In Situ FT-IR Spectroscopic Study of CO₂ and CO Adsorption on Y₂O₃, ZrO₂, and Yttria-Stabilized ZrO₂

Abstract

In situ FT-IR spectroscopy was exploited to study the adsorption of CO₂ and CO on commercially available yttria-stabilized ZrO₂ (8 mol % Y, YSZ-8), Y₂O₃, and ZrO₂. All three oxides were pretreated at high temperatures (1173 K) in air, which leads to effective dehydroxylation of pure ZrO₂. Both Y₂O₃ and YSZ-8 show a much higher reactivity toward CO and CO₂ adsorption than ZrO₂ because of more facile rehydroxylation of Y-containing phases. Several different carbonate species have been observed following CO₂ adsorption on Y₂O₃ and YSZ-8, which are much more strongly bound on the former, due to formation of higher-coordinated polydentate carbonate species upon annealing. As the crucial factor governing the formation of carbonates, the presence of reactive (basic) surface hydroxyl groups on Y-centers was identified. Therefore, chemisorption of CO₂ most likely includes insertion of the CO₂ molecule into a reactive surface hydroxyl group and the subsequent formation of a bicarbonate species. Formate formation following CO adsorption has been observed on all three oxides but is less pronounced on ZrO₂ due to effective dehydroxylation of the surface during high-temperature treatment. The latter generally causes suppression of the surface reactivity of ZrO₂ samples regarding reactions involving CO or CO₂ as reaction intermediates.

1. Introduction

Yttria-stabilized ZrO₂ (YSZ) is already commonly used as a solid oxide fuel cell (SOFC) electrolyte and anode component.¹ However, despite its technological importance, YSZ itself is a very complex material, adopting different crystal structures (monoclinic, tetragonal, and cubic) with varying Y content below 20 mol %.² Of all the associated mixed oxide compounds, YSZ containing 8 mol % yttria is the most technologically important one. Therefore, on the one hand, the understanding of molecular processes on its surface is highly desirable, and on the other hand, insights into the surface reactivity of YSZ are also expected from in-depth studies of the two chemical constituents of the YSZ structure, that is, Y₂O₃ and ZrO₂. The aim for a better understanding of the surface chemistry of the latter two oxides is also fueled by their technological relevance, comprising the use as ceramic materials or as catalysts for a number of chemical reactions.^3,4 However, despite their importance, the use of YSZ and ZrO₂ strongly depends on the particular polymorph used. The most common modification of ZrO₂ is the monoclinic structure that is the only thermodynamically stable phase until ∼1400 K, where a phase change to a tetragonal modification takes place. Above a temperature of ∼2600 K a cubic structure is present.⁴ These polymorphic forms exhibit strikingly different chemical properties, giving rise to also different catalytic behavior. The tetragonal and the cubic phases are catalytically more active than their monoclinic counterpart.⁵ Doping ZrO₂ with Y₂O₃ stabilizes either the cubic or tetragonal phase (strongly depending on production technique) over a large temperature range and creates oxygen vacancies.^2,3 Thus, a lot of progress has been made especially regarding characterization of the surface chemistry of a variety of different ZrO₂ samples. Materialwise, this essentially refers to supported metal/oxide systems⁶⁻⁹ and sulfated ZrO₂ samples.^7,10 Most data are available for pure ZrO₂, with the exception of comparative adsorption studies of tetragonal and monoclinic ZrO₂.¹⁰⁻¹³ Strikingly different, information on the surface chemistry of Y₂O₃ and YSZ mixed oxide phases is virtually nonexistent, and adsorption studies essentially represent no-man’s land. Information on Y₂O₃ is restricted to FT-IR studies of supported Ru and Pd/Y₂O₃ systems and Er-doped Y₂O₃, being used as transparent ceramics.¹⁴ Studies on YSZ by FT-IR spectroscopy are almost exclusively focused on investigating its crystal structure. One notable exception in that respect is the studies on ZrO₂, doped with 3 mol % yttria, which usually serves as a compound to simulate tetragonal ZrO₂.¹⁰

The aim of our studies therefore is to provide a first comparison of the surface chemical properties of technically used YSZ with 8 mol % yttria content and its separated chemical constituents Y₂O₃ and ZrO₂. As representative probe molecules, CO and CO₂ were chosen, due to their importance as reactants in a variety of chemical reactions, including the water–gas shift reaction or other C1 reactions relevant to anodic fuel conversion in solid oxide fuel cells. As the perfect method for monitoring the adsorbed species under technically relevant conditions, a dedicated in situ FT-IR spectroscopy setup was used, allowing static and flowing treatments up to the bar range and elevated temperatures up to 773 K. Exceeding the scientific input from the simple comparison between the three oxidic materials, we also anticipate in-depth information on the reactivity of more complex reactions and materials. Special attention was also paid to a comparison of the determined surface reactivity to literature data, as especially oxide pretreatments are expected to strongly alter the adsorption behavior. Regarding surface studies on oxides, this essentially refers to the hydroxylation state of the surface.

2. Experimental Section

2.1. Materials

Commercial powders of Y₂O₃, ZrO₂, and YSZ were used as starting materials. Cubic (bcc) Y₂O₃ (yttrium(III)oxide, nanopowder, <50 nm particle size) and tetragonal YSZ (zirconium(IV)oxide-yttria stabilized, nanopowder, containing 8 mol % Y₂O₃ as stabilizer, from now on called “YSZ-8”) were supplied by Sigma Aldrich and monoclinic ZrO₂ (with a small amount of tetragonal phase after thermal annealing at 1173 K, zirconium(IV)oxide, 99.978%) by Alfa Aesar. All samples were pretreated by calcination at 1173 K in air and subsequently checked by XRD for structural changes upon annealing. The surface area after pretreatment was determined by nitrogen adsorption at 77 K according to the BET method as 21.7 m²/g (Y₂O₃), 31.6 m²/g (YSZ-8), and 10.4 m²/g (ZrO₂). For BET measurements, a Quantachrome Nova 2000 Surface Area and Pore Size Analyzer was used. Gases were supplied by Messer (CO₂ 4.5 and CO 4.7).

2.2. FT-IR Studies

FT-IR spectra were recorded in transmission mode on a Perkin-Elmer FT-IR System 2000 spectrometer. The powder samples were pressed into thin pellets using a pressure of 2 t (sample mass about 100 mg each). The pellets were subsequently placed inside a homemade in situ reactor cell. The path length of the IR reactor cell amounts to 20 cm. This cell setup allows treatments under static and flowing conditions up to pressures of 1 bar and temperatures up to 873 K. The temperature is controlled by a thermocouple placed next to the pellet. Pretreatment of the powders was performed outside the cell (annealing up to 1173 K), and the pellet was mounted hot into the IR cell. Vacuum was applied immediately (10^–6 mbar). Calcium fluoride is used as window material allowing access to wavelength ranges above 1000 cm^–1. To ensure identical starting conditions, samples were oxidized with 20% oxygen seeded in helium at 873 K for one hour. To ensure a minimal degree of hydroxylation, during the flow mode measurements the gases are cleaned and dried using two liquid nitrogen or liquid nitrogen/ethanol cooling traps. Flowing measurements were performed under ambient conditions. In static mode, the gases are preadsorbed on a 5 Å zeolite trap binding water sufficiently strongly, before the dry gases are desorbed into the evacuated and degassed cell. All reported spectra are corrected with the spectrum of the dry preoxidized oxide pellet prior to adsorption.

3. Results and Discussion

3.1. CO₂ Adsorption

3.1.1. Y₂O₃

Figure 1 shows an overview of effects associated with the static adsorption of dry CO₂ on Y₂O₃ at room temperature using CO₂ pressures up to 10 mbar.

Figure 1.

Infrared spectra of static adsorption of CO₂ on pure Y₂O₃ at room temperature at pressures up to 10 mbar.

As it can be clearly seen, the adsorption leads to formation of a number of distinct (bi)carbonate and hydroxyl species. The assignment of the carbonate species (detailed representation in Figure 3) was based on recent investigations on carbonate formation on different Ga₂O₃ polymorphs,¹⁵ where both experimental and theoretical information is most abundant and thus the assignment in the present case is most straightforward. The structure of the most likely analogous (bi)carbonate species¹⁵ is highlighted in Figure S1 (Supporting Information).

Figure 3.

Assignment of the infrared signals between 1900 and 1160 cm^–1 of the observed (bi)carbonates on Y₂O₃ at room temperature at pressures up to 10 mbar.

A closer look at Figure 3 reveals that at a pressure of 3 × 10^–5 mbar almost no formation of carbonates is observed, whereas raising the pressure up to 1 mbar leads to the formation of a dominant species with signals at 1674, 1397, and 1223 cm^–1. During further pressure increase to 10 mbar, another species with signals at 1652, 1419, and 1223 cm^–1 dominates the spectra. On the basis of ref (15), these peaks can be attributed to the ν_as, ν_s, and δ_OH vibration modes of monodentate and bidentate bicarbonates. The intensification of the latter peaks correlates with the decrease of signals at 3702 and 3676 cm^–1 and an increase of the associated modes at 3658 and 3625 cm^–1 (Figures 2 and 3).

Figure 2.

Details of the room-temperature-collected infrared spectra of adsorbed CO₂ on Y₂O₃ shown in Figure 1 in the wavenumber range between 3750 and 3540 cm^–1 at pressures up to 10 mbar in the region of the hydroxyl stretching modes.

Figure 2 shows that, according to refs (16 and 17), the negative peaks are related to ν(OH) modes of isolated surface hydroxyl groups of the oxide (more than one peak is visible due to the presence of differently coordinated hydroxyl groups), which become consumed by bicarbonate formation, and the accordingly positive signals can be associated with the ν(OH) modes of the newly formed hydroxyl groups of the bicarbonates. The intensity of the hydroxyl group at 3658 cm^–1 correlates with the rise of the signals of the monodentate bicarbonate, whereas the signal at 3625 cm^–1 matches the bidentate bicarbonate species. The analogous appearance of two different bicarbonate species was also observed on ceria samples.¹⁷ The broad feature below 3500 cm^–1 can be assigned to interacting hydroxyl groups of the formed bicarbonates. Hence, the main process associated with chemisorption of a CO₂ molecule on Y₂O₃ is the insertion into a basic surface hydroxyl group leading to the formation of bicarbonates. Note that the experiments probably do not represent true thermodynamically equilibrated processes since the waiting time to reach an adsorption equilibrium had to be balanced against possible contamination of the surface especially at prolonged times.

Following refs (15 and 18) the assignment to the different (bi)carbonate species is shown in Figure 3. Concerning the pattern of the peaks it is apparent that with increasing coordination of the (bi)carbonates a superposition of signals of the different adsorbed features occurs. As pointed out in ref (19) such an effect is due to the presence of different adsorption sites arising from different coordination of the metal atoms on the surface, especially on a powder sample. Also an interaction between different adsorbed species can not be excluded.²⁰

To test the thermal stability of the adsorbates, Figure 4 illustrates the thermal evolution of the (bi)carbonate species on Y₂O₃ in 10 mbar CO₂ up to 873 K under static conditions.

Figure 4.

Infrared spectra collected on Y₂O₃ after exposure of 10 mbar static CO₂ upon heating to 873 K. Infrared spectra have been taken in 100 K steps. Important modes have been marked.

The bicarbonate species persist up to a temperature of 373 K, whereas the intensity of the peaks attributed to bidentate species decreases first. At 473 K the bicarbonates have vanished, and the remaining main species is the bridged carbonate, which subsequently is converted into polydentate carbonate species upon further temperature increase. Thus, at 873 K only signals of polydentate carbonate species are present. Hence, during heating the more weakly bound adsorbates are converted into more strongly bound carbonates. Summarizing, the process of this conversion, i.e., the binding strength of the different kinds of CO₂ adsorbates with increasing temperature on Y₂O₃, follows the sequence monodentate bicarbonate → bidentate bicarbonate → bridged carbonate → polydentate carbonate. Adsorbates on Y₂O₃, resulting from chemisorption of CO₂, can therefore not be removed entirely by heating up to 873 K. Additional experiments (not shown) show that a “carbonate-poisoned” Y₂O₃ surface can only be successfully recovered by heating in dry oxygen up to 873 K, likely because of efficient surface reoxidation.

A final comment on the spectroscopic requirements for studying carbonate formation on Y₂O₃ following CO₂ adsorption should be added. As Y₂O₃ is very prone to hydroxylation, even by traces of H₂O, carbonate formation is in turn highly favorable, but subsequent attribution to distinct carbonate species is not possible due to low resolution of the resulting spectra. This effect was observed in particular under “flowing CO₂” conditions, which under the chosen experimental conditions are less dry. Consequently, only static CO₂ adsorption under as dry as possible conditions, using a zeolite trap close to the IR cell for removing all traces of water from CO₂, permits collecting highly resolved carbonate spectra.

3.1.2. YSZ-8

Experiments on yttria-stabilized ZrO₂ (YSZ-8) under flowing conditions (CO₂ flow = 0.4 mL s^–1) show a behavior similar to pure Y₂O₃ at room temperature. In the case of YSZ-8, flowing conditions were chosen to achieve more intense and thus more reliably interpretable signals because on YSZ-8 the carbonate adsorbates are generally more weakly bound as compared to on Y₂O₃. As apparent from Figure 5, showing also the thermal evolution of the carbonate species, the formation of Y₂O₃-analogous bi- and bridged carbonate species can also be observed.

Figure 5.

Infrared spectra collected during heating YSZ-8 in dry flowing CO₂ (CO₂ flow = 0.4 mL/s) up to 873 K together with the assignment of the observed (bi-) carbonates at room temperature.

The broad features located at 1305 cm^–1 can be ascribed to the ν_s mode of the bridged carbonate, and the other main adsorbate signals at 1646, 1429, and 1225 cm^–1 represent the corresponding bidentate carbonate. We note that the adsorption behavior of YSZ-8 very much resembles that of pure Y₂O₃ (cf. Figure 3) and is crucially different from spectra obtained on YSZ phases with a lower amount of Y, i.e., 3 mol %.¹¹ The latter is usually termed “tetragonal ZrO₂” in the literature and also adsorption-wise closely matches pure tetragonal ZrO₂.¹² In contrast to what was observed on pure Y₂O₃, (bi)carbonates on a YSZ-8 surface can be easily removed by heating, and almost no signals arising from polydentate carbonate species can be detected. Above a temperature of 573 K, these adsorbates decompose completely (Figure 5), although in this experiment flowing conditions with a pressure of 1 bar were chosen.

3.1.3. ZrO₂

On pure monoclinic ZrO₂, only very weak adsorption signals of CO₂ were observed after annealing in CO₂ pressures up to 12 mbar (Figure 6).

Figure 6.

Infrared spectra taken after static adsorption of CO₂ on ZrO₂ at room temperature at pressures up to 12 mbar.

Static conditions with pressures below 12 mbar were enough to saturate the surface with chemisorbed species. Therefore, higher pressures or even flowing conditions do not intensify the very weak (bi)carbonate signals. Nevertheless, the vibrations at 1628, 1431, and 1223 cm^–1 can be associated with a bicarbonate species on the surface. These signals very much coincide with literature-reported spectra taken on pure monoclinic ZrO₂.¹¹ However, in our case only very weak signals are observed, which is essentially due to the strong dehydroxylation treatment, conducted prior to the FT-IR measurements. Spectra shown in ref (11) (Figure 2 of that reference) clearly show peaks of surface hydroxyl groups, which are almost absent in our case. The starting pressure of 2 mbar at room temperature was enough to saturate the surface, and thus any further increase of the CO₂ pressure did not influence the spectra. This corroborates the assumption that hydroxyl groups are of crucial importance for the chemisorption of CO₂. The following heating of ZrO₂ in CO₂ did not lead to reliably interpretable spectra due to the low signal-to-noise ratio.

3.1.4. Comparison of Y₂O₃, YSZ-8, and ZrO₂ upon CO₂ Adsorption

All oxides were pretreated by heating in air up to 1173 K, which leads to effective dehydroxylation of the surface. Only in the case of pure ZrO₂, no rehydroxylation was observed, even in the presence of water vapor at temperatures up to 873 K (see Figure S2, Supporting Information, also showing a comparison of the three oxides upon H₂O adsorption). As a result, ZrO₂ remains almost unreactive for CO₂ adsorption. This emphasizes the fact that surface hydroxyl species are important for effective chemisorption of CO₂.

The comparison of the three oxides Y₂O₃, YSZ-8, and ZrO₂ during exposure to CO₂ at room temperature and 873 K is highlighted in Figure 7 (concerning the assignment to (bi)carbonate species, see Table 1).

Figure 7.

Comparison of the collected infrared spectra upon CO₂ adsorption at room temperature (lower panel) and 873 K (upper panel) on the surfaces of the oxides Y₂O₃, YSZ-8, and ZrO₂.

Table 1. Observed Wave Numbers and Assignment of the Peaks to (Bi-)carbonate Species Following CO₂ Adsorption on Y₂O₃, YSZ-8, and ZrO₂.

oxide	carbonate species	νas(CO3) [cm–1]	νs(CO3) [cm–1]	δ(OH) [cm–1]	ν(OH) [cm–1]
Y2O3	monodentate bicarbonate	1674	1397	1223	3658
	bidentate bicarbonate	1652	1419	1223	3625
	bridged carbonate	1585	1334
	poly dentate carbonate	1452	1388
YSZ-8	bidentate bicarbonate	1646	1429	1225	not determined
	bridged carbonate	not determined	1305
ZrO2	bicarbonate	1628	1431	1223

At room temperature, Y₂O₃ and YSZ-8 show a very similar behavior (with the exception of a different distribution of the (bi)carbonate species), suggesting that the hydroxylated yttrium centers in YSZ-8 are the reactive sites of the mixed oxide. Upon heating YSZ-8 to at least 673 K, the adsorbed carbonate species are completely removed, indicating weaker bonding than on pure Y₂O₃, which is also reflected by the eventual absence of strongly bound polydentate carbonate species. As a conclusion from measurements on pure Y₂O₃, the monodentate bicarbonate must be more strongly bound than the bidentate carbonate, and the conversion to the associated bridged species—bound to two yttrium atoms—is the essential step toward even more strongly bound species, which are hard to remove. Most likely, the monodentate bicarbonate is not only bound to an yttrium atom via one of its oxygen atoms but also stabilized through a hydrogen bond with a neighboring hydroxyl group. In contrast, YSZ-8 obviously does not exhibit these neighboring Y₂O₃ centers. If water binds dissociatively onto a defect caused by a Y³⁺ in the ZrO₂ lattice, a HO^– bound to the yttrium (because of its basicity) is formed. This is the adsorption site for a CO₂ molecule to subsequently form a (bi-)carbonate species. In the case of a bridged carbonate, this species is more likely bound to a yttrium and a zirconium atom than to two adjacent Y³⁺ centers, and this seems to make the difference in the binding strength of this particular species. Comparing the carbonate spectra provided by Pokrovski et al.¹² and Baeza et al.¹¹ obtained on different hydroxylated ZrO₂ polymorphs, we note remarkable similarities in the overall features of the carbonate regions but a slightly different population of the different (bi)carbonate species. On pure Y₂O₃, the bicarbonate species are dominating, whereas on YSZ-8, the relative fraction of bidentate and bridged carbonate species is higher. In close correlation, the spectra of Pokrovski et al. show a higher fraction of bridged species. Most important, the spectra obtained on tetragonal ZrO₂ are strikingly different from those observed on YSZ-8. The main species on tetragonal ZrO₂, as reported by Baeza et al.¹¹ and Pokrovski et al.,¹² are polydentate carbonate species already after adsorption at room temperature (cf. Figure 3 of ref (12)). However, despite the different distribution of bicarbonate, bidentate, and bridged carbonates, no polydentate carbonates are found on YSZ-8, further confirming that the adsorption of YSZ is basically dominated by the hydroxylated Y centers.

3.2. CO Adsorption

Besides molecular adsorption as “intact” CO, for the adsorption of a CO molecule and following reaction to CO₂ on an oxide surface, two mechanisms are in principle possible:^14,21 (i) the CO molecule reacts with an oxygen atom of the lattice to form CO₂ and leaves an oxygen vacancy behind or (ii) it inserts into a surface hydroxyl group, forms a formate, and this intermediate subsequently decomposes into CO₂ (or reacts backward to CO and OH). Figure S3 (Supporting Information) shows the possible formate species on an oxide surface.

3.2.1. Y₂O₃

Figure 8 shows Y₂O₃ heated in 290 mbar CO under static measurement conditions (main panel).

Figure 8.

Infrared spectra taken upon heating Y₂O₃ in 290 mbar CO under static measurement conditions up to 773 K (upper panel) and with details of the wavenumber region above 2700 cm^–1 (right lower panel) and below 1700 cm^–1 (left lower panel).

At room temperature, no adsorption or reaction takes place, but during heating, CO₂ formation starts at 373 K. As an intermediate of this reaction, signals of formates can be detected, with an intensity maximum reached at 573 K. The corresponding vibrations for the ν_as and the ν_s mode of the formate are at 1595 and 1381 cm^–1, and the related ν(CH) vibration is located at 2856 cm^–1. Also the decrease of surface hydroxyl groups (left lower panel) can be detected starting from a temperature of 373 K at 3704 and 3672 cm^–1, which illustrates the analogy to the above-mentioned observations regarding CO₂ adsorption/carbonate formation. Above a temperature of 573 K the formates are rapidly decomposed. Because of the formation of CO₂, vibration modes for carbonates are observed as well—especially in analogy to the measurements in CO₂ at 473 K the bridged carbonate is the main species. At 773 K the polydentate carbonate is again predominant (right lower panel). Accompanied by the already mentioned formate signals, combination modes, namely, ν_1(combi) (ν_as(CO₂ + δ(CH)) at 2972 cm^–1 and ν_2(combi) (ν_s(CO₂) + δ(CH)) at 2727 cm^–1, are observed (lower left panel).^18,21 On the basis of refs (18 and 21) the observed formate species are assigned to a bidentate or bridged species.

Starting from room temperature, weak signals are observed at 1630 and 1294 cm^–1, which may be arising from monodentate formate species. Comparing our results to those of the literature,¹⁹⁻²¹ it is generally assumed that the monodentate species is the more reactive and less stable one. Therefore, the bidentate or bridged formate species could be mainly a spectator below 773 K.

3.2.2. YSZ-8

Figure 9 shows YSZ-8 heated in flowing CO (CO flow = 0.2 mL s^–1).

Figure 9.

Infrared spectra collected upon heating YSZ-8 in flowing CO (CO flow = 0.2 mL s^–1) up to 873 K. Infrared spectra have been taken in 100 K steps.

As for CO₂, flowing conditions were chosen for YSZ-8 because of better interpretable spectra due to increased adsorption of surface species. At room temperature, no signals of any adsorbed molecules are detected. However, the processes on YSZ-8 are again quite similar to those observed on Y₂O₃, with the exception that the spectra are more easy to interpret because of no disturbing (bi)carbonates. CO₂ formation starts at 573 K, and in-parallel typical formate peaks at 2875 cm^–1 for the ν(CH) mode and 1581, 1384, and 1357 cm^–1 representing the ν_as(OCO), the δ(CH), and the ν_as(OCO) mode of the formate arise. The combination modes are less pronounced as compared to Y₂O₃. These formate vibration modes are visible up to a temperature of 675 K, and above 773 K the same spectra fingerprint as for CO₂ adsorption is observed. In other words, no adsorbates are detected anymore. During heating, also a decrease of signals at 3690, 3643, and 3613 cm^–1, attributed to surface hydroxyl groups, occurs. Interestingly, on YSZ-8 no signals for monodentate formate species are visible.

3.2.3. ZrO₂

To ensure full correlation with the CO₂ measurements, static conditions are again chosen. Also by analogy to CO₂ adsorption, pure dehydroxylated ZrO₂ did not show significant signals attributable to surface adsorbates in static CO (Figure 10).

Figure 10.

Infrared spectra collected upon heating ZrO₂ following static adsorption of 248 mbar CO up to 873 K. Infrared spectra have been taken in 100 K steps.

This essentially verifies that no defects (which are usually titrated by molecular CO adsorption⁷) and no substantial amount of hydroxyl groups are present because of the oxidizing high-temperature pretreatment of the sample. However, during heating in 248 mbar CO at 573 K, very weak peaks at 2361 and 2875 cm^–1 being attributable to ν(CH) and/or combination modes of formates are observed (see left inset). The associated broad signals in the region below 1600 cm^–1 are too weak for an appropriate interpretation. We note that all oxides were pretreated by heating up to 1173 K in air to reduce impurities, and in the case of ZrO₂ no rehydroxylation takes place. Because of the lack of surface hydroxyl groups only a minute amount of formates can in turn be formed.

3.2.4. Comparison of Y₂O₃, YSZ-8, and ZrO₂ Regarding CO Adsorption

The comparison of the adsorption behavior during exposure to CO at maximum formate signal, namely, at 573 K, is highlighted in Figure 11.

Figure 11.

Comparison of the infrared spectra of the oxides Y₂O₃, YSZ-8, and ZrO₂ at maximum formate signal (573 K).

On none of the three studied oxides were signals of adsorbed (physisorbed) CO detected, as was previously shown for more defective ZrO₂ samples.¹⁰ We might anticipate that all defects are therefore quenched upon the preannealing in oxygen to 1173 K. Note that our results strongly suggest that in line with the initial question about the mechanism of CO₂ formation following CO adsorption a mechanism including the participation of surface OH-groups is found to be most likely prevailing (mechanism ii). Substantial reduction of the oxides is therefore less likely, and this assumption is also corroborated by additional electric impedance measurements, which only show the reversible formation of thermally excited charge carriers but no substantial formation of surface defects (Figure S4, Supporting Information).

Table 2 summarizes the main formate vibration modes observed on all oxides.

Table 2. Observed Wave Numbers and Assignment of the Peaks to Formate Species Following CO Adsorption of Y₂O₃, YSZ-8, and ZrO₂.

oxide	νas [cm–1]	νs [cm–1]	ν(CH) [cm–1]
Y2O3	1595	1381	2856
YSZ-8	1581	1357	2875
ZrO2	not determined	not determined	2875

Like in the case of CO₂ adsorption, the comparison of the oxides shows that the yttrium amount in YSZ-8 is important for chemisorption reactivity because in contrast to pure ZrO₂ the surface of the mixed oxide can be easily reactivated after dehydroxylation. This again corroborates the assumption that the basic surface hydroxyl species on Y-centers are also important for the conversion of CO toward formates. On YSZ-8, the carbonates—resulting from adsorption of the formed CO₂—are not bound as strongly as on Y₂O₃ and thus do not interfere with the formates.

4. Conclusion

Using FT-IR spectroscopy investigations, surface reaction-induced adsorbates of CO₂ and CO on the technically relevant oxides Y₂O₃, YSZ-8, and ZrO₂ could be identified. Y₂O₃ and YSZ-8 show much higher reactivity toward both CO₂ and CO adsorption than ZrO₂. The reversible formation and basic presence of surface hydroxyl groups is a crucial factor of an active surface, and the yttrium centers in YSZ-8 seem to essentially exhibit this quality. Thus, they are the C1-reactive sites of the mixed oxide YSZ-8. On Y₂O₃ and YSZ-8, several kinds of carbonates are detected, whereby in general on Y₂O₃ the adsorbates are more difficult to remove because of obviously easier conversion to more strongly bound, higher-coordinated species. The mechanism of chemisorption of CO₂ most likely proceeds via an insertion of the CO₂ molecule into a surface hydroxyl group and the following formation of a bicarbonate. Further conversion into more strongly bound carbonate species is observed at higher temperatures. In the case of CO oxidation the formation of formate intermediates is observed on all oxides, but the absence of a significant amount of hydroxyl groups on ZrO₂ leads to very weak signals on this oxide. The present FT-IR spectroscopic study moreover suggests that SOFC-related C1 reactions (water–gas shift reaction, intermediates of CH₄ reforming) on YSZ-based cermet anodes may be mechanistically/kinetically influenced by the basic character and by the pronounced reversibility and degree of hydroxylation of the Y-centers in YSZ-8.

Supporting Information Available

S1: Structure of possible (bi)carbonate species on the surface of an oxide (adapted from ref (15)). S2: IR spectra collected on the three oxides upon adsorption of H₂O(g). Pressures include 21 mbar H₂O for Y₂O₃ and ZrO₂ and 24 mbar for YSZ. S3: Structure of possible formate species on the surface of an oxide (adapted from ref (21)). S4: Comparison of the three oxides by electric impedance measurements in flowing CO, heated to 1073 K. This material is available free of charge via the Internet at http://pubs.acs.org.

The authors declare no competing financial interest.