Dynamic co-catalysis of Au single atoms and nanoporous Au for methane pyrolysis

Nanocatalysts and single-atom catalysts are both vital for heterogeneous catalysis. They are recognized as two different categories of catalysts. Nevertheless, recent theoretical works have indicated that Au nanoparticles/clusters release Au single atoms in CO oxidation, and they co-catalyze the oxidation. However, to date, neither experimental evidence for the co-catalysis nor direct observations on any heterogeneous catalysis process of single-atom catalysts are reported. Here, the dynamic process of nanoporous Au to catalyze methane pyrolysis is monitored by in situ transmission electron microscopy with high spatial–temporal resolutions. It demonstrates that nanoporous Au surfaces partially disintegrate, releasing Au single atoms. As demonstrated by DFT calculation, the single atoms could co-catalyze the reaction with nanoporous Au. Moreover, the single atoms dynamically aggregate into nanoparticles, which re-disintegrate back to single atoms. This work manifests that under certain conditions, the heterogeneous catalysis processes of nanocatalysts and single-atom catalysts are not independent, where their dynamic co-catalysis exists.

certain bright spots with lower intensities in Fig. 1f are Ag single atoms. We supplemented some experiments to analyze the contrast difference between Au and Ag single atoms in a same focal plane. Firstly, we used the magnetron sputtering technique to prepare a new sample with Au and Ag single atoms co-existing on the surface of an ultra-thin carbon film on a Cu grid (Beijing XXBR Technology Co., Ltd), and imaged them with HAADF ( Supplementary Fig. 7a, b). From the contrast intensity analysis results shown in Supplementary Figs. 7c, d, it can be seen that the Au single atoms have higher contrast intensities than the Ag, and their contrast intensities are both obviously higher than that of carbon. respectively. The grey, the cyan and the yellow balls denote C atoms, Au atoms in the outermost surface layer of the Au crystal, and the other Au atoms, respectively. The amorphous carbon cluster consists of twenty-five C atoms, and it is placed on the {311} surface of the Au crystal. The reason to use the {311} plane as the surface is that the surfaces of NPG are uneven and curved, as shown in Figs. 1c, 1e and 2, and the {311} plane is a low-index plane that can give an uneven surface. This consideration has been experimentally proven by previously reported works 1 . The IS is the 13 / 40 configuration of the carbon cluster adsorbed on the Au surface. In the FS, an Au atom is indicated by a red circle, and it strongly interacts with the carbon cluster, by which it has been distant from the Au surface. To reach the FS, the IS needs to evolve firstly into the TS and then into the FS. The energy change from the IS to the TS is the activation energy (Ea), whose value here is calculated to be 0.98 eV. This value is easily achievable under the actual reaction conditions, such as the high temperatures of 580 and 346°C 2 . Therefore, the TS and thus the FS are easily reachable, because the energy of the FS is always lower than that of the TS.
14 / 40 Supplementary Figure 10 Size distribution of sub-nano pores in the amorphous carbon layers, detected by CO 2 adsorption-desorption. Within the scope of our present study, NPG catalyzes CH 4 decomposition in a highly dynamic way. The deposition of amorphous carbon layers is accompanied by tremendous H 2 gas evolution, which results in the formation of multi-modal porous structure in the carbon layers. CO 2 and N 2 gas adsorption-desorption measurements ( Fig. 3a and Supplementary   Fig. 10) demonstrate the existence of both sub-nano pores and nanopores in the carbon layers.
These pores can serve as efficient channels for mass transportation, which are key to the observed co-catalysis behavior in our system. Actually, Fig. 2 indicates that after the NPG surface was fully covered by amorphous carbon layers, the surface still continuously disintegrated, making NPG ligaments continuously slim. This phenomenon manifests that the pores truly worked as efficient channels for mass transportation and the catalytic reaction still occurred on the NPG surface that was covered by the amorphous carbon. Thus, no poisoning effect took place on the NPG surface in our work. Supplementary Note 5 The whole system, containing amorphous carbon, single atoms, clusters and particles, is in a highly dynamic process (Supplementary movies 4 and 5). Thus, the re-orienting of the Au particles might occur during the reaction process. Therefore, we re-checked the process and recorded a new movie (Supplementary Movie 6), some of whose moments are shown in Supplementary Fig. 13a-f. The different orientations in a and d prove that re-orienting indeed occurred on the particle. But, the size reduction of the particle from 8.1 nm in a to 3.2 nm in e is rather large, and at the same moments the particle projection kept the nearly round shape. In consideration of 3D geometry, it is quite unlikely that sole re-orienting may cause such large size reduction while the nearly round projection shape maintains. Thus, it can be concluded that re-orienting and disintegrating simultaneously occurred on the particle. , and even 2D Au thin rafts, etc. we re-examined more than two hundred HAADF images recorded during our research, and found that 2D Au rafts indeed existed, although they were quite rare due to their metastability. Supplementary The IS is a CH 4 molecule (denoted as CH 4 *) adsorbed on a C atom of the amorphous carbon substrate, where an Au SA has been anchored on the substrate. FS1 indicates that CH 4 * loses an H atom and becomes CH 3 * adsorbed on the Au SA. In FS2, CH 3 * moves to and adsorbs on a C atom of the substrate. In FS3, CH 3 * loses an H atom and becomes CH 2 * adsorbed simultaneously on the C atom and the Au SA. In FS4, CH 2 * loses an H atom and becomes CH* adsorbed simultaneously on the C atom and the Au SA. In FS5, CH* loses an H atom and becomes C* adsorbed simultaneously

Supplementary Note 7
The delocalization effects cause lattice images to extend beyond the edges of the Au crystals and therefore mask some details that are occurring on the surfaces/interfaces. In this article, we mainly focus on the dramatic dynamic changes of NPG structure during the catalytic methane pyrolysis. For example, Fig. 2 shows that at the Au/C interfaces, the surfaces of two Au ligaments lost the thicknesses of 2.6 nm (Fig. 2a-c) and 6.1 nm (Fig. 2d-f) within 2 second. In contrast, the fringe images part extending beyond the Au edge in Fig. 2a-c was only~1  Indeed, for too small evolutions of structures, the delocalization effects can cover up the changes.
To address this issue, a TEM instrument with an image corrector (FEI Themis Z) was used. Its image corrector can eliminate the delocalization effects. As shown in Supplementary Fig. 23, the Au ligament surface indicated by the white arrow is located at the appropriate focal plane, and thus its image does not have the delocalization effect. In contrast, the surface indicated by the black arrow is located at a different height, causing that its image has the delocalization effect. Further, Supplementary Fig. 23 shows that the ligament surfaces are truly clean, and the 5-minute irradiation of the electron beam at 200 keV did not induce obvious structural changes on the surfaces, that is, the surfaces are stable. Therefore, we confirm that in the present research, it was the catalyzed pyrolysis of methane, rather than the electron beam irradiation, that induced the surface structure changes of NPG. The electron dose rate used to take the images is the highest used in our experiments (1570 eÅ -2 s -1 ).
The irradiation duration of 68 seconds is much longer than 0.25 seconds of Supplementary Fig. 24.
In contrast to Supplementary Fig. 24, the images here clearly show that when the electron beam irradiation was kept on with no CH 4 introduction, the NPG ligament surface structure was clean and stable at 346°C for 68 seconds, which is much longer than 0.25 seconds. Moreover, Egerton, R. F. et al. have indicated theoretically that the minimum incident-electron energy to knock out Au atoms is approximately 407 keV, much higher than 200 keV of the electron irradiation used in our work 5 . Therefore, we can conclude that the migration of Au atoms from the NPG surfaces in the in-situ methane pyrolysis of our work is not induced by the electron irradiation, rather it is due to the CH 4 pyrolysis catalysis. that CH 4 can be pyrolyzed by pure gold 6 . In our present work, we are not searching for better-activity catalysts, rather, we use the NPG (with the lowest Ag content) as a nice model system to disclose a highly dynamic process of CH 4 Figs. 4 and 11). Therefore, for all the drastic change processes, Au reasonably plays the dominant role. It should be noted that our control experiments using NPG samples with varied residual Ag contents clearly showed that Ag atoms are capable of improving the catalyst performance and thus contribute to the catalytic activity. In our present study, the key finding is a highly dynamic structure evolution process of a nanostructured catalyst during operation.
We demonstrated that NPG ligaments containing massive atoms may rapidly disintegrate during the catalytic methane pyrolysis process at 346°C. The ligament dimension could remarkably decrease from around 13 nm to 5 nm within 2 seconds (Fig. 2d-f). Considering that the residual Ag content is relatively low (1.37 ± 0.38 at%, whose detailed data are given in the Source Data file) in our sample, it is therefore reasonable to conclude that Au plays a dominant role in this drastic structural evolution process involving massive atoms. This structural evolution includes ligament disintegration, Au single atoms formation, and Au nanoparticle formation. Meanwhile, it should also be noted that heterogeneous doping/alloying of other elements such as Ag into Au may modulate Au's catalytic properties, and in some reactions their roles may become dominant, such as CO oxidation 7 . In our present case, however, this is not the key issue for the following two reasons. First, 35 / 40 it has been well acknowledged that pure Au is capable for catalytic C-H activation and CH 4 pyrolysis, based on both experimental and theoretical studies 6,8 , although its real-time structure evolution process during service has never been observed (which is the contribution of our present work).
Second, we have supplemented the same methane pyrolysis experiments using pure Au nanoparticles, and these pure Au nanoparticles were produced by magnetron sputtering with a pure Au (>99.99%) target (see details in Methods and Supplementary Fig. 27). Our in-situ TEM and hydrogen production studies clearly proved that these pure Au nanoparticles can catalyze the CH 4 pyrolysis under the same reaction conditions (Supplementary Fig. 27). Therefore, in our work, it is reasonable to use the NPG sample with the lowest Ag content to monitor its structural evolution because the effect of Ag may be minimized. Atomic-resolution HAADF image of a carbon region of the sample, showing clearly Au single atoms.
Scale bars: a, 500 nm; b, 5 nm. Our samples for the in situ study were prepared via the standard dealloying method and were not treated with any special method such as plasma treatment. After the in situ TEM experiments, the samples were characterized by HAADF, showing clearly the existence of Au single atoms in the carbon layers ( Supplementary Fig. 29b). This result indicates that the Au single atoms were produced by the disintegration of the NPG surface during the CH 4 pyrolysis. Moreover, we have also prepared similar NPG samples with carbon via the ex situ CH 4 pyrolysis reaction. These samples were also confirmed to contain Au single atoms (Fig. 1f). By comparing both in situ (with electron irradiation) and ex situ (without electron irradiation) results, we reveal a highly dynamic process of how Au catalyzes the CH 4 pyrolysis in real space and time.