Active Sites For Methanol Synthesis Research Articles

Recent progress in understanding the nature of active sites for methanol synthesis over Cu/ZnO catalysts

Copper-zinc-alumina (Cu/ZnO/Al2O3) has been utilized as the leading catalyst for industrial methanol synthesis via hydrogenation of CO and CO2 for more than 50 years. Understanding the nature of active Zn sites at the atomic level is vital for gaining insight into the reaction mechanism and rational design of more efficient Cu/ZnO catalysts for methanol synthesis but remains greatly challenging and has been intensively debated for decades. In this mini-review, we first describe the fundamental insights obtained from well-defined model catalysts and then summarize the recent experimental evidence with respect to dynamic structural and electronic changes in the active Zn phase under realistic working conditions by in situ/operando microscopy/spectroscopy and surface site titration using probing molecules. In the following, we discuss the catalytic mechanism of diverse active structures by theoretical calculations and simulations. Therefore, the critical role of interfacial sites between metallic Cu and ZnO, especially those with oxygen defects, in methanol synthesis via CO2 hydrogenation is emphasized. Finally, we discuss the technical challenges and perspectives in understanding the origins of this Cu-ZnO synergy and rational design of next-generation Cu-based methanol synthesis catalysts.

The relationship between structure and performance of Mo-ZrO2 catalysts in CO2 hydrogenation

CO₂ hydrogenation is a structure-sensitive reaction, producing a diverse range of products based on the structure of catalysts. In this study, we present a catalyst Mo-ZrO2 for CO2 hydrogenation and observed the products vary among CO, CH4, and CH3OH as Mo changes in its loading and local environment. Kinetic studies reveal that increasing Mo loading amplifies hydrogenation ability of the catalyst while simultaneously reducing CO2 adsorption on ZrO2. Specifically, Mo loadings under 10 wt% result in Mo integration into ZrO2 support, forming a Mo-O-Zr structure. Conversely, a higher Mo loading leads to the formation of Zr(MoO4)2 clusters on ZrO2 support. The Mo5⁺-Ov-Zr structure, formed in Mo-doped ZrO2 during reduction pretreatment, emerges as an active site for methanol synthesis. In contrast, metallic Mo formed from the reduction of Zr(MoO4)2 clusters loaded on ZrO2 is responsible for methane formation. The evolution of the surface structure from Mo-O-Zr structure to Zr(MoO4)2 clusters on ZrO2 as Mo loading changes, resulting in alterations in the number of active sites for methanol synthesis and methanation, consequently leading to variations in product distribution.

Tuning interfaces between Cu and oxide via atomic layer deposition method for CO2 hydrogenation to methanol

The specific Cu/10c-ZnO/CeO2 catalyst was designed using the atomic layer deposition method and the Cu+–Zn0–Ce4+ structure is the active site for methanol synthesis from CO2 hydrogenation.

Design of Cu/ZnO/Al2O3catalysts with a rich Cu–ZnO interface for enhanced CO2hydrogenation to methanol using zinc-malachite as the precursor

By regulating the precursor structure, CZA-2-R can have a rich Cu–ZnO interface, which exposes more active sites for methanol synthesis.

Boxlike Assemblages of Few-Layer MoS2 Nanosheets with Edge Blockage for High-Efficiency Hydrogenation of CO2 to Methanol

Direct hydrogenation of CO2 into methanol is a promising strategy for reducing excessive dependence on fossil fuels and alleviating environmental concerns. Recently, in-plane sulfur vacancies in two-dimensional MoS2 nanosheets were unveiled as efficient catalytic active sites for methanol synthesis from CO2, whereas edge vacancies facilitated hydrogenation of CO2 to methane. Herein, we developed boxlike assemblages of quasi-single-layer MoS2 nanosheets, which were edge-blocked by ZnS crystallites (denoted as h-MoS2/ZnS) via a metal–organic framework (MOF)-engaged solvothermal route and subsequent heat treatments. The spatial confinement of the ZnS can restrain the growth and aggregation of MoS2 and ensure the stability of few-layer or even single-layer MoS2 in the assemblages. More importantly, the presence of ZnS can prevent reactants from approaching the edge sulfur vacancies of MoS2. With more exposed in-plane sulfur vacancies and less edge sulfur vacancies, the h-MoS2/ZnS exhibits 67.3% methanol selectively, 9.0% CO2 conversion, and a high methanol space-time yield of up to 0.93 gMeOH·gMoS2–1·h–1 at 260 °C, 5 MPa, and 15 000 mL·gcat.–1·h–1. The catalytic activity was stable for at least 120 h. By removing the ZnS phase from h-MoS2/ZnS and thus deliberately creating more edge sulfur vacancies, it was further confirmed that edge sulfur vacancies are active catalytic sites for excessive hydrogenation of CO2 to methane. Furthermore, the reaction mechanism of our catalyst was also investigated by a high-pressure in situ DRIFTS study. Thus, this MOF-templated strategy for assembling and confining quasi-single-layer MoS2 provides insights into the development of highly efficient transition-metal dichalcogenide catalysts for CO2 hydrogenation with excellent stability.

Enhancement of Methanol Synthesis by Oxidative Fluorination of Cu/ZnO Catalysts─Insights from Surface Analyses

A Cu/ZnO catalyst system for methanol synthesis promoted by oxidative fluorination was studied. Gaseous F2 reacts in a first step mainly with CuO to give CuF2 (XPS, thermodynamics). In the active system, the entire fluoride content transforms to ZnF2 and the catalyst system should be formulated as Cu/ZnO1–x/ZnF2 (XPS). Tested for methanol production using a (1 + x) H2/COx syngas (x = 1, ..., 2, eight steps@40 bar, 473 and 513 K), the fluorinated systems have optimal performance at x = 2, that is, a 3H2/CO2 mixture, and inhibit CO2 promotion at low CO2 concentrations. The number of surface ZnO1–x oxygen defect sites and the number of active sites for methanol synthesis increased in the fluorinated systems after H2 reduction (refined chemisorption measurements, XPS, and BET analysis). Concomitantly, the number of active sites for the (reverse) water–gas shift reaction decreased. Both account for the increased methanol activity and selectivity of the fluorinated catalyst systems and imply negligible water inhibition for the fluorinated case.

Mechanochemical Effect in Mixing Sponge Copper with Amorphous ZrO2Creates Effective Active Sites for Methanol Synthesis by CO2Hydrogenation

For efficient methanol synthesis by CO2 hydrogenation, catalysts prepared by mixing sponge copper and amorphous ZrO2 were investigated. The commercial sponge copper itself exhibited catalytic activ...

Unravelling the Structural Modification (Meso-Nano-) of Cu/ZnO-Al2O3 Catalysts for Methanol Synthesis by the Residual NaNO3 in Hydroxycarbonate Precursors

The effects of residual NaNO3 on the modification of Cu/ZnO-Al2O3 catalysts have been extensively documented, but the modification mechanism is so far unclear. This work studies in detail the influence of the residual sodium nitrate present in the hydroxycarbonate precursors on their decomposition during calcination and how it affects to the formation and configuration of the final active sites of the Cu/ZnO-Al2O3 catalysts. Different samples with varying sodium content after washing (from 0.01 to 7.3 wt%) were prepared and studied in detail after calcination and reduction steps. The results of this work demonstrated that NaNO3 affects the decomposition mechanism of the hydroxycarbonate precursors during calcination and produces its decarbonation at low temperature. The enhancement of the decarbonation by NaNO3 leads to segregation and crystallization of CuO and ZnO with loss of mesostructure and surface area in the calcined catalysts. The loss of mesostructure in calcined catalysts affects the subsequent reduction step, decreasing the reducibility and damaging the nanostructure of the reduced catalysts forming large Cu particles in poor contact with ZnOx that results in a significant decrease in the intrinsic activity of the copper active sites for methanol synthesis.

Mechanism and Nature of Active Sites for Methanol Synthesis from CO/CO2 on Cu/CeO2

CO2 hydrogenation to methanol can play an important role in meeting sustainability goals of the chemical industry. Herein, we investigated in detail the role of the Cu-CeO2 interactions for methanol synthesis, emphasizing the role of the copper surface and interface sites between copper and ceria for the hydrogenation of CO2 and CO. A combined CO2-N2O titration approach was developed to quantify the exposed metallic copper sites and ceria oxygen vacancies in reduced Cu/CeO2 catalysts. Extensive characterization shows that copper dispersion is strongly enhanced by strong Cu-CeO2 interactions in comparison to Cu/SiO2. CO2 hydrogenation activity data show that the Cu/CeO2 catalysts displayed higher methanol selectivity compared to a reference Cu/SiO2 catalyst. The improved methanol selectivity stems from inhibition of the reverse water-gas-shift activity. The role of CO in CO2-to-methanol conversion was studied by steady-state and transient co-feeding activity measurements together with (quasi) in situ characterization (TPH, XPS, SSITKA and IR spectroscopy). The Cu-CeO2 interface provides active sites for the direct hydrogenation of CO to methanol via a formyl intermediate. Co-feeding of small amounts of CO2 to a CO/H2 mixture poisons these interfacial sites due to the formation of carbonate-like species. Methanol synthesis proceeds mainly via CO2 hydrogenation in which the metallic Cu surface provides the active sites.

Hydrogen spillover enabled active Cu sites for methanol synthesis from CO2 hydrogenation over Pd doped CuZn catalysts

Surface modification with Pd is an effective way for improved activity in CO2 hydrogenation to methanol over commercial Cu-ZnO catalysts via a so-called hydrogen spillover mechanism. However, there still lacks a quantitative analysis of hydrogen spillover effect and the nature of active sites after Pd modification remains unclear. In this work, we prepared a series of Pd-doped Cu-ZnO catalysts (Pd-CZ-x) with tunable Pd loading by using a facile polyol reduction method for a deep study of the promotion effect of Pd. With the increase of Pd/Cu molar ratio (x) from 0 to 0.04, there emerges a volcano-shaped relationship between methanol space time yield (STY) and Pd loading. 1% Pd doping can increase the methanol STY by 2.5 times and the methanol turnover frequency (TOF) by 3.5 times at 543 K, when compared to undoped Cu-ZnO. Kinetic studies show the activation energy required for methanol synthesis is greatly reduced from 59 kJ mol−1 over Cu-ZnO to 31 kJ mol−1 over Pd-CZ-0.01. Behind the volcano-shaped relationship is a balance between the hydrogen spillover effect of Pd and the reduced surface Cu area caused by Pd blocking. Chemisorption gives a quantitative analysis of an important type of activated Cu sites enabled by hydrogen spillover (Cuhs). Importantly, it is found that the methanol STY correlates linearly with Cuhs surface area, suggesting that the activated Cu sites with the assistance of ZnO are real active sites for methanol synthesis from CO2 hydrogenation.

Comment on "Active sites for CO2 hydrogenation to methanol on Cu/ZnO catalysts".

Kattel et al (Reports, 24 March 2017, p. 1296) report that a zinc on copper (Zn/Cu) surface undergoes oxidation to zinc oxide/copper (ZnO/Cu) during carbon dioxide (CO2) hydrogenation to methanol and conclude that the Cu-ZnO interface is the active site for methanol synthesis. Similar experiments conducted two decades ago by Fujitani and Nakamura et al demonstrated that Zn is attached to formate rather than being fully oxidized.

Bifunctionality of Cu/ZnO catalysts for alcohol-assisted low-temperature methanol synthesis from syngas: Effect of copper content

Alcohol-assisted low-temperature methanol synthesis was conducted over Cu/ZnO_X catalysts while varying the copper content (X). Unlike conventional methanol synthesis, ethanol acted as both solvent and reaction intermediate in this reaction, creating a different reaction pathway. The formation of crystalline phases and characteristic morphology of the co-precipitated precursors during the co-precipitation step were important factors in obtaining an efficient Cu/ZnO catalyst with a high dispersion of metallic copper, which is one of the main active sites for methanol synthesis. The acidic properties of the Cu/ZnO catalyst were also revealed as important factors, since alcohol esterification is considered the rate-limiting step in alcohol-assisted low-temperature methanol synthesis. As a consequence, bifunctionality of the Cu/ZnO catalyst such as metallic copper and acidic properties was required for this reaction. In this respect, the copper content (X) strongly affected the catalytic activity of the Cu/ZnO_X catalysts, and accordingly, the Cu/ZnO_0.5 catalyst with a high copper dispersion and sufficient acid sites exhibited the best catalytic performance in this reaction.

Methanol synthesis from CO2 and H2 over Pd/ZnO/Al2O3: Catalyst structure dependence of methanol selectivity

CO2 hydrogenation to methanol was investigated over Pd/ZnO/Al2O3, focusing on the influence of the preparation method, reduction temperature and Pd loading on the catalytic performance. The structure of each catalyst was thoroughly examined using various techniques. The results indicated that the methanol selectivity depended on the content of PdZn alloy, which changed dynamically with varying the preparation methods and pretreatment procedures of the catalysts. More specifically, the ratio of PdZn/Pd0 in the bimetallic compound increased with the rise of the reduction temperature, which eventually led to the increase of methanol selectivity. Additionally, the EXAFS results revealed the presence of ZnOx modified Pd species when the Pd loading was decreased to be lower than 2wt%. However, the CH3OH selectivity remained almost identical when the reactions were conducted under similar CO2 conversions. Therefore, we conclude that both the PdZn alloy and Pd modified by ZnOx islands are the active sites for methanol synthesis from CO2 hydrogenation.

Methanol synthesis on ZnO(0001¯). I. Hydrogen coverage, charge state of oxygen vacancies, and chemical reactivity

Oxygen vacancies on ZnO(0001) have been proposed to be the catalytically active sites for methanol synthesis on pure ZnO. The charge state and thus the chemical reactivity of such vacancies on this polar O-terminated basal plane of ZnO is expected to be intimately connected to the degree of its hydroxylation in view of its Tasker type(3) unstable character. Here, the interplay between hydrogen adsorption and the thermodynamic stability of O vacancies in various charge states, corresponding formally to F(++), F(+), F(0), F(-), and F(--) centers, is investigated using electronic structure calculations. Assuming thermodynamic equilibrium of the defective surface with a hydrogen containing gas phase the thermodynamically most stable O vacancy type is determined as a function of temperature and pressure. For the adsorption of H(2) molecules at O vacancy sites it is found that the homolytic process leads to energetically more favorable structures than heterolytic adsorption and hydride formation. By homolytic adsorption and desorption one can switch between F(++), F(0), and F(--) or between F(+) and F(-), a process which is believed to occur during methanol synthesis. However, the barrier for heterolytic dissociation of H(2) at O vacancies is significantly lower compared to homolytic cleavage. Furthermore, the barrier for transforming hydridic hydrogen, i.e., ZnH species, to protonic hydrogen, i.e., OH species together with a reduction of ZnO itself, is quite high. This implies that hydridic H(-) species created as a result of heterolytic dissociation might have a long enough lifetime at O vacancies that they will be available for methanol synthesis. ZnH and OH vibrational frequencies have been computed in order to assist future experimental assignments.

Methanol synthesis from sulfur-containing syngas over Pd/CeO2 catalyst

Methanol synthesis from sulfur-containing syngas over Pd/CeO2 catalyst has been investigated in this study, the characterization methods like X-ray powder diffraction (XRD), X-ray fluorescence (XRF), X-ray photoelectron spectroscopy (XPS) and pulse experiments has also been carried out for elucidating sulfur-tolerant mechanism of Pd/CeO2 catalyst. The results show that methanol synthesis from sulfur-containing syngas exhibit stable reaction activity over Pd/CeO2. It was indicated from characterization analysis that the sulfur-tolerance of Pd/CeO2 was due to the capability of CeO2 for transforming H2S to SOx. Thus, Pd active sites for methanol synthesis were protected from sulfur poisoning, which was the main deactivation reason of Pd/Al2O3 catalysts. A sulfur-tolerant model of Pd/CeO2 catalyst for syngas to methanol was proposed according to the reaction performance and characterization results, which might be expanded to other sulfur-containing feed gas reaction systems over metal/ceria-type oxides.

Microstructure and application of mesoporous nanosize zirconia

The mesoporous nanoscale zircoina zeolite was firstly synthesized via solid state—Structure directing method without addition of any stabilizer. The sample bears lamellar or worm pore structures, relatively high surface area compared with that reported. The mesoporous nanosize structure can also resist higher calcination temperature. The introduction of above zirconia to the catalyst of methanol synthesis dedicates the nanosize particle size to the catalyst, which significantly changes the physical structure and electronic effect of the catalyst. The catalyst shows higher catalytic activity and selectivity to methanol. The active sites for methanol synthesis are demonstrated over various catalysts in this paper.

On the issue of the active site and the role of ZnO in Cu/ZnO methanol synthesis catalysts

The problem concerning the active site and the role of ZnO in Cu/ZnO-based methanol synthesis catalysts can be consistently explained based on the literature results by distinguishing CO2 and CO hydrogenations. Although only metallic copper has some activities for methanol synthesis by the hydrogenation of CO2, Cu-Zn alloying in Cu particles is responsible for the major promotional role of ZnO in industrial Cu/ZnO-based catalysts. The morphology effect reported in the literature will probably appear for the system of highly dispersed Cu particles supported on ZnO. As for the hydrogenation of CO, Cu+ species or Cu-O-Zn sites are the active sites for methanol synthesis. The spillover effect of the Cu-ZnO system is not significant compared to the effect of ZnO on the creation of the Cu-O-Zn site.

Oxidation of a Zn-deposited Cu( [formula omitted]) surface studied by XPS and STM

The oxidation of a Zn-deposited Cu(1 1 1) surface was studied by XPS and STM. When oxidizing the Zn/Cu(1 1 1) surface at high temperatures as 600–673 K, hexagonal ZnO islands rotated by 30° with a unit vector of 4.8 Å were formed over the Cu(1 1 1) surface. As precursor species for the formation of the ZnO species on Cu(1 1 1), alternating islands of a Cu–Zn surface alloy and Cu 2O were imaged as a striped structure by STM, in which these two islands coexisted on a single terrace. The terraces were regularly separated by step edges. The oxidation of the Cu–Zn surface alloy into the ZnO species was consistent with the previous results of Cu/ZnO-based powder catalysts, in which the ZnO/Cu interface was regarded as the active site for methanol synthesis by hydrogenation of CO.

The role of ZnO in Cu/ZnO methanol synthesis catalysts — morphology effect or active site model?

We examined whether or not the presence of ZnO changed the morphology of Cu particles in Cu/ZnO methanol synthesis catalysts using physical mixtures of Cu/SiO 2+ZnO/SiO 2. The yield of methanol produced by the CO 2 hydrogenation over the physical mixture increased by the reduction with H 2 at 573–723 K. The promotion of the methanol synthesis activity was due to migration of Zn from ZnO onto Cu/SiO 2, leading to the Cu–Zn active sites, as previously reported. On the other hand, no activity change was observed upon the reduction treatment for the reverse water gas shift (RWGS) reaction over the physical mixture; this result cannot be explained by the morphology change. The activation energy of the methanol synthesis and the RWGS reaction over the physical mixture did not vary upon reduction, further supporting the absence of any morphology change. It was thus clearly proved that the role of ZnO was not to change the morphology of Cu, but to create the Cu–Zn active sites for methanol synthesis only.

Active sites for methanol synthesis on a Zn/Cu(100) catalyst

The active sites for methanol synthesis on a Zn/Cu(100) catalyst are clarified using the dipped adcluster model (DAM) combined with ab initio HF and MP2 calculations. In comparison with that on a Cu(100) catalyst studied previously, it is shown that the Cu–Zn site provides an easier reaction pathway for the rate-limiting step; the energy barriers are all lower and the dioxomethylene intermediate is more stable than that at the Cu–Cu site on a Cu(100) catalyst. The present results prove that the Cu–Zn site is the active site which cooperates with the Cu–Cu site to catalyze methanol synthesis. Electron transfer is the important factor in affecting the reactivity of the catalysts.