Robin Whyman

Identification of active phases in Au–Fe catalysts for low-temperature CO oxidation

In the light of a recent study which identified the beneficial influence of poorly crystallised ferrihydrite Fe5HO8·4H2O on the activity of CO conversion catalysts comprising gold nanoparticles on oxidic iron, we have investigated three series of ferrihydrite-rich samples prepared by coprecipitation. The samples were structurally and chemically characterised using X-ray diffraction and both 57Fe and 197Au Mossbauer spectroscopy, and tested for CO oxidation at room temperature using a microreactor with on-line GC. The highest activity, 100% conversion after 20 min on line, was observed in a dried sample that contained ferrihydrite and a non-crystalline and possibly hydrated gold oxyhydroxide phase, AuOOH·xH2O. The activity of the same materials after calcination, where the gold was transformed to 3–5 nm Au metal particles and the ferrihydrite to hematite, was less than ca. 7%. This is the first report of a synergistic interaction between AuOOH·xH2O and ferrihydrite resulting in an active catalyst for room temperature CO oxidation, and contrasts with previous work which has been interpreted in terms of the requirement for metallic Au nanoparticles.

High-activity Au/CuO–ZnO catalysts for the oxidation of carbon monoxide at ambient temperature

Ambient temperature CO oxidation has been studied with Au/ZnO, Au/CuO and Au/CuO–ZnO catalysts prepared by a coprecipitation method. The catalysts are highly active for CO oxidation and are capable of sustained activity at 20°C. The highest activity is observed with Au/ZnO and this catalyst is shown, using transmission electron microscopy, to be comprised of smaller Au metal particles (2–5 nm) and a higher Au dispersion when compared with the other catalysts.

Preparation and characterisation of solvent-stabilised nanoparticulate platinum and palladium and their catalytic behaviour towards the enantioselective hydrogenation of ethyl pyruvate

Solvent-stabilised Pt and Pd nanoparticles, of size range 2.3–2.8 nm and 2.7–3.8 nm, respectively, have been prepared by metal vapour synthesis routes, characterised by transmission electron microscopy (TEM), and their behaviour as catalysts for the enantioselective hydrogenation of ethyl pyruvate (EP) investigated; comparisons have been effected with the performance of standard supported Pt and Pd catalysts. Cinchona alkaloid-modified Pt nanoparticles display parallel behaviour to that exhibited by their conventional supported counterparts both in terms of the sense of the enantioselectivity in the ethyl lactate product and in the acceleration in reaction rate relative to the unmodified system. With Pd, however, significant differences are noted. Here, the sense of the enantioselectivity relative to that reported previously over conventional supported catalysts is reversed, i.e., an (R)- vs. (S)-enantiomer switch occurs, and a rate acceleration rather than retardation is noted on cinchona alkaloid modification. The Pt particle size distribution shows a higher degree of monodispersity after use in catalysis, although the average particle size remains essentially unchanged, whereas the behaviour of the Pd nanoparticles shows evidence of concentration dependence, lower concentrations showing Pt-like behaviour but more highly concentrated preparations showing evidence of significant aggregation during catalysis. With Pt catalysts, the presence of water as a component of the ketonic solvent system is shown to result in a significant acceleration in overall reaction rate with both conventional supported catalysts and their solvent-stabilised counterparts. In sharp contrast, totally aqueous-based colloidal platinum preparations, obtained by conventional salt reduction, display very low reaction rates and enantioselectivities.

Chemisorption and catalysis by metal clusters: I. Characterisation of materials obtained by impregnation of Os3(CO)12 and Os6(CO)18 onto silica, alumina, and titania

Abstract Os 3 (CO) 12 and Os 6 (CO) 18 were impregnated onto silica, alumina, and titania and characterised in the freshly impregnated state and in states achieved by subjecting the freshly impregnated material to (i) washing, (ii) heating to 523K (temperature-programmed decomposition), and (iii) storage at room temperature. The original clusters interact with the support surfaces and are converted to a family of species A of empirical formula Os n (CO) xn C yn , where the most likely value of n is 12, 2.0 ≤ x ≤ 3.0, and 0.0 ≤ y ≤ 0.4. Retention of osmium-osmium bonding in species A is demonstrated by ultraviolet/visible reflectance spectroscopy and the upper limit of n is suggested by electron microscopy. Infrared spectra of species A contain three bands and indicate the presence of carbonyl ligands bonded to osmium atoms in formal zero, partial negative, and partial positive oxidation states. Species A chemisorbs carbon monoxide and oxygen at 293K, the extent of oxygen chemisorption being the same as that of strong CO chemisorption. A bridged structure for adsorbed-CO is proposed. [ 18 O]CO adsorbed onto species A does not equilibrate, even at high temperatures, with linearly bonded [ 16 O]CO-ligands, confirming that adsorbed-CO and ligand-CO are different states of bound CO. CO 2 is formed, probably by a Boudouard reaction, during temperature-programmed decomposition of all freshly impregnated materials, and hence species A prepared in this way may contain ligand-C. Speculations as to likely cluster structures for species A are presented. Chemisorption and catalytic properties will be described in later papers.

Chemisorption and catalysis by metal clusters: III. Hydrogenation of ethene, carbon monoxide, and carbon dioxide, and hydrogenolysis of ethane catalyzed by supported osmium clusters derived from Os3(CO)12 and from Os6(CO)18

Properties are described for catalysts containing high nuclearity metal clusters (nuclearity ~12) derived from Os3(CO)12 and Os6(CO)18 and supported on silica, alumina, titania, or ceria. Ethene hydrogenation (325–535 K), ethane hydrogenolysis (395–665 K), CO hydrogenation (455–665 K), and CO2 hydrogenation (455–715 K) have been examined in pulsed-flow and static reactors. The high nuclearity osmium clusters, protected against sintering by retained ligand-CO, ligand-C, and a support-cluster interaction, are stable under these conditions and provide highly reproducible activity. Freshly prepared catalysts each exhibit an initial non-steady state, during which hydrocarbon is progressively retained and activity rises, passes through a maximum, and declines to a steady state value. Catalysts in the steady state continue to retain hydrocarbon which is probably branched in structure and unsaturated in character. Such retained hydrocarbon species mediate hydrogen atom transfer to reacting adsorbed species. Their concentrations, which have been determined by infrared spectroscopy, 14C-tracer studies, and material balances, are compared with the known site concentrations associated with fresh cluster-derived catalysts. Catalysts in the steady state exhibit activities the magnitudes of which diminish with increasing support-cluster interaction, viz., silica-supported clusters > titania-supported clusters > alumina-supported clusters. Preliminary measurements using a ceria-supported catalyst suggest that activity versus the strength of the support-cluster interaction exhibits a “volcano” relationship. Adsorption of ethene, ethane, and CO occurs at osmium atom sites on the high nuclearity osmium clusters, and the reaction intermediates are also adsorbed at these sites. CO2, however, is adsorbed and reacts at ligand-C sites. Detailed mechanisms are presented for ethene, CO, and CO2 hydrogenations, of which some aspects have been investigated by use of 14C as an isotopic tracer. Most cluster-derived catalysts show exceptional activity for ethane hydrogenolysis, some apparent turnover numbers being 2 orders of magnitude higher than for supported metallic osmium. The osmium clusters adsorb reactants less strongly than metallic osmium, because of their commitment to bonding with the protective CO-ligands, and this weaker reactant adsorption may provide superior catalytic properties.

Support effects in the ruthenium-catalyzed hydrogenation of carbon monoxide: Ethene and propene addition

Abstract CO hydrogenation catalyzed at about 500 K by 2.6% Ru/silica (I), 1.5% Ru/13X-zeolite (II), 17% Ru/titania (III), and 5% Ru/magnesia (IV) gave methane and 1-alkenes as primary products. 1-Alkene isomerization and hydrogenation gave internal alkenes and alkanes as secondary products. Specific activity varied in the sequence III ⪢ II > I > IV whereas selectivity for methane formation, as opposed to higher hydrocarbon formation, varied in the sequence I > II > III > IV. Comparison of one catalyst with another showed that when the methane yield was high the fraction of higher hydrocarbon appearing as alkane at moderate conversions was also high, and vice versa. Ethene addition to CO hydrogenation over (I) and (II) at low conversions (2 to 15%) markedly increased the rate of higher hydrocarbon formation without greatly influencing the methanation rate, whereas ethene addition over (III) and (IV) enhanced the rate of higher hydrocarbon formation by a factor of less than 2 and reduced the methanation rate. Propene addition to CO hydrogenation over (I) increased the rates both of higher hydrocarbon formation and of C2-hydrocarbon formation, again without markedly affecting the methanation rate. The single most important factor in the determination of the total product distribution is the availability of adsorbed hydrogen which varies from catalyst to catalyst in the sequence I > II > III > IV. The activity sequence is ascribed to various metal-support effects.

The Synthesis of, and Characterization of the Dynamic Processes Occurring in, Pd(II) Chelate Complexes of 2-pyridyldiphenylphosphine

Pd(II) complexes in which 2-pyridyldiphenylphosphine (Ph(2)Ppy) chelates the Pd(II) centre have been prepared and characterized by multinuclear NMR spectroscopy and by X-ray crystallographic analysis. trans-[Pd(kappa(1)-Ph(2)Ppy)(2)Cl(2)] is transformed into [Pd(kappa(2)-Ph(2)Ppy)(kappa(1)-Ph(2)Ppy)Cl]Cl by the addition of a few drops of methanol to dichloromethane solutions, and into [Pd(kappa(2)-Ph(2)Ppy)(kappa(1)-Ph(2)Ppy)Cl]X by addition of AgX or TlX, (X = BF(4)(-), CF(3)SO(3)(-) or MeSO(3)(-)). [Pd(kappa(1)-Ph(2)Ppy)(2)(p-benzoquinone)] can be transformed into [Pd(kappa(2)-Ph(2)Ppy)(kappa(1)-Ph(2)Ppy)(MeSO(3))][MeSO(3)] by the addition of two equivalents of MeSO(3)H. Addition of further MeSO(3)H affords [Pd(kappa(2)-Ph(2)Ppy)(kappa(1)-Ph(2)PpyH)(MeSO(3))][MeSO(3)](2). Addition of two equivalents of CF(3)SO(3)H, MeSO(3)H or CF(3)CO(2)H and two equivalents of Ph(2)Ppy to [Pd(OAc)(2)] in CH(2)Cl(2) or CH(2)Cl(2)-MeOH affords [Pd(kappa(2)-Ph(2)Ppy)(kappa(1)-Ph(2)Ppy)X]X, (X = CF(3)SO(3)(-), MeSO(3)(-) or CF(3)CO(2)(-)), however addition of two equivalents of HBF(4).Et(2)O affords a different complex, tentatively formulated as [Pd(kappa(2)-Ph(2)Ppy)(2)]X(2). Addition of excess acid results in the clean formation of [Pd(kappa(2)-Ph(2)Ppy)(kappa(1)-Ph(2)PpyH)(X)]X(2). In methanol, addition of MeSO(3)H and three equivalents of Ph(2)Ppy to [Pd(OAc)(2)] affords [Pd(kappa(2)-Ph(2)Ppy)(kappa(1)-Ph(2)Ppy)(2)][MeSO(3)](2) as the principal Pd-phosphine complex. The fluxional processes occuring in these complexes and in [Pd (kappa(1)-Ph(2)Ppy)(3)Cl]X, (X = Cl, OTf) and the potential for hemilability of the Ph(2)Ppy ligand has been investigated by variable-temperature NMR. The activation entropy and enthalpy for the regiospecific fluxional processes occuring in [Pd(kappa(2)-Ph(2)Ppy)(kappa(1)-Ph(2)Ppy)(2)][MeSO(3)](2) have been determined and are in the range -10 to -30 J mol(-1) K(-1) and ca. 30 kJ mol(-1) respectively, consistent with associative pathways being followed. The observed regioselectivities of the exchanges are attributed to the constraints imposed by microscopic reversibility and the small bite angle of the Ph(2)Ppy ligand. X-Ray crystal structure determinations of trans-[Pd(kappa(1)-Ph(2)Ppy)(2)Cl(2)], [Pd(kappa(2)-Ph(2)Ppy)(kappa(1)-Ph(2)Ppy)Cl][BF(4)], [Pd(kappa(1)-Ph(2)Ppy)(2)(p-benzoquinone)], trans-[Pd(kappa(1)-Ph(2)PpyH)(2)Cl(2)][MeSO(3)](2), and [Pd(kappa(1)-Ph(2)Ppy)(3)Cl](Cl) are reported. In [Pd(kappa(2)-Ph(2)Ppy)(kappa(1)-Ph(2)Ppy)Cl][BF(4)] a donor-acceptor interaction is seen between the pyridyl-N of the monodentate Ph(2)Ppy ligand and the phosphorus of the chelating Ph(2)Ppy resulting in a trigonal bipyramidal geometry at this phosphorus.

Synthesis and reactivity of palladium hydrido-solvento complexes, including a key intermediate in the catalytic methoxycarbonylation of ethene to methyl propanoate

The sequence of reaction steps and the role of each reactant, required for the transformation of the Pd(0) precursor [Pd(dtbpx)(dba)] [dtbpx = 1,2-(CH2PBut2)2C6H4; dba = trans,trans-(PhCHCH)2CO], 1, into [Pd(dtbpx)H(MeOH)]+, 2a, the active Pd(II)-hydride catalyst for the methoxycarbonylation of ethene to methylpropanoate, have been delineated using a combination of spectroscopic and crystallographic methods. The preparation and characterisation of a variety of related complexes are described including some unusual examples involving bidentate sulfonate complexes and mono-cationic and neutral palladium hydride complexes. X-Ray crystal structures have been determined for [Pd(dtbpx)(η2-O2)], 3, [Pd(dtbpx)(η2-BQ)] (BQ = benzoquinone), 4, [Pd(dcpx)(dbaH)]+ [dcpx = 1,2-(CH2PCy2)2C6H4], 7, and [Pd(dtbpx)(η2-MeSO3)]+, 9b.

Gold nanoparticles a renaissance in gold chemistry

Vapour synthesis techniques have been used to prepare nanoparticulate dispersions of gold and other precious metals in non-aqueous solvents. The dimensions of these solvent-stabilised particles, which can be controlled within the 1–3nm size regime, effectively encompass the areas of molecular chemistry (as typified by high-nuclearity metal clusters) and the smaller colloidal metals. Gold nanoparticles differ from those of the other metals in exhibiting unusual time— and concentration-dependent behaviour. A regime of preparative conditions under which 1–3nm size gold particles, which are stable with respect to aggregation as a function of time, is defined. Some implications for these new developments are indicated.

Stoichiometric hydrogenation of ethene on Rh(111); mechanism, importance of weakly adsorbed ethene, and relationship to homogeneous catalysis

The hydrogenation of ethene is an important reaction in heterogeneous catalysis and, despite its apparent simplicity, many aspects of the reaction mechanism remain unclear. By contrast, the mechanism using homogeneous catalysts such as Wilkinsons catalyst [(RhCl(PPh3)3] is thought to be well understood. To allow a comparison between the homogeneous and heterogeneous reactions we have studied ethene/hydrogen interactions on the (111) plane of rhodium in the temperature range 160–500 K. Under UHV conditions no catalytic reaction was detected. However, we have been able to observe stoichiometric hydrogenation and exchange in the chemisorbed layer. A mixed adlayer of either ethene/deuterium (or perdeuteroethene and hydrogen) was formed at ca. 160 K, and allowed to warm up. From previous spectroscopic studies, ethene is adsorbed at 165 K as partially rehybridised,π bonded species with a C-C bond order of ca. 1.5, similar to ethene in Zeises salt. At 190–210 K we observe coincident desorption of undeuterated ethene — the major species — together with much smaller quantities of deuterated ethane and partially deuterated ethenes. The influence of both hydrogen and ethene pre-coverage has been studied as has the relative extent of hydrogenation and exchange. The ethane formation results parallel those reported by other authors on Pd(110) and Pt(111) and Pt(110). We propose that on all three metals both hydrogenation and exchange follow the same pathway, with a common intermediate for exchange and hydrogenation. This isa weakly held, π bonded species formed during the desorption process, which can be convertedreversibly into an adsorbed ethyl species. A detailed comparison indicates that the mechanism of heterogeneous hydrogenation closely parallels that in the homogeneous phase.