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Experimental and theoretical studies show new autoxidation pathways, providing new routes to highly oxidized organic molecules and secondary organic aerosols, affecting how the atmosphere contributes to climate change.
The fractionalization of magnetic dipoles into monopoles is revealed through a crossover in the relaxation dynamics of spin ice.
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NH 3 and DME dual fuel oxidation and kinetic coupling are experimentally studied by using a supercritical pressure jet-stirred reactor (SP-JSR) at 20 and 100 atm, over a temperature range of 500–900 K, and at fuel-lean and stoichiometric conditions with NH 3 to DME molar ratios of 4 and 0.62, respectively. An HP-Mech model for high-pressure NH3/DME oxidation is developed based on our previous studies and it shows generally better performance than other models in the literature on high-pressure oxidation experiments. Due to DME’s strong low-temperature reactivity, it dramatically enhances the oxidation of NH3 at low temperature. However, the effect of NH 3 on DME oxidation varies with temperature. At low temperatures, NH 3 inhibits low-temperature reactivity by consuming OH radicals. In addition, the NH 2 and NO x formation from NH 3 further suppresses the low-temperature DME reactivity by reducing alkylperoxyl and O 2 QOOH radicals via RO 2 + NH 2 = RO + H 2 NO, RO 2 + NO = RO + NO 2 and O 2 QOOH + NO = 2CH 2 O + HO 2 + NO 2 . At intermediate temperatures, due to enhanced kinetic coupling of NH 2 /NO x /HO 2 chemistry, DME oxidation is significantly promoted. It is found that there are two major NH 2 /NO x /HO 2 coupling pathways for OH radical production brought by NH 3 : (1) NH 2 + HO 2 = H 2 NO + OH and (2) NH 2 + NO 2 = H 2 NO + NO with NO + HO 2 = OH + NO 2 . Moreover, the resulting H 2 NO will further contribute to OH production via the H 2 NO/NO x /HONO coupling pathways: H 2 NO + NO 2 = HNO + HONO, HNO + NO 2 = HONO + NO, and HONO (+M) = OH + NO (+M). These new NH 2 /NO x /HO 2 and H 2 NO/NO x /HONO pathways play a critical role in promoting DME oxidation at intermediate temperatures. Novelty and Significance Statement: Blending NH 3 with DME can significantly enhance the reactivity of NH 3 and facilitate its practical application in advanced internal combustion engines. NH 3 /DME oxidation is studied in a novel supercritical pressure jet-stirred reactor up to 100 atm, a much higher pressure than previous studies. An interesting non-monotonic effect of NH3 addition on fuel oxidation has been found. Unique kinetic couplings of NH2/HO2/NOx and H2NO/NOx/HONO brough by NH3 and DME blending are identified to play important roles in radical generation and reactivity promotion. The valuable experimental data and key kinetics revealed in the current work can tremendously improve our understanding of NH 3 /DME oxidation via NO x /RO 2 and NH 2 / H 2 NO/HO 2 /NO x kinetic couplings at low to intermediate temperatures.
Two-micron lasing is obtained on the {sup 5}I{sub 7} → {sup 5}I{sub 8} transition of Ho{sup 3+} ions in ZrO{sub 2} − Y{sub 2}O{sub 3} −Ho{sub 2}O{sub 3} crystals upon resonance pumping into the {sup 5}I{sub 7} level of these ions by a pulsed laser based on a Tm : LiYF{sub 4} crystal. The efficiency of conversion of pump radiation incident on the crystal to laser radiation and the slope lasing efficiency at a pulse duration of 8 ms and a pulse repetition rate of 10 Hz were 25% and 28%, respectively. (paper)
This review outlines the structure and chemistry of divalent lanthanides and actinides in small molecule systems, focusing on the less accessible divalent lanthanides such as the configurational crossover (Dy 2+ , Nd 2+ ) and non-traditional lanthanides(II) (La 2+ , Ce 2+ , Pr 2+ , Gd 2+ , Tb 2+ , Ho 2+ , Er 2+ , and Lu 2+ ) and the rarely accessible actinides(II). Expansive progress, including recent synthetic firsts in low-oxidation state ƒ-block chemistry, are reviewed and analyzed to build knowledge for future synthetic efforts.
Understanding the low- and intermediate-temperature oxidation chemistry of oxygenated fuels like dimethyl ether (DME) at high pressure is paramount to the development of advanced engines with low carbon emissions. The supercritical pressure jet-stirred reactor (SP-JSR) recently developed at Princeton provides a new platform for conducting kinetic studies at low and intermediate temperatures at extremely high pressures with a uniform temperature distribution and a short flow residence time. This paper uses the SP-JSR to investigate DME oxidation at equivalence ratios of 0.175, 1.0, and 1.72, for pressures of 10 and 100 atm, and temperatures ranging from 400 to 900 K. The results demonstrate weakened NTC behavior at 100 atm relative to 10 atm due to increased flux through QOOH + O 2 = O 2 QOOH relative to QOOH = 2 CH 2 O + OH at 100 atm. Furthermore, the intermediate-temperature oxidation window is shifted to lower temperatures at 100 atm. The experimental data are compared with several chemical kinetic models from the literature. The existing models are seen to agree quite well with the experimental data at 10 atm. However, the models fail to properly capture the NTC behavior at 100 atm. Reaction pathway analyses indicate that both the low- and intermediate-temperature chemistries are controlled by RO 2 consumption pathways. The reaction rates for several of the important reactions, such as DME + OH = CH 3 OCH 2 + H 2 O, H 2 O 2 (+M) = 2 OH (+M), and 2 HO 2 = 2 OH + O 2 are updated in this work. Furthermore, the updated model improves the predictability for all key species compared to the original model.
A rapid compression machine (RCM) has been applied to measure the ignition delay times of NH 3 /CH 3 OH mixtures covering pressures of 20 and 40 bar, equivalence ratios of 0.5, 1.0 and 2.0, and temperatures between 845 and 1100 K. Here the measurements show that the NH 3 /CH 3 OH mixtures become more reactive with increasing methanol addition. Addition of merely 1% (molar basis) of CH 3 OH to NH 3 lowers the ignition temperature around 100 K at 40 bar in comparison to pure NH 3 . The ignition delay is a complex function of fuel mixture and stoichiometry. For the 1% CH 3 OH mixture, the leaner mixtures are more reactive, while the reverse trend is found for mixtures with 5%, 20% and pure CH 3 OH. Analysis of the pressure profiles shows three distinct ignition modes for NH 3/ CH 3 OH mixtures, facilitated by the pre-ignition heat release from NH 3 consumption. A detailed mechanism for ignition of NH 3 /CH 3 OH fuel blends has been developed, capable of reproducing the ignition behavior of mixtures with reasonable accuracy. A subset for amine / methanol interactions was established, with rate constants for the key reaction between NH 2 and CH 3 OH calculated from ab initio theory. A sensitivity analysis indicates that the critical reactions during the auto-ignition process vary with the CH 3 OH mole fraction in the fuel. The ammonia chemistry, namely NH 2 + NO, NH2 + NO 2 and NH 2 + HO 2 , is dominant for the mixture with 1% CH 3 OH, while the reactions related to CH 3 OH and H 2 O 2 are more important for the 20% CH 3 OH mixture. The interaction between ammonia and methanol shows a more prominent effect on auto-ignition for mixtures with 5% CH 3 OH in fuel as compared to those with 1% and 20% CH 3 OH. According to the modeling results, methanol addition is found to enrich the O/H radical pool, consuming ammonia and promoting auto-ignition through different reaction pathways.
Developing advanced low-temperature combustion engines with ammonia-biofuel blends requires a comprehensive understanding of low-temperature flame dynamics and kinetic interactions between ammonia and oxygenated fuels at elevated pressures. This work aims to study the dynamics and kinetics of non-premixed Dimethyl Ether (DME)/Ammonia (NH 3 ) cool and warm flames, and their reignition to hot flames. A counterflow burner is employed to establish DME/NH 3 cool/warm flames at pressures up to 5 atm. The extinction limits of cool flame and the reignition limits of warm flame to hot flame are measured by varying NH 3 concentrations and compared to simulations to quantitatively examine the effects on DME/NH 3 flames. It is found that NH 3 inhibits low-temperature DME oxidation and results in lower cool flame extinction limits. Warm flames in the presence of NH 3 are observed for the first time, revealing a non-monotonic effect of NH 3 addition: a small amount of NH 3 presence enhances warm flame chemistry and promotes reignition to hot flames, while a high NH 3 concentration weakens the warm flame. This trend is further explained by 0-D PSR kinetic simulations and 1-D S-curve flame dynamic calculations. Three flame transition regimes between cool flames (CF), warm flames (WF), and hot flames (HF) by different levels of NH 3 additions at a specific strain rate are identified, namely WFHF reignition, WF-CF transition, and WF extinction. Reaction sensitivity analyses of OH at low temperatures show that NH3 inhibits DME oxidation through OH consumption via H-abstraction and the kinetic couplings of RO 2 /NH 2 , RO 2 /NO x , R/NO x , and O 2 QOOH/NO x further suppress the low-temperature branching. At intermediate-temperatures, NH 2 /NO x /HO 2 coupling promotes warm flames via the pathway NH 2 → H 2 NO → HNO → NO by converting O 2 → HO 2 → OH. At even higher NH₃ concentrations, radical termination reactions of NH 2 + NO/NO 2 and excessive OH consumption via H-abstraction inhibit the flame. The insights into the kinetic coupling between NH 3 and low-temperature chemistry at elevated pressure and its impact on the dynamics of cool-warm-hot flame transitions will contribute to advancing combustion technologies with reduced emissions and improved energy-efficiency.
The kinetics of peroxy radical (RO 2 ) reactions have been of long-standing interest in atmospheric and combustion chemistry. Nevertheless, the lack of kinetic studies at higher temperatures for their reactions with other radicals such as OH has precluded the inclusion of this class of reactions in detailed kinetics models developed for combustion applications. In this work, guided by the limited room temperature experimental studies on selected alkyl-peroxy radicals and literature theoretical kinetics on the prototypical CH 3 O 2 + OH system, we have performed parametric studies on the effect of uncertainties in the rate coefficients and branching ratios to potential product channels for RO 2 + OH reactions at higher temperatures. Literature kinetics models were used to simulate autoignition delays, laminar flame speeds, and speciation profiles in flow and stirred reactors for a variety of common combustion-relevant fuels. Inclusion of RO 2 + OH reactions was found to retard autoignition in fuel-lean (φ = 0.5) mixtures of ethane and dimethyl ether in air. The observed effects were noticeably more pronounced in ozone-enriched combustion of ethane and dimethyl ether. The simulations also examined the influence of ozone doping levels, pressures, and equivalence ratios for both ethane and dimethyl ether oxidation. Sensitivity and flux analyses revealed that the RO 2 + OH reaction is a significant sink of RO 2 radicals at the early stage of autoignition, affecting fuel oxidation through RO 2 ↔ QOOH, RO 2 ↔ alkene + HO 2 , or RO 2 + HO 2 ↔ ROOH + O 2 . Additionally, the kinetic stability of the trioxide formed from RO 2 + OH reactions was investigated using master equation analyses. Last, we discuss other bimolecular reactions that are missing in literature kinetics models but are relevant to hydrocarbon oxidation initiated by external radical sources (plasma-enhanced, ozone-enriched combustion, etc.). In conclusion, the present simulations provide a strong motivation for better characterizing the bimolecular kinetics of peroxy radicals.
Methanol (CH 3 OH) has attracted considerable attention as a renewable fuel or fuel additive with low greenhouse gas emissions. Methanol oxidation was studied using a recently developed supercritical pressure jet-stirred reactor (SP-JSR) at pressures of 10 and 100 atm, at temperatures from 550 to 950 K, and at equivalence ratios of 0.1, 1.0, and 9.0 in experiments and simulations. Here, the experimental results show that the onset temperature of CH 3 OH oxidation at 100 atm is around 700 K, which is more than 100 K lower than the onset at 10 atm and this trend cannot be predicted by the existing kinetics models. Furthermore, a negative temperature coefficient (NTC) behavior was clearly observed at 100 atm at fuel rich conditions for methanol for the first time. To understand the observed temperature shift in the reactivity and the NTC effect, we updated some key elementary reaction rates of relevance to high pressure CH 3 OH oxidation from the literature and added some new low-temperature reaction pathways such as CH 2 O + HO 2 = HOCH 2 O 2 (RO 2 ), RO 2 + RO 2 = HOCH 2 O (RO) + HOCH 2 O (RO) + O 2 , and CH 3 OH + RO 2 = CH 2 OH + HOCH 2 O 2 H (ROOH). Although the model with these updates improves the prediction somewhat for the experimental data at 100 atm and reproduces well high-temperature ignition delay times and laminar flame speed data in the literature, discrepancies still exist for some aspects of the 100 atm low-temperature oxidation data. In addition, it was found that the pressure-dependent HO 2 chemistry shifts to lower temperature as the pressure increases such that the NTC effect at fuel-lean conditions is suppressed. Therefore, as shown in the experiments, the NTC phenomenon was only observed at the fuel-rich condition where fuel radicals are abundant and the HO 2 chemistry at high pressure is weakened by the lack of oxygen resulting in comparatively little HO 2 formation.
Ho(PO)2 crystallizes in the tetragonal I4_1/amd space group. The structure is zero-dimensional and consists of eight Ho(PO)2 clusters. Ho3+ is bonded in a linear geometry to two equivalent O2- atoms. Both Ho–O bond lengths are 2.20 Å. P+0.50+ is bonded in a single-bond geometry to one O2- atom. The P–O bond length is 1.58 Å. O2- is bonded in a water-like geometry to one Ho3+ and one P+0.50+ atom.
Ho(AsO)2 crystallizes in the tetragonal I4_1/amd space group. The structure is zero-dimensional and consists of eight Ho(AsO)2 clusters. Ho3+ is bonded in a linear geometry to two equivalent O2- atoms. Both Ho–O bond lengths are 2.11 Å. As+0.50+ is bonded in a single-bond geometry to one O2- atom. The As–O bond length is 1.78 Å. O2- is bonded in a distorted linear geometry to one Ho3+ and one As+0.50+ atom.
Ho(ReAl5)2 crystallizes in the orthorhombic Cmcm space group. The structure is three-dimensional. there are two inequivalent Ho sites. In the first Ho site, Ho is bonded in a 10-coordinate geometry to four Re and sixteen Al atoms. There are two shorter (3.40 Å) and two longer (3.52 Å) Ho–Re bond lengths. There are a spread of Ho–Al bond distances ranging from 3.06–3.51 Å. In the second Ho site, Ho is bonded in a 4-coordinate geometry to fourteen Al atoms. There are a spread of Ho–Al bond distances ranging from 3.05–3.29 Å. There are two inequivalent Re sites. In the first Re site, Re is bonded in a 10-coordinate geometry to one Ho and ten Al atoms. There are a spread of Re–Al bond distances ranging from 2.56–2.80 Å. In the second Re site, Re is bonded in a 10-coordinate geometry to two equivalent Ho and ten Al atoms. There are a spread of Re–Al bond distances ranging from 2.57–2.74 Å. There are twelve inequivalent Al sites. In the first Al site, Al is bonded in a 2-coordinate geometry to two equivalent Ho, two equivalent Re, and four Al atoms. There are two shorter (2.73 Å) and two longer (3.08 Å) Al–Al bond lengths. In the second Al site, Al is bonded in a 12-coordinate geometry to one Ho, two equivalent Re, and nine Al atoms. There are a spread of Al–Al bond distances ranging from 2.66–3.14 Å. In the third Al site, Al is bonded in a 2-coordinate geometry to two equivalent Ho, two equivalent Re, and eight Al atoms. There are a spread of Al–Al bond distances ranging from 2.67–3.05 Å. In the fourth Al site, Al is bonded in a 2-coordinate geometry to one Ho, two equivalent Re, and nine Al atoms. There are a spread of Al–Al bond distances ranging from 2.81–3.11 Å. In the fifth Al site, Al is bonded in a 12-coordinate geometry to two Ho, two equivalent Re, and eight Al atoms. There are a spread of Al–Al bond distances ranging from 2.63–3.04 Å. In the sixth Al site, Al is bonded in a 2-coordinate geometry to two Ho, two equivalent Re, and six Al atoms. There are a spread of Al–Al bond distances ranging from 2.67–2.97 Å. In the seventh Al site, Al is bonded in a 2-coordinate geometry to one Ho, two equivalent Re, and eight Al atoms. There are two shorter (2.76 Å) and two longer (3.05 Å) Al–Al bond lengths. In the eighth Al site, Al is bonded in a 12-coordinate geometry to one Ho, two equivalent Re, and nine Al atoms. There are a spread of Al–Al bond distances ranging from 2.68–2.90 Å. In the ninth Al site, Al is bonded in a 12-coordinate geometry to two Ho, two equivalent Re, and eight Al atoms. The Al–Al bond length is 2.72 Å. In the tenth Al site, Al is bonded in a 2-coordinate geometry to one Ho, two equivalent Re, and eight Al atoms. Both Al–Al bond lengths are 2.97 Å. In the eleventh Al site, Al is bonded in a distorted linear geometry to two equivalent Ho, two Re, and four Al atoms. In the twelfth Al site, Al is bonded in a 2-coordinate geometry to one Ho, two Re, and four Al atoms.
Ho(Ni5Si)2 crystallizes in the tetragonal P4/nmm space group. The structure is three-dimensional. Ho is bonded in a 12-coordinate geometry to sixteen Ni and four equivalent Si atoms. There are a spread of Ho–Ni bond distances ranging from 2.88–3.06 Å. All Ho–Si bond lengths are 3.16 Å. There are three inequivalent Ni sites. In the first Ni site, Ni is bonded to two equivalent Ho, eight Ni, and two equivalent Si atoms to form NiHo2Si2Ni8 cuboctahedra that share corners with six equivalent SiHo2Ni10 cuboctahedra, corners with twelve NiHo2Si2Ni8 cuboctahedra, edges with four equivalent NiHo2Si2Ni8 cuboctahedra, edges with four equivalent SiHo2Ni10 cuboctahedra, faces with two equivalent SiHo2Ni10 cuboctahedra, and faces with twelve NiHo2Si2Ni8 cuboctahedra. There are four shorter (2.41 Å) and four longer (2.47 Å) Ni–Ni bond lengths. Both Ni–Si bond lengths are 2.33 Å. In the second Ni site, Ni is bonded in a 12-coordinate geometry to one Ho, eleven Ni, and two equivalent Si atoms. There are a spread of Ni–Ni bond distances ranging from 2.40–2.97 Å. Both Ni–Si bond lengths are 2.52 Å. In the third Ni site, Ni is bonded to two equivalent Ho, eight Ni, and two equivalent Si atoms to form distorted NiHo2Si2Ni8 cuboctahedra that share corners with four equivalent SiHo2Ni10 cuboctahedra, corners with fourteen NiHo2Si2Ni8 cuboctahedra, edges with two equivalent SiHo2Ni10 cuboctahedra, edges with five NiHo2Si2Ni8 cuboctahedra, faces with four equivalent SiHo2Ni10 cuboctahedra, and faces with eleven NiHo2Si2Ni8 cuboctahedra. Both Ni–Ni bond lengths are 2.57 Å. Both Ni–Si bond lengths are 2.31 Å. Si is bonded to two equivalent Ho and ten Ni atoms to form distorted SiHo2Ni10 cuboctahedra that share corners with four equivalent SiHo2Ni10 cuboctahedra, corners with fourteen NiHo2Si2Ni8 cuboctahedra, edges with eight NiHo2Si2Ni8 cuboctahedra, faces with four equivalent SiHo2Ni10 cuboctahedra, and faces with ten NiHo2Si2Ni8 cuboctahedra.
Ho(RuP)2 crystallizes in the tetragonal I4/mmm space group. The structure is three-dimensional. Ho is bonded in a 8-coordinate geometry to eight equivalent Ru and eight equivalent P atoms. All Ho–Ru bond lengths are 3.13 Å. All Ho–P bond lengths are 3.10 Å. Ru is bonded in a 12-coordinate geometry to four equivalent Ho and four equivalent P atoms. All Ru–P bond lengths are 2.35 Å. P is bonded in a 9-coordinate geometry to four equivalent Ho, four equivalent Ru, and one P atom. The P–P bond length is 2.38 Å.
Ho(IrSi)2 crystallizes in the tetragonal I4/mmm space group. The structure is three-dimensional. Ho is bonded in a 8-coordinate geometry to eight equivalent Ir and eight equivalent Si atoms. All Ho–Ir bond lengths are 3.22 Å. All Ho–Si bond lengths are 3.13 Å. Ir is bonded to four equivalent Ho and four equivalent Si atoms to form a mixture of distorted edge, corner, and face-sharing IrHo4Si4 tetrahedra. All Ir–Si bond lengths are 2.41 Å. Si is bonded in a 9-coordinate geometry to four equivalent Ho, four equivalent Ir, and one Si atom. The Si–Si bond length is 2.41 Å.