Evolution of Copper Supported on Fe 3 O 4 Nanorods for WGS Reaction

: Rod-shaped Cu 1 Fe 9 O x precursor was successfully prepared through an aqueous precipitation method. The shape and phase composition were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). It was found that Cu 1 Fe 9 O x is composed of CuFe 2 O 4 and Fe 2 O 3 . The reduction performance of Cu 1 Fe 9 O x was studied by in situ XRD and H 2 temperature-programmed reduction (H 2 -TPR). Cu/Fe 3 O 4 nanorod catalyst is obtained through the controllable reduction of Cu 1 Fe 9 O x nanorod, and the formed Cu/Fe 3 O 4 nanorod catalyst does not have low-temperature water gas shift (WGS) activity, but exhibits high-temperature WGS reaction activity. Ambient pressure X-ray photoelectron spectroscopy (AP-XPS) studies showed that the main species of copper is Cu + during the WGS reaction. The interaction between Cu and Fe 3 O 4 rod and phase evolution of Cu species are quite different from Cu/Fe 3 O 4 nanoparticles.


Introduction
Hydrogen is the most promising clean fuel to satisfy energy needs in the future [1]. At present, nearly 95% of the hydrogen supply is produced from reforming crude oil, coal, natural gas, wood, organic wastes, and biomass [2,3]. The water gas shift (WGS) reaction (CO + H 2 O → CO 2 + H 2 ) is essential for hydrogen generation from fuel gas upgrading processes [4]. The reaction has a high reaction rate at high temperature, while it has high conversion of CO at low temperature. The WGS reaction has been studied extensively due to its industrial importance [5][6][7]. Metal and metal oxide catalysts, such as copper, mostly Cu/ZnO/Al 2 O 3 , are typically used for low-temperature water gas shift (LT-WGS) reactions (180-250 • C). Contrarily, iron-based catalysts can promote the water gas shift reaction at moderately high temperatures (350-450 • C), which are always regarded as high-temperature water shift catalysts (HT-WGS).
It was reported that Fe 3 O 4 modified with CuO can improve its HT-WGS activity. Andreev et al. [8] studied the CuO (5 wt.%) effects on Fe-Cr catalysts, and found that the Cu-containing sample showed the best activity at 380 • C. Furthermore, it was found that metallic Cu is the main species for the HT-WGS reaction [9]. Interestingly, Cu is also an active species in the LT-WGS reaction, and the properties of its supports have great influence on its catalytic activity. Supports play very important roles in preventing the sintering of Cu, such as Cu/ZnO, Cu/CeO 2 and Cu/Fe 3 O 4 [10][11][12]. Recently, Cu/Fe 3 O 4 catalysts gain great attention because Cu and Fe 3 O 4 are active phases in LT-and HT-WGS reactions, respectively. However, there is still a lack of studies on the shape effects of Fe 3 O 4 support. Additionally, the precursors of copper are also very important for the stability and activity of copper species. Kameoka et al. [13] proposed that spinel CuFe 2 O 4 was an effective precursor for a high-performance copper catalyst, which showed high thermal stability and activity.
In this study, we investigated the WGS reaction activities over Cu/Fe 3 O 4 nanorod catalyst, which was synthesized through the controllable reduction of rod-shaped Cu 1 Fe 9 O x precursor. Compared with Cu/Fe 3 O 4 nanoparticles, Cu/Fe 3 O 4 rod catalyst exhibited very low activity. The evolution of surface species and structure chemistry during the WGS reaction were studied in detail by employing various characterizations. Moreover, the ambient pressure X-ray photoelectron spectrometer (AP-XPS) patterns indicate the main species of copper over rod-shaped Fe 3 O 4 is Cu + during WGS reaction and the obtained Cu/Fe 3 O 4 nanorod exhibits high activity.

Structure and Morphology of Cu 1 Fe 9 O x Nanorods Catalyst
Phase composition and shape of freshly synthesized samples were characterized by XRD and TEM. Figure 1 shows the XRD patterns of uncalcined and calcined precursors. Distinctive diffraction lines are clearly identified and all peaks are in good agreement with the XRD patterns of CuO (PDF No. 45-0937), β-FeOOH (PDF No. 34-1266), α-Fe 2 O 3 (PDF No. 89-0597) and CuFe 2 O 4 (PDF No. 34-0425). It is obvious that the uncalcined sample is composed of β-FeOOH and CuO, while the calcined sample is composed of CuFe 2 O 4 and α-Fe 2 O 3 . This means that CuO reacted with β-FeOOH to produce CuFe 2 O 4 and excess β-FeOOH is transformed into α-Fe 2 O 3 during the calcination process. It is noteworthy that the CuO phase has not been detected in the calcined sample. It may exist as amorphous phase in the calcined precursor. Figure 2 shows the TEM images of the precursor. The uncalcined precursor displays a rod-like structure with diameter of 30-50 nm and length of 350-500 nm. After calcination, the precursor inherits the rod shape morphology. Interestingly, some mesopores appeared on Cu 1 Fe 9 O x nanorods because of the dehydration of β-FeOOH during the calcination process. The Cu:Fe ratio of the calcined precursor obtained from an ICP result is 1: 8.98, which is consistent with the feed ratios. Thus, the calcined sample is referred to as Cu 1 Fe 9 O x nanorod. The BET surface area of Cu 1 Fe 9 O x nanorod is 48 m 2 /g, which is much higher than Cu 1 Fe 9 O x particles (27 m 2 /g) due to the formed mesopores [14]. Compared with Cu 1 Fe 9 O x nanoparticles [15], it is noteworthy that the main copper species in Cu 1 Fe 9 O x nanoparticles is CuO, while CuFe 2 O 4 is the dominant copper species in Cu 1 Fe 9 O x nanorod. species. Kameoka et al. [13] proposed that spinel CuFe2O4 was an effective precursor for a high-performance copper catalyst, which showed high thermal stability and activity.
In this study, we investigated the WGS reaction activities over Cu/Fe3O4 nanorod catalyst, which was synthesized through the controllable reduction of rod-shaped Cu1Fe9Ox precursor. Compared with Cu/Fe3O4 nanoparticles, Cu/Fe3O4 rod catalyst exhibited very low activity. The evolution of surface species and structure chemistry during the WGS reaction were studied in detail by employing various characterizations. Moreover, the ambient pressure X-ray photoelectron spectrometer (AP-XPS) patterns indicate the main species of copper over rod-shaped Fe3O4 is Cu + during WGS reaction and the obtained Cu/Fe3O4 nanorod exhibits high activity.

Structure and Morphology of Cu1Fe9Ox Nanorods Catalyst
Phase composition and shape of freshly synthesized samples were characterized by XRD and TEM. Figure 1 shows the XRD patterns of uncalcined and calcined precursors. Distinctive diffraction lines are clearly identified and all peaks are in good agreement with the XRD patterns of CuO (PDF No. 45-0937), β-FeOOH (PDF No. 34-1266), α-Fe2O3 (PDF No. 89-0597) and CuFe2O4 (PDF No. 34-0425). It is obvious that the uncalcined sample is composed of β-FeOOH and CuO, while the calcined sample is composed of CuFe2O4 and α-Fe2O3. This means that CuO reacted with β-FeOOH to produce CuFe2O4 and excess β-FeOOH is transformed into α-Fe2O3 during the calcination process. It is noteworthy that the CuO phase has not been detected in the calcined sample. It may exist as amorphous phase in the calcined precursor. Figure 2 shows the TEM images of the precursor. The uncalcined precursor displays a rod-like structure with diameter of 30-50 nm and length of 350-500 nm. After calcination, the precursor inherits the rod shape morphology. Interestingly, some mesopores appeared on Cu1Fe9Ox nanorods because of the dehydration of β-FeOOH during the calcination process. The Cu:Fe ratio of the calcined precursor obtained from an ICP result is 1: 8.98, which is consistent with the feed ratios. Thus, the calcined sample is referred to as Cu1Fe9Ox nanorod. The BET surface area of Cu1Fe9Ox nanorod is 48 m 2 /g, which is much higher than Cu1Fe9Ox particles (27 m 2 /g) due to the formed mesopores [14]. Compared with Cu1Fe9Ox nanoparticles [15], it is noteworthy that the main copper species in Cu1Fe9Ox nanoparticles is CuO, while CuFe2O4 is the dominant copper species in Cu1Fe9Ox nanorod.

Reducibility of Cu1Fe9Ox Nanorod
The reducibility of the rod-shaped Cu1Fe9Ox precursor is investigated by H2-TPR as shown in Figure 3. The H2-TPR profile of pure CuO is also exhibited in Figure 3 for comparison. There are two distinctive reduction regions of the Cu1Fe9Ox nanorod sample, i.e., 50-300 °C and 300-550 °C. Three obvious reduction peaks centered at 113 °C, 225 °C and 266 °C, respectively, were observed in the range of 50-300 °C. Compared with pure CuO, the reduction of Cu species in the Cu1Fe9Ox precursor was obviously shifted to lower temperature. The peak centered at 113 °C should be attributed to the reduction of amorphous CuO, which cannot be detected by XRD. The higher peak at 225 °C is attributed to the reduction of Cu 2+ in CuFe2O4 and the peak centered at 266 °C is the reduction of Fe2O3 to Fe3O4 [16][17][18]. The following high-temperature reduction at 300-550 °C should be due to the reduction of Fe3O4 to Fe 0 [19].    When the reduction temperature rises to 250 °C, the main peak of CuFe2O4 at 35.1° shifts to a low degree, which is attributed to the peak of Fe3O4. In addition, the XRD peaks belonging to Fe2O3 decrease at 250 °C, and completely disappear after reduction at 300 °C for 1 h. Furthermore, after reducing at 300 °C for 2 h, only metallic Cu and Fe3O4 are observed, indicating that Cu1Fe9Ox nanorods are completely reduced to Cu and Fe3O4. Meanwhile, Cu1Fe9Ox nanoparticles can also be reduced to Cu and Fe3O4 at 250 °C [15]. These results reveal that the reduction of Cu1Fe9Ox nanorod

Reducibility of Cu 1 Fe 9 O x Nanorod
The reducibility of the rod-shaped Cu 1 Fe 9 O x precursor is investigated by H 2 -TPR as shown in Figure 3. The H 2 -TPR profile of pure CuO is also exhibited in Figure 3 for comparison. There are two distinctive reduction regions of the Cu 1 Fe 9 O x nanorod sample, i.e., 50-300 • C and 300-550 • C. Three obvious reduction peaks centered at 113 • C, 225 • C and 266 • C, respectively, were observed in the range of 50-300 • C. Compared with pure CuO, the reduction of Cu species in the Cu 1 Fe 9 O x precursor was obviously shifted to lower temperature. The peak centered at 113 • C should be attributed to the reduction of amorphous CuO, which cannot be detected by XRD. The higher peak at 225 • C is attributed to the reduction of Cu 2+ in CuFe 2 O 4 and the peak centered at 266 • C is the reduction of Fe 2 O 3 to Fe 3 O 4 [16][17][18]. The following high-temperature reduction at 300-550 • C should be due to the reduction of Fe 3 O 4 to Fe 0 [19].

Reducibility of Cu1Fe9Ox Nanorod
The reducibility of the rod-shaped Cu1Fe9Ox precursor is investigated by H2-TPR as shown in Figure 3. The H2-TPR profile of pure CuO is also exhibited in Figure 3 for comparison. There are two distinctive reduction regions of the Cu1Fe9Ox nanorod sample, i.e., 50-300 °C and 300-550 °C. Three obvious reduction peaks centered at 113 °C, 225 °C and 266 °C, respectively, were observed in the range of 50-300 °C. Compared with pure CuO, the reduction of Cu species in the Cu1Fe9Ox precursor was obviously shifted to lower temperature. The peak centered at 113 °C should be attributed to the reduction of amorphous CuO, which cannot be detected by XRD. The higher peak at 225 °C is attributed to the reduction of Cu 2+ in CuFe2O4 and the peak centered at 266 °C is the reduction of Fe2O3 to Fe3O4 [16][17][18]. The following high-temperature reduction at 300-550 °C should be due to the reduction of Fe3O4 to Fe 0 [19].   When the reduction temperature rises to 250 °C, the main peak of CuFe2O4 at 35.1° shifts to a low degree, which is attributed to the peak of Fe3O4. In addition, the XRD peaks belonging to Fe2O3 decrease at 250 °C, and completely disappear after reduction at 300 °C for 1 h. Furthermore, after reducing at 300 °C for 2 h, only metallic Cu and Fe3O4 are observed, indicating that Cu1Fe9Ox nanorods are completely reduced to Cu and Fe3O4. Meanwhile, Cu1Fe9Ox nanoparticles can also be reduced to Cu and Fe3O4 at 250 °C [15]. These results reveal that the reduction of Cu1Fe9Ox nanorod  When the reduction temperature rises to 250 • C, the main peak of CuFe 2 O 4 at 35.1 • shifts to a low degree, which is attributed to the peak of Fe 3 O 4 . In addition, the XRD peaks belonging to Fe 2 O 3 decrease at 250 • C, and completely disappear after reduction at 300 • C for 1 h. Furthermore, after reducing at 300 • C for 2 h, only metallic Cu and Fe 3 O 4 are observed, indicating that Cu 1 Fe 9 O x nanorods are completely reduced to Cu and Fe 3 O 4 . Meanwhile, Cu 1 Fe 9 O x nanoparticles can also be reduced to Cu and Fe 3 O 4 at 250 • C [15]. These results reveal that the reduction of Cu 1 Fe 9 O x nanorod is relatively difficult compared with nanoparticles, which may be attributed to the strong interactions between copper species and iron oxide nanorods. is relatively difficult compared with nanoparticles, which may be attributed to the strong interactions between copper species and iron oxide nanorods. In order to elucidate the interactions between Cu species and the Fe3O4 support, temperature-programmed-reduction of oxidized surfaces (s-TPR) was employed, as shown in Figure  5. During the test, Cu1Fe9Ox nanorods and nanoparticles were initially reduced by 5% H2/N2 at 300 °C and 250 °C for 1 h, respectively. Subsequently, Cu on the surface can be oxidized into Cu2O layer exclusively when 10% N2O/He stream was introduced at 90 °C for 0.5 h [20]. Then, H2 was used to quantitatively convert the surface Cu2O layer into metallic copper, and the consumption of H2 with the temperature increasing was profiled using s-TPR as shown in Figure 5. Obviously, the peak area of s-TPR represents the amount of surface Cu atoms, which is also an indicator of Cu dispersion. It can be seen that both curves contain only one peak, and the calculated dispersion of Cu on the surface of Cu/Fe3O4 nanorods and nanoparticles are 18.6% and 14.7%, respectively. In addition, the peak position of s-TPR can indicate the reducibility of Cu2O formed by oxidation of surface Cu 0 , and it is also easy to understand the peak position is dramatically affected by the interaction between Cu and Fe3O4 support. The reduction temperature of Cu2O formed on Cu/Fe3O4 rod (154 °C) is much higher than that of Cu/Fe3O4 nanoparticles (97 °C), which is attributed to the existing strong interaction between Cu and Fe3O4 over Cu/Fe3O4 rod. In order to elucidate the interactions between Cu species and the Fe 3 O 4 support, temperatureprogrammed-reduction of oxidized surfaces (s-TPR) was employed, as shown in Figure 5. During the test, Cu 1 Fe 9 O x nanorods and nanoparticles were initially reduced by 5% H 2 /N 2 at 300 • C and 250 • C for 1 h, respectively. Subsequently, Cu on the surface can be oxidized into Cu 2 O layer exclusively when 10% N 2 O/He stream was introduced at 90 • C for 0.5 h [20]. Then, H 2 was used to quantitatively convert the surface Cu 2 O layer into metallic copper, and the consumption of H 2 with the temperature increasing was profiled using s-TPR as shown in Figure 5. Obviously, the peak area of s-TPR represents the amount of surface Cu atoms, which is also an indicator of Cu dispersion. It can be seen that both curves contain only one peak, and the calculated dispersion of Cu on the surface of Cu/Fe 3 O 4 nanorods and nanoparticles are 18.6% and 14.7%, respectively. In addition, the peak position of s-TPR can indicate the reducibility of Cu 2 O formed by oxidation of surface Cu 0 , and it is also easy to understand the peak position is dramatically affected by the interaction between Cu and Fe 3 O 4 support. The reduction temperature of Cu 2 O formed on Cu/Fe 3 O 4 rod (154 • C) is much higher than that of Cu/Fe 3 O 4 nanoparticles (97 • C), which is attributed to the existing strong interaction between Cu and Fe 3 O 4 over Cu/Fe 3 O 4 rod.

Morphology Structure of As-Prepared Catalyst after Reduction
After reducing at 300 °C, it is obvious that Cu/Fe3O4 can inherit the rod shape of Cu1Fe9Ox nanorods (Figure 6a). The high magnification image (Figure 6b) shows that a lot of nanoparticles are formed on the surface of the nanorods. After analyzing the inter-planar distance of the nanoparticles formed, it is confirmed to be the metallic Cu nanoparticles of 2-10 nm. Accordingly, it can be concluded that Cu1Fe9Ox nanorods can be reduced to Cu 0 /Fe3O4 rods by a controllable reduction process.

Water Gas Shift (WGS) Reaction Kinetics and In Situ Studies
The catalytic activity of Cu/Fe3O4 nanorods is evaluated by WGS reaction, and it has no LT-WGS reaction activity below 250 °C. However, Cu/Fe3O4 nanoparticles with the same contents of Cu exhibit high LT-WGS activity [15]. In order to understand why metallic Cu on the surface of Fe3O4 rods is inactive toward the LT-WGS reaction, surface chemical states of the catalyst during WGS reaction were studied by using a home-made AP-XPS system. Pure gas (here H2 for reduction at 300 °C) or a mixture of reactant gases (H2O vapor and CO for WGS reaction) flows through the Cu1Fe9Ox catalyst when the surface of the catalyst is being examined with AP-XPS. The Cu1Fe9Ox nanorods were first reduced in H2 at 311 °C for 3 h, and followed with the introduction of mixed CO and H2O with a total pressure of 0.8 Torr. Figure 7a shows the Cu 2p3/2 and Auger line Cu LMM spectra of Cu1Fe9Ox nanorod catalyst under WGS reaction conditions. The peak position of Cu 2p3/2 at 932.7 eV indicates the existence of

Morphology Structure of As-Prepared Catalyst after Reduction
After reducing at 300 • C, it is obvious that Cu/Fe 3 O 4 can inherit the rod shape of Cu 1 Fe 9 O x nanorods ( Figure 6a). The high magnification image (Figure 6b) shows that a lot of nanoparticles are formed on the surface of the nanorods. After analyzing the inter-planar distance of the nanoparticles formed, it is confirmed to be the metallic Cu nanoparticles of 2-10 nm. Accordingly, it can be concluded that Cu 1 Fe 9 O x nanorods can be reduced to Cu 0 /Fe 3 O 4 rods by a controllable reduction process.

Morphology Structure of As-Prepared Catalyst after Reduction
After reducing at 300 °C, it is obvious that Cu/Fe3O4 can inherit the rod shape of Cu1Fe9Ox nanorods (Figure 6a). The high magnification image (Figure 6b) shows that a lot of nanoparticles are formed on the surface of the nanorods. After analyzing the inter-planar distance of the nanoparticles formed, it is confirmed to be the metallic Cu nanoparticles of 2-10 nm. Accordingly, it can be concluded that Cu1Fe9Ox nanorods can be reduced to Cu 0 /Fe3O4 rods by a controllable reduction process.

Water Gas Shift (WGS) Reaction Kinetics and In Situ Studies
The catalytic activity of Cu/Fe3O4 nanorods is evaluated by WGS reaction, and it has no LT-WGS reaction activity below 250 °C. However, Cu/Fe3O4 nanoparticles with the same contents of Cu exhibit high LT-WGS activity [15]. In order to understand why metallic Cu on the surface of Fe3O4 rods is inactive toward the LT-WGS reaction, surface chemical states of the catalyst during WGS reaction were studied by using a home-made AP-XPS system. Pure gas (here H2 for reduction at 300 °C) or a mixture of reactant gases (H2O vapor and CO for WGS reaction) flows through the Cu1Fe9Ox catalyst when the surface of the catalyst is being examined with AP-XPS. The Cu1Fe9Ox nanorods were first reduced in H2 at 311 °C for 3 h, and followed with the introduction of mixed CO and H2O with a total pressure of 0.8 Torr. Figure 7a shows the Cu 2p3/2 and Auger line Cu LMM spectra of Cu1Fe9Ox nanorod catalyst under WGS reaction conditions. The peak position of Cu 2p3/2 at 932.7 eV indicates the existence of

Water Gas Shift (WGS) Reaction Kinetics and In Situ Studies
The catalytic activity of Cu/Fe 3 O 4 nanorods is evaluated by WGS reaction, and it has no LT-WGS reaction activity below 250 • C. However, Cu/Fe 3 O 4 nanoparticles with the same contents of Cu exhibit high LT-WGS activity [15]. In order to understand why metallic Cu on the surface of Fe 3 O 4 rods is inactive toward the LT-WGS reaction, surface chemical states of the catalyst during WGS reaction were studied by using a home-made AP-XPS system. Pure gas (here H 2 for reduction at 300 • C) or a mixture of reactant gases (H 2 O vapor and CO for WGS reaction) flows through the Cu 1 Fe 9 O x catalyst when the surface of the catalyst is being examined with AP-XPS. The Cu 1 Fe 9 O x nanorods were first reduced in H 2 at 311 • C for 3 h, and followed with the introduction of mixed CO and H 2 O with a total pressure of 0.8 Torr. Figure 7a shows the Cu 2p3/2 and Auger line Cu LMM spectra of Cu 1 Fe 9 O x nanorod catalyst under WGS reaction conditions. The peak position of Cu 2p3/2 at 932.7 eV indicates the existence of Cu + or/and Cu 0 during catalysis. The Auger Cu LMM was used to distinguish Cu + and metallic Cu. The Auger parameters calculated from the Cu LMM Auger line collected during catalysis with the AP-XPS exhibit the evolution of Cu species on the surface of the Fe 3 O 4 rods. It can be seen that the main species of copper after reducing at 311 • C is metallic Cu. When water vapor and CO were introduced, the dominant copper species is Cu + , suggesting that Cu 0 on Fe 3 O 4 nanorod can be easily oxidized into Cu + by water. Combining the s-TPR results, the strong interaction between Cu and Fe 3 O 4 nanorods is thought to be the main reason why Cu 0 on the Fe 3 O 4 nanorod is much easier to oxidize relative to Cu 0 particles on Fe 3 O 4 nanoparticles. In terms of the catalyst with nanorod structure, there is no obvious difference in photoemission features of Fe 2p among different catalytic temperatures (Figure 7b), which suggest that Fe 2 O 3 was reduced to Fe 3 O 4 upon H 2 reduction and the Fe 3 O 4 phase can be retained during WGS process. Cu + or/and Cu 0 during catalysis. The Auger Cu LMM was used to distinguish Cu + and metallic Cu. The Auger parameters calculated from the Cu LMM Auger line collected during catalysis with the AP-XPS exhibit the evolution of Cu species on the surface of the Fe3O4 rods. It can be seen that the main species of copper after reducing at 311 °C is metallic Cu. When water vapor and CO were introduced, the dominant copper species is Cu + , suggesting that Cu 0 on Fe3O4 nanorod can be easily oxidized into Cu + by water. Combining the s-TPR results, the strong interaction between Cu and Fe3O4 nanorods is thought to be the main reason why Cu 0 on the Fe3O4 nanorod is much easier to oxidize relative to Cu 0 particles on Fe3O4 nanoparticles. In terms of the catalyst with nanorod structure, there is no obvious difference in photoemission features of Fe 2p among different catalytic temperatures (Figure 7b), which suggest that Fe2O3 was reduced to Fe3O4 upon H2 reduction and the Fe3O4 phase can be retained during WGS process.  Figure 8 shows the ratio of Cu (Cu + + Cu 0 )/Fe of Cu1Fe9Ox nanorods and nanoparticles under different conditions. For Cu1Fe9Ox nanorods, the measured Cu/Fe ratio increases with the increasing temperature. This variation indicates that Cu species migrate toward surface during reaction. On the contrary, for Cu1Fe9Ox nanoparticle sample, the measured Cu/Fe ratio increases with increasing temperature and subsequently decreases when the temperature further increases, indicating that metallic Cu on the surface sintered with the temperature increase.  Figure 8 shows the ratio of Cu (Cu + + Cu 0 )/Fe of Cu 1 Fe 9 O x nanorods and nanoparticles under different conditions. For Cu 1 Fe 9 O x nanorods, the measured Cu/Fe ratio increases with the increasing temperature. This variation indicates that Cu species migrate toward surface during reaction. On the contrary, for Cu 1 Fe 9 O x nanoparticle sample, the measured Cu/Fe ratio increases with increasing temperature and subsequently decreases when the temperature further increases, indicating that metallic Cu on the surface sintered with the temperature increase. Although Cu/Fe3O4 nanorod has no LT-WGS activity, it can catalyze the WGS reaction at high temperature as Fe3O4 is active phase for the HT-WGS reaction. Through an in situ reduction approach, Cu/Fe3O4 was formed in a fixed-bed flow reactor in 5 vol.% H2/He at 300 °C for 2 h. When the mixtures of CO and water vapor are introduced, the kinetics of WGS over the Cu/Fe3O4 nanorod is investigated, as shown in Figure 9. The catalyst was tested at a gas hour space velocity (GHSV) of 40,000 mL·g·cat −1 ·h −1 , which made the conversion of CO lower than 10% in the range of 350-425 °C. Figure 9 presents the corresponding Arrhenius plot for the WGS reaction over the Cu/Fe3O4 nanorod and the CO conversions. The activation energy is calculated to be 71.2 kJ/mol. Given the fact that the activation energy of 95-118 kJ/mol and 75-80 kJ/mol can be attributed to the Fe-Cr [21,22] and Fe-Cr-Cu samples [23,24], the addition of Cu to Fe-Cr catalysts can decrease the activation energy by ~20-40 kJ/mol, revealing the promoting role of Cu in the HT-WGS reaction. Therefore, the activation energy of 71.2 kJ/mol for Cu/Fe3O4 nanorod in this work is reasonable, and it indicates that the reduced Cu1Fe9Ox rod exhibits a good HT-WGS reaction activity.   Although Cu/Fe 3 O 4 nanorod has no LT-WGS activity, it can catalyze the WGS reaction at high temperature as Fe 3 O 4 is active phase for the HT-WGS reaction. Through an in situ reduction approach, Cu/Fe 3 O 4 was formed in a fixed-bed flow reactor in 5 vol.% H 2 /He at 300 • C for 2 h. When the mixtures of CO and water vapor are introduced, the kinetics of WGS over the Cu/Fe 3 O 4 nanorod is investigated, as shown in Figure 9. The catalyst was tested at a gas hour space velocity (GHSV) of 40,000 mL·g·cat −1 ·h −1 , which made the conversion of CO lower than 10% in the range of 350-425 • C. Figure 9 presents the corresponding Arrhenius plot for the WGS reaction over the Cu/Fe 3 O 4 nanorod and the CO conversions. The activation energy is calculated to be 71.2 kJ/mol. Given the fact that the activation energy of 95-118 kJ/mol and 75-80 kJ/mol can be attributed to the Fe-Cr [21,22] and Fe-Cr-Cu samples [23,24], the addition of Cu to Fe-Cr catalysts can decrease the activation energy bỹ 20-40 kJ/mol, revealing the promoting role of Cu in the HT-WGS reaction. Therefore, the activation energy of 71.2 kJ/mol for Cu/Fe 3 O 4 nanorod in this work is reasonable, and it indicates that the reduced Cu 1 Fe 9 O x rod exhibits a good HT-WGS reaction activity. Although Cu/Fe3O4 nanorod has no LT-WGS activity, it can catalyze the WGS reaction at high temperature as Fe3O4 is active phase for the HT-WGS reaction. Through an in situ reduction approach, Cu/Fe3O4 was formed in a fixed-bed flow reactor in 5 vol.% H2/He at 300 °C for 2 h. When the mixtures of CO and water vapor are introduced, the kinetics of WGS over the Cu/Fe3O4 nanorod is investigated, as shown in Figure 9. The catalyst was tested at a gas hour space velocity (GHSV) of 40,000 mL·g·cat −1 ·h −1 , which made the conversion of CO lower than 10% in the range of 350-425 °C. Figure 9 presents the corresponding Arrhenius plot for the WGS reaction over the Cu/Fe3O4 nanorod and the CO conversions. The activation energy is calculated to be 71.2 kJ/mol. Given the fact that the activation energy of 95-118 kJ/mol and 75-80 kJ/mol can be attributed to the Fe-Cr [21,22] and Fe-Cr-Cu samples [23,24], the addition of Cu to Fe-Cr catalysts can decrease the activation energy by ~20-40 kJ/mol, revealing the promoting role of Cu in the HT-WGS reaction. Therefore, the activation energy of 71.2 kJ/mol for Cu/Fe3O4 nanorod in this work is reasonable, and it indicates that the reduced Cu1Fe9Ox rod exhibits a good HT-WGS reaction activity.

Materials and Methods
Cu 1 Fe 9 O x nanorod precursor was prepared by using an aqueous precipitation method [25]. The typical synthesis procedure is described as follows. An aqueous solution containing CuCl 2 ·2H 2 O (0.34 g), FeCl 3 ·6H 2 O (4.84 g), NaCl (11.60 g), and PEG (10 mL) with volume of 190 mL was gradually heated to 120 • C. Then, Na 2 CO 3 aqueous solution (200 mL, 0.2 M) was added through a syringe pump at rate of 5.5 mL·min −1 , and the resulting suspension was aged at 120 • C for 1 h. Subsequently, the precipitate was washed with water and ethanol 3 times, followed by drying at 60 • C for 6 h. The resultant Cu 1 Fe 9 O x nanorods precursor was obtained by calcination at 500 • C for 10 h.

Characterization of Catalyst Structure
The crystal structure of synthesized samples was studied by X-ray diffraction (XRD) in a RigakuD/MAX-RB (Rigaku Corporation, Akishima-shi, Japan) diffractometer using CuKa radiation source (λ = 1.54 Å). The average crystallite sizes were calculated by the Scherrer equation [26]. In situ XRD patterns were obtained by using the same instrument and the gas of 5 vol.% H 2 /He was introduced and the heating rate is 5 • C/min.
The morphology and the microstructure of samples were characterized by TEM employing a Hitachi 7700 microscope (Hitachi High-Technologies Corporation, Beijing, China) and HRTEM in a FEI Tecnai G2 F30S-twin microscope (FEI, Hillsboro, OR, USA) operating at 300 kV and JEOL ARM200F (HAADF-STEM) at 200 kV. All samples for the TEM test were first dispersed in ethanol and then coated on Cu grid. The BET surface area of the sample was identified by a Nova 4200e (Quantachrome Instruments, Boynton Beach, FL, USA) physical adsorption instrument. Elemental analysis was examined by inductively coupled plasma atomic emission spectroscopy (ICPS-8100 spectrometer, Shimadzu, Kyoto, Japan).
The temperature-programmed reduction by hydrogen (H 2 -TPR) of Cu 1 Fe 9 O x was tested with an Auto Chem 2920 instrument (Micromeritics, Atlanta, GA, USA). A 40 mg sample was placed in a quartz reactor which was connected to a conventional TPR apparatus. The reactor was heated from 50 • C to 900 • C at a heating rate of 2 • C/min and the amount of H 2 uptake during the reduction was measured by a thermal conductivity detector (TCD). For comparison, 40 mg of pure CuO powder was used.

Ambient Pressure X-Ray Photoelectron SpectroFscopy (AP-XPS) Studies of Evolution of Surface Species
The surface chemistry and structure of the nanocatalysts during catalysis could be different from those of the as-synthesized catalysts before and/or after the catalysis, and the pressure-dependent entropy could restructure the surface of the catalyst during catalysis. In this case, the surface chemistry and structure of catalyst during the WGS reaction at a Torr pressure range were developed at the in-house AP-XPS using monochromator Al Ka [27]. The photoemission feature like the Cu 2p3/2, Fe 2p, Auger line Cu (LMM) and O1s of Cu 1 Fe 9 O x nanorod are collected under WGS conditions at different temperatures. After loading the catalyst in an XPS cell, sample was reduced in 1.5 Torr H 2 at 311 • C for 1 h and no treatment was made in O 2 . Mixtures of CO and H 2 O were then introduced into the XPS cell.

Measurement of Catalytic Performance
The WGS reaction was performed in a fixed-bed continuous flow reactor under atmospheric pressure at 350-425 • C. Prior to the activity test, 100 mg catalyst was loaded and reduced by flowing 5 vol.% H 2 /He gas at a flow rate of 30 mL/min at 300 • C for 2 h. The reaction gas contained 1% CO/3% H 2 O/He. To introduce water, He was used as a carrier gas flowing through deionized water. The effluents were analyzed by online gas chromatography. The conversion of CO is calculated as follows: X % = (CO upstream − CO downstream ) × 100%/CO upstream .

Conclusions
In this study, a Cu 0 /Fe 3 O 4 nanorod was prepared through the controllable reduction of Cu 1 Fe 9 O x nanorod, and it exhibited a high HT-WGS catalytic activity. According to the AP-XPS result, it was found that metallic Cu supported on a Fe 3 O 4 nanorod can be easily oxidized to Cu + after introducing water vapor. Combining the behavior of metallic Cu supported on a Fe 3 O 4 nanorod and nanoparticles during a WGS reaction, it can be found that the interaction between the metallic Cu and Fe 3 O 4 support with different morphology structures is quite different. Furthermore, the activation energy of HT-WGS over Cu/Fe 3 O 4 is 71.2 kJ/mol, which is much lower than the commercial Fe-Cr catalyst.