Evolution of intermetallic GaPd₂/SiO₂ catalyst and optimization for methanol synthesis at ambient pressure

Abstract

The CO 2 hydrogenation to methanol is efficiently catalyzed at ambient pressure by nanodispersed intermetallic GaPd 2 /SiO 2 catalysts prepared by incipient wetness impregnation. Here we optimize the catalyst in terms of metal content and reduction temperature in relation to its catalytic activity. We find that the intrinsic activity is higher for the GaPd 2 /SiO 2 catalyst with a metal loading of 13 wt.% compared to catalysts with 23 wt.% and 7 wt.%, indicating that there is an optimum particle size for the reaction of around 8 nm. The highest catalytic activity is measured on catalysts reduced at 550°C. To unravel the formation of the active phase, we studied calcined GaPd 2 /SiO 2 catalysts with 23 wt.% and 13 wt.% using a combination of in situ techniques: X-ray diffraction (XRD), X-ray absorption near edge fine structure (XANES) and extended X-ray absorption fine structure (EXAFS). We find that the catalyst with higher metal content reduces to metallic Pd in a mixture of H 2 /Ar at room temperature, while the catalyst with lower metal content retains a mixture of PdO and Pd up to 140°C. Both catalysts form the GaPd 2 phase above 300°C, albeit the fraction of crystalline intermediate Pd nanoparticles of the catalyst with higher metal loading reduces at higher temperature. In the final state, the catalyst with higher metal loading contains a fraction of unalloyed metallic Pd, while the catalyst with lower metal loading is phase pure. We discuss the alloying mechanism leading to the catalyst active phase formation selecting three temperatures: 25°C, 320°C and 550°C.

Intermetallic compounds in catalysis – a versatile class of materials meets interesting challenges

“…Intermetallic Ga-Ni [ 110 , 111 ], Ga-Pd [ 112 ] and Zn-Pd [ 113 ] compounds were applied successfully as catalysts to synthesise methanol from CO 2 and hydrogen at normal pressure – thus opening a condition window which enables to store the fluctuating electricity supply from renewable sources like windpower and photovoltaics in chemical form. In this important step of the methanol economy [ 114 ], the CO 2 is ideally sequestered from air and the necessary hydrogen comes from electrochemical water splitting.…”

Mechanistic Study of Carbon Dioxide Hydrogenation over Pd/ZnO‐Based Catalysts: The Role of Palladium–Zinc Alloy in Selective Methanol Synthesis

“…However, limited activity and poor selectivity to methanol due to the reverse water gas shift reaction (RWGS) have forced the investigation of alternative catalyst compositions, for instance: In 2 O 3 based materials;[ 1 , 2 , 3 , 4 ] palladium‐containing catalysts;[ 5 , 6 , 7 ] and gallium‐based intermetallic compounds. [ 8 , 9 , 10 ] Among these Pd/ZnO catalysts, which show good activity and high selectivity to methanol, are of a great scientific interest. [ 11 , 12 , 13 , 14 , 15 ]…”

“…However,limited activity and poor selectivity to methanol due to the reverse water gas shift reaction (RWGS) have forced the investigation of alternative catalyst compositions,f or instance:I n 2 O 3 based materials; [1][2][3][4] palladium-containingc atalysts; [5][6][7] and gallium-based intermetallic compounds. [8][9][10] Among these Pd/ ZnO catalysts,w hich show good activity and high selectivity to methanol, are of ag reat scientific interest. [11][12][13][14][15] In order to improve the performance of catalysts for methanol synthesis from carbon dioxide,anunderstanding of the reaction mechanism, and the structure of the active sites, is of ac onsiderable importance.U nder ah ydrogen rich atmosphere and at elevated temperature (!…”

Mechanistic Study of Carbon Dioxide Hydrogenation over Pd/ZnO‐Based Catalysts: The Role of Palladium–Zinc Alloy in Selective Methanol Synthesis

“…The commercial catalyst for methanol synthesis from syngas (Cu/ZnO/Al 2 O 3 ), as well as other copper‐based materials, such as Cu/ZrO 2 , Cu/CeO 2 , Cu/ZnO/ZrO 2 and Cu/Mo 2 C, are often the first choice for studying carbon dioxide hydrogenation to methanol. However, limited activity and poor selectivity to methanol due to the reverse water gas shift reaction (RWGS) have forced the investigation of alternative catalyst compositions, for instance: In 2 O 3 based materials; [1–4] palladium‐containing catalysts; [5–7] and gallium‐based intermetallic compounds [8–10] . Among these Pd/ZnO catalysts, which show good activity and high selectivity to methanol, are of a great scientific interest [11–15] …”