Red mud is a by-product of the Bayer process: for each ton of aluminum produced approximately 1.9 to 3.6 tons of red mud is produced. It is usually disposed by long-term storage on land; and because of its high alkalinity, it is considered as hazardous with a serious environmental impact. It contains lots of valuable compounds, such as oxides of Fe, Si, Ti, and Al. Thus, its recovery carries an economic impact as well.
In this work, we have converted red mud, kindly supplied by Eti Seydişehir (Konya-Turkey) Aluminum Plant, into a catalyst with exceptionally high and stable performance for COx-free hydrogen production from a carbon-free hydrogen carrier molecule, ammonia. Ammonia does not include any carbon, thus it offers opportunities for COx-free hydrogen storage with a high storage density (17.7 wt%). Different Ru-, Ni-, and Fe-based catalysts have been proposed for the decomposition of ammonia into hydrogen and nitrogen. Among them, Ru-based catalysts are known as the most active ones; however, they suffer from the high cost of ruthenium. Ni- and Fe-based counterparts can be the alternatives to these noble metal catalysts. However, such non-noble metal catalysts require elevated temperatures to be able to reach comparable rates. This requirement leads to sintering and deactivation. Thus, there is a significant need for a cheap catalyst with high and stable performance.
Data presented here showed that an acid treated red mud sample was able to maintain a stable performance for more than 72 hours at hydrogen production rates significantly higher than any other iron-based and/or non-noble metal-based catalysts could provide in the literature. For instance, hydrogen production rates of 72 and 196 mmol H2 min−1 gcat−1 were achieved at 700 °C at a space velocity of 72 000 and 240 000 cm3 NH3 h−1 gcat−1, respectively. Detailed XRD and SEM/EDX characterization of the catalyst before and after the reaction revealed that e-Fe2N nanoparticles that are formed during the induction period are responsible for this record high performance.
Data also illustrate that the significantly high surface area of this modified red mud sample achieved by HCl digestion performed at 220 °C helps to maintain stable activity by enabling a more complete nitriding of iron species to e-Fe2N during NH3 activation as well as making them readily available for the reaction. In the as-received (unmodified) red mud catalyst, on the other hand, iron species were trapped and not readily available to be fully converted into e-Fe2N because of the space limitations; thus this unmodified sample deactivates quickly.
In short, results presented in this study offer opportunities to utilize one of the key hazardous industrial wastes as an eco-friendly, efficient, stable, and almost cost-free catalyst for COx-free hydrogen production from ammonia.
Kurtoğlu SF1,2, Uzun A1,2.Show Affiliations
- Department of Chemical and Biological Engineering, Koç University, Rumelifeneri Yolu, Sariyer, 34450, Istanbul, Turkey.
- Koç University TÜPRAŞ Energy Center (KUTEM), Koç University Rumelifeneri Yolu, Sariyer 34450, Istanbul, Turkey.
Red mud, one of the mostly produced industrial wastes, was converted into a catalyst with exceptionally high and stable performance for hydrogen production from ammonia. Results showed that iron species produced after reduction of the HCl digested red mud were converted into ε-Fe2N during the induction period of ammonia decomposition reaction at 700 °C. The catalytic performance measurements indicated that the modified red mud catalyst provides a record high hydrogen production rate for a non-noble metal catalyst at this temperature. For instance, stable hydrogen production rates were measured as 72 and 196 mmol H2 min(-1) gcat(-1) for the corresponding space velocities of 72 000 and 240 000 cm(3) NH3 h(-1) gcat(-1), respectively, at 700 °C. These results offer opportunities to utilize one of the key hazardous industrial wastes as an eco-friendly, efficient, stable, and almost cost-free catalyst for COx-free hydrogen production from ammonia decomposition.Go To Sci Rep.