Catalytic reduction of N-nitrosodimethylamine with nanophase nickel-boron

Recent work by the authors shows that toxic N-nitrosamines, widely detected in disinfected wastewater and drinking water and recalcitrant towards conventional treatment processes, can be rapidly reduced to less hazardous products by treatment with hydrogen and Raney nickel catalysts. Unfortunately, Raney Ni catalysts are pyrophoric when dry and are readily deactivated when exposed to air, severely limiting their application for water treatment. In this work, a non-pyrophoric and air-tolerant nickel-boron (NiB) catalyst is described, and its reactivity with N-nitrosodimethylamine (NDMA) is quantified. The catalyst, prepared by aqueous reduction of NiCl₂ with NaBH₄, has an elemental composition of 85 wt.% Ni and 7 wt.% B. Nanophase primary catalyst particles are formed and aggregate in micrometer-sized assemblies in solution. The catalyst has a specific surface area of 18 m² g^-1, and the surface contains Ni(0) and B in a mixture of oxidation states (0/+III). Kinetic studies show that NDMA is rapidly reduced via hydrogenation to dimethylamine and ammonia in H_2(g)-saturated (P_H2 = 1 atm) aqueous NiB suspensions, with mass- and surface area-normalized first-order rate constants of 29.5 ± 2.4 L g_Ni^-1 h^-1 and 1.38 ± 0.11 L m_cat^-2 h^-1, respectively. Although the mass-normalized rate constant is less than that measured for Raney Ni, the surface area-normalized rate constant is greater, suggesting higher intrinsic reactivity of the NiB surface if the availability of surface sites for NDMA reactions on either catalyst surface is not significantly affected by aggregation of the catalyst nanoparticles. Kinetic studies show that reactivity of the NiB catalyst with NDMA is unaffected by solution pH and extended pre-exposure of the dry catalyst to air (Raney Ni is immediately deactivated by exposure to air). The NiB catalyst is also more tolerant of dissolved oxygen and low concentrations of dissolved (bi)sulfide than Raney Ni, suggesting potential for greater catalyst longevity during water treatment applications.