Reduction of Nitroarenes Using Efficient PdRu@mSiO2 Nanocatalyst Synthesized by a One‐Pot Approach

In the present work, PdRu@mSiO2 bimetallic core‐shell nanoreactors (NRs) are synthesized for the first time by a fine‐tuning one‐pot method. The obtained NRs are evaluated in the reduction of nitroarenes using 4‐nitrophenol (4‐NP), 1‐chloro‐4‐nitrobenzene (1‐Cl‐4‐NB), 4‐nitrotoluene (4‐NT) and 2,4‐dinitrophenol (2,4‐DNP) as reagents. The mesoporous PdRu@mSiO2 NRs with a Pd1:Ru1 molar ratio present a homogenous spherical morphology with a single nucleus per capsule. Various techniques confirm the formation of Pd‐Ru alloy. The bimetallic NRs show higher catalytic activity and stability compared with the reference catalysts (free and supported nanoparticles (NPs)) and with those reported in the literature. The order of catalytic activity for studied nitroarene compounds is 4‐NP>1‐Cl‐4‐NB>2,4‐DNP>4‐NT. The catalytic activity of NRs is affected by inter and intramolecular interactions between the reagent molecules. The one‐pot method of NRs synthesis is low‐cost and effective, with possible application in the catalytic reduction of various hazardous materials.


Introduction
[6][7][8][9] For example, n-phenols are cited by the United States Environmental Protection Agency (USEPA) as one of the 114 primary organic pollutants that cause health complications like carcinogenesis, teratogenesis, and mutagenesis. [10,11,2]For this reason, exploration of practical methods to degrade nitroarenes is imperative.
[14][15][16][17][18][19] Catalytic hydrogenation of nitroarenes to aminoarenes using noble metals stands out among them due to its ecological aspects, efficiency, and practicality since it may be carried out under mild conditions (ambient temperature and atmospheric pressure). [20]The reaction mechanism consists of the gradual reduction of the nitro group (─NO 2 ) of nitroarene to the amino group (─NH 2 ) of aminoarene by interaction with the atomic hydrogen species located on the metallic sites of the catalyst.Sodium borohydride (NaBH 4 ) is used as the source of hydrogen (reducing agent); it hydrolyzes in aqueous media, releasing ions (BH 4 − ) that act as hydrogen donors through their deprotonation to form hydrogen and borohydride in the gas phase. [21]ubsequently, molecular hydrogen may be adsorbed and dissociated on the metal surface of the catalyst.Once the hydrogen is formed and accumulated on the metallic site of the catalyst, the nitroarene molecule can be transformed into the corresponding aminoarene molecule. [22,23]Hence, the reduction of nitroarenes can only proceed on metallic-based catalysts.39] Due to the synergistic effects of bimetallic NPs, they have been widely applied to different catalytic reactions.For example, Anila Monga et al. [40] reported a two-fold catalytic activity of bimetallic Ag-Au catalysts compared with monometallic Ag NPs in the reduction of nitrobenzene, 3-nitrotoluene and 1-chloro-3nitrobenzene.The effect was attributed to the synergistic effect between Ag and Au and their morphology. [40]In the reduction of nitroarenes, improved catalytic activity has been reported for bimetallic NPs such as Au-Pt and Pt-Ag. [37][43][44][45][46][47][48][49] According to the literature, the catalytic activity was significantly improved when Pd and Ru were combined. [50]For instance, Pitchaimani et al. synthesized a bimetallic catalyst based on PdRu supported on activated carbon (Pd-Ru@CFAC), which proved to be efficient in the reduction of several toxic compounds, including 4nitrophenol (4-NP), highlighting the synergistic effect between both metals. [49]Similarly, Chandan et al. found a higher catalytic performance of the Pd0.5Ru0.5-PVPcatalyst in the reduction of nitroarenes compared with monometallic Pd, Ru, and Rh catalysts. [51]etallic NPs are known to be very active due to the high contribution of unsaturated atoms on their surfaces, corners, and edges. [52,53]However, their catalytic activity may decrease due to particle aggregation. [54]Therefore, NPs are confined into mesoporous capsules to prevent their aggregation and the ensuing loss of active sites.[63][64][65] NRs such as Pd@hTiO 2 , [45] Ru@rGO [47] Au@Fe 3 O 4 , [63] Au@SiO 2 , [66] AuPd@ZrO 2 , [67] and Pd/Au@[Na]-HMAS, [55] are widely used in nitroarene reduction.However, the information available for PdRu bimetallic catalysts applied to nitroarene reduction is limited.To the best of our knowledge, to this date, no reports on PdRu-based NRs have been published.
At present, the elimination of environmental pollutants, such as nitrogen oxides, sulfur compounds, chlorinated, and other organic compounds using commercially applicable economic catalysts is very important.Researchers worldwide are constantly looking for the best catalytic materials for these processes.
The aim of the present work was to search for new materials based on encapsulated PdRu NPs, enclosed in a porous oxide system to prevent their migration and agglomeration, with the purpose of testing them in the catalytic elimination of environmental pollutants such as nitroarenes.

Attenuated Total Reflection -Fourier Transform Infrared (ATR-FTIR) spectroscopy
The thermal decomposition of cetyl trimethyl ammonium bromide (CTAB) was analyzed by ATR-FTIR spectroscopy.Note that the CTAB removal is a significant step because it may be adsorbed on the surface of the NPs, blocking access to the active sites.Figures 1 and S1 (Supporting Information) show the ATR-FTIR spectra of NRs before and after thermal treatment at 450°C.The spectrum for uncalcined PdRu@mSiO 2 nanostructures showed well-detectable bands at c.a. 2922 and 2847 cm −1 (enclosed in the green box in Figure 1; Figure S1, Supporting Information).However, with thermal treatment, these bands disappeared.[70] So, the bands observed herein were assigned to the stretching vibrations of C─H bonds in methylene (─CH 2− ─) groups of the CTAB molecule attached to the surface of uncalcined samples.The disappearance of these bands is directly correlated with CTAB removal from the surface of the NPs, as mentioned in. [71,72]Hence, the spectra for calcined samples confirmed the complete removal of CTAB from the surface of the bimetallic and monometallic NPs, guaranteeing the interaction between the nuclei (the active sites) and the reagents during the reaction.

Transmission Electron Microscopy (TEM)
Figure 2a,b shows typical micrographs of PdRu@mSiO 2 NRs at different close-ups.The dark quasi-spherical spots in the micrographs were assigned to the metallic PdRu NPs, while the  dark-gray zone surrounding the NPs corresponds to the silica that compose the capsules.Note that the silica capsules reproduce the shape of the nuclei (quasi-spherical) and display a notably porous structure, which was attributed to the elimination of organic compounds during the thermal treatment.The obtained samples consisted mainly of uniform and spherical capsules with one nucleus inside each capsule.The bimetallic PdRu nuclei showed an average size of ≈25 ± 3 nm (Figure 2c), while the monometallic Pd and Ru nuclei were on average ≈20 ± 2 nm and ≈10 ± 2 nm (see Figures S2 and S3, Supporting Information), respectively.The bimetallic nuclei displayed an intermediate size between Pd and Ru NPs (Figure 2c; Figures S2c and S3c, Supporting Information).Analysis of high-resolution transmission electron microscopy (HRTEM) images (Figure 3) confirmed homogeneous metal dispersion in the bimetallic nuclei.No metal segregation or lattice planes were detected, which confirmed the PdRu alloy formation.As to the NRs, the diameter of the bimetallic PdRu@mSiO 2 NRs was larger and showed a broader distribution than monometallic NRs (Figure 2c; Figures S2c and S3c, Supporting Information).The average diameter of the bimetallic and monometallic NRs was 102 ± 3, 80 ± 1, and 82 ± 4 nm for PdRu@mSiO 2 , Pd@mSiO 2 and Ru@mSiO 2 , respectively.The capsule thickness was ≈30nm in both cases (see Figure 2 and Table 1; Figures S2 and S3, Supporting Information).[73] No nuclei were observed outside the SiO 2 capsules, supporting the conclusion that all PdRu, Pd, and Ru NPs were confined inside a SiO 2 porous capsule.The PdRu nuclei were covered with CTAB during the synthesis, which acted as a stabilizing agent and a bridge to facilitate the selective formation of silica capsules around the nuclei.It has been reported elsewhere that TEOS hydrolysis in the alkaline medium leads to the formation of negatively charged oligomeric silicate species able to interact strongly with the hydrophilic ends of CTAB. [74,76]At the same time, CTAB polymerized, accompanied by the formation of capsules around the bimetallic nuclei forming PdRu@mSiO 2 core-shell nanostructures.Pore formation was caused by the thermal treatment of the nanomaterials, which resulted in the release of residual organic compounds and atomic diffusion of Si atoms. [75,76]Comparable results were observed for monometallic Pd@mSiO 2 and Ru@mSiO 2 core-shell nanostructures (Figures S2 and S3, Supporting Information).Thus, the bimetallic PdRu@mSiO 2 NRs obtained herein via a one-pot approach were characterized by homogeneous size, and shape, and were mononucleic, that is, they contained one nucleus per capsule.

N 2 Physisorption
The textural properties of the obtained nanostructures were evaluated by nitrogen physisorption.Figure 4a shows the  nitrogen adsorption-desorption isotherms of the synthesized mono and bimetallic NRs.A typical IV-type isotherm with an H2-type hysteresis loop appears at a relative pressure range (P/P 0 ) of 0.4−-1.0.These results are characteristic of mesoporous materials. [71,72]The values obtained for Brunauer-Emmett-Teller (BET) surface area were: 874, 870, and 764 m 2 g −1 for Pd@mSiO 2 , Ru@mSiO 2 , and PdRu@mSiO 2 , respectively (see Table 1).The surface area, pore size, and pore volume of the presently prepared mono and bimetallic NRs were markedly bigger in comparison with SiO 2 nanospheres (@SiO 2 , presented as a reference) obtained by a conventional Stöber method (see Table 1).For instance, the surface area values were increased >40 times, passing from 19.2 m 2 g −1 in SiO 2 nanospheres to ≈870 m 2 g −1 in Pd@mSiO 2 , Ru@mSiO 2 and PdRu@mSiO 2 NRs.The high surface area values for the silica-capsule NRs obtained herein can be attributed to their remarkable porosity (Figure 2b) promoted during the CTAB elimination process.The decrease in the BET surface area value of the PdRu@mSiO 2 sample may be caused by the slight increase in SiO 2 capsule size, as TEM results showed.Note that the modified one-pot method is quite effective for synthesizing small porous PdRu@mSiO 2 nanostructures.
Figure 4b presents the pore size distribution curves for NRs.All NRs showed pore size c.a. 3nm, estimated by the Barrett-Joyner-Halenda (BJH) method.The formed pores were attributed to the CTAB removal and the SiO 2 capsule restructuration due to the thermal treatment of the samples, as discussed above.Therefore, @SiO 2 nanospheres with low porosity were expected.Indeed, the pore size of @SiO 2 nanospheres (34.6nm) corresponds to the voids between the spheres and not to the surface pores (see Table 1).In contrast, the pore sizes of NRs can be associated with pores formed in the capsules, as the TEM micrograph analysis shows.The obtained values of BET surface area and porosity were similar to those of SiO 2 capsule-based NRs [73,76] The porous structure of the SiO 2 capsules is an important factor and is expected to promote mass transport during the catalytic process.

X-Ray Diffraction (XRD)
The structural properties of the prepared samples were analyzed using XRD. Figure 5 shows the low-and high-angle X-ray diffractograms of the PdRu@mSiO 2 , Pd@mSiO 2 , and Ru@mSiO 2 NRs.The low-angle XRD patterns showed a weak peak at 2 = 2.3°(Figure 5b), corresponding to the plane (100), characteristic of SiO 2 mesoporous structures. [77]The structure of @mSiO 2 capsules seemed similar to ordered silica materials such as SBA-15, [78,79] maybe due to the presence and further controlled removal of CTAB in the primary nanostructure.Indeed, the use of CTAB to form ordered channels in oxide matrixes has been reported. [80]A slight peak shift at 2.3°of 2 was observed for PdRu@mSiO 2 NRs compared with Pd@mSiO 2 and Ru@mSiO 2 .The peak shift for bimetallic NRs may be attributed to the formation of Pd-Ru alloy similar to. [81]ll XRD patterns presented a broad peak between 15°to 30°of 2 commonly attributed to the amorphous silica [82] (Figure 5a).A weak and broad peak at 43°of 2 appeared in the diffraction pattern of the Ru@mSiO 2 NRs.This peak can be attributed to the (002) and (101) metallic Ru reflections commonly centered at 42.5°and 44.2°of 2, respectively, according to the JCPDS 06-0663.The shape and intensity of these peaks in Ru-based NRs suggest that the formed Ru NPs were <5nm, as discussed in. [83]In contrast, the diffractograms of the samples containing Pd-based NPs showed well-detectable and narrow peaks at 40.1°a nd 46.5°of 2, accompanied by other less intense peaks.The reflections at 40.1°, 46.5°, 68.2°and 82.2°of 2 in the XRD patterns coincided well with the face-centered cubic (FCC) lattice of metallic Pd; meanwhile, peaks at 33.7°, 36.2°,41.8°and 60.8°of 2 corresponded to PdO according to the JCPDS 05-0681 and JCPDS 0041-1107, respectively (see Figure 5a).It is well-known that metallic Pd in contact with oxygen, even atmospheric air, produces oxidized Pd species. [84]In this case, the presence of unreduced Pd species (PdO) is attributed to the oxidation of palladium atoms located on the surface of the NPs due to contact with air during calcination. [85]o reflections assigned to the metallic Ru phase were observed in the diffraction pattern for PdRu@mSiO 2 .The latter could be attributed to the fine dispersion of Ru species in the bimetallic NPs [86,87] and the low Ru content (≈0.5%) according to ICP analysis.Additionally, phase segregation was not observed, indicating a reasonable degree of alloying between Pd and Ru. [88]oreover, the position of the diffraction peak at 40.1°of 2 for the Pd@mSiO 2 NRs was slightly shifted to a higher 2 degrees (40.3°) when Ru was presented in the structure of Pd-based NRs (see inset in Figure 5a).Similar displacement of the peak at 40.1°t o larger 2 degrees is expected when an alloying phase is formed by the incorporation of Ru into the Pd FCC structure. [89,90]Thus, the synthesis conditions used in this work allowed the fine distribution of metals and the formation of Pd-Ru alloy.

Catalytic Activity
The 4-NP reduction is a model reaction commonly used to analyze catalyst performance. [91]The kinetic analysis of this reaction is easily monitored by UV-vis spectroscopy because both the reagent and the product, 4-NP and 4-aminophenol (4-AP), respectively, present well-detectable characteristic peaks in this spectral region. [92]Excess NaBH 4 as a hydrogen source allows to analyze the reaction as a pseudo-first-order. [93]In this sense, the apparent reaction kinetic constant (k app ) produces the linear slope of the relative absorbance in logarithmic form versus reaction time. [93] is well-known that 4-NP in contact with NaBH 4 leads to immediate 4-NP deprotonation into 4-nitrophenolate ion (4-NPt), causing a red-shift of the 4-NP peak in UV-vis spectrum from 316 to 400nm and a bathochromic effect that also increases its intensity (see Figure S4, Supporting Information).The reaction progress is controlled by a decrease of the peak at 400 nm corresponding to the 4-NPt ion.At the same time, 4-AP formation is detected by the appearance of the peak at 300nm with low intensity due to the difference in the extinction molar coefficient of 4-NPt and 4-AP. [94]or the reaction initiation, the metallic species are required for the appropriate hydrogen activation (formation of hydrogen ions on the metallic atoms). [92,95]Thus, the protocol followed in this work implied the sample reduction before the reagent injection due to the presence of oxidized PdO species, as discussed above.Indeed, the Pd-based NRs initially treated in NaBH 4 presented a notable catalytic activity enhancement compared to none-reduced samples (see Figure S5, Supporting Information).
Figure 6 presents the kinetic analysis of the 4-NPt reduction into 4-AP using mono-and bimetallic NRs.A representative spectral surface obtained during the catalytic reduction of 4-NPt on PdRu@mSiO 2 NRs is shown in Figure 6a.As expected, the consecutive decay of the peak centered at 400 nm (4-NPt) was accompanied by the appearance of another peak at 300 nm (4-AP).Figure 6b shows the change in the relative absorbance at 400 nm as a function of time for the PdRu@mSiO 2 , Pd@mSiO 2 , and Ru@mSiO 2 NRs.Note that the PdRu@mSiO 2 NRs showed faster consumption of the 4-NPt than the Pd@mSiO 2 and Ru@mSiO 2 NRs in terms of k app (Figure 6c).In fact, the estimation of turnover frequency (TOF) values revealed that the activity order was as follows: PdRu@mSiO 2 > Ru@mSiO 2 > Pd@mSiO 2 .The bimetallic NRs revealed more than threefold faster transformation of 4-NPt to 4-AP compared with monometallic Ru and Pd NRs (Figure 6d).A similar tendency has been found for Ru@Pd nanostructures in contrast with their monometallic counterparts. [48]It is well-known that the synergistic effect between metals can drastically benefit the catalytic activity of the materials. [96]Indeed, the @mSiO 2 capsules did not reveal any 4-NPt consumption, evidencing that the reaction had not proceeded (see Figure S6, Supporting Information).
Table 2 shows the analysis of k app and TOF values in the reduction of 4-NP to 4-AP for the catalysts prepared in the present work.The kinetic data are summarized for NRs, free NPs and supported catalysts for comparison.It is well known that many factors affect catalytic activity, such as NPs size, composition, support nature (interphase), and morphology, among others. [97]ono and bimetallic Pd and Ru NPs presented notable catalytic activity in the reduction of 4-NP due to their hydrogenating character.Note that the Ru NPs presented higher TOF value than Pd and PdRu NPs.Indeed, free PdRu NPs showed an intermediate TOF value compared to their individually presented components.Once the NPs were supported or encapsulated in SiO 2 , their catalytic activity changed dramatically.The Pd/SiO 2 catalyst displayed the highest TOF value (see Table 2).Note that the Ru/SiO 2 and PdRu/SiO 2 catalysts were less active than their corresponding free NPs.On the other hand, the encapsulation of NPs into SiO 2 increased the catalytic activity for mono and bimetallic NRs.The most prominent activity was observed for PdRu@mSiO 2 NRs, which showed the highest TOF value (486 min −1 /μmol-metal) and was the most active catalyst analyzed in this work.This high activity may be attributed to the synergetic effect between Pd and Ru in bimetallic catalysts.The formation of Ru metallic species on the Pd surface and its further diffusion into Pd and good mixing led to the formation of Pd-Ru alloy independently of the type of nanostructure (NPs, supported catalysts or NRs).In general, both mono-and bimetallic NRs presented relatively high catalytic activity in the reduction of 4-NP to 4-AP, which was expected due to the stabilizing of active sites inside the capsules, preventing NP agglomeration under reaction conditions and catalyst deactivation.The slight decrease of catalytic activity for monometallic Pd@mSiO 2 Ru@mSiO 2 K is the ratio of the k app to the metal loading (μmol) in the reaction media; b) TOF = K/metal dispersion (D), where D = 8 × radius of Metallic atom (nm)/Metallic core diameter (nm), as in, [ 53] assuming spherical shape for Metallic NPs;  a) Given by authors; b) K is the ratio of the k app to the metal loading (μmol) in the reaction media; c) TOF = K/metal dispersion (D), where D = 8 × radius of Me atom (nm)/Me core diameter (nm), as in, [ 53] assuming spherical shape for Me NPs; NRs may be explained by the partial blocking of the metallic NP surface by the capsule compared to non-encapsulated catalysts.So, the estimated TOF values were remarkably affected by the type of nanostructure rather than by the size of the NPs.For example, free NPs (in colloidal form) showed high efficiency in activating the reagent molecules.In contrast, supported catalysts revealed changes in TOF values that may be attributed to the stability of NPs due to the interaction between the NPs and the support.Apparently, silica as support favored the Pd-silica interaction via a more robust anchoring of the Pd NPs.However, the presence of Ru in the supported mono-and bimetallic catalysts (Ru/SiO 2 and PdRu/SiO 2 ) resulted in the loss of catalytic activity, possibly due to the poor Ru-silica interaction or to NP agglomeration.The encapsulation of Pd, Ru, and PdRu NPs into SiO 2 prevented the aggregation of NP, leading to the increase of catalytic activity of NRs even for larger particles (10−25 nm).
Therefore, a few factors determined the catalytic activity of NRs: i) the presence of the SiO 2 capsule, which avoided NP aggregation, and ii) the synergetic effect between Pd and Ru with the alloy formation.
The catalytic activity of the presently synthesized NRs was compared with similar nanomaterials reported in the literature (see Table 3).Table 3 shows that the systems based on NRs present better catalytic activity than other morphologies, such as free or supported NPs.For example, the Pd@hTiO 2 NRs prepared by a multi-step methodology (sol-gel followed by a chemical etching to remove the template) in [45] display relatively high TOF values.The latter authors attributed to the formation of a Pd-TiO 2 active interface that is favorable for the catalytic reduction of 4-NP.In contrast, the SiO 2 capsules only act as physical barriers, remaining inert to the reaction. [66]A completely different behavior is observed for Ru@rGO NRs when reduced graphene oxide is applied as a capsule material.The Ru@rGO does not present catalytic activity in the reduction of 4-NP to 4-AP. [47]The TOF value estimated for PdRu NPs supported on activated carbon (CFAC) [49] is comparable with that for Ru@Pd NSs (core-shell nanostructures), [48] making evident that CFAC remains inert to the reaction.Note that the TOF value estimated for the presently prepared PdRu@mSiO 2 NRs was more than four times higher than for bimetallic PdRu/CFAC catalysts [49] and Ru@Pd NSs. [48] close analysis of the relevant literature reveals that morphology plays a crucial role in obtaining catalytically active nanostructures.In this work, it seems that the presence of the capsule favored the stability of the bimetallic active sites during the transformation of 4-NP into 4-AP.In contrast, for the nonencapsulated catalysts, the exposure of active sites to the reaction medium can accelerate NP agglomeration, causing the deactivation of catalysts.In conclusion, the PdRu@mSiO 2 catalyst prepared in the present work was the most active material when applied to the 4-NP reduction due to the balanced properties achieved during the synthesis, such as the alloy formation, homogeneity of nuclei dispersion and the stabilization provided by the capsule.
The PdRu@mSiO 2 NPs were selected as the most active nanomaterials to analyze their ability to reduce different nitroarenes compounds.The molecules selected for the test were characterized by diverse functional groups other than OH.In this case, molecules with CH 3 (methyl) and Cl (chlorine) functional groups were chosen, being part of the 4-nitrotoluene (4-NT) and 1-chloro-4-nitrobencene (1-Cl-4-NB) reagent molecules, respectively.Additionally, the effect of the number of substituents in the nitroarene molecule was evaluated using 2,4-dinitrophenol (2,4-DNP) as a reagent.The reactions were carried out under the same conditions used for the 4-NP reduction.
Figure 7 presents the UV−vis spectra collected in situ during the catalytic transformation of the 1-Cl-4-NB, 4-NT, and 2,4-DNP using the PdRu@mSiO 2 NRs. Figure 8 shows the kinetic analysis of the data.According to the estimated TOF values, the catalysts presented the following activity tendency in the reduction of the different nitroarenes: 4-NP > 1-Cl-4-NB > 2,4-DNP > 4-NT (Table 4).The results corresponded well with the molecular configuration of each nitroarene.The presence of the nitro group in the reagent molecules makes them polar and sol- (nm), as in, [ 53] assuming spherical shape for metal NPs.
uble in water.However, the functional group interferes with the charge distribution (see Figure S7, Supporting Information).In this sense, intra and intermolecular interactions play an essential role in driving the chemisorption of reagents at the active sites of the NRs.It can be proposed that the absence of polarity of the methyl group in the 4-NT reagent led to its lower solubility in the reaction medium.As a consequence, it displayed the poorest interaction with the catalyst surface.In contrast, the chlorine and hydroxyl substituents in 1-Cl-4-NB, 4-NP, and 2,4-DNP molecules were highly soluble in the reaction media.Additionally, the difference in electronegativity between chlorine and oxygen atoms could interfere with the intermolecular interactions.As a rule, the electronegativity of oxygen in hydroxyl substituents is higher than that of chlorine.Consequently, weak hydrogen bonds between the 1-Cl-4-NB reagent molecules were probable due to a dispersed electron cloud in chlorine.The latter may affect the catalytic activity.Therefore, slightly higher TOF values were found for the reduction of 4-NP compared with 1-Cl-4-NB.
On the other hand, the catalytic activity of the NRs dramatically decayed (>45%) when a double-substituted nitroarene molecule was used as a reagent (2,4-DNP), in comparison with the singlesubstituted nitroarene molecule (4-NP).The latter may be attributed to the intramolecular forces in the 2,4-DNP molecule.In this case, the electronic resonance caused by the vicinity between the nitro group in position two and the hydroxyl group diminished the intermolecular interactions.In contrast, in the case of 4-NP, the single-substituted nitroarene molecule, the nitro and hydroxyl groups are separated enough.Therefore, the position of the substituent in the nitroarene molecule affects the intra and intermolecular interactions, accompanied by a decrease in catalytic activity due to hindering the interaction between the reagents and the catalyst surface.

Catalytic Stability
Catalyst stability is one of the most desired properties of heterogeneous catalysts because it permits their reusability.NRs attract attention due to their high catalytic stability even after their usage under harsh thermal and chemical reaction conditions.Several NRs, such as Pd@Al-SiO 2 , Pd@hTiO 2 and Ru@SiO 2, have this valuable catalytic property, remaining stable up to at least five consecutive runs in some reactions. [45,71]Usually, the stability test protocol involves separating the catalyst to clean it, and removing the product from the reaction medium before the next catalytic run.However, this action may not warrant the complete recovery of the catalyst.Thus, a loss of catalyst mass is expected between catalytic runs.The best way to evaluate the stability of the catalysts is to apply the conditions that simulate the industrial usage of the catalytic materials, but without catalyst cleaning between several catalytic runs.All samples prepared in the present work (free NPs, supported catalysts and NRs) underwent stability tests using the following protocol: once the reagent (4-NPt) had been completely consumed, another portion of 4-NP (30 mμ, 10 μL) was injected.Note that the catalyst and the product were not separated from the reaction medium, simulating relatively harsh conditions.
Figure 9 shows results of the catalytic stability test for mono and bimetallic PdRu@mSiO 2 NRs. Figure 9a presents the typical curves of changes in the relative absorbance peak at 400 nm, attributed to the 4-NPt.The 4-NPt peak disappeared completely, indicating 100% of 4-NP conversion, after each catalytic run.Figure 9a depicts a gradual increase in the reaction time required for the complete transformation of 4-NP as the reaction runs progressed.Thus, the NRs underwent partial deactivation under the reaction conditions.The estimated TOF values for each catalytic run confirmed the obtained results (Figure 9b).During four consecutive runs, the monometallic Pd@mSiO 2 NRs showed high stability with undetectable activity loss.In contrast, Ru@mSiO 2 and PdRu@mSiO 2 NRs were less stable, displaying almost a 60% activity loss at the end of the fourth run (Figure 9b).It seems that the presence of Ru in NRs favors their destabilization.
A few reasons may cause the decrease of catalytic activity of NRs: i) adsorption of the reaction product (4-AP) on the active  The TOF values estimated for each consecutive catalytic run for Pd@mSiO 2 , Ru@mSiO 2 , and PdRu@mSiO 2 NRs.sites (metallic NPs); ii) partial blocking of the capsule pores by formed product causing mass transport limitations; or, iii) migration of the metallic nuclei to the reaction media.A new catalytic test was proposed to estimate the effect of Ru on the catalytic stability of NRs.In this case, the Ru@mSiO 2 and PdRu@mSiO 2 NRs were mixed with the reaction product, the 4-AP (30 mm, 0.01 mL), during 1800 s prior to the injection of 4-NP in the reaction cell (see Figure S8, Supporting Information).The catalysts used in these reactions were labeled as "dirty".A drastic decrease of the k app value of up to 70% for the Ru@mSiO 2 and ∼53% for the PdRu@mSiO 2 NRs was obtained.Note that the loss of activity is comparable with that recorded after the fourth consecutive catalytic runs carried out for Ru@mSiO 2 and PdRu@mSiO 2 NRs (see Figure 9).In short, the adsorption affinity of Ru to 4-AP negatively impacts the catalytic stability of Ru@mSiO 2 and PdRu@mSiO 2 NRs during the 4-NP catalytic reduction performed under the proposed protocol.A TEM image of the spent PdRu@mSiO 2 NRs was analyzed after the fourth consecutive catalytic run.It confirmed that the NRs preserved their morphology and nucleus size (see Figure S9, Supporting Information).
Finally, in order to confirm the effect of the morphology of nanomaterials on catalytic stability, the bimetallic PdRu NPs, PdRu/SiO 2 supported catalyst and PdRu@mSiO 2 NRs were evaluated.It was observed that the rate constants k app were changed in the following order: PdRu NPs > PdRu@mSiO 2 > PdRu/SiO 2 Figure 10.
Figure 10b shows the estimated TOF values for the three analyzed samples during four consecutive runs to reduce 4-NP.PdRu@mSiO 2 NRs revealed a decline in activity after the first catalytic run.However, the stability of these NRs did not change significantly during three consecutive runs.
The activity loss during the first run of the reaction may be explained by i) the agglomeration of PdRu NPs and ii) the adsorption of reagent product (4-AP) on the catalyst surface.On the contrary, PdRu NPs and PdRu/SiO 2 showed relatively low activity presenting >90% activity loss during four consecutive runs (Figure 10b).Despite the fall in the catalytic activity of PdRu@mSiO 2 NRs during the first catalytic run, it was the most active and stable nanomaterial compared with the PdRu NPs and PdRu/SiO 2 catalysts and similar systems presented in the literature. [48,49]The catalytic stability of monometallic Pd and Ru- based catalysts was studied under similar conditions and displayed in Figure S10 (Supporting Information).

Conclusion
The one-pot method permits the preparation of the bimetallic PdRu-based NRs encapsulated into mesoporous silica with a core-shell structure.The obtained PdRu@mSiO 2 NRs with a Pd1:Ru1 molar ratio showed a mononucleus distribution per capsule and a homogenous spherical morphology.For the first time, the catalytic activity of these sophisticated nanostructures was evaluated in the reduction of hazardous nitroarene compounds such as 4-nitrophenol, 1-chloro-4-nitrobenzene, 4-nitrotoluene and 2,4-dinitrophenol.The synergistic effect between Pd and Ru in bimetallic NRs significantly enhanced their catalytic activity in the reduction of 4-nitrophenol to 4-aminophenol compared with monometallic Pd@mSiO 2 and Ru@mSiO 2 nanomaterials.Bimetallic PdRu@mSiO 2 NRs demonstrated relatively high catalytic stability during four consecutive runs even under harsh conditions, i.e., no cleaning of the NRs before the next catalytic run.It was demonstrated that the affinity of Ru to 4aminophenol adsorption negatively affects the catalytic stability of Ru@mSiO 2 and PdRu@mSiO 2 NRs.The comparative analysis of bimetallic free PdRu NPs, PdRu/SiO 2 supported catalysts and PdRu@mSiO 2 NRs performed in the present work reveals that the morphology affects the catalytic activity being favorable for PdRu@mSiO 2 NRs with core-shell structure.
The development of a low-cost and effective one-pot method for the synthesis of NRs may extend the use of these sophisticated nanostructures to multiple reactions, mainly those for the treatment of hazardous materials.

Figure 4 .
Figure 4. a) N 2 adsorption-desorption isotherms and b) Pore size distribution curves for the mono and bimetallic NRs.

Figure 5 .
Figure 5. a) High-and b) Low-angle XRD patterns for the mono and bimetallic NRs.Inset in (a) shows a close-up of the Pd (111) main peak for the PdRu@mSiO 2 and Pd@mSiO 2 NRs.

Figure 6 .
Figure 6.Kinetic analysis of 4-NP reduction into 4-AP for the prepared NRs.a) UV-vis spectral surface obtained during the in situ monitored 4-NP reduction on the PdRu@mSiO 2 NRs.b) Relative absorbance changes at 400 nm for 4-NPt with reaction time when different NRs are used.c) Apparent reaction rate constants obtained through the linear slope of the changes in the relative absorbance at 400 nm in logarithmic form versus reaction time.d) TOF values obtained for the 4-NP reduction.

Table 4 .
Estimated k app and TOF values for reduction of nitroarenes with an excess NaBH 4 at25°C for prepared PdRu@mSiO 2 NRs.Reagent Molecule k app [min −1 ] K a) [min −1 /μmol-metal] TOF b) [min −1 /μmol-metal] is the ratio of the k app to the metal loading (μmol) in the reaction media; b) TOF = K/metal dispersion (D), where D = 8 × radius of metal atom (nm)/metal core diameter

Figure 8 .
Figure 8. Kinetic analysis of the reduction of different nitroarenes into their corresponding amines using PdRu@mSiO 2 NRs.a) The relative absorbance changes at 298, 285, and 435nm for 1-Ch-4-NB, 4-NT, and 2,4-DNP, respectively, versus time.b) Apparent reaction rate constants obtained through the linear slope of the relative absorbance changes of the characteristic peaks in logarithmic form versus reaction time.c) TOF values obtained for the nitroarenes reduction.

Figure 9 .
Figure9.a) Relative absorbance at 400nm changes during the consecutive reinjection of the reagent after its total consumption monitored in situ for the 4-NP reduction catalyzed by PdRu@mSiO 2 NRs.b) The TOF values estimated for each consecutive catalytic run for Pd@mSiO 2 , Ru@mSiO 2 , and PdRu@mSiO 2 NRs.

Figure 10 .
Figure 10.a) Apparent reaction rate constants obtained through the linear slope of the relative absorbance changes at 400nm for 4-NP in logarithmic form versus reaction time for bimetallic PdRu NPs; PdRu/SiO 2 catalysts and PdRu@mSiO 2 NRs.b) The TOF values obtained for each consecutive catalytic run for the different PdRu catalysts.

Table 1 .
Textural properties and morphological characteristics of synthesized NRs.

Table 2 .
Estimated k app and TOF values for the 4-NP reduction to 4-AP with an excess of NaBH 4 at 25 °C for the presently prepared samples. a)

Table 3 .
Comparison of estimated k app and TOF values for the 4-NP reduction to 4-AP with an excess of NaBH 4 at 25°C for presently prepared nanoreactors and similar Pd-, Ru-and PdRu-based catalysts found in the literature.