Structural changes in noble metal nanoparticles during CO oxidation and their impact on catalyst activity

Abstract

The dynamical structure of a catalyst determines the availability of active sites on its surface. However, how nanoparticle (NP) catalysts re-structure under reaction conditions and how these changes associate with catalytic activity remains poorly understood. Using operando transmission electron microscopy, we show that Pd NPs exhibit reversible structural and activity changes during heating and cooling in mixed gas environments containing O₂ and CO. Below 400 °C, the NPs form flat low index facets and are inactive towards CO oxidation. Above 400 °C, the NPs become rounder, and conversion of CO to CO₂ increases significantly. This behavior reverses when the temperature is later reduced. Pt and Rh NPs under similar conditions do not exhibit such reversible transformations. We propose that adsorbed CO molecules suppress the activity of Pd NPs at lower temperatures by stabilizing low index facets and reducing the number of active sites. This hypothesis is supported by thermodynamic calculations.

Fig. 1. Operando transmission electron microscopy (TEM) of Pd nanocatalysts during CO oxidation.

a Schematic of the experimental setup where metal nanoparticles (NPs) are synthesized within a microfabrication gas cell with an integrated thin-film heater. The reactant gases after passing over the catalysts are analyzed using an inline mass spectrometer. For enlarged images of panels 2, 5, and 6, see Supplementary Fig. 1a. b Sequence of TEM images of a Pd NP under different gas environments and temperatures. c Changes in gas composition (carrier gas is He) obtained from the inline mass spectrometer during temperature changes between 300 and 600 °C. These results correspond to panels 3–6 in the red box shown in b.

Fig. 2. Detailed results for the experiment described in Fig. 1.

a Higher magnification images of the NP in Fig. 1b at 200 °C and under 760 Torr of 9% CO, 18% O₂, and 73% He $\left(\frac{P_{\mathrm{CO}}}{P_{{\mathrm{O}}_2}}=0.5\right)$, and its corresponding fast Fourier transform. The low index facets are highlighted with dashed lines of different colors that correspond to reflections in the Fourier transform. Inset further magnifies the step structure at the vicinal facet on the lower right of the NP. b Higher magnification image of the NP after heating to 500 °C under 760 Torr of 9% CO, 18% O₂, and 73% He gas. Notice the absence of sharp corners in the NP and the steps on the vicinal facet. c Image sequence extracted from a movie recorded during the temperature ramp from 300 to 500 °C (Supplementary Movie 1). The sequence shows the NP changing from a faceted morphology to a rounded one as the temperature increases. d Plots of the measured temperature (orange curve) and heater power (green curve) during the temperature ramp of 2 °C/s. The inset highlights a spike in the measured temperature and the corresponding reduction in heater power needed to compensate for the exothermic release of heat from the NPs (y-axes have been re-plotted for the inset to accommodate both curves). For enlarged images of a and b, see Supplementary Fig. 1a.

Fig. 3. Pd NPs imaged in a gas environment with CO to O₂ ratio of 1.6.

a Measured temperature and heater power profile from the experiment. The inset highlights the temperature spike around the ignition point and the corresponding reduction in heater power expanded from the region demarcated by the dashed box (y-axes have been re-plotted for the inset to accommodate both curves). b Corresponding measurements of the CO, O₂, and CO₂ content in the outlet gas stream during the experiment. c Image sequence describing the morphological changes in a Pd NP at different temperatures in the experiment. Dashed lines highlight the outline of the NP where yellow lines correspond to {111} facets, red lines to {100} facets, and white lines to facets that cannot be indexed from the individual images’ fast Fourier transform.

Fig. 4. A comparison of Pd, Pt, and Rh NPs during CO oxidation.

TEM image sequences of ~10-nm a Pd, b Pt, and c Rh NPs under similar reactions conditions $\left(\frac{P_{\mathrm{CO}}}{{\mathrm{P}}_{O_2}}=0.5\right)$. The Pd NP exhibits the same behavior as other NPs described so far in the manuscript (enlarged images of the NP at 200, 500, and 300 °C are provided in Supplementary Fig. 1b). Dashed lines highlight the outline of the Pd NP where yellow lines correspond to {111} facets, red lines to {100} facets, and white lines to facets that cannot be indexed from the individual images’ fast Fourier transform. The Pt NP only showed subtle changes in its morphology. Light blue dashed lines indicate possible faceting on the Pt NPs at higher temperatures. The Rh NP changes from a faceted oxide NP to a more rounded metallic NP as it reduced oxide to metal over the course of the experiment. The FFTs shown in Supplementary Fig. 11a indicate the presence of both {100} and {111} facets in the reduced state.