Influence of the Support Surface Chemistry on the Catalytic

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J. Phys. Chem. C 2008, 112, 11385–11393

11385

Influence of the Support Surface Chemistry on the Catalytic Performances of PdO/BN Catalysts G. Postole,*,†,§ M. Caldararu,† B. Bonnetot,‡ and A. Auroux§ Institute of Physical Chemistry “Ilie Murgulescu” of the Romanian Academy, Spl. Independentei 202, 060021 Bucharest, Romania, Laboratoire des Multimate´riaux et Interfaces, UMR CNRS 5615, bat Berthollet, UCB Lyon I, 69622 Villeurbanne Cedex, France, and Institut de Recherches sur la Catalyse et l’EnVironnement de Lyon, UMR 5256, CNRS-UCBLyon1, 2 aV. Albert Einstein, 69626 Villeurbanne Cedex, France ReceiVed: May 6, 2008

Two high surface areas BN powders have been prepared to be used as supports for palladium catalysts in propylene oxidation. Two different precursors originating from borazines were used for BN preparation. The BN prepared from trichloroborazine (TCB) had a surface area of 184 m2 g-1 and micrometric spherical (ballshaped) texture. When polytrimethylaminoborazine was used as a precursor, the support obtained (BN1-xOx) presented an oxi-nitride structure with a surface area of 115 m2 g-1 and a glassy aspect. Palladium oxide catalysts were prepared by impregnation of these two supports. Characterization techniques, such as BET, XRD, TEM, XPS, TG, and electrical properties measurements, have been used to study the influence of the surface chemistry of the support on the catalytic performances of the prepared catalysts. The acidic and redox properties of the resulting samples were characterized by TPR and adsorption microcalorimetry. The presence of oxygen in the structure of BN1-xOx support induced different behaviors when compared with pure BN and played an important role in the catalytic activity of PdO/BN1-xOx sample. 1. Introduction Porous solids like zeolites, pillared clays, some polymeric solids, silica gel, inorganic oxides, metal organic frameworks (MOFs), and porous carbon display a large number of applications ranging from absorbents for pollutant gases and purification chromatographic packing to supports for catalytic processes.1–4 For example, noble metal catalysts supported on carbon are used in hydrogenation reactions.5–8 However, their stability in oxidizing atmosphere is low due to the oxidation of the support to form CO and CO2.9 Hexagonal boron nitride (h-BN), a material closely related to graphite structure, frequently used as refractory, electronic, and lubricant material (due to its attractive combination of chemical and thermal stability)1,10–15 started to be recently used as a support in catalysis.14–20 For example, Wu et al. 14 reported that Pt/BN shows better performances than traditional Pt/γ-Al2O3 catalyst with respect to the lifetime and activity in catalytic destruction of volatile organic compounds. Jacobsen17 found that the activity of a Ba-Ru/BN catalyst for ammonia synthesis was significantly higher than that of the traditional catalysts (conventional multipromoted iron catalyst or Ru/MgO catalyst). Boron nitride has been also used as a support for vanadium oxide and tested with promising results in propane oxidation.18 The use of boron nitride as a support significantly increased the selectivity toward acrolein if compared with a V2O5/SiO2. Wu et al. have also presented favorable findings for the selective hydrogenation of R,β-unsaturated aldehydes into unsaturated * To whom the correspondence should be addressed. Fax: (33) 472 44 53 99. Phone: (33) 472 44 53 98. E-mail: georgeta.postole@ ircelyon.univ-lyon1.fr; [email protected]. † Institute of Physical Chemistry “Ilie Murgulescu” of the Romanian Academy. ‡ LMI, UCBLyon 1. § IRCELYON, CNRS-UCBLyon 1.

alcohols by employing BN-supported Pt-Sn19 or BN-supported Pt-Fe20 catalysts. The promising results obtained up to now in catalysis recommend boron nitride as an alternative for carbon and traditional oxide supports. However, our previous studies21,22 indicated that anchoring of the active phases (noble metals or oxides) on the BN support was limited by the specificities of its structure, in relation with the preparation method and the nature of the precursors. The hexagonal BN crystallographic structure consists of three BN groups stacked into a hexagonal framework, leading to BN layers, similar to graphite. These layers are bound together through anisotropic bonds conducting to a very oriented compound. It has been shown that the interaction between pure BN support and the active phase (such as Ga2O3, In2O3, SnO2) is weak,22 which could favor the deposition of oxide aggregates at the support grain border. Similar results have been also reported for the deposition of metallic Pd on pure BN support.21 This paper presents a detailed study on palladium oxide supported on pure BN and oxygen containing BN (BN1-xOx) in order to determine how the oxygen species present on the support surface may influence the anchoring of Pd bonding. The presence of oxygen species on/in BN1-xOx are expected to play a double function: (i) to form anchorage sites for the metal precursor during catalyst preparation and (ii) to act as active centers in the catalytic reaction. For a better understanding of the behavior of the studied catalysts, the two BN supports (BN and BN1-xOx) were fully characterized. The surface dynamics of the supports and catalysts was also studied by electrical conductivity measurements, using the differential steps technique in conditions similar with those encountered in the practical use in catalysis. The catalytic performances of the samples were tested in propylene oxidation as a model reaction for deep oxidation

10.1021/jp803963e CCC: $40.75  2008 American Chemical Society Published on Web 07/09/2008

11386 J. Phys. Chem. C, Vol. 112, No. 30, 2008 process with the aim of proposing BN based catalysts as alternative materials for hydrocarbon removal from air. 2. Experimental Section 2.1. Materials and Synthesis. High surface area boron nitride powders have been prepared from different precursors, namely: (a) trichloroborazine (TCB), containing a large amount of TCB polymers (for BN support)9 and (b) polytrimethylaminoborazines (-(NMe)3-[BNH]3) obtained through the polymerization of trimethylaminoborazines (MeNH[HBNH]3) (for BN1-xOx support). The boron nitride powders were prepared from these precursors by using appropriate thermolysis conditions (ceramisation under ammonia flow up to 1800 °C). Details of the preparation procedures have been published previously.23 To obtain PdO/BN and PdO/BN1-xOx respectively, the boron nitride powders were impregnated by using a classical wet process. The palladium precursor was Pd(NO3)2 hydrate from Strem Chemicals (40 wt % Pd). In a typical experiment, the required quantity of precursor necessary to obtain a 1 wt % Pd on BN (or BN1-xOx) was dissolved in a minimum volume of water containing the immersed support. They were mixed together by stirring at room temperature for 7 h. After drying overnight at 110 °C the samples were calcinated in air flow at 500 °C for 12 h, and then cooled down under the same gas flow. 2.2. Characterization. The physicochemical characteristics of the BN powders and BN supported catalysts were determined by chemical analysis (AES-ICP), Brunauer-Emmett-Teller method (BET), X-ray diffraction (XRD), X-ray photoelectron spectrometry analysis (XPS), thermogravimetry (TG), scanning electron microscopy (SEM), temperature-programmed reduction (TPR), and adsorption microcalorimetry. The concentrations of the supported palladium were determined by chemical analysis using inductively coupled plasma atomic emission spectroscopy (ICP-AES). In order to dissolve them completely, the samples were treated with a mixture of H2SO4 + HNO3 at 250-300 °C. Surface area measurements (SBET) of the samples were obtained by N2 adsorption at -196 °C using the BET method (ASAP 2010 M instrument, from Micromeritics). Prior to surface area determination, the powders were outgassed at 100 °C overnight and then for 6 h at 400 °C. The pore size distribution of each sample was determined from the desorption branch of the N2 isotherm. The crystalline structures of the BN phase and of the obtained catalysts were examined by X-ray diffraction (XRD), using a Bruker (Siemens) D5005 instrument (Cu KR radiation, 0.154 nm). X-ray photoelectron spectroscopy analysis (XPS) was performed by using a Surface Science Instruments 301 spectrophotometer with a monochromatic Al KR radiation. The chemical species present at the surface of the hexagonal boron nitride (h-BN) samples were surveyed using the 1s binding energy of boron. Thermogravimetric experiments were performed in a TGDSC 111 instrument from Setaram, from room temperature up to 600 °C with a heating rate of 5 °C min-1 under air flow. The morphologies of the different samples were examined by scanning and transmission electron microscopies. The SEM images were obtained from a Hitachi S800 (CMEABG UCBLyon1), whereas TEM pictures were recorded using a Jeol 2010 (IRCELYON). Temperature-programmed reduction (TPR) experiments were performed on a homemade apparatus already described in

Postole et al. literature,24 equipped with a U-shaped quartz tube microreactor and surrounded by a furnace controlled by a programmed heating system. Prior to each test, a sample of 0.05 g of catalyst was pretreated in an argon flow at a rate of 20 cm3 min-1 up to 400 °C (2 °C min-1). The reduction was performed in a gas mixture composed of 1% H2 in Ar, at a flow rate of 20 cm3 min-1 and a heating rate of 5 °C min-1 from room temperature up to 600 °C. The hydrogen consumption was monitored using a thermal conductivity detector (TCD). The acid-base properties were studied by adsorption microcalorimetry using NH3 as a probe molecule. The adsorption experiments were performed at 80 °C in a heat flow calorimeter (C80 from Setaram) linked to a conventional volumetric apparatus and equipped with a Barocel capacitance manometer for pressure measurements. The samples were pretreated in the calorimetric quartz cell overnight under vacuum at 400 °C. The differential heats of adsorption were measured as a function of coverage by repeatedly sending small doses of NH3 over the catalysts until an equilibrium pressure of about 67 Pa was reached.25,26 The sample was then outgassed for 30 min at the same temperature and a second adsorption was performed (still at 80 °C) until an equilibrium pressure of about 27 Pa was attained, in order to calculate the amount of irreversibly chemisorbed ammonia at this pressure. The difference between the amounts of gas adsorbed at 27 Pa during the two adsorption runs corresponds to the number of strong acid/basic sites. 2.3. Electrical Conductivity Measurements and Catalytic Test. It is known that the most substantial changes during sintering, phase segregation, adsorption/desorption take place in the surface layers, being directly reflected in changes of the electrical conductivity. Being connected with type and density of structural imperfections, the conductance is a quality of the material and when adequately used can be a tool for in situ study of surface changes. AC electrical conductivity measurements were performed in situ in a cell consisting of two coaxial tantalum cylinders as electrodes, embedded in a Pyrex glass tube. This cell was specially designed to allow simultaneous electrical and catalytic activity measurements in powders, in operando conditions.27–29 The sample powder (fraction between 0.25-0.5 mm) was placed in the annular space between the electrodes and supported on a frit. The electrical conductance G (G ) 1/R, where R is the resistance) and the capacitance C of the powder bed were simultaneously measured at 1592 Hz, in gas flow with a semiautomatic RLC bridge TESLA BM 484 by using the differential step technique (DST) as described previously.27–29 At frequency of the measurements the conductivity of the powder is dominated by surface conduction.30 The differential step technique consists of successive heating (5 °C min-1, between 25 and 400 °C) and cooling (about 10 °C min-1) cycles in different gases according to a specific protocol; this is coupled with the permanent monitoring of the composition of the inlet/ outlet gas by online gas chromatography. Here, the cycles succession was DAr(1-3), DO, DAr4, CT, DAr5, and HO, where DAr abbreviates dry argon, DO and HO abbreviates dry or humid oxygen, respectively, and CT represents the mixture used for the catalytic test (C3H6:air ) 1:22). The overall flow rate in all cases was 69.3 cm3 min-1. Dry gases (except propylene) were obtained by passing the research grade compounds (Linde and SIAD) through molecular sieves units. Humid oxygen was obtained by flushing the gas over the water layer in a saturator at 25 °C. By comparing the plots obtained in successive cycles it is possible to have a deeper insight on the influence of various

Catalytic Performances of PdO/BN Catalysts

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TABLE 1: Pore Characteristics of Boron Nitride Supportsa surface area sample

precursor

BN BN1-xOx

TCB (-(NMe)3-[BNH]3)

2

-1

SBET m g

2

-1

SBJH m g

184 115

168 40

pore volume 2

-1

Smicro m g 58 14

3

-1

Vsingle cm g 0.71 0.11

VBJH cm3 g-1

Vmicro cm3 g-1

0.77 0.16

0.03 0.01

a SBET is the surface area calculated by the BET method; SBJH and VBJH are the cumulative adsorption surface area and pore volume respectively (1.7 < pore diameter < 300 nm) calculated by the BJH method; Smicro and Vmicro are respectively the surface area and pore volume in the micropore range (pore diameter < 2 nm) calculated by the deBoer’s t-plot method; Vsingle is the single point total pore volume (pore diameter < 100 nm).

Figure 2. XPS B1s spectrum of the BN1-xOx support.

TABLE 2: Physicochemical Characteristics of the Catalystsa XPS

Figure 1. SEM micrographs for BN and BN1-xOx supports.

gas atmospheres and on the surface dynamics in conditions similar to those encountered in the practical use in catalysis. The catalytic activity in propylene oxidation was evaluated during the CT run of the electrical conductivity protocol, by periodically sampling the effluent into the gas chromatograph. Gas analysis was performed online with a GC (Pye, TCD detector) equipped with two parallel columns (Porapak Q and molecular sieves 5A) with helium as carrier gas. 3. Results and Discussion Table 1 shows the pore characteristics of the two boron nitride supports prepared. As can be seen, both supports show surface areas higher than 100 m2 g-1, and some microporosity. Some microporosity was also observed by Lindquist et al.,31 who reported a micropore surface area of 43 m2 g-1 and surface area of larger pores of about 307 m2 g-1 for a BN sample heated at 1500 °C for 24 h. Dibandjo et al.32 reported a BN surface area of 540 m2 g-1 using also BET method, and Han et al.1 reported a surface area of 168 m2 g-1 for a porous BN. The micropore surface area and the micropore volume were measured by using DeBoer’s t-plot analysis and the values obtained explain the differences in BN surface areas observed by the BET method. BN sample possesses a higher surface area than BN1-xOx and also a higher pore volume as it can be observed from Table 1. The pore size diameters were also obtained from the adsorption branch of the isotherm using the corrected form of the Kelvin equation by means of the Barrett-Joyner-Halenda (BJH) method and were 18.4 nm for BN and 15.8 nm for BN1-xOx, respectively. The morphological characteristics of the samples have been determined by using SEM. The most significant scanning electron micrographs obtained for the supports are shown in Figure 1. The differences concerning the morphology of these supports are obvious: BN obtained from trichloroborazine seems to be formed of ball shaped particles, while the boron nitride

Pd

TPR

catalyst

SBET m2g-1

ICP Pd/wt %

wt %

at. %

T/°C

H/Pd

PdO/BN PdO/BN1-xOx

233 225

0.97 1.10

16.1 1.7

2.2 0.2

95 135

0.74 0.43

a T (°C) is the temperature of the maximum of the reduction peak; H/Pd ratios were calculated based on the hydrogen uptake.

prepared from polytrimethylaminoborazine (BN1-xOx) presents a glassy surface which leads to a lower surface area. X-ray photoelectron spectroscopy results show a slight oxidation of the surface of BN1-xOx as shown in Figure 2. For BN1-xOx support the B1s binding energies of two kinds of boron bonds were observed. Signal at 190.5 eV is specific of pure BN while that at about 193 eV corresponds to B-O bonds (3% in this case). Using the wet method, the boron nitride powders were impregnated with palladium nitrate, to produce BN-supported PdO (1 wt % Pd) catalysts. The supported catalysts were characterized by some specific methods such as BET, ICP, XRD, XPS, TEM, TPR, and ammonia adsorption microcalorimetry measurements. The response of in situ AC electrical conductivity was used to study surface dynamics of catalysts. Table 2 presents the BET surface area, the amounts of palladium deposited on the supports as determined by chemical analysis and XPS, and the TPR results as obtained when the catalysts were reduced in a mixture of 1% H2/Ar up to 600 °C. As can be seen after impregnation the specific surface areas increased, which can be ascribed to the specificities of the BN structure. When boron nitride is swelled in water, an important fraction of water molecules enter inside the BN layers and open the nanopores during the calcination, leading to an increase of surface area. This process had already been discussed in detail in a previous paper.21 The amount of Pd desired to be deposited was 1 wt % and as it can be noticed in second column of Table 2 (ICP), the amount of metal deposited on the boron nitride supports was found to be close to the required value. The total surface amount

11388 J. Phys. Chem. C, Vol. 112, No. 30, 2008

Postole et al.

Figure 3. Size distribution of palladium oxide particles as deduced from TEM measurements for PdO/BN1-xOx catalyst.

of palladium was also determined by XPS analysis. The results yielding from XPS technique concerning PdO/BN catalyst were very different of the obtained average values from chemical determination. This confirms the presence of palladium mainly on the support surface in case of pure BN. Concerning the PdO/ BN1-xOx, reasonably similar amounts of palladium were obtained from chemical analysis and XPS measurements (even if still slightly higher in XPS). After impregnation with palladium, the XPS results obtained on the PdO/BN1-xOx catalyst showed an increase of the oxygen amount bonded on boron (13%) while no characteristic lines for B-O bonds were observed in the case of BN and PdO/BN samples. As boron oxide is known to be soluble in water, it means that if the existing oxygen would have been enclosed as BOx, it should have diminished on impregnation. Thus, the increase amount of surface oxygen on the PdO/BN1-xOx catalyst must be related with the incorporation of oxygen in place of nitrogen in the framework of the boron nitride hexagons leading to an oxi-nitride structure. Using the hydrogen uptake calculated from the area under the experimental TPR curve (not shown) only 43 wt % of PdO was reduced for PdO/BN1-xOx. This indicates that either part of palladium on the surface is not accessible or has a trend to form stacked aggregates during the thermal oxidation-reduction cycles. For each catalyst the H2 TPR profile shows only one peak of reduction, with a maximum at 95 °C for PdO/BN and 135 °C for PdO/BN1-xOx, respectively. The higher reduction temperature for PdO/BN1-xOx could be related also to the oxinitride structure of the BN1-xOx support; the oxide part can react partially with PdO to form a more stable compound. Transmission electron microscopy (TEM) was employed to determine the dispersion and the size of palladium particles deposited on boron nitride supports. The results obtained concerning PdO/BN catalyst presented elsewhere33 evidenced a homogeneous distribution by size, with an average value for the palladium oxide particles of 3.8 nm. For the PdO/BN1-xOx sample, as it can be seen in the histogram given in Figure 3, the sizes of palladium oxide particles indicated an average value of 4.8 nm with an inhomogeneous size repartition probably due to the formation of aggregates during calcination. Figure 4 represents the differential heats of ammonia adsorption as a function of coverage for the various samples. The acidity of the BN support was almost not affected by the impregnation as it is evidenced by the comparison of the BN and PdO/BN differential curves in Figure 4.

Figure 4. Differential heats of NH3 adsorption at 80 °C vs adsorbed amount on boron nitride supports and PdO supported catalysts.

TABLE 3: Calorimetric Data for NH3 Adsorption at 80 °C on BN Supports and BN-Supported PdO Catalysts sample

Qdiffa kJ mol-1

ntotalb µmol m-2

nreadsc µmol m-2

nirrd µmol m-2

BN PdO/BN BN1-xOx PdO/BN1-xOx

124 124 148 116

0.45 0.32 4.12 2.62

0.19 0.17 1.39 0.72

0.26 0.15 2.73 1.90

a The initial value of the differential heats of ammonia adsorption. b Amount of NH3 absorbed under an equilibrium pressure of 27 Pa. c Amount of NH3 readsorbed after pumping at 80 °C under an equilibrium pressure of 27 Pa. d Amount of irreversibly chemisorbed ammonia.

The deposition of palladium oxide on BN surface resulted in a small decrease of the total number of acidic sites per surface area unit. The acidity measurements performed comparatively on BN supports and the corresponding catalysts indicated that the acidity of BN and PdO/BN is much lower than that of BN1xOx based compounds. The influence of the oxide part on the acidic strength distribution of the BN1-xOx and PdO/BN1-xOx samples is obvious and it is supported by the results presented in Figure 4 and Table 3. For BN1-xOx support the addition of PdO has also an effect but the decrease of strength and in the number of acidic sites is smaller but more evident in this case. The absence of a plateau in the shape of the differential heat curves shows the heterogeneity of the adsorption sites of the samples (excepting PdO/BN1-xOx which presents a small plateau at low ammonia coverage indicating the presence of same strong homogeneous acid sites on this catalyst). Table 3 (which lists calorimetric data for NH3 adsorption at 80 °C on BN supports and BN-supported PdO catalysts),

Catalytic Performances of PdO/BN Catalysts

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Figure 5. (A) Arhenius-type representation of variation of conductivity during heating (5 °C min-1) up to 400 °C in two dry argon runs for BN and BN1-xOx supports and in DAr1 cycle for PdO based catalysts. (B) Water evolution in effluent upon heating in DAr1 and DAr2 runs for BN1-xOx support.

indicates a decrease of surface acidity on palladium deposition on BN1-xOx. PdO probably blocks part of the acidic sites present on the boron nitride supports. We suppose that PdO is preferentially bonded on the oxygen complexes existing of the support surface. The decrease of number of strong acid sites for PdO/BN1-xOx in comparison with the support can be related with a stronger interaction between PdO and BN1-xOx. This result is also supported by the differences showed by TEM and TPR measurements: a better dispersion of PdO was obtained on the BN support; the PdO/BN1-xOx sample was less reducible and PdO was reduced at higher temperature if compared with PdO/ BN catalyst. Since the electrical properties of powder surfaces at low frequency are mainly controlled by intergrain Schottky-type barriers, being dependent on the type and concentration of surface defects in intergrain area, changes of distribution and nature of these defects produced by heating and/or by interaction with reactants will be reflected in the response of the electrical conductivity. As presented in the experimental part, the electrical conductivity was measured in situ, in conditions similar with those involved in catalysis (i.e., in flow system, at atmospheric pressure). In Figure 5A are presented Arrhenius dependences of conductivity σ (nS m-1) for the two BN supports; for comparison, in the figure are also presented the similar plots for the corresponding PdO catalysts in the first cycle in dry inert (i.e., DAr1). As shown, heating in DAr1 produces an U-shaped pattern for the plot of variation of conductivity of the BN1-xOx sample with a minimum at around 130 °C. The decrease of conductivity on heating at low temperature (the

decreasing part of the plot) for this sample was coupled with water evolution in effluent (see also Figure 5B). A similar effect (but in higher extent) was also observed on a γ-alumina sample (minimum of σ around 170-190 °C) and was explained earlier34 as connected with the decrease of the contribution of proton conduction by vehicle mechanism due to water (vehicle) desorption. As it can be seen in Figure 5B (presenting the evolution of water content in the effluent during the first two cycles in inert gas flow for BN1-xOx support), the water evolution in the effluent in DAr1 cycle for BN1-xOx has a maximum located at 265 °C. It is well-known that metal oxide surfaces are usually covered with OH groups which facilitate weak adsorption of molecular water through hydrogen bonding. The physisorbed water can be removed even at room temperature by evacuation or flushing with dry gases, whereas the temperature for the onset of dehydroxylation is a characteristic of the oxide.35 The water evolved at higher temperature (see Figure 5B) could be the result of surface dehydroxylation. BN behaves differently, with rather constant or very slightly increasing conductivity up to approximately 200 °C, and a sharper increase above. The plots of the two BN containing supports present quite identical trends above 190 °C, BN1-xOx showing higher values than BN during all temperature range. On reheating in the next dry inert cycle (DAr2), the behavior of the two samples is also different. BN displays the same pattern as in DAr1, but with slightly lower σ values in the low temperature range. The plot of BN1-xOx support changed dramatically in DAr2, showing now a trend similar to the BN sample (but still at higher σ values). As can be seen in Figure 5B, the amount of water evolved in DAr2 on BN1-xOx is much

11390 J. Phys. Chem. C, Vol. 112, No. 30, 2008 lower and almost constant up to 185 °C; only a very slight increase is observed at higher temperatures. It is obvious that as shown previously34 a correlation exists between the surface water content and the measured conductivity. The increase of the surface content of water corresponds to the increase of σ, while the loss of this water in effluent by desorption, results in a decrease of conductivity (see the low temperature part in Figures 5A and 5B). It is clear that the behavior of BN1-xOx support is related to the presence of the oxi-nitride phase facilitating water adsorption on the surface. This result is confirmed also by thermogravimetric measurements. Thermogravimetric experiments performed in order to compare the level of hydration of the samples indicated that when exposed in air BN support trapped a very small amount of water (mass loss ) 0.45% during heating up to 600 °C) and remains a quite hydrophobic support, while a weight loss of 9.12% was obtained in the same conditions for BN1-xOx. The behavior of the BN1-xOx in the low temperature range in DAr1 cycle can be thus associated with the presence of some weakly adsorbed water species. σ values in DAr2 and DAr3 cycles (not presented here) are almost identical for each support, indicating reproducible surface behavior. As obvious, both plots for PdO based catalysts following DAr1 a profile similar to that of the corresponding support, i.e., a L shaped plot with slightly higher values starting at 315 °C in the case of PdO/BN, and an U shaped plot with lower values in the case of PdO/BN1-xOx. It seems that water is adsorbed on PdO/BN1-xOx on sites with slightly different strength since two inflection points (at 65 and 90 °C, respectively) can be observed on the decreasing arm of the plot. In order to check the influence of moisture on the surface conductivity in presence of oxygen, a cycle was performed in humid oxygen. Figure 6 shows the variation of conductivity during heating in humid oxygen and also, for comparison to the previous dry inert cycle (see the protocol of measurements in the experimental part) for BN1-xOx support. The effect of humidity was practically insignificant for BN sample so for simplicity of the figure the variation of conductivity for this support during HO cycle was not presented here. For BN1-xOx sample, the conductivity increases when flushing humid oxygen at room temperature (not presented), as shown by higher starting values on heating. A peak with a maximum at 90 °C corresponds to desorption of weakly adsorbed water (similar with 100 °C reported for alumina34). Anyway if this result is compared with that obtained previously for γ-Al2O3 (with the same surface area, i.e., ∼100 m2 g-1), the extent of conductivity increase under the influence of adsorbed water was by far larger on alumina.34 As it was demonstrated34 the fast readsorption of water on γ-Al2O3 is only superficial and is mainly limited to weakly (physically) adsorbed water. In the case of boron nitride support, the affinity for water adsorption appears to depend on the presence of oxygen on the surface. Figure 7 presents for comparison, the change of conductivity before and during DO run. Both boron nitride supports present a general n-type behavior (loss of conductivity on DO adsorption with respect to σ values in DAr3, more obvious for BN). The very small influence of oxygen atmosphere on conductivity indicates a very small extent of oxygen adsorption on the two supports. The behavior of both PdO catalysts in DAr1 was shown in Figure 5A. Although for PdO/BN catalyst in the next two cycles in dry inert gas flow no changes in the Arrhenius plot were observed

Postole et al.

Figure 6. Arhenius-type representation of variation of conductivity during heating (5 °C min-1) in DAr5 and HO runs for BN1-xOx support.

(in comparison with DAr1), the σ values measured for PdO/ BN1-xOx sample indicated major changes in the low temperature range conductivity (see Figure 8). Special patterns were obtained in the next two cycles in inert gas flow up to 160 °C. It seems that this catalyst has a trend to trap even traces of moisture from atmosphere at low temperature (up to 110 °C in DAr2 and 135 °C in DAr3) as a slight increase of conductivity can be observed in Figure 8 (another explanation for this behavior could be connected with migration to the surface of capillary water). The sharp decrease of conductivity above these temperatures indicates the loss of these species. Above 160 °C the conductivity values are almost identical, in DAr1-DAr3 cycles with an activation energy of 0.44 ( 0.07 eV indicating a reproducible surface behavior. In DO run (not presented here), for both PdO/BN and PdO/ BN1-xOx catalysts lower σ values were measured with respect to the conductivity values in DAr (indicating for both samples n-type behavior). The variation of conductivity with temperature in DAr4, CT, and HO runs (see the experimental part) for both PdO/BN and PdO/BN1-xOx samples are presented in Figure 9. For both catalysts, the variation of conductivity in DAr4 shows a similar trend as for DAr1-DO runs. During CT (C3H6: air 1:22) cycle some differences are evidenced in Figure 9: between RT and 250 °C almost similar values of conductivity were measured, and then a decrease of the temperature coefficient of σ was noticed with respect to the previous DAr4 cycle. This was followed by a new increase of σ in case of PdO/BN catalyst and the same trend was shown for PdO/BN1-xOx. By taking into account the general n-type behavior, the inflection

Catalytic Performances of PdO/BN Catalysts

J. Phys. Chem. C, Vol. 112, No. 30, 2008 11391

Figure 7. Arhenius-type representation of the variation of conductivity during heating (5 °C min-1) in DAr3 and DO runs for BN and BN1xOx supports.

Figure 8. Arhenius-type representation of the variation of conductivity during heating in DAr1, DAr2, and DAr3 runs for PdO/BN1-xOx catalyst.

point at 250 °C could be an indication of a predominant oxidation process in that temperature region (most probably on palladium sites, since this was not noticed on the supports). As reported by Maillet et al.36 for a PdO/Al2O3 catalyst with a similar palladium loading, a change in the trend of the conversion-temperature plots occurred at 250 °C both in the case of CO and propylene oxidation. As the authors did not observed any difference in catalytic behavior between preoxidized or prereduced samples they stated that probably PdOx is reduced to Pdo before the start of propylene oxidation. We suppose that the process below 250 °C must be related to PdO reduction step/C3H6 adsorption, whereas the process at 250 °C is connected with regeneration of the PdO structure/desorption of some surface species, as the catalytic activity is also suddenly increasing in this temperature range. The higher conductivity observed up to 220 °C in humid oxygen (HO) indicates some humidity effects on PdO/BN catalyst after the catalytic test (also not noticed on the support). For PdO/BN1-xOx catalyst, the influence of humidity is obvious in the low temperature range and, as presented above, it was also proved by thermogravimetric measurement. For this sample, we repeated the TG experiment on the same batch of samples (i.e., on the one previously submitted to TG analysis) after three days of exposure to laboratory air. The results showed again that PdO/BN1-xOx is sensible to atmospheric humidity, and the amount of weight loss was reproducible. The catalytic performances of both catalysts and supports were tested in C3H6 oxidation. Pure BN support was practically inactive in propylene oxidation up to 400 °C. The BN1-xOx sample was inactive up to 300 °C then a conversion of 20%

was registered with propylene being transformed only in CO2 and water. Upon heating at higher temperatures the C3H6 conversion increased up to 57% at 400 °C and small amounts of CO were also detected in the products stream. The calculated conversion (based on the diminution of the propylene peak with respect to the reference mixture, at zero conversion) and the CO and CO2 selectivities for PdO based catalysts are presented in Table 4. The PdO/BN catalyst was inactive in C3H6 oxidation below 200 °C. A sharp increase of activity was detected in propylene oxidation on PdO/BN above 200 °C with propylene transformed mainly to carbon oxides and very small traces of other C3H6 oxidation products. Even if the main oxidation products remain CO2 and H2O, CO was present on products stream on the whole temperature range. PdO/BN1-xOx was more active in propylene oxidation as can be seen in Table 4. Above 300 °C, C3H6 was completely transformed in CO2 and H2O, except at 400 °C when very small amount of other products can be observed. It seems that the presence of oxygen on BN1-xOx has a synergetic effect on propylene combustion, making also the support an active participant of the reaction in this case. The main role of the palladium oxide deposited was to improve the complete oxidation and to shift the onset of oxidation toward a lower temperature. It was experimentally found that the support is not always an inert material, sometimes it might be also involved in the catalytic reaction. It is very often the case of the traditional oxide supports.37 Metal-support interactions exist also in most oxide

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Postole et al.

Figure 9. Arhenius-type representation of variation of conductivity during heating in DAr4, CT and HO runs for (A) PdO/BN and (B) PdO/BN1xOx catalysts.

TABLE 4: Conversion and Carbon Oxides Selectivities for PdO/BN and PdO/BN1-xOx Catalysts in C3H6 Oxidation catalyst PdO/BN PdO/BN1-xOx

temperature/°C C3H6 conversion/% SCO/% SCO2/% 240 330 410 90 204 330 350 375 400

47.8 47.5 61.3 1.0 2.0 100.0 100.0 100.0 100.0

3.2 7.7 22.9

98.6 92.3 77.1 1.0 100.0 100.0 100.0 99.3

supported metal catalysts, having frequently negative effects on the catalytic activity.38–40 In general, BN was inert for the catalytic reaction. Furthermore, as it was demonstrated by Wu and Lin14,19,41 for Pt/BN, BN has a negligible interaction with Pt. The easy migration of Pt particles which occurred on the BN surface might promote metal sintering with positive effect for some catalytic processes (i.e., hydrogenation of R,βunsaturated aldehyde to unsaturated alcohol).41 The sintering of the metal particles deposited as active phase on BN support during catalytic reaction was also proved in case of palladium.33 Even if the negligible interaction with the metal particles has some advantages for using BN as a support in different catalytic processes, it seems to have a negative effect for propylene combustion (i.e., low C3H6 conversion even at 400 °C and presence of CO in the product streams). Good results were

obtained in propylene combustion when a boron nitride presenting an oxi-nitride structure was used as support. This could be due to the ability of this support of maintaining palladium in the oxidized state. PdO/BN1-xOx can be considered as acting like a bifunctional catalyst in which both the guest PdO and host BN1-xOx are accessible to the reactants and consequently participating to the formation of intermediate adsorbed species. This result opens new perspectives in using activated BN supports in catalysis. 4. Conclusions New results were presented here on the use of BN as support for PdO in combustion of propylene at relatively low temperature (room temperature to 400 °C). Two high surface area boron nitride powders (prepared from different molecular borazine-based precursors) and the corresponding PdO catalysts were investigated. The more pure and stable powder was BN, obtained from trichloroborazine; when used as a support for PdO, the catalyst showed a good metal dispersion with PdO particles size of about 3.8 nm. When polytrimethylaminoborazine was used as a precursor, the oxygen-containing BN phase was obtained (BN1-xOx). After impregnation, PdO/BN1-xOx catalyst presented an inhomogeneous size repartition and palladium oxide particles size of about 4.8 nm. The distribution of PdO on the BN supports and the PdO-support interaction were shown to be dependent on the support surface chemistry, that is on the

Catalytic Performances of PdO/BN Catalysts presence of surface oxygen. The presence of some surface oxygen on BN led to a poor distribution of the palladium oxide which can be related to a preferred anchorage of PdO on the oxygen sites. Better performances were obtained in propylene oxidation using PdO/BN1-xOx. Although pure BN was inactive over the whole temperature range, BN1-xOx was found to be an active participant in the catalytic reaction. In this case, several processes occurring on the catalyst surface in this temperature range were evidenced by electrical conductivity measurements. The promising results obtained for PdO/BN1-xOx catalyst opens new perspectives in using an “oxygen-activated” BN support for future applications. Acknowledgment. The authors thank N. Cristin for isotherm measurements, M. Aouine for TEM micrographs, and C. Guimon for XPS analysis. G.P. thanks the NATO project (EAP.RIG.982794) for financial support. References and Notes (1) Han, W.-Q.; Brutchey, R.; Tilley, T. D.; Zettl, A. Nano Lett. 2004, 4, 173. (2) Vinu, A.; Hartmann, M. Catal. Today 2005, 102-103, 189. (3) Ehrburger, P.; Mahajan, O. P.; Walker Jr, P. L. J. Catal. 1976, 43, 61. (4) Vinu, A.; Terrones, M.; Golberg, D.; Hishita, S.; Ariga, K.; Mori, T. Chem. Mater. 2005, 17, 5887. (5) Lu, W.; Chung, D. D. L. Carbon 1997, 35, 427. (6) Han, S.; Kim, S.; Lim, H.; Choi, W.; Park, H.; Yoon, J.; Hyeon, T. Microporous Mesoporous Mater. 2003, 58, 131. (7) Ahn, W. S.; Min, K. I.; Yung, Y. M.; Rhee, H.-K.; Joo, S. H.; Ryoo, R. Stud. Surf. Sci. Catal. 2001, 135, 313. (8) Planeix, J. M.; Coustel, N.; Coq, B.; Brotons, V.; Kumbhar, P. S.; Dutartre, R.; Geneste, P.; Bernier, P.; Ajayan, P. M. J. Am. Chem. Soc. 1994, 116, 7935. (9) Cofer, C. G.; Economy, J. Carbon 1995, 33, 389. (10) Wood, G. L.; Janik, J. F.; Visi, M. Z.; Schubert, D. M.; Paine, R. T. Chem. Mater. 2005, 17, 1855. (11) Alkoy, S.; Toy, C.; Gonul, T.; Tekin, A. J. Eur. Ceramic Soc. 1997, 17, 1415. (12) Paine, R. T.; Narula, C. K. Chem. ReV. 1990, 90, 73. (13) Hoffman, D. M.; Doll, G. L.; Eklund, P. C. Phys. ReV. B 1984, 30, 6051.

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