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. 2024 Mar 4;9(10):11950–11957. doi: 10.1021/acsomega.3c09704

Selective Formation of Cu Active Sites with Different Coordination States on Pseudospinel CuAl2O4 and Their NO Reduction Catalysis

Aoi Okuda †,*, Taniyuki Furuyama ‡, Toshiaki Sakai §, Masato Machida ∥, Hiroshi Yoshida ⊥,*
PMCID: PMC10938440  PMID: 38496955

Abstract

graphic file with name ao3c09704_0010.jpg

In the spinel framework, copper (Cu) in two distinct coordination states exhibits catalytic activity for NO reduction through different mechanisms. However, detailed exploration of their respective catalytic properties, such as the redox behavior of Cu and substrate molecule adsorption, has been challenging due to difficulties in their separate formation. In this study, we present the controlled formation of pseudospinel CuAl2O4, containing exclusively tetrahedrally or octahedrally coordinated Cu, achieved by manipulating aging temperature and O2 concentration. Through these materials, we observed that in the CO–NO reaction, the step primarily determining the rate differs: NO reduction dominates with octahedrally coordinated Cu, whereas carbon monoxide (CO) oxidation is prominent with tetrahedrally coordinated Cu. The lower coordination number of Cu significantly benefits NO reduction but negatively impacts the CO–NO reaction, albeit positively influencing NO reduction in three-way catalytic reactions.

Introduction

Reducing pollutants remains a critical, ongoing challenge in activities producing harmful substances. In the realm of mobile applications, three-way catalysts (TWCs) have played a pivotal role in curbing pollutant emissions from gasoline-powered vehicles.1−5 While platinum group metal (PGM) nanoparticles have been widely used in TWCs, their adoption has been restricted due to their high cost and limited availability. Hence, the pursuit of alternative catalytic materials, leveraging cost-effective and abundant transition metals, has garnered significant interest.6−10 Investigations into various metals’ efficacy, including Cu,11−18 Ni,19−21 and Co22−24 have been conducted.

Automobile exhaust gas predominantly comprises nitrogen oxides (NOx), carbon monoxide (CO), and unburned hydrocarbons like C3H6. Among these components, NOx should be reduced while the other two components are necessary to oxide. While oxidation demands a stoichiometric or excess amount of O2, its presence severely inhibits the NOx reduction process. Additionally, developing catalysts for practical use in automotive emission control systems faces the challenge of thermal durability.18,25−28 Exposure to gasoline engine exhaust at temperatures exceeding 800 °C for prolonged periods leads to severe thermal aging, causing significant sintering of supported metal nanoparticles or changes in the crystal structure, thus degrading the catalyst. To tackle these issues, investigations into solid solution catalyst effectiveness rather than supported metal structures have been explored.16−18,27,29−32

Incorporating Cu into a support material boosts the NOx reduction activity by regulating the redox cycle through the movement of adjacent oxygen atoms. Additionally, this catalyst exhibits remarkable thermal stability, avoiding sintering from thermal degradation. Recent findings reported that Cu, when integrated into the tetrahedral site of γ-Al2O3 (Td-Cu2+), showed heightened catalytic performance in the NO–CO–C3H6–O2 reaction, particularly in NO reduction.18 This occurred due to thermal aging at high temperatures, which prompted Cu2+ incorporation into tetrahedral sites (Td-Cu2+) rather than octahedral ones (Oh-Cu2+).33,34 This Td-Cu exhibited superior redox cycling, transitioning between Td-Cu2+ and Td-Cu+ via CO oxidation and NO reduction, consequently enhancing NO reduction in the three-way catalytic reaction. Enhancing activity further could be achieved by increasing the Cu occupancy within tetrahedral sites on spinel γ-Al2O3, but there are no existing reports on composite materials solely comprising Td-Cu. As Areán and Vinuela reported, site occupancies of Td-Cu2+, Oh-Cu2+, Td-Al3+, and Oh-Al3+ in a spinel CuAl2O4 are 0.649, 0.351, 0.351, and 1.649, respectively, indicating that surpassing a 65% Cu occupancy ratio in the most stable CuAl2O4 is thermodynamically challenging.35 Therefore, developing a catalyst preparation technique to achieve high Cu occupancy within tetrahedral sites is crucial not only for enhancing the three-way catalytic activity but also for deepening our understanding of the relationship between catalytic properties and surface Cu coordination states.

In this study, our focus was on understanding how various reaction conditions—such as temperature, atmosphere, and aging time—affect the fine structure and coordination state when Cu species are incorporated into the framework of γ-Al2O3 through thermal aging. Our aim was to identify suitable preparation conditions to intentionally form Td-Cu on spinel γ-Al2O3. To delve into the detailed structures of the Cu active sites and crystal structure, we employed techniques like X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and diffuse reflectance ultraviolet, visible, and near-infrared spectroscopy (UV–vis–NIR). Additionally, we explored the catalytic activity of both Td-Cu and Oh-Cu in a three-way catalytic reaction. We specifically investigated their NO reduction properties through kinetic analysis and by observing adsorbed species using in situ Fourier transform infrared (FT-IR) spectroscopy techniques.

Materials and Methods

Preparation and Characterization

An Al2O3-supported 6 wt % Cu catalyst (Cu/Al2O3) was prepared by conventional impregnation using aqueous Cu(NO3)2 solutions (99.0% purity, Wako Pure Chemical Industries, Ltd.) and commercially available γ-Al2O3 (99.99% purity, 159 m2 g–1, AKP-G15, provided by the Catalysis Society of Japan). The impregnated samples underwent overnight drying at 110 °C. Subsequently, the sample was thermally treated at 600 °C for 3 h in a gas mixture stream of a mixture of O2 and N2 with varying O2 concentrations (0, 50, and 100%). Further thermal aging occurred at 900 or 1000 °C for 2–25 h in the same gas stream.

The crystal structure of the sample was determined by powder XRD (MiniFlex600, Rigaku) using monochromatic Cu Kα radiation (λ = 1.54 Å). The Brunauer–Emmet–Teller surface area (SBET) was calculated by using N2 adsorption isotherms measured at −196 °C (BELSORPmaxII, MicrotracBEL, Inc.). The oxidation states and surface concentrations of Cu were determined by XPS using monochromatic Al Kα radiation (12 keV, K-Alpha, Thermo Fisher Scientific), and the binding energies were charge-referenced to C 1s at 285 eV. We used XPS as a nondestructive analysis to measure the surface Cu concentration instead of general N2O chemisorption, namely, the absolute surface Cu concentration was defined by assigning the relative surface Cu concentration calculated from XPS to the SBET value. The coordination state of Cu on γ-Al2O3 was confirmed by using a UV–vis–NIR spectrometer (V-570, JASCO Co.) and a diffuse reflectance cell. The obtained spectra were treated according to the Kubelka–Munk theory.

Catalytic Reactions

The catalytic NO–CO–C3H6–O2–H2O reaction was performed in a continuous fixed-bed reactor at atmospheric pressure. The catalyst (0.1 g, 20 mech) was placed in a quartz tube (inside diameter of 4 mm) using quartz wool. The activity assessment involved heating the catalyst bed from room temperature to 600 °C at a rate of 10 °C min–1 while supplying a simulated gas mixture (0.05% NO, 0.50% CO, 0.04% C3H6, 0.40% O2, 10% H2O, and N2 balance, 100 cm3 min–1) at atmospheric pressure. The gas composition corresponded to a stoichiometric air-to-fuel ratio (A/F) of 14.6 based on weight, where all gases can be completely converted into a mixture of CO2, H2O, and/or N2. The effluent gas underwent analysis using nondispersive infrared N2O/CO/C3H6 (VA-5000, HORIBA) and a chemiluminescence detection NO gas analyzer (NOA-7100, SHIMADZU).

The effects of the CO/NO partial pressure on the CO–NO reaction rate were determined at 240 °C, maintaining steady-state conversions below 20% in a differential reactor. The CO partial pressure varied from 0.1 to 0.2 kPa while maintaining a constant NO partial pressure of 0.2 kPa. The NO partial pressure varied from 0.1 to 0.2 kPa while the CO partial pressure was kept constant at 0.2 kPa.

In Situ FT-IR Measurement

The influence of the Cu coordination state on molecular adsorption was systematically investigated by using FT-IR techniques. In situ FT-IR spectra of CO adsorbed on Cu/Al2O3 samples with different Cu coordination states were obtained using an FT-IR spectrometer (FT/IR-4X, JASCO Co.) with a temperature-controlled diffuse reflectance reaction cell and a mercury–cadmium–telluride detector. The sample underwent grinding and was then placed in the cell to form a flat surface. The cell was heated to 200 °C at a constant rate of 10 °C min–1 and held at this temperature for 15 min under a supply of N2. After the mixture was cooled to 50 °C, the background spectrum was collected. The sample was exposed to a gas stream containing 5% CO balanced with N2 to reduce Cu2+ to Cu+ at different temperatures depending on the sample. The CO reduction temperature for each sample was determined from the CO temperature-programmed reduction (CO-TPR) results (Figure S1) prior to the measurements. After removal of gaseous CO by a N2 flow at the same temperature, the sample was cooled to 50 °C.

We first determined the CO temperature-programmed desorption to compare the strength of CO adsorption on Cu+ with different coordination states. After exposure to a 5% CO stream for 30 min, gaseous CO was removed with N2 for 15 min, and the FT-IR spectrum was collected. The temperature was then increased in 10 °C increments, and the spectra were collected at each temperature, and the decrease in the absorption band of CO adsorption with increasing temperature was discussed. The influence of the copresence of NO in the CO adsorption was also evaluated. After pretreatment, the sample was exposed to a stream of 0.2% CO for 30 min and the spectrum was collected in the absence of NO. The NO concentration was then increased in 0.01% increments from 0 to 0.2%, and the spectra were collected under each condition. Their relative intensities to those in the absence of NO were compared.

Results and Discussion

Structure Change of Supported Cu with High-Temperature Treatment under Different Atmospheres

The presence of Cu deposited on Al2O3 as CuO particles below 700 °C is well-documented, while exposure to higher thermal environments triggers a reaction between CuO and the Al2O3 support, resulting in the formation of pseudospinel CuAl2O4.18Figure 1 shows the XRD patterns of Cu/Al2O3 following a brief 2 h thermal aging under different atmospheres. None of the samples exhibited diffraction peaks characteristic of crystalline CuO, signifying the immediate integration of supported Cu2+ ions into the γ-Al2O3 framework during the first stage of thermal aging, irrespective of the atmosphere’s O2 concentration.18,27,36−38

Figure 1.

Figure 1

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XRD patterns of Cu/Al2O3 samples after thermal aging at 900 °C for 2 h under different O2 concentrations (0, 50, and 100%).

The presence of diffraction peaks corresponding to spinel-structured CuAl2O4, alongside those of γ-Al2O3, further corroborated the formation of pseudospinel CuAl2O4 on the outermost surface of γ-Al2O3. Notably, despite sharing the same crystal structure, different specific surface areas (SBET) were observed, contingent upon the concentration of the O2 in the atmosphere. Samples treated in 0 and 50% O2 experienced a decline in SBET to 84–87 m2 g–1 due to thermal sintering. Conversely, the sample treated in 100% O2 exhibited the highest value of 126 m2 g–1, suggesting the potential suppression of thermal sintering in Cu/Al2O3 within high-temperature environments featuring elevated O2 concentrations.

The detailed structure of the Cu sites incorporated in γ-Al2O3 was determined through diffuse reflectance UV–vis–NIR measurements (Figure 2). Two absorption bands, ranging from 400 to 800 nm and 1000 to 1700 nm, were attributed to the d–d transitions in octahedrally (Oh-Cu2+) and tetrahedrally (Td-Cu2+) coordinated Cu2+, respectively.18,30,39,40 Upon thermal aging without O2, the intensity peak of Oh-Cu2+ at 750 nm significantly exceeded that of Td-Cu2+ at 1500 nm. This indicated a predominant occupation of octahedral sites (Oh-Cu2+) within the spinel structure rather than tetrahedral sites (Td-Cu2+). Conversely, spectra from the samples aged under 50 and 100% O2 concentration revealed a marked enhancement in the Td-Cu2+ band at 1500 nm. This shift indicated that thermal aging at high O2 concentrations prompted the incorporation of Cu2+ ions into tetrahedral sites, a phenomenon not previously reported. Thermodynamically, this occurrence aligns with the phase diagram of the Cu–Cu2O–CuO system. At temperatures above 900 °C and under low O2 concentrations, Cu2O predominates due to the thermal decomposition of CuO. Consequently, thermal aging induces the formation of a solid solution between Cu2O and γ-Al2O3, leading to monovalent Cu+ occupying octahedral sites (Oh-Cu+). Conversely, under high O2 concentrations (50 and 100%), CuO stabilization hinders the thermal decomposition of CuO to Cu2O. This condition fosters the formation of a solid solution between CuO and γ-Al2O3, resulting in divalent Cu2+ (Td-Cu2+) occupying both tetrahedral and octahedral sites (Oh-Cu).

Figure 2.

Figure 2

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Diffuse reflectance UV–vis–NIR spectra of Cu/Al2O3 after thermal aging at 900 °C for 2 h under different O2 concentrations (0, 50, and 100%).

After prolonged thermal aging (25 h), the structural changes in Cu/Al2O3 were found to be significantly influenced by the difference in O2 concentration, as shown in Figure 3. When subjected to thermal aging without O2, the sample exhibited the presence of highly crystalline α-Al2O3 and CuAl2O4, leading to a notable decrease in SBET from 84 m2 g–1 (2 h) to 31 m2 g–1 (25 h). However, under conditions of high O2 concentrations, such crystallization was unlikely, and XRD patterns akin to those observed in samples aged for a shorter duration (2 h) (Figure 1) were obtained, accompanied by higher SBET values. For the sample aged under 50% O2 concentration, minor diffraction peaks attributed to α-Al2O3 were noted. Interestingly, no such peaks were evident in the sample aged under 100% O2, indicating that not only crystallization but also the phase transition from γ-Al2O3 to α-Al2O3 can be suppressed under elevated O2 concentrations during thermal aging.

Figure 3.

Figure 3

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XRD patterns of Cu/Al2O3 after thermal aging at 900 °C for 25 h under different O2 concentrations (0, 50, and 100%).

The coordination states of the Cu active sites were assessed by using the UV–vis–NIR technique (Figure 4). Post-thermal aging in the absence of O2, Oh-Cu2+ dominated the sample. However, under high O2 concentrations, the spectra of the thermally aged Cu/Al2O3 strongly suggested larger Td-Cu2+ sites compared to those of Oh-Cu2+ sites. Moreover, longer thermal aging notably amplified the relative intensities of Td-Cu2+ at around 1500 nm. These findings indicate that the coordination state of Cu integrated into the γ-Al2O3 surface can be deliberately modified by regulating the O2 concentration during thermal aging.

Figure 4.

Figure 4

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Diffuse reflectance UV–vis–NIR spectra of Cu/Al2O3 after thermal aging at 900 °C for 25 h under different O2 concentrations (0, 50, and 100%).

Catalytic Properties of Cu Active Sites with Different Coordination States on the Al2O3 Surface

After prolonged thermal aging, we proceeded to assess the catalytic activities of the samples in the stoichiometric NO–CO–C3H6–O2–H2O reaction. Figure 5 shows the light-off curves for NO, CO, and C3H6 conversions over Cu/Al2O3 samples, each featuring Cu active sites with varied coordination states contingent upon the thermal aging atmosphere, as demonstrated in Figure 4. While the light-off curves for CO conversion remained consistent across all samples, catalysts hosting Td-Cu sites exhibited higher activities in C3H6 oxidation and NO reduction. The peak appearance of the NO conversion at around 380 °C can be attributed to the selective catalytic reduction of NO in the presence of hydrocarbon over a Cu catalyst.18,27,36,39,41,42 Chen et al. demonstrated that the NO reduction in the presence of C3H6 over 10 wt % Cu/Al2O3 at 350 °C decreased with further increase in temperature, and they concluded that the complete oxidation of C3H6 with excess O2 at higher temperatures decreased the NO reduction.41 Since similar reaction trends were obtained in Figure 5, we concluded that the peak appearance of NO conversion at around 380 °C is due to the selective catalytic reduction of NO by C3H6. Notably, the catalyst subjected to thermal aging under 100% O2 concentration demonstrated the highest NO conversion rate of 73% at 600 °C. This superior activity can potentially be attributed to the structural disparity in Cu, specifically the prevalence of Td-Cu, indicating its pivotal role in the three-way catalytic activity. Another contributing factor could be the abundance of Cu active sites. However, this latter hypothesis seems less plausible, as both catalysts subjected to high O2 concentrations exhibited identical UV–vis–NIR spectra (Figure 4). To delve into this aspect further, we quantified the surface Cu concentration through XPS analysis and computed the specific surface area of Cu per weight using the SBET value (Figure S2). Our analysis revealed a slightly higher specific surface area of Cu in the catalyst subjected to thermal aging under 100% O2 concentration compared to that aged under 50% O2 concentration, which likely contributes to its heightened NO reduction activity.

Figure 5.

Figure 5

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Light-off curves for a stoichiometric NO–CO–C3H6–O2–H2O reaction over Cu/Al2O3 after thermal aging at 900 °C for 25 h under different O2 concentrations (0, 50, and 100%).

The aforementioned findings uncover two significant revelations. First, the thermal aging of Cu/Al2O3 under high O2 concentrations impedes phase transition and/or crystallization processes. Second, there is a discernible selective formation of Cu active sites, exhibiting distinct coordination states on γ-Al2O3, achieved by precise control over the O2 concentration. Particularly noteworthy is the selective formation of Td-Cu, a state typically less favored concerning crystal field stabilization energy, which holds promise in unraveling the correlation between the coordination state of Cu and catalytic activity.

Given the observed augmentation in the relative intensity of UV–vis–NIR spectra corresponding to Td-Cu with prolonged aging time (Figures 2 and 4), we subjected Cu/Al2O3 to harsher conditions to transform all of the Cu sites into the Td-Cu state. Figure 6 shows the UV–vis–NIR spectra of Cu/Al2O3 after thermal aging at 1000 °C for 25 h under a 100% concentration of the O2 concentration. XRD analyses confirmed the emergence of highly crystalline CuAl2O4 and α-Al2O3 following thermal aging, concomitant with a decrease in the specific surface area to 8 m2 g–1 (Figure S3). The sample showed low catalytic activity in the stoichiometric NO–CO–C3H6–O2–H2O reaction due to the drastic decrease in the specific surface area (Figure S4). Intriguingly, the presence of the band attributed to Oh-Cu2+ at 750 nm was absent, while the absorption band at 1500 nm, indicative of Td-Cu, emerged prominently with high intensity. This outcome signifies the successful synthesis of Cu/Al2O3 wherein all Cu is incorporated solely into the tetrahedral sites (Td-Cu) through thermal aging at 1000 °C under 100% O2 conditions.

Figure 6.

Figure 6

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Diffuse reflectance UV–vis–NIR spectra of Cu/Al2O3 after thermal aging at 1000 °C for 25 h under 100% O2 concentration.

To discern the catalytic characteristics of Td-Cu and Oh-Cu active sites in NO reduction, we examined the dependence of the CO and NO partial pressure within the CO–NO reaction using Al2O3-supported Cu catalysts featuring either Td-Cu or Oh-Cu sites. Model catalysts, namely, the Cu/Al2O3 after thermal aging at 900 °C for 25 h without O2 (Figure 2) and after aging at 1000 °C for 25 h under 100% O2 conditions (Figure 6), were utilized, referred to henceforth as the Oh-Cu sample and Td-Cu sample, respectively.

The investigation involved determining the partial reaction orders concerning CO and NO at 240 °C based on the steady-state reaction rate in varying streams of CO and NO with different partial pressures (Figure 7). The observed positive dependencies on CO and NO suggest that the pivotal step in the reaction involves chemisorbed CO and NO molecules on the catalyst surface.

Figure 7.

Figure 7

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Dependence of the NO reduction rate and reaction order on (a) CO and (b) NO partial pressure in the CO–NO reaction over Td-Cu and Oh-Cu samples at 240 °C.

For the Oh-Cu sample, the reaction orders pertaining to CO and NO were found to be 0.34 and 0.52, respectively. These values indicate a relatively higher dependence of the NO reduction rate on the NO concentrations. In contrast, the Td-Cu sample exhibited a reaction order of 0.91 concerning CO (Figure 7a), significantly surpassing the NO reaction order of 0.63 (Figure 7b).

These findings strongly imply that the rate-determining step governing NO reduction over Cu active sites varies contingent upon the coordination states of Cu. Specifically, it appears that NO adsorption, crucial for the reoxidation of Cu+ to Cu2+, governs the rate-determining step over the Oh-Cu sites. Conversely, for Td-Cu sites, the rate-determining step seems to involve CO adsorption, critical for the reduction of Cu2+ to Cu+, as depicted in Figure 8. This distinction elucidates a significant mechanistic divergence in the NO reduction pathway over Cu active sites contingent upon their specific coordination states, delineating the distinct roles of Oh-Cu and Td-Cu in catalyzing this reaction.

Figure 8.

Figure 8

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Different rate-determining steps of the CO–NO reaction over Td-Cu and Oh-Cu sites by redox cycling of Cu.

After kinetic analysis was conducted, it was discerned that the rate-determining step in NO reduction differs between Td-Cu and Oh-Cu. To delve deeper into their catalytic properties, we systematically investigated the adsorption behavior of substrate molecules on Cu with varying coordination states. Initially, we compared the strength of the adsorption of CO on Td-Cu and Oh-Cu using in situ diffuse reflectance infrared Fourier transform (DRIFT) measurements. The absorption bands associated with linearly adsorbed CO appeared consistently at a wavenumber of 2093 cm–1, irrespective of the Cu coordination state, and those bands can be assigned to the CO adsorbed on the monovalent Cu+.32,43−45 The consistency of the wavenumbers of adsorbed CO implies that the electron back-donation from Td-Cu+ and Oh-Cu+ to the π orbitals of CO is comparable, indicating that the influence of the Cu coordination state on the interaction between the CO molecules and surface Cu+ is not significant. Moreover, the peak intensity demonstrated a gradual decrease with a rise in temperature (Figure S5). This consistent trend in the extent of peak intensity reduction with temperature suggests that there exists no significant disparity in the strength of CO adsorption between Td-Cu and Oh-Cu.

We proceeded to assess the strength of the CO adsorption on Cu sites in the presence of NO. Figure 9a exhibits the DRIFT spectra detailing CO adsorption on Td-Cu while incrementally introducing coexisting NO. In a parallel manner, the spectra depicting CO adsorption on Oh-Cu under the same conditions were also acquired (Figure 9b). To facilitate a comparison, the alteration in their relative intensities was plotted (Figure 9c). The diminishing peak intensity of CO adsorption in the presence of NO suggests competitive adsorption between CO and NO on Cu sites within the CO–NO gas mixture. Notably, the steeper decline in the CO adsorption peak intensity on Td-Cu with rising NO concentration indicates a more pronounced inhibition of CO adsorption compared to Oh-Cu. Consequently, this effect contributes to a rate-limiting step involving the oxidation of Cu2+ to Cu+ during the CO oxidation. Prior research has reported that a decrease in the coordination number of Cu+ correlates with an increased adsorption energy of NO molecules, aligning with our current findings.46,47 Hence, the lower coordination number characteristic of Td-Cu confers a substantial advantage for NO adsorption over that of Oh-Cu. However, this unique property may exhibit a dual impact: while favoring NO reduction in the three-way catalytic reaction (Figure 5), it could potentially hinder CO–NO reactions due to the preferential adsorption of NO, inhibiting CO adsorption. This distinct attribute of Td-Cu, particularly its robust NO adsorption capability, holds promise for the development of PGM-free deNOx catalysts. We anticipate uncovering novel catalytic functions inherent in a Cu catalyst primarily composed of Td-Cu in the near future.

Figure 9.

Figure 9

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In situ DRIFT spectra of CO chemisorbed on (a) Td-Cu+ and (b) Oh-Cu+ sites in the presence of NO at different concentrations. (c) Relative intensity of CO adsorption as a function of NO concentration.

Conclusions

The investigation focused on the formation process of pseudospinel CuAl2O4 on the surface of γ-Al2O3 through the incorporation of Cu2+ into the γ-Al2O3 framework under varying thermal aging conditions. It was observed that the coordination state of Cu on the γ-Al2O3 surface could be manipulated by controlling the O2 concentration during thermal aging. At high temperatures exceeding 900 °C under low O2 concentration, Cu2O dominated due to the thermal decomposition of CuO, fostering the formation of a solid solution between Cu2O and γ-Al2O3. This induced the occupation of the octahedral sites by monovalent Cu+ (Oh-Cu+). Conversely, under high O2 concentrations (50 and 100%), CuO was stabilized, impeding its decomposition into Cu2O. This led to a solid solution formation between CuO and γ-Al2O3, resulting in the occupation of tetrahedral sites by divalent Cu2+ (Td-Cu2+). Additionally, at high temperatures under 100% O2 concentration, thermal aging inhibited both material crystallization and the phase transition from γ-Al2O3 to α-Al2O3, favoring the creation of pseudospinel CuAl2O4. This compound predominantly exhibited a tetrahedrally coordinated surface Cu (Td-Cu) and a high specific surface area.

Successful preparation of pseudospinel CuAl2O4 with Oh-Cu involved thermal aging at 900 °C for 25 h in the absence of O2, while achieving Td-Cu required thermal aging at 1000 °C for 25 h under 100% O2. Kinetic analysis revealed that in the former, the rate-determining step of the CO–NO reaction is NO reduction, while in the latter, it is CO oxidation. This significant difference arises from the relative strength of NO adsorption on Oh-Cu and Td-Cu, where the lower coordination number of Td-Cu offers a considerable advantage for NO adsorption compared to Oh-Cu. Consequently, the strong NO adsorption inhibits the adsorption of CO on Td-Cu, directly impacting its kinetics. While this may negatively affect the CO–NO reaction, it positively influences NO reduction in the three-way catalytic reaction. This material, characterized by a high Td-Cu ratio, emerges as a promising candidate to replace conventional PGM catalysts, showcasing efficient deNOx capabilities alongside high thermal stability.

Acknowledgments

This study was supported, in part, by a Grant-in-Aid from the Japan Society for the Promotion of Science (grant no. 19H02518) and the Iketani Science and Technology Foundation (0351053-A).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.3c09704.

  • Data related to CO-TPR, change in specific surface area of Cu on Cu/Al2O3 after thermal aging under different O2 concentrations, XRD patterns, catalytic activity, and change in the DRIFT spectra of CO (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao3c09704_si_001.pdf (1.8MB, pdf)

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