Formation Of Carbon Dioxide Research Articles

Utilization of depleted heavy oil reservoirs for carbon dioxide storage and sequestration: A molecular level assessment

Hydrocarbon-bearing formations have a life cycle that starts with initial development and ends with abandoning the formation once depleted. The number of abandoned reservoirs is increasing and is expected to do so over the foreseeable future. These reservoirs offer a sustainable solution to mitigate the negative impact of oil and gas industry on the environment. The abandoned reservoirs could serve as natural compartments to store greenhouse gasses, such as carbon dioxide, owing to their geological sealing features, the well-established understanding of their development, and the abundance of drilled wells that could serve as potential carbon dioxide injectors. Among other oil compounds, the abandoned oil phase is enriched with heavy oil components, such as asphaltene. The asphaltene deposition increases with an increased depletion rate and is mostly found as layers coating the rock grains. Owing to their molecular structure, asphaltene can host carbon dioxide in the sorbed form. In this study, realistic asphaltene structures are recreated on a molecular simulation platform and assessed for their carbon dioxide storage capacity. The findings substantiate asphaltene for its carbon dioxide storage capacity as absorbed (dissolved) and adsorbed phases. The presence of asphaltene increases the storage capacity of pore spaces by a factor up to 4 for the considered cases in this study. Furthermore, the sorption capacity has hysteresis favoring the retention of the sorbed phase. This study casts some light on the potential utilization of depleted heavy oil reservoirs for long-term carbon dioxide storage and sequestration.

A Cut Above the Rest: The Historic Perspectives of Circumcision and Anesthesia

Introduction: Circumcision is the oldest known recorded surgical procedure dating to at least 4000 BCE and held religious, cultural, and military significance. Preputial resection connoted contrasting meanings among different cultures; in some, circumcision was and remains an important transtion into infancy and then adulthood. In Bronze Age and early Egyptian cultures, however, circumcision was performed on vanquished enemies and improved upon the morbidity and mortality of phallic resectoin as a war trophy. The circumcision of Jesus was a significant subject for many Medieval and Renaissance artists who often portrated the event as an allegroical commentary on contemporary socio-political events. While there has been much literature, anthropological analysis, and art on circumcision, little is known about the anesthesia provided to those undergoing the procedure, especially those for whom the act was not a punitive militaristic procedure. We aimed to identify descriptions of the anesthesia provided to patients or victims undergoing circumcision and how the anesthesia may have evolved into the modern techniques used in the modern era. Sources and Methods: We performed a literature review via PubMed journal articles, texts, and historical discussions detailing the evolution of circumcision and accompanying anesthesia throughout history. Results: Earliest records of circumcision described religious rather than scientific rationale, marking a boy’s ascension to manhood. Documentation of the earliest practices of anesthesiology are relevant to urology through these traditions. Circumcision is first noted in Egyptian temple hieroglyphics dated to 4000 BCE, depicting young men restrained with a priest performing the cut wielding a knife. As early as 2500 BCE, circumcision in ancient Egypt was the first known surgical procedure utilizing anesthesia. A mixture rendered from calcium carbonate and acetic acid formed carbon dioxide on the prepuce resulting in the first rendition of cryo-analgesia. Egypt was not the only culture performing circumcision during this time period. Assyrian records dating back to 400 BCE describe a similar methodology, but also transcribed another primitive method of anesthesia. Assyrians utilized digital compression of the carotid arteries to produce anesthetic effects of both altered consciousness and decreased procedural pain. Carotid compression was commonplace enough that it influenced the language for which the carotid blood vessels are described in both Greek and Russian with translation as “The Artery of Sleep.” The Jewish tradition of brit milah utilizes a few drops of wine in the mouth of the infant, both as a form of analgesia and symbolic of the sealed covenant. It is hypothesized that the sugar interacts with the opiate receptors in the brain which is enhanced by the suckling action, providing comfort. Conclusions: Since their earliest documented history, anesthetic techniques have contributed to and been intertwined with the practices of circumcision.

Airplane Design Optimization for Minimal Global Warming Impact

This paper presents a method to assess the key performance indicators of aircraft designed for minimum direct operating costs and aircraft designed for minimum global warming impact. The method comprises a multidisciplinary aircraft optimization algorithm capable of changing wing, engine, and mission design variables while including constraints on flight and field performance. The presented methodology uses traditional class-I methods augmented with dedicated class-II models to increase the sensitivity of the performance indicators to relevant design variables. The global warming impact is measured through the average temperature response caused by several emission species (including carbon dioxide, nitrogen oxides, and contrail formation) over a prolonged period of 100 years. The analysis routines are verified against experimental data or higher-order methods. The design algorithm is subsequently applied to a single-aisle medium-range aircraft, demonstrating that a 57% reduction in average temperature response can be achieved as compared to an aircraft optimized for minimal operating costs. This reduction is realized by flying at 7.6 km and Mach 0.60, and by lowering the engine overall pressure ratio to approximately 37. However, to compensate for the lower productivity, it is estimated that 13% more climate-optimized aircraft have to be operated for the hypothetical fleet under consideration.

Theoretical investigation on the formation mechanism of carbonate ion in microbial self-healing concrete: Combined QC calculation and MD simulation

Carbonate ion (CO32–) is essential to the formation of calcium carbonate in microbial self-healing concrete (MSHC). The self-healing effect of MSHC can be improved in practical engineering by exploring the control parameters affecting the CO32– formation. However, investigations on the formation mechanism of CO32– in MSHC are lacking. In this work, the formation mechanism of CO32– in MSHC was investigated by quantum chemical calculation and molecular dynamics simulation. The results showed that carbonic acid (H2CO3) tended to decompose through the concerted mechanism rather than the stepwise mechanism in the neutral condition, and the calculated reaction energy barrier and reaction rate constant were in good agreement with experimental values reported in the literature. In the basic condition of MSHC, H2CO3 was highly prone to react with hydroxyl ion to form CO32– by a two-step reaction rather than react with water to form carbon dioxide, with a rate-determining step energy barrier of 11.56 kcal/mol. The increased temperature could promote the formation rate of CO32–, which was as high as 6.72×104 s−1 at room temperature. The investigation on the formation mechanism of CO32– in this work would provide the theoretical guidance for the better healing ability of MSHC in practical engineering.

Optimization of technological modes of heterogeneous systems at high turbulence

The phenomena of convective heat and mass transfer in turbulent flows, taking into account chemical reactions, are widespread and play an important role in natural phenomena and in various industries. Knowledge of the regularities of such flows is important in the formation of the theory of combustion physics, the development of new physical and chemical technologies, and the solution of problems of thermal power engineering and ecology. In the course of the study, complex combustion processes have been analyzed from the point of view of dependence on the influence of many physical and chemical parameters of the combustion reaction. In the study of phenomena in multiphase systems, the development of the theory of the formation of liquid droplets at high turbulence is of greatest interest. In this paper, the processes of atomization, ignition, and spatial distribution of droplets in heterogeneous two-phase flows have been studied by means of computer simulation methods. These studies were carried out on the basis of current environmental problems, including the problems of air pollution, consisting of harmful toxic metals, sulfuric, nitric acids, carbon oxides, and other harmful substances. The processes of dispersion and combustion of liquid fuels at high turbulence have been studied by using mathematical modeling methods based on the solution of partial differential equations and the optimal technological combustion mode taking into account the formation of carbon dioxide in the combustion chamber has been determined.

Experimental and numerical analysis of CO2 and CH4 hydrate formation kinetics in microparticles: A comparative study based on shrinking core model

The promotion of gas hydrate formation kinetics through mass transfer enhancement has been an important research topic, for which microparticles have been considered an effective method. In this work, carbon dioxide (CO2) and methane (CH4) hydrate formation kinetics in microparticles are investigated using “dry water” particles made of 3, 5 and 8-wt% silica. A modified shrinking-core model is established to study the CO2 and CH4 hydrates formation kinetics. It is the first model that integrates the effects of dissolved gas, the capillary effect of porous hydrate shell, and the volume change from water to hydrate. The experimental results reveal that “dry water” particles with 8-wt% silica has the highest normalized gas uptake due to their small particle size. The simulation results show an initial effective diffusion coefficient of 6.41–6.50×10–14 m2s−1 for CO2 and a slightly higher 6.83×10–14 m2s−1 for CH4 hydrate formation. The average effective diffusion coefficient of gas is higher in smaller particles. The water consumed through capillaries is more prominent in smaller particles, but it only accounts for less than 10% of water consumed at the hydrate–water interface. Furthermore, a decoupled heat transfer model was developed to quantify the effect of heat transfer in gas hydrate formation. The instantaneous temperature gradient in the hydrate shell is of a small magnitude of 10–2 K m−1, indicating that the impact of the heat transfer on hydrate formation kinetics is negligible. This work provides comprehensive insights into gas hydrate formation in microparticles and contributes as a theoretical basis for the improvement of gas hydrate kinetics through mass transfer enhancement.

Magnesium-Promoted Reductive Carboxylation of Aryl Vinyl Ketones: Synthesis of γ-Keto Carboxylic Acids.

Direct reductive carboxylation of easily prepared aryl vinyl ketones under the atmosphere of carbon dioxide led to the selective formation of γ-keto carboxylic acids in 38-86% yields. The reaction is characterized by the carbon-carbon bond formation of carbon dioxide at the β-position of enone, with the use of magnesium turnings that can be easily handled as the reducing agent and the eco-friendly reaction conditions such as no pressuring, no lower or higher reaction temperature, and short reaction time. This protocol showed a wide substrate scope and provided a useful and convenient alternative to access biologically important γ-keto carboxylic acids.

Copper-Palladium Core-Shell Bifunctional Nanoelectrocatalyst for Ethanol Oxidation and Hydrogen Evolution Reactions

Ethanol oxidation reaction (EOR) and hydrogen evolution reaction (HER) are two of the most important technical challenges during the manifestation of ethanol and H2-O2 fuel cells. Herein we explore the electrocatalytic activity of copper-palladium core-shell nanostructures on pencil graphite (Cu@Pd/PGE) for EOR and HER. A maximum peak current density of 261 ± 11 mA cm−2 with excellent retention for 1000 potential cycling is observed for EOR. High performance liquid chromatography and Raman analysis indicate upto 12-electron transfer oxidation with the formation of carbon dioxide or carbonate, which imply the superiority of the catalyst. High HER current near to its thermodynamic potential of ∼0 V vs RHE, together with a stable performance for 1000 voltammetric cycles, make Cu@Pd/PGE as a potential catalyst for HER in acidic media. The catalytic efficiency is also manifested by the well resolved voltammetric features during HER activity with a Tafel slope comparable to commercial Pt/C catalysts. The excellent bifunctional activity of Cu@Pd/PGE provides a new route for commercialisation of low-cost electrocatalysts for fuel cell applications.

An environmental cost basis for regulating aviation NOx emissions

Combustion in aircraft engines results in the formation of nitrogen oxides (NOx) and carbon dioxide (CO2), among other species. NOx impacts air quality and is an indirect contributor to radiative forcing, while CO2 is a long-lived greenhouse gas. The International Civil Aviation Organization sets limits on NOx emissions from commercial aircraft, where for engines with a rated thrust greater than 89 kN the allowable NOx production per unit rated thrust is defined as a function of engine overall pressure ratio (OPR). This definition links the engine thermodynamic cycle, and implicitly fuel burn and CO2 emissions, to allowable NOx levels. These regulations have historically been evaluated and implemented with a focus on reducing adverse air quality impacts around airports, but the thermodynamic efficiency tradeoff with CO2 requires additional analysis to quantify net environmental impacts. This paper introduces a social cost basis for evaluating aviation NOx emissions regulations and quantifies the implied CO2 and NOx attributable air quality damage, climate damage, and fuel costs associated with the emissions standard. We show that higher overall pressure ratio engines operating at the current NOx regulatory limit are allowed more environmental damage per unit rated thrust than lower overall pressure ratio engines, resulting in variable social costs per unit thrust (i.e. fuel and environmental costs combined) across the engine design space. This is a consequence of the definition of the regulation today, where higher pressure ratio engines are allowed higher NOx emissions. Alternative regulation definitions are evaluated which consider the engine cycle and combustor together. Achieving constant social costs requires a regulatory limit where the increase in allowed NOx emissions tapers off at higher pressure ratios, corresponding to the diminishing marginal efficiency improvements due to increasing OPR in that region.

Advanced Catalysts for the Water Gas Shift Reaction

The WGS reaction is an exothermic reaction between carbon monoxide and steam to form carbon dioxide and hydrogen. This reaction, which has been used industrially for more than 100 years, has recently received a great deal of attention from researchers as one of the ways to produce environmentally acceptable hydrogen from fossil fuels in large quantities. For the application of this reaction on an industrial scale, the key is choosing the optimal catalysts that can ensure high CO conversion and have a long lifetime under industrial conditions. Therefore, new types of catalysts are being developed that meet these requirements better than the Fe- and Cu-based catalysts commonly used in the past. The WGSR on a commercial nickel-based catalyst and a laboratory-prepared copper- and cobalt-based catalyst was tested in a laboratory apparatus set up at the University of Chemistry and Technology Prague. The best performance of the laboratory-prepared catalyst was observed for the catalyst with a Cu content of 14.8 wt% and activated in a hydrogen atmosphere. The laboratory-prepared Co-based catalyst showed good WGSR activity in the temperature range of 200–450 °C, although this was always inferior to that of the Cu-based catalyst. When subjected to the feed gas containing 0.4 mole% H2S, the Co-based catalyst showed good resistance to sulphur poisoning. Therefore, Co-based catalysts can be considered good sulphur-tolerant intermediate temperature WGSR catalysts.

Reaction of methane and UV-activated perchlorate: Relevance to heterogeneous loss of methane in the atmosphere of Mars

The absence of significant detectable signatures of organic molecules in the atmosphere and on the surface of Mars is a major unsolved puzzle. One possible explanation is that perchlorate-rich Martian soils, activated by solar ultraviolet (UV) radiation, create an environment favorable for the rapid oxidation of organics such as alkanes (including methane or CH4). In this paper, we measured product formation rates from the methane-perchlorate-UV system at room temperature. Our results show that magnesium perchlorate (Mg(ClO4)2•6H2O) surfaces exposed to UV light at wavelengths reaching the Mars' surface accelerate the decomposition of methane (CH4), resulting in the formation of carbon dioxide (CO2), carbon monoxide (CO), and volatile chlorine oxides. The production rates for CO2 and CO on UV-activated perchlorate surfaces are accelerated by a factor of 2.5 and 4.5, respectively, compared to those in the absence of perchlorate. In addition, with UV radiation exposure, perchlorate (ClO4−) decomposes to chlorate (ClO3−) and chlorine oxides. These results are incorporated into a simple box model to estimate the near-surface atmospheric methane lifetime. The model gives a lower bound of the lifetime on the order of hours to days, substantially shorter than ~300 yrs. calculated from methane loss by gas-phase chemistry alone.

Fused aromatic networks as a new class of gas hydrate inhibitors

Fused aromatic networks (FANs) are attracting considerable interest in the scientific community because of their intriguing electronic properties and superior physiochemical stability due to their fully fused aromatic systems. Here, a three-dimensional (3D) cage-like organic network (3D-CON) and a vertical two-dimensional (2D) layered ladder structure (designated as V2D-BBL structure) were studied as materials for gas hydrate inhibitors because of their outstanding stability in high-pressure/low-temperature and periodically incorporated molecular building blocks. The V2D-BBL structure demonstrated remarkable performance, inhibiting the formation of both methane (CH4) and carbon dioxide (CO2) hydrates, comparable to conventional lactam-based polymers. It was determined that the designed perinone moiety in the V2D-BBL structure enables synergistic interactions with the host (water) and guest (CH4) molecules involved in hydrate nucleation. Given their pre-designability and inherent stability, the FANs hold enormous potential as gas hydrate inhibitors for industrial applications.

Mechanistic insights into benzene oxidation over CuMn2O4 catalyst

The spinel-type CuMn2O4 catalyst exhibits good catalytic activity towards benzene oxidation, but the catalytic oxidation mechanism is not established. Theoretical calculations were implemented to unearth the reaction mechanism of benzene catalytic oxidation over CuMn2O4 catalyst through density functional theory (DFT). The results indicate that benzene adsorption on both Cu-terminated and Mn-terminated surfaces are controlled by the chemisorption mechanism. The Cu-terminated surface is more active for benzene adsorption than the Mn-terminated surface. Cu atom is regarded as the primary active site. During benzene catalytic oxidation, benzene firstly undergoes dehydrooxidation reaction to generate phenoxy group (C6H6* → C6H5* → C6H5O*). Two reaction channels are responsible for the ring-opening and oxidation reactions of phenoxy group, including benzoquinone- and cyclopentadienyl-dominated channels. In the benzoquinone-dominated channel, C6H4O2* is produced from phenoxy dehydrogenation and oxidation, and then decomposes into acetylene via the ring-opening reaction (C6H4O2* → C4H2O2* → C4H2O4* → C2H2*). Compared with the benzoquinone-dominated channel, the cyclopentadienyl-dominated channel is dominant for phenoxy group oxidation. Phenoxy group decomposes to generate cyclopentadienyl. C5H5* is dehydrogenated and oxidized to form cyclopentadienone. Finally, C5H4O* is oxidized to form carbon dioxide through a nine-step reaction pathway. The ring-opening reaction (C5H4O* → C3H2O*) has the highest energy barrier of 283.45 kJ/mol, and is identified as the rate-determining step of benzene catalytic oxidation.

The impact of wormhole generation in carbonate reservoirs on CO2-WAG oil recovery

In this paper, the impact of CO2-carbonate interactions on hydrocarbon recovery from three initially oil-saturated heterogeneous carbonate core samples (with low and mid permeabilities) were investigated. Alternate injection of carbon dioxide (CO2) and synthetic formation brine was conducted after water flooding. X-ray computed tomography scanning of the core samples before and after core flooding was performed at ambient conditions. Extent of carbonate dissolutions and rock matrix modifications were observed from the change in porosities, permeabilities, and wormhole formations. Generally, the CO2-WAG injection resulted in more than 30% extra oil recovery from the three cores after water flooding scenario. As expected, the total recovery factor of the core with lowest initial permeability and porosity was the smallest, however the core permeability increased from 1.45 mD (pre-flood) to 1987.9 mD (post-flood). Considerable amount of oil was left behind in the core with initially low permeability and porosity due to preferentially flow of WAG fluid through the wormholes, bypassing the trapped oil in the pore spaces at the uninvaded region of the rock. The study suggests that low oil recovery from core with low initial permeability and porosity is principally due to formation of wormhole because of rapid rate of rock dissolution and faster rate of CO2-carbonate reactions. Compared to the mid permeability cores, the low permeable core slows down the WAG fluid flow and give more time for CO2-carbonate rock interactions and reactions because of the tightness of the pore spaces. Thus, facilitated more exposure time and effective contact between the carbonate rock fabrics and the CO2-saturated brine.

Formation and Dissociation of CH4 and CO2 Hydrates in Presence of a Sediment Composed by Pure Quartz Mixed with Ti23 Particles.

The present research deals with the formation and dissociation of methane and carbon dioxide hydrates in a confined environment (small—size reactor) and in presence of a porous sediment of pure quartz impregnated with Ti23 particles. This research is part of a wider study aimed at verifying the possibility to use metallic powders, produced via gas-atomization for applications in additive manufacturing, as additives during the production/dissociation of gas hydrates. The porous medium was used to ensure the presence of Ti23 particles in the whole volume and not only in the lowest portion of the internal volume. For both the guest compounds considered, two Ti23 concentrations were explored, respectively, 8.68 and 26.04 wt%. Under the thermodynamic point of view, the dissociation process well approximated the phase equilibrium (defined with values collected from literature) for both compounds. In addition, the amount of gas trapped into hydrates, evaluated as a function of the initial amount of gas inserted inside the reactor, did not show relevant changes. Conversely, the presence of Ti23 was found to reduce the induction time for both components, thus allowing to define it as a kinetic promoter for the process. Such tendency was found to increase with the concentration.

Formation and evolution of C-C, C-O, C[double bond, length as m-dash]O and C-N bonds in chemical reactions of prebiotic interest.

A series of prebiotic chemical reactions yielding the precursor building blocks of amino acids, proteins and carbohydrates, starting solely from HCN and water is studied here. We closely follow the formation and evolution of the pivotal C-C, C-O, C[double bond, length as m-dash]O, and C-N bonds, which dictate the chemistry of the molecules of life. In many cases, formation of these bonds is set in motion by proton transfers in which individual water molecules act as catalysts so that water atoms end up in the products. Our results indicate that the prebiotic formation of carbon dioxide, formaldehyde, formic acid, formaldimine, glycolaldehyde, glycine, glycolonitrile, and oxazole derivatives, among others, are best described as highly nonsynchronous concerted single step processes. Nonetheless, for all reactions involving double proton transfer, the formation and breaking of O-H bonds around a particular O atom occur in a synchronous fashion, apparently independently from other primitive processes. For the most part, the first process to initiate seems to be the double proton transfer in the reactions where they are present, then bond breaking/formation around the reactive carbon in the carbonyl group and finally rupture of the C-N bonds in the appropriate cases, which are the most reluctant to break. Remarkably, within the limitations of our non-dynamical computational model, the wide ranges of temperature and pressure in which these reactions occur, downplay the problematic determination of the exact constraints on the early Earth.

Influence of oxygen on the vinyl acetate elimination pathway and microbial community structure of methanogenic sludge.

Methanogenic-aerobic coupled processes were used to biological degradation of vinyl acetate (VA) to provide evidence of oxygen role for their complete elimination from different angles. First, physiological characterization of a continuous methanogenic-aerobic reactor fed by VA and glucose (G) showed that by adding G, the VA got 100% hydrolyzed to acetate, and then, by adding 1mg·L-1 ·d-1 of dissolved oxygen (DO), this acetate got methanized by 40% and aerobically mineralized by 60%. Second, batch assays in the presence and absence of sodium azide suggest that VA at different concentrations was eliminated by both anaerobic and aerobic metabolic pathways, because without azide and in the presence of 1mg DO·L-1 increased methane and carbon dioxide formation rates at 80% and 75%, respectively. Finally, microbial population dynamics analysis of the reactor by DGGE-sequencing highlighted that Brevibacillus agri (aerobic) and Methanosarcina barkeri (anaerobic) were identified as responsible for VA elimination by up to 98.6%. PRACTITIONER POINTS: Vinyl acetate is removed by simultaneous methanation and aerobic respiration. Methanosarcina barkeri and Brevibacillus agri removed up to 99% of vinyl acetate. DO and VA have a selective effect on the metabolism and population dynamics.

Carbonization of Coconut Shell Biomass in a Downdraft Reactor: Effect of Temperature on the Charcoal Properties

Considering the value of coconut shell biomass as renewable fuels in homes and commercial industries, its effectiveness as a biomass resource has been overlooked by our rural citizens and researchers. Carbonization experiments of coconut shell biomass were conducted in a downdraft carbonization reactor (750 mm height and 67 mm diameter) to determine the effect of temperature (250 to 600 °C, in 50 °C intervals) on the charcoal properties. This was to address the problems of traditional charcoal production methods that include; low yield, environmental pollution, unregulated temperature, and poor quality properties. Yet the scarcity of this information hampers efforts for efficient commercial production. The coconut shell biomass was obtained from a local shop in Malaysia. It was heated in the reactor at a fixed residence time of 60 min and a particle size of 5 mm with nitrogen as a carrier gas. The relationship between temperature and the properties of coconut shell charcoal has been ascertained by the findings of this analysis. The relatively high the temperature, the better the charcoal quality, but the lowest the charcoal yields, since the secondary pyrolysis reaction expends charcoal. It was noted that, up to a final temperature of 500 °C, the yield reduction was rapid; after that, it was slower at 550 °C and almost stable at 600 °C. According to the charcoal’s proximate analysis, the calorific value, fixed carbon content, carbon content and ash content increased with temperature. Whereas, the charcoal density, volatile matter, moisture content, and conversion efficiencies decrease with temperature. The presence of nitrogen gas appears to have reduced combustion reactions that promote the formation of carbon dioxide (CO2). The methods produce the least amount of air pollutants (96 g carbon monoxide (CO), 167 g CO2, and 64 g methane (CH4) per 1 kg of charcoal production). The type of biomass and carbonization kiln has an impact on the production of CO, CO2, and CH4. Thus, the carbonization reactor used in this study has the potentials to produce an eco-friendly charcoal with superior quality properties that can assist in reducing environmental pollution, by proper selections of carbonization temperature.

Effects of Co-adsorbed Water on Different Bond Cleavages of Oxygenates on Pd (111)

Experimental studies have shown that co-adsorbates and solvents affect both the activity and selectivity of heterogeneous catalysts, but how they affect different bond cleavages and a network of parallel reactions is not well understood. Here we present a density functional theory (DFT) calculation of how co-adsorbed water affects different bond cleavages of oxygenates on metal surface, using decomposition of acetic acid over Pd (111) as a model system for oxygenates, with application in biomass conversion. The presence of co-adsorbed water generally enhances O–H bond cleavage while generally inhibiting the OC–O, C–C, and OC–OH bond cleavage. The influence of co-adsorbed water on C–H bond cleavage varies the most and depends on the nature of the transition state and how co-adsorbed water stabilizes the initial and final state. Although these trends are useful as general guidance, they are not sufficient to predict the effect on a complex reaction network such as acetic acid decomposition that has several parallel reaction paths. In the absence of co-adsorbed water, the two lowest energy pathways are decarboxylation and decarbonylation pathways through a common CH2COO intermediate. But through an inhibition of OC–O bond cleavage but enhancement of C–C bond cleavage of CH2COO, one of three exceptions to the general trend of C–C inhibition in the presence of water, the two lowest free energy pathways are decarboxylation forming CO2 in the presence of water. This illustrates how a single reaction step can affect a complex reaction network with many parallel, energetically similar paths. This suggests that the presence of co-adsorbed water makes acetic acid decarboxylation (formation of carbon dioxide) more favorable than acetic acid decarbonylation (formation of carbon monoxide) over Pd (111).

Greenhouse gas formation during the ensiling process of grass and lucerne silage

Silage is an essential global feedstuff and an emitter of greenhouse gases. However, few studies have examined the formation of carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) during the ensiling process. This study aimed to record the course of gas concentrations in forage during the ensiling process and determine the temporal variation in the (microbiological) formation processes. Grass and lucerne, each with two different dry matter (DM) concentrations (four variants, each n = 3), were ensiled in laboratory-scale barrels (120 L). Gas samples were taken from the headspace of the barrels and analysed using a gas chromatograph. The measurement period included the first 49 days of the ensiling process and the measurement interval was 0.5–48.0 h. For all variants, a rapid increase in CO2 concentration and a one-time N2O concentration peak was observed between ensiling hours 36 and 96. Lower DM concentration led to significantly faster CO2 production (p < 0.05). Lucerne forage and higher DM concentrations led to significantly increased N2O concentrations (p < 0.05). The extensive measurements demonstrated that butyric acid formation by clostridia contributes to CH4 formation; thus, lucerne silage had a significantly higher concentration from ensiling day 13 (p < 0.05). Therefore, malfermentation actively contributes to the formation of greenhouse gases. The method described here provides further insights into greenhouse gas formation during the ensiling process and can thus help to improve ensiling research and management.