Temperature Programmed Reaction Mass Spectrometry

Thermal desorption spectroscopy and temperature programmed reaction experiments have provided significant insight into the chemistry of a wide variety of reactions on well characterized surfaces. In such experiments, characterized, adsorbate covered, surfaces are heated at rates of 10-100 K/sec and molecular species which desorb are monitored by mass spectrometry. Typically, several masses are monitored in each experiment by computer multiplexing techniques. Often, in such experiments, the species desorbed are the result of a surface reaction during the temperature ramp. 

Temperature programmed reaction (TPR) experiments were carried out by adsorbing allyl alcohol and allyl iodide on a 9.0 wt% Mo03/Si02 sample and monitoring the evolved products by mass spectrometry. The Raman spectra of the pure liquid reference compounds are shown in Fig. 2. They agree well with those reported earlier (18-20). 

The apparatus which has been described in detail previously (ref 2), allows both temperature programmed desorption (TFD) and temperature programmed reaction (TPRn) studies to be effected. The quantitative analysis of the gas stream eluting from the reactor was accomplished by mass spectrometry. 

Elementary steps in which a bond is broken form a particularly important class of reactions in catalysis. The essence of catalytic action is often that the catalyst activates a strong bond that cannot be broken in a direct reaction, but which is effectively weakened in the interaction with the surface, as we explained in Chapter 6. To monitor a dissociation reaction we need special techniques. Temperature-programmed desorption is an excellent tool for monitoring reactions in which products desorb. However, when the reaction products remain on the surface, one needs to employ different methods such as infrared spectroscopy or secondary-ion mass spectrometry (SIMS). 

The reaction products were analyzed by on-line gas chromatography (HP 5890 GC) equipped with both TCD and FID detectors. GC column used is GS-Q 30 m manufactured by JW Scientific. Temperature program of 5°C/min to 300°C was chosen for the analysis. Liquid products were collected in a cold trap at -3°C and were also analyzed by GC-mass spectrometry. 

Temperature-programmed desorption (or decomposition) with quantitative analysis of gas-phase products (usually by mass spectrometry) has been used to help identify the ligands bonded to a metal in a supported complex. Complications such as reaction of desorbed ligands (e.g., CO) with support groups (e.g., OH) may complicate interpretation of the data (Brenner, 1986). 

Chemical Analysis. Gas chromatography-mass spectrometry (GC-MS) and high-performance liquid chromatography (HPLC) techniques were used to analyze 4-chlorophenol and its oxidation intermediates. For GC-MS analysis, the samples were acetylated in pyridine. The samples were first evaporated to dryness. Then 200 xL of pyridine and 200 (xL of acetic anhydride were added to the dry residue. The samples were heated at 65 °C for 2-3 h to ensure the complete acetylation reaction, and then gently evaporated to dryness in a nitrogen stream. Finally, the residue was redissolved in 0.1 mL of hexane for GC analysis. A GC (HP model 5890) equipped with mass selective detector (HP model 5971) and SPB-5 capillary column (Supelco Co., PA., 25- X 0.2-mm i.d. X 0.33-p.m film thickness) was used. To separate different intermediate products, various oven-temperature programs were performed. The GC-MS interface line was maintained at 300 °C. The mass-... 

NMR and GC/MS Analyses. Reaction products were analyzed by GC-mass spectrometry (MS) and NMR. For GC/MS, silylation of methyl-esterified extract was achieved with a mixture of TMSI + pyridine (1 4, vol/vol) for 30 min at room temperature. GC/MS analysis was performed with a Hewlett-Packard Model 5890 gas chromatograph interfaced with a Model 5971 mass selective detector operating at 70 eV. The capillary column used was a Hewlett-Packard HP-5-MS cross-linked 5% phenylmethyl silicone, 30 m x 0.25 mm i.d., film thickness 0.25 xm. The carrier gas was helium and its flow rate was 0.65 mL/min. The GC column was programmed from 65 to 260°C at a rate of 20°C/min and then kept at 260°C for 20 min. NMR spectra were obtained with a Bruker model ARX-400 spectrometer (Billerica, MA) equipped with a 5-mm dual probe ( C NMR, 100... 

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