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| - Dynamics of CO in Mesoporous Silica Monitored by Time-Resolved Step-Scan andRapid-Scan FT-IR Spectroscopy
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| - Carbon monoxide molecules generated in the channels of mesoporous MCM-41 silica sieve from a precursor(diphenyl cyclopropenone) by photodissociation with a nanosecond laser pulse were monitored by time-resolved Fourier transform infrared (FTIR) spectroscopy using the step-scan and rapid-scan methods. A verybroad absorption of CO is observed in the region 2200−2080 cm-1 at room temperature that decays in abiphasic mode. Two-thirds of the band intensity decays on the hundreds of microsecond scale (lifetime 344± 70 μs). The process represents the escape of the molecules through the mesopores into the surrounding gasphase, and a diffusion constant of 1.5 × 10-9 m2/s is derived (assuming control by intra-MCM-41 particlediffusion). The broad profile of the absorption is attributed to contact of the random hopping CO with siloxaneand silanol groups of the pore surface. Measurements using MCM-41 with the silanols partially capped bytrimethyl silyl groups gave further insight into the nature of the IR band profile. These are the first observationson the diffusion behavior of carbon monoxide in a mesoporous material at room temperature. The residualcarbon monoxide remains much longer in the pores and features distinct peaks at 2167 and 2105 cm-1characteristic for CO adsorbed on SiOH groups C end on and O end on, respectively. The bands decreasewith time constants of 113 ± 3 ms (2167 cm-1) and 155 ± 15 ms (2105 cm-1) suggesting that CO in thesesites is additionally trapped by surrounding diphenyl acetylene coproduct and/or precursor molecules.
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