Silicon Clathrate-Supported Catalysts with Low Work Functions for Ammonia Synthesis

Diamond-type silicon has a work function of ≈4.8 eV, and conventional n- or p-type doping modifies the value only between 4.6 and 5.05 eV. Here, it is shown that the alkali clathrates A_xSi₄₆ have substantially lower work functions approaching 2.6 eV, with clear trends between alkali electropositivity and clathrate cage size. The low work function enables alkali clathrates such as K₈Si₄₆ to be effective Haber-Bosch catalyst supports for NH₃ synthesis. The catalytic properties of Si, Ge, and Sn-based clathrates are investigated while supporting Fe and Ru on the surface. The activity largely scales with the work function, and low activation energies below 60 kJ mol⁻¹ are observed due to strong electron donation effects from the support. Ru metal and Sn clathrates seem to be unsuitable for stability issues. Compared to other similar hydride/electride catalysts, the simple structure and composition combined with stability in air/water make a systematic study of these clathrates possible and open the door to other electron-rich Zintl phases and related intermetallics as low-work function materials suitable for catalysis. The observed low work function may also have implications for other existing electronic applications.