Complete Oxidation of CO, Propene, and Toluene as Model Components of Diesel Engine Exhaust and VOC using Ceria-modified Cu/Hydroxyapatite: Effect of Preparation Methods and Ce Concentration

Abstract

The Ce-modified Cu/HAp catalyst was prepared by successive deposition and coprecipitation methods and was investigated for propene oxidation serving as a model component of diesel engine exhaust and volatile organic components (VOCs). The Cu_0.4/Ce_0.1/HAp catalyst prepared by successive deposition showed better performance than the Cu_0.4–Ce_0.1/HAp catalyst prepared by the coprecipitation method. Furthermore, the Ce concentration has been optimized using successive deposition methods. The catalyst was modified with 0.4 mol Cu and 0.1 mol Ce deposited on HAp and tested for CO, propene, and toluene oxidation. Complete conversion is attained by the Cu_0.4/Ce_0.1/HAp catalyst at approximately 339 °C, highlighting its superior low-temperature activity compared with the remaining Ce-modified Cu/HAp catalyst series. All catalysts were characterized by P-XRD, BET, Raman, XPS, ATR-FTIR, H₂-TPR, FESEM, and HR-TEM techniques. The formation of facile CuO _x species with more Ce3⁺ and adsorbed oxygen was observed in the Cu_0.4/Ce_0.1/HAp catalyst. These species are responsible for the oxidation of propene at lower temperatures compared to the remaining catalyst.

Keywords

Diesel engine exhaust - Hydroxyapatite - Cu–Ce interaction - Lattice defects - VOC

1

Introduction

Volatile organic components (VOCs) and CO are very harmful components in automobile engine exhaust gases.[1] [2] [3] [4] VOCs are produced from biogenic, pyrogenic, and anthropogenic sources. These sources are responsible for the direct and indirect hazards to both the atmosphere and human health.[5] [6] However, very stringent exhaust regulations have been imposed worldwide in recent years to curb the negative effects of diesel engines and VOC emissions on human health and the environment. Therefore, the complete oxidation of CO and VOC has been studied by many researchers.[1] [4] [6] [7] [8] Noble metals like platinum and or platinum supported on alumina are used commercially for total oxidation of diesel engine exhaust and VOC.[9] [10] [11] However, high cost, higher light temperature, and low stability restrict the use of these catalysts.[12] [13] [14] To overcome these problems, metal/metal oxides supported on different metal oxides like CeO₂, Al₂O₃, TiO₂, hydroxyapatite, etc., had been previously reported for the total oxidation of diesel engine exhaust.[7] [9] [10] [15] [16] [17] [18] Thus, metal oxides like Al₂O₃, TiO₂, CeO₂, and hydroxyapatite (HAp) are used as a support for Cu, Mn, and oxides and have been reported for the total oxidation of diesel engine exhaust and VOCs.[3] [15] HAp is known for its very flexible hydrotalcite structure with good thermal stability. HAp is also known for its surface acidity and basicity, exchangeable Ca abilities, and strong adsorption properties.[19] Moreover, ceria has been used in three-way catalysts due to its high oxygen-storage capacity, metal dispersion, and facile redox cycle between the Ce^3+/Ce⁴⁺.[20] [21] [22] [23] Furthermore, copper has shown facile redox properties between Cu⁺/Cu²⁺ in the presence of ceria and has been studied for the oxidation of diesel engine exhaust and VOC.[20] [22] [24] [25] Thus, Cu and Ce showed facile synergistic redox properties and were responsible for the formation of interfacial active sites, which oxidize the CO, propene, and toluene into CO₂.[26] [27] The copper–ceria catalyst showed formation of more Ce³⁺ species, oxygen vacancies, and adsorbed oxygen, responsible for the low-temperature oxidation.[26] [27] [28] [29] However, the effect of Ce on the properties and catalytic activity of Cu supported on HAp is ambiguous and needs to be understood in detail.

The addition of Ce in the Cu/HAp catalyst could improve the low-temperature activation of oxygen by forming a higher oxygen vacancy. The Ce doping could stabilize the structure and improve the redox properties of Cu. The present study deals with the effect of Ce concentration and the preparation methods on the oxidation activity of Cu/HAp with respect to CO, propene, and toluene as a model component of diesel engine exhaust and VOC. The catalyst’s characterizations were studied in detail and correlated to the oxidation activity.

2

Results and Discussion

2.1

Characterization of Ce-promoted Cu/HAp Prepared by Successive Deposition

2.1.1

Powder X-ray Diffraction

The XRD patterns of the Cu/HAp and Ce-promoted Cu/HAp catalysts prepared by the successive deposition methods are shown in [Fig. 1]. The XRD peaks of Ce-promoted Cu/HAp correspond to the HAp (JCPDS no. 09-0432). However, the peaks centered at 2θ = 32.53°, 35.38°, 38.78°, 58.14°, 62.0°, 66.18°, 67.18°, and 67.94° correspond to the CuO (JCPDS no. 48-1548) having a monoclinic structure. HAp peaks at 25.82°, 31.98°, 33.97°, 43.88°, 46.74°, 49.54°, and 53.16° have shifted to a lower angle after deposition of Cu on HAp.[30] The XRD peaks of CuO are observed at 35.3°, 38.2°, and 62.0°. The CuO phase was observed in the Cu/Hap catalyst. However, low intensity of the same peaks was observed in Ce-modified Cu/HAp catalysts. In the case of successive deposition of Ce on HAp followed by Cu (0.1–0.4 mol), no separate peak of Ce was observed due to the overlapping with the peaks of HAp and CuO at 28.34°, 32.86°, 47.34°, and 56.27°. The reduced intensity could be attributed to the dispersion of Cu and Ce species on the surface of HAp. Furthermore, the increase in Cu deposition on Ce/HAp shows the shifting of the HAp peak toward a lower angle. The shifting indicates the interaction of Ce with Cu and HAp, which may lead to the formation of Ce³⁺ species. The successive deposition of Cu²⁺ and Ce on HAp could create the oxygen vacancies (VÖ) responsible for the formation of more Ce³⁺ (2 Ce⁴⁺ ↔ 2Ce³⁺ + VÖ²⁺) ions. The formation of more Ce³⁺ species could be responsible for the activation of gaseous oxygen at lower temperatures. The activation of oxygen leads to the oxidation of propene at lower temperatures.

Fig. 1 Powder X-ray diffraction patterns of HAp, Cu/HAp, and Cu _x /Ce_0.5−x /HAp.

2.1.2

Surface Area Study

The BET surface area was measured by N₂ adsorption and is shown in [Table 1]. The surface area of the catalysts increases with an increase in ceria concentration. The interaction between Cu and Ce reduces the agglomeration and could sustain the active sites beneficial for propene oxidation at lower temperatures.

Table 1 Surface and bulk properties of HAp and Ce-modified Cu/HAp.

Catalyst	2θ°	S BET (m2/g)
HAp	31.98	28.8
Cu0.5/HAp	31.76	40.1
Cu0.1/Ce0.4/HAp	31.82	64.4
Cu0.2/Ce0.3/HAp	31.82	63.6
Cu0.3/Ce0.2/HAp	31.84	62.3
Cu0.4/Ce0.1/HAp	31.88	65.4

2.1.3

ATR-FTIR Study of Ce-promoted Cu/HAp Prepared by the Successive Deposition Method

The attenuated total reflectance Fourier transform infrared spectra are shown in [Fig. 2]. The fundamental vibrational modes for (PO₄)³⁻ in hydroxyapatite were observed at 565, 602, 962, 1030, and 1090 cm⁻¹.[30] [31] The strong bands at 602 and 565 cm⁻¹ are assigned to ʋ₄ vibrational modes of O–P–O, whereas 962 cm⁻¹ ʋ₁ stretching vibrations are associated with a P–O bond. The larger peak at 1030 cm⁻¹ and the smaller peak located at 1090 cm⁻¹ correspond to the vibrational modes of the (PO₄)³⁻ group.

Fig. 2 ATR-FTIR spectra of HAp and Ce-modified Cu/HAp.

The deposition of Cu on the HAp support leads to the shifting of the HAp peaks 562, 1025, and 1088 cm⁻¹ toward a higher wavenumber 564, 1032, and 1090 cm⁻¹, respectively. This shifting indicates the strong interaction of Cu with the O–P–O and P–O bonds of Hap, which is responsible for the increase in strength of the O–P–O and P–O bonds. Whereas, after the addition of ceria by the successive deposition method, catalysts (Cu_0.4/Ce_0.1/HAp) showed bands at 563, 1030, and 1089 cm⁻¹. These results indicate that the IR bands shifted to a lower wavenumber after the addition of Ce in Cu/HAp. The observed shifts in the bands may be attributed to the interaction between Cu and Ce with HAp, leading to the weakening of the O–P–O and P–O bonds.

2.1.4

Raman Spectroscopy Study

Raman spectroscopy can be useful to elucidate the formation of defective sites and the influence of Ce on the structure of Cu/HAp. The Raman scattering spectra of HAp and Ce-modified Cu/HAp are shown in [Fig. 3]. The Raman scattering spectra showed characteristic bands of the HAp phase with vibrational modes of the phosphate group,[32] and the surface defects with the Ce–O band correspond to the Ce-modified HAp catalyst. The prominent band at 962 cm⁻¹ is attributed to the symmetrical stretching of the P–O bonds.[33] Additionally, the distinctive band observed at approximately 456 cm⁻¹, corresponding to the F₂g vibrational mode, is indicative of CeO₂ and is absent in Cu/HAp, confirming the presence of CeO₂ in the sample.[34]

Fig. 3 Raman spectra of Cu/HAp, Ce/Hap, and Ce-modified Cu/HAp.

In the case of Cu/HAp, the shifting of the band was observed from 962 to 954 cm⁻¹. However, Cu_0.4/Ce_0.1/HAp showed the same band at 951 cm⁻¹. The maximum decrease in the frequency of P–O bonds was observed in Cu_0.4/Ce_0.1/HAp. Furthermore, the characteristic F₂g band of CeO₂ has shifted toward the higher frequency (from 440 to 446 cm⁻¹) with an increase in Cu concentration from 0.1 to 0.3 mol (Cu_0.1 to Cu_0.3). However, a further increase in Cu concentration to 0.4 mol (Cu_0.4) leads to the shifting of the F₂g peak of CeO₂ to the lower frequency (from 446 to 433 cm⁻¹) with broadening. Similarly, the B₂g vibrational peak of CuO at 648 cm⁻¹ shifted to 640 cm⁻¹. The decrease in frequencies indicates the weakening of the P–O and Cu–O bonds with the formation of more labile oxygen required for the low-temperature oxidation of propene. Thus, an increase in asymmetry, oxygen vacancy, and distortion in the Ce results in Raman band shifting. The Cu_0.4/Ce_0.1/HAp showed the maximum shifting, indicating the formation of more oxygen vacancies and more labile oxygen, which eventually results in improvement in activity toward propene oxidation at a lower temperature. These results are in good agreement with FTIR and XRD results.

2.1.5

H₂-Temperature Programmed Reduction Study of Ce-modified Cu/HAp Prepared by Successive Deposition

The temperature-programmed reduction (TPR) profiles of both Cu/HAp and Ce-modified Cu/HAp are illustrated in [Fig. 4]. The TPR peaks, evident in the temperature range of 150–300 °C, signify the reduction process of Cu2⁺ to Cu0 through Cu¹⁺. The Cu/HAp catalyst modified with 0.1 mol Ce showed a reduction of Cu²⁺ to Cu⁰ at 194 °C. However, the catalyst with increasing concentration of Ce (0.2–0.4 mol) showed a reduction of Cu²⁺ to Cu⁰ above 203 °C. The Cu/HAp catalyst showed a reduction of larger CuO species at 485 °C, which is absent in Ce-modified Cu/HAp. These results indicate that the Ce-modified Cu/HAp showed the formation of smaller CuO particles/clusters compared to the Cu/HAp. The Cu/HAp catalyst modified with an optimum concentration of Ce showed a reduction of CuO and Cu₂O to Cu at lower temperatures compared to the remaining catalyst.

Fig. 4 H₂-TPR spectra of Cu/HAp and Ce-modified Cu/HAp.

The H₂-TPR peaks of Cu₀.₄/Ce_0.1/HAp prepared by the successive deposition method showed a reduction of Cu and Ce at lower temperatures and indicated the improvement in redox properties of the catalyst after Ce modification. The tendency of lower reduction temperature indicates that the presence of Ce promotes surface reducibility, which could be due to strong interaction with well-dispersed CuO _x . The Cu_0.4/Ce_0.1/HAp showed a weakening of Cu–O bond strength through the strong synergetic interaction between CuO _x and Ce species. The synergistic interaction could enhance the facile redox property of the catalyst, which was responsible for the low-temperature propene oxidation.

2.1.6

X-ray Photoelectron Spectroscopy Study

The XPS spectra of Cu/HAp and Cu_0.4/Ce_0.1/HAp are shown in [Fig. 5]. [Table 2] shows the semiquantitative analysis of the catalysts. The deconvoluted spectra of Cu 2p showed peaks at 932.5 and 954.3 eV, corresponding to Cu¹⁺ and Cu²⁺, respectively ([Fig. 5A]). However, the deconvoluted XPS peaks at 952.15 and 654.28 eV correspond to the Cu 2p_1/2.[1] The four satellite peaks at 940.6, 943.2, 961.6, and 963.1 eV were observed due to the presence of CuO. The shake-up satellite peaks correspond to the major characteristics of Cu²⁺ species, indicating the predominant nature of Cu²⁺ in Cu/HAp and Cu_0.4/Ce_0.1/HAp catalysts. The percentage concentration of Cu²⁺ and Cu¹⁺ is calculated ([Table 5]) by deconvoluted spectra using the following formula

$$\%\mathrm{Cu}\left(\mathrm{I}\right)={\mathrm{A}}_2/\left(\mathrm{A}+\mathrm{B}\right)\times 100$$

$$\%\mathrm{Cu}\left(\mathrm{II}\right)=\left(\mathrm{B}+{\mathrm{A}}_1\right)/\left(\mathrm{A}+\mathrm{B}\right)\times 100$$

where the main emission line A in [Fig. 6]. 9A contains both the Cu (II) (A₁) and Cu (I) (A₂) contributions. However, the satellite intensity (B) is entirely from Cu (II). A is the sum of A₁ and A₂, whereas B is the sum of satellite peaks.[35]

Fig. 5 XPS spectra of Cu/HAp and Cu0.4/Ce0.1/HAp (A) Cu 2p (B) Ce 3d (C) O 1.

Table 2 Semiquantitative analysis of catalyst species with their binding energy.

Catalyst	Binding energy (eV)					Ratio Cu2+/Cu	Relative area of OL/Oa/Os (%)
	Cu+1	Cu+2	OL	Oa	Os
Cu/HAp	932.5	934.3	529.6	531.0	532.2	56.1	13.4/65.9/20.7
Cu0.4/Ce0.1/HAp	932.7	934.5	529.6	530.9	532.2	58.5	13.2/68.5/18.4

Fig. 6 FESEM and HRTEM images of (a, c, e) Cu/HAp and (b, d, f) Cu_0.4/Ce_0.1/HAp.

The ratio of Cu^2+/Cu (Cu²⁺ + Cu¹⁺) for Cu/HAp and Cu_0.4/Ce_0.1/HAp is 56.1 and 58.1, respectively. The addition of Ce in Cu/HAp leads to an increase in the concentration of Cu²⁺. Ce 3d spectra were deconvoluted into three pairs of spin-orbit doublets ([Fig. 5B]). The characteristic peaks are denoted in the range of 877-898 eV and 900-922 eV and assigned as v, v′, v″, v″′ and u, u′, u″, u″′, respectively. The v, v″′, u, and u″′ belong to Ce⁴⁺. However, the peaks assigned as v′/u′ and v″/u″ belong to Ce³⁺ and oxygen vacancies, respectively.[36] [37] The Cu/HAp and Cu_0.4/Ce_0.1/HAp showed an area ratio of Ce³⁺/(Ce³⁺ + Ce⁴⁺) 22.46% and 31.47% respectively, which indicates the presence of more Ce³⁺ species in Cu_0.4/Ce_0.1/HAp. The formation of more Cu²⁺ species indicates the improvement in the redox cycle between Cu and Ce. The facile redox cycle between Cu⁺ and Cu²⁺ depends on the redox cycle of Ce³⁺ ↔ Ce⁴⁺. These results indicate the redox equilibrium of Cu²⁺ + Ce³⁺ ↔ Cu⁺ + Ce⁴⁺, which could be responsible for the lower temperature oxidation of propene.

The XPS deconvoluted spectra of oxygen are shown in [Fig. 5C]. The XPS peaks observed at B.E. at 529.00–530.5, 530–531.5, and 531.5–532.5 eV correspond to the surface lattice oxygen (O_L, O_Lattice, O²⁻), adsorbed oxygen (O_A, O²⁻, O₂ ²⁻, or O⁻), and adsorbed OH groups or molecular water on the surface of the catalysts (O_S, O_surface).[38] [39] [40] The O absorbed peak ratios were quantified by integrating the peak area of all oxygen. The Cu/HAp and Cu_0.4/Ce_0.1/HAp showed 65.90 and 68.46% of OA, respectively. The adsorbed oxygen is reported to be active for the facile redox reaction.[41] The O_A peak was observed at a lower binding energy in Cu_0.4/Ce_0.1/HAp. A higher concentration of adsorbed oxygen on the Cu_0.4/Ce_0.1/HAp catalyst surface was observed compared to the Cu/HAp, which suggested the formation of more labile oxygen after the addition of Ce. The adsorbed oxygen (O_A) and improved redox cycle of Cu and Ce were responsible for the improvement in low-temperature oxidation of propene.

2.1.7

HR-TEM and SEM Study

The particle size and surface morphology of the Cu/HAp and Cu_0.4/Ce_0.1/HAp catalysts were observed by HRTEM and FESEM. Cu/HAp catalyst shows uniform distribution, quasi-spherical morphology, and clusters. Cu_0.4/Ce_0.1/HAp shows the formation of ceria nanorods along with spherical particles and clusters ([Fig. 6a, b]). The formation of nanorods favors the improvement in the redox cycle of Ce.[42] [43] [44] The results indicate that the Cu_0.4/Ce_0.1/HAp showed the formation of highly dispersed CuO _x species with Ce nanorods, which was responsible for the low-temperature propene oxidation. The Cu/HAp showed quasi-spherical and flaky HAp particles. The quasi-spherical HAp could be due to CuO and flakes, or HAp could be due to the HAp support ([Fig. 6c]). Whereas Cu_0.4/Ce_0.1/HAp exhibits quasi-spherical and flake-like HAp particle formation along with Ce nanorods ([Fig. 6d]). The presence of nanorods suggests the interaction of Cu with Ce and HAp. The particle size of Cu/HAp ranges from 15 to 35 nm, whereas Cu_0.4/Ce_0.1/HAp exhibits a decrease in particle size within the range of 3-30 nm (see [Fig. 6c, d]). Additionally, the HR-TEM analysis of Cu/HAp reveals d-spacing values of 0.27 nm (200) and 0.25 nm (200), corresponding to HAp and CuO, respectively (see [Fig. 6e, f]). Whereas, Cu_0.4/Ce_0.1/HAp shows d-spacing of 0.27 nm (200), 0.23 nm (111), and 0.32 nm (111), which corresponds to HAp, CuO, and the presence of Ce rod, respectively. These results indicate that the Cu_0.4/Ce_0.1/HAp showed the formation of highly dispersed, smaller CuOx species with Ce nanorods, which were responsible for the low-temperature propene oxidation.

2.2

Catalytic Activity

2.2.1

Propene Oxidation

2.2.1.1

Activity of Cu/HAp and Ce-promoted Cu/HAp Catalyst Prepared by the Successive Deposition Method (Bimetallic)

The Ce-modified Cu/HAp prepared by the successive deposition method (Cu_0.4/Ce_0.1/HAp) showed maximum propene conversion compared to the catalyst prepared by the coprecipitation method (Cu_0.4/Ce_0.1/HAp). Furthermore, the concentration of Ce in Cu/HAp prepared by successive deposition methods has been optimized for the complete oxidation of propene. The results of catalyst optimization are shown in [Fig. 7] and [Table 3]. The Cu/HAp catalyst modified with 0.1 mol Ce (Cu_0.4/Ce_0.1/HAp) showed T _light–off, T ₅₀, and T ₁₀₀ for propene oxidation to CO₂ at 158, 267, and 339 °C, respectively. However, further increase in the concentration of Ce (0.2–0.4 mol), T _light–off, T ₅₀, and T ₁₀₀ shifted toward higher temperature. Feng Pan et al.[45] reported that the LaFe_0.8Co_0.2O₃ catalyst reached about 90% propene conversion at roughly 345 °C. In contrast, the catalyst developed in the present study achieves full propene oxidation at a slightly lower temperature of 339 °C, highlighting its stronger low-temperature activity.

Fig. 7 Propene oxidation activity of Cu/HAp and Ce-promoted Cu/HAp catalyst prepared by the successive deposition method. Reaction feed: −300 ppm propene, 5% O₂, He balance, GHSV = 50,000 mL g⁻¹ h⁻¹.

Table 3 T _Light–off, T _50, and T ₁₀₀ comparisons of Cu/HAp and Ce-modified Cu/HAp on propene oxidation.

Catalyst	T Light–off Temp. (°C)	T 50 (°C)	T 100 (°C)
Cu/HAp	204	290	368
Cu0.1/Ce0.4/HAp	171	291	384
Cu0.2/Ce0.3/HAp	165	282	370
Cu0.3/Ce0.2/HAp	160	277	352
Cu0.4/Ce0.1/HAp	158	267	339

The catalyst Cu-modified Ce/HAp (Ce_0.3/Cu_0.2/HAp) demonstrated the highest propene conversion. Additionally, the mole of Cu in the Ce/HAp made using the successive deposition approach has been adjusted to investigate the optimal Cu loading for propene oxidation. [Fig. 8] and [Table 4] display the oxidation activity of propene on various molar amounts of Cu and Ce on HAp. The Ce/HAp catalyst modified with Cu displayed T₁₀₀ for the oxidation of propene by the following order, Ce/HAp (>400) > Ce_0.4/Cu_0.1/HAp (398 °C) > Ce_0.1/Cu_0.4/HAp (391 °C) > Ce_0.2/Cu_0.3/HAp (370 °C) > Cu/HAp (368 °C) > Ce_0.3/Cu_0.2/HAp (363 °C). Hence, Cu-modified Ce/HAp shows higher-temperature propene oxidation activity than Ce-modified Cu/HAp due to the more facile redox behavior of Ce and Cu.

Fig. 8 Propene oxidation activity of Cu/HAp and Cu-promoted Ce/HAp catalysts prepared by the successive deposition method. Reaction feed: 300 ppm propene, 5% O₂, He balance, GHSV = 50,000 mL g⁻¹ h⁻¹.

Table 4 Propene oxidation activity of Cu/HAp and Cu-promoted Ce/HAp catalysts prepared by the successive deposition method. Reaction feed: −300 ppm propene, 5% O₂, He balance, GHSV = 50,000 mL g⁻¹ h⁻¹.

Catalyst	T Light–off Temp. (°C)	T 50 (°C)	T 100 (°C)
Cu/HAp	204	290	368
Ce/HAp	292	>400	>400
Ce0.1/Cu0.4/HAp	216	294	391
Ce0.2/Cu0.3/HAp	164	273	370
Ce0.3/Cu0.2/HAp	168	265	363
Ce0.4/Cu0.1/HAp	222	322	398

Table 5 T _Light–off, T _50, and T ₁₀₀ comparison of Cu/HAp and Ce-modified Cu/HAp with and without H₂O.

Catalyst	T Light–off (°C)	T 50 (°C)	T 100 (°C)
Cu/HAp	204	290	368
Cu/HAp + H2O	200	277	375
Cu0.4/Ce0.1/HAp	158	267	339
Cu0.4/Ce0.1/HAp + H2O	160	302	340

2.2.1.2

Effect of H₂O Addition

[Fig. 9] depicts the impact of water on Cu/HAp and Cu_0.4/Ce_0.1/Hap catalysts. After adding H₂O to the reaction feed, the activity of propene oxidation continues to decline. Propene was converted by 50% in Cu_0.4/Ce_0.1/HAp at 268 °C, and with the addition of water, it shifted at 302 °C. This temperature rise is caused by water molecules obstructing the catalysts’ active sites. The FTIR spectra of the catalyst after water addition are shown in [Fig. 10]. The (PO₄)³⁻ vibrations (560 and 1028 cm⁻¹) are shifted toward a lower frequency as a result of the adsorption of water molecules.

Fig. 9 Propene oxidation activity of Cu/HAp and Ce-promoted Cu/HAp catalyst with and without H₂O. Reaction feed: 300 ppm Propene, 0.5% H₂O, 5%O₂, He balance, GHSV = 50,000 mL g⁻¹ h⁻¹.

Fig. 10 ATR-FTIR spectra of HAp, Cu/HAp, and Cu_0.4/Ce_0.1/HAp with and without H₂O.

2.2.1.3

Effect of SO₂ Addition

[Fig. 11] illustrates how SO₂ affects Cu/HAp and Cu_0.4/Ce_0.1/HAp. With the addition of SO₂ to the reaction feed, the activity of propene oxidation keeps declining. In Cu_0.4/Ce_0.1/HAp, complete (T ₁₀₀) of the propene was seen at 339 °C, and with the addition of SO₂, it moved to 360 °C ([Table 6]). This temperature rise is caused by SO₂ molecules poisoning the catalysts’ active sites. As a result, the oxidation activity of the Cu/HAp and Cu_0.4/Ce_0.1/HAp catalysts decreases due to the deterioration of their active centers.

Fig. 11 Propene oxidation activity of Cu/HAp and Ce-promoted Cu/HAp catalyst with and without SO₂. Reaction feed: 300 ppm Propene, 0.5% SO₂, 5% O₂, He balance, GHSV = 50,000 mL g⁻¹ h⁻¹.

Table 6 T _Light–off, T ₅₀, and T ₁₀₀ comparison of Cu/HAp and Ce-modified Cu/HAp with and without SO₂.

Catalyst	T Light–off (°C)	T 50 (°C)	T 100 (°C)
Cu/HAp	204	290	368
Cu/HAp + SO2	210	340	390
Cu0.4/Ce0.1/HAp	158	268	339
Cu0.4/Ce0.1/HAp + SO2	170	315	360

2.2.2

CO Oxidation

[Fig. 12] depicts the toluene oxidation of Cu/HAp and Cu_0.4/Ce_0.1/HAp. The T _light–off, T ₅₀, and T ₁₀₀ for Cu/HAp are recorded at 65, 158, and 200 °C, respectively. After promotion with Ce in Cu/HAp (Cu_0.4/Ce_0.1/HAp), the T _light–off, T ₅₀, and T ₁₀₀ were measured at 50, 140, and 175 °C, respectively ([Table 7]). This low-temperature activity was noticed as a result of the synergistic interaction between Cu and Ce, which generates more labile oxygen and encourages oxidation. In addition, more labile and facile oxygen and Cu species were observed, which move CO oxidation toward low temperature. Waikar et al.[46] reported that their 0.5MnO _x /0.05Ce–0.45Al₂O₃ catalyst achieved full CO conversion around 200 °C. Kim et al.[47] evaluated CO oxidation performance for Mn/Al₂O₃ and Mn–Ce/Al₂O₃ catalysts prepared through impregnation, reporting that the material reached 50% conversion at about 170 °C and complete conversion near 300 °C. In comparison, the catalyst developed in the present study reaches complete oxidation at a lower temperature of 175 °C, indicating a more efficient low-temperature activity.

Fig. 12 The CO oxidation activity of Cu_0.5/HAp and Cu_0.4/Ce_0.1/HAp. Reaction condition: −300 ppm CO, 5% O₂, He balance, GHSV-50,000 mL g⁻¹ h⁻¹.

Table 7 T _Light–off, T ₅₀, and T ₁₀₀ comparison of Cu_0.5/HAp and Cu_0.4/Ce_0.1/HAp on CO oxidation.

Catalyst	T Light–off (°C)	T 50 (°C)	T 100 (°C)
Cu0.5/HAp	65	158	200
Cu0.4/Ce0.1/HAp	50	140	175

2.2.3

Toluene Oxidation

The toluene oxidation of Cu/HAp and Cu_0.4/Ce_0.1/HAp is shown in [Fig. 13]. Following promotion with Ce, the T _light–off, T ₅₀, and T ₁₀₀ for Cu/HAp are observed at 160, 275, and 353 °C, respectively. T ₅₀ and T ₁₀₀ were recorded for Cu_0.4/Ce_0.1/HAp T_light–off at 144, 265, and 325 °C, respectively ([Table 8]). Due to the synergistic interaction between Cu and Ce, which produces more labile oxygen and promotes oxidation, this low-temperature activity was observed.

Fig. 13 The toluene oxidation activity of Cu_0.5/HAp and Cu_0.4/Ce_0.1/HAp. Reaction condition: 300 ppm Toluene, 5% O₂, He balance, GHSV-20,000 mL g⁻¹ h⁻¹.

Table 8 T _Light–off, T ₅₀, and T ₁₀₀ comparison of Cu_0.5/HAp and Cu_0.4/Ce_0.1/HAp on toluene oxidation.

Catalyst	T Light–off (°C)	T 50 (°C)	T 100 (°C)
Cu0.5/HAp	160	275	353
Cu0.4/Ce0.1/HAp	144	265	325

2.2.4

Stability Study

The stability test was performed under stationary conditions at 400 °C to evaluate the long-term stability and reproducibility of the optimized catalyst. The Cu_0.4/Ce_0.1/HAp showed 100% propene, CO, and toluene conversion at 400 °C, and activity was found to be stable up to 1080 min ([Fig. 14]).

Fig. 14 Stability study of Cu_0.4/Ce_0.1/HAp for CO, propene, and toluene.

2.2.5

Effect of Synthesis Methods on the Ce-modified Cu/HAp Catalysts in Propene Oxidation Reaction

The propene oxidation activity comparison of Cu_0.5/HAp, Cu_0.4-Ce_0.1/HAp, and Cu_0.4/Ce_0.1/HAp is shown in [Fig. 15] and [Table 9]. The Cu_0.4/Ce_0.1/HAp catalyst prepared by the successive deposition method (bimetallic) showed a light-off temperature (T _light–off) at 158 °C. However, 50% (T ₅₀) and 100% (T ₁₀₀) propene conversion into CO₂ was observed at 267 and 339 °C, respectively. Whereas, Cu_0.4/Ce_0.1/HAp catalyst prepared by coprecipitation showed light-off temperature, 50% and 100% propene conversion to CO₂ at 171, 287, and 354 °C, respectively. The Cu_0.5/HAp showed T _light–off, T ₅₀, and T ₁₀₀ for propene oxidation at 204, 290, and 368 °C. These results indicate that the catalyst (Cu_0.4/Ce_0.1/HAp) prepared by successive deposition showed higher propene oxidation activity at a lower temperature compared to the catalyst prepared by the coprecipitation method (Cu_0.4/Ce_0.1/HAp) and Cu_0.5/HAp.

Fig. 15 Catalytic propene oxidation of Cu_0.5/HAp, Cu_0.4–Ce_0.1/Hap, and Cu_0.4/Ce_0.1/HAp catalysts. Reaction feed: 300 ppm propene, 5% O₂, He balance, GHSV = 50,000 mL g⁻¹ h⁻¹.

Table 9 T _Light–off, T ₅₀, and T ₁₀₀ comparison of Cu_0.5/HAp, Cu_0.4-Ce_0.1/Hap, and Cu_0.4/Ce_0.1/HAp with different synthesis methods on propene oxidation.

Catalyst	T Light–off	T 50 (°C)	T 100 (°C)
Cu0.5/HAp	204	290	368
Ce0.5/HAp	292	>400	>400
Cu0.4/Ce0.1–HAp DP method	216	327	400
Cu0.4/Ce0.1/HAp Bimetallic	158	267	339
Cu0.4-Ce0.1/HAp CP method	171	287	354
Ce0.1/Cu0.4/HAp Bimetallic	216	294	371

2.2.5.1

Powder X-ray Diffraction and Surface Area Study

The XRD patterns of Ce-promoted Cu/HAp catalysts synthesized by successive deposition and coprecipitation methods ([Fig. 16]) confirmed the hexagonal HAp phase (JCPDS no. 09-0432) with characteristic peaks at 2θ = 22.7–64.06°, along with CuO reflections corresponding to the monoclinic phase (JCPDS no. 48-1548). After Cu incorporation, a slight shift of the HAp peaks toward lower Bragg angles was observed, indicating lattice distortion due to metal deposition. The absence of distinct CeO₂ peaks suggests either low ceria loading (Ce_0.1) or overlap with HAp reflections. A more pronounced peak shift in the Cu_0.4–Ce_0.1/HAp (co-precipitation) catalyst implies Cu2⁺ substitution into the Ce4⁺ lattice, forming a Cu–Ce solid solution and causing lattice contraction. In contrast, the Cu_0.4/Ce_0.1/HAp (successive deposition) catalyst showed minimal peak shifting, suggesting the formation of dispersed CuO species over Ce/HAp, generating surface defects that enhance redox interaction. BET analysis revealed that the coprecipitated sample exhibited a higher surface area (117.7 m² g⁻¹) than the successive deposition catalyst (65.4 m² g⁻¹), likely due to Cu incorporation into the Ce lattice ([Table 10]). The coprecipitated catalyst exhibits a higher surface area because all components form together as fine, uniformly dispersed particles, which prevents pore blocking and maintains an open structure. However, successive deposition tends to coat or partially cover the support surface, leading to pore blockage and the formation of larger aggregates, which reduces the overall surface area. However, the latter’s lower surface area and stronger Cu–Ce interfacial synergy favor the formation of active oxygen species, resulting in superior low-temperature propene oxidation activity.

Fig. 16 Powder X-ray diffraction pattern of HAp, Cu/HAp, Cu_0.4-Ce_0.1/HAp, and Cu_0.4/Ce_0.1/HAp.

Table 10 Textural properties comparison of the Ce-modified catalyst prepared by the successive deposition and coprecipitation method.

Catalyst	2θ	S BET (m2/g)
HAp	31.98	28.8
Cu/HAp	31.76	40.1
Cu0.4-Ce0.1/HAp	31.94	117.7
Cu0.4/Ce0.1/HAp	31.88	65.4

2.2.5.2

ATR-FTIR Spectra Comparison of Cu_0.4-Ce_0.1/HAp and Cu_0.4/Ce_0.1/HAp Catalysts

The attenuated total reflectance Fourier transform infrared spectra are shown in [Fig. 17]. The fundamental vibrational modes for PO₄ ³⁻ in hydroxyapatite were observed around 962, 1030, and 1090 cm⁻¹.[31] [48] The strong bands at 962 cm⁻¹ ʋ₁ were associated with stretching vibrations of the P-O mode. The larger peak at 1030 cm⁻¹ and the smaller peak at 1090 cm⁻¹ correspond to the PO₄ ³⁻ group. However, these bands were also observed for Cu/HAp, Cu_0.4/Ce_0.1/HAp, and Cu_0.4-Ce_0.1/HAp, suggesting that the HAp structure is preserved after deposition of Cu and Ce.

Fig. 17 ATR-FTIR spectra of HAp, Cu/HAp, Cu_0.4-Ce_0.1/HAp, and Cu_0.4/Ce_0.1/HAp.

The peak observed at 562 cm⁻¹ corresponds to the O–P–O bond, which showed shifting to the higher wave number at 565 cm⁻¹, whereas 1025 and 1088 cm^−1, corresponding to the P–O bond showed shifting to the higher wave number at 1032 and 1090 cm⁻¹ after doping of Cu on the HAp support (Cu/HAp). The shifting of these bands suggested the increase in P–O bond strength after the deposition of Cu on HAp. However, the catalyst prepared by successive deposition (Cu_0.4/Ce_0.1/HAp) showed the band at 1030 cm⁻¹, which is lower than Cu/HAp and Cu_0.4-Ce_0.1/HAp (1032 cm⁻¹). The shifting of the band indicates the weakening of the P–O bond due to the successive deposition of Cu and Ce over HAp. The shifting of bands also indicates the interaction of Ce with HAp support and forms the more labile oxygen, favorable for the oxidation of propene at a lower temperature.

2.2.5.3

H₂-Temperature Program Reduction Study of Cu/HAp, Cu_0.4/Ce_0.1/Hap, and Cu_0.4-Ce_0.1/Hap

[Fig. 18] shows the reduction profiles of the Ce-modified Cu/HAp catalyst prepared by successive deposition and coprecipitation methods. The Cu/HAp sample shows the reduction peaks at the small shoulder at 210, 266, 490, and 645 °C. The peaks observed at 210 and 261 °C are attributed to the complete reduction of Cu²⁺ to Cu¹⁺ and Cu¹⁺ to Cu⁰. However, Cu_0.4/Ce_0.1/HAp showed reduction peaks at 194 and 239 °C. Whereas Cu_0.4-Ce_0.1/HAp showed reduction peaks at 266 °C, which corresponds to the reduction of small clusters of CuO _x . The Ce-modified Cu/HAp catalyst prepared by the coprecipitation method showed the formation of smaller CuO _x clusters. The absence of reduction peaks for HAp is in the temperature range of 30 to 800 °C.[49] These suggest that HAp itself does not undergo a significant reduction in this temperature range. The observed peaks between 150 and 300 °C are attributed to the stepwise reduction of CuO to Cu₂O and metallic Cu.[50] [51] The high-temperature Cu reduction peak (250 and 261 °C) was observed due to the small clusters of CuO _x (CuO and Cu₂O). However, the reduction peak at 400 and 485 °C was due to the bulk CuO. The reduction peaks above 600 °C were due to the reduction of Ce. The Cu_0.4/Ce_0.1/HAp showed a reduction of CuO and Cu₂O to Cu at lower temperatures compared to the Cu_0.4-Ce_0.1/HAp and Cu/HAp. The low-temperature reduction peak of Cu was due to the highly dispersed and facile CuO species on the catalyst surface.

Fig. 18 H₂-TPR spectra of Cu/HAp, Cu_0.4-Ce_0.1/HAp, and Cu_0.4/Ce_0.1/HAp.

3

Conclusions

The Ce-modified Cu/HAp catalysts synthesized by successive deposition (Cu_0.4/Ce_0.1/HAp) exhibited significantly higher low-temperature propene oxidation activity than those prepared by coprecipitation (Cu_0.4Ce_0.1/HAp). The optimized Ce concentration in the successive deposition catalyst promoted strong Cu–Ce interactions, enhanced Cu²⁺/Ce³⁺redox behavior, and generated more labile oxygen species. XRD, XPS, and Raman analyses confirmed the presence of smaller CuO particles, Ce³⁺ formation, and weaker Cu–O and P–O bonds, indicating higher oxygen mobility and surface reactivity. In contrast, the coprecipitated catalyst formed a Cu–O–Ce solid solution, reducing the surface-active Cu sites and limiting oxidation efficiency. The improved redox properties and structural synergy in Cu_0.4Ce_0.1/HAp make it highly effective for CO, propene, and toluene oxidation at lower temperatures, although further testing under real diesel exhaust and VOC conditions is recommended.

4

Experimental Section

4.1

Reagents

All chemicals (reagents and solvents) purchased were of analytical grade and procured from S.D. Fine, Sigma-Aldrich, and Loba Chemie and were used without further purifications. The listed chemicals and reagents are Cu (NO₃)₂·3H₂O (99.5%, Sigma-Aldrich), Ca (NO₃)₂·4H₂O (99.5%, S D Fine Chemicals), (NH₄)₂HPO₄ (97%, S D Fine Chemicals), Ce (NO₃)₂·6H₂O (99.9%, Loba Chemie), NH₄OH (25% v/v, S D Fine Chemicals), and NaOH (98%, S D Fine Chemicals). Distilled water was obtained from a Milli-Q purification system.

4.2

Catalyst Preparation

The catalysts Cu _x /HAp, Ce _x /HAp, and Cu _x /Ce_(0.5–x)-HAp were synthesized using a controlled deposition–precipitation (DP) method to ensure uniform incorporation of Cu and Ce into the hydroxyapatite (HAp) matrix. Solution A was prepared by dissolving cerium nitrate hexahydrate and calcium nitrate tetrahydrate in 100 mL of deionized water, while Solution B contained diammonium hydrogen orthophosphate dissolved in 50 mL of deionized water. Both solutions were adjusted to pH 10 using 25% NH₄OH and stirred at 500 rpm for 30 min. Solution B was then added dropwise into Solution A under constant stirring, forming a dense precipitate. The precipitate was washed with ethanol, dried at 100 °C for 12 h, and calcined at 400 °C for 4 h to obtain Ce _x -HAp. For copper incorporation, 2.0 g of HAp or Ce _x -HAp (0.5 mol) was dispersed in 100 mL of deionized water, and an aqueous solution of copper nitrate trihydrate was added dropwise while maintaining pH 10 with 25% NH₄OH. The slurry was stirred at 500 rpm for 24 h, washed thoroughly, dried at 100 °C for 12 h, and calcined at 400 °C for 4 h. This synthesis method demonstrates its potential for crafting catalysts enriched with Copper (Cu) supported on either HAp or Ce _x -HAp, offering promising prospects for diverse applications and thorough investigations in the field.

The bimetallic catalysts denoted as Cu _x /Ce_(0.5–x)/HAp_0.5 were synthesized using a successive impregnation method, based on a previously reported hydroxyapatite (HAp) synthesis procedure.[1] [21] Initially, HAp was prepared following the established method. For bimetallic catalyst preparation (x = 0.1, 0.2, 0.3, 0.4), the required amount of cerium nitrate hexahydrate [Ce(NO₃)₃·6H₂O] was dissolved in 10 mL of distilled water and added to a 0.5 mol aqueous slurry of HAp. The suspension was stirred at 600 rpm for 24 h while maintaining a pH of 10 using 25% NH₄OH. The resulting mixture was filtered, dried at 100 °C for 12 h, and calcined at 400 °C for 4 h to obtain Ce_0.5/HAp. In the second impregnation step, an aqueous solution of copper nitrate trihydrate [Cu(NO₃)₂·3H₂O] was added to a 100 mL slurry of Ce_0.5/HAp under the same conditions (pH 10, 600 rpm). After 24 h of stirring, the mixture was filtered, dried at 100 °C for 12 h, and calcined at 400 °C for 4 h, yielding the Cu _x /Ce_(0.5–x)/HAp_0.5 catalysts with varying Cu loadings (x = 0.1–0.4). Monometallic catalysts (Cu/HAp and Ce/HAp) were also synthesized using the same method by depositing 0.5 mol of either Cu or Ce on HAp. Similarly, the Cu _x /Ce_(0.5–x)/HAp_0.5 catalysts were prepared by reversing the impregnation sequence. Initially, Cu was impregnated on HAp to form Cu_0.5/HAp, followed by Ce incorporation using the same conditions of pH, stirring, drying, and calcination. For comparison, Cu _x Ce_(0.5–x)/HAp catalysts were also synthesized via a coprecipitation (CP) method.[52] In this process, calculated moles of Cu and Ce precursors were dissolved separately in 10 mL of distilled water and added dropwise into an aqueous HAp slurry (0.5 mol). The pH was maintained at 10 using 25% NH₄OH, with continuous stirring at 600 rpm for 24 h. The obtained precipitate was filtered, dried at 100 °C for 12 h, and calcined at 400 °C for 4 h, resulting in the Cu _x Ce_(0.5–x)/HAp catalysts (x = 0.1–0.5).

4.3

Catalytic Activity Testing

The catalytic oxidation of propene as a model diesel engine exhaust and VOC was performed under atmospheric pressure by using a fixed-bed tubular downflow reactor. About 0.5 g of prepared catalyst was diluted with 2.0 g silica gel having a size of 60-120 mesh. Before the propene oxidation, the catalyst was pretreated in 10% O₂ with helium balance for 1 h at 400 °C. The reaction feed consists of 300 ppm (CO/propene/toluene), 5% O₂, and helium balance passed over the catalyst at 50,000 mL g⁻¹ h⁻¹. The catalyst performance was evaluated at 30–400 °C @ 2 °C min⁻¹. The concentration of the gaseous feed was analyzed online by using micro-GC (Agilent 490) equipped with a thermal conductivity detector (TCD) and Porapack Q, MS-5A, and CP Sil 5CB columns. The CO, propene, and toluene conversions were calculated from their initial and final concentrations using the following formula.

$$\%X\text{conversion}=\frac{{\left[X\right]}_{\text{Initial}}-{\left[X\right]}_{\text{Final}}}{\left[X\right]}\times 100\%$$

where, X is CO, propene, and toluene.

4.4

Catalyst Characterizations

The X-ray diffraction patterns of the catalyst were obtained in the range of 2θ 20° to 80° with a step size of 0.02° and scan rate of 4° min⁻¹ (Shimadzu XRD 6100 Cu Kα, λ = 1.54 Å, 40 kV, 30 mA). The nitrogen adsorption was determined on Smart Sorb 92/93 equipment from Smart Instrument Co. The catalyst was preheated at 250 °C in an environment of N₂ purging for 90 min, followed by N₂ adsorption. The Fourier transform infrared spectroscopy (FTIR) systematically monitors the variations of structural characteristic groups. The IR spectra were obtained using a Perkin Elmer (spectrum two) instrument in the range of 400–4000 cm⁻¹. The redox properties of catalyst reducibility can be examined by hydrogen temperature programmed reduction (H₂-TPR). The Micromeritics AutoChem II instrument with thermal conductivity detector (TCD) was used for the H₂-TPR study. In typical TPR, the catalyst was pretreated in 10% O₂ with He balance from room temperature to 400 °C. The catalyst was further heated in the temperature range of 50–800 °C in 5% H₂ with Ar balance @ 20 mL min⁻¹. Furthermore, the Raman spectra were recorded using a Raman spectrometer (Labram HR 800, Horiba Jobin Yvon) with an Ar laser (514.5 nm) in the range of 3000–10 cm⁻¹. The X-ray photoelectron spectroscopy (XPS) was performed on a Kratos Analytical instrument (Model AXIS Supra) having a Monochromatic (AlKα) 600 W X-ray source; 1486.6 eV. The background correction was done by C1s peak at 284.8 eV. The SEM analysis was performed using the JEOL JSM-7600 (FEG-SEM) instrument. The high-resolution transmission electron microscopy (HR-TEM) study was done by using the FEI Tecnai G2 F30 instrument at 300 kV.

Contributors’ Statement

R.M.: Data curation, Formal analysis, Investigation, Writing – original draft. K.P.: Data curation, Writing – original draft. R.P.: Data curation, Writing – original draft. P.M.: Conceptualization, Data curation, Funding acquisition, Project administration, Supervision, Validation, Writing – review & editing.

Conflict of Interest

The authors declare that they have no conflict of interest.

Acknowledgement

Science and Engineering Research Board, Department of Science and Technology, Delhi, India, is acknowledged for financial support provided as an “Empowerment and equity opportunities for excellence in Science” having the sanction order no. EEQ/2016/000264.

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Publication History

Received: 05 November 2025

Accepted after revision: 16 January 2026

Accepted Manuscript online:
16 January 2026

Article published online:
05 February 2026