Hydrogenation of MAX Phase Materials and Catalytic Activity in Ammonia Synthesis
Marcell Toth, Nikita Kudriavykh, Walid Almaksoud, Rohit Rai, José D. Gouveia, Max Avdeev, José R. B. Gomes, Yoji KobayashiABSTRACT
Previously, computational studies have showed MAX phases can be stable with lattice hydride, and some residual amounts of lattice hydride have been observed when prepared from TiH 2 precursor. Separately, MXenes have also been predicted to have activity for N 2 activation. Here, through quantitative H 2 ‐TPD experiments, we show that the MAX phases Ti 2 AlC, Ti 2 AlN, Ti 2 ZnN, V 2 AlC, Cr 2 AlC, Nb 2 AlC, Ti 3 AlC 2 , Ti 3 SiC 2 , and Zr 2 InN can be hydrogenated at elevated temperatures/pressure. They can accommodate up to 0.2 mol of hydrogen per formula unit, although neutron diffraction was unable to clearly identify the coordination site. Despite their compositional similarity to known early transition metal hydride catalysts such as TiH 2 , VH x , NbH x , there is no ammonia synthesis activity on the bare surface. However, by depositing ruthenium metal and cesium as promoter, ammonia synthesis rates up to 17.5 mmol/g/h were attained at 400°C and 50 bar. Kinetic studies on the catalysts revealed no hydrogen poisoning of ruthenium nanoparticles, due to electronic effects from co‐adsorbed hydrogen. The confirmed hydrogen absorption by MAX phases also has implications for various other properties as these materials are used in hydrogen‐containing environments.