L Angheluta

E Jettestuen

J Mathiesen

F Renard

B Jamtveit

Stress-driven phase transformation and the roughening of solid-solid interfaces

Keywords: numbers: 68.35.Ct, 68.35.Rh, 91.60.Hg

The application of stress to multiphase solid-liquid systems often results in morphological instabilities. Here we propose a solid-solid phase transformation model for roughening instability in the interface between two porous materials with different porosities under normal compression stresses. This instability is triggered by a finite jump in the free energy density across the interface, and it leads to the formation of finger-like structures aligned with the principal direction of compaction. The model is proposed as an explanation for the roughening of stylolites -irregular interfaces associated with the compaction of sedimentary rocks that fluctuate about a plane perpendicular to the principal direction of compaction.

Morphological instabilities in systems out of equilibrium are central to most research on pattern formation. A host of processes give rise to such instabilities, and among the most intensively studied are the surface diffusion mediated Asaro-Tiller-Grinfeld instability [1][2][3] in the surfaces of stressed solids in contact with their melts, surface diffusion mediated thermal grooving and solidification controlled by thermal diffusion in the bulk melt [4]. In sedimentary rocks and other porous materials local stress variations typically promote morphological changes via dissolution in regions of high stress, transport through the fluid saturated pore space and precipitation in regions of low stress. This phenomenon is known as pressure solution or chemical compaction. Such processes are often accompanied by the nucleation and growth of thin irregular sheets, interfaces or seams called stylolites [5]. Stylolites form under a wide range of geological conditions as rough interfaces that fluctuate about a plane perpendicular to the axis of compression. They are common in a variety of rock types, including limestones, dolomites, sandstones and marbles, and they appear on scales ranging from the mineral grain scale to meters or greater. A common feature of stylolitic surfaces is small scale roughness combined with large vertical steps in the direction of the compression. Residual unsoluble minerals (i.e. clays, oxides) often accumulate at the interface as stylolites evolve. Despite the considerable attention given to the rich morphology of stylolites there is still no consensus on the mechanism(s) controlling their formation [START_REF] Oelkers | [END_REF][7][START_REF] Aharonov | Modeling stylolite formation[END_REF][START_REF] Koehn | [END_REF]. Here we demonstrate that even if the stylolite is a consequence of pressure solution alone, porosity or other material property gradients may drive the roughening process. In particular, we demonstrate that a compressional load normal to a no-slip solid-solid phase boundary gives rise to a morphological instability. Our setup differs from the commonly studied solid-melt systems, where stresses are applied in the lateral direction and the melt is in a hydrostatic state (see e.g. [10]).

Generally, rocks are heterogeneous bodies with spa-tially variable porosities. The strain energy densities may be larger in regions of high porosity (i.e. low modulus) than in regions of low porosity. Thermodynamically, the total free energy of the system can be reduced by decreasing the porosity variations. In this letter, a simple model for stylolite formation, in which high porosity rock is transformed into low porosity rock at the interface between the low porosity and high porosity materials, is investigated. This solid-solid "phase transformation" is driven by gradients in the free energy per unit volume of mineral, which can be substantial in regions with large porosity variations. The general approach used in this work could be applied to other solid-solid interface roughening phenomena.

We consider a two-dimensional system divided into elastic regions with different but homogeneous porosities (Fig. 1). Without lack of generality, we limit our consideration to two dissimilar materials separated by a single interface. The stress boundary condition is a uniform compression in the vertical direction applied at the top and bottom boundaries. The two phases are separated by a sharp and coherent boundary, i.e. no defects or voids can form along the interface. This translates into continuity of the displacement vector u(r, t) across the interface, [u] = 0. Here, and in other equations the brackets denote the jump in the quantity inside the brackets when the interface is approached from above and below. Under given load conditions the displacement field induced by the compression gives rise, in the linear regime, to a strain tensor of the form ij = 1 2

∂u j ∂x i + ∂u i ∂x j . (1)
The two solid phases are characterized by their Young's moduli, E 1,2 , and Poisson's ratios, ν 1,2 . When E 1 < E 2 , the upper region will be compressed the most and therefore the elastic energy density will be higher in this region. The first step towards a model for the roughening of a solid-solid interface is based on this simple observation. First the elastic parameters of the materials are related to their porosities. Luo and Weng [11] proposed a homogenization method relating the effective bulk and shear moduli to the porosity of the solid. Using this approach, the effective Young's modulus decreases monotonically with the porosity. Consequently, a finite jump in porosity across the interface induces a jump in the elastic energy density, which drives the motion of the interface. Thermodynamically, the evolution of the interface corresponds to a phase transformation from a high to a low energy state. It is assumed that the phase transformation occurs on a time scale that is much longer that the time required for elastic waves to propagate across the system, and the system is therefore always in elastostatic equilibrium. For an isotropic and homogeneous elastic body the elastic equilibrium condition is given by

∂σ ij ∂x j = 0, (2)
together with the uniform uniaxial compression stress boundary condition, σ ij (x, y = ∞) = σ 0 δ i,y δ j,y < 0, and the stress jump across the curved interface due to the effective surface tension is given by

[σ ij n j ] = -γκn i at the interface Γ t , (3)
where κ is the curvature and γ is the local surface tension.

In the limit of negligible surface tension, the stress vector is continuous across the interface ([σ ij n j ] = 0).

For completeness, the basic principles used to derive an equation of motion are presented. When the system approaches an equilibrium configuration, the free energy will be a non-increasing function of time:

d dt V Fdv + Γt Fds ≤ 0, ( 4
)
where F is the free energy per unit volume and F is the interfacial free energy per unit area. Here, the subscript V indicates a volume integration and Γ t indicates integration over the interface. The interfacial energy dissipation is obtained by confining the domain of integration to a narrow zone along the interface and taking the zero thickness limit [12]. This gives

- Γt [F]V n ds + Γt d F dt -κ FV n ds ≤ 0, ( 5
)
where V n is the normal velocity and κ is the local curvature of the interface. This implies the differential form given by

d F dt -(κ F + [F])V n ≤ 0, (6)
where the first term is the total time derivative of the local interfacial energy density. The local free energy is a function of the surface tension only (like fluid-solid interfaces) and thus, it is independent of time. Therefore, the time derivative can be neglected leading to the inequality

-(κ F + [F])V n ≤ 0. ( 7
)
In the linear response regime, this inequality is satisfied when the velocity (the thermodynamic flux) is a linear function of the driving force, (κ F + [F]), namely

V n ∼ c κ F + [F] , (8)
with c ≥ 0. In the absence of surface tension, the normal velocity is simply proportional to the jump in the strain energy, i.e. V n ∼ [F]. While the dynamical law for the interface Eq. ( 8) is very simple, the implementation in a numerical model is more challenging. The model (Fig. 1) of the moving solid-solid interface was numerically implemented using the local force balance and energy dissipation equations (Eqs. ( 3) and ( 8)). The stress field is obtained using the Galerkin finite element discretization of the elastostatic equations and the phase boundary is captured using the level set method. The level set method ( [START_REF] Sethian | Level Set Methods and Fast Marching Methods[END_REF]) is a powerful and reliable technique for tracking surfaces in any number of dimensions. At any time t, the d-dimensional interface Γ t may be defined as the zero level cut through a scalar field ϕ (d+1dimensional surface), namely, ϕ(x, t) = 0 , where x ∈ Γ t . A change in the zero-level cut in response to a change in the scalar field may then be interpreted as a motion of the interface. Therefore the change in the scalar field must correspond to motion of the zero-level cut with a given normal velocity V, this is done by updating the scalar field using a simple advection-like equation

∂ϕ ∂t + V n |∇ϕ| = 0. ( 9
)
The advection of the level set function is solved on a separate lattice using an upwind-scheme. The full dynamical model of the solid-solid phase transformation front is then given by this equation together with Eq. (8).

In the numerical simulations, we have used periodic boundary conditions in the lateral direction to reduce possible finite size effects. The initial interface was generated by a directed random walk (Fig. 2, lower panel). The temporal evolution of the interface (Fig. 2, upper panel) was then recorded for different external stresses, σ 0 , and elastic constants (E, ν). The elastic constants were computed from homogenization relations between elasticity and porosity [11]. As the interface advances increasing lateral stresses may appear behind the tips of the finger like structures. While such lateral stress may cause interface instability, it will only have a secondary effect on the overall transversal evolution.

Initially, the roughness of the interface grows exponentially Γt h(s, t) -h(t) with a characteristic roughening time t * that depends on the external stress and the jump in the elastic properties. To estimate the functional dependence of t * , a set of numerical simulations was performed. First, the external stress σ 0 was systematically varied between 0.005 and 500 MPa for fixed values of the elastic constants (E 1 = 40 GPa, E 2 = 60 GPa, ν 1 = ν 2 = 0.3).

The results shown in Fig. 3 suggests that the characteristic time scales as t * ∼ σ -2 0 , and the prefactor depends on the elastic properties. In order to investigate this type of relation, the elastic constants across the interface were varied (E 2 = 50 GPa, E 1 in the range 5-16 GPa, ν 1 = ν 2 = 0.3). The data for the interface roughening collapses onto a curve which is exponential at small time-scales with a cross-over to a quadratic form at larger times (Fig. 4). Extracting the characteristic time in the exponential growth regime and rescaling it with σ 2 0 , the functional dependence with respect to the jump in the elastic constants across the interface was determined (see Fig. 4, inset). The cross-over from exponential to algebraic roughening depends on the value of the jump. For large jumps in the elastic energy or porosity, the roughening quickly undergoes a transition from exponential to quadratic growth in time. This cross-over time is related to the formation and growth of the finger-like structures shown in Fig. 2. By a simple dimensional argument, we also determined that the generic dependence of t * on the external stress and elastic constants, has the form

t * ∼ L V n ∼ L c[F] ∼ L c 1 [(1 -ν 2 )/E] 1 σ 2 0 , (11)
where [F] = F 1 -F 2 is the jump in the free energy density across the interface. The scaling relation is consistent with the numerical simulation results. The stress field may be calculated analytically with the same boundary and interface conditions for a straight interface perturbed by a sine function with small amplitude A. In linear perturbative analysis (A < < 1) without surface tension, the solution is obtained using the method of Airy's potentials for 2D elastostatics [START_REF] Gao | [END_REF]. The energy jump [E] is proportional to kA 2 , where k = 2π/λ is the wave number. In other words, all the modes are unstable and those with the smaller wavelengths grow faster in the linear regime.

The surface tension adds an ultraviolet cut-off resulting in small scales smoothening of the interface. The system was tested with and without surface tension and in both cases the qualitative behavior was the same -an initial exponential roughening with a crossover to a fingerformation regime. Eventually, these fingers may stabilize due to transport of dissolved minerals and precipitation leading to pore clogging. Stress concentration at the tips is an important characteristic of the system and the pronounced contrast in the energy density between the two phases leads to an enhanced chemical activity. In stylolites the roughening is often accompanied by small fractures aligned with the direction of compression, and this may be explained by the model if the stress concentration at the fingers exceeds the yield strength of the material.

To summarize, a simple solid-solid phase transformation model that predicts a morphological instability of the interface under uniform compressional stress has been developed and investigated. The instability is triggered by a finite jump in the elastic properties across the interface and a concomitant jump in the free energy density. We also showed that the characteristic time of roughening depends on the external applied stress and the elastic parameters jump, in such a way that a higher external compression load or a larger difference between the elastic properties of the phases shortens the time required to roughen the interface. This result allows the roughening time and formation rate of stylolites to be estimated as a function of burial depth in sedimentary basins.

This project was funded by Physics of Geological Processes, a Center of Excellence at the University of Oslo.

FIG. 1 :

 1 FIG. 1: (Color online) Basic setup of the model for a moving interface between two elastic solid phases, characterized by different Young's moduli (E2 > E1) and Poisson's ratios ν1, ν2. The interface boundary propagates with a normal velocity Vn, when the solids are subjected to uniform far-field compressional stresses σ0 in the vertical direction.

FIG. 2 :

 2 FIG. 2: (Color online) Map of the logarithm of the elastic energy in the solids during the roughening process, with E1 = 10 GPa, E2 = 60 GPa, ν1 = ν2 = 0.3, and σ0 = 0.05 MPa. Lower panel: initial h(x) at t = 0. Upper panel: interface at a later stage of roughening.

2 dsFIG. 3 :

 23 FIG. 3: (Color online) Roughness as a function of time for six different external compression stresses σ0 (in units of 10 5 Pa, see legend) for a fixed jump in the Young's modulus and zero surface tension. The root means square height is plotted as a function of time, rescaled with the characteristic roughening time t * , on a semi-logarithmic scale. The data collapse shows the exponential roughening of the interface exp(t/t * ) with a stress independent pre-exponential factor. Inset: the characteristic time as a function of σ0 is given by t * ∼ σ -2 0 (shown by the dashed line).

FIG. 4 :

 4 FIG.4:(Color online) Roughness as a function of time for different jumps in the Young's modulus [1/E] = 1/E1 -1/E2 (measured in units of 10 -10 Pa, see legend) for fixed external stress (σ0 = 5MPa) and zero surface tension. The data is plotted against the time rescaled with t * estimated numerically from the initial exponential growth. At large times, the roughness grows quadratically with increasing time. Inset: Log-log plot of the characteristic time rescaled with the corresponding external stress σ 2 0 • t * as a function of the jump[1/E]. t * ∼ σ 2 0 • [1/E] -α .As guide to the eye, we added a dashed line with exponent α = 3/2.

The authors are grateful to Paul Meakin for fruitful discussions and comments.


M Fathi

F Monnet

Y Schuurman

A H Olmen

C Mirodatos

Reactive Oxygen Species on Platinum Gauzes during Partial Oxidation of Methane into Synthesis Gas

Keywords: partial oxidation, methane, synthesis gas, TAP, platinum, gauzes, bulk oxygen

Platinum gauzes have been studied for the partial oxidation of methane in a high-temperature TAP-2 reactor between 800 and 1100 • C. Two different oxidation procedures were used and the oxygen uptake has been measured. Alternating pulse experiments with oxygen and methane with various pulse intervals have revealed mechanistic information about the platinum catalyzed partial oxidation. It has been shown that carbon monoxide and hydrogen can be produced directly from methane and solid oxidized platinum. In the absence of gas phase oxygen, the maximum selectivities to carbon monoxide and hydrogen were 97 and 96%, respectively, at a methane conversion of 25%. At low surface oxygen concentrations, oxygen that is incorporated in the platinum bulk diffuses back to the surface to react with surface carbon to carbon monoxide. In that case the surface lifetimes of carbon monoxide and hydrogen are much shorter than the oxygen diffusion time and no consecutive oxidation reactions occur. The results of this study show that production of synthesis gas with high yields is possible by a direct route.

INTRODUCTION

Partial oxidation of methane is an attractive way of producing synthesis gas (1)(2)(3). The partial oxidation of methane to synthesis gas can proceed through two routes: (i) indirectly, by total combustion into water and carbon dioxide and subsequent reforming of methane, and (ii) directly, by partial oxidation into hydrogen and carbon monoxide without going through reforming reactions. It has been shown that, under certain conditions and over certain catalysts, carbon monoxide and hydrogen are indeed primary products of the methane oxidation (4)(5)(6)(7)(8)(9)(10). This strongly depends on the availability of oxygen and oxygen-surface bond strengths (9). The problem with the industrial imple-mentation of synthesis gas production through this route is that the primary products are more reactive for further oxidation to water and carbon dioxide with active oxygen surface species than the reactant methane (10). This limits the possible yields of synthesis gas production with methane and oxygen both present in the gas phase without going through reforming. Therefore, recent research on the direct partial oxidation has been directed toward reactions between methane and solid oxygen carriers such as for example CeO 2 (11). The idea is that oxygen supplied from a solid phase is less reactive than oxygen supplied from the gas phase.

In order to address the key question about the nature of selective and nonselective oxygen species, a study of the partial oxidation of methane over platinum gauzes was carried out in a new high-temperature TA P-2 reactor. A few studies report on TA P experiments for the partial oxidation of methane (4,5,9,(12)(13)(14)(15). In most of them rhodium-based catalysts were used. O nly two studies report on platinum catalysts (4,14). Mallens et al. (4) distinguished three different types of oxygen, each being involved in a specific step in the reaction network. Soick et al. (14) carried out a kinetic analysis of the adsorption of methane, carbon monoxide and carbon dioxide over a Pt/MgO catalyst. No further information on the reaction mechanism has been reported. In the present study platinum gauzes are used which provide a unique catalyst because of the extremely short length of the catalytic zone. Moreover, due to the low dispersion of these gauzes the number of platinum surface atoms is equal to or lower than the number of methane molecules per pulse. This avoids gradients in the concentrations of adsorbed species along the catalyst layer which is usually the case in TA P experiments and which can complicate the interpretation of the data. The objective of this study is to clarify the reaction mechanism of the partial oxidation of methane at short contact times over platinum and at high temperatures (800-1100 • C). The current study focuses on the different forms of oxygen species and their specific role in the mechanism.

EXPERIMENTAL

Woven platinum gauzes (supplied by K. A . R asmussen, Norway) were used as catalysts. The wires have a diameter of 90 µm, giving gauzes of 256 meshes/cm 2 . O ne single gauze (70-80 mg) is put in the reactor. The TA P-2 reactor is described elsewhere (16). For these experiments a new high temperature reactor was constructed that allows catalyst temperatures above 1000 • C. The high temperature reactor is a 9.0-cm-high quartz cylinder with an inner diameter of 14 mm. The quartz tube is held between two stainless steel flanges. Two water-cooled viton O -rings are used as seals. A large temperature gradient exists along the reactor axis. H owever, a zone of about 1 cm is uniform in temperature. The reactor is filled with quartz particles (0.2-0.3 mm) in which the gauze is placed about 4.0 cm above the reactor exit. A thermocouple is placed very close to the gauze inside the reactor.

The total number of Pt atoms in the reactor amounts to 0.5 mmol. The number of surface Pt atoms in the reactor has been estimated to be 10 nmol using data from (17) corrected for the different gauze diameters. The number of methane or oxygen molecules admitted per pulse (∼20-60 nmol) is higher than the number of active sites in the reactor (10 nmol). This implies that the reactant pulses largely influence the surface coverage. The alternating pulse experiments were conducted in the following way: oxygen was injected at t = 0 s, and then methane was injected at a later time, referred to as the pulse interval (between 0.05 and 3.0 s). The oxygen/methane ratio was approximately 1. The pulse size was always verified to be small enough to ensure Knudsen diffusion. The catalyst was prereduced in flowing hydrogen for 30 min at 900 • C.

RESULTS

A first experiment was carried out by pulsing methane and oxygen simultaneously over a reduced catalyst at 1000 • C. In Fig. 1 the conversions and yields are shown for the reactants and products as a function of the pulse number. A ll the oxygen is consumed on the first pulse followed by an almost linear decrease in the oxygen conversion until it levels out around 90 pulses. D uring the first 70 pulses the methane conversion is rather low (∼10% ) and hardly any products are observed. D uring this time the baseline at mass 2 increased slowly, indicating the cracking of methane with a slow formation of hydrogen. A t the time the oxygen conversion reaches a minimum (90-110 pulses) the conversion of methane increases rapidly and the production of all products is observed. A t this point the oxygen conversion increases slightly and the signals of all reactants and products remain stable over a long period of time. In all other experiments the catalyst was always preoxidized. Two methods of oxidation were used and the uptake was measured in both cases:

• By a steady flow of oxygen at 1 bar and at 900 • C.

• By pulsing oxygen on the prereduced catalyst while keeping the reactor under vacuum at 900 • C. O xygen was pulsed over the catalyst until the oxygen signal was stable.

U nder vacuum conditions surface oxygen readily desorbs from the platinum. This was observed by a small additional uptake of oxygen (typical 20-50% of a single pulse) after a few minutes under vacuum. The oxygen uptakes in the two cases are compared in Table 1. The amount of oxygen uptake differs significantly for the two pretreatments, but is in both cases far beyond the amount that corresponds to an adsorbed monolayer (assuming a Pt : O = 1 : 1 stoichiometry). The two different oxidation procedures did not result in any difference in catalyst performance for the oxidation of methane.

The partial oxidation of methane was further studied by using alternating pulses of oxygen and methane over preoxidized platinum at 900 • C at vacuum conditions. The only varied parameter was the time between the oxygen and the methane pulses (0.05-2.5 s). Figure 2 shows the calibrated reactant and product responses after an oxygen pulse at t = 0 s and a methane pulse at t = 1.0 s. This figure clearly shows that when methane is injected, the oxygen signal rapidly drops down to zero. The carbon monoxide, carbon dioxide and water signals appear immediately. When the oxygen signal has almost disappeared, the carbon dioxide and water signals start to decrease. The carbon monoxide signal continues to increase, and finally the hydrogen signal appears. The hydrogen production is clearly delayed until the oxygen signal is almost zero. The carbon monoxide and hydrogen signals reach their maxima at about the same time, long after the carbon dioxide and water. A t this time the methane signal is also rapidly decreasing. The decay of the carbon monoxide curve is rather slow and it tails for more than 2 s.

Figure 3 shows the conversion of methane and the selectivities to hydrogen and carbon monoxide as a function of the pulse interval. The methane conversion decreases from approximately 35 to 25% , but the hydrogen and carbon monoxide selectivities are strongly increasing with increasing pulse interval from 25 and 12% to 96 and 97% , respectively. A t a pulse interval of about 2 s, the conversion and selectivity reach steady values.

E xperiments using alternating pulses of oxygen and carbon monoxide over a preoxidized catalyst showed very low activity for the oxidation of carbon monoxide at pulse interval of 2 s. The carbon monoxide conversion was about 7% . A similar experiment with oxygen and hydrogen showed a hydrogen conversion of 2% .

To assess the extent of the reaction between carbon dioxide and methane, another similar alternating pulse experiment was carried out by pulsing oxygen followed 2 s later by a pulse of a mixture of methane (95% ) + carbon dioxide (5% ). No conversion of carbon dioxide was observed and the methane conversion was unaffected by the presence of carbon dioxide.

A lternating pulse experiments similar to those in Fig. 2 were carried out with 18 O 2 to further investigate the reactivity of the different oxygen species. The platinum gauze was preoxidized with pulses of 16 O 2 . For the alternating pulse experiments 18 O 2 was used on the first pulse. Scrambling into Two different pulse responses of C 16 O and C 18 O with two distinct time scales were observed for all pulse intervals as shown in Fig. 4 (curves 1 and 4, respectively). The response of C 16 O is significantly slower than the response of C 18 O and shows a large tailing. The selectivities of C 16 O and C 18 O as a function of the pulse interval are shown in Table 2. There is a clear increase toward C 16 O with increasing pulse interval which slowly levels off at intervals larger than 1 s. Concerning the carbon dioxide formation, C 18 O 2 is formed first, followed a little later by C 16 Its response is shown as curve 3 in Fig. 4. The shape of the response of a pulse of carbon monoxide (curve 3) is rather close to an inert argon pulse shape (curve 2) and does not show any tailing.

The same trend of results as described above was observed in the temperature interval between 800 and 1100 • C.

DISCUSSION

O n the basis of the current data the following reaction scheme is proposed:

C ads + O s CO ads [6] CO ads CO (g) [7] CO ads + O s CO 2 (g).

[8]

The oxygen uptake during the preoxidation of the catalyst was much larger than an adsorbed monolayer for both oxidation procedures (Table 1). This means that, in addition to a dissociative adsorption of oxygen on the platinum surface step [1], there is an uptake of oxygen by bulk platinum as described by step [2]. O xygen uptake in the same order of magnitude has previously been reported during oxygen pulsing over a platinum sponge in the TA P reactor (4). In a series of simultaneous pulses of oxygen and methane over a reduced platinum gauze (Fig. 1), oxygen is initially adsorbed at the platinum surface, followed by diffusion into the bulk rather than reacting with surface carbon. This suggests that surface oxygen is rapidly converted into bulk oxygen through step [2a] to oxidize more than the surface platinum layer. Then a stabilized oxidized state is reached defining the working platinum catalyst under the conditions applied in this study.

The observed oxygen scrambling into 16 O 18 O on pulsing 18 O 2 over a gauze saturated in bulk 16 O supports that 18 O 2 chemisorbs reversibly and dissociatively on platinum (reaction [1]) and that there is an exchange between the bulk and the surface (steps [2a] and [2b]).

The lack of tailing on the methane pulse indicates an irreversible methane adsorption (step [3]). The extent of the dissociation can not be directly deduced from these data. H owever, the marked tailing on the hydrogen signal as shown in Fig. 2 suggests that the partly dehydrogenated carbon species (CH 3 ,CH 2 , or CH ) has a significant surface lifetime. O n subsequent oxygen pulses small amounts of carbon monoxide and carbon dioxide were observed, but no significant amounts of water, indicating that step [3] is completed within a few seconds.

The adsorbed hydrogen atoms can either combine through step [4] into hydrogen gas or can react with surface oxygen to form water (step [5]). The pulse response of hydrogen over oxidized platinum indicated that its dissociative adsorption is fast and reversible. A similar analysis showed a reversible adsorption of water.

Comparison of the responses of C 18 O and C 16 O (Fig . 4, curves 1 and 4) clearly shows the rapid formation of the former, while the production of the latter continues for several seconds. Therefore the reaction between surface carbon and 18 O, mainly located on the surface as it was produced from a recent pulse of gaseous 18 O 2 , is relatively fast, whereas the reaction of surface carbon and 16 O, mainly originating from the bulk, is considerably slower but can continue for a long time due to the large quantities of bulk 16 O available. Surface oxygen is rapidly depleted once the gas phase oxygen concentration goes to zero due to its desorption under vacuum conditions. The difference in rate between the formation of C 18 O and C 16 O suggests a diffusion of 16 O from the bulk to the surface (step [2b]), typically a slow process. A n estimate for the time of oxygen diffusion is given by the difference between the average residence times of C 16 O and C 18 O and amounts to 0.3 s at 900 • C. A n order of magnitude diffusion coefficient of 10 -19 m 2 s -1 can be calculated if a diffusion distance of one platinum monolayer is assumed. This stems from the idea that 16 O, which is most likely to react, is located just underneath the surface. The incorporation of oxygen into the platinum bulk is an activated process since at relatively low temperatures the amount of oxygen uptake corresponds well with the number of surface platinum atoms.

The ratio C 18 O /C 16 O decreases with increasing pulse interval (Table 2). The residence time of surface oxygen is short due to the fast desorption rate at high temperatures under vacuum conditions. Increasing the pulse interval decreases the concentration of surface oxygen at the time of the injection of methane. Bulk oxygen is much more stable at these conditions, indicated by the negligible oxygen uptake that was observed after keeping an oxidized gauze under vacuum. So, by increasing the pulse interval, the diffusion of bulk oxygen and the recombination/desorption of 18 O are increasing, resulting in an increase in the surface concentration of 16 O at the expenses of 18 O .

The reactivity of bulk oxygen toward carbon monoxide and hydrogen is very low. This can be concluded from pulsing CO or H 2 over oxidized platinum without gas phase oxygen present. This resulted in very low conversions of approximately 2-5% , while 30% conversion was obtained for methane under identical conditions. In contrast the reactivity of surface oxygen toward carbon monoxide and hydrogen is very high since at short pulse intervals between O 2 and CO or H 2 , corresponding to high concentrations of surface oxygen, an almost complete conversion of carbon monoxide or hydrogen was observed. These conclusions are also supported by the labeled carbon dioxide product distribution. C 18 O 2 is the first product, due to the fast reaction between surface carbon and surface 18 O followed by the fast reaction between adsorbed C 18 O and surface 18 O . C 18 O 16 O follows a little later in time, due to the slower formation of C 16 O. Very little C 16 O 16 O is formed because of the low probability of the reaction between 16 O originating from the bulk and C 16 O .

A similar analysis for the formation of water was unfortunately impossible due to the overlap between the fragments of all (labeled) reactants and products and those of (labeled) water.

A ll this agrees well with the results of Lauterbach et al. (18), who investigated the properties of oxygen on Pt(111) surfaces by photoemission electron microscopy. They observed the diffusion of oxygen into the bulk platinum and referred to this species as subsurface oxygen. A negligible reactivity of subsurface oxygen with either carbon monoxide or hydrogen was observed. From their experiments as a function of temperature they estimated an activation energy of 63 ± 21 kJ mol -1 for the incorporation of oxygen into the platinum bulk.

The pulse response of carbon dioxide was very close to the response of argon, indicating that its adsorption can be neglected and that the reverse reaction of step ( 8) can be ignored. This can be understood from the fact that under the applied conditions the contact time of carbon dioxide is too short and the oxygen surface concentration is too large for any significant reaction of carbon dioxide to occur.

The maxima of the carbon dioxide and water shown in Fig. 2 are observed prior to those of carbon monoxide and hydrogen. Moreover, the responses of carbon dioxide and water are quite narrow. In contrast to previous interpretations (12,13) this does not imply that carbon monoxide and hydrogen arises from secondary reforming reactions involving carbon dioxide and water. D ue to the high concentration of surface oxygen, initially all the methane is oxidized to carbon dioxide and water. When the surface oxygen is depleted and no more gas phase oxygen is present to replenish the surface, the remaining surface carbon is selectively converted into carbon monoxide by the reaction with oxygen that is diffusing from the bulk to the surface. Therefore carbon monoxide and hydrogen exit the reactor after carbon dioxide and water.

The above developed reaction mechanism explains well the change in selectivity as a function of pulse interval as reported in Fig. 3. A s the concentration of surface oxygen decreases with increasing pulse interval the consecutive oxidation of carbon monoxide and hydrogen diminishes rapidly, resulting in very high selectivities. The product distribution is completely determined by the surface oxygen concentration. A t high surface oxygen concentrations the consecutive reactions of carbon monoxide and hydrogen are much faster than their corresponding desorption steps. H owever, at low surface oxygen concentrations the diffusion of bulk oxygen is slow compared to the residence time of carbon monoxide and hydrogen and negligible complete oxidation occurs. The surface lifetime of carbon is much longer and the diffusion of oxygen becomes then rate determining for the formation of carbon monoxide. The desorption of oxygen originating from the bulk was found to be rather slow under TA P conditions which is due to the slow diffusion rate of bulk oxygen.

Figure 3 shows that the methane conversion is above 35% at high surface oxygen concentrations whereas it approaches 25% when the surface is depleted of oxygen. In line with the above discussion, that difference could arise from the fact that at low oxygen surface coverage, the methane conversion is limited by the oxygen diffusion from the bulk to the surface while at high oxygen surface coverage, another step, faster than bulk oxygen diffusion, would limit the overall process. Since the number of gas molecules is higher than the number of platinum surface atoms and adsorption can be considered as very fast compared to the surface reaction, the number of adsorption sites determines the methane conversion. O r in the form of an equation:

X = N sites N CH 4 .
A t low surface oxygen concentrations the catalytic cycle of the site (the time required for a site to convert a methane molecule) is low due to the slow diffusion of oxygen. By the time the site has become available for reaction, the methane gas concentration has become zero. O n the contrary, at high surface oxygen concentrations the reoxidation of surface carbon is faster than the diffusion time of gas phase methane and the site is again available for methane adsorption, resulting in a higher methane conversion. Thus the ratio of the conversions at high and at low surface oxygen concentrations shows that at a conversion of 0.35 the same sites are working 1.4 times more than at a conversion of 0.25. The residence time of methane in the gas phase inside the catalyst zone for a thin catalyst layers can be calculated according to (20)

τ = ε LL 2 D ,
where τ is the average residence time (s), ε is the bed porosity (m 3 g m -3 r ), L the catalyst zone length (m), L 2 the inert zone after the catalyst (m), and D the Knudsen diffusion coefficient (m 3 g m -1 r s -1 ). For the current reactor configuration a residence time for methane of 0.5 ms was calculated. The apparent rate constant for the oxidation of surface carbon at high surface oxygen concentrations is thus >2000 s -1 .

A s mentioned in the introduction, Mallens et al. (4) reported on the existence of three different kinds of oxygen species on a platinum sponge under vacuum conditions at 800 • C. They referred to these three species as platinum oxide, dissolved oxygen, and chemisorbed oxygen. A ccording to their reaction network direct formation of carbon monoxide occurs via the oxygen present as platinum oxide and dissolved oxygen converts adsorbed carbon and hydrogen into carbon dioxide and water. The consecutive oxidation of carbon monoxide and hydrogen into carbon dioxide and water involves chemisorbed oxygen. The results presented in this study did not indicate the existence of dissolved oxygen. A fter an oxidation of the platinum gauze by a flow of oxygen, no further oxygen uptake was observed during a multipulse experiment, as was the case in the study by Mallens et al. The quantities of dissolved oxygen reported were rather low and the distinction between dissolved oxygen and platinum oxide is not very clear. The small amounts of additional oxygen uptake that was observed after exposure of an oxidized catalyst to vacuum conditions in this study is believed to correspond to a slow desorption of bulk oxygen and not to the formation of a different type of oxygen.

CONCLUSION

The partial oxidation of methane over platinum gauzes in the TA P reactor takes place through a direct path to carbon monoxide and hydrogen. A t high temperatures oxidation of platinum proceeds beyond a surface layer, well into the bulk phase. The product distribution is determined by the surface oxygen concentration. In the presence of gas phase oxygen the surface oxygen concentration is very high and consecutive oxidation of carbon monoxide and hydrogen takes place. In the absence of gas phase oxygen, the diffusion of oxygen from the bulk toward the surface is slower than the desorption of carbon monoxide and hydrogen and syngas is produced with high selectivities. Therefore a strict control of the surface oxygen reactivity is a prerequisite for any highly selective direct oxidation process to syngas over platinum. This can be attained by reducing either its concentration or its mobility. The former requires a process that avoids the simultaneous presence of methane and oxygen in the gas phase. This could be accomplished by nonstationary reactor operation with an alternating feed of methane and oxygen. Preliminary experiments still in progress in the laboratory already demonstrated the validity of that concept which has also been investigated by other research groups over different metal catalysts (21,11). The oxygen mobility can be modified by adding a different metal to the platinum. This is the case for platinum rhodium for which high syngas selectivities have been reported (22).

FIG. 1 .

 1 FIG. 1. Conversions and selectivities during simultaneous pulsing of CH 4 and O 2 over a prereduced Pt gauze at 1000 • C.

FIG. 2 .

 2 FIG. 2. Transient responses of reactants and products during an alternating O 2 -CH 4 pulse experiment over a preoxidized Pt gauze at 900 • C with a pulse spacing of 1.0 s.

16 O 18 O

 1618 (15 to 25% ) was observed in all experiments. No significant amounts16 O 2 were observed in these experiments.

FIG. 3 .

 3 FIG. 3. Conversion of methane and selectivity to synthesis gas as a function of the pulse spacing between O 2 and CH 4 over a preoxidized Pt gauze at 900 • C.

FIG. 4 .

 4 FIG. 4. H eight normalized carbon monoxide and argon signals at 900 • C. (1) The C 18 O transient response from alternating pulses of 18 O 2 followed by CH 4 at t = 0.1 s over a 16 O 2 preoxidized Pt gauze. (2) Transient response of argon. (3) The CO transient response from a pulse of CO over a prereduced Pt gauze. (4) The C 16 O transient response from alternating pulses of 18 O 2 followed by CH 4 at t = 0.1 s over a 16 O 2 preoxidized Pt gauze.

TABLE 1 Oxygen Uptake for the Different Applied Procedures on a Prereduced Pt Gauze at 900

 1

• C Steady flow of O 2 Pulses of O 2 O uptake 1.3 × 10 20 (molec.) 1 × 10 18 (molec.) O /Pt total 0.43 0.003 O /Pt surface 26000 200

TABLE 2 Product Distribution from a Pulse of 18 O 2 Followed by a Pulse of Methane over a Preoxidized ( 16 O) Pt Gauze at 900 • C

 2

	Time interval
	between 18 O 2
	and CH 4 (s)	X CH 4	S C18O	S C16O	S CO
	0.1	0.35	0.14	0.02	0.16
	0.5	0.31	0.26	0.18	0.44
	1.0	0.26	0.39	0.49	0.88
	2.0	0.24	0.34	0.62	0.96
	3.0	0.24	0.31	0.65	0.96

To whom correspondence should be addressed at Institut de R echerches sur la Catalyse, CNR S

, 2 Avenue A lbert E instein, F-69626 Villeurbanne-Cedex, France.

Fax: 33-472445399. E -mail: schuurma@ catalyse.univ-lyon1.fr.

ACKNOWLEDGMENTS

The financial support from the Norwegian R esearch Council is greatly acknowledged.


L B Råberg

Jensen B Morten

Unni Olsbye
email: unni.olsbye@kjemi.uio.no

C Daniel

Stéphane Haag

Claude Mirodatos

Anja Olafsen Sjåstad

M B Jensen

Propane dry reforming to synthesis gas over Ni-based catalysts: Influence of support and operating parameters on catalyst activity and stability

Keywords: Propane dry reforming, Methane dry reforming, Synthesis gas, Ni catalysts, Ni particle size, Support basicity

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Synthesis gas (CO and H 2 ) production represents a step of major importance in the conversion of natural gas to fuels and petrochemical products. Currently, the main process for converting natural gas to synthesis gas is reforming with steam or carbon dioxide. Supported Ni catalysts are the preferred choice as catalysts for the reforming reaction, due to the low cost and easy availability of Ni metal. The main disadvantage of Ni/support catalysts compared with noble metal catalysts is a lower stability with respect to carbon deposition (see [1] and Refs. therein). Many attempts have been made to improve the stability of Ni-based reforming catalysts, with using a basic support among the most successful approaches [1][2][3][4][5][6]. Whether the improved effect is due to oxygen transfer from the basic oxidetothemetal [7][8][9][10], to electronic effects [11] or to a higher dispersion of the metal [12] remains a matter of debate.

Although numerous articles exist on the catalytic dry reforming of methane to synthesis gas [START_REF] Pinaeva | Environmental Challenges and Greenhouse Gas Control for Fossil Fuel Utilization in the 21st Century[END_REF], only few studies have addressed the specificity of propane dry reforming using supported metal catalysts (e.g., Rh, Ru, Re) [START_REF] Solymosi | [END_REF][15][16]. Recently, we extended our previous studies of methane dry reforming over Ni-based catalysts toward propane dry reforming [10,[START_REF] Olafsen | Mirodatos[END_REF] occurring through the following reaction:

C 3 H 8 + 3CO 2 → 6CO + 4H 2 .( 1 )
This choice was based on the lower vapor pressure of propane compared with methane at ambient temperature, making it more suitable for use in, say, fuel cell cars with internal reforming.

In the present work, we address the possible correlation between support basicity and Ni/support activity and stability of catalysts for the dry reforming of propane to synthesis gas by a multitechnique approach. The activity, selectivity, and stability of supported Ni catalysts were investigated by catalytic testing in a conventional fixed-bed reactor. The support systems studied range from weakly acidic (SiO 2 ) to amphoteric [Al 2 O 3 , Mg(Al)O] to basic oxides [MgO, CaO]. The Ni particle size and reduction state was investigated by magnetic measurements performed before and during catalytic testing. Temperatureprogrammed desorption (TPD) using CO 2 as probe molecule was used to rank the supports with respect to basicity. Turnover frequencies (TOFs) for each catalyst were calculated on the basis of metal dispersion as a guideline for addressing the relative importance of support acid-base properties and metal distribution, respectively.

Experimental

Catalyst preparation and characterization

Ni/SiO 2 was synthesized by calcining SiO 2 -40 (Fluka, grain size 0.2-0.3 mm) at 825 • C for 10 h before impregnating it with an aqueous solution of nickel nitrate. After impregnation, the product was dried overnight at 100 • C and calcined at 750 • C for8hinair .

Ni/Al 2 O 3 was prepared by impregnating δ-Al 2 O 3 Puralox SCCa-90 (Condea) with an aqueous solution of nickel nitrate before drying it in air at 100 • C. The material thus obtained was then calcined for 8 h at 750 • Cinair .

Ni/MgO and Ni/CaO were prepared by synthesizing the corresponding hydroxides, Mg 0.98 Ni 0.02 (OH) 2 and Ca 0.98 Ni 0.02 (OH) 2 , by coprecipitation at constant pH (12.0 ± 0.1) at 40 ± 2 • C in a specially designed apparatus. During synthesis, the two reactant solutions A (OH -balanced by K + ; ≈250 ml) and B( M g 2+ + Ni 2+ or Ca 2+ + Ni 2+ balanced by NO - 3 ;8 0m l ) , as well as the product solution C (initially 160 ml of degassed deionized water), were kept at constant temperature in a water bath. The whole system was kept under inert conditions by Ar flushing. The pH was controlled through a feedback loop using a pH meter, which activated or stopped the pump for source solution A. Source solution B was added at a constant rate into the product solution C via a second pump. The mixing of reactant solutions (A and B) to yield the product solution (C) was completed within approximately 3 and OH -balanced with K + ; ≈730 ml) and B (Mg 2+ + Ni 2+ + Al 3+ balanced with NO - 3 ; 250 ml), as well as the product solution (initially containing 500 ml of deionized water), were kept at constant temperature in a water bath. The pH was controlled through a feedback loop using a pH meter, which activated or stopped the pump for the source solution A. Source solution B was added at a constant rate into the product solution via a second pump. The mixing of reactant solutions to yield the product solution was completed within approximately 3 h, followed by stirring for approximately 1 h. The product thus obtained was washed and dried overnight at 90 • C. Powder XRD showed that this hydrotalcite-like material was a pure phase (i.e., no reflections from the potential contaminant KNO 3 or from other phases were observed). The hydrotalcite-like material, Mg 5.88 Ni 0.12 Al 2 (OH) 16 CO 3 •nH 2 O, was subjected to one of several calcination programs, resulting in catalysts designated types I and II. The type I Ni/Mg(Al)O catalyst was obtained by first calcining the corresponding hydrotalcite-like phase in two steps at 400 and 800 • C under vacuum before activating it by Zr reduction using chips of Zr in evacuated sealed silica glass ampules. Zr was positioned in a crucible inside the ampule to separate it from the catalytic material. Both Zr and Ni/Mg(Al)O were held at 600 • C for 7 days before cooling to room temperature. Small amounts of oxygen were finally leached into the ampule to passivate the nickel particles. Ni/Mg(Al)O (type II) was prepared by performing the calcination and activation steps simultaneously by sending a flow of 10% H 2 in N 2 at 650 • C over the hydrotalcite-like material for 14 h in a fluidized-bed reactor before cooling it to ambient temperature under the same atmosphere. Then the material was passivated at room temperature in a 2% O 2 in N 2 flow for 1 h. The main difference in the activation procedures for types I and II is that type I was activated under water-free conditions, whereas type II was activated in the presence of humidity (because calcination and activation occur simultaneously).

Nickel loading (ICP-AES) and specific surface areas (1point BET) of the Ni/support systems (reduced and passivated, see below) are listed in Table 1. . H 2 was added to the feed to keep the catalysts in a reduced state even at moderate conversion, because it had previously been observed that a 1.9 wt% Ni/Mg(Al)O catalyst was oxidized with time on stream under aC 3 H 8 :CO 2 :N 2 (%) = 20:60:20 flow at 600 • C and moderate (<40%) conversion [10]. Water produced during the reaction was collected in a water trap, and the dry effluent was analyzed by a 3 columns Agilent micro gas chromatograph. The effluent was found to contain significant amounts of CO 2 ,C 3 H 8 ,H 2 , N 2 , CO, and CH 4 . In addition, C 2 H 4 was always observed, but at concentrations <1%.

Methane dry reforming was tested over Ni/Mg(Al)O II with CH 4 :CO 2 :H 2 :N 2 (%) = 10:30:10:50, with all other parameters being equal to those in the propane-reforming test.

Magnetic measurements

Magnetic measurements were performed at ambient temperature by the Weiss extraction method [START_REF] Selwood | Chemisorption and Magnetization[END_REF] in an electromagnet providing fields up to 21 kOe. Information on the degree of Ni reduction and on the mean diameter of the Ni metal particles (D) was extracted from the magnetization isotherm [START_REF] Dalmon | Catalyst Characterizaton, in: Physical Techniques for Solid Materials[END_REF].T h e degree of Ni reduction was calculated from the saturation magnetization (M s ). For ferromagnetic materials, a residual magnetic moment remains at zero field after exposure to a magnetic field; however, when the particle diameter drops below a certain size (D c ), the ferromagnetic particles become superparamagnetic [START_REF] Dalmon | Catalyst Characterizaton, in: Physical Techniques for Solid Materials[END_REF]. Absence of remanent magnetization (M r ) indicates superparamagnetic behavior [START_REF] Dalmon | Catalyst Characterizaton, in: Physical Techniques for Solid Materials[END_REF]. Under such conditions, the mean sizes of large (D 1 ) and small (D 2 ) Ni particles can be calculated according to Langevin's equation [START_REF] Dalmon | Catalyst Characterizaton, in: Physical Techniques for Solid Materials[END_REF]. The accuracy of the method is assumed to be within ±20% [START_REF] Selwood | Chemisorption and Magnetization[END_REF]. The mean particle diameter was calculated as D = (D 1 + D 2 )/2. In the applied magnetic setup, it was empirically determined that D c for Ni was 15 nm and that the ratio 2M r /M s gave the fraction of particles with larger size than the critical diameter.

Before the magnetic measurements, Ni/SiO 2 , Ni/Al 2 O 3 , Ni/MgO, and Ni/CaO were activated in a separate reactor at 650 • C for 14 h in a flow of 10% H 2 in N 2 before the reactor was cooled to room temperature and the surface of the generated nickel particles was passivated by a gas flow of 2% O 2 in N 2 for 1 h.

Then ∼200 mg of Ni/SiO 2 , Ni/Al 2 O 3 , Ni/Mg(Al)O (types I and II-c), Ni/MgO-b, and Ni/CaO were loaded into a specially designed fixed-bed reactor that was adapted for both magnetic measurements and catalytic testing [START_REF] Mirodatos | [END_REF]. All of the materials except Ni/CaO were first reduced in situ by heating to 600 • C (10 • C/min) in a flow of 45 Nml/min of 10% H 2 in N 2 (with Ni/CaO reduced in 10% H 2 and 30% CO 2 in N 2 ) before switching to N 2 , isolating the reactor, quenching it to room temperature, and loading it into the electromagnet to collect the magnetization isotherm. Thereafter the reactor was exposed to reforming conditions [600 • C, 45 Nml/min of C 3 H 8 :CO 2 :H 2 : N 2 (%) = 10:30:10:50 (GHSV = 13500 Nml/(h g cat ))] for 1 h. Then a magnetization isotherm was collected again, and so forth. A total of four magnetization isotherms were collected for each catalyst after it was subjected to reforming conditions for 5 min and 1, 2, and 14 h. Each measurement took approximately 1 h, including all of the above-described steps.

CO 2 temperature-programmed desorption

CO 2 TPD measurements were carried out in a quartz reactor loaded with 50 mg of catalyst calcined in situ at 650 • C, reduced under pure hydrogen flow at the same temperature, then cooled to room temperature and contacted with pure carbon dioxide flow for 60 min, and finally flushed with argon flow for 30 min. The TPD ramp under Ar flow was started (20 • C/min) from room temperature to 650 • C, with the evolving gases continuously monitored by online mass spectrometry. No other gases besides CO 2 were detected at the reactor outlet. By integrating the TPD curves, the amount of reversibly adsorbed CO 2 was deduced and normalized to the highest amount corresponding to the Ni/CaO sample.

Calculations

Propane and carbon dioxide conversions were calculated on basis of feed and effluent concentrations, using N 2 as an internal standard. Except for the first analyses in the test, the carbon mass balance was always 100 ± 1%. The carbon monoxide and methane selectivities were calculated on the basis of effluent concentrations.

The turnover frequency (TOF) of a catalyst is defined as the number of reactant molecules converted to products over one active catalyst site per second. In the present case, each Ni atom on the outer surface of the Ni particles is considered an active site. The TOF of the Ni/support catalysts was calculated as follows. The number of exposed Ni surface sites (N A,Ni ) for each catalyst was calculated from the Ni loading as well as the average Ni particle size determined from magnetic measurements (D). It was assumed that the Ni particles are spherical and that all outer surfaces are exposed to the reacting gases, which was demonstrated elsewhere as being a sound model [21]:

(2) m Ni = m cat • (wt% Ni)/100, (3) V tot,Ni = m Ni /ρ Ni , (4) V part,Ni = 4/3 • π • (D/2) 3 , (5)
S Ni = (V tot,Ni /V part,Ni ) • 4 • π • (D/2) 2 , (6) N A,Ni = S Ni /S spec,Ni , and (7) N A,Ni,red = N A,Ni • Reduction%/100,
where D is the average Ni particle diameter calculated from magnetic measurements, ρ Ni is the bulk density of Ni metal, V part,Ni is the volume of each Ni particle, V tot,Ni is the total volume of all Ni particles, S Ni is the exposed Ni surface of all Ni particles, S spec,Ni is the surface area of a Ni atom, and Reduc-tion% is the degree of Ni reduction calculated from magnetic measurements.

The TOF of each catalyst after a given time on stream (TOS) was calculated using the following formula:

(8) TOF (s -1 ) = (C C 3 H 8 /100%) • Flow C 3 H 8 • A/N A,Ni,red ,
where C C 3 H 8 is the propane conversion (in %), Flow C 3 H 8 is the propane flow over the catalyst (mol/s), and A is Avogadro's number.

Results

Catalytic testing

Results from the activity tests of all Ni/support catalysts for propane dry reforming at 600 • C are reported in Figs. 1 and2. Equilibrium conversions and coke (C) selectivity under the applied test conditions were C C 3 H 8 = 100%, C CO 2 = 57%, and S C = 45% [10]. Blank tests using the same feed and either an empty or an Mg(Al)O-filled reactor yielded zero and <0.3% propane conversion, respectively, at 600 • C [10].

Fig. 1 shows significant differences in both initial conversion and stability with time on stream (TOS) among the tested catalysts. The reproducibility of synthesis is good for the impregnated catalysts (Ni/SiO 2 and Ni/Al 2 O 3 ; Fig. 1); however, for the catalysts prepared by coprecipitation, significant variation (i.e., by a factor of two) in activity and stability was observed between different catalyst batches, as shown in Fig. 1 [Ni/Mg(Al)O II and Ni/MgO]. Therefore, a systematic study of the activity-Ni particle size relationship requires performing independent measurements leading to TOF values on the same batch of catalyst, as in the present investigation. Reproducibility tests using the same batch of a Ni/Mg(Al)O variant (heat-treated somewhat differently from types I and II) in different tests gave very similar results (not reported). Thus, the reproducibility of the test procedure, as well as the homogeneity within one batch of catalyst, are considered satisfactory. The results obtained for the Ni/MgO catalysts are similar to those observed for the Ni/Mg(Al)O catalysts. The least active catalyst (Ni/MgO-a) was rather stable during the test. The most active catalyst (Ni/MgO-b) had a period of initial deactivation, followed by a period of stable activity, before the activity dropped to zero after ∼3500 min TOS. Visual inspection of Ni/MgO-b after the test showed that the catalyst changed in color from dark gray to white, indicating Ni oxidation. The Ni/CaO catalyst deactivated slowly throughout the test.

The relationship between product selectivity and propane conversion for the different Ni-based systems is reported in Fig. 2. Note that each plot represents the product selectivity observed during the life duration tests reported in Fig. 1. CO and CH 4 were observed to be main C-containing products for all of the tested catalysts. At very high propane conversions (i.e., at the start of each test), the carbon mass balances were poor, indicating appreciable coking rates. Small amounts of ethene were formed over the catalysts, although the selectivity was always <1%. The theoretical product selectivity (S CO , S CH 4 ) versus propane conversion at equilibrium is shown as dotted lines in Fig. 2. Thermodynamic data were taken from [START_REF]NIST Standard Reference Database 85, NIST/TRC Table Database[END_REF].Only gaseous products were taken into account in these calculations. For Ni/SiO 2 and Ni/Al 2 O 3 , the methane selectivity approached equilibrium throughout the tests (8-100% propane conversion). For all Ni/Mg(Al)O and Ni/MgO samples, the methane selectivity was below equilibrium at all conversions and below 5% at 0-50% conversion. Ni/CaO also gave methane selectivities below equilibrium. However, the Ni/CaO test covered propane conversions only from 2 to 18%, for which even Ni/SiO 2 and Ni/Al 2 O 3 gave low methane selectivity.

The results of a comparative test of methane dry reforming over Ni/Mg(Al)O II-a are presented in Fig. 3. The catalyst used in this experiment originated from the same synthesis batch as the propane dry reforming test performed over Ni/Mg(Al)O II-a, plotted in Fig. 1. A comparison of the two tests shows that methane dry reforming had a higher conversion than propane dry reforming, even though methane dry reforming was thermodynamically limited under the conditions used (i.e., 70% methane conversion and 43% CO 2 conversion at equilibrium), whereas propane conversion was thermodynamically complete.

Magnetic measurements

Nickel particle size and degree of nickel reduction for each catalyst before and during testing are reported in Table 2.F o r the Ni/SiO 2 system, the average Ni particle size was observed to vary between 8.0 and 8.5 nm, whereas the degree of Ni reduc-tion was observed to decrease from 97% to 59% during the test period of 14 h. In addition, about 25-30% of the reduced Ni was found to be located in Ni particles larger than D c = 15 nm. Interestingly, the remanent magnetization was found to decrease slightly with increasing TOS, indicating that a smaller fraction of the reduced Ni particles were larger than D c after being exposed to reforming conditions. Another possible interpretation is that for very large particles, Weiss domains may have formed (equivalent to a collection of smaller particles) and the remanent magnetization may have decreased accordingly. According to Kroll et al. [START_REF] Kroll | [END_REF], the observation of Ni particles larger than D c for Ni/SiO 2 might depend on the pretreatment procedure. To correctly assess the morphological changes suggested by the magnetic measurements, we cross-checked our findings by TEM. Histograms of particle sizes indicated an average Ni particle diameter of 10.2 nm when only particles <15 nm were taken into account and of 13.9 nm when all particles were taken into account. It should be noted that when creating histograms from TEM measurements, all diameters were rounded up, leading to an overestimation of particle size. Therefore, the TEM results support the findings from the magnetic measurements.

The degree of Ni reduction in freshly reduced Ni/Al 2 O 3 was only 60%, most likely due to the formation of NiAl 2 O 4 spinel during the calcination step. According to the literature, spinel formation most likely occurs at elevated temperatures (see, e.g. [24]). The average Ni particle size and degree of Ni reduction of Ni/Al 2 O 3 decreased from 7.5 to 6 nm and from 60 to 37% during the first 2 h on stream. Due to plugging of the reactor by coke, no magnetic measurements were made after 14 h on stream.

For the Ni/Mg(Al)O I system, a slight decrease in the degree of Ni reduction (from 59 to 46%) was observed with TOS. At the same time, the average Ni particle size slightly decreased from 7 to 5.5 nm. No remanence was observed.

The Ni/Mg(Al)O II-c system showed a very low degree of Ni reduction (from 22 to 17%). The average Ni particle was 4 to 5.5 nm throughout the tests. It should be noted that the test duration of magnetic measurements was shorter than the lifetime observed for this catalyst batch in the ordinary test reactor, as presented in Fig. 1.

For Ni/MgO-b, the degree of Ni reduction was also very low throughout the experiment (from 22 to 20%). The average Ni particle size decreased slightly or remained constant between 4.5 and 5 nm during the first 2 h on stream. At TOS = 14 h, no propane conversion was observed, similar behavior to that observed for Ni/MgO-b during the lifetime study (Fig. 1).

The degree of Ni reduction on Ni/CaO was observed to increase slightly with TOS, with a maximum reduction of 89% at TOS = 14 h. The average Ni particle size increased from 8 to 11.5 nm, and the remanence increased from 1 to >13% during the test period.

CO 2 TPD measurements

CO 2 TPD curves obtained over each catalyst are plotted in Fig. 4. Most of the CO 2 TPD curves exhibited a single desorption peak with a maximum temperature within a 50-150 • C interval and spanning from 60 • C to about 500 • C, except for Ni/CaO with a TPD curve shifted to higher temperature, with a T max at 550 • C. Because Fig. 4 1). The normalized concentration of basic sites of supported Ni catalysts derived from the TPD curves integration is reported in Table 3.

TOF

TOF was calculated on the basis on the aforementioned Ni dispersion for each catalyst during testing, as presented in Table 3. Note that due to the very rapid initial deactivation of most catalysts, the first data points (obtained after ca. 2-6 min on stream) should be considered with care. That considered, the data in Table 3 show that after the initial deactivation (<2hon stream), the TOF for each catalyst is nearly constant with increasing TOS.

Plotting the TOF values calculated for the various catalysts after 1 h on stream as a function of the Ni particle diameter (i.e., Ni dispersion) leads to a straightforward relationship, as shown in Fig. 5. A decrease in Ni particle diameter in the range of 11-4.5 nm leads to a dramatic increase in TOF for the supported Ni catalysts, irrespective of the nature of the support. Although Fig. 5 presents the situation after only 1 h on stream, the trend is generally valid throughout the tests.

Comparing the mean stabilized TOFs calculated for the various catalysts as a function of the basicity ranking [SiO 2 < Al 2 O 3 < Mg(Al)O I and II < MgO < CaO] shows no monotonous relationship between intrinsic activity and basicity (Table 3). Instead, a volcano-type relationship is seen, with a clear maximum of the intrinsic activity for the intermediate system Ni/Mg(Al)O II-c (Fig. 6A).

Discussion

Validity of TOF calculations

The conclusions of this study rely strongly on the calculated TOFs, which directly reflect the intrinsic activity of active sites. Thus, a discussion of the validity of TOF calculations is warranted. We made two assumptions during the calculations:

1. We assumed first that only reduced Ni 0 is active for propane dry reforming, whereas nickel oxide (NiO) and nickel carbide (Ni x C), which are not detected by magnetic measure- [25]). For nickel carbide, the TOF calculations infer that the surface and bulk of each Ni particle are in the same zero-reduction state. Some previous reports contradicted this assumption, hypothesizing that whisker carbon Fig. 5. TOF versus D after 1 h on-stream for all tested catalysts. The black markers were calculated using particle diameters from magnetic measurements (cf. Tables 1 and2). The white marker was calculated by using the particle diameter estimated from TEM analysis for Ni/SiO 2 .

formation proceeded by C diffusion through Ni particles, forming a carbide-like phase while the particle surface was still active [26]. However, recent studies [27,28],u s i n g HTEM and EXAFS in combination with catalytic activity measurements, have concluded that whisker formation proceeds at the outer surface of the active metal particle, and that carbide formation inside the particles starts at the onset of catalyst deactivation. Our results (i.e., stable TOF with TOS for coking catalysts) tend to agree with the feature of surface-diffusing carbon with no major bulk-process carbidization of Ni into nickel carbide. However, the effective transfer of carbon atoms from the hydrocarbon dissociation active centers (metallic nickel) to the particle extrusion faces into whiskers might involve interstitial forms of car- 2). At D>D c ,t h e particle size cannot be determined from magnetic measurements. To investigate the effect of including larger particles, we determined TOF values for Ni/SiO 2 from both magnetic measurements and a full size distribution derived from TEM analysis. These two values are reported in Fig. 5 for comparison (with the open symbol representing data obtained by TEM). As can be seen, the TOF for Ni/SiO 2 increased slightly (from 25 × 10 -3 to 34 × 10 -3 s -1 ) when the entire particle size distribution was included. However, including the larger particles found by TEM analysis does not alter the general relationship between Ni dispersion and intrinsic activity shown in Fig. 5, irrespective of the nature of the support.

Support influence

As a general comment, the CO 2 TPD results reported in Table 3 and Fig. 4 suggest a correlation between the normalized concentration of basic sites (able to reversibly adsorb CO 2 ) and basic strength. Note, however, the much stronger basicity for the CaO support, in agreement with theoretical calculations reported previously [29]. The formed carbonate species' high stability and large amount despite the low surface area strongly suggest that in this case, bulk carbonatization was achieved under the prevailing TPD conditions.

Ni particle size

From the data reported in Table 2, together with the basicity measurements given in Table 3, an inverse volcano-type rela- In terms of the degree of reduction of Ni particles, a strong linear coupling between nickel dispersion and degree of reduction is also seen by comparing Figs. 6B and6C. This trend is characteristic of a strong Ni particle-support interaction, which keeps the smallest particles in a partially oxidized state, as in the case of the hydrotalcite-based supports.

Considering the reverse volcano-like relationship observed between Ni dispersion/reduction and surface basicity (Figs. 6B and6C), several reports have suggested a possible inverse correlation between the number of basic or defect support sites and Ni particle diameter [30,31]. In the present study, such a trend of decreased Ni particle diameter with increasing number of basic sites also might be considered when disregarding Ni/CaO. As a matter of fact, the number of basic sites calculated for Ni/CaO should be interpreted with care. CaO is a very basic oxide. Due to its strong basic properties and its very low surface area, CO 2 may interact strongly with sites other than defects, favoring the formation of bulk carbonates and possibly leading to an overestimation of surface defects.

Previous studies [7][8][9] found that Ni particles supported on a highly basic support like lanthanum oxide (La 2 O 3 ), which is almost completely carbonated under dry reforming conditions, are coated with a thin layer of lanthanum carbonate, which prevents sintering of particles and extraction of particles from the surface by carbon filaments during the reaction. In that case, surface area and defects are not concerned but only the faculty for carbonate phases to coat (several monolayers) and stabilize the metal particles.

To conclude, Ni particle distribution may be related to several parameters, most likely partly interconnected, which collectively can be considered a unique effect of metal-support interaction. These parameters include surface area, defect concentration, and basicity via carbonate coating. The dispersion and degree of reduction of nickel can be considered indicators of these various parameters.

Catalyst activity

The results presented in Fig. 5 indicate unambiguously that a catalyst's TOF (i.e., nickel intrinsic activity) is strongly related to Ni particle size. Indeed, the graph displays an exponential shape, with a sharp increase in TOF at average Ni particle diameters below 7 nm. A similar trend was recently reported by Wei and Iglesia [12] for the dry reforming of methane over supported metal catalysts. Our data expand their findings while using a different method for determining exposed Ni area (mag-netic measurements vs H 2 chemisorption) and a different reaction (propane vs methane). In addition, Wei and Iglesia used only one support for Ni catalysts (i.e., MgO), whereas our study covers a wide range of supports. Moreover, the relationship reported by these authors, as well as by Parmaliana et al. [30] in an earlier study of Ni particle size effect in methane steam reforming over Ni/MgO catalysts, was not as exponential as that observed in the present study.

Theoretical and experimental studies of model Ni surfaces have revealed that Ni defects have lower activation barriers toward methane activation than Ni terraces [32]. A recent study that used theoretical calculations to compare the activation barriers of C-C and C-H bond cleavage in ethene over Ni (111) and Ni(211) surfaces demonstrated that the activation barrier of C-C bond cleavage is far more structure-sensitive than that of C-H bond cleavage [33]. The results obtained in this study tend to indicate a stronger surface sensitivity of the propane dry reforming reaction compared with methane dry reforming, suggesting that either C-C bond cleavage or C oxidation is the rate-determining step for propane reforming, whereas only the C-H bond activation is of concern for methane dry reforming.

In this context, it is interesting to observe that the TOF versus D relationship depicted in Fig. 5 is very similar to that reported by Carter et al. for ethane hydrogenolysis over 10 wt% Ni/SiO 2 -Al 2 O 3 with D Ni ≈ 2-10 nm at 267 • C [34]. This result could indicate that the rates of these two reactions are limited by the same reaction step, which would then not involve an oxygen-containing species. The origin of the structure sensitivity of both C-C and C-H bond activation should be related to the higher efficiency of defect Ni sites, which are numerous on small particles, compared with the denser and smoother planes dominant in larger particles. Steric factors favoring the C-C bond cleavage on out-of-plane Ni atoms might be the origin of such structure sensitivity; this effect would be much less prominent for the C-H bond cleavage in the smaller methane molecule.

Catalyst stability

Three types of activity versus TOS behaviors were observed in this study:

1. Ni/SiO 2 and Ni/Al 2 O 3 . As can be seen in Tables 2 and3 1 and2, Ni/CaO displayed a stable reduction state with TOS but with increasing Ni particle size and decreasing TOF throughout the test. This result clearly indicates that the Ni particles sintered with TOS, most likely due to the very poor surface area of that support, preventing any stabilization effect via particle-support interaction. Conversely, the blocking of Ni particles due to coke formation is limited, in line with the above statement because the particles that are not stabilized on the support can be hardly deactivated by encapsulating carbon. The decrease in TOF with an increasing particle size is in agreement with the trend reported in Fig. 5.

Finally, comparing the CH 4 and CO selectivities over the various catalysts during aging tests (Fig. 2), note that the CH 4 selectivity over Ni/SiO 2 and Ni/Al 2 O 3 approached equilibrium throughout the tests, but was far inferior to the equilibrium amount over all catalysts with more basic carrier materials [Mg(Al)O, MgO, CaO]. This difference may be simply related to the surface acidity of the Ni/SiO 2 and Ni/Al 2 O 3 supports. An early effect would be the favoring of direct cracking of propane molecules into methane over the support acid sites, after an initial deactivation of the Ni particles. Another effect would be the favoring of cracking of propane into methane on the metallic surface due to the more pronounced metallic state of the Ni particles on these catalysts and their poor metal dispersion, whereas the partially oxidized state of nickel over the more basic systems prevents cracking of propane into methane, in turn favoring its partial oxidation into carbon monoxide. The relative importance of these two effects is likely to depend on the Ni dispersion and on the effective acidity of the support.

Conclusion

In this work, propane dry reforming over Ni based catalysts was found to be highly structure-sensitive, as demonstrated by an exponential relationship between intrinsic activity (TOF) and mean particle size of metallic nickel particles. The structure sensitivity was more marked than that for methane dry reforming [12,30] due to the specificity of C-C bond cleavage compared with C-H bond cleavage. In parallel, the intermediate basicity observed on such systems as Ni/Mg(Al)O and Ni/MgO ensures the highest stability and catalytic performance of the active Ni phase, including low methane selectivity. This intermediate basicity might correspond to an optimum of CO 2 /carbonate equilibrium, ensuring permanent feeding of the Ni surface with oxygen from carbonate decomposition and sufficiently stable carbonate to protect the Ni particles from deactivation by carbon buildup and metal sintering. Our findings indicate that the best-performing systems displayed the largest nickel dispersion combined with moderate support basicity. Monitoring this complex interconnection of various parameters, including surface area, defect concentration, and basicity, and their varying impacts on dry reforming of propane or methane, is a prerequisite for optimizing catalytic systems and process operating conditions for reforming real feedstocks of natural gas containing methane, higher hydrocarbons, and CO 2 .

  1 h, followed by stirring for approximately 1 h for the Mg 0.98 Ni 0.02 (OH) 2 and 19 h for the Ca 0.98 Ni 0.02 (OH) 2 . The products thus obtained were washed in a diluted NH 4 OH solution and dried overnight at 90 • C. Phase purity of the samples was confirmed by powder X-ray diffraction (XRD). No reflections from KNO 3 or carbonate phases of Mg or Ca were observed. The hydroxides were converted into the corresponding oxides in a fluidized-bed reactor maintained at 650 • C (Mg-Ni-O) and 750 • C (Ca-Ni-O) in N 2 atm for 14 h. The samples were then cooled to room temperature under inert conditions. The Ni/Mg(Al)O catalysts (types I and II) were prepared by first synthesizing the corresponding hydrotalcite-like material with nominal composition Mg 5.88 Ni 0.12 Al 2 (OH) 16 CO 3 •nH 2 O and (Mg + Ni)/Al = 3 by coprecipitation at constant pH (11.5 ± 0.1) and temperature (40 ± 2 • C) in the same apparatus described earlier. During synthesis, the two reactant solutions A (CO 2-

  Fig. 1 further shows that Ni/SiO 2 and Ni/Al 2 O 3 had a strong initial deactivation before their performance stabilized. The activity and stability of the Ni/Mg(Al)O II system varied significantly between different batches [Ni/Mg(Al)O II-a, b, and c]. The most active catalyst, Ni/Mg(Al)O II-c, showed initial deactivation, then reached a plateau of stable conversion, and finally deactivated completely after ∼4000 min TOS. The Ni/Mg(Al)O II-a batch with medium activity showed initial deactivation followed by stable activity, whereas the least active batch (batch b) was stable throughout the test. The activity of the Ni/Mg(Al)O I catalyst (activated by Zr reduction) was similar to that of the corresponding material activated by H 2 reduction, with medium activity [Ni/Mg(Al)O II a]. This sample displayed remarkable stability with TOS over the 8000-min test period.

Fig. 1 .

 1 Fig. 1. Dry reforming of propane activity tests of Ni/support catalysts at 600 • C. Feed composition: C 3 H 8 :CO 2 :H 2 :N 2 (%) = 10:30:10:50.

Fig. 2 .

 2 Fig. 2. Product selectivity versus propane conversion obtained over Ni/support catalysts at 600 • C. Calculated theoretical equilibrium selectivities at each conversion are shown as dotted lines.

Fig. 3 .

 3 Fig. 3. Dry reforming of methane over Ni/Mg(Al)O II-a at 600 • C. Feed composition: CH 4 :CO 2 :H 2 :N 2 (%) = 10:30:10:50.

  presents the CO 2 desorption versus temperature curves, the continued CO 2 desorption from Ni/CaO at 650 • C is not shown in the graph. It can be observed from the graph that the number and strength of basic sites increase from Ni/SiO 2 via Ni/Al 2 O 3 , Ni/Mg(Al)O, and Ni/MgO, to Ni/CaO. It also can be seen that the two close hydrotalcite materials, Ni/Mg(Al)O I and II-c, have different basic site concentrations, in line with the difference in BET surface areas (Table

Fig. 4 .

 4 Fig. 4. CO 2 temperature-programmed desorption curves for the catalysts used in this study.

Fig. 6 .

 6 Fig. 6. (A) TOF after 1 h on-stream for all tested catalysts, (B) average Ni diameter and (C) degree of Ni reduction; all versus normalized concentration of basic sites from CO 2 TPD measurements. The values of Ni particle size and Ni reduction are those reported for reduced catalysts, see Table 2(except for Ni/Mg(Al)O II, where Ni diameter after 2 min TOS is reported). bon (leading to still-ferromagnetic Ni) throughout several surface layers of the Ni particles. 2. A second factor related to TOF calculations is the specific contribution of Ni particles larger than the critical diameter (D c = 15 nm) detected for the case of the two extreme systems Ni/SiO 2 and Ni/CaO (Table2). At D>D c ,t h e particle size cannot be determined from magnetic measurements. To investigate the effect of including larger particles, we determined TOF values for Ni/SiO 2 from both magnetic measurements and a full size distribution derived from TEM analysis. These two values are reported in Fig.5for comparison (with the open symbol representing data obtained by TEM). As can be seen, the TOF for Ni/SiO 2 increased slightly (from 25 × 10 -3 to 34 × 10 -3 s -1 ) when the entire particle size distribution was included. However, including the larger particles found by TEM analysis does not alter the general relationship between Ni dispersion and intrinsic activity shown in Fig.5, irrespective of the nature of the support.

  tionship between Ni particle size and surface basicity can be established, as depicted in Fig. 6B. The smallest Ni particle sizes are obtained for the medium basic Mg(Al)O-II. In turn, larger Ni particle diameters are observed for both the amphoteric Ni/Al 2 O 3 and acidic Ni/SiO 2 and the more basic supports, Ni/MgO and Ni/CaO. The BET surface areas of the catalysts decrease in the order Ni/Mg(Al)O II-c > Ni/Mg(Al)O I > Ni/MgO-b > Ni/Al 2 O 3 ∼ Ni/SiO 2 ≫ Ni/CaO. A similar order is found for the Ni particle size, except for the MgO support, which gives an intermediate Ni particle size between Ni/Mg(Al)O I and Ni/Mg(Al)O II-c despite having a lower specific surface area.

Table 1

 1

	Ni loading and specific surface area of passivated Ni/support systems
		Ni loading (wt%)	Specific surface area (m 2 /g)
	Ni/SiO 2 Ni/Al 2 O 3 Ni/Mg(Al)O I	1.9 1.8 1.6	100 100 131
	Ni/Mg(Al)O II-c	1.8	173
	Ni/MgO-b	1.8	117
	Ni/CaO	2.8	16
	2

Table 2

 2 Nickel phase characteristics extracted from magnetic measurements

		Degree of reduction (%)	D 2 (Å)	D 1 (Å)	D (nm)	2M r /M s
	Passivated	87	Ni/SiO 2 57	114	8.5	0.294
	H 2 /N 2 (10/90)	97		47	111	8	0.285
	C 3 H 8 /CO 2 /H 2 /N 2 10/30/10/50 TOS = 5 min 86 TOS = 1 h 82 TOS = 2 h 96 TOS ≈ 14 h 59	58 52 56 59	113 108 109 109	8.5 8 8 8.5	0.291 0.249 0.253 0.255
	Passivated	53	Ni/Al 2 O 3 58	99	8	0.012
	H 2 /N 2 (10/90)	60		57	97	7.5	0.008
	C 3 H 8 /CO 2 /H 2 /N 2 10/30/10/50 TOS = 1 h 34 TOS = 2 h 37	47 44	80 79	6.5 6	0.024 0.022
	Passivated	51	Ni/Mg(Al)O I 42	89	6.50
	H 2 /N 2 (10/90)	59		51	89	7	0
	C 3 H 8 /CO 2 /H 2 /N 2 10/30/10/50 TOS = 6.5 min 58 TOS = 1 h 50 TOS = 2 h 50 TOS ≈ 14 h 46	41 50 44 49	79 68 69 66	6 6 5.50 0 0 5.50
	Passivated	11	Ni/Mg(Al)O II-c 38	69	5.50
	H 2 /N 2 (10/90)	22		34	n.c.	n.c.	0
	C 3 H 8 /CO 2 /H 2 /N 2 10/30/10/50 TOS = 2 min 17 TOS = 1 h 13 TOS = 2 h 15 TOS ≈ 14 h 17	33 41 44 45	49 48 n.c. n.c.	4 4.50 0.014 n.c. n.c. n.c. n.c.
	Passivated	15	Ni/MgO-b 49	68	6	0
	H 2 /N 2 (10/90)	22		38	55	4.5	0.034
	C 3 H 8 /CO 2 /H 2 /N 2 10/30/10/50 TOS = 5 min 21 TOS = 1 h 24 TOS = 2 h 20	39 48 45	67 50 47	5 5 4.5	0.023 0.032 0.005
	Passivated	72	Ni/CaO 62	114	8	0.0035
	H 2 /N 2 (10/90)	84		62	114	8	0.010
	C 3 H 8 /CO 2 /H 2 /N 2 10/30/10/50 TOS = 5 min 79 TOS = 1 h 82 TOS = 2 h 84 TOS ≈ 14 h 89	74 84 76 95	129 132 133 138	10 11 10.5 11.5n . c . n.c. 0.117 0.134
	n.c. = not calculated.

Table 3 Basicity

 3

	Catalyst	Concentration of basic sites, based on CO 2 TPD data a	TOS (min)	D (nm)	C 3 H 8 conversion (%)	TOF × 10 3 (s -1 )
	Ni/SiO 2	0.02	5 60	8.52 7 8 19	3 7 25
			120	8	17	19
			≈840	8.51 1	2 2
	Ni/Al 2 O 3	0.24	60 120	6.51 6 6 15	4 4 36
	Ni/Mg(Al)O I	0.39	6.5	6	43	71
			60	6	38	73
			120	5.53 6	6 3
			≈840	5.53 4	6 5
	Ni/Mg(Al)O II-c 0.54	2	4	50	172
			60	4.5	37	188
	Ni/MgO-b	0.84	5	5	68	233
			60	5	48	145
			120	4.5	47	153
	Ni/CaO	1.0	5 10	15	18
			60 11	14	18
			120 10.51 2	1 4
			≈840 11.51 0	1 2
	a The concentration of basic sites has been normalized to Ni/CaO.
	ments because they are nonferromagnetic, are not included
	in the calculations. The assumption that NiO is catalyti-

from CO 2 TPD measurements. Turnover frequencies (TOF) versus time-on-stream (TOS) cally inactive is supported by several studies showing that the methane-reforming activity of an unreduced Ni catalyst is far inferior to that of a reduced Ni catalyst (see, e.g.,

  and hence TOF after a rapid initial deactivation, whereas the less active Ni/Mg(Al)O II-b catalyst displayed stable activity throughout the test (Fig.1; Tables2 and 3). The steady-state level was different for each catalyst, increasing with a decrease in average Ni particle size. The rapid initial deactivation observed for these catalysts, as well as for the Ni/SiO 2 and Ni/Al 2 O 3 catalysts, was in all cases accompanied by a poor carbon balance, indicating that the initial deactivation was essentially due to carbide and/or coke formation. Considering now the good steady-state performance of the catalysts that display moderate basicity, it has previously been shown that CO 2 species trapped as unstable surface carbonate on a Mg(Al)O support are able to oxidize the Ni surface during pulse experiments in a temporal analysis of products (TAP) reactor[10]. We thus can deduce that such an oxygen spillover arising from the continuous decomposition of surrounding surface carbonates improves the C/H/O balance on the Ni surface, favoring CO (versus carbon deposits) formation. However, overly efficient oxygen transfer from the support to the Ni surface could contribute to the sudden death of the most active Ni/ MgO-b and Ni/Mg(Al)O II-c catalysts due to Ni oxidation after ∼60 h on stream. 3. Ni/CaO. As can be seen from Tables

	,af-
	ter a rapid initial deactivation, these two catalysts reached
	a stable TOF, but with a prominent decrease in the frac-
	tion of ferromagnetic Ni with TOS, accompanied by a
	slight decrease in the size of the ferromagnetic Ni particles
	for Ni/Al 2 O 3 . This result is consistent with a deactivation process related to coke formation, which has been reported
	to proceed more easily on larger Ni particles [35]. As such,
	this effect could derive from the buildup of an outer nonfer-

romagnetic carbide-like thick layer with TOS, which would decrease the fraction of active metal. It is interesting to note that the TOF remained constant (Ni/SiO 2 ) or increased slightly (Ni/Al 2 O 3 ) with TOS, again supporting the tight correlation between TOF and Ni particle size. 2. Ni/Mg(Al)O I and Ni/Mg(Al)O II-a and b. The two first catalysts reached rapidly steady conversion, Ni particle size distribution,

Acknowledgment

This project was supported by the Norwegian Research Council under the KOSK program (contract 139193/431).