Polycrystalline iron

Paal Z, ErtI G and Lee S B 1981 Interactions of potassium, nitrogen, and oxygen with polycrystalline iron surfaces Appl. Surf. Sc/. 8 231... 

At high potentials in the passive region, the imaging of nickel surfaces proves difficult owing to the formation of thick oxide layers." It was shown by Bhardwaj et that on polycrystalline iron in a borate electrolyte, oxide formation starts as patches on the surface that gradually fuse together to establish a surface oxide fihn. Also, clusters of the hydroxide were seen" on a polycrystalline iron surface obsaved by in situ STM and after potential cycles in an NaOH electrolyte. 

Fig. 8. XPS C(ls) core level spectra for CO adsorbed on polycrystalline iron and Fe(lOO). (a) Clean surface (b) saturation CO coverage at 20°C (c) warmed to 100°C. Lines I and II indicate the C(ls) positions for atomic carbon and carbon in molecular CO, respectively (43).

The adsorption of nitrogen on tungsten 51) was the first nitrogen system to be studied (Fig. 12) by XPS subsequently nitrogen interaction with iron was studied (52, 53), and two distinct N(ls) peaks were observed at 80 K, one at about 405 eV and the other at 400 eV. At room temperature with both single crystals and polycrystalline iron surfaces only a single peak is... 

Over the past several years, Gruen and coworkers have examined the SH response from iron electrodes in alkaline solutions [45, 53, 172]. In their work on polycrystalline iron, they concluded that the potential dependent SH response which was observed during surface oxidation could be attributed to two intermediate phases on the electrode surface between the passive film at oxidative potentials and the reduced metal at hydrogen evolution potentials [53]. They have recently extended this work to Fe(110). In this study [172], they examined the SH rotational anisotropy from this crystal under ambient conditions. They found that the experiments reveal the presence of both twofold and threefold symmetric species at the metal/oxide interface. When their data is fit to the theory of Tom et al. [68], they conclude that the measured three-fold symmetric oxide is found to be tilted by 5° from the Fe(110) plane. The two-fold symmetric structure is aligned with the Fe(110) surface. 

The ionisation potential of a core electron depends, to a small extent, on the chemical environment of the atom in question, and chemical shifts of up to about 10 eV can be observed. For example, the C(ls) XPS signal for molecularly adsorbed carbon monoxide on polycrystalline iron at 290 K shows a peak at 285.5 eV, which is... 

Fig. 12.92. STM image of iron in air. Vbias = -0.298 V. (Reprinted from R. C. Bhardwaj, A. Gonzalez-Mar-tin, and J. O M. Bockris, in Situ Scanning Tunneling Microscopy Studies on Passivation of Polycrystallin Iron in Borate Buffer, J. Electrochem. Soc. 307 1902, 1991. Reproduced with permission of The Electrochemical Society, Inc.)...

Figure 20 Surface topography effects on polycrystalline iron surface after sputtering with 10 keV Kr+ ions at 50° incidence. (From Ref. 76.)...

The above ideas that anion-cation pair sites are the surface sites for CO and CO2 adsorption on magnetite was verified directly by Udovic and Dumesic (43 ). These authors prepared films of magnetite on polycrystalline iron foils and varied the oxidation state of the surface by vacuum-annealing at different temperatures. In short, it was shown by Auger electron spectroscopy and X-ray photo-... 

Carbon deposits have been grown on heated polycrystalline iron specimens in a gas reaction cell in a scanning electron microscope. The complete range of deposits formed from methane at temperatures between 620°C and 850°C have been surveyed, from the first appearance of solid carbon on the surface to the final structural breakdown of the metal. Four characteristic morphologies have been identified. 

Polycrystalline iron foil (99.999% quoted purity, ex-Johnson-Matthey) was used so that any specific activity of the grain boundary intersections at the surface would be apparent. The gas was B.O.C. Ltd. research grade, used without further purification. 

Figure 3. Surface preparation of the polycrystalline iron foil as received, with damage from cold work (a), ion etched, to remove 2 fan of metal (b) after annealing for 2 hat 500°C to remove etch structure (c), and development of step structure after further annealing for 3 hat 600°C (d).

Bhardwaj et al. [2] studied by in-situ real-time STM imaging the passivation of polycrystalline iron in borate buffer. They proceeded by alternating oxidation steps at increasing anodic potentials and reduction steps at cathodic potential. After reduction of the natural oxide at ftie cathodic potential, relatively flat surfaces were produced supposedly corresponding to the metal substrate. Upon oxidation at anodic potential, rougher surfaces were at first produced, with patches or clusters of nanometer dimensions. These patches were observed in the first image after the oxidation step. 

Z. Paal, G. Ertl, and S.B. Lee. Interactions of Potassium, Oxygen, and Nitrogen with Polycrystalline Iron Surfaces. Appl. Surf. Sci. 8 231 (1981). 

Schaefer, H. E., WUurschum, R., Birringer, R., and Gleiter, H., Structure of nanometer sized polycrystalline iron investigated by positron lifetime spectroscopy, Phys. Rev. B, 35(14), 9545 (1988). 

Figure 10. Comparison of product distributions obtained over initially clean polycrystalline iron and rhodium foils...

The initiation of the polymerization of alkylene oxides with iron has been known for some time. A recent patent extends this concept to the alkoxylation of many active hydrogen compounds. The catalyst for this process is a polycrystalline iron oxide (a-iron(lll)oxide). For example, in an autoclave, under nitrogen, n-decanol and 2 wt% of the iron catalyst are treated with ethylene oxide at a pressure of less than 6 bars for 4 hr. The resulting liquid had a degree of polymerization of 5 [42]. 

On Iron. PH3 adsorption on clean polycrystalline iron at 100 K was suggested to be both molecular and dissociative. Near saturation coverage, molecularly adsorbed PH3 desorbs around 170 K as shown by thermal desorption spectra. An activation energy for PH3 desorption of 9.6 kcal/mol was calculated assuming a simple first-order desorption kinetics. The... 

Leygraf, C., Hendewerk, M., Somorjai, G.A. Photodissociation of water by p- and n-type polycrystalline iron oxides by using visible light (<2.7 eV) in the absence of external potential. Proc. Natl. Acad. Sci. U.S.A. 79, 5739 (1982)... 

Emsley AM, HiU MR Kinetics of monticular carbon growth on polycrystalline iron. J Chem Soc Faraday Trans 1983 79 15-26. 

G.P. Siedel, Neutron radiation hardening of polycrystalline iron , Radiat. Ejf, 1969,1,177-190. 

The mechanism of adsorption of a corrosion inhibitor, 1-hydroxy-ethane-1,1-di-phosphonic acid (HEDP), on polycrystalline iron has been studied in neutral solutions. The chemical interactions between inhibitor molecules and different metallic or oxidized surfaces under near-neutral aqueous circumstances were investigated by XPS and XAES surface analytical methods (Felhosi et al., 1999a Karmdn et al., 1998). For quantitative correlation between the components of the oxide layer and the corrosion resistance of the passive film, it is important to determine the ratio of oxygen in different states in the passive layer. 

Pioneering studies by Emmet suggested in 1953 that ethylene acts as a chain initiator in Fe-catalyzed Fischer-Tropsch reactions. Many authors have used isotopi-cally labeled olefins to confirm that 1-alkenes are the primary reaction products (although they are thermodynamically instable under the reaction conditions). For instance, Somorjai s group has shown, using a polycrystalline iron sheet (1 cm ), that the addition of ethylene or propene in the syn gas increases the length of the polymer chain. These olefins readsorb and readily form polymers by insertion into adsorbed C fragments. Thus, the Fischer-Tropsch mechanism involves two steps, namelly the formation of olefins and their polymerization ... 

It should be stressed that not only is there a pressure gap between the UHV studies and the high-pressure measurements, but also a material gap. The UHV studies use carefully cleaned iron metal in the form of single crystals as the catalyst. On the other hand, elemental polycrystalline iron without surface purification does not catalyze ammonia synthesis at high pressure (1 bar), as has been described in Section 2.2. This may be rationalized in terms of the extreme sensitivity of the... 

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