(187k) Bimetallic Pd-Cu Catalysts for CO2 Hydrogenation to Methanol

Xiao Jiang^1,2, Xiaowa Nie^2,3, Xinwen Guo^2,3, Krista Walton¹, Chunshan Song^2,3*

¹ School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Dr., GA 30332, USA

² Clean Fuels & Catalysis Program, PSU-DUT Joint Center for Energy Research, EMS Energy Institute, The Pennsylvania State University, 209 Academic Projects Building, PA 16802, USA

³ PSU-DUT Joint Center for Energy Research, School of Chemical Engineering,

Dalian University of Technology, Dalian 116024, China

Email: csong@psu.edu

Utilization of CO₂ as a carbon source for synthesizing chemical feedstocks and transportation fuels has recently attracted great attention worldwide. The present work involves Pd-Cu bimetallic catalysts for CO₂ hydrogenation to CH₃OH andseeks to clarify if the combination of Pd and Cu could synergetically promote CH₃OH formation from CO₂/H₂ and to develop a fundamental understanding on the composition-structure-alloy-catalytic property relationship.

A strong synergetic effect on promoting methanol formation was observed over Pd-Cu bimetallic catalysts supported on amorphous silica when the Pd/(Pd+Cu) atomic ratios lied within 0.25-0.34 mol mol^-1. The optimal CH₃OH STY and selectivity were obtained on Pd(0.34)-Cu/SiO₂ (e.g., 0.31 μmol g^-1 s^-1 and 34 mol%, respectively), which was almost 3 times as much as the simple summation of those over the corresponding monometallic catalysts. To examine the origin of the synergetic promotion, the reduced catalysts were characterized by TPR, XRD, TEM, STEM/EDS, and H₂-/CO₂-TPD. The results demonstrated that the metallic phases varied with the Pd/(Pd+Cu) atomic ratios, and uniform alloy phases PdCu₃ and PdCu, were found at Pd/(Pd+Cu) atomic ratio of 0.34 with an average crystallite size around 5 nm. TPD results indicated that the combination of Pd and Cu significantly promoted the adsorption towards weakly-bonded H₂ and CO₂ on the catalyst surface, the amount of which appear to correlate with the improved CH₃OH formation rate. The correlation of alloys with the activity performance indicates that the coexistence of nano-sized and uniform Pd-Cu alloy particles, PdCu₃ and PdCu, played a crucial role in the observed bimetallic CH₃OH promotion.

The catalytic performance was also studied at lower metal loading but maintaining the optimal bimetallic composition. Not only did the metal efficiency-based CH₃OH yield well retained when decreasing the metal loading from 18.7 to 2.4 wt%, but it also exhibited almost 2-fold more CH₃OH than the commercial Cu-ZnO-Al₂O₃ catalyst. Albeit such advantage, the Pd-Cu catalysts with lower metal loading yielded more CO, which is attributed to the Cu-rich alloy surface. Evidently, the product distribution strongly depends on the alloy composition.

The alloy composition-dependent behavior was investigated by both computational and experimental work, wherein PdCu alloy composition is superior for both CO₂ and H₂ activation and subsequent hydrogenation to CH₃OH in comparison to PdCu₃ alloy composition.

These findings would be of critical importance for developing new catalysts and catalytic CO₂ hydrogenation process technology and for fundamental understanding of chemistries involving in CO₂ hydrogenation to CH₃OH