Complete oxidation of formaldehyde at ambient temperature over γ-Al2O3 supported Au catalyst

    Complete oxidation of formaldehyde at ambient temperature over γ-Al 2 O3 supported Au catalyst Bing-bing Chen, Xiao-bing Zhu, Mark Crocker, Yu Wang, Chuan Shi PII: DOI: Reference:

S1566-7367(13)00317-8 doi: 10.1016/j.catcom.2013.08.008 CATCOM 3612

To appear in:

Catalysis Communications

Received date: Revised date: Accepted date:

30 May 2013 8 August 2013 13 August 2013

Please cite this article as: Bing-bing Chen, Xiao-bing Zhu, Mark Crocker, Yu Wang, Chuan Shi, Complete oxidation of formaldehyde at ambient temperature over γ-Al2 O3 supported Au catalyst, Catalysis Communications (2013), doi: 10.1016/j.catcom.2013.08.008

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ACCEPTED MANUSCRIPT Complete oxidation of formaldehyde at ambient temperature

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over γ-Al2O3 supported Au catalyst

Key laboratory of Industrial Ecology and Environmental Engineering (MOE), Dalian University of Technology,

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a

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Bing-bing Chen a,b, Xiao-bing Zhu b, Mark Crocker c, Yu Wang a,b, Chuan Shi a,b*

Dalian, People’s Republic of China b

Laboratory of Plasma Physical Chemistry, Dalian University of Technology, Dalian, People’s Republic of China Center for Applied Energy Research, University of Kentucky, Lexington, KY 40511, USA

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c

Abstract

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Au supported on γ-Al2O3 prepared by deposition-precipitation (DP) using urea is found to be a highly active catalyst for the total oxidation of HCHO at room

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temperature under humid air, without the need for a reducible oxide as support. In-situ

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DRIFTS studies suggested that the surface hydroxyl groups played key role in the partial oxidation of HCHO into the formate intermediates, which can be further

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oxidized into CO2 and H2O with participation of nano-Au. This study challenges the traditional idea of supporting noble metals on reducible oxides for HCHO oxidation at

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room temperature.

Keywords: formaldehyde, catalytic oxidation, Au/γ-Al2O3, room temperature 1. Introduction Given that formaldehyde is a major indoor air pollutant, significant efforts have been directed at indoor HCHO removal to meet environmental regulations and human health needs [1, 2]. Catalytic oxidation is recognized as the most promising HCHO removal technology. Indeed, supported Pt catalysts such as Pt/TiO2 and Pt/MnOx-CeO2 have been shown to be active for HCHO complete oxidation at room temperature [3-6]. Moreover, in our previous study, 1% Au/CeO2 catalyst was found *

Corresponding author. Tel.: +86 411 84986083;

E-mail address: [email protected] (Chuan Shi) 1

ACCEPTED MANUSCRIPT to provide 100% HCHO conversion at room temperature (RT). It was suggested that HCHO was partially oxidized into [HCOO]s intermediates on the support, further oxidation of these intermediates requiring the participation of gas phase oxygen

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activated by Au nanoparticles [7]. In general, reducible oxides have been used as the support of choice in past studies because of their high concentrations of oxygen

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defects and their ability to stabilize high dispersions of Pt or Au.

HCHO oxidation aside, γ-Al2O3 is the most common support material for metal

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catalysts, due to its low cost, thermal and chemical stability, high surface area and amphoteric character [8, 9]. However, γ-Al2O3 is considered to be a poor support for

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low temperature HCHO oxidation catalysts due to its irreducibility [10]. It is generally agreed that a reducible oxide such as ceria has strong surface interactions with the supported metal which help to stabilize the latter. Such interaction might

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cause the charge transfer from the supported metal to the support, which leads to the

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weakened bond of the support, such as Ce-O bond [7, 11, 12]. That is the way that the surface oxygen species become very active and normally believed to have key roles in

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low temperature HCHO oxidation [13-15]. Herein, we report for the first time that γ-Al2O3 supported Au is a very active

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catalyst for HCHO oxidation at RT even in the presence of moisture. It is found that although there is no active surface oxygen on γ-Al2O3, surface hydroxyls have the ability to partially oxidize HCHO into formate intermediates, which can be further oxidized into CO2 and H2O by nano-Au. This study challenges the traditional idea of supporting noble metals on reducible oxides for HCHO oxidation at RT. 2. Experimental Section 2.1 Catalyst preparation. Au/γ-Al2O3 catalysts with nominal gold loadings of 0.25, 0.5 and 1 wt% were prepared by the deposition-precipitation method, using urea as precipitant, according to a literature procedure [7]. The supported gold catalysts were dried in air at 80 oC

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ACCEPTED MANUSCRIPT for 16 h, and calcined in air at 200 o C for 4h. 2.2 Catalyst characterization.

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The catalysts were characterized by CO-chemisorption, UV-vis spectra,

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transmission electron microscopy (TEM), H2 temperature-programmed reduction

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(H2-TPR), and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. The detailed procedures used are described in the Supplementary data.

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2.3 Catalytic Activity measurement.

Catalyst tests were performed in a continuous flow fixed bed quartz microreactor

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at atmospheric pressure. 0.25 g catalyst (0.176 mL, 40-60 mesh) was sandwiched between quartz wool layers in the tube reactor. The typical feed gas composition was 80 ppm HCHO and 21 vol. % oxygen, balanced by nitrogen with a relative humidity

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(RH) of 50%. The total flow rate was 100 ml·min-1, corresponding to a gas hourly

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space velocity (GHSV) of 34,000 h-1. Concentrations of CO and CO2 were measured

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online using an infrared absorption spectrometer (SICK-MAIHAK-S710, Germany). In this work, it was not possible to monitor the HCHO concentration directly by Fourier transform-infrared spectroscopy (FT-IR) due to the interfering effects of

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water. Therefore, HCHO was measured by converting it to CO2 in a homemade HCHO-to-CO2 converter (CuO-MnO2/γ-Al2O3 catalyst) at 300 oC and determining the amount of CO2 formed [16, 17]. The detailed explanations are described in the Supplementary data. Conversion of HCHO was calculated as follows: HCHO conversion (%) 

[CO2 ]out vol.%  100 , [HCHO]in vol.%

where [CO2]out is the concentration in the products (vol. %) and [HCHO]in is the HCHO concentration in the feed gas (vol. %). In all cases the carbon balance was near 100%. 3. Results and Discussion 3

ACCEPTED MANUSCRIPT 3.1 Catalytic activity The activity of Au/γ-Al2O3 catalyst for HCHO oxidation at room temperature

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was evaluated by fixing the initial HCHO concentration at 80 ppm and the relative

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humidity (RH) at 50% (25 oC). The performance of Au/γ-Al2O3 catalysts with

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different gold contents in HCHO oxidation is depicted in Fig. 1(A). Over the pure γ-Al2O3 support, almost no HCHO conversion was obtained at room temperature. After the introduction of Au, the conversion of HCHO was seen to increase with the

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Au content. The catalyst with a gold content of 0.25 wt% was much more active than the pure γ-Al2O3 support, the HCHO conversion reaching 40.9%. With 0.5 wt% Au

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addition, the conversion of HCHO increased to 84.9%, while complete conversion of HCHO was achieved over the 1 wt% Au catalyst. These results demonstrate the

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superior properties of Au/γ-Al2O3 catalysts for formaldehyde removal.

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Figure 1(B) investigates the effect of humidity on HCHO oxidation at room temperature. It can be seen that the 1 wt% Au/γ-Al2O3 catalyst showed nearly 90%

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HCHO conversion in the dry feed gas and at 25% RH (25 oC). However, upon increasing the RH to 50% and 75% (25 oC), HCHO was completely oxidized into CO2 and H2O at room temperature, demonstrating that rather than inhibiting the catalyst

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the presence of water actually enhanced the HCHO conversion. Figure S1 (A) compares the conversions of HCHO over the 1 wt% Au/γ-Al2O3 catalyst at different gas hourly space velocities (GHSVs). 100% HCHO conversion was obtained, even at a high GHSV value of 95,500 h-1. A durability test was also conducted at a GHSV of 32,000 h-1 over 0.5 wt% Au/γ-Al2O3. As shown in Fig. S1 (B), no deactivation could be observed after operation for 30 h, indicating that the catalyst is stable under these conditions. These results suggest that the Au/γ-Al2O3 catalyst exhibits high activity and durability for HCHO conversion to CO2, even in wet air. 3.2 Physicochemical properties of the catalysts

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ACCEPTED MANUSCRIPT Fig. 2 presents in situ DRIFT spectra of the γ-Al2O3 and 1 wt% Au/γ-Al2O3 samples under 0.2% CO/He at room temperature. For the γ-Al2O3, adsorption bands at 2170 and 2123 cm-1 can be assigned to CO adsorbed on Al3+ cations and on the

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surface OH groups of the γ-Al2O3, as described elsewhere [18-20]. When the 1 wt%

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Au/γ-Al2O3 catalyst was exposed to CO at RT, three bands immediately appeared at

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2170, 2114 and 2045 cm-1. According to the literature, the band at 2170 cm-1 can be ascribed to CO adsorbed on the support, while the others are attributed to CO adsorbed on Au0 step sites on small particles and CO linearly adsorbed on Au0 [21,

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22]. Hence, the results demonstrate that metallic Au0 species are the main Au species

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on the 1 wt% Au/γ-Al2O3 catalyst.

UV-vis spectra of γ-Al2O3 and 1 wt% Au/γ-Al2O3 are shown in Fig. 3. Besides the contribution of the γ-Al2O3 support, 1 wt% Au/γ-Al2O3 shows a broad band

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centered at 540 nm which is characteristic of metallic Au0 species [23-25]. This result

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is in agreement with the results of CO-chemisorption, as well as H2-TPR results shown in Fig. S2 Specifically, the absence of a reduction peak in the temperature

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range examined (20-200 oC) is consistent with metallic Au0 being the main species present, and also indicates the absence of surface active oxygen species.

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Fig 4 shows a TEM micrograph of the 1 wt% Au/γ-Al2O3 catalyst together with the associated particle size histogram. The gold nanoparticles are highly dispersed on the γ-Al2O3 surface and an average of particle size of 2 nm is determined from TEM images (based on measurement of >100 gold particles). As shown in Fig. S3, the γ-Al2O3 surface is covered by hydroxyl groups. These would be expected to interact with the Au nanoparticles, helping to stabilize them and thereby contributing to their small size [26]. 3.3. In situ DRIFT study In situ DRIFT spectra of the γ-Al2O3 and 1 wt% Au/γ-Al2O3 samples, obtained upon exposure to a stream of HCHO-containing gas at room temperature, are shown in Fig. 5a. Upon exposure to HCHO, bands were observed at 2889, 2868, 1634, 1591, 5

ACCEPTED MANUSCRIPT 1395, 1371 and 1320 cm-1. (Spectra were obtained in the absence of water, because the δ(H-O-H) band of adsorbed water and the formate νas(COO) band appear at the same position.) According to the literature, the bands at 2889 and 2868 cm -1 are due

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to the ν(CH) stretch, the bands at 1630 and 1593 cm-1 can be attributed to the

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asymmetric νas(COO) stretch of formate species, and the other bands are assigned to

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the corresponding symmetric νs(COO) stretch [27]. The intensity of these bands increased with HCHO exposure time. However, no peaks associated with formaldehyde were detected, consistent with immediate oxidation of the HCHO after

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its adsorption. In the case of the 1 wt% Au/γ-Al2O3 catalyst, results were nearly the same as for the γ-Al2O3 sample, bands attributed to formate species being observed,

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while their intensities became stronger with increasing time on stream. Comparing the species formed over the γ-Al2O3 and Au/γ-Al2O3 samples, it is apparent that formate

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species formed over γ-Al2O3 in the absence of gas phase oxygen, suggesting that the

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surface OH groups of the γ-Al2O3 are active in the partial oxidation of HCHO to [HCOO]s according to the following equation:

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[HCHO]s +[OH]s  [HCOO]s +2[H]s

(1)

The consumption of the formates on γ-Al2O3 and 1 wt% Au/γ-Al2O3 upon their exposure to wet air was monitored by in situ DRIFTS as shown in Fig. S4 and Fig. 5b.

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Samples were first exposed to 80 ppm HCHO/21%O2/N2 at room temperature for 90 min. For both samples, the intensity of the band at 1635 cm-1 became stronger after the switch to wet air, resulting from the adsorption of water (δ (H-O-H) band). For γ-Al2O3, no obvious decrease in formate band intensity was observed upon exposure to the wet air for 90 min. In contrast, after 90 min the intensity of the bands due to formate species decreased sharply over the 1 wt% Au/γ-Al2O3 catalyst. These results confirm that formate species could be oxidized over the Au/γ-Al2O3 catalyst at room temperature, which is in agreement with the superior activity of Au/γ-Al2O3 for HCHO oxidation in the presence of H2O, as depicted in Fig. 1(A). These results also demonstrate the role of Au in activation of oxygen for complete oxidation of the intermediates into CO2 and H2O. 6

ACCEPTED MANUSCRIPT The consumption of formate on 1 wt% Au/γ-Al2O3 upon exposure to dry and wet gas streams is compared in Fig. 5b. Samples were first exposed to 80 ppm HCHO/21%O2/N2 at room temperature for 90 min. After the switch to dry air, partial

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consumption of the formates was observed during the first 60 min, although no further change was observed during the subsequent 120 min. In contrast, the formate species

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were completely consumed in the presence of water, this being consistent with the higher HCHO conversions obtained under humid air compared with dry gas as shown in Fig. 1(B). These results confirm that water has a positive effect on consumption of

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the formate intermediates. We postulate that the introduction of water into the feed gas generates hydroxyl groups on the catalyst surface, and these groups can further

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oxidize formate into CO2 and H2O. This promoting effect of water can be expressed as follows:

(2)

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[HCOO]s + [OH]s  CO 2 (g) + H 2O(g)

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In summary, metallic Au0 was found to be the main species dispersed on the γ-Al2O3 support. IR spectroscopy suggested there was a high concentration of surface

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hydroxyl species present on the Au/γ-Al2O3 catalyst, which has the ability to partially oxidize HCHO into formate intermediates. In situ DRIFT results suggested that the rate limiting step for catalytic oxidation of HCHO was deep oxidation of formates

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into CO2 and H2O, which required the presence of highly dispersed Au for activation of O2. Water had a positive effect on formate oxidation, thereby enhancing catalyst activity under humid air. 4. Conclusion In this study, we constructed the bi-functional Au/γ-Al2O3 catalyst for HCHO complete oxidation under wet air at room temperature. In situ DRIFTS results suggested that the surface hydroxyl groups of γ-Al2O3 played key role in the partial oxidation of HCHO into the formate intermediates, but the rate limiting step of HCHO catalytic oxidation was to deep oxidation of the formates into CO2 and H2O, which need the participation of highly dispersed Au activating O2 for complete

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ACCEPTED MANUSCRIPT oxidation of the intermediates. This is the first report on irreducible oxide supported Au catalyst for HCHO complete oxidation under ambient temperature and humid air.

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Acknowledgments

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The work was supported by the National Natural Foundation of China

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(No.20573014 and 21073024), Natural Science Foundation of Liaoning Province (No.201102034) and by the Program for New Century Excellent Talents in University (NCET-07-0136), as well as by the Fundamental Research Funds for the Central

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Universities (No.DUT12LK23).

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ACCEPTED MANUSCRIPT Figure captions Fig. 1 (A) HCHO conversion with various Au contents over Au/γ-Al2O3 catalyst; (B)

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Effect of relative humidity on HCHO conversion over 1% Au/γ-Al2O3 catalyst.

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Reaction conditions: 80 ppm HCHO/21%O2/H2O/N2; R.T.

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Fig. 2 In situ DRIFT spectra of CO adsorbed on γ-Al2O3 and 1% Au/γ-Al2O3 at RT;

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Fig. 3 UV-vis spectra of γ-Al2O3 and 1% Au/γ-Al2O3

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Fig. 4 TEM image and the associated particle size histogram of 1% Au/γ-Al2O3

Fig. 5 (a) In situ DRIFT spectra of HCHO adsorption on γ-Al2O3 and 1% Au/γ-Al2O3

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at RT; reaction conditions: 80 ppm HCHO/N2; (b) In situ DRIFT spectra

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showing formate consumption on 1% Au/γ-Al2O3 in different atmospheres

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following exposure of the catalyst to 80 ppm HCHO/21% O2/N2 for 90 min.

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HCHO conversion to CO2 / %

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Graphical Abstract

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ACCEPTED MANUSCRIPT Highlights

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 Au/γ-Al2O3 catalyst shows superior HCHO oxidation activity at room

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temperature

 Surface hydroxyl groups played key role in the catalytic oxidation of HCHO

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 Complete oxidation of HCHO was firstly reported over γ-Al2O3 supported Au

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catalyst

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