Jessica Scaranto

Facile Synthesis of Palladium Right Bipyramids and Their Use as Seeds for Overgrowth and as Catalysts for Formic Acid Oxidation

Controlling the shape and thus facets of metal nanocrystals is an effective way to enhance their performance in catalytic reactions. While Pd nanocrystals with a myriad of shapes have been successfully prepared with good uniformity and in high yield, Pd right bipyramids (RBPs) that have a singly twinned structure have been elusive. We report a facile route based on polyol reduction for the synthesis of Pd RBPs with purity >90% and sizes controlled in the range 5-15 nm. The success of our synthesis relies on the use of iodide ions to manipulate the strength of an oxidative etchant and selectively cap the Pd{100} facets. The as-prepared RBPs could serve as seeds to generate a set of Pd nanocrystals with novel shapes and structures. The RBPs also exhibited enhanced catalytic activity toward formic acid oxidation, with a current density 2.5 and 7.1 times higher than those of the single-crystal Pd nanocubes (which were also mainly covered by {100} facets) and commercial Pd black, respectively.

A Comprehensive Study of Formic Acid Oxidation on Palladium Nanocrystals with Different Types of Facets and Twin Defects

Palladium has been recognized as the best anodic, monometallic electrocatalyst for the formic acid oxidation (FAO) reaction in a direct formic acid fuel cell. Here we report a systematic study of FAO on a variety of Pd nanocrystals, including cubes, right bipyramids, octahedra, tetrahedra, decahedra, and icosahedra. These nanocrystals were synthesized with approximately the same size, but different types of facets and twin defects on their surfaces. Our measurements indicate that the Pd nanocrystals enclosed by {1 0 0} facets have higher specific activities than those enclosed by {1 1 1} facets, in agreement with prior observations for Pd single‐crystal substrates. If comparing nanocrystals predominantly enclosed by a specific type of facet, {1 0 0} or {1 1 1}, those with twin defects displayed greatly enhanced FAO activities compared to their single‐crystal counterparts. To rationalize these experimental results, we performed periodic, self‐consistent DFT calculations on model single‐crystal substrates of Pd, representing the active sites present in the nanocrystals used in the experiments. The calculation results suggest that the enhancement of FAO activity on defect regions, represented by Pd(2 1 1) sites, compared to the activity of both Pd(1 0 0) and Pd(1 1 1) surfaces, could be attributed to an increased flux through the HCOO‐mediated pathway rather than the COOH‐mediated pathway on Pd(2 1 1). Since COOH has been identified as a precursor to CO, a site‐poisoning species, a lower coverage of CO at the defect regions will lead to a higher activity for the corresponding nanocrystal catalysts, containing those defect regions.

A DFT study of CO adsorbed on clean and hydroxylated anatase TiO2 (001) surfaces

The adsorbate–substrate interaction between carbon monoxide and clean and hydroxylated surfaces of anatase (001) was simulated by periodic DFT calculations. For the clean surface, different periodicities were taken into account in order to investigate the influence of lateral effects. The results show that the molecule interacts only with the penta-coordinated titanium ion when high surface coverages are employed, whereas it interacts with both the titanium and the di-coordinated oxygen ions when the smallest surface coverage is considered. Adsorption on the hydroxylated surface, which was carried out for the first time, was studied by considering the largest used periodicity and focusing attention on the influence of both the terminal and bridging OH groups. The resulting data show that, in the case of adsorption on the hydroxylated surface, no additional bond between the surface oxygen and the carbon atom is present. Therefore, in a regime of very low surface coverage, the molecule behaves in a very different way when it interacts with a clean or a hydroxylated surface.

Influence of the OH groups of hydroxylated rutile (110) surface on the Lewis acidity: an investigation of CO adsorption by quantum-mechanical simulations.

The adsorption of carbon monoxide on a clean and hydroxylated rutile (110) surface has been investigated using a periodic approach at DFT/B3LYP level. The hydroxylated surface was modelled by considering both terminal and bridging OH groups. The variation of the electrophilicity of the Lewis acid site near these groups was evaluated by taking into account the adsorbate–substrate distance, the magnitude of the interaction energy and the blue-shift of the adsorbed CO stretching frequency. The results obtained suggest that the electrophilicity increases in proximity to OH terminal groups, and decreases near the OH bridging groups.

Adsorption of difluoromethane on titanium dioxide: Investigation of the FTIR spectra and quantum-mechanical studies of the adsorbate-substrate structures

The interaction of difluoromethane (CH(2)F(2)) with the TiO(2) surface (P25 Degussa) at room temperature has been studied by Fourier-transform infrared spectroscopy for the first time. From the comparison between the adsorption characteristics and the gas-phase spectra it can be deduced that the molecule adsorbs through an acid-base interaction between one F atom and the surface Lewis acid site (Ti(4+)) and an H-bond between the CH(2) group and the surface Lewis basic site (OH(-) or O(2-)). In order to obtain more information about the orientation geometry and the variation of the molecular structural parameters, a quantum-mechanical investigation at DFT/B3LYP level has been also performed, considering the anatase (101) surface and focusing on the O(2-) as Lewis basic site. The resulting adsorbate-substrate structures involve the formation of an acid-base interaction between one F atom and the Ti(4+) ion and differ for the number of the involved H-bonds. According to the scaled vibrational frequencies, the simulated adsorption model which better agrees with the experimental data corresponds to that in which the CH(2) group interacts with the surface by only one H-bond.

Towards first-principles molecular design of liquid crystal-based chemoresponsive systems

Nematic liquid crystals make promising chemoresponsive systems, but their development is currently limited by extensive experimental screening. Here we report a computational model to understand and predict orientational changes of surface-anchored nematic liquid crystals in response to chemical stimuli. In particular, we use first-principles calculations to evaluate the binding energies of benzonitrile, a model for 4′-pentyl-4-biphenylcarbonitrile, and dimethyl methylphosphonate to metal cation models representing the substrate chemical sensing surface. We find a correlation between these quantities and the experimental response time useful for predicting the response time of cation–liquid crystal combinations. Consideration of charge donation from chemical species in the surface environment is critical for obtaining agreement between theory and experiment. Our model may be extended to the design of improved chemoresponsive liquid crystals for selectively detecting other chemicals of practical interest by choosing appropriate combinations of metal cations with liquid crystals of suitable molecular structure.

IR spectroscopy of CH2CBrF adsorbed on TiO2 and quantum-mechanical studies

The adsorption at room temperature of 1-bromo-1-fluoroethene (CH2CBrF) on TiO2 has been investigated by Fourier-transform infrared spectroscopy after a preventive assignment of the fundamentals in the gas-phase, carried out for the first time. The spectrum resulting from the adsorption has been compared with the gas-phase one and the most important differences consist in a red-shift of the CH2 stretching vibration and in the presence of two distinct absorptions for both the C=C and C–F stretching modes. Basing on the observed features it has been inferred that there is the formation of an H-bond between the CH2 group and a surface Lewis basic site and that the adsorption can occur through the double C=C bond or the F atom. Two proposed adsorbate-substrate structures have been investigated by periodic quantum-mechanical calculations at DFT/B3LYP level considering the rutile (110) surface. In the case of the adsorption by the F atom, also the formation of the H-bond has been considered. The interaction energy resulting from the adsorption through the double C = C bond is smaller than that arising from the interaction by means the F atom and the H-bond. The shifts due to the adsorption of the calculated vibrational frequencies well reproduce the experimental data.

On the Structure Sensitivity of Formic Acid Decomposition on Cu Catalysts

Catalytic decomposition of formic acid (HCOOH) has attracted substantial attention since HCOOH is a major by-product in biomass reforming, a promising hydrogen carrier, and also a potential low temperature fuel cell feed. Despite the abundance of experimental studies for vapor-phase HCOOH decomposition on Cu catalysts, the reaction mechanism and its structure sensitivity is still under debate. In this work, self-consistent, periodic density functional theory calculations were performed on three model surfaces of copper—Cu(111), Cu(100) and Cu(211), and both the HCOO (formate)-mediated and COOH (carboxyl)-mediated pathways were investigated for HCOOH decomposition. The energetics of both pathways suggest that the HCOO-mediated route is more favorable than the COOH-mediated route on all three surfaces, and that HCOOH decomposition proceeds through two consecutive dehydrogenation steps via the HCOO intermediate followed by the recombinative desorption of H2. On all three surfaces, HCOO dehydrogenation is the likely rate determining step since it has the highest transition state energy and also the highest activation energy among the three catalytic steps in the HCOO pathway. The reaction is structure sensitive on Cu catalysts since the examined three Cu facets have dramatically different binding strengths for the key intermediate HCOO and varied barriers for the likely rate determining step—HCOO dehydrogenation. Cu(100) and Cu(211) bind HCOO much more strongly than Cu(111), and they are also characterized by potential energy surfaces that are lower in energy than that for the Cu(111) facet. Coadsorbed HCOO and H represents the most stable state along the reaction coordinate, indicating that, under reaction conditions, there might be a substantial surface coverage of the HCOO intermediate, especially at under-coordinated step, corner or defect sites. Therefore, under reaction conditions, HCOOH decomposition is predicted to occur most readily on the terrace sites of Cu nanoparticles. As a result, we hereby present an example of a fundamentally structure-sensitive reaction, which may present itself as structure-insensitive in typical varied particle-size experiments.

Insights into the interaction between CH2F2 and titanium dioxide: DRIFT spectroscopy and DFT analysis of the adsorption energetics.

Difluoromethane (CH2F2, HFC-32) has been proposed as a valid replacement for both CFCs and HCFCs (in particular HCFC-22), and nowadays it is widely used in refrigerant mixtures. Due to its commercial use, in the last years, the atmospheric concentration of HFC-32 has increased significantly. However, this molecule presents strong absorptions within the 8-12μm atmospheric window, and hence it is a greenhouse gas which contributes to global warming. Heterogeneous photocatalysis over TiO2 surface is an interesting technology for removing atmospheric pollutants since it leads to the decomposition of organic compounds into simpler molecules. In the present work, the adsorbate-substrate interaction between CH2F2 and TiO2 is investigated by coupling experimental measurements using DRIFT spectroscopy to first-principle simulations at DFT/B3LYP level. The experimental results confirm that CH2F2 interacts with the TiO2 surface (∼80% rutile, 20% anatase) through both F and H atoms and show that the DRIFT technique is well suited to study the adsorption of halogenated methanes over semiconductor surfaces. DFT calculations are carried out by considering different periodicities and surface coverages, according to a structure involving an acid-base interaction between the F and Ti(4+) atoms as well as an H-bond between the CH2 group and an O(2-) ion. Lateral effects and energetics are analyzed in the limit of low coverage according to a procedure taking into account the binding, interaction, and distortion energies. The simulation at the different surface coverages and periodicities suggests similar decomposition pathways for the different investigated ensemble configurations.

DFT calculations of carbon monoxide adsorbed on anatase TiO2 (101) and (001) surfaces: correlation between the binding energy and the CO stretching frequency

The adsorption of carbon monoxide (CO) on anatase (101) and (001) surfaces was simulated using periodic density functional theory calculations. The surface Lewis acidity was evaluated by computing the binding energy and the adsorbed CO stretching frequency at surface coverages equal to 1 and 0.25 monolayer (ML). The obtained results, in agreement with the experimental data, indicate that the Ti cation of the (101) surface is more electrophilic than that of the (001) surface, corresponding to a larger surface Lewis acidity. A nearly linear correlation between the calculated binding energy and the CO stretching frequency was found for the first time at the computational level. The effects of slab relaxation on the two surfaces were also investigated and an opposite behaviour was found for the two parameters.