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Article

Highly Carbon-Resistant Y Doped NiO–ZrOm Catalysts for Dry Reforming of Methane

by
Ye Wang
1,2,3,
Li Li
2,
Yannan Wang
2,
Patrick Da Costa
1,* and
Changwei Hu
2,3,*
1
Institut Jean Le Rond d’Alembert, Sorbonne Université, CNRS, 2 Place de la Gare de Ceinture, 78210 Saint-Cyr-L’Ecole, France
2
Key Laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064, China
3
College of Chemical Engineering, Sichuan University, Chengdu 610065, China
*
Authors to whom correspondence should be addressed.
Catalysts 2019, 9(12), 1055; https://doi.org/10.3390/catal9121055
Submission received: 18 November 2019 / Revised: 5 December 2019 / Accepted: 9 December 2019 / Published: 11 December 2019
(This article belongs to the Special Issue CO2 Capture, Utilization and Storage: Catalysts Design)
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Abstract

:
Yttrium-doped NiO–ZrOm catalyst was found to be novel for carbon resistance in the CO2 reforming of methane. Yttrium-free and -doped NiO–ZrOm catalysts were prepared by a one-step urea hydrolysis method and characterized by Brunauer-Emmett-Teller (BET), TPR-H2, CO2-TPD, XRD, TEM and XPS. Yttrium-doped NiO–ZrOm catalyst resulted in higher interaction between Ni and ZrOm, higher distribution of weak and medium basic sites, and smaller Ni crystallite size, as compared to the Y-free NiO–ZrOm catalyst after reaction. The DRM catalytic tests were conducted at 700 °C for 8 h, leading to a significant decrease of activity and selectivity for the yttrium-doped NiO–ZrOm catalyst. The carbon deposition after the DRM reaction on yttrium-doped NiO–ZrOm catalyst was lower than on yttrium-free NiO–ZrOm catalyst, which indicated that yttrium could promote the inhibition of carbon deposition during the DRM process.

Graphical Abstract

1. Introduction

Fischer-Tropsch (F-T) synthesis has become a significant process for producing liquid organic hydrocarbons from syngas (H2 and CO). There are several methods to produce syngas: e.g., steam reforming of methane (Equation (1)), partial oxidation of methane (Equation (2)) and dry reforming of methane (Equation (3)) [1,2,3,4,5]. Among these methods, the dry reforming of methane has a competitive advantage of producing clean hydrogen and carbon monoxide mixture gases with an equimolar ratio (1:1), which best suits for F-T synthesis. Moreover, another significant aspect is the consumption of two greenhouse gases (CO2 and CH4), thereby offering an environmental benefit [3,6].
CH4 + H2O → 3H2 + CO
2CH4 + O2 → 2CO + 4H2
CH4 + CO2 → 2CO + 2H2
In general, noble metal catalysts (Pt, Ir, Rh) exhibit good performance for the dry reforming of methane [7,8,9]. Considering the high cost of noble metals for the industrial scale, many efforts have focused on the Ni-based catalysts, because nickel metal has a high potential for industrial application in DRM [10,11,12,13]. However, it was well known that Ni-based catalysts suffered deactivation caused by carbon deposition and/or sintering (frittage). Therefore, the development of nickel-based catalysts, with the significantly promoted Ni-sintering resistance and coke-tolerance, has been extensively investigated in recent years [14,15,16]. According to the literature [17,18,19], larger Ni particles on the surface of the catalyst led to catalytic selectivity towards methane cracking (Equation (4)), thereby causing carbon deposition. Another approach of the coke deposition was the disproportionation of CO (Equation (5)).
CH4 → 2H2 + C
2CO → CO2 + C
It is found that the adjustment of the catalyst by promoters is an efficient approach to restrain the formation of carbon [20,21,22]. Thus, the modification of the raw catalyst with rare earth elements has been reported to enhance both the activity of the catalyst and inhibition of carbon formation, such as La and Y. The latter has attracted huge interest for reforming catalyst [18,22,23,24]. Taherian et al. [20] found that comparing with mesoporous Ni/SBA-15, Ni/SBA-15 modified by Y2O3 can promote the control of the nickel particle size, and thus enhance the dispersion of nickel. Moreover, the Ni-Y/KIT-6 catalyst exhibited better dispersion of nanosized Ni particles inside the pores of the support, as compared to Ni/KIT-6 material, leading to lower carbon deposition in the initial stability test at 700 °C [19]. Li et al. [21] investigated the effect of different types of impregnation for yttrium introduced on the performance of Ni/Y/Al2O3 catalyst, and observed that compared with the Y-free catalyst, the activity and stability of the Y-modified catalyst increased, because of smaller nickel metallic particle size, more basic sites, the less amount of carbon deposit and the small degree of graphitization. Furthermore, Li et al. [25] also observed high carbon-resistance for the Ni-SBA-15 catalyst modified by yttrium promoter, because of the high dispersion of the nanoparticles of yttrium and nickel. Moreover, more oxygen vacancies were created by the introducing of yttrium.
Except for the active metal and promoter, the support also plays an important role in the catalytic test for DRM. The ZrO2 is considered to be a promising support for the dry reforming of methane, because of the special property, e.g., the reducing and oxidizing abilities, the unexceptionable thermal and chemical stability, and the high oxygen mobilization [22,26,27,28]. These properties of ZrO2 can be improved by adding a cation with valence lower than 4+, e.g., Y3+ [22,23], which results in the formation of solid solution, together with oxygen vacancies. Furthermore, those oxygen vacancies play a key role in gasifying carbon deposits during the activation of CO2 and O2 [23]. Bellido et al. [23] prepared a series of Y2O3–ZrO2 catalysts with different yttrium content (4 to 12 mol %). 5Ni8YZ (mole fraction of Y = 8 mol %) catalyst showed the best activity and stability for DRM. Because Y3+ increased the formation of oxygen vacancies in YZ support. Those modification properties of ZrO2 could improve the performance of catalyst. Asencios et al. [4] investigated the effect of solid solution on performance of NiO–Y2O3–ZrO2 catalyst and found that the formation of NiO–Y2O3 and Y2O3–ZrO2 solid solution could enhance the activity of catalyst for oxidative reforming of model biogas. Moreover, the formation of oxygen vacancies could promote the removal of carbon deposition. Świrk et al. [29,30] also found that the formation of Y2O3–ZrO2 solid solution could improve the stability of catalyst modified by yttrium.
In order to understand the ability of carbon-resistance on the Y-modified ZrO2 catalyst, the NiO–ZrOm and NiO–ZrOm–YOn catalysts were prepared by the one step urea hydrolysis method, and the performance of which was discussed here in detail. This latter catalyst, which was not the most active catalyst in DRM, exhibited, however, a high resistance against carbon deposition. We investigate the ability of carbon resistance on NiO–ZrOm–YOn catalyst, further to understand the relationship between structure and carbon-resistance on Y-modified catalyst.

2. Results and Discussion

2.1. The BET Results of Y-Doped and Y-Free NiO–ZrOm Catalysts

The physical properties of Y-doped and Y-free NiO–ZrOm material was investigated by the N2 adsorption–desorption method (Table 1). Both of the samples display the same narrow pore size of about 2 nm. The NiO–ZrOm catalyst shows a surface area of 113 m2/g and a pore volume of 2 nm; while, the NiO–ZrOm–YOn catalyst exhibits the lower specific surface area (79 m2/g ) and the smaller pore volume (0.1 cm3/g), which may decrease the activity of the catalyst, because the high surface area can enhance the activity of the catalyst [31].

2.2. The Reducibility of Y-Doped and Y-Free NiO–ZrOm Catalysts

The H2-TPR profile of Y-doped and Y-free NiO–ZrOm catalysts is presented in Figure 1A. Two reduction peaks at about 450 and 650 °C, denominated α and β, respectively, are observed on both catalysts. The first peak (α) corresponds to the reduction of the NiO species of weak and strong interaction with Y and/or Zr [23,27]. This first peak shifts to low temperature on NiO–ZrOm–YOn catalyst, as compared to the NiO–ZrOm catalyst. This shift may be attributed to the increase of oxygen vacancies by the addition of the yttrium promoter, because oxygen vacancies can promote the reduction of NiO by weakening the Ni–O bond [32,33]. The β peak on the NiO–ZrOm catalyst is related to the reduction of the solid solution of NiO–ZrO2 and/or ZrO2, while on the NiO–ZrOm–YOn catalyst, the β peak may be assigned to the reduction of solid solution (NiO–ZrO2 and/or NiO–Y2O3), ZrO2 and/or surface-capping oxygen ions of the Y2O3–ZrO2 solid solution [4,34]. On the contrary, the β peak shifts to a higher temperature for NiO–ZrOm-YOn catalyst. Similar phenomenon about the shift was found by Asencios et al. [4] on NiO–Y2O3–ZrO2 catalysts. The yttrium addition could promote the reduction of surface-capping oxygen ions of the NiO–ZrOm–YOn catalyst, leading to the formation of new surface oxygen vacancies at about 700 °C [4]. Therefore, the β peak shifts to about 700 °C. Except for the shift of peak, the total amount of H2 consumption on the NiO–ZrOm catalyst (0.62 mmol H2/g) is higher than that on NiO–ZrOm–YOn catalyst (0.37 mmol H2/g) presented in Table 1. The latter result is consistent with the theoretical value of 0.39 mmol H2/g, while the H2 consumption on this NiO–ZrOm catalyst is higher than the theoretical value of 0.47 mmol H2/g. This phenomenon proves that the ZrO2 is also reduced by hydrogen. Besides, for the α peak, the amount of H2 consumption on our NiO–ZrOm–YOn catalyst decreases by adding the yttrium, which indicates that part of this free NiO would be inserted into the structure of ZrO2 and/or Y2O3 to form NiO–ZrO2 and/or NiO–Y2O3 solid solution. Thus, yttrium can promote nickel embedding into the structure of ZrO2 and/or Y2O3. For the β peak, the amount of H2 consumption on the NiO–ZrOm catalyst is higher than that obtained on NiO–ZrOm–YOn catalyst. In order to understand the reduction of ZrO2, the Zr 3d peak is resolved into two peaks, which is shown in Figure 1B. The peaks at about 181.4 and 182.3 eV are ascribed to Zr3+ and Zr4+, respectively [35,36].
It is very obvious to be observed that most Zr3+ formed on both catalysts, indicating that the most Zr4+ is reduced to Zr3+ after reduction. From Table 1, the content of Zr3+ on the NiO–ZrOm catalyst (90%) is higher than that on the NiO–ZrOm–YOn catalyst (84%), which manifests that more Zr4+ is reduced to Zr3+ on the NiO–ZrOm catalyst, which is conformed to H2-TPR results (Figure 1A and Table 1), that is, this NiO–ZrOm catalyst consumes more H2 during the reduction. These phenomena indicate that the formation of ZrO2 defected by Ni2+ on the NiO–ZrOm catalyst is very easy to be reduced from Zr4+ to Zr3+ in the presence of hydrogen. While the ZrO2 defected by Y3+ is very stable, which is very hard to be reduced. Therefore, the introduction of the Y3+ into ZrO2 lattice can also stabilize the crystal structure, and create new oxygen vacancies.

2.3. Basicity of Y-Doped and Y-Free NiO–ZrOm Catalysts

A CO2 temperature-programmed desorption (CO2 TPD) experiment was conducted to determine the basicity of the Y-doped and Y-free NiO–ZrOm catalysts (Figure 2). The peaks on the NiO–ZrOm–YOn catalyst shift to low temperature, as compared to the NiO–ZrOm catalyst. The total number of basic sites increases from 73 to 100 μmol CO2/g on the NiO–ZrOm catalyst before and after the introducing of yttrium (see in Table 2), which indicates that the yttrium can enhance the total number of basic sites. As already described elsewhere [37,38,39], there are three types of basic sites (weak, medium-strength and strong). The content of weak peak on the NiO–ZrOm–YOn catalyst (36.9%) is higher than that on the NiO–ZrOm catalyst (15.5%), while the content of the strong peak decreases to 13.0% on the NiO–ZrOm–YOn catalyst. Besides, the position of weak and medium-strength peaks on this NiO–ZrOm–YOn catalyst are at about 150 and 240 °C, which are lower than those on the NiO–ZrOm catalyst (185 and 252 °C), respectively. This phenomenon manifests that yttrium can promote the formation of weak basic sites on the NiO–ZrOm–YOn catalyst. Thus, one can note that the addition of yttrium modifies both the distribution and the number of basic sites. A similar phenomenon can be observed in our group’s previous works [37,40]. According to the literature [10,38,39], the weak and medium-strength basic sites can promote the formation of activation carbonate species, thereby enhancing the ability to remove the carbon deposition. Whereas, too strong basic sites lead to too strong CO2 adsorption, thereby promoting more carbon deposition. Therefore, the weak and medium basic sites present on the NiO–ZrOm–YOn catalyst can enhance the ability to eliminate coke.

2.4. Nickel Particle Size and Crystallized Phases of Y-Doped and Y-Free NiO–ZrOm Catalysts

Figure 3 shows X-ray diffraction (XRD) patterns of the NiO–ZrOm–YOn and NiO–ZrOm catalysts after reduction and after catalytic reaction. Tetragonal and/or cubic phase ZrO2 appear in both catalysts. The crystallite sizes of ZrO2 on both catalysts exhibit the same value of 7 nm from the Scherrer Equation, and do not change even after reaction for 8 h (Table 3). The peak at about 44.5° can be attributed to the metallic nickel [41]. The Ni0 size on the NiO–ZrOm catalyst is about 12 nm after reduction, and it increases to 24 nm after reaction. While for the NiO–ZrOm–YOn catalyst, it decreases from 16 nm to 10 nm after reaction for 8 h, which indicates the re-dispersion of Ni0. Similar phenomenon had been reported by other researches [37,38,42]. Nakayam et al. [43] found the re-dispersion of Ni under an alternating condition between H2 reduction and oxidation atmosphere. Under dry reforming of methane condition, CO2 is a source of oxygen. With CO2 adsorption and activation on the surface of catalyst, the nickel metal may be oxidized. The production of H2 and CO as reduction atmosphere may contribute to the reduction of the NiO, and thereby the re-dispersion of nickel particles. On one hand, the Ni0 size is related to the sintering of the nickel. Severe sintering takes place on this NiO–ZrOm catalyst, due to the lower interaction between Ni and Zr, which is comfirmed by the results of H2-TPR. Furthermore, the addition of yttrium can limit the sintering of nickel during the reaction. On the other hand, It is well known that the large nickel particle size may be favored for selective reactions that lead to carbon deposition [18,19]. As a consequence, NiO–ZrOm–YOn can limit the carbon deposition. Besides, NiO–ZrOm–YOn exhibits more basic sites, which could enhance the ability of the adsorption of CO2, thereby promoting the removal of carbon deposition. Both sintering and carbon deposition could contribute to the deactivation of catalyst, thereby reducing the stability of the catalyst.

2.5. The Performance of Y-Doped and Y-Free NiO–ZrOm Catalysts

The catalytic performance of the catalysts was investigated at 700 °C for 8 h (Figure 4). The conversion of methane on both catalysts decreases within 1 h, and finally stabilizes at 67% and 85% for the NiO–ZrOm–YOn and NiO–ZrOm catalyst, respectively. Except for the higher methane conversion on the NiO–ZrOm catalyst, the CO2 conversion and the ratio of H2/CO are higher than those on the NiO–ZrOm–YOn catalyst. The CO2 conversion on the NiO–ZrOm catalyst is about 89% with the H2/CO ratio of 0.95, which is very close to one. When adding yttrium into the NiO–ZrOm catalyst, the CO2 conversion decreases within 60 min and stabilizes at 70% for 8 h time on stream. At the same time, the H2/CO ratio decreases to about 0.85. Because the NiO–ZrOm–YOn catalyst exhibits the lower specific surface area, the smaller pore volume, and the bigger metallic nickel particle size. Those properties could lead to the lower performance of this NiO–ZrOm–YOn catalyst. However, the catalyst activity remained stable during time on stream.

2.6. On the Carbon Deposition on Used Y-Doped and Y-Free NiO–ZrOm Catalysts

2.6.1. Carbon Formation Evidenced by XPS and Raman Spectroscopy

The content of coke on both catalysts are determined by TGA experiment, and the results are shown in Table 3. The content of the coke on the NiO–ZrOm catalyst is about 3.7%, which is higher than that on NiO–ZrOm–YOn catalyst (1.0%), indicating that yttrium can decrease the carbon deposition on the NiO–ZrOm–YOn catalyst. Figure 5A represents the results of C 1s profiles. The intensity of C 1s on the NiO–ZrOm–YOn catalyst is lower than that on our NiO–ZrOm catalyst, showing that the content of surface carbon deposition on this NiO–ZrOm–YOn catalyst is lower, about 63%, While the coke on the surface of the NiO–ZrOm catalyst is about 69%. Figure 5B shows the results of the Raman experiment. Two peaks can be found on the NiO–ZrOm catalyst, and no peak can be observed on the NiO–ZrOm–YOn catalyst, which manifests that no or less coke form on the NiO–ZrOm–YOn catalyst. The peak at about 1328 cm−1 is attributed to the structural imperfections on not-organized carbon materials, namely disorder-induced band (D band), and another peak at about 1585 cm−1 is corresponded to the in-plane C–C stretching vibrations of sp2 atoms in coke, namely graphitic carbon (G band) [16,44,45]. The intensity of peak is named I, while the ratio of IG/ID is about 1.7 (Table 3), indicating that more graphitic carbon forms on the NiO–ZrOm catalyst, which is conformed to the results of XPS, more C–C species on the NiO–ZrOm catalyst. All those phenomena show that the carbon deposition on this NiO–ZrOm catalyst is higher than that on the NiO–ZrOm–YOn catalyst.

2.6.2. On the Study of the Morphology of Carbon Studied by TEM

The morphology structure of carbon on the catalysts after reaction is determined by the transmission electron microscopy experiment (Figure 6). After reaction, the nickel particle size decreases on NiO–ZrOm by adding the yttrium promoter (Figure 6A,C). The nickel particle size formed on NiO–ZrOm–YOn is about 10–15 nm (Figure 6D), while about 15–20 nm becomes formed on NiO–ZrOm (Figure 6B), which is corresponding to the results of XRD. It can be noted that large Ni particles (over 20 nm) can be observed on the NiO–ZrOm catalyst. According to the results of H2-TPR, the interaction between Ni and ZrOm on the NiO–ZrOm catalyst is lower than that on the NiO–ZrOm–YOn catalyst, leading to nickel sintering. Therefore, bigger particles of Ni0 are present on the NiO–ZrOm catalyst. These large nickel particles may be favored for selective reactions that tend to form carbon deposition [18,19], thereby resulting in the formation of carbon deposition. Thus, a lot of carbon deposition can be observed on the NiO–ZrOm catalyst in the form of a carbon nanotube (Figure 6A), and no carbon deposition is observed on the NiO–ZrOm–YOn catalyst (Figure 6C),which indicates that no or little carbon formes on the NiO–ZrOm–YOn catalyst. These carbon nanotubes are graphitic carbon, which is in agreement with the results of the Raman experiment. This phenomenon shows that yttrium can suppress the sintering of nickel particles, and also inhibits the formation carbon during the DRM reaction.

3. Materials and Methods

3.1. Synthesis of Y Doped NiO–ZrOm and NiO–ZrOm Catalysts

5.03 g zirconium(IV) oxynitrate hydrate (Sigma-Aldrich, Saint-Quentin Fallavier, France), 1.14 g nickel(II) nitrate hexahydrate (Emsure, Merck, Fontenay sous Bois, France) 7.06 g urea (Sigma-Aldrich, Saint-Quentin Fallavier, France), 0.86 g yttrium(III) nitrate hexahydrate (Sigma-Aldrich, Saint-Quentin Fallavier, France) (corresponding to a loading of 10 wt %) and 7.96 g of Pluronic P123 (Sigma-Aldrich, Saint-Quentin Fallavier, France) amphiphilic block copolymer were dissolved in 375 mL of distilled water. The mixed liquor was heated from room temperature to 95 °C with constant stirring for 48 h. After that, the mixture was aged at 100 °C for 24 h. Then the slurry was filtered, washed with a little of distilled water and dried at room temperature. The catalysts were calcined in air at 800 °C for 5 h with an increasing rate of 1 °C/min. The obtained materials were denoted as NiO–ZrOm–YOn (Ywt % = 10%). In order to understand the function of yttrium, NiO–ZrOm without yttrium was prepared by the above method and denoted as NiO–ZrOm.

3.2. Activity Test

The activity test was carried out at 700 °C in a fixed-bed flow reactor (id = 12 mm) connected in-line to a gas micro chromatograph (Agilent Varian GC490, Agilent, Les Ulis, France), equipped with a thermal conductivity detector (TCD). The total feed gas flow rate was 100 mL/min with a molar ratio CH4/CO2/Ar = 1/1/8. Considering the volumes of NiO–ZrOm and NiO–ZrOm–YOn catalysts, the total gas hourly space velocity (GHSV) values were 48,000 h−1. Prior to reactions, the sample was reduced with 5% H2/Ar flow at 700 °C for 1 h. The CO2 and CH4 conversion, and H2/CO molar ratio of catalysts were calculated as follows:
X C H 4   =   n C H 4 , i n − n C H 4 , o u t n C H 4 , i n   × 100 %
X C O 2   =   n C O 2 , i n − n C O 2 , o u t n C O 2 , i n   × 100 %
H 2 / C O   =   n H 2 , o u t / n C O , o u t
where X C H 4 and X C O 2 refers to the conversion of CH4 and CO2.

3.3. Catalyst Characterization

The physical property was obtained by the N2 adsorption–desorption method. Before measurement, the sample was pretreated under vacuum conditions at 200 °C for 2 h. Then the test conducted in a Belsorp Mini II apparatus (BEL Japan) instrument under liquid nitrogen temperature (−196 °C). The surface area was calculated by the Brunauer–Emmett–Teller (BET) method, and the pore volume and diameter were calculated by the Barrett–Joyner–Halenda (BJH) method.
Temperature-programmed reduction of H2 (H2-TPR) conducted on a BELCAT-M (BEL Japan, BEL Europe GmbH, Krefeld, Germany) apparatus, equipped with a TCD. The sample (60 mg) was pretreated under helium atmosphere at 150 °C for 30 min. The sample was reduced under 5% H2/Ar mixture flow. The temperature increased from 100 °C to 900 °C with a heating rate of 10 °C/min and held on 30 min at 900 °C.
Temperature-programmed desorption of CO2 (CO2-TPD) was conducted on the same equipment. The sample was reduced under a 5% H2/Ar flow at 700 °C for 1 h. Afterward, CO2 was adsorbed at 80 °C for 1 h under a mixture of 10% CO2 in He. After cleaning the weakly adsorbed CO2 for 30 min, the sample was heated to 900 °C with a heating rate of 10 °C/min under a He flow. The obtained graphs were fitted into three peaks (weak, middle and strong basic sites).
X-ray diffraction (XRD) experiment was carried out on a DX-1000 CSC diffractometer, equipped with Cu Kα radiation source. The data was recorded in a range of 10° < 2θ < 80°, with a scan step size of 0.03°.
Transmission electron microscopy (TEM) experiments were measured on an FEI Tecnai G2 20 Twin instrument at an acceleration voltage of 200 kV.
X-ray photoelectron spectroscopy (XPS) experiment conducted on a KRATOS spectrometer with an AXIS Ultra DLD.
Thermogravimetric analysis (TGA) was used to characterize the carbon deposition of used catalysts. The sample (10 mg) was treated under air atmosphere with a flow rate of 30 mL/min−1 at room tempeature until the scales balanced. Then, the temperature increased from room temperature to 800 °C with a heating rate of 5 °C/min−1. The data was recorded (the weight of sample) from room temperature to 800 °C.
Raman spectroscopy measurements were carried out on an objective (X50LWD) with a Filter of D1, a Hole of 200 µm, a Grating of 600 gr/mm and a Laser of 532.17 nm. The wavenumber values were scaned over the range 40−4000 cm−1 for three times.

4. Conclusions

NiO–ZrOm and NiO–ZrOm–YOn catalysts were prepared by the urea hydrolysis method and were characterized by BET, TPR-H2, CO2-TPD, XRD, TEM and XPS. After adding yttrium, the strong interaction between Ni and ZrOm formed on the NiO–ZrOm–YOn catalyst, resulting in the small size of nickel particle distributed on catalyst after reaction. While for the NiO–ZrOm catalyst, large particles formed on catalyst after reaction, contributing to the deposition of carbon. Besides, a greater amount of weak and medium-strong basic sites formed on the NiO–ZrOm–YOn catalyst, which could enhance the ability of the removal of carbon. Except for the advantages, the NiO–ZrOm–YOn catalyst showed lower specific surface area and the smaller pore volume. Thus, from the results of the activity test at 700 °C for 8 h, our NiO–ZrOm catalyst exhibited higher methane and CO2 conversion, while the NiO–ZrOm–YOn catalyst exhibited high carbon resistance for dry reforming of methane at 700 °C. Therefore, yttrium can modify the interaction between Ni and ZrOm, in enhancing the dispersion of nickel particles during the reaction, and promote the formation of more weak and medium-strength basic sites.

Author Contributions

Y.W. (Ye Wang) did all the tests, the methodology and did the writing of original draft, L.L. participated in the characterization part and participated in the writing of original draft, Y.W. (Yannan Wang) participated in the characterization part and participated in the writing of original draft, P.D.C. participated in the writing of the original draft and the funding acquisition and the supervision, C.H. participated in the writing of original draft and the funding acquisition and the supervision.

Funding

This work was supported by the National Key R&D Program of China (2018YFB1501404), the 111 program (B17030) and Fundamental Research Funds for the Central Universities.

Acknowledgments

Ye Wang acknowledges the financial support of CSC (China Scholarship Council) for her joint-PhD research in Sorbonne Université. We thank Yunfei Tian of analytical & testing center of Sichuan University for XPS experiments.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 09 01055 g001 550
Figure 1. (A) the H2-TPR profiles of NiO–ZrOm–YOn and NiO–ZrOm calcined catalysts, and (B) the Zr 3d profiles from XPS measurements of NiO–ZrOm–YOn and NiO–ZrOm catalysts after reduction.
Figure 1. (A) the H2-TPR profiles of NiO–ZrOm–YOn and NiO–ZrOm calcined catalysts, and (B) the Zr 3d profiles from XPS measurements of NiO–ZrOm–YOn and NiO–ZrOm catalysts after reduction.
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Catalysts 09 01055 g002 550
Figure 2. The CO2-TPD profiles of the NiO–ZrOm–YOn and NiO–ZrOm catalysts.
Figure 2. The CO2-TPD profiles of the NiO–ZrOm–YOn and NiO–ZrOm catalysts.
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Catalysts 09 01055 g003 550
Figure 3. The X-ray diffraction (XRD) profiles of (a) NiO–ZrOm and (c) NiO–ZrOm–YOn catalysts after reduction at 700 °C for 1 h and after reaction at 700 °C for 8 h, and (b) NiO–ZrOm and (d) NiO–ZrOm–YOn catalysts after reaction at 700 °C for 8 h.
Figure 3. The X-ray diffraction (XRD) profiles of (a) NiO–ZrOm and (c) NiO–ZrOm–YOn catalysts after reduction at 700 °C for 1 h and after reaction at 700 °C for 8 h, and (b) NiO–ZrOm and (d) NiO–ZrOm–YOn catalysts after reaction at 700 °C for 8 h.
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Catalysts 09 01055 g004 550
Figure 4. The conversion of CH4 (A) and CO2 (B), and the H2/CO ratio (C) of NiO–ZrOm–YOn and NiO–ZrOm catalysts at 700 °C with the mixed flow of CH4:CO2:Ar = 10:10:80, GSHV = 48,000 h−1.
Figure 4. The conversion of CH4 (A) and CO2 (B), and the H2/CO ratio (C) of NiO–ZrOm–YOn and NiO–ZrOm catalysts at 700 °C with the mixed flow of CH4:CO2:Ar = 10:10:80, GSHV = 48,000 h−1.
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Figure 5. The C 1s (A) and Raman (B) profiles of used NiO–ZrOm and NiO–ZrOm–YOn catalysts.
Figure 5. The C 1s (A) and Raman (B) profiles of used NiO–ZrOm and NiO–ZrOm–YOn catalysts.
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Figure 6. Transmission electron microscopy (TEM) images of used catalysts (A) NiO–ZrOm and (C) NiO–ZrOm–YOn and particle sizes of catalysts (B) NiO–ZrOm and (D) NiO–ZrOm–YOn.
Figure 6. Transmission electron microscopy (TEM) images of used catalysts (A) NiO–ZrOm and (C) NiO–ZrOm–YOn and particle sizes of catalysts (B) NiO–ZrOm and (D) NiO–ZrOm–YOn.
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Table
Table 1. The results of the Brunauer–Emmett–Teller (BET) experiment for NiO–ZrOm and NiO–ZrOm–YOn-calcined catalysts, the specific surface area (SBET) was determined by the BET method; the pore volume (VP) and the pore diameter (DP) determined by the Barrett–Joyner–Halenda (BJH) method. H2 consumption of calcined catalysts determined by hydrogen temperature programmed reduction (H2-TPR). The content of Zr4+ and Zr3+ on both catalysts after reduction determined by X-ray photoelectron spectroscopy (XPS), and the content of Ni on both catalysts determined by Inductively Coupled Plasma (ICP) Spectroscopy.
Table 1. The results of the Brunauer–Emmett–Teller (BET) experiment for NiO–ZrOm and NiO–ZrOm–YOn-calcined catalysts, the specific surface area (SBET) was determined by the BET method; the pore volume (VP) and the pore diameter (DP) determined by the Barrett–Joyner–Halenda (BJH) method. H2 consumption of calcined catalysts determined by hydrogen temperature programmed reduction (H2-TPR). The content of Zr4+ and Zr3+ on both catalysts after reduction determined by X-ray photoelectron spectroscopy (XPS), and the content of Ni on both catalysts determined by Inductively Coupled Plasma (ICP) Spectroscopy.
CatalystDP, nmSBET, m2/gVP, cm3/gH2 Consumption, mmol H2/gZr 3d (%)Ni (wt %)
TotalTheory aαβZr4+Zr3+
NiO–ZrOm–YOn2790.10.370.390.100.27168410
NiO–ZrOm21130.20.620.470.130.49109012
a Caculated by ICP and the comsuption of reduction of pure NiO.
Table
Table 2. Basicity measured by the content of CO2 desorption on CO2-temperature programmed desorption experiment over the NiO–ZrOm–YOn and NiO–ZrOm catalysts.
Table 2. Basicity measured by the content of CO2 desorption on CO2-temperature programmed desorption experiment over the NiO–ZrOm–YOn and NiO–ZrOm catalysts.
CatalystCO2 Peak Identification
Peak 1 (Weak)Peak 2 (Medium-Strength)Peak 3 (Strong)Total Basicity (μmol CO2/g)
Position (°C)Content (%)Position (°C)Content (%)Position (°C)Content (%)
NiO–ZrOm 18515.525264.438020.173
NiO–ZrOm–YOn15036.924055.438013.0100
Table
Table 3. Crystallite sizes of ZrO2 and Ni0 on NiO–ZrOm–YOn and NiO–ZrOm catalysts after reduction at 700 °C for 1 h and after reaction at 700 °C for 8 h, determined by XRD. The particle size of nickel after reaction for 8 h determined by transmission electron microscopy (TEM). The coke content after reaction for 8 h was determined by thermogravimetric analysis (TGA). The C–C content after reaction for 8 h determined by X-ray photoelectron spectroscopy (XPS). The IG/ID after reaction for 8 h determined by Raman spectroscopy.
Table 3. Crystallite sizes of ZrO2 and Ni0 on NiO–ZrOm–YOn and NiO–ZrOm catalysts after reduction at 700 °C for 1 h and after reaction at 700 °C for 8 h, determined by XRD. The particle size of nickel after reaction for 8 h determined by transmission electron microscopy (TEM). The coke content after reaction for 8 h was determined by thermogravimetric analysis (TGA). The C–C content after reaction for 8 h determined by X-ray photoelectron spectroscopy (XPS). The IG/ID after reaction for 8 h determined by Raman spectroscopy.
CatalystZrO2 (nm)Ni0 (nm)ParticleCoke Content (%)C–C Content (%)IG/ID
ReductionReactionReductionReactionSize (nm)
NiO–ZrOm77122415–203.7421.7
NiO–ZrOm–YOn77161010–151.038-

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Wang, Y.; Li, L.; Wang, Y.; Da Costa, P.; Hu, C. Highly Carbon-Resistant Y Doped NiO–ZrOm Catalysts for Dry Reforming of Methane. Catalysts 2019, 9, 1055. https://doi.org/10.3390/catal9121055

AMA Style

Wang Y, Li L, Wang Y, Da Costa P, Hu C. Highly Carbon-Resistant Y Doped NiO–ZrOm Catalysts for Dry Reforming of Methane. Catalysts. 2019; 9(12):1055. https://doi.org/10.3390/catal9121055

Chicago/Turabian Style

Wang, Ye, Li Li, Yannan Wang, Patrick Da Costa, and Changwei Hu. 2019. "Highly Carbon-Resistant Y Doped NiO–ZrOm Catalysts for Dry Reforming of Methane" Catalysts 9, no. 12: 1055. https://doi.org/10.3390/catal9121055

APA Style

Wang, Y., Li, L., Wang, Y., Da Costa, P., & Hu, C. (2019). Highly Carbon-Resistant Y Doped NiO–ZrOm Catalysts for Dry Reforming of Methane. Catalysts, 9(12), 1055. https://doi.org/10.3390/catal9121055

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