Direct observation of catalytic oxidation of particulate matter using in situ TEM

The ability to observe chemical reactions at the molecular level convincingly demonstrates the physical and chemical phenomena occurring throughout a reaction mechanism. Videos obtained through in situ transmission electron microscopy (TEM) revealed the oxidation of catalytic soot under practical reaction conditions. Carbon oxidation reactions using Ag/SiO2 or Cs2CO3/nepheline catalysts were performed at 330 °C under an O2 flow of 0.5 Pa in the TEM measurement chamber. Ag/SiO2 catalyzed the reaction at the interface of the mobile Ag species and carbon, while the Cs species was fixed on the nepheline surface during the reaction. In the latter case, carbon particles moved, remained attached to the Cs2CO3/nepheline surface, and were consumed at the interface by the oxidation reaction. Using this technique, we were able to visualize such mobile and immobile catalysis according to different mechanisms.

or "mobile oxygen" 16,17 . The mobile catalyst, including metal and alkali compounds, has a low melting point. In the reaction atmosphere, the catalyst undergoes solid-liquid phase transformation, leading to an increase in the contact area of the liquefied catalyst with the reactant (PM). Catalysts involving mobile oxygen species such as CeO 2 show high intrinsic activity for PM oxidation, as the active mobile oxygen is easily produced at the catalytic surface/interface with carbon from inside the bulk oxide.
Catalytic PM oxidation occurs under solid (PM, carbon)-solid (catalyst)-gas (O 2 ) triphasic conditions when using mobile species. Unlike other catalytic reactions under heterogeneous conditions (e.g., gas reactant-solid catalyst or liquid reactant-solid catalyst), the solid-solid-gas triphasic reaction proceeds with some difficulty depending on the contact between the two solids. The contact between the PM and catalyst is crucial in promoting the reaction 18 . However, this triphasic reaction and its mechanism in relation to a solid-solid reaction are still unclear; few investigations have been conducted toward a detailed analysis of this process. A catalytic reaction mechanism has been speculated on the basis of the information about the surface before and after the reaction. Real-time investigation of the reaction mechanism on solid catalysts under operating conditions and in the presence of reactants at high temperatures remains a challenge; however, recent studies have used in situ and operando analyses (XRD 19 , IR 20 . To understand the practical reaction mechanism, we focus on in situ analysis by TEM through which we can observe and investigate the actual surface in real time under operating conditions. Several experiments using in situ TEM have been reported: observation of catalytic reactions with gas and solid catalysts 21 , crystal growth processes 22,23 , and investigations of supported metal behavior (e.g., melting point) 24,25 . Herein, we assume a very simple reaction for PM oxidation on solid catalyst, C (s) +O 2(g) →CO 2(g) , where the reaction is triphasic. We report in situ observation of catalytic PM oxidation using TEM and demonstrate how PM oxidation proceeds, as well as catalysts' behavior during oxidation.

Experimental
We initially focused on alkali carbonate catalysts loaded on nepheline, an aluminosilicate that showed high tolerance for hydration and leaching of alkali species [26][27][28][29] . Furthermore, Ag catalysts have been reported and recognized worldwide to show high performance for PM oxidation. In this study, Cs 2 CO 3 -loaded nepheline and Ag supported on SiO 2 were chosen as representative mobile catalysts. Here we have chosen cesium compound rather than potassium or sodium ones because the former showed higher catalytic performance than the latter ones 29 , meaning that lower temperature is expected to be required for the TEM analysis. Also, cesium could be analyzed more clearly than potassium or sodium because of the difference in their atomic weights from the other components such as aluminosilicate nepheline and carbon.
Silica-supported Ag catalyst containing 10 wt% of Ag was prepared with the incipient wetness method using an aqueous AgNO 3 solution (Wako Pure Chemical) and powder SiO 2 (fumed, Aldrich). After heating till complete evaporation of the aqueous phase, the precipitated product was calcined at 500 °C for 3 h in air.
The nepheline-loaded Cs 2 CO 3 catalyst was prepared as follows. A Na form Lind-type A zeolite with Si/Al molar ratio of 1 (Wako Pure Chemical) was sintered at 1000 °C for 4 h in air to give Na-nepheline through solid-solid phase transformation [26][27][28] . Cs 2 CO 3 (Wako Pure Chemical) was then loaded on the Na-nepheline support by impregnation (10 wt%). This was immersed in distilled water, and the solvent was evaporated at 100 °C under vigorous stirring. The precipitated product was calcined at 800 °C for 3 h in air.
The catalytic activity test for PM oxidation was performed using temperature-programmed oxidation on a thermogravimetric-differential thermal analyzer (TG-DTA, Rigaku Thermo Plus TG8120). Carbon black #2600 (Mitsubishi Chemical) of diameter 13 nm was used as the reference for PM particles. The prepared catalyst and carbon were mixed in an agate motor for 10 min, generally called a "tight contact". The catalytic activity test was normally conducted by two types of contacts; one is "loose contact", for which the soot is mixed with the catalyst by using a spatula, and the other is "tight contact", for which the soot and catalyst are mixed in an agate motor. It has long been recognized that the "tight-contact" scenario is more reflective of the intrinsic catalytic performance. Catalyst/carbon weight ratio was set at 10. The mixture was then heated to 800 °C at 10 K min −1 under a flow of dried air. The catalytic activity for the carbon oxidation was evaluated by the temperature T ig , the point at which a distinct exothermic DTA curve derived from the heat of the carbon oxidation starts to appear.
Carbon oxidation on a Cu-microgrid was observed at an accelerating voltage of 1 MeV, under 0.5 Pa O 2 atmosphere at 330 °C on a TEM (JEM-1000K RS; JEOL). A small amount of the tight-contact-mode catalyst and carbon was placed on the Cu-microgrid. This was placed in the sample holder, which equips with the furnace and temperature controller, and inserted in the TEM. First, O 2 gas was introduced into the environmental cell chamber by gradually increasing the pressure surrounding the Cu-microgrid to 0.5 Pa, and then heated to 330 °C. Details of the image recording system for the JEM-1000K RS TEM are described in a previous study 30 .

Results and Discussion
The catalytic performance of carbon oxidation is shown in Fig. 1. Nepheline and SiO 2 (not shown in the figure) showed next to no catalytic activity for the oxidation because their DTA curves closely resemble Scientific RepoRts | 5:10161 | DOi: 10.1038/srep10161 those of the noncatalytic reaction of carbon with gaseous oxygen. The carbon oxidation using Ag/SiO 2 and Cs 2 CO 3 /nepheline occurred at 255 °C and 311 °C, respectively, indicating that the Ag catalyst shows a higher oxidation activity than the Cs 2 CO 3 catalyst. Figures 2 and SI_2 display a series of TEM images obtained during the carbon oxidation using Ag/ SiO 2 at 330 °C with a 0.5 Pa O 2 supply. Images (A)-(G) were acquired over 48 s, with one image captured every 8 s. As shown in image (A), Ag particles can be observed as black dots with their diameters ranging from 5 to 25 nm. During the carbon oxidation, two Ag species were observed: mobile Ag particles on SiO 2 (Ag 1 in (A)) and silent Ag particles on SiO 2 (Ag 2 in (A)). The C-O-Ag interaction may be slightly stronger than the Si-O-Ag one. The mobile Ag on SiO 2 transferred to carbon particles, and oxidation appeared to occur at the active interface of Ag and carbon. As the reaction proceeded, the mobile Ag agglomerated into larger particles. Finally, the mobile, agglomerated Ag terminated the reaction by becoming silent, without any contact with the carbon around the Ag particle. In the case of the silent Ag, from the first reaction stage, the carbon particles around Ag seemed to transfer to the active interface while maintaining contact with Ag during the reaction. To identify the reason for Ag mobility, the behavior of Ag/SiO 2 without the carbon was investigated. By comparing the TEM images before (H) and after (I) the reaction, the morphology and interface/surface of the Ag particle was changed from a crystal-like faceted plane to a spherical shape. Furthermore, the contrast of the Ag particles in image (I) decreased from the surface to the inside, indicating that the particle surface melted and became spherical because of the surface tension.
These results implied that liquefaction may be occurring at the Ag/SiO 2 interface according to the definition of Tammann temperature 31 . Surface premelting of Ag began at 550 °C-650 °C in vacuum depending on the Ag particle size 32 . Coalescence of Ag particles was also observed at 150 °C 33 . In this study, during the carbon oxidation, Ag particles on carbon easily agglomerated, which might be due to the heat of the carbon oxidation at the interface and partial liquefaction of the Ag surface at 330 °C. The mobile range of Ag transferred from the SiO 2 surface onto the carbon particles became limited as carbon was consumed. Consequently, we can deduce that Ag/SiO 2 can be used as a mobile catalyst for PM oxidation. Figures 3(A) and SI_3 show the TEM image of Cs 2 CO 3 /nepheline physically mixed with carbon at 330 °C in vacuum. Images (B) and (C) demonstrate the time course of carbon oxidation at 330 °C, with O 2 flowing at 0.5 Pa, acquired over 28 min. In image (A), the dark spheres correspond to Cs 2 CO 3 /nepheline, and the gray shadows are the agglomerated carbon particles. As explained above, the nepheline itself did not catalyze carbon oxidation at 330 °C; therefore, the oxidation activity derives from the Cs species. Images (A)-(C) led us to conclude that carbon oxidation occurs at the whole surface of the Cs 2 CO 3 / nepheline. This also indicates that the Cs species was highly dispersed on the nepheline surface. During the reaction, carbon particles continuously moved toward the Cs 2 CO 3 /nepheline surface, where carbon was oxidized. Image (D) shows a TEM image of Cs 2 CO 3 /nepheline, and images (E) and (F) display electron energy loss spectroscopy (EELS) mapping images of the Cs M-core at RT and 400 °C, respectively. The Cs species on nepheline was in a high dispersion state at RT, which was maintained even at 400 °C, revealing a specific interaction between the Cs species and nepheline. Images (G)-(I) summarize the TEM time course during the carbon oxidation using Cs 2 CO 3 at 330 °C with an O 2 flow of 0.5 Pa. Black dots derived from Cs cationic species became distinct as carbon oxidation proceeded. Notably, such black dots did not appear on the carbon in the TEM images (A)-(C). This result also indicates the specific interaction between the Cs cationic species and negatively-charged nepheline surface (Coulomb attraction). Alkali metal catalysts are currently categorized as mobile catalysts under PM oxidation conditions owing to their low melting point. However, this study clearly suggests that the Cs species on nepheline maintains its high dispersion state and is silent on the support during the carbon oxidation. Such high stability of alkali metals on nepheline has been demonstrated in our previous study [26][27][28] . Thus, we emphasize that the properties of the supported metals significantly depend on the nature of, and interaction with, the substrate. Therefore, it is evident that Cs 2 CO 3 /nepheline has potential application as a mobile oxygen catalyst for PM oxidation.
Note that the TEM analyses of the PM oxidation performance investigated herein are not simply minor phenomena occurring within the very limited field of view being observed. As indicated in the Supplementary Materials, we compared the kinetics observed by the TEM views and detected by the TG-DTA measurements, and the reaction rate of the carbon oxidation under the same conditions, which showed a comparable order, suggesting that the views by TEM can be recognized as the intrinsic performance of the catalysts reported in this study.

Conclusions
In situ TEM was successfully applied for the elucidation of the PM oxidation mechanism for two catalyst types: Ag/SiO 2 and Cs 2 CO 3 /nepheline. The Ag particles were mobile during the carbon oxidation at 330 °C in an O 2 atmosphere at 0.5 Pa. The carbon was consumed by the mobile Ag particles, causing the Ag particles to agglomerate. The reaction was finalized when all the carbon was consumed from the Ag-C islands. The Cs species were fixed silently on nepheline during the reaction even at 400 °C, which was indicated via EELS mapping. Interestingly, the interface of carbon and nepheline particles was continuously generated where carbon was oxidized. Finally, we show how, by visualizing these reactions at the TEM level, we were able to not only demonstrate the catalytic mechanism but also explain some physical and chemical characteristics of these catalysts. Seeing is not only believing, but also realizing and seizing an actual phenomenon in catalysis.