Impact of Precipitants on the Structure and Properties of Fe-Co-Ce Composite Catalysts

Fe-Co-Ce composite catalysts were prepared by coprecipitation method using CO(NH 2 ) 2 , NaOH, NH 4 HCO 3 , and NH 3 ⋅H 2 O as precipitant agents. The effects of the precipitant agents on the physicochemical properties of the Fe-Co-Ce based catalysts were investigated by SEM,TEM,BET, TG-DTA, andXRD. It was found that the precipitant agents remarkably influenced themorphology and particle size of the catalysts and affected the COD removal efficiency, decolorization rate, and pH of methyl orange for catalytic wet air oxidation (CWAO). The specific surface area of the Fe-Co-Ce composite catalysts successively decreased in the order of NH 3 ⋅H 2 O, NH 4 HCO 3 , NaOH, and CO(NH 2 ) 2 , which correlated to an increasing particle size that increased for each catalyst. For the CWAO of a methyl orange aqueous solutions, the effects of precipitant agents NH 3 ⋅H 2 O and NaOH were superior to those of CO(NH 2 ) 2 and NH 4 HCO 3 . The catalyst prepared using NH 3 ⋅H 2 O as the precipitant agent was mostly composed of Fe 2 O 3 , CoO, and CeO 2 . The COD removal efficiency of methyl orange aqueous solution for NH 3 ⋅H 2 O reached 92.9% in the catalytic wet air oxidation. Such a catalytic property was maintained for six runs.


Introduction
A large amount of wastewater is produced worldwide every year from the ever-expanding printing and dyeing industries.The wastewater originating printing and dyeing industries contain potentially dangerous dyes, dye auxiliaries, acids, alkali metals, and organic compounds [1].Due to the high concentrations of these above-mentioned compounds, complex composition, and poor degradability, it is difficult to treat printing and dyeing wastewater.As a result, current methods and equipment used for treatment have stringent requirements.On the other hand, it is difficult to treat wastewater by conventional methods [2].Catalytic wet air oxidation (CWAO), which is an advanced oxidation technology, has shown great potential for the treatment of biorefractory wastewater.Therefore, researchers have shown profound interest in the research of catalysts [3].It is well known that the noble metal catalysts have excellent catalytic performance.However, owing to their high cost and low natural occurrence, their industrial production and applications are hindered [4].Therefore, many researchers in the field of CWAO have focused on searching for alternative nonnoble metal catalysts with lower price, higher activity, and better stability [5].On the other hand, transition metals are widely used in industrial production and application due to their ubiquitous presence, relatively low cost, and outstanding catalytic performance [6].Rare earth metals can be combined with transition metals to form multicomponent catalysts, which will generate synergistic effects due to their special physical and chemical properties.Therefore, rare earth metals have been increasingly investigated for use with transition metals for researchers investigating catalysts [7].
Currently, the methods for synthesizing catalysts include impregnation [8], coprecipitation method [9], and sol-gel method [10].The coprecipitation method is widely applied in industrial production because of its advantages of simple operation, convenient large-scale manufacturing, and ease of morphology control.The selection of precipitant agents plays an important role for the preparation of composite catalysts [11].For example, Zhang et al. [12] used NH 3 ⋅H 2 O, Na 2 CO 3 , NaHCO 3 , NaOH, and (NH 4 ) 2 CO 3 as precipitant agents, 2 Journal of Nanomaterials respectively, to prepare MgO/SnO 2 /WO 3 catalysts, and they were used for synthesizing epsilon caprolactone.Specifically, the catalyst prepared by the NH 3 ⋅H 2 O precipitant agent exhibited excellent catalytic performance.Park et al. [13] prepared CuO catalysts using NH 3 ⋅H 2 O, NaHCO 3 , Na 2 CO 3 , and NaOH as precipitant agents and found that HCO 3 − and CO 3 2− weaken the Cu-O chemical bond in the presence of Na + on the CuO surface and accelerate the mobility of oxygen and the formation of lattice oxygen.Wang et al. [14] prepared Ni-CeO 2 catalyst by coprecipitation method using Na 2 CO 3 , NaOH, and a Na 2 CO 3 -NaOH mixture (molar ratio of 1 : 1) as a precipitant agent.The mixed precipitant had plenty of oxygen vacancies accompanied with highly dispersed Ni particles and exhibited high catalytic activity.Taken as a whole, it is important to prepare a catalyst with high catalytic performance that is also nonpolluting at the same time.
Building on the previous research, the Fe-Co-Ce composite catalysts were prepared by the coprecipitation method with iron nitrate, cobalt nitrate, and cerium nitrate as metal sources in the present study [15].Transition metals Fe and Co were used as the main components of the catalyst [16,17], and rare earth metal Ce was used as catalyst promoter [18].The present study aims at investigating the effects of CO(NH 2 ) 2 , NaOH, NH 4 HCO 3 , and NH 3 ⋅H 2 O on the particle size and morphology and structure characteristic of the Fe-Co-Ce composite catalysts.The influence of different precipitants on the chemical oxygen demand (COD) removal efficiency, decolorization rate, and pH of methyl orange in CWAO over the Fe-Co-Ce composite catalysts was also investigated.

Experimental
2.1.Catalyst Preparation.The Fe-Co-Ce composite catalysts were prepared by coprecipitation method, with a metal atom ratio of 1 : 1 : 1 in Fe : Co : Ce.A specific amount of Fe(NO 3 ) 3 ⋅9H 2 O, Co(NO 3 ) 2 ⋅6H 2 O, and Ce(NO 3 ) 3 ⋅6H 2 O (AR grade, Aladdin Reagent Co., Ltd.) was weighed, mixed in 100 mL distilled water, and stirred to create a homogenous solution with a concentration of 0.2 M. The precipitant agents CO(NH 2 ) 2 , NaOH, NH 4 HCO 3 , and NH 3 ⋅H 2 O were added to the mixed solution with a concentration of 1.5 M at a speed of 6 drops/min.The sample was magnetically stirred at medium intensity until white precipitants were generated and the pH of the mixed solution reached the equilibrium critical point of 6, 11, 8, and 10.Then, white precipitants were aged at room temperature for 2 h, and the resulting products were poured into a Buchner Funnel, washed twice with distilled water, and collected by vacuum filtration.Subsequently, the samples were dried at 105 ∘ C for 10 h with ventilation and calcined at 550 ∘ C for 3 h at a heating rate of 10 ∘ C/min.This precipitation-calcination was repeated twice to ensure complete loading.Finally, the Fe-Co-Ce composite catalyst powders were ground in a crucible, and they were marked as CAT-1, CAT-2, CAT-3, and CAT-4, corresponding to the precipitant agents CO(NH 2 ) 2 , NaOH, NH 4 HCO 3 , and NH 3 ⋅H 2 O, respectively.

Catalyst Characterization.
The surface morphology and energy dispersive X-ray spectroscopy (EDS) of the samples were observed by means of a scanning electron microscope (ZEISS Auriga FIB, Germany).The surface particle size and structure of the samples were obtained on a transmission electron microscope (TEM, JEOL JEM-3010).The surface and pore structure were determined using a physical adsorption instrument (ASAP2400, Micrometrics, USA) at −196 ∘ C. The samples were treated for 4 h under vacuum and 180 ∘ C before analysis.Thermogravimetry differential thermal analysis (TG-DTA) experiments were performed by a simultaneous thermal analyzer (EXSTAR-7000, Japan).The phase compositions of the samples were measured by a Rigaku D/Max-2400 X-ray diffractometer (XRD) with Cu K radiation at 40 kV, 100 Ma, and 6 ∘ /min.

Catalyst Test.
CWAO experiments treating methyl orange (25 mg/L) were performed in 0.5 L batch autoclave (GS-0.5,Weihai, China) with 2 g/L catalyst.Pure oxygen was pumped into the reactor with continuous heating until the reaction temperature and oxygen partial pressure were 180 ∘ C and 2.5 MPa, respectively.The device had a stirring rate of 500 rpm, and 20 mL aqueous solutions were taken for analyses at 10, 20, 40, 60, and 90 min.The catalytic performance was evaluated by the COD removal efficiency and decolorization rate of aqueous solutions.The COD of water samples was measured by the national standard method of potassium dichromate and absorbance of aqueous solutions was measured using a spectrophotometer.And the decolorization rate of aqueous solutions was calculated by the following formula: where  0 represents the initial absorbance and   stands for the absorbance at time .1(a)-1(d), respectively.Figure 1 shows the four types of catalysts under the above four precipitant agents, which were mainly composed of sphericallike and plate-like grains.The CAT-1 particles displayed obvious agglomeration and larger particle size, whereas the particles of CAT-2 mainly consisted of a flaky structure presumably because the formation of the crystalline grain was rapid under the strong alkalinity of NaOH [19].The particles of CAT-3 were quasi-spherical with a distinct profile and were poorly dispersed.The particles of CAT-4 were quasi-spherical too, but the particle size was smaller than that of CAT-3.The smaller particle size of CAT-4 could be due to the fact that the alkalinity of OH − was higher than that of HCO 3 − in NH 3 ⋅H 2 O which may have resulted in a rapid precipitation process.Figure 1(e) shows that, under the condition of ammonia precipitation, the atomic ratio of  Fe : Co : Ce was approximately 1 : 1 : 1, which was consistent with the mass fraction of Fe, Co, and Ce that was 17.0%, 17.7%, and 44.1%, respectively.

TEM and Particle Size Analyses.
Figure 2 shows the TEM images and the metal nanoparticle size distribution histograms of the Fe-Co-Ce composite catalysts prepared using the four precipitant agents.As shown in Figures 2(a), 2(c), 2(e), and 2(g), the particle size of CAT-1 was larger than that of other samples due to agglomeration with an ambiguous crystal profile having an average particle size of 11.0 nm.The crystalline grain generation rate of CAT-2 was rapid with partial agglomeration due to the strong alkalinity of NaOH, and an average particle size of 10.1 nm was observed.Both particles of CAT-3 and CAT-4 exhibited clear boundaries, and CAT-3 had a relatively complete shape.On the other hand, the particles of CAT-4 were evenly distributed on the catalyst surface, and the average particle sizes of CAT-3 and CAT-4 were 7.3 nm and 6.8 nm, respectively.Notably, using NaOH as the precipitant agent resulted in the introduction of Na + into the aqueous solutions, which adversely affected effluent quality [20].Figure 3(b) shows the decolorization rates of the methyl orange aqueous solutions in the CWAO over the catalysts prepared by the four precipitant agents.The trend of the decolorization rates was consistent with that of the COD removal efficiencies, and the decolorization rates of CAT-1, CAT-2, CAT-3, and CAT-4 were 81.4%, 99.8%, 86.7%, and 99.6%, respectively.Figure 3(c) illustrates that the pH of methyl orange aqueous solutions continually decreased in 90 min when treated with CAT-1 and CAT-3 catalysts.The pH of the methyl orange aqueous solutions increased after an initial decline when treated with CAT-2 and CAT-4.Methyl orange present in water can degrade to an organic acid, resulting in a low pH until the organic acid further decomposed into CO 2 , H 2 O, N 2 , and other small inorganic molecules, which then gradually results in increasing pH.After a treatment time of 90 min, the pH of the samples differed in the order of CAT-2 > CAT-3 > CAT-4 > CAT-1, with end point pH values of 5.7, 5.6, 4.9, and 4.6, respectively.The increase of the catalyst specific surface area was beneficial for increasing the effective contact area between the sample and methyl orange aqueous solution which in turn improved the ability of the catalyst to degrade the pollutants.From the above results, we can conclude that the particle sizes and pore structures of the catalyst affect the degradation of methyl orange.Therefore, NH 3 ⋅H 2 O was determined to be the most appropriate among the above four precipitants used in this study.

Physicochemical Properties of Catalyst Using the Ammonia
Precipitant Agent 3.5.1.TG-DTA Result.In order to further investigate the catalyst structure under optimal precipitation condition, the TG-DTA of the Fe-Co-Ce composite catalyst prepared using NH 3 ⋅H 2 O precipitant agent was analyzed and the results are shown in Figure 4.As shown in the DTA curve, two obvious endothermic peaks were observed at 186.8 ∘ C and 230.4 ∘ C, respectively, in the range of 0 ∘ C to 900 ∘ C. The TG curve was divided into three stages, with a large weight loss in the interval ranging from 0 ∘ C to 186.8 ∘ C, which indicated the loss of the adsorbed water.The second significant weight loss that occurred in the interval 186.8 ∘ C to 230.4 ∘ C was attributed to nitrate decomposition.A small amount of mass loss occurred in the range of 230.4 ∘ C to 900 ∘ C, attributed to the further decomposition of oxides.According to the above analyses, the catalyst was calcined at 550 ∘ C and with a stable chemical composition mainly containing metal oxides.Correspondingly, the decolorization rate decreased from 99.6% to 96.7% after six runs of 90 min each for the catalyst.This preliminary stability study supports the stability, limited activity loss, and repeatability of using the NH 3 ⋅H 2 O precipitant agent to prepare Fe-Co-Ce composite catalyst in methyl orange degradation.

Conclusions
(1) Among the four Fe-Co-Ce composite catalysts prepared using CO(NH 2 ) 2 , NaOH, NH 4 HCO 3 , and NH 3 ⋅H 2 O as the precipitant agent in this study, the catalyst prepared using CO(NH 2 ) 2 exhibited a spherical-like structure with a particle aggregation and larger size.Notably, its nucleation rate was higher than that of the other three catalysts due to its strong alkalinity.When NaOH was used as the precipitant, the particles presented flaky structure and partial aggregation.
Using the NH 4 HCO 3 and NH 3 ⋅H 2 O precipitant agents, the catalysts exhibited spherical-like particles with a distinct grain boundary.Using the NH 4 HCO 3 precipitant, catalyst particles exhibited heterogeneous distribution, while, using the NH 3 ⋅H 2 O precipitant agent, the catalyst particles exhibited a smaller average grain diameter of 6.8 nm and a larger specific surface area of 83.87 m 2 /g.
(2) For the catalytic wet air oxidation of methyl orange aqueous solutions over the Fe-Co-Ce composite catalysts, the effects of the NH 3 ⋅H 2 O and NaOH precipitant agents were superior to those of CO(NH 2 ) 2 and NH 4 HCO 3 precipitant agents.The initial COD removal efficiency of the catalyst prepared using the NH 3 ⋅H 2 O precipitant was 92.9% and the decolorization rate of methyl orange was 99.6%.After six runs using the same catalyst the COD removal efficiency of methyl orange was 90.1% and the decolorization was 96.7%.These data indicate that the Fe-Co-Ce composite catalyst prepared using the NH 3 ⋅H 2 O precipitant exhibited excellent both initial and repeated ability to degrade methyl orange.(3) The active components of the Fe-Co-Ce composite catalyst prepared using NH 3 ⋅H 2 O as the precipitant agent were mainly comprised of Fe 2 O 3 , CoO, and CeO 2 .

Figure 6 :
Figure 6: Recyclability histogram of the Fe-Co-Ce composite catalyst prepared using the NH 3 ⋅H 2 O precipitant.
The SEM images and EDS analyses of the Fe-Co-Ce composite catalysts, prepared using CO(NH 2 ) 2 , NaOH, NH 4 HCO 3 , and NH 3 ⋅H 2 O as a precipitant agent, are shown in Figures 3.1.SEM and EDS Analyses.

Table 1 :
Pore structures of the Fe-Co-Ce composite catalysts prepared using various precipitants.
3.4.Degradation Performance of Methyl Orange.Figures 3(a)-3(c)show the COD removal efficiencies, decolorization rates, and pH for CWAO of methyl orange aqueous solutions over the Fe-Co-Ce composite catalysts prepared using the four precipitant agents, respectively.The COD removal efficiencies for methyl orange aqueous solutions after 90 min were 62.7%, 94.3%, 77.2%, and 92.9%, using CAT-1, CAT-2, CAT-3, and CAT-4, respectively.The COD removal efficiencies of the Fe-Co-Ce composite catalysts prepared using NH 3 ⋅H 2 O or NaOH as the precipitant agent were higher than those prepared using CO(NH 2 ) 2 and NH 4 HCO 3 .
3.5.2.XRD Result.The XRD pattern of the Fe-Co-Ce composite catalyst prepared using the NH 3 ⋅H 2 O precipitant is displayed in Figure5.Diffraction peaks were observed at 28.6 ∘ , 33.1 ∘ , 47.5 ∘ , 56.3 ∘ , 59.1 ∘ , 69.4 ∘ , 76.7 ∘ , 79.1 ∘ , and 88.4 ∘ ∘ (214) were indexed to the crystal planes of Fe 2 O 3 (JCPDS 99-0060).This result suggests that the amount of Fe 2 O 3 was low or the particles of Fe 2 O 3 were uniformly distributed on the catalyst surface.Diffraction peaks characteristic of CoO (JCPDS 02-1217) were observed at 2 = 43.3∘,62.4∘and 79.1 ∘ , indicating that Co exists on the catalyst surface in the form of CoO.Therefore, the catalysts are mainly composed of Fe 2 O 3 , CoO, and CeO 2 , consistent with the results obtained from EDS and TG-DTA analyses.3.5.3.Stability and Recyclability.The stability and recyclability of the Fe-Co-Ce composite catalyst prepared using the NH 3 ⋅H 2 O precipitant agent were investigated by repeatedly using it to degrade methyl orange tainted water.The results are shown in Figure6, which indicates that the COD removal efficiency of methyl orange aqueous solutions reduced from 92.9% to 90.1% after six runs.