Enhanced Methanol Production over Non-promoted Cu–MgO–Al2O3 Materials with Ex-solved 2 nm Cu Particles: Insights from an Operando Spectroscopic Study (original) (raw)
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Enhanced methanol production is obtained over a non-promoted Cu−MgO−Al 2 O 3 mixed oxide catalyst derived from a Cu−Mg−Al hydrotalcite precursor (HT) containing narrowly distributed small Cu NPs (2 nm). Conversions close to the equilibrium (∼20%) with a methanol selectivity of 67% are achieved at 230°C, 20 bar, and a space velocity of 571 mL•g cat −1 •h −1. Based on operando spectroscopic studies, the striking activity of this Cu-based catalyst is ascribed to the stabilization of Cu + ions favored under reaction conditions due to lattice reorganization associated with the "HT-memory effect" promoted by water. Temperature-resolved infrared−mass spectrometry experiments have enabled the discernment of monodentate formate species, stabilized on Cu + as the intermediate in methanol synthesis, in line with the results of density functional theory calculations. These monodentate formate species are much more reactive than bridge formate species, the latter ones behaving as intermediates in methane and CO formation. Moreover, poisoning of the Cu 0 surface by strongly adsorbed species behaving as spectators is observed under reaction conditions. This work presents a detailed spectroscopic study highlighting the influence of the reaction pressure on the stabilization of active surface sites, and the possibility of enhancing methanol production on usually less active non-promoted nano-sized copper catalysts, provided that the proper support is selected, allowing the stabilization of doped Cu +. Thus, a methanol formation rate of 2.6 × 10 −3 mol MeOH •g cat −1 •h −1 at 230°C, 20 bar, and WHSV = 28 500 mL•g cat −1 •h −1 is obtained on the Cu−MgO−Al 2 O 3 HTderived catalyst with 71% methanol selectivity, compared to 2.2 × 10 −4 mol MeOH •g cat −1 •h −1 with 54% methanol selectivity obtained on a reference Cu/(Al 2 O 3 /MgO) catalyst not derived from a HT structure.
On the role of adsorbed atomic oxygen and CO2 in copper based methanol synthesis catalysts
Catalysis Letters, 1994
The role of adsorbed atomic oxygen in methanol synthesis is investigated by a series of transient experiments of the interaction of CO and CO2 with a ternary Cu/ZnO/A1203 catalyst under methanol synthesis conditions. In particular, the response of adding CO and CO2 aspulses and as steps to the reaction gas mixture is studied. Hereby it is possible to study both the formation of CO2 from the reaction of adsorbed atomic oxygen (O-*) with CO, and the dissociation of CO2 in situ, i.e., while the catalyst is producing methanol. The experiments show no evidence of a significant coverage of O-* under methanol synthesis conditions. In addition, it is shown that COz is the main carbon source in methanol synthesis under the given conditions.
ACS Catalysis, 2020
Dispersion of metallic Cu nanoparticles on a metal oxide support increases the number of exposed metallic Cu sites and/or Cu-support interfacial sites, resulting in good catalytic performance for CO 2-to-methanol hydrogenation. However, the formation of highly dispersed Cu nanoparticles is challenging because they are easily sintered. Here, we studied Cu nanoparticle formation by a simple deposition−reduction technique using Cu-doped MgAl 2 O 4 (Mg 1−x Cu x Al 2 O 4). Mg 1−x Cu x Al 2 O 4 possessed the following three types of Cu 2+ species: short O−Cu octahedrally coordinated [CuO 6 ] s , elongated O−Cu octahedrally coordinated [CuO 6 ] el , and tetrahedrally coordinated [CuO 4 ] t. The former two are found in the inverse-spinel-type Mg 1−x Cu x Al 2 O 4 , while the other is found in the normal-spinel-type Mg 1−x Cu x Al 2 O 4. Additionally, by focusing on the difference in the reducibility of the Cu 2+ species, we clarified that their fraction is related to Cu loading. For low Cu loading (x < 0.3), Mg 1−x Cu x Al 2 O 4 mainly contained the [CuO 6 ] s species. On the other hand, for high Cu loading (x ≥ 0.3), the fraction of the [CuO 6 ] el and [CuO 4 ] t species increased. Notably, among the prepared catalysts, H 2-reduced Mg 0.8 Cu 0.2 Al 2 O 4 (x = 0.2) had the largest number of exposed metallic Cu sites, resulting in its good catalytic performance. Hence, the H 2 reduction of [CuO 6 ] s is essential for forming metallic Cu nanoparticles on metal oxides.
Applied Surface Science, 1999
The method of doping trivalent metal ions into a copper-based catalyst for methanol synthesis is effective in modifying the surface structure of the catalyst. The promotion effect and its relation to catalytic activity for hydrogenation of CO to methanol after doping with trivalent metal ions such as Al 3q , Sc 3q , and Cr 3q into Cu-ZnO have been investigated by XRD, ESR, XPS, TPR, and the evaluation of catalytic activity. The results show that doping trivalent metal ions into ZnO assists in the formation of monovalent cationic defects on the surface of ZnO. These monovalent cationic defects both enrich and stabilize monovalent copper on the surface of copper-based catalysts for methanol synthesis during reduction and reaction. They increase catalytic activity for methanol synthesis and extend the life of catalysts.
International Journal of Nanoparticles, 2009
A series of CuO/ZnO/Al 2 O 3 nanocrystalline solid catalysts were prepared by the coprecipitation method at constant temperature. The effect of the change in pH, chemical composition and thermal treatment for all the prepared solids on the physicochemical, surface and catalytic properties was investigated. The crystal structure of the different prepared solids was studied using XRD analysis. The crystallite size calculated from XRD patterns using Scherer equation did not alter effectively by changing the pH of the prepared catalyst precursors. The surface characteristics of various calcined adsorbents were investigated using nitrogen adsorption at -196°C and their catalytic activities were determined using water-gas shift reaction (WGSR) at temperature range between 130°C and 300°C. Only CuO and ZnO were identified for the solids calcined at 350°C. The catalyst with Cu/Zn = 1 and prepared at pH = 7 showed the smallest crystallite size (20 nm) and biggest surface area (S BET = 98m 2 /g). During the catalytic test relatively high conversion of CO into CO 2 at a temperature = 150°C was observed (96%) for the previous catalyst.
CO hydrogenation to methanol on Cu–Ni catalysts: Theory and experiment
Journal of Catalysis, 2012
We present density functional theory (DFT) calculations for CO hydrogenation on different transition metal surfaces. Based on the calculations, trends are established over the different monometallic surfaces, and scaling relations of adsorbates and transition states that link their energies to only two descriptors, the carbon oxygen binding energies, are constructed. A micro-kinetic model of CO hydrogenation is developed and a volcano-shaped relation based on the two descriptors is obtained for methanol synthesis. A large number of bimetallic alloys with respect to the two descriptors are screened, and CuNi alloys of different surface composition are identified as potential candidates. These alloys, proposed by the theoretical predictions, are prepared using an incipient wetness impregnation method and tested in a highpressure fixed-bed reactor at 100 bar and 250-300°C. The activity based on surface area of the active material is comparable to that of the industrially used Cu/ZnO/Al 2 O 3 catalyst. We employ a range of characterization tools such as inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis, in situ X-ray diffraction (XRD) and in situ transmission electron microscope (TEM) to identify the structure of the catalysts.
Unravelling the Zn‐Cu Interaction during Activation of a Zn‐promoted Cu/MgO Model Methanol Catalyst
ChemCatChem, 2021
We report on an inverse model Cu/MgO methanol catalyst modified with 5 % zinc oxide at the Cu surface to elementspecifically probe the interplay of metallic copper and zinc oxide during reductive activation. The structure of copper and zinc was unraveled by in situ X-ray diffraction (XRD) and in situ X-ray absorption spectroscopy (XAS) supported by theoretical modelling of the extended X-ray absorption fine structure and X-ray absorption near-edge structure spectra. Temperatureprogrammed reduction in H 2 during in situ XAS showed that copper was reduced starting at 145°C. With increasing reduction temperature, zinc underwent first a geometrical change in its structure, followed by reduction. The reduced zinc species were identified as surface alloy sites, which coexisted from 200°C to 340°C with ZnO species at the copper surface. At 400°C ZnÀ Cu bulk-alloyed particles were formed. According to in situ XRD and in situ XAS, about half of the ZnO was not fully reduced, which can be explained by a lack of contact with copper. Our experimental results were further substantiated by density functional theory calculations, which verified that ZnO with neighboring Cu atoms reduced more easily. By combining these results, the distribution, phase and oxidation state of Zn species on Cu were estimated for the activated state of this model catalyst. This insight into the interplay of Cu and Zn forms the basis for deeper understanding the active sites during methanol synthesis. This publication is part of a joint Special Collection with ChemElectroChem on "Catalysts and Reactors under Dynamic Conditions for Energy Storage and Conversion (DynaKat)". Please check our homepage for more articles in the collection.
Angewandte Chemie International Edition, 2007
Copper-based catalysts are industrially applied in various reactions including water-gas shift, synthesis of fatty alcohols from fatty acid methyl esters, and methanol synthesis. Today, methanol is produced at low pressures (35-55 bar) and 200-300°C over Cu/ZnO/Al 2 O 3 catalysts. [1] Due to the great commercial relevance, Cu/ZnO-based catalysts have been extensively studied and many different models have been proposed regarding the nature of active sites and the valence of copper under conditions of methanol formation, such as Cu 1+ dispersed in ZnO, [2, metallic copper supported on ZnO, [4] dynamic surface and bulk alloy formation depending on the reduction potential of the synthesis gas, 6] Cuat the so-called Schottky junction between metallic Cu and the semiconductor ZnO, [7] and ZnO segregated on Cu 1+ . The catalytic activity of the binary catalyst has been reported to be several orders of magnitude greater than that of metallic Cu or pure ZnO, respectively, indicating a synergetic interaction of the two components. [9] ZnO is regarded either as provider of atomic spillover hydrogen for further hydrogenation of adsorbed reaction intermediates on Cu sites, or as a structure directing support controlling dispersion, morphology, and specific activity of the metal particles. [14][15] Strong interaction between the metal and the support, especially in the case of large lattice mismatch, is known to cause strain in the metal particles, to which an increase in catalytic performance has been attributed. [19][21] On the other hand, 1-ML-high and thicker Cu islands epitaxially grown on the ZnO (000⎯1) surface were experimentally found to be strain-free. [22] In most of the earlier studies model catalysts with low Cu loadings (Cu/Zn << 1) containing large ZnO single crystals have been investigated, although, usually, in com-mercial catalysts copper represents the main component (Cu/Zn > 1) and the ZnO particles, acting rather as a spacer than as a support, are comparable in size, or even smaller than the Cu particles. In this paper we report the results of TEM and in situ XRD characterization of a series of Cu/ZnO/Al 2 O 3 catalysts exhibiting different catalytic activities. The molar ratio Cu:Zn:Al = 60:30:10 is characteristic of commercial catalysts. [1] The microstructural features of the materials prepared by coprecipitation with sodium carbonate from metal nitrate solution are analyzed after calcination in air at 330°C and subsequent reduction in hydrogen at 250°C. A quantitative estimation of imperfections in metal particles determined by combination of independent TEM and in situ XRD investigations is established. The implications of strain in Cu crystallites and the defect frequency associated therewith on the catalytic activity of Cu/ZnO/Al 2 O 3 catalysts in methanol synthesis are discussed.
Use of the methanol decomposition reaction for identifying surface species in Cu/Co-based catalysts
Journal of Molecular Catalysis, 1994
Methanol decomposition was used as test reaction for characterizing Cu/Co-based catalysts. Three Cu/Co/Al ternary catalysts containing Co/(Cu +Co) molar ratios of 0.34, 0.49, and 0.67, respectively, were prepared. Two binary reference samples containing Co/Al and Cu/Al, respectively, were also prepared. Methanol decomposition on binary Co/Al sample gave predominantly CO, CH,, and higher hydrocarbons; the active sites were metallic Co crystallites, the Co"+ ions being almost inactive. Main products on binary Cu/Al sample were methyl formate, CO, and CO*. Both, Cue and Cu" + sites were active. The methanol decomposition mechanism was interpreted in terms of a reaction scheme based on the initial dehydrogenation of methanol to formaldehyde and the consecutive decomposition of the formaldehyde intermediate to different products according to the nature of the Cu or Co active site involved. The test reaction was specifically used to investigate the effect of both the chemical composition and the calcination temperature on the generation of surface species of ternary Cu/Co/Al samples. Good agreement was found between the conclusions obtained from catalytic testing and those from sample characterization by means of bulk and surface techniques (Xray diffraction, temperature programmed reduction, and X-ray photoelectron spectroscopy). Methanol decomposition proved to be a useful test reaction for discriminating between Cu and Co sites on Cu/ Co-based catalysts.