Hierarchically structured tetragonal zirconia as a promising support for robust Ni based catalysts for dry reforming of methane

Abstract

In this study, nanosheets-accumulating Laminaria japonica-like hierarchically structured tetragonal phase dominant zirconia (t-ZrO₂-lj) with high thermal stability was successfully synthesized for the first time by a facile hexamethylenetetramine–glucose assisted hydrothermal approach. The supported Ni catalyst on t-ZrO₂-lj with tetragonal phase dominant crystalline structure displays much superior activity and stability for synthesis gas production through dry reforming of methane with CO₂ in comparison with a Ni catalyst supported on monoclinic zirconia nanoparticulate (m-ZrO₂-np) prepared by a common hydrothermal method. By employing diverse characterization techniques, including X-ray diffraction (XRD), N₂ adsorption (BET), transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), H₂ temperature-programmed reduction (H₂-TPR), CO chemisorption, CO₂ temperature-programmed desorption (CO₂-TPD), and thermogravimetric analysis (TGA), the natures of the as-synthesized ZrO₂ and the supported Ni catalyst on t-ZrO₂-lj were unambiguously characterized. Correlating the nature of the catalyst to the reaction results, the higher activity of Ni/t-ZrO₂-lj catalyst compared to Ni/m-ZrO₂-np can be ascribed to higher Ni dispersion and better reducibility due to the unique morphology and microstructure of the zirconia support. More interestingly, the developed Ni/t-ZrO₂-lj catalyst demonstrates higher coke resistance during the dry reforming process in comparison with Ni/m-ZrO₂-np, resulting from the higher basicity of tetragonal phase ZrO₂ compared to monoclinic ZrO₂, which endows the developed Ni/t-ZrO₂-lj catalyst with greatly superior stability to Ni/m-ZrO₂-np for dry reforming of methane. This study provides a new avenue for fabricating robust coke-resistent Ni-based catalysts for synthesis gas production. Moreover, the as-synthesized t-ZrO₂-lj with high thermal stability has great potential for applications as an excellent support in diverse transformations, especially for high temperature reactions.

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1. Introduction

Coal and crude oil, as the main fossil fuels, are being depleted rapidly. Meanwhile, the ongoing increase of greenhouse gas emissions is associated with the utilization of coal and crude oil resources, which leads to severe consequences of climate change (e.g., global warming). These concerns have intensified the requirement for alternative sources of energy and chemical production and the reduction of greenhouse gas emissions.

Synthesis gas (H₂ and CO) production from natural gas, waste biomass and natural gas hydrate is being considered as a green and sustainable chemical technology to retard the depletion of crude oil and coal sources and also to create new opportunities for energy storage and conversion. Recently, drying (CO₂) reforming of methane (DRM) has attracted great attention because this process simultaneously consumes the two greenhouse gas (CH₄ and CO₂) and converts them into synthesis gas, which can be further transformed into various chemicals and fuels.^1–3

According to studies, noble metals are more active and less prone to deactivation by coking during the DRM reaction.^4–6 Unfortunately, the high cost and limited availability of precious metal catalysts restrict their large use in industrial applications. Ni-based catalysts have been considered as an ideal alternative. However, Ni is sensitive to deactivation due to carbon deposition and sintering on the active nickel metal after long-term catalytic processes.^7,8 To date, although numerous studies exist on the development of highly efficient Ni-based catalysts for dry reforming of methane, this process has not yet been industrially used for synthesis gas production. The deactivation by coke deposition remains a bottle-neck problem for its industrial application. Therefore, the development of a robust Ni-based catalyst with high stability has been attracting great attention in both academic and industrial aspects of catalysis.

To date, various strategies have been adopted either to avoid or minimize carbon deposition on Ni metal particles.^9–12 It was found that the DRM reaction is strongly dependent on support materials (e.g., acidic and basic properties,^13–16 morphology, crystal phase, and size^17–19). The depressed conversion with time on stream is correlated with the amount of coke deposition (originating from CH₄ decomposition and the Boudouard reaction) during the DRM reaction.²⁰ Therefore, for a good supported Ni catalyst, a bifunctional mechanism is required: CH₄ is activated on the Ni particles, while CO₂ dissociates on the support.¹² The carbon deposition on the catalyst surface depends on the oxidation of carbon species by the released oxygen from the CO₂ absorbed on the catalyst.^21–27 Thus, in order to retard carbon deposition, the enhancement of absorbed CO₂ on the catalyst is necessary. Therefore, much effort has focused on selecting different types of supports.

Recently, zirconia (ZrO₂) support has attracted tremendous attention, owing to its high thermal stability and unique acidic and basic properties.^28–32 The strong anchoring effects and partial activation of CO₂ by Zr⁴⁺ may efficiently enhance the DRM reaction. Therefore, ZrO₂ is a generally used support for this reaction.

ZrO₂ exits in three different structural polymorphs, including monoclinic (m-ZrO₂, room temperature to 1175 °C), tetragonal (t-ZrO₂, 1175 to 2370 °C), and cubic (c-ZrO₂, 2370 to 2680 °C).^33,34 The poor stability of cubic ZrO₂ at room temperature limits its wide use in catalysis in comparison with the monoclinic and tetragonal polymorphs. t-ZrO₂ has been demonstrated to be a superior support to m-ZrO₂ for diverse transformations, including the methanation reaction,³⁵ CO hydrogenation to methanol,³⁶ the water gas shift reaction, and alkylation of phenol.³⁷ Moreover, it was previously reported that t-ZrO₂ can be employed as a support for nickel catalysts in methane reforming. In the reforming process, t-phase ZrO₂ is more desirable than m- or c-phase.^38,39 Unfortunately, the preparation of t-ZrO₂ remains a challenge owing to its thermodynamic instability.

It has been reported that t-ZrO₂ stabilized by heteroatom oxide compounds such as Y₂O₃, CeO₂, La₂O₃ and K₂O can be synthesized.³⁴ However, the introduction of these metal oxides may lead to unwanted reaction products or a possible decrease in reaction activity.^34,40,41 Recently, an efficient method for preparing single-component t-ZrO₂ was developed.³⁰ However, the t-ZrO₂ synthesized by that method has poor thermal stability, and phase transformation from t-ZrO₂ to m-ZrO₂ occurred as it was subjected to heating at 700 °C. Generally, the dry reforming of methane is performed at higher temperatures owing to its endothermic features. Therefore, to satisfy the requirements for catalysts for dry reforming of methane (generally at 700 to 850 °C), the development of a facile and efficient method for synthesizing t-ZrO₂ with high thermal stability (700 to 850 °C) is highly desirable.

In the present work, we firstly synthesized nanosheets-accumulating Laminaria japonica-like hierarchically structured tetragonal phase dominant ZrO₂ (t-ZrO₂-lj) via a hydrothermal method using the low-cost and nontoxic inorganic compound ZrOCl₂·8H₂O as the Zr precursor, water as the solvent, and glucose and hexamethylenetetramine as structure-directing agents. More interestingly, the as-synthesized hierarchically structured t-ZrO₂ exhibits high thermal stability, and no phase transformation from t-phase to m-phase occurs as it undergoes heating at a high temperature of 850 °C, which satisfies the harsh high temperature requirement of dry reforming of methane. Furthermore, the supported Ni catalyst on t-ZrO₂-lj displays greatly superior activity and stability for synthesis gas production through dry reforming of methane with CO₂ in comparison with the supported Ni catalyst on monoclinic zirconia nanoparticulate (m-ZrO₂-np) prepared by a common hydrothermal method. More interestingly, the developed Ni/t-ZrO₂-lj catalyst demonstrates higher coke-resistance during the dry reforming process in comparison with Ni/m-ZrO₂-np. By employing various characterization techniques, the structure–performance relationship was revealed. The excellent catalytic stability of the developed Ni/t-ZrO₂-lj catalyst confers it with great potential for synthesis gas production through DRM reaction. Moreover, the high thermal stability of the Laminaria japonica-like hierarchical structure composed of tetragonal phase ZrO₂ nanosheets makes it an excellent support for other applications in high temperature reactions.

2. Experimental section

2.1 Synthesis of ZrO₂ and Ni-based catalysts

All chemicals (Sinopharm Chemical Reagent Co., Ltd) were used without further purification. In the preparation of t-ZrO₂-lj support, the desired amounts of ZrOCl₂·8H₂O and glucose were dissolved in de-ionized water under strong stirring, followed by adding hexamethylenetetramine aqueous solution to the above aqueous solution. Then, ammonia solution (28%) was added dropwise to the above solution until pH ∼ 11. After further stirring for 3 h, the mixture was transferred into a Teflon-lined autoclave and then was hydrothermally treated at 180 °C for 1 day. The resulting solid was recovered, washed, and then dried at 105 °C overnight. The final nanosheets-accumulating Laminaria japonica-like hierarchically structured tetragonal phase dominant zirconia is denoted as t-ZrO₂-lj. By using a similar process except for the absence of hexamethylenetetramine and glucose, the common ZrO₂ nanoparticulate was prepared and named m-ZrO₂-np.

The NiO/t-ZrO₂-lj and NiO/m-ZrO₂-np samples were prepared by the previously developed L-arginine ligand assisted incipient wetness impregnation method.⁴² The as-synthesized NiO/t-ZrO₂-lj and NiO/m-ZrO₂-np were reduced at 850 °C for 2 h under a mixture of 20% H₂ in N₂ at a flow rate of 30 ml min⁻¹ to obtain the reduced catalysts, which are denoted as Ni/t-ZrO₂-lj and Ni/m-ZrO₂-np, respectively.

2.2 Characterization of the ZrO₂ supports and catalysts

TEM images were obtained using a Tecnai F30 HRTEM instrument (FEI Corp.) at an acceleration voltage of 300 kV. The supports were dispersed into ethanol with ultrasonic treatment for 10 min, and a drop of the suspension was placed on a copper grid for TEM observation. FE-SEM experiments were performed on a JEOL JSM-5600LV SEM/EDX instrument.

Nitrogen adsorption and desorption isotherms were determined on a Beishide apparatus (model 3H-2000PS1) at −196 °C. The specific surface areas were calculated by the Brunauer–Emmett–Teller (BET) method; also, the mesopore size distributions were calculated from the adsorption branch of the isotherm BJH model.

XRD patterns were collected from 10 to 80° at a step width of 0.02° using a Rigaku Automatic X-ray Diffractometer (D/Max 2400) equipped with a CuKα source (λ = 1.5406 Å). The average crystallite sizes were estimated on the basis of the Scherrer formula over the multiple characteristic diffraction peaks using MDI Jade 5 software.

TGA analysis was conducted to study the amount of coke deposited on the spent Ni/t-ZrO₂-lj and Ni/m-ZrO₂-np catalysts using a Perkin-Elmer STA 6000 instrument with a heating rate of 10 °C min⁻¹ from 30 to 800 °C in an air stream.

The Ni dispersion of the two catalysts was measured by CO titration at 35 °C using Builder PCA-1200 Pulsar TPR/TPD equipment. The catalyst (∼100 mg) was reduced in situ with H₂ at 850 °C for 1.5 h, then flushed at 400 °C with He for 50 min. After pre-reduction, the catalyst was cooled to 30 °C and CO chemisorption was carried out. The total metal dispersion of Ni metal was calculated from eqn (1), where D_M, V_S, SF, SW and MW are the metal dispersion (%), volume of active gas chemisorbed (cm³ at STP), stoichiometry factor, metal weight (g), and molecular weight of metallic Ni (g mol⁻¹), respectively:³⁴

H₂-TPR experiments were performed in a Builder PCA-1200 automated system. 100 mg of catalyst was placed in a U-shape quartz tube in a temperature-controlled oven. The catalyst was first purged under 30 cm³ min⁻¹ of Ar flow at 300 °C for 30 min (using a ramp rate of 10 °C min⁻¹) and then cooled to room temperature. After that, the catalyst was reduced with a 10 vol% H₂–Ar mixture (30 ml min⁻¹) by heating to 800 °C at a ramp rate of 10 °C min⁻¹. The amount of hydrogen consumption was measured using a thermal conductivity detector.

CO₂-TPD was performed using the Builder PCA-1200 automated system. The catalyst was pre-reduced under a 10 vol% H₂–Ar mixture gas flow (30 ml min⁻¹) by ramping to 800 °C (10 °C min⁻¹). Then, the system was purged with He at 800 °C for 30 min and then cooled to RT. The adsorption of CO₂ was performed at 50 °C in a pure CO₂ (99.999%) flow with a rate of 30 ml min⁻¹, and the catalyst was purged with a He stream at 50 °C; then, the system was cooled to RT with a He stream until the baseline was steady. Finally, the CO₂-TPD was performed with a ramp of 10 ml min⁻¹ from RT to 800 °C in a 30 ml min⁻¹ He stream.

2.3 Catalytic performance tests

The catalytic performance measurement was performed in a quartz tube fixed-bed continuous reactor (6 mm O.D.) at atmospheric pressure. Typically, 60 mg of catalyst with 40 to 60 mesh particle size was loaded between two quartz wool plugs in the quartz tube. The temperature was measured using K-type thermocouples and controlled by a PID controller. Before the reaction, the catalyst was reduced at 850 °C for 2 h using a mixture of 20% H₂ in N₂ at a flow rate of 30 ml min⁻¹. The reaction feed contained CH₄, CO₂ and N₂ (N₂ was used as an internal standard gas) and was controlled by mass flow controllers. The N₂ was used an internal standard to calculate the CH₄ and CO₂ conversions. The effluent gas was passed through a trap and then analyzed using a gas chromatograph on-line with a molecular sieve column and a Porapaq Q column.

The conversions of CH₄ (X_CH4) and CO₂ (X_CO2) as well as the selectivities of CO (S_CO) and H₂ (S_H2) were calculated using eqn (2)–(5), as follows:

In eqn (2)–(5), F_CH4,in and F_CO2,in are the flow rates towards CH₄ and CO₂ in the feed, whereas F_CH4,out, F_CO2,out, F_H2,out, and F_CO,out are the flow rates corresponding to CH₄, CO₂, H₂, and CO in the effluent, respectively.

3. Results and discussion

3.1 Morphology and crystal structure of ZrO₂ supports

A panoramic view of the as-synthesized t-ZrO₂-lj and m-ZrO₂-np supports was evidenced by FE-SEM, as depicted in Fig. 1; the magnified SEM and TEM images of the developed t-ZrO₂-lj support are also included. As can be seen from Fig. 1a and c, the Laminaria japonica-shaped nanostructure of ZrO₂ was successfully synthesized by the developed hexamethylenetetramine–glucose assisted hydrothermal approach, whereas the ZrO₂ prepared by the common hydrothermal process features nanoparticulate aggregates or single particles (m-ZrO₂-np). Furthermore, from Fig. 1d, the Laminaria japonica-shaped nanostructures of ZrO₂ are composed of nanosheets with particle sizes of approximately 20 to 30 nm (the nanosheet structures can be further confirmed by the presence of slit-shaped pores, as demonstrated in the following BET results). The results demonstrate that the developed ZrO₂ is a hierarchically structured Laminaria japonica-shaped material. Then, the textural properties and crystal structures of the two ZrO₂ supports were investigated by performing N₂ adsorption–desorption and XRD analysis. Fig. 2 presents nitrogen adsorption–desorption isotherms and pore size distributions from the adsorption branch. From Fig. 2a, the isotherm corresponding to the t-ZrO₂-lj support is type IV with a H3 hysteresis loop at the relative pressure range of 0.4 to 0.9, indicating the possible presence of narrow slit-shaped mesopores which are generally associated with nanosheets;^43,44 this is consistent with the TEM observation (Fig. 1d). Compared with the t-ZrO₂-lj support, the isotherm corresponding to m-ZrO₂-np support shifts to a high relative pressure (P/P₀) range, suggesting the formation of large mesopores and macropores. The pore size distribution of the m-ZrO₂-np support in comparison with the t-ZrO₂-lj support can be presented as the inset in Fig. 2b, which results from the accumulation of ZrO₂ nanoparticulates. From Table 1, the notably larger pore volume of m-ZrO₂-np than that of t-ZrO₂-lj may result from the abovementioned accumulating pores. Moreover, it is as expected that the t-ZrO₂-lj support shows a slightly lower surface area (29 m² g⁻¹) than m-ZrO₂-np (33 m² g⁻¹) due to the larger size of the second structures.

Fig. 1 FE-SEM images of t-ZrO₂-lj (a and c) and m-ZrO₂-np (b) supports; TEM image of t-ZrO₂-lj (d). Inset in (d) is the magnified region.

Fig. 2 Nitrogen adsorption–desorption isotherms of the t-ZrO₂-lj (a) and m-ZrO₂-np (b) supports (insets: BJH pore size distribution from the adsorption branch and the specific surface area).

Table 1 Physicochemical properties of the t-ZrO₂-lj and m-ZrO₂-np supports

Support	SBET (m^2 g^−1)	Pore volume (cm^3 g^−1)	CSZrO2^a (nm)	Cell parameter^b (Å)
			550/850	a/b/c
a A comparison of average crystallite sizes of the as-synthesized ZrO2 subjected to calcination processes at 550 and 850 °C, estimated by the Scherrer equation.b Unit cell parameters calculated from the XRD results.
t-ZrO2-lj	29	0.08	14.8/15.5	5.10/5.10/5.25
m-ZrO2-np	33	0.30	12.1/21.9	5.15/5.21/5.32

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Fig. 3 depicts the XRD patterns of the t-ZrO₂-lj and m-ZrO₂-np supports. From Fig. 3, the diffraction peaks appearing at approximately 30.1 and 35.0° can be well resolved and can be assigned to the (011) and (002) planes of the tetragonal phase of t-ZrO₂-lj (JCPDS 17-0923); several small peaks corresponding to the monoclinic structure are also present. However, the m-ZrO₂-np support prepared by the common hydrothermal process shows that the monoclinic facets of the additions of hexamethylenetetramine and glucose in the hydrothermal process for preparing ZrO₂ play a role in inducing the formation of tetragonal phase and that the tetragonal phase dominant nanosheets-accumulating Laminaria japonica-like hierarchically structured zirconia (t-ZrO₂-lj) was successfully prepared. The average crystallite sizes of ZrO₂ for the two samples were estimated by the Scherrer equation and are listed in Table 1. t-ZrO₂-lj exhibits a larger average crystalline size (14.8 nm) than m-ZrO₂-np (12.1 nm). High thermal stability is required for support applications in the DRM reaction. Therefore, we compared the thermal stability of the two ZrO₂ supports by performing XRD analysis on the two forms of ZrO₂ following calcination at 850 °C (Table 1). A remarkable crystal grain growth from 12.1 to 21.9 nm corresponding to m-ZrO₂-np can be clearly seen. However, the average crystalline size of t-ZrO₂-lj only increases from 14.8 to 15.5 nm, suggesting the higher thermal stability of tetragonal ZrO₂ in comparison with the monoclinic form. More interestingly, no phase transformation occurred in the developed hierarchically structured t-ZrO₂-lj while it was subjected to heating at the high temperature of 850 °C, which endows it with great potential as an excellent support for use in the DRM reaction. According to the above experimental results, hierarchically structured t-ZrO₂-lj with a tetragonal plane-dominant crystalline structure has been prepared. In order to further explore the differences in the crystalline structures, the cell parameters were calculated and are listed in Table 1. The results show that m-ZrO₂-np has larger cell parameters corresponding to the a, b, and c values, which further confirms its crystalline structural features. In addition to their textural properties, the crystalline structures of the two supports may affect the properties of the supported Ni catalysts.

Fig. 3 XRD patterns of the t-ZrO₂-lj (a) and m-ZrO₂-np (b) supports.

Scheme 1 illustrates a plausible formation mechanism. On the base of previous reports, the HMTA easily coordinates with the metal ions to generate hydrogen-bonded supramolecular frameworks.⁴⁶ Meanwhile, a very fast polymerization occurs among HMTA molecules via hydrogen bonding reactions to form the polymer.⁴⁷ In this system, in stage 1, a gel quickly forms when the ZrOCl₂·8H₂O aqueous solution is added to HMTA aqueous solution, indicating that the HMTA molecules easily coordinate with the Zr⁴⁺ ions to form supramolecular structures. Subsequently, numerous coordination polymers are formed via HMTA graft copolymerization onto glucose (stage 2). These coordination polymers guide the subsequent self-assembly process via hydrogen bonding reactions. Subsequently, the grafted copolymers undergo decomposition, and the hydroxide is then formed in alkaline atmosphere under hydrothermal conditions. The final hierarchical structure is fabricated by the subsequent calcination process (stage 3).⁴⁵

Scheme 1 Schematic of the formation of Laminaria japonica-like tetragonal phase dominant ZrO₂.

3.2 Effect of support on Ni/ZrO₂ catalysts

Supported Ni catalysts on the as-synthesized t-ZrO₂-lj and m-ZrO₂-np supports with diverse morphologies and crystalline structures were prepared by the previously developed L-arginine ligand assisted incipient wetness impregnation method.⁴² The surface areas, structural features, Ni dispersion, and redox and basic properties of the catalysts were characterized by BET, XRD, CO chemisorption, and CO₂-TPD experiments. The XRD patterns, H₂-TPR, and CO₂-TPD profiles are presented in Fig. 4–6, and the quantitative results are listed in Table 2.

Fig. 4 XRD patterns of the Ni/t-ZrO₂-lj catalyst, (a) the Ni/m-ZrO₂-np catalyst (b) and bulk Ni (c).

Table 2 Characteristics of the as-synthesized supported Ni catalysts

Catalyst	SBET (m^2 g^−1)	CSNi^a (nm)	DNi^b (%)	H2 uptake (mmol g^−1)	DR^c (%)	Coke rate (mg gcat^−1 h^−1)
		Fresh/spent		r-I/r-II
a Average Ni crystallite size of the fresh and spent catalysts, estimated by the Scherrer method based on the (200) plane.b Determined by CO chemisorption, calculated from the eqn (1).c Degree of reduction towards NiO.
Ni/ZrO2-lj	28	14.0/14.5	5.4	1.27/0.76	75.4	6.8
Ni/ZrO2-np	20	13.3/13.3	3.8	1.41/0.52	66.3	8.1

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From Table 2, the Ni/t-ZrO₂-lj catalyst has a much higher surface area (28 m² g⁻¹) than the Ni/m-ZrO₂-np catalyst (20 m² g⁻¹), although the surface area of the t-ZrO₂-lj support is lower than that of m-ZrO₂-np. This may be ascribed to the higher thermal stability of the former, as confirmed above, because the supported Ni catalyst was prepared at high temperature (e.g. the reduction process at 850 °C). Chemisorption is a powerful tool to determine Ni dispersion, which is a direct indicator of accessible Ni for the DRM reaction. From Table 2, Ni/t-ZrO₂-lj shows much higher Ni dispersity (5.4%) than Ni/m-ZrO₂-np (3.8%), which may be a major reason for its higher activity in the DRM reaction. XRD patterns of the t-ZrO₂-lj and m-ZrO₂-np catalysts are presented in Fig. 4, and the estimated average Ni crystalline sizes by the Scherrer equation on the basis of the (111), (200) and (220) planes of Ni are listed in Table 2. From Fig. 4 and Table 2, the metallic Ni phase is identified based on JCPDS card no. 65-2865, and the average crystallite sizes of Ni for the Ni/t-ZrO₂-lj and Ni/m-ZrO₂-np catalysts are estimated. From Table 2, Ni/t-ZrO₂-lj catalyst has a slightly larger crystalline size (14.0 nm) than Ni/m-ZrO₂-np (13.3 nm), in addition to larger cell parameters. From the CO chemisorption results, the slightly smaller Ni crystalline size of Ni/m-ZrO₂-np does not lead to an increase in accessible Ni but to a decrease, due to the decrease in reducibility by the strengthened metal–support interaction, which was confirmed by H₂-TPR as follows.

H₂-TPR analysis was performed to investigate the reducibility of the supported Ni catalysts and the metal–support interaction characteristics. The H₂-TPR profiles are shown in Fig. 5, and the H₂ uptake is listed in Table 2. From Fig. 5, according to a previous report,⁴² the H₂-TPR profiles can be divided into two reduction regions (denoted as r-I and r-II). The r-I region can be attributed to the reduction of free NiO on the support when the NiO has a weak metal–support interaction; moreover, the r-II region can be assigned to the reduction of NiO that has a strong metal–support interaction. From Table 2, the H₂ uptake corresponding to r-II (0.76 mmol g⁻¹) over NiO/t-ZrO₂-lj is much higher than that on Ni/m-ZrO₂-np (0.52 mmol g⁻¹), which may affect the catalytic performance. The peak temperature of r-II shifts to a lower temperature on the catalyst, suggesting that the strength of the metal–support interaction for the developed NiO/t-ZrO₂-lj is slightly weaker than that for NiO/m-ZrO₂-np. The stronger metal–support interaction for Ni/m-ZrO₂-np than NiO/t-ZrO₂-lj may be ascribed to the smaller Ni crystalline size. Moreover, from Table 2, the NiO/t-ZrO₂-lj sample has a higher reduction degree (75.4%) than the NiO/m-ZrO₂-np sample (66.3%), which favors the DRM reaction.

Fig. 5 H₂-TPR profiles of NiO/t-ZrO₂-lj and NiO/m-ZrO₂-np.

The surface basic properties of a catalyst have significant effects on the adsorption and dissociation of CO₂ to produce CO and release active oxygen. As a result, the catalytic activity for DRM can be enhanced, and the coke deposited on supported Ni catalyst can be inhibited. As a subsequence, the coke-resistant stability of a supported Ni catalyst for DRM can be improved. Fig. 6 displays the CO₂-TPD profiles of the two catalysts (the desorbed CO₂ amount is inserted in Fig. 6). As depicted in Fig. 6, the CO₂-TPD profiles show one CO₂ desorption peak centered at a lower temperature of less than 150 °C, representing weak basic sites. However, the CO₂ desorption peak of the Ni/t-ZrO₂-lj catalyst shifts to a higher temperature in comparison with that of Ni/m-ZrO₂-np, suggesting the presence of stronger basic sites on the former. According to previous studies, the Lewis basicity of O²⁻ vacancies in the surface region of ZrO₂ may enable them to behave as basic sites for CO₂ adsorption, and O²⁻ vacancies are more easily formed on the (011) facet of the tetragonal phase than that of the monoclinic phase.^48,49 Therefore, the Ni/t-ZrO₂-lj catalyst with tetragonal phase ZrO₂ as support has more basic sites (71.9 μmol g⁻¹) in comparison with the Ni/m-ZrO₂-np catalyst (57.2 μmol g⁻¹). This feature of the developed Ni/t-ZrO₂-lj catalyst allows it to exhibit higher activity and stability than the common Ni/m-ZrO₂-np catalyst.

Fig. 6 CO₂-TPD profiles of Ni/t-ZrO₂-lj and Ni/m-ZrO₂-np.

The catalytic properties of the as-synthesized Ni/t-ZrO₂-lj and Ni/m-ZrO₂-np catalysts for the DRM reaction were measured and are presented in Fig. 7. From Fig. 7a and b, the developed Ni/t-ZrO₂-lj catalyst demonstrates much higher catalytic activity, corresponding to both CH₄ and CO₂ conversions at different reaction temperatures in comparison with the traditional Ni/m-ZrO₂-np catalyst. Correlated to the characterization results, the greatly superior catalytic activity of the developed Ni/t-ZrO₂-lj catalyst compared to the traditional Ni/m-ZrO₂-np catalyst can be ascribed to the larger surface area, higher Ni dispersion, better reducibility, and increased number of basic sites, which are dependent on the unique morphology and tetragonal crystalline phase of the ZrO₂ support. Moreover, from Fig. 7a and b, higher CO₂ conversion than CH₄ conversion over the two catalysts can be observed; this is ascribed to the possible presence of reverse water gas shift (RWGS) and methanation reactions.⁹ From Fig. 7c and d, the Ni/t-ZrO₂-lj catalyst shows slightly higher H₂ selectivity and H₂/CO, which may be ascribed to possible water gas shift or steam reforming of methane by the formed H₂O by reverse water gas shift. As a consequence, from Fig. 7d, the ratios of H₂/CO over the two catalysts at different reaction temperatures were lower than the theoretical values. The H₂/CO ratio increases as the reaction temperature is increased from 650 to 850 °C, possibly due to the decomposition of methane, water gas shift, and/or carbon gasification.⁹

Fig. 7 CH₄ conversion (a), CO₂ conversion (b), H₂ selectivity (c) and CO selectivity (d) as a function of reaction temperature for the DRM reaction over the Ni/t-ZrO₂-lj and Ni/m-ZrO₂-np catalysts. Reaction conditions: m_cat = 60 mg, CH₄/CO₂/N₂ = 1 : 1 : 1, GHSV = 30 000 ml h⁻¹ g⁻¹, and atmospheric pressure.

Herein, the catalytic stability of the two catalysts for synthesis gas production through the DRM reaction was compared. Fig. 8 demonstrates the catalytic performance as a function of time on stream, and the carbon balance curves are presented in Fig. S1.† From Fig. 8, it can be seen that the developed Ni/t-ZrO₂-lj catalyst exhibits much more stable activity in comparison with the Ni/m-ZrO₂-np catalyst. From Fig. S1,† the carbon balance curves decrease with time on stream, ascribed to coke deposition. Especially, the Ni/m-ZrO₂-np catalyst shows a larger decrease in carbon balance with time on stream than Ni/t-ZrO₂-lj, implying that more coke was deposited on the Ni/m-ZrO₂-np catalyst. Then, we investigated the origin of the significant stability of the supported Ni catalysts on diverse ZrO₂ supports by employing XRD, TEM, and TGA analytical techniques.

Fig. 8 CH₄ conversion (a), CO₂ conversion (b), H₂ selectivity (c) and CO selectivity (d) as a function of time on stream for the DRM reaction over the Ni/t-ZrO₂-lj and Ni/m-ZrO₂-np catalysts. Reaction conditions: m_cat = 60 mg, CH₄/CO₂/N₂ = 1 : 1 : 1, GHSV = 30 000 ml h⁻¹ g⁻¹, T = 850 °C, and atmospheric pressure. The CH₄ equilibrium conversion under these conditions is 97.6%.

Fig. 9 depicts the XRD patterns of the fresh and spent supported Ni catalysts on t-ZrO₂-lj and m-ZrO₂-np with different morphologies and crystalline phases. The average Ni crystalline sizes of the four catalysts are listed in Table 2. From Fig. 8 and Table 2, slight growth from 14.0 to 14.5 for Ni/t-ZrO₂-lj but no change for Ni/m-ZrO₂-np can be observed. The higher Ni sintering resistance on the Ni/m-ZrO₂-np catalyst compared to Ni/t-ZrO₂-lj may be ascribed to the stronger Ni–support interaction as confirmed above. Moreover, the diffraction peak at 26° assigned to graphitic carbon could be observed on both of the spent catalysts. From Fig. 8, the developed Ni/t-ZrO₂-lj catalyst exhibits much higher catalytic stability than Ni/m-ZrO₂-np. Generally, the deposited carbon ultimately forms a barrier on the surface of the catalyst that hinders the access of reacting gases to the active sites and therefore leads to deactivation of the catalyst. Therefore, further TEM and TGA experiments were performed to explore coke deposition behaviors on the spent catalysts.

Fig. 9 XRD patterns of the fresh and spent catalysts and bulk Ni. (a) Fresh Ni/t-ZrO₂-lj, (b) spent Ni/t-ZrO₂-lj, (c) fresh Ni/m-ZrO₂-np, (d) spent Ni/m-ZrO₂-np and (e) bulk metallic Ni.

Fig. 10 depicts TEM images of the two spent catalysts. It can be clearly seen that filamentous carbon exists on both of the catalysts. However, it can be observed that more carbon was deposited on the spent Ni/m-ZrO₂-np catalyst than on Ni/t-ZrO₂-lj. From Fig. 10c and d, the deposited coke on the two catalysts shows a similar tubular morphology. From Fig. 10a and b, the coke on Ni/t-ZrO₂-lj mainly covers the outside of the hierarchical structures; much of the Ni inside the hierarchical structures is not covered. However, the coke on the Ni/m-ZrO₂-np catalyst covers the small particles, and more Ni may be covered by coke; this may be a reason for the significantly lower stability of Ni/m-ZrO₂-np catalyst compared to the developed Ni/t-ZrO₂-lj, although the higher Ni sintering resistance of the former was confirmed by XRD results. The Ni species on the outside of the hierarchical structure for the Ni/t-ZrO₂-lj catalyst and the outside of the nanoparticles for Ni/m-ZrO₂-np mediate the methane decomposition and/or CO Boudouard reactions, which lead to the growth of tubular coke. Due to the strong Ni–support interaction, the Ni was not removed by the growing carbon nanotubes. In addition to the deactivation of the supported Ni catalyst by coke deposition, the produced tubular carbon could destroy the catalyst particles and induce a serious pressure drop within the reactor with time.⁵⁰ Furthermore, the amount of coke deposited on both catalysts was quantitatively measured by TGA-DTG analysis as follows.

Fig. 10 TEM images of the spent Ni/ZrO₂-lj (a) and Ni/ZrO₂-np (b) catalysts. (c) and (d) are magnified images of the selected zones in (a) and (b), respectively.

The TGA profiles of the spent catalysts after 30 h of TOS are presented in Fig. 11, and the quantitative results are listed in Table 2. The general trend of the TGA curves was a decrease as the temperature was increased. However, from Fig. 11, the TGA curves for both of the catalysts experienced a slight increase in the temperature zones from 200 to 600 °C, which may be ascribed to the oxidation of metallic Ni.^42,51 The TGA curves showed that the weight loss of the Ni/t-ZrO₂-lj catalyst owing to coke combustion is much lower than that of the spent Ni/m-ZrO₂-np. The relatively lower coking rate of 6.8 mg g_cat⁻¹ h⁻¹ on the spent Ni/t-ZrO₂-lj catalyst compared to that of Ni/m-ZrO₂-np (8.2 mg g_cat⁻¹ h⁻¹) suggests higher coke resistance, ascribed to the higher number of basic sites. Combining the TGA and TEM images with the catalytic stability results, the higher stability of Ni/t-ZrO₂-lj compared to Ni/m-ZrO₂-np can be ascribed to their different coke behaviors. From the TGA results, the latter shows a slightly higher coke rate than the former; however, significantly decreased conversion on the latter with time on stream can be observed. From the aforementioned TEM images of the spent catalysts, coke coiling on the outside of the hierarchical structure of Ni/t-ZrO₂-lj catalyst and on the nanoparticles of Ni/m-ZrO₂-np can be observed, which may lead to the covering of many more Ni species by coke, although only a slightly greater coke rate for Ni/m-ZrO₂-np than Ni/t-ZrO₂-lj catalyst can be observed. Based on its more stable activity and higher coke resistance, the developed Ni/t-ZrO₂-lj catalyst is a potential candidate for synthesis gas production through the DRM reaction.

Fig. 11 TGA/DTG profiles of the spent Ni/t-ZrO₂-lj (a) and Ni/m-ZrO₂-np (b) catalysts.

4. Conclusions

In summary, this work presents a facile approach for preparing nanosheets-accumulating Laminaria japonica-like hierarchically structured zirconia with tetragonal phase through the established hexamethylenetetramine–glucose assisted hydrothermal method. Owing to the high thermal stability of the as-synthesized hierarchical structure of tetragonal ZrO₂, it can be considered as an excellent support for diverse high temperature reactions, including dry reforming of methane. The supported Ni catalyst on the as-synthesized t-ZrO₂-lj exhibits superior catalytic stability with high coke resistance in comparison with a Ni catalyst using monoclinic zirconia nanoparticulate as support; this can be ascribed to the larger surface area, higher Ni dispersion, more reducible Ni, and the larger number of basic sites of the Ni/t-ZrO₂-lj catalyst in comparison with Ni/m-ZrO₂-np, which are strongly dependent on the morphology and crystalline phase of the ZrO₂ support. Although numerous reports of Ni-based catalysts for dry reforming of methane can be found, deactivation by coke deposition is a bottle-neck problem that prohibits industrial applications for synthesis gas production through this process. This work presents a significant advance in the development of robust Ni-based catalysts with high coke-resistant stability for dry reforming of methane. The developed Ni/t-ZrO₂-lj catalyst can be considered as a promising candidate for the DRM reaction to simultaneously convert two greenhouse gases, CO₂ and methane, to synthesis gas. Moreover, the as-synthesized t-ZrO₂-lj, with its unique morphology and tetragonal crystalline phase, has great potential for application as an excellent support in diverse transformations, especially for reactions performed at high temperature and harsh conditions.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (grant no. 21276041), the Joint Fund of Coal, set up by the National Natural Science Foundation of China and Shenhua Co., Ltd. (grant no. U1261104); it is also sponsored by the Chinese Ministry of Education via the Program for New Century Excellent Talents in University (grant no. NCET-12-0079), the Natural Science Foundation of Liaoning Province (grant no. 2015020200), and the Fundamental Research Funds for the Central Universities (grant no. DUT15LK41).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra12457g

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