Colloidally Engineered Pd and Pt Catalysts Distinguish Surface- and Vapor-Mediated Deactivation Mechanisms

Abstract

Noble metal-based catalysts are ubiquitous because of their high activity and stability. However, they irreversibly deteriorate over time especially in high-temperature applications. In these conditions, sintering is the main reason for deactivation, and understanding how sintering occurs gives the opportunity to mitigate these detrimental processes. Previous studies successfully distinguished between two fundamental sintering modes, namely, particle migration and coalescence (PMC) and Ostwald ripening (OR). However, differentiation between surface- and vapor-mediated Ostwald ripening processes has not been demonstrated yet, even though it is crucial information to tune metal/support interactions and stabilize catalysts. Here, we demonstrate that surface- and vapor-mediated ripening occur in two distinct regimes of temperature with some overlap using Pt and Pd catalysts prepared from colloidal nanocrystals as precursors. By either co-impregnating the two metal nanocrystals on the same grain of alumina support or by physically mixing powders of the two distinct metal catalysts, we tune the intermetal particle distance between nanometers and micrometers. We then use methane complete oxidation as a reporter reaction that occurs at higher rates on pure Pd and lower rates on alloyed Pd/Pt catalysts to trace the movement of Pt in the system. Aging the catalysts at different temperatures allows us to reveal that Pt initially sinters by surface-mediated ripening until ∼750 °C, but at temperatures above 800 °C, vapor-mediated ripening by PtO2 becomes the main sintering mechanism. This work demonstrates how colloidal catalysts allow unique insights into the working and deactivation mechanisms of supported systems.

Biphasic Janus Particles Explain Self-Healing in Pt–Pd Diesel Oxidation Catalysts

“…Such a reaction is plausible given the similar coordination of Pt and Pd in their oxides (see figure S4 showing the structure) and similar ionic sizes for Pt and Pd. Support for such a reaction also comes from recent DFT calculations reported by Oh et al 59 In this work, the authors studied the adsorption of PtO 2 on various surfaces of PdO. They found adsorption to be favorable on PdO (101) provided the surface had vacancies.…”

Palladium Catalysts for Methane Oxidation: Old Materials, New Challenges

“…Pt is known to sinter through the formation of volatile platinum oxides. , At high temperatures in oxygen, PdO can trap mobile Pt species and form Pd/Pt alloys with larger particle sizes. , When Pt alloys with Pd, a drastic decrease in the methane oxidation rate is observed . We hypothesized that we could use the rates of methane oxidation on a Pd catalyst to infer whether Pt was sintering and alloying …”

“…47 We hypothesized that we could use the rates of methane oxidation on a Pd catalyst to infer whether Pt was sintering and alloying. 57 We prepared a coimpregnated catalyst, where Pd and Pt nanoparticles were deposited on the same support grain (Figure 7a,c,d), and a physical mixture sample, where two distinct supported Pd and Pt catalysts were physically mixed (Figure 7b,e,f). In this way, we surmised that surface-and vapormediated ripening could occur in the coimpregnated catalyst, but only surface-mediated ripening could happen in the physical mixture due to the diffusion barrier imposed by the support grains.…”

Controlling the Strong Metal‐Support Interaction Overlayer Structure in Pt/TiO2Catalysts Prevents Particle Evaporation

“…[9][10][11] Those volatile species form at high temperatures because solid PtO 2 becomes thermodynamic unfavored in oxygen at high temperatures and causes the evaporation of the platinum NPs. [12][13][14][15][16][17][18] Thus, treatment strategies that allow a catalyst to be exposed to oxygen at high temperatures, but which prevent particle sintering, have the potential to be employed in a large range of catalytic applications. Exploiting the restructuring of catalysts by targeted (pre)treatment is a key concept of virtually all catalytic processes.…”

“…Loss in active surface area of the noble metal component either due to Ostwald ripening or formation of volatile species is detrimental for catalyst [9–11] . Those volatile species form at high temperatures because solid PtO 2 becomes thermodynamic unfavored in oxygen at high temperatures and causes the evaporation of the platinum NPs [12–18] . Thus, treatment strategies that allow a catalyst to be exposed to oxygen at high temperatures, but which prevent particle sintering, have the potential to be employed in a large range of catalytic applications.…”