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We investigated the oxidation of CH4 on oxygen-pre-covered IrO2(110) surfaces using temperature-programmed reaction spectroscopy (TPRS) and density functional theory (DFT). Our TPRS results show that on-top oxygen (Oot) species hinder CH4 adsorption, providing evidence that CH4 adsorbs on coordinatively unsaturated Ir atoms. We also find that the fractional yield of adsorbed CH4 that reacts during TPRS remains constant at ∼70% as the Oot-coverage increases to about 0.5 monolayer for saturation CH4 coverage, demonstrating that O-rich IrO2(110) surfaces are highly active in promoting CH4 C–H bond cleavage. Our results show that Oot atoms promote CH4 oxidation to CO2 as well as H2O formation while suppressing CO and recombinative CH4 desorption, as evidenced by an increase in the fractional yield of CO2 produced during TPRS and a downshift of CO2 and H2O TPRS peak maxima with increasing Oot-coverage. DFT predicts that initial CH4 bond cleavage is highly facile on both stoichiometric and O-rich IrO2(110) and can occur by either H-transfer to an Oot or a bridging O-atom of the surface. Our calculations also predict that oxidation of the CH x species that result from CH4 activation is more facile on O-rich compared with stoichiometric IrO2(110), and that complete oxidation is strongly favored on the O-rich surface, in good agreement with our experimental findings. According to the calculations, key steps in the CH4 oxidation pathway have significantly lower-energy barriers on O-rich vs stoichiometric IrO2(110) because these steps involve reaction with Oot atoms initially present on the surface rather than the abstraction of more strongly bound Obr species. High coverages of O-atoms also enable adsorbed intermediates to oxidize extensively on O-rich IrO2(110), without the intermediates needing to overcome diffusion barriers to access reactive O-atoms. Our results provide insights for understanding CH4 oxidation on IrO2(110) surfaces under reaction conditions at which Oot atoms and adsorbed CH4 can co-exist.

The adsorption of CO on clean and oxidized Pd(111) surfaces has been investigated using a combination of high-resolution core level spectroscopy (HRCLS), reflection absorption infrared spectroscopy (RAIRS), and density functional theory (DFT) calculations. The HRCLS and RAIRS measurements reveal that CO adsorbs on Pd(111), Pd 5 O 4 and PdO(101) at 100 ± 10 K and that the CO coverage decreases with increasing oxidation state of Pd for the same CO exposures of 10 Langmuirs. Based on the DFT calculations, the CO layer on clean Pd(111) was found to include molecular adsorption in both hollow and bridge sites, whereas CO occupies a combination of bridge and atop sites on the Pd 5 O 4 and PdO(101) surfaces.

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The spread of a source host is the number of distinct destinations that it has sent packets to during a measurement period. A spread estimator is a software/hardware module on a router that inspects the arrival packets and estimates the spread of each source. It has important applications in detecting port scans and DDoS attacks, measuring the infection rate of a worm, assisting resource allocation in a server farm, determining popular web contents for caching, to name a few. The main technical challenge is to fit a spread estimator in a fast but small memory (such as SRAM) in order to operate it at the line speed in a high-speed network. In this paper, we design a new spread estimator that delivers good performance in tight memory space where all existing estimators no longer work. The new estimator not only achieves space compactness but operates more efficiently than the existing ones. Its accuracy and efficiency come from a new method for data storage, called virtual vectors, which allow us to measure and remove the errors in spread estimation. We perform experiments on real Internet traces to verify the effectiveness of the new estimator.