New Cobalt Prussian Blue Catalysts Boost Carbon Monoxide Oxidation

A new study reports a low-cost, noble-metal-free catalyst that can convert toxic carbon monoxide into carbon dioxide at remarkably low temperatures, while retaining useful activity after prolonged exposure to water vapor and sulfur dioxide. Developed by researchers at Leipzig University, Martin Luther University Halle-Wittenberg, and Hochschule Niederrhein, the material is based on cobalt ferrite, CoFe₂O₄, a mixed-metal oxide engineered with a mesoporous structure. The work addresses a long-standing challenge in emissions control: designing catalysts that are both highly active at low temperatures and resistant to the moisture and sulfur compounds found in real exhaust streams.

Carbon monoxide is produced whenever carbon-containing fuels burn incompletely. It is colorless and odorless, and even relatively low concentrations can interfere with the blood’s ability to transport oxygen. Catalytic oxidation offers a direct solution by transforming CO into carbon dioxide in the presence of oxygen. Noble metals such as platinum, palladium, and gold can drive this reaction at very low temperatures, but their cost, limited availability, and sensitivity to poisoning complicate large-scale use. The new research instead focuses on cobalt ferrite, a spinel oxide that uses more abundant elements while reducing the amount of cobalt required compared with pure cobalt oxide catalysts.

The researchers prepared their catalyst through an unusual precursor route involving Prussian blue and a cobalt-containing Prussian blue analogue. Prussian blue, formally Fe₄[Fe(CN)₆]₃, and cobalt Prussian blue analogue, Co₃[Fe(CN)₆]₂, were produced together by co-precipitating cobalt and iron salts with potassium ferricyanide. This approach creates close contact between cobalt and iron ions before the material is heated. During calcination in air, the cyanide framework decomposes and the metals reorganize into cobalt ferrite, CoFe₂O₄. The researchers argue that this atomic-scale proximity helps favor formation of a mixed spinel phase rather than a simple physical mixture of separate cobalt and iron oxides.

The synthesis was refined using two strategies designed to preserve the material’s internal pore network. First, some preparations used a mixture containing dimethylformamide, or DMF, during precipitation. The solvent acted as a capping or crystallization-modifying agent, influencing the size and organization of the precursor particles. Second, the researchers washed the precipitated material with a saturated magnesium sulfate solution rather than pure water. Magnesium sulfate filled the pores before calcination and acted as a temporary protective filler. Without this protection, heating caused small pores and spaces between particles to collapse or merge through sintering, reducing the available surface area. After calcination, the salt was removed by repeated washing, leaving behind a more open mesoporous structure.

The resulting material displayed the characteristics expected of a highly accessible heterogeneous catalyst. Its specific surface area reached approximately 76 square meters per gram, with pores mainly spanning 10 to 70 nanometers and a mesopore volume of about 0.4 cubic centimeters per gram. Electron microscopy showed that the larger particles, measuring roughly 5 to 100 micrometers, were loose agglomerates of much smaller crystallites. Transmission electron microscopy revealed crystallites in the range of 10 to 100 nanometers, with catalytic pores arising primarily from the voids between these crystallites. This hierarchical arrangement gives reactant gases pathways into the catalyst while exposing a comparatively large population of surface sites.

Chemical and structural analyses supported the formation of cobalt ferrite. X-ray diffraction detected the characteristic spinel reflections associated with CoFe₂O₄ or closely related iron-rich spinel phases. Because cobalt ferrite and magnetite, Fe₃O₄, have very similar crystal structures and lattice parameters, diffraction alone could not completely distinguish them. However, inductively coupled plasma measurements showed an overall cobalt-to-iron atomic ratio of approximately 1:2. Energy-dispersive X-ray analysis further indicated a relatively homogeneous distribution of cobalt and iron at the nanoscale, with no significant residual potassium, sulfur, or nitrogen after calcination. Together, the results favored the researchers’ interpretation that cobalt ferrite was the dominant phase, although small quantities of other oxides could not be entirely excluded.

The catalyst’s performance changed dramatically when the pore-protection step was introduced. In a fixed-bed reactor containing dry synthetic air and 1,000 parts per million of carbon monoxide, the untreated material required temperatures around 235 degrees Celsius to achieve 90 percent conversion. By contrast, the magnesium-sulfate-treated samples reached the same conversion below 35 degrees Celsius, with the best preparations falling below the experiment’s lower detection limit of 30 degrees Celsius. Complete conversion was reported at approximately 40 degrees Celsius in repeated tests. The improvement was primarily linked to the preservation of mesopores and the resulting increase in accessible surface area, although the authors note that residual magnesium may also influence the catalyst by affecting metal dispersion, surface oxygen, or oxygen-vacancy formation.

The researchers then moved beyond idealized powder testing and coated the catalyst onto a one-inch alumina honeycomb monolith, a geometry more representative of practical exhaust-treatment systems. The monolith contained 200 cells per square inch and carried approximately 100 grams of catalyst per liter. Under these conditions, the gas passed through the structured support at a space velocity of 50,000 inverse hours, considerably faster than in the powder experiment. As a result, the untreated honeycomb reached 90 percent CO conversion at about 200 degrees Celsius in dry air, demonstrating why laboratory powder results cannot be directly transferred to industrial reactor designs. Nevertheless, complete conversion occurred near 240 degrees Celsius, a notable result for a catalyst containing no precious metals.

Moisture, however, imposed a substantial penalty. When the feed contained 5 percent water vapor, the temperature required for 90 percent conversion increased to approximately 320 degrees Celsius, while complete conversion occurred around 380 degrees Celsius. Water can compete with carbon monoxide and oxygen for adsorption sites, modify surface hydroxyl groups, and alter the reaction pathway on metal oxides. The catalyst nevertheless remained functional after hydrothermal aging for 100 hours at 400 degrees Celsius in air containing water vapor. Exposure to sulfur dioxide was more damaging: after 100 hours in air containing 1 part per million SO₂ at 400 degrees Celsius, the T₉₀ temperature rose to approximately 380 degrees Celsius. Sulfur dioxide can react with reducible surface sites to form stable sulfate species that block active centers, but the material continued to oxidize carbon monoxide rather than becoming completely inactive.

The findings suggest that the catalyst may be especially useful in hot exhaust systems, where temperatures are high enough to overcome the water-related loss in low-temperature activity. The researchers emphasize that cobalt ferrite is not as active as the most optimized pure cobalt oxide catalysts, which can operate at far lower temperatures under specialized laboratory conditions. Its advantage lies instead in the balance between activity, material cost, structural stability, and reduced cobalt content. The Prussian blue analogue route also offers a relatively simple synthesis that could potentially be adapted to other mixed-metal ferrites. Before commercialization, the material will require longer durability studies, testing with realistic mixtures of carbon monoxide, hydrocarbons, nitrogen oxides, particulates, and sulfur compounds, as well as a closer investigation of the roles played by residual magnesium and surface defects. Even so, the study demonstrates how controlling pore architecture during heat treatment can transform an ordinary mixed oxide into a highly active emissions-control catalyst.