Brian Johnson has now retired from his university position

The chemical and physical properties of small metal particles of less than 100 Å in diameter are at present under intense investigation within our group. This interest arises from their situation intermediate between atomic and metallic regimes, and because of their application to a wide range of commercial processes. Within this framework fall the following topics:

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Clusters and Nanocatalysts Anchored Inside Mesoporous Silica

Mixed-metal clusters are used as precursors for bimetallic nanoparticle catalysts. After deposition of the cluster molecules in a support such as the mesoporous silica MCM-41, the nanocatalyst is produced by gentle decarbonylation by heating under vacuum. This method produces homogeneously dispersed particles along the tunnels of the mesopores, of known size and distribution and with the two metals in the same ratio than in the starting cluster.

This strategy has been used successfully to produce efficient Ru-Cu, Ru-Ag, Ru-Pd, Ru-Sn and Ru-Pt hydrogenation catalysts. These materials have been characterised by a combination of in situ infrared and EXAFS spectroscopy, and ex situ high-resolution electron microscopy, and the catalysis of numerous hydrogenation reactions explored. They prove to be remarkbly active and highly selective.

HRTEM image of MCM-41 loaded with [PPN]₂[Ru₆C(CO)₁₆].
The black dots lie along the pore axis, forming a rosary-like pattern.

3D picture of catalytic nanoparticles in MCM-41 obtained by electron tomography.
Nanoparticles are red or blue, MCM is white.

Selected Publications

JM Thomas, BFG Johnson, R Raja, G Sankar, and PA Midgley, Acc. Chem. Res., 2003, 36, 20

JM Thomas, R Raja, BFG Johnson, TJ O'Connell, G Sankar and T Khimyak, Chem. Commun., 2003, 1126

PA Midgley, M Weyland, JM Thomas and BFG Johnson, Chem. Commun., 2001, 907

R Raja, T Khimyak, JM Thomas, S Hermans and BFG Johnson, Angew. Chem. Int. Ed., 2001, 40, 4638

W Zhou, DS Shephard, JM Thomas, T Maschmeyer, BFG Johnson and RG Bell, Science, 1998, 280, 705

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Chiral Catalysts Anchored Inside Mesoporous Silica

Isolated chiral homogeneous catalysts have been tethered to the internal walls of MCM-41 to produce highly effective and highly regio- and enantioselective heterogenised, homogeneous catalysts.

Depiction of the catically active centre bound in a constarined manner inside mesoporous silica.

Selected Publications

Chiral Catalysis at Surfaces (article), M. Jacoby, Chem. Eng. News, 2004, 82, 37

MD Jones, R Raja, JM Thomas and BFG Johnson, Topics in Catalysis, 2003, 25, 71

MD Jones, R Raja, JM Thomas, BFG Johnson, DW Lewis, J Rouzaud and KDM Harris, Angew. Chem. Int. Ed., 2003, 42, 4326

SA Raynor, JM Thomas, R Raja, BFG Johnson, RG Bell and MD Mantle, Chem. Commun., 2000, 1925

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Silsesquioxanes as Homogeneous Models for Heterogeneous Silicate Catalysts

Silica based catalysts play a very important role in heterogeneous catalysis. Their usefulness depends on the strong bonds formed between the silica surface and the catalytically active metal. The trisilanol silsesquioxanes present three silanol functionalities towards the missing vertex of the silicon oxygen cube, and this is the most realistic, soluble model for co-ordination of a metal to a silica surface.

The example shown below is the molecular structure of a model compound for epoxidation catalysis by Ti(IV). Ti(IV) atoms cap the vacant corner of the silsesquioxane, and two molecules are bridged by methoxy ligands. This compound is an active homogeneous epoxidation catalyst in its own right.

 

Molecular structure of a titanosilsesquioxane.

Selected Publications

BFG Johnson, MC Klunduk, TJ O'Connell, C McIntosh and J Ridland, J. Chem. Soc., Dalton Trans. 2001, 1553

EA Quadrelli, JE Davies, BFG Johnson and N Feeder, Chem. Commun., 2000, 1031

MC Klunduk, T Maschmeyer, JM Thomas and BFG Johnson, Chem. Eur. J., 1999, 5, 1481

JM Thomas, G Sankar, MC Klunduk, MP Attfield, T Maschmeyer, BFG Johnson and RG Bell, J. Phys. Chem., 1999, 103B, 8809

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Clusters

Mixed-metal clusters have been shown to be ideal precursors for the preparation of supported bimetallic particles of well-defined sizes and composition. As a consequence, our efforts are directed towards the development of synthetic strategies for the formation of high nuclearity clusters in high yield.

The preparation of polymer precursors leading to the formation of cluster polymers (cluster units bonded to a polymeric backbone) has also been investigated. A route to "electron-hopping" materials has been established, permitting the production of linked nanoparticle arrays such as chains or wires.

Synthetic strategy for Ruthenium-based mixed-metal clusters (left),
and the molecular structure of a linked crown ether cluster compound (right).

Selected Publications

BFG Johnson, S Hermans, T Khimyak, Eur. J. Inorg. Chem., 2003, 1325

Khimyak T, BFG Johnson, Hermans S, Bond AD, Dalton Transactions, 2003, 2651

BFG Johnson, CMG Judkins, JM Matters, DS Shephard and S Parsons, Chem. Commun., 2000, 1549

BFG Johnson, DS Shephard, J Matters and S Parsons, J. Chem. Soc., Dalton Trans., 1998, 2289

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Nanoparticle Reactivity in the Gas Phase

Collision-induced dissociation (CID) in an electrospray ionisation (ESI) source has been successfully used to controllably strip carbonyl clusters of their ligands, creating partially ligated clusters and bare metal nanoparticles in the gas phase. Energy-dependent electrospray ionisation mass spectrometry (EDESI-MS) is used to present all the fragmentation data in a compact 2D format.

Electrospray ionisation Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS) allows the study of the gas-phase reactivity of these nanoparticles. Nanoparticle catalysts on MCM-41 are produced in an analogous manner, and examination of their gas-phase reactivity may provide insight into their action as catalysts in the solid state.

EDESI-MS spectrum of [CoRu₃(CO)₁₃]^-, showing CO ligand loss from the parent cluster to the [CoRu₃]^- core (left),
and FTICR-MSⁿ of [CoRu₃] + CH₄ after SORI activation for 0 s (bottom), 1 s, 5 s and 15 s (right).

Selected Publications

CPG Butcher, A Dinca, PJ Dyson, BFG Johnson, PRR Langridge-Smith and JS McIndoe, Angew. Chem. Int. Ed., 2003, 42, 5752

CPG Butcher, PJ Dyson, BFG Johnson, T Khimyak and JS McIndoe, Chem. Eur. J., 2003, 9, 944

CPG Butcher, PJ Dyson, BFG Johnson, JS McIndoe, PRR Langridge-Smith and C Whyte, Rapid Commun. Mass Spectrom., 2002, 16, 1595

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Carbon Nanotubes

Synthetic methods for high purity single walled carbon nanotubes are being developed within the group. The use of nickel formate as a precursor to nickel seed-nanoparticles as catalysts in the CVD growth process has proved highly successful.

HRTEM image of single walled carbon nanotubes (left),
and the Raman spectrum (excitation beam wavelength ~ 785 nm) (right), showing peaks characteristic of single-walled carbon nanotubes.

Selected Publications

YL Lia, IA Kinlocha, MSP Shaffer, J Geng, BFG Johnson and AH Windle, Chem. Phys. Lett., 2004, 384, 98

J Geng, C Singh , DS Shephard, MSP Shaffer, BFG Johnson and AH Windle, Chem. Commun., 2002, 2666

J Geng, C Ducati, DS Shephard, M Chhowalla, BFG Johnson and J Robertson, Chem. Commun., 2002, 1112

BFG Johnson and J Geng, Abstracts of Papers of the American Chemical Society, 2002, 224, 025-CATL

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The Ligand Polyhedral Model

The Ligand Polyhedral Model (LPM) offers a rationale for the ground-state structures, isomer formation and fluxional behaviour of the binary transition metal carbonyl clusters and their derivatives. For example, the LPM predicts that the carbonyl ligands of the cluster [Co₄(CO)₁₂] will arrange themselves about the metal core in such a way as to form the lowest energy polyhedron with 12 vertices, i.e. an icosahedron. This is in fact the case. Furthermore, the fluxionality of this compound can be explained by two fundamental processes - the low energy libration of the metal core inside the ligand sphere, and a higher energy process in which the ligand shell undergoes reversible change from the icosahedron through a cubeoctahedral complementary geometry.

Libration of the core (left), and polyhedral interconversion of the (CO)₁₂ ligand sphere (right).

Selected Publications

BFG Johnson and S Tay, J. Mol. Cat. A: Chem., 2003, 204-205, 341

BFG Johnson, EA Quadrelli, V Ferrand and AW Bott, J. Chem. Soc., Dalton Trans., 2001, 1063

YV Roberts, BFG Johnson and RE Benfield, Inorg. Chim. Acta, 1995, 229, 221