Relationship Between the Morphology and Catalytic Properties of Mn-Ni Multiphase Nanostructures for the Reduction of 4-Nitrophenol

Abstract

The catalytic reduction of 4-nitrophenol (4-NP) requires highly efficient and cost-effective materials, making Mn-Ni nanostructures a promising candidate. In this study, Mn-Ni nanoparticles with distinct morphologies (specifically spheres, stars, and core–shell structures) were synthesized by tuning the precursor composition. The structural and optical properties of the synthesized catalysts were characterized via electron microscopy and UV-Vis spectroscopy. Kinetic evaluations of the 4-NP reduction demonstrated that the core–shell architecture yielded the highest catalytic activity, outperforming both the star-shaped and spherical nanoparticles. Furthermore, the high-index facets at the tips of the nanostars appeared to have contributed to enhanced catalytic activity by providing additional active surface sites for the reduction process. Ultimately, this work suggests that structural morphology and interfacial interactions play important roles in determining catalytic performance, rather than absolute surface area, providing valuable insights into the design of future bimetallic catalysts.

1. Introduction

4-Nitrophenol (4-NP) is a hazardous anthropogenic pollutant commonly found in industrial wastewater, originating from the production of pesticides, dyes, and explosives [1,2]. Beyond the urgent need for environmental remediation, the catalytic reduction of 4-NP yields 4-Aminophenol (4-AP), a commercially vital intermediate in the synthesis of antipyretic drugs such as paracetamol (acetaminophen) and other analgesic compounds [3]. Consequently, the development of efficient catalytic systems for this transformation is of significant industrial and environmental interest. Historically, noble metal nanoparticles (NPs) such as Au, Pd, and Ag have served as the benchmark catalysts for this reduction due to their exceptional activity and stability [4,5]. Recent studies have further refined the understanding of noble metal-catalyzed 4-nitrophenol reduction at the nanoscale. For example, Pd₆ nanoclusters exhibit high catalytic efficiency, where activity is strongly dependent on ligand environment and surface accessibility. These findings highlight that catalytic performance is not governed solely by composition, but is also strongly influenced by interfacial structure, surface chemistry, and the availability of active sites in ultrasmall metallic systems [6]. However, the high cost and scarcity of these noble metals severely limit their scalability for large-scale wastewater treatment. To address this, significant research has shifted toward earth-abundant 3d transition metals, particularly Ni, Co, and Cu, which offer a sustainable alternative. While cost-effective, monometallic transition metals often suffer from lower intrinsic activity or rapid passivation compared to their noble counterparts [7]. To overcome these limitations, bimetallic engineering and heterostructure design have emerged as a promising strategy. Such approaches not only reduce costs by minimizing or entirely eliminating noble metal content, but they also induce synergistic effects. In these systems, the interaction between two distinct metals modifies the electronic structure and geometric arrangement of the active sites, yielding catalytic performance superior to that of either monometallic component. Previous studies have demonstrated that coupling metals with different electronegativities can induce beneficial charge transfer, which alters the local electronic structure to optimize reactant binding; therefore, we selected Mn and Ni to investigate these synergistic effects [8,9].

Mn-Ni-based materials present a unique and underexplored opportunity because manganese is highly abundant and possesses distinct redox properties [10]. Mn-Ni alloys and related Mn-Ni-based systems have been widely studied for their mechanical and magnetic properties, as well as their applications in sensing and phase transformation control. These systems are synthesized using various methods, including auto-combustion, high-temperature pyrolysis, epitaxy, and ion irradiation [11,12,13,14]. Beyond these physical properties, a key area of research is the catalytic activity of Mn-Ni-based systems [15,16,17]. However, to the best of our knowledge, limited research has addressed their catalytic performance in degrading the widely known pollutant 4-NP. The reduction of 4-NP by NaBH₄ serves as an ideal model reaction for evaluating catalytic activity due to its distinct spectroscopic signature, while simultaneously addressing the remediation of a common industrial pollutant. Consequently, the goal of this work is to fabricate Mn-Ni nanoparticles to understand how varying the bimetallic ratio dictates morphology, and subsequently, to investigate the effect of the Mn-Ni system on the catalytic reduction of 4-NP. Unlike noble metals, the inevitable surface oxidation of Mn species may facilitate the formation of oxides, and not only a pure bimetallic of Mn-Ni which will also be a point of investigation [18].

2. Materials and Methods

2.1. Reagents

Nickel chloride (NiCl₂), manganese chloride (MnCl₂), poly(diallyldimethylammonium chloride) (PDDA), sodium hydroxide (NaOH), 4-nitrophenol (4-NP), diethylhydroxylamine (DEHA), hydrazine, and ethylene glycol (EG). All reagents were bought from Sigma Aldrich (St. Louis, MO, USA) and were used without further purification.

2.2. Synthesis of MnNi NPs

Two precursor solutions (0.1 M NiCl₂ and 0.1 M MnCl₂) were prepared in 1 mL of water. Subsequently, 40 µL of PDDA and 90 µL of NaOH (1 M) were added to 5 mL of ethylene glycol. Strongly basic conditions were established by the addition of NaOH (pH values 9–12). The mixture was then heated to 160 °C for 5 min, followed by heating at 220 °C for 10 min.

Morphology control was achieved by varying the Ni:Mn precursor ratio. For spherical particles, 300 µL NiCl₂ and 150 µL MnCl₂ were added dropwise; for star-like structures, 200 µL NiCl₂ and 200 µL MnCl₂ were added dropwise; and for core–shell structures, 200 µL NiCl₂ and 300 µL MnCl₂ were added dropwise. After 5 min, 50 µL DEHA and 50 µL hydrazine were introduced as reducing agents. The reaction temperature was then increased to 240 °C and maintained for 5 min. The mixture was then cooled to room temperature.

Heating was performed using a conventional heating mantle without measurement of the ramp rate; only target temperatures and dwell times were defined.

The resulting nanoparticles were purified by repeated centrifugation with ethanol (four cycles) to remove residual PDDA and by-products, and finally redispersed in ethanol.

2.3. Characterization

The Mn-Ni NPs were characterized through scanning electron microscopy (SEM) using a Thermo Scientific Phenom Pure G6 Desktop SEM (Waltham, MA, USA) to visualize the morphology. The crystalline structure was determined through transmission electron microscopy (TEM) using a Thermo Scientific Talos F200i (Waltham, MA, USA) microscope operated at 200 keV.

2.4. UV-Vis 4-Nitrophenol Degradation

A stock solution (S1) containing NaBH₄ (0.3 M) and 4-NP (1 mM) was prepared in 5 mL of water. The reaction was conducted in a quartz cuvette by mixing 500 µL of S1 with 1 mL of water. The reaction was initiated by introducing aliquots of Mn-Ni nanostructures, and the reduction of 4-NP to 4-AP was monitored using Thermo Scientific Evolution One Plus UV–Vis Spectrophotometer (Waltham, MA, USA). Completion of the reaction was determined by the disappearance of the absorption peak at ~400 nm.

A control experiment without a catalyst was performed under identical conditions, showing negligible reduction of 4-nitrophenol within the experimental timeframe.

The catalytic experiments were repeated to confirm the consistency of the observed catalytic trends. Surface area analysis was not performed in this study. However, identical synthesis and reaction conditions were used across all samples, allowing comparative evaluation of catalytic performance based on morphology and structure.

3. Results and Discussion

3.1. Morphology and Structural Composition

Mn-Ni nanostructures were synthesized in the presence of a stabilizing agent (PDDA) in an EG solution. Different MnCl₂:NiCl₂ concentrations (1:2, 1:1, and 3:2) and pH values (9–12) were used. Hydrazine was added as a reducing agent. The solution immediately turned black after the process, indicating the formation of Mn-Ni nanostructures. The schematic of the Mn-Ni nanostructure synthesis methodology is shown in Figure 1.

These variations in Ni and Mn composition drove the formation of three distinct structures, core–shell particles, stars, and spheres, captured in the TEM images of Figure 2. The morphological images from the TEM further highlighted nanoscale architecture, showing the biphasic boundary of the core–shell structures and the branched tips characteristic of the star morphology.

The particle size distributions for the three morphologies were determined via statistical analysis of the SEM images (Figure 3). As illustrated in the histograms of Figure 3, the core–shell structures exhibited the largest average diameter (approximately 340 nm) and a relatively broad distribution ranging from 100 to 600 nm. This larger size reflects the total particle diameter, encompassing both the core and the outer shell. In contrast, the star-shaped particles displayed a narrower distribution with a mean diameter of 285 nm. The spheres represented the smallest and most monodisperse structures, characterized by a mean diameter of 150 nm. All samples followed a Gaussian distribution, as indicated by the overlaid fit curves in Figure 3. Contrary to the conventional paradigm where smaller nanoparticles exhibit higher catalytic activity [19,20], our core–shell structures and stars outperformed the smaller spheres despite their larger diameters (340 nm vs. 150 nm). This deviation from the expected surface-area-to-volume relationship suggests that other structural factors are dominant in this system, as discussed later in this work.

The Selected Area Electron Diffraction (SAED) patterns obtained from each of the nanoparticles exhibit distinct Debye-Scherrer rings, characteristic of a polycrystalline nature (Figure 4). In Figure 4a, the reflections from the inner core for the core–shell structure suggest a cubic MnNi₃ structure (space group No. 221). The interplanar spacings (d-spacing) values of 2.52 Å, 2.10 Å, 1.56 Å, and 1.29 Å correspond to the (011), (111), (012), and (022) planes, respectively. Conversely, reflections from the outer shells of the core–shell architecture represented in Figure 4b are consistent with Mn₂O₃ oxide phases; d-spacings of 3.868 Å, 2.289 Å, and 1.933 Å align with the (112), (104), and (224) planes, respectively.

The d-spacing of the stars in Figure 4c are consistent with a disordered FCC solid solution of MnNi 2.06 Å, 1.44 Å and 1.22 Å corresponding to (111), (200), and (220). In addition to the primary reflections associated with the star-structured system, supplementary rings observed at the tips of the stars (Figure 4d) had the following d-spacings of 1.95Å, 1.69 Å, 1.21 Å, and 1.01 Å. These values align closely with the d-spacings of MnO₂ and Mn₃O₄ suggesting partial oxidation of the bimetallic nanoparticles, likely forming a thin oxide shell or secondary phase.

The measured d-spacings of the spheres (Figure 4e) are 3.43 Å, 2.8 Å, 2.12 Å, 1.84 Å and 1.46 Å and correspond closely to the (001), (100), (101) (110) and (102) planes of the tetragonal system with the chemical formula MnNi in the P4/mmm space group. With all these preliminary structures, further stoichiometric analysis is required for definitive phase identification.

3.2. Catalytic Activity and the Reduction of 4-Nitrophenol to 4-Aminophenol

The reduction reaction followed the chemical equation described in Equation (1).

$${\mathrm{C}}_6{\mathrm{H}}_4\left({\mathrm{N}\mathrm{O}}_2\right)\mathrm{O}\mathrm{H}+{3\mathrm{N}\mathrm{a}\mathrm{B}\mathrm{H}}_4+{\mathrm{H}}_2\mathrm{O}\stackrel{\mathrm{M}\mathrm{n}\text{-}\mathrm{N}\mathrm{i}}{\to } {\mathrm{C}}_6{\mathrm{H}}_4\left({\mathrm{N}\mathrm{H}}_2\right)\mathrm{O}\mathrm{H}+{3\mathrm{N}\mathrm{a}\mathrm{B}\left(\mathrm{O}\mathrm{H}\right)}_4$$

(1)

The reduction of 4-NP by NaBH₄ in the presence of Mn-Ni catalysts is commonly described in the literature as proceeding via a Langmuir–Hinshelwood-type mechanism, involving adsorption of both reactants onto the catalyst surface followed by surface-mediated electron transfer [21]. In the present system, we propose that the Mn-Ni surface provides active sites for the adsorption of both borohydride ions and 4-NP, enabling electron transfer and subsequent reduction (Figure 5). However, detailed mechanistic pathways are not directly probed in this study and are therefore discussed qualitatively.

Upon addition of NaBH₄, the characteristic absorption peak of 4-NP at 318 nm shifts to 400 nm due to the formation of 4-nitrophenolate ions under alkaline conditions, as shown in Figure 6a. Following catalyst addition, the intensity of the 400 nm peak progressively decreases, while a new peak emerges at approximately 300 nm, corresponding to the formation of 4-AP [22]. No significant conversion was observed in the absence of a catalyst, confirming that the reaction is catalytically driven by the Mn-Ni nanostructures. This is further supported by the control experiment (4-NP + NaBH₄), where the absorbance at ~400 nm remained constant over time, as shown in the inset of Figure 6a.

Based on these spectral changes, the catalytic performance of the three morphologies was compared. The core–shell nanoparticles exhibit the highest activity, with complete disappearance of the 400 nm peak within ~140 s. Star-like structures show intermediate activity with completion at ~210 s, while spherical nanoparticles require ~340 s for full conversion, as shown in Figure 6b–d.

Within Mn-Ni bimetallic systems, catalytic activity arises from the complementary roles of the two components. Ni-rich phases are expected to facilitate electron transfer due to their metallic conductivity, while Mn species contribute to metal–metal interaction and electronic modulation of the Ni environment. In some morphologies (stars and core–shell structures), partial surface oxidation of Mn is observed, which can further enhance adsorption and activation of reactant molecules. This intrinsic combination enables synergistic catalytic behavior across Mn-Ni nanostructures [23,24].

All synthesized morphologies exhibit Mn-Ni-based catalytic functionality; however, differences in structural architecture and surface chemistry lead to variations in catalytic performance.

3.3. Conversion Efficiency and Stability

The reduction reaction was analyzed using pseudo-first-order kinetics, as described in Equation (2), due to the large excess of NaBH₄ relative to 4-nitrophenol.

$$\ln \left({\displaystyle \frac{A}{A_{max}}}\right)=-k_{obs}t$$

(2)

Kinetic analysis revealed a pronounced, morphology-dependent trend in the reaction rate. The pseudo-first-order rate constants (k_obs) demonstrated that the core–shell nanoparticles exhibited the highest catalytic activity (0.0393 s⁻¹). This value is approximately 1.7 times higher than that of the star-like structures (0.0227 s⁻¹) and over 2.5 times higher than that of the spherical nanoparticles (0.0153 s⁻¹), with all three morphologies achieving near-complete conversion (Figure 7).

The pseudo-first-order kinetic model was applied under excess NaBH₄ conditions to enable comparative evaluation of catalytic activity across different morphologies. The resulting coefficient of determination (R²) values were 0.944, 0.821, and 0.8084 for the respective samples. These values reflect the degree of linearity within the selected kinetic regime and were used to compare apparent reaction rates across the different catalysts. Deviations from ideal linear behavior are commonly observed in heterogeneous catalytic systems, particularly for multiphase nanostructured catalysts, where surface heterogeneity and adsorption phenomena can influence apparent kinetics. Importantly, the kinetic analysis was employed for comparative benchmarking of catalytic performance rather than strict mechanistic validation.

Mass-normalized kinetic constants were not determined due to the difficulty in defining the accessible active surface area across morphologically distinct nanostructures; however, identical catalyst loadings were used for all experiments to ensure valid comparison.

To place the catalytic performance of the Mn-Ni nanostructures in context, a comparison with representative catalysts reported for the reduction of 4-nitrophenol was conducted (

Table 1. Comparison of catalytic performance for 4-NP reduction catalysts reported in the literature.

Catalyst System	Morphology/Structure	k (s−1)	Reference
Au-Ag	Spherical	0.0103	[25]
Au/CoMoS2	Flowerlike nanoclusters	0.01772	[26]
Au/Silica	Core–shells	0.00417	[27]
Ni-MoS2	Flowerlike nanoclusters	0.0181	[28]
Mn-Ni spheres	Spherical	0.0153	This work
Mn-Ni Stars	Branched/Stellated	0.0227	This work
Mn-Ni core–shell	Core–shells	0.0393	This work

). Noble metal systems such as Au-based and Ag-based nanoparticles typically exhibit rapid kinetics with rate constants in the range of 0.01 to 0.1 s⁻¹ under comparable NaBH₄ excess conditions, while earth-abundant transition metal systems generally show lower or comparable activity depending on structure and dispersion. The Mn-Ni core–shell architecture developed in this work exhibits competitive performance within this range, highlighting the effectiveness of morphology and interfacial engineering in enhancing catalytic activity. It should be noted that variations in catalyst loading, reaction conditions, and support effects across literature systems limit strict quantitative comparison; however, the observed trends can provide a meaningful benchmark for evaluating catalytic efficiency.

Conversion efficiency quantifies the extent to which the initial 4-nitrophenol (4-NP) was successfully reduced to 4-aminophenol (4-AP) by the conclusion of the reaction. Assuming a linear relationship between absorbance and concentration as per the Beer-Lambert law, the conversion efficiency was calculated using the relative decrease in the 4-nitrophenolate absorbance peak at 400 nm ($A_0$ and $A_t$ representing initial and instantaneous absorbance, respectively).

These values are determined using Equation (3):

$$Conversion \%=\left({\displaystyle \frac{A_0- A_t}{A_0}}\right)\ast 100$$

(3)

Although the samples exhibited distinct reaction rates (Figure 7a), all three morphologies achieved near-complete conversion (Figure 7b). The core–shell structures reached the 95% conversion threshold within only 100 s, while the stars and spheres required 180 and 290 s, respectively (Figure 7b). These results highlight the high catalytic efficacy of the Mn-Ni systems; furthermore, precise tailoring of morphology presents a clear pathway toward achieving even higher efficiencies and accelerated reaction rates.

Star-like nanostructures, with their arms and branches, provide a higher effective surface area and a greater density of accessible active sites compared to smooth spherical particles, contributing to their intermediate catalytic activity [29]. In contrast to the common expectation that smaller nanoparticles always exhibit higher catalytic activity due to increased surface area, the core–shell architecture shows superior performance despite its relatively larger size. These results highlight clear morphology-dependent differences in catalytic performance. A more detailed discussion of the core–shell mechanism, including structural and interfacial contributions, is provided in the following section.

3.4. Core–Shell Activity

The highest catalytic activity is observed for the core–shell nanoparticles. Structural characterization suggests a heterogeneous architecture comprising a manganese oxide-rich shell and a Ni–Mn core containing an intermetallic compound. Inferred from SAED analysis, the coexistence of multiple crystallographic contributions, including features consistent with Mn₂O₃ and an MnNi₃-type intermetallic structure (Pm-3m, space group 221). These observations support a multiphase core–shell-type architecture rather than a single homogeneous phase. The formation of this structure is likely driven by the higher oxygen affinity of Mn, which promotes preferential surface oxidation during synthesis or post-synthesis exposure, leading to the formation of a Mn₂O₃-enriched shell [30]. Concurrently, the subsurface region becomes enriched in Ni, where Mn and Ni can adopt a more ordered or partially ordered arrangement, consistent with an MnNi₃-type intermetallic core as inferred from SAED analysis. Although direct spatially resolved elemental mapping was not performed, the coexistence of oxide-related and intermetallic-type diffraction features, together with the observed particle morphology, supports the formation of a core–shell-type heterostructure. While electronic structure measurements were not carried out in this study, the presence of an oxide/intermetallic interface is expected to introduce interfacial strain and electronic redistribution. Such interfacial effects have been widely reported to influence adsorption energetics through modification of surface electronic states [31,32,33,34]. We therefore suggest that interfacial coupling between Mn₂O₃ and the underlying Mn-Ni core contributes to the enhanced catalytic activity observed in the core–shell nanoparticles. In particular, the enhanced activity can be attributed to interfacial effects involving both electronic charge transfer and lattice strain at the oxide–metal interface. Charge redistribution between the Mn₂O₃ shell and the Mn-Ni core is expected to modify the electronic structure of surface-active sites, thereby optimizing adsorption strength and facilitating electron transfer during the catalytic reaction. In addition, the Mn oxide phases are proposed to play a dual role, acting as adsorption and activation sites for reactant species while simultaneously modulating the electronic properties of adjacent metallic sites through interfacial coupling. The combination of these effects provides a synergistic framework for the improved catalytic performance of the core–shell architecture.

4. Conclusions and Future Work

This work demonstrated the successful synthesis of diverse Mn-Ni morphologies, including spheres, stars, and core–shell architectures, for the catalytic reduction of 4-NP to 4-AP. The core–shell structures exhibited superior catalytic efficiency, outperforming spherical nanoparticles despite their lower surface area-to-volume ratio. This indicates that catalytic performance in Mn-Ni systems may be governed more strongly by structural architecture and interfacial effects than by surface area alone, particularly in the core–shell configuration. Based on SAED analysis and the proposed structural core–shell model, interfacial strain effects may further contribute to the modulation of the electronic structure of active sites, although the precise contributions of these effects cannot be resolved from the present dataset. While these catalysts show high activity for 4-NP reduction under controlled laboratory conditions, their practical use is more suited to point-source treatment applications such as industrial wastewater remediation rather than complex in situ environmental systems.

Future studies should focus on deeper structural and mechanistic validation of these findings. In particular, post-reaction characterization would be valuable to assess possible phase evolution or surface reconstruction during catalysis. Surface area analysis was not performed in this work; inclusion of such analysis in future work would help further clarify the relative contributions of surface area and interfacial effects. Mass-normalized kinetic parameters were also not determined due to the difficulty in defining accessible surface areas across morphologically distinct nanostructures; future work incorporating surface measurements would enable more rigorous comparison. Furthermore, catalyst recyclability and long-term structural stability were not investigated, as each experiment was conducted using freshly prepared samples. Future work should examine reusability and structural durability. Finally, more detailed spectroscopic or computational studies would be beneficial to further elucidate the respective roles of Mn and Ni and to directly validate proposed interfacial effects such as charge transfer and lattice strain.

Overall, these results demonstrate that structural morphology and interfacial interactions influence the catalytic performance of Mn-Ni systems in the reduction of 4-NP to 4-AP, offering useful insights into the design of future bimetallic catalysts.