Structural evolution in Pt/Ga-Zn-oxynitride catalysts for photocatalytic reforming of methanol
Graphical abstract
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 750850 °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 metalsolution 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)
- et al.
Hydrogen production by photocatalytic steam reforming of methanol on noble metal-modified TiO2
J. Catal.
(2010) - et al.
Preparation and lithium doping of gallium oxynitride by ammonia nitridation via a citrate precursor route
J. Solid State Chem.
(2007) - et al.
Indium and gallium oxynitrides prepared in the presence of Zn2+ by ammonolysis of the oxide precursors obtained via the citrate route
Mater. Res. Bull.
(2010) - et al.
Recent progress in the development of (oxy)nitride photocatalysts for water splitting under visible-light irradiation
Coord. Chem. Rev.
(2013) - et al.
Photocatalytic degradation of polycyclic aromatic hydrocarbons in GaN:ZnO solid solution-assisted process: direct hole oxidation mechanism
J. Mol. Catal. A: Chem.
(2010) - et al.
Photocatalytic hydrogen evolution from CdSZnOCdO systems under visible light irradiation: effect of thermal treatment and presence of Pt and Ru co-catalysts
Int. J. Hydrogen Energy
(2008) - et al.
Pt and Au/TiO2 photocatalysts for methanol reforming: role of metal nanoparticles in tuning charge trapping properties and photoefficiency
Appl. Catal. B: Environ.
(2013) - et al.
Pt nanoparticles on TiO2 with novel metalsemiconductor interface as highly efficient photocatalyst
Mater. Lett.
(2005) Work function electronegativity, and electochemical behaviour of metals. III. Electrolytic hydrogen evolution in acidic solutions
J. Electroanal. Chem.
(1972)- et al.
Nanotechnology for photolytic hydrogen production: colloidal anodic oxidation (review)
Int. J. Hydrogen Energy
(2009)
Rational removal of stabilizer-ligands from platinum nanoparticles supported on photocatalysts by self-photocatalysis degradation
Catal. Today
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.
Effect of post-calcination on photocatalytic activity of (Ga1-xZnx)(N1-xOx) solid solution for overall water splitting under visible light
J. Catal.
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.
Surface compositional changes in GaAs subjected to argon plasma treatment
Appl. Surf. Sci.
The photocatalytic reforming of methanol
Catal. Today
Experimental system for GaN thin films growth and in situ characterization by electron spectroscopic methods
Vacuum
An investigation into the early stages of oxide growth on gallium nitride
Thin Solid Films
An XPS study of the effect of nitrogen exposure time and temperature on the GaAs(001) surface using atomic nitrogen
Appl. Surf. Sci.
Characterization of nitride coatings by XPS
Surf. Coat. Technol.
Microwave plasma oxidation of gallium nitride
Thin Solid Films
Fabrication, characterization, and photocatalysis of GaNGa2O3 core-shell nanoparticles
Mater. Lett.
Effect of electrolyte addition on activity of (Ga1-xZnx)(N1-xOx) photocatalyst for overall water splitting under visible light
Catal. Today
Synthesis and photocatalytic activity of galliumzincindium mixed oxynitride for hydrogen and oxygen evolution under visible light
Chem. Phys. Lett.
Hydrogen: a clean energy source
Int. J. Hydrogen Energy
Gallium oxynitride photocatalysts synthesized from Ga(OH)3 for water splitting under visible light irradiation
J. Phys. Chem. C
Influence of indium doping on the activity of gallium oxynitride for water splitting under visible light irradiation
J. Phys. Chem. C
Solid solution of GaN and ZnO as a stable photocatalyst for overall water splitting under visible light
Chem. Matter
Cited by (10)
Recent advances in sacrificial reagents toward sustainable light-driven photocatalytic hydrogen evolution
2021, Nanostructured Photocatalysts: From Fundamental to Practical ApplicationsStructural transformation of Ga<inf>2</inf>O<inf>3</inf>-based catalysts during photoinduced reforming of methanol
2017, Materials Research BulletinCitation Excerpt :After overnight drying at 90 °C, the sample was heated up to 300 °C in air using 2 °C/min heating rate and calcined for 1 h at 300 °C in an oven. The photoinduced reforming reaction was carried out similarly to our previous study [34]. An UV-Consultig Peschl UV-Reactor System 1 equipped with gas inputs and outputs was used as a photoreactor.
ZnO/ZnGaNO heterostructure with enhanced photocatalytic properties prepared from a LDH precursor using a coprecipitation method
2017, Journal of Alloys and CompoundsCitation Excerpt :A number of methods have been employed to synthesize ZnGaNO solid solution by nitridation of different starting materials/precursors [11]. The starting materials or the precursors include Ga2O3 and ZnO [1,12], Ga2O3 and ZnO plus Zn [13], GaN and ZnO [14], ZnGa2O4 [15,16], ZnGa2O4 and ZnO nanoparticles [16–18], Ga-Zn-hydroxide-like precipitates [19], Ga-Zn-O gel-like precursors [5,20,21], Zn/Ga/CO3 layered double hydroxides (LDHs) [22–25], Ga2O3(ZnO)16 [26], carbon-templated ZnGaO hollow spheres [7], etc. Most of these precursors require complicated processes and long time of nitridation (5–15 h) at high temperatures (600–850 °C) for complete phase transformation.
Nanostructured metal nitrides for photocatalysts
2021, Journal of Materials Chemistry CThermal Treatment of a Keggin-Type Diplatinum(II)-Coordinated Polyoxotungstate: Formation of Hydrophilic Colloidal Particles and Photocatalytic Hydrogen Production
2020, European Journal of Inorganic ChemistryMultimetallic Oxynitrides Nanoparticles for a New Generation of Photocatalysts
2019, Chemistry - A European Journal