Adsorption, Diffusion, and Dissociation of H2S on Fe(100) from First Principles

Sulfide (H2S) Corrosion Modeling of Cr-Doped Iron (Fe) Using a Molecular Modeling Approach

This work presents the use of density functional theory to study the adsorption/dissociation mechanism of the H₂S molecule at the Cr-doped iron (Fe(100)) surface. It is observed that H₂S is weakly adsorbed on Cr-doped Fe; however, the dissociated products are strongly chemisorbed. The most feasible path for disassociation of HS is favorable at Fe compared to Cr-doped Fe. This study also shows that H₂S dissociation is a kinetically facile process, and the hydrogen diffusion follows the tortuous path. This study helps better understand the sulfide corrosion mechanism and its impact, which would help design effective corrosion prevention coatings.

Pt-Anchored CuCrO2 for Low-Temperature-Operating High-Performance H2S Chemiresistors

Recent advances in heterogeneous catalysts indicate that single atoms (SAs), anchored/stabilized on metal oxide nanostructures, exhibit not only high catalyst atom efficiency but also intriguing reactivity and selectivity. Herein, isolated Pt SA-anchored CuCrO₂ (CCO) has been designed by a glycine-nitrate solution combustion synthesis (SCS) route. The density of isolated Pt SAs achieves the highest value of ∼100 μm^-2 for the 1.39 wt % Pt-anchored CCO sample, which results in the drastically boosted H₂S response characteristics, including a high response of 1250 (35 times higher than that of pure CCO) at 10 ppm H₂S and a low operating temperature of 100 °C. Except for CH₄S, the responses of a 1.39 wt % Pt-anchored CCO chemiresistor to diverse vapors with concentrations of 50-100 ppm are less than 2, exhibiting excellent selectivity. Various ex situ characterizations indicate that the spillover catalytic effect of Pt SA sites, other than the conventional sulfuration-desulfuration mechanism, plays a dominant role in the outstanding H₂S response characteristics.

Effect of Impurity Atoms on the Adsorption/Dissociation of Hydrogen Sulfide and Hydrogen Diffusion on the Fe(100) Surface

In the actual environment, impurity atoms significantly affect the adsorption/dissociation of gas molecules on the substrate surface and in turn promote or impede the formation of subsequent products. In this study, we investigate the effects of three kinds of impurity atoms (H, O, and S) on the adsorption/dissociation of hydrogen sulfide (H₂S) and hydrogen (H) diffusion processes by using the density functional theory method. We found that impurity atoms can change the charge density distribution of the surface and thus affect the adsorption/dissociation process of H₂S. The existence of a H atom reduces the dissociation barrier of H₂S. The adsorption site of H₂S near the O atom is transferred from the bridge site to the adjacent top site and the first-order dissociation barrier of H₂S is 0.07 eV, which is prominently lower than that of the pristine surface (0.28 eV). The presence of a S atom transfers the adsorption site of H₂S to a farther bridge site and effectively affects the dissociation process of H₂S. Both O and S atoms hinder the dissociation process of HS. Moreover, the diffusion process of H atoms to the subsurface can be slightly impeded by the O atom. Our work theoretically explains the influence mechanism of impurity atoms on the adsorption/dissociation of H₂S and H diffusion behavior on the Fe(100) surface.

Test Conditions for Pipeline Materials Selection with High Pressure Sour Gas

Acid gases, such as CO2, H2S, and/or sulfur in oil industry’s production fluids, can be responsible for both general and localized corrosion, acting with different mechanisms, which depend on chemical and physical properties of the produced fluids. Materials selection for handling such fluids is performed by combining experience with suggestions from standards and regulations. A good deal of knowledge is available to predict corrosion rates for CO2-containing hydrocarbons, but the effect of high H2S pressure is less understood, mainly due to the difficulty of performing laboratory tests in such challenging conditions. For instance, the so-called NACE solution to assess SSC (Sulfide Stress Cracking) susceptibility of steels is a water-based solution simulating production fluids in equilibrium with one bar bubbling H2S gas. This solution does not represent environments where high gas pressure is present. Moreover, it does not take into account the corrosive properties of sulfur and its compounds that may deposit in such conditions. Besides, properties of high pressure gases are intermediate between those of a gas and those of a liquid: high pressure gases have superior wetting properties and better penetration in small pores, with respect to liquids. These features could enhance and accelerate damage, and nowadays such conditions are likely to be present in many production fields. This paper is aimed to point out a few challenges in dealing with high pressure gases and to suggest that, for materials selection in sour service, a better correspondence of test conditions with the actual field conditions shall be pursued.

Coverage dependent water dissociative adsorption on Fe(110) from DFT computation

Using density functional theory calculations and ab initio atomistic thermodynamics, H2O adsorption and dissociation on the Fe(110) p(4 × 4) surface at different coverages have been computed. At the lowest coverage, the adsorbed H2O, OH, O and H species can migrate easily on the surface. For (H2O)n adsorption, H2O molecules donating H atoms for H-bonding adsorb more strongly than those accepting H atoms for H-bonding. Monomeric H2O dissociation is favored both thermodynamically and kinetically. On nO pre-covered Fe(110) surfaces (n = 1-8), H2O dissociation is accessible for nO + H2O (n = 1-7) both kinetically and thermodynamically, while H2O desorption instead of dissociation occurs at n = 8. With the increased number of surface O atoms, H2 dissociative adsorption energies vary in a narrow range for n = 1-4 and decrease for n = 5-7, while at n = 8, the surface does not adsorb H2. At low OH coverage (n = 2, 4), OH groups are perpendicularly adsorbed without H-bonding, while for n≥ 6, adsorbed OH groups are linearly arranged and stabilized by H-bonding. The maximal OH coverage (n = 12) is 0.75 ML and the reasonable O coverage (n = 7) is 0.44 ML, in line with the experiment. The calculated desorption temperatures of H2O and H2 agree well with the available experimental data. These results provide fundamental insights into water-involved reactions catalyzed by iron and interaction mechanisms of water interaction with metal surfaces.

DFT study on the adsorption and dissociation of H2S on CuO(111) surface

The pathways of the dissociation H₂S on the CuO(111) surface are presented in this work.

Density functional theory studies of the adsorption of hydrogen sulfide on aluminum doped silicane

First principles total energy calculations have been performed to study the hydrogen sulfide (H2S) adsorption on silicane, an unusual one monolayer of Si(111) surface hydrogenated on both sides. The H2S adsorption may take place in dissociative or non-dissociative forms. Silicane has been considered as: (A) non-doped with a hydrogen vacancy, and doped in two main configurations; (B) with an aluminum replacing a hydrogen atom and (C-n; n = 1, 2, 3) with an aluminum replacing a silicon atom at a lattice site. In addition, three supercells; 4x4, 3x3 and 2x2 have been explored for both non-doped and doped silicane. The non-dissociative adsorption takes place in geometries (A), (C-1), (C-2) and (C-3) while the dissociative in (B). Adsorption energies of the dissociative case are larger than those corresponding to the non-dissociated cases. In the dissociative adsorption, the molecule is fragmented in a HS structure and a H atom which are bonded to the aluminum to form a H-S-Al-H structure. The presence of the doping produces some electronic changes as the periodicity varies. Calculations of the total density of states (DOS) indicate that in most cases the energy gap decreases as the periodicity changes from 4x4 to 2x2. The features of the total DOS are explained in terms of the partial DOS. The reported charge density plots explain quite well the chemisorptions and physisorptions of the molecule on silicane in agreement with adsorption energies.

Adsorption, dissociation, penetration, and diffusion of N2 on and in bcc Fe: first-principles calculations

We report first-principles calculations of adsorption, dissociation, penetration, and diffusion for the complete nitridation mechanism of nitrogen molecules on a pure Fe surface (bcc, ferrite phase). The mechanism of the definite reaction path was calculated by dividing the process into four steps. We investigated various reaction paths for each step including the energy barrier based on the climb image nudged elastic band (CI-NEB) method, and the complete reaction pathway was computed as the minimum energy path (MEP). The adsorption characteristics of nitrogen (N) and molecular nitrogen (N2) indicate that nitrogen atoms and molecules are energetically favorable at the hollow sites on pure Fe(100) and (110). The dissociation of the nitrogen molecule (N2) was theoretically supported by electronic structure calculations. The penetration of nitrogen from the surface to the sub-surface has a large energy barrier compared with the other steps. The activation energy calculated for nitrogen diffusion in pure bcc Fe was in good agreement with the experimental results. Finally, we confirmed the rate-determining step for the full nitridation reaction pathway. This study provides fundamental insight into the nitridation mechanism for nitrogen molecules in pure bcc Fe.

A DFT Study of the Adsorption and Dissociation of CO on Sulfur-Precovered Fe(100)

Sulfur is known to be a poison to several catalytic reactions, e.g., the Fischer-Tropsch synthesis (FTS), in which it affects drastically the performance of both iron- and cobalt-based catalysts. However, despite the importance of this industrial process, little is known about what elementary steps are poisoned by sulfur. In the present article, we report, using density functional theory, the effect of sulfur on one of the most relevant reactions in the FTS: the dissociation of carbon monoxide over iron surfaces. We have studied the adsorption and dissociation of CO on Fe(100)-S-p(2 x 2) (theta(S) = 0.25 ML) and on Fe(100)-S-c(2 x 2) (theta(S) = 0.50 ML). We have found surface configurations that correlate well with the desorption features observed in temperature-programmed desorption mass spectroscopy. In addition, we have calculated the activation energy of CO dissociation on Fe(100)-S-p(2 x 2), which, interestingly, is very similar to the activation energy of CO dissociation on the sulfur-free Fe(100) surface. However, the sign of the reaction changes by the presence of sulfur; CO dissociation is highly exothermic on the sulfur-free Fe(100) surface, whereas on the Fe(100)-S-p(2 x 2) surface, it is slightly endothermic. Moreover, according to our results, the influence of sulfur in the CO dissociation seems to be short-ranged.

Prediction of a Highly Activated State of CO Adsorbed on an Al/Fe(100) Bimetallic Surface

Using periodic slab density functional theory, we investigate CO adsorption, diffusion, and dissociation energetics on a monolayer of Al covering Fe(100) [Al/Fe(100)]. We predict a weakly chemisorbed state of CO to exist on Al/Fe(100), with CO adsorbing on the 4-fold hollow site in a very tilted fashion. This state is predicted to have an extremely low CO stretching frequency of only 883 cm(-1), indicating a dramatically weakened CO bond relative to gaseous CO, even though the molecule is predicted to bind to Al/Fe(100) quite weakly. We predict that dissociation of CO starting from this weakly adsorbed state has a barrier of only approximately 0.35 eV, which is approximately 0.70 eV lower than that on Fe(100). To understand how the underlying substrate changes the electronic properties of the supported Al monolayer, we compare CO adsorption on Al/Fe(100) to its adsorption on analogous pure Al(100) surfaces. This highly activated yet weakly bound state of CO on Al/Fe(100) suggests that Al/Fe(100) could be an effective low-temperature bimetallic catalyst in reducing environments.

Effects of Alloying on the Chemistry of CO and H2S on Fe Surfaces

Deleterious gases such as CO and H(2)S can cause degradation of steel by reacting with the metal surface. Here we consider whether alloying the steel surface might be able to inhibit these damaging surface reactions by raising the barriers to molecular dissociation. We employ first-principles density functional theory techniques to investigate the elementary reaction pathways and barriers for CO and H(2)S on FeAl and Fe(3)Si surfaces and compare them with pure Fe surfaces (as a model for steel). We find that H(2)S dissociates on iron surfaces much more easily than CO does. Although FeAl surfaces raise the barriers for H(2)S dissociation, they significantly lower the barriers for CO dissociation. On the other hand, Fe(3)Si surfaces raise the barriers for CO dissociation, but they are as vulnerable as Fe surfaces to H(2)S dissociation. Our findings suggest that alloying iron with Al or Si is unlikely to simultaneously increase its resistance to the initial stages of chemical degradation by CO and H(2)S.