heterogeneous-catalysis.pdf

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