Unlocking Catalyst Secrets: Single Copper Atoms on Ceria Tackle CO e H2

Hey there! Let me tell you about something pretty neat happening in the world of tiny, powerful helpers called catalysts. We’re talking about those clever materials that speed up chemical reactions without getting used up themselves. Specifically, we dove into the fascinating realm of *single atom catalysts*, or SACs. Imagine taking a metal, like copper, and spreading its atoms out so thin that they sit one by one on a support material. In this case, the support is cerium oxide, or CeO₂ – a material famous for being able to store and release oxygen, kinda like a tiny oxygen battery.

Why do we care about single copper atoms on CeO₂? Well, these little guys have unique properties compared to bigger clumps of metal. They can be incredibly efficient and sometimes even selective about which reactions they help along. Our focus here was on two important reactions: cleaning up carbon monoxide (CO) and dealing with hydrogen (H₂), particularly how they get oxidized (basically, react with oxygen). This is super relevant for things like cleaning exhaust fumes or processes where you need to purify hydrogen streams.

To figure out exactly *how* these single copper atoms work their magic, we didn’t head into a lab with beakers and tubes just yet. Instead, we used a powerful computational tool called Density Functional Theory (DFT). Think of it as a super-sophisticated microscope that lets us see and predict how atoms and molecules interact at the quantum level. We wanted to understand where the copper atom likes to sit on the CeO₂ surface, how it changes the surface, and how CO and H₂ behave when they come near it.

What’s the Big Deal with Cu/CeO₂ SACs?

Transition metal oxides, like our friend CeO₂, are workhorses in many applications, from energy storage to sensors and, you guessed it, catalysis. CeO₂ is particularly special because of its ability to easily switch between different oxidation states (Ce⁴⁺ and Ce³⁺), which is key for handling oxygen. People have studied CeO₂ with metal nanoparticles (like gold and copper) for ages, especially for reactions like CO oxidation and the water-gas shift reaction. The secret sauce often involves electron transfer between the metal and the ceria support, creating oxidized metal centers (like Cuδ⁺) and reduced ceria (Ce³⁺). This interaction boosts how molecules stick to the surface and overall catalytic performance.

But putting metals down as single atoms? That’s the newer, exciting frontier! It’s like getting the most bang for your buck from precious or semi-precious metals. SACs have shown some truly extraordinary activity. Various oxides have been used as supports, and CeO₂ is a popular choice. Copper on CeO₂ has already shown promise, especially for oxidation reactions. Researchers have even made single Cu atoms on tiny CeO₂ clusters and tested them for things like converting CO₂ into other chemicals. Our work builds on this, specifically looking at how single Cu atoms on flat CeO₂ surfaces handle CO and H₂ oxidation using our computational magnifying glass.

Getting Down to Basics: How Cu Sits on Ceria

First off, we needed to know where a single copper atom would feel most at home on the CeO₂ surface. CeO₂ has different faces, or “surfaces,” like (111), (110), and (100), each with a slightly different atomic arrangement. Think of them like different sides of a crystal. Previous studies told us the (111) surface is generally the most stable for bare CeO₂, but when you add a single Cu atom, things change a bit.

What we found using our DFT calculations is that a single Cu atom binds favorably on all these surfaces, but it prefers the (100) face the most, followed by (110), and then (111). The binding energy is strongest on (100) at a specific spot called a “bridging oxygen site.”

Here’s where the real magic happens: when a single Cu atom lands on the CeO₂ surface, it doesn’t just sit there idly. It gets oxidized! A neutral copper atom (Cu⁰) gives an electron to a neighboring cerium atom (Ce⁴⁺), turning the copper into a positively charged ion (Cu⁺) and reducing the cerium to Ce³⁺. This charge transfer is super important because it changes the electronic landscape around the copper atom, making it more attractive to certain molecules. We saw this charge transfer clearly in our calculations – the copper loses about half an electron, and the cerium gains it, becoming magnetic (which is a tell-tale sign of Ce³⁺). This matches what others have seen, even with copper on different supports like MXenes!

The Molecules Arrive: CO and H₂ Adsorption

Okay, so we know where the copper likes to sit and that it becomes Cu⁺. Now, what happens when CO and H₂ show up? This is crucial for understanding the catalytic reactions.

We found that CO absolutely *loves* to stick to the single Cu atom on the CeO₂(111) surface. It chemisorbs strongly, meaning it forms a chemical bond. The adsorption energy is quite favorable. The CO molecule sits right on top of the copper, and its own bond length (the distance between the carbon and oxygen atoms) stretches just a tiny bit compared to free CO gas. This strong binding is much more significant than what happens on a bare CeO₂ surface, where CO barely sticks. It seems the positive charge on the Cu⁺ and the electronic structure (specifically, the copper’s *d* orbitals being available near the Fermi level) really help grab onto the CO molecule. Interestingly, the copper atom stays put when CO adsorbs, unlike what’s sometimes seen with other metals like gold on ceria.

Now, let’s look at H₂. Hydrogen behaves quite differently. On most surfaces we looked at (like (110) and (100)), H₂ barely interacts with the copper site. It’s mostly just weak physical adsorption, like a molecule briefly resting on the surface. However, on the Cu/CeO₂(111) surface, something more interesting happens. H₂ still adsorbs weakly compared to CO, but its H–H bond stretches noticeably. This is a sign of *activation*! This type of interaction, where a molecule like H₂ interacts with a metal center without breaking apart initially, is known as *Kubas interaction*. It involves electron donation from H₂ to the metal and back-donation from the metal to H₂, weakening the H–H bond. So, while H₂ doesn’t stick as strongly as CO, the Cu/CeO₂(111) surface does get it ready for action.

In short, the single Cu atom on CeO₂, especially on the (111) surface, is like a super-sticky trap for CO, while H₂ gets a gentler, activating handshake. This difference in how strongly CO and H₂ bind is key to understanding why this catalyst might be good at selectively oxidizing CO even when H₂ is around (a process called CO preferential oxidation, or PROX).

Oxygen Vacancies: The Unsung Heroes

Catalysis on metal oxides often involves defects in the crystal structure, particularly missing oxygen atoms, which we call *oxygen vacancies* (VO). These vacancies aren’t just holes; they create active sites and influence the material’s redox properties. We looked at forming these vacancies near the single copper atom on the CeO₂(111) surface.

Turns out, the single Cu atom makes it a bit harder to form an oxygen vacancy compared to a pure CeO₂ surface. Why? Because the positively charged Cu⁺ atom electrostatically attracts the surrounding lattice oxygen atoms, holding them more tightly. However, when a vacancy *does* form, it’s a big deal. It leaves behind two extra electrons, which get picked up by nearby cerium atoms, reducing them to Ce³⁺. So, in addition to the Ce³⁺ created by the initial copper adsorption, you get two *more* Ce³⁺ sites near the vacancy! These Ce³⁺ sites and the vacancy itself are believed to be crucial for catalytic activity, especially for reactions involving oxygen. While the Cu atom increases the energy needed to *form* the vacancy, the resulting vacancy is thought to be more active for grabbing onto oxygen molecules from the gas phase, which is needed to replenish the surface and keep the catalytic cycle going.

The Main Event: Oxidizing CO and H₂

Now for the exciting part: how do CO and H₂ actually get oxidized on this surface? We explored a common pathway for oxide catalysts called the *Mars-van Krevelen mechanism*. This mechanism involves lattice oxygen atoms (the ones already in the CeO₂ structure) reacting with the adsorbed molecule, creating a vacancy, which is then refilled by oxygen from the gas phase.

For CO oxidation on Cu/CeO₂(111), it starts with CO adsorbing strongly onto the Cu site (we already covered that!). Then, the adsorbed CO molecule reaches out and grabs a lattice oxygen atom from the CeO₂ support right next to the copper. This forms a temporary, bent CO₂-like intermediate. This step requires a certain amount of energy to get over a barrier – what we call the *activation energy*. We calculated this barrier, and it’s a key number because it often determines how fast the reaction goes. After this bent CO₂ forms, it pops off the surface as a CO₂ gas molecule, leaving behind an oxygen vacancy. This vacancy is then quickly filled by an oxygen molecule (O₂) from the gas phase, which gets activated in the process. A second CO molecule can then react with this newly arrived oxygen (or another lattice oxygen), and the cycle continues, producing more CO₂ and regenerating the active site.

For H₂ oxidation, it also starts with H₂ adsorbing onto the Cu site (remember, it’s weaker than CO adsorption, but the H–H bond gets activated). One of the hydrogen atoms then reacts with a lattice oxygen to form a hydroxyl group (OH) on the surface, leaving the other hydrogen atom attached to the copper. This step has a moderate energy barrier. To make water (H₂O), the hydrogen atom on the copper then has to react with the OH group. This second step, turning H and OH into H₂O, is the one that requires the *most* energy to get over its barrier – it’s the *rate-determining step* for H₂ oxidation on this surface. Once H₂O is formed, it desorbs, again creating an oxygen vacancy, which gets refilled by O₂ from the gas, just like in the CO pathway.

Comparing the two pathways, the energy barrier for the rate-determining step in CO oxidation (forming the bent CO₂ intermediate) is significantly lower than the barrier for the rate-determining step in H₂ oxidation (forming H₂O from H and OH). This is a big deal! It means that CO oxidation is energetically much easier and likely faster than H₂ oxidation on this specific catalyst.

Why This Matters

So, what’s the takeaway from all these computational adventures? We’ve seen how anchoring single copper atoms onto cerium oxide surfaces creates really interesting and active sites. The copper atom becomes positively charged (Cu⁺) and causes nearby cerium atoms to be reduced (Ce³⁺). This electronic dance makes the surface much better at grabbing onto molecules like CO compared to plain CeO₂.

We found that CO sticks strongly to the Cu site, while H₂ interacts more weakly, although it does get activated on the (111) surface. This difference in adsorption strength is the first hint that this catalyst might be selective. Then, looking at the oxidation mechanisms, we saw that the steps needed to turn CO into CO₂ require less energy to overcome than the steps to turn H₂ into H₂O. The formation and refilling of oxygen vacancies are key parts of both processes, helping to keep the cycle going.

Ultimately, our computational investigation supports the idea that single copper atoms on CeO₂, particularly on the (111) surface, are promising candidates for catalysis, especially for reactions where you want to oxidize CO efficiently, perhaps even when H₂ is present. It highlights the potential of designing catalysts at the single-atom level to tune their properties for specific tasks. It’s like having a tiny, specialized tool for a particular job!

This kind of detailed, atomistic understanding from DFT is invaluable. It helps us see the reaction pathways, identify the bottlenecks (the rate-determining steps), and understand the role of different parts of the catalyst (the single atom, the support, the vacancies). It’s a powerful way to guide the design of even better catalysts in the future.

Source: Springer