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Article

Strengthen Air Oxidation of Refractory Humic Acid Using Reductively Etched Nickel-Cobalt Spinel Catalyst

by
Qi Jing
,
Jiabai Cai
and
Huan Li
*
Tsinghua Shenzhen International Graduate School, Tsinghua University, Shenzhen 518055, China
*
Author to whom correspondence should be addressed.
Catalysts 2022, 12(5), 536; https://doi.org/10.3390/catal12050536
Submission received: 8 April 2022 / Revised: 1 May 2022 / Accepted: 7 May 2022 / Published: 13 May 2022
(This article belongs to the Special Issue Catalysis in Treatment and Utilization of Organic Waste)
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Abstract

:
Nickel-cobalt spinel catalyst (NCO) is a promising catalyst for air oxidation of humic acid, which is a typical natural refractory organic matter and a precursor of toxic disinfection by-products. In this study, reductive etchers, NaBH4 or Na2SO3, were used to adjust the NCO surface structure to increase the performance. The modified catalyst (NCO-R) was characterized, and the relationship between its intrinsic properties and catalytic paths was discovered. The results of O2-temperature programmed desorption, NH3-temperature programmed desorption, and X-ray photoelectron spectroscopy (XPS) demonstrated that reductant etching introduced oxygen vacancies to the surface of NCO and increased active surface oxygen species and surface acidity. In addition, the modification did not change the raw hollow sphere structure of NCO. The crystallinity and specific surface area of NCO-R increased, and average pore size of NCO-R decreased. XPS results showed that the ratio of Co3+/Co2+ in NCO-R decreased compared with NCO, while the ratio of Ni3+/Ni2+ increased. The results of H2-temperature programmed reduction showed that the H2 reduction ability of NCO-R was stronger. Due to these changes in chemical and physical properties, NCO-R exhibited much better catalytic performance than NCO. In the catalytic air oxidation of humic acid at 25 °C, the total organic carbon (TOC) removal rate increased significantly from 44.4% using NCO to 77.0% using NCO-R. TOC concentration of humic acid decreased by 90.0% after 12 h in the catalytic air oxidation using NCO-R at 90 °C.

1. Introduction

Humic acid (HA) is the most abundant fraction of natural organic matter (NOM) [1], and widely exists in various organic wastewater, e.g., landfill leachate. HA is a typical refractory matter, and it is hardly biodegradable and cannot be decomposed in biological water or wastewater treatment. In membrane separation, HA plays a key role in membrane fouling [2]. In common advanced oxidation processes (AOPs) like chlorination, HA can be degraded into small molecular organics, which are probably the precursors for toxic by-products such as trihalomethane [3]. To some degree, HA has antioxidant properties due to its phenolic groups, which can protect other groups in HA from being oxidized through quenching photons and reactive oxygen species (ROS) [4]. HA also functions as an assignable factor for the removal of other pollutants. Due to its colloidal structure [4], HA is prone to form more toxic complex pollutants with pollutants such as heavy metals [5] and polycyclic aromatic hydrocarbons [6]. HA can terminate free radical chain reactions, inhibiting the degradation of other organic pollutants in natural water or wastewater [7].
The common processes for HA removal mainly include coagulation [8], adsorption [9], AOPs [8,10] and membrane filtration [11,12]. As mentioned above, HA is a negative factor for AOPs or membrane processes due to its oxidation resistance and structural characteristics [4]. Complicated post-treatment processes are needed in adsorption and coagulation processes. Therefore, two or more processes are combined to remove HA. Zhao et al. developed a catalytic CuFe2O4 tailored ceramic membrane with peroxymonosulfate as the oxidant for catalytic filtration of HA, which enhanced HA removal significantly and reduced irreversible fouling resistance in comparison to conventional ceramic filtration [13]. However, the introduction of strong oxidants and accessory equipment inevitably increases the cost of HA treatment. There is an urgent need for new processes that can remove or degrade HA efficiently and economically.
Catalytic air oxidation (CAO) processes, which are operated at temperatures lower than 100 °C and atmospheric pressure, are green and efficient for wastewater treatment. Backed by catalysts with excellent performance on adsorption, interfacial catalytic oxidation, and O2 activation, CAO has a promising development prospect. The catalysts in CAO are usually non-metal transition metals, such as Cu [14,15,16], Ni [16,17], Co [17,18], Fe [19,20], Mo [21], and Mn [22], mainly depending on their excellent interfacial catalytic oxidation ability. CAO is a green, efficient, and economical technology for the treatment of HA. Yang et al. developed a Fe-N-C oxidase-like catalyst for CAO, which made the removal rate of the total organic carbon (TOC) of HA solution reach 57.7% in 24 h (the initial TOC was 4 mg/L) [20]. Bao et al. developed a novel molybdenum-based nanocrystal to decorate ceramic membrane, and the catalyst enhanced the degradation of safranine O and HA simultaneously at room temperatures and 0.1−0.4 bar partial pressure of O2 [21]. In our previous work, urchin-like hollow sphere NiCo2O4 catalyst (NCO) was exploited in CAO of HA and achieved 93.4% of TOC removal (TOC0 28.8 mg/L) after 24 h under 90 °C [17]. However, the current efficiency of CAO is still insufficient to meet the requirements of industrial applications. HA is prone to be adsorbed on metal oxides through van der Waals interactions, acid–base interactions, or electrostatic interactions [13], probably resulting in the blocking of active sites in catalysts and the reduction of their reusability. To guide the modification of catalysts in CAO in theory, an in-depth study of catalytic mechanisms in CAO is indispensable.
The catalytic oxidation of catalysts in CAO can be initiated via the following catalytic pathways. Catalysts in CAO can function as electron tunnels to have a direct redox reaction with adsorbed O2 or organics, and this pathway can be interpreted using the Mars–van Krevelen (MvK) mechanism, the Langmuir–Hinshelwood (L-H) mechanism, or the ligand-to-metal charge transfer (LMCT) mechanism, depending on the patterns of electron-transfer [23]. The intrinsic properties of metal elements in catalysts, including types, valence states, and active surface oxygen (ASO) species, determine the oxidation capacity of catalysts and the electrostatic interaction between catalysts and reactants. Previous researches confirmed that the oxidation of organics caused by high valence metal species, such as Cu2+ in CaxSr1-xCuO3-δ (x = 0, 0.25, 0.5, 0.75 or (1) [14] and Me0.25Sr0.25Cu0.5O (Me referred to Mg or Ce) [15], Ni3+ and Co3+ in NiCo2O4 [17], Co3+ in SCNU-Z2 [18], and Mn4+ in δ-MnO2 [22], is the first step of the whole reactions. However, the selectivity of oxidation reactions is affected by the surface electronic properties of catalysts and organic compounds. The radical path in CAO, which should be another equally notable catalytic pathway, is the generation of ROSs through O2 activation by catalysts at low temperatures [16,17,18,19]. A free radical chain reaction for the oxidation of organics would be initiated by ROSs. Luo et al. firstly proposed that the generation of ROSs from O2 in CAO followed the hopping conduction (HC) mechanism (Equations (1)−(3)) [16]. The Mn–Si–Ca–Cu–Ni complex oxide catalysts, which function as a thermal sensitizer in reaction systems, can generate “hole (h+)-electron (e−)” pairs under room temperature excitation [16]. The catalyst SCNU-Z2 [18] or SrFeO3 [19] also followed the HC mechanism to degrade organic pollutants in darkness. In addition, the generation of ROSs also originated from the activation of surface adsorbed oxygen species based on the oxygen reduction reaction (ORR) mechanism (as shown in Equation (4) to Equation (9)) through the surface electron transfer [17]. The direct electron-transfer path and the radical path are closely related to each other, and may simultaneously play roles in CAO reactions.
To enhance the thermal-activity of catalysts, special structures, such as core-shell [24] and heterojunction [25], can be adopted in catalyst construction. Wan et al. developed a catalyst Cu2O/Ag0@Ag-NPs with high thermo-catalytic performance for degrading Acid Orange 7, where 200 mg/L Acid Orange 7 was decolorized up to 94.4% in 2 h under the dark conditions at room temperature [24]. The Ag-NPs shell exhibited a strong thermal radiation effect, which made the catalyst easy to be activated through absorbing heat energy. Hu et al. developed doped MnO2-Au hybrid catalysts with the formation of heterojunction, which promoted thermally generated electrons in MnO2 transferred to Au [25]. Hu et al. also enhanced the thermal catalytic performance of MnO2-Au through doping Na+ and K+ as the filling ions of MnO2 layers and reduced the reaction temperature by 50 °C and 100 °C when achieving the same reaction effect [25].
Catalyst + heat excitation → h+ + e−
O2 + e− → O2−
H2O + h+ → H+ + OH
Mm+-O2− + H2O + e− → M(m−1)+-OH− + OH−
O2 + e− → *O2− O2
M(m−1)+-OH− + *O2− → Mm+-O-O2− + OH−
Mm+-O-O2− + H2O + e− → M(m−1)+-O-OH− + OH−
M(m−1)+-O-OH− + e− → Mm+-O2− + OH−
Mm+-O-O2− + H2O + e− → M(m−1)+-OH− + HO2−
The thermal activity of a catalyst is related to several catalytic steps, including adsorption of heat energy, thermal excitation of the catalyst itself, and activation of organics or O2. However, the detailed relationship between the catalytic steps and intrinsic properties of catalysts has not been revealed completely. According to previous studies, defective structures of catalysts can promote the adsorption and activation of O2 at the catalyst surface. The Schottky barriers in Cu2O/Ag0@Ag-NPs made electrons easily migrate to the surface and combine with O2 to generate ∙O2− [24]. Wang et al. stressed that oxygen vacancies (OV) of catalysts (Au and Pd nanoparticles deposited on a hybrid of Na2Mo4O13/α-MoO3) accelerated electron-transfer from Au-Pd alloy particles to adsorbed O2 at OVs, stimulating the generation of ∙O2− at low temperatures (even close to 0 °C) [26].
Based on the hypothesis, it is possible to enhance the oxidation ability of NCO by optimizing its structure. Thus, the adsorption and activation of O2 on NCO can be improved, and consequently, more ROSs can be generated. The electron transfer from HA to O2 mediated by catalysts will be accelerated. In this work, we modified NCO by a facial reductant etching method. The new NCO-R was tested comprehensively to discover the effect of surface chemical properties on CAO performance. The results can also provide new insights into the catalytic mechanisms in CAO, which will lay a good foundation for the promotion of CAO processes and the development of efficient CAO catalysts.

2. Results and Discussion

2.1. Characterization of NCO-R

The etching treatment using reduction agents did not change the basic composition of NCO. In Figure 1, the obtained NCO-R had the same phase composition as NCO since the diffraction peaks of NCO-R still accorded with the standard patterns of the spinel NiCo2O4 phase (JCPDF NO. 20-0781) [17]. Nevertheless, the crystallinity of NCO-R increased significantly, as the intensity of peaks in the X-ray diffraction (XRD) profiles of NCO-R became much stronger (Figure 1), and the crystallite size of NCO-R decreased (Table 1). This might improve the dispersion of active sites and facilitate the activation of reactants at the catalyst interface. The existence of the amorphous phase in the NCO-R could be confirmed by the increase in the background on the XRD spectra of NCO-R (Figure 1) [27]. The amorphous structures in NCO-R might increase the defect sites at its surface and change the surface properties, such as surface acidity. The etching treatment mainly affected the morphology and structure of NCO (Figure 2). The fine needle burrs on the surface of NCO-R spheres were weakened, though the hollow sphere structure was maintained. According to the results of energy-dispersive X-ray spectroscopy (EDX) (Table 1), the Ni/Co ratio in NCO was around 0.38, lower than the theoretical Ni/Co ratio of NiCo2O4. The precipitation of Ni2+ during hydrothermal synthesis was harder than that of Co2+. In the follow-up study, the ratio of metal ions and precipitant in the precursor solution needs to be further optimized. The etching of NaBH4 or Na2SO3 to NCO hardly changed the ratio of Ni/Co. The specific surface area (SSA), total pore volume (VT), and average pore diameter (APD) calculated using the Brunauer–Emmet–Teller (BET) equation are listed in Table 1. The SSA of NCO-R was larger than that of NCO, while the VT and APD of NCO-R were smaller than that of NCO. Furthermore, the increment in SSA and the reduction in APD of NCO-RB were much larger than those of NCO-RS. Compared with NCO-RS and NCO, NCO-RB might have a stronger adsorption capacity for the reactants.
The etching of NCO using NaBH4 or Na2SO3 increased the strength of acidic sites on the surface of the catalysts. In the profiles of NH3-temperature-programmed desorption (NH3-TPD), as shown in Figure 3a, temperatures corresponding to the desorption peaks of NH3 reflected the acidic intensity of active sites. For all the catalysts, the desorption peaks of NH3 were centered around 200−300 °C, 300−400 °C, and 500−600 °C, which can be defined as weak acid peak (α), medium acid peak (β), and strong acid peak (γ), respectively [28]. NCO exhibited an intense α peak. After being etched using NaBH4 or Na2SO3, the α peak of NCO-R turned to be weaker, while a broad β peak appeared. An intense γ peak at around 574 °C occurred in the profile of NCO-RB20, while not in other profiles, indicating that strong acid sites were introduced into the surface of NCO-RB20. The acidic sites can be divided into Brønsted acid sites that can affect the surface electrostatic forces between catalysts and reactants, and Lewis acid sites that can accept an electron pair [29]. Therefore, the enhancement of NCO-R acidity may improve the capacity of adsorbing oxygen and activating organic pollutants.
According to the O2-temperature-programmed desorption (O2-TPD) curves (Figure 3b), ASOs on NCO-RB and NCO-RS increased. These species are closely related to surface oxygen mobility, determining the ability to activate O2 and capture electrons from pollutants [30]. ASOs include molecular O2 adsorbed at OV sites and surface oxygen ions (O2−, O−). The O2 desorption peaks appeared below 150 °C (δ peak) and 150−450 °C (θ and λ peaks), respectively [30,31]. Although the oxygen desorbed from NCO was detected in the O2-TPD test, the intensity was weak. Only a quite weak peak at around 500 °C was formed in the O2-TPD curves, which should be assigned to surface lattice oxygen species. The intensities of δ, θ and λ peaks increased significantly in the O2-TPD spectra of NCO-R, especially NCO-RB10, NCO-RB20, and NCO-RS20. The area of O2 desorption peaks in the O2-TPD curves of NCO-RB20 (2427.8) was the largest, compared with NCO-RB10 (1322.0), and NCO-RS20 (533.6). This suggested that the content of ASOs in NCO-RB20 was the highest. Besides, the desorption temperatures of O2 from NCO-R were much lower than those of NCO, indicating the enhancement of surface oxygen mobility of NCO-R. The surface oxygen ions might be the main ASO species of NCO-RB20, indicating its excellent catalytic oxidation activity. NaBH4 changed the surface acidity and oxygen species of NCO through adjusting the valence of metal ions and introducing oxygen vacancies. The reducibility of Na2SO3 was weaker than that of NaBH4, and hence its modification effect on NCO was relatively weaker. However, for NCO-RB25, the peaks in NH3-TPD and O2-TPD were both weak, indicating that excessive NaBH4 was not conducive to the modification of the NCO surface.
Furthermore, surface chemical properties and valence states of elements of NCO and NCO-R were detected (Figure 4). The X-ray photoelectron spectroscopy (XPS) results confirmed that chemisorbed H2O at the surface of NCO-R almost disappeared as the characteristic peak (peak O4, 533.0 eV) disappeared at the O1s spectra. The contents of ASO species of NCO-R, including hydroxyl groups (peak O2, 530.4 eV) and O2 coordination adsorbed at defect sites (peak O3, 531.8 eV), increased significantly, indicating that more OVs were formed due to the etching treatment. The peak O1 (529.3 eV) referred to lattice oxygen species. The ratios of oxygen species in NCO and NCO-R, as well as the ratio of Ni2+ and Co2+, were calculated according to the XPS parameters (Table 2). From NCO to NCO-R, the contents of Co3+ decreased because NaBH4 or Na2SO3 reduced Co(III) species (Figure 4), and the phenomenon was more obvious in NCO-RB. However, the Ni(III) species in NCO might not be reduced by NaBH4 or Na2SO3. The valence proportion of Ni was almost unchanged in NCO-RS20. Notably, the proportion of Ni3+ increased in NCO-RB, probably due to the increased O2 at OVs. It was deduced that the oxidation capacity of NCO-R was better than NCO because the reduction peaks R1 and R2 in the H2 temperature-programmed reduction (H2-TPR) profiles (Figure 3c) of NCO-R shifted to low temperature zones. The R1 peak and the R2 peak were assigned to the reduction from Ni3+/Co3+ to Ni2+/Co2+, and Ni2+/Co2+ to Ni0/Co0, respectively. This was probably attributed to the increase of ASOs in NCO-R. Besides, the increment in the SSAs promoted the reduction of H2, conducive to the decrease of H2 reduction temperature of NCO-R. The results of XPS and H2-TPR further demonstrated that the etching treatment improved the oxygen mobility of NCO-R.

2.2. Oxidation of HA Using NCO-R

For the treatment of HA, NCO-RB20 exhibited the best performance on the whole CAO reactions, because the TOC removal rate of HA at the 25 °C of CAO using NCO-RB20 was the highest (77.0%) (Figure 5). However, in the first 10 min, the HA removal rate reached the maximum when NCO was taken as the catalyst. During this stage, the TOC removal rates of HA followed the order: NCO-RB10 (11.2%) < NCO-RB25 (14.6%) < NCO-RB20 (17.3%) < NCO-RB15 ≈ NCO-RS20 (21.0%) < NCO (26.2%). In the subsequent stage, the oxidation of HA slowed down when using NCO-RS20 and NCO as the catalyst. The modification effect of Na2SO3 on NCO was relatively weaker than that of NaBH4. Especially for NCO, the CAO reaction almost stagnated from the 180th min (37.8%) to the 420th min (44.4%). The TOC removal rates of HA within 180 min followed in the order of NCO-RS20 (32.4%) < NCO0 (37.8%) < NCO-RB25 (43.9%) < NCO-RB15 (48.4%) < NCO-RB10 (52.5%) < NCO-RB20 (59.5%). Heating would promote the catalytic reaction rates of CAO using NCO-R, especially for the first 60 min of the reactions (Figure S1, Supplementary Materials). For CAO using NCO-RB20, the TOC removal reached 42.8% at the 60th min, 73.2% at the 420th min, and 75.6% at the 720th min in the 60 °C of CAO, which had little difference from that in the 25 °C of CAO (Figures S1 and S2). However, the removal rate of HA was raised significantly in the 90 °C of CAO (Figure S1), where the TOC removal reached 54.6% at the 60th min, 85.5% at the 420th min, and 90.0% at the 720th min (Figures S1 and S2). In our previous study, after 24-h CAO at 90 °C, the TOC removal reached 93.4% at an initial TOC of 28.8 mg/L, and 48.0% at an initial TOC of 107.8 mg/L [17]. The weight hourly space velocity (WHSV, which refers to the mass of pollutants treated by per unit mass of catalyst in per unit time) of NCO-R achieved 0.00186 h−1, 3.3 times higher than the WHSV (0.00043 h−1) of the NCO applied in our previous work [17].
The performance of NCO-R in CAO can be explained by the mechanism of catalytic oxidation and the characteristics of NCO and NCO-R. Furthermore, the oxidation of HA was indeed controlled by the electronic structure and the surface structure of the catalysts [12,29]. There should be two paths for the oxidation of HA on NCO and NCO-R: a direct path and a radical path [17].
The pH of the suspensions containing NCO-RB was around 8.8–9.1, higher than the initial pH 8.6 of the HA solution, while the pH of the suspensions containing NCO or NCO-RS20 remained at 8.6 (Table S1). Due to the enhancement in the surface acidity and ASOs, NCO-RB had different surface hydroxylation with NCO. Therefore, the groups of HA combining with catalysts changed, promoting the hydrolysis of HA anions and increasing the pH of suspensions. Along with the removal of HA in CAO at 25 °C, the pH of solution decreased from 8.6 to around 8.0 for the first 10 min and then changed slightly (Figure S3). The solution pH in CAO using NCO-R was higher than that in CAO using NCO at the 420th min. The pH of the solution was unchanged at 60 °C using NCO-RB20. However, at 90 °C using NCO-RB20, the pH of the solution increased to 9.3 in the first 120 min, and then decreased to 8.4 at the 420th min (Figure S3). The change of pH was limited and had little effect on the overall CAO reactions. The above phenomena were probably related to the intermediates in the degradation of HA.
Excitation and emission matrix (EEM) fluorescence spectra of HA solution with the treatment of CAO using different catalysts were exhibited in Figure 6. The fluorescence intensity of each region in the EEM spectra weakened significantly, indicating most components of HA could be eliminated in CAO using NCO or NCO-R. For the untreated HA solution, the distinct peak (EXmax/EMmax = 276 nm/535 nm) and most of the EEM spectra appeared at region IV (EX/EM 250−440/380−450 nm). Through the treatment of CAO using NCO or NCO-R, the EEM peaks of the solution after reaction moved to region II (EX/EM 220−250/330−380 nm) and III (EX/EM 220–250/380–480 nm), indicating that HA was transformed to fulvic-like substances and phenol-like substances [20,32]. A distinct peak (EXmax/EMmax = 250 nm/273 nm) appeared at the EEM spectra of the solution after reaction in CAO using NCO, which could be considered as tryptophan- or protein-like components [33]. The peak did not appear at the EEM spectra of the solution after reaction in CAO using NCO-R. It could be concluded that NCO-R had more catalytic oxidation ability to nitrogenous organics, probably related to the enhancement in the surface acidity of NCO-R. Heating would probably accelerate the oxidation of nitrogenous organic groups of HA at the surface of NCO-R, producing more NH4+ and increasing the solution pH.

2.3. Catalytic Mechanisms of NCO-R

The direct path of the oxidation on NCO and NCO-R played a key role, and this path followed the MVK mechanism [17]. According to the MVK mechanism, the two following reactions occurred cyclically on NCO in a progressive or parallel way: (1) ≡Ni(III) and ≡Co(III) species initiated the oxidation of HA adsorbed on the catalysts; (2) the electrons captured by ≡Ni(III) or ≡Co(III) from HA were then transferred to O2 adsorbed on the catalysts [17,34]. In Figure 5, when using NCO as the catalyst, the oxidation rate at the mid-late stage decreased probably because of the slow rate of the second reaction. In other words, the electron-transfer between NCO and O2 might be the limiting-rate step for the whole CAO reaction. Besides, the intermediates of HA oxidation could cover active sites, and, as a consequence, HA in the bulk solution could not contact the NCO surface for adsorption and oxidation. This also made contributions to the great decline of oxidation rates. The modification through reductant etching enhanced O2 adsorption capacity and surface oxygen mobility of NCO-R, and thus formed a smooth electron-transfer chain from HA to ≡M(III), and then to O2 (M refers to Ni or Co). Therefore, with an enhancement in ORR, the reaction rate in CAO using NCO-R remain stable.
In addition to the surface oxidation, the radical path was also initiated by NCO or NCO-R in the CAO reactions. Figure 7 shows the intensities of DMPO-∙OH signals in the solutions containing air and NCO or NCO-R. The production of ∙OH in the two systems seemed almost the same, and the concentration of ∙OH was quite small at the initial stage. Therefore, the radical path should play a minor role in the oxidation of HA. In other words, the oxidation of HA was mainly completed by the species having oxidation ability on the NCO surface, such as ≡Ni(III), ≡Co(III), and ASOs.
The ROSs combined at the interface of catalysts, the metastable intermediates with strong oxidation produced over the generation of ∙OH, might also be the species involved in the direct oxidation of HA. The generation of ∙OH and other ROSs is attributed to the oxygen reduction reaction (ORR) mechanisms [35,36]. As O2 combines the active sites of catalysts, the following reactions that O2 will participate in will be: O2 → *O2 → *OOH → *O → *OH → H2O, where * stands for active sites. Thus, the metastable intermediates of O2 in ORR are *O2, *OOH, *O, and *OH, which would be released as ROSs (such as ∙O2−, OH, and HO2−) into the bulk solution, because the combination of such intermediates and active sites are weak [35,36]. When ORR was initiated at active sites, the following reactions would occur, as shown in Equation (4) to Equation (9). According to the density functional theory calculation on the catalytic models of ORR by Chen et al. [35], ≡Ni(III) and ≡Co(III) were the active sites of ORR in NiCo2O4, and ≡Co(III) exhibited the highest ORR activity. Therefore, the generation of ROSs on NCO or NCO-R might have a competitive relationship with the direct path for the oxidation of HA. When the adsorption of organics at the catalyst interface is more likely to occur, the active sites ≡Ni(III) and ≡Co(III) will combine the organics and capture electrons. Otherwise, the organics that are hardly adsorbed on NCO or NCO-R would be degraded by those free-state ROSs. As HA has the properties of being easy to combine with metal oxide through electrostatic adsorption, π-π bonds, or complex adsorption, the direct path became the main way to oxidize HA with NCO or NCO-R as the catalysts.
The radical path could also be explained by the HC mechanism [37]. Luo et al. firstly exploited HC to explain the thermal catalytic mechanisms of CAO [16]. They also proposed that the kinetic behavior of the catalytic oxidation by Mn–Si–Ca–Cu–Ni complex oxides conforms to the L-H mechanism [16]. Chen et al. [14], Tummino et al. [19], and other related researchers have also proposed that the catalytic paths of perovskite catalysts (CaxSr1-xCuO3-δ, SrFeO3, etc.) in CAO also followed HC. Therefore, similar to photocatalysts, hole-electron pairs would be produced at NCO and NCO-R as being inspired thermally according to the HC mechanism, followed by the generation of ROSs (Equation (1) to Equation (3)). With strong oxidizability, holes could directly oxidize organic substrates (Equation (10)). As for NCO, the reaction (3) and (10) at NCO might be much faster than reaction (2), resulting in the much faster removal of HA at the initial stage of the reaction than that in the later stages. The OVs in NCO-R promoted the adsorption of O2 and the transfer of “thermo-generated electrons” from NCO-R to O2. The recombination of e− and h+ would therefore be inhibited, which left more free holes at the NCO-R surface. The whole reaction rate of CAO treating HA by the NCO-R was therefore increased. The above inference about the HC mechanism in CAO using NCO and NCO-R also showed a possible correlation between the direct path and the radical path. For the direct path, free holes at the surface of NCO or NCO-R captured electrons from the adsorbed organics, while free electrons were trapped by the adsorbed O2. Thus, the direct path contributed to the oxidation of HA in CAO using NCO or NCO-R, as it was easy to adsorb at metal oxides. The oxidation of organics hard to adsorb at NCO or NCO-R would dominantly rely on a radical path. In other words, the HC mechanism might give an intrinsic explanation for the macro catalytic behaviors of NCO and NCO-R, which are referred to as the direct path and radical path. Therefore, we proposed a new insight about CAO, which unified the MVK mechanism and the HC mechanism from the origin.
h+ + organicsads → intermediates → CO2 + H2O

3. Conclusions

In this work, a series of NiCo spinel catalysts (NCO-R) rich in OVs were synthesized by a facial method of reductant etching. The catalytic performance of NCO-R in CAO treating HA was enhanced owing to the special properties of NCO-R, including the increment of specific surface area, surface acidity, and surface oxygen mobility. Among different NCO-R catalysts, NCO-RB20 exhibited the best performance, achieving a 77.0% TOC removal rate, which was significantly higher than 44.4% using NCO as the catalyst. Unlike a rapid decline of HA removal rate at the mid-late stage in CAO using NCO, the OVs in NCO-R increased the ability of O2 adsorption and O2 activation, ensuring a stable removal rate of HA. The direct path and the radical path co-existed in CAO, and related to each other. This work not only developed a facial method of regulating the OVs of NCO for efficient CAO treatment, but also put forward new insights into the catalytic oxidation mechanism in CAO.

4. Materials and Methods

4.1. Materials

To prepare the HA stock solution (TOC, 2476 mg/L), 10 g sodium humate was dissolved in ultrapure water (UPW), where the insoluble part of the solution was removed by centrifugation (17). All the chemicals used in this study were purchased from Aldrich Chemical Co., Ltd., and they were used as received without further purification.

4.2. Preparation of Catalysts

NCO was prepared basically refers to our previous work [17]. Typically, 96 mmol/L Co(NO3)2·6H2O, 48 mmol/L Ni(NO3)2·6H2O and 2.88 mol/L urea were dissolved in a mixture of isopropanol (IPA, 800 mL) and water (160 mL), followed by solvothermal reaction at 120 °C for 12 h. Then, the obtained precipitate was calcinated at 350 °C in the air for two hours at a ramping rate of 1 °C/min. We reduced the concentration of Co(NO3)2·6H2O, Ni(NO3)2·6H2O, and urea to 90% of that in our previous work [17]. After that, NCO was reduced using NaBH4 or Na2SO3, to generate NCO-RBn or NCO-RSn (collectively referred to as NCO-R). The n represented the concentration of reducing agent used, which is n mmol/L. For example, to prepare NCO-RB10, 5 g as-prepared NCO was put in 10 mmol/L NaBH4 solution, and they fully reacted with magnetic stirring at 200 rpm for 30 min. Then, the products were collected by vacuum filtration, and were washed three times with ultrapure water. After being dried in a vacuum drier at 60 °C, NCO-RB10 was finally obtained. Similar procedures were used for the preparation of NCO-RB15, NCO-RB20, NCO-RB25, and NCO-RS20.

4.3. Characterization of Catalysts

The crystal structure of catalysts was characterized using XRD (Smartlab, Rigaku, Tokyo, Japan) equipped with Cu Kα radiation (λ = 0.15406 nm). Morphology was observed through scanning electron microscopy (SEM, ZEISS SUPRA® 55, Oberkohen, Germany). Metal-element distribution was examined using EDX (X-Max 20, Oxford, Abinden, Oxfordshire, UK). XPS was carried out using PHI5000VersaProbeII (Ulvac-Phi, Kanagawa, Japan). SSA, VT, and APD were tested using an ASAP2020 gas-volumetric apparatus (Micromeritics, Atlanta, GA, USA). H2-TPR (TP-5080, Xianquan, Tianjin, China) was performed on a chemisorption apparatus with a thermal conductivity detector. The H2-TPR profile was recorded from 50 °C to 750 °C at a heating rate of 10 °C/min under a flow of a 10% H2/Ar (30 mL/min) mixture, after samples were pretreated at 300 °C for two hours in Ar flow. NH3-TPD profiles were also recorded to detect the surface acid properties of catalysts after modification. The pretreatment of catalysts in NH3-TPD was the same as that in H2-TPR. After that, catalysts were saturated with 10% NH3/Ar flow at 50 °C for one hour, followed by being flushed with Ar for one hour to remove physiosorbed NH3. Desorption of NH3 was recorded by a thermal conductivity detector from 50 °C to 750 °C with a heating rate of 10 °C/min. Surface adsorbed oxygen species and lattice oxygen species were analyzed through the O2-TPD spectra. Within 10% O2/Ar flow, the procedures of O2-TPD experiments were the same as those of NH3-TPD.

4.4. Assessment of Catalytic Performance

All experiments were conducted in environmental conditions. In a complete experimental process, 5 g/L catalysts and 150 mL HA solution diluted from 7.5 mL the stock solution (TOC 123.8 mg/L) were mixed in a 250 mL glass reactor equipped with a magnetic stirrer and an aeration device. As the mixture of catalysts and solution was stirred at 200 rpm, air was pumped into the reactor through an aeration head. The time point when magnetic stirring was initiated was defined as the beginning of CAO reactions. Solution samples were withdrawn at certain intervals, and filtered with 0.45 μm polyethersulfone membrane to remove the catalysts. The concentrations of target organic substances were reflected using TOC, which was determined using a TOC analyzer (TOC-VCSH, Shimadzu, Kyoto, Japan). The EEM fluorescence spectra of the solutions after reaction were determined by EEM fluorescence spectroscopy (F-7000, HITACHI, Tokyo, Japan). Experiments were conducted in triplicate, and variations are shown as error bars in the figures.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal12050536/s1, Figure S1: Performance of HA degradation in catalytic air oxidation using NCO-RB20 in different reaction temperatures; Figure S2: The TOC removal in 12 h of catalytic air oxidation using NCO-RB20 in different reaction temperatures; Figure S3: The pH of HA solution via catalytic air oxidation at 25 °C using NCO (a), NCO-RB10 (b), NCO-RB15 (c), NCO-RB20 (d), NCO-RB25 (e), and NCO-RS20 (f); at 60 °C using NCO-RB20 (g); at 90 °C using NCO-RB20 (h); Table S1: The pH of the suspensions with catalysts studied in this work (5 g/L) and HA (initial TOC, 123.8 mg/L; initial pH 8.6) before and after the 420 min of catalytic air oxidation (CAO).

Author Contributions

Conceptualization, Q.J. and H.L.; methodology, Q.J. and J.C.; validation, Q.J. and J.C.; formal analysis, Q.J.; investigation, Q.J. and J.C.; resources, Q.J. and J.C.; data curation, Q.J.; writing—original draft preparation, Q.J.; writing—review and editing, J.C. and H.L.; visualization, Q.J. and J.C.; supervision, H.L.; project administration, H.L.; funding acquisition, H.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work is supported by the Stable Support Program for Universities at Shenzhen (Grant number WDZC20200817144218001).

Data Availability Statement

All the relevant data used in this study have been provided in the form of figures and tables in the published article, and all data provided in the present manuscript are available to whom it may concern.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 12 00536 g001 550
Figure 1. X-ray diffraction (XRD) spectra of NCO (a) and modified NCO (b), NCO-RBn refers to the catalyst obtained by NCO being etched by n mmol/L NaBH4 solution (n is 10, 15, 20, or 25); NCO-RS20 refers to the catalyst obtained by NCO being etched by 20 mmol/L Na2SO3 solution.
Figure 1. X-ray diffraction (XRD) spectra of NCO (a) and modified NCO (b), NCO-RBn refers to the catalyst obtained by NCO being etched by n mmol/L NaBH4 solution (n is 10, 15, 20, or 25); NCO-RS20 refers to the catalyst obtained by NCO being etched by 20 mmol/L Na2SO3 solution.
Catalysts 12 00536 g001
Catalysts 12 00536 g002 550
Figure 2. Scanning electron microscopy (SEM) images of NCO (a) and the modified NCO which were etched using the solution of NaBH4 or Na2SO3 etching: NCO-RB10 (10 mmol/L NaBH4) (b), NCO-RB15 (15 mmol/L NaBH4) (c), NCO-RB20 (20 mmol/L NaBH4) (d), NCO-RB25 (25 mmol/L NaBH4) (e), and NCO-RS20 (20 mmol/L Na2SO3) (f).
Figure 2. Scanning electron microscopy (SEM) images of NCO (a) and the modified NCO which were etched using the solution of NaBH4 or Na2SO3 etching: NCO-RB10 (10 mmol/L NaBH4) (b), NCO-RB15 (15 mmol/L NaBH4) (c), NCO-RB20 (20 mmol/L NaBH4) (d), NCO-RB25 (25 mmol/L NaBH4) (e), and NCO-RS20 (20 mmol/L Na2SO3) (f).
Catalysts 12 00536 g002
Catalysts 12 00536 g003 550
Figure 3. The NH3-Temperature Programmed Desorption (TPD) profiles (a), O2-TPD profiles (b) and H2- Temperature Programmed Reduction (TPR) profiles (c) of NCO, NCO-RB10, NCO-RB15, NCO-RB20, NCO-RB25, and NCO-RS20.
Figure 3. The NH3-Temperature Programmed Desorption (TPD) profiles (a), O2-TPD profiles (b) and H2- Temperature Programmed Reduction (TPR) profiles (c) of NCO, NCO-RB10, NCO-RB15, NCO-RB20, NCO-RB25, and NCO-RS20.
Catalysts 12 00536 g003
Catalysts 12 00536 g004 550
Figure 4. The X-ray photoelectron spectroscopy (XPS) profiles of NCO, NCO-RB10, NCO-RB15, NCO-RB20, NCO-RB25, and NCO-RS20: Co2p core level (a), Ni2p core level (b), and O1s core level (c).
Figure 4. The X-ray photoelectron spectroscopy (XPS) profiles of NCO, NCO-RB10, NCO-RB15, NCO-RB20, NCO-RB25, and NCO-RS20: Co2p core level (a), Ni2p core level (b), and O1s core level (c).
Catalysts 12 00536 g004
Catalysts 12 00536 g005 550
Figure 5. Variations of total organic carbon (TOC) in catalytic air oxidation of humic acid (HA) solutions using NCO, NCO-RB10, NCO-RB15, NCO-RB20, NCO-RB25, and NCO-RS20 as the catalysts (initial TOC, i.e., TOC0, 123.8 mg/L; catalysts, 5 g/L; initial pH, 8.6; temperature, 25 °C).
Figure 5. Variations of total organic carbon (TOC) in catalytic air oxidation of humic acid (HA) solutions using NCO, NCO-RB10, NCO-RB15, NCO-RB20, NCO-RB25, and NCO-RS20 as the catalysts (initial TOC, i.e., TOC0, 123.8 mg/L; catalysts, 5 g/L; initial pH, 8.6; temperature, 25 °C).
Catalysts 12 00536 g005
Catalysts 12 00536 g006 550
Figure 6. Excitation and emission matrix (EEM) fluorescence spectra of humic acid (HA) solutions: without any treatment (a); in 420 min catalytic air oxidation using NCO (b), NCO-RB10 (c), NCO-RB15 (d), NCO-RB20 (e), NCO-RB25 (f), and NCO-RS20 (g) as the catalysts; the region I and II refer to aromatic protein components, the region III refers to fulvic-like components, the region IV refers to proteins or polysaccharides, the region V refers to humic-like components a. a The division of region I−IV in (a−g) was based on the method of Tao et al. [32].
Figure 6. Excitation and emission matrix (EEM) fluorescence spectra of humic acid (HA) solutions: without any treatment (a); in 420 min catalytic air oxidation using NCO (b), NCO-RB10 (c), NCO-RB15 (d), NCO-RB20 (e), NCO-RB25 (f), and NCO-RS20 (g) as the catalysts; the region I and II refer to aromatic protein components, the region III refers to fulvic-like components, the region IV refers to proteins or polysaccharides, the region V refers to humic-like components a. a The division of region I−IV in (a−g) was based on the method of Tao et al. [32].
Catalysts 12 00536 g006
Catalysts 12 00536 g007 550
Figure 7. Electron paramagnetic resonance (EPR) spectra of ∙OH in NCO-RB20/air system (a) and NCO/air system (b); the corresponding relative intensity of DMPO-∙OH signals in NCO-RB20/air and NCO/air system (c).
Figure 7. Electron paramagnetic resonance (EPR) spectra of ∙OH in NCO-RB20/air system (a) and NCO/air system (b); the corresponding relative intensity of DMPO-∙OH signals in NCO-RB20/air and NCO/air system (c).
Catalysts 12 00536 g007
Table
Table 1. The specific surface area (SSA), total pore volume (VT), and average pore diameter (APD), crystallite size, and Ni/Co atomic ratio of the origin NCO prepared in this work, and the modified NCO which were prepared by the solution of NaBH4 (10 mmol/L, NCO-RB10; 15 mmol/L NCO-RB15; 20 mmol/L, NCO-RB20; 25 mmol/L, NCO-RB25) or Na2SO3 (20 mmol/L, NCO-RS20) etching.
Table 1. The specific surface area (SSA), total pore volume (VT), and average pore diameter (APD), crystallite size, and Ni/Co atomic ratio of the origin NCO prepared in this work, and the modified NCO which were prepared by the solution of NaBH4 (10 mmol/L, NCO-RB10; 15 mmol/L NCO-RB15; 20 mmol/L, NCO-RB20; 25 mmol/L, NCO-RB25) or Na2SO3 (20 mmol/L, NCO-RS20) etching.
Catalysta SSA (m2/g)a VT (cm3/g)a APD (nm)b Crystallite Size (nm)c The Ni/Co Atomic Ratio
NCO52.70.34426.113.80.387
NCO-RB1069.40.31318.012.00.378
NCO-RB1573.40.31117.011.90.391
NCO-RB2074.30.33017.812.70.389
NCO-RB2575.30.30916.411.90.383
NCO-RS2059.40.32421.812.70.390
a The data of SSA, VT, and APD were obtained from BET tests in an ASAP2020 gas-volumetric apparatus; b the crystallite size of catalysts were calculated based on the strongest peaks [the 311 peak] of the XRD spectra according to the Debye–Scherrer formula [17]; c the Ni/Co atomic ratio of catalysts were obtained from the energy-dispersive X-ray spectroscopy (EDX) tests.
Table
Table 2. The XPS parameters of NCO and NCO-R.
Table 2. The XPS parameters of NCO and NCO-R.
CatalystsNi2+ (%) aCo2+ (%) aO1 (%) aO2 (%) aO3 (%) a
NCO59.531.840.226.09.3
NCO-RB1045.241.336.240.123.7
NCO-RB1539.753.739.831.728.5
NCO-RB2031.861.930.728.640.8
NCO-RB2541.751.429.839.530.7
NCO-RS2047.252.751.419.928.7
a The ratios of Ni2+, Co2+, O1 (lattice oxygen species), O2 (hydroxyl groups), O3 (O2 coordination adsorbed at defect sites), and O4 (chemisorbed H2O) were calculated by Ni2+/(Ni2+ + Ni3+), Co2+/(Co2+ + Co3+) and Ox/(O1 + O2 + O3 + O4) (x = 1, 2, 3, or 4), based on the peak areas of XPS spectra.
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Jing, Q.; Cai, J.; Li, H. Strengthen Air Oxidation of Refractory Humic Acid Using Reductively Etched Nickel-Cobalt Spinel Catalyst. Catalysts 2022, 12, 536. https://doi.org/10.3390/catal12050536

AMA Style

Jing Q, Cai J, Li H. Strengthen Air Oxidation of Refractory Humic Acid Using Reductively Etched Nickel-Cobalt Spinel Catalyst. Catalysts. 2022; 12(5):536. https://doi.org/10.3390/catal12050536

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Jing, Qi, Jiabai Cai, and Huan Li. 2022. "Strengthen Air Oxidation of Refractory Humic Acid Using Reductively Etched Nickel-Cobalt Spinel Catalyst" Catalysts 12, no. 5: 536. https://doi.org/10.3390/catal12050536

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