The present invention relates to a method for synthesizing ammonia from its elements (H₂ and N₂) at atmospheric pressure. This was achieved in a prototype solid state proton (H⁺) conducting cell-reactor.

The development of a successful process for ammonia synthesis from its elements: $${\text{N}}_{\text{2}}{\text{+ 3 H}}_{\text{2}}{\text{<----> 2 NH}}_{\text{3}}$$is considered a landmark in heterogeneous catalysis. The Haber process which involves reaction of gaseous nitrogen and hydrogen on a Fe-based catalyst at high pressures (15 - 30 MPa), was developed at the beginning of the twentieth century after an extensive search for an active catalyst (1).

Even from early studies, it was realized that the conversion is limited by thermodynamics. The gas volume decreases with reaction. Hence, very high pressures have to be used in order to push equilibrium to the right according to the Le Chatelier principle.

The reaction is exothermic (109 kJ/mol at 500°C) and therefore conversion increases with decreasing temperature. In order to achieve however, industrially acceptable reaction rates, the reaction temperature must be high. The trade-off solution is to operate at temperatures in the range of 430 - 480°C, at which the equilibrium conversion is of the order of 10 -15% (1).

The present method refers on an alternative route to ammonia synthesis at atmospheric pressure via the use of solid state proton (H⁺) conductors by which the requirement for operation at high pressures is eliminated. The method involves the process steps of present claim 1.

Solid electrolyte cells have been used so far in heterogeneous catalysis in order to a) study the mechanism of catalytic reactions (2, 3), b) electrochemically alter reaction rates (4,5) and c) cogenerate electricity and useful chemicals (6). The solid electrolytes used in most of the above applications were oxygen ion conductors.

In the last decade however, materials that exhibit protonic conductivity in the solid state have been introduced into catalysis research (7). These H⁺ conductors are particularly useful because they can operate at temperatures in which many industrial hydro- and dehydrogenation reactions take place. Furthermore, in contrast to oxidation reactions, a number of industrial hydrogenations (ammonia, methanol production) are equilibrium limited at the operating conditions.

A model process using solid state proton conductors to obtain conversions higher than those predicted by the reaction equilibrium, has been proposed in the past (8-10). It is possible to use two configurations. The double and the single chamber configuration.

In the double chamber configuration (Figure 1) a vessel 1 has been divided into a hydrogenation reaction chamber 2 and into a chamber containing a hydrogen atmosphere 3, using a proton conducting solid electrolyte (SrCe_0.95Yb_0.05O₃) 4. Two porous polycrystalline palladium (Pd) films served as electrodes. The working electrode 5 was deposited in chamber 2 and served also as catalyst for the reaction of ammonia synthesis. The counter electrode 6 was deposited in the other side of the solid electrolyte, i.e. in chamber 3. These two electrodes are connected with Au wires 7 in a galvanostat - potentiostat 8. The cathode (chamber 2) was exposed to a gaseous stream containing nitrogen diluted in helium while the anode (chamber 3) was exposed to a hydrogen stream. The gaseous H₂ passing over the anode of the proton-conducting cell-reactor, will be converted to H⁺: $${\text{3H}}_{\text{2}}{\text{----> 6H}}^{\text{+}}{\text{+ 6e}}^{\text{-}}$$The protons (H⁺) are transported through the solid electrolyte to the cathode where the half-cell reaction: $${\text{N}}_{\text{2}}{\text{+ 6H}}^{\text{+}}{\text{+ 6e}}^{\text{-}}{\text{----> 2 NH}}_{\text{3}}$$takes place. Thus, reaction [1] is again the overall reaction.

In the single chamber configuration (Figure 2) a reaction vessel 1 contains a proton conducting solid electrolyte (SrCe_0.95Yb_0.05O₃) 2. Two porous polycrystalline palladium (Pd) films were used as electrodes. The working electrode 3 was deposited in the one side of solid electrolyte and served also as catalyst for the reaction of ammonia synthesis. The counter electrode 4 was deposited in the other side of the solid electrolyte. These two electrodes are connected with Au wires 5 in a galvanostat - potentiostat 6. In the following a gaseous mixture containing nitrogen and hydrogen diluted in helium are fed to the reaction vessell. The gaseous H₂ passing over the anode of the proton-conducting cell-reactor, will be converted to H⁺: $${\text{3H}}_{\text{2}}{\text{----> 6H}}^{\text{+}}{\text{+ 6e}}^{\text{-}}$$The protons (H⁺) are transported through the solid electrolyte to the cathode where the half-cell reaction: $${\text{N}}_{\text{2}}{\text{+ 6H}}^{\text{+}}{\text{+ 6e}}^{\text{-}}{\text{----> 2 NH}}_{\text{3}}$$takes place.

Hence, according to another aspect of the present invention, there exists an electrochemical cell as claimed in claim 6.

Specifically, this single chamber configuration is simpler than the double chamber configuration because of the fact that the complexity of the separation of the two chambers ia avoided.

The ceramic material was a strontia-ceria-ytterbia (SCY) perovskite of the form: SrCe_0.95Yb_0.05O₃. This perovskite is a solid with good mechanical strength and with high protonic conductivity [11]. The electrode preparation and characterization procedure has been described in detail elsewhere [12].

Figure 3 shows the dependence of the rate of NH₃ formation in a double chamber cell on I/2F, where I is the imposed current and F is Faraday's constant. Assuming that the SCY is solely a proton conductor, the ratio I/2F is equal to the electrochemical molar flux of hydrogen through the solid electrolyte. The cell was kept at 570°C. A mixture of 1.8% N₂ in He was passing over the cathode at a volumetric flowrate of 8.3x10^-8 m³/s and atmospheric total pressure. A flow of 5.0x10^-7 m³/s of 100% H₂ at atmospheric pressure was maintained over the anode. At I=0, no products were formed. Upon imposing a current through the cell, NH₃ appeared at the cathode and after a transient period of 2-6 minutes, a steady state rate of NH₃ formation was established.

The data points in Figure 3 represent steady state rates. The two dotted lines of Figure 3 are based on thermodynamic calculations and are presented for comparizon of the present results with those that would have been obtained in a conventional catalytic reactor (CCR) in which gaseous H₂ rather than electrochemical H⁺ were used. Specifically, the curve denoted as CCR represents the maximum rate of NH₃ formation attained in a CCR that operates at 570°C and at atmospheric pressure and in which the same amounts of N₂ and H₂ as in the present experiments, are introduced. It can be seen that the NH₃ rates attained experimentally exceed the CCR rates by at least three orders of magnitude. Similarly, the curve denoted as PCCR (pressure in a conventional catalytic reactor) represents the total pressure at which a CCR should operate in order for the NH₃ conversion to be as high as that reported here.

Figure 4 shows the dependence of the rate of NH₃ formation in a single chamber cell on I/2F, where I is the imposed current and F is Faraday's constant. The cell was kept at 600°C. A gas mixture of N₂ (0.5%), H₂ (10%) in He was fed in the reaction vessel at a volumetric flowrate of 3.3x10^-7 m³/s and atmospheric total pressure. At I=0, no products were formed. Upon imposing a current through the cell, NH₃ appeared at the cathode and after a transient period of 2-6 minutes, a steady state rate of NH₃ formation was established.

The data points in Figure 4 represent steady state rates. The two dotted lines of Figure 4 are based on thermodynamic calculations and are presented for comparizon of the present results with those that would have been obtained in a conventional catalytic reactor (CCR) in which gaseous H₂ rather than electrochemical H⁺ were used. Specifically, the curve denoted as CCR represents the maximum rate of NH₃ formation attained in a CCR that operates at 600°C and at atmospheric pressure and in which the same amounts of N₂ and H₂ as in the present experiments, are introduced. It can be seen that the NH₃ rates attained experimentally exceed the CCR rates by at least two orders of magnitude. Similarly, the curve denoted as PCCR (pressure in a conventional catalytic reactor) represents the total pressure at which a CCR should operate in order for the NH₃ conversion to be as high as that reported here.

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