Heterogeneous catalysis Lecture 8 Haber-Bosch synthesis of ammonia 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) https://www.chemguide.co.uk/physical/equilibria/haberflow.gif 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-K2O-Al2O3), 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! https://upload.wikimedia.org/wikipedia/commons/9/95/Back_bonding.png https://upload.wikimedia.org/wikipedia/commons/5/5b/RuA5N2.png Haber-Bosch synthesis of ammonia 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? Figure 4 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 https://pubs.acs.org/na101/home/literatum/publisher/achs/journals/content/jacsat/2001/jacsat.2001.1 23.issue-34/ja010963d/production/images/medium/ja010963df00001.gif 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 Figure 1 •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 Figure 2 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 https://pubs.acs.org/na101/home/literatum/publisher/achs/journals/content/jacsat/2001/jacsat.2001.1 23.issue-34/ja010963d/production/images/medium/ja010963df00001.gif 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 Iron nitrides NOT observed in bulk! Surface? https://pubs.acs.org/na101/home/literatum/publisher/achs/journals/content/jacsat/2001/jacsat.2001.1 23.issue-34/ja010963d/production/images/medium/ja010963df00001.gif 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.