Novel Rh(Pd)-Cu(Ni) supported catalysts for oxy-steam reforming of methanol

2.1 Preparation of the catalytic material

Monometallic copper and nickel catalysts supported on (ZrO₂)_x $·$ (Al₂O₃)_y binary oxide supports were prepared by a wet aqueous impregnation method. Binary oxide ZrO $·$ Al₂O₃ (Zr:Al = 2:1, 1:1, 1:2) systems were prepared by a co-precipitation method. In order to prepare a working range of binary oxides, the following molar ratios of Zr:Al = 2:1, 1:1, 1:2 were used. Aqueous solutions of 1 mol/L zirconium (IV) nitrate and 1 mol/L aluminium nitrate were mixed in appreciate quantities under vigorous stirring at 80 °C. A concentrated ammonia solution was then added dropwise until the pH reached values of between 10 and 11, respectively. Then the mixtures were stirred for another 30 min. The resulting fine precipitates were washed two times in deionised water and then dried at 120 °C for 15 h and calcined for 4 h at 400 °C in air atmosphere. The metal phase (i.e., Cu or Ni) was introduced on the supports using aqueous solutions of copper nitrate (V) or nickel nitrate (V). Copper or nickel loading on the catalyst surface was 20 wt%. The supported catalysts were then dried for 2 h at 120 °C and calcined for 4 h in an air atmosphere at 400 °C. Bimetallic supported catalysts 1% Pd(Rh)–20% Cu/ZrO₂·Al₂O₃ and 1% Pd(Rh)–20% Ni/ZrO₂·Al₂O₃ (Zr:Al = 1:2) were prepared by an impregnation method on the surface of the previously prepared supported copper or nickel catalysts systems.

2.2 Characterisation methods

The specific surface area and porosity of catalytic material were determined by the BET method based on low temperature (−196 °C) nitrogen adsorption in a Sorptomatic 1900 Carlo-Erba apparatus. The pore size distributions of the investigated material were defined based on the BJH method. Temperature programmed reduction TPR-H₂ measurements of supported catalysts were performed in order to study their reducibility. TPR-H₂ measurements were carried out in an automatic apparatus Altamira (AMI-1). The reduction behaviour of all supported copper catalyst systems was studied in the temperature range of 25–900 °C, with a linear heating rate of 10 °C min⁻¹. In each investigation, a sample about 0.1 g was placed in a micro-reactor and was reduced in a mixture of hydrogen in argon stream (5% H₂–95% Ar) with a volumetric flow rate of 40 cm³ min⁻¹. The hydrogen consumption rate was monitored by a thermal conductivity detector (TCD). The TPD-NH₃ system was used to study the acidity of the catalysts. The temperature programmed desorption of NH₃ experiments were performed in the temperature measurements were carried out in a quartz flow micro-reactor using NH₃ as a probe molecule. Before all experiments, the catalyst surface was purified in a flow of He at 600 °C for 60 min. After purification, the NH₃ was adsorbed on the catalyst surface at 50 °C for 30 min. The temperature programmed desorption of NH₃ were performed in the temperature range 100–600 °C using a linear temperature ramp (25 °C min⁻¹). Before each TPD-NH₃ experiment, physically adsorbed NH₃ has been removed from the catalyst surface. All measurements were performed using IR Tracer-100 FTIR (Shimadzu) spectrometer equipped with a liquid nitrogen cooled MCT detector. Before each experiment, a catalyst was reduced at 300 °C in a mixture of 5% H₂–95% Ar mixture. A resolution of 4 cm⁻¹ was used in collecting all spectra. 128 scans were taken in order to achieve a satisfactory signal to noise ratio. The background spectrum was collected at 50 °C after the reduction process of each catalytic material. After the reduction process, a reducing mixture was shifted to a mixture of 1 vol% CH₃OH in argon stream and at the same temperature spectra were collected. Powder X-ray diffraction patterns were recorded on a PANalytical X’PertPro MPD diffractometer in Bragg-Brentano reflecting geometry. Cu K_α radiation (λ = 154.05 pm) from a sealed tube was used in the 2Θ angle range 5–90°. The morphology and composition of the investigated catalyst systems were studied using S-4700 scanning electron microscope HITACHI, equipped with an energy dispersive spectrometer EDS. The XPS spectra were recorded for selected catalysts on a Specs SAGE XPS spectrometer using Mg Kα radiation source (hν = 1253.6 eV) operating at 10 kV and 20 mA. The elements present on the sample surface were identified from a survey spectrum recorded over the energy range 0–1000 eV at pass energy of 100 eV and a spectrum acquisition step of 0.5 eV. The areas under selected photoelectron peaks in the spectrum were used to calculate the percentage of atomic concentrations of each species. High-resolution (spectrum acquisition step of 0.1 eV) spectra were collected for pertinent photoelectron peaks at a pass energy of 20 eV to identify the chemical state of each element. All the binding energies (BEs) were referenced to the C1 s peak (285 eV) coming from adventitious carbon to compensate for the effect of surface charging. The analysis area had a diameter of 0.7 mm. Casa XPS software was used during analysis of the high-resolution spectra.

2.3 Catalytic activity test

OSRM reaction was performed out using a flow quartz micro-reactor under atmospheric pressure in the temperature range 160–300 °C. The following reaction mixture was used in each catalytic test: H₂O/CH₃OH/O₂ = 1/1/0.4 (molar ratio) and the GHSV was 26700 h⁻¹ (calculated at ambient temperature and under atmospheric pressure). The total flow of the reaction mixture was 31.5 cm³/min. Argon was used as a balance gas. The catalytic activity tests were done after two hours of stabilization process performed at each temperature. The mass of the catalysts used in each test was 0.2 g. Before each catalytic activity test copper containing catalysts and all bimetallic systems were activated for 1 h in a mixture of 5% H₂–95% Ar at 300 °C. While, the monometallic supported nickel catalyst was activated under the same conditions as well as at a higher temperature of 500 °C using the same reduction conditions. The analysis of the obtained products in the investigated process was monitored using GC systems. Analysis of the organic products (methanol, methane, methyl formate, dimethylether (DME), and formaldehyde) were performed using chromatograph equipped with FID detector and 10% Carbowax 1500 on Graphpac column. While, CO and CO₂ concentrations were monitored by GC system equipped with TCD detector (150 °C, 60 mA), and Carbosphere 60/80 (50 °C) column. The hydrogen concentration was monitored also by a GC chromatograph equipped with TCD detector (120 °C, 60 mA) and molecular sieve 5a (120 °C) column. Material balances on carbon were calculated for each run to verify the obtained results. The selectivity results towards hydrogen, carbon monoxide, carbon dioxide and DME formation in OSRM was calculated using Eqs. (5)–(8). While, the methanol conversion was calculated using Eq. (9):

where n CH₃OH and n H₂ is the molar flow rate of CH₃OH and H₂, respectively.

where

• nH_2-out – molar flow rate of H₂ feed out,

• nCO_2-out – molar flow rate of CO₂ feed out,

• nCO_out – molar flow rate of CO feed out,

• n₁ⁱⁿ CH₃OH, n₂^out CH₃OH – molar flow rate of CH₃OH feed in and feed out, respectively. Organic compounds such as: methane, formaldehyde and methyl formate formation were not detected in the obtained product. Only carbon monoxide, carbon dioxide, hydrogen and DME were formed as reaction products during the OSRM reaction.

3.1 Catalytic activity measurements

The main goal of this paper was to optimize of the catalyst composition to suit the purpose of the OSRM process. Therefore, in the first step of our catalytic investigations we decided to carry out activity tests for copper catalysts supported on various (ZrO₂)_x $·$ (Al₂O₃)_y binary oxide systems in order to choose the best carrier. The results of the catalytic activity expressed as methanol conversion and selectivity towards hydrogen and other products are given in Table 1. The methanol conversion results showed that the most active catalyst among all studied copper systems supported on Zr and Al was the one with the lowest content of Zr. This catalyst showed the highest methanol conversion at both studied temperatures (i.e., 160 and 200 °C) and also high selectivity towards hydrogen formation in the oxy-steam reforming of methanol process. Furthermore, the reactivity tests showed that increasing of the aluminium content caused an increase in methanol conversion and selectivity towards hydrogen formation. It is worth noting that carbon monoxide was not formed during the reaction which is very advantageous from an application point of view. In summary, it is clear that the activity and selectivity of copper catalysts is a function of catalyst composition according to Abrokwah et al. (2016). The authors studied various monometallic Cu, Co, Ni, Pd, Zn and Sn catalysts supported on MCM-41 in reforming of methanol. They reported that the methanol conversion values and selectivity towards main products depend mainly on the active phase of the catalyst used in the process. They also confirmed that supported copper catalysts exhibited the highest methanol conversion value ∼82% and high selectivity to hydrogen formation. Cu/MCM-41 system also showed the lowest selectivity towards carbon monoxide formation (Abrokwah et al., 2016). Based on the obtained activity results for supported copper catalysts we decided to prepared analogous nickel catalysts supported on ZrO₂·Al₂O₃ (Zr:Al = 1:2) binary oxide and test them for the same reaction. The catalytic activity results obtained for Ni/ZrO₂·Al₂O₃ system clearly showed that this catalyst exhibited lower activity in the low temperature range 160–250 °C. The methanol conversion value for this catalyst at low temperature (160, 200 and 250 °C) was below 15%. Increasing reaction temperature up to 300 °C resulted in a significant increase in the methanol conversion value to about 94% and high selectivity towards hydrogen production. In addition, the results of the catalytic activity in oxy-steam reforming of methanol obtained at 300 °C showed that the carbon monoxide was formed as one of the main products of the reaction (CO selectivity = 25%). On the other hand, in the case of the monometallic supported nickel catalyst, we investigated the effect of higher temperature (500 °C) of the activation process carried out in a mixture of 5% H₂–95% Ar. The results of the catalytic activity showed that the use of a higher reduction temperature before the reactivity test did not improve the activity of the nickel catalyst. The catalysts exhibited lower methanol conversion at 300 °C and selectivity towards hydrogen formation. In addition, large amounts of the dimethyl ether formed at 250 and 300 °C were observed. Abrokwah et al. (2016) also claimed that Ni/MCM-41 exhibited lower activity in the SRM process compared to the monometallic supported copper catalyst. Further, we attempted to improve the catalytic activity of our systems by introducing metallic promoters. Therefore, in the next step of our reactivity studies we prepared and tested bimetallic Pd-Cu(Ni) and Rh-Cu(Ni) catalysts supported on a previously selected carrier ZrO₂·Al₂O₃ (Zr:Al = 1:2). The results of the activity tests performed in oxy-steam reforming of methanol showed that promotion of monometallic nickel catalysts by Pd or Rh significantly improves the activity. Both bimetallic Pd-Ni and Rh-Ni supported catalysts exhibited higher methanol conversion compared to the monometallic systems at 160 and 200 °C. However, carbon monoxide was formed for both catalysts at high temperature. In contrast, DME formation was not observed during the OSRM process at 160 and 200 °C. The catalytic activity tests performed for bimetallic Pd-Cu and Rh-Cu supported catalysts showed that there was an improvement in the catalytic activity. Notably, carbon monoxide formation was not observed in Pd-Cu and Rh-Cu catalytic systems at the reaction temperatures. The comparison of the catalytic activity obtained for bimetallic Pd-Ni and Pd-Cu supported catalysts showed that both systems exhibited practically the same values of methanol conversion and selectivity towards hydrogen formation. On the other hand, bimetallic Rh-Cu catalysts showed the highest activity and selectivity towards hydrogen formation at 200 °C. The catalytic activity tests also showed that the only undesired products which were formed during the reforming process were carbon monoxide and DME. Based on the results of catalytic activity measurements we further optimized the content of rhodium in Rh-Cu bimetallic supported catalysts. Chang et al. (2012) investigated the catalytic properties of copper CuO/ZnO/Al₂O₃ (30/60/10) catalysts promoted by noble metals such as: Pt, Pd, Ru and Rh in oxidative steam reforming of methanol and they reported that addition of noble metals improves the methanol conversion during the reaction in all cases but also increases the formation of CO. The catalytic tests performed by authors in OSRM showed that only copper catalysts promoted by platinum prepared by co-precipitation method exhibited higher methanol conversion and low CO selectivity. We also investigated in this work the influence of the rhodium content on methanol conversion and selectivity results towards H₂, CO, CO₂ and DME. We prepared three bimetallic Rh-Cu catalysts supported on ZrO₂·Al₂O₃ (Zr:Al = 1:2) by an impregnation method and tested for OSRM. The results obtained in oxy-steam reforming of methanol reaction are also given in the same Table 1. The reactivity results clearly indicate that the most active catalyst was the system containing the lowest content of Rh.

The reactivity results clearly indicate that the most active catalyst was the system containing the lowest content of Rh. It is also worth mentioning that this catalyst also exhibited the highest selectivity to hydrogen production among of all bimetallic promoted by Rh catalysts at low temperature i.e. 160 °C. While, at 200 °C, the results of selectivity towards hydrogen formation showed that the 0.5% Rh–20% Cu/ZrO₂·Al₂O₃ (Zr:Al = 1:2) catalyst exhibited also high selectivity towards hydrogen formation similar to the copper catalyst containing 1% wt. of Rh. Further increase of the Rh loading in the catalytic system resulted in decrease of the selectivity to hydrogen production.

3.2 Specific surface area measurements

In the next step of our investigations, we determined the Specific Surface Area (SSA) and average pore size for binary oxides and all tested catalysts. The specific surface area results are presented in Table 2. The results of the SSA measurements clearly show that the binary oxide (ZrO₂·Al₂O₃ (Zr:Al = 1:2)) and catalyst with the highest content of the aluminium among all copper catalysts exhibited also the highest specific surface area. In addition, all investigated catalysts had average pore size below 3 nm.

Notably, the 20% Cu/ZrO₂·Al₂O₃ (Zr:Al = 1:2) system had specific surface area value about 20% higher compared to the rest of the monometallic copper catalysts supported on the same carrier. In contrast, the nickel catalysts supported on ZrO₂·Al₂O₃ system showed significantly lower SSA value compared to the copper catalyst supported on the same support. On the other hand, the SSA measurements obtained for bimetallic supported catalysts showed that the promotions of copper or nickel catalyst by noble metals does not cause significant changes in the specific surface area. In the case of the monolayer capacity values the results indicate that the bimetallic systems exhibited slightly higher values. Whereas, the values of the pore radius obtained for all catalytic systems were below 3 nm in all cases.

3.3 The reducibility of the mono and bimetallic catalysts

Next, we studied the reducibility of the monometallic and bimetallic supported catalysts. TPR-H₂ measurements recorded for copper catalysts supported on various ZrO₂-Al₂O₃ systems are presented in Fig. 2. The reduction measurements obtained for 20% Cu/ZrO₂·Al₂O₃ (Zr:Al = 1:2) catalyst showed two unresolved reduction effects in the temperature range of 150–320 °C (Mierczynski et al., 2013). The maxima of hydrogen consumption peaks at about 210 °C and 280 °C are associated with reduction of CuO species according to the following scheme (Mierczynski, 2016): Cu²⁺ → Cu⁺ → Cu⁰

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Fig. 2 TPR-H₂ profile recorded for monometallic copper catalysts supported on various ZrO₂-Al₂O₃ binary oxide supports.

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The first reduction peak located at 210 °C is assigned to the reduction CuO to Cu₂O species. The next peak with a maximum of hydrogen consumption at 280 °C is associated with the reduction of Cu₂O species to metallic Cu. In the case of the rest of the supported copper catalysts the same reduction stages were visible on the TPR-H₂ profiles as for 20% Cu/ZrO₂·Al₂O₃ (Zr:Al = 1:2) system. Ren et al. (2015) investigated the reducibility of Cu/Al₂O₃ catalyst modified by ZnO, ZrO₂ and MgO. They observed also two reduction stages in the case of all investigated Cu catalysts. TPR-H₂ profile recorded for 20% Cu/Al₂O₃ catalyst modified by ZrO₂ showed two unresolved reduction peaks with maxima of hydrogen consumption peaks at 220 and 270 °C, respectively. These authors reported that a first reduction effect was assigned to the reduction of the highly dispersed CuO phases. The second effect is attributed to the reduction of CuO species strongly interacted with the support. They strongly suggested that the modification of 20% Cu/Al₂O₃ catalyst by ZrO₂ improves the dispersion of CuO species on the catalyst surface. This is due to decrease in the interaction between CuO and support surface which also prevents the migration of metallic copper species onto support surface. Zhu et al. (2015) studied also the reduction behaviour of Cu/ZrO₂/Al₂O₃ catalyst calcined in the temperature range 350–650 °C. The TPR-H₂ profiles recorded for all investigated catalysts showed, similar as in our case, two unresolved reduction effects located in the temperature range 150–300 °C. These reduction stages were assigned to the two steps of the reduction process described by the following scheme Cu²⁺ → Cu⁺ → Cu⁰. It is worth emphasizing that the Cu/ZrO₂/Al₂O₃ catalyst calcined at 350 °C exhibited slightly higher reduction temperature than catalysts calcined at 450 °C. The TPR-H₂ profiles recorded for Cu/ZrO₂/Al₂O₃ catalysts calcined at 750 and 850 °C also showed the reduction effect with the maximum of the hydrogen consumption peak at 370 °C. This reduction peak was attributed to the reduction of the CuAl₂O₄ spinel structure. Fig. 3 presents a comparison of the reducibility of monometallic Cu and Ni and bimetallic Pd-Cu, Rh-Cu, Pd-Ni, Rh-Ni supported catalysts. The TPR-H₂ profile recorded for 20%Ni/ZrO₂ $·$ Al₂O₃ catalyst showed a two-step reduction process (see Fig. 3). The first reduction effect located in the temperature range 350–450 °C is associated with the reduction of unbounded NiO species. The second hydrogen consumption peak located above 450 °C is assigned to the reduction of NiO species differently interacted with support. In the same Fig. 3 the reduction results of bimetallic supported catalysts are also given. The TPR-H₂ profiles recorded for all bimetallic catalysts showed that addition of noble metals into monometallic supported copper and nickel catalysts facilitates their reduction. The TPR-H₂ profiles of bimetallic systems showed the same reduction stages which were observed in the case of monometallic catalysts but shifted towards the lower temperature range. These shifts confirm the facilitated reduction of copper or nickel oxides after introduction of noble metal. In addition, in the case of the 1%Pd-20%Ni/ZrO₂·Al₂O₃ catalyst a low temperature (maximum at about 90 °C) consumption peak was observed in the TPR profile and was assigned to PdO reduction step. Guo et al. (2014) investigated Ni/ZrO₂/Al₂O₃ catalysts with the different ZrO₂ content. The authors reported three various reduction steps assigned to the reduction of α, β and γ nickel oxide species differently interacted with the support present in the TPR-H₂ profiles recorded for these catalytic systems. The α species represent the unbounded NiO which are reduced at low temperature (320–450 °C). The reduction effect located in temperature range 450–720 °C was attributed to β species correspond to NiO interacted with the support. The last high temperature effect is assigned to the reduction of NiAl₂O₄ spinel structure. Furthermore, they reported that increasing the ZrO₂ content in Ni/ZrO₂/Al₂O₃ catalysts leads to the growth of α species which are reduced in low temperature. The reduction properties of supported nickel catalysts were also studied by Richardson at al. (Richardson et al., 1994). The authors observed in the TPR-H₂ profile high temperature reduction effects, also assigned to the reduction of NiAl₂O₄ spinel structure. They have reported that the incorporation of Al³⁺ to the NiO structure or mutual migration of ions leads to the formation of NiAl₂O₄ spinel structure. These processes take place on the support surface during the heat treatment. The reduction studies performed for all catalytic material clearly indicate that all copper catalysts reduced in two steps and are connected with the reduction of CuO and Cu₂O species, respectively (Mierczynski et al., 2015; Mierczynski et al., 2014; Águila et al., 2008).

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Fig. 3 TPR-H₂ profiles of monometallic copper and nickel and bimetallic catalysts supported on ZrO₂-Al₂O₃ binary oxide.

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Fig. 4 presents the influence of the Rh content on the reduction behaviour of supported copper catalysts. The observed TPR profiles recorded for all bimetallic supported catalysts indicate that for all investigated bimetallic catalysts two reduction peaks are visible on the TPR curves. These two steps are connected to the reduction of CuO through Cu₂O intermediates. It is worth noting that in the case of bimetallic catalysts with low Rh loading, the reduction process took place in the temperature range of 100–300 °C. Additionally, the first reduction stage with the maximum of hydrogen consumption located at about 140 °C had the highest intensity compared to the rest of the bimetallic supported catalysts (see Table 3). The highest intensity of the first reduction peak recorded on the TPR-H₂ curve recorded for 0.5% Rh-20% Cu/ZrO₂·Al₂O₃ catalyst means that this system is the easiest reduced catalyst.

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Fig. 4 TPR-H₂ profiles of bimetallic Rh-Cu catalysts supported on ZrO₂-Al₂O₃ binary oxide.

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3.4 Phase composition studies

To further explain the differences in activity we also studied the phase composition of monometallic 20% Cu/ZrO₂·Al₂O₃ and 20% Ni/ZrO₂·Al₂O₃ catalysts being after various treatments. We studied the phase composition of the catalysts calcined in an air atmosphere for 4 h at 400 °C and catalysts after reduction in a mixture of 5%H₂–95%Ar at 300 °C for 1 h and reaction performed in oxy-steam reforming of methanol. The XRD results are given in Figs. 5 and 6. The X-ray diffraction studies were used to determine the changes of the phase composition after various treatments and in order to indicate the interaction between an active phase component and the support. X-ray diffraction curve recorded for 20% Cu/ZrO₂·Al₂O₃ catalyst being after calcination confirmed the amorphous nature of the ZrO₂ (see Fig. 5). The XRD diffraction pattern shows diffraction peak positioned between 30 and 35 theta angles which was attributed to amorphous ZrO₂. While, the diffraction peaks positioned at 36, 38, 48 and 62° were assigned to the CuO phase. In the same XRD curve, γ-Al₂O₃ phase was visible at 2θ angles = 46, 67 and 68°. Whereas, the XRD curve recorded for the same catalyst reduced at 300 °C showed the occurrence of the diffraction peaks assigned to metallic copper and wide diffraction peak positioned between 30 and 35 2θ angle assigned to amorphous zirconia.

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Fig. 5 XRD patterns of monometallic 20% Cu/ZrO₂ · Al₂O₃ catalysts being after calcination process performed at 400 °C in an air atmosphere for 4 h and after reduction at 300 °C in a mixture of 5% H₂–95% Ar and reaction.

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Fig. 6 XRD patterns of monometallic 20% Ni/ZrO₂ · Al₂O₃ catalysts being after calcination process performed at 400 °C in an air atmosphere for 4 h and after reduction at 300 °C in a mixture of 5% H₂–95% Ar and reaction.

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Fig. 6 present the phase composition studies of 20%Ni/ZrO₂ $·$ Al₂O₃ catalyst. The XRD curve recorded for 20%Ni/ZrO₂ $·$ Al₂O₃ catalyst calcined in an air atmosphere at 400 °C shows diffraction peaks positioned at 2θ angles = 36.43, 63, 75 and 79° which are attributed to nickel (II) oxide phase. The XRD pattern recorded for this catalyst also showed a wide XRD peak attributed to amorphous zirconia. However, in the case of the same nickel catalyst after being reduced the diffraction curve showed the presence of peaks assigned to metallic nickel (2 theta angles = 44.52°, 76°), NiO phase (2θ angles = 36.43, 63, 75 and 79°) and amorphous zirconia. Diffraction curve recorded for the reduced nickel catalyst confirmed its partial reduction. The existence of other phases in the diffraction curve was not confirmed by the XRD technique.

3.5 X-ray photoelectron spectroscopy (XPS)

In order to elucidate the differences in activity measurements in OSRM process for bimetallic Rh-Cu supported catalysts the XPS high-resolution spectra of the binding energies between 920 and 970 eV were recorded and the results are given in Fig. 7 and Table 4. The performed surface analysis of the supported copper catalysts showed that in the investigated binding energy range several peaks were visible. Photoelectron peaks visible in the XPS spectrum can be assigned to metallic copper and copper in first and second oxidation states (Kulkarni and Rao, 2003). The presented on each spectra binding energies bands located at 936, 934.5 and 932.4 eV were assigned according to Ertl and co-workers (Ertl et al., 1980) to Cu²⁺, Cu⁰ and Cu⁺, respectively.

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Fig. 7 (A) Cu 2p XPS spectra of reduced 0.5%Rh–20 %Cu/ZrO₂·Al₂O₃ catalyst in 5% H₂–95% Ar at 300 °C (B) Cu 2p XPS spectra of reduced 1%Rh–20 %Cu/ZrO₂·Al₂O₃catalyst in 5% H₂–95% Ar at 300 °C (C) Cu 2p XPS spectra of reduced 2%Rh–20 %Cu/ZrO₂·Al₂O₃ catalysts in 5%H₂–95% Ar at 300 °C mixture.

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The peaks located at about 943 and 963 eV are satellite peaks and are characteristic only of Cu²⁺ species. The detailed analysis of the presented data gave evidence that increasing of the Rh content in the investigated catalyst to 1 and 2% wt. of Rh leads to higher content of metallic copper species present on the catalyst surface (see Table 4).

Cu⁰ and Cu⁺ are active centres in the reaction of methanol reforming, and their number and ratio affect on the catalytic activity. Oxidation reaction of CH₃OH to CH₃O⁻ occurs on metallic copper, while the oxidation of CH₃O⁻ takes place on Cu⁺ in order to formate species creation causing an increase in the conversion of methanol. The presence of Cu⁺ forms on the catalyst surface leads to greater stability of the catalyst in the process of steam reforming of methanol compared to metallic copper which sinters much easier than in the case of Cu₂O species due to its higher Tamman temperature (Mierczynski et al., 2013). In our case the concentration of the Cu⁺ species on the catalyst surface was the highest for the 2%Rh–20%Cu/ZrO₂·Al₂O₃ catalyst. The role of Cu⁰ and Cu⁺ centres in the reforming of methanol process is still unclear but based on the results of the copper species concentration on the catalysts surface it can be assumed that system with the lowest ratio between Cu⁰ and Cu⁺ exhibited the highest activity in OSRM process. In addition, these results also confirmed that the occurrence of Cu⁰ and Cu⁺ species and their ratio is a critical parameter to achieve highly active systems in OSRM (Kulkarni and Rao, 2003). These results agree well with our previous work (Mierczynski et al., 2013) performed for copper catalysts supported on Zn-Al containing systems tested in steam reforming of methanol reaction. Similar results of the catalytic activity were also confirmed by other authors (Oguchi et al., 2005). The increase of metallic copper content in the catalyst surface after reduction agree well with the temperature programmed reduction results carried out for bimetallic catalysts. This also confirmed that increase of the Rh content in the bimetallic catalysts facilitates the reduction of a copper oxide species present on the catalyst surface.

3.6 Acidity measurements of catalysts

In order to explain and understand the difference in activity and selectivity results in the OSRM process, we carried out acidity measurements for all catalytic material. The acidity measurements performed for supports, monometallic and bimetallic supported catalysts are given in Table 5. The results show that all investigated systems exhibited three kinds of the acidic centres on their surface namely weak, medium-strong and strong acid sites. The acidity measurements performed for supported copper catalysts confirmed that the highest total acidity, calculated based on the surface under the peaks, had the system with the highest Al content. This result also indicates that catalysts which showed the highest activity also exhibited the highest total acidity among of all monometallic supported catalyst. The results obtained in this work agree well with our previous investigations (Mierczynski et al., 2016; Mierczynski et al., 2017). The acidity measurements showed that nickel 20% Ni/ZrO₂·Al₂O₃ (Zr:Al = 1:2) system exhibited lower total acidity compared to the monometallic copper catalyst supported on the same support.

While, in the case of the bimetallic supported catalysts also their high total acidity was detected, what suggest also high activity. This tendency can be easily explained by the fact that acidic sites play crucial role during the OSRM process. These centres are indeed responsible for stabilizing of intermediates such as methoxy, monodenate and bidentate formate species and even carbonates, which are then transformed into the main products CO₂ and H₂ (Mierczynski et al., 2016; Hereijgers and Weckhuysen, 2009). These findings are in agreement of other published studies such as this conducted by Hereijgers and Weckhuysen (2009).

3.7 FTIR measurements

To reinforce the observed activity results and hypothesis concerning the important role of the acidity centres during OSRM process, additional experiments were conducted by FTIR and shown in Fig. 8. As we can easily distinguish on all presented spectra that during the sorption process carried out at 50 °C the formation of methoxy (peaks at 2995, 2936, 2919, 2825, 1470, 1443, 1350, 1200 and 1020–1100 cm⁻¹), formate (peaks at 2925, 2850, 1620, 1364, and 1350 cm⁻¹) and carbonate species (peaks at 1620, 1570–1440, and 1220 cm⁻¹) are formed on the catalyst surface. The presented results showed that in the case of the most active systems (·20% Cu/ZrO₂Al₂O₃ (Zr:Al = 1:2)) the highest intensity of the IR bands assigned to methoxy, formate and carbonate species were detected. These results agree well with the hypothesis presented above that catalysts which contain the largest number of acidic centres on its surface has the highest sorption properties with respect to methanol at the studied temperature range. The sorption of methanol is one of the important stages during the oxy-steam reforming process. It is also worth mentioning that catalytic systems which showed the highest intensity of bands assigned to methoxy and formate species during the adsorption process had also the highest activity in the oxy-steam reforming of methanol process.

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Fig. 8 FTIR spectra of adsorbed species taken after reduction of the supported copper catalysts in a mixture of 5%H₂–95%Ar at 300 °C for 1 h and exposure of the catalytic systems to a 1 vol% methanol-argon mixture at 50 °C.

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3.8 SEM-EDS measurements

SEM-EDS measurements were also performed for monometallic and bimetallic copper and supported nickel catalysts. This useful technique allows determining the morphology and composition of the catalyst surface. SEM images and EDS spectra collected for the investigated mono- and bimetallic catalysts supported on the selected support ZrO₂·Al₂O₃ (Zr:Al = 1:2) were present on Figs. 9–11. Fig. 9 and Fig. 10 presented the images and EDS spectra collected for monometallic supported copper and nickel catalysts calcined in an air atmosphere at 400 °C for 4 h, respectively. The presented data clearly confirms the composition of the investigated catalysts. In both spectra the occurrence of the same elements such as Zr, Al, O were confirmed on the catalyst surface. In addition, Cu and Ni were also detected for monometallic copper and nickel catalysts, respectively. Analogical measurements were also performed for bimetallic supported catalysts and the results are given on Fig. 11. The analysis of the bimetallic catalyst confirmed the presence of the same elements which were found on the surface of monometallic systems. The only difference in the case of the bimetallic supported catalyst was the presence of rhodium and palladium for the appropriate bimetallic system.

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Fig. 9 SEM images and EDS spectra collected for the investigated monometallic 20% Cu/ZrO₂·Al₂O₃ catalyst.

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Fig. 10 SEM images and EDS spectra collected for the investigated monometallic 20% Ni/ZrO₂·Al₂O₃ catalyst.

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Fig. 11 SEM images of bimetallic supported catalysts calcined in an air atmosphere at 400 °C for 4 h.

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