28 Search Results

Methanol is considered a promising alternative fuel for internal combustion engines (ICEs) due to its high-octane number, fast laminar flame speed, and elevated latent heat of vaporization, all of which support higher compression ratios and improved thermal efficiency. However, its substantial latent heat of vaporization also poses cold-start challenges, such as misfire and fuel film deposition. This study aims to investigate methanol spray morphology and spray-wall interaction using the Spray M injector from the Engine Combustion Network within a constant-pressure flow vessel. A recently developed unified numerical framework capable of modeling both flash and non-flash boiling sprays is validated againstmore » experimental liquid volume fraction data acquired via 3-D computed tomography. The results reveal that flash boiling significantly alters the spray morphology, leading to smaller droplets and spray collapse due to enhanced air-entrainment-induced turbulence. Quantitative agreement between experiments and simulations confirms this behavior. Coupled 0-D equilibrium and 3-D computational fluid dynamics analyses show that flash boiling accelerates evaporation and reduces fuel residence time, while non-flash conditions maintain a persistent liquid core more susceptible to wall wetting. Wall temperature diagnostics reveal that spray collapse alters heat transfer patterns by shifting cooling effects. Mixture fraction analysis indicates that evaporation is primarily governed by shear-layer turbulence, though deviations from adiabatic equilibrium mixing emerge under low-turbulence conditions. Finally, increasing fuel, ambient, and wall temperatures reduces wall wetting and film thickness, mitigating cold-start risks. These findings enhance the understanding of methanol sprays’ behavior and support its adoption as a viable, alternative fuel for ICEs.« less

Transition metal nitrides (TMNs) have been explored as effective supports for Pt due to their Pt-like electronic properties. However, there is a lack of fundamental understanding regarding the behavior of Pt on different TMNs (Pt/TMN). Herein two TMNs, Mo₂N and TiN, were modified with Pt and compared using methanol decomposition as a probe reaction via both ultrahigh vacuum (UHV) studies on thin films and ambient-pressure batch reactor studies of powder catalysts. Temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS) measurements were conducted under UHV conditions with Mo₂N and TiN thin films. Mo₂N was shown to favor C–H bondmore » scission to form CO with a 56.2% selectivity, while TiN favored C–O bond scission to form CH₄ with a 74.5% selectivity. The addition of 0.9 monolayers (MLs) of Pt increased C–H bond scission selectivity to 89.7% and 49.2% for Mo₂N and TiN respectively. Density functional theory (DFT) calculations on model surfaces revealed that the binding energy of O (BE_*O) was significantly reduced on Pt/TMNs, from −4.02 eV on Mo₂N to −1.31 eV on Pt/Mo₂N and −4.74 eV on TiN to −1.37 eV on Pt/TiN. As a result, C–O bond scission pathways were suppressed, leading to the preferential C–H bond scission that was observed experimentally. The C–O and C–H bond scission trends observed on thin films were then extended to powder catalysts, which demonstrated similar trends toward methanol decomposition. In conclusion, results from the current study establish that by combining UHV studies and DFT calculations over model surfaces, one can effectively predict the catalytic behavior of realistic TMN powder catalysts.« less

Cobalt phthalocyanine (Co(II)Pc) and its derivatives are promising molecular electrocatalysts for the electrochemical reduction of CO₂ to CO and methanol (CH₃OH). Despite increasing interest, a detailed mechanistic understanding of how ligand substituents influence catalytic activity, selectivity, and efficiency remains limited. In this study, we employ density functional theory (DFT) to systematically investigate the influence of electron-donating groups (EDGs) and electron-withdrawing groups (EWGs) on the electronic structure and redox properties of the Co(II)Pc electrocatalyst, and to elucidate CO₂RR mechanistic pathways. Our results reveal that EWGs cause a positive shift in the reduction potentials, favor CO₂ binding over protonation of the Comore » metal center and promote downstream methanol formation at mild potentials. EDGs show opposite trends including favorable protonation steps, promoting a negative shift in the reduction potential, and facilitating the hydrogen evolution reaction (HER), which competes with the desired CO₂RR pathway. Notably, CO dissociation is thermodynamically and kinetically unfavorable across all systems, positioning the redox potential versus CO dissociation energy as a key factor for methanol selectivity. Furthermore, these insights provide a predictive framework for rational catalyst design and underscore the critical role of electronic tuning in advancing molecular electrocatalysts for sustainable CO₂ conversion.« less

Synchrotron-based ambient pressure X-ray photoelectron spectroscopy (AP-XPS) was used to study the surface chemistry of methanol on TiO₂(110), examining the effects of methanol pressure, oxide temperature, and coadsorption with H₂. At 300 K, the adsorption of methanol on TiO₂(110) leads to the formation of CH₃O on the surface with a minor amount of CH₃OH present. The easy cleavage of the O–H bond in the alcohol agrees with the predictions of theoretical calculations, and most of the adsorbed CH₃O was not associated with the presence of Ti³⁺ sites in the oxide substrate. The adsorbed CH₃O was removed from the TiO₂(110) surfacemore » by heating to 600 K without the deposition of CH_x fragments or C on the oxide. The results of temperature-programmed desorption (TPD) showed the evolution of methanol and formaldehyde at 360 and 490 K as a result of a disproportionation reaction: 2CH₃O → CH₃OH + CH₂O. This surface chemistry, where there is no rupture of the C–O bond, and only selective cleavage of O–H and C–H bonds, is very different from that found on metals used in catalysts for methanol reforming, where massive conversion of the alcohol into CO, CH_x and C species is seen. Furthermore, the TPD data indicate that any CH₃O formed on the oxide surface can be hydrogenated and desorbed as CH₃OH at temperatures below 550 K. In this respect, titania is an ideal support for catalysts employed to achieve methanol synthesis through CO₂ hydrogenation.« less

The direct conversion of methane-to-methanol remains a critical challenge in methane valorization. In this study, we unveil the crucial role of PdAu/CeO₂ catalysts in enabling selective methane transformation under mild conditions, using only water as the sole oxidant. Through a combination of experimental techniques, including XPS and catalytic testing, alongside density functional theory (DFT) calculations, we demonstrate that a Pd_0.3Au_0.7/CeO₂ catalyst, which predominantly exposes isolated Pd atoms, achieves remarkable methanol selectivity (∼80%) at 500 K with a 1:1 methane-to-water ratio. While Pd/CeO₂ efficiently activates methane, its tendency for overreaction leads to complete methanol decomposition, thereby limiting selectivity. Alloying Pd withmore » Au on ceria mitigates this over-reactivity, preventing methanol degradation while maintaining sufficient catalytic activity. The PdAu/CeO₂ composite exhibits a synergistic effect: Pd in contact with the ceria support facilitates methane activation and water dissociation, while Au fine-tunes reactivity to promote methanol formation. DFT calculations confirm that isolated Pd sites at the PdAu/CeO₂ interface play a key role in balancing activity and selectivity. This work underscores the importance of alloy/oxide interfaces in controlling selective methane conversion with water and offers valuable insights for designing highly efficient catalysts for methanol synthesis.« less

The continuous development of novel catalytic approaches is crucial for advancing efficient CO₂ hydrogenation processes. Drawing inspiration from single-atom catalysis and 2D materials, we designed a new 2D single-atom catalyst with excellent thermal stability by thermally treating Cu-adsorbed γ-AlOOH nanosheets, which yielded a Cu/γ-Al₂O₃ catalyst with high activity in the hydrogenation of CO₂-yielding methanol (CH₃OH), dimethyl ether (DME), and CO as products. The active Cu sites are monodispersed and highly stable due to their cationic oxidation state and their substitution for pentacoordinated aluminum (Al^P) sites on particle surfaces. This study demonstrates an efficient approach for achieving a high CO₂ hydrogenationmore » rate (30.45 mol mol⁻¹ h⁻¹) using a catalyst system that lacks metallic Cu centers, traditionally considered essential for H₂ dissociation, and employs what was previously thought to be an inert metal oxide (γ-Al₂O₃) for CO and CH₃OH production. Ongoing mechanistic studies aim to elucidate the synergy between cationic Cu single atoms and γ-Al₂O₃, a Lewis acid support, in facilitating hydrogen (H₂) activation and methanol formation.« less

Ambient pressure X-ray photoelectron spectroscopy (AP-XPS) was used to investigate methanol (CH₃OH) adsorption and reaction on the GaP(110) surface. Exposure of CH₃OH to GaP(110) at room temperature led to the formation of at least four different surface species as indicated by analysis of C 1s and O 1s XPS features. By combining AP-XPS data with density functional theory calculations, the surface species were identified as methoxy (CH₃O*), formaldehyde (CH₂O*), and paired methanol (p-CH₃O*H) and methoxy (p-CH₃O*) species, where “paired” means that they belong to a hydrogen-bonded methoxy-methanol complex. Asterisk * here indicates an adsite. The formation of CH₂O* via themore » dehydrogenation of CH₃O* was shown to be limited by the availability of vacant phosphorus (P) sites on GaP(110). With an increase in CH₃OH pressure, the fractional coverage of CH₃O* species reached 0.55, and the surface P sites were completely saturated with hydrogen. Under a constant CH₃OH pressure of 0.5 Torr, the surface concentration of the paired species and of CH₂O* remained constant until 400 K. At higher temperatures, thermally driven reactions led to a significant increase in the concentration of surface CH_x* species, which suggests that C-O bond cleavage of the CH₃O group is the dominant decomposition mechanism on GaP(110). In conclusion, based on the reactivity of GaP(110) toward CH₃OH dehydrogenation, elevated temperatures and CH₃OH pressures may be used to functionalize this surface.« less

Here, we ally steady-state kinetics, kinetic isotope effects, and density functional theory (DFT) calculations to illustrate that Cu-based catalysts remain saturated by H-adatoms (H*) and molecular formic acid (HCOOH**) during CO₂ hydrogenation. High H* coverage under methanol synthesis conditions is evidenced by reverse water-gas shift (RWGS) rates that exhibit positive H₂ reaction orders only at P_H2 ≲ 0.5 bar, above which methanol synthesis and RWGS rates exhibit first and zeroth order dependence on P_H2, respectively. HCOOH** also accumulates on the surface with increasing P_CO2 as informed by the Langmuir-type dependence on P_CO2 (0.25-23 bar) for both methanol synthesis and RWGS.more » As both HCOOH** and H* have one H-atom per site occupied, the two species share the same P_H2 dependence and give rise to CO₂ reaction orders that are independent of P_H2. Surface coverages determined based on kinetic analyses are further corroborated with DFT-derived adsorption energies that show favorable HCOOH** adsorbate-adsorbate interactions as well as repulsive interactions for bidentate formate (HCOO**) on H*-saturated surfaces. Methanol selectivity remains invariant with P_CO2 and P_CO despite CO inhibiting reaction rates, thereby demonstrating methanol synthesis and RWGS occur on the same active site. In contrast, water preferentially inhibits methanol synthesis rates, increases methanol synthesis H₂ reaction order from 1.0 to 1.5, and alters the methanol synthesis H₂/D₂ kinetic isotope effect; the inhibitory effect of H₂O thus cannot be attributed to competitive adsorption alone and instead reflects a change in the rate-determining step for methanol synthesis. The disparate kinetics of methanol synthesis and RWGS evince a branching pathway where methanol is formed from formates and CO is formed from carboxylates. The presented work thus identifies the relevant surface species, underscores the distinct catalytic role of water in branching methanol synthesis and RWGS pathways, and, in doing so, details a mechanistic picture that yields predictable rates and reaction orders for both methanol synthesis and RWGS on Cu-based CO₂ hydrogenation catalysts.« less

Innovative synthetic fuels for advanced propulsion systems, such as methanol and ammonia, and synthetic blended fuels (E00, E10, and E30), known for their high volatility, are often injected directly into combustion chambers. It follows that Eulerian–Lagrangian spray models need to accurately capture the spray collapse as a consequence of flash boiling onset and be capable of proficiently handling the preferential evaporation of multi-component fuels in evaporative scenarios. So, we performed the assessment of an Eulerian–Lagrangian CFD code for simulating methanol and E00 gasoline blend sprays in both early and late injection conditions involving flash boiling conditions and preferential evaporation. Themore » adoption of an effervescent breakup model and of a non-equilibrium phase transition model for the discrete phase allows the adoption of a setup that is almost completely free from specific constant tuning, especially for what concerns the breakup model. We validated the simulations using experimental PLV maps of methanol and E00 sprays issued from the ECN Spray M injector. The results highlight a significantly different morphology of the methanol spray compared to the E00 one under late injection conditions. Under stratified combustion, low-volatile fuels are likely to be ignited first, and the flame propagates toward the high-volatile fuels. In conclusion, the spray collapse was also correctly reproduced, inducing the presence of a low-pressure zone and modifying the spray morphology.« less

Here, high pressure ammonia/methanol oxidation and NOx formations were investigated using a recently developed supercritical pressure jet-stirred reactor (SP-JSR) at 20 and 100 atm with temperatures between 550 and 950 K and equivalence ratios of 0.138 and 1.15. The experimental results show that NH₃ oxidation at high pressure is significantly accelerated by the active OH radicals produced from CH₃OH oxidation. Furthermore, the kinetic interactions between NH₃ and CH₃OH are governed mainly by the reactions CH₃OH + NH₂ = CH₂OH + NH₃, CH₃OH + NH₂ = CH₃O + NH₃, and CH₂O + NH₂ = HCO + NH₃. A HP-Mech model formore » high-pressure NH₃/CH₃OH oxidation was developed in this study. It consists of the most recent NH₃ and CH₃OH models including some new reactions and updated rate constants from the literature as well as NH₃-CH₃OH interactions where rate constants of CH₃OH + NH₂ = CH₂OH + NH₃, CH₃OH + NH₂ = CH₃O + NH₃, NH₂ + CH₂O = NH₃ + HCO, and NH₂ + CH₂O = NH₂CHO + H were theoretically calculated in this study. Our model with these updates improves the prediction for the measured N₂O/NO_x temperature dependence at 100 atm. In addition, the reaction pathway and sensitivity analysis show that N₂O/NO_x/HONO interactions with HO₂ are very important, especially for a fuel-lean mixture at 100 atm. The HONO mole fraction for the fuel-lean mixture at 100 atm was then measured by off-axis integrated cavity output spectroscopy (ICOS) at wavenumber of 6638.26 cm^-1. The experimental data show a significant HONO formation at intermediate temperature that is strongly underpredicted by numerical simulation at 100 atm. Therefore, the HONO related reactions with notable uncertainty at high pressure such as NO + OH (+M) = HONO (+M) and H₂NO + NO₂ = HONO + HNO need deeper exploration in the future.« less