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

TL;DR: In this paper, it was demonstrated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and Xray photoelectron spectroscopy (XPS) that during oxidative activation the oxynitride started to transform into a nitrogen-free Zn-containing Ga- oxyhydroxide.

About: This article is published in Materials Research Bulletin.The article was published on 2016-11-01 and is currently open access. It has received 9 citations till now. The article focuses on the topics: Catalysis & Calcination.

Summary

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.

Figures

Figure 2. TEM images of the Ga-Zn-oxynitrides obtained by nitridation of different precursors (Zn/Ganom=0.32). A: nitridation of the precipitate from Zn- and Ga- nitrate solution; B: nitridation of oxide mixture; C: commercial GaN. Electron diffraction patterns (inserts) reveal the presence of hexagonal wurtzite (# 76-0703).

Figure 10. Pt 4f spectra of a calcined and a used Pt/Ga-Zn-oxynitride sample obtained from nitridation of the oxide mixture. Zn/Ganom=0.14; Pt content= 1 w%.

Table 4. Characteristic binding energies and element ratios obtained from XPS measurements on the spent Pt-loaded GaN-ZnO photocatalysts Sample/co-catalyst formation Zn/Ganom Ga 3d (eV) Ga AP a (eV) N 1s (eV) N/Ga ratio O 1s (eV) Zn 2p Zn/Ga ratio Pt 4f Ga2O3, Pt calcined -- 20.2 1082.7 -- -- 531.1

Figure 11. Influence of the Pt co-catalyst formation method on the initial period of the hydrogen production reaction. The Ga-Zn-oxyinitride was obtained from oxide mixture; Zn/Ganom=0.14. Lines are only guides to the eye.

Figure 7. XRD pattern of recovered catalysts after the photocatalytic H2 production (reaction conditions in Figure 5) a=sample obtained from precipitate, co-catalyst formation by calcination; b= sample obtained from precipitate, co-catalyst formation by hydrogenation, c= sample obtained from precipitate, co-catalyst formation by hydrogenation, blank experiment, d= sample obtained from oxides, co-catalyst formation by calcination; e= sample obtained from oxides, cocatalyst formation by hydrogenation; f= GaN (commercial), co-catalyst formation by calcination, blank experiment.

Figure 8. TEM images of the Pt/Ga-Zn-oxynitride photocatalysts recovered after the photocatalytic H2 production (reaction conditions in Figure 5). A: used catalyst from precipitate, Pt co-catalyst formed by hydrogenation at 450oC; B: used catalyst from oxide mixture, Pt co-catalyst formed by hydrogenation at 450oC; C: used catalyst from precipitate, Pt co-catalyst formed by calcination at 300oC; D: used catalyst from oxide, Pt co-catalyst formed by calcination at 300oC. Consistently with XRD result, arrows in electron diffraction patterns (inserts) point to GaOOH diffractions (# 87-1901). 101 and 110 indices in C indicate hexagonal wurtzite (# 76-0703) contributions.

Table 1. Characteristic binding energies and element ratios obtained from XPS measurements on untreated Ga-Zn-oxynitride samples Sample Zn/Ganom Ga 3d (eV) Ga AP a (eV) N 1s (eV) N/GaXPS O 1s (eV) Zn 2p (eV) Zn/GaXPS Ga2O3 -- 20.5 1082.7 -- -- 531.4

Figure 3. XRD patterns of the Pt loaded Ga-Zn-oxynitrides after different treatments of the Pt precursor. a= nitrided precipitate, Pt formation by calcination; b= nitrided precipitate, calcination after “blank” impregnation (impregnation with no Pt precursor in the solution); c= nitrided precipitate, calcination without impregnation; d= nitrided precipitate, Pt formation by hydrogenation; e= nitrided oxides, Pt formation by calcination; f= nitrided oxides, Pt formation by hydrogenation g= GaN (commercial), Pt formation by calcination. Zn/Ganom: a-d=0.32, e-f=0.14, Pt load (when applicable) = 1%.

Figure 5. Pt 4f spectra of an impregnated, a hydrogen treated and a calcined Pt/Ga-Znoxynitride sample obtained from nitridation of the precipitate. Zn/Ganom=0.14; Pt content= 1 w%.

Table 3. Characteristic binding energies and element ratios obtained from XPS measurements on 1% Pt-loaded Ga-Zn-oxynitride samples Sample/co-catalyst formation Zn/Ganom Ga 3d (eV) Ga AP a (eV) N 1s (eV) N/Ga ratio O 1s (eV) Zn 2p Zn/Ga ratio Pt 4f GaN, Pt calcined - 20.1 1083.4 397.6 0.49 530.9

Figure 12. Evolution of the investigated samples during their lifetime.

Figure 9. O 1s spectra of the reduced and used Pt/GaN-ZnO photocatalysts derived from precipitates (Zn/Ganom=0.32). Top line: reduced, before the photocatalytic reaction; bottom spectrum: the same sample recovered after the photocatalytic reaction

Figure 1. XRD patterns of the Ga-Zn-oxynitrides obtained by nitridation of different precursors. a= sample obtained by nitridation of the precipitate from Zn(NO3)2 and Ga(NO3)3 with Zn/Ganom=0.32 (corundum internal standard added); b= sample obtained by nitridation of the precipitate from Zn(NO3)2 and Ga(NO3)3 with Zn/Ganom=0.14 (corundum internal standard added); c= sample obtained by nitridation of oxide mixture with Zn/Ganom=0.32; d= sample obtained by nitridation of oxide mixture with Zn/Ganom=0.14; e= GaN (commercial)

Figure 4. TEM images of the Pt/Ga-Zn-oxynitrides (Zn/Ganom=0.32) obtained by different formation of Pt co-catalyst. A: sample from precipitate, Pt co-catalyst formed by hydrogenation at 450oC; B: sample from oxide mixture, Pt co-catalyst formed by hydrogenation at 450oC; C: sample from precipitate, Pt co-catalyst formed by calcination at 300oC; D: sample from oxide, Pt co-catalyst formed by calcination at 300oC. Consistently with XRD result, electron diffraction patterns (inserts) revealed the presence of hexagonal wurtzite (# 76-0703) and for C GaOOH diffractions (# 87-1901), which are indicated by arrows.

Frequently asked questions

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.