Carbon monoxide (CO) pollution, generated by incomplete combustion of fossil fuels (e.g., coal and oil), automobile exhaust, and industrial waste gases, has become a health and environmental problem that cannot be ignored because of its high toxicity to the human nervous system. Increasing concerns have prompted chemists to design and develop various catalysts for CO oxidation [1, 2] in air purification, pollution control, and fuel cell technologies. In particular, mesoporous catalysts, including the widely used mesoporous transition metal oxides and noble metals, tend to exhibit superior catalytic activities in some kinds of CO catalytic oxidation [3-7]. Research on highly efficient catalysts as well as the design and optimization of the preparation technology of porous catalysts to complete the oxidation process more effectively is of great significance for basic research as well as for the chemical industry.

Among various metal oxides, ferroferric oxide (Fe₃O₄) has been considered as one of the most promising materials for mesoporous catalysts or supports because of its intriguing properties such as low cost, high chemical and thermal stabilities (remaining stable during sintering), widespread availability, nontoxicity, and environment friendly features [8-12]. Furthermore, the introduction of mesopores into this iron oxide gives rise to a material with more exposed catalytic sites and improved surface functionalities[13]. The enlarged surface areas make this material a promising candidate for application in heterogeneous catalysis. Recent attempts have also focused on the preparation of metal oxides loaded with precious metals (Pd, Pt, Au, etc.)[14-18] for catalytic oxidation of CO. In addition, the pore structure (pore sizes and pore volumes), particle size of the noble metal, and surface area are also the major factors that determine the final catalytic properties. These properties lead to a synergetic effect on the activity of the catalyst in CO oxidation, which is normally higher than that of commercially purchased catalysts.

In general, the typical synthesis strategy of mesoporous materials involves soft templating pathways and hard templating approaches [19, 20]. The common loading processes for noble metals include precipitation [21], sedimentation, impregnation, etc. [22, 23]. Unfortunately, most of these are wet chemical processes with inherent disadvantages such as the requirement for complex steps, the use of excessive solvent, the production of liquid waste, and time- and energy-consuming drying treatments (most require drying or solvent evaporation) [24]. Mechanochemical synthesis has shown advantages in the synthesis of porous solid materials because it benefits from intermolecular interactions that utilize the frictional heat and mechanical momentum generated by the ball milling process. Researchers have already introduced mechanochemistry into the preparation of metal organic frameworks (MOFs) [25-30], ordered mesoporous carbon (OMC) [31, 32], covalent organic frameworks (COFs) [33-35], zeolites [36-38], etc.

Herein, solid-state synthesis, a green strategy, is proposed to synthesize mesoporous Fe₃O₄ and Au@Fe₃O₄ catalysts. In this process, the spontaneous organic-inorganic self-assembly between reactants (e.g., iron nitrate as precursor, and cetyltrimethylammonium bromide (CTAB) as the template) is accomplished with mechanical ball milling in the solid phase, replacing solvent-based technology. Then, sintering is utilized to obtain mesoporous iron oxide powders and noble-metal-loaded catalysts under an air atmosphere. No further wet chemical processes are required. Finally, to further evaluate the catalytic activity of these catalysts, their catalytic performance in a CO oxidation reaction was monitored. It is apparent that such a strategy is comparable to traditional templating methods in industrial manufacturing because it involves easy, solvent-free, less time- and energy-consuming, and environment-friendly preparation [39]. However, some existing deficiencies of current solid-state synthesis strategy still need to be addressed, which will be discussed later.

The principal materials for mechanochemical synthesis of mesoporous Fe₃O₄ are as follows: CTAB (Macklin, purity 99%), ferric nitrate nonahydrate (Fe(NO₃)₃·9H₂O, J & K, purity 98%), ferroferric oxide (Fe₃O₄, General Reagent, 70.5–74.2 wt%), and chloroauric acid (Energy, purity 98%). All the reagents were used as purchased without any further purification. The compressed gas mixture (carbon monoxide, 1%; O₂, 20%; and the remainder, N₂) that we used was commercially purchased from Shanghai Weichuang Standard Reference Gas Analytical Technology Company. The analytical techniques and/or instruments used for the experimental work and their operating parameters include: powder X-ray diffraction (XRD) (PANalytical Empyrean diffractometer, operating at 45 kV and 40 mA, 0.02° per step for scanning step), N₂ adsorption-desorption isotherms (TriStar 3000 volumetric adsorption analyzer, Micromeritics Instrument Corp), scanning transmission electron microscopy (STEM) with high-angle annular dark field (HAADF) capability (The Nion Co., operating at 200 kV, inner angle 80 mrad).

In order to characterize the CO catalytic oxidation, the concentrations of CO and CO₂ in the quartz tube were analyzed by a Buck Scientific 910 gas chromatograph. A dual molecular sieve/porous polymer column (Alltech CTR1) and a thermal conductivity detector were also employed.

In a typical run (Fig. 1), we added a moderate amount of metal salts (ferric nitrate nonahydrate, 1.0 g) and CTAB (0.2 g) into a 25-mL vibrating zirconia ball mill, where they were mixed and ground by using a planetary ball mill (constant frequency, 30 HZ) for 0.5 h without the addition of any solvent; this process gives rise to the mechanochemical self-assembled mixture. We then transferred the obtained mixtures to a muffle furnace; the heating rate was set at 2 ℃/min to the carbonization temperature of 300 ℃ and that temperature was maintained for 2 h to ensure the complete pyrolysis of CTAB into gaseous products. Evidence from the thermal gravimetric analysis (TGA) profiles for Fe₃O₄@CTAB 0.4 and 0.8 confirmed the complete CTAB decomposition at a temperature of ~250℃ (Fig. 2). Finally, we turned off the heat and allowed the products to cool down naturally to room temperature in the oven. For the preparation of mesoporous Fe_xO_y loaded with precious metals (e.g., Au), chloroauric acid (10–20 mg) was added during the initial ball milling process.

The as-prepared samples are denoted as Fe_xO_y@0.1, Fe_xO_y@0.2, Au@Fe_xO_y-10 mg, Au@Fe_xO_y-20 mg, etc., where the root expression such as "Fe_xO_y@0.1" corresponds to 0.1 g of CTAB being used as a template. The designation "Au@Fe_xO_y-10 (20) mg" refers to adding 0.2 g CTAB and 10 (20) mg chloroauric acid.

To explore the crystallinity of as-synthesized Fe₃O₄ samples with surfactant CTAB, XRD was performed. As shown in Fig. 3, while the amount of CTAB as template was adjusted within the range from 0.1 to 1.0 g, the phase transformation from ferric nitrate to Fe₃O₄ at 300 ℃ was verified with the diffraction peaks solely assigned to Fe₃O₄ (JCPDS 72-2303). The different intensity diffraction peaks at 2θ = 30°–45° and 50°–65° were attributed to the (220), (311), (400), (422), (511), and (440) facets, respectively. The average crystallite sizes (14–23 nm) of as-prepared iron oxide samples calculated by the (311) peak at 35.4° were small (Table 1), which is consistent with the broad diffraction peaks in the XRD patterns.

In order to understand the pore structure, N₂ gas sorption characterization was carried out at –196 ℃. The Brunauer-Emmett-Teller (BET) surface area of Fe₃O₄@0.2 (Fig. 4) was 146 m/g, and the Barrett-Joyner-Halenda (BJH) adsorption average pore width (4 V/A) was 5.0 nm. Furthermore, as shown in Fig. 5, N₂ adsorption/desorption isotherms of as-synthesized Fe₃O₄@CTAB (0.1–1.0) measured at liquid nitrogen temperature (–196 ℃) showed characteristic type Ⅳ isotherms of mesoporous materials with obvious hysteresis loops, and the nitrogen uptake kept increasing rapidly above P/P₀ > 0.1, indicating that the samples contained a high degree of mesoporosity. According to the calculated N₂ adsorption parameters (Table 1), the BET surface areas and total pore volumes increased going from Fe₃O₄@0.1 to Fe₃O₄@0.8 and reached their maximum value (0.23 cm³/g, 170 m²/g) when 0.8 g of CTAB was added. Correspondingly, the BJH adsorption average pore size varied in the range of 4.3–7.5 nm. This allows us to conclude that textural properties (specific surface areas, pore volume, and pore size distribution) of mesoporous Fe₃O₄ can easily be tuned by changing the amount of added soft template species. Such high values clearly suggest the rich porosity, which could validate the great potential application of these materials in heterogeneous catalysis.

The catalytic CO oxidation reaction was carried out in a fixed-bed reactor. The catalyst loaded was 30 mg and the feed gas of 1% CO balanced with dry air (20% O₂ and 79% N₂) passed though the catalyst at a gas hourly space velocity (GHSV) of 30 400 mL h^–1 g_cat^–1. The gas phase analysis was carried out every five minutes with gradual heating according to the program. The activity estimation of the catalyst was based on the CO conversion calculated by the percentages of CO₂ and CO after the reaction.

Figure 6 demonstrates the superior CO oxidation activity of the mesoporous iron oxide, as compared with commercial ferroferric oxide. While the commercially purchased Fe₃O₄ (as the standard material) enabled the 100% CO conversion at around 490 ℃, the CO conversion of mesoporous Fe₃O₄ samples can reach 100% even at a relatively low temperature (e.g., Fe₃O₄@0.8, 274 ℃). The significantly enhanced catalytic activity should be attributed to the larger specific surface area, pore volume, and more exposed active sites. Moreover, based on the evolution of the conversion rate curves, mesoporous Fe₃O₄ showed very weak CO oxidation catalytic activity below 135 ℃, with the catalytic behavior not starting until 150 ℃. The Fe₃O₄ @CTAB with a larger specific surface area was significantly more effective, indicating that the catalytic activity is closely related to surface area.

The catalytic stability of as-prepared mesoporous iron oxide on the catalytic oxygenation of CO at 240 ℃ is displayed in Fig. 7. It can be observed that as the reaction time was prolonged, Fe₃O₄@0.4 maintained a high CO conversion reaching up to 80% at 48 h. The CO conversion value is in agreement with the result in Fig. 6, indicating excellent catalysis stability.

Encouraged by the interesting activity of mesoporous Fe₃O₄ catalysts, we undertook a study of CO oxidation reaction at varying gas hourly space velocities (GHSV) in the range of 14400–121600 mL h^–1 g_cat^–1 (Fig. 8). As evidenced by the fact that the flow rate did not show any significant effect on the final 100% CO conversion temperature, the catalytic activity of the Fe₃O₄@0.8 sample was stable at different GHSV levels. On the other hand, Fe₃O₄@0.8 did show a lower initial activation temperature (~127 ℃) for CO oxidation at the higher feed gas space velocity.

In an effort to expand the scope of this mechanochemical solvent-free method, the assembly of iron salt (1 g), CTAB (0.2 g) and HAuCl₄ (10 mg or 20 mg; labelled Au@Fe₃O₄-10mg and Au@Fe₃O₄-20mg, respectively) was carried out by ball milling for 30 min, followed by thermal treatment at 300 ℃ in air.

To gain an insight into the phase morphologies of Au species, Au@Fe₃O₄ samples were examined by XRD. The diffraction pattern indicated the phases of metallic Au (PDF #01-1172) and Fe₃O₄ (Fig. 9). Hence, it is worth emphasizing that the introduction of Au does not affect the crystal structure of the catalyst carrier, thereby providing a simple approach to load gold nanoparticles onto a mesoporous Fe₃O₄ support. In addition, the diffraction peak of Au was relatively wide, indicating that the average particle size was small.

The porosity of Au@Fe₃O₄ by N₂ sorption was examined at –196 ℃. The N₂ adsorption isotherms (Fig. 10) both afforded an increasing uptake with P/P₀ > 0.1, indicating the possible formation of mesopores. The pore size distribution provided additional evidence that the porosity was dominated by mesopores of 2–10 nm. Moreover, increasing the amount of Au on mesoporous Fe₃O₄ did not clearly affect the pore volume and specific surface area of catalysts (65 to 63 m²/g and 0.258 to 0.152 cm³/g, respectively).

Because of the relatively low Tammann temperature and high surface energy, Au nanoparticles are usually prepared by solution-based methods [22, 23]. It is noteworthy that no solvents were used during our entire synthesis process. To verify the structural details of Au@Fe₃O₄-20mg, STEM mapping analysis was carried out in HAADF mode. As seen in Fig. 11, Au nanoparticles with an average size of ~4 nm were highly dispersed on the mesoporous iron oxides. Hence, the current solvent-free assembly by mechanochemistry seems not only to direct mesopores but also control the size of Au nanoparticles and prevent the growth or coalescence of the nanoparticles.

In contrast to existing synthesis methods (soft templating pathways and hard templating approaches) for mesoporous iron oxides, there remains a need for a sustainable method that can consume less solvents, fewer steps, and less time. This work demonstrates a solid-state CTAB-assisted synthesis of mesoporous Fe₃O₄ and suggests a possible method for loading noble metal nanoparticles onto mesoporous metal oxide by mechanochemistry. The specific surface area of Fe₃O₄@0.8 can reach up to 170 cm²/g by adjusting the addition of CTAB, resulting in an enhanced CO oxidation activity compared with commercially purchased Fe₃O₄. During the investigation of noble-metal-loaded catalysts, the as-prepared Au@Fe₃O₄ samples have been explored by XRD, BET, and SEM analysis. Highly dispersed Au nanoparticles on Fe₃O₄ supports, with small particle sizes around 4 nm, were identified. The potential applications and synergistic effects will be studied in detail in ongoing work.