The effect of noble metals on catalytic methanation reaction over supported Mn/Ni oxide based catalysts

3.1.1 Catalytic activity screening of alumina supported nickel oxide based calcined at 400 °C for 5 h

The supported monometallic oxide catalyst (Ni/Al₂O₃, Mn/Al₂O_3, Ru/Al₂O₃ and Pd/Al₂O₃) calcined at 400 °C showed very low catalytic activity towards CO₂/H₂ methanation reaction. Ni/Al₂O₃ catalyst gave a high CO₂ conversion of 13.30% at maximum reaction temperature studied compared to the other oxides catalysts (Table 1). These catalysts did not able to achieve high conversion at low reaction temperature however they showed the capability to be used in methanation reaction. Thus by incorporating manganese and noble metals into system, they would enhance the catalytic activity.

Referring to Table 1, it can be observed that the addition of Mn slightly increased the catalytic performance compared to the monometallic oxide (Ni/Al₂O₃) catalyst. At 400 °C reaction temperature, Mn/Ni(20:80)/Al₂O₃ catalyst gave 17.50% of CO₂ conversion while Mn/Ni(40:60)/Al₂O₃ catalyst was able to obtain 15.30% conversion only. It is probably due to the largest amount of dopant blocking the pores structure of the catalyst and thus decreasing the activity. Besides, the catalytic performance of both Ru/Ni(20:80)/Al₂O₃ and Pd/Ni(20:80)/Al₂O₃ catalysts also increased in a little amount. As can be noticed in Table 1, these bimetallic oxide catalysts have a low percentage of CO₂ conversion (<18%). Thus, alumina supported manganese–nickel oxide based catalyst was modified by incorporating with noble metal, ruthenium and palladium to study their effect towards the catalytic activity.

Incorporating palladium (Pd) into this catalyst (Pd/Mn/Ni(5:35:60)/Al₂O₃) slightly increased the catalytic performance towards CO₂ conversion up to 25.30%. Meanwhile, when ruthenium (Ru/Mn/Ni(5:35:60)/Al₂O₃) was added as a co-dopant further reduction of catalytic performance was observed which only gives 14.00% CO₂ conversion. The decreasing performance of this catalyst could be due to the Ru precursor, RuCl₃.xH₂O used in this research. A small amount of chloride ion present in Ru/Al₂O₃ catalyst could give poisoning effect to the catalyst and thus lead to decrease active sites on the surface of Ru catalyst. A similar finding was concluded by Nurunnabi et al. (2008). The residual chloride ions formed partition between the support and metal and thus, inhibits both CO and hydrogen chemisorption phenomena on the catalyst surface. Chloride precursor can be observed in the as-synthesis of Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst as shown in EDX data (Table 5).

When nickel loading was increased up to 80 wt%, the performance of the catalyst also increased with the increasing temperature reaction. The addition of palladium into this catalyst which is Pd/Mn/Ni(5:15:80)/Al₂O₃, coincidentally enhanced the catalytic activity of CO₂ conversion. Only 5.20% of CO₂ conversion at 100 °C reaction temperature increased to 49.00% at 400 °C reaction temperature. This suggests that a small amount of Pd can play important role in enhancing the catalytic activity. A study by Baylet et al. (2008) found that addition of palladium to the alumina support material gives sufficient absorption for CO₂ dissociation process which is due to the increasing active sites on catalyst surface. As expected, the addition of ruthenium into the catalyst also would increase the catalytic performance but slightly lower than the addition of palladium. Only 32.50% of CO₂ conversion was achieved at maximum studied temperature of 400 °C.

3.1.2 Catalytic activity screening of alumina supported nickel oxide based catalysts calcined at 700 °C for 5 h

The potential catalysts were further studied at 700 °C calcination temperature and the results are summarized in Table 2. At this stage, Ni(1 0 0)/Al₂O₃ catalyst displayed a slight increase in activity compare to the similar catalyst calcined at 400 °C. The addition of manganese into the system (Mn/Ni/Al₂O₃ catalyst), only 20% of CO₂ had been converted.

It can be observed that Pd/Mn/Ni(5:15:80)/Al₂O₃ shows the highest catalytic activity at the maximum reaction temperature of 400 °C. However, Pd/Mn/Ni(5:35:60)/Al₂O₃ showed lower activity (24.40%) compared to the other catalyst (Pd/Mn/Ni(5:15:80)/Al₂O₃). When ruthenium was used as co-dopant in Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst, it presented an increase of catalytic performance from 14.00% to 34.00% of CO₂ conversion when calcined at 400 °C and 700 °C, respectively. In contrast, at similar reaction temperature studied, the performance of Ru/Mn/Ni(5:15:80)/Al₂O₃ catalyst was slightly decreased from 32.50% at calcination temperature of 400 °C to 13.60% at calcination temperature of 700 °C.

This finding was supported by Murata and co-workers (2009) who studied the effect of Ru and Mn concentration on the Fischer–Tropsch reaction. They claimed that by increasing/decreasing Ru or Mn content it will affect the CO₂ conversion. The results showed that the CO₂ conversion and selectivity towards CH₄ were 42.9% and 9.10%, respectively using Ru to Mn ratio of 5:10. They also stated that the high CO₂ conversion was probably due to the Mn species which causes the removal of Cl ions from RuCl₃ precursor and increases the density of active Ru oxide species on the catalyst which resulted in a high catalytic activity. In contrast, Ru/Mn/Ni(5:15:80)/Al₂O₃ catalyst showed a low catalytic performance. It might be due to the calcination temperature applied on this catalyst cannot prevent the coke deposition onto the active site of the catalyst. Branford and Vannice (1998), suggested that reduction temperature more than 1000 °C is necessary to remove most residual Cl from supported catalysts.

3.1.3 Catalytic activity screening of alumina supported nickel based catalysts calcined at 1000 °C for 5 h

Table 3 exhibits the variation of catalytic performance over alumina supported nickel oxide based catalysts which were calcined at 1000 °C. Monometallic and bimetallic oxide catalysts exhibit a similar trend with increasing calcination temperature.

The addition of noble metals resulted in decreasing catalytic activity in both Pd/Mn/Ni(5:35:60)/Al₂O₃ and Pd/Mn/Ni(5:15:80)/Al₂O₃ catalysts. Further reduction might be because of Mn and Pd was not good oxide combination in methanation process and it will retard the process. This is in agreement with Panagiotopoulou et al. (2008) who found that Pd was found to be the least active catalyst which only gave less than 5% CO₂ conversion at 450 °C. The atomic size of Pd (137 pm) is much higher than that of Mn (127 pm). This may cause pore blockage because of the bigger size of Pd which retarded the methanation reaction.

Surprisingly, Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst showed the highest catalytic activity among the catalysts. The catalytic performance of this catalyst keeps on increasing until it reaches the maximum reaction temperature studied (400 °C). At the reaction temperature of 100 °C, only 7.50% CO₂ was converted but the performance turns to increase drastically until it reached 300 °C of reaction temperature. About 99.30% of CO₂ conversion was observed. At the maximum reaction temperature studied (400 °C), the catalytic activity was increased to 99.70% of CO₂ conversion. A similar catalytic behaviour has been observed on the other ratio of Ru/Mn/Ni(5:15:80)/Al₂O₃ catalyst calcined at 1000 °C, whereby the conversion is continuously increasing compared to the performance of similar catalyst calcined at 700 °C.

A research done by Samparthar et al. (2006) claimed that the total pore volume of the calcined samples will decrease as the loading of the transition metal oxides increases. The decreasing behaviour of both surface area and total pore volume with the increasing loading of metal oxides is consistent due to possible blockage of the inner pores especially the smaller ones. However, this finding cannot be proved in our research due to insufficient data. Similar reason can be applied to the Pd/Mn/Ni(5:15:80)/Al₂O₃ catalyst which displays a decreasing trend with the addition of nickel loading.

The above results suggested that the high calcination temperature activates the catalytic centres of the catalyst, thus enhancing the activity. The calcination temperatures are critical for controlling the size of the metal particles and their interaction with Al₂O₃ as suggested by Chen et al. (2009) who investigated the effect of calcination temperatures on nickel catalyst for methane decomposition. It was found that when the calcination temperature increases, the average size of the crystallites increases and it will help to increase the catalytic activity towards CO₂ conversion. In conclusion, Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst was selected as potential catalysts and was further investigated to seek the optimum condition for this catalyst.

3.2.1 Effect of various compositions of prepared catalyst

In order to determine the effect of various compositions towards the catalytic activity, 55–70 wt% of nickel loadings have been used in this research. The detailed trend plot of catalytic performance over Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst towards the percentage CO₂ conversion is displayed in Fig. 2. Generally, all the catalysts prepared showed lower performance of CO₂ conversion at low reaction temperature but started to increase drastically from 200 °C until maximum studied reaction temperature of 400 °C.

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Figure 2

Catalytic performance of CO₂ conversion for CO₂/H₂ methanation reaction over Ru/Mn/Ni/Al₂O₃ catalyst at different compositions calcined at 1000 °C for 5 h.

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It can be seen that Ru/Mn/Ni(5:40:55)/Al₂O₃ catalyst only gave 15.54% CO₂ conversion at 200 °C reaction temperature. By raising the nickel content to 60 wt%, the conversion of CO₂ increased to 25.00%. However, the catalytic performance was reduced to 18.33% with the increasing of Ni loading to 70 wt% in the Ru/Mn/Ni(5:25:70)/Al₂O₃ catalyst. Mostly, these catalysts achieved more than 99% of CO₂ conversion at 300 °C reaction temperature. Catalyst labelled as Ru/Mn/Ni/Al₂O₃ with the ratio of 5:35:60 had been preferred to be the optimum ratio as it showed better performance at low reaction temperature.

From the catalytic performance, it can be concluded that the composition of the catalyst might cause the alteration of catalyst structure which is highly related to the catalytic performance and selectivity towards methanation reaction. Due to the high capability of Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst which contributed to high performance, this catalyst was further studied on the next parameter; the effect of various calcination temperatures.

3.2.2 Effect of different calcination temperatures

This parameter was conducted to determine the effect of various calcination temperatures on the most potential catalysts. Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst was prepared and coated on alumina support and then aged for 24 h before further calcined at three different temperatures of 900, 1000 and 1100 °C. Fig. 3 indicates the trend plot of catalytic activity over Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst at various calcination temperatures.

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Figure 3

Catalytic performance of CO₂ conversion for CO₂/H₂ methanation reaction over Ru/Mn/Ni(5:35:60)/Al₂O₃ catalysts calcined at various calcination temperatures for 5 h.

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All the catalysts show increasing catalytic activity with the rise of reaction temperature. It has been revealed that the highest CO₂ conversion was obtained by Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst which was calcined at 1000 °C. From 25.00% of CO₂ conversion at 200 °C reaction temperature, it increased drastically to 99.70% conversion at the maximum reaction temperature studied (400 °C). However, the percentage of CO₂ conversion was slightly decreased to 99.20% at 400 when the catalyst was calcined at 1100 °C. Meanwhile, at 900 °C calcination temperature, about 96.40% of CO₂ conversion can be obtained at similar reaction temperature.

The high temperature used during calcination could cause agglomeration of catalyst particles thus forming larger crystallite and decreasing the surface area, consequently producing less active catalyst. According to Oh et al. (2007) the growth of crystallite size and morphology of the catalyst surface have strong relationship with calcination temperatures. This was in a good agreement with XRD diffractogram and FESEM morphology as will be discussed in characterization section after this.

Thus, it can be concluded that 1000 °C was the optimum calcination temperature over Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst. Both catalysts were then tested in the presence of H₂S gas in the gas mixtures.

3.2.3 Effect of H₂S gas over Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst

Durability testing of catalyst is an important factor for the practical use of catalysts. Hence, this test was carried out in the H₂/CO₂ gas mixture with a small amount of poison gas (H₂S), which commonly leads to deactivation of the catalyst. In this experiment, the respective catalyst was fed by 1% of H₂S gas during catalytic reaction. Fig. 4 indicates the comparison of catalytic activity with or without the presence of H₂S gas over Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst.

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Figure 4

Effect of the presence of H₂S gas over Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst calcined at 1000 °C for 5 h.

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The Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst was not able to achieve 100% H₂S desulfurization as shown in Fig. 4. It can only convert 41% of H₂S to elemental sulfur at 100 °C reaction temperature and increased up to 86% at the 300 °C reaction temperature studied. After 300 °C reaction temperature, the catalyst started to deactivate due to the sulfur deposition on the catalyst surface. Consequently, the CO₂ conversion over Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst decreased significantly in the presence of hydrogen sulfide gas mixtures from 99.70% (without H₂S) to 7.5% (with H₂S).

The deterioration of the catalyst occurred at higher reaction temperature owing to sulfur formation which had covered the surface catalyst thus avoiding the next flowing H₂S to be converted hence retard the reduction of CO₂ during methanation reaction (Wan Abu Bakar et al., 2011). Moreover, a research done by Dokmaingam et al. (2007) also supports our finding because similar phenomenon occurred in their methane steam reforming reaction in which their activity rate dramatically decreased over Ni/Al₂O₃ and Ni/CeO₂ in the presence of H₂S due to the sulfidation on the surface of the catalysts.

Unexpectedly, carbon monoxide has been observed in FTIR spectrum during reaction in the presence of H₂S gas. It is probably because of incomplete reaction between CO₂ and H₂ which tends to form CO as intermediate species (not shown). No methane peak can be distinguished. The toxic H₂S gas will prevent the catalyst to convert reactant gases; CO₂ and H₂ to produce methane.

3.2.4 Reproducibility test towards potential catalyst

The reproducibility of catalytic activity over Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst was tested using the similar potential catalyst for several times until the catalyst deactivated. Fig. 5 shows the trend plot of reproducibility testing over Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst.

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Figure 5

Trend plot of reproducibility testing over Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst calcined at 1000 °C for 5 h toward CO₂ conversion from methanation reaction.

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Below 200 °C of reaction temperature, it can be seen that the percentage CO₂ conversion was slightly lower than 26.00%. Interestingly, increasing the temperature above 200 °C, a sharp inclination occurred and achieved 99% CO₂ conversion at 280 °C reaction temperature and continuously to do so until it reached the maximum reaction temperature studied (400 °C). It can be distinguished that from 1st test until 7th test, the catalytic performance was almost similar. However, after the seventh testing, catalytic activity slightly decreased to 55% CO₂ conversion at 280 °C reaction temperature but still active at high reaction temperature. It is probably due to the surface of catalyst which was covered by CO₂ thus slightly decreasing the catalytic performance.

3.2.5 Stability testing over the Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst

The catalytic stability of the potential Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst was investigated on stream for 5 h continuously at 250 °C reaction temperature as presented in Fig. 6. The Ru/Mn/Ni(5:35:60)/Al₂O₃ catalysts showed a good stability which was maintained unaffected for 5 h of maximum monitoring reaction time without deterioration by carbon. The CO₂ conversion of Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst was maintained at almost 100% throughout the reaction time.

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Figure 6

Stability test over Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst calcined at 1000 °C for 5 h at 250 °C reaction temperature.

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Even though nickel oxide catalyst is easily deactivated by carbon deposition, the addition of manganese and ruthenium would assist the catalyst to be stable during the reaction. This was in good agreement with Zhao et al. (2012) who found that modifying nickel based with manganese significantly leads to the most stable catalyst compared to the unmodified NiO/Al₂O₃ catalyst. From these results, it can be concluded that the Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst is still active and stable even if it was left on for 5 h under high reaction temperature.

3.2.6 Methane gas formation measurement via gas chromatography

The reactor gas product from FTIR cell was collected and analyzed for CH₄ formation. The methane formation was determined via GC because of low sensitivity of FTIR spectroscopy towards methane stretching region. Table 4 shows the catalytic activity of CO₂/H₂ methanation over the potential Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst.

There are three possible products obtained during CO₂/H₂ methanation reaction namely carbon monoxide, water and methane. A trend could be noticed that the percentage of unreacted CO₂ decreased as the CO₂ was converted to H₂O, CO and CH₄. Besides, the formation of CH₄ also increased as reaction temperature increased. In the Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst, none of methane production has been observed at 100 °C reaction temperature but converted CO₂ tends to yield by- products such as CO and H₂O. The higher methane formation was reached at 400 °C with 72.36%.

These results are in a good agreement with Yaccato et al. (2005) who found that at lower temperature, methanation reaction tends to yield CO and at higher reaction temperature CH₄ was formed. The higher methane formation was reached at 250 °C with 76%. Higher methane has been produced possibly attributed to the rapid hydrogenation of intermediate CO species resulting in higher CO₂ methanation activities at this temperature.

3.3.1 The effect of catalytic testing over Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst calcined at 1000 °C for 5 h by XRD analysis

Fig. 7 shows the XRD patterns for potential Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst which was calcined at 1000 °C for 5 h. XRD diffractograms for used catalysts were found to be similar with fresh catalyst in which owing polycrystalline with some amorphous phase in nature.

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Figure 7

XRD patterns of Ru/Mn/Ni(5:35:60)/Al₂O₃catalysts calcined at 1000 °C for 5 h.

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The XRD pattern over Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst calcined at 1000 °C in fresh condition showed the presence of several oxides on the surface catalyst. High crystallinity of rhombohedral Al₂O₃ can be observed at 2θ of 35.10 (I₁₀₀), 43.34 (I₉₄), 57.48 (I₇₉), 25.56 (I₇₄), 37.70 (I₄₅), 52.50 (I₄₂), 68.17 (I₄₁) and 66.37° (I₂₈) with d values of 2.55, 2.08, 1.60, 3.47, 2.38, 1.74, 1.37 and 1.40 Å (PDF d values of 2.55, 2.08, 1.60, 3.48, 2.38, 1.74, 1.37 and 1.40 Å). However, there is some amorphous character within the crystalline peak which belongs to the alumina cubic indicating the smaller particle sizes owing to the respective catalyst. Interestingly, new peaks attributable to the NiO rhombohedral phase species were observed at 2θ of 43.40 (I₉₈) and 37.38 (I₉₅) with d values of 2.08 and 2.40 Å (PDF d values of 2.08 and 2.41 Å). Meanwhile, RuO₂ tetragonal species intensely located at 2θ of 35.19 (I₁₀₀), 28.10 (I₃₂) and 54.44 (I₁₉) with d values of 2.55, 3.17 and 1.68 Å (PDF d values of 2.55, 3.17 and 1.68 Å)were observed. However, the intensity for MnO₂ tetragonal was very small and hardly distinguished from the background noise. It is probably because of MnO₂ present in low quantities and overlapped with other species thus less sensitive towards XRD analysis. Unexpectedly, two peaks assigned as NiAl₂O₄ species have been detected at 2θ of 37.38 (I₁₀₀) and 65.64°(I₄₃) with d values of 2.40 and 1.42 Å (PDF d values of 2.42 and 1.42 Å) but not obviously can be seen.

It is noteworthy that Al₂O₃ still remains in rhombohedral and cubic phases after catalytic testing (Fig. 7(b)). Meanwhile, NiO species were observed in rhombohedral phase which present in lower intensity. Unexpectedly, NiAl₂O₄, RuO₂ and MnO₂ species were disappeared in both used Ru/Mn/Ni(5:35:60)/Al₂O₃ catalysts suggesting the well dispersion of these species on the surface of the catalysts that below the XRD detection limit. Wan Abu Bakar et al. (2010) revealed that some species collapse after undergoing catalytic testing due to the well dispersion of these particles into the bulk matrix of the catalyst. This phenomenon also can be supported by Zhao et al. (2012) who found no manganese oxide crystalline phase can be detected by XRD analysis after catalytic testing.

The continuous emergence of NiO in Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst (before and after catalytic testing) may suggested that this species can be considered as active species. The recommended active species had increased the percentage removal of CO₂ and at the same time increase the formation of CH₄ as had been discussed before_.

3.3.2 The effect of catalytic testing by FESEM-EDX analysis on Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst calcined at 1000 °C for 5 h

Fig. 8 shows the effect of catalytic testing on the Ru/Mn/Ni(5:35:60)/Al₂O₃ catalysts in various conditions for instance as synthesis (before calcine), fresh (before reaction), used1x and used7x (after reaction) catalysts.

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Figure 8

FESEM micrographs of Ru/Mn/Ni(5:35:60)/Al₂O₃ calcined at 1000 °C for 5 h, (a) as-synthesis, (b) fresh, (c) used1x, (d) used7x.

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These catalysts display inhomogeneous mixtures of aggregated and agglomerated particles in spherical shape. Additionally, these catalysts have been proved to be nano categorised since their particle sizes are in the range of 36–75 nm. Furthermore, these findings were well supported by XRD diffractograms denoted as polycrystalline with some amorphous character for all catalysts (Fig. 7). Smaller particles size lead to higher metal dispersion and thus increase the surface area of the catalyst as well as catalytic activity. From the micrographs, it also can be noted that the average particle size of as-synthesis, fresh and used catalysts remained unchanged suggesting that no significant changes occurred under reaction conditions.

Meanwhile, EDX analysis for all Ru/Mn/Ni(5:35:60)/Al₂O₃ catalysts confirmed the presence of Al, O, Ni, Mn and Ru. As written in Table 5, the percentage of weight ratios for each element in used catalyst was decreased compare to the fresh catalyst except for oxygen atom (O). Al and O contributed the highest percentage of weight ratio since the usage of alumina (Al₂O₃) as support in this research. As-synthesis catalyst showed the presence of 6.16 wt% of Ni, 2.49 wt% of Mn, and 1.43 wt% of Ru as well 3.29 wt% of chloride precursor has been detected.

The fresh Ru/Mn/Ni (5:35:60)/Al₂O₃ catalyst had attained 5.79 wt% of Ni, 4.35 wt% of Mn and 4.08 wt% of Ru. After catalytic testing, the weight ratio of each element became lower. After seventh testing, a small amount of 0.58 wt% and 0.27 wt% of Ru and Mn, respectively, can be detected compared to Ni element. These results are supported by XRD analysis (Fig. 7) in which only NiO in rhombohedral phase is profoundly observed. In contrast, MnO₂ and RuO₂ peaks are hard to distinguish from noise background probably due to the lesser amount of these species as detected by EDX (Table 5).

The reduction in weight ratio of Ni, Mn and Ru of used catalyst is probably due to the well dispersion of these particles onto the support. This phenomenon might explain the migration of Ni, Mn and Ru into the bulk matrix of the catalyst surface resulting in lesser particles that can be detected by EDX analysis on the surface of the catalyst. Besides, this result was in a good agreement with Nurunnabi et al. (2008), who said that the Ru may have been adsorbed into the porous support consequently lowering the concentration of Ru on the surface. Meanwhile, no Cl element was observed in fresh and used catalysts indicating that calcination completely removes chloride precursor.

3.3.3 The effect of catalytic testing on Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst calcined at 1000 °C for 5 h by nitrogen adsorption analysis

The BET surface area and average pore diameter for the potential Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst are listed in Table 6. According to Zhao et al. (2012), surface area for neat alumina support which mainly contributed by the micro/meso-pores and capillary effect plays the dominating role during impregnation. Thus, Al₂O₃ pores offer a space for the access of active Ni and Mn elements.

In this research, alumina has 192 m²/g of surface area. After impregnation process, some alumina pores will be blocked which may contribute to the decreasing surface area and pore diameter of the catalysts. From Table 6, it can be seen that surface area of fresh Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst is smaller about 47 m²/g with 140 Å pore diameter compared to the neat alumina support.

After catalytic testing, the surface area of the catalyst increased to 60 m²/g. This increment is probably due to the smaller particle size contributing to the higher surface area. However, after seventh testing, the surface area of used catalyst was slightly decreased compare to the used1x catalyst. A trend could be observed that by increasing the surface area, the average pore diameter decreases. As a result, average pore diameter became smaller after catalytic testing indicating that some pores are blocked by the larger crystallite.

The nitrogen adsorption–desorption isotherms for both fresh and used of Ru/Mn/Ni(5:35:60)/Al₂O₃ catalysts are shown in Fig. 9. This catalyst exhibits isotherm Type IV with hysteresis loop of type H1 indicating that the catalysts were in mesoporous structure (20 Å < W < 500 Å). Moreover, hysteresis loop of type H1 assigned that the catalysts are in open ended cylindrical channel with uniform shape and size as indicated in FESEM analysis.

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Figure 9

Isotherm plots of Ru/Mn/Ni (5:35:60)/Al₂O₃ calcined at 1000 °C, fresh, (b) used1x, (c) used7x.

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3.3.4 Fourier Transform Infra-Red (FTIR) analysis on Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst calcined at 1000 °C for 5 h

Fig. 10 displays the FTIR spectra of as-synthesis, fresh and used Ru/Mn/Ni(5:35:60)/Al₂O₃ catalysts calcined at 1000 °C for comparison.

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Figure 10

FTIR spectra of Ru/Mn/Ni (5:35:60)/Al₂O₃ calcined at 1000 °C, (a) as-synthesis, (b) fresh, (c) used1x, (d) used7x.

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It can be observed that the FTIR spectra for these catalysts in different conditions remained similar suggesting similar functional groups present in these catalysts. The presence of the OH group from adsorbed water molecule is revealed by absorption peaks at 3390–3445 cm⁻¹ and 1631–1635 cm⁻¹, respectively as OH stretching and bending modes of vibration. These peaks diminished after calcination process. However, the O–H band emerged after seventh testing from water as by-product in the methanation reaction.

The absorption below 1071 cm⁻¹ was assigned as the stretching mode of metal oxide (M⚌O) groups which indicated that the oxide catalysts have been obtained as all impurities have been removed. Meanwhile, some nitrate precursor residues were left in the catalyst due to the strong absorption peak at 1384 cm⁻¹ for the as-synthesis catalyst (Fig. 10(a)). However, nitrate precursor has been removed through calcination process as proven in the fresh and used catalysts. It is believed that nitrate precursor has been completely removed at 1000 °C calcination temperature.

3.3.5 Thermogravimetry Analysis–Differential Thermal Analysis (TGA-DTA) on Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst

The potential Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst in as-synthesis condition which was produced by impregnation method and after aged overnight in oven at 80–90 °C was characterized by TGA–DTA analysis in order to study thermal activity as shown in Fig. 11.

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Figure 11

TGA-DTA thermogram of as-synthesis Ru/Mn/Ni(5:35:60)/Al₂O₃ catalyst.

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Overall, total weight loss that was observed in this sample is 19.66%. At the starting point of temperature studied (60 °C) until 120 °C, there was 2.79% of weight loss which equalled to 0.1325 mg, assigned to the loss of surface free water molecule. On the other hand from 120 °C until 330 °C, the decomposition of surface hydroxyl molecule from the sample was observed. From 330 °C onwards, nitrate compound and surface hydroxyl molecule were decomposed. At 1000 °C, the impurities from the catalyst have been removed and pure metal oxide can be obtained.

DTA analysis supports the weight loss of sample from TGA analysis. A small endothermic peak can be seen below 100 °C which assigned to the dehydration process where the loss of surface water occurred. Then, a broad endothermic peak at 200 °C until 300 °C attributed to the surface hydroxyl was observed. A significant thermal difference can be detected at 450 °C which is due to loss of surface hydroxyl molecule and the decomposition of residual nitrate. This finding was supported by Wan Abu Bakar and co-workers, 2012.

It can be concluded that at 1000 °C calcination temperature is effective in removing all impurities as has been proven by FTIR, EDX and TGA-DTA analysis. Furthermore, a higher activity can be achieved at the optimum 1000 °C calcination temperature.