Pd-Ceria/CNMs Composites as Catalysts for CO and CH4 Oxidation

Abstract

The application of composite materials as catalysts for the oxidation of CO and other toxic compounds is a promising approach for air purification. In this work, the composites comprising palladium and ceria components supported on multiwall carbon nanotubes, carbon nanofibers and Sibunit were studied in the reactions of CO and CH₄ oxidation. The instrumental methods showed that the defective sites of carbon nanomaterials (CNMs) successfully stabilize the deposited components in a highly-dispersed state: PdO and CeO₂ nanoparticles, subnanosized PdO_x and Pd_xCe_1-xO_2-δ clusters with an amorphous structure, as well as single Pd and Ce atoms, are formed. It was shown that the reactant activation process occurs on palladium species with the participation of oxygen from the ceria lattice. The presence of interblock contacts between PdO and CeO₂ nanoparticles has an important effect on oxygen transfer, which consequently affects the catalytic activity. The morphological features of the CNMs, as well as the defect structure, have a strong influence on the particle size and mutual stabilization of the deposited PdO and CeO₂ components. The optimal combination of highly dispersed PdO_x and Pd_xCe_1-xO_2-δ species, as well as PdO nanoparticles in the CNTs-based catalyst, makes it highly effective in both studied oxidation reactions.

Keywords: CO oxidation; carbon nanomaterials; ceria; composite catalysts; palladium.

Figure A1

TEM data for Pd-CeO₂/Al₂O₃: (a,b) HAADF-STEM image and corresponding EDX-mapping pattern; (c) HRTEM image with FTT of the selected region shown in the inset; the interplanar distances corresponding to PdO and CeO₂ are indicated; (d,e) HAADF-STEM image and corresponding EDX-mapping pattern. The square mark in the figure (a) indicates the region of the sample shown in the HRTEM image (c). The EDX elemental maps use the following colors: blue for Al, red for Pd and green for Ce. The maps are presented in background-corrected intensities.

Figure A2

The Pd3d spectra of (a) Pd-CeO₂/CNFs and (b) Pd-CeO₂/Sibunit samples.

Figure A3

Catalytic data for Pd-CeO₂/CNTs catalyst: (a) Temperature dependence of O₂ consumption and CO, CO₂ and H₂O evolution during temperature-programmed oxidation (TPO) in 1%O₂/He; (b) Temperature dependences of CH₄, O₂ conversion and CO, H₂O and CO₂ evolution during CH₄ oxidation; (c) Time dependence of CH₄ conversion at 350 °C.

Figure 1

X-ray diffraction patterns of Pd-CeO₂/CNTs (a), Pd-CeO₂/CNFs (b), Pd-CeO₂/Sibunit (c) and Pd-CeO₂/Al₂O₃ (d) shown as black curves. X-ray diffraction patterns of the corresponding supports are shown in blue; the scattering contribution from the PdO phase is shown in red. The *-sign indicates the reflexes of the CeO₂ phase.

Figure 2

TEM images of (a) Pd-CeO₂/CNTs, (b) Pd-CeO₂/CNFs, (c) Pd-CeO₂/Sibunit and (d) Pd-CeO₂/Al₂O₃. The images are presented at the same magnification.

Figure 3

TEM data for Pd-CeO₂/CNTs catalyst: (a) HRTEM image; (b,c) HAADF-STEM image of the region marked in figure (a), and a corresponding EDX-mapping pattern showing the distribution of carbon (blue), palladium (red) and cerium (green) in the selected region; (d) HAADF-STEM image showing an agglomerate of PdO and CeO₂ nanoparticles on the CNTs’ surface. The insets in figure d show the images of selected areas with brightness/contrast correction for better visualization of single Pd and Ce atoms present on the surface of CNTs.

Figure 4

TEM data for Pd-CeO₂/CNFs catalyst: (a) TEM image; (b) HRTEM image of the region marked in figure (a); (c) HRTEM image and an EDX-mapping pattern (in inset) of the same region; (d–f) TEM image, corresponding EDX-mapping pattern and a HAADF-STEM image of the marked region. The following colors are used for the EDX elemental maps: blue for carbon, red for palladium and green for cerium. The maps are presented in background-corrected intensities. The yellow circles indicate single Pd and Ce atoms on the CNFs surface.

Figure 5

TEM data for Pd-CeO₂/Sibunit: (a–d) HAADF-STEM images and corresponding EDX-mapping patterns; (e) I HRTEM image; (f) HAADF-STEM image, the yellow circles indicate single Pd and Ce atoms on Sibunit surface. The EDX elemental maps use the following colors: blue for carbon, red for palladium and green for cerium. The maps are presented in background-corrected intensities.

Figure 6

The Pd3d spectra of (a) Pd-CeO₂/CNTs and (b) Pd-CeO₂/Al₂O₃ samples.

Figure 7

Temperature dependencies of (a) CO and (b) CH₄ conversion for Pd-CeO₂/CNTs, Pd-CeO₂/CNFs, Pd-CeO₂/Sibunit and Pd-CeO₂/Al₂O₃ catalysts.

Figure 8

CO₂ evolution during TPR-CO for Pd-CeO₂/CNTs, Pd-CeO₂/CNFs, Pd-CeO₂/Sibunit, Pd-CeO₂/Al₂O₃ catalysts and for CeO₂/CNTs sample. The inset shows the initial section of CO₂ evolution curves in the temperature range of −40–100 °C.

Figure 9

A scheme showing the main types of palladium and ceria species formed on carbon supports: the PdO_x clusters and Pd_xCe_1−xO_2−δ particles provide activity in low-temperature CO oxidation; the PdO nanoparticles are essential for CH₄ oxidation.