Improving the Activity and Stability of YSZ-Supported Gold Powder

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Improving the activity and stability of YSZ supported gold powder catalyst by means of ultrathin, coherent, ceria overlayers. Atomic scale structural insights. Ramon Manzorro, William Enrique Celin, José A. Pérez-Omil, José J. Calvino, and Susana Trasobares ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04412 • Publication Date (Web): 25 Apr 2019 Downloaded from http://pubs.acs.org on April 25, 2019

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Improving the activity and stability of YSZ supported gold powder catalyst by means of ultrathin, coherent, ceria overlayers. Atomic scale structural insights. Ramón Manzorro, William E. Celín, José A. Pérez-Omil*, José J. Calvino and Susana Trasobares Departamento de Ciencia de los Materiales e Ingeniería Metalúrgica y Química Inorgánica Facultad de Ciencias, Universidad de Cádiz, Campus Río San Pedro, Puerto Real, 11510 Cádiz, Spain ABSTRACT

A

Au(0.85

wt%)/YSZ

catalyst

was

prepared

through

deposition-

precipitation method and afterward modified with the addition of a very slight amount of CeO2 (3.7 wt%) by incipient wetness impregnation. A prior electron microscopy characterization points out that both catalysts, Au(0.85 wt%)/YSZ and CeO2(3.7 wt%)/Au(0.85 wt%)/YSZ, exhibit a similar Au nanoparticle distribution with most

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particles below 5 nm. The CO oxidation reaction was tested over these catalysts in a heating-cooling cycles experiment, which evidenced a much better stability of the CeO2-modified sample against deactivation under very harsh temperature conditions. The characterization of the catalysts after reaction indicates that the sintering effect of the Au nanoparticles is quite similar in both cases, this suggesting the key role of specific interactions between Au and CeO2 on the performance of the surface modified catalyst. An in-depth Aberration Correction Electron Microscopy study, combining imaging and analytical techniques, allowed us to characterize the details of the spatial distribution and structure at atomic level of CeO2. The formation of atomically-thin CeO2 layers

extending on

detected,

the

particularly

surface in

the

of the YSZ form

of

crystallites was

coherent

monolayers

epitaxially growth on YSZ(111), which guarantee an interaction between ceria and the supported metal phase. Image simulation and Density Functional Theory calculations carried out further confirm the Electron Microscopy observations. A comparison, in terms of stability, to the results observed on a CeO2 modified Au/TiO2 catalyst

of

similar

composition

reveals

both

a

much

better

performance of the catalyst supported on YSZ and neat differences in the nature of the interactions between CeO2 and the support as well as between Au and CeO2. The structural coherence between CeO2 and the cubic YSZ support triggers specific interaction mechanisms


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which differentiate the behavior of CeO2/Au/YSZ catalysts from that of CeO2/Au/TiO2. The whole set of results evidence not only the key role played by highly dispersed and ultrathin ceria surface layers as modifier and stabilizer of the performance of Au-based CO oxidation catalysts but also how advanced, aberration corrected, electron microscopy techniques are a requirement to unveil the structure of such unique nanostructures.

KEYWORDS: YSZ Supported Au catalysts; Advanced electron microscopy; DFT calculations; Ceria monolayers; CO oxidation

1.- INTRODUCTION Since the pioneering work of Haruta revealing that gold nanoparticles below 5 nm were extremely reactive1-2, supported gold nanomaterials have been intensively investigated as promising catalysts in a variety of applications, e.g. as gas sensor or in fuel cells3-5. In fact, gold deposited over reducible metals oxide has become a hot topic because of its high activity for reactions such as oxidation of hydrogen to hydrogen peroxide6 or selective oxidation of different hydrocarbons7. However, the most widely investigated reaction using gold catalyst is, by far, the low temperature CO oxidation


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reaction, in which gold catalysts supported on reducible supports are very active even at temperatures as low as 200 K8-9. Noble metals, such as Rh, Pt, Au or Ru, deposited over different support oxides all exhibit a remarkable behavior in the CO oxidation reaction10-11, but in spite that Au catalysts present the highest activity12, Pt catalysts are the most widely used systems, since they provide an optimum combination of high activity and stability. Although some Au-based catalysts have been recently commercialized at industrial level for processes like the hydrochlorination of acetylene13, the synthesis of vinyl acetate14 or the oxidative esterification of methacrolein to methyl methacrylate15, a more widespread use of gold based catalysts is still missing. A major drawback limits widening the applicability range of these systems, the large susceptibility of Au catalysts to deactivation by sintering16. This particularity does not only affect directly the durability of the catalysts but also, indirectly, the development of viable catalyst preparation methods for a large scale production of highly dispersed catalysts. Unfortunately, highly dispersed Au deactivate much rapidly than some other noble metals, as Rh or Pt, which is very likely motivated by its comparatively low fusion temperature (1337K)17-18. Thus, at temperatures in the 500-


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600K range, i.e. close to the Tamman temperature (1/2 fusion temperature), Au catalysts already suffer intense sintering. Such effect precludes the use of Au catalysts in high temperature applications, like Three Way Catalysts (TWC). These catalytic devices commonly work at temperature regimes between 673 and 1073 K with sudden excursions up to 1273 K19-20, to achieve an efficient conversion of the major pollutants (HC, NOx and CO)21. These working temperatures on TWC demand catalytic components with a high thermal stability for a long time. Thus, although Au could play a more effective role than other, more expensive and critical metals as Rh, Pt or Pd, its very poor stability dismiss it as a candidate for this application. Two different strategies could be followed to deal with this drawback affecting the dispersion of Au nanoparticles, and therefore its catalytic activity. The first concerns the decrease in the TWC working temperature while the second one is related to the stabilization of Au nanoparticles at high temperatures. At this respect, the new design of gasoline and diesel advanced engines have aimed at reducing significantly the pollutant emissions at moderate temperatures, below 500 K22-23. In this case, the incorporation of Au in the formulation of the monolith washcoating becomes feasible to complement the role of other metals like Rh or Pt.


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The second route to implement Au-loaded catalytic formulations in high temperature applications would involve improving the resistance of the catalyst towards deactivation under working conditions. At this respect, several strategies have been proposed to limit Au sintering at high temperatures, which imply enhancing the adhesion of Au to surface sites. Nevertheless these routes involve tedious preparation and support modification processes24-27. For example, Wang et al. synthesized Al2O3 with 3D configuration sheets giving an open structure and rough surface so as to immobilize Au nanoparticles and provide some thermal stability28. More recently Chen et al. modified the surface of zeolites and deposited ultra-small Au nanoparticles of 1 nm in size in the supercage of these zeolites avoiding some sintering effect29. In a much closer approach, Zhu and co-workers achieved the stabilization of Au particles through a simpler postmodification route consisting in the deposition of amorphous silica via impregnation on top of a conventional Au/TiO2 catalyst30. This work revealed an improvement on the catalytic activity on the modified SiO2/Au/TiO2 system when compared to the behavior of a Au supported on SiO2/TiO2, pointing out the influence of the addition order during the synthesis of the SiO2 modified catalyst. Continuing with this strategy, Del Rio et al.


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modified a reference Au/TiO2 catalyst by impregnation with CeO2 and it was observed that the thermal stability of Au nanoparticles was highly improved after the modification and also that the modified catalyst preserved a good catalytic activity, but only up to 973 K31. It is worth mentioning that, apart from the last two papers commented in the paragraph above, most of the Au stabilization strategies studied up to now involve complex preparation methods not amenable to large scale production. Moreover, routes based on the much simpler impregnation method have been only assayed on TiO2 supported catalysts, whose main handicap is the occurrence of a phase transformation from anatase to rutile in reactions taking place at temperatures above 873 K. Yttria-Stabilized Zirconia (YSZ) offers an opportunity to study a system whose structure, cubic or pseudo-cubic32-33, is closer to those of Au fcc and CeO2. Moreover, since the cubic structure remains stable for these oxides even at quite high temperatures, it also provides a chance to estimate the influence of structural changes in the support on the high temperature driven deactivation of supported Au catalysts. Comparison of Au/YSZ catalysts with others based on closely related supports, as it is the case of TiO2, provides also an opportunity to investigate the influence of structural coherence on the stabilization role


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of ceria overlayers, an aspect not considered so far, to which special attention is paid in this work. Although the Au-YSZ system has been widely investigated in the context of electrocatalytic cells, in which Au usually works as electrode and YSZ as the electrolyte34-36, only some previous works have reported on the preparation of powder type Au/YSZ catalyst and their behavior in CO oxidation37. Moreover, to the best of our knowledge none of them focus on high temperature stability issues or the influence of submonolayer modification coatings to improve their deactivation resistance. Indeed, an efficient stabilization of Au nanoparticles, commonly tested in reactions performed at low temperatures, would open an excellent and unique opportunity to widen their application range to reactions where high temperatures are demanded. Therefore, following our previous work30, this contribution explores the synthesis of an Yttria Stabilized Zirconia (YSZ) supported gold catalyst, further modified by addition of submonolayer amounts of CeO2 via the straightforward route of incipient wetness impregnation. To get a deeper understanding about the role of the ceria overlayers in the stabilization effect as well as the advantages in the CO oxidation reaction observed when using the structurally stable ionic conductor YSZ support, a much more detailed characterization at atomic scale


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through a combination of advanced electron microscopy and theoretical DFT calculations has been performed. This study has revealed features only discernible in the sub-angstrom resolution range. Accordingly, the major goals of this work are to analyze the intrinsic catalytic performance of the prepared catalysts, to evaluate the effect on catalytic activity and stability of Au nanoparticles after the addition of CeO2 and understanding the observed stabilization effects on structural grounds. To this end, an in-depth characterization at macroscopic, nanoscopic and atomic scale of the prepared materials, using a wide combination of techniques, as well as a comparison with previous results obtained on closely related TiO2-based catalysts30 are undertaken. Findings obtained in this study demonstrate the high potential of ultra-thin, structurally coherent, surface layers in the design of novel catalytic materials, particularly in terms of promoting activity and stability of noble metal phases. In our view, this is a topic insufficiently understood from a fundamental point of view, when dealing with powder type catalysts close to final applications. Very likely, one of the limiting factors in the advancement of these studies stems from the need to face a characterization using ultra-high resolution


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analytical tools38, as are those available in the AberrationCorrected microscopes employed in this work.

2.- EXPERIMENTAL SECTION 2.1 Sample Preparation. The catalysts were prepared using a commercial Yttria Stabilized Zirconia (YSZ, 8 at% Y) oxide from Sigma Aldrich (CAS Number 114168-16-0). This oxide was calcined in static mode for 2 hours at 1173 K (temperature ramp 10 K/min) in a muffle oven in order to obtain a support texturally stabilized against high temperature conditions. The surface area of the oxide after this treatment was 45 m2/g with pore volumes of 0.21 cm3/g. Au was deposited on the stabilized YSZ surface (5 g.) to a final loading of 0.85 wt% by the deposition-precipitation method (DP) at 60

oC,

using a 5 mM HAuCl4·3H2O aqueous solution and

adding 1 mL per minute for 75 minutes. The synthesis was accomplished under continuous stirring at constant pH 8, achieved with a Na2CO3 0.05 M solution, and the resulting solution was kept for 1 hour under stirring at the same temperature. After Au deposition, the product was filtered and washed at room temperature until the complete elimination of Clspecies was achieved using AgNO3 as precipitating agent. The


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resulting solid was then dried for 12 h at 373 K in an oven. Finally, the Au/YSZ catalyst was calcined in air at 673 K for 2 h. To avoid any further change, the catalyst was stored under inert gas and isolated from sunlight at low temperature in a fridge. The procedure to follow in detailed the synthesis is described elsewhere39. A portion of this Au/YSZ catalyst, 3 g., was further modified by the addition of half a monolayer of CeO2, i.e. 3.7 wt% of Ce. This modification of the initial system was carried out by Incipient Wetness Impregnation (IWI), using 4.2 mL of a 0.58 M Ce(NO3)3·6H2O aqueous solution. To obtain the CeO2/Au/YSZ catalyst, the impregnated solid was submitted to calcination for 1 h in air at 673 K in static mode. This sample exhibits 41 m2/g of SBET and pores volume of 0.18 cm3/g, this indicating that the addition of CeO2 did not change significantly the textural features of the sample. Samples for STEM studies were prepared by depositing a small amount of the powder onto lacey carbon coated Cu grids without the use of solvents in order to avoid contamination. 2.2 Catalytic Activity. The catalytic activity in CO oxidation was evaluated using a flow quartz reactor (more information about the reactor in Figure S1) and 25 mg of sample diluted in 100 mg of SiC to avoid the formation of hot spots in the


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catalyst bed. Activity was monitored following the CO and CO2 signal using a quadrupole Mass Spectrometer (Pfeiffer Vacuum Prisma). Prior to the reaction, the samples were submitted to an activation process consisting in heating the sample at 523 K for 2 hours, the first hour under an O2(5%)/He atmosphere and the second one under He flow. For the CO oxidation reaction, the gas flow was composed of 1:0.6:98.4 of CO, O2 and He respectively, with spatial velocity of 240,000 cm3·h-1·g-1. Light-off curves were recorded using a heating rate of 10 K/min. In fact, catalytic activity was measured during six consecutive heatingcooling cycles going from room temperature up to 523 K, 623 K, 773 K, 923 K, 1223 K-1 and 1223 K-2, from now on referred as to cycles a to f (Figure S2 in supporting information sketches the full experimental procedure as well as a graph depicting the performance in CO oxidation of the SiC used as diluent). At every cycle, the final temperature was kept for 1 hour and then the catalyst was cooled down to room temperature, always under the reaction mixture. After the whole CO oxidation cycling, Au/YSZ-post reaction and CeO2/Au/YSZ-post reaction samples are obtained. Beyond these harsh experiments, in order to study the thermal and textural resistance of the synthesized catalysts under


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stringent conditions, a more severe test was carried out which included several 6 hours cycles at 1223 K under the reaction mixture. During this time, the CO and CO2 signals were continuously monitored. The samples obtained after submitting the Au/YSZ and CeO2/Au/YSZ catalysts to this extreme experiment are labeled as Au/YSZ-long cycle and CeO2/Au/YSZ-long cycle. They have been tested for 24 h and 42 h respectively. 2.3 Characterization Techniques. The catalyst samples were characterized before and after CO oxidation reaction using a variety of Scanning Transmission Electron Microscopy (STEM) techniques. High Angle Annular Dark Field (HAADF-STEM) images were acquired in both a JEOL 2010F (Cs=0.5 mm) and an aberration-corrected monochromated FEI Titan Cubed Themis 60-300 (Cs=0.001 mm and sub-angstrom resolution) microscopes, both operating at 200 kV. Given the large difference in atomic number between gold (Z=79) and the rest of elements in the YSZ support (Zeff=39.85, Egerton40), HAADF-STEM allows an optimum visualization of Au nanoparticles32 on this support. From HAADFSTEM images, Au particle size distributions have been obtained by directly measuring the size of a large number of particles using a software tool developed in our lab. To distinguish Au from Ce (Z=58), and therefore to study the dispersion of Ce over the surface, it was necessary to combine HAADF-STEM images with


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analytical techniques, such as X-Ray Energy Dispersive Spectroscopy (X-EDS) and Electron Energy Loss Spectroscopy (EELS). X-EDS qualitative measurements were performed with the Super X-G2 capabilities of the FEI Titan Cubed Themis 60–300 microscope working in STEM mode, using a beam current of 200 pA and a dwell time per pixel of 128 μs. X-EDS maps were obtained analyzing the Ce-L and Zr-K lines. To improve visualization, the elemental maps were post filtered using a Gaussian blur of 0.8, as provided in the Velox software. Additionally, very high spatial resolution EELS experiments were performed working in the spectrum imaging (SI) mode41, which allows the correlation of analytical and structural information of selected regions of the material under study. In this technique, the EELS and HAADF signals are collected simultaneously while the electron beam is scanned across the selected area of the sample. The SI experiments were acquired in Dual EELS mode using an energy dispersion of 0.25 eV, 50 pA probe current and 50 ms acquisition time per EELS spectrum. In the Dual EELS mode, the zero-loss region is recorded simultaneously with the core-loss signal of the element(s) of interest, which allows a very precise determination of the absolute value of the energies at which the core-loss edges are appearing in the experiment. In our case, Ce-M4,5 (885-905 eV) elemental maps were built after removing the


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background from raw data, using a power law model and a window width of 25 eV. Elemental composition of the samples at macroscopic level was determined by ICP-AES in an Iris Intrepid equipment from Thermo. BET surface areas were obtained via N2 physisorption at 77 K using an Autosorb iQ3 equipment from Quantachrome Instruments. A Bruker B8ADVANCE A25 Davinci equipment was used for powder X-ray diffraction experiments with a Cu source. 2.4 STEM Image Simulations. Cs-corrected HAADF STEM image simulations were performed with TEM-SIM software from E.J. Kirkland42. The following parameters were considered for the calculations: High Voltage 200 kV, Third Order Spherical Aberration 0.001 mm, Fifth Order Spherical Aberration 5 mm, Scherzer Defocus 1.9 nm, and HAADF Detector Geometry 80-200 mrad. A Gaussian Blur with a sigma value of 2 and electronic noise with standard deviation of 25% were applied to the simulated images. 2.5 Density Functional Calculations. The DFT calculations were performed using Quantum Espresso code43. The Perdew-BurkeErnzerhof (PBE) exchange-correlation functional was used within the generalized gradient approximation (GGA)44. For the effect of the cationic cores ultrasoft pseudopotentials were used45 . Ce(5s2,5p6,6s2,4f2), Zr(3s2,3p6,4s2,3d2) and O(2s2,2p4) were treated


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as valence electrons with a plane energy cut-off of 50 Ry, calculated after a convergence study. For K-point integrations a 8x8x8 Monkhorst-Pack grid was used for calculations of the unit cells of bulk systems while 2x4x1 and 2x2x1 grids were used for the calculations of the smaller and larger supercells, respectively. For the cell-ions relaxation the Broyden–Fletcher– Goldfarb–Shanno (BFGS) algorithm for geometry optimization included in Quantum Espresso code was used. 2.6 Supercell modelling. All the supercells for image simulations and DFT calculations were built using Rhodius software which allows to control the orientation, morphology and atomic positions of the nanostructures46. 3.- RESULTS AND DISCUSSION 3.1 Experimental data. XRD patterns of the Au/YSZ and CeO2/Au/YSZ samples (included in Figure S3) indicate the presence of reflections at 30.1o, 34.9o, 50.2o and 59.7o, which could be associated to a cubic or slightly distorted tetragonal YSZ oxide. The volume averaged size of the crystalline domains in these samples, as determined using the Powder-cell software47, was 22 nm. Concerning the metallic component, the characteristic peaks of fcc Au were not observed in any of the samples; this indicating that the DP method led to the formation of a major fraction of nanoparticles with diameter below 5 nm. Moreover,


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CeO2 distinctive peaks did not appear in the CeO2/Au/YSZ diffractogram, this suggesting that the impregnation method allowed incorporating also CeO2 over the support surface in the form of highly dispersed nanostructures.

Figure

1.

HAADF-STEM

images

recorded

on

the

Au/YSZ

(a)

and

CeO2/Au/YSZ (b) starting catalysts. Au particle size distribution histograms of both systems (c). A couple of HAADF images have been appended as supporting information 4 to include more evidences about the size and metal dispersion. Low magnification HAADF images displayed in Figures 1a and 1b illustrate the distribution of Au nanoparticles over the surface of the support in the Au/YSZ and CeO2/Au/YSZ catalysts


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respectively. Note how the metal nanoparticles are clearly observed in the form of high intensity nanometer-sized spots. In both cases, X-EDS and EELS nanoanalysis have been used to determine the spatial distribution of the different elements, confirming that the regions depicting low intensity in the HAADF images correspond to YSZ particles. These particles present sizes in good agreement with those estimated from XRD. Regarding the noble metal, Figure 1c shows the histograms of Au particle size after measuring more than 200 Au nanoparticles in each catalyst (229 particles for the Au/YSZ and 205 for the CeO2/Au/YSZ). Several HAADF images have been included to evidence the metal phase size and distribution (Figure S4). Note that nearly all the particles are below 5 nm in diameter, while most of them fall in the size range between 2 and 3.5 nm, with average size of 2.9 ± 0.6 nm and 3.0 ± 0.9 nm for the Au/YSZ and CeO2/Au/YSZ catalysts respectively. The comparative analysis of the two histograms suggests that the addition of ceria to the Au/YSZ sample enlarges smoothly the Au nanoparticle size, which can be very likely due to the additional calcination treatment carried out after the impregnation of the cerium precursor during the preparation of the CeO2/Au/YSZ. Consequently, a small shift to higher diameters is observed in the gold particles dispersed over the cerium


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modified catalyst, the population of nanoparticles below 3 nm decreasing from roughly 65% in Au/YSZ to 55% in CeO2/Au/YSZ. In the same sense, metallic dispersion drops from 40% to 35%. Apart from these slight differences, it is more important to highlight that the particle size distribution is quite similar in both catalysts. Therefore, any difference in the catalytic behavior of these two catalysts should be attributed to the addition of CeO2. It is important to recall at this respect the recognized contribution of the support and its interaction with Au nanoparticles in the CO oxidation performance9,

48

.

Table 1. T50 and T100 values for each cycle (1 to 6) on Au/YSZ and CeO2/Au/YSZ samples.

Figure 2 illustrates the CO conversion as a function of temperature during the heating-cooling cycles experiments. The vertical dashed line located at 873 K indicates the average working temperature for a TWC catalyst in a gasoline engine. Likewise, Table 1 summarizes the values at which 50% (T50) and


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100% (T100) CO conversion is reached. According to these data, both systems exhibit a similar performance in all the cycles, even during the first cycle at 1223 K (curve e). In both low Au loading YSZ supported catalysts, the unmodified and ceriamodified one, the T50 value remains close to 500 K. The T100 value, though slightly higher in the unmodified Au/YSZ catalyst, does not change too much (curves a to e) until the first cycle at 1223K is run.

Figure 2. CO oxidation catalytic curves on the heating-cooling cycles up to 523 K, 623 K, 773 K, 973 K, 1223 K-1 and 1223 K-2 (a to f respectively) on the Au/YSZ (a) and CeO2/Au/YSZ (b) catalysts. The vertical dotted line on the graphs indicates the average working temperature for a TWC.


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However, a dramatic difference between the two catalysts can be observed when we compare the second cycle at 1223 K, curves labelled with “f” in Figures 2(a) and 2(b). Notice that in the 1223 K-2 cycle the characteristic light-off temperature values experience an important increase in the Au/YSZ system, changing from 519 to 628 (T50) and from 599 to 1089 K (T100), the latter over the average temperature reached in a TWC catalyst. In clear contrast, the CeO2 modified catalyst fully retains its catalytic activity, which remains almost unchanged, with only negligible (T50) or even, surprisingly, negative shifts with respect to the initial values, all of them several hundred degrees below the average temperature in a TWC catalysts. Note that in the cycles e and f the catalyst has been kept for two hours at 1223 K, conditions much more severe than those of eventual excursions up to 1273 K which take place during TWC operation. Consequently, the combination of a high activity and an outstanding stability for the CO oxidation under severe working temperatures nominate the CeO2/Au/YSZ sample as a potential candidate to be incorporated not only in the formulations of gasoline engine catalysts, but also in some other applications requiring high temperatures. The large positive increment (417 K) between the T100 values of Au/YSZ and CeO2/Au/YSZ in the last aging cycle evidences both a


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strong deactivation of the Au/YSZ catalyst and a large stabilizing effect of CeO2, in good agreement with that observed in the CeO2/Au/TiO2 system31. Additionally, the activity of the CeO2/Au/YSZ catalyst is comparable to that of other Au/CeO2 and Au/ZrO2 systems9, which supports the idea of a synergistic effect between CeO2 and YSZ. Table 2. Turn Over Frequency at 423 K for the Au/YSZ and CeO2/Au/YSZ at each cycle.

In order to evaluate the catalytic behavior at low temperature, Turn Over Frequencies (TOF) per Au atom have been calculated for both catalysts at 423 K in each cycle, Table 2 (Figure S5 in supporting information shows the CO conversion vs T plots in the low temperature range). In the Au/YSZ catalyst the highest activity is observed in the first cycle (labelled a). A 30% drop in activity takes place in the cycle at 523K but the last cycle at 1223K is the one that deteriorates activity the most, the TOF value being in this case nearly one order of magnitude lower than the previous one.


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When the successive cycles are run with the modified catalyst, the activity remains almost constant up to the last cycle, after which a remarkable improvement of activity is observed, a behavior which is just opposite to that of the Au/YSZ catalyst. On the other hand, the CeO2/Au/YSZ shows in all cases a higher activity, a factor 1.5-2 times higher in cycles a to e and up to 14 times in the last cycle, this evidencing the positive contribution of CeO2 addition. To understand the structural origins of the differences in the deactivation between Au/YSZ and CeO2/Au/YSZ catalysts, the samples obtained after the CO oxidation reaction were analyzed and compared with those prior to reaction. According to XRD diffractograms (Figure S3), the support crystal size measured in the post reaction samples are close to each other, 19 nm in the unmodified catalyst and 21 nm in the ceria-modified one, and also close to the initial value before reaction, this indicating that the support itself has not been modified despite the harsh experimental conditions. In clear difference with the YSZ support, bulk CeO2 suffers significant sintering at high temperatures, which represents a drawback as catalytic support, since the drop of surface area may eventually result in encapsulation of the supported metal phase.


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Figure 3. HAADF-STEM images representative of the Au/YSZ (a) and CeO2/Au/YSZ (b) catalysts after the CO oxidation experiments. (c) Particle size distribution histogram for the Au/YSZ-post reaction and CeO2/Au/YSZ-post reaction samples. The region encircled in red is zoomed in the inset. Red arrows in the bottom image point to some of the Au particles, for illustration. Figures 3a and 3b show representative HAADF-STEM images of both catalysts after reaction. A first view of these images reveals that the YSZ support has not changed significantly its size or shape, in good agreement with the XRD results. The Au particle size distributions established for the post-reaction catalysts, where analytical techniques were used to discriminate first


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between Au and CeO2 nanostructures, show the presence of a large fraction (above 80%) of small Au nanoparticles, smaller than 5 nm, in both samples, Figure 3c. Nevertheless, and in contrast with that observed in the catalysts prior to reaction, a number of particles appear in both catalysts of very large size, in the range 10-50 nm, which indicates that sintering takes place in both catalysts. Note that the largest particles appear in the Au/YSZ post-reaction sample and that the CeO2-modified catalyst exhibits a slightly larger fraction of particles in the 1-4 nm range, as shown in the zoomed histogram of Figure 3c. Nevertheless, such small differences in the Au nanoparticle size distribution do not properly justify neither the stability observed for CeO2/Au/YSZ nor the differences of this catalyst with the Au/YSZ one. Therefore, specific interactions between Au and CeO2 must instead be at the roots of such observations.


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Figure 4. Representative results of the STEM study of the Cemodified catalyst before and after the CO oxidation reaction: HAADF-STEM images of CeO2/Au/YSZ (a) and CeO2/Au/YSZ-post reaction (d); STEM-XEDS elemental maps (Ce (blue) and Au (yellow)) of CeO2/Au/YSZ (b) and CeO2/Au/YSZ-post reaction (e); (c) and (f) display the Ce and Au intensity profiles along the white arrows marked on (b) and (e). Nanoanalytical electron microscopy techniques have allowed us studying in more detail the spatial distribution of CeO2 in the CeO2/Au/YSZ catalyst before and after reaction, Figure 4. In the catalyst before reaction, the chemical maps, Figure 4b, show Au nanoparticles (yellow) exactly in the regions where the more intense contrasts are observed in the HAADF image, Figure 4a. On


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the other hand, the Ce map (blue) indicates a random dispersion of ceria over the surface with areas of higher intensity nearby the Au nanoparticles. At first glance, the HAADF and elemental maps of the CeO2/Au/YSZ-post reaction catalyst, Figures 4d and 4e, appear quite similar. Nevertheless, a more detailed analysis of these maps using intensity profiles of both elements reveal substantial differences between the two samples. In particular, the comparison of the intensity profiles of Ce and Au extracted from the X-EDS maps, as those shown in Figures 4c and 4f for CeO2/Au/YSZ and CeO2/Au/YSZ-post reaction respectively, point out a neatly different Ce distribution. In the case of CeO2/Au/YSZ, Figure 4e, it can be observed how Ce concentrates at locations nearby the Au nanoparticles, whereas in the CeO2/Au/YSZ-post reaction a much more homogeneous distribution of CeO2 along the surface has been achieved, which improves the dispersion of CeO2 over the catalyst surface. An EELS map which supports the X-EDS results is presented in Figure S6 of supporting information. The EELS results confirm the differences in the CeO2 dispersion between the two samples. This evidence, only available after high spatial resolution chemical analysis, confirms that after the CO oxidation reaction most of the Au nanoparticles dispersed over the support surface are in contact with the CeO2 modification layer. This result clearly differs from that previously described for the CeO2/Au/TiO2 system31, in which the


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dispersion of CeO2 achieved after the reaction was much lower, the CeO2 modifying phase being present in that case in the form of patches of limited extension, which left a significant fraction of Au nanoparticles out of contact with the CeO2 overlayer. According to the characterization performed on the CeO2/Au/YSZpost reaction sample, the improved coverage of the YSZ surface by CeO2 will very likely increase the number of Au||CeO2||YSZ contacts, which could contribute to the negative shift observed in the T100 value in the last cycle (curve f) of the CO oxidation test. CeO2 redispersion, promoted among other factors by the high temperature of the 1223 K cycle and which increases the extent of Au||CeO2||YSZ contacts, could also explain the evolution of the TOF values in table 2; in particular, the TOF increase observed in the last cycle of the ceria modified catalyst. As reported in previous works, nanostructuration of CeO2 in the form of thin layers, as that achieved in the CeO2/Au/YSZ catalyst after redispersion, improves its reducibility with respect to that of the bulk phase49. A better reducibility leads, on its turn, to a higher concentration of oxygen vacancies, which could boost CO oxidation50. The electron microscopy characterization rules out the occurrence of a Strong Metal-Support Interaction (SMSI) by


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decoration of the Au nanoparticles surfaces. This is in good agreement with the results of Gubó et al.

51

which consider that

Au is not a metal prone to SMSI. Decoration by support overlayers has been observed for different noble metals, including gold52, under high temperature reducing environments. Under oxidizing conditions, like those involved in this work, only Liu and coworkers have reported decoration over a ZnO support53, but no evidence has been provided for Au supported on a reducible oxide, e. g. CeO2, in these conditions. Furthermore, several works on Pt nanoparticles, a noble metal very close to Au, deposited over CeO2 have reported the redispersion of the metallic phase after high temperature oxidizing treatments54-55. Further experiments aimed at probing the thermal resistance of the prepared catalysts against extreme working conditions were performed. These experiments consisted in running several, consecutive, long (6h) reaction cycles at 1223 K, as previously described in the experimental section. The resulting curves after 24 hours at 1223 K for both catalyst and after 42 hours in the case of CeO2/Au/YSZ-long cycle are presented in Figure S7 of supporting information. It is worth commenting at this point upon the impact of the severe operating conditions employed in the long-cycle experiments on the structure of the two catalysts. Starting with the YSZ support, the analysis of the


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XRD diagrams reveals important differences. Thus, the Au/YSZlong cycle sample exhibits a YSZ crystal size, 37 nm, which is almost double that determined in the as-prepared catalyst, 22 nm; this pointing out to a remarkable sintering effect after 24 hours under extreme reaction conditions. In clear contrast, the support particles in the CeO2/Au/YSZ-long cycle catalyst suffer only a negligible increase of size, even after 42 hours under the same reaction conditions. In the latter, the average crystal size changes only from 22 nm to 26 nm. These values reveal that the addition of ceria avoids in a very efficient way the sintering of the YSZ support particles. Also important, and regarding now the noble metal phase, ICPAES measurements indicate that the Au load decreases from 0.85 wt% in the as-prepared catalysts to 0.65wt% in the long-cycle tested ones. This could be tentatively related to the fact that the temperature employed in the experiment approaches that corresponding to the fusion temperature (1337 K). At such high temperatures, and taking into account the small size of the Au particles, the equilibrium vapor pressure may rise to values which promote a significant transfer of Au atoms into the gas phase and sweeping out of the catalyst bed by the gas flow. However, in the context of applications in TWCs, the usual working conditions involve only eventual excursions to very high


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temperatures, which is really different to the prolonged aging time used herein to force deactivation. The role of Au corrosion by formation of carbonyl type species cannot be ruled out either, but discerning the actual gold scavenging mechanism which we have detected under the extreme reaction conditions employed in the long-cycle test is out of the scope of the present work. Concerning reaction mechanisms, our results do not allow discriminating which of those most widely accepted in previous literature56-57 operates at the different temperature ranges. Moreover, since the catalytic tests involve in some cases very high temperature conditions, the evolution of the reactive process via radical chemistry, as reported by J. B. Ma et al.58 cannot be fully disregarded.


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Figure 5. (a) HAADF-STEM image recorded on a YSZ crystal of the CeO2/Au/YSZ-long cycle sample; (b) ADF signal of the area, marked with the green box on (a), in which a SI study was performed recording the Ce-M4,5 EELS signal; (c) Ce-M4,5 areal intensity map, where the brighter (yellow) areas correspond to higher cerium concentrations.

(d)

EELS

Ce-M4,5

spectra

extracted

from

the

spectrum imaging experiment at a location in which the electron beam crosses only the crystal surface (black line) and at a location in which the electron beam passes through the bulk (grey line). Proven the outstanding role of the ceria layer to prevent catalyst deactivation, a more in-depth characterization of its


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structure was pursued by means of sub-angstrom resolution analytical techniques, Figure 5. Regarding this question, Figure 5a shows a HAADF-STEM image of the CeO2/Au/YSZ catalyst after the 42h test at 1223 K. A YSZ crystallite, about 25 nm in size, with a flat surface is imaged. A zoom on the surface of this crystallite, which depicts clearly the atomic column structure of the area, is shown in the Annular Dark Field image (ADF) of Figure 5b. The atomically resolved Ce-M4,5 chemical map obtained after processing the Electron Energy Loss spectra in the spectrum-image study performed on this area, Figure 5c, evidences that Ce distributes over the topmost surface of the crystallite in the form of a just one-atom thick layer. The EELS spectra shown in Figure 5d were extracted from two different locations: one at which the electron beam only crossed the surface (line in black) and another in which the beam passed through the bulk of the crystallite (line in grey). The much higher intensity of the Ce-M4,5 peaks in the former confirms the distribution of CeO2 just as a surface coverage. The lower intensity of these peaks in the bulk corresponds to the presence of Ce atoms in the top and bottom crystal surfaces in the direction of the electron beam. These results evidence that there is no diffusion of Ce into the bulk of the YSZ crystallites under the essayed working conditions, as it was the case in the study reported elsewhere59. In fact, after more than


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40 hours of reaction, the distribution of CeO2 on the surface becomes more homogeneous than in the as-prepared CeO2/Au/YSZ catalyst, since a perfect monolayer seems to develop after the forced deactivation essay. Further structural and compositional details of the Cecontaining surface layer were analyzed through a simulation study. To this end, Figure 6 shows a zoom of the HAADF-STEM image of Figure 5a together with some simulated HAADF images and intensity profiles recorded on both type of images. Measurements of the atomic distances, along the direction parallel to the surface, on the two top-most (111) planes of this image, the first one corresponding to the CeO2 monolayer and the second to the first YSZ plane, reveals a perfect match between both phases. If we consider the lattice parameters of CeO2 and the YSZ support, this result indicates the occurrence of a 5.2% compression of the ceria overlayer.


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Figure 6. (a) Atomic model of a YSZ crystal with a (111) CeO2 monolayer deposited on the surface. Cerium and titanium atoms are displayed as pale yellow and blue spheres respectively, while oxygen atoms are shown in red; (b) Zoom of the experimental HAADF image in Figure 5(a), showing a ceria monolayer on top of a YSZ crystal viewed along the [101] zone axis; (c) and (d) illustrate HAADF simulated images of models like that in (a), with and without a correction of the lattice parameter respectively; (e) Intensity profiles

along

a

direction

perpendicular

to

the

crystallite

surface, marked with the white arrows (b) and (d).


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The occurrence of this lattice contraction is better visualized when the experimental HAADF-STEM image (Figure 6b) is compared with simulated images corresponding to two models in which a (111) monolayer of CeO2 is grown on the (111) surface of a cubic YSZ crystallite, Figure 6a; one of them which considers the lattice parameter of bulk CeO2, Figure 6c, and a second one in which the 5.2% shrinkage of the lattice parameter is introduced, Figure 6d. According to the simulations of these two models, the compressed layer exhibits a perfect match with the underlying YSZ structure, whereas a significant misfit is observed in the model which considers the bulk lattice parameter of CeO2. The experimental image resembles much closely the contrasts observed in the simulation corresponding to the compressed layer, Figure 6d, since no shift between atomic columns is observed in this case (c.f. red boxes marked in the experimental and simulated images). The intensity profile, along the perpendicular to the (111) planes, of the 5.2% shrunk simulated HAAD-STEM image, Figure 6e, indicates that the intensity of the top-most ceria monolayer should be higher than that of the underlying YSZ planes. In good agreement, this feature is also observed in the profile recorded on the experimental image when the background is removed in


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order to decrease the effects related to sample thickness variations. Moreover, the comparison of these intensity profiles, indicates that the distance between the (111) type CeO2 monolayer and the first, underlying, (111) YSZ planes is a 7.4% larger than that corresponding to neighboring (111) bulk YSZ planes. This is clearly observed in the shift between the two first peaks of the intensity profiles, as marked by the dashed lines on the left side of Figure 6(e). This feature shows the singular surface nanostructure which is formed after ceria redispersion. Several factors play a key role in the observed CeO2 redispersion on the YSZ surface. First of all, the high temperature reached in the experiments, 1223 K, must provide the required mobility necessary for the migration of cerium cations. Secondly, the excellent structural match between the supported oxide and the support turns out, as shown later, crucial to reach the spread state. Finally, the reaction atmosphere is also critical. At this respect, it is important to recall that in a previous work, the redispersion of 3D CeO2 nanoparticles into a 2 atomic layers-thick mixed ceria-zirconia phase with pyroclore structure has been detected in a CeO2/YSZ catalyst when it was treated in a H2(5%)/Ar mixture at 1173 K49. In contrast, under O2(5%)/He CeO2 diffused into the bulk of YSZ, but only at


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temperatures above 1373K59 . CO oxidation reaction conditions at 1173K favor also wetting of the YSZ surface with CeO2, but in this case in the form of a ceria monolayer. The perfect epitaxial match observed between the YSZ support and the CeO2 monolayer could play an important role in the performance of the CO oxidation reaction. YSZ, widely used as electrolyte in solid oxide fuel cells60-61, is known to be an exceptional ionic conductor with an excess of oxygen vacancies. The presence of CeO2 covering the YSZ surface facilitates the interaction with oxygen, this guaranteeing the entrance and liberation of active oxygen where needed for the CO oxidation process. Moreover, when a perfect structural relationship leads to a coherent interface, as the one observed in this case, an improvement of the ionic mobility could be achieved62. This synergic effect between the YSZ support and the CeO2 monolayer could boost the oxidation process in the essayed reaction. 3.2 DFT calculations. Concerning DFT studies, the lattice parameters for both cubic structures, CeO2 and ZrO2 were first refined. In the case of CeO2, a lattice parameter of 5.48 Å was found after relaxation, a value which is larger than the experimental one but in good agreement with the GGA functional63. The lattice parameter for the relaxed ZrO2 cubic structure, without the effect of any dopants, was 5.11 Å, in very good


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agreement with other calculations performed with hybrid functionals64. The experimental value obtained by X-Ray diffraction for the YSZ support used in this work was 5.13 Å which is very close to the calculation and coherent with the incorporation of a small amount of larger Y3+ cations in the zirconia structure. After these preliminary calculations, different nanostructures were considered. Thus, Figure 7a shows the supercell built to simulate the formation of an isolated ceria (111) monolayer. The supercell, periodic in X and Y, contains 24 atoms with axis along [2-1-1], [01-1] and [111] crystal directions. After a cell-ions relaxation (BFGS), the X and Y dimensions were found to contract to a value of 5.115 Å which is very close to the one calculated for bulk zirconia. This could explain the perfect match between ceria (111) monolayer and zirconia (111) surface reported in this work and shown in figure 6. Apart from this contraction, a 2% expansion was found in the distance between the adjacent Ce and O planes in the O-Ce-O monolayer, which corresponds to a 0.14 Å displacement of the oxygen atoms along the Z axis. Though quite subtle, such change must involve parallel changes in the chemical properties of CeO2. The energy of formation of the (111) monolayer was calculated with respect to bulk ceria using the following equation 1:


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1

EML = N·[E𝐴 ―N·ECeO2]

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(1)

Where EA and ECeO2 correspond to the absolute energy calculated by DFT for the supercell in Figure 7a and bulk CeO2, respectively; and N the number of CeO2 units in the monolayer (8 in this case). After applying this equation, the formation energy was found to be 0.76 eV by CeO2 unit (73.17 kJ/mol). Figure 7b shows the supercell employed in the calculation of a ZrO2 (111) surface. Several calculations were performed in which the number of cationic layers were changed in order to achieve convergence with the dimensions of the nanostructure and supercell. In particular, the supercell shown in figure 7b, periodic in X and Y and with axis along [2-1-1], [02-2] and [111] crystal directions, contains two identical (111) surfaces and 240 atoms. It was created locating one Zr atom in the middle of the supercell. Keeping the dimensions of the supercell fixed, the nanostructure was relaxed until convergence was achieved. After relaxation the topmost surface cationic plane suffers a negligible inwards displacement of just 0.03 Å along the Z direction, in very good agreement with previous calculations reported elsewhere64. This subtle contraction of the lattice parameter falls beyond the accuracy of HAADF-STEM imaging.


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Figure 7. Supercell models representing a ceria (111) monolayer (a), a slab model of zirconia with two (111) surfaces (b), and two ceria (111) monolayers supported on both zirconia surfaces (c). From this model, a (111) surface energy could be calculated using the following equation 2:

ESURF =

1 2A·[E𝐵

―N·EZrO2]

(2)

Where EB and EZrO2 correspond to the calculated absolute energies for the supercell in Figure 7b and bulk zirconia, respectively, N the number of ZrO2 units in the nanostructure, and A the area of one surface. A value of 0.82 J/m2 was found for the (111) surface energy for cubic zirconia, which is significantly smaller than the one reported recently64, probably due to the difference in the relative dimensions of the supercell and the nanostructure. In the present work a larger number of cationic layers and a larger surface area were considered in order to achieve a complete relaxation and energy convergence.


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Finally, the supercell shown in figure 7c was used to consider a zirconia supported ceria (111) monolayer. The orientation and dimensions of this supercell were the same used in the previous calculations. Taking into consideration the expected contraction of the ceria monolayer, a lattice parameter of 5.11 Å was considered for both zirconia and ceria. A monolayer covers the two zirconia (111) surfaces, which finally results in two identical nanostructures. After relaxation, the last cationic plane at the top most surfaces (in this case Ce cations) suffers a displacement outward of 0.25 Å. This displacement is an order of magnitude larger and in the opposite direction than the one found for pure zirconia. According to this data, the distance between the last zirconia plane, that at the interface, and the ceria plane should be an 8.5% larger than that between consecutive bulk zirconia planes (2.95 Å). The experimental expansion (7.4%) measured in Figure 6b is in good agreement with the theoretical one, and is a consequence of the presence of the ceria monolayer on top of zirconia. The slight 1.5% difference between the calculated and experimental expansions along the perpendicular to the interface can be attributed to the presence of dopants (Zr or Y) in the ceria monolayer that were not considered during the DFT calculations.


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To calculate the formation energy of the supported monolayer on the zirconia (111) surface with respect to bulk ceria, we used the following equation: 1

ESML = N·[EC ― EB ―N·ECeO2]

(3)

Where EC and EB correspond to the calculated absolute energies for the supercell in Figure 7b and supercell in Figure 7c respectively, and N the number of CeO2 units in the monolayer, in this case, 32 units. This energy was found to be 0.018 eV/CeO2 (1.76 kJ/mol). If we compare this data with the one calculated for the isolated ceria (111) monolayer (73.17 kJ/mol) it becomes clear that the interaction of the monolayer with the ZrO2 surface, reduces significantly its energy of formation. The interaction energy between the ceria (111) monolayer and the zirconia (111) surface can be calculated by difference. This amounts up to -71.44 kJ/mol, a value which cancels out almost completely (96%) the formation energy of the ceria monolayer. This could explain, from the thermodynamic point of view, the observed tendency of supported ceria to wet the zirconia surface by creating a surface monolayer after several thermal treatments at high temperatures. Furthermore, if we compare the surface energy of zirconia (0.82 J/m2) with the formation energy of the CeO2/ZrO2 nanostructure


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from bulk ceria and zirconia (0.84 J/m2) it is clear that there is just a very small energy barrier for the system to accommodate the ceria phase in the form of a supported bidimensional structure without interrupting the crystal structure of the bulk, in good agreement with the experimental observations reported in this work. YSZ versus Titania supported catalysts. Finally, it is also important establishing a comparison between the catalysts prepared in this work and those in which gold is supported on TiO2, as it is the case of that recently studied31 . In that work, the influence of a sub-monolayer CeO2 addition using IWI on the performance and high temperature stability of the 1.5% Au/TiO2 World Gold Council (WGC) reference catalyst was investigated. The average Au particle size in the Au/TiO2 and CeO2/Au/TiO2 catalysts, 3.2 ± 1.1 and 3.1 ± 1.5 nm respectively, were very close to those in the YSZ based catalysts synthesized in our work. Figure 8 compares the T100 values for CO oxidation reaction, as a function of the consecutive cycles performed at increasing temperatures (523 K, 623 K, 773 K, 923 K, 1223 K and 1223 K again), for the TiO2 and YSZ based catalysts. This Figure indicates that TiO2 based catalysts exhibit a higher activity at low temperatures cycles, which it must be at least partly due to


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the much higher amount of Au (1.5 wt% for TiO2 based against 0.85 wt% for YSZ based). It can also be observed that the two TiO2based catalysts start slightly to deactivate after the fourth cycle, with an important increase of the T100 value after the fifth cycle. In the Au/YSZ catalyst the activity remains constant up to the fifth cycle, a marked deactivation being noticed only in the sixth cycle. Outstandingly, the CeO2/Au/YSZ catalyst experience almost no deactivation throughout the whole set of cycles. Moreover, the performance of the CeO2/Au/YSZ catalyst even overrides that of CeO2/Au/TiO2 at high temperatures cycles. For a better evaluation of the influence of the support on the catalytic activity, the TOF values at 423 K for each cycle were calculated, figure 8b. The values corresponding to the Au/TiO2 and CeO2/Au/TiO2 were obtained from data in reference30, in which the catalytic assays were performed in identical conditions. As observed in this figure the metal sites in the TiO2 supported catalyst are intrinsically more active. Nevertheless, it is also clear that they suffer a more intense loss of efficiency. In the YSZ-based catalysts the activity, although lower in value, remains more stable, particularly in the modified sample, which experiences even an improvement in the final second cycle at 1223K. Moreover, the performance in the last cycle of the


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CeO2/Au/YSZ catalyst improves that of the CeO2/Au/TiO2, as already concluded from the analysis of figure 8a.

Figure 8. (a) T100 value versus the cycle number (1 to 6) on the CO oxidation experiment for the Au/TiO2, CeO2/Au/TiO2, Au/YSZ and CeO2/Au/YSZ samples. (b) TOF value per Au atom at 423 K versus the cycle number (1 to 6) in the CO oxidation experiment for the Au/TiO2, CeO2/Au/TiO2, Au/YSZ and CeO2/Au/YSZ samples. The atomic scale characterization performed on the CeO2/Au/YSZ and CeO2/Au/TiO2 catalysts after reaction provides also a clue about the differences in the stabilization mechanisms operating in both type of systems and allows proposing some hypothesis to rationalize the differences in their performance. Thus, in the YSZ-based system a very high and homogeneous dispersion of CeO2 over the support is achieved, whereas in the TiO2-based one, as it has been previously reported30, the dispersion of CeO2 was


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much lower and more heterogeneous, with areas depicting a high concentration of CeO2 nano-sized patches. Such heterogeneous distribution, results in a significant fraction of Au particles that do not remain in contact with the CeO2 nanostructures. The particles in such state are in general larger in the TiO2-based than in the YSZ-based catalysts. Since the textural properties of the two supports are rather similar and the synthesis procedures have been exactly the same, it can be reasonably hypothesized that the differences in the dispersion of CeO2 onto the surface of the two types of support crystallites must be related to intrinsic crystallographic differences between TiO2 and YSZ. At this respect, it must be recalled that the P25 TiO2 oxide used in the preparation of the WGC catalyst consists in a mixture of the rutile and anatase polymorphs, which are both tetragonal phases. In contrast, YSZ is a cubic, or pseudo-cubic phase, much more amenable to the cubic structure of both CeO2 and fcc Au. A closer structural relationship between the metal and the support may contribute to reduce the mobility of the metal36, hence reducing in some extent the impact of sintering. Moreover, cerium oxide also exhibits a fluorite type cubic structure which improves the distribution and the textural coherence with the YSZ cubic support, hence increasing the interaction between the


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metal phase and ceria. The homogeneous distribution of ceria over the surface of YSZ reached after the reaction cycles on the surface of the CeO2/Au/YSZ catalyst was not observed in the TiO2based catalyst, which only evidenced CeO2 agglomerations nearby Au nanoparticles. In addition, it is well known that the anatase to rutile transition in TiO2 proceeds at temperatures above 773 K65. Such transition is taking place during the high temperature cycles performed with the TiO2 based catalysts, with a concomitant increase of the support crystal size. In contrast, the YSZ oxide used in this work has demonstrated a very high, intrinsic, textural stability, which may be related to the reduced cationic mobility associated to the lack of a phase transition as that verifying in the case of TiO2. Besides these structural differences in the nature of YSZ and TiO2 supports and their influence on the actual CeO2 dispersion attainable in each system, the differences in the ionic conductivity between the two systems should also be taken into account. Thus, the exceptional conductivity of YSZ, which can be further improved by the growth on the surface of an epitaxial CeO2 monolayer under the stringent reaction conditions, could contribute to speeding up the oxidation process and, therefore,


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to improving the performance of the catalyst in CO oxidation, as observed in the catalytic deactivation tests. 4.- CONCLUSIONS A 0.85% Au/YSZ has been prepared and further modified by the addition of a submonolayer coverage of CeO2 by a conventional wetness impregnation method. Such addition not only enhances the catalytic activity of the initial Au/YSZ catalyst in CO oxidation but also its stability against deactivation under prolonged, very high temperature, working conditions. In-depth characterization by a combination of structural and nanoanalytical techniques reveals that the use of an oxide support structurally coherent with both Au and CeO2 facilitates the dispersion of CeO2 and allows an improved contact with Au nanoparticles. High temperature working conditions promote the formation of an atomically thin cerium oxide layer which may incorporate Zr as a dopant. This layer is under a compressive stress with respect to bulk CeO2 but adopts the structure expected from DFT calculations, which is the one expected for an isolated monolayer. The tight contact of most of the Au nanoparticles present on the surface with these epitaxially grown ceria monolayers seems to be the key factor for the outstanding performance of the CeO2/Au/YSZ catalyst in CO oxidation.


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With the aid of Density Functional Theory calculations and the detailed analysis of atomically resolved EELS-STEM and HAADFSTEM images it has been possible to reveal quite subtle details about the first atomic layer present on the (111) surface of the CeO2/Au/YSZ catalyst. Comparison with a CeO2-modified WGC 1.5 wt% Au/TiO2 reference catalyst indicates that the use of the YSZ support results in a catalyst which provides higher activity, in Au per mass terms, and much higher stability under extremely severe working conditions. Such performance, validates CeO2/Au/YSZ as a realistic candidate for high temperature processes, this representing a clear widening of the opportunities of Au nanoparticles in catalytic applications currently unfeasible because of the large sintering tendency of this noble metal. In other words, though nanosized gold is widely recognized as very active, and possibly the best choice among noble metals, in low temperature catalysis, using the strategies described in this paper, and on the basis of the specific interactions particularly developing in the case of ceria-modified YSZ supports, their incursion in other processes is now at reach. Such expansion in the applicability range of supported gold nanoparticles is enabled by the occurrence of unique structural interaction mechanisms, which allow not only maintaining a very


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high dispersion state of the metal under extreme working conditions but also improving the structural (dispersion) and chemical characteristics of the ceria overlayer. Finally, it has also been shown that the stabilization mechanisms differ between YSZ and TiO2 based catalysts and, importantly, that structural coherence is at the roots of such differences.

ASSOCIATED CONTENT Supporting Information. Schemes of the reactor used in the reaction and the reaction procedure followed with a SiC blanc test; X-Ray Diffracton patterns of the Au/YSZ, CeO2/Au/YSZ, Au/YSZ-post reaction, CeO2/Au/YSZ-post reaction, Au/YSZ-long cycle and CeO2/Au/YSZ-long cycle samples; representative HAADF images illustrating direct evidences of Au nanoparticles; CO conversion vs T plots in the low temperature range for the Au/YSZ and CeO2/Au/YSZ catalysts; EELS map depicting the CeO2 distribution before and after the reaction and the graph presenting the CO oxidation performance for the long cycle catalytic tests, structural models used for DFT calculations and HAADF-STEM image simulations.


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AUTHOR INFORMATION Corresponding Author José A. Pérez-Omil: [email protected] ACKNOWLEDGMENT This paper is dedicated to the memory of a wonderful and creative scientist, Professor Malcolm I. Heggie, who passed away in January 2019. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 823717 –ESTEEM3. This work has received also financial support from Junta de Andalucía (FQM334), MINECO/FEDER (Projects MAT2017-87579-R, MAT2016-81720-REDC). STEM studies were performed at the DME Facilities of SCCYT of University of Cádiz. We acknowledge CITI at University of Cádiz for providing access to the Cluster CAI. REFERENCE 1. Haruta, M.; Yamada, N.; Kobayashi, T.; Iijima, S., Gold catalysts prepared by coprecipitation for low-temperature oxidation of hydrogen and of carbon monoxide. J. Catal. 1989, 115, 301-309. 2. Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N., Novel gold catalysts for the oxidation of carbon monoxide at a temperature far below 0.DEG.C. Chem. Lett. 1987, 405-408. 3. Das, M.; Shim, K. H.; An, S. S. A.; Yi, D. K., Review on gold nanoparticles and their applications. J. Toxicol. Environ. Health Sci. 2011, 3, 193-205. 4. Yang, G.; Chen, D.; Lv, P.; Kong, X.; Sun, Y.; Wang, Z.; Yuan, Z.; Liu, H.; Yang, J., Core-shell Au-Pd nanoparticles as


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anatase to rutile phase transformation in TiO2 nanoparticles synthesized by the modified sol–gel method. Mater. Lett. 2012, 82, 195-198.

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Improving the Activity and Stability of YSZ-Supported Gold Powder

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