Specific Interaction of PdOx- and RuOy-Modified Tin Dioxide with CO

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Specific Interaction of PdO and RuO Modified Tin Dioxide with CO and NH Gases: Kelvin Probe and DRIFT Studies 3

Artem V. Marikutsa, Marina N. Rumyantseva, and Alexander M. Gaskov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b02532 • Publication Date (Web): 06 Oct 2015 Downloaded from http://pubs.acs.org on October 7, 2015

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The Journal of Physical Chemistry

Specific Interaction of PdOx and RuOy Modified Tin Dioxide with CO and NH3 Gases: Kelvin Probe and DRIFT Studies Artem Marikutsa*, Marina Rumyantseva, Alexander Gaskov Chemistry Department, Moscow State University, Vorobyevy gory 1-3, Moscow 119991, Russia Keywords. Catalytic clusters, surface modification, gas-solid interaction, nanocrystalline tin dioxide, carbon monoxide, ammonia.

The processes of selective interactions of gas sensitive nanocrystalline tin dioxide modified by catalytic PdOx and RuOy clusters with ppm-range concentrations of CO and NH3 in air was studied. From simultaneous in situ work function and resistance measurements it was deduced that the modifiers promote specific interaction of gas molecules with the materials surface. In situ DRIFT studies revealed a key role of catalytic clusters in the selective gas-solid interactions. Specific CO chemisorption on Pd-sites was observed at room temperature. It is assumed to facilitate further oxidation of the molecules on the surface of SnO2/PdOx providing its low-temperature sensitivity to carbon monoxide. The selectivity of SnO2/RuOy to ammonia is due to the modifier-catalyzed deep oxidation of NH3 molecules, which was evidenced by the evolvement of Ru-bound nitrosyl species during the interaction at raised temperature. The possible routes were suggested for specific SnO2/PdOx – CO and SnO2/RuOy – NH3 interactions as a summary of experimental observations.

Introduction

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Selectivity of gas-solid interactions is a key issue in the fields of gas sensing, heterogeneous 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

catalysis, environmental sciences etc. In semiconductor metal oxide sensors, for example, the lack of selectivity is due to non-specific adsorption of target gas molecules and chemical reaction with materials surface1. The surfaces of metal oxides usually possess a uniform set of active sites, such as undercoordinated cations and anions, adsorbed oxygen and water, hydroxyl groups2. Their RedOx interaction with gas molecules, which determines the sensor behaviour, proceeds via similar routes with different gases. Sensor response is determined by the relevance of target gases to either reducing (CO, NH3, H2S, H2, volatile organic compounds), or oxidizing (nitrogen oxides, ozone, chlorine) type, rather than by individual nature of particular gas molecules1. An efficient approach to improve selectivity of gas sensitive semiconductor oxides is surface modification by catalytic or acid-base additives. This method is to create specific active sites which selectively promote adsorption (acid-base modifiers) and/or conversion (catalytic additives) of gas molecules. Several noble metals (Pd, Pt, Ag, Au, Ru, Rh) and transition metal oxides (CuO, NiO, Cr2O3, La2O3, Fe2O3, V2O5, MoO3), were used for the modification of tin dioxide, which is the most utilized material for semiconductor gas sensors3-5. A systematic approach for choosing a modifier was proposed4. It implies an adjustment of chemical reactivity of the additive to that of a gas molecule, e.g. modification of semiconductor surface by acidic V2O5, MoO3 oxides for detection of basic NH3 molecules or modification by basic CuO to improve the sensing of acidic H2S molecules. For detecting reducing gases with no acid-base reactivity (CO, H2, hydrocarbons) of the highest efficiency are the catalytic noble metal additives – platinum group metals (Pd, Pt, Ru) and gold. Such additives not only increase the materials sensitivity, but also lead to the decrease of temperature of optimal sensitivity6-10. It reflects the catalytic action of modifiers, i.e. lowering activation energy for target gas molecule oxidation. By the way, in heterogeneous catalysis the factors controlling catalyst activity and selectivity in the process of gas molecules conversion are well established. These factors include size, concentration of the catalytic sites and their bond energy with initial gas molecules, intermediates and reaction products, which should balance the rates of gas molecules adsorption, chemical transformation and product desorption11. Application of catalytic 2 ACS Paragon Plus Environment

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modifiers is also advantageous for metal oxide sensitization to basic (ammonia) gas molecules, since 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

it was shown that acidic additives favor the by-process of water adsorption, which rules out selectivity to NH3 in presence of humidity12,13. In our previous works14,15 we showed that modification of nanocrystalline tin dioxide surface by clusters of palladium oxide yields materials highly sensitive to CO at as low as room temperature. On the other hand, modification by ruthenium oxide increased tin dioxide sensitivity to NH3 at raised temperature. Recently it was found that selectivity provided by these modifiers was enough to discriminate CO and NH3 gases mixed together in a ppm-level concentration in air16. The improvement of sensing characteristics agrees with the reported activity and selectivity of palladiumand ruthenium-based catalysts towards CO and NH3 oxidation, respectively, which are due to optimal metal-gas and metal-oxygen bond energies17,18. However, the differences in temperature behaviour and humidity effect on the sensor response implied distinct sensitization routes in these systems. Specific promotion of active sites on the surface of tin dioxide modified by PdOx and RuOy clusters was recognized19. Experimental observations by XPS and oxygen isotopic exchange and indirect results from EPR and catalytic tests suggested that the modifiers interact themselves with target gas and oxygen molecules14,15,20. This paper is to complete our research of active sites and specific interactions of PdOx- and RuOymodified nanocrystalline tin dioxide with CO and NH3 gases by in situ simultaneous work function and DC-conductance measurements and in situ DRIFT spectroscopy. Summarizing the present data with our previous results a schematic representation of the modifiers effects on the interaction routes with CO and ammonia is suggested. Materials and methods The materials of nanocrystalline SnO2, blank and modified by PdOx or RuOy, were used for the study. The samples composition and microstructural characteristics are presented in Table 1. Tin dioxide was synthesized by aqueous deposition route using ammonia from the solution of tin (IV) chloride14. Deposited stannic acid SnO2·nH2O was washed from chloride-ions, dried and annealed in air at 300 0C for 24 h. Tin dioxide surface modification by 1 wt.% of PdOx or RuOy was performed 3 ACS Paragon Plus Environment


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via impregnation technique. The obtained nanocrystalline SnO2 powder was dispersed in ethanol 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

solutions of Pd(acac)2 and Ru(acac)320,21. The impregnated samples SnO2/Pd(acac)2 and SnO2/Ru(acac)3 were calcined at 225 0C and 265 0C, respectively, to decompose the noble metal acetylacetonates. This yielded nanocomposites of agglomerated tin dioxide nanoparticles covered by clusters of mixed-valence oxides of palladium and ruthenium with different oxidation states and crystallinity (Table 1). Table 1. Materials composition and microstructural characteristics Modifier content composition

Sample

Crystalline phasea Total concentrationb , wt.%

and Particle size, nm

Oxidation and SnO2 crystalline statec, at.% (dXRD)a (dTEM) d from total modifier concentration

SnO2

3-6

95 – 100

70 % – PdO (amorphous) SnO2/PdOx

0.94 ± 0.05 SnO2

SnO2/RuOy

Modifiere BET area, m2/g

25 at.% – Pd0

1-3 3-5

5 at.% – Pd3+

0.81 ± 0.05

80 at.% – RuO2 3-5 (structured)

90 – 95 2-8 2-5 90 – 95

20 at.% –Ru3+ a

From XRD and electron diffraction14,15,21,22; bfrom ICP-MS; cestimated by XANES, EXAFS, XPS and EPR14,15,20,22, dfrom TEM14,15,21; eevaluated by HRTEM, STEM and EDX-mapping22. Simultaneous in situ work function (Φ) and DC-resistance measurements were implemented using a laboratory setup including Kelvin probe and a gas sensing microhotplate embedded in a flow chamber15. Two types of samples were prepared for the experiments: Kelvin probe samples – for work function measurements and gas sensor samples – for DC-resistance measurements. The samples of both types were prepared simultaneously and treated under identical conditions. Nanocrystalline powders were ground with a binder (terpeniol) to obtain a paste. For Kelvin probe 4 ACS Paragon Plus Environment

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samples the paste was deposited onto stainless steel substrates, forming a layer with the dimensions 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

of ~5x5 mm and thickness of 14-17 µm. For a gas sensor preparation the same paste was dropdeposited onto alumina microhotplates. The microhotplates were provided with vapor-deposited rectangular-shaped Pt contacts (0.3x0.2 mm) separated by 0.2 mm gap and with embedded Ptmeanders. The formed sensing layer covered area of 1.1x0.6 mm, its thickness was estimated to 5-7 µm. Photographs of sensor substrate before and after sample deposition can be found in Supplementary data. After deposition the Kelvin probe and gas sensor samples were dried in air and annealed in a furnace in air at 200 0C for 24 h. Kelvin Probe S with round gold grid (3 mm diameter) oscillator and Kelvin Control 07 equipment (Besocke Delta Phi GmbH) operated in potential-zeroing mode were used for work function measurements. During the experiments temperature was fixed at a value in the range of 25 – 200 0C and a gas flow (200 ml/min) was in situ switched from background gas (air) to test gas and vice versa. Generator of purified air (model “1,2-3,5”, “Himelectronica”, Russia) was used as the carrier gas source, contaminations level according to the manufacturer guarantee is not higher than: H2O – 10 ppm, CO2 – 2 ppm, hydrocarbons – 0.1 ppm. The test gases were CO (10 – 100 ppm) or NH3 (0.3 – 10 ppm) in air. The sources of analyte gases were certified gas mixtures (“Linde-Gas”, Russia): CO (517 ± 12 ppm):N2 and NH3 (209 ± 8 ppm):air. Electron affinity difference (∆χ) due to test gas exposure was calculated according to the following equation under the assumption that depleted layer conduction model was persistent during the measurements24: ∆Φ = ∆χ – e∆Vs = ∆χ + kT·ln(Rgas/Rair) = ∆χ – kT·lnS

(1),

where Φ is work function; e – elementary charge; Vs – surface potential barrier height; –e∆Vs refers to electron energy barrier height at the surface; k – Boltzmann constant; T – temperature; S – sensor signal defined as S = Rair/Rgas, ∆ implies subtraction of the parameter value under air from that under the test gas. The absence of an impact from gold electrode grid to the work function change in presence of test gases was confirmed via reference Kelvin probe measurements using blank stainless steel as an inert substrate. 5 ACS Paragon Plus Environment


In situ DRIFT spectroscopic measurements were performed using Frontier (Perkin Elmer) FTIR 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

spectrometer equipped with DiffusIR annex with HC900 (Pike Technologies) test camera. The spectrometer works under ambient conditions with automatic H2O/CO2 compensation (spectrometer self-test every 15 min). The test chamber is equipped with gas inlet and outlet, sample crucible heater and ZnSe window for spectra collection. Sample powders (30 mg) were placed into alumina crucible (5 mm diameter) and rammed to smooth the surface, the thickness of sample in crucible was ~1 mm. Before every measurement the samples were preconditioned in the carrier gas at a constant temperature equal to temperature of the subsequent test: the samples in the chamber were kept under purified air flux (100 ml/min) for 1.5 h under until no spectral changes due to water desorption were registered. After preconditioning in air the background spectrum was collected. Starting the test measurement the samples were exposed for 30 min more to air flux to verify that baseline was zero and no further desorption of water occurred. Next, the flux (100 ml/min) was switched to test gas; it was purged for 1 h at a constant temperature in the range of 25 – 200 0C. CO (100 ppm) and NH3 (100 ppm) in air were used as the test gases. During the exposure, relative IR absorbance spectra were registered related to the above mentioned background spectrum in 4000 – 1000 cm-1 wavenumber region with resolution 4 cm-1 and accumulation of 4 scans (0.7 min for a spectrum collection), spectra collection every 5 min. Results and discussion Simultaneous work function and resistance measurements The effect of tin dioxide surface modification on its reactivity to CO and ammonia gases was estimated from in situ Kelvin probe and DC-resistance measurements. Fig. 1 shows dynamic plot of work function change (∆Φ) and DC-resistance responses for modified tin dioxide samples exposed to increasing concentrations of CO at room temperature (Fig. 1a) and NH3 at 200 0C (Fig. 1b). The ∆Φ was defined as the change of work function caused by the sample exposure to the test gases, so that baseline values ∆Φ ~ 0 eV correspond to the samples pretreated in air for 16 h. The exposure to test gases caused reversible resistance decrease, since both CO and ammonia are reducing gases. Such molecules undergo oxidation by ionosorbed oxygen species on the sensor surface. This process 6 ACS Paragon Plus Environment

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is accompanied by releasing electrons into semiconductor bulk, thus increasing its conductivity1. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Along with this, reversible work function decrease was detected during the interaction with target molecules, the response increasing with the test gases concentration. Such behaviour of work function and resistance was observed for all the samples exposed to CO (10 – 100 ppm) and NH3 (0.3 – 10 ppm) in the whole tested temperature range (25 – 200 0C). Notably, ∆Φ recovery in air after the exposure to test gases is rather slow; and in case of room temperature interaction this is due for DC-resistance also (Fig. 1a). The reason for it should be larger layer size and thickness of Kelvin probe samples in comparison with sensor layers for resistance measurements, and also slow kinetics of gas-solid interaction at low temperature. Taking this into account, the response values for resistance and ∆Φ were estimated from extrapolated baseline values in air. To perform it, the baselines in air were recorded for 16 h at each temperature prior to exposure to test gases.

Figure 1. Work function change (∆Φ) and DC-resistance (R) dynamic responses of SnO2/PdOx exposed to CO (10-25-50-100 ppm) in air at T = 25 0C (a) and of SnO2/RuOy exposed to NH3 (0.3-15-10 ppm) in air at T = 200 0C (b). 7 ACS Paragon Plus Environment


In the frame of conduction model describing sensing phenomena in nanoparticular 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

semiconductors24, the variation of work function (∆Φ) during interaction with a gas is a sum of the variation of electron energy barrier height at the surface (–e∆Vs = –kTlnS) and of electron affinity (∆χ). The former parameter determines activation energy for electronic conduction. The observed conduction increase implies electron energy surface barrier decline (–e∆Vs < 0) in presence of CO and NH3. Since the –eVs value is defined by the concentration of ionosorbed surface species, it reflects the intensity of electrochemical interactions at the surface, i.e. reduction of ionosorbed oxygen by CO or NH3. On the other hand, electron affinity (χ) is proportional to the concentration of dipolar (chemisorbed and/or physisorbed) surface species and does not influence electronic conduction23,24. Thus, electron affinity variation (∆χ) in presence of a test gas could be considered as a measure of the intensity of chemical interactions between dipolar surface species and gas molecules (and/or physical adsorption of target molecules at low temperatures). In this work, ∆χ values defined as the difference of electron affinity in presence of target gas from that in air were estimated by eq. (1). At this point the assumption was made that the surface potential barrier-limited conduction mechanism24 was invariable during the experiments. It is reasonable with respect to low concentrations of reducing gases used. Noteworthy, the equation (1) is valid for flat-band conduction model also, i.e. in case of fully depleted semiconductor grains24. In this case the difference would only be in the expression for activation energy of conduction: (Ec,s – Es,s) instead of eVs, where Ec,s is conduction band energy at the surface and Es,s – surface states energy. The relation between ∆Φ, kTlnS and ∆χ values is illustrated on the examples of SnO2/PdOx exposed to CO at room temperature (Fig. 2a) and SnO2/RuOy exposed to NH3 at 200 0C (Fig. 2b), where the highest work function and resistance responses were observed. In these cases the decrease of work function exceeded that of potential barrier, which implied substantial decrease of electron affinity in presence of target gases. Its behaviour was, however, strongly dependent on temperature. Fig. 2c shows electron affinity variation of SnO2/PdOx induced by increasing CO concentration at different temperatures. The ∆χ values were almost independent on the analyte concentration in the 8 ACS Paragon Plus Environment

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tested range (10 – 100 ppm), which agrees with other Kelvin probe studies of SnO2(Pd) – CO system 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

reported by N. Barsan et.al.25. Another coincidence with this work is that presently measured ∆χ vs. CO concentration curves at 150 – 200 0C (Fig. 2c) are similar to those obtained by the authors at 270 0

C under dry air conditions. Yet, the curves like those measured in our work at 25 – 50 0C (Fig. 2c),

in literature25 were observed at 270 0C under 50 % relative humidity. According to the publication and supported by DRIFT data discussed below, the decrease of electron affinity under these conditions can be attributed to surface ОН-groups consumption in the interaction with CO. It is reasonable to assume that increasing humidity at raised temperature (as in25) and lowering temperature at fixed humidity (as in present work) have similar effect on surface hydroxyls. Adsorption of H2O traces (up to 10 ppm in our air source) facilitated by low temperature at 25 – 50 0

C increases the concentration of OH-groups capable to oxidize CO molecules. At raised

temperature because of water desorption there seems to be not enough reactive hydroxyls on the surface, which accounts for low electron affinity variation at 150 – 200 0C (a slightly positive ∆χ ~ 50 meV response at 200 0C is comparable to kT = 41 meV at this temperature). The independence of ∆χ values on the gas concentration in 10 – 100 ppm range means that the amount of OH-groups interacting with CO is limited and corresponds to CO concentration below 10 ppm. Hence, the increase of sensor response with CO concentration (Fig. 2a) should be due to its oxidation by charged surface species, such as ionosorbed oxygen. The electron affinity response of SnO2 and SnO2/RuOy samples to CO was as well independent on concentration and close to zero (lower than kT) in the tested temperature range (Supplementary data), i.e. the interaction with carbon monoxide likely did not involve dipolar species on the surface of these materials. The electron affinity decrease in presence of ammonia was in most cases concentration-dependent in the range of 0.3 – 10 ppm NH3. The largest ∆χ responses were shown by SnO2/RuOy, their dependence on NH3 concentration increasing with temperature (Fig. 2d). For SnO2 and SnO2/PdOx samples the curves looked similarly, but the absolute ∆χ values were smaller (Supplementary data). Temperature plots of ∆χ responses of the samples to a fixed target gas concentration are compared in Fig. 2e,f. On exposure to CO it was SnO2/PdOx that exhibited largest electron affinity 9 ACS Paragon Plus Environment


drop, especially at 25 – 100 0C (Fig. 2e). It agrees with the temperature range of improved CO 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

sensitivity of PdOx-modified tin dioxide14,20. In the interaction with NH3 two temperature regions could be outlined. At 25 – 50 0C for all the samples close ∆χ values about -50 meV were observed (Fig. 2f). As suggested by DRIFT data below, at this temperature ammonia chemisorption on cation sites and on OH-groups is the main process and it proceeds similarly on the samples. The decrease of electron affinity in this case might be due to partial electron density shift towards surface cations either from ammonia directly or from hydroxyl groups upon their association with NH3 molecules. At temperature 100 – 200 0C the largest ∆χ responses were demonstrated by SnO2/RuOy (Fig. 2f), which is in line with its increased NH3 sensitivity at this temperature15. However, in contrast to the SnO2/PdOx – CO system, the interaction of SnO2/RuOy with NH3 at raised temperature is unlikely to involve hydroxyl groups. Work function measurements performed by T. Sahm et.al. indicated that at temperature up to 200 0C molecular chemisorption is the predominant mode of tin dioxide interaction with O2 gas yielding dipolar rather than ionic adsorbates26. Recent study of active sites of modified SnO2 suggested that chemisorbed oxygen is the prevailing type of oxidative sites on the surface and that RuOy clusters facilitate the increase of their concentration19. Thus, the uptake of chemisorbed oxygen species during the interaction with NH3 is assumingly the reason for electron affinity decrease in presence of the target gas at 100 – 200 0C. From the general agreement between temperature trends of ∆χ (Fig. 2e,f) and sensor signals14,15,20 for SnO2/PdOx – CO and SnO2/RuOy – NH3 systems it can be inferred that tin dioxide modification by PdOx and RuOy provides specific enhancement of its surface reactivity to CO and NH3, respectively. Besides the facilitation of electrochemical interactions involving ionosorbed species and resulting in improved conductance sensitivity, this effect includes the promotion of chemical reactions between target gas molecules and chemisorbed surface species evidenced by electron affinity decrease.

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Figure 2. (a-b) Work function (∆Φ), electron affinity (∆χ) and resistance (kTlnS) responses compared on the energy scale of SnO2/PdOx exposed to 10-20-50-100 ppm CO at T = 25 0C (a) and of SnO2/RuOy exposed to 0.3-1-5-10 ppm NH3 at T = 200 0C (b). (c-d) Electron affinity response vs. target gas concentration at different temperatures for: SnO2/PdOx exposed to 10-20-50-100 ppm CO (c) and SnO2/RuOy exposed to 0.3-1-5-10 ppm NH3 (d). (e-f) Electron affinity response vs. temperature for blank and modified tin dioxide to 100 ppm CO (e) and 10 ppm NH3 (f). 11 ACS Paragon Plus Environment


In situ DRIFT study 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 3 shows the DRIFT spectra of samples interacting with 100 ppm CO at room temperature. For the sake of comparison, PtO- and Au-modified (1 wt.% of additive) tin dioxide samples with close microstructural parameters to those in Table 1 were also tested, since these additives are often used to improve CO sensing by tin dioxide8-10. However, it was only on the spectrum of SnO2/PdOx that the peak at 2090 cm-1 and the band at 1910 – 1840 cm-1 appeared. Both the features are indicative of CO chemisorbed on reduced Pd species. The first one is in the region characteristic of linear carbonyl binding (

Specific Interaction of PdOx- and RuOy-Modified Tin Dioxide with CO

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