Characterisation of BN-supported palladium oxide catalyst used for hydrocarbon oxidation

Applied Catalysis A: General 316 (2007) 250–258 www.elsevier.com/locate/apcata

Characterisation of BN-supported palladium oxide catalyst used for hydrocarbon oxidation G. Postole a,b, B. Bonnetot c, A. Gervasini d, C. Guimon e, A. Auroux b, N.I. Ionescu a, M. Caldararu a,* a

‘‘Ilie Murgulescu’’ Institute of Physical Chemistry of the Romanian Academy, Spl. Independentei 202, 060021 Bucharest, Romania b Institut de Recherches sur la Catalyse, CNRS, 2 Av. Albert Einstein, 69626 Villeurbanne Cedex, France c Laboratoire des Multimate´riaux et Interfaces, UMR CNRS 5615, bat Berthollet, UCB Lyon I, 69622 Villeurbanne Cedex, France d Dipartimento di Chimica Fisica ed Elettrochimica & Centro di Eccellenza CIMAINA, Unversita` di Milano, via Golgi 19, I-20133 Milano, Italy e Laboratoire de Physico-Chimie Mole´culaire, Universite´ de Pau, 2 Av. P. Angot, 64053 Pau Cedex 09, France Received 25 April 2006; received in revised form 23 September 2006; accepted 26 September 2006 Available online 25 October 2006

Abstract Hexagonal boron nitride (BN), with a graphite-type structure and with surface area of 184 m2/g was used as a support for palladium oxide (PdO/ BN). About 1 wt% of palladium was deposited on BN by incipient wetness method by using palladium nitrate as precursor. The support and the catalyst were characterized by BET, TEM, XRD, XPS, ICP, TG, TPD, in situ ac electrical conductivity and by ammonia adsorption microcalorimetry. Oxidation of propylene and methane were used as model reactions to study the catalytic properties of the PdO/BN catalyst. The BN support was practically inactive in propylene oxidation up to 400 8C, while the onset of the oxidation was detected around 200 8C on PdO/ BN, which points out the role of the palladium in adsorption of the reactive hydrocarbon species. At the same time, this temperature is coincident with the increase of the electronic conductivity on both BN and PdO/BN samples, which is important for oxygen adsorption/activation as electrophilic species. The catalyst was inactive in methane oxidation below 400 8C. Only about 2% CH4 conversion was observed at 400 8C, increasing sharply up to 87% at 550 8C with methane transformation only to CO2 and water. # 2006 Elsevier B.V. All rights reserved. Keywords: Boron nitride; Palladium catalyst; In situ electrical conductivity; Hydrocarbon oxidation; Acidity

1. Introduction The characteristics of the support play an important role in reactions performed on supported noble metal catalysts. Besides dispersing the metal, the support also improves the thermal stability of the catalyst and, in some cases, might be also involved in the catalytic reaction [1]. Metal–support interactions exist in most oxide supported metal catalysts, having rarely positive effect, but most frequently negative effects on the catalytic activity [2–10]. Since the traditional metal oxide supports, such as Al2O3, SiO2, ZrO2, SnO2, TiO2

* Corresponding author. Tel.: +40 21 3121147; fax: +40 21 3121147. E-mail addresses: [email protected] (G. Postole), [email protected] (B. Bonnetot), [email protected] (A. Gervasini), [email protected] (C. Guimon), [email protected] (A. Auroux), [email protected] (N.I. Ionescu), [email protected] (M. Caldararu). 0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2006.09.026

possess rather low thermal conductivities, this favours the agglomeration of the supported metal particles on hot spots in exothermal reactions. Moreover, these supports are hydrophilic and the catalyst surface can be easily contaminated by water adsorbed at low temperature. The use of ceramic compounds with a higher thermal conductivity such as SiC, Si3N3 or BN [11–13] could increase the stability of the catalysts. Hexagonal boron nitride (BN) exhibits several advantages over the traditional oxide supports [8,11,14–17]. The crystalline and electronic structures of BN are similar to those of graphite; it also possesses excellent chemical and thermal stability, high thermal conductivity and minimum metalsupport interaction. In addition, boron nitride is not attacked by water or any mineral acids except hydrogen fluoride [7,9]. Several recent studies describing the use of BN supported noble metal catalysts for deep oxidation of volatile organic compounds (VOC) [7,9,18] or for ammonia synthesis [19], indicated that their catalytic activity is higher than that of the

G. Postole et al. / Applied Catalysis A: General 316 (2007) 250–258

traditional catalysts. Wu et al. [7] have shown that Pt/BN shows better performances than traditional Pt/g-A2O3 catalyst with respect to the life time and activity; Pt/BN catalyst remained active for 80 h at 185 8C, while the activity of Pt/g-A2O3 at the same temperature was found to decrease continuously with time. Jacobsen [19] found that the activity of a Ba-Ru/BN catalyst for ammonia synthesis was significantly higher than that of traditional catalysts used for this process (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 [20]. The use of boron nitride as a support significantly increased the selectivity towards acrolein if compared with the V2O5/SiO2 catalyst. The catalytic oxidation of hydrocarbons has been investigated and partly commercialized for low and middle temperature applications, such as flameless heaters, catalytic burners, exhaust cleaners, and removal of volatile organic compounds. Palladium is the most active component of the catalysts for combustion of methane and natural gases [1]. It is also extensively used as the active component in several industrial catalytic formulations for the removal of environmentally unfriendly chemicals in gaseous emissions. Similarly to platinum, palladium catalysts show low light-off temperatures in oxidation of hydrocarbons or of other organic chemicals [21]. The catalytic oxidation of hydrocarbons on palladium catalysts occurs by reduction of PdO—reoxidation of Pd. g-Alumina has been widely used as a support for the palladium catalysts due to its high surface area and low cost [1,22–30]. However, the carbon dioxide and water produced in the oxidation reaction act as poisons for the catalyst, especially at low conversions and at low temperatures, which results in deterioration of the activity [1,31]. In this paper we report about the behaviour of palladium oxide supported on hexagonal boron nitride as a catalyst in propylene and methane oxidation reactions. We selected the oxidation of these hydrocarbons as model reactions for investigating the properties of PdO/BN for deep oxidation with the aim of proposing it as an alternative for the catalysts used in the removal of the VOCs from air. To our knowledge, no other group has reported yet about the use of BN supported catalysts for methane and propylene oxidation/combustion. 2. Experimental 2.1. Materials High surface area hexagonal boron nitride powder was prepared from trichloroborazine (TCB), containing a large amount of TCB polymers [11]. The powder was formed of small (1–5 mm) ball shaped grains observed by scanning electron microscopy (SEM) [8]. The BN particles are composed of grains of 10 nm surrounded by a crystalline nano-coating of about 10 BN layers thick [11]. This support (abbreviated in the following as BN) has a surface area of 184 m2/g.

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The impregnation with a palladium salt was done 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 precursor (corresponding to a theoretical loading of 1 wt% Pd) and 1 g of support were introduced in 20 cm3 of water and mixed together by stirring at room temperature for 7 h. After drying overnight at 110 8C the sample was calcinated in air flow (500 cm3/h) at 500 8C for 10 h, and then cooled down under the same gas flow. The sample was used as such, i.e. without any previous reduction, thus containing palladium as palladium oxide phase. This sample will be abbreviated in the following as PdO/BN. 2.2. Characterisation The physico-chemical characteristics of the support and of the catalyst were determined by Brunauer–Emmett–Teller (BET) method, X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), thermogravimetry (TG), temperature-programmed desorption (TPD), in situ ac electrical conductivity and adsorption microcalorimetry. Other characteristics obtained by using scanning electron microscopy (SEM) and temperature-programmed reduction (TPR) were reported previously [8]. The specific surface areas of the samples were measured by N2 adsorption and calculated from the Brunauer–Emmett– Teller (BET) method. Prior to surface area determination, the samples were outgassed at 100 8C overnight and then at 400 8C for 6 h. The pore size distribution (as determined from the desorption branch of the N2 isotherm obtained by using an ASAP 2010 M instrument, from Micromeritics) indicated the existence of some microporosity of the BN support. Two types of pores were observed: large mesopores from 4 to 30 nm and small pores of about 2 nm. Therefore, deBoer’s t-plot analysis was used to obtain the micropore surface area. A surface area of 58 m2/g corresponds to micropores and 126 m2/g to larger pores. A micropore volume of 0.0244 cm3/g was found also by t-method and a total pore volume at P/P0 = 0.98 was found to be 0.82 cm3/g. These values are comparable with those reported in the literature for traditional oxide supports. Most of the materials used for the catalysts manufacture typically possess specific pore volumes of about 0.9–1.3 cm3/g [32]. For example, BET surface areas and pore volumes of conventional g-Al2O3 used as catalyst supports are usually in the ranges 70– 360 m2/g and 0.3–1.5 cm3/g, respectively [33]. The crystalline structure of the hexagonal boron nitride phase and of the PdO/BN catalyst was examined by XRD by using a Bruker (Siemens) D5005 instrument (Cu Ka radiation, 0.154 nm). The final concentration of the palladium in the supported catalyst was determined by chemical analysis by using inductively coupled plasma atomic emission spectroscopy (ICP-AES) with a Spectroflamme-ICP, model D. For this purpose, the sample was treated with a mixture of HCl + HF + HNO3 in order to dissolve it completely. The Pd concentration determined in this way was 0.97 wt%.

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XPS analysis was performed by using a Surface Science Instruments 301 spectrophotometer with monochromatic Al Ka radiation. The 1s binding energy of boron was considered as relevant to survey the surface chemical species of the hexagonal boron nitride sample. The palladium oxide particle size was measured by TEM using a JEOL 2010 instrument. TG experiments were performed in air by heating the samples up to 600 8C with a ramp of 5 8C/min on a TG-DSC 111 instrument from Setaram. The acidic properties of the samples were determined by ammonia adsorption microcalorimetry. Ammonia adsorption was performed at 80 8C 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 a quartz cell by heating overnight under vacuum at 400 8C. The differential heats of adsorption were measured as a function of coverage by repeatedly sending small doses of ammonia onto the sample until an equilibrium pressure of about 67 Pa was reached [34]. The sample was then outgassed for 30 min at the same temperature and a second adsorption was performed at 80 8C until an equilibrium pressure of about 27 Pa was attained in order to calculate the amount of irreversibly chemisorbed ammonia. TPD measurements were performed in the range 25–650 8C under He or air flow (10 cm3/min) for both the support and the catalyst samples as resulted after the calorimetric measurements for ammonia adsorption, by using a Setaram TG-DSC 111 equipment coupled with a mass spectrometer (Thermostar from Pfeifer) and a capillary system. For each experiment, ca. 10 mg of a sample with ammonia adsorbed in the previous microcalorimetric experiment were used. Initially, the samples were purged with helium or air at room temperature for 15 min and then heated with 5 8C/min up to 100 8C. The temperature was kept constant at 100 8C for 15 min and then was linearly increased up to 650 8C with a ramp of 3 8C/min. During this temperature increase, the mass spectrometer was set at m/e = 15 in order to avoid the interference of m/e peaks of water fragmentation. To compare the behaviour of the samples, the areas related to each peak have been calculated by using a simple integration method from the base line estimated for each peak. The measured areas were then normalised for 1 g of sample to get comparable values for each peak area. The same type of TPD experiment was also done on the samples used in propylene oxidation/electrical conductivity measurements; the mass spectrometer was set at m/e = 41 (the significant propylene peak, but also representative for acrolein and acetone). AC electrical conductivity measurements were performed in a Pyrex glass cell specially designed to allow simultaneous electrical and catalytic activity measurements in operando conditions [35–37] in powders. 1.5 cm3 of grains (0.25–0.5 mm diameter) were used. The electrical conductance G (G = 1/R, where R is the resistance) and the capacitance C of the powder bed were simultaneously measured in situ at 1592 Hz, in gas flow with a semiautomatic RLC bridge TESLA BM 484 by

using the differential step technique (DST) as described previously [35–37]. It consists of successive heating (5 8C/min, between 25 and 400 8C)—cooling (about 10 8C/min) 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 on-line gas chromatography. Here, the cycles succession was: DAr(1–3), DO, DAr4, CT, DAr5, HO, where DAr abbreviate dry argon, DO and HO abbreviate 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. 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 8C. 2.3. Catalytic activity 2.3.1. C3H6 oxidation C3H6 oxidation was selected as a model reaction as propylene is a basic molecule allowing the correlation of the activity with surface acidity. At the same time, C3H6 is more reactive than CH4 and a stronger reducer, so when in situ electrical measurements are coupled with the catalytic test it is possible to detect the surface reduction–oxidation. 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. As this testing cycle was followed by DAr5 and HO runs (see above), in the following, by ‘‘used sample’’ we mean the sample removed from the conductivity cell after completion of this protocol. 2.3.2. CH4 oxidation The catalytic test of methane oxidation was performed in a quartz tubular micro-reactor. The samples were prepared by adding 0.05 g of catalyst to 1 g of quartz (1:20 dilution). The powders tested and the quartz used for dilution were ground and sieved separately to obtain particles of 0.2–0.3 mm. The samples were pretreated for 4 h at 400 8C in O2/He (20%, v/v) flow. A set of mass flow controllers (Bronkhorst, Hi-Tec) was used to determine accurately the composition of the reactant mixture: 1500 ppm CH4 and 30,000 ppm O2 in helium. Since our previously reported experiments [17] in dry and respectively wet conditions indicated a higher conversion (about 20%) and better CO2 selectivity in the presence of water, the results reported here were collected only under wet atmosphere by passing the reaction mixture through a humidifier at room temperature. The reaction was carried out at fixed space velocity (GHSV = 42,000 h1) and variable temperature. Seven different temperatures in the interval of 400–700 8C were chosen, each temperature being kept constant for 90 min. The inlet and exit gas streams of the reactor flowed through a gas cell (path length 2.4 m multiple reflection gas cell) in the beam of a FTIR spectrometer (Bio-Rad with DTGS detector). The spectrometer response permitted quantitative analysis for CH4 (1360 cm1), CO (2086 cm1) and CO2

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(721 cm1). The conversion and selectivities were calculated from the measured concentrations of the reagent feed and products formed. 3. Results and discussion In Table 1 are presented the surface areas for the fresh and used samples, respectively. As shown, BN and PdO/BN powders exhibit surface areas higher than 100 m2/g and no changes were detected for the used samples. The distribution by size of the palladium oxide particles on the BN support has been determined. The automatic measurement performed by using a commercial program has not produced reliable results because the automatic choice of the particles to be measured was focused on the most contrasted grains and was not representative for the sample. A manual measurement of 100 particles has been done including all the particles. The histogram given in Fig. 1A evidenced a homogeneous distribution by size, with an average value for the palladium oxide particles of 3.8 nm (standard deviation of 0.9 nm). For the used catalyst the TEM picture was obviously changed, suggesting possible cluster formation. The sizes of palladium oxide particles (measured in the same conditions as for the fresh catalyst) indicated an average value of 6.7 nm (standard deviation of 1.1 nm) with an inhomogeneous size repartition due to the formation of aggregates during the cycles connected with the catalytic test (Fig. 1B). The palladium distribution on the surface obtained by XPS technique for the fresh catalyst (2.2 at.% or 16.1 wt%, Table 2) was different of the average values obtained from chemical analysis (0.97 wt%). These results confirm that palladium remains on the support surface. When TPR was performed in 1% H2/Ar flow up to 600 8C a value of H/Pd = 0.74 was calculated [8]. This result supports the information obtained by XPS and TEM: palladium oxide is rather well dispersed; however, the H2 consumption is lower than expected for the complete reduction of palladium oxide. This indicates that part of palladium is either not accessible or is in the metallic form in the fresh catalyst. In comparison with a standard highly crystalline hexagonal boron nitride [38], the 1s binding energy of boron of the BN support (Table 2) exhibits a positive shift (0.2 eV), which could be due to some residual B–O bonds inducing a positive shift of the binding energy. The residual B–O bonds can play as an anchor for Pd, as Wu et al. [7] have suggested in the case of Pt particles. As shown by XPS data presented in Table 2, the percent of oxygen on the catalyst surface is considerably higher than that found for BN surface, and this must be due to the presence of Table 1 BET surface areas of the BN and PdO/BN samples (fresh and after the use in C3H6 oxidation) Samples

SBET (m2/g) fresh samples

SBET (m2/g) used samples

BN PdO/BN

184 233

189 236

Fig. 1. Size distribution of palladium oxide particles as deduced from TEM measurements (A) PdO/BN fresh catalyst and (B) PdO/BN catalyst used in C3H6 oxidation.

palladium in the oxide form. For the sample used in propylene oxidation this percent is similar with that of the fresh sample (for BN) or lower (for PdO/BN), but after all the heating– cooling cycles in different atmospheres, part of the oxygen species appears bonded in compounds as BNxOy (2 at.% for BN and 3 at.% for PdO/BN). Apparently the surface palladium concentration decreased significantly (to 0.4 at.% or 3.3 wt%) for the used PdO/BN; this result could be explained by the sintering of the palladium containing aggregates coupled with carbon deposition on these clusters. The atomic percent of surface carbon decreases for the used BN support from 4.2 to 1.5%, but increases from 2.5 to 15% for the used PdO/BN catalyst. Species containing C–O, C O, and O–C O (corresponding to binding energies at 285.8, 288, and about 290 eV, respectively) were also detected on this sample (Fig. 2). The deposition of palladium on BN surface resulted in a very small increase of the number of stronger acidic sites (120 < Q < 150 kJ/mol) and an obvious decrease of the number of the weak sites (60 < Q < 120 kJ/mol) compared with BN support, in conditions of the decrease of the total number of the acidic sites (Fig. 3). Thus the relative content of stronger acidic sites is higher on the palladium catalyst surface than on BN support.

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Table 2 XPS characteristics of the fresh and used (in propylene oxidation) of BN and PdO/BN samples Sample

BN (fresh) BN (used) PdO/BN (fresh) PdO/BN (used)

XPS results in atomic percent data B (%)

Pd %

O (%)

C (%)

N (%)

B 1s (eV)

49.3 50.5 49.2 43.2

– – 2.2 0.4

3.6 3.7 5.3 4.6

4.2 1.5 2.5 15.0

42.9 44.2 40.8 36.7

BN: 190.5 (100%) BN: 190.5 (98%); BNxOy: 192.5 (2%) BN:190.5 (100%) BN: 190.5 (97%); BNxOy: 192.5 (3%)

Fig. 2. XPS C1s spectra of the PdO/BN catalyst (a) fresh and (b) used in C3H6 oxidation.

We used the temperature programmed desorption (TPD) as a complementary method to study the strength of interaction of ammonia (and respectively, of propylene, also a basic molecules) with the surface of these samples. NH3-TPD thermograms were obtained by flushing with He (for BN and PdO/BN) or air (for BN) the samples after their use in calorimetric acidity measurements (Fig. 4); the mode of area calculation (as reported in Section 2) is also indicated in Fig. 4A. The temperatures of the ammonia TPD peaks and the corresponding normalised areas are summarized in Table 3. When the TPD measurement was performed under helium, only one broad desorption peak was observed for the BN

Fig. 3. Distribution by strength of acid sites for the support and for the PdO/BN catalyst.

sample, spanned in the range 180–450 8C, asymmetrical on the high temperature side, with the maximum at 252 8C. The observed peak could be related to a high temperature NH3 or NH3-derived species strongly adsorbed on BN surface. When TPD measurement was performed on BN under air, a part of the adsorbed species was more easily removed at 193 8C (Fig. 4B), but an important peak appears at about 570 8C, corresponding to another strongly bonded species. Two peaks were observed for PdO/BN under helium (Fig. 4C): a small one centred at 100 8C and a second peak beginning near 450 8C with a maximum at 533 8C. The area of the high temperature peak was slightly higher for PdO/BN sample than that for BN. The low temperature peak detected in air flow on BN support can be related to a weakly bonded ammonia species; this could be either the same or different of the species desorbing at 100 8C from PdO/BN catalyst. The high temperature peak is possibly related with the presence of a PdO-catalysed oxidation product of ammonia (e.g. NO), particularly as it appears also in the case of the BN support in air flow. Similar TPD experiments were performed under He or air (as carrier gas) on BN and PdO/BN samples previously used in

Fig. 4. TPD after NH3 adsorption on (A) BN support under He, (B) BN support under air and (C) PdO/BN catalyst under He.

G. Postole et al. / Applied Catalysis A: General 316 (2007) 250–258 Table 3 TPD results obtained on the samples previously used in NH3 adsorption microcalorimetry and in C3H6 oxidation Sample

C3H6

NH3 TM (8C)

Speak (a.u./g)

TM (8C)

Speak (a.u./g)

a

252

28.26

325 600

17.73 9.92

BN b

193 570

21.69 27.78

125 455

11.83 23.08

PdO/BNa

100 533

8.92 29.07

100 392 647

6.68 13.09 10.36

84 324 605

6.81 13.99 11.96

BN

PdO/BNb

a b

The experiment was done under helium as carrier gas. The experiment was done under air as carrier gas.

the catalytic activity for propylene oxidation/electrical conductivity measurements (see the protocol in Section 2.2). In this case, the TPD thermograms revealed two or three main peaks at m/e = 41 (Table 3 and Fig. 5). m/e = 41 can also be representative for propylene, acrolein or acetone; thus, except temperature-based reasoning it was impossible in these conditions to assign these peaks to propylene or to one of its products. The low temperature peaks (at about 100 and 84 8C) arising on flushing the PdO/BN sample with He or air, respectively (but also on BN sample in air at 125 8C-not presented here) could be associated with the desorption of a

Fig. 5. TPD after the catalytic test (C3H6 oxidation) on PdO/BN catalyst (A) under He and (B) under air.

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propylene-derived species whose strength of bonding on the surface has been lowered by the final HO cycle of the conductivity protocol. At higher temperatures, the behaviour of the PdO/BN catalyst and of BN support was equivalent, the peak areas being of the same order of magnitude, but the temperature for the maxima (TM) are higher for the former. TM were shifted to lower temperatures in presence of air for both samples, but they remain still much higher for the palladium catalyst. These peaks produced by TPD experiment on the used samples (thus after the flushing in DAr5 and HO run) indicate the existence on the surface of some strongly bonded carbonaceous species. The electrical conductivity was measured in situ, in conditions similar with those involved during operation in catalysis (i.e. in flow system, at atmospheric pressure). It is known that the electrical properties of powder surfaces are mainly controlled by intergrain Schottky-type barriers, being dependent of the type and concentration of surface defects; thus, 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. Both BN and PdO/BN presented n-type behaviour (i.e. lower G on oxygen adsorption in DO with respect to the G values in DAr). The plots in DAr1-3 and DAr4 cycles (not presented here, to let the reader to see more clearly the other plots) indicated almost identical G values (slightly higher in comparison with DO), suggesting a reproducible surface behaviour of BN and PdO/ BN in these conditions. The variation of conductivity with temperature in DO, CT, DAr5 and HO runs for both BN and PdO/BN samples is presented in Fig. 6. As shown all these plots show similar patterns for BN, with rather constant or very slightly increasing conductivity up to approximately 200 8C (220 8C in oxygen), and a sharper increase above (Fig. 6A). The increase of conductivity for BN sample above 200 8C is slightly sharper in CT cycle and in the next ones; in DAr5, the increase of G is associated with continuous evolution of propylene and water in effluent, indicating the presence of some strongly adsorbed species on BN surface after the CT run. By taking into account the general n-type behaviour (lower conductivity on oxygen adsorption), the lower G values in the high temperature part in CT run (Fig. 6B) for the PdO/BN catalyst indicate that in this case the dominant process at high temperature must be the oxygen adsorption/surface oxidation (probably on palladium sites). A very special feature is in evidence in this case in DAr5 (see insert of Fig. 6B): a maximum located at 70 8C was coupled with the evolution of a big amount of water in effluent. At the beginning of this DAr5 experiment traces of propylene were also detected in effluent, but afterwards propylene detection was not anymore possible due to the screening effect produced by this big amount of water evolved (superposed on the propylene GC peak); the same happened in HO run. However, traces of CO2 detected in effluent above 200 8C, indicate clearly the presence of carbon containing residues on the PdO/BN surface. Associated with XPS data and with detection by mass spectrometry of some m/e = 41 peaks in the TPD experiment (when performed on the used sample) these results confirm also the presence of strongly adsorbed

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Fig. 6. The plots of variation of conductivity vs. temperature obtained in DO, CT, DAr5 and HO cycles for (A) BN support and (B) PdO/BN catalyst.

propylene-derived species on the PdO/BN sample. These species seem to be strongly adsorbed, as they cannot be totally removed during the post-catalytic flushing runs (see also below). The slightly higher conductivity in evidence at low temperature (up to 195 8C) in HO indicates some effects of humidity on PdO/BN used surface. The presence of small amounts of water detected (by analysis of the effluent) as a surface contaminant during DAr1 cycle (not presented here) appeared not to have a detectable influence on G values of both fresh BN and PdO/BN samples. The highest effect of water adsorption (increase of conductivity) was noticed at room temperature in HO cycle but it was very small in comparison with the regular effect of moisture at low temperature on oxides [39]). This suggests a low extent of water adsorption on fresh samples and is in agreement with TG results, indicating a weight loss of only 0.45 wt% for the BN sample. However, as indicated by our conductivity data coupled with analysis of effluents, apparently after performing the catalytic experiment (CT cycle), the surface reactivity of both BN and PdO/BN samples was slightly changed (particularly for PdO/BN) and the samples become sensitive to the presence of humidity. It is known that the water adsorption is favoured by the presence of OH groups and/or anion vacancies, which are not currently present on the fresh BN and PdO/BN samples. It can be supposed that the catalytic test in propylene oxidation induces some changes of the surface topography, resulting in increased sensitivity to adsorption.

Pure BN support was practically inactive in propylene oxidation up to 400 8C and also the final carbon content of the used sample is much lower than that measured for the used PdO/BN; this underlines the substantial effect of the deposited PdO (0.97 wt% Pd) which is responsible for the formation of the hydrocarbon adsorption sites. The apparent disagreement between TPD experiment (indicating some adsorbed propylene-derived species on BN) and XPS data (indicating a diminution of the carbon content on the surface of used BN in comparison with the fresh sample) can be reconciled if we take into account the peculiarities of these experiments. Indeed, XPS measurements involve high vacuum cleaning of the surface. This means that the more mobile surface adsorbed species will be removed during this pre-treatment. Thus, it seems that there are two types of propylene-derived residual species adsorbed on the BN and PdO/BN samples: (i) one, less strongly bonded, which can be removed in conditions of TPD experiment (thus on gas flushing during linear heating at atmospheric pressure) and (ii) another one, more stable, detectable by XPS, particularly on PdO/BN catalyst. The formation of this species must be associated with the higher acidic strength of the surface induced by the presence of palladium. A sharp increase of activity was detected in C3H6 oxidation on PdO/BN catalyst above 200 8C with propylene transformed mainly to carbon oxides (and very small traces of other propylene degradation products). This temperature corresponds also to the onset of the increase of conductivity (see Fig. 6B), indicating the increase of electron density at the surface above

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this temperature, facilitating adsorption of oxygen in the form of active species. However, the similar behaviour evidenced for conductivity of BN sample indicated that the increase in electron density is not enough for the catalytic activity. This points to the role of palladium or palladium oxide as the active component in facilitating the hydrocarbon activation/dissociative adsorption. The calculated conversion (based on diminution of the propylene peak with respect to the reference mixture, at zero conversion), and the CO, CO2 selectivities are presented in Table 4. In parentheses are also mentioned the conversion values calculated based on carbon balance (by taking into account as products only the carbon oxides). The much higher values of these last ones indicate also the surface contamination with strongly adsorbed propylene-derived species which at high temperature were burned in presence of oxygen (thus contributing with a fraction to the higher carbon balance). The catalytic performances of the PdO/BN sample were also tested in methane oxidation. This catalytic test was performed only in presence of water since our previous results indicated a higher activity in such conditions [17]. In Table 5 are presented the CH4 conversion values calculated from the residual methane concentration at each reaction time and also the selectivity to CO and CO2. The catalyst was inactive in methane oxidation below 400 8C and this can be easily understood since CH4 molecule is more stable than propylene and needs a higher temperature to be activated. About 2% conversion was observed at 400 8C, increasing then sharply up to 550 8C (87%); at these temperatures only CO2 and H2O were obtained as products. At temperatures ranging between 600 and 700 8C, small concentrations of CO were also detected in the product streams, but the main oxidation products remain CO2 and H2O (Fig. 7). In order to check for the reproducibility of the results, the sample was cooled down to room temperature and kept for 2 h under humid reactive gases atmosphere; further, a second run was performed on the same sample using the same water–saturated gas mixture. The second run was done at only two different temperatures (500 and 550 8C) corresponding to the range of values which include 50% conversion in the first run. The values of the CH4 conversion obtained in this way were 49.6% at 500 8C and 73.4% at 550 8C, respectively, indicating a reasonable good reproducibility of the catalyst activity. It is well known that for the catalytic total oxidation of methane, the support plays an important part in determining the activity and the long-term stability of the catalyst. For example, for g-Al2O3, TiO2 and ThO2 supports, the activity of Pd based catalysts decreases in the order: g-Al2O3 > TiO2 > ThO2 [6].

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Table 5 Methane conversion and selectivity to CO and CO2 on PdO/BN catalyst and CH4 conversion on PdO/g-Al2O3 Temperature (8C)

400 500 550 600 625 650 700

PdO/g-Al2O3

PdO/BN

CH4 conversion (%)

CH4 conversion (%)

SCO (%)

SCO2 (%)

20 96.7 96.7 96.7 nd nd 96.7

2.0 38.9 87.1 89.9 82.1 92.3 100.0

0 0 0 5 14 19 7

100 100 100 95 86 81 93

At the same time, the active alumina-supported catalyst was found to last longer than silica-supported catalyst [40]. Concerning the use of BN as a support for palladium oxide in methane combustion, we observed light-off temperatures (X CH4 ¼ 50%) higher than those reported in the literature for the traditional catalysts (PdO/Al2O3 and PdO/ZrO2) [7,22,23,26,41–43]. Even though many studies have been published on methane oxidation over supported palladium catalysts, they have been carried out under conditions different of ours so the direct comparison of their performances with that of this PdO/BN sample is rather difficult. In order to compare such a catalyst in our experimental conditions we used a PdO/gAl2O3 catalyst with a surface area of 108 m2/g and 0.9 wt% Pd, prepared by a sol-gel method [44] from alumina Degussa and Pd (II) aceylacetonate. The catalytic performances in CH4 oxidation were tested as presented in Section 2. The CH4 conversion values at each reaction temperature are presented in Table 5. The PdO/g-Al2O3 catalyst was also inactive in methane oxidation in our conditions below 400 8C; the activity was obviously much higher above 400 8C in comparison with the PdO/BN sample (97% at 500 8C). Only CO2 and H2O were obtained as products. However, in spite of better results obtained in the case of PdO/g-Al2O3, our previous studies indicated a better stability in time for PdO/BN catalyst, while the activity of PdO/g-A2O3 catalyst decreased steadily during the first 20 h under similar conditions [14].

Table 4 Conversion and carbon oxides selectivities for PdO/BN catalyst in C3H6 oxidation Temperature (8C)

C3H6 conversiona (%)

SCO (%)

SCO2 (%)

240 330 410

47.8 (50.6) 47.5 (58.3) 61.3 (67.2)

3.2 7.7 22,9

96.8 92.3 77.1

a

Calculated based on carbon balance.

Fig. 7. FTIR spectra recorded on the PdO/BN catalyst during methane oxidation.

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4. Conclusions A PdO/BN catalyst with a good active phase dispersion and a homogeneous size repartition of the palladium oxide particles on the support was obtained by using a classical wet process of impregnation of a palladium precursor on a high surface area hexagonal boron nitride; however, after the successive heating– cooling cycles involved in the protocol of the electrical conductivity measurements/catalytic testing in propylene oxidation, PdO particles trend to gather into aggregates. The palladium deposition on BN diminishes the number of weakest acidic sites and increases slightly the number of stronger acidic sites; thus, the relative acidic strength of the catalyst is higher inducing a stronger adsorption of the basic propylene molecule with respect to that on BN support. This correlates well with XPS and TPD data indicating a rather large amount of strongly bonded carbonaceous deposit remaining on the surface of the catalyst tested in C3H6 oxidation up to 400 8C. PdO/BN is active in propylene oxidation at relatively low temperature (about 50% at 240 8C and up to 60% at 400 8C) while the BN support is practically inactive up to 400 8C. This points to the role of palladium in hydrocarbon adsorption/ activation. After the contact with propylene–air mixture, the catalyst is slightly more sensitive to moisture, possibly due to the change of the surface topography induced by the catalytic test. The temperature of the onset of the oxidation corresponds to the temperature at which was observed the sharper increase of the electrical conductivity for both BN and PdO/BN during in situ measurements; this indicates the importance of the electronic properties of the BN support for oxygen adsorption/activation. The catalyst was inactive in methane oxidation below 400 8C since this molecule is more stable than propylene. About 2% conversion was observed at 400 8C, increasing then sharply up to 550 8C (87%). Both methane and C3H6 oxidation on PdO/BN mainly occur to deep oxidation products with dominating selectivity for CO2 and small amounts of CO. In methane oxidation the PdO/BN sample showed a good stability and reproducibility even in presence of humidity. Acknowledgement The authors thank to M. Aouine (IRC-CNRS) for TEM pictures. References [1] [2] [3] [4]

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