Carbon-neutral fuel | EPFL Graph Search

Carbon-neutral fuel is fuel which produces no net-greenhouse gas emissions or carbon footprint. In practice, this usually means fuels that are made using carbon dioxide (CO2) as a feedstock. Proposed carbon-neutral fuels can broadly be grouped into synthetic fuels, which are made by chemically hydrogenating carbon dioxide, and biofuels, which are produced using natural CO2-consuming processes like photosynthesis. The carbon dioxide used to make synthetic fuels may be directly captured from the air, recycled from power plant flue exhaust gas or derived from carbonic acid in seawater. Common examples of synthetic fuels include ammonia and methane, although more complex hydrocarbons such as gasoline and jet fuel have also been successfully synthesized artificially. In addition to being carbon neutral, such renewable fuels can alleviate the costs and dependency issues of imported fossil fuels without requiring either electrification of the vehicle fleet or conversion to hydrogen or other fuels, enabling continued compatible and affordable vehicles. In order to be truly carbon-neutral, any energy required for the process must be itself be carbon-neutral or emissions-free, like renewable energy or nuclear energy. If the combustion of carbon-neutral fuels is subject to carbon capture at the flue, they result in net-negative carbon dioxide emission and may thus constitute a form of greenhouse gas remediation. Negative emissions are widely considered an indispensable component of efforts to limit global warming, although negative emissions technologies are currently not economically viable for private sector companies. Carbon credits are likely to play an important role for carbon-negative fuels. Synthetic hydrocarbons can be produced in chemical reactions between carbon dioxide, which can be captured from power plants or the air, and hydrogen. The fuel, often referred to as electrofuel, stores the energy that was used in the production of the hydrogen. Hydrogen fuel is typically prepared by the electrolysis of water in a power to gas process.

Emission reduction by targeted transient operations in three-way catalysts for heavy-duty natural gas applications

Moyu Wang

To curb the severe effects of climate change, our society needs to radically reduce its CO2 footprint. For the heavy-duty sector, where electrification is difficult, alternative fuels can be the solution. Methane-fueled engines have lower energy-specific CO2 emissions than conventional Diesel-fueled engines. Furthermore, they have the potential of becoming carbon-neutral, when combined with bio-methane/synthetic methane. However, oxidation of unburnt methane in the exhaust gas poses a challenge for aftertreatment systems. The aim of this thesis is to investigate the mechanism of methane abatement and to reveal methods to reduce methane and other hazardous gas emissions. The majority of the experiments were conducted directly on an engine test bench, which is rarely seen in studies focusing on catalytic reaction pathways. Initial investigations focused on methane abatement under steady state and Î»-step transitions. Under steady state, the presence of oxygen was identified as a prerequisite for methane conversion. Reacting with oxygen is the only methane conversion pathway. However, after step transition from oxygen excess conditions (slightly lean) to oxygen-poor conditions (slightly rich), high methane conversion was observed under rich conditions with no oxygen available. This high conversion was attributed to steam reforming (SR), which was activated by the step transition. The SR reaction rate decreased over time when staying at rich conditions, until full deactivation. Investigations in the lab-scale model gas reactor confirmed this analysis. In addition, the reason for SR deactivation was identified by DRIFTS measurements as the accumulation of carbonates on the catalytic surface, blocking the active sites. Based on the identified importance of the SR reaction, targeted Î» oscillations across stoichiometry were introduced, in order to repeatably activate SR and achieve sustainable high methane conversion. During the rich parts of the oscillations, methane was converted via SR, while, during lean parts, the carbonates were periodically removed from the catalyst surface. With these oscillations, methane conversion has been significantly improved, in comparison to steady state. In parallel, a numerical model has been developed in order to simulate the catalyst behavior under oscillating conditions. The model provided insights on the reaction pathways and their distribution along the catalyst axis. The catalytic activity of the different Platinum-group metals has been investigated for the identified reactions. Various catalysts of different compositions were tested under cold start, Î» oscillations and quasi-steady state conditions. Both Pt and Pd activated SR reactions, however SR attenuation was faster in Pt catalysts. In lean conditions, Pt exhibited higher methane oxidation. Rh was identified as important for enhancing NOx reduction and lowering NH3 emissions. The combination of all three metals has improved the overall catalyst performances. In the final part of the thesis, a special aftertreatment system was investigated. It combines a Pd/Rh catalyst subject to stoichiometric conditions with a Pt oxidation catalyst subject to lean conditions. In the Pd/Rh catalyst, methane was removed via Î» oscillations. In the Pt catalyst, the remaining CO, H2 and NH3 were oxidized. The setup provided a novel perspective in reducing the overall environmental impacts.

EPFL2022

Gold-in-copper at low *CO coverage enables efficient electromethanation of CO2

Michael Graetzel, Yang Li, Dan Ren, Jun Li, Yongfeng Xie, Xiaoyan Wang, Yiping Wang, Ziyun Wang, Pengfei Ou, Yi Xu

The renewable-electricity-powered CO2 electroreduction reaction provides a promising means to store intermittent renewable energy in the form of valuable chemicals and dispatchable fuels. Renewable methane produced using CO2 electroreduction attracts interest due to the established global distribution network; however, present-day efficiencies and activities remain below those required for practical application. Here we exploit the fact that the suppression of *CO dimerization and hydrogen evolution promotes methane selectivity: we reason that the introduction of Au in Cu favors *CO protonation vs. C-C coupling under low *CO coverage and weakens the *H adsorption energy of the surface, leading to a reduction in hydrogen evolution. We construct experimentally a suite of Au-Cu catalysts and control *CO availability by regulating CO2 concentration and reaction rate. This strategy leads to a 1.6x improvement in the methane:H-2 selectivity ratio compared to the best prior reports operating above 100mAcm(-2). We as a result achieve a CO2-to-methane Faradaic efficiency (FE) of (562)% at a production rate of (112 +/- 4) mA cm(-2). The electroreduction of CO2 offers a promising approach to produce carbon-neutral methane using renewable electricity. This study shows that the introduction of Au in Cu enables selective methane production from CO2 by regulating *CO availability.

NATURE RESEARCH2021

Exploring Synthetic Strategies for Nanocrystal/Reticular Framework Hybrids and Their Potential as CO2 Reduction Electrocatalysts

Yannick Thomas Guntern

The electrochemical CO2 reduction reaction (CO2RR) into valuable chemicals has the potential to realize a carbon-neutral energy cycle. Developing catalysts that can achieve high selectivity towards one of the possible reduction products is one of the biggest challenges in this field. Catalysts possessing one single type of active sites cannot fulfil this need, yet to establish design rules for bifunctional catalysts is not trivial. To have material platforms which allow to study the sensitivity of the catalytic behaviour to structural and compositional features of the catalyst is of the uttermost importance to inform catalyst design. This thesis aims at combining well-defined colloidal nanocrystals (NCs) and porous reticular frameworks as hybrid materials to explore their synergistic interactions as CO2RR catalysts. These classes of materials were chosen because they possess complementary chemical properties which are appealing towards eventually construct an ideal catalyst with high performance. However, one of the challenges is to obtain these hybrids with tunable structural features in an unrestricted compositional range. The overall objective is to design and explore synthetic strategies for the formation of new multifunctional composites consisting of colloidal NCs and metal-organic/covalent organic frameworks (MOFs/COFs). First, a synthetic approach for the formation of NC/MOF hybrid thin films is proposed, which enables their application as a novel catalytic platform for CO2RR. We demonstrate that a combination of colloidal chemistry, atomic-layer deposition and solvothermal synthesis allows to form an intimate contact between Ag NCs and an Al-PMOF matrix. Our tuneable Ag@Al-PMOF hybrid catalysts show promoted CO2RR which is attributed to electronic changes in the Ag NC surface and minor mass-transport effects. Moreover, the hybrids improve the morphological stability of the Ag NCs under CO2RR conditions. We show that the synthetic approach can be further extended to form hybrids with other metallic NCs, thereby representing a new tool for the preparation of composite catalysts for CO2RR. One of the limitations of thin film composites is that synthesis conditions might need to be adapted for different substrates. To prepare hybrid materials in the form of dispensible colloidal solutions is highly appealing and better suited for mechanistic studies. The latter are important because understanding the chemistry driving the formation of such composites is eventually the key for achieving compositional and structural tunability. In the second part, we introduce a new strategy to encapsulate various colloidal NCs in the imine-linked COF LZU1. The synthetic approach allows us to tune the shell thickness and crystallization of the COF in the presence of the NCs. After understanding the nucleation and growth mechanisms of the hybrids, we extend the developed synthesis to obtain multi-layered structures with controlled spatial distribution of various NCs with different compositions. Altogether, we demonstrate that our approach can be applied to prepare hybrids with new functionalities that derive from the encapsulated NCs. Overall, this thesis proposes two different approaches which combine colloidal and reticular chemistry to synthesize tunable material platforms which might aid the design of CO2RR catalysts as well as contributing to other applications which require multifunctional porous materials.

EPFL2021