Silver-Based Catalysts on Metal Oxides for Diesel Particulate Matter Oxidation: Insights from In Situ DRIFTS

Abstract

Diesel particulate matter (DPM) represents a deleterious environmental contaminant that necessitates the development of effective catalytic oxidation methodologies. This research delineates a comparative analysis of silver-supported metal oxide catalysts (Ag/Al₂O₃, Ag/TiO₂, Ag/ZnO, and Ag/CeO₂), with an emphasis on the effects of silver distribution and the metal-support interaction on the oxidation of DPM. An array of characterization techniques including XRD, HRTEM, XPS, H₂-TPR, TEM, GC-MS, TGA, and in situ DRIFTS was employed. The novelty of this study resides in elucidating the oxidation mechanism through a tripartite pathway and recognizing Ag⁰ as the predominant active species involved in soot oxidation. The Ag/Al₂O₃ catalyst demonstrated superior catalytic performance, achieving a reduction in the ignition temperature by more than 50 °C, attributable to the optimal dispersion of Ag nanoparticles and a balanced metal-support interaction. Conversely, an excessive interaction observed in Ag/ZnO resulted in diminished catalytic activity. The oxidation of DPM transpires through the volatilization of VOCs (<300 °C), the oxidation by reactive oxygen species, and the combustion of soot (>300 °C). This investigation offers significant contributions to the formulation of highly efficient silver-based catalysts for emissions control, with a particular focus on optimizing Ag dispersion and support interactions to enhance catalytic efficacy.

1. Introduction

Reducing the emissions of particulate matter (PM) originating from motor vehicles is of paramount importance, as such emissions account for approximately 5% of the total PM emissions associated with the transportation sector [1]. Vehicles that are characterized by compression ignition or diesel engine configurations represent a significant subset of automobiles that generate PM [2,3]. The primary emissions attributable to diesel engines include hydrocarbons (HCs), carbon monoxide (CO), nitrogen oxides (NOx), and PM [4]. Diesel engines emit diesel particulate matter (DPM) at concentrations that typically fluctuate between 20 and 200 mg/m³ [5]. An exceptionally efficacious approach for alleviating DPM emissions involves the deployment of diesel particulate filters (DPFs). The operation of DPFs is predicated on two fundamental phases: filtration and regeneration [6]. During the filtration phase, DPM particles are sequestered within the porous architecture of the DPF, while the regeneration phase utilizes oxygen to oxidize the accumulated DPM at elevated temperatures, generally surpassing 600 °C [7]. Nevertheless, the exhaust gases produced by diesel engines frequently fail to achieve sufficiently elevated temperatures during routine operations, thereby impeding the regeneration phase [8,9].

The challenge of achieving adequate exhaust temperatures for the oxidation of DPM is addressed through the application of oxidation catalyst technologies. These catalysts facilitate the oxidation of DPM by creating more efficient reaction pathways characterized by reduced energy barriers [10]. Typically, oxidation catalysts are composed of precious metals such as platinum (Pt), ruthenium (Ru), palladium (Pd), gold (Au), and silver (Ag), which are commonly supported on metal oxide matrices including ceria (CeO₂), alumina (Al₂O₃), and silica (SiO₂) [11,12,13]. The incorporation of an oxidation catalyst has been demonstrated to substantially diminish the activation energy requisite for DPM oxidation, thus enabling combustion at lower temperature thresholds [14,15]. Moreover, these catalysts can stimulate the formation of active species, such as highly reactive oxygen and nitrogen dioxide (NO₂), thereby enhancing the overall soot oxidation efficacy. Ai et al. [16] investigated the influence of both surface and bulk palladium additives on the catalytic performance of La₂Sn₂O₇ pyrochlore oxides concerning diesel soot oxidation. The authors provided a mechanistic depiction of NO oxidation and soot combustion, both in the presence and absence of catalysts. Among the array of noble metals investigated for potential application as oxidation catalysts, silver exhibited considerable promise. Research findings indicated that silver catalysts were instrumental in promoting the generation of adsorbed active oxygen species, including peroxide (O₂²⁻) and superoxide (O₂⁻), which were essential for facilitating DPM oxidation [17]. Zeng et al. [6] examined an Ag-assisted CeO₂ catalyst for soot oxidation, revealing that silver not only demonstrated catalytic efficacy but also presented a more cost-effective alternative in comparison to other noble metals.

The augmentation of DPM oxidation efficacy can be achieved through the implementation of metal catalysts in conjunction with a meticulous selection of an appropriate support matrix. Variables such as particle dimensions, spatial distribution, oxidation states, and the synergistic interactions between the support structure and the noble metal are pivotal in ascertaining catalytic performance. Moreover, specific oxide supports enhance the catalytic mechanism by facilitating the generation of active species. For instance, compounds such as CeO₂ [18], Co₃O₄ [19], and MnO [20] contribute to the production of active oxygen, thereby increasing the overall catalytic reactivity. Al₂O₃ has been extensively employed as a support for Pt [21], Cu [22], and Ag [22,23,24]. Sawatmongkhon et al. [25] explored the oxidation of DPM, utilizing silver and ceria-supported alumina as catalytic agents for oxidation. The findings revealed that silver incorporated onto alumina at a concentration of 16 wt% demonstrated exceptional DPM oxidation activity and stability. Furthermore, Aneggi et al. [26] investigated the properties of soot oxidation using Ag deposited on CeO₂, ZrO₂, and Al₂O₃. The incorporation of Ag onto ZrO₂ or Al₂O₃ exhibited considerable soot oxidation activity under both fresh and aged scenarios. Semiconductor materials such as TiO₂, ZnO, and Fe₂O₃ have been utilized as supports to enhance oxidation processes due to their proficiency in oxygen adsorption [27]. Notably, titanium dioxide (TiO₂) is recognized as a quintessential n-type semiconductor with remarkable oxygen adsorption attributes [28]. This inherent characteristic renders TiO₂ particularly beneficial for promoting oxidation reactions. Additionally, TiO₂ has shown effectiveness as an oxide support material for DPM oxidation, demonstrating robust metal-support interaction (SMSI), high thermal stability, and exceptional mechanical integrity [29]. These properties render it a suitable support for various metal catalysts. Zinc oxide (ZnO) is a prominent semiconductor that significantly enhances soot oxidation due to its capacity to adsorb oxygen and generate reactive oxygen species on its surface through the formation of oxygen vacancies [30,31]. CeO₂ is frequently employed as a support for active metals due to its redox properties and oxygen storage capacity (OSC) [32]. The redox cycling between Ce³⁺ and Ce⁴⁺ leads to improved OSC performance, which aids in soot oxidation [33]. The mechanisms governing soot oxidation with CeO₂ consist of two essential phases. Initially, lattice oxygen (Ol) oxidizes the soot, leading to the formation of oxygen vacancies (Ovs) and the reduction of CeO₂ (Ce⁴⁺) to CeO_2−x (Ce³⁺) [34]. Subsequently, gaseous oxygen adsorbs onto the surface of CeO₂, converting into active oxygen [35].

The in situ assessment of the various surface species of catalysts, as well as the identification of oxygen vacancies during reactions performed under authentic operational conditions, can be adeptly executed through the application of diffuse reflectance infrared Fourier transform spectroscopy, widely referred to as DRIFTS, which has been demonstrated to be an indispensable technique within this research arena [36]. This analytical approach holds critical significance for establishing meaningful correlations between surface oxygen vacancies and the intrinsic structural attributes that characterize the catalyst in question. In a distinguished study conducted by Jiacheng Xu and associates [37], the researchers sought to elucidate the influence that oxygen vacancies exert on the efficacy of α-MnO₂, which demonstrates a range of morphologies, particularly concerning the CO oxidation process; this inquiry skillfully employed a combination of DRIFTS and mass spectrometry (MS) to derive their conclusions. The outcomes of their thorough investigation indicated that the rod-shaped variant of the α-MnO₂ catalyst exhibited significantly enhanced performance regarding CO oxidation, a phenomenon ascribed to the increased presence of T-type oxygen (M=O) localized on its surface. The importance of this T-type oxygen species is paramount, as it serves a pivotal function in facilitating the CO oxidation process, chiefly through its capacity to readily adsorb CO molecules, subsequently resulting in the formation of monodentate carbonate species, which in turn accelerates the conversion of these carbonates into gaseous CO₂. Furthermore, the rod-like catalyst was observed to possess a remarkable abundance of T-type oxygen vacancies (M²⁺) under relatively lower temperature conditions, a factor that is essential for comprehending the dynamics of oxidation processes and is crucial for elucidating the catalyst’s functional role in the oxidation of DPM. Additionally, it is imperative to ascertain the contribution rendered by the support material in the context of the silver catalyst, particularly concerning the enhancement of the oxidation reaction associated with DPM. This comprehension is vital for clarifying the complex mechanisms that underlie the facilitation of DPM oxidation.

This investigation scrutinizes the oxidation mechanisms of DPM catalyzed by silver (Ag) deposited on various metal oxide substrates, including alumina (Al₂O₃), titanium dioxide (TiO₂), zinc oxide (ZnO), and cerium dioxide (CeO₂). This study encompasses an evaluation of the mobility of active oxygen species, which is crucial for the oxidation of DPM. The characterization of the catalyst is conducted utilizing sophisticated analytical techniques such as X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), hydrogen temperature-programmed reduction (H₂-TPR), and X-ray photoelectron spectroscopy (XPS). The effectiveness of DPM oxidation is ascertained through the implementation of thermogravimetric analysis (TGA). The fundamental mechanism of DPM oxidation is examined utilizing DRIFTS.

2. Materials and Methods

2.1. Material Preparation

Silver-supported catalysts were synthesized employing the incipient wetness impregnation technique to guarantee a uniform distribution of the metal, as shown in Figure 1a. Silver nitrate (AgNO₃) was solubilized in distilled water and subsequently impregnated onto various metal oxide supports: γ-Al₂O₃ (Ajax Finechem, Taren Point, NSW, Australia, a 142 m²/g BET surface area), TiO₂ (KEMAUS, Cherrybrook, NSW, Australia), ZnO (Quality Reagent Chemical, New Zealand), and CeO₂ (Sigma Aldrich, Burlington, MA, USA). The optimization of silver loadings was conducted, resulting in 16 wt% Ag on Al₂O₃ and 5 wt% Ag on CeO₂. Following the impregnation process, the samples underwent drying at 110 °C for an overnight duration and were calcined at 600 °C for a period of 2 h. The selection of supports encompassed a diverse array of material properties, which included redox-active CeO₂, semiconductor TiO₂ and ZnO, and acidic Al₂O₃, thereby ensuring a comprehensive framework for the evaluation of catalytic performance. The investigation specifically selected a concentration of 16 wt% silver, as extensive experimentation elucidated this condition as optimal for enhancing the combustion efficiency of DPM in conjunction with aluminum oxide. In the scenario of silver supported on ceria (Ag/CeO₂), a reduced concentration of 5 wt% silver was chosen, as this concentration demonstrated the most significant efficacy in facilitating the oxidation of soot among all evaluated conditions.

Figure 2b shows DPM preparation. DPM was procured from the exhaust of a biodiesel-fueled diesel engine operating within the range of 1000–2000 rpm under 25–75% load conditions. A 30 cm stainless-steel mesh (50 mm inner diameter) was inserted into the exhaust pipe to facilitate the collection of particles. The gathered DPM was subjected to drying at 110 °C for 8 h to eliminate moisture and subsequently stored in an airtight container for further analytical procedures.

2.2. Catalyst and DPM Characterization

The crystallinity and phase composition of the catalyst were rigorously analyzed utilizing XRD with a Rigaku Miniflex (Akishima-shi, Tokyo, Japan) 600 apparatus, employing Cu-Kα radiation at a wavelength of 1.5418 Å. The estimation of crystallite dimensions was performed in accordance with Scherrer’s equation. The HRTEM was utilized to evaluate the morphological characteristics of the catalyst, while energy-dispersive X-ray spectroscopy (EDS) facilitated the examination of the elemental composition. The XPS measurements were executed at the BL3.2U beamline of the Synchrotron Light Research Institute (SLRI) in Thailand, employing an excitation energy of 600 eV to scrutinize the binding energies of Ag 3d and O1s. Hydrogen temperature-programmed reduction (H₂-TPR) was conducted using a ChemiSorb 2750 system (Micromeritics Instrument Corp., Norcross, GA, USA) to evaluate the reducibility of the catalyst, with a gas mixture of 10 vol% H₂ in argon, subjected to a heating regimen reaching 800 °C at a rate of 10 °C/min. The volatile organic compounds (VOCs) present in the DPM were analyzed through gas chromatography-mass spectrometry (GC-MS), employing an Agilent (Santa Clara, CA, USA) 7890A/5975C system equipped with an HP-5MS column and helium as the carrier gas at a flow rate of 1 mL/min. The mass spectrometer was operated in full-scan mode covering a mass range of 20–450 atomic mass units (AMUs), wherein VOC identification was facilitated through the utilization of the NIST11 spectral library.

2.3. DPM Oxidation Activity

The TGA apparatus (PerkinElmer Pyris 1, Shelton, CT, USA) was employed to assess the oxidation of DPM. A weight ratio of 1:5 for DPM to catalyst was implemented, accompanied by meticulous mixing in a stainless-steel mortar. Approximately 10 mg of samples were subjected to heating from ambient temperature to 110 °C at a rate of 10 °C/min under a nitrogen atmosphere, prior to transitioning to a gas mixture of 10 vol% O₂ in nitrogen, followed by additional heating to 700 °C at the same rate of 10 °C/min and a flow rate of 50 mL/min.

2.4. In Situ DPM Oxidation on Catalyst

In situ DRIFTS was conducted utilizing a Bruker INVENIO-R (Ettlingen, Baden-Württemberg, Germany) spectrometer, which features potassium bromide (KBr) optical components and a mercury cadmium telluride (MCT) detector. The samples were combined with KBr in a ratio of 1:10 and were subjected to a flow of 10 vol% O₂ in nitrogen at a rate of 50 mL/min. Spectroscopic data were collected at intervals of 50 °C, spanning from 100 to 600 °C, with a spectral resolution of 4 cm⁻¹ and comprising 64 scans per measurement to monitor the dynamics of oxidation.

3. Results

3.1. DPM Characterization

Table 1. Composition of VOCs contained in DPM.

Species	RT (min)	Formula	Area (%)
Propene	1.2319	C3H6	3.17
Nonanal	4.681	C9H18O	1.08
Hexane, 3,3-dimethyl-	7.4215	C8H19	0.48
Hexane, 3,3-dimethyl-	7.8897	C8H20	0.46
Dodecane, 2,7,10-trimethyl-	8.3349	C15H32	1.30
Nonadecane	8.7592	C19H40	2.31
Hexadecanoic acid, methyl ester	8.8722	C17H34O2	10.67
Dibutyl phthalate	9.0827	C16H22O4	3.09
Eicosane	9.164	C20H42	5.02
Heneicosane	9.5519	C21H44	7.67
trans-13-Octadecenoic acid, methyl ester	9.5975	C19H36O2	8.99
Methyl stearate	9.6614	C19H38O2	5.44
Heneicosane	9.923	C21H44	10.01
Heneicosane	10.2785	C21H44	10.63
Butane, 2,2-dimethyl-	10.4964	C6H14	0.44
Tetracosane	10.6203	C24H50	9.41
Heptane, 3,3,4-trimethyl-	10.8288	C10H22	0.37
Pentacosane	10.9488	C25H52	7.33
Phthalic acid, di(2-propylpentyl) ester 2-(1-Butyl-6-methyl-5-(morpholine-4-carbonyl)-2-oxo-1,2,3,4 tetrahydropyridin-3-yl)-	11.1635	C24H38O4	3.86
Heptadecane, 2,6,10,15-tetramethyl-	11.264	C21H44	4.10
Nonadecane Benzamide, 2,5-difluoro-N-(2-phenylethyl)-N	11.264	C19H40	4.16

meticulously details the complex makeup of VOCs, offering an extensive overview of their structural heterogeneity and abundance. The relative abundance of each distinct hydrocarbon was systematically quantified through the rigorous integration of the area under the curve generated from the gas chromatography-mass spectrometry (GC-MS) spectrum, a methodology that yields quantitative insights concerning the concentration of these compounds. The proportion of the total area attributed to each specific compound acts as a crucial metric of its relative abundance within the comprehensive mixture of VOCs, thus enabling a more profound comprehension of the compositional framework. The analytical approach culminated in the identification of a marked predominance of hydrocarbons, wherein the five most prominent VOCs were identified as heneicosane, constituting 28.3% of the total, succeeded by Hexadecanoic acid, methyl ester at 10.7%, tetracosane at 9.4%, trans-13-Octadecenoic acid, methyl ester at 9%, and Pentacosane at 7.3%. Additionally, it is significant to note that a consortium of other hydrocarbons collectively represented 35.3% of the total VOC content, exemplifying the diversity inherent within this chemical category. In relation to molecular size distribution, small hydrocarbons, defined as those containing between one and six carbon atoms, constituted a modest 3.2%, while medium hydrocarbons, characterized as those comprising seven to twelve carbon atoms, were determined to represent a mere 2%; conversely, large hydrocarbons, defined as those containing more than twelve carbon atoms, constituted a substantial majority at an impressive 94.8%. This extensive distribution of hydrocarbon sizes indicates a pronounced dichotomy between lighter and heavier VOCs, with the latter demonstrating a notable predominance, thereby corroborating trends that have been previously documented in the extant scholarly literature [24].

The identification of hydrocarbon types within VOCs is particularly imperative, as it provides critical insights into the specific decomposition mechanisms of DPM. Comprehending the molecular composition enables the targeted application of catalytic materials that optimize the oxidation of distinct hydrocarbon classes, thereby enhancing the overall efficiency of the decomposition process. This is especially pertinent for catalysts that exhibit selective reactivity toward specific VOC structures, such as long-chain alkanes or oxygenated hydrocarbons, which significantly influence the overarching oxidation pathway. Consequently, the capacity to discern VOC composition not only elucidates the fundamental reaction mechanisms but also facilitates the strategic design of catalysts specifically tailored to maximize oxidation efficiency at various reaction stages.

Figure 2 provides an extensive representation of the transmission electron microscopy (TEM) images, energy-dispersive spectroscopy (EDS) data, and the quantification of primary particle dimensions associated with the DPM, generated through diesel engine operations employing biodiesel as the fuel. Figure 2a–c delineate the TEM images that have been precisely captured at the designated magnifications of 200 nm, 100 nm, and 10 nm, respectively, thereby offering a thorough visual elucidation at varying levels of resolution. The representative TEM imagery of soot samples obtained from diesel engines and oxygenated fuel blends illustrates the existence of agglomerated particulate matter that displays a unique morphology typified by a chain-like configuration, as substantiated by antecedent investigations [38]. These particulate entities, synthesized through the combustion processes occurring in diesel engines, exhibit distinctive aggregated formations, aciniform arrangements, and fractal-like geometric configurations that emerge due to interactions, including collisions and subsequent coagulation, among multiple initial particle spherules [39,40]. Additionally, the HRTEM image presented in Figure 2d elucidates a core structure composed of amorphous carbon, which is enveloped by a semi-graphitic shell, thereby accentuating the intricate structural attributes of these particles [41]. In their investigations, Toth et al. [42] documented the oxidation process that transpires in amorphous carbon particles with an initial diameter of approximately 30 nm when subjected to a temperature of 600 °C, wherein the oxidation process commences by targeting the individual carbon cores and subsequently engendering graphene-like sheets, which are expeditiously consumed as the strain on the molecular bonds intensifies. Following the consumption of the core material, a hollow shell remains, which ultimately transforms into a network of intricately entangled multilayer graphene sheets, thus signifying a substantial alteration in the material structure.

To facilitate the effective degradation of soot, it is crucial to comprehend the structural variations present within the particles, as disparate structural phases exhibit distinct resistances to oxidation. Soot oxidation transpires in multiple stages, wherein more robust graphitic structures necessitate a considerably greater amount of active oxygen for decomposition. By discerning these structural phases, it becomes feasible to select an appropriate catalyst specifically tailored to each stage of oxidation, thereby augmenting the efficiency of the soot degradation process. Catalysts proficient in promoting oxygen activation at critical decomposition phases are indispensable for optimizing DPM combustion and emission mitigation strategies.

The elemental composition of soot particles was further scrutinized through EDS analysis, as illustrated in Figure 2e, confirming that carbon constitutes the predominant component of DPM, with trace amounts of ancillary elements potentially arising from fuel contaminants or lubricant additives. The detection of oxygen is attributed to the presence of oxygenates in the biodiesel fuel. Furthermore, the manifestation of copper within the energy-dispersive spectroscopy (EDS) spectra can be attributed to the utilization of copper grids (3.05 mm in diameter, 300 mesh) that were implemented during the preparation of the samples. The distribution of primary particle diameters, depicted in Figure 2f, indicates a mean diameter of 28 nm with a standard deviation of 11 nm, a finding that aligns with previous studies on biodiesel-derived soot, where typical particle sizes range from 20 to 35 nm [8,43]. The interplay between soot structure, oxidation behavior, and catalyst selection underscores the imperative of customizing catalytic strategies to effectively enhance soot oxidation under authentic diesel combustion conditions.

3.2. XRD

Figure 3 provides a detailed illustration that comprehensively delineates the XRD patterns representative of the composite materials Ag/Al₂O₃, Ag/TiO₂, Ag/ZnO, and Ag/CeO₂, thus furnishing critical insights into their structural attributes. The peaks that are distinctly discernible at 2θ values of 38.2°, 44.4°, 64.5°, 77.3°, and 81.4° unequivocally indicate the presence of metallic silver (Ag⁰), a conclusion that is substantiated by the Joint Committee on Powder Diffraction Standards (JCPDS), reference number 04-0783 [18,44,45]. It is particularly noteworthy that silver oxide (Ag₂O) is conspicuously absent in all the catalysts examined via the XRD technique, a phenomenon that can most plausibly be ascribed to the diminutive particle size of approximately 5 nanometers, a scale that unfortunately resides below the detection capabilities afforded by XRD methodologies [26]. The peaks associated with Al₂O₃ at 46° and 67° are specifically linked to the gamma phase of alumina, which is scientifically classified as γ-Al₂O₃ [46,47,48,49]. Furthermore, the XRD analysis revealed the presence of titanium dioxide (TiO₂), as evidenced by peaks recorded at 25.64°, 38.26°, 48.4°, 63°, 69.26°, and 75.16° [50,51,52], which denotes the anatase phase of TiO₂, a polymorphic variant recognized for its remarkable properties [53]. The identification of the various planes of zinc oxide (ZnO) is accomplished through peaks [54,55] observed at the following angles: 31.8° (100), 34.5° (002), 36.2° (101), 47.6° (102), 56.7° (102), 62.9° (110), 66.4° (103), 68° (200), and 69.3° (201), thereby emphasizing the diverse crystalline structures manifested within the ZnO material. In the context of the Ag/CeO₂ composite, the fluorite-like structural characteristics intrinsic to CeO₂ are indicated by the presence of peaks corresponding to the following crystallographic planes [56,57]: 28.5° (111), 33.1° (200), 47.5° (220), 56.3° (311), 59.1° (222), 69.3° (400), 76.6° (331), 79.1° (420), and 88.5° (422). It is particularly significant to note that the intensity of the Ag peak within the Ag/CeO₂ composite is substantially diminished in comparison to the other catalytic materials, a phenomenon that can be attributed to its reduced silver content of merely 5 weight percent (wt%) within the cerium dioxide matrix, in stark contrast to the considerably elevated 16 wt% found in the other samples investigated.

The interplanar distances, along with the mean crystallite dimensions of elemental silver (Ag⁰), were elucidated through the application of Bragg’s law for the assessment of interplanar distances and Scherrer’s equation for the estimation of crystallite sizes, respectively. The evaluated interplanar distances associated with the (111) and (200) crystallographic planes were determined to be approximately 0.236 nanometers and 0.204 nanometers, respectively, thereby furnishing considerable corroboration concerning the existence of Ag⁰ on the supporting materials as cited in antecedent investigations [26,56].

The crystallite dimensions of Ag⁰ were methodically examined, unveiling notable discrepancies across various support substrates: 11 nm for Ag/Al₂O₃, 13 nm for Ag/TiO₂, 15 nm for Ag/ZnO, and 41 nm for Ag/CeO₂. These discrepancies emphasize the considerable impact of metal-support interactions on the distribution and size of Ag⁰ particles. The discerned pattern implies that the selection of support material is crucial in regulating Ag⁰ dispersion, which ultimately influences catalytic efficacy. By elucidating the essential role of Ag⁰ distribution in facilitating the oxidation of DPM, this investigation underscores how variations in Ag⁰ particle dimensions indirectly signify disparities in its dispersion across distinct support substrates. The results demonstrate that the size of Ag⁰ crystallites is profoundly contingent upon the characteristics of the supporting substrate, indicating interactions between Ag and the support material. Such interactions affect the nucleation and growth dynamics of metals during catalyst synthesis, thus modulating the resultant particle size and catalytic activity. Therefore, it can be inferred that the choice of an appropriate support substrate is vital for optimizing Ag⁰ distribution and ensuring superior catalytic efficiency in DPM oxidation. The improved dispersion of Ag⁰, particularly in Ag/Al₂O₃, suggests an increased availability of active sites, thereby promoting oxidation processes. In contrast, the markedly larger Ag⁰ crystallite size noted in Ag/CeO₂ (41 nm) may indicate tendencies toward agglomeration, which could potentially diminish catalytic efficiency.

The spatial arrangement of Ag⁰ within the supported catalysts is of paramount importance in influencing oxidation activity, reaction kinetics, and overall catalytic performance. The spatial configuration of the active metal is hypothesized to play a critical role in modulating oxidation activity, primarily due to its close association with the average crystallite size, which is essential for comprehending catalytic performance. The comparatively small crystallite dimensions of Ag indicate a remarkable level of dispersion within the supported materials, which is favorable for enhanced catalytic activity. Stelmachowski et al. [58] conducted an in-depth investigation into the influence of crystallite size on the effectiveness of iron oxide catalysts, particularly within the context of soot combustion processes. The authors reached a significant conclusion, indicating that catalytic activity exhibited a clear dependence on particle size, specifically within a dimensional range of 5 to 100 nanometers, where the smallest particle dimensions were associated with the highest levels of catalytic activity, which in turn correlated with the lowest established ignition temperature for soot combustion. This correlation highlights the necessity of preserving optimal Ag⁰ dispersion to maximize the oxidation rates while alleviating the sintering effects that could compromise long-term catalyst stability. By establishing a definitive relationship between Ag⁰ particle size, support material characteristics, and catalytic performance, these findings offer valuable insights into the design of high-efficiency catalysts for DPM oxidation.

3.3. HRTEM

Figure 4 illustrates the morphological attributes of silver catalysts that are affixed to various substrates, as discerned through HRTEM. In Figure 4a, the crystalline architecture of Ag⁰ in the (200) orientation is identifiable, demonstrating an interplanar distance of 0.204 nm for the Ag/Al₂O₃ composite. Figure 4b further clarifies two separate interplanar spacings, d₂₀₀ = 0.204 nm and d₁₁₁ = 0.236 nm, indicating the Ag⁰ crystalline framework within the Ag/TiO₂ system. Concerning the Ag/ZnO configuration portrayed in Figure 4c, the interplanar distance attributed to Ag₂O is noted at 0.27 nm, which corresponds to the (111) Miller indices. In Figure 4d, a comparable interplanar distance of 0.27 nm is observed, similarly aligning with the (111) plane of Ag₂O situated within the Ag/CeO₂ substrate. The HRTEM imagery corroborates the exclusive emergence of Ag₂O on the ZnO and CeO₂ substrates, with no observable indication of Ag₂O formation on Al₂O₃ and TiO₂. The predominance of Ag₂O on ZnO and CeO₂ can be ascribed to the vigorous interaction between the silver- and the oxygen-rich surfaces of these substrates, in conjunction with their inherent redox properties and oxygen mobility [59,60]. The pronounced oxygen affinity of ZnO, in conjunction with the extensively documented redox activity of CeO₂, facilitates the oxidation of Ag⁰ to Ag₂O [44,61]. Conversely, the relatively diminished oxygen reactivity exhibited by Al₂O₃ and TiO₂ favors the stabilization of silver in its Ag⁰ state, thereby limiting the formation of Ag₂O [62,63]. This distinction in redox characteristics and oxygen mobility significantly impacts the essential role of the support material in governing the oxidation state of silver within the supported catalytic frameworks.

The HRTEM technique constitutes a critical instrument for probing the nature of metal-support interactions (MSIs) [64]. The methodology implemented in catalyst preparation predominantly dictates the interaction dynamics between Ag and oxide supports [65]. The HRTEM images associated with Ag/Al₂O₃, Ag/TiO₂, and Ag/CeO₂ reveal a weak metal-support interaction (WMSI), likely attributable to the impregnation technique employed during synthesis. These observations are reinforced by the research conducted by Grabchenko et al. [56] in their evaluation of Ag/CeO₂ catalysts, wherein they discerned a weak metal-support interaction primarily in samples synthesized via this methodology. Their investigation assessed catalysts with a silver loading of 10 wt%, which were prepared through three distinct methodologies: impregnation, impregnation of pre-reduced CeO₂, and co-deposition precipitation. Among these methodologies, the conventional impregnation approach distinctly exhibited WMSI, highlighting its significance in applications such as soot oxidation. In contrast, Ag supported on ZnO manifests a strong metal-support interaction (SMSI), a characteristic frequently associated with ZnO when synthesized alongside metals such as Au, Ag, Pd, and Pt [66]. In general, SMSI culminates in the encapsulation of metal nanoparticles, which subsequently diminishes the catalytic efficacy of the supported metal catalysts by obstructing the accessible active metal sites [67].

3.4. XPS

The subsequent application of Gaussian curve fitting to the experimental peaks significantly augments the deconvolution of these peaks, thereby facilitating the discernment of silver species in two separate oxidation states, Ag⁰ and Ag⁺, as illustrated in Figure 5a, with the quantification of these oxidation states thoroughly delineated in

Table 2. Percentage of the oxidation state of Ag (Ag⁰ and Ag⁺) and O1s.

Catalyst	Ag0	Ag+	Ol	Oc	Ov
Ag/Al2O3	52.60	47.40	88.91	11.09	0.00
Ag/TiO2	50.53	49.47	5.07	94.93	0.00
Ag/ZnO	46.29	53.71	39.00	14.02	46.99
Ag/CeO2	43.94	56.06	42.11	18.32	39.56

. Ag⁰ is characterized by the binding energies of 368.4 eV for Ag 3d₅/₂ and 374.5 eV for Ag 3d₃/₂, whereas Ag⁺ exhibits comparatively lower binding energies of 367.8 eV and 373.9 eV, respectively [68]. The analysis elucidates a distinct pattern in which Ag⁰ is predominantly found in Ag/Al₂O₃, progressively diminishing through Ag/TiO₂ and Ag/ZnO, ultimately reaching its nadir in Ag/CeO₂. Conversely, Ag⁺ displays an inverse trend, with its peak concentration noted in Ag/CeO₂, where it correlates with the formation of Ag₂O [69]. This correlation implies that conditions characterized by an elevated ratio of Ag⁺ are also indicative of an increased presence of Ag₂O on the catalyst’s surface. Although Ag₂O is observable across all catalysts, it appears in forms that pose challenges for identification through XRD and HRTEM methodologies, such as poorly crystalline, amorphous, or highly dispersed surface species [70,71]. In certain circumstances, the diffraction peaks of Ag₂O may overlap with those of metallic Ag⁰, particularly when the Ag₂O particles are diminutive or inadequately crystallized [72,73]. Furthermore, its detection via HRTEM hinges upon the arbitrary selection of imaging regions, thereby diminishing the probability of identifying Ag₂O in catalysts where its concentration is relatively low [74], as evidenced in Ag/Al₂O₃ and Ag/TiO₂. The presence of Ag₂O is especially pertinent to hydrogen consumption in catalytic reactions (as examined in the H₂-TPR section) and plays a crucial role in facilitating the oxidation of light VOCs, as elaborated in the TGA section.

Comprehending the relative proportions of Ag⁰ and Ag⁺ across various catalysts furnishes additional evidence that supports materials engaging with Ag, thereby modulating its electronic state and dispersion. This variation in Ag oxidation states considerably influences the mechanisms governing DPM oxidation. The elevated concentrations of Ag⁰ promote soot oxidation, as Ag⁰ functions as an efficient site for electron transfer and the generation of reactive oxygen species (ROS). In contrast, increased concentrations of Ag⁺, particularly in Ag/CeO₂, enhance VOC oxidation, given that Ag₂O is recognized for its capacity to facilitate the activation of oxygen-containing intermediates, thus expediting the decomposition of volatile organic fractions in DPM.

In addition to examining the oxidation states of silver, a comprehensive analysis of the O1s was performed. The XPS spectra were performed to elucidate the oxygen species associated with each catalyst (Figure 5b). The deconvoluted O1s spectra identify three predominant types of oxygen species: lattice oxygen (O_l) at 529.4 eV, oxygen vacancies (O_vs) at 530.4 eV, and adsorbed oxygen (O_c) at 532.3 eV [56]. The characterization of these species, along with the corresponding percentages of the oxygen species, are systematically presented in Table 2. It is noteworthy that the findings for both Ag/TiO₂ and Ag/Al₂O₃ indicate the presence of two principal oxygen species: O_l and O_c. In the scenario of the Ag/TiO₂ catalyst, lattice oxygen constitutes 5.07%, whereas adsorbed oxygen represents 94.93%, thereby highlighting the remarkable ability of TiO₂ to activate surface-associated oxygen species. The elevated proportion of O_c suggests the formidable oxidation capabilities of TiO₂, as it significantly enhances the activation and utilization of surface-bound oxygen for catalytic activities [75]. In contrast, Ag/Al₂O₃ exhibits a distinct distribution of oxygen species, with 87.53% of Oₗ and merely 12.47% of Oc. Al₂O₃, characterized as a non-reducible oxide with a limited oxygen storage capacity, reflects a substantial fraction of lattice oxygen, thereby emphasizing the inherent stability within its oxide framework. The reduced quantity of adsorbed oxygen indicates a diminished capacity to sequester reactive oxygen species, which aligns with Al₂O₃’s restricted potential for oxygen liberation and participation in redox processes. The distribution of oxygen species becomes increasingly complex in the Ag/ZnO and Ag/CeO₂ systems, wherein all three categories of oxygen species—O_l, O_v, and O_c—are present. Notably, Ag/CeO₂ reveals a distribution comprising 42.11% of O_l, 39.56% of O_v, and 18.32% of Oc. The considerable fraction of oxygen vacancies accentuates the oxygen storage capacity (OSC) of CeO₂, which is attributed to its ability to readily transition between the Ce⁴⁺ and Ce³⁺ oxidation states [76]. This heightened Oᵥ content enhances CeO₂’s capability to facilitate ongoing oxygen release and replenishment throughout catalytic reactions. The moderate quantities of Oₗ and O_c imply that CeO₂ proficiently engages both surface and bulk oxygen species in oxidation mechanisms, thereby contributing to its overall redox performance. Similarly, Ag/ZnO displays a well-balanced distribution of oxygen species, comprising 46.98% of O_v, 39% of O_l, and 14.02% of O_c. The relatively elevated Oᵥ content in ZnO emphasizes its reducible characteristics and its aptitude for generating oxygen vacancies, thereby augmenting catalytic activity in redox reactions. The moderate levels of O_l and O_c suggest that ZnO can employ both surface and bulk oxygen species during oxidation processes, thereby reinforcing its catalytic efficacy. These variations in oxygen species and their respective proportions across different supports exert a significant influence on the interaction dynamics between the active metal and the support, which ultimately affects the catalyst’s ability to facilitate oxidation. This behavior is particularly pivotal for the effective combustion of DPM [77].

3.5. H₂-TPR

Figure 6 serves as a significant depiction of the reducibility attributes intrinsic to newly developed catalysts, which have been meticulously examined utilizing the H₂-TPR methodology, a technique that quantitatively evaluates hydrogen consumption as an indicator of the chemical process delineated by the equation, Ag₂O + H₂ → 2Ag⁰ + H₂O [78], alongside the presence of adsorbed oxygen species on diverse support materials such as CeO₂, ZnO, and TiO₂. This transformation is characterized by the release of oxygen from Ag₂O when exposed to temperatures that do not surpass the limit of 300 °C [79]. On the surfaces of these catalysts, one can discern a range of adsorbed oxygen species, including but not limited to O₂⁻, O⁻, and O²⁻ [75], which are pivotal in facilitating oxidation reactions. These species possess the ability to transiently capture oxygen on the catalyst surface, a phenomenon that is particularly evident in oxygen storage materials such as CeO₂ [2]. Catalysts that demonstrate strong metal-oxygen bonding characteristics [75], particularly those where silver is supported by CeO₂, ZnO, and TiO₂, are endowed with the capability to maintain active oxygen in a highly reactive state on their surface, thereby priming it for interactions with incoming reactive species in subsequent chemical reactions.

The H₂-TPR profile for the Ag/Al₂O₃ catalyst reveals a distinct peak at approximately 87 °C, indicating the reduction of Ag₂O and Oc. Thereafter, hydrogen consumption is not detected at temperatures surpassing 100 °C. Moreover, there is an absence of oxygen release during the interaction between Ag and Al₂O₃ [80]. Ousji et al. [81] documented a similar reduction peak around 100 °C in their study of Ag-based catalysts supported on various substrates for the oxidation of formaldehyde. Within the framework of the Ag/Al₂O₃ catalyst, the early release of oxygen at lower temperatures is predominantly attributed to the weak interaction between the metallic component and the support, in conjunction with the limited thermal stability of Ag₂O. The desorbed oxygen is emitted prior to 100 °C due to its insufficient stabilization on the Al₂O₃ surface, which lacks significant oxygen storage capacity or mobility. Furthermore, the decomposition of Ag₂O transpires at reduced temperatures, thereby facilitating oxygen release. In contrast to alternative supports such as TiO₂, ZnO, and CeO₂, which exhibit the ability to stabilize oxygen species and release them across an expanded temperature range, the Ag/Al₂O₃ system expels all active oxygen species at lower temperatures without the substantial participation of lattice oxygen.

The H₂-TPR profile of the Ag/TiO₂ catalyst examined in this investigation reveals two peaks at 80 °C (Oc) and within the temperature range of 150 to 250 °C (Oc and Ag₂O). In a comparative analysis, Kim et al. [10] observed a similar reduction peak for Ag supported on anatase (TiO₂) at elevated temperatures, specifically within the interval of 250 to 330 °C. The consistency in the reducibility behavior of Ag/TiO₂ across these investigations suggests that the peak identified in the present study is likely correlated with the reduction in Ag₂O. This interpretation is further validated by XPS results, which reveal the presence of substantial quantities of Ag₂O. These findings support the decomposition of Ag₂O in the presence of H₂ throughout the reduction process. The XPS analysis indicates a significant presence of Oc on the surface of the Ag/TiO₂ catalyst, implying that a considerable fraction of the surface oxygen is derived from Ag₂O. This observation suggests that Ag₂O likely remains on the TiO₂ surface, thereby contributing to the stabilization and retention of Oc. The elevated levels of Oc detected via XPS are consistent with the presence of surface-bound Ag₂O, which potentially functions as a reservoir for active oxygen species. This underscores the critical role of Ag₂O in enhancing the availability of oxygen on the catalyst surface, as substantiated by the XPS analysis.

In the framework of the Ag/ZnO catalytic system, the release of oxygen encompasses a convergence of Oc, the disintegration of Ag₂O, and the inherent reduction in ZnO at elevated thermal conditions. When temperatures exceed 100 °C, Oc remains present on the surface and contributes to the initial peak observed within the H₂-TPR profile. This peak is further intensified by the decomposition of Ag₂O, which liberates additional oxygen species that participate in reactions with hydrogen. The initial liberation of oxygen from Oc and Ag₂O forms the basis for redox activity at lower thermal conditions. As temperatures rise beyond 300 °C, ZnO initiates oxygen release in a manner analogous to the transformations documented in Cu/ZnO systems, in which the reduction in ZnO is represented by the reaction ZnO + H₂ → Zn⁰ + H₂O [82]. At this point, the ZnO support assumes a critical role in the liberation of oxygen, as the ZnO framework itself undergoes reduction, thereby providing lattice oxygen for ensuing reactions with hydrogen. This reduction at elevated temperatures indicates a transition from the engagement of surface oxygen (Oc and Ag₂O) to the involvement of lattice oxygen from ZnO, thus facilitating further redox reactions. Consequently, the mechanism of oxygen liberation in Ag/ZnO unfolds in two distinct phases: an initial low-temperature oxygen release from Oc and Ag₂O, followed by the release of lattice oxygen from ZnO at temperatures exceeding 300 °C.

In a similar vein, Ag/CeO₂ manifests two peaks of H₂ consumption at approximately 200 °C and 700 °C, which are attributed to the reduction in Oc on CeO₂, Ag₂O, and the lattice oxygen encapsulated within CeO₂. The first peak within the H₂-TPR profile is ascribed to the liberation of Oc and oxygen from Ag₂O at 200 °C. The heightened oxygen storage capacity (OSC) of CeO₂ further amplifies this phenomenon by facilitating the mobility and release of adsorbed oxygen species 31. As elucidated by Grabchenko et al. [54], the initial peak around 200 °C in Ag/CeO₂ is associated with the reduction in both Ag₂O and CeO₂ by H₂. The capability of CeO₂ to oscillate between Ce⁴⁺ and Ce³⁺ oxidation states enables the continuous release and replenishment of oxygen, thereby ensuring sustained redox activity. The synergistic interaction between Ag₂O and the OSC characteristics of CeO₂ fosters efficient oxygen availability at lower temperatures, thereby enhancing the redox performance of the catalyst. Nevertheless, the release of lattice oxygen from CeO₂ occurs beyond the temperature range conducive to DPM oxidation.

The oxygen release properties of silver-supported catalysts across diverse oxide matrices elucidate the functional roles of Ag₂O and Oc in redox reactions. In the scenario of Ag/Al₂O₃, the initial liberation of oxygen is ascribed to a minimal interaction between the metallic component and the support substrate, with an insignificant release of oxygen at heightened temperatures. Ag/TiO₂ exhibits a more prolonged release of Oc, wherein Ag₂O is instrumental in the availability of surface oxygen. The Ag/ZnO system showcases a dual mechanism for oxygen release, commencing with initial contributions from Oc and Ag₂O, which are subsequently followed by the liberation of lattice oxygen from ZnO. Ag/CeO₂ displays augmented oxygen release, catalyzed by the elevated oxygen storage capacity (OSC) of CeO₂ and its ability for oxygen regeneration, thereby rendering it exceptionally active in redox reactions. These observations underscore the significant impact of the support material on the dynamics of oxygen species in silver-supported catalytic systems. The temperature range for the release of active oxygen is pivotal for comprehending how each catalyst facilitates the oxidation of DPM, as it correlates with the respective contributions of Ag⁺ (Ag₂O), Oc, and Oₗ. Given that distinct oxidation reactions transpire within specific temperature intervals, this investigation furnishes critical insights into the selection of catalysts for enhanced DPM oxidation efficacy.

3.6. TGA Result

Figure 7a delineates the oxidative efficiency of DPM alongside the corresponding percentage of weight reduction. Figure 7b exhibits the first derivative of weight loss, a process conventionally termed derivative thermogravimetry (DTG). In the analysis of DPM oxidation conducted without a catalytic agent, our research team identified three significant DTG peaks occurring at approximately 230 °C, 380 °C, and 500 °C. The initial two peaks, which manifest at relatively lower temperature ranges, are correlated with the degradation and combustion of VOCs, with the first peak indicative of lighter VOCs and the second peak signifying heavier VOCs. Within the temperature range of 400–650 °C [83], the oxidation process of solid carbon (soot) is evidenced [19]. The decomposition of DPM can be categorized into three discrete stages based on its constituent materials. The initial stage involves the volatilization of both light and heavy VOCs, whereas the final stage pertains to the oxidation of soot. These stages are distinctly marked by DTG peaks at approximately 271 °C for light VOCs, 400 °C for heavy VOCs, and 600 °C for soot. Huang et al. classified the volatilization of hydrocarbons into four primary categories: high volatility in the absence of oxygen, high volatility in the presence of oxygen, low volatility in the presence of oxygen, and low volatility in the absence of oxygen. The authors underscored that pentadecane, which possesses a boiling point of 268 °C, exemplifies a prototypical case of high-volatility hydrocarbons in the absence of oxygen, whereas tetracosane, characterized by a boiling point of 391 °C, serves as an illustration of low-volatility hydrocarbons that are absent of oxygen. These characteristics are pertinent to light and heavy VOCs, respectively, suggesting that high-volatility hydrocarbons are associated with light VOCs and low-volatility hydrocarbons are associated with heavy VOCs. Moreover, the introduction of oxygen into hydrocarbons augments the oxidation activity. A residual mass of approximately 20% was noted at 700 °C, indicating the incomplete oxidation of soot, likely due to inadequate oxygen availability and inefficient oxidation processes. This finding is congruent with the observation that the exhaust gas temperatures of diesel engines are insufficient to achieve complete soot oxidation, culminating in the obstruction of the DPF.

The integration of silver-based catalysts markedly improves the oxidation of DPM, effectively lowering the necessary combustion temperatures through the generation of active oxygen species. The extent of this enhancement is influenced by factors such as the dispersion of silver, its oxidation states (Ag⁰/Ag⁺), and the oxygen storage capacity (OSC) of the supporting material, as substantiated by XRD, HRTEM, XPS, and hydrogen temperature-programmed reduction (H₂-TPR) analyses. The differential thermogravimetric (DTG) profile associated with the Ag/Al₂O₃ catalyst displays two significant peaks at temperatures of 231 °C and 341 °C, indicating the existence of two distinct phases within the oxidation process. The initial peak correlates with the oxidation and volatilization of lighter VOCs, while the subsequent peak is linked to the oxidation and volatilization of more substantial VOCs and soot. This phenomenon suggests that the reduction in VOC volatilization does not necessarily facilitate the oxidation of lighter VOCs. The observed weight loss and DTG profiles align with the traditional oxidation behavior characteristic of DPM. The Ag/Al₂O₃ catalyst initiates the desorption of adsorbed oxygen starting at 100 °C, a conclusion supported by H₂-TPR analyses. The oxidation of DPM in the presence of the catalyst is analogous to that observed in its absence. Nonetheless, the Ag/Al₂O₃ catalyst significantly augments the combustion of heavier VOCs and soot, which can be ascribed to the generation of active oxygen species by Ag⁰ upon surpassing the temperature threshold of 300 °C. This generation of active oxygen is essential for facilitating the comprehensive oxidation of heavy VOCs and soot particles. The findings from XRD and HRTEM analyses verify that silver particles are highly dispersed on Al₂O₃, resulting in reduced crystallite sizes (~11 nm), which in turn enhances catalytic efficacy. The XPS analysis further corroborates that the formation of Ag⁰ beyond 300 °C promotes soot oxidation, as Ag⁰ facilitates the adsorption of gas-phase oxygen, thereby augmenting oxidation efficiency. However, given that Al₂O₃ exhibits a limited OSC, its oxidation performance is inferior to that of cerium dioxide (CeO₂)- and zinc oxide (ZnO)-based catalysts. This indicates that while Ag/Al₂O₃ is proficient in VOC oxidation, it is less effective for soot oxidation, thus necessitating elevated oxygen concentrations in practical applications.

The DTG data pertaining to the Ag/TiO₂ catalyst elucidate the presence of two distinct combustion phases, which are evidenced by peaks observed at 210 °C and 604 °C. The initial DTG peak at 180 °C signifies the volatilization and subsequent combustion of volatile VOCs, whereas the latter peak at 604 °C is indicative of the combustion of soot. The Ag/TiO₂ catalyst facilitates the oxidation of VOCs through the generation of active oxygen species produced by Ag₂O and the presence of adsorbed oxygen, as corroborated by findings from hydrogen temperature-programmed reduction (H₂-TPR) analysis. The H₂-TPR assessment demonstrates that both Ag₂O and the occluded oxygen (Oc) play a significant role in promoting early oxidation within the temperature range of 80–250 °C. This enhancement in the generation of active oxygen results in a lower oxidation temperature for VOCs relative to the Ag/Al₂O₃ catalyst and the oxidation of DPM. Moreover, the combustion of heavier VOCs and soot is significantly facilitated by Ag⁰ once the temperature exceeds 300 °C, thereby diminishing the requisite temperature for soot combustion. However, the enhancement in soot oxidation under these conditions is relatively less significant than that observed in other catalytic contexts. Consequently, the utilization of Ag/TiO₂ necessitates a higher concentration of oxygen [84]. The XPS data indicate a substantial concentration of Oc on the Ag/TiO₂ surface, reinforcing its capability to stabilize surface-bound active oxygen, which is crucial for the oxidation of VOCs. XRD results reveal a moderate crystallite size of Ag (~13 nm), ensuring an optimal equilibrium between dispersion and catalytic efficacy. Despite its proficiency in the oxidation of VOCs, Ag/TiO₂ exhibits comparatively inferior performance in soot oxidation when juxtaposed with CeO₂- and ZnO-based catalysts. This observation implies that Ag/TiO₂ is most efficacious when utilized in conjunction with elevated oxygen concentrations, which are essential in practical applications.

The Ag/CeO₂ catalyst manifests distinct DTG peaks at 180 °C, 410 °C, and 497 °C, which correspond to the volatilization and oxidation processes associated with light VOCs, heavy VOCs, and soot, respectively. The H₂-TPR analysis elucidates that active oxygen is released from adsorbed oxygen species present on CeO₂ and Ag₂O at temperatures beneath 200 °C. This early liberation of active oxygen engenders the combustion of VOCs at lower temperatures in relation to alternative catalytic systems. The DTG findings suggest that active oxygen initially migrates from the catalyst surface to the DPM, thereby augmenting the catalytic oxidation of VOCs. Subsequently, the oxidation of soot is promoted by the presence of Ag⁰ on the surface of CeO₂. The XRD and high-resolution transmission electron microscopy (HRTEM) analyses indicate that Ag/CeO₂ possesses the largest Ag crystallite size (~41 nm), implying a reduced dispersion yet a robust interaction with CeO₂. The XPS analysis substantiates a high proportion of oxygen vacancies (Oᵥs), enabling CeO₂ to oscillate between Ce⁴⁺ and Ce³⁺ states, thus facilitating continuous oxygen release. The synergistic effect of Ag⁰ and the oxygen storage capacity (OSC) of CeO₂ culminates in the enhanced oxidation of soot at elevated temperatures, rendering Ag/CeO₂ the most effective catalyst for the complete oxidation of DPM.

The Ag/ZnO catalyst exhibits DTG peaks at 174 °C, 415 °C, and 495 °C, which are indicative of the oxidation processes of light VOCs, heavy VOCs, and soot, respectively. The hydrogen temperature-programmed reduction (H₂-TPR) analysis reveals that the reduction in Ag₂O and the adsorbed oxygen on ZnO occur within the temperature range of 150 °C to 400 °C, suggesting that the active oxygen released from Ag₂O is crucial for facilitating the oxidation of both light and heavy VOCs. Following the oxidation of VOCs, the combustion of soot is detected within the temperature interval of 450 °C to 550 °C. Furthermore, the liberation of lattice oxygen from ZnO between 400 °C and 700 °C acts as an effective oxidizing agent. The XRD and HRTEM analyses indicate that Ag/ZnO possesses a moderate crystallite size (~15 nm), which ensures a sufficient interaction between the metal and the support without incurring excessive sintering. Additionally, the XPS data corroborate the significant presence of oxygen vacancies (O_v), signifying that ZnO plays a pivotal role in the generation of active oxygen species. The reduction in Ag₂O by VOCs results in the formation of Ag⁰, which further facilitates the oxidation of soot through gas-phase oxygen adsorption and involvement in redox reactions. These findings imply that Ag/ZnO is exceptionally proficient in the oxidation of both VOCs and soot, thereby positioning it as a compelling candidate for catalytic emission control applications.

By synthesizing findings from XRD, HRTEM, XPS, and H₂-TPR analyses, this study delineates a direct correlation between catalyst architecture, the availability of active oxygen, and oxidation efficacy as evidenced by the TGA and DTG profiles. The Ag/Al₂O₃ catalyst demonstrates an early liberation of adsorbed oxygen at approximately 100 °C, attributable to weak metal-support interactions, rendering it effective for VOC oxidation, albeit limited in its capacity for soot oxidation. Conversely, Ag/TiO₂ reveals moderate silver dispersion and sustained oxygen release, resulting in superior VOC oxidation relative to Ag/Al₂O₃, while exhibiting diminished soot oxidation performance compared to ZnO and cerium oxide (CeO₂). The Ag/ZnO catalyst adheres to a bifurcated mechanism of oxygen release, benefiting from the reducibility of ZnO, thus proving effective for both VOC and soot oxidation. Among the assessed catalysts, Ag/CeO₂ demonstrates the highest oxygen storage capacity (OSC), culminating in the most efficient soot oxidation, as corroborated by the continuous release of oxygen resulting from the Ce⁴⁺/Ce³⁺ redox cycle. These findings affirm that the efficacy of Ag-based catalysts in the oxidation of DPM is significantly influenced by the interactions between the metal and the support, the distribution of oxygen species, and the redox characteristics. Consequently, the selection of an optimal catalyst necessitates a careful balance between silver dispersion, the availability of active oxygen, and the OSC to promote complete oxidation at lower operational temperatures, thereby enhancing the efficiency of DPM oxidation.

Table 3. Comparison of T₉₀ temperatures for soot and DPM oxidation over various catalysts is reported in the literature and in this work.

Type of Catalysts	Oxygen Concentration	T90 (°C)	Reference
Pt-Pd	13% vol in N2	679.4	[85]
Cu/Mn3O4	10% O2 and 10% H2O in N2	550	[86]
Ag/Co3O4	10% vol in N2	340	[19]
Ag/Al2O3	10% vol in N2	476	This work
Ag/TiO2	10% vol in N2	594	This work
Ag/ZnO	10% vol in N2	512	This work
Ag/CeO2	10% vol in N2	511	This work

delineates a comparative analysis of the T₉₀ temperatures corresponding to 90% soot or DPM conversion for an array of catalysts documented in the prior literature, as well as those synthesized in the current investigation. As illustrated, the commercially available Pt-Pd catalyst necessitated an elevated T₉₀ of 679.4 °C for the oxidation of soot within an oxygen-enriched environment (13% O₂ in N₂). Similarly, the Cu/Mn₃O₄ catalyst displayed a comparatively high T₉₀ of 550 °C for Printex-U soot in the presence of both O₂ and H₂O. Conversely, the Ag/Co₃O₄ catalyst exhibited a significantly reduced T₉₀ of 340 °C, thereby indicating a pronounced oxidation capability toward soot. In the present study, Ag-based catalysts, supported on various metal oxides, were meticulously evaluated for their efficacy in oxidizing authentic DPM under a concentration of 10% of O₂ in N₂. Among these catalysts, Ag/Al₂O₃ attained the lowest T₉₀ of 476 °C, thereby demonstrating superior oxidation activity in comparison to other Ag-based systems such as Ag/TiO₂ (594 °C), Ag/ZnO (512 °C), and Ag/CeO₂ (511 °C). These results suggest that the dispersion of silver and the intrinsic characteristics of the support material exert a significant influence on oxidation behavior. The Ag/Al₂O₃ catalyst demonstrated an optimal interaction, thereby facilitating enhanced oxygen mobility for the combustion of DPM.

3.7. In Situ DRIFTS

The oxidative processes associated with DPM are meticulously investigated through in situ DRIFTS, as evidenced in Figure 8. Initially, hydrocarbons are vaporized and subsequently subjected to oxidation by molecular oxygen under thermal conditions that remain below 300 °C. Within the designated temperature range of 100–300 °C, the absorption peaks characteristic of hydrocarbons were identified at wavenumbers of 2840 cm⁻¹ and 2905 cm⁻¹. These spectral characteristics denote the presence of alkene functional groups (C–H stretching), which are conventionally observed within the 2840–3000 cm⁻¹ spectrum [87]. This finding corroborates the results derived from gas chromatography-mass spectrometry (GC-MS), which identified hydrocarbons such as heneicosane and tetracosane as typical volatile organic compounds (VOCs) present within the sample. The peak absorbance associated with an alternative class of hydrocarbons, specifically the methylene group at 1465 cm⁻¹ [87], is recorded at temperatures below 300 °C. This observation further reinforces the volatilization of VOCs. Concurrently, the VOCs are subjected to oxidation by O₂, as evidenced by the CO₂ absorbance peak at 2345 cm⁻¹, which closely aligns with the literature value of 2343 cm⁻¹ for CO₂ [88]. Moreover, the absorbance peak corresponding to the O-H groups, detected within the 3000–3600 cm⁻¹ interval, is also observed at temperatures below 300 °C [89]. In parallel, the adsorption of oxygen onto carbon (soot) is noted, leading to the identification of C–O peaks at 1600 cm⁻¹ [90]. The oxidation of soot becomes evident under thermal conditions exceeding 300 °C, as indicated by an absorption maximum at 2346 cm⁻¹, signifying the generation of CO₂. The observed reduction in the intensity of alkene and methylene signals suggests that the hydrocarbon composition undergoes complete volatilization at temperatures exceeding 300 °C. Furthermore, additional peaks at 1600 cm⁻¹ correlate with the adsorption of oxygen onto carbon, serving as precursors for the subsequent generation of CO₂.

Figure 9 illustrates the complex mechanisms involved in the oxidation of DPM on the silver/alumina (Ag/Al₂O₃) catalyst. The oxidation characteristics demonstrated by hydrocarbons closely replicate the conditions encountered during the oxidation process of DPM. The volatilization and subsequent oxidation of hydrocarbons by molecular oxygen occur at temperatures below 300 °C. This phenomenon is substantiated by the identification of absorbance peaks associated with the methylene group at 1465 cm⁻¹ and the C–H stretching vibrations of alkene groups within the spectral range of 2840–3000 cm⁻¹. Furthermore, the detection of carbon dioxide (CO₂), indicated by an absorbance peak at 2345 cm⁻¹, further corroborates the occurrence of hydrocarbon oxidation. Within the temperature range of 100–300 °C, the Ag/Al₂O₃ catalyst facilitates the adsorption of oxygen onto carbon (soot), as evidenced by the appearance of absorbance peaks at 1600 cm⁻¹ (C–O) and within the 1740–1810 cm⁻¹ range (C=O) [91]. As the temperature approaches 300 °C, a significant absorbance peak for CO₂ at 2345 cm⁻¹ becomes conspicuous, signifying an increased rate of oxidation for heavy VOCs and soot. This elevated oxidation rate is attributed to the activation of Ag⁰, which enhances the transfer of oxygen to hydrocarbons and soot, thereby facilitating their complete oxidation. This assertion is supported by the differential thermogravimetric (DTG) profile (Figure 7b), which displays a peak at 341 °C, correlating with the zenith of oxidation rates for heavy VOCs and soot. The terminal phase of oxidation, which occurs at temperatures exceeding 300 °C, is predominantly characterized by the oxidation of soot. This transition is reflected in the DRIFTS results, which reveal absorbance peaks at 1600 cm⁻¹ (C–O) and 2345 cm⁻¹ (CO₂). The release of active oxygen from Ag⁰ facilitates the conversion of soot into CO₂. The elevated levels of oxidation are further confirmed by the O-H absorbance peak within the 3000–3600 cm⁻¹ range, produced during the oxidation of hydrocarbons. These findings suggest that the Ag/Al₂O₃ catalyst is instrumental in enhancing the oxidation of hydrocarbons and soot, as Ag⁰ is crucial for the generation of reactive oxygen species such as O₂⁻ and O²⁻.

Figure 10 delineates the in-DRIFTS spectra recorded during the oxidative process of DPM, which is notably accelerated by the Ag/TiO₂ catalytic system. In the initial phase, which transpires between 100 and 300 °C, both volatiles of lower and higher molecular weights undergo vaporization. Within this defined thermal range, two significant absorption peaks are observed: one that corresponds to methylene at 1465 cm⁻¹, and another that pertains to alkene functional groups (C–H stretching) located within the interval of 2840 to 3000 cm⁻¹. The presence of silver oxide (Ag₂O) within the Ag/TiO₂ catalyst is instrumental in the generation of active oxygen species, thereby augmenting the oxidation of lighter volatiles. This assertion is corroborated by the appearance of a carbon dioxide (CO₂) peak at 2345 cm⁻¹, which signifies the initiation of oxidation reactions at these specific temperatures. Moreover, the active oxygen species associated with the carbon surface are evidenced by the absorption peaks attributed to carbon-oxygen (C–O) at 1600 cm⁻¹ and carbonyl (C=O) within the range of 1740 to 1810 cm⁻¹. These absorption features suggest that active oxygen interacts with the carbonaceous materials situated on the catalyst surface, thereby instigating oxidation processes. At temperatures exceeding 300 °C, the oxidation of soot is significantly augmented, as illustrated by the marked enhancement of the C–O peak at 1600 cm⁻¹, which demonstrates increased intensity at elevated temperatures. Simultaneously, the CO₂ peak at 2345 cm⁻¹ becomes increasingly pronounced, indicating the progression of oxidation reactions. This heightened oxidation can be attributed to the activation of metallic silver (Ag⁰), which promotes the synthesis of active oxygen. This active oxygen is then absorbed by the soot, facilitating its oxidation and leading to the production of CO₂. At 350 °C, a particularly conspicuous CO₂ peak is recorded, which is ascribed to the buildup of C=O on the surface of the soot. The introduction of active oxygen generated by Ag⁰ at this temperature aids in the oxidation of the accumulated C=O, thus liberating it as CO₂. This sequence of events elucidates the catalytic function of Ag⁰ in the activation of oxygen, which plays a crucial role in the effective oxidation of both volatile organic compounds and soot under elevated thermal conditions.

The oxidation mechanism of DPM on Ag/ZnO demonstrates a remarkable similarity to that observed on Ag/TiO₂, as illustrated in Figure 11. The oxidation process can be systematically categorized into three discrete stages: the vaporization of VOCs, the oxidation of VOCs, and the subsequent oxidation of soot. The hydrocarbons contained within the VOCs are characterized by two significant absorption peaks: the methylene group, which is detected at approximately 1465 cm⁻¹, and the alkene groups, which are identified within the spectral interval of 2840 to 3000 cm⁻¹. The vaporization of VOCs occurs within the thermal range of 100 to 300 °C. The oxidation of VOCs within this temperature range is evidenced by the appearance of a CO₂ peak at 2345 cm⁻¹, attributed to the enhancement of active oxygen facilitated by Ag₂O and surface oxygen species derived from ZnO. This active oxygen acts to accelerate the oxidation of VOCs. Moreover, the adsorption of active oxygen onto carbon (soot) is corroborated by the presence of absorption peaks corresponding to C–O at 1600 cm⁻¹ and C=O within the range of 1740 to 1810 cm⁻¹, indicating initial interactions between oxygen species and carbonaceous materials. At temperatures surpassing 300 °C, the oxidation of soot is notably intensified by the activation of Ag⁰, as confirmed by the persistent C–O peak at 1600 cm⁻¹ and the production of CO₂. A prominent CO₂ peak at 300 °C indicates a substantial rate of soot oxidation. This occurrence is attributed to the accumulation of C=O on the soot surface at this elevated temperature, which is subsequently oxidized into CO₂ with the aid of active oxygen generated by Ag⁰. This underscores the crucial role of Ag⁰ in promoting soot oxidation through the activation and transfer of oxygen species, thereby facilitating the complete combustion of DPM.

Figure 12 elucidates the complexities associated with DPM oxidation on the Ag/CeO₂ catalyst. The oxidation behaviors of hydrocarbons within this framework closely mirror those reported under different DPM oxidation conditions. The volatilization of hydrocarbons and their subsequent oxidation by molecular oxygen occur at temperatures below 300 °C, as evidenced by absorbance peaks corresponding to methylene groups around 1465 cm⁻¹ and alkene groups (C–H stretching) within the range of 2840–3000 cm⁻¹. The identification of CO₂, corroborated by an absorbance peak at 2345 cm⁻¹, further substantiates the occurrence of hydrocarbon oxidation. Within this thermal profile, the oxidation of VOCs is facilitated by the active oxygen generated by Ag₂O and the surface oxygen species derived from CeO₂, thereby enhancing the oxidation mechanism. The Ag/CeO₂ catalyst further promotes the adsorption of oxygen onto carbon (soot), as indicated by absorbance peaks in the vicinity of 1600 cm⁻¹ (C–O) and between 1740 and 1810 cm⁻¹ (C=O). At a temperature of 300 °C, a prominent CO₂ peak is observed, which signifies a considerable oxidation rate of heavy VOCs and soot. This phenomenon is ascribed to the activation of Ag⁰, which catalyzes the generation of active oxygen, thereby facilitating more efficient oxidation. The pronounced CO₂ peak at this temperature arises from the accumulation of C=O groups on the soot surface, which are subsequently converted into CO₂ upon the introduction of active oxygen from Ag⁰. Furthermore, the enhanced oxidation of heavy VOCs is validated by the observation of an O-H absorbance peak within the spectral range of 3000–3600 cm⁻¹, signifying hydrocarbon oxidation. Following this phase, the oxidation activity predominantly transitions into soot oxidation, as reflected by the persistent presence of C–O (1600 cm⁻¹) and CO₂ (2345 cm⁻¹) absorbance peaks. This transition illustrates the catalyst’s ability to sustain oxidation processes, driven by the interaction of active oxygen with both VOCs and soot, ultimately resulting in their complete combustion.

The oxidation mechanisms pertaining to DPM were rigorously examined through in situ DRIFTS, which elucidated distinct oxidation pathways for VOCs and soot, contingent upon the structural and redox characteristics of silver-based catalysts. The DRIFTS spectra substantiated that VOC oxidation transpires at temperatures below 300 °C, as evidenced by the C–H stretching vibrations (2840–3000 cm⁻¹), methylene peaks (1465 cm⁻¹), and CO₂ formation (2345 cm⁻¹), thus corroborating the findings obtained from gas chromatography-mass spectrometry (GC-MS) and TGA, which identified hydrocarbons such as heneicosane and tetracosane as representative VOCs. The oxidation of soot becomes increasingly evident at temperatures exceeding 300 °C, facilitated by the active oxygen species (O₂⁻, O²⁻) derived from Ag⁰, as demonstrated by the augmented CO₂ peak (2345 cm⁻¹) and C–O adsorption (1600 cm⁻¹). The efficacy of each catalyst in promoting oxidation is significantly dictated by its metal-support interactions, the oxidation states of silver (Ag⁰/Ag⁺), and the oxygen storage capacity (OSC), as verified by XRD, high-resolution HRTEM, XPS, H₂-TPR, and TGA analyses. The Ag/Al₂O₃ catalyst showcased early oxygen release (~100 °C) but exhibited limited soot oxidation, whereas the Ag/TiO₂ catalyst demonstrated enhanced VOC oxidation due to sustained oxygen vacancy availability, albeit necessitating elevated temperatures for soot combustion. The Ag/ZnO catalyst adhered to a bifurcated oxygen release mechanism, rendering it effective for both VOC and soot oxidation, while Ag/CeO₂ manifested the highest OSC, thus facilitating efficient soot oxidation via Ce⁴⁺/Ce³⁺ redox cycling. The insights garnered from the DRIFTS analysis are instrumental for the advancement of catalytic materials specifically designed for DPM oxidation, as they furnish a real-time comprehension of the interactions of oxygen species, hydrocarbon transformations, and catalyst performance under operational conditions. These findings highlight the complexities inherent in optimizing catalyst formulations to augment oxidation rates whilst preserving stability under elevated temperature regimes.

4. Conclusions

This research elucidates the oxidative mechanisms that govern the oxidation of DPM, which is facilitated by metal oxide catalysts supported by silver, encompassing Al₂O₃, TiO₂, ZnO, and CeO₂. The results highlight the pivotal function of silver in modulating the dynamics of oxidation through its interactions with these substrates. The characterization of the catalysts reveals pronounced discrepancies in the properties of silver, including mean crystallite size and electronic configuration, which have a direct bearing on the oxidation mechanism and catalytic efficacy. These discrepancies exert an influence on both the reactivity of the catalysts and the efficiency of the oxidation pathways. The oxidation of DPM transpires through three discrete stages. In the initial phase, VOCs, predominantly alkenes such as heneicosane and tetracosane, volatilize at temperatures below 300 °C. This phase is succeeded by the oxidation of VOCs, which is markedly enhanced in the presence of Ag₂O and surface oxygen species, particularly within the Ag/TiO₂, Ag/ZnO, and Ag/CeO₂ catalysts. In the culminating phase, which occurs at temperatures exceeding 300 °C, the oxidation of soot is facilitated by Ag⁰, with Ag/Al₂O₃ exhibiting the highest efficiency attributable to its superior dispersion of silver and the substantial presence of Ag⁰, thereby promoting more effective oxidation. These findings underscore the critical impact of silver loading and interactions with metal oxides in optimizing catalytic performance for applications pertaining to the control of emissions. The insights derived from this research lay the groundwork for the refinement of silver-based catalysts to enhance their efficacy in alleviating harmful emissions from combustion sources. Furthermore, the results indicate potential advancements in hybrid catalytic systems that amalgamate multiple materials to achieve enhanced emission reductions. Future investigations should concentrate on the optimization of catalyst formulations and the exploration of alternative materials to further augment the sustainability and effectiveness of technologies aimed at emission control [92].