Fast Determination of the Lignin Monomer Compositions of Genetic Variants of Poplar via Fast Pyrolysis/Atmospheric Pressure Chemical Ionization Mass Spectrometry.

The proportional content of the phenylpropanoid monomeric units (4-hydroxyphenyl (H), guaiacyl (G), and syringyl (S)) in lignin is of paramount importance in germ plasm screening and for evaluating the results of plant breeding and genetic engineering. This content is usually determined using a tedious and slow (2 days/sample) method involving derivatization followed by reductive cleavage (DFRC) combined with GC/MS or NMR analysis. We report here a fast mass spectrometric method for the determination of the monomer content. This method is based on the fast pyrolysis of a lignin sample inside the ion source area of a linear quadrupole ion trap mass spectrometer. The evaporated pyrolysis products are promptly deprotonated via negative-ion mode atmospheric pressure chemical ionization ((-)APCI) and analyzed by the mass spectrometer to determine the monomer content. The results obtained for the wild-type and six genetic variants of poplar were consistent with those obtained by the DFRC method. However, the mass spectrometry method requires only a small amount of sample (50 µg) and the use of only small amounts of three benign chemicals, methanol, water, and ammonium hydroxide, as opposed to DFRC that requires substantially larger amounts of sample (10 mg or more) and large amounts of several hazardous chemicals. Furthermore, the mass spectrometry method is substantially faster (3 min/sample), more precise, and the data interpretation is more straightforward as only nine ions measured by the mass spectrometer are considered.

Combining Kinetics and Operando Spectroscopy to Interrogate the Mechanism and Active Site Requirements of NOx Selective Catalytic Reduction with NH3 on Cu-Zeolites.

NOx selective catalytic reduction (SCR) with NH3 on Cu-zeolites is a commercial emissions control technology for diesel and lean-burn engines. Mitigating low-temperature emissions remains an outstanding challenge, motivating an improved understanding of the reaction mechanism, active site requirements, and rate-determining processes at low temperatures (<523 K). In this Perspective, we discuss how operando spectroscopy provides crucial information about how the structures, coordination environments, and oxidation states of Cu active sites depend on reaction conditions and sample composition; when combined with kinetic measurements, such operando data provide insights into the Cu site and spatial density requirements for reduction and oxidation steps relevant to the Cu(II)/Cu(I) SCR redox cycle. Isolated Cu ions coordinated to zeolite oxygen atoms ex situ become coordinated to NH3 in situ and dynamically interconvert between mononuclear and binuclear NH3-solvated Cu complexes to catalyze SCR turnovers. We conclude with future research directions that can benefit from combining quantitative kinetic measurements with operando spectroscopy.

Impact of a Supervised Twelve-Week Combined Physical Training Program in Heart Failure Patients: A Randomized Trial.

PURPOSE: The aim of this study was to compare the effects of supervised combined physical training and unsupervised physician-prescribed regular exercise on the functional capacity and quality of life of heart failure patients. METHODS: This is a longitudinal prospective study composed of 28 consecutive heart failure with reduced ejection fraction patients randomly divided into two age- and gender-matched groups: trained group (n = 17) and nontrained group (n = 11). All patients were submitted to clinical evaluation, transthoracic echocardiography, the Cooper walk test, and a Quality of Life questionnaire before and after a 12-week study protocol. Categorical variables were expressed as proportions and compared with the chi-square test. Two-way ANOVA was performed to compare the continuous variables considering the cofactor groups and time of intervention, and Pearson correlation tests were conducted for the associations in the same group. RESULTS: No significant differences between groups were found at baseline. At the end of the protocol, there were improvements in the functional capacity and ejection fraction of the trained group in relation to the nontrained group (p < 0.05). There was time and group interaction for improvement in the quality of life in the trained group. CONCLUSIONS: In patients with heart failure with reduced ejection fraction, supervised combined physical training improved exercise tolerance and quality of life compared with the unsupervised regular exercise prescribed in routine medical consultations. Left ventricular systolic function was improved with supervised physical training.

Exploring the Reaction Mechanisms of Fast Pyrolysis of Xylan Model Compounds via Tandem Mass Spectrometry and Quantum Chemical Calculations.

A commercial fast pyrolysis probe coupled with a high-resolution tandem mass spectrometer was employed to identify the initial reactions and products of fast pyrolysis of xylobiose and xylotriose, model compounds of xylans. Fragmentation of the reducing end by loss of an ethenediol molecule via ring-opening and retro-aldol condensation was found to be the dominant pyrolysis pathway for xylobiose, and the structure of the product-ß-d-xylopyranosylglyceraldehyde-was identified by comparing collision-activated dissociation of the ionized product and an ionized authentic compound. This intermediate can undergo further decomposition via the loss of formaldehyde to form ß-d-xylopyranosylglycolaldehyde. In addition, the mechanisms of reactions leading to the loss of a water molecule or dissociation of the glycosidic linkages were explored computationally. These reactions are proposed to occur via pinacol ring contraction and/or Maccoll elimination mechanisms.

Consequences of exchange-site heterogeneity and dynamics on the UV-visible spectrum of Cu-exchanged SSZ-13.

The speciation and structure of Cu ions and complexes in chabazite (SSZ-13) zeolites, which are relevant catalysts for nitrogen oxide reduction and partial methane oxidation, depend on material composition and reaction environment. Ultraviolet-visible (UV-Vis) spectra of Cu-SSZ-13 zeolites synthesized to contain specific Cu site motifs, together with ab initio molecular dynamics and time-dependent density functional theory calculations, were used to test the ability to relate specific spectroscopic signatures to specific site motifs. Geometrically distinct arrangements of two framework Al atoms in six-membered rings are found to exchange Cu2+ ions that become spectroscopically indistinguishable after accounting for the finite-temperature fluctuations of the Cu coordination environment. Nominally homogeneous single Al exchange sites are found to exchange a heterogeneous mixture of [CuOH]+ monomers, O- and OH-bridged Cu dimers, and larger polynuclear complexes. The UV-Vis spectra of the latter are sensitive to framework Al proximity, to precise ligand environment, and to finite-temperature structural fluctuations, precluding the precise assignment of spectroscopic features to specific Cu structures. In all Cu-SSZ-13 samples, these dimers and larger complexes are reduced by CO to Cu+ sites at 523 K, leaving behind isolated [CuOH]+ sites with a characteristic spectroscopic identity. The various mononuclear and polynuclear Cu2+ species are distinguishable by their different responses to reducing environments, with implications for their relevance to catalytic redox reactions.

Changes in Catalytic and Adsorptive Properties of 2 nm Pt3Mn Nanoparticles by Subsurface Atoms.

Supported multimetallic nanoparticles (NPs) are widely used in industrial catalytic processes, where the relation between surface structure and function is well-known. However, the effect of subsurface layers on such catalysts remains mostly unstudied. Here, we demonstrate a clear subsurface effect on supported 2 nm core-shell NPs with atomically precise and high temperature stable Pt3Mn intermetallic surface measured by in situ synchrotron X-ray Diffraction, difference X-ray Absorption Spectroscopy, and Energy Dispersive X-ray Spectroscopy. The NPs with a Pt3Mn subsurface have 98% selectivity to C-H over C-C bond activation during propane dehydrogenation at 550 °C compared with 82% for core-shell NPs with a Pt subsurface. The difference is correlated with significant reduction in the heats of reactant adsorption due to the Pt3Mn intermetallic subsurface as discerned by theory as well as experiment. The findings of this work highlight the importance of subsurface for supported NP catalysts, which can be tuned via controlled intermetallic formation. Such approach is generally applicable to modifying multimetallic NPs, adding another dimension to the tunability of their catalytic performance.

Tailorable Au Nanoparticles Embedded in Epitaxial TiO2 Thin Films for Tunable Optical Properties.

The unique property of plasmonic materials to localize light into deep sub-wavelength regime has greatly driven various applications in the field of photovoltaics, sensors, and photocatalysis. Here, we demonstrate the one-step growth of an oxide-metal hybrid thin film incorporating well-dispersed gold (Au) nanoparticles (NPs) with tailorable particle shape and diameters (ranging from 2 to 20 nm) embedded in highly epitaxial TiO2 matrix, deposited using pulsed laser deposition. Incorporation of Au NPs reduces the band gap of TiO2 and enhances light absorption in the visible regime owing to the excitation of localized surface plasmons. Optical properties, including the plasmonic response and permittivity, and photocatalytic activities of the Au-TiO2 hybrid materials are effectively tuned as a function of the Au NP sizes. Such optical property tuning is well captured using full-field simulations and the effective medium theory for better understanding of the physical phenomena. The tailorable shape and size of Au NPs embedded in TiO2 matrix present a novel oxide-metal hybrid material platform for optical property tuning and highly efficient plasmonic properties for future oxide-based photocatalytic sensors and devices.

Dynamic multinuclear sites formed by mobilized copper ions in NO x selective catalytic reduction.

Copper ions exchanged into zeolites are active for the selective catalytic reduction (SCR) of nitrogen oxides (NO x ) with ammonia (NH3), but the low-temperature rate dependence on copper (Cu) volumetric density is inconsistent with reaction at single sites. We combine steady-state and transient kinetic measurements, x-ray absorption spectroscopy, and first-principles calculations to demonstrate that under reaction conditions, mobilized Cu ions can travel through zeolite windows and form transient ion pairs that participate in an oxygen (O2)-mediated CuI→CuII redox step integral to SCR. Electrostatic tethering to framework aluminum centers limits the volume that each ion can explore and thus its capacity to form an ion pair. The dynamic, reversible formation of multinuclear sites from mobilized single atoms represents a distinct phenomenon that falls outside the conventional boundaries of a heterogeneous or homogeneous catalyst.

A Discovery of Strong Metal-Support Bonding in Nanoengineered Au-Fe3O4 Dumbbell-like Nanoparticles by in Situ Transmission Electron Microscopy.

The strength of metal-support bonding in heterogeneous catalysts determines their thermal stability, therefore, a tremendous amount of effort has been expended to understand metal-support interactions. Herein, we report the discovery of an anomalous "strong metal-support bonding" between gold nanoparticles and "nano-engineered" Fe3O4 substrates by in situ microscopy. During in situ vacuum annealing of Au-Fe3O4 dumbbell-like nanoparticles, synthesized by the epitaxial growth of nano-Fe3O4 on Au nanoparticles, the gold nanoparticles transform into the gold thin films and wet the surface of nano-Fe3O4, as the surface reduction of nano-Fe3O4 proceeds. This phenomenon results from a unique coupling of the size-and shape-dependent high surface reducibility of nano-Fe3O4 and the extremely strong adhesion between Au and the reduced Fe3O4. This strong metal-support bonding reveals the significance of controlling the metal oxide support size and morphology for optimizing metal-support bonding and ultimately for the development of improved catalysts and functional nanostructures.

A transmission infrared cell design for temperature-controlled adsorption and reactivity studies on heterogeneous catalysts.

A design is presented for a versatile transmission infrared cell that can interface with an external vacuum manifold to undergo in situ gas treatments and receive controlled doses of various adsorbates and probe molecules, allowing characterization of heterogeneous catalyst surfaces in order to identify and quantify active sites and adsorbed surface species. Critical design characteristics include customized temperature control for operation between cryogenic and elevated temperatures (100-1000 K) and modified Cajon fittings for operation over a wide pressure range (10-2-103 Torr) that eliminates the complications introduced when using sealants or flanges to secure cell windows. The customized, hand-tightened Cajon fittings simplify operation of the cell compared to previously reported designs, because they allow for rapid cell assembly and disassembly and, in turn, replacement of catalyst samples. In order to validate the performance of the cell, transmission infrared spectroscopic experiments are reported to characterize the Brønsted and Lewis acid sites present in H-beta and H-mordenite zeolites using cryogenic adsorption of CO (<150 K).

Catalysis in a Cage: Condition-Dependent Speciation and Dynamics of Exchanged Cu Cations in SSZ-13 Zeolites.

The relationships among the macroscopic compositional parameters of a Cu-exchanged SSZ-13 zeolite catalyst, the types and numbers of Cu active sites, and activity for the selective catalytic reduction (SCR) of NOx with NH3 are established through experimental interrogation and computational analysis of materials across the catalyst composition space. Density functional theory, stochastic models, and experimental characterizations demonstrate that within the synthesis protocols applied here and across Si:Al ratios, the volumetric density of six-membered-rings (6MR) containing two Al (2Al sites) is consistent with a random Al siting in the SSZ-13 lattice subject to Löwenstein's rule. Further, exchanged Cu(II) ions first populate these 2Al sites before populating remaining unpaired, or 1Al, sites as Cu(II)OH. These sites are distinguished and enumerated ex situ through vibrational and X-ray absorption spectroscopies (XAS) and chemical titrations. In situ and operando XAS follow Cu oxidation state and coordination environment as a function of environmental conditions including low-temperature (473 K) SCR catalysis and are rationalized through first-principles thermodynamics and ab initio molecular dynamics. Experiment and theory together reveal that the Cu sites respond sensitively to exposure conditions, and in particular that Cu species are solvated and mobilized by NH3 under SCR conditions. While Cu sites are spectroscopically and chemically distinct away from these conditions, they exhibit similar turnover rates, apparent activation energies and apparent reaction orders at the SCR conditions, even on zeolite frameworks other than SSZ13.

High Thermal Stability of La2O3- and CeO2-Stabilized Tetragonal ZrO2.

Catalyst support materials of tetragonal ZrO2, stabilized by either La2O3 (La2O3-ZrO2) or CeO2 (CeO2-ZrO2), were synthesized under hydrothermal conditions at 200 °C with NH4OH or tetramethylammonium hydroxide as the mineralizer. From in situ synchrotron powder X-ray diffraction and small-angle X-ray scattering measurements, the calcined La2O3-ZrO2 and CeO2-ZrO2 supports were nonporous nanocrystallites that exhibited rectangular shapes with a thermal stability of up to 1000 °C in air. These supports had an average size of ∼ 10 nm and a surface area of 59-97 m(2)/g. The catalysts Pt/La2O3-ZrO2 and Pt/CeO2-ZrO2 were prepared by using atomic layer deposition with varying Pt loadings from 6.3 to 12.4 wt %. Monodispersed Pt nanoparticles of ∼ 3 nm were obtained for these catalysts. The incorporation of La2O3 and CeO2 into the t-ZrO2 structure did not affect the nature of the active sites for the Pt/ZrO2 catalysts for the water-gas shift reaction.

Predictive morphology, stoichiometry and structure of surface species in supported Ru nanoparticles under H2 and CO atmospheres from combined experimental and DFT studies.

Further understanding of the chemisorption properties towards CO and H2 on silica-supported Ru nanoparticles is crucial in order to rationalize their high activity towards methanation, Fischer Tropsch and Water Gas Shift reactions. Ru nanoparticles having the same chemisorption properties towards CO and H2 were synthesized on different silica-based supports in order to combine various analytical techniques and obtain complimentary detailed information on their structure; while silica spheres were used in order to obtain high-resolution TEM images of the Ru nanoparticles, high surface area silica-based material (SBA) allowed CO chemisorption to be monitored by (13)C NMR spectroscopy. In addition, a model of the hcp-based Ru nanoparticles observed by HR-TEM was used to predict by ab initio calculations the CO and H2 coverages on the Ru nanoparticle under different conditions of interest in catalysis. For both adsorbates we show and quantify how the adsorption properties of the nanoparticle differ from the commonly used slab models. For the case of CO we show how the top, bridge and hollow sites can be present on the Ru nanoparticle, providing a description at atomistic level in good agreement with the IR spectroscopy measurements.

Mass spectrometric studies of fast pyrolysis of cellulose.

A fast pyrolysis probe/linear quadrupole ion trap mass spectrometer combination was used to study the primary fast pyrolysis products (those that first leave the hot pyrolysis surface) of cellulose, cellobiose, cellotriose, cellotetraose, cellopentaose, and cellohexaose, as well as of cellobiosan, cellotriosan, and cellopentosan, at 600°C. Similar products with different branching ratios were found for the oligosaccharides and cellulose, as reported previously. However, identical products (with the exception of two) with similar branching ratios were measured for cellotriosan (and cellopentosan) and cellulose. This result demonstrates that cellotriosan is an excellent small-molecule surrogate for studies of the fast pyrolysis of cellulose and also that most fast pyrolysis products of cellulose do not originate from the reducing end. Based on several observations, the fast pyrolysis of cellulose is suggested to initiate predominantly via two competing processes: the formation of anhydro-oligosaccharides, such as cellobiosan, cellotriosan, and cellopentosan (major route), and the elimination of glycolaldehyde (or isomeric) units from the reducing end of oligosaccharides formed from cellulose during fast pyrolysis.

Trimethylaluminum and Oxygen Atomic Layer Deposition on Hydroxyl-Free Cu(111).

Atomic layer deposition (ALD) of alumina using trimethylaluminum (TMA) has technological importance in microelectronics. This process has demonstrated a high potential in applications of protective coatings on Cu surfaces for control of diffusion of Cu in Cu2S films in photovoltaic devices and sintering of Cu-based nanoparticles in liquid phase hydrogenation reactions. With this motivation in mind, the reaction between TMA and oxygen was investigated on Cu(111) and Cu2O/Cu(111) surfaces. TMA did not adsorb on the Cu(111) surface, a result consistent with density functional theory (DFT) calculations predicting that TMA adsorption and decomposition are thermodynamically unfavorable on pure Cu(111). On the other hand, TMA readily adsorbed on the Cu2O/Cu(111) surface at 473 K resulting in the reduction of some surface Cu(1+) to metallic copper (Cu(0)) and the formation of a copper aluminate, most likely CuAlO2. The reaction is limited by the amount of surface oxygen. After the first TMA half-cycle on Cu2O/Cu(111), two-dimensional (2D) islands of the aluminate were observed on the surface by scanning tunneling microscopy (STM). According to DFT calculations, TMA decomposed completely on Cu2O/Cu(111). High-resolution electron energy loss spectroscopy (HREELS) was used to distinguish between tetrahedrally (Altet) and octahedrally (Aloct) coordinated Al(3+) in surface adlayers. TMA dosing produced an aluminum oxide film, which contained more octahedrally coordinated Al(3+) (Altet/Aloct HREELS peak area ratio ≈ 0.3) than did dosing O2 (Altet/Aloct HREELS peak area ratio ≈ 0.5). After the first ALD cycle, TMA reacted with both Cu2O and aluminum oxide surfaces in the absence of hydroxyl groups until film closure by the fourth ALD cycle. Then, TMA continued to react with surface Al-O, forming stoichiometric Al2O3. O2 half-cycles at 623 K were more effective for carbon removal than O2 half-cycles at 473 K or water half-cycles at 623 K. The growth rate was approximately 3-4 Å/cycle for TMA+O2 ALD (O2 half-cycles at 623 K). No preferential growth of Al2O3 on the steps of Cu(111) was observed. According to STM, Al2O3 grows homogeneously on Cu(111) terraces.

Fast pyrolysis of 13C-labeled cellobioses: gaining insights into the mechanisms of fast pyrolysis of carbohydrates.

A fast-pyrolysis probe/tandem mass spectrometer combination was utilized to determine the initial fast-pyrolysis products for four different selectively (13)C-labeled cellobiose molecules. Several products are shown to result entirely from fragmentation of the reducing end of cellobiose, leaving the nonreducing end intact in these products. These findings are in disagreement with mechanisms proposed previously. Quantum chemical calculations were used to identify feasible low-energy pathways for several products. These results provide insights into the mechanisms of fast pyrolysis of cellulose.

Isolation of the copper redox steps in the standard selective catalytic reduction on Cu-SSZ-13.

Operando X-ray absorption experiments and density functional theory (DFT) calculations are reported that elucidate the role of copper redox chemistry in the selective catalytic reduction (SCR) of NO over Cu-exchanged SSZ-13. Catalysts prepared to contain only isolated, exchanged Cu(II) ions evidence both Cu(II) and Cu(I) ions under standard SCR conditions at 473 K. Reactant cutoff experiments show that NO and NH3 together are necessary for Cu(II) reduction to Cu(I). DFT calculations show that NO-assisted NH3 dissociation is both energetically favorable and accounts for the observed Cu(II) reduction. The calculations predict in situ generation of Brønsted sites proximal to Cu(I) upon reduction, which we quantify in separate titration experiments. Both NO and O2 are necessary for oxidation of Cu(I) to Cu(II), which DFT suggests to occur by a NO2 intermediate. Reaction of Cu-bound NO2 with proximal NH4(+) completes the catalytic cycle. N2 is produced in both reduction and oxidation half-cycles.

Low-temperature carbon monoxide oxidation catalysed by regenerable atomically dispersed palladium on alumina.

Catalysis by single isolated atoms of precious metals has attracted much recent interest, as it promises the ultimate in atom efficiency. Most previous reports are on reducible oxide supports. Here we show that isolated palladium atoms can be catalytically active on industrially relevant γ-alumina supports. The addition of lanthanum oxide to the alumina, long known for its ability to improve alumina stability, is found to also help in the stabilization of isolated palladium atoms. Aberration-corrected scanning transmission electron microscopy and operando X-ray absorption spectroscopy confirm the presence of intermingled palladium and lanthanum on the γ-alumina surface. Carbon monoxide oxidation reactivity measurements show onset of catalytic activity at 40 °C. The catalyst activity can be regenerated by oxidation at 700 °C in air. The high-temperature stability and regenerability of these ionic palladium species make this catalyst system of potential interest for low-temperature exhaust treatment catalysts.

Palladium nanoparticle formation on TiO2(110) by thermal decomposition of palladium(II) hexafluoroacetylacetonate.

Palladium nanoparticles were synthesized by thermal decomposition of palladium(II) hexafluoroacetylacetonate (Pd(hfac)2), an atomic layer deposition (ALD) precursor, on a TiO2(110) surface. According to X-ray photoelectron spectroscopy (XPS), Pd(hfac)2 adsorbs on TiO2(110) dissociatively yielding Pd(hfac)(ads), hfac(ads), and adsorbed fragments of the hfac ligand at 300 K. A (2 × 1) surface overlayer was observed by scanning tunneling microscopy (STM), indicating that hfac adsorbs in a bidentate bridging fashion across two Ti 5-fold atoms and Pd(hfac) adsorbs between two bridging oxygen atoms on the surface. Annealing of the Pd(hfac)(ads) and hfac(ads) species at 525 K decomposed the adsorbed hfac ligands, leaving PdO-like species and/or Pd atoms or clusters. Above 575 K, the XPS Pd 3d peaks shift toward lower binding energies and Pd nanoparticles are observed by STM. These observations point to the sintering of Pd atoms and clusters to Pd nanoparticles. The average height of the Pd nanoparticles was 1.2 ± 0.6 nm at 575 K and increased to 1.7 ± 0.5 nm following annealing at 875 K. The Pd coverage was estimated from XPS and STM data to be 0.05 and 0.03 monolayers (ML), respectively, after the first adsorption/decomposition cycle. The amount of palladium deposited on the TiO2(110) surface increased linearly with the number of adsorption/decomposition cycles with a growth rate of 0.05 ML or 0.6 Å per cycle. We suggest that the removal of the hfac ligand and fragments eliminates the nucleation inhibition of Pd nanoparticles previously observed for the Pd(hfac)2 precursor on TiO2.

Gas mixing system for imaging of nanomaterials under dynamic environments by environmental transmission electron microscopy.

A gas mixing manifold system that is capable of delivering a stable pressure stream of a desired composition of gases into an environmental transmission electron microscope has been developed. The system is designed to provide a stable imaging environment upon changes of either the composition of the gas mixture or upon switching from one gas to another. The design of the system is described and the response of the pressure inside the microscope, the sample temperature, and sample drift in response to flow and composition changes of the system are reported.