In situ oxidation study of Pd–Rh nanoparticles on MgAl₂O₄(001)

Abstract

Alloy nanoparticles on oxide supports are widely used as heterogeneous catalysts in reactions involving oxygen. Here we discuss the oxidation behavior of Pd-Rh alloy nanoparticles on MgAl2O4(001) supports with a particle diameter from 6-11 nm. As an In situ tool, we employed high energy grazing incidence X-ray diffraction at a photon energy of 85 keV. We find that physical vapor deposited Pd-Rh nanoparticles grow epitaxially on MgAl2O4(001) with a truncated octahedral shape over the whole concentration range. During our systematic oxidation experiments performed at 670 K in the pressure range from 10(-3) to 0.1 mbar, we observe for Rh containing nanoparticles the formation of two different Rh oxide phases, namely RhO2 and a spinel-like Rh3O4 phase. PdO formation is only observed for pure Pd nanoparticles. This oxidation induced segregation behavior is also reflected in the oxidation induced enlargement of the average nanoparticle lattice parameter towards to value for pure Pd. Our results have ramifications for the phase separation behavior of alloy nanocatalysts under varying reducing and oxidizing environments.

In Situ Optical Reflectance Difference Observations of CO Oxidation over Pd(100)

“…More realistic model catalysts consisting of supported nanoparticles were also used for CO oxidation studies. 15,19−26 Even though the first paper on CO oxidation was published as early as 1957, 27 novel aspects of this relatively simple reaction are still discovered frequently.…”

Engineering Functions into Platinum and Platinum–Rhodium Nanoparticles in a One‐Step Microwave Irradiation Synthesis

“…[35][36][37] Similarly,l arger lattice constant values werer eportedf or Rh NPs for sizes < 10 nm, and that was attributed to the surface strain of such small NPs. [38] Upon increasingt he microwave irradiation time to 7, 10, 15, and 20 min, the XRD 2 q peaks are shiftedt oh ighera ngles relative to the values of pure Pt, and the (111)p lane is found at 2 q = 39.9, 40.1, 40.2, and 40.38,r espectively.T he lattice parameter values of the produced NPs at the longer microwave times were calculated by using TOPAS software and are listed in Ta ble 2.…”

“…The lattice parameters of the pure Pt and Rh NPs were calculated by using TOPAS software to be (3.9171±0.0001) and (3.8351±0.0001) Å, respectively, which deviate from the reported bulk values (Pt bulk 3.9231 Å and Rh bulk 3.8032 Å) . Similarly, larger lattice constant values were reported for Rh NPs for sizes <10 nm, and that was attributed to the surface strain of such small NPs …”

High-energy x-ray diffraction from surfaces and nanoparticles

“…The high-energy reciprocal-space map (E = 85 keV) included in Fig. 5(a) was measured on such a sample system with epitaxial Rh nanoparticles grown on MgAl 2 O 4 (001) [20]. The map was probed in a single snapshot with the sample aligned to the (111) Bragg peak of the untilted (here: orange) particles.…”

“…They also involve time-consuming sample and detector movements. * uta.hejral@sljus.lu.se † andreas.stierle@desy.de High-energy surface x-ray diffraction (HESXRD) is a novel tool to investigate the atomic structure of surfaces, interfaces, and ensembles of nanoparticles [16][17][18][19][20], which became feasible with the advent of intense, microfocused hard x-ray beams of high-photon energy (E > 70 keV) at third-generation synchrotron radiation sources [21]. In combination with a large two-dimensional detector, it allows, without scanning the detector, for an immediate and nearly distortion-free mapping of vertical planes in reciprocal space, also containing, among others, CTR and nanoparticle facet diffraction signals.…”