Manufacturing and Physicochemical Characterization of {Pt, Ir}/CeRuO₂ Solid Solutions Tested in CO Oxidation

Abstract

Nanoparticle powders of a Ce_1−xRu_xO₂ mixed oxide (3.0% w/w), were synthesized to be used as catalytic supports, on which Pt and Ir nanoparticles were deposited as the active phase. The catalytic supports were prepared through a route involving microwave heating, while the Pt or Ir nanoparticles were incorporated via the wet incipient method. The {Pt, Ir/Ce_1−xRu_xO₂} catalytic systems were successfully tested as catalysts for low-temperature CO oxidation. To provide adequate support to our results, the compounds were characterized by SEM, EDS, XRD, DRS-UV-vis, and XPS techniques. In addition, BET isotherms were carried out to determine specific surface area features. The CO oxidation evolution was tested in the range of 25–350 °C. Both Pt and Ir supported Ce_1−xRu_xO₂ catalysts that remarkably improved the CO oxidation, reaching and sustaining 100% conversion from 125 °C onwards. Remarkably, the mixed oxide support, by itself, showed outstanding performance, achieving 100% conversion to CO₂, at a temperature of 225 °C.

1. Introduction

Global warming is one of the most important concerns of contemporary society. These disquiets cover a variety of areas, including emissions from mobile sources, gases released by industrial processes, and those caused by everyday human activity. Environmental pollution, affecting either air, water, or soil, has reached alarming levels and, in many cases, has caused irreversible damages. Several specific gases are contributing to global warming, acid rain, melting ice, ocean acidification, disruption of ecosystems and biodiversity, in addition to respiratory diseases. These included CO_x, SO_x, NO_x, N₂O, SF₆, NF₃, and CH₄, among other minor compounds [1]. Amid them, carbon monoxide (CO) and carbon dioxide (CO₂) are of particular interest, as they contribute to environmental hazards, including acid rain and global warming [2]. According to a recent report from M. Yildiz et al. [3], road transportation (36.4%), residential use (28.5%), manufacturing industries (12.6%), and metal production factories (8%) are the major contributors to massive CO production.

From a public health perspective, CO represents a major public health risk, because its continuous exposure, even at low inhalation levels affects the cardiovascular, neurological, and mitochondrial function, and pregnant women may be affected [4]. In fact, low-level CO is recognized as an environmental toxin. It is also considered a cause of short and long-term respiratory morbidity, which under prolonged exposure causes death. The mechanism involves the CO attaching to hemoglobin to form carboxyhemoglobin (COHb), which competes with oxygen for binding sites and thereby reduces the blood’s oxygen-carrying capacity [5]. In general, these lethal and silent compounds are disregarded, thus minimizing their adverse effects. Considering this information, different strategies can be proposed to address the CO production, either by promoting its conversion into CO₂ or incorporating CO into a chemical process, to generate value-added products or chemicals such as methanol, acetic acid, propanoic acid, ethene, propene, gasoline, and diesel fuels, among others [6,7,8]. Photocatalysis has also been explored as a route to convert CO into CO₂ under sunlight, although, the efficiencies attained by this technique are commercially unattractive [9]. Another aspect for CO transformation is catalytic oxidation. A substantial body of scientific literature has focused on the catalytic oxidation of carbon monoxide. Particularly, catalyst efficiency, conversion rates, and thermal stability are critical parameters when assessing their suitability for real-world applications such as industrial exhaust or automotive catalysts. In this case, noble metals like platinum, rhodium, palladium, ruthenium, and even gold have been extensively studied in the past as part of a catalyst for CO oxidation due to their efficiency and conversion performance [10]. The different perspectives addressed for the use of those metals include the study of their shape, size, the inclusion of doping elements, their dispersion on several supports, and the presence of water or hydroxyl groups [11]. Among these materials, cerium oxide (CeO₂) has stood out in carbon monoxide oxidation studies due to its proven ability to promote oxidation and regenerate in the presence of oxygen [12,13], reflecting its intrinsic ability to exchange oxidation/reduction states (Ce³⁺/Ce⁴⁺) with relative easiness. This redox flexibility enables CeO₂ to act as an active catalyst in several reactions [14,15]. It is also used as a support in catalytic oxidation processes like Pt/CeO₂, Pd/CeO₂, Ru/CeO₂ [10,11,15,16], CuO/CeO₂, and Ni/CeO₂ [17]. More complex formulations involve its use as a part of a mixed oxide support in automotive three-way catalysts, including Rh/Ce_xO_y-ZrO₂ [18] and Pd/Ce_xO_y-ZrO₂ [19]. Overall, CeO₂ is known for its stable valence states and high redox capacity associated with the Ce³⁺ to Ce⁴⁺ oxidation states. Nevertheless, pure CeO₂ often shows limited activity in low-temperature CO oxidation, mainly due to moderate oxygen mobility and restricted formation of active oxygen vacancies under reaction conditions. These limitations have motivated researchers to modify ceria to enhance its redox behavior and improve catalytic efficiency [20]. Several strategies have followed to improve the catalytic features of CeO₂; among them, the use of new suitable promoters, supports, pretreatment, best synthesis methods, control of size and dispersion, as well as the incorporation of noble metal catalysts such as Pd, Rh, and Ru have shown favorable effects on reducibility and oxygen activation [21]. One way to control the oxygen vacancy and defect structure is by doping CeO₂ with several metals like In, La, Ru, etc. In this regard, previous studies on Ru-containing CeO₂ compounds have been reported to improve the performance of CO oxidation compared to bare CeO₂, suggesting a favorable synergistic interaction between Ru active species contained in the ceria lattice [22,23]. Once the active metal is incorporated into the CeO₂ lattice, it significantly restricts the oxygen vacancy formation energy on the catalyst surface, resulting in a high catalytic activity at low temperatures.

From this perspective, the present work focuses on the production of {Pt or Ir}/Ce_0.985Ru_0.015O₂ catalysts for its application in the carbon monoxide oxidation reaction. On the one hand, it is expected that the Ce_0.985Ru_0.015O₂ solid solution acts by itself as an active support capable of promoting CO oxidation, while on the other, the Pt or Ir metals, acting as catalysts, are supposed to enhance the redox capabilities of the solid solution, reaching full CO conversion near room temperature.

2. Experimental

2.1. Catalysts Synthesis

The pristine CeO₂ compound was produced from Ce(C₂H₃O₂)₃∙H₂O, 99.99% purity (from Sigma-Aldrich, St. Louis, MO, USA). The Ce(C₂H₃O₂)₃∙H₂O salt was dissolved in deionized water. Afterward, 10 mL of a 10% (w/w) solution of PVP (polyvinylpyrrolidone) surfactant was added. The mixture was heated at 130 °C for 30 min in a Synthos-3000 microwave facility. In the next step, the obtained solution was washed several times, dried at 110 °C, and finally calcined at 400 °C. The Ce_0.985Ru_0.015O₂ solid solution was prepared using 5.4 g of Ce(C₂H₃O₂)₃ (99.9%, Alfa-Aesar, Haverhill, MA, USA) and 0.07 g of Ru₃(CO)₁₂ (99.9%, Sigma-Aldrich), dissolved in distilled water and isopropyl alcohol, respectively. Also, 10 mL of a 10% (w/w) solution of PVP solution was added as surfactant to promote high specific surface area values of the catalytic support. The Ce_0.985Ru_0.015O₂ solid solution was obtained after heating it in a microwave equipment at 150 °C for 30 min. The solution was dried at 120 °C for 24 h, at atmospheric pressure conditions, and further calcined at 500 °C for 3 h.

Pt and Ir catalysts supported on the Ce_0.985Ru_0.015O₂ solid solution were obtained through a modified procedure of the method described in reference [14]. The catalytic support was dispersed in distilled water under constant stirring for 1 h. Subsequently, 5 mL of an 8% PVP solution was added to control the metal particles size. The stirring continued for 1 h. Afterward, either 0.16 g of the precursor salt of Pt (H₂Cl₆Pt·6H₂0), (75% Pt, Alfa Aesar) or 0.02 g of (Ir (CH₃COO)|_n), (49% Ir, Alfa Aesar) was dissolved in distilled water and added to the solution containing the catalytic support and the PVP solution. A total of 10 mL of a citric acid solution was incorporated as a reducing agent. The resulting solution was stirred for 12 h at 85 °C. The Pt/Ce_0.985Ru_0.015O₂ and the Ir/Ce_0.985Ru_0.015O₂ catalysts were dried at 120 °C for 24 h. In a further stage, those compounds underwent a reduction reaction with H₂ using a stream of 30 cm³/min at 400 °C for 3 h. In both cases, the loading of the metals with respect to the Ce_0.985Ru_0.015O₂ support was calculated to adjust 3% (w/w). In the following descriptions, the Ce_0.985Ru_0.015O₂ compound will be abbreviated as CeRuO₂.

2.2. Characterization

Textural properties were measured with a TriStar II 3020 N₂ physisorption equipment (Micromeritics, Norcross, GA, USA). SEM images were obtained with a JEOL JSM 5300 scanning electron microscope (JEOL Ltd., Tokyo, Japan). Transmission electron microscopy (TEM) images were obtained in a JEOL JEM-2100F (STEM) (JEOL Ltd., Tokyo, Japan). X-ray diffraction (XRD) analyses were obtained in a Philips X’Pert analytical diffractometer (Malvern Panalytical, Almelo, The Netherlands) using Cu-Kα as the radiation source, operating at 40 kV and 30 mA of current with a step size of 0.02°/min from 10 to 90 deg (2θ). The X-ray photoelectron spectroscopy (XPS) technique allowed studying the electronic environment of the species in the catalysts employing a Thermo-Scientific device (Thermo Fisher Scientific, Waltham, MA, USA) coupled with an X-ray source, provided by an Al-Kα monochromator. The diffuse reflectance spectroscopy (DRS) UV–Vis spectra, and band-gap energy determination were performed in a Varian Cary-5000 UV/Vis spectrophotometer (Agilent Technologies, Santa Clara, CA, USA). FTIR experiments under reaction conditions were performed in an Agilent 660 spectrophotometer (Agilent Technologies, Santa Clara, CA, USA). In a previous step, the catalysts were submitted to a gas mixture (H₂/N₂ 25% vol H₂), progressively increasing the temperature value up to 300 °C. Once the reduction process was achieved, the flowing gas was shifted for a reaction mixture gas (CO/O₂/N₂ molar ratio 1/1/98). The contact time W/FCO value was 30 g_cat, h mol_CO⁻¹. The temperature of the FTIR reactor started at room temperature and further increased up to 280 °C at a rate of 2.5 °C/min.

2.3. Catalytic Reaction Tests

The CO catalytic oxidation experiments were performed in a temperature range of 25–350 °C using an 8 mm internal diameter reactor at atmospheric pressure conditions. The reagents flow was adjusted for a total flow of 60 cm³/min. The composition consisted of 2% CO, 2% O₂, and 96% N₂, as a balance gas (W/F = 30g hmol⁻¹). A sample of 0.1g of catalyst was used for each test. Previously, samples were reduced in situ at 300 °C with a mixture of 2% H₂ and 98% N₂ for 1 h. The reaction products were analyzed using an on-line gas chromatograph provided with a TCD system (thermal conductivity detector).

3. Characterization Techniques and CO Conversion Results

3.1. Scanning Electron Microscopy and EDS Analyses

Figure 1a shows the CeO₂ compound at a magnification of 50,000×, consisting of flat plates with different sizes. An important feature of those structures is that when observed at a higher magnification of about 100,000×, Figure 1b, they appeared fully porous, allowing gas to pass through the pores. Figure 1c,d shows the grain structure of the CeRuO₂ compound, where plate-like morphologies coexist with small, rounded grains.

Continuing the analysis, Figure 2a,b shows the Pt/CeRuO₂ compound, where flat-plane structures with porous and rough surfaces are clearly distinguished. The EDS analysis in Figure 3b confirmed the presence of Pt, Ce, Ru, and O.

Under a similar approach, the Ir/CeRuO₂ compound exhibits analogous porous plate-like morphologies, as observed in Figure 2c,d. Thus, it can be concluded that the Ir procedure to disperse the nanoparticles does not induce significant morphological changes compared to the CeRuO₂ support. The EDS analysis shown in Figure 3c for the Ir/CeRuO₂ compound revealed characteristic signals of Ir, Ce, and Ru. This structural configuration found in Pt/CeRuO₂ and Ir/CeRuO₂ compounds may be beneficial, as the combination of porosity and flat surfaces can facilitate gas transport across the material.

An EDS analysis of the CeRuO₂ surface detected only Ce, Ru, O, Pt and Ir elements, as can be appreciated in Figure 3, (a) CeRuO₂, (b) Pt/CeRuO₂, and (c) Ir/CeRuO₂ compounds.

3.2. X-Diffraction Studies

X-ray diffraction analyses were performed in samples to ensure the formation of the FCC structure, see Figure 4. Thus, ICCD card number 34-0394 was used to identify that the CeO₂ sample matched properly with the CeRuO₂, Ir/CeRuO₂, and Pt/CeRuO₂ catalysts. The main characteristic planes related to the CeO₂ structure were located at 28.3, 32.96, and 47.30° (2θ), which were ascribed to the (111), (200), (200) planes, respectively, see Figure 4. In addition, a shifting towards lower-angle values was observed in samples doped with ruthenium. Analyzing it in detail, an amplified image in the 25–35 (2θ) range is shown in Figure 5. It shows high-resolution X-ray diffraction patterns performed for the CeO₂ and CeRuO₂ catalysts. As has been previously pointed out, a shift towards low angles is observed in CeRuO₂, as well as in the supported samples.

After calculating the cell parameters for CeO₂ and CeRuO₂ samples, a decrease in cell volume was observed, probably related to the substitution of some Ce sites for Ru ions. Ruthenium has a smaller atomic radius (0.133 nm) compared to Ce (0.183 nm). This observation is consistent with previous reports such as Kurnatowska et al. [20], suggesting that partial substitution occurs. In addition, the crystallite size of the samples was estimated using the Scherrer equation, yielding the results summarized in

Table 1. Crystallite size calculation for CeO₂, CeRuO₂, Pt/CeRuO₂, Ir/CeRuO₂ compounds.

Compound	Crystallite Size (nm)
CeO2	11.11
CeRuO2	8.55
Ir/CeRuO2	6.06
Pt/CeRuO2	7.12

.

$$D={\displaystyle \frac{K\lambda }{W\cos \theta }}$$

(1)

where D: crystallite size, K: 0.94 is the shape constant, λ: 0.15405 nm, the X-ray wavelength for Cu-Kα radiation, W: full width at half maximum of the (111) plane, and θ is the diffraction angle. Accordingly, the higher value for the crystallite size was 13.41 nm for the CeO₂ compound, followed by the CeRuO₂ support. The wet impregnation process used to disperse the Pt or Ir, nanoparticles on the CeRuO₂ support, followed by subsequent hydrogen reduction, promotes a reduction in the crystallite size. This effect, in turn, may enhance the surface/volume ratio, leading to improved catalytic performance.

3.3. Specific Surface Area Determination

The adsorption–desorption isotherms of the supported catalysts are shown in Figure 6 and summarized in

Table 2. Specific surface area and pore size distribution.

Compound	Specific Surface Area (m2/g)	Pore Size (nm)
CeO2	136.62	5.67
CeRuO2	148.3	4.40
Pt/CeRuO2	149.3	5.53
Ir/CeRuO2	123.4	6.85

. The isotherm trends can be ascribed as type IV according to the IUPAC classification, corresponding to a typical mesoporous material. Analysis of the hysteresis loops indicate an H3-type behavior, characteristic of materials containing highly dispersed porous plates, being consistent with SEM analysis discussed earlier. The specific surface area for the CeO₂ catalyst was determined to be 136.62 m²/g, while those corresponding to CeRuO₂ (148.30 m²/g), Pt/CeRuO₂ (149.30 m²/g), and Ir/CeRuO₂ (123.40 m²/g) compounds were relatively similar. The pore size distribution also followed a comparable trend.

3.4. XPS Analyses

XPS analyses were carried out in two stages. Firstly, a general survey was conducted for every sample. In a second stage, high-resolution analyses were carried out to determine the oxidation state of the Ce, Ru, Pt, and Ir elements.

The surface composition of the samples was investigated by X-ray photoelectron spectroscopy (XPS). The survey spectrum of the CeRuO₂ support (Figure 7) shows the characteristic signals of Ce, Ru, and O, confirming the successful incorporation of Ru into the CeO₂ lattice without detectable impurities. The Ce 3d spectrum was deconvoluted into several components corresponding to Ce³⁺ and Ce⁴⁺ species (Figure 8). The peaks located at approximately 880.9, 884.8, 898, and 916 eV were assigned to Ce⁴⁺, while the components at 900.1 and 902.7 eV were attributed to Ce³⁺ species [14]. Based on the integrated peak areas, the relative concentration of Ce³⁺ was estimated to be approximately 15%, whereas Ce⁴⁺ accounted for about 85% of the total cerium species. The presence of Ce³⁺ indicates the formation of oxygen vacancies in the ceria lattice, which are known to enhance oxygen mobility and promote catalytic oxidation reactions.

For the Pt/CeRuO₂ catalyst, the survey spectrum, Figure 9a, displays the expected contributions from Ce, Ru, O, and Pt, while the high-resolution Pt 4f spectrum, Figure 9b, reveals two main peaks at 71.0 eV and 74.9 eV, corresponding to metallic Pt⁰ and oxidized Pt⁴⁺ species, respectively. Although noble metals are preferably obtained in the metallic state (Pt⁰), partial oxidation is common. In this case, the more intense signal associated with Pt⁰ indicates that the reduction treatment was effective in promoting a high fraction of metallic platinum, which is beneficial for catalytic activity. Regarding the Ir/CeRuO₂ catalyst, the survey spectrum, Figure 9c, evidences the presence of Ce, Ru, O, and Ir. The high-resolution Ir 4f spectrum, Figure 9d, shows two well-defined peaks centered at 60.9 eV, characteristic of metallic Ir⁰ species. The absence of contributions from oxidized Ir indicates that the reduction conditions applied during synthesis were sufficient to drastically reduce iridium oxides, favoring the presence of metallic iridium as the dominant active phase. These results confirm that both Pt and Ir are successfully incorporated onto the CeRuO₂ support, with platinum present as a mixture of metallic and oxidized species, while iridium is stabilized in its metallic state.

3.5. UV–Vis Spectroscopy

In Figure 10, the UV–Vis spectra of the samples are presented. The signals corresponding to Ce³⁺ and Ce⁴⁺ are located at 275 and 340 nm, respectively. As can be appreciated, the curves are quite different for every compound. The increase in the absorbance value is evident, following the order CeO₂ > CeRuO₂ > Ir/CeRuO₂ > Pt/CeRuO₂, which can be attributed to the presence of Ir or Pt nanoparticles at the surface’s catalysts In addition, a plasmon signal associated with ruthenium species appears at 480 nm.

3.6. Band-Gap Determination

The energy gap values were determined from diffuse reflectance spectroscopy (DRS) UV–Vis measurements. The absorbance data were transformed into (αhν)² vs. photon energy (hν) plots using the Tauc method, from which the band-gap (Eg) values were obtained and are summarized in Figure 11. The CeO₂ compound showed an energy gap value of 2.97 eV. The energy gap values followed the order CeRuO₂ (2.84 eV) < Pt/CeRuO₂ (2.85 eV) < Ir/CeRuO₂ (2.90 eV). Those values mainly reflect the positive effect of the CeRuO₂ mixed oxide catalytic support.

3.7. FTIR In Situ Studies for the CO Oxidation Reaction

The results of the FTIR spectra for the Pt/CeRuO₂, Ir/CeRuO₂ catalysts are summarized in Figure 12. In Figure 12a,b, signals located at 2360 and 2341 cm⁻¹ are ascribed to CO₂, while the peaks sited among 2193–2186 and 2120–2122 cm⁻¹ are related to CO linearly linked to Lewis’s acids sites of CeO₂, Ce⁴⁺, and Ce³⁺, respectively. On both images, at room temperature, the Ce³⁺-CO signal is more intense than that of Ce⁴⁺. This effect is attributed to the presence of Ru species promoting the reduction at low temperature.

Regarding the Pt/CeRuO₂ catalyst, following the passing of the reaction gas mixture through the chamber at room temperature, signals corresponding to 2183, 2122, 2072, and 1981 cm⁻¹ were detected. When temperature values above 180 °C were reached, a new band appeared at 2186 cm⁻¹, which could be related to asymmetric vibrations of Ir⁺-(CO)₂ di-carbonyl species [21]. This absorption band gradually increased above 180 °C, reaching its highest intensity at 280 °C. The in situ FTIR spectrum for the supported Ir catalysts is shown in Figure 12b. Basically, this compound follows a similar trend to that previously described for the Pt supported catalyst; however, the values where the signals appear are slightly shifted due to the Pt/CeRuO₂ surface charge interaction. Thus, the incorporation of Pt and Ir metals improved gas absorption on the Ce₀RuO₂ support and, consequently, its activity. The evidence of CO₂ production is evident for both compounds as temperature values as low as 60 °C. Furthermore, the addition of the metal increased the number of oxygen vacancies, which are the preferential sites to carry out the catalytic oxidation.

4. CO Oxidation Tests

The CO oxidation tests were carried out on the catalysts and the support, where CeO₂ is the reference material, and the results were condensed into a single graphic image, Figure 13. The reaction reveals that CeO₂ was activated around 175 °C, reaching 90% of conversion at a temperature of 350 °C. This result has been observed and attributed to delayed mobility of lattice oxygen, which appears to be activated after 225 °C. Quite differently, the CeRuO₂ support shows a prominent behavior demonstrating that ruthenium inclusion in CeO₂ as doping element provides a synergistic promoting effect for CO oxidation. As can be seen, this active support achieves 50% of CO conversion at about 100 °C. On the other hand, the Pt/CeRuO₂ catalyst described a similar behavior, being activated near to 60 °C, attaining 100% of CO conversion close to 150 °C, holding that value beyond 350 °C. Regarding the Ir/CeRuO₂ system, it started the CO oxidation at 75 °C, followed by a steep rise in a short temperature interval of 100–110 °C, showing a dramatic change from 25 up to 90% of CO conversion, to finally reach 100% of CO conversion at 125 °C. That means there is an outstanding increase in conversion, where this behavior is attributed to the increase in surface absorption sites, mainly vacant sites where O₂ has been adsorbed, to subsequently dissociate. Simultaneously, the decrease in activation temperature is promoted by the presence of ruthenium species.

5. Discussion

We establish a comparison with previously reported works on CeO₂ doped catalysts for CO oxidation which are condensed in

Table 3. Comparative analysis of CO oxidation using different catalysts or conditions.

Catalyst	Preparation Method	Temperature °C	Maximum Conversion (%)	Ref.
${\mathrm{C}\mathrm{u}/\mathrm{C}\mathrm{e}}_{1-\mathrm{y}}{\mathrm{C}\mathrm{o}}_{\mathrm{y}}{\mathrm{O}}_{\mathrm{x}}$	Coprecipitation	110–125	100	[24]
${\mathrm{C}\mathrm{e}}_{0.9}{\mathrm{C}\mathrm{o}}_{0.1}{\mathrm{O}}_{\mathrm{x}}$	Solution combustion method	125	100	[25]
M-Ce-Ox (M = Fe, Co, Ni, Cu) alloys	microdrop-confined method	110	100	[26]
Pt(N)/CeO2(N)	Precipitation/Wte impregnation	175–190	90	[27]
Pt/CeRuO2 Ir/CeRuO2	Hydrothermal Microwave Heating	125 150	100	This work

; for instance, the preparation of a series of Ce_yIn_1−yO_x compounds, where the indium content was varied has been reported. The results attained reveal that small amounts of indium doping provides lower activation temperature, reaching 100% of CO conversion from 110 to 125 °C [24]. Also, CeO₂ doped cobalt has been tested as a catalyst for CO oxidation, showing similar results, reaching 100% of CO conversion at a temperature of 125 °C [25]. Recently, the preparation of transition metal-embellished cerium oxide (M-Ce-Ox) mesoporous alloys, using Co, Ni, Co, and Fe has been reported. Those catalysts were prepared by means of a micro drop-confined method. This method attempts to promote defects creation and oxygen availability. Their results indicate that catalytic activity is improved in comparison to pristine CeO₂ in the order Cu > Ni > Co > Fe, reaching 100% of CO conversion at about 110 °C [26]. Y. Shen et al. [27] reported the preparation of Pt/CeO₂ catalysts which were modulated in their reactivity through the addition of HNO₃ to produce Pt (N)/CeO₂ (N) catalysts. The HNO₃ was added during support treatment and/or Pt impregnation. Their results indicate that nitric acid weakened the Pt-CeO₂ interaction and facilitates the removal of lattice oxygen from Pt-O-Ce bonds under reaction conditions, generating active ${Pt}^0, {Pt}^1$ species, showing superior Pt dispersion. Their results allowed obtaining 100% of CO conversion at a temperature of 175 °C. In comparison, our results are close to those obtained from these authors; however, we consider it important to emphasize that particularly, ruthenium addition to CeO₂ provides substantial defects creation, which, in turn, enhances the oxygen mobility and long-term stability.

Activation Energy Determination

The activation energy, $E_A$, for the different catalysts was determined at the start of every reaction. To perform the calculation, the Arrhenius equation was used:

$$\ln {-r}_A=\ln A-{\displaystyle \frac{E_A}{R}}\left({\displaystyle \frac{1}{T}}\right)$$

(2)

where r_A: reaction velocity, A: is the frequency factor, E_A: is the activation energy, R: de Rydberg constant of gases, and T is the temperature value, expressed in K degrees. This calculation is valid only for a packed reactor behavior, considering that the gas concentration is the same on the catalyst. It can be used when the conversion from CO to CO₂ is relatively low, under 20%. In

Table 4. Activation energy (Ea) for CeO₂, CeRuO₂, Pt/CeRuO₂, Ir/CeRuO₂ compounds.

Compound	Ea (J/mol)
CeO2	9209.212
CeRuO2	1366.305
Pt/CeRuO2	1963.678
Ir/CeRuO2	4386.761

, the results are summarized relative to those calculations.

As can be seen, the energy value for the CeO₂ compound, E_A = 9209.212 J/mol, is the largest compared to the other compounds. In contrast, the smaller value was found for the CeRuO₂ mixed oxide (E_A = 1366.305 J/mol), confirming the inherent activity of the catalytic support. The Pt/CeRuO₂ catalyst with E_A = 1963.678 J/mol exhibited the lowest activation energy among the metal-supported systems, followed by the Ir/CeRuO₂ compound. At the same time, we should notice that the specific surface area values followed the order CeRuO₂ > Pt/CeRuO₂ > Ir/CeRuO₂ > CeO₂ even though we consider that the area values are not the most important factor to achieve high conversion rates for the present study.

6. Conclusions

In accordance with the objectives initially set, Pt/Ce_1−xRu_xO₂ and Ir/Ce_1−xRu_xO₂ catalysts were successfully synthesized and tested in the CO oxidation reaction. Comparatively, Pt/CeRuO₂, Ir/CeRuO₂ catalysts, and even the CeRuO₂, exhibited higher activity in comparison to CeO₂. It is worth it to express that CeRuO₂ is an active support by itself. However, it performs best when Pt or Ir are dispersed on its surface. When comparing all the systems, the Ir-supported catalyst achieved 90% and 100% CO conversion at lower temperature. On the other hand, the calculated energy activation, under 20% conversion, revealed that Pt/CeRuO₂ (−1963.678 J/mol) owns the lower energy activation, followed by the Ir/CeRuO₂ catalyst (−4386.761 J/mol). However, surprisingly, this catalytic system showed an astonishing behavior with a higher slope in reference to the temperature value, revealing 100% CO conversion at a temperature of about 120 °C. Conclusively, it must be expressed that both Pt and Ir catalysts were synergistically working with the CeRuO₂ active support, reaching 100% CO conversion at relatively low temperatures. In our view, the Ir/CeRuO₂ catalyst deserves a deeper study for better understanding of this system.