Research Article pubs.acs.org/acscatalysis
Core−Shell NiO@PdO Nanoparticles Supported on Alumina as an Advanced Catalyst for Methane Oxidation Xuelin Zou,† Zebao Rui,*,‡ and Hongbing Ji*,† †
School of Chemistry, Sun Yat-sen University, Guangzhou 510275, P.R. China School of Chemical Engineering and Technology, Sun Yat-sen University, Zhuhai 519082, P.R. China
‡
S Supporting Information *
ABSTRACT: An alumina-supported core−shell-structured NiO@PdO catalyst was prepared for lean CH4 combustion. NiO@PdO plays two roles in promoting the reaction. First, the enhanced NiO-PdO interfacial action accelerates the regular tetragonal PdO lattice construction, stabilizes the PdO particles, and suppresses the hydroxyl/water adsorption during the reaction. Second, the dispersion of shell PdO particles over core NiO improves PdO exposure and utilization efficiency. NiO@PdO/Al2O3 with a molar Ni/Pd ratio of 2/1 exhibits a (>)99% CH4 conversion and a good stability at 400 °C with a low 0.2 wt % Pd loading amount, which is among the best of the state-of-the-art Pd-based catalysts with respect to turnover frequency, Pd utilization efficiency, and Ni addition amount. Such interfacepromoted core−shell-structured catalyst design strategy is inspiring for improving noble metal utilization efficiency in CH4 oxidation and other related reaction systems. KEYWORDS: catalytic combustion, methane, PdO, NiO, core−shell
1. INTRODUCTION Catalytic combustion of trace amounts of unburned CH4 from internal combustion engines and natural-gas-powered vehicles is an important issue with respect to environmental sustainability and energy utilization.1−4 Palladium-based catalysts have been widely recognized among the most promising catalysts for the lean methane oxidation.2,5−8 However, their widespread applications are still restricted due to the low Pd utilization efficiency (or high cost) and the poor stability under hydrothermal conditions.1,5,7 As more stringent standards of methane emission are being taken into account, the design of efficient catalytic materials for lean methane combustion remains a challenging subject of continuous research effort. For CH4 oxidation over the PdO catalysts, the dissociative adsorption of CH4 on a Pd-*/PdO site pair (CH4 + Pd−O + Pd-* → Pd−OH + Pd−CH3, here, Pd-* represents an Ovacancy) is usually taken as the rate-controlling step.9,10 Following this step, the recombination of surface hydroxyls yields H2O and the surface vacancy (2Pd−OH → H2O + Pd− O + Pd-*), and PdO is regenerated by oxygen (2Pd-* + O2 → 2Pd−O).11 Hence, the catalyst activity relates to the adsorbed water/hydroxyl and the reoxidation rate of the vacancy (Pd*).11,12 The reoxidation by O exchanging with the support has been demonstrated to promote the reaction,13−15 and consequently, various transition metal (such as Ca,16 Mg,17 Mn,18 Zr,6 Ce,19 Co,20 Ni,21 etc.) oxide promoters have been added to PdO catalysts for CH4 oxidation reactions to enhance this promotion effect. Among them, the promotion by NiO received particular attention, especially in improving the © 2017 American Chemical Society
stability of the alumina-supported PdO catalyst. Normally, Ni specie was introduced by conventional impregnation method, and the introduced Ni was first consumed to form spinel NiAl2O4, which had a stabilizing effect on PdO dispersion but showed no promotion in the catalytic activity.21 In order to improve the interface interaction between NiO and PdO for achieving a notable improvement in the PdO activity, the Ni content in the alumina support must be extremely high, in some cases as high as 36:1 NiO to the alumina support.5,22,23 Alternatively, several attempts have been made to synthesize the bimetallic Pd−Ni composite catalysts to ensure their close contact.5,24 However, the conventional PdNi alloying particles introduced rather limited PdO−NiO interfaces and performance promotion. For example, Ni consumption in spinel formation is still unavoidable because of the exposure of the NiO to the alumina support. In recent years, rational design of the PdO−metal oxide interface has been demonstrated as an effective method to improve the performance of PdO catalysts.2,3,6,25−27 To maximize the PdO−ceria interfacial contact and the catalytic activity of PdO, Cargnello et al.3 developed hierarchical Pd@ CeO2/Al2O3 in which Pd nanoparticles were first produced as a core and subsequently surrounded by a ceria shell. The special configuration led to stable performance for the catalytic CH4 combustion reaction at a high temperature. Following this study, the hierarchical Pd@ZrO2/Si−Al2O3 was reported to be Received: October 31, 2016 Revised: January 17, 2017 Published: January 18, 2017 1615
DOI: 10.1021/acscatal.6b03105 ACS Catal. 2017, 7, 1615−1625
Research Article
ACS Catalysis
0.2 wt % NiO prepared with this procedure, respectively. The reference PdO/NiO/Al2O3(IM) sample with a 0.2 wt % Pd loading amount and a Ni/Pd molar ratio of 2/1 was prepared by the conventional two-step wetness impregnation method. Ni was introduced into the support by impregnating the γ-Al2O3 support in an aqueous solution containing the desired amount of Ni(NO3)2·6H2O (98.0%, Tianjin Damao Chemical Reagent Factory, China) for 1 h under ultrasonication. The asimpregnated sample was dried in an oven at 100 °C overnight and then calcined in air flow at 600 °C for 6 h with a heating rate of 10 °C/min. Then, Pd(NO3)2·2H2O solution (Pd ≥ 39.0%, Aladdin, China) was used to incorporate the Pd to the Ni-modified alumina support by the above-mentioned immersion−drying−calcination procedure. The final concentration of Ni and Pd in the samples was checked by inductively coupled plasma (ICP, IRIS (HR)) element analysis, which showed less than 5% deviation from the marked values. 2.2. Characterization of Catalysts. Transmission electron microscope (TEM, FEI Tecnai G2 Spirit) was operated at an accelerating voltage of 300 kV or lower. Powder X-ray diffraction (XRD) was performed over D-MAX 2200 VPC with monochromatic Cu Kα radiation. XPS measurements were carried out in the UHV chamber (∼2 × 10−7 Pa) of the electron spectrometer ESCALAB 250 (Thermo Fisher Scientific, Al Kα, hv = 1486.6 eV). The spectra calibration was performed by using the C 1s line of adventitious carbon centered at binding energy of 284.8 eV. N2 adsorption was performed at 77 K on a Micromeritics ASAP 2020 instrument to determine the surface area and pore size of the samples. The low-temperature (193 K) CO chemisorption test was performed by a dynamic pulse method. Before the test, the sample was prereduced in H2 stream at 393 K for 3 h. The assumption of unity CO/Pd average stoichiometry was used for the dispersion calculation. The temperature-programmed oxidation (TPO) experiments were carried out by loading the catalyst powders (∼0.1 g, 177−250 μm) in a quartz tube with an inner diameter of 7 mm. The O2 (5.0 vol %) and He gas mixture with a flow rate of 50 mL/min was taken as the feeding gas, and the outlet gas was analyzed with a thermal conductivity detector. The temperature was cycled between 200 and 950 °C at a rate of 10 °C min−1. Raman spectra were measured at room temperature using a Laser Micro-Raman Spectrometer (Renishaw In VIA) equipped with excitation argon ion lasers working at 514.5 nm with a power of 20 mW and spectral resolution of 1 cm−1. In situ DRIFTS studies were performed in a reaction cell on an EQVINOX-55 FFT instrument (Bruker), which was described in detail in our previous work.21 2.3. Performance Test. Catalytic tests of methane combustion were carried out in a vertical continuous-flow fixed-bed quartz reactor with inner diameter of 7 mm at atmospheric pressure. About 200 mg catalysts crushed and sieved to 177−250 μm making up a 0.8 cm-long (0.31 cm3 volume) were held at the center of the reactor with quartz-wool plugs. The reaction gas containing 1 vol % methane in synthetic air was supplied at GHSV (gas hourly space velocity) of 30 000 (or 60 000, 100 000, 120 000 and 300 000 for the effect of GHSV study) ml h−1 g−1. For the performance test under wet conditions, 6 vol % or 9 vol % water vapor was added to the feed stream by passing the gas mixture through a saturator containing water, and the steam amount was controlled by the gas flow rate through it and the temperature of the saturator. For the case of CO2 addition, 20 vol % of CO2 was introduced via a volume flow controller using a high-purity (99.9%) CO2.
stable even when the water vapor coexisted in methane catalytic combustion through strong interaction between the Pd core and ZrO2 shell, which could accelerate the oxygen migration and exchange originally bonded to the support to participate in the reaction.6 Although the formation of PdO@MOx (M = Ce, Zr, Ti, or Si) structure shows unique advantages in maximizing the interfacial action and minimizing deactivation of the catalyst by metal-sintering processes,3,6,7 the encapsulated Pd species by the MOx shell restricts the direct contact between the reactant and the active sites to some extent, which is important to the Pd utilization efficiency, especially for the lean CH4 combustion reaction. In consideration of the promotion effect of NiO on the performance of PdO, enhanced interfacial action of the core− shell structure and the requirement for improving the Pd utilization efficiency, herein, the core−shell-structured NiO@ PdO/Al2O3 catalysts were rationally designed for lean methane oxidation with outstanding performance. A battery of NiO@ PdO/Al2O3(x:y) catalysts with various Pd loadings and Ni to Pd molar ratios (x:y = 10:1, 2:1, 1:3) were synthesized by creating Ni@Pd nanostructure via a seed-mediated strategy followed by the deposition over commercial γ-Al2O3 and calcination in air. The as-developed NiO@PdO/Al2O3(2:1) exhibits a (>)99% CH4 conversion and a good stability at 400 °C with a low 0.2 wt % Pd loading amount and ultralow 0.4 wt % Ni addition, which is among the best of the state-of-the-art Pd-based catalysts with respect to turnover frequency, Pd utilization efficiency, and Ni addition amount. The relation between the performance and the effect of electronic and steric interactions in NiO@PdO/Al2O3 is discussed in detail.
2. EXPERIMENTAL SECTION 2.1. Preparation of Catalysts. The Ni, Pd, and Ni@Pd nanoparticles (NPs) were prepared by a two-step polyol reduction method. In a typical Ni@Pd preparation process, a required quantity of Ni(acac)2 (acac = acetylacetonate) was initially reduced in refluxing glycol in the presence of polyvinylpyrrolidone stabilizers (PVP, MW = 40 000). The as-prepared Ni seeds were coated with Pd particles in a separate deposition sequence by adding required quantity of Pd(acac)2 to the Ni−glycol colloid. The metal concentration and PVP amount in the colloid were 0.005 and 0.2 mmol per mL of glycol, respectively. The mixture was heated to the boiling point in the reaction flask and kept at this temperature for 3 h with vigorous stirring and constant refluxing. The Ni NPs, Pd NPs, and Ni@Pd NPs with Ni/Pd molar ratios of 1/3, 2/1, and 10/1 were synthesized separately with this procedure. The supported catalysts were prepared by adding an appropriate amount of γAl2O3 powders to the colloidal suspension of Ni NPs, Pd NPs, or Ni@Pd NPs. After stirring overnight and drying under vacuum, the as-prepared samples were dried at 100 °C overnight and then calcined at 600 °C for 6 h in air with a heating rate of 10 °C/min. Here, the support γ-Al2O3 was obtained by calcining the commercial γ-Al2O3 (Sinopharm Chemical Reagent Co., China) powders in air at 900 °C for 6 h. Three different Ni/Pd molar ratios (1/3, 2/1, 10/1) with a constant Pd loading amount of 0.2 wt % were loaded, respectively. For simplicity, the as-prepared catalyst was designated according to the Ni/Pd molar ratio and the local structure. For example, NiO@PdO/Al2O3(2:1) represents the catalyst prepared by polyol reduction method with a Pd content of 0.2 wt % and the molar ratio of Ni to Pd of 2:1. PdO/Al2O3 and NiO/Al2O3 refer to γ-Al2O3 supported 0.2 wt % PdO or 1616
DOI: 10.1021/acscatal.6b03105 ACS Catal. 2017, 7, 1615−1625
Research Article
ACS Catalysis
Figure 1. (a−c) Initial activity and Arrhenius plots of the as-prepared catalysts; (d) turnover frequency comparison of NiO@PdO/Al2O3(2:1), PdO/ NiO/Al2O3(IM), and PdO/Al2O3 with typical Pd catalysts reported in the literature: Pd@CeO2/H−Al2O3,3 PdO/CeO2@HZSM-5,27 and Pd/ 0.5NiO/Al2O3;21 (e) stability test of NiO@PdO/Al2O3(2:1), PdO/NiO/Al2O3(IM), and PdO/Al2O3 at GHSV = 30 000 mL h−1 g−1 and T99; and (f) cyclic stability test of the NiO@PdO/Al2O3(2:1) at GHSV = 30 000 mL h−1 g−1.
⎛ E ⎞ r = A exp⎜ − a ⎟[CH4]a [O2 ]b ⎝ RT ⎠
For all cases, the volume concentration of CH4 and O2 were fixed at 1% and 20% in the final inlet gas, respectively. Temperature-programmed tests were conducted by steps of 25 or 50 °C to obtain heating and cooling curves of methane conversion as a function of temperature (light-off curves). The products of the reaction were periodically analyzed online by a GC7890II gas chromatograph (GC) with a TCD detector and a TDX-01 column. Similar to the finding in the literature,3,5,21 the selectivity of CO2 was close to 100% over the Pd-based catalysts under CH4-lean conditions, and the other possible products were under the detective limit of the GC. Turnover frequency (TOF) was calculated by normalizing CH 4 consumption (mol s−1) to the exposed surface active Pd atom (mol) at a low conversion below 10% with moderate reaction temperature in order to eliminate temperature gradient and transport limitations.28 The apparent activation energies (Ea) were calculated when CH4 conversion was lower than 10% using the Arrhenius equations and reaction rate (r) equations:
(1)
When the methane conversion is very low (
