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| The '''water-gas shift reaction''' (WGSR) describes the reaction of [[carbon monoxide]] and [[water vapor]] to form [[carbon dioxide]] and [[hydrogen]] (the mixture of carbon monoxide and hydrogen is known as [[water gas]]):
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| : CO + H<sub>2</sub>O <math>\rightleftharpoons</math> CO<sub>2</sub> + H<sub>2</sub> | |
| The water gas shift reaction was discovered by Italian physicist [[Felice Fontana]] in 1780. It was not until much later that the industrial value of this reaction was realized. Before the early 20th century, hydrogen was obtained by reacting steam under high pressure with iron to produce iron, iron oxide and hydrogen. With the development of industrial processes that required hydrogen, such as the [[Haber-Bosch]] ammonia synthesis, the demand for a cheaper and more efficient method of hydrogen production was needed.<ref name="Lamm Catalysts" /> As a resolution to this problem, the WGSR was combined with the gassification of coal to produce a pure hydrogen product. As the ideal of [[Hydrogen economy]] gains popularity, the focus on hydrogen as a replacement fuel source for hydrocarbons is increasing.
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| ==Applications==
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| The WGSR is an important industrial reaction that is used in the manufacture of [[ammonia]], [[hydrocarbon]]s, [[methanol]], and hydrogen. It is also often used in conjunction with [[steam reforming|steam reformation]] of methane and other hydrocarbons. In the [[Fischer-Tropsch process]], the WGSR is one of the most important reactions used to balance the H<sub>2</sub>/CO ratio. It provides a source of hydrogen at the expense of carbon monoxide, which is important for the production of high purity hydrogen for use in ammonia synthesis. | |
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| The water-gas shift reaction may be an undesired side reaction in processes involving water and carbon monoxide, e.g. the rhodium-based [[Monsanto process]]. The iridium-based [[Cativa process]] uses less water, which suppresses this reaction.
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| The equilibrium of this reaction shows a significant temperature dependence and the equilibrium constant decreases with an increase in temperature, that is, higher carbon monoxide conversion is observed at lower temperatures. In order to take advantage of both the [[thermodynamics]] and [[chemical kinetics|kinetics]] of the reaction, the industrial scale water gas shift reaction is conducted in multiple [[adiabatic process|adiabatic stages]] consisting of a high temperature shift (HTS) followed by a low temperature shift (LTS) with intersystem cooling.<ref name=Smith /> The initial HTS takes advantage of the high reaction rates, but is thermodynamically limited, which results in incomplete conversion of carbon monoxide and a 2-4% carbon monoxide exit composition. To shift the equilibrium toward hydrogen production, a subsequent low temperature shift reactor is employed to produce a carbon monoxide exit composition of less than 1%. The transition from the HTS to the LTS reactors necessitates intersystem colling. Due to the different reaction conditions, different catalysts must be employed at each stage to ensure optimal activity. The commercial HTS catalyst is the iron oxide–chromium oxide catalyst and the LTS catalyst is a copper-based catalyst. The order proceeds from high to low temperature due to the susceptibility of the copper catalyst to poisoning by sulfur that may remain after the steam reformation process.<ref name=Callaghan /> This necessitates the removal of the sulfur compounds prior to the LTS reactor by a guard bed in order to protect the copper catalyst. Conversely, the iron used in the HTS reaction is generally more robust and resistant toward poisoning by sulfur compounds. While both the HTS and LTS catalysts are commercially available, their specific composition varies based on vendor. An important limitation for the HTS is the H<sub>2</sub>O/CO ratio where low ratios may lead to side reactions such as the formation of metallic iron, methanation, carbon deposition, and [[Fischer-Tropsch process|Fischer-Tropsch reaction]].
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| ===Low temperature shift (LTS) ===
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| The typical composition of commercial LTS catalyst has been reported as 32-33% CuO, 34-53% ZnO, 15-33% Al<sub>2</sub>O<sub>3</sub>.<ref name=Callaghan>{{cite journal|last=Callaghan|first=Caitlin|title=Kinetics and Catalysis of the Water-Gas-Shift Reaction|year=2006}}</ref> The active catalytic species is CuO. The function of ZnO is to provide structural support as well as prevent the poisoning of copper by sulfur. The Al<sub>2</sub>O<sub>3</sub> prevents dispersion and pellet shrinkage. The LTS shift reactor operates at a range of 200 <sup>o</sup>C to 250 <sup>o</sup>C. Low reaction temperatures must be maintained due to the susceptibility of copper to thermal sintering. These lower temperatures also reduce the occurrence of side reactions that are observed in the case of the HTS.
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| ===High Temperature Shift Catalysts===
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| The typical composition of commercial HTS catalyst has been reported as 74.2% Fe<sub>2</sub>O<sub>3</sub>, 10.0% Cr<sub>2</sub>O<sub>3</sub>, 0.2% MgO (remaining percentage attributed to volatile components).<ref name="Newsome 1980">{{cite journal|last=Newsome|first=David S.|title=The Water-Gas Shift Reaction|journal=Catalysis Reviews: Science and Engineering|year=1980|volume=21|issue=2|pages=275–318}}</ref> The chromium acts to stabilize the iron oxide and prevents [[sintering]]. The operation of HTS catalysts occurs within the temperature range of 310 <sup>o</sup>C to 450 <sup>o</sup>C. The temperature increases along the length of the reactor due to the exothermic nature of the reaction. As such, the inlet temperature is maintained at 350 <sup>o</sup>C to prevent the exit temperature from exceeding 550 <sup>o</sup>C. Industrial reactors operate at a range from atmospheric pressure to 8375 kPa.<ref name="Newsome 1980" />
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| ===Fuel cells===
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| The WGSR can aid in the efficiency of [[fuel cell]]s by increasing hydrogen production. The WGSR is considered a critical component in the reduction of carbon monoxide concentrations in cells that are susceptible to carbon monoxide poisoning such as the [[proton exchange membrane fuel cell|proton exchange membrane (PEM) fuel cell]].<ref name="Lamm Catalysts" /> The benefits of this application are two-fold: not only would the water gas shift reaction effectively reduce the concentration of carbon monoxide, but it would also increase the efficiency of the fuel cells by increasing hydrogen production.<ref name="Lamm Catalysts" /> Unfortunately, current commercial catalysts that are used in industrial water gas shift processes are not compatible with fuel cell applications.<ref name=Callaghan /> With the high demand for clean fuel and the critical role of the water gas shift reaction ion hydrogen fuel cells, the development of water gas shift catalysts for the application in fuel cell technology is an area of current research interest.
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| Catalysts for fuel cell application would need to operate at low temperatures. Since the WGSR is slow at lower temperatures where equilibrium favors hydrogen production, WGS reactors require large amounts of catalysts, which increases their cost and size beyond practical application.<ref name="Lamm Catalysts">{{cite book|last=Lamm|first=editors, Wolf Vielstich, Hubert Gasteiger, Arnold|title=Handbook of fuel cells : fundamentals, technology, applications|year=2003|publisher=Wiley|location=New York|isbn=0-471-49926-9|edition=Reprinted}}</ref> The commercial LTS catalyst used in large scale industrial plants is also pyrophoric in its inactive state and therefore presents safety concerns for consumer applications.<ref name=Callaghan /> Developing a catalyst that can overcome these limitations is relevant to implementation of a hydrogen economy.
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| ==Reaction Conditions==
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| While the WGSR has been extensively studied for over a hundred years, the mechanism remains under debate. A universal rate expression and mechanistic understanding have proven elusive, reflecting the many reaction variables, vagarities of the catalyst, and the proprietary nature of commercial processes.<ref name=Smith>{{cite journal|last=Smith R J|first=Byron|coauthors=Muruganandam Loganthan, Murthy Shekhar Shantha|title=A Review of the Water Gas Shift Reaction|journal=International Journal of Chemical Reactor Engineering|year=2010|volume=8|pages=1–32}}</ref>
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| ===Temperature Dependence===
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| The water gas shift reaction is a moderately [[Exothermic reaction|exothermic]] reversible reaction. Therefore with increasing temperature the reaction rate increases but the conversion of reactants to products becomes less favorable.<ref>{{cite journal|last=Ratnasamy|first=Chandra|coauthors=Wagner, Jon P.|title=Water Gas Shift Catalysis|journal=Catalysis Reviews|date=September 2009|volume=51|issue=3|pages=325–440|doi=10.1080/01614940903048661}}</ref> Due to its exothermic nature, high carbon monoxide conversion is [[Thermodynamic versus kinetic reaction control|thermodynamically]] favored at low temperatures. Despite the thermodynamic favorability at low temperatures, the reaction is [[Thermodynamic versus kinetic reaction control|kinetically]] favored at high temperatures.
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| The water-gas shift reaction is [[Chemical equilibrium#Effect of temperature|sensitive to temperature]], with the tendency to shift towards reactants as temperature increases due to [[Le Chatelier's principle]]. Over the temperature range 600 – 2000 K, the logarithm of the equilibrium constant for the WGSR is given by the following equation:<ref name=Callaghan />
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| <div align="center"><math>\log K_\mathrm{eq} = -2.4198 + 0.0003855T + \frac{2180.6}{T}</math></div> | |
| The value of ''K''<sub>eq</sub> approaches 1 at {{val|1100|ul=K}}. The following plot depicts the temperature dependence of ''K''<sub>eq</sub> as shown by this equation.<ref name=Callaghan />
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| [[File:WGS Keq.fw.png|400px|thumbnail|center]]
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| ==Mechanism==
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| Two main mechanisms have been proposed: an associative ‘Langmuir–Hinshelwood’ mechanism, and a regenerative ‘redox’ mechanism. While the regenerative mechanism is generally implemented to describe the WGS at higher temperatures, at low temperature both the redox and associative mechanisms are suitable explanations.<ref name="Guiseppe dissertation">{{cite journal|last=Guiseppe|first=Brenna|title=NEW CATALYST FOR THE H2 PRODUCTION BY WATER-GAS SHIFT REACTION PROCESSES|journal=Dissertation|year=2010}}</ref>
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| [[File:WGS mechanism.png|550px|thumb|right|Associative and Redox Mechanism of the Water Gas Shift]]
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| ===Associative Mechanism===
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| In 1920 Armstrong and Hilditch first proposed the associative mechanism. In this mechanism CO and H<sub>2</sub>O are adsorbed onto the surface of the metal catalyst followed by the formation of an intermediate and the desorption of H<sub>2</sub> and CO<sub>2</sub>. In the initial step, H<sub>2</sub>O dissociates into a metal adsorbed OH and H. The hydroxide then reacts with CO to form a carboxyl or formate intermediate which subsequently decomposes into CO<sub>2</sub> and the metal adsorbed H, which ultimately yields H<sub>2</sub>. While this mechanism may be valid under LTS conditions, the redox mechanism which does not involve any long lived surface intermediates is a more suitable explanation of the WGS mechanism at higher temperatures.
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| ===Redox Mechanism===
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| The regenerative ‘redox’ mechanism is the most commonly accepted mechanism for the WGSR. It involves a regenerative change in the oxidation state of the catalytic metal. In this mechanism, H<sub>2</sub>O is activated first by the abstraction of H from water followed by dissociation or disproportionation of the resulting OH to afford atomic O. The CO is then oxidized by the atomic O forming CO<sub>2</sub> which returns the catalytic surface back to its pre-reaction state. Alternatively, CO may be directly oxidized by the OH to form a carboxyl intermediate, followed by the dissociation or disproportionation of the carboxyl. Finally H is recombined to H<sub>2</sub> and CO<sub>2</sub> and H<sub>2</sub> are desorbed from the metal. The principal difference in these mechanisms is the formation of CO<sub>2</sub>. The redox mechanism generates CO<sub>2</sub> by reaction with adsorbed oxygen, while the associative mechanism forms CO<sub>2</sub> via the dissociation of an intermediate. The mechanism of [[decarboxylation]] is debated; it may involve [[β-hydride elimination]], or it may require the action of an external base.
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| ===Homogeneous models===
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| Useful mechanistic insights resulted from the discovery that metal carbonyls catalyze the WGSR.<ref>{{cite journal|last=Jacobs|first=G.|coauthors=B. H. Davis|title=Low temperature water-gas shift catalysts|journal=Catalysis|year=2007|volume=20|pages=122–285|doi=10.1039/B601305H}}</ref> The basic reaction entails nucleophilic attack of water or hydroxide on a M-CO center, generating a [[metallacarboxylic acid]].<ref name="Lamm Catalysts" /><ref>{{cite journal|last=Barakat|first=Tarek|coauthors=Rooke, Joanna C.; Genty, Eric; Cousin, Renaud; Siffert, Stéphane; Su, Bao-Lian|title=Gold catalysts in environmental remediation and water-gas shift technologies|journal=Energy & Environmental Science|date=1 January 2013|volume=6|issue=2|pages=371|doi=10.1039/c2ee22859a}}</ref>
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| ==Reverse Water Gas Shift==
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| [[File:RWGS reactor flowchart.png|420px|thumbnail|right|Reverse Water Gas Shift Flow Cylcle]]
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| Depending on the reaction conditions, the equilibrium for the water gas shift can be pushed in either the forward or reverse direction. The reversibility of the WGSR is important in the production of ammonium, methanol, and Fischer-Tropsch synthesis where the ratio of H<sub>2</sub>/CO is critical. Many other industrial companies exploit the reverse water gas shift reaction (RWGS) reaction as a source of the synthetically valuable CO from cheap CO<sub>2</sub>. Typically, It is done using a copper on aluminium catalyst. The RWGS reaction is also gaining interest in the context of the human missions to Mars primarily for its potential to produce water and oxygen.<ref name="RWGS process">{{cite journal|last=Whitlow|first=Jonathan E.|coauthors=Parish|title=Operation, Modeling and Analysis of the Reverse Water Gas Shift Process|year=2003|doi=10.1063/1.1541409}}</ref><ref>{{cite book|last=Zubrin|first=R.|title=[[The Case for Mars]]|page=153 | isbn = 0-684-83550-9}}</ref> The Mars atmosphere is about 95% CO<sub>2</sub> which can be utilized by the RWGS reaction given a source of hydrogen. Coupling the RWGS with the [[water electrolysis]] process will yield methane and oxygen. Post electrolysis, the hydrogen produced can be recycled back into the RWGS reactor for the continued conversion of CO<sub>2</sub>. Because this reaction is only mildly [[endothermic]], the thermal power needed to drive this reaction can potentially be produced by a [[Sabatier reaction|Sabatier reactor]].
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| ==See also==
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| *[[In-situ resource utilization]]
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| *[[Lane hydrogen producer]]
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| *[[PROX]]
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| *[[Industrial catalysts]]
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| ==References==
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| {{Reflist}}
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| {{DEFAULTSORT:Water Gas Shift Reaction}}
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| [[Category:Inorganic reactions]]
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| [[Category:Chemical engineering]]
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| [[Category:Hydrogen production]]
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PE is significantly from lifestyle-damaging. But individuals that have problems with the condition may well, primarily while in severe humiliation from the clean face together with the impairment, consider or actually feel normally. The condition does cause fantastic anguish, humiliation and anxiousness. It brings pain towards the guy; it leads to extraordinary aggravation on the girl.
As well as associates usually do not talk about premature ejaculation their individual emotions and thoughts and opinions regarding its benefits on the private psyches, the condition might go further than its bodily sizing and cause heavy-sitting down psychological suffering, when the condition proceeds within a like process to get an expanded length. Early climax has been known to be the reason for quite a few association break up-ups.
Matthew Gorden’s curriculum vitae demonstrates experience in exactly the same job areas less than which the fitness of quick ejaculation may very well be labeled. He has acquired rigid as well as-adopting training in the special areas of love-making-education, therapy and remedy, and researching. His deeply expert fascination Ejaculation Trainer By Matt Gorden Pdf (Http://Prematureejaculationtrainers.Com/) with the disorder has moved him into undertaking investigation, and putting together the results of his research projects within a process he has adequately known as Ejaculation Trainer. To his mind, early climax is simply not an ailment that you just allow for to set the path of your health - or your love life, specifically. It is just a state which almost every guy can conquer - considering the perfect training and learning. It is only a couple of “re-training” your body’s re and reactions-developing control over your predisposition to” shed control” or ejaculate too early.
Gorden has a complete program relating to the issue. By so that you are aware of the reasons where the disorder builds up, he starts off. There may be a multitude of explanations and it is rewarding learning what exactly it is just that has made you predisposed to ejaculating too soon once you have sex with your mate.
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Young boys newly awakened towards the sex enjoyment that their systems may go through are oftentimes keen to masturbate. That this action is not culturally acceptable, however they are aware. So, they generally get it done in confidential. And so they make an attempt to climax quickly so that they do not get determined. This rapidly climax can become a learned-reply so much in fact that even just in maturity, and in some cases when performing making love with a loved one, the men is “conditioned” to ejaculate promptly. This pattern could continue regardless of whether both companions are predisposed to look at it as an unwelcome stop to their tryst. Additionally, there are cases when untimely climax could be rooted from the biological identity of mankind. Matt Gorden’s wants to outline guy by using his physical/lovemaking constitution.
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