Diffuse Reflectance Infrared Fourier Transform Spectroscopy Study Research Articles

Bifunctionality of Re Supported on TiO2 in Driving Methanol Formation in Low-Temperature CO2 Hydrogenation.

Low temperature and high pressure are thermodynamically more favorable conditions to achieve high conversion and high methanol selectivity in CO2 hydrogenation. However, low-temperature activity is generally very poor due to the sluggish kinetics, and thus, designing highly selective catalysts active below 200 °C is a great challenge in CO2-to-methanol conversion. Recently, Re/TiO2 has been reported as a promising catalyst. We show that Re/TiO2 is indeed more active in continuous and high-pressure (56 and 331 bar) operations at 125-200 °C compared to an industrial Cu/ZnO/Al2O3 catalyst, which suffers from the formation of methyl formate and its decomposition to carbon monoxide. At lower temperatures, precise understanding and control over the active surface intermediates are crucial to boosting conversion kinetics. This work aims at elucidating the nature of active sites and active species by means of in situ/operando X-ray absorption spectroscopy, Raman spectroscopy, ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Transient operando DRIFTS studies uncover the activation of CO2 to form active formate intermediates leading to methanol formation and also active rhenium carbonyl intermediates leading to methane over cationic Re single atoms characterized by rhenium tricarbonyl complexes. The transient techniques enable us to differentiate the active species from the spectator one on TiO2 support, such as less reactive formate originating from spillover and methoxy from methanol adsorption. The AP-XPS supports the fact that metallic Re species act as H2 activators, leading to H-spillover and importantly to hydrogenation of the active formate intermediate present over cationic Re species. The origin of the unique reactivity of Re/TiO2 was suggested as the coexistence of cationic highly dispersed Re including single atoms, driving the formation of monodentate formate, and metallic Re clusters in the vicinity, activating the hydrogenation of the formate to methanol.

Promoting effect exerted by WOx on Pt/ZrO2 catalysts during NOx reduction using H2 in the excess of O2

This work addresses the reduction of NOx by H2 under O2-rich conditions below 200 °C using Pt/W/ZrO2 catalysts with tungsten loadings from 0 to 14 wt%. Samples were thoroughly characterized by N2 physisorption, temperature-programmed desorption of CO, X-ray diffraction, laser Raman spectroscopy and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) with CO as probe molecule. Catalytic investigations showed the highest efficiency for the sample with 5 wt% W providing NOx conversion of 94 % and N2O selectivity of 15 %. The effect of W promoter is mostly attributed to structural promotion by isolated WO6 entities stabilizing the ZrO2 surface and leading to a high number of active Pt sites. By contrast, negligible electronic interaction between the promoter and Pt sites appears. Moreover, DRIFTS studies suggest that, in addition to the predominate NOx reduction by H2 on Pt sites, a side reaction occurs including NHx species formed at the interface of Pt and WOx/ZrO2.

Evidence of Synergy Effects between Zinc and Copper Oxides with Acidic Sites on Natural Zeolite during Photocatalytic Oxidation of Ethylene Using Operando DRIFTS Studies

In this article, the role of surface sites of modified zeolites with semiconductor nanoparticles as alternative photocatalyts for protecting post-harvest foodstuff from the detrimental effects of ethylene is addressed. Two single and one double catalyst based on zinc and copper oxides supported over modified zeolite samples were prepared. Physical, chemical, and surface properties of prepared materials were studied by several characterization methods. UV-Vis absorption spectra show that the applied modification procedures increase the optical absorption of light in the UV and visible regions, suggesting that an increase in the photocatalytic activity could take place mainly in the obtained co-impregnated catalyst. An ethylene conversion around 50% was achieved when the parent natural zeolite support was modified with both transition metal oxides, obtaining higher removal efficiency in comparison to single oxide catalysts. Adsorption and photocatalytic oxidation experiments were also performed using single and double catalysts supported over fumed silica, attaining lower ethylene conversion and thus highlighting the role of zeolite surfaces as adsorption sites for ethylene during photocatalytic reactions. Operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies reveal that a synergistic mechanism occurs, involving ethylene adsorption at acidic sites of zeolite and its photocatalytic oxidation due to the generation of radicals by the light activation of nanoparticles of zinc and copper oxides.

Realistic Operando‐DRIFTS Studies on Cu/ZnO Catalysts for CO2 Hydrogenation to Methanol – Direct Observation of Mono‐ionized Defect Sites and Implications for Reaction Intermediates

AbstractOur recent study in this journal highlighted misassignments of surface intermediates of diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements of published Cu/ZnO (CZ) catalyst systems for methanol synthesis. Here we investigate a recent and very active CZ system, in part promoted by oxidative fluorination (i. e., a Cu/ZnO1–x/ZnF2 system), with realistic operando‐DRIFTS measurements of a 3H2/CO2 gas feed (30 bar, 513 K). This DRIFTS setup is linked to an online GC analysis system and the catalytic performance of the Cu/ZnO1–x catalyst showed a similar performance of the catalysis process in the DRIFTS cell as in the catalyst test station. In the DRIFTS measurements, a very broad absorption band with a maximum at about 1500 cm−1 (=I1500) is evident. This I1500 band is absent in nitrogen; its intensity increases in pure hydrogen and is particularly high during methanol synthesis. I1500 results from photoionization of an electron residing in a mono‐ionized oxygen vacancy VO+ in the ZnO1–x part of the Cu/ZnO1–x catalyst. Consequently, the I1500 band intensity provides information on the extent of the strong metal‐support interaction SMSI. Therefore, measurement of the I1500 band intensity could be a novel and efficient tool to characterize any CZ‐based catalyst systems online. Mechanistically, the maximum VO+ photoionization I1500 band intensity in a 3H2/CO2 gas stream is coupled to the reaction of CO2 giving the CO2−. radical anion intermediate that is rapidly trapped in the VO2+ site formed. This trapped intermediate may react by hydrogen migration to the well‐known surface formate (1603, 1371, and 1314 cm−1). However, the long‐lived formate with this spectroscopic signature is only a spectator. It is also visible by DRIFTS in catalyst samples that in the same setup do not produce methanol (GC). By contrast, the band intensities of the surface species at 1759, 1691, 1457 and 1398 cm−1 are directly connected to methanol production and the applied WHSV. Therefore, only these surface species are relevant for CO2 hydrogenation to methanol at higher pressure and likely represent true reaction adsorbates/intermediates.

Aldol condensation reaction of acetone on MgO nanoparticles surface: An in-situ drift investigation

Acetone adsorption and catalytic interactions with a MgO nanoparticle surface was studied using in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) at room temperature. Acetone showed adsorption bands at 2965, 1717, 1597, 1422, 1415, 1367, 1238, 1229, and 1204 cm−1. The MgO was found to be a good catalyst for the activation of the aldol condensation reaction of acetone to form di-acetonealcohol (DAA). The OH group role on the MgO surface was found to be a suppression agent to the further dehydration of DAA to mesityl oxide. This is probably because the OH group blocks the active sites on the MgO surfaces that adsorbed the water formed from the dehydration of DAA. This conclusion was confirmed by identifying the basic site concentrations on MgO surface using DRIFTS and TPD studies of CO2 adsorption/desorption on MgO nanoparticles surface. The high-strength basic sites which are required to dehydrate DDA to mesityl oxide was found to represent only 17.5 % of the total basic site concentration, while the low-strength basic sites resulted from hydroxyl surface group represent 22.5 %. On the other hand, the acetone interaction with the MgO surfaces produced several components such as formaldehyde, acetate, ethoxide, as well as other carbonate species such as bicarbonate, bidentate carbonate, and monodentate carbonate. The results of the present investigation supply important essential insights into catalytic reactions resulted from acetone adsorption on MgO nanoparticle surfaces.

Promoting effect of Fe on supported Ni catalysts in CO2 methanation by in situ DRIFTS and DFT study

Bimetallic NiFe catalysts have emerged as a promising alternative to the traditional Ni catalysts for CO2 methanation. However, the promoting effect of Fe on the bimetallic catalysts remains ambiguous. In this study, a series of NiFe catalysts derived from hydrotalcite precursors were investigated. In situ x-ray diffraction (XRD) analysis revealed that small NiFe alloy particles were formed and remained stable during reaction. When Fe/Ni = 0.25, the alloy catalysts exhibited the highest CO2 conversion, CH4 selectivity and stability in CO2 methanation at low temperature of 250–350 °C. The in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) study indicated that the formate pathway was the most plausible reaction scheme on both Ni and NiFe alloy catalysts, while a moderate addition of Fe facilitated the activation of CO2 via hydrogenation to *HCOO. Density functional theory (DFT) calculations further demonstrated that the overall energy barrier for CH4 formation was lower on the alloy surface.

An insight into the promotional effect on Fe-Co bimetallic catalyst in the Fischer Tropsch reaction: A DRIFTS study

Effect of noble metal promotion over the Fischer Tropsch activity of 10Fe/20Co/SiO2 (weight %) bimetallic catalyst was investigated via a detailed experimental study. The addition of noble metal can promote the structural as well as electronic properties of the catalyst, thus affect the activity and product selectivity of the catalyst. The catalyst 10Fe/20Co/SiO2 was impregnated with varying Rh and Ru loading (0.05 to 1 wt%), and their activity was tested under identical reaction conditions. Different catalyst characterization techniques viz. Brunauer–Emmett–Teller surface area, transmission electron microscope, X-ray diffraction, temperature-programmed reduction, O2 titration and H2 pules chemisorption, were used to study the promotional effect on the catalyst structure. The increased metal loading (Ru and Rh) increases the metal dispersion and the reducibility, resulting in higher H2 chemisorption uptake. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to investigate the adsorption properties of CO on Rh and Ru promoted catalyst with CO and H2 as probe molecules. The correlation was observed between DRIFTS results and experimental results obtained from the laboratory reactor. A clear distinction between the adsorption trend of CO over Ru and Rh promoted catalyst was observed. Also, the increased intensity of bridge bonded CO was observed, resulting in higher wax formation in the Ru promoted catalyst. The promoted catalysts (0.5Ru/10Fe/20Co/SiO2 and 0.5Rh/10Fe/20Co/SiO2) were compared with unpromoted bimetallic catalyst at varying temperatures (220–280 °C). The findings also suggest that Rh promotion enhances the water–gas shift reaction, which significantly changes the H2/CO ratio and the overall catalyst performance.

Large anisotropic negative thermal expansion in Cu-TDPAT metal-organic framework: A combined in situ X-ray diffraction and DRIFTS study

Cu-TDPAT (H6TDPAT = 2,4,6-tris(3,5-dicarboxylphenylamino)-1,3,5-triazine), a stable nanoporous metal-organic framework with rht topology, has sparked broad interest as an adsorbent for several chemical separation processes. In this work, in situ synchrotron diffraction experiments followed by sequential LeBail refinements reveal that Cu-TDPAT shows unusually large anisotropic negative thermal expansion (NTE). The PASCal crystallography tool, used to analyze the magnitude of the NTE, reveals an average volumetric thermal expansion coefficient αv = −20.3 MK−1. This value is significantly higher than the one reported for Cu-BTC (also known as HKUST-1), which contains the same Cu-paddlewheel building unit, αv ≈ −12 MK−1. In situ synchrotron single crystal X-ray diffraction and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were employed to shed light on the NTE mechanism. Using these two methods, we were able to elucidate the three main structural motions that are responsible for the NTE effect. The more pronounced NTE behavior of Cu-TDPAT is attributed to the lower symmetry combined with the more complex ligand structure when compared to Cu-BTC. The knowledge obtained in this work is important for understanding the behavior of the adsorbent under transient variable temperature conditions in fixed adsorption beds.

Insights into CO2 Adsorption in M–OH Functionalized MOFs

A series of benzotriazolate MOFs containing nucleophilic transition metal hydroxide (M–OH) groups has been synthesized to compare the effects of framework structure, metal composition, and method of postsynthetic ligand exchange (PSLE) on CO2 adsorption. Analogues of MFU-4 (1a/b-OH, [Zn5(OH)4(bbta)3], bbta2– = benzo-1,2,4,5-bistriazolate) and MFU-4l (2a/b-OH, [Zn5(OH)4(btdd)3], btdd2– = bis(1,2,3-triazolo)dibenzodioxin) were prepared by direct Cl–/OH– ligand exchange (a) or Cl–/HCO3– ligand exchange followed by thermal activation (b). A Ni/Zn heterobimetallic analogue of MFU-4l (2a/b-NiOH) was also synthesized to investigate the effect of metal identity. The products have been characterized by powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). All of the M–OH functionalized MOFs show steep CO2 adsorption at low partial pressures. However, materials synthesized using the direct Cl–/OH– ligand exchange method show greater low-pressure CO2 uptake than those prepared by Cl–/HCO3– PSLE. Notably, the small pore size in 1a/b-OH not only promotes stronger framework–CO2 interactions and higher CO2 uptake than 2a/b-OH but also results in slow adsorption kinetics. The Ni/Zn heterobimetallic analogue 2a-NiOH exhibits the greatest low-pressure CO2 capacity (1.70 mmol g–1 at 2.6 mbar) among the series. In situ DRIFTS studies reveal that both 2a-OH and 2a-NiOH contain weak Zn–OH binding sites that readily desorb CO2 at room temperature. However, 2a-NiOH also contains strong Ni–OH binding sites that are spectroscopically distinct and only desorb CO2 upon heating.

New insight of the influence of acidic surface sites of zeolite on the ability to remove gaseous ozone using operando DRIFTS studies

Abstract Recently, natural zeolites have started to be used as alternative materials for ozone abatement from working environments. In this study, a surface response methodology based on a D-Optimal design is applied to develop a transition-metal-modified natural zeolite that increases ozone removal efficiency. Ozone adsorption and/or decomposition onto natural and cobalt modified natural zeolite were studied by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Results evidenced that ozone is adsorbed and decomposed at strong Lewis acidic sites, whereas ozone adsorption products interact with surface OH groups. Additionally, DRIFTS studies indicate that nitrous species are adsorbed at acidic sites, reducing the capacity to decompose ozone when ozone is generated from air.

Influence of {111} nanofaceting on the dynamics of CO adsorption and oxidation over Au supported on CeO2 nanocubes: An operando DRIFT insight

Abstract CO adsorption and further reaction with O2 on two Au/CeO2 model systems have been investigated by operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) in order to unveil the active sites for CO oxidation. One of the ceria supports was constituted by crystallites depicting a nanocube shape bounded mostly by {100} planes, whilst the second one was consisted of nanocubes whose {110} surfaces were reconstructed into {111}-nanofacets with a zig-zag shape. The deconvolution of the CO adsorption bands and a detailed analysis of the time-evolution of these bands were also carried out. These studies evidenced a fast adsorption of CO over Au° and then Auδ--CO bands appeared during exposure to CO, in parallel with the appearance of Ce3+ features. Such changes suggest that CO adsorption process starts on Au sites on the low coordinated sites of the surface of the Au nanoparticles and their transfer to the interface sites. By comparing the nanostructure of the two catalysts, the Auδ--CO species could be fruitfully related to CO adsorption on sites located at the Au||CeO2 perimeter. The sample containing gold nanoparticles dispersed over the {111}-nanofaceted CeO2 nanocubes, which retained a higher amount of gold nanoparticles, depicts higher contribution of Auδ−−CO bands than the other Au catalyst in the DRIFT spectra. The dynamic DRIFT study clearly revealed that the CO species adsorbed on Auδ- sites were the most reactive ones in both gold supported systems for CO oxidation. This explains much higher TOF values observed on the catalyst prepared on the surface reconstructed support. A detailed analysis on the reactivity was performed by modulation excitation spectroscopy (MES). The presence of peroxide surface species (O22-) in nanofaceted {111} could lead to higher catalytic activity than that of gold on the non-reconstructed catalyst.

Amine-based CO2 capture sorbents: A potential CO2 hydrogenation catalyst

Mechanism of CO2 adsorption on amine loaded SBA-15 sorbents with varying amine coverage has been assessed using thermogravimetric analysis (TGA) and in-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). TGA study showed that sorbent adsorption capacities increased by 122–153% with CO2 concentrations (5–80%) in the gas at 50/75 °C. The DRIFTS studies indicate that low amine loaded sorbents exhibit higher uptakes with an increase in CO2 concentrations which is attributed to the mode of CO2 adsorption i.e. 1:1 CO2 to amine forming carbamic acid. At higher amine loadings CO2 is adsorbed following the 1:2 CO2 to amine forming carbamate-ammonium ions pair resulting in lower adsorption capacities per amine site. The studies also showed an increase in carbamic acid formation with pressure from 100 to 500 kPa at low amine loadings and both carbamic acid and carbamates at higher amine loadings. The paper provides insights into the mechanistic understanding of CO2 adsorption behaviour of the varyingly covered amines on the SBA-15 support with a change in concentration and pressure of CO2. The current work presents the conditions to alter the CO2 adsorption mechanism on amine sites with potential application in CO2 conversion to chemicals.

DRIFT Study on Promotion Effect of the Keggin Structure over V2O5-MoO3/TiO2 Catalysts for Low Temperature NH3-SCR Reaction

Heteropoly acids (HPAs) with the Keggin structure have been widely used in NOx removal. Two kinds of catalysts (those with and without the Keggin structure) are prepared for studying the effect of the Keggin structure on the NH3-SCR reaction. A series of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) analyses are conducted to investigate the surface-adsorbed species on the catalysts during the SCR reaction. The mechanism for enhancing low-temperature activity of the catalysts is proposed. Furthermore, the effect of NH4+ in the Keggin structure is also investigated. Results indicate that both the Langmuir–Hinshelwood (L-H) and Eley–Rideal (E-R) mechanisms occurred in the NH3-SCR reaction over the catalyst with the Keggin structure (Cat-A); in addition, when more acid sites are provided, NOx species activity is improved and more NH4+ ions participate in reaction over Cat-A, thus promoting SCR activity.

Insight into the reaction mechanism of CO2 activation for CH4 reforming over NiO-MgO: A combination of DRIFTS and DFT study

The interaction mechanisms of dry reforming of methane, especially in the part of CO2 activation on the reduced NiO-MgO catalyst, have been systematically investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density functional theory (DFT) calculations. Based on the results, it is indicated that, what more favorable for CO generation is gaseous CO2 reaction with deposited C intermediates, namely E-R type mechanism, rather than CO2 direct dissociation to form CO and atomic O. In addition, with the help of H species, monodentate carbonate, which derives from the adsorbed CO2 on the MgO surface, can be also activated and hydrogenated dissociation to generate CO.

Reactions of SO2 on hydrated cement particle system for atmospheric pollution reduction: A DRIFTS and XANES study

An investigation of the adsorptive property of hydrated cement particle system for sulfur dioxide (SO2) removal was conducted. In situ and ex situ experiments using Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) and X-ray Absorption Near Edge Spectroscopy (XANES) characterization techniques were employed to identify surface species formed during the exposure to SO2. Oxidation of SO2 to sulfate and sulfite species observed during these experiments indicated dominant reaction pathways for SO2 reaction with concrete constituents, such as calcium hydroxide, which were also moderated by adsorption on porous surfaces of crushed aggregates. The impact of variable composition of concrete on its adsorption capacity and reaction mechanisms was also proposed in this work.

Mechanistic Insights and Kinetics of CO Oxidation over Pristine and Noble Metal Modified Fe2O3 Using Diffuse Reflectance Infrared Fourier Transform Spectroscopy

Fe2O3 is an attractive catalyst for CO oxidation because of its low cost and reducible nature. Modification in reducible oxide by a noble metal enhances its reducibility. In this work, the effect of noble metal (NM: Pt, Pd) impregnation and substitution on the reaction mechanism for CO oxidation has been studied. Detailed ex situ (X-ray diffraction, XRD; X-ray photoelectron spectroscopy, XPS; and transmission electron microscopy, TEM) and in situ diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) characterization were performed along with kinetic studies to develop mechanistic models. Remarkably, the Fe2O3-supported NM exhibited superior catalytic activity than the NM-substituted Fe2O3. Consequently, XPS, photoluminescence, and DRIFTS studies were performed to understand this behavior. Fe2O3-supported NM showed the formation of metal carbonyl bands in DRIFTS studies, however, carbonates were only observed over Fe2O3-supported Pt. On the basis of the spectroscopic evidence and kinetic studies, the Eley–Rideal mechanism was proposed for pristine Fe2O3, whereas noncompetitive and competitive Langmuir–Hinshelwood mechanisms were proposed for Fe2O3-supported Pt and Pd, respectively.

BiOBr microspheres for photocatalytic degradation of an anionic dye

BiOBr microspheres were obtained using a solvothermal synthesis route in the presence of ethylene glycol and KBr at 145 °C, for 18 h. BiOBr microspheres were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), nitrogen adsorption-desorption isotherms analysis, diffuse reflectance spectroscopy (DRS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Additionally, the theoretical and experimental isoelectric points (IEP) of BiOBr nanostructured microspheres were determined, and pH's influence on the degradation of an anionic dye (methyl orange) under simulated solar radiation was analyzed. Results show that 97% of methyl orange is removed at pH 2 after 60 min of photocatalytic reaction. Finally, DRIFTS studies permit the proposal of a surface reaction mechanism of the photocatalytic oxidation of MO using BiOBr microspheres.

DRIFTS study of CO adsorption on Pt nanoparticles supported by DFT calculations

Extensive research has been devoted to the assignment of IR bands of CO adsorbed on Pt nanoparticles, which are widely used in heterogeneous and electrocatalysis (e.g. fuel cells). In contrast to single crystal studies, the assignment of CO adsorption to the nanoparticle structure is still controversial. Here we present a case study where we assign CO adsorption bands to the structure of Platinum nanoparticles with a given size distribution. Using a special diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) cell allows to achieve high quality data under in-situ conditions. Temperature dependent CO adsorption spectra are resolved into three bands which depend on the applied flow and pretreatment conditions. Our calculations using Density Functional Theory (DFT) can mimic the experimental findings and link these bands to the particle structure. By explicitly calculating the IR spectra of CO/Pt nanoparticles of different sizes we show that the IR bands are due to a combined size and site effect. For fully covered small nanoparticles the IR bands are attributed to all binding sites. For larger nanoparticles the dominant contribution is related to {111} facets but the other bands are still site independent. Here we provide a tool to assign CO adsorption bands on Platinum nanoparticles with a given size distribution. This can be related to the structure–acitvity relationship which is required for a tailored catalyst design.

In situ DRIFTS study of CO coupling to dimethyl oxalate over structured Al-fiber@ns-AlOOH@Pd catalyst

Pd-catalyzed CO coupling to dimethyl oxalate (DMO) process has been commercialized but its reaction mechanism is still open for debate between COCOOCH3∗ and COOCH3∗ (∗, a surface site). In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies were performed to clarify such controversy on a high-performance Al-fiber@ns-AlOOH@Pd catalyst. Intermediate species consisting of stretching vibrations of CO and CO, and rocking vibration of CH3 was indeed captured, which could be assigned to either COCOOCH3∗ or COOCH3∗. To make discrimination between COCOOCH3∗ and COOCH3∗, methyl oxalyl chloride (CH3OCOCOCl) and methyl chloroformate (CH3OCOCl) were employed to form COCOOCH3∗ and COOCH3∗ species on the catalyst surface for DRIFTS analyses. Interestingly, the characteristic bands of the as-observed intermediate species in the real reaction matched the obtained COCOOCH3∗ species from dissociative adsorption of CH3OCOCOCl. A double carbonylation reaction pathway was thus confirmed for the CO coupling to DMO, i.e., consecutive insertion of two CO molecules into OCH3∗ to form COCOOCH3∗ followed by combining OCH3∗ to yield DMO (CH3OCOCOOCH3).

Combined EXAFS, XRD, DRIFTS, and DFT Study of Nano Copper-Based Catalysts for CO2 Hydrogenation

Highly monodispersed CuO nanoparticles (NPs) were synthesized via continuous hydrothermal flow synthesis (CHFS) and then tested as catalysts for CO2 hydrogenation. The catalytic behavior of unsupported 11 nm sized nanoparticles from the same batch was characterized by diffuse reflectance infrared fourier transform spectroscopy (DRIFTS), extended X-ray absorption fine structure (EXAFS), X-ray diffraction (XRD), and catalytic testing, under CO2/H2 in the temperature range 25–500 °C in consistent experimental conditions. This was done to highlight the relationship among structural evolution, surface products, and reaction yields; the experimental results were compared with modeling predictions based on density functional theory (DFT) simulations of the CuO system. In situ DRIFTS revealed the formation of surface formate species at temperatures as low as 70 °C. DFT calculations of CO2 hydrogenation on the CuO surface suggested that hydrogenation reduced the CuO surface to Cu2O, which facilitated the formation...