The controlled disassembly of mesostructured perovskites as an avenue to fabricating high performance nanohybrid catalysts

Versatile superstructures composed of nanoparticles have recently been prepared using various disassembly methods. However, little information is known on how the structural disassembly influences the catalytic performance of the materials. Here we show how the disassembly of an ordered porous La0.6Sr0.4MnO3 perovskite array, to give hexapod mesostructured nanoparticles, exposes a new crystal facet which is more active for catalytic methane combustion. On fragmenting three-dimensionally ordered macroporous (3DOM) structures in a controlled manner, via a process that has been likened to retrosynthesis, hexapod-shaped building blocks can be harvested which possess a mesostructured architecture. The hexapod-shaped perovskite catalyst exhibits excellent low temperature methane oxidation activity (T90%=438 °C; reaction rate=4.84 × 10−7 mol m−2 s−1). First principle calculations suggest the fractures, which occur at weak joints within the 3DOM architecture, afford a large area of (001) surface that displays a reduced energy barrier for hydrogen abstraction, thereby facilitating methane oxidation.

S-6 X-ray-diffraction (XRD) spectroscopy X-ray diffraction (XRD) analyses were performed on a PANalytical Empyrean II diffractometer with Cu Kα radiation (λ = 0.15406 nm) at 45 kV and 40 mA to identify the crystal phases and to determine the lattice parameters. Scattering intensity was recorded in the range of 8 o < 2θ < 90 o for all samples with a 2θ step of 0.03 o and a count time of 2 s per step. The diffraction patterns were indexed to JCPDS (Joint Committee on Powder Diffraction Standards) files.

Field-emission high-resolution scanning electron microscopy (FE-HRSEM)
Morphologies of the as-prepared samples were observed by scanning electron microscopy (SEM) and field-emission high resolution scanning electron microscopy (FE-HRSEM) using a FEI Nova NanoSEM 450 FE-SEM microscope at an accelerating voltage of 5 kV, with a work distance (WD) between 5 and 8 mm and magnifications in the range of 100−800000.
HRSEM images were obtained using the back-scattered electron detector (BSED). The secondary electron detector (SED) modes show morphologies of the surface structures. To determine chemical compositions of the crystal phases, energy-dispersive spectroscopy (EDS) was used to obtain the EDS spectra by means of an EDS DX-4 analysis system. Prior to analysis the samples were coated with a thin Cr film to provide better conductivity of the sample, for example for the PMMA microspheres.

Field-emission high resolution transmission electron microscopy (FE-HRTEM)
Field-emission high resolution transmission electron microscopic (FE-HRTEM) images as well as the selected-area electron diffraction (SAED) patterns of the typical samples were obtained on a Philips CM200 apparatus. For identifying chemical compositions of the crystal phases, high-angle annular dark-field -scanning transmission electron microscopy energydispersive X-ray spectroscopy (HAADF-STEM-EDS) was employed using a JEOL JEM-S-7 ARM200F STEM to obtain the EDS spectra.

Specific surface area and pore size distribution
The specific surface areas and pore size distributions were obtained using the N 2 physisorption method in conjunction with the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) models, respectively. The N 2 adsorption-desorption isotherms, surface areas, and pore diameters of the samples were determined via N 2 adsorption at 196 o C on a Micromeritics Tristar 3030 adsorption analyser. Before measurement, the samples were degassed at 150 o C for 3 h.

Oxygen temperature-programmed desorption (O 2 TPD)
For the O 2 TPD analysis, 20 mg of freshly prepared sample was loaded into a quartz Ushaped reactor and placed in a Micromeritics Autochem II apparatus equipped with a thermal conductivity detector (TCD) detector. The samples were initially treated with helium at a flow rate of 30 mL/min as the temperature was increased at a rate of 10 °C/min to 150 °C where it was held at this temperature for 30 min (to remove moisture) and then cooled to 50 °C. Then, pure O 2 at a flow rate of 30 mL/min was introduced to the sample, following by 30 mL/min helium flushing. After 30 min equilibrium, the temperature ramped at ramp rate of 10 °C/min from room temperature to 900 °C. The variation in O 2 concentration from the cell effluent was monitored on-line by the TCD detector.

X-ray photoelectron spectroscopy (XPS)
X-ray photoelectron spectroscopy (XPS) was used to determine the La 3d, Mn 2p 3/2 , O 1s, and C 1s binding energies (BEs) of surface species. The analyses were performed on a    Arrhenius plots of methane conversion by the as-prepared catalysts. The oxidised Mn δ+ species are observed to be more active than the metallic (i.e. reduced) Mn 0 species for methane oxidation ( Supplementary Fig. 18). The (001) and (110)  Adsorption energies were calculated using symmetric (9 layer) and asymmetric (8 layer) slabs. To mimic the random distribution of Sr in LSMO, special quasi-random structures (SQS) were used. These were generated using the Monte Carlo SQS tool within the Alloy Theoretical Automated Toolkit (ATAT) 2 . The binding energies of methane at various sites were calculated using Supplementary Equation 1: configurations. Migration barriers were lower on the (001) surface than the (110) surface.

Supplementary
Below are the lowest energy paths on each surface. Supplementary Fig. 19a shows the presence of metastable sites along the migration pathway.
To further resolve the activation barrier on the (001) surface we considered the metastable site as the starting point for the CI-NEB calculation (Supplementary Fig. 19b). Using nine images the refined pathway is about 1.01 eV which is very close to the value obtained originally.
The successive dehydrogenation steps on the (001) surface are shown in Supplementary Fig.   20. The first dehydrogenation step has a higher energy barrier compared to the other steps and thus, constitutes the rate determining step. This observation is in agreement with previous theoretical studies. [4][5][6][7] However, the activation barriers are close in energy to each other which could imply the presence of multiple rate determining steps.

Weisz-Prater analysis
The mass and heat transfer limitations were studied by selecting the most active 3D-hm LSMO catalyst at the CH 4 conversion of 50 % and temperature 360 o C with a GHSV of 50 000 mL/(g h). Supplementary Table 1 lists the parameters used in the calculations. The absence of internal mass diffusion transport resistance was checked by the Weisz-Prater Criterion (N W-P ) 8 .
Based on the above analyses, no significant mass transfer limitation are present under the utilised reaction conditions.

Reaction rate calculation
The method for estimating metal oxide specific reaction rate was similar to that reported by