Oxidation of dichloromethane over Au, Pt, and Pt-Au containing catalysts supported on γ-Al2O3 and CeO2-Al2O3

molecules

Article

Oxidation of Dichloromethane over Au, Pt, and Pt-Au

Containing Catalysts Supported on γ-Al O and

2

3

CeO -Al O

2

2 3

Tuomas K. Nevanperä , Satu Pitkäaho, Satu Ojala and Riitta L. Keiski *

Faculty of Technology, Environmental and Chemical Engineering, University of Oulu, P.O. Box 4300,

FI-90014 Oulu, Finland; tuomas.nevanpera@oulu.ﬁ (T.K.N.); satu.pitkaaho@oulu.ﬁ (S.P.);

satu.ojala@oulu.ﬁ (S.O.)

*

Correspondence: riitta.keiski@oulu.ﬁ; Tel.: +358-40-726-3018

ꢀ

ꢁꢂꢀꢃꢄꢅꢆꢇ

Academic Editor: Leonid Kustov

Received: 31 August 2020; Accepted: 7 October 2020; Published: 12 October 2020

ꢀ

ꢁꢂꢃꢄꢅꢆ

Abstract: Au, Pt, and Pt-Au catalysts supported on Al O and CeO -Al O were studied in the

2

3

2

3

oxidation of dichloromethane (DCM, CH Cl ). High DCM oxidation activities and HCl selectivities

2

were seen with all the catalysts. With the addition of Au, remarkably lower light-oﬀ temperatures

were observed as they were reduced by 70 and 85 degrees with the Al O -supported and by 35 and

2

3

4

0 degrees with the CeO -Al O -supported catalysts. Excellent HCl selectivities close to 100% were

2 2 3

achieved with the Au/Al O and Pt-Au/Al O catalysts. The addition of ceria on alumina decreased

2

3

2

3

the total acidity of these catalysts, resulting in lower performance. The 100-h stability test showed

that the Pt-Au/Al O catalyst was active and durable, but the selectivity towards the total oxidation

2

3

products needs improvement. The results suggest that, with the Au-containing Al O -supported

2

3

catalysts, DCM decomposition mainly occurs via direct DCM hydrolysis into formaldehyde and HCl

followed by the oxidation of formaldehyde into CO and CO₂.

Keywords: environmental catalysis; gold; platinum; bimetallic catalyst; chlorinated volatile organic

compound (CVOC)

1

. Introduction

Chlorinated volatile organic compounds (CVOCs) are widely used in the industry as solvents,

degreasing and cleaning agents, and paint strippers and as additives for paints, inks, and adhesives,

as well as raw materials in the synthesis of chemicals, plastics, and pharmaceuticals [ ].

Dichloromethane (CH Cl , DCM) was chosen as the model CVOC in this work. DCM is one of

1–3

2

the three main chlorinated solvents used in Europe, the others being perchloroethylene (PCE) and

trichloroethylene (TCE) [ ]. DCM is the most stable chlorinated alkane, and therefore, it is very

diﬃcult to be decomposed naturally in the environment [ ]. The 100-year global warming potential

GWP) value for DCM is nine [ ]. In the International Chemical Safety Cards (ICSC), the eﬀects of

4

5

(

6

DCM are identiﬁed as: irritating to the eye, skin, and respiratory tract; causing eﬀects on the central

nervous system, blood, liver, heart, and lungs; exposure at high concentrations causing a lowering of

consciousness and death; and, probably, being a carcinogenic to humans [7].

Among the end-oﬀ pipe technologies, catalytic oxidation is a cost-eﬀective and environmentally

friendly destructive method for the abatement of VOCs [2,8,9]. In CVOC oxidation, selectivity of

the catalyst is of great importance to ensure the production of HCl while restraining the formation

of byproducts and, also, minimizing the deactivation of the catalyst. Besides choosing a durable

and selective catalyst, ensuring an adequate hydrogen amount in the reaction is important [10

–20].

In our previous study [12], we used water as a hydrogen source, and its amount was optimized

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Molecules 2020, 25, 4644

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to be 1.5 vol.%. Among the 15 metallic monoliths containing Pt, Pd, Rh, and V O supported on

2

5

alumina, alumina-titania, and alumina-ceria, 1 wt.% Pt/Al O proved to be the most active and HCl

2

3

◦

selective catalyst, showing 100% DCM conversion at 420 C and over 90% HCl yield at above 540 C.

In addition, the Al O -CeO -supported catalysts showed good activity in DCM oxidation and enhanced

2

3

2

selectivity towards CO , but the HCl yields remained below 90% at the temperature range used in

2

the experiments. [12] In general, platinum group metals have proven to be highly active and durable

catalysts for the oxidation of CVOCs [2,9,21].

In order to develop even more eﬃcient, selective, and sustainable catalysts, the suitability of gold

catalysts in CVOC oxidation also needs to be investigated. Aida et al. [22] studied Au catalysts having

diﬀerent loadings on a number of metal oxides in methyl chloride (CH Cl) oxidation. Another study by

3

Aida et al. [23] included experiments with 1.4 wt.% Au/TiO catalyst in methyl chloride oxidation and

2

5

wt.% Au/Al O catalyst in methyl chloride, DCM, chloroform (CHCl ), and carbon tetrachloride (CCl )

2 3 3 4

oxidation. Based on these studies [22,23], Aida et al. concluded that, after some stability improvement,

the Au/Al O catalyst could be a promising catalyst for the decomposition of halogenated organic

2

3

compounds due to its high activity and HCl selectivity. Chen et al. [24] investigated DCM oxidation

using Au/Co O , Co O , Cr O /Al O , Pd/Al O , and Pt/Al O catalysts. Among all the tested catalysts,

3

4

3

4

2

3

2

3

2

3

2

3

◦

.2 wt.% Au/Co O was the best, showing complete DCM conversion at 350 C. In addition, after a 130-h

3 4

0

stability test, the catalyst proved its durability in laboratory conditions. [24] Mat eˇ jová et al. [25] studied

Au and Pt catalysts supported on ceria-zirconia in the total oxidation of DCM. They concluded that,

even though the support itself showed high activity, the noble metal catalysts exhibited signiﬁcantly

enhanced selectivity towards CO compared to the ceria-zirconia support alone, gold catalysts showing

2

somewhat lower selectivity than platinum in this case. [25] Redina et al. [26] studied the performance

of an Au-Rh/TiO catalyst in DCM oxidation, which was prepared using surface redox reactions. In this

2

study, bimetallic catalysts having metal contents as low as 0.05 wt.% showed high activity and HCl

selectivity. In addition, the selectivity towards CO formation was enhanced when compared to a

2

monometallic Au catalyst with the same Au content. However, strong conclusions on the long-term

stability of the catalyst cannot be drawn, because the authors reported an onstream time of only

1

90 min [26].

In this study, the catalytic performance and selectivity of monometallic Au and bimetallic Pt-Au

catalysts supported on alumina and ceria-alumina were studied in dichloromethane (DCM, CH Cl )

2

oxidation. In addition, the durability of the Pt-Au/Al O catalyst was studied by performing a 100-h

2

3

stability test. Monometallic Pt catalysts, i.e., Pt/Al O and Pt/CeO -Al O , were used as references,

2

3

2

3

as they have proven to be active and selective catalysts in DCM oxidation. To explain the performance

of the catalysts, they were characterized by inductively coupled plasma optical emission spectroscopy

(

ICP-OES), N physisorption, X-ray diﬀraction (XRD), X-ray photoelectron spectroscopy (XPS),

2

high-resolution transmission electron microscope (HR-TEM), temperature-programmed reduction

with hydrogen (H -TPR), temperature-programmed desorption of ammonia (NH -TPD), and carbon

2

3

18

dioxide (CO -TPD), and temperature-programmed O isotopic exchange ( O -TPIE). Based on the

2

results, suggestions related to the reaction mechanism are made for an Au/Al O catalyst.

2

3

2

. Results and Discussion

Table 1 shows the results previously published in References [27,28] and the new results

presented in this study. More detailed characterization results are presented and discussed

in References [27 28], i.e., for monometal and support ICP-OES, BET (Brunauer-Emmett-Teller

method), XRD, H -TPR, and O -TPIE results in Reference [27] and for bimetal ICP-OES, BET-BJH

,

18

2

(

Brunauer-Emmett-Teller/Barrett-Joyner-Halenda method), XRD, XPS, HR-TEM, H -TPR, NH -TPD,

2 3

18

CO -TPD, and O -TPIE results in Reference [28].

2

Molecules 2020, 25, 4644

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Table 1. Summary of the previously published results in References [27

,28] and the new

results presented in this study. DCM: dichloromethane, ICP-OES: inductively coupled

plasma optical emission spectroscopy, BET-BJH: Brunauer-Emmett-Teller/Barrett-Joyner-Halenda

method, XRD: X-ray diﬀraction, XPS: X-ray photoelectron spectroscopy, HR-TEM: high-resolution

transmission electron microscope, H -TPR: temperature-programmed reduction with hydrogen,

2

1

8

O -TPIE: temperature-programmed isotopic exchange (TPIE) of labeled oxygen, NH -TPD:

2

3

temperature-programmed desorption of ammonia, and CO -TPD: temperature-programmed desorption

2

of carbon dioxide.

Results

Monometallic Catalysts

Bimetallic Catalysts

ICP-OES

BET-BJH

XRD

Reference [27]

This study

Reference [27]

This study

Reference [28]

This study

H -TPR

O -TPIE

2

18

XPS

HR-TEM

NH -TPD

This study

This study/Reference [28]

This study

3

CO -TPD

2

Catalytic performance in DCM oxidation

Stability in DCM oxidation

This study

2

.1. Properties of the Catalysts

Metal loadings, BET surface areas (S_B_E_T), pore volumes and average pore sizes, reducibility,

and basicity of the catalysts are summarized in Table 2. Considering the CeO -Al O support,

2

3

the desired weight ratio of 1:4 of ceria to alumina was veriﬁed by ICP-OES and XRF. ICP-OES showed

0.6 wt.% of ceria and 79.4 wt.% of alumina, whereas XRF demonstrated 21.7 wt.% and 78.3 wt.%,

respectively [27].

2

Table 2. Summary of the metal loading, surface area, total pore volume, average pore diameter,

reducibility, and basicity of the tested catalysts. The results were discussed in detail in References [27 28].

The results marked with * are the new results originating from this study.

,

Metal

Loading

Total

Average Pore

Diameter

^H2

a

c

SBET

−1

CO2-TPD

(µmol g

b

Catalyst

Pore Volume

Consumption

2

−1

(

m g

)

3

−1

(µmol g )

(wt.%)

(cm g

)

(nm)

Au

Pt

Al O

Pt/Al

Au/Al

-

0.7

0.8

-

1.1

-

0.9

-

100

105 *

95 *

95

-

2

3

0.51 *

0.48 *

0.49

-

18 *

19

149

52

44

-

100 *

135 *

100

-

Pt-Au/Al

CeO -Al O

65

-

2

3

Pt/Ce-Al

Au/Ce-Al

Pt-Au/Ce-Al

-

0.5

0.9

1.2

-

1.1

85 *

75 *

85

0.38 *

0.35 *

0.41

16 *

17 *

18

326

83

315

85 *

160 *

105

a

5 m g ), ^bcumulative pore volume between diameters of 1.7 nm and 300 nm, and ^cround

2

−1

Round values (

±

−1

values (± 5 µmol g ). - = not reached; SBET—BET surface areas.

In Figures 1 and 2, representative HR-TEM images of the prepared monometallic Au/Al and

Au/Ce-Al catalysts with particle size distribution (PSD) and examples of energy-dispersive X-ray (EDX)

spectra are presented. The HR-TEM results of bimetallic Pt-Au/Al and Pt-Au/Ce-Al can be found in

Reference [28] and the Supplementary Materials. The PSD in each catalyst was calculated based on

Feret’s diameter because of the irregular shape of the particles. Structures resembling needles in the

HR-TEM images are side views of alumina 2D plates.

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Figure 1. High-resolution transmission electron microscope (HR-TEM) images of the Au/Al O₃

2

and Au/CeO -Al O catalysts: (a) corresponding particle size distribution (PSD) data of Au/Al O ,

2 3

2

3

(

b

) example particles in sizes below 10 nm shown with the magniﬁcation of the inset in Figure 1a,

c) corresponding PSD data of Au/CeO -Al O , and ( ) an energy-dispersive X-ray (EDX) spectrum

d

2

3

from the agglomerate (Cu peaks in the spectrum originate from the used sample grid).

The Au/Al catalyst demonstrated homogeneously dispersed Au particles mainly in sizes smaller

than 10 nm (Figure 1a,b). The majority of measured particles were below 5 nm. An EDX spectrum

from the surface of Au/Al catalyst evidenced 1.8 wt.% of Au. The Au particles in the Au/Ce-Al

catalyst were much larger when compared to the Au/Al catalyst (Figure 1c,d), as already discussed

in Reference [27]. These ﬁndings were supported by the XRD and H -TPR results. In comparison

2

with the Au/Al catalyst, the PSD data of the Au/Ce-Al catalyst in Figure 1c show a low number of

distinguishable particles (n = 22) that could be accurately measured and, therefore, do not represent

a satisfactory population for PSD. The Au/Ce-Al catalyst contained unevenly dispersed particles in

smaller sizes similarly to the Au/Al catalyst but, also, large agglomerates of fused particles in sizes of

hundreds of nanometers likely due to sintering (Figure 1d). Islands of smaller particles that were close

to fusing together were observed as well, which supports the occurrence of sintering. These particles

were diﬃcult to distinguish from each other and, therefore, could not be included in the PSD data.

The EDX spectrum from the agglomerate in Figure 1c demonstrates intense Au peaks. Due to the very

poor Au distribution, a representative bulk loading using EDX was not possible.

Figure 2a–d presents typical HR-TEM images of the Pt/Al and Pt/Ce-Al catalysts. Pt particles

were not distinguished clearly from the surface due to contrast similarity, but the EDX spectra

(Figure 2a,c) show evidence of Pt. The Pt particle size is below 10 nm based on Figure 2a–d and other

interpreted images.

Molecules 2020, 25, 4644

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Figure 2. HR-TEM images of the Pt/Al O and Pt/CeO -Al O catalysts: (

a

) EDX spectrum from the

) an example of a 20-nm scale image from the surface of Pt/Al O , ( ) an EDX

) an example of a 10-nm scale image showing the

2

3

2

3

surface of Pt/Al O , (

b

c

2

3

2

3

spectrum from the Pt/CeO -Al O surface, and (

d

2

3

magniﬁcation from the inset in Figure 2c. (Cu peaks in the EDX spectrum originate from the used

sample grid.).

The Pt-Au/Al catalyst contained quite evenly dispersed particles, mostly in sizes between 10 to

0 nm (88%, n = 253), but some larger agglomerates were also observed. The Pt-Au/Ce-Al catalyst

3

contained homogenously dispersed particles mostly in sizes between 10 to 30 nm (77%, n = 112) but,

also, in sizes of 30–40 nm (11%). In addition, some larger isolated islands of agglomerated particles

were observed. See Reference [28] for detailed information.

The total acidity of the catalysts was analyzed by NH -TPD experiments. The temperature range

3

◦

of up to 600 C is especially interesting, since it corresponds to the maximum temperature used in

the light-oﬀ experiments and in the calcination step of catalyst preparation. The obtained NH -TPD

3

proﬁles are shown in Figure 3. In this study, the strengths of the acid sites are determined as weak and

◦

medium/strong in the sites retaining NH at lower than 300 C and at higher than 300 C, respectively.

3

The shapes of the proﬁles and positions of the peaks indicate the presence of acid sites of diﬀerent

◦

strengths, but one main band for all the catalysts centered at 210–250 C corresponding to the weak acid

sites can be seen in Figure 3. The Pt/Al, Au/Ce-Al, and Pt/Ce-Al catalysts show the highest amounts of

weak acid sites, while the Pt-Au/Al, Au/Al, and Pt-Au/Ce-Al catalysts possess the lowest amounts.

The amounts of strong acid sites are rather comparable between the catalysts, except in the case of

Pt-Au/Ce-Al (see Figure 3b), which can, overall, be considered as the least acidic catalyst.

Molecules 2020, 25, 4644

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Figure 3. Temperature-programmed desorption of ammonia (NH -TPD) proﬁles of the Pt, Au, and

3

Pt-Au catalysts supported on (a) Al O and on (b) CeO -Al O , (TCD = thermal conductivity detector).

2 3 2 2 3

The basicity of the catalysts was measured to ﬁnd out if it could explain the results related to

CO formation during DCM oxidation. The total basicity of the catalysts was analyzed by CO -TPD

2

experiments; all results and, also, the new ones for monometallic catalysts, are summarized in

−

1

Table 2. The CO uptake of the Al O -supported catalysts was between 100 to 135

µ

mol g and the

mol g . Au containing catalysts had higher total

basicity than the corresponding Pt catalysts. The basicity of the catalysts was at a low level based on

the CO -TPD [29], which should be advantageous for the desorption of formed CO from the catalyst

2

3

−

1

CeO -Al O -supported catalysts between 85 to 160

µ

2

3

2

surface during DCM oxidation. The basic sites can be assigned low, medium, and high according to

◦

diﬀerent CO desorption ranges at 80–140 C, 160–240 C, and >300 C, respectively [30]. Based on

2

our results, the Pt and Pt-Au catalysts lacked medium-strength basic sites, whereas the Au/Al and

Au/Ce-Al catalysts had low, medium, and high-strength basic sites.

The reducibility of the catalysts was analyzed by H -TPR, and the total hydrogen uptakes between

2

◦

3

5 C and 400 C from References [27

,

28] are summarized in Table 2. Based on the H consumption,

2

the Pt-Au/Al catalyst showed the lowest and Pt/Ce-Al the highest reducibility. Au/Al and Au/Ce-Al

did not show signiﬁcant reduction in the used temperature range, and the H consumption remained

2

−

at a level of less than 50 µmol g . The Pt-Au/Al and Pt/Al catalysts demonstrated both H uptakes at a

2

1

◦

low temperature window of 50–70 C and within a broad temperature range of 150–300 C, both being

caused likely by the reduction of adsorbed oxygen species and/or platinum oxychloride complexes.

◦

The TPR proﬁles of Pt-Au/Ce-Al and Pt/Ce-Al catalysts showed the H uptake maxima at 255 C and

2

◦

at 205 C, which can be assigned to the reduction of Pt species and surface ceria due to a hydrogen

spillover enhanced by Pt particles [31]. The addition of Au in Pt catalysts decreased the reducibility,

while the addition of ceria in the support improved the reducibility. Au might block the H spillover

2

on Pt due to the preparation method used.

XPS measurements were done to identify the oxidation state of Au in the Au/Al and Au/Ce-Al

catalysts. A typical accuracy of the XPS measurements was 0.1 to 0.2 eV. The XPS results for the

bimetallic catalysts (Pt-Au/Al and Pt-Au/Ce-Al) were presented in Reference [28]. A theoretical intensity

0

ratio of I(Au 4f₇_/₂):I(Au 4f₅_/₂) = 4:3 for Au was used as one of the constraints in the ﬁtting of the

data [32]. Figure 4 shows the binding energy of the Au 4f spectra for the Au/Al and Au/Ce-Al catalysts.

Usually, the oxidation states of Au are determined from the Au 4f₇_/₂at roughly 83.9–84.0 eV [33

–

35].

0

1+

The characteristic peaks for diﬀerent Au oxidation states are as follows: Au at 84.0 eV, Au at 84.6 eV,

and Au³at 85.9 eV [36

+

,

37]. Metallic Au is evidenced here based on the main peaks of Au 4f at

7/2

0

83.0 and 83.4 eV and Au 4f₅_/₂at 86.6 and 87.2. The binding energy values were lower than those of

the bulk metallic Au, i.e., Au 4f₇= 84.0 eV and Au 4f₅_/₂= 87.7 eV. Similar results were reported

/2

0

earlier for Au on the surfaces of diﬀerent catalysts. [37

–

40] These diﬀerences can be caused by the

size-dependent peak shifts, the presence of hydroxides or oxides [37], and the metal-support and/or

metal-metal interactions [37–

39]. In addition, the Au/Al catalyst evidenced the presence of Au¹based

+

Molecules 2020, 25, 4644

7 of 25

0

on the peak at 83.6 eV, which is supported by the known peak diﬀerence of 0.6 eV between the Au and

+

1

Au oxidation states [36,37]. In our case, the diﬀerences in peak positions of Au between mono- and

bimetallic catalyst counterparts were relatively small. The peak shapes were similar, and the binding

energy did not diﬀer much. Au was in the metallic state in all Au-containing catalysts, except in the

Au/Al catalyst that contained also Au¹

+

.

0

Figure 4. Au 4f spectra of the Au/Al and Au/Ce-Al catalysts (Au is the theoretical binding energy

0

value of Au ).

Table 3 shows the surface composition and surface metal loading of the catalysts. Pt and Au

contents close to the surface were higher compared to the bulk metal loadings analyzed by ICP-OES,

which could be explained by the very low Au loading and the accuracy of the measurements. In the

case of the Au/Al catalyst, the surface Au loading was 1.6 wt.% based on XPS, which is in accordance

with the EDX result evidenced in HR-TEM (1.8 wt.%). Considering the CeO -Al O support, the bulk

2

3

Al/Ce weight ratio was about 2.5 in comparison to the surface Al/Ce weight ratio of 7.4 and 10.9 for the

Au/Ce-Al and Pt-Au/Ce-Al catalysts, respectively. Since the concentration of ceria on the surface is

higher than in the bulk Au, it can interact with ceria more easily [41].

Table 3. Surface composition of the Au-containing catalysts.

XPS

Catalyst

Surface Composition (wt.%)

Au

Pt

Ce

Al

O

Au/Al

Au/Ce-Al

Pt-Au/Al

1.6

0.6

0.9

2.2

-

1.2

2.1

-

6.3

-

49.6

46.5

51.3

48.1

48.9

46.6

43.2

Pt-Au/Ce-Al

4.4

2

.2. Catalytic Performance in DCM Oxidation

The conversion curves of all the studied catalysts in DCM oxidation are presented in Figure 5a,b.

The T₅₀(temperature at which 50% DCM conversion is observed) and T₉₀(temperature at which

0% DCM conversion is observed) values, together with the HCl and CO yields at T₉₀and at the

9

2

maximum, are listed in Table 4. The results show that the bimetallic Pt-Au/Al and monometallic

Au/Al catalysts were the most active catalysts among the tested catalysts (Figure 5a) based on their T₅₀

and T . The addition of Au resulted in a noticeable improvement in the performance by lowering

90

Molecules 2020, 25, 4644

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◦

the T₅₀by 65 C and 85 C in comparison to the

γ

-Al O support and Pt/Al catalyst, respectively.

2

3

When considering the catalysts supported on CeO -Al O (Figure 5b), similar results can be seen, i.e.,

2

3

◦

gold in the catalyst improving the performance in DCM oxidation. The T₅₀was lowered by 35 C and

0 C in comparison to the CeO -Al O support and Pt/Ce-Al catalyst, respectively. It is noticeable that

◦

4

2

3

the active metal loadings in the Au/Al and Au/Ce-Al catalysts were 0.7 wt.% and 0.5 wt.%, respectively,

which are much lower when compared to the Pt/Al catalyst with 1.1 wt.% and Pt/Ce-Al with 1.2 wt.%

2

−1

(

see Table 2). In addition, the BET surface areas were quite comparable, being from 95 to 105 m g

2

−1

for the Al O -supported catalysts and from 75 to 85 m g for the CeO -Al O -supported catalysts.

2

3

2

3

Therefore, the better performance of Au-containing catalysts cannot be explained by the diﬀerences in

these physical properties.

Figure 5. Dichloromethane (DCM) conversion of the Pt, Au, and Pt-Au catalysts supported on

(a) Al O and on (b) CeO -Al O (DCM 500 ppm, H O 1.5 vol.%, weight hourly space velocity (WHSV)

2 3 2 2 3 2

20 g gcat¹

−

h

−1

).

7

Table 4. Summary of the catalytic performance and selectivity of all the tested catalysts in DCM

−1

h ).

oxidation (DCM 500 ppm, H O 1.5 vol.%, weight hourly space velocity (WHSV) 720 g gcat

2

T

( C)

HCl Yield at

T₉₀(%)

Max. HCl

Yield (%)

CO Yield at

Max. CO₂

Yield (%)

5

0

90

2

Catalyst

◦

(

C)

T₉₀(%)

Al O

Pt/Al

Au/Al

455

460

390

375

470

435

430

-

96 ^a

79

97

76

-

70

2

36

48

70

-

5 ^b

70

30

70

48

77

2

3

555

475

485

585

560

-

79

80

81

76

73

-

Pt-Au/Al

CeO -Al O

2

3

Pt/Ce-Al

Au/Ce-Al

Pt-Au/Ce-Al

76

c

d

80

28

70

570

81

70

a

◦

b

yield at 570 C, ^cDCM conversion at 570 C = 89%,

◦

-

= not reached; DCM conversion at 570 C = 89%, CO

2

d

◦

and CO2 yield at 570 C.

In this study, the presence of Au, alone or together with Pt, enhanced the performance of the

catalyst compared to the catalyst containing only Pt. The addition of Pt improved the selectivity towards

the total oxidation products but not the conversion, which is in agreement with the previous literature.

◦

According to Maupin et al. [19], Pt/Al O catalysts oxidize DCM completely at 380 C, and the

2

3

rate-limiting step takes place on alumina, since neither the loading of Pt nor the dispersion (particle size)

have an eﬀect on the conversion rates and selectivity. Similar observations were seen in our previous

study [12]. The addition of Pt and/or ceria to the catalyst enhanced the selectivity towards CO₂,

but DCM conversion was not inﬂuenced signiﬁcantly [12].

The activity of Au in oxidation reactions is known to be dependent on the particle size and

shape [42,43]. The activation of oxygen by Pt occurs faster on larger particles [44]. The Au/Al catalyst

contained well-dispersed nanoparticles in sizes below 10 nm, whereas, in the Pt-Au/Al catalyst,

Molecules 2020, 25, 4644

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the particles were mainly in sizes between 10 to 30 nm. In addition, the Pt-Au/Ce-Al catalyst had

mostly particles in sizes between 10 to 40 nm, although a few examples of larger agglomerates were

found. The Pt/Al and Pt/Ce-Al catalysts seemed to contain also relatively small particles, but that did

not result in better performance. The good performance of Au/Al and Pt-Au/Al catalysts could be at

least partly related to the particle size distribution.

Based on the literature, both high acidity [11,12,25,45–49] and reducibility [12,25,49] contribute

to enhanced activity in DCM oxidation. The reducibility of the catalysts had a substantial eﬀect

on the performance also in this study but in the opposite way. The catalysts with the lowest H₂

consumption, i.e., Pt-Au/Al and Au/Al (see Table 2), demonstrated the best performance, whereas a

high reducibility correlated with a poorer performance. Overall, the catalysts that demonstrated

◦

reducibility below 300 C due to the presence of ceria (Pt/Ce-Al and Pt-Au/Ce-Al), except the Au/Ce-Al

catalyst, likely because of the large Au particles that are unable to dissociate hydrogen, showed poorer

performance compared to the less reducible catalysts.

Three least-acidic catalysts were among the three most active catalysts in DCM oxidation. When a

weak acidity was considered, the acidity order of the catalysts from the lowest to highest was

Pt-Au/Ce-Al that had the lowest acidity overall < Au/Al followed by the Pt-Au/Al catalyst < Au/Ce-Al

<

Pt/Ce-Al < Pt/Al. The Au/Al catalyst also had the highest amount of strong acid sites, which correlates

with the performance in respect to the previous literature. It is possible that, in our case, the deposition of

Au and/or Pt on ceria-alumina supports compensates the acidity loss to some extent by the introduction

−

of Cl species from the chloride precursors used in the catalyst preparation. In addition, these acid

sites could have diﬀerent characteristics (Brønsted/Lewis acid sites) in diﬀerent proportions as a result

of the deposition, thus inﬂuencing the catalyst performance. The total basicity of the catalysts varied

−

1

between 85 to 160 µmol g , but a direct correlation with the performance could not be seen. However,

it is worth mentioning that the addition of Au in the Pt catalysts increased the basicity, and the DCM

conversion was always higher in comparison to the monometallic Pt catalysts.

By considering the catalytic performance of the studied catalysts on previous ﬁndings, Chen et al.

◦

reported T₅₀and T₉₀values of approximately 220 C and 250 C, respectively, for a 5%Au/Co O catalyst

3

4

−1

in DCM oxidation (DCM 500 ppm, 0.6 wt.% H O, GHSV 15,000 h ) [24]. In a study by Mat

êjová et al.,

2

◦

a 0.3-Au/CeZr catalyst showed T₅₀and T₉₀values of 417 C and 487 C, respectively, in DCM oxidation

3

−1 −1

DCM 1000 ppm, 1.5 vol.% H O, space velocity (SV) 71 m kg h ), with a maximum HCl yield of

2

(

7

7% [25]. Redina et al. showed that an Au-Rh/TiO catalyst reached a DCM conversion of 99% and

2

◦

−1

HCl selectivity of 90% at 400 C in DCM oxidation (DCM 510 ppm, H O 0.25 vol.%, 40,000 h ) [26].

2

.3. Selectivity in DCM Oxidation

In addition to good catalytic performance, high selectivity is also essential in CVOC oxidation

in order to avoid the formation of highly toxic by-products. An analysis of the reaction products

conﬁrmed that the main reaction products detected during DCM oxidation in this study were CO₂,

CO, and HCl. The formation of methyl chloride (CH Cl) and formaldehyde (CH O) were detected

3

2

◦

during the tests over certain catalysts at temperatures above 300 C. The HCl yields of the studied

catalysts are shown in Figure 6a,b and Table 4. The by-product formation for each catalyst is presented

in Figure 7a–f.

Molecules 2020, 25, 4644

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Figure 6. The HCl yields of the Pt, Au, and Pt-Au catalysts supported on (

a) Al O and on

2 3

(

b) CeO -Al O in DCM oxidation (same conditions as in Figure 5).

2

3

Figure 7. The product concentrations of the Pt, Au, and Pt-Au catalysts supported on (

on (d–f) CeO -Al O in DCM oxidation (same conditions as in Figure 5).

a–c) Al O and

2 3

2

3

Molecules 2020, 25, 4644

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The by-product formation for each catalyst is presented in Figure 7a–f. Amongst all the tested

catalysts, Pt-Au/Al and Au/Al catalysts were the most active and HCl selective, both reaching close

◦

to 100% HCl yields at around 550 C. At the same time, the other catalysts were able to reach HCl

yields up to 70–80% during the tests. Even though the monometallic Au/Al and Au/Ce-Al catalysts

were noticeably more active than the Pt catalysts, the formation of CO and other intermediates

were observed to be substantial (Figure 7c,d). The Pt-Au/Al catalyst yielded mostly CO and HCl,

2

but a slight formation of CO and formaldehyde was observed. No Cl-containing intermediates were

observed during the experiment with the Pt-Au/Ce-Al catalyst. The catalysts in this study were

synthesized from chloride-containing precursors and, therefore, may contain residual chlorine as

prepared. Calcination in dry air is ineﬀective to eliminate residual chlorine as HCl. [50,51] Chlorine

stays on alumina-based catalysts and, further, has an eﬀect on hydrocarbon oxidation as an inhibitor.

Residual chlorine is removed during oxidation when water is present, forming HCl, restores the

catalytic activity, and may even increase the activity, because the initial Cl in contact with the active

phase is eliminated completely [50].

The formation of carbon monoxide (CO) was observed in every case between concentrations

of a few ppm up to over 200 ppm, depending on the catalyst. The Pt/Al and Pt/Ce-Al catalysts

◦

showed always the lowest CO formation, the maximum being 27 ppm at 555 C and 30 ppm 530 C,

respectively. Over the Pt-Au/Al and Pt-Au/Ce-Al catalysts, the highest measured CO concentrations

◦

were 78 ppm at 560 C and 62 ppm at 570 C, respectively. The highest concentrations of CO amongst

the studied catalysts were measured with the Au/Al and Au/Ce-Al catalysts, being over 200 ppm at

◦

temperatures higher than 520 C. The CO formation started always at the same temperature levels as

the HCl formation.

The Pt catalysts were the most selective towards the total oxidation product, i.e., carbon dioxide

(

CO ) (Figure 7a,b). The reason might be simply the well-known eﬃciency of Pt in complete oxidation

2

and relatively low tendency to catalyze partial oxidation [52,53]. The bimetallic Pt-Au/Al and

Pt-Au/Ce-Al catalysts were able to oxidize carbon intermediates better in comparison to monometallic

Au catalysts (Figure 7e,f), which is probably due to the presence of Pt.

The selectivities towards HCl were rather high with all the catalysts, as evidenced by the methyl

chloride concentrations, i.e., intermediate of the DCM oxidation reaction, that were relatively low,

varying from zero up to 31 ppm. The highest methyl chloride concentrations were detected with the

◦

Pt-Au/Al catalyst, the maximum being 31 ppm at 405 C followed by the Au/Al catalyst producing

2

◦

7 ppm at the highest at 390 C. The methyl chloride formation was not detected with the Pt-Au/Ce-Al

catalyst at all, and with the other Ce-Al-supported catalysts and the Pt/Al catalyst, the concentrations

were below 8 ppm. Methyl chloride formation was reported over alumina-supported catalysts in

oxidative conditions earlier in several studies [10,11,17–19,45,46]. The presence of ceria seems to

decrease the formation of methyl chloride.

Formaldehyde (CH O) formation was observed with all the catalysts, and the maximum

2

concentrations were from 6 ppm up to 108 ppm. The highest formaldehyde concentrations were

◦

observed over the Au/Al catalyst (>100 ppm between 440–505 C) and the Pt-Au/Al catalyst (74 ppm

◦

at 430 C). The lowest concentrations were seen over the Pt/Ce-Al (6 ppm at 495 C), Pt-Au/Ce-Al

◦ ◦ ◦

(

21 ppm at 445 C), Au/Ce-Al (35 ppm at 470 C), and Pt/Al (36 pp at 510 C) catalysts. In this case,

also, the presence of ceria decreased the formation of the intermediate.

Alumina support alone converted DCM selectively into HCl, reaching yields higher than 90%

◦

at temperatures above 555 C (Table 4). However, methyl chloride (CH Cl) formation up to 60 ppm

3

was observed during oxidation, and carbon was detected as partial oxidation products (CO and

◦

formaldehyde) up to 530 C, after which, CO formation started. The ceria-alumina support alone was

2

also selective to HCl; the detected concentrations of other Cl-containing products were less than 7 ppm

during the light-oﬀ test. In addition, the ﬁnal carbon products were mainly CO and CO. In this case,

2

also, the beneﬁcial eﬀect of ceria was visible. The formaldehyde formation was negligible. However,

Molecules 2020, 25, 4644

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both supports demonstrated low DCM conversion. The alumina support did not reach T , whereas

9

0

◦

with the ceria-alumina support T₉₀was 585 C, as shown in Table 4.

The formation of methyl chloride in oxidative conditions over Al O -supported catalysts has

2

3

been reported previously by several authors [11

,12,18,19,45]. It has also been suggested that methyl

chloride is formed in the presence of Lewis acid sites that are typical for Al O . Formaldehyde is

2

3

reported to be formed on the Brønsted acid sites, which amounts could be increased in the presence

of water [11 48]. The largest diﬀerences were seen in the case of CO formation, since both the Au/Al

,

and Au/Ce-Al catalysts produced relatively high amounts of CO, i.e., over 200 ppm, in addition to the

alumina support that produced CO over 340 ppm. The Pt-containing catalysts demonstrated the most

beneﬁcial product distributions in terms of the intermediate yields. The monometallic Au catalysts

showed the lowest CO yields.

2

Oxygen activation is known to be easier on ceria and noble metals compared to alumina [54],

which enables a faster delivery of reactive oxygen and, thus, accelerates total oxidation. In addition,

surface diﬀusion and the strength of chemisorption on the catalyst inﬂuence the ability of the surface

18

intermediates to move closer to each other for further reactions [12]. The oxygen activation ( O -TPIE),

2

which was discussed in detail for monometallic catalysts in Reference [27] and for bimetallic catalysts

in Reference [28], correlates well with the DCM oxidation in the case of the Au/Al and Pt/Al catalysts.

◦

Oxygen exchange starts at around 310 C with the Au/Al catalyst, i.e., at the same temperature as

◦

the formation of HCl and partial oxidation products (7 ppm of HCl and 10 ppm of CH O at 300 C).

A low formation of O O (0–0.1 mbar) was seen already from the beginning of the experiment

2

18

16

◦

between 200–310 C, after which, it rapidly increased. The Pt/Al catalyst activated oxygen based on

1

8

16

◦

18 16

the formation of O O at around 370 C (ﬁrst observations of O O, i.e., 0–0.1 mbar, already at

◦

around 340 C) and during DCM oxidation; 6 ppm of HCl and 5 ppm of CH O were observed at

40 C. Although the oxygen activation with the Pt-Au/Al catalyst started at a higher temperature of

roughly 380 C, oxidation products during DCM oxidation were observed already at around 290 C

4 ppm of HCl and 11 ppm of CH O), i.e., at the same temperature where the low formation of O O

2

0–0.1 mbar) was seen in the O -TPIE experiment. The Au/Ce-Al catalyst started oxygen activation

progressively at 380 C, but a low formation of O O (0–0.1 mbar) was observed already between 250

to 380 C. During DCM oxidation, the formation of HCl and oxidation products were seen at 305 C

5 ppm of HCl and 6 ppm of CH O). In the case of the Pt/Ce-Al catalyst, the oxygen activation began

gradually at 340 C, and a low formation of O O was observed at above 250 C, approximately.

2

◦

3

◦

16

(

18

2

◦

18 16

◦

(

2

◦

18 16

◦

During DCM oxidation, the formation of HCl and CO was seen at the same temperature (345 C).

2

The oxygen activation seems to have a connection to the initiation of the DCM oxidation reaction

with alumina-supported catalysts. With the Pt-Au/Ce-Al catalyst, the oxygen activation started at

◦

around 390 C, but during DCM oxidation, the formation of HCl, CH O, and CO was seen already

2

◦

18

at 315 C, which cannot be explained by the O -TPIE experiment. This could be related to the

reduction of Pt-Au/Ce-Al, which demonstrated an H uptake between 150–300 C. The behavior of

2

◦

2

the ceria-alumina catalyst could be connected to the reducibility of ceria, i.e., ability to gain electrons

that provide the available lattice oxygen already at lower temperatures before the oxygen activation

starts to increase progressively. Au was in the metallic state in all the Au containing catalysts, except

in the Au/Al catalyst, and no signiﬁcant diﬀerences were evidenced based on the XPS results. Thus,

the oxidation state of Au does not seem to explain the diﬀerences in the catalytic performance of Au/Al

and Pt-Au/Al catalysts in comparison to the Au/Ce-Al and Pt-Au/Ce-Al catalysts.

Carbon balances for the best performing catalysts during the light-oﬀ experiments are shown

◦

in Figure 8a,b. In the case of the Au/Al catalyst, the CO formation started at 450 C, which did not

2

directly have an eﬀect on the CO formation, but formaldehyde formation began simultaneously to

decline. The carbon balance nearly enclosed at the end of the experiment. The Pt-Au/Al catalyst

showed clearly a better performance towards total oxidation, as seen in Figure 8b, compared to the

◦

Au/Al catalyst in Figure 8a. CO formation started at around 340 C, increasing towards the higher

2

temperature area at the end of the experiment. In addition, CO and formaldehyde formation was seen

Molecules 2020, 25, 4644

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during the whole temperature range of the reaction, but both yields were below 16%. Carbon balance

enclosed at the end of the experiment.

Figure 8. DCM oxidation over (

.4. Stability of Pt-Au/Al Catalyst

a) Au/Al and (b) Pt-Au/Al catalysts (the same conditions as in Figure 5).

2

The durability of a catalyst is of great importance, especially in oxidation of chlorinated compounds.

The most active and selective catalyst in this study, i.e., Pt-Au/Al, was chosen for a 100-h stability

test. The experimental conditions were selected based on the T₉₀temperature, but in isothermal

conditions, 90% DCM conversion was reached at a lower temperature. The conditions were as follows:

◦

DCM 500 ppm and a constant temperature of 395 C in moist conditions (1.5 vol.% water). During every

◦

reﬁll of the DCM and water syringes, the temperature increased roughly by 10 C, because no gas was

fed into the reactor (by-pass) but stabilized down to 395 C in a few minutes after the gas stream was

◦

fed back into the reactor.

Based on the results shown in Figure 9, the performance of Pt-Au/Al was stable throughout the

1

00-h test. The DCM conversion remained between roughly 85% and 93%. Only a slight decline in the

DCM conversion was observed after 50 h. At the same time, the HCl yield seemed to increase by a few

percent units on average. The HCl yield ﬂuctuated from 68% to 91% and the CO yield from 13% to

2

3

2

3%. Considering by-products, the CO and formaldehyde yields were from 14% to 30% and 12% to

0%, respectively. Methyl chloride (CH Cl) formation seemed to decline by a few percent.

3

The light-oﬀ experiments performed before and after the stability test (Figure 10a,b) indicated a

slight sintering of active sites and/or support material based on the shape of the curves in Figure 10a [55].

◦

The T₅₀and T₉₀values did not change substantially as the values increased by 5 C and 20 C after

the stability test, respectively. The high-resolution transmission electron microscope (HR-TEM) and

high-angle annular dark-ﬁeld scanning transmission electron microscope (HAADF-STEM) analyses

done after the 100-h stability test provided evidence for the sintering of bimetallic Pt-Au particles.

Based on Figure 9, the CO and CO formation showed a minor increase during the 100-h stability

2

test, but the light-oﬀ experiment performed after the stability test (Figure 10b) showed a lower CO₂

formation. This might be due to the diﬀerences between experiments conducted at constant and

increasing temperatures.

Molecules 2020, 25, 4644

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◦

Figure 9. Stability of the Pt-Au/Al catalyst in DCM oxidation during the 100-h test (395 C, DCM 500 ppm,

−

1

−1

H O 1.5 vol.%, weight hourly space velocity (WHSV) 720 g gcat

h ).

2

Figure 10. (a) DCM conversion, as well as (b) HCl and CO yields, over the Pt-Au/Al catalyst before

2

and after the stability test in DCM oxidation (same conditions as in Figure 5).

Carbon balances for the Pt-Au/Al catalyst during the light-oﬀ tests before and after the 100-h

stability test are shown in Figure 11a,b. Signs of deactivation can be seen based on the yields of CO₂

and CO. The CO yield is slightly lower and, in turn, the CO yield higher in Figure 11b. In this regard,

2

the stability test might cause changes in the catalyst structure, aﬀecting the oxidation ability of the

Pt-Au/Al catalyst. Yet, the methyl chloride (CH Cl) formation was a bit lower, and this was also

3

seen during the stability test (Figure 9), as mentioned earlier. The carbon balance enclosed in both

light-oﬀ experiments.

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Figure 11. Carbon balance in DCM oxidation over the Pt-Au/Al catalyst (

stability test (same conditions as in Figure 5).

a) prior to and (b) after the

Chen et al. [24] studied the stability of a catalyst loaded with 5% of Au on Co O in DCM oxidation

3

4

− ◦

DCM 100 ppm, H O 0.6 wt.%, balance air, GHSV 90,000 h , 350 C). During 130 h on stream, the

2

1

(

Au/Co O catalyst did not show serious signs of deactivation at a DCM conversion level of around

3

4

9

0% to 95%, although a small decline could be observed similarly to our experiment. It is good to note

that, in their study, the DCM concentration was ﬁve times lower and the Au loading ﬁve times higher

compared to our experiments. Aida et al. [22] investigated the stability of Au/Al O (5 wt.% of Au) in

2

3

methyl chloride oxidation (catalyst 1 g, CH Cl 1000 ppm, H O 2 vol.%, balance air, weight/ﬂow (W/F)

3

2

− ◦ ◦

1

3.4 kg s mol , 300 C). At a constant temperature of 300 C, the catalyst lost its activity completely

from around 100% conversion to 5% in 5 h, but it could be regenerated when raising the temperature

◦

above 427 C. Based on thermogravimetric analysis done after the reaction, HCl did not desorb from

◦

the surface at temperatures below 327 C, even after the addition of water, and the active sites suﬀered

from deactivation. Once the temperature was increased above 400 C, the catalyst activity recovered.

◦

The reaction of Cl and water, followed by the formation and further desorption of HCl, closed the

catalytic cycle [22]. It is known that, in the catalytic oxidation of CVOC, it is essential to prevent

the strong interaction of HCl with the catalyst surface, leading to deactivation [53], but at higher

temperatures, another problem arises, since HCl attacks the alumina support [24]. Bond and Rosa [10

]

suggested that the reaction mechanism involves the disruption of the chlorinated reactant at a Lewis

acid site, followed by the formation of metal-chloride bonds. After that, the chemisorption of water

at a similar Lewis acid site and its reaction with the metal-chloride bond to form HCl takes place.

Water and the reactant compete for the same sites, the water molecule more eﬀectively. If the rate of

formation of the Cl-M bonds is too high and the water vapor pressure too low, species such as Cl M

2

and Cl M are formed, and these may react further to give gaseous metal chloride [10].

3

Legawiec-Jarzyna et al. [56] studied the catalytic activity of Pt-Au/Al O catalysts in the

2

3

hydrodechlorination of CCl and suggested that the introduction of Au onto Pt without a noticeable

4

change in metal particle size decreases the aﬃnity to the chloride species. A direct comparison of

◦

free energy of the formation for AuCl and PtCl (at the temperature range of 27–227 C) evidence

3

a much lower metal-Cl bond strength for gold. A higher activation energy for the monometallic

56 vs. ~30 kJ/mol for the bimetallics) indicates that the Cl removal from Pt surface is more diﬃcult

than for the bimetallic Pt-Au surface. Pt particles are very prone to excessive chloriding and, in eﬀect,

(

to a drastic deactivation in the hydrodechlorination of CCl [56].

4

Figure 12a shows a characteristic HR-TEM image with the corresponding PSD data that gave

information on the structural changes in the catalyst after long-term testing. The bright-ﬁeld STEM

image shown in Figure 12b evidences the sintering of the active phases. The Feret’s diameter of the

biggest agglomerates in that image were 150 to 175 nm. Even though the image provided proof of

substantial sintering, the PSD data (n = 450) indicated the contrary. The amounts of particles in sizes

between 0 to 10 nm and 10 to 20 nm increased by 2% and 9%, respectively, whereas the amounts in

Molecules 2020, 25, 4644

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sizes between 20–30 nm decreased by 20% after the stability test. Although, the amounts of larger

particles (i.e., between 30 to 40 nm, 40 to 50 nm, and >50 nm) increased all by a few percent units,

the distribution indicated a larger number of smaller particles, mainly between 10–20 nm (52%) in

comparison to the fresh Pt-Au/Al catalyst with 43% for the same size range. These ﬁndings might

be a result of the removal of the residual chlorine. There might not be enough chlorine present on

the catalyst surface to induce severe sintering of the particles. Au having resistance against highly

electronegative elements and the reduced amount of Cl in the catalyst during the reaction might be

one reason to maintain the catalytic performance in the stability test.

Figure 12. Representative (a) HR-TEM image with PSD data and (b) STEM image of the used Pt-Au/Al

catalyst after the 100-h stability test in DCM oxidation (same conditions as in Figure 9).

2

.5. The Role of Au in the Reaction Mechanism on Alumina-Supported Catalyst

The alumina-supported catalysts were chosen for further consideration regarding the reaction

mechanism, since these were better than ceria-containing analogs. The main reaction products observed

during DCM oxidation in this study were CO , CO, and HCl (see Figure 7). The formation of methyl

2

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chloride (CH Cl) and formaldehyde (CH O) were detected during the tests over the Au/Al and

3

2

Pt-Au/Al catalysts. In the case of Pt/Al, only formaldehyde was observed. To understand better the

role of gold in DCM oxidation, measured gaseous products for the Pt/Al, Au/Al, and Au-Pt/Al catalysts

are shown in Table 5. With the Pt/Al, Au/Al, and Pt-Au/Al catalysts, the formation of formaldehyde

◦

was seen already at about 40 C lower temperature compared to HCl. This could mean that, at lower

temperatures, direct DCM hydrolysis into formaldehyde takes place. Furthermore, methyl chloride

was not observed in the case of Pt/Al, which could mean that the direct hydrolysis reaction rate is

high, with Pt/Al also at a higher temperature. The DCM oxidation over the Pt/Al, Au/Al, and Pt-Au/Al

catalysts in excess hydrogen originating from the water feed can advance via the cleavage of all

chlorines from the carbon atom, thus following the lowest bond energy. Formaldehyde formation is

expected to occur on Brønsted acid sites [48]. According to van den Brink et al. [11], the ﬁrst step in

DCM oxidation over

that produces a chloromethoxyl species, which further reacts to the chemisorbed formaldehyde analog.

The adsorbed formate species decomposes to CO or CO . The chloride displaced after the formation

γ-Al O is the reaction of the adsorbed DCM molecule with a surface OH group

2 3

2

3

+

of the chemisorbed formaldehyde species can react with surface Al to form an aluminum chloride

species or attach to a proton to form HCl. Water is able to regenerate the Al-Cl entities. [11].

Table 5. The gaseous products detected during the light-oﬀ test (same conditions as in Figure 5).

CH2O

CH3Cl

CO

CO2

HCl

DCM Conv.

at 570 C (%)

(

ppm at Temp. (ppm at Temp. (ppm at Temp. (ppm at Temp. (ppm at Temp.

◦

Range, C)

9

0

2–35

at 285–555

2–30

at 340–555

2–230

at 305–575

2–80

2–350

at 290–555

2–150

at 460–575

2–340

2–795

at 330–555

2–960

at 290–575

2–955

Pt/Al

Au/Al

-

at 555

2

–110

at 250–575

–75

at 245–570

2–30

at 300–575

2–30

100

2

Pt-Au/Al

96

at 315–495

at 265–570

at 345–570

at 290–570

Haber et al. and van den Brink et al. focused on the role of Lewis acid sites in DCM oxidation.

Haber et al. suggested that the activity of alumina in DCM oxidation is related to the distribution

of Lewis acid sites, whereas the formation and selectivity to methyl chloride is dependent on the

concentration of Lewis acid sites [45,46]. In the mechanism proposed by van den Brink et al. [11],

both chlorines of DCM are cleaved, and instead of forming HCl or aluminum chloride, Cl can also

re-enter a surface methoxy species by nucleophilic displacement. The formaldehyde species, which was

formed at a lower temperature range, can also disproportionate to form methoxy and formate groups.

The methoxy species can react with HCl to form methyl chloride, which was observed in our case, at a

slightly higher temperature. At higher temperatures, formaldehyde species can decompose rapidly

and desorb more easily, making disproportionation less likely, and if methyl chloride is still formed,

it will be decomposed on γ-Al O [11].

2

3

Maupin et al. [19] suggested a slightly diﬀerent mechanism for methyl chloride formation

compared to that of van den Brink et al. [11]. Firstly, one chlorine atom from DCM is substituted by an

alumina hydroxyl group (step 1), leading to the formation of chloromethoxy species. Secondly, the last

species is transformed into a hemiacetal species (step 2). [19] These steps are in accordance with van

den Brink et al. [11], with the exception that, in the mechanism by van den Brink et al., both chlorines of

DCM are removed and then Cl re-enters on a surface methoxy species. However, the methyl chloride

formation is explained diﬀerently. Maupin et al. [19] suggested that two chloromethoxy species react,

via hemiacetal species, to form a formate species and methyl chloride (step 3), as discussed also by van

den Brink et al., in which the hydride is transferred from the hemiacetal species rather than from the

surface, as suggested by Haber et al. [45]. The chloromethoxy disproportionation is accompanied by

the formation of Al-O-Al bridges that cause a decrease in the number of available hydroxyl groups.

Then, the formed formate species decompose to CO or CO , and ﬁnally, water regenerates the hydroxyl

2

groups (step 4). [19]

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Pinard et al. studied DCM oxidation over pure NaY and PtNaY catalysts [57], NaX and NaY

zeolites [58], and PtHFAU [48] and PtFAU catalysts [59]. They reported that DCM transformation

occurs bifunctionally via DCM hydrolysis on the Brønsted acid sites of the support, resulting in

formaldehyde and HCl, followed by formaldehyde oxidation into CO and H O on the Pt sites.

2

Water is required for the formation of formaldehyde. Formation of CO may result from formaldehyde

oxidation or by decomposition of formate species. The main reactions were suggested as:

CH Cl + H O→CH O + 2HCl and

(1)

(2)

2

1

CH O + O →CO + H O

2

A methyl chloride intermediate was formed at higher temperatures via the Lewis acid sites [48].

The CO formation could result from the direct oxidation of formaldehyde [48], in addition to the

proposed decomposition of formate species on Lewis acid sites, as discussed by van den Brink et al. [11

]

and Maupin et al. [19]. According to Maupin et al. [19], Pt/Al O catalysts oxidize DCM completely at

2

3

◦

3

80 C. The DCM reaction rate limiting step occurs on alumina, since neither the Pt content nor the

dispersion (particle size) inﬂuence the conversion rates and selectivity [19]. Similar observations were

seen in our previous study [12]. The addition of Pt and/or ceria to the catalyst improved the selectivity

towards CO , but the oxidation rates were not aﬀected that much [12]. In our experiments, the addition

2

of Au on alumina resulted in a signiﬁcant decrease in the light-oﬀ temperature, and the addition Au on

Pt/Al O enhanced both DCM conversion and selectivity towards the total oxidation products.

2

3

Au catalysts are known to be eﬀective in selective partial oxidation [42,43,60–62], often succeeding

where other metals fail due to a weaker adsorption of the reactants (such as hydrocarbons) on the

catalyst surface [63]. The too-weak adsorption of reaction intermediates might be one reason for

the lower total oxidation result observed in our experiments with the monometallic Au catalysts.

According to Haruta et al. [42], the surface adsorption and reactivity of Au can be modiﬁed by

generating surface structures via downsizing or scratching. Due to the moderate bonding strength on

the defect sites of Au, which is weaker than that on Pd and Pt, it often occurs that Au catalysts are

better than other noble metal catalysts at low temperatures [42]. This can also be seen in our case,

since reaction products are observed at lower temperatures with the Au-containing catalysts than with

the Pt/Al.

In the presence of water, alumina is strongly hydrated, which facilitates the reaction between

DCM and hydroxyl groups from alumina [19]. It is worth noting that alumina is also hydrophilic

and, thus, attracts water on its surface [64]. In addition, the Pt/Al O catalyst is hydrophilic [53],

2

3

and Au has a hydrophilic nature [42]. Water is weakly adsorbed on Au [65]. The adsorption of

water is followed by water dissociation into protons and OH groups, which are the sites for DCM

adsorption, and, thus, the adsorption is enhanced. We could speculate that the relatively high amounts

of formaldehyde produced by the Au/Al catalyst might result from a high amount of Brønsted acid

sites. Costello et al. [66

,

67] studied an Au/

forms Au -OH via adsorption on Au clusters. Chemisorbed oxygen on Au has a Brønsted base

character [42 65]. On the Au surface, atomic oxygen reacts like a Brønsted base by abstracting a

γ-Al O catalyst in CO oxidation and proposed that water

2 3

+

-

,

hydrogen atom from acidic molecules. Furthermore, oxygen atoms on the Au(110) surface were

observed to react like a nucleophilic base towards formaldehyde. [65] This might explain the product

distributions in the case of the Au-containing alumina catalysts. Further oxidation of an adsorbed

compound (in this case, formaldehyde) is suppressed when the activation energy for desorption is

lower than that for oxidation. Ceria-containing catalysts, however, are able to oxidize carbon products,

further resulting in the formation of CO and CO due to its high oxygen storage capacity and good

2

redox property [68]. The diﬀerent product distribution might be also a consequence of diﬀerent reaction

routes: on alumina, direct DCM hydrolysis into formaldehyde appears, and, on ceria, the Mars-van

Krevelen mechanism prevails.

Molecules 2020, 25, 4644

19 of 25

The role of Au is evident, since formaldehyde, methyl chloride, and CO are formed in comparison

to Pt/Al, which produces mainly CO . Au enhances the DCM conversion and HCl yield but results

2

in partial oxidation products. The results suggest that DCM decomposition over the Au-containing

alumina-supported catalysts proceeds via a bifunctional reaction in which hydrolysis is the ﬁrst step,

followed by oxidation.

3

. Materials and Methods

3

.1. Catalysts

Supported monometallic Pt and Au catalysts were prepared using wet impregnation and

deposition-precipitation with urea, respectively. Bimetallic catalysts were prepared with surface redox

reactions in an aqueous phase, and calcined monometallic Pt catalysts were used as “parent” catalysts

in their synthesis. Commercial aluminum oxide (γ-Al O , Rhodia) was used as the support material

2 3

and as a basis for the preparation of the ceria-alumina (20 wt.% of ceria) support (wet impregnation).

Nomenclature and target compositions of the catalysts are shown in Table 6. Calcination of the catalysts

◦

−1

was done in air for 5 h at 600 C with a heating rate of 5 C min . The preparation steps are described

in detail in the Supplementary Materials.

Table 6. Abbreviations, desired metal loadings, and the used preparation methods for the

studied catalysts.

Catalyst

Support

Desired Metal Loading (wt.%)

Preparation Method

Au

Pt

1

Pt/Al

Au/Al

Pt-Au/Al

Pt/Ce-Al

Au/Ce-Al

Pt-Au/Ce-Al

Al O

Wet impregnation

2

3

Al O

1

Deposition precipitation with urea

Surface redox reactions in aqueous phase

Wet impregnation

Deposition precipitation with urea

Surface redox reactions in aqueous phase

2

Al O

1

2

CeO -Al O

2

3

CeO -Al O

1

2

CeO -Al O

1

2

3

.2. Catalyst Characterization

The prepared catalysts were characterized using ICP-OES (Perkin Elmer Optima 2000 DV, Waltham,

MA, USA), N physisorption (Micromeritics ASAP 2020, Norcross, GA, USA), X-ray diﬀraction (XRD;

2

Siemens D5000, Munich, Germany), X-ray photoelectron spectroscopy (XPS; Thermo Fisher Scientiﬁc

ESCALab 250Xi, Waltham, MA, USA), high-resolution transmission electron microscope (HR-TEM),

temperature-programmed reduction with hydrogen (H -TPR), temperature-programmed isotopic

2

18

exchange (TPIE) of labeled oxygen ( O ), and temperature-programmed desorption of ammonia

2

(

NH -TPD) and carbon dioxide (CO -TPD). Details of the characterization procedures and the results

3 2

are given in References [27,28].

In order to study the properties of the catalysts further, high-resolution transmission electron

microscope (HR-TEM) and high-angle annular dark-ﬁeld scanning transmission electron microscope

(HAADF-STEM) were employed. HR-TEM and HAADF-STEM images were obtained using two

devices: a JEOL JEM-2100 (Tokyo, Japan) and a JEOL JEM-2200FS (Tokyo, Japan), both operated at an

acceleration voltage of 200 kV and equipped with an energy-dispersive X-ray (EDX) spectrometer (JEOL,

Tokyo, Japan). HR-TEM and HAADF-STEM (JEOL JEM-2200FS) were used as the characterization

methods after a 100-h stability test in order to examine the surface of the Pt-Au/Al catalyst.

Temperature-programmed desorption (NH -TPD and CO -TPD) experiments were done for the

3

2

monometallic Au and Pt catalysts supported on alumina and ceria-alumina. The NH -TPD experiments

3

were done using a Quantachrome Chembet Pulsar TPR/TPD device (Boynton Beach, FL, USA) equipped

with a thermal conductivity detector (TCD). The samples (200 mg) were pretreated under an He ﬂow

◦

at 500 C for 30 min and were cooled down to 100 C in an He ﬂow. Then, the samples were treated

◦

−1

with NH for 60 min at 100 C using 120 mL min of 10% NH in He. The physiosorbed NH was

3

Molecules 2020, 25, 4644

20 of 25

◦

removed by feeding He for 30 min at 100 C. Finally, NH desorption was conducted from 100 C to

9

3

◦

−1

50 C (ramp rate of 10 C min ). The CO -TPD experiments were done using an Autochem II device

2

(

Micromeritics, Norcross, GA, USA) equipped with a TCD detector. The samples (50 mg) were ﬂushed

◦

−1

in an He ﬂow at 450 C for 30 min. Next, 5% CO in He (50 mL min ) was adsorbed on the sample

over 60 min at 50 C. After ﬂushing with He for 30 min, the TPD was conducted from 50 to 600 C at a

2

◦

−1

heating rate of 10 C min . XPS measurements were done using a Thermo Fisher Scientiﬁc ESCALab

50Xi spectrometer (Waltham, MA, USA) with an Al K (1486.6 eV) radiation source and a pass energy

of 20 eV. The data were analyzed with Thermo Scientiﬁc Avantage software (v. 5.9904), and the

2

α

™

signals were ﬁtted with the mixed Gaussian–Lorentzian function. Binding energies were referred to

the C1s peak line at 284.8 eV, and the Smart function was used to reduce the background.

3

.3. Catalytic Experiments

Catalytic experiments were performed in a quartz reactor system operated under atmospheric

◦

pressure. The materials used in the experimental set-up (quartz glass, Teﬂon tubings heated to 180 C,

and Teﬂon connectors) are corrosion-resistant due to the nature of reaction products. Liquid DCM and

water were injected into an evaporator using gas tight syringes and mixed with air fed through a mass

ﬂow controller. The preheating oven, ﬁlled with glass spheres to ensure the mixing of gases, was set

◦

to 150 C. More speciﬁc descriptions of the experimental set-up and the Fourier-transform infrared

(

FTIR) analyzer (Gasmet Technologies, Helsinki, Finland) used in the gas analysis can be found in

References [12,69]. The accuracy of the FTIR analysis was 2 ppm.

Catalytic experiments were carried out in a temperature range from 100 C to 600 C, with a

◦

−1

heating rate of 5 C min and a total gas ﬂow of 1 L min equivalent to a weight hourly space

−

1

−1

velocity (WHSV) of 720 g gcat h . The DCM concentration was set to 500 ppm and water feed to

.5 vol.% in all the experiments. A catalyst sample of 100 mg was placed in the vertically aligned

1

tubular ﬁxed-bed reactor between quartz wool plugs to hold it in place. Before each experiment,

◦

the catalysts were pretreated in an air ﬂow from room temperature to 600 C, with the heating rate of

◦

−1

1

0 C min . The light-oﬀ experiments were repeated once to verify the results. The 100-h stability

◦

test was carried out for the Pt-Au/Al catalyst at 395 C, corresponding to ~90% DCM conversion.

The continuous feed of DCM (500 ppm) into the evaporator unit was limited to approximately 5.5 h

before the syringe needed to be reﬁlled. The reﬁlls for DCM and water were done simultaneously,

and their concentrations were let to stabilize before the gas stream (air + DCM + H O) was fed back

2

into the reactor. At the end of each testing day, the feed stream was stopped, and the oven was switched

oﬀ for the night. The following day, after the oven temperature reached the set point, the stabilized gas

stream was introduced into the catalyst bed again.

A Gasmet DX-4000N FTIR analyzer (Gasmet Technologies, Helsinki, Finland), capable of detecting

close to all gas phase compounds except diatomic homonuclear compounds such as chlorine, hydrogen,

oxygen, and nitrogen, was used for gas analysis. The analyzer was calibrated to detect the following

chlorinated compounds: CH Cl , C Cl , C HCl , CHCl , COCl , CH Cl, and HCl. In addition,

2

4

2

3

2

x

3

diﬀerent VOCs (such as formaldehyde, CH O) and CO , CO, NO

, as well as water, were included

2

in the calibrations. Altogether, 41 compounds were analyzed. The conversion of dichloromethane

CH Cl , DCM), and the yields of hydrochloric acid (HCl) and carbon dioxide (CO ), which are the

(

2

desired ﬁnal products, were calculated as follows:

in

out

c

− c

DCM

X_D_C_M= 100 ×

(3)

(4)

in

c

DCM

out

c

HCl

Y_H_C_l= 100 ×

in

2

× c

DCM

Molecules 2020, 25, 4644

21 of 25

out

CO2

c

Y_C_O₂= 100 ×

(5)

in

DCM

c

y

x

where

X

is the conversion (%),

Y

is the yield (%),

c

is the concentration of the compound

x

(ppm)

(

x denoting DCM, HCl, or CO , and y representing the inlet or outlet concentration).

2

4

. Conclusions

Six metal loaded catalysts containing Au, Pt, and Pt-Au supported on Al O and CeO -Al O

3

2

3

2

were studied in the DCM oxidation. The results showed that Au/Al O is active and highly HCl

2

3

selective, even outperforming Pt/Al O , which was found as the most active and HCl selective catalyst

2

3

in our previous study. The reasons might be the small Au particle size (~5 nm), with narrow size

distribution ( 5 nm) and good metal dispersion. The oxygen activation ability seems to lead to the

±

high DCM conversion of Au/Al and Pt-Au/Al catalysts in DCM oxidation. Au changes the product

distribution. The formation of Cl-containing intermediates such as methyl chloride was fairly low

with the Pt-Au/Al catalyst and zero with the Pt-Au/Ce-Al catalyst. Based on our experiments, the total

acidity of the catalysts decreased after the introduction of ceria to alumina, thus decreasing DCM

adsorption and, consequently, the catalytic performance. At the same time, the addition of ceria

eﬀectively improved the selectivity towards the total oxidation products, decreasing the formation

of intermediates such as methyl chloride, carbon monoxide, and formaldehyde. The results suggest

that, with the Au-containing Al O -supported catalysts, DCM decomposition occurs mainly via direct

2

3

DCM hydrolysis into formaldehyde and HCl, followed by the oxidation of formaldehyde into CO and

CO . To get deeper insight on the reaction mechanism, in situ studies are required.

2

The bimetallic Pt-Au/Al O catalyst showed good catalytic performance and stability based on

2

3

◦

the light-oﬀ and 100-h stability experiments at 395 C, but selectivity towards the total oxidation

products in the stability test was not satisfactory. By-product yields, i.e., CO, formaldehyde, and methyl

chloride, between 5% to 30% were observed. The results are signiﬁcant in two respects. Firstly,

they demonstrated the possibility of Au being used eﬀectively in CVOC oxidation (high DCM

conversion and HCl selectivity), and, secondly, the Pt-Au/Al catalyst showed chlorine resistance during

the stability test. The next step is to study whether the DCM oxidation over Au catalysts is size- and/or

shape-dependent and to conﬁrm the role of reducibility and acidity, especially with respect to Brønsted

and Lewis acid sites of the Au catalyst.

Supplementary Materials: The following are available online.

Author Contributions: T.K.N. did most parts of the study (surveying the literature, preparing the catalysts, most

of the characterization, and all catalytic experiments) and was responsible for writing (original draft preparation)

and taking part in the conceptualization. S.P. led the conceptualization and took part in the interpretation of

the results, organization of the manuscript, and writing (review and editing). S.O. and R.L.K. helped with the

interpretation of results and helped in the writing (review and editing). R.L.K., S.O. and S.P. were supervisors

of T.K.N.’s doctoral thesis and contributed to the organization, planning, project administration, and funding

acquisition for this work. All authors have read and agreed to the published version of the manuscript.

Funding: The work was done with the funding of the Council of Oulu Region from the European Regional

Development Fund (A32164), the City of Oulu, and Maj and Tor Nessling Foundation. The Finnish Foundation for

Technology Promotion, KAUTE Foundation, Walter Ahlström Foundation, Tauno Tönning Foundation, and the

University of Oulu Graduate School are acknowledged for their ﬁnancial support for the research reported

in this paper. The research gained funding also from the European Union Seventh Framework Programme

0

(

FP7/2007-2013) under the grant agreement n (PIRSES-GA-2012-317714).

Acknowledgments: Florence Epron and Nicolas Bion are acknowledged for their invaluable help and for accessing

the characterization infrastructure at the University of Poitiers. Shudong Wang and Sheng Wang are thanked for

the use of HR-TEM and NH -TPD equipment at the Dalian Institute of Chemical Physics. Appreciations to Tiina

3

Laitinen, Zouhair El Assal, and Sami Saukko for their assistance in the experimental work.

Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the

study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to

publish the results.

Molecules 2020, 25, 4644

22 of 25

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Sample Availability: Samples of the compounds are not available from the authors.

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Oxidation of dichloromethane over Au, Pt, and Pt-Au containing catalysts supported on γ-Al2O3 and CeO2-Al2O3

DOI: 10.3390/molecules25204644

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