Hydroxyl-Mediated SO2 Boost Enables Efficient CO-Based NOx Reduction on IrIn/Beta

A new study in Nature Communications reports a promising route to clean up nitrogen-oxide pollution by harnessing an unexpected chemical “helper”: hydroxyl-mediated sulfur chemistry. Researchers show that sulfur dioxide (SO₂) is not merely a byproduct to be trapped—it can actively assist catalytic reactions that convert NOₓ into less harmful products. The work targets a longstanding challenge in emission control: achieving high NOₓ reduction efficiency under oxygen-rich conditions, where many catalysts struggle because oxidizing species can disrupt desired reaction pathways.

At the center of the strategy is a catalyst built from iridium and indium supported on zeolite beta (IrIn/Beta). In practical exhaust environments, oxygen is abundant, and this changes the surface chemistry dramatically. The team observed that when oxygen-rich feed conditions include SO₂, the catalytic system can access a reaction network that remains efficient rather than shutting down.

The key appears to be the formation and participation of hydroxyl (–OH) groups. Instead of treating SO₂ as a static sulfur contaminant, the researchers show that SO₂ can be promoted through hydroxyl-mediated transformations on the catalyst surface. Those –OH species help reorganize sulfur-containing intermediates, effectively steering the chemistry toward NOₓ conversion rather than passive sulfate buildup.

Using mechanistic evidence from advanced characterization and guided reaction studies, the authors describe how SO₂-related species evolve into reactive intermediates that can couple with nitrogen oxides. In this oxygen-rich regime, the catalyst’s performance is sustained because the promoted sulfur–hydroxyl chemistry stabilizes beneficial surface states. As a result, carbon monoxide (CO)—a reductant often present or co-injected in abatement scenarios—can more effectively reduce NOₓ.

The findings highlight an “intelligent promotion” concept: by deliberately leveraging a controlled amount of SO₂ chemistry, the catalyst gains access to pathways that tolerate oxidizing conditions. This could be significant for real-world systems where oxygen levels fluctuate and complete removal of SO₂ is not always feasible.

Beyond showing improved NOₓ reduction, the study emphasizes that interfacial functional groups—here, hydroxyls—can act as regulators of reaction selectivity. Such control is particularly valuable in oxygen-rich exhaust, where the balance between oxidation and reduction determines whether NOₓ abatement succeeds.

By linking hydroxyl-mediated SO₂ promotion with efficient CO-driven NOₓ reduction, the work suggests a design principle for next-generation emission catalysts: combine active metals with supports and promoter chemistry that dynamically manage surface intermediates. If translated to scale, this approach could broaden the operating window for NOₓ control technologies and reduce the need for more complex process tuning.