Amine grafted silica supported CrAuPd alloy nanoparticles: superb heterogeneous catalysts for the room temperature dehydrogenation of formic acidby Mehmet Yurderi, Ahmet Bulut, Nurdan Caner, Metin Celebi, Murat Kaya, Mehmet Zahmakiran

Chem. Commun.

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This journal is©The Royal Society of Chemistry 2015 Chem. Commun.

Cite this:DOI: 10.1039/c5cc02371h

Amine grafted silica supported CrAuPd alloy nanoparticles: superb heterogeneous catalysts for the room temperature dehydrogenation of formic acid†

Mehmet Yurderi,a Ahmet Bulut,a Nurdan Caner,a Metin Celebi,a Murat Kayab and

Mehmet Zahmakiran*a

Herein we show that a previously unappreciated combination of CrAuPd alloy nanoparticles and amine-grafted silica support facilitates the liberation of CO-free H2 from dehydrogenation of formic acid with record activity in the absence of any additives at room temperature. Furthermore, their excellent catalytic stability makes them isolable and reusable heterogeneous catalysts in the formic acid dehydrogenation.

Hydrogen (H2) is considered to be a promising energy carrier for our future society as it is light-weight and has a high energy density (142 MJ kg1), almost three times higher than that of natural gas (55 MJ kg1).1 In addition to being light weight and possessing high energy density, hydrogen is also an environmentally friendly energy vector to end users when combined with proton exchange membrane fuel cells (PEMFC), since only water and small amounts of heat are the by-products when it is utilized in PEMFC.2 However, controlled storage and release of hydrogen are still technological barriers in the fuel cell based hydrogen economy.1,2 In this context, formic acid (FA, HCOOH), which is one of the major stable and non-toxic liquid products formed in biomass processing, has attracted recent attention as a suitable hydrogen carrier for fuel cells designed for portable use.3 In the presence of metal catalysts, FA can catalytically be decomposed via dehydrogenation (HCOOH - H2 + CO2) and dehydration (HCOOH- H2O + CO) pathways. 3 The selective dehydrogenation of FA is indispensable for the production of ultrapure H2, since toxic carbon monoxide (CO) produced by the dehydration pathway significantly reduces the activity of the Pt catalyst in PEMFC.4

Recently, serious efforts have been made in the development of homogeneous catalysts for the selective dehydrogenation of FA.5

Even though, the notable activities have been reported by some of these,6 the difficulties met during their isolation-recovery steps hinder their practical application for on-board systems.

With this concern, the current research has been focused on the development of practical heterogeneous catalysts7,8 that exhibit significant activity under mild conditions with painless synthesis and recovery routes. In spite of the tremendous labour, themajority of the reported heterogeneous catalysts for FA dehydrogenation needs extra additives (e.g. HCOONa, LiBF4 etc.,) and elevated temperatures.7 To date, only a few heterogeneous catalysts8 have been found to provide notable activities in the additive-free FA dehydrogenation at low temperatures. In this regard, the development of highly active, selective and reusable solid catalysts that operate at low temperatures for FA dehydrogenation in the absence of additives is of great importance. Herein, we present a facile synthesis of CrAuPd alloy nanoparticles (NPs) supported on 3-aminopropyltriethoxysilane functionalized silica, hereafter referred to as CrAuPd/N-SiO2, and their excellent catalysis in the additive-free dehydrogenation of FA at room temperature.

CrAuPd/N-SiO2 can reproducibly be prepared through a simple impregnation route followed by subsequent sodium borohydride (NaBH4) reduction in water all at room temperature. 9 After centrifugation, copious washing with water, CrAuPd/N-SiO2 was isolated as a gray powder and characterized by multi-pronged techniques. The molar composition of the as-prepared catalyst was found to be Cr0.15Au0.40Pd0.60 (0.21% wt Cr, 1.26% wt Au and 1.65% wt Pd loadings correspond to 4.0 mmol Cr, 6.6 mmol

Au and 16.0 mmol Pd, 0.98 mmol NH2/g SiO2) by inductively coupled plasma-optical emission spectroscopy (ICP-OES) and the ninhydrin method.10 The conventional transmission electronmicroscopy (CTEM), high resolution-TEM (HRTEM), scanning TEMenergy dispersive X-ray spectroscopy (STEM-EDX) and high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) investigations were performed to examine the size, morphology and the composition of the CrAuPd/N-SiO2 catalyst. CTEM images of CrAuPd/N-SiO2 given in Fig. 1(a) and (b) reveal the presence of CrAuPd NPs. The mean particle size for the images given in Fig. 1(a) and (b) was found to be ca. 2.6 nm by the particle size analysis of 4100 non-touching particles a Department of Chemistry, Science Faculty, Yuzuncu Yıl University, 65080, Van,

Turkey. E-mail: zmehmet@yyu.edu.tr; Web: www.nanomatcat.com b Department of Chemical Engineering and Applied Chemistry, Atilim University, 06836, Ankara, Turkey † Electronic supplementary information (ESI) available: Detailed synthesis protocols, experimental procedures, activity results, TEM, FTIR, XPS, and GC characterization.

See DOI: 10.1039/c5cc02371h

Received 21st March 2015,

Accepted 3rd June 2015

DOI: 10.1039/c5cc02371h www.rsc.org/chemcomm

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Chem. Commun. This journal is©The Royal Society of Chemistry 2015 (Fig. 1(a), inset). STEM-EDX analysis of a large number of different domains on the CrAuPd/N-SiO2 surface revealed the presence of Cr,

Au, and Pd in the analyzed regions (Fig. 1(b), inset). The HRTEM image of CrAuPd/N-SiO2 given in Fig. 1(c) displays the highly crystalline nature of the NPs on N-SiO2.

The crystalline fringe distance was measured to be 0.21 nm, which is different from the (111) spacing of the face-centered cubic (fcc) Au (0.235 nm),8e Pd (0.223 nm)8e and the (110) spacing of

Cr (0.263 nm).11 In addition to this, the powder X-ray diffraction (P-XRD) pattern of the as-synthesized catalyst (Fig. 1(d)) exhibits a diffraction peak in the position between Pd(111) and Au(111), whose position also differs from Cr(110).11 The diffuse reflectance UV-vis (DR-UV-Vis) spectrum taken for solid powders of CrAuPd/N-SiO2 (Fig. 1(e)) shows almost no surface plasmon resonance band for Au