Microstructure of Bimetallic PtPd Catalysts under Oxidizing Conditions

Abstract

Diesel oxidation catalysts (DOCs), which decrease the amount of harmful carbon monoxide (CO), nitrogen oxide (NO), and hydrocarbon (HC) emissions in engine exhaust, typically utilize Pt and Pd in the active phase. There is universal agreement that the addition of Pd improves both the catalytic performance and the durability of Pt catalysts. However, the mechanisms by which Pd improves the performance of Pt are less clear. Because these catalysts operate under oxidizing conditions, it is important to understand these catalysts in their working state. Herein, we report the microstructure of PtPd catalysts that are aged in air at 750 °C. After 10 h of aging, EXAFS and XANES analysis show that the Pt is fully reduced but that almost 30 % of the Pd species are present as an oxide. HRTEM images show no evidence of surface oxides on the metallic PtPd particles. Instead, the PdO is present as a separate phase that is dispersed over the alumina support. Within the metallic particles, Pt and Pd are uniformly distributed and there is no evidence of core–shell structures. Therefore, the improved catalytic performance is likely associated with the co‐existence of metallic Pt and Pd on the catalyst surface.

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

“…The compositions of the Pt-Pd particles of these regions on the model catalysts are included in Fig. 6, and the observations are similar to those reported by us previously on a supported catalyst [12], where EXAFS, XRD, and STEM-EDS confirmed that the composition of the individual particles in the aged catalyst mirrors that of the initial catalyst. The results show clearly that Pt and Pd mobile species are emitted from these nanoparticles and the rates of their transport are similar, leading to similar compositions of the larger particles that are formed.…”

“…Accelerated aging of Pt catalysts is generally carried out in air at atmospheric pressure where we see rapid sintering at elevated temperatures [12,13]. Furthermore, air aging leads to similar catalyst deactivation as that seen in vehicle aging [14].…”

“…This apparent ''self-assembly'' of Pt and Pd into bimetallic Pt-Pd particles occurs because PdO acts as a trap for the mobile Pt species. Therefore, metallic Pt-Pd particles are formed under conditions where Pd by itself would transform into stable PdO, and this is why the working state of Pt-Pd diesel oxidation catalysts consists of metallic Pt-Pd alloy particles [12]. What is not understood is how the presence of Pd slows the rates of sintering in these catalysts.…”

Boron Switch for Selectivity of Catalytic Dehydrogenation on Size-Selected Pt Clusters on Al2O3

“…Doping and alloying can be used to tune the properties of bulk Pt. Alloying Pt with Sn 9 and Zn 10,11 has been used for selectivity control and with Pd to reduce sintering 12,13 . Here, our inspiration is drawn from the boration (boron--doping) of extended surfaces of Co and Ni, used in Fischer--Tropsch synthesis and steam me--thane reforming, respectively.…”

“…A similar effect is observed if the Pt 7 /alumina is exposed to a saturation dose of D 2 then heated to 700 K (not shown), consistent with the ISS suggesting thermal restructuring causing a small reduc--tion in the number of exposed Pt sites. 7 /alumina samples that were first exposed to a particular manipulation and then probed by CO TPD (10 L 13 CO expo--sure at 150 K, heating at 3 K/sec to 700 K).…”