Highly nanoporous metal oxides with accessible three-dimensional surface areas offer unprecedented opportunities in catalysis, energy technologies, environmental remediation, etc [1-4].
Al2O3 has attracted great interest in variety of applications due to interesting catalytic, adsorption, optical, and electronic properties; so, the control and improvement of its physical-chemical characteristics is the key subject of ongoing studies [3-5]. Specifically, mesoporous Al2O3 (MA) with unique channels, high surface area, and narrow pore-size distribution is highly desirable for many of industrial and academic usages [6, 7]. A substantial point to achieve the ordered MA structures is solvent evaporation induced self-assembly (EISA) through various synthesis methods which all are mainly based on the sol-gel self assembly processes in the presence of soft (cationic, anionic, and nonionic surfactants) and hard (polymers and carbon molds) templates [6-9]. This allows fine-tuning of the structural properties of the MA-supported structures [10, 11]. Among its various crystalline phases, γ-Al2O3 is the mostly used catalytic support in the automotive and petroleum industries. Surface chemical composition, thermal stability, phase composition, and its local microstructure lead to special acid/base characteristics in γ-alumina .
γ-Alumina has been extensively used to prepare efficient supported noble metal (Pt, Au, Pd, etc) and transition metal (Ni, Co, Fe, etc) catalysts [13-17]. A large number of papers have been discussing the catalytic properties of such materials to promote the dehydrogenation and common oxidation reactions [18-20]. Among the transition and noble metals for being used on the surface, nickel and platinum are the commonly used active agents . Nickel is much cheaper but platinum is much more selective to catalyze the dehydrogenation at low and cracking reactions at high temperatures. According to the reports, the catalytic activity of the deposited Pt nanoparticles is incredibly affected by the microstructure of the support [21, 22]. To clarify this point, various organic and inorganic materials such as silica , activated carbon , carbon nanotubes , nanofibers , zirconia , metal organic frameworks (MOFs) , aluminas [14, 17, 29], zeolites , etc have been investigated and the results confirmed that γ-alumina can be selected as a promising candidate to be used in practical applications.
Because of unique properties including negligible vapor pressures, good electric conductivity, high thermal stability, high solvation interactions with both polar and non-polar compounds, and wide liquid temperature ranges and electrochemical windows, Room Temperature Ionic Liquids (RTILs) have attracted much attention in the synthesis of highly selective structures with a wide range of industrial applications . Besides, RTILs are specifically useful for the preparation of different kinds of nanoparticles with well-controlled morphology, sizes, and shapes, due to their extremely low vapor pressure and capability of dissolving various types of substrates and uniformly-dispersing a variety of solid-like particles. It has been demonstrated that the deposition of noble metals such as Au, Ag, and Pt in RTILs included matrixes results in corresponding metal nanoparticles, easily dispersed in the solution without using any additional stabilizing agent .
Herein, we reports the preparation of Pt loaded γ-alumina nanostructures. Decorating the surface with the [BMIM]PF6 ionic liquid layer was also performed to improve the sorption and catalytic activity. The prepared catalysts, well analyzed by XRD, TEM, EDX, and BET characterization methods, exhibited incredible catalytic activity toward oxidation of AS in the presence of hydrogen peroxide (Fig. 1). The experimental conditions are optimized to get the highest efficiency and a suitable mechanism was proposed for the oxidation reaction.
MATERIALS AND METHODS
Preparation of the catalyst
H2PtCl6.6H2O and Al(NO3)9.H2O (0.001 mol) were dissolved in a solution (water/ethanol: 20/80) contained Pluronic P123 (1 g) and polyvinylpyrrolidone (PVP, 0.5 g). AlCl3 (0.01 mol) was added and the system was stirred at 50 ºC for 3 h, aged for 12 h, and heated at 60 ºC over night to remove the solvent and obtain a xerogel. The product was heated at 550 ºC for 5 h with a heating rate of 5 ºC/min. The product (Pt/γ-Al2O3) was finely powdered and dispersed in a [BMIM]PF6 (in acetonitrile) solution to decorate the surface with ionic liquid layer. After 2 h of stirring at room temperature, the precipitate collected, washed with distilled water and ethanol repeatedly, and dried at 70 ºC overnight.
Catalytic oxidation of AS
The catalytic experiments were performed in a quartz reactor surrounded by a circulating water jacket (Pyrex) to keep the temperature of the reaction medium at 26 ºC. The catalyst powder was dispersed in the AS solution and then, diluted hydrogen peroxide solution (2%) was injected. Sampling was done at different time intervals of the reaction and analyzed by a UV spectrophotometer adjusted at AS lambda max (574 nm).
RESULTS AND DISCUSSION
XRD analysis was carried out to find the crystalline phase of the prepared structures and the results are indicated in Fig. 2. The characteristic peaks at 2theta 37.3, 46.0, 60.9 and 67.0º (black stars) are assigned to the Al2O3 gamma phase.
Pt diffraction peak is also appeared at 39.5º (blue star) which is attributed to the growth of Pt (111) surface orientation . The [BMIM]PF6/Pt/γ-Al2O3 pattern was similar to that of Pt/γ-Al2O3 with a decrease in intensity of Pt (111) characteristic peak which is due to the presence of [BMIM]PF6 layer on the crystalline surfaces. TEM images of the prepared nanostructures are shown in Fig. 3 which confirms the presence of Pt nanoparticles in the chemical texture. TEM images well demonstrate the formation of ionic liquid layer on the surface of Pt/γ-Al2O3.
Furthermore, from Fig. 3, the average size of the catalytic agents is below 80 nm and it’s not easy to allocate a specific regular shape to the Pt nanoparticles in the structure. EDX analysis was performed to find the elemental percentage and the results (Fig. 4 and Table 1) indicated that the prepared catalyst contained Pt, Al, and O as the main constituents.
Moreover, the amount of platinum in the chemical structure of the catalyst was as the same as the quantity used during the preparation steps (~2%). To find the specific surface area and pore size distribution, as important factors to control the catalytic activity, BET and BJH analyses were carried out and the results are represented in Fig. 5a and 4b. BET analysis evaluates specific surface area of materials by nitrogen multilayer adsorption measured as a function of relative pressure using a fully automated analyzer. The technique encompasses external area and pore area measurements to determine the total specific surface area in m2/g. BJH analysis is also employed to determine pore area and specific pore volume using adsorption and desorption techniques. This technique characterises pore size distribution independent of external area.
The hysteresis loop obtained from N2 adsorption-desorption isotherm well demonstrates the high porosity of the Pt/γ-Al2O3catalyst. According to this analysis, the specific surface area was determined to be 315 and 308 m2/g for Pt/γ-Al2O3 and [BMIM]PF6/Pt/γ-Al2O3. Expected to be observed, the small decrease in surface area is due to the [BMIM]PF6 layer on the surface which cover the pores and channels. Furthermore, from Fig. 5b, the maximum in pore size distribution curve is located between 2-50 nm and this indicates that the prepared catalyst is mesoporous.
Rapid oxidation of AS over the prepared catalysts
As a control experiment, 1 mL of H2O2 (2%) solution was added to 50 mL of AS solution (20 ppm) and kept at room temperature to evaluate the likely reactivity of the compounds in the absence of the prepared structures. After 72 h, the solution didn’t show significant decrease in concentration (lower than 2%) and this convinced us to use of the prepared nanostructure to catalyze the reaction. The experiments were performed with different concentrations of AS and catalyst and the results are indicated in Fig. 6a and 6b.
According to Fig. 6, the reaction efficiency enhanced impressively by increasing the catalyst dosage up to 10 mg and then became constant. This typically reminds the needed effective number of catalytic active sites to promote the oxidation reaction on the surface while the further quantities just provide more sorption sites in the medium. Changing the AS concentration had the same effect and we found that 40 ppm was the best concentration. Occupying the most of catalytic active sites by AS organic molecules and interfering with the heterogeneous oxidation pathway on the surface, the higher concentrations of AS decreased the reaction efficiency. To initiate the catalytic oxidation reaction, approaching the substrate and oxidative agent to the surface of the catalyst must be occurred under an equilibrium condition. Out of such condition which is considerably controlled by the substrate concentration and catalyst dosage, the reaction efficiency decreases. H2O2 concentration was also optimized to get the highest efficiency. From the results (Fig. 7), increasing the concentration up to 400 ppm increases the oxidation efficiency.
The reaction efficiency reached to more than 99 % within 10 min when H2O2 concentration increased from 40 to 400 ppm. This is assigned to the generation of a peroxo-intermediate and OH radicals attacking the organic molecules to oxidize them . Introducing as promising candidate for being used in the practical applications, the recycling experiments remarked the role of [BMIM]PF6 layer on the surface of the prepared catalyst. From Fig. 8, [BMIM]PF6/Pt/γ-Al2O3 indicated much better capacity than Pt/γ-Al2O3 during repeated using for AS oxidation. In fact, the catalytic active sites of Pt/γ-Al2O3 particles were easily poisonous due to strong chemical sorption of anionic AS molecules onto the surface. [BMIM]PF6, as a water insoluble ionic liquid, can control the chemical binding of water-solvated AS molecules to the surface of the catalytic particles. This reduces the poisoning of the catalytic active sites of the prepared nanostructure during the repeated uses. In addition, the cationic segment of the IL ([BMIM]+) can act as an absorptive part of the [BMIM]PF6/Pt/γ-Al2O3 nanostructure to attract the sulfonate heads of the AS molecules while they are not strongly adsorbed on the surface in comparison with that of Pt/γ-Al2O3.
As can be seen, the catalytic activity of [BMIM]PF6/Pt/γ-Al2O3 decreased smoothly after 6 times of repeated using. On the other hand, the reaction efficiency significantly descended for Pt/γ-Al2O3 after second time of recycling. In addition, the experimental data demonstrated that the oxidation of AS over the prepared nanostructures followed first order kinetics which means that ln A/A0= f(t) was expected to be linear. From Fig. 9, the rate constant for the reaction under the optimized conditions was determined to be 0.45 min-1.
The theoretical-experimental results released by Lousada et al. confirmed that the H2O2 promoted oxidation mechanism on the surface of suspended solids in aqueous solution includes the existence of an adsorption step .
AS is also adsorbed on the surface and this can be demonstrated by the following equations:
OH radicals generated on the surface can also be converted to other oxidative species which promotes the oxidation reaction from a different pathway:
In addition to above oxidative species, HO2 radicals are the other reactive agents produced from H2O2 dissociation on the surface which act pretty slower than the former types.
The symbol ⸦ in the equations 1-11 is the symbol of “on the surface”. From the proposed mechanism, collisions between the reactants in both aqueous and particulate phases promote the oxidative mechanism and prevention of each step reduces the reaction efficiency significantly.
From the obtained results, it is obvious that the prepared catalyst is highly active to decolorize the AS solution and oxidize a typical dye compound below 15 min of the reaction time which was very faster than the similar reactions investigated by the previous researchers [37, 38].
Mesoporous Pt loaded [BMIM] decorated γ-Al2O3 nanostructures prepared by a sol gel-pyrolysis method using PVP and Pluronic p123 as surfactant and template agents. The results showed that the IL layer on the surface enhanced the sorption capacity and diminished the poisoning rate of catalytic active sites. XRD patterns and EDX analysis well confirmed the presence of Pt (111) nanoparticles on the surface of the support. BET and BJH analyses indicated that prepared catalyst was mesoporous with specific surface area of ~350 m2/g. The results of the H2O2-promoted catalytic experiments implied that the prepared catalyst were incredibly able to oxidize AS in the aqueous phase. 10 mg and 40 ppm were selected as the optimum values for catalyst dosage and AS concentration. According to the recycling experiments, [BMIM]PF6/Pt/γ-Al2O3 exhibited a promising capacity after six times of repeated decolorization of the AS solutions and being used in the practical applications which points out the role of ionic liquid layer on the surface.
CONFLICT OF INTEREST
The authors declare that there are no conflicts of interest regarding the publication of this manuscript.