1. Why this study matters: MXenes move into “real” catalysis

In heterogeneous catalysis, researchers are always hunting for new solid materials that can:

• drive reactions faster

• give better selectivity (fewer side products)

• work under milder and greener conditions

MXenes, a relatively young family of 2D transition metal carbides and nitrides, have already attracted a lot of attention for things like:

• electrocatalysis (HER, OER, ORR, CO₂ reduction)

• energy storage and supercapacitors

• various electronic and biomedical uses

But most of that work has focused on electrocatalysis and photo/electro processes. In contrast, their role as “thermal” catalysts (classical heterogeneous catalysis at elevated temperature) is still barely explored.

This paper focuses on one specific MXene: Nb₂C, made from the parent phase Nb₂AlC. The main message is:

Nb₂C MXene can act as a bifunctional catalyst – it has both acid and base sites and can also catalyze oxidation and hydrogenation reactions, without needing noble metals.

In particular, Nb₂C MXene can:

1. Catalyze aldol condensations (acid–base catalysis)

2. Catalyze oxidative coupling of aniline to azo and azoxy compounds (oxidation catalysis)

3. Catalyze hydrogenation of azoxybenzene to azobenzene (hydrogenation catalysis)

That means a single material can handle quite different reaction types.

2. A quick primer: what are MXenes and what is Nb₂C?

MXenes have a general formula Mₙ₊₁Xₙ, where:

• M = an early transition metal (Ti, Nb, V, etc.)

• X = carbon and/or nitrogen

They are formed by selectively etching out an “A” layer (usually aluminum) from a 3D parent phase called a MAX phase (like Nb₂AlC). After etching away Al³⁺, you’re left with stacked metal–carbon layers that can then be delaminated into 2D sheets.

Typical features of MXenes:

• 2D layered structure, similar in spirit to graphene or other nanosheets

• surface terminations, usually written as “Tₓ” (–O, –OH, –F, etc.)

• good electrical conductivity

• tunable surface chemistry and electronic structure via composition and termination groups

In this study, the MXene of interest is Nb₂C:

• Parent MAX phase: Nb₂AlC

• After Al removal and delamination, the product is Nb₂C MXene, often described as Nb₂C Tₓ (with O and F terminations, plus some intercalated species).

3. How Nb₂C MXene is made in this work

The authors prepared Nb₂C MXene using a hydrothermal etching process:

1. Starting material: commercial Nb₂AlC powder

2. Etching mixture:

   • NaBF₄

   • Concentrated HCl (37%)

   • This combination generates HF in situ, which etches out the Al layer.

3. The mixture is sealed in a Teflon-lined autoclave at 180 °C for 8 hours.

4. After cooling:

   • Filter the resulting black suspension

   • Wash with water until neutral pH

   • Dry at room temperature – this yields an “accordion-like” Nb₂C MXene clay

Next, to delaminate the layered structure into thinner sheets:

1. The clay is first expanded in DMSO for 24 hours.

2. Then it is sonicated in water with a tip sonicator (pulsed mode) to produce dispersed Nb₂C nanosheets.

3. The suspension is centrifuged and washed to remove excess DMSO.

For some samples, they also performed a thermal activation step at 350 °C under nitrogen for 1 hour to modify the surface terminations (e.g., removing some functional groups) and potentially change the number of active sites.

So, in simple terms:

They carved out Al from Nb₂AlC, peeled open the remaining layered structure with DMSO and sonication, and ended up with thin Nb₂C MXene sheets dispersed in water.

4. What does this Nb₂C MXene look like and what are its surface properties?

The team used several standard characterization techniques (all actually reported in the paper) to understand structure and surface chemistry. Let’s go through the main ideas, not every detail.

4.1. Structure and morphology

X-ray diffraction (XRD)

• After Al etching, the original Nb₂AlC peaks largely disappear.

• A new broad low-angle peak appears around 6° 2θ, indicating a larger interlayer spacing (~1.47 nm).

• This is consistent with MXene formation and intercalation (e.g., DMSO between layers).

Raman spectroscopy

• The Nb₂C MXene shows characteristic peaks related to Nb–C vibrations (in-plane and out-of-plane modes).

• There are also D and G bands associated with the carbide/carbon layer, showing some graphitic-like character or disorder in the carbon network.

Electron microscopy (TEM / STEM)

• Images confirm the 2D sheet-like morphology with lateral dimensions ranging from hundreds of nanometers to over a micron.

• High-resolution images show clear lattice fringes (~0.23 nm), indicating high crystallinity.

• Some images also highlight defects in the Nb distribution, which are important later as potential active sites.

Atomic force microscopy (AFM)

• AFM was used to measure sheet thickness.

• Most sheets are about 1.5 nm thick, which corresponds to monolayer or few-layer Nb₂C MXene.

So structurally, the material is indeed a highly crystalline 2D Nb₂C MXene, with monolayer-thick sheets and a layered architecture.

4.2. Surface area and colloidal behavior

As a dry powder, Nb₂C MXene has a very low apparent surface area:

• BET: about 2.5 m²/g

• Langmuir: about 2.7 m²/g

This is typical for MXene powders because the sheets collapse and restack when dried.

However, in aqueous suspension the situation changes:

• Using a methylene blue adsorption method (a dye that binds strongly to negatively charged surfaces), they estimate a surface area of ~55 m²/g.

• Dynamic light scattering shows particles mostly around 300 nm in size in water.

• Zeta potential measurements give about –35 mV at neutral pH, indicating a stable, negatively charged colloid.

This is important because in many catalytic reactions, MXenes will be used in dispersed form, where the “real” accessible surface is much higher than the dry BET value suggests.

4.3. Surface chemistry and defects

X-ray photoelectron spectroscopy (XPS) reveals:

• The Nb₂C MXene contains Nb, C, O, F, and S (the latter from DMSO).

• The estimated formula (considering intercalated DMSO) is close to Nb₂C(F,O)₂·1.5DMSO.

• Surface terminations are a mix of –O, –F, and oxygen-containing groups.

• There is a defective number of surface terminal groups, meaning some vacancies and incomplete coverage.

Depth profiling (sputtering into the material):

• F and O are mainly near the surface.

• Deeper regions have fewer oxygen and fluorine terminations.

These surface terminations and defects are crucial because they define:

• where acidic sites (e.g., Nb–OH, undercoordinated Nb)

• and basic sites (e.g., O atoms with high electron density)
  are located.

4.4. Acid–base and hydrogen interaction (TPD and chemisorption)

H₂ chemisorption and H₂-TPD:

• H₂ pulsed chemisorption shows a relatively large H₂ uptake.

• H₂-TPD shows two desorption peaks (around 166 °C and 392 °C), suggesting two types of hydrogen adsorption sites (different strengths or environments).

• The difference between chemisorption and TPD indicates that some H₂ is only weakly physisorbed, while some is more strongly bound.

NH₃-TPD and CO₂-TPD:

• Both NH₃ (probe for acid sites) and CO₂ (probe for basic sites) show very low uptake (on the order of 0.01–0.03 mmol/g).

• The desorption happens mostly between 300–400 °C, indicating weak to moderate strength sites.

• This means:

  • Nb₂C MXene has few acidic and basic sites per gram,

  • and these sites are not extremely strong, but still well-defined.

Nevertheless, the fact that both acidic and basic sites coexist on the same surface and likely in close proximity is exactly what they need for bifunctional catalysis.

5. Catalytic role #1: Nb₂C MXene as a bifunctional acid–base catalyst for aldol condensations

5.1. Why aldol condensation?

Aldol condensation is one of the most common C–C bond-forming reactions in organic synthesis, typically between:

• an aldehyde

• and a carbonyl compound (often a ketone)

Classically, these reactions are catalyzed by strong homogeneous acids or bases (like KOH, NaOH, AlCl₃), which are effective but create separation and waste-handling problems.

Bifunctional solid acid–base catalysts can:

• activate the aldehyde via acidic sites

• activate the ketone’s α-position via basic sites

• operate under milder conditions and be reused

So they’re attractive as greener alternatives.

5.2. Model reaction: cyclohexanone + benzaldehyde

The authors first studied the condensation of cyclohexanone with benzaldehyde under solvent-free conditions.

Key comparisons:

• Blank reaction (no catalyst): some product forms, but conversion is low.

• Nb₂C MXene: cyclohexanone conversion rises significantly, and both mono-condensation and di-condensation products are formed with good carbon balance (>90%).

• Benchmark catalysts:

  • MgO (stronger basic sites)

  • Hydrotalcite (Mg/Al) (layered double hydroxide)

  • HZSM-5 (strong acid zeolite)

Even though Nb₂C has:

• a lower density of acid and base sites

• and those sites are relatively weak to moderate

it still shows higher turnover frequencies (TOFs) than these benchmark catalysts.

This strongly suggests that:

The combination and proximity of weak-to-moderate acid and base sites on Nb₂C MXene is more important than simply having many strong sites.

Attempts to further “activate” Nb₂C by heating it at 350 °C under N₂ actually made it less active, probably because this treatment partially healed defects and removed some active terminations or vacancies.

5.3. Catalyst stability and recyclability

They ran the cyclohexanone/benzaldehyde condensation five times with the same Nb₂C MXene sample.

• Conversion dropped only slightly (from ~51% to ~46%), likely due to minor catalyst loss during recovery, not real deactivation.

• Selectivity shifted only marginally.

• Metal leaching was very low (Nb leaching < 0.5% of the initial amount).

• TEM, XRD, and XPS after repeated use showed no significant structural changes.

So Nb₂C MXene behaves like a robust and reusable heterogeneous catalyst.

5.4. Reaction scope: other aldehydes

The study extends the aldol reaction to other aldehydes:

• Furfural: gives a mono-condensation product with cyclohexanone and a good TOF.

• Acetaldehyde, propanal, isobutyraldehyde: Nb₂C MXene again shows high activity and often outperforms hydrotalcite.

• Conjugated aldehydes (like 2-butenal, cinnamaldehyde): conversion is lower, which is expected due to electronic and stability issues.

• Propenal: instead of simple aldol, a Michael-type addition is observed at higher temperature.

• Self-condensation of cyclohexanone: Nb₂C can catalyze this to a certain extent.

• Certain electron-rich aromatic aldehydes (like p-(dimethylamino)benzaldehyde) do not react, showing there are limitations.

Overall, though, the data show:

Nb₂C MXene works as a general acid–base catalyst for a variety of aldol-type transformations, often outperforming traditional solid acids and bases.

6. Catalytic role #2: Nb₂C MXene for oxidation of aniline to azo and azoxy compounds

6.1. Why azo and azoxy compounds?

Aromatic azo compounds (like azobenzene) are important:

• dyes and pigments in the textile and food industries

• key components in smart materials, due to reversible E/Z isomerization under light or heat

Traditionally, they are made using stoichiometric oxidizing agents, often metal-based, which generate waste.

A greener route is:

• use molecular oxygen (air) as the oxidant

• use a solid catalyst, ideally without noble metals

6.2. Oxidative coupling of aniline

The authors studied the aerobic oxidative coupling of aniline to:

• azobenzene (Azo)

• azoxybenzene (Azoxy)

They used:

• Aniline in toluene

• Nb₂C MXene as the catalyst

• Temperature: 80–140 °C

• Air under autogenic pressure in a sealed reactor

Findings:

• Without catalyst: no detectable azo or azoxy products.

• With Nb₂C MXene: both azobenzene and azoxybenzene form with high carbon balance (>95%).

• Increasing temperature and time increases conversion.

• The ratio of azobenzene to azoxybenzene depends on reaction conditions:

  • Higher temperature tends to favor more azoxybenzene at a given conversion.

Again, the catalyst is stable:

• It can be reused with only minor loss in activity.

• Structural analyses before and after catalysis show that the 2D structure and surface states are largely preserved.

• This contradicts the simplistic notion that “MXenes always quickly oxidize and degrade in the presence of oxygen”; in organic solvents and under these conditions, Nb₂C MXene remains stable.

7. Catalytic role #3: Using the same Nb₂C MXene for hydrogenation of azoxybenzene

A particularly nice aspect of this work is showing that the same Nb₂C MXene can also act as a hydrogenation catalyst.

After performing the oxidative coupling of aniline:

1. Air is removed and replaced with H₂ (5 bar).

2. The reaction is continued at elevated temperature in the same reactor and with the same catalyst.

Under these conditions:

• Azoxybenzene is selectively hydrogenated to azobenzene.

• The overall selectivity to azobenzene can reach up to ~98%.

So using one catalyst, in a one-pot, two-step sequence, they can:

1. Oxidize aniline with air to a mixture of azo and azoxy compounds.

2. Hydrogenate the azoxy intermediate to azobenzene, greatly improving azobenzene selectivity.

This showcases Nb₂C MXene as a dual-function oxidation/hydrogenation catalyst, not just an acid–base solid.

8. What do calculations say about how the oxidative coupling works?

To understand how Nb₂C MXene catalyzes the oxidative coupling of aniline, the authors performed theoretical calculations (DFT combined with a neural network potential approach).

They used a model of Nb₂CO₂ MXene with:

• Surface oxygen terminations, and importantly

• Two neighboring oxygen vacancies on the surface

Key mechanistic insights (in simple terms):

1. Aniline adsorption and activation

   • Aniline first adsorbs on a site with an oxygen vacancy.

   • The surface can abstract a hydrogen atom from aniline, forming a surface OH group and a deprotonated aniline.

2. O₂ activation

   • O₂ adsorbs at a nearby oxygen vacancy.

   • One oxygen atom binds strongly to the vacancy; the other becomes activated and participates in forming nitrosobenzene-like intermediates.

3. Formation of nitrosobenzene-type species

   • The activated oxygen reacts with deprotonated aniline to form a Ph–N–O intermediate (nitroso species) bound to the surface.

4. Regeneration of vacancies via water formation

   • Surface OH groups can combine to form water, which then desorbs.

   • This regenerates oxygen vacancies, which serve as active sites.

5. Second aniline and N–N bond formation

   • A second aniline molecule adsorbs near the nitrosobenzene intermediate.

   • After deprotonation, this second aniline attacks the nitroso intermediate, forming the N–N bond and yielding a Ph–NH–NPh species.

6. Final dehydrogenation and azobenzene release

   • A final hydrogen transfer from the N–H group to the surface occurs.

   • Water is again formed and desorbs.

   • Azobenzene is released, and oxygen vacancies are regenerated.

The calculations suggest:

• The cooperation of two neighboring O-vacancies is crucial. A single isolated defect does not support a reasonable full mechanism.

• The rate-determining step is associated with desorption and regeneration of vacancies (i.e., the last step where azobenzene leaves and surface water is removed).

Experimentally, it is consistent with the idea that etching and surface treatment of MXenes create patches of defects, including groups of neighboring vacancies. These patches become highly active catalytic centers.

9. Overall conclusions and bigger picture

This study demonstrates that Nb₂C MXene is far more than a passive support or purely electrochemical material:

• It is an active thermal catalyst for:

  • Aldol condensations (via bifunctional acid–base chemistry)

  • Aerobic oxidative coupling of aniline (oxidation with air)

  • Hydrogenation of azoxybenzene (reduction with H₂)

Several key points stand out:

1. Bifunctional acid–base behavior

   • Even though Nb₂C MXene has low densities of acid and base sites and those sites are only weak–moderate in strength, its TOFs are high, often higher than reference solid acids and bases.

   • The synergy and proximity of acid and base sites on the same 2D surface are more important than the sheer number or strength of single-type sites.

2. Multi-functional redox catalyst

   • The same Nb₂C MXene promotes both oxidation (with air) and hydrogenation (with H₂) in the same system.

   • This enables convenient one-pot sequences, like aniline → azo/azoxy mixture → pure azobenzene.

3. Defects as active sites

   • Calculations support a mechanism where pairs of neighboring oxygen vacancies act as the true active sites.

   • This suggests that controlling defect density and distribution in MXenes could be a powerful way to tune their catalytic properties.

4. Stability under realistic conditions

   • Under the conditions studied (organic solvents, elevated temperatures, air or hydrogen), Nb₂C MXene remains structurally stable, reusable, and shows minimal leaching.

5. Implications for the wider MXene family

   • There are over 70 known MXenes, with many combinations of metals and terminations.

   • This work shows that MXenes are promising candidates for thermal catalysis, not just electro- or photocatalysis.

   • With smart design (choice of metal, surface terminations, defect engineering), MXenes could become a versatile platform to replace or complement traditional oxides, zeolites, and supported metal catalysts.

In short

The paper tells a clear story:

• Nb₂C MXene can function as:

  • a bifunctional acid–base catalyst for aldol condensations,

  • an oxidation catalyst for aniline to azo/azoxy products using air,

  • and a hydrogenation catalyst for converting azoxybenzene to azobenzene with H₂.

• Its activity comes not from high surface area or very strong acidity/basicity, but from:

  • smart 2D architecture,

  • balanced acid–base sites, and

  • defect-rich Nb–O surface chemistry.

This opens the door to systematic exploration of MXenes as thermal catalysts, where composition, terminations, and defects can all be tuned to design catalysts for many different organic transformations.