Ag-Pt/Al₂O₃-WOx Catalysts Supported on Cordierite Honeycomb for the Reduction of NO with C₃H₈, CO, and H₂

Abstract

Selective catalytic reduction (SCR) of NO using various reducing agents is a critical area of research for mitigating environmental pollution. In this study, the influence of active phase loading was investigated in four bimetallic Pt-Ag/Al₂O₃-WO_x catalysts, one monometallic Ag/Al₂O₃-WO_x catalyst, and one Pt-Ag/Al₂O₃-WO_x catalyst subjected to high-severity air-SO₂ pretreatment. All catalysts were supported on cordierite monoliths, and their performance in NO SCR was evaluated using H₂, C₃H₈, and CO as reducing agents. An increase in the active phase loading (Pt-Ag/Al₂O₃) from 10.7 wt% to 17.4 wt% resulted in higher conversions of NO, C₃H₈, and H₂, as well as improved N₂ selectivity. However, CO conversion decreased as the active phase loading increased, which was attributed to competitive reduction by H₂, since both reactions occur within the same temperature range (100–200 °C). The presence of N₂O below 6 ppm was observed in some catalysts. Furthermore, higher active phase loadings led to increased carbon deposition; the Ag/Al₂O₃-WO_x catalyst exhibited the highest carbon content (5 wt%). The deposited carbon was identified as ordered graphitic carbon. In the Pt-Ag catalysts, the presence of Ag⁺ and Agⁿδ⁺ species, as well as the Ag° plasmon, was identified by UV-Vis spectroscopy. STEM analysis showed Ag-Pt crystallites with an average size of 24 nm, which may have contributed to the higher NO conversion observed at 350 °C and the improved N₂ selectivity at 100 °C in the Pt-Ag bimetal catalysts, compared to the activity of the Ag/Al₂O₃-WOx catalyst.

1. Introduction

Diesel engines contribute to increasing greenhouse gas emissions; they emit NOx, CO₂, CO, and unburned hydrocarbons. However, NOx and particulate matter (PM) are of great concern due to their effects on human health and the environment. This has led to a further increase in legislation to reduce NOx emissions in recent decades [1,2,3].

Catalytic processes have been designed and developed to convert NOx into nitrogen and reduce environmental pollution. The selective catalytic reduction (SCR) of NOx with hydrocarbons and oxygenates has remained among environmental catalysis research processes as a promising method for the reduction of NOx in the exhaust gases of diesel and lean combustion engines [4,5,6].

However, not only have hydrocarbons (HC) been studied as reducers, but also alcohols such as methanol, ethanol, and butanol [6,7,8]. Some of the most used reducers (HC) are methane, propene, n-propane, n-butane, and n-octane [7,8,9]. The SCR-NOx system with HC (or HC-SCR) is of great interest because, in the presence of an oxidizing atmosphere, just as diesel engines work, it is possible to eliminate NOx [10,11].

Among the various catalytic systems proposed in SCR-NOx technology with organic reducing agents, there are noble metals (Rh, Ru, and Pt) supported on metal oxides, obtaining promising results with hydrocarbons of short chain (HC) [12]. However, since the early work of Held et al. [13] and Iwamoto et al. [14], a large number of different materials have been proposed and tested for HC-SCR. Among these, Ag/Alumina has shown high activity both in the laboratory and in real-scale tests [15].

Ag-alumina is of great interest due to its high activity, good selectivity to form N₂, good stability against water vapor, and tolerance to SO₂ [6,7,16,17]. Various studies that have been carried out mainly with C₃H₆ and through impregnation methods of the Ag precursor in Al₂O₃, have identified that the appropriate concentration of Ag on the support is between 1 and 3 wt% [18].

Studies have also been carried out on the nature and role of active Ag species during NOx reduction in the presence of C₃H₆ and water [9,10,11,12,13,14,15,16,17,18,19]. The SCR of NO with Ag/Al₂O₃, catalysts in the presence of O_2, depends on the concentration and type of Ag on the surface in the form of Ag⁺ cations and Ag_n nanoclusters (n = 8). The Ag⁰ nanoparticles have catalyzed the oxidation of total hydrocarbons or alcohols to CO₂ and H₂O [16].

On the other hand, the Ag/Al₂O₃ catalyst presents some limitations, such as its high activity in a narrow operating temperature window, and the activity decreases below 400 °C in the case of SCR with light hydrocarbons [6,16].

However, despite these limitations, several studies have investigated the expansion of the operating temperature window through the addition of H₂ [19,20]. The results indicate that enhanced NO reduction is mainly achieved in two distinct temperature ranges: a low-temperature range (80–180 °C) and a high-temperature range (180–480 °C). In these regions, NO is effectively reduced by C₃H₈ in the presence of H₂ and CO.

In the search for other reducers, the mixture of H₂ and NH₃ together has also been investigated [21,22], obtaining good reduction results at low temperatures, although there may be NH₃ emissions. In a study carried out by Shang Z. et al. [23], the combined effect of CO in the presence of C₃H₈ was analyzed, and the temperature window was expanded, but a catalyst of 5 wt% Ag/Al₂O₃ was required.

For its part, the addition of noble metals such as Pt to Ag [24] is useful to obtain high conversions of NO at low temperatures, using octane as a reducing agent. The catalytic composition that showed the greatest activity for the reduction of NOx was a catalyst with 2wt% Ag/Al₂O₃ doped with 500 ppm of Pt. This catalyst showed a greater capacity to adsorb and partially oxidize the hydrocarbon (reductant), where the Pt has a predominant role.

Other studies of NO reduction were investigated with Pt, Rh, and Ag/Al₂O₃ with C₃H₆, and Ag was the most active at higher temperatures (300 °C), while Pt and Rh were active at lower temperatures (200–250 °C) [25]. The Pt catalysts supported on WO₃/ZrO₂ with the presence of H₂ without Ag in the reduction of NO [26] found high activity at temperatures below 200 °C and high selectivity to N₂ (90%). Furthermore, the catalyst showed excellent hydrothermal stability as well as resistance to SO₂.

Previous studies have shown that the addition of small amounts of WOx (<1 wt%) to Al₂O₃ or a Pt/Al₂O₃ catalyst results in increased thermal stability of both the Al₂O₃ and the Pt catalyst, increasing the resistance to noble metal sintering [27,28].

This is the reason why we have adopted these WOx, especially to preserve the thermal stability of the Pt species, in order to increase the resistance to deactivation by sintering, stabilizing the porous structure of the Al₂O₃. This effect helps keep the support area after high-temperature heat treatments (up to 800 °C), and also this effect could extend to Ag species (Ag⁰ and Ag⁺ cations and Ag_n nanoclusters).

In this study, we focused on obtaining monolithic catalysts based on solid honeycomb cordierite supports containing M-Al₂O₃-WOx (M = Ag, Pt [29], or Ag-Pt) catalytic systems. As mentioned previously, the study contributes to improving the performance of the Ag/Al₂O₃ catalyst by adding minimal amounts of Pt and WOx.

The catalytic activity of the Ag-Pt/Al₂O₃-WOx catalyst in the reduction of nitrogen monoxide (NO) in the presence of C₃H₈ and subsequent addition of H₂ as a reducing gas has allowed obtaining a higher conversion of NO at low temperature (120 °C) than when H₂ was not added. The study combines the presence of the reductants H₂, CO, and C₃H₈ and the presence of small amounts of Pt, using a synthetic mixture of emission gases (C₃H₈, CO, CO₂, NO, O₂, and water vapor) similar to those emissions from diesel engines.

2. Results and Discussion

2.1. Characterization of Catalysts

2.1.1. Textural Properties

The Al₂O₃-WOx support deposited on cordierite monoliths (γ-Al₂O₃-WOx/Cordierite) showed type IV isotherms (IUPAC) (Figure 1a), which indicate the presence of multilayer physical adsorption, characteristic of mesoporous solids [30]. The γ-Al₂O₃-WOx/Cordierite sample presented a high area and the largest pore volume (

Table 1. Texture and area properties of the catalysts/monoliths calcined at 500 °C for 6 h and after reaction in the C₃H₈-SCR of NO at 350 °C.

Catalyst Name	Key	Fresh			Evaluated
		SBET (a) (m2/g)	Vp (b) (cm3/g)	Dp (c) (Å)	SBET (m2/g)	Vp (cm3/g)	Dp (Å)
Al2O3-WOx/Cordierite	AW/CRTA	55.89	0.09	28.59	---	---	---
0.1Pt-2Ag/Al2O3-WOx/Cordierite	CAT.1	20.90	0.03	66.62	10.41	0.02	38.94
0.1Pt-2Ag/Al2O3-WOx/Cordierite	CAT.2	20.64	0.03	55.26	10.39	0.02	48.13
0.1Pt-2Ag/Al2O3-WOx/Cordierite	CAT.3	21.00	0.03	59.00	15.20	0.02	33.45
0.1Pt-2Ag/Al2O3-WOx/Cordierite	CAT.4	18.39	0.06	42.91	12.74	0.04	39.02
2Ag/Al2O3-WOx/Cordierite	CAT.5	47.55	0.07	63.58	26.33	0.04	32.43
0.1Pt-2Ag/Al2O3-WOx/Cordierite	CAT.6	25.54	0.04	39.00	12.91	0.03	36.65

^(a) Specific surface area; ^(b) pore volume; ^(c) pore diameter.

), and a unimodal pore distribution was observed, as reported by Neimark et al. [31], in the alumina synthesized for use as a support in the reaction of SCR of NO with C₁₀H₂₂.

The fresh catalysts CAT.1 with Pt-Ag and CAT.5 with only Ag also presented type IV isotherms. The characteristic of this type of isotherm is the presence of a hysteresis cycle, which is associated with the capillary condensation phenomenon that occurs in the mesopores (Figure 1a) [32].

The pore volume distribution is shown in Figure 1b; the support (γ-Al₂O₃-WOx/Cordierite) showed the greatest variation (ΔV/ΔDp) located at 24 Å, which is between the range of micropores and mesopores [33].

The variation (ΔV/ΔDp) of the CAT.1 and CAT.5 catalysts drops drastically (2/3) with respect to the support (γ-Al₂O₃-WOx/Cordierite) due to the impregnation of the Pt and Ag precursors, respectively. The pore size distribution was also unimodal, indicating that there were no drastic changes in pore size distribution during the preparation of these catalysts.

After the catalytic evaluation, the type IV isotherm persists in the CAT.3, CAT.4, and CAT.6 catalysts (Figure 2a) and has H2-type hysteresis [32,34]. This type of cycle is characteristic of pores and solids formed by small spherical particles [32,34]. The pore size distribution was unimodal (Figure 2b).

The CAT.2 catalyst shows an H3-type hysteresis (Figure 2a), associated with aggregates of particles in the form of parallel plates that give rise to slit-shaped pores [35]. This material presented a bimodal pore distribution (Figure 2b), where the formation of large pores (63 Å) corresponds to the residual spaces between particles existing or formed after the reaction [36]. The presence of small pores did not change (24 Å) as expected.

Table 1 shows the areas and pore size distribution of fresh catalysts (calcined at 500 °C for 6 h and reduced at 500 °C for 3 h) and evaluated in the NO reduction reaction (at 350 °C for 4 h).

2.1.2. Crystalline Properties

The X-ray diffraction pattern of the CAT.1 catalyst (Figure 3a) shows the intense and sharp peaks characteristic of crystalline cordierite [37]; eleven diffraction peaks were found and observed at 10.35°, 18.36°, 19.11°, 21.63°, 26.38°, 28.37°, 29.58°, 33.75°, 38.45°, 43.12°, and 54° with Miller (hkl) plans corresponding to the following: (100), (110), (002), (102), (112), (202), (211), (212), (310), and (312), respectively. The crystalline phase assigned was α-cordierite (JCPDS card no. 84-1222). In addition, there are two small peaks, either in the CAT.1 catalyst or the cordierite (calcined at 1350 °C), close to 37° and 39.7°, which could be related to the corundum phase of ceramics.

The XRD for the powder catalysts (Figure 3b AW, Figure 3c 0.1PtAg/AW, and Figure 3d 1PtAg/AW) showed three broad, low-intensity reflections with 2θ = 37°, 46°, and 67°, corresponding to the (311), (400), and (440) planes of Al₂O₃ in its cubic phase (JCPDS card no. 10-0425). The characteristic peaks indicate amorphous samples with low crystallinity. This agrees with what was reported by Aguado et al. [38] and Hernández-Terán et al. [20] for γ-Al₂O₃.

The characteristic reflections of Pt and Ag were not found in the CAT.1 diffraction pattern (Figure 3a); it could also be due to the limited resolution of the XRD, since the concentrations of 0.1 wt% Pt and 2 wt% Ag were below the detection limit of the equipment.

However, some authors, such as Richter et al. [39], have observed the presence of Ag₂O at 2θ = 38.7, 50.5, and 66.8 in a catalyst with 5 wt% Ag/Al₂O₃, and although the peaks have a metastable structure, it can be said that Ag₂O is present on the surface of their catalysts.

The powder catalysts, Al₂O₃-WOx (AW) (Figure 3b), 0.1PtAg/AW (Figure 3c), and 1PtAg/AW (Figure 3d), did not show signals related to Pt and Ag species, again, due to the low concentration of these metals in the sample.

2.1.3. Scanning Electron Microscopy (SEM/EDX) Before Catalytic Evaluation

SEM images of the inner walls of the support and the catalysts supported on the surface of the cordierite monoliths are shown in Figure 4. SEM images of the cleaned cordierite monolith are shown in Figure 4a–c at different magnifications (25X, 500X, and 20,000X, respectively). Figure 4c shows macropores on the cordierite surface, revealing scale-like structures that could harbor a certain amount of boehmite.

The secondary coating of the support (γ-Al₂O₃) obtained using the dip-coating method (Al₂O₃-WOx/Cordierite), Figure 4d–f, shows the coating within the monolith channels. In this coating, good surface homogeneity was observed (Figure 4e), reaching a thickness of approximately 20 µm. After coating the support, approximately 17.4% by weight of γ-Al₂O₃-WOx was obtained on the cordierite monolith, using the dip-coating method with a boehmite suspension. In all these figures, the porosity type was very different from the type observed in fresh cordierite.

The secondary support coating formed a root-shaped layer within the soil or, in this case, within the monolith’s channels, which helped improve the adhesion or interaction with the active layer (Ag and PtAg) compared to the interaction with the clean monolith.

Sandeeran Govender and Holger B. Friedrich [40] mention that suspension coating can be ideal for high weight percentage coatings and that the particle size of the coated alumina can be controlled. Suspension coating can offer fewer recoats and good alumina adhesion.

Using this technique on the CAT.1 (PtAg/Al₂O₃-WOx/Cordierite) catalyst, Figure 4g,h were very similar to the figures where only Al₂O₃-WOx was added (Figure 4d,e). In contrast, spherical particles, or groups of them, deformed between 150 nm and 350 nm, are already observed on the CAT.1 catalyst (Figure 4i).

In the case of Figure 4j,k of the CAT.5 catalyst (Ag/Al₂O₃-WOx/Cordierite), no significant differences are observed with the corresponding Al₂O₃-WOx/Cordierite samples (Figure 4d,e) and the CAT.1 sample (Figure 4g,h).

On the other side, at higher resolution, spherical particles with sizes between 250 nm and 310 nm were observed for the CAT.5 catalysts (Figure 4l). Similar particles in shape and texture are again observed, but in the CAT.1 catalyst (Figure 4i).

Two different zones of the clean cordierite were analyzed by energy dispersive spectroscopy (EDX). Figure 5a shows the results of the qualitative chemical analysis and the presence of the elements detected in these two areas. The presence of Au and Pd in the spectrum of the cordierite monolith (Figure 5a) was due to the coating that the sample received before characterization to make it conductive.

The presence of oxygen, silicon, aluminum, and magnesium in a greater proportion was observed; this is indicating that cordierite is indeed found, since, as mentioned by Bueno-López et al. [41], cordierite is a mineral belonging to the ternary system MgO, Al₂O₃, and SiO₂. The most accepted global formula for this inorganic compound is 2MgO₂ Al₂O₃.5SiO₂ with a stoichiometric composition of 51.36% SiO₂, 34.86% Al₂O₃, and 13.78% MgO.

Qualitative EDX analysis of the Al₂O₃-WOx/Cordierite support (Figure 5b) indicates the presence of cordierite. O, Si, Al, and Mg are present in major proportions, while Fe, Ti, and Ca are present in smaller proportions.

Figure 5c shows the spectrum of the CAT.5 catalyst (Ag/Al₂O₃-WOx/Cordierite); the elements that compose cordierite (O, Al, Si, and Mg) are also present, as well as the active metal Ag. Figure 5d shows the spectrum of the CAT.1 catalyst (AgPt/Al₂O₃-WOx/Cordierite), which indicates the presence of cordierite, as well as the elements W, Pt, and Ag.

The elemental mapping of the CAT.1 catalyst (PtAg/Al₂O₃-WOx/Cordierite) coated on the monolith is shown in Figure 6a–f by EDX. The Al₂O₃-WOx layer with a thickness of between 15.3 and 28.3 µm grew uniformly on the channel walls of the cordierite monolith.

Figure 6a shows the image of the analyzed section of the catalyst. Figure 6b,c show the densest atomic distributions of O₂ and Al, while Figure 6d shows a fairly good density of Ag. Next, we have that in Figure 6e for Pt, the lowest density was observed. Finally, Figure 6f shows the distribution of W, also in a small amount. These densities of Pt atoms compared to those of W correspond approximately to the amounts of 0.1 wt% Pt and 0.5 wt% W.

2.1.4. Scanning Transmission Electron Microscopy (HAADF-STEM)

For the powder-calcined catalyst (0.1PtAg/AW), bright Ag nanoparticles were found in Figure 7a. These metal particles with high density contrasted with the Al₂O₃ support have round shapes without being perfectly spherical.

Figure 7b shows the particle diameter histogram, obtaining an average particle size of 35.3 nm in monomodal form with sizes ranging from 20 to 140 nm. The shape of these particles was similar (shape and intensity) to those obtained by Kondratenko et al. [42], Hernández y Fuentes [20], and Gauthard et al. [43].

Kondratenko et al. [42] have mentioned that they correspond to polycrystalline silver oxide (Ag₂O). In the case of Gauthard et al. [43], PtAg/Al₂O₃ bimetallic catalysts by TEM found a monomodal distribution of particle sizes that varied from 10 to 200 nm and with a composition by EDX that varied from 20 to 80 at% Ag.

Hernández et al. [20] have also found similar particles by HRTEM with a 2 wt% Ag/Al₂O₃ catalyst, having an average particle size of 11 nm. These authors identified AgO, Ag₂O, and Ag⁰.

In the case of the 1 wt% Pt-2 wt% Ag/Al₂O₃ (1PtAg/AW catalyst), Figure 7c also shows spherical particles with sizes similar to the sizes found in the previous sample. Figure 7d shows the particle size histogram of this catalyst, where a monomodal distribution with an average size of 18.47 nm was found.

The particles analyzed for this catalyst showed concentrations of Pt and Ag evaluated by EDX that were correlated with the particle diameter observed in the analysis zone within a selected region of the STEM image.

In the case of the calcined CAT.1 catalyst, Figure 7e shows Ag nanoparticles (metal with high density) are observed, which contrast with the Al₂O₃ support, having round shapes without being perfectly spherical. The STEM analysis of the CAT.1 catalyst is shown in Figure 7e, where Ag (high-density metal) nanoparticles are observed that contrast with the Al₂O₃ support, presenting round shapes without being perfectly spherical. Figure 7f shows the particle size histogram, obtaining an average particle size of 24.04 nm in monomodal form with sizes ranging from 10 to 60 nm.

The shape of these particles is like that obtained in the 1PtAg/AW catalyst (Figure 7b) and may correspond, as already mentioned, to polycrystalline silver oxide (Ag₂O) [42].

2.1.5. Temperature-Programmed Reduction (H₂-TPR)

The TPR results of the Pt-Ag/Al₂O₃-WOx catalysts are presented in Figure 8 and

Table 2. Reduction of peak temperatures and hydrogen consumption (µmol H₂/g) observed in the TPR experiments.

Catalyst	Peak 1	Peak 2	Peak 3	Total, H2 (μmolH2/g)	H2/(Pt or Ag)
1PtAg/AW	------	208 °C (112.94)	430 °C (8.35)	121.3	2.36
0.25PtAg/AW	65 °C (3.1)	120 °C (25.52)	430 °C (4.21)	32.8	2.5
0.1PtAg/AW	83 °C (9.16)	137 °C (2.6)	477 °C (0.166)	11.93	2.34
0.4Pt/A	83 °C (49)	------	------	49	2.24
0.4Pt/AW	------	180 °C (46)	------	46	2.39
Ag/AW	101 °C (41.7)	291 °C (29)	------	70.7	0.378

. The TPR analysis of the 2%Ag/Al₂O₃ catalyst (Ag/AW) is shown in Figure 8a. Two reduction peaks were observed: the first at 117 °C, corresponding to the reduction of AgO clusters [44], and the second at 291 °C, which may be associated with the reduction of well-dispersed Ag₂O clusters [45]. The hydrogen consumption for the first peak was 41.7 µmol H₂/g, while for the second it was 29 µmol H₂/g (Table 2). Only these two silver oxides, AgO and Ag₂O, were identified in this catalyst. The reduction temperatures for AgO standards range from 100 to 120 °C, whereas for Ag₂O they range from 140 to 160 °C. On various supports, the reduction temperature maxima can shift by several degrees, even between γ-Al₂O₃ and η-Al₂O₃ [20]. The reduction reactions of these oxides are as follows:

2AgO + H₂ → Ag₂O + H₂O

(1)

Ag₂O + H₂ → 2Ag⁰ + H₂O

(2)

Based on the 2 wt% Ag content in the catalyst, the theoretical H₂ consumption is 185 µmol H₂/g, whereas the TPR-derived consumption from the two peaks was 71.3 µmol H₂/g. This indicates that only 38% of the theoretical hydrogen was consumed. Such a result suggests that a fraction of the silver was already present in the metallic state (Ag⁰) after calcination. These findings are consistent with those reported by Kannisto et al. [46] and Richter et al. [41].

In the case of the monometallic 0.4Pt/AW catalyst prepared with H₂PtCl₆, a peak at 280 °C was observed (Figure 8b) after calcination at 500 °C. This peak has been reported by Lieske et al. [47] at approximately 290 °C and is attributed to a platinum oxychloride species of the type [Pt⁴⁺(O)xCly]. The measured H₂ consumption was 2.39 µmolH₂/g (Table 2), which is consistent with platinum being present predominantly in the Pt⁴⁺ oxidation state.

For the monometallic 0.4 wt% Pt/Al₂O₃ catalyst (0.4Pt/A), which contains no WOx and serves solely as a reference catalyst in the TPR study and which was prepared using Pt(NO₃)₂, a peak was observed at 83 °C (Figure 8c). This peak corresponds to a hydrogen consumption of approximately 2.24 µmol H₂/Pt (Table 2), a value similar to that of the 0.4Pt/AW catalyst. In this case, the oxide present is PtO₂, consistent with platinum in the Pt⁴⁺ oxidation state. This peak coincides with that reported by Ivanova et al. [48] at 85 °C.

In the case of the 0.1PtAg/A₂W catalyst (Figure 8d), two reduction peaks were observed, one at 83 °C and another at 137 °C. The peak at 83 °C is the most intense and corresponds to the reduction of PtO₂, with a hydrogen consumption of H₂/Pt = 9.16 (Table 2), representing 77% of the total H₂ consumed. For comparison, the TPR profile of a catalyst prepared from Pt(NO₃)₂, designated as 0.4Pt/A (Figure 8c), is included. This reference catalyst displays a peak at 83 °C with an H₂/Pt consumption of 2.24, a value consistent with platinum in the Pt⁴⁺ oxidation state.

On the other hand, the peak at 137 °C may be attributed to the reduction of AgO clusters, similar to what was observed in the TPR profile of the Ag/AW catalyst, which exhibited a peak at 117 °C. In the 0.1PtAg/AW catalyst, a small peak was also detected at 430 °C, which could correspond to the reduction of AgO₂ species strongly bound to the Al₂O₃ support. A peak near 420 °C has previously been reported by Zhang and Kaliaguine [49] for an Ag/Al₂O₃ catalyst. Alternatively, this 430 °C feature may be associated with the reduction of a platinum oxychlorocomplex strongly anchored to the support, as suggested by Lee et al. [50].

In the case of the 0.25PtAg/AW catalyst (Figure 8e), two reduction peaks were observed, one at 65 °C and the other at 102 °C. The appearance of these two peaks in the TPR profile may be associated with different dispersions of Pt oxides, a phenomenon previously reported by Ivanova et al. [48] using HRTEM on Pt/Al₂O₃ catalysts. In their study, two reduction peaks (at 44 and 85 °C) were observed for platinum in an oxidation state close to PtO₂ (see H₂/Pt = 2.5 values in Table 2).

Additionally, a third peak was detected at 430 °C, likely corresponding to the reduction of a Pt oxide species strongly bound to the support [50], the intensity of which increases with increasing Pt concentration.

In the case of the 1PtAg/AW catalyst (Figure 8f), two reduction peaks were observed, one at 208 °C and another at 430 °C. These two features may be interpreted as the reduction of Pt–O–Pt and Pt–O–Al species, as proposed by Lee et al. [50] for Pt/Al₂O₃ catalysts. Based on STEM and TPR studies of a 1% Pt/Al₂O₃ catalyst, the authors concluded that the H₂ consumption peak at 150 °C corresponds to the reduction of two-dimensional Pt oxide rafts (Pt–O–Pt). The high-temperature peak they reported (380 °C) was attributed to the reduction of atomically dispersed Pt species, which can only possess a Pt–O–Al bonding configuration, with no Pt–O–Pt interactions.

Based on previous studies, the formation of atomically dispersed Pt clusters has been observed at temperatures above 300 °C under an H₂ atmosphere [51]. The peak at 430 °C could also be attributed to the reduction of AgO₂ species strongly bound to the Al₂O₃ support, as suggested by Zhang and Kaliaguine [49]. However, we consider it more appropriate to assign this peak to the reduction of a Pt oxychlorocomplex, since this feature increases with increasing Pt loading up to 1%. Moreover, when the H₂ consumption values from the peaks are summed, an H₂/Pt ratio of 2.36 is obtained (Table 2), which supports this interpretation.

2.1.6. Pt Dispersion (H₂-Chemisorption at 25 °C)

Table 3. Pt dispersion evaluated by H₂ chemisorption at 25 °C, before and after catalytic evaluation. Reaction conditions: 500 ppm NO, 1750 ppm C₃H₈, 6 vol% O₂, 600 ppm CO, 10 vol% CO₂, 12 vol% H₂O, and N₂ mixture; GHSV of 12,000 h⁻¹.

Catalysts	%DPt (a)	%DPt (b)
CAT.1	76.8	73.47
CAT.2	70.8	34
CAT.3	35.4	30.65
CAT.4	25.3	23.41
CAT.6 (c)	7.07	4.55

^(a) Pt dispersion of the catalyst reduced to 500 °C for 3 h; ^(b) Pt dispersion of the catalyst after reaction at 350 °C for 4 h; ^(c) high-severity test (gas mixture of 25 ppm SO₂/Air at 750 °C for 16 h).

presents the Pt dispersions of the catalysts prepared on cordierite monoliths. Higher Pt dispersions were observed in catalysts CAT.1 (76.8%) and CAT.2 (70.8%) compared to CAT.3 (35.4%), CAT.4 (25.3%), and CAT.6 (7.07%). In all cases, the Pt and Ag loadings were the same, whereas catalyst CAT.5 contained no Pt.

When examining the pretreatment of each catalyst, it was observed that the dynamic calcination conditions had a significant effect on Pt dispersion. Catalysts CAT.1 and CAT.2, which were calcined under a continuous air flow (30 mL/min), exhibited dispersions of 76.8% and 70.8%, respectively. In contrast, catalysts CAT.3, CAT.4, and CAT.6, which were calcined in a muffle furnace or under static conditions, showed lower Pt dispersions.

A possible explanation could be that the flow calcination improves the mass transfer coefficients of O₂ so that they can oxidize the Pt oxychlorocomplexes on Al₂O₃ towards the formation of PtO₂ and PtOxCly (Pt oxychlorides), thereby achieving a better reduction to metallic Pt than when these Pt complexes containing chlorine are not completely oxidized [47,52].

For Pt/Al₂O₃ catalysts without Ag, some authors, such as Lietz and Lieske [47,52], found that if the calcination temperature in air is 500 or 600 °C, a peak is obtained by TPR at 290 °C due to the reduction of PtClxOy (Pt⁴⁺ to Pt°), whereby the reduction at 500 °C allows for obtaining good Pt dispersions, as has been demonstrated in the literature even in the presence of WOx with alumina [26,27].

In the case of catalysts CAT.3 and CAT.4, they showed a low dispersion of Pt, probably due to a partial transformation towards Pt oxychlorocomplexes (PtOxCly) due to an insufficient supply of O₂ during static calcination (in a muffle).

This may be related to the fact that chlorinated compounds in alumina of the [PtCl₃/Al₂O₃] type require higher temperatures for their reduction to Pt° than well-oxidized, chlorine-free compounds such as PtO₂/Al₂O₃.

The CAT.6 catalyst exhibited the lowest Pt dispersion due to the high-severity pretreatment, where the catalyst received 25 ppm of SO₂ in air at 750 °C for 16 h. This low dispersion was related to the adsorption of toxic SO₂ at the Pt sites.

On the other hand, the determination of Ag dispersion and particle diameter in catalysts from 1.28 to 6 wt% Ag in Al₂O₃ was carried out with O₂ chemisorption at 170 °C by Arve et al. [53].

The samples were reduced previously in H₂ flow up to 250 °C. The authors validated the stoichiometry of O₂ chemisorption (O₂/Ag = 2) by comparing the average particle size using O₂ chemisorption techniques, bright-field TEM, and high-angle annular dark field (HAADF), and the values of active Ag dispersion that they found were as follows: 57.6% (for 1.28 wt% Ag); 51% (for 1.91 wt% Ag); 44.8% (for 2.88 wt% Ag); and 51.2% (for 6 wt% Ag). The particle sizes were as follows: 2.63 nm, 2.62 nm, 3 nm, and 2.63 nm, respectively, in the percentages of Ag mentioned above.

In addition, Hu et al. [54] studied the effect of the type of Pt precursor, the heating rate during calcination, and the way of calcining in a static air atmosphere in a muffle and dynamically with an air flow. Their investigations demonstrated that calcination in air flow and slow heating rates (2 °C/min) improved the Pt dispersion.

In order to determine the dispersion of Pt and Ag in the catalysts, we applied the pulsed chemisorption method in a flow of He in the CAT.1 catalyst. We found preliminary values of 0.43 µmol H₂/µmol Pt and 0.139 µmol O₂/µmol Pt at 25 °C and 0.596 µmol H₂/µmol Pt and 0.00212 µmol O₂/µmol Ag at 170 °C, in accordance with Arve et al. [53]. We have not yet determined the dispersion of each metal independently; we will be able to do so in the near future.

2.1.7. Ex Situ UV-Vis Spectra of the Pt-Ag/Al₂O₃-WOx/Cordierite Catalysts

Figure 9A shows the spectrum of the Pt-Ag/Al₂O₃-WOx/Cordierite catalyst. The bands are between 200 and 230 nm, corresponding to the electronic transition of Ag⁺ ions. In this same sample, a shoulder was observed around 240 and 270 nm, assignable to oxidized silver clusters (Ag_n⁺) [55,56]; however, studies carried out by Contreras et al. [57] showed this band could also be a ligand-metal charge transfer band of the single W-O ligand, because this same band was also observed for the Al₂O₃-WOx support deposited in cordierite.

Following the H₂–C₃H₈–SCR reaction (Figure 9B), the signals associated with the Ag^δ+n and Ag^_n1 clusters weakened, while the Ag^_n2 signal disappeared entirely. A new, broad absorption band emerged in the visible region, which is attributed to the surface plasmon resonance (SPR) of metallic silver (Ag⁰), indicating the formation of Ag⁰ nanoparticles [58]. These observations demonstrate that the interaction with the gas-phase components and the temperature variations induce structural modifications that alter the distribution of the different Ag species. Overall, the results indicate that the Ag/γ-Al₂O₃ catalyst possesses a complex and highly labile surface structure.

In the case of CAT.1, CAT. 2, and CAT.3 catalysts, the presence of Ag⁺ is observed, and also the bands above 390 nm (or 400 nm) were due to the formation of metallic silver Ag⁰ [24], and a plasmon was observed in these catalysts, and it was almost imperceptible in the CAT.4 catalyst.

This Ag plasmon did not appear in the CAT.5 and CAT.6 catalysts. As can be seen, the F(R) signal decreases in the following order: CAT.1 > CAT.2 > CAT.3 > CAT.4, which could probably be related to the NO conversion (see Figure 9B).

2.1.8. Raman Spectra of the Evaluated Pt-Ag/Al₂O₃-WOx/Cordierite Catalysts

Raman spectroscopy revealed the presence of carbon-containing species (Figure 10). However, it was necessary to quantify the carbon content by thermogravimetric analysis for all catalysts, as shown in Figure 11, where the water content was slightly higher than the carbon content.

It is well known that the deposition of carbon-containing species [59] contributes to decreases in both conversion and product selectivity. However, this effect was not significant in the samples, with the exception of catalyst CAT.5 (without Pt), which exhibited a noticeable decrease in NO conversion (Figure 12) above 300 °C. Moreover, as shown in Figure 12, catalyst CAT.5 produced the highest amounts of both water and carbon.

In the case of catalyst CAT.6, which was subjected to the high-severity test (exposure to an SO₂/air flow at 700 °C for 16 h), only 4.5% carbon and 6% H₂O were detected—values that were consistent with the average behavior of the other catalysts. This indicates that no significant decrease in NO conversion above 350 °C was observed (Figure 12) compared with catalysts CAT.4 and CAT.5. For this same catalyst (CAT.6), it was not possible to obtain a Raman spectrum due to luminescence caused by the presence of sulfur.

Raman spectroscopy of the evaluated catalysts (Figure 10) shows that the band located at 1322 cm⁻¹ corresponds to the D band, which arises from the A₁g vibrational mode and is attributed to disorder- or defect-related signals in sp²-hybridized carbon within aromatic rings. This type of carbonaceous species is present in all catalysts. However, an additional band at approximately 1600 cm⁻¹ is observed for CAT.1, corresponding to the G band, which is associated with the E₂g vibrational mode arising from C–C bond stretching in sp²-hybridized carbon of aromatic rings and olefinic species.

This information provides insight into structural properties such as defect density, graphitization degree, and carbon disorder [20,39,40,41,42,43,44,45,46,47,48,49,50]. These results are consistent with the quantified carbon content, as reflected in the relative intensities of the Raman features that indicate carbon deposition on the surface of the monoliths. The G band, associated with graphitic carbon, is observed in catalysts CAT.1 and CAT.5, whereas the D band—associated with amorphous carbon and structural defects—is detected in catalysts CAT.1, CAT.2, and CAT.3, confirming the presence of carbonaceous deposits on their surfaces.

Overall, the behavior of catalysts CAT.2 and CAT.3 in NO conversion above 450 °C was similar, with both achieving an average conversion of approximately 97% (Figure 12). The carbon produced on these catalysts was 2.5% and 2%, respectively.

On the other hand, when comparing the performance of catalyst CAT.1 with that of all the other catalysts evaluated, a superior activity is observed; at 500 °C, CAT.1 still maintains an NO conversion of 91% (Figure 12). This enhanced performance may be related to an optimal balance between Ag and Pt active sites. Notably, this catalyst exhibited the highest Pt dispersion (73.47%).

2.2. Catalytic Tests

NO Conversion of the Catalysts in the H₂-Assisted C₃H₈–SCR Reaction

During the NO conversion as a function of temperature for catalysts CAT.1, CAT.2, and CAT.3, a maximum conversion of 93% was observed at 350 °C (Figure 12). The performance of the three catalysts was very similar, with a shoulder appearing between 150 and 200 °C, corresponding to a conversion of approximately 40%. The activity ranking among the three catalysts was found to be as follows: CAT.1 > CAT.2 > CAT.3. This trend can be explained by considering the Al₂O₃ content within the complete monolith, which was 2.97%, 2.33%, and 2.04 wt%, respectively. The BET surface areas were 21, 20.6, and 21 m²/g, respectively.

Although the active sites for this reaction are primarily associated with Ag, the addition of small amounts of Pt (<0.1 wt% Pt) has been shown to enhance NO conversion and broaden the effective temperature window, particularly at low temperatures (100–180 °C) [29].

These advantages have also been reported by Gunnarsson et al. [24]. In their study, Pt concentrations of 500 ppm and 2% Ag supported on Al₂O₃, which was in turn deposited on cordierite monoliths, were used. The superior performance of their catalyst was attributed to the ability of Pt to adsorb hydrocarbons and partially oxidize them on the surface of bimetallic Pt-Ag particles. They further noted that the presence of Pt could lower the barrier for dissociative hydrocarbon adsorption, as well as modify the oxidation potential, effects that could be attributed to the addition of Pt particles.

In the case of catalyst CAT.4, which contained the lowest amount of Al₂O₃-WOx (1.827 g) with a BET surface area of 18.4 m²/g and low Pt dispersion (25%), a lower NO conversion was observed at 350 °C, reaching a maximum of 83%. On the other hand, when catalyst CAT.5 was prepared with Ag only (2% Ag/Al₂O₃-WOx/cordierite) without Pt, a maximum NO conversion of 78.8% at 350 °C was obtained. The Al₂O₃-WOx loading in this case was 1.997 g with a BET surface area of 47.5 m²/g. This higher BET area is due to the absence of H₂PtCl₆ in the preparation, as we have observed that the BET surface area of Al₂O₃ is almost halved when H₂PtCl₆ is used.

In the case of the Pt-Ag catalyst CAT.6, which was subjected to high-severity treatment (750 °C under an SO₂ atmosphere), the lowest NO conversion at 350 °C was observed, reaching 74.4%. This result is consistent with the high-severity pretreatment and the deactivation caused by SO₂ poisoning.

During C₃H₈ oxidation (Figure 13), the steeper conversion profile of CAT.1 (containing Pt-Ag) compared to CAT.5 (containing only Ag) is particularly evident. The change in conversion with temperature for CAT.1 is several times greater than that observed for CAT.5, which can be attributed to the presence of Pt and to the higher metal dispersion of CAT.1 (76.8%) among all the bimetallic Pt-Ag catalysts.

It is clear that CAT.1 facilitates faster C₃H₈ oxidation than CAT.5, a phenomenon also reported by Gunnarsson et al. [24]. As shown in Figure 13, between 200 and 300 °C, a region of rapid conversion increase is observed for CAT.1, which may be related to enhanced chemisorption of C₃H₈ due to the presence of small amounts of Pt on the Ag particles, as well as the corresponding oxidation of NO to NO₂. It appears that CAT.1 is able to convert a greater fraction of C₃H₈ species adsorbed on its surface (Figure 13). This behavior could be associated with modifications of the metallic surface of the Ag particles, affecting both their surface structure and the bulk of their lattice.

In the case of CO conversion, catalyst CAT.1 exhibits the lowest conversion at low temperatures between 25 and 200 °C (Figure 14), whereas all other catalysts show higher CO conversion in this range. This behavior suggests stronger inhibition of CO chemisorption on the Pt-Ag particles. The order of CO inhibition can be summarized as follows: CAT.1 > CAT.2 > CAT.3 > CAT.5. These results indicate that bimetallic Pt-Ag catalysts may chemisorb H₂, O₂, or NO more readily than CO during the reaction.

However, it is well known that CO is efficiently oxidized at low temperatures [60] over Pt/Al₂O₃ catalysts, more effectively than over other noble metals such as Pd, Rh, Ru, or Ir. In other words, Pt acts as a catalyst capable of promoting the following redox reactions:

2NO + 2CO → N₂ + 2CO₂

(3)

2CO + O₂ → 2 CO₂

(4)

In our reaction system, where O₂ is in excess, it competes with NO to oxidize the reducing agent CO [61]. It has been shown that reaction (4) predominates over a number of Pt-supported catalysts. However, when NO is added to the CO + O₂ mixture, the rate of CO₂ formation is drastically reduced [62].

Additionally, both Pt and Pd are highly effective for the combustion of CO and hydrocarbons.

Regarding H₂ conversion at 150 °C, the values for the catalysts were as follows (Figure 15, see the dotted line): CAT.1, 91.5%; CAT.2, 91.5%; CAT.3, 84.7%; CAT.4, 82.1%; CAT.5, 66%; and CAT.6, 58.9%. The activity trend appears to correlate with Pt dispersion, although in some cases this correlation is not straightforward, as observed for CAT.1 and CAT.2, which exhibit nearly identical values. It is evident that catalysts CAT.1 and CAT.2 (which are almost overlapping) outperform the other catalysts due to their higher Pt dispersion.

The selectivity toward N₂ (S(N₂)) was calculated according to the following equation reported by Jablonska et al. (2025) [63]:

S(N₂) = [(N₂)/((N₂) + (N₂O) + 0.5(NO₂))]100

(5)

where

• (N₂) is the N₂ concentration (mol/L), calculated based on the molar concentrations of inlet NO, outlet NO, NO₂, and N₂O, using the mass balance equation proposed by Richter et al. [39] as follows:

$$({\mathrm{N}}_2)={\displaystyle \frac{1}{2}}[(NO)°-(NO)-({NO}_2)-2(N_{2}O)]$$

(6)

where

• (NO)° = concentration of NO fed to the reactor (mol/L);
• (NO) = concentration of NO at the reactor outlet (mol/L);
• (NO₂) = concentration of NO₂ at the reactor outlet (mol/L);
• (N₂O) = concentration of N₂O at the reactor outlet (mol/L).

The N₂ selectivity for all catalysts is shown in Figure 16. Above 300 °C, selectivity reaches 100%, although this comes at a higher energetic cost. At 100 °C, most catalysts exhibit selectivities between 33% and 70.3%, due to the formation of NO₂. A comparison of catalyst selectivity at this temperature follows the order: CAT.1 > CAT.2 > CAT.5 > CAT.4 > CAT.3 > CAT.6.

At 200 °C, CAT.1 achieves a selectivity of 94.5%, followed by the remaining catalysts (CAT.2–CAT.6), with CAT.4 reaching 90%. These results indicate that CAT.1, containing the highest Al₂O₃-WOx loading and the highest Pt dispersion, is the most selective toward N₂ formation. CAT.2 and CAT.5 (without Pt) follow closely. In contrast, CAT.6, which underwent high-severity treatment, exhibits the lowest N₂ selectivity.

To provide a more complete overview of catalyst behavior as a function of temperature, the formation of N₂, NO₂, N₂O, and unconverted NO is presented in Figure 17. Direct analysis of the maximum N₂ concentrations per catalyst yields the following order: CAT.1 (232 ppm N₂) = CAT.3 (232 ppm N₂) > CAT.2 (228 ppm) > CAT.6 (216 ppm) > CAT.5 (208 ppm) > CAT.4 (203 ppm).

3. Materials and Methods

3.1. Synthesis of Structured Catalysts

The catalyst was synthesized using a cordierite ceramic monolith with a density of 400 cells per square inch (cpsi) and dimensions of 25.4 mm in diameter and 50 mm in length, corresponding to a volume of 25.3 cm³ (Ganzhou Dingchang New Materials Co. Ltd., Ganzhou, China). The average monolith weight was 17 g, and approximately 1.0 g of catalyst was deposited using the dip-coating method. The catalyst composition was 2 wt% Ag and 0.1 wt% Pt supported on Al₂O₃ and promoted with 0.5 wt% W. The details of the catalyst preparation are described below.

A suspension of boehmite (AlO(OH)), used as the precursor of Al₂O₃, together with ammonium metatungstate in aqueous solution, was prepared by co-precipitation of Al(NO₃)₃·9H₂O (99%, Fermont, Monterrey, Mexico) and (NH₄)₁₂W₁₂O₄₀·5H₂O (66.55% W, Sigma-Aldrich Quimica, S.A. Company, Toluca, Mexico) as the promoter, in order to obtain a nominal W content of 0.5 wt%. NH₄OH (30 wt%, J.T. Baker, Phillipsburg, NJ, USA) was used as the precipitating agent to adjust the suspension to pH 9. The resulting AlO(OH) suspension was maintained under continuous stirring for 12 h at 25 °C, and the monoliths were impregnated by total immersion using the dip-coating method (see Figure 18).

Considering important impregnation variables [64], such as the concentration of boehmite (AlO(OH)), the withdrawal speed of the monolith from the suspension, the number of immersion cycles, and the drying and calcination temperatures—which influence both the deposited coating load and the washcoat adhesion—six samples were prepared and designated as CAT.1 through CAT.6. Prior to impregnation, the monoliths were heated at 500 °C in an airflow (100 mL/min) for 4 h to stabilize the structure and remove environmental moisture and impurities.

The boehmite impregnation process was carried out in the following stages: first, the monolith was slowly immersed in the boehmite suspension to allow sufficient time for the air within the channels to be displaced (Figure 18a). The monolith was kept immersed for 1–1.5 h until all air was completely expelled. It was then slowly withdrawn and positioned to allow the excess suspension to drain from the channels by gravity (Figure 18b).

When the monoliths were immersed in the boehmite suspension for 1 to 1.5 h, a deposit of 2.3 to 5.8 wt% Al₂O₃-WOx was obtained. After each immersion step (each cycle), the monoliths were dried at 110 °C for 24 h and subsequently weighed (see Figure 19). Once the weight remained constant, the calcination stage was performed. The first three catalysts (CAT.1, CAT.2, and CAT.3) were calcined at 500 °C at a heating rate of 5 °C/min under an air flow of 100 mL/min for 6 h. Catalysts CAT.4, CAT.5, and CAT.6 were calcined in a static atmosphere, also at 500 °C and a heating rate of 5 °C/min for 6 h.

With the impregnation of the Al₂O₃-WOx support onto the cordierite monoliths, the effects of the boehmite suspension concentration, the number of immersion cycles and their frequency, as well as the use of 3 wt% SiO₂ colloidal silica (LUDOX, 40 wt% suspension, Aldrich, Burlington, MA, USA), on the adhesion and resulting mechanical stability of the coating were evaluated.

The coating process using the dip-coating method resulted in an increase in the weight of the cordierite monoliths after two to four immersion cycles (denoted I2 to I4) for catalysts CAT.1, CAT.2, CAT.3, CAT.5, and CAT.6 (Figure 19). However, this behavior was not observed for catalyst CAT.4 (Figure 19). The poor adhesion of the support during the first immersion (I1) is attributed to the possible formation of an air film that hindered the penetration of the boehmite suspension and prevented its homogeneous adhesion within the monolith channels.

It was observed that as the number of immersion cycles increased, the viscosity of the suspension also increased (from 500 to 900 mPa·s), resulting in reduced adhesion of boehmite onto the monolith. Therefore, the structured catalysts were prepared using four immersion cycles (I4). Under these conditions, a homogeneous and stable support layer was obtained, with the thickness of the Al₂O₃-WOx layer on cordierite ranging from 15.3 to 28.3 µm (see

Table 4. Amount and thickness of the Al₂O₃-WOx deposit (film) on the monolithic catalysts from CAT.1 to CAT.6.

Catalyst	Al2O3-WOx (g)	Average Thickness (µm)	Visualization of the Deposit, Morphology, and Texture	Color
CAT.1	2.97 (17.4%)	28.3	Greater layer uniformity; the layers are well-defined and homogeneous	Black and beige
CAT.2	2.33 (13.74%)	20.0	Greater layer uniformity; the layers are well-defined and homogeneous.	Beige
CAT.3	2.04 (12.0%)	18.6	The initial layers are uniform, while the final layer exhibited efflorescence due to the sudden water discharge	Black and beige
CAT.4	1.82 (10.7%)	15.3	The coating is very thin; the layers are well-defined and homogeneous.	Black and beige
CAT.5	1.99 (11.7%)	17.1	The coating is extremely thin, with layers that are clearly distinguishable and homogeneous	Black and beige
CAT.6	1.90 (11.2%)	18.3	Layer uniformity is maintained; the final layer exhibits the same effect observed in CAT.3, although to a lesser extent	Gray and beige

).

Bimetallic Pt-Ag/Al₂O₃-WOx/cordierite catalysts (CAT.1, CAT.2, CAT.3, CAT.4, and CAT.6) were prepared using monoliths previously coated and calcined with the Al₂O₃-WOx layer, following the methodology described by González Hernández et al. (2020) [29]. The impregnation of the metal precursors was carried out sequentially, starting with the impregnation of H₂PtCl₆ using the incipient wetness method. The monolith was then dried at 110 °C for 12 h and calcined at 500 °C for 5 h. Subsequently, the Ag precursor (AgNO₃, 99%, Aldrich) was impregnated at room temperature, followed by drying at 110 °C for 12 h and calcination at 500 °C for 5 h (see Figure 20).

For the 2 wt% Ag/Al₂O₃-WOx/cordierite catalyst (CAT.5), 25 mL of an aqueous AgNO₃ solution (99%, Aldrich) with a concentration of 4 mg Ag/mL was used per 2 g (±0.25 g) of Al₂O₃-WOx coated on the monolith. The material was dried at 110 °C for 12 h and subsequently calcined at 500 °C with a heating rate of 5 °C/min for 5 h.

For Pt impregnation, 32.89 mL of an aqueous H₂PtCl₆ solution (99%, Aldrich) was added to 1.8–3 g (±0.25 g) of Al₂O₃-WOx coated on cordierite (pH = 2.5) to obtain solids with a Pt content of 0.1 wt%. The solids were dried at 110 °C for 24 h and subsequently calcined following two different protocols: the first three catalysts (CAT.1, CAT.2, and CAT.3) were calcined under flowing air, while the others were subjected to a static calcination procedure. Finally, all catalysts were reduced in a flow of H₂ (30 cm³/min) at 500 °C for 3 h.

3.2. Synthesis of Powder Catalysts

Two powdered catalysts were synthesized: one with a high Pt concentration (1 wt% Pt-2 wt% Ag/Al₂O₃-WOx), abbreviated as 1PtAg/AW, and another with a low Pt concentration (0.1 wt% Pt-2 wt% Ag/Al₂O₃-WOx), abbreviated as 0.1PtAg/AW. First, a stirred aqueous mixture containing 0.44 mg/mL of Al(NO₃)₃·9H₂O (Fermont, Mexico) and the required amount of (NH₄)₁₂WO₄·5H₂O to achieve a nominal W content of 0.5 wt% was prepared. A 30% NH₄OH solution (J.T. Baker, Phillipsburg, NJ, USA) was added dropwise until a boehmite (AlO(OH)) precipitate formed at pH 9–10. The suspension was stirred for 12 h at 25 °C, then filtered, dried at 110 °C for 24 h, and calcined at 500 °C for 6 h.

For Pt impregnation, the 1PtAg/AW catalyst was treated with 131.56 mL of H₂PtCl₆ solution in 5 g of Al₂O₃-WOx (pH = 2.5), while the 0.1PtAg/AW catalyst was treated with 13.15 mL of H₂PtCl₆ solution in 5 g of Al₂O₃-WOx (pH = 2.8). The solids were dried at 110 °C for 12 h and calcined at 500 °C for 6 h. Subsequently, 5 g of the calcined solids were impregnated with 25 mL of an aqueous AgNO₃ solution (4 mg Ag/mL, Aldrich, Milwaukee, WI, USA) to achieve a nominal Ag content of 2 wt%. The mixture was maintained at 60 °C for 2 h, then dried at 110 °C for 12 h and calcined at 500 °C for 6 h.

3.3. Characterization

N₂ physisorption using a Micromeritics ASAP-2460 Version 2.01 instrument (Norcross, GA, USA) was carried out at 77 K to determine the textural properties of the catalysts. The samples were heated at 300 °C for 3 h under vacuum (1 × 10⁻³ torr). The surface area values were obtained by application of the BET equation (absolute error ± 1.5 m²/g). The micropore volume of the samples was calculated by means of the t-plot method. The calculation of the pore size was carried out by applying the BJH model to the desorption branch of the isotherm, assuming cylindrical pore geometry.

X-Ray Diffraction (XRD) conducted the structural analysis of the synthesized catalysts (CAT.1, CAT.2, CAT.3, CAT.4, CAT.5, and CAT.6) on a Bruker D8FOCUS analytical instrument with Cu Kλ radiation (Billerica, MA, USA). XRD patterns were made using angles of 2Ѳ from 10 to 70°, with a step size of 0.02° and a counting time of 2 s; the absolute error was ±0.1°. The identification of the different crystalline phases (after and before calcination at 500 °C) was performed by comparison with the corresponding JCPDS diffraction data cards.

The surface morphology, particle size, and dispersion were analyzed by scanning electron microscopy (SEM) in a JEOL (model JFM-6701-F) instrument equipped with an energy dispersive X-ray detector (EDX) and by scanning transmission electron microscopy (HAADF-STEM, JEOL, Akishima, Japan). The particle size distribution of Ag-Pt and Ag in the catalyst samples was obtained at an 8 × 10⁵ magnification. The size distribution of particles and the mean diameter of Ag particles were calculated from images taken from different areas of the sample. The qualitative and quantitative chemical analyses were obtained by coupling the energy dispersive spectroscopy of X-ray (EDS) to a microscope; the absolute error was ±0.1 keV. For the observation, each sample was spread on a graphite tape to make it conductive, and Pd and Au atoms were also deposited.

Pt dispersion was obtained by H₂ chemisorption in glass volumetric adsorption equipment with pressure sensors (Cole-Parmer EW-68073, from Cole-Palmer Mexico, Mexico City, Mexico). The materials were previously reduced at 500 °C for 3 h with a constant flow of H₂ and degassed (1 × 10⁻³ Torr) at 400 °C for 2 h in an adsorption cell and stabilized at 25 °C. The sample weight was 0.5 g, and the H₂ adsorption was performed at room temperature (25 °C). Gas pressure at adsorption equilibrium was about 550 torr. The dead volume of the apparatus was 16.942 cm³ for the estimation of Pt metal dispersion, and the absolute error was ±5 mmol of H₂. The molar ratio of adsorbed hydrogen atom to the surface Pt atom was considered as one (H/Pt = 1) on the basis of the previous reports [27,28].

The temperature-programmed reduction (TPR) profiles of the calcined powder catalysts of 0.1PtAg/AW and 1PtAg/AW were obtained on a Bel Japan Equipment (Belcat model) equipped with a thermal conductivity detector (TCD). A gas mixture of 10 vol% H₂ in Ar was used, with 30 mg of sample at a flow rate of 25 cm³/min. The temperature was increased from 25 to 800 °C at a heating rate of 10 °C/min. The H₂ consumption was monitored by a TCD (absolute error ± 0.15 mmol/g). The amount of H₂ consumed was obtained by the deconvolution and integration of the TPR peaks using the Peak Fit program. The calibration was performed by measuring the change in weight due to a reduction in H₂ of 20 mg of CuO using an electrobalance Cahn-RG (Canh-Instruments, Cerritos, CA, USA). The TPR signal of CuO was made and correlated with the stoichiometric H₂ consumption.

Ex situ UV-visible spectra were obtained at 25 °C from 0.5 g of the samples calcined at 500 °C. The monolith samples evaluated at 450 °C were also analyzed in a UV-vis spectrophotometer (GBC model Cintra 20, manufactured by GBC Scientific Equipment Ltd. from Melbourne, Australia) equipped with a Praying Mantis diffuse reflectance accessory (Harrick Scientific Products Inc., from Pleasantville, NY, USA). The range studied was 200–800 nm at a 1 nm resolution and a 500 nm min⁻¹ scan rate (error 0.5% absorbance).

The Raman spectra of the evaluated catalysts were recorded under ambient conditions using powder samples. Measurements were performed with a Renishaw InVia Raman spectrometer (Wotton-Under-Edge, Gloucestershire, UK) equipped with a 532 nm (green) laser over the spectral range of 25–4000 cm⁻¹.

3.4. Catalytic Tests

The activity of the monolithic catalysts was evaluated using a cylindrical stainless-steel reactor, 22 cm in length and 25.4 mm in internal diameter (Figure 21). Three thermocouples were employed: two of them measured the initial and final temperatures of the oven and were placed in contact with the reactor, while the third was positioned at the center of the reactor to measure the temperature in the middle of the monolithic catalyst. These three temperatures were used to calculate the average longitudinal temperature.

The monolith of cordierite had approximately 1 g of catalyst deposited with a wall thickness of 20 µm. The feed of exhaust gases was composed of 500 ppm NO, 1750 ppm C₃H₈, 6 vol% O₂, 600 ppm CO, 10 vol% CO₂, 12 vol% H₂O, and N₂.

The cordierite monolith contained approximately 1.8–3 g of catalyst deposited, with the thickness of the Al₂O₃-WOx layer ranging from 15.3 to 28.3 µm. The exhaust gas feed consisted of 500 ppm NO, 1750 ppm C₃H₈, 6 vol% O₂, 600 ppm CO, 10 vol% CO₂, 12 vol% H₂O, and N₂.

The reaction products were analyzed by gas chromatography using a silica gel column (18 m × 1/8″ ID) and a Porapak Q column (9 m × 1/8″ ID) (from Ohio Valley Specialty Company from Marietta, OH, USA) for the analysis of NO₂ and N₂O. A Gow-Mac 550 gas chromatograph equipped with a thermal conductivity detector (from Gow-Mac Instruments Co., Bethehem, PA, USA) was used, with H₂ as the carrier gas.

Prior to catalytic testing, the CAT.6 catalyst was subjected to a high-severity treatment using a gas mixture of 25 ppm SO₂ in air at 750 °C for 16 h. The conditions for this severity test were adopted from the literature [65].

The catalytic performance was evaluated via selective catalytic reduction (SCR) of NO using a hydrogen stream containing 1 vol% H₂, generated from the ethanol reforming reaction. A gaseous diluent was used to mix H₂ with the main gas stream containing NO. The total pressure in the reaction system was maintained at 590 Torr, with a gas hourly space velocity (GHSV) of 70,000 h⁻¹. The reaction temperature was varied from 50 to 500 °C over a period of 6 h.

Simultaneous analysis of NO, CO, CO₂, C₃H₈, and O₂ was performed using a Consino automotive emissions gas analyzer (model FGA-4100, Shenzhen, China). Conversion curves as a function of temperature were obtained from 50 °C to 500 °C over 6 h at a total flow rate of 900 mL·min⁻¹, while maintaining a constant feed composition.

External diffusion effects were considered significant and were taken into account when calculating NO conversion as a function of monolith length using a one-dimensional model (see Supplementary Information).

4. Conclusions

The effect of the active loading (Pt-Ag/Al₂O₃-WOx) by weight in monoliths, composed of four bimetallic catalysts (0.1 wt% Pt-2 wt% Ag/Al₂O₃-WOx), one monometallic catalyst (2 wt% Ag/Al₂O₃-WOx), and one bimetallic catalyst (Pt-Ag), with a high-severity air-SO₂ flow pretreatment calcined at 750 °C for 16 h on cordierite monoliths, was studied in the SCR reaction of NO using H₂, C₃H₈, and CO as reducing agents. An increase in the conversion of NO, C₃H₈, the selectivity to N₂ was found when the active loading within the monolith increased from 10.7% to 17.4%. However, CO conversion decreased with increasing active loading due to the competitive reaction of H₂, as both reduction reactions occur within the same temperature range (100–200 °C). The presence of N₂O was detected with concentrations below 6 ppm.

In the Pt-Ag bimetallic catalysts evaluated in the reaction, an increase in carbon content was observed by ATG as the active loading increased, with the Ag catalyst without Pt exceeding all the Pt-Ag bimetallic catalysts in carbon content (5 wt%). The type of carbon observed by Raman spectroscopy was ordered graphitic. In the evaluated Pt-Ag catalysts, Ag⁺, Ag_n^δ+ species, and the Ag plasmon were found by UV-Vis spectroscopy. AgPt crystallites with an average size of 24 nm were also found by STEM. These AgPt crystallites with high dispersions could be contributing to the fact that the bimetallic Pt-Ag catalysts showed the highest NO conversions at 350 °C, as well as the highest selectivities to N₂ at 100 °C, exceeding the catalytic performance of the monometallic Ag catalyst without Pt.

The Pt-Ag catalyst subjected to high-severity heat treatment (750 °C) and with air plus SO₂ showed the lowest NO conversion and selectivity to N₂ and H₂. The low Pt dispersion caused by Pt sintering in this catalyst (7%) and the presence of adsorbed SO₂ contributed significantly to these results. A method for preparing monolithic catalysts is proposed, involving impregnating up to three layers of boehmite promoted with WOx oxides (0.5 wt% W).