Perovskite membranes in ammonia oxidation: towards process intensification in nitric acid manufacture.

Ammonia is oxidized over PtRh alloy gauzes to form NO as the first step in the industrial production of nitric acid, a process that has remained practically unchanged for over 80 years. The reaction typically yields 94–96% NO and 4–6% by-products (N2O and N2) at 1073–1223 K. [1] Major drawbacks associated with the platinum-based catalysts are: 1) high production cost, 2) metal loss in the form of volatile oxides necessitate efficient metal recovery (Pd catchment) and refining systems, and 3) the production of N2O, an environmentally harmful gas. Nitric acid manufacture is the largest single source of N2O in the chemical industry (125 10 t CO2-equiv per annum), and the development and implementation of abatement technology for this gas is required. The above aspects have stimulated research towards replacing noble metals by oxide catalysts for NH3 oxidation. Oxides may offer the advantage of lower investment, simpler manufacture, and reduced N2O emission. [2,3] A vast number of patent applications have claimed promising performance of spinels or perovskites in the reaction, preferably containing Co, but also Fe, Mn, Bi, or Cr. Laboratory, pilot, and industrial tests have typically been carried out in fixed-bed reactors with oxides in the form of particles, pellets, or monoliths. Several key aspects have prevented the industrial implementation of oxide catalysts: 1) relatively low NO selectivity (< 90%), 2) rapid deactivation under relevant reaction conditions, and 3) the lower optimal operating temperature compared to noble metal catalysts causes difficulties with the steam balance in a revamped plant. Herein we present the first lanthanum ferrite-based perovskite membranes for ammonia oxidation, with which NO selectivities up to 98% and no N2O formation were attained. Our strategy was to combine in a single membrane reactor the separately reported properties of the above perovskites as oxygen conductors and catalysts for NH3 oxidation. Accordingly, the applied configuration, depicted in Figure 1, integrates the separation of O2 from air at the feed side by transport through oxygen vacancies in the mixed conducting membrane, and the reaction of oxygen with ammonia on the membrane surface at the permeate side. The surfaces of the membrane at the feed and permeate sides function as reduction (O2!2O ) and oxidation (NH3!NO) catalysts, respectively. This gives rise to a radically intensified process for nitric acid manufacture, as large amounts of inert N2 (2/3 of the total flow in today s plants) are excluded. Important features of perovskite membranes in ammonia oxidation include oxygen flux J(O2), selectivity to NO S(NO), and chemical stability in reducing and oxidizing atmosphere at high temperature. These interrelated parameters can be tailored by tuning the degree of calcium and strontium substitution in lanthanum ferrite-based perovskites La1 x(Sr,Ca)xFeO3 d. A higher degree of substitution x increases the number of oxygen vacancies d and thus the oxygen flux, but at the expense of lower chemical stability. 7] Furthermore, La substitution by alkaline earth cations in the perovskite structure is also known to influence the catalytic properties of these mixed oxides. This can be caused by variation in the charge and/or coordination of 3d cations, changes in surface chemical composition, and development of microheterogeneities at the catalyst surface. The preparation and characterization of the perovskite powders and the applied procedure for testing themembranes in the form of dense disks are described in the Experimental Section. For a La0.8Sr0.2FeO3 d membrane disk as an illustrative example, Figure 2 shows the typical dependence of NO selectivity and O2 flux on the inlet NH3 flow at different temperatures. In the temperature range investigated, NO selectivities in the range of 90–100% can be obtained by adjusting the inlet NH3 flow. No N2O was formed (< 10 ppm) in these experiments, and N2 was the only N-containing byproduct. Ammonia conversion (80–95%) increases with