CO Saturation Research Articles

Exploring coverage-dependent chain-growth mechanisms on Ru(111) for Fischer–Tropsch synthesis

The chain growth mechanisms of Fischer–Tropsch synthesis (FTS) remain a controversial topic, especially considering the much stronger adsorption ability of CO over H2, resulting in CO pre-adsorption or saturation.

Correlating CO Coverage and CO Electroreduction on Cu via High-Pressure in Situ Spectroscopic and Reactivity Investigations.

The absolute coverage of CO has been a missing piece in the mechanistic puzzle of the CO reduction reaction (CORR) on Cu. For the first time, we revealed the upper bound of the CO coverage under electrocatalytic conditions to be 0.05 monolayer at atmospheric pressure and the saturation CO coverage to be ∼0.25 monolayer by conducting surface enhanced infrared spectroscopy at CO pressures up to 60 barg in a custom-designed spectroelectrochemical cell. CORR activities on Cu were also determined in the same pressure range. Calculated reaction orders of C2+ products with respect to adsorbed CO are substantially less than unity, clearly indicating that the coupling of adsorbed CO is not the rate-determining step leading to multicarbon products. The increase in CO coverage can reduce the C affinity on the Cu surface and favor the selectivity towards oxygenates, especially acetate, over ethylene. Uncommon products, including ethane, glycolaldehyde, and ethylene glycol, were detected in appreciable amounts, likely due to a new C-C coupling mechanism taking place at elevated CO pressures.

PLASMA LEVEL OF ANTIMONY CORRELATES WITH PULMONARY HYPERTENSION SEVERITY

TOPIC: Pulmonary Vascular Disease TYPE: Original Investigations PURPOSE: Heavy metals are linked to cardiovascular disease, but the relationship between heavy metals and pulmonary arterial hypertension (PAH) is not clear. We hypothesized that there may be a correlation between heavy metal levels and the severity of pulmonary hypertension METHODS: A prospective, single center pilot study recruited patients from our pulmonary hypertension center over a period of 3 months. Patients with PAH who were ≥ 18 years old were eligible to participate. We obtained IRB approval and all the participated patients signed an informed consent. Blood and urine samples were collected to measure essential and non-essential metal levels in blood, plasma, and urine using an X Series II quadrupole inductively coupled plasma mass spectrometry (ICP-MS, Thermo Fisher Scientific). Also, we collected demographics, hemodynamics, last 6 minute walk distance (6MWD), last available BNP level, and REVEAL Lite 2 risk score. Statistical analysis was performed using statistical software R and p value<0.05 was considered significant. RESULTS: A total of 20 patients with PAH were included in the study. The mean age was 57.6±11.8 years, 80% were female and 6 patients had idiopathic PAH. All the patients were receiving PAH combination therapies including a prostanoid agent (13 on parenteral treprostinil, 1 on epoprostenol, 5 on inhaled treprostinil, and 1 on selexipag) with a mean parenteral treprostinil dose of 67.8±28.9 ng/kg/min. Mean last 6MWD was 312.7m±96.2 and mean last BNP level was 118.3±129.6 pg/ml. RVEAL Lite 2 risk score was low in 9 patients, intermediate in 6, and high in 4 patients. Mean hemodynamic values were mRAP 5.9±4.4 mmHg, mPAP 45.6±13.9 mmHg, PAWP 8±3.5 mmHg, CO 5.1±1.4L/min, CI 2.86±1.1 L/min/m2, and PVR 8.32±4.4 WU. There was significant correlation between plasma level of 121Sb (Antimony) and mRAP (r=0.47, p=0.036), CO (r=-0.50, p=0.026), CI (r=-0.54, p=0.014), PVR (r=0.52, p=0.019), and PA saturation (r=-0.54, p=0.016). CONCLUSIONS: We found significant correlations between plasma level of 121Sb and prognostic hemodynamic markers of disease severity in PAH patients without known prior history of occupational heavy metal exposure. CLINICAL IMPLICATIONS: Antimonial compounds are still used in treatment of Leishmaniasis with cardiac toxicity being one of the known potential side effects. Ventricular myocytes of guinea-pigs treated with trivalent antimony showed impaired contraction responses to changes in stimulus frequency, reduced calcium current, and elongated action potential. However, significant correlation between serum 121Sb levels with the prognostic hemodynamic markers of disease severity in PAH patients has not been reported. This finding may suggest that environmental 121Sb can have a potential role in pulmonary hypertension and right ventricular dysfunction. More studies are needed to further elucidate this role and its mechanism. DISCLOSURES: No relevant relationships by Ozan Akca, source=Web Response No relevant relationships by Lu Cai, source=Web Response No relevant relationships by Oscar Chen, source=Web Response Speaker/Speaker's Bureau relationship with United therapeutics Please note: $5001 - $20000 by Karim El-Kersh, source=Web Response, value=Honoraria Removed 04/21/2021 by Karim El-Kersh, source=Web Response Advisory Committee Member relationship with United therapeutics Please note: $1001 - $5000 by Karim El-Kersh, source=Web Response, value=Consulting fee Removed 04/21/2021 by Karim El-Kersh, source=Web Response Advisory Committee Member relationship with Actelion Please note: $5001 - $20000 by Karim El-Kersh, source=Web Response, value=Consulting fee Removed 04/21/2021 by Karim El-Kersh, source=Web Response Advisory Committee Member relationship with United Therapeutics Please note: Current Added 04/23/2021 by Karim El-Kersh, source=Web Response, value=Consulting fee Advisory Committee Member relationship with Actelion Please note: 2019 and 2020 Added 04/23/2021 by Karim El-Kersh, source=Web Response, value=Consulting fee Speaker/Speaker's Bureau relationship with United Therapeutics Please note: Current Added 04/29/2021 by Karim El-Kersh, source=Web Response, value=Honoraria Advisory Committee Member relationship with United Therapeutics Please note: Current Added 04/29/2021 by Karim El-Kersh, source=Web Response, value=Honoraria Advisory Committee Member relationship with Actelion Please note: 2019 and 2020 Added 04/29/2021 by Karim El-Kersh, source=Web Response, value=Honoraria Consultant relationship with Acceleron Pharma Please note: Current Added 04/29/2021 by Karim El-Kersh, source=Web Response, value=Consulting fee No relevant relationships by C. Danielle Hopkins, source=Web Response Consultant relationship with GE Healthcare Please note: 2019-Present Added 04/27/2021 by Jiapeng Huang, source=Web Response, value=Consulting fee Research grant relationship with Gilead Sciences Please note: 2020-2022 Added 04/27/2021 by Jiapeng Huang, source=Web Response, value=Grant/Research Support Consultant relationship with GE Healthcare Please note: 2019 to Present Added 04/27/2021 by Jiapeng Huang, source=Web Response, value=Consulting fee Research Funding relationship with Gilead Sciences Please note: 2020-2022 Added 04/27/2021 by Jiapeng Huang, source=Web Response, value=Grant/Research Support No relevant relationships by Caitlin Wessel, source=Web Response

Reduced Carbon Monoxide Saturation Coverage on Vicinal Palladium Surfaces: the Importance of the Adsorption Site.

Steps at metal surfaces may influence energetics and kinetics of catalytic reactions in unexpected ways. Here, we report a significant reduction of the CO saturation coverage in Pd vicinal surfaces, which in turn is relevant for the light-off of the CO oxidation reaction. The study is based on a systematic investigation of CO adsorption on vicinal Pd(111) surfaces making use of a curved Pd crystal. A combined X-ray Photoelectron Spectroscopy and DFT analysis allows us to demonstrate that an entire row of atomic sites under Pd steps remains free of CO upon saturation at 300 K, leading to a step-density-dependent reduction of CO coverage that correlates with the observed decrease of the light-off temperature during CO oxidation in vicinal Pd surfaces.

Study of the adsorption of heavy metal ions from binary and multicomponent solutions by ceramic chips

The adsorption of heavy metal ions from binary solutions was studied using Co (II) as an example in the concentration range from 1 to 50 mg/dm3 and the adsorption of heavy metal ions from multicomponent solutions using Co (II), Ni (II) and Cu (II) by fractions of ceramic chips of 0.5-1.0 mm and 1.0-2.0 mm. It was established that the value of the ultimate saturation of Co (II) ions with ceramic chips depends on the pH value. The maximum value of the ultimate saturation for Co (II) ions is reached at pH = 9, the smallest – at pH = 3. An increase in temperature from 273 to 353 K leads to an increase in the ultimate saturation for Co (II) ions. Experimental data on the sorption of metal ions from multicomponent solutions show that the rate of achievement of the ultimate saturation of the sorbent in a metal depends on the value of the ionic radius. The ultimate saturation limit for a metal at the same concentration of metals and the presence of other metals is greatest for Cu (II), the smallest for Co (II). The results of the study indicate the high efficiency of ceramic chips as a sorbent of heavy metal ions from wastewater of various industries, which indicates the possibility of practical use of ceramic chips for wastewater treatment from heavy metals, especially taking into account the availability and the possibility of using it without regeneration with recycling in production pigments.

Morpho-Butterfly Inspired Lamella-based Optical Sensors for Measuring Percent Level Concentrations of H2 and CO with Au and CeO2

Morpho-Butterfly inspired lamella structures were fabricated to make a multivariable optical sensor for measuring percent level concentrations of H2 and CO in a high temperature, zero-O2 environment. The SiO2 and Si3N4 3D structures were covered with gold nanoparticles (AuNPs) and cerium IV dioxide (CeO2) and exposed to varying concentrations of H2 and CO at 300°C. Experiments with AuNP sizes with an average diameter of 11 ± 3 nm and 3 ± 0.5 nm (larger and small AuNPs, respectively) showed that the larger AuNPs had a stronger response to the same concentrations of H2 and CO with less CO saturation and baseline drift. Further testing in the presence of 1% hydrocarbons (HCs) as an interfering gas showed excellent response to up to 17% H2 and CO in a zero-O2 environment with changes in reflected intensity of -13.1 ± 0.4% for H2 and -5.8 ± 0.3% for CO. The presence of HCs did not induce any baseline drift, with the total amount of drift being less than 0.5% over 13 hours of testing at 300°C.

Kinetics study of CO molecules adsorption on Al2O3/Zeolite composite films prepared by roll-coating method

ABSTRACT CO gas adsorption has become a topic of great significance and worthy of investigation. The purpose of this study was to explore the CO adsorption of Al2O3/Zeolite composite films prepared by a roll coating method. XRD, FESEM, and EDX analyses have been used to study the structural, morphological, and elemental properties of Al2O3/Zeolite composite films. A gas analyzer was used to probe CO gas adsorption on the composite films at a constant temperature and pressure. The concentration of inlet CO gas and saturation level was 190 and 4 mg L−1, respectively. The maximum values of adsorption efficiency and uptake capacity were measured at 97.89% and 213.9 mg g−1. To determine the kinetic study, three models were investigated: pseudo-first-order, pseudo-second-order, and intra-particle diffusion. The pseudo-second-order model was suitable for CO adsorption by Al2O3/Zeolite adsorbent. Additionally, the rate-controlling step for Al2O3/Zeolite adsorbent was identified through the intra-particle diffusion model.

Toward a quantitative theoretical method for infrared and Raman spectroscopic studies on single-crystal electrode/liquid interfaces.

In situ electrochemical infrared spectroscopy and Raman spectroscopy are powerful tools for probing potential-dependent adstructures at solid/liquid electrochemical interfaces. However, it is very difficult to quantitatively interpret the observed spectral features including potential-dependent vibrational frequency and spectral intensity, even from model systems such as single-crystal electrode/liquid interfaces. The clear understanding of electrochemical vibrational spectra has remained as a fundamental issue for four decades. Here, we have developed a method to combine computational vibrational spectroscopy tools with interfacial electrochemical models to accurately calculate the infrared and Raman spectra. We found that the solvation model and high precision level in the self-consistent-field convergence are critical elements to realize quantitative spectral predictions. This method's predictive power is verified by analysis of a classic spectroelectrochemical system, saturated CO molecules electro-adsorbed on a Pt(111) electrode. We expect that this method will pave the way to precisely reveal the physicochemical mechanism in some electrochemical processes such as electrocatalytic reactions.

Mid-infrared sensing of CO at saturated absorption conditions using intracavity quartz-enhanced photoacoustic spectroscopy

The sensitivity of quartz-enhanced photoacoustic spectroscopy (QEPAS) can be drastically increased using the power enhancement in high-finesse cavities. Here, low noise resonant power enhancement to 6.3 W was achieved in a linear Brewster window cavity by exploiting optical feedback locking of a quantum cascade laser. The high intracavity intensity of up to 73 W mm−2 in between the prongs of a custom tuning fork resulted in strong optical saturation of CO at 4.59 µm. Saturated absorption is discussed theoretically and experimentally for photoacoustic measurements in general and intracavity QEPAS (I-QEPAS) in particular. The saturation intensity of CO’s R9 transition was retrieved from power-dependent I-QEPAS signals. This allowed for sensing CO independently from varying degrees of saturation caused by absorption induced changes of intracavity power. Figures of merit of the I-QEPAS setup for sensing of CO and H2O are compared to standard wavelength modulation QEPAS without cavity enhancement. For H2O, the sensitivity was increased by a factor of 230, practically identical to the power enhancement, while the sensitivity gain for CO detection was limited to 57 by optical saturation.

Effect of microstructure on corrosion of welded joints of X80 steel in water saturated supercritical CO2

Purpose The purpose of this paper is to research whether microstructure varieties of zones at welded joints pose a tremendous effect on its corrosion in SC-CO2. Design/methodology/approach The interrelation between the corrosion and microstructure of base metal (BM), fine grain heat affected zone (FHAZ), coarse grain heat affected zone and weld metal (WM) in welded of X80 steel in water saturated supercritical CO2 was studied by using optical microscope, weight loss test, electrochemical measurements and surface analytical techniques. Findings The all subzones of X80 weld joints were attacked by SC-CO2 corrosion and showed flower-like corrosion scale spots consisted of granular FeCO3. The most severe corrosion appeared at WM due to lower proportion of ferrite to pearlite, but the slightest corrosion displayed at BM. GHAZ with larger grains and more polygonal ferrite exhibited more severe corrosion than that at BM. Due to its smaller grain, FHAZ displayed comparatively more severe corrosion to that at BM. Originality/value There exists close interrelation between the corrosion and microstructure of the welded carbon steel in water saturated supercritical CO2.

CORROSION AND FATIGUE OF HEAT TREATED MARTENSITIC STAINLESS STEEL 1.4542 USED FOR GEOTHERMAL APPLICATIONS

During capture and storage technology (CCS) as well as in geothermal energy production steels need to withstand the corrosive environment such as: heat, pressure, salinity of the aquifer and CO 2 -partial pressure. 1.4542 shows unusual corrosion phenomena, but is still sufficiently resistant in corrosive environments. To better understand its behaviour differently heat treated coupons of 1.4542 and for comparison X20Cr13 and X46Cr13 were kept in the artificial brine of the Northern German Basin at T=60 °C. Ambient pressure as well as p=100 bar for 700 h - 8000 h in water saturated supercritical CO 2 and CO 2 -saturated synthetic aquifer environment was applied. Fatigue tests were performed via push-pull tests with a series of 30 specimens from 150 MPa to 500 MPa (sinusoidal dynamic test loads, R=-1; resonant frequency ~ 30 Hz). FeCO 3 and FeOOH are corrosion products also after dynamic corrosion tests. Martensitic microstructure offers good corrosion resistance in geothermal environment. The S-N-curve showing no typical fatigue strength and very steep slopes of possible fatigue strength for finite life. Possible influencing artefacts, such as Al-inclusions could not be correlated to early rupture despite specimens containing inclusions at the fracture surface and cross section reached lower number of cycles. Applied potential proofed to enhance fatigue life tremendously. Article DOI: https://dx.doi.org/10.20319/mijst.2019.51.138158 This work is licensed under the Creative Commons Attribution-Non-commercial 4.0 International License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc/4.0/ or send a letter to Creative Commons, PO Box 1866, Mountain View, CA 94042, USA.

Volumetric mass transfer coefficient for carbon monoxide in a dual impeller stirred tank reactor considering a perfluorocarbon–water mixture as liquid phase

Overall volumetric mass transfer coefficient (kLa) of carbon monoxide in a perfluorocarbon–distilled water mixture was evaluated in a dual impeller stirred tank bioreactor (Rushton-type and Smith-type). Two agitation speeds (300 and 500 rpm), three specific gas flow rates (2.0, 2.5 and 2.7 vvm), two working volumes (0.75 and 1 L) were tested, as well as four different liquid phase compositions: pure distilled water; distilled water and perfluorodecalin (PFC); distilled water and Tween® 80; and distilled water, PFC and Tween® 80. CO concentration in liquid phase was determined using a myoglobin bioassay technique and kLa and saturated CO concentration were determined using Maximum Likelihood Estimation (MLE) with a hybrid optimization strategy. A maximum kLa of 603.48 h−1 was obtained at 500 rpm, 2.7 vvm and distilled water, PFC and Tween® 80 as liquid phase. Tween® 80 was responsible for increasing mass transfer when mixing these three components comparing to a liquid phase composed of PFC and distilled water (7 time increase at 2.7 vvm and 500 rpm), and pure distilled water (2.9 time increase at 2.0 vvm and 1.6 times at 2.5 vvm). Moreover, MLE proved to be a valuable tool to critically analyse experimental uncertainty in parameter estimation.

Single-Atom Electroplating on Two Dimensional Materials

Catalyst doped with a single-atom noble metal displays distinctive catalytic behavior from the bulk counterparts, with tunable electronic structures and spatial versatilities, which excels in today’s heterogeneous catalysis. To deposit noble metals in a single atomic level requires a restricted chemical environment and precise thermodynamic control. Electroplating methods are commercially used to deposit uniform and conformal metal thin films on different hardware surfaces. Yet the atomic level electroplating has never been achieved. Herein we demonstrate a voltage gauged electrochemical deposition method to synthesize single-atom Pt, Au, and Pd on MoS2 and other two-dimensional (2D) materials. The surface atomic doping level for Pt, Au, and Pd can reach 1.1, 7.0, and 14%, respectively, and the doping sites are precisely positioned at Mo- and S-vacancies. The monodispersed noble atoms show enhanced hydrogen evolution activity and saturated CO tolerance, as explained by density functional theory calculatio...

The Marine Carbonate System at Maluku and Sulawesi Seas

Maluku and Sulawesi Sea s are parts of Indonesian Large Marine Ecosystem in the east area which has an important role for Indonesian fisheries activities. Those areas are facing the climate change phenomena that could change their marine ecosystem quality. Marine carbonate system is very important for controlling the circulation of CO 2 between the atmosphere and the ocean as well as regulating the pH value of waters. We conducted in situ measurement of marine carbonate system parameters (pH, Total Alkalinity, Total CO 2 (TCO 2 ) and pCO 2 ) during IJEP cruise on September 2016 to analyses spatial variations of the marine carbonate system in the Northwest Monsoon period. The results of marine carbonate system showed that in general Maluku and Sulawesi Sea s are under super saturation of CO 2 or acts as source of CO 2 which have range value of sea surface partial of CO 2 (pCO 2 ) between 280 – 500 µatm. Spatial distribution of sea surface pH values measured with optical sensor at study sites showed varied between 8,03 – 8,15. Measurement results of Total alkalinity (TA) by using titration method varied between 2300 – 2400 µmol/Kg meanwhile range value of TCO 2 was 2300 – 2500 µmol/Kg at surface waterand it’s increased through the deep water with the range value of TCO 2 was 2500 – 2800 µmol/Kg at deeper layer of water.Characteristic of marine carbonate system between Maluku and Sulawesi Sea s was different in the Southeast monsoon period indicates complex marine biogeochemistry processes influences those system .

CO adsorption on nanoislands: Ni on Au(111).

The adsorption behavior of CO on Ni islands grown on Au(111) was studied with a combination of temperature programmed desorption, Fourier transform infrared spectroscopy, and surface resistivity measurements. The Au(111) herringbone reconstruction provides a template for the growth of ordered Ni islands, with evidence for the presence of strain and Au atoms within the islands. The islands grow radially until θNi ≈ 0.3 ML, after which subsequent Ni atoms contribute primarily to a second layer. We study saturated CO adsorption at 227 K over a range of Ni island sizes and find layer-dependent adsorption properties. For single-layer islands at low Ni coverage, CO adsorbs primarily in the atop position and desorbs at lower temperatures than on pure Ni, with a saturation CO coverage of about 0.5 CO/Ni. As second layer Ni grows, saturated CO coverages on the Ni approach unity, with higher desorption temperatures, but still with primarily atop CO. Based on previous studies, we propose that in the first Ni layer, ligand effects from the Au substrate and possibly Au in the islands and strain due to the Ni/Au lattice mismatch affect the Ni-CO bonds. CO adsorption behavior on the two-layer islands is qualitatively explained by a decrease in Au nearest neighbors and the presence of a more expanded/corrugated structure.

Reaction and Diffusion Paths of Water and Hydrogen on Rh Covered Black Titania

The reactions of H2O, H2 D2 and CO with clean and rhodium covered black titania have been investigated by TDS, AES and sensitive temperature programmed work function (TP-WF) measurements to elucidate the complex interactions with this narrow bandgap material promising for visible light energy harvesting. Water formed molecular and dissociative adsorption states with positive outward dipole moments on the reduced, r–TiO2 (110). Surface hydroxyl groups decomposed to H2 and recombined to H2O in a broad temperature range, characterized by TDS peaks at 300, 355–377 and 470 K, which has been associated with surface inhomogeneity. On a strongly reduced, sr–TiO2 (110), a part of H atoms arising from OHa species dissolved in the titania at 200–500 K, and desorbed as H2O with Tp = 570, 670 and 750 K. Sub-monolayer TiOx films produced by stepwise heating on r–TiO2 (110) supported Rh particles suppressed the adsorption of hydrogen, but allowed its spillover to the support. Co-adsorption experiments with D2, H2, H2O and CO on the Rh covered r–TiO2 (110) were also performed. Saturating the Rh by CO at 330 K blocked the uptake of hydrogen on the metal, eliminating its spillover to the support. At 270 K saturation CO exposure removed the pre-adsorbed hydrogen, while at 200 K replaced a part of it decreasing the adsorption bond energy of the rest remained adsorbed. Co-adsorption data proved that the hydrogen desorption states with Tp = 470 and 570 K belong to the decomposition of hydroxyl groups on the r–TiO2 (110) support and at the Rh–TiOx interface, respectively. It is remarkable that the latter state can evolve in the presence of adsorbed CO, which exhibits reactivity towards the same interface.

DFT study on bimetallic Pt/Cu(1 1 1) as efficient catalyst for H2 dissociation

To design a catalyst for the dissociation of H2 with better CO-tolerance performance is very important for proton exchange membrane fuel cells (PEMFCs) towards high efficiency. With slab model, the catalytic properties of overlayer Pt on Cu substrate (Pt/Cu) are analyzed by first-principle calculations. The CO saturation coverage (40%) on Pt2/Cu is found to be lower than that of pure Pt (about 75%). The dissociation barrier from H2 to H is less than 0.4 eV under the saturation coverage of CO. On the basis of kinetics of proton formation, the CO-tolerance ability on double-layer Pt with Cu is found to be greatly improved compared with that on pure Pt. It is expected that Pt overlayer on Cu(1 1 1) is a potential anode material with lower cost for PEMFCs.

Ab InitioInvestigation of the Role of CO Adsorption on the Physical Properties of 55-Atom PtCo Nanoalloys

The knowledge of the physical and chemical properties of PtCo nanoparticles as a function of the Pt/Co composition and atomic distribution is crucial for several potential applications, which includes catalysis, anticorrosion, data storage, etc. However, our current atom-level understanding is far from satisfactory, in particular due to the challenges to take into account chemical environment effects. In this work, we report a density functional theory investigation of the structural, energetic, and electronic properties of binary 55-atom PtCo particles at a saturated CO atmosphere (31 molecules), i.e., (CO)31/PtnCo55–n. For PtCo in the gas phase, which adopts an icosahedron-like (ICO-like) structure in the lowest-energy configurations for all studied compositions, we found a rough correlation between stability and the number of bonds among the Pt and Co species; i.e., the stability (excess energy) increases (decreases) by increasing the number of Pt and Co bonds and with a minimum at about n = 28–42 (Pt-...

Catalytic CO Oxidation by CO-Saturated Au Nanoparticles Supported on CeO2: Effect of CO Coverage

Understanding the reaction mechanism and the nature of the reactive species of heterogeneous catalysts under reaction conditions is the first step in the design of more consistent, reliable, and practical catalysts. We used density functional theory (DFT) calculations to study the mechanism of CO oxidation catalyzed by CeO2-supported Au nanoparticles (NPs) under reaction conditions by considering the sequential CO adsorption onto and CO saturation of Au NPs. We found that the Au9 NPs supported by CeO2(100) and CeO2(111) bind as many as eight or four CO molecules, respectively. The last-bound CO molecule opens the fast CO oxidation pathway. The CO oxidation pathways constructed on both systems show that the reaction occurs at the Au–CeO2 interface via the Mars–van Krevelen mechanism. We found that the most important O–C–O-type intermediate was spontaneously formed at the Au–CeO2(100) interface upon the sequential binding of CO molecules onto the Au NPs. The reaction pathway therefore becomes relatively sim...

The parsec–scale relationship between ICO and AV in local molecular clouds

We measure the parsec-scale relationship between integrated CO intensity (I_CO) and visual extinction (A_V) in 24 local molecular clouds using maps of CO emission and dust optical depth from Planck. This relationship informs our understanding of CO emission across environments, but clean Milky Way measurements remain scarce. We find uniform I_CO for a given A_V, with the results bracketed by previous studies of the Pipe and Perseus clouds. Our measured I_CO-A_V relation broadly agrees with the standard Galactic CO-to-H2 conversion factor, the relation found for the Magellanic clouds at coarser resolution, and numerical simulations by Glover & Clark (2016). This supports the idea that CO emission primarily depends on shielding, which protects molecules from dissociating radiation. Evidence for CO saturation at high A_V and a threshold for CO emission at low A_V varies remains uncertain due to insufficient resolution and ambiguities in background subtraction. Resolution of order 0.1 pc may be required to measure these features. We use this I_CO-AV relation to predict how the CO-to-H2 conversion factor (X_CO) would change if the Solar Neighborhood clouds had different dust-to-gas ratio (metallicity). The calculations highlight the need for improved observations of the CO emission threshold and HI shielding layer depth. They are also sensitive to the shape of the column density distribution. Because local clouds collectively show a self-similar distribution, we predict a shallow metallicity dependence for X_CO down to a few tenths of solar metallicity. However, our calculations also imply dramatic variations in cloud-to-cloud X_CO at subsolar metallicity.