Catalysts Deactivation Loss of activity with time on stream Time Activity rigeneration

Catalysts Deactivation Loss of activity with time on stream Poisoning Coking Sintering Phase Transformation

1. Poisoning Loss of activity due to the strong chemisorption on the active sites of impurities present in the feed stream Geometric effect: blocking an active site Electronic effect: alter the adsorptivity of other species Chemical effect: alter the chemical nature of the active site Reconstruction: formation of new compounds

1. Poisoning Reversible: not too strongly adsorbed Irreversible: irreversible damages Selective: contribution of the characteristics of the active site Non-Selective: poison occurs in a uniform manner Affected Selectivity Not Affected (Selective Poisoning)

1b. Preventing Poisoning Best method: Decrease to acceptable levels the poison content in the feed stream HDS + H2S absorption Methanation of COx in NH3 synthesis Another approch: Choose proper catalyst formulation and design

Catalytic reactions involving HC or CO 2. Coking Catalytic reactions involving HC or CO Formation of carbonaceous residues Coke Carbon or Coking or condensation of HC CHx: 0.5 < x < 1 CO disproportionation 2 CO C + CO2 Physically cover the active sites Coke or Carbon Pore blocking

2. Coking Mechanism of coke formation Olefin polymerization Olefin cyclization to substituted benzenes Formation of polynuclear aromatics from benzenes via carbonium ions intermediates Analytical techniques IR EPR TG-DTA UV-vis 13C-NMR Evolution of combustion products (CO2 and H2O)

2. Coking Coke deposition Preferencially on the exterior of the particles Resistance to the transport of reactants and products in the pores Pore blocking

Appropriate combination of process conditions 2b. Preventing Coking Appropriate combination of process conditions The gas mixture composition is kept as far as possible from conditions under which carbon formation is thermodynamically favored High H2 pressure in catalytic reforming Low HC / H2O ratios in steam reforming over Ni catalysts Optimal catalyst composition Promoters or additives that enhance the rate of gasification adsorben carbon or coke precursors and / or depress the carbon - forming reactions. K in Ni - based steam - reforming catalysts Pt - Re / Al2O3

Metal catalysts (supported or unsupported) 3. Sintering Loss of active surface area via structural modificaion of the catalyst. Metal catalysts (supported or unsupported) Agglomeration and coalescence of small metal crystallites into larger ones with low surface - to - volume ratio. Crystalline migration Atomic migration Result: Particle size growth Low Temperature High Temperature

Coalescence and growth of bulk oxide cristallites 3. Sintering Oxide catalysts Coalescence and growth of bulk oxide cristallites 1250°C 1400°C 1600°C 1550°C Result: Increase in crystallite dimension Increase in grain dimensions Decrease of surface area and porosity SEM images of CeO2 sintered at different temperatures Zhang T. et al. Materials Letters 57 (2002), 507 - 512

Key factors: Temperature Reaction atmosphere 3. Sintering Sinterisation is a thermic activated process Reaction atmosphere Water vapor accelerates crystallization and structural change in oxide catalysts.

4. Solid Phase Transformations Transformation of one phase into a different one Metal supported catalysts Incorporation of metal into the support Ni/Al2O3 active for reforming Rh/Al2O3 Three - Way catalyst NiAl2O4 inactive for reforming Rh2Al2O4 Lean conditions

4. Solid Phase Transformations Metal oxide catalysts Transformation of one crystalline phase into a different one with a step - wise decrease in the internal surface area Limited by rate of nucleation Foreign compounds in the bulk or even on the surface can influence this process: Na2O enhances sintering of g - Al2O3 BaO, CeO2, La2O3, SiO2, ZrO2 improve the stability of g - Al2O3

Ammonia Synthesis A: Steam reforming CH4 + H2O = CO + 3H2 NiO/Al2O3 B: High temperature water-gas shift CO + H2O = CO2 + H2 NiO/Al2O3/ CaO C: Low temperature water-gas shift Zn/Cu D: CO2 absorption basic solution / homogeneous catalyst E: Methanation (CO poison) CO + 3H2 = CH4 + H2O Ni/Al2O3 F: Ammonia synthesis N2 + 3H2 = 2NH3 Fe/Al2O3/K2O/CaO G: NH3 separation.

Ammonia Synthesis The catalyst beds in a reactor for production of 1500 t NH3/day are, in round numbers, 100 m3 and contain 250 t of catalyst. 1010 kg / year of NH4NO3 for USA fertiliser and explosives industry 4NH3 + 5O2  4NO + 6H2O Pt or Pt alloy NO + 1/2 O2  NO2 NO2 + 1/2 H2O  HNO3

Ammonia Synthesis Dn = -2 DH773K = -109 KJmol-1 Rate ? T = 400-500 °C 1/2 N2 + 3/2 H2  NH3 T = 300 °C T = 400 °C T = 500 °C Dn = -2 DH773K = -109 KJmol-1 Rate ? XNH3 T = 400-500 °C P = 100-300 atm Pressure

Ammonia Synthesis i = 0.5 - 0.7 R = k q q = f (T, flow rate and gas composition, catalyst….) q = 4/10

Heterogeneous reactions Surface boundary layer 1. Diffusional transport of reactants from bulk phase to external catalyst surface 2. Pore diffusion of reactants to internal surface 3. Adsorption of reactants 4. Chemical reaction 5. Desorption of products 6. Pore diffusion of products to external surface 7. Diffusional transport of products to bulk phase Bulk phase reactants 3, 4, 5 1 Catalyst pore 6 7 2 products 3, 4, 5 External catalyst surface

Transport phenomena Porous solid Intrareactor transport Intrapellet Stagnant gas film Pores Solid Porous solid Intrapellet transport Intrareactor transport Interphase transport Stagnant gas film Flowing bulk gas

Ammonia Synthesis Molecular route Atomic route N2* + 6H*  2NH3 * + 4* N* + 3H*  NH3 * + 3* N2  N2(ads)  2N (ads) N2  N2(ads)  2N (ads)  NH3  surface saturation NH3 steady- state [N(ads)] = k Thermal stability H2(g) + 2*  2H* Tdes < 200°C N2(g) + *  N2* Tdes <-100°C N2 * + *  2N* Tdes 450°C NH3(g) + *  NH3* Tdes <100°C Fe(111) 310 °C in vacuum after reaction

Ammonia Synthesis 1 H2(g) + 2*  2H* 2 N2 (g) + *  N2* 3 N2* + *  2 N* slow 4 N* + H*  NH* + * 5 NH* + H*  NH2* + * 6 NH2* + H*  NH3* + * 7 NH3*  NH3(g) + *

Ammonia Synthesis

Ammonia Synthesis Cat. BASF: FeOX / K2O / Al2O3 / CaO Reduction at 400° C Fe met / K2O / Al2O3 / CaO XPS: X-ray Photoelectron Spectroscopy AES: Auger Electron Spectroscopy

Ammonia Synthesis p* s Promoter (K) Poison N2 Fe N N Fe K

Ammonia Synthesis The reactor may contain 2 catalyst beds and 2 internal heat exchangers inside a pressure shell: Upper heat exchanger Lower heat exchanger First bed Second bed

Ammonia Synthesis A: The main flow (pale blue) first flows along the pressure shell and cools it. The gas is then heated in the lower heat exchanger and quench gas (green) is added. B: In the first bed, both the temperature and the concentration increases. At the outlet from the first bed, the state of the gas is very near equilibrium (red curve). C: The gas is cooled in the upper heat exchanger and the third inlet gas stream (dark blue) is added. This inlet stream was preheated in the upper heat exchanger. D:In the second bed, both the temperature and the concentration increases. E: The gas is cooled in the lower heat exchanger and leaves the reactor as the outlet stream (red).