Activation of CO2 and CH4 on MgO surfaces: mechanistic insights from first-principles theory

Effect of fit-for-purpose biochars on rumen fermentation, microbial communities, and methane production in cattle

Introduction

Biochar has gained significant attention as a possible anti-methanogenic supplement for ruminants due to its potential to reduce methane (CH₄) emissions from enteric fermentation. However, its effects on rumen methanogenesis have been inconsistent and, in some cases, contradictory. These variations are likely influenced by factors such as the type of biochar used, its source material, and how it is administered, including the form in which it is provided and the dosage needed to achieve desired outcomes. This study aimed to examine the effects of two fit-for-purpose biochars on rumen fermentation, CH4 emissions, and the rumen microbiome of cattle-fed roughage-based diets. Two experiments were conducted to assess the potential of biochar in mitigating CH4 emissions.

Experiment 1

This was a controlled pen trial conducted over 56 days, involving 12 steers that were fed Rhodes grass hay ad libitum. The animals were assigned to one of four treatment groups: control (no biochar, only molasses), low dose (50 g biochar/animal/day), mid dose (100 g biochar/animal/day), or high dose (200 g biochar/animal/day). Two types of biochar, Biochar 1 and Biochar 2, were administered with molasses (200 mL per animal/day). Methane emissions were measured using open-circuit respiration chambers, and rumen fluid samples were collected for analysis of the rumen microbial community and fermentation metabolite.

Experiment 2

In this trial, 45 heifers were selected and grazed together in a single paddock for 60 days to assess the effects of biochar on productivity and CH4 emissions under grazing conditions. The animals were allocated to one of three treatment groups (15 animals per group): control (no biochar, only molasses), Biochar 1, or Biochar 2. Each group was administered biochar at an estimated single dose of 100 g per animal/day mixed with molasses. Methane emissions were measured using GreenFeed systems in the field to monitor CH₄ production from individual animals.

Results

In the controlled pen trial (Experiment 1), biochar supplementation resulted in a reduction of CH₄ emissions by 8.8-12.9% without any negative effects on rumen fermentation or dry matter intake (DMI). Minor changes were observed in the rumen bacterial community, particularly in the Christensenellaceae and Prevotellaceae families. However, in the grazing trial (Experiment 2), no significant differences in CH₄ emissions or productivity were detected with biochar supplementation.

Conclusion

While the results from controlled feeding conditions suggest that biochar has the potential to reduce enteric CH₄ emissions, the lack of significant findings under grazing conditions highlights the need for further research. Future studies should focus on identifying biochar types, doses, and delivery methods that are effective in reducing CH₄ emissions in grazing systems without compromising cattle productivity.

Mechanism of Autocatalytic Reduction of CO2 over MgCO3 to High Value-Added Chemicals: A DFT & AIMD Study

Calcination of MgCO3 is an important industrial reaction, but it causes significant and unfavorable CO2 production. Calcination in a reducing green hydrogen atmosphere can substantially reduce CO2 release and produce high value-added products such as CO or hydrocarbons, but the mechanism is still unclear. Here, the in situ transformation process of MgCO3 interacting with hydrogen and the specific formation mechanism of the high value-added products are thoroughly investigated based on reaction thermodynamic, ab initio molecular dynamics (AIMD) simulations, and density functional theory (DFT) calculations. The reaction thermodynamic parameters of MgCO3 coupled with hydrogen to produce CO or methane are calculated, revealing that increasing and decreasing the thermal reductive decomposition temperature favors the production of CO and methane, respectively. Kinetically, the energy barriers of each possible production pathway for the dominant products CO and methane are further calculated in conjunction with the AIMD simulation results of the transformation process. The results suggest that CO is produced via the MgO catalytic-carboxyl pathway (CO2*→ COOH*trans→ COOH*cis→ CO*→ CO), which is autocatalyzed by MgO derived from the thermal reductive decomposition of MgCO3. For the mechanism of methane formation, it prefers to be produced by the stepwise interaction of carbonates in the MgCO3 laminates with hydrogen adsorbed on their surfaces (direct conversion pathway: sur-O-CO → sur-O-HCO → sur-O-HCOH → sur-O-HC → sur-O-CH2 → sur-O-CH3 → sur-O + CH4*).

Electrostatically tunable interaction of CO2 with MgO surfaces and chemical switching: first-principles theory

Electric field-assisted CO2 capture using solid adsorbents based on basic oxides can immensely reduce the required energy consumption compared to the conventional processes of temperature or pressure swing adsorption. In this work, we present first-principles density functional theoretical calculations to investigate the effects of an applied external electric field (AEEF) within the range from -1 to 1 V Å-1 on the CO2 adsorption behavior on various high and low-index facets of MgO. When CO2 is strongly adsorbed on MgO surfaces to form carbonate species, the coupling of electric fields with the resulting intrinsic dipole moment induces a 'switch' from a strongly chemisorbed state to a weakly chemisorbed or physisorbed state at a critical value of AEEF. We demonstrate that such 'switching' enables access to different metastable states with variations in the AEEF. On polar MgO(111) surfaces, we find a distinct feature of the adsorptive dissociation of CO2 towards the formation of CO in contrast to that on the non-polar MgO(100) and MgO(110) surfaces. In some cases, we observe broken inversion symmetry because of the AEEF that results in induced polarity at the interaction site of CO2 on MgO surfaces. Our results provide fundamental insights into the possibility of using AEEFs in novel solid adsorbent systems for CO2 capture and reduction.