Electrochemical Mechanism of Tellurium Reduction in Alkaline Medium

Abstract

A systematic electrochemical study was conducted to investigate the reduction of tellurium (Te) in alkaline solutions. The effect of various parameters, including tellurite ion concentration, applied potential, and pH was investigated by both linear sweep voltammograms (LSVs) and electrochemical quartz crystal microbalance (EQCM). EQCM was essential to understand the reduction of Te(0) to soluble Te 2− 2 (-I) or Te 2− (-II). The Tafel slopes for two Te reduction reactions [i.e., Te(IV) to Te(0) and Te(0) to Te(-I)] indicated that the electrochemical reduction of Te is strongly dependent on solution pH, whereas it is independent of the concentration of TeO 2− 3. At relatively weaker alkaline solutions (i.e., pH ≤ 12.5), the discharge of Te(OH) + 3 was determined to be the rate-limiting step during the reduction of Te(IV) to Te(0). For the reduction of Te(0) to Te(-I), the reaction follows a four-step reaction, which consisted of two discharge and two electrochemical reactions. The second discharge reaction was the rate-limiting step when pH ≤12.5 with the Tafel slope of 120 mV/decade. At a higher pH of 14.7, the Tafel slope was shifted to be 40 mV/decade, which indicated that the rate-limiting step was altered to the second electrochemical reaction. Te(0) deposits were found either on the surface of an electrode or in the solution depending on pH due to the different rate-limiting reactions, revealing that pH was a key parameter to dictate the morphology of the Te(0) deposits in alkaline media.

Role of Tellurium Ions for Electrochemically Synthesized Zinc Telluride 2D Structures on Nonconductive Substrate

“…Without an applied potential, tellurium ions (IV) (formed from the dissolution of TeO 2 in the electrolyte) were absorbed on the gold electrode and the SiO 2 . This adsorption is strong on a variety of electrode and oxide surfaces [ 53–55 ] With the application of a negative potential (Figure 1A), tellurium ions (HTeO 2 + (IV)) in the electrolyte were reduced to zero‐valent tellurium metal (0) on gold electrodes by the following reaction [ 50,52,55–60 ] $$\[ \begin{array}{*{20}{c}}{{\rm{HTeO}}_2^ + + 3{{\rm{H}}^ + } + 4{{\rm{e}}^ - } \to {\rm{Te}} + 2{{\rm{H}}_2}{\rm{O}}}\end{array} \]$$…”

“…TeO 2 in the electrolyte) were absorbed on the gold electrode and the SiO 2 . This adsorption is strong on a variety of electrode and oxide surfaces [53][54][55] With the application of a negative potential (Figure 1A), tellurium ions (HTeO 2 + (IV)) in the electrolyte were reduced to zero-valent tellurium metal (0) on gold electrodes by the following reaction [50,52,[55][56][57][58][59][60] HTeO 3H 4e Te 2H O 2 2…”

“…In the presence of telluryl ions (IV), this redox reaction rapidly occurs due to the instability of Te (−II) substances. [52,59,61] Also, the strong adsorption of telluryl ions (IV) on SiO 2 would help Te (−II) react mainly on the surface of the SiO 2 . As a result of this redox reaction on the SiO 2 , the tellurium metal is deposited on the surface of the SiO 2 insulator without direct current transport.…”

“…These studies observed Te chemically formed by indirect electrical analyses [50,51] or color change of electrolyte near the electrode coming from the black Te particle formation. [52] Although the Te particles were synthesized in the electrolyte homogeneously or on the electrode heterogeneously, chemically generated Te has been the focus of investigations into its deposition mechanisms rather than practical device applications.…”

Bioelectrocatalytic Reduction of Tellurium Oxyanions toward Their Cathodic Recovery: Concentration Dependence and Anodic Electrogenic Activity

“…Tellurium metal­(loid)­s are scarce in nature. , Tellurium principally occurs in four oxidation states as telluride (−II, Te 2– ), tellurite (+IV, TeO 3 2– ), tellurate (+VI, TeO 4 2– ), and elemental tellurium (0, Te 0 ). − Tellurium and its composites are mostly applicable in petroleum refining, electronics, electroplating, optics and glass, and sensor industries. ,, Tellurium exhibits various physicochemical properties, viz., catalytic activity, photoconductivity, gas sensing, and thermoelectric activity, which makes it essential for the above-mentioned specified industrial applications. , Tellurium has no known biological role in humans, but it is metabolized as a volatile gaseous compound and excreted in sweat with a characteristic “garlic” odor . The medical applications of Te 0 compounds include treatment of leprosy, dermatitis, cystitis, and eye infections, while they are also biologically involved in the inhibition of T-cell cytokine production and anti-sickling. , Expanded usage of tellurite oxyanions (TeO 3 2– and Te 4+ ) is highly toxic and leads to environmental contamination. − Te 4+ oxyanions (tellurite-TeO 3 2– and tellurate-TeO 4 2– ) are toxic and soluble, while the tellurium (Te 0 ) element is less toxic and insoluble in water. , Tellurite (TeO 3 2– ) is a strong oxidant and is essential to be detoxified/decontaminated by converting to the tellurium element (Te 0 /Te). ,− Te 0 is worth of its recovery using innovative and economically considerable processes due to its scarce nature.…”