Nanosized Au Catalysts Supported on Mg(OH)2-CeO2 for Preferential Oxidation of CO in Hydrogen Stream

Preferential oxidation of carbon monoxide in the 
presence of hydrogen (PROX) is a promising method to 
remove CO from a hydrogen-containing gas mixture. Nanosized gold catalyst 
supported on CeO2 and modified with Mg(OH)2 was used for 
preferential oxidation of carbon monoxide in hydrogen-rich stream in this 
study. Mg(OH)2 was added on CeO2 by incipient-wetness 
impregnation. Au was loaded on Mg(OH)2-CeO2 by 
deposition-precipitation method. PROX reaction was carried out in a continuous 
flow, fixed bed reactor. CO/O2 feed ratio was fixed at 1 to magnify 
the difference of various catalysts. The catalysts were characterized by N2 sorption, TEM, HR-TEM and XPS. Mg(OH)2 formed a thin layer on the 
surface of CeO2. CeO2 was in the crystalline phase and 
Mg(OH)2 was amorphous. Au particles were homogeneously dispersed on 
the support with a size of 2 - 5 nm. Using CeO2 as a support could increase the dispersion of Mg(OH)2 and thus increase the 
interaction between Au and Mg(OH)2. Adding Mg(OH)2 on Au/CeO2 could 
suppress H2 oxidation and therefore increase CO oxidation activity.


Introduction
The hydrogen containing mixture used to feed portable power units based on proton-exchange membrane fuel cells (PEMFCs) is obtained through the conversion of hydrocarbon with steam reforming. These mixtures usually contain 0.5 -2 vol. % of CO, 15 -25 vol. % of CO 2 , and 5 -10 vol. % of water vapor. Because CO is a poison to PEMFC anodes, its concentration should be reduced to a DOI: 10.4236/mrc.2019.82002 12 Modern Research in Catalysis level of less than 10 ppm. The preferential oxidation of carbon monoxide in the presence of hydrogen (PROX) is a promising method for removing CO from hydrogen-containing gas mixture [1]- [10]. It contains two reactions: 2 2 2CO O 2CO = = (1) Reaction (2) of hydrogen oxidation leads to a loss of fuel for a PEMFC and lowers the efficiency of process; it is therefore desirable to suppress this reaction. Supported gold nanoparticles are promising catalysts for the preferential oxidation of CO. The catalysts which are able to selectively oxidize CO in the presence of excess H 2 should possess high CO oxidation activity as well as inhibit oxidation of H 2 at the temperature region of the PEM fuel cells (80˚C -100˚C) [1] [2] [3] [4] [5].
The promotional effect of oxidation of CO in the existence of hydrogen has been reported by many researchers [11] [12]. CO adsorption and oxidation on Au/MgO have been studied extensively [11]- [17]. Cunningham et al. [11] suggested that the CO was directly reacted with hydroxyl radicals located at the interface of gold and Mg(OH) 2 . These active hydroxyl radicals on the Mg(OH) 2 surface strongly interacted with the gold. Tompos et al. [14] reported that Pb, Sm and V-promoted Au/MgO catalysts were very active and selective for CO oxidation in the presence of hydrogen. However, high Au metal loading (>2.8 wt%) was used in their study. In addition, commercial MgO was used as a support and it had low surface area.
Au supported on MgO (001) was studied using various DFT approaches with either cluster or periodic geometries [15]. The vacancy defects were associated with the surfaces of Au nanoparticles embedded in MgO, indicating that Au atoms intend to attach to the defect sites. Cunningham et al. [11] used Al 2 O 3 as a support for MgO and Au. However, Al 2 O 3 is not good a support for Au, since its isoelectric point is too low and it lacks oxygen vacancy.
MgO has low surface area. If one can prepare MgO thin film with many defect sites on the surface of CeO 2 , it would have high surface area of MgO and many defect sites. One can deposit Au on Mg(OH) 2 -CeO 2 which has the similar properties as Au/MgO. In a previous study, one of the authors [18] has reported that Au/MgO x -TiO 2 demonstrated high PROX performance. However, Au supported on Mg(OH) 2 -CeO 2 has never been reported in the literature.
In this study, Au catalysts supported on Mg(OH) 2 -CeO 2 with various Mg(OH) 2 contents were prepared. Various techniques were used to characterize the catalysts. The aim of this study was to investigate the effects of Mg content in Au/Mg(OH) 2 -CeO 2 catalyst on the activity and selectivity of oxygen reacting with CO in PROX reaction. co-precipitation method [1]- [10] using magnesium nitrate and cerium nitrate as the starting materials. The catalyst was dried overnight in air at 80˚C for 8 h, and then calcined at 550˚C for 4 h in air. Deposition-precipitation method (DP) was used in this study to deposit gold on the support. The detailed preparation method has been described in previous paper [18]. An aqueous solution of HAuCl 4 (2.55 × 10 -3 M) was slowly added into the solution containing support under vigorous stirring at a temperature 65˚C. NH 4 OH was used to adjust the pH value of the solution to 7. After aging for 2 h, the sample was filtered and the filtration cake was washed until no chloride ions were detected. Finally, the sample was dried overnight at 80˚C, and was calcined at 180˚C for 4 h. This temperature was high enough to reduce Au 3 = cation and low enough to prevent Au from sintering. The nominal loading of Au was 1 wt%. The sample was denoted as Au/Mg(OH) 2 -CeO 2 (x) where x is the atomic ratio of Mg/(Mg + Ce).

Catalysts Characterization
The catalysts were characterized with ICP-MS, N 2 -sorption, transmission electron micros copy (TEM), high resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), and X-ray Spectroscopy (XPS). The characterization technique has been described in the previous paper [18]. The real Au contents of the catalysts were measured by ICP-MS. The surface area of the samples was measured using a Micromeritics ASAP 2010 by Brunauer-Emmett-Teller (BET) method in the range P/P 0 = 0.05 -0.3. Prior to the experiments, the sample was treated at 120˚C for 8 h at the vacuum pressure below 1 × 10 -5 torr.
XRD analysis was performed using a Siemens D500 powder diffractometer using Cu K α1 radiation (0.15405 nm) at a voltage and current of 40 kV and 40 mA, respectively. The sample was scanned over the range of 2θ = 20˚ -70˚ at a rate of 0.05˚/min. The catalysts were characterized by TEM on a JEM-2000 EX II operated at 120 kV and HRTEM on a JEOL JEM-2010 operated at 160 kV. A small amount of sample was placed into the sample tube filled with a 95% ethanol solution. After agitating under ultrasonic environment for 3 h, one drop of the sample was dipped on a carbon-coated copper mesh (300#) (Ted Pella Inc., CA, USA), then dried in vacuum overnight. Images were recorded digitally with a Gatan slow scan camera (GIF).
XPS analysis was carried out with a Thermo VG Scientific Sigma Prob spectrometer. The XPS spectra were collected using Al K α radiation at a voltage and current of 20 kV and 30 mA, respectively. The spectrometer was operated at 23.5 eV pass energy and the binding energy was corrected by contaminant carbon (C 1s = 285.0 eV). Peak fitting was done using XPSPEAK 4.1 with Shirley background and 30:70 Lorentzian/Gaussian convolution product shapes.

PROX Reaction
The PROX reaction was carried out in a fixed-bed glass reactor. 0.1 g catalyst heated in a furnace with a heating rate of 2˚C/min. After reaching the reaction temperature for 5 min, the product was analyzed by a gas chromatograph equipped with a thermal conductivity detector using MS-5A packed column. CO conversion and selectivity of oxygen reacting with CO were calculated by the following equations: In all experiments, the carbon imbalance did not exceed ±1%.

BET Surface Area
The BET surface areas of Au/Mg(OH) 2 -CeO 2 catalysts are listed in Table 2. It shows that the addition of Mg(OH) 2 in Au/CeO 2 increased the BET surface area of the catalyst. Since CeO 2 was crystalline phase, it had low surface area. Mg(OH) 2 was in amorphous phase and had higher surface area. Adding Mg(OH) 2 in CeO 2 increased the BET surface area of the catalyst, as expected.

XRD
In the XRD pattern of catalysts with different magnesium contents ( Figures   1(b) Mg(OH) 2 can be converted to MgO as the temperature was higher than 400˚C.
Since the calcination temperature of the sample was 180˚C, Mg would be in the form of Mg(OH) 2 . There was no significant difference in particle size of cerium oxide with different Mg contents.

TEM
As shown in the TEM photos in Figure 2, the gold particles were observed as small dark spots. The gray bulk mass was ceria support, while Mg(OH) 2 was surrounded on ceria with a lighter color. It shows that Mg(OH) 2 Table 3. The average size of Au on these catalysts was between 3.5 nm and 5 nm, the Au particle size decreased as the content of Mg increased.
The XPS spectra of Au 4f are shown in Figure 6, and the concentrations of various electronic states of Au on Mg(OH) 2 -CeO 2 with different Mg/(Mg + Ce) atomic ratios are listed in Table 6. Gold displayed two peaks of Au 4f 7/2 and Au  Table 6 shows that the higher concentration of metallic gold species was observed with        Figure 7 and Figure 8 show the effect of Mg content on the performance of PROX over Au/Mg(OH) 2 -CeO 2 catalysts. CO conversion increased with increasing temperature until 40˚C or 60˚C, depending on sample, and then level off. At high temperature, H 2 would compete with CO to react with O 2 , and resulted in low CO selectivity. At lower reaction temperature, the existence of magnesium did not facilitate the CO conversion; instead, lower CO conversion was observed, except the sample with Mg/(Mg + Ce) ratios of 6.5%. Figure 8 shows that the samples modified with Mg(OH) 2    Au/Mg(OH) 2 -CeO 2 (20%) catalyst showed the highest activity among all catalysts, it also possessed the highest ratio of hydroxyl species. This confirms that the OH species between the interface of gold and support is the species to activate CO oxidation reaction. Molina and Hammer [19] reported that the Au-Au coordination determined the local reactivity of the Au atoms and the presence of the MgO support that, besides providing excess electrons to the Au clusters, forms ionic bonds to the peroxo part of the CO-O 2 reaction intermediate. They [19] reported that the type of interface boundary likely to be predominant for medium-sized nanoparticles provides the optimal degree of low-coordinated Au atoms in the neighborhood of the MgO support. Our results are in accord.

PROX Reaction
The metallic gold was believed to be the active site for PROX reaction [20]- [28]. The amount of Au 0 in the catalysts increased with the increase of magnesium content as shown in Table 5. However, overdoes of Mg(OH) 2 would retard the CO adsorption and suppress CO oxidation. It also suppressed H 2 oxidation and thus there was enough O 2 to react with CO [13] [21]. The results clearly demonstrate that adding Mg(OH) 2 in Au/CeO 2 could suppress H 2 oxidation and therefore increase CO oxidation activity.

Conclusion
A series of Au/Mg(OH) 2 -CeO 2 catalysts were prepared. Mg(OH) 2 would form a thin layer on the surface of CeO 2 . CeO 2 was in the crystalline phase and Mg(OH) 2 was amorphous. Using CeO 2 as a support could increase the dispersion of Mg(OH) 2 and thus increase the interaction between Au and Mg(OH) 2 . The amount of Au 0 in the catalysts increased with the increase of magnesium content. However, overdoes of Mg(OH) 2 would retard the CO adsorption and suppress CO oxidation. It also suppressed H 2 oxidation, and thus there was enough O 2 to react with CO. The results clearly demonstrate that adding Mg(OH) 2 in Au/CeO 2 could suppress H 2 oxidation and therefore increase CO oxidation activity.