Ultrahigh temperature water gas shift catalysts to increase hydrogen yield from biomass gasification

Noble metal (Rh, Pt, Pd, Ir, Ru, and Ag) and Ni catalysts supported on CeO₂–Al₂O₃ were investigated for water gas shift reaction at ultrahigh temperatures. Pt/CeO₂–Al₂O₃ and Ru/CeO₂–Al₂O₃ demonstrated as the best catalysts in terms of activity, hydrogen yield and hydrogen selectivity. At 700 °C and steam to CO ratio of 5.2:1, Pt/CeO₂–Al₂O₃ converted 76.3% of CO with 94.7% of hydrogen selectivity. At the same conditions, the activity and hydrogen selectivity for Ru/CeO₂–Al₂O₃ were 63.9% and 85.6%, respectively. Both catalysts showed a good stability over 9 h of continuous operation. However, both catalysts showed slight deactivation during the test period. The study revealed that Pt/CeO₂–Al₂O₃ and Ru/CeO₂–Al₂O₃ were excellent ultrahigh temperature water gas shift catalysts, which can be coupled with biomass gasification in a downstream reactor.     

Kinetics of the water–gas shift reaction over nanostructured copper–ceria catalysts

Water–gas shift reaction was studied over two nanostructured Cu_xCe_1−xO_2−y catalysts: a Cu_0.1Ce_0.9O_2−y catalyst prepared by a sol–gel method and a Cu_0.2Ce_0.8O_2−y catalyst prepared by co-precipitation method. A commercial low temperature water–gas shift CuO–ZnO–Al₂O₃ catalyst was used as reference. The kinetics was studied in a plug flow micro reactor at an atmospheric pressure in the temperature interval between 298 and 673 K at two different space velocities: 5.000 and 30.000 h⁻¹, respectively. Experimentally estimated activation energy, E_af, of the forward water–gas shift reaction at CO/H₂O = 1/3 was 51 kJ/mol over the Cu_0.1Ce_0.9O_2−y, 34 kJ/mol over the Cu_0.2Ce_0.8O_2−y and 47 kJ/mol over the CuO–ZnO–Al₂O₃ catalyst. A simple rate expression approximating the water–gas shift process as a single reversible surface reaction was used to fit the experimental data in order to evaluate the rate constants of the forward and backward reactions and of the activation energy for the backward reaction.     

Effects of tar model compounds on commercial water gas shift catalysts

One attractive application of the catalytic water gas shift (WGS) reaction is the production of a syn-gas with high hydrogen concentration from gasification of solid combustibles. The catalyst's behavior can be affected by the hydrocarbons with high molecular weight (tar) still present in the gas after its purification. In this work the effect of some tar model compounds on two typical commercial WGS catalysts were investigated. A low temperature catalyst composed of Cu/Zn/Al and a high temperature catalyst based mainly on Fe and Cr were tested. N-hexadecane, fluorene, phenol, and octanol were used as tar model compounds. Besides these compounds, the effect of biodiesel was also investigated. In all cases a concentration of 0.5 g/Nm³ was used. Generally it was observed that a low concentration is often sufficient to produce a rapid deactivation. Catalysts were characterized by means of BET and XRD.     

Kinetics of the water–gas shift reaction on Pt catalysts supported on alumina and ceria

We report the kinetic parameters for the water–gas shift (WGS) reaction on Pt catalysts supported on ceria and alumina under fuel reformer conditions for fuel cell applications (6.8% CO, 8.5% CO₂, 22% H₂O, 37.3% H₂, and 25.4% Ar) at a total pressure of 1 atm and in the temperature range of 180–345 °C. When ceria was used as a support, the turnover rate (TOR) for WGS was 30 times that on alumina supported Pt catalysts. The overall WGS reaction rate (r) on Pt/alumina catalysts as a function of the forward rate (r_f) was found to be: r = r_f(1 − β), where r_f = k_f[CO]^0.1[H₂O]^1.0[CO₂]^−0.1[H₂]^−0.5, k_f is the forward rate constant, β = ([CO₂][H₂])/(K_eq[CO][H₂O]) is the approach to equilibrium, and K_eq is the equilibrium constant for the WGS reaction. The negative apparent reaction orders indicate inhibition of the forward rate by CO₂ and H₂. The surface is saturated with CO on Pt under reaction conditions as confirmed by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The small positive apparent reaction order for CO, in concert with the negative order for H₂ and the high CO coverage is explained by a decrease in the heat of adsorption as the CO coverage increases. Kinetic models based on redox-type mechanisms can explain the observed reaction kinetics and can qualitatively predict the changes in CO coverage observed in the DRIFTS study.     

Water-gas shift: steady state isotope switching study of the water-gas shift reaction over Pt/ceria using in-situ DRIFTS

The stability of surface formates generated by reaction of bridging OH groups with CO is an important first criterion supporting the idea that the rate limiting step of WGS involves formate decomposition. The second important factor is that, in the presence of water, shown directly by the measurements obtained during this steady state isotope switching study, the forward decomposition of surface formates to CO₂ and H₂ is strongly auto-catalyzed by H₂O, in agreement with the findings of Shido and Iwasawa. Based on a normal kinetic isotope effect previously observed with H₂O:D₂O switching and the response of surface formate coverages to the WGS rate under steady state conditions when a high H₂O:CO ratio is employed, the conclusion is drawn that a surface formate mechanism is likely operating for the low temperature water gas shift reaction.     

Evaluation of carbon monoxide oxidation over CeO2/Co3O4 catalysts: Effect of ceria loading

Modification of cobaltic oxide (obtained from the reduction of high-valence cobalt oxide and assigned as R230, S_BET = 100 m² g⁻¹) with different loading of ceria was proceeded using the impregnation method (assigned as CeX/R230, X = 4, 12, 20, 35 and 50 wt%). The CeX/R230 catalysts were characterized by X-ray diffraction (XRD), nitrogen adsorption at −196 °C, temperature-programmed reduction (TPR) and transmission electron microscopy (TEM). Their catalytic activities towards the CO oxidation were studied in a continuous flow micro-reactor. The results revealed that the optimal modification, i.e., Ce20/R230, can increase the surface area (S_BET = 109 m² g⁻¹) of cobaltic oxide, further weaken the bond strength of CoO and lower the activation of CO oxidation among CeX/R230 catalysts due to the combined effect of cobaltic oxide and ceria. The Ce20/R230 catalyst exhibited the best catalytic activity in CO oxidation with T₅₀ (temperature for 50% CO conversion) at 88 °C.     

Support‐dependent activity of noble metal substituted oxide catalysts for the water gas shift reaction

The water gas shift reaction was carried out over noble metal ion substituted nanocrystalline oxide catalysts with different supports. Spectroscopic studies of the catalysts before and after the reaction showed different surface phenomena occurring over the catalysts. Reaction mechanisms were proposed based upon the surface processes and intermediates formed. The dual site mechanism utilizing the oxide ion vacancies for water dissociation and metal ions for CO adsorption was proposed to describe the kinetics of the reaction over the reducible oxides like CeO₂. A mechanism based on the interaction of adsorbed CO and the hydroxyl group was proposed for the reaction over ZrO₂. A hybrid mechanism based on oxide ion vacancies and surface hydroxyl groups was proposed for the reaction over TiO₂. The deactivation of the catalysts was also found to be support dependent. Kinetic models for both activation and deactivation were proposed. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

Cr2O3 promoted skeletal Cu catalysts for the reactions of methanol steam reforming and water gas shift

Promotional effects of chromia on the structure and activity of skeletal copper catalysts for methanol steam reforming and water gas shift have been studied. Catalysts were prepared by leaching CuAl₂ alloy particles in aqueous NaOH solutions containing sodium chromate at various concentrations. XPS spectra showed that the surface of the resulting catalysts mainly consisted of Cr³⁺ compounds and Cu⁰. Cu⁺ and/or Cu²⁺ were not observed by XPS.

Increasing the concentration of chromate in the leach liquor resulted in decreases in pore diameter and copper crystallite size but significant enhancement of BET surface area was observed while the total pore volume was maintained. The addition of small amounts of chromate to the leach liquor significantly enhanced the Cu surface area. However, higher concentrations of chromate in the leach liquor decreased the Cu surface areas although the total surface areas increased.

The activities of Cr₂O₃ promoted skeletal copper catalysts for both methanol steam reforming and water gas shift reactions were determined separately. The results indicated that deposition of Cr₂O₃ on skeletal copper catalysts significantly improved the specific activities for these reactions. Chromia is found to act as a structural and catalytic promoter for these reactions.     

Hydrogen production for fuel cell by oxidative reforming of diesel surrogate: Influence of ceria and/or lanthana over the activity of Pt/Al2O3 catalysts

A series of Pt catalysts supported on Al₂O₃ (Pt/A), Al₂O₃-CeO₂ (Pt/A-C), Al₂O₃-La₂O₃ (Pt/A-L) and Al₂O₃-La₂O₃-CeO₂ (Pt/A-L-C) have been prepared and tested in the oxidative reforming of diesel surrogate with the aim of studying the influence of ceria and lanthana additives over the activity and stability toward hydrogen production for fuel cell application. Several characterization techniques, such as adsorption-desorption of N₂, X-ray diffraction, X-ray photoelectron spectroscopy, temperature programmed reduction, H₂ chemisorption, and thermogravimetric analysis, have been used to define textural, structural, and surface properties of catalysts and to establish relationships with their behaviour in reaction. This physicochemical characterization has shown that lanthana inhibits the formation of α phase in alumina support and decreases ceria dispersion. Activity results show a better performance of ceria-loaded catalysts, being the Pt/A-C sample the system that offers higher H₂ yields after 8 h of reaction. The greater H₂ production for ceria-loaded catalysts, particularly in the case of the system Pt/A-C, is attributed to the Pt-Ce interaction that may change the electronic properties and/or the dispersion of active metal phase. Also, the Ce^III form of Ce^IV/Ce^III redox pair enhances the adsorption of oxygen and water molecules, thus increasing the catalytic activity and also decreasing coke deposition over surface active Pt phases. Stability tests showed that catalysts in which Pt crystallites are deposited on the alumina substrate covered by a lanthana monolayer, give rise to an increase in stability toward H₂ production.     

Promotion effect of Re additive on the bifunctional Ni catalysts for methanation coupling with water gas shift of biogas: Insights from activation energy

The cheap manganese sand was first modified by H₂O₂ and was further creatively utilized as Ni-based catalyst support. In order to enhance the catalytic performance, Re was added into the Ni-based catalyst and the promotion effect of Re on the methanation coupling with water gas shift of biogas was investigated from the perspective of activation energy. It was found that CH₄ and CO₂ formation rates, which separately represented the reaction rate of methanation and water gas shift, were both enhanced after Re addition compared to non-added catalyst. Two kinetics models including empirical model and K-model were employed and from the results of calculation, it showed that Re selectively decreased the activation energy of methanation reaction and had little impact on the activation energy of water gas shift. The increased CO₂ formation rate was owing to the assistance of accelerated H₂O production from methanation rather than the activation energy change in water gas shift.     

CO preferential oxidation over Au/MnOx-CeO2 catalysts prepared with ultrasonic assistance: Effect of calcination temperature

The Au/MnO_x-CeO₂ catalysts used for CO preferential oxidation were prepared by deposition-precipitation with ultrasonic assistance. The effect of calcination temperature (150-350 °C) on the structures and catalytic performance of the catalysts was systematically investigated. It is found that the catalyst Au/MnO_x-CeO₂ calcined at 250 °C exhibits the best catalytic performance, giving not only the highest CO conversion of 90.9% but also the highest selectivity of oxygen to CO₂ at 120 °C. The results of XRD, TEM and XPS indicate that this catalyst possesses the smallest particle size, the highest dispersion of Au species and the largest amount of surface adsorbed oxygen species, which are favorable to CO oxidation. The H₂-TPR results reveal that the selectivity of oxygen to CO₂ is mainly determined by the reducibility of Au species in the catalysts. The strong interaction between Au species and the support in Au/MnO_x-CeO₂-250 decreases its capability for H₂ dissociation and oxidation, leading to high selectivity of oxygen to CO₂.     

Influence of the catalyst surface area on the activity and stability of Au/CeO2 catalysts for the low-temperature water gas shift reaction

The influence of the support surface area on the activity and stability/deactivation of Au/CeO₂ catalysts (2.7 wt% Au) in the water gas shift reaction in dilute water gas were investigated by kinetic measurements and in situ Diffuse Reflectance IR spectroscopy. For ceria support surface areas between 24 and 284 m² g⁻¹, the gold particle size is independent on the catalyst surface area (about 2.1 nm) up to 188 m² g⁻¹, and we found increased amounts of (i) Auⁿ⁺, (ii) Ce³⁺, (iii) OH groups, and (iv) carbon containing adsorbed side products such as formates and carbonates for increasing surface area supports. Consequences of these results on the mechanistic understanding of the reaction are discussed.     

Low Temperature Water–Gas Shift/Methanol Steam Reforming: Alkali Doping to Facilitate the Scission of Formate and Methoxy C–H Bonds over Pt/ceria Catalyst

Doping Pt/ceria catalysts with the Group 1 alkali metals was found to lead to an important weakening of the C–H bond of formate and methoxy species. This was demonstrated by a shift to lower wavenumbers of the formate and methoxy ν(CH) vibrational modes by DRIFTS spectroscopy. Li and Na-doped Pt/ceria catalysts were tested relative to the undoped catalyst for low temperature water–gas shift and methanol steam reforming using a fixed bed reactor and exhibited higher catalytic activity. Steaming of formate and methoxy species pre-adsorbed on the catalyst surface during in-situ DRIFTS spectroscopy suggested that the species were more reactive for dehydrogenation steps in the catalytic cycle for the Li and Na-doped catalysts relative to undoped Pt/ceria. However, with increasing atomic number over the series of alkali-doped catalysts, the stability of a fraction of the carbonate species was found to increase. This was observed during TPD-MS measurements of the adsorbed CO₂ probe molecule by a systematic increase of a high temperature peak for a fraction of the CO₂ desorbed. This result indicates that alkali-doping is an optimization problem—that is, while improving the dehydrogenation rates of methoxy and formate species, the carbonate intermediate stability increases, making it difficult to liberate the CO₂. Infrared spectroscopy results of CO adsorbed on Pt and ceria suggest that the alkali dopant is located on, and electronically modifies, both the Pt and ceria components. The results not only lend further support to the role that methoxy and formate species play as intermediates in the catalytic mechanisms, but also provide a path forward for improving rates by means other than resorting to higher noble metal loadings.     

Mechanism of deactivation of Au/Fe2O3 catalysts under water–gas shift conditions

The stability of Au supported catalysts for the water–gas shift reaction was studied. Two types of continuous flow experiments were performed, i.e. temperature-programmed and long-term isothermal stability test. The highest initial rate was obtained for catalysts used without any calcination or other high-temperature treatment. The continuous flow experiments showed that all Au/Fe₂O₃ catalysts deactivated under water–gas shift conditions. The deactivation trend occurred independently on the Au loading, the method of preparation, calcination or high-temperature treatment. The various causes on the deactivation, i.e. the formation of carbon-containing species, the change of Au particles or changes of the support were investigated in terms of DRIFTS coupled with MS, TGA, TEM, N₂ physisorption, ICP, and XRD. Even though stable carbonate and carbonyl surface species were found on the spent catalysts, the quantity of these species indicated that they are not the main cause of the deactivation. Furthermore, the agglomeration of the Au particles was not severe and was not observed for all Au catalysts. The deactivation of Au/Fe₂O₃ was mainly caused by the change of the support. A significant reduction of the surface area of the support is taking place during the water–gas shift reaction as a function of time on stream. This decrease of the surface area can almost solely explain the decrease on the catalytic activity.     

Comparison of natural ilmenites as oxygen carriers in chemical-looping combustion and influence of water gas shift reaction on gas composition

Chemical-looping combustion (CLC) is a promising technology for CO₂-capture for storage or reuse as a method to mitigate CO₂ emissions from the use of fossil fuels. In a CLC system the oxygen carrier is of great importance. Environmentally sound and low cost materials seem to be preferable especially for CLC of solid fuels. The natural occurring ore ilmenite has already been the target of different studies in order to work out its feasibility as oxygen carrier for different fuels. The initial part of this work is a screening of five commercial available ilmenite minerals as oxygen carrier, crushed and sieved to 125–180 μm. The screening includes an examination of the sulfur released during the first heat up and the activation of the oxygen carrier, indicated by the fuel conversion using alternating reduction (syngas 50 vol.% CO in H₂) and oxidation conditions (10 vol.% O₂ in N₂). The five first cycles were carried out at 850 °C to avoid initial agglomeration whereas the main activation cycles have been performed at 950 °C in a tubular quartz reactor under fluidized bed conditions. From these experiments it is concluded that rock ilmenites are preferable as oxygen carriers since they revealed an improved fuel conversion, although offering a higher sulfur content, which is released during the initial heat up.     

Study of the water gas shift reaction on Fe in a high temperature proton conducting cell

The water gas shift (WGS) reaction was studied in a double-chamber high temperature proton conducting cell (HTPC). The proton conductor was a strontia–ceria–ytterbia (SCY) disk of the form: SrCe_0.95Yb_0.05O₃₋ and the working electrode was a polycrystalline Fe film. The reaction temperature and the inlet partial pressure of CO varied between 823 and 973 K, and between 1.0 and 10.6 kPa, respectively. The inlet partial pressure of steam (P_H2O) was kept constant at 2.3 kPa. An increase in the production of H₂ was observed upon “pumping” protons away from the catalyst surface. The Faradaic efficiency (Λ) was lower than unity, indicating a sub-Faradaic effect. The highest value of rate enhancement ratio (ρ) was approximately 3.2, at T = 823 K. The proton transport number (PTN) varied between 0.45 and 1.0. An up to 99% of the produced H₂ was electrochemically separated from the reaction mixture.     

The CO–H2 and CO–H2O reactions over TiO2 nanotubes filled with Pt metal nanoparticles

We have investigated the catalytic behavior of Pt encapsulated TiO₂ nanotubes for the water gas shift reaction as well as the hydrogenation of CO. Pt–TiO₂ nanotube catalysts were prepared by employing fine fiber shaped crystals of [Pt(NH₃)₄](HCO₃)₂ complex as a structure determining template material. The turnover frequencies (TOF) of these nanotube catalysts were more than one order of magnitude larger than conventional impregnation Pt/TiO₂ catalysts, and the selectivity for methanol in CO–H₂ reaction was extraordinary high compared to the impregnation catalysts. The XPS and XRD analyses of the nanotubes revealed characteristic electronic state of reduced TiO₂ (Ti³⁺ in rutile structure) with zerovalent Pt even after the calcination at 773 K. In WGS reaction, electron rich Ti³⁺ on the nanotube wall may play an important role to activate water molecules for the oxidation of CO. In CO–H₂ reaction, similar promotion effect of Ti³⁺ species may be operating for selective methanol formation by supplying active OH(a).     

A comparative study of water gas shift reaction over gold and platinum supported on ZrO2 and CeO2–ZrO2

In this study platinum- and gold-based catalysts supported on ZrO₂ and ceria–zirconia solid solution have been characterized by several techniques (TPR, XRD, BET, HRTEM) and tested in the water gas shift (WGS) reaction under feed conditions typical of an autothermal reformer outlet. Platinum and gold catalysts behave differently especially in the range of 423–513 K, with gold being superior than platinum. The possibility of modifying the redox and structural characteristics of zirconia with the insertion of ceria allowed us to conclude that the bulk redox properties of the support play a secondary role, while the key parameter for an active WGS catalyst is the nature of metal support interface. This, in turn, depends on the metal particle distribution and on the structural and morphological properties of support. It has been found that the synergism between precious metals and support can be designed with an appropriate choice of the parameters of synthesis and the characteristics of support.     

The role of cationic Au and nonionic Au species in the low-temperature water–gas shift reaction on Au/CeO2 catalysts

The roles of cationic and nonionic Au species in the water–gas shift (WGS) reaction on Au/CeO₂ catalysts were studied by comparing the reaction behavior of a cyanide leached catalyst, after removal of the Au nanoparticles by cyanide leaching, with that of non-leached catalysts, following the technique introduced by Q. Fu et al. [Science 301 (2003) 935]. Using rate measurements as well as in situ spectroscopic and structure-sensitive techniques, we found that based on the Au mass balance, cyanide leaching removed all Au except for ionic Au³⁺ species, and that leaching resulted in a pronounced decay of the catalyst mass normalized activity to 1–25% of that of a non-leached catalyst. The extent of the activity loss strongly depended on the post-treatment of the leached catalyst. Both the catalyst treatment after leaching and, in particular, the WGS reaction resulted in considerable reformation of Au⁰ species by thermal decomposition of Au oxides (Au³⁺) and subsequent nucleation and growth of very small Au⁰ aggregates and metallic Au⁰ nanoparticles, as indicated by Au(4f) signals at 85.9 eV (Au³⁺), 84.0–84.6 eV (up-shifted signal of small Au⁰ aggregates), and 84.0 eV (metallic Au⁰). In this work, correlations between ionic and nonionic Au species and between total WGS activity and activity for the formation/decomposition of bidentate formate species are evaluated, and the role of the respective Au species in the WGS reaction on Au/CeO₂ catalysts is discussed.