Tracking the shape-dependent sintering of platinum–rhodium model catalysts under operando conditions

Abstract

Nanoparticle sintering during catalytic reactions is a major cause for catalyst deactivation. Understanding its atomic-scale processes and finding strategies to reduce it is of paramount scientific and economic interest. Here, we report on the composition-dependent three-dimensional restructuring of epitaxial platinum–rhodium alloy nanoparticles on alumina during carbon monoxide oxidation at 550 K and near-atmospheric pressures employing in situ high-energy grazing incidence x-ray diffraction, online mass spectrometry and a combinatorial sample design. For platinum-rich particles our results disclose a dramatic reaction-induced height increase, accompanied by a corresponding reduction of the total particle surface coverage. We find this restructuring to be progressively reduced for particles with increasing rhodium composition. We explain our observations by a carbon monoxide oxidation promoted non-classical Ostwald ripening process during which smaller particles are destabilized by the heat of reaction. Its driving force lies in the initial particle shape which features for platinum-rich particles a kinetically stabilized, low aspect ratio.

Combining synchrotron light with laser technology in catalysis research

“…This beamline is designed for material studies with hard X-ray radiation (King et al, 2014). The high-energy X-rays enable full surface-structure determination, where each detector image contains the projection of a large area of the reciprocal space, on a time scale suitable for operando studies Shipilin et al, 2014Shipilin et al, , 2016Hejral et al, 2016Hejral et al, , 2017. The incident angle of the X-rays was set to 0.04 , close to the critical angle of Pd(100) at the photon energy used (85 keV).…”

Time Resolved Operando X-ray Techniques in Catalysis, a Case Study: CO Oxidation by O2 over Pt Surfaces and Alumina Supported Pt Catalysts

“…Approaches Using High Energy (>50 keV): SXRD and Total X-ray Scattering/PDF High energy scattering methods are also coming of age, as hopefully the study shown in Section 2.7 has made clear in the case of supported Pt catalysts for fast PDF studies. However, SXRD [177,178] made upon metal surfaces and a sophisticated variant of it due to Stierle and co-workers [179][180][181][182] using high energy X-rays to investigate supported nanoparticles has much to offer; and there are good reasons to think that in the near future both high energy PDF, XRD and even high energy SAXS measurements are going to be able to play ever more important roles in the wider scheme of catalytic science.…”

“…If one then combines this general idea with a combinatorial (graduated particle sizes [180] or compositions [182]) as Stierle and co-workers have succeeded in doing, then a very powerful, structurally deterministic operando approach rises. Importantly in these cases, crystal truncation rods that arise from the arrays of selected and supported metal nanoparticles used in such studies, become measurable.…”

“…From these, precise assertions regarding the morphology-perhaps the most difficult aspect of nanoparticle structure to be precise about-and structural behavior of these nanoparticles in operando (using the evolution of the reactor due to Ferrer et al in its commercialized flow version [118,119]) becomes possible. Indeed, in [182] this has even been achieved for arrays of PtRh nanoparticles undergoing CO oxidation to show that the more platinum rich these nanoparticles are the more structurally labile they become in terms of morphology and sintering, and that this structural fluidity is related to the energy released into the particles by the CO oxidation events themselves.…”

Chemically Induced Sintering of Nanoparticles

“…Sintering of nanometer-sized noble metal particles accounts for many important chemical phenomena in industrial production, such as inkjet printing of electronic circuits [1][2][3] and deactivation of nanostructured heterogeneous catalysts. [4][5][6][7][8] Nanoparticles are prone to sinter or coalesce into larger particles even under mild conditions owing to their large surface-to-volume ratio and consequent high surface energy.The growth of nanoparticles is acomplex physical and chemical process governed by multiple factors,i ncluding particle size,metal type,temperature,chemical environment, and nature of the substrate.D espite extensive research on sintering over the past decade,t he detailed mechanisms of sintering,i np articular chemical circumstances (e.g., in oxidative vs.r eductive atmospheres), are still far from fully understood. [9,10] Ad eep understanding of the mechanisms of nanoparticle sintering (and the consequent changes in plasmonic behavior) in the presence of reactive gases will permit the development of new applications of this interesting chemical process in areas ranging from microelectronics fabrication, 3D printing,a nd heterogeneous catalysis to applications in chemical sensing and environmental monitoring [11] and even facial recognition.…”