Morphology and Selectivity in Catalysis by Metals

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1 Dennis Dowden Commemoration University of Durham, 3rd & 4th April 2013 Morphology and Selectivity in Catalysis by Metals Peter B. Wells Emeritus Professor, University of Hull Department of Chemistry, Cardiff

2 Ethylene Yields in Acetylene Hydrogenation Butene Yields in Butadiene Hydrogenation Ru Rh Pd 75% 80% 99%+ 75% 80% 99%+ Os Ir Pt 40% 30% 90% 40% 20% 90%

3 Ethylene Yields in Acetylene Hydrogenation Butene Yields in Butadiene Hydrogenation SPd>SPt>SRh>SRu>SOs>SIr Ru This selectivity pattern is Rh Pd observed in hydrogenations 75% 80% of all di-unsaturated 99% 75% aliphatic hydrocarbons 80% 99% Os Ir Pt 40% 30% 90% 40% 20% 90%

4 Dennis s perspective in1960 Of course catalytic activity is the combined result of the electronic structure of the metal and the physical arrangement of the surface atoms; and, of course physical arrangements include the various crystal planes exposed together with their imperfections.

5 Dennis Dowden appointed Honorary Visiting Professor University of Hull?

6 Honorary Professorship at the University of Hull Three lecture courses 1. Electron Theory of Metals

7 Honorary Professorship at the University of Hull Three lecture courses 1. Electron Theory of Metals 2. Catalyst Preparation

8 Honorary Professorship at the University of Hull Three lecture courses 1. Electron Theory of Metals 2. Catalyst Preparation 3. Catalysis by Sulphides

9 Snapshot 1: Variation of activity with electronic structure Exchange of H and D in benzene H H H H + D D D D X X X X H H D D X X [X = H or D] Gas phase reactions at 273 K were catalysed by evaporated films of most of the transition metals A rate coefficient for the overall exchange, k, was determined from the time-dependencies of the decays of C6H6 and C6D6, and the rise and subsequent decay of each of the deuteriobenzenes R.G. James, R.B. Moyes, Faraday I, 1977,

10 log10k Ni V Cr Fe Co Dependence of the rate coefficient for benzene exchange, k, on the density of states at the Fermi level (for first row transition metals) Ti Mn 0.0 Sc /N(EF)

11 log10k Co Ni Re w Fe V Mn Dependence of the rate coefficient for benzene exchange, k, on the density of states at the Fermi level for first and third row transition metals 0.0 Cr Ta Hf [P.B. Wells, Proc. Symp. Electroctalysis (ed. M.W. Breiter, The Electrochemical Society, Princeton) ] log10n(ef) Ti Sc

12 Snapshot 2: Selectvity and electronic structure Hydrogenation of 1,3-butadiene + H +H * Me +H * Me [J.J. Phillipson, P.B. Wells, G.R. Wilson, J. Chem. Soc. (A), 1969, ]

13 80 Selectivity for 1-butene formation from 1,3-butadiene over films of second and third row transition metals Au 70 Pt 60 1-butene yield / % Mo Re Pd Rh Ir Y Hf Nb Zr Ta W Pauling electronegativity Ag [R.B. Moyes, P.B. Wells, Y. Salman, Appl. Catal. A General. 2002, 229, ]

14 Snapshot 3: Selectivity and bulk structure (example 1) Butadiene Hydrogenation catalysed by MoS 2 40 Butenes /% Conversion / % MoS2 exists in two forms rhombic hexagonal In every respect - specific activity, kinetics, product composition, decay charactreristics, the two forms behaved identically! [M.R. Blake, M. Eyre, R.B. Moyes, P.B. Wells, Bull. Soc. Chim. Belg., 1981, 90, ]

15 Snapshot 4: Selectivity and bulk structure (example 2) SPd > SPt > SRh > SRu > SOs > SIr Metal powders prepared by reduction of salts in H2 were found to occlude hydrogen. [Extent of H- occlusion measured by exchange with D2] IrH0.25 > OsH0.09 > RuH0.04 >RhH0.03 > PtH0.006 Extent of occlusion is a measure of bulk disorder Extent of disorder is determined by the mobility of the metal atoms at the reduction temperature Rule of thumb: Surface metal atoms become mobile at one-third of their Kelvin M.Pt. (Huttig s Rule)

16 -2.5 Pt ln x = E(TH Tred) THTRed ln x Rh Ru -1.0 Os -0.5 Ir ln x = E(TH Tred) / THTRed [P.B. Wells, J. Catal., 1978, 52, 498]

17 If this is correct, then Ir should be selective if it is prepared by reduction above its Huttig Temperature (2716/3) i.e. 905 K Ir/silica reduced at 1100 K and used to catalyse 1,3-butadiene hydrogenation at room temperature gave a butene yield of 97.5% (cf. Pt = 98% under the same conditions) [A.G. Burden, J. Grant. J. Martos, R.B. Moyes, P.B. Wells Disc. Faraday Soc., 1981, 72, ]

18 Snapshot 5: Selectivity and site congestion CH2=CH-CH2-CH2-CH3 * 1-pentene + H -H(s) CH3-CH-CH2-CH2-CH3 * -H (i) cis/% highly congested single-atom site less congested single-atom site relatively uncongested multiple-atom sites i/ (i + s) CH3-CH=CH-CH2-CH3 * cis- & trans-2-pentene Catalysts Single Ni-atom sites in NiH[P(OEt)3]4 Single Ni-atom sites in Ni phthalocyanine Multiple Ni-atom sites in Ni/alumina Multiple Ni-atom sites in Ni film + [D. McMunn, R.B. Moyes, P.B. Wells, J. Catal, 1978, 52, 1519]

19 1-Pentene isomerisation at 353 K Effect of dilution of catalyst RuHCl(CO)(PPh3) 3 [RuHCl(CO)(PPh3)3] cis-yield/% trans-yield/% cis/trans micromol/l ratio 1060 (307 K) [D. Bingham, D,E, Webster, P.B. Wells, J. Chem. Soc. Dalton, 1974, ]

20 Snapshot 6: A facile aspect of the Metal-Support-Interaction The FTIR and EXAFS spectra of Os6(CO)18/alumina varies with time as the bi-capped tetrahredron of Os atoms unfolds P.B. Wells, P. Worthington, M.S. Roberts, R.B. Moyes, J. Physique, 1986, 47, C

21 Snapshot 7: Edge and terrace site contributions to enantioselectivity MeCOCOOEt + H2 Pt + cinchonidine 30 bar MeC*H(OH)COOEt R- and S- enantiomers Enantiomeric excess /% = 100[(R-) (S-)] / [(R-) + (S-)] Typical values: Pt/alumina 75 85% (Zurich group) Pt/silica 65 75% (Hull group, EUROPT-1) Pt/graphite 40 45% (Cardiff group) Pt/carbon 20% (Orito, Japan) heat-treated Pt/carbon ~60% (Orito, Japan) sintered Pt/silica 78% (Hull group) sintered Pt/graphite 63%* (Cardiff group) Exceptional value: Colloidal Pt 95% [*G.A. Attard, K.G. Griffin, D.J. Jenkins, P.Johnston, P.B.Wells, Catal. Today, 2006, 114, ]

CV of as-received Pt/graphite 400 200 (a) (b) (c) (d) Current / A 0-200 (d) at 0.47 V = {111} terrace -400-600 0.0 0.1 0.2 0.3 0.4 0.

22 CV of as-received Pt/graphite (a) (b) (c) (d) Current / A (d) at 0.47 V = {111} terrace E (Pd/H Reference) / V (a) at 0.06 V = {111} x {111} step (b) at 0.20 V = {100} x {111} step (c) at 0.28 V = {100} terrace

23 Effect of adding Bi (step-site blocker) to Pt/graphite and to Pt/silica ee /%R %Pt/graphite ee /%R %Pt/silica [EUROPT-1] Bi coverage Volume of modifier solution /ml Conclusion: Step sites give high ees, terrace sites give low ees

24 Reverting to Snapshot 4 What sites are responsible for unwanted alkane formation?

25 What sites are responsible for alkane formation? H2C=C=CHMe + D2 reaction suggested Me CH2Me Hence CH3 in ethyne hydrogenation C * CH [R.G. Oliver, P.B. Wells, J. Catal., 1977, 47, 364] * Professor Gary Attard is presenting a paper at the Faraday Discussion in Berlin this month, in which he presents evidence for this type of intermediate and, by Raman spectroscopy and cyclic voltammetry, identifies the step sites at which it is formed.

26 Snapshot 1 Snapshot 2 Snapshots 3,4 Snapshot 5 Snapshot 6 Snapshot 7 Summary Activity and electronic structure Selectivity an electronic structure Selectivity and bulk structure Selectivity and site congestion Metal-support interaction and structure Edge and terrace site contributions to selectivity

(C) Pavel Sedach and Prep101 1

(C) Pavel Sedach and Prep101 1 (C) Pavel Sedach and Prep101 1 (C) Pavel Sedach and Prep101 2 (C) Pavel Sedach and Prep101 2 (C) Pavel Sedach and Prep101 3 (C) Pavel Sedach and Prep101 3 (C) Pavel Sedach