WGS Reaction Research Articles

A general rate equation theory for non– and catalytic coupling gas–solid reactions and its application to sorption-enhanced water gas shift

Sorption-enhanced water gas shift (SEWGS) involves the catalytic WGS reaction of CO with H2O and the non-catalytic gas–solid carbonation reaction of CO2 with CaO. This work proposes a first-principal rate equation theory for non– and catalytic coupling gas–solid reactions. The theory starts from quantum chemistry calculation and extends to larger scales to predict SEWGS experiments conducted in the reactor result without any empirical hypothesis. The coupling mechanism of non– and catalytic reactions is illustrated in a mesoscopic way, based on which the kinetic transition behavior and morphology change on the catalyst surface for WGS can be obtained. The hierarchical framework bridges the scale gap between elementary reactions and pilot plants and the prediction results of and gas profile at the exit of the bubbling bed and carbonation conversion are validated by experiments. The multiscale kinetics is promising to apply to non– and catalytic coupling gas–solid reaction kinetics studies.

A comprehensive thermodynamic analysis of hydrogen and synthesis gas production from steam reforming of propionic acid: Effect of O2 addition and CaO as CO2 sorbent

A thermodynamic analysis was performed for the production of synthesis gas and hydrogen gas from steam reforming of propionic acid, one of the constituents of the bio-oil, using ASPEN Plus v8.8. The aim of the study was to perform a global analysis on steam reforming of propionic acid focusing on several parameters such as steam-to-propionic acid ratio, reaction temperature, addition of O2 in the feed stream and presence of CaO as a CO2 sorbent in the reaction environment. The results showed that higher amounts of steam/propionic acid ratio resulted in higher H2 and CO2 yields. It was also observed that high H2O/PA molar ratios led to reduced coke production as WGS reaction was favored with excess steam, causing CO in the reaction mixture to be consumed through the WGS reaction rather than the Boudouard reaction. Addition of O2 in the reaction environment was observed to negatively affect yield of H2. Yet, coke formation was deprived and increasing the O2 amount in the feed stream decreased the energy required for the process. Furthermore, the presence of CaO enhanced H2 yield and energy requirements of the process compared to conventional steam reforming process. 100 % yield of H2 production with a heat generation of −83.4 MJ/kmol of PA and almost zero formation of CO2, CO, CH4 and coke is achieved at 600 °C for sorption enhanced steam reforming of propionic acid.

Pre-pilot scale study of hydrogen production from biomass syngas via water-gas shift in Pd–Ag catalytic membrane reactor and dedicated hydrogen permeation unit

The water-gas shift reaction (WGS) was evaluated in two Pd–Ag membrane reactors operating in parallel and capable of processing 0.25 Nm3·h−1 of syngas. Various configurations, syngas compositions, and steam flow rates were explored within the temperature range of 300–350 °C and pressure range of 4–8bar. The performances were evaluated in a coupled configuration, consisting of a permeator and a membrane reactor operating in series, and single configuration of membrane reactor. Two syngas mixtures were fed and treated under different combinations of temperature, pressure, and steam/CO ratio. The syngas composition used was, in one case that typically produced by updraft gasifiers operated with air and having a content of H2 19.1 vol% and, in the second case, having content of 36.8 vol%, typically achievable by oxy-steam gasification. Carbon monoxide conversion, hydrogen permeation, and hydrogen permeability of membranes were determined. The use of these membranes effectively enhances the WGS reaction, overcoming the performances of a Gibbs reactor, and produces streams of ultrapure hydrogen. Competitive strong adsorption of CO was suggested from the permeability analysis and comparison with existing literature.

Catalysis/sorption enhanced pyrolysis-gasification of biomass for H2-rich gas production: Effects of various nickel-based catalysts addition and the combination with calcined dolomite

The effects of addition of various Ni-based catalysts and their combination with calcined dolomite on H2-rich gas production were investigated during catalysis/sorption enhanced pyrolysis-gasification of sawdust. It was found that catalyst support greatly affected the catalytic performance of Ni-based catalysts, the highest H2 concentration and yield was obtained by NiO/SiO2, followed by NiO/γ-Al2O3 and NiO/ZSM-5. Additional active sites on Ni-based catalyst would be formed by introduction of secondary metals onto NiO/γ-Al2O3. Positive synergistic effect between secondary metals and Ni was observed, especially for Mg and Co, resulting in further improved H2 production. Calcined dolomite was further introduced into the process to enhance H2 production, simultaneous catalysis and sorption was found to be better for H2 production than sequential catalysis and sorption. The addition of calcined dolomite could offset the catalytic performance difference for varied Ni-based catalysts due to its catalysis/sorption enhancing effect. Comparatively, bimetallic catalysts were more effective than monometallic catalysts when combined use with calcined dolomite, the H2 concentration was significantly improved despite of the slight changes in H2 yield. Ni-Cu/γ-Al2O3 and Ni-Zn/γ-Al2O3 with good performance on WGS reaction showed better synergistic effect with calcined dolomite, a highest H2 concentration of 87.06 vol.% was achieved by Ni-Cu/γ-Al2O3 catalyst.

In situ/operando spectroscopic evidence on associative redox mechanism for periodic unsteady-state water–gas shift reaction on Au/CeO2 catalyst

Cyclic and repeatable CO2/H2 formation under periodic CO ↔ H2O feeds, that is unsteady-state water–gas shift reaction (uss-WGS), was found to be catalyzed by a gold nanoparticles-loaded CeO2 (Au/CeO2) catalyst. Kinetics of the CO-reduction of Ce4+ to Ce3+ and Ce3+ reoxidation by H2O in combination with transient CO2/H2 formation over Au/CeO2 were studied by operando ultraviolet–visible (UV–vis) and infrared (IR) spectroscopies at 175 °C. The Ce4+–OH species were reduced by CO to give Ce3+-□-Ce3+ (□: oxygen vacancy) and gas phase products (H2, CO2). The Ce3+-□-Ce3+ species were oxidized by H2O to give H2 and Ce4+–OH species. The reduction and reoxidation rates of Ce4+/Ce3+ redox couple were close to the rates of transient CO2/H2 formation. The X-ray absorption spectroscopy results showed that the oxidation states of Au remained unchanged during the redox reactions. These results indicate that the uss-WGS is primary driven by the Ce4+/Ce3+ redox couple. Combined with the observation of adsorbed carboxylate intermediates as a precursor of H2 and CO2, associative redox mechanism is proposed as the main pathway for the unsteady-state WGS reaction on Au/CeO2 at 175 °C.

Study of NiO/Al2O3 and NiO/Zn-Al2O3 catalysts for water gas shift reaction

Nickel-based catalysts are of great importance for the generation of hydrogen from natural gas. Within this process, the conditions in which commercial NiO/Al2O3 is converted to Ni0/Al2O3 catalyst should be further investigated. A widely used technique to identify Ni2+ reduction conditions and the different compounds or types of particles in which this element is present is temperature-programmed reduction. In this work, the type of nickel oxide particles occurring on Al2O3 and ZnO-Al2O3-based supports were studied by different techniques, such as energy-dispersive X-ray spectroscopy, FTIR, the BET method, X-ray diffraction (XRD), and Temperature Programmed Reduction (TPR). All materials were evaluated in the water gas shift reaction (WGS), and the influence of their characteristics on the catalytic activity was assessed. Solids were prepared at different temperatures and Ni/Zn molar ratios. The results showed the presence of NiO in all materials, as well as the presence of ZnO, NiAl2O4, and ZnAl2O4 in materials prepared at higher temperatures. In all the materials calcined at the lowest temperature, the formation of NiO particles that fail to interact with the supports was prioritized. As the calcination temperature increased, NiO aggregates were formed, which, to a greater degree, interacted with the supports, whereby nickel aluminate was detected in all materials prepared at 750 °C. Zinc increased the selectivity but decreased specific surface area and activity through the WGS reaction. The solid labeled AZ15-500 showed higher activity and selectivity, reaching values of 100% for the water gas shift reaction.

A new catalytic dehydration strategy by coupling chloride hydrate dehydration with water–gas shift reaction

Thermal dehydration process is an indispensable unit operation involving most of the fields in industry. Driven by the imperious demands in energy consumption reduction and quality improvement, we developed a coupling catalytic dehydration strategy, in which the water in the chloride hydrate was directly used as the reactant to participate in water–gas shift reaction. The autocatalytic process is depended on the chemical reactivity of water interacted with the surrounding atoms in certain degree. The thermodynamic minimum temperatures of dehydration could be reduced significantly by coupling WGS reaction compared to the direct dehydration. With the guidance of thermodynamic calculation, an extremely challenging process of bischofite dehydration was performed experimentally. Study demonstrates that the residual crystallized water number was reduced to 0.64 by coupling WGS reaction at 433 K, while there is still 1.54 via the direct dehydration under the N2 atmosphere. Meanwhile, the amount of by-product MgOHCl is maintained at quite low level, which well solves the seesaw relationship between dehydration and hydrolysis side reaction in traditional process. Subsequently, the autocatalytic mechanism was revealed that CO reacted with the surficial OH group ligated with magnesium to product CO2 and H2, in which the formate species serve as active intermediate species. Furthermore, the universality of such novel strategy was revealed by coupling CaCl2·2H2O dehydration with WGS reaction.

Continuous Unsteady-State De-NOx System via Tandem Water-Gas Shift, NH3 Synthesis, and NH3-SCR under Periodic Lean/Rich Conditions.

The development of urea-free and platinum group metal (PGM)-free catalytic systems for automotive emission control is a challenging task. Herein, we report a new de-NOx system using cyclic feeds of rich and lean gas mixtures with PGM-free catalysts. Initial catalyst screening tests showed that Cu/CeO2 with 5 wt % Cu loading was the most suitable for the water-gas shift reaction (WGS, CO + H2O → CO2 + H2), followed by the selective NH3 synthesis by the NO + H2 reaction. The unsteady-state system under alternating feeds of rich (0.1% NO + 0.5% CO + 1% H2O) and lean (0.1% NO + 2% O2 + 1% H2O) gas mixtures over a mixture of Cu/CeO2 and Cr-exchanged mordenite (CrMOR) showed higher NOx conversion than the steady-state (0.1% NO + 0.35% CO + 0.6% O2 + 1% H2O) reaction between 200 and 500 °C. The de-NOx mechanism under periodical rich/lean conditions was studied by operando infrared (IR) experiments. In the rich period, the WGS reaction on the Cu/CeO2 catalyst yield H2, which reduces NO to NH3 on the Cu/CeO2 catalyst. NH3 is then captured by the Brønsted acid sites of CrMOR. In the subsequent lean period, the adsorbed NH3 acts as a reductant for the selective catalytic reduction of NOx catalyzed by the Cr sites of CrMOR. This study demonstrates a new urea-free and PGM-free catalytic system that can provide an alternative de-NOx technology for automotive catalysis under periodic rich/lean conditions.

Understanding the promotion effect of Pt in the reaction system of water gas shift reaction catalyzed by Pt/α-MoC from theoretical perspectives

Pt/α-MoC is an outstanding catalyst for the low-temperature water gas shift (WGS, CO + H2O → H2 + CO2) reaction, exhibiting elevated activity, stability, and selectivity. In this study, three significant active site models (α-MoC-111, Pt-111, and Pt1/α-MoC-111) in Pt/α-MoC catalyst were established to investigate the promotion effect of Pt on the WGS reaction catalyzed by Pt/α-MoC. The results indicate that a synergistic effect at the metal-carbide interface enhances chemical activity compared to pure Pt-111 and α-MoC-111. Firstly, the surface of Pt1/α-MoC-111 exhibits strong metal-support interaction between Pt single atom and α-MoC-111 surface. Compared to the α-MoC-111 surface, the introduction of Pt strengthens the Mo−Mo bond while weakening the Mo−C bond. Secondly, adding a Pt atom significantly increases adsorption strength for CO and H2O, while decreasing it for products (CO2), oxides (O atom), and by-products (C atom). The advantage of suppressing surface carbon deposition and oxidation is observed on the surface of Pt1/α-MoC-111. Furthermore, elementary reactions from the three mainstream reaction pathways of the WGS reaction are systematically calculated to evaluate catalytic performance among these three active sites. On the Pt1/α-MoC-111 surface, activation energy barriers for H2O dissociation and CO oxidation are reduced. This provides theoretical ideas for designing high-efficiency α-MoC based catalysts in the future.

Boosting reactivity of water-gas shift reaction by synergistic function over CeO2-x/CoO1-x/Co dual interfacial structures

Dual-interfacial structure within catalysts is capable of mitigating the detrimentally completive adsorption during the catalysis process, but its construction strategy and mechanism understanding remain vastly lacking. Here, a highly active dual-interfaces of CeO2-x/CoO1-x/Co is constructed using the pronounced interfacial interaction from surrounding small CeO2-x islets, which shows high activity in catalyzing the water-gas shift reaction. Kinetic evidence and in-situ characterization results revealed that CeO2-x modulates the oxidized state of Co species and consequently generates the dual active CeO2-x/CoO1-x/Co interface during the WGS reaction. A synergistic redox mechanism comprised of independent contribution from dual functional interfaces, including CeO2-x/CoO1-x and CoO1-x/Co, is authenticated by experimental and theoretical results, where the CeO2-x/CoO1-x interface alleviates the CO poison effect, and the CoO1-x/Co interface promotes the H2 formation. The results may provide guidance for fabricating dual-interfacial structures within catalysts and shed light on the mechanism over multi-component catalyst systems.

Investigation of the light-off characteristics and secondary pollutants production mechanism in Pd/Rh catalyst for natural gas engines

To investigate the influence mechanism of the light-off characteristics and secondary pollutants (N2O and NH3) of the Pd/Rh catalytic converter for natural gas engines, a global model of the catalytic converter and a more comprehensive catalytic reaction mechanism were first established. Further, the model and the catalytic reaction mechanism were validated by using experimental data. Then, the effects of exhaust temperature and exhaust composition (H2O, O2, and H2 concentrations) on the catalytic converters were investigated. The results showed that a higher temperature reduced the light-off time t50 of NO and CO. The production of N2O decreased to 0 at high temperatures due to the reduction reactions of R14 (N2O+H2 = H2O+N2) and R16 (N2O+CO = CO2 + N2). NH3 would form at the temperature above 825 K, and the required H2 originated from SR and WGS reactions. Increasing the H2 concentration promoted the conversion of NO and CO. When the temperature exceeds 600 K, increasing the H2 concentration decreases the production of N2O. H2O has the opposite effect on the production of N2O and NH3 at high temperatures. Increasing the content of H2O increased the conversion of NO and inhibited the conversion of CO. O2 concentration at 4000 × 10−6 is beneficial to reduce secondary pollutants of N2O and NH3. At the condition of 730–820 K and 4000 × 10−6 O2, the productions of N2O and NH3 were reduced to 0. This study contributes to the efficient application of Pd/Rh catalytic converters and further enhances the emission reduction of natural gas engines.

Hydrophobic RWGS catalysts: Valorization of CO2-rich streams in presence of CO/H2O

Nowadays, the majority of the Reverse Water Gas Shift (RWGS) studies assume somehow model feedstock (diluted CO2/H2) for syngas production. Nonetheless, biogas streams contain certain amounts of CO/H2O which will decrease the obtained CO2 conversion values by promoting the forward WGS reaction. Since the rate limiting step for the WGS reaction concerns the water splitting, this work proposes the use of hydrophobic RWGS catalysts as an effective strategy for the valorization of CO2-rich feedstock in presence of H2O and CO. Over Fe-Mg catalysts, the different hydrophilicities attained over pristine, N- and B-doped carbonaceous supports accounted for the impact on the activity of the catalyst in presence of CO/H2O. Overall, the higher CO productivity (4.12 μmol/(min·m2)) attained by Fe-Mg/CDC in presence of 20 % of H2O relates to hindered water adsorption and unveil the use of hydrophobic surfaces as a suitable approach for avoiding costly pre-conditioning units for the valorization of CO2-rich streams based on RWGS processes in presence of CO/H2O.

Hydrogen generation from glycerol steam gasification over cobalt loaded MgO–Al2O3 hydrotalcite supports

The objective of this research was to determine the effect of the composition of MgO–Al2O3 in hydrotalcite (HT) on the performance of cobalt catalysts in glycerol steam reforming (GSR) for hydrogen. The co-precipitation method was used to prepare five hydrotalcite supports with varying MgO:Al2O3 compositions (1:1, 1:3, 3:1, 1:5, and 5:1), which were then loaded with a 20 wt% Co using the incipient wetness impregnation method. All the catalysts were systematically characterized by X-ray diffraction and temperature-programmed studies (H2 reduction, both CO2 and NH3 desorption). The impregnation of cobalt oxide on hydrotalcite produced different types of cobalt oxides depending on the hydrotalcite composition. HT with high MgO to Al2O3 (3:1, 5:1) ratios favored the formation of a Co-(Mg)–O solid solution, while HT with low MgO to Al2O3 (1:3 and 1:5) ratios favored the formation of Co2AlO4 and CoAl2O4. On the other hand, HT with a 1:1 mol ratio of MgO to Al2O3 minimized the formation of Co-(Mg)–O solid solution and cobalt aluminates while encouraging the formation of mixed phases (Co3O4, Co2AlO4, and CoAl2O4). Interlinked mesopores (N2-physisorption) were observed in the 20 wt% Co/(1:1) MgO:Al2O3 catalyst, and its surface was discovered to be amphoteric (from NH3 and CO2 TPD). Each catalyst was investigated for hydrogen production from glycerol in the range of 400–700 °C. The best hydrogen yield and glycerol conversion to vapor-phase achieved at 600 °C on the 20 wt% Co/(1:1) MgO–Al2O3 catalyst are due to high reducibility and an adequate amount of cobalt available on the surface. The amphoteric behavior of the catalyst prevented hydrogen consumption in the methanation reaction. It is also speeding up the WGS reaction and enhancing the hydrogen yield.

Production of high-purity H2 through sorption-enhanced water gas shift over a combination of two intermediate-temperature CO2 sorbents

Sorption-enhanced water gas shift (SEWGS) that integrates the WGS reaction and in situ CO2 removal in one reactor is a promising technology for producing high-purity hydrogen. In this study, a series of Mg–Al hydrotalcite-based CO2 sorbents were prepared, characterized, and evaluated at low CO2 pressure (0.01 bar). It shows that the layered double oxide prepared at pH = 10 (abbr. LDO_10) has a higher CO2 adsorption uptake at 30–400 °C than other LDO_x, owing to its large surface area. After doping with M2CO3 (M = Li, Na, K, or Cs), the resulting M-LDO_10 sorbents exhibit different CO2 uptake, with Li-LDO_10 the lowest and K-LDO_10 the highest. It is considered that the high total basicity contributed to the high CO2 uptake of K-LDO_10. A single-layered reactor (physically mixed Cu/Ce0·6Zr0·4O2 (catalyst) and K-LDO_10) can realize a stable production of high-purity H2 in 10 SEWGS cycles, but the duration time is rather short (≤1 min). By combining our previously developed MgO-based sorbent (AMS-Mg95Ca5) with K-LDO_10 in a four-layered configuration, i.e., three (Cu/Ce0·6Zr0·4O2|AMS-Mg95Ca5) layers followed by one (Cu/Ce0·6Zr0·4O2 + K-LDO_10) layer, high-purity H2 (>99.9%) without CO contamination is stably produced with extended duration time (22–31 min) in 10 SEWGS cycles (300 °C, 12 bar, and H2O/CO molar ratio of 1.5).

Water-gas-shift reaction over Au single-atom catalysts with reversible oxide supports: A density functional theory study

The low-temperature water-gas-shift (LT-WGS) reaction has shown remarkable activity for Au single-atom supported on reducible oxide catalysts. The water dissociation step of the WGS reaction is calculated using density functional theory (DFT) over four single Au atom (Au1)-pristine reversible oxides support systems Au1/MaOb-Ov (Au1/TiO2-x, Au1/ZrO2-x, Au1/CeO2-x and Au1/Co3O4-x) system and four Au1-supports with O vacancy (Ov) systems Au1/MaOb (Au1/TiO2, Au1/ZrO2, Au1/CeO2 and Au1/Co3O4). According to its greatest H2O adsorption energy, lowest water dissociation barrier, and slightest structural distortion, Au1/CeO2-x is chosen as the most beneficial catalyst for the WGS process. Afterwards, for Au1/CeO2-x, three reaction pathways (redox path, formate path and carboxyl path) are calculated. The predominant reaction pathway is the carboxyl pathway, and hydrogen production is the rate-determining step (RDS). For the purpose of designing single metal-support catalysts for the LT-WGS reaction, this paper gives information on the strong metal-support interaction (SMSI).

CeO2/Pt/Al2O3 catalysts for the WGS reaction: Improving understanding of the Pt-O-Ce-Ox interface as an active site

In this work, we demonstrate that knowledge of the structural parameters of Pt nanoparticles (PtNp) supported on ceria/alumina is essential for understanding the surface structure, the electronic properties of PtNp, and the effect of ceria on the catalytic activity in the water-gas shift reaction. The similar Pt-Pt and Pt-O bond distances for different PtNp sizes suggested similar electronic density of the Pt sites, indicating that changes of the electronic properties of the Pt sites, due to the size effect, were compensated by the presence of Pt-O species. The turnover frequency based on Pt0 sites varied between 2 and 10 times higher, compared to unpromoted catalysts, and was dependent on the PtNp size and morphology, which determined the density of oxygen on the surface and the density of Pt-O-Ce sites. The effect of ceria in increasing the activity could be explained by the creation of active Pt-O-Ce sites at the surface.

Sr(II) and Ba(II) Alkaline Earth Metal-Organic Frameworks (AE-MOFs) for Selective Gas Adsorption, Energy Storage, and Environmental Application.

Two new alkaline earth metal-organic frameworks (AE-MOFs) containing Sr(II) (UPJS-15) or Ba(II) (UPJS-16) cations and extended tetrahedral linker (MTA) were synthesized and characterized in detail (UPJS stands for University of Pavol Jozef Safarik). Single-crystal X-ray analysis (SC-XRD) revealed that the materials are isostructural and, in their frameworks, one-dimensional channels are present with the size of ~11 × 10 Å2. The activation process of the compounds was studied by the combination of in situ heating infrared spectroscopy (IR), thermal analysis (TA) and in situ high-energy powder X-ray diffraction (HE-PXRD), which confirmed the stability of compounds after desolvation. The prepared compounds were investigated as adsorbents of different gases (Ar, N2, CO2, and H2). Nitrogen and argon adsorption measurements showed that UPJS-15 has SBET area of 1321 m2 g-1 (Ar) / 1250 m2 g-1 (N2), and UPJS-16 does not adsorb mentioned gases. From the environmental application, the materials were studied as CO2 adsorbents, and both compounds adsorb CO2 with a maximum capacity of 22.4 wt.% @ 0 °C; 14.7 wt.% @ 20 °C and 101 kPa for UPJS-15 and 11.5 wt.% @ 0°C; 8.4 wt.% @ 20 °C and 101 kPa for UPJS-16. According to IAST calculations, UPJS-16 shows high selectivity (50 for CO2/N2 10:90 mixture and 455 for CO2/N2 50:50 mixture) and can be applied as CO2 adsorbent from the atmosphere even at low pressures. The increased affinity of materials for CO2 was also studied by DFT modelling, which revealed that the primary adsorption sites are coordinatively unsaturated sites on metal ions, azo bonds, and phenyl rings within the MTA linker. Regarding energy storage, the materials were studied as hydrogen adsorbents, but the materials showed low H2 adsorption properties: 0.19 wt.% for UPJS-15 and 0.04 wt.% for UPJS-16 @ -196 °C and 101 kPa. The enhanced CO2/H2 selectivity could be used to scavenge carbon dioxide from hydrogen in WGS and DSR reactions. The second method of applying samples in the area of energy storage was the use of UPJS-15 as an additive in a lithium-sulfur battery. Cyclic performance at a cycling rate of 0.2 C showed an initial discharge capacity of 337 mAh g-1, which decreased smoothly to 235 mAh g-1 after 100 charge/discharge cycles.

The Nature of Interfacial Catalysis over Pt/NiAl2O4 for Hydrogen Production from Methanol Reforming Reaction.

Reforming of methanol is one of the most favorable chemical processes for on-board H2 production, which alleviates the limitation of H2 storage and transportation. The most important catalytic systems for methanol reacting with water are interfacial catalysts including metal/metal oxide and metal/carbide. Nevertheless, the assessment on the reaction mechanism and active sites of these interfacial catalysts are still controversial. In this work, by spectroscopic, kinetic, and isotopic investigations, we established a compact cascade reaction model (ca. the Langmuir-Hinshelwood model) to describe the methanol and water activation over Pt/NiAl2O4. We show here that reforming of methanol experiences methanol dehydrogenation followed by water-gas shift reaction (WGS), in which two separated kinetically relevant steps have been identified, that is, C-H bond rupture within methoxyl adsorbed on interface sites and O-H bond rupture within OlH (Ol: oxygen-filled surface vacancy), respectively. In addition, these two reactions were primarily determined by the most abundant surface intermediates, which were methoxyl and CO species adsorbed on NiAl2O4 and Pt, respectively. More importantly, the excellent reaction performance benefits from the following bidirectional spillover of methoxyl and CO species since the interface and the vacancies on the support were considered as the real active component in methanol dehydrogenation and the WGS reaction, respectively. These findings provide deep insight into the reaction process as well as the active component during catalysis, which may guide the design of new catalytic systems.

Influencia del pretratamiento en los nanocompuestos de CeO2 y Au/CeO2 para mejorar la creación de defectos superficiales que permitan modificar la interbanda óptica

Cerium oxide as a material within heterogeneous catalysis remains a relevant research topic due to its redox behavior, directly related to the presence of oxygen vacancies (Vo-) which are responsible for ceria’s high oxygen storage capacity. CeO2 also have been employed as a support for active noble metal particles; in particular, supported gold catalyst has been proposed as a candidate to be used in different reactions such as CO oxidation, catalytic combustion of hydrocarbons, WGS reaction, among others. Despite its relevance, few works have a detailed understanding of the pretreatment effect on these catalysts. For this reason, in this work, we study the in situ behavior of CeO2 and Au/CeO2 during different pretreatments in temperature and atmosphere by Raman and Diffuse reflectance UV-Vis. These techniques allow us to follow in real-time the surface changes of Au nanoparticles and CeO2. We demonstrate a direct correlation lattice structural defects of CeO2 with modifications formed by electronic states in the optical interband and, the deposition of Au nanoparticles on the surface of CeO2 allows to improve the properties formed by the electronic states between the valence band and the conduction band by increasing more than twice the structural defects compared to CeO2 alone.

NiAlxFe2−xO4 mixed oxide catalysts for methane reforming with CO2: Effect of Al vs Fe contents and precursor salts

NiAlxFe2−xO4 (0 ≤x ≤ 2) was synthesized, characterized and evaluated in methane reforming with CO2 without any pretreatment. It was shown that the nature of the precursor has a significant effect on the structural, textural and reactivity properties. The cell parameters decrease with increasing Al-content which can be explained by the partial substitution of Fe3+ ions (∼0.63 Å in tetrahedral site and ∼0.78 Å and high spin in octahedral site) by Al3+ ions (∼0.53 Å) either in the tetrahedral or the octahedral sites. If, on the one hand, the catalytic activity of the aluminum compound exhibits high CH4 and CO2 conversions as well as excellent syngas selectivity, on the other hand, a poor activity is observed for NiFe2O4 whatever the precursors. For low temperatures (T < 750 °C), powders prepared with chlorides are more active than powders prepared with nitrates. However, this behavior mitigates at high temperature. NiAl2O4 issued from nitrate precursors shows the best catalytic performances. Negligible contributions to WGS Reaction are illustrated by H2/CO ratios close to stoichiometry. The intermediate FeAl2O4 spinel phase plays an important role as catalytic precursor in the in-situ production of Ni° nanoparticles, highly dispersed and less prone to coke formation in spite of the severe reaction conditions. A lack of affinity of Al-species to combine with Ni° to form detrimental Ni-Al alloy is proposed to explain the good performances. Strong interactions developed between metallic active sites and mixed oxides substrates allow the stabilization and improvement of catalytic performances in dry reforming of methane.