Elsevier

Materials Research Bulletin

Volume 83, November 2016, Pages 65-76
Materials Research Bulletin

Structural evolution in Pt/Ga-Zn-oxynitride catalysts for photocatalytic reforming of methanol

https://doi.org/10.1016/j.materresbull.2016.05.012Get rights and content

Highlights

  • Wurtzite-like Ga-Zn-oxynitrides were obtained from oxide mixtures and precipitates.

  • Pt/Ga-Zn-oxynitrides were active in methanol photocatalytic reforming reaction.

  • Pt co-catalyst obtained by calcination had higher activity than that by reduction.

  • Ga-Zn-oxynitrides transformed to oxyhydroxides during the photocatalytic reaction.

  • In situ metallic Pt formation was observed during the photocatalytic reaction.

Abstract

Products of microwave-assisted urea-induced co-precipitation of Ga(NO3)3 and Zn(NO3)2 or Ga2O3 and ZnO were nitridated in order to obtain Ga-Zn-based photocatalysts. Irrespectively to the starting material, wurtzite-like Ga-Zn-oxynitride phases formed. The preparation was completed by deposition of a Pt co-catalyst, which was activated by either reduction in hydrogen or calcination in air. It was demonstrated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) that during oxidative activation the oxynitride started to transform into a nitrogen-free Zn-containing Ga- oxyhydroxide. Regardless to the structure of the catalysts after the activation step, almost complete oxynitride to oxyhydroxide transformation was observed during the methanol photocatalytic reforming reaction, accompanied by complete reduction of the Pt co-catalyst to metallic state. The observations of this study point to the importance of phase transitions under reaction conditions in the development of the active ensemble in the Ga,Zn-based photocatalysts.

Introduction

Photocatalytic hydrogen production is a promising approach for storing solar energy in chemical form. High gravimetric energy density, abundance and storage potential make hydrogen a potential energy carrier [1]. It has great promise for utilizing in clean, high-efficiency power generation systems such as fuel cells.

Methanol is a good hydrogen resource because of its high hydrogen/carbon ratio. Significant efforts have been made for photo-induced reforming of methanol. The idea is to use an efficient photocatalyst together with solar energy to promote H2 formation from methanol according to Eq. (1) [2]:CH3OH + H2O ⿿ 3H2 + CO2

Ga-oxynitride [3], [4], [5] and Ga-Zn-oxynitride [4] photocatalysts can be efficiently applied for utilizing visible light [6], they are good candidates for photocatalytic reforming of methanol [5], [7]. Both materials are semiconductors, their band gaps are reduced [5] in comparison to GaN (⿼3.4 eV), ZnO (⿼3.2 eV) and Ga2O3 (⿼4.6 eV). Ga-oxynitrides have a chemical formula of (Ga1-xx)(N,O) and can adopt the wurtzite-type structure of the hexagonal GaN (GaNh), in which O substitutes for N and the octahedral sites are randomly occupied by Ga and vacancies [3]. In Refs. [8], [9], [10] a range of (Ga1-xZnx)(N1-xOx) materials were synthesized; these wurtzite-type phases can be regarded as solid solutions of the two constituents GaN and ZnO, in which Ga and Zn randomly occupy the octahedral cation sites. The Ga-Zn-oxynitride ((Ga1-xZnx)(N,O)) structure can be related to the GaN-ZnO solid solution structure by incorporation of more O to the N sites and compensating vacancies to the cationic sites, resulting in an imperfect wurtzite-type material.

Ga-oxynitrides, Ga-Zn-oxynitrides and GaN-ZnO solid solutions, which are isostructural with GaNh, are commonly synthesized via high temperature ammonolysis of the appropriate oxide [8], [9], [10] or hydroxide precursors [4], [5]. However, many parameters of the preparations, among others temperature and duration of the ammonolysis, geometry of the reactor, the gas flow and even the pre-calcination of the precursor ZnO [8], can significantly influence the properties, thus the activity of the photocatalysts. In particular, the degree of the crystallinity in Ga-Zn-oxynitrides increases with increasing nitridation time and in parallel the Zn and O concentration decreases because the significant part of the ZnO precursor is removed as a result of reduction and volatilization of the Zn [9]. The decrease of nitridation temperature, as a method of controlling ZnO concentration, results in Ga-Zn-oxynitrides with poor crystallinity, phase separation and mixed surface oxide formation leading to poor photocatalytic activity [11]. Literature data suggests that Ga(OH)3 behaves as a more suitable precursor for Ga-oxynitride synthesis than Ga2O3 because its crystal lattice contains unoccupied 12-coordinate sites, which facilitate the ionic transport during the nitridation [5]. The abundance of vacancies at the octahedral sites in Ga-oxynitrides can be reduced by increasing the nitridation temperatures in the range of 750⿿850 °C [3] and the vacancies can be eliminated by introducing Zn2+ into the structure, during which a complete solid solution of (Ga1-xZnx)(N1-xOx) forms [4].

The photocatalytic activity of the semiconductors can be significantly improved by applying co-catalysts [12], [13], [14], [15]. The H2 formation in the methanol photocatalytic reforming reaction increases several orders of magnitude if co-catalysts have been introduced onto the surface of the semiconductor [16], [17], [18]. In the absence of co-catalysts, semiconductors induce poor H2 evolution even in the presence of any sacrificial electron donor [13]. Co-catalysts promote the charge separation and suppress the recombination of the photogenerated electron-hole pair [13], [14]. Another less emphasized but important role of the co-catalyst is to provide reaction sites for elementary reaction steps subsequent to light absorption, such as formation of molecular hydrogen and its desorption from the surface. If the surface reaction is too slow to consume the charges, the probability of charge recombination increases [13].

Noble metals such as Ag, Au, Pt and oxides such as RuO, NiOx are considered to be effective co-catalyst. Regarding the photocatalytic hydrogen production, Pt with the largest work function is not only the best co-catalyst for electron trapping but it shows excellent catalytic activity for H+ reduction and promotes the combination of surface hydrogen atoms into molecular H2 as well [13]. According to literature data, Pt has the lowest activation energy for H2 evolution [19]. In addition, Pt not only drains electrons from the semiconductor but transfers them to the solution, while other metals such as Au, Ag and Cu store the excess electrons on the metal surface due to the Helmholtz capacitance of the metal⿿solution interface, rather than transport electrons directly to the solvent [20], [21]. Consequently, Pt is considered as the most suitable hydrogen evolution co-catalyst because of its excellent electronic and catalytic properties [13].

In order to load co-catalysts on the surface of the semiconductors several methods are available. Commonly used techniques include in situ photodeposition [5], [12], [22] and deposition of pre-prepared metal colloids [23], [24], [25], [26]. An easy and effective way for preparing co-catalysts is impregnation with the appropriate metal salt followed by calcination. A series of metal oxide co-catalysts (such as NiOx, RuO2, RhOx, and PtOx) can be built on the surface of Ga-Zn-oxynitrides by means of this method [27]. RuO2 can also be loaded via impregnation by tetrahydrofuran solution of Ru3(CO)12 followed by drying and calcination [9]. Calcination of the Ga-Zn-oxynitride support itself is preferred because the treatment eliminates the traces of metallic Zn, which are potential recombination centers [28]. According to [29], [30], the Rhx-Cry-oxide pair, obtained by co-impregnation (co-evaporation) followed by calcination, is one of the most effective co-catalysts in the overall water splitting on Ga-Zn-oxynitrides. However, this co-catalysts/semiconductor system is less effective in the methanol photocatalytic reforming reaction [31]. High temperature reduction of the metal precursor [32] in H2 is one of the most commonly used methods to prepare supported metal catalysts, but it is believed unsuitable for oxynitrides because of their limited thermal stability compared to oxides [33].

In this contribution the structural stability of the Ga-Zn-oxynitride/Pt co-catalyst system is discussed. The effect of the synthesis steps on the structural properties of the catalysts is compared in case of two distinct Ga-Zn precursor compounds. The study is completed by exploring the structural evolution of the catalyst systems during the reaction conditions of photocatalytic H2 production.

Section snippets

Materials

Ga(NO3)3 (Aldrich), Ga2O3 (Aldrich), ZnO (Aldrich), GaN (Aldrich), Pt(NH3)4(NO3)2 (Aldrich), Zn(NO3)2 ÿ 6H2O (Fluka), urea (Molar Chemicals Ltd., Hungary) were used as received. Methanol and NH4OH solution were purchased from Reanal (Hungary). Double distilled water (18 MΩ) was used in every experiments.

Synthesis of the photocatalysts

We assumed that homogeneous distribution of the components was favorable to retain Zn during the nitridation process. Therefore a co-precipitation method starting from Ga and Zn salts was chosen to

Characterization of oxynitrides

All of the samples recovered after high temperature nitridation, obtained either from the precipitates or from the oxide mixtures, had a color of dark yellow to orange. Their corresponding XRD patterns exhibited exclusively the characteristic lines of the hexagonal wurtzite-type structure (see Fig. 1) similar to GaN (# 76-0703) and ZnO (zincite) (# 36-1451). None of the prepared oxynitride samples contained crystalline Ga2O3 (# 87-1901) or cubic ZnO. We interpreted the elevated background as

Conclusion

In Fig. 12 a graphical summary of this study is shown.

Ga-Zn-based photocatalysts were prepared by high temperature nitridation of either Ga-Zn-hydroxide-like precipitates obtained from nitrates or mixtures of Ga2O3 and ZnO. A combination of bulk and surface characterization methods revealed that irrespective to the starting material, the product of the synthesis was a wurtzite-like Ga-Zn-oxynitride phase. After synthesis, a Pt co-catalyst was added by impregnation. The oxynitride turned out to

Acknowledgements

This project has been supported by the National Development Agency, grant No. KTIA_AIK_12-1-2012-0014. Financial support by the OTKA-projectK77720 (András Tompos) and K100793 (Zoltán Pászti) is greatly acknowledged. Szabolcs Bálint and Péter Németh acknowledge support from János Bolyai Research Fellowship. The authors thank Ms. Ildikó Turi for the technical assistance.

References (57)

  • Z. Jiang et al.

    Rational removal of stabilizer-ligands from platinum nanoparticles supported on photocatalysts by self-photocatalysis degradation

    Catal. Today

    (2015)
  • K. Maeda et al.

    Improvement of photocatalytic activity of (Ga1-xZnx)(N1-xOx) solid solution for overall water splitting by co-loading Cr and another transition metal

    J. Catal.

    (2006)
  • K. Maeda et al.

    Effect of post-calcination on photocatalytic activity of (Ga1-xZnx)(N1-xOx) solid solution for overall water splitting under visible light

    J. Catal.

    (2008)
  • X. Sun et al.

    Preparation of (Ga1-xZnx)(N1-xOx) solid-solution from ZnGa2O4 and ZnO as a photo-catalyst for overall water splitting under visible light

    Appl. Catal. A: Gen.

    (2007)
  • C.C. Surdu-Bob et al.

    Surface compositional changes in GaAs subjected to argon plasma treatment

    Appl. Surf. Sci.

    (2001)
  • L.S. Al-Mazroai et al.

    The photocatalytic reforming of methanol

    Catal. Today

    (2007)
  • V. Matolin et al.

    Experimental system for GaN thin films growth and in situ characterization by electron spectroscopic methods

    Vacuum

    (2004)
  • S.D. Wolter et al.

    An investigation into the early stages of oxide growth on gallium nitride

    Thin Solid Films

    (2000)
  • P. Hill et al.

    An XPS study of the effect of nitrogen exposure time and temperature on the GaAs(001) surface using atomic nitrogen

    Appl. Surf. Sci.

    (1998)
  • I. Bertóti

    Characterization of nitride coatings by XPS

    Surf. Coat. Technol.

    (2002)
  • S. Pal et al.

    Microwave plasma oxidation of gallium nitride

    Thin Solid Films

    (2003)
  • H. Xiao et al.

    Fabrication, characterization, and photocatalysis of GaN⿿Ga2O3 core-shell nanoparticles

    Mater. Lett.

    (2012)
  • K. Maeda et al.

    Effect of electrolyte addition on activity of (Ga1-xZnx)(N1-xOx) photocatalyst for overall water splitting under visible light

    Catal. Today

    (2009)
  • K. Kamata et al.

    Synthesis and photocatalytic activity of gallium⿿zinc⿿indium mixed oxynitride for hydrogen and oxygen evolution under visible light

    Chem. Phys. Lett.

    (2009)
  • L.P. Bicelli

    Hydrogen: a clean energy source

    Int. J. Hydrogen Energy

    (11 1986)
  • C.C. Hu et al.

    Gallium oxynitride photocatalysts synthesized from Ga(OH)3 for water splitting under visible light irradiation

    J. Phys. Chem. C

    (2010)
  • C.C. Hu et al.

    Influence of indium doping on the activity of gallium oxynitride for water splitting under visible light irradiation

    J. Phys. Chem. C

    (2011)
  • K. Maeda et al.

    Solid solution of GaN and ZnO as a stable photocatalyst for overall water splitting under visible light

    Chem. Matter

    (2010)
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