**3. Results**

## *3.1. Catalyst Characterization*

The XRD patterns of Ni/M*x*O*y* catalysts are shown in Figure 1. The characteristic peaks located at the diffraction angles of 32.7◦, 37.7◦, 39.9◦, 45.8◦ and 67.5◦ were appeared for Ni/Al2O3 which are attributed to (220), (311), (222), (400) and (440) facets of cubic Al2O3 (JCPDS Card No. 10-425), respectively. The XRD spectra recorded for Ni/CeO2 catalyst consisted of peaks located at 2θ = 28.79◦, 33.26◦, 47.62◦, 56.34◦, 59.1◦, 69.42◦, 76.66◦, 79.11◦ attributed to (111), (200), (220), (311), (222), (400), (331) and (420) planes of the cubic CeO2 (JCPDS Card No. 2-1306), whereas in the case of Ni/ZrO2 catalyst numerous peaks were detected on the XRD pattern. In particular, the peaks detected at 24.2◦, 24.5◦, 28.5◦, 31.7◦, 34.5◦, 35.5◦, 38.9◦, 41.0◦, 41.5◦, 45.1◦, 45.8◦, 49.5◦, 50.5◦, 54.4◦, 55.7◦, 57.5◦, 60.2◦, 62.1◦, 63.2◦, 66.0◦, 71.6◦, 75.3◦ correspond to (011), (−110), (−111), (111), (002), (200), (021), (−211), (−121), (112), (211), (022), (−221), (202), (013), (212), (−302), (113), (311), (−321), (−104), (−140) planes of monoclinic ZrO2 (JCPDS Card No. 13-307). When Ni was supported on YSZ, the XRD pattern was characterized by reflections at 30.4◦, 35.1◦, 50.4◦, 59.9◦, 62.9◦ and 74.0◦ attributed to (111), (200), (220), (311), (222) and (400) planes of YSZ (JCPDS Card No. 82-1246), respectively.

**Figure 1.** XRD patterns obtained from 5 wt.% Ni catalysts supported on the indicated commercial metal oxide carriers. The reflection planes of anatase (◦) and rutile (\*) TiO2 phases are indicated in the diffractogram of Ni/TiO2 catalyst.

Results obtained from Ni/TiO2 catalyst showed that the sample consisted of TiO2 in both its anatase and rutile form exhibiting peaks at 2θ = 25.6◦ (101), 37.2◦ (103), 38.2◦ (004), 38.9◦ (112), 48.4◦ (200), 54.3◦ (105), 55.4◦ (211), 63.1◦ (204), 69.3◦ (116), 70.7◦ (220), 75.4◦ (215) and 76.4◦ (301) for anatase (JCPDS Card No. 21-1272), and at 2θ = 27.6◦ (110), 36.3◦ (101), 41.6◦ (111), 44.3◦ (210), 54.6◦ (211), 56.9◦ (220) and 64.3◦ (310) for rutile (JCPDS Card No. 21-1276).

In the case of Ni/TiO2 and Ni/YSZ catalysts an additional weak peak located 44.5◦ was appeared corresponding to Ni (111) plane (JCPDS Card No. 04-0850). The absence of peaks corresponding to metallic Ni for the rest catalysts investigated is due to the low Ni loading and/or particle size. The primary crystallite size of the supports was estimated according to Scherrer's formula at the diffraction angles corresponding to (440) plane for Al2O3, (−111) plane for ZrO2, (111) plane for CeO2, (111) plane for YSZ and (101) plane for TiO2, and it was found to be 6.0 nm for Ni/Al2O3, 10.5 nm for Ni/CeO2, 15.0 nm for Ni/ZrO2, 20.9 nm for Ni/YSZ and 21.8 nm for Ni/TiO2 (Table 1).

The SSAs of Ni catalysts supported on metal oxide (M*x*O*y*) carriers were estimated equal to 39 m2/g for Ni/ZrO2, 11 m2/g for Ni/YSZ, 66 m2/g for Ni/Al2O3, 39 m2/g for Ni/CeO2 and 41 m2/g for Ni/TiO2 (Table 1).

**Table 1.** Physicochemical properties of supported Ni (5 wt.%) catalysts and their apparent activation energies for propane steam reforming reaction.


(a) Specific surface area, estimated with the BET method. (b) Primary crystallite size of <sup>M</sup>*x*O*y*, estimated from XRD line broadening. (c) Dispersion and mean particle size of Ni, estimated from selective chemisorption measurements.

Results of Ni dispersion (DNi) and mean particle size (dNi) estimated from CO chemisorption meaurements are summarized in Table 1. Generally, low Ni dispersions were estimated for all the investigated catalysts, most possibly due to the high Ni content (5 wt.%) in agreemen<sup>t</sup> with previous studies [4,6]. Higher Ni dispersion of 11.9% and smaller particle size of 8.5 nm was found for Ni/CeO2 catalyst, whereas Ni/TiO2 exhibited the lowest value of Ni dispersion of 2.8% and the largest particle size of 36.1 nm.

Figure 2 shows representative TEM images and selected area electron diffraction (SAED) patterns obtained from Ni/YSZ, Ni/CeO2 and Ni/TiO2 catalysts. In all cases Ni particles appear as fairly homogeneously distributed spherical particles with average sizes of 20 nm for Ni/YSZ, 10 nm for Ni/CeO2 and 30 nm for Ni/TiO2, in agreemen<sup>t</sup> with those estimated according to CO chemisorption measurements (Table 1).

**Figure 2.** TEM images and Selected Area Electron Diffraction (SAED) patterns obtained for (**A**) 5%Ni/YSZ, (**B**) 5%Ni/CeO2 and (**C**) 5%Ni/TiO2 catalysts. Ni particles are indicated with red arrows.

It should be noted that, based on the results of Table 1, the mean particle size of Ni is similar to the average size of the corresponding metal oxide used as support for all the investigated catalysts. This may hinder distinguishing between Ni particles and the MxOy carrier in TEM images. Thus, SAED analysis was performed to calculate the d-spacing in an attempt to further discerned Ni particles from those of metal oxide support (Figure 2, Table 2). Results indicated that, in all cases, Ni particles are present in TEM images as evidenced by the appearance of the (111) plane of Ni (dspacing = 0.21 nm, JCPDS No 1-1258). The appearance of the (101) (dspacing = 0.35 nm, JCPDS No 1-562) and (103) (dspacing = 0.24 nm, JCPDS No 1-562) planes of TiO2, the (111) (dspacing = 0.30 nm, JCPDS No 30-1468) and (200) (dspacing = 0.26 nm, JCPDS No 30-1468) planes of YSZ, as well as the (111) (dspacing = 0.31 nm, JCPDS No 1-800) and (200) (dspacing = 0.27 nm, JCPDS No 1-800) planes of CeO2 also confirmed the presence of the metal oxides.


**Table 2.** Selected area electron diffraction (SAED) analysis of TEM images obtained for 5%Ni/YSZ, 5%Ni/CeO2 and 5%Ni/TiO2 catalysts.

#### *3.2. Influence of the Nature of the Support on Catalytic Activity*

The influence of the nature of the support on catalytic performance for the propane steam reforming reaction has been investigated using Ni catalysts (5 wt.%) supported on five different commercial metal oxide powders (ZrO2, YSZ, TiO2, Al2O3, CeO2). The results obtained are shown in Figure 3A, where propane conversion is plotted as a function of reaction temperature. It is observed that, among the investigated catalysts, Ni/ZrO2 is the most active one, exhibiting measurable C3H8 conversions at temperatures higher than 400 ◦C and achieving complete conversion at 750 ◦C. Although Ni/YSZ is activated at similar temperatures as Ni/ZrO2, the conversion curve of propane is shifted toward higher temperatures. This is also the case for Ni/Al2O3 and Ni/CeO2 catalysts, which present similar performance. The latter catalysts are less active than Ni/YSZ below 550 ◦C, but are able to reach higher *X*C3H8 at higher temperatures. The titania-supported catalyst becomes active above 500 ◦C, with the propane conversion curve being shifted at remarkably higher temperatures. In all examined cases, the carbon balance was satisfactory, with a deviation of <1%.

Results of specific reaction rate measurements are presented in the Arrhenius diagram of Figure 3B, where it is observed that the TOF of propane conversion increases in the order Ni/TiO2 < Ni/CeO2 < Ni/Al2O3 < Ni/YSZ < Ni/ZrO2, with its value at 450 ◦C being more than one order of magnitude higher when Ni is dispersed on ZrO2 compared to TiO2, and approximately 2.5 times higher than that of Ni/Al2O3. It should be mentioned that, as discussed above, the mean particle size of Ni varies significantly for the investigated catalysts from 8.5 nm for Ni/CeO2 to 36.1 nm for Ni/TiO2. If the Ni particle size were similar for this set of catalysts then the order of catalytic activity could be somewhat different. Interestingly, no trend was observed between the specific reaction rate and Ni particle size or MxOy crystallite size or MxOy surface area. This indicates that either

these parameters do not affect catalytic activity or most possibly each of them contributes in a different manner to the reaction rate, resulting in the observed catalyst ranking. It should be noted that all catalysts have been reduced at 400 ◦C prior to physicochemical characterization measurements. Although the values of SSA or dMxOy or dNi may were different if catalyst pre-reduction was carried out at 750 ◦C, which is the onset reaction temperature for catalytic performance experiments, the trend of catalytic properties with respect to the nature of the support is not expected to vary due to the catalyst pretreatment at different temperatures, at least to such an extent that would affect the catalyst ranking for the propane steam reforming reaction.

**Figure 3.** (**A**) Conversions of C3H8 as a function of reaction temperature and (**B**) Arrhenius plots of turnover frequencies of C3H8 conversion obtained over Ni catalysts (5.0 wt.%) supported on the indicated commercial oxide carriers. Experimental conditions: Mass of catalyst: 150 mg; particle diameter: 0.15 < dp < 0.25 mm; Feed composition: 4.5% C3H8, 0.15% Ar, 44% H2O (balance He); Total flow rate: 250 cm<sup>3</sup> min−1.

The apparent activation energies (Ea) of the propane steam reforming reaction were calculated from the slopes of the fitted lines of Figure 3B. The results showed that the nature of the metal oxide carrier significantly affects Ea, which takes values between 102 kJ/mol for Ni/CeO2 and 154 kJ/mol for Ni/ZrO2 without presenting any trend with respect to catalytic activity (Table 1). This can be explained taking into account that, as it will be discussed below, several reactions run in parallel under the present experimental conditions each one of which is influenced by the nature of the support in a different manner resulting in the observed random variation of Ea with catalytic activity. The results are in agreemen<sup>t</sup> with our previous study where it was found that the apparent activation energy for the reaction of steam reforming of propane over Rh catalysts supported on a variety of metal oxides does not present any trend with the activity order [11].

Figure 4 shows the selectivities toward reaction products as a function of temperature over the supported Ni catalysts investigated. In all cases the main products detected were H2, CO2, CO and CH4 with their selectivities being significantly varied with temperature. In particular, for Ni/ZrO2 catalyst (Figure 4A), both hydrogen (*S*H2) and CO2 (*S*CO2) selectivities decrease from 99 to 78% and from 98 to 58%, respectively, with increasing temperature in the range of 390–505 ◦C followed by an increase of methane selectivity (*S*CH4) up to 32.5%, indicating the occurrence of CO2 methanation reactions. Carbon dioxide consumption continues with further increase of temperature above 505 ◦C contrary to *S*H2 which progressively increases reaching 99% at 720 ◦C. Consumption of CO2 is followed by production of CO providing evidence that the reverse WGS (RWGS) reaction is enhanced at high temperatures. Moreover, *S*CH4 decreases above 505 ◦C and becomes practically zero at 720 ◦C, implying that the reaction of methane steam reforming occurs contributing to the observed increase of both *S*H2 and *S*CO. It should be noted that selectivity toward reaction products containing carbon was defined as the concentration of each product containing carbon at reactor effluent over the concentration of all products containing

carbon (5), whereas SH2 was defined as the concentration of hydrogen produced divided by the concentration of all products containing hydrogen (6). Therefore, the values of *S*cn and *S*H2 cannot be correlated based on the stoichiometry of the reactions taking place under propane steam reforming conditions.

**Figure 4.** Selectivities toward reaction products as a function of reaction temperature obtained over Ni catalysts (5.0 wt.%) supported on (**A**) ZrO2, (**B**) YSZ, (**C**) Al2O3, (**D**) CeO2 and (**E**) TiO2. Experimental conditions: same as in Figure 3.

Qualitatively similar results were obtained for the rest of the investigated Ni catalysts, with the main differences being related to the values of the selectivities toward the reaction products, which reflect the extent of each reaction taking place with respect to the nature of the support. In particular, the observed decrease of *S*H2 and *S*CO2 at low temperatures and the simultaneous increase of *S*CH4 are higher for the most active Ni/ZrO2 (Figure 4A) and Ni/YSZ (Figure 4B) catalysts, followed by Ni/Al2O3 (Figure 4C) and Ni/CeO2 (Figure 4D), whereas it is eliminated for Ni/TiO2 (Figure 4E). As a result methane production increases in the order of Ni/TiO2 < Ni/CeO2 < Ni/Al2O3 < Ni/YSZ < Ni/ZrO2 which is consistent

with the order of catalytic activity. This can be clearly seen in Figure 5 where the TOF at 450 ◦C is plotted as a function of methane selectivity obtained at the same temperature for all the investigated catalysts. It is observed that the specific reaction rate increases from 0.018 s<sup>−</sup><sup>1</sup> to 0.33 s<sup>−</sup><sup>1</sup> following the above catalyst ranking and accompanied by a parallel increase of *S*CH4 from 0 to 29%. The results indicate that there is a clear relationship between catalytic activity and methane production.

It should be noted that besides CO2 hydrogenation, CH4 can be also produced via CO hydrogenation. However, the contribution of the latter reaction does not seem to be significant for the results of the present study taking into account the low *S*CO below 500 ◦C and its progressive increase with temperature. Moreover, methane formation may also take place via hydrogenation of CH*x* species formed following the dissociative adsorption of propane on Ni surface and the subsequent hydrogenation of the so-formed C3Hx species [29,30]. As it will be discussed below, the formation of CHx species intermediates may be the key reaction since it has been proposed that they interact with the hydroxyl groups or lattice oxygen of the support producing CO or CO2 and H2 [3,29,30].

The mass of H2 produced per propane mass unit contained in the feed was calculated at 550 ◦C and it was found to be higher for Ni/ZrO2 (23.2 wt.%) followed by Ni/CeO2 (21.2 wt.%), Ni/Al2O3 (20.6 wt.%) and Ni/YSZ (19.7 wt.%), whereas Ni/TiO2 exhibits the lowest H2 production (5.2 wt.%).

The influence of the nature of the support on the activity of Ni catalysts for propane steam reforming reaction was also investigated by Harshini et al. [16], who found that Ni/LaAlO3 was more active than Ni/Al2O3, while Ni/CeO2 exhibited intermediate performance. The optimum activity of the former catalyst was attributed to the small Ni nanoparticles dispersed on LaAlO3 surface. Although the effect of the support nature on propane steam reforming activity has not been widely studied over Ni catalysts, certain properties of metal oxide carriers may help explain the results of Figure 3. For example, the use of YSZ as support, which exhibited high activity in the results of the present study, has been found to suppress carbon deposition over Rh-Ni catalysts by providing lattice oxygen, which facilitates carbon removal and enhances the dissociation of C-C bond under reaction conditions [3]. The prevention of coke formation, occurring either via hydrocarbons decomposition or CO dissociation, by the lattice oxygen of the support has been also demonstrated over Ni/CeO2-Al2O3 [19]. Moreover, the addition of manganese oxide on Ni/Al2O3 was found to act as an oxygen donor that is transferred to Ni particles leading to rapid decomposition and oxidation of C3H8 and CH4 or C2H4 that may be produced under reaction conditions, resulting in further H2 production and improvement of the catalyst lifetime [9]. It has been also found that activation of steam followed by H2 formation may be favored over metal catalysts supported on "reducible" metal oxides through generation of oxygen defects, resulting in improved propane steam reforming activity and resistance to coke formation [3,13,31]. Based on previous studies, the reducibility of the supports used in the present study is expected to vary significantly. It is well known that Al2O3 is a hardly reducible metal oxide characterized by low oxygen storage capacity contrary to TiO2 and CeO2, which are strongly reducible metal oxides, or ZrO2 and YSZ, which are characterized by intermediate oxygen mobility [32]. Based on the above, Ni/TiO2 should be also active for the title reaction, taking into account that titania support is characterized by high oxygen storage capacity [32,33]. However, the results of Figure 3 clearly show that: (a) this catalyst was the least active one and (b) the catalytic activity is increased in the order Ni/TiO2 < Ni/CeO2 < Ni/Al2O3 < Ni/YSZ < Ni/ZrO2, which cannot be correlated with the reducibility of the support. Therefore, it is evident that support reducibility is not among the key parameters affecting the catalytic activity of Ni according to the results of the present study. The low activity of Ni/TiO2 catalyst may be related to the fact that Ni/TiO2 has lower Ni dispersion and larger Ni particles, which was previously suggested to suppress both propane steam reforming [15,16], and the intermediate (CO2 or CHx) hydrogenation reactions, in excellent agreemen<sup>t</sup> with the results of our previous study [28]. However, large Ni particles may not be solely responsible for the low activity of Ni/TiO2

taking into account that no trend was observed between TOF and Ni particle size for the investigated catalysts.

**Figure 5.** Turnover frequencies of C3H8 conversion as a function of selectivity toward CH4 obtained at 450 ◦Cover supported Ni catalysts.

It should be mentioned that a different activity order was reported in our previous study over supported Rh catalysts, where it was found that Rh/TiO2 was the most active catalyst with TOF being one order of magnitude higher compared to that measured for Rh/CeO2 [11]. This implies that the nature of the metallic phase may affect metal/support interactions leading to variations on propane steam reforming activity and/or possibly changes on the type of active sites on the catalyst surface.

#### *3.3. Long Term Stability Test*

The long-term stability of Ni/ZrO2 catalyst, which exhibited the highest activity, was investigated at 650 ◦C using the same experimental conditions as those used in catalytic performance tests. In this experiment, the catalyst was reduced in situ at 300 ◦C under 50%H2/He flow followed by heating at 650 ◦C. The flow was then switched to the reaction mixture and determination of the conversion of propane and product selectivity started. The system was shut down overnight, while the catalyst was kept at room temperature under a He flow. The next day the catalyst is heated to 650 ◦C in the He flow, followed by switching of the flow to the reaction mixture and determination of *X*C3H8 and the product selectivity as a function of time. Results obtained are shown in Figure 6, where *X*C3H8 and *S*H2, *S*CO2, *S*CO and *S*CH4 are plotted as functions of time-on-stream. As it can be seen the catalyst presents excellent stability for more than 30 h-on-stream. Propane conversion and hydrogen selectivity were varied in the range of 95–99% and 97–98%, respectively. The selectivity toward methane was low (3–4%) whereas *S*CO and *S*CO2 exhibited similar values ranging between 46 and 50%. The carbon balance was found to be satisfactory during the stability test, with a deviation lower than 1–2%.

**Figure 6.** Long-term stability test of the 5%Ni/ZrO2 catalyst at 650 ◦C: Alterations of the conversion of C3H8 and selectivities toward reaction products with time-on-stream. Experimental conditions: Same as in Figure 3. Dashed vertical black lines indicate shutting down of the system overnight.

#### *3.4. DRIFT Studies*

The interaction of selected catalysts with the reaction mixture was also investigated employing in situ FTIR spectroscopy. Experiments were conducted in the temperature range of 100–500 ◦C using a feed composition of 0.5%C3H8 + 5%H2O (in He) and the results obtained are shown in Figure 7. It is observed that the spectrum recorded at 100 ◦C (Figure 7A, trace a) for the pre-reduced Ni/TiO2 catalyst is characterized by two negative bands at 3787 and 3676 cm<sup>−</sup><sup>1</sup> which can be attributed to losses of *ν* (OH) intensity of at least two different types of free hydroxyl groups, which are either originally present on TiO2 surface or created via H2O adsorption. Two weak peaks were also detected in the *ν* (C-H) region, located at 2987 and 2966 cm<sup>−</sup><sup>1</sup> (trace a) due to C-H stretching vibrations in methyl groups (CH3,ad) and to symmetric C-H vibrations in methylene groups (CH2,ad), respectively [20,31,34–36]. These peaks are more obvious in Figure 8A (trace a) where selected spectra in the narrow range of 3200–2400 cm<sup>−</sup><sup>1</sup> are presented. Moreover, a band at 1642 cm<sup>−</sup><sup>1</sup> followed by a shoulder at 1560 cm<sup>−</sup><sup>1</sup> can be discerned, which have been previously assigned to carbonate species associated with TiO2 support [37–42]. An increase of temperature results in progressive separation of the latter two bands which are both shifted toward lower wavenumbers. A new band at 1430 cm<sup>−</sup><sup>1</sup> can be also discerned in the spectra obtained at 350 ◦C (trace f) which is also due to carbonate species. This peak may be also present in the spectra obtained at lower temperatures but couldn't be clearly observed due to the low signal-to-noise ratio in the region below 1700 cm<sup>−</sup>1. The intensities of bands assigned to carbonate species are progressively decreased above 200 ◦C. This decrease is accompanied by the detection of a weak peak at 2021 cm<sup>−</sup><sup>1</sup> [38,43–46], which is characteristic of linear-bonded CO on reduced nickel sites (Ni◦), indicating that carabonate species are further decomposed yielding CO and most possibly also CO2 in the gas phase. The weak bands in the *ν* (C-H) region are present on the spectra obtained up to 500 ◦C (Figure 7A, trace i) implying that CH*x* species are thermally stable and remained adsorbed on the catalyst surface.

**Figure 7.** DRIFT spectra obtained over the (**A**) Ni/TiO2 and (**B**) Ni/ZrO2 catalysts following interaction with 0.5% C3H8 + 5% H2O (in He) at 100 ◦C for 15 min and subsequent stepwise heating at 500 ◦C.

**Figure 8.** DRIFT spectra obtained in the region 3200–2400 cm<sup>−</sup><sup>1</sup> over the (**A**) Ni/TiO2 and (**B**) Ni/ZrO2 catalysts following interaction with 0.5% C3H8 + 5% H2O (in He) at 100 ◦C and 450 ◦C for 15 min.

A similar experiment was conducted over the most active Ni/ZrO2 catalyst and the results obtained are presented in Figure 7B. It is observed that the interaction of catalyst with the reformate mixture at 100 ◦C (trace a) results in the appearance of bands corresponding to bicarbonate species (1640 cm<sup>−</sup>1) [47–49], CHx species (2983 and 2966 cm<sup>−</sup>1) [31,36,47] as well as by negative bands (3749 and 3675 cm<sup>−</sup>1) related to the consumption of surface OH groups [31,36,48]. Increase of temperature at 200 ◦C (trace c) leads to the progressive development of two bands at 1540 and 1425 cm<sup>−</sup><sup>1</sup> due to bicarbonate species [47–49]. The intensity of the latter bands increases with increasing temperature up to 300 ◦C and diminishes upon further heating at 350–400 ◦C. This is also the case for the band at 1640 cm<sup>−</sup>1, indicating that bicarbonate species are decomposed above 450 ◦C. At temperatures higher than 400 ◦C (traces h-i) two new broad bands seem to be developed at ca 1540 and 1350 cm<sup>−</sup>1. The broadness of these bands implies that they may contain contributions from more than one species with their corresponding bands being overlapped.

This can be clearly seen in the spectrum obtained at 500 ◦C (trace i) where four bands can be clearly discerned located at 1558, 1520, 1370 and 1331 cm<sup>−</sup>1. Those detected at 1520 and 1331 cm<sup>−</sup><sup>1</sup> have been previously attributed to bidentate carbonates [49,50], whereas those located at 1558 and 1370 cm<sup>−</sup><sup>1</sup> can be assigned to bidentate formate species [51] adsorbed on ZrO2 surface. The appearance of the latter bands is accompanied by evolution of three peaks in the *ν*(CO) region due to CO linearly adsorbed on reduced Ni sites (2021 cm<sup>−</sup>1) and bridged bonded CO (1909 and 1858 cm<sup>−</sup>1) [43,46,52–54].

Interestingly, CH*x* species are eliminated from the spectra obtained above 400 ◦C followed by evolution of CH4 in the gas phase, as evidenced by the detection of the 3016 cm<sup>−</sup><sup>1</sup> band (traces h–i). Production of CH4 at the expense of CHx species can be clearly seen in Figure 8B where the spectra obtained at 100 and 450 ◦C in the wavenumber range of 3200–2400 cm<sup>−</sup><sup>1</sup> are presented.

Based on the above it can be suggested that the reaction of steam reforming of propane over Ni/ZrO2 catalyst proceeds via a dissociative adsorption of propane on metallic Ni leading to the formation of C3Hx species, which are further decomposed toward CHx species and probably carbon oxides due to the presence of H2O adsorbed on the support surface. This may result in the formation of the bicarbonate species (1640, 1540 and 1425 cm<sup>−</sup>1) detected at low temperatures on the surface of the support [20]. Part of CHx species are hydrogenated above 400 ◦C, yielding methane in the gas phase (band at 3016 cm<sup>−</sup>1) whereas the rest interact with adsorbed water producing formates (bands at 1558 and 1370 cm<sup>−</sup>1) and, eventually, CO species adsorbed on the Ni surface (bands at 2021, 1909 and 1858 cm<sup>−</sup>1). Formates may also interact with hydroxyl groups producing H2 and carbonate species (1520 and 1331 cm<sup>−</sup>1), which are further decomposed to CO2 [20]. It should be noted, however, that, under certain conditions, CH*x* species may be also dehydrogenated producing C and H2. Surface carbon is either accumulated on the catalyst surface resulting in catalyst deactivation (which is not the case here) or it interacts with the hydroxyl groups or the lattice oxygen of the support yielding CO or CO2 [3,16,19,31,55].

The ability of CH*x* species to be converted to methane and/or formates on the surface of Ni/ZrO2 may imply that CH*x* species are more weakly adsorbed and/or more reactive on the surface of this catalyst, thus resulting in higher overall activity for the propane steam reforming reaction. This agrees well with results of Figure 5, where it was shown that catalytic activity increases progressively with increasing methane selectivity. If as discussed above the origin of adsorbed CO is formate species, the high reactivity of CH*x* species over Ni/ZrO2 is also indirectly confirmed by the significantly higher population of carbonyl species observed over this catalyst. On the other hand, although CHx species were also detected on the surface of Ni/TiO2 catalyst, no band due to gas phase methane or formate species was observed, indicating that CHx species cannot be further converted in the temperature range investigated. The increase of SCH4 in parallel with the increase of catalytic activity (Figure 5) indicates that the reactivity of CHx species is strongly related to the conversion of propane to the desired products. Although detailed mechanistic studies should be conducted to further explore the reaction pathway, it can be suggested that CHx species are key reaction intermediates for the reaction of propane steam reforming.
