Catalysis of Organic..

O’Keefe, Jiang, Ng and Rempel 371and mesityl oxide probe molecules were obtained to ascertain their adsorptionproperties. The adsorption DRIFT spectra at ambient temperature of carbonmonoxide probe molecules on the Pd/Al 2 O 3 catalyst, which was pre-reduced inhydrogen for 1h at 120°C, are illustrated in Figure 3. The band at 2075 cm -1corresponds to linearly adsorbed carbon monoxide. The bands at 1977 and 1916 cm -1correspond to bridged and multibridged carbon monoxide, respectively. The doubletbands at 2173 cm -1 and 2119 cm -1 correspond to gaseous carbon monoxide over thecatalyst. The existence of linear and bridged carbon monoxide not only confirms theexistence of Pd 0 but is also indicative of the metal crystallite structure since thebridged species requires larger crystallite sizes while the linearly adsorbed carbonmonoxide is characteristic of smaller more highly dispersed crystallites. Moreover,the prevalence of bridged and multibridged carbon monoxide in the specimen ofFigure 3 indicates the existence of adjacent sites, which may facilitate thediadsorption of mesityl oxide, as we have previously proposed (2). Additionalexperiments in which the catalyst was reduced in situ over various temperatures priorto carbon monoxide adsorption revealed that the crystallites became increasinglyaggregated with increasing reduction temperature up to 250°C, beyond which nomorphological change was observed.reflectance (a.u.)reflectance (a.u.)CO pulse ads.CO Δt= pulse 10 sec ads.Δt = 10 sec21732119time1916increases2075207519772300 2200 2100 2000 1900 1800wavenumber (cm -1 )-1 )Figure 3 DRIFT spectra ofcarbon monoxide probemolecule pulse adsorbed ona commercial Pd/Al 2 O 3catalyst reduced in H 2 for 1hat 120°C (spectra wererecorded in 10 sec intervalafter pulse).The hydrogenation of mesityl oxide over the commercial Pd/Al 2 O 3 catalyst wascarried out in situ in the FTIR cell and was found to be facile proceeding readily atambient temperature as illustrated in Figure 3. The spectra in the top half of Figure 4correspond to mesityl oxide pulse adsorbed onto the catalyst. The bands at 1677 and1610 cm -1 correspond to the carbonyl and olefin groups respectively. Note that ashydrogen is pulsed into the IR cell as illustrated in the bottom half of Figure 3, theband corresponding to the olefin group of mesityl oxide begins to disappear and theband corresponding to the ketone group begins to shift from 1677 to 1700 cm -1 ,characteristic of the ketone group of MIBK, indicating that the C=C species inmesityl oxide is easy to hydrogenate. Note that MIBK and mesityl oxide are furtherdistinguished by their characteristic CH x stretching vibrations as shown in Figure 3.