CO Adsorption on Supported Gold Nanoparticle Catalysts: Application of the Temkin Model

The adsorption of CO on the supported gold nanoparticle catalysts Au/TiO2, Au/Fe2O3, and Au/ZrO2 was examined using infrared transmission spectroscopy to quantify the isobaric CO coverage as a function of temperature. The Temkin adsorbate interaction model was then applied to account for the adsorption behavior. To test the general applicability of the Temkin model, this treatment was also applied to three data sets from the literature. This included another real-world catalyst and two model catalysts. All data sets were accurately represented by the Temkin adsorbate interaction model. The resulting thermodynamic metrics are consistent with previous determinations and reflect a particle size-dependence. In particular, the intrinsic adsorption enthalpy at zero CO coverage varies almost linearly with Au particle size, and this trend appears to be correlated with the abundance of low-coordinate Au sites (cf., CN = 6 and 7 for corners and edges, respectively). For very small particles with mostly CN = 6 corner sites, the enthalpy reflects strong binding (cf., −ΔH0 ≈ 78 kJ/mol), while for large particles with mostly CN = 7 edge sites, the enthalpy reflects weaker binding (cf., −ΔH0 ≈ 63 kJ/mol). The results also suggest that these sites are coupled. This study demonstrates that the Temkin adsorbate interaction model accurately represents adsorption data, yields meaningful metrics that are useful for characterizing nanoparticle catalysts, and should be applicable to other adsorption data sets

Characterization of a pneumococcal meningitis mouse model

<p>Abstract</p> <p>Background</p> <p><it>S. pneumoniae </it>is the most common causative agent of meningitis, and is associated with high morbidity and mortality. We aimed to develop an integrated and representative pneumococcal meningitis mouse model resembling the human situation.</p> <p>Methods</p> <p>Adult mice (C57BL/6) were inoculated in the cisterna magna with increasing doses of <it>S. pneumoniae </it>serotype 3 colony forming units (CFU; n = 24, 10⁴, 10⁵, 10⁶and 10⁷CFU) and survival studies were performed. Cerebrospinal fluid (CSF), brain, blood, spleen, and lungs were collected. Subsequently, mice were inoculated with 10⁴CFU <it>S. pneumoniae </it>serotype 3 and sacrificed at 6 (n = 6) and 30 hours (n = 6). Outcome parameters were bacterial outgrowth, clinical score, and cytokine and chemokine levels (using Luminex^®) in CSF, blood and brain. Meningeal inflammation, neutrophil infiltration, parenchymal and subarachnoidal hemorrhages, microglial activation and hippocampal apoptosis were assessed in histopathological studies.</p> <p>Results</p> <p>Lower doses of bacteria delayed onset of illness and time of death (median survival CFU 10⁴, 56 hrs; 10⁵, 38 hrs, 10⁶, 28 hrs. 10⁷, 24 hrs). Bacterial titers in brain and CSF were similar in all mice at the end-stage of disease independent of inoculation dose, though bacterial outgrowth in the systemic compartment was less at lower inoculation doses. At 30 hours after inoculation with 10⁴CFU of <it>S. pneumoniae</it>, blood levels of KC, IL6, MIP-2 and IFN- γ were elevated, as were brain homogenate levels of KC, MIP-2, IL-6, IL-1β and RANTES. Brain histology uniformly showed meningeal inflammation at 6 hours, and, neutrophil infiltration, microglial activation, and hippocampal apoptosis at 30 hours. Parenchymal and subarachnoidal and cortical hemorrhages were seen in 5 of 6 and 3 of 6 mice at 6 and 30 hours, respectively.</p> <p>Conclusion</p> <p>We have developed and validated a murine model of pneumococcal meningitis.</p

CO as a promoting spectator species of CxHy conversions relevant for Fischer-Tropsch chain growth on cobalt: evidence from temperature-programmed reaction and reflection absorption infrared spectroscopy

Cobalt-catalyzed low temperature Fischer-Tropsch synthesis is a prime example of an industrially relevant reaction in which CxHy intermediates involved in chain growth react in the presence of a large quantity of COad. In this study, we use a Co(0001) single-crystal model catalyst to investigate how CO, adsorbed alongside CxHy adsorbates affects their reactivity. Temperature-programmed reaction spectroscopy was used to determine the hydrogen content of the CxHy intermediates formed at different temperatures, and infrared absorption spectroscopy was used to obtain more specific information on the chemical identity of the various reaction intermediates formed. Ethene, propene, and but-1-ene precursors decompose below 200 K. The 1-alkyne adsorbate is identified as a major product, and some alkylidyne species form as well when the initial alkene coverage is high. The surface hydrogen atoms produced in the low temperature decomposition step start leaving the surface >300 K. When an alkyne/Had-covered surface is heated in the presence of CO, the alkyne adsorbates are hydrogenated to the corresponding alkylidyne at temperatures <250 K. This finding shows that CxHy surface species react differently in the presence of COad, a notion of general importance for catalytic reactions where both CO and CxHy species are present. In the context of Fischer-Tropsch synthesis, the observed CO-induced reaction is of specific importance for the alkylidyne chain growth mechanism. In this reaction, scheme hydrocarbon chains grow via coupling of CHad with a (Cn) alkylidyne adsorbate to produce the (Cn+1) alkyne. A subsequent hydrogenation of the alkyne product to the corresponding alkylidyne is required for further growth. The present work shows that this specific reaction is promoted by the presence of CO. This suggests that the influence of CO spectators on the stability of CxHy surface intermediates is beneficial for efficient chain growth

The effect of C-OH functionality on the surface chemistry of biomass-derived molecules: Ethanol chemistry on Rh(100)

The adsorption and decomposition of ethanol on Rh(100) was studied as a model reaction to understand the role of C-OH functionalities in the surface chemistry of biomass-derived molecules. A combination of experimental surface science and computational techniques was used: (i) temperature programmed reaction spectroscopy (TPRS), reflection absorption infrared spectroscopy (RAIRS), work function measurements (Kelvin Probe-KP), and density functional theory (DFT). Ethanol produces ethoxy (CH3CH2O) species via O-H bond breaking upon adsorption at 100 K. Ethoxy decomposition proceeds differently depending on the surface coverage. At low coverage, the decomposition of ethoxy species occurs via β-C-H cleavage, which leads to an oxometallacycle (OMC) intermediate. Decomposition of the OMC scissions (at 180-320 K) ultimately produces CO, H2 and surface carbon. At high coverage, along with the pathway observed in the low coverage case, a second pathway occurs around 140-200 K, which produces an acetaldehyde intermediate via α-C-H cleavage. Further decomposition of acetaldehyde produces CH4, CO, H2 and surface carbon. However, even at high coverage this is a minor pathway, and methane selectivity is 10% at saturation coverage. The results suggests that biomass-derived oxygenates, which contain an alkyl group, react on the Rh(100) surface to produce synthesis gas (CO and H2), surface carbon and small hydrocarbons due to the high dehydrogenation and C-C bond scission activity of Rh(100)

Benzene adsorption and oxidation on Ir(111)

Adsorption, decompn. and oxidn. of benzene on Ir(1 1 1) was studied by high resoln. (synchrotron) XPS, temp. programmed desorption and LEED. Mol. adsorption of benzene on Ir(1 1 1) is obsd. between 170 K and 350 K. Above this temp. both desorption and decompn. of benzene take place. An ordered adsorbate structure was obsd. upon adsorption around 335 K. Decompn. involves C-C bond breaking as the formation of CHad is obsd. The presence of a satd. Oad layer (0.5 ML) weakens mol. benzene adsorption and suppresses decompn. [on SciFinder (R)

Methanol decomposition and oxidation on Ir(111)

The adsorption, decompn., and oxidn. of methanol (CH3OH) has been studied on Ir(111) using temp.-programmed desorption and high-energy resoln. fast XPS. Mol. methanol desorption from a methanol-satd. surface at low temp. shows three desorption peaks, around 150 K (alpha ), around 170 K (beta 1), and around 220 K (beta 2), resp. The alpha peak is assigned to methanol adsorbed on top of the first, chemisorbed layer, whereas beta 1 and beta 2 are both assigned to methanol directly coordinated to the metal surface atoms (chemisorbed). The CH3OHad responsible for the beta 2 desorption peak appears as a sep. component in the C 1s core level spectra. A part of the initially adsorbed methanol decomps. into COad and Had around (or even below) 175 K. Intermediate CHxO species of CH3OH decompn. were not obsd. The formation of a small amt. of CHxad indicates that (Hx)C-O(H) bond scission occurs as well. Temp.-programmed desorption expts. confirm that CHxad species form, as evidenced by a high-temp. (500 K) H2 formation peak due to decompn. of CHad. The presence of Oad causes a downward shift in the C 1s and O 1s BEs of molecularly adsorbed methanol, but the desorption barrier for mol. methanol desorption is not significantly influenced by the presence of Oad. A stable reaction intermediate, most probably methoxy (CH3Oad), was obsd. in the presence of Oad, between 160 and 220 K. It is an intermediate in the formation of both formate (HCO2ad) and COad, which occurs around 220 K. Formate decomps. around 350 K into CO2 (g) and Had (which reacts with the remaining oxygen to H2O), whereas the COad reacts with Oad around 400 K. [on SciFinder (R)

Ethanol adsorption, decomposition and oxidation on Ir(111) : a high resolution XPS study

Ethanol (C2H5OH) adsorption, decomposition and oxidation is studied on Ir(111) using high-energy resolution, fast XPS and temperature-programmed desorption. During heating of an adsorbed ethanol layer a part of the C2H5OHad desorbs molecularly, and another part remains on the surface and decomposes around 200 K; these two decomposition pathways are identified, as via acetyl (H3CCO) and via COad+CH3ad, respectively. Acetyl and CH3ad decompose around 300 K into CHad (and COad). CHad decomposes forming Cx and H2 around 520 K. In the presence of Oad an acetate intermediate is formed around 180 K, as well as a small amount of CH3ad and COad. Acetate decomposes between 400-480 K into CO2, H2(/H2O) and CHad