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CeO2-Al2O3-TiO2 catalysts
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Methanol total oxidation as model reaction for the effects of different Pd content on Pd-Pt/CeO2 -Al2 O3 -TiO2 catalysts Yuyu Guo, Shen Zhang, Wentao Mu, Xingying Li, Zhe Li ∗ College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024 Shanxi, China
a r t i c l e
i n f o
Article history: Received 8 October 2016 Received in revised form 28 November 2016 Accepted 29 November 2016 Available online xxx Keywords: Methanol total oxidation Pd content Room temperature Adsorbed oxygen
a b s t r a c t A series of Pd-Pt/CAT1 catalysts with different Pd content were prepared by stepwise impregnation methods for methanol total oxidation. The influences of Pd content on the physicochemical properties of catalysts were investigated by XRD, Nitrogen adsorption-desorption, noble metal dispersion, XPS, SEM and H2 -TPR techniques. Under the reaction conditions (1.0 vol.% CH3 OH, 2.0 vol.% O2 , GHSV: 35,000 h−1 ), 0.8% Pd-Pt/CAT catalyst converts 35% methanol in room temperature (27 ◦ C) and T50 , T90 are 35 ◦ C and 58 ◦ C, respectively. Until the conversion reaches 100%, the heat is only provided by experimental heat releasing. The results of characterizations show that the content of active components has an apparent effect on the exposed surface area as well as the TiO2 structures and thus influences the catalytic performance. The higher contents of adsorbed oxygen over 0.8% Pd-Pt/CAT catalyst are more conducive to rate determining step and subsequent reactions. The larger proportion of high-valence TiOx and Ti4+ are prone to “attack” water to produce −OH and then form the Pt-OHads or Pd-OHads to react with COads . Furthermore, the decrease of grain size or the reduction of crystallinity indicates that the surface of catalyst is more amorphous and will provide more vacancy for active sites. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Volatile organic compounds (VOCs) are usually recognized as major contributors to air pollutions which emitted from many industry processes as well as from indoor sources [1,2]. With the more stringent emission standards, the reduction of VOCs is becoming more important. Methanol, one of the dominating VOCs, is widely used as solvents of many chemical products and the basic material to produce some important industrial chemicals like formaldehyde, acetic acid [3,4]. Specially, the uses of methanol in automotive fuels become more common because of its reduction of the emission of conventional pollutants such as CO [5]. So many applications will directly lead to the increase emission of residual methanol. Methanol has remarkable impact on human health, such as stomachache, eye irritation and headache. So the removal of methanol is inevitable to protect the atmospheric environment and human health [6–8]. Several techniques of physical and chemical adsorption, photocatalytic and catalytic oxidation methods are currently used. Among them, the catalytic oxidation to carbon dioxide and water is the most ideal and effective technique on
∗ Corresponding author. E-mail address: [email protected] (Z. Li). 1 CAT: CeO2 -Al2 O3 -TiO2 .
account of advantages like high removal efficiency, low light-off temperature, widespread applications, simple equipment and no secondary pollution [9–11]. The catalysts with high performance are indispensable and the research of them has become a hot topic, particularly at low temperature. Various kinds of catalysts such as single or mixed metal oxides and Pt or Pd supported catalysts were applied for the total oxidation of VOCs [12]. The supported noble-metal catalysts are more durable and more efficient compared to metal oxide based catalysts in the oxidation of VOCs [13,14]. The Pd supported catalysts have been extensively studied, since palladium has been found to be promising for practical applications in the total combustion of VOCs due to its high activity at relatively low temperature, tolerance to moisture [15] and higher resistance to thermal sintering in an oxidizing environment than platinum [16]. Álvarez-Galván et al. [17] reported that the addition of Pd on Mn/Al2 O3 catalyst could decrease the total oxidation temperature of the mixture of HCHO and methanol. Hosseini et al. [18] found that the Au/Pd catalysts showed higher activity than the monometallic Au or Pd catalysts. Platinum metal is recognized as the most active metal for hydrocarbon combustion [19]. Chuang et al. [20] showed the Pt based-catalysts for VOC oxidation is complete at low temperature. In terms of the superior properties of Pd and the high performance of Pt, we want to get a
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catalyst with higher performance which combines with the merits of Pd and Pt. The support properties also play an important role in the efficiency of catalysts [21]. Supports were expected to be an important component of the catalyst, since they often profoundly affected the generation of active species and the catalytic performance of Pd [12]. Nanometer TiO2 belongs to an important semiconductor material and has been widely used in many fields [22]. Ordered porous TiO2 possess the general characteristics of porous materials, such as uniform aperture size, narrow pore size distribution, the orderliness of the pore structure and high porosity. But deficiencies of low specific surface area and weak surface acidity also exist in this material. The porous Al2 O3 -TiO2 composites precisely make up the disadvantages of single TiO2 because of the large surface area and strong surface acidity of Al2 O3 [23]. CeO2 is a crucial component in catalysts for its role in oxygen storage, redox properties and enhancing the dispersion of noble metals and the thermal stability of catalysts which always plays the role of promoter in support [21,24]. Based on the previous researches and conclusions of my laboratory [25,26], CeO2 -Al2 O3 -TiO2 was chosen as the support in which the weight percent of Ce was 1%. Pd was used as the first active component from 0.5 wt.% to 1.2 wt.% and 0.5 wt.% Pt was loaded as another active component. The aim of this work was to understand the structure-function relationship dependent on Pd loading for methanol total oxidation. On this basis, find one catalyst with high performance through changing the content of Pd. Several characterization methods were used to study the physicochemical properties of the catalysts for the purpose of explaining the relationship between the catalytic activity and the structures of catalysts.
The catalytic properties of the catalysts were evaluated in a continuous-flow, atmospheric pressure, fixed-bed reactor (Ø = 6 mm) which was a straight quartz tube and shrank in the middle to hold the catalyst by two quartz wool plugs. The catalyst powders were pressed, crushed and sieved to the fraction of 40–60 mesh. 300 mg of catalysts (approximate 0.25 mL) were utilized every time. The reactants were fed to the reactor by replenishing a part of the nitrogen stream through a glass saturator containing methanol (room temperature, 26 ± 1 ◦ C). The CH3 OH, O2 and N2 were mixed before they reached the reactor. In order to achieve accurate and stable controlling of the gas flow rates, mass flow controllers were adopted in experiments. The feed gas consisted of 1.0 vol.% CH3 OH, 2.0 vol.% O2 and a balance of N2 . The total flow rate through the reactor was 145 mL min−1 , which gave a gas hourly space velocity (GHSV) of 35,000 h−1 . The reaction temperature was in the control of a tubular electric furnace and the temperature of the catalyst bed was monitored automatically by thermocouples. The compositions of reactants and products were analyzed by an online gas chromatograph (HXSP, GC-950) with two FID detectors and the one incorporated a methanator. The one FID detector was applied to analyze organic compounds composed of methane, formaldehyde, dimethyl ether and methanol. The other one was used to detect CO, CH4 and CO2 . Methanol conversion (Xi ) and the selectivity (Si ) of products were calculated according to the following formulas:
2. Experimental
Xi = [(Ci,in −Ci,out )/Ci,in ] × 100%
2.1. Preparation of catalysts
where Ci,in and Ci,out are the peak areas of methanol before reaction and in combustion products.
2.1.1. Synthesis of catalyst support (CAT) Aluminum isopropoxide (1.979 g) and tetrabutyl titanate (40 mL) were taken as precursors of aluminum and titanium. Both the alumina and titania sols were prepared by sol–gel method [25]. In the process of preparing titania sols, cerous nitrate (0.316 g) and alumina sols which had been prepared in advance were dumped into it. Then they were mixed together to obtain 1% CeO2 -Al2 O3 TiO2 and the weight content ratio of Al2 O3 to TiO2 was 1/19. The weight percentage of Ce was 1%, which was calculated on the amount of Al2 O3 -TiO2 as a benchmark. The mixed sols were dried at 50 ◦ C and 110 ◦ C in an oven for 12 h, respectively, then calcined at 500 ◦ C in air for 5 h at the heating rate of 2 ◦ C min−1 . The catalyst support (denoted as CAT) was in the form of yellow powder. 2.1.2. Synthesis of Pd/CAT catalysts Different weight of the Pd component were taken according to the proportion in experimental design and loaded on CAT by the method of impregnation [25]. Certain amounts of PdCl2 were dissolved by deionized water, then a portion of CAT and two drops of glacial acetic acid were put into PdCl2 solution. The solution was stirred continuously at room temperature for 24 h and then dried by the method of evaporating to dryness in a water bath at 80–85 ◦ C and in an oven for 12 h at 110 ◦ C. Subsequently, the dry power were grinded and then calcined at 500 ◦ C in air for 5 h (the rate of heating was 2 ◦ C min−1 ). Pd contents of different samples were 0.5%, 0.6%, 0.8%, 1.0% and 1.2% of mass percentage. 2.1.3. Preparation of Pd-Pt/CAT catalysts Another active component (Pt) was loaded after Pd by using H2 PtCl6 ·H2 O through the same method and Pt content of all the
samples were 0.5 wt.%. The aforementioned weight percentage of Ce, Pd and Pt were contents of element. With the increase of the Pd content, the catalysts were marked as C1, C2, C3, C4 and C5. 2.2. Oxidation of methanol
Si = [nj ·Cj,out /
nj ·Cj,out ] × 100%
where Cj,out are the concentrations of products, nj is the products and reactants carbon atom ratios. 2.3. Characterizations of catalysts X-ray powder diffraction (XRD) analysis of the catalysts were performed with a Rigaku D/max 2500 powder diffractometer operated at 40 KV and 100 mA using Ni-filtered Cu-K␣ radiation. The intensities of the diffraction peaks were recorded in the 2 range of 5–80◦ with a step size of 0.03◦ , and the scanning speed was 8◦ min−1 . N2 adsorption-desorption isotherms for C1, C2, C3, C4 and C5 were measured on a Micromeritics ASAP 2020 analyzer. The specific surface areas of the catalysts were calculated from the adsorption curve using the Brunauer-Emmett-Teller (BET) method, and pore volume and pore size distribution were determined by using Barrett-Joyner-Halenda (BJH) desorption theoretical model. All samples were degassed at 200 ◦ C and 6.67 × 10−2 Pa vacuum pressure for 4 h before measurement. Noble metal dispersion was evaluated with a TP-5085 metal dispersion instrument. The catalysts were put into the tube and the temperature was raised to 500 ◦ C and kept constant for 2 h in flowing H2 . Then the samples were cooled to room temperature under H2 and blown by N2 . After that, the catalysts were treated with O2 -titration. Blown by N2 once again, H2 -titration was carried out finally. X-ray photoelectron spectra (XPS) were carried out using ESCALAB 250 multifunctional electronic energy spectrometer with
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Table 1 T50 , T90 and S99 of methanol oxidation for C1: 0.5% Pd-Pt/CAT, C2: 0.6% Pd-Pt/CAT, C3: 0.8% Pd-Pt/CAT, C4: 1.0% Pd-Pt/CAT and C5: 1.2% Pd-Pt/CAT.
Fig. 1. Methanol conversion as a function of reaction temperature for C1- C5 from 25 ◦ C to 200 ◦ C, Reaction conditions: 1.0 vol.% CH3 OH, 2.0 vol.% O2 , GHSV: 35,000 h−1.
Al-K␣ (150 W) X-ray source. All the element binding energies were referenced to the C 1 s line situated at 284.6 eV. All measurements were subject to an estimated error of ±0.1 eV. The surface morphologies of catalysts were detected by scanning electron microscopy (SEM) on TESCAN Maia 3 LMH equipment. Meanwhile, content and dispersion maps of different elements were investigated by energy dispersive spectrometer (EDS) on the same instrument utilizing OXFORD EDS Inca X-Max. H2 -TPR experiments of the catalysts were obtained on an AutoChem 2920 chemisorption analyzer (Micromeritics, Norcross, GA, USA) equipped with a thermal conductivity detector. For the analysis, 50 mg of catalysts were pretreated by flowing N2 at a rate of 50 mL min−1 for 40 min at 400 ◦ C. After cooling to room temperature, a feed gas containing 5 vol.% H2 in N2 was fed to the catalysts at a flow rate of 50 mL min−1 and the temperature of reactor was raised to 950 ◦ C (heating rate: 10 ◦ C min−1 ).
Temperatures
C1
C2
C3
C4
C5
T50 /◦ C T90 /◦ C S99 /◦ C
50 105 50
50 90 50
35 58 59
60 75 100
55 70 60
in which the conversion reached 100%, the heat was only provided by experimental heat releasing instead of external heating. The other four catalysts required external heating to start reaction. Furthermore, the selectivity of CO2 also owned high behavior over C3 catalyst. In order to compare conveniently, some special temperatures were listed. Table 1 shows the light-off temperature (T50 ), complete conversion temperature (T90 ) and the temperature of CO2 selectivity reached 99% (S99 ) of different catalysts. T50 of these catalysts were below 60 ◦ C and T90 were under 90 ◦ C except C1 (105 ◦ C). The results illustrated that all these catalysts were active at low temperature. Under 60 ◦ C, the CO2 selectivity reached 99% except C4 (100 ◦ C). The data indicated that methanol was completely oxidized and the products of reactions were almost CO2 . The distributions of products were in accordance with requirements of green chemistry. T50 and T90 of C3 catalyst were 35 ◦ C and 58 ◦ C which were the lowest in these catalysts. The activity sequences were C3 > C5 > C4 > C2 > C1, which showed different rules from the order of Pd content reduced. The results revealed that the Pd content did have effects on catalytic performance and the contents of Pd and Pt over C3 catalyst were more appropriate. 3.2. XRD analyses
The tendency of CH3 OH conversion and CO2 selectivity along with temperature increasing were shown in Figs. 1 and 2 from 25 ◦ C to 200 ◦ C and Fig. S.1–S.2 was from 25 ◦ C to 500 ◦ C, respectively. CH3 OH conversion and CO2 selectivity of all catalysts monotonically increased at beginning and kept steadily after they reached 100% until 500 ◦ C. Compared with the other four catalysts, C3 catalyst showed higher performance in the temperature range from room temperature (27 ◦ C) to 200 ◦ C and lowered the light-off temperature and complete conversion temperature. It was worth mentioning that until the temperature rose to approximately 62 ◦ C
For investigate the changes of crystalline phase and grain sizes after active components loaded, XRD analyses were used and the patterns of catalyst support and five catalysts are reported in Fig. 3. The diffraction peaks appeared at 2 = 25.4◦ , 37.8◦ , 48.2◦ , 53.9◦ and 62.9◦ were indexed to diffractions from lattice planes (101), (004), (200), (105) and (204) [27] respectively, corresponded well to the anatase phase (PDF#73-1764). Only the diffraction peaks of the anatase structure were evident in catalyst support and all the catalysts. PtOx , PdO, CeO2 and Al2 O3 did not appear might be because of their low content or high dispersion [28]. The 2, FWHM and crystalline size of anatase in various samples are summarized in Table 2. The average crystalline size was estimated according to the Scherrer equation [29]. Compared with the catalyst support, values of full width at half-maximum (FWHM) of the strongest diffraction peak around 25.4◦ increased when loaded by active components. For all the samples, the crystalline sizes of
Fig. 2. CO2 selectivity as a function of reaction temperature for C1-C5 from 25 ◦ C to 200 ◦ C, Reaction conditions: 1.0 vol.% CH3 OH, 2.0 vol.% O2 , GHSV: 35,000 h−1 .
Fig. 3. XRD patterns of CAT: Catalyst Support, C1: 0.5% Pd-Pt/CAT, C2: 0.6% PdPt/CAT, C3: 0.8% Pd-Pt/CAT, C4: 1.0% Pd-Pt/CAT and C5: 1.2% Pd-Pt/CAT.
3. Results and discussion 3.1. Activity of methanol oxidation
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Table 2 2, FWHM and crystalline size of anatase in catalyst support and various catalysts. Samples
2 (◦ )
FWHM (◦ )
Crystalline Size (nm)
Catalyst Support: CAT C1: 0.5% Pd-Pt/CAT C2: 0.6% Pd-Pt/CAT C3: 0.8% Pd-Pt/CAT C4: 1.0% Pd-Pt/CAT C5: 1.2% Pd-Pt/CAT
25.40 25.40 25.40 25.40 25.40 25.37
0.31 0.52 0.53 0.38 0.33 0.34
25.7 15.4 15.3 21.2 24.4 24.0
Fig. 6. Calculated by parameters of Pd Calculated by parameters of Pt, Noble metal dispersions of C2: 0.6% Pd-Pt/CAT, C3: 0.8% Pd-Pt/CAT and C4: 1.0% Pd-Pt/CAT.
Fig. 4. Pore size distributions of C1-C5.
Fig. 5. N2 adsorption-desorption isotherms of C1: 0.5% Pd-Pt/CAT, C2: 0.6% PdPt/CAT, C3: 0.8% Pd-Pt/CAT, C4: 1.0% Pd-Pt/CAT and C5: 1.2% Pd-Pt/CAT.
anatase phase were roughly concentrated on 15–25 nm. When the active components were loaded, the grain sizes decreased in different degree. Moreover, the decreased peak intensity suggested the reduction of crystallinity. The results certified that XRD peaks became less sharp when the grain sizes decreased or the crystallinity reduced [30]. 3.3. Nitrogen adsorption-desorption results N2 adsorption-desorption were adopted to analyze the pore structure parameters of samples, therefore, Figs. 4 and 5 exhibit pore size distributions and N2 adsorption-desorption isotherms.
The pore sizes of all the catalysts concentrated on the scope range from 5 to 15 nm. The shape of these isotherms showed a type IV isotherm (IUPAC classification) with a H4 hysteresis loop [31] located at 0.40 < P/P0 < 1.0. All the features indicated that these catalysts belonged to typical mesoporous materials. The physical properties consisted of BET surface area, total pore volume and average pore diameter are listed in Table 3. Sizes of Pd and Pt ions were much less than 8 nm which the average pore diameter centered around. It meant that the two ions could easily enter into the interior of catalyst pores. In other words, the more active contents were introduced, the more spaces of pores were covered possibly. As shown, the surface area (from 48.79 m2 g−1 to 27.69 m2 g−1 ) and total pore volume (from 0.098 cm3 g−1 to 0.057 cm3 g−1 ) decreased one after another from C1 to C4 with the active components contents increased. The C5 catalyst should own the smallest surface area and total pore volume in theory. But the surface area (35.06 m2 g−1 ) was similar to C3 (34.26 m2 g−1 ) and total pore volume (0.089 cm3 g−1 ) was slight smaller than C2 (0.092 cm3 g−1 ). This abnormal phenomenon exhibited might be owing to the active particles of C5 were aggregated instead of separate. The average pore diameter of C5 (10.2 nm) was larger than others, which was interpreted by the collapse of pore structures [25] after several calcination. These factors were harmful to reaction and led to the worse catalytic performance of C5.
3.4. Noble metal dispersion In order to gain the specific dispersions of active components, the characterization of noble metal dispersion was used. The results of this technique were co-dispersions of Pd and Pt instead of separate. In the calculation of dispersion, molar mass, density and some parameters were required. Therefore, Fig. 6 shows dispersions calculated by Pd and Pt, respectively. The higher dispersion (0.16 and 0.30) demonstrated the better distribution of active contents over C3 catalyst. Values of C4 (0.06 and 0.11) were much smaller than the other two, which might be because the existence of aggregation over C4 catalyst.
Table 3 Pore structure parameters of C1-C5. Catalysts
BET surfacearea/(m2 g−1 )
Total porevolume/(cm3 g−1 )
Average porediameter/(nm)
C1: 0.5% Pd-Pt/CAT C2: 0.6% Pd-Pt/CAT C3: 0.8% Pd-Pt/CAT C4: 1.0% Pd-Pt/CAT C5: 1.2% Pd-Pt/CAT
48.79 45.58 34.26 27.69 35.06
0.098 0.092 0.072 0.057 0.089
8.6 7.5 8.4 8.3 10.2
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Fig. 7. XPS spectra of Pd 3d for C1: 0.5% Pd-Pt/CAT, C2: 0.6% Pd-Pt/CAT, C3: 0.8% Pd-Pt/CAT, C4: 1.0% Pd-Pt/CAT and C5: 1.2% Pd-Pt/CAT.
Fig. 8. XPS spectra of Ti 2p for C1: 0.5% Pd-Pt/CAT, C2: 0.6% Pd-Pt/CAT, C3: 0.8% Pd-Pt/CAT, C4: 1.0% Pd-Pt/CAT and C5: 1.2% Pd-Pt/CAT.
3.5. XPS evaluate To obtain the valence states and relative concentrations of different elements, the XPS was introduced and spectra of Pd 3d of various catalysts are displayed in Fig. 7. After fitting, the peak of Pd 3d5/2 was decomposed into single one around 336.4 eV which was attributed to Pd2+ [32–34] over all the catalysts. The peak of Al overlapped with Pt and the content of Pt was much less than the Al possessed which resulted in the Pt did not appear separately [35]. At.% of different elements and values of Al/Ti over C1-C5 are summarized in Table 4. The Pd content ranged from 0.12% to 0.40%, and the maximal At.% of Pd (0.40%) meant that the C3 catalyst surface had more content of Pd than other four catalysts. According to our designs, the content of Ti was much larger than Al possessed. Results showed the At.% of Al was slight bigger than Ti which was interpreted by the enrichment of Al on the surface. The greater values of Al/Ti demonstrated that Ti which located in the skeleton of support surface was replaced by some active components. These kinds of active particles turned stably and were unprofitable to the reactivity. Value (1.79) of C5 illustrated that more quantity of active components entered into the skeleton, by contrast, the quantity of high-dispersion contents on the surface reduced. So the smaller values of Al/Ti demonstrated that the proportion of disperse active components over C2 and C3 catalysts were higher. It was in good agreement with the conclusions of noble metal dispersion. Changes of TiO2 which were the major components of catalysts played important roles in affecting the performance. To deep investigate, the XPS spectra of Ti 2p of C1-C5 are presented in Fig. 8. Table 5 shows the peak fitting parameters of Ti 2p spectra for C1-C5. The regions were decomposed into several contributions according to Shirley background and Caussian-Lorentzian fitted method corresponded to different oxidation states of Ti, respectively. The ratio of the area of the two peaks A(Ti 2p1/2 )/A(Ti 2p3/2 ) is equal to 1/2 and the binding energy separation between Ti 2p1/2 and Ti 2p3/2 in the same valence conditions was 5.7 eV, which was in agreement with
the literatures [36,37]. Peaks at 457.8 eV (2p3/2 ) and 463.5 eV (2p1/2 ) were attributed to the existence of Ti3+ [38], 458.9 eV (2p3/2 ) and 464.6 eV (2p1/2 ) stood for Ti4+ [39], and the peaks around 460.9 eV (2p3/2 ) and 466.6 eV (2p1/2 ) were also observed. OCAL [40] believed that the peak around 460.2 eV was ascribed to TiOx /Ti. In our study, Pd and Pt possessed higher electronegativity (Pd-2.20, Pt-2.28) than Ti (1.62), which resulted in the electron transfer from Ti to Pd and Pt [41–43]. So the proportion of high-valence TiOx increased and the binding energy of TiOx /Ti peak for all the catalysts shifted to a higher position (by 0.6–0.8 eV). The positive shift of 0.3 eV (Ti3+ , 457.8 → 458.1 eV) and 0.2 eV (Ti4+ , 458.9 → 459.1 eV) for C3 catalyst when compared with C5 catalyst meant the existence of highervalence Ti-ion over C3 surface. These ions were ascribed to TiOx which enhanced the content of TiOx /Ti (19.32 → 21.50). The bigger value of shift resulted in the larger content of C2 (22.03). In addition, the C3 catalyst also possessed larger concentrations of Ti4+ (26.18). The states of oxygen species and relative contents of different oxygen species also had effects on the catalytic activity. The XPS peaks of O 1s after fitting are displayed in Fig. 9. As shown, the O 1s curves of all catalysts were fitted into three peaks. Peaks around 529.2–529.4 eV were attributed to the lattice oxygen from metal oxides, labeled as O␣ . The middle peaks at 530.8–531.3 eV were assigned to adsorbed oxygen, donated as O . And the final peaks at 532.2–532.8 eV belonged to the surface oxygen by adsorbed water species, named as O␥ [44–46]. Generally, the adsorbed oxygen (O ) was considered to be more reactive in oxidation reaction because of its higher mobility, and the high relative concentration of O on catalyst surface could be correlated with high activity [47]. The specific information about the peak positions and relative contents of different oxygen species are summarized in Table 6. Due to the largest proportion of Ti3+ (70.29%) over C4, the lattice oxygen content (57.29%) was much higher than others corresponded to the lattice oxygen of Ti2 O3 . The lattice oxygen was inactive in reactions and high content of O␣ might be one reason for the poor activity of C4. The contents of O increased at first (C1 to C3) and then
Table 4 At.% of different elements and values of Al/Ti over C1-C5.
Table 5 Peak fitting parameters of Ti 2p spectra for C1-C5.
Catalysts
C1: 0.5% Pd-Pt/CAT C2: 0.6% Pd-Pt/CAT C3: 0.8% Pd-Pt/CAT C4: 1.0% Pd-Pt/CAT C5: 1.2% Pd-Pt/CAT
Catalysts
At.% of different elements Pd 3d
Ce 3d
Al 2p
Ti 2p
Al/Ti
0.21 0.28 0.40 0.14 0.12
0.23 0.23 0.22 –0.15
14.53 11.41 13.48 4.05 4.68
10.27 10.63 10.52 2.72 2.62
1.41 1.07 1.28 1.49 1.79
C1: 0.5% Pd-Pt/CAT C2: 0.6% Pd-Pt/CAT C3: 0.8% Pd-Pt/CAT C4: 1.0% Pd-Pt/CAT C5: 1.2% Pd-Pt/CAT
Binding Energy/eV
Relative Contents/%
Ti3+
Ti4+
TiOx /Ti
Ti3+
Ti4+
TiOx /Ti
458.2 458.2 458.1 457.7 457.8
458.8 459.8 459.1 459.3 458.9
460.9 461.0 460.8 460.8 460.9
57.28 62.16 52.32 70.29 56.67
31.64 15.81 26.18 17.49 24.01
11.08 22.03 21.50 12.22 19.32
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Table 7 Elemental weight contents from EDS of Support and C2-C5. Catalysts
C
O
Al
Ti
Pd
Catalyst Support: CAT C2: 0.6% Pd-Pt/CAT C3: 0.8% Pd-Pt/CAT C4: 1.0% Pd-Pt/CAT C5: 1.2% Pd-Pt/CAT
3.79 3.59 2.36 0.89 4.17
43.38 54.74 40.38 39.22 50.57
2.66 2.38 2.15 2.82 1.95
50.17 38.67 54.34 56.28 42.45
–0.62 0.77 0.79 0.86
decreased from C3 to C5. The content of O␥ changed little from C1 to C3 and then decreased in C4 (19.32%). Finally, the value turned into the largest one (37.20%) over C5 catalyst. 3.6. SEM and EDS
Fig. 9. XPS spectra of O 1 s for C1: 0.5% Pd-Pt/CAT, C2: 0.6% Pd-Pt/CAT, C3: 0.8% Pd-Pt/CAT, C4: 1.0% Pd-Pt/CAT and C5: 1.2% Pd-Pt/CAT.
Table 6 Peak fitting parameters of O 1 s spectra for C1-C5. Catalysts
C1: 0.5% Pd-Pt/CAT C2: 0.6% Pd-Pt/CAT C3: 0.8% Pd-Pt/CAT C4: 1.0% Pd-Pt/CAT C5: 1.2% Pd-Pt/CAT
Binding Energy/eV
Relative Contents/%
O␣
O
O␥
O␣
O
O␥
529.3 529.4 529.4 529.4 529.2
531.0 531.0 530.8 531.3 531.0
532.2 532.3 532.2 532.5 532.8
48.89 48.81 38.96 57.29 27.82
29.26 26.66 37.51 23.39 34.98
21.85 24.53 23.53 19.32 37.20
In order to have an intuitive observation of the dispersion of active components, SEM and EDS characterization methods were adopted. Fig. 10 demonstrates the SEM images of support and different catalysts. The support was composed of compact and uniform particles. When loaded with Pd and Pt, the surface of catalysts presented some particles with lighter color which carried active components. The catalysts surface was still compact and uniform, which illustrated the basic structures of support were unchanged after Pd and Pt loading. The elemental weight contents listed in Table 7 are concluded from EDS analyses of support and C2-C5 catalysts in Fig. S.3–S.7. As shown, in regions we randomly selected, Pd content of C2, C3 were 0.62% and 0.77%, respectively. They were very close to experimental designs of 0.6% and 0.8%. The results showed the homogeneous distribution of active components over C2 and C3. 0.79% of C4 (much
Fig. 10. SEM images of CAT: Catalyst Support, C2: 0.6% Pd-Pt/CAT, C3: 0.8% Pd-Pt/CAT and C4: 1.0% Pd-Pt/CAT.
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Fig. 11. EDS maps of C3: 0.8% Pd-Pt/CAT.
less than 1.0%) was in good agreement with the aggregated phenomenon. In order to confirm the existence of aggregation, the EDS analysis of C5 catalyst was also adopted. The Pd content of C5 (0.86%) was far less than 1.2%, which was explained by the aggregated phenomenon of active components and proved the accuracy about the conclusions in Nitrogen adsorption-desorption results of C5. Fig. 11 shows the EDS maps of C3 catalyst. Maps of Ti and Pd reflected the outline of chosen region. It illustrated that not only the support but also the active components distributed well [48]. Owing to the too scattered distribution of Pt, the map of it did not show any inerratic shape. In this condition, it was difficult to catch the Pt species and there was no one catalyst displayed the content of Pt in elemental analyses of EDS.
shoulder peaks. These features suggested the formation of Pd-PtTiOx structures between active components oxidations and TiO2 . According to the area sizes, there were more such structures over C3 than others. The low content or the peak was covered by other reduction peaks might be the reasons why the reduction peak of ceria did not appear. 3.8. Discussion Obviously, all the catalysts were active in methanol total oxidation, especially at low temperatures shown in Fig. 1. The conversion of methanol reached 100% under 100 ◦ C, which was suitable for many emissions in eliminating the residual methanol. After Pd and Pt loading, although the surface area and the total pore vol-
3.7. H2 -TPR measurements H2 -TPR measurements were carried out to identify the oxides state and changes of reduction temperature, and profiles are shown in Fig. 12. In contrast to CAT, the negative spikes of C2, C3 and C4 presented at 90 ◦ C were generally ascribed to the decomposition of palladium hydride [49,50]. This part of Pd corresponded to high dispersion of palladium on the support surface. The overlapped reduction peaks (200–800 ◦ C) of support suggested the stepwise reduction of TiO2 to low-valence [26]. The addition of Pd and Pt were beneficial to the reduction of other oxidations. Therefore, in comparison to support, the TiO2 reduction peaks of catalysts shifted to lower temperature. Wang [26] considered that the reduction peak around 850 ◦ C belonged to the reduction of Pd-TiOx lattice oxygen. In our study, the last peaks were located in about 800 ◦ C which was between 700 ◦ C (TiO2 of support) and 850 ◦ C (Pd-TiOx lattice oxygen). Furthermore, the last peaks did not show apparent
Fig. 12. H2 -TPR profiles of CAT: Catalyst Support, C2: 0.6% Pd-Pt/CAT, C3: 0.8% PdPt/CAT and C4: 1.0% Pd-Pt/CAT.
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ume decreased with the Pd increasing, the structures of anatase remained intact in XRD. The results illustrated that the addition of active components did not change the basic crystal form and the decreased intensities of peaks implied the interactions between support and Pd, Pt oxides [51,52]. The interactions made the active components more stable in reaction process. Furthermore, the decrease of grain size or the reduction of crystallinity meant that the surface of catalysts were more amorphous and provided more vacancy for active sites. These factors were conducive to enhance the performance of catalysts. When combined the excellent performance of Pd and Pt in methanol oxidation with the above-mentioned factors, these catalysts performed so well in methanol total oxidation. The abnormal BET surface area and total pore volume of C5 integrated with results of EDS showed the aggregated phenomenon on C4 and C5 catalysts. The aggregations would lead to the decrease of active sites, generally, the less quantity the active sites were, the less active the catalyst was. It might be one reason why C4 and C5 did not perform better with their higher contents of Pd. The greater value of dispersion, maximal At.% of Pd in XPS and 0.77 wt.% of Pd in EDS showed the active components of C3 catalyst did distributed better than others. The higher dispersion of active components resulted in better performance. There is also controversy about the first step of methanol oxidation whether the C O bond breaks or the O H bond breaks. Although it is not clear about the specific mechanism, the reactants are determinate. The first step is methanol reacts with adsorbed oxygen to generate adsorbed methoxy group and hydroxyl. The adsorbed oxygen and hydroxyl will continue to take part in the subsequent reactions, so the more quantity of them is beneficial to performance. In another aspect, the H2 O belongs to products of several reactions [53,54]. Every steps of the reaction belongs to reversible process, therefore, too large content of O␥ (37.20%, adsorbed water) over C5 catalyst was harmful to reaction process. The higher content of O (37.51%) and less O␥ (23.53%) over C3 catalyst was more profitable for the catalytic performance. On the active components surface, the strong interactions between the adsorbed CO with the active components make it difficult to desorption and further oxidation of CO [55]. Bock [56] and Long [57] believed that the Pt-COads were removed by reacting with Ru-OHads to produce CO2 on Pt-Ru catalysts. The larger proportion of high-valence TiOx and Ti4+ over C3 catalyst are prone to “attack” water to produce −OH and then form the Pt-OHads or PdOHads to react with COads . Finally, the Pd-Pt-TiOx structures might be conducive to the transmission of intermediate or the electron, which further accelerated the speed of reaction. All of these factors facilitated the superior performance of C3 catalyst in methanol ´ et al. [15] found that T50 and T90 of total oxidation. M. Jabłonska 1.5%Pd/HY catalyst were 90 ◦ C and 120 ◦ C for the methanol incineration in 20 cm3 min−1 of total flow rate. Merte [58] et al. proved that the 1%Pt/␥-Al2 O3 catalyst showed high performance in the oxidation of 0.15% CH3 OH at low temperature. Compared with the other researches, the C3 catalyst showed higher performance in one or more conditions among the methanol concentration (1.0 vol.% CH3 OH), GHSV (35,000 h−1 ), T50 (35 ◦ C) or T90 (58 ◦ C). The C3 catalyst possessed superior comprehensive performance among the similar catalysts types.
4. Conclusions In summary, we demonstrated that the physicochemical properties and catalytic activities of catalysts in methanol total oxidation were influenced by the content of Pd. The results revealed that the contents of Pd and Pt over C3 catalyst (0.8% Pd-Pt/CAT) were appropriate which resulted in higher performance in methanol total oxidation and CO2 selectivity. Results of characterizations
present that the active components disperse well in low content and the aggregated phenomena appear when the Pd content reach 1.0%. The higher contents of adsorbed oxygen over C3 catalyst are more conducive to rate determining step and subsequent reactions. The larger proportion of high-valence TiOx and Ti4+ are prone to “attack” water to produce −OH and then form the Pt-OHads or PdOHads to react with COads . Furthermore, the decrease of grain size or the reduction of crystallinity indicates that the surface of catalyst is more amorphous and will provide more vacancy for active sites. This finding also provides a new constituent and highly reactive catalyst for the removal of methanol in VOCs, especially at low temperature. Acknowledgements This work was financially supported by the State Key Program of National Natural Science of China (No.21336006) and the Shanxi Province Scientific and Technological Project (No.20140313002-2). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.molcata.2016.11. 041. References [1] W.B. Li, J.X. Wang, H. Gong, Catalytic combustion of VOCs on non-noble metal catalysts, Catal. Today 148 (2009) 81–87. [2] Q. Niu, B. Li, X.L. Xu, X.J. Wang, Q. Yang, Y.Y. Jiang, Y.W. Chen, S.M. Zhu, S.B. Shen, Activity and sulfur resistance of CuO/SnO2 /PdO catalysts supported on ␥-Al2 O3 for the catalytic combustion of benzene, RSC Adv. 4 (2014) 51280–51285. [3] C. H. Guo, Y. Chen, J. Xiao, Z. Wang, D. Fan, Y. Sun Li, Influence of preparation method on the surface and catalytic properties of sulfated vanadia–titania catalysts for partial oxidation of methanol, Fuel Process. Technol. 106 (2013) 77–83. [4] M. Haruta, A. Ueda, S. Tsubota, R.M. Torres Sanchez, Low-temperature catalytic combustion of methanol and its decomposed derivatives over supported gold catalysts, Catal. Today 29 (1996) 443–447. [5] A. Mirzaei, S.G. Leonardi, G. Neri, Detection of hazardous volatile organic compounds (VOCs) by metal oxide nanostructures-based gas sensors: a review, Ceram. Int. 42 (2016) 15119–15141. [6] J.M. Tatibougt, Methanol oxidation as a catalytic surface probe, Appl. Catal. A: Gen. 148 (1997) 213–252. [7] S. Zhao, J. Bennici, Surface and catalytic properties of V2 O5 –TiO2 /SO4 2− catalysts for the oxidation of methanol prepared by various methods, J. Mol. Catal. A: Chem. 309 (2009) 28–34. [8] Y. Zhao, Z. Qin, G. Wang, M. Dong, L. Huang, Z. Wu, W. Fan, J. Wang, Catalytic performance of V2 O5 /ZrO2 –Al2 O3 for methanol oxidation, Fuel 104 (2013) 22–27. [9] L. Delannoy, K. Fajerwerg, P. Lakshmanan, C. Potvin, C. Méthivier, C. Louis, Supported gold catalysts for the decomposition of VOC: Total oxidation of propene in low concentration as model reaction, Appl. Catal. B: Environ. 94 (2010) 117–124. [10] S.C. Kim, W.G. Shim, Properties and performance of Pd based catalysts for catalytic oxidation of volatile organic compounds, Appl. Catal. B: Environ. 92 (2009) 429–436. [11] M. Ousmane, L.F. Liotta, G. Pantaleo, A.M. Venezia, G. Di Carlo, M. Aouine, L. Retailleau, A. Giroir-Fendler, Supported Au catalysts for propene total oxidation: study of support morphology and gold particle size effects, Catal. Today 176 (2011) 7–13. [12] K. Okumura, T. Kobayashi, H. Tanaka, M. Niwa, Toluene combustion over palladium supported on various metal oxide supports, Appl. Catal. B: Environ. 44 (2003) 325–331. [13] S.S.T. Bastos, J.J.M. Órfão, M.M.A. Freitas, M.F.R. Pereira, J.L. Figueiredo, Manganese oxide catalysts synthesized by exotemplating for the total oxidation of ethanol, Appl. Catal. B: Environ. 93 (2009) 30–37. [14] J. Peng, S. Wang, Performance and characterization of supported metal catalysts for complete oxidation of formaldehyde at low temperatures, Appl. Catal. B: Environ. 73 (2007) 282–291. ´ ˛ K. Tarach, V. Girman, L. Chmielarz, K. [15] M. Jabłonska, A. Król, E. Kukulska-Zajac, Góra-Marek, Y. Zeolites, modified with palladium as effective catalysts for low-temperature methanol incineration, Appl. Catal. B: Environ. 166-167 (2015) 353–365. [16] E.M. Cordi, J.L. Falconer, Oxidation of volatile organic compounds on Al2 O3 Pd/Al2 O3 , and PdO/Al2 O3 catalysts, J. Catal. 162 (1996) 104–117.
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Methanol total oxidation as model reaction for the effects of different Pd content on Pd-Pt/CeO2 -Al2 O3 -TiO2 catalysts Yuyu Guo, Shen Zhang, Wentao Mu, Xingying Li, Zhe Li ∗ College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024 Shanxi, China
a r t i c l e
i n f o
Article history: Received 8 October 2016 Received in revised form 28 November 2016 Accepted 29 November 2016 Available online xxx Keywords: Methanol total oxidation Pd content Room temperature Adsorbed oxygen
a b s t r a c t A series of Pd-Pt/CAT1 catalysts with different Pd content were prepared by stepwise impregnation methods for methanol total oxidation. The influences of Pd content on the physicochemical properties of catalysts were investigated by XRD, Nitrogen adsorption-desorption, noble metal dispersion, XPS, SEM and H2 -TPR techniques. Under the reaction conditions (1.0 vol.% CH3 OH, 2.0 vol.% O2 , GHSV: 35,000 h−1 ), 0.8% Pd-Pt/CAT catalyst converts 35% methanol in room temperature (27 ◦ C) and T50 , T90 are 35 ◦ C and 58 ◦ C, respectively. Until the conversion reaches 100%, the heat is only provided by experimental heat releasing. The results of characterizations show that the content of active components has an apparent effect on the exposed surface area as well as the TiO2 structures and thus influences the catalytic performance. The higher contents of adsorbed oxygen over 0.8% Pd-Pt/CAT catalyst are more conducive to rate determining step and subsequent reactions. The larger proportion of high-valence TiOx and Ti4+ are prone to “attack” water to produce −OH and then form the Pt-OHads or Pd-OHads to react with COads . Furthermore, the decrease of grain size or the reduction of crystallinity indicates that the surface of catalyst is more amorphous and will provide more vacancy for active sites. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Volatile organic compounds (VOCs) are usually recognized as major contributors to air pollutions which emitted from many industry processes as well as from indoor sources [1,2]. With the more stringent emission standards, the reduction of VOCs is becoming more important. Methanol, one of the dominating VOCs, is widely used as solvents of many chemical products and the basic material to produce some important industrial chemicals like formaldehyde, acetic acid [3,4]. Specially, the uses of methanol in automotive fuels become more common because of its reduction of the emission of conventional pollutants such as CO [5]. So many applications will directly lead to the increase emission of residual methanol. Methanol has remarkable impact on human health, such as stomachache, eye irritation and headache. So the removal of methanol is inevitable to protect the atmospheric environment and human health [6–8]. Several techniques of physical and chemical adsorption, photocatalytic and catalytic oxidation methods are currently used. Among them, the catalytic oxidation to carbon dioxide and water is the most ideal and effective technique on
∗ Corresponding author. E-mail address: [email protected] (Z. Li). 1 CAT: CeO2 -Al2 O3 -TiO2 .
account of advantages like high removal efficiency, low light-off temperature, widespread applications, simple equipment and no secondary pollution [9–11]. The catalysts with high performance are indispensable and the research of them has become a hot topic, particularly at low temperature. Various kinds of catalysts such as single or mixed metal oxides and Pt or Pd supported catalysts were applied for the total oxidation of VOCs [12]. The supported noble-metal catalysts are more durable and more efficient compared to metal oxide based catalysts in the oxidation of VOCs [13,14]. The Pd supported catalysts have been extensively studied, since palladium has been found to be promising for practical applications in the total combustion of VOCs due to its high activity at relatively low temperature, tolerance to moisture [15] and higher resistance to thermal sintering in an oxidizing environment than platinum [16]. Álvarez-Galván et al. [17] reported that the addition of Pd on Mn/Al2 O3 catalyst could decrease the total oxidation temperature of the mixture of HCHO and methanol. Hosseini et al. [18] found that the Au/Pd catalysts showed higher activity than the monometallic Au or Pd catalysts. Platinum metal is recognized as the most active metal for hydrocarbon combustion [19]. Chuang et al. [20] showed the Pt based-catalysts for VOC oxidation is complete at low temperature. In terms of the superior properties of Pd and the high performance of Pt, we want to get a
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catalyst with higher performance which combines with the merits of Pd and Pt. The support properties also play an important role in the efficiency of catalysts [21]. Supports were expected to be an important component of the catalyst, since they often profoundly affected the generation of active species and the catalytic performance of Pd [12]. Nanometer TiO2 belongs to an important semiconductor material and has been widely used in many fields [22]. Ordered porous TiO2 possess the general characteristics of porous materials, such as uniform aperture size, narrow pore size distribution, the orderliness of the pore structure and high porosity. But deficiencies of low specific surface area and weak surface acidity also exist in this material. The porous Al2 O3 -TiO2 composites precisely make up the disadvantages of single TiO2 because of the large surface area and strong surface acidity of Al2 O3 [23]. CeO2 is a crucial component in catalysts for its role in oxygen storage, redox properties and enhancing the dispersion of noble metals and the thermal stability of catalysts which always plays the role of promoter in support [21,24]. Based on the previous researches and conclusions of my laboratory [25,26], CeO2 -Al2 O3 -TiO2 was chosen as the support in which the weight percent of Ce was 1%. Pd was used as the first active component from 0.5 wt.% to 1.2 wt.% and 0.5 wt.% Pt was loaded as another active component. The aim of this work was to understand the structure-function relationship dependent on Pd loading for methanol total oxidation. On this basis, find one catalyst with high performance through changing the content of Pd. Several characterization methods were used to study the physicochemical properties of the catalysts for the purpose of explaining the relationship between the catalytic activity and the structures of catalysts.
The catalytic properties of the catalysts were evaluated in a continuous-flow, atmospheric pressure, fixed-bed reactor (Ø = 6 mm) which was a straight quartz tube and shrank in the middle to hold the catalyst by two quartz wool plugs. The catalyst powders were pressed, crushed and sieved to the fraction of 40–60 mesh. 300 mg of catalysts (approximate 0.25 mL) were utilized every time. The reactants were fed to the reactor by replenishing a part of the nitrogen stream through a glass saturator containing methanol (room temperature, 26 ± 1 ◦ C). The CH3 OH, O2 and N2 were mixed before they reached the reactor. In order to achieve accurate and stable controlling of the gas flow rates, mass flow controllers were adopted in experiments. The feed gas consisted of 1.0 vol.% CH3 OH, 2.0 vol.% O2 and a balance of N2 . The total flow rate through the reactor was 145 mL min−1 , which gave a gas hourly space velocity (GHSV) of 35,000 h−1 . The reaction temperature was in the control of a tubular electric furnace and the temperature of the catalyst bed was monitored automatically by thermocouples. The compositions of reactants and products were analyzed by an online gas chromatograph (HXSP, GC-950) with two FID detectors and the one incorporated a methanator. The one FID detector was applied to analyze organic compounds composed of methane, formaldehyde, dimethyl ether and methanol. The other one was used to detect CO, CH4 and CO2 . Methanol conversion (Xi ) and the selectivity (Si ) of products were calculated according to the following formulas:
2. Experimental
Xi = [(Ci,in −Ci,out )/Ci,in ] × 100%
2.1. Preparation of catalysts
where Ci,in and Ci,out are the peak areas of methanol before reaction and in combustion products.
2.1.1. Synthesis of catalyst support (CAT) Aluminum isopropoxide (1.979 g) and tetrabutyl titanate (40 mL) were taken as precursors of aluminum and titanium. Both the alumina and titania sols were prepared by sol–gel method [25]. In the process of preparing titania sols, cerous nitrate (0.316 g) and alumina sols which had been prepared in advance were dumped into it. Then they were mixed together to obtain 1% CeO2 -Al2 O3 TiO2 and the weight content ratio of Al2 O3 to TiO2 was 1/19. The weight percentage of Ce was 1%, which was calculated on the amount of Al2 O3 -TiO2 as a benchmark. The mixed sols were dried at 50 ◦ C and 110 ◦ C in an oven for 12 h, respectively, then calcined at 500 ◦ C in air for 5 h at the heating rate of 2 ◦ C min−1 . The catalyst support (denoted as CAT) was in the form of yellow powder. 2.1.2. Synthesis of Pd/CAT catalysts Different weight of the Pd component were taken according to the proportion in experimental design and loaded on CAT by the method of impregnation [25]. Certain amounts of PdCl2 were dissolved by deionized water, then a portion of CAT and two drops of glacial acetic acid were put into PdCl2 solution. The solution was stirred continuously at room temperature for 24 h and then dried by the method of evaporating to dryness in a water bath at 80–85 ◦ C and in an oven for 12 h at 110 ◦ C. Subsequently, the dry power were grinded and then calcined at 500 ◦ C in air for 5 h (the rate of heating was 2 ◦ C min−1 ). Pd contents of different samples were 0.5%, 0.6%, 0.8%, 1.0% and 1.2% of mass percentage. 2.1.3. Preparation of Pd-Pt/CAT catalysts Another active component (Pt) was loaded after Pd by using H2 PtCl6 ·H2 O through the same method and Pt content of all the
samples were 0.5 wt.%. The aforementioned weight percentage of Ce, Pd and Pt were contents of element. With the increase of the Pd content, the catalysts were marked as C1, C2, C3, C4 and C5. 2.2. Oxidation of methanol
Si = [nj ·Cj,out /
nj ·Cj,out ] × 100%
where Cj,out are the concentrations of products, nj is the products and reactants carbon atom ratios. 2.3. Characterizations of catalysts X-ray powder diffraction (XRD) analysis of the catalysts were performed with a Rigaku D/max 2500 powder diffractometer operated at 40 KV and 100 mA using Ni-filtered Cu-K␣ radiation. The intensities of the diffraction peaks were recorded in the 2 range of 5–80◦ with a step size of 0.03◦ , and the scanning speed was 8◦ min−1 . N2 adsorption-desorption isotherms for C1, C2, C3, C4 and C5 were measured on a Micromeritics ASAP 2020 analyzer. The specific surface areas of the catalysts were calculated from the adsorption curve using the Brunauer-Emmett-Teller (BET) method, and pore volume and pore size distribution were determined by using Barrett-Joyner-Halenda (BJH) desorption theoretical model. All samples were degassed at 200 ◦ C and 6.67 × 10−2 Pa vacuum pressure for 4 h before measurement. Noble metal dispersion was evaluated with a TP-5085 metal dispersion instrument. The catalysts were put into the tube and the temperature was raised to 500 ◦ C and kept constant for 2 h in flowing H2 . Then the samples were cooled to room temperature under H2 and blown by N2 . After that, the catalysts were treated with O2 -titration. Blown by N2 once again, H2 -titration was carried out finally. X-ray photoelectron spectra (XPS) were carried out using ESCALAB 250 multifunctional electronic energy spectrometer with
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Table 1 T50 , T90 and S99 of methanol oxidation for C1: 0.5% Pd-Pt/CAT, C2: 0.6% Pd-Pt/CAT, C3: 0.8% Pd-Pt/CAT, C4: 1.0% Pd-Pt/CAT and C5: 1.2% Pd-Pt/CAT.
Fig. 1. Methanol conversion as a function of reaction temperature for C1- C5 from 25 ◦ C to 200 ◦ C, Reaction conditions: 1.0 vol.% CH3 OH, 2.0 vol.% O2 , GHSV: 35,000 h−1.
Al-K␣ (150 W) X-ray source. All the element binding energies were referenced to the C 1 s line situated at 284.6 eV. All measurements were subject to an estimated error of ±0.1 eV. The surface morphologies of catalysts were detected by scanning electron microscopy (SEM) on TESCAN Maia 3 LMH equipment. Meanwhile, content and dispersion maps of different elements were investigated by energy dispersive spectrometer (EDS) on the same instrument utilizing OXFORD EDS Inca X-Max. H2 -TPR experiments of the catalysts were obtained on an AutoChem 2920 chemisorption analyzer (Micromeritics, Norcross, GA, USA) equipped with a thermal conductivity detector. For the analysis, 50 mg of catalysts were pretreated by flowing N2 at a rate of 50 mL min−1 for 40 min at 400 ◦ C. After cooling to room temperature, a feed gas containing 5 vol.% H2 in N2 was fed to the catalysts at a flow rate of 50 mL min−1 and the temperature of reactor was raised to 950 ◦ C (heating rate: 10 ◦ C min−1 ).
Temperatures
C1
C2
C3
C4
C5
T50 /◦ C T90 /◦ C S99 /◦ C
50 105 50
50 90 50
35 58 59
60 75 100
55 70 60
in which the conversion reached 100%, the heat was only provided by experimental heat releasing instead of external heating. The other four catalysts required external heating to start reaction. Furthermore, the selectivity of CO2 also owned high behavior over C3 catalyst. In order to compare conveniently, some special temperatures were listed. Table 1 shows the light-off temperature (T50 ), complete conversion temperature (T90 ) and the temperature of CO2 selectivity reached 99% (S99 ) of different catalysts. T50 of these catalysts were below 60 ◦ C and T90 were under 90 ◦ C except C1 (105 ◦ C). The results illustrated that all these catalysts were active at low temperature. Under 60 ◦ C, the CO2 selectivity reached 99% except C4 (100 ◦ C). The data indicated that methanol was completely oxidized and the products of reactions were almost CO2 . The distributions of products were in accordance with requirements of green chemistry. T50 and T90 of C3 catalyst were 35 ◦ C and 58 ◦ C which were the lowest in these catalysts. The activity sequences were C3 > C5 > C4 > C2 > C1, which showed different rules from the order of Pd content reduced. The results revealed that the Pd content did have effects on catalytic performance and the contents of Pd and Pt over C3 catalyst were more appropriate. 3.2. XRD analyses
The tendency of CH3 OH conversion and CO2 selectivity along with temperature increasing were shown in Figs. 1 and 2 from 25 ◦ C to 200 ◦ C and Fig. S.1–S.2 was from 25 ◦ C to 500 ◦ C, respectively. CH3 OH conversion and CO2 selectivity of all catalysts monotonically increased at beginning and kept steadily after they reached 100% until 500 ◦ C. Compared with the other four catalysts, C3 catalyst showed higher performance in the temperature range from room temperature (27 ◦ C) to 200 ◦ C and lowered the light-off temperature and complete conversion temperature. It was worth mentioning that until the temperature rose to approximately 62 ◦ C
For investigate the changes of crystalline phase and grain sizes after active components loaded, XRD analyses were used and the patterns of catalyst support and five catalysts are reported in Fig. 3. The diffraction peaks appeared at 2 = 25.4◦ , 37.8◦ , 48.2◦ , 53.9◦ and 62.9◦ were indexed to diffractions from lattice planes (101), (004), (200), (105) and (204) [27] respectively, corresponded well to the anatase phase (PDF#73-1764). Only the diffraction peaks of the anatase structure were evident in catalyst support and all the catalysts. PtOx , PdO, CeO2 and Al2 O3 did not appear might be because of their low content or high dispersion [28]. The 2, FWHM and crystalline size of anatase in various samples are summarized in Table 2. The average crystalline size was estimated according to the Scherrer equation [29]. Compared with the catalyst support, values of full width at half-maximum (FWHM) of the strongest diffraction peak around 25.4◦ increased when loaded by active components. For all the samples, the crystalline sizes of
Fig. 2. CO2 selectivity as a function of reaction temperature for C1-C5 from 25 ◦ C to 200 ◦ C, Reaction conditions: 1.0 vol.% CH3 OH, 2.0 vol.% O2 , GHSV: 35,000 h−1 .
Fig. 3. XRD patterns of CAT: Catalyst Support, C1: 0.5% Pd-Pt/CAT, C2: 0.6% PdPt/CAT, C3: 0.8% Pd-Pt/CAT, C4: 1.0% Pd-Pt/CAT and C5: 1.2% Pd-Pt/CAT.
3. Results and discussion 3.1. Activity of methanol oxidation
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Table 2 2, FWHM and crystalline size of anatase in catalyst support and various catalysts. Samples
2 (◦ )
FWHM (◦ )
Crystalline Size (nm)
Catalyst Support: CAT C1: 0.5% Pd-Pt/CAT C2: 0.6% Pd-Pt/CAT C3: 0.8% Pd-Pt/CAT C4: 1.0% Pd-Pt/CAT C5: 1.2% Pd-Pt/CAT
25.40 25.40 25.40 25.40 25.40 25.37
0.31 0.52 0.53 0.38 0.33 0.34
25.7 15.4 15.3 21.2 24.4 24.0
Fig. 6. Calculated by parameters of Pd Calculated by parameters of Pt, Noble metal dispersions of C2: 0.6% Pd-Pt/CAT, C3: 0.8% Pd-Pt/CAT and C4: 1.0% Pd-Pt/CAT.
Fig. 4. Pore size distributions of C1-C5.
Fig. 5. N2 adsorption-desorption isotherms of C1: 0.5% Pd-Pt/CAT, C2: 0.6% PdPt/CAT, C3: 0.8% Pd-Pt/CAT, C4: 1.0% Pd-Pt/CAT and C5: 1.2% Pd-Pt/CAT.
anatase phase were roughly concentrated on 15–25 nm. When the active components were loaded, the grain sizes decreased in different degree. Moreover, the decreased peak intensity suggested the reduction of crystallinity. The results certified that XRD peaks became less sharp when the grain sizes decreased or the crystallinity reduced [30]. 3.3. Nitrogen adsorption-desorption results N2 adsorption-desorption were adopted to analyze the pore structure parameters of samples, therefore, Figs. 4 and 5 exhibit pore size distributions and N2 adsorption-desorption isotherms.
The pore sizes of all the catalysts concentrated on the scope range from 5 to 15 nm. The shape of these isotherms showed a type IV isotherm (IUPAC classification) with a H4 hysteresis loop [31] located at 0.40 < P/P0 < 1.0. All the features indicated that these catalysts belonged to typical mesoporous materials. The physical properties consisted of BET surface area, total pore volume and average pore diameter are listed in Table 3. Sizes of Pd and Pt ions were much less than 8 nm which the average pore diameter centered around. It meant that the two ions could easily enter into the interior of catalyst pores. In other words, the more active contents were introduced, the more spaces of pores were covered possibly. As shown, the surface area (from 48.79 m2 g−1 to 27.69 m2 g−1 ) and total pore volume (from 0.098 cm3 g−1 to 0.057 cm3 g−1 ) decreased one after another from C1 to C4 with the active components contents increased. The C5 catalyst should own the smallest surface area and total pore volume in theory. But the surface area (35.06 m2 g−1 ) was similar to C3 (34.26 m2 g−1 ) and total pore volume (0.089 cm3 g−1 ) was slight smaller than C2 (0.092 cm3 g−1 ). This abnormal phenomenon exhibited might be owing to the active particles of C5 were aggregated instead of separate. The average pore diameter of C5 (10.2 nm) was larger than others, which was interpreted by the collapse of pore structures [25] after several calcination. These factors were harmful to reaction and led to the worse catalytic performance of C5.
3.4. Noble metal dispersion In order to gain the specific dispersions of active components, the characterization of noble metal dispersion was used. The results of this technique were co-dispersions of Pd and Pt instead of separate. In the calculation of dispersion, molar mass, density and some parameters were required. Therefore, Fig. 6 shows dispersions calculated by Pd and Pt, respectively. The higher dispersion (0.16 and 0.30) demonstrated the better distribution of active contents over C3 catalyst. Values of C4 (0.06 and 0.11) were much smaller than the other two, which might be because the existence of aggregation over C4 catalyst.
Table 3 Pore structure parameters of C1-C5. Catalysts
BET surfacearea/(m2 g−1 )
Total porevolume/(cm3 g−1 )
Average porediameter/(nm)
C1: 0.5% Pd-Pt/CAT C2: 0.6% Pd-Pt/CAT C3: 0.8% Pd-Pt/CAT C4: 1.0% Pd-Pt/CAT C5: 1.2% Pd-Pt/CAT
48.79 45.58 34.26 27.69 35.06
0.098 0.092 0.072 0.057 0.089
8.6 7.5 8.4 8.3 10.2
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Fig. 7. XPS spectra of Pd 3d for C1: 0.5% Pd-Pt/CAT, C2: 0.6% Pd-Pt/CAT, C3: 0.8% Pd-Pt/CAT, C4: 1.0% Pd-Pt/CAT and C5: 1.2% Pd-Pt/CAT.
Fig. 8. XPS spectra of Ti 2p for C1: 0.5% Pd-Pt/CAT, C2: 0.6% Pd-Pt/CAT, C3: 0.8% Pd-Pt/CAT, C4: 1.0% Pd-Pt/CAT and C5: 1.2% Pd-Pt/CAT.
3.5. XPS evaluate To obtain the valence states and relative concentrations of different elements, the XPS was introduced and spectra of Pd 3d of various catalysts are displayed in Fig. 7. After fitting, the peak of Pd 3d5/2 was decomposed into single one around 336.4 eV which was attributed to Pd2+ [32–34] over all the catalysts. The peak of Al overlapped with Pt and the content of Pt was much less than the Al possessed which resulted in the Pt did not appear separately [35]. At.% of different elements and values of Al/Ti over C1-C5 are summarized in Table 4. The Pd content ranged from 0.12% to 0.40%, and the maximal At.% of Pd (0.40%) meant that the C3 catalyst surface had more content of Pd than other four catalysts. According to our designs, the content of Ti was much larger than Al possessed. Results showed the At.% of Al was slight bigger than Ti which was interpreted by the enrichment of Al on the surface. The greater values of Al/Ti demonstrated that Ti which located in the skeleton of support surface was replaced by some active components. These kinds of active particles turned stably and were unprofitable to the reactivity. Value (1.79) of C5 illustrated that more quantity of active components entered into the skeleton, by contrast, the quantity of high-dispersion contents on the surface reduced. So the smaller values of Al/Ti demonstrated that the proportion of disperse active components over C2 and C3 catalysts were higher. It was in good agreement with the conclusions of noble metal dispersion. Changes of TiO2 which were the major components of catalysts played important roles in affecting the performance. To deep investigate, the XPS spectra of Ti 2p of C1-C5 are presented in Fig. 8. Table 5 shows the peak fitting parameters of Ti 2p spectra for C1-C5. The regions were decomposed into several contributions according to Shirley background and Caussian-Lorentzian fitted method corresponded to different oxidation states of Ti, respectively. The ratio of the area of the two peaks A(Ti 2p1/2 )/A(Ti 2p3/2 ) is equal to 1/2 and the binding energy separation between Ti 2p1/2 and Ti 2p3/2 in the same valence conditions was 5.7 eV, which was in agreement with
the literatures [36,37]. Peaks at 457.8 eV (2p3/2 ) and 463.5 eV (2p1/2 ) were attributed to the existence of Ti3+ [38], 458.9 eV (2p3/2 ) and 464.6 eV (2p1/2 ) stood for Ti4+ [39], and the peaks around 460.9 eV (2p3/2 ) and 466.6 eV (2p1/2 ) were also observed. OCAL [40] believed that the peak around 460.2 eV was ascribed to TiOx /Ti. In our study, Pd and Pt possessed higher electronegativity (Pd-2.20, Pt-2.28) than Ti (1.62), which resulted in the electron transfer from Ti to Pd and Pt [41–43]. So the proportion of high-valence TiOx increased and the binding energy of TiOx /Ti peak for all the catalysts shifted to a higher position (by 0.6–0.8 eV). The positive shift of 0.3 eV (Ti3+ , 457.8 → 458.1 eV) and 0.2 eV (Ti4+ , 458.9 → 459.1 eV) for C3 catalyst when compared with C5 catalyst meant the existence of highervalence Ti-ion over C3 surface. These ions were ascribed to TiOx which enhanced the content of TiOx /Ti (19.32 → 21.50). The bigger value of shift resulted in the larger content of C2 (22.03). In addition, the C3 catalyst also possessed larger concentrations of Ti4+ (26.18). The states of oxygen species and relative contents of different oxygen species also had effects on the catalytic activity. The XPS peaks of O 1s after fitting are displayed in Fig. 9. As shown, the O 1s curves of all catalysts were fitted into three peaks. Peaks around 529.2–529.4 eV were attributed to the lattice oxygen from metal oxides, labeled as O␣ . The middle peaks at 530.8–531.3 eV were assigned to adsorbed oxygen, donated as O . And the final peaks at 532.2–532.8 eV belonged to the surface oxygen by adsorbed water species, named as O␥ [44–46]. Generally, the adsorbed oxygen (O ) was considered to be more reactive in oxidation reaction because of its higher mobility, and the high relative concentration of O on catalyst surface could be correlated with high activity [47]. The specific information about the peak positions and relative contents of different oxygen species are summarized in Table 6. Due to the largest proportion of Ti3+ (70.29%) over C4, the lattice oxygen content (57.29%) was much higher than others corresponded to the lattice oxygen of Ti2 O3 . The lattice oxygen was inactive in reactions and high content of O␣ might be one reason for the poor activity of C4. The contents of O increased at first (C1 to C3) and then
Table 4 At.% of different elements and values of Al/Ti over C1-C5.
Table 5 Peak fitting parameters of Ti 2p spectra for C1-C5.
Catalysts
C1: 0.5% Pd-Pt/CAT C2: 0.6% Pd-Pt/CAT C3: 0.8% Pd-Pt/CAT C4: 1.0% Pd-Pt/CAT C5: 1.2% Pd-Pt/CAT
Catalysts
At.% of different elements Pd 3d
Ce 3d
Al 2p
Ti 2p
Al/Ti
0.21 0.28 0.40 0.14 0.12
0.23 0.23 0.22 –0.15
14.53 11.41 13.48 4.05 4.68
10.27 10.63 10.52 2.72 2.62
1.41 1.07 1.28 1.49 1.79
C1: 0.5% Pd-Pt/CAT C2: 0.6% Pd-Pt/CAT C3: 0.8% Pd-Pt/CAT C4: 1.0% Pd-Pt/CAT C5: 1.2% Pd-Pt/CAT
Binding Energy/eV
Relative Contents/%
Ti3+
Ti4+
TiOx /Ti
Ti3+
Ti4+
TiOx /Ti
458.2 458.2 458.1 457.7 457.8
458.8 459.8 459.1 459.3 458.9
460.9 461.0 460.8 460.8 460.9
57.28 62.16 52.32 70.29 56.67
31.64 15.81 26.18 17.49 24.01
11.08 22.03 21.50 12.22 19.32
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Table 7 Elemental weight contents from EDS of Support and C2-C5. Catalysts
C
O
Al
Ti
Pd
Catalyst Support: CAT C2: 0.6% Pd-Pt/CAT C3: 0.8% Pd-Pt/CAT C4: 1.0% Pd-Pt/CAT C5: 1.2% Pd-Pt/CAT
3.79 3.59 2.36 0.89 4.17
43.38 54.74 40.38 39.22 50.57
2.66 2.38 2.15 2.82 1.95
50.17 38.67 54.34 56.28 42.45
–0.62 0.77 0.79 0.86
decreased from C3 to C5. The content of O␥ changed little from C1 to C3 and then decreased in C4 (19.32%). Finally, the value turned into the largest one (37.20%) over C5 catalyst. 3.6. SEM and EDS
Fig. 9. XPS spectra of O 1 s for C1: 0.5% Pd-Pt/CAT, C2: 0.6% Pd-Pt/CAT, C3: 0.8% Pd-Pt/CAT, C4: 1.0% Pd-Pt/CAT and C5: 1.2% Pd-Pt/CAT.
Table 6 Peak fitting parameters of O 1 s spectra for C1-C5. Catalysts
C1: 0.5% Pd-Pt/CAT C2: 0.6% Pd-Pt/CAT C3: 0.8% Pd-Pt/CAT C4: 1.0% Pd-Pt/CAT C5: 1.2% Pd-Pt/CAT
Binding Energy/eV
Relative Contents/%
O␣
O
O␥
O␣
O
O␥
529.3 529.4 529.4 529.4 529.2
531.0 531.0 530.8 531.3 531.0
532.2 532.3 532.2 532.5 532.8
48.89 48.81 38.96 57.29 27.82
29.26 26.66 37.51 23.39 34.98
21.85 24.53 23.53 19.32 37.20
In order to have an intuitive observation of the dispersion of active components, SEM and EDS characterization methods were adopted. Fig. 10 demonstrates the SEM images of support and different catalysts. The support was composed of compact and uniform particles. When loaded with Pd and Pt, the surface of catalysts presented some particles with lighter color which carried active components. The catalysts surface was still compact and uniform, which illustrated the basic structures of support were unchanged after Pd and Pt loading. The elemental weight contents listed in Table 7 are concluded from EDS analyses of support and C2-C5 catalysts in Fig. S.3–S.7. As shown, in regions we randomly selected, Pd content of C2, C3 were 0.62% and 0.77%, respectively. They were very close to experimental designs of 0.6% and 0.8%. The results showed the homogeneous distribution of active components over C2 and C3. 0.79% of C4 (much
Fig. 10. SEM images of CAT: Catalyst Support, C2: 0.6% Pd-Pt/CAT, C3: 0.8% Pd-Pt/CAT and C4: 1.0% Pd-Pt/CAT.
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Fig. 11. EDS maps of C3: 0.8% Pd-Pt/CAT.
less than 1.0%) was in good agreement with the aggregated phenomenon. In order to confirm the existence of aggregation, the EDS analysis of C5 catalyst was also adopted. The Pd content of C5 (0.86%) was far less than 1.2%, which was explained by the aggregated phenomenon of active components and proved the accuracy about the conclusions in Nitrogen adsorption-desorption results of C5. Fig. 11 shows the EDS maps of C3 catalyst. Maps of Ti and Pd reflected the outline of chosen region. It illustrated that not only the support but also the active components distributed well [48]. Owing to the too scattered distribution of Pt, the map of it did not show any inerratic shape. In this condition, it was difficult to catch the Pt species and there was no one catalyst displayed the content of Pt in elemental analyses of EDS.
shoulder peaks. These features suggested the formation of Pd-PtTiOx structures between active components oxidations and TiO2 . According to the area sizes, there were more such structures over C3 than others. The low content or the peak was covered by other reduction peaks might be the reasons why the reduction peak of ceria did not appear. 3.8. Discussion Obviously, all the catalysts were active in methanol total oxidation, especially at low temperatures shown in Fig. 1. The conversion of methanol reached 100% under 100 ◦ C, which was suitable for many emissions in eliminating the residual methanol. After Pd and Pt loading, although the surface area and the total pore vol-
3.7. H2 -TPR measurements H2 -TPR measurements were carried out to identify the oxides state and changes of reduction temperature, and profiles are shown in Fig. 12. In contrast to CAT, the negative spikes of C2, C3 and C4 presented at 90 ◦ C were generally ascribed to the decomposition of palladium hydride [49,50]. This part of Pd corresponded to high dispersion of palladium on the support surface. The overlapped reduction peaks (200–800 ◦ C) of support suggested the stepwise reduction of TiO2 to low-valence [26]. The addition of Pd and Pt were beneficial to the reduction of other oxidations. Therefore, in comparison to support, the TiO2 reduction peaks of catalysts shifted to lower temperature. Wang [26] considered that the reduction peak around 850 ◦ C belonged to the reduction of Pd-TiOx lattice oxygen. In our study, the last peaks were located in about 800 ◦ C which was between 700 ◦ C (TiO2 of support) and 850 ◦ C (Pd-TiOx lattice oxygen). Furthermore, the last peaks did not show apparent
Fig. 12. H2 -TPR profiles of CAT: Catalyst Support, C2: 0.6% Pd-Pt/CAT, C3: 0.8% PdPt/CAT and C4: 1.0% Pd-Pt/CAT.
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ume decreased with the Pd increasing, the structures of anatase remained intact in XRD. The results illustrated that the addition of active components did not change the basic crystal form and the decreased intensities of peaks implied the interactions between support and Pd, Pt oxides [51,52]. The interactions made the active components more stable in reaction process. Furthermore, the decrease of grain size or the reduction of crystallinity meant that the surface of catalysts were more amorphous and provided more vacancy for active sites. These factors were conducive to enhance the performance of catalysts. When combined the excellent performance of Pd and Pt in methanol oxidation with the above-mentioned factors, these catalysts performed so well in methanol total oxidation. The abnormal BET surface area and total pore volume of C5 integrated with results of EDS showed the aggregated phenomenon on C4 and C5 catalysts. The aggregations would lead to the decrease of active sites, generally, the less quantity the active sites were, the less active the catalyst was. It might be one reason why C4 and C5 did not perform better with their higher contents of Pd. The greater value of dispersion, maximal At.% of Pd in XPS and 0.77 wt.% of Pd in EDS showed the active components of C3 catalyst did distributed better than others. The higher dispersion of active components resulted in better performance. There is also controversy about the first step of methanol oxidation whether the C O bond breaks or the O H bond breaks. Although it is not clear about the specific mechanism, the reactants are determinate. The first step is methanol reacts with adsorbed oxygen to generate adsorbed methoxy group and hydroxyl. The adsorbed oxygen and hydroxyl will continue to take part in the subsequent reactions, so the more quantity of them is beneficial to performance. In another aspect, the H2 O belongs to products of several reactions [53,54]. Every steps of the reaction belongs to reversible process, therefore, too large content of O␥ (37.20%, adsorbed water) over C5 catalyst was harmful to reaction process. The higher content of O (37.51%) and less O␥ (23.53%) over C3 catalyst was more profitable for the catalytic performance. On the active components surface, the strong interactions between the adsorbed CO with the active components make it difficult to desorption and further oxidation of CO [55]. Bock [56] and Long [57] believed that the Pt-COads were removed by reacting with Ru-OHads to produce CO2 on Pt-Ru catalysts. The larger proportion of high-valence TiOx and Ti4+ over C3 catalyst are prone to “attack” water to produce −OH and then form the Pt-OHads or PdOHads to react with COads . Finally, the Pd-Pt-TiOx structures might be conducive to the transmission of intermediate or the electron, which further accelerated the speed of reaction. All of these factors facilitated the superior performance of C3 catalyst in methanol ´ et al. [15] found that T50 and T90 of total oxidation. M. Jabłonska 1.5%Pd/HY catalyst were 90 ◦ C and 120 ◦ C for the methanol incineration in 20 cm3 min−1 of total flow rate. Merte [58] et al. proved that the 1%Pt/␥-Al2 O3 catalyst showed high performance in the oxidation of 0.15% CH3 OH at low temperature. Compared with the other researches, the C3 catalyst showed higher performance in one or more conditions among the methanol concentration (1.0 vol.% CH3 OH), GHSV (35,000 h−1 ), T50 (35 ◦ C) or T90 (58 ◦ C). The C3 catalyst possessed superior comprehensive performance among the similar catalysts types.
4. Conclusions In summary, we demonstrated that the physicochemical properties and catalytic activities of catalysts in methanol total oxidation were influenced by the content of Pd. The results revealed that the contents of Pd and Pt over C3 catalyst (0.8% Pd-Pt/CAT) were appropriate which resulted in higher performance in methanol total oxidation and CO2 selectivity. Results of characterizations
present that the active components disperse well in low content and the aggregated phenomena appear when the Pd content reach 1.0%. The higher contents of adsorbed oxygen over C3 catalyst are more conducive to rate determining step and subsequent reactions. The larger proportion of high-valence TiOx and Ti4+ are prone to “attack” water to produce −OH and then form the Pt-OHads or PdOHads to react with COads . Furthermore, the decrease of grain size or the reduction of crystallinity indicates that the surface of catalyst is more amorphous and will provide more vacancy for active sites. This finding also provides a new constituent and highly reactive catalyst for the removal of methanol in VOCs, especially at low temperature. Acknowledgements This work was financially supported by the State Key Program of National Natural Science of China (No.21336006) and the Shanxi Province Scientific and Technological Project (No.20140313002-2). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.molcata.2016.11. 041. References [1] W.B. Li, J.X. Wang, H. Gong, Catalytic combustion of VOCs on non-noble metal catalysts, Catal. Today 148 (2009) 81–87. [2] Q. Niu, B. Li, X.L. Xu, X.J. Wang, Q. Yang, Y.Y. Jiang, Y.W. Chen, S.M. Zhu, S.B. Shen, Activity and sulfur resistance of CuO/SnO2 /PdO catalysts supported on ␥-Al2 O3 for the catalytic combustion of benzene, RSC Adv. 4 (2014) 51280–51285. [3] C. H. Guo, Y. Chen, J. Xiao, Z. Wang, D. Fan, Y. Sun Li, Influence of preparation method on the surface and catalytic properties of sulfated vanadia–titania catalysts for partial oxidation of methanol, Fuel Process. Technol. 106 (2013) 77–83. [4] M. Haruta, A. Ueda, S. Tsubota, R.M. Torres Sanchez, Low-temperature catalytic combustion of methanol and its decomposed derivatives over supported gold catalysts, Catal. Today 29 (1996) 443–447. [5] A. Mirzaei, S.G. Leonardi, G. Neri, Detection of hazardous volatile organic compounds (VOCs) by metal oxide nanostructures-based gas sensors: a review, Ceram. Int. 42 (2016) 15119–15141. [6] J.M. Tatibougt, Methanol oxidation as a catalytic surface probe, Appl. Catal. A: Gen. 148 (1997) 213–252. [7] S. Zhao, J. Bennici, Surface and catalytic properties of V2 O5 –TiO2 /SO4 2− catalysts for the oxidation of methanol prepared by various methods, J. Mol. Catal. A: Chem. 309 (2009) 28–34. [8] Y. Zhao, Z. Qin, G. Wang, M. Dong, L. Huang, Z. Wu, W. Fan, J. Wang, Catalytic performance of V2 O5 /ZrO2 –Al2 O3 for methanol oxidation, Fuel 104 (2013) 22–27. [9] L. Delannoy, K. Fajerwerg, P. Lakshmanan, C. Potvin, C. Méthivier, C. Louis, Supported gold catalysts for the decomposition of VOC: Total oxidation of propene in low concentration as model reaction, Appl. Catal. B: Environ. 94 (2010) 117–124. [10] S.C. Kim, W.G. Shim, Properties and performance of Pd based catalysts for catalytic oxidation of volatile organic compounds, Appl. Catal. B: Environ. 92 (2009) 429–436. [11] M. Ousmane, L.F. Liotta, G. Pantaleo, A.M. Venezia, G. Di Carlo, M. Aouine, L. Retailleau, A. Giroir-Fendler, Supported Au catalysts for propene total oxidation: study of support morphology and gold particle size effects, Catal. Today 176 (2011) 7–13. [12] K. Okumura, T. Kobayashi, H. Tanaka, M. Niwa, Toluene combustion over palladium supported on various metal oxide supports, Appl. Catal. B: Environ. 44 (2003) 325–331. [13] S.S.T. Bastos, J.J.M. Órfão, M.M.A. Freitas, M.F.R. Pereira, J.L. Figueiredo, Manganese oxide catalysts synthesized by exotemplating for the total oxidation of ethanol, Appl. Catal. B: Environ. 93 (2009) 30–37. [14] J. Peng, S. Wang, Performance and characterization of supported metal catalysts for complete oxidation of formaldehyde at low temperatures, Appl. Catal. B: Environ. 73 (2007) 282–291. ´ ˛ K. Tarach, V. Girman, L. Chmielarz, K. [15] M. Jabłonska, A. Król, E. Kukulska-Zajac, Góra-Marek, Y. Zeolites, modified with palladium as effective catalysts for low-temperature methanol incineration, Appl. Catal. B: Environ. 166-167 (2015) 353–365. [16] E.M. Cordi, J.L. Falconer, Oxidation of volatile organic compounds on Al2 O3 Pd/Al2 O3 , and PdO/Al2 O3 catalysts, J. Catal. 162 (1996) 104–117.
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