Bulk Au has been regarded as being chemically inert toward chemisorption of reactive molecules such as oxygen and hydrogen ^[1-5]. However, in the last decade, it has been widely proved that Au nanoparticles deposited on appropriate supports show high catalytic activity in many reactions ^[6], e.g., hydrogen purification for fuel cell applications ^[7-9], combustion of CO ^[10-13] and volatile organic compounds ^[14-16], elimination of NO ^[17], oxidation of toluene ^[18], and oxidation of formaldehyde ^[19]. Soot emitted from diesel engines is one of the most serious pollutants because it adversely affects the human respiratory system by penetrating lung cells and can cause death ^[20]; its removal is therefore an important research topic. Catalysts based on Au nanoparticles are promising for use in oxidation ^[21-23]. It has been reported that CeO₂-Au interactions are important because they enhance Au particle dispersion and provide active oxygen species ^[24]. However, CeO₂ is not a suitable support for catalytic applications because of its poor thermal stability against sintering and loss of its oxygen-storage/release properties ^[25-28]. These drawbacks can be overcome by using an Al₂O₃ support to stabilize CeO₂. Al-Ce-O solid solutions are formed in CeO₂/Al₂O₃ catalysts, resulting in the creation of extrinsic oxygen vacancies. The formation of extrinsic oxygen vacancies caused by the presence of Al³⁺ in the CeO₂ structure has been reported in the literature ^[29]. These vacancies are different from the intrinsic oxygen vacancies formed by the reduction of Ce⁴⁺ to Ce³⁺ and removal of oxygen, and lead to improved oxygen-transport properties ^[30]. A prerequisite for the enhancement of intrinsic oxygen vacancies is the presence of a noble metal, which promotes CeO₂ redox behavior and therefore promotes soot combustion ^[31]. Au nanoparticle catalysts supported on CeO₂/Al₂O₃, which are expected to give good performance in soot combustion, therefore need to be studied.

The contact conditions between soot and a catalyst is a key factor and can affect the catalytic activity in soot combustion. The pores of conventional catalysts are smaller ( < 10 nm) than soot particles (> 20 nm) ^[32]. It is therefore difficult for soot particles to enter the inner pores of these catalysts. Catalytic soot combustion is limited by poor soot-catalyst contact and the restricted number of active sites. Control of the catalyst porosity is therefore important in combustion of diesel soot.

Three-dimensionally ordered macroporous (3DOM) materials, a class of important porous materials, have become a hot topic in porous material research ^[33]. Reactant species are efficiently transferred by 3DOM materials with uniform ordered macroporous structures. Consequently, the number of active points between soot particles and the catalyst is significantly increased. There have been many studies of soot combustion over 3DOM materials, and good catalytic performance has been achieved ^{[21-23, 32]}.

The objective of this work was to prepare Au/CeO₂/3DOM Al₂O₃ catalysts and to investigate their structures, electronic properties, reduction, and catalytic performance in soot oxidation. For comparison,3DOM Al₂O₃, CeO₂, CeO₂/3DOM Al₂O₃, and Au/3DOM Al₂O₃ were also studied. The catalysts were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), ultraviolet-visible diffuse-reflectance spectroscopy (UV-vis DRS), X-ray diffraction (XRD), N₂ adsorption-desorption analysis, X-ray photoelectron spectroscopy (XPS), and temperature-programmed hydrogen reduction (H₂-TPR). The nature of the interactions between the supports and Au nanoparticles and their effects on the catalytic performance in soot combustion were also investigated.

2 Experimental

2.1 Catalyst preparation

2.1.1 Synthesis of monodispersed poly(methyl methacrylate) (PMMA) colloidal spheres

Monodispersed PMMA colloidal crystal spheres of average diameter 350 nm were synthesized using a previously reported soap-free-emulsion polymerization method ^[23]. A certain amount of MMA monomer and distilled water were added to a 2000 mL four-necked round-bottomed flask. Ar was introduced to remove the air inside the flask, and the mixture was magnetically stirred. The temperature of the reaction system was raised to 80 ℃. An appropriate amount of potassium persulfate solution was added to the mixture to initiate polymerization. The reaction mixture was held at this temperature for 2 h, cooled to room temperature, and filtered to obtain monodispersed PMMA microspheres.

2.1.2 Assembly of 3DOM PMMA colloidal crystal templates

PMMA colloidal crystal templates were assembled using a centrifugation method. A certain amount of PMMA latex was ultrasonicated to form a uniform emulsion and centrifuged at a rate of 3000 r/min for 10 h. After full precipitation of the PMMA colloidal spheres, the supernatant was removed. The centrifuge tube containing the well-arranged PMMA colloidal spheres was dried in a vacuum oven at 40 ℃ to obtain highly ordered PMMA colloidal crystal templates for use in synthesizing 3DOM supports.

2.1.3 Synthesis of 3DOM Al₂O₃

The 3DOM Al₂O₃ support was prepared via a precursor thermal decomposition assisted colloidal crystal templating method. Typically, a certain amount of citric acid was dissolved in ethanol (95 wt%,10 mL) for 30 min, and then the stoichiometric amount of Al(NO₃)₃·9H₂O was dissolved in the mixed solvent to obtain the precursor solution. The PMMA templates were permeated into the precursor solution for 2 h and the solution was filtered and dried. The dried PMMA colloidal crystal templates were mixed with quartz sand (10-15 mesh) and calcined in air; the temperature was linearly increased at 1 ℃/min to 310 ℃, held at that temperature for 4 h, and then linearly increased at a rate of 1 ℃/min to 600 ℃ and held at 600 ℃ for 5 h.

2.1.4 Synthesis of 3DOM CeO₂

The 3DOM CeO₂ was synthesized using a modified version of a previously reported method ^[34]. Eqimolar amounts of citric acid and Ce(NO₃)₃·6H₂O were dissolved in ethanol (95%,10 mL) to form a precursor solution. Citric acid, which forms stable complexes with rare-earth ions, was used as a chelating ligand, to provide a higher-melting point and good compatibility with the PMMA template. The obtained PMMA colloidal crystal templates filled with citric acid and Ce(NO₃)₃·6H₂O precursor solution were annealed at 400 ℃ at a heating rate of 1 ℃/min for 5 h to remove the PMMA colloidal templates and form 3DOM CeO₂.

2.1.5 Synthesis of CeO₂/3DOM Al₂O₃ catalyst

The CeO₂/3DOM Al₂O₃ catalyst was synthesized by micropore-diffusion precipitation (MDP) using ammonia solution as the precipitating agent ^[21]. The MDP apparatus was a ceramic membrane reactor with a core consisting of four ceramic membrane tubes. The abundant nanometer holes in the walls of the ceramic membrane tube enabled even deposition of the precipitating agent. The typical procedure for the preparation was as follows. Stoichiometric amounts of Ce(NO₃)₃·6H₂O, the 3DOM Al₂O₃ support, and distilled water were mixed and stored in a precursor tank (beaker I). The ammonia solution, which was used as the precipitating agent, was stored in another tank (beaker Ⅱ). The precursor solution (beaker Ⅱ) was diffused into the membrane reactor via the holes (40 nm) distributed on the walls of the ceramic membrane tubes, using a constant-flow pump. Precipitation of Ce³⁺ occurred in the membrane reactor, and Ce(OH)₃ was deposited on the surface of the Al₂O₃ support. The deposited slurry mixture was filtered and dried at 80 ℃, and then calcined at 550 ℃ for 4 h in static air to give CeO₂/3DOM Al₂O₃.

2.1.6 Synthesis of supported Au nanoparticle catalysts

Supported Au nanoparticle catalysts were synthesized by a reduction-deposition method using HAuCl₄·4H₂O as the precursor. An appropriate amount of polyvinylpyrrolidone dissolved in water was used as a protecting agent to prevent aggregation. A stoichiometric amount of HAuCl₄·4H₂O (the theoretical Au loading was 2%) was added. After stirring for 5 min, potassium borohydride was added dropwise to the mixed solution to reduce the Au³⁺ ions. The mixture was then stirred for 1 h. The support, i.e.,3DOM Al₂O₃ or CeO₂/3DOM Al₂O₃ (0.3 g), was introduced into the solution. The reaction system was vigorously stirred for 4 h. After aging and filtration, the product was washed several times with deionized water and ethanol, respectively. The resulting material was dried at 50 ℃ for 24 h and then calcined at 350 ℃ for 1 h in static air to give the desired Au/3DOM Al₂O₃ and Au/CeO₂/3DOM Al₂O₃ catalysts.

2.2 Catalyst characterization

The surface morphologies of the catalysts were examined using SEM (Quanta 200 F); the accelerating voltage was 5 kV. The samples were dusted on an adhesive conductive carbon belt attached to a Cu disk and coated with Au prior to observations.

The BET surface areas were determined based on N₂ adsorption-desorption isotherms (Micromeritics ASAP 2020). The samples were degassed at 300 ℃ for 3 h.

XRD patterns were obtained with a powder X-ray diffractometer (Shimadzu XRD 6000) using Ni-filtered Cu K_α radiation (λ = 0.154184 nm); the scanning range was 5°-90° and the scanning rate was 4°/min. Phase identification was performed by comparing the patterns with JCPDS reference data.

TEM and high-resolution TEM (HRTEM) images were obtained using a JEOL JEM 2100 electron microscope at an accelerating voltage of 200 kV.

UV-vis DRS (Hitachi U-4100) was performed in the range 200-800 nm with BaSO₄as a reference.

The redox properties of the catalysts were investigated using H₂-TPR; the experiments were conducted in a conventional flow apparatus connected to a thermal conductivity detector. The sample was pretreated in an Ar stream at 300 ℃ for 1 h and then cooled to 30 ℃. The flow was then switched to 10% H₂-90% Ar (40 mL/min). The temperature was increased from 30 to 900 ℃ at a rate of 10 ℃/min.

XPS was performed using a Perkin-Elmer PHI-1600 ESCA spectrometer with an Mg K_α (hv = 1253.6 eV,1 eV = 1.603 × 10⁻¹⁹ J) X-ray source. All binding energies (BEs) were calibrated using the C 1s peak of contaminant carbon (BE = 284.6 eV) as an internal standard.

2.3 Catalytic activity tests

The activities of the catalysts were evaluated based on a temperature-programmed oxidation (TPO) reaction. The reaction temperature was raised from 150 to 650 ℃ at a rate of 2 ℃/min. The model soot (Printex-U) used in this work was purchased from Degussa. The loose contact mode (physical contact by shaking soot and a catalyst in a sample bottle is referred to as loose contact ^[35]) was reproduced using a mixture of soot and catalyst (0.11 g) in a weight ratio of 1:10 using a spatula, as described by Jelles et al. ^[36]; this provides a model of diesel soot particles flowing through a catalytic filter. The simulated flue gases were a mixture of 10% O₂,0.2% NO, and balance Ar. The gas concentrations were measured continuously using a flue gas analyzer (Sp-3420, Beijing, China) with a flame ionization detector. The T₁₀,T₅₀, and T₉₀ values, i.e., the temperatures at which 10%,50%, and 90% soot conversions, respectively, were achieved during TPO, were used as an index of the catalytic activity. The selectivity for CO₂ formation (S_CO2) was calculated by S_CO2 = C_CO2/(C_CO + C_CO2) × S_CO2^m, where S_CO2^m was defined as S_CO2 at the temperature at which the highest rate of soot burning occurred. The intrinsic activities of the catalysts were determined based on their turnover frequencies (TOFs) ^[26].

3 Results and discussions

3.1 Catalyst characterization

3.1.1 SEM

The morphologies and macroporous structures of the samples were investigated using SEM. Fig. 1(a) shows that the obtained PMMA colloidal microspheres are highly uniform, with a narrow size distribution, and are closely packed in a highly ordered 3D arrangement. The average size of the PMMA microspheres in the colloidal crystal arrays are estimated to be around 350 nm. Use of the colloidal crystal arrays as templates enabled creation of a 3DOM Al₂O₃ support via a precursor complexation process. Fig. 1(b) shows that the formed Al₂O₃ has a well-defined 3DOM structure. The 3DOM Al₂O₃ pore sizes are approximately 260 nm, as shown in Fig. 1(b); this is smaller than the original PMMA colloidal microsphere templates because of template melting and sintering of the produced metal oxides. Interconnected networks of the ordered macroporous structures of 3DOM Al₂O₃ can be clearly observed. Fig. 1(c) shows that removal of the PMMA template gave 3DOM CeO₂, in which uniform pores are arranged in the same fashion as those in the template. The high-magnification image shows that each pore is connected to its 12 neighbors by a small “window”. It clearly shows that CeO₂/3DOM Al₂O₃ prepared by MDP has a well-defined 3DOM structure. The inverse-opal long-range order is retained. This structure is favorable for mass transfer because of the pore connectivity.

Fig. 1

Fig. 1. SEM images of the template and the catalysts. (a) The assembled PMMA colloidal crystal template; (b) 3DOM Al2O3; (c) 3DOM CeO2; (d) CeO2/3DOM Al2O3; (e) Au/3DOM Al2O3; (f) Au/CeO2/3DOM Al2O3.

Au/3DOM Al₂O₃ and Au/CeO₂/3DOM Al₂O₃ with 2% Au loadings were synthesized via a reduction-deposition method. Fig. 1(e) and (f) shows that the long-range ordered structures are still present in the supported Au catalysts, with no blocking by Au nanoparticles, indicating good distribution of Au nanoparticles on the 3DOM support. Interconnected networks in the supported Au catalysts can be clearly observed, confirming that Au/3DOM Al₂O₃ and Au/CeO₂/3DOM Al₂O₃ have well-defined 3D ordered hexagonal structures.

3.1.2 Textural properties

The BET surface areas, pore volumes, and average pore diameters are listed in Table 1. The data show that the surface area and pore volume varied, depending on the CeO₂ loading on the 3DOM Al₂O₃ support, from 76 m²/g for the Al₂O₃ substrate to 126 m²/g for CeO₂/3DOM Al₂O₃. This is because abundant interconnected mesopores are formed by aggregation of CeO₂ crystallites. The textural properties of Al₂O₃ are considerably modified by the presence of CeO₂ in CeO₂/3DOM Al₂O₃, and the CeO₂ content strongly affects these parameters. The macropores are sufficiently large and are not blocked by CeO₂ crystallites. This is in agreement with the SEM results, which show that the CeO₂/3DOM Al₂O₃ catalyst has a completely macroporous structure. Deposition of Au nanoparticles does not significantly change the surface area and pore diameter of CeO₂/3DOM Al₂O₃.

表 1 (Table 1)

Table 1 Physicochemical properties of as prepared catalysts. Catalyst Surface area a (m2/g) Total pore volume b (cm3/g) Pore size c (nm) 3DOM Al2O3 76 0.20 9.3 CeO2/3DOM Al2O3 126 0.25 7.6 Au/CeO2/3DOM Al2O3 131 0.23 7.6 a Calculated by the BET method. b Calculated by the BJH adsoption cumulative volume of pores between 1.7 and 300 nm diameter. c Calculated by the BJH adsorption average pore diameter.

Table 1 Physicochemical properties of as prepared catalysts.

N₂ adsorption-desorption isotherms of the samples are shown in Fig. 2. The isotherms of the 3DOM catalysts are type Ⅱ, according to the International Union of Pure and Applied Chemistry classification, i.e., typical of macroporous materials with a type H3 hysteresis loop in the relative pressure (p/p₀) range 0.5-1.0. An almost linear region can be observed in the low-pressure area in the middle section of the isotherm; this is attributed to unrestricted monolayer-multilayer adsorption, suggesting that the sample is a macroporous adsorbent. A macroporous structure is important for effective contact between soot and the catalyst. The large increase in adsorption at high relative pressure is characteristic of a wall consisting of tenuously assembled solid-solution clusters, i.e., there are many mesopores within the macroporous wall. These mesopores, of about 8 nm in size, contribute to the high BET surface area.

Fig. 2

Fig. 2. Nitrogen adsorption and desorption isotherms of as-prepared catalysts. (1) 3DOM Al2O3; (2) CeO2/3DOM Al2O3; (3) Au/CeO2/3DOM Al2O3.

3.1.3 XRD

XRD was performed to confirm the formation and phase structures of the prepared catalysts; the patterns are shown in Fig. 3. All the Al₂O₃ reflections are typical of the γ-Al₂O₃ phase (PDF 01-1303,2θ = 37.3°,45.8°, and 66.8°). The diffraction peaks at 2θ = 28.5°,33.1°,47.5°, and 56.3° correspond to the (111),(200),(220), and (311) lattice faces, respectively, of face-centered cubic (fcc) fluorite-type CeO₂. These diffraction peaks are in good agreement with the data in the standard file (PDF 65-2975). For the CeO₂/3DOM Al₂O₃ catalyst, four main diffraction peaks are observed and no separate single oxides, i.e., CeO₂ and Al₂O₃, are detected. These results suggest the formation of an Al-Ce-O solid solution. The diffraction peaks shift slightly from 28.5° to 28.7°. Many investigations have shown that CeO₂ and Al₂O₃ interact strongly, but the nature of the interactions is still under discussion, with a focus on possible Al³⁺ incorporation into the CeO₂ lattice. The radius of Al³⁺ (0.057 nm) is much smaller than that of Ce⁴⁺ (0.097 nm), and therefore incorporation of Al³⁺ ions into the CeO₂ lattice leads to lattice shrinkage ^[37-40].

Fig. 3

Fig. 3. XRD patterns of the prepared catalysts. (1) 3DOM Al2O3; (2) 3DOM CeO2; (3) CeO2/3DOM Al2O3; (4) Au/3DOM Al2O3; (5) Au/CeO2/3DOM Al2O3.

It should be noted that the Au diffraction peak at 38.2° is not clearly observed for the Au-based catalysts. This could be because the Au particles are small and highly dispersed over the CeO₂/3DOM Al₂O₃ or Al₂O₃ support. A comparison of the patterns for CeO₂/3DOM Al₂O₃ and Au/CeO₂/3DOM Al₂O₃ shows that the Au/CeO₂/3DOM Al₂O₃ catalyst has a peak from γ-Al₂O₃, at 37.3°. This suggests phase separation between γ-Al₂O₃ and CeO₂ after calcination and during Au deposition. We cannot exclude the presence of broad Au diffraction peaks that are superimposed on those of the γ-Al₂O₃ phase.

3.1.4 TEM and HRTEM

Fig. 4 shows the TEM and HRTEM images, and size distributions of 3DOM Al₂O₃, CeO₂, CeO₂/3DOM Al₂O₃, and the Au-based catalysts. It shows that all the prepared catalysts have 3DOM structures with overlapping pores. The pore size of the 3DOM Al₂O₃ support is ca. 260 nm, and the voids are interconnected through open windows of diameter 60 nm; this is in agreement with the SEM images. The HRTEM image in Fig. 4(c) clearly indicates 0.317,0.273, and 0.193 nm lattice fringes, indexed to CeO₂(111),(200), and (220), respectively. A 0.313 nm lattice fringe is observed for the CeO₂/Al₂O₃ catalyst, and this is indexed to the fcc (111) crystal face of CeO₂. The results indicate that Al incorporation into the fluorite-type lattice could result in Al-Ce-O solid-solution formation. This is consistent with the XRD results.

Fig. 4

Fig. 4. TEM, HRTEM images, and size distribution of Au nanoparticles of 3DOM Al2O3 (a),3DOM CeO2 (b-c), CeO2/3DOM Al2O3 (d-f), Au/3DOM Al2O3 (g-h), and Au/CeO2/3DOM Al2O3 (i-j).

Fig. 4(g-h) clearly shows precipitation of spherical Au nanoparticles on the surface of the 3DOM Al₂O₃ support; all the Au particles are uniform and highly dispersed. The Au particle sizes are in the range 1-5 nm, with a narrow distribution, and the mean diameter is 3.1 nm. The HRTEM image of Au/CeO₂/3DOM Al₂O₃ (Fig. 4(j)) shows hemispherical Au nanoparticles of crystal size 3.4 nm. Also, single Au nanoparticles are in intimate contact with the CeO₂/3DOM Al₂O₃ support, which indicates strong metal-support interactions. A nanocrystalline Al-Ce-O solid-solution support and Au nanoparticles in the active component may have a strong synergetic effect on the redox properties because the composite increases the mobility of lattice oxygens in the catalysts. The particle size and structure of the Au nanoparticles are similar in Au/3DOM Al₂O₃ and Au/CeO₂/3DOM Al₂O₃, indicating that the effect of Au particle size can be ignored in determining the influence of the support on the catalytic activity.

3.1.5 UV-vis -DRS

Fig. 5 shows the UV-vis DR spectra of 3DOM γ-Al₂O₃, CeO₂, CeO₂/3DOM Al₂O₃, Au/3DOM Al₂O₃, and Au/CeO₂/3DOM Al₂O₃. The broad bands at around 550 nm are attributed to surface plasmon resonance bands for Au particles; this is characteristic of nanocrystalline Au particles (diameter < 5 nm) ^[23]. Fig. 5(4) and (5) shows that the band intensities do not change and the bands do not shift toward higher energies. We can therefore assume that the Au particles on Al₂O₃ and CeO₂/Al₂O₃ are similar in size. This is in accordance with the size distribution of the supported Au nanoparticles calculated from the HRTEM images. The intense absorption bands with a maximum at around 300 nm are ascribed to Ce³⁺ ← O^2- charge transfer and overlap of the Ce⁴⁺ ← O^2- charge-transfer and inter-band transitions. It has been reported that Al₂O₃ can increase the band gap energy E_g of CeO₂ in combinations of these materials ^[41]. These results suggest strong interactions or possible substitutions of Al³⁺ by Ce⁴⁺ ions in the Al₂O₃ network. This is in agreement with the XRD results, which show the formation of an Al-Ce-O solid solution in the CeO₂/Al₂O₃ catalyst.

Fig. 5

Fig. 5. UV-vis diffuse reflectance spectra of the catalysts. (1) 3DOM Al2O3; (2) 3DOM CeO2; (3) CeO2/3DOM Al2O3; (4) Au/3DOM Al2O3; (5) Au/CeO2/3DOM Al2O3.

3.1.6 H₂-TPR

For metal oxide catalysts, H₂-TPR measurements simultaneously reflect the reducibility of high-valent metal ions to low-valent ions or metal atoms, and the potential to remove or take up oxygen, i.e., the mobility of lattice oxygen species (O^2-). The reduction peak temperature (T_red) is used to evaluate the redox ability of the catalyst ^[42]. The lower T_red is, the stronger the redox ability of the catalyst. In practical applications, the low-temperature rate of soot oxidation is more important than the high-temperature rate, and therefore we focus on the lowest T_red peak. The H₂-TPR profiles of 3DOM Al₂O₃, CeO₂, CeO₂/3DOM Al₂O₃, and Au/CeO₂/3DOM Al₂O₃ are shown in Fig. 6. No hydrogen consumption is observed for the H₂-TPR profile of pure Al₂O₃. For pure CeO₂, the H₂-TPR profile shows two hydrogen consumption regions: a low-temperature region (430-560 ℃), which is associated with the reduction of surface oxygen and the outmost layers of Ce⁴⁺, and a high-temperature region (above 700 ℃), which is attributed to the bulk reduction of CeO₂ to Ce₂O₃ by elimination of O^2- anions from the lattice. The two reduction peaks for CeO₂/3DOM Al₂O₃ shift to lower temperatures compared with that for bare CeO₂, and the peak at high temperature becomes weak, which indicates enhanced reducibility of the CeO₂ support prepared using MDP. The peak centered at 362 ℃ originates from the reduction of small CeO₂ crystallites and/or surface Ce atoms. The high-temperature peak at 518 ℃ is associated with the reduction of Ce⁴⁺ to Ce³⁺ with formation of CeAlO₃. These results are in agreement with the XRD data, which show the formation of Al-Ce-O solid solutions.

Fig. 6

Fig. 6. H2-TPR profiles of 3DOM Al2O3 (1),3DOM CeO2 (2), CeO2/3DOM Al2O3 (3), Au/3DOM Al2O3 (4), and Au/CeO2/3DOM Al2O3 (5).

Fig. 6(3) shows that the H₂-TPR profile of Au/3DOM Al₂O₃ differs significantly from that of bare Al₂O₃. After Au deposition, the reduction peak for Al₂O₃ shifts to about 565 ℃. Highly dispersed Au promotes Al₂O₃ reduction by spillover hydrogen; this is in agreement with the TEM results. For the as-prepared Au/CeO₂/3DOM Al₂O₃ catalyst, the reduction peak temperature gradually shifts to lower temperature, and there is a clear reduction peak centered at 322 ℃, which is related to reduction of chemisorbed oxygen species on the highly dispersed Au on the support Au (Au-O_x to Au) or at the interface between Au particles and the support (Ce-O_x-Au to Au), which can weaken the Ce-O bonds induced by Au atoms ^[22]. The results suggest strong interactions between Au nanoparticles and the support in the Au/CeO₂/3DOM Al₂O₃ catalyst, and different support-surface compositions can result in Au behaving differently in hydrogen oxidation, resulting in different synergetic effects between CeO₂ and Au nanoparticles. Among these catalysts, Au/CeO₂/3DOM Al₂O₃ displays the best reaction behavior for low-temperature hydrogen oxidation because of the strong metal (Au)-support (CeO₂) interactions, indicating that it will give a better catalytic performance in soot combustion.

3.1.7 XPS

XPS is an effective technique for determining surface element compositions, metal oxidation states, and adsorbed species for solid materials ^[43]. Au/CeO₂/3DOM Al₂O₃ was studied using XPS; the results are shown in Fig. 7. The Au 4f XPS spectrum of Au/CeO₂/3DOM Al₂O₃ is shown in Fig. 7(a). The Au 4f signals at BE = 84.3 and 88.0 eV indicative the presence of surface Au^{0 [44]}, and the peaks at BE = 86.2 and 89.9 eV are attributed to positively charged Au species (Au^δ+). The surface Au^δ+/Au⁰ molar ratio of Au/CeO₂/Al₂O₃ is 0.034. Au^δ+ species are present on Au/CeO₂/Al₂O₃, probably as a result of interactions between Au⁰ and Ce⁴⁺ via the reaction Au^δ+ + Ce³⁺ → Au⁰ + Ce⁴⁺. Au^δ+ species are more active than Au⁰ species ^[33].

Fig. 7

Fig. 7. XPS spectra of Au 4f (a), O 1s (b) and Ce 3d (c) regions for Au/CeO2/3DOM Al2O3 catalyst.

Fig. 7(b) shows that the O 1s spectrum of the catalyst can be fitted to three BE peaks. The lower BE peak, at 529.7 eV, corresponds to lattice oxygen species (O^2-), and the two higher BE peaks, at 531.3 and 532.2 eV, are indexed to chemisorbed oxygen species, i.e., O^- and O₂^-, respectively ^[32]. The presence of surface active oxygen species (O^- and O₂^-) can greatly enhance the catalytic performance in deep oxidation.

Fig. 7(c) shows the Ce 3d XPS spectrum. The Ce 3d_5/2 and Ce 3d_3/2 levels contain various states. The XPS peaks denoted by u3 (916.7 eV), u2 (908.1 eV), u (901 eV), v3 (898.6 eV), v2 (889.2 eV), and v (882.7 eV) are assigned to Ce⁴⁺ species, and u1 (903.6 eV), u0 (897.4 eV), v1 (885.2 eV), and v0 (881.3 eV) are assigned to Ce³⁺ species ^{[45, 46]}. The Ce³⁺ species contribute to the interactions between Ce and the surrounding atoms, which leads to an increase in the amount of oxygen vacancies. We can therefore deduce that there are a strong interaction between the CeO₂/Al₂O₃ support and Au. These results are in good agreement with the H₂-TPR measurements.

3.2 TPO reaction results and TOF values

Diesel soot oxidation is a typical heterogeneous catalytic reaction with solid particles as a reactant. The catalytic activities and selectivities were determined in TPO reactions using a NO/O₂ mixture as the oxidizing agent. TheT₁₀,T₅₀, and T₉₀ values for soot combustion with and without catalysts under conditions of loose contact between soot and the catalyst are listed in Table 2. For bare soot,T₅₀ is 558 ℃. The data clearly show that all the 3DOM catalysts promote soot combustion. The catalytic activity of pure CeO₂ is higher than that of the pure Al₂O₃ support. This is because CeO₂ has the advantages of a high oxygen-storage capacity and excellent redox properties, which favor diesel soot oxidation. The catalytic activity of CeO₂/3DOM Al₂O₃ is higher than that of pure CeO₂ because of the small particle size.

表 2 (Table 2)

Table 2 The catalytic activities of 3DOM catalysts for soot combustion Catalyst T10/℃ T50/℃ T90/℃ SCO2m/% 3DOM Al2O3 362 530 586 57.5 3DOM CeO2 318 379 418 92.8 CeO2/3DOM Al2O3 293 379 425 79.6 Au/3DOM Al2O3 298 453 513 96.9 Au/CeO2/3DOM Al2O3 273 364 412 97.6 Soot 449 558 605 39.4

Table 2 The catalytic activities of 3DOM catalysts for soot combustion

It is clear that the presence of Au can affect the activity and selectivity of the catalyst under loose contact conditions. The catalytic activity in soot oxidation is significantly enhanced by depositing Au nanoparticles on the surface of 3DOM Al₂O₃ or CeO₂/3DOM Al₂O₃; in particular, the T₁₀ temperature decreases greatly and S_CO2 increases greatly. The T₁₀ of Au/3DOM Al₂O₃ is significantly lower, by 64 ℃, than that of 3DOM Al₂O₃. Among the obtained catalysts, Au/CeO₂/3DOM Al₂O₃ shows the best ignition catalytic performance, i.e.,T₁₀ is 273 ℃. This could be caused by a synergetic effect at the interface between the Au nanoparticles and the Al-Ce-O solid solution. This interfacial interaction promotes dioxygen activation. The higher activity of Au/CeO₂/3DOM Al₂O₃ is probably also related to the increased lattice oxygen mobility resulting from formation of a defective CeO₂ structure; Ce ions are usually regarded as anchoring points for the stabilization of deposited metal nanoparticles. The oxygen vacancies in the nanosized defective CeO₂ act as nucleation sites for Au anchoring ^{[34, 47]}.

Table 3 lists the reaction rates, active redox site densities, and TOF values for soot combustion under O₂ at 280 ℃ over the CeO₂/3DOM Al₂O₃ and Au/CeO₂/3DOM Al₂O₃ catalysts. The TOF value for Au/CeO₂/3DOM Al₂O₃ is slightly higher than that for CeO₂/3DOM Al₂O₃. However, the number of active redox sites is significantly large. Such active oxygen species promote activation of soot particles and therefore lower the temperature of soot combustion. These results are in good agreement with the TPO results.

表 3 (Table 3)

Table 3 Reaction rate, active oxygen (O*) density, and TOF values for soot combustion with O2 at 280 ℃ over the CeO2/3DOM Al2O3 and Au/CeO2/3DOM Al2O3 catalysts. Catalyst v a (10-7 mol s-1 g-1) v b (10-9 mol s-1 m-2) O amount c (10-5 mol g-1) O density d (nm-2) TOF (10-4 s-1) CeO2/3DOM Al2O3 0.31 0.25 3.79 0.18 8.18 Au/CeO2/3DOM Al2O3 0.47 0.36 5.54 0.25 8.48 a Reaction rate. b Specific reaction rate per unit BET surface area. c Number of active redox sites. d Density of active redox sites.

Table 3 Reaction rate, active oxygen (O*) density, and TOF values for soot combustion with O2 at 280 ℃ over the CeO2/3DOM Al2O3 and Au/CeO2/3DOM Al2O3 catalysts.

3.3 Discussion

3.3.1 Influence of support on catalytic performance

The intrinsic properties of a catalyst play a dominant role in soot oxidation ^{[48, 49]}. The inherent properties, including the phase structure and redox properties, of a catalyst are controlled by the composition, valence state of ions, and oxygen vacancies. Fig. 6 shows that 3DOM Ce-rich catalysts have good redox properties at low temperatures, e.g., the low-temperature reduction peak of the CeO₂/3DOM Al₂O₃ catalyst is centered at 362 ℃. It can be deduced that the lattice oxygen mobility increases from the bulk to the surface for a moderately Al-doped CeO₂ lattice ^[50]. This leads to changes in the lattice and increases in the channel diameter for lattice oxygen migration, because the ionic radius of Al³⁺ (0.057 nm) is smaller than that of Ce⁴⁺ (0.097 nm). The effectiveness of the catalyst is related to its ability to deliver oxygen from the lattice to the soot surface over a wide temperature range, and Ce-rich samples have a stronger ability to donate oxygen for soot oxidation. A comparison of the catalytic activities of Au/3DOM Al₂O₃ and Au/CeO₂/3DOM Al₂O₃ shows that better redox support properties improve the catalytic activity in soot combustion, especially at low temperatures.

Another factor to be considered in the design of catalysts for soot combustion is the soot-catalyst contact, which significantly affects catalytic combustion ^[51]. Catalytic combustion of soot is a typical heterogeneous catalytic reaction, which takes place at the three-phase boundary among a solid catalyst, a solid reactant (soot), and gaseous reactants (O₂ and NO). The catalytic efficiency is therefore strongly affected by the contact between the soot and the catalyst. Generally, only the outer surfaces of catalysts are available for soot combustion because the pore diameters (usually < 10 nm, or even non-porous) are too small to allow entry of soot particles. In this work,3DOM materials with a uniform pore size (> 50 nm) and periodic voids interconnected through open windows were designed and synthesized using a colloidal crystal template method. The soot particles can easily enter the interior of 3DOM catalysts, and there is less resistance to their passage through the catalyst structure. Fig. 8 shows that the 3DOM structure of the catalyst was intact up to 250 ℃, and soot particles could enter the inner pores of the 3DOM catalyst with the help of an airflow. Fig. 8(c) shows that the voids in the 3DOM Al₂O₃ catalyst were filled with soot particles, indicating that the 3DOM structure is a desirable feature for diesel soot combustion. The contact efficiency between soot and the catalyst is also improved. The factors that aid soot combustion can be summarized as follows: pores large enough to permit diesel soot to enter the inner pores, and uniform macroporous networks that allow easy mass transfer and have less diffusional resistance when soot particles pass through the catalyst structure. These properties are favorable for the catalytic combustion of soot particles.

Fig. 8

Fig. 8. TEM images of soot (a),3DOM Al2O3 (b-c) and Au/CeO2/3DOM Al2O3 (d-f) after the TPO reaction until 250 ℃. Reaction conditions: reaction gas 10% O2-0.2% NO/Ar (50 mL/min), soot/catalyst mass ratio 1:10.

3.3.2 Influence of Au nanoparticles on catalytic performance

The catalytic activity results show that the introduction of Au nanoparticles on the surface of the 3DOM support significantly affects the oxidation of soot, especially T₁₀ and S_CO2. Generally, the catalytic performance of Au-based catalysts depends strongly on the particle size because Au nanoparticles can activate molecular oxygen. The average sizes of the Au particles in the Au-based catalysts were determined based on XRD, TEM, and UV-vis DRS analyses, and all the values are in good agreement. In this study, the average size of the supported Au particles is around 3 nm. The TPO results show that the activities in soot combustion of catalysts with different supports clearly differ. This indicates that the support properties, as well as the Au particle size, are important in catalyzing soot oxidation. Based on the H₂-TPR and XPS results, we can deduce that the active oxygen species over the Au/CeO₂/3DOM Al₂O₃ catalyst can be derived via two routes: direct activation of oxygen on the Au nanoparticle surfaces, and from the synergetic effect of metal (Au)-support (Ce) interactions, which may result in the Ce-based support serving as an oxygen reservoir for the oxidation reaction ^[23]. The support-Au interactions can lead to transfer of lattice oxygens in the support or adsorption of oxygen from support vacancies at low temperature. The active oxygen species derived from metal-support interactions are important in soot combustion. Au/CeO₂/3DOM Al₂O₃ gives a good catalytic performance in soot combustion because of its strong ability to donate oxygen from Ce-rich samples.

Fig. 8 shows that Au nanoparticles supported on the inner surfaces of 3DOM catalysts can further increase the contact between soot and the catalyst by “wetting” because of the surface mobility of low-melting point Au nanoparticles. The contact between soot and Au nanoparticles is tight in solid-solid reactions. The supported Au nanoparticles act as centers for tight contact between the CeO₂/Al₂O₃ support and soot particles; this enhances the catalytic performance in soot combustion because the active oxygen species can migrate from the catalyst to the soot particles via spillover. The 3DOM Au-based catalysts therefore give good catalytic performance at low temperature. Fig. 9 shows that the peaks of the CO₂ concentration profiles for soot combustion over Au-based catalysts shift to lower temperatures compared with that for the 3DOM support; for example, the peak temperature for CO₂ concentration over the CeO₂/3DOM Al₂O₃ catalyst is 390 ℃. After Au deposition, the peak temperature for CO₂ concentration shifts to 371 ℃. This confirms that the Au nanoparticles play an essential role in soot oxidation.

Fig. 9

Fig. 9. CO2 concentration profiles for soot oxidation over the catalysts. (1) 3DOM Al2O3; (2) 3DOM CeO2; (3) CeO2/3DOM Al2O3; (4) Au/3DOM Al2O3; (5) Au/CeO2/3DOM Al2O3.

4 Conclusions

We synthesized 3DOM pure Al₂O₃ and CeO₂ catalysts with large pores and interconnected macroporous structures using a colloidal crystal templating method. The 3DOM catalyst structure promotes diffusion of solid reactants (soot particles) through the materials, and the contact efficiency between the catalyst and soot is improved. We also fabricated a CeO₂/3DOM Al₂O₃ catalyst using MDP, with ammonia solution as the precipitating agent. A synergetic effect between CeO₂ and Al₂O₃ leads to the formation of an Al-Ce-O solid solution. Au/3DOM Al₂O₃ and Au/CeO₂/3DOM Al₂O₃ catalysts were synthesized using a reduction-deposition method. Au nanoparticles are highly dispersed on the inner walls of uniform macropores. The catalytic activity in soot oxidation is greatly enhanced by supporting Au nanoparticles on the surface of 3DOM Al₂O₃ or CeO₂/3DOM Al₂O₃ catalysts. Among all the obtained catalysts, Au/CeO₂/3DOM Al₂O₃ gives the best catalytic performance in soot oxidation, e.g., the initiation temperature is 273 ℃. This is attributed to strong interactions between the Au nanoparticles and the Al-Ce-O solid solution. The synergetic effect between CeO₂ and Al₂O₃ is favorable for transfer of active oxygen in the support, and for increasing the amounts of oxygen vacancies and active oxygen species; this is crucial to the catalytic performance.

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