triple-oxide-support (TOS) oxidation catalyst

Applied Catalysis A: General 489 (2015) 24–31

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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata

Strong synergism between gold and manganese in an Au–Mn/triple-oxide-support (TOS) oxidation catalyst Arshid M. Ali a , Muhammad A. Daous a , Ahmad A.M. Khamis a , Hafedh Driss a , Robbie Burch b , Lachezar A. Petrov c,∗ a

Department of Chemical and Materials Engineering, Faculty of Engineering, King Abdulaziz University, Jeddah, Saudi Arabia School of Chemistry and Chemical Engineering, Queens University Belfast, Northern Ireland, UK c SABIC Chair in Catalysis, King Abdulaziz University, Jeddah, Saudi Arabia b

a r t i c l e

i n f o

Article history: Received 5 May 2014 Received in revised form 2 October 2014 Accepted 4 October 2014

a b s t r a c t A strong synergism between Au and Mn substantially increased the catalytic activity of the Au–Mn/TOS catalyst for the complete propane oxidation as compared to the activity of mono metal Au/TOS and Mn/TOS catalysts. The high activity of Au–Mn/TOS catalyst is a result of the formation of Au2 Mn or Au5 Mn2 phases and enhanced mobility of the oxygen in the support oxides. XPS, TPR, TEM and XRD studies were used to characterise the catalysts. Based on the results, a possible mechanism for the participation of the non-stoichiometric and lattice oxygen is also proposed. It is suggested that a joint effect of the Au and Mn increased the non-stoichiometric and lattice oxygen mobility of the Au–Mn/TOS catalyst, potentially being dominated by Mn. © 2014 Published by Elsevier B.V.

1. Introduction In the last two decades, much attention has been paid to the treatment, management and eradication of toxic and hazardous materials [1] and effluent gases [2] from various sources. Volatile organic compounds (VOCs), such as propane, are one of the pollutants of concern [3]. Direct combustion has been used to eliminate VOCs, but this approach is expensive [4]. More recently, most studies [5–12] in attempting to reduce VOC emissions and meet the increasingly stringent environmental quality standards (EQS) have focused on catalytic oxidation. Various combinations of metal oxides and supported metals (both precious and nonprecious) have been investigated for their potential to act as efficient oxidation catalysts. Among these combinations, ceriumoxide-supported catalysts [5,13–16] are the most popular choice for the catalytic oxidation of VOCs. The mixed ceria–zirconia oxides have proved to be efficient catalyst carriers [17] because of the high surface area and the good redox properties. Since the discovery of the high catalytic activity of Au nanoparticles supported on high surface area metal oxide supports,

∗ Corresponding author at: SABIC Chair of Catalysis, King Abdulaziz University, P.O. Box 80204, Jeddah 21589, Kingdom of Saudi Arabia. Tel.:+966 507589382; fax: +966 26952257. E-mail addresses: [email protected], [email protected], [email protected] (L.A. Petrov). http://dx.doi.org/10.1016/j.apcata.2014.10.006 0926-860X/© 2014 Published by Elsevier B.V.

supported Au has been extensively studied as a heterogeneous catalyst in both the gaseous and liquid phases. Supporting of Au nanoparticles on ceria–titania [18] and ceria–zirconia [19] has been found to be effective for the elimination of VOC compounds because of the activity and influence of Au on the reduction of ceria–titania or ceria–zirconia supports. However, Au catalysts for this reaction are less efficient than Pt catalysts, so further improvement is needed. The availability of highly dispersed Au nanoparticles on the metal oxide supports could increase the lattice oxygen mobility and the weakening of the metal oxide bonds. This may lead to the release of the lattice or mobile oxygen [20,21] that could participate in an oxidation reaction via a Mars–Van Krevelen mechanism [20,22–25]. Like many transition metal oxides, the manganese oxides MnO, MnO2 , Mn2 O3 and Mn3 O4 have labile oxygen atoms that may participate in oxidation processes. Lahousse et al. established that ␥-MnO2 catalyst is active for the abatement of VOCs [3]. Some studies have also stressed the importance of the positive role of manganese in Mn/CeO2 catalysts [26]. Some of the published papers [17–20,27–29] have established that gold as well as manganese are components of active oxidation catalysts for VOC. Therefore, it was logical to expect that supported bi-component gold–manganese system could have the potential to become active oxidation catalysts. Our preliminary experimental data had shown that the Au–Mn system supported on a mixed oxide carrier composed of CeO2 ,

A.M. Ali et al. / Applied Catalysis A: General 489 (2015) 24–31

ZrO2 and TiO2 had high catalytic activity in the total oxidation of propane. Therefore, to try to elucidate the effect of the individual “active” components, either acting alone or in combination, we have investigated Au alone, Mn alone, and the combination of Au and Mn supported on a triple-oxide support of CeO2 , ZrO2 and TiO2 were the object of our study. The samples were characterised by a number of methods before and after the propane oxidation reaction. To the best of our knowledge, this is the first study to report on the catalytic properties of this system which shows a remarkable synergy between Au and Mn on this triple oxide support.

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using consecutively 0.5 and 0.1 M HNO3 .The pH tuning was done by adding specified amounts of the Mg-citrate solution. The ageing of the suspension was carried out for 1 h at 60 ◦ C and 180 rpm. The freshly prepared sludge was rinsed multiple times with lukewarm DIW under vacuum. Again, the removal of chloride ions was tested using AgNO3 as a probe, to yield a chloride-ion-free sludge which was dried at 120 ◦ C and then calcined at 525 ◦ C for 4 h. The catalyst grains (250–500 ␮m) were then prepared by grinding the pelletised catalysts. For convenience, the catalysts are labelled as follows: 1% Au on the mixed triple oxide is Au/TOS; 1% Mn is Mn/TOS; and the 1% Au + 1% Mn is Au–Mn/TOS.

2. Experimental 2.1. Materials The following chemicals were used: cerium (IV) oxide (99.9%, Acros Organics), zirconium (IV) oxide (Sigma-Aldrich), titanium (IV) oxide (Sigma-Aldrich), potassium hydroxide (86%, Fluka), hydrogen tetrachloroaurate (III) hydrate (Acros Organics), manganese (II) chloride tetrahydrate (extra pure, Scharlau), magnesium citrate tri-basic nonahydrate (Fluka), nitric acid (69%, Fisher Scientific) and deionised water (DIW, ELGA). 2.2. Catalyst preparation The Au–Mn-containing catalysts were prepared in three steps: (i) preparation of the triple-oxide support, (ii) Mn impregnation and (iii) Au deposition-precipitation. The Mn-only catalysts were prepared using steps (i) and (ii), while the Au-only catalysts were prepared using steps (i) and (iii). The preparation details are summarised as follows. 2.2.1. Support preparation The triple-oxide support (TOS) used is a mechanical mixture of CeO2 (22 nm), ZrO2 (70 nm) and TiO2 (73 nm). Based on initial trial experiments, accurately weighed amounts of each oxide (5.5 g of CeO2 , 2.5 g of ZrO2 and 2.0 g of TiO2 ) were ultrasonically mixed for 1 h in a Power Sonic 410 instrument at a high sonication frequency at room temperature. The well-mixed catalyst support was dried in an oven for 2 h at 120 ◦ C before use in the impregnation step. 2.2.2. Manganese impregnation A solution (120 cm3 ) containing the stoichiometric amount of Mn obtained from the precursor of extra-pure MnCl2 in deionised water (DIW) was added to the Rotavapor flask containing 10 g of the support. Both the MnCl2 solution and the support were preheated to 65 ◦ C. Impregnation was performed using a Hiedolph Hei-VAP Rotavapor at 65 ◦ C and 75 rpm. The 1%-Mn-impregnated support (Mn/TOS) was then thoroughly rinsed using slightly warmed DIW to remove the chloride ions. AgNO3 was used to test for the complete removal of chloride ions. Mn/TOS was then dried for 3 h in an oven at 120 ◦ C before further use as a catalyst, or for preparation of gold-containing catalysts by the deposition-precipitation method. 2.2.3. Gold deposition-precipitation on the manganese-impregnated support The deposition-precipitation of gold on Mn/TOS was carried out in a Mettler Toledo Labmax reactor, which permits the precise control of the pH, temperature, liquid reagent delivery and mixing of the suspension. First, 13.5 cm3 of 0.1 M KOH and 3.86 cm3 of DIW per gram of the freshly prepared Mn/TOS were added to a Labmax reactor flask and mixed at room temperature at 180 rpm at pH 12.7. The reactor temperature was raised to 60 ◦ C before adding the precisely calculated amount of 1% Au in 10 cm3 DIW using HAuCl4 as the gold source. Very soon after the addition of Au, the pH was decreased first to 10.4 and then to 8 within the next 5 min

2.3. Catalyst testing The catalytic activity experiments were carried out in a PID Microactivity Reference reactor. All of the catalytic tests for propane (0.5 vol.% propane in He) oxidation were carried out in a 4-mmdiameter quartz glass reactor charged with 0.5 g of the calcined (525 ◦ C in air) catalyst at temperatures between 50 and 500 ◦ C and 12,000 GHSV. In each experiment, the temperature was increased in 50 ◦ C steps from 50 to 500 ◦ C at a ramping rate of 10 ◦ C/min. The steady-state regime at each temperature was achieved after 45 min. The gas flow rates were controlled by Bronkhorst mass flow controllers with an accuracy of 0.1% of the fixed value. The temperature was kept constant within 0.5 ◦ C and monitored throughout the catalytic experiment using PID Microactivity Reference software. The reaction products were periodically analysed using an intermittently connected GC. Three analyses of reaction products were recorded for each reaction condition to monitor the repeatability of the experimental measurements. In addition, all of the catalysts were used at least twice to check the reproducibility and were found to be highly reproducible (standard variance ±1) under the set parameters of the PID Microactivity apparatus. C3 H8 , O2 , CO2 and H2 O were the only components of the reaction mixture at the reactor exit in all experiments. An Agilent 7890A GC system equipped with a flame ionisation detector (FID) and thermal conductivity detector (TCD) was used to identify and monitor reaction products. Data processing was performed using GC ChemStation® B.04.03 (54). A 0.5-cm3 gas sample, Agilent DB1/122-1063 (1 ␮m, 60 m × 0.25 mm) and HayeSep Q (8 m) columns and He and N2 carrier gases were used for FID and TCD, respectively. 2.4. Catalyst characterisation X-ray photoelectron spectroscopy (XPS) measurements were carried out in an ultra-high vacuum multi-technique surface analysis system (SPECS GmbH, Germany) operating at base pressures of 5 × 10−9 mbar. A standard dual-anode X-ray source (SPECS XR50, Al K␣, 1486.6 eV) was used to irradiate the sample surface (13.5 kV, 100 W) at a 90◦ electron take-off angle relative to the sample surface plane. The wide-scan survey spectra and highenergy-resolution narrow-scan spectra were recorded at room temperature with a 180◦ hemispherical energy analyser (PHOIBOS150 with nine single-channel electron multipliers, MCD-9). The analyser was operated in fixed analyser transmission (FAT) and medium-area lens modes with a 40-eV pass energy, 1-eV step size (for survey scans) and 20-eV pass energy and a 0.025-eV step size (for narrow scans) with a 0.1 s dwell time. The adventitious hydrocarbon C 1s line (285 eV), corresponding to the C–C bond, was used as the binding energy reference for charge correction. The highenergy-resolution spectra were obtained under analysis conditions that yielded an FWHM = 0.85 eV for the Ag 3d5/2 signal of a freshly argon-ion-etched silver (Ag) sample.

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A.M. Ali et al. / Applied Catalysis A: General 489 (2015) 24–31 100 90

Au-Mn/TOS

80

Au/TOS Mn/TOS

% Conversion

70

TOS 60 50 40 30 20 10

0 150

200

250

300

350

400

450

500

Temperature (º C) Fig. 1. Catalytic activity comparison for the propane total oxidation by Au/TOS, Mn/TOS and Au-Mn/TOS and TOS catalysts.

The transmission electron microscope (TEM) analysis was performed using a Tecnai G2 F20 Super Twin at 200 kV with a LaB6 emitter. The microscope was fully equipped for analytical work with an energy-dispersive X-ray (EDX) detector with S-UTW window and a high-angle annular dark-field (HAADF) detector for STEM imaging. Unless stated otherwise, the scanning transmission electron microscopy (STEM) imaging and all analytical work were performed with a probe size of 1 nm resulting in a beam current of about 0.5 nA. TEM images and selected area diffraction (SAD) patterns were collected on an Eagle 2K HR 200 kV CCD camera. The HAADF-STEM EDX and CCD line traces were collected fully automatically using the Tecnai G2 User Interface and processed with the Tecnai Imaging and Analysis (TIA) software Version 1.9.162. X-ray diffraction (XRD) was performed using an Inel EQUINOX 1000 powder diffractometer equipped with CPS 180 detector (filtered Cu K␣1 radiation, 30 kV, 30 mA, spinning sample holder). The powder pattern analyses were processed using Match© Crystal Impact software (v.1.11e) for phase identification (using both COD and ICDD databases), IMAD-INEL© XRD software (v.4.8) for the graphical illustrations and MAUD© software for the Rietveld analysis. All of the data were collected under the same conditions within 24 h. H2 -TPR was performed on ChemBet Pulsar TPR/TPD/TPO instrument. 0.2 g of the calcined (525 ◦ C in air) sample was first dried to 150 ◦ C for 60 min and then cooled to 40 ◦ C. H2 -TPR was performed using a mixture of 10% hydrogen in nitrogen. Temperature was ramped to 600 ◦ C at a rate of 10 ◦ C/min. The consumption of hydrogen was measured with a TCD detector. 3. Results 3.1. Catalytic activity A detailed comparison of the catalytic activity of the Au-Mn/TOS, Au/TOS and Mn/TOS catalysts and of the TOS support is provided in Fig. 1. The results show that although the TOS alone had good catalytic activity at high temperatures, at 300 ◦ C less than 5% propane conversion was achieved. This value reached 10% at 400 ◦ C and then increased to nearly 90% at 450 ◦ C. The catalytic activity of the Mn/TOS catalyst at temperatures up to 500 ◦ C is practically identical to that of the support. These results show that at the level of Mn used in our promoted Au catalysts, the Mn in the Mn/TOS catalyst contributes almost nothing to the activity of the catalyst at temperatures up to 500 ◦ C.

The Au/TOS catalyst has moderate catalytic activity. Oxidation began above 200 ◦ C, and about 75% propane conversion was observed at 400 ◦ C. The catalytic activity is substantially higher than that found for the Mn/TOS catalyst. However, at 95% conversion the curves for both the Au/TOS and Mn/TOS catalysts converge. These results indicate that the Au in the Au/TOS catalyst is making a significant contribution to the activity of the Au/TOS combination. The catalytic conversion of propane on the supported Au–Mn/TOS catalyst began somewhere below 150 ◦ C and then strongly accelerated at temperatures above 225 ◦ C. Indeed, 95% conversion was observed at 375 ◦ C and complete conversion of propane occurred with the Au–Mn/TOS catalyst at 400 ◦ C. Based on these results, it may be concluded that the joint presence of Mn and Au in the Au–Mn/TOS catalyst leads to the formation of a system with strongly enhanced catalytic activity for the propane oxidation. Therefore, this points to a strong synergism between Au and Mn. The observed synergism resulting from the interaction of Au, Mn and the triple-oxide support has not been reported in the previous literature. The nature and mechanism of this interaction has yet to be understood. In comparison to the other studies [30–33] on the total propane oxidation by using either oxides catalysts or supported noble metal catalyst, none of the study, as per author knowledge, is reported under similar catalytic reaction conditions. However, based on GHSV (12,000), 50% propane catalytic conversion by Au–Mn/TOS is achieved at 310 ◦ C as compared to 50% catalytic conversion by transition metal oxides catalyst (SAS 10 vol% water derived catalyst) at 175 ◦ C and GHSV (6000) by Marin et al. [30]. Wu et al. [32] concluded 50% propane catalytic conversion nearly at 240 ◦ C by using 200 mg of supported platinum and tungsten catalyst (Pt/WOx /Al2 O3 )–wellestablished fact on platinum group metals (PGM), comparatively very expansive, acts faster than the Au. Also, as per author’s knowledge, none of the study has yet identify or emphasised on the possible synergism between metals, either PGM or Au, with the other transition metal. 3.2. XPS measurements To investigate the interaction between Au and Mn and the possible impacts of each of these elements on the various sources of mobile oxygen from the TOS components CeO2 , ZrO2 or TiO2 , detailed XPS analyses were carried out before and after carrying out the reaction of propane oxidation. 3.2.1. XPS spectra of the catalysts before and after the propane oxidation reaction To investigate the oxidation state of the various components in the catalyst, XPS spectra were recorded for fresh and used catalysts. The results are shown in Fig. 2. There are no dramatic differences between the samples and it is difficult to see subtle changes in the spectra in these broad-spectrum profiles. However, enlarged deconvoluted sections of the spectra will be shown and discussed in the discussion section. 3.3. XRD analysis An XRD study was conducted for all of the catalysts and TOS before and after the catalytic reactions in order to find possible changes in the bulk structure of the TOS support. The results are summarised in Figs. 3 and 4. It can be clearly seen (Fig. 3A) that the XRD diffraction patterns remained entirely the same for the Au/TOS and Mn/TOS catalysts as well as for the TOS. However, when compared to the TOS, a notable change in the Au–Mn/TOS catalyst is observed in the 2 range of 37–39◦ (see Fig. 3B). This difference is maintained in the used catalyst as shown in Fig. 4 where the XRD fingerprints of the fresh and

Au-Mn Catalyst Au-Mn/TOS

8

x 100000

x 100000

A.M. Ali et al. / Applied Catalysis A: General 489 (2015) 24–31

A

7

4

Ti LMM

O KLL Ce 4s Ce 4p

O 1s

Ce 3d Ce MNN

Au 4f

3

0 1200

Mn 2p Ti 2s

Mn 2s

2 1

8 7

Support TOS

Ti 2p

Before Reaction After Reaction

O 2s

Zr 3s

Zr 3p C 1s

Zr 3d

800

600

0

Ce MNN

O 1s

3 2

200

O KLL Ti LMM Ce 3d

4

Ce 4dTi 3p

400

Ce 3d

5

1

1000

B

C KLL

6

5

Intensity (cps)

Intensity (cps)

6

27

Ti 2p

Ti 2s

Before Reaction After Reaction

Zr 3p C 1s Zr 3d Ti 3p Ce 4p Ce 4d O 2s

Zr 3s

0

Binding Energy (eV)

1200

1000

800

600

400

200

0

7

Au Catalyst Au/TOS

C

C KLL

x 100000

8

6 O KLL

Intensity (cps)

5

C KLL Ce 4s

Ce MNN

Ce 4p Au 4f

3 Ti 2s

2 1 0

Ti 2p

Ti 3p Zr 3p

Zr 3s

Before Reaction After Reaction 1200

1000

D

7

O 1s

Ce 3d

4

Mn Catalyst Mn/TOS

8

6

Ti LMM

C 1s

Zr 3d

O 2s

800

600

400

200

5

O KLL Ti LMM

4

Ce MNN

3

Ti 2s

0

1

Ce 4s

O 1s

Ce 3d

2

Ce 4d Ti 3s

Binding Energy (eV)

Intensity (cps)

x 100000

Binding Energy (eV)

Ti 2p

Mn 2p

Before Reaction After Reaction

O 2s Zr 3d Zr 3p Zr 3s C 1s Ce 4p Ce 4d Ti 3p

0 1200

1000

800

600

400

200

0

Binding Energy (eV) Fig. 2. Comparison of the overlaid survey XPS spectra before and after catalytic reaction. (A) Au-Mn/TOS, (B) TOS, (C) Au/TOS and (D) Mn/TOS catalysts.

Fig. 3. (A) Comparison of the overlaid XRD diffractograms: (a) Au–Mn/TOS, (b) Au/TOS, (c) Mn/TOS and (d) TOS catalysts. (B) Magnified diffractograms.

Fig. 4. Au–Mn phases present in the difference of XRD fingerprints between Au–Mn/TOS and TOS catalysts.

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A.M. Ali et al. / Applied Catalysis A: General 489 (2015) 24–31 60

Au-Mn/TOS Catalyst Mn/TOS Catalyst Au/TOS Catalyst TOS

Intensity (a.u)

45

30

15

0

0

100

200

300 Temperature (ºC)

400

500

600

Fig. 6. Comparison of H2 consumption of TPR profile of the Au–Mn/TOS, Au/TOS, Mn/TOS and TOS catalysts.

shown in Fig. 1. The most active catalyst for propane oxidation is also the most active catalyst for self-reduction by hydrogen. 4. Discussion Fig. 5. TEM images of Au–Mn/TOS catalysts. Presence of Au2 Mn compound on TOS surface, gold dispersion and particle size.

used Au–Mn/TOS catalyst were found to be identical. It seems clear that in combination, the Au and Mn generate new diffraction lines. The difference in the XRD patterns of the Au–Mn/TOS catalyst and the TOS consisted of five peaks (Fig. 4), most probably belong˚ Based on the Au–Mn lattice ing to a cubic system with a = 4.05 A. parameter of the Au solid solution as a function of Mn composition [34], the expected structure contained between 25 and 35% Mn. From the Au–Mn phase diagram [34], two probable structures, Au5 Mn2 or Au2 Mn, can be identified. They have an average crystallite size of 7 nm. Hence, based on the XRD results, the enhanced catalytic activity of the Au–Mn catalyst could be due to the presence of Au5 Mn2 and/or Au2 Mn. 3.4. TEM analysis To investigate the gold nanoparticle dispersion on the highly active gold containing catalyst, TEM analysis of Au–Mn/TOS was performed and the results are shown in Fig. 5. The images show well-dispersed gold nanoparticles (≈5–10 nm) on the TOS surface. The presence of Au–Mn together was established by EDX analysis. These particles were about 7 nm in size and seemed to be preferentially located near to the CeO2 and TiO2 surfaces. Quantification of the EDX results showed that the combined Au–Mn were in the ˚ which agreed with form of Au2 Mn (with cell parameter a = 4.053 A) results obtained from XRD calculations. 3.5. Temperature-programmed reduction (TPR) study The TPR studies were also performed to determine the effect of the presence of Au and/or Mn in the Au–Mn/TOS, Au/TOS and Mn/TOS catalysts on the reduction behaviour of triple oxide support. The TPR spectra are presented in Fig. 6. The TPR spectra showed that the H2 consumption for each of the Au–Mn/TOS, Au/TOS, Mn/TOS and TOS catalysts were different, although the Mn/TOS and TOS were quite similar in terms of the overall TPR profile. The amount of H2 consumption in each of the catalysts decreased in the following order: Au–Mn/TOS > Au/TOS > Mn/TOS ≈ TOS. The most noticeable difference is that the consumption of hydrogen in the temperature region around 150–350 ◦ C closely follows the differences in activity as

4.1. Role of the Ce, Au and Mn in the reaction of propane oxidation As shown in Section 2, the combination of catalytic and structural information from XRD, TPR and HRTEM point to an interaction between Au and Mn, and probably with ceria as important in explaining the enhanced activity of Au–Mn/TOS. To try to further define the nature of the interaction between the active components, we now examine in more detail the XPS results. 4.2. Impact of Au and Mn on the mechanism of lattice oxygen participation in the propane oxidation Based on the XRD results, discussed earlier, it was proposed that the joint presence of Au and Mn enhanced the catalytic activity of the Au–Mn/TOS system via the possible formation of Au5 Mn2 and/or Au2 Mn compounds. However, it was not clear how the presence of each of Mn and Au affects the available sources of oxygen present within the catalyst. To investigate this aspect, a study of the deconvoluted XPS oxygen spectra of the Au–Mn/TOS catalyst before and after the catalytic reaction was conducted (Fig. 7). For the TOS, four main oxygen peaks, corresponding to nonstoichiometric lattice oxygen, OTi (oxygen belonging to TiO2 ), OCe (oxygen belonging to CeO2 ), OZr (oxygen belonging to ZrO2 ) and an OH− peak were detected. The calculated atomic concentrations of each of the oxygen are summarised in Table 1. The oxygen distribution in the Au/TOS catalyst is different from the oxygen distribution in the triple-oxide support TOS. This result is due to the presence of Au, which had attracted part of the nonstoichiometric and lattice oxygen (see Table 1). At the same time, the OTi atomic content decreased, while the OCe atomic content was increased. Based on this observation, it can be assumed that both Au and Ce attracted the lattice oxygen mostly from TiO2 . The amount of OZr oxygen nearly remained the same. In the case of the Mn/TOS catalyst, part of the non-stoichiometric and lattice oxygen was attracted by Mn. However, the amount of oxygen attracted by Mn is larger than the amount of oxygen attracted by Au, which can be estimated by comparing the OMn atomic content with the OAu in the case of Au/TOS catalyst. This increase in the OMn was mostly at the expense of CeO2 only, as both the OTi and OZr contents remained unchanged. Therefore, both Au/TOS and Mn/TOS catalysts have different distributions of non-stoichiometric and lattice oxygen. The presence

A.M. Ali et al. / Applied Catalysis A: General 489 (2015) 24–31

B

A

TOS Support Intensity (cps)

Intensity (cps)

Support TOS

Mn catalyst Mn/TOS

Au-Mn /TOS Au-Mn ca talyst

534

Mn/TOS catalyst Mn

Au-Mn /TOS Au-Mn catal yst

Au/TOS Au catalyst

Au catalyst Au/TOS

535

29

533

532

531

530

529

528

527

535

534

533

532

531

530

529

528

527

526

525

Binding Energy (eV)

Binding Energy (eV)

Fig. 7. Comparison of the overlaid deconvoluted oxygen XPS spectra before (A) and after (B) reaction.

of Mn in the Mn/TOS catalyst affects the oxygen content of CeO2 while the Au in the Au/TOS catalyst has more affinity towards the oxygen of TiO2 . The activity tests have shown that the Au catalyst is more active than the Mn catalyst. However, the amount of oxygen attracted by Au in the Au/TOS catalyst is 2.5 times less than that of Mn in the Mn/TOS catalyst. In the Au–Mn/TOS catalyst, both Au and Mn attracted the nonstoichiometric, as well as the lattice oxygen belonging to CeO2 and ZrO2 , with the former being most strongly affected. Part of the free lattice oxygen was attracted also by TiO2 , which lead to the increased ratio OTi . Due to the combined action of Au and Mn, both are acquiring non-stoichiometric and lattice oxygen in a ratio 1:6. The total amount of oxygen attracted by joint action of Au–Mn is higher than the oxygen attracted in monometallic catalysts. However, the amount of oxygen attracted by Au in the Au–Mn/TOS catalyst was smaller by a factor of 2.5 times than the amount of oxygen attracted by Au in the Au/TOS catalyst while Mn in the Au–Mn/TOS catalyst attracted the same amount of oxygen Table 1 Summary of the atomic concentration of oxygen associated with each of the metal before and after reaction. Catalyst

Type of oxygen

Oxygen concentration in atomic % Before reaction

After reaction

Free lattice OTi

2.036 30.073

1.697 32.296

TOS

OCe OZr OH− OMn OTi

23.003 10.481 3.813 9.294 30.451

21.609 10.493 3.748 8.927 30.779

Mn/TOS catalyst

OCe OZr OH− OAu OTi

16.873 10.805 3.033 3.666 27.543

17.318 10.113 3.089 2.154 26.535

Au/TOS catalyst

OCe OZr OH− OAu OMn

26.578 10.129 3.514 1.49 8.898

28.619 10.447 3.529 1.479 8.03

OTi OCe OZr OH−

33.374 14.761 8.794 2.804

31.382 15.395 8.831 3.541

Au–Mn/TOS catalyst

as Mn in the Mn/TOS catalyst. Based on this, it is fair to conclude that Mn is promoting the mobility of the oxygen toward the Au+1 where it participates in propane oxidation. The role of ZrO2 in the triple oxide system is mainly to be a source of additional non-stoichiometric and lattice oxygen. The consumed oxygen is recovered by interaction of the gas-phase oxygen with oxides in the support. The probable mechanism of participation in the reaction of the lattice oxygen and other oxidant sources is summarised in Fig. 8. The TPR spectra of the samples confirm the conclusions reached based on XPS analysis. The TPR spectrum of the TOS shows that the H2 consumption begins at a temperature of 120 ◦ C. Up to 475 ◦ C, the amount of consumed H2 increased. In this temperature interval, only one welldefined intensive peak with a maximum at 529 ◦ C was detected. The Mn/TOS TPR spectrum has the same features as the TPR spectrum of TOS. The H2 consumption begins at 115 ◦ C and is slightly more intensive than consumption of TOS. In the temperature interval 115–450 ◦ C, a low-intensity peak was detected at about 200 ◦ C. This spectrum contains also one well-defined intensive TPR peak, which is slightly shifted to lower temperatures and its maximum appears at 510 ◦ C. Its area is about 20% larger than the area of the TOS reduction peak (see Table 2). The TPR spectrum of Au/TOS shows that the H2 consumption begins at a temperature of 35 ◦ C, much lower than for the TOS. The TPR spectrum of the Au/TOS catalyst contains a large single TPR peak. However, the maximum of this peak is registered at about 250 ◦ C much lower than the TPR peak of the Mn/TOS which was at 510 ◦ C. The area, an indirect representation of H2 consumption, of the Au/TOS TPR peak is 2.26 and 1.90 times larger than the area of TPR peak areas of the TOS and the Mn/TOS catalysts, respectively (see Table 2). The shift in the reduction peak of the Au/TOS towards lower temperature indicated that the presence of Au alone has a significant effect on the availability of the oxygen from the TOS. The TPR peak of the Au–Mn/TOS catalyst has complex character and it can possibly be deconvoluted into four smaller peaks (at temperature of 217, 255, 265 and 279 ◦ C, respectively). A large TPR Table 2 Summary of the consummated H2 in TPR studies. Catalyst

TPR peaks area (a.u.)

TPR peaks areas ratio (catalyst/TOS)

50% Catalytic conversion temperature (◦ C)

TOS Mn/TOS Au/TOS Au–Mn/TOS

270 321 612 1322

1.0 1.19 2.26 4.89

425 425 360 310

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A.M. Ali et al. / Applied Catalysis A: General 489 (2015) 24–31

Fig. 8. Schematic of the joint and individual impact of Au and Mn on the non-stoichiometric and lattice oxygen: (A) Au/TOS, (B) Mn/TOS and (C) Au–Mn/TOS on the oxygen mobility of the support.

peak was recorded in the spectrum of the Au–Mn/TOS catalyst at 279 ◦ C. The total area of this complex peak is 2.26 times larger than the TPR peak of the Au/TOS and 4.1 times larger than the TPR peak of the Mn/TOS sample (see Table 2). Based on the TPR reduction peak temperature, the peak obtained at 255 ◦ C could be because of the presence of Au only on the TOS—nearly the same temperature as we noticed in the case of the Au/TOS, whereas peaks at 217, 265 and 279 ◦ C could be because of the joint presence of Au and Mn. In comparison, the impact of Mn alone in the Mn/TOS catalyst, on the H2 consumption is very small, which means that the presence of Mn does not appear to influence substantially the oxygen mobility in any of the oxides of the support. This is not in agreement with the XPS for oxygen deconvolution above which shows that in Mn/TOS the Ce has lost a lot of oxygen. However, overall the TPR show that the most active catalyst (Au–Mn/TOS) is the one where the largest amount of labile (reactive) oxygen is observed in the temperature region where the activity of the catalyst is increasing rapidly with increasing temperature. The joint presence of Au and Mn shows both in the TPR and in the catalytic activity measurements, a clear indication of synergism between Au and Mn, which may be related to enhanced mobility of oxygen within the catalytic system and explain why the Au–Mn/TOS is active at lower temperatures.

5. Conclusions The catalytic activity of the Au/TOS catalyst in the total propane oxidation was higher than that of the Mn/TOS catalyst, which was in turn the same as that of the TOS. The Au–Mn/TOS catalyst exhibited the highest catalytic activity. In general, the high catalytic activity of the Au–Mn/TOS catalyst is reflected in a strong effect on the oxidation states of the Ce and Mn. XRD results showed that the joint presence of Au and Mn was in the form of bimetallic particles with an average size of 7 nm. TEM showed that this joint association between Au and Mn is preferably near the CeO2 and TiO2 surfaces of the TOS. The addition of Au to the Mn/TOS catalyst promotes the mobility of the non-stoichiometric and lattice oxygen of all of the support oxides, which enhanced the catalytic activity of the Au-Mn/TOS catalyst. The enhanced non-stoichiometric and lattice oxygen mobility was promoted by the formation of Aux Mny compounds—a clear indication of the strong synergism between Mn and Au. Based on the results, a possible mechanism involving the participation of the non-stoichiometric and lattice oxygen is also proposed—a joint effect of the Au and Mn being to increase the non-stoichiometric and lattice oxygen mobility.

Acknowledgements The authors would like to acknowledge the Deanship of Scientific Research of King Abdulaziz University in Jeddah, Saudi Arabia, for funding this project under grant No. D-005/431. The authors, therefore, kindly acknowledge the financial and technical support of university authorities. The authors would also like to acknowledge Raoof Ahmad for his technical support and assistance.

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