How Gold Turns Deadly Carbon Monoxide into Harmless Carbon Dioxide
From inert metal to chemical activator
Imagine a metal so inert that for centuries it was considered catalytically useless, relegated to jewelry and currency. Now imagine this same metal becoming one of the most remarkable catalysts of our time, capable of purifying air and saving lives. This is the story of gold—a noble metal that revealed its hidden talents when scientists looked at it through a different lens: the nanoscale world.
Carbon monoxide binds to hemoglobin in our blood more than 200 times more effectively than oxygen, causing poisoning that leads to thousands of deaths annually.
The combustion of carbon monoxide (CO) represents more than just a chemical reaction—it's a critical process for human health and environmental protection. Catalytic combustion—the process of converting CO to CO₂ using a catalyst—provides an elegant solution to this deadly problem. For decades, scientists have searched for efficient catalysts to drive this reaction at lower temperatures with greater efficiency. The most surprising discovery emerged in the late 20th century: gold, traditionally considered catalytically inactive, outperformed all other metals when prepared in nanoparticle form 5 9 .
How nanotechnology transformed a noble metal into a catalytic powerhouse
For centuries, gold's chemical inertness made it the ideal material for jewelry and currency. Bulk gold doesn't tarnish, resist oxidation, and remains unchanged even when exposed to various corrosive environments. This very stability led chemists to dismiss gold as a potential catalyst for most of the 20th century—while platinum and palladium stole the spotlight in catalytic applications.
Catalytically inert, used primarily for jewelry and currency
Highly catalytically active, enables CO oxidation at sub-zero temperatures
The revolution began in the late 1980s when Japanese scientist Masatake Haruta made a startling discovery. By depositing gold as nanoparticles on metal oxide supports, he created a material that could catalyze the oxidation of carbon monoxide at temperatures as low as -73°C (-99°F) 5 . This finding was nothing short of revolutionary—it contradicted everything the scientific community believed about gold's catalytic properties.
"The work on gold catalysts illustrates how exploring materials at the nanoscale can reveal properties that transform our technological capabilities."
The key to gold's dramatic transformation lay in two factors: particle size and support interaction. When divided into nanoparticles smaller than 5 nanometers (approximately 1/10,000 the width of a human hair), gold exhibits fundamentally different properties than bulk gold. At this scale, quantum effects become significant, and a much larger proportion of gold atoms are exposed on the surface, available to participate in chemical reactions 8 9 .
Unveiling how gold catalyzes CO combustion
The astonishing catalytic performance of gold nanoparticles left scientists with a pressing question: how does it work? unraveling the reaction mechanism became crucial for optimizing catalyst design and performance.
Both CO and O₂ adsorb onto the catalyst surface before reacting—this is the most widely accepted pathway for gold catalysts 2 .
Oxygen from the support material participates in the reaction, creating oxygen vacancies that are subsequently replenished by gas-phase oxygen 6 .
One reactant adsorbs to the surface while the other reacts directly from the gas phase.
For gold-based catalysts, the Langmuir-Hinshelwood mechanism appears dominant under most conditions. In this process, CO molecules adsorb to the gold nanoparticles, while oxygen molecules adsorb either to the gold or at the gold-support interface. The proximity of these adsorbed species allows them to react and form CO₂, which then desorbs from the surface, making room for more reactants 3 8 .
| Support Material | Role in Catalysis | Advantages | Limitations |
|---|---|---|---|
| TiO₂ (Titania) | Provides adsorption sites for oxygen; creates active interfaces with gold | High activity at low temperatures; relatively low cost | Can suffer from deactivation in moist environments |
| Fe₂O₃ (Iron Oxide) | Acts as oxygen reservoir; stabilizes gold nanoparticles | Good stability; resistant to sintering | Less active than other supports at very low temperatures |
| CeO₂ (Ceria) | Excellent oxygen storage capacity; readily provides lattice oxygen | Creates strong metal-support interactions; high thermal stability | Can be expensive compared to other supports |
| Al₂O₃ (Alumina) | Provides high surface area; stabilizes small gold nanoparticles | Excellent thermal stability; low cost | Limited oxygen mobility compared to reducible oxides |
Isolating gold's active species for CO oxidation
For decades after Haruta's discovery, a vigorous debate persisted within the catalysis community: what form of gold is most active for CO oxidation? Was it individual gold atoms, small clusters of gold atoms, or larger nanoparticles? Resolving this question was crucial for designing optimal catalysts, but the heterogeneous nature of most gold catalysts—which typically contained a mixture of these species—made answers elusive.
Using specialized deposition techniques, the team created three different ceria-supported gold catalysts with controlled gold structures.
The researchers employed aberration-corrected HAADF-STEM to visualize the gold species directly at atomic resolution.
Using X-ray absorption fine structure (XAFS) spectroscopy, the team determined the chemical state and local environment of gold in each catalyst.
The catalytic performance for CO oxidation was evaluated in a flow reactor under identical conditions for all three catalysts.
The results were striking and definitive. The catalytic activity per gram of gold decreased in the order: Au_particle > Au_cluster >> Au_atom 8 . The nanoparticle catalyst was approximately twice as active as the cluster catalyst and orders of magnitude more active than the single-atom catalyst.
| Gold Species | Size Range | Primary Chemical State | Relative Activity | Stability |
|---|---|---|---|---|
| Single Atoms | <0.3 nm | Ionic (Au⁺ or Au³⁺) | Very Low | Low (tend to aggregate into clusters) |
| Clusters | 0.3-2 nm | Metallic (Au⁰) | High | Moderate (some sintering over time) |
| Nanoparticles | 2-5 nm | Metallic (Au⁰) | Highest | High (stable with proper support) |
These findings resolved a long-standing debate in gold catalysis and provided crucial guidance for catalyst design: maximizing the number of metallic gold clusters and small nanoparticles rather than striving for single-atom dispersion.
Practical applications and future directions
The fundamental understanding gained from studies of gold catalysis has paved the way for practical applications. Gold-based catalysts are now being employed or considered for several important technologies:
Gold catalysts effectively remove trace CO from indoor air, especially in confined spaces like submarines, spacecraft, and underground parking facilities 9 .
The preferential oxidation of CO (PROX) in hydrogen-rich streams is crucial for proton exchange membrane fuel cells, which are extremely sensitive to CO poisoning 9 .
Gold catalysts can be integrated into industrial processes to reduce CO emissions, particularly in applications where low-temperature operation is advantageous 1 .
Miniature CO sensors incorporating gold catalysts can detect dangerous CO levels in homes and industrial settings 5 .
Future research focuses on enhancing the durability and cost-effectiveness of gold catalysts. A significant challenge is preventing the sintering of gold nanoparticles—the tendency of small particles to agglomerate into larger, less active ones at elevated temperatures.
The story of gold catalysis exemplifies how scientific paradigms can shift dramatically with new insights and techniques. What was once considered a catalytically inert metal has become the foundation for some of the most active catalysts known today—all because researchers looked at it from a different perspective, literally at the nanoscale.
As we face growing challenges of air pollution and energy sustainability, gold-based catalysts offer promising solutions for cleaning emissions and enabling cleaner energy technologies. The once-humble noble metal has truly earned its place as a catalytic champion, turning the deadly carbon monoxide into harmless carbon dioxide and making our environment safer for all.