Alexis T. Bell

An Investigation of Thin-Film Ni–Fe Oxide Catalysts for the Electrochemical Evolution of Oxygen

A detailed investigation has been carried out of the structure and electrochemical activity of electrodeposited Ni-Fe films for the oxygen evolution reaction (OER) in alkaline electrolytes. Ni-Fe films with a bulk and surface composition of 40% Fe exhibit OER activities that are roughly 2 orders of magnitude higher than that of a freshly deposited Ni film and about 3 orders of magnitude higher than that of an Fe film. The freshly deposited Ni film increases in activity by as much as 20-fold during exposure to the electrolyte (KOH); however, all films containing Fe are stable as deposited. The oxidation of Ni(OH)2 to NiOOH in Ni films occurs at potentials below the onset of the OER. Incorporation of Fe into the film increases the potential at which Ni(OH)2/NiOOH redox occurs and decreases the average oxidation state of Ni in NiOOH. The Tafel slope (40 mV dec(-1)) and reaction order in OH(-) (1) for the mixed Ni-Fe films (containing up to 95% Fe) are the same as those for aged Ni films. In situ Raman spectra acquired in 0.1 M KOH at OER potentials show two bands characteristic of NiOOH. The relative intensities of these bands vary with Fe content, indicating a change in the local environment of Ni-O. Similar changes in the relative intensities of the bands and an increase in OER activity are observed when pure Ni films are aged. These observations suggest that the OER is catalyzed by Ni in Ni-Fe films and that the presence of Fe alters the redox properties of Ni, causing a positive shift in the potential at which Ni(OH)2/NiOOH redox occurs, a decrease in the average oxidation state of the Ni sites, and a concurrent increase in the activity of Ni cations for the OER.

Enhanced activity of gold-supported cobalt oxide for the electrochemical evolution of oxygen.

Scanning electron microscopy, linear sweep voltammetry, chronoamperometry, and in situ surface-enhanced Raman spectroscopy were used to investigate the electrochemical oxygen evolution reaction (OER) occurring on cobalt oxide films deposited on Au and other metal substrates. All experiments were carried out in 0.1 M KOH. A remarkable finding is that the turnover frequency for the OER exhibited by ∼0.4 ML of cobalt oxide deposited on Au is 40 times higher than that of bulk cobalt oxide. The activity of small amounts of cobalt oxide deposited on Pt, Pd, Cu, and Co decreased monotonically in the order Au > Pt > Pd > Cu > Co, paralleling the decreasing electronegativity of the substrate metal. Another notable finding is that the OER turnover frequency for ∼0.4 ML of cobalt oxide deposited on Au is nearly three times higher than that for bulk Ir. Raman spectroscopy revealed that the as-deposited cobalt oxide is present as Co(3)O(4) but undergoes progressive oxidation to CoO(OH) with increasing anodic potential. The higher OER activity of cobalt oxide deposited on Au is attributed to an increase in fraction of the Co sites present as Co(IV) cations, a state of cobalt believed to be essential for OER to occur. A hypothesis for how Co(IV) cations contribute to OER is proposed and discussed.

Identification of highly active Fe sites in (Ni,Fe)OOH for electrocatalytic water splitting

Highly active catalysts for the oxygen evolution reaction (OER) are required for the development of photoelectrochemical devices that generate hydrogen efficiently from water using solar energy. Here, we identify the origin of a 500-fold OER activity enhancement that can be achieved with mixed (Ni,Fe)oxyhydroxides (Ni(1-x)Fe(x)OOH) over their pure Ni and Fe parent compounds, resulting in one of the most active currently known OER catalysts in alkaline electrolyte. Operando X-ray absorption spectroscopy (XAS) using high energy resolution fluorescence detection (HERFD) reveals that Fe(3+) in Ni(1-x)Fe(x)OOH occupies octahedral sites with unusually short Fe-O bond distances, induced by edge-sharing with surrounding [NiO6] octahedra. Using computational methods, we establish that this structural motif results in near optimal adsorption energies of OER intermediates and low overpotentials at Fe sites. By contrast, Ni sites in Ni(1-x)Fe(x)OOH are not active sites for the oxidation of water.

Theoretical investigation of the activity of cobalt oxides for the electrochemical oxidation of water

The presence of layered cobalt oxides has been identified experimentally in Co-based anodes under oxygen-evolving conditions. In this work, we report the results of theoretical investigations of the relative stability of layered and spinel bulk phases of Co oxides, as well as the stability of selected surfaces as a function of applied potential and pH. We then study the oxygen evolution reaction (OER) on these surfaces and obtain activity trends at experimentally relevant electro-chemical conditions. Our calculated volume Pourbaix diagram shows that β-CoOOH is the active phase where the OER occurs in alkaline media. We calculate relative surface stabilities and adsorbate coverages of the most stable low-index surfaces of β-CoOOH: (0001), (0112), and (1014). We find that at low applied potentials, the (1014) surface is the most stable, while the (0112) surface is the more stable at higher potentials. Next, we compare the theoretical overpotentials for all three surfaces and find that the (1014) surface is the most active one as characterized by an overpotential of η = 0.48 V. The high activity of the (1014) surface can be attributed to the observation that the resting state of Co in the active site is Co(3+) during the OER, whereas Co is in the Co(4+) state in the less active surfaces. Lastly, we demonstrate that the overpotential of the (1014) surface can be lowered further by surface substitution of Co by Ni. This finding could explain the experimentally observed enhancement in the OER activity of Ni(y)Co(1-y)O(x) thin films with increasing Ni content. All energetics in this work were obtained from density functional theory using the Hubbard-U correction.

Fischer-Tropsch synthesis over reduced and unreduced iron oxide catalysts

An investigation has been carried out of Fischer-Tropsch synthesis over potassium-promoted and unpromoted iron oxide catalysts. The distribution of products over reduced and unreduced Fe2O3 are similar. The primary products are methane and linear, terminal olefins. Some C+2 paraffins are produced in parallel with olefins, but most result from hydrogenation of the olefins. Isomerization of terminal to internal olefins also occurs as a secondary reaction. The Anderson-Schulz-Flory distribution of products is characterized by two branches differing in the value of α. The average molecular weight of the product increases with decreasing temperature, decreasing H2 partial pressure, and increasing CO partial pressure. Methanol is the only oxygenated product formed over unpromoted Fe2O3. Promotion of Fe2O3 with potassium suppresses the rates of olefin hydrogenation and isomerization, and the rate of methanol formation, but enhances the rates of formation of C+2 hydrocarbons, branched hydrocarbons, and aldehydes. The kinetics for the synthesis of C1C7 hydrocarbons were determined for both promoted and unpromoted catalysts. The data show that in general the kinetics of formation of an individual product cannot be represented by simple power-law rate expressions. Based on X-ray diffraction analyses of used catalysts, it is concluded that the phase of iron active for Fe2O3 Fischer-Tropsch synthesis is a mixture of χ- and e′-iron carbides, and a small amount of α-iron.

Catalytic Synthesis of Hydrocarbons over Group VIII Metals. A Discussion of the Reaction Mechanism

Abstract A broad range of hydrocarbons and oxygenated products can be synthesized catalytically from CO and H2 produced by the gasification of coal, Since the earliest examples of such chemistry were reported by Fischer and Tropsch [1, 2] in 1926, the nonselective generation of organic compounds via the hydrogenation of CO has become known as Fischer-Tropsch synthesis. In the intervening 54 years, many efforts have been made to identify the mechanism by which reactants are converted to products. These studies have been motivated in large measure by the desire to understand how catalyst composition and reaction conditions govern the distribution of products formed. A brief review of the mechanistic hypotheses developed prior to 1970 will be presented here to serve as background for a discussion of more recent ideas, Details concerning the earlier studies can be found in a number of reviews [3–15] published previously.

Efficient methods for finding transition states in chemical reactions : Comparison of improved dimer method and partitioned rational function optimization method

A combination of interpolation methods and local saddle-point search algorithms is probably the most efficient way of finding transition states in chemical reactions. Interpolation methods such as the growing-string method and the nudged-elastic band are able to find an approximation to the minimum-energy pathway and thereby provide a good initial guess for a transition state and imaginary mode connecting both reactant and product states. Since interpolation methods employ usually just a small number of configurations and converge slowly close to the minimum-energy pathway, local methods such as partitioned rational function optimization methods using either exact or approximate Hessians or minimum-mode-following methods such as the dimer or the Lanczos method have to be used to converge to the transition state. A modification to the original dimer method proposed by [Henkelman and Jonnson J. Chem. Phys. 111, 7010 (1999)] is presented, reducing the number of gradient calculations per cycle from six to four gradients or three gradients and one energy, and significantly improves the overall performance of the algorithm on quantum-chemical potential-energy surfaces, where forces are subject to numerical noise. A comparison is made between the dimer methods and the well-established partitioned rational function optimization methods for finding transition states after the use of interpolation methods. Results for 24 different small- to medium-sized chemical reactions covering a wide range of structural types demonstrate that the improved dimer method is an efficient alternative saddle-point search algorithm on medium-sized to large systems and is often even able to find transition states when partitioned rational function optimization methods fail to converge.

An XPS study of metal-support interactions on Pd/SiO2 and Pd/La2O3

Abstract The electronic properties of palladium supported on SiO 2 and La 2 O 3 were investigated by X-ray photoelectron spectroscopy. Spectra were collected after each stage of catalyst preparation: deposition of the palladium chloride precursor, oxidation in air at 623 K, and reduction in H 2 at 573 K. The Pd 3d 5 2 binding energies recorded following precursor deposition and oxidation were the same on both catalysts. However, after reduction the Pd 3d 5 2 binding energies of the Pd La 2 O 3 samples shifted below the corresponding values for metallic Pd, while the Pd 3d 5 2 binding energies of the Pd SiO 2 samples did not. Furthermore, the binding energies for the Pd La 2 O 3 samples decreased with increasing metal loading, the largest Pd particles apparently exhibiting the greatest interaction. A maximum shift of −0.7 eV relative to the Pd foil value was observed for 8.8% Pd La 2 O 3 . This binding energy shift is interpreted as arising from a change in the chemical state of Pd, i.e., that Pd supported on La 2 O 3 is more electronegative than zero-valent Pd alone. A model is proposed which suggests that a thin covering of the La 2 O 3 support lies on a portion of the Pd surface. This covering is partially reduced during H 2 treatment at 573 K and, because of the electropositive nature of La, the additional charge is distributed amongst the surface Pd atoms. It should be noted that the La 2 O 3 support surface after H 2 reduction is a complex mixture of La(OH) 3 , LaO(OH), La 2 O 3 , and La 2 (CO 3 ) 3 . The exact composition of the support depended strongly on the reduction temperature and the amount of Pd deposited.

Quantitative structural analysis of dispersed vanadia species in TiO2(anatase)-supported V2O5

V2O5-TiO2(anatase) catalysts have been studied under oxidizing and reducing conditions using in situ laser Raman spectroscopy (LRS) and temperature-programmed reduction (TPR) and oxidation (TPO). Quantitative Raman and TPO analysis of the oxidized samples show that these materials are comprised of a distribution of monomeric vanadyls, polymeric vanadates, and crystallites of V2O5. At low loadings, the predominant species are monomeric vanadyls, with the remaining vanadia being present in the form of polymeric vanadates. As the surface concentration of V2O5 increases, a maximum in the concentration of the polymeric vanadates is detected. Crystallites of V2O5 form at the expense of the polymeric vanadates as the loading is raised above the dispersive capacity of the TiO2(anatase) support. An equilibrium polymerization model is proposed to account for the observed concentration of vanadia species, which leads to an initial polymer size of ∼2 at low loadings, consistent with the formation of dimeric species. Raman and TPR/TPO studies of the reduction process indicate that the terminal V = 0 groups of the monomeric and polymeric vanadia species are removed preferentially to the bridging oxygen atoms of the polymeric species. The maximum loss of oxygen upon reduction is one oxygen atom per vanadium atom.

A growing string method for determining transition states: Comparison to the nudged elastic band and string methods

Interpolation methods such as the nudged elastic band and string methods are widely used for calculating minimum energy pathways and transition states for chemical reactions. Both methods require an initial guess for the reaction pathway. A poorly chosen initial guess can cause slow convergence, convergence to an incorrect pathway, or even failed electronic structure force calculations along the guessed pathway. This paper presents a growing string method that can find minimum energy pathways and transition states without the requirement of an initial guess for the pathway. The growing string begins as two string fragments, one associated with the reactants and the other with the products. Each string fragment is grown separately until the fragments converge. Once the two fragments join, the full string moves toward the minimum energy pathway according to the algorithm for the string method. This paper compares the growing string method to the string method and to the nudged elastic band method using the alanine dipeptide rearrangement as an example. In this example, for which the linearly interpolated guess is far from the minimum energy pathway, the growing string method finds the saddle point with significantly fewer electronic structure force calculations than the string method or the nudged elastic band method.