Combining in situ NEXAFS spectroscopy and CO2 methanation kinetics to study Pt and Co nanoparticle catalysts reveals key insights into the role of platinum in promoted cobalt catalysis.

The mechanistic role of platinum and precious metals in promoting cobalt hydrogenation catalysts of the type used in reactions such as Fischer-Tropsch synthesis is highly debated. Here we use well-defined monometallic Pt and Co nanoparticles (NPs) and CO2 methanation as a probe reaction to show that Pt NPs deposited near Co NPs can enhance the CO2 methanation rate by up to a factor of 6 per Co surface atom. In situ NEXAFS spectroscopy of these same Pt NP plus Co NP systems in hydrogen shows that the presence of nearby Pt NPs is able to significantly enhance reduction of the Co at temperatures relevant to Fischer-Tropsch synthesis and CO2 methanation. The mechanistic role of Pt in these reactions is discussed in light of these findings.

Superenantioselective chiral surface explosions.

Chiral inorganic materials predated life on Earth, and their enantiospecific surface chemistry may have played a role in the origins of biomolecular homochirality. However, enantiospecific differences in the interaction energies of chiral molecules with chiral surfaces are small and typically lead to modest enantioselectivities in adsorption, catalysis, and chemistry on chiral surfaces. To yield high enantioselectivities, small energy differences must be amplified by reaction mechanisms such as autocatalytic surface explosions which have nonlinear kinetics. Herein, we report the first observations of superenantiospecificity resulting from an autocatalytic surface explosion reaction of a chiral molecule on a naturally chiral surface. R,R- and S,S-tartaric acid decompose via a vacancy-mediated surface explosion mechanism on Cu single crystal surfaces. When coupled with surface chirality, this leads to decomposition rates that exhibit extraordinarily high enantiospecificity. On the enantiomorphs of naturally chiral Cu(643)(R&S), Cu(17,5,1)(R&S), Cu(531)(R&S) and Cu(651)(R&S) single crystal surfaces, R,R- and S,S-tartaric acid exhibit enantiospecific decomposition rates that differ by as much as 2 orders of magnitude, despite the fact that the effective rates constants for decomposition differ by less than a factor of 2.

Exploring surface science and restructuring in reactive atmospheres of colloidally prepared bimetallic CuNi and CuCo nanoparticles on SiO2 in situ using ambient pressure X-ray photoelectron spectroscopy.

Bimetallic nanoparticles (to approximately 11 nm diameter) of CuNi and CuCo were prepared by a new synthetic route and the 1:1 atomic ratio of their constituent elements confirmed using STEM-EDS at a single particle level. These nanoparticles, supported on the native oxide layer of a silicon wafer, were studied in situ in a series of reactive gas atmospheres (H2 --> CO or CO/H2 --> O2 --> H2) using ambient pressure X-ray photoelectron spectroscopy (AP-XPS). Despite the deliberate similarity of nickel and cobalt with respect to copper, their restructuring behaviour is different. CuNi nanoparticles were found to surface segregate nickel in H2, but copper in O2 reversibly, while CuCo nanoparticles were found to surface segregate copper irreversibly when exposed to O2, such that the surface remains copper rich when re-exposed to H2. Both systems also restructure in opposition to the behaviour predicted by the surface free energies and enthalpies of oxide formation of the elements from which they are comprised--factors that are seen to control restructuring and surface segregation in many similar systems.

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.

High structure sensitivity of vapor-phase furfural decarbonylation/hydrogenation reaction network as a function of size and shape of Pt nanoparticles.

Vapor-phase transformations of furfural in H(2) over a series of Pt nanoparticles (NPs) with various particle sizes (1.5-7.1 nm size range) and shapes (rounded, cubes, octahedra) encapsulated in poly(vinylpyrrolidone) (PVP) and dispersed on MCF-17 mesoporous silica were investigated at ambient pressure in the 443-513 K temperature range. Furan and furfuryl alcohol (FFA) were two primary products as a result of furfural decarbonylation and hydrogenation reactions, respectively. Under conditions of the study both reactions exhibited structure sensitivity evidenced by changes in product selectivities, turnover rates (TORs), and apparent activation energies (E(A)'s) with Pt particle size and shape. For instance, upon an increase in Pt particle size from 1.5 to 7.1 nm, the selectivity toward FFA increases from 1% to 66%, the TOR of FFA production increases from 1 × 10(-3) s(-1) to 7.6 × 10(-2) s(-1), and E(A) decreases from 104 kJ mol(-1) to 15 kJ mol(-1) (9.3 kPa furfural, 93 kPa H(2), 473 K). Conversely, under the same experimental conditions the decarbonylation reaction path is enhanced over smaller nanoparticles. The smallest NPs (1.5 nm) produced the highest selectivity (96%) and highest TOR values (8.8 × 10(-2) s(-1)) toward furan formation. The E(A) values for decarbonylation (∼62 kJ mol(-1)) was Pt particle size independent. Furan was further converted to propylene via a decarbonylation reaction, but also to dihydrofuran, tetrahydrofuran, and n-butanol in secondary reactions. Furfuryl alcohol was converted to mostly to 2-methylfuran.

Size-controlled model Co nanoparticle catalysts for CO2 hydrogenation: synthesis, characterization, and catalytic reactions.

Model cobalt catalysts for CO(2) hydrogenation were prepared using colloidal chemistry. The turnover frequency at 6 bar and at 200-300 °C increased with cobalt nanoparticle size from 3 to 10 nm. It was demonstrated that near monodisperse nanoparticles in the size range of 3-10 nm could be generated without using trioctylphosphine oxide, a capping ligand that we demonstrate results in phosphorus being present on the metal surface and poisoning catalyst activity in our application.