VOCs in industrial waste gas mostly exist in the form of mixtures, such as coexisting benzene series, esters and alkanes. Competitive adsorption widely occurs among different components on the catalyst surface. Some refractory components can inhibit the catalytic combustion efficiency of easily oxidized components. Catalyst formulation can rationally designed to enhance the synergism of simultaneous elimination for multi-component VOCs.

1. Competitive Adsorption and Inhibitory Effect of Multi-Component VOCs

1.1 Essence and rules of competitive adsorption

Due to the differences in polarity, molecular size, adsorption enthalpy and functional groups, various VOC molecules undergo Langmuir competitive adsorption on catalyst active sites (metal sites, oxygen vacancies, Lewis/Brønsted acid sites). Molecules that occupy priority sites will repel other components.

Common order of adsorption strength:

Aromatic hydrocarbons (benzene, toluene) > Esters (ethyl acetate) > Ketones > Short-chain alkanes (propane, n-hexane)

Core mechanisms:

1. Active site competition: Strongly adsorbed components such as toluene preferentially occupy metal active sites and oxygen vacancies, blocking the adsorption and activation of weakly adsorbed components like alkanes.
2. Intermediate coverage: Partial oxidation intermediates of refractory components (e.g., benzene) such as phenols and benzaldehyde strongly adsorb on the catalyst surface, forming carbon deposition precursors and covering active sites.
3. Electronic interference: Electron-rich aromatic hydrocarbons alter the surface electron density of catalysts and weaken the activation capacity for electron-poor alkanes.

1.2 Inhibition of easily oxidized components by refractory components

Typical inhibition phenomena:

• Benzene/toluene (refractory) inhibits the degradation of ethyl acetate/acetone (easily oxidized). The strong adsorption of aromatics prevents esters and ketones from accessing active sites, raising the T90 conversion temperature by 30–80 ℃ and significantly reducing conversion efficiency.
• Propane (high C–H bond energy, hard to oxidize) inhibits toluene and ethyl acetate. Although propane shows weak adsorption, its high oxidation barrier easily generates intermediates such as formaldehyde and formic acid, poisoning active sites and suppressing deep oxidation of aromatics and esters.

Two inhibition modes:

1. Competitive inhibition (dominant): Strongly adsorbed VOCs monopolize active sites and exclude weak components from adsorption.
2. Non-competitive inhibition (poisoning): Intermediates or by-products of refractory components (carbon deposition, chlorinated derivatives) irreversibly occupy or destroy active centers.

1.3 Promotion effect in special cases

Synergistic promotion occurs under specific conditions:

• Ethyl acetate/acetone provide hydrogen/oxygen species at low temperature to promote benzene ring opening of toluene, reducing its T50 by over 50 ℃.
• Highly exothermic components such as methanol release heat during combustion to raise local temperature and accelerate the conversion of refractory components.

2. Catalyst Design Strategies for Enhancing Multi-Component Synergistic Removal

2.1 Construction of multiple active sites (core strategy)

Realize division of labor for different VOCs to avoid competition on single active centers.

• Precious metal alloys (Pt–Pd, Pt–Ru, Pd–Au)Pt preferentially activates aromatic hydrocarbons; Pd is suitable for oxidizing esters, ketones and alcohols; Au improves low-temperature activity and anti-CO poisoning performance and weakens strong adsorption.Pt–Pd/TiO₂ reduces the T90 of toluene-ethyl acetate mixture by 40–60 ℃ with negligible mutual inhibition.
• Precious metal combined with transition metal oxides (Pt/Co₃O₄–MnOₓ, Pd/CeO₂–MnO₂)Precious metals achieve low-temperature ignition and aromatic activation; MnOₓ/Co₃O₄ oxidize esters, ketones and alkanes and supply active oxygen; CeO₂ accelerates Ce³⁺/Ce⁴⁺ redox cycling to enhance oxygen migration and vacancy regeneration.
• Single-atom or diatomic alloysHighly dispersed sites avoid agglomeration; electronic synergy activates C–H bonds and O₂ simultaneously, achieving T90 below 200 ℃ for propane-toluene mixtures.

2.2 Support and promoter engineering: weaken competition and strengthen oxygen transfer

Support selection:

• Al₂O₃: high specific surface area and good thermal stability; strong acidity favors adsorption of aromatics and polar VOCs.
• Anatase TiO₂: abundant oxygen vacancies benefit aromatic ring opening and anti-poisoning performance.
• CeO₂–ZrO₂ solid solution: excellent oxygen storage and release capacity accelerates oxygen supplement and relieves site competition.
• Molecular sieves (ZSM-5, Y, Beta): hierarchical pore structure and shape-selective effect spatially separate macromolecular aromatics and small-molecule esters/alkanes to reduce competitive adsorption.

Typical dopants (Ce, La, Nb, W, Zr):

Ce induces oxygen vacancies and promotes oxygen overflow and carbon resistance; La stabilizes crystal phase and modulates surface acid-base properties; Nb/W enhances surface acidity and activates C–H bonds of alkanes and aromatics.

2.3 Precise regulation of surface properties

Acid-base matching:

Acidic sites adsorb non-polar/weakly polar aromatics and alkanes; alkaline sites adsorb polar oxygen-containing components such as esters and ketones. Composite acid-base supports realize independent adsorption of different VOCs and avoid competition.

Hydrophobic modification:

Industrial waste gas usually contains water vapor. Hydrophobic modification reduces competitive adsorption of H₂O and improves long-term stability for mixed VOCs.

2.4 Optimization of morphology and pore structure

Core-shell and gradient loading structures allocate different active components for different VOCs: shell layers for easily oxidized esters/ketones, core layers for refractory aromatics/alkanes, realizing graded oxidation without mutual interference.

Micro-mesoporous composite structure reduces mass transfer resistance and avoids overload competition on local active sites.

2.5 Anti-interference and anti-poisoning design

Coating and confinement structures protect active sites from S, Cl and siloxane poisoning and extend service life under complex working conditions.

Adsorption-catalysis bifunctional composites realize pre-separation followed by graded oxidation, fundamentally eliminating competitive adsorption.

3. Practical Catalyst Formulation Example (Benzene Series + Esters + Alkanes)

Formula: Pt–Pd/CeO₂–ZrO₂–ZSM-5 (0.2% Pt + 0.3% Pd, Ce/Zr molar ratio = 1:1, ZSM-5 silica-alumina ratio = 200)

Pt activates toluene and xylene; Pd oxidizes ethyl acetate and acetone; CeO₂–ZrO₂ accelerates oxygen transfer; ZSM-5 provides shape-selective adsorption and hydrophobicity.

At 280–320 ℃, the conversion rate of toluene, ethyl acetate and propane all exceeds 98% with no obvious mutual inhibition.

4. Summary

Competitive adsorption is ubiquitous in multi-component VOCs systems. Strongly adsorbed and refractory components commonly inhibit the oxidation of easily oxidized components.

The core of synergistic design lies in multi-active site collaboration, enhanced oxygen transfer of supports, dual acid-base surface regulation and spatial separation via pore structure.

Engineering application adopts the composite system of precious metal-transition metal oxides, ceria-based solid solution and molecular sieve support to achieve high-efficiency synergistic removal of multi-component VOCs.