Materials Science

[Recusant]

While the previous item shows an intriguing possibility for materials science, this one is just cool, in my opinion.

"Radical Theory Suggests Earthquakes Spark Gold Nuggets Into Existence" | Science Alert

New findings by scientists in Australia could challenge what we thought we knew about the way gold nuggets bloom in vast reefs beneath our feet.

Under pressures of hundreds of megapascals (tens of thousands of pounds per square inch) and boiling hot temperatures, water squeezed up from the depths of Earth's crust carries dissolved gases, metals, and minerals to the surface with every quake and shudder of a seismic event.

As any good prospector knows, buried seams of crystalized silicon dioxide – better known as quartz – are fertile ground for gold mining, with both materials precipitating out of solution under strikingly similar conditions.

Though the basic mechanisms behind the precious ore's formation have been understood for some time, certain details have never quite added up, and new research from scientists at Australia's Monash University, the CSIRO, and the Australian Nuclear Science and Technology Organisation challenge the conventional views on how gold forms.

[. . .]

Silicon dioxide is an incredibly unique material. Where other crystals are relatively symmetrical, quartz forms with a bias that produces a voltage when stressed – a phenomenon known as the piezoelectric effect.

With every tremor of Earth's crust, seams of quartz will crackle with static currents as voltages emerge and electrons rebalance.

This charge jump is unlikely to move very far given quartz is an insulating material. Gold, on the other hand, is a great conductor of electricity, raising the possibility that electrochemical reactions within quartz seams might serve as a catalyst, drawing sufficient gold from solution in concentrated spots through repeated cycles of tiny shakes.

[. . .]

What was simulated in the lab using concentrated solutions and extensive periods of shaking would of course take far longer in the real world with dilute solutions and occasional tremors.

On geological timescales, however, the process could be relatively rapid. Without the added zap of stressed quartz, it's difficult to even explain how gold might accumulate in such rich deposits in the first place.

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The paper is behind a paywall.

Abstract:

Gold nuggets occur predominantly in quartz veins, and the current paradigm posits that gold precipitates from dilute (<1 mg kg⁻¹ gold), hot, water ± carbon dioxide-rich fluids owing to changes in temperature, pressure and/or fluid chemistry.

However, the widespread occurrence of large gold nuggets is at odds with the dilute nature of these fluids and the chemical inertness of quartz. Quartz is the only abundant piezoelectric mineral on Earth, and the cyclical nature of earthquake activity that drives orogenic gold deposit formation means that quartz crystals in veins will experience thousands of episodes of deviatoric stress.

Here we use quartz deformation experiments and piezoelectric modelling to investigate whether piezoelectric discharge from quartz can explain the ubiquitous gold–quartz association and the formation of gold nuggets. We find that stress on quartz crystals can generate enough voltage to electrochemically deposit aqueous gold from solution as well as accumulate gold nanoparticles.

Nucleation of gold via piezo-driven reactions is rate-limiting because quartz is an insulator; however, since gold is a conductor, our results show that existing gold grains are the focus of ongoing growth. We suggest this mechanism can help explain the creation of large nuggets and the commonly observed highly interconnected gold networks within quartz vein fractures.

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Direct observation of palladium acting as a catalyst to synthesize water.

"Watch The Smallest Water Droplet Ever Seen Grow, Molecule by Molecule" | Science Alert

We all know the ingredients for water. You take two atoms of hydrogen and one of oxygen, moosh them together just so, and voila – you have a molecule of the most important compound to life on Earth.

Now, for the first time, scientists have observed this process in action up close. In real-time, and on a molecular scale, materials scientist Vinayak Dravid of Northwestern University and his colleagues watched as the tiniest bubble of water ever seen bloomed seemingly out of thin air.

How did they do it? By harnessing the strange powers of palladium, a metal that is known to catalyze the two elements to synthesize water.

"Think of Matt Damon's character, Mark Watney, in the movie The Martian," Dravid says. "He burned rocket fuel to extract hydrogen and then added oxygen from his oxygenator. Our process is analogous, except we bypass the need for fire and other extreme conditions. We simply mixed palladium and gasses together."

We've known about palladium's strange ability to synthesize significant amounts of water in a relatively short amount of time. But how exactly it works has been tricky to pin down. This is because it's hard to observe the process on the scales at which it occurs.

"You really need to be able to combine the direct visualization of water generation and the structure analysis at the atomic scale in order to figure out what's happening," explains materials scientist Yukun Liu of Northwestern University.

To overcome this significant obstacle, the team developed a membrane that traps molecules inside tiny, hexagonal nanoreactor cells. This makes it easier to image molecular processes using transmission electron microscopy, down to the nanometer scale.

Even so, the researchers were not sure that their attempt to directly observe the palladium reaction would be successful. They added hydrogen and oxygen atoms to the surface of a 20 nanometer-wide piece of palladium, and used their membrane to capture the ensuing interaction.

A single water molecule is less than a third of a nanometer across, while the atoms they consist of are far, far smaller. To 'see' the bonding of these tiny compounds, the researchers used a form of electron microscopy that recorded energy lost by scattering electrons.

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The paper is behind a paywall.

Abstract:

Palladium (Pd) catalysts have been extensively studied for the direct synthesis of H₂O through the hydrogen oxidation reaction at ambient conditions. This heterogeneous catalytic reaction not only holds considerable practical significance but also serves as a classical model for investigating fundamental mechanisms, including adsorption and reactions between adsorbates.

Nonetheless, the governing mechanisms and kinetics of its intermediate reaction stages under varying gas conditions remain elusive. This is attributed to the intricate interplay between adsorption, atomic diffusion, and concurrent phase transformation of catalyst. Herein, the Pd-catalyzed, water-forming hydrogen oxidation is studied in situ, to investigate intermediate reaction stages via gas cell transmission electron microscopy.

The dynamic behaviors of water generation, associated with reversible palladium hydride formation, are captured in real time with a nanoscale spatial resolution. Our findings suggest that the hydrogen oxidation rate catalyzed by Pd is significantly affected by the sequence in which gases are introduced. Through direct evidence of electron diffraction and density functional theory calculation, we demonstrate that the hydrogen oxidation rate is limited by precursors' adsorption. These nanoscale insights help identify the optimal reaction conditions for Pd-catalyzed hydrogen oxidation, which has substantial implications for water production technologies. The developed understanding also advocates a broader exploration of analogous mechanisms in other metal-catalyzed reactions.