Palladium-Copper Bimetallic Catalysts for Selective Carbon Dioxide Hydrogenation to Methanol

Open Access
Author:
Jiang, Xiao
Graduate Program:
Energy and Mineral Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 09, 2015
Committee Members:
  • Chunshan Song, Dissertation Advisor
  • Chunshan Song, Committee Chair
  • Sarma V Pisupati, Committee Member
  • Randy Lee Vander Wal, Committee Member
  • Michael John Janik, Committee Member
Keywords:
  • CO2 hydrogenation
  • Methanol
  • Pd-Cu bimetallic catalysts
  • Pd-Cu alloy.
Abstract:
Catalytic CO2 hydrogenation for synthesis of methanol has attracted significant attention recently as a way of recycling carbon dioxide as a resource. In the past decades, most prior works focused on Cu-based and supported Pd-based catalysts to hydrogenate CO2 to CH3OH. Cu and Pd were proposed to have different affinities towards the adsorption of CO2 and H2, respectively. However, little attention has been paid to the effect of Pd-Cu bimetallic catalysts on the methanol synthesis from CO2 hydrogenation. Thus, this work aims at studying the effect of combining Pd and Cu on the activity and selectivity of methanol synthesis via CO2 hydrogenation and developing a fundamental understanding on composition-structure-activity relationship. The Pd-Cu bimetallic catalysts with various compositions were prepared and examined in CO2 hydrogenation at relatively mild reaction conditions (523 K and 4.1 MPa). A strong synergistic effect was observed over Pd–Cu bimetallic catalysts supported on silica at specific compositions as evidenced from the superior methanol formation rate and selectivity in comparison to monometallic catalysts, and the optimal Pd/(Pd+Cu) atomic ratios lied in the range of 0.25-0.34. The methanol formation rate over Pd(8.7)-Cu(10)/SiO2 was almost three times higher than the simple sum of those over monometallic Cu and Pd catalysts. To investigate the composition-structure-activity relationship, the Pd-Cu bimetallic catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning transmission electron microscopy coupled with energy dispersive X-ray spectroscopy (STEM/EDS), X-ray photoelectron spectroscopy (XPS), hydrogen temperature-programmed reduction (H2-TPR), and temperature-programmed desorption (H2-, CO2-, & CO-TPD). Detailed characterization results demonstrated the importance of two well-dispersed Pd–Cu alloy particles (PdCu3 and PdCu) for the observed methanol promotion over Pd–Cu bimetallic catalysts. DRIFTS spectra revealed that the incorporation of Pd and Cu greatly improved the formation of formate and CO species during the CO2 hydrogenation. CO-TPD profiles confirmed the existence of three forms of chemisorbed CO species, and the bonding strength increased in the following order: COL (linear) < COB (di-coordinated bridging) < COH (triple-coordinated bridging). Among them, chemisorbed COB was suggested as a potential intermediate for methanol synthesis because of its moderate bonding strength and desorption temperature range (proximity to reaction temperature). Quantitative analyses showed that the Pd-Cu could promote the chemisorbed COB at specific composition and thus gave rise to the possibility of methanol formation via the following hydrogenation. The selectivity-conversion profile of the Pd–Cu/SiO2 catalyst suggested that CO2 was a primary carbon source for methanol synthesis at lower CO2 conversion, and chemisorbed CO species contributed at higher CO2 conversion within the conversion range examined.