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

Authors: 
Tian, P., East China University of Science and Technology
Sun, Y., East China University of Science and Technology
Ding, D., State Key Laboratory of Chemical Engineering, East China University of Science and Technology
Tian, F., East China University of Science and Technology
Xu, J., East China University of Science and Technology
Han, Y. F., East China University of Science and Technology

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 Xu1* and Yi-Fan
Han1,2* 

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

margin-left:0cm;line-height:125%">*Corresponding
authors: yifanhan@ecust.edu.cn; xujing@ecust.edu.cn

margin-left:0cm;text-align:justify;text-justify:inter-ideograph;line-height:
125%">Keywords: hydrogen
peroxide; direct synthesis; palladium; semi-continuous process

text-justify:inter-ideograph;line-height:125%">Introduction

inter-ideograph;line-height:125%"> line-height:125%;font-family:" times new roman>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.

inter-ideograph;line-height:125%"> line-height:125%;font-family:" times new roman>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.

justify;text-justify:inter-ideograph;line-height:125%">Methods

inter-ideograph;line-height:125%"> line-height:125%;font-family:" times new roman>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.

justify;text-justify:inter-ideograph;line-height:125%">Results and Discussion

inter-ideograph;line-height:125%"> line-height:125%;font-family:" times new roman>DFT
calculations suggested that line-height:125%;font-family:" times new roman> 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. W font-family:" times new roman>e 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 125%;font-family:" times new roman>TiO2 font-family:" times new roman>supported Pd-Te catalyst by finely tuning the Pd/Te atomic ratio. The 125%;font-family:" times new roman> 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. font-family:" times new roman>As a result, the side reactions are markedly restrained, leading to
the enhancement in H2O2 selectivity.

text-align:center;line-height:125%"> line-height:125%;font-family:" times new roman>

line-height:125%"> font-family:" times new roman>Figure 1. Structure-function
correlation of Pd catalysts for direct H2O2 synthesis.

line-height:125%"> font-family:" times new roman>

line-height:125%"> font-family:" times new roman>Figure 2. Effect of the Te/Pd
atomic ratio on the H2O2 decomposition and hydrogenation.

justify;text-justify:inter-ideograph;line-height:125%">Conclusion

inter-ideograph;line-height:125%"> line-height:125%;font-family:" times new roman>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.

justify;text-justify:inter-ideograph;line-height:125%">References

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