Co3 O4 -CeO2 Nanocomposites for Low-Temperature CO Oxidation.

In an effort to combine the favorable catalytic properties of Co3 O4 and CeO2 , nanocomposites with different phase distribution and Co3 O4 loading were prepared and employed for CO oxidation. Synthesizing Co3 O4 -modified CeO2 via three different sol-gel based routes, each with 10.4 wt % Co3 O4 loading, yielded three different nanocomposite morphologies: CeO2 -supported Co3 O4 layers, intermixed oxides, and homogeneously dispersed Co. The reactivity of the resulting surface oxygen species towards CO were examined by temperature programmed reduction (CO-TPR) and flow reactor kinetic tests. The first morphology exhibited the best performance due to its active Co3 O4 surface layer, reducing the light-off temperature of CeO2 by about 200 °C. In contrast, intermixed oxides and Co-doped CeO2 suffered from lower dispersion and organic residues, respectively. The performance of Co3 O4 -CeO2 nanocomposites was optimized by varying the Co3 O4 loading, characterized by X-ray diffraction (XRD) and N2 sorption (BET). The 16-65 wt % Co3 O4 -CeO2 catalysts approached the conversion of 1 wt % Pt/CeO2 , rendering them interesting candidates for low-temperature CO oxidation.

Introduction

Ceria (CeO2) has applications in many catalytic reactions, such as methane dry reforming, hydrocarbon and diesel soot oxidation, organic synthesis and especially environmental catalysis. Ceria is a crucial component of three‐way‐catalysts (TWCs), serving together with alumina as support for dispersed noble metal (e. g., Pt (or Pd) and Rh) nanoparticles, preventing their sintering at high temperatures. Additionally, ceria regulates the surface oxygen concentration under fuel lean and rich conditions, due to its oxygen buffer (storage/release) capacity (OBC/OSC), associated with the fast Ce4+/Ce3+ redox cycle. Recently, advanced synthesis methods allowed to develop nanostructured ceria of various morphologies (shapes), with high specific surface area (SSA) and improved Ce3+/Ce4+ ratio.[ , , ] Furthermore, noble metals supported on ceria exhibit remarkable catalytic activity in preferential CO oxidation (PROX) and water gas shift (WGS), attributed to active sites at the metal/support interface and the availability of lattice oxygen (oxygen vacancies). CeO2 is thus considered an active (“non‐innocent”) support.

Increasingly stringent emission regulations require continuous innovations, especially regarding engine cold start emissions at temperatures when noble metals are CO‐poisoned and inactive. For noble metals, temperatures around 100–200 °C are typically required to initiate CO oxidation (“ignition”),[ , ] which is why ∼80 % of the emissions result from the cold‐start period. Therefore, many studies focused on low‐temperature CO oxidation, for example, over Au nanoparticles (2–4 nm size) on reducible oxides (e. g., TiO2, CeO2). These are less prone to CO poisoning, but Au nanoparticles tend to sinter. Co3O4 has been shown to be very active,[ , ] but fully replacing CeO2 by Co3O4 would require substantial modifications of the existing TWC technology, as the CeO2 support is important for both oxidation and reduction. However, Co3O4‐modified CeO2 nanocomposites may be a compromise, combining the favorable activity of cobalt oxide with the high oxygen storage capacity of ceria. Most importantly, for such mixed or supported oxides the limitations of availability and costs of noble metals apparently do not apply.

However, apart from the mere basic composition, the performance of Co3O4−CeO2 catalysts strongly depends on the applied synthesis route, which determines the dispersion, morphology/microstructure, surface composition, redox and catalytic properties of the oxides. The materials reported so far in the literature were mainly prepared by conventional impregnation and coprecipitation methods followed by high temperature calcination, which does not yield catalysts with the desired high surface area and high dispersion of oxide phases, which are prerequisites for high catalytic activity in CO oxidation.

Previously, we have presented a new feasible route for preparation of Co3O4‐modified CeO2 catalysts, based on sol‐gel synthesis combined with solvothermal processing, permitting the direct crystallization of the gel without the need of annealing at high temperatures to induce crystallization. Using this approach, we have been able to synthesize Co3O4‐modified CeO2 nanocomposites with high specific surface area and highly dispersed cobalt oxide nanoparticles. This maximizes the number of accessible active sites and results in high CO oxidation activity of Co3O4‐modified CeO2 catalysts, comparable to that of pure Co3O4 and, in terms of activation energy, even to Pt/ and Pd/CeO2. Au/CeO2 shows activity at lower temperature, due to lower CO binding energy,[ , , ] but a coinage metal is apparently required.

Based on the effect of different synthesis routes on the structure of Co3O4‐modified CeO2 reported previously, herein the available oxygen species, redox properties and catalytic performance are examined in detail. Furthermore, an optimum Co3O4 loading of CeO2 is determined. Pure CeO2 and Co3O4, as well as Pt/CeO2, are included for comparison. The Co3O4‐CeO2 nanocomposites turned out to be promising candidates for low‐temperature CO oxidation and could be implemented by moderate modifications of TWC manufacture.

Results and Discussion

Table 1 summarizes the three synthesis routes and the different resulting structures of the Co3O4 modified CeO2 catalysts, as reported previously. Herein, the focus is on their reduction by CO, reflecting the active oxygen species and ability for oxygen vacancy formation, which are important for the catalytic flow reactor performance. Then, the Co3O4 loading is tuned, including further characterization and activity measurements. The catalytic performance of the Co3O4‐CeO2 nanocomposites is finally contrasted to those of the pure oxides and Pt/CeO2.

Table 1

Overview of three different sol‐gel routes for synthesis of Co3O4‐CeO2 nanocomposites and of corresponding structures (details please see the Supporting Information).

Route	Procedure	Structure [17]
1
		small (3–5 nm) Co3O4 particle aggregation (black), forming layers on the surface of larger (10–20 nm) agglomerated CeO2 particles (yellow): “supported Co3O4”
2
		coexisting (3–5 nm) Co3O4 (grey) and (20–30 nm) CeO2 nanoparticles (yellow): “intermixed oxides”
3
		homogeneous distribution of cobalt within CeO2 (via Co‐O−Ce network): “Co‐doped CeO2”

Structure

CO‐TPR of pure CeO2 and Co3O4

First, the pure oxides were characterized by CO temperature programmed reduction, with the results being in line with the literature.[ , , ] For pure CeO2, prepared by the combination of sol‐gel and solvothermal methods in ethanol (STE), and further air calcined (AC) at 500 °C (i. e., STE‐AC), the CO2 evolution in CO‐TPR occurs in three regions (Figure 1a, red line): (I) 250–425 °C, due to removal of surface lattice oxygen (OSL); CO+OSL→CO2; (II) 425–625 °C, due to water gas shift (WGS) between CO and surface OH groups (CO+OH→CO2 +1/2 H2); (III)>625 °C, due to extraction of bulk lattice oxygen (OBL). Accordingly, due to WGS the H2 evolution on CeO2 exhibits a main peak in region II (Figure 1b). Still, the reducibility of ceria is much lower than that of Co3O4.

Figure 1

CO‐TPR of pure CeO2 (STE‐AC) and commercial Co3O4: (a) CO2 and (b) H2 evolution.

Co3O4 is reduced at lower temperature and several studies reported a two‐step process: Co3+→Co2+→Co0. This was confirmed by CO‐TPR of commercial Co3O4, as evident from Figure 1 (black lines). The CO2 evolution (Figure 1a) occurs in three stages: (i) <270 °C, via OSL removal; CO+OSL→CO2; Co3+ SL→Co2+ SL; (ii) 270–450 °C, OBL removal of bulk lattice oxygen (CO+OBL→CO2), causing Co3+ BL→Co2+ BL, and further reduction of Co2+→Co0. In the 450–670 °C region, CO disproportionation (2CO CO2+C) or CO dissociation (CO C+O) may occur on Co0, resulting in minute CO2 desorption. Additionally, some H2 evolution (Figure 1b) is detected around 300 °C, pointing to WGS of CO with surface OH groups. Some OH groups only react around 500 °C. Both for CeO2 and Co3O4 mass 18 was also recorded, but no water desorbed over the entire temperature range. CO‐TPR spectra (mass 44 and 2) of a 1 : 1 physical mixture of CeO2/Co3O4 were simply a superposition of the spectra of the individual oxides, indicating that oxide/oxide interactions were absent in this case.

CO‐TPR of 10 wt % Co3O4‐modified CeO2 synthesized via routes 1–3

For Co3O4–modified CeO2 different and more complex CO‐and H2‐TPR profiles were obtained, that were not just a sum of the profiles of the individual oxides. First, new low temperature peaks indicate additional surface oxygen species and, second, the “individual oxide peaks” are shifted by 20 or more degrees to higher temperature. Both indicate synergetic interfacial interactions between cobalt oxide and ceria, affecting the Co3+/Co2+ and Ce4+/Ce3+ redox properties, which seems to promote the reactive oxygen species. This also holds true for H2 evolution.[ , ]

Accordingly, CO‐TPR was performed for all six samples (three STE (Figure 2) and the corresponding STE‐AC (Figure 3) samples). The different profiles clearly show that the interaction with CO/reducibility (CO2 evolution) strongly depends on the preparation route and the heat‐treatment (calcination), as both affect the Co distribution. Based on the CO‐TPR results in Figure 1 and the literature,[ , , , ] the peaks of all synthesized nanocomposites, except from 3‐STE, can be assigned to five regions:

Figure 2

CO‐TPR of 10.4 wt. % Co3O4 modified CeO2 STE samples: (a) CO2 and (b) H2 evolution.

Figure 3

CO‐TPR of 10.4 wt. % Co3O4 modified CeO2 STE‐AC samples: (a) CO2 and (b) H2 evolution.

Region I (<240 °C): Reaction of CO with reactive oxygen species, very likely on Co3O4, present after the oxidative pretreatment. Molecularly adsorbed oxygen on Co2+ dissociates to atomic oxygen (O−), transforming Co2+ to Co3+. CO may also react with this oxygen species to carbonates, that decompose below 200 °C.

Region II (240‐270 °C): Reaction of OSL at Co3O4/CeO2 interfaces and reduction of Co3+→Co2+: CO+OSLCo3O4 →CO2;

Region III (270‐350 °C): Reaction of OBL near Co3O4/CeO2 interfaces: CO+OBLCo3O4→CO2; Co3+→Co2+. Reaction of OBL of CoO, interacting with CeO2, producing Co0: CO+OBLCoO → CO2; Co2+→Co0. WGS reaction of OH on Co3O4, including both isolated Co3O4 (at slightly lower temperature) and the Co3O4/CeO2 interface (at slightly higher temperature): CO+OH→1/2 H2+CO2;

Region IV (350–500 °C): removal of OSL of CeO2 interacting with Co3O4; CO+OSL→CO2;

Region V (>500 °C): Reaction of OBL of CeO2: CO+OBLCeO2→CO2; Ce4+→Ce3+.

The CO2 evolution in region III is related to Co3O4 surface patches, from which the surface abundance of the modified Co3O4 phase can be deduced. For STE samples, only sample 1‐STE has a sharp peak in this region (Figure 2a), while sample 2‐STE has a weak shoulder and sample 3‐STE has no peak. This indicates that the amount of the available Co3O4 on or near the ceria surface is 1‐STE>2‐STE>3‐STE (in line with HRTEM results). For sample 3‐STE, the intense peak of CO2 evolution (432 °C: Figure 2a) is caused by the decomposition of the POBC ligand, which is still present after the solvothermal treatment. This is reflected in the lower temperature shoulder of the 534 °C peak of H2 evolution (Figure 2b). The latter peak and the one at 348 °C once more indicate WGS of CO with surface OH groups.

To examine how high temperature calcination influences reactivity/reducibility, the STE samples were air‐calcined at 500 °C for 2 h (STE‐AC). After calcination, all STE‐AC samples showed low temperature peaks in region I, and especially sharp peaks in region III. The different intensities in region III indicate that part of the Co in sample 2‐STE‐AC and 3‐STE‐AC sintered and/or segregated to the surface (Figure 3). The higher temperature peaks are related to ceria. Once more, the intensity of the peaks in regions I–III indicates the amount of Co oxide that is available for CO oxidation. Compared to pure Co3O4 (Figure 1) and uncalcined nanocomposites (Figure 2), the peaks in region III are even more shifted to higher temperature, indicating an increased interaction with CeO2. For 2‐STE‐AC and 3‐STE‐AC, though some Co aggregated, some Co is still in the bulk of CeO2, and may thus not participate in CO oxidation. The H2 evolution of STE‐AC samples in region III (Figure 3b), due to WGS, showed a similar trend. Sample 1‐STE‐AC formed the most H2, while 2‐STE‐AC and 3‐STE‐AC produced only half the H2 amount. Thus, it can be deduced that 1‐STE‐AC exhibited most active Co3O4 interacting with CeO2, while the other two samples had only about half the amount of available CoOx/CeO2. This is consistent with high‐resolution transmission electron microscopy (HR‐TEM) micrographs of the AC samples.

Catalytic performance of 10 wt % Co3O4‐modified CeO2 synthesized via routes 1–3

The catalytic activity of the six samples (three STE samples by different routes and the corresponding STE‐AC samples) in CO oxidation (5 % CO, 10 % O2, He balance) was evaluated at different temperatures (Figure 4). Trends were tabulated previously; herein, more detailed catalytic tests are contrasted to the characterization discussed above. For practical applications, apart from the catalytic activity, the thermal stability of catalysts is most crucial. To detect a potential loss of catalytic activity, the CO conversion was thus recorded upon heating, upon subsequent cooling and upon re‐heating, without intermittent catalyst reactivation. Catalysts that underwent such cycling are stable in isothermal reactions up to 200 °C, typically over hundreds of hours.

Figure 4

CO oxidation over 10.4 wt % Co3O4 modified CeO2 samples. (a) route 1, (b) route 2, (c) route 3 and (d) direct comparison of 1st heat‐up.

Samples 1‐STE (Figure 4a) and 2‐STE (Figure 4b) (prepared by two individual precursors) did not show any hysteresis or loss of activity within several test cycles. In contrast, for sample 3‐STE (Figure 4c) the CO conversion upon heating and subsequent cooling do not coincide anymore. Interestingly, the CO conversion at a given temperature became apparently higher upon cooling and then remained the same during re‐heating. This behavior of 3‐STE (for which a single source precursor was used) can be explained by the decomposition of residual organics (e. g., POBC ligand) upon heating to ∼250 °C in oxidative atmosphere (which is supported by previous thermogravimetric analysis (TGA)). The highest activity (at a given temperature) of the catalyst prepared by route 1 can be explained by the more active and more abundant Co3O4 on the CeO2 surface (layer structure).

In order to determine the effect of calcination, the STE‐AC samples were also tested in CO oxidation (Figure 4). 1‐STE‐AC exhibited somewhat lower CO conversion than its STE pendant (Figure 4a). In contrast, when comparing the other STE samples with the corresponding STE‐AC samples (Figure 4b, c), at a given temperature 2‐STE‐AC and especially 3‐STE‐AC had a higher CO conversion than the related STE samples. Apparently, the significant amount of organic residues, which had remained after solvothermal treatment, blocked reaction sites, resulting in lower activity. Thus, the high temperature calcination is beneficial for 2‐STE and 3‐STE to remove organic residues. Nevertheless, the high temperature calcination also caused a structure collapse and reduction in surface area (from 216∼217 m2 g−1 to 25∼96 m2 g−1), which is why annealing of 1‐STE (with the smallest amount of organic residues) resulted in somewhat lower catalytic activity of 1‐STE‐AC. Therefore, for 1‐STE the post‐synthesis calcination should be avoided. The performance of all catalysts in the first heating cycle is directly compared in Figure 4d.

The apparent activation energy (E) of all STE and STE‐AC samples was calculated from Arrhenius‐type plots using kinetic rates below 30 % conversion (Figure 5). 1‐STE has the lowest E of 47.4 kJ mol−1, while 3‐STE shows the highest E of 77.5 kJ mol−1. The E of 1‐STE is similar to that of noble metals supported on ceria, such as 0.5 wt % Pd/CeO2 (48–52 kJ mol−1, 1 wt % Pd/CeO2 (40 kJ mol−1)), 0.5 wt % and 1 wt % Pt/CeO2 (42–63 and 44 kJ mol−1, respectively) and Au/CeO2 (46–56 kJ mol−1). Thus, the combination of sol‐gel and solvothermal methods allows obtaining very active cobalt oxide‐modified ceria nanocomposites, which could be used as a low‐temperature‐active additive to noble metal loaded CeO2.

Figure 5

Arrhenius‐type plots for CO oxidation over STE and STE‐AC samples (error: ±2 kJ mol−1).

The promising activity of 10.4 wt % Co3O4−CeO2 catalysts (route 1) is attributed to the favorable phase distribution, i. e., a high dispersion and small crystallite size of the active Co3O4 phase on CeO2. [29] The oxygen availability seems promoted by synergetic interfacial interactions between cobalt and ceria (Co‐O−Ce bonds), modifying the Co2+/Co3+ and Ce3+/Ce4+ redox properties and producing more active oxygen species.[ , ] Along these lines, the Co3O4 loading on CeO2 catalysts was varied, as described in the following.

Optimizing the Co3O4 loading on CeO2 (via route 1)

To further improve the performance of Co3O4‐modified CeO2, the number of Co3O4 surface sites accessible to the reaction was optimized via the Co3O4 loading. As route 1 produced the material with the highest CO oxidation activity, additional catalysts were prepared following this route, but with different amounts of cobalt precursor. The molar percentage of Co(Oac)2/(CeB+Co(Oac)2) used for all samples was 10, 20, 30 and 80 %, translating to Co3O4/(Co3O4+CeO2) wt. % ratios of 4.9, 10.4, 16.6 and 65.1 wt %, respectively.

The 1‐STE samples with different Co3O4 loading were characterized by X‐ray diffraction (XRD) and N2 sorption (Figure 6). The XRD of 4.9 wt % Co3O4/CeO2 showed only diffraction peaks of CeO2, as Co was highly dispersed. The higher loadings exhibited features characteristic of both Co3O4 and CeO2, with the intensity of the Co3O4 diffraction peaks increasing with Co loading. The CeO2 crystals were in the size range of 2.5–3.5 nm, whereas that of Co3O4 was about 25 nm (similar to commercial Co3O4).

Figure 6

(a) XRD patterns, (b) N2 adsorption–desorption isotherms, and (c) pore size distributions of Co3O4‐CeO2 nanocomposites (1‐STE). The data of the 10.4 wt % sample were adapted with permission from ref. [17]. Copyright 2015, Wiley.

The textural properties of the differently loaded 1‐STE catalysts were characterized by N2 adsorption. Adsorption‐desorption isotherms, the resulting specific surface area (SSA) and the pore size distributions are plotted in Figure 6 b,c. The specific surface area (SBET) decreased with increasing loading. While SBET was about 220 m2/g for 4.9 wt % Co3O4/CeO2, it was only ∼120 m2/g for 65.1 wt % Co3O4/CeO2. Nevertheless, this is still large compared to the SSA of commercial Co3O4 (37 m2 g−1). Up to 16.6 wt % Co3O4/CeO2, the isotherms indicate mainly mesopores and a small proportion of macropores, with narrow pore size distributions. In contrast, the 65.1 wt % Co3O4/CeO2 sample is mostly macroporous with a small amount of mesopores. Thus, a suitable ratio between Co3O4 and CeO2 is crucial to preserve mesoporosity and to obtain a high specific surface area with a high dispersion of Co3O4 on CeO2. The structural data are summarized in Table 2.

Table 2

Crystallite size (P), specific surface area (S), and activity data for Co3O4‐modified CeO2 (1‐STE), pure CeO2 and Co3O4, as well as 1 wt % Pt/CeO2.

Co3O4 loading [wt %]	PCeO2 [a] [nm]	PCo3O4 [b] [nm]	SBET [c] [m2 g−1]	T10% [d] [°C]	T90% [e] [°C]	r100 °C [f] [mol s−1 g−1]	R100 °C [g] [mol s−1 m−2]	RCo100 °C [h] (mmol CO/mmol Co h−1]
0 (Ceria)	<3	/	277.0	253	398	/	/	/
4.9	3.0	/	218.9	138	197	3.90×10−6	1.78×10−8	22.9
10.4	3.5	/	216.5	117	155	7.78×10−6	3.59×10−8	21.6
16.6	2.5	27.5	187.4	105	148	1.23×10−5	6.56×10−8	21.4
65.1	3.3	23.7	122.7	105	134	1.21×10−5	9.87×10−8	7.4
100 (Co3O4)	/	28	37	84	114	4.14×10−5	1.12×10−6	11.9
0 (Pt/CeO2)	13.5	/	43.2	100	124	1.15×10−5	2.68×10−7	/

[a] CeO2 crystal particle size calculated by Scherrer equation from XRD (JCPDS card number of CeO2: 34–0394) [b] Co3O4 crystal particle size calculated by Scherrer equation from XRD (JCPDS card number of Co3O4: 42–1467) [c] BET surface area from N2 sorption [d] Reaction temperature for 10 % CO conversion [e] Reaction temperature for 90 % CO conversion [f] Reaction rate of CO oxidation at 100 °C per gram [g] Normalized specific reaction rates of CO oxidation on a unit surface area at 100 °C [h] Reaction rates per unit amount of Co at 100 °C.

/

/

/

/

/

/

100 (Co

/

0 (Pt/CeO

/

/

Catalytic performance of different Co3O4 loadings on CeO2 (via route 1)

The different 1‐STE nanocomposites (without air‐calcination) were subsequently tested in CO oxidation, and contrasted to pure CeO2, pure Co3O4 and 1 wt. % Pt/CeO2 (the latter with a mean Pt particle size of 1.7 nm, according to CO chemisorption). Figure 7 shows the temperature‐dependent CO conversion of the different pretreated 1‐STE samples and pretreated reference catalysts (10 mg each). The temperatures of 10 % CO conversion (T10%) for 1 STE samples are: 138 °C (4.9 wt %) >117 °C (10.4 wt %) >105 °C (16.6 wt %)=105 °C (65.1 wt %) >100 °C (1 wt % Pt/CeO2)>84 °C (Co3O4); the temperatures of 90 % CO conversion T(90%) are: 197 °C (4.9 wt %) >155 °C (10.4 wt %) >148 °C (16.6 wt %) >134 °C (65.1 wt %) >124 °C (1 wt % Pt/CeO2) >114 °C (Co3O4).

Figure 7

CO conversion vs. temperature of Co3O4‐modified CeO2 (STE) for different loadings. CeO2, Co3O4, and Pt/CeO2 serve as reference. Conditions: 5 % CO, 10 % O2, He balance; 10 mg catalyst.

Apparently, pure CeO2 is the least active (30 % conversion at 300 °C), but already adding ∼4.9 wt. % Co3O4 drastically increased activity. This trend continued for 10.4, 16.6, and 65.1 wt %, the latter approaching the activity of Pt/CeO2. Clearly, the higher the Co percentage in the synthesis and thus the final Co3O4 loading are, the higher the resulting catalytic activity is. Pure Co3O4 is the most active, but for possible TWC‐applications, ∼16‐65 wt % Co3O4‐modified CeO2‐based catalysts seem the best. This agrees with a similar higher activity of Co3O4 catalysts impregnated by 10 wt. % CeO2 in preferential CO oxidation.

To compare the different samples, the catalytic activity/rate of the catalysts at 100 °C was normalized by weight (r100 °C), by specific surface area (R100 °C) and by the unit amount of Co (RCo100 °C), as listed in Table 2 (the CO conversion at 100 °C was below 20 % for all samples). Up to 16.6 wt % Co3O4, the values of r100 °C and R100 °C increased almost in direct proportion to the Co3O4 loading. For example, the r100 °C and R100 °C values of 4.9 % Co3O4 are 3.90×10−6 mol s−1 g−1 and 1.78×10−8 mol s−1 m−2, respectively. The corresponding values of 10.4 % Co3O4 are 7.78×10−6 mol/s g and 3.59×10−8 mol/s m2, i. e., each almost exactly double.

This is consistent with the RCo100 °C values, as up to 16.6 wt % Co3O4 the samples have almost the same RCo100 °C value of 22±1 mmol CO/mmol Co⋅h (Table 2). This indicates that Co3O4 is well dispersed, forming increasingly larger islands on the CeO2 surface. Further increasing the Co3O4 loading to 65 wt % hardly increased the specific activity and even decreased the rate normalized by the Co amount to 7.4 mmol CO/mmol Co⋅h, as agglomerated particles or thicker layers have less dispersion. This is clearly evident from the low temperature range (Figure 7), with the conversion of the 16.6 and 65 wt % samples being almost the same. Pure Co3O4 nanoparticles are characterized by a similar value (11.9 mmol CO/mmolCo⋅h), as the Co3O4 dispersion is was lower than that of thin Co3O4 layers.

Even though Co3O4 is known for high CO oxidation activity at low temperature, the use of pure Co3O4 in catalytic convertors is not feasible, as ceria has oxygen storage/release (buffer) capacity, in addition to preventing sintering. It is thus important to preserve the main CeO2 phase, but its low‐temperature activity could be boosted by well‐dispersed Co3O4 overlayers. As modern engines run under oxygen‐rich conditions to increase mileage, the exhaust gas is oxygen rich too, which stabilizes the Co3O4 phase, so that no reduction would occur (deactivation by adsorbed water may be a problem at lowest temperatures, though). The re‐oxidation of (metallic) cobalt starts above 200 °C, at 300 °C it is oxidized to CoO and Co3O4, and at 400 °C Co3O4 is the most stable phase. Clearly, operando examination of the nanocomposites would add to the mechanistic understanding and Co3O4 may also be beneficial for NOx reduction.

Conclusions

The current results demonstrate that the sol‐gel synthesis of Co3O4−CeO2 nanocomposites, in combination with solvothermal treatment, allows obtaining well‐dispersed cobalt oxide nanoparticles on high surface area CeO2. The obtained materials are thermally more stable and very active in CO oxidation, and are comparable to supported Pt−, Pd− and Au−CeO2 catalysts. The catalytic performance of Co3O4‐modified CeO2 strongly depends on the number of the Co3O4 surface sites accessible for the CO oxidation reaction, which is controlled by the route of introducing Co cations and by the Co loading. Considering the high activity of the presented catalysts in CO oxidation, with that of 16 wt % Co3O4−CeO2 approaching that of Pt/CeO2, it is anticipated that further optimization of the layered CeO2−Co3O4 nanocomposites may allow obtaining prototypes with even better low‐temperature TWC performance. In terms of a practical application in TWCs, the long‐term stability and activity in hydrocarbon oxidation and NO reduction in realistic exhaust gas feeds should also be investigated.

Conflict of interest

The authors declare no conflict of interest.

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