Napkin ring problem: Difference between revisions
Jump to navigation
Jump to search
en>David Eppstein Undid revision 503691266 by Joshiansmith (talk) I think it made more sense with the sphere |
en>Myasuda m →References: added missing diacritic |
||
| Line 1: | Line 1: | ||
[[Image:Electrocatalyst ANL.jpg|thumb|175px|A platinum cathode electrocatalyst's stability is measured.]] | |||
An '''electrocatalyst''' is a [[Catalysis|catalyst]] that participates in [[Electrochemical reaction mechanism|electrochemical reactions]]. Catalyst materials modify and increase the rate of chemical reactions without being consumed in the process. Electrocatalysts are a specific form of catalysts that function at [[electrode]] surfaces or may be the electrode surface itself. An electrocatalyst can be [[heterogeneous]] such as a platinum surface or [[nanoparticles]],<ref name="CNT"> | |||
{{cite web | |||
| last = Wang | |||
| first = Xin | |||
| title = CNTs tuned to provide electrocatalyst support | |||
| publisher = Nanotechweb.org | |||
| date = 19 January 2008 | |||
| url = http://nanotechweb.org/cws/article/tech/37366 | |||
| accessdate = 27 February 2009}} | |||
</ref> or [[homogeneous]] like a [[coordination complex]] or [[enzyme]]. The electrocatalyst assists in transferring electrons between the electrode and reactants, and/or facilitates an intermediate chemical transformation described by an overall [[half-reaction]]. | |||
== Context == | |||
There are multiple ways for many transformations to occur. For example, [[hydrogen]] and [[oxygen]] can be combined to form water through a [[free radical reaction|free radical mechanism]] commonly referred to as [[combustion]]. Useful energy can be obtained from the thermal heat of this reaction through an [[internal combustion engine]] with an upper efficiency of 60% (for compression ratio of 10 and specific heat ratio of 1.4) based on the [[Otto cycle|Otto]] [[thermodynamic cycle]]. It is also possible to combine the hydrogen and oxygen through redox mechanism as in the case of a [[fuel cell]]. In this process, the reaction is broken into two half-reactions which occur at separate electrodes. In this situation the reactant's energy is directly converted to electricity.<ref>{{Cite journal | |||
| doi = 10.1002/anie.200903603 | |||
| volume = 48 | |||
| issue = 49 | |||
| pages = 9230–9237 | |||
| last = Kunze | |||
| first = Julia | |||
| coauthors = Ulrich Stimming | |||
| title = Electrochemical Versus Heat-Engine Energy Technology: A Tribute to Wilhelm Ostwald's Visionary Statements | |||
| journal = Angewandte Chemie International Edition | |||
| accessdate = 2009-12-04 | |||
| year = 2009 | |||
| pmid = 19894237 | |||
}}</ref><ref name="video">{{cite web | |||
| last = Haverkamp | |||
| first = Richard | |||
| title = What is an electrocatalyst? | |||
| work = | |||
| publisher = Science learning New Zealand | |||
| date = 3 June 2008 | |||
| url = http://www.sciencelearn.org.nz/contexts/nanoscience/sci_media/video/what_is_an_electrocatalyst | |||
| format = QuickTime video and transcript | |||
| accessdate = 27 February 2009}} | |||
</ref> | |||
{| cellpadding="10" class="wikitable sortable" | |||
|- | |||
! [[Half-reaction]] | |||
! ''E°'' (V) | |||
|- | |||
|-class="sortbottom" | |||
|{{hs|H}} '''H<sub>2</sub>(''g'') <math>\rightleftarrows</math> 2H<sup>+</sup> + 2''e''<sup>−</sup> ''' | |||
|{{hs|0}} '''≡ 0''' | |||
|-class="sortbottom" | |||
| O<sub>2</sub>(''g'') + 4{{hsp}}H<sup>+</sup> + 4{{hsp}}''e''<sup>−</sup> <math>\rightleftarrows</math> 2{{hsp}}H<sub>2</sub>O | |||
| +1.23 | |||
|-class="sortbottom" | |||
|} | |||
This process is not governed by the same thermodynamic cycles as combustion engines, it is governed by the total energy available to do work as described by the [[Gibbs free energy]]. In the case of this reaction, that limit is 83% efficient at 298K.<ref>[http://www.worldenergy.org/focus/fuel_cells/377.asp Fuel Cell efficiency]</ref> This half-reaction pair and many others don't achieve their theoretical limit in practical application due to lack of an effective electrocatalyst. | |||
One of the greatest drawbacks to [[galvanic cell]]s, like [[fuel cell]]s and various forms of [[electrolytic cell]]s, is that they can suffer from high activation barriers. The energy diverted to overcome these activation barriers is transformed into heat. In most exothermic combustion reactions this heat would simply propagate the reaction catalytically. In a redox reaction, this heat is a useless byproduct lost to the system. The extra energy required to overcome kinetic barriers is usually described in terms of low [[faraday efficiency|faradaic efficiency]] and high [[overpotential]]s. In the example above, each of the two [[electrode]]s and its associated [[half-cell]] would require its own specialized electrocatalyst. | |||
Half-reactions involving [[Electrochemical reaction mechanism|multiple steps]], multiple electron transfers, and the evolution or consumption of gases in their overall chemical transformations, will often have considerable kinetic barriers. Furthermore, there is often more than one possible reaction at the surface of an electrode. For example, during the [[electrolysis of water]], the anode can oxidize water through a two electron process to [[hydrogen peroxide]] or a four electron process to oxygen. The presence of an electrocatalyst could facilitate either of the reaction pathways.<ref name="electromethods"> | |||
{{Cite document | |||
| last1=Bard | |||
| first1=Allen J. | |||
| author1-link= Allen J. Bard | |||
| last2=Faulkner | |||
| first2=Larry R. | |||
| date=January 2001 | |||
| title=Electrochemical methods: fundamentals and applications | |||
| publisher=[[John Wiley & Sons|Wiley]] | |||
| publication-place=[[New York]] | |||
| isbn=978-0-471-04372-0 | |||
| url=http://www.amazon.co.uk/gp/reader/0471043729/ref=sib_dp_pt#reader-link | |||
| accessdate = 27 February 2009}} | |||
</ref> | |||
Like other catalysts, an electrocatalyst lowers the [[activation energy]] for a reaction without altering the [[Chemical equilibrium|reaction equilibrium]]. Electrocatalysts go a step further than other catalysts by lowering the excess energy consumed by a redox reaction's activation barriers. | |||
== Ethanol fuel cells == | |||
An electrocatalyst of [[platinum]] and [[rhodium]] on carbon backed tin-dioxide nanoparticles can break [[Carbon-carbon bond|carbon bonds]] at room temperature with only [[carbon dioxide]] as a by-product, so that [[ethanol]] can be oxidized into the necessary hydrogen ions and electrons required to create electricity.<ref name="Booze-power"> | |||
{{cite web | |||
| last = Harris | |||
| first = Mark | |||
| title = Booze-powered cars coming soon | |||
| publisher = techradar.com | |||
| date = 26 January 2009 | |||
| url = http://www.techradar.com/news/world-of-tech/booze-powered-cars-coming-soon-513666 | |||
| accessdate = 27 February 2009}} | |||
</ref> | |||
==References== | |||
{{Reflist}} | |||
[[Category:Electrochemistry]] | |||
[[Category:Catalysis]] | |||
Latest revision as of 16:37, 17 February 2013
An electrocatalyst is a catalyst that participates in electrochemical reactions. Catalyst materials modify and increase the rate of chemical reactions without being consumed in the process. Electrocatalysts are a specific form of catalysts that function at electrode surfaces or may be the electrode surface itself. An electrocatalyst can be heterogeneous such as a platinum surface or nanoparticles,[1] or homogeneous like a coordination complex or enzyme. The electrocatalyst assists in transferring electrons between the electrode and reactants, and/or facilitates an intermediate chemical transformation described by an overall half-reaction.
Context
There are multiple ways for many transformations to occur. For example, hydrogen and oxygen can be combined to form water through a free radical mechanism commonly referred to as combustion. Useful energy can be obtained from the thermal heat of this reaction through an internal combustion engine with an upper efficiency of 60% (for compression ratio of 10 and specific heat ratio of 1.4) based on the Otto thermodynamic cycle. It is also possible to combine the hydrogen and oxygen through redox mechanism as in the case of a fuel cell. In this process, the reaction is broken into two half-reactions which occur at separate electrodes. In this situation the reactant's energy is directly converted to electricity.[2][3]
| Half-reaction | E° (V) |
|---|---|
| Template:Hs H2(g) 2H+ + 2e− | Template:Hs ≡ 0 |
| O2(g) + 4Template:HspH+ + 4Template:Hspe− 2Template:HspH2O | +1.23 |
This process is not governed by the same thermodynamic cycles as combustion engines, it is governed by the total energy available to do work as described by the Gibbs free energy. In the case of this reaction, that limit is 83% efficient at 298K.[4] This half-reaction pair and many others don't achieve their theoretical limit in practical application due to lack of an effective electrocatalyst.
One of the greatest drawbacks to galvanic cells, like fuel cells and various forms of electrolytic cells, is that they can suffer from high activation barriers. The energy diverted to overcome these activation barriers is transformed into heat. In most exothermic combustion reactions this heat would simply propagate the reaction catalytically. In a redox reaction, this heat is a useless byproduct lost to the system. The extra energy required to overcome kinetic barriers is usually described in terms of low faradaic efficiency and high overpotentials. In the example above, each of the two electrodes and its associated half-cell would require its own specialized electrocatalyst.
Half-reactions involving multiple steps, multiple electron transfers, and the evolution or consumption of gases in their overall chemical transformations, will often have considerable kinetic barriers. Furthermore, there is often more than one possible reaction at the surface of an electrode. For example, during the electrolysis of water, the anode can oxidize water through a two electron process to hydrogen peroxide or a four electron process to oxygen. The presence of an electrocatalyst could facilitate either of the reaction pathways.[5]
Like other catalysts, an electrocatalyst lowers the activation energy for a reaction without altering the reaction equilibrium. Electrocatalysts go a step further than other catalysts by lowering the excess energy consumed by a redox reaction's activation barriers.
Ethanol fuel cells
An electrocatalyst of platinum and rhodium on carbon backed tin-dioxide nanoparticles can break carbon bonds at room temperature with only carbon dioxide as a by-product, so that ethanol can be oxidized into the necessary hydrogen ions and electrons required to create electricity.[6]
References
43 year old Petroleum Engineer Harry from Deep River, usually spends time with hobbies and interests like renting movies, property developers in singapore new condominium and vehicle racing. Constantly enjoys going to destinations like Camino Real de Tierra Adentro.
- ↑ Template:Cite web
- ↑ One of the biggest reasons investing in a Singapore new launch is an effective things is as a result of it is doable to be lent massive quantities of money at very low interest rates that you should utilize to purchase it. Then, if property values continue to go up, then you'll get a really high return on funding (ROI). Simply make sure you purchase one of the higher properties, reminiscent of the ones at Fernvale the Riverbank or any Singapore landed property Get Earnings by means of Renting
In its statement, the singapore property listing - website link, government claimed that the majority citizens buying their first residence won't be hurt by the new measures. Some concessions can even be prolonged to chose teams of consumers, similar to married couples with a minimum of one Singaporean partner who are purchasing their second property so long as they intend to promote their first residential property. Lower the LTV limit on housing loans granted by monetary establishments regulated by MAS from 70% to 60% for property purchasers who are individuals with a number of outstanding housing loans on the time of the brand new housing purchase. Singapore Property Measures - 30 August 2010 The most popular seek for the number of bedrooms in Singapore is 4, followed by 2 and three. Lush Acres EC @ Sengkang
Discover out more about real estate funding in the area, together with info on international funding incentives and property possession. Many Singaporeans have been investing in property across the causeway in recent years, attracted by comparatively low prices. However, those who need to exit their investments quickly are likely to face significant challenges when trying to sell their property – and could finally be stuck with a property they can't sell. Career improvement programmes, in-house valuation, auctions and administrative help, venture advertising and marketing, skilled talks and traisning are continuously planned for the sales associates to help them obtain better outcomes for his or her shoppers while at Knight Frank Singapore. No change Present Rules
Extending the tax exemption would help. The exemption, which may be as a lot as $2 million per family, covers individuals who negotiate a principal reduction on their existing mortgage, sell their house short (i.e., for lower than the excellent loans), or take part in a foreclosure course of. An extension of theexemption would seem like a common-sense means to assist stabilize the housing market, but the political turmoil around the fiscal-cliff negotiations means widespread sense could not win out. Home Minority Chief Nancy Pelosi (D-Calif.) believes that the mortgage relief provision will be on the table during the grand-cut price talks, in response to communications director Nadeam Elshami. Buying or promoting of blue mild bulbs is unlawful.
A vendor's stamp duty has been launched on industrial property for the primary time, at rates ranging from 5 per cent to 15 per cent. The Authorities might be trying to reassure the market that they aren't in opposition to foreigners and PRs investing in Singapore's property market. They imposed these measures because of extenuating components available in the market." The sale of new dual-key EC models will even be restricted to multi-generational households only. The models have two separate entrances, permitting grandparents, for example, to dwell separately. The vendor's stamp obligation takes effect right this moment and applies to industrial property and plots which might be offered inside three years of the date of buy. JLL named Best Performing Property Brand for second year running
The data offered is for normal info purposes only and isn't supposed to be personalised investment or monetary advice. Motley Fool Singapore contributor Stanley Lim would not personal shares in any corporations talked about. Singapore private home costs increased by 1.eight% within the fourth quarter of 2012, up from 0.6% within the earlier quarter. Resale prices of government-built HDB residences which are usually bought by Singaporeans, elevated by 2.5%, quarter on quarter, the quickest acquire in five quarters. And industrial property, prices are actually double the levels of three years ago. No withholding tax in the event you sell your property. All your local information regarding vital HDB policies, condominium launches, land growth, commercial property and more
There are various methods to go about discovering the precise property. Some local newspapers (together with the Straits Instances ) have categorised property sections and many local property brokers have websites. Now there are some specifics to consider when buying a 'new launch' rental. Intended use of the unit Every sale begins with 10 p.c low cost for finish of season sale; changes to 20 % discount storewide; follows by additional reduction of fiftyand ends with last discount of 70 % or extra. Typically there is even a warehouse sale or transferring out sale with huge mark-down of costs for stock clearance. Deborah Regulation from Expat Realtor shares her property market update, plus prime rental residences and houses at the moment available to lease Esparina EC @ Sengkang - ↑ Template:Cite web
- ↑ Fuel Cell efficiency
- ↑ Template:Cite document
- ↑ Template:Cite web