“…Nonetheless, the CVs acquired in all supporting electrolytes display an anodic peak (or pair of peaks, in the case of acetate) at potentials < −0.9 V, which corresponds to the oxidation of Fe, dissolution of Fe II from the Fe nanoparticle surface, and formation of insoluble Fe(OH) 2 at the surface, along with competitive adsorption of Fe II −anion complexes. This voltammetric response is similar to that seen for iron electrodes in supporting electrolytes containing acetate anions, where Fe(OH) 2 precipitation has been shown to precede the competitive anion-adsorption step, giving rise to a pair of observed peaks rather than a single peak. − 2 CVs of nondoped CNTs collected after a 30 s potential hold at −1.5 V (−1.3 V for phosphate, bottom scan) versus Hg/Hg 2 SO 4 in various 1 M solutions of supporting electrolytes (labeled). Scan rate: 30 mV/s.…”
Section: Resultssupporting
confidence: 64%
“…This voltammetric response is similar to that seen for iron electrodes in supporting electrolytes containing acetate anions, where Fe-(OH) 2 precipitation has been shown to precede the competitive anion-adsorption step, giving rise to a pair of observed peaks rather than a single peak. [54][55][56][57] Along with the large anodic peaks observed upon Fe(OH) 2 precipitation and Fe II dissolution, changes in Fe II/III peaks were also evident, as shown in Figure 2 in citrate, phosphate, and acetate electrolytes. The peak potentials for Fe II/III in Figure 2 agree with those presented in Table 1.…”
Catalytically synthesized carbon nanotubes (CNTs) such as those prepared via chemical vapor deposition (CVD) contain metallic impurities including Fe, Ni, Co, and Mo. Transition metal contaminants such as Fe can participate in redox cycling reactions that catalyze the generation of reactive oxygen species and other products. Through the nature of the CVD growth process, metallic nanoparticles become encased within the CNT graphene lattice and may still be chemically accessible and participate in redox chemistry, especially when these materials are utilized as electrodes in electrochemical applications. We demonstrate that metallic impurities can be selectively dissolved and/or passivated during electrochemical potential cycling. Anomalous Fe dissolution and passivation behavior is observed in neutral (pH=6.40+/-0.03) aqueous solutions when using multiwalled CNTs prepared from CVD. Fe particles contained within these CNTs display intriguing, potential-dependent Fe redox activity that varies with supporting electrolyte composition. In neutral solutions containing dibasic sodium phosphate, sodium acetate, and sodium citrate, FeII dissolution and surface confined FeII/III redox activity are significant despite Fe being encapsulated within CNT graphene layers. However, no apparent Fe dissolution is observed in 1 M potassium nitrate solutions, suggesting that the electrolyte composition plays an important role in observing FeII dissolution, passivation, and surface confined FeII/III redox activity. Between potentials of 0 and -1.1 V versus Hg/Hg2SO4, the primary redox-active Fe species are surface FeII/III oxides/oxyhydroxides. This FeII/III surface oxide redox chemistry can be completely suppressed by passivating Fe through repeated cycling of the CNTs in supporting electrolyte. By increasing the potential to more negative values (>-1.3 V), FeII dissolution may be induced in electrolyte solutions containing acetate and phosphate and inhibited by addition of sodium benzoate, which adsorbs on exposed Fe particles, effectively passivating them. Finally, we observe that the FeII/III redox chemistry or subsequent passivation does not affect the onset of oxygen reduction at nitrogen-doped CNTs, suggesting that the surface-bound FeII species is not the primary catalytically active site for oxygen reduction in these materials.
“…Nonetheless, the CVs acquired in all supporting electrolytes display an anodic peak (or pair of peaks, in the case of acetate) at potentials < −0.9 V, which corresponds to the oxidation of Fe, dissolution of Fe II from the Fe nanoparticle surface, and formation of insoluble Fe(OH) 2 at the surface, along with competitive adsorption of Fe II −anion complexes. This voltammetric response is similar to that seen for iron electrodes in supporting electrolytes containing acetate anions, where Fe(OH) 2 precipitation has been shown to precede the competitive anion-adsorption step, giving rise to a pair of observed peaks rather than a single peak. − 2 CVs of nondoped CNTs collected after a 30 s potential hold at −1.5 V (−1.3 V for phosphate, bottom scan) versus Hg/Hg 2 SO 4 in various 1 M solutions of supporting electrolytes (labeled). Scan rate: 30 mV/s.…”
Section: Resultssupporting
confidence: 64%
“…This voltammetric response is similar to that seen for iron electrodes in supporting electrolytes containing acetate anions, where Fe-(OH) 2 precipitation has been shown to precede the competitive anion-adsorption step, giving rise to a pair of observed peaks rather than a single peak. [54][55][56][57] Along with the large anodic peaks observed upon Fe(OH) 2 precipitation and Fe II dissolution, changes in Fe II/III peaks were also evident, as shown in Figure 2 in citrate, phosphate, and acetate electrolytes. The peak potentials for Fe II/III in Figure 2 agree with those presented in Table 1.…”
Catalytically synthesized carbon nanotubes (CNTs) such as those prepared via chemical vapor deposition (CVD) contain metallic impurities including Fe, Ni, Co, and Mo. Transition metal contaminants such as Fe can participate in redox cycling reactions that catalyze the generation of reactive oxygen species and other products. Through the nature of the CVD growth process, metallic nanoparticles become encased within the CNT graphene lattice and may still be chemically accessible and participate in redox chemistry, especially when these materials are utilized as electrodes in electrochemical applications. We demonstrate that metallic impurities can be selectively dissolved and/or passivated during electrochemical potential cycling. Anomalous Fe dissolution and passivation behavior is observed in neutral (pH=6.40+/-0.03) aqueous solutions when using multiwalled CNTs prepared from CVD. Fe particles contained within these CNTs display intriguing, potential-dependent Fe redox activity that varies with supporting electrolyte composition. In neutral solutions containing dibasic sodium phosphate, sodium acetate, and sodium citrate, FeII dissolution and surface confined FeII/III redox activity are significant despite Fe being encapsulated within CNT graphene layers. However, no apparent Fe dissolution is observed in 1 M potassium nitrate solutions, suggesting that the electrolyte composition plays an important role in observing FeII dissolution, passivation, and surface confined FeII/III redox activity. Between potentials of 0 and -1.1 V versus Hg/Hg2SO4, the primary redox-active Fe species are surface FeII/III oxides/oxyhydroxides. This FeII/III surface oxide redox chemistry can be completely suppressed by passivating Fe through repeated cycling of the CNTs in supporting electrolyte. By increasing the potential to more negative values (>-1.3 V), FeII dissolution may be induced in electrolyte solutions containing acetate and phosphate and inhibited by addition of sodium benzoate, which adsorbs on exposed Fe particles, effectively passivating them. Finally, we observe that the FeII/III redox chemistry or subsequent passivation does not affect the onset of oxygen reduction at nitrogen-doped CNTs, suggesting that the surface-bound FeII species is not the primary catalytically active site for oxygen reduction in these materials.
“…This values also are according values present in literature which are of 1.8 x 10 -5 cm 2 s -1 for Co(II) 25 and 3.0 x10 -6 cm 2 s -1 for Fe. 37 The values of N and AN ¥ listed in Table 1, were used to plot the dependence of ln N or ln AN ¥ vs. E ( Figure 5). This figure shows that an increased E leads to a higher N and AN ¥ , but with a diminishing gradient and when E is high enough, N and AN ¥ will eventually saturate.…”
Section: Analysis Of the Current Maximummentioning
A eletrodeposição de Fe, Co e ligas de Fe-Co foi investigada a partir de soluções de cloreto em pH 2.0. As ligas Fe-Co foram depositadas a partir de soluções com razão molar de 1:10, 1:1 e 10:1. O processo de eletrocristalização dos metais foi estudado através de saltos potenciostáticos. O modelo de Scharifker e Hills foi utilizado para analisar os transientes de corrente. Os resultados experimentais obtidos para Co e ligas de Fe-Co, depositadas a partir da solução de 1:10 ajustaramse ao de um mecanismo de nucleação progressiva. No caso de Fe puro e ligas de Fe-Co, depositadas a partir das soluções 1:1 e 10:1, o processo de nucleação ocorreu pelo mecanismo de nucleação instantânea. A densidade do número de núcleos calculados variou de 2.6 a 19.1 x 10 3 cm-2 para Fe e a taxa de formação de núcleos para Co de 0.10 a 15.8 x 10 6 cm-2 s-1. The electrodeposition of Fe, Co and Fe-Co alloys onto Pt has been investigated from chloride solutions at pH 2.0. The Fe-Co alloys were deposited from solutions with molar ratio of 1:10, 1:1 and 10:1. Electrocrystallization of metals was studied by potentiostatic steps. The Scharifker and Hills model was used to analyze current transients. A progressive nucleation mechanism was found to fit the experimental results for both Co and Fe-Co alloys deposited from 1:10 solution. In the case of pure Fe and Fe-Co alloy, deposited from 1:1 and 10:1 solutions, the nucleation process occurred by the instantaneous mechanism. The nuclei number densities and rate of nucleation were calculated and their values were 2.6 to 19.1 x 10 3 cm-2 for Fe and 0.10 to 15.8 x 10 6 cm-2 s-1 for Co deposition.
“…This is one of the oldest and most widely used techniques in chemical reactions engineering. The rotating disk apparatus, also known as a rotating disk electrode (RDE) in the iron and steel industry, was used for iron dissolution and corrosion rate analysis. − Today there are different types of RDAs which are used in different scientific applications . The RDE was widely used before its application in the oil and gas industry to study the rock–acid reaction when designing stimulation jobs.…”
How best to measure molecular diffusion in a mixture is a topic that has been studied widely and is an essential element of various scientific disciplines. In the oil and gas industry, accurate acid diffusion measurement is vital to the design of acid treatments. In fact, predictions of acid penetration distance in hydraulic fractures and wormhole length during matrix acidization are not possible without accurate diffusion coefficient data or correlations. The present study explains and critically compares the various methods used to measure the acid diffusion coefficient, including the rotating disk apparatuses, core flooding, nuclear magnetic resonance, annular reactors, parallel plates, and diffusion cells. This study also summarizes the impacts of acid type, chemical additives, minerals, temperature, and pressure on diffusion measurement. Tabulated summaries of the diffusion coefficient values associated with various acid types are also provided. Popular correlations for predicting the acid diffusion coefficient are also thoroughly discussed. Finally, recommendations are provided regarding best practices for obtaining accurate acid diffusion measurement data.
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