Some New Insights into the Sensing Mechanism of Palladium

J. Phys. Chem. B 2001, 105, 5737-5742

5737

Some New Insights into the Sensing Mechanism of Palladium Promoted Tin (IV) Oxide Sensor Shik Chi Tsang,* Colin D. A. Bulpitt, Philip C. H. Mitchell, and Anibal J. Ramirez-Cuesta Surface and Catalysis Research Centre, Department of Chemistry, UniVersity of Reading, Whiteknights, Reading, Berkshire, RG6 6AD, U.K. ReceiVed: January 17, 2001; In Final Form: April 6, 2001

The combination of techniques including switching experiments, temperature programed reduction and in situ neutron scattering-conductivity are used to investigate the sensing mechanism of 1% Pd/SnO2 toward hydrogen-containing gas mixtures. In particular, the use of the in situ neutron scattering-conductivity for the first time allows the simultaneous monitoring of electrical conductivity and inelastic neutron scattering spectra of the sensor material. Direct evidence is obtained on a reversible migration of hydrogenic species from and to the metal and the underlying tin oxide surface, i.e., reversible hydrogen spillover. As a result of this spillover, a dramatic change in electrical conductivity of the Pd doped tin oxide material is observed. We confirm, in accordance with the known mechanism, that the change of conductivity is based upon the creation or destruction of negatively charged adsorbed oxygen species on the sensor surface. In addition, we report a new but important sensing mechanism, the spillover hydrogen species behaving like a shallow donor to the semiconductor oxide as the direct source of conductivity occurs concurrently with the known mechanism.

Introduction For many years there has been intense interest in exploring the properties of semiconductor oxides, in particular tin (IV)oxide, SnO2, as gas sensitive resistors for toxic and explosive gas leaks for commercial and industrial applications.1,2 The conductivity of SnO2 is low in air, but markedly increases when air is mixed with a reducing gas (H2, CO and organic molecules). The recent increasing demand for small sensors to monitor pollutants, leaking hydrogen (a potential future fuel) and energy performance in automobile is particularly noted. Tin oxide based sensors are generally inexpensive, simple and fast to operate. It has been shown that the gas sensing properties of the tin oxide can be much improved by decorating the oxide surface with noble metals,3 such as Pd,4,5 Pt,6 and Ru.7 However, the working principle for of this type of sensor is not yet well understood despite of its wide applications. So far, the generally accepted sensing mechanism is based on the change of electrical conductance of the tin oxide upon exposure to different gases. When a reducing agent is not present, the atmospheric oxygen ties up the electronic carriers as it is adsorbed on the surface of the tin oxide.8 This reduces the sensor conductivity as the electrons from the bulk of the semiconductor near the oxide surface are attracted toward the adsorbed oxygen9 forming a depletion layer (space charge layer). When the noble metal-tin oxide sensor is in contact with hydrogen or a hydrocarbon in air, the reducing gas is first activated by the metal surface. Some forms of surface hydrogenic and hydrocarbon moieties species from created on the noble metal surface will react with the adsorbed, charged oxygen molecules on the tin oxide underneath support Via a spillover process at elevated temperatures. This reaction will lead to the re-injection of the localized electrons back into the bulk, thus rendering the tin oxide material more conducting. As a result, the quantity of reducing gas in air will * To whom all correspondence should be addressed. Tel (fax): 00 44 118 9316346. E-mail: [email protected].

be reflected by the change in magnitude of the conductivity of the sensor material. The noble metal promoter in the sensor material readily activates hydrogen or hydrocarbon dissociatively at low temperature generating reductant species on the metal surface. Thus, in the spillover process the hydrogenic or hydrocarbon species are transported to the tin oxide surface crossing the phase boundary between metal and support. It is, however, the role of spillover as constituting the key element in chemical sensing is still highly obscure. Circumstantial evidence for spillover in catalysis is provided by, for example, temperature-programmed reduction, FT-IR spectroscopies.10,11 However, direct evidence of spillover in relation to chemical sensing is not established. The aim of our work is to elucidate the sensing mechanism of the Pd-SnO2 sensor by using a combination of techniques. Recent developments in the inelastic neutron scattering technique allied with the very high neutron scattering cross section of hydrogenic species could enable us to observe directly and to characterize spillover hydrogen species on a sensor surface. Here we report the direct observation of spillover hydrogen atoms entrapped by the surface oxygen species of tin oxide forming surface hydroxyl intermediate, where the spillover hydrogen atoms are initially generated on the surface of Pd/ SnO2 under hydrogen atmosphere. In combination with the conductivity data obtained by the in situ cell at different temperatures the relationship of sensing to the hydrogen spillover and the nature of oxygen species involved in the sensing is revealed. Experimental Section Tin dioxide, SnO2, (AnalaR, purity > 99.9%) and bis(2,4pentanedionato)palladium(II), [Pd(CH3COCHCOCH3)2] (palladium acetylacetonate, [Pd(acac)2]), were obtained from Aldrich. Pd/SnO2 powder carrying 1% w/w Pd was prepared by adding the appropriate amount [Pd(acac)2] solution in de-ionized

10.1021/jp010175a CCC: $20.00 © 2001 American Chemical Society Published on Web 05/26/2001

5738 J. Phys. Chem. B, Vol. 105, No. 24, 2001 water to SnO2 followed by oven drying at 100 °C. The dried powder was then crushed and calcined in air at 500 °C for 24 h. For ex situ conductivity measurements the calcined powder was cold pressed at 10 tonne into a pellet (area ) 1 cm2) using an infrared press. The pellet (0.6 g of 1% Pd/SnO2) was sandwiched between two platinum disk electrodes and housed in a temperature controllable reactor. This was comprised of an airtight silica reactor tube (containing the pressed sensor material) housed within a programmable furnace with one end connected to gas supply via Swagelock couplings and the other connected to a rotary pump. Gas was passed over the sensors at a total flow rate of 100 mL min-1 controlled by Bronkhorst Hi-Tec mass flow controllers. The gas compositions under investigation were, 5% H2/Ar (BOC), 10% hydrocarbon (BDH) in air or in pure N2 (BOC). The diluted hydrocarbon gas was prepared at the 10% level by mixing the pure gas with pure N2 at the ratio of 9:1. All experiments were carried out between 30 and 300 °C. ac impedance measurements (swept from 0.05 Hz to 100 000 Hz at 500 mV) were taken using a Solatron 1260 analyzer. The resistance of a sample in ohm value at variable frequency was measured by sandwiching the sample between two 1 cm2 identical Pt disks. Because all the samples as in form of a thin film of a very similar thickness the impedance value of the sample is defined as (Ω cm2). This value is found only to be inversely proportion to the surface area if a sample with a smaller cross section area is used. Neutron scattering experiments were carried out at the ISIS Facility at the Rutherford Appleton Laboratory (RAL) pulsed neutron source. We used the electron volt spectrometer (eVS) which determines the neutron Compton scattering (NCS) spectrum of a material. In neutron Compton scattering, the momentum distributions of atomic nuclei are measured by the deep inelastic scattering of high-energy neutrons. In the eVS experiment we measure a large number of scattering events and build up a profile of the momentum and energy transfer to the scatterer. From the momentum and energy transfer the mean kinetic energies of the scattering nuclei and from the scattering intensities, the scatterer concentrations are derived. The kinetic energies reflect the chemical environment of the scatterer, for example, hydrogen atoms whether bound to oxygen or palladium. Thus, the neutron scattering experiments in our work provide directly two critical pieces of information: the relative concentrations of, in particular, H, O, and Sn and the chemical environment of H. Details of the eVS technique, the treatment of the spectra and the data analysis have been reported previously.12 The NCS spectra of the Pd/SnO2 sensor materials under different gas atmospheres were recorded simultaneously with the impedance spectra in situ. The in situ cell was built inhouse at Reading University to meet the specifications of the neutron beam at RAL. It consisted of two rectangular slabs of stainless steel, 115 mm × 100 mm × 20 mm, each with a central aluminum foil window of 60 mm diameter held together by nonelectrically conducting plastic bolts and plastic washers and sealed with an insert of heat resistant plastic sheet. The two halves of the cell were therefore insulated from each other. The thin aluminum foils served as windows for the neutron beam and as electrodes in contact with the sensor powder. To heat the cell 2 rod-heaters, 8 mm i.d. × 100 mm length, were inserted into either side of the slabs. A thermocouple was fixed into the cell so that the direct temperature could be measured at all times. Swagelock gas inlets and outlets of 1/8 inch were fixed into the cell at the top and bottom of the slabs so that gas could be

Tsang et al. TABLE 1: 1% Pd/SnO2 Exposed with Pulses of Air (or N2) Followed by 1%butane-air at 300 °C gas mixture

Impedance/107 Ω cm2 at 0.5 Hz

air 1%butane in air air/butane sensitivity* N2 1% butane in air N2/butane sensitivity*

9.755 0.0125 780.4 0.1153 0.011 33 1.02

sensitivity/106

780.4 10.02

purged through at one end and the other end connected to a rotary pump. Thus gases could be continuously flowed through the SnO2 sensor throughout the experiment. The impedance was measured via 2 extended cables from either side of the cell. For comparison reasons all the eVS measurements were conducted in the presence of diluted hydrogen (5% H2 in Ar) at 30 °C after various sample treatments. The measuring procedure was as follows. Any signal from the in situ cell packed with pure SnO2 with a minimum gas dead volume in a flow of 30 mL/min 5% H2/Ar at 30 °C was regarded as the background. We found that there was no difference in the background signals whether the in situ cell was purged with hydrogen or nitrogen gas (with no vacuum). Thus, a small amount of gaseous hydrogen does not critically affect the measurements as compared with the large amount of hydrogen adsorbed on the material surface. A sample of 1% Pd/SnO2 of same particle size was then packed into the cell and was first purged with dry nitrogen (150 °C) to eliminate any physisorbed water. The sample was then allowed to cool and the gas was switched to 5% H2/Ar at 30 °C. eVS spectra were collected under the diluted hydrogen for the next 12 h, at 30 °C. ac impedance spectra (in the diluted H2) were measured at regular intervals (every 4 h) during the 12 h period. The sample was then purged evacuated to enhance all the hydrogen desorption and then purged with nitrogen for 30 min where and another impedance spectrum (in N2) was recorded. The gases were switched to 5% H2/Ar, temperature ramped and kept at 120 °C, for 60 min and then cooled back to 30 °C. The same procedures as before were applied. The temperature was further ramped and kept at 200 °C for 60 min and then cooled back to 30 °C. Again, the same procedure was applied. The temperature was ramped and kept at 250 °C for 60 min and then cooled to 30 °C. The same procedures as before were applied. The temperature was finally ramped and kept at 300 °C for 60 min and then cooled back to 30 °C. Once again, the same procedures were applied. Results Ex Situ Experiments. Before we conducted any detailed characterizations or mechanistic studies, we tested our sensor material to confirm the sensing response to butane gas. It is noted from Table 1 that the recorded real impedance value (Bode value) was 9.755 × 107 Ω cm2 measured across the 0.6 g 1% Pd/SnO2 disk in a flowing stream of air at 300 °C from our apparatus. The impedance value of this sensor material is in good agreement with the literature.13 When air was switched to the stream of 1% n-butane in air the sensor impedance decreased to 1.25 × 105 Ω cm2. It was reported previously that the impedance response was in proportion to the concentration of butane in air.13 It is significant that the identical sensor when stabilized in pure N2 instead of air for 2 h gave a much lower impedance (1.153 × 106 Ω cm2). Hence, when we switched to

Palladium Promoted Tin (IV) Oxide Sensor

Figure 1. TPR profile for 1% Pd/SnO2 showing reduction peaks at 175 and 230 °C.

Figure 2. Effect of initially testing 5% hydrogen and then 10% cis2-butene under a dynamic vacuum at 30 °C over 1% Pd/SnO2.

the butane/air stream a considerably lower sensitivity was obtained (10 instead of 780). This observation supports the idea that some weakly bound oxygen species are responsible for constructing a depletion layer (mobile electrons in the semiconductor oxide were localized by the surface adsorbed oxygen species) giving a high impedance value in air. Under our sensing conditions these oxygen species are well desorbed in the pure nitrogen stream. Hence, without the large oxygen depletion layer, a low impedance value of the sensor material in nitrogen is therefore recorded. To gain a greater understanding of the stability of the particular surface oxygen species, temperature-programmed reduction of the sensor material using dilute hydrogen was conducted. As shown in Figure 1, the Pd/SnO2 sensor material exhibits a negative slope starting from room temperature (peak maximum at 50 °C) indicating some hydrogen liberation. It is known that PdO can be reduced by H2 at subambient temperature and can form PdHx at very low temperature. It is believed that the unstable hydride decomposed at above room temperature giving out the hydrogen gas. The TPR experiment shows clearly that two reactive forms of surface adsorbed oxygen species are reduced at 170 °C and 230 °C before the lattice oxygens of tin oxide are reduced at above 350 °C. For pure SnO2 we observed two reduction peaks before the lattice oxygen reduction at higher temperatures, namely, 260 and 325 °C, respectively (not shown). In agreement with the literature, the lower reduction temperatures of the two reactive oxygen species on Pd/SnO2 surface support the fact that spillover hydrogen on a Pd metal surface facilitates surface oxygen reduction. We have measured the change in conductivity of our sensor material (a new disk) at low temperatures by passing diluted hydrogen gas at 30 mL min-1 (5% H2 in Ar) over the 1% Pd/ SnO2 sensor material. These experiments exhibited some very interesting results as shown in Figure 2 and Figure 3. First, at 30 °C, the presence of H2 greatly reduced the observed

J. Phys. Chem. B, Vol. 105, No. 24, 2001 5739

Figure 3. Recovery in impedance of 1% Pd/SnO2 after H2 tests for a range of temperatures.

impedance (resistance). The values fell from 3 × 108 Ω cm2 to 3-5 Ω cm2 in our compressed Pd/SnO2 disk within 20 s. (Figure 2). The low impedance in the flow of hydrogen can be reproduced and there was no short circuit between the Pt electrodes. As shown by Figure 1 there was no detectable oxygen species that can be reduced by the hydrogen at below 175 °C; hence, no destruction of oxygen depletion layer should be expected. Thus, the important question is why the material switches to give high conductivity in flowing hydrogen at 30 °C. To investigate whether the attenuation of impedance is reversible or not a vacuum pump (a dynamic vacuum of 1 × 10-2 Torr can be created) was used in order to pull the hydrogen species from the sensor surface. The sensor material quickly returned to its prehydrogen level when the diluted hydrogen gas was switched off with simultaneously turning on the vacuum pump. A flow of 10% cis 2-butene was also used to substitute 5% of hydrogen and see whether a similar effect can be created. In the case of cis-but-2-ene, the impedance also immediately fell within 30 s from 108 Ω cm2 to a stable value of 1.2 × 105 Ω cm2. It is interesting that the hydrocarbon does not create the huge decrease in impedance shown by the diluted hydrogen. However, we show that the change is also a reversible one. In Figure 3 we show the effect of temperature on impedance in the presence and absence of hydrogen (under a dynamic vacuum). At 100 °C, a pattern similar to that at 30 °C was observed. Again, when H2 was passed the impedance fell within the first 20 s and then stabilized at around 3-5 Ω cm2. Upon removal of hydrogen, the impedance of the sensor rapidly increased back to the original prehydrogen level. The reproducible observation of the 100 °C experiment also discounts any effect due to PdHx which should well be decomposed at the reaction temperature. This clearly shows that the hydrogen effect on impedance is a reversible phenomenon depending on the presence or absence of gaseous hydrogen. It is noted that this effect is very different from the known sensing mechanism that involves high temperature construction or destruction of oxygen depletion layers. At 200 °C and 300 °C similar decrease in impedance values of 3-5 Ω cm2 was also observed when the sensor was exposed to diluted hydrogen gas. However, there was a marked difference in impedance recovery upon removal of hydrogen. Full recovery was not observed and the recovery curves were much more progressive and slow as compared to 30 °C and 100 °C experiments. At 200 °C the recovered impedance was about 5 × 106 Ω cm2 and at 300 °C, a steady value of 9 × 105 Ω cm2 was observed after a prolonged recovery time. (60 min). It is interesting to point out that the variation in H2 concentration showed marginal effect on the magnitude of

5740 J. Phys. Chem. B, Vol. 105, No. 24, 2001

Figure 4. H/O atomic ratios and impedance values determined by neutron Compton scattering and in situ conductivity measurements, respectively at 30 °C. (A) 2.05 × 109 Ω cm2 in N2, 2468 Ω cm2 in H2; (B) 8.91 × 108 Ω cm2 in N2, 2898 Ω cm2 in H2; (C) 4.86 × 107 Ω cm2 in N2, n/a in H2; (D) 8.07 × 106 Ω cm2 in N2, 3283 Ω cm2 in H2; (E) 8.95 × 105 Ω cm2 in N2, 1809 Ω cm2 in H2.

impedance loss or recovery as concentrations of 5, 2.5, 1.25 and 0.315% H2 was employed throughout the testing. Table 2 shows the effect of concentration on the impedance values. In Situ Experiments. Figure 4 shows the in situ impedance data and the measured H/O ratios of the 1% Pd/SnO2 sample, pretreated with hydrogen at different temperatures. The oxygen/ tin ratio was derived from the backscattering, which was proportional to the total oxygen content (bulk) and the H/Oxygen ratio was derived from the forward scattering bank. It is noted that the H signal obtained was only possible from the surface adsorbed hydrogenic species (OH, O2H-, O3H-, H2O, etc.). Interestingly, the 30 °C and 120 °C properly dried samples show incredibly high H/O ratios (0.017-0.021). In addition, we observed no significant difference in H/O ratios when the sample was switched to a flow of pure nitrogen (no vacuum application for enhanced hydrogen desorption) after the sample was prereduced in hydrogen at these temperatures. This implies that the gaseous hydrogen contributed insignificantly to the data. From the kinetic energy analysis, we found an average energy, within the harmonic approximation of 1390.7 ( 100 cm-1 for hydrogen and of 592 ( 50 cm-1 for oxygen atoms. These match well with the -OH vibrational frequencies suggesting that most of the hydrogen species are bonded to surface oxygen species. It is noted that the H/O ratios are much higher than the Pd/O ratio in the 1% Pd/SnO2 sample (see discussion) and the Pd predominantly exists as the Pd metal but not oxide in the presence of hydrogen. This high hydrogen content associated with the surface oxygen species on tin oxide supports hydrogen spillover. Further hydrogen pretreatments at higher temperatures rendered the H/O ratios lower; this suggests that there were changes in the relative concentrations of the surface hydrogen to the total oxygen content. It was noted that an abrupt decrease in the H/O ratios was observed between the temperature range of 100-200 °C, matching the observation of reduction peaks with the TPR experiments (Figure 2) and the dramatic changes in the sensor impedance recovery (after the hydrogen exposure) in the switching experiments (Figure 3), over the same temperature region. Thus, it is evident that the decrease in H/O ratios is associated with the reduction of surface oxygen species with the adsorbed hydrogenic species. Regarding to the impedance values, the sensor material displayed consistently low impedance values in diluted H2 (2,000-3,000 Ω cm2) and high

Tsang et al.

Figure 5. O/Sn ratios determined from neutron Compton scattering spectra.

impedance values (107-108 Ω cm2) in N2 under this particular cell geometry. It is very interesting to note that there was a progressive decrease in the impedance values collected in N2 at 30 °C (after all the hydrogen was pumped away) along with the higher hydrogen pretreatment temperatures. The largest decrease in the impedance values was again recorded when the sample was heated from 120 °C to 200 °C. (8.91 × 108 Ω cm2 at 120 °C; 4.86 × 107 Ω cm2 at 200 °C) In Figure 5 we show the ratios of the backscattering signals of oxygen (bulk) to tin (bulk). The result shows that the O/Sn ratio is 2 as expected for the SnO2 based material. For the SnO2 the ratio was maintained over the sample pretreated with hydrogen at different temperatures, except the data at 300 °C, where lattice oxygen reduction occurred in the prolonged hydrogen pretreatment. Thus, at below 300 °C, the experiments confirm that the principal drop in the H/O ratios shown in Figure 4 are mainly due to the loss of surface hydrogen associated with its reduction of surface oxygen (-OH de-hydroxylation), rather than any change in the bulk O or Sn content under the different pretreatment conditions. Discussion A New Sensing Mechanism. The basic principle of chemical sensing involves transduction of a chemical species into a detectable signal, the magnitude of which is proportional to the species concentration. For semiconductor oxide sensors, such as the tin oxide based material, a successful commercial example, the nature of its signal transduction is still very obscure. The change of the macroscopic electrical conductivity of this sensor material (signal) upon different gaseous environments reflects the change of the properties (surface and band structures) of the individual tin oxide particles and their intergranular characteristics. It is generally believed that some form of adsorbed oxygen species localize mobile electron from the bulk n-type semiconducting tin oxide in air creating a depletion layer at the skin of individual particles and intergranular regions while the reaction of reducing gas with the charged oxygen destroys the electron localization process. According to our ex situ switching experiments, we observed a dramatic decrease in impedance (and sensitivity) when the sensor was exposed to nitrogen at 300 °C. Our TPR work and the reported TPD work in the literature14 both suggested that the reactive oxygen species responsible for chemical sensing be readily removed in hydrogen or nitrogen at 300 °C. These facts are in a good agreement with the above mechanism since desorption of the weakly bound oxygen on tin oxide in nitrogen

Palladium Promoted Tin (IV) Oxide Sensor (their transient existence in air) could equally cause the destruction of depletion layer hence reducing the sample impedance value. Apart from confirming the well-known mechanism at elevated temperatures we made very important new observations from our switching experiments described in this work. First, gaseous diluted hydrogen and to a small extent, cis-2-butene, can cause a dramatic decrease in the 1% Pd/SnO2 sensor impedance, even at 30 °C, much lower temperature than this sensor material operates. Second, the lost impedance can be recovered (reversible) by enhancing desorptions of adsorbates under evacuated conditions, without involving any reoxidation process (no gaseous oxidant is needed for this observation). As far as we are aware that this observation obviously relating to the noble metal promoter has not been properly addressed before. It is noted that no observation is made over pure tin oxide. We believe that this observation at low temperatures is mainly arisen because of the reversible hydrogen spillover process between metal promoter and the tin oxide support underneath. Direct evidence for the spillover mechanism over the 1% Pd/SnO2 can now be provided by our in situ eVS experiments, which can directly measure the nature and quantity of surface hydrogenic species and the corresponding impedance values. The surprisingly high H/O ratios of the 30 °C and 100 °C samples (one surface H atom per every 47-58 atoms of the total bulk O content) cannot be properly explained without incorporating the spillover mechanism. It is noted that the contributions from water on the material (extensively dried sample), hydrogen from PdHx (decomposed over 50 °C) and from the gas-phase dihydrogen, to the overall signal were found to be insignificant. The argument is as follows: We know that the average Pd particle size on the tin oxide is of about 2.5 nm diameter, with the exposed metal surface area of about 0.1 m2 g-1 at 5.1% dispersion level, (TEM and hydrogen chemisorption experiments, not shown). Thus, the maximum hydrogen carried by the Pd metal (Pd:H ) 1:1), is only 0.1 m2 multiplied by 1 × 1020 atoms/m2 (maximum surface close packing density of Pd metal atoms in lattice) which equals 1 × 1019 H atom per gram of the 1% Pd/SnO2 used. The total O content of 2 × 6.023 × 1023/(118.69 + 16 × 2) ) 7.99 × 1021 O atoms). Hence, the theoretical ratio of the H/O is 1 × 1019/7.99 × 1021 ) 1.25 × 10-3 (1 [H] atom per 800 [O] atom), which is at least 15 times lower than the measured eVS value, with the assumption that no hydrogen is spilled from the Pd metal to the tin oxide. However, the actual measured ratios are far above this number suggesting spillover hydrogen indeed occurs. Also, it was evident that the large quantity of hydrogen measured by the eVS is associated in the -OH form but not the Pd-H (by the kinetic energy analysis), which indisputably supports the transfer of hydrogen from metal to some oxygen species on the tin oxide surface. We also showed that this spillover process could easily be reversed. In addition, TPR experiment shows that when hydrogen is passed over the 1% PdO/SnO2 sensor, Pd was formed from PdO via PdHx at very low temperatures. The Pd metal induces the surface and lattice oxygen reductions of Pd promoted tin oxide at much lower temperatures, as compared to the pure tin oxide. The experiments thus provide some indirect evidence to support the hydrogen spillover mechanism. In light of this direct and indirect evidence, we present a simple model in Figure 6. This describes how the adsorbed hydrogen species (or hydrocarbon moieties) on the Pd metal surface can be reversibly transferred from the metal to the substrate tin oxide.

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Figure 6. Spillover of H2 on the 2 component Pd/SnO2 system showing hydrogen atom migration over the SnO2 surface.

At low temperature (i.e., 30-120 °C), it is evident that the surface oxygen species cannot be reduced by the H species (maintaining the high H/O ratios). At high temperature, the reduction of the surface oxygen species is clearly demonstrated (the observations of reduction peaks by TPR experiment and the lowering of the H/O values in the in situ experiments) through catalytic combustion. Hence, the in situ impedance values measured in nitrogen could therefore reflect the extent of depletion layer remained with the sample after its hydrogen pretreatments at the particular temperatures. This could be seen clearly that when the temperatures were of above 120 °C there were dramatic decrease in impedance values because of the reductive destruction of the surface oxygen species. However, the in situ impedance values measured in the presence of hydrogen do not follow the H/O ratios. The hydrogen (or hydrocarbon) exposure to the 1% Pd/SnO2 at all temperatures, can cause similar degree of impedance change switching from the very high impedance values in nitrogen to extremely low impedance in hydrogen (108 Ω cm2 in N2 to a few hundred Ω cm2 in H2). There was no significant difference between all the impedance values measured in H2. In addition, Figure 4 indicates a considerable quantity of surface H was still detected (H/O ) 0.007) when the sample was previously exposed to or above 250 °C whereby the weakly bound oxygen species were readily destructed. This clearly suggests that not all the surface H species are associated with the weakly bound oxygen. We envisage that some hydrogen species are spilled and are stabilized by a much more stable oxygen environment. Hence, the adsorption of the hydrogen on this particular site on the tin oxide surface at all temperatures rendered the material of extremely low impedance. Thus, the process of electron donation to the tin oxide conduction band during the hydrogen spillover process must have taken place. The nature of stable oxygen is not yet known; however, it could be some kind of defective oxygen sites (positively charged) which facilitate electron transfer. We therefore believe that the spilled hydrogen atoms can directly contribute to the band states of the material and this process is totally reversible at low temperatures. However, it is not clear how and when this happens (refer to the model on Figure 6). These results agree with one school of thought in that the hydrogen spillover mechanism, which involves protonic species (H+) movement to the oppositely charged oxygen species, coupled with electron flow in the substrate n-type of semiconductor oxide. It is noted that a very recent theoretical work also arguing that adsorbed hydrogen atoms can act as direct shallow electron donor to zinc oxide (another n-type semiconductor)15 As a result, our in situ measurements of H/O ratios and corresponding impedance values in hydrogen and in nitrogen provide the first direct experimental support. It is noted that the impedance change would not follow the gaseous hydrogen concentration (Table 2) indicating that the fixed amounts of stable receptive oxygen species will be subsequently saturated by the spillover hydrogen. It is logical for one to assume that at elevated temperatures,

5742 J. Phys. Chem. B, Vol. 105, No. 24, 2001 TABLE 2: The Effect of Concentration on Impedance Values after 1 min concentration of H2

impedance value/ω cm2

5% 2.5% 1.25% 0.315%

3.0747 3.6173 3.6774 4.0626

these H species can also be destroyed by bombardment from mobile surface oxygen species or gaseous molecular oxygen directly through catalytic combustion at a dynamic equilibrium. Hence, according to this work the actual sensing mechanism may involve two independent routes. One is via the traditional way of destruction/construction of depletion layer associated with weakly bound oxygen, the other via oxidative removal/ reductive addition of spillover species that interfere with the direct electron donation to the sensor. Consequently, the overall impedance value of the material will then be a function of the gaseous reductant and oxygen concentration. Insights On Hydrogen Reverse Spillover. The switching experiments have also revealed one fundamental phenomenon. The reverse hydrogen spillover process on 1% Pd/SnO2 seemed to be critically affected by the temperature. We found that the higher the temperature used, the slower the reverse spillover rate observed (the greatest effect was also observed between 100-200 °C). From the TPR result and sensor test in nitrogen, it can be established that some weakly bound surface oxygen species, which are related to sensing at elevated temperatures by the conventional mechanism, could be removed by hydrogen. It is therefore envisaged that reduction of these oxygen species (proton acceptor) impairs the reverse hydrogen spillover process. The precise route of hydrogen migration on the tin oxide surface is not known. Whether it needs other surface sites (lattice oxygen or surface vacancies) for the migration still remains unclear. However, most of the hydrogen signals from the eVS spectra can be attributed to -OH, which decreases substantially upon the hydrogen pretreatment at 200°C. Without any new hydrogenic species identified, it is thought that the same oxygen species is also responsible as the surface sites for the hydrogen transfer from one location to the other. The reduction of its concentration causes the decrease in the rate of the reverse spillover process. It is therefore proposed that the surface proton migration during the spillover process may be somewhat akin to the protonic migration through water (hopping through the oxygen species as shown in the model, Figure 6). It is also interesting to consider another possibility. The hydrogen spill-

Tsang et al. over on this material could also be accounted by the movement of this labile oxygen species to the hydrogen source (Pd) rather than involving the hydrogen migration; hence, the reduction of its surface oxygen concentration slows down the entire process. A similar conclusion was drawn by Li et al.16 who studied the hydrogen spillover over Pt/TiO2 catalyst using TPD and TPR techniques. Acknowledgment. The authors wish to thank Dr W. Vermeer for helpful discussions, to Dr Stewart Parker and Dr Jerry Mayer (RAL) for help with the eVS experiments. Financial support from EPSRC and the award of beamtime at RAL to S.C.T. is acknowledged. References and Notes (1) Seiyama, T.; Kato, A.; Fujiishi, K.; Nagatani, M. Anal. Chem. 1962, 34, 1502-1510. (2) Henshaw, G.; Williams, D. J. Chem. Soc., Faraday Trans. 1996, 92 (18), 3411-3417. (3) Lim, C.; Oh, S. Sens. Actuators B 1996, 30, 223-231. (4) Chiorino, A.; Ghiotti, G.; Prinetto, F.; Carotta, M.; Martinelli, G.; Merli, M. Sens. Actuators B 1997, 44, 474-482. (5) Kim, J.; Jun, H.; Huh, J.; Lee, D. Sens. Actuators B 1997, 45, 271277. (6) Kappler, J.; Barsan, N.; Weimar, U.; Dieguez, A.; Alay, J.; Gopel, W. Fresenius J. Anal. Chem. 1998, 361, 110-114. (7) Chaudhary, V.; Mulla, I.; Vijayamohanan, K. Sens. Actuators 1999, 55, 127-133. (8) Morrison, S. IEEE Circuits and DeVices 1991, 7, 32. (9) Yamazoe, N.; Fuchigami, J.; Kishikawa, M.; Seiyama, T. Surf. Sci. 1979, 86, 335-341. (10) Sermon, P.; Bond, G. Catal, ReV, 1973, 8, 211-239. (11) Conner W. C.; Falconer, J. L. Chem. ReV. 1995, 95, 759-788. (12) Mitchell, P. C. H.; Green, D. A.; Payen, E.; Evans, A. C. J. Chem. Soc., Faraday Trans. 1995, 91, 4467-4469. Mayers, J.; Evans, A. C. Measurement of Atomic Momentum Distributions by Neutron Compton Scattering. Rutherford Appleton Laboratory Report RAL-91-048; Rutherford Appleton Laboratory: Oxfordshire, 1991. Postorino, P.; Fillaux, F.; Mayers, J.; Tompkinson, J.; Holt, R. S. J. Chem. Phys. 1991, 4411-4415. Mayers, J.; Burke, T. M.; Newport, R. J. Neutron Compton Scattering from Amorphous Hydrogenated Carbon. Rutherford Appleton Laboratory Report RAL-93-052; Rutherford Appleton Laboratory: Oxfordshire, 1993. Lovesey, S. W. Neutron Compton Scattering. Rutherford Appleton Laboratory Report RAL-94-051; Rutherford Appleton Laboratory: Oxfordshire, 1994. (13) Bulpitt, C.; Tsang, S. C. Sens. Actuator B 2000, 69, 100-107. (14) Egashira, M.; Nakashima, M.; Kawasumi, S. Fundamentals and Applications of Chemical Sensors; American Chemical Society: Washington, DC, 1996; Chapter 4, pp 71-81. (15) Chris, G.; Walle, Van de. Phys. ReV. Lett. 2000, 85, 1012-1015. (16) Li, X. S.; Li, W. Z.; Chen, Y. X.; Wang, H. L. Catal. Lett. 1995, 32, 31-42. (17) Sermon, P.; Keryou, K. Stud. Surf. Sci. Catal. 1997, 112, 251. (18) Li, X. S.; Li, W. Z.; Chen, Y. X.; Wang, H. L. Catalysis Lett. 1995, 32, 31-42.