Sulfur & water resistance: core bottleneck in CO oxidation commercialization

CO catalytic oxidation technology faces its greatest hurdle in moving from laboratory to industrial application not in catalytic activity itself, but in catalyst durability under real flue gas conditions—especially simultaneous resistance to SO₂ and water vapor. Industrial sintering flue gas typically contains about 30 ppm SO₂ and around 10% water vapor, and their synergistic effect can irreversibly cover and destroy active sites within hours. Even after desulfurization pretreatment, residual SO₂ is sufficient to cause continuous performance decay. Sulfur and water resistance directly determines whether a catalyst can operate stably over the long term in the field—and that is the decisive factor for large‑scale commercialization of CO catalytic oxidation. The following four sections examine the harshness of industrial flue gas environments, the microscopic deactivation mechanisms, the practical challenges in engineering validation, and current technology breakthroughs, to explain how this bottleneck forms and how it can be overcome.

1. Industrial Flue Gas Environment: The Real‑World Test Bed for Catalysts

Under laboratory conditions, CO catalytic oxidation can proceed efficiently in clean simulated atmospheres—temperature precisely controlled, no impurities, stable reaction conditions. But real industrial flue gas is far more complex.

Iron and steel sintering flue gas is a major source of CO emissions, and its composition is extremely complex: besides CO, it contains SO₂ (about 30 ppm), NOx, dust, and about 10% water vapor. Based on China’s 2023 crude steel output of 1.029 billion tons and sinter production of about 1.38 billion tons, the total annual CO emissions from sintering flue gas could reach 13.8 million tons. Such massive emissions mean that a single treatment system handles enormous gas volumes—a 435 m² sintering machine can produce a flue gas flow of 1.6 million m³/h. The challenge of maintaining stable catalyst performance under these high‑flow, high‑humidity, sulfur‑containing conditions far exceeds anything seen in laboratory simulations.

Of particular concern is the synergistic poisoning effect of water vapor and SO₂. Studies have shown that under SO₂‑only conditions, some catalysts retain reasonable activity; but when both water vapor and SO₂ are present, SO₂ oxidizes to SO₃ and reacts with water to form sulfuric acid, which deposits as sulfate species on the catalyst surface, blocking pores and covering active sites. Some research indicates that certain catalysts can maintain activity under 250 ppm SO₂ and 5% water vapor—but sintering flue gas typically contains 10%–20% water vapor, far exceeding laboratory test conditions. This gap between “laboratory success and field failure” is a direct manifestation of why sulfur and water resistance is a critical industrialization bottleneck.

CO oxidation catalysts currently fall into two broad categories: noble‑metal catalysts (Pt, Pd, Ru, etc.) and non‑noble‑metal catalysts (copper‑manganese oxides, etc.). Noble‑metal catalysts are widely studied for their high efficiency and better resistance to water and sulfur; non‑noble‑metal catalysts are cheaper but face greater challenges in poisoning resistance. For either category, durability in industrial flue gas environments is the key indicator of engineering applicability.

2. Microscopic Deactivation Mechanisms: From Physical Coverage to Chemical Destruction

The complexity of deactivation mechanisms is what makes sulfur and water resistance such a formidable bottleneck—it is not merely physical blockage but involves multiple failure pathways, including chemical bond breaking, crystal structure damage, and electronic structure alteration.

For non‑noble‑metal catalysts (taking Hopcalite as an example), the active components are mainly MnO₂ and CuO, and the catalytic cycle depends on lattice oxygen migration between copper and manganese oxides. When sulfur compounds (such as H₂S or SO₂) are present in the flue gas, they react with CuO and MnO₂ to form CuS, MnS, and sulfate species. Sulfur poisoning deactivates the catalyst through two primary pathways: first, sulfides and sulfates form a physical overlayer that blocks CO molecules from accessing active sites; second, chemical reactions with copper and manganese directly destroy the Cu‑O‑Mn bonding network, weakening the electronic synergy between the two metals. Experimental data show that when the H₂S concentration in the gas exceeds 5 ppm, the activity of Hopcalite catalyst drops by more than 90% within 40 hours. Moreover, Hopcalite is highly sensitive to water vapor—when ambient humidity exceeds 10%, water vapor occupies active sites and causes a marked decrease in catalytic efficiency; at relative humidity above 45%, the activity declines significantly.

For noble‑metal catalysts (taking Pt‑based catalysts as an example), SO₂ poisoning pathways are equally complex. In Pt/Al₂O₃ catalysts, the primary deactivation route under coexisting H₂O and SO₂ is “spillover difficulty” of sulfate intermediates—sulfur species adsorb on Pt sites but cannot transfer effectively to the Al₂O₃ support, leading to continuous coverage of active Pt sites. By tuning the oxidation state and coordination environment of Pt, the spillover of sulfate intermediates from Pt to Al₂O₃ can be promoted, thereby enhancing sulfur resistance.

Interestingly, water vapor does not always play a purely negative role in poisoning. Recent studies on Pt/TiO₂ catalysts have revealed that water vapor actually promotes the migration of sulfur species from the Pt surface to the TiO₂ support—sulfur first adsorbs as SO₃ on Pt and then migrates as SO₄²⁻ to TiO₂, thereby preserving the CO oxidation activity of the Pt surface. This finding suggests that understanding and exploiting the role of water vapor in poisoning mechanisms is a key breakthrough point for designing sulfur‑ and water‑resistant catalysts.

Ru‑based catalysts exhibit different sulfur‑tolerance characteristics. Studies show that Ru/TiO₂ catalysts can remain stable in SO₂‑ and H₂O‑containing atmospheres under certain conditions (e.g., high CO concentration). These differences among noble‑metal systems offer diversified options for catalyst design.

3. Practical Engineering Challenges: The Last Mile from Lab to Field

Sulfur and water resistance is not just an academic issue—it is a real engineering concern. Operating data from several CO catalytic purification projects illustrate the severity of this bottleneck from both positive and negative angles.

On one hand, catalyst poisoning is indeed a core obstacle to commercialization. The approximately 30 ppm SO₂ and about 10% water vapor in sintering flue gas are known to poison catalysts, making the development of water‑ and sulfur‑tolerant CO oxidation catalysts “urgently needed”. The industrialization bottleneck for CO catalytic oxidation technology lies precisely in developing catalyst systems that combine high activity, excellent poisoning resistance, and low cost.

On the other hand, catalysts modified for poisoning resistance have shown feasibility in engineering practice. In one 435 m² sintering machine project with full‑flow CO catalytic purification, noble‑metal honeycomb catalysts were installed in the spare layer of the existing denitrification tower, achieving both CO reduction and heat recovery without additional external equipment. Over four months of continuous operation, the overall performance remained stable, with CO conversion efficiency of 76%–85% and outlet CO concentrations reduced to 1070–2365 mg/m³. In another 426 m² sintering machine project, the catalyst operated stably at a low temperature of 270–280°C, with a service life exceeding three years and stable CO removal efficiency with low decay.

These engineering practices demonstrate that the degree of progress in sulfur and water resistance directly determines the industrial service life and economic viability of the catalyst. A catalyst with a lifespan of more than three years versus one that lasts only a few months makes a world of difference in overall project economics. In practice, catalyst deactivation is rarely an abrupt “all‑or‑nothing” failure but rather a continuous performance decline—meaning that catalysts require regular monitoring, evaluation, and replacement, which directly impacts system operating costs and reliability.

4. Pathways to Breakthrough: From Passive Protection to Active Design

Researchers are tackling the sulfur and water resistance challenge from multiple directions. The core philosophy behind these approaches is to shift from “passively enduring poisoning” to “actively designing for resistance”.

Support engineering is an important route. For Pt/TiO₂ catalysts, tuning the ratio of anatase to rutile phases in the TiO₂ support significantly improves sulfur and water resistance. Studies show that anatase TiO₂ has strong SO₂ adsorption but weak CO adsorption, whereas rutile shows the opposite behavior—the phase interface effectively reduces SO₂ adsorption and oxidation on the catalyst surface, suppressing TiOSO₄ deposition. When the anatase‑to‑rutile ratio is optimized, the catalyst achieves 93% CO conversion at 160°C and maintains over 95% conversion for 46 hours at 220°C under 8% water vapor and 0.016% SO₂.

Element doping is equally effective. CoOx doping of Pt/TiO₂ catalysts significantly improves catalytic activity and resistance to sulfur and water. Sm‑doped Cu‑Ce catalysts also show promise in sulfur resistance. Cobalt‑based spinel catalysts with low‑level doping (Co, Fe, Ni, Cu) exhibit enhanced water and sulfur resistance in low‑temperature CO oxidation. In addition, phosphotungstic acid (PWA) modification of TiO₂ supports, where PWA decomposes at high temperature to form Pt‑WO₃ anchored on TiO₂, improves sulfur resistance without sacrificing catalytic activity.

Hydrophobic modification offers another effective strategy. By constructing a hydrophobic surface on Pt/CeO₂ composites, researchers have achieved CO oxidation under water‑ and SO₂‑containing conditions with ultra‑low noble‑metal loadings. The hydrophobic surface effectively prevents water vapor adsorption on the catalyst, cutting off the poisoning chain: “SO₂ → SO₃ → sulfuric acid → sulfate deposition.”

Carbon‑shell coating provides a novel approach. Introducing a carbon shell into Pt/CeO₂ catalysts effectively suppresses SO₂‑induced deactivation—the carbon layer maintains the valence state of Pt species, enabling CO oxidation at 180°C in the presence of SO₂ and H₂O. This approach offers a new perspective for improving SO₂ tolerance in exhaust‑gas treatment catalysts.

High‑entropy alloys and composite oxides represent a more frontier direction. High‑entropy metal oxide catalysts, through multi‑metal synergy, are expected to maintain high stability in complex flue gas environments. Additionally, Pt/TiO₂‑ZrO₂ catalysts prepared by solid‑state mixing show excellent sulfur and water resistance at relatively low temperatures.

In the journey from laboratory to industrialization of CO catalytic oxidation, sulfur and water resistance has always been the core obstacle between theory and engineering. The synergistic poisoning effect of SO₂ and water vapor in industrial flue gas attacks catalysts at multiple levels—physical coverage, chemical bond destruction, and crystal structure collapse—causing rapid deactivation under real conditions. Recent engineering practice has proven that when poisoning‑resistant designs achieve breakthroughs, CO catalytic purification can operate stably in the field—cases with catalyst lifetimes exceeding three years and stable CO removal efficiency are now documented. Future technology directions will continue to explore support interface engineering, elemental doping, hydrophobic surface construction, carbon‑shell protection, and high‑entropy alloy design, with the goal of enabling catalysts not only to “survive” but also to “live long and perform well” in complex industrial flue gases. Continued progress in sulfur and water resistance is precisely the key to transforming CO catalytic oxidation from a “laboratory sample” into a “true industrial product.”

author：Gloria
date:2026-07-06