Heterogeneous catalysis Lecture 3 Catalysts preparation Types of catalysts •Bulk catalysts •Supported catalysts •Malaxed-agglomerated catalysts Types of catalysts •Bulk catalysts •Supported catalysts •Malaxed-agglomerated catalysts –Intermediate category –Catalysts obtained by mixing active species with support (powders+precursors) –Ill defined catalysts – broad application in industry Types of catalysts •Bulk catalysts •Supported catalysts •Malaxed-agglomerated catalysts Preparation of bulk catalysts •The whole body of the catalyst is made of one active substance •= It must be cheap, thermally, mechanically, and chemically stable, porous,…! –Silica-alumina (cracking) –Raney nickel, Co molybdate (hydrogenation) –Catalyst supports: silica, alumina, silica-alumina, zeolites, TiO2, MgO, carbon – Preparation of bulk catalysts •Ceramic route (heat and beat) –e.g. Molybdates Bi2O3 + MoO3 → Bi2MoO6 –Used as selective oxidation catalysts (propylene to acrolein) –Homogeneity, time, temperature •(Co-)Precipitation –E.g. NH4VO3 + Al(NO3)3 → V-Al oxynitride (used in propene ammoxidation to acrylonitrile) –change of pH leads to precipitation •Sol-gel method •Combustion synthesis •Solvothermal method –e.g. zeolites Preparation of bulk catalysts •Ceramic route, (co-)precipitation, sol-gel method, combustion synthesis, and solvothermal method: #InorgMatChem •Heat and beat quickly and effectively? •Sol-gel continuously??? • Preparation of bulk catalysts Preparation of bulk catalysts Preparation of bulk catalysts •◊ γ-AlOOH •● α-Al2O3 •Δ α-AlOOH Preparation of bulk catalysts Preparation of bulk catalysts Types of catalysts •Bulk catalysts •Supported catalysts •Malaxed-agglomerated catalysts Preparation of supported catalysts •Properties of catalyst supports –Cheap –Porous –Thermally, mechanically, and chemically stable –… •Role of catalyst supports (non-innocent) –Porosity/hydrophobicity vs. Activity/selectivity –O, H, N,…bank (MvK mechanism) –Electron donor/acceptor (Haber-Bosch) –… – Preparation of supported catalysts •Three types of interaction between support and active phase –Weak –Electrostatic –Covalent Preparation of supported catalysts •Weak interaction = Impregnation methods –Wet impregnation •Suspension of support (e.g. SiO2 in water) •Solution of metal salt (e.g. Cu(NO3)2 in water) •Mixed together (suspension) •Cu2+ cations diffuse into the pores of SiO2 support (=we need to wait for enough long time, Fick laws) •Drying, calcination (= formation of Cu2O NPs on SiO2), (reduction of Cu2O to Cu NPs) •Disadvantages: Time, homogeneity. Preparation of supported catalysts •Weak interaction = Impregnation methods –Dry impregnation (Incipient wetness impregnation) •Dry support, pores full of gas (e.g. SiO2 in ambient air) •Solution of metal salt, V = pore volume (e.g. Cu(NO3)2 in water) •Mixed together (paste) •Capillary elevation (fast) brings Cu2+ solution directly into the pores (concentration gradients might occur – pore size!) •Drying, calcination (= formation of Cu2O NPs on SiO2), (reduction of Cu2O to Cu NPs) •Solubility limits! • Preparation of supported catalysts •Weak interaction = Impregnation methods –Dry impregnation (Incipient wetness impregnation) •Big issue: Gas trapped inside the pores Pore collapse Mechanical degradation Attrition … Preparation of supported catalysts •Weak interaction = Impregnation methods –Dry impregnation (Incipient wetness impregnation) •Big issue: Gas trapped inside the pores Possible solutions: -No gas (impregnation under vacuum) -Lower surface tension (addition of a surfactant) -Wait until gas dissappears and impregnation is completed (either gas dissolves in solution or bubbles move out from the pores) Preparation of supported catalysts •Weak interaction = Impregnation methods –Dry impregnation (Incipient wetness impregnation) •Big issue: Gas trapped inside the pores Preparation of supported catalysts •Three types of interaction between support and active phase –Weak –Electrostatic –Covalent Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Classic ion exchangers: clays, layered hydroxides, zeolites –Oxides with the help of pH Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Zeolites, how does it work? – – – – –Cation compensating the negative charge? –Na+ from the synthetic step Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Zeolites, how does it work? –Na+ → H+ directly? –Na+ → ??? → H+ Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Zeolites, how does it work? –Na+ → NH4+ → H+ – –Zeolite NaY –Up to 73 % of Na+ can be exchanged at R.T. –4 consecutive steps or continuous ion exch. ΔT Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Enhancement…trap for leaving ion (shifting equilibrium)! –e.g. Exchange of Ni2+ for H+ in zeolite mordenite –Ni(NO3)2 vs. Ni(OAc)2 Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Trap •OAc- + H+ → AcOH (rarely dissociated) •OH- + H+ → H2O (rarely dissociated) •SCN- + Ag+ → AgSCN (rarely dissociated) Salt - trap Support Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Classic ion exchangers: clays, layered hydroxides, zeolites –Oxides with the help of pH Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Oxides with the help of pH Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Oxides with the help of pH •Isoelectric point Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Oxides with the help of pH •Exchange capacity: the farther you are from isoelectric point, the higher the exchange capacity. Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Oxides with the help of pH •Stability of support at different pH! •Precursor stability at different pH! Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Oxides with the help of pH •Competitive ion exchange • • • • • •Example: very high affinity between Al2O3 and Pt •Result: heterogeneous distribution of Pt (only outer surface) Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Oxides with the help of pH •Competitive ion exchange • • • • • •NH4OH dissociates readily, NH4+ competes with Pt for sites (= inverse to trap) •Result: homogeneous distribution of Pt through the support 2– Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Oxides with the help of pH •Competitive ion exchange [NH4+] Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Oxides with the help of pH •Competitive ion exchange Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Oxides with the help of pH •Competitive ion exchange Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Oxides with the help of pH •Competitive ion exchange: Note •We say homogeneous Pt distribution… •But how homogeneous??? •Latest research from de Jong group (University Utrecht, HR-TEM, perfect control of Pt deposition on atomic scale) shows that uniform Pt homogeneity is not ideal for hexane isomerization! Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Oxides with the help of pH •Detailed knowledge of isoelectric point, precursor behavior, control over pH: Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Oxides with the help of pH •Detailed knowledge of isoelectric point, precursor behavior, control over pH: Preparation of supported catalysts •Electrostatic interaction = Ion exchange –Oxides with the help of pH Preparation of supported catalysts •Three types of interaction between support and active phase –Weak –Electrostatic –Covalent Preparation of supported catalysts •Covalent bonds = grafting –Formation of metal oxide on top of catalyst support – – – – – – –Is calcination necessary? More in lecture on single site catalysts… Preparation of supported catalysts •Covalent bonds = grafting –Formation of metal NPs on top of catalyst support Preparation of supported catalysts •Covalent bonds = grafting –NPs by impregnation vs. grafting calcination Covalent bonds Uniform NPs Preparation of supported catalysts •Covalent bonds = grafting –Formation of metal NPs on top of graphene oxide – – – – – – Content of COOH groups increased by oxidation (HNO3, H2O2,O3,…) Preparation of supported catalysts •Covalent bonds = grafting –Formation of metal NPs on top of graphene oxide –Last step: Pd reduction – – – – – – Preparation of supported catalysts •Covalent bonds = Deposition/precipitation –Starts as a dry impregnation –Metal salt is then selectively precipitated at the catalyst surface –Precipitation mainly by pH change – hydrolysis/condensation – – – – –