Imperial College London

DrEhsan AleemAhmad

Faculty of Natural SciencesDepartment of Chemistry

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109Molecular Sciences Research HubWhite City Campus

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Publications

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11 results found

Ahmad EA, Chang H-Y, Al-Kindi M, Joshi GR, Cooper K, Lindsay R, Harrison NMet 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.

Journal article

Rafols i Belles C, Selim S, Harrison NM, Ahmad EA, Kafizas Aet 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

Journal article

Joshi GR, Cooper K, Zhong X, Cook AB, Ahmad EA, Harrison NM, Engelberg DL, Lindsay Ret 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.

Journal article

Tileli V, Ahmad E, Webster R, Mallia G, Duchamp M, Stoerzinger K, Shao-Horn Y, Dunin-Borkowski R, Harrison Net al., 2016, Decoupling of valence and coordination number contributions at perovskite surfaces, Pages: 934-935

Journal article

Ahmad EA, Tileli V, Kramer D, Mallia G, Stoerzinger KA, Shao-Horn Y, Kucernak AR, Harrison NMet 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.

Journal article

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.

Journal article

Ahmad EA, Mallia G, Kramer D, Kucernak AR, Harrison NMet al., 2013, The stability of LaMnO3 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

Journal article

Ahmad EA, Mallia G, Kramer D, Kucernak AR, Harrison NMet al., 2013, The stability of LaMnO3 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

Journal article

Ahmad EA, Mallia G, Kramer D, Tileli V, Kucernak AR, Harrison NMet 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

Journal article

Ahmad EA, Liborio L, Kramer D, Mallia G, Kucernak AR, Harrison NMet al., 2011, Thermodynamic stability of LaMnO3 and its competing oxides: A hybrid density functional study of an alkaline fuel cell catalyst, PHYSICAL REVIEW B, Vol: 84, ISSN: 2469-9950

Journal article

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