Umwelt und Klimawandel

Aenert. Research Laboratory news
With energy demand rising around the globe and efforts being made to produce the greater part of the energy consumed from renewables because of their obvious environmental benefits, scientists around the world supported by local governments are seeking to find means which can fulfill both conditions: reduce emissions as well as produce sufficient amounts of energy for the industry as well as society. Utilisation of CO₂ released into the atmosphere during industrial processes is one possible method to reduce emissions, but also generate additional energy which can be used on the spot in industrial production cycles. One such process involves dry reforming of methane for producing hydrogen with the help of CO₂ released during the process. Dry reforming of methane is a suitable means to generate valuable chemicals from syngas while reducing the emission of two potent carbon-based greenhouse gases, methane and CO₂. For this purpose, however, efficient catalysts are needed to facilitate different kinds of reactions as well as start complex chemical processes.

Now (2023), researchers at Brookhaven National Laboratory have found a very effective catalyst for methane dry reforming and researched it thoroughly to find out how it worked in the course of reaction, where two greenhouse gases, methane and carbon dioxide, are simultaneously converted into a mixture of hydrogen molecules and carbon monoxide which is then used for the preparation of high-value chemicals and fuels.

The catalyst consists of palladium (Pd), cerium (Ce), and oxygen (O), where the Ce and O take the form of cerium oxide, CeO₂. The molecular structure of CeO₂ is such that it easily incorporates clusters of palladium atoms. The interacting of the CeO₂ and the palladium atoms, a process which is driven by a grinding method called “ball milling,” is essential to the successful functioning of the catalyst.

Ball milling is a dry method used to make highly active and selective catalyst powders. It does not require the use of solvents which makes it cheaper and less energy-intensive. This feature has sparked renewed interest in ball milling, with a view to making lots of unique and highly active catalysts for various catalytic processes.

The catalyst was studied using several state-of-the-art experiments, including x-ray studies performed by means of the Quick X-ray Absorption and Scattering (QAS) beamline and Argonne’s Advanced Photon Source. In both facilities beams of highly focused x-rays are used for studying the molecular-level behaviours and structures of a huge variety of materials.

The synchrotron x-ray techniques performed “in situ” enabled studying of the changing atomic structure of the catalyst as it interacted with the reacting gases. To this end, a flow cell was used, incorporating the catalyst sample and the methane/carbon dioxide mixture which was passed over it. Then the cell was heated to temperatures nearing 700 degrees Celsius, which comes close to the experimental limits of the in-situ technique.

The experiments showed that palladium played a major role in the catalytic process and the cerium oxide component had a critical supporting role. The palladium atoms formed into nanoparticles, deposited themselves on the CeO₂ surface and bound to oxygen atoms. This enabled the Pd nanoparticles to be more strongly attached to the CeO₂ surface and disperse more evenly on it. The methane (CH₄) interacting with the nanoparticles dissociated into hydrogen (H₂) and carbon (C). Each carbon atom then picked up an oxygen atom and turned into carbon monoxide (CO). This happed either by first taking oxygen from the nearby CeO₂ or by starting the dry reforming reaction where the carbon is oxidized by way of the carbon dioxide, which dissociates into carbon monoxide and oxygen when passing over the catalyst. These findings were experimentally analysed using in situ infrared spectroscopy.

The reaction also had an unexpected intermediate product, CO bound to Pd atoms, resulting from the direct oxidation of methane. Therefore, the group of scientists also want to explore other catalysts according to this method to see whether unexpected properties could be revealed in them too and their benefits used for other chemical systems.

Image: Synchrotron x-ray techniques—performed “in situ”



Source: Juan D. Jiménez, Luis E. Betancourt, Maila Danielis, Hong Zhang, Feng Zhang, Ivan Orozco, Wenqian Xu, Jordi Llorca, Ping Liu, Alessandro Trovarelli, José A. Rodríguez, Sara Colussi*, and Sanjaya D. Senanayake/ Identification of Highly Selective Surface Pathways for Methane Dry Reforming Using Mechanochemical Synthesis of Pd–CeO₂/ ACS Catal. 2022, 12, 20, 12809–12822, October 7, 2022/ doi.org/10.1021/acscatal.2c01120/ Open Access This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0)

Producing carbon-neutral fuels is one of the chief interests of today’s science. In 2021, scientists found that the functionalised graphene quantum dots were able to control CO₂ to CH₄ conversion with simultaneous high selectivity and production rate. The electron-donating groups yielded CH₄ from CO₂ electro-reduction whereas electron-withdrawing groups reduced CO₂ electro-reduction. The yield of CH₄ on electron-donating group functionalized graphene quantum dots could be linked to the electron-donating ability and content of electron-donating group. The graphene quantum dots could achieve Faradaic efficiency of 70.0% for CH₄ at −200 mA cm⁻² partial current density of CH₄. The scientists found that the superior yield of CH₄ on the electron-donating group- over the electron-withdrawing group-functionalized graphene quantum dots was possibly caused by maintaining higher charge density of potential active sites and the interaction between the electron-donating group and key intermediates. The research gave insights into the design of active carbon catalysts at the molecular scale for the CO₂ electro-reduction.

Image: Characterization results of the two –NH₂ functionalized GQDs



Source: Tianyu Zhang, Weitao Li, Kai Huang, Huazhang Guo, Zhengyuan Li, Yanbo Fang, Ram Manohar Yadav, Vesselin Shanov, Pulickel M. Ajayan, Liang Wang, Cheng Lian, Jingjie Wu/ Regulation of functional groups on graphene quantum dots directs selective CO₂ to CH₄ conversion/ Nature Communications volume 12, Article number: 5265 (2021), 06 September 2021/ doi.org/10.1038/s41467-021-27919-9/ Open Access This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0)

In 2022, scientists developed a catalytic method to directly transfer solid biomass to bio-natural gas suitable for the current energy infrastructure. They designed a catalyst with an Ni₂Al₃ alloy phase which enabled nearly complete conversion of various agricultural and forestry residues. The total carbon yield of gas products reached almost 93% after several hours at relative low-temperature (300 degrees Celsius). The catalyst exhibited good capability for the production of natural gas during thirty cycles. It had a low-carbon footprint. The production process was found to be economically competitive.

Image: Characterization of the nickel-based catalyst with Ni₂Al₃ alloy phase and reaction pathways of ethanol compound



Source: Xiaoqin Si, Rui Lu, Zhitong Zhao, Xiaofeng Yang, Feng Wang, Huifang Jiang, Xiaolin Luo, Aiqin Wang, Zhaochi Feng, Jie Xu & Fang Lu/ Catalytic production of low-carbon footprint sustainable natural gas/ Nature Communications volume 13, Article number: 258 (2022), 11 January 2022/ doi.org/10.1038/s41467-021-27919-9/ Open Access This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0)

The new catalyst has many potential advantages: The mechano-chemical synthesis can be scaled up and easily expanded to the industrial level. This, in turn, will make a valuable contribution to the energy infrastructure worldwide. Also, the conversion of greenhouse gases into useful chemicals and materials to avoid emission into the atmosphere is a main target of carbon-negative strategies.

The current research is part of the “Carbon-Negative Shot”, one of the six thrusts of the DOE Energy Earthshots Initiative, whose aim is to address climate change by advancing sustainable clean-energy solutions.

The ball milling catalyst synthesis approach could be applied much more broadly in industry. It can also change the field of sustainable chemistry, aimed at designing chemicals and processes that reduce the use or generation of hazardous substances. This may initiate a whole new approach to sustainable chemistry.

By the Editorial Board