The adsorption of CO on the Fe(001) surface has been investigated by ab initio density functional methods. The surface is modeled using both an infinite periodic slab and a finite cluster which allows a detailed comparison of these two approaches. We have studied CO adsorption at three different surface locations and find that the energetically preferred site for CO adsorption corresponds to the 4-fold hollow site (R 3 ) followed by binding to the on-top (R 1 ) and 2-fold (R 2 ) positions. Our study is in good agreement with experiments, which show that the R 3 site is the preferred binding site. The theoretical value for the tilting angle in the R 3 position (54°in both slab and cluster calculations) agrees with the experimental value of 45 ( 10°, while the greatly increased theoretical CO bond length of 1.30 Å (compared to 1.11 Å in the gas phase) demonstrates the activation of the CO bond in the R 3 position and correlates well to the observed CO stretch frequency of 1200 cm -1 as opposed to the gas-phase value of 2143 cm -1 . Both the R 2 site and R 1 position are known to exist experimentally, but the precise energetic ordering of these sites has not been established unambiguously. In these sites the theoretical CO bond length increases to about 1.18-1.20 Å in correspondence to the experimentally observed vibrational features that are shifted to lower frequencies of about 2010 cm -1 . In our calculations we find that the R 1 CO is bound more strongly than the R 2 CO by 7.6 kcal/mol.
The effect of the reduction of the native surface oxide of Fe on the binding of imidazole (as a corrosion inhibitor) with Fe in an aqueous brine solution has been addressed here. The surface interactions and corrosion inhibition efficiency were studied using X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). It was shown that imidazole dissolved in brine bonds with the unreduced iron oxide surface via pyrrole-type nitrogen. However, surface interactions with Fe occur via both pyridine-type and pyrrole-type nitrogen atoms when imidazole is added to brine containing a cathodically reduced iron surface. The packing density of imidazole is found to be higher in the latter case with a corresponding increase in the corrosion inhibition efficiency.
The effect of H 2 S at the parts per million level on the early stage of iron corrosion in 3 wt % NaCl solutions saturated with CO 2 is investigated using electrochemical and surface science techniques. Small H 2 S concentrations ͑5 ppm͒ have an inhibiting effect on corrosion in the presence of CO 2 . At higher H 2 S concentrations ͑up to 500 ppm͒, the corrosion rate is higher but below the H 2 S-free case. The characterization of the iron surfaces after corrosion uses photoelectron spectroscopy and electron microscopy. For Fe exposed to H 2 S-containing solutions, a sulfur peak ͑S 2p͒ appears at a binding energy of 162.1 eV, attributable to disulfide ͑S 2 2− ͒ adsorption. The Fe 2p 3/2 peak attributed to Fe͑II͒ at the surface shifts by about 1 eV in the presence of 5 ppm H 2 S compared to the H 2 S-free case due to bonding of Fe to S and O. At higher H 2 S concentrations, the formation of a S-rich surface is indicated. Morphological changes are observed on the iron surface after exposure to H 2 S-containing solutions. A thin protective film forms at 5 ppm H 2 S, while at higher H 2 S concentrations a thicker, more porous surface phase forms. A mechanism for corrosion inhibition of iron in different concentrations of H 2 S in CO 2 /brine is proposed.In the search for new sources of oil and gas, operational activities have moved to harsher environments in deeper high pressure/high temperature wells and remote offshore locations. This has created increased challenges to the economy of project development and execution where operational integrity and an accurate prediction of material performance are becoming paramount. The economic move toward multiphase transportation through subsea completions and long in-field flow lines has increased the risk of corrosion. [1][2][3][4][5][6][7][8][9] Corrosion, therefore, remains a major operational obstacle to successful hydrocarbon production, and its optimum control and management is regarded essential for the cost-effective design of facilities and their safe operations. This has wide-ranging implications on the integrity of many materials used in the petroleum industry. 1-9 Oilfield corrosion manifests itself in several forms, among which CO 2 corrosion ͑sweet corrosion͒ and H 2 S corrosion ͑sour corrosion͒ in the produced fluids and oxygen corrosion in water injection systems are by far the most prevalent forms of attack encountered in oil and gas production. The majority of oilfield failures result from CO 2 corrosion of carbon and low alloy steels primarily due to inadequate knowledge and predictive capability and the insufficient resistance of carbon and low alloy steels to this type of attack. 1-12 The understanding, prediction, and control of this corrosion are key challenges to sound facilities design, operation, and subsequent integrity assurance.CO 2 corrosion..-CO 2 corrosion, or "sweet corrosion," of carbon and low alloy steels is not a new problem. It was first recorded in the U.S. oil and gas industry in the 1940s, followed by several studies since then. 1-8 A...
A combination of first-principles thermodynamics and density functional theory (DFT) was applied for the prediction of sulfur-poisoned monomeric Cu/Fe species formed in the SSZ-13 catalyst framework under selective catalytic reduction (SCR)-relevant conditions in the presence of sulfur dioxide, ammonia, oxygen, and water. Differences in fresh and sulfurpoisoned species were found for Cu-and Fe-SSZ-13 catalysts containing one Al (1Al sites) or two Al (2Al sites) in 6-membered rings (6MRs) or 8membered rings (8MRs). The impact of ammonia concentration during lowand high-temperature sulfur-poisoning on Cu-and Fe-speciation was also investigated. SCR-relevant concentrations of ammonia in the gas mixture led to the formation of ammonium sulfates over copper in 2Al and 1Al sites of Cu-SSZ-13, while bisulfate and sulfuric acid species were predicted at these copper sites either in the absence of ammonia or at negligible concentrations of ammonia during low-and high-temperature poisoning. The absence of ammonia in the gas mixture led to the formation of iron-bisulfates at 2Al sites of Fe-SSZ-13 during lowtemperature poisoning, while the formation of ammonium sulfates was favorable under SCR-relevant conditions. In contrast to the facile formation of ammonium sulfates at copper sites of Cu-SSZ-13, only ammonium-free iron-sulfates formed at 1Al sites in Fe-SSZ-13 under realistic operational conditions. The regeneration of 2Al sites of Cu-SSZ-13 was predicted to occur at higher temperatures compared to 2Al sites in Fe-SSZ-13, whereas the opposite was predicted for 1Al sites. The analysis of fresh and regenerated Cu/Fe species was carried out as well. These theoretical results on model catalysts provide a first step in the understanding of sulfur-poisoning in Fe-SSZ-13 catalysts, supporting further experimental investigations to improve NH 3 -SCR catalysts for meeting future emission standards.
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