Nanoporous Nickel Composite Catalyst for the Dry Reforming of Methane.

The development of efficient catalysts with high activities and durabilities for use in the dry reforming of methane (DRM) is desirable but challenging. We report the development of a nanoporous nickel composite (nanoporous Ni/Y2O3) via a facile one-step dealloying technique, for use in the DRM. Focusing on the low-temperature DRM, our composite possessed remarkable activity and durability against coking compared with conventional particle-based Ni catalysts. This was attributed to the aluminum oxides present on the Ni surface, which suppress pore coarsening. In addition, the inert bundled Y2O3 nanowires are suitable for use as substrates for nanoporous Ni.

Introduction

As the major components of natural and greenhouse gases, methane (CH4) and carbon dioxide (CO2) are key to achieving a more sustainable society. Accordingly, the dry reforming of methane (DRM) into “syn gas”, which is represented by the transformation CH4 + CO2 → 2H2 + 2CO, could be a potential technological solution; however, this reaction requires high temperatures (550–1000 °C), which results in significant catalyst degradation due to material sintering and coke deposition.[1−3] To date, many heterogeneous (typically Ni-based) catalysts have been evaluated to determine their stabilities and performances and the modification of interactions between Ni and oxide supports[4−8] and/or the structural design of multicore–shell shaped species[9−22] have been proposed. Overall, the enhancement of catalyst performances is the key target, in particular, in the context of lowering reaction temperatures.

Dealloying is an electrochemical process commonly employed for the leaching of less noble elements from precursor alloys to form bicontinuous nanoporous materials.[23,24] Recently, nanoporous metals with excellent chemical and physical properties have been prepared via dealloying, thereby leading to cross-interdisciplinary research related to batteries, catalysis, sensing, and biotechnological applications.[25−30] However, nanopore coarsening at high temperatures can cause a degradation in performance and the suppression of such coarsening is a challenging issue. We herein report the preparation of nanoporous Ni composites via the one-step dealloying of a NiYAl alloy and compare the DRM performances and durabilities of these composites with those of conventional Ni catalysts prepared via chemical routes. In particular, we focus on the low-temperature DRM (450 °C) where carbon coking tends to be significant for Ni catalysts and we wish to demonstrate that our nanoporous Ni composites exhibit structural durability and resistance to coking. We expect that the nanoporous Ni composites reported herein will widen the application of nanoporous metals and inspire the design of novel DRM catalysts.

Results and Discussion

As outlined in Figure a, we initially prepared the nanoporous Ni composite catalyst from the dealloying of a precursor alloy by Al leaching using a 30 wt % solution of NaOH, whereas Ni does not dissolve in the strongly alkalic solution. In addition, we compared the performances of products obtained from two precursors, namely, Al4NiY and Al2NiY intermetallics, with Al4NiY being the more desirable of the two, as discussed later. It should be noted here that the nanoporous Ni/Y2O3 refers to the dealloyed Ni12.5Y12.5Al75 (Al4NiY). Thus, we selected the nanoporous Ni composites prepared from the Al4NiY precursor for extensive evaluation. Interestingly, this one-step dealloying procedure employed NiYAl powders to yield bundled urchinlike nanoporous Ni particles bearing yttrium hydroxide, Y(OH)3. Although the tangled urchinlike Y(OH)3 structure was unexpectedly obtained, we do note that hexagonal yttrium hydroxide nanowires have been previously prepared via hydrothermal routes.[31] The driving force for growth was attributed to the crystal structure of yttrium hydroxide, and the formation mechanism was attributed to complex interactions between the OH– and Y3+ ions.[32]Figure b,c shows the low-magnification transmission electron microscopy (TEM) images of the prepared samples, and the high-resolution image given in Figure d indicates that the nanoporous Ni regions contain fine pores of ∼10–20 nm diameter. In addition, X-ray analysis (Figure e) confirmed the uniform distribution of Ni and Al. Indeed, the X-ray diffraction (XRD) patterns showed that changes in the crystal structure took place during dealloying, with conversion from the intermetallic Al4NiY precursor to the fcc Ni and Y(OH)3 compounds being followed by the conversion of Y(OH)3 to Y2O3 subsequent to the DRM reaction. This process is outlined in Figure S1. From inductively coupled plasma (ICP) analysis, the nominal elemental composition was determined to be Ni20.8Y19.2Al3.5O56.5 (atom %). In addition, the Brunauer–Emmett–Teller (BET) surface area was calculated to be 8.3 m2/g. For comparison with conventional catalysts, Ni/Al2O3 and Ni/Y2O3 composites were synthesized via a hydrothermal method and the TEM images of these composites are shown in Figures S2 and S3. For the conventional catalysts, Ni particle sizes were comparable to those of the nanoporous Ni/Y2O3 prepared herein, and to maximize the performance, the volumes of the Ni and oxide components were similar.

Figure 1

(a) Schematic illustration of the experimental procedure; (b, c) low-magnification TEM images; (d) high-magnification TEM image showing the fine pores; (e) scanning TEM (STEM) image and energy-dispersive X-ray spectrometry (EDS) chemical maps of the selected area, showing the distributions of Ni (red) and Al (green).

As shown in Table , Figures , and S4, the catalytic performances of the prepared composites were examined for the DRM over 100 h at 450 °C and the results compared with those of conventional Ni/Al2O3 and Ni/Y2O3. The product obtained from the Ni12.5Y12.5Al75 precursor gave a slightly better performance than that obtained from Ni25Y25Al50, as shown in Table , and it was found by ICP analysis that the dealloying of Al was incomplete for the Ni25Y25Al50 precursor (Table S1), thereby suggesting that the Al-rich precursor (Ni12.5Y12.5Al75) preferentially forms the desired nanostructure during the dealloying process. In the initial stages of the reaction, Ni/Al2O3 and Ni/Y2O3 were more active than our prepared samples but both caused significant carbon coking and increased the reactor pressure due to stacking. Indeed, to prevent breakdown of the reactor, it was necessary to terminate the tests after ∼15 h. In contrast, the nanoporous Ni/Y2O3 composite (i.e., dealloyed Ni12.5Y12.5Al75) prepared herein maintained a moderately high performance over 100 h and the reactor pressure remained relatively constant, thereby indicating that carbon coking was suppressed. It should be noted that an ideal equilibrium conversion of 59% was calculated[33] for CH4 and CO2 at 450 °C, assuming that no side reactions take place, thereby indicating that there is still scope for further improvement.

Table 1

DRM Performance of the Ni/Y2O3 Composites Dealloyed from Ni12.5Y12.5Al75 and Ni25Y25Al50 and Compared with the Performances of Ni/Al2O3 and Ni/Y2O3 Prepared via Chemical Routesa

samples	CH4 conv. (%)	CO2 conv. (%)	CH4 consumpt. rate (10–5 mol h–1)	CO2 consumpt. rate (10–5 mol h–1)	H2 formation rate (10–5 mol h–1)	CO formation rate (10–5 mol h–1)	H2/CO rate
nanoporous Ni/Y2O3 (Ni12.5Y12.5Al75-dealloyed)	37	37	100	100	197	142	1.4
nanoporous Ni/Y2O3 (Ni25Y25Al50-dealloyed)	30	34	80	91	158	141	1.1
Ni/Al2O3	54	40	143	106	285	136	2.0
Ni/Y2O3	48	36	130	97	255	134	1.9

Measurements were taken at 450 °C after 6 h (from Figure ).

Figure 2

DRM performance plotted against time-on-stream and comparison with conventional Ni/Al2O3. Nanoporous Ni/Y2O3: solid red box, CH4 conversion; solid green box, CO2 conversion; upward solid blue triangle, reactor pressure; Ni/Al2O3: open red ring, CH4 conversion; open green ring, CO2 conversion; upward open blue triangle, reactor pressure.

Coking suppression in the nanoporous Ni/Y2O3 composite was confirmed by in situ Fourier transform infrared (FTIR) experiments, as shown in Figure a,b. More specifically, H2 and CO were generated at both 450 and 700 °C without any significant catalytic degradation being observed and the absence of significant signals indicated that little carbon coking took place. However, in the case of the conventional Ni/Al2O3, the presence of abundant absorbance signals between 1000 and 4000 cm–1 was attributed to carbon coking (note that the bands from the CO2 phase at 2360 and 2340 cm–1 are overlapped) and these signals increased in intensity with increasing reaction time. Furthermore, the scanning electron microscopy images confirmed that graphitic carbon nanotubes covered the surface following the FTIR experiments, as shown in Figure S5.

Figure 3

(a) Concentrations of CH4, H2, and CO during the in situ FTIR experiment plotted against the reaction time for the nanoporous Ni/Y2O3. Open red ring, H2; solid green box, CO; and upward solid blue triangle, CH4. (b) FTIR spectra of the nanoporous Ni/Y2O3 and Ni/Al2O3 (Ni) materials.

The deposited carbon present following the DRM process was also evaluated by thermal gravimetric differential thermal analysis (TG-DTA) under ambient conditions. The mass differences between the nanoporous Ni/Y2O3 (i.e., dealloyed Ni12.5Y12.5Al75) and the conventional Ni/Al2O3 samples after the DRM are shown in Figure . For comparison, the data from the as-prepared samples that were not subjected to the DRM are also shown. The sharp mass loss above 500 °C corresponds to the combustion of carbon,[5,22] thereby indicating that carbon coking is indeed suppressed for the nanoporous Ni/Y2O3.

Figure 4

TGA analysis of the spent and as-prepared catalysts. Nanoporous Ni/Y2O3 after 6 h at 450 °C and Ni/Al2O3 after 6 h at 450 °C.

Upon comparison of the heat durability with that of the bare nanoporous Ni dealloyed from a NiMn precursor at 650 °C (Figure S6) in our previous study[34] and with that of other bare nanoporous metals, we found that the prepared composite exhibited an excellent durability against heat. To determine the origin of this improved durability, the microstructure of the nanoporous Ni/Y2O3 composite (i.e., dealloyed Ni12.5Y12.5Al75) was examined following the DRM test at 650 °C for 3 h and the duration test at 450 °C for 100 h (Figure a,b) and the region of interest in Figure b is probed on the nanoporous Ni area that does not contain Y oxide substrate; hence, Y mapping is not shown/detected here. Representative low-magnification images are also shown in Figure S7. In both cases, the nanostructured Ni/Y2O3 with fine nanopores (i.e., ∼20–30 nm diameter) was retained. We also observed slight pore coarsening during the DRM test, which corresponds to the decay taking place during the DRM (Figure ). Interestingly, energy-dispersive X-ray spectrometry (EDS) mapping clearly indicated that oxides of aluminum were formed on the nanoporous Ni surface and that this suppressed pore coarsening. Indeed, as surface diffusion is the dominant mechanism of pore coarsening, a novel solution for suppressing such surface diffusion involves the formation of a nanometer-thick layer of alumina by atomic layer deposition (ALD).[35] In this study, the nanostructure transformed into a more active and durable nanostructure without the requirement for the expensive ALD process, as the solid-solute Al (Figure e) diffused to the surface and underwent oxidation on the nanoporous Ni. The reaction-driven transformation of porous nanostructures into active nanostructures can also be found in other nanoporous metals[36] and intermetallic compounds.[37]

Figure 5

STEM image and EDS chemical maps of the selected area, showing the elemental distributions after the DRM tests of the nanoporous Ni/Y2O3 composite (i.e., dealloyed Ni12.5Y12.5Al75): (a) 650 °C for 3 h and (b) 450 °C for 100 h.

We subsequently performed the in situ observation of nanoporous Ni/Y2O3 under the DRM reaction atmosphere using TEM with an environmental cell to gain further insight into the resistance of this material to coking. The initial microstructure present prior to the reaction is shown in Figure a. Upon increasing the temperature to 600 °C under vacuum, no change in the microstructure was observed. In addition, no carbon coking or pore coarsening was observed for the nanoporous Ni regions following the introduction of a mixture of CH4 and CO2 (see Figure b), thereby indicating the improved coking resistance of the nanoporous structure compared to that of the Ni particles. In the case of conventional Ni particles, carbon deposition and nanotube growth were catalytically triggered on the Ni surface[38,39] during in situ TEM observations of the conventional Ni/Al2O3 catalyst (see Figure S8). It therefore appears that the high structural stability of the aluminum oxide-coated nanoporous Ni could be key to understanding the improved coking resistance. In addition, although we presume the formation of a complex mixture between the carbon layer and the Y2O3 surface during in situ observation of nanoporous Ni/Y2O3 under the DRM reaction atmosphere, as indicated in Figures d,e and S9 showing the layer-by-layer motion and the tubelike feature, the whole Y2O3 substrate was stable, which was also confirmed in the practical DRM tests (Figure S7). The structurally robust nature of the inert Y2O3 wire may also account for the improved heat durability, thereby rendering this a suitable substrate for nanoporous Ni.

Figure 6

In situ TEM observations during the DRM reaction at 600 °C. (a) Low-magnification TEM image and the region of interest, as marked. (b) Region of nanoporous Ni showing the least amount of coking after 5 min. Growth of the carbon layer on Y2O3: (c) the initial stage and after (d) 2 min and (e) 5 min.

Conclusions

In summary, we successfully developed a nanoporous Ni/Y2O3 catalyst for use in the dry reforming of methane (DRM). This nanoporous nickel composite was prepared via a one-step dealloying process, and the aluminum oxide from the solid-solute Al present on the nanoporous Ni surface was found to be key to understanding the structural stability of the catalyst against heat and coking, due to its role in preventing pore coarsening. We found that the inert and urchinlike bundled Y2O3 was a suitable and robust substrate for nanoporous Ni. As we expect that the production of these nanoporous catalysts could easily be scaled up, they can be considered a potential alternative to traditional Ni particle catalysts for application in the DRM and may inspire the design of novel DRM catalysts.

Methods

Preparation of Nanoporous Metals

Nanoporous Ni Composite

Ni12.5Y12.5Al75 and Ni25Y25Al50 (atom %) ingots were prepared by melting pure Ni, Y, and Al (purity >99.9 atom %), using an Ar-protected arc melting furnace, where Ni12.5Y12.5Al75 was the optimal composition for dealloying. The prepared ingots were ground using a mortar and pestle and then sieved to obtain powdered samples with an average size of 20 μm. The precursor powders were dealloyed in a 30 wt % NaOH (97% Wako, Japan) solution for 4 h at 50 °C and then rinsed thoroughly with water and dried under air.

Conventional Ni/Al2O3 and Ni/Y2O3 Composites

The Ni/Al2O3 and Ni/Y2O3 composites were prepared by a conventional impregnation method. More specifically, following the dissolution of Ni(NO3)2·6H2O (0.8 g, Sigma-Aldrich) in ethanol (20 mL), either Al2O3 (0.3 g, Sigma-Aldrich) or Y2O3 (0.3 g, Sigma-Aldrich) powder was added to the alcoholic solution. The resulting mixture was stirred for 8 h, and then ethanol was removed by evaporation at 353 K. The desired Ni/Al2O3 and Ni/Y2O3 catalysts were obtained following calcination in a H2–Ar gas mixture (5 vol % H2) at 873 K over 4 h.

Microstructural Characterization

The microstructures of the obtained catalysts were characterized by transmission electron microscopy (TEM, JEM-2100F, JEOL, equipped with aberration correctors, for the image- and probe-forming lens systems, CEOS GmbH) and energy-dispersive X-ray spectrometry (EDS, JED-2300T, JEOL). High-resolution TEM and scanning TEM (STEM) observations were conducted at an accelerating voltage of 200 kV, with the Cs correctors optimized for point-to-point resolutions of 1.3 and 1.1 Å for TEM and STEM, respectively. The samples were transferred onto a Cu grid without the use of a uniform carbon support film. X-ray diffraction profiles were obtained using a Rigaku SmartLab X-ray diffractometer with Cu Kα radiation.

For in situ TEM analyses, a 1000 kV JEM-1000K RS TEM (JEOL) equipped with an environmental cell designed at Nagoya University (Japan) was employed, along with a point-to-point resolution of 1.5 Å. All samples were observed in a CH4 + CO2 gas mixture (50:50, vol %) at 600 °C over a wide range of total pressures (1–30 Pa). The current flux was measured as 0.23 A cm–2 using a Faraday gauge.

Inductively coupled plasma analysis was performed using an IRIS Advantage DUO instrument (Thermo Fisher Scientific).

The deposited carbon present after the DRM process was evaluated by using thermal gravimetric differential thermal analyzer (TG-DTA, NETZCH, STA 2500) under air. The sharp mass loss above 500 °C corresponded to the combustion of carbon.

Catalytic Experiments

The desired sample (100 mg) was loaded into a 4 mm quartz tube and tested using a continuous-flow fixed-bed microreactor under atmospheric pressure. The quantities of CH4, CO, H2, and CO2 were monitored and evaluated using an on-line gas analyzer (BELMass, MicrotracBEL) and a gas chromatograph (GC-2014, Shimadzu, Japan) equipped with thermal conductivity detectors. The reactant gas containing 1 vol % CH4, 1 vol % CO2, and Ar for balance was introduced into the reactor at a space velocity of 100 cm3 min–1 (W/F = 0.06 g s cm–3). The calculation details for the DRM performance are as followswhere [...]in and [...]out represent the gas concentrations in the feed gas and effluent gas, respectively.

FTIR spectra of the catalyst surfaces were measured at the operating temperature using a JASCO 6100 FTIR system equipped with a heat chamber (ST-Japan). Each sample (5 mg) was loaded onto the sample stage, and the reactant gas containing 1 vol % CH4, 1 vol % CO2, and Ar for balance was introduced into the environmental cell at a rate of 10 cm3 min–1. The concentrations of the feed gas and generated gas components were determined using micro gas chromatography (Inficon, 3000 Micro-GC).

Surface Area Measurements

The Brunauer–Emmett–Teller (BET) surface areas of the samples were measured at 77 K using a BELSORP-MAX II (MicrotracBEL Japan, Inc.). Each sample was heated at 80 °C under vacuum for 24 h prior to measurement, and the mass of each sample was measured using a balance.