Preparation and catalytic evaluation of Au/γ -Al2O3 nanoparticles for the conversion of 4-nitrophenol to 4-aminophenol by spectrophotometric method

A set of catalysts having gold nanoparticles deposited on γ -Al2O3 ( Au/ γ -Al2O3) with lowest effective amount of gold content were prepared by successive impregnation and hydrogen reduction method. The structural features of prepared catalysts were analysed by X-ray diffraction (XRD), N2 physisorption, scanning electron microscopy (SEM), and Fourier transform infrared (FTIR). The catalytic activity was evaluated for the reduction of an organic pollutant 4-nitrophenol (4NP) to 4-aminophenol (4AP) by spectrophotometric analysis. Supported catalyst presented excellent catalytic ability to convert 4NP to 4AP in the presence of sodium borohydride (SBH) due to synergistic effect of Au NPs and mesoporous γ -Al2O3 support. The reduction reaction was also performed at a range of temperatures to calculate kinetic parameters. The development of highly stable Au/γ -Al2O3 catalysts with lowest noble metal content and recyclability made the process cost effective and may promote their applications in various fields including removal of organic pollutants in industrial waste water and high-temperature gas-phase reactions.


Introduction
Supported gold catalysts are currently being used to catalyse various reactions having environmental and industrial significance. The incredible catalytic activity of supported gold nanoparticles (NPs) was first identified by Haruta et al. [1]. Since then, gold NPs have been used as a catalyst in many reactions involving electron transfer process [2][3][4][5][6].
Industrially important catalytic processes mainly require stable supported metal or metal oxide catalysts [7]. The advantages of supported metal catalysts are the uniform metal distribution, no agglomeration, and deposition of less metal content on the support, easy recovery of expensive noble metal, multiple uses of the catalysts, and cost economics [8]. The catalytic performance of a metal deposited on a support depends on the properties of support as well as active metal [9,10]. The support can play a crucial role in understanding the catalytic phenomenon including diffusion of reactants [11,12], metal-support interactions [13,14], hydrophilic character of catalyst [15] etc. Gold NPs have been supported on multiple supports including silica, titania, mixed oxides, zeolites, boehmite, and alumina [16][17][18][19][20]. The most commonly used catalytic support is γ -Al 2 O 3 due to its high surface area and porosity, optimum mechanical strength, and uniform pore size distribution [21]. * Correspondence: r_quresh@qau.edu.pk This work is licensed under a Creative Commons Attribution 4.0 International License.
Typically, impregnation method is employed for the preparation of supported catalysts. It is based on soaking the support into metal salt solution followed by evaporation and metal reduction. A surfeit of reports on the catalytic activity of supported gold NPs has explored the redox behaviour of Au NPs [12][13], acid-base properties of gas-phase gold, and gold-oxide [14][15][16]. To date several theories have been put forward to explore an extraordinary catalytic activity of supported gold NPs; nonetheless these theories are somewhat contentious [22][23][24][25]. The outcomes of these theories showed that the size of gold NPs, choice of support, and method of preparation played an important role in delivering a highly efficient catalyst [26].
Pal et al. for the first time introduced catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) [27][28][29][30]. This reaction takes place in aqueous system on the surface of a catalyst. Reduction of this organic pollutant can be conveniently monitored by decrease of strong UV-visible absorption spectra of 4-nitrophenolate anion at 400 nm [31,32]. Au nanoparticles supported on Al 2 O 3 and SiO 2 are reported to be thermally much more stable than Au nanoparticles on TiO 2 . For example, thermal treatment of Au nanoparticles up to 700°C , on SiO 2 , let particles grow from 4 to 6 nm while on TiO 2 from 3 to 13 nm. Also, Al 2 O 3 provides a nonreducible support and a nonoxidizing atmosphere for stable supported Au nanoparticle [33]. To the best of our knowledge we obtained lowest effective nobel metal loading as compared to previous reports [33][34][35][36].
In the present work, a set of Au/γ -Al 2 O 3 catalysts were prepared by successive impregnation method.
For this purpose, γ -Al 2 O 3 support in granular shape (mesh size = 0.6-1 mm and BET surface area = 175 m 2 g − 1 ) was prepared by sol gel route. All the prepared samples were characterized by XRD, surface area analysis, SEM, and FTIR. The aim of this work was to synthesize cost efficient supported gold catalysts with low noble content and evaluate their catalytic activity for the reduction of 4NP to 4AP using SBH as a reducing agent by UV-visible spectroscopy. Interestingly, the prepared catalysts showed excellent activity towards the conversion 4-NP to 4AP, owing to good metal dispersion and metal-support interactions.

Preparation of supported catalysts
Alumina support was produced by sol gel method using 0. Cl − , Na + , NH + 4 etc.). If these impurities remain in the support, they can affect the efficiency of catalyst. Alumina gel was converted to the granules by a method described earlier [37][38]. In order to obtain gamma phase, granules were heat treated in programmable muffle furnace up to 750°C for 16 h. Granular support ( γ -Al 2 O 3) was obtained having mesh size, between 0.6 and 1 mm and BET surface area of 175 m 2 g − 1 .
A series of Au/γ -Al 2 O 3 catalysts were prepared by wet impregnation method with composition of 0.2%, 0.4%, 0.6%, 0.8%, 1% by weight. Impregnation was accomplished on predried γ -Al 2 O 3 granular support using required volume of HAuCl 4 precursor. All catalysts were soaked overnight, dried in oven at 120°C for 2 h and further calcined at 550°C for 3 h. Finally, catalysts were reduced in H 2 gas (99.99 %) flow at 550°C for 3 h.

Characterization
X-ray diffraction (XRD) patterns of the synthesized materials packed in an aluminium glass holder were recorded at room temperature using Philips PW-1840 diffractometer with CuK α radiation ( λ = 0.154 nm) in 2 θ range of 20°-80°with scan rate of 0.01 θ s − 1 .
Surface area analysis (SAA) of supported catalyst was carried out on KELVIN 1042. Degassing of the samples was carried out at 250°C with N 2 as carrier gas. BET surface area was obtained from nitrogen adsorption-desorption isotherms measured at liquid nitrogen temperature (-196°C). Pore size and total pore volume were obtained by employing BJH method.
Fourier transform infrared (FTIR) spectroscopy analysis was carried out on Thermo Scientific, model Shimadzu spectrophotometer) with wavelength range of 190-1100 nm was utilized to collect spectral data.

Catalytic activity test
The catalytic performance of Au/γ -Al 2 O 3 catalysts was evaluated with the help of a well-studied reaction of conversion of 4NP to 4AP using SBH as a reducing agent and UV-visible spectra were recorded during the reaction [32]. In a typical reaction, 5 mg of sample in a standard quartz cuvette was taken and then 100 µL of 1mM 4NP solution was added. The volume was adjusted to 3.5 mL with water and finally ice-cold SBH (100 µL, 100 mM) was added to the above quartz cuvette. The decrease in absorbance of 4NP was recorded with a time interval of 3-5 min (according to the reaction conditions). Rate constants were observed to be dependent upon the wt% of Au nanocatalyst and temperature. The catalytic reaction was carried out at various temperatures to calculate kinetic parameters (rate constant and activation energy).

Results and discussion 3.1. Characterization of support and catalysts
The nitrogen adsorption-desorption isotherms (Figure 1a) of 0.6Au/γ -Al 2 O 3 sample with type IV shape designated the presence of mesopores with uniform pore size distribution [37,38]. Table 1 presents the composition, BET surface area, pore volume, average pore diameter, and crystallite sizes of γ -Al 2 O 3 support and Au/ γ -Al 2 O 3 catalysts. Figure 1b shows unimodal distribution of pore for Au/ γ -Al 2 O 3 catalyst which exhibited with maxima centred at~7 nm. BET surface area and pore volume of Au catalysts were found to decrease with respect to γ -Al 2 O 3 support. It can be correlated to occlusion of alumina pores with Au particles during impregnation process. In addition, with further increase in metal content, BET surface area of the catalysts was examined to decrease which might be due to metal-on-metal deposition besides the blockage of alumina pores with metal. After the deposition of gold NPs on γ -Al 2 O 3 support the surface area of the support decreased from 175 m 2 g − 1 to 129 m 2 g − 1 and pore volume decreased from 0.47 cm 3 g − 1 to 0.310 cm 3 g − 1 . This reduction in surface area and pore volume of the pure support after metal loadings provides an evidence of the successful deposition of the gold NPs by impregnation method.

Samples
Composition Surface properties Au a (wt%)  where K is particle shape factor (0.9 for cubic system), λ is wavelength of X-ray beam (0.154 nm), β is peak broadening (FWHM), and θ is diffraction angle.
D XRD values of all samples were calculated by using high intensity diffraction peak and were found~6 nm. To investigate the morphology of synthesized materials, SEM images were taken. Figure 3a presents SEM image of γ -Al 2 O 3 support calcined at 750°C. With a coarse surface, γ -Al 2 O 3 was observed having particles of irregular size and shape. In contrast, 0.6Au/γ -Al 2 O 3 catalyst (Figure 3b-3d) presented homogenous distribution of Au NPs over the surface of γ -Al 2 O 3 support at different resolutions. Au particles were found spherical in shape having uniform size. Here, high metal dispersion of this optimal composition was corresponded for its excellent activity towards conversion of 4NP to 4AP.    Figure 5 shows the typical UV-visible spectra for the conversion of 4NP to 4AP. A significant decrease in peak intensity was observed upon the conversion. In the spectrum 4-nitrophenol was depicted λ max at~400 nm and was shifted to~295 nm during the catalytic reaction thus confirming the formation of 4-aminophenol.

Catalytic testing of prepared catalysts
The solution (4-Nitrphenol) is light yellow in colour, while 4-aminophenol is colourless. The physical change from light yellow colour to a colourless solution was also examined during the reaction, depicting a complete transformation from 4-nitrophenol to 4-aminophenol. We selected 0.6wt% Au/γ -Al 2 O 3 for complete catalytic testing. The reason for selecting this wt% is its ability to catalyse reaction at reduced time as compared to lower (0.2 and 0.4wt%) and higher (0.8 and 1wt%) values. Lower wt% has less deposition of gold nanoparticles and higher wt% block the mesopores in γ -Al 2 O 3 , which in turn decreases the availability of active sites of nanogold. Thus, out of the 5 prepared catalysts, 0.6Au/ γ -Al 2 O 3 was chosen to proceed for further studies.  Temperature was found to have a pertinent effect on the catalytic performance. The catalysed reaction showed change in its kinetics on increasing the temperature from 25°C to 40°C as can be seen in Figure 7a The rate constant was calculated using the following equation: lnA/A°= -kt Eq (1) lnA-ln A°= -kt Eq (2) ln A = ln A°-kt Eq (3) Where, A is absorbance of 4-NP at time t, A°is initial absorbance of 4-NP, k is the rate constant, and t is the time interval for UV-visible data collection.
The plot of ln A versus t (Figure 7a) at different temperatures produces a straight line with the slope equivalent to rate constant. The rate constant value was observed to increase 5.4 times on increasing the temperature from 25°C to 40°C. The activation energy (E a ) was calculated using Arrhenius equation. Figure 7b shows the Arrhenius plot of rate constant versus the reciprocal of temperature (both in logarithmic scale) with a straight line. The activation energy was found to be 97 Kjmol − 1 . This value of E a is high as compared to the E a for free colloidal suspension of gold nanoparticles as given in our previous results . [31,39]. Although gold NPs deposited on γ -Al 2 O 3 were well dispersed as can be seen in Figures 3b-3e, still all the active sites of these supported gold NPs were not available for catalysis. Since we see how the physical state (colloidal or supported) of a catalyst affects its catalytic performance, we can conclude that size and physical state of the catalyst play a very crucial role in their catalytic performance.  Supported gold catalyst was well stable during the course of reaction and even after many cycles in comparison to the reported colloidal alloy nanoparticles [39], which show aggregation during and after the catalysed reaction. It is reported that capping material is usually stripped off during the catalytic reaction, so this may contribute towards the nanocatalyst aggregation which is not observed in the present case for the supported catalyst. Present catalyst is also recyclable, it was used 6 times with appreciable efficiency ( Figure   8). The AuNPs supported on the surface of mesoporous γ -Al 2 O 3 were found to be stable and did not leach out into the solution after heating at 60°C for 1h. This was confirmed by taking UV-visible of solution which shows no SPR for free AuNPs or gold salt, also SEM imaging, which showed no free AuNPs in the solution ( Figure 3e).

k = A exp (-E a /RT) Eq (4)
The dependence of the reduction of 4-NP by BH as a function of temperature can be modelled in terms of Langmuir-Hinshelwood model [40][41][42]. According to this model both reactants must be adsorbed to the surface of the nanocatalyst to react and this reaction was found to be kinetics controlled where the transportation of the reactants through the solution was not the rate determining step, rather the formation of 4-AP was the rate determining step. The adsorbed species can then react, and the product will desorb from the surface of supported gold catalyst (Figure 9). Comparative performance of Au/γ -Al 2 O 3 catalyst with available literature is compiled in Table 3.

Conclusions
In the present work, a sol-gel method was utilized to synthesize mesoporous γ -Al 2 O 3 in granular shape with BET surface area of 175 m 2 g − 1 , which was reduced to 125 m 2 g − 1 after gold deposition. Au/γ -Al 2 O 3 catalysts with 0.2-1wt% metal contents were prepared via impregnation method. The results of SAA indicated a decrease in BET specific surface area of Au catalysts due to occlusion of γ -Al 2 O 3 pores with metal particles while pore diameter increased up to 8 nm at the expense of sintering process. XRD results depict the formation of single phase γ -Al 2 O 3 NPs. SEM depicted a uniform dispersion of Au particles on the surface of γ -Al 2 O 3 support. The metal particles were found spherical. The catalytic activity of Au/ γ -Al 2 O 3 (0.6wt%) catalyst