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Rhodium(0) nanoparticles supported on hydroxyapatite nanospheres and further stabilized by dihydrogen phosphate ion: A highly active catalyst in hydrogen generation from the methanolysis of ammonia borane

Ozhava, Derya
Özkar, Saim
Rhodium(0) nanoparticles, supported on nanosized hydroxyapatite (Rh(0)/nanoHAP), were prepared by ion exchange of Rh+3 ions with Ca+2 ions of hydroxyapatite, followed by reduction of the resulting Rh+3/nanoHAP precatalyst during the catalytic methanolysis of ammonia borane (AB) in the presence of tetrabutylammonium dihydrogen phosphate (TBAP) at room temperature. Rh(0)/nanoHAP were characterized by a combination of advance analytical techniques including ICP-OES, XRD, TEM, EDX, XPS, ATR-IR and N-2 adsorption-desorption. Rh(0)/nanoHAP with an average particle size of 4.7 +/- 0.8 nm were found to be highly active catalyst in hydrogen generation from the methanolysis of AB liberating 3.0 equivalent H-2 per mole of AB. They provide 26,000 turnovers in hydrogen generation from the methanolysis of AB over 23 h before deactivation and an initial TOF value of 147 min(-1) which is the highest TOF value ever reported for the methanolysis of AB using rhodium catalyst at 25.0 +/- 0.5 degrees C. Carbon disulfide poisoning experiment demonstrates that Rh(0)/nanoHAP catalyzed methanolysis of AB is a heterogeneous catalysis. This study also covers the detailed kinetics of the methanolysis of AB catalyzed by Rh(0)/nanoHAP depending on stabilizer concentration, catalyst concentration and temperature. The apparent activation energy of the catalytic reaction was calculated from the evaluation of temperature dependent kinetic data: E-aapp = 56 +/- 2 kJ/mol. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.