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Kinetics, Catalysis, and Reaction Engineering

A novel hierarchical RuNi/Al2O3–carbon nanotubes/Ni foam catalyst for selective removal of CO in H2-rich fuels Dan Ping, Chaojie Dong, Hua Zhao, and Xinfa Dong Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00390 • Publication Date (Web): 05 Apr 2018 Downloaded from http://pubs.acs.org on April 5, 2018

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Industrial & Engineering Chemistry Research

A novel hierarchical RuNi/Al2O3–carbon nanotubes/Ni foam catalyst for selective removal of CO in H2-rich fuels Dan Ping, Chaojie Dong, Hua Zhao, Xinfa Dong* School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, PR China

Corresponding author. Tel: +86 20 87110659; E-mail addresses: [email protected]

1

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT A novel hierarchical RuNi/Al2O3–carbon nanotubes/Ni foam (RuNi/Al2O3–CNTs/NF) catalyst

is

prepared

from

RuNiAl-layered

double

hydroxides/CNTs/NF

(RuLDH/CNTs/NF) precursor and applied in CO selective methanation (CO-SMET) for hydrogen purification. Results show that the RuNi/Al2O3–CNTs/NF hierarchical catalyst has an excellent catalytic performance towards CO-SMET (i.e., CO outlet concentration less than 10 ppm and CO-SMET selectivity greater than 50%) over a wide reaction temperature window of 190–250 °C. Furthermore, this catalyst also shows good catalytic stability during a long-term durability test (120 h). The excellent catalytic performance is mainly attributed to the high specific surface area and superior electronic conductivity of CNTs, the highly dispersed and thermally stable RuNi nanoparticles from the RuLDH precursor, and the good thermal conductivity of the NF substrate. Such a hierarchically structured catalyst proposed herein may open a new window in the efficient hydrogen purification for fuel cells and can be a promising material in other structure-sensitive reactions. Keywords: layered double hydroxides; carbon nanotubes; Ni foam; catalyst; hydrogen

purification; fuel cell

2


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1. INTRODUCTION The purification of hydrogen produced from steam reforming of hydrocarbons is mandatory for the practical applications of proton exchange membrane fuel cells (PEMFCs).1 In the present, CO selective methanation (CO-SMET) has been considered as an efficient technology for CO deep removal (< 10 ppm) from H2-rich reforming gases.2 Since large amount of CO2 is also present in the reforming gases, the side reactions of CO2 methanation and reverse water gas shift (r-WGS) taking place at relatively high temperatures (> 260 °C) may occurred, thereby leading to a huge consumption of H2. Moreover, the exothermic character of both CO and CO2 methanation could cause thermal runway problem in reactor, further resulting in the increase of CO2 conversion. Therefore, the suitable CO-SMET catalyst must be developed, able to deep-remove the outlet CO concentration to below 10 ppm especially at low temperatures and suppress the conversion of CO2. For this process, numerous studies have been focused on the Ru and Ni catalysts supported on metal oxides.3-6 It is found that the catalytic performance is highly correlated with the size and dispersion of active species, and the morphology/structure of the support.5,6 Currently, three-dimensional (3D) hierarchical composites constituted by the combination of 2D materials and 1D nanostructures (typically nanotubes or nanowire) have attracted increasing attention on account of their excellent performance in various fields.7-10 As a representative of 2D materials, layered double hydroxides (LDH), a class of highly ordered inorganic compounds comprising positively-charged layers and negatively-charged interlayers, has been 3



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extensively investigated due to their potential applications as efficient catalysts or catalyst supports in heterogeneous catalysis.11 Specifically, uniformly dispersed, thermally stable metal catalysts can be generated after calcination and reduction.5,12 Nevertheless, the intrinsic low specific surface area (SSA) and preferential agglomeration of the LDH particles considerably affect their catalytic performance. To overcome this drawback, the LDH/carbon composites with enhanced catalytic performance have been studied.13-16 For example, Gao et al.

13

have reported a Co–

Ru/C catalyst used for the transfer hydrogenation of furfural via the in situ self-reduction of the CoAlRu–LDH–C precursor. Jia et al. 16 have synthesized CoMn– LDH/CNTs and NiMn–LDH/carbon nanotubes (CNTs) hierarchical composites by a chemical bath deposition method for efficient electrocatalytic water oxidation. In addition, NiAl–LDH@graphene oxide composite was also prepared and used as a precursor to form the N-doped carbon@Ni–Al2O3@graphene oxide composite for hydrogen evolution reaction.14 In the composite, LDH acts as a catalyst precursor to provide a uniformly distributed, thermally stable active metal nanoparticles, while carbon can help improve the dispersion of LDH particles, contributing to the improvement in the composite properties.17 Among the reported carbon materials, CNTs have been considered to be a superior catalyst material due to their large specific surface area (SSA), unique electronic structure, and mechanical and thermal properties.18 Therefore, these unique 3D composite materials constructed by the hybridization of 2D LDH and 1D CNTs are expected to show superior performance compared with that of the individual components. 4


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Despite the several advantages of the hybridization of LDH and CNTs, the LDH (as well as their derivatives) and CNTs are typically obtained as powder, which suffers from several drawbacks such as severe aggregation, poor morphology control, and high pressure drop in fixed-bed reactors,19 considerably limiting their practical applications. Hence, it is highly desirable to incorporate excellent powder samples on structured supports (e.g., SiC foam,20 anodic alumina oxide,21 carbon cloth,22 and Ni foam (NF) 23) to fabricate a hierarchical composite catalyst. Inspired by above mentioned results, herein, a series of RuNiAl LDH/CNTs (RuLDH/CNTs) hierarchical nanomaterials on NF structured support was proposed and fabricated for the first time. The fabrication (Scheme 1) involved (ⅰ) the in situ growth of CNTs on NF (CNTs/NF) by chemical vapor deposition (CVD) technology by utilizing a NiAl LDH film on NF (LDH/NF) as the catalyst, and (ⅰ) the deposition of the RuLDH nanosheet on this CNTs backbone (RuLDH/CNTs/NF) via a facile hydrothermal method. By virtue of the benefits provided by each of the building blocks, the as-prepared RuLDH/CNTs/NF hierarchical composite exhibited enhanced SSA, better stability, and increased heat and electron conductivities. After subsequent calcination and reduction, the resultant RuNi/Al2O3–CNTs/NF hierarchical composite with high metal dispersion was obtained and used as an efficient catalyst for CO-SMET in H2-rich reforming gases for fuel-cell applications. On account of the synergistic effects of well-dispersed RuNi particles from the RuLDH precursor and unique properties of CNTs and NF, the derived RuNi/Al2O3–CNTs/NF hierarchical catalyst exhibited excellent catalytic performance, which can deep-remove CO 5



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concnetration from a high level of 1 vol.% to below 10 ppm with the selectivity greater than 50% over a wide reaction temperature window (190–250 °C).

Scheme 1. Schematic illustration of the preparation of RuNi/Al2O3–CNTs/NF catalyst from RuLDH/CNTs/NF hierarchical composite. 2. EXPERIMENTAL 2.1. Catalyst Preparation Chemicals and materials: All of the chemicals and reagents, including Ni(NO3)2·6H2O,

Al(NO3)3·9H2O,

NiCl2·6H2O,

RuCl3·nH2O,

urea,

Polyvinylpyrrolidone (PVP, K=30), absolute ethanol, acetone, HNO3 and NaOH were of analytical grade and directly used as received. All gases (N2, H2, CH4) of high purity (99.999%) were used. Deionized water was homemade. NF (110 PPI, 2 mm thick, Shenzhen Hanbo Co., Ltd.) had a porosity greater than 95%. Prior to catalyst preparation, the NF (with a dimension of 30 mm ×20 mm × 2 mm) was ultrasonically degreased in acetone for 30 min, and then rinsed thoroughly with deionized water and absolute ethanol, finally dried overnight. The preparation of NiAl LDH/NF: The LDH nanosheets on NF was synthesized via 6


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a facile hydrothermal method. Briefly, a certain amount of Ni(NO3)2·6H2O, Al(NO3)3·9H2O and urea were dissolved into 70 ml deionized water with [Ni2+]+[Al3+]=0.1 M, [Ni2+]:[Al3+]=2:1, [urea]/[Ni2++Al3+]=3.3. Then the mixture was poured into a 100 ml Teflon-lined autoclave, and the pretreated NF was immersed in the solution. Finally, the autoclave was treated at 120 °C for 10 h to obtain the resultant LDH/NF product. The growth of CNTs/NF: The growth of CNTs on NF was conducted in a quartz tube reactor via CVD method by utilizing LDH/NF as catalyst. Before reaction, the LDH/NF catalyst was calcinated at 500 °C for 2 h under a N2 atmosphere, and then reduced at 500 °C for 1 h in a mixed gas of 50% H2/N2. After reduction, H2 was switched off and the reactor was heated up to 650 °C. The 50% CH4/N2 was then introduced into the reactor for 1 h to produce the CNTs on NF. The prepared CNTs was pretreated at 350 °C in air atmosphere for 2 h, and then put in 4.5 M NaOH for 1 h and 4.5 M HNO3 for 45 min, follwed by washing and drying to generate the target CNTs/NF product. The fabrication of RuLDH/CNTs/NF: The prepared CNTs/NF was employed as backbone for the RuLDH film formation via hydrothermal method. Typically, a certain amount of Ni(NO3)2·6H2O, Al(NO3)3·9H2O, RuCl3·nH2O, urea and 1 g PVP were dissolved into 70 ml deionized water with [Ni2+]+[Al3+]+[Ru3+]=0.1 M, [Ni2+]:[Al3+]:[Ru3+]=2:1:0.15, [urea]/[Ni2++Al3++Ru3+]=3.3. The mixture was then poured into the autoclave and a piece of CNTs/NF was added in the solution. After being heated at 100 °C for 10 h, the final RuLDH/CNTs/NF-0.1 hierarchical 7



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composite was obtained, where the number denoted the total metal concentration of Ni2+, Al3+ and Ru3+ in the solution. In addition, RuLDH/CNTs/NF-0.05 and RuLDH/CNTs/NF-0.15 were also prepared by changing the total metal concentration to 0.05 and 0.15 M. The load of RuLDH in the composite determined by the weight difference between RuLDH/CNTs/NF and CNTs/NF were approximately 20.2% for RuLDH/CNTs/NF-0.05,

25%

for

RuLDH/CNTs/NF-0.1,

and

28.9%

for

RuLDH/CNTs/NF-0.15, respectively. The

synthesis

of

RuNi/Al2O3–CNTs/NF

catalyst:

The

obtained

RuLDH/CNTs/NF-0.1 hierarchical composite was calcinated in N2 atmosphere at 500 °C for 2 h. The resultant product was denoted as RuNi/Al2O3–CNTs/NF catalyst. For comparision, RuNi/CNTs/NF with loading of 10% was prepared by impregnating CNTs/NF with a mixed solution containing NiCl2·6H2O and RuCl3·nH2O

([Ni2+]:[Ru3+]=2:0.15), followed by calcination and

reduction.

RuNi/Al2O3/NF was also prepared via a similar treatment of RuLDH/NF precursor. 2.2. Activity Test The catalytic activity towards CO-SMET was performed in a micro-reactor with the reaction chamber dimensions of 30 × 20 × 1.8 mm (L × W × H). The catalyst (about 350 mg) was loaded in the chamber and a graphite gasket was applied to seal the reactor. Before catalyst evaluation, a preliminary experiment was conducted to determine the suitable gas flow rate of the reactants so as to eliminate the influence of diffusion resistance. Typically, the catalysts were first reduced with 50%H2/N2 at 350 °C for 90 min. Then, the feed gas mixture consisting of 1 vol.% CO, 20% vol.% 8


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CO2 and 79 vol.% H2 (dry basis),6,12,24,25 which simulates the reforming gas in a practical condition, was introduced into the reactor with a gas hourly space velocity (GHSV) of 1400 h–1. The reaction was conducted at temperatures from 140 to 320 °C, and the effluent gas was analyzed online by a gas chromatograph (GC, Agilent 7820A) equipped with a methanator, a thermal conductivity detector (TCD) and a flame ionization detector (FID). Each experimental result demonstrated a good carbon balance (> 97%) for CO-SMET. The selectivity for CO methanation (SCO) was measured using the following expression: SCO =

inlet outlet -CCO ×Voutlet Cinlet CO × outlet Coutlet CH4 ×V

×100%

outlet Where Cinlet CO and CCO are the CO concentrations in the feed and effluent gas, inlet respectively, and Coutlet and Voutlet CH4 is the CH4 concentration in the effluent gas. V

denote the volumetric flow rates of feed and effluent gas, respectively. Generally, the SCO of greater than 50% is an important parameter for measuring the catalytic performance24-26 which indicates the outlet CH4 concentration of less than 2% (i.e., CO2 conversion of below 5%) when the inlet CO concentration is not greater than 1%. Therefore, the selectivity for CO methanation can also be well represented by the CH4 concentration in the effluent gas. 2.3. Characterization Powder X-ray diffraction (XRD) patterns were obtained on a rotation anode X-ray diffractometer (Bruker D8 Advance, Germany) by using Cu-Kα radiation (λ = 0.154 nm) and operating at 40 kV and 40 mA. X-ray photoelectron spectroscopy (XPS) was conducted on a Kratos Axis Ultra DLD spectrometer by using Al Kα radiation as 9



excitation source. The morphology of materials were characterized by field-emission scanning electron microscopy (FE-SEM) (merlin, Germany) performed at a voltage of 20 kV and transmission electron microscopy (TEM) (JEM-2100F, Japan) operated at an voltage of 100 kV. The chemical composition was analyzed by the coupled energy dispersive X-ray spectrometer (EDS). Nitrogen adsorption/desorption experiments were performed on a Micromeritics ASAP2010 instrument at 77 K. The specific surface area (SSA) was measured from the adsorption isotherm using the Brunauer–Emmett–Teller (BET) method, and the pore size distribution was determined from the desorption branch of isotherm using the Barrett–Joyner–Halenda (BJH) model. The Fourier transform infrared (FT-IR) spectroscopy (Bruker, Vertex 33) were conducted in the range of 4000–400 cm–1. The Raman signal was collected using LabRam HR with a wavelength of 532 nm over the range of 1000–2000 cm–1. Thermogravimetric (TG) analysis (NETZSCH, Germany) was performed in air atmosphere with a heating rate of 10 °C min–1. H2 temprature-programmed reduction (H2-TPR) analysis was investigated on Micromeritics AutoChem II 2920 instrument from 80 to 800 °C. 3. RESULTS AND DISCUSSION 3.1. Growth of CNTs on NF The growth of CNTs on NF includes the preparation of the LDH/NF catalyst precursor and the subsequent catalytic growth of CNTs. In the XRD pattern of the 10


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LDH/NF composite (Figure S1a), weak but distinct diffraction peaks were observed at 11.7°, 22.8°, 34.9°, and 61.2°, which well corresponded to the (003), (006), (012), and (110) crystal planes reported for NiAl LDH,27 respectively, indicative of the successful synthesis of LDH. The interplanar spacing of the LDH film was approximately 0.75 nm, indicative of the formation of CO32− anions in the LDH interlayer.12 After calcination and reduction, the obtained NF-supported Ni catalyst was reported as an efficient catalyst for the growth of CNTs via CVD.28-30 As indicated in Figure S1b, the XRD patterns belonging to the LDH phase disappeared, and a new peak was observed at 26.4°, corresponding to carbon formation. In addition, the Raman spectra shown in Figure 4b confirmed the result, showing two apparent peaks at around 1356 (D band) and 1600 cm−1 (G band) characterized for carbon. The TEM image (Figure 1e) further confirmed that the formed carbon materials are CNTs. SEM and TEM were utilized to examine the structure and morphology of LDH/NF and CNTs/NF. Bare NF exhibited a smooth surface and a typical porous structure (Figure 1a). After hydrothermal treatment, the NF surface was densely coated by vertically aligned, highly interconnected LDH nanosheets (Figure 1b and c). The Ni/Al molar ratio estimated from EDS analysis was around 2.3, slightly greater than the theoretical value (Ni/Al = 2) due to the partial dissolution of NF in the solution. The cross-sectional SEM image of LDH/NF revealed an approximately 290-nm-thick LDH film (Figure 1b). The corresponding TEM image (Figure S2) indicated that LDH exhibits an ultrathin property with an estimated diameter of approximately 110 nm. During subsequent CVD process, the CNTs film was obtained. The detailed growth 11



mechanism has been described in a study reported previously by our group.31 The carbon yield calculated from the mass difference of catalyst before and after the reaction was approximately 45.9%. As shown in Figure 1d, several microns long CNTs were randomly oriented and densely coated on the NF surface. The corresponding diameter distribution (inset of Figure 1d) indicated that the average diameter of the as-grown CNTs is 33.1 nm and that the diameter distribution ranges from 27 to 43 nm. The TEM image shown in Figure 1e revealed that the formed CNTs exhibit a rope-like nanotubular structure, and a majority of the Ni catalyst particles are observed at the end of the CNTs, indicative of a base-growth mechanism.32

Figure 1. SEM images of NF based materials: (a) bare NF, (b) the side-view and (c) the top-view of LDH/NF, and (d) CNTs/NF and the corresponding outer diameter distributions of CNTs (inset). (e) The TEM image of prepared CNTs/NF material. 12


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3.2. Characterization of RuLDH/CNTs/NF hierarchical composites Based on the presence of abundant oxygen-containing functional groups on CNTs, the above-prepared CNTs/NF composite was then taken as the nucleation center and a hollow backbone for the preparation of RuLDH/CNTs/NF hierarchical composites by the hydrothermal method. Previously, the growth mechanism has been reported to involve the the nucleation and growth of RuLDH on the CNTs surface at the early stage, followed by the predominant longitudinal growth of RuLDH along the CNTs.33,34 The low-magnification SEM image of the RuLDH/CNTs/NF-0.1 hierarchical composite (Figure 2a) showed that a densely packed film of RuLDH/CNTs is uniformly covered on the NF surface without distinct detachment. The high-magnification SEM images of the RuLDH/CNTs/NF-0.1 hierarchical composite (Figure 2b and c) indicated that the CNTs surface are uniformly and completely coated with RuLDH nanosheets, and isolated RuLDH crystallites are seldom detected on and out of CNTs. In addition, these nanosheets were interconnected with each other, which creats several pores and crevices, thereby ensuring a large SSA for the composite.35 The SEM-EDS analysis indicated that the atomic ratio of Ni/Al/Ru in the composite is 2.6/1/0.13. The TEM image of the composite in Figure 2d further indicated that RuLDH nanosheets are homogeneously coated on the CNTs surface, and self-isolated aggregates are barely observed after ultrasonication, indicating the strong interaction between RuLDH and CNTs. Furthermore, the influence of the RuLDH content on the morphology of the RuLDH/CNTs/NF

hierarchical

composites

have

13


been

investigated.

For


RuLDH/CNTs/NF-0.05 with a lower RuLDH content, only part of CNTs were coated by RuLDH particles, while bare CNTs were clearly observed in the composite (Figure S3). On the other hand, for RuLDH/CNTs/NF-0.15 with a higher RuLDH content, the CNTs were completely covered by RuLDH, except that a few isolated RuLDH agglomerates were observed outside the CNTs surface (Figure S3).

Figure 2. SEM images (a, b and c) of RuLDH/CNTs/NF-0.1 composite with different magnifications. TEM image (d) of RuLDH/CNTs/NF-0.1 composite. Considering the extremely low SSA of pristine NF (0.01–0.1 m2·g–1), the development of a NF-based material with increased SSA is hence highly desirable for catalytic applications. Table 1 summarizes the SSA and average pore size distribution of each composite. As can be seen, the SSA of the prepared CNTs/NF material after 14


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CVD process considerably increased to 40.1 m2·g–1, and the subsequent deposition of RuLDH

crystallites

further

slightly

increased

the

SSA of

the

obtained

RuLDH/CNTs/NF hierarchical composite. The SSA of the RuLDH/CNTs/NF hierarchical composite slightly increased (and the pore size decreased) with the increase of RuLDH content, and the maximum SSA (51.2 m2·g−1) was obtained over the RuLDH/CNTs/NF-0.1 composite. With the further increase in the RuLDH content (RuLDH/CNTs/NF-0.15), the SSA considerably decreased probably because of the formation of the RuLDH aggregates in the composite, as confirmed by the TEM analysis (Figure S3). This indicated that the enhancement in the SSA for the hierarchical composite is mainly ascribed to the growth of CNTs, and the subsequent RuLDH deposition can help to prevent the stacking and aggregation of CNTs, further contributing to the increase in SSA.36 Table 1. Physicochemical properties of the NF-based samples. Sample

Specific surface

Pore volume

Average pore

area (m2·g–1)

(cm3·g–1)

size (nm)

bare NF

0.01~0.1

~

~

CNTs/NF

40.1

0.13

16.1

RuLDH/NF

13.2

0.04

13.5

RuLDH/CNTs/NF-0.05

43.5

0.18

21.3

RuLDH/CNTs/NF-0.1

51.2

0.18

16.1

RuLDH/CNTs/NF-0.15

42.9

0.15

21.1

Besides, the highly porous structure generated by the interconnected RuLDH 15



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nanosheets also contributed to the increased SSA, as confirmed by the N2 adsorption/desorption analysis (Figure S4). Notably, both samples of CNTs/NF and RuLDH/CNTs/NF-0.1 exhibited a typical IUPAC type IV pattern with an H3-type of hysteresis loop, implying the existence of a mesopores. No limiting adsorption can be noted at high P/P0, implying the presence of interconnected macropores.37 The corresponding pore size distribution of CNTs/NF exhibited two peaks centered at approximately 2.8 nm and 110 nm, which can be ascribed to the internal pores within the

CNTs

and

the

macropores

between

the

CNTs,

respectively.38

The

RuLDH/CNTs/NF-0.1 composite exhibited a similar pore size distribution of CNTs/NF, albeit it shrunk to a smaller size range. Figure 3A illustrates the XRD patterns of the obtained RuLDH/CNTs/NF hierarchical composites with different RuLDH content. All of the RuLDH/CNTs/NF-x composites exhibited similiar characteristic peaks containing the (002) reflection of CNTs and the (003), (006), (012), and (110) reflections of RuLDH. The co-existence of CNTs and LDH in the composite indicated that the RuLDH crystals have been successfully deposited on CNTs. The d-spacing of the (003) plane of RuLDH (d003) in the composite was approximately 0.804 nm; this value is apparently greater than that of pristine RuLDH (d003 = 0.768) in the RuLDH/NF material. This phenomenon is related to the strong interactions between RuLDH with CNTs, which weakened the electrostatic interactions between positively charged layers and negatively charged interlayers (CO32−).10 In addition, compared with that of pristine CNTs/NF, the intensity of the RuLDH diffraction peaks in the RuLDH/CNTs/NF hierarchical 16


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composites gradually increased with the RuLDH content. The result was confirmed from TG analysis according to the mass loss of RuLDH in the composite. As observed in Figure 3B, the weight loss for pristine RuLDH/NF apparently involved two stages: the first one at low temperatures (< 250 °C) is related to the elimination of physically adsorbed and intercalated water molecules in RuLDH; the second one at 250−500 °C is ascribed to the dehydroxylation of the layers and the decomposition of carbonate anions in interlayers.12,39 The subsequent mass increase observed at high temperatures (>500 °C) corresponded to the oxidation of NF substrate.32 The weight loss for pristine CNTs/NF was predominantly observed at 500−680 °C, corresponding to the gasification of CNTs. With respect to the RuLDH/CNTs/NF hierarchical composites, the thermal behavior included both the decomposition of RuLDH and the gasification of CNTs. No clear mass gain was observed in both CNTs/NF and the RuLDH/CNTs/NF composite, probably due to the overlapping between the oxidation of NF and the gasification of CNTs. The amount of residue in the composite apparently increased with the increase of RuLDH content in the composite, as was also observed for other synthesized hybrid materials.39,40 Furthermore, compared with that of pristine RuLDH/NF, the decomposition temperature of RuLDH in the RuLDH/CNTs/NF hierarchical composites markedly increased, indicative of an enhanced thermal stability of RuLDH. This result is ascribed to the strong interaction between the positively charged RuLDH and negatively charged CNTs, which could mitigate the agglomeration of RuLDH during calcination and generate an increased number of active sites, thereby contributing to a 17



more active and stable catalyst material.39

Figure 3. XRD patterns (A) and TG curves (B) of prepared materials: (a) RuLDH/NF, (b) CNTs/NF, (c) RuLDH/CNTs/NF-0.05, (d) RuLDH/CNTs/NF-0.1, and (e) RuLDH/CNTs/NF-0.15. The FT-IR and Raman analyses of the CNTs/NF and RuLDH/CNTs/NF-x (x = 0.05, 0.1, 0.15) composites were displayed in Figure 4. In FT-IR spectra (Figure 4A), the observed strong vibrations at 3415 and 1631 cm−1 were ascribed to the O-H stretching vibration and bending vibration from brucite-like layers and interlayer water molecules, respectively.12,41 Absorption bands simultaneously observed around 1734, 1379, and 1062 cm−1 were related to the C=O, C-OH and C-O vibrations, respectively,42 indicative of the considerable oxidation of the CNTs after treatment with alkali and acid. For the RuLDH/CNTs/NF-x composites, the intense band located at approximately 1371 cm–1 was attributed to the stretching vibration of interlayer CO32−,33 confirming the formation of CO32−-intercalated RuLDH. Bands observed in the low-frequency region (< 800 cm−1) mainly corresponded to the metal–oxygen vibrations in the RuLDH structure.33 In Raman spectra (Figure 4B), two prominent peaks at approximately 1346 18


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(D-band) and 1577 cm–1 (G-band) were observed in all samples. Basically, the D-band mainly corresponded to the the vibrations of disorder or defects in the graphene sheets, and the G-band corresponded to the ordered graphite structure on CNTs.31,32,43 Apparently, the D and G bands for RuLDH/CNTs/NF-x composites were shifted slightly to higher wavenumber compared with those of the CNTs/NF sample, possibly related to the interactions between RuLDH and CNTs.43 Usually, the ID/IG intensity ratio is used to evaluate the structural changes of CNTs in the composites. In our case, the ID/IG ratio of the CNTs/NF composite was 0.685; however, the value increased to 0.876, 0.934, and 1.029 for RuLDH/CNTs/NF-0.5, RuLDH/CNTs/NF-0.1, and RuLDH/CNTs/NF-0.15, respectively, indicating that an increased number of defects or structural disorders have been introduced on the CNTs surface during the growth of RuLDH, which plays an important part in the promotion of nucleation and prevention of the agglomeration of RuLDH crystallites.44

Figure 4. FT-IR (A) and Raman (B) spectra of prepared materials: (a) CNTs/NF, (b) RuLDH/CNTs/NF-0.05, (c) RuLDH/CNTs/NF-0.1 and (d) RuLDH/CNTs/NF-0.15. 3.3 Catalytic performance of the derived catalysts The derived RuNi/Al2O3–CNTs/NF composite from the RuLDH/CNTs/NF-0.1 19



precursor via calcination and reduction was employed as an efficient catalyst for CO-SMET. The real contents of Ru, Ni, Al2O3 and CNTs in the prepared catalyst layer determined by SEM-EDS analysis were about 2.5 wt%, 36.7 wt%, 25.3 wt% and 30.5 wt%, respectively. For comparison, the catalytic performance of RuNi/Al2O3/NF and RuNi/CNTs/NF catalysts was also investigated. The SSA and cumulative pore volume of the RuNi/Al2O3–CNTs/NF hierarchical catalyst were 68.63 m2·g−1 and 0.27 cm3·g−1, respectively (Figure S5), which were greatly larger than the corresponding values for RuNi/Al2O3/NF (14.26 m2·g−1 and 0.04 cm3·g−1) and RuNi/CNTs/NF catalysts (16.15 m2·g−1 and 0.07 cm3·g−1). The reason was mainly attributed to the increased SSA and developed pore volume of hierarchical RuLDH/CNTs/NF-0.1 precursor due to the deposition of RuLDH which not only prevent the aggregation of CNTs, but also create new pores on the composite.45 Figure 5 shows the SEM and TEM images of each prepared catalyst. Pure RuLDH with a thin-sheet-like morphology grew uniformly and vertically on NF (Figure S6). After calcination and reduction, the layered structure of RuLDH collapsed, resulting in a well-dispersed RuNi catalyst supported on NF (Figure 5a and d). The selected area electron diffraction (SAED) patterns in the inset of Figure 5d revealed a polycrystalline structure of the catalyst, containing both the metallic Ru and Ni phases. However, the relatively low SSA and inferior electron conductivity considerably restrained its catalytic performance. For the RuNi/CNTs/NF catalyst (Figure 5b and e), nanoparticles with a relatively larger particle size agglomerated on the CNTs surface, 20


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which was considered unfavorable for the CO-SMET catalytic performance.46 The corresponding SAED pattern (inset of Figure 5b) also revealed the polycrystalline nature of the catalyst, and the appearance of strong diffraction spots further confirmed the presence of large-size particles. On the other hand, for the RuNi/Al2O3–CNTs/NF hierarchical catalyst (Figure 5c and f) obtained from the RuLDH/CNTs/NF precursor, plate-like morphology comprising small particles was firmly coated on the CNTs surface, and some exposed CNTs were clearly observed (green arrows). The TEM image of RuNi/Al2O3– CNTs/NF (Figure 5f) further showed that these particles are uniformly dispersed on CNTs without obvious aggregation. Because of the large SSA and unique electronic propertities of CNTs, as well as the uniformly distributed metal nanoparticles form the LDH precursor, the derived RuNi/Al2O3–CNTs/NF hierarchical catalyst is expected to exhibit excellent catalytic performance (Figure 8). The high-magnification TEM image of RuNi/Al2O3–CNTs/NF (Figure 5g) revealed a lattice spacing of 0.35 nm, belonging to the (002) plane of CNTs. Meanwhile, the nanoparticles of single Ni and Ru were also observed, which plays a crucial role in the catalytic performance for CO-SMET. The corresponding high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) in Figure 5h and the energy-dispersive X-ray (EDX) elemental mapping images in Figure 5i further confirmed the high dispersion of nanoparticles on the CNTs surface.

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Figure 5. The SEM images of (a) RuNi/Al2O3/NF, (b) RuNi/CNTs/NF, (c) RuNi/Al2O3–CNTs/NF catalysts. The TEM images of (d) RuNi/Al2O3/NF, (e) RuNi/CNTs/NF, (f, g) RuNi/Al2O3–CNTs/NF catalysts. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) (h) and EDX element mapping (i) of RuNi/Al2O3–CNTs/NF catalyst. The chemical states of the supported metal particles and the interactions between the metal particles and support were examined by XPS. Figure S7 shows the XPS survey (wide-scan) spectra referenced to the C 1s signal: Ni, Ru, C, and O components were observed in the composite. Cl-containing species were notdetected, indicative of the complete decomposition of the Ru precursor during calcination. As 22


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can be seen in Figure 6A, three peaks at around 853.0, 856.5 and 862.5 eV were observed in the XPS spectra of Ni 2p3/2, corresponding to the metallic Ni, NiO and the satellite, respectively.47 This observation indicated the incomplete reduction of the NiO species, which is in accordance with the TEM observations. In addition, the XPS spectra of C 1s around 284.65 eV and Ru 3d5/2 around 281 eV were shown in Figure 6B. The high-intensity C 1s peak for RuNi/CNTs/NF and RuNi/Al2O3–CNTs/NF was related to the presence of CNTs in the catalysts. The binding energies of Ni 2p3/2 and Ru 3d5/2 for RuNi/Al2O3–CNTs/NF were shifted to higher binding energies compared with those of RuNi/Al2O3/NF and RuNi/CNTs/NF, indicative of the change in the electronic properties of Ni and Ru species in the hierarchical composite. The reason can be attributed to the strong interactions between Ru and Ni nanoparticles with adjacent metal oxide support13 and to the high dispersion of nanoparticles that are easily oxidized with oxygen at room temperature.48

Figure 6. The XPS spectra of prepared catalysts: (a) RuNi/Al2O3/NF, (b) RuNi/CNTs/NF and (c) RuNi/Al2O3–CNTs/NF. H2-TPR analysis (Figure 7) was further conducted to study the reduction behavior of the prepared catalysts after calcination. It can be observed that the bare CNTs/NF 23



showed a small peak at low temperature of 280 °C and a broad peak at high temperature of 600 °C, which were ascribed to the reduction of bulk NiO from the NF substrate and the methanation of CNTs, respectively.49,50 For the prepared catalysts, each showed three reduction peaks at temperatures below 400 °C. In particular, the first peak observed at approximately 135 °C was ascribed to the reduction of Ru oxides,3 and the second peak around 230 °C was assigned to the reduction of small-sized NiO particles.24 The third peak observed at around 300 °C was related to the reduction of larger NiO species from catalyst particles and NF. It is noted that the reduction peak corresponding to the small NiO particles in the RuNi/Al2O3–CNTs/NF hierarchical catalyst exhibited a considerably higher peak area compared to that of the larger NiO particles as against the other two catalysts, implying its higher metal dispersion. The shift in the reduction temperature has been reported to mainly depend on the metal–support interaction and the size and/or the location of nanoparticles. In our case, the decreased reduction temperature observed for the RuNi/Al2O3–CNTs/NF catalyst was possibly due to the formation of smaller-sized nanoparticles. In addition, compared with CNTs/NF and RuNi/CNTs/NF, the methanation temperature of CNTs over RuNi/Al2O3–CNTs/NF hierarchical catalyst shifted greatly to lower temperatures, indicating the higher catalytic ability for the highly-dispersed supported Ru and Ni nanoparticles.50 Therefore, according to the XPS and H2-TPR results, smaller-sized and well-dispersed nanoparticles are obtained over the RuNi/Al2O3–CNTs/NF hierarchical catalyst, contributing to better catalytic activity.

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Figure 7. The H2-TPR profiles of prepared samples: (a) CNTs/NF, (b) RuNi/Al2O3/NF, (c) RuNi/CNTs/NF and (d) RuNi/Al2O3–CNTs/NF. The catalytic performance for CO-SMET over the prepared catalysts was investigated (Figure 8). The outlet CO concentrations of all catalysts first decreased rapidly with increasing temperature and achieved a minimum, then slightly increased because of the competitive reaction of r-WGS (Figure 8A).51 Similar phenomena have been reported previously for CO-SMET.6,12 For the RuNi/Al2O3/NF catalyst, the CO concentration decreased only to 55 ppm at 240 °C, which cannot meet the requirement of high-purity hydrogen for fuel cells. For the RuNi/CNTs/NF catalyst, the CO concentration decreased to less than 10 ppm at a high temperature of 260 °C and then increased to 20 ppm at 280 °C, indicative of insufficient activity. On the other hand, for the RuNi/Al2O3−CNTs/NF hierarchical catalyst derived from the RuLDH/CNTs/NF precursor, the temperature for optimal CO removal (

Al2O

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