From Lying to Standing: Anchoring Naphthoquinone via Pd─S Bonds on Pd Nanoclusters to Suppress Over-Hydrogenation for Mild H2O2 Synthesis.
Xu Qinghua Q, Tian Guolong G, Yang Zhe Z, Li Lei L et al.
The industrial anthraquinone (AQ) process for H2O2 production is limited by undesired deep hydrogenation of quinone working compound, promoted by cofacial adsorption of its aromatic π-system on Pd surfaces. Here, we report a Pd─S anchoring strategy that constrains a dithiolated naphthoquinone ligand (SNQ), whose quinone moiety serves as a simplified structural model of AQ, in a standing configuration on low-valent Pd nanoclusters. This directional coordination minimizes direct contact between the aromatic framework and Pd while retaining access of the carbonyl groups to activated hydrogen. Supported on XC-72 affords a catalyst that produces H2O2 through sequential H2 hydrogenation and air oxidation under mild conditions (0.2 MPa H2, 30°C), achieving 300 mmol gPd -1 h-1 and maintaining its initial activity over eight cycles. X- ray absorption spectroscopy supports the coexistence of Pd─Pd and Pd─S coordination, whereas in situ diffuse reflectance infrared Fourier transform spectroscopy reveals a reversible quinone/hydroquinone-type redox cycle. Density functional theory calculations further identify carbonyl hydrogenation as energetically favored over aromatic-ring hydrogenation. The Pd─S interface thus electronically promotes H2 activation and geometrically restricts the quinone π-system, spatially decoupling H2 activation from aromatic-ring adsorption and enabling selective, durable quinone-mediated H2O2 synthesis.