Introduction

The production of syngas (a mixture of H2 and CO) through dry reforming of methane is an excellent strategy to reduce the global warming effects of carbon dioxide (CO2) and methane (CH4). Noble metals such as palladium (Pd), platinum (Pt), and ruthenium (Ru) have been used for this purpose, but costly precursors and instability of catalyst, at high reaction temperature around 800 °C, have limited their application1. On the other hand, cost-effective nickel (Ni) metal, supported on an appropriate supports such as alumina2, mesoporous silicates3,4,5,6,7, and zirconia8,9,10, has been found to withstand at this reaction temperature (800 °C). In this context, many researchers have followed the surface modification methodology to optimise the catalyst performance because Ni-based catalyst is also prone to deactivation. The first series of modifications were carried out over alumina supports with K11,12, Mg, Ca, Ba, Sr 13,14,15,16, Y17, La18, Ce19, K-Ce20, Ti21, Zr22,23, Mo, W21, Mn24, Co & Cu25, Zn26, B27, Si21, and Sn14. Due to the extended pore network (from micro to meso) and easy pore tunable synthetic methodology of silicates, silica support is preferable over alumina support28. Therefore, the second series of modifications were carried out over mesoporous silicates supports with Li29, K30, Mg31,32, Ca30, Ba33, La34, Ce30,35,36, Zr37,38, Mn38, Co39,40, Cu41,42, Zn40, Al43 and Sn44. However, neither alumina nor silica supports can utilize their lattice oxygen during carbon deposit oxidation at the surface, but zirconia support does and is thus are used as oxygen carrier materials. Zirconia support is characterized by its thermal stability, an expanded network, and the ability to utilize its mobile oxygen during the surface reaction45. The third series of modifications were carried out over zirconia supports with K46, Mg47,48,49,50, K-MgO51, Ca52,53, La54,55, and Ce56,57,58. A brief literature survey of promoter/modifiers that were utilized over Ni-doped different supports is given in Table 1.

Table 1 A brief literature survey of promoter/modifier Ni-doped different supports.
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Use of strong solid base as CaO and MgO showed significant improvement and facilitated the catalytic performance with prompt adsorption of slightly acidic CO2 during dry reforming reaction over Ni-based catalysts. CaO coprecipitated Ni supported ZrO2 was well studied for different types of carbon species deposited over the catalyst surface during dry reforming of methane59. MgO modified Ni system is known for outstanding coking tolerance60. Chunwen Sun et al. showed that MgO modification might help to stabilize the lattice oxygen sites of NiO which effectively decrease the carbon deposition or graphitic layer formation61. Garcia et al. prepared the Ni/MgO–ZrO2–MgO (MgO loading in the range of 1–5 wt%) catalysts by co-precipitation method and found out that the CO2 and CH4 conversions were less than 35%47. Asencios and Assaf loaded Ni and Mg with different ratios on zirconia support by wet impregnation method and found out that catalyst with 20 wt% Ni and 20 mol% Mg has the best performance, where the activity was less than 80% in the oxidative reforming of methane49. Most of the research outputs in the literature used high loading of Ni or MgO (as high as 35 mol%) for the dry reforming reaction as Montoya et al. via sol–gel method56 and Titus et al. via melt impregnation50.

Herein, we prepared four catalysts via incipient wetness impregnation method, where the support was mesoporous zirconia, nickel as the active catalyst, and magnesium oxide as a promoter. We varied the amount of magnesium oxide to find its optimum loading for the best catalytic performance. Furthermore, we optimised the performance by varying reaction temperature. Catalysts were characterized by TGA, N2 physisorption, XRD, H2-TPR, and CO2-TPD. To understand the surface chemistry in optimizing the catalytic activity along with the stability of the modified catalyst, CO2-TPD, H2-TPD and O2-TPO of spent catalyst were also performed. A very fine-tuning, among catalytic activity and characterization results were performed; this will help to better understand the surface behaviour towards syngas production from dry reforming of methane.

Results and discussion

The catalytic activity of 5NixMgZr catalysts (x = 0, 3, 5, 7) in terms of CH4 conversion, CO2 conversion, and H2/CO mole ratio at 700 °C are shown in Fig. 1(A–C) and at 800 °C are shown in Fig. 1(D–F). The TGA results of spent catalysts are shown in Fig. 1(G,H), respectively. It is worth noting that without magnesium oxide modification, catalyst 5Ni/ZrO2 shows lower catalytic activity than that of magnesium oxide modified catalyst in all cases. At the reaction temperature of 700 °C, 5Ni/xMg–ZrO2 catalysts showed approximately 50–60% CH4 conversion and 65–75% CO2 conversion which were comparable to those in the recent publications10,47,49,54,62,63. The TGA results of these spent catalysts also showed carbon deposition. Interestingly, when the reaction temperature was set at 800 °C, it gave a stable performance with constant high conversion up to 500 min in the time-on-stream test (TOS) and no noticeable carbon deposition. Over 5Ni/3Mg–ZrO2 catalyst, 85% CH4 conversion, 92% CO2 conversion and 0.94 H2/CO ratios were achieved constantly up to 500 min in the TOS. On the target of H2/CO = 1, the performance of the 5Ni/5Mg–ZrO2 catalyst was found to be the best as it showed 82% CH4 conversion, 87% CO2 conversion. The 5Ni/7Mg–ZrO2 catalyst performance was a little bit lower than that of 5Ni/5Mg–ZrO2 (78% CH4 conversion, 86% CO2 conversion and H2/CO = 0.98). Whether to check the thermal decomposition of CH4 at 800 °C, reaction without catalyst was carried out with substrate CH4. It gave 3% CH4 conversion with 1.8% H2 yield in 3 h time on stream. Further again, a blank reaction was carried out with substrates CH4 and CO2 together at 800 °C. It resulted in 1.6% CH4 conversion, 3.6% CO2 conversion, H2 yield 0.63%, CO yield 4.25% and H2/CO = 0.14. As our catalytic systems are highly active towards DRM, so the thermal decomposition of CH4 as an intermediate step in DRM could be neglected.

Figure 1
figure 1

Catalytic activity profiles for methane dry reforming over various catalysts (A–F); (A) CH4 conversion at 700 °C reaction temperature, (B) CO2 conversion at 700 °C reaction temperature, (C) H2/CO mole ratio at 700 °C reaction temperature, (D) CH4 conversion at 800 °C reaction temperature, (E) CO2 conversion at 800 °C reaction temperature, (F) H2/CO mole ratio at 800 °C along with TGA curves of spent catalysts, (G) TGA results of spent catalyst carried out at 700 °C reaction temperature, (H) TGA results of spent catalysts carried out at 800 °C reaction temperature.

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To understand the surface behaviour of the DRM reaction, we characterised the catalyst thoroughly and discussed the characterization results herein. The surface area analysis indicated that after the addition of MgO, type IV adsorption–desorption curve with H1 hysteresis loop (Figure S1) was built up. It indicates the narrow distribution of mesopores.

XRD patterns of 5NixMgZr catalysts (x = 0, 3, 5, 7) are shown in Fig. 2(A–D). The diffraction lines at 2θ = 24.2°, 28.34°, 31.45°, 34.2°, and 55.4° were attributed to the monoclinic zirconia (m-ZrO2) whereas diffraction lines at 2θ = 30.48° and 50.24° were attributed to tetragonal zirconia (t-ZrO2). Cubic nickel oxide showed diffraction lines at 2θ = 37.2°, 43.28° and 62.9° for (111), (200) and (220) crystallographic planes, respectively. After the addition of basic promoter 3 wt% MgO, the crystalline peak intensity of ZrO2 remarkably increased as well as the selected plane of NiO (200) about 43.28° bragg angle also intensified and shifted to the lower angle 43.12°. It indicated the rapid growth of NiO-MgO solid solution50 after addition of MgO. Further addition of MgO did not show such a rapid rise of NiO-MgO solid solution.

Figure 2
figure 2

XRD of catalyst samples: m = monoclinic zirconia (m-ZrO2), t = tetragonal zirconia (t-ZrO2), n = NiO.

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The H2-TPR surface reduction profiles of fresh 5Ni/xMg–ZrO2 catalysts are shown in Fig. 3A. 5Ni/ZrO2 has one small reduction peak in the temperature range of 140–200 °C that attributed to the free NiO species, a shoulder reduction peak at the temperature range of 200–300 °C for “NiO weakly interacted with ZrO2 support” and a strong peak at 300–450 °C for “NiO that interacted strongly with ZrO2 support”. After the addition of 3.0 wt% MgO, these three peaks diminished and reduction peaks in the intermediate and high-temperature ranges appeared. The high reduction temperature for MgO modified samples could be correlated to the high inherent stability expected for NiO–MgO-solid solution with respect to pure NiO. From the XRD results, also after MgO modification, NiO–MgO-solid solution was found50. The intermediate temperature reduction peak in the range of 450–700 °C could be attributed to “NiO–MgO-solid solution weakly interacted with ZrO2 support” whereas high-temperature reduction peak in the range of 700–900 °C could be claimed to “NiO–MgO-solid solution strongly interacted with ZrO2 support”. As MgO loading was increased from 3.0 wt% to 5.0 wt%, the TCD signal intensity of the intermediate temperature reduction peak was decreased and high-temperature reduction peak was increased. These observations indicated that a higher amount of “NiO-MgO-solid solution strongly interacted on ZrO2” was present in 5Ni/5Mg–ZrO2 than 5Ni/3Mg–ZrO2, thus 5 wt% MgO was the optimum loading. At 7 wt% MgO loading, both types of high-temperature peaks were suppressed in comparison to those for 5Ni/5Mg-ZrO2. The H2-TPR surface reduction profile of spent 5Ni/3Mg–ZrO2 is shown in Fig. 3B. It showed that TPR peaks in the intermediate and high-temperature regions had got suppressed. Also, it was noticeable that a lower reduction temperature peak (0–400 °C) remained preserved as well as shifted to a lower temperature. The H2-TPR surface reduction profile of spent 5Ni/5Mg–ZrO2 indicated the suppression and shifting of high-temperature region peaks to intermediate temperature regions (Fig. S2). These observations indicated that NiO supported on ZrO2 was less involved whereas “NiO-MgO-solid solution interacted with ZrO2 support” are significantly involved in DRM. Apart from that, the elimination of carbon deposit by hydrogen gas during methane gasification reaction (C + 2H2 → CH4) over spent catalyst system was also possible64.

Figure 3
figure 3

(A) The H2-TPR profile of 5Ni/xMg-ZrO2, (B) the H2-TPR profile of fresh and spent 5Ni/3Mg-ZrO2, (C) the CO2-TPD surface reduction profile of 5Ni/xMg-ZrO2, (D) CO2-TPD and O2-TPO TPD profile of fresh and spent 5Ni/3Mg-ZrO2 catalyst.

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The CO2-TPD profiles of 5Ni/xMg–ZrO2 are shown in Fig. 3C. Without magnesium oxide modification, the catalyst showed a sharp peak at lower temperature (weak basic sites) region and in intermediate temperature (medium basic sites) regions, but a broad peak in higher temperature regions (strong basic sites). This profile indicated a wide distribution of basic sites. However, after loading of 3.0 wt% MgO, only weak basic sites remained preserved; the rest disappeared. Surprisingly, basic modifier addition caused the disappearance of basicity. XRD of the same sample showed the appearance of NiO–MgO-solid solution as well as the rise of ZrO2 crystallinity. This means that after the addition of basic 3.0 wt% MgO, basic MgO was engaged in the nurture of NiO–MgO solid solution and supported the crystallinity, thus it caused the disappearance of basicity. It caused the removal of intermediate strength as well as strong strength basic sites from the surface. Again, at 5 wt% MgO loading, peak reappeared in the intermediate temperature region whereas it broadened in high-temperature regions. As the TGA profile of the spent catalyst did not show markable carbon deposition, it is interesting to observe the basic profile of the spent catalyst.

The CO2-TPD profile of fresh as well as spent 5Ni/3Mg–ZrO2 & 5Ni/5Mg–ZrO2 catalyst are shown in Fig. 3D and Figure S3 respectively. Figures 3D and S3 also include O2-TPO and “CO2-TPO followed by O2-TPO” of spent 5Ni/3Mg–ZrO2 and 5Ni/5Mg–ZrO2 catalysts, respectively. It is obvious from the fresh and spent CO2-TPD samples that there was a significant decrease in the intensity of basic sites after the reaction over the spent catalysts. However, unlike the fresh samples, the spent catalysts showed a small peak in CO2-TPD. Again, a consumption (negative) peak in O2-TPO of spent 5Ni/3Mg–ZrO2 and spent 5Ni/5Mg–ZrO2 catalyst samples were also seen at about the same temperature region. Interestingly, O2-TPO (carried out after CO2-TPD) of spent 5Ni/3Mg–ZrO2 and spent 5Ni/5Mg–ZrO2 catalysts had no such O2 consumption peak. It can be explained that O2 consumption peak in O2-TPO was due to oxidation of residual carbon by O2 into CO2. So, the small evolution peak in CO2-TPD profile also indicated the oxidation of residual carbon deposit by CO2. As the carbon deposit on the surface of the catalyst was already oxidized by CO2 during CO2-TPD profile so when O2-TPO was carried out after CO2-TPD, no evolution peak was found. This confirmed the oxidation of the carbon deposit by CO2 over the surface of the catalyst45,55. Oxidation of carbon deposit by lattice oxygen of ZrO2 and thereafter simultaneous compensation of the oxygen vacant sites by CO2 (through losing one of its oxygen to the vacant site) might be a possible route of oxidation of carbon deposit by CO2.

To study the conditions and sites of CH4 decomposition, CH4-temperature programmed surface reaction (CH4-TPSR) experiment over ZrO2, 5Ni/ZrO2 and 5Ni/3Mg–ZrO2 were carried out (Fig. 4). It shows a decrease in the methane concentration with temperature over catalysts due to methane decomposition reaction on the surface. For ZrO2, a single prominent consumption peak at 870 °C temperature was noticed due to CH4 interaction at ZrO2 surface53. After the addition of Ni, apart from the high-temperature peak, a lower temperature CH4 consumption peak at about 350 °C and an intermediate temperature broad peak in the range of 400–800 °C were observed. Low temperature and intermediate temperature peaks could be claimed to the catalytic decomposition of CH4 over Ni active sites as well as Ni–Zr interface53. MgO containing catalysts (i.e. 5Ni/3Mg–ZrO2) also showed the intense peak at high temperature (about 800 °C), attributed to the effect of the temperature. At higher reaction temperature (about 800 °C), an endothermic feature of DRM reaction promotes more efficient catalytic decomposition of CH4 over Ni and Ni–Zr interface over 5Ni/3Mg–ZrO2 catalyst systems. This could explain the excellent CH4 conversion over the magnesium modified catalyst system. It is worth noting that the high-temperature peak is near to the reaction temperature region according to the CH4-TPSR profiles. That means if dry reforming of methane was carried out in the temperature region of 700 °C, an advantage of high temperature favourable endothermic feature (about 800 °C) of DRM reaction would be missing as shown in Fig. 4. It might be an indication of lower catalytic conversion at the lower reaction temperature.

Figure 4
figure 4

CH4-TPSR profile of catalysts.

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Discussion

Thermal decomposition of CH4 and thereby oxidation of carbon deposits by CO2 towards dry reforming of methane is albeit possible with little activity i.e. 1.6% CH4 conversion, 3.6% CO2 conversion, H2/CO = 0.14. So, the catalytic role is utmost demanded in DRM. The summary of the catalytic activity of different catalysts towards dry reforming of methane is shown in Fig. 5. At 700 °C reaction temperature, comparable CH4 conversion, and CO2 conversion, were observed. At high reaction temperature, about 800 °C, an endothermic feature of DRM reaction was ruled over. It efficiently promotes catalytic decomposition of CH4 over Ni and Ni–Zr interface and thereafter oxidation of deposit by CO2. So, at 800 °C, all catalysts showed high CH4 and CO2 conversion as well as nearly no carbon deposit over the surface of the catalysts. Yang et al.60 also claimed MgO modified Ni system as outstanding coking tolerance and Chunwen et al.61 explained the effective reduction of carbon deposit by MgO modified Ni system by stabilization of lattice oxygen sites of NiO by MgO.

Figure 5
figure 5

Summary of catalytic activity for different catalyst systems.

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5Ni/ZrO2 had free NiO species, “NiO species interacted with support” and a wide range of basicity. CO2 uptake at basic sites, catalytic decomposition of CH4 at Ni and Ni–Zr and oxidation of deposits by CO2 pivoted the way of high-performance dry reforming reaction. It showed a constant 76% CH4 conversion, constant 84% CO2 conversion and 0.99 H2/CO ratios for 130 min, then a ratio of 0.98 for 300 min and finally a ratio of 0.96 for 500 min.

After modifying the catalyst with 3.0 wt% MgO, NiO–MgO-solid solution was built up. With a wide range of NiO–MgO-solid solution interaction (weakly as well as strongly with support ZrO2), 5Ni/3 Mg–ZrO2 promoted the efficient catalytic decomposition of CH4 over Ni, Ni–Zr interface and thereafter oxidation of deposit by CO2. Thus, 5Ni/3Mg–ZrO2 showed high 85% CH4 conversion and 92% CO2 conversion with H2/CO ratio ~ 0.96. The CO2-TPD, as well as O2-TPO profile of spent catalysts, showed an extra peak in TPD and a negative (consuming) peak in TPO, respectively which both related to the oxidation of residual carbon deposits on the surface of the catalyst. The CO2-TPD along with the O2-TPO results showed that CO2 is capable of oxidizing carbon deposit over the surface of the catalyst. Removal of carbon deposits by hydrogen gas through methane gasification reaction (C + 2H2 → CH4) is also possible64. It resulted in stable catalytic activity up to 500 min in the TOS test. Furthermore, modifying the catalyst with 5 wt%MgO in 5Ni/5Mg–ZrO2, it showed more amount of “NiO–MgO solid solution strongly interacted with ZrO2 support” as well as a wide variety of basic sites. That catalyst showed a constant conversion (82% CH4 conversion and 89% CO2 conversion) as well as H2/CO ratio = 1 for 250 min in the TOS then slightly decreased to 0.99 for another 250 min, with overall 500 min TOS. Thus, it could be concluded that 5 wt% MgO loading is optimum loading for an active and stable catalyst for methane dry reforming reaction. Further increase in magnesium oxide loading to 7 wt% MgO caused a decrease in NiO–MgO-solid solution that interacted weakly or strongly with the ZrO2 support and consequently the loss of strong basic sites. Thus, decreasing the CH4 conversion to 79% as well as CO2 conversion to 86% and H2/CO ratio to 0.98 were noticed.

Conclusion

Magnesium promoted NiO supported mesoporous zirconia, 5Ni/xMg–ZrO2 (x = 0, 3, 5, 7) were prepared and tested for the methane dry reforming reaction. Higher activity was found at 800 °C than that at 700 °C due to favourable endothermic feature of DRM reaction which promotes efficient CH4 decomposition over Ni and Ni–Zr interface and successive oxidation of carbon deposits by CO2. By modifying the catalyst (5Ni/ZrO2) with MgO as a promoter, NiO–MgO-solid solution was formed. It was found that for high constant CH4 and CO2 conversions, NiO–MgO-solid solution played a significant role during the DRM. The 5Ni/3Mg–ZrO2 catalyst showed a constant 85% CH4 conversion and 92% CO2 conversion up to 500 min on stream at H2/CO mole ratio ~ 0.96. The highly constant performance of magnesium oxide modified catalysts was due to the ability of CO2 to oxidize the carbon deposits during the DRM, thus maintaining the catalytic stability. However, with a further loading (> 5.0 wt% Mg) such as in 5Ni/5Mg–ZrO2 which showed a higher amount of “NiO–MgO-solid solution strongly interacted with ZrO2 support” along with a wide variety of basic sites as well. Thus, it showed a constant 82% CH4 conversion and 89% CO2 conversion and H2/CO mole ratio ~ 1. It is hoped that these findings could inspire finding more stable and less expensive synthesis gas production catalysts, including from two potent greenhouse gases emissions such as methane and carbon dioxide.

Experimental

Materials

Nickel nitrate hexahydrate [Ni (NO3)2.6H2O, 98%, Alfa Aesar], magnesium acetate tetra-hydrate [Mg(O2CCH3)2.4H2O, 99.5–102.0%, Merck], mesoporous zirconia (meso-ZrO2, 1/8" pellets, Alfa Aesar) were commercially available and were used without further purification. Ultrapure water was acquired from a Milli-Q water purification system (Millipore).

Catalyst preparation

A two-step procedure, based on incipient wetness impregnation as described elsewhere21, was followed for synthesizing the desired catalysts. The first step was to dope the support with a metal oxide promoter, while the second step was to load nickel oxide over the promoted support. The detailed description of each synthesis step is given below.

Synthesis of mesoporous zirconia promoted with magnesia (MgO-meso-ZrO2)

The required amount of Mg (CH3-CO2)2.4H2O for 3.0, 5.0, or 7.0 wt/wt% loading of MgO was mixed and pulverized with the required amount of meso-ZrO2. To this resultant solid mixture, drops of ultrapure water were added until the formation of a colourless paste, which was mechanically stirred until complete dryness at room temperature. The addition of water and drying processes were performed three times to ensure homogeneous distribution of Mg (CH3CO2)2 within the matrix of meso-ZrO2. The solid mixture was then grounded and calcined in a muffle furnace, at 600 °C for 3 h in the static air atmosphere. The resultant materials were designated as xMg–ZrO2 catalysts where x is wt% of MgO (x = 0, 3, 5, 7).

Synthesis of mesoporous zirconia supported nickel oxide promoted with magnesia (NiO/MgO-meso-ZrO2)

The required amount of Ni (NO3)2.6H2O to obtain 5.0 wt/wt% of NiO loading was mixed and was crushed with the required amount of MgO-meso-ZrO2 of the desired MgO wt/wt% loading, forming a green solid mixture. Drops of ultrapure water were then added to get a paste. By continuous mechanical stirring, the paste was dried at room temperature. The wetting and drying processes were repeated three times. Afterwards, calcination was performed at 600 °C for 3 h in static air atmosphere. Overall, 5 wt% NiO loaded catalyst sample is designated as 5Ni/xMg–ZrO2 catalysts where x is wt% of MgO (x = 0, 3, 5, 7).

Catalyst characterization

The details of instrument specifications and procedures are described in the supporting information and described elsewhere21.

Catalyst test

DRM was carried out in a fixed-bed stainless steel tubular micro-reactor (ID = 9 mm) at atmospheric pressure. A load of 0.10 g catalyst was activated under 20 SCCM H2 flow at 800 °C for 60 min. Then 20 sccm of N2 was fed to the reactor for 20 min at 800 °C to remove adsorbed H2. Afterwards, CH4, CO2, and N2 were dosed at flow rates of 30, 30 and 5 sccm, respectively. A GC (GC-2014 Shimadzu) unit, equipped with a thermal conductivity detector and two columns, Porapak Q and Molecular Sieve 5A, was connected in series/bypass connections to have a complete analysis of the reaction products. The following equations were used to calculate the conversion of each reactant and the H2/CO mole ratio, respectively21.

$$\begin{aligned} & {\text{CH}}_{4} \;{\text{conversion}} = { }\frac{{{\text{CH}}_{{4,{\text{in}}}} - {\text{CH}}_{{4,{\text{out}}}} }}{{{\text{CH}}_{{4,{\text{in}}}} }}{ } \times { }100{\text{\% }} \\ & {\text{CO}}_{2} { }\;{\text{conversion}} = { }\frac{{{\text{CO}}_{{2,{\text{in}}}} - {\text{CO}}_{{2,{\text{out}}}} }}{{{\text{CO}}_{{2,{\text{in}}}} }}{ } \times { }100{\text{\% }} \\ & \frac{{{\text{H}}_{{2{ }}} }}{{{\text{CO}}}}{ } = { }\frac{{{\text{mole}}\;{\text{ of }}\;{\text{H}}_{2} \;{\text{produced}}}}{{{\text{mole }}\;{\text{of }}\;{\text{CO }}\;{\text{produced}}}} \\ \end{aligned}$$