Influence of metal-support interaction on nitrate hydrogenation over Rh and Rh-Cu nanoparticles dispersed on Al₂O₃ and TiO₂ supports

1 Introduction

A great challenge in catalytic field is to prepare catalysts, showing enhanced activity and selectivity, based on well-defined metallic and bimetallic nanoparticles (NP's). An important component of the research development is to acquire better understanding on the catalytic systems exhibiting strong-metal support interactions (SMSI).

Metal-support interaction was investigated mostly in the case of monometallic NP's (Sadeghi and Henrich, 1984a,b; Linsmeier and Taglauer, 2011). For example, it is disclosed that the interaction of Pt clusters with basic support favors the catalysis due to the increase in electron density on metal sites (Gates, 1995). However, in spite of intense researches, lack of agreement still exists on the role played by substrate on the activity of NP's. In addition, metal-support interaction is less studied for bimetallic nanoparticles. Study dedicated to this issue would be important because bimetallic NP's show in many cases superior catalytic activity and selectivity compared to monometallic ones due to the synergistic effect between the component metals (Monyoncho et al., 2015). The colloidal deposition method of well defined NP's is a valuable alternative to obtain model catalysts in which is easier to discriminate between support and particle size effects. The influence of the support on the formation of mono and bimetallic nanoparticles is therefore eliminated as the metal nanoparticles are generated before they are dispersed on the support.

Structure sensitive model reactions are the best selection to evidence the distinct influence of size and support on metal(s) catalytic performances. The nitrate catalytic reduction (involving the formation of N—N bonds) is a suitable model reaction because is sensitive to the structure (size and shape) of supported metal particles (Gates, 1995). In addition, nitrate reduction is a good example for the beneficial effect of secondary metal addition on catalytic reduction of nitrate ion (Vorlop and Tacke, 1989).

The reduction of nitrate at ppm level in drinking water has a practical relevance only if the selectivity to nitrogen is close to 100%. Up to date, no efficient catalysts for selective conversion of nitrate and its derivatives for N₂ were identified but researches on this field are still in progress (Barrabés and Sá, 2011). The selective and complete conversion of nitrate to ammonia may have impact eventually on energy conservation field. Ammonia was found recently to be a very attractive alternative to H₂ as energy storage media (Chakraborty et al., 2010). For ammonia large scale production, with no additional carbon foot print generation, liquid wastes with high concentration of nitrates and nitrites should be used.

Well-defined Rh and Rh-Cu NP's were considered in this research as the best candidates to investigate catalytic reduction of NO₃⁻ with H₂. One of the reasons is that Rh-based nanoparticles were reported to exhibit good hydrogenation activity. Photocatalytically generated Rh nanoparticles, capped by CTAB, exhibit excellent catalytic activity towards the reduction of aromatic nitro compounds by NaBH₄ to their corresponding amino derivates (Kundu et al., 2009). Nitrobenzaldehyde is selectively hydrogenated to aminobenzaldehyde over colloidal Rh-Cu NP's (Sharif et al., 2014). Another reason for our material selection is that, to the best of our knowledge, the hydrogenation of nitrate and/or nitrites over well-defined Rh-based mono/bimetallic nanoparticles, either in colloidal form or dispersed on oxide supports, was not explored yet.

The hydrogenation of nitrate over physical mixtures with Rh and Cu supported on active carbon was reported by Soares et al. (2011). The activity of co-impregnated Rh-Cu/C catalysts was also investigated (Soares et al., 2008, 2009, 2011). Witońska et al. (2007) studied the kinetic of nitrate hydrogenation in water using impregnated Rh/Al₂O₃ and Rh–Cu/Al₂O₃ catalysts.

Thus, the investigation of these well-defined Rh-based mono/bimetallic nano-materials would bring new knowledge in catalytic field from fundamental and practical points of view. The aims of our work were to investigate the (i) activity of Rh and Rh-Cu NP's for NO₃⁻ hydrogenation, (ii) effect of TiO₂ and Al₂O₃ supports on catalytic behavior of mono and bimetallic nanoparticles, (iii) impact of nanoparticles morphology and chemical state on catalytic activity.

2 Material and methods

3 Results and discussion

Fig. 1A shows the TEM images of PVP protected Rh nanoparticles. The average size of very fine Rh nanoparticles is centered at 1.6 nm (see the inset containing the associated histogram). At a closer look, few atomic rows can be distinguished at individual nanoparticles.

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Figure 1

TEM image of PVP-protected Rh (A) and Rh-Cu (B) nanoparticles and the associated size histograms. The insets evidence magnified individual Rh and Rh-Cu nanoparticle.

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The characteristic TEM image of bimetallic Rh-Cu nanoparticles is presented in Fig. 1B. The average size, derived from the inset histogram, is ≈1.3 nm. From TEM analysis it is unambiguous that the polyol synthesis method is able to yield reproductively, fine and well-dispersed Rh as well as Rh-Cu nanoparticles which can be used further to catalyze various chemical reactions.

The TEM images of Rh and Rh-Cu NP's dispersed on Al₂O₃ and TiO₂ supports are presented in Fig. 2A–C. The supported mono and bimetallic nanoparticles capped by PVP were calcined and then reduced at 400 °C for 1 h.

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Figure 2

TEM images of supported Rh and Rh-Cu nanoparticles: Rh/Al₂O₃ (A), Rh/TiO₂ (B), Rh-Cu/Al₂O₃ (C) and Rh-Cu/TiO₂ (D).

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The TEM micrographs of conditioned catalysts reveal the influence, especially in the case of Rh-Cu NP's, of supporting material nature on dispersion (average size). The average size of Al₂O₃ and TiO₂-supported NP's increased to some extent after conditioning procedures (calcination and reduction steps). The only exception was Rh on Al₂O₃ which preserved its initial size of 1.6 nm. The Rh on TiO₂ appeared slightly larger (1.8 nm) than of the precursor nanoparticles (1.6 nm). The Rh-Cu NP's deposited on Al₂O₃ and on TiO₂ underwent to coarsening process, the average diameter increasing from 1.3 nm (the size of colloidal batch NP's) to 2.1 and 2.8 nm, respectively (see the histograms in Fig. 2C and D). The arrows in TEM micrograph 2C points to Rh-Cu NP's characterized by deformed spherical (ovoid) or by hemispherical shapes as result of particle interaction with Al₂O₃ support. The side view of hemispherical Rh-Cu nanoparticles, indicated by arrows, in intimate (epitaxial) contact with TiO₂ support can be visualized in Fig. 2D. The flattening of Rh-Cu NP's on TiO₂ suggests that strong interaction takes place between metal and support (SMSI). Earlier published works on other metals confirm this hypothesis. For example, Comotti et al. (2006) advanced the idea that the change in shape of Au particles from spherical to hemispherical one, due to the interaction with the TiO₂ support, is an important factor to decide catalytic activity. Similarly, the wetting-like phenomenon of support by Rh-Cu NP’s is likely to create reactive perimeters with catalytic activity (Fujitani et al., 2009).

Fig. 3 displays the XRD patterns of colloidal nanoparticles as well as of the supported catalysts. The colloidal Rh and Rh-Cu nanoparticles are XRD amorphous, in spite that few crystalline fringes are visible at some of the individual nanoparticles (see the insets in Figs. 1 and 2). The absence of the characteristic XRD reflections can be explained by the very small size of Rh and Rh-Cu nanoparticles (d = 1.3–1.6 nm). Molecular dynamic simulation demonstrates the formation of amorphous structure is inevitable for metallic NP's on reducing the diameter down to ≈1 nm (Sun et al., 2007). As expected, the supported catalysts show only the characteristic diffraction lines of oxides (alumina and titania’s anatase and rutil).

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Figure 3

Comparative XRD patterns of colloidal nanoparticles as well as of the catalysts. ■-γ-Al₂O₃; ●-anatase; □-rutile.

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Relevant surface information on the chemical state of metals and support oxides was obtained from XPS investigation. The atomic sensitivity factors were derived from the electronic cross-section of each element using the transmission and inelastic mean free path corrections.

The XPS analysis was carried out on the as prepared PVP-capped nanoparticles as well as on the conditioned supported catalysts. The XPS results, summarized in Table 1, reflect the complex composition and chemical state of mono and bimetallic nanoparticles. Rhodium is present in mixed metallic and oxide states in all investigated catalysts systems. The Rh/Rh³⁺ atomic ratio for Rh and Rh-Cu NP's capped with PVP is 1.5 and 1.1, respectively. The oxidized form of rhodium in Rh-Cu nanoparticles dispersed on TiO₂ prevailed over the metallic state (Rh/Rh³⁺ = 0.7) in contrast to the same bimetallic nanoparticles supported on Al₂O₃ (Rh/Rh³⁺ = 1.5). In the case of simple Rh nanoparticles, the choice of support seems to have an opposite effect on their oxidation state compared to Rh-Cu system. The TiO₂ support favors the metallic state for Rh (Rh/Rh³⁺ = 3) compared to Al₂O₃ where Rh/Rh³⁺ ratio is close to unity.

Metal-support interaction can be viewed as a redox reaction (implying electron transfer) at metal(s) - oxide support interface (Fu and Wagner, 2007). Charge will flow from the component with higher Fermi level to the lower one (Tang and Henkelman, 2009). The interactions are strongly dependent on the surface properties of oxide substrate. Surface stoichiometry, surface terminations, and surface defects are the most important factors influencing the metal-oxide interactions (Fu and Wagner, 2007). Chemical reactions, occurring between reactive metals (i.e. Nb) and TiO₂, lead to oxidation of few layers of metal in parallel with partial reduction of TiO₂ layer of nanometer order near the metal-support junction (Marien et al., 2000). Oxygen mass transport commonly accompanies the chemical reactions at metal-oxide junctions. For Rh-Cu NP's deposited on TiO₂ it can be assumed that electron transfer from metal to support favors the increase of Rh³⁺ (Rh/Rh³⁺ = 0.7) compared to Al₂O₃ support (Rh/Rh³⁺ = 1.5) where electron conduction cannot take place because of alumina insulating characteristics. This corresponds to electronic configuration E_F(metal) > E_F(TiO₂) and results in built-up of negative space charges in TiO₂ and downward bending of bands (Fu and Wagner, 2007). Probably that copper intermediates the electronic processes at metal-support interfaces but its precise role remains obscure at present.

As anticipated, aluminum was in fully oxidized state (Al³⁺) in alumina support (see Fig. 4A and B). For Rh/TiO₂ (Fig. 4C) as well as for simple TiO₂ (not shown here), Ti⁴⁺ was the only species identified. On the other hand, large amount of Ti³⁺ was detected at Rh-Cu/TiO₂ catalyst. From the XPS spectrum presented in Fig. 4D, the estimated surface fraction of Ti³⁺ is quite important, about 42.5 at.%. To explain the extensive formation of Ti³⁺ on the surface of the catalyst, hydrogen spillover from metal NP's to support, during the reduction at 400 °C, should be considered. This mechanism, valid only for reducible and semiconducting oxides (Prins, 2012), implies that H₂ molecules dissociate to H atoms on Rh-Cu nanoparticles. According to our TPR results, H₂ starts to be activated over Rh-Cu nanoparticles dispersed on TiO₂ at room temperature. The resulted H atoms generate electrons (e⁻) and protons (H⁺) at the interface with TiO₂. The electrons located at Ti⁴⁺ sites produce Ti³⁺ (which are in fact negative lattice defects, Ti'_Ti) at the metal-support interface while the protons binds to O²⁻ in the vicinity of Ti³⁺ forming OH⁻ (O_O^•, positive lattice defects). The formation of Ti³⁺ lattice point defects, creating locally electron-rich environments at metal-support perimeter, may promote catalytic activity of deposited metal nanoparticles (Cai et al., 2013). The extent of the reduction of metal oxide at interface with metal particles depends on the activation energy required for proton-electron migration. In the case of Pt/TiO₂, the proton-electron migration is limited to the immediate environment of the metal particles due to the relative high migration activation energy (Huizinga and Prins, 1981). Extensive reduction of oxide was observed for materials characterized by low migration activation energy. For example on MoO₃, with activation energy of 6–12 kJ mol⁻¹ for electrons and protons, the reduction moves away from the metal support interface and the support is extensively reduced (Prins, 2012). At higher temperatures, two adjacent OH⁻ groups are reversibly eliminated to form one oxygen vacancy (V_O^••) bearing two positive charges and one H₂O molecule. The negative charges of two Ti³⁺ lattice defects are compensated by positive charges oxygen vacancy. The formation of Ti³⁺ by hydrogen spillover mechanism was reported also for Pd-Cu/TiO₂ catalysts (Kim et al., 2013). In fact, the formation of Ti³⁺ is indicative that Rh-Cu nanoparticles posses high hydrogenation activity when are supported on suitable oxide material enabling H₂ spillover. In the case of Rh/TiO₂ catalyst, although H₂ can be activated by Rh NP's at low temperature (see TPR results), the electron and proton transfer to TiO₂ is not important because the formation of Ti³⁺ was not evidenced by XPS analysis.

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Figure 4

XPS core-level spectra of Al2p, Ti2p regions measured for Rh/Al₂O₃ (spectrum A), Rh-Cu/Al₂O₃ (spectrum B), Rh/TiO₂ (spectrum C) and for Rh-Cu/TiO₂.

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TPR measurements were carried out on freshly calcined catalysts to obtain information on the oxidation state of supported metals. The TPR results (hydrogen consumption) are synthetically presented in Table 2. In addition, information concerning the oxidation state of metals on catalysts reduced in H₂ at 400 °C for 1 h is displayed for comparison in the same table. The Rh/Rh³⁺ proportion, depending on the nature of support, shows similar tendency for calcined and reduced catalysts. The Rh/Rh³⁺ ratio increases for all investigated catalysts after reduction with H₂.

The interesting feature of Rh-Cu-based catalysts is that they exhibit only one reduction peak, located at 55 °C for Rh-Cu/TiO₂ and at 70 °C for Rh-Cu/Al₂O₃. The atypical, simultaneous reduction of CuO_x and RhO_x species strongly suggests that a synergetic effect takes place between the component metals. Mendes and Schmal (1997) reported that rhodium interaction with copper promotes reduction of copper oxide. Normally, CuO_x is reduced at higher temperature compared to Rh₂O₃ (Guerrero-Ruiz et al., 1997). The second observation is that in spite of high hydrogen consumption (0.33 mmol g⁻¹), the oxidized Rh-Cu NP's on TiO₂ are reduced fast in one step process at low temperature (55 °C, see Fig. 5). The two metals behave in fact like one, homogeneous oxide compound. The H₂ consumption for Rh-Cu on Al₂O₃ is smaller (0.24 mmol g⁻¹), and the broad reduction maximum is placed at higher temperature (70 °C). The sharp reduction peak at low temperature of Rh-Cu/TiO₂, consistent with high hydrogen consumption rate, suggests excellent catalytic activity for hydrogenation reactions. In contrast to Rh-Cu, supported Rh NP's exhibit distinct low and high temperatures reduction peaks. The reduction maxima of Rh/TiO₂ and Rh/Al₂O₃ are positioned at 68, 237 °C and at 70, 105, 270 °C, respectively. According to the published data, the low-temperature TPR peak belongs to well-dispersed Rh₂O₃, which is also easily formed, while the high-temperature TPR peak belongs to the reduction of bulk like, crystalline Rh₂O₃ (or larger Rh₂O₃ particles) (Vis et al., 1985).

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Figure 5

Comparative TPR traces obtained for Rh/Al₂O₃, Rh/TiO₂, Rh-Cu/Al₂O₃ and Rh-Cu/TiO₂.

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The number of exposed rhodium atoms was assessed by H₂ chemisorption. The assumption was that all the hydrogen adsorption occurred selectively only at Rh sites (Anderson et al., 1999). The particle size of Rh supported on Al₂O₃ of 1.7 nm (see Table 3), derived from H₂ chemisorption measurements, matches well with the average diameter observed by TEM (d_TEM = 1.6 nm). For Rh NP's deposited on TiO₂, the particle size calculated from H₂ chemisorption data is bigger (d_H2 = 2.7 nm) than observed by TEM (d_TEM = 1.8 nm). The decrease in Rh surface area from 2.9 m² g⁻¹ (corresponding to Rh particles of about 1.8 nm) to 1.8 m² g⁻¹, probed by H₂ chemisorption, can be interpreted as beginning of metal encapsulation by support oxide. Previous experiments showed that encapsulation of Rh occurs on defected TiO₂, subjected to high reduction temperature or Ar+ sputtering (Sadeghi and Henrich, 1984a; Berkó et al., 1998). The encapsulation process requires the outward diffusion of Ti_iⁿ⁺ to TiO₂ surfaces and an electronic configuration of E_F(TiO₂) > E_F (Rh). The n-type TiO₂ and noble metals with large work function (i. e. Pt, Pd, Rh) are necessary for encapsulation reactions (Fu and Wagner, 2007). In our case the encapsulation of Rh nanoparticles by TiO₂, leading to decrease in surface of exposed metal surface area, was limited by the relatively low reduction temperature (400 °C). The encapsulation of Rh by TiO₂ was reported to charge negatively Rh and to oxidize Ti³⁺ defects to Ti⁴⁺ (Berkó et al., 1998). This observation is in line with our XPS results showing that the formation of Ti³⁺ lattice defects do not take place for Rh/TiO₂.

The data in Table 3 show significant lower rhodium surface area for supported Rh-Cu NP's compared to Rh NP's. The CuO_x species formed on the surface of Rh nanoparticle core can be a pertinent explanation for these chemisorption results. The XPS and TPR data support also the idea that copper is extensively oxidized to CuO. The TEM and HRTEM images revealed that particle coarsening occurs in the case of alumina and TiO₂-supported Rh-Cu nanoparticles (d_TEM of Rh-Cu on Al₂O₃ and TiO₂ is 2.1 and 2.8 nm, respectively). The particle coarsening, suggested by H₂ chemisorption, contributes to a decrease in Rh surface area. The encapsulation of Rh-Cu nanoparticles by TiO₂ cannot be probed by chemisorption measurements as it competes with the covering of Rh nanoparticle with CuO_x. The correlation between surface energy (γ) and work function (ϕ) was made and the results suggest that encapsulation is expected for ϕ > 5.3 eV and γ > 2 j m⁻² (Fu et al., 2005). Metals with large γ values such as Pt, Pd, Rh, Ir are likely to experience encapsulation reactions while Cu, Ag, Au with small γ value are not expected to be encapsulated. In light of this information it is likely that copper presence prevents the encapsulation of Rh-Cu nanoparticles by TiO₂. Thus, the surface of bimetallic nanoparticles remains available for catalytic reactions. This hypothesis is confirmed by the excellent catalytic activity of Rh-Cu nanoparticles deposited on TiO₂. The SMSI may contribute to the low value of exposed Rh surface area of Rh-Cu/TiO₂ catalyst. The SMSI phenomenon led to a decreased H₂ (or CO) chemisorbed on the metal either due to a decrease in the number of surface metal (partially covered by the reduced support oxide) or due to a change in electron density in metal surface (electron flow from metal to support), which cause a decrease in the metal and adsorbate bond strength (Santos et al., 1983; Chou and Vannice, 1987). Sa et al. (2007) observed that SMSI takes place for Pd/TiO₂ at relatively low temperatures (350 °C) with the formation of Ti₄O₇ stable phase, even at exposure to atmospheric pressure. The formation of TiO_x species are a consequence of support interaction with H₂ dissociated on the noble metals. This finding is consistent with our XPS results showing the formation of large amount of stable Ti³⁺ after reduction treatment of Rh-Cu/TiO₂.

The time courses of NO₃⁻ reactant and of products (NO₂⁻ and NH₄⁺) are comparatively presented in Fig. 6. The evolution of pH with time is presented in the same figure. The activity of Rh NP's in colloidal form and as supported NP's is represented by open symbols whereas that of Rh-Cu, either in colloidal or supported forms, by closed symbols. The first glance at results depicted in Fig. 6 evince that Rh-Cu NP's, either supported or in colloidal form, perform better than the monometallic Rh NP's in terms of NO₃⁻ and NO₂⁻ conversions. The second general observation is that the support, regardless its nature, enhances the catalytic performances of mono/bimetallic nanoparticles. The third remark is that deposition of NP's on TiO₂ is far a better choice than on Al₂O₃. The Rh-Cu/TiO₂ catalyst showed the best performances in term of NO₃⁻ and NO₂⁻ conversion and selectivity for NH₄⁺. The NO₃⁻ and NO₂⁻ (reaction intermediate) were transformed completely in 90 and 120 min, respectively. Assuming the general accepted step way reaction mechanism $({\text{NO}}_3^{-}\stackrel{{\text{K}}_1}{\to }{\text{NO}}_2^{-}\stackrel{{\text{K}}_2}{\to }{\text{NH}}_4^{+}\text{,}{\text{N}}_2)$ (Miyazaki et al., 2015) the calculated reaction rate constants for the hydrogenation of NO₃⁻ and NO₂⁻ were k₁ = 3.5 × 10⁻² min⁻¹ and k₂ = 4.3 × 10⁻² min⁻¹, respectively. The NH₄⁺ was the only ionic compound at the end of denitration reaction over Rh-Cu/TiO₂ catalyst. The selectivity for NH₄⁺ and N was 76% and 24%, respectively. The formation of Ti³⁺ in connection to activity enhancement of Rh-Cu nanoparticles dispersed on TiO₂ are in our view the result of SMSI effect. The Ti³⁺ lattice defects should play an essential role in mechanism of hydrogenation reaction. Probably H₂ molecules dissociate at the reactive bimetal-support region formed by SMSI. Similar mechanism for hydrogen dissociation over Au nanoparticles on TiO₂ was reported by Fujitani et al. (2009). Then hydrogen atoms react with NO₃⁻ (as well as with NO₂⁻) at the perimeter of bimetallic nanoparticles. The protons generation by spillover mechanism, close to the reacting sites (metal-support interface), helps the adsorption of negatively charged anion reactants (NO₃⁻ and NO₂⁻) even at basic pHs (see Fig. 6D) when it is supposed that the support should be charged negatively. Usually, the increase of pH in the liquid phase during the reaction course by formation of OH⁻ results in development of negative charge which inhibits the adsorption of nitrate and nitrite ions on the surface of catalyst. One possible cause for ammonia formation are the rise of the pH during reaction and/or as suggested in the literature isolated metal atoms, which can act as active sites (Yoshinaga et al., 2002). Catalytic studies pointed out that the formation of Ti³⁺ lattice defects is effective to promote in some cases oxidation processes. According to Li et al. (2014), the highly defective structure of Co₃O₄/TiO₃ due to the formation of Ti³⁺, is responsible for the high catalytic activity observed for CO oxidation.

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Figure 6

Comparative time course of NO₃⁻ (A), NO₂⁻ (B) and NH₄⁺ (C) concentrations for Rh/Al₂O₃, Rh-Cu/Al₂O₃, Rh/TiO₂ and Rh-Cu/TiO₂ catalysts. The figure presents also the time course evolution of pH (D) during the reaction (NO₃⁻ + 4 H₂ = NH₄⁺ + H₂O + 2 OH⁻).

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The Rh-Cu nanoparticles supported on Al₂O₃ exhibited satisfactory performance for reduction of NO₃⁻ (conversion was ≈94%) but on the other hand, the hydrogenation rate of NO₂⁻ intermediate was very low (see Fig. 6). The concentration of NO₂⁻ reached a maximum at 0.57 mmol L⁻¹, then decreases to 0.53 mmol L⁻¹. The reaction selectivity for NO₂⁻, NH₄⁺ and N at end of reaction time was 28.2, 32.9 and 38.9%, respectively. It is documented that the H₂ spillover does not occur for insulators, such as alumina (Prins, 2012). The absence of H₂ spillover may explain the limited catalytic activity of the Rh-Cu nanoparticles (originating from solely one batch) when they are supported onto the insulating Al₂O₃. The simple Rh nanoparticles, performed better on TiO₂ than on Al₂O₃ (see Fig. 6). The Rh/Al₂O₃ catalyst was practically inactive to hydrogenate NO₃⁻.

Fig. 7A shows that Rh-Cu NP's on TiO₂ show the highest yields to nitrogen as well as to NH₄⁺(Fig. 7B). The non-supported Rh-Cu NP's produce only NH₄⁺. It is clear that by supporting Rh-Cu NP's, both on Al₂O₃ and TiO₂, active sites for selective hydrogenation of NO₃⁻ to N₂ are created. Zhang et al. (2008) advanced the idea that the selectivity of nitrate hydrogenation depends generally on the size of active phase. On the bimetallic ensembles with size below to 3.5 nm the exposed palladium particle becomes too small to adsorb and activate two N-containing species simultaneously for the formation of nitrogen. Since the formation of ammonium required only one N, the size effect is not remarkable, and ammonium as by-product is always formed. This observation is in line with our results showing that over the very small, non-supported, Rh-Cu NP's of 1.6 nm, NH₄⁺ is formed selectively. The Rh-Cu NP's coarsens when deposited on Al₂O₃ or on TiO₂ to 2.1 and 2.8 nm, respectively. In this case, small amounts of N₂ are formed along with the main reaction product, NH₄⁺. In light of this hypothesis, the increase in size can be one of the reasons for N₂ formation.

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Figure 7

Comparative NO₃⁻ hydrogenation yield to N₂ (N) and NH₄⁺ over supported and non-supported Rh and Rh-Cu nanoparticles.

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4 Conclusions

The catalytic behavior of colloidal and of supported Rh and Rh-Cu nanoparticles in a model hydrogenation reaction (NO₃⁻ hydrogenation) was investigated.

The most important findings of this research can be summarized as follow.

1. Oxide supports bust up the catalytic activity of metallic NP's. The non-supported NP's are either inactive or posses only low hydrogenation activity.

2. Two metals are required to obtain good hydrogenation activity. The Rh NP's dispersed on Al₂O₃ are inactive whereas on TiO₂ they show only modest performances. The synergetic effect between Rh and Cu enhances considerably the catalytic activity.

3. The strong metal-support interaction is a key point in determining hydrogenation activity. The Rh-Cu NP's dispersed on TiO₂ semiconductor are extremely active for NO₃⁻ and NO₂⁻ (intermediate) deep hydrogenation, with high selectivity for NH₄⁺. When the same Rh-Cu NP's are supported on Al₂O₃ insulator the hydrogenation activity is hindered considerably, the main products of NO₃⁻ hydrogenation being NO₂⁻ intermediate.

Future research plans are aiming to identify a new class of hydrogenation reactions with high practical importance to valorize the excellent hydrogenation activity of Rh-Cu nanoparticles. New catalytic semiconducting supports will be also screened to maximize the catalytic performances of Rh-based NP's through SMSI mechanism. The exploration of synthesis strategies leading to bimetallic nanoparticles with controlled structure (core-shell, inverse core shell, alloy) as well as the accurate control of nanoparticle size is worth of attention in time to come.