Catalyst passivation through carbon poisoning is a common and costly problem as it reduces the lifetime and performance of the catalyst. Adding oxygen to the feed stream could reduce poisoning but may also affect the activity negatively. We have studied the dehydrogenation, decomposition, and desorption of naphthalene co-adsorbed with oxygen on Ni(111) by combining temperature-programmed desorption (TPD), sum frequency generation spectroscopy (SFG), photoelectron spectroscopy (PES), and density functional theory (DFT). Chemisorbed oxygen reduces the sticking of naphthalene and shifts H2 production and desorption to higher temperatures by blocking active Ni sites. Oxygen increases the production of CO and reduces carbon residues on the surface. Chemisorbed oxygen is readily removed when naphthalene is decomposed. Oxide passivates the surface and reduces the sticking coefficient. But it also increases the production of CO dramatically and reduces the carbon residues. Ni2O3 is more active than NiO.

The aromatic hydrocarbons (AHs) fluorobenzene, naphthalene, and 1-fluoronaphthalene are introduced as promising alternatives to xenon as propellant for in-space electric propulsion (EP). These storable molecules have similar mass, lower cost, and lower ionization energies compared to xenon, as well as the critical advantage of low post-ionization fragmentation compared to other molecular propellant candidates. The ionization characteristics of AHs are compared with those of xenon and the diamondoid adamantane, previously evaluated as a molecular propellant for EP. Quantum chemical calculations and BEB theory together with 25 eV electron ionization mass spectrometry (EI-MS) measurements have been used to predict the fragmentation of the AHs and adamantane when ionized in a plasma with an electron temperature of 7 eV (a typical electron temperature in EP plasmas). A high fraction (81 − 86%) of the detected AH ions originate from intact molecules, compared to 34% for adamantane, indicating extraordinarily low fragmentation for the selected AHs. The ionization potential of the AHs is similar to that of adamantane but lower compared to xenon (8.14–9.2 eV for the AHs, 9.25 for adamantane and 12.13 eV for xenon). BEB calculations have also been used to predict total ionization cross sections. The calculated ionization cross section of the AHs is comparable to that of adamantane but 3–5 times higher than that of xenon, which together with the low ionization potential can contribute to more efficient ionization. The AHs may have the potential to perform better than xenon, despite the absence of fragmentation in xenon.

There are several difficulties when experimentally determined reaction mechanisms are applied from model systems to real catalysis. Besides the infamous pressure and material gaps, it is sometimes necessary to consider impurities in the real reactant feedstock that can act as promoters or catalyst poisons and alter the reaction path. In this study, the effect of sulfur on the dehydrogenation of naphthalene on Ni(111) is investigated by using X-ray photoelectron spectroscopy and scanning tunneling microscopy. Sulfur induces a (5 root 3 x 2) surface reconstruction, as previously reported in the literature. The sulfur does not have a strong effect on the dehydrogenation temperature of naphthalene. However, the presence of sulfur leads to a preferred formation of carbidic over graphitic carbon and a strong inhibition of carbon diffusion into the nickel bulk, which is one of the steps of destructive whisker carbon formation described in the catalysis literature.

Clostridium thermocellum is a cellulolytic thermophile that is considered for the consolidated bioprocessing of lignocellulose to ethanol. Improvements in ethanol yield are required for industrial implementation, but the incompletely understood causes of amino acid secretion impede progress. In this study, amino acid secretion was investigated via gene deletions in ammonium-regulated, nicotinamide adenine dinucleotide phosphate (NADPH)-supplying and NADPH-consuming pathways as well as via physiological characterization in cellobiose-limited or ammonium-limited chemostats. First, the contribution of the NADPH-supplying malate shunt was studied with strains using either the NADPH-yielding malate shunt (Δppdk) or a redox-independent conversion of PEP to pyruvate (Δppdk ΔmalE::Peno-pyk). In the latter, branched-chain amino acids, especially valine, were significantly reduced, whereas the ethanol yield increased from 46 to 60%, suggesting that the secretion of these amino acids balances the NADPH surplus from the malate shunt. The unchanged amino acid secretion in Δppdk falsified a previous hypothesis on an ammonium-regulated PEP-to-pyruvate flux redistribution. The possible involvement of another NADPH-supplier, namely, NADH-dependent reduced ferredoxin:NADP+ oxidoreductase (nfnAB), was also excluded. Finally, the deletion of glutamate synthase (gogat) in ammonium assimilation resulted in the upregulation of NADPH-linked glutamate dehydrogenase activity and decreased amino acid yields. Since gogat in C. thermocellum is putatively annotated as ferredoxin-linked, a claim which is supported by the product redistribution observed in this study, this deletion likely replaced ferredoxin with NADPH in ammonium assimilation. Overall, these findings indicate that a need to reoxidize NADPH is driving the observed amino acid secretion, likely at the expense of the NADH needed for ethanol formation. This suggests that metabolic engineering strategies that simplify the redox metabolism and ammonium assimilation can contribute to increased ethanol yields.

We present a new velocity map imaging instrument for studying molecular beam surface scattering in a near-ambient pressure (NAP-VMI) environment. The instrument offers the possibility to study chemical reaction dynamics and kinetics where higher pressures are either desired or unavoidable, adding a new tool to help close the "pressure gap " between surface science and applied catalysis. NAP-VMI conditions are created by two sets of ion optics that guide ions through an aperture and map their velocities. The aperture separates the high pressure ionization region and maintains the necessary vacuum in the detector region. The performance of the NAP-VMI is demonstrated with results from N2O photodissociation and N-2 scattering from a Pd(110) surface, which are compared under vacuum and at near-ambient pressure (1 x 10(-3) mbar). NAP-VMI has the potential to be applied to, and useful for, a broader range of experiments, including photoelectron spectroscopy and scattering with liquid microjets.

The temperature dependent dehydrogenation of naphthalene on Ni(111) has been investigated using vibrational sum-frequency generation spectroscopy, X-ray photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory with the aim of discerning the reaction mechanism and the intermediates on the surface. At 110 K, multiple layers of naphthalene adsorb on Ni(111); the first layer is a flat lying chemisorbed monolayer, whereas the next layer(s) consist of physisorbed naphthalene. The aromaticity of the carbon rings in the first layer is reduced due to bonding to the surface Ni-atoms. Heating at 200 K causes desorption of the multilayers. At 360 K, the chemisorbed naphthalene monolayer starts dehydrogenating and the geometry of the molecules changes as the dehydrogenated carbon atoms coordinate to the nickel surface; thus, the molecule tilts with respect to the surface, recovering some of its original aromaticity. This effect peaks at 400 K and coincides with hydrogen desorption. Increasing the temperature leads to further dehydrogenation and production of H-2 gas, as well as the formation of carbidic and graphitic surface carbon. 

We report measurements to investigate the effects of mechanical strain on the binding energy of carbon monoxide (CO) on the (111) surface of a 16 nm thin film of palladium (Pd) grown on rutile titanium dioxide (r-TiO2). The lattice mismatch between Pd and the r-TiO2 leads to a tensile mechanical in-plane stress in the Pd layer of approximately 0.38 GPa. We observe an increase of (40 +/- 10) kJ mol(-1) in the CO binding energy for the 16 nm sample compared to a bulk Pd(111) crystal, which is in qualitative agreement with expectations based on the d-band model.

Catalysts are widely used to increase reaction rates. They function by stabilizing the transition state of the reaction at their active site, where the atomic arrangement ensures favourable interactions 1. However, mechanistic understanding is often limited when catalysts possess multiple active sites - such as sites associated with either the step edges or the close-packed terraces of inorganic nanoparticles 2-4 - with distinct activities that cannot be measured simultaneously. An example is the oxidation of carbon monoxide over platinum surfaces, one of the oldest and best studied heterogeneous reactions. In 1824, this reaction was recognized to be crucial for the function of the Davy safety lamp, and today it is used to optimize combustion, hydrogen production and fuel-cell operation 5,6. The carbon dioxide products are formed in a bimodal kinetic energy distribution 7-13 ; however, despite extensive study 5, it remains unclear whether this reflects the involvement of more than one reaction mechanism occurring at multiple active sites 12,13. Here we show that the reaction rates at different active sites can be measured simultaneously, using molecular beams to controllably introduce reactants and slice ion imaging 14,15 to map the velocity vectors of the product molecules, which reflect the symmetry and the orientation of the active site 16. We use this velocity-resolved kinetics approach to map the oxidation rates of carbon monoxide at step edges and terrace sites on platinum surfaces, and find that the reaction proceeds through two distinct channels 11-13 : it is dominated at low temperatures by the more active step sites, and at high temperatures by the more abundant terrace sites. We expect our approach to be applicable to a wide range of heterogeneous reactions and to provide improved mechanistic understanding of the contribution of different active sites, which should be useful in the design of improved catalysts.

Time-resolved kinetics of CO oxidation on Pd(100) are studied using near-ambient pressure velocity map imaging (NAP-VMI) with a pulsed molecular beam. We observed two types of bimodel activities to CO oxidation at different oxygen exposures. To explain this behavior, we have developed two kinetic models with two different surface configurations. Three reaction channels are discovered on Pd(100) under oxygen rich environment, representing CO oxidation on different metastable surfaces. We assign those reaction channels to: CO oxidation (1) on a pristine metal surface, (2) on an epitaxial multilayer PdO(101), and (3) on a domain boundary between Pd(100) and (√5 × √5) single layer surface oxide. All reaction channels can be described in the Langmuir–Hinshelwood mechanism. This is a direct evidence of the coexistence of multiple surface activities to the CO oxidation on the Pd(100) surface. 

Tar is an undesirable byproduct of biomass gasification, which can be removed through catalytic reforming to syngas components. Iron is a promising, abundant alternative to highly active but toxic nickel catalysts. The results observed so far in catalytic studies with iron have been mixed. In this paper, the decomposition of naphthalene, a representative model compound of tar, was studied on the catalytic Fe(110) surface using temperature programmed desorption (TPD), sum frequency generation spectroscopy (SFG), and X-ray photoelectron spectroscopy (XPS). Napthalene adsorption, dehydrogenation and the formation of surface carbon were investigated, as well as the influence of oxygen. In comparison with previous studies on Ni(111), a similar dehydrogenation activity was found for Fe(110) with two main desorption peaks at 450 K and 550 K. The reaction of naphthalene on Fe(110) resulted in the predominant formation of carbidic and atomically adsorbed carbon on the surface, which did not dissolve into the bulk even at high temperatures. A moderately carbon-covered surface was shown to still be active toward naphthalene decomposition. Similarly to Ni(111), large amounts of oxygen inhibited the reaction but, at low oxygen doses, very high hydrogen yields were observed, suggesting that Fe(110) could be a valid alternative for tar decomposition.