A low temperature organic synthesis of monodispersed NiRu nanocrystals for CO₂ methanation

Abstract

In this study, monodispersed NiRu nanocrystals with a diameter of 3 nm were synthesized at 90 °C via a tuning hot-inject method to lower the temperature of the organic phase synthesis of monodispersed nanomaterials. The key factor for the nanocrystalline formation of NiRu alloy nanocrystals was summarized in detail. Simultaneously, the synergistic effect of Ni and Ru in CO₂ methanation was explored. Doping trace Ru can significantly improve the conversion rate of CO₂ methanation and CH₄ selectivity. The underlying mechanism was studied in detail via X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature-programmed hydrogen reduction (H₂-TPR) and desorption (H₂-TPD) tests, and temperature-programmed desorption of CO₂ (CO₂-TPD). This study gives out a new way for the general synthesis of monodisperse nickel-based nanocrystals and provides a reference for the development and application of monodispersed nanoparticles for CO₂ methanation.

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1. Introduction

Carbon dioxide (CO₂) is a well-known greenhouse gas, which has led to a global climate change.^9–12 CO₂ capture and recovery is an effective method to deal with the increasing greenhouse effect.^13–15 CO₂ methanation for producing hydrocarbons is one of the effective ways of converting waste into resources and energy to solve the problem of CO₂ emissions caused by environmental and energy crisis.^16–20 Besides, CO₂ methanation could be a good route for hydrogen storage and transportation, which could make full use of green hydrogen produced by renewable energy, such as solar energy or by industrial water gas reaction,^9,21,22 contributing to the further realization of a low-carbon society in the future. A lot of research has already been done on CO₂ methanation using supported nickel-based catalysts due to their low cost and easy availability.^{9,17,18,23–27} The addition of a second metal is an effective way to enhance the stability and catalytic activity of the nickel-based catalysts. Ruthenium is identified as more active in CO_x methanation, providing a larger range of catalytic reaction temperature compared to nickel, but is also considerably more expensive. Thus, it is necessary to investigate the CO₂ methanation performance of NiRu bimetallic nanocatalysts, which is likely to obtain better catalytic activity.

In the traditional catalyst preparation system, due to the limitations of synthetic methods and technologies, catalysts have numerous complex and diverse interfacial forms. For example, the existence of different grain sizes and complex surface morphology of the carrier makes it difficult to create a single regulatory variable process in the catalyst development, which is not conducive to establish the relationship between the microscopic size and the catalytic reaction at the mechanism level. The monodispersed nanoparticles can well solve this problem. The bulk-building material of monodispersed nanoparticles (generally sub-10 nm in size) with highly structured morphology and concentrated size distribution provides an ideal basis for studying catalytic reactions. In the traditional monodispersed Ni-based bimetal composite magnetic nanoparticles, most of them are prepared using oleylamine and oleic acid surface adsorbents^1,2 via the high-temperature pyrolysis of precursors.^3,4 However, in this synthesis system, as the synthesis temperature is generally higher than 150 °C,^5,6 it is difficult to achieve large-scale and high yield preparation. Therefore, it is of great significance to develop the synthesis system of medium and low-temperature monodispersed nickel-based nanoparticles. In the monodisperse nickel-based nanoparticle synthesis system, the precursor used for pyrolysis usually from carbonyl compounds is expensive and made up of special complexes.⁷ The high cost and high toxicity make it difficult to popularize this method. Therefore, it is necessary to explore synthesis methods for monodispersed nickel-based materials with a harmless and low-cost precursor. To prepare nickel-based monodisperse alloy nanoparticles, it is necessary to overcome the difficulties of different reduction potentials of different precursors. With the traditional high-temperature pyrolysis method, it is difficult to prepare alloy nanoparticles with uniform distribution of elements, but it is easier to form core–shell structures.⁸

As a result, a tuning hot-inject method was improved in order to reduce the temperature of the organic phase synthesis of monodispersed NiRu nanocrystals. The mechanism is shown in Scheme 1. NaBH₄ was introduced to achieve the co-reduction of Ni and Ru rather than via the pyrolysis of organometallic precursors method at a high temperature (>150 °C). DMF was introduced to serve as a solvent for NaBH₄ due to the limited solubility of NaBH₄ in oleylamine. As a result, when injecting the DMF solution of NaBH₄ into the oleylamine solution of Ni(acac)₂ and RuCl₃, NaBH₄ will quickly diffuse into the oleylamine solvent owing to the excellent solubility of DMF. With the release of hydrogen protons, Ni and Ru atoms are rapidly reduced and enter the process of common nucleus detonation. After that, the co-nucleated NiRu NPs will be covered by oleylamine molecules that inhibit the agglomeration of nanoparticles at the nuclear growth stage.

Scheme 1 Schematic of the preparation procedure of monodispersed NiRu NPs for CO₂ methanation.

In summary, Ni and Ru achieve the simultaneous reduction despite their high reducing potential difference because of the high reducing properties of NaBH₄ in our proposed synthetic system. Monodispersed NiRu alloy nanoparticles with uniform elemental and particle size distributions were obtained and loaded onto an Al₂O₃ support as an ideal-building bulk material for CO₂ methanation.^28,29 The NiRu/Al₂O₃ catalysts show enhanced performance than that reported in the existing literature (Table S1†) and exhibit obvious synergistic effects.

2. Experimental

2.1 Materials

All chemicals were used as-received without any further purification: nickel(II) acetylacetonate (Ni (acac)₂, 95%, Aladdin), oleylamine (C₁₈H₃₇N, 80–90%, Aladdin), ruthenium(III) chloride anhydrous (RuCl₃, 45–55% Ru content, Aladdin), N,N-dimethylformamide (DMF, 99.5%, Sinopharm Chemical Reagent), ethanol (C₂H₆O, ≥99.5%, Aladdin), cyclohexane (C₆H₁₂, 99.9%, Sinopharm Chemical Reagent), acetone (C₆H₁₂, 99.9%, Sinopharm Chemical Reagent).

2.2 Preparation of NiRu NPs

NiRu NPs (Ni/Ru atomic ratio of Ni₁Ru₀, Ni_0.95Ru_0.05, Ni_0.9Ni_0.1, Ni_0.8Ru_0.2 = 10 : 0, 9.5 : 0.5, 9 : 1, 8 : 2) were prepared by a tuning hot-inject method. Taking Ni_0.9Ni_0.1 NPs as an example, 0.9 mmol Ni(acac)₂ and 0.1 mmol RuCl₃ were dissolved in 15 mL oleylamine and transferred into a 50 mL three-necked flask, and then heated in an oil bath at 90 °C under N₂ atmosphere for 1 h. 100 mg NaBH₄ dissolved in 2 mL DMF was pre-heated in a 70 °C oven for 10 min was injected into the above solution. Numerous bubbles were released and the solution turned black immediately. After 1 h, the three-mouth flask was lifted from the oil bath and cooled to room temperature. After that, cyclohexane and acetone were added to the product solution and centrifuged for washing. The process was repeated five times to remove oleylamine as far as possible from the surface, and black solid particles were obtained. Finally, the colloidal particles were dissolved in 10 mL cyclohexane for further use. To explore the influence of temperature. Ni₁Ru₀ nanocrystals were synthesized at 80 °C, 90 °C, 100 °C, and 110 °C, respectively, as an example.

2.3 Preparation of 10 wt% NiRu/Al₂O₃

γ-Al₂O₃ was synthesized by the hydrothermal synthesis method based on the reported literature.³⁰ 60 mL H₂O, 7.5 g aluminum nitrate and 6 g urea were transformed into a 100 mL Teflon-lined stainless-steel autoclave, which was sealed and maintained at 120 °C for 24 h. After cooling to room temperature, the samples were washed 3 times and dried for 6 h in an air atmosphere of 70 °C. The dried samples were calcined at 550 °C for 6 h, and the temperature was lowered to room temperature to obtain a final willow leaf-like γ-Al₂O₃ product for further use. γ-Al₂O₃ was dispersed in 40 mL cyclohexane and stirred evenly via ultrasonication. Then, 10 mL of the Ni_xRu_(1−x) cyclohexane solution obtained in the previous experimental step was poured into the solution. A uniform brown color was obtained, and the solution was ultrasonically stirred for 2 h. After standing for 30 min, the solution was separated into two layers. The upper layer was transparent cyclohexane, and the lower layer was loaded with Ni_xRu_(1−x)/γ-Al₂O₃ flocculation, indicating that the Ni_xRu_(1−x) colloid particles that were originally soluble in cyclohexane were completely loaded onto the γ-Al₂O₃ carrier. After the loading was completed, the suspension was centrifuged once again at a low rotation speed (8000 rpm, 3 min), dried in a 70 °C oven for 4 h.

2.4 Catalytic experiments

CO₂ methanation catalytic activity was evaluated in an 8 mm fixed-bed steel tube microreactor at atmospheric pressure. 0.1 g catalyst powder was mixed with 0.2 g quartz sand (60–70 mesh sieved) and then reduced at 350 °C for 2 h in 25 mL min⁻¹ H₂ flow before the run. Next, the sample was cooled from 350 to 200 °C. Following the catalyst pre-treatment, the reactant gas containing 15% CO₂: 60% H₂: 25% N₂ was passed through the reactor bed at a total flow rate of 26.67 mL min⁻¹ (WHSV of ca. 12 000 mL h⁻¹ g⁻¹). In order to explore the effects of Ru doping on NiRu/Al₂O₃, the catalytic performance of the NiRu/Al₂O₃ catalyst with Ru contents of 0%, 5%, 10%, and 20% were investigated under the condition of 200–400 °C temperature at atmospheric pressure (each sampling temperature point was stable for 1 h to obtain a relatively stable data). Product analysis was performed with Varian CP-3800 chromatography equipped with a TCD detector.

2.5 Characterization

Transmission Electron Microscopy (TEM) was carried out on a Hitachi HT7700 with an accelerating voltage of 120 kV (10 microliters of NiRu NPs in cyclohexane solution were dropped onto the carbon support film and dried at room temperature). The crystallinity of catalysts was determined by X-ray diffraction (XRD) using X'Pert Pro (Cu Kα radiation, 40 kV, 30 mA, λ = 1.5418 Å). In the H₂-TPR method, 50 mg of the samples were heated from 50 to 600 °C (10 °C min⁻¹ heating rate, 30 mL min⁻¹ 5% H₂/N₂). Before the TPR measurement, each sample was flushed with He flow at 200 °C for 60 min and cooled to 30 °C. In the H₂-TPD method, 50 mg of the samples were pretreated at 350 °C under 5% H₂/He for 2 h, purged with He for 0.5 h, and then cooled to 50 °C to adsorb H₂ (50 m min⁻¹, 40 min). After they were purged with He for 30 min, the samples were heated to 800 °C at a rate of 10 °C min⁻¹. In the CO₂-TPD method, 50 mg of the samples were reduced at 350 °C under 5% H₂/He for 30 min, treated with He flow at 350 °C for 15 min, and then cooled to 100 °C. CO₂ (50 mL min⁻¹) was adsorbed on the samples for 1 h at 100 °C and cooled to 50 °C. The temperature was then raised to 500 °C at a heating rate of 10 °C min⁻¹ under He flow. N₂ adsorption and desorption isotherms were measured by an automatic physical adsorption analyzer (Micromeritics ASAP2020). The samples were degassed at 150 °C for 2 h to remove physically absorbed water and impurities on the surface before the measurement. The specific surface area was calculated by the Brunauer–Emmett–Teller (BET) method and pore size distribution was analyzed by the Barrett–Joyner–Halenda (BJH) method. XPS spectra of the catalysts were obtained using a KRATOS AXIS-NOVA (Shimadzu Corporation) instrument with a monochromatic Al-Kα as an X-ray source. The binding energy shift caused by the relative surface charging was corrected using the C 1s level as an internal standard. ICP-OES was performed to analyse the metal content of NiRu/Al₂O₃ catalysts.

3. Results and discussion

3.1 Standard characterization

3.1.1 TEM analysis. As shown in Fig. 1 a–d, the monodisperse Ni_xRu_(1−x) with different Ru doping contents was successfully synthesized, and the particle diameter was about 2.5 nm. In contrast, Ni₁Ru₀ shows a bigger particle size than others and is easier to agglomerate according to the TEM results; while Ru doping in the synthesis process reduces the diameter of the size of the nanoparticles (Fig. 1h) and significantly improves the dispersion and uniformity of nanocrystals. The experimental results showed that after the introduction of Ru, the generation rate of hydrogen improved when the DMF solution containing NaBH₄ was injected into the oleylamine solution containing the NiRu precursor. This indicates that the introduction of Ru was likely to promote the decomposition of NaBH₄ and accelerate the release of reducing hydrogen protons, then promoting the nucleation process and thus leading to a more concentrated nanometer size of Ni_xRu_(1−x) doped by Ru. This synthesis system covers a wide temperature window. To investigate the influence of temperature during the synthesis process of NiRu nanoparticles, Ni₁Ru₀ nanocrystals were synthesized at different temperatures ranging from 80 to 110 °C. According to the TEM results (Fig. 1e–h), the agglomeration of nickel nanoparticles was more severe at low temperature (80 °C). The diameter of nanoparticles decreases from 2.7 nm to 2.3 nm with the increase in the synthesis temperature (80–100 °C), but the particle size increased to 3 nm when the synthesis temperature was 110 °C. The reason is that temperature rising (below 100 °C) can increase the diffusion rate of NaBH₄ at DMF and promote the release of hydrogen protons, which increased the nucleation concentration leading to a smaller nanoparticle diameter. However, when the temperature is above 100 °C, Ostwald ripening will be the rate determining step for the diameter of the nanoparticles.

Fig. 1 TEM results of (a–d) Ni₁Ru₀, Ni_0.95Ru_0.05, Ni_0.9Ru_0.1 and Ni_0.8Ru_0.2 nanoparticles at a synthetic temperature of 90 °C (scan bar: 100 nm). (d–g) Ni₁Ru₀ nanoparticles at synthesis temperatures of 80, 100 and 110 °C. (h) Particle size of Ni₁Ru₀ nanoparticles at different synthesis temperatures.

3.1.2 XRD analysis. X-ray diffraction (XRD) patterns of NiRu NPs are shown in Fig. 2a. Ni₁Ru₀, Ni_0.95Ru_0.05, Ni_0.9Ru_0.1 and Ni_0.8Ru_0.2 all show characteristic peaks of Ni (JCPDS 47-1049) and NiO (JCPDS 4-0850), but no characteristic peaks of Ru. It indicated that NiRu nanocrystals exhibit the nickel crystal structure. Among them, Ni₁Ru₀ and Ni_0.95Ru_0.05 showed significant NiO diffraction peaks at positions 37°, 43° and 63°, which should be caused by the oxidation of Ni on the surface of Ni₁Ru₀ and Ni_0.95Ru_0.05 by oxygen present in the air during the drying process. In contrast, Ni_0.9Ru_0.1 and Ni_0.8Ru_0.2 showed almost pure Ni characteristic peaks with no diffraction peaks of NiO. XRD results of NiRu/Al₂O₃ are shown in Fig. 2b; only a weak diffraction peak of the Al₂O₃ support can be observed due to the very small particle size of NiRu NPs. The XRD patterns of NiRu/Al₂O₃ after H₂ reduction showed almost the same peak position and intensity compared with NiRu/Al₂O₃ even after the catalytic performance evaluation, indicating that there were no crystal changes during the CO₂ methanation process. Ni₁Ru₀/Al₂O₃ showed an obvious face-centered cubic (hcp) Ni crystal structure (JCPDS no. 45-1027). The intensity of the Ni diffraction peak decreases with the increase in the ruthenium introduction, which might be because Ru doping could inhibit the NiRu nanocrystal growth under high-temperature CO₂ methanation conditions.

Fig. 2 XRD results of (a) NiRu NPs with Ar treatment at 300 °C, (b) NiRu/Al₂O₃, (c and d) NiRu/Al₂O₃ after H₂ reduction and after the catalytic performance test.

3.2 CO₂ methanation catalytic activity study

Fig. 3 shows CO₂ conversion and CH₄ selectivity of NiRu/Al₂O₃ catalysts as a function of the reaction temperature. All of the NiRu/Al₂O₃ shows better performance than Ni/Al₂O₃ catalysts. The CH₄ selectivity was significantly improved for NiRu/Al₂O₃ with almost 100% CH₄ selectivity, particularly at low temperature (<350 °C) compared with that for Ni/Al₂O₃. Ni_0.9Ru_0.1/Al₂O₃ and Ni_0.95Ru_0.05/Al₂O₃ showed almost similar CO₂ conversion and CH₄ selectivity when the temperature was below 300 °C. Ni_0.9Ru_0.1/Al₂O₃ shows better performance at 325 °C and achieves the highest CO₂ conversion (89.7%) at 350 °C, superior to that of most reported nickel-based CO₂ methanation catalysts (Table S1†). Comparative tests were conducted to illustrate the key role of the DMF solvent for NaBH₄ and the effect of the low solubility of NaBH₄ in oleylamine. DMF and 1 : 1 volume ratio DMF : oleylamine were two contrasting solvents for NaBH₄. NaBH₄ completely dissolved in DMF. By contrast, some crystals precipitated and could be observed in the oleylamine and DMF solvent mixture. According to the catalytic performances of the catalysts synthesized under two different solvents for NaBH₄, as shown in Fig. 3 a–d, NiRu/Al₂O₃ with DMF as a solvent for NaBH₄ showed better performance than that prepared in the oleylamine and DMF solvent mixture. Therefore, DMF plays a crucial role in this synthesis system. When the DMF solution of NaBH₄ was injected into the oleylamine solution of Ni(acac)₂ and RuCl₃, the DMF solution of NaBH₄ would rapidly diffuse into the oleylamine solvent and rapidly dissolve. Therefore, NaBH₄ was decomposed and Ni(acac)₂ and RuCl₃ were reduced before their precipitation. After that, NiRu NPs were coated by oleylamine and entered the nuclear growth stage. Finally, the monodispersed NiRu alloy NPs were obtained.

Fig. 3 (a) CO₂ conversion rate curve, (b) CH₄ selectivity curve, (c) CO selectivity curve, (d) CH₄ yield curve of NiRu/Al₂O₃ at different temperatures. (e–h) The catalytic performance of CO₂ methanation Ni_xRu_(1−x)/γ-Al₂O₃ prepared with different solutions of NaBH₄.

3.3 TPR, TPD, N₂ adsorption and desorption isotherms, XPS analyses

H₂-TPR, H₂-TPD, CO₂-TPD, BET characterization and XPS studies were carried out to study the reasons for the improved performance of CO₂ methanation on Ni_0.9Ru_0.1/Al₂O₃ compared with Ni/Al₂O₃. Fig. 4a shows the H₂-TPR profiles of Ni₁Ru₀/Al₂O₃ and Ni_0.9Ru_0.1/Al₂O₃, it can be observed that there were three main peaks centered at 144–205 °C (peak A), 205–284 °C (peak B) and 284–387 °C (peak C) for Ni_0.9Ru_0.1/Al₂O₃. In contrast, only two main peaks centered at 123–192 °C (peak D), 200–346 °C (peak E) and a weak peak centered at 73.6 °C were detected for Ni₁Ru₀/Al₂O₃. According to the H₂-TPR results, the peak located at 73.6 °C should be attributed to oxygen adsorption on the surface of Ni NPs or the surface of NiO, and peaks A and D should be attributed to NiRuO_x strongly interacting with Al₂O₃.³¹ It matched with the XRD results (Ni₁Ru₀ can be easily oxidized under the air condition). Peak C and F are attributed to the subsurface oxygens of Al₂O₃. It is worth noting that Ni_0.9Ru_0.1/Al₂O₃ has an extra peak B. We believe that the peak B should be attributed to the weakly interacting NiRuO_x and the H-spillover effects. For Ni_0.9Ru_0.1/Al₂O₃, the NiRu site is tightly bound to Al₂O₃ and will be reduced first, and the weakly interacting NiRuO_x will then be reduced. After that, the hydrogen overflow happens on the reduced NiRu surface and reacted with the subsurface oxygens. It can explain why the area of peak C is smaller than that of peak E, and consistent with the H₂-TPD results (Ni_0.9Ru_0.1/Al₂O₃ shows higher temperature location of hydrogen desorption peaks at 443.9 °C compared to the 437.3 °C peak of Ni₁Ru₀/Al₂O₃). The hydrogen desorption quality was calculated from the areas of the corresponding desorption peaks. Ni_0.9Ru_0.1/Al₂O₃ showed a higher H₂ desorption quantity (21.795 cm³ g⁻¹) compared to that by Ni₁Ru₀/Al₂O₃ (7.636 cm³ g⁻¹), which indicates that Ni_0.9Ru_0.1/Al₂O₃ has stronger hydrogen adsorption capability than Ni₁Ru₀/Al₂O₃. Fig. 4c shows the CO₂-TPD profile of Ni₁Ru₀/Al₂O₃ and Ni_0.9Ru_0.1/Al₂O₃. Ni_0.9Ru_0.1/Al₂O₃ shows a huge distinctive CO₂-TPD peak compared with Ni₁Ru₀/Al₂O_3. Two desorption peaks at 200–350 °C and 350–450 °C can be observed for Ni₁Ru₀/Al₂O₃ that correspond to weak basic sites and medium-strength basic sites. In contrast, only one desorption peak at 350–500 °C, corresponding to medium-strength basic sites was detected for Ni₁Ru₀/Al₂O₃. In CO₂ methanation reactions, medium-strength basic sites are more active than the weak basic site. Ni_0.9Ru_0.1/Al₂O₃ also showed higher CO₂ desorption quantity (1068 cm³ g⁻¹) than Ni₁Ru₀/Al₂O₃ (405 cm³ g⁻¹), which indicated that Ni_0.9Ru_0.1/Al₂O₃ has better CO₂ chemisorption than that by Ni₁Ru₀/Al₂O₃. Ni₁Ru₀/Al₂O₃ and Ni_0.9Ru_0.1/Al₂O₃ show similar N₂ adsorption and desorption isotherms displaying the IV type isotherms with an apparent hysteresis loop according to the IUPAC classification, the type H₂ hysteresis loop appears in the region of 0.4–0.7p/p_0, indicating a narrow mesopore size distribution. In addition to the high specific surface area, Ni_0.9Ru_0.1/Al₂O₃ shows approximately the same physical parameters compared with Ni₁Ru₀/Al₂O₃; therefore, the synergistic effects is the core factor affecting the catalytic performance (Table 1). XPS characterization was performed to acquire more information on the surface species of the catalysts that completed the CO₂ methanation activity test. In the Ni 2p XPS spectra (Fig. 4f), the peaks located at 855.6, 873.6 eV can be assigned to Ni²⁺ 2p_3/2 and 2p_1/2, respectively, and the peak at 852.5 eV belongs to Ni⁰. Compared to Ni₁Ru₀/Al₂O₃, the Ni 2p_3/2 peak of Ni²⁺ largely increased for Ni_0.9Ru_0.1/Al₂O₃.

Fig. 4 (a) H₂-TPR, (b) H₂-TPD, (c) CO₂-TPD profile of NiRu/Al₂O₃, (d) N₂ adsorption–desorption isotherms of NiRu/Al₂O₃. (e) Survey scan. (f) High-resolution Ni 2p spectra. (g) High-resolution Al 2p spectra. (h) High-resolution O 1s spectra.

Table 1 Physical properties of Ni₁Ru₀/Al₂O₃ and Ni_0.9Ru_0.1/Al₂O₃

a Calculated from the BET equation.b BJH desorption pore volume.c BJH desorption average pore diameter.d Calculated from ICP-OES results.
Catalyst	Ni1Ru0/Al2O3	Ni0.9Ru0.1/Al2O3
SBET^a/(m^2 g^−1)	336.2	360.9
Vp^b (m^3 g^−1)	0.575	0.576
Dp^c (nm)	3.38	3.38
Ni^d (wt%)	9.95	8.96
Ru^d (wt%)	0	1.003

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3.4 CO₂ methanation scheme of NiRu/Al₂O₃

The possible pathways for CO₂ methanation are summarized in Fig. S1,† and the kinetics reaction of CO₂ methanation was found to follow the direct C–O bond cleavage pathway (Fig. 5) for Ni/γ-Al₂O₃ catalysts, in which the rate-limiting step was the subsequent dissociation of adsorbed *CO.³³ Surface metallic Ni can be oxidized by CO₂,³² and we believe that the Ru site easily absorbs CO and neighbouring Ni site adsorbs dissociated O during the CO₂ decompose process for the NiRu crystal surface. It can be supported from CO₂ TPD s and XPS results (more Ni²⁺ content). Alternatively, Ru doping promotes the CO₂ decomposition, which leads to improved CO₂ methanation activity. On the other hand, the rate-limiting step for CO production through the reverse water gas shift reaction was identified as *CO desorption.³⁴ Ru has a stronger adsorption capacity to *CO, which inhibits the desorption of the *CO, leading to enhanced CH₄ selectivity for the NiRu/Al₂O₃ catalyst. In summary, the addition of Ru promotes the dissociation of carbon dioxide and inhibits the formation of CO. The acceleration of dissociated hydrogen species promotes the production of methane and finally makes the catalyst show a better catalytic performance of the methanation.

Fig. 5 CO₂ methanation scheme on NiRu/Al₂O₃.

4. Conclusions

The monodispersed NiRu nanocrystals were prepared at a low temperature by modified heat injection with NaBH₄ as the reducing agent and loaded onto Al₂O₃ to be an ideal building block for CO₂ methanation. Affected by the compositional effect, Ni_0.9Ru_0.1/Al₂O₃ exhibits the best performance for both CO₂ conversion and CH₄ selectivity. The underlying mechanism was carefully studied by XRD, ICP, H₂-TPR, H₂-TPD, CO₂-TPD, BET, and XPS. All the characterization results indicate that Ru doping is an effective means to improve the performance of the Ni-based CO₂ methanation catalyst, which could promote hydrogen adsorption, desorption and H-spillover effects, and promote CO₂ chemisorption. This work has put forward a new strategy for the synthesis of monodispersed nickel-based nanocrystals and provides a fresh idea for the development and application of precious metals in nickel-based catalysts for CO₂ methanation.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (NSFC) (General Program No. 21571012).

Notes and references

1. O. Metin, V. Mazumder, S. Ozkar and S. Sun, Monodisperse nickel nanoparticles and their catalysis in hydrolytic dehydrogenation of ammonia borane, J. Am. Chem. Soc., 2010, 132(5), 1468–1469.
2. A. P. LaGrow, B. Ingham, S. Cheong, G. V. M. Williams, C. Dotzler, M. F. Toney, D. A. Jefferson, E. C. Corbos, P. T. Bishop, J. Cookson and R. D. Tilley, Synthesis, Alignment, and Magnetic Properties of Monodisperse Nickel Nanocubes, J. Am. Chem. Soc., 2012, 134(2), 855–858.
3. Q. Li and S. Sun, Recent advances in the organic solution phase synthesis of metal nanoparticles and their electrocatalysis for energy conversion reactions, Nano Energy, 2016, 29, 178–197.
4. A. P. LaGrow, B. Ingham, M. F. Toney and R. D. Tilley, Effect of Surfactant Concentration and Aggregation on the Growth Kinetics of Nickel Nanoparticles, J. Phys. Chem. C, 2013, 117(32), 16709–16718.
5. M. Shviro and D. Zitoun, Tip Enhanced Silver Growth on Shaped Controlled Nickel Nanocrystals, J. Phys. Chem. C, 2014, 118(19), 10455–10462.
6. K. E. Marusak, A. C. Johnston-Peck, W.-C. Wu, B. D. Anderson and J. B. Tracy, Size and Composition Control of CoNi Nanoparticles and Their Conversion into Phosphides, Chem. Mater., 2017, 29(7), 2739–2747.
7. M. Heilmann, H. Kulla, C. Prinz, R. Bienert, U. Reinholz, A. Guilherme Buzanich and F. Emmerling, Advances in Nickel Nanoparticle Synthesis via Oleylamine Route, Nanomaterials, 2020, 10(4), 713.
8. H. Lv, Z. Xi, Z. Chen, S. Guo, Y. Yu, W. Zhu, Q. Li, X. Zhang, M. Pan, G. Lu, S. Mu and S. Sun, A New Core/Shell NiAu/Au Nanoparticle Catalyst with Pt-like Activity for Hydrogen Evolution Reaction, J. Am. Chem. Soc., 2015, 137(18), 5859–5862.
9. M. Cai, J. Wen, W. Chu, X. Cheng and Z. Li, Methanation of carbon dioxide on Ni/ZrO₂-Al₂O₃ catalysts: effects of ZrO₂ promoter and preparation method of novel ZrO₂-Al₂O₃ carrier, J. Nat. Gas Chem., 2011, 20(3), 318–324.
10. H. Liu, X. Zou, X. Wang, X. Lu and W. Ding, Effect of CeO₂ addition on Ni/Al₂O₃ catalysts for methanation of carbon dioxide with hydrogen, J. Nat. Gas Chem., 2012, 21(6), 703–707.
11. T. A. Le, J. Kim, Y. R. Jeong and E. D. Park, CO₂ Methanation over Ni/Al@MAl₂O₄ (M = Zn, Mg, or Mn) Catalysts, Catalysts, 2019, 9(7), 599.
12. Z.-H. Huang, F.-F. Sun, M. Batmunkh, W.-H. Li, H. Li, Y. Sun, Q. Zhao, X. Liu and T.-Y. Ma, Zinc–nickel–cobalt ternary hydroxide nanoarrays for high-performance supercapacitors, J. Mater. Chem. A, 2019, 7(19), 11826–11835.
13. A. Beuls, C. Swalus, M. Jacquemin, G. Heyen, A. Karelovic and P. Ruiz, Methanation of CO₂: further insight into the mechanism over Rh/γ-Al₂O₃ catalyst, Appl. Catal., B, 2012, 113–114, 2–10.
14. E. H. Cho, W. Kim, C. H. Ko and W. L. Yoon, Enhanced CO₂ Methanation Reaction in C₁ Chemistry over a Highly Dispersed Nickel Nanocatalyst Prepared Using the One-Step Melt-Infiltration Method, Catalysts, 2020, 10(6), 643.
15. J. B. Branco, R. P. da Silva and A. C. Ferreira, Methanation of CO₂ over Cobalt-Lanthanide Aerogels: Effect of Calcination Temperature, Catalysts, 2020, 10(6), 704.
16. T. Sakakura, J.-C. Choi and H. Yasuda, Transformation of Carbon Dioxide, Chem. Rev., 2007, 107(6), 2365–2387.
17. I. Rossetti, C. Biffi, C. L. Bianchi, V. Nichele, M. Signoretto, F. Menegazzo, E. Finocchio, G. Ramis and A. Di Michele, Ni/SiO₂ and Ni/ZrO₂ catalysts for the steam reforming of ethanol, Appl. Catal., B, 2012, 117–118, 384–396.
18. H. C. Wu, Y. C. Chang, J. H. Wu, J. H. Lin, I. K. Lin and C. S. Chen, Methanation of CO₂ and reverse water gas shift reactions on Ni/SiO₂ catalysts: the influence of particle size on selectivity and reaction pathway, Catal. Sci. Technol., 2015, 5(8), 4154–4163.
19. S. Tada, T. Shimizu, H. Kameyama, T. Haneda and R. Kikuchi, Ni/CeO₂ catalysts with high CO₂ methanation activity and high CH₄ selectivity at low temperatures, Int. J. Hydrogen Energy, 2012, 37(7), 5527–5531.
20. Y. Guo, S. Mei, K. Yuan, D.-J. Wang, H.-C. Liu, C.-H. Yan and Y.-W. Zhang, Low-Temperature CO₂ Methanation over CeO₂-Supported Ru Single Atoms, Nanoclusters, and Nanoparticles Competitively Tuned by Strong Metal–Support Interactions and H-Spillover Effect, ACS Catal., 2018, 8(7), 6203–6215.
21. Q. Zhang, Z. Yang, B. Chen and X. Liang, Phase-competition-driven formation of hierarchical FeNiZn-MIL-88B-on-MOF-5 octapods displaying high selectivity for the RWGS reaction, Chem. Commun., 2019, 55(58), 8450–8453.
22. R.-P. Ye, Q. Li, W. Gong, T. Wang, J. J. Razink, L. Lin, Y.-Y. Qin, Z. Zhou, H. Adidharma, J. Tang, A. G. Russell, M. Fan and Y.-G. Yao, High-performance of nanostructured Ni/CeO₂ catalyst on CO₂ methanation, Appl. Catal., B, 2020, 268, 118474.
23. G. Du, S. Lim, Y. Yang, C. Wang, L. Pfefferle and G. L. Haller, Methanation of carbon dioxide on Ni-incorporated MCM-41 catalysts: the influence of catalyst pretreatment and study of steady-state reaction, J. Catal., 2007, 249(2), 370–379.
24. G. Garbarino, P. Riani, L. Magistri and G. Busca, A study of the methanation of carbon dioxide on Ni/Al₂O₃ catalysts at atmospheric pressure, Int. J. Hydrogen Energy, 2014, 39(22), 11557–11565.
25. Z. Li, B. Li, Z. Li and X. Rong, The promoter action of CeO₂ for the Ni/Al₂O₃-catalyzed methanation of CO₂, Kinet. Catal., 2015, 56(3), 329–334.
26. V. Shanmugam, S. Neuberg, R. Zapf, H. Pennemann and G. Kolb, Effect of Support and Chelating Ligand on the Synthesis of Ni Catalysts with High Activity and Stability for CO₂ Methanation, Catalysts, 2020, 10(5), 493.
27. L. Wang, J. Hu, H. Liu, Q. Wei, D. Gong, L. Mo, H. Tao and C. Zhang, Three-Dimensional Mesoporous Ni–CeO₂ Catalysts with Ni Embedded in the Pore Walls for CO₂ Methanation, Catalysts, 2020, 10(5), 523.
28. H. Nagase, R. Naito, S. Tada, R. Kikuchi, K. Fujiwara, M. Nishijima and T. Honma, Ru nanoparticles supported on amorphous ZrO₂ for CO₂ methanation, Catal. Sci. Technol., 2020, 10(14), 4522–4531.
29. A. A. A. Mohammed, M. A. H. S. Saad, A. Kumar and M. J. Al-Marri, Synthesis of fumed silica supported Ni catalyst for carbon dioxide conversion to methane, Greenhouse Gases: Sci. Technol., 2020, 10(4), 715–724.
30. S. Liu, X. Liang, J. Zhang and B. Chen, Temperature sensitive synthesis of γ-Al₂O₃ support with different morphologies for CoMo/γ-Al₂O₃ catalysts for hydrodesulfurization of thiophene and 4,6-dimethyldibenzothiophene, Catal. Sci. Technol., 2017, 7(2), 466–480.
31. J. Lin, C. Ma, J. Luo, X. Kong, Y. Xu, G. Ma, J. Wang, C. Zhang, Z. Li and M. Ding, Preparation of Ni based mesoporous Al₂O₃ catalyst with enhanced CO₂ methanation performance, RSC Adv., 2019, 9(15), 8684–8694.
32. C.-c. Yu, Y. Lu, X.-j. Ding and S.-k. Shen, Studies on Ni/Al₂O₃ catalyst for CO₂ reforming of CH₄ to synthesis gas—a combined research of TPD, TPPR and XPS, in Studies in Surface Science and Catalysis, ed. M. dePontes, R. L. Espinoza, C. P. Nicolaides, J. H.Scholtz and M. S. Scurrell, Elsevier, 1997, vol. 107, pp. 503–510.
33. S. Kattel, P. Liu and J. G. Chen, Tuning Selectivity of CO₂ Hydrogenation Reactions at the Metal/Oxide Interface, J. Am. Chem. Soc., 2017, 139(29), 9739–9754.
34. T. Avanesian, G. S. Gusmão and P. Christopher, Mechanism of CO₂ reduction by H₂ on Ru(0001) and general selectivity descriptors for late-transition metal catalysts, J. Catal., 2016, 343, 86–96.

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Footnote

† Electronic supplementary information (ESI) available: Literature comparison. See DOI: 10.1039/d0ra08562f

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