Publications
16 results found
Ahmad EA, Al-Kindi M, Aboura Y, et al., 2023, Sweet corrosion scale: Structure and energetics of siderite facets, APPLIED SURFACE SCIENCE, Vol: 635, ISSN: 0169-4332
Restuccia P, Ahmad EA, Harrison NM, 2022, A transferable prediction model of molecular adsorption on metals based on adsorbate and substrate properties, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 24, Pages: 16545-16555, ISSN: 1463-9076
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- Citations: 3
Kousar K, Walczak MS, Ljungdahl T, et al., 2021, Corrosion inhibition of carbon steel in hydrochloric acid: Elucidating the performance of an imidazoline-based surfactant, Corrosion Engineering Science and Technology, Vol: 180, Pages: 1-8, ISSN: 0007-0599
A combination of electrochemical measurement and interface analysis have been applied to characterise the interaction of OMID, an exemplar imidazoline-based corrosion inhibitor, with carbon steel in 1 M hydrochloric acid. Corrosion inhibition efficiency data indicate that excellent performance is achieved well below the critical micelle concentration. High resolution X-ray photoelectron spectra demonstrate that, as the corrosion rate decreases, the interface evolves towards one comprising OMID bound to film-free carbon steel. This latter result provides key input for those researchers attempting to predict corrosion inhibitor functionality through atomic scale interfacial modelling, and so identify next generation chemistries.
Acres MJ, Hussain H, Walczak MS, et al., 2020, Core level photoemission line shape selection: Atomic adsorbates on iron, Surface and Interface Analysis, Vol: 52, Pages: 507-512, ISSN: 0142-2421
Robust fitting of core level photoemission spectra is often central to reliable interpretation of X‐ray photoelectron spectroscopy (XPS) data. One key element is employment of the correct line shape function for each spectral component. In this study, we consider this topic, focusing on XPS data from atomic adsorbates, namely, O and S, on Fe(110). The potential of employing density functional theory (DFT) for generating adsorbate projected electronic density of states (PDOS) to support line shape selection is explored. O 1s core level XPS spectra, acquired from various ordered overlayers of chemisorbed O, all display an equivalent asymmetric line shape. Previous work suggests that this asymmetry is a result of finite O PDOS in the vicinity of the Fermi level, allowing O 1s photoexcitation to induce a weighted continuum of final states through electron‐hole pair excitation. This origin is corroborated by O DFT‐PDOS generated for an optimised five‐layer Fe(110)(2 × 2)‐O slab. Adsorbate DFT‐PDOS were also computed for Fe(110) urn:x-wiley:01422421:media:sia6770:sia6770-math-0001 ‐S. As, similar to adsorbed O, there is a significant continuous distribution of states about the Fermi level, it is proposed that the S 2p XPS core levels should also have asymmetric profiles. S 2p XPS data acquired from Fe(110) urn:x-wiley:01422421:media:sia6770:sia6770-math-0002 ‐S, and their subsequent fitting, verify this prediction, suggesting that DFT‐PDOS could aid line shape selection.
Ahmad EA, Chang H-Y, Al-Kindi M, et al., 2019, Corrosion protection through naturally occurring films: new insights from iron carbonate, ACS Applied Materials and Interfaces, Vol: 11, Pages: 33435-33441, ISSN: 1944-8244
Despite intensive study over many years, the chemistry and physics of the atomic level mechanisms that govern corrosion are not fully understood. In particular, the occurrence and severity of highly localized metal degradation cannot currently be predicted and often cannot be rationalized in failure analysis. We report a first-principles model of the nature of protective iron carbonate films coupled with a detailed chemical and physical characterization of such a film in a carefully controlled environment. The fundamental building blocks of the protective film, siderite (FeCO3) crystallites, are found to be very sensitive to the growth environment. In iron-rich conditions, cylindrical crystallites form that are highly likely to be more susceptible to chemical attack and dissolution than the rhombohedral crystallites formed in iron-poor conditions. This suggests that local degradation of metal surfaces is influenced by structures that form during early growth and provides new avenues for the prevention, detection, and mitigation of carbon steel corrosion.
Ignatans R, Mallia G, Ahmad EA, et al., 2019, The effect of surface reconstruction on the oxygen reduction reaction properties of LaMnO3, The Journal of Physical Chemistry C: Energy Conversion and Storage, Optical and Electronic Devices, Interfaces, Nanomaterials, and Hard Matter, Vol: 123, Pages: 11621-11627, ISSN: 1932-7447
Perovskites have been widely studied for electrocatalysis due to the exceptional activity they exhibit for surface-mediated redox reactions. To date, descriptors based on density functional theory calculations or experimental measurements have assumed a bulk-like configuration for the surfaces of these oxides. Herein, we probed an initial exposed surface and the screened subsurface of LaMnO3 particles, demonstrating that their augmented activity toward the oxygen reduction reaction (ORR) can be related to a spontaneous surface reconstruction. Our approach involves high energy resolution electron energy loss spectroscopy for the fine structure probing of oxygen and manganese ionization edges under electron beam conditions that leave the structure unaffected. Atomic multiplet and density functional theory calculations were used to compute the theoretical energy loss spectra for comparison to the experimental data, allowing to quantitatively demonstrate that the particle surface layers are La-deficient. This deficiency is linked to equivalent tetrahedral Mn2+ sites at the reconstructed surface, leading to the coexistence of +3 and +2 oxidation states of Mn at the surface layers. This electronic and structural configuration of the as-synthesized particles is indirectly linked to strong adsorption pathways that promote the ORR on LaMnO3, and thus, it could prove to be a valuable design feature in the engineering of catalytic surfaces.
Rafols i Belles C, Selim S, Harrison NM, et al., 2019, Beyond band bending in the WO3/BiVO4 heterojunction: insight from DFT and experiment, Sustainable Energy and Fuels, Vol: 3, Pages: 264-271, ISSN: 2398-4902
Heterojunction photocatalysts can significantly enhance the efficiency of photocatalytic water splitting. It is well known that the key to such improvements lies at the interfacial region where charge separation occurs. Understanding the origins of this interfacial enhancement can enable the design of better performing water splitting devices. Therefore, in this work, a novel theoretical–experimental approach is developed for the study of photocatalytic heterojunctions using the model system – WO3/BiVO4, where it has been shown that the quantum efficiency of water splitting can approach unity at certain wavelengths. Our photoelectrochemical measurements of this heterojunction show a significantly enhanced performance over its separate components when illuminated through the BiVO4 side but not the WO3 side. This is indicative of more efficient electron transfer (i.e. from BiVO4 to WO3) than hole transfer (i.e. from WO3 to BiVO4) across the junction. Our classical band bending model of this junction predicts noticeable interfacial barriers, but could not explain the reduced performance under back illumination. Our atomistic model was used to investigate the effect of interfacial reconstructions and chemical interactions on the electronic structure of the system. The model reveals a non-staggered valence band, in contrast to the staggered conduction band, due to strong hybridization of valence band orbitals in both materials across the interface. This non-staggered valence band does not provide an energetic driving force for charge separation for hole transfer (i.e. from WO3 to BiVO4 under back illumination). Hence, a significant improvement in performance is only observed under front illumination. This combined approach, using both experiment and theory, results in a more complete understanding of a heterojunction photocatalyst system and provides unique insight into the interfacial effects that arise when two semiconductor materials are brought together
Joshi GR, Cooper K, Zhong X, et al., 2018, Temporal evolution of sweet oilfield corrosion scale: Phases, morphologies, habits, and protection, Corrosion Science, Vol: 142, Pages: 110-118, ISSN: 0010-938X
Electrochemical measurements and substrate analysis have been employed to study the corrosion of iron in sweet solution (pH = 6.8, T = 80 °C) over a period of 288 h. Correlated with decreasing corrosion rate, diffraction, microscopy, and spectroscopy data reveal the evolution of adhered sweet corrosion scale. Initially, it is comprised of two phases, siderite and chukanovite, with the latter affording little substrate protection. Subsequently, as the scale becomes highly protective, siderite is the sole component. Notably, siderite crystals are concluded to display a somewhat unexpected habit, which may be a trigger for local breakdown of protective sweet scales.
Tileli V, Ahmad E, Webster R, et al., 2016, Decoupling of valence and coordination number contributions at perovskite surfaces, Pages: 934-935
<jats:p> Perovskite oxide nanostructures are on the forefront of technology due to the wide spectrum of possible applications pertinent to renewable energy sources, such as water‐splitting, solar cells, fuel cells, batteries, and catalysis. In particular, the exceptional properties for the oxygen reduction reaction in catalysis have been detailed recently in a volcano plot and the results reveal that orthorhombic, Jahn‐Teller distorted LaMnO <jats:sub>3</jats:sub> perovskite nanoparticles are the leading, non‐noble metal candidate for enhanced catalytic activity on the cathode electrode of fuel cells [1]. Since the functional properties of these nanoparticles lie on their active surfaces, our approach involves a detailed structural and chemical evaluation of the surfaces on the atomic scale to determine what/where the reaction centres are. Subsequently, the morphology of the particles can be optimised to maximise the number of these reaction centres, allowing us to attain the highest possible performance of perovskite catalysts. </jats:p> <jats:p> From structural transmission electron microscopy (TEM) data it was determined that polar facets exist on crystallites, which lead to assumptions on possible surface reconstruction/relaxation. However, high resolution TEM indicated that the atomic terminations of several surfaces remained defect‐free up to the very surface with no visible reconstruction taking place [2], as shown in Figure 1. Next, the surface and subsurface of the working perovskite catalyst was probed by high spatial and temporal resolution electron energy‐loss spectroscopy (EELS) in scanning TEM mode. The results revealed that the surface shows different character than the bulk. Tan <jats:italic>et al.</jats:italic> has previously shown that different oxidation states of Mn can be probed at neighbouring sites in the same compound
Ahmad EA, Tileli V, Kramer D, et al., 2015, Optimizing Oxygen Reduction Catalyst Morphologies from First Principles, Journal of Physical Chemistry C, Vol: 119, Pages: 16804-16810, ISSN: 1932-7455
Catalytic activity of perovskites for oxygen reduction (ORR) wasrecently correlated with bulk d-electron occupancy of the transition metal. Weexpand on the resultant model, which successfully reproduces the high activity ofLaMnO3 relative to other perovskites, by addressing catalyst surface morphology asan important aspect of the optimal ORR catalyst. The nature of reaction sites onlow index surfaces of orthorhombic (Pnma) LaMnO3 is established from FirstPrinciples. The adsorption of O2 is markedly influenced by local geometry andstrong electron correlation. Only one of the six reactions sites that result from experimentally confirmed symmetry-breakingJahn−Teller distortions is found to bind O2 with an intermediate binding energy while facilitating the formation of superoxide, animportant ORR intermediate in alkaline media. As demonstrated here for LaMnO3, rational design of the catalyst morphology topromote specific active sites is a highly effective optimization strategy for advanced functional ORR catalysts.
Kucernak ARJ, 2015, Electrochemical Characterization and Quantified Surface Termination Obtained by LEIS and XPS of Orthorhombic and Rhombohedral LaMnO<sub>3</sub> Powders, Journal of Physical Chemistry C, Vol: 119, Pages: 12209-12217, ISSN: 1932-7455
LaMnO3 powder synthesized by glycine combustion synthesis with the rhombohedral and orthorhombic structures has been characterized by the combination of low energy ion scattering (LEIS) and X-ray photoelectron spectroscopy (XPS), while the electrocatalytic activity for the oxygen reduction reaction is measured with the rotating disk electrode (RDE) method. Quantification of the surface terminations obtained by LEIS suggests that the orthorhombic LaMnO3 crystallites are near thermodynamic equilibrium as surface atomic ratios compare well with those of equilibrium morphologies computed by a Wulff construction based on computed surface energies. Both rhombohedral and orthorhombic structures present the same La/Mn atomic ratio on the surface. Electrochemical activity of the two structures is found to be the same within the error bar of our measurements. This result is in disagreement with results previously reported on the activity of the two structures obtained by the coprecipitation method [Suntivich et al. Nat. Chem. 2011, 3 (7), 546], and it indicates that the preparation method and the resulting surface termination might play a crucial role for the activity of perovskite catalysts.
Ahmad EA, Mallia G, Kramer D, et al., 2013, Erratum: The stability of LaMnO<inf>3</inf> surfaces: A hybrid exchange density functional theory study of an alkaline fuel cell catalyst (Journal of Materials Chemistry A (2013) (DOI:10.1039/c3ta11382e)), Journal of Materials Chemistry A, Vol: 1, ISSN: 2050-7488
Ahmad EA, Mallia G, Kramer D, et al., 2013, The stability of LaMnO<sub>3</sub> surfaces: a hybrid exchange density functional theory study of an alkaline fuel cell catalyst, JOURNAL OF MATERIALS CHEMISTRY A, Vol: 1, Pages: 11152-11162, ISSN: 2050-7488
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- Citations: 25
Ahmad EA, Mallia G, Kramer D, et al., 2013, The stability of LaMnO<sub>3</sub> surfaces: a hybrid exchange density functional theory study of an alkaline fuel cell catalyst (vol 1, pg 11152, 2013), JOURNAL OF MATERIALS CHEMISTRY A, Vol: 1, Pages: 15555-15555, ISSN: 2050-7488
Ahmad EA, Mallia G, Kramer D, et al., 2012, Comment on "2D Atomic Mapping of Oxidation States in Transition Metal Oxides by Scanning Transmission Electron Microscopy and Electron Energy-Loss Spectroscopy", PHYSICAL REVIEW LETTERS, Vol: 108, ISSN: 0031-9007
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- Citations: 7
Ahmad EA, Liborio L, Kramer D, et al., 2011, Thermodynamic stability of LaMnO<sub>3</sub> and its competing oxides: A hybrid density functional study of an alkaline fuel cell catalyst, PHYSICAL REVIEW B, Vol: 84, ISSN: 2469-9950
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- Citations: 37
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