Next Article in Journal
Solid-Liquid Phase C-Alkylation of Active Methylene Containing Compounds under Microwave Conditions
Previous Article in Journal
Cyclopropanation of 5-(1-Bromo-2-phenyl-vinyl)-3-methyl-4-nitro-isoxazoles under Phase Transfer Catalysis (PTC) Conditions
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Catalysts
Volume 5
Issue 2
10.3390/catal5020606
catalysts-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Pt/MOx/SiO2, Pt/MOx/TiO2, and Pt/MOx/Al2O3 Catalysts for CO Oxidation

by
Hongmei Qin
,
Xiaoshuang Qian
,
Tao Meng
,
Yi Lin
and
Zhen Ma
*
Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP3), Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, China
*
Author to whom correspondence should be addressed.
Catalysts 2015, 5(2), 606-633; https://doi.org/10.3390/catal5020606
Submission received: 5 February 2015 / Revised: 3 April 2015 / Accepted: 13 April 2015 / Published: 20 April 2015
Browse Figures
Versions Notes

Abstract

:
Conventional supported Pt catalysts have often been prepared by loading Pt onto commercial supports, such as SiO2, TiO2, Al2O3, and carbon. These catalysts usually have simple metal-support (i.e., Pt-SiO2) interfaces. To tune the catalytic performance of supported Pt catalysts, it is desirable to modify the metal-support interfaces by incorporating an oxide additive into the catalyst formula. Here we prepared three series of metal oxide-modified Pt catalysts (i.e., Pt/MOx/SiO2, Pt/MOx/TiO2, and Pt/MOx/Al2O3, where M = Al, Fe, Co, Cu, Zn, Ba, La) for CO oxidation. Among them, Pt/CoOx/SiO2, Pt/CoOx/TiO2, and Pt/CoOx/Al2O3 showed the highest catalytic activities. Relevant samples were characterized by N2 adsorption-desorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), H2 temperature-programmed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS), CO temperature-programmed desorption (CO-TPD), O2 temperature-programmed desorption (O2-TPD), and CO2 temperature-programmed desorption (CO2-TPD).

1. Introduction

Supported metal catalysts have been widely used in a number of reactions for the synthesis of chemicals, the ablation of environmental pollutants, and the processing of fossil, hydrogen, and biomass fuels [1]. These catalysts are usually prepared by impregnating solid supports with soluble and decomposable metal salts followed by calcination (thermal decomposition) and appropriate reduction. Alternatively, metal colloids (nanoparticles) can be deposited onto supports. The commonly used supports include SiO2, TiO2, Al2O3, Fe2O3, CeO2, and carbon. These supported metal catalysts usually have simple metal-support (i.e., Pt-SiO2) interfaces.
A way to tune the local structure and functionality of supported metal catalysts is to put an additive such as a small amount of metal oxide, either before or after loading metals onto supports. In many circumstances, the properties (e.g., thermal stability, redox property, acid-base property) of supported metal catalysts can be changed accordingly [2,3,4].
One case in point is the development of metal oxide-promoted Au catalysts. It is often recognized that Au nanoparticles supported on TiO2 support can sinter under elevated temperatures, thus leading to decreased catalytic activity. To circumvent this problem, Yan et al. modified TiO2 support with some Al2O3 additives, and then loaded Au onto the modified support (Al2O3/TiO2) via deposition-precipitation [5]. The resulting Au/Al2O3/TiO2 showed better thermal stability than the conventional Au/TiO2, and therefore retained significant catalytic activity after being treated at 500 °C. Ma extended this modification to the development of a series of Au/MxOy/TiO2 (M = Ca, Fe, Ni, Cu, Zn, Ga, Y, Zr, Mo, W, Bi, La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Yb) catalysts, and found that some additives can stabilize Au nanoparticles and lead to better catalytic activity in CO oxidation [6] and the water-gas shift reaction [7]. This modification strategy is general, as demonstrated in numerous studies on the development of modified Au catalysts [8,9,10,11,12].
Supported Pt catalysts are useful in various reactions related to chemical synthesis, environmental catalysis, and fuel conversions. The previous research has focused on the influence of preparation methods and details on the performance of Pt catalysts in a wide range of reactions. There are numerous papers on the application of modified Pt catalysts in many reactions [13,14,15,16,17,18,19]. For instance, Nieuwenhuys and co-workers developed a number of promoted Pt catalysts for automotive pollution control [20,21,22]. Kondarides and co-workers developed Pt/MOx/Al2O3 and Pt/MOx/TiO2 (M = Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, La, Ce, Nd, Sm, Eu, Gd, Ho, Er, Tm) catalysts, and found that most of these composite catalysts are more active than the corresponding Pt/Al2O3 and Pt/TiO2 in the water-gas shift reaction [23]. However, there are few comprehensive reports on the development of several series of modified Pt catalysts for CO oxidation.
In this work, we modified commercial SiO2, TiO2, and Al2O3 supports with metal oxide (MOx, M = Al, Fe, Co, Cu, Zn, Ba, La) additives, and then loaded Pt onto the modified supports by conventional impregnation. These catalysts were screened using catalytic CO oxidation as a probe reaction. Among them, Pt/CoOx/SiO2, Pt/CoOx/TiO2, and Pt/CoOx/Al2O3 catalysts showed the highest activities and were subjected to detailed characterization.

2. Results and Discussion

2.1. Effect of Pretreatment in 4% H2/He

All the Pt catalysts reported here were prepared using H2PtCl6 as the precursor via impregnation. H2PtCl6 is a relatively cheap Pt precursor, and has been commonly used in the preparation of supported Pt catalysts. Impregnation is a convenient way to prepare supported metal catalysts in the laboratories and in industry. In the literature, as-prepared Pt catalysts (prepared by calcining H2PtCl6/supports) were sometimes pretreated in H2 at elevated temperatures to reduce cationic Pt and remove residual Cl [24,25,26].
Results from our initial experiments involving Pt/SiO2, Pt/TiO2, and Pt/Al2O3 indeed show that it is necessary to pretreat the catalysts in H2 prior to catalytic testing. As shown in Figure S1 in the Supporting Information, Pt/SiO2, Pt/TiO2, and Pt/Al2O3 catalysts pretreated in 4% H2 at 300 °C are active in CO oxidation, achieving 50% CO conversion at 176, 142, and 117 °C, respectively. On the other hand, Pt/SiO2, Pt/TiO2, and Pt/Al2O3 (prepared by calcining H2PtCl6/SiO2, H2PtCl6/TiO2, and H2PtCl6/Al2O3 at 500 °C) not pretreated in 4% H2 are much less active. Therefore, in the flowing experiments, all the catalysts were pretreated in 4% H2 prior to reaction testing.

2.2. Pt/MOx/SiO2 Catalysts

Figure 1 shows the CO conversions on Pt/MOx/SiO2 catalysts as a function of reaction temperature. Pt/SiO2 starts to show CO conversion above 75 °C, and achieves 50% and 100% CO conversion at 141 and 150 °C, respectively. The catalytic activities of various catalysts, in terms of their T50 (temperature required for 50% conversion) values, follow the sequence of Pt/CoOx/SiO2 (T50 = 32 °C) > Pt/BaO/SiO2 (T50 = 121 °C) > Pt/La2O3/SiO2 (T50 = 136 °C) > Pt/SiO2 (T50 = 141 °C) > Pt/CuOx/SiO2 (T50 = 143 °C) > Pt/Al2O3/SiO2 (T50 = 151 °C) ~ Pt/ZnO/SiO2 (T50 = 151 °C) > Pt/FeOx/SiO2 (T50 = 170 °C). The trend is also clearly seen from the positions of the conversion curves in Figure 1. In particular, Pt/CoOx/SiO2 is the most active and can achieve complete CO conversion at 55 °C.
Figure 2 shows the XRD patterns of Pt/MOx/SiO2 catalysts collected after reaction testing. These catalysts are referred to as spent Pt/MOx/SiO2 catalysts. We picked out spent catalysts for XRD characterization because they are closer to the working catalysts after being exposed to the reaction ambient. Pure SiO2 (reference sample) shows a broad peak centered at 22°, indicating its amorphous nature. For Pt/SiO2, five additional peaks appear at 2θ = 39.7°, 46.2°, 67.4°, 81.2°, and 85.7°, assigned to metallic Pt. The Pt peaks are sharp, indicating the presence of big Pt particles. As shown in Figure 2, the addition of FeOx or ZnO does not decrease the intensities of Pt peaks obviously, whereas the addition of Al2O3, CoOx, CuOx, BaO, or La2O3 attenuates the Pt peaks significantly, indicating the stabilization of Pt nanoparticles on the latter MOx/SiO2 supports.
Catalysts 05 00606 g001 1024
Figure 1. CO conversions on Pt/MOx/SiO2 catalysts as a function of reaction temperature.
Figure 1. CO conversions on Pt/MOx/SiO2 catalysts as a function of reaction temperature.
Catalysts 05 00606 g001
Catalysts 05 00606 g002 1024
Figure 2. XRD patterns of Pt/MOx/SiO2 catalysts collected after reaction testing. The XRD pattern of SiO2 support is shown for comparison. (*: Pt).
Figure 2. XRD patterns of Pt/MOx/SiO2 catalysts collected after reaction testing. The XRD pattern of SiO2 support is shown for comparison. (*: Pt).
Catalysts 05 00606 g002
From Figure 2, some peaks other than the characteristic peaks of Pt and SiO2 can be seen. Therefore, these data were analyzed more carefully. As shown in Figure S2, the Pt/FeOx/SiO2 collected after 4% H2 pretreatment and reaction testing has three sets of peaks corresponding to Pt, Fe2O3, and SiO2. The spent Pt/CoOx/SiO2 contains Pt, CoO, Co3O4, and SiO2. The spent Pt/CuOx/SiO2 contains Cu3Pt, Cu2O, Cu, and SiO2. The spent Pt/ZnO/SiO2 contains Pt, ZnO, and SiO2.
The MOx content of MOx/SiO2 supports prepared by incipient wetness impregnation was intended to be fixed at 0.05 g MOx per g SiO2, i.e., the content of MOx in the catalyst was intended to be 4.76 wt.%. The Pt content of the catalysts prepared by incipient wetness impregnation was intended to be 2 wt.%. However, some loss of Pt was found to occur during the calcination of the H2PtCl6/MOx/SiO2 precursors in porcelain crucibles. The inner walls of porcelain crucibles become shinny in some cases, indicating the coating of the inner walls by metallic Pt. In addition, after drying the slurry containing H2PtCl6 and MOx/SiO2 in a beaker, some dried H2PtCl6 may adhere to the bottom or wall of the beaker. A similar phenomenon can happen for the preparation of MOx/SiO2 supports.
We then analyzed the Pt and M contents of Pt/MOx/SiO2 catalysts by ICP. Here the contents of M rather than MOx were presented due to the oxidation states of some MOx are unknown at this stage. As shown in Table 1, the M contents of Pt/MOx/SiO2 catalysts are in the range of 1.14–4.4 wt.%, and the Pt contents are in the range of 0.31–1.78 wt.%. The atomic Pt:M ratios are in the range of 1:3.3 and 1:24. From Figure 1 and Table 1, it is clear that the addition of CoOx can dramatically increase the catalytic activity in CO oxidation. We will present more characterization data in later sections.
Table
Table 1. ICP results, T50, and T100 for Pt/MOx/SiO2 catalysts.
Table 1. ICP results, T50, and T100 for Pt/MOx/SiO2 catalysts.
SamplePt (wt.%)M (wt.%)Atomic Pt:MT50 (°C)T100 (°C)
Pt/SiO20.90-1:0141150
Pt/Al2O3/SiO21.782.341:9.5151155
Pt/FeOx/SiO20.951.141:4.2170175
Pt/CoOx/SiO20.481.801:12.43255
Pt/CuOx/SiO20.312.421:24143150
Pt/ZnO/SiO20.962.651:8.2151160
Pt/BaO/SiO20.464.401:13.6121125
Pt/La2O3/SiO21.002.341:3.3136140

2.3. Pt/MOx/TiO2 Catalysts

Figure 3 shows the CO conversions on Pt/MOx/TiO2 catalysts as a function of reaction temperature. Pt/TiO2 begins to show some activity above 120 °C, and achieves 50% and 100% CO conversions at 176 and 180 °C, respectively. Apparently, Pt/TiO2 is less active than Pt/SiO2 reported above. From Figure 3, it is also clear that all the MOx additives promote the catalytic activity, and the most effective promoter is still CoOx. Pt/CoOx/TiO2 achieves 50% and 100% CO conversion at 91 and 115 °C, respectively.
Figure 4 shows the XRD patterns of spent Pt/MOx/TiO2 catalysts. Comparing the XRD patterns of Pt/TiO2 and TiO2, it is clear that Pt/TiO2 shows additional peaks at 2θ = 39.7°, 46.2°, 67.4°, 81.2°, and 85.7°, corresponding to metallic Pt. In particular, the peak at 2θ = 39.7° is the sharpest. These metallic Pt peaks are not as obvious as those shown in Figure 2, because the SiO2 support in Figure 2 is amorphous and its XRD peaks have low intensities, whereas the TiO2 support in Figure 4 is crystalline (mainly anatase) and its XRD peaks have high intensities. As seen from Figure 4, the addition of FeOx does not attenuate the metallic Pt peaks, and the addition of CoOx attenuates the metallic Pt peaks slightly, whereas the addition of other oxide additives lead to obviously attenuated Pt peaks, indicating the stabilization of Pt nanoparticles. No additional peaks can be observed.
Catalysts 05 00606 g003 1024
Figure 3. CO conversions on Pt/MOx/TiO2 catalysts as a function of reaction temperature.
Figure 3. CO conversions on Pt/MOx/TiO2 catalysts as a function of reaction temperature.
Catalysts 05 00606 g003
Catalysts 05 00606 g004 1024
Figure 4. XRD patterns of Pt/MOx/TiO2 catalysts collected after reaction testing. The XRD pattern of TiO2 support is shown for comparison. (*: Pt).
Figure 4. XRD patterns of Pt/MOx/TiO2 catalysts collected after reaction testing. The XRD pattern of TiO2 support is shown for comparison. (*: Pt).
Catalysts 05 00606 g004
The Pt and M contents of Pt/MOx/TiO2 catalysts are shown in Table 2. The M contents of Pt/MOx/TiO2 catalysts are in the range of 2.4–3.66 wt.%, and the Pt contents are in the range of 0.9–1.74 wt.%. From Figure 3 and Table 2, it is clear that the addition of CoOx can dramatically increase the catalytic activity in CO oxidation.
Table
Table 2. ICP results, T50, and T100 for Pt/MOx/TiO2 catalysts.
Table 2. ICP results, T50, and T100 for Pt/MOx/TiO2 catalysts.
SamplePt (wt.%)M (wt.%)Atomic Pt:MT50 (°C)T100 (°C)
Pt/TiO21.20-1:0176180
Pt/Al2O3/TiO21.502.401:11.6135140
Pt/FeOx/TiO21.742.901:5.8128135
Pt/CoOx/TiO20.902.791:10.291115
Pt/CuOx/TiO21.643.661:6.8140150
Pt/ZnO/TiO21.683.261:5.8167175
Pt/BaO/TiO21.673.541:3.0126130
Pt/La2O3/TiO21.203.021:3.5133150

2.4. Pt/MOx/Al2O3 Catalysts

Figure 5 shows the CO conversions on Pt/MOx/Al2O3 catalysts as a function of reaction temperature. Pt/Al2O3 starts to show some activity above 75 °C, and achieves 50% and 100% CO conversions at 121 and 125 °C, respectively. Apparently, Pt/Al2O3 is more active than Pt/SiO2 and Pt/TiO2. For the Pt/MOx/Al2O3 catalysts, the activities follow the sequence Pt/CoOx/Al2O3 > Pt/BaO/Al2O3 > Pt/Al2O3 > Pt/CuOx/Al2O3 ~ Pt/FeOx/Al2O3 > Pt/ZnO/Al2O3 > Pt/La2O3/Al2O3. In particular, Pt/CoOx/Al2O3 shows complete CO conversion at room temperature (RT).
Catalysts 05 00606 g005 1024
Figure 5. CO conversions on Pt/MOx/Al2O3 catalysts as a function of reaction temperature.
Figure 5. CO conversions on Pt/MOx/Al2O3 catalysts as a function of reaction temperature.
Catalysts 05 00606 g005
Figure 6 shows the XRD patterns of Pt/MOx/Al2O3. The Al2O3 used in this study is crystalline (α-Al2O3). Pt/Al2O3 shows characteristic α-Al2O3 and metallic Pt peaks. The addition of Fe2O3 does not lead to attenuated metallic Pt peaks, whereas the addition of other metal oxide additives can attenuate the metallic Pt peaks, indicating the stabilization of Pt nanoparticles.
The Pt and M contents of Pt/MOx/Al2O3 catalysts are shown in Table 3. The M contents of these catalysts are in the range of 0.08–4.00 wt.%, and the Pt contents are in the range of 0.33–0.92 wt.%, indicating that the Pt contents are relatively low on the MOx/Al2O3 supports.
Catalysts 05 00606 g006 1024
Figure 6. XRD patterns of Pt/MOx/Al2O3 catalysts collected after reaction testing. The XRD pattern of α-Al2O3 support is shown for comparison. (*: Pt).
Figure 6. XRD patterns of Pt/MOx/Al2O3 catalysts collected after reaction testing. The XRD pattern of α-Al2O3 support is shown for comparison. (*: Pt).
Catalysts 05 00606 g006
Table
Table 3. ICP results, T50, and T100 for Pt/MOx/Al2O3 catalysts.
Table 3. ICP results, T50, and T100 for Pt/MOx/Al2O3 catalysts.
SamplePt (wt.%)M (wt.%)Atomic Pt:MT50 (°C)T100 (°C)
Pt/Al2O30.75-1:0121125
Pt/FeOx/Al2O30.690.081:0.4143155
Pt/CoOx/Al2O30.330.391:3.9<RT<RT
Pt/CuOx/Al2O30.383.731:30.1142160
Pt/ZnO/Al2O30.922.791:9.0156160
Pt/BaO/Al2O30.414.001:13.97985
Pt/La2O3/Al2O30.802.421:4.2159165

2.5. Control Experiments

It is clear from Figure 1, Figure 3, and Figure 5 that the addition of certain MOx additives can significantly enhance the activity in CO oxidation. To see whether the activity is due to MOx/SiO2, MOx/TiO2, and MOx/SiO2 alone (without Pt), these modified supports were tested in CO oxidation. As shown in Figure S3, among MOx/SiO2 supports, only CoOx/SiO2 shows some activity above 140 °C. Among MOx/TiO2 supports, only CoOx/TiO2 and CuOx/TiO2 show some activity above 130 °C. Among MOx/Al2O3 supports, only CoOx/Al2O3 and CuOx/TiO2 show some activity above 75 and 100 °C, respectively. These data underscore the importance of integrating three components (Pt, MOx, and oxide support) in improving the catalytic activity.
Figure 7 compares the catalytic activities of Pt/CoOx/SiO2, Pt/CoOx/TiO2, Pt/CoOx/Al2O3 and their corresponding supports. Although CoOx/SiO2, CoOx/TiO2, and CoOx/Al2O3 show significant activities, the high activities of Pt/CoOx/SiO2, Pt/CoOx/TiO2, and Pt/CoOx/Al2O3 are not contributed from CoOx/SiO2, CoOx/TiO2, and CoOx/Al2O3 alone.
Catalysts 05 00606 g007 1024
Figure 7. CO conversions on Pt/CoOx/SiO2, Pt/MOx/TiO2 and Pt/CoOx/Al2O3 catalysts as a function of reaction temperature.
Figure 7. CO conversions on Pt/CoOx/SiO2, Pt/MOx/TiO2 and Pt/CoOx/Al2O3 catalysts as a function of reaction temperature.
Catalysts 05 00606 g007
To put the work in perspective, an additional sample, Pt/CoOx was synthesized by decomposing Co(NO3)2·6H2O at 500 °C followed by loading Pt via impregnation. The CoOx prepared by thermal decomposition of Co(NO3)2·6H2O has a BET surface area of 4.5 m2/g, and the Pt content of Pt/CoOx is determined by ICP as 0.30 wt.%. The catalyst was tested after being pretreated in 4% H2 at 300 °C. As seen in Figure S4, Pt/Co3O4 starts to show CO conversion above 25 °C, and reaches 50% and 100% conversions at 126 and 135 °C, respectively.

2.6. More Characterization of CoOx-Promoted Pt Catalysts

2.6.1. TEM Data

Figure 8A,B shows the TEM images of Pt/SiO2 collected after reaction testing. The majority of the amorphous SiO2 spheres are micrometer-sized. There are a portion of small-sized SiO2 spheres. This feature is consistent with the low surface area of the SiO2 support (0.4 m2/g). In addition, some wind-like amorphous SiO2 substances attach to SiO2 spheres. Pt/SiO2 contains a wild range of Pt particles sizes, normally on the order of 5–20 nm. The average Pt particle size is 13.0 nm.
Figure 8C,D shows the TEM images of spent Pt/CoOx/SiO2. The BET surface area of CoOx/SiO2 support is 1.4 m2/g. It seems that Pt nanoparticles tend to attach to the wind-like amorphous features rather than SiO2 spheres. In addition, Pt nanoparticles on CoOx/SiO2 support are quite small and the mean particle size is 2.4 nm. The size distributions of Pt particles are shown in Figure 8E,F.
Catalysts 05 00606 g008 1024
Figure 8. TEM images of the spent Pt/SiO2 (A,B) and Pt/CoOx/SiO2 (C,D) catalysts. Size distributions of Pt particles of spent Pt/SiO2 (E) and Pt/CoOx/SiO2 (F).
Figure 8. TEM images of the spent Pt/SiO2 (A,B) and Pt/CoOx/SiO2 (C,D) catalysts. Size distributions of Pt particles of spent Pt/SiO2 (E) and Pt/CoOx/SiO2 (F).
Catalysts 05 00606 g008
Figure 9A,B shows the TEM images of spent Pt/TiO2. The TiO2 support is crystalline (mainly anatase) and has a particle-like morphology. The BET surface area of TiO2 support is determined by N2 adsorption-desorption as 9.5 m2/g. Pt nanoparticles on TiO2 are usually large. The addition of CoOx into the catalyst formula leads to additional amorphous phase on the catalyst. The BET surface area of CoOx/TiO2 support is 8.4 m2/g. Some of the Pt nanoparticles are adjacent to the amorphous substances (Figure 9C,D). The size distributions of Pt particles are shown in Figure 9E,F.
Catalysts 05 00606 g009 1024
Figure 9. TEM images of the spent Pt/TiO2 (A,B) and Pt/CoOx/TiO2 (C,D) catalysts. Size distributions of Pt particles of spent Pt/TiO2 (E) and Pt/CoOx/TiO2 (F).
Figure 9. TEM images of the spent Pt/TiO2 (A,B) and Pt/CoOx/TiO2 (C,D) catalysts. Size distributions of Pt particles of spent Pt/TiO2 (E) and Pt/CoOx/TiO2 (F).
Catalysts 05 00606 g009
Figure 10 shows the TEM images of spent Pt/Al2O3 and Pt/CoOx/Al2O3. Here the Al2O3 refers to α-Al2O3 support with a BET surface area of 5.7 m2/g, and the BET surface area of CoOx/Al2O3 is 5.4 m2/g. As shown in Figure 10, Pt/Al2O3 and Pt/CoOx/Al2O3 both exhibit very small Pt nanoparticles with sizes of ~2 nm. There are occasionally some bigger Pt particles (Figure S5), which explains the appearance of small XRD peaks corresponding to metallic Pt (Figure 6). Normally CoOx coatings or amorphous substances are not clear to see. The size distributions of Pt particles are shown in Figure 10E,F.
Catalysts 05 00606 g010 1024
Figure 10. TEM images of the spent Pt/Al2O3 (A,B) and Pt/CoOx/Al2O3 (C,D) catalysts. Size distributions of Pt particles of spent Pt/Al2O3 (E) and Pt/CoOx/Al2O3 (F).
Figure 10. TEM images of the spent Pt/Al2O3 (A,B) and Pt/CoOx/Al2O3 (C,D) catalysts. Size distributions of Pt particles of spent Pt/Al2O3 (E) and Pt/CoOx/Al2O3 (F).
Catalysts 05 00606 g010

2.6.2. H2-TPR

Figure 11 shows the TPR profiles of the Pt/CoOx/SiO2, Pt/CoOx/TiO2, and Pt/CoOx/Al2O3 catalysts and some reference samples. There is no obvious H2 consumption peak on Pt/SiO2, Pt/TiO2, and Pt/Al2O3, probably due to the low Pt loading on these catalysts. The reduction peaks on CoOx/SiO2, CoOx/TiO2, and CoOx/Al2O3 can be attributed to the reduction of CoOx to metallic Co [27]. For Pt/CoOx/SiO2, the first peak at 110 °C can be attributed to the reduction of Pt species [28] and the second peak is due to the reduction of CoOx to metallic Co. Two peaks appear for Pt/CoOx/TiO2; the first small peak is contributed to the reduction of Pt species and Co3+ to Co2+, and the second one is caused by the reduction of Co2+ [29,30,31,32]. Pt/CoOx/Al2O3 has much closer peaks at the lowest temperature around 150 °C. By comparing the peak positions, it is clear that the presence of Pt can facilitate the reduction of CoOx at lower temperatures.
Catalysts 05 00606 g011 1024
Figure 11. H2-TPR profiles of SiO2, TiO2, and Al2O3-based samples.
Figure 11. H2-TPR profiles of SiO2, TiO2, and Al2O3-based samples.
Catalysts 05 00606 g011

2.6.3. XPS Characterization of Pt Species

The oxidation states of Pt species of the calcined, reduced, and spent Pt/CoOx/SiO2, Pt/CoOx/TiO2, and Pt/CoOx/Al2O3 were studied by XPS. Here, the calcined sample refers to the sample calcined in air at 500 °C, the reduced sample refers to the sample obtained by reducing the calcined sample in 4% H2 at 300 °C, and the spent sample refers to the catalyst collected after reaction testing.
Figure 12 shows the XPS data of Pt/CoOx/SiO2 samples. The Pt4f profiles were deconvoluted into three peaks centered at around 74.3, 72.5, and 71.2 eV, corresponding to Pt4+ [33], Pt2+ [34,35,36], and Pt0 [37], respectively. As seen from Figure 12 and Table 4, the Pt species in the calcined Pt/CoOx/SiO2 mainly exist in the form of Pt2+ (60.5%, relative proportion among Pt species) and Pt4+ (39.5%).
Catalysts 05 00606 g012 1024
Figure 12. Pt-4f XPS spectra of the calcined, reduced, and spent Pt/CoOx/SiO2.
Figure 12. Pt-4f XPS spectra of the calcined, reduced, and spent Pt/CoOx/SiO2.
Catalysts 05 00606 g012
Table
Table 4. The relative proportions (peak areas) of different Pt species as observed from the Pt-4f/Pt-4d XPS spectra of the calcined (I), H2 pretreated (II) and spent (III) Pt/CoOx/SiO2, Pt/CoOx/TiO2, Pt/CoOx/Al2O3.
Table 4. The relative proportions (peak areas) of different Pt species as observed from the Pt-4f/Pt-4d XPS spectra of the calcined (I), H2 pretreated (II) and spent (III) Pt/CoOx/SiO2, Pt/CoOx/TiO2, Pt/CoOx/Al2O3.
SampleTreatment StateRelative Proportions (%)
Pt0Pt2+Pt4+
Pt/CoOx/SiO2I-60.539.5
II35.948.116.0
III51.642.26.2
Pt/CoOx/TiO2I7.660.531.9
II46.246.17.7
III8.827.264.0
Pt/CoOx/Al2O3I-7.392.7
II16.665.517.9
III14.866.918.3
After reduction in 4% H2 at 300 °C, the Pt species mainly exist in the form of Pt0 (35.9%) and Pt2+ (48.1%), whereas the relative proportion of Pt4+ is 16.0%, indicating the transformation of significant portions of Pt2+ and Pt4+ to Pt0.
For Pt/CoOx/SiO2 collected after reaction testing, the relative proportions of Pt0, Pt2+, and Pt4+ are 51.6%, 42.2%, and 6.2%, respectively, indicating the further reduction of small portions of Pt2+ and Pt4+ during the course of CO oxidation.
Figure 13 shows the XPS data of the calcined, reduced, and spent Pt/CoOx/TiO2. For calcined Pt/CoOx/TiO2, the relative proportions of Pt0, Pt2+, and Pt4+ are 7.6%, 60.5%, and 31.9%, respectively.
After reduction in 4% H2 at 300 °C, the relative proportions change to 46.2%, 46.1%, and 7.7%, respectively, indicating the reduction of part of Pt2+ and Pt4+ to Pt0. The trend is consistent with the trend in the case of Pt/CoOx/SiO2.
Catalysts 05 00606 g013 1024
Figure 13. Pt-4f XPS spectra of the calcined, reduced, and spent Pt/CoOx/TiO2.
Figure 13. Pt-4f XPS spectra of the calcined, reduced, and spent Pt/CoOx/TiO2.
Catalysts 05 00606 g013
After reaction testing, the relative proportions of Pt0, Pt2+, and Pt4+ of Pt/CoOx/TiO2 are 8.8%, 27.2%, and 64.0%, respectively, indicating the oxidation of significant portions of Pt0 and Pt2+ during CO oxidation.
Figure 14 shows the XPS data of the calcined, reduced, and spent Pt/CoOx/Al2O3. Here, Pt4d rather than Pt4f data were analyzed, due to the strong interference caused by the Al2p peak [15,38]. The raw data were deconvoluted into three peaks assigned to Pt4+ [39,40,41], Pt2+ [42], and Pt0 [40]. As shown in Figure 14 and Table 4, the calcined Pt/CoOx/Al2O3 contains Pt2+ (7.3%) and Pt4+ (92.7%), but no Pt0. The reduced sample contains Pt0 (16.6%), Pt2+ (65.5%), and Pt4+ (17.9%), indicating the significant reduction of Pt4+ to Pt2+ and Pt0. For the spent catalyst, the relative proportions of Pt0, Pt2+, and Pt4+ become 14.8%, 66.9%, and 18.3%, respectively, indicating the partial oxidation of Pt0 during the course of CO oxidation.
Catalysts 05 00606 g014 1024
Figure 14. Pt-4d XPS spectra of the calcined, reduced, and spent Pt/CoOx/Al2O3.
Figure 14. Pt-4d XPS spectra of the calcined, reduced, and spent Pt/CoOx/Al2O3.
Catalysts 05 00606 g014

2.6.4. XPS Characterization of Co Species

The Co-2p regions of the XPS spectra were analyzed. The XPS spectra contain Co-2p3/2 and Co-2p1/2 regions with relatively low and high binding energies. The Co-2p3/2 region contains the peaks due to Co3+ (780.2 eV), Co2+ (781.8 eV), and a Co2+ satellite located at 786.3 eV [43,44]. The energy difference between the corresponding Co-2p3/2 and Co-2p1/2 peaks is approximately 15 eV [43,44,45,46,47].
As seen from Figure 15 and Table 5, the Co species in the calcined Pt/CoOx/SiO2 are Co3+ (57.4%) and Co2+ (42.6%), which means that the CoOx in this sample is in the form of Co3O4. After reduction in 4% H2 at 300 °C, the proportions of Co3+ and Co2+ are 40.5% and 59.5%, respectively, indicating the reduction of a portion of Co3+ to Co2+. For spent Pt/CoOx/SiO2, the relative proportions of Co3+ and Co2+ are 50.2% and 49.8%, indicating the oxidation of a small portion of Co2+ during the course of CO oxidation.
Figure 16 shows the XPS data of the calcined, reduced, and spent Pt/CoOx/TiO2. For the calcined Pt/CoOx/TiO2, the relative proportions of Co3+ and Co2+ are 64.5% and 35.5%. After reduction in 4% H2 at 300 °C, the relative proportions change to 18.8% and 81.2%, indicating the reduction of a significant portion of Co3+ to Co2+. After reaction testing, the relative proportions of Co3+ and Co2+ are 26.0% and 74.0%, indicating the oxidation of part of Co2+ to Co3+ during CO oxidation.
Figure 17 shows the XPS data of the calcined, reduced, and spent Pt/CoOx/Al2O3. The calcined Pt/CoOx/Al2O3 contains Co3+ (65.0%) and Co2+ (35.0%). The reduced sample contains Co3+ (26.4%) and Co2+ (73.6%), indicating the reduction of a significant portion of Co3+ to Co2+. For the spent catalyst, the relative proportions of Co3+ and Co2+ become 44.3% and 55.7%, respectively, indicating the partial oxidation of Co2+ during the course of CO oxidation. This trend is consistent with the trend seen above.
Catalysts 05 00606 g015 1024
Figure 15. Co-2p XPS spectra of the calcined, reduced, and spent Pt/CoOx/SiO2.
Figure 15. Co-2p XPS spectra of the calcined, reduced, and spent Pt/CoOx/SiO2.
Catalysts 05 00606 g015
Table
Table 5. The binding energies and relative proportions (peak areas) of different Pt species as observed from the Co-2p XPS spectra of the calcined (I), H2 pretreated (II) and spent (III) Pt/CoOx/SiO2, Pt/CoOx/TiO2, Pt/CoOx/Al2O3.
Table 5. The binding energies and relative proportions (peak areas) of different Pt species as observed from the Co-2p XPS spectra of the calcined (I), H2 pretreated (II) and spent (III) Pt/CoOx/SiO2, Pt/CoOx/TiO2, Pt/CoOx/Al2O3.
SampleTreatment StateRelative Proportions (%)
Co3+Co2+
Pt/CoOx/SiO2I57.442.6
II40.559.5
III49.850.2
Pt/CoOx/TiO2I64.535.5
II18.881.2
III26.074.0
Pt/CoOx/Al2O3I65.035.0
II26.473.6
III44.355.7

2.6.5. CO-TPD

Figure 18 shows the CO-TPD profiles of Pt/CoOx/SiO2, Pt/CoOx/TiO2, Pt/CoOx/Al2O3 and some reference samples. These samples were all pretreated in 4% H2 at 500 °C prior to CO adsorption and temperature-programmed desorption.
Catalysts 05 00606 g016 1024
Figure 16. Co-2p XPS spectra of the calcined, reduced, and spent Pt/CoOx/TiO2.
Figure 16. Co-2p XPS spectra of the calcined, reduced, and spent Pt/CoOx/TiO2.
Catalysts 05 00606 g016
Catalysts 05 00606 g017 1024
Figure 17. Co-2p XPS spectra of the calcined, reduced, and spent Pt/CoOx/Al2O3.
Figure 17. Co-2p XPS spectra of the calcined, reduced, and spent Pt/CoOx/Al2O3.
Catalysts 05 00606 g017
Catalysts 05 00606 g018 1024
Figure 18. CO-TPD profiles of SiO2, TiO2, and Al2O3-based samples.
Figure 18. CO-TPD profiles of SiO2, TiO2, and Al2O3-based samples.
Catalysts 05 00606 g018
As shown in Figure 18, a common feature of the data is that there is virtually no desorption of CO from CoOx/SiO2, CoOx/TiO2, and CoOx/Al2O3 supports, indicating that the adsorption of CO on these supports at room temperature is very limited. The CO-TPD profiles of Pt/CoOx/SiO2, Pt/CoOx/TiO2, and Pt/CoOx/Al2O3 are similar to those of Pt/SiO2, Pt/TiO2, and Pt/Al2O3, respectively, but quite different from those of CoOx/SiO2, CoOx/TiO2, and CoOx/Al2O3, indicating that CO adsorbs mainly on Pt.

2.6.6. O2-TPD

Figure 19 shows the O2-TPD profiles of Pt/CoOx/SiO2, Pt/CoOx/TiO2, Pt/CoOx/Al2O3 and some reference samples. There is no clear desorption of O2 from Pt/SiO2, Pt/TiO2, and Pt/Al2O3 catalysts, indicating that the adsorption of O2 at 50 °C on these samples is limited. The desorption of O2 from CoOx/SiO2, CoOx/TiO2, and CoOx/Al2O3 is also limited. On the other hand, there is enhanced desorption of O2 from Pt/CoOx/SiO2, Pt/CoOx/TiO2, and Pt/CoOx/Al2O3, indicating that O2 can adsorb more on these catalysts at 50 °C. In particular, the effect is the most obvious for Pt/CoOx/Al2O3, consistent with that fact that Pt/CoOx/Al2O3 shows the highest activity at room temperature.

2.6.7. CO2-TPD

It is known that enhanced basicity of support can enhance the activity of supported Pt catalysts for CO oxidation [48,49,50]. CO2 is a weak acid, and the temperature-programmed desorption of CO2 can indicate the basicity of catalysts. As shown in Figure S6, there is virtually no desorption of CO2 from Pt/SiO2, Pt/TiO2, Pt/Al2O3, and their CoOx-modified counterparts, indicating that the promotion of activity by CoOx additives is not due to the creation of more basic sites.
Catalysts 05 00606 g019 1024
Figure 19. O2-TPD profiles of SiO2, TiO2, and Al2O3-based samples.
Figure 19. O2-TPD profiles of SiO2, TiO2, and Al2O3-based samples.
Catalysts 05 00606 g019

2.6.8. Catalyst Stability

Figure S7 shows CO conversions on six Pt catalysts (pretreated in 4% H2 at 300 °C) as a function of reaction time on stream. Normally 0.25 g catalyst was loaded in a U-type tube in each test, and the reaction temperature was chosen to be no higher than T100. The only exception is that only 0.1 g Pt/CoOx/Al2O3 was used because of its super high activity at room temperature. As shown in Figure S7, Pt/SiO2 and Pt/Al2O3 are stable on stream, whereas Pt/TiO2 shows quick deactivation at the initial stage of reaction. The deactivation of Pt/CoOx/SiO2, Pt/CoOx/TiO2, and Pt/CoOx/Al2O3 is more obvious, especially during the initial stage of reaction. Nevertheless, the catalytic activities seem to be stabilized during the later stage of reaction, which means that in practical applications, 100% CO conversion can still be maintained if more catalyst is used or the reaction temperature is higher. The deactivation of catalysts here may be due to the accumulation of carbonate species or the partial oxidation of Pt0, Pt2+ and/or Co2+ during the reaction. More work is needed in the future to understand this process.

2.7. Discussion

A supported metal catalyst is usually composed of an active metal and a solid support. The active metal, dispersed on support surfaces, plays a main role in catalyzing chemical reactions, whereas the solid support can not only disperse metal nanoparticles and assure their thermal stability, but also influence the catalytic performance in different ways. A series of papers concerning modified platinum catalysts have been published [13,14,15,16,17,18,19,20,21,22,23]. The addition of new components may sometimes stabilize Pt particles and produce new interfaces beneficial to the catalytic performance.
Here we have found that the activity of supported Pt catalysts can be tuned by the presence of different MOx additives. Some MOx additives can facilitate the dispersion of Pt nanoparticles (Figure 2, Figure 4, and Figure 6), keeping the Pt particles in a small size. The MOx additives can increase the interaction between Pt and support, minimizing the sintering of Pt.
The most interesting result in this study is that among three series of catalysts (Pt/MOx/SiO2, Pt/MOx/TiO2, and Pt/MOx/Al2O3), CoOx-promoted Pt catalysts are the most active. In particular, Pt/CoOx/Al2O3 shows complete CO conversion at room temperature. These CoOx-promoted Pt catalysts are more active than the corresponding CoOx-promoted supports and Pt/CoOx prepared for reference, indicating that the “Pt-CoOx-support” three-component system is important for enhancing the catalytic activity in CO oxidation.
The addition of CoOx reduces the mean size of Pt nanoparticles from 13 nm for Pt/SiO2 to 2.4 nm for Pt/CoOx/SiO2 (Figure 8). It is recognized that small Pt particles are necessary for achieving high activity in CO oxidation, although the Pt particle size is not the only factor that determines the activity.
CoOx additives also affect the chemical features of supported Pt catalysts. For calcined catalysts, Pt2+ and Pt4+ are the main Pt species, but in the catalysts pretreated by 4% H2 at 300 °C, Pt0 and Pt2+ become the main Pt species. Prereduced samples show better catalytic activity in CO oxidation, implying that Pt0 and Pt2+ are the main working species for CO oxidation. Pt0 and Pt2+ have better CO adsorption ability than Pt4+ [51,52,53,54].
The proportion of Co3+ and Co2+ is almost 2:1 on the surface of calcined catalysts, indicating that the CoOx is Co3O4. The relative proportions of Co2+ among Co species increase to 59.5%, 81.2%, and 73.6% for prereduced Pt/CoOx/SiO2, Pt/CoOx/TiO2, and Pt/CoOx/Al2O3 catalysts. Co2+ ions can dissociate the O2 to O atom [21,55]. CoOx serves as an O-supplier for the reaction. The presence of more active oxygen species on CoOx-promoted Pt catalysts may account for the higher activities of these catalysts in CO oxidation.
From our CO-TPD data (Figure 18), it is clear that CO does not adsorb on CoOx/SiO2, CoOx/TiO2, and CoOx/Al2O3. A comparison of “Pt/support” and “Pt/CoOx/support” shows the presence of CoOx does not have an obvious effect on the adsorption of CO. CO tends to adsorb on Pt.
In this paper, the presence of CoOx on supported Pt catalysts can enhance the adsorption of O2, whereas the presence of only CoOx on SiO2, TiO2, and Al2O3 supports (without Pt) does not lead to enhanced O2 adsorption (Figure 19), indicating the interface between Pt and CoOx is important for O2 adsorption in this reaction.
In conclusion, the oxidation of CO over Pt/CoOx/support may follow this process: CO is adsorbed on Pt. O2 is adsorbed on the Pt-CoOx interface and dissociated by CoOx. COa on Pt will react with Oa on cobalt to form CO2 which is then desorbed into the gas phase. This reaction may take place at the interface between Pt and CoOx. It is also possible that Oa migrates from the CoOx to the Pt, in which case the reaction proceeds on the Pt surface.
After discussing the common promotional effect of CoOx additives in this study, we then turn to the question of why Pt/CoOx/Al2O3 is the most active. In Table 4, H2-pretreated Pt/CoOx/TiO2 has a higher relative proportion of Pt0 (46.2%) than Pt/CoOx/Al2O3 (16.6%). As for oxidation of CO, Co2+ is preferred over Co3+ for CO oxidation. The relative proportions of Co2+ of H2-pretreated Pt/CoOx/TiO2 and Pt/CoOx/Al2O3 are 74.0% and 73.6%, respectively. However, Pt/CoOx/Al2O3 is still the most active. That is because the activity of catalysts is not solely determined by the proportion of Pt0 and Co2+. As we can see, Pt/CoOx/Al2O3 has smaller Pt particle size (Figure 10) and the CoOx/Al2O3 is inherently more active than CoOx/TiO2 and CoOx/SiO2 (Figure 7), so the loading of small Pt nanoparticles onto CoOx/Al2O3 can make the final catalyst more active. In addition, more CO and O2 can adsorb on Pt/CoOx/Al2O3 (Figure 18 and Figure 19), which is beneficial for CO oxidation.
The objective of this work is to identify the MOx modifiers that can promote supported Pt catalysts. It should be mentioned that current synthesis adopts impregnation of a soluble Pt salt, which is conventionally adopted in the laboratories and in industry. As we can see from our reported results, in particular the XRD results (Figure 2, Figure 4, and Figure 6), the Pt particle sizes on different modified and unmodified supports are quite different, which makes the comparison less straightforward, i.e., there exist many complex/interrelated factors that can influence the activity of a final catalyst. Although here the sizes of Pt particles are not the same using the conventional impregnation, we believe that after the catalyst screening is done, further research can be carried out by using catalysts prepared by depositing Pt colloids with controlled sizes onto supports.

3. Experimental Section

3.1. Synthesis of Modified Supports

H2PtCl6·6H2O (Sinopharm Chemical Reagent, AR, Shanghai, China), SiO2 (Aladdin, 99.99%, Shanghai, China), TiO2 (ACROS, 98%+, Fair Lawn, NJ, USA), Al2O3 (Aladdin, 99.99%), Al(NO3)3·9H2O (Sinopharm Chemical Reagent, AR), Fe(NO3)3·9H2O (ACROS, 99%+), Co(NO3)2·6H2O (ACROS, 98%+), Cu(NO3)2·3H2O (ACROS, 99%+), Zn(NO3)2·6H2O (Sinopharm Chemical Reagent, AR), Ba(NO3)2 (Sinopharm Chemical Reagent, AR), and La(NO3)3·9H2O (Aladdin, 99.9%,) were used as received.
Metal oxide-modified SiO2, TiO2, Al2O3 supports were prepared via incipient wetness impregnation. A calculated amount of metal nitrate was dissolved in deionized water (typically 10 mL water per gram of support) in a 250 mL beaker, and a support (SiO2, TiO2, or Al2O3) was subsequently added. The intended loading was 0.05 g MOx per g support. Here MOx refers to a metal oxide additive with the highest oxidation state of M. The actual M contents were determined by ICP analysis. The slurry was stirred using a glass rod, kept static at room temperature overnight, and then dried at 80 °C for 12 h. The solid product deposited at the bottom of the beaker was collected and ground into fine powders, and further calcined in air at 500 °C for 3 h.

3.2. Synthesis of Catalysis

Pt was loaded onto commercial and modified supports by incipient wetness impregnation. In a typical synthesis, 3.92 g support was immersed into 21 mL H2PtCl6 solution (0.01 g/mL) in a 250 mL beaker. The Pt loading was intended to be 2 wt.%, and the actual value was determined by ICP analysis. The slurry was stirred using a glass rod, kept static at room temperature overnight, and dried at 80 °C for 12 h. The obtained solid was grid into fine powders, and further calcined in a muffle oven under static air at 500 °C for 3 h.

3.3. Characterization

ICP analyses of catalysts were conducted using an Optima 8000 spectrometer (PerkinElmer, Waltham, MA, USA) after dissolution of the sample in aqua and appropriate dilution.
BET surface areas were measured by N2 adsorption at 77 K using a Micrometrics ASAP 2020 M+C surface area and porosity analyzer (Micromeritics, Norcross, GA, USA). Before the measurement, a sample (0.1–0.2 g) was degassed at 200 °C for 6 h, the weight of the sample was measured again, and the sample was then subjected to N2 adsorption-desorption.
XRD experiments were conducted on a PW3040/60X’Pert PRO (PANalytical, Almelo, Netherlands) X-ray diffractometer with Cu Kα radiation. The scanning was conducted at 2θ = 20°−90° and the scanning rate was 6°/min.
TEM experiments were conducted on a JOEL JEM2100F field-emission transmission electron microscope (JOEL Ltd., Tokyo, Japan) with an acceleration voltage of 200 kV. Prior to recording TEM images, a small portion of sample was dispersed in ethanol, and a few drops were dropped onto a Cu grid-supported carbon films and dried under an infrared lamp.
XPS spectra were recorded on a PerkinElmer PHI 5000C ESCA (Perkin Elmer, Waltham, MA, USA) system operating at a voltage of 14 kV and a current of 20 mA, with MgKα radiation source. The obtained binding energies were corrected by using the binding energy of C1s as a reference, and the XPS data were processed using XPS PEAK41 software. The Co-2p spectra in Figure 15 were obtained using Kratos Axis Ultra-DLD (Kratos, Beijing, China) system.
H2-TPR experiments were performed in a FINESORB-3010 instrument (FINETEC, Hangzhou, China). A sample (0.12 g, 40–60 mesh) was loaded into a U-shaped quartz tube. A reducing gas (10% H2/Ar) was introduced at a flow rate of 30 mL/min. The temperature was hold on 30 °C for 2 h and then increased to 800 °C at a rate of 10 °C/min. The H2 consumption was detected by a TCD (thermal conductivity detector) and recorded as a function of temperature.
CO-TPD, O2-TPD, and CO2-TPD experiments were performed in a FINESORB-3010 instrument (FINETEC, Hangzhou, China). A sample (0.25 g, 40–60 mesh) was loaded into a U-shaped quartz tube, 4% H2 (balance He, total flow rate 50 mL/min) was then used to sweep the sample, and the temperature was ramped from room temperature to 300 °C (at a rate of 10 °C/min) in the presence of 4% H2, and kept at 300 °C for 2 h. Then the sample was cooled down to 30 °C (CO-TPD, O2-TPD) or 50 °C (CO2-TPD), and exposed to 2% CO/He (CO-TPD) or pure O2 (O2-TPD) or 5% CO2/He (CO2-TPD) for 1 h to allow for adsorption. The flowing gas was then switched to He and swept by the flow for 3h. The temperature was then ramped to 500 °C (CO-TPD, O2-TPD) or 600 °C (CO2-TPD) at a rate of 10 °C/min, while the desorbed gases were monitored using a TCD detector.

3.4. Catalytic Testing

Catalytic CO oxidation was conducted in a fixed bed reactor (FINESORB-3010 temperature-programmed chemsorber, FINETEC, Hangzhou, China). The experimental conditions and procedures were reported previously [50]. Briefly, A catalyst (0.25 g) loaded into the reactor was pretreated in flowing 4% H2 (balance He) at 300 °C for 2 h, cooled down to room temperature, and the gas stream was switched to 1% CO (balance air, flow rate: 50 mL/min). The catalyst was maintained at room temperature for 1 h, and then heated to 200 °C at a rate of 0.5 °C/min. The exiting stream was analyzed by GC (Agilent 7890A) every 10 min to record CO and CO2 peaks. The CO conversion was calculated as
([CO]in − [CO]out)/[CO]in × 100%

4. Conclusions

Three series of metal oxide-modified Pt catalysts (i.e., Pt/MOx/SiO2, Pt/MOx/TiO2, and Pt/MOx/Al2O3, where M = Al, Fe, Co, Cu, Zn, Ba, La) were prepared and tested for CO oxidation. Among them, Pt/CoOx/SiO2, Pt/CoOx/TiO2, and Pt/CoOx/Al2O3 exhibited the highest catalytic activities. The presence of the “Pt-CoOx-support” three-component system is important because the catalytic activity of any two-component system was found to be significantly lower. The higher activity of CoOx-promoted Pt catalysts is ascribed to the greater supply of active oxygen. Although here we chose CO oxidation as a probe reaction, we believe that the Pt/MOx/support catalysts may find applications in other reactions, considering the functionality furnished by both Pt and MOx-modified supports.

Acknowledgments

Zhen Ma acknowledges the financial support from the National Natural Science Foundation of China (Grant No. 21177028).

Author Contributions

Hongmei Qin and Xiaoshuang Qian conducted all of the synthesis and the catalytic testing. Tao Meng and Yi Lin carried out the XRD characterization. Hongmei Qin conducted the TPR/TPD experiments. Tao Meng helped to maintain the catalytic testing apparatus. Zhen Ma guided the TEM characterization. Hongmei Qin and Zhen Ma interpreted the results and wrote the manuscript. All contributed to the discussions.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Ma, Z.; Zaera, F. Heterogeneous catalysis by metals. In Encyclopedia of Inorganic Chemistry; Scott, R.A., Ed.; John Wiley & Sons: Chichester, UK, 2014. [Google Scholar] [CrossRef]
  2. Cao, A.M.; Lu, R.W.; Veser, G. Stabilizing metal nanoparticles for heterogeneous catalysis. Phys. Chem. Chem. Phys. 2010, 12, 13499–13510. [Google Scholar] [CrossRef] [PubMed]
  3. Ma, Z.; Dai, S. Development of novel supported gold catalysts: A materials perspective. Nano Res. 2011, 4, 3–32. [Google Scholar] [CrossRef]
  4. Wu, B.H.; Zheng, N.F. Surface and interface control of noble metal nanocrystals for catalytic and electrocatalytic applications. Nano Today 2013, 8, 168–197. [Google Scholar] [CrossRef]
  5. Yan, W.F.; Mahurin, S.M.; Pan, Z.W.; Overbury, S.H.; Dai, S. Ultrastable Au nanocatalyst supported on surface-modified TiO2 nanocrystals. J. Am. Chem. Soc. 2005, 127, 10480–10481. [Google Scholar] [CrossRef] [PubMed]
  6. Ma, Z.; Overbury, S.H.; Dai, S. Au/MxOy/TiO2 catalysts for CO oxidation: Promotional effect of main-group, transition, and rare-earth metal oxide additives. J. Mol. Catal. A 2007, 273, 186–197. [Google Scholar] [CrossRef]
  7. Ma, Z.; Yin, H.F.; Dai, S. Performance of Au/MxOy/TiO2 catalysts in water-gas shift reaction. Catal. Lett. 2010, 136, 83–91. [Google Scholar] [CrossRef]
  8. Grisel, R.J.H.; Weststrate, C.J.; Goossens, A.; Craje, M.W.J.; van der Kraan, A.M. Oxidation of CO over Au/MOx/Al2O3 multi-component catalysts in a hydrogen-rich environment. Catal. Today 2002, 72, 123–132. [Google Scholar] [CrossRef]
  9. Qian, K.; Huang, W.X.; Jiang, Z.Q.; Sun, H.X. Anchoring highly active gold nanoparticles on SiO2 by CoOx additive. J. Catal. 2007, 248, 137–141. [Google Scholar] [CrossRef]
  10. Yan, W.F.; Ma, Z.; Mahurin, S.M.; Jiao, J.; Hagaman, E.W.; Overbury, S.H. Novel Au/TiO2/Al2O3·xH2O catalysts for CO oxidation. Catal. Lett. 2008, 121, 209–218. [Google Scholar] [CrossRef]
  11. Guczi, L.; Beck, A.; Frey, K. Role of promoting oxide morphology dictating the activity of Au/SiO2 catalyst in CO oxidation. Gold Bull. 2009, 42, 5–12. [Google Scholar] [CrossRef]
  12. Qian, K.; Lv, S.S.; Xiao, X.Y.; Sun, H.X.; Lu, J.Q.; Luo, M.F.; Huang, W.X. Influences of CeO2 microstructures on the structure and activity of Au/CeO2/SiO2 catalysts in CO oxidation. J. Mol. Catal. A 2009, 306, 40–47. [Google Scholar] [CrossRef]
  13. Törncrona, A.; Skoglundh, M.; Thormählen, P.; Fridell, E.; Jobson, E. Low temperature catalytic activity of cobalt oxide and ceria promoted Pt and Pd: Influence of pretreatment and gas composition. Appl. Catal. B 1997, 14, 131–145. [Google Scholar] [CrossRef]
  14. Thormählen, P.; Skoglundh, M.; Fridell, E.; Andersson, B. Low-temperature CO oxidation over platinum and cobalt oxide catalysts. J. Catal. 1999, 188, 300–310. [Google Scholar] [CrossRef]
  15. Navarro, R.M.; Álvarez-Galván, M.C.; Sánchez-Sánchez, M.C.; Rosa, F.; Fierro, J.L.G. Production of hydrogen by oxidative reforming of ethanol over Pt catalysts supported on Al2O3 modified with Ce and La. Appl. Catal. B 2005, 55, 229–241. [Google Scholar] [CrossRef]
  16. Jain, S.K.; Crabb, E.M.; Smart, L.E.; Thompsett, D.; Steele, A.M. Controlled modification of Pt/Al2O3 for the preferential oxidation of CO in hydrogen: A comparative study of modifying element. Appl. Catal. B 2009, 89, 349–355. [Google Scholar] [CrossRef]
  17. Tomita, A.; Shimizu, K.; Kato, K.; Tai, Y. Pt/Fe-containing alumina catalysts prepared and treated with water under moderate conditions exhibit low-temperature CO oxidation activity. Catal. Commun. 2012, 17, 194–199. [Google Scholar] [CrossRef]
  18. Kim, M.Y.; Choi, J.S.; Toops, T.J.; Jeong, E.S.; Han, S.W.; Schwartz, V.; Chen, J.H. Coating SiO2 support with TiO2 or ZrO2 and effects on structure and CO oxidation performance of Pt catalysts. Catalysts 2013, 3, 88–103. [Google Scholar] [CrossRef]
  19. Tomita, A.; Shimizu, K.; Tai, Y. Effect of metal oxide promoters on low temperature CO oxidation over water-pretreated Pt/alumina catalysts. Catal. Lett. 2014, 144, 1689–1695. [Google Scholar] [CrossRef]
  20. Mergler, Y.J.; van Aalst, A.; van Delft, J.; Nieuwenhuys, B.E. Promoted Pt catalysts for automotive pollution control: Characterization of Pt/SiO2, Pt/CoOx/SiO2, and Pt/MnOx/SiO2 Catalysts. J. Catal. 1996, 161, 310–318. [Google Scholar] [CrossRef]
  21. Mergler, Y.J.; van Aalst, A.; van Delft, J.; Nieuwenhuys, B.E. CO oxidation over promoted Pt catalysts. Appl. Catal. B 1996, 10, 245–261. [Google Scholar] [CrossRef]
  22. Mergler, Y.J.; Nieuwenhuys, B.E. NO reduction by H2 over promoted Pt catalysts. Appl. Catal. B 1997, 12, 95–110. [Google Scholar] [CrossRef]
  23. Panagiotopoulou, P.; Kondarides, D.I. A comparative study of the water-gas shift activity of Pt catalysts supported on single (MOx) and composite (MOx/Al2O3, MOx/TiO2). Catal. Today 2007, 127, 319–329. [Google Scholar] [CrossRef]
  24. Lieske, H.; Lietz, G.; Spindler, H.; Völterb, J. Reactions of platinum in oxygen- and hydrogen-treated Pt/γ-Al2O3 catalysts: I. Temperature-programmed reduction, adsorption, and redispersion of platinum. J. Catal. 1983, 81, 8–16. [Google Scholar] [CrossRef]
  25. Gardner, S.D.; Hoflund, G.B.; Schryer, D.R.; Upchurch, B.T. Characterization study of silica-supported platinized tin oxide catalysts used for low-temperature carbon monoxide oxidation: Effect of pretreatment temperature. J. Phys. Chem. 1991, 95, 835–838. [Google Scholar] [CrossRef]
  26. Gracia, F.J.; Miller, J.T.; Kropf, A.J.; Wolf, E.E. Kinetics, FTIR, and controlled atmosphere EXAFS study of the effect of chlorine on Pt-supported catalysts during oxidation reactions. J. Catal. 2002, 209, 341–354. [Google Scholar] [CrossRef]
  27. Arnoldy, P.; Moulijn, J.A. Temperature programmed reduction of CoO/Al2O3 catalysts. J. Catal. 1985, 93, 38–54. [Google Scholar] [CrossRef]
  28. Huizinga, T.; Vangrondelle, J.; Prins, R. A temperature programmed reduction study of Pt on Al2O3 and TiO2. Appl. Catal. 1984, 2, 199–213. [Google Scholar] [CrossRef]
  29. Sun, S.; Tsubaki, K.; Fujimoto, K. The reaction performances and characterization of Fischer–Tropsch synthesis Co/SiO2 catalysts prepared from mixed cobalt salts. Appl. Catal. A 2000, 202, 121–131. [Google Scholar] [CrossRef]
  30. Tiernan, M.J.; Fesenko, E.A.; Barnes, P.A.; Parkes, G.M.B.; Ronane, M. The application of CRTA and linear heating thermoanalytical techniques to the study of supported cobalt oxide methane combustion catalysts. Thermochim. Acta 2001, 379, 163–175. [Google Scholar] [CrossRef]
  31. Xue, L.; Zhang, C.B.; He, H.; Teraoka, Y. Catalytic decomposition of N2O over CeO2 promoted Co3O4 spinel catalyst. Appl. Catal. B 2007, 75, 167–174. [Google Scholar] [CrossRef]
  32. Liu, Y.Y.; Hanaoka, T.; Miyazawa, T.; Murata, K.; Okabe, K.; Sakanishi, K. Fischer–Tropsch synthesis in slurry-phase reactors over Mn- and Zr-modified Co/SiO2 catalysts. Fuel Process. Technol. 2009, 90, 901–908. [Google Scholar] [CrossRef]
  33. Matolin, V.; Matolinova, I.; Vaclavu, M.; Khalakhan, I.; Vorokhta, M.; Fiala, R.; Pis, I.; Sofer, Z.; Poltierova-Vejpravova, J.; Mori, T.; et al. Platinum-doped CeO2 thin film catalysts prepared by magnetron sputtering. Langmuir 2010, 26, 12824–12831. [Google Scholar] [CrossRef] [PubMed]
  34. Kiss, G.; Josepovits, V.K.; Kovacs, K.; Ostrickb, B.; Fleischerb, M.; Meixnerb, H.; Réti, F. CO sensitivity of the PtO/SnO2 and PdO/SnO2 layer structures: Kelvin probe and XPS analysis. Thin Solid Films 2003, 436, 115–118. [Google Scholar] [CrossRef]
  35. Wu, G.; Chen, Y.S.; Xu, B.Q. Remarkable support effect of SWNTs in Pt catalyst for methanol electro oxidation. Electrochem. Commun. 2005, 7, 1237–1243. [Google Scholar] [CrossRef]
  36. Wang, D.L.; Lu, S.F.; Jiang, S.P. Tetrahydrofuran-functionalized multi-walled carbon nanotubes as effective support for Pt and PtSn electrocatalysts of fuel cells. Electrochim. Acta 2010, 55, 2964–2971. [Google Scholar] [CrossRef]
  37. Şen, F.; Gökağaç, G. Different sized platinum nanoparticles supported on carbon: An XPS study on these methanol oxidation catalysts. J. Phys. Chem. C 2007, 111, 5715–5720. [Google Scholar] [CrossRef]
  38. Riguetto, B.A.; Damyanova, S.; Gouliev, G.; Marques, C.M.P.; Petrov, L.; Bueno, J.M.C. Surface behavior of alumina-supported Pt catalysts modified with cerium as revealed by X-ray diffraction, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy of CO adsorption. J. Phys. Chem. B 2004, 108, 5349–5358. [Google Scholar] [CrossRef]
  39. Bouwman, R.; Biloen, P. Valence state and interaction of platinum and germanium on γ-Al2O3 investigated by X-ray photoelectron spectroscopy. J. Catal. 1977, 48, 209–216. [Google Scholar] [CrossRef]
  40. Shyu, J.Z.; Otto, K. Identification of platinum phases on γ-alumina by XPS. Appl. Surf. Sci. 1988, 32, 246–252. [Google Scholar] [CrossRef]
  41. Serrano-Ruiza, J.C.; Huberb, G.W.; Sánchez-Castillob, M.A.; Dumesicb, J.A.; Rodríguez-Reinosoa, F.; Sepúlveda-Escribanoa, A. Effect of Sn addition to Pt/CeO2-Al2O3 and Pt/Al2O3 catalysts: An XPS, 119Sn Mössbauer and microcalorimetry study. J. Catal. 2006, 241, 378–388. [Google Scholar] [CrossRef] [Green Version]
  42. Wang, X.D.; Yu, H.B.; Hua, D.Y.; Zhou, S.H. 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. [Google Scholar] [CrossRef]
  43. Epling, W.S.; Hoflund, G.B.; Weaver, J.F. Surface characterization study of Au/α-Fe2O3 and Au/Co3O4 low-temperature CO oxidation catalysts. J. Phys. Chem. 1996, 100, 9929–9934. [Google Scholar] [CrossRef]
  44. Pietrogiacomi, D.; Tuti, S.; Campa, M.C.; Indovina, V. Cobalt supported on ZrO2: Catalysts characterization and their activity for the reduction of NO with C3H6 in the presence of excess O2. Appl. Catal. B 2000, 28, 43–54. [Google Scholar] [CrossRef]
  45. Kang, M.; Song, M.W.; Lee, C.H. Catalytic carbon monoxide oxidation over CoOx/CeO2 composite catalysts. Appl. Catal. A 2003, 251, 143–156. [Google Scholar] [CrossRef]
  46. Xie, S.H.; Dai, H.X.; Deng, J.G.; Liu, Y.X.; Yang, H.G.; Jiang, Y.; Tan, W.; Ao, A.S.; Guo, G.S. Au/3DOM Co3O4: Highly active nanocatalysts for the oxidation of carbon monoxide and toluene. Nanoscale 2013, 5, 11207–11219. [Google Scholar] [CrossRef] [PubMed]
  47. Li, J.; Lu, G.Z.; Wu, G.S.; Mao, D.S.; Guo, Y.L.; Wang, Y.Q.; Guo, Y. Effect of TiO2 crystal structure on the catalytic performance of Co3O4/TiO2 catalyst for low-temperature CO oxidation. Catal. Sci. Technol. 2014, 4, 1268–1275. [Google Scholar] [CrossRef]
  48. Lee, C.H.; Chen, Y.W. Effect of basic additives on Pt/Al2O3 for CO and propylene oxidation under oxygen deficient conditions. Ind. Eng. Chem. Res. 1997, 36, 1498–1506. [Google Scholar] [CrossRef]
  49. Chen, G.X.; Zhao, Y.; Fu, G.; Duchesne, P.N.; Gu, L.; Zheng, Y.P.; Weng, X.F.; Chen, M.S.; Zhang, P.; Pao, C.W.; et al. Interfacial effects in iron-nickel hydroxide-platinum nanoparticles enhance catalytic oxidation. Science 2014, 344, 495–499. [Google Scholar] [CrossRef] [PubMed]
  50. Qian, X.S.; Qin, H.M.; Meng, T.; Lin, Y.; Ma, Z. Metal phosphate-supported Pt catalysts for CO oxidation. Materials 2014, 7, 8105–8130. [Google Scholar] [CrossRef]
  51. Arteaga, G.J.; Anderson, J.A.; Rochester, C.H. Effects of oxidation-reduction and oxychlorination-reduction cycles on CO adsorption by Pt-Sn/Al2O3 catalysts. J. Catal. 2000, 184, 268–279. [Google Scholar] [CrossRef]
  52. Chong, F.K.; Anderson, J.A.; Rochester, C.H. Effects of oxidation/reduction and oxychlorination/reduction cycles on CO adsorption by Pt-Re/Al2O3 catalysts. J. Catal. 2000, 190, 327–337. [Google Scholar] [CrossRef]
  53. Shen, S.A.; Wang, X.L.; Ding, Q.; Jin, S.Q.; Feng, Z.C.; Li, C. Effect of Pt cocatalyst in Pt/TiO2 studied by in situ FTIR of CO adsorption. Chin. J. Catal. 2014, 35, 1900–1906. [Google Scholar] [CrossRef]
  54. Vicerich, M.A.; Oportus, M.; Benitez, V.M.; Reyes, P.; Pieck, C.L. Influence of Na content on the catalytic properties of Pt-Ir/Al2O3 catalysts for selective ring opening of decalin. Appl. Catal. A 2014, 480, 42–49. [Google Scholar] [CrossRef]
  55. Meunier, G.; Garin, F.; Schmitt, J.; Maire, G.; Roche, R. Reactivity studies of automobile exhaust catalysts in presence of oxidizing or reducing conditions. Stud. Surf. Sci. Catal. 1987, 30, 243–255. [Google Scholar]

Share and Cite

MDPI and ACS Style

Qin, H.; Qian, X.; Meng, T.; Lin, Y.; Ma, Z. Pt/MOx/SiO2, Pt/MOx/TiO2, and Pt/MOx/Al2O3 Catalysts for CO Oxidation. Catalysts 2015, 5, 606-633. https://doi.org/10.3390/catal5020606

AMA Style

Qin H, Qian X, Meng T, Lin Y, Ma Z. Pt/MOx/SiO2, Pt/MOx/TiO2, and Pt/MOx/Al2O3 Catalysts for CO Oxidation. Catalysts. 2015; 5(2):606-633. https://doi.org/10.3390/catal5020606

Chicago/Turabian Style

Qin, Hongmei, Xiaoshuang Qian, Tao Meng, Yi Lin, and Zhen Ma. 2015. "Pt/MOx/SiO2, Pt/MOx/TiO2, and Pt/MOx/Al2O3 Catalysts for CO Oxidation" Catalysts 5, no. 2: 606-633. https://doi.org/10.3390/catal5020606

Article Metrics

Back to TopTop