(Ag)Pd-Fe3O4 Nanocomposites as Novel Catalysts for Methane Partial Oxidation at Low Temperature

Abstract

Nanostructured composite materials based on noble mono-(Pd) or bi-metallic (Ag/Pd) particles supported on mixed iron oxides (II/III) with bulk magnetite structure (Fe₃O₄) have been developed in order to assess their potential for heterogeneous catalysis applications in methane partial oxidation. Advancing the direct transformation of methane into value-added chemicals is consensually accepted as the key to ensuring sustainable development in the forthcoming future. On the one hand, nanosized Fe₃O₄ particles with spherical morphology were synthesized by an aqueous-based reflux method employing different Fe (II)/Fe (III) molar ratios (2 or 4) and reflux temperatures (80, 95 or 110 °C). The solids obtained from a Fe (II)/Fe (III) nominal molar ratio of 4 showed higher specific surface areas which were also found to increase on lowering the reflux temperature. The starting 80 m² g^-1 was enhanced up to 140 m² g^-1 for the resulting optimized Fe₃O₄-based solid consisting of nanoparticles with a 15 nm average diameter. On the other hand, Pd or Pd-Ag were incorporated post-synthesis, by impregnation on the highest surface Fe₃O₄ nanostructured substrate, using 1-3 wt.% metal load range and maintaining a constant Pd:Ag ratio of 8:2 in the bimetallic sample. The prepared nanocomposite materials were investigated by different physicochemical techniques, such as X-ray diffraction, thermogravimetry (TG) in air or H₂, as well as several compositions and structural aspects using field emission scanning and scanning transmission electron microscopy techniques coupled to energy-dispersive X-ray spectroscopy (EDS). Finally, the catalytic results from a preliminary reactivity study confirmed the potential of magnetite-supported (Ag)Pd catalysts for CH₄ partial oxidation into formaldehyde, with low reaction rates, methane conversion starting at 200 °C, far below temperatures reported in the literature up to now; and very high selectivity to formaldehyde, above 95%, for Fe₃O₄ samples with 3 wt.% metal, either Pd or Pd-Ag.

Keywords: Ag; EDS; Fe3O4; Pd; Raman; TG in air; TG in hydrogen; XRD; electron microscopy; formaldehyde; heterogeneous catalysis; low-temperature activity; magnetite iron oxide; methane; nanocomposite; oxidation catalysis; palladium; silver.

Figure 1

XRD patterns of magnetite solids prepared by coprecipitation employing a Fe³⁺/Fe²⁺ aqueous molar ratio of 2 (a–c) or 4 (d–f), at different synthesis temperatures: 110 °C (a,d), 95 °C (b,e), or 80 °C (c,f). Symbols: Fe₃O₄ (- - -), α-Fe₂O₃ (‒ · ‒ · ‒).

Figure 2

(a) Field emission scanning electron microscopy (FESEM) micrograph of magnetite solid precipitated from a reflux synthesis at 80 °C using a nominal Fe³⁺/Fe²⁺ molar ratio of 4. Image conditions: in-lens mode, work distance 2.7 mm, energy selective backscattered (EsB) grid of 300 V and electron high tension (EHT) of 1 kV. (b) Statistic distribution of particle diameter from several FESEM micrographs of the magnetite solid shown in (a).

Figure 3

XRD patterns of nanocomposite catalysts based on magnetite (from 85 °C synthesis using Fe³⁺/Fe²⁺ molar ratio of 4) doped with Pd or Pd_-Ag: 1% Pd-Fe₃O₄ (a), 2% Pd-Fe₃O₄ (b), 3% Pd-Fe₃O₄ (c), 3% AgPd-Fe₃O₄ (d). Symbols: (- - -) Fe₃O₄ (ICSD: 01-075-1609); the highlighted strip defines the region where the highest intensity diffractions for pure Pd, pure Ag, and AgPd alloys are found [41,42,43,44].

Figure 4

Z-contrast micrographs obtained by high-resolution field emission scanning electron microscopy (HRFESEM) using an energy selective backscattered (EsB) electron in-lens detector for the Fe₃O₄-based catalysts doped with 1% Pd (a), 2% Pd (b), and 3% Pd (c).

Figure 5

HAADF-STEM micrograph of 3% AgPd-Fe₃O₄ sample, with SA-EDS analysis spots marked and numbered: chemical analyses in brighter particles (1–5 spots) showed AgPd wt.% >12, with an averaged Ag:Pd atomic ratio of 0.2:0.8 (0.1 SD), while in 6–10 spots AgPd wt.% <0.9.

Figure 6

Catalytic activity (a) and selectivity to formaldehyde (b) for methane partial oxidation over the (Ag)Pd-Fe₃O₄ nanocomposite catalysts at 200 °C (light color bars) and 250 °C (dark color bars). Reaction conditions: 100 mg catalyst mass, CH₄/O₂/He molar ratio of 32/4.3/63.7, and contact time (W/F, at standard conditions) of 2.6 g_cat. h mol_CH4⁻¹.

Figure 7

(a) Raman spectra of pristine Fe₃O₄-based substrate (a1), 3% Pd-Fe₃O₄ (a2), and 3% AgPd-Fe₃O₄ (a3) nanocomposite catalysts, collected at 0.6 mW. (b) High-resolution Raman spectrum and its second derivative by the Savitzky–Golay algorithm, using 81 convolution points of the Fe₃O₄-based substrate submitted to identical reduction treatment (250 °C/2 h in H₂) as the noble metal-containing catalysts. Acronyms from (a): Mh (maghemite), H (hematite), and M (magnetite) label the dotted lines according to the assignment ranges found in the literature (references at the end of the footnote). Symbols from (b): numerical ranges together with the black and grey labels define the regions where maxima have been reported in the literature for maghemite [33,61,62,63,64,65] and magnetite [33,34,58,59,60,61,62,63].

Figure 8

Thermogravimetry analysis curves collected in H₂ for the different (Ag)Pd-Fe₃O₄ catalysts prepared and for the pristine metal-free Fe₃O₄ substrate (a), and their first derivative (b).

Figure 9

The first derivative of the thermogravimetry (TG) curves collected under synthetic air flow for the monometallic 3% Pd-Fe₃O₄ and the bimetallic 3% AgPd-Fe₃O₄ catalysts, as well as for the pristine metal-free Fe₃O₄-based substrate.