An X-ray flash illuminates a molecule.
Newswise — The utilization of brief bursts of X-ray illumination moves scientists significantly nearer to enhancing catalysts for converting the methane greenhouse gas into a less detrimental substance. The findings, documented in the journal Science, unveil the mechanism of carbon-hydrogen bond rupture and catalyst functionality in this reaction, marking a groundbreaking achievement.
The emission of methane, a highly potent greenhouse gas, is escalating due to livestock farming and the ongoing thawing of permafrost. Converting methane and longer alkanes into more benign and valuable chemicals presents an opportunity to mitigate these hazards while providing a vast resource for the chemical industry. Nevertheless, the conversion of methane requires the initial disruption of a C-H bond, which is among the most robust chemical connections found in nature.
Four decades ago, the scientific community unearthed molecular metal catalysts capable of effortlessly cleaving C-H bonds. Surprisingly, a simple burst of visible light was found to activate the catalyst, resulting in the seemingly magical breaking of sturdy C-H bonds in proximity to it, with minimal energy consumption. Despite the significance of this C-H activation reaction, the precise mechanism by which the catalyst accomplishes this function remained a mystery throughout the years.
Researchers from Uppsala University, in partnership with the Paul Scherrer Institute in Switzerland, Stockholm University, Hamburg University, and the European XFEL in Germany, spearheaded the investigation. In a groundbreaking achievement, the team successfully observed the catalyst in action for the first time, uncovering the intricate process by which it cleaves the C-H bonds.
In a pair of experiments conducted at the Paul Scherrer Institute in Switzerland, the researchers accomplished the meticulous observation of electron exchanges between a rhodium catalyst and an octane C-H group during the bond-breaking process. Leveraging the extraordinary capabilities of two of the world's most powerful sources of X-ray flashes, the SwissFEL X-ray laser and the Swiss Light Source X-ray synchrotron, the entire reaction was traced from its inception to its culmination. The measurements unveiled the catalyst's prompt activation triggered by light within a remarkable timespan of 400 femtoseconds (0.0000000000004 seconds), leading to the ultimate breaking of the C-H bond after 14 nanoseconds (0.000000014 seconds).
"The time-resolved X-ray absorption experiments we conducted required the use of advanced facilities such as SwissFEL and the Swiss Light Source, equipped with exceptionally intense and brief X-ray pulses. Immersed in a concentrated octane solution, the catalyst was scrutinized from the metal's viewpoint, enabling us to selectively target the precise C-H bond, among hundreds of thousands, destined to undergo rupture," elucidates Raphael Jay, a researcher at Uppsala University and the study's principal experimentalist.
In order to decipher the intricate experimental data, theoretical experts from Uppsala University and Stockholm University collaborated and conducted sophisticated quantum-chemical calculations.
"Through our calculations, we gain a precise understanding of the precise distribution of electronic charge between the metal catalyst and the C-H group, ensuring an optimal balance. We observe that charge transfer from the metal to the C-H bond facilitates the cohesion of the two chemical groups. Conversely, charge transfer in the opposite direction acts as a metaphorical scissor, ultimately causing the separation of the C and H atoms," elucidates Ambar Banerjee, a postdoctoral researcher at Uppsala University and the study's principal theoretician.
This study successfully unravels a perplexing enigma that persisted for four decades: how an activated catalyst can effectively rupture robust C-H bonds through precise fractional electron exchanges, without necessitating extreme temperatures or pressures. Equipped with this newfound knowledge, the researchers now aim to further their understanding of directing electron flow. Their goal is to develop enhanced catalysts for the chemical industry, enabling the transformation of methane and other alkanes into valuable substances, thereby harnessing their potential utility.
Facts
The study builds on the pioneering work of grandfather, father and son Manne, Kai, and Per Siegbahn.
Manne Siegbahn (Uppsala University), who received the Nobel Prize in Physics in 1924, pioneered how different elements can be distinguished by X-rays.
Kai Siegbahn (Uppsala University), who received the Nobel Prize in Physics in 1981, pioneered how different chemical environments of the same element can be distinguished by X-rays.
Per Siegbahn (Stockholm University) theoretically predicted the concerted exchange of electronic charge required for breaking a C-H bond.
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