Dynamics of CO in Mesoporous Silica Monitored by Time ResolvedStep-Scan and Rapid-Scan FT-IR Spectroscopy
Carbon monoxide molecules generated in the channels ofmesoporous MCM-41 silica sieve from a precursor (diphenyl cyclopropenone)by photodissociation with a nanosecond laser pulse were monitored by timeresolved FT-infrared spectroscopy using the step-scan and rapid-scanmethods. A very broad absorption of CO is observed in the region 2200 to2080 cm-1 at room temperature that decays in a biphasic mode. Two-thirdsof the band intensity decays on the hundreds of microsecond scale(lifetime 344 + 70 ?s). The process represents the escape of themolecules through the mesopores into the surrounding gas phase, and adiffusion constant of 1.5 x 10-9 m2/sec is derived. The broad profile ofthe absorption is attributed to contact of the random hopping CO withsiloxaneand silanol groups of the pore surface. Measurements usingMCM-41 with the silanols partially capped by trimethyl silyl groups gavefurther insight into the nature of the infrared band profile. These arethe first observations on the diffusion behavior of carbon monoxide in amesoporous material at room temperature. The residual carbon monoxideremains much longer in the pores and features distinct peaks at 2167 and2105 cm-1 characteristic for CO adsorbed on SiOH groups C end on and Oend on, respectively. The bands decrease with time constants of 113 + 3ms (2167 cm-1) and 155 + 15 ms (2105 cm-1) suggesting that CO in thesesites is additionally trapped by surrounding diphenyl acetyleneco-product and/or precursor molecules.
- Research Organization:
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- DE-AC02-05CH11231
- OSTI ID:
- 922812
- Report Number(s):
- LBNL-61020; R&D Project: 1; BnR: YN0100000; TRN: US200803%%537
- Journal Information:
- Journal of Physical Chemistry B, Vol. 110; Related Information: Journal Publication Date: 10/18/2006
- Country of Publication:
- United States
- Language:
- English
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