CHAPTER IVEFFECT OF SUPPORT COMPOSITION AND METAL LOADING O N

Au CATALYST ACTIVITY IN STEAM REFORMING OF METHANOL*

4.1 Abstract

Gold (Au) supported on Ce0 2 -Fe2 0 3 catalysts prepared by the deposition- coprecipitation technique were investigated for steam reforming of methanol (SRM). The 3 wt% Au/Ce0 2 -Fe2 0 3 sample calcined at 400 °c achieved 100 % methanol conversion and 74 % hydrogen yield due to a strong Ce-Fe interaction in the active solid solution phase, CexFei-x0 2 . The sintering of All particles was observed when the highest metal content of 5 wt% was registered, which worsened the SRM activity. According to the TPR and TPO analysis, it was found that the transformation of the a-FcoGj structure in the mixed oxides and the coke ,deposition were the main factors for the rapid deactivation of the catalyst.

Keywords: Steam Reforming; Methanol; Flydrogen; Au catalyst; CeCb; FepOi

4.2 Introduction

Due to the demand for new energy carriers worldwide, many methods for efficient hydrogen production are still being investigated to find the best candidate [1-3]. Methanol (CH3OH) is widely acknowledged as a good reactant for the high- purity of hydrogen due to its low boiling point and high H/C ratio, which reduces the soot and coke formation [4-7]. Normally, hydrogen production based on a methanol source can be classified into three main reactions: methanol decomposition (DM), steam reforming (SRM), and partial oxidation (POM). SRM is also an endothermic reaction which obtains the highest H2 yield (3 moles FF/l mole CH3OH) with lower amounts of CO and low temperature operation (200-400 °C), as shown in Eq (4.1).

CFI3OH + H20 -> CO2 + 3FF AH(298 K) = 50 kJ moF1 (4.1)

* International Journal of Hydrogen Energy, 37 (2012) 14072-14084.

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However, the activity of SRM reaction depends on the type of catalyst used. Accordingly, it is necessary to produce high H2 purity which follows the requirement in PEM fuel cells (CO < 10 ppm) [8-10],

Many previous investigations mainly focused on Cu-based catalysts due to their high activity for SRM reaction. Unfortunately, the pyrophoric characteristic seems to be a crucial issue, which causes the rapid deactivation and sintering of these catalysts when the catalyst is carried out at high temperatures (> 270 °C) [11-13]. To avoid this problem, there has been significant activity to develop a new catalyst for this in recent years. Only a few studies related to hydrogen production from methanol over Au-based catalysts are reported; however, this catalyst is well known as an active catalyst in many reactions including preferential CO oxidation (PROX)[14] and water-gas shifted reaction (WGSR) [15]. Yi et al found that the Au/CeO: catalyst could obtain full methanol conversion at 300 °c over the SRM reaction with no deactivation [1 1 ].

The activity of gold catalysts strongly depends on the All dispersion. Au natural site, All particle size, and interaction between All and support (or by itself)[16]. Our previous work found an improvement in Au-Au interaction of 5 wt% Au/Ce0 2 catalysts, which facilitated extreme catalytic activity in the oxidative methanol reforming (OSRM) [17]. Ceria (CeÛ2) is defined as an interesting support because it has high oxygen storage capacity (OSC) or oxygen vacancy, which allows itself to store and release active oxygen to provide good performance in SRM reaction [18,19], In further studies, the chemical properties of ceria were successively developed by the incorporation of triple cations, M3+ (M = Fe, La, etc), into the ceria lattice [20-22]. This incorporation can enhance the formation of vacancies in the anion sublattice during the charge balancing, where the redox properties in ceria can be improved. The addition of another interesting support, Fe2().i, with CeCL has been receiving much attention due to the creation of an active solid solution phase (CexFei_x0 2 ), which could be beneficial for many applications [20,23,24], The Au/Ce0 2 -Fe2 0 3 showed high activity for the PROX and WGSR when the strong Ce-Fe interaction was formed in solid solution [25,26], Based on

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previous studies, there are a number of possibilities for catalyst modification by creating oxygen vacancies on the support [21].

In this study, a series of Au/CeOj-FejCb catalysts prepared by the deposition-precipitation technique (DP) were investigated for SRM in the temperature range of 200 to 400 ๐c.

4.3 Experimental

4.3.1 Catalyst preparationVarious ratios of mixed oxides (CeCb-FeiCb) supports—Ce:Fe = 8:1,

1:1, and 1:8—were prepared by a co-precipitation method, while the pure CeCb and Fe: 0 3 supports were prepared by a precipitation method. The Au/CeCE, Au/Fe2 0 3 , and Au/CeC>2-Fe2 0 3 catalysts were prepared by the deposition-precipitation technique. For support preparation, cerium (III) nitrate hexahydrate (Ce(N0 3 )3.6 H20 ) (Aldrich), iron (III) nitrate nonahydrate (Fe(N0 3 )3-9 H2 0 ) (Aldrich), and Na2C0 3 (Riedel-de Haen) were mixed under vigorous stirring condition at 80 °c and pH 8. Afterward, the precipitate was washed, dried, and calcined in air at 400 °c for 4 h in order to obtain the Ce0 2 “Fe2 0 3 supports.

Gold was loaded on the supports (CeC>2, Fe2C>3, and CeCrt-FejCb). An aqueous solution of HA11CI4.3 H2O (Alfa AESAR) was heated at 80 ๐c and adjusted to pH 8 with Na2C03. After the resulting solution was stirred for 1 h, the suspension was washed with warm deionized water to eliminate the residue ions. The deionized precipitate was dried at 110 ๐c and calcined in air at various temperatures (200-400 °C) for 4 h. The Ce02-Fe2C>3 support at each composition (Ce:Fe atomic ratio) was designated as CeXFeY, where X and Y are the atomic ratio of Ce and Fe, respectively.

4.3.2 Catalyst characterization

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The XRD measurement was carried out on a JEOL X-Ray diffractometer system (JDX-3530) with a CuKa (1.5406 Â) X-ray source operating at 40 kv and 30 mA.

The size and distribution of the All particles deposited on the supports were directly observed by a transmission electron microscope (JEOL, JEM 2010) at an accelerating voltage of 200 kV in bright field mode. Before being transferred into the TEM chamber, the samples were dispersed in ethanol and were then dropped onto a copper grid. The volume-area average Au particle size diameter (diEM ) was calculated from the following formula: djEM = ร(njdj3)/(njdj2), where ท, is the number of All particles of diameter dj.

DR/UV-vis spectroscopy experiments checked for the presence of different states of oxidation of the contained metals (which were recorded on a Shimadzu u v spectrophotometer 2550). The measurements were performed on air- exposed samples between 200 and 800 ททไ at ambient temperature. The absorption intensity was expressed using the Kubelka-Munk function, F(Roo) = (l-Rco)2/(2Rco), where Roo is the diffuse reflectance from a semi-infinite layer. An X-Ray Fluorescence Spectrometry, XRF (AXIOS PW4400) was used to determine the actual surface (Au, Ce, and Fe) composition.

Temperature-programmed reduction (TPR) was utilized for the evaluation of the reducibility of the catalysts and was employed by using 10 % FL in Ar at 30 mL/min as a reducing gas in a conventional TPR reactor equipped with a thermal conductivity detector. The reduction temperature was raised from 30 to 850 °c at a ramp rate of 10 °c/min.

The amount of carbon formation of the spent catalysts was measured by means of temperature-programmed oxidation (TPO). Approximately 50 mg of the powdered samples was packed in a quartz tube reactor before being heated from 1 0 0

°c with a heating rate of 12 ๐c/min to 900 ๐c under a flow of 2 % Cb/He using a gas flow rate of 30 mL/min.

4.3.3 Catalytic activity measurements

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The SRM reaction was carried out in a fixed-bed reactor with 80 to 120 mesh holes containing 100 mg of the Au/CeCb-FeiCf catalysts under atmospheric pressure with a reaction temperature of 200 to 400 °c. A mixture of distilled water and methanol in a syringe was injected continuously by a syringe pump at a rate of 1.5 mL h' 1 to a vaporizer to produce a vapor of methanol and steam, which was mixed with the He carrier gas before entering the catalytic reactor. The H2O/CH3OH molar ratio was fixed constantly at 2/1 since it has been defined as the suitable feed composition for the high catalytic activity of Au catalyst [17], The All contents and calcination temperatures were varied from 1 to 5 พt% and 200 to 400 °c. respectively. Finally, the stability of the prepared catalysts was tested for 10 h. The gas hourly space velocity (GHSV) was kept at 21 000 mL/g-cat. h. The product gases (e.g. IT, CO, CO2, and CH4) from the reactor were analyzed both qualitatively and quantitatively by auto-sampling in an on-line gas chromatograph, Agilent 6890N, with a packed carbosphere (80/100 mesh) column (10 ft X 1/8 inch) and a thermal conductivity detector (TCD). The selectivity of each product gas was defined by the mole percentage in the product stream. No methane formation was observed from this study.

4.4 Results and discussion

4.4.1 Catalyst characterizationThe chemical and physical properties of the series of Au/Ce0 2 -Fe2 0 3

catalysts are summarized in jab le 4.1. The lattice constant of CeC>2 in Ce0 2 -Fe2 0 3

(0.540-0.543 run) is smaller than that of CeC>2 (0.544), indicating that the presence of Fe2Û3 resulted in the incorporation of Fe3+ (0.064 11m) into the Ce4+ (0.101 nm) to form a solid solution [25]. However, with high Fe contents (Ce:Fe = 1:8). these values increased since the segregation of Fe3+ from the Ce4+ lattice might be more favorable than the Fe3+-Ce4+ combination [25], suggesting that high Fe content might prevent solid solution formation.

Table 4.1 Chemical-physical properties of the Au/CeCL-FeoCb catalystsC a t a ly s t C a lc in a t io n T e m p e r a t u r e

( ° C )

A u

( w t % )

C e

( \v t% )

F e

( w t % )

C r y s t a l l i t e s i z e 3

( n m )

L a t t ic e c o n s t a n t o f C e 0 2b

( n m )

A u c r y s t a l l i t e s i z e ( n m )

d u t d 200 d 2 2 0

3 w t% A u /C e 0 2 4 0 0 2 .7 4 9 7 .2 6 - 7 .6 5 0 .5 4 4 < 5 _

3 w t% A u /C e 8 F e l 4 0 0 2 .2 7 9 2 .0 0 5 .7 3 6 .7 8 0 .5 4 3 7 .7 -1 w t% A u /C e lF e l 4 0 0 1 .59 6 1 .0 2 3 7 .3 9 6 .1 8 0 .5 4 0 10 .4 -3 w t% A u /C e lF e l 4 0 0 2 .4 9 6 1 .8 8 3 5 .6 3 6 .2 5 0 .5 4 0 11 .9 -3 w t% A u /C e lF e l 3 0 0 2 .6 0 6 1 .3 0 3 6 .1 0 6 .8 2 0 .5 4 0 11.6 -3 w t% A u /C e lF e l 200 2 .5 9 6 4 .4 6 3 2 .9 5 6 .0 9 0 .5 4 0 12.1 -5 w t% A u /C e lF e l 4 0 0 4 .6 9 5 8 .0 0 3 7 .3 1 6.68 0 .5 4 0 13 .9 12.1 6 .6

3 w t% A u /C e lF e 8 4 0 0 2 .5 9 7 6 .0 9 2 1 .3 2 1 1.96 0 .5 4 2 11 .3 -3 w t% A u /F e 20 3 4 0 0 2 .5 7 - 9 7 .4 3 - 10 .4 -

“Mean crystallite sizes were calculated from the average values of Ce02 plane (111), (220), and (311). bUnit cell parameter calculated from CeOi (220) with the Bragg’s equation.

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The XRD patterns of all studied catalysts are shown in Fig. 4.1 A. The strongest diffraction peak of Ce02 at 20 = 28.5° represented the fluorite structure[24], Other positions of weak peaks were also detected, at 33.08, 47.47, 56.33, 59.08, 69.40, 76.69, and 79.07°, corresponding to (200), (220), (311), (222), (400). (331), and (420) for the CuKa (1.5406 Â) radiation, respectively [28,29], The diffraction patterns of Fe20 3, at 24.1, 33.1, 35.6, 40.8, 49.4, 54.0, 57.5, 62^6, and 64.0° (20) were due to the presence of hexagonal a-Fe2C>3 phases that were, respectively, assigned to hematite crystallite planes of (012), (104), (110), (113), (024), (116),(018), (214), and (300) [30,31]. At 3 wt% All loading, compared to the Au/Ce02, the intensities of the Ce02 decreased with increasing Fe content into the mixed oxides (Ce:Fe = 8:1), while the presence of very weak hematite intensities could be associated with the well-dispersed Fe3+ incorporating inside the ceria lattice to form Ce02Fess or CexFei.x0 2 solid solution [25,32], It can be seen that the peaks of the solid solution supports become broader and less intense, suggesting the existence of mixed phases, which dominated the reduction of crystallite sizes [20], The intensities of hematite peaks became well-defined after increasing the Fe content (Ce:Fe of 1:1), while extreme amounts of Fe added (Ce:Fe = 1:8) provided the sharp peaks of oc- Fe20 3 . As can be seen from curve e in Fig. 4.1 A, the peaks attributed to the spinel structure, appeared in the sample of Ce:Fe = 1:1 and 1:8. This correlated to the crystallinity improvement of the free hematite particles without incorporation of Ce4+ inside [25], Indeed, the insertion of Fe caused a shifting of ceria peaks toward higher diffraction angles, whilst no shifting in hematite phases toward lower diffraction angles was observed in this experiment. Especially for highly concentrated Fe (Ce:Fe = 1:8), in the ranges of 47.5-50° and 56-58°, the intensities of hematite peaks became more pronounced and started to shift away from the overlapped regions of mixed phases, evidenced by the segregation of Fe3+ from the ceria lattice to form the pure hematite phase. Consequently, the lattice constant was increased with the excess Fe, which exhibited more segregated or free a-Fe20 3.

Interestingly, the significant difference in peak intensities of Au (111 ) at 38.5° became detectable at high Fe contents probably due to the variation in gold dispersion affected by Fe concentration. Based on the Scherrer equation, the

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crystallite sizes of the Au (111) were detectable in the high Ce concentration (Ce:Fe = 8:1).Moreover, high Fe amounts could enlarge the All crystallite sizes (10-11 nm), leading to speculation that the strong intermetallic bonding of Au-Fe could form the Au cluster (Aun, 1 < ท < 10) which enhanced the Au crystallinity [33,34], When the amount of All loading reached 5 wt%, a significant increase in crystallinity of the All(111) was clearly observed in Fig. 4.IB, including the appearance of Au (200) and All (220) reflections at 20 = 44.4° and 64.6° [35], respectively. This result suggested that an agglomeration of the gold particles occurred, and then resulted in larger All particle size. The All crystallite size increased from 10.4 nm to 13.9 nm with an increasing of All loading from 1 to 5 wt%, indicating the growth of All nanoparticles[36].

In Fig. 4.1C, the high crystallinity of both Au (111) and CeCb peaks were observed when the calcination temperature was lower than 400 °c. In addition, the appearances of high a-Fe2C>3 inteusities were also observed at these low temperatures. A possible explanation could be the generation of free a-FeiCb particles separating from the solid solution phase and then becoming soluble with All to facilitate more All clusters or All crystallinity, as reported previously. As a result, the calcination temperature of 400 °c seemed to broaden the ceria diffraction peaks, including lowering crystallinity, representing the best incorporation of Fe1+ in Ce4+ for homogeneous solid solution creation. The similar explanation of broaden peaks was also reported in mixed oxides, such as CeCb-ZrO? [37], In addition, the broadening of the ceria is attributed to either a decrease of ceria crystallite size or oxygen vacancies.

20 (degree)

Signal

(a.u.

)Sig

nal (a

.u.)

Signal

(a.u.

)Sig

nal (a

.n.)

CTs

65

Figure 4.1 XRD patterns of the Au/Ce0 2 -Fe2 0 3 catalysts: (A) Effect of support compositions calcined at 400 ๐C; (a) CeC>2, (b) 3 wt% Au/CeOi, (c) 3 wt% Au/Ce8 Fel, (d) 3 wt% Au/CelFel, (e) 3 wt% Au/CelFe8 , (f) 3 wt% Au/Fe2 0 3 , and(g) Fe2Û3. (B) Effect of Au loadings; (a) 1 wt% Au/CelFel, (b) 3 wt% Au/CelFel, and (c) 5 wt% Au/CelFel. (C) Effect of calcination temperatures on 3 wt% Au/CelFel; (a) calcined at 200 °c, (b) calcined at 300 °c, and (c) calcined at 400 °c. (D) Comparison between (a) fresh catalyst and (b) spent catalyst.

4.4.2 Catalytic activity4.4.2.1 Effect o f support composition

Fig. 4.2 (A, B) presents the catalytic activities of supports and supported Au catalysts. In the absence of Au deposition, the pure CeC>2 and Fe2Û3

supports showed low catalytic activities in the reaction temperatures studied, whilst CeC>2-Fe2 0 3 (Ce:Fe = 1:1) significantly enhanced either methanol conversion or product gases concentrations. Focusing on the supported All catalysts with 3wt% Au, an increase of methanol conversion as the functional reaction temperature was observed in all samples. Interestingly, All catalysts demonstrated much better catalytic performance than those of the supports without Au loading, indicating that the presence of the active Au metal was necessary to efficiently promote catalytic activity. Ill terms of product composition in the All catalysts, it was found that the H2

and CO concentration (%) increased with reaction temperature, where the methanol decomposition could appear at high temperatures during the SRM reaction. The mole percentage of CO was still quite low in the range of 0-3.2 %, while the FE concentration seemed to have the same trends as the methanol conversion, and its maximum value of 38 % was observed at the highest temperature.

In terms of gas selectivity (Fig. 4.2 bar graph), the roles of All loading was clearly observed in decreasing CO selectivity and increasing FR selectivity in the entire range of reaction temperatures. Indeed, when compared to another catalyst, CuO-Ce0 2 [1], our catalysts were considered as more active catalysts with less CO formation. This could be implied that the All catalyst plays an important role in CO reduction in SRM. The support composition (Ce:Fe) directly

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influenced the SRM activity as ordered: 1:1 >8:1 > 1:0 > 1:8 > 0:1. High catalytic activity was observed when the amounts of Fe2(>3 were initially registered, denoted as Ce:Fe (1:1) and (8:1). For the catalysts with a high amount of FejCF (Ce:Fe = 1:8 and pure Fe^Oa), the catalytic behavior was suppressed when the Fe concentration was higher than the optimum value, Au/Fe2Û3 displayed the lowest activity. The results obtained from the prepared catalysts agree perfectly with the trends reported by Yonggang et al. 1 who concluded that its high activity could be attributed to the solid solution formation and good dispersion of Fe2Û3 on the catalyst surface [38]. As mentioned above, the Au/Ce0 2 -Fe2 0 3 (1:1) was the best catalyst, which obtained 100 % methanol conversion, 74 % hydrogen yield, and 38 % mole H2 at 400 °c. In this experiment, the hydrogen yield was not shown since it followed the same trend as the methanol conversion.

Gas s

electiv

ity (%

) แ2

conce

ntrati

on (v

oI%)

CHjO

H con

versio

n (%)

67

Temperature (°C) Temperature (°C)

Figure 4.2 Effect of support composition on methanol conversion and product compositions over: (A) support without Au deposition; and (B) Au/CeCh-FeiOj. (Reaction conditions: H2O/CH3OH, 2/1; calcination temperature, 400 °c.)

CO co

ncentr

ation

(niol

e%)

68

0 100 200 300 400 500 600 700 800 900Temperature (°C)

Figure 4.3 TPR profiles of the samples: (A) without Au deposition; (a) Ce02, (b) Ce8 Fel, (c) CelFel, (d) CelFe8 . and (e) Fe2C>3. (B) with Au deposition; (a) Ce02,(b) 3 wt% Au/Ce02, (c) 3 wt% Au/Ce8 Fel, (d) 3 wt% Au/CelFel, (e) 3 wt% Au/CelFe8 , (f) 3 wt% Au/Fe20 3 , and (g) Fe20 3 -

According to the TPR profiles of the supports (Fig. 4.3A), the ceria presented a broader peak situated close to 489 °c, which is typically related to an oxygen surface reduction process, and the other at a higher temperature (around 850 °C), which is linked to the bulk reduction from the ceria structure (Ce02 to Ces0 4 ) [39^11], In the two-step reduction of Fe20 3 , the first peak at 432 °c was

69

associated with the hematite-to-magnetite transformation (Fe2C>3 -> FejOA, which then transformed to metallic iron (F'CjCh -/► FeO Fe) at 698 °c [42-44]. In comparison to the pure CeC>2 and Fe2C>3, the mixed oxides showed three reduction peaks; the first peak was related to the hematite-to-magnetite state or the reduction of Fe3+ located on the surface [45]; the second peak was possibly associated with the overlapping of the reduction of Ce4+ and the reduction of Fe2+; and the last peak was attributed to the reduction of bulk CeC>2 [46],

It is well known that the generation of oxygen vacancies could result in active oxygen releasing. The presence of Fe could enhance reducibility of the catalysts by shifting reduction peaks toward a lower temperature, when compared to the pure CeCb. This shifting behavior was evidenced by the strong Ce-Fe interaction where the hematite-like solid solution could be formed [25,47], Besides, the intensity of the peak at 400 °c gradually strengthened with increasing Fe content, implying that the more free a-FejCb generated in the rich-Fe samples, the more Fe3+ reductive species were created, as evidenced in the increased intensity of Fe2Û3 diffractions in the XRD [45], According to the TPR and XRD results, the coexistence of solid solution and free a-Fe2Û3 particles in Ce:Fe of 1:1 could indicate the interaction between the free a-Fe2 0 3 particles and solid solution, which exhibited the best redox properties. Some authors also found a similar interaction between NiO particles and Ce-Ni solid solution, playing a significant role in the reducibility improvement of the catalyst [48]. In contrast, the Ce:Fe of the 1:8 sample had no solid solution (according to the high lattice constant), but it was reduced faster than the pure CeC>2, and the intensity of the Fe3+ peak at 390 °c became pronounced. This infers that the interaction between free a-Fe2Û3 particles or free Fe3' species and CeC>2 is still possible for the reduction behavior in the Ce-Fe system [45],

From the TPR profiles of Au/CeC>2-Fe2 0 3 catalysts (Fig. 4.3B), the Tmax at very low temperatures (117-142 °C) illustrated the reduction of the AuxOy species (or All hydroxide) to Au metal (Au°) [49,50], The deposition of All particles could improve the reducibility of all supports where the shifting reduction peaks toward lower temperatures correlated to the strong metal-support interactions (Au-Fe and Au-Ce) [30,44,51], The improvement in reducibility of Au/Fe2 0 3

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catalysts has been proposed by the kinetic mechanisms [42,52];furthermore, some authors signified that the weakening of the Fe-0 bond by another metal as the main reason for simple reduction [53]. The simplest reducibility of Au nanoparticles belonged to the Au/CeCb catalyst with the highest quantity of แ 2 consumption (or area under its reduction peak), when compared with Au/CeCf-Fe^Ch. According to small amount of hydrogen consumed, it has been suggested that All could form very' strong intermetal lie bonds with Fe due to the slight solubility of Fe in gold[33,34]Therefore, the gold clusters (Aun) with less reducibility than Au5+ might be favored [33] and possibly increase the Au crystallite size. This was in line with the appearance of Au (111) at high Fe concentrations in the XRD patterns. Apparently, the trends of Ce-Fe interaction were similar to those without All deposited, implying the similarity of solid solution form. The optimal ratio of Ce:Fe at 1:1 still gave the three closest reduction peaks; hematite-to-magnetite reduction (317 °C); the likely- combination between Fe2+ and Ce4+ reduction (578 ๐C); and magnetite-to-metallic iron (779 °C) where the strongest Ce-Fe interaction in Cei-xFexC>2 solid solution occurred [25].

When focusing on the Au-support interaction, the AuxOy peak shifted toward higher temperatures with increasing Fe content, and stayed closer to the Fe3+ reduction (316-325 °C), as shown in the Ce:Fe sample of 1:1, 1:8, and pure FeoOj. This suggested an improvement in the interaction between the AuxOy and CeOi-l^WOj support. However, the Au/CeCb-FeiOj (1:1) catalyst obviously indicated a stronger interaction between the AuxOy and the Fe3+ in the solid solution phase, which strongly facilitated the catalytic activity. Although the strength of the Au-Fe3+ interaction in Ce:Fe of 1:8 and pure FctO; were still high, the Fe3+ interaction mainly came from the free a-Fe2 0 3 , as evidenced in the increasing intensity of the Fe3+ reduction, which was less active than the solid solution. Consequently, the strongest Au-free Fe3+ interaction in Ce:Fe of 1:8 and pure Fe20 3

did not always enhance SRM activity since All sintering and the lack of solid solution phase dominated the activity instead. The evolution of free Fe2C>3 particles, via XRD, has been identified as Fe segregation from the solid solution, which reduced the active catalysts by weakening the Ce-O-Fe interaction [54], We

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postulated that the strong Au-Fe3+ interaction in the solid solution, not free Fe3+, seemed to increase the reaction in this case.

To provide a uniform solid solution, the limitation of Fe3+ doping must be considered [55,56]. It was strongly noted that the solubility limit of iron in the ceria lattice depends on many conditions, such as the preparation technique [54], calcination temperature [45], and precursor [54], However, our optimal catalyst conditions, with a Ce:Fe ratio of 1:1 calcined at 400 °c, prepared by the coprecipitation technique demonstrated the solubility limit of iron in the ceria matrix, while the excess Fe caused Fe3+ segregation from the ceria lattice.

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350 400 450 500 550 600 650 700 750Wavelength (nin)

Figure 4.4 D if f u s e r e f le c ta n c e U V - v is s p e c t r a o f th e A ll s p e c ie s : (A ) E ffe c t o f s u p p o r t c o m p o s i t io n s , (B ) E f f e c t o f A ll lo a d in g s , a n d (C ) E f fe c t o f c a lc in a t io n te m p e ra tu re .

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U V -v is sp e c tra m e a s u r e m e n ts w e re u s e d to id e n t i fy th e A ll s p e c ie s , w h ic h w e re r e s p o n s ib le fo r th e c a ta ly t ic a c t iv i ty , a s s h o w n in F ig . 4 .4 A . T h e p la s m a b a n d th a t c e n te re d a t a ro u n d 3 4 0 n m c o u ld b e r e la te d to th e CeC>2 , a n d th e F e 2 C>3 th a t ty p ic a l ly s h o w s tw o p la s m a b a n d s a t 3 4 5 n n i a n d 5 6 4 n n r [3 1 ] . F o r th e m ix e d o x id e , th e b r o a d e n C eC b p e a k a n d th e s h i f t in g in F e 2 Û 3 p e a k w ith lo w in te n s i ty a t 541 ru n in d ic a te d th e in c o r p o ra t io n o f F e 3+ in th e C e 4+ fo r s o l id s o lu t io n c re a t io n . A s im ila r o b s e rv a t io n o f th is m ix e d p h a s e h a s b e e n r e c o r d e d w ith th e s a m e te c h n iq u e [2 3 ] . N o rm a l ly , th e p la s m a b a n d in th e r a n g e o f 5 0 0 - 6 0 0 lira a n d < 2 5 0 n m a re d e f in e d a s th e g o ld p la s m o n o r A ll m e ta ll ic (A ll0) s p e c ie s [5 7 ] a n d A ll3 ' s p e c ie s , r e s p e c t iv e ly [5 8 ] , a n d A u c lu s te r s ( A u n) c a n b e o b s e r v e d a t 2 8 0 - 3 8 0 n m [5 9 ] . O n ly th e A ll11 s p e c ie s in th e A u /C e 0 2 c a ta ly s t w a s d e te c ta b le , w h e re a s th o s e o f A u /F e 2 0 3 a n d A u /C e 0 2 - F e 2 0 3 c a ta ly s ts w e re im p o s s ib le to d e te c t d u e to th e in te r fe r e n c e o f s u p p o r ts a n d th e r e s o n a n c e b a n d o f v a r io u s g o ld s p e c ie s .

4.4.2.2 Effect o f Alt contentIt is w e ll k n o w n th a t th e a m o u n t o f A ll c o n te n t c a u s e s a

s t r o n g e f fe c t o n c a ta ly t ic a c tiv ity . In th is e x p e r im e n t , c a ta ly t ic a c t iv i ty in c re a s e d s ig n if ic a n t ly w i th in c re a s in g a m o u n ts o f A ll f ro m 1 w t% to 3 w t% , w h i le th e h ig h e s t A u c o n te n t (5 w t% ) c o n v e r s e ly e x h ib i te d lo w e r a c t iv i ty , a s i l lu s t r a te d in F ig . 4 .5 . T h e m o le p e r c e n ta g e o f C O w a s s ti ll o b s e rv e d in th e m in u te v a lu e s o f 0 - 3 .2 % , w h ile th e g a p d i f f e re n c e o f % m o le FT b e c a m e la rg e r , th e s a m e a s in th e m e th a n o l c o n v e r s io n , a f te r in c re a s in g th e A u lo a d in g to 3 % a n d 5 % , r e s p e c tiv e ly . T h e s e r e s u lt s r e v e a le d th a t th e a c t iv e A ll m e ta l is v e ry e f fe c tiv e in S R M a c tiv i ty . T h e 3 w t% A ll s a m p le w a s d e te rm in e d to b e th e m o s t s u i ta b le c o n te n t fo r S R M r e a c t io n , w h ic h p r o m o te d h ig h d is p e r s io n , in c lu d in g o p t im u m g o ld p a r t ic le s iz e . It is n o t in g o o d a g r e e m e n t w ith r e c e n t f in d in g s w ith 1 w t% All in o u r p r e v io u s w o rk , w h ic h e x h ib i te d th e h ig h e s t a c t iv i ty fo r P R O X [6 0 ] .

74

ร®4 ®ไ

" o

B

3 5

■I200 250 300 350 400

T e m p e r a tu r e ( ° C )

Figure 4.5 E ffe c t o f A u lo a d in g o n m e th a n o l c o n v e r s io n a n d p r o d u c t c o m p o s i t io n s o v e r A u /C e 0 2 - F e 2 0 3 . (R e a c tio n c o n d i t io n s : H 2 O /C H 3 O H , 2 /1 ; c a lc in a t io nte m p e ra tu re , 400 °c.)

A s i l lu s t ra te d in F ig . 4 .6 A , T P R p ro f i le s o f A ll c a ta ly s t w i th d i f f e re n t A u lo a d in g s r e v e a le d th a t th e h e m a t i te r e d u c t io n s h i f te d to w a rd lo w e r a te m p e ra tu re (2 7 5 ° C ) , w h i le tw o r e m a in in g r e d u c t io n p e a k s s h i f t e d to w a rd h ig h e r te m p e ra tu re s (591 a n d 8 0 5 ๐C ) a f te r d e p o s i t in g th e lo w e s t A ll c o n te n t . T h is c o u ld b e re fe r re d to e i th e r w e a k e r s u p p o r t - s u p p o r t ( C e - F e ) in te r a c t io n in f re e a - F e 2 0 3 o r le ss d is p e r s e d F e in th e m ix e d o x id e s , r e s u l t in g in in s u f f ic ie n t s i te s o f s o l id s o lu tio n . A s im ila r k in d o f fre e m e ta l o x id e w ith le s s in te r a c t io n o n th e c a ta ly s t s u r fa c e w a s a lso

75

r e p o r te d [3 7 ] . O n ly a s l ig h t c h a n g e in A u r e d u c t io n te m p e ra tu re w a s o b s e rv e d w h e n th e A lt lo a d in g w a s h ig h e r th a n 1% . W ith - th e d e c r e a s e in th e a m o u n t o f F F w h ic h w a s c o n s u m e d in A u r e d u c t io n p e a k , th e q u a n t i t ie s o f v a r io u s A u s p e c ie s c o u ld be c o r r e la te d w i th in c re a s in g A ll lo a d in g . T h e a m o u n t o f A u ° s p e c ie s w as h ig h in th e p re s e n c e o f 3 w t% a n d 5 w t% A u , w h ile 1 w t% A u c o n ta in e d m a n y A ll6"1" sp e c ie s .

O u r s p e c u la t io n is th a t 3 w t% A u c re a te d s u i ta b le A ll0 a n d A u fi+ c o n te n t to fo rm th e o p t im u m A ll p a r t i c le s iz e . W h e n th e a m o u n t o f 5 w t% A ll w a s d e p o s i te d , th e d im in u t io n o f FF c o n s u m e d in d ic a te d th e d e p le t io n o f th e c o n c e n t r a t io n o f A u /C e C b - F e 2 0 3 in te r p h a s e s i te s b e c a u s e a n a g g lo m e ra t io n o f A ll n a n o p a r t ic le s ( p ro m o tin g u l tra A u ° c o n te n t) p o s s ib ly in h ib i te d th e c a ta ly t ic a c t iv i ty o f A u c a ta ly s t [4 1 ] , T h is e x p la n a t io n a lso f o l lo w e d th e t r e n d s o f X R D p a t te rn s . N e v e r th e le s s , th e v a r ie ty o f A u s+, A u ° , a n d A u n a p p e a ra n c e s is r e g a rd e d as th e k e y p a r a m e te r fo r P R O X , w h ile W G S re a c t io n h a s n o t b e e n y e t c la r i f ie d [1 6 ] . S o m e a u th o r s m e n tio n th a t th e c o ­e x is te n c e o f A u 11 a n d A ll0 h a s n o m o r e .a c tiv e th a n o n ly th e A l l0 sp e c ie [6 1 ] . It h a s b e e n s u g g e s te d th a t th e a c t iv i ty o f th e c a ta ly s ts s t ro n g ly d e p e n d o n th e A u ° p re s e n te d o n th e s u r fa c e a n d th e s tr e n g th o f e a c h in te r a c t io n ty p e a s w e ll.

B a s e d o n th e U V -s p e c tr a s h o w n in F ig . 4 .4 B , th is te c h n iq u e c a n b e u s e d to a n a ly z e q u a l i ta t iv e ly th e A ll s in te r in g b y c o n s id e r in g th e in c re a s e ill in te n s i ty ( o r a re a ) o f th e A u ° p la s m a b a n d in 5 w t% A u . A d d i t io n a l ly , th e T E M im a g e s w e re a ls o u s e d to e s t im a te th e A ll p a r t ic le s iz e — th e d a rk s p o ts o n th e s u p p o r ts — a n d th e ir d is t r ib u t io n s , a s im a g e d in F ig . 4 .7 ( a - c ) . T h e m e a n A u particle- s iz e s a t 1 , 3 , a n d 5 w t% A ll lo a d in g s w e re 2 3 .5 3 , 4 3 .0 7 , a n d 6 2 .5 7 n m , re sp e c tiv e ly . T h e r e s u l t s a g re e d w i th m a n y p r e v io u s c h a r a c te r iz a t io n s , w h e re 5 w t% A ll p re s e n te d th e la r g e s t A ll p a r t i c le s iz e s , a t t r ib u t in g to c o m p le te s in te r in g . I t sh o u ld b e n o te d th a t th e s u i ta b le A u p a r t i c le s iz e fo r th e S R M a c tiv i ty w a s a p p ro x im a te ly 43 n m .

76

0 100 200 300 400 500 600 700 800 900

T e m p e r a tu r e ( ° C )

Figure 4.6 T P R p ro f i le s o f th e A u/C eC > 2- F e 2 0 3 c a ta ly s ts : (A ) c a lc in e d a t 400 °c w ith v a r io u s A u lo a d in g s ; (a ) 1 w t% A u / C e l F e l , (b ) 3 w t% A u / C e l F e l , a n d (c ) 5 w t% A u / C e l F e l . (B ) w ith d i f f e r e n t c a lc in a t io n te m p e ra tu re s ; ( a ) c a lc in e d a t 200 ๐c, (b ) c a lc in e d a t 300 °c, a n d (c ) c a lc in e d a t 400 °c.

77

Figure 4.7 T E M im a g e s a n d A ll p a r t ic le s iz e d i s t r ib u t io n s ( in s e ts ) : (a ) 1 w t% A u / C e l F e l ; (b ) 3 w t% A u / C e l F e l ; (c ) 5 w t% A u / C e l F e l ; (d ) 3 w t% A u /C e l F e l c a lc in e d a t 200 °C; a n d (e ) 3 w t% A u /C e l F e l c a lc in e d a t 300 °c.

78

4.4.2.3 Effect o f calcination temperatureA s r e p o r te d in F ig . 4 .8 , th e h ig h a c t iv i ty w ith % m o le FF o f 3

w t% A u /C e O i- F e iC h (1 :1 ) w a s n o t o b s e rv e d w h e n th e c a lc in a t io n te m p e ra tu re w a s lo w e r th a n 4 0 0 °c. In a d d i t io n , th e lo w e s t p e r c e n ta g e o f m o le C O w a s fo u n d in th e

c a ta ly s t c a lc in e d a t 4 0 0 °c, w h ile th e h ig h e r C O c o n c e n t r a t io n s w e re o b s e rv e d a t lo w e r th e c a lc in a t io n te m p e ra tu re s . H o w e v e r , th e h ig h e s t % m o le C O w a s s till le ss th a n 5 % . I t c o u ld a ls o b e im p l ie d th a t th e p e r f o rm a n c e d u r in g th e p r o d u c t io n o f H t g a s a n d th e r e d u c t io n o f C O d u r in g th e S R M re a c t io n w a s p o s s ib ly a f fe c te d b y th e th e rm a l t r e a tm e n t . T h e s u i ta b le c o n d i t io n fo r th e rm a l t r e a tm e n t s h o u ld b e 4 0 0 °c s in c e th is t e m p e ra tu re p r o v id e d s u f f ic ie n t in te r a c t io n a n d c r y s ta l l iz a t io n fo r so lid s o lu t io n . W e p o s tu la te d th a t h ig h e r c a lc in a t io n te m p e ra tu re s w e re s ig n if ic a n t fo r th e c a ta ly s t s o l id s o lu t io n p ro p e r t ie s in th e S R M re a c tio n .

T h e T P R p ro f ile s , p r e s e n te d in F ig . 4 .6 B , s h o w tw o m o re s e p a r a t io n p e a k s o f f re e a - F e 2 C>3 re d u c t io n , c o r r e s p o n d in g to tw o ty p e s o f f re e a - F e iC f p a r t ic le , a t lo w c a lc in a t io n te m p e ra tu re . T h e f i r s t ty p e c o u ld b e c la s s i f ie d a s la r g e r f re e F ctCF p a r t ic le s th a t h a v e le ss s u p p o r t - s u p p o r t in te r a c t io n (o r s tro n g F e - F e in te r a c t io n ) , w h ic h th e n r e q u i r e th e lo w e r r e d u c t io n te m p e ra tu re s (2 5 3 a n d 2 4 6 °C ).

A n o th e r ty p e a t a h ig h e r r e d u c t io n o f te m p e ra tu re (3 1 9 a n d 311 °C ) h a s s m a ll F ctCT p a r t ic le s a s s o c ia te d w i th a s t r o n g e r in te r a c t io n . C o n v e rs e ly , th e s tro n g m e ta l - s u p p o r t in te r a c t io n b e tw e e n A u a n d th e la rg e f re e F e jO j p a r t i c le s c o r r e s p o n d e d to th e o v e r la p p in g o f th e r e d u c t io n p e a k s a t a lo w c a lc in a t io n te m p e ra tu re . A c c o rd in g ly , m o re A u n m ig h t b e g e n e ra te d a s w e ll. I n te re s t in g ly , th e 2 0 0 ° c t r e a tm e n t g a v e th e s h a r p e s t p e a k o f th e la rg e f re e a - F e iC b p a r t ic le s c o m p a re d w i th th e b r o a d e r p e a k a t th e c a lc in a t io n te m p e ra tu re o f 300 °c. T h is m ig h t b e d u e to th e la c k o f h o m o g e n e o u s d i s p e r s io n o f th e h e m a t i te p h a s e in th e m ix e d o x id e s u p p o r t , w h ic h c a n b e e s t im a te d f ro m th e w id th o f th e h e m a t i te p e a k [3 7 ] , so th e c h a n c e o f f re e u - f e T O j g e n e ra t io n w o u ld b e h ig h e r w i th d e c r e a s in g c a lc in a t io n te m p e ra tu re . It s h o u ld b e n o te d th a t th e b r o a d e r th e r e d u c t io n p e a k th e f re e a - F e 2 Û 3 h a s , th e b e t te r th e d is p e r s io n a n d in te r a c t io n o f h o m o g e n e o u s f re e a - F e 2 Û 3 b e c o m e s . In c o n c lu s io n , th e in c re a s e in c a lc in a t io n te m p e ra tu re le d to th e fu lly d e v e lo p e d s o l id s o lu t io n p h a s e h o m o g e n e ity

79

w h e re tw o ty p e s /p e a k s o f f re e a - F e o O j tu rn e d to c o m b in e c o m p le te ly in th is e x p e r im e n t.

6

5 Î -J U '■Ô

4 รo

0

T e m p e r a tu r e ( ° C )

Figure 4.8 E f f e c t o f c a lc in a t io n te m p e ra tu re o n m e th a n o l c o n v e r s io n a n d p ro d u c t c o m p o s i t io n s o v e r 3 w t% A u / C e l F e l . ( R e a c t io n c o n d i t io n : FI2 O /C H 3 O H , 2 /1 )

A s m e n t io n e d a b o v e , th e X R D p a t te r n s (F ig . 4 .1 C ) s ig n if ie d th e d is p e r s io n o f h o m o g e n e o u s a -F c z C b o n th e c e r ia la t t ic e w h e re th e in te n s i t ie s o f c e r ia p e a k s w e r e b ro a d e n e d w i th in c re a s in g c a lc in a t io n te m p e ra tu re . I n p a r t ic u la r , o n e d i f f r a c t io n a n g le o f a - F e iC b a t 4 0 .8 ° a p p e a re d a t th e lo w e s t c a lc in a t io n te m p e ra tu re b e c a u s e th e f a c t th a t th e la rg e f re e a - F e 2 0 _ 3 p a r t ic le w a s f a v o re d a n d e a s i ly d e te c te d o n th e s u r fa c e o f th e c a ta ly s t . T h is k in d o f o b s e rv a t io n h a s b e e n

80

r e p o r te d in f re e N iO c a ta ly s ts [3 7 ] . O n th e o th e r h a n d , th e in te n s i ty a n d c ry s ta l l i te s iz e o f A ll (1 1 1 ) w a s th e m o s t p ro n o u n c e d a t th e lo w e s t te m p e ra tu re , c o r r e s p o n d in g to th e la r g e r A u p a r t ic le s iz e s w h ic h m ig h t c o m e f ro m th e c o m b in a t io n b e tw e e n A u n a n d A u ° s p e c ie s .

F o c u s in g o n th e บ V - s p e c t r a in th e F ig . 4.4C, th e s h if t in g in s p e c t ru m p e a k s ( f ro m 541 to 564 n m ) w e re o b s e rv e d f o r lo w c a lc in a t io n te m p e ra tu re s . T h is r e p r e s e n te d th e c r e a t io n o f f re e a - F e 2 0 3 w i th o u t h o m o g e n e o u s s o l id s o lu t io n f o m i, C ei_xF e x0 2 . F ro m T E M re s u lts in F ig . 4 .7 ( b , d , a n d e ) , th e m e a n p a r t ic le s iz e s o f th e A u , w i th c a lc in a t io n te m p e ra tu re s o f 200, 300, a n d 400 °c, w e re , r e s p e c t iv e ly , 46.87, 44.71, a n d 43.07 n m . A l th o u g h A ll c a ta ly s ts c a lc in e d a t 300 a n d 400 °c w e r e a s im ila r p a r t i c le s iz e s . T h e la r g e s t A u p a r t i c le c a lc in e d a t 200 °c s h o w e d s l ig h t a g g lo m e r a t io n , w h ic h w a s in l in e w i th th e A u „ f o r m a t io n o r ig in a t in g f ro m th e s t r o n g in te r a c t io n o f A u - F e ( la rg e f re e a - F c i C h p a r t ic le ) . W e , th e re fo re , e lu c id a te d th e h o m o g e n e i ty o f s o l id s o lu t io n p h a s e a n d th e A ll s in te r in g a s th e m a in fa c to r s f o r lo w e r in g th e c a ta ly t ic a c tiv ity .

4 .4 .3 S ta b i l i ty te s t in gA s i l lu s t r a te d in F ig . 4 .9 , th e s ta b i l i ty o f 3 w t% A u /C e 0 2 - F e 2 0 3

c a lc in e d a t 4 0 0 ° c w ith o u t c>2 p r e tr e a tm e n t w a s te s te d fo r S R M a t a H 2 O /C H 3 O H m o la r r a t io o f 2/1 a t 4 0 0 °c f o r 10 h . T h e m e th a n o l c o n v e r s io n a n d h y d ro g e n y ie ld w e re fo u n d to d r o p r a p id ly f ro m 100 % a n d 7 4 % ( a t th e in i t ia l t im e ) to th e a v e ra g e v a lu e s o f 2 5 .9 % a n d 19 .3 % , r e s p e c tiv e ly , a f te r 9 6 m in u te s . T h e s e le c t iv i ty o f FI2 , C O , a n d C O 2 g a s e s w e re 7 5 .5 , 2 0 .5 , a n d 5 % , r e s p e c t iv e ly . T h e d e c l in e in c o n v e r s io n c o u ld b e a t t r ib u te d to th e c o k e fo rm a t io n [2 9 ] v ia th e c a r b o n m o n o x id e d is p r o p o r t io n a t io n r e a c t io n o r th e r a p id c o n s u m p t io n o f s u r f a c e - a d s o rb e d o x y g e n d u r in g th e e x p e r im e n t . I n a d d i t io n , it is w e ll k n o w n th a t g o ld - b a s e d c a ta ly s ts a re m o r e s u s c e p t ib le to th e rm a l s in te r in g th a n o th e r c o m m o n ly u s e d c a ta ly s ts , r e s u lt in g in th e s in te r in g o f th e g o ld c r y s ta l l i te s , a n d c o n s e q u e n t lo s s o f c a ta ly t ic a c tiv ity .

81

0 60 120 180 240 300 360 420 480 540 600

T im e o n s t r e a m (m in .)

Figure 4.9 S ta b i l i ty o b s e r v a t io n o f 3 w t% A u / C e l F e l c a ta ly s t c a lc in e d a t 4 0 0 °c. ( R e a c t io n c o n d i t io n s : H 2 O /C H 3 O H , 2 /1 ; r e a c t io n te m p e r a tu r e , 4 0 0 ° C ; a n d t im e -o n - s t r e a m p e r s a m p le , 1 0 h )

T o a n a ly z e th e a m o u n t o f c o k e d e p o s i te d o n th e c a ta ly s t s u r fa c e , th e T P O p r o f i l e s o f s p e n t c a ta ly s ts , a s i l lu s t ra te d in F ig . 4 .1 0 , s h o w tw o d i s t in c t p e a k s o f c o k e f o rm e d : i) lo w - te m p e r a tu re o x id a t io n a t 2 8 5 , 3 1 2 , a n d 3 2 0 °c a s s ig n e d to th e o x id a t io n o f th e p o o r ly p o ly m e r iz e d c o k e d e p o s i te d o n th e m e ta l p a r t ic le s a n d ii) h ig h - te m p e r a tu r e o x id a t io n a t 4 9 2 , 5 1 2 , a n d 5 9 3 ° c a t t r ib u te d to h ig h ly p o ly m e r iz e d c o k e d e p o s i te d n e a r th e m e ta l - s u p p o r t in te rp h a s e [6 2 ] . T h e ty p e s o f c o k e fo rm e d o n

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th e c a ta ly s t s u r fa c e h a v e b e e n c la s s i f ie d b a s e d o n th e r e a c t io n te m p e ra tu re a n d th e ty p e o f r e a c t io n [63-65]. In th is w o rk , th e r e s u lt s r e v e a le d th a t th e a m o u n t o f c o k e d e p o s i te d o n th e m e ta l w a s h ig h e r th a n th a t o n th e s u p p o rt , a n d th e c o k e p e rc e n ta g e in th e s p e n t c a ta ly s t w a s 2.60 w t% . H o w e v e r , th e f re s h c a ta ly s t s h o w e d th e lo w e s t te m p e ra tu re a t 106 °c, w h ic h w a s r e la te d to th e o x id a t io n o f c a r b o n a te s p e c ie s , n o t th e c o k e [ 1 1 ].

T e m p e r a t u r e ( ° C )

Figure 4.10 T P O p r o f i le s o f s p e n t 3 w t% A u / C e l F e l c a ta ly s t a f te r e x p o s u re to r e a c t io n c o m p a re d w i th th e f re s h c a ta ly s t . ( R e a c t io n c o n d i t io n s : H 2 O /C H 3 O H , 2 /1 ; r e a c t io n te m p e ra tu re , 4 0 0 ๐C ; a n d t im e - o n - s t r e a m p e r s a m p le , 10 h )

T h e X R D p a t te r n s o f th e s p e n t c a ta ly s ts a re p r e s e n te d in F ig . 4 . ID . T h e u n c h a n g e d C eC b d i f f r a c t io n p e a k s w e re d e te c te d a n d s o m e d i f f r a c t io n p e a k s o f a - F e iC b d is a p p e a re d , im p ly in g th a t th e c o k e p a r t i a l ly c o v e re d th e a - F e 2 0 3 s u r fa c e . In a d d i t io n , s o m e a n g le s o f a - F e 2 C>3 (3 5 .6 ° a n d 6 2 .6 ° ) s e e m e d to le a d to im p ro v e d c r y s ta l l in i ty , w h ic h p o s s ib ly c a m e f ro m th e s in te r in g o f F e 2 0 3 p a r t i c le s d u r in g th e s ta b i l i ty te s t . O n th e o th e r h a n d , th is m ig h t r e p r e s e n t th e s e g r e g a t io n o f th e fre e F e 2 Û 3 p a r t ic le s f ro m th e s o l id s o lu t io n p h a s e , in d ic a t in g th e la c k o f s o l id s o lu t io n d u r in g th e S R M r e a c t io n . P a r t ic u la r ly , th e p e a k o f c a rb o n i r o n c h e m ic a l c o m p o u n d s

83

( F e xC ) w a s a lso v i s ib le . T h is a p p e a ra n c e c o u ld c o n f i rm th a t c o k e r e m a in e d o n th e s p e n t c a ta ly s t s u r fa c e . T o s u p p o r t th e a b o v e e x p la n a tio n . K n o g z h a i et al. a lso d e f in e d th e F e 3C as m a s s iv e c a r b o n d e p o s i t io n s o n th e s p e n t c a ta ly s t , d e te c te d b y a n X R D m a c h in e [20 ]. A c c o rd in g ly , th e fo rm a t io n o f Fexc in o u r w o rk m ig h t b e th e c o n s e q u e n c e o f th e r e d u c t io n o f F e 2 0 3 v ia th e d e p o s i te d c a r b o n , o r v ia th e H 2

p r o d u c t g as , f o l lo w e d b y th e c a rb o n in te r a c t io n d u r in g th e r e a c t io n . In c o n c lu s io n , w e s p e c u la te d th a t th e fa c to rs fo r th e A u /C e 0 2- F e 2 0 3 d e a c t iv a t io n w e re m a in ly f ro m th e la c k o f th e h o m o g e n e o u s s o l id s o lu tio n p h a s e a n d c o k e d e p o s i t io n .

4.5 Conclusions

T h e A u /C e 0 2- F e 2 0 3 c a ta ly s ts h a v e b e e n d e v e lo p e d fo r S R M to p ro d u c e h ig h p u r i ty Ha in th e te m p e ra tu re ra n g e o f 200 to 400 °c. I n i t ia l c a ta ly t ic a c t iv i ty w as s u c c e s s fu l ly a c h ie v e d b y m ix in g F e 2 C>3 a n d C e 0 2 e q u a lly w i th th e C e :F e a to m ic ra tio o f 1:1 to fo rm th e a c t iv e s o l id s o lu t io n p h a s e . M o d e r a te a m o u n ts o f A u d e p o s i te d (3 w t% ) o n th e m ix e d o x id e s s u p p o r t c a lc in e d a t 400 ๐c s e e m e d to b e th e o p tim u m c o n d i t io n w ith o u t p r o v id in g A u s in te r in g . N e v e r th e le s s , th e s in te r in g o f h e m a ti te p a r t i c le s w a s fu r th e r c o n s id e re d a s o n e o f th e p a r a m e te r s fo r th e lo s s in a c t iv i ty o f th e A u /C e 0 2- F e 2 0 3 c a ta ly s t , w h ile A u s in te r in g w a s n o t in v o lv e d in th is c a se . O u r f in d in g s s u g g e s t th a t th e r e d u c ib i l i ty o f th e s u p p o r t m a y a f f e c t th e c a ta ly s t p ro p e r t ie s a n d th e c a ta ly t ic a c t iv i t ie s fo r S R M .

4.6 Acknowledgements

T h e a u th o r s a c k n o w le d g e th e c o n tr ib u t io n s a n d f in a n c ia l s u p p o r t o f th e f o l lo w in g o r g a n iz a t io n s : th e T h a i la n d R e s e a r c h F u n d th r o u g h th e R o y a l G o ld e n J u b i le e P h .D . P ro g r a m (G ra n t N o . P H D /0 2 8 2 /2 5 5 2 ) ; th e N a t io n a l C e n te r o f E x c e l le n c e fo r P e t r o le u m , P e tro c h e m ic a ls , a n d A d v a n c e d M a te r ia ls , C h u la lo n g k o m U n iv e r s i ty ; a n d T h e N a t io n a l R e s e a rc h U n iv e r s i ty P r o je c t o f C H E a n d th e R a tc h a d a p h is e k s o m p h o t E n d o w m e n t F u n d ( E N 2 7 6 B ) .

84

4.7 References

[1] U d a n i P P C , G u n a w a rd a n a P V D S , L e e H C , K im D H . S te a m r e fo rm in g a n d o x id a t iv e s te a m r e fo rm in g o f m e th a n o l o v e r C u O - C e O i c a ta ly s ts . In t. J. H y d ro g e n E n e r g y 2 0 0 9 ; 3 4 : 7 6 4 8 - 5 5 .

[2] P a te l ร , P a n t K K . K in e t ic m o d e l in g o f o x id a t iv e s te a m r e fo rm in g o f m e th a n o l o v e r C rP Z n O /C e C h /A L C L c a ta ly s t . A p p l . C a ta l ., A 2 0 0 9 ; 3 5 6 : 1 8 9 - 2 0 0 .

[3] M a n z o l i M , C h io r in o A , B o c c u z z i F . D e c o m p o s i t io n a n d c o m b in e d re fo rm in g o f m e th a n o l to h y d ro g e n : a F T IR a n d Q M S s tu d y o n C u a n d A ll c a ta ly s ts s u p p o r te d o n Z n O a n d T iC h . A p p l. C a ta l . , B 2 0 0 4 ; 5 7 : 2 0 1 - 9 .

[4] L iu N , Y u a n z, W a n g ร , Z h a n g c, W a n g ร , L i D . C h a ra c te r iz a t io n a n d p e r f o n n a n c e o f a Z n O -Z n C i^ C V C e C L -Z rC L m o n o l i th ic c a ta ly s t fo r m e th a n o l a u to - th e n n a l r e fo rm in g p ro c e s s . In t. J . H y d ro g e n E n e r g y 2 0 0 8 ; 3 3 : 1 6 4 3 -5 1 .

[5] H o n g X , R e n ร . S e le c t iv e h y d r o g e n p r o d u c t io n f ro m m e th a n o l o x id a t iv e s te a m re fo rm in g o v e r Z n - C r c a ta ly s ts w i th o r w i th o u t C u lo a d in g . J. M o l. C a ta l. A : C h e m . 2 0 0 8 ; 3 3 : 7 0 0 - 8 .

[6 ] S h ish id o T , Y a m a m o to Y , M o r io k a H , T a k e h ir a K . P r o d u c t io n o f h y d ro g e n f ro m m e th a n o l o v e r C u /Z n O a n d C u /Z n O /A F O i c a ta ly s ts p r e p a r e d by h o m o g e n e o u s p re c ip i ta t io n : S te a m r e fo rm in g a n d o x id a t iv e s te a m re fo rm in g . J. M o l. C a ta l. A : C h e m . 2 0 0 7 ; 2 6 8 : 1 8 5 - 9 4 .

[7] P a te l ร , P a n t K K . H y d r o g e n p r o d u c t io n b y o x id a t iv e s te a m re fo m i in g o f m e th a n o l u s in g c e r ia p r o m o te d c o p p e r - a lu m in a c a ta ly s ts . F u e l P ro c e s s . T e c h n o l . 2 0 0 7 ; 8 8 : 8 2 5 - 3 2 .

[ 8 ] N a k n a m P , L u e n g n a ru e m i tc h a i A , W o n g k a s e m ji t ร . P re f e re n t ia l C O o x id a tio n o v e r A u /Z n O a n d A u /Z n 0 - F e 2 0 3 c a ta ly s ts p re p a re d b y p h o to d e p o s i t io n . In t. J. H y d ro g e n E n e r g y 2 0 0 9 ; 3 4 : 9 8 3 8 - 4 6 .

[9] Y a n g Y F , S a n g e e th a p , C h e n Y W . A u /T i0 2 c a ta ly s ts p r e p a r e d b y p h o to ­d e p o s i t io n m e th o d fo r s e le c t iv e C O o x id a t io n in H i s tr e a m . In t. J . H y d ro g e n E n e rg y 2 0 0 9 ; 3 4 : 8 9 1 2 - 2 0 .

[1 0 ] H u a n g T J , C h e n H M . H y d r o g e n p r o d u c t io n v ia s te a m r e f o m i in g o f m e th a n o l o v e r C u /( C e ,G d ) 0 2 -x c a ta ly s ts . In t. J. H y d r o g e n E n e r g y 2 0 1 0 ; 35 : 6 2 1 8 - 2 6 .

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[1 1 ] Y i N , S i R , S a l ts b u rg H . S te p h a n o p o u lo s M F . S te a m r e fo rm in g o f m e th a n o l o v e r c e r ia a n d g o ld -c e r ia n a n o s h a p e s . A p p l . C a ta l.. B 2 0 1 0 ; 9 5 : 8 7 - 9 2 .

[1 2 ] T w ig g , M V , S p e n c e r M S . D e a c t iv a t io n o f c o p p e r m e ta l c a ta ly s ts f o r m e th a n o l d e c o m p o s i t io n m e th a n o l s te a m r e f o n n in g a n d m e th a n o l s y n th e s is . T o p . C a ta l. 2 0 0 3 ; 2 2 : 1 9 1 - 2 0 3 .

[1 3 ] H o n g X , R e n ร . S e le c t iv e h y d ro g e n p r o d u c t io n f ro m m e th a n o l o x id a t iv e s te a m r e fo rm in g o v e r Z n -C r c a ta ly s ts w i th o r w i th o u r t C u lo a d in g . In t. J . H y d ro g e n E n e rg y 2 0 0 8 ; 3 3 : 7 0 0 - 8 .

[1 4 ] G rise l R , พ e s ts t ra te K J , G lu h o i A , N ie u w e n h u y s B E . C a ta ly s is b y g o ld n a n o p a r t ic le s , G o ld B u ll. 2 0 0 2 ; 3 5 : 3 9 - 4 5 .

[1 5 ] L e n i te B A , G a l le t t i c, S p e c c h ia ร . S tu d ie s o n A ll c a ta ly s ts fo r w a te r g a s sh if t r e a c tio n . In t. J . H y d r o g e n E n e rg y 2 0 1 1 ; 3 6 : 7 7 5 0 -8 .

[16] B o n d G C , L o u is c, T h o m p s o n D T . C a ta ly s is b y G o ld , v o lu m e 6, Im p e ria l C o lle g e P re s s , L o n d o n , 2006.

[17] P o ja n a v a ra p h a n c, L u e n g n a ru e m itc h a i A , G u la r i E . H y d r o g e n P ro d u c t io n b y O x id a tiv e S te a m R e f o r m in g o f m e th a n o l o v e r A u /C e 0 2 c a ta ly s ts . C h e m . E n g .J . 2 0 1 2 ; 19 2 : 1 0 5 -1 3 .

[1 8 ] S r is ir iw a t N , T h e r d th ia n w o n g ร , T h e r d th ia n w o n g A . O x id a t iv e s te a m r e fo rm in g o f e th a n o l o v e r N i /A L 0 3 c a ta ly s ts p r o m o te d b y CeC>2 , Z rO i a n d C e C L -Z rC L . In t. J . H y d r o g e n E n e r g y 2 0 0 9 ; 34 : 2 2 2 4 - 3 4 .

[1 9 ] B e r a p , H e g d e M S . C h a ra c te r iz a t io n a n d c a ta ly t ic p r o p e r t ie s o f c o m b u s t io n s y n th e s iz e d A u /C e 0 2 - C a ta l . L e tt. 2 0 0 2 ; 7 9 : 7 5 - 8 1 .

. [2 0 ] K o n g z h a i L I , H u a พ , Y o n g g a n g พ , M in g c h u n L . P re p a ra t io n a n d c h a r a c te r iz a t io n o f C e i-xF e x 0 2 c o m p le x o x id e s a n d its c a ta ly t ic a c t iv i ty fo r m e th a n e s e le c t iv e o x id a t io n . J . R a re E a r th s 2 0 0 8 ; 2 6 : 2 4 5 - 9 .

[2 1 ] K a m im u r a Y , S a to ร , T a k a h a s h i R , S o d e s a w a T , A k a s h i T . S y n th e s is o f 3 - p e n ta n o n e f ro m 1- p ro p a n o l o v e r C e 0 2 - F e 2 0 3 c a ta ly s ts . A p p l . C a ta l . , A 2 0 0 3 ; 2 5 2 : 3 9 9 - 4 1 0 .

[2 2 ] M a lu f ss , N a s c e n te P A P , A s s a f E M . C u O a n d C u O - Z n O c a ta ly s ts s u p p o r te d o n C e 0 2 a n d C e 0 2 - L a 0 3 f o r lo w te m p e ra tu re w a t e r - g a s s h if t r e a c t io n . F u e l P ro c . T e c h . 2 0 1 0 ; 9 1 : 1 4 3 8 -4 5 .

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[2 3 ] P ra d h a n G K , P a n d a K M . F a b r ic a t io n o f i r o n -c e r iu m m ix e d o x id e : a n d e f f ic ie n t p h o to c a ta ly s t fo r d y e d e g ra d a t io n . In t. J . E n g . S c i. T e c h n o l . 2 0 1 0 ; 2 : 5 3 -6 5 .

[2 4 ] N e r i G , B o n a v i ta A , R iz z o G , G a lv a g n o ร , C a p o n e ร , S ic i l ia n o p . M e th a n o l g a s - s e n s in g p r o p e r t ie s o f C e 0 2 - F e 2 0 3 th in f i lm s . S e n s . A c tu a to r s , B 2 0 0 6 ; 114: 6 8 7 - 9 5 .

[2 5 ] T a b a k o v a T , A v g o u r o p o u lo s G , P a p a v a s i l io u J , M a n z o l i M , B o c c u z z i F , T e n c h e v K , V in d ig n i F , lo a n n id e s T . C O -f re e h y d r o g e n p r o d u c t io n o v e r A u /C e 0 2 - F e 2 0 3 c a ta ly s ts : P a r t 1. Im p a c t o f th e s u p p o r t c o m p o s i t io n o n th e p e r f o rm a n c e fo r th e p r e f e re n t ia l C O o x id a t io n r e a c t io n . A p p l. C a ta l . , B 2 0 1 1 ; 101: 2 5 6 - 6 5 .

[2 6 ] T a b a k o v a T , M a n z o l i M , P a n e v a D , B o c c u z z i F , I d a k ie v V , M i to v I. C O -f re e h y d ro g e n p r o d u c t io n o v e r A u /C e 0 2 - F e 2 0 3 c a ta ly s ts : P a r t 2 . Im p a c t o f th e s u p p o r t c o m p o s i t io n o n th e p e r f o rm a n c e in th e w a te r - g a s s h i f t r e a c t io n . A p p l. C a ta l ., B 2 0 1 1 ; 1 0 1 :2 6 6 - 7 4 .

[2 7 ] W id m a n n D , L iu Y , S c h ü th F , B e h m R J . S u p p o r t e f f e c ts in th e A u -c a ta ly z e d C O o x id a t io n - C o r r e l a t io n b e tw e e n a c t iv i ty , o x y g e n s to ra g e c a p a c ity , a n d s u p p o r t r e d u c ib i l i ty . J . C a ta l . 2 0 1 0 ; 2 7 6 : 2 9 2 - 3 0 5 .

[2 8 ] K u n m in g J , H u il i Z , W e n c u i L . E f f e c t o f m o r p h o lo g y o f th e c e r ia s u p p o r t o n th e a c t iv i ty o f A u /C e 0 2 c a ta ly s ts fo r C O o x id a t io n . C h in . J. C a ta l . 2 0 0 8 ; 2 9 : 1 0 8 9 -9 2 .

[2 9 ] K im C H , T h o m p s o n L T . D e a c t iv a t io n o f A u /C e O x w a te r g a s s h i f t c a ta ly s ts . J . C a ta l. 2 0 0 5 ; 2 3 0 : 6 6 - 7 4 .

[3 0 ] C h a n g F W , R o s e l in L S , O u T C . H y d r o g e n p r o d u c t io n b y p a r t ia l o x id a t io n o f m e th a n o l o v e r b im e ta l l ic A u -R u /F e 2 Û 3 c a ta ly s ts . A p p l . C a ta l . , A 2 0 0 8 ; 3 3 4 : 1 4 7 -5 5 .

[3 1 ] J ia n g X C , Y u A B . S y n th e s is o f P d / a - F e 2 0 3 n a n o c o m p o s i te s fo r c a ta ly t ic C O o x id a t io n . J . M a te r . P ro c e s s . T e c h n o l . 2 0 0 9 ; 2 0 9 : 4 5 5 8 - 6 2 .

[3 2 ] H o n g y a n L IN , Z h iq ia n g M A , L in g D IN G , J ie s h a n Q IU , C h a n g h a i L IA N G . P re p a ra t io n o f n a n o s c a le C e xF e i_x 0 2 s o l id s o lu t io n c a ta ly s t b y th e te m p la te m e th o d a n d Its c a ta ly t ic p r o p e r t ie s f o r e th a n o l s te a m re fo rm in g . C h in . J . C a ta l. 2 0 0 8 ; 2 9 : 4 1 8 - 2 0 .

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[3 3 ] A n d r e e v a D , T a b a k o v a T , I d a k ie v V , C h r is to v P . G io v a n o l i R . A u /a -F e iC L c a ta ly s t fo r w a t e r - g a s s h if t r e a c t io n p re p a re d b y d e p o s i t io n - p r e c ip i ta t io n . A p p l. C a t a l , A 1 9 9 8 ; 169: 9 - 1 4

[3 4 ] H a r u ta M , K o b a y a s h i T . I i j im a ร , D e la n n a y F . P ro c e e d in g 9 th In t. C o n g r . C a ta l . , C a lg a r y , T h e C h e m ic a l In s t i tu te o f C a n a d a , O tta w a , 1 9 8 8 ; 2 : 1206 .

[3 5 ] F a n L , I c h ik u n i N , S h im a z u ร , U e m a ts u T . P re p a ra t io n o f A u /T iO i c a ta ly s ts b y s u s p e n s io n s p ra y r e a c t io n m e th o d a n d th e ir c a ta ly t ic p ro p e r ty fo r C O o x id a tio n . A p p l. C a ta l . , A 2 0 0 3 ; 2 4 6 : 8 7 - 9 5 .

[3 6 ] A r e n a F , F a m u la r i p , T ru n f io G , B o n u ra G , F ru s te r i F , S p a d a ro L. P ro b in g th e f a c to rs a f f e c t in g s t r u c tu r e a n d a c t iv i ty o f th e A u /C eC L s y s te m in to ta l a n d p r e f e r e n t ia l o x id a t io n o f C O . A p p l . C a ta l ., B 2 0 0 6 ; 6 6 : 8 1 - 9 1 .

[3 7 ] B is w a s p , K u n z r u D . S te a m r e fo rm in g o f e th a n o l f o r p r o d u c t io n o f h y d r o g e n o v e r N i /C e 0 2 - Z r 0 2 c a ta ly s t : E f f e c t o f s u p p o r t a n d m e ta l lo a d in g . In t. J . H y d r o g e n E n e r g y 2 0 0 7 ; 3 2 : 9 6 9 - 8 0 .

[3 8 ] X in g Z-, H u a พ , Y o n g g a n g พ , K o n g z h a i L , X ia n m in g c. H y d r o g e n a n d s y n g a s p r o d u c t io n f ro m tw o - s te p s te a m r e fo rm in g o f m e th a n e o v e r C e 0 2 - F e 2 Ü ; o x y g e n c a r r ie r . J . R a re E a r th s 2 0 1 0 ; 2 8 : 9 0 7 - 1 3 .

[3 9 ] J a c o b s G . R ic o te ร , P a t te r s o n P M , G ra h a m U M , D o z ie r A, K h a lid ร . R h o d u s E,

D a v is B H . L o w te m p e ra tu re w a te r - g a s sh ift: E x a m in in g th e e f f ic ie n c y o f A ll as a p r o m o te r f o r c e r ia -b a s e d c a ta ly s ts p re p a re d b y C V D o f a A ll p re c u r s o r . A p p l. C a ta l . , A 2 0 0 5 ; 2 9 2 : 2 2 9 - 4 3 .

[4 0 ] J a c o b s G , G r a h a m U M , C h e n u E , P a t te r s o n P M , D o z ie r A , D a v is B H . L o w - te m p e r a tu r e w a t e r - g a s sh if t : im p a c t o f P t p r o m o te r lo a d in g o n th e p a r tia l r e d u c t io n o f c e r ia a n d c o n s e q u e n c e s fo r c a ta ly s t d e s ig n . J. C a ta l . 2 0 0 5 ; 2 2 9 :4 9 9 - 5 1 2 .

[41] C a m p o B, P e ti t c, V o lp e M A . H y d r o g e n a t io n o f c ro to n a ld e h y d e o n d i f f e re n t A u /C e C L c a ta ly s ts . J . C a ta l . 2 0 0 8 ; 2 5 4 : 7 1 -8 .

[4 2 ] I l ie v a L I , A n d r e e v a D H , A n d r e e v A A . T P R a n d T P D in v e s t ig a t io n o f A u /a - F e iC ft. T h e r m o c h im . A c ta 1 9 9 7 ; 2 9 2 : 1 6 9 -7 4 .

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[4 4 ] K h o u d ia k o v M , G u p ta M C , D e e v i ร . A u /F e 2 C>3 n a n o c a ta ly s ts fo r C O o x id a tio n : A c o m p a ra t iv e s tu d y o f d e p o s i t io n - p r e c ip i ta t io n a n d c o p re c ip i ta t io n te c h n iq u e s . A p p l. C a ta l ., A 2 0 0 5 ; 2 9 1 : 151-61.

[4 5 ] K o n g z h a i L , H u a พ , Y o n g g a n g พ , D o n g x ia Y . S y n g a s p ro d u c t io n f ro m m e th a n e a n d a ir v ia a r e d o x p ro c e s s u s in g C e - F e m ix e d o x id e s a s o x y g e n c a r r ie r s . A p p l . C a ta l . , B 2 0 1 0 ; 9 7 : 3 6 1 - 7 2 .

[4 6 ] K o n g z h a i L , H u a พ , Y o n g g a n g พ , D o n g x ia Y . T ra n s fo rm a t io n o f m e th a n e in to s y n th e s is g a s u s in g th e r e d o x p ro p e rty o f C e - F e m ix e d o x id e s : E f fe c t o f c a lc in a t io n te m p e ra tu re . In t. J . H y d ro g e n E n e r g y 2 0 1 1 ; 3 6 : 3 4 7 1 - 8 2 .

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[4 8 ] S h a n พ , L u o M , Y in g p . S h e ll พ , L i c. R e d u c t io n p r o p e r ty a n d c a ta ly t ic a c t iv i ty o f C e i - \ N i x 0 2 m ix e d o x id e c a ta ly s ts f o r C H 4 o x id a t io n . A p p l. C a t a l , A 2 0 0 3 ; 2 4 6 : 1 -9 .

[4 9 ] F u Q , K u d r ia v ts e v a ร , S a l ts b u rg H , S te p h a n o p o u lo s M F . G o ld - c e r ia c a ta ly s ts fo r lo w - te m p e r a tu r e w a te r - g a s s h i f t re a c tio n . C h e m . E n g . J. 2 0 0 3 ; 9 3 : 4 1 - 5 3 .

[5 0 ] H u a n g x s , รนท H , W a n g L C , L iu Y M , F an K N . C a o Y . M o rp h o lo g y e f fe c ts o f n a n o s c a le c e r ia o n th e a c t iv i ty o f A u /C e C b c a ta ly s ts fo r lo w - te m p e r a tu re C O o x id a t io n . A p p l. C a ta l . , B 2 0 0 9 ; 2 3 9 : 2 2 4 - 3 2 .

[5 1 ] A n d r e e v a D , I d a k ie v V , T a b a k o v a T , A n d re e v A . L o w - te m p e ra tu r e w a te r g a s s h i f t r e a c t io n o v e r A u /a - F e 2 0 3 . J . C a ta l. 1 9 96 ; 158 : 3 5 4 - 5 .

[5 2 ] M u n te a n u G , I l ie v a L , A n d r e e v a D . K in e tic p a r a m e te r s o b ta in e d f ro m T P R d a ta f o r a - F e 2 0 3 a n d A u /a - F e 2 0 3 s y s te m s . T h e rm o c h im . A c ta 1 9 9 7 ; 2 9 1 : 1 7 1 -7 7 .

[53] S c ir è ร , M in ic ô ร , C r is a fu l l i c , G a lv a g n o ร . C a ta ly t ic c o m b u s t io n o f v o la t i le o r g a n ic c o m p o u n d s o v e r g ro u p IB m e ta l c a ta ly s ts o n F e 2 0 3 . C a ta l . C o m m u n . 2 0 0 1 ; 2 : 2 2 9 - 3 2 .

[5 4 ] A n e g g i E , L e i te n b u rg c, D o lc e t t i G , T ro v a re l l i A . P ro m o tio n e f fe c t o f r a re e a r th s a n d t r a n s i t io n m e ta ls in th e c o m b u s tio n o f d ie s e l s o o t o v e r C eC T a n d C e 0 2 - Z r 0 2 . C a ta l . T o d a y 2 0 0 6 ; 114: 4 0 -7 .

[5 5 ] L i G , S m ith R L , J r , I n o m a ta H . S y n th e s is o f N a n o s c a le C e i-xF e x 0 2 S o lid S o lu t io n s v ia a lo w - te m p e r a tu re a p p ro a c h . J . A m . C h e m . S o c . 2 0 0 1 ; 123: 1 1 0 9 1 - 2 .

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[57] Z a n e l la R , G io rg io ร , S h in C H , I-ienry C R , L o u is c. C h a ra c te r iz a t io n a n d r e a c t iv i ty in C O o x id a t io n o f g o ld n a n o p a r t ic le s s u p p o r te d o n T iO l p re p a re d b y d e p o s i t io n - p r e c ip i ta t io n w ith N a O H a n d u re a . J . C a ta l . 2 0 0 4 ; 2 2 2 : 357-67.

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[5 9 ] P a rk ร , Y o o K , P a rk H J. L e e J C , L e e J H . R a p id g o ld io n r e c o v e r y f ro m w a s te w a te r b y p h o to c a ta ly t ic Z n O n a n o p o w d e r s . J . E le c t ro c e ra m 2 0 0 6 ; 17: 8 3 1 - 3 4 .

[60] M e n g M , T u Y , D in g T , รนท z, Z h a n g L . E f f e c t o f s y n th e s is p H a n d A u lo a d in g o n th e C O p re fe re n tia l o x id a t io n p e r f o rm a n c e o f A u /M n O x- C e 0 2

c a ta ly s ts p r e p a r e d w ith u l t r a s o n ic a s s is ta n c e . In t. J . H y d r o g e n E n e rg y 2 0 1 1 ; 36 : 9 1 3 9 - 5 0 .

[6 1 ] H a r u ta M , T s u b o ta ร , K o b a y a s h i T , K a g e y a m a H , G e n e t M , D e lm o n B . L o w -

T e m p e r a tu r e O x id a t io n o f C O o v e r G o ld S u p p o r te d o n T i 0 2 , a - F e 2 0 3 , a n d C 0 3 O 4 . J . C a ta l . 1 9 9 3 ; 144: 1 7 5 - 9 2 .

[6 2 ] L u e n g n a ru e m itc h a i A , K a e n g s i la la i A . A c t iv i ty o f d i f f e r e n t z e o l i te - s u p p o r te d N i c a ta ly s ts fo r m e th a n e r e fo rm in g w ith c a r b o n d io x id e . C h e m . E n g . J. 2 0 0 8 ; 144 : 9 6 - 1 0 2 .

[6 3 ] G u is n e t M , M a g n o u x p . O rg a n ic c h e m is t ry o f c o k e f o rm a t io n . A p p l. C a ta l . , A 2 0 0 1 ; 2 1 2 : 8 3 - 9 6 .

[6 4 ] F a u n g n a w a k i j K , K ik u c h i R , E g u c h i K . T h e r m o d y n a m ic e v a lu a t io n o f m e th a n o l s te a m r e fo rm in g f o r h y d ro g e n p r o d u c t io n . J . P o w e r S o u rc e s 2 0 0 6 ; 161 : 8 7 - 9 4 .

[6 5 ] A r m o r .IN. T h e m u l t ip le ro le s fo r c a ta ly s is in th e p r o d u c t io n o f H 2 . A p p l. C a ta l . , A 2 0 0 8 ; 176 : 1 5 9 -7 6 .