(765c) High-Selectivity Synthesis of H2O2 from H2 and O2 over Palladium-Based Catalysts at Ambient Pressure

High-Selectivity
Synthesis of H₂O₂ from H₂ and O₂ over
Palladium-Based Catalysts at Ambient Pressure

Pengfei Tian¹, Doudou Ding¹,
Yang Sun¹, Feixiang Tian¹, Jing Xu^1* and Yi-Fan
Han^1,2* 

¹ State Key Laboratory of
Chemical Engineering, East China University of Science and Technology, Shanghai
200237, China

² Research Center of
Heterogeneous Catalysis and Engineering Sciences, School of Chemical
Engineering and Energy, Zhengzhou University, Zhengzhou 450001, China

*Corresponding
authors: yifanhan@ecust.edu.cn; xujing@ecust.edu.cn

Keywords: hydrogen
peroxide; direct synthesis; palladium; semi-continuous process

Introduction

Hydrogen
peroxide (H₂O₂), as an effective and environmentally benign
oxidant, is applied in textile, waste water treatment, and pharmaceutical
industries. The current method for industrial production of H₂O₂
is the Riedl-Pfleiderer process, while it inherently needs heavy capital and
operating costs.^[1] The synthesis of H₂O₂ directly
from H₂ and O₂ over palladium (Pd) based catalysts is a
promising atom-efficient alternative.^[2-8] However, the direct
process has not yet been used in industrial production of H₂O₂,
since its performance cannot meet the industrial standards. Poor understanding
in the structure of active sites and reaction mechanism restrains the
development of high-selectivity catalysts, raising the need for fundamental
studies of structure-function correlation of Pd-based catalysts at moderate
conditions.

In this work,
density functional theory (DFT) calculations and multiple characterization
techniques including X-ray diffraction (XRD), bright-field transmission
electron microscopy (BF-TEM), high-angle annular dark-field scanning
transmission electron microscopy (HAADF-STEM), X-ray absorption near-edge
structure (XANES), extended X-ray absorption fine structure (EXAFS), in situ
diffuse reflectance infrared Fourier transform spectroscopy of CO adsorption (in
situ CO-DRIFT) and X-ray photoelectron spectroscopy (XPS) were combined to
investigate reaction mechanism and the structure-performance relationship of
Pd-based catalysts. The origin of size effects and support effects is revealed,
and highly selective H₂O₂ synthesis at moderate
conditions (283 K, 0.1 MPa) using a semi-continuous reactor is reported over
Pd-Te and Pd-Bi catalysts.

Methods

Catalysts
were prepared by incipient wetness impregnation using an aqueous solution. All
reactions were carried out in a modified micro triphase semi-batch reactor at
283 K and atmospheric pressure using 50 mg of catalyst; H₂
conversion and H₂O₂ selectivity were analyzed with a gas
chromatograph and a UV¨CVis spectrophotometer, respectively. DFT calculations
were performed using the Vienna ab-initio simulation package (VASP). Cluster
models and slab models were adopted to simulate sub-nanometer clusters and extended
surfaces, respectively.

Results and Discussion

DFT
calculations suggested that the performance of Pd sites is determined primarily by both
coordination and site geometry. The coordinative unsaturated sites generally
possess larger activities and lower selectivities (Fig. 1). Thus, with the
decline in the size of Pd nanoparticles, the increase in low-coordinated active
sites results in the drop of H₂O₂ selectivity. Moreover,
the dissociation of O₂ is the primary factor resulting in the poor
performance of monometallic Pd catalysts. To suppress the dissociative
activation of O₂ over Pd sites, Te, Sb, Se and Bi were added to Pd
to prepare Pd-based bimetallic catalysts. We found that Pd-Te and Pd-Bi
catalysts showed remarkably better performance for H₂O₂
synthesis. In particular, H₂ selectivity of >90% toward H₂O₂
was obtained on a TiO₂ supported Pd-Te catalyst by finely tuning the Pd/Te atomic ratio. The degradation of H₂O₂
by H₂ hydrogenation was also suppressed with the addition of Te (Fig.
2).Transmission electron microscopy (TEM) measurements indicated that these
second metals can promote the dispersion of Pd particles, evidenced by in situ
CO-DRIFTS. Te and Bi are inert to the adsorption and activation of surface
species containing O-O bonds, and they thus have dilute effects on continuous
Pd sites. As a result, the side reactions are markedly restrained, leading to
the enhancement in H₂O₂ selectivity.

Figure 1. Structure-function
correlation of Pd catalysts for direct H₂O₂ synthesis.

Figure 2. Effect of the Te/Pd
atomic ratio on the H₂O₂ decomposition and hydrogenation.

Conclusion

Monometallic
Pd sites, especially coordinative unsaturated sites, are high-reactivity sites
for O-O bonds dissociation and unfavorable for H₂O₂
synthesis. By adding second metals as Te and Bi, the dissociative activation of
O₂ is effectively suppressed, leading to the rise of H₂O₂
selectivity. This work makes great advance on adding second nonprecious metal
to Pd to acquire catalysts with high efficiency for H₂O₂
synthesis. Meanwhile, it would benefit the rational design of catalysts for reactions
involving hydrogenation and oxidation, such as oxygen reduction reactions and
fuel cell.

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