High CO Coverage Research Articles

Equity in newborn care, evidence from national surveys in low- and middle-income countries

BackgroundHigh coverage of care is essential to improving newborn survival; however, gaps exist in access to timely and appropriate newborn care between and within countries. In high mortality burden settings, health inequities due to social and economic factors may also impact on newborn outcomes. This study aimed to examine equity in co-coverage of newborn care interventions in low- and low middle-income countries in sub-Saharan Africa and South Asia.MethodsWe analysed secondary data from recent Demographic and Health Surveys in 16 countries. We created a co-coverage index of five newborn care interventions. We examined differences in coverage and co-coverage of newborn care interventions by country, place of birth, and wealth quintile. Using multilevel logistic regression, we examined the association between high co-coverage of newborn care (4 or 5 interventions) and social determinants of health.ResultsCoverage and co-coverage of newborn care showed large between- and within-country gaps for home and facility births, with important inequities based on individual, family, contextual, and structural factors. Wealth-based inequities were smaller amongst facility births compared to non-facility births.ConclusionThis analysis underlines the importance of facility birth for improved and more equitable newborn care. Shifting births to facilities, improving facility-based care, and community-based or pro-poor interventions are important to mitigate wealth-based inequities in newborn care, particularly in countries with large differences between the poorest and richest families and in countries with very low coverage of care.

Reveal the nature of particle size effect for CO2 reduction over Pd and Au

Small cluster and periodic surface models with low coverages of intermediates are frequently employed to investigate reaction mechanisms and identify active sites on nanoparticles (NPs) in density functional theory (DFT) studies. However, diverse active sites on NPs cannot be sufficiently represented by these simple models, hampering the in-depth insights into the catalytic behavior of NPs. This paper describes the crucial roles of both model and coverage effect on understanding the nature of active sites for CO2 reduction over Au and Pd NPs using DFT calculations. Terrace sites exhibit higher selectivity for CO than edge sites on Au NPs, which is opposite to the results on Au periodic surfaces. This contradiction reveals the computational model effect on clarifying active site properties. For Pd catalysts, the coverage effect is more significant. On bare Pd NPs and periodic surfaces, the selectivity for CO at edge sites is nearly identical to that at terrace sites, whereas edge sites display higher selectivity for CO than terrace sites in the case of high CO coverages. Through considering the more realistic models and the coverage effect, we successfully describe the size effect of Au and Pd NPs on CO selectivity. More importantly, this work reminds us of the necessity of reasonable models in DFT calculations.

On the structure sensitivity of and CO coverage effects on formic acid decomposition on Pd surfaces

Using density functional theory calculations, the Pd-catalyzed vapor-phase formic acid decomposition was studied, with a focus on the structure sensitivity and CO coverage effects. A comprehensive reaction network was developed on both the (111) and (100) facets of Pd, at CO coverages of 0 and 5/9 monolayer (ML). Pd(100) was determined to be more reactive than Pd(111) at both CO coverages. The introduction of 5/9 ML CO decreased the activity of both facets significantly, due to destabilization of the surface intermediates and transition states on the CO-decorated surfaces. Three reaction pathways were explored on the clean surfaces: the formate (HCOO) pathway, the carboxyl (COOH) pathway leading to the formation of CO2, and the COOH pathway leading to the formation of CO (COOH→CO). Based on the DFT-derived energetics alone, it appears that all three pathways contribute to the reaction on clean Pd, whereas the presence of 5/9 ML of CO inhibits the HCOO pathway on both facets and favors the COOH→CO pathway on the (111) facet, but the COOH→CO2 one on the (100) facet. Moreover, at high CO coverages, alternative spectator CO-assisted adsorbate decomposition pathways were discovered, which could potentially play a role in formic acid decomposition on Pd catalysts under realistic reaction conditions.

Interplay between CO Disproportionation and Oxidation: On the Origin of the CO Reaction Onset on Atomic Layer Deposition-Grown Pt/ZrO2 Model Catalysts.

Pt/ZrO2 model catalysts were prepared by atomic layer deposition (ALD) and examined at mbar pressure by operando sum frequency generation (SFG) spectroscopy and near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) combined with differentially pumped mass spectrometry (MS). ALD enables creating model systems ranging from Pt nanoparticles to bulk-like thin films. Polarization-dependent SFG of CO adsorption reveals both the adsorption configuration and the Pt particle morphology. By combining experimental data with ab initio density functional theory (DFT) calculations, we show that the CO reaction onset is determined by a delicate balance between CO disproportionation (Boudouard reaction) and oxidation. CO disproportionation occurs on low-coordinated Pt sites, but only at high CO coverages and when the remaining C atom is stabilized by a favorable coordination. Thus, under the current conditions, initial CO oxidation is found to be strongly influenced by the removal of carbon deposits formed through disproportionation mechanisms rather than being determined by the CO and oxygen inherent activity. Accordingly, at variance with the general expectation, rough Pt nanoparticles are seemingly less active than smoother Pt films. The applied approach enables bridging both the “materials and pressure gaps”.

Mechanistic insight into carbon-carbon bond formation on cobalt under simulated Fischer-Tropsch synthesis conditions

Facile C-C bond formation is essential to the formation of long hydrocarbon chains in Fischer-Tropsch synthesis. Various chain growth mechanisms have been proposed previously, but spectroscopic identification of surface intermediates involved in C-C bond formation is scarce. We here show that the high CO coverage typical of Fischer-Tropsch synthesis affects the reaction pathways of C2Hx adsorbates on a Co(0001) model catalyst and promote C-C bond formation. In-situ high resolution x-ray photoelectron spectroscopy shows that a high CO coverage promotes transformation of C2Hx adsorbates into the ethylidyne form, which subsequently dimerizes to 2-butyne. The observed reaction sequence provides a mechanistic explanation for CO-induced ethylene dimerization on supported cobalt catalysts. For Fischer-Tropsch synthesis we propose that C-C bond formation on the close-packed terraces of a cobalt nanoparticle occurs via methylidyne (CH) insertion into long chain alkylidyne intermediates, the latter being stabilized by the high surface coverage under reaction conditions.

Constraining CO coverage on copper promotes high-efficiency ethylene electroproduction

The availability of inexpensive industrial CO gas streams motivates efficient electrocatalytic upgrading of CO to higher-value feedstocks such as ethylene. However, the electrosynthesis of ethylene by the CO reduction reaction (CORR) has suffered from low selectivity and energy efficiency. Here we find that the recent strategy of increasing performance through use of highly alkaline electrolyte—which is very effective in CO2RR—fails in CORR and drives the reaction to acetate. We then observe that ethylene selectivity increases when we constrain (decrease) CO availability. Using density functional theory, we show how CO coverage on copper influences the reaction pathways of ethylene versus oxygenate: lower CO coverage stabilizes the ethylene-relevant intermediates whereas higher CO coverage favours oxygenate formation. We then control local CO availability experimentally by tuning the CO concentration and reaction rate; we achieve ethylene Faradaic efficiencies of 72% and a partial current density of >800 mA cm−2. The overall system provides a half-cell energy efficiency of 44% for ethylene production. The electrocatalytic upgrading of CO to higher-value feedstocks provides a promising route to multicarbon products. Here, the authors show that high ethylene selectivity can be achieved by constraining CO availability on copper, with an ethylene Faradaic efficiency of 72% and a partial current density of >800 mA cm−2.

Facile Fischer–Tropsch Chain Growth from CH2 Monomers Enabled by the Dynamic CO Adlayer

Numerous chain growth mechanisms, namely CO insertion and carbide, and active sites (flat and stepped surfaces) have been proposed to explain how hydrocarbons are formed from syngas during the Fischer–Tropsch reaction on Ru catalysts, particularly active and selective toward long-chain products. While these reaction pathways are supported by density functional theory (DFT) calculations, computational models often considered surfaces at rather low adsorbate coverage. A systematic comparison of chain growth mechanisms including the CO adlayer present on the catalyst’s surface under reaction conditions is therefore not available due to the challenging representation of co-adsorbate interactions in DFT models. Here, we show that the high coverage of chemisorbed CO on the metal surface favors the carbide mechanism on flat surfaces according to ab initio molecular dynamics simulations, which introduce the complex adlayer effects at the reaction temperature of 200 °C. At the considered CO and H coverages (0.50–0.72 and 0.24 monolayer, respectively) hydrocarbon formation involves CH2 monomers yielding ethylene and propylene as primary products, consistent with the selectivity observed in experiments. Such mechanism is favored by the presence of the CO adlayer. Indeed, in the absence of co-adsorbed CO, methane may be formed on the flat surface and the first C–C bond occurs preferentially on stepped surfaces via CH and CH2 monomers with a higher free-energy barrier (55 kJ mol–1) compared to the coupling of two CH2 species at high CO coverage on the flat surface (20 kJ mol–1). Therefore, the CO adlayer strongly modulates the nature of chain growth monomers and active sites and drives the formation of hydrocarbons during Ru-catalyzed Fischer–Tropsch. Overall, these results show how adsorbate–adsorbate interactions dictate reaction mechanisms operating in adlayers, ubiquitous in heterogeneous catalysis.

The effect of CO on CO2 methanation over Ru/Al2O3 catalysts: a combined steady-state reactivity and transient DRIFT spectroscopy study

The reactivity of Ru/Al2O3 catalysts in the hydrogenation of CO/CO2 gas stream is investigated in this work to assess the possibility of carrying out CO2 methanation even in the presence of CO in the feed stream. Such a goal is pursued by conducting reactivity studies at process conditions of industrial interest (i.e., at high COx per-pass conversion and with concentrated COx/H2 streams) and by monitoring the surface species on the catalyst through transient DRIFTS-MS analysis. The catalyst shows gradual deactivation when the methanation is carried out in the presence of CO in the gas feed at low temperatures (200–300 °C). However, stable performance is observed at higher temperatures, showing CH4 yields even higher than those observed during methanation of a pure CO2 feed. DRIFTS-MS experiments agree with a CO2 methanation pathway where CO2 is adsorbed as bicarbonate on Al2O3 and successively hydrogenated to methane on Ru, passing through formate and carbonyl intermediates. In the presence of CO at low temperature, the catalyst shows a higher CO coverage of the Ru sites, a larger formate coverage of the alumina sites and the presence of adsorbed carbonaceous species, identified as carboxylate and hydrocarbon species. By carrying out the CO2 hydrogenation on the deactivated catalyst, carboxylates remain on the surface, effectively blocking CO2 adsorption sites. However, the catalyst deactivation at low temperature is reversible as thermal treatment (>350 °C) is able to restore the catalytic activity. Notably, working above the carboxylate decomposition temperature ensures a clean catalyst surface without high CO coverage, resulting in stable and high performance in CO/CO2 methanation.

High-Coverage CO Adsorption and Dissociation on Ir(111), Ir(100), and Ir(110) from Computations

CO adsorption on the perfect Ir(111) as well as the unreconstructed Ir(100) and Ir(110) surfaces at different coverages has been computed at the level of periodic density functional theory. General...

Weakening the strength of CO binding on subsurface alloyed Pt(111)

The interaction of CO with Pt/3d/Pt(111) subsurface alloys (3d: Ni, Co, Fe) was investigated by combination of vibrational spectroscopy and temperature programmed desorption. The binding strength of CO is significantly reduced on Pt/Fe/Pt(111) and Pt/Co/Pt(111) and is sorted in the following order: Pt>Ni>Co≈Fe. Our analysis shows that the 3d admetal is stable in the subsurface region and CO is linearly bonded to the atop sites of the Pt(111) surface. At high CO coverage, compressed structures are obtained. At low CO coverage, the desorption activation energy of CO (∼143 kJ/mol for Pt(111)) drops to 124 kJ/mol for Pt/Ni/Pt(111), and goes further down to 111 and 110 kJ/mol for Pt/Co/Pt(111) and Pt/Fe/Pt(111), respectively. The enhancement in the surface activity is attributed to the modified backdonation to the antibonding states of adsorbed CO due to the downshifted 5d-band center of Pt upon embedding 3d metals in the subsurface region.

Polarization-Dependent SFG Spectroscopy of Near Ambient Pressure CO Adsorption on Pt(111) and Pd(111) Revisited

Polarization-dependent sum frequency generation (SFG) vibrational spectroscopy was employed to examine CO overlayers on Pt(111) and Pd(111) single crystal surfaces at room temperature. Utilizing different polarization combinations (SSP and PPP) of the visible and SFG light allows to determine the molecular orientation (tilt angle) of interface molecules but the analysis of the measured I_{text{ppp}}/I_{text{ssp}} is involved and requires a proper optical interface model. For CO/Pt(111), the hyperpolarizability ratio left( {R={beta _{aac}}/{beta _{ccc}}={beta _{bbc}}/{beta _{ccc}}} right) is not exactly known and varying R in the range 0.1–0.5 yields tilt angles of 40°–0°, respectively. Based on the known perpendicular adsorption of CO on Pt, an exact R-value of 0.49 was determined. Polarization-dependent SFG spectra in the pressure range 10−4 to 36 mbar did not indicate any change of the tilt angle of adsorbed CO. Modeling also indicated a strong dependence of {I_{{text{ppp}}}}/{I_{{text{ssp}}}} on the incidence angles of visible and IR laser beams. Complementing previous low temperature/low pressure data, room temperature CO adsorption on Pd(111) was examined from 10−6 to 250 mbar. The absolute PPP and SSP spectral intensities on Pt and Pd were simulated, as well as the expected {I_{{text{ppp}}}}/{I_{{text{ssp}}}} ratios. Although CO on Pt and Pd should exhibit similar intensities (at high CO coverage), the higher {I_{{text{ppp}}}}/{I_{{text{ssp}}}} ratio for Pd (48 vs. 27 on Pt) renders the detection of adsorbed CO in SSP spectra difficult. The presence or absence of CO species in SSP spectra can thus not simply be correlated to tilted or perpendicular CO molecules, respectively. Careful modeling, including not only molecular and interface properties, but also the experimental configuration (incidence angles), is certainly required even for seemingly simple adsorbate–substrate systems.

Modification of Surface Reactivity by CO: Effects on Decomposition and Polymerization of Acetaldehyde on Ru(0001)

The adsorption and reaction of acetaldehyde on the clean and CO pre-covered Ru(0001) surfaces have been investigated using temperature programmed desorption method. On the clean Ru(0001) surface, the decomposition of acetaldehyde is the main reaction channel, with little polymerization occurring. However, on the CO pre-covered Ru(0001) surface, the decomposition of acetaldehyde is inhibited considerably with increasing CO coverage. Whereas, the polymerization occurs efficiently, especially at high CO coverage (θCO>0.5 ML), which is strongly CO coverage dependent. Combined with previous studies, the well-ordered hexagonal structure of CO layer formed on the Ru(0001) surface at high CO coverage that matches the configuration of paraldehyde is likely to be the origin of this remarkable phenomenon.

Theoretical Study on the Catalysis Effect of Platinum Cluster during Carbon Monoxide Polymer Growth

AbstractThe polymerization of carbon monoxide on Pt55 cluster has been studied using density functional theory. Firstly, some randomly generated oxocarbons, ranging from C2 species to C9 species, are obtained on the Pt cluster at high CO coverage. The high stability of those adsorbed oxocarbons is demonstrated by ab initio molecular dynamics simulations, and the high CO coverage can hinder the decomposition of oxocarbons especially for C2O2. By comparing the favorite structures of C4O4, C5O5 and C6O6 before and after adsorbed on the Pt cluster, it is found that all the oxocarbons preferred to be linear rather than cyclic or lactone‐like when adsorbed on the Pt cluster because of the stronger interaction between linear oxocarbons and Pt surface. Several different mechanisms of oxocarbon growth are also investigated, and we find that small oxocarbons prefer to couple with co‐adsorbed CO to form larger oxocarbons with overcoming an energy barrier of ∼ 1 eV. The high stability and low energy barrier of oxocarbon growth indicate the utilization of Pt cluster can significantly overcome the drawbacks of CO polymerization in diamond anvil cell: extreme condition and very low yield.

Simultaneous time-resolved ATR-SEIRAS and CO-charge displacement experiments: The dynamics of CO adsorption on polycrystalline Pt

The high sensitivity of ATR-SEIRAS allowed simultaneous recording of time-resolved infrared spectra of the Pt-electrolyte interface and of the current flowing during CO-charge displacement experiments. These experiments revealed the dynamics of CO adsorption on Pt, as well as of the rearrangement of the interfacial water layer. Our experiments show that COL sites are populated first, COB appearing only at slightly higher coverages. At potentials sufficiently separated from the potential of zero total charge, the charge displaced upon CO adsorption on Pt and the integrated intensity of the COL band run approximately parallel, as also do the current transient and the time derivative of the integrated intensity of the COL band. This suggests that, for low to medium CO coverage, the current flowing at a given instant during the CO-charge displacement experiment corresponds to the instantaneous rate of CO adsorption, while the charge displaced when a CO submonolayer is formed is a good estimate of the corresponding relative coverage. Although the structure of the interfacial water layer is affected at low coverages, the band corresponding to interfacial water on top of a CO-covered Pt surface only appears at a relatively high CO coverage, suggesting that this kind of interfacial water is linked to the presence of compact islands of adsorbed CO.

Investigating dry reforming of methane with spatial reactor profiles and particle‐resolved CFD simulations

Dry reforming of methane (DRM) over nickel in a fixed‐bed reactor of spheres was studied experimentally and with CFD simulations. Temperature and mole fraction profiles were measured in a dedicated profile reactor as function of axial coordinate. Particle‐resolved CFD simulations took into account conjugate heat transfer, surface‐to‐surface radiation, and surface reactions described by microkinetics. Energy transport of CFD simulations were verified by studying heat transfer without chemical reactions. DRM experiments could not be reproduced with the original microkinetics formulation, even with the axial temperature profile applied. A detailed analysis of the microkinetics showed that thermodynamic inconsistencies are present, which are amplified by high surface coverage of CO*. After modifying the mechanism the experiments could be reproduced. This study shows how complex interactions between local transport phenomena and local kinetics can be quantified without relying on transport correlations. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4436–4452, 2016

Comparison of mechanistic understanding and experiments for CO methanation over nickel

A reaction mechanism based on Density Functional Theory (DFT) calculated energies was built using the Ni(211) step as representative for the active sites in Ni-based methanation catalysts and an analytical kinetic model based on two rate determining steps was used to evaluate a comprehensive experimental dataset over a wide range of conditions.Micro-kinetic analysis by means of the degree of rate control showed that CO splitting through a COH intermediate is the rate determining step of the catalytic reaction at high temperatures where the CO coverage is low, while CO splitting competes with CH hydrogenation at lower temperatures and high CO coverage. The inhibiting effect of carbon monoxide at high CO pressures could be attributed to carbon buildup at the B5-site at Ni(211). The model predicts the experimental shift in the hydrogen reaction order from αH2≈1/2, at low CO pressure to αH2≈1 at high CO pressures. This behavior is explained by removal of CO induced carbon from the B5-site as the hydrogen pressure is raised. The effects of temperature and total pressure are also well-described by the new two-step model.

High coverage CO adsorption and dissociation on the Co(0001) and Co(100) surfaces from DFT and thermodynamics

The adsorption, dissociation and desorption of CO at different coverage over two Co surfaces and the corresponding equilibrium phase diagrams under different temperatures and partial pressures have been systematically investigated; here, the results are obtained using density functional theory calculations and atomistic thermodynamics together with the periodic slab model. Our results show that the saturated adsorption on both Co(0001) and (100) surface are 7/9 ML; meanwhile, on these two surfaces, the lateral repulsive interaction has an effect on the adsorption structures and the corresponding stepwise adsorption energies of these adsorbed CO molecules, and become stronger with the increasing of CO coverage, which further leads to CO migration over Co surface; moreover, the adsorption energies decrease gradually with the increasing of CO coverage until to the saturated adsorption. According to the stepwise adsorption configurations, further calculations on CO dissociation show that only molecular adsorption CO is favored over (0001) surface; whereas on Co(100) surface, when the coverage is 1/9 ML, CO dissociation is more facile than its desorption; when the coverage is from 2/9 to 5/9 ML, only the first CO dissociation is preferred, while desorption is more favored for the left CO molecules; when the coverage is equal to or greater than 6/9 ML, CO desorption will be more favorable than its dissociation. Further, the adsorption and activation of CO under different temperatures and partial pressures from atomistic thermodynamics method well illustrate the relationship between the stable CO adsorption with the temperatures and CO partial pressure on Co(0001) and Co(100) surface, respectively. Therefore, on the basis of above results, we can obtain that the surface structures mainly affect the adsorption form of CO due to the different activity towards CO, while CO coverage will affect its adsorption energies and configurations, as well as CO dissociation on Co surfaces. The calculated stretching frequencies of CO molecule at different coverage agree with the available experimental data. This study provides a more accurate and detailed information for the process of CO adsorption and activation on Co surface.

CO oxidation over supported Pt clusters at different CO coverage

CO adsorption and oxidation over supported Pt14 with different CO coverage on TiO2(110) surface were investigated using density functional theory (DFT) calculations and thermodynamic analysis. According to the phase diagram, Pt14/TiO2(110) and 11CO@Pt14/TiO2(110) were chosen to represent the low and high CO coverage of Pt clusters, respectively. Our study shows that the high coverage of CO can induce the structural change of supported Pt clusters and weaken the interaction between Pt clusters and TiO2 support. The CO adsorption and oxidation mechanism depends on the CO coverage, which is determined by the experimental reactant composition, pressure, and temperature. At low CO coverage, the dissociated oxygen is active specie to form CO2 by reacting with CO. At high coverage, the molecular oxygen can directly react with CO via the formation of OOCO intermediate. Our proposed mechanisms provide useful information for understanding the CO oxidation over Pt clusters with different CO coverage. © 2016 Wiley Periodicals, Inc.

Theoretical insights into the CO dimerization and trimerization on Pt nanocluster

CO dimerizaiton and trimerization on icosahedral Pt55 cluster.

The Impact of Specifically Adsorbed Ions on the Copper-Catalyzed Electroreduction of CO2

Electrolytic ions are known to impact the rates of electrocatalytic reactions, though a molecular understanding of the mechanism of such impact is not well understood. We employ density functional theory to investigate the specific adsorption of potassium and iodine ions on Cu (111), Cu (100) and Cu (211) electrode surfaces under CO2 electroreduction (CO2 ER) conditions and explore their effect on the reaction energetics and binding strength of reaction intermediates. The calculated equilibrium potentials for K* and I* adsorption (* denotes surface-adsorbed species) suggest that a low coverage of adsorbed K* and/or I* is thermodynamically favorable, at different pH, at CO2 ER overpotentials. Co-adsorbed I* serves to primarily weaken CO* binding and may promote CO* desorption or subsequent formation of C-H bonds, phenomena that are typically obstructed by a high surface coverage of CO*. Co-adsorbed K* preferentially increases the binding strength of O-terminated species, including CO* and CHO* relative to COH*. The presence of K* will shift the selectivity and kinetics of CO* reduction towards the CHO* pathway by destabilizing the COH* formation transition state. The impact of solvation in conjunction with the effects of ion specific adsorption on the energetics of CO* reduction are also discussed.