1
X-ray absorption spectroscopy under reaction conditions: suitability of different reaction cells for combined catalyst characterization and time-resolved studies

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

2
Structure–activity correlations of solid catalysts and time-resolved studies generally require that the structure within a solid catalyst is probed simultaneously and at the same location where the catalysis and the structural changes occur.

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

3
These requirements lead to a compromise between the spectroscopic arrangement and the optimum design for an in situ reactor cell.

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

4
Opportunities and limitations of in situ and time-resolved X-ray absorption spectroscopy (XAS) combined with gas analysis are critically analysed with the help of two different cell designs, an in situ EXAFS cell designed for solids in the form of pressed wafers and a capillary cell where the catalyst is packed similarly to a plug flow reactor.

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

5
On the basis of three examples, the reduction of CuO/ZnO, the reduction of PdO/ZrO2 and methane oxidation over PdOx/ZrO2, criteria are developed which allow to judge the appropriate cell design in solid–fluid reactions.

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

6
The prerequisites for the design of an in situ cell including the catalyst shape strongly depend on the time resolution required.

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

7
Important issues embrace the type of reaction to be investigated (slow vs. fast), the reaction medium, and the porosity of the catalyst material.

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

8
Criteria for assessing the role of pore and film diffusion in in situ studies are of paramount importance for a proper experimental design.

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

Introduction

9
Time-resolved spectroscopic studies and the combined investigation of the structure and activity are important in the field of heterogeneous catalysis.1–11

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

10
While time-resolved studies can elucidate information on intermediates on the catalyst surface, the combination of structure and activity aims at possible relationships between the active centres and the reactivity of the catalyst, being a first step to rational catalyst design.

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

11
Both goals require in situ investigations and the design of appropriate in situ cells for simultaneous structural and catalytic studies.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot3

12
Ex situ studies have been performed to a great extent but it is generally accepted nowadays2,3,10 that such studies provide structure–activity relationships only in selected cases, i.e. if it has been ruled out in advance that the structure does not change if the catalyst is exposed to ambient atmosphere or reaction conditions.

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

13
Even cooling from the reaction temperature to room temperature may change the catalyst structure (cf. example in ref. 12).

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

14
In particular, techniques based on X-rays (X-ray diffraction, scattering techniques, X-ray absorption) have been used,4,8,9,13–16 since in situ cells can be constructed that mimic conditions close to the reaction conditions, even in liquid phase or at high pressure.17–19

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot4

15
This can be traced back to the fact that a number of cell window materials exist (Kapton, Al foil, thin diamond, Be and quartz windows), which have sufficient low absorbance of hard X-rays: thus the absorption of the probing beam by reaction gasses or window materials can be held marginal.

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

16
For the characterization of heterogeneous catalysts, the EXAFS technique is advantageous,4,20,21 since it can detect element-specifically both amorphous and crystalline structures.

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

17
In order to interrelate the structure and activity of the catalysts, the flow properties in the EXAFS cell have to be similar to those usually met in the reactor and for time resolved studies, diffusion (film and pore diffusion) should be faster than the corresponding chemical reaction.

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

18
Different designs have been reported, such as a disk of pelletized catalyst powder (also denoted as pressed wafer) placed in an environmental cell (cf. refs. 22 and 23).

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

19
Another approach is the use of a powdered catalyst in an in situ cell as, e.g., proposed by Bazin et al24. and Clausen and Topsøe.25

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

20
Moreover, Clausen et al26. and Sankar et al27. used a capillary cell for combined XRD–EXAFS studies.

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

21
This cell design has recently also been used to obtain both structure and activity data on heterogeneous catalysts.28,29

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

22
However, none of the studies compared the advantages and limitations of the different cell designs to provide truly in situ EXAFS studies, both under dynamic and static conditions.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot5

23
Following the different concepts, we have constructed two kinds of in situ setups (using either an environmental chamber for a plate-like pellet/pressed wafer or a capillary cell for spectroscopic studies), which are shortly described in this paper.

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

24
Their characteristics are illustrated using exemplary case studies.

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

25
In the first case, the reduction of CuO/ZnO is investigated as an example for time-resolved studies since Cu+ species occur as an intermediate during the reduction of CuO/ZnO which is important for the activation of such catalysts.30–32

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

26
In addition, the reduction behaviour of PdO/ZrO2 at room temperature was investigated in dependence of the material used for pressing a mechanically stable catalyst pellet.

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

27
In the subsequent chapter, examples based on the PdOx/ZrO2 catalyst are presented during methane combustion at 500 °C.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj7

28
Both structure and catalytic activity were determined at the same time under reaction conditions.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj8

29
For comparison, reduction of PdO/ZrO2 by methane and re-oxidation by oxygen are shown.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj9

30
In situ studies of such catalysts are important, since several mechanisms for the oxidation of methane have been proposed33–38 and the structure of Pd under reaction conditions is still a matter of discussion.35

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot6

31
Based on these examples, the setup of in situ cells in gas–solid, liquid–solid and dense fluids (e.g., supercritical fluids)–solid reactions are discussed with respect to catalyst shape, influence of external film and pore diffusion, catalyst efficiency and time-resolved studies.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj10

Experimental

Design of in situ EXAFS cells

32
Different in situ cell types that have been reported in literature are schematically gathered in Figs. 1a–c.

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

33
The first category of cells is based on a pressed wafer placed in an environmental chamber, which has been widely used for in situ studies.

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

34
The gas bypasses the pressed wafer either from one or from two sides.

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

35
An alternative design is shown in Fig. 1b.

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

36
In this case, the gas stream passes through the catalyst pellet instead of just by-passing it.

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

37
Another possibility is the use of a design that is similar to a plug flow reactor.

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

38
Since the wall thickness of normal glass reactors is too high, a glass tube with thin windows (0.5–2 mm diameter, 0.01 mm thick windows)—typically a glass capillary—has been proposed for such experiments.26,27

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

39
Alternative proposals using powdered catalysts have been made in literature as well.24,25,39

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

40
Based on the designs in literature, we have constructed two in situ cells that correspond to Fig. 1a (“pellet cell”) and Fig. 1c (“capillary cell”).

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

41
While the first cell is optimal with respect to spectroscopy (homogeneous sample), the second cell should be ideal for on-line catalytic studies (optimal thickness d/length l, no dead volume).

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

42
The in situ cell applicable for the investigation of self-supporting pellets has been designed according to Fig. 1a, embedding the cell inside an oven with X-ray transparent windows.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj11

43
A catalyst pellet (diameter of 13 mm) was pressed (ca.

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

44
1 ton) and then loaded into the stainless steel cell.

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

45
The thickness of the pellet was ca.

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

46
2 mm, and the total X-ray path through the cell ca.

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

47
6 mm.

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

48
The gas could pass along the pellet from both sides.

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

49
From these dimensions, a dead volume of about 0.5 cm3 resulted.

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

50
Three mass flow controllers (Brooks, 0–50 ml min−1) were used to control the feed of different pre-mixed gasses through the in situ cell.

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

51
The outlet of the reactor cell was connected to a mass spectrometer (Balzers).

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

52
The whole in situ setup was mounted on an x,z,θ-table (x = translation, z = height, θ = angle to X-ray axis), directly fixed on an X95-profile (Newport) usually applied at synchrotron beamlines.

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

53
The setup according to Fig. 1c was constructed on the basis of a previous successful design26 and some more details are given in Fig. 2.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp9

54
Focus was put on an in situ cell that allows both XAS studies and the determination of the catalytic performance of the catalysts.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj12

55
The catalyst was loaded between two glass wool plugs in a glass reactor (quartz capillary, Markröhrchen, Hilgenberg GmbH) of 1–1.5 mm diameter.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp10

56
The capillary is heated by a hot gas stream (here air, which can be substituted also by nitrogen) with a controlled air flow adjusted by a mass flow controller (Brooks, 0–2000 ml min−1).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp11

57
The temperature of the heater (Tcontrol) was measured by a thermocouple at the heating element (ring cartridge, SUVAG, Zürich, 300 W/230 V) in order to control a constant temperature ramp.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp12

58
The actual sample temperature (Tsample) was monitored just below the sample.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp13

59
The heater was embedded in glass wool and the outer side of the oven could be cooled by water or air in order to prevent the surroundings from warming up (i.e. important for ionization chambers).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp14

60
The capillary was enclosed in a Kapton cap just above the heater where the hot stream of air passed out of the oven.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp15

61
Similar to the pellet cell, the outlet of the capillary was connected using gas-tight Swagelok fittings to a mass spectrometer (Balzers).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp16

62
The whole assembly (in situ cell, heater) was mounted on a small x,z-table (x = translation, z = height) to align the sample in the X-ray beam.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp17

Materials and treatment of the samples

Reduction of 30% CuO/ZnO in 5% H2/He

63
The 30% CuO/ZnO catalyst was prepared by controlled co-precipitation of Cu(NO3)2 and Zn(NO3)2 (precipitation pH = 9.2) followed by filtration through a 0.45 μm cellulose acetate membrane filter.40

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp18

64
After drying at 90 °C and grinding, 1.0 g of the precursor powder was calcined in an oven with a temperature ramp of 3 °C min−1 and a final temperature of 250 °C for 10 h.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp19

65
The sample was diluted 1∶3 with boron nitride (Aldrich, hexagonal, 99.9%) during dry grinding and pressed either to a pellet of 13 mm diameter or loaded in a capillary cell (see above, 80–120 μm sieved fraction).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp20

66
For the reduction, 5% H2/He was used.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp21

Reduction of PdO/ZrO2 in 5% H2/He and re-oxidation in 2.5% O2/He

67
The PdO/ZrO2 material was prepared from amorphous Pd33Zr67 by oxidation in air at 400 °C.41

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp22

68
The sample was diluted with boron nitride (2∶7, Aldrich, hexagonal, 99.9%) or Al2O3 (2∶7, Engelhard) and pressed to a pellet of 13 mm diameter (after dry grinding).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp23

69
For experiments in the capillary cell, a sieved fraction of about 80–120 μm particles (diluted ca.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp24

70
1∶3 with Al2O3) was filled into a 1.5 mm diameter capillary.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp24

71
The reduction was performed at room temperature in 5% H2/He, monitoring the outlet gas by a mass spectrometer.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp25

72
Re-oxidation of the samples was performed in 2.5% O2/He to 500 °C with a ramp rate of 5 °C min−1.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp26

Methane combustion over PdOx/ZrO2

73
In the first experiment the PdO/ZrO2 material—prepared from amorphous Pd33Zr67 by oxidation in air at 400 °C41—was heated in the reaction mixture 1% CH4/4% O2/He to 500 °C with 5 °C min−1.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp27

74
Both the pellet and plug flow reactor cell were utilized using samples prepared in a similar way as for the reduction in 5% H2/He.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp28

75
The space velocity was adjusted to the conversion degree of methane over the catalyst in order to study the in situ activation of the catalyst.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp29

76
Also, the reaction temperature was varied between 500 and 550 °C.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp30

77
Afterwards, the catalyst was reduced by 5% H2/He at room temperature.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp31

78
In the next step, the reduced Pd/ZrO2 sample was studied during temperature programmed reaction in 1% CH4/4% O2/He.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp32

XAS experiments

79
The reduction of CuO/ZnO was performed at the Swiss Norwegian Beamline (SNBL) at the European Synchrotron Facility (ESRF) in Grenoble, France.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp33

80
The electron energy was 6.0 GeV and the ring current between 150 and 200 mA.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp34

81
A Si(111) crystal was used as channel-cut monochromator.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp35

82
Harmonic rejection was performed by a double-bounce gold coated mirror system.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp36

83
EXAFS data were collected in the transmission mode at room and elevated temperatures.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp37

84
Three ionization chambers were used for detecting the intensity of the X-rays—of the incoming I0 (filled with 3% Ar and 97 % N2, 17 cm length), of the transmitted It (filled with 50% Ar and 50% N2, 31 cm length) and of the through the reference transmitted Iref (filled with 50% Ar and 50% N2, 31 cm length) X-rays.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp38

85
The spectra were taken in a step scanning mode around the Cu K-edge (for fast spectra 8.96 – 9.08 keV, 3 min per scan).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp39

86
Characterization of the beam behind the sample in the capillary and the pellet cell was performed by an X-ray eye of Micro Photonics (Allentown, PA, USA).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp40

87
Using the X-ray eye, the homogeneity of the samples was checked and the alignment of the in situ cell was optimised.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp41

88
The experiments with PdO/ZrO2 were performed at SNBL, at ESRF, and at beamline X1 at HASYLAB (DESY, Hamburg).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp42

89
In particular, for faster acquisition time (20–80 s) the QEXAFS mode at the Hamburger Synchrotronlabor (HASYLAB) at Deutsches Elektronen Synchrotron (DESY in Hamburg, Germany) at beamline X1 was applied using a Si(311) double crystal monochromator.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp43

90
The typical beam current was 80–120 mA (operating positron energy at 4.5 GeV).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp44

91
Higher harmonics were effectively removed by detuning of the crystals to 70% of the maximum intensity.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp45

92
Three ionisation chambers filled with Ar were used to record the intensity of the incident and the transmitted X-rays (in situ reactor cell located between the first and second ionisation chamber, a Pd-reference foil for energy calibration between the second and third ionization chamber).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp46

93
Under stationary conditions EXAFS spectra were taken around the Pd K-edge in the step scanning mode between 24 000 and 25 800 eV.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp47

94
Faster scans around the Pd K-edge were recorded using the normal step scanning EXAFS mode (ca.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp48

95
5 min per scan) or the continuous EXAFS scanning mode between 24 300 and 24 800 eV (80 s per scan, QEXAFS, cf42.).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp48

96
During reduction with hydrogen faster scans were recorded (24 310–24 490 eV, 20 s per scan).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp49

97
The raw data were energy-calibrated (Pd K-edge energy of the Pd-foil: 24 350 eV, first inflection point), smoothed, background corrected, and normalized using the WINXAS 2.1 software.43

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp50

98
In order to quantify the relative ratio of Cu2+/Cu+/Cu0 and Pd2+/Pd0, respectively, linear combination analysis of the XANES region (8.97 to 9.03 keV for the Cu K-edge and 24.33–24.45 keV for the Pd K-edge) around the edge was performed, using the spectra of the completely reduced (after hydrogen treatment) and the oxidized (as prepared) materials as model spectra.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp51

99
Additionally, the XANES spectrum of Cu2O was included in the fit.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp52

Results and discussion

Characterization of the X-ray beam behind the catalyst sample in the in situ cells

100
Fig. 3 gives a comparison of the X-ray beam behind the plug-flow reactor and the pellet reactor in situ cell.

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

101
Figs. 3a and 3d represent the optimal position of the in situ cells.

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

102
While the beam is homogeneous in the image of Fig. 3d (pellet cell), several shadows of the particles can be seen in the image of Fig. 3a (capillary cell).

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

103
Hence, some inhomogeneity of the sieved catalyst in the capillary cell can be seen with the X-ray eye.

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

104
Images 3b and 3c (capillary cell) show what happens if the capillary is not properly aligned: the intensity is higher at the outer region (Fig. 3b) or at the bottom of the capillary (Fig. 3c).

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

105
However, no pinhole is present as, e.g., shown in image 3e (pellet cell) where the beam bypasses the catalyst pellet.

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

106
The results show that inhomogeneities in the sample may lead to inhomogeneous penetration of the X-ray through the sample.

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

107
These inhomogeneities can be minimized using a small sieved fraction.

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

108
They lead in principle to a lower μd, and hence a lower sensitivity of the method.

Type: Method | Advantage: No | Novelty: New | ConceptID: Met6

109
This effect is not so relevant, unless the beam moves during the experiment (or the beam is inhomogeneous in intensity and the particles move).

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

110
Also, the use of a position sensitive detector, as applied, e.g., in the DEXAFS technique,44,45 may be difficult if the sieved fraction is not small enough.

Type: Method | Advantage: No | Novelty: New | ConceptID: Met6

111
For this technique, it is important to use a sample as homogeneous as possible, which is best achieved with finely ground powder pressed as a wafer, as is visible in Fig. 3d.

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

112
QEXAFS is not so sensitive to inhomogeneities in the sample since it probes the whole sample at all energies as an average32,45 and was thus exclusively used in these studies.

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

Reduction of CuO/ZnO in 5% H2/He

113
The time-resolved reduction of CuO/ZnO in the form of powder or pressed wafers has been subject to a number of studies.30–32

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

114
Here, we compare the reduction of a 30% CuO/ZnO sample (diluted in boron nitride) when using it as pressed wafer (pellet cell, Fig. 1a) or as sieved fraction (capillary cell, Fig. 1c).

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj13

115
Fig. 4 shows the results.

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

116
On the left, normalised spectra at the Cu K-edge are shown for illustration (note that there are less “intermediate” spectra in Fig. 4a than in 4b).

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

117
The right part of the figure shows that in both cases the reduction started at about 175–180 °C.

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

118
Accordingly, on-line gas analysis uncovered the consumption of hydrogen and the formation of water (not shown).

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

119
Although hydrogen was only consumed to a small extent during reduction (<30%) and the temperature ramp was the same (1 °C min−1) in both cases, the reduction in the capillary cell was faster than that observed with the pellet cell.

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

120
Analysis of the XANES region by linear combination of spectra originating from Cu0, Cu+, and Cu2+ showed that in both cases Cu+ is formed as an intermediate (region 8.97–9.03 keV).

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

121
The major part of the copper oxide is, however, directly reduced to Cu0.

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

122
This is in agreement with recent studies.30–32

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

123
The fraction of Cu+ is also slightly different and principal component analysis gave similar results.

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

124
Despite the low heating rate, a different behaviour was found which is attributed to the different design of the in situ cells.

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

125
The reduction of CuO/ZnO starts in both cases at the same temperature but the reduction time is markedly different.

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

126
Since hydrogen was used in excess and in the mass spectrometer a conversion of <30% was observed, the difference can be attributed to mass transfer limitations or different flow conditions in the pellet cell.

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

127
A rough estimate of the significance of the various steps of the gas–solid reaction (external film diffusion, pore diffusion, chemical reaction) can be obtained by applying a shrinking core model with constant size (thickness) of the plate-like pellet/particle.46–49

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

128
The duration for complete reduction τt can be expressed aswhere τfilm, τpore, and τchem are the times for external mass transfer, pore diffusion and the chemical reaction, respectively, necessary if the corresponding steps control the overall reaction.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod2

129
Models describing the fluid–solid reaction for different rate limiting steps have been summarized by Levenspiel both for flat plates and spheres.47

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

130
Assuming a laminar flow in case of the pellet cell (200 °C, isothermal pellet), the different contributions to the complete reduction time (τt) can be calculated using the expressions developed for the shrinking core model of unchanging size for flat plates:where ρCuO is the molar density of copper in the pellet, d the thickness of the pellet, cH2 the concentration of hydrogen and km the mass transfer coefficient.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod3

131
The different geometric parameters and fluid properties used for the evaluation are summarized in Table 1.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod4

132
In order to estimate the mass transfer coefficient km, we consider a laminar flow across a flat plate of length L = 13 mm, which can be described and analytically solved using a boundary layer model50 resulting in an average mass transfer coefficientwhere ReL is the Reynolds number for a plate of length L, Sc the Schmidt number and DH2 the diffusion coefficient of H2 in He (Table 1).

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod5

133
By using eqn. (2), a time for complete reduction of 6.5 s is estimated if external mass transport (film diffusion) is rate-determining.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs17

134
Thus, diffusion through the gas film cannot be the limiting step during reduction of CuO/ZnO in the pellet cell.

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

135
The control by the chemical reaction cannot explain the difference between the two reaction cells either.

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

136
Thus, pore diffusion can be assumed to be the limiting step.

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

137
Pore diffusion coefficients in porous solids depend on the porosity of the solid and slightly on the temperature.

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

138
Assuming an effective diffusion coefficient De between 10−6 and 10−8 m2 s−1 and a pellet thickness of d = 2 mm, reduction times can be calculated according to:47(disk of 2 mm diameter, molar density of copper in the pellet ρCuO = 67 mol m−3, concentration of hydrogen cH2 = 1 mol m−3, compare with Table 1).

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod6

139
For De = 10−6 and 10−8 m2 s−1 reduction times of 33 s and 56 min, respectively, can be calculated and thus τt can strongly depend on the pore diffusion.

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

140
In comparison, using a sieved fraction of 100 μm in a capillary microreactor under continuous flow conditions the terms for both film diffusion and pore diffusion are less significant as the following calculation shows.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs18

141
Applying the expressions for spherical particles (constant particle size), the contribution from film diffusion in a plug flow reactor can be calculated as:where Rp is the radius of the spherical particle.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod7

142
Mass transfer coefficients (km) for fluids flowing around spheres in packed-bed reactors have been widely estimated for different cases (e.g., refs. 50–52).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac20

143
For Reynolds numbers in the range 0.1 < Rep < 50 applied in this study (Table 1), a suitable relation is reported in ref. 50 and 52:

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac21

144
The calculation of the mass transfer coefficient from the Reynolds number and the Schmidt number is given in Table 1.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs19

145
Calculation of the reduction time, assuming that film diffusion is rate-limiting (eqn. (5)), results in a τfilm of 1.7 ms.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs20

146
Assuming pore diffusion instead of film diffusion being the rate limiting step, the corresponding reduction times for round-shaped particles can be calculated according to:

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod8

147
Also, these reduction times are much smaller with 0.02–2 s for a diffusion coefficient De of 10−6–10−8 m2 s−1, respectively.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs21

148
As long as the CuO/ZnO reduction takes more than 1 s at a De > 10−7 m2 s−1, the rate-limitation results from the chemical reaction.

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

149
Hence, the differences between the two in situ cells is expected to be due to pore diffusion effects.

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

150
Note that here we assumed different pore diffusion coefficients for hydrogen.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod9

151
The same accounts for the gaseous product (water formation) and it has recently been shown that the reduction of CuO/ZnO is strongly affected by the presence of water.53

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac22

152
Hence, also the pore diffusion of water could be limiting.

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

153
In addition, we can conclude from these estimations that for gas–solid reactions in the subsecond region,4,32,54,55 intermediates may not only be a consequence of the reaction itself but also be indirectly affected by film and/or pore diffusion.

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

154
This is, in particular, the case if the flow properties are not as good as in a plug flow reactor or the particle diameter is too high.

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

155
Moreover, differences can be more significant if larger molecules and, in particular, liquids are used (smaller diffusion coefficient).

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

156
Both in situ cells can give similar results during temperature programmed reaction if the ramp rate is small, the flow properties are good and the pore diffusion of the molecules is sufficiently fast.

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

157
The latter is also strongly dependent on the material used to dilute the catalyst sample as shown in the next part.

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

Comparison of differently prepared PdO/ZrO2 samples during reduction

158
Reduction of an oxidized PdO/ZrO2 catalyst by hydrogen turned out to lead to a more active methane combustion catalyst.41,56

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac23

159
Reduction occurs very fast at room temperature.37

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac24

160
Fig. 5 shows the reduction of PdO/ZrO2 (capillary cell, diluted 1∶3 with Al2O3) in 5% H2/He using the QEXAFS (20 s per scan) mode.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs22

161
On-line gas analysis showed that the response time inside the reactor is <5 s, while the reaction occurs over a time interval of 2 min.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs23

162
The XANES region (24.33–24.45 keV) of all spectra could be reconstructed from the spectrum of the reduced and the oxidized form of the sample.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp53

163
The fraction α of oxidized or metallic palladium is shown in Fig. 5b.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs24

164
Note that some palladium hydride formed as well, since EXAFS analysis indicated the formation of some Pd hydride due to an increased Pd–Pd distance to 2.80 Å.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs25

165
The analysis in Fig. 5b reveals that the reduction occurs via an induction and acceleration period.

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

166
Fig. 6 shows the comparison of two different pellets in the pellet cell.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs26

167
Using a pressed wafer of PdO/ZrO2 diluted once with BN (thickness d = 1.5 mm) and once with Al2O3 (thickness d = 2 mm), shows a striking difference in the reduction behaviour.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp54

168
The same reduction conditions were used in both cases and also the response time of the pellet cell was only a few seconds.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs27

169
Nevertheless, both the start of the PdO/ZrO2 reduction, as well as the reduction rate, are negatively affected in the case of the dilution with boron nitride.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs28

170
This shows that not only the catalyst shape (sieved catalyst powder vs. pellet) but also the material plays an important role.

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

171
BET measurements of the reduced pellets revealed that the surface area and the pore structure of the BN- and Al2O3-pellet are significantly different.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs29

172
While the specific pore volume of the used Al2O3 pellet is 0.327 cm3 g−1 (the BET surface area is ca.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs30

173
133 m2 g−1), the pore volume of the BN pellet is only 0.036 cm3 g−1 and the BET surface area 2 m2 g−1.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs30

174
Hence, using BN for pressing the pellets led to pellets with significantly lower porosity, and these results suggest the use of high porosity materials.

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

The oxidation state of palladium during catalytic combustion of methane over PdOx/ZrO2

175
Several mechanisms concerning the oxidation of methane over Pd/ZrO2 catalysts have been proposed.35,37

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac25

176
Interestingly, Pd/ZrO2 (pre-reduced in hydrogen) performs better than PdO/ZrO2 in this reaction.56

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac26

177
Hence, the question arises on the role which metallic Pd plays in the combustion of methane.

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

178
Once again, results over PdOx/ZrO2 in a pellet and a capillary cell are considered.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj14

179
As a basis, the re-oxidation of a (reduced) Pd/ZrO2 sample in 2.5% O2/He and reduction in 1% CH4/He are discussed.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj15

Re-oxidation of Pd/ZrO2 in oxygen and reduction of PdO/ZrO2 in methane

180
Figs. 7 and 8 show the oxidation of Pd/ZrO2 in 2.5% O2/He and the reduction of PdO/ZrO2 in 1% CH4/He, respectively.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs31

181
While reduction of PdO/ZrO2 in hydrogen already occurs at room temperature (Figs. 5 and 6), the re-oxidation only partly occurs at room temperature.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs32

182
Interestingly, this oxidation has not been observed by TPO on Pd/ZrO2.57

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac27

183
Some shift in the edge position was observed as well, indicating the decomposition of the Pd hydride phase (also supported by EXAFS analysis).

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

184
Above 320 °C, most of the palladium is re-oxidized and XANES/EXAFS analysis reveals no detectable metallic palladium after oxidation at 500 °C, supported by thermal analysis (not shown).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs33

185
The reduction by methane also occurs only at higher temperatures (>275 °C) and results in a completely reduced catalyst.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs34

186
No reduction took place up to 275 °C.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs35

187
Above this temperature, a very sudden reduction within 1 scan (50 s per scan) was observed.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs36

188
It seems that once methane can be activated on the PdO/ZrO2 catalyst a fast reaction occurs.

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

189
PdO was rapidly reduced once metallic particles started to be formed, which seem to catalyse the activation of methane.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs37

190
This is in agreement with temperature programmed reduction and Raman studies,57 where it was also found that a certain temperature is needed to activate methane.

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

191
Once metallic palladium is formed, dissociative adsorption of methane is enhanced (on the metallic Pd particles) and further reduction occurs.

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

192
The fact that an autocatalytic reduction occurs at 275 °C is also supported by the fact that dissociative adsorption of methane is reported to occur already at 200 °C58.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac28

Reaction in 1% CH4/4% O2/He

193
The oxidation behaviour of Pd/ZrO2 during temperature programmed reaction in 1% CH4/4% O2/He (Fig. 9) uncovers in the low temperature region (<350 °C) a similar behaviour as in 2.5% O2/He.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs38

194
The methane combustion (according to on-line gas analysis) sets in at 300–320 °C.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs39

195
Likewise, in the reduction of the CuO/ZnO, both the plug-flow reactor cell (based on the capillary) and the cell using a pelletized sample (disk-shape) were compared.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj16

196
In both setups, methane combustion was more effective over Pd/ZrO2 (pre-reduced) than PdO/ZrO2 (oxidized form by direct oxidation of amorphous PdZr alloy).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs40

197
During the temperature programmed reaction the conversion of methane over Pd/ZrO2 reached already its maximum well below 500 °C, whereas only partial conversion of methane was observed over the PdO/ZrO2.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs41

198
As Table 2 summarizes, the conversion over the Pd/ZrO2 pellet only reached about 80%, while total conversion was observed over Pd/ZrO2 in the capillary cell.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs42

199
This is traced back to the dead volume of the pellet cell, allowing some of the methane to pass through the cell without contact with the catalyst (by-passing).

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

200
The observed conversions and rates are summarized in Table 2.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs43

201
In both cases, it was observed that PdO/ZrO2 was not reduced to an extent measurable by EXAFS/XANES, while starting from Pd/ZrO2 evidenced that Pd was not completely re-oxidized or possessed a different structure (Fig. 10), both if either the pellet or the glass capillary cell was used.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs44

202
The “whiteline” in the XANES spectra was slightly different and EXAFS analysis indicated a different number of coordination shells in the PdO/ZrO2 and the Pd/ZrO2 catalyst after methane combustion at 500 °C.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs45

203
While the PdO/ZrO2 sample had mainly contributions at 2.02 Å (Pd–O) and at 3.02 Å (Pd–Pd in oxide), a significantly higher contribution at 2.76 Å (Pd–Pd shell) was found in the Pd/ZrO2 catalyst (cf. more details and similar results in ref. 37).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs46

204
Hence, the findings on the structure of the catalyst are similar in both in situ studies, though the catalytic activity is different but with the same tendency in the two cells.

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

205
Nevertheless, the significantly lower methane oxidation rate over the catalyst pellet compared to the corresponding particles in the capillary reactor requires an explanation.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj17

Comparison of the effectiveness of the catalyst in a plug flow and a pellet in situ cell

206
One reason for the lower catalytic activity of disk-shaped catalyst samples compared to the sieved catalyst powder is by-passing of the gas due to the dead volume in the pellet reactor cell.

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

207
This would, however, still lead to a representative result for both the catalytic activity and the active species if the whole catalyst contributes to the catalytic activity.

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

208
In order to verify this hypothesis for methane combustion, the influence of pore diffusion in both kinds of cells is considered.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj18

209
The pore diffusion resistance can dramatically influence the effectiveness of heterogeneous catalysts.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac29

210
The phenomenon has been intensively investigated in kinetics and reaction engineering studies in heterogeneous catalysis.47,59,60

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac30

211
This concept can be directly transferred to estimate the influence of the catalyst shapes (porosity, characteristic length) in the two cell designs used in the present study.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj19

212
According to Thiele61 and Aris,62 for a plate-like pellet and a first order reaction the effectiveness factor η can be described in terms of the Thiele modulus ψ47

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod10

213
For a plug flow reactor, the equations are correspondingly given (see Table 3), and for a rough estimation the same relation as given in eqn. (8) can be used with a characteristic length: Lc = dp/6.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod11

214
For estimating the effectiveness factor in the methane oxidation reaction over a cylindrical platelet of thickness d = 2 mm and over spherical particles of dp = 80 μm both the reaction rate and the diffusion coefficient were taken from a recent work on methane combustion over Pd/Al2O3 catalysts by Groppi et al.63

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod12

215
The calculations summarized in Table 3, show that the effectiveness factor of the plate-like pellet is only 0.03%, which means that due to pore resistance only a thin layer on each side is contributing to the reaction.

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

216
For an efficiency of 50% over a Pd/ZrO2 catalyst pellet, a very thin platelet of d ≤ 0.06 mm is required.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs47

217
Fig. 11 shows the estimated concentration gradient in the pellet, it dropped by 90% already at 70 μm.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs48

218
If this drop in reactant concentration can be tolerated in the middle of the pellet, a Thiele modulus of about ψ = 3 is required, corresponding in the case of a flat plate to a thickness of d = 160 μm.

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

219
For the capillary cell the effectiveness factor is much larger but still only 88% of the catalyst are effectively contributing to the reaction (due to the fast reaction rate).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs49

220
The concentration gradient of the reactants inside a catalyst particle is flat, as Fig. 11 shows.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs50

221
Hence, the structure monitored by the bulk technique EXAFS is particularly representative if the catalyst is used as powder.

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

222
For pellet-like shapes a more surface-sensitive technique (e.g., electron yield XAS, grazing incidence XAS) would be required.

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

Discussion

223
The experiments in this study have shown the importance of appropriate cell designs for in situ X-ray absorption spectroscopy and related operando spectroscopic techniques.

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

224
The EXAFS technique offers a number of opportunities in the field of time-resolved studies and for combining structural analysis with catalytic activity measurements.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot7

225
Here, the reduction of CuO/ZnO and PdO/ZrO2 was uncovered in a time-resolved manner and the structure of PdOx/ZrO2 was monitored during methane combustion.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj20

226
The reduction of CuO occurs via Cu+ species, as also supported in .refs. 30–32

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac31

227
However, it seems that the reduction is dependent on the cell design/sample preparation which may also influence the occurrence of Cu+ species.

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

228
In case of methane oxidation over PdOx/ZrO2 catalysts, the pre-reduced catalyst was more active under reaction conditions, in accordance with literature, and some of the palladium remained in reduced state at 500 °C under reaction conditions.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs51

229
Reduction of PdO/ZrO2 to Pd/ZrO2 by hydrogen occurred within 5 min at room temperature, while re-oxidation was only to a small extent observed up to 250 °C.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs52

230
Above this temperature, the palladium constituent was continuously and completely oxidized.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs53

231
These studies support the conclusions in literature that reduced palladium is one of the active species in methane combustion.37,57,64

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

232
Studies with an X-ray eye uncovered that the X-ray intensity behind a sample pressed as pellet was more homogeneous than behind a catalyst sample used in a sieved powder fraction.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs54

233
Thus, a pellet will lead to a better signal/noise ratio (due to a higher μd), in particular, if the position of the beam varies slightly.

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

234
The use of pellets also allows to irradiate a larger area resulting in a better signal/noise ratio.

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

235
A further limitation of the capillary cell used in this study is that the temperature is measured below the catalyst sample and not in the catalyst bed.

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

236
Temperature measurement in the catalyst sample may become of paramount importance for highly exothermic or endothermic reactions, particularly at high conversion.

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

237
Moreover, the catalyst composition may change in such a plug flow reactor, so that local probing of the catalyst composition over the catalyst bed would be interesting as we recently proposed.65

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac32

238
Effects due to pore diffusion resistance can be encountered both in time-resolved studies and in catalytic studies, in particular in the case of pressed wafers.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac33

239
Eqns. (2), (4), (5) and (7) show that time-resolved changes on heterogeneous catalysts depend on the gas–solid mass transfer coefficient and the effective diffusion of the reactants/products in the solid.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod13

240
Effects caused by pore diffusion limitations can be eliminated or at least mitigated by the use of macroporous materials and/or small particles or very thin pellets.

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

241
Our study has shown two extreme cases: the pore structure/volume of BN and Al2O3 are very different.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs55

242
This shows that high porosity materials, such as Al2O3 used in this study, are preferable.

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

243
Since boron nitride is a favoured material in X-ray absorption spectroscopy due to the light elements, the use of a high porosity form would be of interest for spectroscopic purposes—here hexagonal BN with low porosity was used.

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

244
Nevertheless, in slow gas–solid reactions the pore diffusion is relatively fast compared to the chemical reaction rate so that using a disk may be preferred due to the better signal/noise ratio in spectroscopy.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con10

245
We suggest therefore a comparison of the measured reaction time τt with the time τfilm expected if film resistance is rate-limiting or with the time τpore if pore diffusion is limiting, using eqns. (2)–(7), respectively.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj21

246
These effects become even more relevant in liquid/solid reactions.

Type: Object | Advantage: Yes | Novelty: New | ConceptID: Obj21

247
Recently, we observed in the reduction of PdO/Al2O3 by benzyl alcohol that the reduction of palladium occurred within less than 2 min using PdO/Al2O3 as powder, while the reduction over a PdO/Al2O3 pellet took more than 45 min.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs56

248
Assuming a diffusion coefficient of De = 1 × 10−9 m2 s−1 of the alcohol in a fluid and a typical viscosity of 0.33 × 10−3 m−1 kg s−1, it already turns out with eqns. (2) and (5) that the external film resistance (fluid–solid) in liquid phase is much higher than in gas phase.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod14

249
In case of film diffusion controls the reduction, some example calculations are given in Table 4 for reduction of a catalyst pellet (platelet) and a catalyst powder (spherical particles) of 5% Pd/Al2O3 in toluene.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs57

250
Also, pore diffusion resistance is supposed to be considerably higher than in gas phase.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac34

251
Hence, combination of on-line catalytic monitoring combined with EXAFS analysis has to be performed on finely sieved catalysts, as recently reported in refs. 66 and 67.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con11

252
Another possibility is the use of shell-impregnated catalysts.

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

253
We applied this approach in the field of supercritical (or dense) fluids, since sufficiently large particles are required for proper flow conditions.68

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj22

254
A recent ATR-IR study69 has proven that the supercritical fluid inside the pores has a similar density to the corresponding liquid and it can thus be assumed to have a similar effective diffusion coefficient as in liquid carbon dioxide.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac35

255
The fluid–solid mass transfer coefficient is, however, much higher than in liquid phase.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac35

256
Another important consequence of the presented influence of film and pore resistance is the appropriate design of cells for time-resolved studies on heterogeneous catalysts.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con12

257
Both the QEXAFS and DEXAFS techniques are used with the aim to identify possible intermediates and/or reveal the mechanism of reduction/oxidation.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj23

258
Considering the reduction of a catalyst by hydrogen on a pressed wafer, it turns out according to eqn. (2) that studies on the second scale will even be influenced by the gas film diffusion using the in situ cell sketched in Fig. 1c.

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

259
The calculations using a plug flow reactor with catalyst particles of mean particle size of 100 μm show that in this case the response time is ca.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs58

260
1 ms.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs58

261
Hence, 1 ms seems to be the lower reasonable limit of time-resolved studies in a fluid/solid reaction.

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

262
Not only during time-resolved studies but also in case of reactions, such as methane oxidation, pore resistance may influence the spectroscopic results, in particular if the diffusion of reactants inside the pores is slow compared to the chemical reaction.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con13

263
In this case, only a fraction of the catalyst may be involved in the reaction.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con13

264
Though methane oxidation is a rapid reaction, such limitations by pore/film diffusion have also been observed in other studies.70

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac36

265
A possible solution may be the use of electron yield XAS.71

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

266
However, special care has to be taken concerning the influence of the reaction mixture on the detection technique.

Type: Method | Advantage: No | Novelty: Old | ConceptID: Met8

267
Another possibility is once again the use of particles with small characteristic length Lc.

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

268
It is suggested to calculate the Thiele modulus ψ

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod15

269
If ψ < 0.5, the effectiveness factor is higher than 90%.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs59

270
For ψ > 3 the reactant concentration drops to less than 10% inside the particle.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs60

271
Thus sufficiently thin pellets or particles (resulting in a small characteristic length Lc) will lead to η > 0.9.

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

272
In the case when the reaction rate kr cannot be estimated, the Weisz modulus can be used instead as well known from mass transfer literature (cf. ref. 59).

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod16

273
As pointed out above, also shell-impregnated particles can be used for the studies.

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

274
In this way, operando studies become feasible not only for gas/solid but also for liquid/solid or dense fluid/solid reactions.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con14

Conclusions

275
In summary, the different examples show that, as in kinetic studies, special care has to be taken to account for possible film and pore diffusion limitations during time-resolved or operando spectroscopy studies.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con15

276
Limitations due to film and pore diffusion may occur due to flow conditions, catalyst shape, the chosen reactor design or material to dilute the catalyst to an optimal absorption length μd.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con16

277
Low porosity materials, such as hexagonal boron nitride, are not ideal for this purpose, even though samples can be easily pelletized and its low absorption coefficient of hard X-rays makes it a preferable material for spectroscopy.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con17

278
The use of a high porosity material is desired.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con18

279
Both the cell design and the catalyst system need to be adapted to the reaction of interest (kind of fluid, reaction rate regime, diffusion coefficient, pore size distribution, etc.).

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con19

280
In order to find a balance between optimal design for spectroscopy and catalysis we suggest to estimate the importance of film diffusion (τfilm), pore diffusion (τpore) and/or the effectiveness factors (Thiele modulus), as discussed in this work.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con20

281
These criteria are important for describing and estimating the reaction conditions during operando spectroscopy and time-resolved studies, not only in X-ray absorption spectroscopy but in situ spectroscopy in general such as XRD, Raman or IR spectroscopy (transmission mode).

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con21

282
The discussed criteria will make both in situ spectroscopic and time-resolved studies more powerful for elucidating the working mechanism of heterogeneous catalysts, as exemplified here for two well-known catalyst systems (Cu/ZnO for methanol synthesis and PdOx/ZrO2 for methane combustion).

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con22