Tandem electrocatalytic-thermocatalytic conversion could help offset emissions of potent greenhouse gas by locking carbon away in a useful material
串聯之電催化與熱催化的轉化，藉由將碳鎖藏於有用材料中，可能有助於抵消強有力之溫室氣體的排放。
圖1. 科學家們業已想出一種，將來自大氣的二氧化碳(CO2)轉化成，有價值之奈米碳纖維的策略。該過程使用串聯之電催化(藍環)與熱催化(橙環)的反應，來將二氧化碳(鳧藍色與銀分子)加水(紫色與鳧藍色)轉化成"固定的"碳奈米纖維(銀色)。產生氫氣(H2，紫色)作為一種有益的副產品。碳奈米纖維可能被用來，增強諸如水泥等建築材料，及鎖藏碳達數十年。
Scientists have devised a strategy for converting carbon dioxide (CO2) from the atmosphere into valuable carbon nanofibers. The process uses tandem electrocatalytic (blue ring) and thermocatalytic (orange ring) reactions to convert the CO2 (teal and silver molecules) plus water (purple and teal) into "fixed" carbon nanofibers (silver), producing hydrogen gas (H2, purple) as a beneficial byproduct. The carbon nanofibers could be used to strengthen building materials such as cement and lock away carbon for decades.
Scientists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory and Columbia University have developed a way to convert carbon dioxide (CO2), a potent greenhouse gas, into carbon nanofibers, materials with a wide range of unique properties and many potential long-term uses.
於美國能源部(DOE)所屬布魯克海文國家實驗室，及哥倫比亞大學的科學家們，業已開發出一種，將強有力之溫室氣體，二氧化碳(CO2)轉化成奈米碳纖維的方法。這是具有廣泛獨特屬性，及諸多潛在長期用途的材料。
Their strategy uses tandem electrochemical and thermochemical reactions run at relatively low temperatures and ambient pressure. As the scientists describe in the journal Nature Catalysis, this approach could successfully lock carbon away in a useful solid form to offset or even achieve negative carbon emissions.
他們的策略使用了，於相對低的溫度及周遭壓力下，運作之串聯電化學與熱化學的反應。如同此些科學家，於《自然•催化作用》期刊的描述。此方法能成功地，以一種有用的固態形式，將碳鎖藏住，以抵消或甚至達成負的碳排放。
“You can put the carbon nanofibers into cement to strengthen the cement,” said Jingguang Chen, a professor of chemical engineering at Columbia with a joint appointment at Brookhaven Lab who led the research. “That would lock the carbon away in concrete for at least 50 years, potentially longer. By then, the world should be shifted to primarily renewable energy sources that don’t emit carbon.”
具有哥倫比亞大學與布魯克海文國家實驗室聯合任命的化學工程學教授，Jingguang Chen宣稱：「能將奈米碳纖維置入水泥中，以增強水泥。那能將碳鎖藏於混凝土中，達至少50年，可能更長。到那時候，世界應該轉向，本質上不排放碳的可再生能源。」
As a bonus, the process also produces hydrogen gas (H2), a promising alternative fuel that, when used, creates zero emissions.
作為一種附贈品，該過程也產生氫氣(H2)。當被使用時，這是一種創造零排放之有指望的替代燃料。
The idea of capturing CO2 or converting it to other materials to combat climate change is not new. But simply storing CO2 gas can lead to leaks. And many CO2 conversions produce carbon-based chemicals or fuels that are used right away, which releases CO2 right back into the atmosphere.
捕獲二氧化碳或將其轉化成其他材料，來對抗氣候變遷，並非新構想。不過，僅儲存二氧化碳氣體，會導致洩漏。因此，許多二氧化碳轉換，產生立即被使用之以碳為基礎，將二氧化碳釋放回大氣中的化學物質或燃料。
“The novelty of this work is that we are trying to convert CO2 into something that is value-added but in a solid, useful form,” Chen said.
Chen宣稱：「該項研究的新穎處是，我們正嘗試將二氧化碳轉化成，具有附加價值的物質，不過以一種固態、有用的形式。」
Such solid carbon materials—including carbon nanotubes and nanofibers with dimensions measuring billionths of a meter—have many appealing properties, including strength and thermal and electrical conductivity. But it’s no simple matter to extract carbon from carbon dioxide and get it to assemble into these fine-scale structures. One direct, heat-driven process requires temperatures in excess of 1,000 degrees Celsius.
這種固態碳材料(包括具有尺寸度量為十億分之一公尺的碳奈米管及奈米纖維)具有諸多吸引人的屬性，包括強度、導熱性及導電性。不過，從二氧化碳提取碳，並將其組合成此些精細結構物，並非一件簡單的事。一種直接、熱驅動的過程，需要超過攝氏1千度的溫度。
“It’s very unrealistic for large-scale CO2 mitigation,” Chen said. “In contrast, we found a process that can occur at about 400 degrees Celsius, which is a much more practical, industrially achievable temperature.”
Chen宣稱：：「就大規模模二氧化碳減量而言，這是極不切實際的。相較之下，我們發現一種，能在攝氏400度左右發生的過程。這是一種更實用、工業上可達成的溫度。」
圖2. 用於生產奈米碳纖維(CNF：Carbon Nanofiber)之電催化與熱催化的串聯策略，藉由將CO2與水共同電解成合成氣(CO及H2)，與一種隨後在溫和條件(370-450°C，周遭壓力)下之熱化學過程相結合，規避了熱力學的限制。這產生了高的CNF生產率。這種鐵鈷(FeCo)合金及額外金屬Co的最佳協同作用，增強了合成氣的解離活化，而促進生產CNF的碳-碳鍵形成。
The electrocatalytic-thermocatalytic tandem strategy for CNF production circumvents thermodynamic constraints by combining the co-electrolysis of CO2 and water into syngas (CO and H2) with a subsequent thermochemical process under mild conditions (370-450 °C, ambient pressure). This yields a high CNF production rate. The optimal synergy of iron-cobalt (FeCo) alloy and extra metallic Co enhanced the dissociative activation of syngas, promoting carbon-carbon bond formation for CNF production.
The trick was to break the reaction into stages and to use two different types of catalysts—materials that make it easier for molecules to come together and react.
此訣竅是將反應分解成幾個階段，並使用兩種不同類型的觸媒。也就是，使分子更容易聚集在一起，並發生反應的材料。
“If you decouple the reaction into several sub-reaction steps you can consider using different kinds of energy input and catalysts to make each part of the reaction work,” said Brookhaven Lab and Columbia research scientist Zhenhua Xie, lead author on the paper.
該項論文首要撰文人，哥倫比亞大學兼布魯克海文國家實驗室的研究科學家，Zhenhua Xie宣稱：「倘若將反應拆解成幾個子反應步驟，則能考慮使用不同類型的能量輸入及觸媒，來使反應的每一部分起作用。」
The scientists started by realizing that carbon monoxide (CO) is a much better starting material than CO2 for making carbon nanofibers (CNF). Then they backtracked to find the most efficient way to generate CO from CO2.
此些科學家因瞭解到，就製造奈米碳纖維(CNF)，一氧化碳(CO)是比二氧化碳更佳的起始材料，而感到驚訝。因此，他們回溯去尋找，從二氧化碳產生一氧化碳的最有效方法。
Earlier work from their group steered them to use a commercially available electrocatalyst made of palladium supported on carbon. Electrocatalysts drive chemical reactions using an electric current. In the presence of flowing electrons and protons, the catalyst splits both CO2 and water (H2O) into CO and H2.
來自其團隊的早期研究引導了他們使用，由碳承載之鈀所製成，商業上可資使用的電觸媒。電觸媒利用電流驅動化學反應。於存在流動的電子及質子下，該種觸媒將CO2與水(H2O)分解成CO及H2。
For the second step, the scientists turned to a heat-activated thermocatalyst made of an iron-cobalt alloy. It operates at temperatures around 400 degrees Celsius, significantly milder than a direct CO2-to-CNF conversion would require. They also discovered that adding a bit of extra metallic cobalt greatly enhances the formation of the carbon nanofibers.
為了第二個步驟，此些科學家轉向一種，由鐵-鈷合金所製成，經熱激活的熱觸媒。這在大約攝氏400度的溫度下運作，顯著比直接將CO2轉化成CNF，會需要的溫度較溫和。他們也發現，添加一點額外的金屬鈷，大大增強了奈米碳纖維的形成。
“By coupling electrocatalysis and thermocatalysis, we are using this tandem process to achieve things that cannot be achieved by either process alone,” Chen said.
Chen宣稱：「藉由結合電催化及熱催化，我們正在使用這種串聯的過程，來獲得單獨任一過程，無法獲得的東西。」
圖3. 高解析度的透射式電子顯微鏡技術(TEM)顯示了，在鐵鈷/氧化鈰(FeCo/CeO2)熱觸媒上，從而產生之奈米碳纖維的頂端(左)。使用掃描透射式電子顯微鏡技術(STEM)，科學家們繪製了，新形成之奈米碳纖維(右)的結構及化學成分、高角度的環形暗場(HAADF)造影及能量分散的X-射線光譜(EDS)圖。(比例尺代表8奈米)。此些圖像顯示，奈米纖維由碳(C)製成，並揭露催化的金屬鐵(Fe)及鈷(Co)，被推離催化的表面並積聚於奈米纖維的頂端。
High-resolution transmission electron microscopy (TEM) shows the tip of the resulting carbon nanofiber (left) on the iron-cobalt/cerium oxide (FeCo/CeO2) thermocatalyst. Scientists mapped the structure and chemical composition of newly formed carbon nanofibers (right) using scanning transmission electron microscopy (STEM), high-angle annular dark field (HAADF) imaging, and energy-dispersive x-ray spectroscopy (EDS) (scale bar represents 8 nanometers). The images show that the nanofibers are made of carbon (C), and reveal that the catalytic metals, iron (Fe) and cobalt (Co), are pushed away from the catalytic surface and accumulate at the tip of the nanofiber.
To discover the details of how these catalysts operate, the scientists conducted a wide range of experiments.
為了知曉這些觸媒如何運作的細節，此些科學家進行了廣泛的實驗。
These included computational modeling studies, physical and chemical characterization studies at Brookhaven Lab’s National Synchrotron Light Source II (NSLS-II)—using the Quick X-ray Absorption and Scattering (QAS) and Inner-Shell Spectroscopy (ISS) beamlines—and microscopic imaging at the Electron Microscopy facility at the Lab’s Center for Functional Nanomaterials (CFN).
此些包括了，於布魯克海文實驗室，使用快速X-射線吸收暨散射(QAS)與內殼光譜學(ISS)光束線之國家同步加速器光源II (NSLS-II)的計算模型製作研究、物理與化學特性描述的研究，及於該實驗室功能性奈米材料中心 (CFN)之電子顯微鏡設施的顯微鏡造影。
On the modeling front, the scientists used “density functional theory” (DFT) calculations to analyze the atomic arrangements and other characteristics of the catalysts when interacting with the active chemical environment.
在模型製作前面，此些科學家使用了“密度泛函理論”(DFT)計算，來分析當與活性化學環境交互作用時，觸媒的原子排列及其他特性。
“We are looking at the structures to determine what are the stable phases of the catalyst under reaction conditions,” explained study co-author Ping Liu of Brookhaven’s Chemistry Division who led these calculations. “We are looking at active sites and how these sites are bonding with the reaction intermediates. By determining the barriers, or transition states, from one step to another, we learn exactly how the catalyst is functioning during the reaction.”
布魯克海文國家實驗室化學部，領導此些計算的該項研究合撰人，Ping Liu解釋：「我們正在探究此些結構，以確定在反應條件下，該種觸媒的穩定相，是怎麼樣。我們正在探究活性位點，及這些位點如何與反應居間物結合。藉由確定從一個步驟到另一個步驟的壁壘(也就是過渡態)，我們切確地瞭解了，於該反應過程期間，這種觸媒如何發揮作用。」
X-ray diffraction and x-ray absorption experiments at NSLS-II tracked how the catalysts change physically and chemically during the reactions. For example, synchrotron x-rays revealed how the presence of electric current transforms metallic palladium in the catalyst into palladium hydride, a metal that is key to producing both H2 and CO in the first reaction stage.
以NSLS-II進行之X-射線衍射及吸收的實驗追蹤了，於反應過程期間，觸媒物理及化學上如何變化。譬如，同步加速器X-射線揭露了，存在的電流如何將觸媒中之金屬鈀轉化成，於第一反應階段中，產生H2及CO的關鍵金屬─氫化鈀。
For the second stage, “We wanted to know what’s the structure of the iron-cobalt system under reaction conditions and how to optimize the iron-cobalt catalyst,” Xie said. The x-ray experiments confirmed that both an alloy of iron and cobalt plus some extra metallic cobalt are present and needed to convert CO to carbon nanofibers.
為了第二階段，Xie宣稱：「我們想知曉，在反應條件下，什麼是鐵-鈷體系的結構及如何最佳化該種鐵-鈷觸媒。此些X-射線實驗證實了，將二氧化碳轉化成奈米碳纖維，鐵與鈷之合金加上一些額外的金屬鈷，兩者是即刻可用且必需的。」
“The two work together sequentially,” said Liu, whose DFT calculations helped explain the process.
Liu宣稱：「這兩種金屬依序一齊起作用。」他的密度泛函理論計算，協助解釋了此過程。
“According to our study, the cobalt-iron sites in the alloy help to break the C-O bonds of carbon monoxide. That makes atomic carbon available to serve as the source for building carbon nanofibers. Then the extra cobalt is there to facilitate the formation of the C-C bonds that link up the carbon atoms,” she explained.
她解釋：「根據我們的研究，於該種合金中的鈷-鐵位點，有助於斷開一氧化碳的C-O鍵。那使得原子碳可資充當，形成奈米碳纖維的來源。然後在那裡，額外的鈷是促進連結碳原子之C-C鍵的形成。」
“Transmission electron microscopy (TEM) analysis conducted at CFN revealed the morphologies, crystal structures, and elemental distributions within the carbon nanofibers both with and without catalysts,” said CFN scientist and study co-author Sooyeon Hwang.
奈米碳纖維(CFN)科學家兼該項研究合撰人，Sooyeon Hwang宣稱：「以CFN 進行的透射式電子顯微鏡技術(TEM)分析揭露了，於奈米碳纖維中，具有及沒有觸媒時的形態、晶體結構及元素分佈。」
The images show that, as the carbon nanofibers grow, the catalyst gets pushed up and away from the surface. That makes it easy to recycle the catalytic metal, Chen said.
Chen表示，此些影像顯示，當奈米碳纖維生長時，這種觸媒被向上且推離表面。 那使得容易回收該種催化的金屬。
“We use acid to leach the metal out without destroying the carbon nanofiber so we can concentrate the metals and recycle them to be used as a catalyst again,” he said.
他宣稱：「我們使用酸來溶濾出該種金屬，而沒有破壞奈米碳纖維。這樣我們能濃縮此些金屬並回收它們，再度被作為一種觸媒使用。」
This ease of catalyst recycling, commercial availability of the catalysts, and relatively mild reaction conditions for the second reaction all contribute to a favorable assessment of the energy and other costs associated with the process, the researchers said.
此些研究人員表示，該種觸媒回收的容易性、此些觸媒商業上的可用性及對第二個反應，相對溫和的反應條件，皆有助於與該過程之能源及其他成本相關的有利評估。
“both are really important—the CO2 footprint analysis and the recyclability of the catalyst” said Chen. “Our technical results and these other analyses show that this tandem strategy opens a door for decarbonizing CO2 into valuable solid carbon products while producing renewable H2.”
Chen宣稱：「對實際的諸多應用而言，二氧化碳足跡分析及該種觸媒的可回收性，兩者皆非常重要。我們的技術成果及其他分析顯示，這種串聯的策略，為從二氧化碳脫碳成為，有價值的固態碳產品，同時生產可再生的氫氣，打開了一種途徑。」
If these processes are driven by renewable energy, the results would be truly carbon-negative, opening new opportunities for CO2 mitigation.
倘若此些過程是由可再生能源所驅動，此些結果會是真正的負碳。這為減少二氧化碳，開闢了諸多新良機。
網址：https://www.bnl.gov/newsroom/news.php?a=121635
翻譯：許東榮
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