Temperature-programmed SIMS

Figure 7.15. Temperature-programmed SIMS experiment showing the surface reaction between adsorbed C and N atoms to give a surface cyanide species at 475 and 600 K decomposition of CN into C -t N, followed by instantaneous desorption of N2, occurs at...

Fig. 4.10 Dissociation of NO on the Rh(100) surface monitored in real time by temperature-programmed SIMS and desorption (TPSIMS and TPD), showing the desorption of N2 and NO, and Rh2(NO)+/Rh2+ and Rh2N+/Rh2+ TPSSIMS ion intensity ratios, representing the surface coverages of NO and N-atoms respectively, at low, middle and high initial NO coverage. (Adapted from [38]).

Fig. 5.11. Temperature programmed SIMS and desorption measurements of NO adsorbed on rhodium shows that NO dissociates completely aroimd 300 K at low coverages (left), whereas dissociation at higher coverages (right) is retarded to temperatures where NO desorbs and creates empty sites on the surface (adapted from Borg et al. [52]).

Studies to determine the nature of intermediate species have been made on a variety of transition metals, and especially on Pt, with emphasis on the Pt(lll) surface. Techniques such as TPD (temperature-programmed desorption), SIMS, NEXAFS (see Table VIII-1) and RAIRS (reflection absorption infrared spectroscopy) have been used, as well as all kinds of isotopic labeling (see Refs. 286 and 289). On Pt(III) the surface is covered with C2H3, ethylidyne, tightly bound to a three-fold hollow site, see Fig. XVIII-25, and Ref. 290. A current mechanism is that of the figure, in which ethylidyne acts as a kind of surface catalyst, allowing surface H atoms to add to a second, perhaps physically adsorbed layer of ethylene this is, in effect, a kind of Eley-Rideal mechanism. 

Figure 3.13 shows the thermal stability of immobilized ODN and PNA. The signal for the Thy- and Cyt-bases obtained with temperature-programmed (TP) SIMS starts to decrease at approximately 150 °C for ODN and 200 °C for PNA. This variance is caused by the different strengths of binding between the bases and the sugar-phosphate and peptide backbones, respectively. 

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). 

Figure 7.12. Dissociation of NO N + O in a temperature-programmed desorption and static SIMS experiment, along with Monte Carlo simulations, showing the effect of lateral interactions (see text for explanation). The bottom part shows representative arrangements of NO molecules (grey), and...

Similar SSIMS and TDS results were obtained for rhodium on tltanla and fiir hydrogen chemisorption on both substrates. In a blank experiment Involving i o metal over layer, temperature programming while following the T1 and TIO SIMS signals (Fig. 4) shows that the tltanla thin film does not begin to change until the temperature reaches about 760 K, well beyond the 615 K where Tl was first noted to Increase on the systems with thin metal overlayers. ... 

Spectroscopic developments have accelerated advances in the field of catalysis. This volume analyzes the impact on catalyst structure and reactivity of EXAFS, SIMS, MSssbauer, magic-angle spinning NMR (MASNMR), and electron-energy-loss vibrational spectroscopy. Many of these techniques are combined with other analytical tools such as thermal decomposition and temperature-programmed reactions. 

The concept of intact emission of adsorbed molecular species for identifying reaction intermediates is also well illustrated in several recent studies. Benninghoven and coworkers (2-4,12) used SIMS to study the reactions of H2 with O2, C2H4 an< 2H2 on P°ly polycrystalline Ni. For the C2H /Ni interaction, for example, direct relationships could be established between characteristic secondary ions and the presence of specific surface complexes (12). In another study, Drechsler et al. (13) used SIMS to identify NH(ads) as the active intermediate during temperature-programmed decomposition of NH3 on Fe(110). 

The application of heterogeneous catalysis plays a key role in technological processes. Engineering of the catalytic activities requires the study of the complex chemistry between absorbate and the catalyst at the surface. Static SIMS has been used to determine the surface composition and properties of solid catalysts before and after the catalytic actions by several groups.138-140 In addition, the dissociation kinetics of NO on Rh (111) surfaces have been studied by temperature programmed static SIMS.139... 

The term 1 or h indicates low or high coverage of adsorbed ethene, as inferred from ethene exposures.h TPD, temperature-programmed desorption LITD, laser-induced thermal desorption 1 FT-MS, Fourier-transform mass spectrometry SIMS, secondary-ion mass spectrometry MS, mass spectrometry T-NEXAFS, transient near-edge X-ray absorption fine structure spectroscopy RAIRS, reflection-absorption infrared spectroscopy. d Data for perdeut-erio species.1 Estimated value. 

The interpretation of the spectra of surface-adsorbed species, on singlecrystal surfaces in particular, is helped by complementary evidence derived from diffraction methods (LEED, PED) and from other nonvibrational spectroscopies (UPES, XPES, NEXAFS, SIMS, etc.). In particular, temperature-programmed desorption (TPD) is often measured in parallel with... 

ISS ion-scattering spectroscopy LEIS low-energy ion scattering SEM scanning electron microscopy SIMS Secondary ion mass spectrometry TEM transmission electron microscopy TP temperature-programmed XANES X-ray absorption near edge spectroscopy. 

As NO dissociation produces two atoms from one molecule, the reaction can only proceed when the surface contains empty sites adjacent to the adsorbed NO molecule. In addition, the reactivity of the molecule is affected by lateral interactions with neighboring species on the surface. Figure 4.10 clearly illustrates all of these phenomena [38]. The experiment starts at low temperature (175 K) with a certain amount (expressed in fraction of a monolayer, ML) of NO on the Rh(100) surface. During temperature programming, the SIMS intensities of characteristic ions of adsorbed species are followed, along with the desorption of molecules into the gas phase, as in temperature-programmed desorption (TPD) or temperature-programmed reaction spectroscopy (TPRS) (see Chapter 2). 

Figure 17. SIM chromatograms of TPTM (a), TPTE (b), and methylvendex derivative (v) on 3% OV-7 with temperature programming to 265°C...

Temperature programmed GC on capillary columns has been mostly used for determination of CYMS and CYMD congeners. Mean relative retention times from non-polar column are presented in Table 1. Detectors used have been ECD [10, 21, 24], FID [13], and MS [11, 17-20]. Generally, the most sensitive and selective determination was achieved by selected ion monitoring (SIM) mass spectrometry [17]. 

Fig. 5.16. Temperature programmed reaction spectroscopy of ethylene and NO coadsorbed on a rhodium surface reveals the many reactions that are possible. Note the formation of CO and CN species on the surface as visible in SIMS, and the formation of HCN as a gas phase product. The stability of the CN species is the reason that desorption of N2 occurs at very high temperatures only...

Figure 4.19 GC/MS chromatogram of total ions recorded in SIM mode in the TCA and TCP analysis of a wine extract. Chromatographic conditions Injector and detector temperatures 200 and 240 °C, respectively oven temperature program 50°C for 5min, 1.5°C/min until 100°C, isotherm for 3min, 30°C/min until 250 °C, isotherm for 5 min. Carrier gas He column head pressure 8 psi. (Reproduced from J. Agric. Food Chem., 2002, 50, 1032-1039, Soleas et al., with permission of the American Chemical Society)...

Thermal desorption of CO, Auger electron spectroscopy, and temperature programmed oxidation all show that the carbon layer 1) is Immobile below 550 K 2) forms a more densely packed surface phase at temperatures of 550-1150 K and 3) dissolves into the bulk at 1350 K. SIMS measurements of isotope mixing in the ions confirm formation of dense-phase (graphitic) islands after heating the carbon layer to 923 K. SIMS spectra also demonstrate that at 520 K, CO dissociates on Ru(OOl). The oxygen-free carbon layer that forms behaves similarly to the carbon from ethylene. Both SIMS and thermal desorption results show no positive interaction between adsorbed CO and D but significant attraction between and C formed by CO dissociation. 

Temperature Programmed Oxidation. These measurements characterize both the amount and chemical nature of the carbon on the surface. After a surface is exposed to ethylene and pretreated as desired, it receives a 6 L dose of O2 at 323 K. The TPO spectrum is the CO desorption signal at a 6 K/sec programming rate. CO2 accounts for less than 1% of the oxidation, so the CO signal accounts for essentially all of the carbon removed. O2 dosing is repeated until no further CO is evolved during heating. SIMS results show that all carbon has been removed from the surface at the TPO end point. 

A 30 m x 0.25 mm id x 0.25 i.m film thickness HP-5ms capillary column was used, with temperature programming from an initial temperature held at 90°C for 2 min before commencing a 7°Cmin-1 rise to 285°C, with a final time of 20 min. The split/splitless injector was held at 280°C and operated in the splitless mode, with the split valve closed for 1 min following sample injection. The split flow was set at 40 ml min-1, and the mass spectrometer transfer line was maintained at 280°C. Electron impact ionization at 70 eV, with the electron multiplier voltage set at 1500 V, was used, while operating in the single-ion monitoring (SIM) mode. 

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