Deactivation of Pd Catalysts by Water during Low Temperature Methane Oxidation Relevant to Natural Gas Vehicle Converters

Effects of H2O on the activity and deactivation of Pd catalysts used for the oxidation of unburned CH4 present in the exhaust gas of natural-gas vehicles (NGVs) are reviewed. CH4 oxidation in a catalytic converter is limited by low exhaust gas temperatures (500–550 °C) and low concentrations of CH4 (400–1500 ppmv) that must be reacted in the presence of large quantities of H2O (10–15%) and CO2 (15%), under transient exhaust gas flows, temperatures, and compositions. Although Pd catalysts have the highest known activity for CH4 oxidation, water-induced sintering and reaction inhibition by H2O deactivate these catalysts. Recent studies have shown the reversible inhibition by H2O adsorption causes a significant drop in catalyst activity at lower reaction temperatures (below 450 °C), but its effect decreases (water adsorption becomes more reversible) with increasing reaction temperature. Thus above 500 °C H2O inhibition is negligible, while Pd sintering and occlusion by support species become more important. H2O inhibition is postulated to occur by either formation of relatively stable Pd(OH)2 and/or partial blocking by OH groups of the O exchange between the support and Pd active sites thereby suppressing catalytic activity. Evidence from FTIR and isotopic labeling favors the latter route. Pd catalyst design, including incorporation of a second noble metal (Rh or Pt) and supports high O mobility (e.g., CeO2) are known to improve catalyst activity and stability. Kinetic studies of CH4 oxidation at conditions relevant to natural gas vehicles have OPEN ACCESS Catalysts 2015, 5 562 quantified the thermodynamics and kinetics of competitive H2O adsorption and Pd(OH)2 formation, but none have addressed effects of H2O on O mobility.


Introduction
Natural gas, an abundant energy resource with worldwide proven reserves of over 204.7 trillion m 3 [1], is used primarily for electricity generation and heating.The composition of natural gas (NG) is highly variable, but CH4 typically accounts for 80-90% of the components of NG.CH4 has the highest H/C ratio among all hydrocarbon fuels and during combustion, generates the lowest amount of CO2 per unit of energy.The amount of SO2 generated during NG combustion is also relatively low because the S content of NG is significantly lower than that of gasoline or diesel fuels.These environmental benefits, together with a relatively low cost of NG, have resulted in an increased interest in its use as a transportation fuel.Currently there are >16 million natural gas vehicles (NGVs) in operation around the world, and their numbers are growing at about 20% annually [2].However, a significant concern for the wide-spread implementation of NG as a fuel for combustion engines is that unburned CH4, expelled in the engine exhaust, is a significant greenhouse gas with potency more than 25xs that of CO2.
The transportation sector is a major contributor to air pollution through the combustion of gasoline and diesel fuels, accounting for ~77% of CO emissions, ~47% of hydrocarbon emissions and ~60% of NOx emissions in the USA [3].The exhaust gas of a conventional gasoline powered spark-ignition internal combustion engine (SI-ICE) consists mostly of N2 (70-75%), CO2 (11-13%) and water (10-12%) with about 1-2% of pollutants, specifically unburned hydrocarbons, CO and NOx [4,5].The pollutants must be removed before the exhaust gas is emitted to the atmosphere so as to meet increasingly stringent worldwide emission standards.The pollutants present in the engine exhaust are dependent on the engine air/fuel (A/F) ratio.For example, if the A/F ratio is above the stoichiometric value for complete combustion (A/F = 14.6), the concentration of reducing agents (hydrocarbons and CO) in the exhaust gas decreases whereas the concentration of oxidizing agents (O2 and NOx) increases.Consequently, several different strategies have been developed to control engine emissions, depending on the operating conditions and the target emission levels [5].Typically, a gasoline engine management system controls the A/F ratio or the exhaust gas composition (using an oxygen sensor connected to a secondary air supply) near the stoichiometric value.A single three-way catalyst (TWC) bed, placed in the exhaust gas flow, ensures simultaneous oxidation of the CO and hydrocarbons and the reduction of the NOx.Alternatively, dual-bed systems combine a NOx reduction catalyst bed with a separate oxidation catalyst and secondary air to remove the CO and hydrocarbons.Under lean-burn conditions a gasoline engine may operate with sufficiently high A/F ratios so as to obtain a significant reduction in CO and NOx emissions and improved fuel efficiency.The function of the catalyst in this case is limited to the oxidation of mainly hydrocarbons, while the NOx emissions are captured using a NOx trap followed by desorption and reduction in a TWC during an occasional near stoichiometric excursion of the engine.Although lean-burn engines improve fuel efficiency, the exhaust gas temperature is significantly lower than from conventional gasoline powered engines, and consequently, catalysts with high oxidation activity at relatively low temperatures are needed for this application [5].
Modern TWC converters used in gasoline ICEs contain Pt, Rh and Pd, dispersed on a washcoat applied to a cordierite ceramic monolith or metal monolith [3,5].The monolith usually has a honeycomb structure with 1 mm square channels to accommodate the high gas throughputs from the exhaust with minimal pressure drop.The washcoat, a mix of several metal oxides (Al2O3, CeO2, ZrO2), is applied to increase the metal support surface area (Al2O3), to improve thermal stability (ZrO2) and to provide enhanced oxygen storage capacity (CeO2) that widens the operating range for optimal oxidation and reduction by the catalyst.The metal composition of the converter varies with application but typically contains 5-20:1 of Pt:Rh with a total metal loading of 0.9-2.2g L −1 .Pd may be used to replace all or part of the Pt for cost savings [5].
Exhaust gas emissions from NGVs are difficult to control because low concentrations of CH4 (400-1500 ppmv) must be oxidized in the presence of high concentrations of H2O (10-15 vol.%) and CO2 (15 vol.%) at relatively low exhaust gas temperatures (450-550 °C).The greater strength of the C-H bond in CH4 (450 kJ/mol) relative to other hydrocarbons [6] implies that catalysts with high CH4 oxidation activity must be used.NGVs operate near the stoichiometric point or under lean-burn conditions [7,8].Stoichiometric NGV engines are primarily used in light-duty passenger cars, whereas lean-burn engines are more common in heavy-duty vehicles such as buses.Over the past ~20 years, conventional converter technologies have been adapted for NGVs using Pd catalysts (which have the highest activity for CH4 oxidation [7,9,10]) to adequately reduce (by 50-60%) the CH4 content in NGV exhausts at <500 °C in the presence of high H2O concentrations.Commercial catalysts for SI-NG engines also typically incorporate a CeO2/ZrO2 solid solution for high O2 adsorption capacity, which serves to buffer O2 concentration during the rapid switching between slightly oxidizing and reducing conditions close to a stoichiometric mixture (e.g., [7,11]).
Mechanisms and kinetics of CH4 oxidation over Pd/PdO catalysts have elicited continued debate in the literature [6,13,14,23], for which data interpretation is complicated by the transitions that Pd catalysts undergo during thermal pre-treatment and reaction [24].Furthermore, the high concentration of H2O in the NGV exhaust and the typically transient reaction conditions that result from cycling between oxidizing and reducing conditions in the NG engine [6,11] are known to significantly impact catalyst activity and stability.
The present review is focused on the inhibition and deactivation effects of H2O, especially at the relatively low temperatures representative of CH4 oxidation over Pd catalysts in a NG engine.Although previous reviews have addressed the issue of Pd catalyst stability in the presence or absence of H2O [4,12,20,25], and several catalyst deactivation mechanisms are possible at the exhaust gas conditions [26], several unresolved issues remain.More recent studies of the past decade have provided new insights into the effects of H2O, especially at lower temperatures, and these are the focus of the present review.Note, however, that in many cases, fresh catalysts in powder form have been evaluated using ideal fixed-bed micro-reactors and simulated exhaust gas under steady state operating conditions.Tests of monolith catalysts with promoters suitably aged and operated with A/F frequency and amplitude modulation that occur in a vehicle are few [7,11].Nonetheless, interpretation of data from ideal catalyst studies allows direct links to be drawn between fundamental catalyst properties and catalyst performance for CH4 oxidation, whereas in real systems this may be more difficult to achieve.

Effects of H 2 O on CH 4 Oxidation over Pd Catalysts
Water is a major component of the engine exhaust and is also a product of the combustion that occurs in the catalytic converter.In TWCs, H2O acts as an oxidizing agent for CO conversion by the water-gas-shift reaction and for steam reforming of hydrocarbons [4].H2O also significantly affects the thermal stability of the metals (Pt, Rh and Pd) present in the TWC as well as the support, mostly through sintering mechanisms [4,27,28] and by changes in the Pd oxidation state during hydrothermal aging [29].Water may also act as a reaction inhibitor by adsorption onto the catalyst.
Bounechada et al. [11] reported on the activity of a Pd-Rh (Pd/Rh = 39/1) TWC converter supported on stabilized Al2O3, promoted with Ce-Zr (Zr/Ce = 3.5) and wash coated on a ceramic honeycomb monolith, tested under fuel-lean (λ > 1), stoichiometric (λ = 1.00), and fuel-rich (λ < 1) conditions (gas composition: 0.15 vol.%CH4, 0.6% CO, 0.1% H2, 10% H2O, 10.7% CO2, 0.13% NO, 0-1.14% O2; λ was varied by changing feed O2 concentration; GHSV = 50,000 h −1 ).At stationary conditions (constant λ; steady-state experiment), the CH4 conversion was observed to continuously decrease under both stoichiometric (52 to 43% after 0.5 h reaction) and fuel-lean (from 62 to 59% after 0.5 h reaction) conditions, even though injecting a fuel-rich pulse during fuel-lean stationary operation increased the CH4 conversion to its initial value at the onset of reaction.The authors attributed the deactivation under fuel-lean conditions to the inhibition effect of H2O on the CH4 oxidation reaction, whereas under stoichiometric conditions, partial reduction of PdO due to the lack of oxygen, may lead to a loss in PdO active sites for CH4 oxidation.The authors also claimed that the presence of high oxygen capacity metals (Ce and Zr) in the catalyst made the reduction of PdO improbable under stoichiometric conditions.Under fuel-rich conditions, H2O acts as an oxidant through water-gas shift and steam reforming reactions.

Water Concentration and Reaction Temperature Effects on CH4 Oxidation Activity of Pd Catalysts
With the growing interest in NGVs, recent studies have focused on effects of H2O on Pd catalysts during CH4 combustion [16,18,[30][31][32][33][34][35][36][37][38].Deactivation or inhibition effects of H2O are dependent upon several factors including catalyst formulation, reaction temperature, catalyst time-on-stream history, and H2O concentration.Table 1 summarizes selected data that show effects of H2O added to the feed gas during CH4 light-off experiments over Pd catalysts.The light-off temperature (here reported as the temperature corresponding to 30% CH4 conversion during temperature programmed reaction, T30) increases as the H2O concentration increases, showing a clear inhibition effect that increases in magnitude with increasing H2O concentration.
In several cases the effects of H2O have been examined by measuring the CH4 conversion at steady-state, with and without H2O added to the feed gas.A typical set of data, reported by Persson et al. [35], is shown in Figure 1 using several Pd/Al2O3 catalysts reacted at 500 °C.These data also show that added H2O significantly suppresses CH4 conversion, but the effect is at least partially reversible.Similar effects of H2O addition have been reported in the literature, as summarized in Table 2.These reports confirm that H2O acts as an inhibitor of CH4 oxidation over Pd catalysts and that upon removal of the H2O from the CH4/O2 reactant, the inhibition is partially reversible [31,33].
Reaction temperature is another key variable affecting the role of H2O addition.Although the data of Table 2 cannot be compared directly because of the different operating conditions, they do show that at 600 °C, the decrease in CH4 conversion with H2O addition is much less significant than at lower temperatures (400 °C).Several authors have proposed that the deactivation is related to the reaction of H2O with active PdO sites [16,18,31,40,41], PdO + H2O→Pd(OH)2, resulting in the formation of inactive Pd(OH)2, as first proposed by Cullis et al. [40].Burch et al. [31] also reported a strong inhibitory effect of water on Pd catalysts up to 450 °C.However, at higher temperatures the negative influence of water on the activity was very small, suggesting that above 450 °C the reverse reaction (Pd(OH)2→PdO + H2O) occurs.Eriksson et al. [41] observed a significant decrease in CH4 conversion over a much wider range of temperatures (200-800 °C) after adding 18% H2O to a CH4/O2 feed over a Pd/ZrO2 catalyst, which was likely due to the relatively high H2O concentration used in this study.Different results were reported by Kikuchi et al. [16] when adding 1 vol.%H2O during CH4 oxidation over a Pd/Al2O3 catalyst, i.e., a decrease in activity was observed up to about 450 °C and no H2O inhibition was observed at higher temperatures.However, during addition of 20 vol.% H2O, the inhibiting effect could be observed up to 600 °C, in qualitative agreement with Eriksson et al. [41].Further insight into the H2O adsorption/desorption phenomena on Pd/ZrO2 catalysts has been obtained using pulsed-flow experiments [42,43].Accordingly, pulses of CH4/O2/He (1:4:95 vol %) were passed over a Pd/ZrO2 catalyst at various temperatures and the products monitored by mass spectrometer.The time at which the peak maximum for H2O appeared in each spectrum, compared to other products, was reported as the delay in the H2O peak.The data (Figure 2) show that the H2O generated during CH4 oxidation lags other products, suggesting a slow H2O adsorption/desorption equilibrium which might include spillover to the support.As the temperature increases above 450 °C (723 K), the desorption rate of H2O increases and the delay in the H2O peak compared to the other products is insignificant.This behavior is in agreement with observations from other studies [30,31,44] that the desorption rate of H2O produced during CH4 oxidation is slow and on the order of seconds below 450 °C, even though CO2, the other product of reaction, desorbs very quickly.Increasing temperature above 450 °C removes the desorption time gap between CO2 and H2O, and thus, no inhibition by H2O occurs.Ciuparu et al. [42] also pulsed gas containing 3.45 vol.%H2O/O2/He but no CH4 (and hence no reaction) through the same catalyst bed (Figure 2), showing that the H2O generated from CH4 oxidation lags the H2O added to the feed.These data demonstrate that the adsorption/desorption of H2O from the Pd catalyst surface is temperature dependent and reaches equilibrium at temperatures above ~450 °C (723 K), even for H2O added in the gas phase.
Figure 3 compares temperature-programmed-reaction (TPR) profiles for CH4 oxidation obtained over a Pd/ZrO2 catalyst, from both pulsed and continuous flow experiments with or without H2O added [42,43].The pulsed flow TPR profile was obtained by injecting pulses of the reaction mixture (1/4/95:CH4/O2/He for the "dry" feed and 1/4/95:CH4/O2/He saturated with ~2% H2O for the "wet" feed) into a He stream every 3 min while ramping the temperature at 0.5 K min −1 .Between consecutive pulses the catalyst was purged in flowing He.The pulsed flow data of Figure 3 show that at temperatures above 450 °C (723 K), there is no H2O inhibition, since the conversions of "dry" and "wet" reaction mixtures are essentially the same.At <450 °C, inhibition is observed due to a low H2O desorption rate.When H2O is added to the gas phase, the H2O adsorption rate is enhanced and the rate of desorption is further decreased.With continuous flow of reactants and a higher H2O concentration, H2O inhibition occurs at high temperatures due to re-adsorption.The addition of H2O to the feed directs the equilibrium towards more H2O adsorption on the surface and hence a greater decrease in catalyst activity during CH4 oxidation.The above observations are consistent with the following hypotheses: (1) product inhibition of CH4 oxidation by H2O on PdO catalysts occurs at temperatures below 450 °C; (2) product inhibition by H2O is enhanced by its slow rate of desorption from the PdO catalyst relative to a higher rate of CH4 oxidation; (3) PdO and H2O may interact via the reversible reaction: PdO + H2O↔Pd(OH)2 yielding inactive Pd(OH)2 and thus reversibly deactivating PdO as first proposed by Cullis et al. [40]; and (4) the extent of the CH4 oxidation reaction increases with increasing temperature but is reduced with increasing H2O concentration in the gas phase.

Catalyst Sintering by H2O
The possibility that addition of H2O may degrade Pd catalysts through a sintering mechanism [26] has also been investigated.According to Hansen et al. [45], the sintering rate of metal nanoparticles depends on their size.For nanoparticles <3 nm in diameter, Ostwald ripening is the most likely sintering mechanism.For larger particles (3-10 nm), both Ostwald ripening and particle migration and coalescence may occur, but the sintering rate is much slower than for the smaller particles [45].The particle sintering rate has also been shown to correlate with the vapor pressure of the surface species [4].Pd is unique among the PGMs in that the oxide (PdO) has a much lower vapor pressure than the metal (Pd), and consequently, one would expect a very low sintering rate of PdO by Ostwald ripening [4].The rate of sintering is also dependent on the support.Lamber et al. [46] suggested that on SiO2 in the presence of H2O, the formation of silanol (Si-OH) groups favors the migration and coalescence of Pd, whereas in the absence of H2O, Ostwald ripening is favored.Sintering suppression has been demonstrated for Pt catalysts using supports that enhance metal-support interactions [28].Nagai et al. [47] demonstrated a correlation between the O electron density of the support, the strength of the Pt-O interaction and the resulting crystallite size.Thus, supports with a stronger metal-support interaction have a higher O electron density and yield smaller Pt crystallites in the order SiO2 < Al2O3 < ZrO2 < TiO2 < CeO2 [28,47].
Xu et al. [48] reported that the main deactivation mechanism of Pd/Al2O3 catalysts following exposure to 10 (v/v)% H2O/N2 at 900 °C for up to 200 h is Pd sintering.A substantial decrease in Pd dispersion from 3.7% to 0.9% over 7 wt.%Pd/Al2O3 and similar decreases at other Pd loadings after 96 h hydrothermal aging, were observed.As noted by Xu et al. [48], aging the catalyst at 900 °C ensures that PdO decomposition to Pd 0 is complete and consequently the more rapid sintering observed is relevant to the behavior of Pd 0 rather than PdO.
Escandon et al. [49] examined effects of hydrothermal aging at lower temperatures, where PdO is thermodynamically stable [6].A 1 wt.%Pd/ZrO2-Ce catalyst was hydrothermally aged at 300, 425, and 550 °C in 2% H2O/Air for 30 h, before being evaluated for CH4 oxidation under lean-burn conditions (5000 ppmv CH4 in dry air).The results, shown in Figure 4, are compared with the same catalyst, thermally aged at 550 °C in dry air for 30 h (identified as Pd/ZrO2-Ce-550 in Figure 4) [49].A significant irreversible decrease in CH4 conversion occurs and the extent of catalyst deactivation increases with aging temperature (Figure 4).The T50% increases from 375 °C for the fresh oxidized catalyst (identified as Pd/ZrO2-Ce in Figure 4), to 450 °C for the air-aged catalyst and to > 550 °C for the hydrothermally aged catalyst.Pd dispersion and BET surface area of the aged catalysts did not change [49].Comparing the activity results of the catalyst thermally aged in air (Pd/ZrO2-Ce-550) with that aged in 2% H2O/air at 550 °C (Pd/ZrO2-Ce-550h), confirms that catalyst deactivation rate increases in the presence of H2O.The stability of the hydrothermally aged catalysts during reaction was also evaluated, using both isothermal deactivation experiments at 500 °C and light-off measurements made after 50 h reaction with 5000 ppmv CH4 in air.The catalysts aged in the presence of H2O at 300 °C underwent a significant deactivation whereas the catalyst aged in the presence of H2O at 425 °C was much more resistant to deactivation, and after 25 h time-on-stream was the most active of all the catalysts examined.XRD analysis of the catalysts showed that the more stable catalysts are associated with the most stable supports [49].In another study of CH4 oxidation at low temperature (250-450 °C), a change in PdO dispersion was suggested as the main cause of deactivation of 0.5% Pd/Al2O3 and 0.5% Pd/SiO2 catalysts [50].Dispersion decreased from 10% for the unused 0.5% Pd/SiO2 catalyst to 5.6% for the catalyst reacted in 1% CH4/air feed at 450 °C for 7 h, whereas for the 0.5% Pd/Al2O3 catalyst the corresponding changes in dispersion were 67% to 6.3%, respectively.These observations are in good agreement with that of Narui et al. [51], in which the PdO dispersion of a 0.5% Pd/Al2O3 catalyst decreased from 14% to 11% after 6 h reaction at 350 °C.Zhang et al. [52] investigated Pd catalysts supported on ZSM-5 and reported that catalyst stability is improved when CH4 oxidation is carried out in the presence of H2O at 430-480 °C, compared to the reaction in a dry feed.In both cases, the loss in catalyst activity could be related to reduced PdO dispersion, as determined by the Pd/Si ratio measured by XPS, but the loss in dispersion is smaller in the presence of H2O [52].By contrast, Araya et al. [53] reported an insignificant drop in PdO dispersion (from 31.7% to 28.2%) of a Pd/SiO2 catalyst after 96 h of reaction at 325 °C in 1.5% CH4/6% O2 in He, despite a significant decrease in CH4 conversion from 32% to 22%.The extent of catalyst deactivation was found to further increase in the presence of 3% H2O added to the feed.
Several studies have demonstrated that catalyst sintering can be reduced by encapsulating Pd/PdO nanoparticles in support materials.Sinter-resistant Pd catalysts have been prepared by atomic layer deposition of Al2O3 overlayers on Pd [54], as well as by the synthesis of Pd/SiO2 core-shell structures [55,56].Cargnello et al. [22] reported a Pd/CeO2 core-shell catalyst supported on Al2O3 for CH4 oxidation that is about 200xs more active than an equivalent Pd-CeO2/Al2O3 catalyst prepared by wet impregnation.The authors demonstrated that the Pd cores remain isolated even after heating the catalyst to 850 °C and that the CH4 light-off curves (measured at GHSV of 200,000 h −1 in a feed gas of 0.5% CH4, 2% O2 in Ar) are the same for the fresh catalyst and one that has been aged at 850 °C for 12 hours.The Pd nanoparticles encapsulated by CeO2 enhance the metal-support interaction that leads to exceptionally high CH4 oxidation activity and good thermal stability [22].

Effects of Support
The data of Table 1 show that the inhibition of CH4 oxidation by H2O on Pd catalysts is dependent upon the support.Pd/Al2O3 shows significantly more inhibition with 10% H2O added to the feed than either the Pd/SnO2 or Pd/ZrO2 catalysts.More detailed data from Kikuchi et al. [16] comparing CH4 light-off curves for a 1.1 wt.% Pd/Al2O3 catalyst and a 1.1 wt.% Pd/SnO2 catalyst with H2O added to the feed over a range of concentrations (1-20 vol.%), are shown in Figures 5 and 6.By increasing the H2O concentration, the CH4 light-off curves for both catalysts shift to higher temperatures.However, the temperature shift is larger over the Pd/Al2O3 catalyst than the Pd/SnO2.The authors completed a simplified kinetic analysis of the CH4 oxidation rate data to show that the enthalpy of adsorption of H2O is strongest on the Pd/Al2O3 catalyst (∆Had ~ −49 kJ/mol), from which they concluded that the significant loss in activity of the Pd/Al2O3 in the presence of H2O is due to a high coverage of the active sites by H2O [16].These results could also be interpreted according to the more recent proposals by Schwartz et al. [44,57], that hydroxyl accumulation on the support hinders oxygen migration and exchange, and hence CH4 oxidation.The strong adsorption of H2O determined by kinetic analysis on the Pd/Al2O3 catalyst [16] is consistent with a large hydroxyl accumulation on the catalyst surface that could inhibit the O exchange.The rate of deactivation during CH4 oxidation in the presence of H2O has been shown to be reduced by using a support with high oxygen surface mobility.At temperatures below 450 °C, Ciuparu et al. [30] reported the inhibition effect of H2O to be dependent upon the oxygen mobility of the support.Comparing PdO supported on oxides with increasing surface oxygen mobility: Al2O3 < ZrO2 < Ce0.1Zr0.9O2,they show that the resistance to H2O inhibition during CH4 oxidation increases in the same order.The deactivation rate of PdO was also compared over Al2O3, MgO, and TiO2 supports by Schwartz et al. [44,57] at temperatures <450 °C.Deactivation is shown to be a consequence of reduced oxygen mobility due to hydroxyl adsorption.They also reported that PdO/MgO catalyst has a slower deactivation rate compared with Al2O3 and TiO2 supports because of the higher oxygen surface mobility on the MgO [44,57].However, Pd catalysts dispersed on other supports such as MCM-41, which have high surface area (1113 m 2 /g) and lower oxygen mobility than MgO and Al2O3, did not deactivate either, suggesting that other factors also play a role, depending on the catalyst and the support.
Another study compared the stability of Pd/SiO2 and Pd/ZrO2 during CH4 oxidation using a dry feed gas [53].The data (Figure 7) show that the Pd/ZrO2 is stable after 40 h time-on-stream, while the CH4 conversion over the Pd/SiO2 catalyst increases from 13% to 32% in the first 3 h, and then decreases to 22% after 96 h (see Figure 7).Although the Pd/ZrO2 catalyst is more stable than the Pd/SiO2 catalyst, its conversion is lower than for the Pd/SiO2 catalyst.The lower deactivation rate observed on the Pd/ZrO2 is consistent with the higher oxygen mobility of this catalyst compared to Pd/SiO2, as noted above.
Metal-support interactions, support stability and the tendency of the support to encapsulate Pd, may also play a role in the deactivation of Pd catalysts during CH4 oxidation.Gannouni et al. [58] compared Pd catalysts supported on silica and mesoporous aluminosilicas and showed that, according to the light-off curves measured with 1% CH4, 4% O2 in He, CH4 oxidation activity is enhanced on the pure silica support, whereas on the aluminosilica, the beneficial effect of Al 3+ on metal dispersion and catalytic activity is counterbalanced by partial metal encapsulation.Above 500 °C in the presence of H2O, the structural collapse of the support, metal sintering, and metal encapsulation by the support all occur [58].Similar effects were reported with SiO2 supports by Zhu et al. [59].SiO2 desorbs chemisorbed H2O (silanol groups -Si-OH) at ~397 °C [46] and the formation of hydroxides according to the reaction: SiO 2 (s) + 2H 2 O (g) ↔ Si(OH) 4 (g) is feasible at temperatures above 700 °C [60,61].
Hydroxyl mobility can change the extent of metal-support interactions [45,46].Zhu et al. [59] reported the encapsulation of PdO by SiO2 during CH4 oxidation at only 325 °C.The authors suggested that silica migration by (i) formation of a palladium silicide during H2 reduction at 650 °C that is subsequently oxidized during CH4 oxidation and (ii) migration of SiO2 during CH4 oxidation caused by the water formed during reaction, are important related factors facilitating the encapsulation of PdO by the SiO2.Migration of SiO2 onto the metal crystallites in other catalyst systems containing H2O has also been reported in the literature [46,62].Yoshida et al. [63] also examined the effects of various metal oxide supports of Pd on the low temperature oxidation of CH4 as summarized in Table 3.The catalytic activity varies with the support, but the support oxides with moderate acid strength (Al2O3 and SiO2) give maximum CH4 conversion.For these catalysts higher activity corresponds to a higher oxidation state of Pd (bulk PdO).The lower activity of Pd on basic supports is attributed to the formation of binary oxides from PdO and the support (such as Pd/MgOx), in spite of a high Pd oxidation state.
The effect of metal oxides added to Pd/Al2O3 to improve the hydrothermal stability has been reported by Liu et al. [36] who showed in particular, that the addition of NiO or MgO improved the hydrothermal stability of Pd/Al2O3 through the formation of NiAl2O4 and MgAl3O4 spinel structures.According to the authors, the spinel results in weakened support acidity that suppresses the formation of Pd(OH)2 during hydrothermal aging.A comparison of initial CH4 oxidation activity as a function of temperature for Pd-Pt catalysts on Al2O3, ZrO2, LaMnAl11O19, Ce-ZrO2, and Y-ZrO2 was reported by Persson et al. [64].Monolith catalysts were tested in a tubular quartz flow reactor at atmospheric pressure in 1.5 vol.%CH4 in dry air and at a space velocity of 250,000 h −1 .In steady-state experiments, reaction temperature was set initially at 470 °C and then increased to 720 °C stepwise in 50 °C increments, with 1-h holds at each temperature.The Pd-Pt/Al2O3 catalyst had the highest activity at lower temperatures (470-570 °C), while the Pd-Pt/Ce-ZrO2 catalyst had the highest activity between 620 °C and 800 °C [64].The authors suggested that the higher surface area of the Al2O3 compared to the other supports (e.g., 90 m 2 /g for Al2O3 versus 10 m 2 /g for Ce-ZrO2) accounts for the higher activity of Pd-Pt/Al2O3 at lower temperatures, due to higher dispersion of Pd-Pt oxides, while at higher reaction temperatures the Pd-Pt catalyst probably undergoes reduction to the metal.A combination of lower activity for Pd metal and its propensity for rapid sintering probably explain the lower activity.The authors also suggested that the Ce-ZrO2 likely enhances the stability of the PdO, similar to the enhanced stability observed on CeO2 [30].In addition, ZrO2 has high oxygen mobility [30] and the ability to re-oxidize metallic Pd into PdO should be higher.Indeed, Pd/alumina is re-oxidized very slowly, whereas Pd supported on ceria-stabilized ZrO2, is re-oxidized more rapidly.
Since H2O adsorption on the Pd and/or the support is an important step in inhibiting CH4 oxidation over Pd, support hydrophobicity may be expected to impact the inhibition effect of H2O.Araya et al. [53] studied this effect on the deactivation of Pd-based catalysts by preparing 1 wt.%Pd on two different commercial silicas, Aerosil130 and Aerosil R972.The Aerosil R972 is hydrophobic since the OH groups have been replaced by methyl groups.Both 1% Pd/A130 and 1% Pd/R972 were tested at 325 °C in a gaseous mixture of 1.5% CH4 and 6% O2 in He at a total flow rate of 90 cm 3 min −1 with addition of 3% H2O after 2 h As shown in Figure 8, the effect of H2O addition to the feed gas is approximately the same for the hydrophobic silica, Pd/R972, and the hydrophilic Pd/A130.In both cases, a large decrease in CH4 conversion is observed with the introduction of H2O to the reactor.The authors reported a reaction order with respect to H2O of −0.25 for both Pd/A130 and Pd/R972, emphasizing that the hydrophobicity of the support does not affect the extent of H2O inhibition observed on either catalyst.

H2O Inhibition and Hydroxyl Formation
Although Pd(OH)2 has been postulated as a cause for deactivation of PdO catalysts in the presence of H2O [18,31,32,40], and while this mechanism is consistent with many of the observations discussed above, recent evidence obtained from FTIR and isotopic labeling experiments that monitor the formation and conversion of hydroxyls on the catalyst surface during reaction, suggest an alternative mechanism of deactivation.
Using DRIFTS, Persson et al. [35] reported an increase in signal intensity from surface hydroxyls weakly H-bonded to the support (3200-3800 cm −1 ) [65] after introducing 1.5% CH4 in air to a PdO/Al2O3 catalyst at low temperature (200 °C; Figure 9).The peak at 3016 cm −1 in Figure 9a, assigned to gas phase CH4, increases with time-on-stream because of catalyst deactivation.The hydroxyls have characteristic adsorptions at 3733, 3697, 3556 and 3500 cm −1 , with the hydroxyls at 3697 and 3733 cm −1 assigned to bridged and terminal isolated hydroxyl species, respectively.Upon CH4 removal from the feed (Figure 9b), the peaks associated with OH species remain, highly consistent with a slow desorption of OH species produced during CH4 oxidation.Hence, Persson et al. [35] suggested that catalyst deactivation on PdO/Al2O3 might be due to the formation and accumulation of hydroxyls on the catalyst surface, bound either to the PdO, Al2O3 or the interface between the two [30].Gao et al. [32] reported similar hydroxyl bands at 3733, 3697, 3556 and 3500 cm −1 during lean-burn CH4 oxidation (0.4% CH4 in air) at 250 °C.The FTIR spectra from reaction with 2 vol.%H2O added to the CH4-O2 feed also yield a broad band at 3445 cm −1 that is associated with OH species on Al2O3 [32].Ciuparu et al. [30] also identified three well-defined peaks at 3732 (OHI), 3699 (OHII), and 3549 (OHIII) cm −1 associated with surface hydroxyls generated during CH4 oxidation on a PdO/Al2O3 catalyst (3.5 wt % Pd) at 350 °C using a feed gas of 0.128% CH4 and 17.3% O2 in He/N2 (Figure 10).The spectrum was compared to that measured at the same temperature when injecting pulses of ~3% H2O into an air flow over the PdO/Al2O3 catalyst and the Al2O3 support (see Figure 10).Since Al2O3 has been shown to have a significantly lower hydroxyl coverage compared to PdO/Al2O3 when injecting H2O pulses at 350 °C (the spectrum of Al2O3 is magnified by a factor of 15 in Figure 10), they concluded that the three peaks are associated with the presence of OH adsorbed on the PdO catalyst surface.The higher hydroxyl coverage during CH4 oxidation compared to pulse injection of H2O onto the PdO/Al2O3 catalyst, indicates that (1) adsorbed H2O is dissociated on the surface of PdO/Al2O3 and (2) hydroxyls formed from H2O pulses are less strongly bound to the surface than hydroxyls produced by the CH4 oxidation reaction.
Since the frequencies of the OHI and OHII species are shifted to higher wave numbers for OH species more weakly bound to Pd, Ciuparu et al. [30] suggested that the high frequency peaks (OHI, OHII) can be assigned to terminal and bridged hydroxyl species, respectively, and the low frequency peak at ~3549 cm −1 with broad maximum values can be associated with OH species bound to different sites (multi-bound OHs; OHIII) (Figure 10).Transient temperature experiments show that the hydroxyl binding energy increases in the order OHI < OHII < OHIII [30].
The peak areas of the terminal, bridged, and multi-bound hydroxyls were monitored with time-on-stream at different temperatures during reaction, as illustrated by Figure 11 for reaction at 175 °C [30].Upon removal of CH4 from the feed, the peak areas for the bridged and multi-bound OH species continue to increase, whereas the area of the terminal OH species decreases (Figure 11).This decrease is attributed to the conversion of terminal OH species to bridged or multi-bound OH species.Based on the intensities of the various hydroxyl species at different temperatures, the authors proposed the inter-conversion among the OH species as: OH III ↔ OH II ↔ OH I → H 2 O (g) where only terminal OH species recombine and desorb as H2O and the transformation of bridged OH species to terminal OH species is the rate determining step (RDS) for hydroxyl desorption and hence low temperature CH4 oxidation [30].Importantly the authors show that the surface coverage by the hydroxyls (Figure 11) correlates with the activity loss at low temperature, meaning that the activity loss and surface coverage have similar timescales, from which they conclude that the former is likely an effect of the latter [30].FTIR spectra measured during CH4 oxidation at 325 °C with 0.1% CH4/4%O2 in He over a series of 3 wt.%PdO catalysts supported on Al2O3, MgO, TiO2 and MCM-41 [44] show that the hydroxyl coverage is dependent on the support.On Al2O3, well defined peaks similar to those identified by Ciuparu et al. [30] are observed, but no common peak among all catalysts that would provide evidence for Pd-OH bond formation, are present.Furthermore the large contribution from OH bonding on the supports makes it impossible to directly identify the presence of Pd(OH)2 on these supports [32,44].However, by using 18 O isotopic labeling and FTIR, the authors demonstrate that peaks associated with the accumulation of hydroxyls on PdO are not present at 325 °C.Hence, the more recent evidence suggests that deactivation by Pd(OH)2 formation is unlikely, in agreement with the experimental observation that Pd(OH)2/C decomposes in N2 at about 250 °C [66].In addition, evidence from temperature-programmed desorption studies of H2O adsorbed on PdO(101) thin films, suggests the formation of an OH-H2O complex at low temperature (<127 °C) and low coverage (< ½ monolayer), whereas H2O preferentially chemisorbs in molecular form at higher coverages [67].
Schwartz et al. [44] showed, however, that catalyst deactivation during CH4 oxidation correlates with hydroxyl accumulation on the oxide support.The redox mechanism for CH4 combustion on Pd/PdO generally assumes dissociation of a CH4 molecule to yield a methyl fragment and a hydroxyl group (CH4 + Pd-O + Pd-*→Pd-OH + Pd-CH3, where Pd-* represents a vacancy) [68,69].H atoms are abstracted sequentially from the methyl group by neighboring Pd-O to form surface hydroxyl groups (Pd-OH).Recombination of surface hydroxyls yields water and a surface vacancy (2Pd-OH→H2O + Pd-O +Pd-*), that is regenerated by oxygen (2Pd-* + O2→2Pd-O) [68,69].Based on their experimental studies, Schwartz et al. [44,57], proposed that during lean-burn CH4 oxidation, O2 molecules dissociate on Pd-* sites and exchange with oxygen on the support so that Pd active sites are re-oxidized with oxygen atoms from the support during the catalytic reaction as follows: and overall: where S represents the support, S-* is an O vacancy on the support and O s represents an O atom associated with the solid oxide.This proposed mechanism suggests the possibility that a primary cause for catalyst deactivation is hydroxyl accumulation on the support, which hinders oxygen migration and exchange processes.
Evidence for O exchange with the support is provided by the isotopic labeling experiments summarized in Figure 12, during which Pd 18 O/Al2 16 O3 and Pd 18 O/Mg 16 O were exposed to 18 O2/He flow at 400 °C [57].An increase in 16 O 18 O signal intensity with time is proposed to arise from oxygen exchange with the catalyst support [44].The 16 O 18 O signal (see lower, separate dashed line in Figure 12) is reduced when H2 16 O is injected to the feed and is recovered when H2 16 O is removed.Apparently, hydroxyl groups tend to migrate to the oxide support rather than desorb.By increasing the concentration of hydroxyl groups, through addition and dissociation of H2O, oxygen exchange of Pd-* active sites with the oxide support (S-O s ) is interrupted.Thus, the number of PdO sites participating in the CH4 oxidation reaction decreases with time, as H2O dissociates and OH coverage of the support increases, with a consequent decrease in CH4 conversion [44].This proposed mechanism of catalyst deactivation is believed to occur at temperatures below 450 °C.Finally, the authors note that the rate of deactivation on Pd/Al2O3 catalysts, with higher concentrations of hydroxyl during reaction, is higher than on catalysts containing a support with higher oxygen mobility (Pd/MgO) [44,57].
Ciuparu et al. [70] also reported on pulsed experiments with 18 O2 over pure Pd and Pd/ZrO2 catalysts, oxidized before reaction, to clarify the effect of hydroxyls on the surface oxygen exchange.They determined that due to the slow recombination of hydroxyls and hence H2O desorption from the Pd catalyst surface during CH4 oxidation (2Pd-OH→H2O + Pd-O +Pd-*), the isotopic exchange of oxygen with the Pd sites (see Figure 13) occurs before H2O desorption from the surface.The oxygen vacancies on the PdO surface resulting from H2O desorption are thus rapidly filled by oxygen from the PdO bulk or oxide support (Pd-* + S-O s ↔Pd-O s + S-*).In fact, in this unsteady-state experiment, the labeled oxygen pulsed through the catalyst bed, is purged from the reactor before H2O is desorbed [70].These observations are in agreement with the studies of Schwartz et al. [44,57] already discussed and confirm that the accumulation of hydroxyls on the Pd catalyst surface impedes the oxygen exchange and limits Pd catalyst activity.  1O2/He at 400 °C.H2 16 O was injected at some time to probe its effect on oxygen exchange.Reproduced with permission from [44].Copyright ©2012 American Chemical Society.

The Use of Pd-Bimetallic Catalysts for CH 4 Oxidation
Pd-bimetallic catalysts have been studied to improve stability of Pd catalysts CH4 oxidation [19,51,71,72].Pd-bimetallic catalysts are usually less active than Pd alone [64,[73][74][75] simply because they contain less Pd, the most active metal for CH4 oxidation [20,25].The lower activity of the bimetallic compared to Pd alone may also be due to the presence of smaller amounts of PdO as a result of alloy formation between Pd and Pt [64], or the transformation of PdO to Pd metal [76].According to Ozawa et al. [77] the addition of Pt improves PdO/Al2O3 catalyst stability by preventing the growth of PdO and Pd-Pt particles during CH4 oxidation at high temperature (800 °C) [77].
Several studies have reported higher initial activity of Pd-bimetallic catalysts compared to Pd alone [17,19,51,78].These researchers suggest that the second metal added to Pd dissociates O2 and the resulting O atoms are adsorbed by Pd, helping to maintain PdO active sites.Ishihara et al. [78] reported a T50 of 533 °C for a 1 wt.%Pd/Al2O3 catalyst, whereas for a Pd-Ni/Al2O3 catalyst (Pd:Ni = 9:1) T50 was 380 °C.In another study, it was reported that a higher dispersion of PdO on PdO-Pt/α-Al2O3 catalyst (27%) compared to PdO/α-Al2O3 (14%) results in higher initial activity and higher stability of the bimetallic catalyst [51] .After exposing the PdO/α-Al2O3 catalyst to the reaction feed stream for 6 h at 350 °C, an increase in average particle size from 8 to 11 nm is observed, whereas the average particle size does not change significantly for the PdO-Pt/α-Al2O3 catalyst [51].
Persson et al. [73] examined a series of Pd-bimetallics supported on Al2O3 finding that the metallic phase structure has a significant influence on the catalyst stability.For example, in several bimetallic systems (PdAg, PdCu, PdRh, and PdIr) separate phases of each metal oxide are formed after calcination (at 1000 °C for 1h followed by 1000 °C for 2h after loading the supported metal oxide powders onto a cordierite monolith) and this enhances catalyst stability in the case of the PdCu and PdAg (as measured stepwise at temperatures from 400-800 °C in 1.4% CH4 in dry air at a space velocity of 250,000 h −1 ).Formation of a Co or Ni aluminate spinel in PdCo and PdNi bimetallics, however, does not improve catalyst stability, whereas alloy formation in PdPt and PdAu on Al2O3 increases hydrothermal stability in the presence of 15% H2O/air at 1000 °C for 10 h.In another study by Persson et al. [64], Pd-Pt bimetallic catalysts on various supports (alumina, zirconia) were shown to have higher thermal stability than monometallic Pd during CH4 oxidation in dry air (1.5% CH4 in air at a GHSV 250,000 h −1 ).The stability of the Pd-Pt catalysts improved at lower temperatures (up to 620 °C).At temperatures of 520 °C and 570 °C CH4 conversion on Pd-Pt catalysts increased with time-on-stream.Above 620 °C (especially at 670 °C and 720 °C) conversion decreased with time-onstream.Those catalysts with higher initial activity also had higher deactivation rates.The deactivation cannot be attributed to PdO decomposition because the initial activity test showed that PdO decomposition started at higher temperature (770 °C with 1.5 vol.%CH4 in air).According to XRD results, no PdO decomposition was observed at temperatures below 800 °C for the Pd/Al2O3, although PdO decomposition at ~700 °C may have yielded Pd that was not detectable by XRD (due to low concentration or high dispersion).
The amount of second metal added to the Pd can also affect the stability of the bimetallic catalyst.Persson et al. [74] reported that Pd-Pt bimetallic catalysts with Pd:Pt ratios of 2:1 and 1:1 are stable.Time-on-stream CH4 oxidation experiments (in 1.5% CH4 in air at a space velocity of 250,000 h −1 ) for both a 5 wt.%Pd/Al2O3 and a 2:1 Pd:Pt/Al2O3 bimetallic with total metal loading of 5 wt.% were studied over a wide range of temperatures (470-720 °C) [64].The temperature was increased from 470 °C to 720 °C stepwise by 50 °C and held for 1 h at each temperature.CH4 conversion over the Pd/Al2O3 and Pd-Pt/Al2O3 catalyst decreased during the 1 h reaction time at each temperature.However, the decrease in conversion was lower for the bimetallic catalyst compared to the Pd catalyst.The decrease in activity was higher at higher temperatures (670 °C and 720 °C), especially for the Pd catalyst.In situ XRD spectra of the Pd-Pt bimetallic catalysts are shown in Figure 14.At room temperature, a sharp peak corresponding to Pd-Pt (111) and a small peak corresponding to PdO (101) are observed for the PdPt-Al2O3 catalyst.By increasing the temperature to 300 °C, the PdO peak disappears and then reappears at 500 °C.The Pd-Pt peak intensity reaches a maximum at 700 °C while the PdO peak disappears at this temperature.The formation of Pd-Pt instead of PdO is consistent with deactivation of the bimetallic catalyst at high temperature (700 °C).Steady-state experiments using a 18.7 wt.% Pd/Al2O3 catalyst with different loadings of Pt (1.6, 3.1 and 3.9 wt.%) (Figure 15) reported by Ozawa et al. [77], also provide some insight into the improved stability of bimetallic catalysts as Pt content increases.In this study, reaction temperature was held at 800 °C and CH4 combustion rate was measured over a 10 h period using a 1% CH4 in air feed gas at a GHSV of 1,500,000 mL/(gcat-h).Deactivation rate is shown to decrease as the Pt loading of the Pd-Pt bimetallics increases.For example, the combustion rate for the 18 wt.%Pd-3.9 wt.% Pt/Al2O3 decreases from 710 μmol s −1 g −1 to 460 μmol s −1 g −1 after 10 h, whereas it decreases to 400 μmol s −1 g −1 for the 18.4 wt.% Pd-1.6 wt.% Pt/Al2O3 catalyst.
XRD analysis of the catalysts studied by Ozawa et al. [77] after 10 h reaction indicates PdO to be present in the Pt-doped catalysts while no Pd 0 is observed.However, Pd 0 is present in the Pd monometallic catalyst, likely because of the decomposition of PdO at the high temperature of the reaction (800 °C).In addition, the crystallite size of the PdO (101) in the Pd catalyst is larger than for the Pd-Pt catalysts.Table 4 compares changes in PdO particle size and BET surface area before and after 10 h reaction for the same Pd and Pd-Pt catalysts.From these data it is clear that the extent of sintering of the Pd catalyst is greater than for the Pd-Pt catalysts.The time-on-stream conversion data reported by Ozawa et al. [77] (Figure 15) were fitted to a deactivation equation with two terms, the first representing rapid transformation of PdO to Pd 0 of the Pd-Pt alloy phase, and the second associated with the slow growth of the PdO crystallite [77].The deactivation is affected more by the second term suggesting that particle growth of the PdO is the main cause of catalyst deactivation at the chosen reaction conditions [77].These results are in a good agreement with the results reported by Yamamoto et al. [72] in which a Pd-Pt bimetallic catalyst was more active for CH4 conversion than Pd (as measured by the temperature required for 50% CH4 conversion) and the conversion was maintained following 2500 h time-on-stream at 385 °C.XRD analyses showed that the crystallite growth as a function of time for both Pd (111) and PdO (101) was faster on the Pd (10 g/L)/Al2O3 catalyst than the Pd(10 g/L)-Pt (10 g/L)/Al2O3 catalyst.Hence one concludes that the presence of Pt retards the sintering of PdO.
Effects of H2O on deactivation of Pt versus Pt-Pd catalysts have also been reported, at both thermal and hydrothermal aging conditions [17,19,71].Pieck et al. [17] reported that the T50 of a 0.4% Pt-0.8%Pd/Al2O3 catalyst after thermal treatment at 600 °C for 4 h in wet air (60 cm 3 min −1 air flow with 0.356 cm 3 h −1 water ), is ~50 °C lower than that obtained over a Pd catalyst.Lapisardi et al. [19] reported that a fresh Pd0.93-Pt0.07/Al2O3catalyst (total metal loading 2.12 wt.% with Pd:Pt molar ratio of 0.93:0.07) is as active as a fresh Pd/Al2O3 catalyst in a dry feed [19].Interestingly, the Pd0.93-Pt0.07/Al2O3catalyst is less affected by addition of 10 vol.% steam to the feed stream than the 2.2 wt.% Pd/Al2O3 catalyst.The T50 for the Pd-Pt bimetallic increases from 320 °C to 400 °C when 10 vol.% steam is added to the feed stream, whereas the corresponding increase in T50 for the Pd/Al2O3 catalyst is from 320 °C to 425 °C.Thus, the Pd-Pt bimetallic, containing only 0.26 wt % Pt is more active and stable than the Pd catalyst for CH4 oxidation in the presence of steam.
The stabilities of Pt and Pt-Pd catalysts each loaded on a wash coated monolith have also been reported [71].A feed stream with 4067 ppmv CH4 in air was reacted over these catalysts as reaction temperature increased from 300 to 700 °C stepwise in 50 °C increments.CH4 conversion was monitored for a period of 1 h at each temperature.Subsequently the temperature was decreased to 300 °C also in 50 °C steps, again holding at each temperature for 1 h.The conversion of CH4 was compared for both heating and cooling cycles.The results show that the Pt-Pd catalyst is more active than the Pt catalyst.The comparison between the heating and cooling cycles was also done for steam-aged catalysts, in which the catalysts were exposed to the feed stream at 650 °C with 5 vol.% water for 20 h.Table 5 lists the T50 for both fresh Pt and Pd-Pt catalysts, the steam-aged catalysts during tests in a dry feed and the steam-aged catalysts tested in a wet feed, containing 5 wt % H2O.The data show that the fresh Pd-Pt catalyst is more active than the fresh Pt catalyst.Higher activities are also observed for steam-aged Pd-Pt catalysts tested in dry or wet feed gas.Table 5. T50 for fresh and steam aged Pd and Pt-Pd catalysts operated in dry and wet feed.Combustion conditions: 4067 vol.ppm CH4; total flow rate of 234.5 cm 3 /min; 500 mg catalyst; 5 vol.% water in wet feed.Adapted from [71].

Kinetic Consequences of H 2 O on CH 4 Oxidation over Pd Catalysts
The rate of CH4 oxidation over Pd catalysts is influenced by temperature, reactant partial pressures, the state of the Pd at reaction conditions (Pd 0 , PdO or a sub-oxide), possibly Pd crystallite size (i.e., may be structure-sensitive), and inhibition by products H2O and CO2.Consequently, kinetic parameters reported in the literature vary over wide ranges; this is especially true of the apparent activation energy for CH4 oxidation [20].As noted by Carstens et al. [79], rate data must account for the inhibition effect of H2O when determining the activation barrier, but Ciuparu et al. [43] has shown that the correction is complicated by the fact that the effect of H2O inhibition is temperature dependent.For example, the apparent activation energy for CH4 oxidation over a Pd/ZrO2 catalyst is estimated to be 180 kJ/mol from data measured at temperatures below 192 °C, whereas a value of 87 kJ/mol is obtained at temperatures above 192 °C [42].The higher value of the apparent activation energy at lower temperatures is attributed to the strong inhibiting effect of H2O on the Pd catalyst.
Zhu et al. [80] reported kinetic parameters for CH4 oxidation over a series of model Pd and PdO surfaces and foils, and compared the values to literature data on supported Pd catalysts (Table 6).From Table 6 the reaction orders for CH4 and O2 are probably not sensitive to the structure of the Pd catalyst, although on the supported catalysts the reaction orders for H2O vary from −0.25 to −1.3.Taking account of the error in the Ea estimates (±20 kJ/mol), Zhu et al. [80] concluded that on the large single-crystal model catalysts, the activation energies are similar and the combustion of CH4 over Pd or PdO is not sensitive to the structure of the catalyst.Larger Ea values are reported for the Pd/oxide-supports (150-185 kJ/mol) corrected for the effect of H2O (assuming an order of −1) [79], whereas the much smaller Ea for the Pd/zeolite catalysts (72-77 kJ/mol) are possibly associated with the high acidity and high OH surface concentration of zeolites, in obvious contrast to the observed inhibition by OH groups for PdO supported on conventional supports.The negative orders of reaction for H2O are indicative of the varying degrees of inhibition of CH4 oxidation by H2O on Pd and PdO surfaces and catalysts.The role of H2O in the inhibition of PdO catalysts during CH4 oxidation has been documented in this review to relate to the adsorption and slow desorption of H2O on active sites during reaction.Kikuchi et al. [16] proposed a kinetic model assuming competitive adsorption between H2O and CH4 on PdO sites, where dissociative CH4 adsorption was assumed to be the rate determining step (RDS) and the coverage by C-species was assumed to be negligible.The main elementary steps of the reaction are postulated as follows: from which the following rate expression is derived [16]: where r is the reaction rate, k r is the rate constant for H abstraction, K H 2 O is the H2O adsorption equilibrium constant, and P CH 4 and P H 2 O are the partial pressures of CH4 and H2O, respectively.K H 2 O is exponentially dependent upon the H2O adsorption enthalpy (∆  ).To increase the activity and durability of the Pd catalysts in the presence of H2O, K H 2 O should be small according to the above reaction model.Based on the measured ∆  values for water on supported Pd catalysts, water adsorbed on Pd/Al2O3 has the highest negative adsorption enthalpy (∆  = −49 kJ mol −1 ) compared to Pd/SnO2 (−31 kJ mol −1 ) and Pd/Al2O3-36NiO (−30 kJ mol −1 ) (Table 7) despite the lower activation energy calculated for Pd/Al2O3 (see Table 7) [16].A higher ∆   implies stronger H2O adsorption on the surface and is evidence of a higher coverage of active sites by H2O molecules on Pd/Al2O3 catalysts and consequently lower catalyst activity.However, the larger negative enthalpy also predicts a more rapid decrease in KH2O with increasing temperature for Pd/Al2O3.The larger negative value in the order of H2O for the 1% Pd/ZrO2 catalyst, compared to the Pd/SiO2 catalyst, as reported by Araya et al. [53] (Table 6), reflects stronger H2O adsorption on ZrO2 than on the SiO2 [53].Hurtado et al. [83] observed a change in the power-law reaction order of H2O from −1.3 to −0.9 as temperature increased from 300 °C to 350 °C using a H2O-CH4-O2 reactant mixture and a commercial 0.5 wt.% Pd/γ-Al2O3 catalyst.Considering the equation proposed by Kikuchi et al. [16], with K H 2 O P H 2 O ≫ 1, the H2O reaction order will reduce to −1 but if K H 2 O P H 2 O is small, the H2O reaction order reduces to a value approaching zero.
Hurtado et al. [83] also attributed the inhibition effect of H2O during reaction to the adsorption of H2O on Pd catalysts.Based on this assumption the authors examined several Eley-Rideal, Langmuir-Hinshelwood and Mars-van Krevelen kinetic models finding that by considering competitive adsorption between H2O and CH4 on Pd oxide sites and slow desorption of products, the following kinetic model could be derived: where k1, k2, and k3 are the rate constants for (1) irreversible oxygen adsorption, (2) surface reaction with CH4, and (3) product desorption steps in the mechanistic sequence, respectively.This model provides the best fit of their measured rate data.The ∆ ads for water estimated from equation ( 7) is −54.5 kJ/mol, in agreement with the data of Table 7.The inhibiting effects of H2O are assumed to be a consequence of a competitive adsorption between CH4 and H2O on PdO sites.Deactivation by H2O was previously thought to be due to formation of inactive Pd(OH)2 that does not participate in the CH4 oxidation reaction and is reversible at temperatures above 250 °C [66].Hurtado et al. [83] also note that the formation of Pd(OH)2 is thermodynamically favored from PdO sites rather than from Pd 0 .However, the more recent mechanism involving H2O inhibition of the O exchange between Pd sites and oxide supports, proposed by Schwartz et al. [44,57] (see earlier discussion) appears to be supported by more definitive data.

Conclusions
Studies of the past decade provide new insights into the effects of H2O on Pd catalysts during CH4 oxidation, especially at lower temperatures.The principal effects of H2O are: (a) reaction inhibition by H2O adsorption (b) deactivation due to formation of Pd(OH)2 and (c) H2O-assisted sintering at high reaction temperatures (>500 °C) Reaction inhibition by H2O increases with (a) decreasing reaction temperature at <500 °C and (b) higher H2O concentrations, while this effect is generally negligible at >500 °C.O surface mobility of supports apparently influences H2O inhibition, i.e., high O mobility (on CeO2 and ZrO2) results in less inhibition by H2O than for Al2O3.
The main cause of partially reversible deactivation has been related to hydroxyl adsorption on the support and PdO.Although earlier studies suggested that formation of inactive Pd(OH)2 could be the cause of deactivation, recent studies provide definitive evidence that adsorbed hydroxyls suppress O exchange between the support and Pd active sites causing suppression of catalyst activity.
H2O-assisted sintering of supported Pd catalysts is observed at >500 °C.Catalysts with stabilized supports or core-shell structures have higher resistance to hydrothermal sintering.Several studies show that Pd bimetallic catalysts also improve catalyst stability, although explanations for the role of the second metal are not well-defined.Suppression of PdO sintering, enhanced oxygen mobility and suppression of hydroxide formation are postulated to play a key role in higher stability of Pd bimetallic catalysts.
Rate expressions from kinetic studies of CH4 oxidation at conditions relevant to natural gas vehicles are based on the assumptions of (a) product inhibition by H2O is a consequence of a competitive adsorption mechanism between CH4 and H2O on PdO sites; and (b) deactivation by H2O is due to the formation of inactive Pd(OH)2 .None of the previous kinetic studies have linked the observed kinetic effects of H2O to O mobility that recent studies show is critical during CH4 oxidation.

Figure 3 .
Figure 3. Temperature-programmed reactions during pulsed or continuous flow of reactants over Pd/ZrO2 with or without H2O in the feed.Reproduced with permission from [42].Copyright © 2001 Elsevier.

Table 4 .
[77]ges in Pd and Pt-Pd catalyst properties before and after aging.Adapted from[77].

Table 6 .
Kinetic parameters for CH4 oxidation over Pd catalysts.
a E a determined under dry reaction conditions, correction for H 2 O inhibition.