Uncovering Structure-Activity Relationships in Pt/CeO2 Catalysts for Hydrogen-Borrowing Amination

The hydrogen-borrowing amination of alcohols is a promising route to produce amines. In this study, experimental parameters involved in the preparation of Pt/CeO₂ catalysts were varied to assess how physicochemical properties influence their performance in such reactions. An amination reaction between cyclopentanol and cyclopentylamine was used as the model reaction for this study. The Pt precursor used in the catalyst synthesis and the properties of the CeO₂ support were both found to strongly influence catalytic performance. Aberration corrected scanning transmission electron microscopy revealed that the most active catalyst comprised linearly structured Pt species. The formation of these features, a function result of epitaxial Pt deposition along the CeO₂ [100] plane, appeared to be dependent on the properties of the CeO₂ support and the Pt precursor used. Density functional theory calculations subsequently confirmed that these sites were more effective for cyclopentanol dehydrogenation-considered to be the rate-determining step of the process-than Pt clusters and nanoparticles. This study provides insights into the desirable catalytic properties required for hydrogen-borrowing amination but has relevance to other related fields. We consider that this study will provide a foundation for further study in this atom-efficient area of chemistry.

Effect of oxygen storage materials on the performance of Pt-based three-way catalysts

Pt supported on oxygen storage materials (CeO2 and CeO2–ZrO2) as effective three-way catalysts.

Oxygen chemistry of halogen-doped CeO2(111)

We study substitutional fluorine, chlorine and bromine impurities at CeO₂(111), and their effects on the oxygen chemistry of the surface, using density functional theory. We find that impurity formation results in a halide ion and one Ce³⁺ ion for all three halogens, although the formation energy depends strongly on the identity of the halogen; however, once formed, all three halogens exhibit a similar propensity to form impurity-impurity pairs. Furthermore, while the effects of halogen impurities on oxygen vacancy formation are marginal, they are more significant for oxygen molecule adsorption, due to electron transfer from the Ce³⁺ ion which results in an adsorbed superoxide molecule. We also consider the displacement of a halide ion on to the surface by half of an oxygen molecule, and find that the energy required to do so depends strongly not only on the identity of the halogen, but also on whether or not a second halogen impurity, with its associated Ce³⁺ ion, is present; if it is, then the process is greatly facilitated. Overall, our results demonstrate the existence of a rich variety of ways in which the oxygen chemistry of CeO₂(111) may be modified by the presence of halogen dopants.

Mechanistic study and catalyst development for selective carbon monoxide methanation

As for selective CO methanation over heterogeneous catalysts, numerous investigations of the reaction mechanism and catalyst development are reviewed.

Reduction of NO by CO using Pd-CeTb and Pd-CeZr catalysts supported on SiO2 and La2O3-Al2O3

The catalytic activity of Pd catalysts supported on Ce0.73Tb0.27Ox/SiO2, Ce0.6Zr0.4Ox/SiO2, Ce0.73Tb0.27Ox/La2O3-Al2O3 and Ce0.6Zr0.4Ox/La2O3-Al2O3 was studied using the reduction of NO by CO. The catalysts were characterized by X-ray fluorescence, surface area, X-ray diffraction, temperature-programmed reduction, CO chemisorption and oxygen storage capacity. Temperature-programmed reduction results indicated that Tb or Zr incorporation improves the reducibility and oxygen storage capacity. CO chemisorption data suggested the presence of large PdO particles due to the low CO/Pd ratio. No significant differences were obtained in light off temperatures (TLight off) for all Pd catalysts and the most active was 1.5%Pd/Ce0.6Zr0.4Ox/SiO2.

Designed synthesis of CeO2 nanorods and nanowires for studying toxicological effects of high aspect ratio nanomaterials

While it has been shown that high aspect ratio nanomaterials like carbon nanotubes and TiO(2) nanowires can induce toxicity by acting as fiber-like substances that damage the lysosome, it is not clear what the critical lengths and aspect ratios are that induce this type of toxicity. To answer this question, we synthesized a series of cerium oxide (CeO(2)) nanorods and nanowires with precisely controlled lengths and aspect ratios. Both phosphate and chloride ions were shown to play critical roles in obtaining these high aspect ratio nanostructures. High-resolution TEM analysis shows that single-crystalline CeO(2) nanorods/nanowires were formed along the [211] direction by an "oriented attachment" mechanism, followed by Ostwald ripening. The successful creation of a comprehensive CeO(2) nanorod/nanowire combinatorial library allows, for the first time, the systematic study of the effect of aspect ratio on lysosomal damage, cytotoxicity, and IL-1β production by the human myeloid cell line (THP-1). This in vitro toxicity study demonstrated that, at lengths ≥200 nm and aspect ratios ≥22, CeO(2) nanorods induced progressive pro-inflammatory effects and cytotoxicity. The relatively low "critical" length and aspect ratio were associated with small nanorod/nanowire diameters (6-10 nm), which facilitates the formation of stacking bundles due to strong van der Waals and dipole-dipole attractions. Our results suggest that both length and diameter components of aspect ratio should be considered when addressing the cytotoxic effects of high aspect ratio materials.

Structure and phase stability of nanocrystalline Ce(1-x)Ln(x)O(2-x/2-delta) (Ln = Yb, Lu) in oxidizing and reducing atmosphere

The structure and phase evolution of nanocrystalline Ce(1-x)Ln(x)O(2-x/2-delta) (Ln = Yb, Lu, x = 0 - 1) oxides upon heating in H(2) was studied for the first time. Up to 950 degrees C the samples were single-phase, with structure changing smoothly with x from fluorite type (F) to bixbyite type (C). For the Lu-doped samples heated at 1100 degrees C in the air and H(2), phase separation into coexisting F- and C-type structures was observed for ~0.40 < x < ~0.70 and ~0.25 < x < ~0.70, respectively. It was found also that addition of Lu(3+) and Yb(3+) strongly hinders the crystallite growth of ceria during heat treatment at 800 and 950 degrees C in both atmospheres. Valency of Ce and Yb in Ce(0.1)Lu(0.9)O(1.55-delta) and Ce(0.95)Yb(0.05)O(1.975-delta) samples heated at 1100 degrees C was studied by XANES and magnetic measurements. In the former Ce was dominated by Ce(4+), with small contribution of Ce(3+) after heating in H(2). In the latter, Yb existed exclusively as 3+ in both O(2) and H(2).