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Back | Show only these items: | In situ XRD and HRTEM studies on the evolution of the Cu/ZnO methanol synthesis catalyst during its reduction and re-oxidation

1
In situ XRD and HRTEM studies on the evolution of the Cu/ZnO methanol synthesis catalyst during its reduction and re-oxidation

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

During the formation of the CuZnO solid solutions, the foreign anions in anion-modified (a.m.-) oxides give rise to: (i) extended stacking faults of (002) ZnO lattice plane, which are occupied by copper ions in the form of small clusters; and (ii) vacant inner holes of the a.m.-ZnO crystal.

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

The main part of copper ions in the clusters is reduced to Cu⁰ with hydrogen at 473 K. According to HRTEM studies, the reduction of Cu_0.08Zn_0.92O results in the formation of copper metal species of two types: (i) particles of 3–10 nm in size on the a.m.-ZnO surface; and (ii) small (no more than 3 nm in size) atomic copper metal clusters in defect voids of the a.m.-ZnO structure.

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

The copper metal clusters are coherent inclusions in the bulk of the ZnO, and the large copper metal particles are epitaxially bonded to the surface of the ZnO matrix.

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

Copper metal particles on the surface of a.m.-ZnO are reoxidized to Cu⁺² at 523 K in the helium flow containing 0.05 vol% oxygen, and they come back to the extended stacking faults.

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

The copper metal clusters in the holes of a.m.-ZnO are inaccessible to the oxygen and are not reoxidized.

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

Introduction

Methanol is used as the principal feedstock for production of many organic compounds, among which are formaldehyde, acetic acid, methyl-methacrilate MMA, dimethyl-terephthalate DMT, methyl-tert-butylether MTBE and chloromethanes.

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

Due to its high energetic density and no problem in its storage and handling, methanol is also considered an important liquid power-supplier in fuel cells.

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

Copper–zinc and copper–zinc–aluminium oxide catalysts, which are very selective and active enough at low-pressure conditions, are mainly used for the methanol synthesis.

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

A variety of techniques were applied by numerous researchers^1–10 for studying the active state of the copper–zinc catalysts but the nature of the active sites is still disputable in literature.

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

The present research is focused on the structure of the active state of the methanol synthesis catalysts by means of studying the model copper–zinc catalysts, including the as-prepared sample, as well as those after hydrogen reduction and mild re-oxidation.

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

The chosen samples are the solid solutions of copper ions in anion-modified zinc oxide Cu_0.08Zn_0.92O and solid solutions of copper and aluminium ions in anion-modified zinc oxide Cu_0.2Al_0.1Zn_0.7O.

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

They are known to be the precursors for active catalysts for synthesis of methanol.^11–14

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

It has been shown before that these solid solutions of copper (and aluminium) cations in the zinc oxide structure can only be obtained via low-temperature (not higher than 773–823 K) decomposition of Cu(Al)Zn hydroxycarbonates of hydrozincite or aurichalcite clay structure^12,15 to produce OH- and CO₃-containing zinc oxide (anion-modified, a.m.-ZnO).

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

The ad-anions provide stabilization of copper cations in the zinc oxide structure, i.e. dissolution of the copper cations in the a.m.-ZnO.¹⁶

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

Al³⁺ cations dissolved in zinc oxide cause an increase of the clusters in number thus preventing formation of the copper oxide phase as the copper concentration increases.¹³

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

ES studies of the solid solutions at 5000–50000 cm⁻¹ showed copper cations forming oxygen-containing small clusters Cu²⁺–O–Cu²⁺ with the strongly distorted, almost flat-square, oxygen coordination of Cu²⁺.¹⁷

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

The hydrogen treatment at temperatures below 573 K results in the reduction of the majority of the copper ions from the a.m.-ZnO followed by the copper diffusion to the a.m.-ZnO surface.

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

XRD data allow the hypothesis that metallic copper forms 3–4 nm crystallites which are epitaxially bonded to the ZnO.¹⁸

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

The epitaxy of Cu^_⁰ crystallites over ZnO support was directly shown by the in situ HREM.⁹

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

The data of ^{refs. 18 and 19} also give the grounds for the supposition that the reduction of copper cations from a.m.-ZnO solid solution is reversible: the epitaxial Cu⁰ particles are easily re-oxidizable and can return to the a.m.-ZnO structure under the action of low partial pressures of oxygen.

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

The reversible reduction–oxidation of copper–zinc oxide was also observed recently in .^{ref. 20}

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

Meanwhile, the air treatment transforms the epitaxial Cu⁰ particles to CuO phase.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac14

The goal of the present study is to give a better understanding of the structural evolution of the methanol synthesis catalysts during their reduction and re-oxidation by means of in situ XRD and HRTEM.

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa1

The nature of the copper species migration is in the focus of the investigation.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj4

The catalytic activity of both Cu/ZnO and CuAl/ZnO catalysts is proportional to the copper concentration in the a.m.-ZnO.¹¹

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac15

This allows to assume a similar nature of the Cu/ZnO and CuAl/ZnO catalysts action in the methanol synthesis, while introduction of Al³⁺ contributes to increasing the solubility of Cu²⁺ in a.m.-ZnO and improves the thermal stability of the catalyst.

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

Since the electron microscopic studies can give more clear results for the double oxide system (i.e., without aluminium), these studies were first performed for the Cu_0.08Zn_0.92O sample.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj5

Meanwhile, we considered it important to present in this paper the in situ XRD data for the reduction–oxidation evolution of the Cu_0.2Al_0.1Zn_0.7O sample as well.

Type: Object | Advantage: None | Novelty: None | ConceptID: Obj6

The study uses exactly the same samples of Cu_0.08Zn_0.92O and Cu_0.2Al_0.1Zn_0.7O, which were studied earlier in ^{refs. 11–13 and 17–19}.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac16

Experimental procedures

Samples of a.m.-CuO, a.m.-ZnO, a.m.-Cu_0.08Zn_0.92O and a.m.-Cu_0.2Al_0.1Zn_0.7O were prepared by calcination of the corresponding hydroxycarbonates in flowing air at 623 K for 4 h.

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

Hydroxycarbonate precursors of Cu/ZnO and CuAl/ZnO catalysts were synthesized via the continuous co-precipitation method.

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

Solution of sodium carbonate and mixture of appropriate quantities of 0.5 M solutions of copper, zinc and aluminum nitrates were added dropwise at 343–348 K and pH = 6.8–7.0 to an appropriate buffer solution.

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

Dwell time was 30 min.

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

Copper nitrate 3-hydrate (Saki Chemical Plant, Russia, >99% pure), zinc nitrate 6-hydrate (Donetsk Chemical Plant, Ukraine, >98% pure) and aluminum nitrate 9-hydrate (Donetsk Chemical Plant, Ukraine, >99% pure) were used.

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

The precipitates were thoroughly washed to remove Na⁺ and NO₃⁻ ions, filtered out, and dried under IR-lamp overnight.

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

Reductive treatments were performed in flowing hydrogen at 473 K for 1 h, heating rate −2 K min⁻¹.

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

The re-oxidations were performed in the flowing mixture of helium and oxygen (0.05% O₂) at 2 K min⁻¹, 523 K, until the termination of oxygen absorption.

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

A Siemens D-500 and a Bruker-D8 diffractometers (Germany) with Cu Kα radiation were used for in situ X-ray diffraction (XRD) studies.

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

A high-temperature reactor chamber²¹ was linked to the gas feeding system.

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

At first, in this chamber the full diffraction patterns were acquired for the as-prepared samples.

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

Then, the samples were reduced in flowing hydrogen at 2K min⁻¹ 473 K until the diffraction patterns (monitored by the intensity of Cu⁰ (111) line) stopped changing, and the full diffraction patterns were acquired.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met5

Then the reduced samples were reoxidized in the flowing mixture of helium and oxygen (0.05% O₂) at 523 K until the diffraction patterns stopped changing.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met6

Samples treated under present conditions were studied using high resolution transmission electron microscopy (HRTEM).

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met7

Electron micrographs were obtained with JEM-2010 instrument with lattice resolution 1.4 Å and accelerating voltage 200 kV.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met8

Periodic images of lattices structures were analyzed using digital Fourier transformation.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met9

Local elemental analysis was performed with EDX method on Energy-dispersive X-ray Phoenix spectrometer equipped with Si (Li) detector with energy resolution not worse than 130 eV.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met10

Samples were fixed on ≪holey≫ carbon films supported on copper or molybdenum grids and investigated with the electron microscope.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met11

In situ thermogravimetric (STA) data were obtained by means of the Netzsch STA 409 thermobalance.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met12

The sensitivity of the weight measurements was about 0.01 mg (ca. 0.02% of the sample weight).

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met13

The heating rate was 5 K min⁻¹).

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met14

Results and discussion

Studies of the as-prepared samples

XRD and STA studies

Fig. 1 shows the diffraction patterns of as-prepared samples: a.m.-CuO (curve 1), a.m.-ZnO (curve 2), a.m.-Cu_0.08Zn_0.92O (curve 3).

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

The diffraction pattern of as-prepared a.m.-Cu_0.2Al_0.1Zn_0.7O is presented by curve 1 in Fig. 2.

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

The lines of zinc oxide are only seen in the diffraction patterns acquired both for a.m.-Cu_0.08Zn_0.92O and a.m.-Cu_0.2Al_0.1Zn_0.7O samples, as well as for a.m.-ZnO.

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

So, only one phase with ZnO structure is observed in a.m.-Cu_0.08Zn_0.92O and a.m.-Cu_0.2Al_0.1Zn_0.7O.

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

No copper oxide phase is seen.

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

The STA method provides additional evidence for the absence of a copper oxide phase.

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

Fig. 3 shows a TPR profile recorded for the a.m.-Cu_0.08Zn_0.92O sample in flowing hydrogen.

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

Two effects are observed at 435 and 680 K. According to XRD data, the first effect is caused by the reduction of copper and formation of metallic particles.

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

The weight loss of 1.44% weight corresponds to the reduction of ca. 92% of the Cu²⁺.

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

The second weight loss at ca. 680 K is related to the removal of the ad-anions and the recrystallization of the metallic copper particles as well as of the a.m.-ZnO phase giving the regular zinc oxide.

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

The peculiarities of the transformation of the anion-modified oxides into regular oxides were discussed earlier in ^{refs. 22 and 23}.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac17

The existence of the weight loss at 680 K in hydrogen flow proves that the sample Cu_0.08Zn_0.92O after calcination at 625 K contains the ad-anions, i.e. represents itself as the anion-modified mixed oxide.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

The weight loss at above 750 K is accounted for by the reduction of ZnO and evaporation of metallic zinc.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res14

Electron microscopic studies

HRTEM micrograph of anion-modified zinc oxide, a.m.-ZnO, is shown in Fig. 4 (left).

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

There are seen (002) planes of ZnO crystal lattice with a lattice spacing of 0.26 nm (the (002) planes are oriented normally to the plane of the micrograph).

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

Microvoids are seen in the crystal bulk as rounded 2–3 nm spots of slightly light contrast.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res15

There is no visible anomalous contrast or distortion of the planes due to imperfection of the crystal lattice.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res16

Fig. 4 (right) presents a HRTEM image of a.m.-Cu_0.08Zn_0.92O as-prepared.

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

As was said above, this is the solid solution of copper ions in a.m.-ZnO.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res17

Here, two types of the structural defects are observed in the micrograph.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res18

The interplanar spacing ZnO (002) is unchanged (0.26 nm), i.e. the planes are also perpendicular to the plane of image.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res19

However, some planes are seen broken and shifted as a result of (002) plane shear dislocations.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res20

These shearing dislocations cause the formation of stacking faults of (002) lattice planes to substitute for the atomic plane alternation characteristic of the hexagonal lattice.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res21

The stacking faults are visible as dark bands along planes (002).¹²

Type: Result | Advantage: None | Novelty: None | ConceptID: Res22

The stacking faults are linked with the large pores of 5 to 10 nm in size, exposed to the external crystal surface.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res23

The defects of the second type are microvoids that are seen in the crystal bulk as light-contrasted 2–3 nm rounded spots.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res24

These defects are similar to those observed in a.m.-ZnO.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res25

From the electron micrography data, when considered along with the data on formation of the sample under consideration, it can be proposed that the observed elongated defects such as stacking faults are the residence regions of impurity anions and copper cations bound to them.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res26

The microvoids may form in the crystal bulk as a result of the vacancy sink in the anion lattice during formation of the a.m.-ZnO crystal.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res27

The microvoids don’t seem to include copper ions.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res28

So, the performed study of Cu/ZnO catalysts has confirmed the formation of copper and copper and aluminium ion solutions in a.m.-ZnO via the low-temperature decomposition of the appropriate hydroxocarbonate.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res29

A.m.-Cu_0.08Zn_0.92O and a.m.-Cu_0.2Al_0.1Zn_0.7O samples represent themselves as solid solutions on the basis of a.m.-ZnO and don’t contain the CuO phase.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res30

The HRTEM data allow discussing the structural peculiarities of a.m.-ZnO based solid solutions and the regularities of copper ions allocation.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res31

It is worth emphasizing once more that there is no visible anomalous contrast or distortion of the planes due to imperfection of the crystal lattice in a.m.-ZnO as-prepared, but microvoids are only seen in the crystal bulk of ZnO as the rounded 2–3 nm spots.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res29

Meanwhile, in a.m.-Cu_0.08Zn_0.92O as-prepared sample shearing dislocations cause the formation of stacking faults of (002) lattice planes in addition to the microvoids, similar to those observed in a.m.-ZnO.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res32

Supposedly, these are the observed stretched defects such as stacking faults, where the ad-anions and copper cations are allocated.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res33

Studies of patterns after reduction by H₂

XRD studies

Diffraction patterns of a.m.-Cu_0.08Zn_0.92O and a.m.-Cu_0.2Al_0.1Zn_0.7O after reduction in flowing hydrogen at 473 K are shown in Figs. 1 (curve 4) and 2 (curve 2), accordingly.

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

The lines assigned to copper metal appear.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res34

The coherent scattering domains allow the estimation of the Cu⁰ crystallite size as 3.5–5.0 nm and 3–4 nm, accordingly.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res35

The observed Cu⁰ XRD band intensities ratio shows the plausible imperfection of copper metal particles.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res36

Electron microscopic studies

Fig. 5 shows the micrographs of a.m.-Cu_0.08Zn_0.92O reduced with hydrogen.

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

The general view of the picture is changed: (i) stacking faults have less contrast; and (ii) two kinds of Cu⁰ particles are observed.

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

Some of Cu⁰ particles are 3 to 10 nm in size on the surface of zinc oxide and there is also the fraction of small clusters almost equal in size (2–3 nm) which are seen as dark spots in the microvoids of zinc oxide.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res37

The small clusters are shown at a higher magnification in Figs. 5 (right) and 6 (left).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs10

The dark spots in the HRTEM micrograph are seen to be made up of Moiré fringes parallel to ZnO (002) lattice plane with periodicity of about 1 nm.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res38

They result from superposition of ZnO (002) and cluster Cu⁰ (111) planes (d₀₀₂ ZnO = 0.26 nm, d₁₁₁Cu = 0.21 nm).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res39

Hexagonal packing is characteristic of these planes in both crystal lattices.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res40

The fact that Moiré fringes of the fine Cu⁰ particles are parallel to ZnO (002) lattice plane indicates the uniform orientation and the coherent lattices of Cu⁰ intrusions and the ZnO crystal matrix.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res41

100

Two types of reflections are seen in the Fourier pattern (Fig. 6, right) of the coherent inclusion: one related to ZnO (the reflections are shown by downward arrows) and another – to Cu⁰ (upward arrows).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res42

101

Reflections of Cu⁰ 111 and 100 are broadened that indicates the presence of stacking faults of (111) metal planes.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res43

102

This structural feature of Cu⁰ clusters may happen because the FCC (face centred cubic) structure of Cu is incorporated into HP (hexagonal packing) structure of ZnO.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res44

103

So, it follows from the obtained XRD results that the reduction of both a.m.-Cu_0.08Zn_0.92O and a.m.-Cu_0.2Al_0.1Zn_0.7O leads to the copper metal formation in the form of imperfect particles of small size.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res45

104

It is in good agreement with our earlier data on Cu/ZnO catalysts.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res46

105

HRTEM studies make it possible to reveal that the reduction of a.m.-Cu_0.08Zn_0.92O results in the formation of copper metal species of two types: (i) particles of 3–10 nm in size on the a.m.-ZnO surface; and (ii) small (no more than 3 nm in size) metal copper clusters in microvoids of the a.m.-ZnO structure.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res47

106

The copper clusters are coherent inclusions in the bulk of zinc oxide, and the large particles are epitaxially bonded to the surface of the ZnO matrix in accordance with earlier data.¹⁸

Type: Result | Advantage: None | Novelty: None | ConceptID: Res48

107

It should be noted that if such a microvoid is situated in the subsurface layer of ZnO, it seems to be observable by LEIS as a metallic copper particle covered with ZnO.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res49

108

Such a state was observed in ^{ref. 7}.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac18

Studies of samples oxidized in the flow of oxygen-containing (0.05 vol% O₂) helium after reduction with hydrogen

XRD studies

109

Diffraction patterns of a.m.-Cu_0.08Zn_0.92O and a.m.-Cu_0.2Al_0.1Zn_0.7O samples after reduction with hydrogen and oxidation by the oxygen–helium mixture are shown in Figs. 1 and 2, respectively.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs11

110

The changes observed in the diffraction patterns of reduced samples in the course of oxidation (Fig. 1, curves 5–7 and Fig. 2, curves 3–4) are the same: the lines with d = 0.208 nm decrease in intensity to disappear within ca. 2 h.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs12

111

Some difference in the behavior of the samples is that the line with d = 0.208 nm is shifted to d = 0.212 nm as its intensity decreases in the diffraction pattern of the a.m.-Cu_0.2Al_0.1Zn_0.7O sample.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res50

112

The line with d = 0.212 nm is the most intense in the pattern of Cu₂O.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res51

113

After re-reduction with hydrogen of the samples at 500 K, the diffraction pattern (Fig. 2, curve 5) is identical to those of the primarily reduced samples (Fig. 1, curve 4 and Fig. 2, curve 2).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs13

114

When the samples are re-oxidized again (re-re-oxidation) in the flowing oxygen-containing helium, the characteristic lines of copper metal disappear again (Fig. 2, curves 6–8).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs14

115

Thus, Figs. 1 and 2 illustrate the experimental observations of the reversible changes in copper state in the samples under study.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs15

116

Copper ions in the structure of a.m.-ZnO are reduced with hydrogen to form copper metal, then Cu⁰ atoms are oxidized in the flowing oxygen-containing helium to form Cu²⁺ ions, which return back to the zinc oxide structure.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res52

117

The obtained results on the evolution of the diffraction patterns during re-oxidation of the reduced a.m.-Cu_0.08Zn_0.92O and a.m.-Cu_0.2Al_0/1Zn_0.7O samples are the direct proof of the reversible red-ox copper behaviour.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res53

118

This phenomenon for Cu/ZnO sample has been shown by us before^18,19 and recently reported elsewhere.²⁰

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac19

119

For a.m.-Cu-Al-ZnO, it is for the first time shown in the present work.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res54

Electron microscopic studies

120

Fig. 7 shows a HRTEM micrographs of the a.m.-Cu_0.08Zn_0.92O sample oxidized in the flow of oxygen containing (0.05 vol%) helium after the reduction with hydrogen.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs16

121

In the micrographs, some elongated defects of stacking faults type are seen in the a.m.-ZnO structure, which are similar to the defects in the pre-reduced sample (see Fig. 4, right) and 2–3 nm clusters of copper metal in the micro-voids of the a.m.-ZnO crystal (the second type of copper metal in the reduced samples).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res55

122

The surface copper metal particles (first type of copper metal in the reduced samples) are not observed.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res56

123

It is important that many defective regions, containing stacking faults of ZnO (002) planes, are present.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res57

124

They are located in regions near the copper clusters (Fig. 7).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res58

125

Thus, HRTEM studies make it possible to reveal that oxidation of surface copper metal particles by oxygen (at its low partial pressure) brings major copper back to the bulk of a.m.-ZnO crystal.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res59

126

The copper cations are localized at the stacking faults in the a.m.-ZnO crystal.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res60

127

Copper metal particles of 2–3 nm in size in the microvoids in the bulk of the a.m.-ZnO structure are inaccessible to the oxygen.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res61

128

The portion of preserved Cu⁰ after the treatment in O₂-containing mixture is not so high, since it is not detected by XRD.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res62

129

Hence, HREM data also argue for the reversible reduction of copper–zinc samples, i.e. the main part of copper particles formed in the sample upon treatment with hydrogen can be re-oxidized to return to the structure of defective zinc oxide.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res63

130

The quantitative evaluation of the amount of Cu⁰ in the re-oxidized samples needs the use of the other investigation techniques, e.g. the measurements of the temperature dependence of the magnetic susceptibility.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res64

131

We expect to present the relevant data on the magnetic susceptibility of the CuZn and CuAlZn samples in one of our next publications.

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa2

Conclusions

132

The solid solutions of copper and aluminum ions in anion-modified ZnO are formed in the course of low temperature decomposition of appropriate hydroxocarbonates.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res65

133

Copper metal as a defect particles of small size is formed as a result of the reduction with hydrogen.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res66

134

Copper cations reduction is reversible: the return of copper back to the a.m.-ZnO structure occurs during re-oxidation with oxygen at low partial pressures.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res67

135

The HRTEM studies allow to reveal the sites of copper cations placed in the solid solution.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res68

136

Important details of the reduction and reoxidation processes can be understood, among which are the following:

Type: Result | Advantage: None | Novelty: None | ConceptID: Res69

137

(i).