Ceria-supported Pd is a promising heterogeneous catalyst for CO oxidation relevant to environmental cleanup reactions. Pd loaded onto a nanorod form of ceria exposing predominantly (111) facets is already active at 50 °C. Here we report a combination of CO-FTIR spectroscopy and theoretical calculations that allows assigning different forms of Pd on the CeO2(111) surface during reaction conditions. Single Pd atoms stabilized in the form of PdO and PdO2 in a CO/O2 atmosphere participate in a catalytic cycle involving very low activation barriers for CO oxidation. The presence of single Pd atoms on the Pd/CeO2-nanorod, corroborated by aberration-corrected TEM and CO-FTIR spectroscopy, is considered pivotal to its high CO oxidation activity.

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A novel microfluidic reactor for in situ small-angle X-ray scattering (SAXS) and X-ray absorption fine structure spectroscopy (XAFS) of Pd colloidal nanoparticles synthesis is reported. The microreactor allows time resolution ranging from milliseconds to several minutes with a residence time of over an hour. The synthesis of colloidal Pd nanoparticles is investigated in the presence of oleylamine and trioctylphosphine ligands. For both ligands, SAXS results show the synthesis proceeds through slow, continuous nucleation as evidenced by a continuous increase in the number of particles. Growth is autocatalytic and fast at the early times; then, despite the availability of a large percentage of unreacted Pd precursor, growth slows dramatically and the Pd nanoparticle diameter reaches a plateau while more nanoparticles continue to form. In situ XAFS reveals an increase in Pd−P coordination coinciding with the time of slowed growth. The combined SAXS and XAFS results strongly suggest that the capping ligands play an important role in slowing growth by binding to the nanoparticle surface leading to a self-limiting nanoparticle size. Despite the slow continuous nucleation and overlapped fast autocatalytic growth, 1 nm Pd nanoparticles with narrow size distribution (±20%) can be synthesized using strong capping ligands (e.g., trioctylphosphine).

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In this Environmental Transmission ElectronMicroscopy (ETEM) study we examined the growth patterns of uniform distributions of nanoparticles (NPs) using model catalysts. Pt/SiO 2 was heated at 550 °C in 560 Pa of O 2 while Pd/carbon was heated in vacuum at 500 °C and in 300 Pa of 5%H 2 in Argon at temperatures up to 600 °C. Individual NPs of Pd were tracked to determine the operative sintering mechanisms. We found anomalous growth of NPs occurred during the early stages of catalyst sintering wherein some particles started to grow significantly larger than the mean, resulting in a broadening of the particle size distribution (PSD). The abundance of the larger particles did not fit the log-normal distribution. We can rule out sample nonuniformity as a cause for the growth of these large particles, since images were recorded prior to heat treatments. The anomalous growth of these particles may help explain PSDs in heterogeneous catalysts which often show particles that are significantly larger than the mean, resulting in a long tail to the right. It has been suggested previously that particle migration and coalescence could be the likely cause for such broad size distributions. We did not detect any random migration of the NPs leading to coalescence. A directed migration process was seen to occur at elevated temperatures for Pd/ carbon under H 2 . This study shows that anomalous growth of NPs can occur under conditions where Ostwald ripening is the primary sintering mechanism.

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This paper reports the successful synthesis of true two-dimensional silicon carbide using a top-down synthesis approach. Theoretical studies have predicted that 2D SiC has a stable planar structure and is a direct band gap semiconducting material. Experimentally, however, the growth of 2D SiC has challenged scientists for decades because bulk silicon carbide is not a van der Waals layered material. Adjacent atoms of SiC bond together via covalent sp3 hybridization, which is much stronger than van der Waals bonding in layered materials. Additionally, bulk SiC exists in more than 250 polytypes, further complicating the synthesis process, and making the selection of the SiC precursor polytype extremely important. This work demonstrates, for the first time, the successful isolation of 2D SiC from hexagonal SiC via a wet exfoliation method. Unlike many other 2D materials such as silicene that suffer from environmental instability, the created 2D SiC nanosheets are environmentally stable, and show no sign of degradation. 2D SiC also shows interesting Raman behavior, different from that of the bulk SiC. Our results suggest a strong correlation between the thickness of the nanosheets and the intensity of the longitudinal optical (LO) Raman mode. Furthermore, the created 2D SiC shows visible-light emission, indicating its potential applications for light-emitting devices and integrated microelectronics circuits. We anticipate that this work will cause disruptive impact across various technological fields, ranging from optoelectronics and spintronics to electronics and energy applications.