(105b) Ni-CaO-ZrO2 Nanocomposite Catalyst for CO2 Reforming of CH4

1. 1.       Introduction

Recently, utilization of natural gas to fuels and chemicals receives more attention due to its abundance over the world and less CO₂ emission. The production of syngas by CO₂ reforming of methane is one of the attractive routes to reach this goal. Compared with conventional steam reforming, CO₂ reforming reaction tends to easier coking formation and needs extra 20% heat to obtain similar conversion. Nickel-based catalysts have shown an excellent behavior in this reaction with a comparable activity to noble metal catalysts [1]. However, coke formation  and sintering of nickel particles at a high temperature condition are often observed on Ni catalysts [2].  Developing a high-temperature stable catalyst is highly interesting for this reaction by controlled synthesis of catalyst. In this contribution, a stable Ni-CaO-ZrO₂ nano-composite catalyst for CO₂ reforming of CH₄ is prepared by a coprecipitation method. The effect of preparation condition is carefully investigated. The relationships between catalyst structures and activity of nanocomposite catalysts are discussed.

1. 2.       Experimental

Ni-CaO-ZrO₂ catalysts were prepared by a coprecipitation method with a reflux-digestion process. After precipitation, the obtained slurry was washed and refluxed in de-ionized water, then filtered, dried and calcined at 600 ^oC for 5 h. The catalysts prepared at different reflux-digestion condition were assigned as NC-X-Y, where X and Y are the reflux temperature and time, respectively. The catalyst without reflux was assigned as NC-0. The reforming reaction was conducted at 850-900 °C in a quartz fixed-bed reactor at atmospheric pressure. Obtained catalysts are tested by feeding the reaction mixture of CO₂ and CH₄ (molar ratio 1.2: 1) without dilution and a gas hourly space velocity (GHSV) of 79 000 ml/(hg) . In addition, comparison of calciantion procedure and coking quantity was performed at different operating conditions.

1. 3.       Results and discussion

3.1 Physical and chemical properties of the Ni-CaO-ZrO₂ catalysts

The pore structure parameters are collected in Table 1. The catalyst without reflux-digestration has the smallest specific surface area (63 m²/g) and pore volume (0.09 cm³/g). With the increase of the reflux time, the specific surface area and pore volume of the catalysts increased. NC-100^oC-24h exhibits the largest surface area of 245 m²/g. Reflux temperatures also have a significant effect on the pore structures of the catalysts. At the same reflux time, the specific surface areas of the catalysts increased with increasing the reflux temperature.

Table 1.  Pore structure parameters of the catalysts

The XRD patterns of the catalysts prepared at different reflux-digestion condition are shown in Figure 1. The catalyst without reflux has strong diffusion peaks of ZrO₂ and NiO. As the increase of the reflux time and temperature, diffusion peaks assigned to ZrO₂ and NiO became weaker and wider. This result indicates that reflux digestion improves the dispersion of NiO and decreases the size of ZrO₂ crystals in the catalysts.

TPR profiles of Ni-CaO-ZrO₂ catalysts are illustrated in Figure 2. The peak or shoulder at about 400 ^oC is attributed to the reduction of large NiO particles, while the peaks at 600 ^oC and 700 ^oC may belong to the reduction of NiO that have the weak and strong interactions with ZrO₂, respectively. For the catalyst without reflux, NiO mostly has the weak interaction with the carrier or is in the form of bulk particles. At the reflux temperature of 100 ^oC, the interaction between NiO and ZrO₂ became stronger with the increase of reflux times (Figure 2 a). However, when the reflux temperature is lower than 100 ^oC, the interaction between NiO and carrier was not significantly enhanced (Figure 2 b). So the high reflux temperature and long reflux time are key factors to prepare the catalysts with strong interaction between metal and the carrier.

Figure 1.  XRD patterns of the catalysts     Figure 2. TPR profiles of the catalysts

3.2 Activity of the Ni-CaO-ZrO₂ catalysts for CO₂ reforming of methane

Results of the catalytic tests at 850 ^oC with a high GHSV (CO₂/CH₄=1.2:1) of 79, 000 ml/(h·g) are shown in Figure 3(a). The catalyst without reflux has a low activity with the CH₄ conversion of about 70 %. In addition, CH₄ conversion of this catalyst decreased with time and only 55% (conversion) is achieved after reaction on stream for 150 h. A higher conversion of CH₄ and stability (very close to equilibrium) is observed on the catalyst refluxing at 100^oC for 24 h. Therefore, prolonging the reflux time will increase activity of Ni-CaO-ZrO₂ catalysts for CO₂ reforming of methane. The catalyst with 24 h reflux-digestion (NC-100 ^oC-24h) exhibits a highly stable activity over 1000 h without obvious deactivation at a GHSV of 79, 000 ml/(hg) at850 ^oC (Data are not shown) .

Figure 3.  Catalytic performance of catalysts for CO₂ reforming of methane

(a)     Effect of reflux temperature and time; (b) Effect of calcination temperature; (c) Effect of calcination procedure

Furthermore, the calcination temperature effect on the catalyst activity is carried out at similar operating conditions. It is found that the activity decreases when the sample is calcined at static air at a higher temperature. The NC-100-24h sample calcined at 900^oC presents a very low activity (Fig.3b) . XRD examination indicates that monoclinic ZrO₂ is gradually formed and particle size of ZrO₂  becomes larger. TPR measurement indicates no obvious peaks for NiO reduction for the sample calcined at 900^oC. These observations indicate that the sintering occurs at high temperatures. However, if the catalyst is directly reduced in the mixtures of H₂ and N₂ at 850^oC or 900^oC for 3 h, the conversion of CH₄ still can maintain a high CH₄ conversion (Fig. 3c),  implying that the calcination procedure has a profound impact on the activity of catalyst. Interestingly, the catalyst performing at 900^oC gives a lower carbon deposition than 850 and 800^oC after time on stream for 200 h. The full operating window will be given by a more systematic study afterwards.

1. 4.       Conclusions

A high-temperature stable Ni-CaO-ZrO₂catalyst is developed by the coprecipitation method with a reflux-digestion process. Reflux digestion can improve the dispersion of NiO and enhance the strong interaction between metal and support. The obtained Ni-CaO-ZrO₂ catalyst can be further improved by modifying the calciantion procedure.

References

[1] J. R. Rostrup-Nielsen, J. H. B. Hansen, Journal of Catalysis 144 (1998).

[2] C. J.Liu, J. Y. He, J. J. Jiang, Y. X.Pan, ChemCatChem 3 (2011)