Surface Properties and Microstructure of Catalytically Active Ni70Ga30, Ni70Sn30 and Ni70In30 Melt Spun Intermetallics Subjected to Oxidation-Reduction Heat Treatment

P. Czaja; J. Przewoźnik; P. Ozga; M. Marzec; K. Stan-Głowińska; D. Duraczyńska; E. Serwicka; L. Lityńska-Dobrzyńska

doi:10.2320/matertrans.MT-MA2022018

Abstract

Melt spun Ni₇₀Ga₃₀, Ni₇₀Sn₃₀ and Ni₇₀In₃₀ show different oxidation behaviour following heat treatment at 900°C in air. The Ni₇₀Ga₃₀ ribbon develops a continuous layer of Ga₂O₃ oxide, which passivates the surface and prevents further oxidation of the Ni₃Ga bulk phase. On the contrary, the Ni₇₀Sn₃₀ and Ni₇₀In₃₀ ribbons undergo considerable segregation and decomposition and aside from the Ni and NiO and the primary Ni₃Sn and Ni₃In also comprise SnO₂ and In₂O₃ phases. Results are conformed from XRD, TEM and XPS analysis. The oxidation behaviour especially in the case of the Ni₇₀Sn₃₀ ribbon is promising with view for development of fine Ni particles on the Ni₃Sn support.

1. Introduction

Intermetallic compounds comprising two or more metallic elements have recently turned out catalytically active for a range of chemical reactions including carbon monoxide oxidation, methanol decomposition, selective hydrogenation of unsaturated and dehydrogenation of saturated hydrocarbons etc.¹⁾ Their catalytic activity largely derives from a combination of structural and electronic properties afforded by a unique elemental make up.²⁾ Most frequently, such intermetallics come in a form of fine nanoparticles synthesised by means of wet chemical methods.³^,⁴⁾ While wet chemical methods are advantageous in providing a sufficient amount of free surface, they are insufficient due to poor control of the desired stoichiometry and often require sequential annealing steps to moderate the fine size of the nanoparticles through agglomeration and clustering. Hence, a viable alternative to wet chemical synthesis represented by melt spinning technique has been recently proposed for production of catalytically active intermetallics.⁵^,⁶⁾ Melt spinning is easily scalable and it furthermore yields brittle, easily pulverisable ribbon flakes with dimensions of 2–7 mm in length and 20–60 µm in thickness. Given the ease and fast production rate as well as composition controllability, melt spinning as such can be easily applied for catalyst screening.

In previous work we have studied microstructure, thermal stability and catalytic activity of such intermetallic, monolithic catalysts based on melt spun Ni–Al, Ni–Ga, Ni–Sn and Ni–In ribbons. We have demonstrated appreciable conversion rates on the order of 40% and selectivity reaching up to 80% after 4 h reaction time in the reaction of selective hydrogenation of phenylacetylene to styrene.⁷⁾ Better conversion output is still expected through etching or controlled oxidation-reduction heat treatment as has been outlined for Ni₃Al monolithic catalysts.⁸⁾ In this work we therefore examine the surface properties and microstructure evolution of Ni₇₀Ga₃₀, Ni₇₀Sn₃₀ and Ni₇₀In₃₀ melt spun ribbons following oxidation in air at 900°C for 1 h in order to shed light on microstructure behaviour under exposure to air at elevated temperature. Considerable differences in the oxidation behaviour and in the makeup of the outer oxide layer are revealed through XRD, SEM, STEM/TEM, TPR and XPS analysis. The results are important from a standpoint of material stability, its sensitivity to oxidation heat treatment and overall for the development of feasible routes for enhancing catalytic activity.

2. Experimental

The Ni₇₀Ga₃₀ and Ni₇₀Sn₃₀ and Ni₇₀In₃₀ ribbons have been obtained by melt spinning.⁵⁾ Oxidation in air has been conducted in a furnace at 900°C for 1 h. Scanning Electron Microscopy (SEM) with aid of FEI E-SEM XL30 and SCIOS 2 Dual Beam ultra-high-resolution, analytical Focused Ion Beam (FIB)-SEM has been used to examine the surface morphology, composition and to cut thin FIB lamella for transmission electron microscopy (TEM) analysis. TEM inspection has been carried out by Thermo Fischer Titan Themis G2 200 Probe Cs Corrected Scanning Transmission Electron Microscope (STEM) equipped with high angle annular dark field (HAADF) detector and energy dispersive X-ray (EDX) EDAX microanalyser. Conventional thin foils for TEM inspection have been prepared by Tenupol-5 double jet electropolisher operating at −30°C and under 15 V (the electrolyte was made up of nitric acid and methanol −1:3 vol.). The phase composition and structure analysis was further confirmed by X-ray diffraction (XRD). The XRD patterns have been collected with a Panalytical Empyrean diffractometer and an X’Celerator linear detector using Cu Kα radiation (λ = 1.540598 Å) and a rear graphite monochromator. The powdered ribbons were placed on a reflectionless silicon (“zero background”) sample holder. The data obtained has been analyzed using the profile fitting program FullProf based on the Rietveld method. The background intensity was approximated with a polynomial function and the peak shape was fitted with a pseudo-Voigt function. The XPS analyses were carried out in a PHI VersaProbeII Scanning XPS system using monochromatic Al Kα (1486.6 eV) X-rays focused to a 100 µm spot and scanned over the area of 400 µm × 400 µm. The photoelectron take-off angle was 45° and the pass energy in the analyzer was set to 117.50 eV for survey scans and 46.95 eV to obtain high energy resolution spectra for the C 1s, O 1s, Ni 2p_3/2, Sn 3d_5/2, In 3d_5/2, Ga 3d, Bi 4f and Pb 4f regions. A dual beam charge compensation with 7 eV Ar⁺ ions and 1 eV electrons were used to maintain a constant sample surface potential regardless of the sample conductivity. All XPS spectra were charge referenced to the unfunctionalized, saturated carbon (C–C) C 1s peak at 284.8 eV. The operating pressure in the analytical chamber was less than 3 × 10⁻⁹ mbar. Deconvolution of spectra was carried out using PHI MultiPak software (v. 9.9.0.8). Spectrum background was subtracted using the Shirley method.

3. Results and Discussion

Phase composition as determined by XRD is illustrated on the XRD profiles in Fig. 1. The phases identified, lattice parameters and their relative volume fractions are given in the Table 1. From the XRD measurements it has been found out that the Ni₇₀Ga₃₀ oxidized ribbon is composed of the tetragonal Ni₃Ga and trigonal Ga₂O₃ phases. The Ni₇₀Sn₃₀ air oxidized ribbon has been identified with the hexagonal Ni₃Sn, tetragonal SnO₂ and cubic NiO with minor volume fraction (0.8%) of Ni. While the Ni₇₀In₃₀ ribbons after oxidation develops cubic In₂O₃ phase, Ni, NiO with cubic structure and Ni₃In with hexagonal structure. These results are further well confirmed with TEM.

Fig. 1

Room temperature X-ray diffraction profiles of Ni₇₀Ga₃₀ (a), Ni₇₀Sn₃₀ (b) and Ni₇₀In₃₀ (c) melt spun ribbons subjected to oxidation in air for 1 h at 900°C.

Table 1 The ribbon alloy, space group, lattice parameters a, b, c and relative volume fraction Vol. % of the phases identified in as oxidized ribbons following XRD measurements.

Prior to TEM, surface microstructure of as oxidized ribbons has been examined with SEM. Typical SEM micrographs are shown for illustration in the Fig. 2. Following oxidation heat treatment the ribbons develop a coarse like surface morphology with well-defined crystallites, typical for granular oxides particles. Their size varies depending on the ribbon sample. The largest crystallites are formed on the Ni₇₀Sn₃₀ ribbon. They also poses the best developed shape morphology. The average size of crystallites in the case of the Ni₇₀Ga₃₀ ribbons is lower than that in the Ni₇₀Sn₃₀ but it is still larger than in the case of Ni₇₀In₃₀ ribbon, which shows the finest surface microstructure features among the three studied ribbons.

Fig. 2

SEM micrographs of Ni₇₀Ga₃₀, Ni₇₀Sn₃₀ and Ni₇₀In₃₀ ribbons following oxidation heat treatment in air at 900°C lasting for 1 h.

The results of TEM analysis are in accord with XRD. An example STEM-HAADF micrograph (a) with the corresponding elemental map distribution of each element (b)–(d) for the Ni₇₀Ga₃₀ oxidized ribbon is shown in the Fig. 3. From the STEM-HAADF image it is seen that grain sized increased remarkably and only three grains can be seen within the entire body of the thin foil. It is also well seen that matrix is stable and does not show any signs of decomposition or precipitation, the composition is uniform. This is most likely due to a Ga₂O₃ oxide layer, which has developed and passivated the ribbon surface. The thickness of the layer is around 500 nm. The phase identity of the bulk ribbon and of the surface layer are confirmed with selected area electron diffraction patterns (SADP). The Ni₃Ga phase within the body of the ribbon is pictured in the BF image (e) and it is confirmed along the [011] zone axis what is evidenced on the corresponding SADP (f). Whereas, the Ga₂O₃ phase visible in the BF image (g) is confirmed along the [145] zone axis determined based on the corresponding SADP (h).

Fig. 3

STEM-HAADF micrograph (a) and the corresponding elemental map distributions (b)–(d), BF images (e), (g) and the corresponding selected area electron diffraction patterns (f), (h) taken from the Ni₇₀Ga₃₀ ribbon after oxidation heat treatment.

A different situation is observed in the case of Ni₇₀Sn₃₀ ribbon. From the STEM-HAADF image (a) it is clear that the ribbon has decomposed into Sn-rich and Ni-rich phases and Ni is found to segregate to the outer ribbon surface, where it produces a continuous and thick, of approximately 1 µm, NiO layer. The extent of decomposition and oxidation is considerable as is evidenced by elemental map distributions (b)–(d). The presence of oxide phase in agreement with XRD is confirmed from electron diffraction. Figure 4(e) shows a BF image and the corresponding SADP of the NiO phase indexed along the [001] zone axis (f). The Fig. 4(g) on the other hand depicts a BF image taken from the SnO₂ phase, which is confirmed from SADP along the [001] zone axis.

Fig. 4

Similar decomposition behaviour to Ni₇₀Sn₃₀ is also noticed in the case of the Ni₇₀In₃₀ ribbon (Fig. 5). From the STEM-HAADF image (Fig. 5(a)) and the corresponding element distribution maps (Fig. 5(b)–(d)) one can easily see phase separation and the extent of oxidation. The In₂O₃ phase shown on the BF images (e), (g) is confirmed from the corresponding SADPs along the [112] and [120] zone axes (Fig. 5(f), (h)).

Fig. 5

Surface concentrations of chemical bonds obtained from fitting XPS data for all analyzed samples are listed in Table 2. Within the experiment geometry the information depth of analysis was about 5 nm. Spectra collected in O 1s region are similar for all samples and can be fitted with two lines centered at 529.6 eV and 531.5 eV (Fig. 6–8). First peak can be attributed to lattice oxygen in metal oxides⁹^,¹⁰⁾ whereas second peak to defective oxygen and/or organic contamination.¹⁰^,¹¹⁾ The spectra collected at Ni 2p_3/2 region are similar for all samples. For Ni₇₀Sn₃₀ spectrum is fitted with seven components with first line centered at 852.6 eV which indicate metallic state of Ni, and next six lines which indicate Ni²⁺ oxidation state like in NiO.¹²^,¹³⁾ These six lines within energy range of 853.6–867 eV are due to the multiplet splitting phenomena and additionally point out the existence of Ni²⁺ state. The Ni₇₀In₃₀ and Ni₇₀Ga₃₀ samples are fitted only by components arising from NiO. The Sn 3d_5/2 spectrum for Ni₇₀Sn₃₀ sample was fitted two components: first minor peak at 484.2 eV which indicate metallic state of Sn and second major peak at 485.9 eV which points out existence of either SnO or SnO₂ states of Sn.¹⁴⁾ The Ga 3d spectra was fitted with three components: first peak at 18.1 eV indicate metallic state of gallium, second at 19.8 eV indicate GaO and/or Ga₂O₃ oxides¹⁵⁾ and third peak is a superposition of a O 2s peak.

Table 2 Surface concentrations of chemical bonds as obtained from fitting XPS data.

Fig. 6

XPS spectra of the Ni₇₀Ga₃₀ ribbon for O 1s, Ni 2p and Ga 3d after oxidation at 900°C.

Fig. 7

XPS spectra of the Ni₇₀Sn₃₀ ribbon for O 1s, Ni 2p, Sn 3d and Pb 4f after oxidation at 900°C.

Fig. 8

XPS spectra of the Ni₇₀In₃₀ ribbon for O 1s, Ni 2p and Ga 3d after oxidation at 900°C.

The In 3d_5/2 spectrum for Ni₃In sample is decomposed into two peaks where first peak lies at 444.1 eV indicating In₂O₃ state of In and second peak at 445.2 eV which point out the existence of In(OH)₃ component. For samples Ni₇₀Sn₃₀ and Ni₇₀In₃₀ a small amount of Pb was found and the corresponding spectra were similar – fitted with single 4f doublet structure with main 4f_7/2 peak positioned at 138.0 eV which indicate PbO existence. For sample Ni₇₀In₃₀ a small amount of Bi was found – the Bi 4f spectrum was fitted with two 4f doublet structures with first 4f_7/2 peak at 157 eV indicating metallic state of Bi and second 4f_7/2 peak at 159 eV originating from Bi₂O₃. The small carbon content on the surface is typical for air-handled samples and comes from contamination.

TPR measurements have been performed on all the oxidized ribbons in order to confirm the reduction temperature. The results of TPR experiments are shown in the Fig. 9. The Ni₇₀In₃₀ and Ni₇₀Sn₃₀ ribbons show strong reduction peaks at 750°C and 450°C. The observed peaks should correspond to reduction of NiO to metallic Ni.⁸⁾ On the contrary the Ni₇₀Ga₃₀ ribbon shows a small peak at around 100°C. The lack of distinct reduction peak for the Ni₇₀Ga₃₀ ribbon is due to minor amount of NiO present on the ribbon surface, which is covered with solid and continuous Ga₂O₃ layer in consistence with XRD and TEM analysis.

Fig. 9

TPR profiles of Ni₇₀Ga₃₀, Ni₇₀Sn₃₀ and Ni₇₀In₃₀ ribbons oxidized in air at 900°C.

Preliminary results for the reaction of selective hydrogenation of phenylacetylene to styrene run at room temperature for 4 h are shown in Fig. 10. Before the reaction the ribbons have been reduced at the determined reduction temperatures with exception for Ni₇₀Ga₃₀ because of the limitations of the reduction temperature set up, thus we only present results for the Ni₇₀Sn₃₀ and Ni₇₀In₃₀ ribbons. Following reduction and prior to catalytic testing the ribbons have been pulverised and etched with NaBH₄. The results for oxidized ribbons are compared with results for unoxidized ribbons. From the Fig. 10 one can notice that the selectivity and conversion slightly deteriorate following oxidation heat treatment, selectivity also decreases when the results are compared with the same output but obtained on unoxidized ribbons. For instance in the case of Ni₇₀Sn₃₀ the conversion rate prior to oxidation is 28% and after 25%, similar for the Ni₇₀In₃₀ before oxidation conversion hits 34% while afterwards 28%. This is most likely due to the solid and continuous oxide layer, which develops on the outer ribbons surface. The results demonstrate that oxidation leading to a layer of fine Ni particles supported on intermetallic base might pave the way for enhanced conversion rate and selectivity. More work is now required to optimise etching scheme for producing more mesoporous like surface layer.

Fig. 10

Molar yield, selectivity and conversion of phenylacetylene to styrene over Ni₇₀Sn₃₀ and Ni₇₀In₃₀ oxidized and unoxidized ribbons. The reaction has been conducted at 60°C and prior to the reaction all the ribbons had been etched with NaBH₄.

4. Conclusions

Exposure to air at 900°C reveals profound differences in oxidation behaviour between Ni₇₀Ga₃₀, Ni₇₀Sn₃₀ and Ni₇₀In₃₀ intermetallic ribbons. The Ni₇₀Ga₃₀ ribbons are easily passivated through development of a continuous Ga₂O₃ oxide layer, which prevents the bulk from further oxidation. Whereas the Ni₇₀Sn₃₀ and Ni₇₀In₃₀ undergo oxidation and decomposition throughout the entire ribbon body. Aside from the primary Ni₃Sn and Ni₃In phases, the oxidized Ni₇₀Sn₃₀ and Ni₇₀In₃₀ ribbons comprise Ni, NiO, SnO₂ and Ni, NiO, In₂O₃ phases, respectively. Most importantly the outer ribbon surface of the two latter ribbons is covered with NiO, which is then reduced to metallic Ni. The results of catalytic performance for the reaction of selective hydrogenation of phenylacetylene to styrene show slight deterioration following heat treatment in comparison to pre-oxidation state. This deterioration is most likely due to the evolution of a solid and continuous layer of Ni on the outer ribbon surface. More work is needed to determine optimum etching treatment of the outer Ni layer to produce a mesoporous structure with enhanced free surface area. Controlled oxidation-reduction heat treatment appears as a feasible way for improving catalytic activity of intermetallic melt spun ribbons.

Acknowledgments

This work was financially supported by the National Science Centre (NCN) Poland within the project No. 2017/25/B/ST8/02804.

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