4. Synergistic Effect of Ceria Morphology and Copper Doping for Water-Gas Shift Reaction
4.3. Results and Discussion
4.3.4. Reducibility of Pd Catalysts by Morphology Change and Cu Doping
TOF showed increased reducibility by morphology engineering and Cu doping (Figure 4.2c), but whether the TOF was increased by the reducibility improvement remains to be precisely determined.
Thus, further analyses were performed to characterize the reducibility. Temperature-programmed reduction (H2-TPR) was performed to observe the catalytic properties in the reduction conditions (Figure 4.4a). In H2-TPR, the oxygen species can be identified by reduction peaks as a function of temperature. Pd/CeO2-C (59.8 °C) shows a reduction peak at a lower temperature than Pd/CeO2-O (299.3 °C). These surface reduction peaks are due to the Olatt near the Pd. Furthermore, Cu doping of Pd/CeO2-C and Pd/CeO2-O amplified the peak and shifted it slightly even to a lower temperature (58 °C for Pd/CDC-C, 271.5 °C for Pd/CDC-O). This change is evidence of increased reducibility of CeO2; PdO reduction occurred below room temperature (RT),13, 27 and therefore did not show in the spectra.
Figure 4.4. Reducibility tests of tuned CeO2-supported Pd catalysts. (a) H2-TPR profile. (b) Time- resolved CO oxidation. (c) Lattice expansion by CO-TPR and in-situ XRD. XPS spectra of (d) Ce 3d, (e) O 1s.
The concentration of Ce3+ (CSD) is related to the amount of VO on the surface, or to the reducibility of the support;39 when an Olatt desorbs, it leaves two excess electrons, which fill the empty 4f orbitals of Ce4+ to yield Ce3+.39, 40 Ce 3d core level X-ray photoelectron spectrometer (XPS) spectra of all samples were de-convoluted into 10 peaks (Figure 4.4d). Then CSD was derived by comparing the areas under them. CSD was higher for cubes (Pd/CDC-C and Pd/CeO2-C) than for octahedra (Pd/CDC-O and
Pd/CeO2-O); this difference indicates that the amount of surface defects depends on the exposed facet.
Cu doping further increased CSD of both morphologies.
The Oads/Olatt ratio also indicates the degree of VO formation, where the Oads is a surface-adsorbed oxygen that is involved in the formation of VO. Surface-active oxygen species can be identified by O 1s core level spectra (Figure 4.4e). The Oads/Olatt was higher in Pd/CeO2-C than in Pd/CeO2-O, and the ratio was increased by Cu doping; this result is consistent with the CSD analysis, and may be due to the difference in the Olatt-binding strengths of the facets. Cu doping facilitated formation of defects; this observation is consistent with the literature, which has shown Evf lowering by Cu doping in CeO2.39
For reactions that involve Olatt, VO healing is faster than VO formation in supported metal catalysts.
The oxygen adsorbed on VO transforms into active oxygen species by accepting delocalized electrons from the vacancies;41 so surface-active oxygen species are short-lived and difficult to observe under steady-state reaction conditions.42, 43 Therefore, a time-resolved CO-oxidation experiment was performed at 300 °C to semi-quantitatively measure the amount of this oxygen species (Figure 4.4b).
This experiment allows estimation of the degree of surface reduction by the reaction with CO. Pd/CeO2- C produced more CO2 than Pd/CeO2-O did; this difference means that the cube has more available active oxygens than the octahedron does. Cu doping increased the amount of CO2 produced, because Cu doping lowered the Evf, and thereby increased the reducibility. Therefore, the results of time-resolved CO oxidation concur with the analyses of CSD and Oads/Olatt from XPS, that the increase of reducibility by morphology control and Cu doping.
In-situ XRD was combined with CO-TPR to estimate the reducibility of the support. During CO- TPR, CO reacts with Olatt to form CO2, and Olatt desorbs to leave a VO on the surface, while passing an electron to an adjacent Ce4+ cation, which therefore becomes Ce3+. A Ce3+ is larger than a Ce4+, so the overall lattice parameter of CeO2 increases; this expansion can be calculated from the results of in-situ XRD, and be used to quantify the surface reduction. The lattice expansion was calculated by the CO- TPR and in-situ XRD results in the presence of CO (Figure 4.4c). As the temperature increased, the lattice parameter of the Pd/CDC-C was increased more than that of Pd/CDC-O. This comparison is further evidence that the improvement of Pd/CDC-C by morphology engineering and Cu doping yielded increased reducibility.
DFT calculations were performed to identify how morphology modification and Cu doping affected VO formation at the atomic scale. DFT-calculated Evf that is required to create a VO was 1.54 eV for CeO2(100) and 2.92 eV for CeO2(111); the difference shows that morphology control increases the reducibility. Calculated Evf were -1.06 eV for CDC(100) and -0.13 eV for CDC(111); these results also indicate that that Cu doping increases improves the reducibility, and corroborate the evidence for reducibility increase obtained in the experimental analyses in previous sections. They also demonstrate that the VO formation is easiest in Pd/CDC-C, as a result of morphology engineering and Cu doping.
Figure 4.5. Characterizations of Cu dopants and the dependence of reducibility on surface Cu. (a) Difference in TOFs. (b) XPS Cu 2p and (c) Pd 3d spectra of Pd/CDC-C and Pd/CDC-O. ToF-SIMS surface and depth profile of (d) CDC-C and (e) CDC-O. (f) Evf of CeO2(100) and (111) depending on the atomic ratio of surface Cu (NCu/(NCu+NCe)); yellow region: reasonable range of NCu/(NCu+NCe) around 1.49 mol % from ICP-OES; l, k, j, i denote the Evf of CDC(100), CeO2(100), CDC(111), and CeO2(111), respectively, with larger NCu/(NCu+NCe) on CDC(100) than that on CDC(111).
4.3.5 Reinforcement of Cu Doping Effect and CeO2 Facet
The WGSR activity of CeO2 catalysts can be increased by Cu doping. Moreover, even with the same amount of Cu (CCu), the TOF is increased more for the cubic structure than for the octahedral structure (Figure 4.5a),; this difference indicates that Cu-induced increase in the catalytic activity can be reinforced by modification of morphology. For CeO2 catalysts, the extent of dopant migration to the surface depends on the migrating direction (or facet), and the amount of dopant at the surface affects the catalytic activity.39, 44 The reinforcement is reasonably attributed to the difference in the segregation tendency of Cu on the two types of CeO2 facet. The XPS analysis supports this argument. Although a pair of peaks and a satellite due to Cu appear in the spectrum of Pd/CDC-C, only a small peak appears in the spectrum of Pd/CDC-O (Figure 4.5b). In Pd 3d spectra, the peak intensity regarding Pd‒Cu bonding was stronger in Pd/CDC-C than in Pd/CDC-O (Figure 4.5c). Considering that almost the same CCu was included in both catalysts, as verified by ICP-OES, these results indicate that the Cu can segregate and be placed near the surface more easily in cubes than in octahedra.
To further investigate the distribution of the Cu inside the CeO2 surfaces of different morphology, time-of-flight secondary ion mass spectroscopy (ToF-SIMS) analysis was performed (Figure 4.5d and 4.5e). The surface profile shows the distribution of Cu and Ce ions within 20 nm of the surface. The mass-to-charge ratio (m/z) that corresponds to Cu ions (m/z ~60) was more common and m/z ~140 of Ce ions was less common in the CDC-C surface (Figure 4.5d) than in CDC-O (Figure 4.5e); i.e., the proportion of Cu ions at the surface was higher in CDC-C than in CDC-O. Depth profiles were also analyzed to obtain detailed information on Cu distribution within CeO2 (Figure 4.5d and 4.5e). The ToF-SIMS results show that in CDC-C the amount of the Cu and Ce ions stays at a certain level, but in CDC-O the amount of Cu decreases as it approaches to the surface. This comparison indicates that the segregation of Cu towards octahedron’s surface is harder than towards the cube’s surface.
To understand the segregation of Cu towards the CDC surfaces at an atomic scale, Cu segregation energy (ECu,seg) of CDC(100) and (111) slab models, which represent cube and octahedron surfaces, respectively, were obtained by DFT calculations. A lower ECu,seg means easier Cu segregation towards the surface. CDC(100) had ECu,seg = -1.89 eV, whereas CDC(111) had ECu,seg = -1.69 eV; the difference means that Cu segregation along [100] is thermodynamically preferred to that along [111]. This energetics analysis supports the arguments derived from XPS and ToF-SIMS analyses that the Cu more easily migrates to the surface of the cubic CeO2 than of octahedral CeO2.
Figure 4.6. Quantitative analysis of WGSR rate tuned by morphology engineering and Cu doping. (a) Doping sensitivity of rate and ratio of doping sensitivity of cube to octahedron sample as functions of CCu. (b) WGSR rate of cube and octahedron and morphology effect on the rate as functions of CCu. (c) Schematics of the mechanism for the increase of CeO2 support reducibility and Pd metal dispersion by morphology engineering and Cu doping.
Cu exposed at the surface facilitates VO formation, so the increased amount of surface Cu would lead to increased reducibility, which is crucial for the WGSR actvity. To test this possibility, DFT calculations were performed to obtain Evf of (100) and (111) slab models by increasing the number of surface Cu from zero to four (CeO2 → CDC-4Cu) (Figure 4.5f). As surface Cu was increased, the Evf
of both facets decreased: 1.54 → -2.64 eV for (100), and 2.92 → -0.98 eV for (111); this shows the downshifting trend from (111) to (100) facet (Figure 4.5f). These results are consistent with XPS and H2-TPR results of CeO2 nanoparticles with different Cu concentration. Within a reasonably low region of Cu density, NCu/(NCu+NCe), which is larger on (100), the Evf (l > k > j > i) also explains the reducibility tendency of CDC-C > CeO2-C > CDC-O > CeO2-O, obtained by H2-TPR (Figure 4.4a); NCu (NCe): the number of Cu (Ce) atoms at the first cation layer of a CeO2 slab model. Thus, it is demonstrated that the Cu doping effect can be further increased by using cubic CeO2 because Cu segregation is easier in cubes than in octahedra, and thereby increases the amount of surface Cu, which is essential for reducibility.
4.3.6 Quantification of Morphology and Cu Doping Effects on WGSR
Morphology engineering and Cu doping increase the catalytic activity by increasing the CeO2
support reducibility and DPd. To quantitatively understand the increase of WGS reactivity by these methods, the WGS reaction rate is theoretically analyzed in this section. For the analysis, a simple reaction rate model is constructed using experimentally measured properties such as the CSD.
The reaction rate is related to DPd and TOF by the equation,
Pd Pd ,
rate n D TOF (1)
where nPd is the number of Pd atoms (nPd = 1.16 × 10-6 mol for all samples) and DPd is the dispersion of Pd [%]. TOF is a function of CSD, so the combination of CSD and DPd determines the reaction rate.
To quantify the increase of CSD and DPd by the morphology engineering and Cu doping, the quantities were modeled as functions of CCu for each morphology i (= cube, octahedron):
S D , C u , S D ,
p u r e
i i i i
C a C C (2)
P d , C u , P d ,
p u re
i i i i
D b C D (3)
where CSD ,purei and DPd,purei are the CSD and DPd of undoped CeO2 surface with morphology i, respectively. ai and bi respectively are coefficients that represent sensitivities of CSD and DPd to CCu in morphology i. Using measured values of the two properties with respect to CCu, the sensitivities were calculated as acube = 1.69, aocta = 0.805, bcube = 4.19, and bocta = 1.34. Comparison of the coefficients indicates that Cu doping of cubes increases CSD and DPd 2.10 (= acube/aocta) and 3.13 (= bcube/bocta) times more than Cu doping of octahedra does, respectively. Higher doping sensitivity of CSD (acube) can be attributed to the lower ECu,seg and Evf of the cube surface than of the octahedron surface. Easier increase
of DPd (bcube) can be explained by lower ECu,seg of cube than that of octahedron, and by the increase of Pd binding by Cu (Eads< 0 upon Cu doping). Inserting equations (2) and (3) into (1) yields the rate equation as a function of CCu:
2
Cu, Cu,
(0.0484 1.59 0.0745),
octa Pd octa octa
rate n C C
(4)2
Cu, Cu,
(0.318 4.99 6.30).
cube Pd cube cube
rate n C C
(5)The derivatives of the rates in terms of CCu, Cu d
d cube rate
C
and Cu
d
d octa rate C
, are named as “doping
sensitivity of rate” (Figure 4.6a). For the same CCu, C u C u
d d
d cube/ d octa
rate rate
C C
(ratio of doping sensitivity) is larger than 3.13, converging to 6.57 as the CCu increases (Figure 4.6a). Thus, the doping sensitivity of rate in the cube is always > 3.13 times higher than in the octahedron.
The effect of morphology on the rate,
cube octa
rate rate
, is named as “morphology effect” (Figure 4.6b).
Practically for CCu < 20.0 mol % (solubility limit),39 the morphology effect is larger than 4.18, and also approaches to 6.57 as the CCu increases (Figure 4.6b). Thus, the morphology effect improves the reaction rate of octahedron > 4.18 times for the same CCu. Although the rate of octahedron can be improved by Cu doping even without the morphology engineering, the rate can be more significantly increased by Cu doping when the morphology is changed into cube (Figure 4.6b) because the doping sensitivity of rate is higher in the cube than in the octahedron (Figure 4.6a). Owing to the simplicity of the rate equations above, the quantitative analysis shown here can be easily employed to assess the contribution to each of surface atom control methods (e.g. morphology engineering, metal doping) on the improvement of materials properties (e.g. reducibility, metal dispersion) and reactivity of catalytic materials in the form of supported metal.
4.4. Conclusion
As key properties to enhance the catalytic activity of WGSR, we controlled the exposed surface atoms of the supported metal catalyst. The reducibility and DPd of Pd/CDC were effectively boosted by morphology engineering and Cu doping. To verify, Cu-doped CeO2 was synthesized in cubic and octahedral shapes (CDC-C and CDC-O), and Pd was loaded to yield an improved WGSR catalyst.
Under WGSR conditions, the cubic form (Pd/CeO2-C) was more active than the octahedral form (Pd/CeO2-O), and Cu doping increased the activity of both catalysts(activity: Pd/CDC-C > Pd/CeO2-C
> Pd/CDC-O > Pd/CeO2-O). EXAFS, DRIFTS, and GA-DFT calculations showed that both the morphology engineering and Cu doping increase DPd. DFT calculations clarified that the increased DPd
on cube is attributed to the stronger binding of Pd to the (100) surface than to the (111) surface. Even
without the effect of DPd,TOF still showed that these tuning methods increase the activity for WGSR catalysis. H2-TPR, XPS, time-resolved CO oxidation, in-situ XRD, and DFT calculations indicate that the reducibility was increased because the two methods lowered Evf. XPS and ToF-SIMS with DFT calculations of ECu,seg further revealed that the two support-tuning strategies can yield activity increase larger than just the sum of two effects, because increased ease of Cu segregation in the cube reinforces the effect of Cu doping; thus, simultaneous application of morphology engineering and doping was clearly verified to synergistically increase the efficiency of supported metal catalyst for reactions involving redox process. For many essential reactions like TWC reactions, CO PROX, and WGSR, the knowledge of rational design will help to guide the development of cutting-edge supported metal catalysts.
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Chapter 5
Interface Control between Spinel Oxides and Ceria to Understand the Role of Ceria during the Oxidation Reaction
This chapter includes the published contents:
Yoon, S.; Jo, J.; Jeon, B.; Lee, J.; Cho, M. G.; Oh, M. H.; Jeong, B.; Shin, T. J.; Jeong, H. Y.; Park, J.
Y.; Hyeon, T.; An, K. ACS Catal. 2021, 11, 1516–1527. DOI: 10.1021/acscatal.0c04091. Reproduced with permission. Copyright © 2021 American Chemical Society.
5.1. Introduction
The interfacial effect is a critical factor for improving the catalytic performances of heterogeneous catalysts, which typically consist of noble metal nanoparticles (NPs) supported on oxides.1⁻4 In particular, charge transfer at the interface of the supported catalyst is important for determining the structure of the adsorbate and surface intermediates and understanding the reaction mechanism.5⁻7 Several reducible oxides (CeO2, TiO2, Co3O4, and NiO) are known to enhance the catalytic activity through charge transfer at the interface when noble metals (Pt, Au, and Pd) are supported.8⁻10 Among these, CeO2, which exhibits a high oxygen storage capacity and the facile formation of oxygen vacancies, is an excellent oxide support that can reversibly reduce Ce4+ ions to Ce3+ ions at the interface with metals.11,12 CO oxidation has been extensively studied as a model reaction to investigate the interfacial effects.13,14 Since CO is more strongly adsorbed to noble metals than O2, CeO2 plays a crucial role in improving the CO oxidation rate by delivering oxygen at the interface, particularly under oxygen-deficient conditions.15,16 However, if a spinel oxide rather than a noble metal is used as the main active species, the change in the oxidation state at the oxide-oxide interface becomes more complex, and the factors that affect the reactivity are not easily characterized.
Spinel oxides such as Co3O4 and Mn3O4 are excellent supports for noble metal catalysts and significantly improve the CO oxidation rate, in addition to exhibiting CO oxidation reactivity even in the absence of noble metals.17⁻19 Xie et al. reported that spinel Co3O4 nanorods predominantly exposing {110} planes showed high activities in the low-temperature CO oxidation reaction due to the abundant exposure of active Co3+ sites.18 Co3O4-catalyzed CO oxidation follows a typical Mars–van Krevelen (MvK) mechanism whereby CO molecules react with the surface lattice oxygen atoms coordinated to