A Brief Study of Chromium Dioxide

 

While writing another web page involving chromium(IV) chemistry the subject of chromium dioxide resurfaced . I've discussed chromium dioxide before (in my ebook and in the Solids web page ; underlined blue hyperlinks can be clicked when online to retrieve the article cited . The requested document will open in a new window) . I thought it might be useful to revisit the topic in a new web page dedicated to it .

Chamberland has described the chemistry of chromium dioxide in considerable detail ; his report remains the definitive review of the subject (the references are presented at the end of the text , below) . As a chemistry writer and not a researcher I can't personally add any new experimental data to the repertoire of this remarkable material . Nevertheless , I hope the following brief study will be interesting , helpful , and thought provoking for the reader , and will stimulate further investigation of CrO2 by researchers .

 

An Overview of Chromium Dioxide

 

Chromium dioxide has been described as a black microcrystalline powder (Chamberland) , or brown-black (CRC Handbook of Chemistry and Physics) . It has no discrete melting point but decomposes to Cr2O3 and O2 at around 400 ºC .

Pure , crystalline CrO2 is ferromagnetic ; its Curie temperature , TC , is 118 ºC (391 K) . Its magnetic susceptibility meff at room temperature is 2.0 BM (Bohr magnetons) . The simple formula

meff = [n (n+2)]1/2

relates the magnetic susceptibility of magnetically dilute compounds with n , the number of their unpaired electrons . When n = 1 , meff = 1.73 BM ; when n = 2 , meff = 2.83 BM . This suggests that the ferromagnetism in CrO2 is derived from a single unpaired chromium valence electron ; cooperative ferromagnetic ordering in the lattice , while relatively weak compared to the more powerful FM ordering in iron , cobalt , and nickel , is probably responsible for the increase in the overall magnetic susceptibility of CrO2 from the theoretical 1.73 BM value to the observed 2.0 BM .

Chromium dioxide is very easily magnetized and demagnetized , and this property has led to its principal commercial application as the magnetic material (coating) in recording media such as in reel-to-reel tapes and compact cassettes . In fact , I have several such “chrome oxide” cassettes here at home . I scanned one of them to produce the following illustration of a consumer product containing chromium dioxide :

The main competitor to CrO2 in these compact cassettes is iron(III) oxide , Fe2O3 . These “ferro” cassettes were usually considered technically inferior (although cheaper) than the higher performance chrome oxide cassettes . However – as the reader is doubtless well aware – in the past decade or so such magnetic data storage media have been rapidly disappearing , being replaced in the electronic consumer marketplace by photonic media (CDs and DVDs) and flash memory devices (USB “keys”, which use billions of microtransistors on printed circuits) .

Chromium dioxide has the rutile crystal structure , with a tetragonal symmetry . The extended rutile structure is portrayed in the following sketch (based on Hiroi's graphic) :

A “polyhedral” version of this extended rutile crystal structure is presented in the excellent drawing from the highly recommended website , The Fascination of Crystals and Symmetry :

My thanks to the author (and presumably copyright owner) of this graphic , which has been slightly annotated . The Wikipedia sketch of vanadium(IV) oxide is somewhat similar to this one .

In both illustrations the octahedral and trigonal planar coordinations of the chromium and oxygen atoms , respectively , are clearly apparent .

Chromium dioxide is a metallic solid , with an excellent electrical conductivity over a wide range ; moreover , its conductivity is that of a true metal (inverse electrical conductivity–temperature relationship) , similar to that of the common metallurgical metals . Chamberland's graph of this property clearly shows this :

(my thanks to the author and/or copyright holder of this sketch)

I redrew Chamberland's sketch in terms of the corresponding electrical conductivity (rather than resistivity) values , as follows :

This latter graph looks quite similar to those of the electrical conductivities of many common metallurgical metals over a wide temperature range . Chromium dioxide's ambient electrical conductivity is ~ 2500 ohm-1cm-1 , rising to > 50,000 ohm-1cm-1 below 100 K . Of course , because of its ferromagnetism it never becomes superconducting .

The electronic structure of chromium dioxide has been described by several authors . For example , its Molecular Orbital Theory diagram has been presented by Goodenough and reproduced in Chamberland's review , from which the following sketch was copied :

(my thanks to the author and/or copyright holder of this sketch)

The following critique disagrees with this MOT electronic structure . First , we should immediately note that CrO2 is a covalent-metallic solid ; it is NOT an ionic solid , as implied by the Cr4+ and O2- at the top of the sketch . Most tetravalent metal atoms are covalently and not ionically bonded ; this is especially true of the smaller Transition metal cations , eg. Ti(IV) . The crystal ionic radius of Cr(IV) is 0.55 Å (six-coordinate , per Shannon-Prewitt) ; that , and its high positive charge , guarantee its covalent bonding with oxygen in CrO2 . There are no ionic Cr(IV) salts , to the best of my knowledge . Chromium dioxide is an inorganic polymer with Cr–O covalent bonds ; additionally , there is a metallic bond with itinerant , free electrons delocalized throughout its crystal lattice .

Second , the metallic bond in the material (defined in the MOT diagram above as the “narrow band p* ” immediately above the Fermi level , EF) almost certainly isn't comprised of such antibonding molecular orbitals . Metallic bonds are bonding , not antibonding in nature ; and ABMOs are nodal , which would result in any XO (crystal orbital = “polymerized molecular orbital” = conduction band = metallic bond) having pseudometal characteristics , ie. like semiconductors , which CrO2 certainly isn't :

Pseudometals (GIF image , 41 KB , from the Solids web page) have a direct electrical conductivity–temperature relationship ; as noted above , CrO2 has an inverse relationship . This electrical conductivity behaviour in pseudometals is a direct consequence of the periodic nodes in their metallic bond XOs . True metals have nodeless XOs ; their electrical conductivities are controlled by the scattering of the free electrons off the atomic kernels , rather than by electrons tunneling through nodes , as with the pseudometals .

Valence Bond Theory sketches are closely correlated with the actual chemical and physical properties of the materials they describe . They present a simple , straightforward picture of the chemical bonding (especially of covalent bonds , and in ground-state conditions) of those materials . This is true of the following Valence Bond sketch of the electronic structure of chromium dioxide :

As with all Valence Bond descriptions , priority is given to the formation of the strong , low energy Cr–O covalent bonds in the rutile crystal structure . As an early Transition metal element , chromium tends to use all of its d orbitals and valence electrons in the formation of an octahedral hybrid orbital for use in the six Cr–O covalent bonds per CrO2 formula unit . In this case it blends the five 3d orbitals and the 4s native atomic orbital to form the octahedral d5s hybrid orbital . It fills the lobes of this orbital with pairs of electrons , using its own valence electrons and those from the oxygen atoms to which it bonds . Twelve electrons are required for the six Cr–O covalent bonds . The two oxygens can supply four electrons each (for eight) ; the chromium provides the remaining four electrons . That completes the CrO2 rutile skeleton .

However , chromium has six valence electrons , so two are “leftover”. They have to be located in any available , empty frontier orbitals (LUMOs , in MO theory) . These are chromium's 4p orbitals , which are at about the same energy level as its 3d and 4s orbitals . One electron goes into the 4px or 4py orbital ; the other goes into the 4pz orbital . It so happens that in the rutile structure the chromium 4pz and oxygen 2pz orbitals have the same shape , symmetry , and orientation in the structure ; they can overlap throughout the lattice to form a Cr 4pz1 – O 2pz2 pi XO , which is predicted to be the Cr–O bilayer metallic bond in the solid . This pi XO is nodeless , and permits chromium dioxide to behave as a true metal .

It would be interesting to conduct an ARPES (angle-resolved photoelectron spectroscopy) analysis of a sample of pure , crystalline CrO2 . ARPES has been used to study the orbital state of the valence electrons (in our case , the metallic bond) in various metallic , superconducting solids . An ARPES spectrum of CrO2 might provide some support for this Valence Bond picture of its metallic bond .

Chromium dioxide is often referred to as a half-metallic ferromagnet . We see from the above analysis that CrO2 is actually fully metallic ; there is probably Pauli paramagnetism in the pi XO metallic bond free electrons , although this feeble magnetism would be obscured by the much stronger ferromagnetism . Iron , cobalt , and nickel are three “fully metallic” metals that are also simultaneously ferromagnetic . They likely have a veiled , weak , Pauli paramagnetism in their metallic bonds , which are comprised of their 4 s-p orbitals . The strong ferromagnetism in these three metals is produced by the organized , cooperative spins of their 3d orbital electrons .

There are no corresponding 2px and 2py orbitals on the oxygen trigonal planar sp2 hybrid orbital , so the second “leftover” chromium valence electron in the 4px,y orbitals is pinned (localized) in them . It so happens these latter pinned electrons have a parallel spin orientation throughout the lattice ; their cooperative magnetic fields produce the observed ferromagnetism (the 2.0 BM magnetic susceptibility mentioned above) in the material .

 

Syntheses of Chromium Dioxide

 

Chamberland has described a number of syntheses of chromium dioxide in his comprehensive review . The most popular method seems to be the carefully controlled thermal decomposition of chromium trioxide , usually from 300–500 ºC . This pyrolysis chemistry is quite complex , and a pure , single phase CrO2 is difficult to produce by it . A typical example is provided by the experimental descriptions in U.S. Patent 7276226 (Bajpai and Nigam , 2007 ; see also the report by Verma and co-workers , 2012) . The deep brownish-red liquid , chromyl chloride (CrO2Cl2 , b.p. 117 ºC) can also be pyrolysed to form CrO2 (eg. de Vries , 1967) .

One preparative method noted by Chamberland was the oxidation of Cr(OH)3 by chromic acid , in effect reproportionating Cr(III) with Cr(VI) to obtain Cr(IV) . This reaction , apparently first investigated in 1938 , was more recently studied by Ye and co-workers (2003) , who claimed to have prepared their ferromagnetic material at “ambient pressure and temperature” (although they admitted that annealing the product at 300 ºC for 5 hours enhanced the magnetic properties of the material) . Chamberland suggested that the brown-colored CrO2 produced in the 1938 reaction might have been Cr5O9 , which is CrO1.8 , or [Cr3+0.4Cr4+0.6(O2-)1.8] , a mixed-valent compound .

Ye and co-workers commented ,

“It has been reported that the CrO2 particles used in magnetic recording tapes are chemically unstable and react with the organic binder , leading to a noticeable loss in magnetization” (p. 6856) .

That might be a partial explanation for why my several dozen compact cassettes , of both the iron oxide and chrome oxide varieties , with many hours of music recorded on them , have deteriorated after several decades to the point where they are now essentially worthless junk . The music on them has greatly deteriorated , and is now faded and distorted to such an extent that it's almost painful to listen to . While compact cassettes (and reel-to-reel tape decks) certainly had their golden era two or three decades ago , clearly their day has ended and we see them now as a rather inferior data storage medium , satisfactory perhaps for short-term use , but a complete failure for long-term data storage . To be fair, I've also noted – with some dismay – fading and loss of data (ie. unreadability) in CD-R discs over a decade or so ; and even small scratches on the polycarbonate plastic layer on CR-ROMs (eg. commercial music discs) can ruin them . And a recently purchased 64 GB flash memory (USB) key proved to be defective , becoming write-resistant after being only 40% filled with data . I despair of ever finding the ideal , foolproof data storage device or medium !

Another way to prepare CrO2 is the reduction of dichromate by iodide anion , which is oxidized to iodine as a by-product :

K2Cr2O7 + 4 KI + 3 H2O ---------> 2 CrO2 + 2 I2 + 6 KOH .

This reaction was apparently investigated by Ridley in 1924 , and again by Sconzo in 1933 , who refuted Ridley's claim to have prepared CrO2 by it (according to Chamberland ; I haven't read the original research reports . I revised the chemical equation provided by Chamberland on his p. 7 , which was incorrect) . Reduction of Cr(VI) to Cr(IV) is an attractive approach to the synthesis of chromium dioxide , even though Ridley's reaction seems a rather clumsy way to accomplish it . Ramesha and Gopalakrishnan (1999) retooled this iodide–Cr(VI) reduction method into an impressive , efficient , ambient pressure synthesis of CrO2 :

CrO3 + 2 NH4+ X- -------- [120–150 ºC , one atm. , quantitative yield] ------->

CrO2 (amorphous , nonmagnetic) + 2 NH3 + H2O + X2 ; X = Br , I ;

CrO2 (amorphous) ----- [anneal at 195 ºC for 1 week] ------> CrO2 (crystalline , ferromagnetic) .

These researchers attributed their success to the solid state nature of their reaction ; analogous aqueous reactions invariably yield only Cr(III) products :

“This is in contrast to reactions between CrVI and I- /Br- in aqueous media , which proceed directly to give CrIII ; CrIV is not accessible as a stable species in aqueous media by redox reactions” (p. 1174) .

Interestingly , their initial CrO2 product was amorphous and nonmagnetic ; a prolonged annealing of pellets of the material at 195 ± 5 ºC for one week was required to convert it into crystalline , ferromagnetic CrO2 with the rutile crystal structure .

 

Several Proposed New Preparative Methods for Chromium Dioxide

 

There are two basic problems to devising new , efficient methods of synthesizing CrO2 . The first is that it is a metastable compound ; that is , its Cr(IV) can easily disproportionate into the more thermodynamically stable Cr(III) and Cr(VI) . Second , CrO2 itself is a very powerful oxidizing agent , stronger in fact than the Cr(VI) in chromic acid :

CrO2 + 4 H+ + e- --------> Cr3+ + 2 H2O ; E0red = 1.48 V ;

HCrO4- + 7 H+ + 3 e- --------> Cr3+ + 4 H2O ; E0red = 1.35 V .

Thus , when Cr(VI) is reduced to Cr(IV) , the latter species is then preferentially reduced to Cr(III) by the added reducing agent instead of the remaining Cr(VI) in the reaction vessel . To avoid this problem , the researcher must devise an experimental method to remove the desired CrO2 continuously from the reaction as it's being formed (for example , by precipitation from a solvent in which it's insoluble) .

Chromic acid (usually a solution of CrO3 in water or sulfuric acid) is often used by organic chemists to remove the residual tarry “gunk” from their reaction flasks during clean-up after a reaction . The chromic acid “fries” [oxidizes] the organic matter , presumably converting it into CO2 . We see that CrO2 is an even stronger oxidizing agent than chromic acid . No wonder CrO2 is gradually degraded by the organic matrix in magnetic recording tapes and chrome oxide compact cassettes . Perhaps the traditional Mylar™® (polyethylene terephthalate , PETE) tape material should be replaced by a polytetrafluoroethylene (PTFE , Teflon™®) ribbon for use in such magnetic recording media !

Several authors have stated that CrO2 can be successfully synthesized only under high pressures and high temperatures ; for example ,

“The importance of pressure vs. temperature diagrams was underlined in the preparative section , in which it was observed that pure CrO2 could only be prepared under presssure conditions at low temperature . Chromium dioxide is metastable , and pressure facilitates its formation” (Chamberlands's review , pp. 7 & 9) .

“As far as we know , all other Sr–Cr(IV)–O phases with Cr(IV) in an octahedral environment require high pressure and high temperature conditions to be obtained . In particular the technically important binary Cr(IV) oxide , CrO2 , with the rutile structure is a high pressure phase ....” (Castillo-Martínez and Alario-Franco , p. 564) .

This general consensus may require some revision . I would suggest that high temperature syntheses of CrO2 do indeed require high pressures to stabilize the Cr(IV) against disproportionation into Cr(III) and Cr(VI) , since at one atmosphere CrO2 is well known to decompose at ~ 400 ºC . However , preparations of the material at lower temperatures and ambient pressure CAN succeed , as demonstrated by Ramesha and Gopalakrishnan's results , as noted above . Granted , it may not be possible to produce crystalline (rutile) CrO2 under ambient pressure ; the amorphous Cr–O initial product might have to be annealed at higher temperatures to promote its crystallization to the desired ferromagnetic rutile phase .

With these considerations in mind , possible new synthesis routes to chromium dioxide are described below . Several of the reactions would undoubtedly be very exothermic , and would best be carried out in the appropriate high temperature–high pressure apparatus . Several more the proposed syntheses , however , might be conducted at STP (even with cooling by an icewater bath) and could thus be considered as chemie douce preparations of CrO2 .

1 . Reproportionation of Cr(II) and Cr(VI)

Chromium(II) is a mild natural reducing agent , and chromium(VI) is a powerful oxidizing agent :

Cr2+ – e- --------> Cr3+ ; E0ox = 0.407 V ;

HCrO4- + 7 H+ + 3 e- --------> Cr3+ + 4 H2O ; E0red = 1.35 V ;

Cr2+ + CrO42- --------> Cr2+(CrO42-) --------> “Cr2O4” --------> 2 CrO2 ; E0T ~ 1.76 V .

Overall chemical reaction :

CrCl2 (aq) + K2CrO4 (aq) --------> Cr2+(CrO42-) --------> 2 CrO2 (ppt) + 2 KCl (aq) .

This reaction is predicted to be thermodynamically spontaneous at STP , and indeed could be quite exothermic ; its solvent , water , could act as a moderator , combined with cooling in an icewater bath if necessary . Several drops of an acid , eg. conc. HCl , might be needed as a reaction catalyst (chromate reduction is accomplished in an acidic environment ; note , however , that Cr2+ apparently can reduce H+ to H0 , ie. H2 gas) . To ensure the correct stoichiometry of the product , exact equivalents of the CrCl2 and K2CrO4 would be used .

As noted by Ramesha and Gopalakrishnan , aqueous reductions of Cr(VI) to Cr(IV) usually fail , with the production only of Cr(III) compounds . They thought that only solid-state reactions could produce CrO2 . The above reaction might be tried as a solid-state metathesis reaction :

CrCl2 (1.00 eq , m.p. 824 ºC) + K2CrO4 (1.00 eq , m.p. 974 ºC) + 2 NaCl (2.00 eq , m.p. 801 ºC)

--------> Cr2+(CrO42-) --------> 2 CrO2 (s) + 2 KCl–NaCl eutectic (m.p. 650 ºC) .

Frankly , I would be rather nervous about the very high E0T ~ 1.76 V for this reaction , which indicates it might be quite exothermic . I'll therefore copy and paste my standard precaution for it :

Any researcher attempting this reaction (CrCl2 + K2CrO4) would be advised to : (1) use only semi-micro quantities of reagents , only a few grams , perhaps at the 0.01 mole scale ; (2) observe all available safety measures : eye protection by shatterproof safety glasses is imperative ; (3) a shatterproof safety visor for the face and/or a safety shield would be advisable ; (4) heavy leather gloves or asbestos mittens should be worn to protect the hands . The reaction mixture of chromium(II) chloride and potassium chromate (ground thoroughly together ; take these safety precautions for the grinding process , too !) would be placed in an over-sized porcelain crucible , held in a clay-pipe triangle on a stand . The researcher would cautiously heat the crucible with a Bunsen or Meker burner , or a propane torch , until some indication of the commencement of the reaction was observed . It might also be a good idea to conduct the reaction outside , on the lawn in back of the laboratory , and have a fire extinguisher on hand . Be very careful !

This hypothetical metathesis might resemble a Thermite sort of reaction , very exothermic and producing a molten mass of reactants and products . If the temperature of the reaction mixture exceeded 400 ºC , any CrO2 product would be pyrolysed to Cr2O3 and O2 . It might be more fruitfully attempted in a high pressure–high temperature apparatus capable of withstanding the considerable exotherm and pressures generated by it . The molten KCl (m.p. 771 ºC) by-product could act as a reaction flux and moderator for the reaction , and promote the growth of very pure , acicular crystals of CrO2 . NaCl (m.p. 801 ºC) could also be added to the reaction mixture to form the 50 : 50 KCl–NaCl eutectic , which melts at 650 ºC (Kimura , molten salt synthesis , p. 75) .

A related solid-state metathesis reaction worthy of mention – even though it might be impractical – is the combination of Cr2+ cation with peroxide anion , O22- , to form CrO2 directly :

2 KCl (m.p. 771 ºC) + CrCl2 (m.p. 824 ºC) -------> K2CrCl4 ------- [Na2O2 , m.p. 460 ºC , dec.]

-------> CrO2 + 2 KCl–NaCl eutectic (m.p. 650 ºC) .

The Merck Index (8th edition , 1968) says this about sodium peroxide :

“In contact with organic matter or readily oxidizable substances ignition and explosion may take place” (p. 963) .

Since the Cr2+ in CrCl2 and K2CrCl4 qualifies as a “readily oxidizable substance” the solid-state combination of K2CrCl4–Na2O2 might well behave like a gunpowder mixture and go BANG ! when heated . A safer method of carrying out this reaction might be in water solution . The Merck Index says that sodium peroxide is “Freely soluble in water , forming sodium hydroxide and hydrogen peroxide , the latter quickly decomposing into oxygen and water”. Small quantities of solid Na2O2 would be slowly tapped from a spatula into a solution of CrCl2 in cold water , with vigorous mechanical stirring and with additional cooling in an ice/water bath . Hopefully the Cr2+ would intercept the O22- before it has a chance to hydrolyze into hydrogen peroxide . The appearance of a fine , black precipitate would indicate the probable formation of CrO2 , although it might be amorphous and require subsequent thermal annealing for transformation into the desired rutile product .

2. Oxidation of Cr(III) to Cr(IV)

In trying to prepare CrO2 from a Cr(III) reagent by oxidation we are faced with the same problem as in trying to prepare it from a Cr(VI) source , such as chromate , by reduction . Scrolling back up this web page to the two redox reactions involving Cr(III) , Cr(IV) , and Cr(VI) , we can quickly see that any attempt to selectively prepare Cr(IV) from Cr(III) would be futile . The oxidation of Cr(III) to Cr(VI) has a lower E0ox [– 1.35 V] than does the oxidation of Cr(III) to Cr(IV) [– 1.48 V] . So , it takes less energy for the former reaction than the latter . Thus , any attempt to preferentially form Cr(IV) from Cr(III) will end in failure ; only Cr(VI) will be obtained .

This prohibition apparently doesn't apply to a redox process involving reproportionation . For example , Cr(OH)3 can be oxidized successfully to CrO2 by H2CrO4 , as mentioned above . However , this oxidation is really the reproportionation of Cr(III) and Cr(VI) to Cr(IV) .

Generally , the higher the oxidation state of a metal atom , the stronger a chemical oxidizer it is . Conversely , the lower its oxidation state , the better a chemical reducer it is . This is mostly true for chromium : Cr(II) is a mild natural reducer , Cr(III) is low energy (neither reducer nor oxidizer) , and Cr(VI) is a powerful oxidizer . Perversely , Cr(IV) bucks the trend and is a stronger oxidizer than Cr(VI) , and that makes things difficult for the chemist trying to devise an efficient synthesis of Cr(IV) compounds . This problem is illustrated in the following examples .

Two common , yet very strong oxidizers might be examined for the conversion of Cr3+ to CrO2 : acidic hydrogen peroxide and acidic hypochlorite . The redox reactions for the first system are :

2 x [Cr3+ + 2 H2O – e- --------> CrO2 + 4 H+] ; E0ox = – 1.48 V ;

H2O2 + 2 H+ + 2 e- --------> 2 H2O ; E0red = 1.776 V ;

Net : 2 Cr3+ + H2O2 + 2 H2O --------> 2 CrO2 + 6 H+ ; E0T = 0.296 V .

In the competing reaction , the Cr3+ would instead be oxidized to chromate :

2 x [Cr3+ + 4 H2O – 3 e- --------> HCrO4- + 7 H+] ; E0ox = – 1.35 V ;

3 x [H2O2 + 2 H+ + 2 e- --------> 2 H2O] ; E0red = 1.776 V ;

Net : 2 Cr3+ + 3 H2O2 + 2 H2O --------> 2 HCrO4- + 8 H+ ; E0T = 0.426 V .

Comparing the two sets of equations , we see that it's easier to oxidize Cr3+ to chromate (– 1.35 V) than it is to CrO2 (– 1.48 V) , and that more chemical energy is released in forming chromate (0.426 V) than CrO2 (0.296 V) . Conclusion : we aren't going to get any CrO2 from this acidic peroxide system !

As is well-known , even to non-chemists , the mixing of a hypochlorite solution , eg. Javelle bleach , with an acid , eg. vinegar , will release chlorine gas . The chlorine actually comes from the decomposition of hypochlorous acid , HOCl , which is a remarkably strong oxidizer , and could conceivably oxidize Cr3+ to CrO2 :

Cr3+ + 2 H2O – e- --------> CrO2 + 4 H+ ; E0ox = – 1.48 V ;

HOCl + H+ + e- --------> ½ Cl2 + H2O ; E0red = 1.611 V ;

Net : Cr3+ + HOCl + H2O --------> CrO2 + ½ Cl2 + 3 H+ ; E0T = 0.131 V .

The competing reaction to produce chromate :

Cr3+ + 4 H2O – 3 e- --------> HCrO4- + 7 H+ ; E0ox = – 1.35 V ;

3 x [HOCl + H+ + e- --------> ½ Cl2 + H2O] ; E0red = 1.611 V ;

Net : Cr3+ + 3 HOCl + H2O --------> HCrO4- + 1½ Cl2 + 4 H+ ; E0T = 0.261 V .

Once again chromate trumps chromium dioxide . We see from these examples that , yes , it might be feasible to prepare CrO2 in these reactions . Unfortunately , it's MORE feasible to obtain chromate using the same acidic peroxide and hypochlorite oxidizers with Cr3+ .

While reviewing the redox chemistry of CrO2 I wondered if its powerful oxidizing nature might be tapped in an energy storage system , that is , in an electrical battery . Everyone is familiar with the common Leclanché dry cell (and this web page) , which uses manganese dioxide as the oxidizing agent and the surrounding zinc casing of the battery as its reducing agent :

MnO2 + 4 H+ + 2e- --------> Mn2+ + 2 H2O ; E0red = 1.224 V ;

Zn0 – 2e- --------> Zn2+ ; E0ox = 0.7618 V ;

Net : MnO2 + 4 H+ + Zn0 --------> Mn2+ + Zn2+ + 2 H2O ; E0T = 1.9858 V .

If CrO2 is substituted for MnO2 in the Leclanché dry cell , a higher battery output voltage might be obtained , but at the cost of twice the consumption of CrO2 compared to MnO2 :

2 CrO2 + 8 H+ + 2e- --------> 2 Cr3+ + 4 H2O ; E0red = 1.48 V ;

Zn0 – 2e- --------> Zn2+ ; E0ox = 0.7618 V ;

Net : 2 CrO2 + 8 H+ + Zn0 --------> 2 Cr3+ + Zn2+ + 4 H2O ; E0T = 2.2418 V .

CrO2 is an excellent electrical conductor (~ 2500 ohm-1cm-1 , as mentioned above) , and so could result in a low internal electrical resistance in batteries in which it is used . Manganese dioxide is in contrast a poor electrical conductor , with an ambient conductivity of ~ 20 ohm-1-cm-1 (GIF image , 55 KB) . Chromium dioxide might also be examined as the oxidizer in the modern alkaline dry cells , which have mostly displaced the older acidic type of manganese dioxide–zinc batteries with an aqueous ammonium chloride electrolyte .

Replacement of zinc (0.7618 V) by the inexpensive aluminum (1.662 V) in the above Leclanché dry cell with CrO2 would result in a theoretical maximum cell potential of E0T = 3.142 V , which is quite substantial . This novel CrO2 electrochemistry might make an interesting research project for university students !

3. Deoxygenation of chromium trioxide

It might be possible to remove one of the oxygen atoms from a molecule of CrO3 , to obtain CrO2 . Crystalline chromium trioxide consists of linear chains of –O–Cr(O)2–O– polymer :

CrO3 can form molecular complexes with electron pair donors such as pyridine . Pyridine–CrO3 complexes are stable in certain organic solvents , in particular methylene chloride (CH2Cl2 , b.p. 40 ºC) :

Ratcliffe and Rodehorst (1970) considered this deep red-brown solution of pyridine–CrO3 in CH2Cl2the reagent of choice in almost all situations calling for the oxidation of an alcohol (p. 4001) . An extract from Ratcliffe's Organic Synthesis procedure using the reagent to oxidize 1-decanol to 1-decanal is reproduced above . Interested readers should consult Ratcliffe's full reports on the pyridine–CrO3 complex . His method of preparing the solution apparently is safe , but earlier procedures by other researchers occasionally led to fires as the CrO3 ignited the pyridine .

Organic phosphines and phosphites are often used in organic chemistry syntheses as deoxygenating reagents , as phosphorus is known to have a particularly powerful bonding affinity with oxygen . Triphenylphosphine (Ph3P) is used as the deoxygenating reagent in the Wittig olefination reaction , while phosphite esters such as trimethyl and triethyl phosphite [(RO)3P ; R = Me , Et] are the deoxygenating reagents in the Horner-Wadsworth-Emmons phosphonate modification of the Wittig reaction . Ph3P and (RO)3P are used in non-aqueous solvents , and they might be effective in deoxygenating CrO3 to CrO2 :

Addition of one equivalent of Ph3P or (RO)3P to a methylene chloride solution of one equivalent of pyridine–CrO3 complex , with rapid mechanical stirring and cooling (if necessary) , should result in the replacement of the pyridine ligands on the CrO3 with the Ph3P or (RO)3P ligands ; these would form the strong phosphorusoxygen bonds in the R3P+–O–Cr(=O)–O- complex . Cr(VI) in the CrO3 would be reduced to Cr(IV) by the added lone pair of electrons from the P(III) , which then becomes P(V) in the resulting phosphorane adduct . The unstable intermediate Cr(IV) chromo–P(V) phosphorane cyclo-complex would then split into the two chemically stable fragments , CrO2 and triphenylphosphine oxide (or trialkyl phosphate) , by analogy with the reaction pathway of the Wittig reaction . However , CrO2 is completely insoluble in CH2Cl2 and would precipitate from the reaction mixture , thus being removed from any further reduction by the R3P [which is unlikely anyway , since CrO2 has an infinite atomic lattice with inaccessible Cr(IV) atoms , unlike the molecular CrO3 in the pyridine complex with readily accessible Cr(VI) atoms] .

The resulting CrO2 precipitate would be thoroughly washed with CH2Cl2 and dried at ~ 110 ºC . It might be amorphous , as with the material produced in Ramesha and Gopalakrishnan's procedure . If so , it would have to be annealed at an elevated temperature , as recommended by them , to convert it into the ferromagnetic , crystalline rutile form .

Sulfides (thioethers) can be oxidized by hydrogen peroxide and organic peroxides to the corresponding sulfoxides and sulfones . That is , their sulfur atoms are acting as oxygen atom acceptors ; like phosphorus , sulfur also has a strong bonding affinity with oxygen . By analogy with peroxides , possibly dimethyl sulfide (b.p. 38 ºC) and dimethyl sulfoxide (b.p. 189 ºC) might similarly be able to deoxygenate CrO3 to CrO2 :

2 pyridine–CrO3 /CH2Cl2 --------- [add 1.0 eq. H3C–S–CH3 / CH2Cl2 , stirring , cooling] ------->

2 CrO2 (ppt) + 2 pyridine + H3C–SO2–CH3 ;

pyridine–CrO3 /CH2Cl2 --------- [add 1.0 eq. H3C–SO–CH3 / CH2Cl2 , stirring , cooling] ------->

CrO2 (ppt) + pyridine + H3C–SO2–CH3 .

Phosphorus and hypophosphorus acids (H3PO3 and H3PO2 respectively) are also excellent deoxygenating reducing agents ; however , they are water-based reagents , and might not be suitable for the reduction of the Cr(VI) , as mentioned above . Nevertheless , the following experiment would still be quite interesting (fun !) to try :

Na2CrO4 (aq) + H3PO3 (aq) ------- [mix the two solutions together in a beaker with

rapid mechanical stirring , cooling if necessary] -------> CrO2 (ppt) + H2O + Na2HPO4 (aq) .

The pertinent redox half reactions for this chemical reaction are as follows :

3 x [H3PO3 + H2O – 2 e- --------> H3PO4 + 2 H+] ; E0ox = 0.276 V ;

2 x [HCrO4- + 7 H+ + 3 e- --------> Cr3+ + 4 H2O] ; E0red = 1.35 V ;

Net : 3 H3PO3 + 2 HCrO4- + 8 H+ --------> 3 H3PO4 + 2 Cr3+ + 5 H2O ; E0T ~ 1.626 V .

The overall cell potential , E0T , is quite substantial for this hypothetical reaction , suggesting that it is both spontaneous at STP and quite exoergic . Note that the chromate standard reduction potential is for Cr(VI) to Cr3+ and not Cr(IV) , but it would probably be approximately the same in either case . It might be necessary to chill the Na2CrO4 and H3PO3 solutions in icewater before mixing them . If the reaction was too vigorous (i.e. boiling , volcanic eruption) the reagents could be diluted with more water .

The chemistry of the equations indicates that the solution of starting reagents would be acidic to begin with , but would become essentially neutral upon completion (0.025 M aqueous Na2HPO4 has a pH of 6.865 at 25 ºC : CRC Handbook of Chemistry & Physics) . This would be a very simple , neat synthesis method for CrO2 ..... if it worked , of course . Admittedly , the competing chromate reduction to Cr3+ might be more favorable . Pure phosphorus acid (also called “phosphonic acid”) is a solid at room temperature (m.p. 74 ºC) . It's commercially available at a modest cost , eg. from Aldrich and Alfa-Aesar .

In a simpler version of the reduction , equimolar quantities of anhydrous chromium trioxide and phosphorus acid would be mixed together and gently warmed (if necessary) to initiate the reaction :

CrO3 + H3PO3 --------> CrO2 + H3PO4 .

I suspect this might be a very violent , explosive , hazardous reaction ; a safer way to investigate it would be to mix dilute (and chilled) aqueous solutions of H2CrO4 and H3PO3 . Unfortunately , Cr3+ and not CrO2 would most likely be the product of such a water-based reaction .

Oxalic and formic acids , (COOH)2 and HCOOH respectively , are also reducing agents :

(COOH)2 – 2 e- --------> 2 CO2 (g) + 2 H+ ; E0ox = 0.43 V ; so possibly ,

CrO3 + HOOCCOOH (neat , m.p. 190 ºC, dec.) --------> CrO2 + 2 CO2 (g) + H2O ; and ,

HCOOH – 2 e- --------> CO2 (g) + 2 H+ ; E0ox = 0.199 V ; so possibly ,

CrO3 + H–COO–H (neat , b.p. 100 ºC) --------> CrO2 + CO2 (g) + H2O .

These three reactions would probably be very exothermic , explosively violent , and very hazardous . In a more manageable , safer modification their pyridine salts might be dissolved in CH2Cl2 and added dropwise to the pyridine–CrO3 complex in CH2Cl2 with mechanical stirring , and with cooling if necessary . Such a non-aqueous environment might favor the formation of CrO2 over Cr3+ .

4. Dehalogenation of Chromyl Chloride

Another organic chemie douce method might be successfully applied to the Cr(VI) reagent chromyl chloride , CrO2Cl2 . This is a dark , brownish-red liquid at room temperature , b.p. 117 ºC , that looks very much like bromine . It's commercially-available , eg. from Alfa-Aesar , but can also be easily prepared by the distillation of a mixture of a chromate or dichromate with ordinary salt (NaCl) and a substantial excess concentrated sulfuric acid :

Na2CrO4 + 2 NaCl + 2 H2SO4 (conc. , excess) ------ [distil product into a cooled receiver] ------>

CrO2Cl2 (l) + 2 H2O + 2 Na2SO4 .

I actually did this experiment a long , long time ago (when I was ~ 12 years old) with my chemistry set , distilling the deep red CrO2Cl2 from one test tube to another (cooled in icewater) , using a small alcohol burner . The extremely corrosive chromyl chloride vapors turned the cork in the reaction test tube into a soft mush , but I was able to collect a few ml of fairly pure CrO2Cl2 product .

Chromyl chloride has been used as an oxidizing agent in organic syntheses , such as in the Étard reaction (conversion of an aromatic methyl group into an aldehyde group) . It might be reduced to CrO2 by the strong one- and two-electron reducing agent tetrakis(dimethylamino)ethylene , TDAE :

TDAE is a pale yellow liquid , b.p. 59 ºC (0.9 mm) , which is strongly luminescent with air” ; it is quickly oxidized by atmospheric oxygen and must be handled under nitrogen or argon . TDAE is said (Hoffmann , 1968) to be a reducing agent with an oxidation potential comparable to that of zinc metal (E0ox = 0.762 V) . It's commercially-available , eg. from the Aldrich Chemical Co. .

A solution of TDAE (1.0 equivalent) in dry CH2Cl2 would be added slowly , dropwise , from a pressure-equalizing funnel under nitrogen or argon to a solution of CrO2Cl2 (1.0 equivalent) in dry CH2Cl2 , with vigorous mechanical stirring and cooling in an icewater bath . The TDAE would donate two of its electrons to the CrO2Cl2 , reducing its Cr(VI) atoms to Cr(IV) . That would cause the chlorine atoms to “pop off” it as chloride anions . They would remain associated with the by-product TDAE cations :

CrO2Cl2 (1.0 equiv.) + TDAE (1.0 equiv.) -------- [CH2Cl2 solvent , mechanical stirring , N2 atmos.]

--------> CrO2 (ppt) + TDAE2+(Cl-)2 .

The chromium dioxide precipitate would be collected by filtration , washed with additional CH2Cl2 , and oven-dried . Once again , it would be thermally annealed to form the crystalline material if an X-ray diffraction analysis showed it to be amorphous .

TDAE reduction might also be successful with other covalent inorganic oxychlorides (and maybe chlorides) . Analogous reductions might include MoO2Cl2 to MoO2 , WO2Cl2 to WO2 , and UO2Cl2 to UO2 ; and possibly WCl6 to WCl4 , TiCl4 to TiCl2 , and SnCl4 to SnCl2 , for example [also the synthetically impractical SO2Cl2 to SO2 (g)] .

The anomalous redox behaviour of the Cr(IV) in chromium dioxide and its thermodynamic metastability make its synthesis an interesting challenge . I hope the ideas presented in this web page will be mentally stimulating and beneficial to chemists and technologists engaged in CrO2 research .

 

References and Notes

 

The standard redox potentials cited are versus the SHE at STP , and were from : D.R. Lide (ed.) , CRC Handbook of Chemistry and Physics , 87th edition , CRC Press / Taylor & Francis , Boca Raton (FL) , 2006 ; P. Vanýsek (ed.) , “Electrochemical Series”, pp. 8-20 to 8-25 . Another useful reference with redox potentials is : A.J. Bard (ed.) , Encyclopedia of Electrochemistry of the Elements , various volumes , Marcel Dekker , New York , ca. 1973-1986 .The oxalic acid standard oxidation potential was from the Wikipedia web page “Table of Standard Electrode Potentials”. For a convenient tabulation of oxidizing metal oxides and their E0red values , see this GIF image (45 KB) .

Chamberland : B.L. Chamberland , “The Chemical and Physical Properties of CrO2 and Tetravalent Chromium Oxide Derivatives”, CRC Crit. Rev. Solid State Mater. Sci. 7 (1) , pp. 1-31 (1977) . This key review of chromium dioxide is available from the publisher as a PDF download (DOI) , which however is very expensive if your Science Library doesn't have an online subscription for that journal . I've prepared a home-made PDF copy of the review (3210 KB) from scanned photocopied pages from our Science Library's hard copy journal . Interested readers can contact me for a copy of the review , which I'll be happy to furnish as an attachment to an email reply .

Hiroi's graphic : Z. Hiroi , “Structural Instability of the Rutile Compounds and Its Relevance to the Metal–Insulator Transition of VO2”, ArXiv.org , March 8th , 2015 ; see Fig. 1 , p. 2 [PDF , 5282 KB] .

Goodenough : J.B. Goodenough , “Transition Metal Oxides with Metallic Conductivity”, Bull. Soc. Chim. France (4) , pp. 1200-1206 (1965) ; CrO2 is discussed on pp. 1204-1205 .

ARPES : A.F. Santander-Syro , “Introductory Lectures on Angle-Resolved Photoemission Spectroscopy (ARPES) and Its Application to the Experimental Study of the Electronic Structure of Solids” [PDF , 1742 KB] ; ARPES Research Group , University of British Columbia [web page] ; A. Damascelli , “Probing the Electronic Structure of Complex Systems by ARPES”, Physica Scripta T109 , pp. 61-74 (2004) [PDF , 1859 KB] ; R. Comin and A. Damascelli , “ARPES : A Probe of Electronic Correlations”, ArXiv.org web page (ArXiv PDF , 4054 KB) ; Shen Laboratory, Stanford University , CA [web page] ; EE290f - Lecture 19 : “Angle Resolved Photoemission and Non-ARPES”, given by Dr. Eli Rotenberg , ALS/LBNL [YouTube video , FLV , run time 1:07:35 ; a high speed Internet connection is recommended , if not mandatory , for viewing this video !] .

various metallic , superconducting solids : In a study of the ferropnictide superconductors I presented a Valence Bond sketch of doped LaOFeAs , the prototype compound of the family :

As you can see , the metallic bond in this compound was predicted to be in the iron atoms' 4p frontier orbitals , resulting in the metallic bond being a FeFe monolayer pi XO . Some time after the web page was posted on Chemexplore , a research report by Zabolotnyy and co-workers was published , concerning their ARPES analysis of the ferropnictide superconductor Ba1-xKxFe2As2 (Tc = 38 K at x = 0.4 , PDF , 750 KB) . It seemed to confirm my simple Valence Bond electronic structure of the ferropnictides : V.B. Zabolotnyy et al. , p, p Electronic Order in Iron Arsenide Superconductors”, Nature 457 (7229) , pp. 569-572 (2009) [PDF , 495 KB] . Note that typical MOTDOS calculations [eg. PDF , 240 KB] place the metallic bond in the iron 3d orbitals for these compounds .

In a second interesting example , an ARPES analysis of the superconductor magnesium diboride (MgB2 , Tc = 39 K) revealed it had two metallic bonds [conduction bands] , a broad sigma band and a narrower pi band : S. Souma et al. , The Origin of Multiple Superconducting Gaps in MgB2, Nature 423 (6935) , pp. 65-67 (May 1, 2003) . The pi band can be readily attributed to the pi XO over the boron rings (cf. benzene and other aromatic compounds) . As suggested in the Layered web page , the sigma band might originate from a mixed-valent Mg0Mg2+ state in the compound's magnesium atoms , which might not have transferred 100% of their 3s2 electrons to the boron atoms . The magnesium 3s orbitals , overlapping continuously in the lattice , and with a small population of free electrons , could be forming a sigma XO as the second metallic bond in the material .

half-metallic ferromagnet : H. van Leuken and R.A. de Groot , Electronic Structure of the Chromium Dioxide (001) Surface, Phys. Rev. B 51 (11) , pp. 7176-7178 (1995) [PDF , 462 KB] ; J.M.D. Coey and M. Venkatesan , Half-Metallic Ferromagnetism : Example of CrO2 (Invited), J. Appl. Phys. 91 (10) , pp. 8345-8350 (2002) . These latter authors have provided a classification of metallic solids (ten classes) , based on the spin orientation of the free , mobile electrons at the Fermi level , EF : Table I , “Summary of the Classification of Half-Metals”, p. 8346 .

Bajpai and Nigam : A. Bajpai and A.K. Nigam , “Chromium Dioxide (CrO2) and Composites of Chromium Dioxide and Other Oxides of Chromium Such as CrO2 /Cr2O3 and CrO2 /Cr2O5 and Process for Manufacturing the Same, U.S. Patent 7276226 , 29 pp. (October 2 , 2007) [PDF , 340 KB] . Note : this file can be opened only with Adobe Acrobat Reader v. 6 or later . If desired , this application can be downloaded for free from Oldversion.com .

Verma and co-workers : V. Verma , S, Ahmad , A. Dar , and R. Kotnala , “An Inexpensive Route to Synthesize High-Purity CrO2 for EMI Shielding in X-Band Frequencies”, ISRN Mater. Sci. 2012 , article 948219 , 4 pp. [PDF , 1326 KB] .

de Vries : K. J. de Vries , “Preparation of Chromium(IV)-Oxide from Chromylchloride”, Naturwiss. 54 (21) , p. 563 (1967) [PDF , 130 KB] .

Ye and co-workers : H. Ye et al. , “Method for the Synthesis of CrO2 at Ambient Pressure and Temperature”, J. Appl. Phys. 93 (10) , Pts. 2&3 , pp. 6856-6858 (2003) [PDF , 112 KB] .

Ramesha and Gopalakrishnan : K. Ramesha and J. Gopalakrishnan , “A New Method for the Synthesis of Chromium(IV) Oxide at Ambient Pressure”, Chem. Commun. 1999 (13) , pp. 1173-1174 [PDF , 132 KB] .

Castillo-Martínez and Alario-Franco : E. Castillo-Martínez and M.Á. Alario-Franco , “Revisiting the Sr–Cr(IV)–O System at High Pressure and Temperature with Special Reference to Sr3Cr2O7, Solid State Sci. 9 (7) , pp. 564-573 (2007) [PDF , 1998 KB] .

chemie douce : F.J. DiSalvo , “Solid State Chemstry”, Solid State Commun. 102 (2-3) , pp. 79-85 (1997) ; see pp. 81-82 ; A. Stein , S.W. Keller , and T.E. Mallouk , “Turning Down the Heat : Design and Mechanism in Solid-State Synthesis”, Science 259 (5101) , pp. 1558-1564 (1993) ; C.N.R. Rao , Chemical Approaches to the Synthesis of Inorganic Materials , Wiley Eastern Ltd. , New Delhi , India , 1994 ; pp. 11-13 (and elsewhere in this textbook) ; C.N.R. Rao and J. Gopalakrishnan , New Directions in Solid State Chemistry , 2nd edition , Cambridge University Press , Cambridge (UK) , 1997 ; p. 128-137 ; “Chemie Douce (Soft Chemistry)” [web page] , Prof. Patrick Woodward , Ohio State University .

solid-state metathesis reaction : J.B. Wiley and R.B. Kaner , “Rapid Solid-State Precursor Synthesis of Materials”, Science 255 (5048) , pp. 1093-1097 (1992) ; R.E. Treece , G.S. Macala , and R.B. Kaner , “Rapid Synthesis of GaP and GaAs from Solid-State Precursors”, Chem. Mater. 4 (1) , pp. 9-11 (1992) ; R.B. Kaner et al. , “Rapid Solid-State Methathesis of Refractory Materials”, U.S. Patent 5110768 , 8 pp. (May 5 , 1992) [PDF , 368 KB] ; R.B. Kaner , C.H. Wallace , and T.K. Reynolds , “Instantaneous Synthesis of Refractory Nitrides from Solid Precursors”, U.S. Patent 6096282 , 11 pp. (August 1 , 2000) [PDF , 127 KB] ; R.B. Kaner and C.H. Wallace , “Process for Rapid Solid-State Formation of Refractory Nitrides”, U.S. Patent 6120748 , 8 pp. (September 19 , 2000) [PDF , 89 KB] . Note : these three patent files can be opened only with Adobe Acrobat Reader v. 6 or later .

molten salt synthesis : T. Kimura , “Molten Salt Synthesis of Ceramic Powders”, Ch. 4 , pp. 75-100 in Advances in Ceramics – Synthesis and Characterization , Processing and Specific Applications , C. Sikalidis (ed.) , InTech , Shanghai , China (2011) [PDF , 1233 KB] . Note : this file can be opened only with Adobe Acrobat Reader v. 6 or later .

Ratcliffe and Rodehorst : R. Ratcliffe and R. Rodehorst , “Improved Procedure for Oxidations with the Chromium Trioxide-Pyridine Complex”, J. Org. Chem. 35 (11) , pp. 4000-4002 (1970) ; R.W. Ratcliffe , “1-Decanal”, Org. Syn. Coll. Vol. 6 , pp. 373-375 (1988) [PDF , 142 KB] .

easily prepared : A detailed description of the dichromate method of preparing a sample of chromyl chloride is provided in this web page . See also this YouTube video [FLV , 9109 KB , 2:19 run time] of a chemistry set demonstration of its preparation . An improved procedure for preparing larger quantities of CrO2Cl2 was described by Law and Perkin in 1907 :

CrO3 (50 g) ------ (1) dissolve in conc. HCl (170 cc) ------- (2) add conc. H2SO4 (100 cc) in 20 cc aliquots with cooling ------- (3) remove lower dense layer in a separatory funnel -------> CrO2Cl2 ;

H.D. Law and F.M. Perkin , “Preparation of Chromyl Dichloride”, J. Chem. Soc. Trans. 91 , pp. 191-192 (1907) [download from this web page , PDF , 157 KB] .

TDAE : R.L. Pruett et al. , “Reactions of Polyfluoro Olefins. II. Reactions with Primary and Secondary Amines”, J. Amer. Chem. Soc. 72 (8) , pp. 3647-3650 (1950) ; the preparation of TDAE is described on p. 3649 .

Hoffmann : R.W. Hoffmann , “Reactions of Electron-Rich Olefins”, Angew. Chem. Internat. Ed. Engl. 7 (10) , pp. 754-765 (1968) ; N. Wiberg , “Tetraaminoethylenes as Strong Electron Donors”, Angew. Chem. Internat. Ed. Engl. 7 (10) , pp. 766-779 (1968) .

 

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