Dr Melis S. Duyar

Molybdenum phosphide (MoP) catalyzes the hydrogenation of CO, CO2, and their mixtures to methanol, and it is investigated as a high-activity catalyst that overcomes deactivation issues (e.g., formate poisoning) faced by conventional transition metal catalysts. MoP as a new catalyst for hydrogenating CO2 to methanol is particularly appealing for the use of CO2 as chemical feedstock. Herein, we use a colloidal synthesis technique that connects the presence of MoP to the formation of methanol from CO2, regardless of the support being used. By conducting a systematic support study, we see that zirconia (ZrO2) has the striking ability to shift the selectivity towards methanol by increasing the rate of methanol conversion by two orders of magnitude compared to other supports, at a CO2 conversion of 1.4% and methanol selectivity of 55.4%. In situ X-ray Absorption Spectroscopy (XAS) and in situ X-ray Diffraction (XRD) indicate that under reaction conditions the catalyst is pure MoP in a partially crystalline phase. Results from Diffuse Reflectance Infrared Fourier Transform Spectroscopy coupled with Temperature Programmed Surface Reaction (DRIFTS-TPSR) point towards a highly reactive monodentate formate intermediate stabilized by the strong interaction of MoP and ZrO2. This study definitively shows that the presence of a MoP phase leads to methanol formation from CO2, regardless of support and that the formate intermediate on MoP governs methanol formation rate.

Fischer–Tropsch synthesis (FTS) is an essential approach to convert coal, biomass, and shale gas into fuels and chemicals, such as lower olefins, gasoline, diesel, and so on. In recent years, there has been increasing motivation to deploy FTS at commercial scales which has been boosting the discovery of high performance catalysts. In particular, the importance of support in modulating the activity of metals has been recognized and carbonaceous materials have attracted attention as supports for FTS. In this review, we summarised the substantial progress in the preparation of carbon-based catalysts for FTS by applying activated carbon (AC), carbon nanotubes (CNTs), carbon nanofibers (CNFs), carbon spheres (CSs), and metal–organic frameworks (MOFs) derived carbonaceous materials as supports. A general assessment of carbon-based catalysts for FTS, concerning the support and metal properties, activity and products selectivity, and their interactions is systematically discussed. Finally, current challenges and future trends in the development of carbon-based catalysts for commercial utilization in FTS are proposed.

Supported bimetallic catalysts have become an important class of catalysts in heterogeneous catalysis. Although well-defined bimetallic nanoparticles (BNPs) can be synthesized by seeded-growth in liquid phase, uniform deposition of these BNPs onto porous supports is very challenging. Here, we develop a universal nanoreactor strategy to directly fabricate the PdAu BNPs in the solid support of coral-like nitrogen-doped mesoporous polymer (NMP) with uniform dispersion in a large scale. This strategy is based on coordination chemistry to introduce the high-quality seeds of Pd nanoclusters and the Au ions into the NMP, and thus to be used as a nanoreactor for seeded growth of PdAu BNPs in solid state during thermal reduction. Many other supported Pd-based BNPs (diameters ranging from 2 to 3 nm) have also been successfully synthesized by adoption of this strategy, including PdRu, PdCo, PdNi, PdZn, PdAg and PdCu BNPs. As an example, the as-synthesized Pd1Au1/4 sample shows enhanced catalytic performance in formic acid (FA) dehydrogenation compared with the monometallic analogues, indicating the synergistic effect between Pd and Au. In addition, the Pd1Au1/4 product is molded into monolith without any binders due to its coral-like structure. The Pd1Au1/4 monolith shows considerable activity in FA dehydrogenation with a turnover frequency (TOF) value of 3684 h−1 at 333 K, which is recycled five times without changes in activity. We believe that the nanoreactor strategy provides an effective route to synthesize various supported bimetallic catalysts that have potential for applications in green and sustainable catalytic processes.

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.

A -Ni5Ga3/SiO2 catalyst, which is highly active and stable for thermal CO2 hydrogenation to methanol, was investigated to understand its surface dynamics during reaction conditions. The catalyst was prepared, tested and characterized using a multitude of techniques, including ex-situ XRD (X-ray Diffraction), TEM (Transmission Electron Microscopy), H2-TPR (Temperature Programmed Reduction), CO chemisorption, along with in-situ ETEM (Environmental Transmission Electron Microscopy), APXPS (Ambient Pressure X-ray Photoelectron Spectroscopy) and HERFD-XAS (High Energy Resolution Fluorescence Detected X-Ray Absorption Spectroscopy). Upon air exposure Ga migrates from the subsurface region to the surface of the nanoparticles forming a Ga-oxide shell surrounding a metallic core. The oxide shell can be reduced completely only at high temperatures (above 600 °C); the temperature of the reducing activation treatment plays a crucial role on the catalytic activity. HERFD-XAS and APXPS measurements show that an amorphous Ga2O3 shell persists during catalysis after low temperature reductions, promoting methanol synthesis.

In2O3 has recently emerged as a promising catalyst for methanol synthesis from CO2. In this work, we present the promotional effect of Pd on this catalyst and investigate structure-performance relationships using in situ X-ray spectroscopy (XAS), ex-situ characterization, and microkinetic modeling. Catalysts were synthesized with varying In:Pd ratios (1:0, 2:1, 1:1, 1:2, 0:1) and tested for methanol synthesis from CO2/H2 at 40 bar and 300°C. In:Pd(2:1)/SiO2 shows the highest activity (5.1 μmol MeOH/gInPds) and selectivity toward methanol (61%). While all bimetallic catalysts had enhanced catalytic performance, characterization reveals methanol synthesis was maximized when the catalyst contained both In-Pd intermetallic compounds and an indium oxide phase. Experimental results and density functional theory suggest the active phase arises from a synergy between the indium oxide phase and a bimetallic In-Pd particle with a surface enrichment of indium. We show that the promotion observed in the In-Pd system is extendable to non-precious metal containing binary systems, in particular In-Ni, which displayed similar composition-activity trends to the In-Pd system. Both palladium and nickel were found to form bimetallic catalysts with enhanced methanol activity and selectivity relative to indium oxide.

Molybdenum phosphide (MoP) catalysts have recently attracted attention due to their robust methanol synthesis activity from CO/CO2. In this study, synthesis strategies are employed to steer MoP selectivity towards higher alcohols, by investigating promotion effects of alkali (K), CO‐dissociating (Co, Ni) and non CO‐dissociating (Pd) metals. A systematic study with transmission electron microscopy (TEM), X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS) and X‐ray Absorption Spectroscopy (XAS) revealed that critical parameters governing activity of MoP catalysts are P/Mo ratio and K loading, both facilitating MoP formation. Kinetic studies of mesoporous silica‐supported MoP catalysts show a two‐fold role of K, which also acts as electronic promoter by increasing the total alcohol selectivity and chain length. Palladium (Pd) increases CO conversion, but decreases alcohol chain length. The use of mesoporous carbon (MC) support had the most significant effect on catalyst performance and yielded a KMoP/MC catalyst that ranks among the state‐of‐the‐art in terms of selectivity to higher alcohols.

Methanol is a major fuel and chemical feedstock currently produced from syngas, a CO/CO2/H2 mixture. Herein we identify formate binding strength as a key parameter limiting the activity and stability of known catalysts for methanol synthesis in the presence of CO2. We present a molybdenum phosphide catalyst for CO and CO2reduction to methanol, which through a weaker interaction with formate, can improve the activity and stability of methanol synthesis catalysts in a wide range of CO/CO2/H2feeds.

Dual function materials (DFMs) for CO2 capture and conversion couple the endothermic CO2  step of a traditional  with the exothermic hydrogenation of CO2 over a catalyst in a unique way; a  operating at an  temperature (320 °C) and pressure (1 atm) can capture CO2 from , and release it as upon exposure to  hydrogen. This combined CO2 capture and utilization eliminates the energy intensive CO2 desorption step associated with conventional CO2  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.  remains the catalyst of choice due to its lower price and excellent  performance. The role of CO2  on the final  capacity of the DFM has also been investigated by testing several new . Two novel DFM compositions are reported here (5% Ru 10% K2CO3/Al2O3and 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 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 CO2species. 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 CO20.34. 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.

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  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 CO2adsorbent, 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 methanation has been evaluated as a means of storing intermittent renewable energy in the form of synthetic natural gas. A range of process parameters suitable for the target application (4720 h−1 to 84,000 h−1 and from 160 °C to 320 °C) have been investigated at 1 bar and H2/CO2 = 4 over a 10% Ru/γ-Al2O3 catalyst. Thermodynamic equilibrium was reached at T ≈ 280 °C at a GHSV of 4720 h−1. Cyclic and thermal stability tests specific to a renewable energy storage application have also been conducted. The catalyst showed no sign of deactivation after 8 start-up/shut-down cycles (from 217 °C to RT) and for total time on stream of 72 h, respectively. In addition, TGA-DSC was employed to investigate adsorption of reactants and suggest implications on the mechanism of reaction. Cyclic TGA-DSC studies at 265 °C in CO2 and H2, being introduced consecutively, suggest a high degree of short term stability of the Ru catalyst, although it was found that CO2 chemisorption and hydrogenation activity was lowered by a magnitude of 40% after the first cycle. Stable performance was achieved for the following 19 cycles. The CO2 uptake after the first cycle was mostly restored when using a H2-pre-treatment at 320 °C between each cycle, which indicated that the previous drop in performance was not linked to an irreversible form of deactivation (sintering, permanent poisoning, etc.). CO chemisorption on powder Ru/γ-Al2O3 was used to identify metal sintering as a mechanism of deactivation at temperatures higher than 320 °C. A 10% Ru/γ-Al2O3//monolith has been investigated as a model for the design of a catalytic heat exchanger. Excellent selectivity to methane and CO2conversions under low space-velocity conditions were achieved at low hydrogenation temperatures ( = 240 °C). The use of monoliths demonstrates the possibility for new reactor designs using wash-coated heat exchangers to manage the exotherm and prevent deactivation due to high temperatures.

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.