The ammonia oxidation reaction on Pt{ 100) has been investigated over the temperature range 300-800 K, using molecular beams under UHV conditions. The reaction is biphasic, with N2 being the major product below 600 K and NO being the major product above 600 K. It is found that product selectivity can be controlled by varying the beam composition as well as by varying the surface temperature. The efficiency of the reaction to NO can be significantly increased by preadsorption of oxygen on the crystal. Coadsorption and isothermal experiments translate the beam composition dependence into a surface oxygen coverage dependence, with high oxygen coverages resulting in the suppression of N2 production. N2 is believed to be produced mainly from the dissociation of NO produced by oxidation of adsorbed N H 3 . The observed oxygen coverage dependence of product formation is explained by a sharp fall in the heat of adsorption of the dissociated N(a) and O(a) with increasing O(a) coverage. At high surface oxygen coverages the suppression of NZ production arises from the resulting inhibition of NO dissociation. The observed surface temperature dependence of product formation is explained by competition between NO desorption and NO dissociation.
We have made the first dynamic measurement of the (1 xl)-CO island growth rate and the simultaneous CO coverage on the quasihexagonal reconstructed (hex-/?) phase during the CO-induced hex-/? -(1 xl) phase transformation on Pt{l00}. The island growth rate is described by a strongly nonlinear power law with respect to the local CO coverage on the hex-/? phase at surface temperatures between 380 and 410 K, with an apparent reaction order of 4.5 ±0.4. These kinetics manifest themselves as a strongly flux dependent net sticking probability.PACS numbers: 68.35.Bs, 68.45.Da Numerous observations have now been made of surface structural phase changes induced by the presence of adsorbates. Despite this, there are no studies available which establish the growth mechanism of the islands of the new phase from the stable clean surface phase. In this Letter we present a study of the conversion of the stable surface of clean Pt{l00}, described as hex-/?, to a (lxl) configuration during CO adsorption. It represents the first dynamic measurement of the (1 x l)-CO island growth rate and the simultaneous CO coverage on the hex-/? phase during the surface structural transformation. The results show that 4 to 5 CO molecules are involved in the restructuring process, leading to a strongly nonlinear growth dependence. This is crucial to the understanding of the nucleation and growth mechanism and also oscillatory reactions involving CO on Pt{l00} [1,2].The most stable phase of the clean Pt{l00} surface is a reconstructed phase in which the surface layer of Pt atoms has a quasihexagonal structure. Following Heilmann, Heinz, and Muller [3], we will refer to this surface as Pt{l00}-hex-/?0.7° or simply hex-/?. A metastable (lxl) clean surface (the bulk truncation structure) can be prepared which reconstructs irreversibly above =^ 400 K [4,5] to form the "hex" surface, which in turn reconstructs to form the hex-/? surface at ^ 1100 K. The hex and hex-/? surfaces differ by a rotation of the hexagonal layer by -0.7° with respect to the bulk. The hex and hex-/? reconstructions are lifted by the adsorption of CO, NO, O2, and other adsorbates to yield the (lxl) phase. The driving force of the CO-induced hex-* (lxl) surface phase transition was identified by Thiel et al. [6] from a low energy electron diffraction (LEED) study to be the higher heat of adsorption of CO on the (lxl) phase than on the reconstructed phase. Thiel et al. also proposed that the transformation occurs by sequential steps of adsorption on the reconstructed surface followed by migration and "trapping" of CO onto growing "islands" of the (lxl) surface, initially formed by nucleation. It is known that a critical CO coverage of ca. 0.05 monolayer (ML) on the hex phase is required to induce the hex-* (lxl) phase transformation [7,8] [where
Mammalian oocytes and early cleavage-stage embryos are critically dependent on their ˜100,000 mitochondria to develop from ovulation to compacted morula stage. They rely almost solely on oxidative phosphorylation of multiple intracellular substrates- namely pyruvate, fatty acids and glutamine- for production of ATP. Increasing evidence exists for the requirement of both fatty acids and pyruvate for mammalian developmental potential. Fatty acids are stored as neutral lipids in lipid droplets, which are liberated into the cytoplasm as free fatty acids and taken up into mitochondria for metabolism. Different mammalian species exhibit different amounts of stored and free lipid, while the types of lipid present tend to remain constant. It is thought that the amount of lipid contained in the oocytes of mammalian species reflects the extent of β-oxidation, but it is unclear why large differences are seen in lipid content. Maternal high fat diet or obesity causes negative intracellular effects such as the ER stress response, and oxidative mitochondrial and DNA damage. While some mechanisms have been established, it is still unclear exactly how high fat leads to compromised oocyte and embryo quality. It is proposed that healthy mammalian oocyte mitochondria require a balance of pyruvate and fatty acid oxidation in order to maintain a low level of otherwise damaging ROS production. This balance is disrupted in conditions of excess or insufficient substrate.
Mammalian oocytes contain lipid droplets that are a store of fatty acids, whose metabolism plays a substantial role in pre-implantation development. Fluorescent staining has previously been used to image lipid droplets in mammalian oocytes and embryos, but this method is not quantitative and often incompatible with live cell imaging and subsequent development. Here we have applied chemically specific, label-free coherent anti-Stokes Raman scattering (CARS) microscopy to mouse oocytes and pre-implantation embryos. We show that CARS imaging can quantify the size, number and spatial distribution of lipid droplets in living mouse oocytes and embryos up to the blastocyst stage. Notably, it can be used in a way that does not compromise oocyte maturation or embryo development. We have also correlated CARS with two-photon fluorescence microscopy simultaneously acquired using fluorescent lipid probes on fixed samples, and found only a partial degree of correlation, depending on the lipid probe, clearly exemplifying the limitation of lipid labelling. In addition, we show that differences in the chemical composition of lipid droplets in living oocytes matured in media supplemented with different saturated and unsaturated fatty acids can be detected using CARS hyperspectral imaging. These results demonstrate that CARS microscopy provides a novel non-invasive method of quantifying lipid content, type and spatial distribution with sub-micron resolution in living mammalian oocytes and embryos.
Absolute coverages and hysteresis phenomena associated with the COinduced Pt(100) hex(1×1) phase transition A molecular beam study of the catalytic oxidation of CO on a Pt(111) surface
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