Ni-Phosphide catalysts as versatile systems for gas-phase CO2 conversion: Impact of the support and evidences of structure-sensitivity

We report for the first time the support dependent activity and selectivity of Ni-rich nickel phosphide catalysts for CO2 hydrogenation. New catalysts for CO2 hydrogenation are needed to commercialise the reverse water–gas shift reaction (RWGS) which can feed captured carbon as feedstock for traditionally fossil fuel-based processes, as well as to develop flexible power-to-gas schemes that can synthesise chemicals on demand using surplus renewable energy and captured CO2. Here we show that Ni2P/SiO2 is a highly selective catalyst for RWGS, producing over 80% CO in the full temperature range of 350–750 °C. This indicates a high degree of suppression of the methanation reaction by phosphide formation, as Ni catalysts are known for their high methanation activity. This is shown to not simply be a site blocking effect, but to arise from the formation of a new more active site for RWGS. When supported on Al2O3 or CeAl, the dominant phase of as synthesized catalysts is Ni12P5. These Ni12P5 catalysts behave very differently compared to Ni2P/SiO2, and show activity for methanation at low temperatures with a switchover to RWGS at higher temperatures (reaching or approaching thermodynamic equilibrium behaviour). This switchable activity is interesting for applications where flexibility in distributed chemicals production from captured CO2 can be desirable. Both Ni12P5/Al2O3 and Ni12P5/CeAl show excellent stability over 100 h on stream, where they switch between methanation and RWGS reactions at 50–70% conversion. Catalysts are characterized before and after reactions via X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), temperature-programmed reduction and oxidation (TPR, TPO), Transmission Electron Microscopy (TEM), and BET surface area measurement. After reaction, Ni2P/SiO2 shows the emergence of a crystalline Ni12P5 phase while Ni12P5/Al2O3 and Ni12P5/CeAl both show the crystalline Ni3P phase. While stable activity of the latter catalysts is demonstrated via extended testing, this Ni enrichment in all phosphide catalysts shows the dynamic nature of the catalysts during operation. Moreover, it demonstrates that both the support and the phosphide phase play a key role in determining selectivity towards CO or CH4.Financial support for this work was provided by the Department of Chemical and Process Engineering at the University of Surrey and CO2ChemUK through the EPSRC grant EP/P026435/1 as well as the Royal Society Research Grant RSGR1180353. This work was also partially sponsored by Ministry of Science and Innovation through the projects PID2019-108453 GB-C21 and JC2019-040560-I. This work was also partially sponsored by the European Commission through the H2020-MSCA-RISE-2020 BIOALL project (Grant Agreement: 101008058. SASOL is also acknowledged for kindly providing the Al2O3-based supports

Engineering exsolved catalysts for CO2 conversion

Introduction: Innovating technologies to efficiently reduce carbon dioxide (CO2) emission or covert it into useful products has never been more crucial in light of the urgent need to transition to a net-zero economy by 2050. The design of efficient catalysts that can make the above a viable solution is of essence. Many noble metal catalysts already display high activity, but are usually expensive. Thus, alternative methods for their production are necessary to ensure more efficient use of noble metals. Methods: Exsolution has been shown to be an approach to produce strained nanoparticles, stable against agglomeration while displaying enhanced activity. Here we explore the effect of a low level of substitution of Ni into a Rh based A-site deficienttitanate aiming to investigate the formation of more efficient, low loading noblemetal catalysts. Results: We find that with the addition of Ni in a Rh based titanate exsolution is increased by up to ∼4 times in terms of particle population which in turn results in up to 50% increase in its catalytic activity for CO2 conversion. Discussion: We show that this design principle not only fulfills a major research need in the conversion of CO2 but also provides a step-change advancement in the design and synthesis of tandem catalysts by the formation of distinct catalytically active sites

Dual function materials for CO₂ capture and conversion using renewable H₂

The accumulation of CO₂ emissions in the atmosphere due to industrialization is being held responsible for climate change with increasing certainty by the scientific community. In order to prevent its further accumulation in the atmosphere, CO₂ must be captured for storage or converted to useful products. Current materials and processes for CO₂ capture are energy intensive. We report a feasibility study of dual function materials (DFM), which capture CO₂ from an emission source and at the same temperature (320 °C) in the same reactor convert it to synthetic natural gas, requiring no additional heat input. The DFM consists of Ru as methanation catalyst and nano dispersed CaO as CO₂ adsorbent, both supported on a porous γ-Al₂O₃ carrier. A spillover process drives CO₂ from the sorbent to the Ru sites where methanation occurs using stored H₂ from excess renewable power. This approach utilizes flue gas sensible heat and eliminates the current energy intensive and corrosive capture and storage processes without having to transport captured CO₂ or add external heat

CO₂ utilization with a novel dual function material (DFM) for capture and catalytic conversion to synthetic natural gas: An update

Dual function materials (DFMs) for CO₂ capture and conversion couple the endothermic CO₂ desorption step of a traditional adsorbent with the exothermic hydrogenation of CO₂ over a catalyst in a unique way; a single reactor operating at an isothermal temperature (320 °C) and pressure (1atm) can capture CO₂ from flue gas, and release it as methane upon exposure to renewable hydrogen. This combined CO₂ capture and utilization eliminates the energy intensive CO₂ desorption step associated with conventional CO₂ capture systems as well as avoiding the problem of transporting concentrated CO₂ to another site for storage or utilization. Here DFMs containing Rh and dispersed CaO have been developed (˃1% Rh 10% CaO/γ-Al₂O₃) which have improved performance compared to the 5% Ru 10% CaO/γ-Al₂O₃ DFM (0.50 g-mol CH₄/kg DFM) developed previously. Ruthenium remains the catalyst of choice due to its lower price and excellent low temperature performance. The role of CO₂ adsorption capacity on the final methanation capacity of the DFM has also been investigated by testing several new sorbents. Two novel DFM compositions are reported here (5% Ru 10% K₂CO₃/Al₂O₃ and 5% Ru 10% Na₂CO₃/Al₂O₃) both of which have much greater methanation capacities (0.91 and 1.05 g-mol CH₄/kg DFM) compared to the previous 5% Ru 10% CaO/γ-Al₂O₃ DFM

Kinetics of CO₂ methanation over Ru/γ-Al₂O₃ and implications for renewable energy storage applications

Kinetics of CO₂ hydrogenation over a 10% Ru/γ-Al₂O₃ catalyst were investigated using thermogravimetric analysis and a differential reactor approach at atmospheric pressure and 230–245 °C. The data is consistent with an Eley–Rideal mechanism where H2 gas reacts with adsorbed CO₂ species. Activation energy, pre-exponential factor and reaction orders with respect to CO₂, H₂, CH₄, and H₂O were determined to develop an empirical rate equation. Methane was the only hydrocarbon product observed during CO₂ hydrogenation. The activation energy was found to be 66.1 kJ/g-mole CH₄. The reaction order for H₂ was 0.88 and for CO₂ 0.34. Product reaction orders were essentially zero. This work is part of a larger study related to capture and conversion of CO₂ to synthetic natural gas

In situ CO2 capture using CaO/γ-Al₂O₃ washcoated monoliths for sorption enhanced water gas shift reaction

In situ capture of CO₂ allows the thermodynamically constrained water gas shift (WGS) process to operate at higher temperatures (i.e., 350 C) where reaction kinetics are more favorable. Dispersed CaO/γ-Al₂O₃ was investigated as a sorbent for in situ CO₂ capture for an enhanced water gas shift application. The CO₂ adsorbent (CaO/γ-Al₂O₃) and WGS catalyst (Pt/γ-Al₂O₃) were integrated as multiple layers of washcoats on a monolith structure. CO₂ capture experiments were performed using thermal gravimetric analysis (TGA) and a bench scale flow through reactor. Enhancement of the water gas shift (EWGS) reaction was demonstrated using monoliths (400 cells/in.2) washcoated with separate layers of dispersed CaO/γ-Al₂O₃ and Pt/γ-Al₂O₃ in a flow reactor. Capture experiments in a reactor using monoliths coated with CaO/γ-Al₂O₃ indicated that increased concentrations of steam in the reactant mixture increase the capture capacity of the CO₂ adsorbent as well as the extent of regeneration. A maximum capture capacity of 0.63 mol of CO₂/kg of sorbent (for 8.4% CaO on γ-Al₂O₃ washcoated with a loading of 3.45 g/in.3 on monolith) was observed at 350 C for a reactant mixture consisting of 10% CO₂, 28% steam, and balance N₂. Hydrogen production was enhanced in the presence of monoliths coated with a layer of 1% Pt/γ-Al2O₃ and a separate layer of 9.4% CaO/γ-Al₂O₃. A greater volume of hydrogen compared to the baseline WGS case was produced over a fixed amount of time for multiple cycles of EWGS. The CO conversion was enhanced beyond equilibrium during the period of rapid CO₂ capture by the nanodispersed adsorbent. Following saturation of the adsorbent, the monoliths were regenerated (CO₂ was released) in situ, at temperatures far below the temperature required for decomposition of bulk CaCO₃. It was demonstrated that the water gas shift reaction could be enhanced for at least nine cycles with in situ regeneration of adsorbent between cycles. Isothermal regeneration with only steam was shown to be a feasible method for developing a process

Low-pressure methanol synthesis from CO2 over metal-promoted Ni-Ga intermetallic catalysts

Ni-Ga and M-Ni-Ga (M = Au, Co, Cu) catalysts were evaluated for methanol synthesis from CO2 at 10 bar and 200−270 °C. The following trend in turnover frequency (TOF) for CO2 hydrogenation was observed: AuNiGa > CuNiGa > NiGa > CoNiGa, where TOF increased with decreasing catalyst affinity for CO. The presence of a third metal was found to influence both the formation of the Ni-Ga intermetallic phase as well as the number of available sites for CO chemisorption. Phase formation, catalyst composition and stability were evaluated using therm ogravimetric analysis (TGA), X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning transmission electron microscopy and energy dispersive X-ray spectroscopy (STEM-EDX). Au-Ni-Ga, which showed a nearly 4-fold improvement in TOF at 263 °C and 10 bar compared to Ni-Ga, consisted of Ni3Ga particles decorated with Au, as evidenced by post catalysis characterization

Catalytic processes for fuels production from CO2-rich streams: Opportunities for industrial flue gases upgrading

Carbon dioxide (CO2) can be used as a C1 building block for the production of fuels and chemicals. The use of catalysts in the decarbonization of CO2-rich industries via the utilization of CO2 is fundamental. Herein, the aim of this chapter is to describe the catalytic upgrading of CO2-rich industrial flue gases. Initially, the CO2 capture technologies including precombustion, postcombustion, and oxyfuel are discussed. In addition, the industries producing CO2-rich streams which are cement, steel and iron, biogas, and brewery are described. Moreover, the effect of impurities during CO2 capture and conversion into high-value chemicals and fuels are examined. Finally, the reaction pathways during which CO2 is converted into added-value fuels and chemicals are explained by emphasizing the desirable catalytic characteristics and the current catalytic limitations.N

The Need for Flexible Chemical Synthesis and How Dual-Function Materials Can Pave the Way

Since climate change keeps escalating, it is imperative that the increasing CO2 emissions be combated. Over recent years, research efforts have been aiming for the design and optimization of materials for CO2 capture and conversion to enable a circular economy. The uncertainties in the energy sector and the variations in supply and demand place an additional burden on the commercialization and implementation of these carbon capture and utilization technologies. Therefore, the scientific community needs to think out of the box if it is to find solutions to mitigate the effects of climate change. Flexible chemical synthesis can pave the way for tackling market uncertainties. The materials for flexible chemical synthesis function under a dynamic operation, and thus, they need to be studied as such. Dual-function materials are an emerging group of dynamic catalytic materials that integrate the CO2 capture and conversion steps. Hence, they can be used to allow some flexibility in the production of chemicals as a response to the changing energy sector. This Perspective highlights the necessity of flexible chemical synthesis by focusing on understanding the catalytic characteristics under a dynamic operation and by discussing the requirements for the optimization of materials at the nanoscale.University of Surrey EP/X000753/1European Union 101008058Ministerio de Ciencia e Innovación PID2021-126876OB-I0