Dynamic chemical state conversion of nickel species supported on silica under CO–NO reaction conditions

Accepted Manuscript Title: Dynamic Chemical State Conversion of Nickel Species Supported on Silica under CO–NO Reaction Conditions Authors: Shohei Yamashita, Yusaku Yamamoto, Hisataka Kawabata, Yasuhiro Niwa, Misaki Katayama, Yasuhiro Inada PII: DOI: Reference:

S0920-5861(17)30521-7 http://dx.doi.org/doi:10.1016/j.cattod.2017.07.028 CATTOD 10942

To appear in:

Catalysis Today

Received date: Revised date: Accepted date:

2-6-2017 25-7-2017 29-7-2017

Please cite this article as: Shohei Yamashita, Yusaku Yamamoto, Hisataka Kawabata, Yasuhiro Niwa, Misaki Katayama, Yasuhiro Inada, Dynamic Chemical State Conversion of Nickel Species Supported on Silica under CO–NO Reaction Conditions, Catalysis Todayhttp://dx.doi.org/10.1016/j.cattod.2017.07.028 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Dynamic Chemical State Conversion of Nickel Species Supported on Silica under CO–NO Reaction Conditions Shohei Yamashita a, Yusaku Yamamoto a, Hisataka Kawabata a, Yasuhiro Niwa b, Misaki Katayama a, Yasuhiro Inada a,* a

Department of Applied Chemistry, College of Life Sciences, Ritsumeikan University, 1-1-1

Noji-Higashi, Kusatsu 525-8577, Japan b

Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research

Organization, 1-1 Oho, Tsukuba 305-0801, Japan

Graphical abstract

Highlights 

The chemical state conversions of the Ni species supported on SiO2 under the CO and NO atmosphere were investigated.



The CO–NO reaction was mediated by the redox cycles of the supported Ni species between NiO and Ni(0).



The transient NiO state was directly detected by the time-resolved XAFS measurement.

Abstract The chemical state conversion of Ni species supported on SiO 2 under CO and NO atmosphere was observed by in situ XAFS techniques, and the reversible and quantitative chemical state conversions between NiO and Ni(0) were clarified during the temperature-programmed reduction of

NiO by CO and the temperature-programmed oxidation of Ni(0) by NO. The evaluated chemical state of the SiO2-supported Ni species indicated the catalytic CO–NO reaction conditions using the Ni/SiO2 catalyst. The actual catalytic experiments demonstrated the achievement of the catalyzed CO–NO reaction at 873 K. The dynamic chemical state analysis by means of the time-resolved dispersive XAFS method revealed the existence of a transient Ni species, which was partially oxidized by the faster reaction with NO. The formed NiO species can oxidize the CO molecule, and thus the CO–NO reaction is driven by the redox conversion of the supported Ni species. Keywords nickel catalyst, CO–NO reaction, redox reaction, in situ XAFS, time-resolved dispersive XAFS 1. Introduction The chemical state analysis of the catalytically active species will provide the necessary information not only to understand correctly the catalytic reaction scheme but also to develop catalysis systems with enhanced activity. The in situ X-ray absorption fine structure (XAFS) technique is the most powerful for this purpose because of its applicability to samples under the actual reaction conditions, specifically, the reaction and product gas flow at elevated temperatures [1]. An in situ investigation into Ni species supported on SiO2 has been published recently under H2 and O2 atmosphere as the most basic oxidative and reductive reaction/product gas [2]. Static XAFS analysis has been applied to precisely elucidate the chemical state of the Ni species and its conversion between NiO and metallic Ni at elevated temperatures. Furthermore, the dynamic observation of the redox reactions of the Ni species by means of the time-resolved wavelength-dispersive XAFS (DXAFS) technique has provided the mechanistic information for the chemical conversion in the reaction with gaseous molecules [2-10]. The kinetic analyses for the reduction of NiO particles by H2 and the oxidation of Ni(0) particles by O2 have revealed that the reactions are initiated by the fast surface reduction/oxidation process and proceed by the rate-determining oxide ion migration to continue the redox reactions at the inner part of the particles [2,3]. The so-called CO–NO reaction, Equation (1), is a useful catalytic reaction to reduce the toxic gaseous emissions in the environment by simultaneously achieving the oxidation of CO and the reduction of NO [11]. 2CO + 2NO

→ 2CO2 + N2

(1)

The three-way catalyst using Pt, Pd, and Rh is widely used as an active catalyst for a similar purpose for automobile emissions with the assistance of the CeO2–ZrO2 promoter. In this case, it is known that the Pt, Pd, and Rh species simultaneously achieve the oxidation of CO, the reduction of

NO, and the oxidation of hydrocarbon [12-16]. A Pd catalyst supported on a perovskite oxide is also used for the chemical conversion of harmful gases, and the supporting material has the role of preventing the sintering of the active Pd particles, resulting in the retention of the rare metal materials [12]. Numerous investigations into the catalytic reactions of CO and NO are continuously being conducted using the more widely available metal elements, such as Ni, Co, and Cu [11,17-21], whereas the most basic knowledge for such metal species, the chemical states of the supported metal species under the CO and NO gas environment at elevated temperatures, unfortunately remains yet unclear. The oxidation reaction of CO has been studied mainly using Pt and Pd catalysts, while the catalytic activities of Rh and Ir catalysts have been investigated for the NO reduction reaction. Advanced analysis techniques have recently been applied to observe the catalytic reaction under operando conditions [21-23]. Zhou et al. have reported the CO oxidation activity of the Pt catalyst supported on various facets of the TiO2 crystal and its effect on the stability and dispersion of the active Pt particles [21]. A number of investigations have also been carried out on the catalytic CO– NO reaction. Some reaction pathways have been proposed by Ueda et al. to form the N2 molecule according to mechanistic study of the metallic Ir surface [24]. Wang et al. have reported the CO– NO reaction activity for Ni particles supported on CeO2, -Al2O3, and TiO2, and they pointed out the contribution of the redox reactions of the Ni particle surface to the conversion performance [25]. A relationship between the CO–NO reaction activity and the shape of supporting CeO2 particles has been investigated and the enhanced activity was reported by the shape control [26]. The CO–NO reaction has been examined using an alloy catalyst composed of Fe and Co, and it has been evaluated that the highest activity is achieved by the alloy composition of Fe 0.8Co0.2 [27]. The perovskite catalysts have applied to the CO–NO reaction, and Izadkhah et al. have reported the enhancement of the number of surface active sites due to the existence of the supported metal species [28]. In this study, the chemical state of the Ni species supported on SiO 2 has been directly evaluated by means of the in situ XAFS technique under a CO or NO gas environment at elevated temperatures. The reversible and quantitative conversion between the NiO and Ni(0) species has been demonstrated by the redox reactions of NiO with CO and Ni(0) with NO. The temperature-programmed in situ XAFS measurements have precisely revealed the composition of the supported Ni species, and the composition changes versus temperature during the temperature-programmed reduction (TPR) and oxidation (TPO) processes have been compared with those of the corresponding processes using H2 or O2 as the reactant gas. The information obtained regarding the chemical state of the Ni species is useful to construct the CO–NO reaction conditions using the SiO2-supported Ni catalyst, and the actual catalytic activity of the CO–NO reaction has been examined. Furthermore, the dynamic chemical state changeover of the SiO2-supported Ni

species has been directly observed after rapid injection of the gas mixture of CO and NO by means of the time-resolved DXAFS technique. The transient Ni species under the CO–NO reaction conditions has been clarified, and the achievement of the CO–NO reaction catalyzed by the Ni species has been demonstrated. 2. Experimental 2.1. Ni catalyst preparation and characterization The Ni catalyst was prepared by the impregnation method at a Ni loading of 5 wt.% using SiO2 (JRC-SIO-10) obtained from the Catalysis Society of Japan. The SiO2 powder was added into an aqueous solution of Ni(II) nitrate. The suspension was stirred for 1 h and dried at 343 K for 72 h. The obtained powder was calcined at 873 K in air for 3 h. The sample was reduced under a diluted CO flow (10 vol.% balanced by He, total flow rate of 200 cm3/min) at 1023 K for 1 h. The reduced sample was re-oxidized under a dilute NO flow (10 vol.% balanced by He, total flow rate of 200 cm3/min) at 873 K for 1 h. X-ray diffraction (XRD) patterns were measured using an Ultima IV diffractometer (Rigaku) equipped with a linear detector using Cu K radiation. The diffraction intensities were recorded over the 2 range between 10 ° and 80 °. Transmission electron microscopy (TEM) observations were performed on a JEOL2010 microscope (JEOL) for the reduced sample. 2.2. XAFS measurement and analysis The in situ XAFS measurements at the Ni K edge were carried out at the BL-9C station of the Photon Factory (High Energy Accelerator Research Organization, Japan). The higher-order harmonics were removed by detuning the double-crystal monochromator to attenuate the incident X-ray intensity to 70 % of the maximum at the Ni K edge. A flow-type in situ XAFS cell was used to observe the XAFS spectra at elevated temperatures under the flow of CO or NO (10 vol.%) diluted with He at a total flow rate of 200 cm3/min. The sample temperature was increased from 300 K to 1023 K at a rate of 10 K/min. Because it is known that CO and NO molecules have potential as the reducing and oxidizing agents, respectively, only the TPR process of the supported NiO species was monitored under the CO atmosphere, while only the TPO process was measured for the reduced Ni(0) species under the flow of dilute NO gas. The metallic Ni(0) species was quantitatively formed after the TPR process, and the sample was cooled down to room temperature while maintaining the dilute CO gas flow. In order to avoid the formation of toxic Ni(CO) 4, the flow of CO gas was stopped during the cooling process once the temperature had reached 573 K. The reduced Ni(0) species produced by CO was used as the sample for the following TPO process with NO.

The time-resolved DXAFS measurements were performed at the NW2A station of the Photon Factory Advanced Ring (High Energy Accelerator Research Organization, Japan) [9]. A Bragg-type Si(111) curved crystal with a bending radius of 2 m was used as the polychromator. The polychromatic X-rays were detected using a linear photodiode array detector equipped with a scintillator (CsI:Tl). The detected X-ray energy was calibrated based on the observed XAFS spectrum of Ni foil. The sample was placed in the batch-type observation cell [10], and the reaction gas (CO, NO, or a mixture thereof) was rapidly introduced into the evacuated cell at 873 K to observe the chemical state conversion of the supported Ni species. The normalized XANES spectrum was obtained by subtraction of the background absorbance estimated in the pre-edge region using the Victoreen equation and by normalization using the smooth absorbance in the post-edge region estimated by cubic spline smoothing. The EXAFS oscillation (k) was obtained by the subtraction of the smooth absorbance and the conversion of X-ray energy to the photoelectron wave number k [29]. The k3-weighted (k) function was used for the Fourier transformation, and the structure parameters were determined by the least squares fitting of the theoretical EXAFS function of Equation (2) using the FEFFIT package [30], 𝑁𝑗 2𝑅𝑗 2 2 ( ) ( )} 𝜒(𝑘) = 𝑆0 2 ∑ ) 2 𝐹𝑗 𝑘 sin{2𝑘𝑅𝑗 + 𝛿𝑗 𝑘 exp (−2𝜎𝑗 𝑘 − 𝜆𝑗 (𝑘) 𝑘𝑅𝑗

(2)

𝑗

where Fj(k) is the scattering amplitude from each of the Nj scatters at distance Rj from the X-ray absorbing atom, j(k) is the total phase shift, j(k) is the mean free path of the photoelectron, j is the Debye–Waller factor, and S02 is the amplitude reduction factor. The values of Fj(k), j(k), and

j(k) were estimated by the FEFF8 code for the hypothetical Ni–O and Ni–Ni atomic clusters with the interatomic distances of bulk NiO and Ni, respectively. The S02 values for the Ni–O interaction of the NiO species and the Ni–Ni interaction of metallic Ni were determined by fixing the values of Nj to 6 and 12, respectively. The edge-energy correction term E0 was included as one of the optimization parameters. 2.3. Catalytic test The CO–NO reaction activities were tested using a flow-type reactor made of silica glass. In a typical experiment, 150 mg of the calcined catalyst was placed in a U-shaped sample tube and this was positioned in a tubular furnace. The catalyst was preheated at 873 K for 1 h under the dilute H2 (10 vol.%) atmosphere balanced by Ar to quantitatively reduce the NiO species before the catalytic run. The sample temperature was kept at 873 K, and the gas mixture of CO (5 vol.%) and NO (5 vol.%) diluted by Ar was fed to the Ni catalyst at a total flow rate of 100 cm3/min. The gas composition was analyzed on a gas chromatograph (GC-8A, Shimadzu) equipped with a ShinCarbon ST capillary column and a thermal conductivity detector. The CO and NO conversions were estimated from the changes in their peak areas.

3. Results and discussion 3.1. Characterization of supported Ni species and particle size The XRD patterns for the calcined and reduced Ni catalysts are shown in Fig. 1. The pattern of the calcined sample was perfectly consistent with that of NiO, and the formation of metallic Ni(0) was confirmed by the XRD pattern of the reduced sample. Although the reduction treatment was carried out under dilute CO atmosphere, the NiO particles were perfectly converted to the Ni(0) particles. The Scherrer equation was applied to estimate the crystallite sizes of 24 nm and 17 nm for the NiO and Ni(0) particles, respectively. After the reduction treatment with CO, the sample was re-oxidized by treatment under dilute NO gas flow, and the XRD pattern assigned to NiO was regenerated as seen in Fig. 1. The crystallite size (17 nm) of the regenerated NiO particles was almost in agreement with that in the calcined sample. The particle size distribution according to the TEM results was analyzed for the reduced Ni(0) sample. Figure 2 shows a histogram of the particle size and a representative TEM image. The average size was estimated to be 17 nm, and this value was consistent with the estimated crystallite size on the basis of the XRD data. 3.2. Chemical state conversion of the supported Ni species under CO atmosphere The experimental results for the in situ XAFS analysis during the TPR process by CO are summarized in Fig. 3. The initial spectrum before the TPR process was consistent with that of NiO, and during the TPR process the XANES spectrum changed to that of metallic Ni as shown in Fig. 3(A). The slight difference between the final spectrum and that of Ni foil was ascribed to the size effect for the small Ni(0) particles and the temperature effect due to the atomic disorder [31,32]. The change in XANES spectrum exhibited obvious isosbestic points, and it was thus reasonably accepted that the chemical conversion from NiO to Ni(0) proceeded without any stable intermediates. The composition analysis of the Ni species was performed on the basis of the linear combination approximation of the component species, NiO and Ni(0), using the observed XANES spectra. The XANES spectra of the NiO powder and the Ni foil were used as the component species for the analysis. The mole fractions of the component species were determined by a curve-fitting procedure using the observed XANES spectra. The determined values are plotted in Fig. 3(B) as a function of temperature. The CO gas atmosphere promoted the chemical conversion from NiO to Ni(0) at around 820 K, and this conversion temperature was higher by ca. 200 K than that for the corresponding TPR process by H2, indicating the lower reducing ability of CO relative to H2 and the possible inhibition due to the adsorption of CO on the NiO particles [33].

The k3(k) data and the Fourier transform functions at various representative temperatures are shown in Figs. 3(C) and 3(D), respectively. The conversion from NiO to Ni(0) during the TPR process was clearly observed. The nearest Ni–O interaction for NiO was observed at ca. 1.6 Å without the phase-shift correction at the beginning of the TPR process, and was replaced with the Ni–Ni interaction assigned to the metallic Ni(0) species at above 770 K. The EXAFS data were analyzed using Equation (2) to determine the structure parameters, and all values were given in Table S1 (Supporting Information). Because the NiO and Ni(0) species coexist during the TPR process, the optimized values of N for the Ni–O and Ni–Ni interactions of NiO and Ni(0), respectively, are the product of the actual number of the interaction multiplied by its mole fraction. The variations of the N values are given in Fig. 3(E) as a function of temperature. The initial NNi–O value is almost in accordance with the rock salt structure with the NiO6 octahedron around the Ni(II) center. Its decrease at ca. 820 K represents the reduction to Ni(0), and this is compensated for the increase in the NNi–Ni value. The NNi–Ni value reached 10 at ca. 900 K, indicating the completion of the reduction reaction. The final NNi–Ni value of 10 was evidently smaller than the corresponding value of 11 for the Ni(0) particles supported on SiO2 after the reduction treatment using the H2 gas. The adsorption of CO on the Ni(0) particles is considered to promote the dispersion, resulting in the reduced particle size. 3.3. Chemical state conversion of the supported Ni species under NO atmosphere The XANES spectra, the composition change of the Ni species, the k3(k) data, the Fourier transform functions, and the average interaction number around the Ni center for the TPO process of the Ni(0) species under NO atmosphere are shown in Fig. 4. The determined structure parameters were given in Table S2. A change in the XANES spectra opposite to that in Fig. 3(A) was observed during the TPO process, and the initial Ni(0) species was quantitatively oxidized to NiO. The same isosbestic points observed for the TPR process were also observed for the TPO process. As seen in Fig. 4(B), half of the Ni species was oxidized to NiO at ca. 750 K, and the conversion temperature from Ni(0) to NiO was shifted higher relative to that for the corresponding TPO process in the presence of O2. The oxidation reaction of the Ni(0) particles proceeded mildly at relatively lower temperatures, which was in marked contrast to the sudden progress of the reduction reaction for the NiO particles (see Figs. 3(B) and 4(B)). The oxidation of the Ni(0) particles is considered to proceed from the particle surface, and the gradual increase of the NiO composition may be attributable to the development of the NiO shell thickness formed around the Ni(0) core. The formation of this outer NiO shell may prevent the lattice expansion of the inner Ni(0) core to permit the insertion of the oxide ion, leading to the tailing of the oxidation reaction at the higher temperature range. The chemical conversion from Ni(0) to NiO was also observed in the EXAFS data given in Figs.

4(C) and 4(D). The Ni–O interaction peak appeared at above 750 K, accompanying the disappearance of the Ni–Ni interaction. The conversion temperature was consistent with the composition change given in Fig. 4(B). The NNi–Ni value remained almost constant until the temperature reached 580 K, as shown in Fig. 4(E). The partial oxidation of the Ni(0) particle was indicated by the reduction of NNi–Ni, and the NNi–O value correspondingly increased. The NNi–O value of ca. 5 that was finally reached was almost consistent with the initial value before the TPR process. The present in situ XAFS analyses confirmed that the oxidation of Ni(0) by NO and the reduction of NiO by CO proceeded reversibly and quantitatively. It is worth noting that the reduction of NiO to Ni(0) by CO and the oxidation of Ni(0) to NiO by NO can proceed simultaneously at temperatures above 850 K in the presence of both CO and NO gases. This means that the oxidation of CO and the reduction of NO are mediated by the redox cycle of the supported Ni species. This expectation has been assessed by the analysis of the reaction gas for the catalytic test, as described in the next section. 3.4. Gas analysis for the CO–NO reaction An example of the measured gas chromatogram of the product gas during the CO–NO reaction at 873 K is shown in Fig. 5. The CO and NO mixture gas fed to the Ni/SiO2 catalyst was apparently converted to CO2 and N2 as the product gases of the oxidation of CO and the reduction of NO, respectively. The CO and NO conversions were about 60 %, which is almost comparable to the corresponding value of 60 % reported for the LaMnO3 catalyst operating at 873 K [28]. The present investigation demonstrates the applicability of the Ni/SiO2 catalyst to the CO–NO reaction driven by the redox cycle of the supported Ni species on the basis of the chemical state analysis under the CO and NO atmosphere. In addition to N2, a trace amount of N2O was also detected in the product gas mixture due to the incomplete reduction of NO. When the NO molecule is converted to N 2O instead of N2 by the reaction with CO, the reaction can be expressed as in Equation (3), and thus the stoichiometry between CO and NO is shifted from 1:1 for the standard CO–NO reaction (Equation (1)) to 1:2. CO + 2NO

→

CO2 + N2 O

(3)

This means that an excess amount of NO relative to CO is consumed by the reaction and a certain amount of CO is expected to remain when the gas mixture of CO and NO with a composition of 1:1 is reacted over the Ni/SiO2 catalyst in a closed batch-type reactor. 3.5. Dynamic chemical state conversion of the supported Ni species The time-resolved DXAFS measurements revealed the dynamic chemical state conversion, which was initiated by the rapid injection of CO or NO at 873 K. Figure 6 shows the time-resolved XANES changes for the reactions of NiO with CO and Ni(0) with NO. Similar XANES changes as

in Fig. 3(A) for the TPR of NiO and in Fig. 4(A) for the TPO of Ni(0) were observed during the time-resolved measurements for the reduction of NiO by CO and the oxidation of Ni(0) by NO, although the spectral energy resolution was degraded because of the limitation of the X-ray optics used in the DXAFS instrument. The time-course change of the X-ray absorbance at the white line peak of NiO (8.347 keV) depicted in the inset of Fig. 6 indicated that the initial oxidation rate of the Ni(0) species by NO was faster than the initial reduction rate of NiO by CO under the same injection pressure of 15.8 kPa. This result suggests that the injection of a mixed gas of CO and NO to the Ni(0) catalyst at 873 K causes the faster oxidation of Ni(0) by NO and the formation of NiO on the particle surface. The formed NiO species must oxidize the coexisting CO molecule to drive the CO–NO catalysis reaction. Time-resolved DXAFS measurements were thus conducted for the reaction between the Ni(0) species and the gas mixture of CO and NO at 873 K. A typical example of the XANES change is shown in Fig. 7(a) at the gas pressures of 5.0 kPa for both CO and NO. Because the initial rate of the oxidation by NO was faster than the reduction by CO (vide supra), partial oxidation of the Ni(0) species was first observed within ca. 120 s under these conditions. The higher composition of NiO would increase the reverse reduction rate from NiO to Ni(0) by CO present in the atmosphere. The unchanged XANES spectra around 100–130 s suggested the establishment of a steady state between the two reactions on the Ni species, and the oxidation rate of Ni(0) and the reduction rate of NiO were considered to be comparable. Because the product gas analysis revealed that N2O was formed, the generated NiO species then reacted with the excess CO when the initial gas pressures of CO and NO were equivalent. As seen in the later period of the XANES spectra shown in Fig. 7(a), the transient NiO state was finally reduced to regenerate the initial Ni(0) species. The above-mentioned reaction scheme was supported by the response when the initial gas ratio was varied. The time-course changes of the X-ray absorbance at 8.347 keV are given in Fig. 7(b) for the reaction of Ni(0) with gas mixtures of CO and NO at various mixing ratios. The X-ray absorbance of ca. 0.08 before the gas injection at 0 s indicates that the Ni species existed in the Ni(0) state, and the initial increase of the absorbance corresponds to the partial oxidation of the Ni(0) particles due to the faster reaction with NO. Because the absorbance reached for the transient state was related to the formed quantity of NiO species, it was found that the quantity of NiO generated increased when the composition of NO was increased because of the enhanced oxidation rate of Ni(0). In addition, the period of time of the transient steady state became longer when the composition of NO in the reaction gas was increased. A reasonable interpretation is that the regeneration of the Ni(0) species due to the reduction by excess CO was delayed upon increasing the amount of NO injected. The final regeneration of the Ni(0) species under CO and NO gas pressures of 4.5 and 5.5 kPa, respectively, also supports the excess consumption of NO to form N 2 O under the current experimental conditions. These results can give some information about the

steady-state CO–NO reaction where CO and NO are constantly supplied. The faster oxidation of Ni(0) by NO than the reduction of NiO by CO under the same pressures suggests the coexistence of the Ni(0) and NiO species on the particle surface. The redox reactions of CO and NO are thus mediated by the chemical state conversions of the surface Ni species. 4. Conclusions The atmospheric environment largely affects the chemical state of the active Ni species, and elucidating the in situ speciation of the supported Ni species is thus crucial to understanding the catalytic reaction and determining the guiding principles for performance improvement. The chemical state changeover between NiO and Ni(0) has been clarified in this study by means of the in situ XAFS technique for a SiO2-supported Ni catalyst during the TPR process using CO and the TPO process using NO. It has been confirmed that the reduction of NiO to Ni(0) by CO and the oxidation of Ni(0) to NiO by NO proceed simultaneously at temperature above 850 K under the CO–NO reaction conditions, indicating that the CO–NO reaction is mediated by redox reactions of the supported Ni species. The actual gas analysis demonstrated the formation of CO 2 and N2 by the CO–NO reaction catalyzed by the Ni/SiO2 catalyst. The transient change of the chemical state between Ni(0) and NiO was directly clarified for the supported Ni species during the reaction with the CO and NO gas mixture. The increased formation quantity and the prolonged lifetime of the transient NiO state support the CO–NO reaction driven by the redox reactions of the catalytically active Ni species.

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Figures

Fig. 1. XRD patterns for the calcined Ni catalyst (a), its reduced sample upon the treatment under the CO atmosphere (b), and its re-oxidized sample upon the treatment under the NO atmosphere (c). The corresponding pattern for SiO2 and the reference data for NiO and Ni metal are included for comparison.

Fig. 2. Histogram of the Ni(0) particle size and a representative TEM image for the reduced Ni catalyst after treatment under the CO atmosphere (inset).

Fig. 3. XANES spectral change (A), the composition change of the Ni species versus temperature (B), the k3-weighted (k) curves (C), the Fourier transform functions (D), and the variation of the number of Ni–O and Ni–Ni interaction versus temperature (E) for the TPR process under the CO atmosphere. In (B), the solid lines represent the current experimental data, and the dotted lines depict the corresponding composition change under the H2 atmosphere.

Fig. 4. XANES spectral change (A), the composition change of the Ni species versus temperature (B), the k3-weighted (k) curves (C), the Fourier transform functions (D), and the variation of the number of Ni–Ni and Ni–O interaction versus temperature (E) for the TPO process under the NO atmosphere. In B, the solid lines represent the current experimental data, and the dotted lines depict the corresponding composition change under the O2 atmosphere.

Fig. 5. Representative gas chromatogram of the product gas during the CO–NO reaction at 873 K. An expansion of the region from 10 to 20 min is also shown (inset).

Fig. 6. Time-resolved XANES changes for the reduction reaction of NiO with CO (A) and the oxidation reaction of Ni(0) with NO (B) measured at 873 K. The X-ray absorbance at 8.347 keV for the reduction (A) and the oxidation (B) processes is also plotted as a function of time (inset).

Fig. 7. Representative time-resolved XANES change for the reaction of Ni(0) with the gas mixture of CO (5.0 kPa) and NO (5.0 kPa) measured at 873 K (a). The time-course changes of the X-ray absorbance at 8.347 keV for the reaction of Ni(0) with gas mixtures of CO and NO at various mixing ratios (b). The partial gas pressures (PCO:PNO) are given in units of kPa.