Low-cost catalyst materials for effective VAM catalytic oxidation

Application of ventilation air methane (VAM) thermal oxidiser requires high temperatures (over 1000°C), which causes sintering and ceramic corrosion with CaO generated from stone dust decomposition over 950°C and requires special and high cost ceramic materials for the bed. Alternatively, catalytic thermal oxidation is an effective way to remarkably lower the temperature of VAM complete oxidation. The use of a catalyst reduces the operating temperature of a VAM mitigator to 350-600°C, which is well below the stone dust decomposition temperature of 900-1050°C, thus avoiding sintering and ceramic corrosion with CaO. The lower operating temperature is also very beneficial for safety management. Moreover, catalytic oxidation is able to process a much higher ventilation air (VA) flow than the thermal oxidation unit, leading to smaller-size mitigation units and a smaller footprint. The catalytic-version VAM mitigator requires a lower minimum operating methane concentration, making it self-sustaining at lower VAM concentrations.

Palladium (Pd)-based catalysts are well known as the most active catalyst material for catalytic methane oxidation. However, the main drawback of Pd-based catalysts for the VAM abatement application is their significantly high cost. Considering the large-scale application of catalytic mitigator, which requires large-size units for treating significant VA flows, the material cost for catalysts is significant.

Alternative catalyst materials with low cost and high catalytic activity and stability are highly sought after in the development of catalytic VAM mitigators for cost-effective VAM abatement with enhanced safety management. Within this context, some nanostructured transition metal oxides are particularly promising due to their good intrinsic catalytic activity and significantly low cost compared to noble metal Pd. Previously we have developed a new range of nanostructured copper oxide (CuO) catalysts for methane oxidation. The results shows that our CuO nanobelt (CuO-NB) catalysts exhibit high activity for methane oxidation, even close to that of Pd-based catalysts, making nanostructured metal oxides a promising replacement for expensive Pd-based catalysts.

This project developed novel low cost high performance catalyst materials for catalytic VAM oxidation, which will lead to opportunities for significantly lowering the cost of VAM mitigation and speeding up its development. The project work was initially focused to modify the CuO-NB catalysts to further improve their catalytic properties of methane oxidation under wet VAM conditions and evaluate the catalytic performance of other promising transition metal oxides catalysts such as spinel nickel cobalt oxides (NCO) for VAM oxidation. Preliminary experimental results showed that the modified CuO nanobelts exhibit improved catalytic stability against water vapour and high reaction temperature. In particular, the NCO catalysts were found to possess superior catalytic activity in VAM catalytic tests, being even significantly more active than the modified CuO. Based on the promising performance obtained from the spinel NCO catalysts, our efforts have been focused to optimise the catalytic activity and stability of the spinel NCO catalysts for VAM catalytic oxidation and elucidate the reaction mechanisms and pathways of catalytic methane oxidation over metal oxides catalysts through combined experimental and theoretical studies. Significant experimental work was undertaken in preparation and characterisation of catalyst materials, and evaluation of their catalytic activity and stability under the simulated VA conditions with various methane concentrations and VA flow rates.

The modified CuO nanobelt catalyst with surface loading of ceria nanoparticles, denoted as CuO-NB-CeOx, exhibits enhanced resistance to the elevated reaction temperature and moisture. A long-term catalytic stability test of CuO-NB-CeOx was conducted with humid VAM at 570 °C for 260 hours, indicating that after a slight decrease in methane conversion at the initial stage, the catalytic activity remains constant with methane conversion of 80-81% for over 240 hours. The results of catalyst characterisation by H2-TPR and CH4-TPR show that loading of CeOx nanoparticles improves the oxygen mobility and reducibility of CuO-NB, resulting in a lower energy barrier required for oxygen dissociation over CuO-NB-CeOx as revealed by the theoretical DFT calculations.

A range of nanostructured spinel NCO catalysts (denoted as NCO-A to NCO-E) with various types of crystalline structures and materials morphologies were prepared and examined for catalytic VAM oxidation under both dry and humid conditions. Overall, all the NCO samples outperform the CuO catalysts in terms of catalytic activity. The NCO nanoparticles (NCO-A) and nanotubes (NCO-B) previously reported to be highly active in methane oxidation by others were found not to perform well in our evaluation of catalytic oxidation of simulated VAM. To investigate the effect of crystalline surface facets on the catalytic performance, catalysts NCO-C and NCO-D were prepared, both of which have a similar nanoplate morphology and surface area but different crystalline surface facets. The crystalline surfaces of (111) and (112) are predominant in NCO-C and NCO-D, respectively. The NCO-D catalyst exhibits greater activity than NCO-C, which is attributed to the significantly lower energy barriers of intermediate reactions of methane oxidation over the dominant surface (112) of NCO-D than over (111) of NCO-C, as revealed by DFT calculations. Catalyst NCO-E comprising nanosheets shows the second-best catalytic activity after NCO-D, because of its relatively high catalytic reducibility and large numbers of active metal sites. The relationships between the catalytic properties of various spinel NCO catalysts and their structural features were investigated by detailed catalyst materials characterisation, catalytic reaction evaluation and theoretical DFT calculations. It was found that the exposed crystalline facets and the number of accessible surface active metal species are the key factors in determining the catalytic activity.

It was found that the degradation of catalytic activity is significant in some of prepared catalysts when reacting with humid VAM. Our efforts were also focused to understand how water molecules interact with methane oxidation over the developed low-cost catalysts. Based on the new understandings of the interaction of water molecules obtained with the combined experimental and theoretical studies in the project, we have successfully developed a novel noble-metal-free catalyst through architecture engineering of nanostructured catalysts to mitigate the deactivation of the catalyst by water. This catalyst exhibits outstanding catalytic activity with a light-off temperature of T10 (temperature for 10% of methane conversion) as low as 280 °C and T90 of 433 °C under the simulated wet VAM condition. Moreover, it possesses exceptional catalytic stability attaining 93% methane conversion at 500 °C with the wet VAM for an extensive testing period of 260 hours. The superior catalytic performance is attributed to its novel nanostructure, which offers significant amounts of exposed active sites for methane oxidation and meanwhile the separate active sites for water molecules to minimise the unfavourable interaction of water molecules with methane catalytic oxidation.