Unveiling the H2 production mechanism in methanol decomposition on Ni3Sn4 surfaces

Hydrogen production from direct methanol decomposition reaction (CH3OH ⇆ 2H2 + CO) is a valuable alternative to the steam reforming of methane, commonly exploited at an industrial level. Given the urgent need to find viable alternatives to the use of fossil fuels for economic and environmental issues, methanol owns the great advantage of being one of the most important chemical feedstocks obtained from several sources, including biomass. Because of the high H content, methanol is regarded as a viable solution to the problems related to H2 transport. The methanol decomposition reaction brings about the production of the so-called “syngas mixture” (H2 + CO), which can be used for a wide range of applications, including the use of H2 as a fuel. Among the state-of-the-art catalysts for methanol decomposition reaction, Ni-based catalysts combine stability, efficient catalytic activity, and low cost of raw materials, thus representing ideal candidates for industrial scaleup. One of the major challenges when using Ni-based catalysts regards the optimization of the catalytic activity and selectivity with respect to undesired secondary reactions that can occur in parallel with the methanol decomposition, with the consequent production of CO2, CH4, CH2O, H2O and C. In this context, Ni−Sn intermetallic compounds deserve particular attention, considering that the incorporation of Sn atoms can be beneficial for hydrogen selectivity and makes CO adsorption energetically unfavorable, thus preventing the poisoning of the catalyst.

In this study, we investigated the methanol decomposition mechanism of Ni3Sn4 at temperatures between 250 and 300°C. This hitherto unexplored temperature range, in addition to being extremely advantageous for the industrial implementation of methanol decomposition, would also allow for the recovery of waste heat in methanol-fueled vehicles. To this aim, we combined operando ambient-pressure NEXAFS with synchrotron-based in situ X-ray photoemission spectroscopy (XPS), both performed at the APE-HE beamline of Elettra, complemented by High Resolution Transmission Electron Microscopy (HR-TEM) and density functional theory (DFT) calculations. The use of operando and in situ spectroscopic techniques using synchrotron radiation proved to be particularly appropriate to monitor the status of the catalyst surface during the reaction. The in situ XPS technique allowed us to detect a selective oxidation of Sn (Fig. 1a) atoms when the Ni3Sn4 surface is exposed to an oxidizing atmosphere. This result indicated the presence of a Sn-rich surface oxide skin emerging from the natural interaction of the catalyst surface with the ambient atmosphere, also imaged by HR-TEM (Fig. 1b). The electronic structure modifications of Ni atoms were monitored by means of operando NEXAFS upon realistic reaction conditions, i.e., exposing the sample to methanol at 250°C and 300°C at the total pressure of 1 bar while detecting the reaction products with a micro gas chromatograph. The results we obtained indicate that the Sn-rich oxide skin plays a pivotal role in the catalytic mechanisms. Indeed, by a comparison with a Ni3Sn2 compound, we understood that Sn surface content helps preserve the underlying Ni atoms from irreversible electronic structure modifications that poison the catalyst surface (Fig. 1c). This was confirmed analyzing the gas products detected with the online micro gas chromatograph, finding an important increase in H2 selectivity in pre-oxidized Ni3Sn4 (Fig. 1d) with respect to pre-oxidized Ni3Sn2. Similar experiments performed with pre-reduced samples (Fig. 1c-d) confirm the important role of the Sn-rich oxide skin naturally formed on the NixSny surfaces in oxidizing conditions, paving the way for further catalyst optimization. The DFT calculations confirmed the experimental results, indicating that H2 and CO desorption are favored on the surface of the pre-oxidized Ni3Sn4 compound.

Figure 1 of top-story by Mauri et. al. J Phys. Chem. Lett. 2023

Figure 1: a) Ni 3p and Sn 3d XPS spectra for Ni3Sn4 in different oxidizing environments. b) HR-TEM micrograph of a Ni3Sn4 grain. An oxide skin with an average thickness of ∼1.5 nm is indicated by the two arrows. c) Operando NEXAFS spectra of Ni L3 edge acquired during the exposition to CH3OH. Black dotted, red dotted, and blue spectra denote spectra acquired before introducing the CH3OH in the NEXAFS reaction cell, after having removed CH3OH from the reaction cell, and during the exposition to CH3OH, respectively. d) Online micro gas chromatography results obtained by detecting the gas products of the CH3OH decomposition catalyzed by Ni3Sn4 and Ni3Sn2, after sample pre-oxidation and pre-reduction, at 250 and 300°C.

This research was conducted by the following research team:

Silvia Mauri1,2, Gianluca D'Olimpio3, Corneliu Ghica4, Luca Braglia1, Chia-Nung Kuo5,6, Marian Cosmin Istrate4, Chin Shan Lue5,6, Luca Ottaviano3, Tomasz Klimczuk7, Danil W. Boukhvalov8, Antonio Politano3, Piero Torelli1.
1 CNR - Istituto Officina dei Materiali, TASC, Trieste, Italy.
2 Department of Physics, University of Trieste, Trieste, Italy.
3 Department of Physical and Chemical Sciences, University of L’Aquila, L’Aquila, Italy.
4 National Institute of Materials Physics, Atomistilor, Magurele, Romania  
5 Department of Physics, National Cheng Kung University, Tainan, Taiwan
6 Taiwan Consortium of Emergent Crystalline Materials, Ministry of Science and Technology, Taipei, Taiwan
7 Department of Solid-State Physics, Gdansk University of Technology, Poland
8 College of Science, Institute of Materials Physics and Chemistry, Nanjing Forestry University, Nanjing, P. R. China.

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S. Mauri, G. D’Olimpio, C. Ghica, L. Braglia, C.N. Kuo, M.C. Istrate, C.S. Lue, L. Ottaviano, T. Klimczuk, D.W. Boukhvalov, A. Politano and P. Torelli, “Hydrogen Production Mechanism in Low-Temperature Methanol Decomposition Catalyzed by Ni3Sn4 Intermetallic Compound: A Combined Operando and Density Functional Theory Investigation”, J. Phys. Chem. Lett. 14, 1334-1342 (2023); DOI: 10.1021/acs.jpclett.2c03471.

Last Updated on Saturday, 15 April 2023 16:23