Enhanced ammonia-borane decomposition by synergistic catalysis using CoPd nanoparticles supported on titano-silicates

Abstract

The decomposition of small molecules (such as formic acid or ammonia-borane (AB)) is one of the most promising alternatives for the in-situ H 2 generation towards a H 2 scenario implementation. 1-3 AB is claimed as one of the inorganic compounds with the highest hydrogen content (19.6 H 2 wt. %). This, added to its high reactivity with noble metals (such as Ru, Pd or Pt) makes it one of the best candidates for its use as H 2 feed in a PEMFC. [3][4][5][6][7][8] The total decomposition of this compound in liquid phase using water as solvent produces 3 mol of H 2 per mol of AB according to the following reaction equation.Ammonia borane is a liquid-phase chemical hydrogen storage material with great current interest. Its decomposition on a wide range of catalysts has been extensively reported in the literature. Among these, noble metals such as Rh, Ir, Ru, and Pt, have shown interesting catalytic activities, 10 but they are unsuitable for widespread practical applications due to their availability and price. Previous studies have demonstrated that bimetallic nanoparticles combining a noble metal and a firstrow transition metal forming an alloy structure could be promising candidates for the design of catalysts for the hydrolysis of ammonia borane. 11In order to assess the beneficial effect of these noble metal/first-row transition metal combinations for this application, we have used Pd due to its moderate activity among noble metals 12 and Co because it shows the highest activity among non-noble metal catalysts. 11,13,14The enhancement of the catalytic activity of the supported noble metal can be attained by different routes; i) increasing the metal dispersion on the support by the synthesis of very small NPs, 15 ii) alloying the noble metal with a transition metal, (which is also attractive due to the cost reduction of the catalysts) 11,16,17 and iii) supporting the NPs on an UV-Vis active support to upgrade the electron-transfer from the support to the NPs. 1,18 In the present study, the synthesis of Pd and Co-PdNPs by the reduction-by-solvent methodology has been applied with successful results in the production of H 2 by AB decomposition. The synthesis conditions allow a perfect control over size and morphology of the NPs (both mono-and bimetallic). Pure or alloyed cobalt (as oxide or reduced form) has been addressed in the recent literature as a promising metal in the AB decomposition reaction due to its low cost (compared with noble metals) and its high activity in relevant reactions. 12,16,[19][20][21] Due to their different reduction potential (compared with Pd), when M-Pd (i. e. Cu or Co) are wellalloyed there is a charge transfer from the M to Pd increasing the electron density on the Pd atom and its activity in the AB decomposition is enhanced. 1,18 On the other hand, the use of an active support (under UV-Vis conditions), such as titania or a titano-silicate (Ti-SiO 2 ), is an available strategy to enhance the catalytic activity of the final catalytic system. 22,23In the present work, Pd and Co x Pd 1-x...

Steering the Selectivity in Gold–Titanium-Catalyzed Propene Oxidation by Controlling the Surface Acidity

“…The 0.1Ti-SiO 2 , with a nominal loading of 0.4 wt % Ti, shows two absorption maxima, at 221 and 252 nm, respectively. These maxima correspond to isolated tetrahedral Ti-OH sites and hydrated isolated Ti-OH sites as reported by García-Aguilar et al . , The absorption maximum of 0.5Ti-SiO 2 at 1.9 wt % Ti shows a red-shift, most likely due to the presence of dimers or small oligomers of TiO x . For both supports, the absorbance maxima and optical edge energies (see Figures S2–S4) are in good agreement with isolated hydrated Ti-sites, but the oligomerization of Ti-sites for the 0.5Ti-SiO 2 support cannot be ruled out. ,,, The X-ray diffractogram (Figure S5) does not contain any diffraction lines that can be ascribed to crystalline TiO 2 .…”

New Approaches Toward the Hydrogen Production From Formic Acid Dehydrogenation Over Pd-Based Heterogeneous Catalysts

“…The search for solid and liquid-state hydrogen storage materials has been the focus of fruitful investigations in the last decades. Examples of some representative hydrogen storage molecules include ammonia borane (NH 3 BH 3 ) (García-Aguilar et al, 2016a;Navlani-García et al, 2016a;Navlani-garcía et al, 2018), ammonia (NH 3 ) (Lan et al, 2012;Afif et al, 2016), methanol (CH 3 OH) (Nielsen et al, 2013;Monney et al, 2014), sodium borohydride (NaBH 4 ) (Chowdhury et al, 2015;Demirci, 2015), magnesium hydride (MgH 2 ) (Lillo-Ródenas et al, 2008;El-Eskandarany et al, 2016) and formic acid (HCOOH) (Navlani-García et al, 2015a;Mori et al, 2017b;Wu et al, 2017) with hydrogen contents of 19.5, 17.6, 12.6, 10.8, 7.6, and 4.4 wt.% of hydrogen, respectively. However, the practical application of some of these hydrogen storage molecules is greatly limited due to their low kinetics for reversible H 2 adsorption-desorption reactions, thermodynamic stability, low inherent thermal conductivity, high price, and toxicity (Czaun et al, 2014;Zhong et al, 2018).…”

Hydrolytic Dehydrogenation of Ammonia Borane Attained by Ru-Based Catalysts: An Auspicious Option to Produce Hydrogen from a Solid Hydrogen Carrier Molecule

“…Both homogeneous and heterogeneous catalytic systems have been explored, but the advantages of heterogeneous catalysts make these systems preferred from a practical point of view. There is vast literature reported on the hydrolytic dehydrogenation of AB, highlighting those contributions made by Yamashita et al [46][47][48][49][50][51][52][53][54], Özkar et al [55][56][57][58], and Xu et al [44,[59][60][61][62].…”