6. Dry Reforming of CH4 with CO2. Dry reforming of CH4 was carried out in a fixed bed reactor. In a typical reaction, the NixCoy/HZrO2 nanocatalyst (20 mg) was placed in the quartz tube and reduced at 700 °C for 60 min under the flow of H2/Ar mixing gas (5%). Then, the feed gases (CH4/CO2/Ar = 1/1/9) were made to flow through the catalyst bed at a flow rate of 91.6 mL/min, which corresponded to a gas hourly space velocity (GHSV) of 275 L·gcat−1 h−1. The effluent gases were analyzed by an online gas chromatograph equipped with a TDX-01 column and a TCD. The flow rate of each component of feed gases and effluent gases, such as r(CH4), could be measured by the gas chromatography peak areas. The conversions of CH4 and CO2 were calculated according to the following formulas: Conv.(CH4) =

3. RESULTS AND DISCUSSION The Ni/H-ZrO2 particles were synthesized by wet chemistry reactions, including sol−gel coating of mesoporous ZrO2 on SiO2 particles, etching of the SiO2 core, and loading of Ni nanocatalysts (Figure 1a). First, the SiO2@ZrO2 particles were prepared by hydrolysis of zirconium(IV) butoxide (ZrB) in the presence of HDA. In the hydrolytic reaction, HDA acted as a soft template to form a mesoporous ZrO2 shell, and it also guided the hydrolysis on SiO2 particles to form a core−shell nanostructure because of the promoted electrostatic interaction between HDA and SiO2. Second, the SiO2 core was etched by NaOH and HDA was removed by calcination, which 24080


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ACS Applied Materials & Interfaces produced ZrO2 hollow particles with clean surface. As shown in the scanning electron microscopy (SEM) image (Figure 1d), this inorganic hollow structure has good mechanical stability and it did not collapse even after being vacuumed for some time. Third, about 4.76% of Ni nanocatalysts were loaded to the shell of H-ZrO2 particles through impregnation. Eventually, the Ni/H-ZrO2 nanocatalysts were obtained after calcination in air and reduction in H2 atmosphere subsequently. In addition to the change of morphologies, the chemical composition and nanostructure also changed accordingly in the above synthesis, which was investigated by XRD patterns and N2 AD isotherms. At the first sight, the XRD patterns of SiO2@ZrO2 particles, H-ZrO2 hollow particles, and Ni/HZrO2 catalyst particles were close to each other, which showed characteristic peaks of monoclinic zirconia (M-ZrO2, PDF#371484) and tetragonal zirconia (T-ZrO2, PDF#49-1642) (Figure 1f) because the amorphous SiO2 does not show XRD peaks, and the weak diffraction signal of the small amount of Ni nanoparticles were hard to be identified in the presence of strong diffraction signals of crystalline ZrO2. When the XRD patterns were amplified, a weak and broad peak at 44.5° corresponding to the (111) plane of Ni (PDF#04-0850) could be observed, which suggested that nanosize Ni particles had been successfully loaded onto H-ZrO2 particles. On the other hand, the N2 AD isotherms and pore distributions for the three samples were quite different (Figure 1g−h). The AD curves of H-ZrO2 and Ni/H-ZrO2 were type IV isotherms with hysteresis loops in the medium range of relative pressure, which could be ascribed to the capillary condensation in intercrystalline mesoporous structures and thereby used to confirm the mesopore materials. However, the isotherm of SiO2@ZrO2 particles showed no characteristics of mesopores as they were not yet emptied through removal of SiO2. The pore volume and pore size increased, which were consistent with the removal of HDA. The BET surface area for SiO2@ ZrO2, H-ZrO2, and Ni/H-ZrO2 particles was measured to be 29.0, 42.6, and 44.0 m2/g, respectively, which suggested that the hollow structure was favorable to the increase of surface area and the Ni catalyst loaded by impregnation did not fill all the mesopores. Using similar synthetic procedures, the NixCoy/H-ZrO2 alloy nanocatalysts with precise control of Ni/Co ratios could be prepared by adjusting the dosage of Ni(NO3)2 and Co(NO3)2 during the synthesis. The XRD patterns of all NixCoy/H-ZrO2 alloy catalysts were identical to each other, which were majorly composed of diffraction signals of crystalline zirconia (Figure 2). Similar to the XRD patterns of Ni/H-ZrO2, all the weak diffraction peaks around 45° could be ascribed to the (111) crystal face of cubic Co (PDF#150806), cubic Ni (PDF#04-0850), or their alloys.54 With the increase of Ni/Co molar ratios, the diffraction peak gradually shifted from 44.2° to 44.5°, which confirmed the formation of NixCoy alloy and fine tuning of their chemical composition by the current synthetic method. Table 1 shows the physicochemical properties of these NixCoy/H-ZrO2 alloy catalysts. The grain size of deposited Ni, Co, and NixCoy alloys was calculated according to the Scherrer equation and full width at half-maxima of (111) diffraction peaks from XRD patterns. The practical weight percentage of Ni and Co was measured by ICP-AES, and the mesopore characteristics were obtained by the N2 AD isotherms (Table 1, Figure S1). The measurement results indicated that the NixCoy/H-ZrO2 nanocatalysts had very close surface area (41.9−44 m2/g), mesopore diameter

Figure 2. (a,b) XRD patterns of NixCoy/H-ZrO2 nanocatalysts with different Ni/Co molar ratios.

(3.15−3.4 nm), pore volume (0.164−0.176 cm3/g), and grain size for the loaded metal nanoparticles (8.0−9.6 nm) but continuously altered Ni/Co ratio, which provided an ideal material system for the study of alloy catalysts. The aforementioned multistep synthesis eventually produced highly dispersed NixCoy nanoparticles supported on the mesoporous shell of well-crystalline hollow ZrO2 particles. The TEM images of the Ni0.8Co0.2/H-ZrO2 catalyst showed that all particles had a typical hollow structure with a 20 nm shell of ZrO2 (Figure 3). Linear scanning of energy-dispersive spectra of Zr also confirmed the formation of the hollow structure. Because of the high crystallinity, the hollow structure had good mechanical stability to prevent itself from collapse in general sample treatments. The high-resolution TEM (HRTEM) image revealed that the shell was composed of ZrO 2 nanocrystals and mesopores existed between these nanocrystals. Because of the close capability to scatter the electron for Zr, Ni, and Co, it was hard to directly distinguish the alloy nanoparticles on H-ZrO2 by contrast. However, the lattice image did indicate that the alloy nanoparticles with an approximate size of 5 nm were dispersed on H-ZrO2 without apparent aggregations. The HAADF-STEM image and the EDS mapping of Ni0.8Co0.2/H-ZrO2 nanocatalysts also confirmed the good dispersibility of alloy nanoparticles. The overlap in the distribution of Ni and Co suggested the formation of NiCo alloy but not independent deposition of Ni and Co nanoparticles. Although EDS analysis reported a lower weight percentage of Ni0.8Co0.2 alloy in the catalyst, the atomic ratio of Ni and Co calculated by their atomic percentages was well consistent with the designed alloy composition (Table S1). In short, the NixCoy/H-ZrO2 nanocatalysts possessed highly dispersed alloy nanoparticles supported by hollow ZrO2 particles with mesoporous shells, which would have great potential as nanocatalysts in many heterogeneous catalytic processes, including DRM explored in this work. Dry reforming of CH4 with CO2 was an extremely endothermic reaction accompanied by side reactions, which 24081


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When a series of NixCoy alloy nanocatalysts was used in the DRM reaction, the Ni0.8Co0.2/H-ZrO2 catalysts presented the highest activity and long-term stability compared to others. Here, all DRM reactions were performed at 700 °C with GHSV set as 275 L·gcat−1 h−1 to test and compare the activity of these alloy catalysts (0.02 g) (Figure 4). When the Ni/Co

Figure 3. (a,b) Bright-field TEM images and (c) HRTEM images of the Ni0.8Co0.2/H-ZrO2 nanocatalyst. (d) HAADF-STEM image; (e) EDS mappings of Ni, Co, Zr, and O elements; (f) EDS linear scanning of Zr across the particle; and (g) complete EDS spectrum of the Ni0.8Co0.2/H-ZrO2 particle.

required proper high temperature and active and long-life catalysts to realize high energy efficiency in the production of syngas. Previous research works based on the Gibbs free energy minimization method indicated that CH4 and CO2 conversions were maximized at low pressure and high temperature.55,56 According to the Gibbs free energies, the DRM reaction (eq 1) became spontaneous at a temperature higher than 645 °C. In our experiments, the conversions of CH4 and CO2 did increase with the elevation of reaction temperatures (Figure S2). However, because of the existence of side reactions and change of catalysts at high temperature, the practical DRM was much more complicated than an endothermic reaction, which presented several urgent problems to be solved. First, the activity of metal catalysts usually decreased at high temperature because of the agglomeration of metal nanoparticles. Second, the carbon depositions produced by CH4 cracking (eq 2) above 555 °C and CO disproportionation (eq 3) below 700 °C would cover the active sites of metals and let them lose the catalytic activities. Third, the produced H2 would be consumed by the reverse water gas shift (RWGS, eq 4), so that the H2/CO ratio was less than 1 for the generated syngas. Because of these considerations, an active and long-life catalyst that was resistant to agglomeration and carbon depositions at high temperature was highly desired for the DRM reaction. CH4 + CO2 → 2CO + 2H 2

(1)

CH4 → C(s) + 2H 2

(2)

2CO → CO2 + C(s)

(3)

CO2 + H 2 → H 2O + CO

(4)

Figure 4. (a,b) Conversions of CH4 and CO2 in DRM catalyzed by NixCoy/H-ZrO2 with Ni/Co molar ratios of (1) 1:0, (2) 0.8:0.2, (3) 0.6:0.4, (4) 0.4:0.6, (5) 0.2:0.8, and (6) 0:1. (c,d) Conversions of CH4 and CO2 and (e) H2/CO ratio of the products in long-term DRM catalyzed by Ni0.8Co0.2/H-ZrO2 and Ni/H-ZrO2 catalysts. Reaction conditions: CH4/CO2/Ar = 1/1/9, GHSV = 275 L·gcat−1 h−1, 700 °C.

ratio in the catalyst is less than 1, the NixCoy catalysts gradually deactivated completely in 2−3 h, which could be attributed to the surface oxidation of Co by excessive oxygen adatoms and thereby the blocking of DRM reactions.57,58 The Ni/Co ratio being adjusted to be larger than 1, the activity of nanocatalysts increased steadily and the conversion of both CH4 and CO2 was maintained at a high level during the 6 h reaction. The conversion presented a volcano distribution along with the Ni/ Co ratio, and a highest conversion of CH4 (92.8%) and CO2 (93.0%) could be reached by Ni0.8Co0.2/H-ZrO2 catalysts, which was close to the equilibrium values at that temperature. The Ni/H-ZrO2 and Ni0.8Co0.2/H-ZrO2 nanocatalysts, which showed good activity in the above experiments, were further tested in a long-term reaction to evaluate their stabilities. For DRM catalyzed by Ni/H-ZrO2, the conversion of CH4 and CO2 declined from 79.8 to 51.6% and from 86.5 to 61.0%, respectively, in 80 h. For reactions catalyzed by Ni0.8Co0.2/HZrO2, the conversion of CH4 and CO2 declined from 92.8 to 69.1% and from 93.0 to 75.7%, respectively. In a word, there was a 35 and 29% decrease in the production of CH4 and CO2 for Ni/H-ZrO2 catalysts after 80 h but a 25.5 and 18.6% decrease in the production of CH4 and CO2 for Ni0.8Co0.2/H24082


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change of electron densities and valence states of the metal catalyst during the reaction (Figure 6). In the XPS spectrum of

ZrO2 catalysts. The decrease in conversion was possibly caused by the agglomeration of part of the metal or alloy nanoparticles at high temperature. Although the conversions declined in both cases, a relatively slower decrease in conversion suggested that the Ni0.8Co0.2/H-ZrO2 catalyst had better stability for long-time reaction. As for the H2/CO ratio in the products of DRM reactions after 80 h, which is another important value to evaluate the catalyst performance, the Ni0.8Co0.2/H-ZrO2 catalyst also showed a higher value (0.78) than that of the Ni/H-ZrO2 catalysts (0.75). The H2/CO ratio usually could not reach the theoretical value (1.0) because part of H2 was consumed by the RWGS reaction, and a H2/CO ratio around 0.78 for the Ni0.8Co0.2 /H-ZrO 2 catalyst is reasonable considering the activity it presented. TGA of these NixCoy/H-ZrO2 catalysts indicated that carbon deposits increased with the increase of Ni content in the alloy, which suggested that CH4 preferred to be adsorbed on the surface of Ni (Figure 5). After the DRM reaction for 24

Figure 6. XPS spectra of (a) Ni 2p in Ni/H-ZrO2 and Ni0.8Co0.2/HZrO2 catalysts and (b) Co 2p in Co/H-ZrO2 and Ni0.8Co0.2/H-ZrO2 catalysts. Figure 5. TGA of Co/H-ZrO2, Ni0.4Co0.6/H-ZrO2, Ni0.8Co0.2/HZrO2, and Ni/H-ZrO2 catalysts after DRM reaction.

Ni/H-ZrO2, the binding energies of Ni 2p3/2 and Ni 2p1/2 were measured to be around 854 and 872 eV, and these two broad XPS peaks can be further fitted to 853.1 eV (Ni0), 855.1 eV (Ni2+), 871.5 eV (Ni0), and 873.4 eV (Ni2+), respectively. Similarly, the binding energies of Co 2p3/2 and Co 2p1/2 were measured and fitted to be 780.1 eV (Co0) and 781.7 eV (Co2+), and 795.7 eV (Co0) and 797.3 eV (Co2+), respectively. The ratio of metallic state and oxidative state was basically the same from the 2p3/2 and 2p1/2 XPS spectra. Therefore, only the bind energies of Ni 2p3/2 and Co 2p3/2 would be analyzed to study the change of the electronic state of surface Ni and Co. For Ni0.8Co0.2 alloy, the binding energy of Ni 2p3/2 and Co 2p3/2 was measured to be close values as those of Ni in the Ni/ H-ZrO2 catalyst and Co in the Co/H-ZrO2 catalyst, but the ratio of Ni0/Ni2+ increased and the ratio of Co0/Co2+ decreased in alloy catalysts compared to the metal catalysts. It suggested that the electrons were further enriched around Ni, and Co became more electron-deficient. After the reaction, the binding energies of Ni remained almost unchanged while those of Co increased 1.5 eV compared to the unreacted catalyst. Because the increase of binding energies suggested a more difficult escaping of electrons and a decrease of electron density on the metal surface, it could be concluded that more electron-deficient Co had appeared during the reaction. The electron-deficient Co, compared to the electron-rich Ni, could be more suitable to bond with CO2. In addition, in situ IR spectra for Ni/H-ZrO2 and Co/HZrO2 catalysts under sealed CO2 atmosphere at 550 °C were recorded to monitor the CO2 activation on metals (Figure 7). At a temperature lower than the practical reaction temperature, CO2 was already converted to CO by the Co/H-ZrO2 catalyst, but none of it was converted by Ni/H-ZrO2, which proved that CO2 was activated by Co. TPD of CO2 on Ni/H-ZrO2 and

h, the Co/H-ZrO2, Ni0.4Co0.6/H-ZrO2, Ni0.8Co0.2/H-ZrO2, and Ni/H-ZrO 2 catalysts were analyzed in air by a thermogravimetric analyzer to determine the adsorbed chemical species on the surface of the catalyst. It should be noted that the catalysts were collected after 24 h of reaction, but not 6 or 80 h of reaction, simply because the weight loss due to carbon residual was not obvious in the former case and the obtaining of all catalyst samples for TGA test was timeconsuming for the latter case. The TGA curve of Ni/H-ZrO2 was similar in trend to that of Ni0.8Co0.2/H-ZrO2, where the weight loss from 25 to 250 °C could be attributed to the desorption of physically adsorbed water and gases in air and the weight loss from 500 °C and 600 °C could be attributed to the burning of deposited carbon on the surface of the catalyst. On the other hand, the TGA curves of Ni0.4Co0.6/H-ZrO2 and Co/H-ZrO2 were close to each other, in which no weight loss due to carbon burning was observed. With the increase of Ni/ Co ratio in the alloy, the carbon residuals on the catalyst increased accordingly. Because the carbon residuals were produced by the decomposition of CH4, it was reasonable to conclude that CH4 is preferentially adsorbed and decomposed on Ni compared to Co. As another reactant in DRM reaction, CO2 was found to be preferentially adsorbed on Co in the alloy according to the XPS, XRD, CO2-TPD, and FTIR characterizations. At high temperature, CO2 acted as an oxidative reactant which preferred to bond with an electron-deficient metal species to form an active intermediate. Therefore, the XPS spectra for Ni and Co elements in the Ni/H-ZrO2, Co/H-ZrO2, and Ni0.8Co0.2/H-ZrO2 catalysts were recorded to investigate the 24083


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Figure 8. Synergistic reaction mechanism for DRM catalyzed by the NixCoy/H-ZrO2 catalyst. Step (1) decomposition of CH4, step (2) reduction of CO2, and step (3) removal of carbon by O adatom.

on the Ni surface. On the other hand, the adsorbed CO2 on Co was reduced to CO and left O species on the Co surface. The carbon deposits on Ni was quickly oxidized by the active oxygen left on Co, which released CO molecules and emptied the metal surface for the next-round reaction. For the Ni/HZrO2 catalyst shown in Figure 4, the CH4 cracking is faster than CO2 reduction, so that the carbon deposits gradually accumulate on the catalyst surface and the conversion decreases accordingly. However, for Co-rich catalysts, CO2 reduction is apparently faster than CH4 cracking. The excessive O adatoms produced by the reduction of CO2 on Co will cause surface oxidation of Co at high temperature, which blocks the continuous generation of O adatoms, their reaction with the carbon species, and thereby the whole DRM reaction. Only with optimized Ni/Co ratio and dynamic equilibrium between CH4 cracking and CO2 reduction, the coke accumulation and catalyst oxidation could be avoided at the same time, which ensured a fast and stable conversion for the DRM reaction. On the basis of the above mechanism and the other structural characteristics, a higher activity and better stability for NixCoy/ H-ZrO2 alloy catalysts could be explained by the following reasons. The higher activity and better stability of NixCoy alloy catalysts, compared to that of Ni catalysts, first originated from the strong metal−support interactions. Here, the TPR of the oxidized NixCoy/H-ZrO2 by H2 (H2-TPR) was used to study the interactions between NixCoy-active centers and ZrO2 hollow particles (Figure 9). In the TPR profile, the peaks usually indicated different metal oxide species and sometimes

Figure 7. Time evolution of FTIR spectra for (a) Ni/H-ZrO2 and (b) Co/H-ZrO2 catalysts under sealed CO2 atmosphere at 550 °C.

Co/H-ZrO2 catalysts also indicated that CO2 had stronger interaction with Co as more CO2 was desorbed at 200−400 °C (Figure S3). After reaction, a slight oxidation of Co in the Co/ H-ZrO2 catalyst could be proved by its XRD pattern, but the Ni in Ni/H-ZrO2 was not oxidized under the same condition (Figure S4). All the above results suggested that CO2 molecules were preferentially adsorbed to the Co surface to form an active intermediate for the DRM reaction. To further clarify the function of Ni and Co in an alloyed Ni−Co catalyst compared with pure Ni, we have performed first-principles calculations based on model Ni−Co bimetallic surfaces (see the Supporting Information for computational details). The activation barriers for the dissociative adsorption of CH4 and CO2 have been calculated on pure and Co-doped Ni(111), each modeled by a 2 × 2 surface supercell with four atomic layers. The NiCo(111) surface is modeled by replacing one-fourth of Ni atoms in the topmost layer by Co atoms. CH4 and CO2 adsorption and dissociation can take place at different sites of the NiCo surface. According to the calculation, introduction of Co atoms has little effects on the adsorption of CH4, but CO2 adsorbs more strongly on the Co-doped Ni surface, and the energetically most favorable site is the one with the adsorbing oxygen atom in direct contact with a Co atom (Figure S6), which agrees well with the conclusions from XPS and XRD experiments. Furthermore, it is found that the formation of NiCo bimetallic surfaces has different effects on CH4 and CO2 dissociation: the free-energy barrier for CH4 dissociation increases by 20 kJ/mol and that of the CO2 dissociation decreases by about 17 kJ/mol (Table S3). The theoretical calculations therefore clearly indicate that introducing Co into Ni surfaces can enhance CO2 adsorption and CO2 dissociation to form chemically active O atoms. On the basis of the above analysis, a synergistic mechanism was summarized to describe the kinetics for the DRM reaction catalyzed by NixCoy/H-ZrO2 alloy catalysts (Figure 8). At the beginning, CH4 and CO2 could be preferentially adsorbed by Ni and Co when the feeding gas was introduced to the reactor. CH4 decomposed on Ni, which released H2 and left C species

Figure 9. TPR of oxidized NixCoy/H-ZrO2 nanocatalysts. 24084


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surface of NixCoy, so that the catalyst presented the best activity and stability. Third, the current Ni0.8Co0.2/H-ZrO2 catalyst also possessed hollow structures for effective mass transportation, which improved the catalytic activity and stability. Here, Ni/SiO2@ ZrO2 particles and Ni/H-ZrO2 particles with the same Ni loading were used as a typical “solid” and “hollow” nanocatalyst to evaluate the contribution of hollow structures (Figure 10). The DRM reaction was performed at 700 °C, and

the same metal oxide with different size or in different chemical/physical environment. The TPR peaks appearing at a higher temperature indicated a stronger interaction between the metal and the support, and a larger peak area indicated a larger amount of such metals in the sample. As shown in the TPR profile, the oxidized Ni/H-ZrO2 catalyst had a strong and broad reduction peak around 465 °C and a weak peak around 650 °C, both of which were ascribed to the reduction of NiO species as Ni2+ is directly reduced to Ni0 without any intermediate oxidation state. It suggested that a small amount of Ni nanoparticles had stronger interactions with the ZrO2 hollow particles. A similar situation could be observed in the TPR profile of the oxidized Co/H-ZrO2 catalyst, where the reduction peak around 313 and 356 °C was associated with the reduction of Co3O4 and CoO, and the peak at 680 °C might be attributed to a very small amount of CoO with strong interactions with the support. When 20% of Co was introduced to form alloy nanoparticles during the synthesis, the asprepared oxidized Ni0.8Co0.2/H-ZrO2 catalyst showed a much higher reduction peak at 686 °C, which should be attributed to the reduction of NiO−Co3O4 composites. The reduction peak at a lower temperature zone was not observed at all, which suggested that all the Ni0.8Co0.2 alloy nanoparticles had strong interaction with the H-ZrO 2 . However, with further introduction of Co species, the reduction peaks at lower temperature appeared and became dominant compared to the weak peaks at higher temperature, which suggested that the interaction between most of the alloy nanoparticle and H-ZrO2 was weakened. For metal catalysts, a higher reduction temperature in the TPR profile usually indicated a stronger interaction between the metal and the support, and thereby an enhanced sintering-resistant capability for metal nanoparticles, which would be very helpful to the high conversion of substrate molecules on the active sites of the catalyst. The TEM images of the Ni/H-ZrO2 and Ni0.8Co0.2/H-ZrO2 catalysts after reaction for 80 h did indicate that less agglomeration of metal nanoparticles was formed in the latter sample (Figure S5). The difference in the reduction temperature of NixCoy alloy nanocatalysts was consistent with the sequence of their activities. It proved that the introduction of 20% Co effectively increased the metal−support interactions and catalyst activities. Second, the coexistence of Ni and Co in Ni0.8Co0.2 catalysts helped to balance the number of carbon species derived from CH4 decomposition and oxygen species from the reduction of CO2, which avoided loss of activity due to coke accumulation and catalyst oxidation. As mentioned in the mechanism, the reforming was accomplished by several steps, including CH4 cracking to form H2 and C deposits, reduction of CO2 to form CO and O adatoms, and the removal of C deposits with O adatoms. Because the CH4 cracking was more favorable on Ni, the increase of Ni species would accelerate the decomposition of CH4 and the carbon residual will block the catalyst from the reactant. On the other hand, CO2 was preferentially adsorbed on the cobalt surface because of its strong oxophilicity, and the increase of Co species would produce excessive O adatoms covering the metal surface, which was also unfavorable to the DRM reaction. Only with an appropriate Ni/Co ratio, a matched speed for CH4 cracking and CO2 reduction would be achieved, and both the carbon deposition and the surface oxidation could be avoided. In this case, the adsorption, reaction, and desorption took place continuously on the

Figure 10. Conversion of CH4 and CO2 in DRM reaction catalyzed by Ni/H-ZrO2 (hollow) and Ni/SiO2@ZrO2 (Solid) catalysts with GHSV set to be (a,b) 137.5 L·gcat−1 h−1 and (c,d) 275 L·gcat−1 h−1.

the GHSV of feeding gas was set to be 275 and 137.5 L·gcat−1 h−1. Under low GHSV, the “hollow” Ni/H-ZrO2 catalyst showed better activity in conversion of CH4 and CO2, and it also presented slightly better stability compared to the “solid” Ni/SiO2@ZrO2 catalyst as the reaction proceeded. However, under high GHSV, the “solid” Ni/SiO2@ZrO2 catalyst apparently lost its activity in 6 h as the conversion of CH4 and CO2 decreased by 19 and 18%. However, for the “hollow” Ni/H-ZrO2 catalyst, the conversion of CH4 and CO2 only decreased by 6.2 and 4.1%, which suggested that the hollow nanostructure did improve the stability of the catalyst especially at higher GHSV. The hollow structure of the Ni/ H-ZrO2 catalyst promoted the CH4 cracking and CO2 reduction by quickly transporting the feeding gas to the active sites on the mesoporous ZrO2 shell, and it also facilitated the combination of carbon deposits and active oxygen species by removing the generated CO in time. Therefore, the enhancement of the mass transfer for both reactants and products effectively improved the catalyst stability in long-term reactions.

4. CONCLUSIONS In summary, NixCoy/H-ZrO2 alloy nanocatalysts were synthesized by multistep reactions including coating of ZrO2 on SiO2 colloids, etching of the SiO2 core, and deposition of metal via impregnation. The as-prepared catalysts were composed of highly dispersed NixCoy alloy nanoparticles supported by the mesoporous shell of hollow ZrO2 particles, which was favorable to the catalysis of DRM. Through a series of DRM reactions, the Ni0.8Co0.2/H-ZrO2 catalyst was found 24085


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to have the best activity and stability compared to others. TG analysis and XPS characterization of the used NixCoy/H-ZrO2 nanocatalysts after reaction indicated that CH4 and CO2 were preferentially adsorbed on Ni and Co. A synergistic mechanism was then proposed to explain the kinetics of the DRM reaction, where Ni and Co were catalysts for CH4 cracking and CO2 reduction, and the induced carbon deposits and active oxygen combined to release CO, thus regenerating the metal surface for the next-round reaction. The as-prepared Ni0.8Co0.2/HZrO2 catalyst showed high activity and good stability at a relatively lower temperature and higher GHSV compared to previous results in the literature (Table S2). The higher activity and better stability for Ni0.8Co0.2/H-ZrO2 catalysts, compared to the other NixCoy/H-ZrO2 catalysts, could be explained in three aspects, including the strong metal−support interactions, the effective mass transportation for hollow structures, and the avoidance of carbon accumulation or catalyst oxidation under balanced CH4 cracking and CO2 reduction with coexistence of Ni and Co.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b05822.

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N2 AD, DRM at different temperatures, TPD of CO2 adsorbed on the catalyst, XRD patterns and TEM images of the used catalyst, EDS analysis of Ni0.8CO0.2/ H-ZrO2 particle, calculation of adsorption and dissociation energy on Ni and Co, and comparison of the TOF values in the literature (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H. Jiang). *E-mail: [email protected] (J.P. Ge). ORCID

Jianping Ge: 0000-0002-4366-1226 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Program of China (2016YFB0701103) and National Natural Science Foundation of China (21671067). The authors also thank Dr. Teng Xue in the Shanghai Key Laboratory of Green Chemistry and Chemical Processes for his great help to the characterization of nanocatalysts.

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NixCoy Nanocatalyst Supported by ZrO2 Hollow Sphere for Dry

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