Forward Water Gas Shift Research Articles

Reverse water-gas shift: Na doping of m-ZrO2 supported Pt for selectivity control

Reverse water-gas shift (RWGS) is a vital step in producing syngas for the chemical conversion of CO2 to liquid transportation fuels and chemicals. Na-doping of m-ZrO2 supported Pt catalysts allowed selectivity control by systematically increasing the ratio of relative rates of rCO/rCH4. This was achieved by facilitating the formation of formate intermediate species, which precedes CO formation, and by suppressing the metallic Pt0 active sites responsible for CH4 formation. A 2.5%Na-2%Pt/m-ZrO2 catalyst was first tested for the forward water-gas shift (FWGS) reaction and found to have 50% higher CO conversion at 285 °C compared to the undoped catalyst. Results of DRIFTS spectroscopy of adsorbed CO confirmed a formate ν(CH) band shift to lower wavenumbers (2870–2802 cm−1) with the addition of Na and more rapid forward formate decomposition in steam to H2 and carbonate species, the precursor to CO2. This is consistent with C-H bond breaking being the rate limiting step of a FWGS mechanism occurring at the metal-support junction. Consistent with this, DRIFTS of RWGS in 4%CO2 + 60%H2 showed more facile formation of formate for the Na-doped catalyst and, once again, the ν(CH) band was shifted to lower wavenumbers (2874–2803 cm−1) with Na-doping. In addition, Na doping resulted in a systematic decrease in the Pt-carbonyl band in DRIFTS of adsorbed CO as well as DRIFTS of in-situ RWGS reaction tests, suggesting that Na blocked a fraction of on-top Pt sites, breaking up ensembles of Pt0 responsible for methanation. The selectivity of the 2.5%Na-doped catalyst, unlike its undoped counterpart, was remarkably resistant to methanation (e.g., selectivity < 0.2% CH4 with pressures of up to 20 bar).

Supercritical water gasification of oil-containing wastewater with a homogeneous catalyst: Detailed reaction kinetic study

Supercritical water gasification (SCWG) is a promising technology for oil-containing wastewater treatment. This paper aims to establish a reaction kinetic model to provide better guidance for optimal industrial reactor design. The model is developed based on the experimental results obtained from K2CO3-catalyzed SCWG of diesel (the model compound of oil containment in wastewater) at various conditions of 500–650 °C and 15.23–64.45 s. Then the model validation by using the experimental data from other conditions. The validation results showed that the kinetic model can predict the gas concentration with an acceptable deviation. Afterward, the indicators of carbon gasification efficiency and gas yield versus residence time are predicted. The results show that the required residence time for the complete gasification is varied from 214.2 to 2150.8 s when the temperature changes from 500 to 650 °C. Moreover, the reaction rate analysis result indicates that the two reactions contributed most to the hydrogen production are the forward water-gas shift and the reverse CO methanation reaction. Additionally, the sensitivity analysis result reveals that the hydrolysis reaction of diesel has a significant influence at the initial stage, while the CO and CO2 methane reactions play a vital role at the late stage for gas production.

Operating parametric analysis and kinetic modeling of methanol gasification in supercritical water

Experimental analysis and quantitative kinetic modeling were performed on methanol gasification in supercritical water using continuous reactor at different reaction temperatures (550–650 ℃), pressures (21–25 MPa), residence times (6–60 s), and initial methanol concentrations (0.05–0.2 molL−1) to investigate the mechanism underlying methanol gasification and conversion. The kinetic model consisted of eight separate reaction pathways. Simultaneously, reaction rate and sensitivity analyses were conducted. Higher temperatures, longer residence times and lower feedstock concentrations were beneficial for the yields of H2 and CO2. Kinetic analysis showed that H2 and CO2 generation mainly occurred through the hydrolysis of methanol. CO was mainly consumed during the forward water-gas shift reaction, and the production of CH4 depended primarily on methanol methanation. Additionally, methanol hydrolysis was identified as an important elementary reaction for CH4 production by sensitivity analysis.

Kinetics of Water Gas Shift Reaction on Au/CeZrO4: A Comparison Between Conventional Heating and Dielectric Barrier Discharge (DBD) Plasma Activation

The kinetics of the low-temperature forward water gas shift (LT-WGS) reaction have been studied over a 2 wt% Au/CeZrO4 (Au/CZO) catalyst using both thermal and dielectric barrier discharge plasma heterogeneous catalyst systems. Using the energy density (e), the apparent activation energy has been calculated under plasma and thermally activated conditions. A substantially lower apparent activation energy is observed in the plasma activated system (9.5 kJ/mol) compared with the thermal-catalysed reaction (132.9 kJ/mol). Different kinetic isotope effect (KIE) values for water were found in thermal (1.43) and plasma (1.89) activated catalytic systems which infer different mechanisms between the two activation processes and also shows the importance of water activation. Furthermore, negative and positive reaction orders with respect to CO and H2O are found for both conditions which are − 1.30, 0.28 under thermal and − 1.53, 0.35 under plasma processes, respectively. The reaction order with respect to H2O and KIE studies demonstrate that the bond cleavage in H2O molecule is a rate determining step in the plasma-assisted LT-WGS, similar to that in the thermal-assisted reaction.

Comparison of CuO-MO x (M=Ce, Zn, Cr and Zr) catalysts in various water-gas shift reactions

The water-gas shift (WGS) reaction in the temperature range of 100–350 °C for various feed compositions simulating forward, reverse and real WGS conditions was studied for a series of coprecipitated mixed metal oxide catalysts of 30 wt% of CuO and 70 wt% of metal oxide (CeO2, ZnO, Cr2O3, and ZrO2) as well as for a commercial WGS catalyst (ICI 83-3). The catalysts were characterized using BET, XRD, H2-TPR and N2O dissociation studies. Among the tested catalysts, CuO-Cr2O3 showed the best activity in the forward WGS, while the commercial catalyst was the best catalyst in the real and reverse WGS reactions. The effect of Cu content in the catalyst was also studied and, in the case of the real WGS, 50 wt% CuO-Cr2O3 was more active than 30 wt% CuO-Cr2O3. H2 and CO2 were found to inhibit the forward WGS, decreasing the reaction rate substantially, particularly at temperatures below 200 °C. The inhibition effect varied depending on the tested catalyst and increased with increasing H2 or CO2 concentration. As the inhibition effect was reversible, the competitive adsorption of H2 or CO2 on the active sites has been suggested to be responsible for the effect. The high activity of the commercial catalyst in the H2 rich real WGS could be described by the difference in the H2 inhibition between the catalysts. An easily reducible copper species was found in CuO-Cr2O3 and could be attributed to the high activity in the forward WGS. Keywords:Mixed Metal Oxide Catalysts, Water Gas Shift Reaction, H2 Inhibition, CO2 Inhibition, Copper Chromate Catalyst

A kinetic analysis of the partial oxidation of C3H8 over a 2% Rh/Al2O3 catalyst in annular microreactor

In this paper, we report a detailed kinetic investigation of C3H8 catalytic partial oxidation (CPO) over a Rh/α-Al2O3 catalyst. We applied an isothermal microreactor to collect kinetically relevant data with the aim of identifying the prevalent kinetic dependences and extending our C1 molecular kinetic scheme to the description of the C3H8 CPO process. The kinetic study consisted of CPO and steam reforming tests with diluted feeding mixtures in a wide range of temperature, composition and space velocity. The experimental results showed that H2 and CO are formed through consecutive steps of total oxidation and steam reforming of the fuel. The syngas composition is also affected by the water gas shift reaction, as well by the methanation of COx. In analogy with CH4 CPO, the rate of total oxidation of C3H8 and the rate of steam reforming of C3H8 were found to be proportional to the concentration of fuel, but independent from the concentration of the co-reactant. The rate of the forward water gas shift reaction was found to be first order dependent on H2O and independent from CO, while the rate of methanation was assumed to be proportional to H2 partial pressure. A preliminary micro-kinetic analysis showed that C3H8 adsorption likely involves the formation of CHx intermediates with x≤2.

Dry reforming of methane has no future for hydrogen production: Comparison with steam reforming at high pressure in standard and membrane reactors

The methane dry-reforming and steam reforming reactions were studied as a function of pressure (1–20 atm) at 973 K in conventional packed-bed reactors and a membrane reactors. For the dry-reforming reaction in a conventional reactor the production yield of hydrogen rose and then decreased with increasing pressure as a result of the reverse water-gas shift reaction in which the hydrogen reacted with the reactant CO2 to produce water. For the steam reforming reaction the production yield of hydrogen kept increasing with pressure because the forward water-gas shift reaction produced additional hydrogen by the reaction of CO with water. In the membrane reactors the methane conversion and the hydrogen production yields were higher for both the dry-reforming and steam reforming reactions, but for the dry reforming at high pressure half of the hydrogen was transformed into water. Thus, the dry-reforming reaction is not practical for producing hydrogen.

Thermal resistance effect on methanol-steam reforming performance in micro-scale reformers

We numerically investigate hydrogen production based on methanol-steam reforming (MSR) using a micro-scale cylindrical packed bed reformer. The reformer wall is included in the physical model. The heat required for the reforming reaction is supplied either internally using a heating rod placed along the center of the reformer or externally by a heat flux applied at the reformer outer wall. Our results show that the thermal resistance from the heat source to the reformer environment plays an important role in the reformer performance. This thermal resistance depends on the reformer geometry, wall material and heat transfer coefficients inside the catalyst bed and outside the reformer. Based on our numerical results, it is suggested that better methanol conversion and hydrogen yield can be obtained using reformer wall material with low thermal conductivity and thin thickness. For both internal and external heating under the same heat rate supply, no significant difference in reformer performance was found. A water gas shift (WGS) reaction model was included in the present numerical model. In the reformer low-temperature zone the forward WGS reaction was clearly demonstrated, resulting in a decrease in carbon monoxide (CO) selectivity. In the high temperature zone the backward WGS reaction was also clearly demonstrated in which CO selectivity increases with the increase in temperature. For both internal and external heating under the same heat rate supply, our results indicated that CO selectivity is about thirty times lower when the WGS reaction is neglected.

Cycles Across an Equilibrium: A Kinetic Investigation of the Reverse and Forward WGS Reaction over a 2% Pt/CeO 2 Catalyst (Experimental Data and Qualitative Interpretation)

The forward and reverse water–gas-shift (WGS)-reactions over 2%Pt/CeO 2 catalyst are studied under steady-state conditions using a procedure which entails adjusting feed gas concentrations to cycle back and forth across the equilibrium line. The kinetic data were collected on “both sides” of the equilibrium under varying feed gas compositions (CO and CO 2 concentrations) and temperatures. The obtained data are interpreted using the concept of the apparent “kinetic resistance” developed recently based on the original non-linear theory [see Lazman, M.Z., Yablonsky, G.S., 2008. Overall reaction rate equation of single-route complex catalytic reaction in terms of hypergeometric series. Advances in Chemical Engineering 34, 47–102].

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

Water–gas shift reaction was studied over two nanostructured CuxCe1−xO2−y catalysts: a Cu0.1Ce0.9O2−y catalyst prepared by a sol–gel method and a Cu0.2Ce0.8O2−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 673K at two different space velocities: 5.000 and 30.000h−1, respectively. Experimentally estimated activation energy, Eaf, of the forward water–gas shift reaction at CO/H2O=1/3 was 51kJ/mol over the Cu0.1Ce0.9O2−y, 34kJ/mol over the Cu0.2Ce0.8O2−y and 47kJ/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.

Uncatalyzed and wall‐catalyzed forward water–gas shift reaction kinetics

AbstractThe kinetics of the high‐temperature (1070–1134 K), low‐ and high‐pressure gas‐phase forward water–gas shift reaction (fWGSR) were evaluated in an empty quartz reactor and a quartz reactor packed with quartz particles. The power‐law expression for the reaction rate was consistent with the Bradford mechanism and was invariant with respect to pressure. The experimental rate constant was lower than that published by Graven and Long, and slightly higher than estimates obtained using the reaction rate expression derived from the Bradford mechanism in conjunction with values of reaction rate constants obtained from the GRI database. Similar experiments conducted using a reactor composed of Inconel® 600, a representative reactor shell material, exhibited substantially enhanced rates of reaction. A simple power‐law rate expression was incorporated into a surface‐catalyzed plug flow reactor (PFR) model to correlate the results between 600 and 900 K. Palladium and palladium–copper alloy surfaces, representative of hydrogen membranes, were also shown to enhance the fWGSR rate, but not as much as the Inconel® 600 surfaces. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Relating catalyst structure and composition to the water–gas shift activity of Cu–Zn-based mixed-oxide catalysts

In order to investigate the effect of cerium oxide on Cu–Zn-based mixed-oxide catalysts four catalyst samples were characterized by means of XRD, in situ XANES and thermogravimetric analysis. The activity of the catalyst samples was tested for the forward water–gas shift reaction. Cerium oxide was found to increase the crystallinity of the ZnO phase indicating a segregation of the Cu and ZnO phases. The TOF of the water–gas shift reaction based on chemisorption data was found to be independent of composition and preparation conditions of the four catalyst samples. In contrast, the catalyst stability depends on composition and preparation conditions. Cerium oxide impregnated before calcination of the hydrotalcite-based Cu–Zn precursors leads to a more stable water–gas shift catalyst.

Investigation of the kinetic properties for the forward and reverse WGS reaction by energetic analysis

The kinetic properties of forward water gas shift reaction (CO+H 2O→CO 2+H 2) and reverse water gas shift reaction (CO 2+H 2→CO+H 2O) over the copper surface have been analyzed by UBI-QEP approach. For the ‘surface redox’ mechanism, the energies analysis results show that the rate controlling step in FWGS is the dissociation of adsorbed H 2O, and the dissociation of adsorbed CO 2 is the rate controlling step of the RWGS reaction. The RWGS reaction is more structure sensitivity than that of FWGS reaction over the Cu single crystal surfaces. Both the activity of FWGS reaction and RWGS reaction over the Cu (111) surface are higher than that of in Au (111) surface. In the meantime, the reason why the FWGS reaction and RWGS reaction is easily poisoned by sulphur has been explained, namely, the activation barrier of rate controlling step increases with the increasing of the coverage of sulphur.

Kinetics of the reverse water-gas shift reaction over Cu(110)

The reverse water-gas shift reaction (CO 2 + H 2 → H 2O + CO) has been studied over a clean Cu(110) single-crystal model catalyst at temperatures between 573 and 723 K. The steady-state kinetic measurements were carried out at medium pressures (10–2000 Torr) in a microreactor after cleaning and characterization of the sample under UHV conditions. The H 2/CO 2-pressure ratios varied from 1000: 1 to 1 : 10. The product buildup was monitored with a gas chromatograph (GC). The apparent activation energy is about 18 kcal/mol, and the reaction orders in H 2 and CO 2 depend strongly on the H 2/CO 2 ratio and temperature. The steady-state kinetics are compared favorably with the rates of elementary steps potentially involved in a “surface redox” reaction mechanism of the reverse and forward water-gas shift reaction involving the formation and removal of oxygen adatoms. Kinetic evidence that is tentatively attributed to a hydrogen-induced surface phase transition that affects the reaction rate, is also presented.

A model study of alkali promotion of water-gas shift catalysts: Cs/Cu(111)

The kinetics of the forward water-gas shift reaction (H2O+CO→H2+CO2) have been measured on a Cs-doped Cu(111) single-crystal surface at pressures near 40 Torr. The Cs is dosed from aqueous CsOH solution and dried in air. The surface structures have been characterized by XPS, AES, LEED, and ISS both before and after reaction. The activity of the Cu(111) surface increases with increasing Cs coverage until it reaches a maximum at θCs⋍0.13, where the rate is fifteen times that for Cs-free Cu(111). At higher coverages the rate decreases. The as-dosed surface is oxidized to Cu2O to a depth ≥20 Å. After brief treatments (<40 s) under reaction conditions, the surface is completely reduced to metallic Cu(111), with a Cs overlayer showing an O:Cs atomic ratio of about unity. This overlayer demonstrates a p(2×2) LEED pattern when θCs exceeds 0.13 (where the rate maximizes), which is interpreted in terms of islands of an oxidic or hydroxidic Cs overlayer of local coverage θCs=1/4. The promotion by Cs is attributed to a mechanism where the surface Cs/O complex participates directly in the dissociation of adsorbed H2O, which is the rate-determining step.

H 2S/Cu(111): A model study of sulfur poisoning of water-gas shift catalysts

The interaction of H2S with Cu(111) has been studied with XPS, AES, ISS and LEED; and the effects of sulfur adatoms upon the kinetics of the medium-pressure water-gas shift reaction over Cu(111) have been measured. At 120 K, H2S adsorption is partially molecular. By 200 K, some of the H2S desorbs and the rest dissociates to produce sulfur adatoms bonded on top of the Cu(111) surface. The surface saturates at room temperature in a (7x7) structure (θs=0.43) characteristic of a nearly close-packed sulfur overlayer, very similar to layers in (111)-oriented Cu2S. Sulfur adatoms are quite stable to heating in vacuum or H2, with a reaction probability for removal per H2 collision with the surface of ≤10−9 at 636 K. The rate of the forward water-gas shift reaction (H2O+CO→H2+CO2) decreases linearly with sulfur coverage as (1–2.6θs). This poisoning is attributed to steric blocking by the sulfur adatoms of the sites required for dissociative water adsorption. The site ensemble required for this step is small (1–2 Cu atoms). The well-known sensitivity of shift catalysts to sulfur poisoning is discussed in the light of these results.