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Article

Electro-Oxidation of Ammonia over Copper Oxide Impregnated γ-Al2O3 Nanocatalysts

by
Safia Khan
1,
Syed Sakhawat Shah
1,
Mohsin Ali Raza Anjum
2,*,
Mohammad Rizwan Khan
3 and
Naveed Kausar Janjua
1,*
1
Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan
2
Chemistry Division, Directorate of Science, Pakistan Institute of Nuclear Science and Technology, Nilore, Islamabad 45650, Pakistan
3
Advanced Materials Research Chair, Chemistry Department, College of Science, King Saud University, Riyadh 11451, Saudi Arabia
*
Authors to whom correspondence should be addressed.
Coatings 2021, 11(3), 313; https://doi.org/10.3390/coatings11030313
Submission received: 31 January 2021 / Revised: 26 February 2021 / Accepted: 5 March 2021 / Published: 9 March 2021
(This article belongs to the Special Issue Synthesis, Chracterization and Applications of Coated Composite Materials for Energy Applications)
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Abstract

:
Ammonia electro-oxidation (AEO) is a zero carbon-emitting sustainable means for the generation of hydrogen fuel, but its commercialization is deterred due to sluggish reaction kinetics and the poisoning of expensive metal electrocatalysts. With this perspective, CuO impregnated γ-Al2O3 (CuO/γ-Al2O3) hybrid materials were synthesized as effective and affordable electrocatalysts and investigated for AEO in alkaline media. Structural investigations were performed via different characterization techniques, i.e., X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electrochemical impedance spectroscopy (EIS). The morphology of γ-Al2O3 support as interconnected porous structures rendered the CuO/γ-Al2O3 nanocatalysts with robust activity. The additional CuO impregnation resulted in the enhanced electrochemical active surface area (ECSAs) and diffusion coefficient and spiked the electrocatalytic performance for NH3 electrolysis. Owing to good values of diffusion coefficient for AEO, low bandgap, and availability of ample ECSA at higher CuO to γ-Al2O3 ratio, these proposed electrocatalysts were proved to be effective in AEO. Due to good reproducibility, electrochemical stability, and higher activity for ammonia electro-oxidation, CuO/γ-Al2O3 nanomaterials are proposed as efficient promoters, electrode materials, or catalysts in ammonia electrocatalysis.

1. Introduction

Ammonia (NH3) is a corrosive, pungent and carcinogenic inorganic gaseous pollutant and is being produced from both biogenic and anthropogenic sources, i.e., livestock waste, animal agriculture, refrigeration, nitrogen fertilizer and petroleum refining industries [1,2]. Many efforts have been devoted to removing NH3 from gaseous and waste streams through chemical, biological, and physical methods, but all have their limitations [2,3,4]. Recently, electrocatalytic oxidation of ammonia (AEO) has attracted much attention of researchers and scientists because, as a hydrogen-rich carrier, it possesses 70% higher volumetric hydrogen content than pure liquid hydrogen [5,6]. Theoretical and experimental investigations reveal that hydrogen generation via ammonia electro-oxidation (AEO) is a cost-effective approach compared to water electrolysis because it requires much lower oxidation potential (0.06 V) than water (−1.23 V), as described in Equations (1–3) [7,8]. Theoretical ammonia electrolysis consumes energy of about ~1.55 Wh·g−1 H2 at standard conditions, which is 95% less than water electrolysis [9]. Due to fast reaction kinetics and modest operating conditions, ammonia electro-oxidation is considered a promising future technology to produce hydrogen from ammonia polluted wastewaters [10,11]. At 25 °C, the ammonia electrooxidation potential is −0.77 V vs. Standard Hydrogen Electrode (SHE), merely 0.06 V less negative than the oxidation potential of −0.83 V vs. SHE for hydrogen evolution in alkaline solution [12].
Anode reaction: NH3(aq) + 3OH− → 1/2N2 + 3H2O + 3e− → Eanode = −0.77 V vs. SHE
Cathode reaction: 3H2O + 3e− → 3/2H2 + 3OH− → Ecathode = −0.83 V vs. SHE
Overall reaction: NH3(aq) → 1/2N2 + 3/2H2 → ΔE = 0.06 V
Electro-oxidation of ammonia prompts the inhibition of the oxygen evolution reaction (OER) because of adsorption of the products in result of ammonia oxidation on the electrode surface. The OER inhibition boosts the AEO reaction, which is the main reaction. The amino radicals as intermediates, formed in the course of ammonia oxidation, trigger a reaction chain where dissolved molecular oxygen is involved in the AEO [13]. Nitrogen present as ammonia or ionized ammonium or both forms deplete dissolved oxygen through oxidation in aqueous solutions, which boost the AEO reaction [14].
Moreover, innovation in the design and fabrication of reasonable and proficient electrocatalysts for AEO are essential for its successful implementation. To commercialize this direct ammonia fuel cell (DAFC) technology, an efficient, stable, and economical electrocatalyst is required. Thus far, expensive noble metal-based (Pt, Ru, Ir, Rh, Pd) alloys are considered as the best performing AEO catalysts [5,6,7,8,15,16]. Sluggish reaction kinetics of AEO, high cost and low resistance to poisoning by reaction intermediates of these catalysts hinder this technology in commercialization. Recently, some effort has been devoted to developing non-noble metal electrocatalysts including NiO-TiO2 [17] and CuO-TiO2 [1] for AEO, but they require higher overpotential. Similarly, Ni oxides and hydroxides-based catalysts have displayed better performance towards AEO; however, they get easily corroded and deactivated in ammonia solution [9,17,18]. Although it is proved theoretically that copper displays comparable activity to Pt, it forms too weak a bond with Ni than Pt, leading to a very high overpotential [19]. Therefore, it displays poor catalytic performance toward AOR experimentally [20]. Cupric oxide (CuO) is a promising p-type semiconductor and has been investigated in various applications such as photocatalytic hydrogen production [21], dye degradation [22], electro-oxidation of hydrazine [23,24,25], methanol oxidation [26], and so on. Recently, the oxidation behavior of CuO-modified TiO2 was investigated for ammonia (0.5 M H2SO4 + 0.1 M NH3) by using linear sweep voltammetry [1]. However, extensive research is required to explore the cost-effective and efficient electrocatalysts to replace the incumbent Pt-based alloys [27]. Porous materials behave as potential electrodes owing to their high conductivity [28].
Furthermore, catalyst support materials exhibit great influence on the cost, performance, and durability of polymer electrolyte membrane (PEM) fuel cells. Due to high chemical/electrochemical stability, surface area and versatility, the high surface area metal oxides including alumina (γ-Al2O3), silica (SiO2), titania (TiO2) and zirconia (ZrO2) are considered as better catalyst supports than conventional carbon materials [29,30,31,32]. Besides, multiple types of transition alumina are recurrently used as supports for developing heterogeneous catalysts which consist of an active phase dispersed on a carrier or support. Alpha (α), beta (β) and gamma (γ) are three different types of alumina. The α-Al2O3, also known as nano-alumina, is a white puffy powder. It has lower specific surface area, limited high temperature resistance and it is inert, therefore it does not belong to activated alumina, and hence displays almost no catalytic activity. β-Al2O3 is hexagonal, with lamellar structure and the unit cell contains two alumina spinel-based block. It also exhibits low catalytic and strengthening properties. Among the different known alumina types, γ-Al2O3 is perhaps the most important with direct application as a catalyst support in the several industries. γ-Al2O3 possesses high purity and provides excellent dispersion and high specific surface area, offering commendable resistance to high temperature and high activity. It is porous; hence, it is said to be activated alumina and used as catalyst support as well as adsorbent [33]. The efficacy of γ-Al2O3 can be regarded to an auspicious combination of its textural possessions, e.g., surface area, pore volume, pore-size distribution and acid/base characteristics, which are mainly related to local microstructure, surface chemical composition and phase composition [34]. The highly porous γ-Al2O3 can be synthesized at moderate temperatures; therefore, it is widely used as support in many applications [22,26,35,36].
Herein, cost-effective and easily synthesized electrocatalysts (CuO/γ-Al2O3) were investigated for efficient ammonia electro-oxidation reaction in alkaline media. The CuO impregnated γ-Al2O3 nanomaterials were prepared by a simple impregnation-annealing process [26]. In such systems, impregnation of γ-Al2O3 with an aqueous active metal salt, preceded by drying and calcination, typically results in diffusion of active metal ions into the support surface, forming interaction species. Depending on the calcination temperature and time, only a finite amount of metal ions can be accommodated in the vacant lattice sites of the support. Once all of the available lattice sites are saturated, further addition of metal ions can be accommodated only by segregation of a separate metal oxide phase. The morphology of γ-Al2O3 support was modulated from plate-type to network-like by altering the CuO contents in compositions [37]. The electrocatalytic performance of as-synthesized CuO/γ-Al2O3 was observed by using cyclic voltammetry as the investigation tool. The prepared nanomaterials displayed high electrochemical active surface areas (ECSA), diffusion coefficients, and electrocatalytic activity for NH3 electro-oxidation. Also, the conductive and stable catalytic performance towards ammonia electro-oxidation is indebted to their low bandgaps. Increasing the CuO contents in CuO/γ-Al2O3 nanomaterials enhanced the catalytic performance because of the suppressed formation of active reaction intermediates as observed via CV profiles. The electrochemical stability and higher performance towards the ammonia electro-oxidation rendered the CuO/γ-Al2O3 nanomaterials as efficient electrocatalysts.

2. Experimental

2.1. Preparation of CuO/γ-Al2O3 Electrocatalysts

All chemicals were purchased from Sigma Aldrich, St. Louis, MO, USA. and were used without further purification. Firstly, the catalyst’s support alumina (γ-Al2O3) was prepared by calcination of pre-precipitated aluminum hydroxide (Al(OH)3). Typically, 36.2 g of aluminum nitrate nonahydrate (Al2(NO3)3·9H2O) was dissolved in distilled water (50 mL) and aqueous ammonia (35%) was added dropwise to obtain the white precipitates of aluminum hydroxide. Finally, as-prepared vacuum dried (80 °C) powder was calcined in NEY 2-525 furnace at 800 °C in the air for two hours to obtain γ-Al2O3. The copper oxide coated alumina (CuO/γ-Al2O3) nanomaterials were synthesized in two-steps; in the first step, as-synthesized γ-Al2O3 was impregnated in the required amount of aqueous solution of copper nitrate trihydrate (Cu(NO3)2·3H2O) for 48 h. and dried in an oven at 200 °C. The X-CuO/Al2O3 (X = 4, 8, 12, 16 and 20 wt.% of CuO) impregnated catalysts were obtained by calcination at 500 °C in the air for two hours as shown in Figure 1. The gradual variation in color of synthesized catalysts from bluish-white to bluish green was observed with incremental CuO content.

2.2. Electrochemical Investigations

Electrochemical analysis of all prepared electrocatalysts was carried out by using Gamry potentiostat interface 1000 and three-electrode system in which silver/silver chloride (Ag/AgCl), silver wire and modified glass carbon electrode are used as a reference, counter and the working electrode, respectively [38,39,40]. Ag/AgCl does not interfere with the system under investigation as it has been used as a reference electrode in ammonia and other basic media [41,42,43]. Silver wire is also used as a conductive counter electrode without affecting the system performance [44]. The glassy carbon electrode was polished with alumina slurry and cleaned with ethanol before dropping the catalyst ink that was prepared in ethanol while 5% Nafion (2.0 μL) solution was poured on the powder catalyst as a binder. The electrochemical active surface areas (ECSAs) of all electrocatalysts were determined in a standard redox solution (5.0 mM K4[Fe(CN)6] + 3.0 M KCl) at a scan rate of 100 mVs−1. Ammonia electro-oxidation was conducted by cyclic voltammetry (CV) in 1.0 M solution of NH3 prepared in 0.1 M KOH solution.

3. Results and Discussion

3.1. Structural Characterization

The phase purity and crystal structure of all prepared samples was determined by powder X-ray diffraction (PXRD) with a scan rate 0.4° per minute in 2θ window of 10°–70° via continuous scan type using PANalytical X’PERT High Score’s diffractometer, Malvern, UK. As shown in Figure 2a, the diffraction peaks at 32.4° and 36.7° are assigned to CuO which become more prominent as the loading of CuO is increased from 4% to 20%, while 37°, 61.5° and 68.1° corresponds to Al2O3. All these peaks have been indexed for pure cubic system and corresponded to JCPDS card No. 45-0937 [26]. There is no visible diffraction peak shift due to addition of Cu contents which indicates CuO species on the alumina surface exist as highly dispersed species [45]. Moreover, if 20%-CuO/Al2O3 is compared with pure alumina and 4%-CuO/Al2O3, an increase in peak intensity can be analyzed. Particle size increased from 2.5 in γ-Al2O3 to 16.2 nm in 20%-CuO/γ-Al2O3. However, this increment in particle size is not too much to affect the crystalline integrity of γ-Al2O3 matrix. The additional broadness of diffraction peaks at lower copper percentages is dedicated to more amorphous phase of composites [46].
Debye-Scherrer formula was used to estimate the average crystallite size, Equation (4):
Dav = K·λ/(β·cos θ)
Here, Dav is the average crystallite size in nm, λ is the wavelength of X-ray (1.5418 nm) while using Cu as an anode, θ is Bragg’s angle and β is the full width of the diffraction peak at half of the maximum. The calculated crystallite size for CuO is 14.6 nm which lies in the range between 10 to 30 nm as already reported [47,48,49,50]. The crystallite sizes of alumina increase from 2.5 nm (for pure γ-Al2O3) to 16.2 nm (20%-CuO/γ-Al2O3) as shown in Table 1. This increment in crystallite size was already observed in CuO/Al2O3 materials, which might be due to the insertion of CuO items into the crystal lattice of γ-Al2O3 [44]. In all samples, the CuO impregnation and calcination steps resulted in the increased crystallite size.
Fourier-transform infrared (FTIR) spectra of all materials were obtained by using Nicolet 5PC, Nicolet Analytical Instrument Protea, Cambridgeshire, UK and compared with that of the bare CuO in Figure 2b. The absorption signals at ~500, ~1300 and ~1670 cm−1 indicate the stretching vibration of the Cu–O bond [51]. This analysis further confirms the successful impregnation of CuO into γ-Al2O3 support. Our results indicate that, at low copper loadings (below 10%), Cu2+ ions form a well-dispersed interaction species with the support. At high copper loadings (after saturation of the support, i.e., >16% Cu), segregation of bulk-like CuO occurred. The crystallite size of this CuO phase increases with metal loading [52].

3.2. Surface Characterization

The surface morphology, porosity and effect of CuO impregnation on the textural nature of Al2O3 in all synthesized materials were investigated by scanning electron microscopy (SEM) using TESCAN (Brno, Czech Republic) MAIA3, i.e., an ultra-high-resolution SEM. As displayed in Figure 3, γ-Al2O3 showed plate-like morphology with large pores (Figure 3a–c at different magnifications) and significant dendrites distributed over the surface similar to the earlier reports [53,54]. Interestingly, these plates were converted to a porous network-like structure when the contents of CuO were increased in materials as shown in Figure 3d–h). The CuO nanoparticles are heterogeneously distributed on the surface of Al2O3 nanostructures. The energy dispersive spectrum corresponds to the presence and elemental purity of Al, Cu and O, as displayed in Figure 3h.
Low-magnification TEM nanostructures shown in Figure 4a–f are taken using TEM, JEOL 2100F (Tokyo, Japan), 200 kV to assess the surface picture of as-synthesized γ-Al2O3 and 16%-CuO/γ-Al2O3 matrices and if any agglomeration observed in the nanoparticles. TEM images for γ-Al2O3 shown in Figure 4 a and b displayed the globular structure of particles and the images corresponds with the already reported TEM nanostructures [55,56]. Figure 4c–f represents the TEM images for 16%-CuO/γ-Al2O3 and it is evident that incorporation of CuO into γ-Al2O3 matrix alters the surface structure. The modification in the γ-Al2O3 structure and a reconstructed composite surface is associated with dispersion of fine metal particles in catalysts [56]. Also, it can be perceived that the composite oxide is assembled from nearly uniform particles exhibiting similar shape, and significant porosity, and no big blocks are observed [57]. In such catalysts, CuO dispersed on the surface of the flake-like γ-Al2O3 generate synergy and coupling effects for ammonia adsorption [58].

3.3. Electrochemical Impedance Spectroscopy EIS

The electron transfer properties of all as-prepared nano-electrodes were studied via EIS in 1 M NH3 and 0.1 M KOH. The Nyquist plots witnessed for CuO/γ-Al2O3 modified glassy carbon electrodes are displayed in Figure 5, and associated EIS parameters are tabulated in Table 2. Systematically, the electron transfer resistance decreased with increase in copper content while again undergoing a decrease after an optimum composition. It endorsed that 16% CuO/γ-Al2O3 is optimal for electrochemical catalysis. A significantly lower value of Rct referred to the superior conductivity and much efficient electrocatalytic activity of 16%-CuO/γ-Al2O3, comparative to other composites of the series. The differences in electrochemical behavior of the as-synthesized electrocatalysts depend upon the relative feasibility of electron transferal [26].
The nature of electrodes exhibits no influence on solution resistance (Rs) and Warburg resistance (Rw) because these are features of electrolyte and diffusion of electroactive specie that are common in all observations. However, the charge-transfer resistance (Rct) and constant phase element (CPE) are influenced by modification of electrodes, as they are associated with conductive properties of the active material. α represents capacitance and surface roughness, respectively and its value varies from 0 to 1. Herein, currently modified electrode systems have α value ranging from 0.56 to 0.87, revealing that catalysts depicted enough surface roughness, which also correlates with the SEM and TEM observations. The electron-transfer rate constant kapp for as-proposed catalysts was calculated by Equation (5) [59].
kapp = RT/F2·Rct·C
Here, F represents the Faraday’s constant, C corresponds to concentration of analyte and R is universal constant in SI units. It is obvious from Table 2 that the value of kapp for 16%-CuO/γ-Al2O3 is greatest among the series of nanomaterials, referring to its highest capacity to assist the AEO reaction.

3.4. Active Surface Area of the CuO/γ-Al2O3 Modified Electrodes

The electrochemical active surface area (ECSA) of a catalyst is an important performance indicator of any electrochemical reaction; therefore, cyclic voltammograms of all prepared electrocatalysts were recorded in a standard redox solution (5.0 mM K4[Fe(CN)6] + 3 M KCl) at 100 mVs−1 for ECSA estimation. Peak current (ip) increment with CuO contents in the observed CV profile corresponded to a reversible one-electron transfer process using the synthesized nanomaterials as electron mediators in modified electrodes in K4[Fe(CN)6] electrolyte (Figure 6a). This observation of a reversible CV profile points to the facile electro kinetics in the model redox couple, which correlates the electrocatalytic behavior of the used materials. The peak currents corresponding to [Fe(CN)6]4− oxidation and peak current of [Fe(CN)6]3− reduction increase with the increase in the concentration of active CuO thus overall a diffusion-controlled process [60]. The ECSA of electrodes was calculated by applying the Randles-Sevcik Equation (6) [26].
ip = 2.69 × 105·n3/2·A·D1/2·υ½·C
Here, ip is the peak current, n is the number of electrons transferred, A is the electrochemical active surface area (cm2), D is the diffusion coefficient (0.76 × 10−5 cm2·s−1 at 25 °C), [61] υ is the scan rate (V·s−1) and C is the concentration of the analyte. ECSA of catalysts are compared in Figure 6b that increases in the following order: 4%-CuO/γ-Al2O3 < 8%-CuO/γ-Al2O3 < 12%-CuO/γ-Al2O3 < 20%-CuO/γ-Al2O3 < 16%-CuO/γ-Al2O3. This little drop in ECSA of 20%-CuO/γ-Al2O3 may affect from the agglomeration and consequent phase-out of CuO nanoparticles at its higher loading.

3.5. Electrochemical Studies

Electrocatalytic responses of all CuO/γ-Al2O3 materials towards ammonia electro-oxidation (AEO) was investigated by cyclic voltammetry in 1.0 M NH3 and 0.1 M KOH as displayed in Figure 6c,d. The peaks observed in both the forward (anodic) and reverse (cathodic) scans correspond to oxidative and reductive removal of chemisorbed species of ammonia, respectively [62]. The height of the anodic peak represents the electro-oxidation of ammonia on the surface of the electrodes. The anodic peak current increases with CuO contents in CuO/γ-Al2O3 nanomaterials; however, 16%-CuO/γ-Al2O3 shows the best performance towards AEO compared to other serial materials (Figure 6c), bare CuO and γ-Al2O3 (Figure 6d). The onset potential for AEO is found to be about −0.35 V vs. Ag/AgCl (i.e., −0.2 V vs. NHE) which is comparable to the Pt electrode [5,63,64,65].
To examine the electrode kinetics and the I–V responses towards AEO, voltammograms were also recorded for each catalyst at various scan rates from 10 to 100 mVs−1. As shown in Figure 7a–e, a linear increase in peak current of AEO at ~0.2 V is observed with a scan rate that indicates a facilitated electron transfer process of ammonia electro-oxidation. Therefore, the proposed nanocatalysts behave like the adsorptive species on the electrode surface [9]. The electrochemical response of each material towards AEO was enhanced with varying contents of active component, i.e., CuO in CuO/γ-Al2O3. This further confirms that CuO species present at the surface of catalysts play important role in AEO, more active sites available at the surface will maximize the ammonia adsorption thus leading to AEO high performance. The AEO strongly depends on the adsorption/desorption of NH3 species and the number of available active sites of active components at the surface of the electrode [66]. Similarly, two peaks in the cathodic curve are attributed to desorption of ammonia and proton at the surface of the catalysts, Figure 7 and Figure 8 [67]. The first cathodic peak is attributed to the reduction of reaction intermediates. Besides, the anodic and cathodic peak currents are also increased with the number of available active sites due to CuO. An anodic shoulder peak at −0.4 V can be attributed to the structural sensitivity of catalysts towards oxidation of pre-adsorbed hydrogen or nitrogen-containing intermediates at the surface of the electrode [68]. However, the anodic shoulder peak is suppressed by the ammonia oxidation peak when the contents of CuO are increased from 4% to 20% while the cathodic shoulder peaks increase because of the conversion/reduction of reaction intermediates. In this way, it can be said that irreversibility character increases with an increase in copper content up to an optimum level. As observed in Randles-Sevcik plots (Figure 7f), the peak current (ip) exhibits a linear relationship with the square root of scan rate (υ1/2), which is an indication of a diffusion-controlled process for AEO [67,68].
ECSA can be affected by the change of electrode layer structure; thus, 16%-CuO/γ-Al2O3 shows a higher peak current output than 20%-CuO/γ-Al2O3. After a certain limit, a further increase in CuO percentage leads towards agglomeration. Also, more ammonia molecules lead to an increase in diffusion-layer thickness, which results in a lower catalytic response in 20%-CuO/γ-Al2O3 [69]. The 16%-CuO/γ-Al2O3 is inferred to be the optimal loading of CuO for AEO under the optimal conditions. Subsequently, compositions above 20%-CuO/γ-Al2O3 were not studied to avoid the agglomerated material, as depicted by ECSA. Also, the same composition has been proved as the promising optimal loading of CuO onto γ-Al2O3 support for glucose sensing and methanol electro-oxidation [26].
To further assess the electrochemical activity of catalysts towards AEO, the electrochemical response of 16%CuO/γ-Al2O3 electrode was profiled by recording the cyclic voltammograms in pure 0.1 M KOH and 1.0 M NH3 solution. As displayed in Figure 8, no significant oxidation peak current is observed in 0.1 M KOH compared to 1.0 M NH3 as an analyte. This endorses the theory that the major peak corresponds to ammonia electro-oxidation rather than the electrolyte at the electrode surface [70]. The optimal onset potential for AEO is −0.35 V and the maximum oxidation peak current is observed at ~0.1 V. The electrochemical stability is a vital element in the commercialization of an electrocatalyst; therefore, it was determined by repeating CV cycles in 1.0 M NH3 + 0.1 M KOH at 100 mV·s−1. No significant current loss was observed after 10 CV cycles in Figure 8b, which indicates that the used catalysts give the stable performance towards AEO and can be commercialized for industrial applications. Concerning low CuO percentages, it is observed from Figure 8c that it also oxidizes ammonia to an extent, although current response is lower than that for high copper contents. Figure 8d gives a validation that a bare glassy carbon electrode when modified with 12%-CuO/γ-Al2O3, gave an oxidation output. Hence, it can be inferred that even a small amount of CuO can electro-catalyze ammonia oxidation.

3.6. Diffusion Coefficients for AEO on CuO/γ-Al2O3 Modified Electrodes

Diffusion coefficient (D) is an important parameter to study mass transfer kinetics, and it depends on various factors including analyte size and the concentration of electrolyte. Thus, the diffusion coefficients for ammonia (DNH3) in 0.1 M KOH were determined by applying the Randles-Sevcik formula (Equation (5)), which shows a linear relationship between the peak current and square root of the scan rate (υ1/2). The adsorption of ammonia occurs until the surface achieves saturation. Adsorbed nitrogen species cover and hence block the surface, whereas NH2(ads) and NH(ads) seem to be the active species, which recombine to form N2Hx (x = 2–4) species as reaction intermediates. These finally dehydrogenate to form N2. The recombination of NHx species is proposed to be the rate-determining step [71]. Generalized from overall ammonia electro-oxidation reaction (Equation (3)), three electrons (i.e., n = 3) are required for AEO. Moreover, quite similar ammonia oxidation voltammograms are reported earlier, describing it as a three-electron process [72]. The DNH3 using all the catalysts can be estimated from the slope of the Randles-Sevcik (ip vs. υ1/2) plot at a constant scan rate (100 mV·s−1) at 25 °C [73]. Resultantly, 16%-CuO/γ-Al2O3 gives the highest value of the diffusion coefficient as compared to the other materials Ni/Pt [65] and Ni/Ni(OH)2 [18], as enlisted in Table 3. Usually, the diffusion coefficient for AEO in KOH electrolyte (10−9) is much lower than that of water (2.4 × 10−5) [66] due to the presence of hydroxyl (OH−) ions in the electrolyte [20].

3.7. Estimation of Bandgap Values of CuO/γ-Al2O3 for Ammonia Electro-Oxidation

The electrochemical bandgap (Eg) and frontier orbitals energy levels (EHOMO and ELUMO) are important factors to understand the electrical and electrochemical properties of any material [74]. Although it is difficult to quantify the exact values for bandgaps, it can be estimated from onset potentials, as is widely done in the literature [75,76,77]. As described in mathematical expressions (Equations (7) and (8)), the onset potentials of oxidation (anodic) curve, i.e., (Eonset)ox and reduction (cathodic) curve, i.e., (Eonset)red linearly correlate with energies of frontier orbitals, HOMO (EHOMO) and LUMO (ELUMO), respectively [78].
EHOMO = −[(Eonset)ox + 4.4] eV
ELUMO = −[(Eonset)red + 4.4] eV
Here, the onset potentials are calibrated regarding the saturated calomel electrode [79]. As presented in Table 4, the estimated bandgap narrows by increasing the copper contents in materials from 0.98 eV (4%-CuO/γ-Al2O3) to 0.22 eV (16%-CuO/γ-Al2O3), which results in maximizing the conducting properties. Also, the bandgap decreases with the increase of crystallite size due to the quantum confinement effect [80,81]. Furthermore, the conductive electrocatalysts let ample electrons speed up the electrochemical reactions [82,83]. Therefore, in general, the materials containing higher CuO contents give better AEO performance due to the rapid transfer of electrons from the conduction band. Accordingly, the most conductive response is provided by the catalyst of optimal composition, i.e., 16%-CuO/γ-Al2O3.

4. Conclusions

The copper oxide modified γ-Al2O3 electrocatalysts (CuO/γ-Al2O3) were synthesized by the facile co-impregnation and calcination process. The CuO nanocrystals were successfully grown on γ-Al2O3 supports, as depicted by X-ray diffraction and FTIR. The shape of γ-Al2O3 support was changed from irregular to network-like by increasing the CuO contents, as observed in SEM images. The network-like structures coincided with the electrochemical active surface areas (ECSAs), which also varied with the CuO contents. The surface coarseness and homogeneity were seen in TEM images. Electrochemical characterization involved cyclic voltammetry and EIS that conferred to the absolute catalytic behavior of as-proposed electrocatalysts. The maximum ECSA and minimum charge-transfer resistance have been displayed by 16%-CuO/γ-Al2O3, which makes it the optimal composition. The effect of CuO contents was investigated in the catalytic performance of ammonia electro-oxidation (AEO) in alkaline media. It was observed that the AEO is an irreversible and diffusion-controlled process under alkaline conditions on the surface of CuO/γ-Al2O3 electrodes. Additionally, the suppressing of catalysts by nitrogenous species can be significantly reduced by increasing the CuO contents, as displayed by the continuous disappearance of anodic shoulder peak from lower to higher CuO loading. The successive increment in diffusion coefficients for NH3 with increasing CuO to γ-Al2O3 ratio showed that these materials can effectively electro-oxidize the NH3 due to facile electron transfer. Correspondingly, the prepared electrocatalysts demonstrated good electrocatalytic activity, reproducibility, and stability towards the ammonia electro-oxidation. Recent investigation of CuO/γ-Al2O3 electrocatalysts has revealed a large synergistic effect towards AEO at ambient temperatures. Therefore, these nanomaterials could be used as efficient electrocatalysts or promoters in AEO due to the higher diffusion coefficient for NH3, low bandgap, and chemical/electrochemical characteristics.

Author Contributions

S.K. and N.K.J. envisioned the study scheme and S.K. performed the experiments and the electrochemical measurements at Department of Chemistry, Quaid-I-Azam University Islamabad. N.K.J., M.A.R.A. and S.K. analyzed, discussed the results and wrote the manuscript. S.S.S. and N.K.J. are the supervisor and co-supervisor of S.K., respectively and did proof reading. SEM and EDS mapping were performed by M.A.R.A. while N.K.J. and M.A.R.A. are the corresponding authors of this paper. M.R.K. performed the TEM analysis. All authors have read and agreed to the published version of the manuscript.

Funding

The authors acknowledge the Fuel Cell Lab, Department of Chemistry, Quaid-i-Azam University, Islamabad that furnished this research. Also grateful to the Deanship of Scientific Research, King Saud University for this collaborative project through Vice Deanship of Scientific Research Chairs.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data is contained within article.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Dong, C.-D.; Chen, C.-W.; Kao, C.-M.; Hung, C.-M. Synthesis, characterization, and application of CuO-modified TiO2 electrode exemplified for ammonia electro-oxidation. Process Saf. Environ. Prot. 2017, 112, 243–253. [Google Scholar] [CrossRef]
  2. Yuan, M.-H.; Chen, Y.-H.; Tsai, J.-Y.; Chang, C.-Y. Ammonia removal from ammonia-rich wastewater by air stripping using a rotating packed bed. Process Saf. Environ. Prot. 2016, 102, 777–785. [Google Scholar] [CrossRef]
  3. Sheng, Y.; Fang, L.; Zhang, L.; Wang, Y. Aimed at building a healthy living environment: An analysis of performance of Clean-Air Heat Pump system for ammonia removal. Build. Environ. 2020, 171, 106639. [Google Scholar] [CrossRef]
  4. Ulu, F.; Kobya, M. Ammonia removal from wastewater by air stripping and recovery struvite and calcium sulphate precipitations from anesthetic gases manufacturing wastewater. J. Water Process. Eng. 2020, 38, 101641. [Google Scholar] [CrossRef]
  5. Barbosa, J.R.; Leon, M.N.; Fernandes, C.M.; Antoniassi, R.M.; Alves, Q.C.; Ponzio, E.A.; Silva, J.C.M. PtSnO2/C and Pt/C with preferential (100) orientation: High active electrocatalysts for ammonia electro-oxidation reaction. Appl. Catal. B Environ. 2020, 264, 118458. [Google Scholar] [CrossRef]
  6. Katsounaros, I.; Figueiredo, M.C.; Calle-Vallejo, F.; Li, H.; Gewirth, A.A.; Markovic, N.M.; Koper, M.T. On the mechanism of the electrochemical conversion of ammonia to dinitrogen on Pt (1 0 0) in alkaline environment. J. Catal. 2018, 359, 82–91. [Google Scholar] [CrossRef] [Green Version]
  7. Diaz, L.A.; Botte, G.G. Mathematical modeling of ammonia electrooxidation kinetics in a Polycrystalline Pt rotating disk electrode. Electrochimica Acta 2015, 179, 519–528. [Google Scholar] [CrossRef] [Green Version]
  8. Bayati, M.; Liu, X.; Abellan, P.; Pocock, D.; Dixon, M.; Scott, K. Synergistic Coupling of a Molybdenum Carbide Nanosphere with Pt Nanoparticles for Enhanced Ammonia Electro-Oxidation Activity in Alkaline Media. ACS Appl. Energy Mater. 2020, 3, 843–851. [Google Scholar] [CrossRef] [Green Version]
  9. Zhang, H.; Wang, Y.; Wu, Z.; Leung, D.Y. An ammonia electrolytic cell with NiCu/C as anode catalyst for hydrogen production. Energy Procedia 2017, 142, 1539–1544. [Google Scholar] [CrossRef]
  10. Li, L.; Liu, Y. Ammonia removal in electrochemical oxidation: Mechanism and pseudo-kinetics. J. Hazard. Mater. 2009, 161, 1010–1016. [Google Scholar] [CrossRef]
  11. Marinčić, L.; Leitz, F.B. Electro-oxidation of ammonia in waste water. J. Appl. Electrochem. 1978, 8, 333–345. [Google Scholar] [CrossRef]
  12. Botte, G.G.; Benedetti, L.; Gonzalez, J. Electrolysis of ammonia: An in situ hydrogen production process. In Proceedings of the 2005 AIChE Annual Meeting, Cincinnati, OH, USA, 30 October–4 November 2005. [Google Scholar]
  13. Michels, N.-L.; Kapałka, A.; Abd-El-Latif, A.; Baltruschat, H.; Comninellis, C. Enhanced ammonia oxidation on BDD induced by inhibition of oxygen evolution reaction. Electrochem. Commun. 2010, 12, 1199–1202. [Google Scholar] [CrossRef]
  14. Mahvi, A.H.; Ebrahimi, S.J.A.D.; Mesdaghinia, A.; Gharibi, H.; Sowlat, M.H. Performance evaluation of a continuous bipolar electrocoagulation/electrooxidation–electroflotation (ECEO–EF) reactor designed for simultaneous removal of ammonia and phosphate from wastewater effluent. J. Hazard. Mater. 2011, 192, 1267–1274. [Google Scholar] [CrossRef] [PubMed]
  15. Allagui, A.; Sarfraz, S.; Ntais, S.; Al Momani, F.; Baranova, E.A. Electrochemical behavior of ammonia on Ni98Pd2 nano-structured catalyst. Int. J. Hydrog. Energy 2014, 39, 41–48. [Google Scholar] [CrossRef]
  16. Zhang, Q.; Zhang, T.; Xia, F.; Zhang, Y.; Wang, H.; Ning, P. Promoting effects of acid enhancing on N2 selectivity for selectivity catalytic oxidation of NH3 over RuOx/TiO2: The mechanism study. Appl. Surf. Sci. 2020, 500, 144044. [Google Scholar] [CrossRef]
  17. Almomani, F.; Bhosale, R.; Khraisheh, M.; Kumar, A.; Tawalbeh, M. Electrochemical oxidation of ammonia on nickel oxide nanoparticles. Int. J. Hydrog. Energy 2020, 45, 10398–10408. [Google Scholar] [CrossRef]
  18. Kapałka, A.; Cally, A.; Neodo, S.; Comninellis, C.; Wächter, M.; Udert, K.M. Electrochemical behavior of ammonia at Ni/Ni(OH)2 electrode. Electrochem. Commun. 2010, 12, 18–21. [Google Scholar] [CrossRef]
  19. Xu, W.; Lan, R.; Du, D.; Humphreys, J.; Walker, M.; Wu, Z.; Wang, H.; Tao, S. Directly growing hierarchical nickel-copper hydroxide nanowires on carbon fibre cloth for efficient electrooxidation of ammonia. Appl. Catal. B Environ. 2017, 218, 470–479. [Google Scholar] [CrossRef]
  20. HHerron, J.A.; Ferrin, P.; Mavrikakis, M. Electrocatalytic oxidation of ammonia on transition-metal surfaces: A first-principles study. J. Phys. Chem. C 2015, 119, 14692–14701. [Google Scholar] [CrossRef]
  21. Kum, J.M.; Yoo, S.H.; Ali, G.; Cho, S.O. Photocatalytic hydrogen production over CuO and TiO2 nanoparticles mixture. Int. J. Hydrog. Energy 2013, 38, 13541–13546. [Google Scholar] [CrossRef]
  22. Hua, L.; Ma, H.; Zhang, L. Degradation process analysis of the azo dyes by catalytic wet air oxidation with catalyst CuO/γ-Al2O3. Chemosphere 2013, 90, 143–149. [Google Scholar] [CrossRef]
  23. Hosseini, S.R.; Kamali-Rousta, M. Preparation of electro-spun CuO nanoparticle and its application for hydrazine hydrate electro-oxidation. Electrochim. Acta 2016, 189, 45–53. [Google Scholar] [CrossRef]
  24. Firdous, N.; Janjua, N.K. CoPtx/γ-Al2O3 bimetallic nanoalloys as promising catalysts for hydrazine electrooxidation. Heliyon 2019, 5, e01380. [Google Scholar] [CrossRef] [Green Version]
  25. Ma, Y.; Li, H.; Wang, R.; Wang, H.; Lv, W.; Ji, S. Ultrathin willow-like CuO nanoflakes as an efficient catalyst for electro-oxidation of hydrazine. J. Power Sources 2015, 289, 22–25. [Google Scholar] [CrossRef]
  26. Mujtaba, A.; Janjua, N.K. Fabrication and electrocatalytic application of CuO@ Al2O3 hybrids. J. Electrochem. Soc. 2015, 162, H328–H337. [Google Scholar] [CrossRef]
  27. Firdous, N.; Janjua, N.K.; Qazi, I.; Wattoo, M.H.S. Optimal Co–Ir bimetallic catalysts supported on γ-Al2O3 for hydrogen generation from hydrous hydrazine. Int. J. Hydrog. Energy 2016, 41, 984–995. [Google Scholar] [CrossRef]
  28. Zhang, S.; Feng, D.; Shi, L.; Wang, L.; Jin, Y.; Tian, L.; Li, Z.; Wang, G.; Zhao, L.; Yan, Y. A review of phase change heat transfer in shape-stabilized phase change materials (ss-PCMs) based on porous supports for thermal energy storage. Renew. Sustain. Energy Rev. 2021, 135, 110127. [Google Scholar] [CrossRef]
  29. López, M.; Palacio, R.; Mamede, A.-S.; Fernández, J.J.; Royer, S. Hydrodeoxygenation of guaiacol into cyclohexane over mesoporous silica supported Ni–ZrO2 catalyst. Microporous Mesoporous Mater. 2020, 309, 110452. [Google Scholar] [CrossRef]
  30. Ramesh, A.; Tamizhdurai, P.; Krishnan, P.S.; Ponnusamy, V.K.; Sakthinathan, S.; Shanthi, K. Catalytic transformation of non-edible oils to biofuels through hydrodeoxygenation using Mo-Ni/mesoporous alumina-silica catalysts. Fuel 2020, 262, 116494. [Google Scholar] [CrossRef]
  31. Thou, C.Z.; Khan, F.S.A.; Mubarak, N.M.; Ahmad, A.; Khalid, M.; Jagadish, P.; Walvekar, R.; Abdullah, E.C.; Khan, S.; Khan, M.; et al. Surface charge on chitosan/cellulose nanowhiskers composite via functionalized and untreated carbon nanotube. Arab. J. Chem. 2021, 14, 103022. [Google Scholar] [CrossRef]
  32. Kashif, M.; Ngaini, Z.; Harry, A.V.; Vekariya, R.L.; Ahmad, A.; Zuo, Z.; Sahari, S.K.; Hussain, S.; Khan, Z.A.; Alarifi, A. An experimental and DFT study on novel dyes incorporated with natural dyes on titanium dioxide (TiO2) towards solar cell application. Appl. Phys. A 2020, 126, 1–13. [Google Scholar] [CrossRef]
  33. Paranjpe, K.Y. Alpha, Beta and Gamma Alumina as Catalyst. Pharma. Innov. J. 2017, 6, 236–238. [Google Scholar]
  34. Trueba, M.; Trasatti, S.P. γ-Alumina as a support for catalysts: A review of fundamental aspects. Eur. J. Inorg. Chem. 2005, 3393–3403. [Google Scholar] [CrossRef]
  35. Badri, M.T.; Barati, M.; Rasa, S.H. A kinetic study of acetone acidic oxidation with KMnO4 in the absence and presence of CuO/-Al2O3 as a heterogeneous nano-catalyst. Sci. Iran. 2020, 27, 1234–1242. [Google Scholar]
  36. Bradu, C.; Frunza, L.; Mihalche, N.; Avramescu, S.-M.; Neaţă, M.; Udrea, I. Removal of Reactive Black 5 azo dye from aqueous solutions by catalytic oxidation using CuO/Al2O3 and NiO/Al2O3. Appl. Catal. B Environ. 2010, 96, 548–556. [Google Scholar] [CrossRef]
  37. Firdous, N.; Janjua, N.K.; Wattoo, M.H.S. Promoting effect of ruthenium, platinum and palladium on alumina supported cobalt catalysts for ultimate generation of hydrogen from hydrazine. Int. J. Hydrog. Energy 2020, 45, 21573–21587. [Google Scholar] [CrossRef]
  38. Minakshi; Pundir, C.S. Construction of an amperometric enzymic sensor for triglyceride determination. Sens. Actuators B Chem. 2008, 133, 251–255. [Google Scholar] [CrossRef]
  39. Hooda, V.; Gahlaut, A. Amperometric cholesterol determination using HRP incorporated carbon paste electrode. Biosci. Biotechnol. Res. Asia 2020, 17, 53–64. [Google Scholar] [CrossRef]
  40. Odeyemi, C.S.; Awodugba, A.O. Improving the efficiency of titanium dioxide based dye sensitized solar cell (DSSC) using silver surface counter electrode. Futa J. Eng. Eng. Technol. 2018, 12, 1–6. [Google Scholar]
  41. Xu, W.; Du, D.; Lan, R.; Humphreys, J.; Miller, D.N.; Walker, M.; Wu, Z.; Irvine, J.T.; Tao, S. Electrodeposited NiCu bimetal on carbon paper as stable non-noble anode for efficient electrooxidation of ammonia. Appl. Catal. B Environ. 2018, 237, 1101–1109. [Google Scholar] [CrossRef]
  42. Strehlitz, B.; Gründig, B.; Kopinke, H. Sensor for amperometric determination of ammonia and ammonia-forming enzyme reactions. Anal. Chim. Acta 2000, 403, 11–23. [Google Scholar] [CrossRef]
  43. Kang, Y.; Wang, W.; Li, J.; Li, Q.; Liu, S.; Lei, Z. A highly efficient Pt-NiO/C electrocatalyst for ammonia electro-oxidation. J. Electrochem. Soc. 2017, 164, F958. [Google Scholar] [CrossRef]
  44. Schechner, P.; Kroll, E.; Bubis, E.; Chervinsky, S.; Zussman, E. Silver-plated electrospun fibrous anode for glucose alkaline fuel cells. J. Electrochem. Soc. 2007, 154, B942–B948. [Google Scholar] [CrossRef]
  45. Luo, M.F.; Fang, P.; He, M.; Xie, Y.L. In situ XRD, Raman, and TPR studies of CuO/Al2O3 catalysts for CO oxidation. J. Mol. Catal. A Chem. 2005, 239, 243–248. [Google Scholar] [CrossRef]
  46. Bagtache, R.; Saib, F.; Abdmeziem, K.; Trari, M. A new hetero-junction p-CuO/Al2O3 for the H2 evolution under visible light. Int. J. Hydrog. Energy 2019, 44, 22419–22424. [Google Scholar] [CrossRef]
  47. Azam, A. Size-dependent antimicrobial properties of CuO nanoparticles against Gram-positive and-negative bacterial strains. Int. J. Nanomed. 2012, 7, 3527–3535. [Google Scholar] [CrossRef] [Green Version]
  48. Li, Y.; Liang, J.; Tao, Z.; Chen, J. CuO particles and plates: Synthesis and gas-sensor application. Mater. Res. Bull. 2008, 43, 2380–2385. [Google Scholar] [CrossRef]
  49. Ren, G.; Hu, D.; Cheng, E.W.; Vargas-Reus, M.A.; Reip, P.; Allaker, R.P. Characterisation of copper oxide nanoparticles for antimicrobial applications. Int. J. Antimicrob. Agents 2009, 33, 587–590. [Google Scholar] [CrossRef] [PubMed]
  50. Baqer, A.A.; Matori, K.A.; Al-Hada, N.M.; Shaari, A.H.; Kamari, H.M.; Saion, E.; Chyi, J.L.Y.; Abdullah, C.A.C. Synthesis and characterization of binary (CuO)0.6(CeO2)0.4 nanoparticles via a simple heat treatment method. Results Phys. 2018, 9, 471–478. [Google Scholar] [CrossRef]
  51. Gholami, T.; Salavati-Niasari, M. Effects of copper: Aluminum ratio in CuO/Al2O3 nanocomposite: Electrochemical hydrogen storage capacity, band gap and morphology. Int. J. Hydrog. Energy 2016, 41, 15141–15148. [Google Scholar] [CrossRef]
  52. Strohmeier, B.R.; Levden, D.E.; Field, R.S.; Hercules, D.M. Surface spectroscopic characterization of CuAl2O3 catalysts. J. Catal. 1985, 94, 514–530. [Google Scholar] [CrossRef]
  53. Keshavarz, A.R.; Rezaei, M.; Yaripour, F. Nanocrystalline gamma-alumina: A highly active catalyst for dimethyl ether synthesis. Powder Technol. 2010, 199, 176–179. [Google Scholar] [CrossRef]
  54. Liu, Y.; Ma, D.; Han, X.; Bao, X.; Frandsen, W.; Wang, D.; Su, D. Hydrothermal synthesis of microscale boehmite and gamma nanoleaves alumina. Mater. Lett. 2008, 62, 1297–1301. [Google Scholar] [CrossRef] [Green Version]
  55. Schmücker, M.; Albers, W.; Schneider, H. Mullite formation by reaction sintering of quartz and α-Al2O3—A TEM study. J. Eur. Ceram. Soc. 1994, 14, 511–515. [Google Scholar] [CrossRef]
  56. Rozita, Y.; Brydson, R.; Scott, A.J. An investigation of commercial gamma-Al2O3 nanoparticles. J. Phys. Conf. Ser. 2010, 241, 012096. [Google Scholar] [CrossRef]
  57. Nie, M.-X.; Li, X.-Z.; Liu, S.-R.; Guo, Y. ZnO/CuO/Al2O3 composites for chloroform detection. Sens. Actuators B Chem. 2015, 210, 211–217. [Google Scholar] [CrossRef]
  58. Duan, N.; Liu, J.; Li, Q.; Xiao, H. Enhanced adsorption performance of CuO-Al2O3 composite derived from cotton template. Can. J. Chem. Eng. 2015, 93, 2015–2023. [Google Scholar] [CrossRef]
  59. Maximiano, E.M.; de Lima, F.; Cardoso, C.A.L.; Arruda, G.J. Incorporation of thermally activated zeolite into carbon paste electrodes for voltammetric detection of carbendazim traces in milk samples. J. Appl. Electrochem. 2016, 46, 713–723. [Google Scholar] [CrossRef]
  60. Muhammad, S.; Banin Zahra, U.; Ahmad, A.; Shah, L.A.; Muhammad, A. Understanding the basics of electron transfer and cyclic voltammetry of potassium ferricyanide-an outer sphere heterogeneous electrode reaction. J. Chem. Soc. Pak. 2020, 42, 813. [Google Scholar]
  61. Cossar, E.; Houache, M.S.; Zhang, Z.; Baranova, E.A. Comparison of electrochemical active surface area methods for various nickel nanostructures. J. Electroanal. Chem. 2020, 870, 114246. [Google Scholar] [CrossRef]
  62. Zhan, T.; Sun, X.; Wang, X.; Sun, W.; Hou, W. Application of ionic liquid modified carbon ceramic electrode for the sensitive voltammetric detection of rutin. Talanta 2010, 82, 1853–1857. [Google Scholar] [CrossRef]
  63. Zhang, X.-B.; Han, S.; Yan, J.-M.; Shioyama, H.; Kuriyama, N.; Kobayashi, T.; Xu, Q. Electrochemical oxidation of ammonia borane on gold electrode. Int. J. Hydrog. Energy 2009, 34, 174–179. [Google Scholar] [CrossRef]
  64. Le Vot, S.; Roué, L.; Bélanger, D. Synthesis of Pt–Ir catalysts by coelectrodeposition: Application to ammonia electrooxidation in alkaline media. J. Power Sources 2013, 223, 221–231. [Google Scholar] [CrossRef]
  65. Li, Z.-F.; Wang, Y.; Botte, G.G. Revisiting the electrochemical oxidation of ammonia on carbon-supported metal nanoparticle catalysts. Electrochim. Acta 2017, 228, 351–360. [Google Scholar] [CrossRef] [Green Version]
  66. Matinise, N.; Mayedwa, N.; Ikpo, C.O.; Hlongwa, N.W.; Ndipingwi, M.M.; Molefe, L.; Dywili, N.; Yonkeu, A.L.D.; Waryo, T.; Baker, P.G.L.; et al. Bimetallic nanocomposites of palladium (100) and ruthenium for electrooxidation of ammonia. J. Nano Res. 2016, 44, 100–113. [Google Scholar] [CrossRef]
  67. Álvarez-Ruiz, B.; Gómez, R.; Orts, J.M.; Feliu, J.M. Role of the metal and surface structure in the electro-oxidation of hydrazine in acidic media. J. Electrochem. Soc. 2002, 149, D35–D45. [Google Scholar] [CrossRef]
  68. Zhou, L.; Cheng, Y. Catalytic electrolysis of ammonia on platinum in alkaline solution for hydrogen generation. Int. J. Hydrog. Energy 2008, 33, 5897–5904. [Google Scholar] [CrossRef]
  69. Zhu, P.; Zhao, Y. Effects of electrochemical reaction and surface morphology on electroactive surface area of porous copper manufactured by Lost Carbonate Sintering. RSC Adv. 2017, 7, 26392–26400. [Google Scholar] [CrossRef] [Green Version]
  70. da Silva, S.W.; Navarro, E.M.O.; Rodrigues, M.A.S.; Bernardes, A.M.; Pérez-Herranz, V. The role of the anode material and water matrix in the electrochemical oxidation of norfloxacin. Chemosphere 2018, 210, 615–623. [Google Scholar] [CrossRef] [PubMed]
  71. Moran, E.; Cattaneo, C.; Mishima, H.; De Mishima, B.A.L.; Silvetti, S.P.; Rodriguez, J.L.; Pastor, E. Ammonia oxidation on electrodeposited Pt–Ir alloys. J. Solid State Electrochem. 2008, 12, 583–589. [Google Scholar] [CrossRef]
  72. Yao, K.; Cheng, Y. Electrodeposited Ni–Pt binary alloys as electrocatalysts for oxidation of ammonia. J. Power Sources 2007, 173, 96–101. [Google Scholar] [CrossRef]
  73. Khan, M.; Janjua, N.K.; Khan, S.; Qazi, I.; Ali, S.; Saad Algarni, T. Electro-oxidation of ammonia at novel Ag2O-PrO2/γ-Al2O3 Catalysts. Coatings 2021, 11, 257. [Google Scholar] [CrossRef]
  74. Diaz, L.A.; Valenzuela-Muñiz, A.; Muthuvel, M.; Botte, G.G. Analysis of ammonia electro-oxidation kinetics using a rotating disk electrode. Electrochim. Acta 2013, 89, 413–421. [Google Scholar] [CrossRef]
  75. Cheng, Y.-J.; Yang, S.-H.; Hsu, C.-S. Synthesis of conjugated polymers for organic solar cell applications. Chem. Rev. 2009, 109, 5868–5923. [Google Scholar] [CrossRef] [PubMed]
  76. Admassie, S.; Inganäs, O.; Mammo, W.; Perzon, E.; Andersson, M.R. Electrochemical and optical studies of the band gaps of alternating polyfluorene copolymers. Synth. Met. 2006, 156, 614–623. [Google Scholar] [CrossRef]
  77. Loganathan, K.; Pickup, P.G. Poly(Δ4,4′-dicyclopenta[2,1-b:3,4-b′] dithiophene–co-3,4-ethylenedioxythiophene): Electrochemically generated low band gap conducting copolymers. Electrochim. Acta 2005, 51, 41–46. [Google Scholar] [CrossRef]
  78. Jessop, I.; Zamora, P.P.; Díaz, F.R.; del Valle, M.A.; Leiva, A.; Cattin, L.; Makha, M.; Bernède, J.C. New polymers based on 2, 6-di (thiophen-2-yl) aniline and 2, 2’-(thiophen-2, 5-diyl) dianiline monomers. preparation, characterization and thermal, optical, electronic and photovoltaic properties. Int. J. Electrochem. Sci. 2012, 7, 9502–9517. [Google Scholar]
  79. Lohrman, J.; Zhang, C.; Zhang, W.; Ren, S. Semiconducting single-wall carbon nanotube and covalent organic polyhedron-C60 nanohybrids for light harvesting. Chem. Commun. 2012, 48, 8377–8379. [Google Scholar] [CrossRef]
  80. Rehman, S.; Mumtaz, A.; Hasanain, S.K. Size effects on the magnetic and optical properties of CuO nanoparticles. J. Nanoparticle Res. 2011, 13, 2497–2507. [Google Scholar] [CrossRef]
  81. Phoka, S.; Laokul, P.; Swatsitang, E.; Promarak, V.; Seraphin, S.; Maensiri, S. Synthesis, structural and optical properties of CeO2 nanoparticles synthesized by a simple polyvinyl pyrrolidone (PVP) solution route. Mater. Chem. Phys. 2009, 115, 423–428. [Google Scholar] [CrossRef]
  82. Nasir, M.H.; Janjua, N.K.; Santoki, J. Electrochemical performance of carbon modified LiNiPO4 as Li-ion battery cathode: A combined experimental and theoretical study. J. Electrochem. Soc. 2020, 167, 130526. [Google Scholar] [CrossRef]
  83. Butt, T.M.; Janjua, N.K.; Mujtaba, A.; Zaman, S.A.; Ansir, R.; Rafique, A.; Sumreen, P.; Mukhtar, M.; Pervaiz, M.; Yaqub, A.; et al. B-site doping in lanthanum cerate nanomaterials for water electrocatalysis. J. Electrochem. Soc. 2020, 167, 026503. [Google Scholar] [CrossRef]
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Figure 1. Graphical abstract for the synthesis of CuO/γ-Al2O3 nanomaterials for ammonia electro-oxidation.
Figure 1. Graphical abstract for the synthesis of CuO/γ-Al2O3 nanomaterials for ammonia electro-oxidation.
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Figure 2. X-ray diffraction patterns (a) and Fourier-transform infrared (FTIR) spectra (b) of pure γ-Al2O3 and CuO/Al2O3 nanomaterials.
Figure 2. X-ray diffraction patterns (a) and Fourier-transform infrared (FTIR) spectra (b) of pure γ-Al2O3 and CuO/Al2O3 nanomaterials.
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Figure 3. SEM micrographs for pure Al2O3 at 20 µm (a), 5 µm (b) and 2 µm (c) and for 4%–20%-CuO/γ-Al2O3 materials respectively at 1 µm (d–h) and (i) Energy dispersive spectrum for 4%-CuO/γ-Al2O3.
Figure 3. SEM micrographs for pure Al2O3 at 20 µm (a), 5 µm (b) and 2 µm (c) and for 4%–20%-CuO/γ-Al2O3 materials respectively at 1 µm (d–h) and (i) Energy dispersive spectrum for 4%-CuO/γ-Al2O3.
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Figure 4. TEM images for pure Al2O3 at 300 nm (a), 500 nm (b), and for 16%-CuO/γ-Al2O3 materials at 500 nm (c), 100 nm (d), 50 nm (e) and 5 nm (f).
Figure 4. TEM images for pure Al2O3 at 300 nm (a), 500 nm (b), and for 16%-CuO/γ-Al2O3 materials at 500 nm (c), 100 nm (d), 50 nm (e) and 5 nm (f).
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Figure 5. Electrochemical impedance spectroscopy (EIS) spectra for 4%–20%-CuO/γ-Al2O3 electrodes recorded in 1 M NH3 + 0.1 M KOH aqueous solutions. Inset represents the model fit for calculating parameters.
Figure 5. Electrochemical impedance spectroscopy (EIS) spectra for 4%–20%-CuO/γ-Al2O3 electrodes recorded in 1 M NH3 + 0.1 M KOH aqueous solutions. Inset represents the model fit for calculating parameters.
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Figure 6. Cyclic voltammetric profiles on 4%–20%-CuO/γ-Al2O3 modified electrodes materials in (5.0 mM K4[Fe(CN)6] + 3.0 M KCl) solution at scan ate of 100 mV·s−1 (a) and comparison of ECSA (b), Comparison of AEO in 0.1 M KOH + 1.0 M NH3 at 100 mV·s−1 (c) and 16%-CuO/γ-Al2O3 with bare CuO and γ-Al2O3 (d).
Figure 6. Cyclic voltammetric profiles on 4%–20%-CuO/γ-Al2O3 modified electrodes materials in (5.0 mM K4[Fe(CN)6] + 3.0 M KCl) solution at scan ate of 100 mV·s−1 (a) and comparison of ECSA (b), Comparison of AEO in 0.1 M KOH + 1.0 M NH3 at 100 mV·s−1 (c) and 16%-CuO/γ-Al2O3 with bare CuO and γ-Al2O3 (d).
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Figure 7. Cyclic voltammograms for 4%–20%-CuO/γ-Al2O3 recorded in (1.0 M NH3 + 0.1 M KOH) solution; scan rate from 10–100 mV·s−1 (a–e) and Randles-Sevcik plots derived from cyclic voltammetry (CV) curves of each catalyst (f).
Figure 7. Cyclic voltammograms for 4%–20%-CuO/γ-Al2O3 recorded in (1.0 M NH3 + 0.1 M KOH) solution; scan rate from 10–100 mV·s−1 (a–e) and Randles-Sevcik plots derived from cyclic voltammetry (CV) curves of each catalyst (f).
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Figure 8. Comparison of CV curves for 16%-CuO/γ-Al2O3 recorded pure 0.1 M KOH and solution of 1.0 M NH3 in 0.1 M KOH (a), Multiple cycles for 20%-CuO/γ-Al2O3 (b), Comparison of NH3 free voltammetry for 12%-CuO/γ-Al2O3 (c) and Comparison of 12%-CuO/γ-Al2O3 with bare glassy carbon electrode (d).
Figure 8. Comparison of CV curves for 16%-CuO/γ-Al2O3 recorded pure 0.1 M KOH and solution of 1.0 M NH3 in 0.1 M KOH (a), Multiple cycles for 20%-CuO/γ-Al2O3 (b), Comparison of NH3 free voltammetry for 12%-CuO/γ-Al2O3 (c) and Comparison of 12%-CuO/γ-Al2O3 with bare glassy carbon electrode (d).
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Table
Table 1. Average crystallite size calculated from Scherrer formula.
Table 1. Average crystallite size calculated from Scherrer formula.
ElectrocatalystDav (nm)
CuO14.6
γ-Al2O32.5
4%-CuO/γ-Al2O37.1
8%-CuO/γ-Al2O39.1
12%-CuO/γ-Al2O311.5
16%-CuO/γ-Al2O315.1
20%-CuO/γ-Al2O316.2
Table
Table 2. Parameters estimated from EIS analysis of electrocatalysts.
Table 2. Parameters estimated from EIS analysis of electrocatalysts.
ElectrocatalystsRs
(Ω)		Rct
(kΩ)		CPE
(µF)		αWo
(Ω)		kapp/10−9
(cms−1)
4%-CuO/γ-Al2O3816.023.922.570.8017.010.4
8%-CuO/γ-Al2O3770.622.72.120.8716.78.81
12%-CuO/γ-Al2O3968.022.22.260.8216.010.8
16%-CuO/γ-Al2O3736.03.9255.40.5613.961.2
20%-CuO/γ-Al2O3934.227.94.770.8423.68.73
Table
Table 3. Diffusion coefficients values for AEO using CuO/γ-Al2O3 modified electrodes in 0.1 M KOH at 25 °C.
Table 3. Diffusion coefficients values for AEO using CuO/γ-Al2O3 modified electrodes in 0.1 M KOH at 25 °C.
ElectrocatalystDNH3 10−9 (cm2s−1)
4%-CuO/γ-Al2O31.0
8%-CuO/γ-Al2O32.6
12%-CuO/γ-Al2O32.7
16%-CuO/γ-Al2O34.1
20%-CuO/γ-Al2O33.8
Ni/Pt [55]1.2
Ni/Ni(OH)2 [16]2.8
Ionic liquids [16]0.1
Table
Table 4. Electrochemical bandgap values derived from onset potentials vs. sat’d calomel electrode for AEO on the CuO/γ-Al2O3 materials.
Table 4. Electrochemical bandgap values derived from onset potentials vs. sat’d calomel electrode for AEO on the CuO/γ-Al2O3 materials.
CatalystE(ons)oxi (V)E(ons)red (V)EHOMO (eV)ELUMO (eV)Eg (eV)
4%-CuO/γ-Al2O3−0.64−0.25−3.16−4.140.98
8%-CuO/γ-Al2O3−0.43−0.13−3.56−4.310.75
12%-CuO/γ-Al2O3−0.53−0.21−3.98−4.690.71
16%-CuO/γ-Al2O3−0.41−0.18−3.90−4.200.22
20%-CuO/γ-Al2O3−0.32−0.06−3.89−4.300.41
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Khan, S.; Shah, S.S.; Anjum, M.A.R.; Khan, M.R.; Janjua, N.K. Electro-Oxidation of Ammonia over Copper Oxide Impregnated γ-Al2O3 Nanocatalysts. Coatings 2021, 11, 313. https://doi.org/10.3390/coatings11030313

AMA Style

Khan S, Shah SS, Anjum MAR, Khan MR, Janjua NK. Electro-Oxidation of Ammonia over Copper Oxide Impregnated γ-Al2O3 Nanocatalysts. Coatings. 2021; 11(3):313. https://doi.org/10.3390/coatings11030313

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Khan, Safia, Syed Sakhawat Shah, Mohsin Ali Raza Anjum, Mohammad Rizwan Khan, and Naveed Kausar Janjua. 2021. "Electro-Oxidation of Ammonia over Copper Oxide Impregnated γ-Al2O3 Nanocatalysts" Coatings 11, no. 3: 313. https://doi.org/10.3390/coatings11030313

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