Small but mighty: unlocking the catalytic power of individual iridium atoms on titanium oxide

Single atom catalysts (SACs) have emerged as a rapidly developing field of catalysis research, with great potential for improving the efficiency and selectivity of many chemical reactions. SACs consist of isolated metal atoms dispersed on a support material, providing a unique and well-defined atomic structure and composition, allowing for precise control over their properties. In particular, iridium SACs are applied in numerous reactions, from electrocatalytic to photocatalytic applications. By the other hand, titanium oxide is a semiconductor with important applications as a reducible support for different catalyst, widely used in different reactions because of its high activity and stability. This review covers recent developments and frontiers in the particular system of Ir – TiO₂ SACs. It discusses the importance, synthesis, characterization techniques such as XPS, STEM, Differential Reflectance Infrared Fourier Transform, and XAS, and the applications of Ir – TiO₂ SACs. The review also explores the stability and durability of single-atom catalysts and the importance of understanding their structure-activity relationships to optimize their performance. A key dimension emphasized in this review is the importance of investigating the iridium-titania system. Iridium exhibits superior properties compared to other metals, particularly in maintaining stability as a single atom, owing to its resistance to sintering. Gaining a comprehensive understanding of and optimizing these factors are instrumental in unlocking the full potential of Ir – TiO₂ SACs. This route offers a promising trajectory towards enhancing catalytic performance across a spectrum of applications and facilitating the discovery of novel chemical reactions.

GRAPHICAL ABSTRACT

GRAPHICAL ABSTRACT | Ir catalyst supported by TiO₂.

1 Introduction

Catalysis plays a central role in shaping our lives, over 90% of all chemical products have at least one catalytic step in their manufacture (de Vries and Jackson, 2012). Supported metal nanostructures are the most used form of heterogeneous catalyst in the industrial processes. Considerable endeavors have been dedicated to enhancing the effectiveness of supported metal catalysts downsizing the metal particles (Yang et al., 2013). The size of metal particle is a critical factor in determining the effectiveness of the catalysts, new findings from both theoretical and experimental studies have shown that clusters with sizes smaller than a nanometer exhibit enhanced catalytic activity and/or selectivity compared to particles of nanometer-scale dimensions (Herzing et al., 2008; Turner et al., 2008). There have been reports indicating that active sites often correspond to low coordination sites, such as unsaturated atoms (Remediakis et al., 2005). The size reduction also benefits the metal-support interactions, this phenomenon stems from the chemical bonding effect between the metal and the supports, as well as the associated interface, along with the charge transfer taking place between metal species and supports (Yang et al., 2013).

Late transition metals that are dispersed at the atomic level are often referred to as single atom catalysts (SACs). The SACs can be defined as a catalyst with isolated single atoms anchored onto its surface, capable of driving a catalytic reaction (Qiao et al., 2011). Generally, the single atoms are randomly and uncontrollably dispersed on the substrates. The interaction of these single-atoms with a metal oxide support can be different depending on its environment. The type of metal-support interactions regulates the electronic structure of catalysts, affecting the intrinsic activity of active sites (Samantaray et al., 2020). SACs can differentiate from other related single-site catalysts such as organometallic catalysts, in which in the latter case an organometallic complex are used to form the single-atom site, but keeping part of their ligands to achieve the catalytic reaction (Shan et al., 2022). SACs epitomize the utmost dispersion of a metal on a surface, with all the atoms being exposed. This holds paramount importance for the expensive and scarce noble metal catalysts, such as Ir, Pd, and Pt, which not only find the most applications but also attract significant interest for research purposes.

The significance of developing SACs becomes evident. Although the concept of having uniform active sites is highly appealing, the support structure in the vicinity of the single atom (SA) introduces heterogeneity to the active sites of the single atom catalyst (SAC, perhaps better conveys that the individual metal atoms are situated on the surface of another material) (Kottwitz et al., 2021). Consequently, the ideal uniform activity, which is characteristic of homogeneous catalysts, cannot be achieved. The combination of isolated single metals atoms with supports, gives rise to a distinct classification of catalysts. The synthesis of SAC presents a challenge, especially regarding the proper characterization of these materials, which is known to be inherently challenging owing to maximized atom utilization and precisely defining active centers, the direct observation of single atomic sites was not realized until the use of advanced characterization techniques such as atomic-resolution high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) which allowed imaging with atomic resolution (Li et al., 2019) that can directly measure the single atom to confirm the structure and conformation of the single metal atom, the chemical state of the metal center, and the conformation environment (Zonghua et al., 2020).

Supports play a critical role since they provide stability and enhance the efficiency of the catalysts. A common support is TiO₂ which has exhibited remarkable and intriguing properties. TiO₂ is a metal oxide with important applications in photocatalysis and oxidation reactions. Iridium, in turn, possesses excellent catalytic properties but its main limitation is its scarceness and high cost. Hence, iridium-based SAC improves the use of resources and, in combination with TiO₂ supports, represent excellent alternatives for the enhancement of catalytic activities towards different important reactions.

In this mini review, we centered in the recent developments and frontiers in the synthesis and characterization of SACs using TiO₂ as support, focusing on the stability and durability of SAC on TiO₂ and the importance of understanding their structure-activity relationships to optimize their performance. Despite the wide literature reviewing the field of SACs on different supports, this review gains importance in highlighting the contributions of TiO₂ supports forming SACs, in which much more limited information is available. First, a general view of the experimental finding about Ir as SACs is provided, reviewing its importance in catalytic reaction. Some common synthetic approaches to obtain SACs are summarized. Then, the experimental findings on Ir – TiO₂ SACs are introduced, discussing the common characterization techniques. Finally, applications and challenges of Ir – TiO₂ SACs and other metal oxide supports are reviewed.

2 Experimental findings about iridium as SACs

Fabrication of SACs is hard due to the tendency of single atoms to aggregate adds an additional layer of complexity, making it an even greater challenge. Elements with extremely low abundance on Earth are often the most active in catalytic reactions. This is precisely why optimizing the utilization efficiency of metal atoms and providing a greater number of exposed sites for reaction activation are crucial factors.

Iridium, the element with the 77th position on the periodic table, is classified among the 5 days transition metals. It crystallizes in a face-centered cubic (FCC) structure, but it stands as one of the most incompressible elements. Notably, iridium shares this distinction with other common metal centers that exhibit activity in numerous catalytic reactions. However, despite its potential, iridium has been limited in its exploration due to its scarcity in the Earth’s crust and its high cost. As a result of their ability to undergo easy changes in oxidation state, iridium and its complexes find extensive use in catalysis for a wide range of industrial processes.

Single atom catalysts (SACs) offer a remarkable advantage in achieving 100% atomic utilization, serving as an exemplary strategy to significantly reduce noble-metal content without any compromise on catalytic efficiency. Hence, the study of metals, such as atomically dispersed iridium, does not impede the sustainable utilization of the metal. This exceptional property allows it to maintain its mechanical properties even at temperatures exceeding 2000°C and pressures up to 1.4 Mbar. Furthermore, iridium exhibits outstanding resistance to corrosion, making it highly suitable for a wide range of applications (Monteseguro et al., 2020).

Noble metals of group VIII, such as Pd, Rh, Pt, and Ir, present outstanding catalytic activities, that together with transition metal oxide support catalyzes a wide number of reactions. In particular, iridium possesses a higher atomic mass compared to Pd and Rh, and it is the group with higher melting point. It possesses a versatile coordination chemistry, facilitating its dispersion in different metal oxide supports, such as MgO, FeOx, or MgAl₂O₄, with a strong interaction with the support (Lin et al., 2013; Pascarelli et al., 2019). For instance, Ir single-atom coordination with reduced Fe₂O₃, MgO, and TiO₂, corresponds to a substitutional configuration on the cation vacancies, with Ir coordinated with two oxygen atoms, which number could change after reaction (Xiao et al., 2019). A synergetic affect take place between the single atoms and their supports, lowering the reduction temperature of the support and generating a large amount of oxygen vacancies which contributes to their activity. The reducibility of the support also contributes to the stability of the single atoms, exhibiting high activity and selectivity.

Despite of this, few studies exist on Ir SACs in comparison with these other metals of the group, finding most applications in electrocatalysis. In this review, we aim to highlight the significance of studying this uncommon element when it is considered in its atomic form.

The pioneers in the field of catalysis utilizing iridium as an active center have posited that the success in fabricating single atom catalysts with iridium may be attributed to the remarkably low loading of the metal (Lu et al., 2019a). The probability of encountering short-range and long-range iridium-iridium interactions is estimated to be as minimal as 0.001 atoms nm^-1. Furthermore, upon increasing the iridium loading to 0.32%, a corresponding rise in observation frequency within the range of 0.5–1 nm was noted. Remarkably, no aggregation to sizes exceeding 1 nm was observed (Cao et al., 2019) therefore, it is advantageous to propose monodispersed iridium catalysts.

2.1 Summary of iridium as single atoms

In Table 1, examples of synthesis are presented where iridium has been used as a metal center on various supports, along with the synthesis routes employed and their applications.

TABLE 1

TABLE 1. Summary of catalysts using iridium and different supports.

Currently, there is ongoing research into synthesis methodologies aimed at creating Single Atom Catalysts, with wet impregnation being identified as the most promising approach. Although wet impregnation offers simplicity and scalability, it is important to acknowledge certain limitations. These include the challenge of achieving optimal metal loading and precise control over the anchoring position of isolated metal species on substrates, which can impact both catalytic efficiency and the feasibility of practical industrial applications.

2.2 Synthesis

The exploration and development of synthetic methodologies for single atom catalysts have become one of the most crucial research focuses. Fabricating single atom catalysts and maintaining the atomic dispersion of metal species under realistic synthesis and reaction conditions pose significant challenges. From a practical perspective, an alternative and preferable approach is the development of wet-chemistry synthetic methods for single atom catalysts (SACs). This method offers easy operation and the potential for large-scale manufacturing. In wet-chemistry synthesis, mononuclear metal species are typically used as precursors. Therefore, implementing synthetic strategies to achieve atomically dispersed separation and isolation of the precursor, as well as preventing the migration and agglomeration of the formed single atoms, becomes crucial for successful SAC synthesis. These considerations are fundamental in ensuring the efficient and controlled production of SACs (Chen et al., 2018). Ir precursors are deposited onto the substrate surface using various wet-chemistry methods. This involves the metal precursors that are dispersed onto substrates through deposition-precipitation, coprecipitation, or wet-impregnation. Subsequently, reduction or activation procedures are carried out to form Ir-based catalysts with atomically dispersed Ir species. These steps are crucial in achieving the desired dispersion and isolation of individual Ir atoms on the substrate, enabling the synthesis of highly efficient catalysts with unique properties and reactivity (Pham et al., 2023).

2.2.1 Advantages and disadvantages of various synthetic approaches of SAC’s

In Table 2, some advantages and disadvantages of various synthesis methods for obtaining SACs are presented (Pham et al., 2023).

TABLE 2

TABLE 2. A comparison of different synthetic strategies, offering a comprehensive overview of their individual advantages and disadvantages.

The low metal content is mentioned as a disadvantage; however, for SACs, it is a notable advantage due to the reduced use of precious metals. Additionally, the possibility of industrial scalability is emphasized. Additionally, the potential for industrial-scale production is highlighted.

2.2.2 Deposition-precipitation with urea

Deposition-precipitation is a synthesis method commonly used in the preparation of catalysts. It involves the deposition of metal precursors onto a support material, followed by the precipitation of the metal species in the presence of a precipitating agent. This method allows for controlled and uniform distribution of the metal species on the support, resulting in catalysts with well-defined structures and properties. Deposition-precipitation offers advantages such as easy scalability, versatility, and the ability to tailor the catalyst composition by adjusting the deposition and precipitation conditions (Zanella et al., 2005; Qin et al., 2015).

The deposition-precipitation technique has been widely used to fabricate gold catalysts (Haruta et al., 1987; Haruta, 1997). In this method, the pH of an aqueous solution containing HAuCl₄ is carefully adjusted within the range of 6–10, taking into consideration the isoelectric points (IEP) of the metal oxide supports. This pH adjustment is crucial due to the amphoteric properties of Au(OH)₃, ensuring the formation of stable and well-dispersed gold species on the support material. The DP method offers ease of handling and provides control over the catalyst’s composition and properties, making it a preferred choice in industrial catalyst production (Milone et al., 2010; Bokhimi et al., 2011; Qin et al., 2015).

2.2.3 Impregnation

The impregnation method is a technique used to deposit an active compound onto a porous surface or support. It involves immersing the support into a solution containing the active compound and allowing it to impregnate the porous surface through absorption. After impregnation, the support is subjected to drying or thermal treatment to remove the solvent and fix the active compound onto the support. This method offers great flexibility as it allows for control of the loading of the active compound and adjustment of the catalyst properties to specific application needs. It is widely employed in catalyst synthesis, particularly in the preparation of supported catalysts on porous solids (Van Dillen et al., 2003).

2.2.4 In situ techniques

Metal cations are chemically reduced to their metallic state by certain agents. In this state, they act directly as catalysts, facilitating chemical reactions involving the same agents. This process is straightforward and requires no additional purification. Consequently, it prevents potential catalyst loss or oxidation during the transfer process. By embracing this methodology, potential pitfalls associated with ligand presence, challenges stemming from uncontrollable variations in catalyst particle sizes, and concerns regarding mass loss or metal oxidation during purification and transfer procedures are all expertly sidestepped (Erdoğan et al., 2009; Liang et al., 2012; Cui et al., 2022; Zhang et al., 2022; Sun et al., 2023).

3 Experimental findings TiO₂ as support for Ir

Support materials play a crucial role in the stability and catalytic performance of single atom catalyst. These catalysts heavily rely on the support to provide a solid foundation for the deposition of active metal species. The choice of an appropriate support is of paramount importance as it directly influences the dispersion, accessibility, and reactivity of the active sites. Additionally, the support acts as a vital stabilizing agent, preventing the coalescence or aggregation of individual metal atoms and ensuring their sustained single-atom state during catalytic reactions. By offering a substantial surface area for the immobilization of metal atoms, the support facilitates efficient interactions with reactants, thereby enhancing catalytic reactions. Thus, meticulous selection and thoughtful design of suitable support materials are indispensable for the successful utilization of single atom catalysts in diverse catalytic applications.

Titanium is a metallic element that occupies the 22nd position in the periodic table. It is highly abundant in the Earth’s crust and exists in three main crystal forms: anatase, rutile, and brookite. Titanium is known for its exceptional strength-to-weight ratio, corrosion resistance, biocompatibility, affordability, and eco-friendliness (Ramos-Delgado et al., 2016). These properties make it a valuable material in various industries, as it offers a cost-effective and environmentally friendly solution. Titanium also serves a significant role in catalysis. TiO₂ are employed both as supports for catalysts and as catalysts themselves. These catalysts, including metal/TiO₂ and metal oxide/TiO₂ composites, play a significant role in diverse reactions such as hydrogenations, hydrodesulfurizations, selective oxidations, reductions, and Fischer-Tropsch processes. An additional notable feature lies in its capacity as a photocatalyst, which stems from its inherent semiconducting properties (Kominami et al., 1997; Scirè et al., 2021).

The proper selection of a catalyst support is crucial for enhancing the dispersion of active components and modulating the catalytic functionalities through metal-support interactions. The catalyst support significantly influences the overall performance and stability of the catalyst system, allowing for improved control over reaction rates and selectivity. It acts as a substrate that facilitates the anchoring and distribution of active species, providing a stable environment for catalytic reactions (Palcheva et al., 2013). The interaction between metal/TiO₂ has been studied as SMSI effect. The finding of heteroatomic metal-metal bonding suggested that titanium cations at surfaces might be capable of bonding to metal cations or metal atoms in a supported phase due to of the possibility of an interaction between the d orbital electrons of the surface cations and those of the supported metal atoms (Tauster et al., 1981).

As mentioned previously, other supports such as MgO, ZIF-8, and Al₂O₃ have been extensively explored in the literature for supporting iridium catalysts. However, titanium dioxide (TiO₂) has not been the preferred choice as a support material, which presents an area of opportunity for further investigation. TiO₂ offers distinct advantages, including its large surface area and high stability. Moreover, the use of the rutile phase of TiO₂ has been found to enhance the dispersion of the metal compared to other phases. The rutile phase provides a favorable environment for the dispersion of iridium species (Kim et al., 2018) promoting their accessibility and reactivity in catalytic reactions. This unique characteristic of the rutile phase, combined with the advantageous properties of TiO₂ as a support, makes it a promising candidate for supporting iridium catalysts. Exploring the potential of TiO₂, particularly its rutile phase, as a support material for iridium catalysts holds great promise in the development of highly efficient and selective catalytic systems.

4 Comprehending the nature of catalytic sites on the surface

X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), including aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), diffuse reflectance infrared Fourier-transform spectroscopy (DRIFT), and X-ray absorption spectroscopies (XAS) are the key techniques employed for the analysis and authentication of iridium single atom catalysts (SACs). XPS allows for the investigation of surface composition and chemical states. HAADF-STEM provide atomic-scale resolution to directly examine the structure and coordination environments of isolated iridium atoms. DRIFT enables the study of molecular adsorption and surface species reactivity. With XAS it is possible to investigate a broad spectrum of surface structures. These techniques collectively play a pivotal role in the comprehensive characterization and understanding of iridium-based catalysts.

4.1 X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) is a highly valuable technique employed in catalysis research for the analysis of catalyst surfaces. It serves the purpose of examining the surface composition and chemical states of catalyst materials. By utilizing X-ray photons to stimulate the ejection of inner-shell electrons from atoms within the catalyst, XPS enables the measurement of electron kinetic energy and intensity. This data provides crucial insights into the elemental composition, oxidation states, and bonding environments present on the catalyst surface (Greczynski and Hultman, 2020).

In the realm of catalysis, XPS plays a pivotal role in elucidating surface chemistry and catalytic reactivity. It facilitates the investigation of active sites and their electronic structures, which are fundamental to catalytic processes. By employing XPS, researchers can explore changes in oxidation states of catalytic metals under reaction conditions, observe adsorbed species or reaction intermediates, and analyze the effects of environmental factors such as temperature and pressure on the catalyst surface (Oswald et al., 2013).

Moreover, XPS proves to be a valuable tool for pre- and post-reaction analysis of catalysts, allowing the detection of surface modifications, elucidation of catalyst deactivation mechanisms, and determination of catalyst stability. The technique can also be utilized for in situ characterization, enabling real-time monitoring of catalyst performance during catalytic reactions.

In summary, X-ray photoelectron spectroscopy is an indispensable technique in catalysis research, offering valuable insights into the surface chemistry and reactivity of catalysts. Its ability to probe the atomic-scale properties of catalyst surfaces provides crucial information for understanding catalytic mechanisms, optimizing catalyst design, and advancing the development of more efficient and selective catalytic processes. XPS enables the determination of the oxidation state of iridium, providing valuable insights into its chemical state.

In Figure 1A, Yiming Zhu et al. shows the XPS spectrum of IrO₂ displays a characteristic doublet at 62.5 and 65.4 eV, corresponding to the presence of Ir⁴⁺ species. On the other hand, for Ir-Co₃O₄, the Ir 4f spectrum exhibits two sets of doublets centered at 61.8/63.7 eV and 62.4/65.3 eV. These doublets can be attributed to the coexistence of Ir⁴⁺ and Ir³⁺ species, respectively.

FIGURE 1

FIGURE 1. (A) XPS spectrum of characteristic Ir⁴⁺. Adapted from Zhu et al. (2022b), licensed under CC BY 4.0. (B,C) Spectra of Ir⁰, Ir⁴⁺, and the overlap of Ti 3s. Adapted from Lee et al. (2018), licensed under CC BY 4.0.

In Figures 1B, C Lee et al. (2018) shows the spectra of the Ir 4f_5/2 and 4f_7/2 core level peaks are presented in Figures 1B, C. The deconvolution of the peak profile revealed the presence of both Ir⁴⁺ and Ir⁰ components in the Ir:SrTiO₃ film. By measuring spectra at different takeoff angles, the depth distribution of the Ir⁴⁺ and Ir⁰ components can be estimated due to the limited inelastic mean free path of photoelectrons in XPS measurements. Measurements performed at higher takeoff angles, which are more surface-sensitive, demonstrated a higher Ir⁰ ratio (∼13%) compared to measurements taken at a 45° angle (∼10%). Interestingly, while tetravalent Ir⁴⁺ is expected based on DFT calculations for iridium atoms substituting Ti, the presence of metallic Ir⁰ was observed.

X-ray photoelectron spectroscopy (XPS) analysis reveals the presence of metallic iridium supported on TiO₂, which holds significant importance as it signifies the catalytically active state of the material. The observation of metallic iridium on the surface of TiO₂ confirms its availability as an active species for catalytic reactions. Furthermore, the ability to deposit isolated iridium atoms at low metal loadings underscores the potential for achieving single-atom dispersion. This discovery unveils promising avenues for leveraging the distinctive reactivity and selectivity of single atom catalysts supported on TiO₂, thereby offering compelling prospects for catalytic applications.

4.2 Aberration-corrected scanning transmission electron microscopy

Transmission electron microscopy (TEM) is an advanced scientific instrument that employs a focused and accelerated beam of electrons to obtain detailed and high-resolution images of samples at the micro and nanoscale. By passing the electron beam through the sample, TEM can generate valuable information about the sample’s internal structure, composition, and morphology. This technique relies on the interaction between the electrons and the atoms in the sample, producing signals that are captured by detectors and transformed into visual representations. With its exceptional resolving power, TEM enables researchers to explore the intricate details of materials, including the arrangement of atoms and the presence of defects or nanoscale features. There are two main transmission modes in TEM: the conventional TEM and the scanning transmission electron microscopy (STEM).

In conventional TEM, electrons coming from the electron source are focused on the sample in a wide and quasi-parallel beam, formed by the illumination system composed by electromagnetic condenser lenses, Figure 2A. These lenses are the responsible of forming the incoming beam and the manner of how it hits on the sample. Once the electron beam passes through the sample, the transmitted beam is collected by the objective lens and projected to the screen. An image of the sample is instantaneously produced of all the illuminated area.

FIGURE 2

FIGURE 2. Sketch of the two main modes in TEM: conventional TEM (A) and STEM (B).

The incident electrons are described as a plane electron wave $\psi \left(r\right)$, which interacts with the sample and undergoes different dispersion phenomena. The exit electron wave at the end of the specimen plane, described as ${\psi }_{exit}\left(r\right)$, is formed by interference of the waves scattered by the atoms of the sample. These scattered waves are collected by the objective lens and focused at its back focal plane (BFP). If the lens is ideal and in focus, the recovered wave at the image plane, ${\psi }_{im}\left(r\right)$, should be:

${\psi }_{im}\left(r\right)={\psi }_{exit}\left(r\right)$

And the intensity distribution will be

$I_{im}\left(r\right)={\psi }_{im}\left(r\right){\bullet \psi }_{im}^{*}\left(r\right)=1$

However, there is no perfect lenses, so the wave at the image plane will be:

${\psi }_{im}\left(r\right)={\psi }_{exit}\left(r\right)\otimes OL\left(r\right)$

where OL(r) is a function describing the behavior of the objective lens (Pennycook and Nellist, 2011; Deepak et al., 2015).

The phase shift produced by the lens can be described as (Carter, 2009):

$\chi \left(u\right)=\pi ∆f\lambda u^2+\frac{1}{2}\pi C_s{\lambda }^3{u}^4$

Therefore, it is observed that the final image will depend on how the electron beam interacted with the sample and the phase shift produced by the objective lens, which in turn depends directly on the defocus Δf, the wavelength of the incoming electrons λ, and the spherical aberration constant, Cs. The interpretation of high-resolution TEM images is not straightforward because the image contrast will depend on our optical system, defocus conditions, sample thickness, and aberrations.

In STEM, instead of a wide and parallel beam, a highly focused convergent beam of electrons is scanned over the sample, Figure 2B. The electron probe is formed by the interference of an infinite number of plane waves at all the different convergence angles forming the illumination cone. No lenses are present below the sample, so the electrons that have interacted with the sample are collected directly by different detectors placed below the sample plane. The coherent beam is focused on each point of the sample, so the diffracted beams form discs that interfere to each other and provides the contrast interpreted as images, Figure 3A.

FIGURE 3

FIGURE 3. (A) The convergent electron probe form diffracted discs. (B) Schematics of the reciprocal and real space of the electron probe formed by coherent plane waves passing through an aperture defining a semi angle α. (C) Diffracted electrons from the sample are collected by detectors placed at different angles.

The amplitude of the waves is moderated by the partial coherence of the electron beam, which depend on the convergence angle and aperture size which limits the space from $-{\mathbf{k}}_{\max }\text{\hspace{0.17em}to\hspace{0.17em}}{\mathbf{k}}_{\max }\left({\mathbf{k}}_{\max }=\mathrm{\lambda }/\mathrm{\alpha }\right)$, Figure 3B. In real space, the amplitude of the electron probe $P\left(r\right)$ can be described as:

$P\left(r\right)=\int A\left(k\right)\exp \left[i2\pi kr\right]dk$

where $A\left(\mathbf{k}\right)$ corresponds to a function of the circular aperture defining the convergence angle. In STEM, the upper objective lens is the one responsible of forming the electron probe, and will be affected by aberrations. The aberration coefficients have dimensions of length and can be expressed in power series of scattering angle $\theta$ as (Carter and Williams, 2016):

$\gamma \left(\theta \right)=\frac{1}{2}∆f{\theta }^2+\frac{1}{4}{C}_3{\theta }^4+\frac{1}{6}{C}_5{\theta }^6+\frac{1}{8}{C}_7{\theta }^8+\dots$

With $\mathrm{\chi }=2\pi \gamma$. The C_s coefficients are positive values. The amplitude of the STEM probe can be described as:

$P\left(r\right)=\int A\left(k\right)\exp \left[i2\pi kr\right]\exp \left[-i\chi \left(k\right)\right]dk$

And the probe intensity is its square ${\left|P\left(r\right)\right|}^2$.

In an uncorrected probe, the third order spherical aberration (C₃) can be compensated by negative defocus. In a spherical-aberration corrected probe, the next higher order aberration C₅ can be compensated by a slight negative C₃ (Carter and Williams, 2016). The optimal resolution for an uncorrected system can be expressed as:

$d_{opt}=0.43{\lambda }^{3/4}{C}_3^{1/4}$

and for a Cs corrected system, the limiting resolution due to the nth order aberration:

$d_{opt}=0.45{\lambda }^{5/6}{C}_3^{1/6}$

In a 200 keV aberration-corrected microscope, C₃ can be set to positive or negative values close to cero, while for an uncorrected high-resolution microscope C₃ is close to 0.5 mm. The electron probe diameter in STEM can be reduced through the aberration-corrected system, which consists of a hexapole-corrector with two multipole stages and it compensates all aberrations up to third order (CESCOR, 2023). Then, this small probe is scanned through the sample and interact with its atoms, which can be seen as an arrangement of individual scatters. Now the highly-convergent electron beam interacts individually with each scatter (instead of a wide parallel beam interacting with a large number of scatters in TEM). Each atom (a spike of potential) will scatter in proportion to the local probe intensity. Therefore, the intensity of the image can be written as:

$I\left(r\right)=O\left(r\right)\otimes {\left|P\left(r\right)\right|}^2$

i.e., the intensity at the image is the convolution of the object O(r) (array of scatters) and the probe intensity profile P(r). The sharper the probe, the clearer the atoms are seen.

In STEM, when the beam is scanned over the sample, the collection of scattered electrons is done by the detectors placed below the sample, Figure 3C. The angle of scattering will depend on the atomic number Z of its constituent elements. Hence, detectors are placed to collect the scattered electrons at different angles, thus collecting different information of the sample. A bright-field detector (BF) collects those electrons scattered at low angles (∼<10 mrad), and the images are similar to conventional TEM. An annular dark-field detector (ADF) can be placed to collect electrons scattered at higher angles (∼10–50 mrad), which contain information of the chemical composition of the sample. A high-angle annular dark-field detector (HAADF) collects those electrons passing close to the atoms nucleus electrons, being scattered at much high angles (>50 mrad). These high-angle scattered electrons contain pure Z-contrast information from the sample and possess incoherent characteristics (Carter and Williams, 2016; Plascencia-Villa et al., 2020; Plascencia-Villa and Mendoza-Cruz, 2022).

STEM and TEM are powerful techniques widely used in catalysis research. One of the key features of these techniques is their ability to correct spherical aberration, allowing for high-resolution imaging at the atomic scale. With the advent of advanced electron microscopy instruments, aberration-corrected STEM offers the capability to visualize individual atoms and atomic structures in catalyst materials. This level of resolution is particularly valuable in the study of single atom catalysts (SACs), where the dispersion and arrangement of individual metal atoms play a critical role in their catalytic activity.

The aberration-corrected STEM/TEM imaging allows researchers to directly observe the presence and distribution of single atoms on the catalyst surface, and with the advantage of Z-contrast, atomic-resolution structural and chemical information can be obtained in a single image, Figure 4. This information is essential for understanding the relationship between atomic structure and catalytic properties, as well as for investigating the dynamics and behavior of single atoms during catalytic reactions.

FIGURE 4

FIGURE 4. AC-HAADF-STEM measurements were employed to visually inspect the distribution of Ir in the 0.25% Ir/TiO₂ catalyst. Notably, no Ir nanoclusters were observed, and individual Ir atoms were identified as bright dots highlighted in yellow.

Advanced AC-HAADF-STEM measurements were employed to visually inspect the distribution of Ir in the 0.25% Ir/TiO₂ catalyst. Notably, no Ir nanoclusters were observed, and individual Ir atoms were identified as bright dots highlighted in yellow. These Ir atoms were consistently located on the Ti sites of the TiO₂ support.

Individual Ir atoms were observed by AC-HAADF-STEM. The Ir sacs were resolved using an intensity profile since the observation was complicated due to the thickness of TiO₂ crystal. Nevertheless, it is possible to analyze the intensity profile and distinguish the presence of iridium atoms, Figure 5. These Ir atoms were consistently located on the Ti sites of the TiO₂ support.

FIGURE 5

FIGURE 5. AC-HAADF-STEM of Ir in the 0.25% Ir/TiO₂ catalyst and its intensity profile.

4.3 IR/DRIFT

Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) is a powerful analytical technique used in catalysis research for the characterization of catalysts and the investigation of surface species and adsorbed molecules. It utilizes infrared radiation to probe the vibrational modes of molecules on the catalyst surface. By measuring the changes in infrared absorption and reflection, DRIFTS provides valuable information about the surface chemistry, adsorption properties, and catalytic reactions occurring on the catalyst surface. This technique offers insights into the nature of active sites, the interaction between the catalyst and reactants, and the mechanism of catalytic reactions. It is a versatile tool for understanding the structure-function relationships of catalysts and designing more efficient catalytic systems.

DRIFTS works by irradiating the catalyst sample with infrared radiation and analyzing the resulting diffusely reflected light. The infrared radiation consists of a range of wavelengths that corresponds to the vibrational frequencies of chemical bonds in molecules. When the infrared light interacts with the catalyst surface, it is absorbed by the molecules present, causing them to vibrate and undergo changes in their dipole moment. These changes in dipole moment result in the scattering and reflection of the infrared light (Mitchell, 1993).

The diffusely reflected light is collected by a detector, such as a Fourier Transform Infrared (FT-IR) spectrometer, which measures the intensity of the reflected light as a function of wavelength. This spectrum provides information about the vibrational modes of the molecules on the catalyst surface, allowing for the identification of specific functional groups and the determination of surface species (Díaz et al., 2011).

By comparing the DRIFTS spectra of the catalyst before and after exposure to reactants or under different reaction conditions, researchers can gain insights into the adsorption and desorption processes, the formation of reaction intermediates, and the overall catalytic activity. The conducted studies have revealed the absorption band of iridium between 2075 and 2068 cm^-1 (Díaz et al., 2011).

In Figure 6, the DRIFT spectra presented in this study demonstrate the adsorption of CO on catalysts that were reduced in situ in a hydrogen flow at 300°C. The iridium-based catalysts, namely, Ir/TiO₂ and Au-Ir/TiO₂-S, exhibited strong absorption bands in the range around 2100 cm^-1. These bands are characteristic of CO molecules adsorbed on iridium metal. The monometallic Ir catalyst showed a particularly intense absorption band centered at approximately 2068 cm^-1, accompanied by a broad contribution in the low-frequency side. These absorption bands in the region can be attributed to CO linearly adsorbed on various Ir⁰ sites. Additionally, bands characterizing cationic iridium species were also observed in the spectral region between 2000 and 2107 cm^-1 (Gómez-Cortés et al., 2009).

FIGURE 6

FIGURE 6. DRIFT spectra of Ir/TiO₂ 3.7 %wt (Adapted from Gómez-Cortés et al. (2009), with permission from Copyright © 2009 American Chemical Society).

Rojas et al. (2015) showed that the Ir/TiO₂ catalyst exhibited a prominent absorption band with a pronounced peak at 2073 cm⁻¹, indicating the adsorption of CO on Ir⁰ sites. Furthermore, it has been demonstrated that it is possible to deposit isolated atoms on TiO₂.

DRIFT is a valuable technique that plays a crucial role in the comprehensive characterization of the support surface. In the specific case of iridium catalysts supported on TiO₂, DRIFT analysis enables the direct observation of CO absorption bands, providing valuable insights into the presence and reactivity of iridium sites on the catalyst surface. The detection of CO absorption bands signifies that the iridium species maintain their reactivity and availability for catalytic reactions, despite the strong interaction and bonding with the TiO₂ support. This finding highlights the remarkable reactivity of iridium, which positions it as a highly promising catalyst for various catalytic applications. The ability to maintain its reactivity and accessibility on the support surface makes iridium an attractive choice for catalytic transformations, offering opportunities for enhanced catalytic performance and selectivity in various chemical processes.

4.4 X-ray absorption spectroscopies

X-ray absorption spectroscopy (XAS) is currently being utilized to investigate a broad spectrum of surface structures. In the extended X-ray absorption fine structure (EXAFS) regime, single scattering is typically employed, enabling the straightforward determination of near-neighbor distances (R) and coordination numbers (A).

XANES is commonly affected by intra-molecular scattering, potentially providing straightforward insights into molecular orientation, intramolecular distances, and, consequently, the specifics of bonding with the surface. Moreover, it can reveal the atom’s oxidation state, the chemical environment, and electronic transitions occurring near the central atom (Norman, 1986). XAS will only be shown in this review to observe the possibility of iridium binding in TiO₂.

EXAFS specializes in the examination of atomic arrangements around a central atom within a material, offering a meticulous investigation of its local atomic structure. This analytical method delves into the energy region situated beyond the critical point of X-ray absorption. EXAFS has a unique advantage over more standard methods like X-ray diffraction because one can directly determine the location of atoms surrounding each constituent separately (Sayers et al., 1970). In contrast, XANES concentrates on furnishing insights into the valency of atoms and their chemical environment, closely exploring the region proximate to the X-ray absorption edge. It is widely acknowledged that XANES exhibits sensitivity to the adsorption of various substances. Initially, we isolate changes in the metal’s XANES data due to the presence of the adsorbate. This involves obtaining two separate XANES spectra: one for the metal with the adsorbate and another for the metal under different conditions, such as altered potential, current, or in a vacuum. The latter serves as the reference baseline. This technique can be viewed as a subtractive approach, aimed at isolating the influence of the adsorbate. Consequently, the resulting spectral shape provides insights into the chemistry, site symmetry, and quantity of adsorbed species on the surface (Ramaker and Koningsberger, 2010).

These two techniques synergistically complement each other and are jointly employed to achieve a comprehensive understanding of a material’s structure and chemistry (Koningsberger and Prins, 1987).

X-radiation, when passing through matter, is absorbed through various mechanisms, including the photoelectric process, which involves the direct excitation of occupied core electrons to unoccupied levels see Figure 7 (Norman, 1986). The origins of EXAFS stem from fluctuations in the photoelectric cross-section resulting from the scattering of the emitted photoelectron by neighboring atoms encircling the absorbing atom (Sayers et al., 1971).

FIGURE 7

FIGURE 7. Schematic of the X-ray absorption spectrum, highlighting the different regions in which is divided.

XANES is commonly affected by intra-molecular scattering, potentially providing straightforward insights into molecular orientation, intramolecular distances, and, consequently, the specifics of bonding with the surface. XANES directly examines the angular momentum of unoccupied electronic states, which can encompass bound or unbound, discrete or broad, atomic or molecular characteristics (Vaithianathan et al., 2006). Moreover, it can reveal the atom’s oxidation state, the chemical environment, and electronic transitions occurring near the central atom (Norman, 1986). XAS will only be shown in this review to observe the possibility of iridium binding in TiO₂.

The chemical coordination environments of the Ir single atom species were investigated by acquiring extended X-ray absorption fine structure (EXAFS) data at the L₃-edge of the Ir species, using the Ir/AC nano catalyst for comparison with other standard Ir compounds.

Porous organic polymers based on aminopyridine show that Ir’s closest neighbors are Ir-Ir and Ir-O. This result can explain the presence of Ir metal on the surface. The fitted EXAFS data indicate the presence of Ir-Cl, suggesting that the Ir species existed as isolated metal atoms and possibly maintained their high coordination number as a result of ligand exchange with the support. In summary, EXAFS data show that mononuclear Ir is situated on the framework using -Cl₂, -O₂, or -OH groups as ligands. The Ir single atom species was evidently coordinated by two neighboring oxygen nuclei associated with the support (Shao et al., 2019b).

In atomically-dispersed iridium on tin oxide, they performed ex situ and in situ X-ray absorption spectroscopy and have shown data in R-space that correspond to Ir-O, with a lack of strong contributions from Ir-Ir or Ir-O-Ir interactions. They conducted this study to investigate the oxygen evolution reaction (OER) mechanism, for which they computed the mechanisms using the structures of Ir-_SAC-ITO. In these structures, they’ve demonstrated the binding of Ir sites with oxygen. In this study, they observed IrO₂ (Ir_IV). The oxidation states of Ir were assigned as Ir_III, Ir_IV, and Ir_V (Lebedev et al., 2020).

In Figure 8, the researchers constructed computational models based on Ir-SAC, and these models illustrated the coordination of Ir atoms with oxygen sites present on the support material. Within the proposed mechanism, the researchers further elucidated the bonding interactions by showcasing how the Ir sites form bonds with hydroxyl (OH) groups. This detailed analysis provides valuable insights into the intricate chemical processes occurring at the atomic level, shedding light on the catalytic behavior of Ir-_SAC in various reactions, including the oxygen evolution reaction. Computed studies of iridium nanoparticles using density functional theory calculations have shown that the planar configurations are more stable than the three-dimensional ones, with each arranged in decreasing order of stability (Pawluk et al., 2005).

FIGURE 8

FIGURE 8. Computed OER with the structures of Ir-_SAC-ITO. Reproduced from Lebedev et al. (2020), with permission from Copyright © 2023 American Chemical Society, licensed under CC BY 4.0.

In the analysis of Ir/TiO₂ for CO oxidation, both EXAFS and XANES techniques were used to understand the rate-controlling steps for CO oxidation. Ir single-atom sites within the catalyst were found to be uniformly distributed.

DRIFT spectrum of the Ir₁/TiO₂ catalyst in Figure Xb shows two CO bands at 2077 and 1995 cm⁻¹, attributed to Ir gem-dicarbonyl (Ir(CO)_2-support).

In Figure 9 (Xanes region), the EXAFS spectra reveal a dicarbonyl state (two Ir-CO). Besides, the model fit indicates two Ir-O bonds from the support and one Ir-Ti bond. XAS results indicate Ir initially exists as Ir(CO₂(O)₂ after reduction and gets oxidized to Ir(CO) (O)₃ during/after reaction. For the Density Functional Theory (DFT) calculations (Figure 9), we opted for adsorbed Ir single atoms to represent the experimental structure. This choice was made because these Ir single atoms can potentially form a dicarbonyl in the CO-reducing environment. It is important to note that Ir single atoms embedded within the TiO₂ lattice could not maintain a stable dicarbonyl structure.

FIGURE 9

FIGURE 9. Ir/TiO₂ Characterization. (A) Scanning Transmission Electron Microscopy (STEM) image of the 0.1 wt% Ir/TiO₂ catalyst post-CO oxidation kinetic measurements at 170°C followed by CO reduction. (B) Differential Reflectance Infrared Fourier Transform (DRIFT) spectra recorded after CO oxidation at 170°C (blue) and post-reduction in CO at 170°C (red). (C) Ir-L₃ X-ray Absorption Near Edge Structure (XANES) spectra of the catalyst, showcasing the active state (blue) and the reduced state (red). (D) Magnitude component of the Fourier-transformed k3-weighted χ(k) data (Δk = 2.5−12 Å−1). For XANES/EXAFS, the reduced-state spectra were acquired in CO at 180°C, whereas the active-state spectra were recorded in CO and O₂ at 35°C, following CO oxidation at 195°C. (E) Representation of the local structures of Ir₁/TiO₂, combining experimental and theoretical insights. Color coding: O, red; Ti, silver; Ir, blue; C, gray. Reproduced from Coogan et al. (2023), with permission from Copyright © 2023 American Chemical Society.

It is imperative to underscore that this investigation was undertaken using TiO₂ in the anatase phase. Consequently, it is strongly advised to contemplate a parallel inquiry employing rutile TiO₂, as the variations in the crystalline phase can yield distinctive outcomes.

5 Exploring the catalytic capabilities of iridium: applications

Ir-_SACs have been applied widely in numerous applications, in particular, in electrocatalytic reactions, such as ORR, water splitting, CO₂ reduction, HER, epoxidation of ethylene (Yang et al., 2023). Ir on TiO₂ as a SAC, is a more specific system that has been applied in oxidation and reduction reactions, photocatalysis, electrocatalysis. Here, we review some application of Ir SACs supported on metal oxides.

5.1 Oxidation reactions

Carbon monoxide (CO) and ammonia oxidation are important reactions, since they are harmful residual gases from the incomplete combustion of industrial compounds. By the other hand, CO oxidation is a model reaction for the fundamental understanding of catalytic processes which can be applied to generate products of industrial relevance. Hence, efforts to improve this reaction in which Ir-based SACs play an important role have been done.

Jian Lin and coworkers designing an Ir-Fe(OH)x catalyst which stabilized Ir single atoms for the oxidation of CO. HAADF imaging showed that Ir single atoms distributed uniformly over the support, with small clusters below 1 nm. The adsorption and activation of O₂ was promoted by the support, reflecting its erole in the activity of Ir-based catalysts for the CO oxidation (Lin et al., 2012). Lu Dai and collaborators, reported the promotion of CO oxidation of Ir-TiO₂ catalysts. The promotion was achieved by the tuning of the metal-support interaction, induced by the incorporation of small amounts of CeO₂ into the TiO₂ structure. The presence of a small amount of CeO₂ promoted the CO oxidation significantly, and the activities enhanced with the increase of CeO₂ amount, being Ir/Ce_0.2Ti the sample with the highest activity (Lu et al., 2023).

Ir-TiO₂ single-atom catalyst were prepared by Y Wang et al. The catalyst showed excellent low-temperature selective catalytic oxidation of ammonia. They found that rutile phase performed better than it counterpart anatase. Ir atoms had a better dispersion on rutile. The activity was related to the stronger electronic metal-support interaction with rutile than other phases 15 (Li et al., 2020).

C.B. Thompson studied the rate-controlled elementary steps for CO oxidation of the Ir-TiO₂ system. They showed that the supported Ir SACs were catalytically active for CO oxidation at atmospheric pressure and temperature ranges from 150°C to 200°C in 0.1%–10% CO and 1%–17% kPa O₂, providing insight into the reaction mechanism (Coogan et al., 2023).

Y. Lu studied SAC of Ir on MgAl₂O₄ for the CO oxidation reaction. They applied operando infrared and X-ray absorption spectroscopies and quantum chemical calculations to identify the Ir single-atom complex formed during CO oxidation. They found that Ir(CO) is the active complex, and the formation of Ir single-atom complex promotes the CO oxidation via an Eley–Rideal mechanism where Ir(CO) (O) is the resting state of the catalyst. Their results showed that strong adsorption by a ligand does not necessarily lead to catalyst poisoning due to the ability of single-atoms to bind to more than one ligand. DFT results indicated that Ir single atoms had a critical role in facilitating O₂ activation, while the CO ligand lowers the barrier for the Eley–Rideal rate-limiting step (Lu et al., 2019b).

5.2 Hydrogenation reactions

One of the first reports on single atom catalyst was done by Gates (Uzun et al., 2010). They reported the ethene hydrogenation by isolated atoms of Ir supported on MgO at room temperature and atmospheric pressure.

A dual single atom catalysts made of IrMo/TiO₂ was reported by J Fu and collaborators. The dual catalyst was consisted in discrete Ir single atoms and Mo single atoms anchored on TiO₂, showing great catalytic activity and chemoselectrivity in the hydrogenation of 4-nitrostyrene nad 4-vinylaniline that their monometallic counterparts. According to their density funcional theory calculations, they demonstrate that Ir sites were responsible for H₂ activation, while Mo sites were active site for the adsorption of 4-nitrostyrene via the nitro group (Fu et al., 2021).

A. Jia et al. (2023) reported the selective hydrogenation of crotonaldehyde (CROL) over Ir/TiO₂ catalyst. They studied the effect of metal size, from single atoms to nanoclusters (NC) to nanoparticles (NP), on the selective hydrogenation of this compound. Ir single atoms, clusters and nanoparticles were prepared on anatse TiO2. The reaction rates and TOFs of the different Ir/TiO₂ catalysts followed the order Ir(NP) > Ir(NC) > Ir(SA), while the selectivity was higher than 80% for Ir(SA) supported on TiO₂(101).

5.3 H₂ production

X Zhou and coworkers reported an approach to trap and stabilize Ir SACs on TiO₂ nanotubes, using the high density of Ti³⁺ - oxygen vacancies surface defect which served as highly effective SA iridium traps. The stably trapped SACs showed turnover frequencies of 4 × 10⁶ h^-1 in the photocatalytic H₂ production (Zhou et al., 2021).

5.4 Water gas shift

Ir₁/FeOx Single atom catalyst had a remarkable performance for water gas shift reaction, being superior to its Pt-based counterpart. Ir single atoms enhanced the reducibility of FeOx and generation of oxygen species (Luet al., 2019a).

5.5 Oxygen evolution reaction

MQ Yang et al. (2022) reported an strategy to improve the activity of Ir SACS by anchoring atomic Ir on oxygen vacancies of a CoNiO₂ support. The prepared SACs presented an overpotential of 183 mV at the current density was of 10 mA cm^-2, which was lower than a catalyst using Ir clusters.

The oxygen vacancies provided abundant active sites for the adsorption of OH* for OER. Based on Density Functional Theory (DFT) calculations, the enhanced activity of iridium (Ir) could be attributed to the Ir-S moiety in Ir₁/NFS, which is evident in the favorable formation of the *OOH intermediate during the Oxygen Evolution Reaction (OER) process (Lei et al., 2022).

L. Suhadolnik and collaborators prepared nanotubular TiOxNy-supported Ir single atoms, combined with clusters, as thin-film electrocatalysts. The nanomaterials consisted of a porous morphology. The catalyst exhibited very high oxygen evolution reaction activity in 0.1 M HClO₄, reaching 1,460 Ag-₁Ir at 1.6 V versus a referente hydrogen electrode (Suhadolnik et al., 2023).

Yin et al. (2020) reported a strategy for the preparation of Ir single atoms supported on ultrathin NiCo₂O₄ porous nanosheets. The samples were produced by a co-elecgtrodeposition method. The catalysts showed higher OER activity and stability in acidic media due to the coupling of Ir single atoms with oxygen vacancies. An ultralow overpotencial of 240 mV a j = 10 mAcm^-2 was reported, with a long-term stabilty of 70 h. The surface electronic exchange-and-transfer activities of Ir atoms incorporated on the intrinsic oxygen vacancies of the metal support is the responsible to the high OER performance.

An influential work was done by Q. Wang and collaborators, who developed a catalyst consisted of a nickel oxide matrix, loaded with ultra-high content of Ir single atoms, up to 18 wt%. The catalyst showed enhanced OER in alkaline electrolyte, with an overpotential of 215 mV at 10 mAcm^-2, surprasing the activities of pure NiO and IrO2 catalyts. XPS and XANES results on the chemical state of Ir single atoms indicated that Ir was highly oxidized, with an oxidation state close to 4⁺. After Ir doping, the oxidation state of Ni also modified, being higher than NiO. Ir atoms were well dispersed on the metal oxide surface with chemical bonding with the NiO substrate in a six-coordinated Ir – O (Wang et al., 2020).

5.6 Theoretical research

Undeniably, theoretical work on the interaction between metal atoms and molecules, as well as supports, have broaden the understanding of the behavior and stability of SACs. The calculations of their coordination environment and interactions have delivered important information on the single atom’s contribution to the catalytic activity for different reactions.

For instance, Lin et al. (2020) studied the molecular adsorption properties of CH₄ on single atoms supported on anatase TiO₂(101) using the first-principles method based on density functional theory (DFT). The calculated formation energy of Ir-TiO₂ was de lowest, compare to other noble metal systems. Their results showed that Ir was the most stable system.

In their theoretical work, V. Fung and collaborators reported that single atoms dispersed on TiO₂ acomplish strong methane chemisorption and facle C-H activations. The calculations for chemisorption of methane were done by replacing a surface Ti atom (110) with the single metallic atom, coordinated to four surface oxygens and one subsurface oxygen. The coordination to the rutile TiO₂ surface modified the electronic structure of the single atoms site for chemisorption. They showed that Ir single atoms and other SACs on rutile TiO₂(119) can active C-H of methane at low temperature. Methane adsorption was found to be stronger on the single atom site on the rutile (110) surface than on the anatase (101) surface, with Ir and Pt single atoms being the metals with stronger CH₄ adsorption (Fung et al., 2018).

6 Future challenges

Several challenges exist for the practical implementation of Ir SACs, with a primary concern being the enhancement of the metal-support interaction. Potential strategies include investigating the DPU synthesis method, employing the impregnation method, and exploring in situ-synthesis techniques with low metal loading. Furthermore, a comprehensive characterization of the Ir-TiO_2-rutile surface through XAS, XPS, HR-STEM, and DRIFT studies is essential to improve the catalyst’s performance. Additionally, an evaluation of its activity, such as CO conversion, for comparative analysis with existing Ir/TiO₂ systems is crucial. Finally, the application of DFT studies to examine Iridium-SACs in various facets of TiO₂ may yield valuable insights into their catalytic behavior. It is imperative to continue studying the mechanisms involved during the catalytic processes to gain a better understanding of the contributions of Ir SACs. There are still few reports on this area and further research is of great importance.

7 Conclusion

In conclusion, despite the challenges involved in the synthesis and characterization of single atom catalysts (SACs), this study demonstrates the successful deposition of iridium as isolated atoms on TiO₂. The presence of these iridium atoms on the catalyst surface has been confirmed, highlighting the potential of TiO₂ as a suitable support for SACs. Furthermore, the reactivity of these isolated iridium atoms in catalytic reactions has been observed, emphasizing their functional role in driving catalytic processes.

The ability to deposit and retain iridium atoms as isolated entities on the TiO₂ support opens up new possibilities for the design and development of highly efficient catalysts. The presence of these reactive iridium species on the catalyst surface offers enhanced catalytic performance and selectivity. Despite the challenges associated with the synthesis and characterization of SACs, the findings from this study demonstrate the feasibility of achieving and maintaining isolated iridium atoms on the TiO₂ support, providing valuable insights for future catalyst design and optimization.

Overall, the successful deposition of isolated iridium atoms on TiO₂, their retention on the support surface, and their reactivity in catalysis highlight the promising potential of SACs in various catalytic applications. Further research and development efforts are warranted to fully explore and harness the capabilities of SACs supported on TiO₂, with the aim of advancing catalytic technologies and addressing current environmental and energy challenges.

Author contributions

MM-T: Conceptualization, Investigation, Writing–original draft. OH-C: Supervision, Validation, Writing–review and editing. RM-C: Conceptualization, Funding acquisition, Supervision, Validation, Writing–review and editing.

Funding

The authors declare financial support was received for the research, authorship, and/or publication of this article. The research was founded by Dirección General de Asuntos del Personal Académico (DGAPA) through grant PAPIIT IA-106623.

Acknowledgments

Acknowledgments to Dr. Gabriela Díaz-Guerrero, Dr. Rodolfo Zanella Specia, Dr. Raúl Herrera, Dr. Antonio Gómez-Cortés, Phys. Josué Romero, Dr. Lourdes Bazán, Dr. Omar Novelo, M.S Ana Karla Bobadilla, Phys. Lázaro Huerta Arcos, M.S. Adriana Tejeda, M.S. Viridiana Maturano Rojas, M.S. Alejandro Pompa-García, Dr. Samuel Tehuacanero-Cuapa, and Eng. Cristina Zorrilla for all the scientific and technical support provided.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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