Excess nitrate concentration in drinking water appears to correlate to blue baby syndrome, bladder cancer, and thyroid cancer, which are major public health hazards. Various methods, such as chemical degradation, physical separation, and biodegradation, are studied to remove nitrate from groundwater. However, current nitrate removal technologies have drawbacks like catalyst degradation, low efficiency, waste disposal problem, high cost, etc. For these reasons, we need to find new materials for nitrate removal.We first explore nitrate hydrogenation over nanoscale Ni2P. Ni2P, the first non-noble metal-based nitrate hydrogenation catalyst, hydrogenates nitrate to ammonium near ambient conditions with up to 96% selectivity. It can be recycled with limited loss of activity. We study the mechanism of Ni2P-catalyzed nitrate hydrogenation. Similar to conventional bimetallic catalysts, NO2- and NO are intermediates and are also hydrogenated. Density functional theory (DFT) shows that nitrite hydrogenation is the reaction bottleneck. The reconstructed Ni2P (001) surface plays a key role in adsorbing reaction species and initiating hydrogenation. Next, we develop Ni2P-modified semiconductors for photocatalytic reduction of nitrate. We present the synthesis of Ni2P/Ta3N5, Ni2P/TaON, Ni2P/TiO2. As made Ni2P/Ta3N5 and Ni2P/TaON display as high as 79% and 61% NO3- conversion under visible light (419 nm) within 12 h. Visible light active semiconductors Ta3N5 and TaON exhibit higher activities. This suggests that the dominant migration pathway of photo-generated electrons in these materials is from semiconductor to Ni2P, while elevation of Fermi level provides the driving force for the reaction. Inspired by the activity of binary nickel phosphide, we try using nickel phosphide clathrates for nitrate hydrogenation. Nickel phosphide clathrates contain Ni8P16 cages, display activity for nitrate hydrogenation, and achieve up to 82% nitrate conversion with 100% selectivity towards ammonium under near ambient conditions in 12 h. The activity of these bulk nickel phosphide clathrates indicates that nanosized clathrates may be good candidates for nitrate hydrogenation. This dissertation makes progress in chemical nitrate removal and better understands the basic science behind these reactions. We hope this will lead to improvements over the current nitrate removal methods for water purification.