Pt-group metals supported on oxides or carbon are of great importance in many industrial chemical processes as heterogeneous catalysts [1-3]. They usually exhibit high efficiency in both oxidation and reduction reactions [4-7]. However, large scale application of Pt-based catalysts is still limited by their high cost [8-11]. Moreover, active Pt sites cannot discriminate the differences of functional groups, causing a low catalytic selectivity [12-14]. Take the hydrogenation of nitroarenes as an example, nitro groups and other reducible groups (e.g., C=C, C=O, –X, etc.) can be simultaneously hydrogenated over Pt metal active sites [15-18]. Some solutions for improving the selectivity include adding metal salts or organic compounds as additives, or loading a less active component (e.g., Au, Ag, etc.) [19-23]. The role of additives is to avoid the accumulation of hazardous aromatic hydroxylamines or lower the flat adsorption of nitroarenes with the benzene ring to decrease a simultaneous exposure of the reducible groups toward the active sites. However, most of them are at the cost of catalytic activity, and additional metal salts also face with the post-processing and new environmental problems.

Recent research shows that rational design of Pt-group centers and/or proper regulating the environment around Pt sites could obtain highly chemoselective catalysts for hydrogenation of nitroarenes [24-29]. For example, Zhang's group [24] reported that the catalyst with single-atom and pseudo-single-atom Pt supported on FeO_x exhibits high activity and chemoselectivity for hydrogenation of functional nitroarenes. Corma et al. [25] showed that the chemoselectivity can be significantly improved by generating nanosized Pt with special exposed crystal faces on the surface of a TiO₂ support. Qu and his coworkers [26] used porous CeO₂ nanorods supported sub-nanometric Pd clusters as catalysts to improve chemoselectivity. However, practical applications of these catalysts still face great challenges due to the dependence of activity on the amount of Pt loading [28]. Reducing the Pt sites within the catalysts has a significantly negative effect on the catalytic activity. Moreover, the aggregation of Pt nanoparticles and stability of the oxide supports under strong reduction atmosphere also deserve concern.

In this work, we attempt to carry out the strategy of spatial separating the active sites to develop low cost and highly efficient catalysts for chemoselective hydrogenation of nitroarenes. It is expected to introduce the participation of oxide supports to enhance the utilization efficiency of Pt sites. This strategy is mainly based on the H₂-spillover mechanism. The challenges for realizing this strategy include the efficiency of H₂ dissociation over Pt centers, and the capacity of H diffusion and reactants adsorption over oxide supports [30-32]. More important is the synergism of these performances over Pt and oxide supports. Therefore, the fabrication of Pt sites and choosing well-matched oxides supports are the key factors for this strategy.

For achieving above goals, a colloid method is adopted to prepare Pt active centers, which has been proved in our previous work that this method could effectively control the sizes and the dispersion states of Pt particles [4, 33]. Considering the strong electron-withdrawing properties of nitro groups, several metal oxides with Lewis-basic sites are screened as catalyst supports, including Fe₂O₃, CeO₂, Al₂O₃ MgO and TiO₂. They could generally provide large number of electron-rich sites to interact with nitro groups [34, 35]. Interestingly, Pt/Fe₂O₃ with Pt-loading amount as low as 0.2 wt% exhibits high catalytic activities and chemoselectivities in the hydrogenation of a variety of nitroarenes. It should be noted that the catalytic tests are carried out under quite mild reaction conditions (30 ℃, 5 bar H₂). Besides, both the Pt nanoparticles and Fe₂O₃ support exhibit a high stability under the strong reduction atmosphere. The highly efficient H₂ dissociation over Pt nanoparticles (Pt⁰), and the capacity of H diffusion and reactant adsorption over α-phase Fe₂O₃ should be co-responsible for the excellent performance of Pt/Fe₂O₃ catalysts.

Hexachloroplatinic(Ⅳ) acid hexahydrate, Fe(NO₃)₃·9H₂O, Ce(NO₃)₃·6H₂O and sodium carbonate were purchased from Sinopharm Chemical Reagent Co., Ltd. Sodium hydroxide was obtained from Beijing Chemical Work. Titania P25 (TiO₂) was purchased from Evonik Degussa (ca. 50 m²/g, anatase ca. 80% and rutile ca. 20%). Magnesium oxide (MgO) was obtained from Tianjin Fuchen Chemical Reagents Factory. Alumina oxides (γ-Al₂O₃) were purchased from Longkou Chemical Paking Co., Ltd. All chemical reagents used in the experiment were of commercially available analytical grade, which were used without further purification.

The precursor of the Fe₂O₃ support was prepared via a precipitation reaction of an aqueous solution of Fe(NO₃)₃·9H₂O with sodium carbonate. In a typical preparation, a Na₂CO₃ aqueous solution was obtained by dissolving 23 g of Na₂CO₃ in 100 mL of water. 100 mL of an aqueous solution containing 56 g of Fe(NO₃)₃·9H₂O was added dropwise into above solution. Then, additional aqueous solution of Na₂CO₃ (0.1 mol/L) was added into the mixture until the pH value reached 9.0. After aging for 2 h, the precursor was collected by filtration and washing with distilled water. Finally, the Fe₂O₃ support was obtained by calcining the precursor at 500 ℃ for 3 h. CeO₂ was prepared by thermal treating Ce(NO₃)₃·6H₂O at 350 ℃ for 4 h.

Platinum colloids were prepared by a polyol reduction method which was reported in our previous work [4, 33]. Typically, 13.5 mL glycol solution of sodium hydroxide (0.34 mol/L) and 30 mL glycol solution of H₂PtCl₆·6H₂O (8.5 × 10^-4 mol/L) were mixed under vigorous stirring for 20 min, and then heated at 140 ℃ for 30 min under N₂ flow.

All the oxides-supported Pt catalysts were prepared using a colloidal deposition method. Taking Pt/Fe₂O₃ catalyst as an example, a certain amount of Fe₂O₃ was added to the Pt colloids. After stirring for about 2 h at room temperature, the mixture was heated at 80 ℃ for 12 h. Then, the resulting mixture was filtered and washed several times with distilled water and dried at 100 ℃ overnight. Before characterizations and activity tests, the obtained solid was calcined in 20% O₂/Ar at 200 ℃ for 2 h. The loading amount of Pt in Pt/Fe₂O₃ was tuned by the content of Pt colloids in the mixture, and finally identified with an inductively coupled plasma atomic emission spectrometer (ICP). According to the ICP results, the samples were denoted as 0.1 wt% Pt/Fe₂O₃, 0.2 wt% Pt/Fe₂O₃ and 0.5 wt% Pt/Fe₂O₃. The preparation of other oxides-supported Pt catalysts also used the above colloidal deposition method. The loading amount of Pt was identified by ICP and shown in Table 1.

Powder X-ray diffraction (XRD) patterns were recorded with a Rigaku X-ray diffractometer using Cu K_α radiation (λ = 0.15418 nm) at 40 kV and 40 mA. Elemental analysis for Pt was obtained using a PLASMA-SPEC(Ⅰ) inductively coupled plasma atomic emission spectrometer (ICP). Transmission electron microscopy (TEM) images were obtained on a JEOL JEM-2010 electron microscope with an operating voltage of 200 kV. Temperature-programmed reduction (TPR) was performed using a Tianjin Xianquan TP-5079 adsorption analyzer. Before detection, the catalysts were treated in an Ar (99.99%) flow at 120 ℃ for 30 min. After the sample was cooled down to 30 ℃, the flowing gas was switched to 5 vol% H₂/Ar and the sample was heated to 800 ℃ with a temperature ramp rate of 10 ℃/min. N₂ adsorption-desorption isotherms were measured at –196 ℃, using a Micromeritics ASAP 2010N analyzer. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo ESCA LAB 250 system with Mg K_α source (1254.6 eV). Binding energies were obtained by referencing to the C 1s binding energy of carbon (peak at 284.6 eV). In-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) spectra were recorded on a Nicolet 6700 spectrometer. Among them, in-situ CO adsorption DRIFT measurement was carried out to investigate the state of Pt. The samples were pretreated in N₂ flow at 100 ℃ for 10 min. After the system was cooled to room temperature, a background spectrum was collected. Then the sample was exposed to 1 vol% CO/Ar flow before recording the spectra. In-situ DRIFT spectra of nitrobenzene adsorption and hydrogenation were obtained as following. Firstly, the samples were treated in 30 mL/min N₂ flow at 100 ℃ for 10 min. Then, the nitrobenzene gas was carried by N₂ flow and introduced into the sample cell for adsorption. In the hydrogenation measurement, the mixture of hydrogen and the nitrobenzene gases was simultaneously introduced into the sample cell, and the spectra were in-situ recorded.

The liquid-phase hydrogenation reaction of nitroarenes was performed in a magnetically stirred 50 mL stainless steel autoclave. A quantity of catalyst (50 mg) was added to the autoclave containing nitroarene (1 mmol) and toluene (15 mL). Then, the autoclave was flushed with 10 bar hydrogen five times. After being sealed, the autoclave was charged with H₂ until 5 bar and then it was kept at 30 ℃. A rotation rate of 1000 r/min was used in this work, which can effectively exclude the influence of mass transfer on the reactions. The reactants and products were analyzed with a gas chromatograph (GC-112A, FID detector) equipped with an HP-5 column (30 m). In the recycling experiments, the solid catalyst was separated by simple decantation and directly used for the next cycle without any further treatment.

The hydrogenation of nitrobenzene was carried out to screen suitable oxide supports for supported Pt catalysts. Five oxides, Fe₂O₃, CeO₂, Al₂O₃, MgO and TiO₂, were chosen based on their the surface acid-base properties and reducibility [36]. All the supported Pt catalysts were prepared by a colloid deposition method. For achieving the aim of low cost, the Pt contents in these catalysts were controlled at 0.1 wt%-0.5 wt%. No Pt diffraction peaks can be observed in the XRD patterns of these supported Pt catalysts (Fig. 1 and Fig. S1). Table 1 shows the catalytic performance of these catalysts in hydrogenation of nitrobenzene. Without a catalyst or in the presence of the Fe₂O₃ support only, nearly no products can be detected (Table 1, entries 1 and 2). Under the same reaction conditions, Pt/Fe₂O₃ and Pt/CeO₂ exhibit obviously higher activities than Pt/MgO, Pt/Al₂O₃ and Pt/TiO₂. Compared with Pt/CeO₂, Pt/Fe₂O₃ possesses the advantage of high stability under the reaction conditions (Table 1, entries 3 and 4). It can be recovered from the reaction mixture through simple filtration, and there is no obvious change in conversion of nitrobenzene and selectivity to aniline after five successive runs (Fig. 2A). It should be noted that mass transfer can be excluded in the above tests. Because of the bulk nature of these oxides (Fig. S2), the active sites are nearly all located on the external surface of the supports. In addition, the influence of stirring rate on the catalytic performance was also investigated. The same reaction rates under 800 and 1000 r/min over the Pt/Fe₂O₃ catalyst further show that mass transfer has little influence on the reaction (Fig. S3). The different activities over these catalysts should be mainly due to the surface properties of the oxide supports, which will be discussed below.

Fig. 2B shows the catalytic activities of Pt/Fe₂O₃ with different Pt loadings. The conversion of nitrobenzene improves with the increase of Pt contents. According to the initial reaction rates of these catalysts, the activities are normalized by Pt loading, which is 2627, 3170 and 2063 mol_conv. h^-1 mol_Pt^-1 for 0.1 wt% Pt/Fe₂O₃, 0.2 wt% Pt/Fe₂O₃ and 0.5 wt% Pt/Fe₂O₃, respectively. It should be noted that such high activities are obtained under quite mild reaction conditions (30 ℃, 5 bar). Compared with the literature results under the similar reaction conditions, Pt/Fe₂O₃ exhibits obvious advantage (Table S1). Compared with the commercial Pt/C catalyst, Pt/Fe₂O₃ also possesses advantage. The conversion rate of nitrobenzene over 0.5 wt% Pt/Fe₂O₃ and 0.2 wt% Pt/Fe₂O₃ is also obviously higher than Pt/C under the same reaction conditions (Fig. 2B). It is known that the loading amount of Pt in the commercial Pt/C catalyst is 5 wt%, much higher than that of Pt/Fe₂O₃.

In addition, it is found that Pt/Fe₂O₃ also displayed good performance when extended to substituted nitroarenes. We mainly focus on nitroarenes with reducible groups such as –C=O, O=C–O and –X because they are with grand challenge for the application of selective hydrogenation. Table 2 shows that Pt/Fe₂O₃ exhibits high selectivity to the corresponding anilines. Even for halonitrobenzene, Pt/Fe₂O₃ catalyst is also with relatively high chemoselectivity. Previously, it has been reported that most Pt-based catalysts suffer from the hydrogenolysis of weak carbon-halogen bond [24]. In our case, the high chemoselectivity should be mainly due to the spatial separation of H₂ dissociation and hydrogenation active sites. As for commercial Pt/C catalyst, a relatively low chemoselectivity is detected in the hydrogenation of substituted nitroarenes under the same reaction conditions (Table 2). Moreover, a decrease of chemoselectivity can also be observed over Pt/Fe₂O₃ when Pt loading is larger than 1 wt%, which should be due to the increase of contact possibility between Pt sites and reducible functional group.

Comparing the nitrobenzene conversion rates normalized by the Pt loading amount, it can be seen that these values are at the same order of magnitude, which should mainly ascribe to the uniform Pt nanoparticles highly dispersed on the surface of Fe₂O₃ and playing the similar role in the reaction process. It can be observed that the value over 0.5 wt% Pt/Fe₂O₃ is relatively low (2063 mol_conv. h^-1 mol_Pt^-1), showing that there is no linear relationship between activity and the Pt loading. This result could give strong support to our strategy that separating the hydrogenation active sites from Pt to the supports. It is known that the Pt loading of 0.5 wt% is also a low content value compared with many literature and the contrast sample in this case. Given that the Pt particles are the hydrogenation active sites, the average contribution of Pt would not decrease from 3170 mol_conv. h^-1 mol_Pt^-1 (0.2 wt% Pt) to 2063 mol_conv. h^-1 mol_Pt^-1 (0.5 wt% Pt). It would maintain such high conversion efficiency at least in this loading range. The results in our case show that Pt sites only serve as hydrogen dissociation centers. The hydrogenation process should occur on the surface of Fe₂O₃ supports. According to above results, a reaction mechanism concerning H₂ dissociation, nitroarene adsorption and hydrogenation is proposed (Scheme 1). The Fe₂O₃ support provides adsorption sites for the nitroarenes and should also function as the reaction area for the hydrogenation.

To verify this hypothesis, a series of characterizations were carried out to detect the physicochemical properties of Pt/Fe₂O₃ and in-situ reaction process of nitroarene hydrogenation over this catalyst. XRD patterns (Fig. 1) show that only diffraction peaks assigned to α-Fe₂O₃ phase could be observed on the patterns of both samples with different Pt contents and the one after five cycles. It shows that α-Fe₂O₃ possesses a relatively high stability. Both the reducibility of Pt colloids in the preparation process and the strong reduction atmosphere in the reaction process have little influence on the phase structure of α-Fe₂O_3. No diffraction peaks assigned to Pt can be observed, suggesting that Pt nanoparticles are highly dispersed on the surface of α-Fe₂O₃.

TEM images further confirm the above results. Fig. 3b and 3c shows that almost all Pt particles in the Pt/Fe₂O₃ catalyst are consistent with the size of Pt particles in the colloids (Fig. 3a). The size of 3–4 nm is the most probable distribution of these particles. It should be noted that Pt nanoparticles are highly dispersed on the surface of the Fe₂O₃ support, which might be correlated with the low loading amount. The shortest distance between two Pt nanoparticles is about 12 nm in the detected region. As for the spatial separation of the active sites, enough reaction area is needed, which could effectively reduce the contact possibility between the reducible groups of nitroarenes and Pt sites. Considering the largest molecule in the reactants is about 6 Å, it can be speculated that Pt/Fe₂O₃ provides enough spaces to achieve the spatial separation of H₂ dissociation and hydrogenation active sites. Besides, the TEM images of the sample after five cycles are also shown in Fig. 3. Both the sizes and dispersion states of Pt are almost the same as that of the fresh one. No particle aggregation can be observed, suggesting that Pt nanoparticles possess a high stability during the reaction process.

Pt 4f XPS was measured to analyze the surface chemical state of Pt nanoparticles (Fig. 4). Because of the detection limit, Pt signals in low Pt-content samples are hard to detect, so 0.5 wt% Pt/Fe₂O₃ was used as the representative sample for testing. Two main peaks centered at about 71.5 and 74.6 eV can be observed, which are corresponding to the spin-orbit split doublet of Pt 4f_7/2 and Pt 4f_5/2, respectively [33, 37-39]. The binding energy of this original peak center is consistent with the Pt⁰ electron binding energy. Although the spectra still could be deconvoluted into the components of metal Pt (Pt⁰) and oxide states Pt (Pt²⁺), the amount of oxide states Pt (Pt²⁺) is quite limited. Metallic Pt should be the absolutely predominant one in the sample.

Furthermore, in-situ DRIFT spectra of adsorbed CO at room temperature were also recorded over the samples of 0.1 wt% Pt/Fe₂O₃, 0.2 wt% Pt/Fe₂O₃ and 0.5 wt% Pt/Fe₂O₃ to confirm the chemical state of Pt. It is known that v_co > 2100 cm^-1 is assigned to the linear adsorption of CO on oxide states Pt, while v_co < 2100 cm^-1 is ascribed to CO adsorbed on metallic Pt [40, 41]. Fig. 5 shows that a relatively strong band at 2085 cm^-1 appears in all three samples, which can be ascribed to CO adsorbed on metallic Pt. This result confirms that Pt mainly presents as metallic state in Pt/Fe₂O₃, and a small amount of Pt²⁺ detected by Pt 4f XPS should be located at the interface between Pt nanoparticles and supports, which can be reflected from the shoulder peak of CO adsorption. This metallic Pt possesses strong H₂ dissociation ability. H₂-TPR profiles show that the reduction peaks corresponding to Fe³⁺ to Fe²⁺ shift significantly to the relatively low temperature in the presence of Pt nanoparticles (Fig. 6). It decreases at least 120 ℃ compared with that of pure Fe₂O₃ support. Increasing the Pt contents could further decrease the reduction temperature. In addition, the low reduction temperature and narrow reduction peak could also show that Fe₂O₃ support possesses excellent hydrogen spillover capability. It could facilitate the diffusion of hydrogen on the surface of Fe₂O₃. In our case, the strong H₂ dissociation ability of Pt nanoparticles and hydrogen spillover capability of Fe₂O₃ support should be two important factors that catalyze the hydrogenation reaction under mild conditions (30 ℃, 5 bar).

For understanding the adsorption behavior and hydrogenation of nitroarenes over the Pt/Fe₂O₃ catalyst, in-situ DRIFT measurements were carried out. Nitrobenzene is introduced by a N₂ flow to the Fe₂O₃ support and 0.2 wt% Pt/Fe₂O₃. Two bands appeared at 1531 and 1350 cm^-1 could be attributed to asymmetric stretching and symmetric stretching vibrations of the nitro group, respectively [28]. There is no difference between Fe₂O₃ support and Pt/Fe₂O₃ (Fig. 7a and 7c), suggesting that the nitro group mainly adsorbed on the surface of Fe₂O₃ support. Upon introducing H₂ to 0.2wt% Pt/Fe₂O₃, a new broad band can be observed at 1589–1620 cm^-1 (Fig. 7d), which is attributed to the vibrations of –NO and –NH₂ species [28, 42, 43]. This suggests that hydrogenation of nitrobenzene occurs over 0.2 wt% Pt/Fe₂O₃. In contrast, when H₂ is introduced to the Fe₂O₃ support, no signal assigned to –NH₂ species can be observed (Fig. 7b). This is mainly due to the lack of H₂ dissociation sites on the Fe₂O₃ support.

The strong nitrobenzene adsorption behavior for Fe₂O₃ should origin from the presence of oxygen vacancies. The O 1s XPS spectrum (Fig. 4) indicates that a large amount of oxygen vacancy (531 eV) is present on the surface of Pt/Fe₂O₃ [44, 45]. These oxygen vacancies are electron-rich sites, which could have a relatively strong interaction with nitro groups (strong electron-withdrawing group). Correspondingly, a relatively broad Fe 2p XPS peak (711 and 724.3 eV) can be observed (Fig. S4). It shows that most of iron species on the surface of the samples are present as a valence of +3, combining with a small amount of iron species with a valence of +2 [10, 46]. This property should facilitate both the reactant adsorption and the hydrogen spillover.

It should be noted that the high stability of α-Fe₂O₃ under the reduction condition is also a critical factor for Pt/Fe₂O₃ to become an excellent catalyst. We had ever detected the performance of ferrihydrite (Fe(OH, H₂O) and composite iron oxides co-presence of (Fe(OH, H₂O) and α-Fe₂O₃ (Fig. S5). It is well known that the presence of water could accelerate the diffusion of hydrogen atoms across solid oxide surfaces. Although the catalysts prepared with above two oxides exhibit a little higher initial activity than α-Fe₂O₃ (Fig. S6), their poor performance in the following reaction cycles shows that they cannot become efficient catalysts (Fig. S7). As for the support of α-Fe₂O₃, it exhibits not only relatively high activity but also high stability in the hydrogenation of nitroarenes.

Combined with the catalytic tests and characterization results, it can be confirmed that the supports play an important role in the reaction process. The catalyst performance directly correlates with their capability of reactant adsorption and hydrogen spillover. Reducible oxides have higher capability of hydrogen spillover than irreducible oxide supports, which reflects on not only the spillover speed but also the migrated distance [30]. It should be the main reason that Al₂O₃- and MgO-supported Pt catalysts exhibit low activities. The stability of the supports under the reduction condition should also be concerned. The poor recyclability of CeO₂ and ferrihydrite supported samples should be mainly due to the stability of the oxide support in the presence of H₂. Actually, both CeO₂ and ferrihydrite have higher reducibility than α-Fe₂O₃ because of the high activity of surface oxygen species, but these active species are easily damaged in the strong reduction atmosphere of H₂.

In addition, the adsorption behavior of the supports is another important factor that affects the catalytic performance of these catalysts. It is directly affected by the surface electronic properties of the supports. Some literature has reported that modifying the catalysts with vanadium salts or organic thiol compounds could effectively tune the adsorption and desorption behavior of the catalysts [19, 20]. The role of vanadium salts is to avoid accumulation of hazardous aromatic hydroxylamines, while organic thiol compounds could avoid the flat adsorption of nitroarenes via the benzene ring. Flat adsorption would cause a simultaneous exposure of the reducible groups toward the active sites. In our case, the suitable surface chemical state of α-Fe₂O₃ facilitates the adsorption of nitro group of nitroarenes. The unique properties of α-Fe₂O₃ realize its high performance cooperated with Pt nanoparticles for chemoselective hydrogenation of niroarenes.