A review on transition metal oxides in catalysis

Abstract

Transition Metal Oxides (TMOs) have drawn significant attention due to their diverse range of properties and applications. The partially filled d orbitals of the transition metal ions, with highly electronegative oxygen atoms, give rise to unique electronic structures that lead to multiple applications due to their magnetic, optical, and structural properties. These properties have a direct influence on chemical reactions that enable tailoring materials for specific applications in catalysis, such as electrocatalysis and photocatalysis. While the potential of TMOs is promising, their development for enhanced functional properties poses numerous challenges. Among these challenges, identifying the appropriate synthesis processes and employing optimal characterization techniques are crucial. In this comprehensive review, an overview of recent trends and challenges in the synthesis and characterization of highly functional TMOs as well as ceramics will be covered with emphasis on catalytic applications. Mesoporous materials play a key role in augmenting their functionality for various applications and will be covered. Ab-initio modeling aspects for the design and development of novel TMO will be also discussed.

Keywords: coating; density functional theory; electrocatalysis; high temperature ceramic composites; photocatalysis; porous transition metal oxides.

FIGURE 1

Structure and nanostructure of the h’-WO₃ framework. (A) SEM and (B) HRTEM images showing nanosized platelets. (C) The tungsten octahedra along the c axis. Figure adapted from Ref. Lin et al. (2021).

FIGURE 2

Mechanochemical nano casting route for the synthesis of porous metal oxide.

FIGURE 3

Mechanism of photoinduced electron transfer and charge recombination routes of (A) WO₃, (B) Ov-WO₃, and (C) Ov-WO₃-Pt. The blue, green and magenta colors indicate the valence band, conduction band and surface oxygen-vacancy states, respectively. The figure is adapted from Ref. Zhang et al. (2018).

FIGURE 4

XRD patterns (left panel) and SEM images (right panel) of synthesized α-MoO₃ with multiple etching durations. Sample-1 is the pristine MoO₃ (no etching), Sample-2: etching for 10 min, Sample-3: etching for 20 min. The XRD patterns are plotted around (020) diffraction peak (Zhang et al., 2018). Figure adapted from Ref. Solsona et al. (2001).

FIGURE 5

Cross sectional SEM image of carbon fiber with SiC coating of CMC.

FIGURE 6

Schematic diagram of TBCs and EBCs (Thickness of the layers are not depicted in correct proportions).

FIGURE 7

(A) Stable adsorption configuration of benzylamine molecule on meso-Co-MnO_x(100) surface (Dutta et al., 2018). (B) Reaction pathways for water (O–H) bond breaking on spinel sulfide (red line) and spinel oxide (green line) surfaces with the corresponding transition state geometries (Miao et al., 2017a). (C) Charge density difference for adsorbed water on FeS₂ (100) surface (Miao et al., 2017b). Red and turquoise balloons denote the charge accumulation and charge depletion regions, respectively. (D) The electronic density of states of Pd-CeO₂(111) after H₂ adsorption (Sahoo et al., 2021). (E) Methane cracking reaction demonstrating the multi-state reactivity with Fe catalyst Sahoo et al. (2016).

FIGURE 8

The energy diagram demonstrating the change in metal d-band in the 2p-states of adsorbed oxygen at a transition metal surface. As shown in the left panel, prior to the mixing with metal d-band, the O 2p-states hybridize with the metal sp-band, resulting in the O 2p-state. These states interact with the metal d-band as shown in the right panel, to form bonding and antibonding states, which are projected onto the O 2p-states or metal d-states as indicated by the green peaks and orange peaks, respectively.