X-ray absorption fine structure studies on nickel phosphide catalysts for the non-oxidative coupling of methane reaction using a theoretical model

https://doi.org/10.1016/j.radphyschem.2021.109727Get rights and content

Highlights

  • High performance catalysts for non-oxidative coupling of methane (NOCM).

  • Nickel phosphide catalysts.

  • X-ray absorption fine structure (XAFS) analysis.

  • Theoretical reference for XANES and EXAFS.

  • Crystal structure identifications using theoretical model.

Abstract

SiO2-supported Ni phosphide catalysts are highly active toward the non-oxidative coupling of methane (NOCM) reaction, and their catalytic activity is strongly dependent on their Ni:P ratio. We analyzed Ni phosphide catalysts using X-ray absorption fine structure (XAFS) to elucidate the structure-catalytic activity relationship. Because only Ni2P was available as a reference material, we calculated the theoretical XAFS based on the reference crystal structures using the FEFF program and compared them with the experimental spectra of Ni phosphide catalysts. We demonstrated that catalysts with Ni to P ratios of 1:1, 2:1, and 3:1 consisted mainly of the Ni2P, Ni12P5, and Ni3P, respectively. We found that Ni2P exhibited the highest activity toward NOCM because of its optimum balance of C–H cleavage activity and coke formation. This analysis demonstrated that the theoretical XAFS simulation could be used to identify the structure of supported catalysts based on the crystal structure of reference compounds.

Introduction

Transition-metal Ni phosphides have attracted much attention as highly active catalysts for hydrodesulfurization (HDS) (Oyama, 2003; Oyama et al., 2009; Rodriguez et al., 2003), hydrodenitrogenation (HDN) (Oyama, 2003; Oyama et al., 2009), hydrodeoxygenation (HDO) (Cho et al., 2014; Iino et al., 2019; Zhao et al., 2011), hydrogen evolution reaction (HER) (Hu et al., 2020; Liu and Rodriguez, 2005; Moon et al., 2015; Popczun et al., 2013; Vij et al., 2017), dehydrogenation of cyclohexane (Li et al., 2014), the water-gas shift (WGS) reaction (Liu et al., 2009), water splitting (Menezes et al., 2017), and hydrodechlorination (Liu et al., 2008).

Methane (CH4) is the main ingredient in natural and shale gases. The efficient conversion of CH4 to higher hydrocarbons (C2H2, C2H4, C2H6, C3H6, and C6H6) as feedstock is a desirable catalytic reaction for CH4 as a petroleum alternative (Kondratenko et al., 2017; Wang et al., 2017). Non-oxidative coupling of methane (NOCM) reactions have attracted intensive interest for converting CH4 to higher hydrocarbons (Borkó and Guczi, 2006; Soulivong et al., 2008; Xu and Lin, 1999). Yamanaka and coworkers found that SiO2-supported Ni phosphide binary catalysts with a Ni to P ratio of 1:1 exhibited the highest catalytic performance for NOCM among various NiX (X = various elements) compounds (Dipu et al., 2018, 2020, 2021). In a previous study, we investigated the structure of Ni phosphide catalysts by analyzing SiO2-supported Ni phosphide compound catalysts using X-ray absorption fine structure (XAFS) (Al Rashid et al., 2020).

XAFS is classified into two energy regimes: X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). XANES is the structure in the X-ray absorption spectrum near the edge up to 50 eV region. EXAFS refers to the oscillation that appears 50 eV or more above the X-ray absorption edge. EXAFS arises from the interference between outgoing photoelectrons and singly scattered electrons (Iwasawa and Asakura, 2017). EXAFS analysis provides details about the local structure, such as the coordination number and bond length. EXAFS data are usually analyzed by curve fitting (CF) (Teo, 1986). Although EXAFS is a well-established technique, it suffers from several drawbacks. First, the signals are damped quickly at 500–1000 eV if the sample is highly disordered or if the surrounding atoms are low-Z elements. Second, it can only provide one-dimensional bonding information. Third, the amount of information is limited by Nyquist theory in the CF analysis (Kido et al., 2020; Stern, 1993). Consequently, a straightforward analysis of complex systems is difficult.

Ni phosphides have different phases: Ni3P, Ni12P5, Ni2P, Ni5P4, NiP, NiP2, and NiP3 (Massalski et al., 1990; Ren et al., 2007). These compounds have complicated structures with different Ni and P sites and different Ni–P and Ni–Ni bond lengths, as shown in Fig. S1(a). For example, the two different types of Ni sites in Ni2P have different Ni–P and Ni–Ni bond lengths (Fig. S1(b)) (Bando et al, 2011, 2012, 2011; Kawai et al., 2003; Wada et al., 2012a, 2012b, 2012b; Yuan et al., 2015; Contreras-Mora et al., 2018). In our previous two-shell fitting EXAFS analysis of Ni phosphide NOCM catalysts with Ni:P ratios of 1:1, 2:1, and 3:1, we found that the Ni–P and Ni–Ni bond lengths differed depending on the Ni:P ratio (Table S1). The Ni–Ni bond length decreased with increasing Ni:P ratio. Interestingly, we found that Ni phosphide catalysts on SiO2 with Ni:P = 3:1 had a shorter Ni–Ni bond length (RNi–Ni = 2.45 Å) than that in Ni foil (RNi–Ni = 2.48 Å). However, analysis by CF could not easily specify the structures. By comparing the results with those for Ni2P reference compounds, we deduced that the Ni phosphide with Ni:P = 1:1 had the Ni2P structure. However, the structure of other Ni phosphide catalysts with Ni:P ratios of 2:1 and 3:1 was difficult to determine because of a lack of appropriate reference compounds.

In the near-edge region, strong and characteristic XANES signals appear. XANES occurs because of multiple scattering or transitions to empty bound states (Ankoudinov, 1996; Gunter et al., 2002). XANES analysis is a powerful technique for investigating the local structure of complex systems because it is more sensitive to three-dimensional structures and electronic states (Koningsberger et al., 1988). Because theoretical calculations using the FEFF program have enabled us to reproduce the X-ray absorption fine structure spectra in both the XANES and EXAFS regions (Bosman and Thieme, 2009; Rehr and Albers, 2000), we attempted to analyze the structure of Ni phosphide catalysts by comparing with the theoretical XANES and EXAFS spectra of reference compounds based on their crystal structures instead of by comparing with their experimental XANES and EXAFS spectra. In the present work, we carried out structural analyses of Ni phosphide catalysts on SiO2 with initial Ni:P ratios of 1:1, 2:1, and 3:1 using theoretical models without the use of experimental reference compounds.

Section snippets

Experimental

SiO2-supported Ni phosphide catalysts (Ni–P/SiO2) with different Ni and P ratios of 1:1, 2:1, and 3:1 were prepared from Ni(NO3)2·6H2O and (NH4)2HPO4 precursors by a conventional impregnation method. The preparation details were available elsewhere (Dipu et al., 2018). The samples are denoted by their initial Ni:P ratio; for example, the notation Ni–P/SiO2 (Ni:P = 1:1) indicates that the molar ratio of Ni to P was 1:1.

X-ray absorption fine structure (XAFS) spectra were recorded at the BL9C

Results and discussion

Fig. 1 compares Ni K-edge XANES, inversely Fourier Transform (IFT) and Fourier Transform (FT) of the Ni2P reference sample with the FEFF8-calculated theoretical spectra. The experimental and theoretical XANES spectra of Ni2P show the same three features (labeled A, B, and C in Fig. 1).

To estimate the systematic error, we compared the peak positions in the experimental and calculated Ni2P spectra. The differences between the experimental and calculated peak energies were 0.0, −0.4, and 0.0 eV

Conclusions

In the present work, we analyzed Ni K-edge XANES and EXAFS spectra of Ni phosphide catalysts and determined the crystal structure by comparison with theoretically calculated XANES and EXAFS spectra. The catalytic activity of SiO2-supported Ni phosphide catalysts in NOCM followed the order Ni3P < Ni12P5 < Ni2P. The theoretical XANES and EXAFS simulation approach predicted the unknown structure of supported metal catalysts with complex structures. This approach may open a new way to reveal the

Author statement

Any data will be available upon request.

Authors’ contributions

Md Harun Al Rashid , Shinchi Nagamatsu, Daiki Kido, Bing Hu, Kiyotaka Asakura.

Role: XAFS measurement and analysis.

Arnoldus Lambertus Dipu, Yuta Nishikawa, Hitoshi Ogihara, Yuta Inami, Shoji Iguchi, Ichiro Yamanaka.

Role: XAFS measurement and preparation of sample.

Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Kiyotaka Asakura reports financial support was provided by Japan Science and Technology Agency.

Acknowledgments

We performed these experiments under the CREST project “Innovative Catalysts” JPMJCR15P4 of Japan Science and Technology (JST). All the XAFS work was performed at KEK-PF under proposal numbers 2016G546 and 2018G628. We thank all XAFS team members at the Photon Factory for their technical support.

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