New insights into structure-activity relationships for propane hydrogenolysis over Ni-Cu bimetallic catalysts

Article abstract of DOI:10.1039/c5ra07433a

Propane hydrogenolysis has been investigated on Ni-Cu/SiO₂ model catalysts under elevated pressure conditions. The surface Ni active sites on Ni-Cu/SiO₂ were measured by selective hydrogen adsorption under ultrahigh vacuum (UHV) conditions. The specific activity of propane hydrogenolysis shows a slight increase and then decrease with increasing Cu coverages on Ni-Cu/SiO₂ bimetallic catalysts, and varies within the 0.6 s^-1 to 2.0 s^-1 range. On the contrary, many previous studies show a three-to-five order decrease in the specific activity of ethane or propane hydrogenolysis over Ni-Cu bimetallic catalysts. The significant difference was explained by the possible over counting of active Ni sites in the selective hydrogen chemisorptions under atmospheric pressure conditions due to hydrogen spillover from Ni to Cu. New structure-activity relationships for Ni-Cu bimetallic catalysts in propane hydrogenolysis were established based on the present work. Furthermore, possible carbon deposits in this reaction were examined by post reaction Auger electron spectroscopy.

Full text of DOI:10.1039/c5ra07433a

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DOI: 10.1039/C5RA07433A
New Insights into Structure-Activity Relationships for Propane
Hydrogenolysis over Ni-Cu Bimetallic Catalysts
Yunxi Yao,* D. Wayne Goodman†
Department of Chemistry, Texas A&M University, College Station, TX 77843, USA
* Corresponding author. E-mail address: yunxiyao@gmail.com (Y.Yao)
† Dr. Goodman deceased on February 27, 2012.
Abstract
Propane hydrogenolysis has been investigated on Ni-Cu/SiO2 model catalysts under
elevated pressure conditions. The surface Ni active sites on Ni-Cu/SiO2 were
measured by selective hydrogen adsorption under ultrahigh vacuum (UHV)
conditions. The specific activity of propane hydrogenolysis shows a slight increase
and then decrease with increasing Cu coverages on Ni-Cu/SiO2 bimetallic catalysts,
and varies within 0.6 to 2.0 s-1 range. On the contrary, many previous studies show a
three-to-five order decrease in the specific activity of ethane or propane
hydrogenolysis over Ni-Cu bimetallic catalysts. The significant difference was
explained by the possible over counting of active Ni sites in the selective hydrogen
chemisorptions under atmospheric pressure conditions due to hydrogen spillover from
Ni to Cu. New structure-activity relationships for Ni-Cu bimetallic catalysts in
propane hydrogenolysis were established based on the present work. Furthermore,
possible carbon deposits in this reaction were examined by post reaction Auger
spectroscopy.
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DOI: 10.1039/C5RA07433A
Keywords: Ni-Cu; bimetallic catalyst; hydrogenolysis; propane; hydrogen spillover;
ensemble effect;
1. Introduction
In the past 40 years, great research interest has been devoted in bimetallic
catalysts, which usually show chemical and catalytic properties different from the
monometallic counterparts.1-10 The distinct properties of bimetallic catalysts from
pure metals are generally explained by ensemble effects and ligand or electronic
effects.1-4 In most cases both effects play a role in the surface adsorption and catalytic
reaction on bimetallic catalysts.11 There is one typical bimetallic system composed of
a Group VIII metal and a Group IB metal, such as Ni-Cu,12-16 Ni-Au,17-20 Pd-Ag21,
Pd-Au,3, 22 Ru-Cu,15, 23, 24 where the active Group VIII metal is diluted by the less
active Group IB metal. Ensemble effects usually are considered to play a more
significant role in such systems.21, 22 Our focus in this paper is to study the surface
structure-activity relationships of Ni-Cu bimetallic catalysts.
Ni-Cu bimetallic catalysts have become a classic system since the pioneering
work of Sinfelt and co-workers on Ni-Cu bimetallic catalysts in 1972 when they
found there was a four-to-five order decrease in the specific activity of the Ni-Cu
catalysts for ethane hydrogenolysis reactions from pure Ni to Ni-Cu bimetallic
catalysts with 60 atomic% Cu.13 Similar phenomena were also observed on Ru-Cu
bimetallic catalysts.14 Sinfelt proposed an ensemble effect to explain the sharp
decrease of the specific activity of ethane hydrogenolysis on Ni catalysts caused by
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15
adding of Cu.1,
Sinfelt suggested active multiplets are needed forDOeI:t1h0a.1n03e9/C5RA07433A
hydrogenolysis on Ni-Cu bimetallic catalysts, and the concentration of such an
ensemble will decline sharply because the active Ni metal atoms are diluted with
inactive Cu metal atoms.1, 15 Dalmon and Martin had proposed that that about
12-to-18 adjacent nickel atoms are required for ethane, propane and butane
hydrogenolysis on Ni-Cu/SiO2 catalysts.25 Alstrup and coworkers found the sulfur
modification effect on nickel catalysts in propane hydrogenolysis cannot be explained
by the advanced ensemble model, which was proposed to interpret the hydrogenolysis
results over Ni-Cu bimetallic catalysts by sinfelt and others,1, 15, 25 and they suggested
breaking of the C-C bonds may need special sites.26
These conclusions are entirely based on the specific activity of the Ni-Cu
catalysts for hydrogenolysis reactions, which is usually obtained with the active Ni
sites measured by selective hydrogen adsorption assuming no hydrogen uptake on Cu
sites on Ni-Cu bimetallic catalysts.1, 15, 25, 26 It is true there is no hydrogen uptake on
pure Cu at room temperature due to the activated dissociative hydrogen adsorption on
Cu.27 But Cu sites could be possibly occupied by atomic hydrogen by hydrogen
spillover from Ni to Cu on bimetallic catalysts, as Crucq et al. observed hydrogen
spillover from Ni to Cu on Ni-Cu alloys in their hydrogen adsorption isotherms.28
Recently we got direct evidence that hydrogen spillover from oversaturated Ni sites to
Cu sites12. As suggested by Goodman and coworkers the possible hydrogen spillover
on bimetallic catalysts could results in over counting active sites and failure in the
hydrogen selective chemisorptions on bimetallic catalysts12, 29. However, our previous
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study shows the selective hydrogen adsorption on Ni sites is still valid under ultrahigh
vacuum (UHV) conditions. 12
Another issue in Ni-Cu bimetallic catalysts is that the Cu sites are traditionally
considered just as a diluting agent due to its lower activity compared with Ni.13-15
Although pure copper is not active for rupture of C-C, or H-H bonds, the Cu sites
surrounding active metals could act as receivers for activated hydrogen or cleaved
carbon-hydrogen species by the surrounded active metal sites.30-32
Inspired by these new research findings, we herein re-investigated the surface
structure-activity relationships of Ni-Cu bimetallic catalysts for propane
hydrogenolysis reactions on a Ni-Cu model catalyst under technique relevant reaction
conditions. The total reaction rate of propane hydrogenolysis over Ni-Cu bimetallic
catalysts sharply decreased with the increase of Cu coverage. However, the specific
surface activity of propane hydrogenolysis (number of propane molecules reacted per
Ni active site per second) changed within only one-order, unlike the reported results
that a three-to-five order decrease in the specific activity of ethane or propane
hydrogenolysis over Ni-Cu bimetallic catalysts. The significant differences between
our studies and the literature results are discussed, and new insights of the
structure-activity relationship of Ni-Cu bimetallic catalysts were obtained.
2. Experimental Section
All the experiments were carried out in a stainless steel ultrahigh vacuum
(UHV) chamber with a coupled high pressure cell for adsorption and reactions at
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elevated pressures.12, 33-35 Briefly, the system was equipped with Auger electron
spectroscopy (AES), low energy electron diffraction (LEED) and a UTI 100 mass
spectrometer for temperature programmed desorption (TPD) measurement. The
bottom of the UHV chamber was coupled with a high pressure cell which could be
isolated from the main chamber by differentially pumped sliding seals for high
pressure studies. The base pressures for both the main chamber and the high pressure
cell are lower than 2.0×10-10 Torr. A linear ramp of 10 K/s was used for H2 TPD
measurements.
DOI: 10.1039/C5RA07433A
The preparation of Ni-Cu bimetallic catalysts supported on SiO2/Mo(110) was
described previously in details12, 33, 35. Briefly, Ni and Cu were evaporated on thin
SiO2 films supported on Mo(110) substrates via resistive heating of a tantalum
filament wrapped with respective high pure metal wires. The supported Ni-Cu
bimetallic catalysts were annealed at 700 K for 5 min before any measurements. For
all the measurements, the rear sides of Mo(110) substrates were covered with SiO2
thin films without metal depositions. The amount of metal deposited was controlled
by depositing time, calibrated with AES spectra, and was given in the unit of
monolayer equivalent (MLE, 1 MLE = 1.43×1015 atom per cm2).12, 35 Ni-Cu will grow
into nanoparticles on SiO2 thin films. It has been measured the average particle size is
around 4 nm for 5 MLE Ni/SiO2.33
Ultrahigh purity H2 was further purified by passing through a LN2 cooling trap.
Propane was purified by repeated freeze-pump-thaw cycles before mixed with
hydrogen. The propane hydrogenolysis reactions were carried out in the high pressure
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cell as a batch reactor (1.2 L) using C3H8/H2 (1:5 ) mixtures at a total pressuDreOI:o1f0.160039/C5RA07433A
Torr at 600 K for 20 min. The conversions of propane were controlled below 5%. The
reaction products were analyzed with a HP 5890 gas chromatograph equipped with a
flame ionization detector and a Porak Q column. Blank experiments on SiO2 films
showed that the rate of propane hydrogenolysis was four orders lower than
monometallic Ni/SO2 under the same reaction conditions.
Fig. 1. (a) Reaction rates of propane hydrogenolysis in 60 Torr C3H8/H2 (1:5) at 600
K as a function of Cu coverage on Cu-5MLE Ni/SiO2 catalysts. The reaction rate of
propane hydrogenolysis on 5 MLE Cu/SiO2 was shown as a comparison. (b)
Selectivity S = R2/(R1+R2) versus Cu coverage on Cu-5MLE Ni/SiO2 catalysts. The
solid line is a guide for the eye.
3. Results and Discussion
3.1 Reaction rates of propane hydrogenolysis over Ni-Cu/SiO2
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Fig. 1(a) shows the reaction rates of propane hydrogenolysis at 600 DKOI: 1o0v.10e3r9/C5RA07433A
Ni-Cu/SiO2 bimetallic catalysts as a function of Cu coverage on Cu-5MLE Ni/SiO2.
The reaction rates of propane hydrogenolysis sharply decrease by two orders in
magnitude with increasing Cu coverages from 0 to 3 MLE. The reaction rate of
propane hydrogenolysis on pure Cu (5 MLE Cu/SiO2) was also measured, which is
about four orders lower than that of the monometallic Ni (5 MLE Ni/SiO2), within the
same order as background reaction on clean SiO2 thin films. These observations agree
well with previous reported results that the reaction rate of ethane hydrogenolysis
decreased markedly with addition of Cu to Ni catalysts.13-15 There are two possible
reactions for propane hydrogenolysis: (i) rupture of both the C-C bonds in the propane
to form methane; (ii) rupture of only one C-C bond in the propane to form methane
and ethane. The selectivity of reaction (ii) was shown in Fig. 1(b), which is consistent
with reported results that the selectivity for partial hydrogenolysis increases with
increasing Cu coverage.25 On clean Ni surface, the selectivity is only 5%, which
indicates almost all the C-C bonds are fully ruptured due to the strong interaction
between propane and Ni surfaces. The increased selectivity can be explained by
ensemble effect. The decreased Ni-Ni adjacent site number by adding of Cu will also
account for the decreased possibility for propane complete hydrogenolysis.
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DOI: 10.1039/C5RA07433A
Fig. 2. (a) H2 TPD on Cu-5MLE Ni/SiO2 as a function of Cu coverage. Ni-Cu/SiO2
bimetallic catalysts were prepared by depositing indicated coverage of Cu onto 5
MLE Ni/SiO2 and subsequently annealing at 700 K for 5 min. Saturated H2 adsorption
was performed by exposing Ni-Cu/SiO2 to 1×10-7 Torr H2 for 10 min at about 100 K.
A linear ramp of 10 K/s was used for TPD measurement. (b) Estimate Ni active sites
by H2 TPD on Ni-Cu/SiO2 compared with H2 TPD on 10 ML Ni/Mo(110) films. The
surface atomic density of the 10 ML Ni/Mo(110) was assumed to be 1.8×1015 /cm2.
The error bar represents 20% uncertainty in the Ni active site measurements. The
fitted solid line is a guide for the eye.
It has been shown that the active Ni sites can be measured by selective H2
adsorption under UHV conditions without the possible hydrogen spillover effect from
Ni to Cu.12 H2 TPD on Ni-Cu/SiO2 as a function of Cu coverage, as shown in Fig.
2(a), indicates all hydrogen desorbs from Ni sites without H2 desorption features on
Cu sites, which will appear at lower desorption temperatures.12 One interesting feature
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from Fig. 2a is that the H2 desorption peaks first increase to 372 K with increasing Cu
coverage from 0 to 2 MLE, and then slowly decrease to 354 K at 5 MLE Cu coverage.
The first increase of the hydrogen desorption temperature may be caused by the
decrease of molecule-molecule interaction by adding Cu on the surface. When Ni
sites are highly isolated by Cu atoms at high Cu coverages, the adsorption of
hydrogen on Ni sites can be weakened due to the electronic modification effect by Cu
to Ni. The surface Ni active sites could be estimated by comparing the H2 TPD
desorption intensity on Ni-Cu/SiO2 with H2 TPD desorption intensity on a 10 ML
Ni/Mo(110) film, as shown in Fig. 2(b). Since multilayer Ni films on a Mo(110)
mainly expose (111) facet, it is reasonable to assume the surface atomic density of the
10 ML Ni/Mo(110) equal to that of the Ni(111) surface (1.8×1015 atoms/cm2 )33.
Combined the H2 TPD results with the known surface area of the Mo(110) (0.78 cm2),
the active sites could be estimated. The H2 TPD intensity on 1 MLE Cu-5 MLE
Ni/SiO2 is just about 5% of that on 5 MLE Ni/SiO2, which indicates Cu enriches on
the Ni-Cu alloy surfaces due to the lower surface free energy of Cu than Ni35-38. After
that, the H2 desorption intensity remains almost constant independent of the Cu
coverage. The trend of the evolution of H2 TPD intensity with Cu coverage looks
similar to the reaction rate of propane hydrogenolysis as reported in Fig. 1(a).
In order to get more clear surface structure-activity relationship, specific surface
activity of propane turn over frequency (TOF, number of propane molecules
consumed per active site per second) as a function of Cu coverage for propane
hydrogenolysis reaction over Ni-Cu/SiO2 was plotted in Fig. 3(a) by using the
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measured reaction rates of propane hydrogenolysis (Fig. 1) and the estimated Ni
active sites (Fig. 2). The propane TOF slightly increases from 1.2 s-1 on clean 5 MLE
Ni/SiO2 to 2.0 s-1 at 1 MLE Cu coverage, and then decreases to 0.6 s-1 at 3 MLE Cu
coverage. The first increase in the propane TOF is mainly because the increase of
selectivity of ethane products with adding of Cu on Ni. If the specific reaction rate is
plotted as the turn over frequency of C-C bond ruptures, as shown in Figure 3(b), the
turn over frequency of C-C rupture changes only from 2.3 s-1 on pure 5 MLE Ni/SiO2
to 2.9 s-1 on 1 MLE Cu-Ni/SiO2. The specific activity of Ni toward C-C rupture does
not change with Cu coverages up to 1 MLE. But the complete hydrogenolysis of
propane to methane is hindered by adding of Cu, which results in the slight increase in
the specific activity of propane hydrogenolysis. With Cu coverages above 1 MLE, the
continuous decrease of propane TOF may be caused by the decreased activity of Ni
when surrounded and isolated by Cu atoms at high Cu coverages. The electronic
effect leads to the decrease of the specific activity.39 It has been reported that the
adsorption energy of CHx decreases on Ni when alloyed with Cu40. Our results show
the specific activity of Ni sites for propane hydrogenolysis could be tuned by adding
of Cu, but varies just in a small relative range (0.4 to 2.0 s-1). It has been reported that
there was a three-to-five order decrease in the specific reaction rate of ethane or
propane hydrogenolysis over Ni-Cu bimetallic catalysts with 50 atomic% Cu
compared with that on pure Ni catalysts.13-15, 25, 26 Our data presented here are
significantly different from those reported results. The uncertainties in the Ni active
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site measurement from H2 TPD are estimated less than 20%,41 which will not change
DOI: 10.1039/C5RA07433A
the trend observed in the propane hydrogenolysis specific activity.
Fig. 3. (a) Propane turnover frequency (TOF) as a function of Cu coverage on Cu-5
MLE Ni/SiO2. Reaction conditions: C3H8:H2 = 1:5, total pressure = 60 Torr at T = 600
K for 20 min. (b) Specific C-C rupture reaction rate as a function of Cu coverage on
Cu-5 MLE Ni/SiO2. One C-C bond rupture in propane hydrogenolysis to ethane and
methane, and two C-C bond ruptures in propane complete hydrogenolysis to methane.
The error bars in (a) and (b) represent 20% uncertainty from Ni active site
measurement by H2 TPD.
The difference is probably caused by the over counting of Ni sites in the active
site measurement by selective hydrogen chemisorptions usually conducted at
atmospheric pressure conditions for technique bimetallic catalysts characterizations7,
where hydrogen spillover from Ni to Cu is kinetically possible.12 Our recent reports
show direct evidence that saturated atomic hydrogen on Ni could spill over to Cu on
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Ni-Cu bimetallic catalysts12. Crucq et al. observed hydrogen spillover from Ni to Cu
on Ni-Cu alloys in their hydrogen adsorption isotherms.28 On Ni-Cu bimetallic
catalysts, Cu is enriched on the surface due to its lower surface free energy35. From
clean monometallic Ni catalysts to Ni-Cu bimetallic catalysts with high Cu
concentrations, several order decrease in the surface Ni sites can be expected, which
is the main reason for sharp decrease in the total reaction rate in hydrogenolysis. The
correctness in the measurement of the specific reaction rate highly depends on the
reliability of the surface active site measurements. Any adsorbate spillover or surface
segregation induced by adsorption can lead to large discrepancy in the counting of
surface active sites via selective chemisorptions.12, 35 Goodman et al. had reported
that hydrogen spillover from Ru to Cu happens on Ru-Cu bimetallic catalysts,
DOI: 10.1039/C5RA07433A
resulting in the active site over counted by selective hydrogen chemisorptions23, 24, 29
.
Yao and Goodman have shown CO adsorption is not suitable for Ni surface site
measurement on Ni-Cu bimetallic catalysts even under UHV conditions due to CO
adsorption induced Ni surface segregation.
Moreover, our results here indicate the proposal that a large ensemble of Ni
atoms is required for hydrogenolysis reactions over Ni-Cu bimetallic catalysts may
be not true1, 25. Certain atom ensembles, not limited to adjacent Ni sites, may be
required to accommodate the C-C rupture in the hydrogenolysis reactions. Cu atoms
around Ni sites could play a more important role rather than traditionally considered
as diluting inert agents, such as acting as receivers for hydrogen or carbon hydrogen
species activated by Ni sites30, 31. Recently Medlin et al. reported synergistic effect
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was observed in Cu4Ni alloy catalysts in hydrogenation reactions due to a hDyOdIr: o10g.1e0n39/C5RA07433A
spillover effect from Ni to Cu.32 The determination of the smallest atomic ensembles
for the C-C rupture in the hydrogenolysis reactions is out of the scope of this paper.
But a large ensemble of 12-to-18 adjacent nickel atoms proposed by Dalmon and
Martin is not necessary for this reaction based on our present results.25
Fig. 4. Post-reaction AES spectra on Cu-5 MLE Ni/SiO2 as a function of Cu coverage.
The AES spectra on fresh SiO2, 5 MLE Ni/SiO2 and 5 MLE Cu/SiO2 were also shown
for comparison. After 20 min reaction of propane hydrogenolysis at 600 K, the
reaction cell was evacuated and pumped down to UHV. The samples were transferred
to analysis chamber under UHV conditions and flashed to 600 K before AES
measurements.
The possible carbon deposits in the propane hydrogenolysis over Ni-Cu/SiO2 were
studied by post-reaction AES spectra, as shown in Fig. 4. After annealing to 600 K
(reaction temperature) in UHV, there are no carbon deposits detected on Ni-Cu/SiO2
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bimetallic catalysts. This results show under the reaction conditions there is nDoOsI:t1r0o.1n0g39/C5RA07433A
adsorbed carbon species deposited on the Ni-Cu surfaces excluding the possible
influence of carbon deposits in the hydrogenolysis reactions.25 But there is relative
amount carbon deposits detected on the monometallic Cu catalysts of 5 MLE Cu/SiO2,
which agrees with that carbon layers could form on Cu surfaces by cracking of carbon
hydrogen species.42 This result indicates carbon deposits could take place on Cu rich
Ni-Cu surfaces due to the lack of active Ni sites to remove carbon deposits by
hydrogenation reactions.
Another effect on the surface reactivity of Ni-Cu/SiO2 bimetallic catalysts is the
possible surface segregation of Ni under propane hydrogenolysis reaction conditions.
We have shown CO adsorption could induce Ni surface segregation due to strong
interaction between CO and Ni.35 Molecular propane has weak interaction with Ni,
but the dissociated carbon hydrogen species could have strong interaction with Ni.
However, these reaction intermediates will be quickly hydrogenated and have low
surface coverages. Our present results do not show obvious Ni surface segregation
effect in propane hydrogenolysis reactions, but the possibility cannot be excluded.
4. Conclusions
Propane hydrogenolysis reactions were investigated on Ni-Cu/SiO2 model
catalysts. The total reaction rate of propane hydrogenolysis sharply decreases on
Ni-Cu bimetallic catalysts as a function of Cu coverage due to the surface enrichment
of Cu. The selectivity for ethane formation in propane hydrogenolysis is enhanced by
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adding of Cu to Ni on Ni-Cu bimetallic catalysts. Specific activity of propane
hydrogenolysis is slightly affected by adding of Cu to Ni on Ni-Cu bimetallic
catalysts, which varies in the range of 0.6 to 2.0 s-1. The significant differences of the
Specific activity of propane hydrogenolysis over Ni-Cu bimetallic catalysts between
the present study and those reported in literature are mainly because the possible over
counting of the active Ni sites in the selective hydrogen chemisorptions, where
hydrogen spillover from Ni to Cu could occur under atmospheric pressure conditions.
This research gains new insights into the surface structure-activity relationships for
Group VIII-Group IB bimetallic catalysts, e.g. Ni-Cu, which will benefit the designs
of highly selective catalysts.
Acknowledgements
This work was financially supported by the US Department of Energy, Office of
Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and
Bio-sciences (DE-FG02-95ER-14511).
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