(310d) Cellulose As a New Templating Agent for Combustion Synthesis of NiO-MgO Solid Solution Catalysts for the Dry Reforming of Methane

Due to the similar crystalline structures and lattice parameters NiO and MgO can form substitutional solid solutions, which, upon reduction, can be an effective catalysts for the dry reforming of methane (DRM). It is known that Ni supported on NiO-MgO solid solution catalysts are quite stable towards carbon formation. This stability is attributed to the strong metal support interactions and the basicity of MgO, which enables the adsorption/activation of carbon dioxide with subsequent oxidation of deposited carbon [1]. However, the preparation methods of NiO-MgO solid solutions usually include prolonged calcination steps at elevated temperatures which decreases the surface area of the obtained powders, resulting in lower catalytic activity [2].

In this work we present a new route to prepare NiO-MgO solid solutions, namely by solution combustion (SC) synthesis method. We showed that it is possible to obtain high surface area NiO-MgO active and stable catalysts when using glycine as a combustion fuel for redox reaction with metal nitrates assisted by cellulose as a templating agent. Time on stream (TOS) studies showed that the obtained catalysts exhibit high DRM activity comparable to the Ni-Pt catalysts reported in the literature, and have enhanced stability towards carbon formation. We found that under certain conditions, cellulose acts as a template and additional fuel, thus resulting in structured catalysts with up to 150 m²/g BET surface area. The mechanism of combustion synthesis reaction and the main factors responsible for the templating effect and high surface area are also reported.

Materials and Methods

Nickel nitrate and magnesium nitrate hexahydrates, with a ratio 10, 20 and 30 wt. % of Ni in final powders were dissolved in deionized water and impregnated on cellulose paper (Whatman, grade 4 filter paper). Upon drying at 80 ^oC, the impregnated cellulose sheets were ignited to initiate the combustion reaction resulting in the formation of NiO-MgO solid solutions. The resulting powders were first calcined and then reduced with 5% H₂ at 600 ^oC. This treatment segregated some Ni to the surface and resulted in catalysts consisting of Ni supported on NiO-MgO. DRM catalytic activity was measured in the temperature range of 500-700 ^oC in a continuous flow fixed bed quartz reactor at atmospheric pressure (total flow - 120 ml/min, CH₄, CO₂ = 20%, balanced He). The combustion synthesis reaction was monitored by High Speed Infrared Camera, and in-situ TG and DSC measurements, were performed to evaluate the phase formations. BET adsorption experiments and SEM/TEM microscopy were used to determine the key structural features of the catalysts. In addition, catalysts were also characterized by TPR, chemisorption, XRD and XPS methods.

Results and Discussion

The application of cellulose paper during combustion synthesis greatly influences the surface area of the catalysts. The regular solution combustion (SC) resulted in a NiO-MgO solid solutions with a BET surface area of up to 23m²/g, while the paper assisted (PA) combustion synthesis method allowed to obtain catalysts with up to 140 m²/g surface area. Upon reduction, smaller Ni crystallites segregated to the surface resulting in a higher catalytic activity. The highest methane conversion was obtained for 10%Ni/NiO-MgO (PA) catalysts, reaching 60% at 600 ^oC and 75% at 700 ^oC.

Figure 1 shows the microstructures of the initial filter paper and the combustion synthesis products. It can be seen that cellulose paper (1a) has a fibrous structure which is reflected on the structure of final powders. The templating effect is more pronounced for NiO (1b) showing fiber-like structure after the combustion reaction, whereas for MgO (1c) the structure is mostly porous. For 10%NiO-MgO solid solution (1d) these two effects combine with each other resulting in a fibrous, porous structures responsible for the high surface area of final catalysts.

Significance

The use of cellulose as a templating fuel enables fast and energy efficient synthesis of active and stable catalysts for the DRM. This method can be used also for the synthesis of high surface area materials for other applications, which include the formation of solid solutions and metal-support complex interactions.

References

1. Ruckenstein, E., Hu, Y.H., Appl. Cat. A 154, 185-205 (1997).
2. Zanganeh, R.M., Rezaei, M., Zamaniyan, A. Adv. Powder Technol. 25, 1111-1117 (2014).