XPS detection

The state of tin in Pt/Sn/alumina catalysts was investigated bv Li and Shia (25) via Mossbauer spectroscopy (i/9Sn enriched isotopes) and XPS. The former technique indicated the presence of Sn+, Sn+2 and Sn, in proportions that depended on the method of preparation, but in all cases the Sn+4 component dominated. These conclusions were confirmed by the XPS experiments. Additional TPR tests on the reduced catalyst and on samples exposed to air showed that reoxidation of Pt/Sn/alumina reduced preparations was rather slow, confirming our EXAFS observations. The presence of zero valent tin in similar preparations, using the acetone complexation procedure, was recently confirmed by Li, Stencel and Davis (12) in an extended XPS investigation. For reduced samples, with a Pt Sn ratio 1 5, these authors estimated that approximately 68% of the tin was in the metallic state. However, they observed that exposure of the sample to air for 10 minutes entirely eliminated the XPS detectable Sn°. Their data also indicated that upon reduction, chlorine migrated from the surface to the alumina. Thus, XPS which measures surface composition indicates a higher sensitivity to oxidation than was demonstrated by our EXAFS experiments, which is a bulk diagnostic. 

Post-reaction XPS detected considerable amounts of CH (about 0.7 ML), indicating that CHx may again have been involved in the reaction, and also a Pd3d BE shift (approximately 0.6 eV), which points to a partial oxidation of the palladium nanoparticles during the reaction (297,504) (Fig. 55c). It is inferred that the oxidation was not complete, because the frequencies of adsorbed CO were still characteristic of CO on metallic palladium (Fig. 55b), and full oxidation to PdO particles would result in BE shifts of about + 1.5eV (514). The observed BE shift (characteristic of oxidized palladium) could be (partly) reversed by reaction... 

Detection of elements in concentrations as low as 10-6, while AES or XPS detection limits are concentration levels of 0.1 atom% ... 

A model Phillips catalyst for ethylene polymerization has been prepared by spin coating of a Cr(III) precursor (Cr(acac)3) on a flat silicon wafer (100) covered by amorphous silica. The spin coating parameters were chosen in order to obtain a homogeneous film. The model catalyst was submitted to an activation process. The surface concentration of Cr decreased from about 0.8 to 0.4 Cr atom/nm as the temperature increased from 150 to 550°C. Direct information concerning the surface molecular species and the environment of Cr was provided by ToF-SIMS and XPS. At 350°C, the catalyst precursor was decomposed Cr species were in the form oxide and surface-anchored chromates. Upon final activation at 650°C for 6 h, Cr species were below the XPS detection limit however the model catalyst was active for ethylene polymerization at 160°C and 2 bar pressure. 

The last step of the activation process, 6 h at 650°C, reflecting industrial conditions, induced also modifications on the surface of the model catalyst. The Cr species were below the XPS detection limit. This can possibly be attributed to two phenomena (i) desorption due to a decrease of chromate stability, as a result of dehydroxylation and strain induced in the surface Si-0 bond surface reorganization of chromate species [45,46], forming Cr203 aggregates, which are known to be inactive versus ethylene polymerization [7]. 

After 6 h activation under dry air flow at 650°C, the Cr density falls under the XPS detection limit. Nevertheless, the model catalyst is active for polymerization of ethylene at 160°C under 2 bar pressure. From these observations, it is concluded that the model catalyst prepared from Cr(III) precursor behaves like its industrial counterpart it contributed to a better understanding of its activation process at a molecular scale. 

Table 1 also shows that no significant influence on the surface area is observed in any case. The surface chlorine content was below the XPS detection limit on most of the samples except for the sample prepared with HAuCh and NH4OH. 

Acid-base properties of zeolites are probed by studying their interactions with basic/acidic molecules by appropriate techniques (IR, NMR, calorimetry, TPD). For the external surface region, analogous methods based on XPS detection were developed following the pioneering work of Defosse and Canesson [65]. The probe molecules used are pyridine [55,56,66-72], ammonia [21,43,44,73], and pyrrole and chloroform for basic sites [70,74,75]. Kaliaguine [59] has published a review of the results. 

Apart from the traditional applications of XPS (detection of extra-zeolite material, assessment of reduction degrees of alloy components), an attempt has been made to prove the formation of alloy particles by observation of alloy shifts with the signals of the metals involved [47]. For this purpose, however, EXAFS is by far superior. 

The dissociation adsorption of N2 is supported by many experimental results (i) Only NH and N were detected after N2 and H2 were adsorbed on dual-promoter iron catalyst at reaction temperature and 101.3kPa and vacuumed at 200°C (ii) N2 adsorption state has never been found by XPS detection. Heat desorption data provided by Toyashima and Xiamen University showed that N2 is not the main adsorption species under the conditions of ammonia S3mthesis at 400°C-450°C (iii) ErtP° provides a powerful support for the dissociation adsorption of N2 by the following results from energy spectroscopy ... 

Areas in which X-ray photoelectron spectroscopy might be expected to perform best are the detection of surface effects (e.g. blooming), surface active additives (e.g. release and slip agents, lubricants, surfactants, etc.), rapidly migrating additives (e.g. plasticisers), or thin-film contaminants. XPS detection limits for additives (0.5 vol.%) are unfavourable for... 

Jia [98] observed the complete degradation of a glass-reinforced silicone seal in a fuel cell stack, with silicon from the seal detectable throughout the MEA. Schulze et al. [99] detailed the degradation of silicon-based seals, and using XPS detected residues of silicone in the anode CL and cathode GDL. They concluded that the direction of movement of the silicone traces was from the anode to the cathode, due to the electrical field, and that it was blocked by the PEM. However, traces of decomposition products from the sealing material in both the membrane and electrodes were detected by Du et al. [100], as shown in Fig. 11.13. 

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