Rational design of metal nitride redox materials for solar-driven ammonia synthesis

Fixed nitrogen is an essential chemical building block for plant and animal protein, which makes ammonia (NH3) a central component of synthetic fertilizer for the global production of food and biofuels. A global project on artificial photosynthesis may foster the development of production technologies for renewable NH3 fertilizer, hydrogen carrier and combustion fuel. This article presents an alternative path for the production of NH3 from nitrogen, water, and solar energy. The process is based on a thermochemical redox cycle driven by concentrated solar process heat at 700-1200°C that yields NH3 via the oxidation of a metal nitride with water. The metal nitride is recycled via solar-driven reduction of the oxidized redox material with nitrogen at atmospheric pressure. We employ electronic structure theory for the rational high-throughput design of novel metal nitride redox materials and to show how transition-metal doping controls the formation and consumption of nitrogen vacancies in metal nitrides. We confirm experimentally that iron doping of manganese nitride increases the concentration of nitrogen vacancies compared to no doping. The experiments are rationalized through the average energy of the dopant d-states, a descriptor for the theory-based design of advanced metal nitride redox materials to produce sustainable solar thermochemical ammonia

Solar thermochemical production of ammonia from water, air and sunlight: Thermodynamic and economic analyses

Ammonia is an important input into agriculture and is used widely as base chemical for the chemical industry. It has recently been proposed as a sustainable transportation fuel and convenient one-way hydrogen carrier. Employing typical meteorological data for Palmdale, CA, solar energy is considered here as an inexpensive and renewable energy alternative in the synthesis of NH3 at ambient pressure and without natural gas. Thermodynamic process analysis shows that a molybdenum-based solar thermochemical NH3 production cycle, conducted at or below 1500 K, combined with solar thermochemical H2 production from water may operate at a net-efficiency ranging from 23 to 30% (lower heating value of NH3 relative to the total energy input). Net present value optimization indicates ecologically and economically sustainable NH3 synthesis at above about 160 tons NH3 per day, dependent primarily on heliostat costs (varied between 90 and 164 dollars/m2), NH3 yields (ranging from 13.9 mol% to stoichiometric conversion of fixed and reduced nitrogen to NH3), and the NH3 sales price. Economically feasible production at an optimum plant capacity near 900 tons NH3 per day is shown at relative conservative technical assumptions and at a reasonable NH3 sales price of about 534 ± 28 dollars per ton NH3

Thermochemical production of ammonia using sunlight, air, water and biomass

Doctor of PhilosophyDepartment of Chemical EngineeringPeter H. PfrommApproximately 45% of the global hydrogen production (from fossil fuels such as natural gas or coal totaling 2% of the global energy generation) is absorbed as feedstock in the synthesis of over 130 million metric tons ammonia (NH[subscript]3) annually. To achieve food security for a growing world population and to allow for additional uses of the nitrogen-fertilizer for production of bio-energy feedstock or as combustion fuel or H[subscript]2 carrier - demand for NH[subscript]3 is projected to increase. This work pursues the synthesis of ammonia at atmospheric pressure and without fossil fuel. Conceptually, concentrated solar radiation is utilized to transfer electrons from the lattice oxygen of a transition metal oxide to the metal ion. This yields a metallic reactant that provides the reducing power for the subsequent six-electron reductive cleavage of N[subscript]2 forming a transition metal nitride. In a second reaction, the generated lattice nitrogen is hydrogenated with hydrogen from H[subscript]2O to NH[subscript]3. This furnishes the transition metal oxide for perpetuated NH[subscript]3 synthesis. Theory and experimentation identified manganese nitride as a promising reactant with fast diffusion characteristics (8 ± 4 x 10[superscript]-9 cm[superscript]2 s [superscript]-1 apparent nitrogen diffusion constant at 750 degree C) and efficient liberation of 89 ± 1 mol% nitrogen via hydrolysis at 500 degree C. Opposed to only 2.9 ± 0.2 mol% NH[subscript]3 from manganese nitride, 60 ± 8 mol% of the nitrogen liberated from molybdenum nitride could be recovered as NH[subscript]3. Process simulation of a Mo-based NH[subscript]3 synthesis at 500-1200 degree C estimates economically attractive production under fairly conservative process and market conditions. To aid the prospective design of a Mn or Mo-based reactant, correlating the diffusion constants for the hydrolysis of seven nitrides with the average lattice nitrogen charge (9.96-68.83%, relative to an ideal ionic solid) indicates the utility of first-principle calculations for developing an atomic-scale understanding of the reaction mechanism in the future

Computational screening of perovskite redox materials for solar thermochemical ammonia synthesis from N2 and H2O

To circumvent the scaling relations of activation energies and adsorption energies at catalytic surfaces limiting their catalytic activity, perovskites are investigated for a solar-driven production of ammonia (NH3) from N2 and H2O via a two-step redox cycle. The cycle consists of an endothermal reduction of N2 at 1400 °C using solar process heat, followed by an exothermal hydrolysis forming NH3 at 400 °C. Both steps are carried out at ambient pressure. Electronic structure computations are employed to assess the stability and surface activity of oxygen vacancies and lattice nitrogen at the (001) facet of nitrogen-doped perovskites. The results are compared to the activities of Mo2N(100), Mo2N(111), and Mn2N(0001) reference models. We find producing oxygen vacancies at high temperature that are active in N2 reduction is the energetically limiting reaction step of the redox cycle. The redox energetics can be tuned by the perovskite composition and are most sensitive to the type of transition metal at the B site terminating the surface. Promising perovskites contain Co or Mn at the surface and Co doped with Mo or W in the bulk, such as CaCoO3-terminated La0.5Ca0.5Mo0.5Co0.5O3, SrCoO3-terminated Sr0.5La0.5Co0.5W0.5O3, and CaMnO3-terminated Sr0.5Ca0.5MnO3. Trade-offs in the redox energetics are quantified to guide future experimental work.ISSN:0920-5861ISSN:1873-430

Thermodynamics of metal reactants for ammonia synthesis from steam, nitrogen and biomass at atmospheric pressure

Catalytic ammonia synthesis at approximately 30 MPa and 800 K consumes about 5% of the global annual natural gas production causing significant CO2 emissions. A conceptual solar thermochemical reaction cycle to produce NH3 at near atmospheric pressure without natural gas is explored here and compared to solar thermochemical steam/air reforming to provide H2 used in the Haber-Bosch process for NH3 synthesis. Mapping of Gibbs free energy planes quantifies the tradeoff between the yield of N2 reduction via metal nitridation, and NH3 liberation via steam hydrolysis vs. the temperatures required for reactant recovery from undesirably stable metal oxides. Equilibrium composition simulations suggest that reactants combining an ionic nitride-forming element (e.g., Mg or Ce) with a transition metal (e.g., MgCr2O4, MgFe2O4, or MgMoO4) may enable the concept near 0.1 MPa (at maximum 64 mol % yield of Mg3N2 through nitridation of MgFe2O4 at 1,300 K, and 72 mol % of the nitrogen in Mg3N2 as NH3 during hydrolysis at 500 K). </p

Trends in the phase stability and thermochemical oxygen exchange of ceria doped with potentially tetravalent metals

Ceria is among the most prominent materials for generating clean fuels through solar thermochemical CO2 reduction and water splitting. The main optimization parameter for ceria in solar reactors is the oxygen exchange capacity (OEC, Dd), which can be notably improved through various dopant types. Among them, tetravalent dopants excel through the formation of active vacancies which lead to particularly high OEC values. We thus performed a comprehensive screening evaluation of all dopants in the periodic table which have been reported to adopt an oxidation state of +IV. All thermally stable doped ceria samples, M0.1Ce0.9O2d (M ¼ Si, Ti, V, Cr, Zr, Nb, Rh, Hf, Ta, Nb, V, Pr, and Tb), were first analyzed for Dd improvement with thermogravimetric analysis (TGA). Dopant solubility limits and behavior in the ceria host lattice was evaluated with scanning electron microscopy (SEM-EDX) and powder X-ray diffraction techniques. No indications for carbonate side product formation were found. Hf-, Zr-, and Ta-doped ceria display higher OEC than pristine ceria, and Raman spectroscopy indicated that their improved performance is accompanied by a higher versatility in the underlying vacancy formation processes. Furthermore, the effective dopant radii are close to an optimal dopant radius around 0.8 °A for maximum Dd according to TGA cycling experiments. These experimentally derived trends for doped ceria were supported by density functional theory (DFT) calculations which analogously correlate Dd with the partial electronic charge of the metal dopants

Solar-driven thermochemical splitting of CO2 and in-situ separation of CO and O2 across a ceria redox membrane reactor

ISSN:2542-4351ISSN:2542-478

An Ionicity Rationale to Design Solid phase Metal Nitride Reactants for Solar Ammonia Production

Ammonia provides the basis of nutrition for a large portion of the human population on earth and could be used additionally as a convenient hydrogen carrier. This work studies a solar thermochemical reaction cycle that separates the reductive N₂ cleavage from the hydrogenation of nitrogen ions to NH₃ without using electricity or fossil fuel. The hydrolysis of binary metal nitrides of magnesium, aluminum, calcium, chromium, manganese, zinc, or molybdenum at 0.1 MPa and 200–1000 °C recovered up to 100 mol % of the lattice nitrogen with up to 69.9 mol % as NH₃ liberated at rates of up to 1.45 × 10^–3 mol NH₃ (mol metal)⁻¹ s^–1 for ionic nitrides. These rates and recoveries are encouraging when extrapolated to a full scale process. However, nitrides with lower ionicity are attractive due to simplified reduction conditions to recycle the oxidized reactant after NH₃ formation. For these materials diffusion in the solid limits the rate of NH₃ liberation. The nitride ionicity (9.96–68.83% relative to an ideal ionic solid) was found to correlate with the diffusion constants (6.56 × 10^–14 to 4.05 × 10^–7 cm² s^–1) suggesting that the reduction of H₂O over nitrides yielding NH₃ is governed by the activity of the lattice nitrogen or ion vacancies, respectively. The ionicity appears to be a useful rationale when developing an atomic-scale understanding of the solid-state reaction mechanism and when designing prospectively optimized ternary nitrides for producing NH₃ more sustainably and at mild conditions compared to the Haber Bosch process

Concentration Of The baculovirus Autographa californica M nucleopolyhedrovirus (AcMNPV) by ultrafiltration

Concentration and retention of a rod-shaped virus during tangential flow ultrafiltration (UF) was assessed to evaluate the potential of membrane-based downstream methods with advantages such as easy scale-up for industrial processes. A recombinant baculovirus of the non-spherical Autographa californica M nucleopolyhedrovirus (AcMNPV), vHSGFP, expressing egfp was filtered using polyethersulfone membranes ranging from 30 to 1000 kDa molecular weight cut-off (MWCO). A 20-fold virus concentration was achieved when a membrane cut-off range of 100 to 1000 kDa was tested. Fouling was observed and cake formation and pore plugging were postulated as concurrent causes with different impact depending on the MWCO. A reduction of virus concentration in the range of 2 to 5 log units in the permeate was observed illustrating the potential of membrane-based virus filtration as a useful unit operation in downstream processing

Solar-driven co-thermolysis of CO2 and H2O promoted by in situ oxygen removal across a non-stoichiometric ceria membrane

We report on the first ever experimental demonstration of simultaneous thermolysis of CO2 and H2O with in situ separation of fuel and oxygen in a solar-driven membrane reactor. Gaseous CO2/H2O mixtures at molar ratios from 3 : 4 to 2 : 1 were fed to a mixed ionic–electronic conducting non-stoichiometric ceria (CeO2−δ) membrane enclosed in a solar cavity receiver and exposed to simulated concentrated solar radiation of up to 4200 suns. Reaction rates were measured under isothermal and isobaric conditions in the range of 1723–1873 K and 0.2–1.7 Pa O2, yielding a maximum combined CO and H2 fuel production rate of 2.3 μmol cm−2 min−1 at 1873 K and 0.2 Pa O2 at steady state, which corresponded to a conversion of reactants of 0.7%. Under all conditions tested, CO production was favored over H2 production, as expected from theory. Experimental results followed the same trends as the thermodynamic equilibrium limits of membrane-assisted thermochemical fuel production.ISSN:2058-988