Ihab M. Abdelrehim

Thiophene decomposition on Pd(111) forms S and C4 species : a laser-induced thermal desorption/Fourier transform mass spectrometry study

Abstract The data presented here show that Pd(111) can directly activate thiophene decomposition resulting in the deposition of sulfur and the formation of C 4 species, most likely C 4 H 4 or possibly C 4 H 5 , on the surface. Temperature programmed reaction (TPR) studies of a 0.2 L exposure of thiophene show some reversible, but primarily irreversible adsorption. No C- or S-containing reaction products desorb during TPR. However, laser induced thermal desorption (LITD) with Fourier transform mass spectrometry (FTMS) can be used to monitor the surface composition prior to conventional desorption. LITD/FTMS shows that thiophene is stable to approximately 280 K. Above 300 K, 1,3-butadiene is observed. The yield of 1,3-butadiene on the surface, as observed by LITD/FTMS, is estimated to be 30% of the initial thiophene signal.

Benzene and thiophene formation from acetylene on sulfided Pd(111) studied by LITD/FTMS

We report the first laser-induced thermal desorption (LITD) studies of acetylene on Pd(111). LITD coupled with Fourier transform mass spectrometry (FTMS) probes the surface molecular composition. Our results show simultaneous formation of thiophene and benzene at 120 K after a 6 L dose of acetylene at 80 K on a Pd(111) surface with 0.06 ML of sulfur. Simultaneous formation implies that formation of the C4H4 intermediate is the slow step in the formation of both cyclic products. The relative amounts of thiophene and benzene observed with LITD/FTMS are comparable, while thermal desorption spectroscopy (TDS) yields integrated thiophene signals that are < 2% as intense as for benzene. This indicates that thiophene primarily decomposes upon heating. Low coverage (0.5 L) results confirm reports that the presence of sulfur enhances benzene production.

Laser-induced thermal desorption with Fourier transform mass spectrometry for the time resolved analysis of catalyst and biomedical implant model surface molecular composition

Laser-induced thermal desorption with Fourier transform mass spectrometry is used to analyze the surface molecular composition of several samples containing complex mixtures of adsorbates. Data are presented for the formation of benzene and thiophene from acetylene on a sulfur-treated Pd(111) surface showing the utility of this technique for monitoring changes in surface composition on catalyst models. Also, time dependent data are shown for the formation of benzene from acetylene on clean Pd(111). The data are first order, lending insight into the mechanism. Finally, a new technique for deflection of laser-formed ions to allow analysis of the coadsorbed organics is described and used to analyze the surface composition of oxidized Ti foils used as biomedical implant models.

An ultrahigh vacuum surface analysis instrument incorporating a Fourier transform mass spectrometer and a Fourier transform infrared spectrometer

An ultrahigh vacuum chamber equipped with Fourier transform reflection absorption infrared spectroscopy, Fourier transform mass spectrometry, laser-induced thermal desorption, Auger electron spectroscopy, and low energy electron diffraction is described. The marriage of the various techniques has led to novel designs for sample manipulation and incorporation of the instrumentation. A new ion deflection technique is also described. Some results from studies of hydrocarbon reactions on Pd(111) surfaces, such as desorption kinetics for propene and the kinetics and mechanisms of acetylene cyclization to benzene and thiophene, are discussed, as are analyses of oxidized Ti foils exposed to the ambient environment. The performance of this instrument is thus evaluated.

Deflection of Laser-Produced Ions in Laser- Induced Thermal-Desorption/Fourier- Transform Mass Spectrometry for Surface Analysis

A method for deflecting ions, such as K+, produced outside a Fourier-transform mass spectrometer cell during laser-induced thermal desorption, is described. This technique has been shown to deflect laser-generated K and Ti ions from two Ti foil samples (biomedical implant model surfaces), yielding mass spectra of coadsorbed organic species. Further studies characterizing the laser desorption/deflection parameters have shown that ion deflection improves with higher deflection voltages and greater sample to Fourier-transform mass spectrometry cell separation. Higher laser power densities resulted in greater surface ion production; hence higher deflection voltages were necessary. A 6% increase in laser power necessitated a fourfold increase in deflection voltage for the Ti sample.