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The reaction of Al and H2O is a promising method for the renewable production of H2 (an environmentally friendly fuel whose combustion produces only water), because it does not directly include fossil fuels conversion. This reaction was studied at extreme pH values as low as 1 and as high as 13.5 with HCl, H2SO4, and NaOH, and at low hydrothermal temperatures of 40-100 °C. Factors such as pH, temperature, and solution medium influenced H2 production, which was considerably greater at higher temperatures and more extreme pH (acidic or alkaline). Alkaline conditions consistently favored more rapid H2 production than acidic conditions. Under the most extreme conditions, the activation energy was ∼60 kJ mol−1 for both acidic and alkaline reactions. A model predicting H2 production in acidic reactions was derived from the reaction mechanism and kinetics. The model yielded a good fit to on-site measurements at the Tamagawa and Zao hot springs in northeast Japan. This study would aid the development of industrial H2 production systems using natural acidic hot springs or alkaline industrial wastewater.

MXenes have become one of the most actively studied emerging materials. They hold a great promise as efficient catalysts for chemical conversion and environmental purification, due to their unique layered structures, high specific surface areas, abundant active sites, and excellent chemical stability. Recent breakthroughs demonstrate MXenes and their derivatives in adsorbing, activating, and converting nitrogen (N2) to ammonia (NH3) via electrocatalytic and photocatalytic nitrogen reduction reaction (NRR). The NRR is considered a green and environmentally friendly NH3 synthesis utilizing N2 and water molecules under mild conditions, unlike the current-day Haber-Bosch technology that directly uses fossil with intensive energy processes. However, the NH3 yield and efficiency of NRR using MXene-based catalysts remain too low to meet practical applications. Therefore, a fundamental understanding of NRR mechanisms and their significant challenges should be addressed for future development. In this review, we summarize a theoretical investigation of the promising properties of MXenes as NRR catalysts and discuss the recent development of MXenes and their derivatives in NRR and their enhancement strategies. The synthesis protocols to obtain tunable morphology, nanocomposite-based, and derivatives of MXenes are also presented. Furthermore, this review will provide new perspectives on future research to realize clean and sustainable NH3 production.

Hydrogen is gaining attention as an energy source, but its production through fossil-fuel use is not environmentally friendly. A more sustainable source could be the hydrothermal reaction involving aluminum and water reaction, which owns technical issues on aluminum passivation and material sources for the upscale application. This study analyzes the aluminum-water hydrothermal reaction at laboratory and field scales and includes an environmental assessment of H2 fuel production involving hot-spring water with extremely low pH (~1) and boiling temperature (~373 K). Acidic hot-spring alternates water sources, and its unique feature activates aluminum surface by attacking oxide layer, hence enables H2 generation. Aluminum waste sources include dross and cutting chips, which could replace primary aluminum metal. The highest H2 yield could be obtained of about ~55 mmol H2 gAl−1 for the chip (almost reached theoretical yield) and a smaller amount of ~20 mmol H2 gAl−1 for dross. The environmental assessment comprises carbon dioxide emissions and energy consumption from the overall H2 fuel system resulting in a reduced environmental impact. The proposed hydrogen production method encourages hydrogen energy development from the local scale. In addition, the use of aluminum waste materials is a new and useful waste management strategy than direct disposal, and hot-springs use advances its direct utilization.