Influence of Size-Induced Oxidation State of Platinum Nanoparticles on Selectivity and Activity in Catalytic Methanol Oxidation in the Gas Phase

Abstract

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.

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

“…For example, as the Pt size was increased from 1.5 to 7.1 nm, the selectivity toward furfuryl alcohol increased from 1 to 66 % and turnover rates of the furfuryl alcohol production remarkably increased from 1 9 10 -3 to 7.6 9 10 -2 s -1 , while activation energies decreased gradually. In an oxidation reaction of methanol, the Pt sizedependent selectivity change was exhibited for the production of either formaldehyde or carbon dioxide, by the partial or full oxidation, respectively [61]. While the 2, 4, and 6 nm Pt nanoparticles showed similar catalytic activity and selectivity, the 1 nm Pt nanoparticles exhibited a significantly higher selectivity toward formaldehyde, but a lower total turnover frequency due to the strong oxidation tendency of small nanoclusters.…”

Comparing the Catalytic Oxidation of Ethanol at the Solid–Gas and Solid–Liquid Interfaces over Size-Controlled Pt Nanoparticles: Striking Differences in Kinetics and Mechanism

“…1−3 For example, methanol oxidation by molecular oxygen over Pt nanoparticles with 1−6 nm sizes exhibited higher turnover frequency (TOF) values and higher selectivity toward carbon dioxide in the case of particles with larger sizes. 4 Alcohol oxidation reactions are used in industrial processes for energy conversion and as starting materials for synthesis of organic chemicals as well as for pharmaceuticals. For example, several highly effective fuel cells are based on the complete oxidation of low molecular weight alcohols, 5,6 while complex alcohols are used for production of drugs and fine chemicals.…”

Solid State Morphology and Size Tuning of Nanostructured Platinum Using Macromolecular Complexes

“…The first way, solution methods, in spite of that to allow the size and morphology control, their incorporation to a solid state device could be not easy. In solution, small nanoparticles as small as 1 nm have reported [8] and using the here reported solid-state method, nanoparticles of 6 nm have observed, although with a low dispersity, however the latter aspect is a common characteristic of the solid-state methods to prepare nanoparticles [52].…”

“…All of these applications use Pt finely divided [6]. It has also been established that both, the reactivity and selectivity of nanostructures of Pt catalysis, are highly dependent on the size and morphology of the nanoparticles of Pt [7][8][9]. In this context, crystal planes exposed to the surface of the nanoparticles are crucial in the catalytic activity [10][11][12][13].…”