Clusters, surfaces, and catalysis

Abstract

The surface science of heterogeneous metal catalysis uses model systems ranging from single crystals to monodispersed nanoparticles in the 1-10 nm range. Molecular studies reveal that bond activation (C-H, H-H, C-C, CO) occurs at 300 K or below as the active metal sites simultaneously restructure. The strongly adsorbed molecules must be mobile to free up these sites for continued turnover of reaction. Oxide-metal interfaces are also active for catalytic turnover. Examples using C-H and CO activation are described to demonstrate these properties. Future directions include synthesis, characterization, and reaction studies with 2D and 3D monodispersed metal nanoclusters to obtain 100% selectivity in multipath reactions. Investigations of the unique structural, dynamic, and electronic properties of nanoparticles are likely to have major impact in surface technologies. The fields of heterogeneous, enzyme, and homogeneous catalysis are likely to merge for the benefit of all three.

Fig. 1.

Model surfaces for surface science and catalysis studies. (Upper) Three single crystal surface diagrams representing the (111), (100), and stepped (557) surfaces of a face-centered cubic crystal lattice. (Lower) A 2 × 2-μm atomic force microscopy image of a platinum nanoparticle array supported on a thin film of alumina. This array was fabricated with electron beam lithography.

Fig. 2.

The structure of adsorbed ethylene and the C–H activation induced restructuring of the platinum surface. (a) The best fit structure of di-σ-bonded ethylene on Pt(111) is a mixture of 60% at fcc sites and 40% at hcp sites. The symbols b₁ and b₂ represent Pt–C bond lengths, and bu represents buckling in the top layer. These data were obtained by LEED surface structure analysis. (b) The restructuring of the Pt(111) crystal face, when ethylidyne forms from ethylene by C–H bond breaking, is obtained by LEED surface structure analysis. Shading distinguishes buckled metal atoms in each layer.

Fig. 3.

Temperature-dependent rearrangement of adsorbed ethylene as monitored by SFG on Pt(111) surfaces. (Left) SFG spectra showing conversion from di-σ-bonded ethylene to ethylidyne with increasing temperature stepwise from 243 to 352 K. These spectra are taken after a 4-langmuir dose of ethylene onto Pt(111) single crystal. (Right) Proposed mechanism for this surface transformation.

Fig. 4.

The H₂ pressure dependence of ethylene surface species as monitored by SFG on Pt(111) surfaces. (a) SFG spectrum of the Pt(111) surface during ethylene hydrogenation with 100 torr of H₂, 35 torr of ethylene, and 615 torr of He at 295 K. (b) SFG spectrum of the Pt(111) surface during ethylene hydrogenation with 727 torr of H₂ and 60 torr of ethylene.

Fig. 5.

CO top-site frequency as a function of temperature for Pt(111), Pt(557), and Pt(100) under 40 torr of CO. The observed frequency redshift before CO dissociation is attributed to a harmonic coupling to the frustrated translational mode.

Fig. 6.

Shown are 100 × 100-Å STM images of a Pt(111) single crystal after the sequential addition of 20 mtorr of H₂ (Left Upper), 20 mtorr of C₂H₄ (Center Upper), and 5.6 mtorr of CO (Right Upper and Lower). The catalytically active mobile adsorbate layer becomes immobile upon catalyst deactivation caused by coadsorption of CO.

Fig. 7.

STM images of active and CO-poisoned Pt(111) catalyst surfaces during cyclohexene hydrogenation/dehydrogenation. (a) A 75 × 75-Å image of Pt(111) in the presence of 200 mtorr of H₂ and 20 mtorr of cyclohexene at 300 K. No discernable order is present. (b) A 90 × 90-Å STM image of Pt(111) in the presence of 200 mtorr of H₂, 20 mtorr of cyclohexene, and 5 mtorr of CO at 300 K. The surface forms an ordered CO structure, and the catalyst is deactivated.

Fig. 8.

Model catalyst system to study the effect of the oxide–metal interface on CO₂ hydrogenation. (a) Diagram of submonolayer metal oxide islands formed on Rh foil. (b) Effect of different metal oxides, as a function of coverage, on the rate of methane formation from CO₂ and H₂ over Rh foil.

Fig. 9.

The oxide–metal interface (highlighted by the red and black arcs) is catalytically active.

Fig. 10.

Transmission electron microscopy of platinum nanoparticles synthesized by two different techniques to obtain size and shape control of the particles. (a) Monodispersed platinum nanoparticles in the 1–8 nm range are synthesized in solution. They are capped with a polymer coating (polyvinylpyrrolidone in this case) that prevents their aggregation. (b) In the presence of silver ion, the shape of the platinum nanoparticles is altered because of preferential adsorption on one of the crystal surfaces.

Fig. 11.

The polymer-capped monodispersed platinum nanoparticles are compressed using a Langmuir–Blodgett trough and captured on an oxide surface to form 2D arrays of different density. The surface density of nanoparticles is controlled by the surface pressure.

Fig. 12.

Monodispersed platinum nanoparticles are encapsulated in mesoporous silica with a channel structure (SBA-15) to form a 3D model catalyst system. The particle size is varied while keeping the platinum loading at 1%.