Water-Induced Deactivation and Resistance of VOCs Catalysts

1. Introduction

Catalytic oxidation of volatile organic compounds (VOCs) is widely recognized as one of the most promising abatement technologies due to its high efficiency, economic viability, and environmental friendliness. However, the vast majority of studies have focused on the oxidation of single VOCs under ideal conditions, whereas real industrial exhaust streams are far more complex. Water vapor, an inevitable component in industrial flue gas, exerts a significant impact on catalyst performance and represents a core bottleneck limiting the industrial application of catalytic oxidation.

Water vapor content in industrial exhaust typically ranges from 3% to 8%, depending on fuel type, combustion conditions, and reaction temperature. The influence of water vapor on VOCs oxidation efficiency, stability, and selectivity varies considerably across different industrial operating conditions. Understanding the underlying mechanisms of water vapor effects and developing effective water‑resistance strategies have therefore become crucial research topics. In recent years, more than 390 relevant papers have been published between 2013 and 2024, systematically investigating the role of water vapor in VOCs catalytic oxidation.

2. Sources and Characteristics of Water Vapor in Industrial Exhaust

Water vapor in industrial exhaust originates from multiple sources. Combustion products (especially from fossil fuels) are a primary source, as hydrogen in the fuel combines with oxygen to form water. In addition, process water carryover (e.g., from washing, extraction, and other unit operations in chemical production) and end‑of‑pipe treatment facilities such as wet desulfurization and wet dust removal introduce substantial amounts of water vapor into the gas stream. Water vapor levels differ significantly across industrial sectors: coal‑fired flue gas typically contains 3%–8% water vapor; chemical tail gases exhibit wider fluctuations depending on the process; and spray‑painting exhaust varies with the solvent‑to‑water ratio in coatings.

Water vapor does not exist in isolation. Industrial exhaust often contains inorganic gases such as SO₂, NOx, CO₂, and CO, along with multiple VOCs. Water vapor may have synergistic or antagonistic interactions with these coexisting components – it can promote sulfate deposition on catalyst surfaces, thereby exacerbating SO₂ poisoning, while it may also facilitate the desorption of coke or other by‑products through a "cleaning" effect. Operating conditions (reaction temperature, space velocity, water vapor concentration, etc.) critically determine the net effect of water vapor, and the response of a given catalyst to water vapor can differ markedly at different temperatures.

3. Mechanisms of Water Vapor Effects on VOCs Catalytic Oxidation

The influence of water vapor on VOCs catalytic oxidation is not simply inhibitory; it plays a dual role. On one hand, water molecules can dissociate on active sites to form hydroxyl groups (–OH), which may promote VOCs conversion and CO₂ selectivity. On the other hand, water molecules compete with VOCs for adsorption on active sites ( competitive adsorption ), and because of their higher polarity, they preferentially occupy sites, thereby reducing the available sites for VOCs.

Based on recent research, the inhibitory/promoting effects can be categorized into the following five mechanisms:

3.1 Competitive Adsorption

Water molecules and VOCs compete for the same active sites on the catalyst surface. Water, being more polar, preferentially adsorbs and blocks the sites, diminishing VOCs adsorption and subsequent reaction.

3.2 Surface Hydroxyl Formation

Water vapor dissociates on metal oxide surfaces to generate –OH groups, which cover active sites and alter the surface chemical state, thereby reducing catalytic activity.

3.3 Lattice Oxygen Replenishment Blockage

Water vapor occupies oxygen vacancies and impedes the replenishment of lattice oxygen, leading to irreversible efficiency loss. According to the Mars‑van Krevelen (MvK) mechanism, lattice oxygen consumed during VOCs oxidation must be regenerated by gas‑phase oxygen. The dissociative adsorption of water molecules at oxygen vacancies hinders this regeneration process.

3.4 Reaction Pathway Alteration

The presence of water may change the reaction pathway and product distribution of VOCs catalytic oxidation.

3.5 Cleaning Effect

Under certain conditions, water vapor can promote the desorption of coke or other by‑products from the catalyst surface, acting as a "cleaning" agent that helps sustain catalytic activity.

The net outcome of these five mechanisms depends on key factors including catalyst composition, reaction temperature, water vapor concentration, VOC type, and space velocity. In practice, multiple mechanisms often operate simultaneously and compete with each other, so the final promoting or inhibiting effect is governed by the combined interplay of these variables.

4. Quantitative Characterization and Representative Data of Water‑Induced Deactivation

The inhibitory effect of water vapor on catalyst performance can be quantified by experimental data. Several representative examples are given below:

1. Upon introducing 5% water vapor into a simulated flue gas, the oxidation efficiency of unmodified MnOx/γ‑Al₂O₃ toward o‑xylene decreased by 20%. After water vapor was removed, the catalyst still exhibited an irreversible loss of about 9% – indicating that the inhibition is partially irreversible.

2. At 300°C with 5% water vapor, the same unmodified catalyst showed a 26% decrease in o‑xylene oxidation efficiency, demonstrating that even at elevated temperatures, water vapor inhibition remains substantial.

3. Single‑atom Pt‑based catalysts are highly susceptible to deactivation under humid conditions, severely restricting their practical application in industrial exhaust treatment. The strong adsorption of water molecules on single‑atom sites is the primary cause of deactivation.

4. A siloxane‑modified MnOx catalyst (MnOx‑Si₀.₉/γ‑Al₂O₃) maintained stable catalytic efficiency for o‑xylene oxidation for more than 200 minutes under humid conditions.

These data indicate that water‑induced deactivation is universal (affecting various catalyst systems), temperature‑dependent (the degree of inhibition varies with temperature), and partially irreversible (activity cannot be fully restored after water vapor removal).

5. Design Strategies and Research Progress on Water‑Resistant Catalysts

To address water‑induced catalyst poisoning, extensive research has been conducted, leading to several effective strategies.

5.1 Modification by Introducing Additional Elements

Doping with a second metal component can modulate the electronic structure and active sites, effectively enhancing water resistance. For example, in Cu‑doped MnOx catalysts – researchers developed a catalyst (Cu₀.₁Mn₁‑Al₂O₃(NF)) where Cu‑doped MnOx was dispersed on a flower‑like hollow‑sphere support. The introduction of Cu⁺ suppressed the dissociation of H₂O on metal sites and the formation of –OH, attributed to the weaker H₂O dissociation ability at Mn‑O_v‑Cu oxygen vacancies. Experiments showed that under 5% water vapor, this catalyst lost only 5% of its oxidation efficiency, and the irreversible loss after water removal was merely 2%. The water resistance of this catalyst surpassed that of most reported MnOx catalysts.

5.2 Special Structure and Morphology Design

Special nanostructures such as flower‑like hollow spheres can reduce hydrogen‑bonded water adsorption. The H₂O desorption endothermic temperature of Cu₀.₁Mn₁‑Al₂O₃(NF) was 8°C lower than that of the unmodified catalyst, indicating superior water desorption capability. Core‑shell structures and hydrophobic zeolites also show promise in water‑resistant catalyst development.

5.3 Preparation of Hydrophobic Materials

Modifying catalyst supports with hydrophobic materials such as inorganic siloxanes can effectively reduce water adsorption and promote water desorption. Molecular adsorption energy calculations indicate that siloxane modification suppresses water adsorption while promoting organic adsorption. Hydrophobic surfaces also minimize sulfate deposition, thereby mitigating the synergistic poisoning effect of SO₂. The MnOx‑Si₀.₉/γ‑Al₂O₃ catalyst showed only 5% efficiency loss under humid conditions, far superior to the 26% loss of the unmodified catalyst.

5.4 Catalyst Regeneration Technology

Based on the H₂O‑involved MvK mechanism, researchers have developed an air‑purge regeneration protocol that enhances catalyst operational stability under wet flue gas conditions. This approach provides a feasible pathway for long‑term industrial operation.

6. Conclusions and Outlook

Water‑induced deactivation remains a core obstacle in translating VOCs catalytic oxidation from laboratory to industrial applications. The unavoidable moisture in real industrial exhaust degrades catalyst performance through multiple mechanisms including competitive adsorption, surface hydroxyl coverage, and lattice oxygen replenishment blockage, with partial irreversible loss.

Designing water‑resistant catalysts requires a balance among activity, stability, and economy. Current progress has been made through element doping, morphology engineering, hydrophobic material preparation, and regeneration technologies. Future directions include the synergistic combination of multiple anti‑water strategies, development of smart responsive water‑resistant materials, and further efforts in cost control, scalable preparation, and long‑term validation to bridge the gap between fundamental research and industrial deployment.

author: Gloria
date:2026/6/30