Heterogeneous catalysis (C9981) Lecture 8 Haber-Bosch Synthesis of Ammonia styskalik@chemi.muni.cz styskalik.sci.muni.cz Haber-Bosch synthesis of ammonia • The nitrogen problem – atmospheric N2 fixation (breaking N≡N bonds) • Fertilizers – modern agriculture feeding billions of people on Earth • 176 millions tons ammonia produced in 2016 worldwide • 1–2 % of the world‘s entire energy supply; 2–5 % of total natural gas production 0.5 N2 + 1.5 H2 → NH3 (ΔH298 = –46.22 kJ mol–1) 0.5 N2 + 1.5 H2 → NH3 (ΔH298 = –46.22 kJ mol–1) Reactions at surfaces: From atoms to complexity; Noble prize lecture 2007; Gerhardt Ertl Haber-Bosch synthesis of ammonia • Temperature: a compromise between thermodynamics (exothermic rxn) and kinetics (low rxn rates at low temperatures) • Pressure: a compromise between thermodynamics (Le Chatelier‘s principle) and costs (high pressure reactors) Haber-Bosch synthesis of ammonia • „Low“ temperature – exothermic rxn • High pressure – no. of molecules in gas phase decreases Haber-Bosch synthesis of ammonia • Catalyst? – Early 1800s: ammonia decomposition over Fe and other metals is known – 1908, Haber: reaction of hydrogen with nitrogen over catalyts based on Os and U (500 °C, 150–200 atm) – 1908–1922, Bosch and Mittasch (BASF): 2500 different catalyst formulations (almost all elements of PT tried), 6500 runs (already in 1911), finished in 1922 after a total of 22 000 tests (trial-and-error) – Serendipity: Gallivare magnetite from Sweden (naturally containing K, Ca, and Al as impurities) used as catalyst with excellent results – 1913, BASF: first ammonia production plant, catalyst = „promoted“ Fe Haber-Bosch synthesis of ammonia • Catalyst? Haber-Bosch synthesis of ammonia • Catalyst? – „promoted“ Fe – Why? – A third compromise: better results with Ru, Os, but price has to be considered in large-scale industrial processes →Fe – Moreover Ru suffers from to strong H2 chemisorption (i.e. H2 effectively works as catalyst poison, no spot for N2 chemisorption at high pressures) →Fe – Fe-based catalysts stable up to 15 years time-on-stream →Fe Haber-Bosch synthesis of ammonia • Catalyst? – 1913: Promoted Fe by Mittasch? – Nowadays: Promoted Fe by Mittasch (only slight changes!) – Promoted Fe? – Fe-K2O-Al2O3 (also low amounts of Ca, Mg, Si may occur) – Commercially available „ammonia catalyst“ is an oxidized form of this formulation based on magnetite (Fe3O4-K2OAl2O3), reduction necessary! Active component, α-Fe Activity promoter „structural promoter“ – „porosity stabilizer“ Haber-Bosch synthesis of ammonia • Fe-K2O-Al2O3, example of composition BASF S6-10 ammonia catalyst Fe [at%] K [at%] Al [at%] Ca [at%] O [at%] Bulk – unreduced 40.5 0.35 2.0 1.7 53.2 Surface – unreduced (XPS) 8.6 36.2 10.7 4.7 40.0 Surface – reduced (XPS) 11.0 27.0 17.0 4.0 41.0 Surface – cat. active spot (AES)* 30.1 29.0 6.7 1.0 33.2 *Auger electron spectroscopy, similar to EDAX in SEM, comparable results Ullman‘s Encyclopedia of Industrial Processes; Max Appl; Ammonia, 2. Production processes Haber-Bosch synthesis of ammonia • Fe-K2O-Al2O3: Industrial production Haber-Bosch synthesis of ammonia • Fe-K2O-Al2O3, reduction before use Haber-Bosch synthesis of ammonia • Fe-K2O-Al2O3 – 30 nm primary crystallites, grain/particle size 6–10 nm – Pore volume 0.09–0.1 cm3 g–1, bimodal pore size distribution 10 nm and 25–50 nm, surface area ca. 15 m g–1, pores represent 44–46 % of total volume – Porosity originates in the reduction of originally nonporous Fe3O4 and is stabilized by Al2O3 (stability against sintering of particles) Ullman‘s Encyclopedia of Industrial Processes; Max Appl; Ammonia, 2. Production processes Haber-Bosch synthesis of ammonia • Fe-K2O-Al2O3 – Why Fe (and Ru and Os) are active??? – Why K improves activity??? Active component, α-Fe Activity promoter „structural promoter“ – „porosity stabilizer“ Haber-Bosch synthesis of ammonia • Fe-K2O-Al2O3 – Why Fe (and Ru and Os) are active??? – Why K improves activity??? Active component, α-Fe Activity promoter „structural promoter“ – „porosity stabilizer“ • Why Fe, Ru, Os? N2 as a ligand in complexes!* Haber-Bosch synthesis of ammonia *Further reading on CO as a ligand can be found in Inorganic Chemistry, C.E. Housecroft, chapters 2, 20.4, and 24.2.2 Haber-Bosch synthesis of ammonia • Why K? • Electropositivity = ability to donate electrons • Basicity (e.g. KOH) • No 1. mechanism: We need to push electrons back to N2 = weakening of N≡N triple bond • No 2. mechanism: We need to desorb NH3 from catalyst surface. Basic NH3 desorbs well from basic surface. Haber-Bosch synthesis of ammonia • A method to follow the extent of π back e- donation? doi.org/10.1002/cctc.202001141 Haber-Bosch synthesis of ammonia • Ways of improvement, thorough studies – Influence of Fe crystal planes – Back to Ruthenium – Alloys of metals with strong and weak interaction with N2 – Electron-donating supports („Electrides“, hydrides, oxides, carbon) – Metal nitrides as catalyst supports (MvK) – Electrocatalytic NH3 synthesis Haber-Bosch synthesis of ammonia • Fe-K2O-Al2O3: Industrial production • Comparison of catalytic activity on different Fe crystal planes (111 vs. 100 vs. 110) Haber-Bosch synthesis of ammonia Figure 1 Calculated turnover frequencies for ammonia synthesis as a function of the adsorption energy of nitrogen. The synthesis conditions are 400 °C, 50 bar, gas composition H2:N2 = 3:1 containing 5% NH3. Haber-Bosch synthesis of ammonia • Alloying of metals with strong and weak interaction with N2 Haber-Bosch synthesis of ammonia • Back to Ruthenium – Let‘s forget about price (stable and active catalyst might pay back) – Ru suffers from to strong H2 chemisorption (i.e. H2 effectively works as catalyst poison, no spot for N2 chemisorption at high pressures) – In 1992 new catalyst patented: Ru-Ba-K/C (British Petroleum); minor but industrial use – New activity benchmark for catalytic studies • Electron donating catalyst supports („Electrides“) – E.g. Mayenite: 12CaO.7Al2O3 – 2x12CaO.7Al2O3 + 4 H2 → [Ca24Al28O64]4+.4(H-) + 2 H2O – 2x12CaO.7Al2O3 + 2 Ca → [Ca24Al28O64]4+.4(e-) + 2 CaO Haber-Bosch synthesis of ammonia Science 301 (5633), 626-629. • Electron donating catalyst supports („Electrides“) – E.g. Mayenite: 12CaO.7Al2O3 – 2x12CaO.7Al2O3 + 4 H2 → [Ca24Al28O64]4+.4(H-) + 2 H2O – 2x12CaO.7Al2O3 + 2 Ca → [Ca24Al28O64]4+.4(e-) + 2 CaO Haber-Bosch synthesis of ammonia • Electron donating catalyst supports („Electrides“) – Ru/[Ca24Al28O64]4+.4(e-): N≡N triple bond dissociation is not a rate determining step anymore! – N−H bond formation becomes RDS Haber-Bosch synthesis of ammonia Nature 2012 DOI: 10.1038/NCHEM.1476 • Electron donating catalyst supports (Hydrides) – LiH, CaH2, TiH2, CaFH, Ca2NH, BaCeO3-xNyHz – H− are strongly electron donating species – Moreover, they can release some H− ion from the lattice and refill it with hydrogen from H2 in the reaction mixture – Deactivation: Highly reactive hydrides (e.g. LiH) form surface layer of imides and nitrides = deactivation. Some extra hydrogen needed to prevent it. Haber-Bosch synthesis of ammonia doi.org/10.1002/cctc.202001141 • Electron donating catalyst supports (others) – Oxides based mostly reduced CeO2 – Carbon Haber-Bosch synthesis of ammonia Haber-Bosch synthesis of ammonia • Metal nitrides – Nitrogen can be released from their lattice and later on refilled from N2 in the reaction mixture (MvK) – Fe, Mo: Strong N2 chemisorption. Formation of Fe, Mo nitrides in situ? Haber-Bosch synthesis of ammonia • Metal nitrides – promoted Fe, the industrial catalyst: Big question! – Nitridation of Fe by NH3 known from steel industry (steel hardening) – Thermodynamic data suggest Fe nitrides stable at H-B rxn conditions Postcatalyticrunneutrondiffraction Insiturunneutrondiffraction Iron nitrides NOT observed in bulk! Surface? Haber-Bosch synthesis of ammonia • Metal nitrides – Mo? Its interaction with N2 even stronger than in Fe! – Mo nitrides formation observed! – CoMo alloy? Co3Mo3N! Electrocatalytic NH3 synthesis • Single Au atoms deposited on N-doped graphene layers Enzymatic N2 fixation and its transformation to NH3 • Nature – Nitrogenase – RT – Ambient pressure – It works! And much better than all our „tailored“ catalysts.