Gas-Phase Synthesis of Ni-CeOx Hybrid Nanoparticles and Their

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Gas-Phase Synthesis of Ni-CeOx Hybrid Nanoparticles and Their Synergistic Catalysis for Simultaneous Reforming of Methane and Carbon Dioxide to Syngas Teng-Yun Liang, Chih-Yuan Lin, Fang-Chun Chou, Meiqi Wang, and De-Hao Tsai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00665 • Publication Date (Web): 09 May 2018 Downloaded from http://pubs.acs.org on May 10, 2018

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The Journal of Physical Chemistry

Gas-Phase Synthesis of Ni-CeOx Hybrid Nanoparticles and Their Synergistic Catalysis for Simultaneous Reforming of Methane and Carbon Dioxide to Syngas

Teng-Yun Liang, Chih-Yuan Lin, Fang-Chun Chou, Meiqi Wang, De-Hao Tsai*

Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan, R.O.C.

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ABSTRACT

We develop a new Ni-CeOx hybrid nanoparticle as a high-performance catalyst for dry reforming of methane with carbon dioxide (DRM) using a facile gas-phase synthetic approach. The crystallites of Ni and CeO2 were homogenously self-assembled as a hybrid nanostructure, creating a large amount of NiCe-O interface for a strong metal-support interfacial interaction. Chemical composition and oxidation state of the hybrid nanostructure were tunable directly in the aerosol state. The results show that the starting temperature of catalysis by Ni-based catalysts reduced by ≈ 150 ˚C through the hybridization with CeO2 followed by a direct H2-reduction in the aerosol state. In comparison to Ni-only nanoparticle, the Ni-CeOx hybrid nanoparticle showed stable and high conversion for CH4 and CO2 with a remarkably turnover frequency at low temperature (0.1 s-1 at 450 °C). The amount of coke formation greatly reduced by 18×, whereas the H2/CO ratio was constant at ≈ 0.8 by a 1.5× of the increase of CO2/CH4 ratio. The work demonstrated a facile route for controlled synthesis of Ni-CeOx hybrid nanoparticles with a very high catalytic activity and stability of DRM. The findings of this study can shed a light on the mechanism of Ni-Ce-O synergistic catalysis, which can be especially useful for methane-based energy applications.


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1. INTRODUCTION Syngas (i.e., CO + H2) is highly advantageous for industrial production of commodity chemicals (e.g., ammonia and methanol).1-6 Dry reforming of methane with CO2 (DRM; CH4 + CO2 2CO + 2H2) arises a great interest for the synthesis of syngas. Firstly, the product ratio, H2/CO, can be close to unity, which is useful for the production of liquid hydrocarbons and oxygenated compounds by means of Fischer–Tropsch synthesis production.1-3, 7-8 Moreover, DRM has shown to be an environmentalfriendly route for the synthesis of value feedstock with a simultaneous reduction of two greenhouse gases, CH4 and CO2.2-4, 9 The generation of hydrogen via the DRM also shows the promise of providing a clean source for power generation.7 Transition metal-based catalysts, especially the Ni-based catalyst at nanoscale, are preferred for the industrial use in the catalysis of DRM.1-2, 6, 8, 10-11 In comparison to the noble metal-based catalyst, Nibased nanoparticle (NP) is economical, highly available, and has a comparably high catalytic activity and selectivity toward DRM.2, 12-13 The potential in the catalytic DRM can be further enhanced by reducing the required reaction temperature (i.e., less heat required). However, the deactivation has shown to be a great challenge for the Ni-based NP, which can hinder the use as a long-term catalyst in industrial applications.2, 8-9, 11 The major routes of deactivation of Ni-based possibly are from (1) the sintering at a high operating temperature,7-9,

11, 14

and (2) the coke deposition on the active site of

catalyst during reaction.2, 4-5, 13, 15-18 In this regard, a combination of a support material or a second metal with the Ni-based catalysts is a promising strategy.1, 5, 8-9, 11, 15, 19 CeO2 was considered as a useful additive to the Ni-based catalyst.7, 20-21 The strong interfacial metal-support interaction (SMSI) in the Ni-Ce-O hybrid nanostructure provides a synergistic catalytic route for DRM.1, 7, 15, 20 The redox pair of Ce4+/Ce3+ promotes the oxidation of dissociated methane and the adsorption of CO2,22-23 which can inhibit coke formation on the catalyst surface to offer a high operation stability.1, 5, 7, 20 Besides, thermal stability of Ni-based catalysts are also able to be improved by using CeO2 for the dispersion of active ACS Paragon Plus Environment

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metal.7 A variety of synthetic methods of Ni-based hybrid NP have been explored to date (e.g., coimpregnation and co-precipitation), which have shown to be critical to the catalytic activity of DRM.10, 15, 24

In this work, we employ a gas-phase evaporation-induced self-assembly (EISA) method followed by a two-stage thermal treatment to develop Ni-based hybrid nanoparticles by design.25-28 As depicted in Figure 1, a stable, isotropic aqueous solution of precursors is nebulized to become droplets containing Ni and Ce precursors. Through a fast evaporation, the droplets become aerosol particles of dried precursor crystallites with a homogeneous elemental distribution.27-30 After 1st-stage gas-phase calcination, the aerosols of dried precursor crystallites converted to hybrid nanostructures of mixed oxides (NiO+CeO2). The aerosols of hybrid NP transformed to be Ni-CeO2 hybrid nanoparticles through a 2nd-stage H2 reduction. In comparison to the liquid-phase approaches reported previously (e.g., traditional impregnation method),24, 31-34 the gas-phase EISA proposed here demonstrates superior advantages.28,

35-37

Firstly, a continuous two-stage gas-phase flow reactor was employed for the

controlled synthesis of hybrid NPs, which provides the ability of direct tuning of oxidation state of NP (e.g., surface reduction). In the continuous gas-phase process, initial homogeneous dispersions of the multiple components can be maintained at their molecular level homogeneity in solutions.28,

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Moreover, the gas-phase EISA can also effectively avoid the issues related to the restrictions of solution-based chemistry (e.g., restriction in the properties of solvent, addition of surfactants).28 Complementary characterization approaches, including differential mobility analysis (DMA), highresolution transmission electron microscopy (HR-TEM), scanning electron microscopy (SEM), and Xray diffractometry (XRD) are employed to provide information of physical size, elemental composition, morphology and crystallinity, respectively. Our objective is to fabricate Ni-CeO2 hybrid NPs with controllable material properties (size, morphology, composition, oxidation state) and high


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catalytic activity and stability. The results can be used to provide mechanistic understanding of catalytic DRM at nanoscale.

Figure 1. Cartoon depiction of gas-phase synthesis of Ni-CeOx hybrid nanoparticles via two stages of thermal treatment.

2. MATERIALS AND METHODS 2.1. Materials 6-hydrate nickel nitrate (98%. Showa Chemical Industry Co., Ltd., Tokyo, Japan) and 6-hydrate cerium nitrate (99%. Showa Chemical Industry Co., Ltd.) were used as precursors of Ni and Ce, respectively. Biological grade 18.2 MΩ•cm deionized water (Millipore, Billerica, MA, USA) was used to prepare the precursor solutions.

2.2. Nanoparticle Synthesis and In-situ Differential Mobility Analyses A customized nebulizer was used to convert precursor solutions to aerosolized droplets, by using a compressed filtered N2 at a flow rate of 1.5 L/min. Through a fast evaporation, aerosol particles of ACS Paragon Plus Environment

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dried precursors formed and simultaneously self-assembled in the gas phase state with a homogeneous distribution. After calcination at 500 °C (i.e., the 1st stage thermal treatment) for 4 s in a quartz-made flow reactor, the aerosols of dried precursor crystallites thermally decomposed to NPs of mixed oxides with a homogenous elemental distribution. Then, the aerosols were delivered to the second-stage temperature-programmed quartz-made flow reactor for the H2 reduction at a flow rate of H2 of 0.25 L/min. The temperature of H2 reduction (Td2) ranged from room temperature (RT; 25 ˚C) to 800 ˚C, and the residence time of H2 reduction was 6.8 s. After the 2-stage thermal treatment, the synthesized NP were collected by a 47-mm in-line aerosol filter holder (All-Field Enterprise Corp, Taipei, Taiwan, ROC) equipped with a mixed cellulose ester membrane (0.2 µm, Advantec. Tokyo, Japan). Table 1 summarizes the sample identification having different molar fractions of Ni and Ce in the catalysts (denoted as nNi and nCe, respectively). The mass concentration of Ni precursor was constant at 16 wt%. The mass concentration of Ce precursor was ranging from 0 wt% to 2.4 wt%. The corresponding molar ratio of active component (i.e., Ni) was > 0.8, which was preferable for achieving high conversion ratios by the synergistic catalysis.28 Prior to the sample collection, the mobility diameter (dp,m) and the number-based mobility size distributions of aerosol particles were characterized in-situ by differential mobility analysis (DMA).28, 38 The flow rate of sheath air was set at 10 L/min, the step size was 4 nm, and the step time was 10 s.

Table 1. Notation of the samples. nNi and nCe are the molar fractions of Ni and Ce in the catalyst, respectively. Sample

Td2 (℃)

nNi (%)

nCe (%)

NiCeOx-NP-RT

RT

91

9


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NiCeOx-NP-300

300

91

9

NiCeOx-NP-600

600

91

9

NiCeOx-NP-800

800

91

9

Ni-only-NP-RT

RT

100

0

Ni-only-NP-300

300

100

0

Ni-only-NP-600

600

100

0

Ni-only-NP-800

800

100

0

2.3. Ex-situ Nanoparticle Characterization The morphologies of Ni-only-NP and NiCeOx-NP samples were analyzed using a field emission high-resolution transmission electron microscopy (HR-TEM, JEM-2100F, JEOL, Tokyo, Japan) equipped with an energy dispersive spectrometer (EDS) operated at an acceleration voltage of 300 kV, and a field emission scanning electron microscope (FE-SEM, Hitachi SU8010, Hitachi, Tokyo, Japan) operated at 10 kV. The synthesized NPs in the form of aerosol were delivered to a customized electrostatic precipitator with an electric field of -10 kV cm−1, where NPs deposited onto a copper grid or a silicon chip at a flow rate of ≈ 1.5 L min−1.28, 38 The X-ray diffraction (XRD) was performed using a X-ray powder diffractometer (Ultima IV, Rigaku Cooperation, Japan), with Cu-Kα radiation (λ= 1.5406 Å) operated at 40 kV, 20 mA, and a scanning rate of 2.0 °/min. The crystallite size of NP, dc, was calculated using Debye-Scherrer equation (details were shown in Section S1 of Supporting Information). Additionally, the X-ray photoelectron spectroscopic analysis (XPS; Microlab350, VG Scientific, East Grinstead, UK) was performed to ACS Paragon Plus Environment

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obtain high-resolution spectra of C 1s region. The thermogravimetric analysis (TGA, SDT Q600, TA Instruments, DE, USA) was employed to measure the amount of coke in the sample after the catalysis of DRM. The sample weight was 18 mg. The temperature of TGA increased from 25 °C to 800 °C at a constant heating rate of 10 °C/min. The air flow rate of TGA was 100 ml/min.

2.4. Catalytic performance tests of dry reforming The catalytic performance tests of DRM were carried out in a fixed-bed reactor system. The temperatures of activity tests ranged from RT to 900 °C. The sample weight was 100 mg. The flow rates of CH4, CO2, and N2 were 10 mL/min, 10 mL/min, and 180 mL/min, respectively. Based on the stoichiometry of total DRM (i.e., CH4 + CO2 => 2CO + 2H2), we defined that the molar ratio of CO2 to CH4 (Ω) equaled to 1 as the CO2-sufficient condition and Ω = 1.5 as the CO2-rich condition. The concentrations of CH4 (CCH4) and CO2 (CCO2) in the flow of gaseous mixture were directly measured by a CH4 non-destructive infrared spectrometers (ND-IR; CI-IR 10, Chang-Ai Inc., Shanghai, China) and a CO2 ND-IR (Rosemount™ NGA 2000, Emerson, St. Louis, MO, USA), respectively. Prior to the catalytic performance tests, both CCH4 and CCO2 measured by ND-IR were calibrated (a comparison of the accuracy of ND-IR versus gas chromatography previously used was shown in Section S2 of the Supporting Information). The retention time for each temperature step was 5 minutes, and the data of CCH4 and CCO2 were record at a steady state. The conversion ratios of methane and carbon dioxide, XCH4 and XCO2, were calculated using Eq. 1 and Eq. 2, respectively: XCH4 (%) =100 – CCH4/CCH4,0 × 100

(1).

XCO2 (%) =100 – CCO2/CCO2,0 × 100

(2).


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Here CCH4 and CCO2 were measured at a given surrounding temperature (Tsur) and reaction time (t). CCH4,0 and CCO2,0 were the CCH4 and CCO2 prior to the catalytic DRM. The turnover frequency (TOF; unit: s-1) of the catalyst was calculated by normalizing the observed reaction rate, R [= mol of CH4/(s*g of catalyst)], to the number of Ni atoms on surface of NP per gram of catalyst, SNi (unit: mol Ni/g of catalyst).9 The H2/CO ratio was calculated using Eq. 3, by assuming the reverse water gas shift reaction (RWGS; CO2+H2 => H2O + CO) is the dominant side reaction (details of derivation of TOF and H2/CO ratio, with an experimental validation using a gas chromatography, were shown in Section S2 of the Supporting Information). H2/CO = (3XCH4 - Ω*XCO2)/(XCH4 + Ω*XCO2)

(3).

In this study, the errors bars of the conversion ratios and the H2/CO ratios present one standard deviation of at least two replicate measurements.

3.

RESULTS AND DISCUSSION

3.1. Material Characterization of Ni-only-NP and NiCeOx-NP Firstly, SEM and DMA were used complementarily to provide information of morphology and particle size of the NPs synthesized by the 2-stage aerosol-based method. Figure 2a shows the representative SEM images of Ni-only-NP samples with different Td2 (additional SEM images with histogram analyses were shown in Section S3 of the Supporting Information). The morphology shown to be spherical for all of the Ni-only-NP samples, and the primary diameter (d0) was relatively constant at (110-115) nm. From the mobility size distributions measured in-situ by DMA (Fig. 2b), the peak dp,m (i.e., representing the primary diameter) was ≈95 nm for all of the samples treated under different Td2. The results of SEM and DMA analyses both indicate that the primary diameter of Ni-only-NP was ACS Paragon Plus Environment

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independent of Td2. Note that the decreasing trend of full width at half maximum (FWHM) versus Td2 shown in Fig. 2b implies a gradual sintering of Ni-only-NP by the H2-reduction at a high Td2. Figure 2c shows the XRD patterns of the Ni-only-NP samples with different Td2. For Ni-only-NP-RT and Ni-only-NP-300, only crystalline NiO was shown in the XRD diffractogram. The NiO crystalline transformed to a mixture of (Ni + NiO) by increasing Td2 to 600 ºC, and the dc of NiO and Ni were calculated as 19.7 nm and 16.0 nm, respectively. For Ni-only-NP-800, only metallic Ni crystalline presented in the diffractogram with a dc of 21.2 nm. The results illustrate that NiO reduced to Ni when Td2 > 600 ºC, and the increase of dc of Ni with Td2 implies a sintering of individual crystallites in the Nionly-NP samples.


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Figure 2. Analyses of morphology, physical size and crystallinity of Ni-only-NP samples with different Td2. (a) Representative SEM images. 1: Ni-only-NP-RT; 2: Ni-only-NP-300; 3: Ni-only-NP600; 4: Ni-only-NP-800. The scale bars were 500 nm. (b) Mobility size distributions in-situ measured by DMA. (c) XRD patterns.


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The morphology and the particle size of NiCeOx-NPs were also examined using SEM and DMA. Similar to Ni-only-NP (Figure 3a), the morphology of NiCeOx-NP with different Td2 was also spherical. The d0 was constant at ≈ (97±3) nm, close to the peak dp,m in-situ measured by DMA [≈ (90-100) nm; Fig. 3b]. The results show that the primary diameters of NiCeOx-NP samples were unchanged by the H2 reduction over various Td2. Unlike Ni-only-NP, the FWHM of NiCeOx-NP was relative constant over various Td2, indicating no remarkable sintering of NiCeOx-NP by the H2-reduction at a higher temperature (i.e., Td2 < 800 °C). Figure 3c shows the XRD diffractograms of the samples of NiCeOx-NP with different Td2. For the samples of NiCeOx-NP-RT and NiCeOx-NP-300, both NiO and CeO2 were clearly identified in the diffractograms with dc of (13-14) nm and (13-15) nm, respectively. By increasing Td2 to 600 ºC, a mixture of (Ni + NiO + CeO2) crystalline was observed, and the dc of NiO, Ni and CeO2 were 22.7 nm, 16.0 nm and 13.2 nm, respectively. By further increasing Td2 to 800 ºC, only metallic Ni and CeO2 crystalline presented in the diffractogram, and the dc of Ni and CeO2 were 17.7 nm and 12.7 nm, respectively. The results indicate that NiO reduced to Ni in the sample of NiCeOx-NP when Td2 > 600 ºC, close to the required Td2 of Ni-only-NP. Note that no further phase segregation was found during the synthesis of Ni-CeO2 hybrid nanostructure (i.e., mixed oxides crystallites). The slight increase of dc of Ni with Td2 implies that a less sintering of Ni crystallites occurred within individual primary particles of NiCeOx-NP than in the primary particles of Ni-only-NP, which did not significantly affect the mobility diameter of Ni-CeO2 hybrid nanostructure.


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Figure 3. Analyses of morphology, physical size and crystallinity of NiCeOx-NP samples with different Td2. (a) Representative SEM images. 1: NiCeOx-NP-RT; 2: NiCeOx-NP-300; 3: NiCeOx-NP600; 4: NiCeOx-NP-800. The scale bars were 500 nm. (b) Mobility size distributions in-situ measured by DMA. (c) XRD patterns.


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HR-TEM image analyses were employed to examine the structure and the homogeneity of NiCeOxNP. As shown in Figure 4a (i.e., using NiCeOx-NP-600 as an example), the crystallites of Ni and NiO were clearly identified on the surface of CeO2 crystallite, implying the formation of Ni-Ce-O interface. From elemental mapping (Figure 4b), Ni and Ce atoms were homogeneously distributed in the primary particle, indicating that the crystallites of Ni and CeO2 were uniformly dispersed in the hybrid nanostructure. The results suggest that a massive amount of new active sites were able to create by the formation of Ni-Ce-O interfaces homogenously distributed in the NiCeOx-NP.

Figure 4. HR-TEM images (a) with elemental mapping (b) of Ni and Ce. Sample: NiCeOx-NP-600.


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3.2. Catalytic activity tests of dry methane reforming with CO2 In this section, we evaluate the catalytic performance of Ni-only-NP and NiCeOx-NP samples with different Td2 for DRM. Figure 5a shows the conversion ratios of reactant gases versus the temperature of environment (Tsur) during the catalysis of DRM (additional data of catalytic bed temperature were shown in Section S4 of the Supporting Information). The trends of XCH4 (5a-1) and XCO2 (5a-2) versus Tsur were shown to be very similar, implying that DRM was the dominant reaction. For Ni-only-NP600 and Ni-only-NP-800, the reaction of DRM started at a lower Tsur than Ni-only-NP-RT and Ni-onlyNP-300, indicating that Ni was more active than NiO for starting the catalysis of DRM. Note that the RWGS occurred simultaneously with the DRM at the range of temperature between 600 °C and 700 °C, resulting in a higher XCO2 than XCH4 shown in Fig. 5a.5, 7 A transient decrease of catalytic activity by the elevation of Tsur was observed for all of the Ni-onlyNP samples, and the loss of catalytic activity was possibly attributed to the sintering of Ni crystallites and also the formation of moss-like coke on the surface of catalyst.1 As shown in Figure 5b, only the metallic Ni crystalline was found in the Ni-only-NP samples after the activity tests. For Ni-only-NP600 and Ni-only-NP-800 (i.e., reduced by H2 prior to the activity tests), the dc of Ni increased from 16 nm and 21 nm to 25 nm and 36 nm, respectively, after the activity tests. The results indicate that NiO crystallites converted to Ni during the DRM by the reductive gases (H2 and CO) generated in the reaction. The results also show that the samples with a lower Td2 (i.e., the oxidized Ni) deactivated at a higher Tsur, and the enhanced reaction stability was attributed to that a higher Tsur was required to obtain metallic Ni from NiO. Note that the small amount of moss-like deposited carbon will inhibit the conversion of methane. Hence the formation of coke on the Ni-only-NP was unable to be identified in the XRD diffractogram (Figure 5b).


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Figure 5c shows a cartoon depiction of the possible mechanisms of DRM catalyzed by Ni-only-NP using the data of Figure 5a-b for the mechanistic understanding. Firstly, CH4 adsorbs to the Ni surface and dissociates, where the hydrogen atoms are abstracted from the adsorbed methane and then desorb from the surface of catalysts as hydrogen gaseous molecules. Simultaneously, the adsorbed CO2 molecule interacts with the carbon atom of the dissociated methane, resulting in a reduction to two CO molecules desorbed from the surface (Figure 5c-1).39 Since the dissociation rate of methane is higher than the rate of CO2 reduction (C + CO2 => 2CO), some of the deposited carbon atoms (i.e., the mosstype amorphous carbon)1,

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are unable to be removed and form moss-like coke on the surface of

catalyst (Figure 5c-2). The active surface area gradually decreases by the carbon deposition and the sintering of Ni.


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Figure 5. DRM catalyzed by Ni-only-NP samples with different Td2. (a) Activity tests. 1: XCH4 versus Tsur; 2: XCO2 versus Tsur. (b) XRD patterns after the activity tests. (c) Cartoon depictions of catalytic DRM by Ni-only-NP. 1: Early-stage catalysis (no coke formation). 2: Catalysis with coke formation.


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Figure 6a shows the XCH4 (6a-1) and XCO2 (6a-2) versus Tsur using NiCeOx-NP samples as catalysts. Similar trends of XCH4 and XCO2 versus Tsur were observed, confirming that DRM was the dominant reaction. Both XCH4 and XCO2 increased with Tsur and reached 100 % at Tsur = 700 ºC for all NiCeOx-NP samples. In comparison to Ni-only-NP (Figure 5a), the results show that hybridization of CeO2 significantly increased catalytic activity and reaction stability of Ni-based NP toward DRM (i.e., no transient decrease of activity). The lower starting Tsur (400-450 ºC) and a higher TOF at a low temperature (≈ 0.1 s-1 at Tsur = 450 ºC) were observed for the NiCeOx-NP-600 and the NiCeOx-NP-800 than the NiCeOx-NP-RT and the NiCeOx-NP-300 (TOF ≈ 0 s-1 at Tsur = 450 ºC), indicating that the H2reduction during the gas-phase synthesis of NiCeOx-NP samples effectively reduced the required Tsur for starting their catalysis of DRM.


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Figure 6. DRM catalyzed by the NiCeOx-NP samples with different Td2. (a) Activity tests. 1: XCH4 versus Tsur; 2: XCO2 versus Tsur. (b) XRD patterns after the activity tests. (c) Representative SEM image of NiCeOx-NP-600 after the activity test. The scale bar was 200 nm. (d) XPS analyses of NiCeOx-NP600 after the activity test. 1: C 1s region. ACS Paragon Plus Environment

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XRD was employed for the analysis of crystalline state of the NiCeOx-NP after the activity tests. For all of the NiCeOx-NP samples with different Td2 (Figure 6b), metallic Ni and CeO2 crystalline were observed after the activity tests, with dc of (22-35) nm and (20-23) nm, respectively. The results illustrate that NiO reduced to metallic Ni by the syngas produced during DRM,2 whereas CeO2 remained in a complete oxidation state. The increase of crystallite sizes implies a sintering of catalysts, which did not show any remarkably impact on the catalytic activity of the NiCeOx-NP samples over the experimental conditions studied. Besides, clearly a formation of carbon crystalline with a dc of (20-23) nm was identified in all of the NiCeOx-NP samples after the catalysis of activity tests. As can be seen from the SEM image (Fig. 6c; NiCeOx-NP-600 as an example), hollow carbon fibers were found and deposited on the surface of NiCeOx-NP catalysts. The diameter of these carbon fibers mainly ranged from 15 nm to 50 nm (additional SEM images of carbon fibers and the EDS analyses were shown in Section S5 of the Supporting Information). The C 1s XPS spectrum (Figure 6d) indicates two types of deposited carbon on the surface of Ni-CeOx-NP-600: amorphous carbon and carbon nanotubes.1 The growth of carbon fiber during the catalysis of DRM has been reported previously,1,

5, 7, 15, 40

showing that the

perpendicular growth of carbon was initiated by the hydrocarbon dissociation on the surface of Ni, after which a chemical vapor deposition-type reaction occurred for the growth of carbon fiber.1, 15, 40-41 As a result, the formation of these carbon fibers will increase the diffusion resistance of reactant gases toward the active surface of the Ni-based catalysts and will also reduce the amount of total active surface area for catalysis. Since the rate-determining step of catalytic DRM is surface reaction (e.g., adsorption and dissociation of methane and CO2),16, 42-43 the increase of diffusion resistance through a vertical growth of carbon deposits will not result in a significant loss of catalytic activity (i.e., only a partial loss of active surface area). ACS Paragon Plus Environment

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3.3. Operation stability tests The operation stability of NiCeOx-NP versus Ni-only-NP during the catalysis of DRM was investigated at a constant Tsur of 700 oC. Figure 7a shows the XCH4 and XCO2 versus the reaction time (t) catalyzed by NiCeOx-NP-600 at Ω = 1 (i.e., a CO2-sufficient condition). The XCH4 decreased from 100 % to 80 % in the first 2 hours and was constant at ≈ 78 % in the rest of the 8-h reaction. On the other hand, the XCO2 was constant at ≈ 94 % in the 8-h reaction. By calculation, H2/CO was constant at ≈ 0.79 after 2 h. The results show that the catalysis of DRM by NiCeOx-NP-600 was relatively stable during the 8-h reaction. The higher XCO2 also implies that the RWGS occurred at Tsur = 700 oC. In comparison, the XCH4 and XCO2 decreased dramatically from 53 % and 65 % to 0 % and 5 %, respectively, in 2 hours using Ni-only-NP-600 as the catalyst (see Figure 7b). The results illustrate that the synergistic catalysis at the Ni-Ce-O interface demonstrates a much higher operation stability than the catalysis by the Nionly-NP sample. To our knowledge, our study reported the first time of controlled gas-phase synthesis of Ni-CeO2 hybrid nanostructure for catalytic DRM to achieve high activity and stability. Figure 7c shows the XCH4 and XCO2 versus t catalyzed by NiCeOx-NP-600 at Tsur = 700 ˚C, Ω = 1.5. The XCH4 and XCO2 were constant at ≈ 94 % and ≈ 79 %, respectively, corresponding to a TOF of 0.6 s-1 at Tsur = 700 oC. By calculation, the H2/CO ratio was constant at 0.77 over the 8-h reaction. The results show that both the catalytic activity and stability increased by the increase of Ω from 1 to 1.5.


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Figure 7. Stability tests of the Ni-only-NP-600 and NiCeOx-NP-600 for the 8-h catalysis of DRM. Tsur = 700 °C. (a) The conversion ratios and H2/CO ratio versus t using NiCeOx-NP-600 as catalyst. Ω = 1. (b) The conversion ratios versus t using Ni-only-NP-600 as catalyst. Ω = 1. (c) The conversion ratios and H2/CO ratio versus t using NiCeOx-NP-600 as catalyst. Ω = 1.5.

The difference in the catalytic performance over different Ω was mainly attributed to the effect of coke formation. As shown in the SEM images (Figure 8a-b), we identify that the amount of coke reduced significantly by increasing Ω from 1 (Fig. 8a) to 1.5 (Fig. 8b). The XRD analyses (Figure 8c) also identified a formation of carbon crystalline in the NiCeOx-NP-600 after 8-h catalysis of DRM at Ω = 1. By increasing Ω to 1.5, the relative intensity of the carbon peak decreased. Using TGA analyses ACS Paragon Plus Environment

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(Figure 8d), the amount of carbon formed on the sample of NiCeOx-NP-600 was 34.6 wt% and 1.9 wt% at Ω = 1 and Ω = 1.5, respectively. The results indicate that increasing the amount of CO2 inhibited the coke formation, which was attributed to the acceleration of the oxidation of surface-bound carbon species on catalysts (i.e., C+CO2 => 2CO). Note that the carbon peak shown in the XRD patterns of NiCeOx-NP-600 disappeared after the TGA analyses, confirming the reduction of sample mass measured by the TGA was mainly attributed to the removal of coke. The dc of Ni of NiCeOx-NP600 increased from 16 nm to 25 nm and 23 nm, whereas the dc of CeO2 of NiCeOx-NP-600 increased from 13.3 nm to 20.4 nm and 17.5 nm at Ω = 1 and Ω = 1.5, respectively, after 8-h catalysis of DRM at Tsur = 700 °C. The results imply that the sintering of crystallites did not affect the stability of catalysis. Figure 8e demonstrates a cartoon depiction of catalytic DRM by NiCeOx-NP using the aforementioned data shown in Figure 7 and 8a-d. The CO2 molecule will adsorb to CeO2, dissociate, and then react to the carbon of dissociated methane on the surface of catalyst (Figure 8e-1).22-23 Unlike Ni-only-NP showing that a fast dissociation of the methane results in the coke formation on Ni surface and the evitable deactivation of the Ni-based catalyst (Figure 5c), the synergistic effect between Ni and CeO2 increases the rate of oxidation of deposited carbon especially at the Ni-Ce-O interface, which also provides a coke-free zone for a stable catalysis.22-23, 39 Therefore, the operation stability of Ni-based NPs for catalytic DRM can be effectively enhanced after hybridization with CeO2. With an increasing amount of CO2 in the reactant gases (i.e., increase of Ω), the rates of methane adsorption and its subsequent dissociation (i.e., the cracking of methane) are reduced.15 Simultaneously,, the rate of oxidation of carbon species at the surface of catalyst increases. Therefore, the amount of coke deposition at the active sites decreases, and the operation stability of Ni-based catalysts can then be improved significantly. During the long-time catalysis at a CO2-sufficeint atmosphere, the carbon will migrate and accumulate at the center of Ni crystallite (i.e., a bottom-up chemical vapor deposition of carbon) to form a hollow fiber-type structure (Figure 8e-2).40-41 Our study ACS Paragon Plus Environment

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here demonstrates the first time that the SMSI of Ni-CeOx-NP, especially at a higher ratio of CO2/CH4, can effectively prevent the deactivation of Ni-based catalyst by simultaneous changing the type of carbon deposited and suppressing the amount of coke formation. To our knowledge, this is the first study to shed a light on the mechanism of DRM via Ni-Ce-O interface versus the Ni-only surface at nanoscale.


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Figure 8. Material properties of NiCeOx-NP-600 after 8-h catalysis of DRM at Tsur = 700 ˚C. (a) The representative SEM image of NiCeOx-NP-600 at Ω = 1. (b) The representative SEM image of NiCeOxNP-600 at Ω = 1.5. (c) XRD patterns of NiCeOx-NP-600 before and after the TGA analyses shown in (d). (d) TGA analyses of NiCeOx-NP-600 after the stability tests. (e) Cartoon depictions of catalytic DRM by NiCeOx-NP. 1: Catalysis without coke formation (high Ω). 2: Catalysis with coke formation (low Ω).

4.

CONCLUSIONS Dry reforming of methane with carbon dioxide (DRM) to syngas can be achieved using a synergistic

Ni-Ce-O nanocatalysis. The gas-phase self-assembly approach demonstrates a facile route of effectively producing a homogenous Ni-CeOx hybrid nanostructure with tunable chemical composition and oxidation state. The Ni-CeOx hybrid nanostructure shows a very high catalytic activity and stability in comparison to Ni-only-NP: a low starting catalytic temperature with remarkably turnover frequency ACS Paragon Plus Environment

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(0.1 s-1 at 450 °C), high conversion ratios of reactant gases and stable for 8 h of catalysis. The amount of coke formation can be reduced significantly by the increase of the CO2/CH4 ratio. The work demonstrated a facile, prototype method of gas-phase controlled synthesis of Ni-CeOx hybrid nanostructures for the synergistic catalysis of DRM. The mechanistic understanding, especially in the inhibition of coke formation, is useful for improving the design of nanocatalyst to simultaneously produce valuable chemical products and reduce environmental pollutions.

ASSOCIATED CONTENT Supporting Information. Calculation of crystallite size of nanoparticle, derivation of turnover frequency (TOF) and determination of H2/CO ratio for activity tests, additional SEM images with histogram analyses of gas-phase synthesized nanoparticles, catalytic bed temperature during the activity test, and additional SEM images with EDS mapping of the samples after catalysis of DRM. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Phone: 886-3-5169316

ACKNOWLEDGMENT The authors thank Ministry of Science and Technology (MOST) of Taiwan, R.O.C. MOST 106-26228-007-017, MOST 106-2813-C-007-003-E, and Office of Naval Research Global, U.S.A. (ONRG N62909-17-1-2040) for financial support. ACS Paragon Plus Environment

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SYNOPSIS


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Gas-Phase Synthesis of Ni-CeOx Hybrid Nanoparticles and Their

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