Heterogeneous Catalysis PDF
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Uploaded by CheaperBlueLaceAgate
The University of Zambia
Richard Walton
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This document provides lecture notes on heterogeneous catalysis, covering definitions, surface reactions with solid catalysts, and industrial applications like ammonia synthesis and hydrogen production. It details various aspects of catalytic processes and presents insights into the optimization of heterogeneous catalysis. The document is from a university course in inorganic chemistry.
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lOMoARcPSD|6077384 Heterogeneous catalysis Advanced Inorganic Chemistry and Laboratory (The University of Warwick) Studocu is not sponsored or endorsed by any college or university Downloaded by Anushaya Jeyabalan ([email protected]) lOMoARcPSD|6077384 Richard Walton – Inorganic Segment 1 - defin...
lOMoARcPSD|6077384 Heterogeneous catalysis Advanced Inorganic Chemistry and Laboratory (The University of Warwick) Studocu is not sponsored or endorsed by any college or university Downloaded by Anushaya Jeyabalan ([email protected]) lOMoARcPSD|6077384 Richard Walton – Inorganic Segment 1 - definitions Heterogenous catalysis: multi-phase i.e. gas and liquid Homogeneous: single phase Heterogenous catalysts: Uniform: single phase i.e. crystalline particle regular structures Multi-phase: active species + support i.e. metal nanocrystals on carbon support/matrix Active sites: sites for adsorption I.e. bronstead acid sites, lewis acid sites Segment 2: Binding of molecules at surface with solid catalysts 1. Transport of substrate to surface 2. Surface interaction: adsorption process a. Either physisorption van der waals, 10-50kjmol-1 b. Chemisorption formation of chemical bond, strong interaction >100kjmol-1 3. Chemical reaction 4. Loss of product from surface 5. Transport of product from surface empty active sites produced Examples of surface binding modes: Optimization of heterogeneous catalysis - sabatier principle Must understand that there is a balance to be struck: too favourable formation of surface complex- product wont be released But then weak binding leads to no interaction and no reaction Unstable intermediate is formed between substrate and catalyst surface I.E. decomposition of formic acid volcano plot HCOOH CO+H2O Optimum catalyst at the top of the peak Weak catalysts at the bottom – left to right increases in bond strength/interaction Low sticking at left Strong interactions at right No chemical reactions 2x positions for gold, that we will look at Segment 3: Surface reactivity e.g. hydrodesulfurization of dibenzothiophene Bimolecular reactions A (g) + B (g) C (g) Downloaded by Anushaya Jeyabalan ([email protected]) lOMoARcPSD|6077384 Langmuir-Hinshelwood Eley-Rideal Are all crystal surfaces the same? Dependent on how we slice to give the unit cell crystal has a mixture of faces Miller indices Plane (hkl) is one of a set that runs parallel through the origin Surface reactivity depends on crystal face Closest metal contact for Ni FCC 100 is 2.48 angstroms All atoms at same level for 110 No 3.51 angstroms for 111 For ethene adsorption on FCC Ni metal: dehydrogenation of ethene to form C=-C + H2 We see straining of molecule, feta, and this depends on the Ni-Ni distance. If d is larger, then there is greater strain at the surface of the adsorbed species more reactive Hence, 110 presents lots of 3.51 A and so rate of reaction is greater than the other 2 None seen in 11 Some 3.51 A for 100 but not many as its along the diagonal –no stacking 110>100>111 We could then produce a volcano plot using this kind of data Segment 4: Surface area How do we prepare high surface area solid catalysts? Ball milling: mechanical grinding of large particles Solution growth of inorganic particles: rapid ppt, hydrothermal (100degrees Celsius, from water + in enclosed vessel) Gas phase: rapid combustion of volatile reagents – flame pyrolysis Porous materials: micro or miso porous – internal surface area Surface area calculation 1 10x10x10nm3 cube Compare the surface area of this cube to 1x1x1nm3 cube that occupy the same volume SA of 1st cube = 100nm2 x 6 =600nm2 SA of 2nd cube = 1nm2 x 6 = 6nm2 Volume = 1000nm3 – occupy the same volume 6x 1000 = 6000nm3, which is 10x more SA Surface area calculation 2 Downloaded by Anushaya Jeyabalan ([email protected]) lOMoARcPSD|6077384 What is the specific Fe2O3 surface area made up of 1nm3 cubes? P=5.24gcm-3 = Specific SA = S.A. per gram of material Mass = pv = 5.24 x 1 = 5.24x10-21g (convert units to cm3) 107nm = 1cm So SA= 6nm2 P=5.24x10-21g/nm-3 Units =m2g-1 So specific SA = 6x10-18/5.24x10-21g = 1145m2g-1 Advantages and disadvantages of heterogeneous catalysts Segment 5 – examples of industrial catalysts 1 Ammonia synthesis N2 +H2 2NH3 enthalpy of forward: 92kjmol-1 Production of artificial fertilizer Haber-Bosch process: Fe catalyst, 400 degrees, 15MPa – mechanism in the notes N=-N: 942kjmol-1 H-H: 432 kjmol-1 Fe(111) surface most reactive Fe is BCC 3-way catalysis: automotive exhausts 1. Oxygen concentration – mechanism in notes 2. Role of precious metal: multi-phase catalysts allow each phase to take place 1. Rh: CO oxidation 2. Pt/Al2O3: hydrocarbon oxidation 3. Pt: NO reduction to nitrogen 3. Support Downloaded by Anushaya Jeyabalan ([email protected]) lOMoARcPSD|6077384 High S.A. allows for dispersion of catalyst and gas flow optimized over catalsy High T >500 degrees Celsius Segment 6: Industrial catalyst 2 Hydrogen production and purification Steam reforming of hydrocarbons: CnHm + H2O nCO +(n+m/2)H2 under high t Ni catalyst, with an Al2O3 support at 450-950 degrees Both molecules influencing reaction 3D volcano plot langmuir hindelwood Ni, Rh, Ir, Ru are optimum catalysts as they are in the red zone spenny Hydrogen purification – remove CO ‘water gas shift’ CO +H2O CO2 +H2 Enthalpy of forward reaction = -41 kjmolTypes of catalysts: High temp shift catalyst: traditional, oxides of iron or chromium. 400-500 degrees Celsius. CO reduced to 2-5% Low temp shift catalyst: copper oxides, zinc oxide, alumina, 200 and 400 degrees Celsius. CO reduced to 1% -better yield Au-CeO2 further reduction in temperature, new state of the art catalysis homogeneous type o See notes for mechanism o 1) adsorption of CO o 2) CO CO2 leaves system, leaving behind a vacant site o 3) Water fills the vacant site o 4) Proton transfer o 5) 2nd proton transfer o 6) loss of H2 – pristine catalyst regenerated o Redox chemistry: 4+ 4+/3+ (4-2x)+ (4-x)+ Oxidation catalysis: large scale organic chemistry Vanadium oxide oxidation catalysis: Water/steam 1. V 5+ Dimethyl benzene coordinates to active species causing a 5+ 4+ reduction 2. V 4+ Rearrangement/loss of water leads to less crowded coordinated species and therefore space for 2 dimethyl benzenes molecules 3. V 4+ Another dimethyl benzene coordinates 4. V 3+ product defuses away – phallic anhydride product don’t need the mechanism for this Key points for heterogenous catalysis: Downloaded by Anushaya Jeyabalan ([email protected]) lOMoARcPSD|6077384 Solid/liquid or solid/gas reactions used in industrial scale Uniform or multiphase catalysis with specific active sites Various sites are available i.e. Lewis or Bronstead, redox, metal surfaces (V) Kinetic models/mechanisms derived from adsorption of molecules at surfaces Sabatier principle/volcano plots show the affinity relation to catalysts EQM shifted by T and P Surface binding sites on metals dependent on which crystal faces present. Recall (110,100,111) Problem 1 1. Re is a poor catalyst because the CO bond breaks too easily (exo) and so the bond formed between Re-CO is too strong and so the reaction will not continue. Pt, Pd has too strong bonds so there will be no interaction(endothermic) at all with the catalyst, weak binding/low sticking probability. Ru and Co are the best catalysts with the highest amount of activity. 2. As H2 (gas, not adsorbed). Elley-Rideal mechanism 3. Increase the surface area, crystal faces, facetted crystals, allows Segment 7 Porous materials 1D pores 2D pores: perpendicular pipe system, void/gage like spaces OR zig-zag 3D pores Classification of porosity Diameter (d) of pore defines porosity Microporous: d