Research Journal of Chemical Sciences ______________________________________________ ISSN 2231-606X Vol. 3(3), 30-33, March (2013) Res. J. Chem. Sci. International Science Congress Association 30 Pt, Pd Supported on Niobium Phosphates as Catalysts for the Hydrogen Oxidation Lisnyak V.V.1*, Ischenko E.V., Stratiichuk D.A., Zaderko A.N., Boldyrieva O.Yu., Safonova V.V.and Yatsymyrskyi A.V.Kyiv National Taras Shevchenko University, 01601 Kyiv, UKRAINEV.N. Bakul Institute for Superhard Materials, 04074 Kyiv, UKRAINEAvailable online at: www.isca.in Received 27th November 2012, revised 29th January 2013, accepted 24th February 2013Abstract Niobium phosphates NbP and NbPO were synthesized by using convenient synthetic routs. The catalysts, namely, Pt or Pd metals supported on NbP and NbPO, were prepared by the impregnation method and characterized by means of miscellaneous techniques. Activity measurements of the H oxidation reaction were performed over PGM/NbP, PGM/NbPO, and PGM/Al (PGM = Pt or Pd) catalysts, the latter were used for comparison reasons, in the gas mixture with oxygen excess (H:O:Ar = 1:20:79 vol.%). It was found that for catalysts with the same platinum group metal (PGM) loading of 0.5 wt. %, the catalytic activity of Pt and Pd supported on the niobium phosphates was higher than that of Pt and Pd supported on alpha-Al, correspondingly. The results showed that the conversion of H is dependent on chemical nature of supports, and the activity of the catalysts decreases in the order of PGM/NbP � PGM/NbPO � PGM/Al. The enhanced activity of Pt/NbP and Pd/NbP catalysts is attributed to the presence of Nb4+/Nb5+states in the NbP. These states can promote the generation of easily reduced oxygen species, which are active in the catalytic H oxidation. Keywords: Hydrogen oxidation, niobium phosphates, catalytic activity, supported platinum group metal, XPS. Introduction The development of alternative energetic involves finding ways of fuel switching to biofuel1,2, biogas or H-sourced fuel4,5 for restructuring of energy-intensive industries. It should be mentioned that from the economic point-of-view hydrogen fuel is the most promising alternative source of energy among available ones. However, the release of hydrogen and subsequent accumulation of hydrogen-oxygen flammable gas mixtures may occur at the operation of H-source engine. So, for safety reasons and acting in the paradigm of clean up technology6,7, catalytic converters, that work with hydrogen-air gas mixtures at low temperatures and low concentrations of hydrogen, can be used to for recombination of leaks of hydrogen. For functioning of these converters new highly active catalysts for the oxidation of H should be developed. The use of such catalyst in these converters could improve the recombination efficiency and could reduce the severity level of leakage to an acceptable level of risk. Platinum group metals (PGM), in particular Pt or Pd, supported on Al and SiO2 oxides, i.e. on the supports which are inactive for the red-ox reactions, are traditionally used as effective solid red-ox reactions catalysts. It is known that the chemical nature of the support significantly affects the catalyst performance, so, the selection of the support is crucial for successful design of efficient catalyst. Despite the large number of known catalytic systems8,9, a relatively small number of transition metals phosphates (TMP) have been applied for design of the reduction or the total oxidation (combustion) catalysts. It could be found from the literature, that Pt/B1–PO10 and Pt/AlPO11 catalysts show high activity in the reduction of NO to N by hydrocarbons for gas mixtures with oxygen excess. The (Pt/NbPO)/carbon12 and Pt/FePO13,14 composites are efficient catalysts of the electrochemical reduction of oxygen. Pt/FePO15catalysts display prominent activity in the oxidation of methanol and CO as the product of the methanol selective oxidation. Among different TMP niobium phosphates, namely, NbP7.5Nb1.912.8212 as P-deficient Nb2–(PO3 and -NbPO, are of considerable interest to be used as supports for preparation of oxidation catalysts, as these niobium phosphates have a certain catalytic activity in the oxidative dehydrogenation reactions16,17. So, in this communication we report characterization and catalytic activity for H oxidation of Pt, Pd supported on NbPand NbPO5 (NbOPO) niobium phosphates, which were prepared by convenient synthetic routs. Material and MethodsPreparation of catalysts: Niobic acid (NbO) was obtained by hydrolysis rout18. Niobium acid was mixed with 85 wt. % HPO to form slurry. This slurry was calcinated at 1073 K for 3 h to prepare NbPO. NbP was synthesized from NbCl148HO and phosphoric acid19. 0.5 wt. % load of Pt or Pd metal was supported over NbPO and NbP. The same load of Pt or Pd was supported over -Al3 sp = 3.6 m/g), to be used for comparison. The supporting of platinum and palladium metals was performed as follows. The NbPO, NbP and Al3 powders were impregnated with the preset quantity of 8 mM aqueous HPtCl or PdCl solution. These solutions were Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 3(3), 30-33, March (2013) Res. J. Chem. Sci. International Science Congress Association 31 evaporated from the samples at 353 K and wet powders obtained were dried at 373 K for 1 h20. Dried powders of NbPO, NbP7 and -Al, which contain adsorbed Pt4+ or Pd2+, were packed within quartz wool layers inside a continuous-flow U-type reactor and form a uniform catalyst bed. The packed in the reactor powders were reduced in a stream of hydrogen-argon gas mixture (10:90 vol. % H:Ar) at 673 K for 1 h. The reduced samples, denoted further as 0.5 wt. % PGM/NbPO, 0.5 wt. % PGM/NbP7 and 0.5 wt. % PGM/Al, where PGM = Pt or Pd, were cooled and stored in a desiccator with P under ambient conditions. Characterization of catalysts: The samples specific surface area (sp) was determined by means of Ar physisorption at 77 K. Nb and P element analyses were carried out on fused glass beads by X-ray fluorescence spectrometry (XRF) on a Phillips X’Unique PW1480 XRF spectrometer. The content of PGM in the samples was determined by inductively coupled plasma-mass spectrometry (ICP-MS) on an Agilent 7500 quadrupole ICP-MS instrument. The crystalline phases in the samples were identified by matching experimental powder X-ray diffraction (PXRD) patterns, which were collected using a Philips PW 3710 diffractometer (u -radiation), to the powder diffraction file (PDF)21. XPS spectra of the catalysts were recorded using a Kratos AXIS spectrometer with a monochromated radiation source (Al K, h = 1486.7 eV). It was considered that the Nb core level XP spectra are the sum of Nb5+ and Nb4+ valence levels. So, the Nb4+/Nb5+ ratio originated from different amounts of Nb5+ and Nb4+ valence states in the Nb-containing catalysts was determined by deconvolution of the Nb core level XP spectra. Catalytic activity: The catalytic tests were performed in the same continuous-flow reactors which were used for the PGM reduction in the H-Ar gas mixture. The fraction size of the catalysts powders was 0.2–0.5 mm; the mass of all catalysts was 0.5 g. The catalytic tests were done at an atmospheric pressure. Analytical grade H, O and Ar gases were used as inlet gases, which flow was controlled by calibrated mass-flow controllers. The total gas inlet was 0.1 L/min, containing 1 vol. % H, 20 vol. % O(oxygen excess), rest Ar. The hydrogen and oxygen contents in inlet and outlet gas mixtures were determined on a gas chromatograph LHM 8MD (Yagot, Moscow, Russia). This chromatograph was equipped with thermal conductivity detector and with a steel column (Ø = 3 mm, 3 m) packed with SKT-active carbon. The temperature of the column, injector port, and conductivity detector was 100°C. Argon was used as a gas-carrier with a flow rate of 40 mL/min. The volume of the gas sample was 2 mL. The catalytic activity in the H oxidation was examined in the temperature range 273–380 K for the supported PGM catalysts and at 273–380 K for the supports. The temperature control at the catalytic tests was performed by means of a thermometer connected to a digital thermal sensor. The thermometer sensor was situated in the center of the packed catalyst bed to determine the temperature inside the catalyst layer. The reactor was gradually heated from 273 K to a temperature at which complete H conversion ((H)) was observed and then cooled down. This procedure was then repeated for at least three times in order to reach a steady temperature at a certain H conversion X%(H), which was used as a measure of the catalytic activity. Results and DiscussionNb and P content obtained by XRF confirms formation of NbPO (Nb, 45.2%; P, 15.0%) and NbP (Nb, 34.2%; P, 23.0%), ICP-MS data proves 0.5 wt. % of Pt or Pd metals load in the catalysts prepared. The PXRD patterns of PGM supported catalysts are similar to that of respective supports, namely, pseudo-cubic NbP14and -NbPO phases15. For example indexed PXRD patterns of the 0.5 wt. % Pt/NbPO (1) and 0.5 wt. % Pt/NbP7 (2) catalysts were depicted on figure-1. Figure-1 Indexed PXRD patterns of catalysts: 1) 0.5 wt. % Pt/NbPO and 2) 0.5 wt. % Pt/NbP Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 3(3), 30-33, March (2013) Res. J. Chem. Sci. International Science Congress Association 32 XPS dataindicate that Pt and Pd in thecatalystsstudied are present in themetallic state. The catalysts characteristics sp, X%(H) and Nb4+/Nb5+ ratio taken from XPS data are listed in table-1. The catalysts are characterized by sp values lesser than 4 m/g. Pure NbPO and NbP phosphates exhibit no low-temperature activity in the H oxidation. 20%(H) of NbPO and NbPreaches a value of 680 K and 630 K, correspondingly. No prominent H conversion was observed over pure -Al at these temperatures. The temperature dependence of Hconversion over Pt and Pd supported metal catalysts characterized by the temperature hysteresis, as it can be seen on figure-2 for the Pt catalysts. As on can see form the table-1 and the figure-2, that X%(H) over 0.5 wt. % PGM/NbP is about on 20–30 lower than that over 0.5 wt. % PGM/Al. The Pd catalysts are less active than Pt ones. The temperature hysteresis loop width for PGM/NbP and PGM/NbPO is narrowed in comparison with PGM supported on Al3 catalysts. The activity of catalysts decreases in a sequence of PGM/NbP � PGM/NbPO � PGM/Al3 (PGM = Pt or Pd). The platinum group metals supported over NbP are more active in H oxidation than supported over inert carrier Al. It should be suggested that a minor amounts of reduced Nb4+ in NbPO, which are found from XPS data, can not play determining role for H oxidation. The enhanced activity of Pt/NbP and Pd/NbP catalysts is attributed to interplay between Nb4+/Nb5+ states in the NbPaffects on the surface red-ox reactions. Figure-2 conversion against the temperature over catalysts: 1) 0.5 wt. % Pt/NbP, 2) 0.5 wt. % Pt/NbPO, 3) 0.5 wt. % Pt/AlTable-1 The catalyst composition, specific surface area (sp), the temperature at a certain H conversion (X%(H)) and the ratio of Nb4+/Nb5+Catalyst composition sp, m/g X% (H 2 ), K Ratio of Nb4+/Nb5+ T 30% , K 100% , K NbPO 5 1.8 744 - 0/100 NbP 2 O 7 2.1 704 36/64 0.5 wt. % Pt/Al 2 O 3 3.6 337 373 - 0.5 wt. % Pd/Al 2 O 3 3.6 358 394 - 0.5 wt. % Pt/NbPO 5 3.3 323 353 8/92 0.5 wt. % Pd/NbPO 5 3.0 332 362 5/95 0.5 wt. % Pt/NbP 2 O 7 3.8 313 342 42/58 0.5 wt. % Pd/NbP 2 O 7 3.5 314 353 38/62 Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 3(3), 30-33, March (2013) Res. J. Chem. Sci. International Science Congress Association 33 Conclusion Oxidation of hydrogen over PGM/NbP, PGM/NbPO and PGM/Al3 (PGM = Pt or Pd) catalysts was studied. 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