Structure Sensitivity in Pt Nanoparticle Catalysts for Hydrogenation of 1,3-Butadiene:In SituStudy of Reaction Intermediates Using SFG Vibrational Spectroscopy

Abstract

The product selectivity during 1,3-butadiene hydrogenation on monodisperse, colloidally synthesized, Pt nanoparticles was studied under reaction conditions with kinetic measurements and in situ sum frequency generation (SFG) vibrational spectroscopy. SFG was performed with the capping ligands intact in order to maintain nanoparticle size by reduced sintering. Four products are formed at 75 °C: 1-butene, cis-2-butene, trans-2-butene, and n-butane. Ensembles of Pt nanoparticles with average diameters of 0.9 and 1.8 nm exhibit a ∼30% and ∼20% increase in the full hydrogenation products, respectively, as compared to Pt nanoparticles with average diameters of 4.6 and 6.7 nm. Methyl and methylene vibrational stretches of reaction intermediates observed under working conditions using SFG were used to correlate the stable reaction intermediates with the product distribution. Kinetic and SFG results correlate with previous DFT predictions for two parallel reaction pathways of 1,3-butadiene hydrogenation. Hydrogenation of 1,3-butadiene can initiate with H-addition at internal or terminal carbons leading to the formation of 1-buten-4-yl radical (metallocycle) and 2-buten-1-yl radical intermediates, respectively. Small (0.9 and 1.8 nm) nanoparticles exhibited vibrational resonances originating from both intermediates, while the large (4.6 and 6.7 nm) particles exhibited vibrational resonances originating predominately from the 2-buten-1-yl radical. This suggests each reaction pathway competes for partial and full hydrogenation and the nanoparticle size affects the kinetic preference for the two pathways. The reaction pathway through the metallocycle intermediate on the small nanoparticles is likely due to the presence of low-coordinated sites.

Conquering Catalyst Complexity: Nanoparticle Synthesis and Instrument Development for Molecular and Atomistic Characterisation Under In Situ Conditions

“…For example, using SFG it has recently been possible to show that during 1,3-butadiene hydrogenation the ratios of different intermediates (resulting from addition of the first hydrogen atom at an internal or terminal position on the carbon chain) vary as a function of nanoparticle size. [56] The variation in the intermediates observed correlates well with both the final products obtained and DFT calculations for two possible reaction pathways that can occur for this reaction.…”

Nanocatalysis I: Synthesis of Metal and Bimetallic Nanoparticles and Porous Oxides and Their Catalytic Reaction Studies

“…Many hydrogenation reactions of small molecules including pyrrole [53], furan [54], crotonaldehyde [55], butadiene [56], furfural [57], methylcyclopentane [58], cyclohexene [59], and nhexane [60] have been proven to change their selectivity by Pt nanoparticle size or shape. For example, in 1,3-butadiene hydrogenation, 0.9 and 1.8 nm Pt nanoparticles increased the production of n-butane by full hydrogenation, whereas 4.6 and 6.7 nm Pt catalysts favored 1-butene by partial hydrogenation [56]. From the study of calculated intermediate structures, the 0.9 and 1.8 nm Pt nanoparticles provided low coordination adsorption sites facilitating H-insertion at the internal carbon as well as terminal carbon, while larger Pt catalysts favored H-insertion only at the terminal carbon as observed on Pt bulk materials.…”

Electrochemically dealloyed platinum with hierarchical pore structure as highly active catalytic coating

“…The selectivity obtained e.g. 59 The selectivities obtained on our porous Pt samples Pt-A, Pt-B and Pt-C (Fig. 4A).…”

“…4B compares the butadiene The observed reaction products were in all cases butane, 1-butene, trans-2-butene as well as cis-2-butene. 59 They reported selectivities of 50% butane, 30% 1-butene, 15% trans-2-butene and 8% cis-2-butene for Pt nanoparticles below 2 nm diameter (75°C, 10× H 2 excess). for sample Pt-C at 35°C amounted to 53% butane, 34% 1-butene, 10% trans-2-butene and 5% cis-2butene (Fig.…”