XPS and TPR studies of 2%(Ru1/2-Sn1/2)/Al2O3catalyst synthesized by microwave techniques

X-Ray photoelectron spectroscopy (XPS) and Temperature-programmed Reduction (TPR) of 2%(Ru1/2-Sn1/2)/Al2O3 prepared by the microwave-assisted method have been studied. 2%(Ru1/2-Sn1/2)/Al2O3 have previously been evaluated as a catalyst in the reduction reaction of 4-nitrophenol. XPS and TPR studies of 2%(Ru1/2-Sn1/2)/Al2O3 was performed to confirm the metal state of Ru and Sn and reducibility of the sample, respectively. XPS data suggested that the formation of metal Ru(0) and Sn(0) in all samples. The XPS spectra of Ru 3p3/2 and Sn 3d5/2 shows binding energy peak at ∼ 461.0 ± 0.2 eV and ∼ 484,1 eV, respectively. The correlation of TPR and XPS characterization will be discussed further.


Introduction
Nitro-aromatic compounds such as 4-nitrophenol are the most important class of industrial chemicals which is widely used in the synthesis of many diverse products, including dyes, polymers, pesticides, and explosives. Their extensive use has led to environmental contamination of soil and groundwater. US Environmental Protection Agency has listed the aromatic compound as "priority pollutant" for environment [1]. Borohydride reduction of 4-nitrophenol to 4-aminophenol by metal nanoparticles has received increasing attention because this reaction can be performed in aqueous solutions under mild condition. Nanoparticles serve as better catalysts in the facile reduction process of pollutants like 4nitrophenols due to the greater accessibility to surface atoms and low coordination number. The characteristics and the structures of photocatalysts were depended on the preparation processes. In the other words, the preparation method plays a key role in a successful synthesis of catalysts.
Various Ru-Sn/Al 2 O 3 catalysts which had been prepared according to different metal reduction sequencesunder microwave irradiation was evaluated in the process of 4-nitrophenol hydrogenation [2]. The results of catalytic measurements obtained at room temperature revealed that differently prepared catalysts and the physical mixture containing Ru-Sn/Al 2 O 3 with the same Ru/Sn ratio exhibited similar activities [2]. The structure of Ru-Sn bimetallic catalysts prepared by microwave irradiation method has not been reported.The structure of these solids has so far been studied by means of EXAFS, XANES, temperature-programmed reduction (TPR), CO adsorption and FTIR analyses [10][11][12]. The TPR of Pd-Cu/KL-zeolite bimetallic catalysts was studied by Fernandez-Garc´ıa et al. [11] using XANES and infrared spectroscopy with the probe molecule CO. The authors showed that complete alloying with electronic perturbation of both components was reached over the full range of compositions. They also reported that the effect of reduction temperature on the nature of the surfaces exposed by the active phases, as well as the sintering processes.
In this paper, Ru-Sn based nanocatalyst was characterized by XPS and TPR to examine the oxidation states of metals prepared in a different route.

Sample Preparation
The synthesis of 2%(Ru 0.5 Sn 0.5 )/Al 2 O 3 bimetallic nanoparticles was adopted from the method reported [3], [4] with some modifications in preparation steps and washing procedures [5]. In detail, sample preparation procedures were previously reported elsewhere[2].

X-ray Photoelectron Spectroscopy (XPS) and TPR characterization
X-ray photoelectron spectra (XPS) were acquired by KRATOS AXIS ULTRA spectrometer equipped with a hemispherical electron analyzer an Al Kα (E = 1486.4 eV, 1 eV = 1.6302 x 10 -19 J) 300-W of the X-ray source. The powder sample was mounted on a double-sided copper adhesive tape and then placed on a sample rod in a pre-treatment chamber and then transferred to the analysis chamber. Before recording the spectra, the sample was maintained in the analysis chamber until a residual pressure of ca. 5x10 -7 Nm -2 was reached. The narrow scan spectra of the particular element were collected at the pass energy of 20 eV.
Normally, the binding energy of C1s was used as a standard for calibration. In this study, we examined Ru 3p instead of Ru 3d because the Ru 3d spectra are overlapped with the region of C1s [4], [5]. For that reason, we also calibrated the reading using Al 3+ binding energy (74.4 eV) as reference [6]. Spectral background calibration used Shirley or Linear method for every peak and adjusted using the Gaussian function.
The characterization of the sample using TPR is done using ThermoFinnigan TPD/R/O 1100 apparatus. Before being characterized, the sample was treated in a flow N 2 gas stream with a flow rate of 5 cc/min for 5min at room temperature. Then the temperature was raised at a warming rate of 10 ºC/min to a temperature of 150°C and held for 5 min. At the time of TPR characterization, 5% hydrogen gas in argon with flow rate (25 cc/min) flowed into the sample at a temperature rate of 10 ºC/min to 450 ºC. H 2 gas consumption is read at every temperature increase.

X-Ray Spectroscopy
To confirm the reduction of Ru and Sn, the irradiated samples were characterized by X-ray Photoelectron Spectrometer as shown in Figure 1. From XPS analysis, all samples contain ruthenium in its metallic state. It is characterized by the binding energy of Ru 3p 3/2 ~ 461.0±0.2 eV [4]. Other signals at higher binding energy were assigned to the higher oxidation states of ruthenium (Ru 4+ and Ru 6+ ) i.e. 463.1±0.1 eV and 466±0.2 eV.The surface oxidation of ruthenium was actually possible  [4], [5]. A narrow scan of the Sn 3d 5/2 XPS spectra provides some information. Sn at metallic oxidation state was detected for RS-2 and RS-3 samples which are noticed at 484.1 eV. However, Sn(II) and Sn (IV) are unable to be differentiated [7]. According to literature, Sn 3d 5/2 peak of Sn at ~ 486.5 might be assigned to Sn(II) and/or Sn(IV) [6]. The peak at ~ 484.1 eV corresponds to Sn o . The RS-1 sample shows two Sn 3d 5/2 peaks centered around 484.1 eV and 486.8 eV. The first peak is most probably due to tin Snº species.
According to the literature [6], [8], this binding energy is not much different from the binding energy of Sn(II) ~ 484.8 eV [8]. The results were contrary with the results observed from uncalcinedRu-Sn bimetallic prepared by the co-impregnation technique where the metallic tin cannot be observed [6]. In our work, the as-prepared sample obtained by microwave irradiation technique exhibits Sn in the metallic state for all samples as listed in Table 1. From the results, cc As we can see from XPS analysis, bimetallic PVP-stabilized Ru-Sn/Al 2 O 3 catalyst obtained via microwave irradiation are majority composed of ruthenium and Sn(II) species. In all samples, the oxidation state of Sn is in major concentration. The tin is tightly bonded (via oxygen) in the outer surface of the γ-Al2O3 particles and may act as electronic promoter. Intensity, a.u. (c)

Temperature-Programmed Reduction
The XPS resultswere then correlated with the characterization of TPR. The effect of the preparation route to the TPR profile of the Ru-Sn catalyst sample is supported with Sn/Ru atom ratio of 1.0 and compared with Sn / Ru = 0 sample shown in Figure 2. The differences in H 2 use patterns for different samples after preparation indicate a change in decline behavior. The emergence of a peak of hydrogen use was seen in the 1% Ru/Al 2 O 3 TPR pattern at temperatures around (~ 223 o C). The intensity is related to the free Ru (0) formed in support [6]. TPR sample RS-1 samples show peak intake of H 2 at around ~ 140 o C with a reduction rate of 160-190 o C. There is also a peak at 330 o C and another peak above 450 o C. The peak at 140 o C is the reduction in RuCl 3 [9]. It also implies that there is still chlorine content in the sample, in accordance with the analysis using XPS in Table 1. The decrease in hydrogen consumption after temperatures around 160 and 200 o C is derived from the reduction of Ruthenium  [9]. The peak above the 450 o C temperature comes from Sn (IV) which does not interact with Ru (0) [6], [10]. The previous studies showed that the Ru-Sn preparation route drastically alters 2%(Ru-Sn)/Al 2 O 3 catalytic properties[2]. In the case of RS-2 catalysts, provided by a consecutive reduction, the peak of the TPR reading was observed near 100 o Cdue to the reduction of ruthenium oxide (RuO 2 ) to the Ru metal [11]. Another small peak seen at 180 o C is also due to RuO 2 [12]. The use of low hydrogen by RS-2 sample shows that it can be easily reduced. This is contrary to other findings in conventional Ru-Sn cases that found that Sn prevented access to hydrogen for reduced ruthenium [13]. Sample prepared from in-situ reductionof metal salts with the presence of Al 2 O 3 and ethylene glycol (RS-3) has a similar profile with RS-2. It exhibits a wide peak at 179 o C given to RuO 2 [12]. The RS-2 and RS-3 profiles of hydrogen usetemperature decline at the range of approximately 270 o C. The peak at 270 o C comes from another Ru n+ with interactions toSn n+ [6]. Low-temperature reduction of RS-2 and RS-3 samples from low peaks also show a strong interaction of Ru and Sn species to alumina support [13].

Conclusions
XPS studies of 2%(Ru-Sn)/Al 2 O 3 by three different preparation route under microwave irradiation, contain ruthenium and tin at their metallic state. Ruthenium and Tin exist majorly in an oxidation state ofRu(0) andSn(II) on the surface of reduced samples, respectively. TPR studies of the catalysts showed the most reactive sample most probably due to its high content of Ru(0) with no interaction with Sn.