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Journal ArticleDOI

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

01 Nov 2016-Materials Research Bulletin (Pergamon)-Vol. 83, pp 65-76

AbstractProducts of microwave-assisted urea-induced co-precipitation of Ga(NO 3 ) 3 and Zn(NO 3 ) 2 or Ga 2 O 3 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.

Summary (1 min read)

Introduction

  • Photocatalytic hydrogen production is a promising approach for storing solar energy in chemical form.
  • 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.
  • This co-catalysts/semiconductor system is less effective in the methanol photocatalytic reforming reaction [31].
  • Double distilled water (18 MΩ) was used in every experiments.
  • Energy Dispersive X-ray Spectrometry (EDX) analysis was performed by an INCA (Oxford Instruments Ltd.) detector and an INCA Energy software package attached to a ZEISS EVO 40XVP Scanning Electron Microscope (accelerating voltage: 20kV, Wfilament, working distance 10 mm).

3. Results and discussion

  • 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.
  • The Zn retention during the nitridation was larger in the samples obtained from precipitates than those from oxides (cf. Zn/Ganom and Zn/GaEDX values in Table 2).

4. Conclusion

  • 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.
  • This transformation became complete during the photocatalytic methanol reforming reaction, accompanied by reduction of the Pt co-catalyst until the fully metallic state.
  • Eventually, it was demonstrated that Pt was an effective catalyst in surface transformation of the oxynitride to oxyhydroxide both by thermal and photo activated processes.
  • The finding that, irrespectively to synthesis routes of the photocatalyst, metallic Pt- loaded Ga-Zn-oxyhydroxide formed under the reaction conditions suggests that this Ptoxyhydroxide system could play an important role in the photocatalytic reaction.

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1
Structural evolution in Pt/Ga-Zn-oxynitride catalysts for photocatalytic reforming of
methanol
Materials Research Bulletin, Volume 83, November 2016, Pages 65-76
Ádám Vass, Zoltán Pászti, Szabolcs Bálint, Péter Németh, Gábor P. Szíjjártó,
András Tompos, Emília Tálas
http://dx.doi.org/10.1016/j.materresbull.2016.05.012
ISSN: 00255408 CODEN: MRBUA Source Type: Journal Original
language: English
DOI: 10.1016/j.materresbull.2016.05.012 Document Type: Article
Publisher: Elsevier Ltd
Corresponding author: Emília Tálas
Graphical abstract:

2
Structural evolution in Pt/Ga-Zn-oxynitride catalysts for photocatalytic reforming of
methanol
Ádám Vass, Zoltán Pászti, Szabolcs Bálint, Péter Németh, Gábor P. Szíjjártó, András
Tompos, Emília Tálas
*
Institute of Materials and Environmental Chemistry, Research Centre for Natural Sciences,
Hungarian Academy of Sciences, H-1117 Budapest, Magyar tudósok körútja 2, Hungary
*
Corresponding author, Tel.: +36 1 382 6916, email: talas.emilia@ttk.mta.hu, address: H-1519 Budapest,
P.O.Box 286, Hungary (Emília Tálas)

3
Abstract
Products of microwave-assisted urea-induced co-precipitation of Ga(NO
3
)
3
and Zn(NO
3
)
2
or
Ga
2
O
3
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.
Keywords: A.nitrides; C.X-ray diffraction; C.photoelectron spectroscopy; D.catalytic
properties; D.surface properties

4
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 H
2
formation from
methanol according to equation (1) [2]:
(1)
Ga-oxynitride [3-5] 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 Ga
2
O
3
(~4.6 eV). Ga-oxynitrides have a
chemical formula of (Ga
1-x
x
)(N,O) and can adopt the wurtzite-type structure of the
hexagonal GaN (GaN
h
), in which O substitutes for N and the octahedral sites are randomly
occupied by Ga and vacancies [3]. In Ref [8-10] a range of (Ga
1-x
Zn
x
)(N
1-x
O
x
) 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 ((Ga
1-x
Zn
x
)(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 GaN
h
, are commonly synthesized via high temperature ammonolysis of the
appropriate oxide [8-10] 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

5
photocatalytic activity [11]. Literature data suggests that Ga(OH)
3
behaves as a more suitable
precursor for Ga-oxynitride synthesis than Ga
2
O
3
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
o
C [3] and the vacancies can be
eliminated by introducing Zn
2+
into the structure, during which a complete solid solution of
(Ga
1-x
Zn
x
)(N
1-x
O
x
) forms [4].
The photocatalytic activity of the semiconductors can be significantly improved by
applying co-catalysts [12-15]. The H
2
formation in the methanol photocatalytic reforming
reaction increases several orders of magnitude if co-catalysts have been introduced onto the
surface of the semiconductor [16-18]. In the absence of co-catalysts, semiconductors induce
poor H
2
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, NiO
x
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 H
2
as well [13]. According to literature data, Pt has the lowest activation
energy for H
2
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 [23-26]. 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 NiO
x
, RuO
2
, RhO
x
, and PtO
x
) can be built on the surface
of Ga-Zn-oxynitrides by means of this method [27]. RuO
2
can also be loaded via

Figures (14)
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Abstract: In this study, relationships between preparation conditions, structure, and activity of Pt-containing TiO₂ photocatalysts in photoinduced reforming of glycerol for H₂ production were explored. Commercial Aerolyst® TiO₂ (P25) and homemade TiO₂ prepared by precipitation-aging method were used as semiconductors. Pt co-catalysts were prepared by incipient wetness impregnation from aqueous solution of Pt(NH₃)₄(NO₃)₂ and activated by calcination, high temperature hydrogen, or nitrogen treatments. The chemico-physical and structural properties were evaluated by XRD, ¹H MAS NMR, ESR, XPS, TG-MS and TEM. The highest H₂ evolution rate was observed over P25 based samples and the H₂ treatment resulted in more active samples than the other co-catalyst formation methods. In all calcined samples, reduction of Pt occurred during the photocatalytic reaction. Platinum was more easily reducible in all of the P25 supported samples compared to those obtained from the more water-retentive homemade TiO₂. This result was related to the negative effect of the adsorbed water content of the homemade TiO₂ on Pt reduction and on particle growth during co-catalyst formation.

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Cites background from "Structural evolution in Pt/Ga-Zn-ox..."

  • ...In case of co-catalyst formation by calcination, in situ reduction of platinum has been found during the photoinduced H2 production from methanol-water reaction mixture [29,42]....

    [...]

  • ...formation by calcination was favorable for the hydrogen production in the photocatalytic reaction of methanol [29,42]....

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  • ...Our recent results revealed that the working conditions of the methanol photocatalytic reforming reaction may result in significant changes of the structure of certain metal oxide–semiconductor catalyst systems involving both the semiconductor [54] and co-catalyst [15] compared to the fresh state....

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TL;DR: The research shows that loading suitable dual cocatalysts on semiconductors can significantly increase the photocatalytic activities of hydrogen and oxygen evolution reactions, and even make the overall water splitting reaction possible.
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Q1. What are the contributions in "Structural evolution in pt/ga-zn-oxynitride catalysts for photocatalytic reforming of methanol" ?

In this paper, the structural evolution in Pt/Ga-Zn-oxynitrides catalysts for photocatalytic reforming of methanol was studied.