Diphosphine-Protected Au22 Nanoclusters on Oxide Supports Are Active for Gas-Phase Catalysis without Ligand Removal.

Investigation of atomically precise Au nanoclusters provides a route to understand the roles of coordination, size, and ligand effects on Au catalysis. Herein, we explored the catalytic behavior of a newly synthesized Au22(L8)6 nanocluster (L = 1,8-bis(diphenylphosphino) octane) with in situ uncoordinated Au sites supported on TiO2, CeO2, and Al2O3. Stability of the supported Au22 nanoclusters was probed structurally by in situ extended X-ray absorption fine structure (EXAFS) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and their ability to adsorb and oxidize CO was investigated by IR absorption spectroscopy and a temperature-programmed flow reaction. Low-temperature CO oxidation activity was observed for the supported pristine Au22(L8)6 nanoclusters without ligand removal. Density functional theory (DFT) calculations confirmed that the eight uncoordinated Au sites in the intact Au22(L8)6 nanoclusters can chemisorb both CO and O2. Use of isotopically labeled O2 demonstrated that the reaction pathway occurs mainly through a redox mechanism, consistent with the observed support-dependent activity trend of CeO2 > TiO2 > Al2O3. We conclude that the uncoordinated Au sites in the intact Au22(L8)6 nanoclusters are capable of adsorbing CO, activating O2, and catalyzing CO oxidation reaction. This work is the first clear demonstration of a ligand-protected intact Au nanocluster that is active for gas-phase catalysis without the need of ligand removal.

Thiolate ligands as a double-edged sword for CO oxidation on CeO2 supported Au25(SCH2CH2Ph)18 nanoclusters.

The effect of thiolate ligands was explored on the catalysis of CeO2 rod supported Au25(SR)18 (SR = -SCH2CH2Ph) by using CO oxidation as a probe reaction. Reaction kinetic tests, in situ IR and X-ray absorption spectroscopy, and density functional theory (DFT) were employed to understand how the thiolate ligands affect the nature of active sites, activation of CO and O2, and reaction mechanism and kinetics. The intact Au25(SR)18 on the CeO2 rod is found not able to adsorb CO. Only when the thiolate ligands are partially removed, starting from the interface between Au25(SR)18 and CeO2 at temperatures of 423 K and above, can the adsorption of CO be observed by IR. DFT calculations suggest that CO adsorbs favorably on the exposed gold atoms. Accordingly, the CO oxidation light-off temperature shifts to lower temperature. Several types of Au sites are probed by IR of CO adsorption during the ligand removal process. The cationic Au sites (charged between 0 and +1) are found to play the major role for low-temperature CO oxidation. Similar activation energies and reaction rates are found for CO oxidation on differently treated Au25(SR)18/CeO2 rod catalysts, suggesting a simple site-blocking effect of the thiolate ligands in Au nanocluster catalysis. Isotopic labeling experiments clearly indicate that CO oxidation on the Au25(SR)18/CeO2 rod catalyst proceeds predominantly via the redox mechanism where CeO2 activates O2 while CO is activated on the dethiolated gold sites. These results point to a double-edged sword role played by the thiolate ligands on Au25 nanoclusters for CO oxidation.

CO oxidation on supported single Pt atoms: experimental and ab initio density functional studies of CO interaction with Pt atom on θ-Al2O3(010) surface.

Although there are only a few known examples of supported single-atom catalysts, they are unique because they bridge the gap between homogeneous and heterogeneous catalysis. Here, we report the CO oxidation activity of monodisperse single Pt atoms supported on an inert substrate, θ-alumina (Al2O3), in the presence of stoichiometric oxygen. Since CO oxidation on single Pt atoms cannot occur via a conventional Langmuir-Hinshelwood scheme (L-H scheme) which requires at least one Pt-Pt bond, we carried out a first-principles density functional theoretical study of a proposed pathway which is a variation on the conventional L-H scheme and inspired by the organometallic chemistry of platinum. We find that a single supported Pt atom prefers to bond to O2 over CO. CO then bonds with the oxygenated Pt atom and forms a carbonate which dissociates to liberate CO2, leaving an oxygen atom on Pt. Subsequent reaction with another CO molecule regenerates the single-atom catalyst. The energetics of the proposed mechanism suggests that the single Pt atoms will get covered with CO3 unless the temperature is raised to eliminate CO2. We find evidence for CO3 coverage at room temperature supporting the proposed mechanism in an in situ diffuse reflectance infrared study of CO adsorption on the catalyst's supported single atoms. Thus, our results clearly show that supported Pt single atoms are catalytically active and that this catalytic activity can occur without involving the substrate. Characterization by electron microscopy and X-ray absorption studies of the monodisperse Pt/θ-Al2O3 are also presented.

Adsorption and reaction of methanol over CeO(X)(100) thin films.

Methanol was adsorbed on oxidized and reduced CeOX(100) thin films to probe the active sites and reaction selectivity of these surfaces compared to those of CeOX(111). Roughly twice as much methoxy was formed on oxidized CeO2(100) compared to that formed on CeO2(111). In addition to more methoxy, hydroxyl is also more stable on CeO2(100). Unlike on CeO2(111), however, methanol on CeO2(100) produced CO, CO2, and H2 in addition to water and formaldehyde. The behavior of CeO2(100) is related to its surface structure, which provides greater access to Ce cations and therefore more active adsorption sites and more highly undercoordinated Ce and O. The undercoordinated O may explain the enhanced dehydrogenation activity leading to CO and H2 formation. The reduction of ceria leads to increased methanol uptake on both CeO2 - X(100) and CeO2 - X(111). However, although the uptake doubled on reduced CeO2 - X(111) compared to the oxidized surface, it increased by only 10% on reduced CeO2 - X(100) compared to that on fully oxidized CeO2(100). Reduction of both surfaces leads to a greater production of CO and H2. Reaction on all surfaces progresses rapidly from methoxy to products. There is no spectroscopic evidence of formyl or formate intermediates. On CeOX(100), carbonate is detected that decomposes into CO2 at high temperature.

Development of a novel capillary electrophoresis method for quantitative measurements of intracellular recombinant protein titer.

The measurement of volumetric titer is an integral step in the assessment and selection of a production cell line and cell culture process. The production of monoclonal antibodies (mAbs), a major class of therapeutic proteins, in Chinese Hamster Ovary (CHO) cell lines is challenging due to the clone-to-clone variations in the intrinsic capability to secrete a biologically complex protein. The measurement of intracellular mAb concentration could be a valuable tool to determine the ratio of intracellular to secreted product and be part of the evaluation of potential mAb productive cell lines. High throughput automation is a valuable tool that is used in bioprocess development to reduce work intensive steps. When coupled with the Simple Western (Wes) platform, automated capillary electrophoresis is an efficient method to measure recombinant protein concentration. In this study, we demonstrate the utility of using the automated Wes to rapidly measure intracellular titer and then compare the intracellular titer, volumetric titer and specific productivity between high and low production CHO clones expressing a model human IgG1 mAb.

Oxygen vacancy-assisted coupling and enolization of acetaldehyde on CeO2(111).

The temperature-dependent adsorption and reaction of acetaldehyde (CH(3)CHO) on a fully oxidized and a highly reduced thin-film CeO(2)(111) surface have been investigated using a combination of reflection-absorption infrared spectroscopy (RAIRS) and periodic density functional theory (DFT+U) calculations. On the fully oxidized surface, acetaldehyde adsorbs weakly through its carbonyl O interacting with a lattice Ce(4+) cation in the η(1)-O configuration. This state desorbs at 210 K without reaction. On the highly reduced surface, new vibrational signatures appear below 220 K. They are identified by RAIRS and DFT as a dimer state formed from the coupling of the carbonyl O and the acyl C of two acetaldehyde molecules. This dimer state remains up to 400 K before decomposing to produce another distinct set of vibrational signatures, which are identified as the enolate form of acetaldehyde (CH(2)CHO¯). Furthermore, the calculated activation barriers for the coupling of acetaldehyde, the decomposition of the dimer state, and the recombinative desorption of enolate and H as acetaldehyde are in good agreement with previously reported TPD results for acetaldehyde adsorbed on reduced CeO(2)(111) [Chen et al. J. Phys. Chem. C 2011, 115, 3385]. The present findings demonstrate that surface oxygen vacancies alter the reactivity of the CeO(2)(111) surface and play a crucial role in stabilizing and activating acetaldehyde for coupling reactions.

Egyptian blue: from pigment to battery electrodes.

Herein we report on using Egyptian blue as an anode material for Li-ion batteries. A 1st cycle lithiation capacity of 594 mA h g-1 and reversible capacity of 210 mA h g-1 at 20 mA g-1, and at 500 mA g-1 a reversible capacity of 120 mA h g-1 (stable over 1000 cycles) were achieved with coulombic efficiency more than 99.5%. Using X-ray diffraction, and FTIR and X-ray absorption spectroscopies we found that the material goes through a conversion reaction during the 1st cycle that results in the formation of amorphous mixed oxides with copper nanoclusters.

Identification of hydrophobic amino acids required for lipid activation of C. elegans CTP:phosphocholine cytidylyltransferase.

CTP:phosphocholine cytidylyltransferase (CCT), critical for phosphatidylcholine biosynthesis, is activated by translocation to the membrane surface. The lipid activation region of Caenorhabditis elegans CCT is between residues 246 and 266 of the 347 amino acid polypeptide, a region proposed to form an amphipathic alpha helix. When leucine 246, tryptophan 249, isoleucine 256, isoleucine 257, or phenylalanine 260, on the hydrophobic face of the helix, were changed individually to serine low activity was observed in the absence of lipid vesicles, similar to wild-type CCT, while lipid stimulated activity was reduced compared to wild-type CCT. Mutational analysis of phenylalanine 260 implicated this residue as a contributor to auto-inhibition of CCT while mutation of L246, W249, I256, and I257 simultaneously to serine resulted in significantly higher activity in the absence of lipid vesicles and an enzyme that was not lipid activated. These results support a concerted mechanism of lipid activation that requires multiple residues on the hydrophobic face of the putative amphipathic alpha helix.

Temperature evolution of structure and bonding of formic acid and formate on fully oxidized and highly reduced CeO2(111).

Adsorption of formate on oxide surfaces plays a role in water-gas shift (WGS) and other reactions related to H(2) production and CO(2) utilization. CeO(2) is of particular interest because its reducibility affects the redox of organic molecules. In this work, the adsorption and thermal evolution of formic acid and formate on highly ordered films of fully oxidized CeO(2)(111) and highly reduced CeO(x)(111) surfaces have been studied using reflection absorption infrared spectroscopy (RAIRS) under ultra-high vacuum conditions, and the experimental results are combined with density functional theory (DFT) calculations to probe the identity, symmetry, and bonding of the surface intermediates. Disordered ice, ordered alpha-polymorph and molecular formic acid bonded through the carbonyl are observed at low temperatures. By 250 K, desorption and deprotonation lead to formate coexisting with hydroxyl on CeO(2)(111), identified to be a bridging bidentate formate species that is coordinated to Ce cations in nearly C(2v) symmetry and interacting strongly with neighboring H. Changes in the spectra at higher temperatures are consistent with additional tilting of the formate, resulting in C(s)(2) or lower symmetry. This change in bonding is caused primarily by interaction with oxygen vacancies introduced by water desorption at 300 K. On reduced CeO(x), multiple low-symmetry formate states exist likewise due to interactions with oxygen vacancies. Isotopic studies demonstrate that the formyl hydrogen does not contribute to H incorporated in hydroxyl on the surface, and that both formate oxygen atoms may exchange with lattice oxygen at 400 K. The combined experimental and theoretical results thus provide important insights on the surface reaction pathways of formic acid on ceria.

Promoting Pt catalysis for CO oxidation via the Mott-Schottky effect.

CO oxidation is an important reaction both experimentally and industrially, and its performance is usually dominated by the charge states of catalysts. For example, CO oxidation on the platinum (Pt) surface requires a properly charged state for the balance of adsorption and activation of CO and O2. Here, we present "Mott-Schottky modulated catalysis" on Pt nanoparticles (NPs) via an electron-donating carbon nitride (CN) support with a tunable Fermi level. We demonstrate that properly-charged Pt presents an excellent catalytic CO oxidation activity with an initial conversion temperature as low as 25 °C and total CO conversion below 85 °C. The tunable electronic structure of Pt NPs, which is regulated by the Fermi level of CN, is a key factor in dominating the catalytic performance. This "Mott-Schottky modulated catalysis" concept may be extended to maneuver the charge state on other metal catalysts for targeted catalytic reactions.

Rh-promoted methanol decomposition on cerium oxide thin films.

Methanol adsorption and reaction have been studied on Rh-deposited cerium oxide thin films under UHV conditions using temperature-programmed desorption and synchrotron soft X-ray photoelectron spectroscopy. The methanol behavior was examined as a function of the Ce oxidation state, methanol exposure, and Rh particle size and coverage. When Rh nanoparticles were deposited on the ceria films, methanol decomposed on Rh to CO and H below 200 K. H atoms recombined and desorbed between 200 and 300 K. CO evolved from Rh deposited on fully oxidized ceria between 400 and 500 K. However, on reduced ceria films, the CO on Rh further decomposed to atomic C. Methanol adsorbed on the ceria films deprotonated to form methoxy as the only intermediate on the surface. This methoxy decomposed and desorbed as CO and H2 at higher temperatures regardless of the ceria oxidation state. Compared with the methanol reaction on Rh-free ceria thin films, formaldehyde formation from methoxy was completely suppressed after Rh deposition. Our results indicate that Rh can promote the decomposition of methoxy adsorbed on the ceria and that decomposition of methoxy intermediates occurred at the metal/oxide interfaces. On the other hand, the reduced ceria can promote total methanol decomposition on Rh.

Heterostructured BaSO4-SiO2 mesoporous materials as new supports for gold nanoparticles in low-temperature CO oxidation.

Nanosized BaSO4-based mesoporous hybrid materials have been developed and identified as new efficient inorganic salt-based support systems for ultrastable gold nanoparticles in low-temperature CO oxidation.