Pd/ZnO catalysts for direct CO2 hydrogenation to methanol

Development and Application of Carbon-Layer-Stabilized, Nitrogen-Doped, Bamboo-Like Carbon Nanotube Catalysts in CO2 Hydrogenation

Nitrogen-doped, bamboo-like carbon nanotubes (BCNTs) were synthesized from butylamine by catalytic chemical vapor deposition (CCVD method). The nanotubes were oxidized by H2SO4/HNO3 treatment and used to prepare calcium alginate gelled BCNT spheres. These beads were first carbonized and then Pd, Rh and Ni nanoparticles were anchored on the surface of the spheres. These systems were then applied as catalysts in CO2 hydrogenation. The BCNT support was examined by Raman spectroscopy, dynamic light scattering (DLS) and X-ray photoelectron spectroscopy (XPS). The prepared catalysts were characterized by HRTEM and SEM. The oxidation pretreatment of BCNTs was successful, with the electrokinetic potential of the water-based dispersion of BCNTs measuring -59.9 mV, meaning the nanotube dispersion is stable. Pyridinic and graphitic types of incorporated nitrogen centers were identified in the structure of the nanotubes, according to the XPS measurements. The Pd-containing BCNT sphere catalyst was the most efficient in the catalytic studies. The highest conversion was reached on the Pd catalyst at 723 K, as well as at 873 K. The difference in the formation rate of CO was much less at 873 K between the Pd and Rh compared to the 723 K values. Accordingly, the application of Pd-containing BCNT/carbon-supported catalyst favored the generation of CO. However, the Ni-BCNT/carbon catalyst leads to the formation of CH4 as the major product.

Highly Dispersed Metal Carbide on ZIF-Derived Pyridinic-N-Doped Carbon for CO2 Enrichment and Selective Hydrogenation

Catalytic conversion of CO₂ into chemicals is a critical issue for energy and environmental research. Among such reactions, converting CO₂ into CO has been regarded as a significant foundation to generate a liquid fuels and chemicals on a large scale. In this work, zeolitic imidazolate framework-derived N-doped carbon-supported metal carbide catalysts (M/ZIF-8-C; M=Ni, Fe, Co and Cu) with highly dispersed metal carbide were prepared for selective CO₂ hydrogenation. Under the same metal loadings, catalytic activity for CO₂ hydrogenation to CO follows the order: Ni/ZIF-8-C≈Fe/ZIF-8-C>Co/ZIF-8-C>Cu/ZIF-8-C. These catalysts are composed of carbide or metal supported on pyridinic N sites within the N-doped carbon structure. ZIF-8-derived pyridinic nitrogen and carbide effect CO₂ adsorption, whereas dispersed Ni or Fe carbide and metal species serve as an active site for CO₂ hydrogenation. The supported Ni catalyst exhibits extraordinary catalytic performance, which results from high dispersion of the metal and exposure of the carbide. Based on high-sensitivity low-energy ion scattering (HS-LEIS) and line scan results, density functional theory (DFT) was used to understand reaction mechanism of selective CO₂ hydrogenation over Ni/ZIF-8-C. The product CO is derived mainly from the direct cleavage of C-O bonds in CO₂ * rather than decomposition of COOH*. The CO* desorption energy on Ni/ZIF-8-C is lower than that for further hydrogenation and dissociation. Comparison of Ni/ZIF-8-C with ZIF-8-C indicates that the combined effects of the highly dispersed metal or carbide and weak CO adsorption result in high CO selectivity for CO₂ hydrogenation.

Solvent Free Synthesis of PdZn/TiO2 Catalysts for the Hydrogenation of CO2 to Methanol

Catalytic upgrading of CO₂ to value-added chemicals is an important challenge within the chemical sciences. Of particular interest are catalysts which are both active and selective for the hydrogenation of CO₂ to methanol. PdZn alloy nanoparticles supported on TiO₂ via a solvent-free chemical vapour impregnation method are shown to be effective for this reaction. This synthesis technique is shown to minimise surface contaminants, which are detrimental to catalyst activity. The effect of reductive heat treatments on both structural properties of PdZn/TiO₂ catalysts and rates of catalytic CO₂ hydrogenation are investigated. PdZn nanoparticles formed upon reduction showed high stability towards particle sintering at high reduction temperature with average diameter of 3–6 nm to give 1710 mmol kg⁻¹ h of methanol. Reductive treatment at high temperature results in the formation of ZnTiO₃ as well as PdZn, and gives the highest methanol yield.

Condensed-phase low temperature heterogeneous hydrogenation of CO2 to methanol

A low-temperature CH₃OH synthesis was achieved at 120–170 °C using tertiary amine and alcohol in the presence of a Cu/ZnO/Al₂O₃ catalyst by CO₂ hydrogenation.

Highly selective PdZn/ZnO catalysts for the methanol steam reforming reaction

Catalysts were prepared by impregnation-based method involving excess Cl⁻ anion addition to the metal chloride precursors which resulted in improved mixing of metals and led to formation of highly ordered PdZn alloys responsible for high catalytic selectivity.

Bimetallic catalysts for green methanol production via CO2 and renewable hydrogen: a mini-review and prospects

This mini review discusses the recent advancements in the use of bimetallic catalysts for green methanol production via CO₂ hydrogenation.

DFT study of In2O3-catalyzed methanol synthesis from CO2 and CO hydrogenation on the defective site

The activity of CO hydrogenation is higher than that of CO₂ hydrogenation on the defective In₂O₃ surface.

Sustainable Conversion of Carbon Dioxide: An Integrated Review of Catalysis and Life Cycle Assessment

CO₂ conversion covers a wide range of possible application areas from fuels to bulk and commodity chemicals and even to specialty products with biological activity such as pharmaceuticals. In the present review, we discuss selected examples in these areas in a combined analysis of the state-of-the-art of synthetic methodologies and processes with their life cycle assessment. Thereby, we attempted to assess the potential to reduce the environmental footprint in these application fields relative to the current petrochemical value chain. This analysis and discussion differs significantly from a viewpoint on CO₂ utilization as a measure for global CO₂ mitigation. Whereas the latter focuses on reducing the end-of-pipe problem "CO₂ emissions" from todays' industries, the approach taken here tries to identify opportunities by exploiting a novel feedstock that avoids the utilization of fossil resource in transition toward more sustainable future production. Thus, the motivation to develop CO₂-based chemistry does not depend primarily on the absolute amount of CO₂ emissions that can be remediated by a single technology. Rather, CO₂-based chemistry is stimulated by the significance of the relative improvement in carbon balance and other critical factors defining the environmental impact of chemical production in all relevant sectors in accord with the principles of green chemistry.

A highly selective and stable ZnO-ZrO2 solid solution catalyst for CO2 hydrogenation to methanol

Although methanol synthesis via CO hydrogenation has been industrialized, CO₂ hydrogenation to methanol still confronts great obstacles of low methanol selectivity and poor stability, particularly for supported metal catalysts under industrial conditions. We report a binary metal oxide, ZnO-ZrO₂ solid solution catalyst, which can achieve methanol selectivity of up to 86 to 91% with CO₂ single-pass conversion of more than 10% under reaction conditions of 5.0 MPa, 24,000 ml/(g hour), H₂/CO₂ = 3:1 to 4:1, 320° to 315°C. Experimental and theoretical results indicate that the synergetic effect between Zn and Zr sites results in the excellent performance. The ZnO-ZrO₂ solid solution catalyst shows high stability for at least 500 hours on stream and is also resistant to sintering at higher temperatures. Moreover, no deactivation is observed in the presence of 50 ppm SO₂ or H₂S in the reaction stream.

Abiotic Synthesis with the C-C Bond Formation in Ethanol from CO2 over (Cu,M)(O,S) Catalysts with M = Ni, Sn, and Co

We demonstrate copper-based (Cu,M)(O,S) oxysulfide catalysts with M = Ni, Sn, and Co for the abiotic chemical synthesis of ethanol (EtOH) with the C-C bond formation by passing carbon dioxide (CO₂) through an aqueous dispersion bath at ambient environment. (Cu,Ni)(O,S) with 12.1% anion vacancies had the best EtOH yield, followed by (Cu,Sn)(O,S) and (Cu,Co)(O,S). The ethanol yield with 0.2 g (Cu,Ni)(O,S) catalyst over a span of 20 h achieved 5.2 mg. The ethanol yield is inversely proportional to the amount of anion vacancy. The kinetic mechanism for converting the dissolved CO₂ into the C₂ oxygenate is proposed. Molecular interaction, pinning, and bond weakening with anion vacancy of highly strained catalyst, the electron hopping at Cu⁺/Cu²⁺ sites, and the reaction orientation of hydrocarbon intermediates are the three critical issues in order to make the ambient chemical conversion of inorganic CO₂ to organic EtOH with the C-C bond formation in water realized. On the other hand, Cu(O,S) with the highest amount of 22.7% anion vacancies did not produce ethanol due to its strain energy relaxation opposing to the pinning and weakening of O-H and C-O bonds.

Challenges in the Greener Production of Formates/Formic Acid, Methanol, and DME by Heterogeneously Catalyzed CO2 Hydrogenation Processes

The recent advances in the development of heterogeneous catalysts and processes for the direct hydrogenation of CO₂ to formate/formic acid, methanol, and dimethyl ether are thoroughly reviewed, with special emphasis on thermodynamics and catalyst design considerations. After introducing the main motivation for the development of such processes, we first summarize the most important aspects of CO₂ capture and green routes to produce H₂. Once the scene in terms of feedstocks is introduced, we carefully summarize the state of the art in the development of heterogeneous catalysts for these important hydrogenation reactions. Finally, in an attempt to give an order of magnitude regarding CO₂ valorization, we critically assess economical aspects of the production of methanol and DME and outline future research and development directions.

CuMnOS Nanoflowers with Different Cu+/Cu2+ Ratios for the CO2-to-CH3OH and the CH3OH-to-H2 Redox Reactions

A conservative CO₂-Methanol (CH₃OH) regeneration cycle, to capture and reutilize the greenhouse gas of CO₂ by aqueous hydrogenation for industry-useful CH₃OH and to convert aqueous CH₃OH solution by dehydrogenation for the clean energy of hydrogen (H₂), is demonstrated at normal temperature and pressure (NTP) with two kinds of CuMnOS nanoflower catalysts. The [Cu⁺]-high CuMnOS led to a CH₃OH yield of 21.1 mmol·g⁻¹catal.·h⁻¹ in the CuMnOS-CO₂-H₂O system and the other [Cu⁺]-low one had a H₂ yield of 7.65 mmol·g⁻¹catal.·h⁻¹ in the CuMnOS-CH₃OH-H₂O system. The successful redox reactions at NTP rely on active lattice oxygen of CuMnOS catalysts and its charge (hole or electron) transfer ability between Cu⁺ and Cu²⁺. The CO₂-hydrogenated CH₃OH in aqueous solution is not only a fuel but also an ideal liquid hydrogen storage system for transportation application.

Formation and stability of an active PdZn nanoparticle catalyst on a hydrotalcite-based support for ethanol dehydrogenation

A subsequent hydrogen–air treatment prior to reaction is important for a highly active innovative nanoparticle PdZn catalyst for ethanol dehydrogenation.

PdZn catalysts for CO2 hydrogenation to methanol using chemical vapour impregnation (CVI)

The formation of PdZn bimetallic alloys on ZnO, TiO₂ and Al₂O₃ supports was investigated, together with the effect of alloy formation on the CO₂ hydrogenation reaction. The chemical vapour impregnation (CVI) method produced PdZn nanoparticles with diameters of 3–6 nm. X-ray photoelectron spectroscopy and X-ray diffraction revealed the changes in the structure of the PdZn alloy that help stabilise formate intermediates during methanol synthesis. PdZn supported on TiO₂ exhibits high methanol productivity of 1730 mmol kg_cat⁻¹ h⁻¹ that is associated with the high dispersion of the supported PdZn alloy.

The effect of sodium species on methanol synthesis and water–gas shift Cu/ZnO catalysts: utilising high purity zincian georgeite

The effect of sodium species on the physical and catalytic properties of Cu/ZnO catalysts derived from zincian georgeite has been investigated. Catalysts prepared with <100 ppm to 2.1 wt% Na⁺, using a supercritical CO₂ antisolvent technique, were characterised and tested for the low temperature water–gas shift reaction and also CO₂ hydrogenation to methanol. It was found that zincian georgeite catalyst precursor stability was dependent on the Na⁺ concentration, with the 2.1 wt% Na⁺-containing sample uncontrollably ageing to malachite and sodium zinc carbonate. Samples with lower Na⁺ contents (<100–2500 ppm) remained as the amorphous zincian georgeite phase, which on calcination and reduction resulted in similar CuO/Cu particle sizes and Cu surface areas. The aged 2.1 wt% Na⁺ containing sample, after calcination and reduction, was found to comprise of larger CuO crystallites and a lower Cu surface area. However, calcination of the high Na⁺ sample immediately after precipitation (before ageing) resulted in a comparable CuO/Cu particle size to the lower (<100–2500 ppm) Na⁺ containing samples, but with a lower Cu surface area, which indicates that Na⁺ species block Cu sites. Activity of the catalysts for the water–gas shift reaction and methanol yields in the methanol synthesis reaction correlated with Na⁺ content, suggesting that Na⁺ directly poisons the catalyst. In situ XRD analysis showed that the ZnO crystallite size and consequently Cu crystallite size increased dramatically in the presence of water in a syn-gas reaction mixture, showing that stabilisation of nanocrystalline ZnO is required. Sodium species have a moderate effect on ZnO and Cu crystallite growth rate, with lower Na⁺ content resulting in slightly reduced rates of growth under reaction conditions.

A review of the catalytic hydrogenation of carbon dioxide into value-added hydrocarbons

We review the indirect and direct routes for CO₂ hydrogenation to hydrocarbons and recent developments in catalyst design, performance and mechanism.

DFT study of CO2 conversion on InZr3(110) surface

The InZr₃ alloy is a potential candidate catalyst for methanol and methane synthesis from CO₂ hydrogenation.