Recent progress in heterogeneous ammonia synthesis

Abstract

Ammonia is feeding nearly half the world population and also holds the promise as a carbon-free energy carrier to store and transport renewable energy. However, the catalytic synthesis of ammonia from N2 and H2 is currently realized industrially under harsh conditions (10-25 MPa, 350-550℃), consuming 1%-2% of the power produced globally. This fossil fuelbased process also results in the emissions of 670 million tonnes of CO2. The development of more sustainable and environmentally benign processes is highly desirable. To this end, heterogeneous catalysts that allow ammonia synthesis under lower temperature and pressure, solid materials for chemical looping ammonia synthesis, as well as electron-, photon-and plasma-driven routes have been actively explored in recent years. In this review article, some recent progress on these topics are summarized and discussed. Major effort has been devoted to searching for more active heterogeneous catalysts under milder conditions. The tailoring of particle size and shape of transition metals as well as the control of metal-support and -promoter interactions are key factors for activity enhancement. Surface science and theoretical results reveal that the scaling relations among the adsorption energies of reacting species place limitations on the activity enhancement of a catalytic process. To develop more active catalysts, the scaling relations should be circumvented or broken. Some strategies including the introduction of a non-transition metal site, or construction of surface cluster site have been shown to be effective approaches. Chemicallooping ammonia synthesis may provide an alternative approach, which enables ammonia production under ambient pressure by decoupling a catalytic process into several separated reactions. This process circumvents the scaling relations and the competitive adsorption of N2 and H2 or H2O on metal catalysts. Challenges arise from tuning the thermodynamic and kinetic properties of all of the separated steps by selecting materials containing H, N, O, etc. The electron-, photon-and plasma-driven routes for ammonia synthesis have been received increasing attention recently. The poor selectivity and pretty low ammonia production rates are the two key challenges in electro-or photon-chemical processes. The ammonia synthesis rates should be carefully measured and quantified by combining multiple methods. The strategy for designing catalysts with high efficiency may also be focused on the modification of catalyst surface structural and electronic properties to circumvent the scaling relations. Through these efforts, processes that are relied on renewable energy, which are quite different from the conventional Haber-Bosch process, may come to the fore in the future.