Extract of natural plants is one of the most important metallic corrosion inhibitors. They are readily available, nontoxic, environmentally friendly, biodegradable, highly efficient, and renewable. The present project focuses on the corrosion inhibition effects of Peganum Harmala leaf extract. The equivalent circuit with two time constants with film and charge transfer components gave the best fitting of impedance data. Extraction of active species by sonication proved to be an effective new method to extract the inhibitors. High percent inhibition efficacy IE% of 98% for 283.4 ppm solutions was attained using impedance spectroscopy EIS measurements. The values of charge transfer Rct increases while the double layer capacitance Cdl values decrease with increasing Harmal extract concentration. This indicates the formation of protective film. The polarization curves show that the Harmal extract acts as a cathodic-type inhibitor. It is found that the adsorption of Harmal molecules onto the steel surface followed Langmuir isotherm. Fourier-transform infrared spectroscopy FTIR was used to determine the electron-rich functional groups in Harmal extract, which contribute to corrosion inhibition effect. Scanning electron microscopy SEM measurement of a steel surface clearly proves the anticorrosion effect of Harmal leaves.

Recently, carbon nanostructures are of high importance due to their unique characteristics and interesting applications. Pyrolysis of anthracene with cobalt complex Co(2,2′-bipy)Cl2(1), where (2,2′-bipy) is 2,2′-bipyridine, in the absence and presence of silica gave in high yield cobalt-carbon nanocomposite CoCNC (2) and CoCNC@SiO2(3) at 600 °C and 850 °C, respectively. They were characterized using SEM, TEM, PXRD, Raman and XPS. (3) and (2) contain core–shell cobalt(0)/cobalt oxide-graphite with or without silica support. PXRD indicates that (2) contains crystalline hexagonal α-Co and cubic β-Co phases while (3) contains only cubic β-Co phase and silica. The structure of (2) is 3D hierarchical carbon architecture wrapping spherical and elliptical cobalt nanoparticles. (3) consists of graphitized structures around cobalt nanoparticles embedded in the silica matrix. XPS reveals that the nanocomposites contain oxygen functional groups that enhance uptake of cationic dyes. CoCNC@SiO2(3) has higher capacity and thus is better adsorbent of Basic Violet 3 than CoCNC (2). The Langmuir adsorption capacity of (3) is 19.4 mg g−1 while column capacity is 12.55 mg g−1 at 25 °C. Freundlich isotherm and pseudo-second-order kinetic models fit well the adsorption data. Thermodynamics indicate that adsorption(3) is exothermic. Column regeneration was tested for three cycles and Yan et al. was found the best kinetic model.

A 3D hierarchical graphitic carbon nanostructure encapsulating cobalt(0)/cobalt oxide nanoparticles (CoGC) has been prepared by solid-state pyrolysis of a mixture of anthracene and cobalt 2,2′-bipyridine terephthalate complex at 850 °C. Based on the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods, the prepared material has high surface area (186.8 m 2 g –1 ) with an average pore width of 205.5 Å. XPS reveals the functionalization of carbon with different oxygen-containing groups, such as carboxylic acid groups. The presence of metallic cobalt nanoparticles with cubic and hexagonal crystalline structures encapsulated in graphitized carbon is confirmed using XRD and TEM. Raman spectroscopy indicates a graphitization degree of I D / I G = 1.02. CoGC was cast onto a glassy carbon electrode and used for urea electrooxidation in an alkaline solution. The electrochemical investigation shows that the newly prepared CoGC has a promising electrocatalytic activity toward urea. The specific activity is 128 mA cm –1 mg –1 for the electrooxidation of 0.3 M urea in 1 M KOH at a relatively low onset potential (0.31 V vs Ag/AgCl). It can be mainly attributed to the morphological structure of carbon and the high reactivity of cobalt nanoparticles. The calculated charge-transfer resistance, R ct , of the modified electrode in the presence of urea (10.95 Ω) is significantly lower than that in the absence of urea (113.5 Ω), which indicates electrocatalytic activity. The value of charge-transfer rate constant, k s , for the anodic reaction is 0.0058 s –1 . Electrocatalytic durability in 1000 s chronoamperometry of the modified electrode suggests high structure stability. The modified electrode retained about 60% of its activity after 100 cycles as indicated by linear sweep voltammetry.

3D hierarchical graphitic carbon nanowalls encapsulating cobalt nanoparticles HPC-Co were prepared in high yield from solid-state pyrolysis of cobalt 2,2′-bipyridine chloride complex. Annealing of HPC-Co in air gave HPC-CoO, which consists of a mixture of crystallite Co3O4 nanospheres and nanorods bursting out of mesoporous carbon. Both nanocomposites were fully characterized using SEM, TEM, BET, and powder X-ray diffraction. The elemental composition of both nanocomposites examined using SEM elemental mapping and TEM elemental mapping supports the successful doping of nitrogen. The powder X-ray diffraction studies supported the formation of hexagonal cobalt in HPC-Co, and cubic crystalline Co3O4 with cubic cobalt in HPC-CoO. HPC-Co and HPC-CoO can be used as a modified carbon electrode in cyclic voltammetry experiments for the detection of fructose with limit of detection LOD 0.5 mM. However, the single-frequency impedimetric method has a wider dynamic range of 8.0–53.0 mM and a sensitivity of 24.87 Ω mM−1 for the electrode modified with HPC-Co and 8.0–87.6 mM and a sensitivity of 1.988 Ω mM−1 for the electrode modified with HPC-CoO. The LOD values are 3 and 4 mM, respectively. The effect of interference increases in the following order: ascorbic acid, ethanol, urea, and glucose. A simple method was used with negligible interference from glucose to measure the percentage of fructose in a corn syrup sample with an HPC-CoO electrode. A specific capacitance of 47.0 F/g with 76.6% retentivity was achieved for HPC-Co and 28.2 F/g with 87.9% for HPC-CoO for 3000 charge–discharge cycles. Thus, (1) has better sensitivity and specific capacitance than (2), because (1) has a higher surface area and less agglomerated cobalt nanoparticles than (2).

The corrosion inhibition effect of the three extracts from Harmal roots (HRE), leaves (HLE), and flowers (HFE) were studied for carbon steel corrosion inhibition in 0.25 M H2SO4 solution. The electrochemical impedance study indicated that the three types of extracts decreased corrosion effectively through a charge transfer mechanism. Harmal roots and leaf extracts showed inhibition values of 94.1% and 94.2%, while it was 88.7% for Harmal flower extract at the inhibitor concentration of 82.6 ppm. Potentiodynamic polarization data revealed that Harmal extracts acted through predominant cathodic type inhibition. Both the corrosion current density and corrosion rate decreased significantly in the presence of Harmal extracts compared to blank solution. The corrosion rate (mpy) value was 63.3, 86.1, and 180.7 for HRE, HLE, and HFE, respectively. The adsorption-free energy change ΔGads (kJ·mol−1) values calculated from the Langmuir adsorption isotherm plots were for HRE (−35.08), HLE (−33.17), and HFE (−33.12). Thus, corrosion inhibition occurred due to the adsorption of Harmal extract on the carbon steel surface via the chemisorption mechanism. Moreover, a computational investigation using B3LYP/6-311G++(d,p) basis set in both gaseous and aqueous phases was performed for the major alkaloids (1–8) present in the Harmal extract.

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