Dynamics of Propane in Silica Mesopores Formed upon Propylene Hydrogenation over Pt Nanoparticles by Time-Resolved FT-IR Spectroscopy

Abstract

Propylene hydrogenation over Pt nanoparticles supported on mesoporous silica type SBA-15 was monitored by time-resolved FT-IR spectroscopy at 23 ms resolution using short propylene gas pulses that joined a continuous flow of hydrogen in N 2 (1 atm total pressure). Experiments were conducted in the temperature range 323 -413 K.Propane was formed within 100 milliseconds or faster. The CH stretching region revealed distinct bands for propane molecules emerging inside the nanoscale channels of the silica support. Spectral analysis gave the distribution of the propane product between support and surrounding gas phase as function of time. Kinetic analysis showed that the escape of propane molecules from the channels occurred within hundreds of milliseconds (3.1 + 0.4 s -1 at 383 K). A steady state distribution of propane between gas phase and mesoporous support is established as the product is swept from the catalyst zone by the continuous flow of hydrogen co-reactant. This is the first direct spectroscopic observation of emerging products of heterogeneous catalysis on nanoporous supports under reaction conditions.2

Observation of Parahydrogen‐Induced Polarization in Heterogeneous Hydrogenation on Supported Metal Catalysts

“…An alkene (e.g., CH 3 CH = CH 2 ) is then physisorbed or chemisorbed on the metal surface, harvests a hydrogen atom from the surface pool to form a surface alkyl moiety ((CH 3 ) 2 CHÀM or CH 3 CH 2 CH 2 ÀM), and then another H atom to yield the final alkane product (CH 3 CH 2 CH 3 ). [11][12][13] In some studies, however, it is argued that mostly the H atoms dissolved in the metal lattice and emerging at the surface are reactive enough to hydrogenate surface alkenes. [14,15] Observation of PHIP effects is commonly regarded as evidence that addition of the two H atoms occurs pairwise.…”

Hyperpolarized NMR Spectroscopy: d‐DNP, PHIP, and SABRE Techniques

“…The most commonly used catalysts for heterogeneous hydrogenation and hydrogenolysis processes are nanoscale metal particles that are dispersed over porous oxide or carbon supports. It is widely accepted that dissociative hydrogen chemisorption takes place on such catalysts, and that the hydrogenation of unsaturated compounds occurs as a successive addition of two hydrogen atoms with the formation of intermediate surface alkyl species . It is also known that adsorbed hydrogen atoms can rapidly move over the surface of metal nanoparticles, embed into the crystal lattice of the metal, and migrate onto the support (also known as the spillover phenomenon) .…”

“…It is widely accepted that dissociative hydrogen chemisorptiont akes place on such catalysts, [134] and that the hydrogenationofu nsaturated compounds occurs as as uccessive addition of two hydrogen atoms with the formation of intermediate surface alkyl species. [135][136][137][138][139] It is also known that adsorbed hydrogen atoms can rapidly move over the surfaceo fm etal nanoparticles, [140] embed into the crystal lattice of the metal, [141][142][143] and migrate onto the support (also knowna st he spillover phenomenon). [144] Therefore, the probability of ap airwise addition of two hydrogen atoms from the same H 2 molecule to as ubstrate molecule should be extremelyl ow.F urthermore, after the dissociative chemisorption of aH 2 molecule, the adsorbed Hatoms become magnetically inequivalent, therebyl eading to the rapid loss of spin correlation between them.…”

Heterogeneous Catalysis and Parahydrogen‐Induced Polarization

“…It is believed that the chemisorption of H 2 on such catalysts is dissociative, [45] and the hydrogenation of unsaturated compounds proceeds as a sequential addition of two hydrogen atoms via the formation of an alkyl surface intermediate. [46,47] It is also known that adsorbed hydrogen atoms can quickly migrate on the surface of a metal nanoparticle, [48] embed into the crystal lattice of a metal, [49][50][51] and also spill over to the support. [52,53] For example, hydrogen atoms adsorbed on nickel, on average, move every 2.7 • 10 À 9 s over a distance of 3 Å at 150°C.…”