Low-Pressure Hydrogenation of CO

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Low-Pressure Hydrogenation of CO to CHOH Using Ni-InAl/SiO Catalyst Synthesized via a Phyllosilicate Precursor 2

Anthony R. Richard, and Maohong Fan ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00848 • Publication Date (Web): 19 Jul 2017 Downloaded from http://pubs.acs.org on July 19, 2017

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Low-Pressure Hydrogenation of CO2 to CH3OH Using Ni-In-Al/SiO2 Catalyst Synthesized via a Phyllosilicate Precursor Anthony R. Richard,† Maohong Fan, *,†,‡ †

Department of Chemical Engineering, University of Wyoming, Laramie, WY 82071, USA

‡

School of Energy Resources, University of Wyoming, Laramie, WY 82071, USA

ACS Paragon Plus Environment

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ABSTRACT. The overall objective of this research is to convert the increasingly concerning CO2 and renewable H2 to highly demanded methanol (CH3OH), which creates a win-win scenario for simultaneous climate change prevention and sustainable economic development. The key to the success of this targeted CO2 utilization technology is the development of lowpressure methanol synthesis catalysts (NiaInbAl/SiO2, a: 0-8.3; b: 0-9.1) by means of a phyllosilicate precursor allowing for formation of well-dispersed metallic particles with an average diameter of 2.5 – 3.5 nm. The catalysts were characterized with various methods including ICP-OES, N2 physisorption, XRD, SEM, TEM, TGA, H2 TPR, DRIFTS, and XPS. The performances of the NiaInbAl/SiO2 catalysts and conventional catalyst were compared under various evaluation temperatures at ambient pressure. It was found that catalysts with Ni/In ratios of 0.4 – 0.7 showed the highest activity. Ni3.5In5.3Al/SiO2 (NIA-0.7) with 15% metal loading was the best among the tested NiaInbAl/SiO2 catalysts with activity of 0.33 mol h-1 mol catalyst metal1

compared to the benchmark Cu/ZnO/Al2O3 (CZA) catalyst at 0.17. Several NiaInbAl/SiO2

catalysts also showed similar CO2 conversions compared to the CZA catalyst. Infrared studies using DRIFTS determined that CO2 hydrogenation on NiaInbAl/SiO2 catalysts proceeds through monodentate carbonate before further conversion to monodentate and bidentate formate. With a feed of CO/H2 instead of CO2/H2 the primary hydrocarbon product changes from methanol to propane accompanied by a lack of formate and monodentate carbonate IR signals.

KEYWORDS. Methanol synthesis, CO2 hydrogenation, heterogeneous catalysis, phyllosilicate, nickel indium catalyst


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1. INTRODUCTION Rising atmospheric carbon dioxide levels have continued to motivate CO2 capture and utilization research due to the significance of CO2 impacts on climate change.1 A particularly attractive form of utilization of CO2 is to convert the greenhouse gas to liquid fuels or chemicals.2 The CO2 captured from natural or industrial sources can serve as a cost-effective carbon source for material production, which not only avoids direct atmospheric release but also the need for expensive sequestration. One potential option is the production of methanol (or MeOH) which had global demand of approximately 70 million metric tons in 2015.3 The overall reaction of methanol (CH3OH) synthesis from CO2 and H2 is described by R1. CO2 + 3H2 → CH3 OH + H2 O

(R1)

The required H2 can be obtained using renewable energy resources such as biomass and solar based water splitting. Thus, the methanol produced in this way is renewable with the potential to be a fossil fuel replacement.4 In industrial settings, methanol is typically produced with CO, CO2, and H2 over a catalyst containing Cu, Zn, and Al at 230-300 °C and 50-100 bar.5 There have also been numerous studies involving other catalyst metals, including precious metals such as Pt and Pd.6 Even though CH3OH has been synthesized from CO2 directly, and indirectly through CO,7 the conventional Cu/ZnO/Al2O3 (CZA) catalyst is mainly optimized for high-pressure conditions using both CO2 and CO.5b Thus, the development of alternative catalysts is needed to more effectively utilize CO2. An additional technical hurdle is the low performance of the


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conventional catalyst for MeOH synthesis at low pressure, a topic which has been gaining high attention recently.4a,

4b, 8

De-centralized methanol production or applications such as fuel cells

can benefit from low-pressure MeOH synthesis, and so it is imperative to develop new low pressure catalysts for realizing MeOH synthesis with CO2 soon to be captured worldwide. The methanol synthesis catalysts presented here are designed to effectively utilize CO2 for methanol synthesis at low pressure with high stability, and without precious metals. The new composite catalysts, NiaInbAl/SiO2, contain nickel and indium as co-catalysts and are synthesized via a phyllosilicate precursor. Many silica-supported Ni catalysts have been reported in literature,4a, 9 although the use of silica as a support in this work is motivated by the ability to form a Ni phyllosilicate precursor during deposition-precipitation synthesis. Phyllosilicate structures are composed of sheets of SiO4 tetrahedra and cation-containing octahedra (usually Mg, Al, Fe, or Ni). Synthesized Ni phyllosilicates have two common structures which are denoted as the ratio of tetrahedral to octahedral layers. The ideal 1:1 structure (one tetrahedral sheet per octahedral) has the formula of Ni3Si2O5(OH)4, and the 2:1 structure which is an octahedral sheet sandwiched between two tetrahedral sheets has the formula of Ni3Si4O10(OH)2.10 Phyllosilicate precursor catalysts have shown high stability due to decreases in sintering and carbon deposition, and improved reactivity.10-11 The synthesis of nickel phyllosilicates when employing the deposition-precipitation method for silica supported nickel catalyst preparation as in this work has been well documented.11b, 12 While phyllosilicate formation when using urea as the precipitating base is commonly reported 12b-g, other basic solutions of sodium hydroxide 12h-j, ammonia

11b, 12k-m

, sodium bicarbonate

12n

, and (as in this work) sodium carbonate

been reported as well.


12a, 12o

have

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Any successful catalysts lie in their activity and stability. The metals used in this study were chosen for both their catalytic and structural attributes. In addition to the facile Ni phyllosilicate formation, the use of Ni as a catalyst has a wide range of applications due to its high interaction with hydrogen and relatively low cost compared to precious metals.13 For CO2 hydrogenation reactions, nickel catalysts are typically used for CH4 production,14 although methanol synthesis catalysts have also been reported.4a, 15 Indium, in addition to its use in semiconductor materials, has also attracted attention as a catalyst,16 and was recently shown to have activity for methanol synthesis from CO2/H2.17 Aluminum is a well-known structural promoter and was included to increase stability by protecting against sintering.5a, 18 With all the above-mentioned factors considered, a family of new catalysts, NiaInbAl/SiO2, were designed to produce methanol at low pressure with high stability. The success in developing low pressure CO2 and H2 based MeOH synthesis catalysts means synthesis that is not only less energy-intensive, but can also be beneficial to development of other energy technologies such as direct methanol fuel cells.4a, 8c 2. EXPERIMENTAL 2.1

Catalyst Preparation and Characterization

NiaInbAl/SiO2 catalysts were prepared using the deposition-precipitation method, as reported by others.18-19 The silicon dioxide support (99.5%, 325 mesh, Sigma-Aldrich) was placed into 100 mL of 0.05 mol/L solution of nickel (II) nitrate hexahydrate (99%, Sigma-Aldrich), indium (III) nitrate hydrate (99.999%, Alfa Aesar), and aluminum nitrate (99.9%, J.T. Baker) at the designated molar ratio which was prepared in a stirred glass reactor at room temperature (~20 °C). A 1 mol/L Na2CO3 (99.5%, Fisher Scientific) solution was added to the reactor by pumping


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at a rate of 0.3 mL/min until a pH of 10 was reached, which typically took 30 min, followed by a 30 min aging during which the pH was maintained by addition of carbonate solution. After the aging period, catalysts were filtered and washed three times with deionized water and subsequently dried overnight at 90 °C. CZA catalyst was prepared using the coprecipitation method similar to that described by Baltes et al.20 This method produces catalysts with a Cu:Zn:Al ratio of approximately 6:3:1 and equivalent or higher activity compared to industrial catalysts.20 CZA catalysts produced with this method also have been used as a benchmark for comparison in a number of studies.4a,

8g, 17b, 20

A 1 mol/L solution of copper (II) nitrate

pentahydrate (98%, Alfa Aesar), zinc nitrate hexahydrate (98%, Sigma-Aldrich), and aluminum nitrate nonahydrate at a molar ratio of 6:3:1 for Cu:Zn:Al was pumped at a flowrate of 5 ml/min into a heated and stirred glass reactor prepared with 30 mL of deionized water at 70 °C while a 1 mol/L solution of Na2CO3 was simultaneously added to the reactor to maintain a pH of 6.5 (±0.1). Addition of metal nitrate solution was discontinued after 20 mL had been added to the reactor, followed by an aging period of 30 min during which the pH was maintained through the addition of carbonate solution or metal nitrate solution, while the temperature was maintained in the stirred reactor. After drying, catalysts were ground and sieved through a No. 120 sieve to obtain particles no larger than 125 micrometers, followed by calcination for 4 h at a designated temperature under dry air flowing at 120 mL/min. CuZnAl catalyst was calcined at 300 °C, and NiInAl/SiO2 catalysts were calcined at 400 °C. Elemental analyses of the catalysts were performed with a PerkinElmer Optima 8300 Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) equipped with a Teflon sample introduction system. A Quantachrome Instruments Autosorb iQ was used to perform physisorption and temperature programmed reduction (TPR). Nitrogen (99.999%, U.S. Welding)


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physisorption was performed at -196 °C after outgassing samples under flowing helium (99.999%, U.S. Welding) at 200 °C. Multi-point BET analysis was used for surface area (SBET), and BJH desorption for pore volume (Vp), pore diameter (dp), and pore size distribution. Hydrogen TPR was performed on calcined samples from 40 to 1100 °C ramping at 5 °C/min after outgassing in flowing helium at 200 °C. Scanning electron microscopy (SEM) was performed using a Quanta 450 FEG operated at 5 kV. Scanning transmission electron microscopy (STEM), high resolution TEM (HRTEM), and energy-dispersive X-ray spectroscopy (EDS) analysis were performed using a 300 kV FEI Titan 80-300. Thermogravimetric analysis (TGA) was obtained using a TA Instruments SDT Q600 from 20 to 800 °C at a heating rate of 5 °C/min under flowing nitrogen at 100 ml/min. X-ray photoelectron spectroscopy (XPS) was completed using a Thermo Scientific ESCALAB 250 Xray Photoelectron Spectrometer calibrated to C 1s binding energy of 284.8 eV. X-ray diffraction (XRD) was conducted using a Rigaku Smartlab X-ray diffractometer with monochromatic Cu Kα radiation, operating at conditions of 40 kV and 40 mA for 2θ of 10° to 70° with a step size of 1 degree per minute. Reduced samples analyzed by SEM, STEM, TGA, XPS, and XRD were treated using 20 ml/min of 50% H2 in N2 for two hours. CZA catalyst was reduced at 250 °C and all other catalysts were reduced at 390 °C. Fourier transform infrared spectroscopy (FTIR) was performed using a Thermo Scientific Nicolet iS50 FTIR spectrometer equipped with a Praying Mantis device (Harrick Scientific) with ZnSe windows. All samples were reduced in situ at ambient pressure under H2 flow (100 ml/min) at 390 °C for 13 hours, followed by cooling to the required temperature while purging with He (100 ml/min). Temperature-programmed adsorption (TPA) experiments were performed on pretreated samples by first purging the reaction chamber with the adsorbate (either CO or


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CO2) at 40 ml/min for two hours at 10 °C, followed by heating of the reaction chamber to 450 °C at a rate of 3 °C/min while maintaining adsorbate flow. For in situ, isothermal reactions using CO2/H2 (H2:CO2 = 3:1) or CO/H2 (H2:CO = 2:1), after reduction and He purging, the gas flow was switched to the reactant mixture (40 ml/min) at 5 °C and was allowed to flow for 30 min before being pressurized to 250 psi. The higher pressure is used to increase the signals which were weak at atmospheric pressure. The temperature was then increased to 260 °C and held for the remaining test period. Temperature-programmed surface reaction (TPSR) experiments were performed by first pre-loading samples with adsorbate (either CO or CO2 at 100 ml/min) at 30 °C and ambient pressure for one hour, after which the gas feed was adjusted to contain the stoichiometric ratio of reactants (H2:CO2 = 3:1 or H2:CO = 2:1). The temperature was then raised to 450 °C at 3 °C/min, and then subsequently lowered to 30 °C at the same rate, all while flowing the reactant mixture. 2.2

Catalyst Evaluation

Activity experiments were performed at ambient pressure in a tubular glass reactor with an inside diameter of 3.6 mm, and heated by a furnace with temperature monitored by an internal thermocouple. The reactor was loaded with 0.1 g (±0.002 g) of catalyst mixed with 0.3 g sand (50-70 mesh, Sigma-Aldrich) and held stationary with quartz wool. Preceding activity tests, catalysts were reduced in situ using 20 ml/min of 50% H2 in N2 for two hours, with CZA catalyst reduced at 250 °C and all other catalysts reduced at 390 °C. During activity tests, a mixture of H2, CO2, and N2 was fed to the reactor at a ratio of 6.75:2.25:1 at 20 ml/min (H2:CO2 = 3:1). The effluent stream was measured using an Inficon 3000 Micro GC with PLOT U and molecular sieve packed columns with TCD detectors and an SRI GC with metal capillary column and FID detector. During experiments, temperatures were ramped by 15 °C after reaching steady state


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with temperature range prescribed to include the methanol activity peak temperature determined experimentally. Gas flow was controlled by Porter instruments flow controllers, and the reaction conditions yield a space velocity of approximately 4,000 h-1. Catalyst activity experiments were performed at least three times for each reported data point, and error bars represent standard errors. The experimental apparatus is shown in Figure 1.

Figure 1. Experimental apparatus used for catalyst activity evaluation.

3. RESULTS AND DISCUSSION 3.1 3.1.1

Catalyst Characterization Elemental Analysis

To investigate the effect of Ni/In ratio, NiaInbAl/SiO2 catalysts with a: 0-8.3; b: 0-9.1 were synthesized. Metal loading was also investigated by synthesizing selected catalysts at


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approximate loadings of 5, 10, 15, and 20 wt%. ICP-OES was used to obtain elemental analysis data for catalysts in this work, and the results are shown in Table 1. Catalysts containing Ni, In, or Al are denoted with N, I, or A, respectively, followed by a number representing the approximate Ni/In ratio where applicable. Table 1. Elemental analysis for NiaInbAl/SiO2 and CZA catalysts determined by ICP-OES, and BET surface area (SBET) determined by N2 physisorption. Loading (wt%)

Ni

In

Al

IA NIA-0.3 NIA-0.4 NIA-0.5

10.6 10.8 10.5 5.2 9.5 15.5 18.8 4.9 9.8 14.4 19.1 9.6 9.2 15.0

0 2.0 2.6 3.0 3.1 3.0 3.1 3.3 3.6 3.5 3.5 5.2 8.3 3.5

9.1 7.1 6.7 5.6 6.2 5.9 6.1 4.8 5.3 5.3 5.3 3.5 0 5.3

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0

0 0.28 0.39 0.54 0.51 0.51 0.51 0.69 0.67 0.66 0.66 1.46 ∞ 0.66

Cu

Zn

Al

Cu/Zn ratio

1.0

2.22

NIA-1.5 NA NI-0.7 SiO2

CZA 63 5.8 2.6 Molar ratios normalized with Al = 1 except NI-0.7.

3.1.2

SBET

Catalyst

NIA-0.7

a

Molar ratioa Ni/In ratio

(m2/g) 268.5 259.0 351.6 268.5 338.2 371.0 359.5 346.2 372.4 406.3 411.4 296.6 469.0 n.d. 510.6 77.4

Nitrogen Physisorption

Nitrogen physisorption was performed to probe surface area, pore diameter, pore volume, and pore size distribution. The BET surface area (SBET) for NiaInbAl/SiO2 catalysts, SiO2 support, and


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CZA are shown in Table 1, while pore volume (Vp), and pore diameter (dp) are listed in Table S1 and pore size distributions are shown in Figure S1b. As Table 1 shows, the surface area of the NiaInbAl/SiO2 catalysts is lower than that of the SiO2 support. Some have observed an increase in surface area after phyllosilicate formation,12b, decreases in surface area,11b,

21

12f, 12m, 19b

but there have also been observed

and the support SiO2 used in this work has high surface area

before catalyst synthesis (510.6 m2/g). The isotherms obtained for the NiaInbAl/SiO2 catalysts at 10 wt% loading, shown in Figure S1a, all exhibit a type IV isotherm shape. The shape of the hysteresis loops for all NIA catalysts is a superposition of types H1 and H3 resulting from both the slit-shaped pores formed by the phyllosilicate arrangement (H3) and the porous silica support (H1),12m,

22

The hysteresis loop for the NA catalyst is H3 due to the increased nickel

phyllosilicate formation, and the IA catalyst, which lacks phyllosilicate, has an H1 type hysteresis loop similar to the SiO2 support. As can be seen in Table 1, as the loadings are increased for the NIA-0.5 and NIA-0.7 catalysts, an increase in surface area is observed until 20 wt% where the NIA-0.5 shows a decrease and the NIA-0.7 shows almost no increase over the 15 wt%. This behavior suggests that at 15 – 20 wt% metal loading, phyllosilicate formation is approaching a limit with respect to the synthesis and aging time used (1 hour in total). Previous studies have shown that the proportion of phyllosilicate is dependent upon aging time, and the increased growth of phyllosilicate structures causes an increase in surface area.23 Figures S2a and b show the isotherms for NIA-0.5 and NIA-0.7 at various loadings. As loading is increased, the hysteresis loops show a shift from spherical to slit-like pores.11b,

24

Figures S2c and d show the pore size distribution for different loadings of NIA-0.7 and NIA-0.5, respectively, and for both catalysts the distribution narrows as loading is increased. A


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corresponding decrease in pore radius can also be observed as loading is increased for NIA-0.5 and NIA-0.7. 3.1.3

XPS analysis

XPS analyses were performed on both calcined and reduced NiaInbAl/SiO2 catalysts to probe chemical characteristics and to examine the nature of the nickel phase. Notably, the binding energies for Ni 2p3/2, O 1s, and Si 2p can be used to aid in characterization of Si-supported Ni catalysts. Representative spectra of both calcined and reduced NIA-0.7 and NA catalysts are shown deconvoluted in Figure 2, and Table 2 lists binding energies of calcined NiaInbAl/SiO2 catalysts along with binding energies for reference compounds. All NiaInbAl/SiO2 containing nickel show strong satellite structures which are due to electron shake-up.12j,

12k

As Table 2

shows, the Ni 2p3/2 binding energies for calcined NiaInbAl/SiO2 catalysts are in the range of 856.7 – 857.2 eV, which is near the binding energy for 2:1 Ni phyllosilicate, but certainly higher than binding energies observed for nickel oxide and hydroxide species. The Ni 2p3/2 satellite splitting (∆Esat), which is the separation between the primary and satellite lines, falls between 5.6 and 5.8 eV for NiaInbAl/SiO2 catalysts. This is consistent with Ni phyllosilicate and as well as sesquioxide or hydroxide compound, but these types of oxide compounds can be ruled out due to thermal stability (vide infra). The binding energy for O 1s has also been used to further examine the nature of the Ni phase in Ni/SiO2 materials, and it can be seen in Table 2 that the O 1s binding energies for NiaInbAl/SiO2 catalysts (532.3 – 532.8 eV) are slightly above the range for Ni phyllosilicates but in the range of SiO2, which is expected because of the high SiO2 content. However, the O 1s peak is asymmetric (not shown) indicating multiple oxygen species, one of which is the SiO2 support. Examination of the Si 2p, and more importantly the difference between the binding energies of Ni 2p3/2 and Si 2p (∆ENi-Si), have been used to aid in Ni species


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identification.12j, 25 Nickel silicates have been reported to give a ∆ENi-Si in the range of 752.8 – 753.8 eV with other Ni compounds having much smaller separation.12j, 12k, 26 The high ∆ENi-Si separation observed for NiaInbAl/SiO2 catalysts at 754.0 – 754.3 eV further suggests the presence of Ni phyllosilicate. XPS results for reduced NiaInbAl/SiO2 catalysts are shown in Table 3, and as can also be seen in Figure 2, the reduced NIA catalysts exhibit an additional Ni 2p3/2 peak at lower binding energy (852.0 – 852.4 eV) which is due to metallic Ni. While this low range of values is below that shown for Ni0 in Table 2, contributions by other Ni compounds can most likely be ruled out accordingly. The Ni 2p3/2 phyllosilicate peak ca. 857 eV can be observed after reduction, but has shifted to lower binding energy for all NIA catalysts. This demonstrates that a partial reduction of the catalyst has taken place, and that a portion of the phyllosilicate structure remains after the reduction process. The values of ∆Esat and ∆ENi-Si for reduced catalysts shown in Table 3 also support this observation. The In 3d5/2 binding energies for calcined and reduced NiaInbAl/SiO2 catalysts are shown in Table S2 with values ranging from 444.8 to 445.2 eV for calcined catalysts and 444.4 to 445.2 eV for reduced catalysts. These values are consistent with previous reports of binding energies for oxidized and reduced indium, as is the characteristically small shift of binding energy upon reduction.27 XPS data for aluminum are not useful due to overlap of Ni 3p satellite structure with the Al 2p line and low aluminum concentration.12k


ACS Catalysis

Intensity (a.u.)

a

857.2 862.8

I = 500

Calcined NIA-0.7

856.4 852.4

862.1 Reduced NIA-0.7 Acquisition Envelope

Background Fit component

857.0

b I = 2000

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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862.8

856.8

Calcined NA 862.5 Reduced NA 872

868 864 860 856 852 Photoelectron Binding Energy (eV)

Figure 2. Deconvolution of Ni 2p3/2 XPS binding energies for calcined and reduced (a) NIA-0.7 10 wt% and (b) NA 10 wt% catalysts. Table 2. XPS results for calcined NiaInbAl/SiO2 catalysts at 10 wt% loading and binding energies for reference compounds. Calcined Catalyst

Ni 2p3/2 (eV)

∆Esata (eV)

O 1s (eV)

Si 2p (eV)

∆ENi-Sib (eV)

IA NIA-0.3 NIA-0.4 NIA-0.5 NIA-0.7 NIA-1.5 NA Reference Compoundc

857.0 857.1 856.9 857.2 856.7 857.0

5.7 5.8 5.7 5.6 5.7 5.7

532.8 532.7 532.7 532.5 532.7 532.3 532.5

103.0 103.0 103.1 102.8 103.1 102.8 103.0

754.1 754.3 754.2 754.2 754.0 754.2


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Ni0 NiO Ni2O3 Ni(OH)2 1:1 Ni phyllosilicate 2:1 Ni phyllosilicate

102.5 103.1 - 103.3

SiO2

103.2 – 103.7

852.8 - 852.9 854.0 - 854.9 6.5 - 7.2 529.6 – 530.4 855.7 - 855.8 5.6 530.0 – 531.7 855.5 - 856.5 5.8 – 5.9 531.0 – 532.0 855.6 - 855.7 5.9 – 6.0 531.1 – 531.5 856.9 - 857.0 n.a. 532.1 – 532.5 d d 856.1 5.6 532.2 – 533.1 a ∆Esat = separation between the primary and satellite lines. b ∆ENi-Si = Ni 2p3/2 - Si 2p. c Reference values collected by Lehmann et al.12j d Determined from Ashok et al.28

Table 3. XPS results for reduced NiaInbAl/SiO2 catalysts at 10 wt% loading. Reduced Catalyst

Ni0 2p3/2 (eV)

Ni 2p3/2 (eV)

∆Esata (eV)

O 1s (eV)

IA 532.5 NIA-0.3 852.4 856.5 532.3 5.6 NIA-0.4 852.4 856.6 532.1 5.6 NIA-0.5 852.4 856.4 532.0 5.7 NIA-0.7 852.4 856.4 532.4 5.7 NIA-1.5 852.0 856.3 532.6 5.8 NA 856.8 532.4 5.7 a ∆Esat = separation between the primary and satellite lines. b ∆ENi-Si = Ni 2p3/2 - Si 2p.

3.1.4

Si 2p (eV)

∆ENi-Sib (eV)

103.1 102.8 102.6 102.6 102.8 103.0 102.8

753.7 753.9 753.8 753.5 753.3 754.0

Electron microscopy analysis

Analysis of the phyllosilicate phase of NIA-0.7 15 wt% was conducted by TEM and SEM, and Figure 3a and b are SEM and STEM images showing the phyllosilicate structures on the porous SiO2 support surface. HRTEM of the phyllosilicate structure is shown in Figure 3c where lattice is visible in contrast to the amorphous SiO2 support illustrating the layered nature of the phyllosilicate phase. An energy dispersive X-ray spectroscopy (EDS) line scan across the


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Page 16 of 46

phyllosilicate structure is indicated on Figure 4 as a yellow line, and the corresponding results are shown in the bottom half of the figure. The line profile reveals an increase in Ni, Si, O, and to a much lesser extent, In signals as the scan reaches the structure (marked as a vertical red dashed line) which suggests that the structure is primarily nickel phyllosilicate.

Figure 3. Phyllosilicate structure detail of calcined NIA-0.7 15 wt%. (a) SEM, (b) STEM, and (c) HRTEM.


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ACS Catalysis

Figure 4. EDS line scan across phyllosilicate structure of NIA-0.7 15 wt%. STEM image (top) includes yellow line indicating location of scan, and corresponding fractional composition data (bottom). Very similar morphology can be seen for nickel phyllosilicate, nickel carbonate, and nickel hydroxide in the turbostratic form when analyzed by TEM, but further characterization can rule out both the carbonate and hydroxide forms.12j, 29 Ghuge et al. found that at high SiO2/Ni ratios (~2) phyllosilicate was the dominant form over carbonate,12o and since the ratios for catalysts in this work are higher (≥ 6 for SiO2:NiInAl), it is unlikely that the dominant form is carbonate. In addition, a weight loss should occur upon heating between 200 and 500 °C for nickel carbonate, and TGA data for NIA-0.7 catalyst show no such loss (vide infra). EDS (Figure 4) shows a high fraction of silicon in the structures which suggests phyllosilicate rather than turbostratic nickel hydroxide. Additionally, thermal stability can aid in determining if structures present are hydroxide because nickel hydroxide decomposes into NiO at temperatures above 200 °C.30 Since


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Page 18 of 46

all NA, NIA, and IA catalysts in this work have been calcined at 400 °C, the hydroxide structure can most likely be ruled out as well. Examination of the pre-reduced catalysts using TEM reveals that after reduction the phyllosilicate structures have decreased in size, and particles are observed on the surface illustrating that a low-temperature reduction facilitates formation of well-dispersed particles while maintaining a portion of the phyllosilicate.10 These can be seen in Figure 5 accompanied by particle size distributions which show the greatest proportion of particle diameters near the 2.5-3.5 nm range for all catalysts. The range of particles sizes for NA is much narrower than the NIA catalysts, and the NIA-0.7 has the widest distribution. Additionally, a small shift to slightly larger particle size is observed as the Ni fraction is increased, and for NIA catalysts with higher Ni fractions, a greater amount of phyllosilicate is observed. In Figure 6 (top), a HRTEM image of a metallic particle from reduced NIA-0.7 15 wt% shows spherical morphology and has observable lattice structure. An EDS line scan across a metallic particle reveals composition that is mainly nickel and indium, but also a small concentration of aluminum is detected. Since the phyllosilicate structures contain mainly nickel, the particles containing high concentrations of both nickel and indium means that the nickel atoms freed by decomposition of the silicate layers during reduction agglomerate with indium and aluminum atoms on the surface to form larger particles.


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ACS Catalysis

Figure 5. STEM images and particle size distributions for reduced (a) NIA-0.3, (b) NIA-0.4, (c) NIA-0.5, (d) NIA-0.7, (e) NIA-1.5, and (f) NA catalysts.


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Figure 6. HRTEM of metallic particle from NIA-0.7 15 wt% (top), EDS line scan across metallic particle in STEM image with yellow line indicating location of scan (middle), and corresponding fractional composition data (bottom). 3.1.5

XRD Characterization Figure 7 shows XRD powder diffraction patterns for both calcined (a) and reduced (b)

catalysts at 10 wt%. Phyllosilicate peaks are visible for the calcined NA catalyst near 2θ angles of 34.5°, 35.8° and 60.4°.10,

31

The (002) phyllosilicate peak is not observed, but at lower

synthesis temperatures the degree of crystallization is low which results in smaller or absent XRD peaks, particularly the (002) peak which is absent here.10, 12b, 12d, 12j For the calcined NIA-


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ACS Catalysis

0.7 and NIA-0.5 catalysts, the peak at 60.4° is not distinguishable and the peaks at 34.5° and 35.8° are present as a single broad peak that is almost imperceptible. After reduction, the phyllosilicate peaks for NA are still visible, but the amorphous silica peak at 21.9° is larger, indicating the loss of some of the phyllosilicate. The already small peaks observed for the calcined NIA-0.7 and NIA-0.5 are not apparent after reduction. For NA and NIA catalysts, after loss of phyllosilicate from reduction there are no new peaks observed for metallic Ni, In, or Al. The small size of the metallic particles observed in Figure 5 for reduced samples cause peak broadening which makes observation of peaks difficult, particularly when considering the amorphous nature of the support. As observed with TEM in Figure S4, the IA catalyst (no Ni) does not form phyllosilicate, and this is supported by the lack of phyllosilicate XRD peaks for the calcined catalyst and presence of metallic In XRD peaks (PDF#00-005-0642) after reduction as seen in Figure 7b.


ACS Catalysis

+

a

SiO2 PS

IA

+

NIA-0.4

+

NIA-0.5 NIA-0.7 +

Intensity (a.u.)

NIA-0.3

+

+

NIA-1.5 NA

10

+

+

20

30 40 50 2θ (degrees)

60

SiO2 In0 PS

NIA-0.3 NIA-0.4 NIA-0.5 NIA-0.7 +

Intensity (a.u.)

IA

♦ ♦

♦

♦ ♦

+

♦

♦

b

70

+

+

NIA-1.5 +

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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+

NA

10

20

30 40 50 2θ (degrees)

60

70

Figure 7. X-ray diffraction patterns for (a) calcined and (b) reduced NiaInbAl/SiO2 catalysts at 10 wt% loading. PS denotes phyllosilicate peaks. Test conditions: Cu Kα radiation, 40 kV and 40 mA, 1°/ min. 3.1.6

Hydrogen TPR

H2 TPR was performed on NiaInbAl/SiO2 catalysts at 10 wt% loading to probe reducibility, and the profiles are shown in Figure S5. The reduction peak for the NA catalyst is at 650 °C and the catalysts containing both nickel and indium exhibit a reduction peak ca. 500 °C but with a secondary peak at 660 °C. The profile for IA catalyst has multiple peaks, which could be caused


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ACS Catalysis

by the presence of different sized In particles where smaller, highly dispersed particles are reduced at lower temperatures compared to larger particles.32 The NIA catalysts demonstrate a decrease in reduction temperature compared to NA and IA which allows for lower-temperature partial reduction (390 °C) and partial retention of the phyllosilicate substrate while inducing metal particle formation in situ. The remaining phyllosilicate-supported particle morphology obtained through this type of partial reduction has been shown to aid in dispersion, and guard against sintering and carbon deposition through the availability of surface OH groups.10, 12m, 33 The reduction curves also demonstrate the lack of nickel hydroxide or nickel oxide since there are no reduction peaks below 400 °C.10,

34

In terms of examining the particular form of Ni

phyllosilicate, the 2:1 form should exhibit a higher reduction peak temperature compared to the 1:1 form, but the reduction peak is influenced by the synthesis temperature.12b,

12j

Nickel

phyllosilicates synthesized at room temperature similar to those used by this work showed peaks near 385 °C and 450 °C for 1:1 and 2:1 forms, respectively,12b and Burattin et al. found that the reduction peak of nickel phyllosilicates prepared at 90 °C was 450-650 °C for the 1:1 form and 690-760 oC for the 2:1 form.12b, 12c In addition, the reduction temperature is also influenced by the degree of crystallization of the Ni phyllosilicates.35 The reduction peak temperature for the NA catalyst in this work is near 650 °C which is much higher than the results for roomtemperature synthesized Ni phyllosilicates previously mentioned. A higher reduction temperature could suggest the 2:1 form, while the lower reduction temperature observed for the NIA catalysts might suggest 1:1, but the XPS data show very little difference between the Ni in the NA and NIA catalysts. It is possible that the higher Ni content and absence of In in the NA catalyst could account for the higher degree of crystallization of Ni phyllosilicate, which would result in a higher reduction temperature.


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The shape of the reduction peak has also been found useful in distinguishing phyllosilicate forms. Many studies have shown that 2:1 phyllosilicate produces an asymmetric reduction peak with tailing on the lower temperature side, while the 1:1 form does not.10, 12b, 12j, 35 The reduction peak for the NA catalyst, seen in Figure S5, appears to somewhat fit this profile, however it has also been shown that 1:1 phyllosilicate synthesized at 80 °C produces a less-defined asymmetric reduction peak which also resembles the peak for the NA catalyst.10 In addition, a mixture of phyllosilicate type cannot be ruled out. When the phyllosilicate phase is ill-crystalized, distinction between the phyllosilicate forms becomes increasingly difficult. Because of this, the TPR curves alone are not able to reveal the specific phyllosilicate type. Other parameters have also been found to influence the type of phyllosilicate that forms, such as synthesis method and aging time. When synthesis is performed at the conditions in this work (room temperature, short aging times (< 100 min), deposition-precipitation method), the phyllosilicate phase that forms has been shown to be 1:1.12b However, identification when the phyllosilicate is synthesized in the presence of In is clearly less straightforward.

3.2 3.2.1

Catalytic Behavior Methanol synthesis activity

NiaInbAl/SiO2 catalysts were evaluated for activity of methanol synthesis, and the results for catalysts with varied Ni/In ratios at 10 wt% loading are shown in Figure 8. For activity calculations, catalyst metal refers to the total amount of MeOH synthesis catalyst metal present as determined by ICP-OES, which does not include promoter or support material. For CZA, the catalyst metal is based on the total amount of copper, and for the NIA catalysts the catalyst metal is based on the total amount of both the nickel and indium. It can be seen that the NA catalyst


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ACS Catalysis

with indium absent produces very little methanol, while the IA catalyst with nickel absent did not produce methanol. This shows that the Ni and In are individually poor methanol synthesis catalysts at these concentrations, but perform well when combined. As the Ni/In ratio changes, the activity peaks at ratios of 0.5 to 0.7 (0.26 and 0.27 mol h-1 mol catalyst metal-1, respectively, compared to 0.17 mol h-1 mol catalyst metal-1 for CZA). The activity for NiaInbAl/SiO2 catalysts at 10 wt% loading increases in the series: IA < NA < NIA-4.0 < NIA-1.5 < NIA-1.0 < NIA-0.3 < NIA-0.4 < NIA-0.7 < NIA-0.5. The Ni/In ratio also influences the temperature of peak activity and, as seen in Figure 8, the peak activity temperature increases as the Ni/In ratio decreases. It is worth mentioning that for the NIA catalysts at 10 wt% loading, there is a correlation between high activity and high surface area. When activity results are normalized for surface area as shown in Figure S3, the trend of the results is similar to when normalized by moles which shows that the performance is not exclusively due to surface area effects. The effect of total metal loadings (5, 10, 15, and 20 wt%) on catalytic activity was investigated. The activity of NIA-0.5 and NIA-0.7 at different loadings as well as CZA are shown in Figure 9. The greatest methanol yield was achieved by NIA-0.7 with 15 wt% metal loading at 0.33 mol h-1 mol catalyst metal-1, which is approximately 1.9 times the activity of CZA. Recent studies examining NiGa/SiO2 and GaPd/SiO2 for ambient pressure methanol synthesis yielded results equivalent to CZA activity (NiGa/SiO2)

4a

and 1.6 times higher than

CZA (GaPd/SiO2).8g Figure 9 also shows that there is a decrease in the peak activity temperature with higher metal loading. At the peak activity temperature for CZA (215 °C), NIA-0.5 at 15 and 20 wt% have similar activity to that of CZA, while the activities of NIA-0.7 catalysts with the same loadings are superior to that of the CZA at the same temperature. The temperatures of peak activities for


ACS Catalysis

all NIA catalysts are higher than that for CZA, with the NIA-0.5 and NIA-0.7 at 15 wt% loading peaking approximately 45 °C higher than CZA. The activity, conversion, and selectivity for NiaInbAl/SiO2 and CZA catalysts are summarized in Table 4. All NIA catalysts show very little production of hydrocarbon side products (only methane was detected) up to approximately 260 °C, whereas the CZA selectivity starts to decrease significantly above 215 °C. It should be noted that the methane produced is a small fraction of the methanol production, so at low methanol production levels, methane levels drop

0.27

0.10

0.2

0.16

0.3

0.17

NA NIA-1.5 NIA-0.7 NIA-0.5 NIA-0.4 NIA-0.3 CZA

0.25

0.26

below the detectable limit.

0.1 0.01

MeOH formation rate (mol h-1 mol catalyst metal-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 46

0.0 185

200

215

230

245 260 Temperature (°C)

275

290

305

Figure 8. Methanol production from NiaInbAl/SiO2 catalysts at 10 wt% loading and CZA catalysts as a function of temperature at ambient pressure. Peak activity for each catalyst is indicated with a star followed by the peak activity rate.


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ACS Catalysis

Figure 9. Methanol production from NIA-0.5 and NIA-0.7 at 5, 10, 15, and 20 wt% loading, as well as CZA catalyst, as a function of temperature at ambient pressure. Peak activity for each catalyst is indicated with a star followed by the peak activity rate.

Table 4. Catalytic performance summary for NIA and CZA catalysts. Catalyst

Loading

Tpeaka

MeOH activityb

CO2 conversion

(wt%)

(°C)

(mol h-1 mol catalyst metal-1)

(%)

CZA NIA-0.3 NIA-0.4 NIA-0.5

Selectivity (%) CO

MeOH

CH4

63 215 0.17 3.1 87.8 12.2 0.04 10 290 0.16 1.6 98.9 1.1 10 275 0.25 3.1 98.5 1.5 10 275 0.27 2.1 98.2 1.8 15 260 0.32 2.9 97.4 2.6 20 260 0.33 3.2 97.3 2.7 NIA-0.7 10 260 0.26 2.5 98.2 1.8 15 260 0.33 3.8 97.7 2.3 0.01 20 245 0.30 2.3 96.4 3.6 NIA-1.5 10 260 0.10 3.7 99.5 0.5 NA 10 230 0.01 2.2 89.6 0.2 10.2 a Temperature of peak MeOH activity. b MeOH activity at Tpeak (N2:H2:CO2 = 6.75:2.25:1 (H2:CO2 = 3:1), 20 mL/min, SV = 4000 h-1).


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To examine the thermodynamic equilibrium associated with hydrogenation of CO2 to methanol, the following equilibria are considered:

CO2 + 3H2 ⇔ CH3 OH + H2 O

(E1)

CO2 + H2 ⇔ CO + H2 O

(E2)

CO + 2H2 ⇔ CH3 OH

(E3)

CO2 + 4H2 ⇔ CH4 + 2H2 O

(E4)

To determine the equilibrium constants (K), the associated equilibrium equations (E5 – E8) are used, and to calculate experimental constants (k), concentration data from activity experiments are used in E5 – E8 yielding values denoted as k1 – k4. = =

[CH3 OH] [H2 O]

[CO2 ] [ ]

(E5)

[CO] [H2 O] [CO2 ] [ ]

(E6)

[CH OH] ]

= [CO] 3[ =

(E7)

[CH4 ] [ ]

(E8)

[CO2 ] [ ]

Since experiments are performed with reaction gases at atmospheric pressure, the pressure terms in K1, K3, and K4 are unity. The resulting equilibrium and experimental constants are shown in Figure 10 revealing that all experimental values are below equilibrium. At 215 °C and below, methanol production from CZA is greater than NIA-0.7 15 wt% (k1 and k3), but decreases at elevated temperatures and is surpassed for k3 at 230 °C, and k1 above 260 °C by NIA. It also appears that k1 for NIA increases up to 275 °C unlike the CZA which decreases above 230 °C.


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For k2, which is the reverse water gas shift reaction (E2), the k values for CZA are greater throughout the experimental temperature range.

100

K and k

10-2 10-4

Eq. K1 Eq. K2 Eq. K3 CZA k1 CZA k2 CZA k3 NIA k1 NIA k2 NIA k3

10-6 10-8 10-10 185 200 215 230 245 260 275 Temperature (°C)

Figure 10. Equilibrium constants (solid lines) and experimental constants for CZA (dashed lines) and NIA-0.7 15 wt% (dotted lines, denoted as NIA on graph). (Δ) CO2 + 3H2 ⇌ CH3OH + H2O, (□) CO2 + H2 ⇌ CO + H2O, (○) CO + 2H2 ⇌ CH3OH. The products formed when using CO as the carbon source in lieu of CO2 with NIA-0.7 15 wt% were examined and the activity values are shown in Figure 11. The primary hydrocarbon product was found to be propane (C3H8) when using a CO/H2 feed (H2:CO = 2:1) under the same

Formation rate (mol h mol catalyst metal-1)

reaction conditions as the CO2/H2 activity tests that produced methanol.

-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

0.10 0.08

CH4 C3H8

0.06 0.04 0.02 0.00 200 215 230 245 260 275 Temperature (°C)


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Figure 11. Hydrocarbon production from NIA-0.7 15 wt% as a function of temperature using CO/H2 feed at ambient pressure.

3.2.2

Stability

To examine the stability of the catalysts in this work compared to CZA, tests were performed on NIA-0.7 at 10 and 15 wt% loading and NI at 15 wt% loading at the temperature of peak activities of the catalysts, which were determined from previous experiments. As shown in Figure 12a, the activity of both the 10 and 15 wt% NIA-0.7 catalysts show no noticeable decreases within the first 120 hours of reaction, while the CZA shows a near 27% decline. After 120 hours on stream, the catalysts were subjected to a mid-cycle reduction which caused an increase in the activity but a decrease in stability, particularly for the NIA-0.7 15 wt%. This behavior supports that partial reduction occurred after the initial reduction treatment, but further reduction (mid-cycle) resulted in additional loss of phyllosilicate. This causes an initial increase in activity due to newly available metal particles, but decreased sintering protection from the loss of phyllosilicate structure results in the formation of larger particles and lower stability. Figure 12b is a STEM image of the used NIA-0.7 15 wt% catalyst which shows the formation of additional metal particles but less visible phyllosilicate structure compared to the reduced catalyst as shown in Figure 5. Before the mid-cycle reduction, the NI-0.7 15 wt% catalyst (no aluminum) experiences an activity loss of approximately 17%, demonstrating lower stability than the catalysts containing aluminum. Moreover, the NI-0.7 15 wt% has lower activity than the aluminum-containing NIA-0.7 15 wt%.


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ACS Catalysis

Figure 12. (a) Stability of NIA-0.7 10 and 15 wt%, and NI-0.7 15 wt% catalysts compared to CZA; (b) STEM image of NIA-0.7 15 wt% after stability testing; (c) TGA profiles for NIA-0.7 catalyst in calcined, reduced, and used form. “Used” refers to catalyst after stability testing. The calcined, reduced, and used NIA-0.7 15 wt% catalysts were also examined by using TGA, the results of which are shown in Figure 12c. The calcined catalyst exhibits a small weight loss below ~150 °C due to the loss of adsorbed water, and an additional loss up to ~500 °C resulting from the loss of OH groups.10, 12o The reduced NIA-0.7 15 wt% catalyst experiences only a very small loss of water, and the used catalyst has almost no weight loss at all which, together with the results of the stability test, indicates that carbon deposition is negligible. No significant losses are observed above 500 °C which demonstrates the thermal stability of the catalysts after reduction.


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3.3 3.3.1

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FTIR in situ DRIFTS studies CO2 and CO temperature programmed adsorption on NIA-0.7 15 wt%

To examine the effects of CO2 sorption on NIA catalysts and to identify surface species, in situ DRIFTS temperature programmed adsorption (TPA) tests were performed using NIA-0.7 15 wt% due to its high activity. TPA was examined in the range from 10 to 450 °C under flowing CO2, and the results can be seen in Figure 13. Identification of surface active species can be aided by examining not only the position of the bands, but also the CO3- ν3-band splitting width (∆ν3 = νas - νs) as well as thermal stability. After sorption of CO2 at 10 °C, peaks are observed in the region of 1300 to 1800 cm-1 at 1676, 1549, 1412, and 1384 cm-1 (Figure 13). The peaks at 1676 and 1412 cm-1 can be seen to decrease in intensity as the temperature is increased, and are no longer present above 200 °C. Adsorption of CO2 on similar materials commonly forms various (bi)carbonate species, and low thermal stability is typically indicative of bicarbonate species (HCO3-).36 The position of these bands also suggests the presence of surface bicarbonates36b,

37

and in addition, ∆ν3 of approximately 260 cm-1 is consistent with HCO3-

species.36b, 38 Based on these considerations, the peaks at 1676 and 1412 cm

-1

are assigned to

νas(CO3) and νs(CO3) of surface bicarbonate, respectively. The peaks at 1549 and 1384 cm-1, with ∆ν3 = 165 cm-1, show an increase in intensity up to 200 °C, followed by a decrease at higher temperatures. The positions of these bands are similar to those observed for monodentate carbonate (m-CO32-) on Ni/MgO39 as well as others,40 and ∆ν3 near 165 cm-1 has been observed for m-CO32- on a variety of surfaces.36a,

39-40

Additionally, monodentate carbonate should be

more thermally stable than bicarbonate,41 and thusly, the peaks at 1549 and 1384 cm-1 are ascribed to νas(CO3) and νs(CO3) modes of monodentate carbonate.


Page 33 of 46

As the temperature is increased above 200 °C new bands emerge at 1597, 1478, and 1437 cm-1. The band at 1597 cm-1 is attributed to the νas(CO3) stretching mode of bidentate carbonate (bCO32-) which has higher thermal stability than m-CO32- and has been observed in this position.36a The peaks at 1478 and 1437 cm-1 appear and grow together as the temperature is increased to 500 °C, and this high thermal stability suggests multiple bonded CO2.42 Polydentate carbonate (p-CO32-) has similar band positions and ∆ν3 of less than 100 cm-1, as well as high thermal stability compared to other species,37b,

38, 41-42

and so the peaks at 1478 and 1437 are due to

νas(CO3) and νs(CO3) stretching modes of p-CO32-. Therefore, the sequence of binding strengths of the CO2 adsorbates as temperature increases is as follows: HCO3- < m-CO32- < p-CO32- < bCO32-. -

500 °C 400 °C 300 °C 200 °C 100 °C 10 °C

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

HCO3 2-

2-

b-CO3

2-

p-CO3

m-CO3

2-

m-CO3

-

HCO3

A = 0.05

1800

1700 1600 1500 Wavenumber (cm-1)

1400

Figure 13. Infrared spectra of CO2 TPA measured from 10 to 500 °C after in situ reduction and purging the IR cell with He, after which the background spectrum was taken.

Carbon monoxide TPA was also performed on NIA-0.7 15 wt% under the same conditions as the CO2 TPA tests, and the results are shown in Figure 14. The initial CO sorption at 10 °C


ACS Catalysis

results in multiple bands in the region from 1800 – 2100 cm-1 with bands above ~2000 cm-1 attributed to linearly adsorbed CO, and those below ascribed to bridged CO.39, 43 The band at 2064 cm-1 is due to the formation of nickel carbon monoxide hydroxyl species, which is a linearly adsorbed CO species with associated hydroxyl (Ni-C=O-OH).28 The peaks at 2010 and 1956 cm-1 are assigned to linear and bridged CO of bi-nuclear Ni2(CO)y.39 As the temperature is increased, the three bands at 2064, 2010, and 1956 cm-1 decrease and disappear above 200 °C, meanwhile new peaks emerge at 2081 and 2033 cm-1. The band at 2081 cm-1 can be ascribed to the formation of subcarbonyl species, Ni(CO)y (y=2, 3), and the lower frequency peak at 2033 cm-1 is due to linearly adsorbed CO species.28, 39 Above 300 °C the peak at 2081 cm-1 vanishes while the linear CO species at 2033 cm-1 remains. Small amounts of CO2 are also observed beginning at 50 °C and increase with temperature, and in addition to the CO species, carbonate species are observed at 200 °C and above with both bidentate and polydentate carbonate present.

500 °C 400 °C 300 °C 200 °C 100 °C 10 °C

A = 0.1

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 46

CO (g)

2200

linear CO

bridged CO

2- p-CO 23

b-CO3

2000 1800 1600 Wavenumber (cm-1)

1400

Figure 14. Infrared spectra of CO TPA measured from 10 to 500 °C after in situ reduction and purging the IR cell with He, after which the background spectrum was taken.


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ACS Catalysis

3.3.2

CO2/H2 isothermal reaction on NIA-0.7 15 wt%

The time dependent infrared spectra for isothermal reaction of flowing CO2 and H2 on NIA-0.7 15 wt% at 250 psi are shown in Figure 15. In the presence of H2, there are new peaks observed compared to those found with CO2 TPA (Figure 13). In the first few minutes, peaks appear at 1662, 1597, 1576, and 1377 cm-1 and are accompanied by bands at 3015, 2959, 2904, and 2857 cm-1. The peaks at 1576, 1377, and 2857 cm-1 are assigned to νas(CO2), νs(CO2), and ν(CH) of bidentate formate (b-HCOO).36b, 38, 42-43, 44 A ∆ν near or below 220 cm-1 is indicative of either bridging or bidentate formate, and the ∆ν of 199 cm-1 observed here is very similar to splitting observed by other groups for b-HCOO.36b, 38, 42, 44a, 44b In addition to the ν(CH) peak at 2904 cm-1, the smaller peak at 1662 cm-1 on the high frequency side of the νas(CO2) b-HCOO peak is attributed to monodentate formate (m-HCOO) which has a higher wavenumber νas(CO2) peak compared to b-HCOO and has been observed in this position.36b, 42 At later times, the carbonate species observed for CO2 TPA also appear (m-CO32-, b-CO32-, p-CO32-). In the CH-stretching region the peak at 3015 cm-1 emerges first belonging to gaseous CH4, the presence of which was confirmed in the activity tests via GC.43a,

45

Several other peaks are observed and are due to

multiple CH modes and combination bands associated with the various compounds present.


ACS Catalysis

A = 0.05

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

b-CO32-

200 min. 80 min. m-HCOO 30 min. 15 min. 5 min. 3 min. 2 min. 1 min.

1800

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A = 0.01 b-HCOO

3000 m-CO32-

2900

2800

m-CO32p-CO32-

1700 1600 1500 Wavenumber (cm-1)

b-HCOO

1400

Figure 15. Time-evolved infrared spectra of isothermal CO2/H2 reaction (Prior to test, sample was reduced in situ and the IR cell was purged with He, followed by collection of the background spectrum).

3.3.3

CO2 sorption followed by CO2/H2 TPSR on NIA-0.7 15 wt%

Following adsorption of CO2 at 30 °C for one hour, temperature programmed surface reaction was performed on NIA-0.7 15 wt% using a CO2/H2 feed (H2:CO2 = 3:1) from 30 to 450 °C at 3 °C/min, after which the temperature was decreased back to 30 °C at the same rate. The IR spectra for both the temperature increase and decrease are presented in Figure 16. After CO2 sorption at 30 °C, the spectrum looks very similar to CO2 adsorption at 10 °C seen with CO2 TPA (Figure 13). When the feed is changed to CO2/H2, there is no observable effect on the existing (bi)carbonate peaks. Before increasing the temperature under the CO2/H2 flow, the spectrum at 30 °C exhibits peaks associated with HCO3- (νas(CO3) = 1676 cm-1, νs(CO3) = 1412 cm-1) and a small m-CO32- peak (νas(CO3) = 1549cm-1), all of which vanish when heated above 150 °C (Figure 16, top). Above 200 °C, peaks due to formate species begin to appear with the


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bidentate configuration emerging first (ν(CH) = 2857 cm-1, νas(CO2 = 1576 cm-1) followed closely by the monodentate formate peaks (ν(CH) = 2904 cm-1, νas(CO2 = 1662 cm-1). The bands associated with the formate species increase upon heating until reaching a maximum at 300 °C, and subsequently decrease until vanishing above 325 °C (m-HCOO) and 400 °C (b-HCOO). The spectra obtained during the temperature decrease are shown in the lower half of Figure 16. When decreasing the temperature, the peaks due to b-formate appear below 425 °C and those attributed to m-HCOO emerge at 325 °C, with all formate peaks increasing as the temperature decreases. The negative peak observed at 1635 cm-1 is attributed to the O-H-O bending mode of molecular water.43a, 46 This peak is seen to decrease with increasing temperature during the ramp up, and shows a partial recovery during ramp down. This negative molecular water peak is not observed during the CO2/H2 isothermal reaction (Figure 15). During both the temperature ramp up and down, the peak for gaseous methane (3015 cm-1) is considerably smaller than the peak observed during the CO2/H2 isothermal reaction (Figure 15, 260 °C) and is only observed above 400 °C. Additionally, during the temperature ramp down there are no (bi)carbonate species observed as in the isothermal reaction. This suggests that the formate species (more competitively) occupy the same sites as the (bi)carbonate species.


ACS Catalysis

-

HCO3 Ramp up (30-450°C)

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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b-HCOO m-HCOO 2m-CO3 2b-CO3

450 °C 400 °C 300 °C 200 °C 100 °C 30 °C Ramp down (450-30°C)

HCO3

-

A = 0.01

3000

2800

A = 0.01

A = 0.1 3000

1800

1700 1600 1500 Wavenumber (cm-1)

2800

1400

Figure 16. Infrared spectra of CO2/H2 TPSR measured as temperature is increased from 30 to 450 °C (top) and decreased from 450 to 30 °C (bottom). Before temperature was increased, the sample was reduced, purged with He, and loaded with CO2 at 10 °C.

3.3.4

CO/H2 isothermal reaction on NIA-0.7 15 wt%

Isothermal reaction was performed using flowing CO/H2 at 250 psi with the resultant IR spectra shown in Figure 17. As was observed in the activity tests in Section 3.2.1, when using CO as the carbon source instead of CO2 the hydrocarbon product profile shifts from methanol to propane and methane (Figure 11). The spectrum collected after one minute (Figure 17, 1 min) is similar to the spectrum obtained after CO adsorption at 10 °C (Figure 14) showing no adsorption in the range of 1300 to 1800 cm-1. After 2 minutes, CO2 production is observed and peaks begin to emerge due to b-CO32- and p-CO32- which are soon followed by the appearance of the peak at 3015 cm-1 attributed to gaseous CH4 after five minutes. After 120 minutes, the peaks associated with m-HCOO at 1664 and 2904 cm-1 appear and continue to increase with time, followed by the


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appearance of a shoulder at 1576 cm-1 associated with b-HCOO. A negative peak associated with water is also observed at 1635 cm-1, which decreases with time.

A = 0.05

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

1800

b-CO3

2-

b-HCOO

p-CO3

2-

m-HCOO

360 min. 240 min. 120 min. 30 min. 15 min. 5 min. 3 min. 2 min. 1 min.

A = 0.05

H2 O 3000 2900 2800

1700 1600 1500 1400 Wavenumber (cm-1)

Figure 17. Time-evolved infrared spectra of isothermal CO/H2 reaction. Prior to test, sample was reduced in situ and the IR cell was purged with He, followed by collection of the background spectrum.

3.3.5

Discussion of results from FTIR studies

Table 5 summarizes the assignments to signals for (bi)carbonate and formate. The behavior observed in the IR spectra from the isothermal reaction with CO/H2, which was observed to produce almost no methanol in activity tests, differs from the behavior observed when using CO2/H2 as the feed in several ways. First, m-CO32- is not detected under CO/H2 feed conditions, but was observed when using CO2/H2 feed. CO2/H2 TPSR revealed that m-CO32- decreases at temperatures above 200 °C with a concomitant emergence of the formate bands, suggesting that the hydrogenation of CO2 involves conversion through a m-CO32- intermediate species. Additionally, the monodentate formate bands observed with CO2/H2 appear very small and only


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at later times when using CO/H2, and the strong bidentate bands that can be seen with CO2/H2 are also much smaller and appear later for CO/H2. This suggests that the interconversion between formate species under reaction conditions (CO/H2 flow, 260 °C) is not favored, and that formate is a key intermediate for methanol synthesis with b-HCOO as the primary species hydrogenated to eventually form methanol.

Table 5. Observed infrared signals and assignments for (bi)carbonate and formate species on NIA-0.7 15 wt%. (Bi)carbonate species

νas(CO3) [cm-1]

νs(CO3) [cm-1]

HCO3m-CO32b-CO32p-CO32Formate species

1676 1549 1597 1478 νas(CO2) [cm-1]

1412 1384 not determined 1437 νs(CO2) [cm-1]

ν(CH) [cm-1]

m-HCOO b-HCOO

1662 1576

not determined 1377

2904 2857

4. CONCLUSIONS NiaInbAl/SiO2 was successfully synthesized from a phyllosilicate precursor using the deposition-precipitation method. Characterization tests (TPR, TGA) reveal the absence of nickel oxide, hydroxide, and carbonate, and XPS binding energies show the presence of Ni phyllosilicate. The specific nature of the phyllosilicate phase is unclear due to synthesis in the presence of indium, and the low degree of crystallization found with the catalysts in this work. EDS revealed that the phyllosilicate phase is primarily nickel phyllosilicate, and the layered nature of the structure was shown via HRTEM. Examination of the particles formed after reduction showed that they are composed of Ni, In, and Al and have spherical morphology. As the Ni/In ratio increases for NIA catalysts, average particle size shifts to slightly larger diameter


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ACS Catalysis

by ~0.5 nm, and larger amounts of phyllosilicate remain after reduction. As seen in Figure 5, the Ni/In ratio of 0.7 produces a catalyst with the most beneficial proportion of both particles and phyllosilicate structure after reduction. TPR tests showed that catalysts containing both nickel and indium are reducible at lower temperatures than those with indium absent. This allows for partial reduction at lower temperatures resulting in retention of some phyllosilicate while inducing metal particle formation. Because catalysts in this work have temperature-sensitive structural properties, they are well-suited for lower-temperature (< 400 °C) use, as in methanol synthesis. Methanol synthesis activity tests revealed that Ni and In function as co-catalysts, with neither able to catalyze methanol synthesis effectively in the absence of the other. NiaInbAl/SiO2 catalysts with Ni/In ratios of 0.4 – 0.7 showed the highest activity, and NIA-0.7 at 15% metal loading demonstrated the greatest activity of 0.33 mol h-1 mol catalyst metal-1 compared to the conventional Cu/ZnO/Al2O3 catalyst at 0.17. Several of the NIA catalysts exhibit better CO2 conversion than the benchmark CZA catalyst, and NIA-0.7 was found to have better stability than CZA. Activity was found to be higher when loading was increased to 15 wt%, but did not significantly improve for 20 wt% loading. IR experiments led to identification of several (bi)carbonate and formate species and revealed that CO2 hydrogenation on NiaInbAl/SiO2 catalysts proceeds through monodentate carbonate, followed by further conversion to monodentate and bidentate formate. When the reactant feed was changed to CO/H2 instead of CO2/H2, the primary hydrocarbon product changed from methanol to propane. This was accompanied by a large decrease in formate and absence of monodentate carbonate IR signals.


ACS Catalysis

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Two major factors determine the activity of the NIA catalysts. First, the combination of nickel and indium as co-catalysts is key to methanol synthesis as demonstrated by activity tests which revealed the lack of methanol production from NA and IA catalysts. Second, the ratio of Ni/In in the catalyst is important to achieving catalyst structure and particles of optimal composition after reduction. This is controlled by partial decomposition of the phyllosilicate that releases Ni atoms and allows for formation of highly dispersed particles containing Ni, In, and Al.

AUTHOR INFORMATION Corresponding Author *

(M. Fan) Email: [email protected]; Phone: 1 307 766 5633

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ASSOCIATED CONTENT The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Figure S1-S5, Table S1-S2.

ACKNOWLEDGMENT The authors greatly appreciate the support from the State of Wyoming for this research. REFERENCES 1. Solomon, S.; Plattner, G.-K.; Knutti, R.; Friedlingstein, P., Proc. Natl. Acad. Sci. U. S. A. 2009, 106, 1704-1709.


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Low-Pressure Hydrogenation of CO

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