Insights into the Mechanism of Tandem Alkene Hydroformylation over a Nanostructured Catalyst with Multiple Interfaces.

The concept of tandem catalysis, where sequential reactions catalyzed by different interfaces in single nanostructure give desirable product selectively, has previously been applied effectively in the production of propanal from methanol (via carbon monoxide and hydrogen) and ethylene via tandem hydroformylation. However, the underlying mechanism leading to enhanced product selectivity has remained elusive due to the lack of stable, well-defined catalyst suitable for in-depth comprehensive study. Accordingly, we present the design and synthesis of a three-dimensional (3D) catalyst CeO2-Pt@mSiO2 with well-defined metal-oxide interfaces and stable architecture and investigate the selective conversion of ethylene to propanal via tandem hydroformylation. The effective production of aldehyde through the tandem hydroformylation was also observed on propylene and 1-butene. A thorough study of the CeO2-Pt@mSiO2 under different reaction and control conditions reveals that the ethylene present for the hydroformylation step slows down initial methanol decomposition, preventing the accumulation of hydrogen (H2) and favoring propanal formation to achieve up to 80% selectivity. The selectivity is also promoted by the fact that the reaction intermediates produced from methanol decomposition are poised to directly undergo hydroformylation upon migration from one catalytic interface to another. This synergistic effect between the two sequential reactions and the corresponding altered reaction pathway, compared to the single-step reaction, constitute the key advantages of this tandem catalysis. Ultimately, this in-depth study unravels the principles of tandem catalysis related to hydroformylation and represents a key step toward the rational design of new heterogeneous catalysts.

Selective amplification of C=O bond hydrogenation on Pt/TiO2: catalytic reaction and sum-frequency generation vibrational spectroscopy studies of crotonaldehyde hydrogenation.

The hydrogenation of crotonaldehyde in the presence of supported platinum nanoparticles was used to determine how the interaction between the metal particles and their support can control catalytic performance. Using gas-phase catalytic reaction studies and in situ sum-frequency generation vibrational spectroscopy (SFG) to study Pt/TiO2 and Pt/SiO2 catalysts, a unique reaction pathway was identified for Pt/TiO2, which selectively produces alcohol products. The catalytic and spectroscopic data obtained for the Pt/SiO2 catalyst shows that SiO2 has no active role in this reaction. SFG spectra obtained for the Pt/TiO2 catalyst indicate the presence of a crotyl-oxy surface intermediate. By adsorption through the aldehyde oxygen atom to an O-vacancy site on the TiO2 surface, the C=O bond of crotonaldehyde is activated, by charge transfer, for hydrogenation. This intermediate reacts with spillover H provided by the Pt to produce crotyl alcohol.

Influence of size-induced oxidation state of platinum nanoparticles on selectivity and activity in catalytic methanol oxidation in the gas phase.

Pt nanoparticles with various sizes of 1, 2, 4, and 6 nm were synthesized and studied as catalysts for gas-phase methanol oxidation reaction toward formaldehyde and carbon dioxide under ambient pressure (10 Torr of methanol, 50 Torr of oxygen, and 710 Torr of helium) at a low temperature of 60 °C. While the 2, 4, and 6 nm nanoparticles exhibited similar catalytic activity and selectivity, the 1 nm nanoparticles showed a significantly higher selectivity toward partial oxidation of methanol to formaldehyde, but a lower total turnover frequency. The observed size effect in catalysis was correlated to the size-dependent structure and oxidation state of the Pt nanoparticles. X-ray photoelectron spectroscopy and infrared vibrational spectroscopy using adsorbed CO as molecular probes revealed that the 1 nm nanoparticles were predominantly oxidized while the 2, 4, and 6 nm nanoparticles were largely metallic. Transmission electron microscopy imaging witnessed the transition from crystalline to quasicrystalline structure as the size of the Pt nanoparticles was reduced to 1 nm. The results highlighted the important impact of size-induced oxidation state of Pt nanoparticles on catalytic selectivity as well as activity in gas-phase methanol oxidation reactions.

Preparation of mesoporous oxides and their support effects on Pt nanoparticle catalysts in catalytic hydrogenation of furfural.

Mesoporous SiO(2), Al(2)O(3), TiO(2), Nb(2)O(5), and Ta(2)O(5) were synthesized through a soft-templating approach by a self-assembled framework of Pluronic P123 and utilized for the preparation of 3-dimensional catalysts as supports. Colloidal Pt nanoparticles with an average diameter of 1.9 nm were incorporated into the mesoporous oxides by sonication-induced capillary inclusion. The Pt nanoparticles supported on mesoporous oxides were evaluated in the hydrogenation reaction of furfural (70 torr furfural and 700 torr H(2) with a balance of He) to study the effect of catalyst supports on selectivity. In the temperature ranges of 170-240°C, the major products of this reaction were furan, furfuryl alcohol, and 2-methyl furan through a main reaction pathway of either decarbonylation or carbonyl group hydrogenation. While Pt nanoparticles with the size ranges of 1.5-7.1 exhibited strong structure-dependent selectivity, various supports loaded with only 1.9 nm Pt nanoparticles produced dominantly furan as a major product. Compared to the inert silica support, TiO(2) and Nb(2)O(5) facilitated an increase in the production of furfuryl alcohol via carbonyl group hydrogenation as a result of a charge transfer interaction between the Pt and the acidic surface of the oxides. The same trend was confirmed on 2-dimensional type catalysts, in which thin films of SiO(2), Al(2)O(3), TiO(2), Nb(2)O(5), and ZrO(2) were prepared as supports. When furfural hydrogenation was conducted (1 torr furfural, 100 torr H(2), and 659 torr He) over Pt nanoparticle monolayers deposited on oxide substrates, only TiO(2) was shown to increase the production of furfuryl alcohol, while other oxides produced furan.

Furfuraldehyde hydrogenation on titanium oxide-supported platinum nanoparticles studied by sum frequency generation vibrational spectroscopy: acid-base catalysis explains the molecular origin of strong metal-support interactions.

This work describes a molecular-level investigation of strong metal-support interactions (SMSI) in Pt/TiO(2) catalysts using sum frequency generation (SFG) vibrational spectroscopy. This is the first time that SFG has been used to probe the highly selective oxide-metal interface during catalytic reaction, and the results demonstrate that charge transfer from TiO(2) on a Pt/TiO(2) catalyst controls the product distribution of furfuraldehyde hydrogenation by an acid-base mechanism. Pt nanoparticles supported on TiO(2) and SiO(2) are used as catalysts for furfuraldehyde hydrogenation. As synthesized, the Pt nanoparticles are encapsulated in a layer of poly(vinylpyrrolidone) (PVP). The presence of PVP prevents interaction of the Pt nanoparticles with their support, so identical turnover rates and reaction selectivity is observed regardless of the supporting oxide. However, removal of the PVP with UV light results in a 50-fold enhancement in the formation of furfuryl alcohol by Pt supported on TiO(2), while no change is observed for the kinetics of Pt supported on SiO(2). SFG vibrational spectroscopy reveals that a furfuryl-oxy intermediate forms on TiO(2) as a result of a charge transfer interaction. This furfuryl-oxy intermediate is a highly active and selective precursor to furfuryl alcohol, and spectral analysis shows that the Pt/TiO(2) interface is required primarily for H spillover. Density functional calculations predict that O-vacancies on the TiO(2) surface activate the formation of the furfuryl-oxy intermediate via an electron transfer to furfuraldehyde, drawing a strong analogy between SMSI and acid-base catalysis.

Solid-state charge-based device for control of catalytic carbon monoxide oxidation on platinum nanofilms using external bias and light.

Using a Pt/Si catalytic nanodiode, we externally control the rate of CO oxidation on a Pt nanofilm. The catalytic reaction can be turned on and off by alternating between bias states of the device. Additionally, the reaction rate is sensitive to photocurrent induced by visible light. The effects of both bias and light show that negative charge on the Pt increases catalytic activity, while positive charge on the Pt decreases catalytic activity for CO oxidation.