Introduction

Presently, diesel engines are widely used in heavy-duty vehicles because of their low operating costs, excellent durability, superior fuel efficiency, and reliability under lean conditions [1,2,3]. However, soot particulates emitted from diesel engines have caused severe damage to human health and the environment [4]. Catalytic combustion technology using oxidation catalysts combined with a diesel particulate filter is a promising after-treatment strategy to trap and eliminate soot particulates in the range of 200–500 °C [5, 6].

Numerous catalysts for soot combustion have been reported using noble metals [7,8,9], perovskites [10, 11], spinel-type oxides [12, 13], hydrotalcites [14, 15], alkaline metal oxides [16, 17], transition metal oxides [18], and rare earth metal oxides [19]. Ag-based catalysts are promising candidates for catalytic soot oxidation reactions because of their low price among noble metals, and particularly the Ag0 species has a high ability to activate oxygen [20,21,22]. Interestingly, the Ag0 species can be automatically obtained by directly supporting Ag salt on reducible metal oxides (Co3O4, CeO2, MnO2, etc.) because of the reaction between silver and variable valence cations (for example, Ag+  + Co2+  = Ag0 + Co3+) [7]. However, because of the increased cation valence on supports (such as Co2+  → Co3+), the number of oxygen vacancies on the surface of reducible supports will inevitably be reduced. Nevertheless, oxygen vacancies play important roles in anchoring metal sites, increasing metal dispersion, and tuning the metal–support interaction, which significantly influence the activity and stability of catalysts; moreover, they are beneficial to promoting oxidation reactions [23,24,25,26]. Thus, before loading metals, a possible way to solve the problem is by pre-reducing reducible supports to construct more available oxygen vacancies. To date, few reports have used this strategy to develop efficient catalysts for soot combustion [27].

Some reducible, noble metal-free metal oxides, such as CeO2 and MnO2 and particularly Co3O4, are active in soot oxidation [27]. Moreover, Co3O4 exhibits a high ability for NO oxidation to NO2 [22, 28], which is a more effective oxidant than O2 for soot oxidation. Thus, Co3O4 becomes a good candidate for supporting noble metal catalysts in this reaction.

In addition, soot is a type of particulate with a diameter of 25–100 nm, which makes touching the inner surface of mesopores and micropores in traditional powder catalysts difficult during a reaction, resulting in low soot–catalyst contact efficiency and unsatisfactory catalytic performance [29]. To solve this problem, Zhao’s research group [12, 30, 31] developed a series of three-dimensional ordered macroporous catalysts using the colloidal crystal template method for catalytic soot elimination, which can greatly improve the contact efficiency and catalytic soot oxidation performance. Zhang’s group [32, 33] focused on multiple strategies to decrease the ignition temperature of soot combustion. In addition, our group [1, 5, 34] has developed a series of three-dimensional monolithic catalysts by in situ growth of nanostructured active components on monolithic substrates, which can provide a sufficient open macroporous structure to increase soot–catalyst contact opportunities and remarkably lower soot elimination temperatures.

In this work, we designed and synthesized Ag catalysts supported on pre-reduced Co3O4 nanowires (Co-NW) using Ni foams as the monolithic substrate for soot oxidation. The structure–activity relationship was revealed through various characterization techniques, such as scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction by soot (soot-TPR), temperature-programmed oxidation by NO (NO-TPO), and CO temperature-programmed desorption (CO-TPD) measurements. The pre-reduction of the Co-NW support enhances metal–support interactions on Ag/Co-NW-R and then increases the metal dispersion and the number of oxygen vacancies and improves the turnover frequency (TOF) and catalytic performance for soot oxidation.

Experimental

Catalyst Preparation

Ag/Co3O4 nanowires on Ni foam were synthesized via a simple hydrothermal and incipient wetness impregnation method [1], as illustrated in Scheme 1.

Scheme 1
scheme 1

Schematic diagram of the synthesis of the as-prepared catalysts on monolithic Ni foam

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The Ni foam (thickness: ca. 1.5 mm, porosity: ≥ 98%, and mechanical strength: ≥ 1 MPa) was purchased from LANKE battery materials Co. Ltd. The Ni foam was immersed in 2 mol/L HCl in an ultrasound bath for 5 min to remove the surface oxide layers and then rinsed with deionized water and absolute ethanol for 5 min, respectively. In a typical synthesis, 1 mmol of Co(NO3)2·6H2O, 2 mmol of NH4F, and 5 mmol of CO(NH2)2 were dissolved in 50 mL of water. After intense stirring for 30 min, the solution was transferred into a 100-mL Teflon-lined stainless steel autoclave. Subsequently, a piece of the clean Ni foam was immersed in the reaction solution. The autoclave was sealed and maintained at 120 °C for 5 h and cooled naturally to room temperature. The as-synthesized precursor was ultrasonically cleaned with water for 5 min and rinsed with absolute ethanol three times. After drying at 60 °C, the precursor was calcined at 500 °C in air for 2 h and denoted as Co-NW. The clean Ni foam substrate was also calcined for 2 h at 500 °C for comparison and denoted as Ni foam.

Ag/Co-NW was prepared using the impregnation method. The Co-NW precursor was impregnated with an aqueous solution of silver nitrate, followed by drying at 60 °C, and then annealed at 500 °C in air for 2 h. This catalyst was designated as Ag/Co-NW. For comparison, Co-NW was pre-reduced at 200 °C in 8% H2/N2 flow, and then Ag was impregnated on Co-NW-R with the same procedure of Ag/Co-NW, and the obtained catalyst was denoted as Ag/Co-NW-R. The pre-reduced Co-NW was also calcined for 2 h in 500 °C for comparison and denoted as Co-NW-R. According to the inductively coupled plasma (ICP) method (Vista MPX instrument), the loading of silver in the prepared catalysts amounted to 5 wt% (the mass fraction of Ag in the catalyst without the Ni foam substrate).

Catalyst Characterizations

The samples were characterized by XRD on a Rigaku D/MAX-2500 diffractometer (Cu Kα radiation), XPS on an ESCALAB instrument, SEM on a Hitachi S-4800 scanning electron microscope, and transmission electron microscopy (TEM) and EDS mapping on a JEOL-JEM-2100F electron microscope.

On the basis of the diameter (d) of Ag on catalysts determined by TEM, the dispersity (DAg) of Ag crystallites was calculated from the expression DAg (%) = 1.34/d, assuming that they had a spherical morphology [35].

Soot temperature-programmed reduction (soot-TPR) tests were performed in a fixed-bed reactor at a heating rate of 5 °C/min in N2 (100 mL/min) on a CO–CO2 IR analyzer. NO temperature-programmed oxidation (NO-TPO) experiments were measured on the same reactor with 250 mg of catalyst in 600-ppm NO/10% O2/N2 (100 mL/min) at a rate of 5 °C/min and recorded using a chemiluminescence NO − NO2 − NOx analyzer (Model 42i-HL, Thermo Scientific).

H2 temperature-programmed reduction (H2-TPR) tests were performed on a TP-5080 instrument (50-mg sample, 8% H2/N2, 10 °C /min). CO temperature-programmed desorption (CO-TPD) experiments were performed on the same instrument equipped with a mass spectrometer (Hiden). Before CO-TPD, the catalysts were pretreated at 100 °C in He flow and then adsorbed 7% CO/N2 for 1 h. After cooling to room temperature, the system was switched to He flow and heated to 900 °C at a rate of 10 °C/min.

Catalytic Activity Measurement

The catalytic activities for soot oxidation were evaluated by soot temperature-programmed oxidation (soot-TPO) using Printex-U soot (Degussa) as a model in a fixed-bed flow reactor. To achieve the loose contact condition between catalysts and soot particulates, 15 mg of soot was dispersed in 25 mL of ethanol by ultrasound to obtain a suspended soot-ethanol solution, which was gradually dropped on the monolithic catalyst via a precisely controlled pipette gun, and then the mixture was dried at 60 °C for 2 h. The mass ratio of soot/catalyst (excluding the weight of the Ni foam substrate) was approximately 1:10. For each reaction, the monolithic catalyst was heated from 200 to 650 °C at a heating rate of 2 °C/min in the reactant gas flow (100 mL/min) of 0- or 600-ppm NO and 10% O2 balanced with N2. The products of CO2 and CO were online analyzed by an IR analyzer. The catalytic activities were evaluated using the temperatures at 10% (T10) and 50% (T50) soot conversion during the soot-TPO reaction. The CO2 selectivity (\(S_{\text{CO}_2}\)) was defined as \(S_{\text{CO}_2}\) = \(C_{\text{CO}_2}\)/(CCO + \(C_{\text{CO}_2}\)), where CCO and \(C_{\text{CO}_2}\) were the concentrations of CO and CO2, respectively.

Isothermal Kinetic Measurements

The isothermal kinetic experiments (isothermal reactions and isothermal anaerobic titrations) were performed to calculate reaction rates, active oxygen amounts, and TOF values of the monolithic catalysts [36]. To ensure that all reactions occurred in the dynamic region with a low soot conversion (< 10%), the isothermal oxidation tests were conducted in a 10% O2/N2 flow at 280 °C (150 mL/min). When the CO2 concentration became steady, the 10% O2/N2 flow was switched to a pure N2 flow at the same volume flow rate, and the isothermal anaerobic titration began. The outlet gas was monitored online by an infrared gas analyzer. The reaction rate (ν) was calculated according to the following equation:

$$\nu = - dn/\left( {mdt} \right) = Q \times c/m$$
(1)

where Q is the molar gas flow rate (mol/s); c is the molar fraction of CO2 estimated by isothermal reactions; and m is the mass of the catalyst (g); n is the amount of substance of CO2 (mol).

The number of active oxygen species (O∗) was quantified by integrating the diminishing rate of CO2 concentration with time. The TOF was obtained as follows:

$$O*{\text{amount}}\left( {{\text{mol}}/{\text{g}}} \right) = 2P_{0} \times V \times A \times 10^{{{-}6}} /\left( {R \times T \times m} \right)$$
(2)
$${\text{TOF}}\;\left( {s^{ - 1} } \right) = \nu /O*$$
(3)

where P0 represents the atmospheric pressure (Pa);V represents the volumetric flow rate (m3/s);A represents the integral of the CO2 concentration curves as a function of time during the isothermal anaerobic titration (s); R represents the ideal gas constant; T represents the room temperature (K).

Results and Discussion

Catalyst Characterizations

Structural Properties

XRD patterns of the monolithic catalysts are shown in Fig. 1. For all as-prepared catalysts, three strong diffraction peaks are observed at 2θ = 44.5°, 51.8°, and 76.2°, which belong to metallic Ni (JPCDS 04–0850) [34]. Meanwhile, three weak peaks associated with the NiO (JPCDS 47–1049) phase are observed at 37.2°, 43.2°, and 62.6°. As for Co-NW and Ag/Co-NW, the four peaks located at 31.1°, 36.8°, 59.2°, and 65.1° can be ascribed to the (220), (311), (511), and (400) lattice planes of Co3O4 (JPCDS 43–1003) [7], respectively. Figure 1b shows a zoom-in of Fig. 1a, and no diffraction peak related to Ag is observed, which may result from the low loading amount and small crystallite size of Ag on the catalysts.

Fig. 1
figure 1

XRD patterns of the as-prepared catalysts: Ni foam, Co-NW, Ag/Co-NW, and Ag/Co-NW-R. a Full patterns and b zoom-in

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In Fig. 2, the structure of the as-prepared catalysts, the average diameter of silver on the Co-NW surface, and the features of Ag-Co3O4 composites are investigated using SEM and TEM. Figure S1 displays the SEM images of the Ni foam, which possesses a 3D-macroporous structure with diameters ranging from 200 to 500 µm. In Fig. 2a, d and g, the monolithic catalysts exhibit nanowire morphology, which is well-distributed on the skeleton of the Ni foam substrate. The nanowires are self-assembled into grass-like clusters, which can provide enough macroporous space for soot deposition [5]. The HRTEM images in Fig. 2c, f and i demonstrate that these nanowires are composites of Co3O4 [21].

Fig. 2
figure 2

Electron microscope characterization of the catalysts a1–a3 Co-NW, b1–b3 Ag/Co-NW, and c1–c3 Ag/Co-NW-R; a1–c1 SEM images, a2–c2 TEM and SEM images, a3–c3 HRTEM images

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After loading Ag, the crossed nanowire morphology of Ag/Co-NW and Ag/Co-NW-R remain unchanged, except that the surface of the nanowire becomes slightly rough in Fig. 2 a, d and g. The lattice spacing of 0.236 nm belonging to Ag (111) in Fig. 2f, i confirms the presence of metallic Ag on the catalysts [21]. In Fig. S2, Ag nanoparticles (NPs) are homogeneously dispersed on Co3O4 nanowires with a low content from TEM-EDS mapping. Moreover, as shown in Fig. S3j, k, the average size of Ag NPs is approximately 5.8 nm and 4.4 nm for Ag/Co-NW and Ag/Co-NW-R, respectively. We calculated the dispersion of Ag based on the above results and found that it was approximately 1.5-fold larger on Ag/Co-NW-R compared to Ag/Co-NW, as shown in Table S1.

Redox Properties

The soot-TPR and CO-TPD experiments were performed to investigate the active oxygen species of the catalysts. As shown in Fig. 3, the soot-TPR curves of the catalysts consist of three temperature ranges, representing three types of oxygen species. The low-temperature soot-TPR reduction peak at 200–400 ºC can be assigned to interfacial active oxygen species in the case of Ag–O–Co, which is probably caused by the strong interaction between Ag and Co3O4. These oxygen species can be assigned to surface-adsorbed oxygen species. In addition, the peaks at 400–650 °C originate from surface lattice oxygen (O2−), and the peaks above 650 °C are ascribed to the bulk lattice oxygen (O2−) of the catalysts, which has little impact on the catalytic activity [5]. Compared with soot, CO is more easily oxidized, so CO-TPD can be a better technology than soot-TPR for revealing the activity of oxygen species on the catalyst [37]. Generally, the adsorbed CO reacts with the reactive oxygen species on the catalyst and then is oxidized to CO2 during the temperature-programmed process [37, 38]. Figure 4 shows the CO-TPD profiles of the samples. The desorption peaks below 200 °C are attributed to the desorption of CO2 produced directly by the reaction of adsorbed CO with surface-adsorbed oxygen species. The broad peaks between 200 and 350 °C for all three catalysts belong to the desorption of CO2 oxidized by surface lattice oxygen species. No obvious difference in the shape of the peaks is apparent for all catalysts below 350 °C. The peaks above 350 ºC can be ascribed to the desorption of CO2 oxidized by bulk lattice oxygen species. According to the temperature range (200–600 °C) for soot oxidation, surface-adsorbed oxygen species and surface lattice oxygen species are the main active oxygen species over these catalysts [1]. For soot-TPR and CO-TPD, the peaks of surface-adsorbed oxygen species and bulk lattice oxygen species for the Ag-loaded catalysts shift to lower temperatures, particularly for Ag/Co-NW-R, which illustrates that Ag-Co3O4 interactions can activate surface oxygen species and promote bulk lattice oxygen mobility. In Figs. 3, 4, the peak areas of the catalysts follow the order of Ag/Co-NW-R > Ag/Co-NW > Co-NW, which illustrates that Ag/Co-NW-R possesses more active oxygen species than other catalysts.

Fig. 3
figure 3

Soot-TPR profiles of the catalysts

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Fig. 4
figure 4

CO-TPD profiles of the catalysts

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Surface Chemical States

Figure 5 and Table 1 display the XPS results of the samples. In Fig. 5a, Ag species display Ag 3d5/2 and Ag 3d3/2 peaks at binding energies of 368.0, 368.3, 374.0, and 374.3 eV, respectively, with a splitting value of 6.0 eV, which demonstrates that Ag species exist in the metallic state on the catalysts [25, 39]. Notably, the peaks of Ag/Co-NW-R shift to higher binding energy (BE) compared with Ag/Co-NW, demonstrating the enhanced metal–support interaction on Ag/Co-NW-R, which is also suggested by the decreased size of Ag NPs [27] in Fig. 2.

Fig. 5
figure 5

XPS spectra of the catalysts: a Ag 3d, b Co 2p, and c O 1 s

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Table 1 BE of Co 2p and O 1 s core levels and the ratio of Co2+/(Co2+ + Co3+) and Oads/(Oads + Oads) for the as-prepared catalysts
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In Fig. 5b, the Co 2p XPS spectra of the catalysts show spin–orbit splitting into Co 2p1/2 and Co 2p3/2, which can be deconvoluted into Co3+, Co2+, and two weak satellite peaks at relatively high binding energies (800–810 and 786–791 eV, respectively) [1]. As listed in Table 1, Co species exist as Co3+ (779.5, 794.3 eV) and Co2+ (781.4, 795.6 eV) [40]. Additionally, the spin–orbit splitting energy between the two peaks is 15.2 eV, which indicates that the surface cobalt species exist as Co3O4 [41]. The values of surface Co2+/(Co2+ + Co3+) for all samples were calculated according to the corresponding peak areas in the XPS spectra. Notably, Co-NW-R possesses a higher Co2+ concentration than Co-NW, indicating that the reduction pretreatment indeed created many Co2+ species on Co-NW-R, as we proposed. Loading Ag on the supports consumed Co2+ cations according to Ag+  + Co2+  = Ag0 + Co3+. However, we interestingly observed an increased value of Co2+/(Co2+ + Co3+) on Ag/Co-NW and particularly on Ag/Co-NW-R. This result indicates that the enhanced Ag–Co3O4 interaction can generate more Co2+ species on the catalyst surface, which commonly accompanies the generation of oxygen vacancies to generate active oxygen species [7].

In Fig. 5c, the O 1 s XPS spectra are deconvoluted into three peaks. The peak at approximately 529.4 eV corresponds to surface lattice oxygen (Olat), and the peaks at approximately 531.0 and 532.8 eV agree with oxygen species such as O2− and O22− adsorbed on oxygen vacancies (Oads) [42]. In Table 1, compared with Co-NW, Co-NW-R exhibits a higher ratio of Oads/(Oads + Olat), demonstrating that more oxygen vacancies were constructed on the surface of Co-NW-R by H2 reduction. Notably, loading Ag greatly increases the ratio of Oads/(Oads + Olat) on the catalyst surface, as shown in Table 1, particularly for Ag/Co-NW-R. Additionally, the O 1 s peaks of the Ag-loaded catalysts shift to higher BEs, i.e., in the direction of the Oads peaks. All of these results demonstrate that the enhanced metal–support interaction benefits the creation of surface-adsorbed oxygen species. It also coincides with the results of Co 2p XPS, soot-TPR, and CO-TPD.

Catalytic Soot Combustion Performance

Figure 6 and Table 2 display the soot-TPO activities and the corresponding CO2 concentration profiles of the as-prepared monolithic catalysts in the absence or presence of 600-ppm NO. The blank experiment without any catalysts shows that T10, T50, and the CO2 selectivity are 490 °C, 565 °C, and 55%, respectively. For all other samples, the only product of soot oxidation is CO2, as listed in Table 2. Compared with the blank experiment, Ni foam can decrease the ignition temperature by 62 °C. Because of the excellent redox ability of Co3O4, Co-NW and Co-NW-R exhibit much higher catalytic soot oxidation activity than Ni foam alone. Evidently, after loading Ag NPs on them, T50 decreases remarkably, which indicates that Ag loading greatly promotes soot oxidation. The activities of the catalysts follow the order of Ag/Co-NW-R > Ag/Co-NW > Co-NW-R > Co-NW > Ni foam > Blank with and without NO. Introducing 600-ppm NO significantly lowers T10 and T50 for all as-prepared catalysts but particularly for Ag/Co-NW-R. Notably, Co-NW-R exhibits only slightly higher catalytic activity than Co-NW, but the activity of Ag/Co-NW-R is much higher than that of Ag/Co-NW with and without NO. This result can be attributed to the enhanced metal–support interaction, high dispersion of Ag, and presence of more active oxygen species on Ag/Co-NW-R.

Fig. 6
figure 6

a, c CO2 concentration profiles and b, d soot conversion of the as-prepared catalysts during the soot-TPO reaction in a, b 10% O2 balanced by N2 and c, d 600-ppm NO and 10% O2 balanced by N2 under the loose contact condition

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Table 2 Activities of the as-prepared catalysts in the presence or absence of 600-ppm NO for soot combustion
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As a more effective oxidant than O2, NO2 can remarkably lower the oxidation temperature of soot particulates and plays a crucial role in the soot combustion process [43]. Thus, the ability to oxidize NO to NO2 is a key factor influencing soot oxidation. Figure 7 shows the NO-TPO profiles of the catalysts. Apparently, loading Ag significantly lowers the NO oxidation temperatures and produces more NO2 than the support alone, particularly for Ag/Co-NW-R.

Fig. 7
figure 7

Profiles of NO oxidation to NO2 over the catalysts in a NO/O2/N2 atmosphere

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Kinetic Study

To further understand the intrinsic activity of the catalysts, the isothermal anaerobic titration processes were carried out at 280 °C. Figure 8 shows the CO2 concentration as a function of time before and after removing O2 from the reactant flow. Table 3 and Fig. 8 display the corresponding kinetic results for the reaction rate, amount of O*, and TOF. Obviously, these values follow the order of Ag/Co-NW-R > Ag/Co-NW > Co-NW-R > Co-NW, which is consistent with the catalytic performance. Meanwhile, the parameters of Co-NW-R are only slightly larger than those of Co-NW, suggesting that the positive effect of the pre-reduced treatment of Co-NW on soot oxidation is limited. According to the Ag wt% measured by ICP, the reaction rate per gram of Ag on Ag/Co-NW-R is nearly 1.4-fold that on Ag/Co-NW, as shown in Table 3, demonstrating the higher utilization efficiency of Ag on Ag/Co-NW-R. Additionally, as shown in Table 3, compared with the supports, the Ag-loaded catalysts not only generate more active O* species but also achieve higher intrinsic activity (TOF), demonstrating the important effect of Ag-Co3O4 interactions on soot oxidation. Here compared with Ag/Co-NW, Ag/Co-NW-R has an only slightly higher TOF but an approximately 1.4-fold higher reaction rate. This result indicates that the primary function of the Ag-Co3O4 interaction is to improve the dispersion of Ag sites to increase the catalyst’s apparent activity.

Fig. 8
figure 8

Curves of CO2 concentration as a function of time during isothermal soot oxidation at 280 °C under the loose contact conditions over the as-prepared catalysts a Co-NW, b Co-NW-R, c Ag/Co-NW, d Ag/Co-NW-R before and after O2 is removed from the reactant flow

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Table 3 Active oxygen (O*) amounts, reaction rates (ν, νAg), and the TOF of the catalysts calculated from the results in Fig. 8
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Catalytic Stability and Water Resistance

The catalytic stability and H2O resistance of catalysts are crucial factors for soot combustion under practical conditions. Figure 9a displays five consecutive cycles of soot-TPO tests in NO/O2/N2 over Ag/Co-NW-R under loose contact conditions. During the reaction, the catalyst and the soot were mixed in the same proportion to ensure as far as possible the same conditions for each test. T50 of Ag/Co-NW-R is very similar in each cycle, illustrating the high stability of this catalyst for soot oxidation. The soot-TPO experiment was performed over Ag/Co-NW-R in 10% H2O/NO/O2/N2 to further investigate the impact of water vapor on soot combustion under the loose contact conditions. In Fig. 9b, introducing water vapor in the reactant flow improves the catalytic performance of Ag/Co-NW-R, which agrees with some previous reports [44, 45].

Fig. 9
figure 9

a Stability of Ag/Co-NW-R in 600-ppm NO/10% O2/N2 and b water resistance of Ag/Co-NW-R in 600-ppm NO/10% O2/N2

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Discussion

Soot combustion is a redox reaction in nature. The active oxygen species play an important role in catalytic soot oxidation. In our work, the XPS results demonstrate that Ag species exist on the catalyst surface as Ag0. For the catalytic soot oxidation in a N2 atmosphere, as considered in Fig. 3, soot can only be oxidized by active oxygen species on the catalysts. In the range of 200–300 °C, little CO2 is produced by oxidizing soot closely contacted with active sites. Compared with Co-NW, the CO2 peak of the Ag-loaded catalysts shifts to lower temperatures, indicating that the oxidation ability of Ag sites is higher than that of Co-NW at low temperatures, particularly for Ag/Co-NW-R. With increasing temperature (400–600 °C), a large amount of soot is oxidized to CO2 by surface lattice oxygen species, and the difference among the as-prepared catalysts is insignificant. However, when the temperature increases to 600 °C, soot is oxidized to CO2 by bulk lattice oxygen species of as-prepared catalysts. The Ag-loaded catalysts exhibit a larger CO2 peak area due to the higher mobility of bulk lattice oxygen promoted by the Ag–Co3O4 interaction compared with their supports.

Conversely, after introducing gaseous oxygen, O2 can be continuously activated and converted into reactive oxygen species at active sites. Compared with the reaction in the N2 atmosphere, considered in Fig. 3, the soot oxidation performance of the catalysts improves substantially, and the shape of the CO2 curves sharpens over the reaction temperature range (200–600 °C) in Fig. 6a, indicating a faster reaction rate in an O2 atmosphere. Moreover, the CO2 concentrations of the supports are quite low below 350 °C, while that of the Ag-loaded catalysts increases remarkably, demonstrating that the major function of Ag0 sites is to activate gaseous oxygen molecules to oxidize soot. According to the results of soot-TPR, CO-TPD, O 1 s XPS, and TOF, Ag/Co-NW-R possesses the most surface oxygen species and the highest TOF value due to the enhanced metal–support interaction and thus shows the lowest ignition temperature and the highest apparent activity.

After being introduced to the reactant gas, NO is readily oxidized to NO2, which is a better oxidant than O2. Additionally, T10 and T50 of the as-prepared catalysts for catalytic soot oxidation continue to decrease in Fig. 6c, particularly for Ag/Co-NW-R. According to the NO-TPO results in Fig. 7, Ag/Co-NW-R shows the highest ability for oxidizing NO to NO2 due to the promoted generation of active oxygen species by an enhanced metal–support interaction.

The above results indicate that the pre-reduction treatment on reducible supports effectively anchors metal sites, improves the metal dispersion, and enhances metal–support interactions, thereby increasing the number of active oxygen species and improving the intrinsic activity of catalysts for oxidation reactions.

Conclusions

In summary, in this work, we facilely synthesized Ag catalysts supported on Co3O4 nanowires in situ grown on monolithic Ni foam substrates. The designed catalyst exhibits high activity, stability, and water resistance for soot oxidation. Here, the Ag catalyst exists as Ag0 due to the automatic reduction of Ag+ by Co2+. Interactions between Ag and Co3O4 improve the generation of active oxygen species to approximately twice that on the support alone. Through the reduction pretreatment of Co3O4 to generate more oxygen vacancies, Ag NPs with smaller sizes were highly dispersed and anchored thereon. In particular, this pretreatment not only increases the number of active oxygen species but also improves the intrinsic activity for soot oxidation. In addition, the macropores of the monolithic catalysts with crossed Co3O4 nanowires can provide more soot–catalyst contact sites and increase the soot capturing efficiency. These advantages make the catalyst a possible candidate for industrial applications.

Supporting information

SEM images of the monolithic Ni foam (Fig. S1), the TEM-EDS mapping images of the Ag/Co-NW-R catalyst (Fig. S2), the corresponding size distribution of Ag NPs determined by HRTEM images (Fig. S3), and the average diameter (dAg) and dispersion (DAg) values of Ag NPs on monolithic catalysts (Table S1).