Synthesis and Characterization of Nitrogen-Doped Carbon

Article pubs.acs.org/IECR

Synthesis and Characterization of Nitrogen-Doped Carbon Nanospheres Decorated with Au Nanoparticles for the Liquid-Phase Oxidation of Glycerol Sonia Gil,*,† Pedro Joaquín Lucas,‡ Antonio Nieto-Márquez,∥ Luz Sánchez-Silva,‡ Anne Giroir-Fendler,† Amaya Romero,‡,§ and José Luís Valverde‡ †

Université Lyon 1, CNRS, UMR 5256, IRCELYON, Institut de recherches sur la catalyse et l’environnement de Lyon, 2 avenue Albert Einstein, F-69626 Villeurbanne, France ‡ Facultad de Ciencias Químicas/§Escuela Técnica Agrícola and Departamento de Ingeniería Química, Universidad de Castilla-La Mancha, 13071 Ciudad Real, Spain ∥ Departamento de Química Industrial y Polímeros, EUIT Industrial, Universidad Politécnica de Madrid, Ronda de Valencia, 3, 28012 Madrid, Spain ABSTRACT: Carbon and nitrogen-doped carbon nanospheres (CNS) were prepared by thermal pyrolysis of benzene (CNSB), aniline (CNSAN), and nitrobenzene (CNSNB) and used as supports for gold nanoparticles. Gold-based catalysts were prepared by the gold-sol method. The catalysts were checked in liquid-phase glycerol oxidation. The nature of the N-containing groups influenced both the acid−base properties of the supports and the Au particle deposition. As a consequence, an enhanced metal sintering by enriching the surface electron density of the support, essentially in the quaternary form, was observed. Both the glycerol conversion and glyceric acid selectivity increased with decreasing gold particle size. Moreover, catalyst Au/CNSB promoted the formation of glyceric acid, whereas Au/CNSAN and Au/CNSNB catalysts favored the oxidation of the secondary hydroxyl group of glycerol. Results clearly confirmed the influence of the support properties on the catalytic performance of gold in selective glycerol oxidation.

1. INTRODUCTION Glycerol has attracted attention as a useful starting material because of its ready availability (biosustainable sources) and its high level of functionalization.1 Numerous studies have been conducted, and innovative uses for glycerol are the subject of research, especially those that take glycerol as a raw material in the manufacture of value-added products (glyceric acid, hydroxypyruvic acid, glycolic acid, mesoxalic acid, oxalic acid, tartronic acid, etc.).1 The majority of these products are produced through either nonenvironmentally stoichiometric oxidation or poor selectivity and low productivity fermentation processes.2−4 The use of gold catalysts in oxidation reactions is principally due to the specific characteristics of this metal compared to more classical Pd- or Pt-based catalysts when O2 is used as the oxidant, being less prone to deactivation.1,5,6 Moreover, gold catalysts show high selectivity toward the oxidation of primary with respect to secondary alcohols.5,6 Nevertheless, the activity of gold catalysts is often discussed in terms of particle sizes, particle shapes, and support properties, but to date, conclusive studies about the origin of the catalytic activity are lacking.7 Among all the variables postulated to modulate gold activity, the stability and the role of support appear to have a greater influence.8,9 Carbon materials have proven to be better catalytic supports than oxides in liquid-phase reactions10−12 because of their specific properties, such as acid and base resistance, porosity, and surface chemistry control and the possibility of recovering the metals by the combustion of the supports.10−14 For this © 2014 American Chemical Society

reason, carbon materials have been widely employed as catalyst supports in a vast range of liquid-phase oxidation reactions,2,15−22 being highly active and selective in these situations. In this context, the challenge is to elucidate the influence of carbon, such as porosity and surface functionality. This can restrict diffusion of liquid-phase reactants from effects intrinsic to the gold catalyst such as particle sizes, exposure to metal surfaces, and blocking species.22−25 The discovery of novel carbon nanostructures, such as carbon nanotubes (CNT), carbon nanofibers (CNF) and carbon nanospheres (CNS) has led to a growing interest in their potential catalytic applications.26−35 In contrast to activated carbons which exhibit many inaccessible active sites within the micropores, CNT have only external surface area; therefore, metal nanoparticles are expected to be exposed and accessible to reactants, thus improving the efficiency of the catalysts.20,36 CNF present a large amount of edges in the lattice and basal regions, providing increased metal−support interactions,31 and lower mass-transfer constraints associated with their mesoporous character.37−40 CNS are typically isolated as a conglomeration of spherical bodies with low surface areas41 but a high surface chemical activity provided by the “unclosed” graphitic layers, reactive open edges, and “dangling bonds”, which can enhance reactant adsorption.42 Received: Revised: Accepted: Published: 16696

July 18, 2014 October 2, 2014 October 3, 2014 October 3, 2014 dx.doi.org/10.1021/ie502873x | Ind. Eng. Chem. Res. 2014, 53, 16696−16706

Industrial & Engineering Chemistry Research

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Brunauer−Emmett−Teller (BET) method. Mesoporosity was evaluated by the Barret−Joyner−Halenda (BJH) method. Elemental composition of the carbon supports was determined using a LECO CHNS-932 unit. The carbon (ca. 2 mg) combustion (at 1223 K) products were analyzed by IR (for C and H content) and TCD (for N content). The different distribution of nitrogen functionalities was analyzed by X-ray photoelectron spectroscopy (XPS). These analyses were performed in a SPEC Phoibos system operating with Al Kα radiation. Peak areas were determined using Shirley’s method, and the spectra were fitted with Gaussian curves. Sensitivity factors for peaks C 1s and N 1s were 1 and 1.8, respectively. Chemical states of the atoms in the catalyst surface were investigated by XPS on an AXIS Ultra DLD spectrometer marketed by Kratos Analytical, operating with Al Kα radiation. XPS data were calibrated using the binding energy of C 1s (284.6 eV) as the standard. Acid−base titrations were performed in a Metrohm 686 titrator with a Dosimat 665 automatic system. The sample (25 mg) was immersed in 50 cm3 of solution of 0.1 M NaCl, acidified to pH ca. 3 with HCl (0.1 M) with constant stirring under a He atmosphere. A 0.1 M NaOH solution was used as the titrant and added dropwise (1.8 cm3 h−1). The starting NaCl solution served as a blank. XRD analyses were conducted with a Philips X’Pert instrument using nickel-filtered Cu Kα radiation; the samples were scanned at a rate of 0.02° step−1 over the range 5° ≤ 2θ ≤ 90° (scan time, 2 s step−1). Diffractograms were compared with the JCPDS-ICDD references.57 The metal particle size was estimated after hydrogen reduction with H2 (60 cm3 min−1) at 5 K min−1 to 660 K for 2 h, using the Scherrer equation:

In this sense, although CNS have been proposed as potential catalyst supports,22,42−45 there is a dearth of studies dealing with their use in oxidation reactions. In addition, the use of carbon materials as catalyst supports for metal nanoparticles requires control of porosity, defects, and chemical state of the surface. These factors become critical parameters because they influence the active phase dispersion, which in turn affect the catalytic activity of the metals.46,47 Doping carbonaceous structures with heteroatoms, such as nitrogen or boron, is an effective means of modifying their surface and electronic properties and drastically influences the metal dispersion on the surface.48−51 Indeed, it has been established that the incorporation of nitrogen in CNT results in enhanced conductivity, polarity, and basicity, while modifying the surface hydrophilicity. 52 On the other hand, the incorporation of oxygen and nitrogen functionalized in CNF, by treatment with gaseous NH3 at different temperatures, was recently demonstrated to be a good alternative for Au nanoparticles in liquid-phase applications, improving drastically the metal distribution. As a consequence, an increase in the catalytic activity with the basicity of CNF was observed, whereas the selectivity appeared most related to the types of surface groups.25 Accordingly, the aim of this study was the use of Ausupported on CNS catalysts in the liquid-phase oxidation of glycerol. The study tested the influence of nitrogen doping of this material and the type of N-containing group on the deposition of gold particles and the basicity of the materials, and consequently their catalytic response. To the best of our knowledge, this work is the first study to be related to the use of nitrogen-doped CNS as a catalytic support in this kind of reaction.

ds =

2. EXPERIMENTAL SECTION 2.1. Support and Catalyst Preparation. The three CNS employed in this study were prepared, as described in detail elsewhere,53 via the thermal pyrolysis (2 h, 1223 K) of benzene (CNSB), aniline (CNSAN), and nitrobenzene (CNSNB). Gold catalysts were prepared via the gold-sol method with THPC (tetrahydroxymethyl phosphonium chloride) as the reducing agent using HAuCl4·3H2O (Sigma-Aldrich) as the metal precursor salt.1 The THPC acts as a reducing agent, producing the reduction of the chloroauric solution through its partial hydrolysis.15,18,54,55 In addition, according to Sobczak et al.,56 THPC used during the gold-sol procedure stabilizes the colloid gold solutions. Gold particle sizes can be small and, consequently, dispersed. The suspension was filtered, washed with deionized water to obtain a chloride-free filtrate (AgNO3 test), dried at 383 K for 24 h, and sieved into a sieve of CNSAN > CNSNB. The incorporation of nitrogen in the carbon structure led to a disruption of lattice defects in the graphene layers, consistent with a decrease in maximum oxidation temperature (TPOmax) and in the number of graphene planes in the crystallites (npg, ratio between average crystallite size and interlayer spacing in the (002) direction). Regarding the acid− base character (Figure 1), only in the case of CNSAN was the

Table 2. Physicochemical Properties of the Au Catalysts total Au loading (wt %)a TPR Tmax (K) TPR-H2 consumption (μmol H2 gcat−1) (experimental/ theoretical) dAu (nm) (TEMb/XRDc) BET surface area (m2 g−1) total pore volume (nm) Au loading on the surface before reaction (wt %)d Au loading on the surface after reaction (wt %)d

Au/CNSB

Au/CNSAN

Au/CNSNB

0.4 639 2.1/13.5

0.4 664 4.5/13.5

0.4 656 11.1/13.5

4.2/6.7 14 0.05 0.07

10.5/10.2 11 0.03 0.38

13.1/12.1 9 0.03 0.38

0.06

0.38

0.18

a

Actual Au loading determined by AA analysis. bAverage diameter of Au particles determined by counting around 400 particles on the TEM images using the equation d̅Au = [(∑ini·di3)/(∑ini·di2)] where ni is the number of particle of diameter di. Au particle size was estimated as the average between the longest and the shortest segments that intersect at the particle center. cAverage Au particle sizes determined by XRD for the half-width of the main Au peaks. dAu loading on the surface before and after glycerol oxidation reaction determined by XPS analysis.

state (Au0). Comparable Au reduction temperatures, in the range of 573−623 K, have been described elsewhere for Au− carbon systems prepared under similar conditions.1,39,62 Theoretical H2 consumption values (e.g., H2 amount needed to reduce Au3+ to Au0) were always higher than the corresponding experimental ones (see Table 2), indicating that part of the Au was in the metallic form (Au0) before TPR reduction. These results were expected because the THPC used in the sol-gold method also acts as a reducing agent. The second hydrogen consumption peak (Tmax around 990 K), common to all the catalysts and supports, was attributed to the hydrogen uptake by the support upon decomposition of the oxygen surface groups.27,63 This assignment was investigated by temperature-programmed decomposition (TPD) analyses in He after reduction of the catalysts at 990 K (temperature corresponding to the second TPR reduction peak). This technique provides qualitative information related to the presence of oxygen surface groups in the carbon structure, i.e., the release of CO + CO2 resulting from the decomposition of carboxylic anhydride groups.64,65 TPD profiles (Figure 3) demonstrated that the desorption response for the reduced catalysts was weaker than that for the corresponding supports44 because of the decomposition of the oxygen surface groups (in He at T ≥ 990 K). Thus, the majority of the signal was formed as a consequence of the CO and CO2 release resulting from the decomposition of the oxygen surface groups corresponding to water, carboxylic, lactone, anhydride, phenol, carbonyl, and quinone groups.64−66 According to the obtained TPR results, 660 K was chosen as a suitable reduction temperature to ensure metal activation without considerably affecting the surface properties of the support. XRD patterns corresponding to the Au-based catalysts before and after H2 reduction are shown in Figure 4. XRD-derived particle sizes are listed in Table 2. Diffraction peak around 2θ ≈ 26° corresponds to the (002) graphite plane of carbon (JCPDS-ICDD Card 41-1487). Peaks observed at 38°, 44°, 64° and 77° correspond to the (111), (200), (220), and (311) planes, respectively, of metallic gold (JCPDS-ICDD Card 011172), which was consistent with a face-centered cubic lattice.56,57,67,68 Results clearly indicated that before H2

Figure 1. (a) Acid/base titration curves associated with (●) CNSB, (■) CNSAN, and (▲) CNSNB. Solid line corresponds to the blank. (b) Expanded equivalence point region of titrations.

titration curve slightly shifted left from the blank, suggesting some measurable basicity. This result is in a good agreement with the presence of a pyridinic (lone electron pair) component, according to the XPS analyses (Table 1). 3.2. Catalyst Characterization. The final catalysts were prepared by introducing the Au metal phase by the gold-sol method using THPC as the reducing agent. Final metal loading in the catalysts is given in Table 2. TPR analyses were performed to select the optimum temperature at which to carry out the catalyst reduction. Figure 2 shows the TPR profiles of the catalysts and their corresponding supports. The first hydrogen consumption peak (Tmax around 660 K), common to all the catalysts and absent in the supports, was attributed to the complete reduction of the oxidized-Au form to the metallic 16699

dx.doi.org/10.1021/ie502873x | Ind. Eng. Chem. Res. 2014, 53, 16696−16706


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Figure 2. TPR profiles associated with catalysts (a) Au/CNSB, (b) Au/CNSAN, and (c) Au/CNSNB (solid line) and the corresponding supports (dotted line). Experimental conditions: 0.1 g sample, heated (5 K min−1) to 1273 K, in a reducing atmosphere (17.5% v/v H2/Ar, 60 cm3 min−1).

Figure 4. XRD patterns of the Au-based catalysts before (dashed line) and after (solid line) reduction, associated with (I) Au/CNSB, (II) Au/ CNSAN, and (III) Au/CNSNB.

reduction, part of the Au species was in the metallic form (Au0). After H2 reduction, the rest of the oxidized Au species were reduced to the Au0 form. This fact could also be corroborated by XPS analysis (Figure 5). Regarding the oxidation state of gold in the reduced catalysts, XPS of the Au 4f region showed in the three catalytic systems the characteristic Au0 peaks represented by a doublet with peaks at 84.0 and 87.7 eV.69−74 Furthermore, the C 1s spectrum confirmed the graphitic character of the different support. The values of BET surface area and total pore volume associated with the reduced catalysts (Table 2) were also quite similar to those of the supports, observing negligible changes in both the pore diameter and the BET surface area. Representative TEM micrographs and derived particle size distributions, presented in Figures 6 and 7, were used to determine the size (d̅Au, Table 2) and morphology of the Au0 particles.75 Catalyst Au/CNSB exhibited small and welldispersed gold particles with surface area-weighted Au diameters around 4.2 nm. These relatively small and welldispersed Au particles are diagnostic of a high metal−support interaction76 because of its high electrochemically accessible surface area and high electron density. This could be attributed to the high crystallinity of the support, enhancing the electron transfer from the support to the metal, favored by the location

Figure 3. TPD profiles associated with catalysts (a) Au/CNSNB, (b) Au/CNSAN, and (c) Au/CNSB; Au after activation at 1100 K (solid line) and the corresponding supports (dotted line). (Oxygen groups: 1 and 2, carboxylic groups; 3, lactone groups; 4, anhydride groups; 5, phenol groups; 6, carbonil and quinone groups). Experimental conditions: 0.1 g sample, heated (5 K min−1) to 1273 K, in a flow of He (100 cm3 min−1).

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Figure 5. (a) XPS of the Au 4f region and (b) XPS of the C 1s region, associated with the reduced Au/CNS (●) after reduction and before glycerol oxidation reaction and (■) after glycerol oxidation reaction.

of the particles along the unclosed graphitic flakes, as has been reported by other authors.77,78 This fact could be corroborated by TPR and XPS analyses taking into account the higher reduction temperature and the negligible changes between the amount of surface gold before and after the glycerol oxidation reaction (see Table 2). Nevertheless, under an identical metal incorporation method and comparable metal loading (Table 2), larger Au particles

were supported on nitrogen-doped CNS and catalysts Au/ CNSAN and Au/CNSNB (average Au diameters centered at around 10.5 and 13.1 nm, respectively), suggesting a weaker metal−support interaction.76 This fact could be also corroborated by the lower reduction temperature and higher leaching in the reaction medium (see Table 2). Thus, the previously mentioned differences in particle size must be linked to the different metal mobility during activation, in which those 16701



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Figure 6. Representative (a) low- and (b,c) high-magnification TEM micrographs associated with the reduced catalysts (I) Au/CNSB, (II) Au/ CNSAN, and (III) Au/CNSNB.

spheres played an important role in the deposition of Au particles. In addition, XPS analyses (Table 2) showed that the amount of surface gold (0.07 wt %) was minimal with respect to the total Au loaded on the Au/CNSB catalyst (0.4 wt %). This fact could be attributed to the fact that the major part of the Au active sites were probably located in the relatively small pores of the support. In the case of catalysts Au/CNSAN and Au/CNSNB, the major part of the Au was located on the surface (0.38/0.4 wt %) because the pores of the supports were considerably smaller than the size of the Au particles. 3.3. Catalytic Tests. Catalytic performances of Au supported on CNS catalysts were evaluated for the liquidphase selective oxidation of glycerol. The reaction conditions used were optimized in a previous paper:81 glycerol solution concentration, 0.3 M; glycerol/Au, 3500 mol mol−1; PO2 = 5 bar; reaction temperature, 333 K; agitation speed, 1000 rpm; and NaOH/glycerol, 2 mol mol−1. Figure 8 represents glycerol conversion (XGLY) and glyceric acid selectivity (SGLYA) as a function of the time-on-stream, where the glycerol conversion increases with the progress of the reaction. Obtained results show that the catalytic activity increases when the Au particle size decreases, following the sequence Au/CNSB > Au/CNSAN > Au/CNSNB. This fact could be attributed to the higher surface exposure of the active sites, as has been shown by other authors.82,83 Thus, the curved structure in the CNSB-based catalyst, with many open edges at the surface, results in the best catalytic activity because of the presence of the smallest Au particles and therefore the best dispersed particles. These small, welldispersed, and strongly anchored to the support Au particles

Figure 7. Gold particle size distributions associated with the activated catalysts (a) before and (b) after glycerol oxidation reaction.

particles more weakly attached to the support must sinter to a greater extent. It is commonly accepted that the introduction of nitrogen atoms into a graphitic matrix lowers the electron work function of the carbon surface, resulting in enhanced electron mobility if compared to that of pure carbon.79,80 This mobility, being essentially enhanced when nitrogen is incorporated as quaternary nitrogen (NQ), would enrich the density of π electrons in the aromatic system, enhancing metal motion and subsequent sintering.80 To summarize, the following sequence of Au particle sizes (nanometers) was obtained: Au/CNSB (4.2) < Au/CNSAN (10.5) < Au/CNSNB (13.1) This result was in good agreement with the high amount of quaternary nitrogen present in the support. Therefore, obtained results clearly indicated that the nitrogen-doping of carbon nano16702



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Figure 8. Variation of glycerol conversion and glyceric acid selectivity as a function of the time-on-stream. Reaction conditions: 0.3 M glycerol solution, glycerol/Au = 3500 mol/mol, PO2 = 5 bar, 60 °C, 1000 rpm, and NaOH/glycerol = 2 mol/mol.

would promote the proton abstraction from glycerol, increasing the catalytic conversion.9 This could be attributed to the greater stability of small particles due to the charge transfer to the metal,77,78 favoring the glycerol adsorption and subsequent oxidation. One might expect that the N-doped catalysts with larger particle sizes presented catalytic activity profiles that are less pronounced than that of catalyst Au/CNSB. This fact is shown in the catalyst Au/CNSNB, which presented a low catalytic activity, attributed to the larger particle size, weak metal− support interactions, and smaller active surface exposure. Thus, the weak metal−support interaction could be responsible for the leaching into the reaction medium, which corresponds to a 49% loss of the amount of Au contained in the catalyst after reaction (0.21 wt %/0.4 wt %), as has been observed by Atomic Absorption (AA) Spectroscopy. These results were corroborated by XPS profiles (Figure 5 and Table 2), in which, unlike the other catalysts, a significant Au surface leaching for the Au catalysts supported on CNSNB was noted. Moreover, the TEM micrographs and derived particle size distributions (Figures 7 and 9) show an increase in Au particle size for catalyst Au/ CNSNB after the glycerol oxidation reaction. Again, this is unlike the other catalysts, for which the particle size remained constant. This sintering must be linked to the different metal mobility during reaction. Thus, the smaller metal−support interaction attributed to this catalyst could favor the higher metal motion and subsequent sintering. Thus, Au particles could tend to be located close to each other because reactant and product adsorption, along with operating pressure, could modify the binding energy of the gold atoms, giving rise to this greater Au particle size after reaction. Nevertheless, in the case of catalyst Au/CNSAN, the catalytic activity was very similar to that of catalyst Au/CNSB, which definitely shows that the support has an important influence in the rate of glycerol oxidation. This fact may be attributed to the degree of metal−support interaction. As mentioned above, the high metal−support interaction could favor the greater stability of the Au particles by electron transfer from the support to the metal, increasing the glycerol adsorption, which facilitates the

Figure 9. Representative (a) low- and (b) high-magnification TEM micrographs associated with the reduced catalysts after glycerol oxidation reaction (I) Au/CNSB, (II) Au/CNSAN, and (III) Au/ CNSNB.

proton abstraction from glycerol. Thus, the higher crystallinity of the CNSAN support compared to that of the CNSNB support (Table 1) could be responsible for the greater stability of the Au particles, increasing the glycerol conversion.77,78 This could be corroborated by the higher reduction temperature and the negligible Au particles leached (3.8 ppm) in the reaction medium, as has been observed by elemental analysis (AA spectrophotometry) and XPS analysis before and after glycerol oxidation reaction (Table 2 and Figure 5). In addition, the high performance could be attributed to the amount of basic sites present in the N-doped CNS, as has been reported in the literature.25,22,84 Thus, the high activity obtained with the catalyst based on CNSAN may be caused by the presence of pyridinic nitrogen, Np (Table 1), and thus by its basicity (Figure 1). Moreover, the glyceric acid selectivity increased with decreasing Au particle size, being greater in the case of Au catalysts supported on CNSB and similar to Au catalysts supported on nitrogen-doping carbon CNS with comparable Au particle size (Figure 8). Table 3 lists the product selectivity obtained with the three different catalytic systems, in which glyceric and glycolic acids (GLYA and GLYCA) were mainly obtained, followed by some amount of mesoxalic acid (MOXALA), hidroxypyruvic acid (HPYA), tartronic acid (TARAC), and oxalic acid (OXALA). Obtained results showed that catalyst Au/CNSB promoted the selectivity to C3 products (glyceric acid) by oxidation of primary hydroxyl group of glycerol, whereas Au/CNSAN and Au/CNSNB favored the C−C bond cleavage, leading to the formation of glycolic acid and decreasing the glyceric acid selectivity. This agrees with different studies showing that selectivity could be ruled by a more complex set of factors, particularly the textural and 16703



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Table 3. Distribution of Reaction Products in the Selective Oxidation of Glycerola catalyst

time (h)b

SGLYA35 (%)c

SMOXALA35 (%)c

SGLYCA35 (%)c

SHPYA35 (%)c

STARAC35 (%)c

SOXALA35 (%)c

TOF (h−1)d

TOF (h−1)e

Au/CNSB Au/CNSAN Au/CNSNB

1 1 2

68 51.4 48.2

3.7 4.6 4.2

24.2 33.1 38.1

1.4 7.5 5.9

1.9 2.6 2.7

0.9 0.7 0.8

587.2 542.1 337.3

224.5/224.7 204.4/204.1 146.7/205.7

Reaction conditions: 0.3 M glycerol solution; glycerol/Au = 3500 mol mol−1; PO2 = 5 bar; 333 K; 1000 rpm; and NaOH/glycerol = 2 mol mol−1. Reaction time needed to get the 35% of glycerol conversion. cSelectivity to glyceric acid (GLYA), mesooxalic acid (MOXALA), glycolic acid (GLYCA), hydroxypyruvic acid (HPYA), tartronic acid (TARAC), and oxalic acid (OXALA) at 35% of glycerol conversion. dTOF (h−1) after 1 h of reaction. Determined by eq 5. eTOF (h−1) after 7 h of reaction, using the initial/final Au particle size, where the final Au particle size is the average diameter of Au particles determined by counting around 400 particles on the TEM images of the used catalyst, after reaction. a b

catalyst. Obtained results have shown that structural nitrogen present in CNS, essentially in its quaternary form, led to an electron-enriched carbon surface, promoting the mobility of the metal and subsequent sintering. Thus, it could be claimed that the glycerol oxidation reaction was structure-sensitive in the presence of the different Au/CNS catalytic systems here tested. In addition, it was observed that the catalytic activity increased with the metal−support interaction and the amount of basic sites present in the catalysts. Furthermore, Au catalysts supported on N-doped CNS, Au/CNSAN and Au/CNSNB, favored the oxidation of the secondary hydroxyl group of glycerol, whereas catalyst Au/CNSB promoted the oxidation of the primary hydroxyl group of glycerol, increasing the selectivity to glyceric acid. The results showed that the physicochemical properties of the supports played an important role in the deposition of the Au particles and, as a consequence, in its glycerol conversion and product distribution.

chemical properties of the materials used as supports.22,45,82,83,85 Thus, the lower selectivity to glyceric acid presented by catalyst Au/CNSNB could be attributed to its lower catalytic activity due to the presence of larger gold particles. However, Au catalysts supported on CNSB and CNSAN with similar conversion rates showed different selectivity. This could be attributed to the relationship between the gold particle size and the surface properties of the supports, as described elsewhere.78,86−88 Therefore, the major part of the metal actives sites was probably located in the relatively small pores of the support, CNSB, because of the small sizes of the Au particles. Regarding the molecular size of glycerol (0.620 nm), it is evident that a strong steric effect could be generated resulting in a well-confined form of the glycerol molecules that favored the oxidation of the primary alcohol group of glycerol (Scheme 1). Nevertheless, in the case of N-doped-based catalysts, Au/

■

Scheme 1. Surface Morphology of CNS and n-Doped CNS: The Relationship between the Gold Particle Size and the Surface Properties of the Supports

AUTHOR INFORMATION

Corresponding Author

*Tel.: +33-472431054. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the Regional Government of Castilla-La Mancha (Project PCI080020-1239). Mimoun Aouine, Laurence Burel and Laurence Massin of IRCELYON (Université Claude Bernard Lyon 1, France) are gratefully acknowledged for assistance in TEM and XPS measurements and for stimulating discussions.

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REFERENCES

(1) Demirel-Gülen, S.; Lucas, M.; Claus, P. Liquid phase oxidation of glycerol over carbon supported gold catalysts. Catal. Today 2005, 102, 166−172. (2) Carrettin, S.; McMorn, P.; Johnston, P.; Griffin, K.; Hutchings, G. J. Selective oxidation of glycerol to glyceric acid using a gold catalyst in aqueous sodium hydroxide. Chem. Commun. (Cambridge, U.K.) 2002, 696−697. (3) Pollington, S. D.; Enache, D. I.; Landon, P.; Meenakshisundaram, S.; Dimitratos, N.; Wagland, A.; Hutchings, G. J.; Stitt, E. H. Enhanced selective glycerol oxidation in multiphase structured reactors. Catal. Today 2009, 145, 169−175. (4) ten Brink, G. J.; Arends, I. W. C. E.; Sheldon, R. A. Green, catalytic oxidation of alcohols in water. Science 2000, 287, 1636−1639. (5) Carrettin, S.; McMorn, P.; Johnston, P.; Griffin, K.; Kiely, C. J.; Attard, G. A.; Hutchings, G. J. Oxidation of glycerol using supported gold catalysts. Top. Catal. 2004, 27, 131−136. (6) Porta, F.; Prati, L. Selective oxidation of glycerol to sodium glycerate with gold-on-carbon catalyst: An insight into reaction selectivity. J. Catal. 2004, 224, 397−403.

CNSAN and Au/CNSNB, the metal active sites were located on the surface of the supports, as has been demonstrated by the XPS analysis, without apparent steric effect. Consequently, the binding of glycerol over the gold surface may be in the adequate conformation for the oxidation of the secondary alcohol group of glycerol, requiring more space to react (shape selectivity), Scheme 1. Obtained results clearly demonstrate the textural and chemical properties of the supporting materials influence the catalytic activity and selectivity in the liquid-phase oxidation of glycerol.

4. CONCLUSIONS In the present work, gold supported on carbon and nitrogendoped carbon nanospheres was employed in the liquid-phase oxidation of glycerol. Nitrogen doping had an important impact on the surface and electronic properties of the support and, as consequence, on the Au particles’ dispersion on the final 16704



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Synthesis and Characterization of Nitrogen-Doped Carbon

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