Complex Concentrated Alloy Catalyst of AlCrFeCoNi for Heterogeneous Degradation of Rhodamine B

The Fe-based catalysts have attracted good attention due to their earth abundance and low toxicity with good Fenton-like performance. However, the narrow pH working range and iron-containing sludge produced during the reaction drove the necessary of developing a potential catalyst in the corresponding application. High entropy alloy that now expands to complex concentrated alloy (CCA) represents a new class of material owing to a broader range of functional and structural properties. A new application of CCA as a catalyst for catalytic degradation of azo dyes has already been a scientific research hotspot. AlCrFeCoNi CCA powder has been successfully synthesized by mechanical alloying (MA) method using a vertical planetary ball mill. Based on the characterization, the catalyst possessed a spherical morphology with a particle size range of 3.5-12.6  m. The catalyst exhibited photo-Fenton performance up to 85.3% which would be a promising Fenton-like catalyst for wastewater treatment.


Introduction
Indonesia is among the world's largest textile manufacturers in the top ten.Indonesia's textile industry accounted for about 6% growth in the manufacturing sector [1].In the textile industry, the dyeing and finishing steps generated around 17-20% of total wastewater effluent [2].It has been reported that more than 300.000 tons of dyes are discharged annually into treatment facilities [3].
One of the most widely used chromophores in dye chemistry is azo dyes, accounting for more than 60% of synthetic dyes in the market [4].The color of azo dyes is determined by the electronwithdrawing functional groups (chromophores), such as N=N, C=O, −CH=N, NO2, NO, NOH, C=N, C≡N, C=C, and C≡C; and electron-releasing groups (auxochromes), such as SO3H, OH, COOH, NH2, NH3, NHCH3, and N(CH3)2 groups.The stable structure of dyes present in wastewater makes their degradation difficult.Therefore, it is essential to understand the practical methods to deal with textile wastewater to save the environment.
Various techniques for azo dye removal in wastewater have beeln attempted, such as physical adsorption, chemical and biological degradation, coagulation, and chemical oxidation by Fenton and hypochlorite methods [5][6][7][8].Among those techniques, Fenton method (Fe 2+ /Fe 3+ +H2O2) has been a more effective approach.Its powerful oxidation capability can quickly degrade organic dyes with different chemical structures (xanthene, thiazine, triarylmethane, and azo dyes) [9,10], but it demands highly acidic conditions.Thus, developing a new type of heterogeneous catalyst is urgently needed for efficient dye degradation and environmental safety.
Over this last decade, high-entropy alloys (HEAs) have received extensive attention and achieved tremendous evolvements in the materials science community.HEA is defined as the five or more principal elements in an equiatomic ratio (concentration between 5-35%) with no distinction between solute and solvent [11,12].HEAs have been reported to be promising catalysts because of their highly complex atomic structure, high configurational entropy, and potential energy.A new term for complex concentrated alloys (CCA) is an expansion of HEA, which allows elemental concentration of less than 5% or excess than 35%, including single phase intermetallic alloys with any number of intermetallic phases and solid solution [13].CCA has been commonly synthesized by wet chemistry method, thermal spraying, and mechanical alloying (MA) [14].
CCA of AlCoCrTiZn and AlCrFeMn have been explored and exhibit prominent efficiency in degrading azo dye Direct Blue 6 (DB6).Both research only explored the zero valence metal (ZVM) reduction method for azo groups degradation, not involving a Fenton-like process, which can work for more complicated structures of dyes.The CCA catalysis degradation has yet to be widely studied and the literary studies are limited.
In this work, AlCrFeCoNi CCA synthesized by MA was conducted to study the behaviour of CCA in Fenton-like activity of dye degradation.The elements of Fe, Co, Ni, Cu and Cu have a direct contribution to possessing Fenton-like activity [15].In contrast while Al and Cr elements were used to promote the formation of amorphous alloys.
Several researchers focused on Fe-based metallic glass catalyst for azo dyes degradation.Although novel HEA has been developed in recent years, but less works reported about dye degradation behaviour of equiatomic HEA and of course the CCA catalyst.This study aims syntesize AlCrFeCoNi CCA by MA process using planetary ball mill and investigate the performance in Fenton-like process dye degradation of Rhodamine B (RhB).In this work, AlCrFeCoNi CCA were successfully prepared by one-step ball milling method and then used as heterogeneous photo-Fenton catalysts.The catalytic performances were investigated by decomposition of RhB, which usually hard to be decomposed by ZVM reduction method.

Catalyst Synthesis
The alloy catalysts of AlCrFeCoNi were synthesized by ball milling using corresponding pure metal powders as raw materials.The mixed pure powders of Al (5.39 g), Cr (10.39 g), Fe (11.17 g), Co (11.79 g), Ni (11.74 g) and the amount of stearic acid (2 wt%) as control agent were loaded into a 500 mL stainless steel jar together with stainless steel balls which have different diameters (5, 8, 10, and 15 mm), among them, the ratio of ball to powder weight ratio was 10:1.The MA process was carried out in the vertical planetary ball mill (TENCAN, Changsa Tianchuang Powder Technology, China) at 300 rpm rotation speed for 60 h.To avoid the phenomenon of overheating, the rotation direction was changed every hour with a pause of 10 min.The synthesized catalyst was then calcined at 400 °C under argon gas.

Materials Characterization
The crystal structure of the as-milled AlCrFeCoNi CCA powders was analyzed by X-ray diffraction (XRD, Bruker D8 Advance) using Co source.Thermo gravimetric (TG) analysis / Differential Scanning Calorimeter (DSC) was carried out on 60 h milled AlCrFeCoNi CCA powder in Linseis STA Platinum Series of TG/DSC with a heating rate of 10 °C/min from 30 to 1200 °C.The morphology characteristics the samples were investigated by scanning electron microscopy (SEM, Hitachi High-Tech Co. Ltd, Japan).

Catalytic activity
The catalytic of the as-prepared AlCrFeCoNi CCA was examined for the degradation of RhB at room temperature and various reaction pH.In a typical experiment, the mixture of 50 mL of RhB solution (10 ppm), 1 wt% catalyst, and 1 v% H2O2 was magnetically stirred under a UV lamp ( = 250-400 nm).Before the degradation, the whole suspension was stirred in the dark for 30 min to ensure the establishment of the adsorptiondesorption equilibrium).The extent of photocatalytic degradation was evaluated using a UV-Vis spectrophotometer (Agilent Technology) by measuring the change in maximum absorbance of 554 nm.The photocatalytic degradation efficiency (%) of AlCrFeCoNi CCA at any time t after visible light irradiation was calculated according to the following equation: where, PDE is photodegradation efficiency (%), C0 (mg.L −1 ) and Ct (mg.L −1 ) are the concentration of RhB dye in the test solution at initial and after time, respectively.

Characterization of AlCrFeCoNi CCA
The AlCrFeCoNi CCA powder was prepared from room temperature MA of high-purity precursor metals through ball milling process with stearic acid, as PCA as illustrated in Figure 1.
Differential thermal analysis (DTA) measurements were conducted on 60 h milled AlCrFeCoNi CCA powder samples.Thus, the phase transformation temperatures upon heating can be predicted, especially at the low temperature.Figure 2 shows the DTA and thermographimetric (TG) curves of 60 h mechanically alloyed AlCrFeCoNi CCA powder with a heating rate of 10 °C/min and up to 1200 °C.The slight exothermic curve was observed in a temperature range of about 150 to 300 °C, which may relieve the internal stresses during the highenergy ball milling process.The exothermic peaks above 450 °C, approximately 600 °C, were associated with the energy released during the phase transformation process.Oxidation of metal occurred to form metal oxide which is indicated by the weight gain curve present in TG analysis by heating the sample above 450 °C.To avoid the oxide form of AlCrFeCoNi CCA powder, the calcination process of the catalyst was conducted at 400 °C.In addition, the stearic acid as PCA is decomposed at 361 °C; thus all the PCA can be removed at calcination temperature.
Figure 3 illustrates the SEM morphologies of the AlCrFeCoNi CCA powder and corresponding particle size distribution (Figure 3(a-b)) after treated calcination at 400 °C to remove stearic acid.As revealed in this figure, spherical morphology appeared, and the particle size exhibits a relatively uniform distribution in the range of 3.5-12.6m.The catalyst samples with small average particle sizes (less than 20 m) are caused by the high entropy effects due to the MA process.The milling balls impact the phenomena of dynamic balance between the fracture occurred in individual particles and cold welding among particles.Furthermore, the ductile property Al powders caused the brittle particles occluded and trapped into it.Thus, the particle size cannot reach nano size.These regular patterns suggest Figure 1.Room temperature ball milling synthesis of CCA AlCrFeCoNi powder.the small particle size and spherical round block morphology are caused by the high entropy effect, thus the catalyst's specific surface area and active sites may expect to significantly increase [16].
The composition of each element in the CCA was quantitatively analyzed by the EDS spectrum of 60 h milled sample (Figure 3(c)).It confirms that the composition is nearly equiatomic.The lower amount of Al element detected through EDS may be caused by its lower atomic mass resulted in less easy to detect.The CCA sample with wide range of atomic numbers, the peak size detectors cannot precisely represent the exact atomic ratio of elements present.However, those EDS results verify that the actual composition of AlCrFeCoNi CCA is close to the nominal equiatomic composition.In Figure 3(d-i) indicates that all five elements (Al, Cr, Fe, Co, Ni) are distributed homogeneously.
The XRD patterns of AlCrFeCoNi CCA are shown in Figure 4, the reduction and broadening of the peaks are attributed to the decrease in crystallite size and diffusion of five elements during milling, leading to the formation of solid solution.The pattern peaks demonstrate simple solid-solution phases (FCC and BCC).The initial five multi phases crystal structure of each element is reduced into two phases after MA process.This phenomenon indicates that the five elements are alloyed during MA process.The lattice parameter of FCC is close to Ni, which may be caused by the Al and Co elements diffusing with Ni to form FCC solid solution.Meanwhile, the lattice parameter of BCC is close to that of Fe and Cr, but the melting point of Cr is higher and is less active in the diffusion.In this case, Cr acts as the solvent in the formation of BCC solid solution [17].
After calcination at 400 °C, the peaks remained similar, except for a small peak (33.5°) that may be the intermetallic of NiAl.The NiAl is generally obtained through liquid-solid-phase transformation via crystal nucleation and subsequent crystal growth [18].This indicates that solid solution was formed after milling and due to heating treatment under argon at 400 °C, intermetallic of NiAl appeared.

Degradation of Rhodamine B dye by AlCrFeCoNi CCA
To test the catalyst activity, first, we compare the efficiency of two oxidation processes, Fenton and photo-Fenton.In the Fenton process, a mixture of H2O2, AlCrFeCoNi CCA, and a model compound of RhB 10 ppm was stirred at room temperature.In the Photo-Fenton process, a similar mixture was stirred at room temperature under uv radiation, and the result is found in Figure 5. Photo-Fenton reaction worked more efficiently to degrade RhB with a photodegradation efficiency (PDE) of 85.7%.Under Fenton reaction, the degradation efficiency was about half compared to that of Photo-Fenton.Henceforward, the degradation reaction was conducted in Photo-Fenton reaction.
During the RhB degradation process, a double effect mechanism is occurred on the surface of CCA where the heterogeneous photo-Fenton and semiconductor CCA promote each other.M 3+ can capture the photo-induced electrons in CCA and it plays role in producing the combination rate of photoelectrons (e − ) and holes (h + ) to increase the photocatalytic activity.This process produces high active •OH that reveals superior catalytic efficiency and ensures the catalyst's cycle performance.For example, Fe 2+ /Co 2+ /Ni 2+ further reacts with H2O2 to generate Fe 3+ /Co 3+ /Ni 3+ utilizing UV light, which can effectively increase the Photo-Fenton.The probable mechanism of the photodegradation of RhB is represented in Figure 6.
The experiment of RhB decomposition carried out catalyzed by AlCrFeCoNi and H2O2 as shown in Figure 6.In the decomposition of RhB with only AlCrFeCoNi catalyst (Figure 7(a)) resulted in slight decrease of the intensity peak in absorption spectra.So did the decomposition of RhB catalyzed by H2O2 (Figure 7(b)), the intensity peak in the absorption spectra does not decrease a lot.Applying the AlCrFeCoNi catalyst together with H2O2 implied that the degradation reaction worked effectively (Figure 7(c)).The photodegradation reaction was also conducted at different pH of 2, 3, and 7 (Figure 7(d)).The Photo-Fenton reaction shows higher activity in an acidic environment to reach up to 97.5% PDE after 60 min reaction time.However, at around neutral conditions, the photodegradation reaction still worked well to give a %PDE of 85.7.Thus, realizing highly efficient degradation of RhB without adjusting reaction conditions at low acidity can avoid secondary pollution to the environment.speed up and increase the effectiveness of the process by accelerating the development of hydroxyl radicals.The catalyst exhibited photo-Fenton performance up to 85.3% which would be a promising Fenton-like catalyst for wastewater treatment.The result of this study can be applied in treating RhB dye for wastewater management, especially in the textile industry, to fulfil SDG number 9, clean water and sanitation.

Figure 2 .
Figure 2. DTA and TG curve of a 60 h milled AlCrFeCoNi CCA powder.
the elements alloyed during the MA process as indicated by the formation of simple solid-solution phases (FCC and BCC).This work assessed the capability of AlCrFeCoNi CCA as a catalyst in the degradation of RhB dye in the Photo-Fenton process.The AlCrFeCoNi CCA can Figure 4. XRD patterns of AlCrFeCoNi CCA powder.

Figure 5 .
Figure 5.The Fenton and photo-Fenton reaction in degradation RhB.