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Understanding structure function relationships in zeolite supported Pd catalysts for oxidation of ventilation air methane Hadi Hosseiniamoli, Glenn Bryant, Eric M. Kennedy, Karina Mathisen, David Graham Nicholson, Gopinathan Sankar, Adi Setiawan, and Michael Stockenhuber ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04462 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on May 1, 2018

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Understanding structure function relationships in zeolite supported Pd catalysts for oxidation of ventilation air methane Hadi Hosseiniamoli a Glenn Bryant a, Eric M. Kennedy a Karina Mathisen b, David Nicholson b Gopinathan Sankar c Adi Setiawan d and Michael Stockenhuber a a

The University of Newcastle, Newcastle, 2308, Australia Norwegian University of Science & Technology, Trondheim, N-7491, Norway c Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, U.K d Mechanical Engineering Department, Universitas Malikussaleh, Lhokseumawe, 24352, Indonesia b

Abstract Catalytic combustion of ventilation air methane (VAM) is a potential solution for abatement of this greenhouse gas. In this study, we evaluate the combustion of VAM (with methane concentrations below 1%) spanning over 100 h time on stream (TOS) during reaction over a Pd/HZSM-5 catalyst. The aim is to understand the structural changes that lead to catalyst deactivation. We observe the formation of carbonaceous deposits even under oxygen rich conditions which are an important contributor to deactivation. X-Ray absorption spectroscopic (XAS) investigation shows that, in addition to carbon deposits, the growth of Pd oxide clusters leading to a reduced number of accessible sites and in turn intrinsic activity. STEM-EDS analysis disclosed the presence of the carbonaceous deposit on the surface of the used catalyst, and TGA confirmed the presence of different carbon species on the used catalyst under very lean conditions. Structural changes show that Pd-O / acid-base interactions have a significant influence on the structure of the active site. This assertion is consistent with findings from acid-base characterisation experiments. Although the catalyst displayed a high level of stability over the first 10 h of VAM combustion, long-term reaction, in the presence of water vapour, is associated with a partial rearrangement of the zeolite, accompanied with a gradual deactivation of the catalyst. This rearrangement is associated with a decrease in surface area and pore volume which is consistent with the significant changes observed in the Al-X-Ray absorption near-edge spectroscopic (XANES) analysis. A comparison of the NH3-TPD of fresh and used Pd/HZSM-5 catalysts shows that the strength of the acid sites are significantly reduced. This is a consequence of the changing nature of transition metal interaction with the zeolite which is accompanied by the dealumination of the zeolite support, thereby enhancing Pd agglomeration and the emergence of two low index surface orientation facet planes identified as PdO (101) and PdO (100). A higher turnover frequency (TOF) (0.031 s-1) for reactivated Pd/HZSM-5 after removing all carbonaceous material compared to TOF (0.024 s-1) for used Pd/HZSM-5 was observed. The catalyst regained 75% of its initial catalytic activity after removing carbonaceous compound from the used catalyst. We propose the formation of palladium carbonaceous complex manifesting itself in carbonate and carbonyl group observed in used Pd/HZSM-5. These species act as an important contributor to catalyst deactivation and cause partial reversible deactivation during long-term VAM combustion.

Keywords: VAM, Pd/HZSM-5, XAFS, Pd-Pd, Pd-O-Pd, agglomeration, acid site, dealumination 1

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1. Introduction The oxidation of methane, when it is present at concentrations well below its lower flammability limit, is challenging because of the difficulty activating the methane molecule. This is a particular problem in underground coal mining, where high volumes of methane, diluted with humid ventilation air constantly purges through the mine and is eventually emitted into the atmosphere. Although diluted to a concentration of less than 1%, these emissions represent a significant point source of a potent greenhouse gas. Combustion of CH4 to CO2 results in a net benefit because of the high CO2(e) of CH4 estimated to be 23. The use of conventional combustion processes to consume the methane is hindered because the methane concentration is well below the lower flammability limit in air and as a consequence requires high temperatures for autoignition. Due to the resulting high cost of treatment and the generation of additional CO2 emissions, the addition of fuels to the VAM gas is not a practical option to generate temperatures high enough to sustain homogenous combustion of the VAM. However catalytic combustion is a potential solution to treat such lean methane mixtures. In this process, methane is converted to carbon dioxide at low temperatures (making the process self-sustaining) over active metals, most often palladium, that are supported on supports such as Al2O3, SiO2, ZrO2 and zeolites 1-7. The catalytic activity of methane over these catalysts depends on a number of factors, including the dispersion and size of palladium particles, the coordination geometry and the associated electronic structure of the active site, and the nature of the support and its interactions with the palladium metal 710

. Compared with other supports, zeolites stand out because their well-defined structure and they are

characterised by a symmetric pore structure in addition to the presence of highly dispersed aluminium atoms incorporated into the framework of the zeolite 11-12. The porous nature of zeolites results in high surface areas and they possess properties that enable small metal oxide clusters with narrow size distribution to be anchored in the zeolite pore 13. Framework aluminium atoms present in the zeolite structure imparts a strong affinity to water, which in turn leads to reversible deactivation of the Pd/zeolite catalyst during the oxidation of methane 14-16. Hydrothermal stability is improved for methane combustion by increasing the Si/Al ratio in those catalysts with low loadings of palladium 8-9, 17. For these reasons, the activity of Pd methane combustion depends, at least in part, on the structure and concentration of Al in the zeolite support 12.

2


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The catalytic oxidation of methane depends on the interaction between the support and the Pd and PdO phases

17-18

. It appears that the catalytic performance of metal or metal oxide ensembles that are

positioned within the pores of the zeolite is influenced by the acid-base nature of the material and that this influence manifests itself by inducing structural changes in Pd and PdO phases that can be either conducive or deleterious to catalyst performance 12, 17, 19. The surface acid sites of HZSM-5 in particular play an important role in controlling the valence state and level of dispersion of palladium and hence the formation and anchoring of active PdO species on the zeolite

8-9

. M’Ramadj et al. proposed that the

active site for dissociating methane in the Pd/ HZSM-5 system is [AlO]Pd2+

20

. These researchers

prepared variants of Pd/HZSM-5 by adopting impregnation and ion-exchange techniques and found that the order of activity is directly related to the Brønsted acidity of the catalyst, with catalysts prepared with impregnation techniques being more active than those prepared by ion-exchange processes. In a recent paper, Lou and co-workers report that the stable structures of HZSM-5 supported palladium appear to be [AlO2]Pd-ZSM-5, [AlO2]Pd[AlO2]-ZSM-5, [AlO2]PdOH-ZSM-5, and [AlO2]Pd(OH)2-ZSM-5 8. They suggest that the active site for the catalytic combustion of methane could be [AlO2]PdOH-ZSM-5. Their method for revealing the relationship between the acidity of HZSM-5 and the local structure around palladium was x-ray absorption fine structure (XAFS) 12. Despite there being numerous studies on the catalytic oxidation of methane by Pd/zeolite systems, there is a dearth of structural data which are aimed at explaining the mechanism of deactivation that is often observed during time-on-stream studies. Structural information, together with attendant electronic structures, is essential for rationalising the activity and stability of the catalyst during the oxidation of methane in the presence of water vapour. Although there are several reports on stability tests on Pd/ZSM-5, they are limited in scope by a lack of structural analysis 15, 21-22. An exception is the XAFS study by Okumura et al. 9 on the zeolitic systems mordenite and ZSM-5 that supported palladium. Of particular interest is also the observed formation of carbon deposits on the catalyst under the very lean conditions and its relationship to deactivation. XAS (ie. XANES and XAFS) has been proven to be a valuable tool for monitoring and studying the structural changes (electronic and geometrical) 23-25 that result from the deactivation of catalytic active sites. Accordingly, we report our investigation of the deactivation and long term stability of a Pd/HZSM-5 catalyst, including the role played by the active palladium phase(s) and the zeolite support during the oxidation of methane in a humid simulated VAM gas stream. This includes examining the effect of the valence state of palladium, the dispersion and clustering of that metal on the surface of the catalyst 3


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before and after 100 h of VAM oxidation. Complementary methods of characterisation, such as Al-X-Ray near edge absorption spectroscopy (XANES), nitrogen adsorption NH3-Temperature Programmed Desorption (TPD) and Scanning Transmission Electron Microscopy (STEM), were all used to assist in identifying aspects of the mechanism of deactivation.

2. Experimental 2.1. Catalyst Preparation HZSM-5 (Si/Al=140, Zeolyst International, Kansas, USA) and palladium (II) nitrate (Sigma-Aldrich) were used as received. 1 wt% palladium was loaded on calcined HZSM-5 via the incipient wetness impregnation method26, using a Pd(II) nitrate solution (10 wt% in 10 wt% nitric acid). Following impregnation, the sample was dried at 110°C overnight and calcined at 550°C for 2 h in air.

2.2. Characterization Microscopic images of the catalyst were acquired using a JEOL 2100 Energy dispersive X-ray spectroscopy and Scanning Transmission Electron Microscope (STEM) at the acceleration voltage of 200 kV. The quantity of palladium loaded on catalysts was determined using a Varian 715-ES inductively coupled plasma optical emission spectrometer (ICP-OES) following nitric acid digestion. Powder X-Ray diffraction patterns were collected with a Philip X’Pert diffractometer by Cu Kα radiation. Surface area measurements were performed using a Micromeritics Tristar surface area analyser using nitrogen adsorption and desorption at 77 K. A Micromeritics Vacprep 061 for sample degassing was used. An indigenous (Temperature-programmed desorption) TPD apparatus with a Pfeiffer Prisma quadrupole mass analyser was used for detection of NH3 desorbing from acid sites present on the zeolite. Thermogravimetric analysis experiments were performed in a Mettler Toledo TGA/DCS 1 STARe instrument. The samples were placed in cylindrical alumina crucibles (height = 5 mm, internal diameter = 4 mm) and then heated over the range of 25˚C-1000˚C with temperature ramping of 5˚C min-1 over 7.0 mg of sample in Argon flow. XAFS measurement of the Pd K-edge (24.4 keV) was carried out at the Swiss-Norwegian beamline (SNBL) at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. The storage ring electron energy was 6 GeV and operated with a current of 200 mA. A Si (111) double crystal was used to obtain the monochromatic X-ray beam. Three ionization chambers were used 4


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for detecting incident intensity (I0), transmitted intensity (I) and reference (Pd) foil transmitted intensity (Iref). The Fourier transform of the k3-weight XAFS oscillation from k space and r space were performed. The analysis of XAFS data was performed using Athena and excurv98 code. Also, X-ray absorption nearedge (XANES) spectra at the Pd, edge was recorded at the wiggler XAS Beamline and Al and C edge at the Soft X-ray beamline of the Australian Synchrotron in Melbourne, Victoria, where the storage ring is operated at 3 GeV with a maximum beam current of 200 mA.

2.3. Catalyst Test Catalytic combustion of lean methane gas mixtures was performed in a tubular stainless steel fixed bed micro reactor. The inlet methane concentration was fixed at 7000 ppm with 30 000 ppm water balance in air with a total gas hourly space velocity (GHSV) of 100 000 h-1. Methane conversion was monitored using a gas chromatograph (Shimadzu GC-2014) equipped with a packed Hysep and Molecular Sieve dual column and a thermal conductivity detector (TCD). The experimental setup used has been described in detail elsewhere 26. For kinetic measurements of the methane oxidation over the catalysts, an inlet feed with the same condition was used. Since the oxidation reaction is independent of the partial pressure of CO2, CO2 was not added to the inlet feed 27. The turn-over frequency (TOF) was calculated under the condition of methane conversion below 12%. The number of active sites on the surface of the catalyst was calculated by Pd dispersion measurement.

3. Results and discussion 3.1. Catalyst Activity and Stability The stability of the 1% Pd/HZSM-5 was evaluated under a humid simulated VAM feed (7000 ppm CH4) at a constant temperature of 400°C. In addition, methane conversion as a function of bed temperature over fresh and used catalyst was determined. The results of the activity testing over fresh and used catalyst in Figure 1 showed a significant difference between the temperature of 90% conversion in which, lean methane over fresh and 100h used catalyst reached to 90% conversion at 390o and 470oC, respectively. Additionally, Figure 1 shows that the fresh catalyst is very stable during the first 10 hours on stream, operating at a conversion level of 90 ±1%. The stability under these conditions shows that the catalyst has excellent initial resistance towards poisoning by water vapour. This is attributed to the high Si/Al=140 ratio of the HZSM-5 support with attendant highly hydrophobic properties and high thermal stability. These characteristics are consistent with the low concentration of (hydrophilic) 5


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protons and the weak interaction with water of the oxygen of the Si-O-Si groups

28-29

. In contrast,

decreasing the Si/Al ratio in the zeolite leads to an increase in water affinity. Otsuka et al 15 used ZSM-5 (Si/Al2=16) in their study and observed that the catalyst rapidly deactivates when water vapour is introduced into the feed. However, while the Pd/HZSM-5 (Si/Al=140) catalyst displayed a high degree of stability under the humid feed during the first 10 h on stream, a rapid deactivation is observed between 10 h and 50 h of reaction time. From 50 h to 100 h of reaction, the catalyst activity was lower but had stabilised. 2nd run

Mtehane Conversion (%)

90 80 70 60 50

100

Methane conversion %

1st run

100

80 60 40 Fresh Catalyst Used Catalyst Reactivation

20 0 330

40

380

430

Temperature (℃)

480

30 20 10 0 0

20

40

60

80

100

Time(h)

Fig 1. 1st run: methane conversion as a function of time on stream at constant bed temperature at 400 ℃ over fresh catalyst, 2nd run: methane conversion as a function of bed temperature for fresh, 100 h used and reactivated catalyst, 7000 ppm CH4, 30 000 ppm H2O(v) balanced in air, GHSV: 100 000 h-1

Fresh Pd/HZSM-5

-2.3

Used Pd/HZSM-5 -3.1

Ln (rate)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Ea=144 KJmol-1

-3.9

-4.7

-1

Ea= 207 KJmol

-1

Ea= 188 KJmol -5.5 1.52

1.55

1.58 1.61 1000/T (K)

1.64

1.67

6


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Fig.2. The Arrhenius plot and activation energy (Ea) of fresh, used and reactivated of Pd/HZSM-5 catalysts, 7000 ppm methane, 30 000 ppm H2O(v) balance air (CH4 conversion < 12%) Arrhenius plots derived from steady state low conversion (below 12%) measurements are shown in Figure 2 for fresh, used and reactivated catalyst evaluated under humid conditions. In addition, TOF values and reaction rates for methane conversion under humid feed are summarized in Table 1. Under saturated water feed conditions, the apparent activation energy increased from 144 kJ/mol for the fresh Pd/HZSM-5 to 207 kJ/mol for the used catalyst and following reactivation of catalyst at 550 °C in air, the activation energy decreased to 188 kJ/mol. This change of activation energy after long-term VAM oxidation has also been reported in previous publication26. As the intrinsic data calculation showed that the TOF over the fresh catalyst at a selected temperature of 360℃ decreased from 0.041 s-1 to 0.024 s-1 to used catalyst and after reactivation reached to 0.031 s-1 (Table 1).

Table 1. Kinetic data for fresh, used and reactivated Pd/HZSM-5 Rate (mol·gcat -1·s-1) 1.4 E-06

Samples Fresh Pd/HZSM-5

TOF (s-1) 0.041

Ea (kJ/mol) 144

Used Pd/HZSM-5

2.7 E-07

0.024

207

Reactivated Pd/HZSM-5

5.9 E-07

0.031

188

3.2 Highly dispersed PdO on the surface of catalyst and the role of acid site Figure 3 shows the k3 weighted XAFS measured at the palladium K-edge following calcination of the catalyst. Included in the figure is a comparison of the corresponding XAFS of bulk PdO, which highlights the significant differences between the bulk and supported systems. The Fourier transform spectrum of the bulk material contains three peaks, centred on 2.0, 3.0 and 3.4 Å. These peaks represent the Pd—O coordination (first shell), second (Pd—O—Pd) and third (Pd—O-Pd-O) shells respectively of the bulk material. These features differ from those in the spectrum of freshly calcined Pd/HZSM-5, namely an increased intensity of the Pd—O peak and decreased intensity of the Pd—Pd peak. These changes are quantified by fitting the k-space where three or four shells formed the input (Table 2). These finding accord with the supposition that PdO is highly dispersed over the surface of calcined and fresh

7


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Pd/HZSM-5 catalysts, and is supported by the TEM images (Figure 4) that show the presence of highly dispersed, 2 – 4 nm palladium particles on the surface of the catalyst.

Table. 2: Curve-fitting Analysis Pd K-Edge XAFS Data using EXCURV98 software for freshly calcined and used Pd/HZSM-5 samples under VAM oxidation conditions Sample

Treatment

Pd/HZSM-5

Pd/HZSM-5

Pd/HZSM-5

PdO

PdO

e

g

e

f

a

R/A˚

b

2

Ơ /Å

2 c

Shell

CN

Pd-O

4.0

2.00 ± 0.007

0.007

Pd-Al

1.7

3.34 ± 0.058

0.019

Pd-Pd

2.5

3.06 ± 0.011

0.013

Pd-Pd

3.4

3.40 ± 0.017

0.017

Pd-O

3.3

1.99 ± 0.011

0.007

Pd-Pd

3.4

3.04 ± 0.010

0.015

Pd-Pd

7.9

3.39 ± 0.013

0.014

Pd-O

2.4

1.95 ± 0.025

0.008

Pd-C

3.0

2.09 ± 0.036

0.008

Pd-Pd

3.4

3.04 ± 0.012

0.014

Pd-C

3.4

3.09 ± 0.052

0.011

Pd-Pd

8.1

3.38 ± 0.013

0.013

Pd-O

4

2.02 ± 0.009

0.008

Pd-Pd

4

3.05 ± 0.008

0.013

Pd-Pd

8

3.42 ± 0.012

0.016

Pd-O

4

2.02

Pd-Pd

4

3.04

Pd-Pd

8

3.42

Calcined

VAM - 100 h

VAM - 100 h

EF/eV

4.3

d

FI

0.00048

4.8

0.00050

6.7

0.00036

3.9

0.00041

_

a. Coordination number. b. Bond distance. c. Debye-Waller factor. (According to standard statistical 2 2 30 methods, sensible values lie in the range 0.005 < Ơ /Å < 0.025 ) d. Difference in the origin of photoelectron energy between the reference and the sample. e. Curve-fitting of used catalyst without introducing carbon to the second shell. f. Curve-fitting of used catalyst with introducing carbon to the second shell. g. Data from X-ray crystallography.

8


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Fig. 3 k3-weighted Pd K-edge XAFS spectra for fresh and used Pd/HZSM-5 and PdO as reference, right: kspace (Å-1) and left: R-space (Å)

A good fit to the Pd K-edge XAFS of fresh Pd/HZSM-5 was achieved by introducing a T atom into the second shell. This T atom is most probably Al at 3.34 Å, although it is possible that this atom is Si, though the fit index (FI) are slightly lower for Al and the binding site is most likely Al 30. However, we could not distinguish with certainty between Si and Al atoms. Moreover, the presence of aluminium in the vicinity of palladium has been reported previously in XAFS results for Pd/HZSM-5 12. The interaction of palladium with the active sites (associated with aluminium atoms in the zeolite framework) and the formation of dispersed nano-particulate PdO species can enhance the stabilisation of the Pd/HZSM-5 catalyst, as was observed during the first 10 hours on stream during catalysts stability testing (Figure 1).

9


Pd

24 20 16 12 8 4 0 1 2 3 4 5 6 7 8 9 Particle size (nm)

Pd 10 8 Frequency %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Frequency %

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6 4 2 0 1

4 7 10 13 16 19 Particle size (nm)

Fig 4. TEM images and palladium particle distribution for fresh Pd/HZSM-5 (top) and TEM images of 100 h used Pd/HZSM-5(bottom)

10


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Fig. 5 NH3-TPD desorption over unmodified HZSM-5, Pd/HZSM-5 and 100 h used Pd/HZSM-5

XAFS suggest that besides the larger nanoparticles observed, the Pd species are also stabilised in cationic positions in the vicinity of aluminium atoms. Hence, in agreement with the literature 8, 16, the Brønsted acid sites of HZSM-5 are crucial to anchoring the PdOx species to the active sites. The NH3-TPD experiments (Figure 5) produced results which are consistent with this. In the case of HZSM-5 (Si/Al=140), two peaks were detected, one at low temperature (LT) is assigned to a weak Brønsted sites 31-32

and the high temperature (HT) one is attributed to a strong Brønsted sites 8, 33. The HT peak (Figure

5) for the NH3-TPD desorption over palladium oxide on HZSM-5 shifted to lower temperatures and its intensity decreased in comparison with the corresponding peak for pure HZSM-5, indicating a reduction in the strength and the number of strong acid sites. Previous studies have reported that post modification of HZSM-5 by oxides can affect the surface acid site’s strength8, 16, 34. This is consistent with the assertion that strong Brønsted acid sites are exchanged by Pd(Ox) species. Pd(Ox) cationic species are

11


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exchanged for protons, removing acid sites

16

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consistent with TPD analysis. This observation concurs

with that reported for NH3-TPD measurements on pure HZSM-5 and Pd/HZSM-58 and our XAFS measurements. Lewis acid sites ( the cations) are generated from Brønsted acid sites by this process as has also been suggested by others

8, 35

. The concentration of adsorbed NH3 on used Pd/HZSM-5 is

considerably lower than that observed in fresh Pd/HZSM-5. This is related to dealumination resulting in the presence of extra-framework aluminium species and their agglomeration (see below).

Cluster A

Cluster B

Pd

Pd

C

C

12


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O

O

Fig 6. EDS investigation of Palladium and carbon distribution on used catalyst for two different clusters

3.2 Structural changes of catalyst during long-term VAM oxidation under humid condition The Fourier transforms of the Pd K-edge XAFS of Pd/HZSM-5 before and after 100 h time on stream during VAM oxidation are displayed in Figure 3. Although the Fourier transforms are somewhat similar, there are subtle differences between the calcined catalyst and PdO. During the TOS, it is evident that the originally nano sized palladium oxide, which was distributed throughout the catalyst, becomes increasingly similar to that of the bulk oxide and which is indicative of the agglomeration of smaller to larger particle sizes. At the same time, there is no evidence of the presence of metallic phase palladium on the surface of the used catalyst. After 100 hours, there is an increase in the peak intensity of the Pd— Pd oxide (Pd—O—Pd) distance, which can be interpreted as an increase in the Pd—Pd distances of 3.0 Å from 2.5 to 3.4 but at 3.4 Å increased even more, from 3.4 to 7.9 for the fresh and used catalysts respectively. These results are consistent with an increasing level of particle agglomeration of palladium oxide, which is confirmed by TEM-EDS. Figure 6 shows the distribution of palladium for two different clusters on used Pd/HZSM-5 catalyst. According to the EDS result, the agglomeration of palladium as large sized clusters is clear. Also, it was found that carbon deposits are located in the same spatial positions as palladium on the catalyst. This result suggests that a palladium-carbon species formed on the surface of the used catalyst during long-term methane oxidation. This speculation is investigated further by EXAFS analysis. In table 2, the result of curve fitting used catalyst in presence and absence of carbon atom is presented. A (statistically) significantly improved fit to the Pd K-edge XAFS of used Pd/HZSM-5 was achieved by introducing a C atom into the second shell (fit index decreases from 0.00050 to 0.00036). The Pd—C bond was observed at 2.09 Å and 3.09 Å. This range of Pd-C distance is reported previously for organo palladium compounds. In addition, carbon

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XANES result showed the presence of carbon bonding with palladium on the used catalyst comparison with fresh catalyst. Vicente et al 36 investigated palladium complexes and their crystal structure by X-ray and IR data. They found the Cl—Pd—C—O and Cl—Pd—C—C bonds with the Pd-C at bond distances of 1.85 Å and 2 Å, respectively36, which is consistent with the current findings. Feltham et al 37 synthesized carbonyl complexes of palladium in which the palladium atoms were located at the corners of a distorted tetrahedron in which five of the six edges are bridged by the carbonyl ligands. They reported different distances of Pd—C—O bond with the Pd—C at the distance of 1.99 Å, 2.096 Å and 2.047 Å. Also, Davies et al 38 reported that the O—Pd—C—C bond with the Pd-C at distance of 1.98 Å, 2.14 Å and 2.28 Å for different membered transition state. Therefore, it is suggested that according to these XAFS fitting results, a Pd carbon bond length of 2.09 Å is feasible. Also, there is a Pd-C bond distance of 3.09 Å which is slightly higher than the Pd-C distance for palladium carbide, which is reported in the range of 2.74 Å to 2.78 Å based on the reported EXAFS curve fitting39. Thus, in addition to agglomeration occurring during long term reaction on stream, carbon deposition and dealumination occurs in parallel and contribute to the deactivation process. Moreover, carbon deposition on the used catalyst was investigated by thermogravimetric analysis (TGA) in an argon (non-oxidative) flow. Figure 7 displays the CO2 signal detected by a mass spectrometer at the inert gas of TG analysis of used catalysts. As is clear from the data, CO2 is generated over three temperature ranges: 200 °C-300 °C, 300 °C -400 °C and over 450 °C. The CO2 peaks at high temperatures suggest the presence of carbon on the surface of the used catalyst. The CO2 released at the lower temperature range (200 °C -300 °C) may be due to physically adsorbed carbon dioxide which formed in the pores of the catalyst during the methane combustion or as a result of decomposition of metal carbonyl complexes. This argument is consistent with Fillman’s work on thermogravimetric analysis of transition metal carbonyl complexes [30]. The CO2 evolution peaks observed at a relatively high temperature range (300-400°C) and over 450°C could be assigned to decomposition of metal carbonato complexes similar to the peaks reported in the literature [31]. The peak occurred over 450°C is unlikely to belong to carbide since palladium carbide with metastable Pd-C phase decomposes above 600°C in an inert atmosphere [19]. Therefore, these TGA give support to the presence of palladium carbonaceous complex on the surface of the used catalyst. This speculation was further substantiated by FTIR analysis, assigning carbonato monodendate and carbonyl stretching on the surface of used Pd/HZSM-5 catalyst which will be discussed in detail in a subsequent publication.

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Intensity(a.u)

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.

100 200 300 400 500 600 700 800 900 1000 Temperature(°C)

Fig.7 TG in argon result of CO2 desorption from used Pd/HZSM-5 catalyst According to these finding, it is proposed that the presence of carbon in the vicinity of palladium promotes the deactivation of the catalysts. As the intrinsic data showed that the TOF of fresh catalyst at a selected temperature of 360 °C decreased from 0.041 s-1 to 0.024 s-1 for used catalyst and increased to 0.031 s-1 following reactivation (Table 1). Since the TOF of the used catalyst was less than that over the fresh catalyst it can be concluded that the chemical environment, presumably the carbon deposits, play an important role in influencing the intrinsic catalytic activity. The Arrhenius plots indicated a higher rate of oxidation per active site for reactivated Pd/HZSM-5 compare to used Pd/HZSM-5 indicating the catalyst could partially retain its initial activity after removing carbon from the surface of the used catalyst. Therefore, it can be concluded that carbon deposition is an important contributor to catalyst deactivation and causes a partial but reversible deactivation during long-term VAM combustion. The additional process of agglomeration, with concomitant migration of PdO, is supported by the TEM images (Figures 4). Fresh, calcined catalyst contains small nanoparticles (2 to 4 nm) of palladium oxidecontaining particles and cationic species that are highly dispersed over the zeolite surface (including the pores). According to nitrogen adsorption-desorption isotherms, both mesopores and micropores are present in the structure of the zeolite pore size ranging from 1.9 nm to 21.1 nm based on estimation of the BJH adsorption distribution of pores. Hence, during the preparation of catalyst by loading palladium nitrate solution, there is a possibility for Pd2+ cations to transport inside the micropores as well as on the surface of the zeolite. A shift of desorption curve (Fig. S1) at a relative pressure ranging from 0.1 to 0.35 and 0.45 to 0.95 is observed, which suggests the coexistence of micro and mesopore within our 15


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sample. The coexistence of micro and mesopores on HZSM—5 is consistence with previous reports in the literature 40. As a result, the specific surface area and pore volume of Pd oxide-modified on HZSM-5 decreased which is consistent with the assertion that some PdO particles entered the mesopore channels of support. This finding is compatible with data about textural properties of metal oxide modified on HZSM-5 catalyst reported in a recent publication34. The palladium particle distribution of fresh catalyst and used catalyst is also shown in Figure 4. Statistical analysis reveals the distribution of Pd particles in the range of 1 nm to 9 nm on fresh catalyst while over the used catalyst the range increases up to 20 nm. The dispersion and particle size are consistent and, since the latter have fundamentally different properties from bulk particles (because their surface areas increasingly dominate over volume as size decreases from around 10 nm), exhibiting enhanced chemical activity

41

. Conversely, in the used catalyst, the small nanoparticles agglomerate into much larger

particles (up to 20 nm) accompanied by a lower level of dispersion (Figure 4). Note the TOF also changes, suggesting a size-independent deactivation. XRD and TEM analysis of the support suggest a variety of palladium particle size and level of metal dispersion. The change is quite evident, as shown in the TEM Figure 4 and in the diffractograms (Figure 8). The reflection at around 33.8°-33.9° 2θ degree has been assigned to the [101] plane of PdO 42-43 and establishes the presence of the oxide in the used catalyst. Furthermore, the observation of a reflection in the used catalyst (after 100 hours) shows that particle sizes increase. The XRD results are complemented by the TEM images which, more explicitly, shows this evolution. The presence of excess oxygen in the VAM stream renders the palladium to be in the oxidized form. The accumulation of PdO was shown by increasing intensity of Pd-Pd oxide phase in Pd K-edge spectra in Figure 3 and PdO reflection in XRD results for 100 h used catalyst. Table 3 shows the surface area, pore volume for calcined HZSM-5, fresh and used Pd/HZSM-5 as determined by nitrogen adsorption experiments. A significant variation in the textural properties of the catalyst was observed after 100h time on stream (TOS) in which catalyst lost approximately 50% of its surface area and pore volume. The gradual deactivation following 10 hours of reaction, with a decreasing surface area can be explained by gradual degradation of the pore structure in HZSM-5. For a better understanding of the structural changes of the catalyst, Al XANES was used to examine the change in the Al sites of the HZSM-5 support.

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Fresh Pd/HZSM-5 100h Used Pd/HZSM-5 Support HZSM-5 calcined

Intensity(a.u)

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31

33

35

37

39

41

2ϴ, degrees

Fig 8. XRD pattern of calcined HZSM-5, fresh and 100 h used Pd/HZSM-5, 2θ range 31-41°

Table 3. Surface are and pore volume of calcined and used Pd/HZSM-5 catalyst

Langmuir surface Sample

2

Pore volume

Area(m /g)

(cm³/g)

HZSM-5, calcined

473

0.193

Pd/HZSM-5, Fresh

457

0.188

Pd/HZSM-5, Used 100 h

228

0.076

3.3 Al XANES investigation of fresh and used catalyst Figure 9 illustrates the significant change in the Al-XANES before and after long-term reaction. The XANES spectrum of aluminium for the fresh catalyst is similar to HZSM-5, and the spectrum of the used catalyst is similar to that of alumina

44

which suggest that dealumination has occurred in the used

catalyst. It seems likely that exposing Pd/HZSM-5 to a high level of super-heated steam over the course of a long-term reaction in the VAM oxidation leads to a loss of lattice aluminium. Although the extent of dealumination is usually minimal at low temperatures, it would otherwise be consistent with previous reports on the dealumination of HZSM-5 by steaming

45-46

. Steaming conditions, such as temperature

and duration, also affects the dealumination of HZSM-5 47-48. Zhang et al 48 examined the dealumination of HZSM-5 at several temperatures (400°C, 500°C and 700 °C) with 100% water vapour for 2 h. They found that for all samples the extent of dealumination depends on temperature and duration of the 17


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treatment. It is therefore suggested that in the present study the dealumination of Pd/HZSM-5 takes place at 400°C under conditions of high humidity and that an increase in the duration of the reaction up to 100 h results in additional loss of aluminium species from the lattice. The Al-XANES spectrum of fresh Pd/HZSM-5 is almost identical to that of fresh HZSM-5, indicating the presence of typical tetrahedrally coordinated aluminium atoms 49-50. During the hydrothermal treatment of the catalyst, the tetrahedrally coordinated lattice aluminium in the HZSM-5 frameworks changed to lower symmetry aluminium species. The left arrow at 1567 eV indicates the position of the tetrahedrally coordinated aluminium while the right arrow shows the position of an octahedrally coordinated aluminium species 49. Comparison of the spectra of the fresh and used catalysts shows that the intensity of the peak at 1570-1575 eV for the former is higher than that at 1567 eV for the latter thereby indicating a change from tetrahedral to octahedral coordination. Therefore dealumination involves rearranging the alumina-silicate framework. This leads to destruction of acid sites in the catalyst accompanied by palladium oxide species separating from their framework aluminium anchors. This process accelerates migration and agglomeration of palladium species at the surface of the catalyst. The agglomeration of palladium oxide has been confirmed by TEM, XRD and XAFS, vide infra. It is suggested that this dealumination process occurs slowly during hydrothermal stability testing of catalyst and caused the gradual deactivation of Pd/HZSM-5 in presence of water vapour during TOS experiment. The dealumination of Pd/HZSM-5 during VAM oxidation is supported by the NH3-TPD experiments as shown in Figure 5. The intensity of the NH3-desorption peaks for used Pd/HZSM-5 is significantly lower compared with fresh Pd/HZSM-5. For the former, the intensity of the Brønsted acid sites decreases due to the dealumination of the support and the elimination of Brønsted acid sites and the intensity of the Lewis acid sites also decreases because of agglomeration of extra framework aluminium during 100 h reaction. This finding is compatible with previous observations 47, 51. Moreover, dealumination caused reconstruction of palladium species along with agglomeration of palladium on the surface of the HZSM-5. To better understand this reconstruction of the palladium oxide (carbonyl/carbonato) phase, the diffraction pattern of palladium particles was investigated by TEM analysis.

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Fig 9. The XANES at the Al k-edge of fresh and used Pd/HZSM-5

3.4 Diffraction pattern of palladium species Diffraction experiments on nanoparticles was performed and disclosed the inequivalent low-index surface orientations of the tetragonal structure of PdO in fresh and used Pd/HZSM-5. The small size of the palladium nanocrystallites in the fresh catalyst do not exhibit long range order and therefore do not diffract. The Fast Fourier Transform (FFT) of the TEM image (Figure 10) enables extraction of the atomic arrangement for the fresh catalyst. In order to improve statistics, 5 different zones were examined. Figure 10 shows that when palladium particles are exposed on the surface of catalyst and calcined at 550 °C, the (111) crystalline planes develop preferentially for PdO at the surface Pd-O-Al nanocrystallites. This is consistent with the results reported by Colussi et al.

52

of a HRTEM analysis on

calcined Pd/CeO2 in which the PdO particles developed essentially from (111) planes in the nano-surface structure of Pd-O-Ce. Due to the increased palladium oxide crystallite sizes in the used catalyst, the nano-diffraction technique is better suited for studying the crystal planes. The palladium-containing phases in the fresh catalyst are dominated by (111) planes while the used catalyst displays three diffraction planes namely (111), (101) and (100) planes in its structure (Figure 11, 12 and 13). It is concluded that the zeolite gradually degrades when undergoing steaming during the 100 h VAM oxidation thereby promoting the agglomeration of palladium species with formation some other low index surface orientation of carbon 19


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containing Pd[O][C] species with low index planes similar to PdO (101) and PdO (100) which are suggested to be inactive palladium species, most likely associated with carbon deposits as observed with microprobe analysis.

Fig. 10 FFT pattern of a palladium particle in Fresh Pd/HZSM-5

Used PdO 111

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Fig. 11. Diffraction pattern of a palladium particle in used Pd/HZSM-5

Used PdO 101 Fig. 12. Diffraction pattern of a palladium particle in used Pd/HZSM-5

Used PdO 100

Fig. 13 Diffraction pattern of a palladium particle in used Pd/HZSM-5 21


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4. Conclusions In this paper we investigated the structural changes observed in Pd/HZSM-5 during long-term VAM combustion. A contribution of two different mechanisms for catalyst deactivation were concluded. One involved a reversible deactivation caused by carbon deposition which resulted in significant reduction of the activity (TOF) of Pd. The other, a more gradual deactivation of the catalyst (VAM combustion over 100 h) was coincident with the agglomeration and migration of PdO species of the used catalyst to the limited number of active sites, which has been detected by TEM analysis. Therefore, the reaction of CH4 over the highly dispersed PdO during long-term reaction results in the significant change in local structure of Pd, as shown in XAFS analysis. The interaction between PdO and acid sites of H-ZSM-5 through anchoring Pd2+ cations near Al in the structure of acid sites of zeolite was confirmed by XAFS and NH3-TPD analysis. Long-term reaction in presence of water vapour also caused a notable loss in surface area and pore volume as determined by N2 physisorption as well as a significant change in Al XANES due to loss of the tetrahedrally coordinated aluminium and formation of octahedrally coordinated aluminium. Evidence of carbon deposition is based on EDS, TGA, XAS and IR spectroscopy. Removal of all carbonaceous compound through reactivation of catalyst resulted in a higher TOF (0.031 s-1) compare to TOF (0.024 s-1) for used Pd/HZSM-5 indicating the catalyst could regain 75% of its initial activity. We propose the formation of a palladium carbonaceous complex involving carbonate and carbonyl group present in used Pd/HZSM-5 which acts as an important contributor of catalyst deactivation and causes partial reversible deactivation during long-term VAM combustion.

5. Supporting Information The Supporting Information is available free of charge on the ACS Publications website: Nitrogen desorption-adsorption isotherm for HZSM-5 as support Thermogravimetric analysis of used Pd/HZSM-5 catalyst under air flow

5. Corresponding Author Michael Stockenhuber [email protected] Telephone: (+61) 2 4985 4433 22


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6. Acknowledgment We would like to acknowledge the Australian Coal Assessment Research Program (ACARP) for their financial support of this study. Special thanks goes to the ESRF for providing beamtime and the Australian Synchrotron for a travel grant. HH gratefully acknowledges financial support in the form of a postgraduate scholarship provided by University of Newcastle. Also, we would appreciate Jenny Zobec and Huiming Zhang from the X-ray Unit at UON for their help in TEM analysis; and special thanks to Luke Harvey and Matthew Drewery for their help in XAFS experimental work at the ESRF (Grenoble); and Bruce Ravel for Athena code and Norman Binsted for excurv98 code used for analysis of XAFS data.

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