Sintering Ag33 Nanoclusters on TiO2 Nanoparticles as an Efficient Catalyst for Nitroarene Reduction

At a Glance

Metadata	Details
Publication Date	2024-12-14
Journal	Materials
Authors	Weihua Zhang, Wenwen Yang, Jianglu Yuan, Huiping Zhao, Qing‐Wen Han
Institutions	Wuhan Institute of Technology, Hefei General Machinery Research Institute (China)
Analysis	Full AI Review Included

Executive Summary

This research details a novel, highly efficient silver catalyst derived from sintered Ag₃₃ nanoclusters (NCs) supported on TiO₂, designed for the selective reduction of nitroarenes.

Core Achievement: Successful preparation of polydispersed Ag species anchored on oxygen-deficient TiO₂ (TiO_2-x) via a controlled calcination (sintering) process.
Precursor Strategy: Atomically precise Ag₃₃ NCs, protected by organic ligands (p-BMTC and PPh₃), were used as precursors to ensure uniform starting material and controlled decomposition.
Activation Mechanism: Calcination under N₂ atmosphere removes the organic ligands by extracting lattice oxygen from the TiO₂ support, generating CO₂, SO₂, and H₂O vapor. This process simultaneously creates abundant oxygen vacancies (TiO_2-x) and sinters the Ag NCs in situ.
Optimal Performance: The catalyst calcined at the optimal temperature of 400 °C achieved complete conversion of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in only 30 seconds, demonstrating superior catalytic activity.
Active Site Identification: The high efficiency is attributed to the polydispersed Ag species (nanoparticles, clusters, and single atoms) anchored on the oxygen-deficient TiO₂ surface, which act as stable active centers.
Stability and Reusability: The catalyst exhibited impressive stability, maintaining its high catalytic efficiency even after nine consecutive reaction cycles.
Engineering Value: Provides a facile, scalable calcination method using ligand-protected NCs to fabricate highly active metal/metal oxide nanocomposites with engineered surface defects (oxygen vacancies).

Technical Specifications

Parameter	Value	Unit	Context
Optimal Calcination Temperature	400	°C	Temperature yielding maximum catalytic activity.
Calcination Atmosphere	N₂	N/A	Used for ligand removal and oxygen vacancy formation.
N₂ Flow Rate	100	mL/min	Gas flow during the sintering process.
4-NP Conversion Time (Optimal)	30	s	Time required for complete conversion (400 °C catalyst).
Ag Species Size (Fresh NCs)	less than 5	nm	Diameter of Ag₃₃ nanodots before calcination.
Ag 3d XPS Binding Energy Shift	~0.6 (Red Shift)	eV	Shift after calcination, indicating Ag-O bond formation/interaction with TiO₂.
Ti 2p/O 1s XPS Binding Energy Shift	~0.2 (Red Shift)	eV	Shift after calcination, indicating oxygen vacancy formation (TiO_2-x).
Oxygen Vacancy EPR Signal	2.003	N/A	Characteristic g-factor signal observed after N₂ calcination.
TGA Mass Loss Period 1	6	wt%	Associated with removal of alkane chains (max at 200 °C).
TGA Mass Loss Period 2	2.5	wt%	Associated with removal of remaining C-S species (optimal at ~500 °C).
AIMD Simulation Temperature	1500 and 3000	K	Unrealistic temperatures used to accelerate simulation of ligand removal and sintering.

Key Methodologies

The preparation and activation of the Ag species-modified TiO₂ catalyst involved precise synthesis and controlled thermal treatment:

Ag₃₃ Nanocluster Synthesis: Ag₃₃(p-BMTC)₂₄(PPh₃)₄ NCs were synthesized via a standard reduction method using AgNO₃ and 4-chlorobenzyl mercaptan, followed by reduction with aqueous NaBH₄ at 10 °C.
Catalyst Loading (Ag₃₃/TiO₂): The Ag₃₃ NCs were loaded onto commercial TiO₂ (P25) powder using a simple evaporation method. Ag₃₃ was dissolved in dichloromethane (DCM), mixed with TiO₂, and the solvent was completely evaporated at 30 °C.
Thermal Activation (Sintering): The Ag₃₃/TiO₂ powder was placed in a tubular furnace and heated at a rate of 5 °C/min under a continuous flow of nitrogen gas (100 mL/min).
Optimization: Samples were calcined at various temperatures (200 °C, 300 °C, 400 °C, 500 °C, 600 °C) for 2 hours to determine the optimal sintering conditions (400 °C).
Catalytic Evaluation: The reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) using NaBH₄ was monitored via UV-vis spectrophotometry by tracking the characteristic absorption peak changes at 400 nm.
Structural Characterization: TEM/HAADF confirmed the decomposition of NCs and the formation of polydispersed Ag species. XPS and EPR confirmed the formation of oxygen vacancies (TiO_2-x) and the chemical state of the anchored Ag species.
Computational Modeling: Ab initio Molecular Dynamics (AIMD) simulations were performed to model the structural degradation of the Ag₃₃ cluster and the formation of Ag-O bonds on the TiO₂ surface during high-temperature ligand removal.

Commercial Applications

This technology is highly relevant to industries requiring efficient, selective, and sustainable chemical synthesis processes, particularly those focused on heterogeneous catalysis and advanced material engineering.

Pharmaceutical and Fine Chemical Synthesis: Production of aniline derivatives, which are crucial intermediates for manufacturing drugs, agrochemicals, and specialty chemicals. The high selectivity and conversion rate (30 s) offer significant process intensification benefits over traditional stoichiometric methods.
Green Catalysis and Sustainability: Provides a highly reusable, non-precious metal-based catalyst (Ag instead of Pd, Au, or Ru) for hydrogenation reactions, reducing reliance on expensive precious metals and minimizing hazardous solid waste associated with traditional reducing agents (e.g., tin or iron).
Advanced Materials Engineering: The method uses atomically precise nanoclusters as building blocks for designing highly efficient catalysts. This approach can be generalized to fabricate other metal species-loaded metal oxide nanocomposites (like TiO₂, SiO₂, or SiC) with tailored surface defects (oxygen vacancies) for various catalytic or photocatalytic applications.
Industrial Hydrogenation Processes: Applicable to various selective hydrogenation reactions where the nitroarene contains other reducible groups (C=O, C=C, C-Br), leveraging the high selectivity often associated with supported Ag catalysts.

View Original Abstract

Polydispersed Ag species-modified TiO2 samples with abundant oxygen vacancies were successfully prepared through the calcination of atomically precise Ag33 nanocluster-loaded TiO2 at an optimal temperature under a nitrogen atmosphere. The ligands of the Ag33 nanoclusters are removed by extracting lattice oxygen from TiO2 during the calcination, leading to the formation of CO2, SO2, and H2O vapor. This process simultaneously induces Ag species sintering on the surface of TiO2. The resulting nanocomposites exhibited excellent catalytic activity for the reduction of nitroarenes with NaBH4 as the reductant. This is attributed to the produced Ag species on the oxygen-deficient TiO2, which act as active centers for the catalytic process.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma17246120

References

2023 - Electronic interaction and oxygen vacancy engineering of g-C3N4/α-Bi2O3 Z-scheme heterojunction for enhanced photocatalytic aerobic oxidative homo-/hetero-coupling of amines to imines in aqueous phase [Crossref]
2020 - Pd immobilization on the multi-amine functionalized halloysite as an efficient catalyst for hydrogenation reaction: An experimental and computational study [Crossref]
2022 - Cooperation of Pt and TiOx in the hydrogenation of nitrobenzothiazole [Crossref]
2022 - Theoretical study on nitrobenzene hydrogenation by N-doped carbon-supported late transition metal single-atom catalysts [Crossref]
2019 - One-step rapid and facile synthesis of subnanometer-sized Pd6(C12H25S)11 clusters with ultra-high catalytic activity for 4-nitrophenol reduction. ACS Sustain
2018 - Review on selective hydrogenation of nitroarene by catalytic, photocatalytic and electrocatalytic reactions [Crossref]
2024 - Reversible interconversion between Ag2 and Ag6 clusters and their responsive optical properties
2022 - Identifying the real chemistry of the synthesis and reversible transformation of AuCd bimetallic clusters [Crossref]
2024 - Octahedral vs tiara-like Pd6(SR)12 clusters
2019 - Pd-mediated synthesis of Ag33 chiral nanocluster with core-shell structure in T point group [Crossref]