Relating adatom emission to improved durability of Pt–Pd diesel oxidation catalysts

Abstract

a b s t r a c tSintering of nanoparticles is an important contributor to loss of activity in heterogeneous catalysts, such as those used for controlling harmful emissions from automobiles. But mechanistic details, such as the rates of atom emission or the nature of the mobile species, remain poorly understood. Herein we report a novel approach that allows direct measurement of atom emission from nanoparticles. We use model catalyst samples and a novel reactor that allows the same region of the sample to be observed after short-term heat treatments (seconds) under conditions relevant to diesel oxidation catalysts (DOCs). Monometallic Pd is very stable and does not sinter when heated in air (T 6 800°C). Pt sinters readily in air, and at high temperatures (P800°C) mobile Pt species emitted to the vapor phase cause the formation of large, faceted particles. In Pt-Pd nanoparticles, Pd slows the rate of emission of atoms to the vapor phase due to the formation of an alloy. However, the role of Pd in Pt DOCs in air is quite complex: at low temperatures, Pt enhances the rate of Pd sintering (which otherwise would be stable as an oxide), while at higher temperature Pd helps to slow the rate of Pt sintering. DFT calculations show that the barrier for atom emission to the vapor phase is much greater than the barrier for emitting atoms to the support. Hence, vapor-phase transport becomes significant only at high temperatures while diffusion of adatoms on the support dominates at lower temperatures.

Preparation of CuO/CeO2 Catalyst with Enhanced Catalytic Performance for Water–Gas Shift Reaction in Hydrogen Production

“…From the XRD patterns of the CuO/CeO 2 catalysts after catalytic stability test (Figure S2, Supporting Information), the diffraction peaks of metallic Cu are sharp and intense, and the calculated Cu crystal sizes are larger than those of the reduced catalysts (Table ), indicating the agglomeration of Cu nanoparticles occurs during the thermal treatment. The agglomeration process can be explained by the Ostwald ripening mechanism, that is, the emission of mobile species from small particles and capture by large particles. Jones et al .…”

“…From the XRD patterns of the CuO/CeO 2 catalysts after catalytic stability test ( Figure S2, Supporting Information), the diffraction peaks of metallic Cu are sharp and intense,a nd the calculated Cu crystal sizes are larger than those of the reduced cata- (Table 1), indicating the agglomeration of Cu nanoparticles occurs during the thermal treatment. Thea gglomeration process can be explained by the Ostwald ripening mechanism, [35] that is,t he emission of mobile species from small particlesa nd captureb yl arge particles.J ones et al [36] reported that ceria with differente xposed surface facets has different impact on anchoring the mobile metal. However, in these cases,t he surface facets of ceria are the same for all the three catalysts,t herefore the different degrees of agglomerationa re attributed to their distincta bility of CeO 2 to prevent the emission of small Cu nanoparticles.A ss een in the XRD results,t he lattice strain values of the CuO/CeO 2 catalysts followt he order of CuCe-30N > CuCe-80 > CuCe-80N, which is the same as their catalyst stabilities.T herefore, the strong CuO-CeO 2 synergetic interaction is believed to be effective in anchoring the Cu nanoparticles on the support, and thusi mproving the thermal stability of CuO/CeO 2 catalyst in the WGS reaction.…”

Three‐Dimensional Probing of Catalyst Ageing on Different Length Scales: A Case Study of Changes in Microstructure and Activity for CO Oxidation of a Pt–Pd/Al2O3 Catalyst

“…Previous studies have shown that Pt is very mobile at high temperatures in oxidising environments and can be effectively trapped by PdO nanoparticles ,. The studies demonstrated that upon heat treatment of specimens with initially separated Pt and PdO nanoparticles, the highly mobile Pt is trapped by the more stable PdO nanoparticles and alloyed Pt–Pd nanoparticles are formed, which are thermodynamically more stable and slow down further sintering by Ostwald ripening as well as the emission of Pt into the vapour phase . It has also been shown that bulk PdO basically does not sinter at temperatures below the oxide decomposition temperature which lies above 800 °C, because sintering mainly occurs in the metallic form …”

“…If the Pd was fixed on the surface and only Pt would sinter and be trapped by PdO nanoparticles, there should be a larger number of smaller nanoparticles after ageing (larger nanoparticles than in the fresh sample, but much smaller than what we observe). The reason behind the sintering of Pd is that it is not present in the form of PdO, but alloyed with Pt, which keeps it in a metallic state ,. In this way, Pt increases the rate of the Pd sintering.…”

In‐situ Transmission Electron Microscope Techniques for Heterogeneous Catalysis

“…After the oxidation treatment, Pt nanoparticles were successfully redispersed into a large number of smaller nanoparticles, but Pd preferred sintering into ultra‐large particles. The difference is mainly attributed to the chemical stabilities of Pt and Pd oxides and the energy barrier for their diffusion to the vapor phase (1.6 eV for PtO in contrast to 4.2 eV for PdO) . In the case of Ag catalysts for soot oxidation, the situation was more complicated.…”