Nanostructuration

As mentioned above, the process of nanostructuration involves the modification of a structure to contain a region or elemental building block in the nanoscale.

Such is the case with the forming tunnels of cryptomelane (tunnel dimensions of 4.6 Å by 4.6 Å) or layers of birnessite (layer separation of 7 Å).

This nanostructuration process was observed during initial testing of the effects of surface potassium promotion of manganese, iron and cobalt spinels.

Investigating the nanostructuration process

Initially, a series containing manganese, iron and cobalt spinels were promoted with potassium to explore the benefits of alkali promotion on soot combustion.

The materials were calcined at a temperature of 400°C for 4 h, which was sufficient to initiate the process of nanostructuration of the manganese-based catalyst, allowing for the formation of the new phase identified as birnessite (Fig. 11).

The nanostructuration process was not observed over iron and cobalt oxides, even with high alkali loading, as the formation of the appropriate nanostructured phases occurs at high temperatures (700-900°C) through a solid-state reaction.

The nanostructuration of the manganese oxide could only proceed to a certain extent, which was determined by the concentration of potassium, forming a core-shell structure with the birnessite phase located on the surface and spinel remaining within. Thus, with lower potassium concentrations, the effect is not observed by XRD, as their concentration of the new phase is not sufficient to be measured. As alkali cations are mobile, even in the stable nanostructures, they are known to desorb from the active phases. The low temperature, at which the nanostructuration of the manganese spinel occurred, is extremely beneficial, as it allows the recreation of the active nanostructured phase and the re-entry of the mobile alkali back into the tunnels or layers of the catalyst. [Paper I]

33 Fig. 11 Diffractograms of the manganese spinel promoted with surface K2CO3, with cryptomelane and birnessite diffractogram references

The nanostructuration process does not only occur for manganese spinels, but also for cobalt and iron oxides. As the formation of the birnessite phase was beneficial, lowering the temperature of soot combustion compared to the parent spinel, pure phases of nanostructured materials were prepared to examine the effects of the process on the transition metal oxides. For iron, two phases were prepared: monoferrite (KFeO2), which contains intersecting tunnels containing the stabilizing potassium cations, and betaferrite (K2Fe22O24), the counterpart of birnessite due to its layered structure stabilized by alkali and oxygen bridges (Fig. 12). Nanostructuration of cobalt oxides leads to the formation of a layered cobaltate phase (AxCo2O4), which can contain different concentrations of alkali as well as different alkali elements (A = Na, K). While all the aforementioned nanostructured phases can be prepared through a solid-state reaction, only birnessite and cryptomelane can be synthesized through more benign methods such as sol-gel or hydrothermal. Nonetheless, when compared to their unaltered oxide counterparts, all nanostructured phases displayed superior catalytic activity, which was explained through enhanced alkali mobility (birnessite, monoferrite and betaferrite) or through the stabilization of higher oxidation states of the transition metal cation (cobaltate). [Papers II and III]

34 Fig. 12 Structures of iron and cobalt analogues of birnessite and cryptomelane with comparative soot conversion (tight contact) with the appropriate metal spinel

Direct synthesis is not the only way to achieve a nanostructured phase.

As mentioned above, surface promotion of a manganese spinel and calcination were sufficient to produce a partly nanostructured phase. The thermal transformation of birnessite to cryptomelane is also a well-documented process.

In fact, studies have shown that the formation cryptomelane from birnessite is due to a collapse of the layer framework around the interstitial cations ¹⁰⁴. As birnessite stoichiometrically contains a larger quantity of potassium than cryptomelane, the excess must be removed, either by washing out with aqueous solutions or through thermal treatment, during which the mobility of the cations allows for the layers to collapse into tunnels ¹⁰⁵. Long term exposure to high temperatures (400 – 650°C) can be used to enable the thermal transformation of birnessite to cryptomelane. Though birnessite is often the more active phase for catalytic soot combustion, the forming cryptomelane has a higher activity than its parent material in loose contact (Fig. 13). At first glance, one might assume this is due to potassium being located not only in the tunnels but also on the surface of the catalyst. To test this theory the formed cryptomelane catalyst was thoroughly washed, revealing the high activity remains even when surface potassium is removed, suggesting the properties of the newly formed cryptomelane phase

35 (lower work function than parent birnessite) or its morphology (enhanced alkali mobility) are responsible for the activity. [Paper V]

Fig. 13 Soot conversion (loose contact) over untreated and thermally treated birnessite

Effects of nanostructuration on the pro-catalytic properties

The nanostructuration process has many effects on the parent material apart from the alteration of the structure. One major difference between the tunnel and layered nanostructured manganese oxides is their typical morphology.

The formation of cryptomelane particles is controlled by its tunnelled structure, which run in parallel and promote growth in one direction, leading to the formation of long, rod-shaped particles (Fig. 14 A). On the other hand, the layers of birnessite do not require such specific organization and the typical morphology is that of plate-like, irregular shapes (Fig. 14 B).

36 Fig. 14 TEM images showing the typical nanorod morphology of cryptomelane (A) and the unordered plate-like structures of birnessite (B)

One property which can be modified is specific surface area (SSA). While the surface area of a promoted spinel may change upon surface promotion and calcination, a drastic change occurs during the thermal transformation of birnessite to cryptomelane. From the initial surface area of 16 m²/g for the birnessite parent, the SSA nearly doubles to 36 m²/g after 12 h treatment in 425°C, with over 70%

of the birnessite transforming to cryptomelane. Longer treatment or one at a higher temperature is needed to complete the transformation, which in turn lowers the SSA due to sintering. The irregular morphology of birnessite allows for the particles to easily clump together, minimizing the expose surface (Fig. 15 - untreated). The increase of SSA is due to the reorganization of the birnessite parent and formation of the typical nanorod morphology of cryptomelane, exposing organized surfaces and minimizing any amorphous content (Fig. 15 – 425 and 550°C). [Paper V]

Fig. 15 TEM images showing the transformation of birnessite to cryptomelane

37 As mentioned above (see 3.2, 3.3), the average oxidation states of the catalysts are 3.8 for cryptomelane and 3.5-3.9 for birnessite. This is an important change from the +2/+3 oxidation state of manganese cations in the spinel and +4 in manganese oxide (IV), as the mixed valences allow for enhanced redox properties and increase the ability of lattice oxygen to take part in oxidation reactions.

An important parameter describing the surface reactivity of materials is work function. As the catalytic oxidation process exhibits a redox character, low work function is preferred for facile electron transfer from the catalyst surface to gas phase oxygen, forming reactive oxygen species (usually the first step in catalytic combustion). Indeed, in several series of catalytic materials (such as alkali-bulk promoted tungsten bronzes or even alkali oxides) the correlation between catalytic activity and soot combustion is evident ^106,107. The electrodonor properties are not only important during electron transfer, but also are involved in the charged desorption of other species such as K⁺ or O^-108. Additionally, in this work a correlation was observed for the cryptomelane-based series. It should be also noted, that low work function does not immediately determine high activity, as upon surface promotion of the manganese spinel with potassium, the work function was found to increase from 4.0 eV to 4.2 eV.

The work function of birnessite and cryptomelane also tend to be higher than that of the spinel, with measured values ranging from 4.1 to 4.65 eV for potassium birnessite and cryptomelane. Despite an increase in work function, the nanostructured phases are always more catalytically active than the unpromoted spinel. The results suggest, that the increased alkali mobility and redox properties characteristic of the tunnelled and layered structures provide increased catalytic activity, though the work function may still be the determining property in the mechanism over the tested series and a correlation between it and catalytic activity may be present. Thus, the changes in work function should be measured in a similar series (morphology/structure) instead of comparing distinct materials. [Paper I and V]

The result of the aforementioned influence of the nanostructuration process is the enhancement of catalytic activity. The effect of surface promotion of manganese oxides with potassium varies with the promoter’s concentration, with small amounts decreasing activity while larger concentrations increase catalytic activity. This can be in part due to the partial nanostructuration of the oxides. Nonetheless, compared to surface or unpromoted manganese

38 oxides, pure nanostructured phases display superior catalytic activity in soot combustion (Fig. 16). [Paper I]

Fig. 16 Soot conversion (tight contact) over manganese oxides, surface promoted manganese spinel, birnessite and cryptomelane

The increased activity is the result of enhanced redox properties of the catalyst, beneficial morphology but also of alkali mobility. For instance, while potassium is more stable inside the tunnels or layers than it would be on the materials surface, high temperatures allow the cations to migrate in the tunnels/layers, segregate on the catalysts surface and, in higher temperature, desorb from the catalysts surface. Depending on the properties and surface of the catalyst, potassium may desorb in a number of states. First, if the catalyst has a low work function and can readily pass an electron to a potassium cation, the desorption will occur in the form of an atom. Should no electron be passed to the cation, then the potassium will desorb in its charged state. A third option is the desorption of a Rydberg atom or cluster. Rydberg atoms are formed when one or more of an atom’s electrons is excited to a high principal quantum number (n > 10), which results in a large electronic orbit of the excited (and highly energetic) electron. Upon contact with soot particles or gas phase molecules, this electron (and its high energy) can easily be transferred, initiating a reaction or activating the receiving particle/molecule. Rydberg atoms are not the only high-energy states, which can desorb. Several Rydberg atoms may condense into Rydberg matter when the orbits of the excited electrons align planarly and the electrons move in a coherent motion. These clusters have a very long lifetime and, due to the large radii of the excited electrons, have a large cross-section making them suitable for the activation of reactants. The desorption of potassium in the form of atoms, ions, Rydberg atoms and clusters was documented in Paper IV. Similar desorption of potassium species was measured for a potassium birnessite material. [Paper IV]

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