Identification of the nearby hydroxyls role in ... - ACS Publications

Subscriber access provided by - Access paid by the | UCSB Libraries

Kinetics, Catalysis, and Reaction Engineering

Identification of the nearby hydroxyls role in promoting HCHO oxidation over a Pt catalyst Ying Huo, Xuyu Wang, Zebao Rui, Xiaoqing Yang, and Hongbing Ji Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01547 • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 29, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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

Industrial & Engineering Chemistry Research

Identification of the nearby hydroxyls role in promoting HCHO oxidation over a Pt catalyst

Ying Huo #, Xuyu Wang #, Zebao Rui *, Xiaoqing Yang, Hongbing Ji *

School of Chemical Engineering and Technology (Zhuhai 519082), Fine Chemical Research Institute, School of Chemistry (Guangzhou 510275), Sun Yat-sen University, P.R. China

#

Equal contribution

*Correspondence to the authors can be sent to Z. B. Rui ([email protected]) and H. B. Ji ([email protected])

1

ACS Paragon Plus Environment

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

ABSTRACT

Insight into the relationship between catalytic trend and physicochemical properties of the composite nanoparticles is essential for their rational design. Herein, a series of 3d-M (M = Mn, Fe, Co, Ni) metal hydroxides promoted PtM(OH)x/Al2O3 catalysts are developed and well-characterized for establishing the catalytic HCHO oxidation reactivity trend as a function of more fundamental properties, like hydroxyls concentration and adsorption strength. The reactivity of PtM(OH)x/Al2O3 exhibits an increasing trend of Mn< Fe< Co< Ni, which is governed by their OH–M2+δ bond strength (Ni < Co < Fe < Mn) and surface hydroxyls concentration (Mn < Fe < Co < Ni). Both PtCo(OH)x/Al2O3 and PtNi(OH)x/Al2O3 exhibit a (>)95% HCHO conversion and (>)100 h performance stability at 30 oC with a low 0.2 wt.% Pt loading amount. The identification of these catalytic trends provides foundations for composite active sites design for HCHO oxidation and other hydroxyls involved reactions.

Keywords: Composite active sites; Metal hydroxide; Platinum; Formaldehyde oxidation; Hydroxyl promotion

2


Page 2 of 23

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


INTRODUCTION Catalytic formaldehyde oxidation to CO2 and H2O is an important reaction in the 1-9

envrionmental field due to the harmfulness and wide range existence of HCHO

.

Although supported noble metal catalysts have been demonstrated to exhibit a high HCHO oxidation activity, their practical application are still restricted by either heating or high noble metal loading requirement

2-8

. A raional catalyst design based

on the understanding of the reaction mechanism continues to be a great chanllage in this field. There are generally two pathways for catalytic HCHO oxidation over a supported noble metal catalyst. One is the direct HCHO oxidation route (HCHO→ HCOO- → CO → CO2), in which the formate decomposing into CO being the rate determining step

10-13

.

This route is mainly popular for those

surface

hydroxyls-lacking noble metal catalysts, for which the performace is usually tailored through manipulating the chemisorbed oxygen and metal-support interaction

12-14

. In

the presence of efficient hydroxyls nearby the noble metal active sites, HCHO oxidation mainly proceeds through the hydroxyls promoted route (HCHO → HCOO2, 15-21

→ H2O + CO2)

and the direct formate oxidation with hydroxyls is usually

regarded as the controlling step 2, 21. In addition, the hydroxyls promoted pathway was found to be a more efficient HCHO oxidation route, especially for those irreducible material supported noble metal catalysts 19-21. Varies strategies have been proposed to introduce surface hydroxyls for improving the performance of the supported metal catalysts

2, 17, 19, 21

. Zhang et al. 2 reported that the addition of alkali-metal ions (such

as Na+) to Pt/TiO2 stabilized Pt-O(OH)x–alkali-metal composite species on the catalyst surface, remarkably promoting the HCHO oxidation activity. Xu et al.

19

found nanostructured AlOOH supported Pt catalysts were more active for the 3



Page 4 of 23

oxidative decomposition of HCHO than the conventional commercial Al2O3 supported Pt catalyst. More recently, our group constructed nickel hydroxide promoted PtNi(OH)x/Al2O3 and achieved a good HCHO oxidation performance comparable with those reducible oxide supported Pt catalysts through employing the interface talioring strategy

21

. Indeed, the rational design and synthesis of complex

active sites composing of precious metals and non-precious metal oxides/hydroxides is an effectve strategy in developing efficient noble metal based catalysts 21-24. In spite of the great progress in the development of hydroxyls promoted noble metal catalysts and their HCHO oxidation application, their design foundations, such as hydroxyls density, hydroxyls adsorption strength and the preferable combination between noble metal and non-precious metal hydroxides, are still unclear. Identification of the relationship between the catalytic trend and these design foundations is essential for their rational design. Herein, a series of 3d-M metal hydroxide promoted PtM(OH)x/Al2O3 (M = Mn, Fe, Co, Ni) catalysts were developed and well-characterized for exploring the catalytic HCHO oxidation reactivity trend as a function of hydroxyls concentration and adsorption strength.The relationship exhibits reactivity trend in PtM(OH)x/Al2O3 of Mn< Fe< Co< Ni, which is governed by hydroxyl adsorption strength or OH–M2+δ bond strength (Ni< Co< Fe< Mn) in association with surface hydroxyls concentration (Mn< Fe< Co< Ni). The clear identification of these catalytic trends brings valuable foundations for rational design of composite active sites for hydroxyls involved reactions.

EXPERIMENTAL SECTION Materials

Synthesis.

The

chemicals

n-butylamine

(98%),

platinum

acetylacetonate [Pt(acac)2, 97%], nickel acetylacetonate [Ni(acac)2, 95%], cobalt 4


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


acetylacetonate [Co(acac)3, 98%], iron acetylacetonate [Fe(acac)3, 98%], manganese acetylacetonate [Mn(acac)2, 97%] and oleylamine (OAm, 80-90%) were bought from Aladdin and used without further treatment. In a typical PtNi nanoparticles (NP) synthesis process, about 10 mg Pt(acac)2, 5 mg Ni(acac)2 and 5 mL oleylamine solution were mixed together and heated at 343 K in a round-bottomed flask for 10 min aiming to obtain a Pt/Ni molar ratio of 1.3 and form a homogeneous yellow solution. The sealed flask was then charged with CO to 1 bar, heated from 343 K to 513 K with a ramping rate of 3 oC/min and kept at 513 K for 60 min. After cooling to room temperature, the NP were precipitated by addition of ethanol, isolated via centrifugation and further washed using an ethanol-cyclohexane solution. The PtCo NP, PtFe NP and PtMn NP with a Pt/M (M = Mn, Fe, Co, Ni) molar ratio of 1.3 and the reference Pt NP were synthesized with the same procedure. The oleylamine-capped PtM NP and Pt NP were respectively dissolved in n-butylamine solution to obtain a homogeneous suspension composed of ~10 mg Pt per 25 mL n-butylamine. The required amount of γ-Al2O3 support, which is purchased from Sinopharm Chemical Reagent Co. (China) and pre-calcined at 900 oC for 6 h in air, was then added into the solution and stirred for three days in a room environment. The sample was then precipitated by addition of ethanol, isolated via centrifugation, washed with additional ethanol to remove the excess n-butylamine and dried under vacuum at 70 oC overnight. Finally, the samples were calcined at 200 oC for 1 h with a ramping rate of 30 oC/h in air. The as-prepared catalysts were respectively denoted as Pt/Al2O3, PtMn/Al2O3, PtFe/Al2O3, PtCo/Al2O3, and PtNi/Al2O3 with a Pt loading amount number in front for simplicity. Before characterization and catalytic test, the samples were reduced by HCHO solution. In a typical HCHO reduction process, about 0.5 g catalyst was mixed with 20 mL deionized water and 5 mL ~35 wt. % 5



HCHO solution and stayed for 3 h under reflux conditions and vigorous stirring at 70 o

C. Then, the suspension was separated and dried in air at 120 oC for 6 h for use. Final

contents of Pt, Mn, Fe, Co and Ni in each sample were confirmed by flame atomic absorption spectrometry (Z-2000, Hitachi) and presented in Table 1. Characterization of Samples. The morphology of the NP and catalysts was observed by transmission electron microscope (TEM). Powder X-ray diffraction (XRD) was used to characterize the crystal structure of the samples. The physical structure of the samples was measured by N2 adsorption at 77 K. Pt NP dispersion over the samples was evaluated by CO chemisorption. The chemical states of the catalysts surface were checked by X-ray photoelectron spectroscopy (XPS) and fourier transform infrared spectra (FTIR) characterization. In situ Diffuse Reflectance Infrared Fourier Transformed Spectroscopy (DRIFTS) analysis was employed to follow the change of the catalyst surface species during the reaction process. The effluent species in the in situ DRIFTS study were analyzed by a mass spectrometry (MS). Catalytic tests of HCHO oxidation were performed under atmospheric pressure in a quartz tubular (i.d.= 7 mm) fixed-bed reactor. The reactant contains 80 mL/min simulated air stream (N2/O2 = 4) and ~30 ppm HCHO with ~35% relative humidity (or RH). A gas hourly space velocity of 24000 ml h-1 g-1 was used. In the case of dry reaction experiment (< 5% RH), the reactant was treated by a CaCl2 fixed bed before entering the reactor. Gaseous HCHO was generated by bubbling in a HCHO solution (35 wt. % HCHO) with a stream of simulated air. HCHO amount in the inlet or outlet gas stream was measured by phenol spectrophotometric method 12. HCHO conversion was calculated according to its concentration change. The experimental details of characterization and catalytic tests are described in Supporting Information. Density functional theory calculations. Density functional theory (DFT) 6


Page 6 of 23

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


calculations were performed using the Cambridge Sequential Total Energy Package (CASTEP)

25

gradient

approximation

and Materials Studio software (Accelrys, Inc.) within the generalized (GGA)

26-28

.

Perdew-Wang

1991

(PW91)

exchange-correlation functional and ultrasoft pseudopotentials were used for the periodic plane-wave approach. The self-consistent field (SCF) tolerance in the structural optimizations was 2.0×10-6 eV/atom and the energy cutoffs used was 400 eV. A 2 × 2 × 1 grid of k-points was used to sample the Brillouin zone 29. Four atomic layers with a 4 × 4-unit cell were used to model the Pt (111) surface, and a 12 Å vacuum layer was used to minimize the interactions between surfaces of adjacent slabs. A M (M= Mn, Fe, Co or Ni) atom was respectively loaded and optimized on the top of Pt (111) slabs to construct PtM nanoparticles model (Figure S1) for following hydroxyl adsorption and reaction calculation study.

RESULTS AND DISSCUSSION Structural properties

Figure 1. HRTEM images and EDS elemental mapping of PtM nanoparticles: (a) 7



PtMn, (b) PtFe, (c) PtCo and (d) PtNi HRTEM images and EDS analysis in Figure 1 demonstrate the formation of PtM (M = Mn, Fe, Co and Ni) composites with NP size around 10~20 nm. XPS spectra of Pt and PtM NP in Figure 2 present a Mn 2p3/2 peak centered at 641.0 eV attributed to Mn3+ in PtMn NP 9, a Fe 2p3/2 peak at 711.0 eV assigned for Fe3+ in PtFe NP 30, the peaks around 785.8, 780.2 and 778.1 eV ascribed to Co3+ (785.8, 780.2 eV) and Co0 (778.1 eV) in PtCo NP

31, 32

, and the Ni 2p3/2 peaks at 855.4 and 852.2 eV

attributed to the Ni2+ and Ni0 species in PtNi NP 21, 33. The Pt 4f7/2 peaks of these NP are around 71.0~71.2 eV demonstrating the metallic state of Pt in PtM NP 34, 35. The O 1s spectra of Pt and PtM NP all show two peaks at 532.6~533.1 eV ascribed to oxygen of OH (OH -M)

15

and 530.2-530.7 eV related to oxo-groups (M-O-M) 20, 21

with a OH-M/O-M peak intensity ratio of 3.53, 4.29, 4.44, 6.79 and 7.21 for Pt, PtMn, PtFe, PtCo and PtNi, respectively, which indicates a surface hydroxyls concentration trend of Mn < Fe < Co < Ni in PtM (M = Mn, Fe, Co, Ni) NP. These findings demonstrate the formation of M-hydroxide promoted PtM(OH)x (M = Mn, Fe, Co, Ni) NP.

8


Page 8 of 23

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


Figure 2. XPS spectra of Pt and PtM (M = Mn, Fe, Co, Ni) nanoparticles: M 2p, Pt 4f and O 1s

Table 1. Specific surface area, Pt apparent dispersion and particle size, O1s XPS data of PtM/Al2O3 (M = Mn, Fe, Co, Ni) Pt Content (wt.%) b

BET Sample

surface area

(%) a

(m2/g)

a

O-OH

Pt dispersion

XPS

Theoretical

actual

(O-O-)

O-OH/O-O-

peaks

ratio c

(eV)

Al2O3

94

-

-

-

PtMn/Al2O3

99

51.5

0.20

0.19

PtFe/Al2O3

101

57.9

0.20

0.18

PtCo/Al2O3

92

50.7

0.20

0.19

PtNi/Al2O3

97

55.6

0.20

0.18

Determined by CO chemisorption; b

Determined by ICP;

c

531.9 (530.7) 531.9 (530.7) 531.9 (530.7) 531.9 (530.7)

0.96

1.04

1.16

1.17

Calculated from the corresponding areas of XPS

fitted peaks

The Pt and PtM NP (M = Mn, Fe, Co, Ni) were then respectively loaded on γ-Al2O3. Homogeneous metal particles distribution was obtained over PtM/Al2O3 (M = Mn, Fe, Co, Ni) (TEM images in Figure S2). Due to the good distribution and low Pt or M (M = Mn, Fe, Co and Ni) content in the catalysts, XRD patterns of these Pt-based catalysts (Figure 3a) only display characteristic peaks of γ-Al2O3. There is no obvious difference in BET-surface area and Pt dispersion among these samples (Table 1). FTIR spectra of fully dried Pt/Al2O3 and PtM/Al2O3 (M = Mn, Fe, Co, Ni) in Figure 3b shows a strong band at ca. 3450 cm-1 assigned to the stretching vibrations of surface hydroxyls and a sharp band at ca. 1640 cm-1 ascribed to the bending vibrations of surface hydroxyl groups 21, 23, respectively. The 3450 cm-1 and 1640 cm-1 9



Page 10 of 23

bands of the reference γ-Al2O3 are extremely weak. PtCo/Al2O3 and PtNi/Al2O3 hold more intense hydroxyls vibration bands or larger surface hydroxyls amount than Pt/Al2O3, PtMn/Al2O3 and PtFe/Al2O3. XPS analysis was used to characterize the chemical states of surface elements in Pt/Al2O3 and PtM/Al2O3 (M = Mn, Fe, Co, Ni). The O 1s spectra of these catalysts in Figure 3c present two peaks at ~530.7 and 531.9 eV assigned to lattice oxygen (O-O-) of γ-Al2O3 or bridging oxo-groups (M-O-M) and oxygen of surface hydroxyl (OOH-) groups

15, 20

, respectively. Semi-quantitative

analysis by calculating peak area ratio (AOH-/A-O-) indicates that the surface OH concentration of PtCo/Al2O3 and PtNi/Al2O3 are higher than that of PtMn/Al2O3 and PtFe/Al2O3 (Table 1), which agree well with the FTIR results. Due to the overlap between Pt 4f5/2 peak (ca.71~74.3 eV) and Al 2p peak of Al2O3 (Figure S3) 19, Pt 4d5/2 XPS spectra are recorded (Figure S4), which show the Pt0 characteristic peak at 314.2~314.4 eV

34, 35

. The M (M = Mn, Fe, Co, Ni) XPS spectra (Figure S5)

demonstrate Mn3+ species (~641 eV) in PtMn/Al2O3 9, Fe3+ species (~711 eV) in PtFe/Al2O3

30

, Co3+ species (ca.784.8 and 780.2 eV) in PtCo/Al2O3

species (ca. 855.7 and 861.4 eV) in PtNi/Al2O3

31, 32

and Ni2+

21, 33

. These characterization results

confirm that the formation of hydroxide promoted PtM(OH)x NP enriches the surface OH concentration of PtM/Al2O3 (M = Mn, Fe, Co, Ni).

Figure 3. Structural characterization of Pt/Al2O3 and PtM/Al2O3 (M = Mn, Fe, Co, Ni) 10


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


catalysts: (a) XRD patterns, (b) FTIR spectra and (c) O 1s XPS spectra

Catalysts activity test The catalytic HCHO oxidation activity comparison in Figure 4a demonstrates the superior activities of PtCo/Al2O3 and PtNi/Al2O3 than those of PtFe/Al2O3 PtMn/Al2O3 and Pt/Al2O3. With a low Pt loading amount of 0.2 wt. %, the initial HCHO conversions at 30 oC are 50%, 70 %, 79 %, 96 % and 98 % over Pt/Al2O3, PtMn/Al2O3, PtFe/Al2O3, PtCo/Al2O3 and PtNi/Al2O3, respectively. The well performance stability of PtCo/Al2O3 and PtNi/Al2O3 is demonstrated through 100 h test at 30 oC, showing a (>) 95 % and (>) 97 % HCHO conversion (Figure 4b), respectively. The effect of humidity and function of surface hydroxyls on the performance of PtCo/Al2O3 were checked. As shown in Figure 4c, after flushing 40 h with dry reactant gas (relative humidity or RH < 5 %), PtCo/Al2O3 holds a lower activity at 30 oC with a HCHO conversion of 60 % in comparison that measured at RH= 35% with a HCHO conversion of 96 %. Under this dry feeding condition, the HCHO conversion first slightly increases and then decreases with increasing reaction temperature, resulting a low HCHO conversion of 44% at 70 oC (Stage 1). Subsequently, the desiccant was abandoned and the RH value returned to ~35%. At this feeding condition and 70 oC, the HCHO conversion gradually recovers up to 68 % after 72 hours on stream reaction (Stage 2). The promotion effect of appropriate humidity is related to the function of surface hydroxyls

18, 21

. FTIR spectra

comparison of fresh, Stage 1 and Stage 2 PtCo/Al2O3 (Figure S6) indicates the identical trend of surface OH groups amount (or OH vibration bands intensity) with 11



activity. The H2O promotion effect on PtCo /Al2O3 indirectly shows the importance of surface or interface OH groups and their tailoring on the performance of PtM/Al2O3 (M=Mn, Fe, Co, Ni).

Figure 4. (a) Dependence of HCHO conversion on reaction temperature over Pt/Al2O3 and PtM/Al2O3 (M = Mn, Fe, Co, Ni), (b) Long term test over PtCo/Al2O3 and PtNi/Al2O3 at 30 oC and (c) Effect of humidity on HCHO conversion at various temperatures of 30 oC~70 oC over PtCo/Al2O3

In-situ DRIFT test In-situ DRIFTS study was carried out to compare the performance of PtMn/Al2O3, PtFe/Al2O3, PtCo/Al2O3 and PtNi/Al2O3 for HCHO oxidation with respect to the behavior of adsorbed species on the catalyst surface. Fig. 5 shows the dynamic changes in the DRIFTS spectra of the catalysts as a function of time in a flow of O2 + HCHO + He at 30 oC. After exposing the samples to O2 + HCHO + He mixture gas, the bands at 1570, 1690, 1770, 2340, 2837, 3265 cm-1 appear over PtMn/Al2O3, PtFe/Al2O3, PtCo/Al2O3 and PtNi/Al2O3. The band at ca. 1570 cm-1 is assigned for ν(as)COO of formate species. The 1690 cm-1 band is ascribed to surface carbonate species, originating from the formate species oxidation over the catalyst. The band at 1770 cm-1 is attributed C=O stretch of HCOOH. The band at ca. 2837 cm-1 was ascribed to CH2 antisymmetric stretching vibration of adsorbed HCHO. The bands at ca. 3700-3000 cm-1 were attributed to OH group bonded to the catalyst surface or the 12


Page 12 of 23

Page 13 of 23

adsorbed H2O. The formation of adsorbed CO2 (band at 2340 cm−1) was observed, while no found CO (linearly adsorbed on Pt, Ca. 2042 cm−1), indicating that HCHO oxidation over PtM/Al2O3 (M = Mn, Fe, Co, Ni) follows the hydroxyls promoted route 2, 21.

(a) PtMn/Al2O3

(b) PtFe/Al2O3 1690

Kubelka Munk

Kubelka Munk

3265 2837 2340

min 60

1770

1570

20 5 1

4000

3200

2400

1600

Wavenumber (cm 3265 2837 2340

20 5 1

3200

2400

3265 2837 2340 1690 1770 1570

20 5 1

3200

3265 2837 2340 1770 1570

(d) PtNi/Al2O3

60

4000

1690

2400

Wavenumber (cm

min

)

-1

)

1690 1770

1570

60 20 5 1

4000

1600

-1

1600

Wavenumber (cm

Kubelka Munk

min

min 60

4000

-1 )

(c) PtCo/Al2O3

Kubelka Munk

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


3200

2400

1600

Wavenumber (cm-1)

Figure 5. In situ DRIFTS spectra as a function of exposure time in O2 + HCHO + He gas mixture adsorption at 30 oC: (a) PtMn/Al2O3, (b) PtFe/Al2O3, (c) PtCo/Al2O3 and (d) PtNi/Al2O3 Figure 6 shows in situ DRIFTS spectra of O2 + HCHO + He gas mixture reaction over these samples for 60 min at various temperatures. Generally, the intensity of the intermediates band decreases with increasing reaction temperature. While the adsorbed CO2 (2340 cm-1) and carbonate species (1690 cm-1) bands intensity gradually enhances along with formate species consumption. The effluent CO2 flux 13



during the in situ DRIFTS test determined by online MS (Figure S7) gives an increase trend with increasing adsorption temperature, providing obvious evidence for the deep HCHO oxidation over these catalysts. Since decomposition or oxidation formate species is generally accepted as the key step in HCHO oxidation over Pt based 2, 21

catalysts

, a semi-quantitative analysis by calculating the integrated formate

species band (1570 cm-1) area was carried out (Figure S8). Under the same test conditions, the accumulated formate species over these catalysts holds a sequence of PtMn/Al2O3 > PtFe/Al2O3 > PtCo/Al2O3 > PtNi/Al2O3 which is inverse to their activity sequence. In consideration with the activity comparison in Figure 3a, we can propose that both the formate species formation rate and decomposing/oxidation rate over PtCo/Al2O3 and PtNi/Al2O3 are faster than those over PtMn/Al2O3 and PtFe/Al2O3, and the HCHO oxidation controlling step over these catalysts is the formate species decomposing/oxidation step, which agrees well with the findings in the literature 2, 21.

(b) PtFe/Al2O3

(a) PtMn/Al2O3

4000

3750

1570

3750 3265 2837

oC

1690 2340

120 80 60 40

3200

2400

Kubelka Munk

Kubelka Munk

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

Page 14 of 23

4000

1600

Wavenumber (cm

-1

)

3265

1570 oC 1690 120

2837 2340

80 60 40

3200

2400

1600

Wavenumber (cm-1)

14


(c) PtCo/Al2O3

(d) PtNi/Al2O3

1570

3750

Kubelka Munk

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


1690 oC

3265 2837

2340

120

80 60 40

4000

3200

2400

1570

3750

1600

Kubelka Munk

Page 15 of 23

4000

Wavenumber (cm-1)

1690

oC

2340 3265 2837

120 80 60 40

3200

2400

1600

Wavenumber (cm-1)

Figure 6. In situ DRIFTS spectra as a function of temperature in O2+He+HCHO gas mixture with an adsorption time of 60 min: (a) PtMn/Al2O3, (b) PtFe/Al2O3, (c) PtCo/Al2O3 and (d) PtNi/Al2O3

Effect of hydroxyl adsorption strength DFT calculations were performed to evaluate the effect of hydroxyl adsorption strength on the performance of PtM (M= Mn, Fe, Co, Ni) based catalysts for HCHO oxidation. A hydroxyl group is connected to the M site of the as-developed PM structures and then relaxed to their lowest energy configurations to form PtM-OH structures (Figure S9). The adsorption energy of OH on PtM surface (Ead) is calculated as follow,

E ad = E s + EOH − Etotal Here, Es is the total energy of PtM, EOH refers to the total energy of a hydroxyl group in the vacuum, and Etotal is the total energy of PtM bonding with hydroxyl group. All energies are given with zero points energy (ZPE) corrections. As compared in Figure 7a, the hydroxyl group adsorption energy over PtM has a sequence of PtMn> PtFe> PtCo> PtNi. The function of the adsorbed OH during the HCHO oxidation process was evaluated by calculating its oxidation ability to the key intermediates formate species (CHO2 + OH → CO2 + H2O). The reactant (or the CHO2 adsorption over M site of PtMOH) and product (or the CO2 and H2O co-adsorption over M site of PtM) 15



of this reaction are firstly relaxed to their lowest energy configurations. Then, the transition state (TS) of this reaction is searched at the same theoretical level with the complete Linear Synchronous Transit/Quadratic Synchronous Transit (LST/QST method) implemented in CASTEP. The stable reactant, product and TS configurations, as well as the reaction barrier energies of CHO2 + OH → CO2 + H2O over PtM are presented and compared in Figure 7b & Figure S9. The barrier energy trend of this key reaction step over PtM is identical to the OH bonding energy trend over PtM, that is PtMn> PtFe> PtCo> PtNi, which is inverse to their HCHO oxidation activity sequence.

Figure 7. (a) Hydroxyl group adsorption energies and (b) barrier energies of formate-OH reaction step CHO2 + OH→ CO2 + H2O over PtM (M = Mn, Fe, Co, Ni) nanoparticles. The corresponding adsorption and reaction configurations on PtNi nanoparticles are included in each figure for reference.

Discussion Herein, PtM nanoparticles and PtM/Al2O3 catalysts (M= Mn, Fe, Co, Ni) with abundant of interface hydroxyl groups were synthesized. The surface hydroxyls concentration nearby the active Pt sites has a trend of Mn < Fe < Co < Ni for both PtM and PtM/Al2O3 (Figure 3b&c, Table 1). While the energetic OH–M2+δ strength 16


Page 16 of 23

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


has an inverse trend of Mn> Fe> Co> Ni (Figure 7a) The hydroxyl involved HCHO oxidation route (HCHO → HCOO-M + OH-M → H2O + CO2 + 2M, M is noble metal) 2, 21

was demonstrated over PtM/Al2O3. For example, no peak associated to

molecularly adsorbed CO was observed in in situ HCHO oxidation DRIFTS spectra over these catalysts (Figure 5&6). Such hydroxyl promoting effect on PtM/Al2O3 shows trend of Mn < Fe < Co < Ni (Figure 4a), which is proposed to relate to both the surface hydroxyls concentration nearby the active sites and the energetic OH–M2+δ strength. The identical variation trend of the HCHO oxidation activity (Figure 4c) and surface hydroxyl amounts over PtCo/Al2O3 (Figure S7) with changing the humidity in the reactants demonstrates the importance of hydroxyl concentration for its promotion effect on HCHO oxidation. The hydroxyl bonding strength effect was evaluated by fixing one hydroxyl to PtM nanoparticles and comparing their formate species oxidation ability through DFT calculation, which is taken as the rate-controlling step in HCHO oxidation process

2, 18, 21

. It is found that a low hydroxyl bonding energy is

beneficial for CHO2 + OH → CO2 + H2O oxidation, and an oxidation ability sequence of PtMn < PtFe < PtCo < PtNi is demonstrated (Figure 7), which is identical to the trend found by in situ HCHO oxidation DRIFTS study over PtM/Al2O3 (Figure S8). In short, the enriched and tenderly adsorbed hydroxyls over PtCo/Al2O3 and PtNi/Al2O3 lead to their more active performance for HCHO oxidation than that of PtMn/Al2O3 and PtFe/Al2O3.

CONCLUSIONS A series of 3d-M (M= Mn, Fe, Co, Ni) metals hydroxide promoted PtM(OH)x/Al2O3 catalysts were developed and well-characterized to obtain intrinsic insight into the hydroxyls promotion effect on HCHO oxidation over Pt based 17



catalysts. The catalytic HCHO oxidation activity for these PtM(OH)x/Al2O3 catalysts shows an increasing trend Mn< Fe< Co< Ni with decreasing OH–M2+δ bond strength and increasing surface hydroxyls concentration. A (>)95% HCHO conversion and (>)100 h performance stability were obtained over PtCo(OH)x/Al2O3 and PtNi(OH)x/Al2O3 with a low 0.2 wt.% Pt loading amount at 30 oC. The identified catalytic trend and its relationship with more fundamental properties, such as hydroxyls density and adsorption strength, provide the foundation for rational design of hydroxyls involved reactions.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. (HR)TEM, XPS data, FTIR of the samples; DFT configurations of Pt111-M and reaction (CHO2 + OH→ CO2 + H2O) over Pt111-M.

ACKNOWLEDGEMENTS The financial support of Natural Science Foundation of China (No. 21576298, 21776322, U1663220 and 21425627) Science and Technology Program of Guangzhou (201804010154) and Natural Science Foundation of Guangdong Province (2014A030308012).

REFERENCES (1) Salthammer, T; Mentes. S; Marutzky. R., Formaldehyde in the indoor environment. Chem. Rev. 2010, 100, 2536-2572. 18


Page 18 of 23

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


(2) Zhang, C; Liu, F; Zhai, Y; Ariga, H; Yi, N; Liu, Y; Asakura, K; Flytzani-Stephanopoulos, M; He. H., Alkali-metal-promoted Pt/TiO2 opens a more efficient pathway to formaldehyde oxidation at ambient temperatures. Angew. Chem. Int. Edit. 2012, 51, 9628-9632. (3) Sang, J. P.; Bae, I.; Nam, I. S.; Cho, B. K.; Jung, S. M.; Lee, J. H., Oxidation of formaldehyde over Pd/Beta catalyst. Chem. Eng. J. 2012, 195, 392-402. (4) Chen, D; Qu, Z; Sun, Y; Gao, K; Wang. Y., Identification of reaction intermediates and mechanism responsible for highly active HCHO oxidation on Ag/MCM-41 catalysts. Appl. Catal. B: Environ. 2013, 142, 838-848. (5) Ma, Y.; Zhang, G., Sepiolite nanofiber-supported platinum nanoparticle catalysts toward the catalytic oxidation of formaldehyde at ambient temperature: Efficient and stable performance and mechanism. Chem. Eng. J 2016, 288, 70-78. (6) Chen, Y.; Gao, J.; Huang, Z.; Zhou, M.; Chen, J.; Li, C.; Ma, Z.; Chen, J.; Tang, X., Sodium rivals silver as single-atom active centers for catalyzing abatement of formaldehyde. Environ. Sci. Technol. 2017, 51, 7084-7090. (7) Wu, H. C.; Chen, T. C.; Chen, Y. C.; Lee, J. F.; Chen, C. S., Formaldehyde oxidation on silica-supported Pt catalysts: The influence of thermal pretreatments on particle formation and on oxidation mechanism. J. Catal. 2017, 355, 87-100 (8) Zhang, L.; Chen, L.; Li, Y.; Peng, Y.; Chen, F.; Wang, L.; Zhang, C.; Meng, X.; He, H.; Xiao, F., Complete oxidation of formaldehyde at room temperature over an Al-rich Beta zeolite supported platinum catalyst. Appl. Catal. B: Environ. 2017, 219, 200-208 (9) Wang, J.; Li, J.; Jiang, C.; Zhou, P.; Zhang, P.; Yu, J., The effect of manganese vacancy in birnessite-type MnO2 on room-temperature oxidation of formaldehyde in air. Appl. Catal. B: Environ. 2017, 204, 147-155 19



(10) Zhang, C.; He, H.; Tanaka, K., Catalytic performance and mechanism of a Pt/TiO2 catalyst for the oxidation of formaldehyde at room temperature. Appl. Catal. B: Environ. 2006, 65, 37-43 (11) Huang, H.; Leung, D. Y. C.; Ye, D., Effect of reduction treatment on structural properties of TiO2 supported Pt nanoparticles and their catalytic activity for formaldehyde oxidation. J. Mater. Chem. 2011, 21, 9647-9652. (12) Chen, H.; Rui, Z.; Ji, H., Monolith-like TiO2 nanotube array supported Pt catalyst for HCHO removal under mild conditions. Ind. Eng. Chem. Res. 2014, 53, 7629-7636. (13) Chen, H.; Tang, M.; Rui, Z.; Ji, H., MnO2 promoted TiO2 nanotube array supported Pt catalyst for formaldehyde oxidation with enhanced efficiency. Ind. Eng. Chem. Res. 2015, 54, 8900-8907. (14) Lin, M.; Yu, X.; Yang, X.; Li, K.; Ge, M.; Li, J., Highly active and stable interface derived from Pt supported on Ni/Fe layered double oxides for HCHO oxidation. Catal. Sci. Technol. 2017, 7, 1573-1580. (15) Nie, L.; Meng, A.; Yu, J.; Jaroniec, M., Hierarchically macro-mesoporous Pt/gamma-Al2O3 composite microspheres for efficient formaldehyde oxidation at room temperature. Sci. Rep. 2013, 3, 3215. (16) Chen, B.; Zhu, X.; Crocker, M.; Wang, Y.; Shi, C., Complete oxidation of formaldehyde at ambient temperature over γ-Al2O3 supported Au catalyst. Catal. Commun. 2013, 42, 93-97. (17) Bai, B; Li, J., Positive effects of K+ ions on three-dimensional mesoporous Ag/Co3O4 catalyst for HCHO oxidation. ACS. Catal. 2014, 4, 2753-2762. (18) Kwon, D. W.; Seo, P. W.; Kim, G. J.; Hong, S.C.; Characteristics of the HCHO oxidation reaction over Pt/TiO2 catalysts at room temperature: The effect of relative humidity on catalytic activity. Appl. Catal. B: Environ. 2015, 163, 436-443. 20


Page 20 of 23

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


(19) Xu, Z.; Yu, J.; Jaroniec, M., Efficient catalytic removal of formaldehyde at room temperature using AlOOH nanoflakes with deposited Pt. Appl. Catal. B: Environ.

2015, 163, 306-312. (20) Chen, H.; Rui, Z.; Wang, X.; Ji, H., Multifunctional Pt/ZSM-5 catalyst for complete oxidation of gaseous formaldehyde at ambient temperature. Catal. Today

2015, 258, 56-63. (21) Yang, T.; Huo, Y.; Liu, Y.; Rui, Z.; Ji, H., Efficient formaldehyde oxidation over nickel hydroxide promoted Pt/γ-Al2O3 with a low Pt content. Appl. Catal. B: Environ.

2017, 200, 543-551. (22) Subbaraman, R.; Tripkovic, D.; Chang, K. C.; Strmcnik, D.; Paulikas, A. P.; Hirunsit, P.; Chan, M.; Greeley, J.; Stamenkovic, V.; Markovic, N. M., Trends in activity for the water electrolyser reactions on 3d M(Ni, Co, Fe, Mn) hydr(oxy)oxide catalysts. Nat. Mater. 2012, 11, 550-557. (23) Chen, G.; Zhao, Y.; Fu, G.; Duchesne, P. N.; Gu, L.; Zheng, Y; Weng, X.; Chen, M.; Zhang, P.; Pao, C. W.; Lee, J. F.; Zheng. N., Interfacial effects in iron-nickel hydroxide-platinum nanoparticles enhance catalytic oxidation. Science 2014, 344, 495-499. (24) Zou, X.; Rui. Z.; Ji. H., Core-shell NiO@PdO nanoparticles supported on alumina as an advanced catalyst for methane oxidation. ACS. Catal. 2017, 7, 1615-1625. (25) Segall, M. D.; Lindan, P. J. D.; Probert, M. J.; Pickard, C. J.; Hasnip, P. J.; Clark, S. J.; Payne, M. C., First-principles simulation: ideas, illustrations and the CASTEP code. J. Phys-Condens Mat. 2002, 14, 2717-2744. (26) Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865-3868. 21



(27) Dal Corso, A.; Pasquarello, A.; Baldereschi, A.; Car. R., Generalized-gradient approximations to density-functional theory: A comparative study for atoms and solids. Phys. Rev. B 1996, 53, 1180-1185. (28) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C., Atoms, molecules, solids, and surfaces: Applications of the generalized gradient approximation for exchange and correlation. Phys. Rev. B 1992, 46, 6671-6687. (29) Monkhorst, H. J.; Pack, J. D., Special points for Brillouin-zone integrations, Phys. Rev. B 1976, 13, 5188-5192. (30) Xu, Y.; Ruban, A. V.; Mavrikakis, M., Adsorption and dissociation of O2 on Pt-Co and Pt-Fe alloys. J. Am. Chem. Soc. 2004, 126, 4717-4725. (31) Wang, Z.; Wang, W.; Zhang, L.; Jiang, D., Surface oxygen vacancies on Co3O4 mediated catalytic formaldehyde oxidation at room temperature. Catal. Sci. Technol.

2016, 6, 3845-3853. (32) Liu, H.; Lin, Y.; Ma, Z., Rh2O3/mesoporous MOx-Al2O3 (M = Mn, Fe, Co, Ni, Cu, Ba) catalysts: Synthesis, characterization, and catalytic applications, Chinese. J. Catal.

2016, 37, 73-82. (33) Deivaraj, T. C.; Chen, W.; Lee, J. Y., Preparation of PtNi nanoparticles for the electrocatalytic oxidation of methanol. J. Mater. Chem. 2003, 13, 2555-2560. (34) Wang, X.; Yu, H.; Hua, D.; Zhou. S., Enhanced catalytic hydrogenation activity and selectivity of Pt-MxOy/Al2O3 (M = Ni, Fe, Co) heteroaggregate catalysts by in situ transformation of PtM alloy nanoparticles. J. Phys. Chem. C 2013, 117, 7294-7302. (35) Ivanova, A. S.; Slavinskaya, E. M.; Gulyaev, R.V.; Zaikovskii, V. I.; Stonkus, О. А.; Danilova, I. G.; Plyasova, L. M.; Polukhina, I. A.; Boronin, A. I., Metal–support interactions in Pt/Al2O3 and Pd/Al2O3 catalysts for CO oxidation. Appl. Catal. B: 22


Page 22 of 23

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


Environ. 2010, 97, 57-71. (36) Busca, G.; Lamotte, J.; Lavalley, J. C.; Lorenzelli, V., FT-IR study of the adsorption and transformation of formaldehyde on oxide surfaces. J. Am. Chem. Soc.

1987, 109, 5197-5202. (37) Raskó, J.; Kecskés, T.; Kiss, J., Formaldehyde formation in the interaction of HCOOH with Pt supported on TiO2. J. Catal. 2004, 224, 261-268.

For TOC only

23


Identification of the nearby hydroxyls role in ... - ACS Publications

Recommend Documents