Physicochemical properties of Pt-SO4/Al2O3 alkane oxidation catalysts

A series of sulfated alumina catalysts were synthesised by wet impregnation with sulfate-containing solutions.

The degree of surface sulfation and corresponding surface acidity could be readily tuned by varying the molarity of impregnating solution.

Strong acid treatments (>0.1 M) induced aluminium-sulfate crystallisation with a concomitant decrease in porosity and surface acidity.

Platinum-doped sulfated aluminas showed enhanced activity towards methane, ethane and propane combustion.

Activity scaled with the degree of accessible surface sulfate and platinum loading, however C–H bond scission appeared rate-limiting over both pure and presulfated aluminas.

The magnitude of sulfate-promoted propane oxidation was greatest under heavily oxidising conditions (C3H6∶O2 > 1∶20) but independent of Pt loading, confirming that support-mediated alkane activation is the dominant factor in the promotional mechanism.


The importance of heterogeneously catalysed hydrocarbon chemistry can hardly be overestimated, ranging from processes such as petroleum reforming1,2 to fine chemicals synthesis and pollution control.3

Despite these successes C–H activation of saturated hydrocarbons remains a challenge both for their utilisation as an alternative feedstock, and their combustion for power generation and the regulation of environmental emissions.4,5

Low-temperature activation of short chain alkanes for chemical synthesis is conventionally achieved via strong solid acids, which are able to protolyse the C–H or C–C bond.6

Sulfated metal-oxides represent one of the most versatile classes of solid acid, possessing high activity, good hydrothermal stability and tuneable surface acidity and porosity.7–9

The latter is important in their application to large organic substrates wherein mass-transport diffusion limitations hinder microporous zeolitic materials.10

Sulfated metal-oxides find application in a range of gas phase reactions including alkane isomerisation and cracking11 and more recently liquid phase alkylation and acylation chemistry.12,13

Their catalytic properties can be further tuned by metal doping which can enhance both catalyst activity and stability, e.g. in butane isomerisation, with C–H activation observed at temperatures as low as 60 °C.12,14

Sulfated zirconia is probably the most widely studied system, in which the interaction of sulfate ions with the zirconia surface can generate superacidic sites.

These materials exhibit both Lewis and Brønsted acidity dependent on the degree of surface hydration.12

We have recently demonstrated that wet impregnation of zirconia by dilute H2SO4 affords a simple means to regulate the surface acidity and catalytic performance of mesoporous sulfated zirconia.15

In contrast the physical properties and catalytic applications of sulfated aluminas have been scarcely explored;9,16,17 this despite alumina being the most widely used catalyst support material.

Gas phase sulfation of alumina is known to enhance the Pt catalysed combustion of propane for automotive emission control.18,19

Enhanced C–H activation may occur via support-mediated propane adsorption over sulfated alumina, and subsequent spillover of alkylsulfate intermediates formed at the Pt-support interface.20,21

However the nature and strength of acid sites in SO4/Al2O3 is poorly understood, while the interaction with transition-metal dopants has been hardly touched upon.22–24

Surprisingly sulfated aluminas have not been exploited to promote low-temperature C–H activation in other catalytic applications.

Here we report the preparation and characterisation of well-defined SO4/Al2O3 and their applications as support materials in Pt-SO4/Al2O3 catalysts for light alkane combustion.


Catalyst preparation

Sulfated aluminas were prepared by incipient wetness impregnation of 1 g of γ-Al2O3 (Degussa aluminium oxide C) with a 10 cm3 solution of 0.01–2.5 M H2SO4 (Fisher 98%).

The resultant slurry was dried at 80 °C in air for 12 h and then calcined in flowing O2 at 550 °C for 3 h.

Reference materials were also prepared by the same procedure using 0.1 M solutions of (NH4)2SO4 (Aldrich 99%) or Al2(SO4)3 (Aldrich 99.99%).

Samples were subsequently stored in air.

Platinum was added via the incipient wetness technique using 1 cm3 of (NH4)4PtCl2 (Johnson Matthey, 55.24 wt% Pt assay) as an aqueous solution per gram of support.

The resultant paste was air-dried at 80 °C for 12 h and then calcined in flowing O2 at 500 °C for 2 h followed by a 2 h reduction in flowing H2 at 400 °C.

Gas-phase sulfation was performed either prior to reaction, using a flow of 0.1 vol% SO2∶1 vol% O2 in He mix (10 cm−3) at 400 °C for 2 h followed by cooling under He, or via introduction of 0.1 vol% SO2/He (1 cm3 min−1) along with the reactant stream.

Catalyst characterisation

Porosity and surface area measurements were performed via N2 adsorption on a Micromeritics ASAP 2010 instrument.

Surface areas were calculated using the BET equation over the pressure range P/P0 = 0.02–0.2, where a linear relationship was maintained.

The final sulfur content was determined by elemental analysis using a Carlo Erba 1108 CHN/S instrument (quoted percentages refer to total S levels).

Pt loadings were measured using a Perkin–Elmer P40 emission ICP-MS instrument.

XRD patterns were acquired using a Siemens D5000 diffractometer and Cu Kα radiation.

S 2p and Pt 4d XP spectra were also measured at 40 eV pass energy using a Kratos AXIS HSi instrument, equipped with charge neutraliser and Mg Kα X-ray source.

Two-point energy referencing was employed using adventitious carbon at 285 eV and the valence band, and in all instances only SO4 groups were observed with a characteristic binding energy of 170 eV.

Thermogravimetric analysis (TGA) was performed in a Stanton–Redcroft STA-750 system using 50 mg sample in an alumina crucible under flowing nitrogen and a ramp of 10 °C min−1.

Temperature-programmed reduction (TPR) was carried out in a quartz tube using 50 mg sample under a 5% H2/Ar flow (E&W 99%) with a ramp of 11 °C min−1.

DRIFTS spectra were obtained using a Bruker Equinox FTIR spectrometer.

Samples were diluted with KBr powder (10 wt% in KBr), then loaded into an environmental cell and subjected to additional drying under vacuum at 110 °C for 10 min prior to measurements to remove moisture physisorbed during air exposure.

Ex situ pyridine adsorption was performed by incipient wetness impregnation, with excess pyridine evaporated prior to sample loading and evacuation as described above.

Acidity was determined by Hammett indicators; ∼25 mg of sample was shaken with 1 cm3 of a solution of Hammett indicator diluted in 10 cm3 cyclohexane and left to equilibrate for 2 h.

Catalyst testing

Catalyst testing was performed in a fixed-bed quartz reactor using 100 mg catalyst.

The total gas flow rate was 21 cm3 min−1 and stoichiometric mixtures were employed using 1 cm3 min−1 of alkane together with the appropriate oxygen flow with helium added as an inert diluent.

This equated to gas mixes of 5 vol% HC and 10–25 vol% O2 in He.

Gases used were CH4 (E&W 99.995%), C2H6 (E&W 99.5%) and C3H8 (E&W 99.9%).

Light-off measurements were performed with a ramp rate of 15 °C min−1 with the catalyst bed temperature measured with a coaxial thermocouple.

Reaction was monitored via on-line mass spectrometry (VG 200 amu QMS).

The sole reaction products were CO2 and H2O.

The overall error in conversion was ±4%.

Blank runs showed negligible gas-phase contributions to alkane combustion below 700 °C.

Results and discussion

Sulfated aluminas

In order to elucidate the respective roles of metal and support effects in sulfate-promoted alkane activation, the physico-chemical properties of a series of sulfated alumina support materials were characterised prior to platinum doping.

Fig. 1 shows the total and surface sulfur content as a function of molarity of impregnating solution.

It is evident that the surface sulfur loading increased rapidly for low sulfuric acid concentrations demonstrating efficient S adsorption.

Quantification was achieved from the integrated S 2p XP intensity and overall surface composition, incorporating appropriate elemental response factors (RF) according to the equation below:The surface loading roughly mirrored the total sulfur content for all samples, although a slight plateau in the former is apparent around 0.1 M. The surface loading rose again above >1 M H2SO4 to give a common surface and total S loading of ∼14 wt% S for the 2.5 M impregnated sample.

This is equivalent to the S content in Al2(SO4)3·18H2O.

Complete surface sulfation thus required bulk sulfur loadings in excess of 1.7 wt%.

Surface areas of impregnated aluminas were a strong function of sulfur loading as shown in Fig. 2.

For sulfate concentrations below 0.1 M the surface area was close to that of untreated γ-alumina at around 140 m2 g−1.

This fell dramatically for higher concentrations reaching only 10 m2 g−1 following impregnation by 2.5 M H2SO4.

These changes are likely associated with crystallization and pore collapse upon formation of bulk Al2(SO4)3.

Formation of crystalline sulfated alumina was investigated by powder X-ray diffraction.

Fig. 3 shows the uncalcined alumina precursor contained several weak reflections in the range 2θ = 32–38°, with two stronger reflections at 46 and 67.4° characteristic of γ-alumina.

Sulfation sharpened the weaker alumina features which resolved into peaks at 33, 34.6, 37 and 39.7°, and for concentrations ≥0.5 M H2SO4 also resulted in new reflections at 25.6 and 29°.

The latter are characteristic of anhydrous aluminium sulfate with contributions from Al2(SO4)3·14H2O and the monohydrate Al2(SO4)3·H2O.25,26

Simple peak shape analysis assuming a stoichiometric, non-defective structure suggested the initial alumina phase comprised crystallites of ∼145 Å.

Sulfation was accompanied by significant crystallite growth with Al2(SO4)3 particle sizes estimated from the 25.6° reflection exceeding 400 Å following impregnation by 0.5 M H2SO4.

The thermal stability of impregnated sulfated aluminas was examined by TGA.

The γ-alumina precursor showed negligible weight loss at temperatures below 900 °C, Fig. 4.

Following sulfation a single high temperature weight loss was observed above 600 °C, indicative of a relatively well-defined sulfoxy species, the magnitude of this loss increased with acid molarity.

The loss profile for the 2.5 M impregnated alumina resembles that of bulk aluminium sulfate,25 which exhibited a similar high-temperature drop above 600 °C attributable to SO2 desorption.

A small low temperature loss was also observed between 100 and 300 °C from the Al2(SO4) standard, associated with loss of physisorbed water and surface dehydroxylation.25

Corresponding TPR measurements shown in Fig. 5 revealed a similar trend.

Low sulfate loadings gave rise to a broad, high temperature reduction peak centred around 500 °C.

As the sulfate content rose this peak shifted to higher temperature, reaching 600 °C for 2.5 M H2SO4, while a second minor state emerged at 420 °C.

Hence increased thermal stability is accompanied by a decrease in the homogeneity of sulfoxy environments.

The total reduction signal scaled with sulfur loading, approaching that of pure Al2(SO4)3 for the highest molarity sample.

γ-Alumina was unreduced below 800 °C, hence all the features in Fig. 5 must be associated with sulfate reduction.

Al2(SO4)3 also exhibited two principal reduction states: one at 600 °C matching that from the 2.5 M H2SO4 impregnated alumina; the second at 420 °C matching that for molarities ≤0.1 M. These may correspond to bulk and surface sulfate reduction processes respectively.

The intensity ratio of the two reduction peaks for the impregnated series is inconsistent with a dual-stage reduction process in which SO4 → SOx followed by SOx → S for which a 1∶3, 2∶2 or 3∶1 low∶high temperature ratio is expected.

Our observations suggest that a single SO4 species predominates following impregnation with 0.1–0.5 M H2SO4, beyond which sulfur is incorporated within various subsurface environments.

This results in a progressive recrystallisation of the γ-Al2O3 substrate, as observed by XRD, coupled with pore collapse and a loss of surface area.

Table 1 presents the acid strength of the sulfated aluminas as determined using Hammett indicators.

Such measurements use a series of neutral basic indicators which change to the colour of their conjugate acid when interacting with acid sites of a critical pKa.

Quantitative acid site distributions can be determined by UV spectroscopy by titration of the suspended solid with n-butylamine, according to the Hammett–Bertolacini method.27

However determination of superacidity and acid site distributions of porous solid acid catalysts by such measurements remains controversial due to solvent–support interactions28 and accessibility of the probe molecule within micropores.29

Provided the polarity of the solvent used to suspend the solid and indicator is similar to that of the reaction media, Hammett indicators therefore provide a semi-quantitative value for the maximum solid acid strength.

Table 1 shows that pure gamma alumina displayed only weak basic characteristics.

Sulfation increased the support surface acidity, however sulfated aluminas remained only weakly acidic and did not show superacid properties.

Maximum surface acidity was achieved following 0.5 M H2SO4.

Stronger impregnating solutions actually led to a drop in acidity associated with a transformation to bulk aluminium sulfate (the latter exhibited amphoteric properties).

None of these impregnated materials showed superacidity in contrast to their sulfated zirconia counterparts.15

Note that subsequent Pt doping of these materials had no effect on their H0 values.

Surface functionalisation of the sulfated aluminas was further explored by DRIFTS.

Sulfation resulted in the appearance of new features in the region 1400–900 cm−1 (Fig. 6a) which fall in the range expected for vibrations of coordinated sulfate species.30,31

The generally accepted modes of SO4 binding to oxide surfaces involve mono- or bidentate coordination environments, which exhibit 3 or 4 vibrational modes, respectively.

For chelating and bridged bidentate SO4 species, modes attributed to the OSO and O–S–O stretches are observed in the order νa(OSO) > νs(OSO) > νa(O–S–O) > νs(O–S–O).

Considering first the aluminium sulfate standard, the well defined peak at 1320 cm−1 can be assigned to the asymmetric OSO stretch of bidentate surface sulfate, consistent with previous observations ∼1370 cm−1.32

The peaks at 1280, 1190 and 1125 cm−1 are assigned to νs(OSO), νa(O–S–O) and νs(O–S–O) respectively in agreement with the observed frequency range observed for (NH4)2SO4 and Al2(SO4)3 impregnated Al2O3.32,33

Only three bands were observed for low sulfur loadings suggestive of a monodentate adsorption geometry.

Indeed the relative frequencies of these peaks at 1060 cm−1, 1160 cm−1 and 1250 cm−1 are in good accord with the corresponding ν(O-SO3) and νs(O-SO) and νa(O-SO3) modes.

A weak feature observed at 1360 cm−1 in both the alumina support and all low loading samples may arise from a weak lattice mode associated with octahedral Al3+ in γ-Al2O3.34

Higher sulfur loadings attenuated these monodentate characteristics, concomitant with the emergence of features (notably the 1320 cm−1 band) associated with bidentate SO4 species.

The spectrum from alumina impregnated with 2.5 M3 H2SO4 bears a striking resemblance to that of pure Al2(SO4)3, demonstrating crystallisation extends throughout the selvedge.

It is important to note the precise frequencies of surface SO4 vibrational modes are sensitive to the degree of surface hydration, accounting for a wide variance in literature values.

Pyridine adsorption was subsequently employed to titrate Brønsted and Lewis acid sites.

As anticipated from our Hammett measurements the alumina precursor showed no detectable surface acidity, whereas all sulfated aluminas exhibited sharp peaks at 1485, 1533, 1604 cm−1 and 1633 cm−1 characteristic of predominantly Brønsted acid sites (Fig. 6b (inset)).35

A small contribution from Lewis acid sites at 1485 and 1604 cm−1 cannot be discounted, however the absence of any characteristic Lewis band at ∼1450 cm−1 suggests the Lewis contribution is negligible.

A high ratio of Brønsted versus Lewis sites is to be expected as a result of moisture adsorption during storage in air.

The strength of the principal pyridinium-ion vibration at 1533 cm−1 is also shown as a function of total S content in Fig. 6b for SO4/Al2O3 and bulk Al2(SO4)3.

The acid site density increased with sulfur loading up to 0.5 M, but rapidly decreased for higher loadings following the trend in H0 values.

These DRIFTS measurements suggest crystalline Al2(SO4)3 possesses no surface acid sites.

Platinum-doped sulfated aluminas

The potential of SO4/Al2O3 as a support material in alkane combustion was first investigated over doped sulfated aluminas with low 0.05 wt% Pt loadings (selected to maximise possible promotional effects).

The catalytic performance was screened in propane light-off under stoichiometric conditions (1∶5 C3H8∶O2) as shown in Fig 7.

Pure alumina was inert below 800 °C.

Unsulfated Pt/Al2O3 exhibited the poorest combustion activity with light-off commencing above 350 °C and a T50 = 510 °C (temperature for 50% conversion).

The light-off temperature decreased with increasing alumina sulfation reaching a minimum T50 = 350 °C for 0.1 M impregnated SO4/Al2O3.

Further sulfation lowered catalyst activity with the T50 increasing to 410 °C for the 2.5 M sample, this despite a 14-fold increase in the total S content, revealing a dramatic drop in the activity per mole S. This variation in T50 correlates directly with the accessible surface sulfate.21

The increased light-off temperature for heavily sulfated aluminas (>0.5 M) reflects the drop in surface area and acidity.

Alumina impregnation with 0.1 M H2SO4 gave the optimal promotion.

This promotional phenomenon was independent of the nature of sulfate precursor, Fig. 8, and depended solely on the concentration of sulfate ion.

The influence of Pt loading on sulfate promoted propane combustion was subsequently studied over a 0.1 M SO4/Al2O3 support (chosen to provide the greatest enhancement).

Fig. 9 shows the resulting bulk and surface Pt loadings determined by XPS and elemental analysis using the equation given below.Pt wt% = IPt × RFPt / [(IPt × RFPt) + (IS × RFS) + (IO × RFO) + (IC × RFC)]The surface Pt coverage initially rose rapidly over both pure and sulfated aluminas as the total Pt content increased.

However the fraction of exposed Pt (i.e. metal dispersion) fell dramatically for total loadings above 1 wt% as shown in Fig. 9 (inset).

In all cases the Pt dispersion was lower over sulfated alumina consistent with the formation of large Pt crystallites.36

The structural evolution of Pt clusters was also explored via XPS.

Fig. 10 shows the growth of the Pt 4d3/2,5/2 doublet (spin–orbit splitting = 16.7 eV) with increasing total platinum loading over pure alumina.

The binding energy decreased significantly from 332.0 to 330.6 eV as the loading rose from 0.05 to 25 wt% Pt (Fig. 10, inset).

The same components and peak shifts were observed over a 0.1 M SO4/Al2O4 support.

This shift has two possible origins.

Core-level binding energies are often observed to decrease with increasing metal particle size, characteristic of a cluster → bulk transition and associated improved final-state relaxation and core-hole screening.37,38

However such behaviour is only expected for Pt clusters on weakly-interacting supports such as carbon and silica.39

Over substrates possessing localised orbitals with binding energies that overlap the cluster d-orbitals metal–support interactions actually produce the opposite core-level shift.40

Further complications also arise due to cluster charging over insulating substrates,39,40 hence the present shift cannot be simply attributed to electronic effects.

An alternative explanation is suggested by our recent EXAFS measurements, which have demonstrated a dramatic change in Pt oxidation state with loading over alumina.36

The shifts in Fig. 10 are thus fully consistent with, and attributable to, a transformation from small PtOx clusters to large metallic Pt crystallites.

The emergence of large metallic Pt clusters ≥1 wt% was confirmed by XRD as shown in Fig. 11.

No Pt reflections were observed for lower loadings, consistent with a high dispersion and particle sizes <30 Å.

Higher Pt loadings resulted in the appearance of fingerprint fcc Pt {111} and {200} reflections at 39.9 and 46.5° respectively.

Peak fitting yields a volume averaged particle size of ∼400 Å for the 5 wt% Pt-SO4/Al2O4 sample.

The influence of surface platinum on the reducibility of the 0.1M sulfated alumina support was examined by TPR.

Fig. 12 shows that the presence of even trace Pt influences the reduction profiles, causing a broadening and decrease in the sulfate reduction temperature.

This observation likely reflects surface SO4 destabilisation via platinum-mediated H2 dissociation and spillover at the metal–support interface as observed for Pt-SO4/ZrO2.41

For Pt loadings ≥0.25 wt% an additional low temperature reduction peak emerges around 350 °C.

This lies within the range of 200–400 °C typically observed for PtOx reduction in Pt/Al2O3 catalysts;42–44 the absence of this state for lower loadings reflects the TPR sensitivity limit.

Reduction profiles for the highest Pt loadings contain a number of poorly resolved contributions arising from the direct reduction of surface aluminium sulfate, Pt-mediated reduction of interfacial sulfate and PtO2 reduction.

The effect of Pt loading on propane combustion under stoichiometric conditions over bare and 0.1 M sulfated alumina is shown in Fig. 13.

A strong loading dependence was observed for both supports, with the highest Pt loadings (largest particles) exhibiting the lowest light-off temperature (highest activity).

Alumina presulfation enhanced the performance of all catalysts with the magnitude of promotion roughly independent of the Pt loading.

This observation is in line with our recent finding that support-effects are the dominant factor in sulfate-promoted propane combustion.

The reaction profiles follow sigmoidal curves however kinetic analysis suggests a compensation effect operates, with activation barriers rising from 70 to 127 kJ mol−1 as the Pt loading falls from 5 to 0.05 wt% over bare alumina, accompanied by a 107 drop in the preexponential factor.

A similar trend occurred over the sulfated alumina support.

Sulfate promotion of Pt/Al2O3 catalysts via gas-phase SO2 is sensitive to oxygen concentration.

In order to examine whether this dependence extended to presulfated aluminas, the influence of air–fuel ratio on an unsulfated 0.05 wt% Pt/Al2O3 catalyst was first isolated (Fig. 14).

Propane light-off was a strong function of reactant composition, showing a dramatic increase (i.e. reduced activity) upon switching from fuel-rich to fuel-lean mixtures.

This can be ascribed to competitive adsorption between oxygen and propane for Pt sites, coupled with the intrinsically low sticking probability of propane versus O2.45

Fig. 15 shows that propane combustion over Pt-SO4/Al2O3 catalysts also exhibited a strong dependence on air–fuel ratio.

As for the untreated supports, optimum propane conversion required fuel-rich conditions.

However the magnitude of sulfate promotion is greatest under oxygen-rich conditions; the threshold temperature for propane light-off falls by 185 °C versus only 74 °C under excess hydrocarbon.

This demonstrates the active species (present during reaction) generated via support presulfation require an oxidising environment—consistent with surface sulfoxy/sulfate.

It also highlights the possibility that sulfate reduction may occur in-situ under reducing reaction conditions.

It is interesting to compare the magnitude of promoted propane combustion achievable via different sulfation methodologies.

Fig. 16 reveals that enhancement was essentially independent of sulfation protocol, i.e. wet-chemical presulfation of an alumina support was as efficient as gas-phase sulfation by SO2 (introduced either prior to or during propane combustion).

Sadowski and Treibmann have reported a similar finding.46

This result also demonstrates that sulfate promotion is irreversible, and indeed all our sulfated catalysts could be stored and reused without measurable decrease in light-off performance.

Light alkane reactivity

Fig. 17 compares the light-off curves for methane, ethane and propane combustion over sulfated and unsulfated 0.05 wt% Pt/Al2O3.

The alkane light-off temperatures decreased in the order propane < ethane < methane over both catalysts in accord with their respective C–H bond energies of 401, 420 and 440 kJ mol−1; C–H bond activation is widely accepted as rate-limiting in alkane oxidation over Pt/Al2O3.

Alkane combustion was sensitive to both metal dispersion and reaction conditions, and the light-off temperatures for our unsulfated catalyst are in good agreement with literature T50 values for propane,18 ethane and methane47 under fuel-lean conditions.

Activation energies were also in line with typical literature values which range between 113–146, 80–114 and 71–104 kJ mol−1 for methane,48,49 ethane47,50 and propane,18,19 respectively.

Alumina presulfation reduced the light-off temperature for all light alkanes, with the magnitude of promotion increasing dramatically from methane to ethane, while ethane and propane exhibited similar enhancements of 178 and 153 °C, respectively.

Although the influence of sulfoxy species on the catalytic combustion of hydrocarbons has generated significant interest, previous studies have focused on enhanced propane combustion.

Propane light-off is variously reported as lowered by between 50–250 °C over Pt/Al2O3 catalysts,18,19,51,52 consistent with the present value.

However no systematic correlations between catalyst structure, reaction conditions and the degree of promotion have been previously identified.

SO2-promoted methane combustion has only been previously reported by Meeyoo et al. over a Pt/Al2O3 cordierite monolith,53 wherein a ∼30 °C improvement was observed, in accordance with Fig. 17.

Our recent letter21 and the present paper represent the first report of sulfate-promoted ethane combustion.

Higher Pt loadings facilitated lower temperature light-off (Fig. 18), in line with the documented structure sensitivity of alkane combustion over Pt/Al2O3 catalysts,49 but gave the same order of alkane reactivity.

The former finding supports our earlier predictions that large metallic Pt clusters favour C–H bond scission,36 while the latter indicates alkane activation proceeded via a common rate-limiting step over all catalysts.

Alumina presulfation again facilitated low-temperature light-off; however the magnitude of the promotional effect was independent of Pt loading (Fig. 19).

This suggests the promotional phenomenon originates principally from sulfate-related changes in the alumina support, and not the platinum.

The results presented in Fig. 7, wherein the Pt loading was fixed and the surface aluminium sulfate coverage varied, confirm that support-mediated chemistry indeed plays the dominant role in promoting alkane oxidation.21

Catalyst sulfation did not significantly alter the apparent activation barriers to combustion, in agreement with Hubbard et al.,19 but did increase the associated pre-exponential factors.

This suggests alkane oxidation occurs via a common reaction pathway over both fresh and sulfated Pt/Al2O3, with sulfation generating a vast number of new surface sites active towards low temperature C–H activation.

The nature of these sites remains speculative, however our model studies have shown that sulfate enhances the dissociative sticking probability of propane over Pt{111}/Al2O3 single-crystal surfaces.54

This enhancement directly correlated with interfacial sulfate formation, and may involve spillover of a propylsulfate intermediate, formed at the periphery of Pt clusters, onto bare metal sites for subsequent oxidation.

Such alkylsulfate intermediates may provide an alternative low energy pathway to C–H bond activation.55

Fast XPS measurements recently provided direct evidence for alkylsulfate formation during the reaction of alkenes over model SO4/Pt{111} surfaces.56

This hypothesis is supported by the observation that SO2 does not enhance combustion of long chain alkanes (e.g. cyclohexane) which have intrinsically higher sticking probabilities.46

Sulfate-enhanced propene activation was also recently reported over a full single-crystal model Pt/γ-Al2O3/NiAl{110} catalyst,57 demonstrating this phenomenon extends across both the pressure and structure gaps.

It should be noted that direct activation of propane over SO4/Pt{111} surfaces is possible under vacuum conditions, even in the absence of alumina.45

However the low saturation coverage (θSO4 = 0.25 ML) and thermal stability (<200 °C) of surface SO4 achievable over Pt sites alone,56 suggests this is a minor pathway in the present systems.


Sulfated aluminas possessing well-defined surface sulfate species and tuneable acid properties can be prepared by simple direct impregnation by aqueous sulfate-ion solutions.

Low molarity solutions (<0.1 M SO42−) only modify the surface of γ-alumina while higher concentrations induce the crystallisation of non-acidic bulk Al2(SO4)3.

Thermally stable surface sulfate groups promote the adsorption of light alkanes and subsequent low temperature C–H activation.

Pt-promoted SO4/Al2O3 catalysts exhibit enhanced C1–C3 alkane combustion over their Pt/Al2O3 counterparts, and activity scales with both surface aluminium sulfate coverage and metal loading.

Alkane reactivity increases with chain length in all cases suggesting C–H bond scission remains rate-limiting over sulfated catalysts.

The magnitude of sulfate-promotion was independent of Pt loading and greatest under fuel-lean conditions.

Support-mediated alkane activation over SO4/Al2O3 and subsequent spillover of alkyl moieties onto Pt sites is the principal origin of this enhanced catalytic combustion.