Preparation of Nickel-Gallium based catalysts for carbon dioxide hydrogenation to methanol

Abstract

Abstract Catalytically converting CO2 to methanol by hydrogenation offers a method to effectively reduce the excessive CO2 emission in the atmosphere and produces value-added chemicals simultaneously. Thus, the investigations on catalysts in methanol synthesis reaction has gained attraction in the past few years. Commercial catalysts based on Copper, Zinc, and Zirconium are popular, but increasingly, researchers are looking for other options with superior conversion and selectivity. From prior literature, catalysts based on Nickel and Gallium, specifically a Ni5Ga3 bimetallic catalyst exhibits a similar CO2 conversion and higher methanol yield compared with commercial Copper-based catalysts. However, the purities of Ni5Ga3 catalysts were found to be restricted during the reported synthesis process. Thus, a simpler and reproducible method to prepare highly pure Ni5Ga3 is desirable. In this study, we developed a method to synthesize highly pure Ni5Ga3 catalyst from hydrotalcite-like compounds (HTlc) precursors for CO2 hydrogenation to methanol. A series of Ni-Ga HTlc precursor was synthesized in the temperature range between 90 C and 150 C. The results indicated the HTlc phase in the nickel-gallium precipitant became better crystallized and the structure became more stable as the synthesis reaction temperature increased. Bimetallic alloy Ni5Ga3 was obtained by reducing the as-prepared HTlc precursors in a hydrogen atmosphere. X-ray absorption spectroscopy (XAS) investigation confirmed that a stable and complete HTlc precursor structure assisted in the synthesis of a steady and perfectly structured Ni5Ga3 alloy, where the bond distance of Ni-Ga and cell volume increased with temperature. Ni-Ga HTlc precursor prepared at a hydrothermal temperature of 110 C resulted in the formation of bimetallic alloy, Ni5Ga3, which demonstrated characteristics such as smaller crystal size and stable structure under optimized conditions. The enhanced performance was demonstrated by an endurance test with a constant CO2 conversion and 100% methanol selectivity at 200 C, and the turnover frequency reached 0.27 s-1. Metal oxide promoters are well known to enhance catalytic properties, thus, a modification by incorporating promoters, such as Mg, Zn and Zr, was investigated. A new series of Ni-Ga-X HTlc precursors (X represented Mg, Zn and Zr) were prepared by a similar synthesis procedure, followed by a H2 reduction process. The results revealed that the main Ni-Ga phase transformed from Ni5Ga3 to Ni3Ga when promoters were incorporated in the Ni-Ga catalytic system, due to an unstable HTlc structure as additional elements were incorporated in the parent precursor. Mg and Zr were present as metal oxides, while ZnGa2O4 structure was present in Zn-promoted Ni-Ga catalysts. The BET surface area was measured for all prepared Ni-Ga-X catalysts, and the surface area exhibited a sharp increase after the promoter modifications. Among all samples, the Ni-Ga-Zr revealed the highest BET surface area. TEM-mapping measurements, for Ni-Ga-Zr catalyst, showed Ni-Ga assembly as a core, while Zr surrounded the core, which isolated and separated Ni-Ga catalysts. Thus, the average particle sizes of Ni-Ga-Zr catalysts were considerably decreased compared with other samples, resulting in a relatively large surface area. However, the promotion effect was not obvious in other samples, because Mg could not be completely precipitated in the catalysts and ZnGa2O4 was formed instead. Furthermore, ZrO2 also facilitated the reduction of Ni-Ga-Zr HTlc precursor due to an enhanced electron transfer. Additionally, incorporation of promoters generated additional strong basic sites in the catalytic system, as demonstrated by CO2-TPD measurement. The catalytic properties were evaluated, and a maximum methanol yield (3.8%) was obtained over a Zr-modified Ni3Ga catalyst at 300 C, 30 bar, which exhibited a similar reactivity of commercial Cu-based catalysts. The Ni-Ga-Zr catalysts were subsequently mixed with a commercial high-temperature CO2 adsorbent (MG50). The Ni-Ga-Zr (NGZr) and MG50 were well-mixed, as revealed from SEM images, and the Ni3Ga phase did not change when MG50 was introduced in the Ni-Ga-Zr catalytic system. A series of mixed samples, with different ratios of MG50 and NGZr, was prepared. The corresponding CO2 conversion exhibited a mild decrease as the amount of Ni-Ga-Zr decreased due to loss of active sites, however, the methanol space-time yield was greatly improved as MG50 increased, which suggested that the catalytic property was considerably promoted in the presence of MG50. The highest space-time yield was observed in 25%NGZr/MG50 mixture, with 123.5 gmeth/gcat/h at 300 C. The promotion was ascribed to enhanced CO2 adsorption on MG50 adsorbent, resulting in higher CO2 concentration adjacent to NGZr active sites, contributing to a higher reaction rate and CO2 conversion. Despite the great improvement in methanol space-time yield in NGZr/MG50, the overall CO2 conversion was lower than that of Cu-based catalysts under moderate temperatures, such as 200 C - 250 C. Thus, the NGZr catalysts were subsequently modified by optimizing the Zr amount in the NGZr catalytic system. The TEM-mapping revealed that once the ZrO2 concentration increased above 15%, ZrO2 experienced a severe agglomeration between the Ni-Ga particles instead of surrounding them. Consequently, the interactions between Ni3Ga and ZrO2 was not further increased as Zr content increased from 15% to 25%. The batches of NGZr catalysts were tested for catalytic performance, respectively. The Ni3Ga catalysts with 15% Zr content exhibited a higher CO2 conversion under the entire reaction temperature range when compared to Cu-based catalysts, which indicated that the Ni-Ga-Zr (15%) catalyst area promising candidate for future catalytical CO2 conversion to methanol.