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Thread: Ammonia + Aluminum

  1. #1 Ammonia + Aluminum 
    Forum Sophomore Tharghana's Avatar
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    I know one of Products of this reaction is Hydrogen, but what is exactly taking place in this reaction? The the Solution is 90% H2O, 10% NH3



    I've done this reaction myself, which made Hydrogen Gas,I checked by setting it off, and left the Aluminum covered in a Black Coating.

    One source ive read says that the Ammonia is a catalyst and the Aluminum reacts with the water to form Aluminum Oxide



    And Anther Source Says it reacts with the Ammonia to Form Aluminum Nitride



    So Which one is it?


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    Aluminum Nitride seems more plausible. The reaction would be a redox reaction and not a oxidation one.


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    Quote Originally Posted by the man of science
    Aluminum Nitride seems more plausible. The reaction would be a redox reaction and not a oxidation one.
    what determines whether the reaction is redox or not?
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    redox reactions involve the loss of electrons and oxidation state changes. so do oxidation reactions but they involve the gaining of electrons. they are very similar reactions but dont mistake oxidation reaction for an oxidization reaction
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    The second reaction is a definite per a paper (see abstract below) but requires a high temperature reaction.

    "Nitridation reaction of aluminum powder in flowing ammonia

    by Yu Qiu and Lian Gao,
    State Key Lab of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People's Republic of, China
    Received 14 June 2002; revised 16 December 2002; accepted 1 January 2003. ;
    Abstract
    AlN powder was prepared by the nitridation of metal Al in flowing NH3. The effects of reaction temperature and the temperature gradient of the reaction zone on the nitridation of Al were investigated. Comparative analysis of products formed in different reaction zones and reaction temperatures suggested that the nitridation reaction of liquid Al particles in flowing NH3 was through the following mechanisms: NH3 dissociated into reactive nitrogen (N) and hydrogen (H) radicals at the surface of Al particles. N reacted with Al to form AlN, while H promoted the decomposition of NH3, which provided enough energy for the dissociation of NH3. All of the experimental results had been discussed on the basis of this model, which indicated high reaction temperature or positive temperature gradient was favorable for the nitridation of Al."

    The first reaction is also correct in my opinion. Per one source (see the "Concise Encyclopedia Chemistry" by deGruyter, page 48), Aluminium, on having its protective oxide layer removed (for instance by amalgamation), is highly reactive (think of Sodium). With water, it rapidly forms Al(OH)3 and releases H2. With O2, Al2O3 rapidly reforms. Apparently, my speculation is that NH3 (aq) is NOT acting as a catalyst, but is directly and slowly reacting with Al2O3 as it does with many metal oxides (MxO):

    NH3 + MxO --> H2O + N (or on occasion NO) + M (or on occasion MN)

    In our case with the Aluminium oxide and NH4OH at room temperature with time:

    2 NH4OH + Al2O3 --> 5 H2O + N2 + 2 Al

    and immediately:

    2 Al + 6 H2O ---> 2 Al(OH)3 + 3 H2
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  7. #6  
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    Almost all AL alloys contain Si unless its pure Al like Al foil, theblack stuff is Si left on the surface after the al is eaten away.
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    AJKOER here again. I decided to put my speculation that NH3 reacts slowly with Al2O3 (thereby exposing Al of its protective layer to react with water) to an actual home experiment. I burned cheap Aluminium foil and added it to household ammonia. I was expecting nothing to a possible slow reaction.

    WRONG: Surprisingly, I observed quite an effervescent reaction with tiny bubbles contained in large white plums (it actually looks like a forest fire with Al2O3 acting as the trees, the smoke in air is plums of white gelatinous Aluminium Hydroxide).

    Combing two reactions from my earlier post:

    2 NH4OH + Al2O3 + H2O --> 2 Al(OH)3 + N2 + 3 H2

    However, after two hours, the white solution becomes clear and the Al2O3 appears completely dissolved except for a black salt (Silicon residue?). This observation is normally consistent with the formation of a double salt or NH3 complex (like Al(OH)4-, as the Al(OH)6 occurs only in highly alkaline solutions, but this is speculation on my part given the lack of purity of household ammonia and even the Al foil).

    Thus, it appears that Aluminium metal in NH4OH is efficiently stripped of its Al2O3 protective coat. The reactive Aluminium further immediately reacts with the water itself and/or possibly any dissolved oxygen or CO2 in the water.
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  9. #8  
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    Also note that Al is amphoteric
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  10. #9  
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    The similar reaction using sodium hydroxide also leaves a black residue, so I suspect it is impurities in the aluminum, probably silicon.
    (Not sure if this guy is using pure sodium hydroxide or drain cleaner, but I have seen black residue myself)
    http://www.youtube.com/watch?v=16dshZ5GHZw
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    AJKOER again. I noticed that the reaction of NH4OH on freshly prepared Al2O3 was much more energetic and effective than simply trying to dissolve Aluminium foil in NH4OH. But Why? After some research, to quote from "Adhesion of Coatings to Aluminum Foil" dated September 1, 2003 by Günter Schubert, Hydro Aluminum Deutschland GmbH (reference link supplied below):

    "After rolling, a continuous amorphous oxide layer forms immediately due to the reaction between oxygen and humidity from the environment and the freshly generated metal surface. During annealing at temperatures of approximately 300°C, this oxide grows thicker due to elevated diffusion of oxygen through the oxide and reactivity with the metal in the oven atmosphere. The oxide growth occurs at the interface between oxide and metal. The oxide on top changes during annealing accompanied by a loss of water leading to a more compact oxide of generally higher resistance."

    And also (same source):

    "Ethylene acrylic acid (EAA) films with 6% acrylic acid were melt coated onto this foil in a laboratory oven at 200°C by continuously laying a molten film on the foil as in slow motion extrusion coating."

    However, there is hope for those trying to dissolve Al foil as per the same article:

    "Even at room temperature, acetic acid can easily pass through a coating infiltrating chemical bonds by advanced attack and leading to adhesion failure."

    So would priming the Aluminium foil in acetic acid produce better results?

    I tried by placing Al foil strips in acetic acid for 12 hours. Very very little activity, one might say NOTHING. There is reputedly a long "induction" period. But, I decided to stop. After draining off the acetic acid, I rinsed the test foil strips 3 times before placing them into NH4OH. And then, bubbles and more, and in minutes the solution contained white jelly like Al2O3! The reaction was not quite was effective as using freshly prepared Al2O3, but close!

    Reference Link:
    http://pffc-online.com/the_place/pap...ings_aluminum/
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    I previously speculated and some would say most likely erroneous so: " Apparently, my speculation is that NH3 (aq) is NOT acting as a catalyst, but is directly and slowly reacting with Al2O3 as it does with many metal oxides (MxO):

    NH3 + MxO --> H2O + N (or on occasion NO) + M (or on occasion MN)

    In our case with the Aluminium oxide and NH4OH at room temperature with time:

    2 NH4OH + Al2O3 --> 5 H2O + N2 + 2 Al"

    Well, I just came across a full (and free) article that examines the low temperature oxidation of NH3 with a Ag/Al2O3 catalyst, just to add some support to my conjecture. The article is : "Mechanism of selective catalytic oxidation of ammonia to nitrogen over Ag/Al2O3" by Li Zhang, Hong He, based on their work at the State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, PR China.

    Here is the abstract:

    "The mechanism of selective catalytic oxidation (SCO) of NH3 over Ag/Al2O3 was studied by NH3 temperature-programed oxidation, O2-pulse adsorption, and in situ DRIFTS of NH3 adsorption and oxidation. The essence which affects the low temperature activity of Ag/Al2O3 has been elucidated through the mechanism study. Different Ag species on Ag/Al2O3 significantly influence O2 uptake by catalysts; while different oxygen species affect the activity of NH3 oxidation at low temperature. The activated –NH could react with the atomic oxygen (O) at low temperatures (<140 C); however, the –NH could also interact with the O2 at temperatures above 140 C. At low temperatures (<140 C), NH3 oxidation follows the –NH mechanism. However, at temperatures above 140 C, NH3 oxidation follows an in situ selective catalytic reduction (iSCR) mechanism (two-step formation of N2 via the reduction of an in situ-produced NOx species by a NHx species)."

    Now if a reaction can proceed at under 140 C, with time a slow breakdown of NH4OH by freshly prepared Al2O3 alone in a warm solution is looking a bit more likely. Note, at least one old chemistry text reports that by shaking fine copper filings (far less reactive then Al) in NH4OH, a white cloud is visible at room temperature, even though Cu (and CuO) catalyst to decompose NH3 now normally operate at much higher temperatures to achieve efficient yields.

    This link gives the full free text article and I would graciously want to thank the People Republic of China for their apparent policy of distributing important work in environmental studies freely to all for the benefit of all.

    http://hehong.rcees.ac.cn/bookpic/20101301643540128.pdf
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  13. #12  
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    the solution to your problem appears rather simple. you have a reaction in which you add a metal to a basic solution. you wish to test whether the metal is reacting with the base or the water. if water is used up in a reaction in the way you described then the concentration of your base will increase.

    if the base is used up in the reaction then the concentration of base will increase.

    in some reactions you would have to adjust for reactants that are acids or bases, however if either of the reactions proposed by the OP is correct then one can determine which it is by comparing the Ph of the solution before and after the reaction.
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    To be clear, there is my proposed reaction between NH4OH, Al2O3 and H2O namely:

    2 NH4OH + Al2O3 + H2O --> 2 Al(OH)3 + N2 + 3 H2

    versus NH3 is unchanged (a 'catalyst'), but somehow the Al2O3 is dissolved to facilitate the reaction. I would propose for this side of the argument the possible creation of the Al(OH)4- complex given the amphoteric nature of Aluminium and the fact that a reaction does occur:

    Equation 1. 2 NH4OH + Al2O3 + 3 H2O --> 2 NH4[Al(OH)4]

    and with excess NH4OH, I also propose the creation of NH4[Al(OH)4]:

    Equation 2. 3 NH4OH + Al2O3 + H2O --> 2 NH4[Al(OH)4] + N2 + 3 H2

    Now the problem with Equation 1 (the NH3 is not decomposed argument) versus my proposed Equation 2 is the very apparent observation that the Al2O3 reaction with NH4OH does produce at least one gas!

    Another possibility is the creation of Al(OH)6 complex with the liberation of H2 from H2O. This also would be interesting as this complex is only believed to be created in high pH environments.

    As a side note, it appears that MgSO4 combines with NH4[Al(OH)4] to form Ammonium aluminiumsulfate, NH4AlSO4, and Mg(OH)2 in a somewhat unique double precipitate reaction.
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    I believe I found the answer. Aluminum even when thoroughly burned apparently has still has some unreacted Al along with AlN in the Al2O3. So the most likely reaction are:

    2 NH4OH (aq) + Al2O3.Al (s)+ 3 H2O (l) --> 2 NH4[Al(OH)4] (aq) + Al (s)

    and immediately, the reaction with reactive Al and water is:

    2 Al (s) + 6 H2O (l) ---> 2 Al(OH)3 (aq) + 3 H2(g)

    The reactive Al may also react with the base NH4OH, being amphoteric to form an aluminate and release H2:

    2Al(s) + 2NH4OH(aq) + 6H2O → 2NH4+(aq) + 2[Al(OH)4]- + 3H2(g)

    Also, the reaction of any Aluminium nitride formed in water is reportedly slow with the release of NH3 gas:

    AlN + 3H2O --> Al(OH)3 + NH3

    The reference below implies to me that the complete combustion of Aluminium, even as a fine powder, is difficult to accomplish with the final combustion products consisting of Al2O3, AlN (Aluminium Nitride) and Al (around 10%). My attempts to burn Al in air most likely has even higher amounts of unreacted Al.

    Reference: "Study of aluminum nitride formation by superfine aluminum powder combustion in air" by Alexander Gromov and Vladimir Vereshchagina at Chemical Department, Tomsk Polytechnic University, 30, Lenin Ave., Tomsk, 634050, Russia (available online 18 November 2003).

    "ABSTRACT
    An experimental study on the combustion of superfine aluminum powders (average particle diameter as0.1 μm) in air is reported. Formation of aluminum nitride during combustion of aluminum in air is focused in this study. Superfine aluminum powders were produced by wire electrical explosion (WEE) method. Such superfine aluminum powder is stable in air but, if ignited, it can burn in self-sustaining way. During the combustion, temperature was measured and actual burning process was recorded by a video camera. SEM, XRD, TG-DTA and chemical analysis were executed on initial powders and final products. It was found that powders, ignited by local heating, burned in two-stage self-propagating regime. The products of the first stage consisted of unreacted aluminum (70 mass%) and amorphous oxides with trace of AlN. After the second stage AlN content exceeded 50 mass% and residual Al content decreased to 10 mass%. A qualitative discussion is given on the probable mechanism of AlN formation in air."
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  16. #15 Reference Request 
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    Hi Tharghana:

    Could I have your reference on the reaction where the NH3 is acting as a reputed catalyst in the NH4OH dissolving of Al producing Al(OH)3 and H2?

    I want to see if I can dig further.

    Thanks
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    I found an interesting reference on one of the slides titled "Presence of Aluminum Nitride in Salt Cake" presented by the Global Symposium on Recycling, Waste Treatment and Clean Technology in Ocober 2008, Cancun, Mexico. In essence, the dissolving of AlN produces ammonia that raises the pH which "dissolves the alumina film on unrecovered Aluminum particles surface, thereby exposing the Al surface to the reaction Al + H2O --> Al(OH)3 + H2". Note, I did not balance the author's quoted equation as presented on his slide.

    One could read this as the NH3 is reputedly directly dissolving the Al2O3 as the pH is raised, which is almost a complete support for my position. I say almost because the dissolving of the Al2O3 does not necessarily imply the creation an Ammonium Aluminum complex (the process by which the Alumina dissolves may not be fully understood, for example). However, since I have witness a clear solution, this would support the possible formation of NH4[Al(OH)4].

    Link: http://www.es.anl.gov/Energy_systems...h/industrial_m...
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