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Back | Show only these items: | Promotion of reduction in Ce0.5Zr0.5O2: the pyrochlore structure as effect rather than cause?

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Promotion of reduction in Ce_0.5Zr_0.5O₂: the pyrochlore structure as effect rather than cause?

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp1

Evidence is presented that the well-established phenomenon of promotion of Ce_0.5Zr_0.5O₂ reduction by suitable treatment procedures may be controlled by adjusting the conditions; and that such reduction leads to the formation of the pyrochlore structure at extremely low temperature.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res1

The redox properties of ceria–zirconia mixed oxides (CZMOs) have been extensively investigated due to their use in automotive three-way catalysts (TWCs), in which CZMOs are included as the oxygen storage component.¹

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

Correlation between structure and ease of reduction, which has received particular interest, remains an open question.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot1

The Ce_1–xZr_xO₂ phase diagram² may be summarised as follows: for x ≤ 0.15 a cubic, fluorite-type phase is formed, while for x ≥ 0.85, a monoclinic phase is present.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac2

At intermediate compositions, various phases (t, t′, t″, κ and t*) have been identified.^2–4

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac2

Oxygen deficient structures, such as the pyrochlore structure (A₂B₂X₇), may also be formed.⁵

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

This is an ordered defect structure, related to that of fluorite, with ⅛ of the anions missing.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

In practice, pyrochlores are often not stoichiometric and are more correctly represented as A₂B₂X_7+y.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

Current interest in the pyrochlore structure in CZMOs is strong as its formation has been suggested to enhance oxygen storage behaviour.^3,4,6,7

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot2

For CZMOs, they are generally reported to form under severe reducing conditions.⁵

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac4

Here, we report evidence for the formation of a pyrochlore for Ce_0.5Zr_0.5O₂ during low temperature reduction of the sample.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

Ce_0.5Zr_0.5O₂, BET surface area 40 m² g^–1 after calcination at 773 K (5 h), was prepared by a previously reported citrate route.⁸

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

An in-situ cleaning procedure (CP) was applied before each set of experiments.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

Temperature programmed reduction (TPR) profiles (25 mg of sample) were recorded in a system equipped with a thermal conductivity detector: CP-O₂ at 773 K (1 h), then cool, first to 423 K in O₂, then to room temperature (rt) in Ar; TPR-H₂ (10 or 1%) in Ar at rt (30 min), ramp to 1173 or 1273 K, hold 15 min, switch to Ar (15 min).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp3

TPR profiles are designated as severe reduction (SR) procedures; after TPR, cool to 700 K and oxidise (mild oxidation—MO) in O₂ (1 h), or oxidise at 1173 or 1273 K (severe oxidation—SO).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp4

O₂ was pulsed in all cases (100 µl, every 75 s).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp4

Flow and ramp rates: 25 ml min^–1 and 10 K min^–1 respectively.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp5

Powder XRD spectra were collected on a Siemens Kristalloflex Mod.F (Ni-filtered Cu Kα).

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met1

TPR-Raman spectra were obtained on a Renishaw Raman System 1000, with an Ar laser (514 nm, 25 mW) fitted with an in-situ cell that allows control of temperature and gas flow.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met2

Reduction was performed by introducing the reducing gas (1 or 10% H₂ in Ar), heating in a stepwise manner (50 K intervals) and recording the spectra during the isothermal steps (15 min).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp6

Oxidation was performed in the same cell at the same temperatures as for TPR experiments.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp7

It is known that the temperature at which CZMOs release oxygen under reducing conditions may be decreased or increased upon application of SR/MO or SO treatment regimes.^3,4,8,9

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac5

This effect is reported in Fig. 1, which shows a series of TPR profiles obtained using 10 and 1% H₂.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs1

SR and SO were conducted at both 1173 K (A) and 1273 K (B).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp8

Due to the different response to 1 and 10% H₂, the profiles shown have been normalised to the total area detected.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

Profiles A1, A2, B1 and B2 were obtained after: SR–MO–SR–SO.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met3

All show H₂ uptake at relatively high temperature, the maximum of which depends on H₂ concentration.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

The latter is an established TPR phenomenon.¹⁰

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac6

After intermediate MO procedures, further TPR profiles (A1′, A2′, B1′ and B2′) were recorded.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met4

The most striking feature of this data set is that, for SR and SO at 1173 K (A), the procedure followed results in low temperature reduction with 10% H₂ but not with 1% H₂.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs2

It should be noted that the profile always remains on-scale with 1% H₂.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

Thus, reduction is not limited by H₂ concentration.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

For SR and SO at 1273 K (B), low temperature reduction is observed with both H₂ concentrations, but the effect is more pronounced with 10% H₂, in the sense that the relative intensities of the two peaks observed indicate more uptake in the low temperature feature.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res4

Cycling between the related profiles in Fig. 1B may be achieved by further SO and SR–MO treatments.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs3

Thus, aspects of the behaviour reported in Fig. 1 are consistent with previous observations.^8,9

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con1

However, here we show that the effect is also strongly dependent on H₂ concentration and temperature, in addition to the known dependence on the oxidation conditions (SO vs. MO).⁹

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con1

Rietveld analysis of the XRD profile after SO at 1173 and 1273 K initially indicated that the structure may be fitted equally well using cubic (Fm3m) or tetragonal (P4₂/nmc) models.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs4

Careful collection of XRD data revealed the presence of a low intensity characteristic t′ reflection (2θ = 42°).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs5

Similar analysis after subsequent SR (corresponding to 1A2 and 1B1)–MO showed only a small cell volume decrease from the SO-treated samples, suggesting that the procedures do not induce significant changes in the cation sub-lattice, at least on the XRD scale of detection (see ESI).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res5

Fig. 2 shows two series of in-situ TPR-Raman spectra obtained during reduction of Ce_0.5Zr_0.5O₂ using 1% H₂ (A) and 10% H₂ (B).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs6

These Raman spectra offer a representative study of the TPR profiles A2′ and B1′ shown in Fig. 1.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs6

In both cases, there was negligible change in the position and a small decrease in intensity of the peaks between rt and 473 K (first spectra shown).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs6

It is important to underline the possibility of superimposed luminescence signals, derived from rare earth impurities at ppm levels, in these spectra.¹¹

Type: Result | Advantage: None | Novelty: None | ConceptID: Res6

The intensity of such peaks strongly depends on factors such as temperature and environment, so great care must be taken when comparing spectra for structural analysis.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac7

For the sample treated to 1173 K in 1% H₂ (Fig. 2A), the first spectrum shows peaks at 225, 306, 470, and 623 cm^–1 which are consistent with the presence of a t′ phase.¹²

Type: Result | Advantage: None | Novelty: None | ConceptID: Res7

As the reduction temperature increases, the peak intensities decrease, until, at temperatures corresponding to complete reduction of the sample (Fig. 1A2′), a featureless spectrum is observed.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res7

No Raman features could be observed at 1173 K and above due to the blackbody radiation, which overwhelms the Raman signal.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res7

In the case of the sample treated to 1273 K in 10% H₂, the first spectrum shows peaks at 184, 301, 470, 540 and 623 cm^–1, which are also consistent with the presence of a t′ phase.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res8

Between 523 and 573 K, after commencement of sample reduction (Fig. 1B1′), peak intensities decrease strongly.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs7

At 623 K a weak peak appears at 295 cm^–1, which becomes more intense and shifts to lower wavenumber as the temperature increases, while others appear at 380 and 495 cm^–1.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs7

These peaks are consistent with the formation of a pyrochlore phase.³

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con2

It is clear that the temperature at which the pyrochlore structure becomes visible (673 K, Fig. 2) is well above the start of the reduction profile (<500 K, Fig. 1B1′).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs8

The difference in experimental procedure (ramped vs. stepped temperature increase) should mean that, at a given temperature, the sample is more reduced during TPR-Raman than during TPR.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp2

Finally, after a TPR-Raman series corresponding to Fig 1B1, the sample was cooled to 700 K and spectra were measured before and after oxidation.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj2

No evidence of the pyrochlore or related structures was found, indicating that the pyrochlore was not formed at high temperature during the previous TPR.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res9

The metastable nature of CZMOs with intermediate compositions has created difficulty in establishing certainties about the structure of such materials; which has generally complicated structural analysis of TPR behaviour.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac8

Despite this, the observation of promoted TPR profiles has recently been linked to a newly reported κ phase, formed by oxidation (873 K) of Ce_0.5Zr_0.5O₂ with the pyrochlore structure and destroyed by high temperature oxidation (1323 or 1423 K) to yield another new phase (t*).^3,4

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac9

The pyrochlore was initially formed by high temperature reduction of a t′ phase.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac9

Here we present evidence of formation of the pyrochlore phase during low temperature reduction.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj3

Clearly, the preceding treatments have already conferred upon the sample the potential to undergo low temperature reduction, which in turn leads to very low temperature pyrochlore formation.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con3

Neither effect is observed with 1% H₂, SR and SO 1173 K, thereby implying a link between promoted reduction and low temperature pyrochlore formation.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con4

Table 1 reports the degree of reduction for the eight TPR profiles of Fig. 1, as measured by O₂ uptake during MO.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs9

Comparable and high degrees of reduction were achieved in all experiments suggesting that formation of pyrochlore should be equally favoured.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res10

Despite this, pyrochloric structure was observed only in Fig. 2B, corresponding to TPR profile B1′, thereby indicating that extent of reduction is not directly responsible for pyrochlore formation.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con5

Finally, the Raman spectra recorded after the CP, in Fig. 2 and at the start of TPR profile B1 (not shown) may all be assigned to t′.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res11

We find no clear correlation between the appearance of the Raman spectra and reduction profile (or formation of pyrochlore), although it must be reiterated that the possible presence of impurity peaks makes identification of any such correlation difficult.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con6

Instead, we believe that a more complex situation than a simple structure–TPR relationship may be inferred for the TPR behaviour of this sample.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con6

The formation of surface domains which promote reduction of the sample could instead offer an explanation.⁸

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp3

The kinetics of the pyrochlore formation at high temperature has not been carefully studied.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac10

Therefore, as suggested by an anonymous referee, it cannot be excluded that the pyrochlore nucleation process occurs in high temperature conditions (its rate being increased by increasing hydrogen pressure), while the growth process could occur with a significant rate at moderate temperatures.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con7

Once formed superficially at high temperature, a very thin pyrochlore layer (the nuclei) could be transformed into the κ phase under mild conditions, which in turn gives the pyrochlore structure (as in Fig. 2b) at low temperature, first on the surface then inwards of the particle (growth process).

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con7

Such a process would be difficult to observe given the detection limits of the techniques employed here.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac11

In addition, the existence of antiphase domain boundaries may further weaken the XRD peaks due to the κ phase.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac12

On the other hand while the Raman technique proved to be sensitive in detecting the κ phase,¹³ no evidence for its presence was observed here.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con8

Further studies are in progress in order to ascertain these aspects.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac13

Consistent with the suggestion of the importance of surface modifications, studies carried out on the present sample indicated that the scrambling capacity between D₂ and surface hydroxyl groups is affected by the redox treatments.⁸

Type: Result | Advantage: None | Novelty: None | ConceptID: Res12

In summary, formation of a bulk pyrochloric structure, i.e. an ordered system, is not a necessary prerequisite to achieve promoted redox properties in Ce_0.5Zr_0.5O₂.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con9