Reduction of CO<Subscript>2</Subscript> to CO via reverse water-gas shift reaction over CeO<Subscript>2</Subscript> catalyst

CeO₂ catalysts with different structure were prepared by hard-template (Ce-HT), complex (Ce-CA), and precipitation methods (Ce-PC), and their performance in CO₂ reverse water gas shift (RWGS) reaction was investigated. The catalysts were characterized using XRD, TEM, BET, H₂-TPR, and in-situ XPS. The results indicated that the structure of CeO₂ catalysts was significantly affected by the preparation method. The porous structure and large specific surface area enhanced the catalytic activity of the studied CeO₂ catalysts. Oxygen vacancies as active sites were formed in the CeO₂ catalysts by H₂ reduction at 400 °C. The Ce-HT, Ce-CA, and Ce-PC catalysts have a 100% CO selectivity and CO₂ conversion at 580 °C was 15.9%, 9.3%, and 12.7%, respectively. The highest CO₂ RWGS reaction catalytic activity for the Ce-HT catalyst was related to the porous structure, large specific surface area (144.9 m²?g^?1) and formed abundant oxygen vacancies.     

Pulse-response TAP studies of the reverse water–gas shift reaction over a Pt/CeO2 catalyst

The temporal analysis of products (TAP) technique was successfully applied for the first time to investigate the reverse water–gas shift (RWGS) reaction over a 2% Pt/CeO₂ catalyst. The adsorption/desorption rate constants for CO₂ and H₂ were determined in separate TAP pulse-response experiments, and the number of H-containing exchangeable species was determined using D₂ multipulse TAP experiments. This number is similar to the amount of active sites observed in previous SSITKA experiments. The CO production in the RWGS reaction was studied in a TAP experiment using separate (sequential) and simultaneous pulsing of CO₂ and H₂. A small yield of CO was observed when CO₂ was pulsed alone over the reduced catalyst, whereas a much higher CO yield was observed when CO₂ and H₂ were pulsed consecutively. The maximum CO yield was observed when the CO₂ pulse was followed by a H₂ pulse with only a short (1 s) delay. Based on these findings, we conclude that an associative reaction mechanism dominates the RWGS reaction under these experimental conditions. The rate constants for several elementary steps can be determined from the TAP data. In addition, using a difference in the time scale of the separate reaction steps identified in the TAP experiments, it is possible to distinguish a number of possible reaction pathways.     

Positive Effect of Water Vapor on CO Oxidation at Low Temperature over Pd/CeO2–TiO2 Catalyst

CO oxidation at low temperature over Pd/CeO₂–TiO₂ catalyst was carried out in the feed containing different contents of water vapor (H₂O). A positive effect of H₂O was observed on the catalytic performance of Pd/CeO₂–TiO₂ in CO oxidation at low temperature. The extent of this effect depends on the content of H₂O in the feed; with a H₂O content being 2.5 vol%, the catalyst Pd/CeO₂–TiO₂ exhibits the highest stability (longest life time for CO oxidation at 80 °C). The results of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and temperature-programmed reaction (TPReaction) reaction illustrated that H₂O in the feed supplies sufficient OH groups in the presence of O₂, which can react with adsorbed CO on Pd species to produce CO₂. Moreover, H₂O may also enhance the adsorption of CO and suppress the formation of some carbonate species.     

Ethanol Steam Reforming on Co/CeO2: The Effect of ZnO Promoter

A series of ZnO promoted Co/CeO₂ catalysts were synthesized and characterized using XRD, TEM, H₂-TPR, CO chemisorption, O₂-TPO, IR-Py, and CO₂-TPD. The effects of ZnO on the catalytic performances of Co/CeO₂ were studied in ethanol steam reforming. It was found that the addition of ZnO facilitated the oxidation of Co⁰ via enhanced oxygen mobility of the CeO₂ support which decreased the activity of Co/CeO₂ in C–C bond cleavage of ethanol. 3 wt% ZnO promoted Co/CeO₂ exhibited minimum CO and CH₄ selectivity and maximum CO₂ selectivity. This resulted from the combined effects of the following factors with increasing ZnO loading: (1) enhanced oxygen mobility of CeO₂ facilitated the oxidation of CH _x and CO to form CO₂; (2) increased ZnO coverage on CeO₂ surface reduced the interaction between CH _x /CO and Co/CeO₂; and (3) suppressed CO adsorption on Co⁰ reduced CO oxidation rate to form CO₂. In addition, the addition of ZnO also modified the surface acidity and basicity of CeO₂, which consequently affected the C₂–C₄ product distributions.     

Adsorption of carbon dioxide and water vapor on fly-ash based ETS-10

CO₂ capture from humid flue gas is always costly due to the irreplaceable pretreatment of dehydration in current processes, which creates an urgent demand for moisture-insensitive adsorbents with considerable CO₂ uptakes as well as remarkable H₂O tolerances. In the present work, the microporous titanium silicate molecular sieve ETS-10 was synthesized with coal fly ash as the only silica source. The as-synthesized ETS-10 was characterized by X-ray diffraction, scanning electronic microscopy and infrared spectroscopy to verify its crystal morphology, in which neither impurity nor aggregation was observed. The following CO₂ adsorption experiments on the thermal gravimetric analyzer demonstrated its similar CO₂ adsorption capacity yet dramatical adsorption kinetics among some other microporous materials, e.g., potassium chabazite. These specific properties consequently guaranteed its favorable CO₂ adsorption capacity even at high temperatures (1.35 mmol/g at 393 K) and shortened the breakthrough time of single CO₂ flow to less than 20 s. In CO₂/H₂O binary breakthrough experiments, the as-obtained ETS-10 still maintained excellent CO₂ uptake of 0.81 mmol/g at 323 K, regardless of the presence of water vapor, making it a promising substitute for direct CO₂ separation from humid flue gases at practical conditions of post-combustion adsorption.     

Thermodynamic analysis and optimization of RWGS processes for solar syngas production from CO2

Process systems were investigated for syngas production from CO₂ and renewable energy (solar) by the reverse water‐gas shift (RWGS) and the reverse water‐gas shift chemical looping (RWGS‐CL) process. Thermodynamic analysis and optimization was performed to maximize the solar‐to‐syngas (StS) efficiency η_StS. Special emphasis was laid on product gas separation. For RWGS‐CL, maximum StS efficiencies of 14.2 and 14.4% were achieved without and with heat integration, respectively. The StS efficiency is dictated by the low overall efficiency of H₂ production. RWGS‐CL is most beneficial for the production of pure CO, where the StS efficiency is one percent point higher compared to that of the RWGS process with heat integration. Heat integration leads to significant reductions in external heat demand since most of the gas phase process heat can be integrated. The StS efficiencies for RWGS and RWGS‐CL achieve the same level as the reported values for solar thermochemical syngas production. © 2016 American Institute of Chemical Engineers AIChE J, 63: 15–22, 2017     

Reverse water–gas shift reaction over ceria nanocube synthesized by hydrothermal method

A well-defined ceria nanocube with six (100) planes was successfully prepared by a facile hydrothermal method. Hydrogenation tests on the carbon dioxide, and several advanced analysis techniques, were used to investigate the catalytic performance of ceria nanocube for reverse water–gas shift (RWGS) and understand the governing reaction mechanism. The results demonstrated that the obtained ceria was a typical mesoporous material with a fluorite structure, and mainly had cerium with + 4 valence oxidation state. As-obtained ceria nanocube showed good performance for RWGS reaction, while nickel on ceria evidently promoted the hydrogenation of CO₂. An oxygen-transformation and metal-dissociation mechanism for RWGS reaction was proposed. The dissociation of carbon dioxide over ceria by directly oxidized oxygen vacancy was considered as a main reaction pathway of RWGS. Meanwhile, dissociated adsorption of CO₂ and hydrogen over nickel surface directly formed CO and supplied spillover hydrogen to nearby oxygen vacancies, respectively. The neighboring oxygen vacancies at the interface of nickel and ceria were considered as efficient active sites for CO₂ hydrogenation.     

A catalytic membrane reactor for water-gas shift reaction

We conducted the WGS reaction on a catalytic membrane reactor consisting of a WGS catalyst bed, Pt/CeO₂ and thin, defect-free, Pd-Cu alloy membranes. The presence of CO and other gases with H₂ reduced the H₂ permeation through the membrane by more than 50% and the effect of the other gases on the permeation reduction decreased in the following order: CO>CO₂>N₂. In a catalytic membrane reactor with helium sweep gas, the CO conversion was improved by about 65% compared with the catalyst without any membrane, and the CH₄ formed from an undesirable side reaction was significantly reduced. Although the H₂ permeation was severely reduced by surface phenomena such as blocking of available H₂ dissociation sites by CO, CO₂ and steam, the CO conversion was notably improved by the membrane presence. Moreover, the CO conversion was maintained at 98% even after 60 h of reaction and our Pd-Cu-Ni alloy membrane withstood the exposure of CO and the other gases. However, for separation of pure H₂, a newly designed, catalyst-membrane system is required with better sealing and the ability to withstand the high operating pressure that drives the H₂ permeation.     

Syngas Production via Reverse Water‐Gas Shift Reaction over a Ni‐Al2O3 Catalyst: Catalyst Stability,Reaction Kinetics,and Modeling

The synthesis of liquid fuels from CO₂, e.g., separated from flue gases of power plants, and H₂ from renewables, i.e., water electrolysis, is a concept for substituting fossil fuels in the transport sector. It consists of two steps, syngas production via reverse water‐gas shift (RWGS) and synfuel production by Fischer‐Tropsch synthesis. Research is concentrated on the RWGS using a Ni‐catalyst. The catalyst shows an appropriate performance in catalyzing the RWGS. The catalyst is stable at technically relevant temperatures. The intrinsic and effective kinetics were determined and considerations on a technical application of the process are proposed.     

Effect of three processes on CO2 and O2 simultaneously reforming of coke oven gas to syngas

CO₂ and O₂ simultaneously reforming of coke oven gas (COG) in three processes including non-catalytic process (NCP), catalytic process (CP), and two-stage process (TSP) was investigated under two important operating conditions, CO₂/CH₄ and O₂/CH₄, over Ni-based catalyst in a fixed bed reactor. It was found that the technical indexes depend strongly on CO₂/CH₄ and O₂/CH₄ in different processes. CO₂ can adjust H₂/CO ratio in a wider range (0.52–3.83) in the presence of O₂. The conversions of CH₄ increase in overall COG reforming processes by adding O₂. Also, a little O₂ promotes CO₂ conversions in NCP and restrains CO₂ conversions in CP and TSP. The addition of O₂ can also adjust H₂/CO ratio of syngas, which is actually at the cost of H₂ consumption by oxidation rather than reverse water gas shift (RWGS) reaction. In addition, H₂ combustion in the first-stage of TSP provides heat to drive the endothermic CH₄ reforming reactions and RWGS reaction in the second-stage of TSP to achieve higher CH₄ and CO₂ conversions. Therefore, TSP precedes significantly NCP and CP in the reforming of COG. When H₂/CO ratio is 2.10, the conversions of CH₄ and CO₂ are 98.96 and 62.32% respectively; and, oxygen consumption is 0.13 m³ per COG m³ at gas hour space velocity 9256 h⁻¹ in TSP.     

Characteristics of as-prepared biochar derived from catalytic pyrolysis within moderate-temperature ionic liquid for CO2 uptake

A promising biochar as solid adsorbent for CO₂ uptake was prepared by the catalytic pyrolysis of coconut shell in moderate-temperature ionic liquid (IL). Then, it was characterized by means of SEM, EDS, BPEA, BET, NLDFT, FTIR, and TG-DSC, and a mechanism interpretation of the porous biochar formation was conducted. In addition, the adsorption characteristics of CO₂ on the as-prepared biochar, such as adsorption capacity, adsorption potential, isosteric heat, and static selectivity at different adsorption temperatures and pressures, were systematically evaluated. The results indicated that the as-prepared biochar exhibited an adequate CO₂ adsorption with a capacity of 4.5 mmol/g at 273 K and 100 kPa. Then, a significant number of slit-like pores were revealed to exist on the as-prepared biochar with a peak pore size between a range of 0.6 nm-2 nm. The porous structure formation was ascribed to the release of carbon-, hydrogen-, oxygen-, sulphur-, and nitrogen-containing compounds during biochar preparation. Meanwhile, both the adsorption potential and isosteric heat of the CO₂ uptake under the tested conditions decreased with an increase in the adsorption capacity, which ranged from 33 kJ/mol-21 kJ/mol and 23 kJ/mol-7 kJ/mol, respectively. Therefore, the isosteric heat could be considered as a piecewise function of adsorption capacity. In addition, the molar ratios of CO₂ over N₂ adsorbed under the tested conditions were above 11 and were accompanied by molar ratio peaks of 26 at 273 K and 19 at 298 K, respectively. Moreover, an interesting phenomenon occurred: the static adsorptive selectivity of CO₂ over N₂ first increased and then decreased and there was an increase in the adsorption pressure at the tested adsorption temperatures.     

Promoting effect of CeO2 addition to Ni/AC catalyst for vapor phase carbonylation of ethanol to propionic acid

Ni/AC catalysts promoted with or without CeO₂ for vapor phase carbonylation of ethanol to propionic acid were tested and investigated by CO chemisorption, XRD and H₂-TPR techniques. The catalytic test results showed that the proper amount of CeO₂ addition could remarkably enhance the activity and stability of Ni/AC catalyst. The characterization results indicated that CeO₂ added can improve the dispersion and metal area of Ni on the catalyst, suppress the sintering of Ni crystallite and benefit the reduction of Ni, which are closely related to high performance of the Ni/AC catalyst promoted with CeO₂.     

Fe-promoted CuO/CeO2 catalyst: Structural characterization and CO oxidation activity

In the present paper, effect of iron on activity of catalyst CuO/CeO₂ in selective CO oxidation in H₂-containing gases mixture was investigated. Catalysts were prepared by wet impregnation and calcination at 400 °C, characterized by ICP, BET, H₂-TPR, XRD and TEM. The addition of iron to Cu/CeO₂ catalyst improved the catalytic activity and selectivity for CO oxidation. Discussion of the results showed that the synergistic effect is correlated to better reducibility and dispersion of copper in the presence of the iron metal additive.     

Selective catalytic reduction of NOx by NH3 over MoO3-promoted CeO2/TiO2 catalyst

The addition of MoO₃ enhanced the activity of CeO₂/TiO₂ catalyst for the selective reduction of NO_x with NH₃. The MoO₃-promoted CeO₂/TiO₂ exhibited higher activity than CeO₂/TiO₂ even in the co-presence of H₂O and SO₂. This is because the introduction of Mo to the Ce10Ti catalyst can inhibit the adsorption of H₂O and SO₂ as well as the formation of sulfate species on the catalyst surface, thus alleviating the poisoning effect of H₂O and SO₂.     

Catalytic syngas production from greenhouse gasses: Performance comparison of Ru-Al2O3 and Rh-CeO2 catalysts

In this work, 3% Ru-Al₂O₃ and 2% Rh-CeO₂ catalysts were synthesized and tested for CH₄-CO₂ reforming activity using either CO₂-rich or CO₂-lean model biogas feed. Low carbon deposition was observed on both catalysts, which negligibly influenced catalytic activity. Catalyst deactivation during temperature programmed reaction was observed only with Ru-Al₂O₃, which was caused by metallic cluster sintering. Both catalysts exhibited good stability during the 70 h exposure to undiluted equimolar CH₄/CO₂ gas stream at 750 °C. By varying residence time in the reactor during CH₄-CO₂ reforming, very similar quantities of H₂ were consumed for water formation. Reverse water-gas shift (RWGS) reaction occurred to a very similar extent either with low or high WHSV values over both catalysts, revealing that product gas mixture contained near RWGS equilibrium composition, confirming the dominance of WGS reaction and showing that shortening the contact time would actually decrease the H₂/CO ratio in the syngas produced by CH₄-CO₂ reforming, as long as RWGS is quasi equilibrated. H₂/CO molar ratio in the produced syngas can be increased either by operating at higher temperatures, or by using a feed stream with CH₄/CO₂ ratio well above 1.     

Oxygen-enhanced water gas shift on ceria-supported Pd–Cu and Pt–Cu bimetallic catalysts

Aiming at enhancing H₂ production in water gas shift (WGS) for fuel cell application, a small amount of oxygen was added to WGS reaction toward oxygen-enhanced water gas shift (OWGS) on ceria-supported bimetallic Pd–Cu and Pt–Cu catalysts. Both CO conversion and H₂ yield were found to increase by the oxygen addition. The remarkable enhancement of H₂ production by O₂ addition in short contact time was attributed to the enhanced shift reaction, rather than the oxidation of CO on catalyst surface. The strong dependence of H₂ production rate on CO concentration in OWGS kinetic study suggested O₂ lowers the CO surface coverage. It was proposed that O₂ breaks down the domain structure of chemisorbed CO into smaller domains to increase the chance for coreactant (H₂O) to participate in the reaction and the heat of exothermic surface reaction helping to enhance WGS kinetics. Pt–Cu and Pd–Cu bimetallic catalysts were found to be superior to monometallic catalysts for both CO conversion and H₂ production for OWGS at 300 °C or lower, while the superiority of bimetallic catalysts was not as pronounced in WGS. These catalytic properties were correlated with the structure of the bimetallic catalysts. EXAFS spectra indicated that Cu forms alloys with Pt and with Pd. TPR demonstrated the strong interaction between the two metals causing the reduction temperature of Cu to decrease upon Pd or Pt addition. The transient pulse desorption rate of CO₂ from Pd–Cu supported on CeO₂ is faster than that of Pd, suggesting the presence of Cu in Pd–Cu facilitate CO₂ desorption from Pd catalyst. The oxygen storage capacity (OSC) of CeO₂ in the bimetallic catalysts indicates that Cu is much less pyrophoric in the bimetallic catalysts due to lower O₂ uptake compared to monometallic Cu. These significant changes in structure and electronic properties of the bimetallic catalysts are the result of highly dispersed Pt or Pd in the Cu nanoparticles.     

Effects of zirconia promotion on the activity of Cu/SiO2 for methanol synthesis from CO/H2 and CO2/H2

The effect of zirconia promotion on Cu/SiO₂ for the hydrogenation of CO and CO₂ at 0.65 MPa has been investigated at temperatures between 473 and 573 K. With increasing zirconia loading, the rate of methanol synthesis is greatly enhanced for both CO and CO₂ hydrogenation, but more significantly for CO hydrogenation. For example, at 533 K the methanol synthesis activity of 30.5 wt% zirconia-promoted Cu/SiO₂ is 84 and 25 times that of unpromoted Cu/SiO₂ for CO and CO₂ hydrogenation, respectively. For all catalysts, the rate of methanol synthesis from CO₂/H₂ is higher than that from CO/H₂. The apparent activation energy for methanol synthesis from CO decreases from 22.5 to 17.5 kcal/mol with zirconia addition, suggesting that zirconia alters the reaction pathway. For CO₂ hydrogenation, the apparent activation energies (~12 kcal/mol) for methanol synthesis and the reverse water-gas shift (RWGS) reaction are not significantly affected by zirconia addition. While zirconia addition greatly increases the methanol synthesis rate for CO₂ hydrogenation, the effect on the RWGS reaction activity is comparatively small. The observed effects of zirconia are interpreted in terms of a mechanism which zirconia serves to adsorb either CO or CO₂, whereas Cu serves to adsorb H₂. It is proposed that methanol is formed by the hydrogenation of the species adsorbed on zirconia.     

Influence of CeO2 morphology on the catalytic oxidation of ethanol in air

Nano-CeO₂ catalysts of different shapes were synthesized at different hydrothermal crystallization temperatures from an alkaline aqueous solution. X-ray diffraction (XRD), transmission electron microscope (TEM), and H₂ temperature-programmed reduction (H₂-TPR) were used to study the synthesized nano-CeO₂ catalyst samples. The catalytic properties of the prepared nano-CeO₂ catalysts for the catalytic oxidation of ethanol in air were also investigated. TEM analysis showed that CeO₂ nanorod and nanocube catalysts have been synthesized at hydrothermal crystallization temperatures of 373 K and 453 K, respectively. XRD results showed that the synthesized nano-CeO₂ catalysts have similar cubic fluorite structures. H₂-TPR results indicated that CeO₂ nanorod and nanocube catalysts exhibit different reduction behaviors for H₂ and that the nanorod catalyst has better low-temperature reduction performance than the nanocube catalyst. Ethanol catalytic oxidation results indicated that oxidation and condensation products (including acetaldehyde, acetic acid, CO₂, and ethyl acetate) have been produced from the prepared catalysts. The ethyl acetate and acetic acid can be ignited by ethanol at low temperature on the CeO₂(R) catalyst to give low catalytic combustion temperature for ethyl acetate and acetic acid molecules. CeO₂ nanorods gave ethanol oxidation conversion rates above 99.2% at 443 K and CO₂ selectivity exceeding 99.6% at 483 K, while CeO₂ nanocubes gave ethanol oxidation conversion rates of about 95.1% until 508 K and CO₂ selectivity of only 93.86% at 543 K. CeO₂ nanorod is a potential low-cost and effective catalyst for removing trace amounts of ethanol to purify air.     

Catalytic Activity of Y Zeolite Supported CeO2 Catalysts for Deep Oxidation of 1, 2-Dichloroethane (DCE)

Three Y zeolites supported CeO₂ catalysts (CeO₂/USY, CeO₂/HY, CeO₂/SSY) were prepared and used for deep oxidation of 1,2-dichloroethane (DCE) in low concentration (about 1,000 ppm). The catalysts were characterized by XRD, N₂ adsorption/desorption and H₂-TPR. The results showed that the catalytic activity of the supported CeO₂ catalysts was much higher than that of Y zeolites, in particular, CeO₂/USY exhibited the highest activity, T_98% values of DCE was about 270 °C. And the catalytic activity was strongly related to the interaction between CeO₂ and Y zeolites.     

Cu/Ni‐Loaded CeO2‐ZrO2 Catalyst for the Water‐Gas Shift Reaction: Effects of Loaded Metals and CeO2 Addition

Two different types of metals (Cu and Ni) and the effect of CeO₂ addition to produce a CeO₂‐ZrO₂ co‐supporter were investigated through the water‐gas shift (WGS) reaction. It was found that the WGS activity could be enhanced with CeO₂ addition. At relatively high temperature, Ni‐loaded catalysts exhibited higher CO conversion while Cu‐loaded catalysts demonstrated better performance at low temperatures. The stability and yield of the CO₂ and H₂ products of the Cu catalysts were higher than those of the Ni catalysts. These results may be caused by an irreversible adsorption of CO on Ni and the reverse WGS reaction occurring on the Ni catalysts. In situ diffuse‐reflection infrared Fourier transform spectroscopy data suggests that the WGS mechanism likely proceeded via formate species.