Catalytic reduction of nitrocompounds is actively used in the cleaning of the environment from nitroarenes, including explosive nitrocompounds [1
], and in industrial manufacturing of amino compounds [2
]. Toxic properties of nitroarenes are presented in many publications [3
]. There are several methods to reduce nitrocompounds, namely, chemical reduction [1
], biological reduction [5
], photocatalytic degradation [6
], electrochemical methods [7
], etc. However, the transformation of the nitrogroup into an aminogroup by catalytic hydrogenation is the most widely used, since it has low energy-intensiveness and does not use harmful organic solvents [8
]. The liquid-phase reduction of nitroarenes has a significant environmental impact on wastewater purification and push for the decision by using new sorbents [9
], catalysts and technical solutions [10
Despite the fact that there have been many studies of the catalytic hydrogenation of nitroaromatics, many catalysts do not meet the requirements for practical use, and there are still several challenges:
1) Due to the high cost of Pd and Pt catalysts, the development of catalysts without Pt-group metals, such as Au-based ones, is required.
2) Carrying out selective hydrogenation with the preservation of other functional groups. In addition, controlling the degree of recovery of the nitrogroup to produce a fully hydrogenated amino group is of great interest.
3) Obtaining a material with a high catalytic performance, since the selectivity increase often leads to a decrease in activity.
Therefore, the elaboration of active, selective, and environmentally benign catalysts to synthesize amines is essential.
Catalysts based on noble metals [11
] are used to reduce nitrocompounds under mild conditions. Contrary to other noble metals, silver is a cheaper raw material that possesses high chemical activity [12
]. Recently, Liao et al. [15
] overviewed the features of p-nitrophenol reduction over Ag catalysts. It was shown that several factors determine the catalytic properties of the deposited Ag particles: dispersion (particle size), particle shape, pretreatment conditions, the nature of the precursor, support, etc.
In our previous work, we showed that for Ag/CeO2
, the metal–support interaction contributes to enhancing the catalytic activity in CO and soot oxidation [16
]. For CeO2
support, in addition to its unique redox properties, the strong metal–support interaction (SMSI) and electronic metal–support interaction (EMSI) are characteristic phenomena having an impact on the following oxidation processes: electro-oxidation of methanol [17
], CO oxidation [18
], hydrogenation of quinolones [19
], photocatalytic reactions [20
]. The role of the features of the metal–support interaction in reduction processes is poorly discussed in the literature [21
], especially for Ag/CeO2
In the present work by varying some experimental conditions with respect to the Ag/CeO2
series previously reported [16
], we have synthesized three Ag–CeO2
catalysts with relatively high Ag loading (20 mol. % corresponding to ~13.6 wt. %). Different techniques have been used in order to control Ag oxidation state and particle size distribution, as well as Ag–CeO2
interfacial interaction. Depending on the preparation method, the catalysts were labeled as follows: Ag–CeO2
(by co-precipitation method), Ag/CeO2
(impregnation of the as-prepared ceria) and Ag/CeO2(red)
(impregnation of pre-reduced ceria).
The p-nitrophenol reduction into p-aminophenol in aqueous media with NaBH4 under mild conditions (room temperature, atmospheric pressure) was studied over the prepared Ag–CeO2 catalysts.
We found that the prepared silver-based catalysts exhibited different activity in the p-nitrophenol reduction under ambient conditions in accordance with the effects of the preparation method that affected the oxidation state of silver and the size of metal particles, their dispersion on the surface or within the catalyst, and interaction with the ceria support.
prepared by the co-precipitation method is characterized by the formation of spherical agglomerates. The XRD data shows that both small and large metallic Ag particles are formed in this catalyst, owing to the redox reaction between silver and ceria precursors during the co-precipitation. This catalyst showed the lowest activity. According to Ref. [15
], the activity of silver-based catalysts in nitrophenol reduction is mainly determined by silver active sites dispersed on the catalyst surface. Accordingly, we can assume that the relatively low activity of the Ag–CeO2
catalyst is connected with the Ag distribution both on the surface and inside the agglomerates, and only a part of silver is accessible for the reaction. Additionally, the dispersion of silver should be considered, and the high amount of relatively large silver particles in Ag–CeO2
leads to a decreased active surface of silver and, therefore, to low activity.
The catalysts prepared by the impregnation techniques, especially for Ag/CeO2, are characterized by higher activity than the co-precipitated one. TEM and XRD data show that silver is stabilized predominantly in a highly dispersed form, i.e., the active surface of silver is higher than that for Ag–CeO2 catalyst. In the TPR profiles, the increased intensity of the low-temperature peaks for the catalysts prepared by impregnation indicates that silver is well dispersed, likely present as silver oxide and/or as mixed silver-cerium oxide species, and it is in good contact with the ceria surface. The area of these peaks is connected with the amount of easily reducible silver oxide-like species. It is expected that the reduction of this oxidized silver species by NaBH4 occurs during the catalytic reaction with the formation of highly active metallic silver species well interacting with ceria surface. The highest amount of this species for Ag/CeO2 catalyst correlates with the highest activity of this catalyst.
As for the Ag/CeO2(red), the additional reductive pretreatment of ceria support before the impregnation leads to the partial reduction of silver during the impregnation and formation of relatively large silver particles. This explains the lower activity of the Ag/CeO2(red) catalyst as compared to the one of Ag/CeO2. Thus, the activity of Ag/CeO2 catalysts is mainly determined by the active surface of silver, and Ag stabilization in a highly dispersed state appears favorable when Ag was deposited by the classical impregnation technique that resulted suitable for the preparation of Ag/CeO2 catalyst with high activity in the p-nitrophenol reduction.
4. Materials and Methods
4.1. Synthesis of Catalysts
Ceria-based catalysts were prepared by two methods: precipitation [25
] and impregnation [40
]. The Ag–CeO2
catalyst was prepared by co-precipitation method using mixed aqueous solution of AgNO3
(2.1 g) and Ce(NO3
O (21.8 g), where the diluted ammonia solution (11.6 mL of 25% NH4
OH solution diluted with 36.2 ml of H2
O) was added at room temperature immediately with a rotary stirrer (350 rpm). Stirring for 1 min was used, then the co-precipitate was evenly heated by steam for 10 min in an autoclave at 120 °C. The co-precipitate was centrifuged, washed by distilled water, dried overnight at 120 °C, and calcined at 500 °C for 5 h in air. The Ag loading in the synthesized catalysts was 20 mol. % (corresponding to ~13.6 wt. %). The CeO2
support was synthesized by the same method via precipitation of cerium(III) nitrate hexahydrate with ammonia solution.
catalysts with Ag loading of 20 mol. % were synthesized with impregnation techniques using ceria prepared by precipitation and calcined at 500 °C for 5h. The as-prepared ceria or the one pre-reduced at a temperature of 500 °C for 30 min in H2
/Ar flow (10 vol. % H2
) were impregnated with an AgNO3
aqueous solution. Then the samples were dried at 120 °C overnight and calcined at 500 °C for 1 h. The catalysts were denoted as Ag/CeO2
(as-prepared ceria as a support) and Ag/CeO2(red)
(pre-reduced ceria as support), respectively. The reductive pretreatment of ceria support yields surface Ce3+
species. The subsequent impregnation with AgNO3
results in the redox reaction between Ce3+sur.
to yield Ce4+
and Ag, and the enhanced interfacial Ag–CeO2
]. The Ce3+
ions participate in the same redox reaction during the co-precipitation of the corresponding nitrates by ammonia solution.
4.2. Materials Characterization
The porous structures of the obtained materials were investigated using the low-temperature N2 adsorption (−196 °C) at an automatic gas adsorption analyzer TriStar 3020 (Micromeritics, Norcross, GA, USA). The multipoint BET method using the flattening of the adsorption isotherms (the p/p0 range was from 0.05 to 0.30) was applied to calculate specific surface area (SBET). The BJH-Desorption method accompanied by the analysis of the desorption branches of the N2 adsorption–desorption isotherms was employed to estimate the pore size distributions. Prior to the experiments, the degassing of the samples (50–80 mg) at 200 °C and 100 mtorr for 2 h was used.
The powder X-ray diffraction (XRD) analysis was carried out on the Miniflex 600 diffractometer Riguku (Austin, TX, USA) with CuKα radiation (λ = 1.5418 Å) equipped with a monochromator to study the phase composition of the prepared materials. The scanning rate was 0.2 deg/min, and the 2Θ range was 2–90°. The PCPDFWIN databases and the full-profile analysis program POWDER CELL 2.4 were used. The coherent scattering region (CSR) and Scherrer equation were used to estimate the particle sizes.
The electron microscope JEM-2200 FS (JEOL, Tokyo, Japan) was applied to investigate the sample structure with the high-resolution transmission electron microscopy (HR TEM). The resolution was 0.1 nm, the accelerating voltage was 200 kV. The DigMicrograph (GATAN) software allowed determining the crystal lattice parameters by applying the Fourier transform.
The temperature-programmed reduction in hydrogen (H2-TPR) on the chemisorption analyzer ChemiSorb 2750 (Micromeritics, Norcross, GA, USA) made it possible to study the features of the sample reduction. The thermal conductivity detector (TCD signal) was used. The heating rate was 10°/min, the flow comprised an argon–hydrogen mixture (10 vol. % H2). The flow rate was 20 mL/min.
4.3. Catalytic Activity Test
The room-temperature (25 °C) p-nitrophenol reduction with NaBH4 was used to study the catalytic properties of the prepared materials in aqueous media and at atmospheric pressure. Firstly, 28.9 mg of NaBH4 were added into 50 mL of aqueous solution of p-nitrophenol (0.15 mmol/L) under rapid magnetic stirring to produce a homogeneous solution. The stirring rate was 700 rpm and it was previously optimized to carry out the reaction under these conditions in a kinetic mode. Then 6 mg of the as-synthesized catalyst was added to the mixture under stirring. The yellow color of the solution was due to the light absorption by p-nitrophenolate ion (maximum at 400 nm). The ion was formed through the p-nitrophenol dissociation under basic conditions. During the catalytic experiments, the color change from bright yellow to colorless was observed due to the p-nitrophenolate conversion into p-aminophenol. The spectrometer Solar PB 60 was used to measure the UV-Vis spectra and control the reaction progress.